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T. P. Pareek

Publications and source records attributed to T. P. Pareek.

At least 19 recordsLinked to original sources
arXiv2018

Effect of boundary scattering on spin-hall effect

The spin dependent reflection in quasi-two-dimensional electron gas from an impenetrable barrier in presence of Rashba and Dresselhaus spin-orbit coupling is analyzed in detail. It is shown that due to spin-orbit effects the reflected beam split in two beams gives rise to multiple reflection analogous to phenomena birefringence. The interplay between Rashba and Dresselhaus spin-orbit coupling gives rise to anisotropy in Fermi energy surface and a non-zero net spin-polarized current oscillating with two frequencies for all the values of incident angle except at $45^{o}$ when averaged over all components of reflected beam. It is also shown that in over critical region, all the three polarization components as well as net polarization has non-zero values and are exponentially decaying as distance from the barrier increases which in turns spin-accumulation near the barrier is an important consequence of spin-hall effect.

cond-mat.mes-hall↗
arXiv2010

Unified quaternionic description of charge and spin transport and intrinsic non-linearity of spin currents

We present a unified theory of charge and spin transport using quaternionic formalism. It is shown that both charge current and spin currents can be combined together to form a quaternionic current. The scalar and vector part of quaternionic currents correspond to charge and spin current respectively. We formulate a unitarity condition on the scattering matrix for quaternionic current conservation. It is shown that in presence of spin flip interactions a weaker quaternionic unitarity condition implying charge flux conservation but spin flux non conservation is valid. Using this unified theory we find that spin currents are intrinsically non linear. Its implication for recent experimental observation of spin generation far away from the boundaries are discussed.

cond-mat.mes-hall↗
arXiv2008

Multi-terminal Spin Transport: Non applicability of linear response and Equilibrium spin currents

We present generalized scattering theory for multi-terminal spin transport in systems with broken SU(2) symmetry either due to spin-orbit interaction,magnetic impurities or magnetic leads. We derive equation for spin current consistent with charge conservation. It is shown that resulting spin current equations can not be expressed as difference of potential pointing to {\it non applicability of linear response for spin currents} and as a consequence equilibrium spin currents(ESC) in the leads are non zero. We illustrate the theory by calculating ESC in two terminal normal system in presence of Rashba spin orbit coupling and show that it leads to spin rectification consistent with the non linear nature of spin transport.

cond-mat.mes-hall↗
arXiv2007

Generation and Measurement of Non Equilibrium Spin Currents in Two Terminal Systems

Generation and measurement of non-equilibrium spin current in two probe configuration is discussed. It is argued and shown that spin current can be generated in two terminal non-magnetic system. Further it is shown that these spin currents can be measured via conductance in two probe configuration when the detector probe is ferromagnetic.

cond-mat.mes-hall↗
arXiv2005

Spin orbit induced torque in collinear spin valve structuer and associated entropy

We predict that due to spin-orbit(SO) interaction there is a torque even for the parallel configuration of spin valve structure (F1/2DEG/F2). This torque arises due to spin orbit interaction. We develop a scattering theory for spin density matrix which allows us to study this effect quantitatively. Further we show that the von-Neumann entropy associated with transport of polarization can show non-linear behavior as a function of absolute angle and oscillator behavior as a function of SO interaction strength.

cond-mat.mes-hall↗
arXiv2005

Measuring Rashba spin orbit interaction strength zero magnetic field

We propose a multi-terminal(Non-magnetic) mesoscopic electrical transport measurement for measuring Rashba spin orbit strength. The method proposed is different from the usual magneto transport measurements, which require magnetic fields for e.g. Shubnikov-de Hass oscillations or weak anti-localization measurements. Our methods uses three terminal mesoscopic device, in which one of the terminal acts as a voltage probe. We show that the Voltage measured at the non-magnetic voltage probe depends on the strength of Rashba spin-orbit interaction.

cond-mat.mes-hall↗
arXiv2004

Absolute spin valve effect

We study charge transport in a two dimensional hybrid systems consisting of nonmagnetic two dimensional electron gas with spin-orbit interaction sandwiched between a Ferromagnetic lead and a normal metallic lead (FM/2DEG/NM). An {\it absolute spin valve effect} is shown to exist. It is shown that the conductance of such a hybrid system changes upon rotating the magnetization such that it always stays perpendicular to the current direction. An ASVC ({\it Absolute Spin Valve Coefficient}) is defined to quantify this effect and its dependence on various parameter is studied.

cond-mat.mes-hall↗
arXiv2003

Pure spin currents and associated electrical voltage

We present a generalize Landauer-Büttiker transport theory for multi-terminal spin transport in presence of spin-orbit interaction. It is pointed out that the presence of spin-orbit interaction results in {\it equilibrium spin currents}, since in presence of spin-orbit interaction spin is not a conserved quantitative. Further we illustrate the theory by applying it to a three terminal Y-shaped conductor. It is shown that when one of the terminal is a potential probe, there exist {\it nonequilibrium pure spin currents} without a n accompanying charge current. It is shown that this pure spin currents causes a voltage drop which can be measured if the potential probe is ferromagnetic.

cond-mat.mes-hall↗
arXiv2002

Carrier induced ferromagnetism in diluted magnetic semi-conductors

We present a theory for carrier induced ferromagnetism in diluted magnetic semi-conductor (DMS). Our approach treats on equal footing quantum fluctuations within the RPA approximation and disorder within CPA. This method allows for the calculation of $T_c$, magnetization and magnon spectrum as a function of hole, impurity concentration and temperature. It is shown that, sufficiently close to $T_c$, and within our decoupling scheme (Tyablicov type) the CPA for the itinerant electron gas reduces to the Virtual Crystal Approximation. This allows, in the low impurity concentration and low density of carriers to provide analytical expression for $T_c$. For illustration, we consider the case of $Ga_{1-c}Mn_{c}As$ and compare our results with available experimental data.

cond-mat.str-el↗
arXiv2001

Anisotropic spin and charge transport in Rashba Hamiltonian

We explore spin and charge transport phenomena in two dimensional electron gas in presence of Rashba spin-orbit coupling connected to two ideal Ferromagnetic leads. In particular we show through a combination of analytical and numerical calculation that the spin polarization which is transported depends on the Magnetization direction of ferromagnet even if the magnetization of both FM's are parallel.Conductance is also shown to be anisotropic. These anisotrpies present in spin and charge transport are a consequence of breaking of rotational invariance due to Rashba spin-orbit interaction and are present irrespective of the Hamiltonian considered being an effective mass Hamiltonian or tight binding model Hamiltonian.

cond-mat.mes-hall↗
arXiv2000

Magnetic Scanning Tunneling Microscopy with a Two-Terminal Non-Magnetic Tip: Quantitative Results

We report numerical simulation result of a recently proposed \{P. Bruno, Phys. Rev. Lett {\bf 79}, 4593, (1997)\} approach to perform magnetic scanning tunneling microscopy with a two terminal non-magnetic tip. It is based upon the spin asymmetry effect of the tunneling current between a ferromagnetic surface and a two-terminal non-magnetic tip. The spin asymmetry effect is due to the spin-orbit scattering in the tip. The effect can be viewed as a Mott scattering of tunneling electrons within the tip. To obtain quantitative results we perform numerical simulation within the single band tight binding model, using recursive Green function method and Landauer-Büttiker formula for conductance. A new model has been developed to take into account the spin-orbit scattering off the impurities within the single-band tight-binding model. We show that the spin-asymmetry effect is most prominent when the device is in quasi-ballistic regime and the typical value of spin asymmetry is about 5%.

cond-mat.mes-hall↗
arXiv1998

Blocking of inter-subspace tunneling by intra subspace inelastic scattering

In recent years the notion of intrinsic decoherence and dephasing of a particle interacting with its environment is being investigated intensively. This has an important bearing on a plausible causal connection between incoherent c-axis resistivity and high-temperature superconductivity. In our work we study the tunnel supression and incoherent motion of a particle tunneling between two sites. The bosonic excitations of the environment are coupled to only one site inducing on-site spin flips. We show that this on-site spin flip scattering makes the tunnel motion incoherent. In the high-temperature limit incoherent rate or hopping rate has been calculated. We also briefly discuss the renormalization of the effective tunneling by environmental coupling at zero temperature following Wegner's renormalization group procedure.

cond-mat.supr-con↗
arXiv1998

C-axis resistivity and high Tc superconductivity

Recently we had proposed a mechanism for the normal-state C-axis resistivity of the high-T$_c$ layered cuprates that involved blocking of the single-particle tunneling between the weakly coupled planes by strong intra-planar electron-electron scattering. This gave a C-axis resistivity that tracks the ab-plane T-linear resistivity, as observed in the high-temperature limit. In this work this mechanism is examined further for its implication for the ground-state energy and superconductivity of the layered cuprates. It is now argued that, unlike the single-particle tunneling, the tunneling of a boson-like pair between the planes prepared in the BCS-type coherent trial state remains unblocked inasmuch as the latter is by construction an eigenstate of the pair annihilation operator. The resulting pair-delocalization along the C-axis offers energetically a comparative advantage to the paired-up trial state, and, thus stabilizes superconductivity. In this scheme the strongly correlated nature of the layered system enters only through the blocking effect, namely that a given electron is effectively repeatedly monitored (intra-planarly scattered) by the other electrons acting as an environment, on a time-scale shorter than the inter-planar tunneling time. Possible relationship to other inter-layer pairing mechanisms proposed by several workers in the field is also briefly discussed.

cond-mat.supr-con↗
arXiv1997

Comparison between the two models of dephasing in mesoscopic systems

In mesoscopic systems to study the role of inelastic scattering on the phase coherent motion of electrons two phenomenological models have been proposed. In the first one, due to Büttiker, one adds a voltage probe into the system (or in the scattering matrix). The second model invokes the complex (or optical) potential in the system Hamiltonian. Studying a simple geometry of a metallic loop in the presence of Aharonov-Bohm magnetic flux, we show that the two probe conductance is symmetric in the reversal of the magnetic field in Büttiker's approach. Whereas the two probe conductance within the complex potential model is asymmetric in the magnetic flux reversal contrary to the expected behavior.

cond-mat.mes-hall↗
arXiv1997

Stochastic resonance and nonlinear response in a dissipative quantum two-state system

We study the dynamics of a dissipative two-level, system driven by a monochromatic ac field, starting from the usual spin-boson Hamiltonian. The quantum Langevin equations for the spin variables are obtained. The amplitude of the coherent oscillations in the average position of the particle is studied in the high temperature limit. The system exhibits quantum stochastic resonance in qualitative agreement with earlier numerical results.

cond-mat.stat-mech↗
arXiv1997

Normal state c-axis resistivity of high T_c cuprate superconductors

It is shown that a strong intraplanar incoherent scattering can effectively block the interplanar coherent tunneling between the weakly coupled planes of the highly anisotropic but clean (intrinsic) materials such as the optimally doped high-T_c layered cuprate superconductors. The calculated normal-state C-axis resistivity ρ_c(T) then follows the metal-like temperature dependence of the ab-plane resistivity ρ_{ab}(T) at high temperatures. At low enough temperatures, however, ρ_c(T) exhibits a non-metal like upturn even as ρ_{ab}(T) remains metallic. Moreover, in the metallic regime, ρ_c(T) is not limited by the maximum metallic resistivity of Mott-Ioffe-Regel. This correlation between the intrinsic ρ_c(T) and ρ_{ab}(T) is observed in the normal state of the high-T_c stoichiometric cuprates.

cond-mat.supr-con↗
arXiv1996

Enslaving random fluctuations in nonequlibrium systems

Several physical models have recently been proposed to obtain unidirectional motion of an overdamped Brownian particle in a periodic potential system. The asymmetric ratchetlike form of the periodic potential and the presence of correlated nonequilibrium fluctuating forces are considered essential to obtain such a macroscopic motion in homogeneous systems. In the present work, instead, inhomogeneous systems are considered, wherein the friction coefficient and/or temperature could vary in space. We show that unidirectional motion can be obtained even in a symmetric nonratchetlike periodic potential system in the presence of white noise fluctuations. We consider four different cases of system inhomogeneity We argue that all these different models work under the same basic principle of alteration of relative stability of otherwise locally stable states in the presence of temperature inhomogeneity.

cond-mat↗