Gautam I. Menon

Conservation laws and integrability of a one-dimensional model of diffusing dimers

We study a model of assisted diffusion of hard-core particles on a line. Our model is a special case of a multispecies exclusion process, but the long-time decay of correlation functions can be qualitatively different from that of the simple exclusion process, depending on initial conditions. This behavior is a consequence of the existence of an infinity of conserved quantities. The configuration space breaks up into an exponentially large number of disconnected sectors whose number and sizes are determined. The decays of autocorrelation functions in different sectors follow from an exact mapping to a model of the diffusion of hard-core random walkers with conserved spins. These are also verified numerically. Within each sector the model is reducible to the Heisenberg model and hence is fully integrable. We discuss additional symmetries of the equivalent quantum Hamiltonian which relate observables in different sectors. We also discuss some implications of the existence of an infinity of conservation laws for a hydrodynamic description.

Magnetic phase diagram of anisotropic superconductor 2H-NbSe2

Abstract We report the magnetic phase diagram of a clean single crystal of 2H-NbSe 2 and present an update on the determination of its intrinsic superconducting parameters.

On the magnetic study of the peak effect in the anisotropic superconductor 2H-NbSe2 evidence for reentrant behavior

We present the results of DC and AC magnetization measurements of the peak effect (PE) at low fields in several samples of an anisotropic superconductor, 2H-NbSe 2. Close to the To(H) line, the PE makes itself manifest as a dip feature in the in-phase (X) AC susceptibility data. From our temperature- and field-dependence studies we construct the loci of this dip feature for H [I c and H II a in the H-T plane. We find that the PE temperature (Tpc~k) increases as H decreases from the high-field end to about 200 de. However, between 200 de to about 30 de, Tr~k decreases as H decreases from about 150 de to 50 de. Relying on the recent observations that the PE phenomenon in the 2H-NbSe 2 system can be associated with the melting of its soft flux-line lattice, we interpret our central result, namely, the two intersections of Tr~k(H) curve with a fixed T line, in terms of reentrant behavior of the flux-lattice melting line at low fields. We believe that such a reentrance of the melting line, first proposed on theoretical grounds by D.R. Nelson in 1988, has not been unambiguously experimentally elucidated in any superconducting system prior to our new data on 2H-NbSe 2.

Melts of Semi-Flexible, Living Polymers: A Lattice Model

A lattice model for the ordering of melts of living polymers, incorporating semi-flexibility, vacancies and open ends, is studied in two dimensions by Monte Carlo simulations and the analysis of exactly solvable limits. We present evidence for high-temperature disordered, and low-temperature ordered phases separated by a continuous transition. If the density of open ends is small, strong crossover effects, arising because of the proximity of the power law phase in the F-model limit of our model, dominate our simulations. The distribution of polymer lengths always has an exponential tail; however, in the crossover regime, it decays with a power law for small and intermediate lengths. The relevance of our study to other lattice models for melts of semi-flexible polymers is discussed.

Thermodynamic behaviour of two-dimensional vesicles revisited.

We study pressurised self-avoiding ring polymers in two dimensions using Monte Carlo simulations, scaling arguments and Flory-type theories, through models which generalise the model of Leibler, Singh and Fisher (Phys. Rev. Lett. 59, 1989 (1987)). We demonstrate the existence of a thermodynamic phase transition at a non-zero scaled pressure

Asymptotic Behavior of Inflated Lattice Polygons

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Measurement and understanding of the temperature dependence of the lower critical field in YNi2B2C

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Sponge phase transitions from a lattice model

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Reentrant peak effect via magnetization studies in NbSe2

= Np/4

Density-functional theory of flux-lattice melting in high-Tc superconductors.

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