Papers by Chandan Dasgupta
Physical review, Sep 1, 1999
We study muon-spin rotation (µSR) spectra in the mixed phase of highly anisotropic layered superc... more We study muon-spin rotation (µSR) spectra in the mixed phase of highly anisotropic layered superconductors, specifically Bi 2+x Sr 2-x CaCu 2 O 8+δ (BSCCO), by modeling the fluid and solid phases of pancake vortices using liquid-state and density functional methods. The role of thermal fluctuations in causing motional narrowing of µSR lineshapes is quantified in terms of a first-principles theory of the flux-lattice melting transition. The effects of random point pinning are investigated using a replica treatment of liquid state correlations and a replicated density functional theory. Our results indicate that motional narrowing in the pure system, although substantial, cannot account for the remarkably small linewidths obtained experimentally at relatively high fields and low temperatures. We find that satisfactory agreement with the µSR data for BSCCO in this regime can be obtained through the ansatz that this "phase" is characterized by frozen short-range positional correlations reflecting the structure of the liquid just above the melting transition. This proposal is consistent with recent suggestions of a "pinned liquid" or "glassy" state of pancake vortices in the presence of pinning disorder. Our results for the high-temperature liquid phase indicate that measurable linewidths may be obtained in this phase as a consequence of density inhomogeneities induced by the pinning disorder. The results presented here comprise a unified, first-principles theoretical treatment of µSR spectra in highly anisotropic layered superconductors in terms of a controlled set of approximations.
Physica D: Nonlinear Phenomena, Jul 1, 1992
We present the results of two numerical studies of the glass transition. In the first, we show th... more We present the results of two numerical studies of the glass transition. In the first, we show that the Ramakrishnan-Yussouff density functional for hard spheres has, at large enough packing fraction, a large number of metastable minima corresponding to frozen, inhomogeneous, but not periodic or quasiperiodic density configurations. These minima are mean-field glassy states. The second work is a molecular dynamics simulation of Lennard-Jones mixtures, which finds a complete absence of spatial scale-dependence in the growth of relaxation times near the glass transition. This is evidence against the interpretation of the glass transition as a continuous phase transition.
Physical Review Letters, Feb 26, 2016
Temporal relaxation of density fluctuations in supercooled liquids near the glass transition occu... more Temporal relaxation of density fluctuations in supercooled liquids near the glass transition occurs in multiple steps. The short-time β-relaxation is generally attributed to spatially local processes involving the rattling motion of a particle in the transient cage formed by its neighbors. Using molecular dynamics simulations for three model glass-forming liquids, we show that the β-relaxation is actually cooperative in nature. Using finite-size scaling analysis, we extract a growing length-scale associated with β-relaxation from the observed dependence of the β-relaxation time on the system size. Remarkably, the temperature dependence of this length scale is found to be the same as that of the length scale that describes the spatial heterogeneity of local dynamics in the long-time αrelaxation regime. These results show that the conventional interpretation of β-relaxation as a local process is too simplified and provide a clear connection between short-time dynamics and long-time structural relaxation in glass-forming liquids.
Bulletin of the American Physical Society, Mar 15, 2010
Spatiotemporal chaos with shear banding in a driven nematogenic fluid DEBARSHINI CHAKRABORTY, CHA... more Spatiotemporal chaos with shear banding in a driven nematogenic fluid DEBARSHINI CHAKRABORTY, CHANDAN DASGUPTA, SRIRAM RA-MASWAMY, AJAY SOOD, Indian Institute of Science -We present the results of a numerical study of a model of the hydrodynamics of a sheared nematogenic fluid in which spatial variations are allowed only in the gradient direction and the effects of order parameter stresses on the velocity profile are taken into account. When the value of a dimensionless viscosity parameter is so chosen that the order parameter stress is comparable to the bare viscous stress, the system exhibits steady states with the characteristics of shear banding, In addition, a non-zero choice of a parameter that governs the effect of the velocity field on stretching the nematic order parameter leads to the appearance of a new steady state in which the features of both spatiotemporal chaos and shear banding are present.
Physical review, Feb 21, 2020
Thermal conductivity of a model glass-forming system in the liquid and glass states is studied us... more Thermal conductivity of a model glass-forming system in the liquid and glass states is studied using extensive numerical simulations. We show that near the glass transition temperture, where the structural relaxation time becomes very long, the measured thermal conductivity decreases with increasing age. Secondly the thermal conductivity of the disordered solid obtained at low temperatures depends on the cooling rate with which it was prepared, with lower cooling rates leading to lower thermal conductivity. Our analysis links this decrease of the thermal conductivity with increased exploration of lower-energy inherent structures of the underlying potential energy landscape. Further we show that the lowering of conductivity for lower-energy inherent structures is related to the high frequency harmonic modes associated with the inherent structure being less extended.
Nucleation and Atmospheric Aerosols, 2018
Superconductivity in the cuprates, discovered in the late 1980s and occurring at unprecedentedly ... more Superconductivity in the cuprates, discovered in the late 1980s and occurring at unprecedentedly high temperatures (up to about 140K) in about thirty chemically distinct families, continues to be a major problem in physics. In this article, after a brief introduction of these square planar materials with weak interlayer coupling, we mention some of the salient electronic properties of hole doped cuprates such as the pseudogap phase and the Fermi arc . We then outline a phenomenological, Ginzburg Landau like theory developed by some of us for the emergent d-wave symmetry superconductivity in these materials, and confronted successfully with a large amount of experimental information. A more recent application of the approach to fluctuation diamagnetism and to the anomalously large Nernst effect is also discussed.
Langmuir, Sep 14, 2018
The ubiquitous nature of water invariably leads to a variety of physical scenarios that can resul... more The ubiquitous nature of water invariably leads to a variety of physical scenarios that can result in many intriguing properties. We investigate the thermodynamics and associated phase transitions for a water monolayer confined within a quasi-two-dimensional nanopore. An asymmetric nanopore constructed by combining a hydrophilic (hexagonal Boron Nitride) sheet and a hydrophobic (Graphene) sheet leads to an ordered water structure at much higher temperatures compared to a symmetric hydrophobic nanopore consisting of two graphene sheets. The discontinuous change in the thermodynamic quantities, potential energy (U) and entropy (S) of confined water molecules computed from the all-atom molecular dynamics simulation trajectories, uncovers a first-order phase transition in the temperature range of T = 320 K to T = 330 K. Structural analysis reveals that water molecules undergo a disorder-to-order phase transformation in this temperature range with a 4-fold symmetric phase persisting at lower temperatures. Our findings predict a novel confinement system which has the melting transition for monolayer water above the room temperature, and provide a microscopic understanding which will have important implications for other nanofludic systems as well.
Physical Review Letters, Apr 12, 2007
We present magnetization (M) and magnetoresistance (MR) data for a series of Sr 2 FeMoO 6 samples... more We present magnetization (M) and magnetoresistance (MR) data for a series of Sr 2 FeMoO 6 samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in Sr 2 FeMoO 6 is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M.
arXiv (Cornell University), Sep 27, 2021
The fast flow rate of water through nanochannels has promising applications in desalination, ener... more The fast flow rate of water through nanochannels has promising applications in desalination, energy conversion, and nanomedicine. We have used molecular dynamics simulations to show that the water molecules passing through a wide single-walled carbon nanotube (CNT) cavity get aligned by flow to have a net dipole moment along the flow direction. With increasing flow velocity, the net dipole moment first increases and eventually saturates to a constant value. This behavior is similar to the Langevin theory of paraelectricity with the flow velocity acting as an effective aligning field. We show conclusively that the microscopic origin of this behavior is the preferential entry of water molecules with their dipole vectors pointing inward along the CNT axis.
Biological Cybernetics, Feb 1, 1993
A simple neural network model is proposed for kindling -the phenomenon of generating epilepsy by ... more A simple neural network model is proposed for kindling -the phenomenon of generating epilepsy by means of repeated electrical stimulation. The model satisfies Dale's hypothesis, incorporates a Hebb-like learning rule and has low periodic activity in absence of shocks. Many of the experimental observations are reproduced and some new experiments are suggested. It is proposed that the main reason for kindling is the formation of a large number of excitatory synaptic connections due to learning.
Physical review, Nov 1, 1994
The dynamic behavior of a dense hard-sphere liquid is studied by numerically integrating a set of... more The dynamic behavior of a dense hard-sphere liquid is studied by numerically integrating a set of Langevin equations that incorporate a free energy functional of the Ramakrishnan-YussoufF form. At relatively low densities, the system remains, during the time scale of our simulation, in the neighborhood of the metastable local minimum of the free energy that represents a uniform liquid. At higher densities, the system is found to Buctuate near the uniform liquid minimum for a characteristic period of time before making a transition to an inhomogeneous minimum of the free energy. The time that the system spends in the vicinity of the liquid minimum before making a transition to another one de6nes a new time scale of the dynamics. This time scale is found to decrease sharply as the density is increased above a characteristic value. Implications of these observations on the interpretation of experimental and numerical data on the dynamics of supercooled liquids are discussed.
Scientific Reports, Jul 24, 2015
Journal of Chemical Physics, Mar 14, 2013
The breakdown of the Stokes-Einstein (SE) relation between diffusivity and viscosity at low tempe... more The breakdown of the Stokes-Einstein (SE) relation between diffusivity and viscosity at low temperatures is considered to be one of the hallmarks of glassy dynamics in liquids. Theoretical analyses relate this breakdown with the presence of heterogeneous dynamics, and by extension, with the fragility of glass formers. We perform an investigation of the breakdown of the SE relation in 2, 3 and 4 dimensions, in order to understand these interrelations. Results from simulations of model glass formers show that the degree of the breakdown of the SE relation decreases with increasing spatial dimensionality. The breakdown itself can be rationalized via the difference between the activation free energies for diffusivity and viscosity (or relaxation times) in the Adam-Gibbs relation in three and four dimensions. The behavior in two dimensions also can be understood in terms of a generalized Adam-Gibbs relation that is observed in previous work. We calculate various measures of heterogeneity of dynamics and find that the degree of the SE breakdown and measures of heterogeneity of dynamics are generally well correlated but with some exceptions. The two dimensional systems we study show deviations from the pattern of behavior of the three and four dimensional systems both at high and low temperatures. The fragility of the studied liquids is found to increase with spatial dimensionality, contrary to the expectation based on the association of fragility with heterogeneous dynamics.
Glass transition in the hard sphere system
Springer eBooks, Apr 7, 2008
ABSTRACT
Journal of Chemical Physics, Jul 20, 2016
Extensive computer simulations are performed for a few model glass-forming liquids in both two an... more Extensive computer simulations are performed for a few model glass-forming liquids in both two and three dimensions to study their dynamics when a randomly chosen fraction of particles are frozen in their equilibrium positions. For all the studied systems, we find that the temperaturedependence of the α relaxation time extracted from an overlap function related to the self part of the density autocorrelation function can be explained within the framework of the Random First Order Transition (RFOT) theory of the glass transition. We propose a scaling description to rationalize the simulation results and show that our data for the α relaxation time for all temperatures and pin concentrations are consistent with this description. We find that the fragility parameter obtained from fits of the temperature dependence of the α relaxation time to the Vogel-Fulcher-Tammann (VFT) form decreases by almost an order of magnitude as the pin concentration is increased from zero. Our scaling description relates the fragility parameter to the static length scale of RFOT and thus provides a physical understanding of fragility within the framework of the RFOT theory. Implications of these findings for the values of the exponents appearing in the RFOT theory are discussed.
arXiv (Cornell University), Apr 10, 2014
arXiv (Cornell University), Jul 19, 2010
We propose and develop here a phenomenological Ginzburg-Landau-like theory of cuprate hightempera... more We propose and develop here a phenomenological Ginzburg-Landau-like theory of cuprate hightemperature superconductivity, one of the central problems of modern condensed matter physics. The cuprate free energy is expressed as a functional F of the complex spin-singlet pair amplitude ψij ≡ ψm = ∆m exp(iφm) where i and j are nearest-neighbor sites of the square planar Cu lattice in which the superconductivity is believed to primarily reside and m labels the site located at the center of the bond between sites i and j. The system is modeled as a weakly coupled stack of such planes and (∆m, φm) are the real magnitude and phase of the pair amplitude at site m. We hypothesize a simple form, for the functional, where m and n are nearest-neighbor sites on the bond lattice. This is superficially like the original continuum Ginzburg-Landau free energy; the coefficients A, B and C are determined from comparison with experimental results. A combination of analytic approximations, numerical minimization and Monte Carlo simulations of finite two-dimensional systems is used to work out a number of consequences of the proposed functional for specific choices of A, B and C as functions of hole density x and temperature T . There can be a rapid crossover of ∆m from small to large values as A changes sign from positive to negative on lowering T ; this crossover temperatures Tms(x) is identified with the observed pseudogap temperature T * (x). The thermodynamic superconducting phase-coherence transition occurs at a different temperature Tc(x), and describes superconductivity with d-wave symmetry (or 'antiferromagnet-like' order in a magnetic analogy) for the positive C used here. We calculate Tc(x) as a function of x; this has the observed parabolic shape, being strongly influenced by the coupling between ∆m and φm present in the functional. The superfluid density ρs(x, T ), the local gap magnitude ∆m , the specific heat Cv(x, T ) (with and without a magnetic field) as well as vortex properties are obtained using this functional. We compare our results successfully with experiments. We also obtain the electron spectral density as influenced by the coupling between the electrons and the correlation function of the pair amplitude calculated from the Ginzburg-Landau functional. Characteristic features such as temperature dependent Fermi arcs, antinodal pseudogap filling temperature T an (x), gapped or pseudogapped density of states in different momentum regions of the Fermi surface and 'bending' of the energy gap versus momentum on the Fermi surface emerge from the theory. All these compare well with recent high-resolution ARPES measurements. For the specific heat, vortex structure and electron spectral density, only some of the final results are presented here; the details are in subsequent papers.
Physical Review E, Dec 3, 2010
A Neural Network Model for Kindling of Focal Epilepsy
WORLD SCIENTIFIC eBooks, Nov 1, 1996
Physical review, Dec 23, 2002
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Papers by Chandan Dasgupta