S. Bhattacharya

Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

Anomalous peak effect in CeRu2 and 2H-NbSe2: Fracturing of a flux line lattice

CeRu 2 and 2H-NbSe 2 display remarkable similarities in their magnetic response, reflecting the manner in which the weakly pinned flux line lattice (FLL) loses spatial order in the peak-effect (PE) regime. We present evidence for discontinuous changes in the screening response near the onset of the PE in these systems, and demonstrate history-dependent effects. We attribute these features to a disorder-induced fracturing and entanglement of the FLL, as an alternative to the appearance of a spatially modulated ground state for CeRu 2 .

Theory of size-driven transitions in displacive and order-disorder ferroelectrics

We present a simple theory for structural transitions in displacive ferroelectrics of the perovskite type. As in earlier approaches, our model is based on the competition between the elastic energy cost for the displacement of the B-ion from the centrosymmetric position, and the energy gain due to ferroelectric ordering of the dipoles. We use an effective single-site approximation which leads naturally to a first-order transition. This transition takes place at a certain temperature as the temperature is varied, and at a certain size as the size of the system is varied. The transition temperature is suppressed as the sample size is reduced, and vanishes for samples below a certain size. This is in accordance with recent observations on nanocrystalline perovskites. For systems with a continuous bulk transition, which can also be treated within our theory, we show that size effects are highly suppressed, a result that is also borne out by experiments on order-disorder ferroelectrics with a continuous transition.

Percolation Transition in the Heterogeneous Vortex State of NbSe2

A percolation transition in the vortex state of a superconducting 2H-NbSe2 crystal is observed in the regime where vortices form a heterogeneous phase consisting of ordered and disordered domains. The transition is signaled by a sharp increase in critical current that occurs when the volume fraction of disordered domains reaches the value P(c) = 0.26 +/- 0.04. Measurements on different vortex states show that, while the temperature of the transition depends on history and measurement speed, the value of P(c) and the critical exponent characterizing the approach to it, r = 1.97 +/- 0.66, are universal.

Metastable states of a flux-line lattice studied by transport and small-angle neutron scattering

Flux-line lattice (FLL) states have been studied using transport measurements and small-angle neutron scattering in low-T{sub c} materials. In Pb-In, the bulk dislocations in the FLL do not influence the transport properties. In Fe-doped NbSe{sub 2}, transport properties can differ after a field cooling (FC) or a zero field cooling (ZFC) procedure, as previously reported. The ZFC FLL is found ordered with narrow Bragg peaks and is linked to a linear V(I) curve and to a superficial critical current. The FC FLL pattern exhibits two Bragg peaks and the corresponding V(I) curve shows an S-shape. This can be explained by the coexistence of two ordered FLLs slightly tilted from the applied field direction by different superficial currents. These currents are wiped out when the transport current is increased.

Elucidation of amorphization of flux line lattice in Yb3Rh4Sn13

Abstract The characteristic details of the path dependence in the in-phase ac susceptibility data are elucidated via provision of an external stimulus and the process of thermal cyclings across the multiple steps comprising the peak effect (PE) phenomenon in a single crystal of Yb3Rh4Sn13.

Structure of the flux lines lattice in NbSe2: Equilibrium state and influence of the magnetic history

We have performed small-angle neutron scattering of the flux line lattice (FLL) in a Fe-doped

Tata Institute for Fundamental Research

{text{NbSe}}_{2}

Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi-2212 crystals?

sample which presents a large peak effect in the critical current. The scattered intensity and the width of the Bragg peaks of the equilibrium FLL indicate an ordered structure in the peak effect zone. The history dependence in the FLL structure has been studied using field-cooled and zerochar21{}field-cooled procedures, and each state shows the same intensity of Bragg scattering and good orientational order. These results strongly suggest that the peak effect is unrelated to a bulk-disordering transition and confirm the role of a heterogeneous distribution of screening current.

Phenomenological model for history effects and metastability in weakly pinned superconductors

Your article entitled Indian lab looks to home-grown talent (June p7) reports on the process and choice of a new director the Tata Institute of Fundamental Research (TIFR) in Mumbai, India. The article cites events related to a proposed restructuring by the current director as evidence of his non-consultative style and, by implication, less-than-transparent dealings with the governing council. I would like to point out that the article contains distorted views and incorrect facts.