arXiv subjects

Sthitadhi Roy

Publications and source records attributed to Sthitadhi Roy.

67 records · Page 4Linked to original sources

arXiv2016

Disordered Chern insulator with a two step Floquet drive

We explore the physics of a Chern insulator subjected to a two step Floquet drive. We analytically obtain the phase diagram and show that the system can exhibit different topological phases characterized by presence and chirality of edge-modes in the two bulk gaps of the Floquet quasienergy spectrum, around $0$ and $π$. We find that the phase of the system depends on the mean but not on the amplitude of the drive. The bulk topological invariants characterizing the phases can be extracted by mapping the unitary evolution within a time period to an energetically trivial but topologically non-trivial time evolution. An extensive numerical study of the bulk topological invariants in the presence of quenched disorder reveals new transitions induced by strong disorder (i) from the different topological to trivial insulator phases and (ii) from a trivial to a topological Anderson insulator phase at intermediate disorder strengths. Careful analysis of level statistics of the quasienergy spectrum indicates a `levitation-annihilation' mechanism near these transitions.

cond-mat.dis-nn↗

arXiv2016

Emergent Weyl nodes and Fermi arcs in a Floquet Weyl semimetal

When a Dirac semimetal is subject to a circularly polarized laser, it is predicted that the Dirac cone splits into two Weyl nodes and a nonequilibrium transient state called the Floquet Weyl semimetal is realized. We focus on the previously unexplored low-frequency regime, where the upper and lower Dirac bands resonantly couple with each other through multi-photon processes, which is a realistic situation in solid state ultrafast pump-probe experiments. We find a series of new Weyl nodes emerging in pairs when the Floquet replica bands hybridize with each other. The nature of the Floquet Weyl semimetal with regard to the number, locations, and monopole charges of these Weyl nodes is highly tunable with the amplitude and frequency of the light. We derive an effective low energy theory using Brillouin-Wigner expansion and further regularize the theory on a cubic lattice. The monopole charges obtained from the low-energy Hamiltonian can be reconciled with the number of Fermi arcs on the lattice which we find numerically.

cond-mat.mes-hall↗

arXiv2016

Generalized Dyson model: nature of zero mode and its implication in dynamics

We study the role of the anomalous $E=0$ state in dynamical properties of non-interacting fermionic chains with chiral symmetry and correlated bond disorder in one dimension. These models posses a diverging density of states at zero energy leading to a divergent localization length at the band center. By analytically calculating the localization length for a finite system, we show that correlations in the disorder modify the spatial decay of the $E = 0$ state from being quasilocalized to extended. We numerically simulate charge and entanglement propagation and provide evidence that states close to $E=0$ dominate the dynamical properties. Remarkably, we find that correlations lead to subdiffusive charge propagation, whereas the growth of entanglement is logarithmically slow. A logarithmic scaling of entanglement saturation with system size is also observed, which indicates a behavior akin to quantum critical glasses.

cond-mat.dis-nn↗

arXiv2016

Chern numbers and chiral anomalies in Weyl butterflies

The Hofstadter butterfly of lattice electrons in a strong magnetic field is a cornerstone of condensed matter physics, exploring the competition between periodicities imposed by the lattice and the field. In this work we introduce and characterize the Weyl butterfly, which emerges when a large magnetic field is applied to a three-dimensional Weyl semimetal. Using an experimentally motivated lattice model for cold atomic systems, we solve this problem numerically. We find that Weyl nodes reemerge at commensurate fluxes and propose using wavepackets dynamics to reveal their chirality and location. Moreover, we show that the chiral anomaly -- a hallmark of the topological Weyl semimetal -- does not remain proportional to magnetic field at large fields, but rather inherits a fractal structure of linear regimes as a function of external field. The slope of each linear regime is determined by the difference of two Chern numbers in gaps of the Weyl butterfly and can be measured experimentally in time of flight.

cond-mat.quant-gas↗

arXiv2016

Response of fermions in Chern bands to spatially local quenches

We study the dynamical evolution of Chern-band systems after subjecting them to local quenches. For open-boundary systems, we show for half-filling that the chiral nature of edge states is manifested in the time-dependent chiral response to local density quenches on the edge. In the presence of power-law traps, we show how to mimic the half-filling situation by choosing the appropriate number of fermions depending on the trap size, and explore chiral responses of edges to local quenches in such a configuration. We find that perturbations resulting from the quenches propagate at different group velocities depending on the bulk band gap. Our results provide different routes to check dynamically the non-trivial nature of Chern bands.

cond-mat.quant-gas↗

arXiv2016

Pseudospin-valve effect on transport in junctions of three-dimensional topological insulator surfaces

We show that the surface states of pristine 3D topological insulators (TIs) are analogs of ferromagnetic half metals due to complete polarization of an emergent momentum independent pseudospin (SU(2)) degree of freedom on the surface. To put this claim on firm footing, we present results for TI surfaces perpendicular to the crystal growth axis, which clearly show that the tunneling conductance between two such TI surfaces of the same TI material is dominated by this half metallic behavior leading to physics reminiscent of a spin-valve. Further using the generalized tunnel magnetoresistance derived in this work we also study the tunneling current between arbitrary TI surfaces. We also perform a comprehensive study of the effect of all possible surface potentials allowed by time reversal symmetry on this spin-valve effect and show that it is robust against most of such potentials.

cond-mat.mes-hall↗

arXiv2016

Wavepacket dynamics on Chern band lattices in a trap

The experimental realization of lattices with Chern bands in ultracold-atom and photonic systems has motivated the study of time-dependent phenomena, such as spatial propagation, in lattices with nontrivial topology. We study the dynamics of gaussian wavepackets on the Haldane honeycomb Chern-band lattice model, in the presence of a harmonic trap. We focus on the transverse response to a force, which is due partly to the Berry curvature and partly to the transverse component of the energy band curvature. We evaluate the accuracy of a semiclassical description, which treats the wavepacket as a point particle in both real and momentum space, in reproducing the motion of a realistic wavepacket with finite extent. We find that, in order to accurately capture the wavepacket dynamics, the extent of the wavepacket in momentum space needs to be taken into account. The dynamics is sensitive to the interplay of band dispersion and Berry curvature over the finite region of momentum (reciprocal) space where the wavepacket has support. Moreover, if the wavepacket is prepared with a finite initial momentum, the semiclassical analysis reproduces its motion as long as it has a large overlap with the eigenstates of a single band. The semiclassical description generally improves with increasing real-space size of the wavepacket, as long as the external conditions (e.g., external force) remain uniform throughout the spatial extent of the wavepacket.

cond-mat.quant-gas↗

arXiv2016

Transport signatures of surface potentials on three-dimensional topological insulators

The spin-momentum locked nature of the robust surface states of three dimensional topological insulators (3D TI) make them promising candidates for spintronics applications. Surface potentials which respect time reversal symmetry can exist at the surface between a 3D TI and the trivial vacuum. These potentials can distort the spin texture of the surface states while retaining their gapless nature. In this work, the effect of all such surface potentials on the spin textures is studied. Since, a tunnel magnetoresistance signal carries the information of the spin texture, it is proposed that spin-polarized tunneling of electrons to a 3D TI surface can be used to uniquely identify the surface potentials and quantitatively characterize them.

cond-mat.mes-hall↗

arXiv2015

Probing surface states exposed by crystal terminations at arbitrary orientations of three-dimensional topological insulators

The topological properties of the bulk band structure of a three-dimensional topological insulator (TI) manifest themselves in the form of metallic surface states. In this paper, we propose a probe which directly couples to an exotic property of these surface states, namely the spin-momentum locking. We show that the information regarding the spin textures, so extracted, for different surfaces can be put together to reconstruct the parameters characterizing the bulk band structure of the material, hence acting as a hologram. For specific TI materials like, $\text{Bi}_2\text{Se}_3, \text{Bi}_2\text{Te}_3 \text{and Sb}_2\text{Te}_3$, the planar surface states are distinct from one another with regard to their spectrum and the associated spin texture for each angle ($θ$), which the normal to the surface makes with the crystal growth axis. We develop a tunnel Hamiltonian between such arbitrary surfaces and a spin polarized STM which provides a unique fingerprint of the dispersion and the associated spin texture corresponding to each $θ$. Additionally, the theory presented in this article can be used to extract value of $θ$ for a given arbitrary planar surface from the STM spectra itself hence effectively mimicking X-ray spectroscopy.

cond-mat.mes-hall↗

arXiv2015

Tunnel Magnetoresistance scan of a pristine three-dimensional topological insulator

Though the Fermi surface of surface states of a 3D topological insulator (TI) has zero magnetization, an arbitrary segment of the full Fermi surface has a unique magnetic moment consistent with the type of spin-momentum locking in hand. We propose a three-terminal set up, which directly couples to the magnetization of a chosen segment of a Fermi surface hence leading to a finite tunnel magnetoresistance (TMR) response of the nonmagnetic TI surface states, when coupled to spin polarized STM probe. This multiterminal TMR not only provides a unique signature of spin-momentum locking for a pristine TI but also provides a direct measure of momentum resolved out of plane polarization of hexagonally warped Fermi surfaces relevant for $Bi_2Te_3$, which could be as comprehensive as spin-resolved ARPES. Implication of this unconventional TMR is also discussed in the broader context of 2D spin-orbit (SO) materials.

cond-mat.mes-hall↗

arXiv2014

Dynamical localization in a chain of hard core bosons under a periodic driving

We study the dynamics of a one-dimensional lattice model of hard core bosons which is initially in a superfluid phase with a current being induced by applying a twist at the boundary. Subsequently, the twist is removed and the system is subjected to periodic $\de$-function kicks in the staggered on-site potential. We present analytical expressions for the current and work done in the limit of an infinite number of kicks. Using these, we show that the current (work done) exhibit a number of dips (peaks) as a function of the driving frequency and eventually saturates to zero (a finite value) in the limit of large frequency. The vanishing of the current (and the saturation of the work done) can be attributed to a dynamic localization of the hard core bosons occurring as a consequence of the periodic driving. Remarkably, we show that for some specific values of the driving amplitude, the localization occurs for any value of the driving frequency. Moreover, starting from a half-filled lattice of hard core bosons with the particles localized in the central region, we show that the spreading of the particles occurs in a light-cone-like region with a group velocity that vanishes when the system is dynamically localized.

cond-mat.stat-mech↗

arXiv2013

Fidelity, Rosen-Zener Dynamics, Entropy and Decoherence in one dimensional hard-core bosonic systems

We study the non-equilibrium dynamics of a one-dimensional system of hard core bosons (HCBs) in the presence of an onsite potential (with an alternating sign between the odd and even sites) which shows a quantum phase transition (QPT) from the superfluid (SF) phase to the so-called "Mott Insulator" (MI) phase. The ground state quantum fidelity shows a sharp dip at the quantum critical point (QCP) while the fidelity susceptibility shows a divergence right there with its scaling given in terms of the correlation length exponent of the QPT. We then study the evolution of this bosonic system following a quench in which the magnitude of the alternating potential is changed starting from zero (the SF phase) to a non-zero value (the MI phase) according to a half Rosen Zener (HRZ) scheme or brought back to the initial value following a full Rosen Zener (FRZ) scheme. The local von Neumann entropy density is calculated in the final MI phase (following the HRZ quench) and is found to be less than the equilibrium value ($\log 2$) due to the defects generated in the final state as a result of the quenching starting from the QCP of the system. We also briefly dwell on the FRZ quenching scheme in which the system is finally in the SF phase through the intermediate MI phase and calculate the reduction in the supercurrent and the non-zero value of the residual local entropy density in the final state. Finally, the loss of coherence of a qubit (globally and weekly coupled to the HCB system) which is initially in a pure state is investigated by calculating the time-dependence of the decoherence factor when the HCB chain evolves under a HRZ scheme starting from the SF phase. This result is compared with that of the sudden quench limit of the half Rosen-Zener scheme where an exact analytical form of the decoherence factor can be derived.

cond-mat.str-el↗

arXiv2011

Relative velocity of sliding of microtubules by the action of Kinesin-5

Kinesin-5, also known as Eg5 in vertebrates is a processive motor with 4 heads, which moves on filamentous tracks called microtubules. The basic function of Kinesin-5 is to slide apart two anti-parallel microtubules by simultaneously walking on both the microtubules. We develop an analytical expression for the steady-state relative velocity of this sliding in terms of the rates of attachments and detachments of motor heads with the ATPase sites on the microtubules. We first analyse the motion of one pair of motor heads on one microtubule and then couple it to the motion of the other pair of motor heads of the same motor on the second microtubule to get the relative velocity of sliding.

physics.bio-ph↗