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Muktish Acharyya

Publications and source records attributed to Muktish Acharyya.

At least 73 records · Page 4Linked to original sources
arXiv2014

Polarised Electromagnetic wave propagation through the ferromagnet: Phase boundary of dynamic phase transition

The dynamical responses of ferromagnet to the propagating electromagnetic field wave passing through it are modelled and studied here by Monte Carlo simulation in two dimensional Ising ferromagnet. Here, the electromagnetic wave is linearly polarised in such a way that the direction of magnetic field is parallel to that of the magnetic momemts (spins). The coherent propagating mode of spin-clusters is observed. The time average magnetisation over the full cycle (time) of the field defines the order parameter of the dynamic transition. Depending on the value of the temperature and the amplitude of the propagating magnetic field wave, a dynamical phase transition is observed. The dynamic transition was detected by studying the temperature dependences of the dynamic order parameter, the variance of the dynamic order parameter, the derivative of the dynamic order parameter and the dynamic specific heat. The phase boundaries of the dynamic transitions were drawn for two different values of the wave lengths of the propagating magnetic field wave. The phase boundary was observed to shrink (inward) for lower speed of propagation of the EM wave. The divergence of the releavant length scale was observed at the transition point.

cond-mat.stat-mech↗
arXiv2013

Dynamic-Symmetry-Breaking Breathing and Spreading Transitions in Ferromagnetic Film Irradiated by Spherical Electromagnetic Wave

The dynamical responses of a ferromagnetic film to a propagating spherical electromagnetic wave passing through it are studied by Monte Carlo simulation of two dimensional Ising ferromagnet. For a fixed set of values of the frequency and wavelength of the spherical EM wave, and depending on the values of amplitude of the EM wave and temperature of the system, three different modes are identified. The static pinned mode, the localised dynamical breathing mode and extended dynamical spreading mode are observed. The nonequilibrium dynamical-symmetry-breaking breathing and spreading phase transitions are also observed and the transition temperatures are obtained as functions of the amplitude of the magnetic field of EM wave. A comprehensive phase diagram is drawn. The boundaries of breathing and spreading transitions merge eventually at the equilibrium transition temperature for two dimensional Ising ferromagnet as the value of the amplitude of the magnetic field becomes vanishingly small.

cond-mat.stat-mech↗
arXiv2013

Nonequilibrium Phase Transition in the Kinetic Ising model: Absence of tricritical behaviour in presence of impurities

The nonequilibrium dynamic phase transition, in the two dimensional site diluted kinetic Ising model in presence of an oscillating magnetic field, has been studied by Monte Carlo simulation. The projections of dynamical phase boundary (surface) are drawn in the planes formed by the dilution and field amplitude and the plane formed by temperature and field amplitude. The tricritical behaviour is found to be absent in this case.

cond-mat↗
arXiv2013

Noneqilibrium Phase Transition in Kinetic Ising Model: Absence of Tricritical behaviour in Presence of Impurities

The nonequilibrium dynamic phase transition, in the two dimensional site diluted kinetic Ising model in presence of an oscillating magnetic field, has been studied by Monte Carlo simulation. The projections of dynamical phase surface are drawn in the planes formed by the dilution and field amplitude and the plane formed by temperature and field amplitude. The tricritical behaviour is found to be absent in this case which was observed in the pure system.

cond-mat.stat-mech↗
arXiv2013

Random field Ising model swept by propagating magnetic field wave: Athermal nonequilibrium phase diagram

The dynamical steady state behaviour of the random field Ising ferromagnet swept by a propagating magnetic field wave is studied at zero temperature by Monte Carlo simulation in two dimensions. The distribution of the random field is bimodal type. For a fixed set of values of the frequency and wavelength of propagating magnetic field wave and the strength of the random field, four distinct dynamical steady states or nonequilibrium phases were identified. These four nonequilibrium phases are characterised by different values of structure factors. State of first kind, where all spins are parallel (up). The second one is, the propagating type, where the sharp strips formed by parallel spins are found to move coherently. The third one is also propagating type, where the boundary of the strips of spins is not very sharp. The fourth kind, shows no propagation of stripes of magnetic spins, forming a homogeneous distribution of up and down spins. This is disordered phase The appearance of these four dynamical phases or modes depends on the value of the amplitude of propagating magnetic field wave and the strength of random (static) field. A phase diagram has also been drawn, in the plane formed by the amplitude of propagating field and the strength of random field. It is checked that, the existence of these dynamical phases is neither a finite size effect nor a transient phenomenon.

cond-mat.stat-mech↗
arXiv2012

Bose-Einstein condensation in generalised d dimensions

The density of bosonic states are calculated for spinless free massive bosons in generalised d dimensions. The number of bosons are calculated in the lowest energy state. The Bose Einstein condensation was found in generalised dimensions (on and above d = 3) and the condensation temperature was observed to drop abruptly above three dimensions and decreases monotonically as the dimensionalities of the system increases. The rate of fall of the condensation temperature decreases as the dimensionality increases. This article is aimed to the University graduate students.

cond-mat.stat-mech↗
arXiv2012

Bose - Einstein condensation in arbitrary dimensions

The density of bosonic states are calculated for spinless free massive bosons in generalised d dimensions. The number of bosons are calculated in the lowest energy state. The Bose Einstein condensation was found in generalised d dimensions (on and above d = 3) and the condensation temperature is calculated which is observed to drop abruptly above three dimensions and decreases monotonically as the dimensionalities of the system increases. The rate of fall of the condensation temperature decreases as the dimensionality increases. Interestingly, in the limit of infinite dimensions, the condensation temperature is observed to approach a nonzero finite value.

cond-mat.stat-mech↗
arXiv2011

Nonequilibrium phase transition in the kinetic Ising model driven by propagating magnetic field wave

The two dimensional ferromagnetic Ising model in the presence of a propagating magnetic field wave (with well defined frequency and wavelength) is studied by Mone Carlo simulation. This study differs from all of the earlier studies done so far, where the oscillating magnetic field was considered to be uniform in space. The time average magnetisation over a full cycle (the time period) of the propagating magnetic field acts as the dynamic order parameter. The dynamical phase transition is observed. The temperature variation of the dynamic order parameter, the mean square deviation of the dynamic order parameter, the dynamic specific heat and the derivative of the dynamic order parameter are studied. The mean square deviation of the dynamic order parameter, dynamic specific heat show sharp maxima near the transition point. The derivative of dynamic order parameter shows sharp minimum near the transition point. The transition temperature is found to depend also on the speed of propagation of the magnetic field wave.

cond-mat.stat-mech↗
arXiv2011

Monte Carlo study of dynamic phase transition in Ising metamagnet driven by oscillating magnetic field

The dynamical responses of Ising metamagnet (layered antiferromagnet) in the presence of a sinusoidally oscillating magnetic field are studied by Monte Carlo simulation. The time average staggered magnetisation plays the role of dynamic order parameter. A dynamical phase transition was observed and a phase diagram was plotted in the plane formed by field amplitude and temperature. The dynamical phase boundary is observed to shrink inward as the relative antiferromagnetic strength decreases. The results are compared with that obtained from pure ferromagnetic system. The shape of dynamic phase boundary observed to be qualitatively similar to that obtained from previous meanfield calculations.

cond-mat.stat-mech↗
arXiv2011

Pauli spin paramagnetism and electronic specific heat in generalised d-dimensions

The temperature variations of Pauli spin paramagnetic susceptibility and the electronic specific heat of solids, are calculated as functions of temperature following the free electron approximation, in generalised d-dimensions. The results are compared and become consistent with that obtained in three dimensions. Interestingly, the Pauli spin paramagnetic susceptibility becomes independent of temperature only in two dimensions.

cond-mat.stat-mech↗
arXiv2010

Nonequilibrium Magnetisation Reversal by Periodic Impulsive Field in Ising Meanfield Dynamics

We studied the nonequilibrium magnetisation reversal in kinetic Ising ferromagnets driven by periodic impulsive magnetic field in meanfield approximation. The meanfield differential equation was solved by sixth order Runge-Kutta-Felberg method. The periodicity and strength of the applied impulsive magnetic field play the key role in magnetisation reversal. We studied the minimum strength of impulsive field required for magnetisation reversal at any temperature as a function of the periodicity of the impulsive field. In the high temperature and small period this is observed to be linear. The results are compared with that obtained from Monte Carlo simulation of a three dimensional Ising ferromagnet.

cond-mat.stat-mech↗
arXiv2010

Noninteracting Fermions in infinite dimensions

Usually, we study the statistical behaviours of noninteracting Fermions in finite (mainly two and three) dimensions. For a fixed number of fermions, the average energy per fermion is calculated in two and in three dimensions and it becomes equal to 50 and 60 per cent of the fermi energy respectively. However, in the higher dimensions this percentage increases as the dimensionality increases and in infinite dimensions it becomes 100 per cent. This is an intersting result, at least pedagogically. Which implies all fermions are moving with Fermi momentum. This result is not yet discussed in standard text books of quantum statistics. In this paper, this fact is discussed and explained. I hope, this article will be helpful for graduate students to study the behaviours of free fermions in generalised dimensionality.

cond-mat.stat-mech↗
arXiv2010

Critical Slowing Down Along the Dynamic Phase Boundary in Ising Meanfield Dynamics

We studied the dynamical phase transition in kinetic Ising ferromagnets driven by oscillating magnetic field in meanfield approximation. The meanfield differential equation was solved by sixth order Runge-Kutta-Felberg method. The time averaged magnetisation plays the role of the dynamic order parameter. We studied the relaxation behaviour of the dynamic order parameter close to the transition temperature, which depends on the amplitude of the applied magnetic field. We observed the critical slowing down along the dynamic phase boundary. We proposed a power law divergence of the relaxation time and estimated the exponent. We also found its dependence on the field amplitude and compared the result with the exact value in limiting case.

cond-mat.stat-mech↗
arXiv2007

Inflection point as a manifestation of tricritical point on the dynamic phase boundary in Ising meanfield dynamics

We studied the dynamical phase transition in kinetic Ising ferromagnet driven by oscillating magnetic field in meanfield approximation. The meanfield differential equation was solved by sixth order Runge-Kutta-Felberg method. We calculated the transition temperature as a function of amplitude and frequency of the oscillating field. This was plotted against field amplitude taking frequency as a parameter. As frequency increases the phase boundary is observed to become inflated. The phase boundary shows an inflection point which separates the nature of the transition. On the dynamic phase boundary a tricritical point (TCP) was found, which separates the nature (continuous/discontinuous) of the dynamic transition across the phase boundary. The inflection point is identified as the TCP and hence a simpler method of determining the position of TCP was found. TCP was observed to shift towards high field for higher frequency. As frequency decreases the dynamic phase boundary is observe to shrink. In the zero frequency limit this boundary shows a tendency to merge to the temperature variation of the coercive field.

cond-mat.stat-mech↗
arXiv2005

Nonequilibrium multicritical behavior in anisotropic Heisenberg ferromagnet driven by oscillating magnetic field

The Heisenberg ferromagnet (uniaxially anisotropic along z-direction), in the presence of time dependent (but uniform over space) magnetic field, is studied by Monte Carlo simulation. The time dependent magnetic field was taken as elliptically polarised in such a way that the resulting field vector rotates in the XZ plane. In the limit of low anisotropy, the dynamical responses of the system are studied as functions of temperature and the amplitudes of the magnetic field. As the temperature decreases, it aws found that the system undergoes multiple dynamical phase transitions. In this limit, the multiple transitions were studied in details and the phase diagram for this observed multicritical behaviour was drawn in the field amplitude and temperature palne.The natures (continuous/discontinuous) of the transitions are determined by the temperature variations of fourth order Binder cumulant ratio and the distributions of the order parameter near the transition points. The transitions are supported by finite size study. The temperature variations of the variances of dynamic order parameter components (for different system sizes) indicate the existence of diverging length scale near the dynamic transition points. The frequency dependences of the transition temperatures of the multiple dynamic transition are also studied briefly.

cond-mat.stat-mech↗