arXiv subjects

Sudhansu S. Mandal

Publications and source records attributed to Sudhansu S. Mandal.

At least 55 records · Page 3Linked to original sources

arXiv2006

`Spin-Spin' Hall Effect in Two-Dimensional Electron Systems with Spin-Orbit Interaction

We calculate spin Hall conductivities in the two dimensional electron systems with Rashba spin-orbit interaction. The salient feature is that, apart from the usual spin-Hall conductivity σ^z_{xy} which corresponds to the induction of out-of-plane spin-current due to the application of transverse charge current, there is a novel spin-Hall conductivity σ^\perp_{xy} which arises due to the induction of transverse spin-polarized current in the transverse direction by the application of in-plane spin-polarized current. This phenomenon which we call as `spin-spin' Hall effect is a spin analog of conventional Hall effect, but with no magnetic field. This contribution may be understood through the spin-diffusive equation.

cond-mat.mes-hall↗

arXiv2006

Spin analog of the controlled Josephson charge current

We propose a controlled Josephson spin current across the junction of two non-centrosymmetric superconductors like CePt_3Si. The Josephson spin current arises due to direction dependent tunneling matrix element and different momentum dependent phases of the triplet components of the gap function. Its modulation with the angle ξbetween the noncentrosymmetric axes of two superconductors is proportional to \sin ξ. This particular dependence on ξmay find application of the proposed set-up in making a Josephson spin switch.

cond-mat.supr-con↗

arXiv2003

Partially spin polarized quantum Hall effect in the filling factor range 1/3 < nu < 2/5

The residual interaction between composite fermions (CFs) can express itself through higher order fractional Hall effect. With the help of diagonalization in a truncated composite fermion basis of low-energy many-body states, we predict that quantum Hall effect with partial spin polarization is possible at several fractions between $ν=1/3$ and $ν=2/5$. The estimated excitation gaps are approximately two orders of magnitude smaller than the gap at $ν=1/3$, confirming that the inter-CF interaction is extremely weak in higher CF levels.

cond-mat.mes-hall↗

arXiv2003

Two-dimensional electron system in high magnetic fields: Wigner crystal vs. composite-fermion liquid

The two dimensional system of electrons in a high magnetic field offers an opportunity to investigate a phase transition from a quantum liquid into a Wigner solid. Recent experiments have revealed an incipient composite fermion liquid in a parameter range where theory and many experiments had previously suggested the Wigner crystal phase, thus calling into question our current understanding. This Letter shows how very small quantitative corrections (< 1%) in the energy due to the weak interaction between composite fermions can cause a fundamental change in the nature of the ground state, thus providing insight into the puzzling experimental results.

cond-mat.mes-hall↗

arXiv2002

Theoretical Search for Nested Quantum Hall Effect of Composite Fermions

Almost all quantum Hall effect to date can be understood as {\em integral} quantum Hall effect of appropriate particles, namely electrons or composite fermions. This paper investigates theoretically the feasibility of nested states of composite fermions which would lead to a quantum Hall effect that cannot be understood as integral quantum Hall effect of composite fermions. The weak residual interaction between composite fermions will play a crucial role in the establishment of such quantum Hall states by opening a gap in a partially filled composite-fermion level. To treat the problem of interacting composite fermions, we develop a powerful method that allows us to obtain the low energy spectra at composite fermion fillings of $ν^*=n+\bar ν$ without making any assumption regarding the structure of composite fermions in the topmost partially filled level. The method is exact aside from neglecting the composite-fermion Landau level mixing, and enables us to study rather large systems, for example, 24 particles at a total flux of 62 $hc/e$, for which the dimension of the lowest Landau level Hilbert space is $\sim 10^{17}$. We have investigated, for fully spin polarized composite fermions, several filling factors between 1/3 and 2/5 using this approach. The results indicate that any possible incompressibility at these fractions is likely to have a fundamentally different origin than that considered earlier.

cond-mat.mes-hall↗

arXiv2002

Activation energies for spin-reversed excitations in the fractional quantum Hall effect

The activation energy measured in transport experiments in the fractional Hall regime corresponds to the energy required to create a far separated particle hole pair. We calculate, for several fractional quantum Hall states, the energy gaps for the excitation in which the excited composite fermion reverses its spin quantum number. Our results indicate that such spin reversed excitations may be relevant in experimentally accessible regimes.

cond-mat.mes-hall↗

arXiv2002

Relevance of inter-composite fermion interaction to the edge Tomonaga-Luttinger liquid

It is shown that Wen's effective theory correctly describes the Tomonaga-Luttinger liquid at the edge of a system of non-interacting composite fermions. However, the weak residual interaction between composite fermions appears to be a relevant perturbation. The filling factor dependence of the Tomonaga-Luttinger parameter is estimated for interacting composite fermions in a microscopic approach and satisfactory agreement with experiment is achieved. It is suggested that the electron field operator may not have a simple representation in the effective one dimensional theory.

cond-mat.mes-hall↗

arXiv2002

Ground State Vortex Lattice Structures in d-wave Superconductors

We show in a realistic $d_{x^{2}-y^{2}}$ symmetry gap model for a cuprate superconductor that the clean vortex lattice has discontinuous structural transitions (at and near T=0), as a function of the magnetic field $B$ along the c-axis. The transitions arise from the singular nonlocal and anisotropic susceptibility of the $d_{x^{2}-y^{2}}$ superconductor to the perturbation caused by supercurrents associated with vortices. The susceptibility, due to virtual Dirac quasiparticle-hole excitation, is calculated carefully, and leads to a ground state transition for the triangular lattice from an orientation along one of the crystal axis to one at 45$^o$ to them, i.e, along the gap zero direction. The field scale is seen to be 5 Tesla $ \sim (Δ_{0}/ta)^{2}Φ_{0}$, where $Δ_{0}$ is the gap maximum, $t$ is the nearest neighbour hopping, $a$ is the lattice constant, and $Φ_{0}$ is the flux quantum. At much higher fields ($\sim 28T$) there is a discontinuous transition to a centred square structure. The source of the differences from existing calculations, and experimental observability are discussed, the latter especially in view of the very small (a few degrees $K$ per vortex) differences in the ground state energy.

cond-mat.supr-con↗

arXiv2001

How universal is the fractional-quantum-Hall edge Luttinger liquid?

This article reports on our microscopic investigations of the edge of the fractional quantum Hall state at filling factor $ν=1/3$. We show that the interaction dependence of the wave function is well described in an approximation that includes mixing with higher composite-fermion Landau levels in the lowest order. We then proceed to calculate the equal time edge Green function, which provides evidence that the Luttinger exponent characterizing the decay of the Green function at long distances is interaction dependent. The relevance of this result to tunneling experiments is discussed.

cond-mat.mes-hall↗

arXiv2001

Testing the Landau Fermi liquid description of the fractional quantum Hall effect

We study theoretically the dispersion of a single quasiparticle or quasihole of the fractional quantum Hall effect, obtained by injecting or removing a composite fermion. By comparing to a free fermion system, we estimate the regime of validity of the Landau-Fermi-liquid-type description and deduce the Landau mass.

cond-mat.mes-hall↗

arXiv2001

Study of Low Energy Spin Rotons in the Fractional Quantum Hall Effect

Motivated by the discovery of extremely low energy collective modes in the fractional quantum Hall effect (Kang, Pinczuk {\em et al.}), with energies below the Zeeman energy, we study theoretically the spin reversed excitations for fractional quantum Hall states at $ν=2/5$ and 3/7 and find qualitatively different behavior than for $ν=1/3$. We find that a low-energy, charge-neutral "spin roton," associated with spin reversed excitations that involve a change in the composite-fermion Landau level index, has energy in reasonable agreement with experiment.

cond-mat.mes-hall↗

arXiv1997

Specific heat in the integer quantum Hall effect: An exact diagonalization approach

We have studied the integer quantum Hall effect at finite temperatures by diagonalizing a single body tight binding model Hamiltonian including Aharonov-Bohm phase. We have studied the energy dependence of the specific heat and the Hall conductivity at a given temperature. The specific heat shows a sharp peak between two consecutive Hall plateaus. At very low temperatures, the numerical results of the temperature variations of specific heat (in the plateau region) are in good agreement with the analytical results.

cond-mat.mes-hall↗

arXiv1997

Skyrmions in arbitrarily polarized quantum Hall states

We derive an effective non-linear sigma model for quantum hall systems with arbitrary polarizations, by employing the recently proposed doublet model. We study the topological excitations, in particular, the skyrmions, as a function of the filling fraction as well as the polarization. We determine the relationship between the topological charge density and the electronic charge density, and the statistics of skyrmions. We also estimate the value of spin stiffness by using the dispersion relations that we have obtained recently by employing the time dependent Hartree-Fock approximation for the doublet model. Finally, we point out how the skyrmionic excitations reveal information directly on the number of flux tubes that get attached to the electrons in order to form composite fermions.

cond-mat↗

arXiv1996

Spin-flip and spin-wave excitations in arbitrarily polarized quantum Hall states

We study spin-flip and spin-wave excitations for arbitrarily polarized quantum Hall states by employing a fermionic Chern-Simons gauge theory in the low Zeeman energy limit. We show that the spin-flip correlation functions do not get renormalized by the fluctuations of Chern-Simons gauge field. As a consequence, the excitations for a given integer quantum Hall state are identical to fractional quantum Hall states in the lowest Landau level having the same numerator equal to the integer quantum Hall state. Fully and partially polarized states possess only spin-wave excitations while spin-flip excitations are possible for all states, irrespective of their polarizations.

cond-mat.mes-hall↗

arXiv1996

Activated resistivities in the integer quantum Hall effect

We have determined the off-diagonal and diagonal conductivities for a quantum Hall effect system at exactly integer filling at finite temperatures and in the presence of weak short ranged disorder potential within the self consistent Born approximation. We find that there is a finite temperature contribution to off-diagonal conductivity $σ_{xy}$ which is `anomalous' in nature as it survives even in the zero impurity limit. The diagonal conductivity $σ_{xx}$ survives only when both temperature and disorder is non zero. At low temperatures, $σ_{xx}$ activates with a temperature dependent prefactor. Inverting the conductivity matrix, we determine the resistivities. The deviation of the off-diagonal resistivity $ρ_{xy}$ from its zero temperature value and the diagonal resistivity $ρ_{xx}$ activate with a temperature dependent prefactor at low temperatures, in agreement with experiments. Further, we find two physical regimes both of which are at low temperatures and low broadening, which provide the experimentally observed linear relationship between the deviation of $ρ_{xy}$ and the $ρ_{xx}$ with different signs. We have also estimated the effective masses from the experimental data of $ρ_{xy}$ and find them to be reasonable. Finally, our result on compressibility as a function of temperature shows that there is no phase transition involved in the system as far as the temperature is concerned.

cond-mat↗

arXiv1995

Direct test of composite fermion model in quantum Hall systems

We show that neutron scattering and Raman scattering experiments can unambiguously determine a composite fermion parameter, viz., the effective number of Landau Levels filled by the composite fermions. For this purpose, one needs partially polarized or more preferably unpolarized quantum Hall states. We further find that spin correlation function acts as an order parameter in the spin transition.

cond-mat↗

arXiv1995

Wave functions for arbitrarily Polarized Quantum Hall States

We determine the wave functions for arbitrarily polarized quantum Hall states by employing the doublet model which has been proposed recently to describe arbitrarily polarized quantum Hall states. Our findings recover the well known fully polarized Laughlin wave functions and unpolarized Halperin wave function for the filling fraction $ν=2/5$. We have also confirmed by an explicit One-loop computation that the Hall conductivity does indeed get quantized at those filling fractions that follow from the model. Finally, we have given a physical picture for the non-analytic nature of the wave functions, and shown that quantum fluctuations restore the Kohn mode.

cond-mat↗

arXiv1995

Theory of arbitrarily polarized quantum Hall states

We propose a global model which accounts for all the observed quantum Hall states in terms of an abelian doublet of Chern-Simons gauge fields, with the strength of the Chern-Simons term given by a coupling matrix. The model is employed within the composite fermion picture.

cond-mat↗