Upcoming seminars


We have resumed the seminar series online. Welcome back!

______________________________________________________________________________

______________________________________________________________________________


Dr. Kartik Venkataraman




Realizing Atomic Resolution Vibrational Spectroscopy


Vibrational spectroscopy has become a ubiquitous characterization tool in materials science, revealing compositional information and yielding spectral fingerprints for atomic bonding arrangements. Traditional vibrational spectroscopies based on infrared photon absorption, visible photon scattering, and inelastic neutron scattering boast of sub-meV energy resolution but suffer from poor (~1 μm) spatial resolution. Near-field infrared and optical vibrational spectroscopies have demonstrated significant improvement in spatial resolution, routinely yielding ~50 nm. Such spatial resolution still limits characterization of individual nanoparticles or atomic to nanoscale heterogeneities like defects and interfaces in materials. In 2014, a novel vibrational spectroscopy was realized which combines 1Å spatial resolution routinely achievable in an aberration-corrected scanning transmission electron microscope (STEM) with 3 – 8 meV energy resolution achievable using monochromated electron energy-loss spectroscopy (EELS). Since its realization, vibrational spectroscopy with monochromated STEM EELS has seen tremendous progress like performing damage-free nanoscale detection for both organic and inorganic specimens, measuring temperature in nanometer-sized areas with 1 K precision, determining the phonon dispersion in individual nanoparticles, demonstrating atomic resolution, etc., which has impacted a wide variety of important scientific problems. In this talk, I will present my contribution to this field interlaced with seminal contributions from researchers all around the world.

Personal Page



Past seminars


Year 2021

Prof. Collin Broholm




Emergent quasi-particles in frustrated magnets


Crystalline solids with Interacting local moments that do not fully order at low temperatures can support a variety of strongly correlated electronic phenomena. With no static magnetism or symmetry breaking, the quantum spin liquid (QSL) represents an important albeit rare limit [1]. A much larger variety of materials has been found to have spin liquid like properties in restricted temperature and frequency regimes. I shall describe experiments that explore such materials by scattering neutrons from their emergent magnetic quasi-particles. The materials range from insulating rare earth oxides through transition metal oxides near the metal insulator transition to Kondo semi-metals and heavy fermions systems. Conducted using the latest neutron scattering instrumentation, the experiments offer detailed atomic scale insights into strongly correlated phenomena that challenge conventional understanding of magnetism in solids.

[1] C. Broholm, R. J. Cava, S. A. Kivelson, D. G. Nocera, M. R. Norman, and T. Senthil, Science, Vol. 367, Issue 6475, (2020).

Personal Page

Prof. Sergey Frolov




How do we discover Majorana particles in nanowires?


I have been on a ten year journey to gather evidence of Majorana particles, a quarter of my life. For me personally this came from theorists: frist Carlo Beenakker and his students who presented at our group meetings, and then - during a 30 second conversation with Roman Lutchyn at the 2010 March meeting, when I realized that we could try this in semiconductor nanowires which I was working with in the Delft lab as a postdoc. (Wires came from Sebastien Plissard and Erik Bakkers in Eindhoven).
I fleshed out ideas for how we would do it and ghost-written a grant proposal with my advisor Leo Kouwenhoven. It gave us money to hire two amazing PhD students: Kun Zuo and Vincent Mourik. With my faculty job in Pittsburgh secure by that time, there was no risk in trying to hunt for Majorana, and we went for it with force. By 2012 we found promising Majorana signatures: zero-bias peaks in conductance that were sticky: did not move from zero voltage bias when we changed electric or magnetic fields. A lot of work lay ahead to verify, improve and find additional evidence. Yet we - and lots of other people - had little doubt that we were on track to demonstrate Majorana.
But nature prepared a surprise for us: in subsequent years other researchers, Eduardo Lee and Silvano de Franceschi from Grenoble, demonstrated that sticky zero-bias peaks can also come from Andreev states, not only Majorana. It is very rare that a phenomenon so carefully mapped out can also have another compelling explanation. At that time my old team in Delft, and my new team in Pittsburgh were struggling to find further Majorana signatures. It took time for us to embrace the fact that Majorana have not been discovered: some are still working through that.
Yet in a decade we have learned a lot. First, we learned how to tell Andreev from Majorana. Even if all we measured so far is Andreev, we are sure that we will know Majorana when we see it. Second, we improved our nanowire materials by deploying advanced growth and nanofabrication techniques. We developed elaborate computational models that capture many subtleties of nanowire devices, with superconducting shells. The future is bright: because Majorana is based on a solid theoretical foundation, we should be able to make the discovery. If society still has the patience to let us pursue it. What of quantum computing applications? That will have to wait until we figure out the basic physics.

Personal Page

Dr. Debangshu Mukherjee




Gigabytes of data, picometers of precision:

How electron microscopy and data analytics is enabling quantitative materials science

There has been a hardware revolution in transmission electron microscopy in recent years as advances in aberration correction and electron detectors have pushed both the achievable resolution and the information that can be extracted. While aberration correction has resulted in routine measurements of picometer scale displacements in materials, high-speed direct electron detectors now enable the entire diffraction pattern to be captured from every scan point. Such multidimensional datasets, also known as 4D-STEM, are the starting points for high precision strain measurements with nanobeam diffraction, electric field measurements with differential phase-contrast microscopy, and super-resolution microscopy through ptychography. This talk will focus on using these techniques to image ferroelectric displacements across domain walls, charge accumulation at domain walls in polar metals, and picometer precision strain measurements in catalyst nanoparticles. The associated software toolkits developed for these analyses will also be presented, along with upcoming developments that combine advanced computing with the microscope to speed up data analysis bottlenecks.

Personal Page

Dr. Avik Dutt




Synthetic dimensions: harnessing internal degrees of freedom of light for quantum and topological photonics


The dimensionality of a physical system strongly influences its properties. Moreover, richer topological features typically arise with increasing dimensionality, which necessitate complicated high-dimensional spatial structures. Recently, “synthetic dimensions” have emerged to circumvent the challenge of realizing such high-dimensional structures, by replacing one or more spatial dimensions with intrinsic properties of photons such as frequency, spin or temporal modes. Synthetic dimensions enable very simple photonic structures to be used for classical and quantum simulation of high-dimensional physics, complementing the recent surge in studying low-dimensional physics in quantum materials, 2D materials and cold atoms.

In this talk, I will discuss how dynamically modulated photonic cavities provide a fertile ground to realize synthetic dimensions and demonstrate complex topological effects. We introduce and demonstrate a synthetic-dimension spectroscopy technique to directly read out band structures from a time-resolved transmission. Using this technique, we probed 2D quantum Hall physics in a single cavity by simultaneously harnessing two synthetic dimensions of frequency and spin, thus elucidating how higher-dimensional physics can be implemented in simpler, experimentally feasible lower-dimensional structures. In such a cavity, neutral photons experience an artificial magnetic field, allowing us to observe a wide variety of phenomena such as spin-orbit coupling, spin-momentum locking, chiral edge currents and a Meissner-to-vortex phase transition at room temperature. Examples of the extreme tunability of synthetic-space photonic circuits to realize flexibly reprogrammable long-range complex coupling and reconfigurable lattice Hamiltonians will also be provided, in a manner that is challenging in real-space architectures. The talk will conclude with prospects for studying new phases of light and matter such as higher-order topological insulators and non-Hermitian phases, and also provide an outlook for using such concepts for quantum and nanophotonic technologies.

Personal Page

Year 2020

Prof. Anshu Pandey


  • Venue: Online
  • Date : Dec 08, 2020
  • Time: 3.00 pm
  • Note: Different day and time
Talk Video

Unconventional Properties of Au-Ag Nanostructures


This presentation will describe our observations of unconventional behavior in engineered Au-Ag nanostructures. Structurally, these comprise of ~1 nm Ag nanoclusters embedded into an Au matrix. These materials behave unconventionally and fail to exhibit metal-like behavior on several fronts. In particular, these materials show unusual transitions to a highly conductive, strongly diamagnetic state. Further, in their optical properties, a suppression of Mie-like behavior is observed along with the emergence of a broad, surprisingly non-dissipative, scattering resonance.

Group Page

Prof. Allan MacDonald


  • Venue: AG 66
  • Date : Jan 28, 2020
  • Time: 2.30 pm
  • Note: Different day and venue

Superconductivity in Magic Angle Twisted Bilayer Graphene


When twisted to a magic [1] relative orientation angle near 1 degree, the moiré superlattice minibands of bilayer graphene become extremely narrow, electronic correlations become strong, and superconductivity occurs over a wide range of twist angles and gating conditions. I will discuss a theoretical examination of the ansatz [2] that superconductivity is mediated by the graphene sheet’s optical phonons, emphasizing the local nature of pairing that this mechanism implies and the role of low-energy flavor fluctuations.

[1] Moire bands in twisted double-layer graphene, R. Bistritzer and A.H. MacDonald, PNAS 108, 12233 (2011).
[2] Theory of phonon-mediated superconductivity in twisted bilayer graphene, F Wu, A.H. MacDonald and I Martin, Phys. Rev. Lett. 121, 257001 (2018).

Group Page

Public Lecture: Prof. Allan MacDonald


  • Venue: AG 66
  • Date : Jan 27, 2020
  • Time: 5.30 pm
Talk Video

Strong Correlation Physics in Moiré Superlattices formed in Two-Dimensional Crystals


Moiré patterns are ubiquitous in layered van der Waals materials, and can now be fabricated with considerable control by combining mechanical exfoliation of van der Waals layers with tear and stack device fabrication techniques. I will explain why the electronic and optical properties of two-dimensional semiconductors and semimetals are strongly altered in long-period moiré superlattices, focusing in particular on the remarkable example of twisted bilayer graphene. When twisted to a magic [1] relative orientation angle near 1 degree the moiré superlattice minibands of bilayer graphene become extremely narrow and electronic correlations become strong.

[1] Moiré bands in twisted double-layer graphene, R. Bistritzer and A.H. MacDonald, PNAS 108, 12233 (2011).

Group Page

Year 2019

Prof. Jeremy Levy


  • Venue: AG 69
  • Date : Nov 4, 2019
  • Time: 2.30 pm
Talk Video

Correlated Nanoelectronics


The study of strongly correlated electronic systems and the development of quantum transport in nanoelectronic devices have followed distinct, mostly non-overlapping paths. Electronic correlations of complex materials lead to emergent properties such as superconductivity, ma gnetism, and Mott insulator phases. Nanoelectronics generally starts with far simpler materials (e.g., carbon-based or semiconductors) and derives functionality from doping and spatial confinement to two or fewer spatial dimensions. In the last decade, these two fields have begun to overlap. The development of new growth techniques for complex oxides have enabled new families of heterostructures which can be electrostatically gated between insulating, ferromagnetic, conducting and superconducting phases. In my own research, we use a scanning probe to “write” and “erase” conducting nanostructures at the LaAlO3/SrTiO3 interface. The process is similar to that of an Etch-a-Sketch toy, but with a precision of two nanometers. A wide variety of nanoscale devices have already been demonstrated, including nanowires, nanoscale photodetectors, THz emitters and detectors, tunnel junctions, diodes, field-effect transistors, single-electron transistors, superconducting nanostructures and ballistic electron waveguides. These building blocks may form the basis for novel technologies, including a platform for complex-oxide-based quantum computation and quantum simulation.

Group Page

Dr. Ajaya K. Nayak


  • Venue: AG 69
  • Date : July 22, 2019
  • Time: 2.30 pm
Talk Video

Imaging magnetic anti-skyrmions using Lorentz TEM


Chiral magnetic solitons, such as, domain walls, skyrmions, etc, are subjects of extreme research interests due to the ever increasing demand in high density data storage devices. Skyrmions, in particular, due to their topologically protected structure are considered to be the most exciting candidates for spintronic applications. In chiral magnets with broken inversion symmetry skyrmions are stabilized by competition between the Heisenberg exchange and the Dzyaloshinskii-Moriya interaction (DMI). In recent study using Lorentz Transmission Electrom Microscopy (LTEM), it has been demonstrated that the D 2d crystal symmetry based tetragonal Heusler compounds can exhibit a new type of skyrmion, named antiskyrmions [1]. The presence of antiskyrmions in the bulk Heusler compounds is demonstrated using various noble techniques, such as, magnetic entropy change [2], topological Hall effect and ac- susceptibility measurements [3]. We have also shown that by systematically tailoring the sub- lattice moments it is possible to achieve a fully compensated magnetic state that can result in compensated antiskyrmions. These compensated antiskyrmions are supposed to exhibit zero skyrmion Hall effect and hence are perfect candidates for their use in racetrack memory.

[1] A. K. Nayak et al., Nature 548, 561 (2017).
[2] Sk Jamaluddin et al., Adv. Funct. Mater. 29, 1901776 (2019).
[3] S. Sen et al., Phys. Rev.B 99, 134404 (2019).

Group Page

Prof. Mohit Randeria


  • Venue: AG 69
  • Date : July 15, 2019
  • Time: 2.30 pm
Talk Video

Upper bounds on the superconducting transition temperature:

Applications to twisted-bilayer graphene and ultra-cold Fermi gases


Understanding the material parameters that control the superconducting transition temperature Tc is a problem of fundamental importance. We use sum rules to derive a rigorous upper bound on the superfluid phase stiffness Dsvalid in any dimension. This in turn leads to an upper bound on Tcin two dimensional (2D) systems, which holds irrespective of mechanism, strength of pairing interaction, or order-parameter symmetry. While this bound is of general validity, it is particularly useful and leads to stringent constraints for the strongly correlated regime of low-density and narrow-band systems, where conventional the BCS-Eliashberg approach fails. For a simple parabolic band, we find that kBTccan never exceed EF/8 in 2D. We show that this bound is close to being saturated in 2D ultracold Fermi gases in the strongly interacting regime of the BCS-BEC crossover. Applying our multi-band bound to magic-angle twisted bilayer graphene (MA-TBG), we find that the available electronic structure results already constrain the maximum possible Tcto be close to the experimentally observed value. Finally, I will discuss the theoretical challenges in deriving rigorous upper bounds on Tcin three dimensions (3D) and the experimental evidence for or against such a 3D bound.

Group Page

Dr. Abhinav Kandala


  • Venue: AG 69
  • Date : Apr 26, 2019
  • Time: 2.30 pm
  • Note: Different day
Talk Video

Quantum computation with noisy superconducting qubits


Improvements in the control and coherence of superconducting qubits have enabled the development of noisy intermediate-scale quantum (NISQ) processors, and the exploration of problems addressable by NISQ devices that are intractable to classical computation. In this talk, I shall present a brief summary of superconducting qubit technology and discuss results from small-scale demonstrations of algorithms for quantum simulation [1,2] and machine learning [3]. These experiments highlight the detrimental effect of incoherent and readout errors on computations with noisy devices. While this can be remedied, in theory, with quantum error correction, the resources required for experimental implementations are prohibitively large for the near term. In this context, I shall introduce “error mitigation” techniques [4] that enable access to noise-free estimates of expectation values after the application of a short depth quantum circuit, without requiring any additional hardware resources. In particular, I shall demonstrate how our implementation of a zero-noise extrapolation method extends the computational capability of a noisy quantum processor [2].

[1] A. Kandala, et al Nature, 549, 242 (2017)
[2] A. Kandala, et al Nature, 567, 491 (2019)
[3] V. Havlicek, et al Nature, 567, 209 (2019)
[4] K. Temme, et al PRL, 119, 180509 (2017)

Group Page

Prof. Piers Coleman


  • Venue: AG 69
  • Date : Mar 22, 2019
  • Time: 2.30 pm
  • Note: Different day

Order Fractionalization


Right across condensed matter research today: from pump-probe spectroscopy, to time-crystals, research has been energized by a new appreciation of the role of the time domain. Order fractionalization is the proposal that the time domain can play a new role in broken symmetry phases of matter, through the observation that the development of fermionic bound-states can cause classical Weiss molecular fields to break up dynamically into spinors that act at widely separated times, leading to a new class of dynamical broken symmetry.

It has long been assumed that electronic order involves a condensation of bosons composed of an even number of electrons and holes, carrying integer spin and even charge. Here we propose the existence of a new class of quantum phase in which quantum observables fractionalize into half-integer bosons that condense.

Using the numerical renormalization group studies we demonstrate that fractionalized order can be induced in quantum impurity models by the application of an external field. The mechanism involves a fractionalization of fermionic bound-states into a half-integer molecular field and a residual “dark fermion”. In lattice quantum systems, the molecular fields created by this process are expected to sustain a stable, spontaneous order fractionalization with long range order, and the resulting novel phases will exhibit an observable coincidence of broken symmetry and fractionalized excitations.

Ramifications of this idea may affect a broad class of quantum materials. The possibility of order fractionalization in the context of particle physics will also be briefly discussed.

[1] Y. Komijani, Anna Toth, Premala Chandra, Piers Coleman, arXiv:1811.11115

Group Page

Year 2018

Dr. Lara Benfatto


  • Venue: AG 69
  • Date : October 29, 2018
  • Time: 2.30 pm
Talk Video

New frontiers in the optical detection of superconducting collective modes


Spontaneous symmetry breaking across the superconducting critical temperature is characterized by the emergence of a finite order parameter and by collective electronic excitations connected to its fluctuations. The amplitude fluctuations are usually named Higgs mode for their analogy with the massive boson of the standard model, while phase fluctuations identify the Goldstone massless excitation expected when the broken symmetry is a continuous one. Their description needs to go beyond BCS theory, which nonetheless explains with great accuracy the conventional spectroscopies in standard superconductors. The reason is that these collective excitations are spectroscopically inert in ordinary single-band BCS superconductors. In the last few years, a number of experiments have been proposed to detect and identify the superconducting Higgs and Goldstone mode. Here I will review our theoretical understanding of the optical properties of superconducting collective excitations in a variety of systems, considering the peculiar effects due to space inhomogeneity, coexisting states and non-linear optical response.

Group Page

Prof. Mikko Möttönen


  • Venue: AG 69
  • Date : October 22, 2018
  • Time: 2.30 pm
Talk Video

In-situ-tunable dissipators for superconducting quantum circuits --- and a glimpse of electron pumps and magnetic monopole analogues


I will focus on our efforts on implementing superconducting quantum electric circuits such as superconducting qubits. For quantum computers, fast and accurate initialization of qubits, it is of utmost importance to be able to quickly remove any unwanted qubit excitations on demand. Furthermore, a reduction of excess photon population in qubit-coupled resonators is important in tackling shot-noise-induced dephasing. To this end, we recently introduced two devices: (i) a quantum-circuit refrigerator (QCR) [1, 2] and (ii) a tunable heat sink [3]. The QCR is a stand-alone component that can be integrated with most superconducting quantum electric devices without major compromises in their other design criteria. In our experiments, we show how we can tune the dissipation of a superconducting resonator by orders of magnitude just by applying a bias voltage on the refrigerator. The time scale for switching the dissipation on and off is in the nanosecond range. We also observe a tunable broad-band Lamb shift [4] owing to the dissipation induced by the refrigerator. At high bias voltages, we observe that instead of refrigeration, we heat up the resonator mode up to 2.5 K providing an incoherent cryogenic microwave source [5]. For the heat sink [5], we observe that the quality factor of the resonator may be reduced from above 100,000 to a few thousand at 10 GHz in good quantitative agreement with the theoretical model. In the future, we aim to integrate these components with Xmon qubits and to demonstrate fast and accurate initialization [6]. If time permits, I will briefly discuss our efforts in establishing a single-electron pump for the realization of the new quantum standard of electric current [7, 8] and give an overview of our recent results on creating magnetic monopole analogues, quantum knots, and 3D skyrmions in spin-1 Bose-Einstein condensates [9-12].



  • [1] K. Y. Tan, et al., Nat. Commun. 8, 15189 (2017). https://dx.doi.org/10.1038/ncomms15189
  • [2] M. Silveri et al., Phys. Rev. B 96, 094524 (2017). https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.094524
  • [3] M. Partanen et al., Sci. Rep. 8, 6325 (2018). https://doi.org/10.1038/s41598-018-24449-1
  • [4] M Silveri et al. arXiv:1809.00822 (2018). https://arxiv.org/abs/1809.00822
  • [5] S. Masuda et al., Sci. Rep. 8, 3966 (2018). https://www.nature.com/articles/s41598-018-21772-5
  • [6] J. Tuorila et al., npj Quant. Inf. 3, 27 (2017). https://www.nature.com/articles/s41534-017-0027-1
  • [7] A. Rossi et al., Nano Lett. 14, 3405 (2014). https://pubs.acs.org/doi/abs/10.1021/nl500927q
  • [8] R. Zhao et al., Phys. Rev. Appl. 8, 044021 (2017). https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.8.044021
  • [9] M. W. Ray et al., Nature 505, 657 (2014). https://www.nature.com/articles/nature12954
  • [10] M. W. Ray et al., Science 348, 544 (2015). http://science.sciencemag.org/content/348/6234/544
  • [11] D. S. Hall et al., Nature Phys. 12, 478 (2016). https://www.nature.com/articles/nphys3624
  • [12] W. Lee et al., Science Adv. 4, eaao3820 (2018). http://advances.sciencemag.org/content/4/3/eaao3820.full
Group Page

Dr. Shouvik Chatterjee


  • Venue: AG 69
  • Date : October 1, 2018
  • Time: 2.30 pm

Shining Light on Quantum Materials: a combined MBE-ARPES approach


Quantum materials provide an exciting platform to understand emergent phenomena in condensed matter systems. Our ability to synthesize these materials with atomic precision using molecular-beam epitaxy (MBE) and simultaneously visualize its momentum-space electronic structure via angle-resolved photoemission spectroscopy (ARPES) allows us to gain insights into material systems that has remained out of reach of traditional experimental approaches. It also opens up new possibilities of engineering novel functionalities via materials design.

In this talk, I describe the efficacy of such an approach in three different material systems viz. inter-metallic mixed valence/heavy fermions, topological half-Heusler compounds, and rare-earth monopnictides.

I show how local valence fluctuations connect to momentum space concepts of band filling and Fermi surface topology in a prototypical mixed-valent Kondo lattice system YbAl 3 , where a decrease in Yb valence leads to a Lifshitz transition and an increased itinerancy of the Yb 4f states. Furthermore, we utilize the observed temperature dependence of the Yb 4f states to demonstrate enhancement of the spin hall effect by f electrons, where a giant spin hall conductivity of 12,000 ħ/2e S/cm is observed at 37 K. Next, I present experimental signatures of topological surface states in PtLuSb, a compound belonging to the multi-functional Heusler family. Utilizing substitutional alloying of Pt with Au in metastable Pt x Au 1-x LuSb epitaxial thin films, we are able to mitigate unintentional p-type doping in the parent compound and tune the chemical potential close to the Dirac point. Finally, I present our recent results on dimensional confinement in a compensated semi-metal LuSb that exhibits large magnetoresistance. By fabricating ultra-thin films, I show that it is possible to controllably create an imbalance in the band fillings of electron and hole-like carriers in this otherwise compensated semi-metal. I conclude with broad prospects of our approach and future directions.

Web Page

Dr. Ish Dhand


  • Venue: AG 69
  • Date : September 24, 2018
  • Time: 2.30 pm
Talk Video

Tensor networks for fun and profit


Tensor networks are a powerful parametrisation of quantum states. In this talk, I will give an accessible introduction to tensor networks and will give a flavour for two application of tensor networks in quantum many-body physics. First, I will describe how tensor networks can be used to perform efficient tomography of a quantum many-body system. I will also talk about recent and ongoing work on simulating open quantum systems such as the spin-boson and spin-fermion models and the central spin problem in the presence of dephasing using tensor network methods.

Efficient tomography: arXiv:1612.08000 (Nature Physics volume 13, pages 1158–1162);
Simulation: arXiv:1706.01315 (Phys. Rev. B 96, 174436) and arXiv:1710.01508 (Science Advances, 4(8), eaat8978)

Web Page

Prof. Michael Hatridge


  • Venue: AG 69
  • Date : June 12, 2018
  • Time: 2.30 pm
  • Note: Different day
Talk Video

Quantum Hamiltonian Engineering with parametric drives


One prerequisite for the construction of large-scale quantum machines is rapid, high fidelity qubit measurement. This challenge is now regularly met in superconducting qubits read out with parametrically driven microwave amplifiers. However, their near-quantum limited performance comes at the cost of virtually every other performance metric, including dynamic range, bandwidth, and directionality. In recent work, we have demonstrated the role played by fourth-order Kerr nonlinearities in shifting mode frequency with input power [1] and thus limiting device dynamic range. I will present new results demonstrating our ability to cancel these terms by shunting the Josephson junctions in the three-mode Josephson Parametric Converter (JPC) [2]. I will also discuss how multiply parametrically pumping this device, as recently proposed by Metelmann et al.[3], can result in a new form of amplification which is both directional, has higher saturation powers, and a fixed, broad bandwidth.

[1] Liu, et al. Appl. Phys. Lett. 111, 202603 (2017).
[2] Bergeal, et al. Nat. Phys. 6, 296 (2010).
[3] Metelmann and Clark, Phys. Rev. X 5, 021025 (2015)

Group Web Page

Prof. Prabhat Mandal


  • Venue: AG 69
  • Date : June 4, 2018
  • Time: 2.30 pm
Talk Video

Topological insulators and 3D Dirac/Weyl semimetals: A novel electronic state of matter


The electronic properties of graphene have opened up a new window in condensed matter physics research and device applications. The surface states of several materials such as B1-xSbx, Bi2Se3, Sb2Te3 and Bi2Te3 have striking similarities with graphene. In these materials, known as topological insulators, the bulk has a full insulating energy gap, whereas the surface possesses gapless highly conducting states. As a result, the electrical conduction is dominated by the surface. Following the discovery of topological insulators, another new class of materials, 3D Dirac semimetal, which is 3D analog of graphene has been reported recently. In this seminar, I present some of our recent works on electronic transport properties of topological insulators and 3D Dirac semimetals.

Group Web Page

Prof. Justin Song, Dept. of Physics, NTU, Singapore


  • Venue: AG 69
  • Date : Mar 29, 2018
  • Time: 2.30 pm
  • Note: Different day

Bloch-band quantum geometry and new collective modes

The internal structure of quasiparticles (such as spin or valley quantum numbers), while conventionally obscured from view, can dramatically alter their behavior when they couple with kinematic degrees of freedom. Featuring prominently in topological materials, this “spin-orbit”-type coupling can lead to unusual classes of behavior such as topological edge states, as well as quasiparticles that can acquire geometric phases as they propagate skewing their trajectories and enriching their dynamics.
I will discuss how the unusual carrier dynamics in topological materials lead to a new range plasmons, the collective modes of an electron liquid. Examples include anomalous Hall (Berry) plasmons that acquire a chirality even in the absence of an applied magnetic field in anomalous Hall metals, as well as collective oscillations of carriers in topological edge states that exhibit unusual spectral properties (e.g., hyperbolicity, extended lifetimes) owing to their edge-state origin.
If time permits, I will also discuss how plasmon quasiparticles can also possess and emergent internal (hidden) structure. This emergent non-trivial texture arises from the local current density configuration of plasmons and can take on an intricate pattern when a magnetic field is applied, exhibiting a non-trivial winding. As a result, plasmons to pick up non-trivial geometric phases when they scatter and dramatically alters their dynamics yielding new means for directing plasmonic beams, as well as a window into the hidden lives of plasmon oscillations.

Group Web Page

Public Lecture: Prof. Philip Kim


  • Venue: Homi Bhabha Auditorium
  • Date : January 8, 2018
  • Time: 5.00 pm
Talk Video

Stacking atomic layers: quest for new materials and physics


Modern electronics has heavily relied on the technology to confine electrons in the interface layers of semicoductors. In recent years, scientists discovered that various atomically thin materials including graphene, a single atomic carbon layer, can be isolated. In these atomically thin materials, quantum physics allows electrons to move only in an effective 2-dimensional (2D) space. By stacking these 2D quantum materials, one can also create atomic-scale heterostructures with a wide variety of electronic and optical properties. I will discuss the creation of new heterostructures based on atomically thin materials and emerging new physics with technological implications therein.

Group Web Page

Prof. Philip Kim


  • Venue: AG 66
  • Date : January 9, 2018
  • Time: 2.30 pm
  • Note different day and venue
Talk Video

Electronic and Optoelectronic Physics in the van der Waals Heterojunctions of 2-dimensional Materials


Heterogeneous interfaces between two dissimilar materials are an essential building block for modern semiconductor devices. The 2-dimensional (2D) van der Waals (vdW) materials and their heterostructures provide a new opportunity to realize atomically sharp interfaces in the ultimate quantum limit for the electronic and optoelectronic processes. By assembling atomic layers of vdW materials, such as hexa boronitride, transition metal chalcogenide and graphene, we can construct atomically thin novel quantum structures. Unlike conventional semiconductor heterostructures, charge transport in of the devices is found to critically depend on the interlayer charge transport, electron-hole recombination process mediated by tunneling across the interface. We demonstrate the enhanced electronic optoelectronic performances in the vdW heterostructures, tuned by applying gate voltages, suggesting that these a few atom thick interfaces may provide a fundamental platform to realize novel physical phenomena. In addition, spatially confined quantum structures in TMDC can offer unique valley-spin features, holding the promises for novel mesoscopic systems, such as valley-spin qubits. We report the fabrication of the gate-defined quantum structures formed in atomically thin TMDC heterostructures, exhibiting quantum transport phenomena and optoelectronic processes.

Group Web Page

Prof. Aveek Bid


  • Venue: AG 69
  • Date : January 15, 2018
  • Time: 2.30 pm
Talk Video

Superconductivity in Oxide heterostructure


The mutual interplay of point group symmetry, charge inversion symmetry, U(1) gauge symmetry and spin rotation symmetry in heterostructure of complex perovskite oxides lead to the co-existence of a host of intriguing properties - ferroelasticity, ferroelectricity, super-conductivity and ferromagnetism. Superconductivity and magnetism are generally considered to be incompatible with each other and hence the observation of the co-existence of these two phases in the conducting electronic layer formed at the interface of two insulating oxides LaAlO3 (LAO) and SrTiO3 (STO) has opened up a new direction of research in condensed matter physics. Despite intensive research over the last decade, there is no clear understanding of the origin of superconductivity and ferromagnetism in this system. There is now overwhelming evidence that superconductivity is mediated by phonons and is conventional BCS-like. Scanning superconducting quantum interference device (SQUID) measurements have revealed that the superconductivity in these systems is probably spatially inhomogeneous although more recent experiments may suggest otherwise. While torque measurements show a large in-field magnetization, scanning SQUID experiments show that there are only spatially inhomogeneous patches of local moments with no net magnetization. Although various scenarios have been invoked to reconcile these apparently contradictory experimental observations, a clear picture of the magnetization and superconducting behaviour of this system is yet to emerge.

We have observed a novel magnetic field assisted apparent transient superconducting state in the two-dimensional electron gas existing at the interface of LaAlO3/SrTiO3 heterostructure. This transient state appears when a relaxing normal magnetic field reduces the magnetization of the system to a value such that electron pairing becomes energetically favorable. The metastable state thus reached depends critically on the doping density in the parent compound. We find that at ultra-low temperatures and at moderate doping levels, superconductivity is a hidden order and is masked by the in-plane magnetization - appearing only when the net magnetization is sufficiently low. Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems.

Group Web Page

Prof. Umesh Waghmare


  • Venue: AG 69
  • Date : January 22, 2018
  • Time: 2.30 pm
Talk Video

Functional Properties of Materials at Nano-scale:First-principles Theory


Materials exhibit a remarkable diversity in their properties, which are determined fundamentally by their electronic and atomic structure, and involve processes that occur at many length, energy and time-scales. As a result, these properties evolve in interesting manner as the size of a material is reduced to nano-scale. As most modern technologically important materials (from semiconductors to structural materials used in jet engines) are nano-structured, it is important to understand how their specific functional properties depend on size and structure. In this talk, we will first introduce a "standard model" of materials that allows prediction of their structure and properties with no empirical knowledge in principle. Deriving a material-specific model with input from this first-principles microscopic description and analyzing it with Monte Carlo or Molecular Dynamics simulations, we determine its multi-scale behavior. In particular, we show how functional properties like ferroelectricity in oxides, superconductivity in Ta, and mechanical behavior of metals evolve from their bulk to nano-scale form. The mechanisms uncovered through such theoretical analysis can be very valuable in design and prediction of novel materials and technologies.

Group Web Page

Prof. Roderich Moessner, MPIPKS, Dresden, Germany


  • Venue: AG 66
  • Date : Feb 27, 2018
  • Time: 2.30 pm
  • Note: Different day and venue

Magnetic Majorana Fermions

Condensed matter systems provide emergent mini-universes in which quasiparticles may exist which do not correspond to any experimentally detected elementary particle. Topological quantum materials have been particularly productive in this regard, with the present search focussing on Majorana fermions, known theoretically already for decades. Here, we discuss manifestations of magnetic Majorana fermions in the Kitaev model. We place particular emphasis on their fate when perturbations, such as Heisenberg terms, are added to the ideal model system, and address experimental signatures of their vestiges in phases adjacent to the spin liquid.

Group Web Page

Dr. Vivishek Sudhir, LIGO Laboratory, MIT, USA


  • Venue: AG 69
  • Date : Mar 19, 2018
  • Time: 2.30 pm
Talk Video

Quantum measurement and metrology with a mechanical oscillator

The precision measurement of position has a rich heritage in physics. A 100 years ago, debate surrounding the atomic hypothesis was settled by careful measurements of the thermal (“Brownian”) motion of particulate matter. Today, similar measurements of random position fluctuations, but with a 17 orders of magnitude improvement in sensitivity, witness the vacuum fluctuations (“zero-point motion”) of nanoscopic objects. These advances, rooted in the pursuit of gravitational wave detection, hope to measure the position of a macroscopic object with the minimum noise allowed by the principles of quantum mechanics.
I shall present details of a series of experiments, performed over the past few years, where this hope has largely been realized. In these experiments, the fundamental vibrational mode of a glass nanostring is measured using an interferometric sensor with a precision that is sufficient to force a confrontation with the predictions of the uncertainty principle. We are able to observe the concomitant back-action of the measurement, and use feedback control to cancel it. We are also able to observe the generation of quantum correlations in the light exiting the interferometer. These non-classical correlations form a useful resource that can be used for metrological gain: we demonstrate that they can be used for a quantum enhancement of the force sensitivity of a room-temperature interferometer, via coherent back-action cancellation.
Some of these lessons learnt from a table-top experiment are now being scaled-up to improve the sensitivity of the current generation of interferometric gravitational wave detectors

Google Scholar Page

Prof. Prerna Sharma, IISc, Bangalore


  • Venue: AG 69
  • Date : Mar 26, 2018
  • Time: 2.30 pm

Curvature Instability of Chiral Colloidal Membranes on Crystallization

Crystallization is a well understood disorder-order phase transition in bulk three-dimensions. Faceted and flat single crystals appear during seeded growth from a melt. However, crystal morphology changes dramatically when the same transition is confined to two-dimensions. We show the existence of a curvature instability that arises during the crystallization of finite-sized monolayer membranes of chiral colloidal rods. While the bulk of the membrane crystallizes, its edge remains fluid like and exhibits chiral ordering. The resulting internal stresses cause the flat membrane to buckle macroscopically and wrinkle locally. Our results demonstrate an alternate pathway based on intrinsic stresses instead of the usual external ones to assemble non-Euclidean sheets at the colloidal length scale.

Group Web Page

Year 2017

Prof. T Senthil, MIT


  • Venue: AG 69
  • Date : July 4, 5 & 6 2017
  • Time: 11 am
Talk Video

Symmetry Protected Topological Phases of Quantum Matter

A major advance In the physics of the last decade is the theoretical prediction and experimental discovery of topological insulators. Though the topological insulator was initially discussed within the framework of free electron band theory, recent work has focused attention on the topological insulator phenomenon in interacting many particle systems. It is now recognized that free fermion topological insulators are a special case of a more general class of phases of matter known as Symmetry Protected Topological (SPT) phases. In these lectures I will describe these SPT phases with a focus on their physical (rather than mathematical) characterization. Studies of SPT phases have increasingly played a central role in quantum many body physics and have lead to deep and surprising connections between many different research topics. Time permitting, I will describe some of these as well.

Group Web Page

Prof. Jason Petta, Princeton


  • Venue: AG 69
  • Date : July 14, 2017
  • Time: 2.30 pm
Talk Video

Generating light with single electrons


Cavity-coupled double quantum dots (DQDs) allow the investigation of non-equilibrium physics in strongly-driven quantum systems. I will describe three quantum optics experiments involving semiconductor quantum dots, which are nanoscale systems that allow single electrons to be confined in three spatial dimensions. First, we have demonstrated a new type of maser that is driven by single electron tunneling events. Semiconductor double quantum dots serve as a gain medium and are placed inside of a high quality factor microwave cavity. Maser action is verified by comparing the statistics of the emitted microwave field above and below the maser threshold. 1 Second, we demonstrate a periodically-driven “Sisyphus” light source. In analogy with Sisyphus, a single electron is repeatedly pushed uphill in energy, only to relax to the ground state by emitting a microwave frequency photon. Since the device controls one and the same electron, photons are emitted in the absence of a dc current. 2 Lastly, using ultra-coherent silicon quantum materials, we have recently demonstrated coherent coupling of a single electron in silicon to a single microwave frequency photon, which results in the paradigmatic vacuum Rabi splitting. 3

1. Y.-Y. Liu, J. Stehlik, C. Eichler, M. J. Gullans, J. M. Taylor, J. R. Petta, “Semiconductor double quantum dot micromaser,” Science 347, 285 (2015).
2. J. Stehlik, Y.-Y. Liu, C. Eichler, T. R. Hartke, X. Mi, M. J. Gullans, J. M. Taylor, and J. R. Petta, “Double quantum dot Floquet gain medium,” Physical Review X 6, 041027 (2016).
3. X. Mi, J. V. Cady, D. M. Zajac, P. W. Deelman, J. R. Petta, “Strong coupling of a single electron in silicon to a microwave photon,” Science 355, 156 (2017).

Group Web Page

Prof. Takis Kontos


  • Venue: AG 69
  • Date : July 17, 2017
  • Time: 2.30 pm
Talk Video

Hybrid quantum circuits with carbon nanotubes


Carbon nanotubes are low dimensional conductors which can be used to implement various types of model system. They can be used to investigate various aspects of condensed matter system ranging from atomic like systems (i.e. quantum dots) to strongly correlated electron systems. In the first part of my talk, I will present our recent work in which we combine superconducting contacts with a magnetic texture proximal to a carbon nanotube. We demonstrate a large synthetic spin orbit interaction which deeply modifies the induced superconducting correlations in the carbon nanotube. We also observe a zero bias conductance peak which is the hallmark of Majorana zero modes. Recently, we have also demonstrated that nanotube based devices could be coupled to microwave cavities. These hybrid electron-photon system are interesting for quantum information processing but also to study fermion-boson problems relevant for condensed. I will show how we can strongly couple a quantum dot circuit to a microwave cavity but also use cavity photons to unravel the freezing of charge dynamics which accompanies the Kondo effect.

Group Web Page

Prof. Kater Murch


  • Venue: AG 69
  • Date : July 31, 2017
  • Time: 2.30 pm
Talk Video

Measurement and control in superconducting qubits: from the quantum Zeno effect to quantum enhanced metrology


The quantum Zeno effect is a feature of measurement-driven quantum evolution where frequent measurement inhibits the decay of a quantum state. We will explore how the opposite effect; the anti-Zeno effect - where frequent measurement accelerates decay - can also emerge from frequent measurement,. The emergence of one effect or the other elucidates the nature of quantum measurement and the role measurement plays in controlling quantum evolution. In a second experiment, we investigate how control over a single qubit can be used achieve a quantum speedup in the precision of frequency measurements, demonstrating a frequency sensitivity that improves as 1/T^2, where T is the duration of the experiment.

Group Web Page

Prof. Surajit Dhara


  • Venue: AG 69
  • Date : August 14, 2017
  • Time: 2.30 pm
Talk Video

Self-assembly of liquid crystal colloids via elasticity and topological defects


Topological defects have been objects of intense studies in various disciplines starting from cosmology to condensed matter, optics and more recently in active matter. In liquid crystals (LCs) they are created during the isotropic to nematic phase transition. The defects can also be induced by dispersing foreign nano- and micro-particles in LCs. The embedded particles create elastic distortions in the LC medium inducing topological defects, and they interact via long-range anisotropic elastic forces so generated. These forces obviously have no analogues in regular colloidal systems in an isotropic dispersive medium. An interesting manifestation of such novel forces is the ability of the colloidal system to self-assemble. In an experiment, such a process can be conveniently guided to create 2d and 3d colloidal crystals, with complex architectures. In this talk we will briefly present some of our recent results on transformations of such topological defects associated with the colloidal particles, such as hyperbolic hedgehogs, Saturn-rings and boojums, across the nematic to smectic-A and smectic-C phase transitions. We show that the divergence of bend elastic constant and the emergence of smectic layering have profound effects on these defects, in terms of the colloidal pair-interactions and their resulting two-dimensional assemblies. Finally we will discuss driven active-colloids in which the motion of the colloidal particles is powered by externally applied electric fields.

Group Web Page

Prof. Rajesh Ganapathy


  • Venue: AG 69
  • Date : September 4, 2017
  • Time: 2.30 pm
Talk Video

Deconstructing the Structural Glass Transition Through Experiments on Colloidal Suspensions


The dramatic slowing down of dynamics, without a concomitant change in structure, as a liquid approaches the glassy state is perhaps the most-puzzling and enduring problem in condensed matter physics [1]. While there exist many competing theoretical frameworks that attempt to explain the slowing down, distinguishing between them in atomic/molecular experiments continues to remain a challenge. This talk will describe results from particle-resolved colloid experiments that allowed us to critically assess competing theoretical frameworks of the glass transition [2,3]. We will show that the dynamical slowing down is accompanied by a growth in ‘amorphous order’. We will show that the amorphous order is associated with domains that rearrange cooperatively and quantify the interfacial tension of amorphous-amorphous interfaces [4].
Towards the end of my talk, I will describe results on encoding and retrieval of mechanical memory in sheared amorphous solids [5].

References
[1] Shreyas Gokhale, A K Sood and Rajesh Ganapathy, Advances in Physics 65, 363 (2016)
[2] Hima K Nagamanasa, Shreyas Gokhale, A K Sood and Rajesh Ganapathy, Nature Physics 11, 403 (2015)
[3] Shreyas Gokhale, Hima K Nagamanasa, Rajesh Ganapathy and A K Sood, Nature Communications 5, 4685 (2014)
[4] Divya Ganapathi, A K Sood and Rajesh Ganapathy (submitted 2017)
[5] Srimayee Mukherji, A K Sood and Rajesh Ganapathy (in preparation, 2017)

Group Web Page

Prof. Surjeet Singh


  • Venue: AG 69
  • Date : September 18, 2017
  • Time: 2.30 pm
Talk Video

Disordered Quantum Spin-1/2 Chains


In this talk, I will show some experimental results concerning the effect of dilute disorder on the ground state and excitations of quantum spin 1/2 chains. The disorder is induced by doping either magnetic or non-magnetic impurity in two representative Heisenberg spin 1/2 chain systems. The specific results that I will show include: (i) finite-size effects arising due to spin 0 or spin 1 impurities leading to gapping of the spin excitations; and (ii) anisotropic spin 1/2 impurities leading to Heisenberg to Ising crossover.

Group Web Page

Prof. Krishnendu Sengupta


  • Venue: AG 69
  • Date : October 16, 2017
  • Time: 2.30 pm
Talk Video

Translational symmetry broken Mott states of ultracold bosons


In this talk I am going to survey a class of translational symmetry broken Mott states which has been recently experimentally realized using ultracold bosonic atoms in optical lattices. The theoretical prediction and experimental realization of such states using ultracold bosons shall be charted out. This will be followed by a discussion of their out-of-equilibrium dynamics which would reveal the potential of such ultracold boson systems to serve as experimental platform for realization of several dynamical phenomena such Kibble-Zureck scaling and dynamic freezing.

Group Web Page

Prof. Anindya Das


  • Venue: AG 69
  • Date : October 23, 2017
  • Time: 2.30 pm
Talk Video

Andreev reflection in Graphene


Andreev reflection (AR) is the underlying quantum phenomena by which the current flows from a normal region into a superconductor at the normal-superconductor junction. This process is quite different for relativistic electrons in graphene. Theoretically, it has been predicted for both retro as well as specular type of AR in graphene. Despite extensive search for about a decade, specular Andreev reflection is only recently realized in bilayer graphene-superconductor interface. In this talk the evolution from the typical retro type Andreev reflection to the unique specular Andreev reflection in a van der Walls interface of single layer graphene and NbSe2 superconductor will be discussed. We find that the conductance becomes suppressed as we pass through the Dirac cone via tuning the Fermi level and bias energy. The suppression indicates the blockage of Andreev reflection beyond a critical angle of the incident electron with respect to the normal between the single layer graphene and the superconductor junction. In the second part of talk I will discuss about the AR in quantum Hall edge state of graphene. The coupling of a quantum Hall state and a superconductor has been proposed as a novel route for creating even more exotic topological entities, such as non-abelian Majorana, parafermion or Fibbonacci particles. As a step toward that goal, we demonstrate AR at the junction of a QH state in a single layer graphene and a two dimensional NbSe2 superconductor. We see characteristic signatures of Andreev reflection, such as: enhanced conductance inside the superconducting gap; oscillations in the conductance as a function of the magnetic field or the back gate voltage; and also an anomalous finite-temperature peak located precisely at the Dirac point, which provides a compelling evidence for inter-Landau level Andreev reflection.

Group Web Page

Prof. Satish Patil


  • Venue: AG 69
  • Date : November 6, 2017
  • Time: 3.00 pm (Note change in time)
Talk Video

Rational Design of Semiconducting Polymers for Electron Transport


A rapid improvement in the performance of organic solar cells, light-emitting diodes and field-effect transistors largely originate from the successful development of new conjugated polymers. However, the fundamental question still remains related to the unequal mobility of hole and electron in π-conjugated polymers. A rational design of polymers is necessary to target n-channel stable polymers, which can work in ambient processing conditions. Our laboratory employs a molecular engineering approach to develop high charge carrier n-channel semiconducting polymers for enhancing the performance of optoelectronic devices. In this seminar, the integrated approach to materials design for enhanced electron mobility will be discussed. Specific emphasis will be placed on the guideline principle of the donor-acceptor approach to rationally design the low band-gap polymers with minimum defect, optimum energy levels and high electron mobility.

References:
1) Mallari A. Naik, N. Venkatramaiah, Catherine Kanimozhi, and Satish Patil*, J. Phy. Chem-C, 2012, 116, 26128–26137
2) Kanimozhi, C.; Yaacobi-Gross, N.; Chou, K. W.; Amassian, A.; Anthopoulos, T. D.; Satish Patil, J. Am. Chem. Soc. 2012, 134, 16532

Group Web Page

Prof. Arnab Das


  • Venue: AG 69
  • Date : November 13, 2017
  • Time: 2.30 pm

Physics of Periodically Driven Closed Quantum Systems: Some Recent Elementary Developments


Non-equilibrium dynamics of driven many-body quantum systems is a largely unexplored but rapidly expanding subject. In this lecture we will discuss few elementary recent developments in the field, in the context of periodically driven closed (i.e., in absence of external bath) quantum systems. The talk is aimed to provide a broad theoretical overview of the general scenario.

Group Web Page

Prof. Subir Sachdev


  • Venue: AG 66
  • Date : November 17, 2017
  • Time: 11.00 am
  • Note: Different day, time and venue
Talk Video

Topological order in quantum matter


The powerful new idea of topological order has significantly advanced our understanding of the phases of quantum many-particle systems. Topological order characterizes multi-particle quantum entanglement in the ground state, and leads to gauge fields (abelian or non-abelian) in the description of the excitations. I will give examples of topological order in models of the high temperature superconductors, and connect them to recent observations and numerical computations on the “pseudogap” phase.

Group Web Page

Prof. Amit Ghosal


  • Venue: AG 69
  • Date : December 11, 2017
  • Time: 2.30 pm
Talk Video

Signature of glassy behavior associated with melting of Coulomb clusters


We present responses of a small number of Coulomb-interacting particles in two-dimensional confinements, across the crossover from their solid- to liquid-like behaviors. Here, irregular confinements emulate the role of disorder.
Focusing first on the thermal melting, where zero-point motion of the particles are frozen, we explore the signatures of a hexatic-glass like behavior. While static correlations, that investigate the translational and bond orientational order [1,2], indicate a hexatic-like phase at low temperatures, dynamical correlations show considerably slow relaxations. Using density correlations we probe intriguing inhomogeneities arising from the interplay of the irregularity in the confinement and long-range interactions. The relaxation at multiple time scales show stretched-exponential decay of spatial correlations in irregular traps [1,3]. Temperature dependence of characteristic time scales, depicting the structural relaxation of the system, show strong similarities with those observed for the glassy systems. Our results indicate that some of the key features of supercooled liquids emerge in irregular confined systems. The analysis of the normal modes [4] elucidates how long time behavior of the system is encoded in the quasi-localized modes.
Subsequently, we extend our studies to include the effects of quantum fluctuations. Our results, using quantum Monte Carlo techniques for Boltzmann particles, seem to indicate complementary mechanisms for the quantum and thermal crossovers in Wigner molecules [5]. Both thermal and quantum crossovers are associated with production of defects. However, these defects appear to play distinct roles in driving the quantum and thermal “melting”. We will also discuss our recent analyses upon including the effects of quantum statistics.

1. B. Ash, J. Chakrabarti and A. Ghosal, Phys. Rev. E 96, 042105 (2017).
2. D. Bhattacharya and A. Ghosal, Eur. Phys. J. B 86, 499, (2013).
3. B. Ash, J. Chakrabarti and A. Ghosal, Euro. Phys. Lett., 114, 4, (2016).
4. B. Ash, C. Dasgupta and A. Ghosal (unpublished)
5. D. Bhattacharya, A. V. Filinov, A. Ghosal and M.Bonitz, Eur. Phys. J. B 89, 60, (2016).

Group Web Page

Prof. Sumathi Rao


  • Venue: AG 69
  • Date : December 18, 2017
  • Time: 2.30 pm
Talk Video

Spin mode switching in the quantum Hall effect


Along with a brief introduction to the phenomenon of the integer quantum Hall effect, I will briefly review edge reconstruction due to Coulomb interactions and changes in the smoothness of the edge potential. Then I will describe some recent work where we show how exchange interactions can cause a switching of the spin of the edge modes, causing like spins to come close to each other.

Group Web Page

Dr. Shantanu Debnath


  • Venue: AG 69
  • Date : December 19, 2017
  • Time: 11.30 am
  • Note: Different day and time
Talk Video

Computations on a Programmable Quantum Processor Based on Trapped Atomic Ions


Realizations of small quantum computers have been achieved so far by engineering experimental systems to meet specific requirements of particular algorithms. I will present a quantum processor based on trapped atomic ions that allows a user to program any quantum algorithm in the software while staying blind to the underlying hardware [1]. The processor consists of a linear chain of trapped Ytterbium ions that can be manipulated selectively using an array of optical addressing Raman beams. I will discuss how the collective vibrations of the chain mediates long range interactions between qubits, which are engineered to realize a fully connected graph of two-qubit native gates [2]. We combine these techniques to realize a quantum computation architecture where programmed algorithm sequences are de-composed into native gate operations effected by shaped laser pulses. Using this device, we implement several algorithms that are based on the quantum Fourier transform, the Grover search, and the fault-tolerant encoding of a logical qubit. We then extend the capabilities of the processor to simulate a Hubbard like system of bosons by accessing the local vibrational (phonon) modes of individual ions in a chain [3]. We study the free hopping of phonons between sites as well as its suppression by selectively applying programmable blockades at individual sites.

Work done at: Joint Quantum Institute, Department of Physics, University of Maryland, College Park, MD 20742, USA
Currently at: Department of Physics, University of California, Berkeley, CA 94720, USA.

[1] S. Debnath, N. M. Linke, C. Figgatt, K. A. Landsman, K. Wright, and C. Monroe Nature 536, 63 (2016).
[2] T. Choi, S. Debnath, T. A. Manning, C. Figgatt, Z.-X. Gong, L.-M. Duan, and C. Monroe Phys. Rev. Lett. 112, 19502 (2014).
[3] S. Debnath, et. al. (in preparation).

Group Web Page

Dr. Sajal Dhara


  • Venue: AG 69
  • Date : December 27, 2017
  • Time: 2.30 pm
  • Note: Different day
Talk Video

Light-matter interaction in two dimensional systems


Graphene-like two dimensional semiconductors are promising materials for their intriguing electronic as well as optical properties. In this talk I will give an overview of some of the recent development in this field. I will consider atomically thick layers of transition metal dichalcogenides, which are direct bandgap semiconductors with large binding energy of longlived excitons. Strong light-matter interaction can be achieved using an optical microcavity to realize exciton-polaritons in this system. The ultralow mass of polaritons makes it a promising candidate to realize the polariton condensation and superfluidity at temperatures close to room temperature. I will present some of our experimental results where we directly probe the polariton energy band dispersion by angle resolved spectroscopy. A finite electron density in the system give rise to charged-excitons or trions which are like stable hydrogen ions. We observed that the dispersion of trion-polariton branch deviates from its expected characteristics as understood from a coupled harmonic oscillator model. The anomalous dispersion that we observed can be understood by considering interaction between the polariton branches mediated by free electrons in the system. The exciton-polaritons in this new material system can provide a playground to study non-equilibrium manybody physics.

Group Web Page