arXiv subjects

Aditi Mitra

Publications and source records attributed to Aditi Mitra.

At least 73 records · Page 4Linked to original sources

arXiv2014

Quench dynamics of one-dimensional interacting bosons in a disordered potential: Elastic dephasing and critical speeding-up of thermalization

The dynamics of interacting bosons in one dimension following the sudden switching on of a weak disordered potential is investigated. On time scales before quasiparticles scatter (prethermalized regime), the dephasing from random elastic forward scattering causes all correlations to decay exponentially fast, but the system remains far from thermal equilibrium. For longer times, the combined effect of disorder and interactions gives rise to inelastic scattering and to thermalization. A novel quantum kinetic equation accounting for both disorder and interactions is employed to study the dynamics. Thermalization turns out to be most effective close to the superfluid-Bose glass critical point where nonlinearities become more and more important. The numerically obtained thermalization times are found to agree well with analytic estimates.

cond-mat.str-el↗

arXiv2013

Hydrodynamic long-time tails after a quantum quench

After a quantum quench, a sudden change of parameters, generic many particle quantum systems are expected to equilibrate. A few collisions of quasiparticles are usually sufficient to establish approximately local equilibrium. Reaching global equilibrium is, however, much more difficult as conserved quantities have to be transported for long distances to build up a pattern of fluctuations characteristic for equilibrium. Here we investigate the quantum quench of the one-dimensional bosonic Hubbard model from infinite to finite interaction strength U using semiclassical methods for weak, and exact diagonalization for strong quenches. Equilibrium is approached only slowly, as t^{-1/2} with subleading corrections proportional to t^{-3/4}, consistent with predictions from hydrodynamics. We show that these long-time tails determine the relaxation of a wide range of physical observables.

cond-mat.quant-gas↗

arXiv2013

Quench dynamics of one-dimensional bosons in a commensurate periodic potential: A quantum kinetic equation approach

Results are presented for the dynamics arising due to a sudden quench of a boson interaction parameter with the simultaneous switching on of a commensurate periodic potential, the latter providing a source of non-linearity that can cause inelastic scattering. A quantum kinetic equation is derived perturbatively in the periodic potential and solved within the leading order gradient expansion. A two-particle irreducible formalism is employed to construct the stress-momentum tensor and hence the conserved energy. The dynamics is studied in detail in the phase where the boson spectrum remains gapless. The periodic potential is found to give rise to multi-particle scattering processes that relaxes the boson distribution function. At long times the system is found to thermalize with a thermalization time that depends in a non-monotonic way on the amount of energy injected into the system due to the quantum quench. This non-monotonic behavior arises due to the competing effect of an increase of phase space for scattering on the one hand, and an enhancement of the orthogonality catastrophe on the other hand as the quench amplitude is increased. The approach to equilibrium is found to be purely exponential for large quench amplitudes, and more complex for smaller quench amplitudes.

cond-mat.stat-mech↗

arXiv2013

Correlation functions in the prethermalized regime after a quantum quench of a spin-chain

Results are presented for a two-point correlation function of a spin-chain after a quantum quench for an intermediate time regime where inelastic effects are weak. A Callan-Symanzik like equation for the correlation function is explicitly constructed which is used to show the appearance of three distinct scaling regimes. One is for spatial separations within a light-cone, the second is for spatial separations on the light-cone, and the third is for spatial separations outside the light-cone. In these three regimes, the correlation function is found to decay with power-laws with nonequilibrium exponents that differ from those in equilibrium, as well as from those obtained from quenches in a quadratic Luttinger liquid theory. A detailed discussion is presented on how the existence of scaling depends on the properties of the initial state before the quench.

cond-mat.str-el↗

arXiv2013

Time-evolution and dynamical phase transitions at a critical time in a system of one dimensional bosons after a quantum quench

A renormalization group approach is used to show that a one dimensional system of bosons subject to a lattice quench exhibits a finite-time dynamical phase transition where an order parameter within a light-cone increases as a non-analytic function of time after a critical time. Such a transition is also found for a simultaneous lattice and interaction quench where the effective scaling dimension of the lattice becomes time-dependent, crucially affecting the time-evolution of the system. Explicit results are presented for the time-evolution of the boson interaction parameter and the order parameter for the dynamical transition as well as for more general quenches.

cond-mat.str-el↗

arXiv2012

Thermally-Assisted Spin-Transfer Torque Magnetization Reversal in Uniaxial Nanomagnets

We simulate the stochastic Landau-Lifshitz-Gilbert (LLG) dynamics of a uniaxial nanomagnet out to sub-millisecond timescales using a graphical processing unit based micromagnetic code and determine the effect of geometrical tilts between the spin-current and uniaxial anisotropy axes on the thermally assisted reversal dynamics. The asymptotic behavior of the switching time ($I\rightarrow 0$, $<τ>\propto\exp(-ξ(1-I)^2)$) is approached gradually, indicating a broad crossover regime between ballistic and thermally assisted spin transfer reversal. Interestingly, the mean switching time is shown to be nearly independent of the angle between the spin current and magnet's uniaxial axes. These results have important implications for modeling the energetics of thermally assisted magnetization reversal of spin transfer magnetic random access memory bit cells.

cond-mat.mes-hall↗

arXiv2012

Thermalization and dissipation in out of equilibrium quantum systems: A perturbative renormalization group approach

A perturbative renormalization group approach is employed to study the effect of a periodic potential on a system of one-dimensional bosons in a non-equilibrium steady-state due to an initial interaction quench. The renormalization group flows are modified significantly from the well known equilibrium Berezinski-Kosterlitz-Thouless form. They show several new features such as, a generation of an effective temperature, generation of dissipation, as well as a change in the location of the quantum critical point separating the weak coupling and strong coupling phases. Detailed results on the weak-coupling side of the phase diagram are presented, such as the renormalization of the parameters and the asymptotic behavior of the correlation functions. The physical origin of the generated temperature and friction is discussed.

cond-mat.str-el↗

arXiv2011

Mode coupling induced dissipative and thermal effects at long times after a quantum quench

An interaction quench in a Luttinger liquid can drive it into an athermal steady state. We analyze the effects on such an out of equilibrium state of a mode coupling term due to a periodic potential. Employing a perturbative renormalization group approach we show that even when the periodic potential is an irrelevant perturbation in equilibrium, it has important consequences on the athermal steady state as it generates a temperature as well as a dissipation and hence a finite life-time for the bosonic modes.

cond-mat.str-el↗

arXiv2011

A random phase approximation study of one-dimensional fermions after a quantum quench

The effect of interactions on a system of fermions that are in a non-equilibrium steady state due to a quantum quench is studied employing the random-phase-approximation (RPA). As a result of the quench, the distribution function of the fermions is highly broadened. This gives rise to an enhanced particle-hole spectrum and over-damped collective modes for attractive interactions between fermions. On the other hand, for repulsive interactions, an undamped mode above the particle-hole continuum survives. The sensitivity of the result on the nature of the non-equilibrium steady state is explored by also considering a quench that produces a current carrying steady-state.

cond-mat.str-el↗

arXiv2011

Current driven defect unbinding transition in an XY ferromagnet

A Keldysh-contour effective field theory is derived for magnetic vortices in the presence of current flow. The effect of adiabatic and non-adiabatic spin transfer torques on vortex motion is highlighted. Similarities to and differences from the superconducting case are presented and explained. Current flow across a magnetically ordered state is shown to lead to a defect-unbinding phase transition which is intrinsically nonequilibrium in the sense of not being driven by a variation in effective temperature. The dependence of the density of vortices on the current density is determined.

cond-mat.str-el↗

arXiv2011

Domain wall dynamics in integrable and chaotic spin-1/2 chains

We study the time evolution of correlation functions, spin current, and local magnetization in an isolated spin-1/2 chain initially prepared in a sharp domain wall state. The results are compared with the level of spatial delocalization of the eigenstates of the system which is measured using the inverse participation ratio. Both integrable and non-integrable regimes are considered. Non-integrability is introduced to the integrable Hamiltonian with nearest neighbor couplings by adding a single site impurity field or by adding next-nearest-neighbor couplings. A monotonic correspondence between the enhancement of the level of delocalization, spin current and magnetization dynamics occurs in the integrable domain. This correspondence is however lost for chaotic models with weak Ising interactions.

cond-mat.stat-mech↗

arXiv2010

Quenched dynamics in interacting one-dimensional systems: Appearance of current carrying steady states from initial domain wall density profiles

We investigate dynamics arising after an interaction quench in the quantum sine-Gordon model for a one-dimensional system initially prepared in a spatially inhomogeneous domain wall state. We study the time-evolution of the density, current and equal time correlation functions using the truncated Wigner approximation (TWA) to which quantum corrections are added in order to set the limits on its validity. For weak to moderate strengths of the back-scattering interaction, the domain wall spreads out ballistically with the system within the light cone reaching a nonequilibrium steady-state characterized by a net current flow. A steady state current exists for a quench at the exactly solvable Luther-Emery point. The magnitude of the current decreases with increasing strength of the back-scattering interaction. The two-point correlation function of the variable canonically conjugate to the density reaches a spatially oscillating steady state at a wavelength inversely related to the current.

cond-mat.str-el↗

arXiv2010

Current induced decoherence in the multichannel Kondo problem

The properties of a local spin S=1/2 coupled to K independent wires is studied in the presence of bias voltages which drive the system out of thermal equilibrium. For K >> 1, a perturbative renormalization group approach is employed to construct the voltage dependent scaling function for the conductance and the T-matrix. In contrast to the single-channel case, the Kondo resonance is split even by bias voltages small compared to the Kondo temperature T_K, V << T_K. Besides the applied voltage V, the current induced decoherence rate Γ<< V controls the physical properties of the system. While the presence of V changes the structure of the renormalization group considerably, decoherence turns out to be be very effective in prohibiting the flow towards new nonequilibrium fixed points even in variants of the Kondo model where currents are partially suppressed.

cond-mat.mes-hall↗

arXiv2010

Quantum Quenches in an XXZ Spin Chain from a Spatially Inhomogeneous Initial State

Results are presented for the nonequilibrium dynamics of a quantum $XXZ$-spin chain whose spins are initially arranged in a domain wall profile via the application of a magnetic field in the $z$-direction which is spatially varying along the chain. The system is driven out of equilibrium in two ways: a). by rapidly turning off the magnetic field, b). by rapidly quenching the interactions at the same time as the magnetic field is turned off. The time-evolution of the domain wall profile as well as various two-point spin correlation functions is studied by the exact solution of the fermionic problem for the $XX$ chain and via a bosonization approach and a mean-field approach for the $XXZ$ chain. At long times the magnetization is found to equilibrate (reach the ground state value), while the two-point correlation functions in general do not. In particular, for quenches within the gapless $XX$ phase, the transverse spin correlation functions acquire a spatially inhomogeneous structure at long times whose details depend on the initial domain wall profile. The spatial inhomogeneity is also recovered for the case of classical spins initially arranged in a domain wall profile and shows that the inhomogeneities arise due to the dephasing of transverse spin components as the domain wall broadens. A generalized Gibbs ensemble approach is found to be inadequate in capturing this spatially inhomogeneous state.

cond-mat.quant-gas↗

arXiv2010

Nonequilibrium dynamics in a two-channel Kondo system due to a quantum quench

Recent experiments by Potok et al. have demonstrated a remarkable tunability between a single-channel Fermi liquid fixed point and a two-channel non-Fermi liquid fixed point. Motivated by this we study the nonequilibrium dynamics due to a sudden quench of the parameters of a Hamiltonian from a single-channel to a two-channel anisotropic Kondo system. We find a distinct difference between the long time behavior of local quantities related to the impurity spin as compared to that of bulk quantities related to the total (conduction electrons + impurity) spin of the system. In particular, the local impurity spin and the local spin susceptibility are found to equilibrate, but in a very slow power-law fashion which is peculiar to the non-Fermi liquid properties of the Hamiltonian. In contrast, we find a lack of equilibration in the two particle expectation values related to the total spin of the system.

cond-mat.str-el↗

arXiv2010

AC- and DC-driven noise and I-V characteristics of magnetic nanostructures

We study a structure consisting of a ferromagnetic (F) layer coupled to two normal metal (N) leads. The system is driven out of equilibrium by the simultaneous application of external dc and ac voltages across the N/F/N structure. Using the Keldysh diagrammatic approach, and modeling the ferromagnet as a classical spin of size S >> 1, we derive the Langevin equation for the magnetization dynamics and calculate the noise correlator. We find that the noise has an explicit frequency dependence in addition to depending on the characteristics of the ac and dc drive. Further, we calculate the current-voltage characteristics of the structure to O(1/S^2) and find that the nonequilibrium dynamics of the ferromagnetic layer gives rise to corrections to the current that are both linear and nonlinear in voltage.

cond-mat.mes-hall↗

arXiv2009

1/N expansion of the nonequilibrium infinite-U Anderson Model

Results are presented for the nonequilibrium infinite-U Anderson model using a large-N approach, where $N$ is the degeneracy of the impurity level, and where nonequilibrium is established by coupling the level to two leads at two different chemical potentials so that there is current flow. A slave-boson representation combined with Keldysh functional integral methods is employed. Expressions for the static spin susceptibility $χ_S$ and the conductance $G$ are presented to ${\cal O}(\frac{1}{N})$ and for an applied voltage difference $V$ less than the Kondo temperature. The correlation function for the slave-boson is found to be significantly modified from its equilibrium form in that it acquires a rapid decay in time with a rate that equals the current induced decoherence rate. Physical observables are found to have a rather complex dependence on the coupling strength to the two leads which can lead to asymmetric behavior $χ_S(V) \neq χ_S(-V)$, $G(V)\neq G(-V)$ both in the mixed valence and in the Kondo regime.

cond-mat.str-el↗

arXiv2008

Dissipative and nonequilibrium effects near a superconductor-metal quantum critical point

We present a microscopic derivation of the effect of current flow on a system near a superconductor-metal quantum critical point. The model studied is a 2d itinerant electron system where the electrons interact via an attractive interaction and are coupled to an underlying normal metal substrate which provides a source of dissipation, and also provides a source of inelastic scattering that allows a nonequilibrium steady state to reach. A nonequilibrium Keldysh action for the superconducting fluctuations on the normal side is derived. Current flow, besides its minimal coupling to the order parameter is found to give rise to two new effects. One is a source of noise that acts as an effective temperature $T_{eff} = e E v_F τ_{sc}$ where $E$ is the external electric field, $v_F$ the Fermi velocity, and $τ_{sc}$ is the escape time into the normal metal substrate. Secondly current flow also produces a drift of the order-parameter. Scaling equations for the superconducting gap and the current are derived and are found to be consistent with previous phenomenological treatments as long as a temperature $T \sim T_{eff}$ is included. The current induced drift is found to produce additional corrections to the scaling which are smaller by a factor of ${\cal O}(\frac{1}{E_F τ_{sc}})$, $E_F$ being the Fermi energy.

cond-mat.str-el↗