Sushanta Dattagupta

Memory in nanomagnetic systems : Superparamagnetism versus spin-glass behavior

The slow dynamics and concomitant memory (aging) effects seen in nanomagnetic systems are analyzed on the basis of two separate paradigms: superparamagnets and spin glasses. It is argued that in a large class of aging phenomena it suffices to invoke superparamagnetic relaxation of individual single domain particles but with a distribution of their sizes. Cases in which interactions and randomness are important in view of distinctive experimental signatures are also discussed.

Structure Factor for Neutron Scattering from Tunneling Systems in Metals

We consider the effect of conduction electrons on the cross-section for neutron scattering from tunneling centers in metals. For a scatterer tunneling between two trap sites a crossover between inelastic scattering at low temperatures and quasi-elastic scattering at higher temperatures is obtained. The temperature dependence of the spectrum arises from the nonadiabatic response of the electrons to the tunneling transitions.

A stochastic theory of anelastic creep

Abstract Creep (the time-dependent yielding of a material under stress) arises from the stress-modulated motion of defects. We develop, for the case of linear anelasticity, a fundamental theory that exploits the randomness of defect motion leading to creep. The random motion of defects causes fluctuations in the strain, and the formalism of linear response theory for classical variables is brought to bear on the problem to obtain the fluctuation-dissipation (FD) theorem for anelasticity: formulae are derived for the complex compliance and the creep function in terms of the autocorrelation of the strain fluctuations in zero external stress. These formulae are also expressed in terms of the power spectrum of the fluctuating strain, as this is the quantity of potential experimental interest. Moment theorems are derived for the power spectrum, to constrain and to give physical meaning to the parameters introduced in empirical network models of anelasticity. A generalized stochastic equation is derived connect...

The Langevin dynamics of vibrated powders

We present a microscopic theory of the relaxational behaviour of a granular pile submitted to vibration, elucidating the different roles of collective and independent-particle relaxation. We write down and solve Langevin equations for these processes, which have an explicit coupling. The analysis of the solution in terms of independent-particle and collective relaxations provides a consistent framework for the interpretation of experimental results.

The structure factor for neutron scattering from a two-state system in metals

The authors consider the dynamics of a defect tunnelling between two trap sites in a metal. The interaction with conduction electrons is shown to influence strongly the defect motion at low temperatures. They calculate the structure factor for inelastic neutron scattering. The inelastic scattering peaks found at very low temperatures are shown to merge into a single quasi-elastic peak at higher temperatures. The width of the quasi-elastic peak narrows as the temperature is increased further. This behaviour results from the damping of the defect motion through the non-adiabatic response of the screening cloud. The present paper extends the authors previous results for symmetric two-state systems to the asymmetric case.

Crystal fields and second‐order magnetic hyperfine interactions in TmFe2Si2

We report the measurement of Mossbauer hyperfine spectra of TmFe2Si2 using the 8.4 keV transition in 169Tm. The detailed temperature dependence of the quadrupole interaction has permitted us to evaluate the crystalline electric fields (CEF) acting on Tm atom. The analysis indicates the ground state of Tm to be a singlet state. However, the hyperfine coupling of the 169Tm with the excited CEF states generates a second order magnetic hyperfine interaction which has been observed in our experiment. An account of the spin relaxation of the second‐order magnetic hyperfine interaction is presented.

Low-temperature thermodynamics in the context of dissipative diamagnetism.

We revisit here the effect of quantum dissipation on the much studied problem of Landau diamagnetism and analyze the results in the light of the third law of thermodynamics. The case of an additional parabolic potential is separately assessed. We find that dissipation arising from strong coupling of the system to its environment qualitatively alters the low-temperature thermodynamic attributes such as the entropy and the specific heat.

Transverse vibrations driven negative thermal expansion in a metallic compound GdPd3B0.25C0.75

We have observed negative thermal expansion (NTE) in a metallic, polycrystalline, and structurally ordered cubic compound GdPd3B0.25C0.75. Our analysis suggest that the NTE observed in this compound does not stems from valence or magnetic instability of lattice ions, which is in general the case of metallic compounds exhibiting such an anomaly. We propose a possible alternative mechanism, namely, the transverse vibrations at low temperatures arising from site anisotropy, that induce the lattice contraction thereby resulting in isotropic NTE. The observed NTE also reflects its effect on the electrical transport properties of this compounds.

Kinetics of ordering

The kinetics of certain complex ordering processes during structural phase transitions in alloys necessitates a fresh look at the validity of the widely used phenomenological equations of the time-dependent Ginzburg-Landau and Cahn-Hilliard types. We start with a longish introduction so as to motivate the need for studying mixed kinetics in which both Glauber and Kawasaki processes occur, as well as segregation of an AB mixture in the presence of anisotropic fields, e.g., gravity. We argue then that the most systematic method of tackling these problems is via the master equation approach to kinetic Ising models. Using such an approach we derive order parameter rate equations for both mixed kinetics and segregation under gravity. Our results show that only under certain limiting conditions do these equations reduce to the usual phenomenological forms. The consequences of complex ordering processes are also discussed.

Effect of collisions on spectral lines in gases

Collisions between an emitter and the surrounding buffer gas particles influence the emission lines. A collision may perturb the emitter in its excited and ground states, cause direct transitions between the levels, and at the same time, change the velocity of the emitter. In this paper, we present a model which deals for the first time with all these effects when they occursimultaneously in each collision. The model assumes the emitter to be subject to random collisions by the surrounding gas particles which are taken to constitute a heat bath in thermal equilibrium. The collisions are assumed to be binary, instantaneous, and to occur with a probabiity given by the Poisson distribution. These assumptions are shown to be equivalent to the widely used impact approximation in the collision broadening theory. We discuss several special cases of the general result for the line shape obtained in the paper.