Impact of high-frequency pumping on anomalous finite-size effects in three-dimensional topological insulators

Lowering of the thickness of a thin-film three-dimensional topological insulator down to a few nanometers results in the gap opening in the spectrum of topologically protected two-dimensional surface states. This phenomenon, which is referred to as the anomalous finite-size effect, originates from hybridization between the states propagating along the opposite boundaries. In this work, we consider a bismuth-based topological insulator and show how the coupling to an intense high-frequency linearly polarized pumping can further be used to manipulate the value of a gap. We address this effect within recently proposed Brillouin-Wigner perturbation theory that allows us to map a time-dependent problem into a stationary one. Our analysis reveals that both the gap and the components of the group velocity of the surface states can be tuned in a controllable fashion by adjusting the intensity of the driving field within an experimentally accessible range and demonstrate the effect of light-induced band inversion in the spectrum of the surface states for high enough values of the pump.Comment: 6 pages, 3 figure

Tensor train optimization of parametrized quantum circuits

We examine a particular realization of derivative-free method as implemented on tensor train based optimization to the variational quantum eigensolver. As an example, we consider parametrized quantum circuits composed of a low-depth hardware-efficient ansatz and Hamiltonian variational ansatz for addressing the ground state of the transverse field Ising model. We further make a comparison with gradient-based optimization techniques and discuss on the advantage of using tensor train based optimization, especially in the presence of noise.Comment: 7 pages, 5 figure

Fractional Marcus-Hush-Chidsey-Yakopcic current-voltage model for redox-based resistive memory devices

We propose a circuit-level model combining the Marcus-Hush-Chidsey electron current equation and the Yakopcic equation for the state variable for describing resistive switching memory devices of the structure metal-ionic conductor-metal. We extend the dynamics of the state variable originally described by a first-order time derivative by introducing a fractional derivative with an arbitrary order between zero and one. We show that the extended model fits with great fidelity the current-voltage characteristic data obtained on a Si electrochemical metallization memory device with Ag-Cu alloy.Comment: 7 pages, 3 figure

Spin dynamics of two-dimensional systems in strong-coupling regime

A possibility to use the spin properties of the particles opens an alternative path for the creation of new devices for information storage and transfer, which in comparison to the currently existing charge-based devices have several manufacturing benefits such as a compact size of the systems and ability of easy-going state control. Additionally apart from strongly applied motives, the study of particle spin dynamics has a significant fundamental potential for the understanding of the nature of spin-induced effects. Speaking about the difficulties of spin usage, note that initial spin alignment can be lost with time under the multiple scattering events and various interactions in the absence of external influences, which regarding device manufacturing results in the information loss. For that reason, the study of the spin dynamics and the search for alternative particles, which can carry the information for relatively long time are the topics of high priority nowadays. The current thesis is devoted to the theoretical study of the spin transport effects in two-dimensional systems in strong-coupling regime. Following discussion covers several aspects of the spin dynamics and, in particular, includes: investigation of the spin and the transport properties of two-dimensional electrons dressed by laser field; investigation of polariton spin dynamics in diffusive regime; investigation of the effect of spin control in polariton systems using the effect of polariton Rabi oscillations.DOCTOR OF PHILOSOPHY (SPMS

Another view on Gilbert damping in two-dimensional ferromagnets

A keen interest towards technological implications of spin-orbit driven magnetization dynamics requests a proper theoretical description, especially in the context of a microscopic framework, to be developed. Indeed, magnetization dynamics is so far approached within Landau-Lifshitz-Gilbert equation which characterizes torques on magnetization on purely phenomenological grounds. Particularly, spin-orbit coupling does not respect spin conservation, leading thus to angular momentum transfer to lattice and damping as a result. This mechanism is accounted by the Gilbert damping torque which describes relaxation of the magnetization to equilibrium. In this study we work out a microscopic Kubo-Streda formula for the components of the Gilbert damping tensor and apply the elaborated formalism to a two-dimensional Rashba ferromagnet in the weak disorder limit. We show that an exact analytical expression corresponding to the Gilbert damping parameter manifests linear dependence on the scattering rate and retains the constant value up to room temperature when no vibrational degrees of freedom are present in the system. We argue that the methodology developed in this paper can be safely applied to bilayers made of non- and ferromagnetic metals, e.g., CoPt

Antichiral ferromagnetism

Here, by combining a symmetry-based analysis with numerical computations, we predict a different kind of magnetic ordering-antichiral ferromagnetism. This term aims to reflect that spontaneous modulation of the magnetization direction m(r) appears in a way that both types of chirality (handedness) exist simultaneously, and alternate in space. Without loss of generality, we focus our investigation on crystals with full tetrahedral symmetry where chiral interaction terms-Lifshitz invariants-are forbidden by symmetry. However, we demonstrate that the leading chirality-related term leads to nontrivial smooth magnetic textures exhibiting antichirality. In addition to the unconventional ground state, the revealed ordering gives rise to rich phenomena such as unique magnetic domains and skyrmions