arXiv subjects

K. A. Muttalib

Publications and source records attributed to K. A. Muttalib.

At least 37 records · Page 2Linked to original sources

arXiv2008

Spin-wave mediated quantum corrections to the conductivity in thin ferromagnetic gadolinium films

We present a study of quantum corrections to the conductivity of thin ferromagnetic gadolinium films. In situ magneto-transport measurements were performed on a series of thin films with thickness d < 135A. For sheet resistances R0 < 4011 Ohm and temperatures T < 30K, we observe a linear temperature dependence of the conductivity in addition to the logarithmic temperature dependence expected from well known quantum corrections in two dimensions. We show that such a linear T-dependence can arise from a spin-wave mediated Altshuler-Aronov type correction.

cond-mat.dis-nn↗

arXiv2008

Solutions for correlations along the coexistence curve and at the critical point of a kagomé lattice gas with three-particle interactions

We consider a two-dimensional (d=2) kagomé lattice gas model with attractive three-particle interactions around each triangular face of the kagomé lattice. Exact solutions are obtained for multiparticle correlations along the liquid and vapor branches of the coexistence curve and at criticality. The correlation solutions are also determined along the continuation of the curvilinear diameter of the coexistence region into the disordered fluid region. The method generates a linear algebraic system of correlation identities with coefficients dependent only upon the interaction parameter. Using a priori knowledge of pertinent solutions for the density and elementary triplet correlation, one finds a closed and linearly independent set of correlation identities defined upon a spatially compact nine-site cluster of the kagomé lattice. Resulting exact solution curves of the correlations are plotted and discussed as functions of the temperature, and are compared with corresponding results in a traditional kagomé lattice gas having nearest-neighbor pair interactions. An example of application for the multiparticle correlations is demonstrated in cavitation theory.

cond-mat.stat-mech↗

arXiv2007

Disorder and temperature dependence of the Anomalous Hall Effect in thin ferromagnetic films: Microscopic model

We consider the Anomalous Hall Effect (AHE) in thin disordered ferromagnetic films. Using a microscopic model of electrons in a random potential of identical impurities including spin-orbit coupling, we develop a general formulation for strong, finite range impurity scattering. Explicit calculations are done within a short range but strong impurity scattering to obtain AH conductivities for both the skew scattering and side jump mechanisms. We also evaluate quantum corrections due to interactions and weak localization effects. We show that for arbitrary strength of the impurity scattering, the electron-electron interaction correction to the AH conductivity vanishes exactly due to general symmetry reasons. On the other hand, we find that our explicit evaluation of the weak localization corrections within the strong, short range impurity scattering model can explain the experimentally observed logarithmic temperature dependences in disordered ferromagnetic Fe films.

cond-mat.dis-nn↗

arXiv2007

Power law eigenvalue density, scaling and critical random matrix ensembles

We consider a class of rotationally invariant unitary random matrix ensembles where the eigenvalue density falls off as an inverse power law. Under a new scaling appropriate for such power law densities (different from the scaling required in Gaussian random matrix ensembles), we calculate exactly the two-level kernel that determines all eigenvalue correlations. We show that such ensembles belong to the class of critical ensembles.

cond-mat.stat-mech↗

arXiv2007

Weak localization correction to the anomalous Hall effect in polycrystalline Fe films

In situ transport measurements have been made on ultrathin ($<$100 Å thick) polycrystalline Fe films as a function of temperature and magnetic field for a wide range of disorder strengths. For sheet resistances $R_{xx}$ less than $\sim 3kΩ$, we find a logarithmic temperature dependence of the anomalous Hall conductivity $σ_{xy}$ which is shown for the first time to be due to a universal scale dependent weak localization correction within the skew scattering model. For higher sheet resistance, granularity becomes important and the break down of universal behavior becomes manifest as the prefactors to $σ_{xx}$ and $σ_{xy}$ decrease at different rates with increasing disorder.

cond-mat.dis-nn↗

arXiv2005

Anomalous Hall effect in ferromagnetic disordered metals

The anomalous Hall effect in disordered band ferromagnets is considered in the framework of quantum transport theory. A microscopic model of electrons in a random potential of identical impurities including spin-orbit coupling is used. The Hall conductivity is calculated from the Kubo formula for both, the skew scattering and the side-jump mechanisms. The recently discussed Berry phase induced Hall current is also evaluated within the model. The effect of strong impurity scattering is analyzed and it is found to affect the ratio of the non-diagonal (Hall) and diagonal components of the conductivity as well as the relative importance of different mechanisms.

cond-mat.dis-nn↗

arXiv2005

Family of solvable generalized random-matrix ensembles with unitary symmetry

We construct a very general family of characteristic functions describing Random Matrix Ensembles (RME) having a global unitary invariance, and containing an arbitrary, one-variable probability measure which we characterize by a `spread function'. Various choices of the spread function lead to a variety of possible generalized RMEs, which show deviations from the well-known Gaussian RME originally proposed by Wigner. We obtain the correlation functions of such generalized ensembles exactly, and show examples of how particular choices of the spread function can describe ensembles with arbitrary eigenvalue densities as well as critical ensembles with multifractality.

cond-mat.other↗

arXiv2004

Conductance distribution in 3D Anderson insulators: deviation from log-normal form

We show how a recent proposal to obtain the distribution of conductances in three dimensions (3D) from a generalized Fokker-Planck equation for the joint probability distribution of the transmission eigenvalues can be implemented for all strengths of disorder by numerically evaluating certain correlations of transfer matrices. We then use this method to obtain analytically, for the first time, the 3D conductance distribution in the insulating regime and provide a simple understanding of why it differs qualitatively from the log-normal distribution of a quasi one-dimensional wire.

cond-mat.mes-hall↗

arXiv2003

Conductance distribution in quasi-one-dimensional disordered quantum wires

We develop a simple systematic method, valid for all strengths of disorder, to obtain analytically the full distribution of conductances P(g) for a quasi one dimensional wire within the model of non-interacting fermions. The method has been used in [1-3] to predict sharp features in P(g) near g=1 and the existence of non-analyticity in the conductance distribution in the insulating and crossover regimes, as well as to show how P(g) changes from Gaussian to log-normal behavior as the disorder strength is increased. Here we provide many details of the method, including intermediate results that offer much insight into the nature of the solutions. In addition, we show within the same framework that while for metals P(g) is a Gaussian around g >>1, there exists a log-normal tail for g << 1, consistent with earlier field theory calculations. We also obtain several other results that compare very well with available exact results in the metallic and insulating regimes.

cond-mat.mes-hall↗

arXiv2002

Conductance distribution in disordered quantum wires: Crossover between the metallic and insulating regimes

We calculate the distribution of the conductance P(g) for a quasi-one-dimensional system in the metal to insulator crossover regime, based on a recent analytical method valid for all strengths of disorder. We show the evolution of P(g) as a function of the disorder parameter from a insulator to a metal. Our results agree with numerical studies reported on this problem, and with analytical results for the average and variance of g.

cond-mat.mes-hall↗

arXiv2002

Non-analyticity in the distribution of conductances in quasi one dimensional wires

We show that the distribution P(g) of conductances g of a quasi one dimensional wire has non-analytic behavior in the insulating region, leading to a discontinuous derivative in the distribution near g=1. We give analytic expressions for the full distribution and extract an approximate scaling behavior valid for different strengths of disorder close to g=1.

cond-mat.dis-nn↗

arXiv2002

Generalization of the DMPK equation beyond quasi one dimension

Electronic transport properties in a disordered quantum wire are very well described by the Dorokhov-Mello-Pereyra-Kumar (DMPK) equation, which describes the evolution of the transmission eigenvalues as a function of the length of a multichannel conductor. However, the DMPK equation is restricted to quasi one dimensional systems only. We derive a generalized DMPK equation for higher dimensions, containing dependence on the dimensionality through the properties of the transmission eigenvectors, by making certain statistical assumptions about the transfer matrix. An earlier phenomenological generalization is obtained as a special case.

cond-mat.mes-hall↗

arXiv2002

S=1 kagomé Ising model with triquadratic interactions, single-ion anisotropy and magnetic field: exact phase diagrams

We consider a S=1 kagomé Ising model with triquadratic interactions around each triangular face of the kagomé lattice, single-ion anisotropy and an applied magnetic field. A mapping establishes an equivalence between the magnetic canonical partition function of the model and the grand canonical partition function of a kagomé lattice-gas model with localized three-particle interactions. Since exact phase diagrams are known for condensation in the one-parameter lattice-gas model, the mapping directly provides the corresponding exact phase diagrams of the three-parameter S=1 Ising model. As anisotropy competes with interactions, results include the appearance of confluent singularities effecting changes in the topology of the phase diagrams, phase boundary curves (magnetic field vs temperature) with purely positive or negative slopes as well as intermediate cases showing nonmonotonicity, and coexistence curves (magnetization vs temperature) with varying shapes and orientations, in some instances entrapping a homogeneous phase.

cond-mat.stat-mech↗

arXiv2001

q-Random Matrix Ensembles

Theory of Random Matrix Ensembles have proven to be a useful tool in the study of the statistical distribution of energy or transmission levels of a wide variety of physical systems. We give an overview of certain q-generalizations of the Random Matrix Ensembles, which were first introduced in connection with the statistical description of disordered quantum conductors.

cond-mat.dis-nn↗

arXiv2000

Electric field induced memory and aging effects in pure solid N_2

We report combined high sensitivity dielectric constant and heat capacity measurements of pure solid N_2 in the presence of a small external ac electric field in the audio frequency range. We have observed strong field induced aging and memory effects which show that field cooled samples may be prepared in a variety of metastable states leading to a free energy landscape with experimentally ``tunable'' barriers, and tunneling between these states may occur within laboratory time scales.

cond-mat.dis-nn↗

arXiv1999

`One-sided' log-normal distribution of conductances of a disordered quantum wire

We develop a simple systematic method, valid for all strengths of disorder, to obtain analytically for the first time the full distribution of conductance P(g) for a quasi one dimensional wire in the absence of electron-electron interaction. We show that in the crossover region between the metallic and insulating regimes, P(g) is highly asymmetric, given by an essentially `one sided' log-normal distribution. For larger disorder, the tail of the log-normal distribution for g > 1 is cut off by a Gaussian.

cond-mat.dis-nn↗

arXiv1999

Conductance distribution of disordered quasi one-dimensional wires

We determine analytically the distribution of conductances of quasi one-dimensional disordered electron systems, neglecting electron-electron interaction, for all strengths of disorder. We find that in the crossover region between the metallic and insulating regimes, P(g) is highly asymmetric, given by ``one-sided'' log-normal distribution. For larger disorder, the tail of the log-normal distribution is cut-off for g > 1 by a Gaussian.

cond-mat.mes-hall↗

arXiv1999

Generalized Fokker-Planck Equation For Multichannel Disordered Quantum Conductors

The Dorokhov-Mello-Pereyra-Kumar (DMPK) equation, which describes the distribution of transmission eigenvalues of multichannel disordered conductors, has been enormously successful in describing a variety of detailed transport properties of mesoscopic wires. However, it is limited to the regime of quasi one dimension only. We derive a one parameter generalization of the DMPK equation, which should broaden the scope of the equation beyond the limit of quasi one dimension.

cond-mat.dis-nn↗