Simultaneous ferromagnetism and ferroelectricity in elemental Selenium

Multifunctional materials in which ferromagnetism and ferroelectricity not only coexist but are intrinsically coupled are of great technological interest and have generated intense research. The mutually exclusive nature of ferromagnetism and ferroelectricity is ascribed to symmetry restrictions as well as conflicting chemical requirements. In most of the known multiferroic systems, magnetism and ferroelectricity originate from different sublattices (e.g., in BiFeO3) or functional groups and are therefore weakly coupled. In this context, our observation that magnetic and ferroelectric orders coexist in elemental, single crystalline Selenium microtubes below 40K is very significant. The onset of the bi-ferroic ordering appears to be triggered by an order-disorder transition. This is the first time that multiferroicity has been observed in a simple element. A first-principles theoretical analysis reveals that this unexpected multiferroic behavior originates from Se being a weak topological insulator. It thus exhibits stable electronic states at its surface, and magnetism emerges from their spin polarization.

Finite Size Effects on Superconductivity

We have studied the size dependence of the superconducting transition temperature (TC), upper critical field (HC2) and energy gap in conventional superconductors such as Nb and Pb. We show that superconductivity persists far below the coherence length and is destabilized at a critical size predicted by the Anderson criterion. HC2 shows a remarkable enhancement with decreasing size. We have also suggested a simple formalism for understanding the role of superconducting proximity effect in granular multi-component systems. Nanocrystalline Nb exhibits another very interesting property. While well-dispersed Fe atoms in bulk Nb are non-magnetic, they acquire a magnetic moment with a decreasing size of the host (Nb) lattice due to an increase in the local density of states at the Fermi level.

Emergent behavior in Metal Nanorod Arrays

Emergent behavior (which arises essentially due to the collective nature of the nanorod arrays) leads to very interesting and technologically important properties, such as remarkable improvements in field emission, laser-induced x-ray emission and gas discharge characteristics. We have also shown that the surface of a clustered metal nanorod array can be made superhydrophobic by suitably controlling the cluster size and average inter-cluster separation.

Stabilization of novel crystallographic phases of Silver

In the late 1980s, we were among the few to initiate studies of size-induced changes in the crystal structure. Continuing this investigation, we have recently stabilized a novel, hexagonal polytype of silver, with strikingly different physical and chemical properties. Unlike normal silver, the new phase is poorly metallic, highly reactive and has a quasi-2D structure. We have carried out a detailed experimental study of the structural, electronic, transport and optical properties of this new form of silver. These observations are supported by first principles DFT calculations (in collaboration with Prof. Waghmare's group at JNCASR)

Nanoscale Alloying of Immiscible Metals

Certain pairs of metals with a positive enthalpy of mixing (e.g. Cu-Nb and Cu-Ag) show an unexpected tendency to alloy. Using atom probe tomography (at UNT, Texas) and EXAFS (at APS, Argonne), we detected nanometer-scale phase segregation in these alloys. We also studied the thermal evolution of these novel nano-amorphous structures and their electrical transport properties.

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