Vijay B. Shenoy

Effect of strain on the thermal conductivity of solids

We present a systematic study of the effect of strain (equivalent to uniform pressure) on the thermal conductivity of an insulating solid. Following a theoretical analysis that uncovers the dependence of the thermal conductivity on temperature and strain, we present classical molecular dynamics calculations of the thermal conductivity. We find that the molecular dynamics results closely match the theoretical result.

Resonance energy transfer from a fluorescent dye to a metal nanoparticle

A quantum mechanical theory of the rate of excitation energy transfer from a fluorescent dye molecule to the surface plasmonic modes of a spherical metal nanoparticle is presented. The theory predicts the distance dependence of the transfer rate to vary as 1/d(sigma), with sigma=3-4 at intermediate distances, in partial agreement with the recent experimental results. Försters 1/d(6) dependence is recovered at large separations. The predicted rate exhibits nontrivial nanoparticle size dependence, ultimately going over to an asymptotic, a(3) size dependence. Unlike in conventional fluorescence resonance energy transfer, the orientational factor is found to vary between 1 and 4.

Edge state magnetism of single layer graphene nanostructures

We study edge state magnetism in graphene nanostructures using a mean field theory of the Hubbard model. We investigate how the magnetism of the zigzag edges of graphene is affected by the presence of other types of terminating edges and defects. By a detailed study of both regular shapes, such as polygonal nanodots and nanoribbons, and irregular shapes, we conclude that the magnetism in zigzag edges is very robust. Our calculations show that the zigzag edges that are longer than three to four repeat units are always magnetic, irrespective of other edges, regular or irregular. We, therefore, clearly demonstrate that the edge irregularities and defects of the bounding edges of graphene nanostructures do not destroy the edge state magnetism.

Possible high-temperature superconducting state with a d plus id pairing symmetry in doped graphene

Superconductivity at room temperatures, with its multifarious technological possibilities, is a phenomenon that is yet to be realized in a material system. The search for such systems has lead to the discovery of “high temperature” superconducting materials such as the fascinating cuprates, MgB2 3 and most recently, Fe-based pnictides. Graphene is a remarkable two dimensional conductor. A newly discovered method to cleave and isolate single or finite number of atomic layers of graphene, its mechanical robustness and novel electrical properties has caught the attention of the scientific and nanotechnology communities. Undoped graphene is a semimetal and does not superconduct at low temperatures. However, on “doping optimally” if graphene supports high Tc superconductivity it will make graphene even more valuable from basic science and technology points of view. Here we build on a seven year old suggestion of Baskaran (GB) of an electron correlation based mechanism of superconductivity for graphite like systems and demonstrate theoretically the possibility of high temperature superconductivity in doped graphene. GB suggested the possibility of high temperature superconductivity in graphene based on an effective phenomenological Hamiltonian that combined band theory and Pauling’s idea of resonating valance bonds (RVB). The model predicted a vanishing Tc for undoped graphene, consistent with experiments. However, for doped graphene superconducting estimates of Tc ’s were embarrassingly high. Very recently Black-Schaffer and Doniach used GB’s effective Hamiltonian and studied graphitic systems and found that a superconducting state with d + id symmetry to be the lowest energy state in a mean field theory. The mean field theory also predicts a rather high value of the optimal Tc. Other authors have studied possibility of superconductivity based on electron-electron and electron-phonon interactions. While there is an encouraging signal for high Tc superconductivity in the phenomenological GB model, it is important to establish this possibility by the study of a more basic and realistic model. Since the motivation for GB model arose from a repulsive Hubbard model, here we directly analyse this more basic repulsive Hubbard model that describes low energy properties of graphene. We construct variational wavefunctions motivated by RVB physics, and perform extensive Monte Carlo study incorporating crucial correlation effects. This approach which has proved to be especially successful in understanding the ground state of cuprates, clearly points to a superconducting ground state in doped graphene. Further support is obtained from a slave rotor analysis which also includes correlation effects. Our estimate of the Kosterlitz-Thouless superconducting Tc is of the order of room temperatures, and we also discuss experimental observability of our prediction of high temperature superconductivity in graphene. Low energy electrical and magnetic properties of graphene are well described by a tight binding Hubbard model defined on a honeycomb lattice with a single 2pz orbital per carbon atom:

Electronic phase separation and other novel phenomena and properties exhibited by mixed-valent rare-earth manganites and related materials

Transition metal oxides, such as the mixed-valent rare-earth manganites Ln(1−x)AxMnO3 (Ln, rare-earth ion, and A, alkaline-earth ion), show a variety of electronic orders with spatially correlated charge, spin and orbital arrangements, which in turn give rise to many fascinating phenomena and properties. These materials are also electronically inhomogeneous, i.e. they contain disjoint spatial regions with different electronic orders. Not only do we observe signatures of such electronic phase separation in a variety of properties, but we can also observe the different ‘phases’ visually through different types of imaging. We discuss various experiments pertaining to electronic orders and electronic inhomogeneities in the manganites and present a discussion of theoretical approaches to their understanding. It is noteworthy that the mixed-valent rare-earth cobaltates of the type Ln(1−x)AxCoO3 also exhibit electronic inhomogeneities just as the manganites.

Dewetting of glassy polymer films.

Dynamics and morphology of hole growth in a film of power hardening viscoplastic solid [ yield stress approximately (strain-rate)(n)] is investigated. At short times the growth is exponential and depends on the initial hole size. At long times, for n>1 / 3, the growth is again exponential but with a different exponent. However, for n<1 / 3 the hole growth slows and the hole radius approaches an asymptotic value at long times. The rim shape is highly asymmetric, the height of which has a power law dependence on the hole radius (exponent close to unity for 0.25<n<0.4). The above results explain recent intriguing experiments of Reiter [Phys. Rev. Lett., 87, 186 101 (2001)] on dewetting.

Coulomb Interactions and Nanoscale Electronic Inhomogeneities in Manganites

We study electronic inhomogeneities in manganites using simulations on a microscopic model with Coulomb interactions amongst two electronic fluids-one localized (polaronic), the other extended-and dopant ions. The long range Coulomb interactions frustrate phase separation induced by the large on site repulsion between the fluids. A single phase ensues which is inhomogeneous at a nanoscale, but homogeneous on mesoscales, with many features that agree with experiments. This, we argue, is the origin of nanoscale inhomogeneities in manganites, rather than phase competition or disorder effects.

How does a synthetic non-Abelian gauge field influence the bound states of two spin-

We study the bound states of two spin1 2 fermions interacting via a contact attraction (characterized by a scattering length) in the singlet channel in 3D space in presence of a uniform non-Abelian gauge field. The configuration of the gauge field that generates a Rashba type spin-orbit interaction is described by three coupling parameters (λx, λy, λz). For a generic gauge field configuration, the critical scattering length required for the formation of a bound state is negative, i.e., shifts to the “BCS side” of the resonance. Interestingly, we find that there are special high-symmetry configurations (e.g., λx = λy = λz) for which there is a two body bound state for any scattering length however small and negative. Remarkably, the bound state wave functions obtained for high-symmetry configurations have nematic spin structure similar to those found in liquid He. Our results show that the BCS-BEC crossover is drastically affected by the presence of a non-Abelian gauge field. We discuss possible experimental signatures of our findings both at high and low temperatures.

1/2

In the presence of a synthetic non-Abelian gauge field that produces a Rashba-like spin?orbit interaction, a collection of weakly interacting fermions undergoes a crossover from a Bardeen?Cooper?Schrieffer (BCS) ground state to a Bose?Einstein condensate (BEC) ground state when the strength of the gauge field is increased (Vyasanakere et al 2011 Phys.?Rev.?B 84 014512). The BEC that is obtained at large gauge coupling strengths is a condensate of tightly bound bosonic fermion pairs. The properties of these bosons are solely determined by the Rashba gauge field?hence called rashbons. In this paper, we conduct a systematic study of the properties of rashbons and their dispersion. This study reveals a new qualitative aspect of the problem of interacting fermions in non-Abelian gauge fields, i.e. that the rashbon state ceases to exist when the center-of-mass momentum of the fermions exceeds a critical value that is of the order of the gauge coupling strength. The study allows us to estimate the transition temperature of the rashbon BEC and suggests a route to enhance the exponentially small transition temperature of the system with a fixed weak attraction to the order of the Fermi temperature by tuning the strength of the non-Abelian gauge field. The nature of the rashbon dispersion, and in particular the absence of the rashbon states at large momenta, suggests a regime in parameter space where the normal state of the system will be a dynamical mixture of uncondensed rashbons and unpaired helical fermions. Such a state should show many novel features including pseudogap physics.

fermions?

ABSTRACT With the help of simulations based on energy minimization, we have studied the effect of roughness of a rigid contactor with sinusoidal and step patterns on the adhesion-debonding cycle of a soft thin elastic film. The surface instability engendered by attractive forces between the contactor and the film produces a regularly spaced array of columns in the bonding phase. The inter-column spacing is governed largely by periodicity of the contactor pattern. Decreased periodicity of the pattern favors intermittent collapse of columns rather than a continuous peeling of contact zones. An increase in the amplitude of roughness decreases the maximum force required for debonding and increases the snap-off distance. The net effect results in a reduced work for debonding. Introduction of noise and increased step-size in simulations decreases the pull-off force and the snap-off distance, as in the case of a smooth contactor. Finally the study reveals that a patterned contactor can be used as a potential template in the patterning of soft interfaces.