arXiv subjects

Sayak Dasgupta

Publications and source records attributed to Sayak Dasgupta.

13 recordsLinked to original sources

arXiv2022

Tuning the Hall response of a noncollinear antiferromagnet via spin-transfer torques and oscillating magnetic fields

The kagome lattice antiferromagnets Mn$_3$X(= Sn, Ge) have a noncollinear 120$^\circ$ ordered ground state, which engenders a strong anomalous Hall response. It has been shown that this response is linked to the magnetic order and can be manipulated through it. Here we use a combination of strain and spin-transfer torques to control the magnetic order and hence switch deterministically between states of different chirality. Each of these chiral ground states has an anomalous Hall conductivity tensor in a different direction. Furthermore, we show that a similar manipulation of the strained sample can be obtained through oscillating magnetic fields, potentially opening a pathway to optical switching in these materials.

cond-mat.mes-hall↗

arXiv2022

Strain tuning the magnetic and transport properties of Mn$_3$Ge

The kagome lattice antiferromagnet Mn$_3$Ge has a local hexagonal symmetry with a 120$^\circ$ ordered ground state. This non-colinear ground state engenders a strong anomalous Hall response. The main goal of this work is to understand the effect of strain on this response. We derive an effective model for the Hall vector which functions as our order parameter. Using this model we show how both intra and inter planar strains can be used to switch the sign of the Hall response at a constant magnetic field. Further, we also investigate the effect of this strain on the spin wave band gaps and show that strain can be used to effectively manipulate them, which can be used to tune the magnon responses in this system.

cond-mat.mes-hall↗

arXiv2022

Majorana zero modes in a magnetic and superconducting hybrid vortex

We propose and investigate a new platform for the realization of Majorana zero modes in a thin-film heterostructure composed of an easy-plane ferromagnet and a superconductor with spin-orbit coupling. The system can support an energetically favorable bound state comprising a magnetic and a superconducting vortex. We show that a hybrid vortex thus created can host a robust zero-energy Majorana bound state at its core over a wide range of parameters, with its partner zero mode located at the outer boundary of a disk-shaped topological region. We identify a novel mechanism underlying the formation of the topological phase that, remarkably, relies on the orbital effect of the magnetization field and not on the usual Zeeman effect. The in-plane components of magnetization couple to electrons as a gauge potential with non-zero curl, thus creating an emergent magnetic field responsible for the gapped topologically non-trivial region surrounding the vortex core. Our construction allows the mobility of magnetic vortices to be imposed on the Majorana zero mode at the core of the superconducting vortex. In addition, the system shows a rich interplay between magnetism and superconductivity which might aid in developing future devices and technologies.

cond-mat.supr-con↗

arXiv2021

Zeeman term for the Néel vector in a two sublattice antiferromagnet using Dzyaloshinsky-Moriya interaction and magnetic field

We theoretically investigate the dynamics of solitons in two sublattice antiferromagnets under external perturbations, focusing on the effect of Dzyaloshinsky-Moriya (DM) interactions. To this end, we construct a micromagnetic field theory for the antiferromagnet in the presence of the external magnetic field, DM interaction, and spin-transfer torque. In particular, we show external magnetic field and spin current couple to Néel vector in a Zeeman-like manner when DM interactions present, which can be used to efficiently drive antiferromagnetic solitons of different dimensions. Besides, we study the effect of straining the local lattice. It can serve as an external handle on the Néel field inertia and thus dynamical properties. Our findings may find applications in antiferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2020

Ambipolar magneto-optical response of ultra-low carrier density topological insulators

We have investigated the THz range magneto-optical response of ultralow carrier density films of Sb$_2$Te$_3$ using time-domain THz polarimetry. Undoped Sb$_2$Te$_3$ has a chemical potential that lies inside the bulk valence band. Thus its topological response is masked by bulk carriers. However, with appropriate buffer layer engineering and chemical doping, Sb$_2$Te$_3$ thin films can be grown with extremely low electron or hole densities. The ultralow carrier density samples show unusual optical properties and quantized response in the presence of magnetic fields. Consistent with the expectations for Dirac fermions, a quantized Hall response is seen even in samples where the zero field conductivity falls below detectable levels. The discontinuity in the Faraday angle with small changes in the filling fraction across zero is manifestation of the parity anomaly in 2D Dirac systems with broken time reversal symmetry.

cond-mat.mes-hall↗

arXiv2020

Theory of spin waves in a hexagonal antiferromagnet

We construct a field-theoretic description of spin waves in hexagonal antiferromagnets with three magnetic sublattices and coplanar $120^\circ$ magnetic order. The three Goldstone modes can be separated by point-group symmetry into a singlet $α_{0}$ and a doublet $(β_x,β_y)$. The $α_0$ singlet is described by the standard theory of a free relativistic scalar field. The field theory of the $(β_x,β_y)$ doublet is analogous to the theory of elasticity of a two-dimensional isotropic solid with distinct longitudinal and transverse "speeds of sound". The well-known Heisenberg models on the triangular and kagome lattices with nearest-neighbour exchange turn out to be special cases with accidental degeneracy of the spin-wave velocities. The speeds of sound can be readily calculated for any lattice model. We apply this approach to the compounds of the Mn$_3$X family with stacked kagome layers.

cond-mat.mes-hall↗

arXiv2020

Quantum statistics of vortices from a dual theory of the XY ferromagnet

We extend the well-known mapping between the easy-plane ferromagnet and electrostatics in $d=2$ spatial dimensions to dynamical and quantum phenomena in a $d=2+1$ spacetime. Ferromagnetic vortices behave like quantum particles with an electric charge equal to the vortex number and a magnetic flux equal to the transverse spin of the vortex core. Vortices with half-integer core spin exhibit fermionic statistics.

cond-mat.mes-hall↗

arXiv2018

Energy-momentum tensor of a ferromagnet

The energy-momentum tensor of a ferromagnet derived according to the standard prescription of Noether's theorem has a major flaw: the term originating from the spin Berry phase is gauge-dependent. As a consequence, some physical quantities computed from the tensor show unphysical behavior. For example, the presence of a spin-polarized current does not affect the energy of the domain wall in the commonly accepted gauge, which implies-incorrectly-the absence of the adiabatic spin torque. In other gauges, the spin torque shows unphysical glitches occurring when the plane of magnetization crosses the Dirac string associated with a magnetic monopole in spin space. We derive a gauge-invariant energy-momentum tensor that is free from these artifacts but requires the addition of an extra spatial dimension, with the ferromagnet living on its boundary. It can be obtained most directly from the Wess-Zumino action for spins, which relies on the same extra dimension.

cond-mat.mes-hall↗

arXiv2017

Gauge fields and related forces in antiferromagnetic solitons

We derive equations of motion for topological solitons in antiferromagnets under the combined action of perturbations such as an external magnetic field and torque-generating electrical current. Aside from conservative forces, such perturbations generate an effective "magnetic field" exerting a gyrotropic force on the soliton and an induced "electric field" if the perturbation is time-dependent. We apply the general formalism to the cases of a domain wall and of a vortex. An antiferromagnetic vortex can be effectively moved by combined applications of a magnetic field and an electric current.

cond-mat.mes-hall↗

arXiv2016

Dynamical manipulation of Dirac points in the Kitaev honeycomb model

We study the effect of a half wave rectified sinusoidal electromagnetic (EM) wave on the Kitaev honeycomb model with an additional magneto-electric coupling term {arising due to induced polarization of the bonds. Within the framework of Floquet analysis, we show that merging of a pair of Dirac points in the gapless region of the Kitaev model leading to a semi-Dirac spectrum is indeed possible} by externally varying the amplitude and the phase of the EM field.

cond-mat.stat-mech↗

arXiv2014

Phase Transition in the periodically pulsed Dicke Model

We study the effect of pulsed driving and kicked driving of the interaction term on the non-equilibrium phase transition in the Dicke Model. Within the framework of Floquet theory, we observe the emergence of new non-trivial phases on impingement by such periodic pulses. Notably, our study reveals that a greater control over the dynamical quantum criticality is possible through the variation of multiple parameters related to the pulse, as opposed to a single parameter control in a monochromatic drive. Furthermore, the probability of the system remaining trapped in a metastable state during the observed first order transition from the super-radiant to normal phase is found to be higher for small number of kicks (or pulses) in comparison to the sinusoidal perturbation.

cond-mat.stat-mech↗

arXiv2014

A simple experiment demonstrating the connection between chaos and music

In this pedagogical paper we study the dripping pattern of a leaking faucet which we analyse on the basis of a musical procedure which we outline and match the power spectral density of these drops (which are recorded as noise signals over time) to verify the $\frac{1}{f^β}$ power law which characterizes the nature of such systems with non linear characteristics.

physics.ed-ph↗

arXiv2014

Exploring chaos in Dicke Model using ground state fidelity and Loschmidt echo

We study the quantum critical behaviour of the Dicke Hamiltonian, with finite number of atoms and explore the signature of quantum chaos using measures like the ground state fidelity and the Loschmidt echo. We show that both these quantities clearly point to the chaotic nature of the system in the super-radiant phase.

cond-mat.stat-mech↗