arXiv subjects

Michael Thorwart

Publications and source records attributed to Michael Thorwart.

At least 55 records · Page 3Linked to original sources

arXiv2017

Skyrmion-Antiskyrmion pair creation by in-plane currents

Magnetic skyrmions can be considered as topologically protected localized vortex-like spin textures. Due to their stability, their small size, and the possibility to move them by low electric currents they are promising candidates for spintronic devices. Without violating topological protection, it is possible to create skyrmion-antiskyrmion pairs, as long as the total charge remains unchanged. We derive a skyrmion equation of motion which reveals how spin-polarized charge currents create skyrmion-antiskyrmion pairs. It allows to identify general prerequisites for the pair creation process. We corroborate these general principles by numerical simulations. On a lattice, where topological protection becomes imperfect, the antiskyrmion partner of the pairs is annihilated and only the skyrmion survives. This eventually changes the total skyrmion number and yields a new way of creating and controlling skyrmions.

cond-mat.mes-hall↗

arXiv2017

Second quantization of Leinaas-Myrheim anyons in one dimension and their relation to the Lieb-Liniger model

In one spatial dimension, anyons in the original description of Leinaas and Myrheim are formally equivalent to locally interacting bosons described by the Lieb-Liniger model. This admits an interesting reinterpretation of interacting bosons in the context of anyons. We elaborate on this parallel, particularly including the many-body bound states from the attractive Lieb-Liniger model. In the anyonic context these bound states are created purely by quantum-statistical attraction and coined quantum-statistical condensate, which is more robust than the Bose-Einstein condensate. We introduce the second quantization formalism for the present anyons and construct the generalized Jordan-Wigner transformation that connects them to the bosons of the Lieb-Liniger model.

cond-mat.mes-hall↗

arXiv2016

Nature does not rely on long-lived electronic quantum coherence for photosynthetic energy transfer

During the first steps of photosynthesis, the energy of impinging solar photons is transformed into electronic excitation energy of the light-harvesting biomolecular complexes. The subsequent energy transfer to the reaction center is understood in terms of exciton quasiparticles which move on a grid of biomolecular sites on typical time scales less than 100 femtoseconds (fs). Since the early days of quantum mechanics, this energy transfer is described as an incoherent Forster hopping with classical site occupation probabilities, but with quantum mechanically determined rate constants. This orthodox picture has been challenged by ultrafast optical spectroscopy experiments with the Fenna-Matthews-Olson protein in which interference oscillatory signals up to 1.5 picoseconds were reported and interpreted as direct evidence of exceptionally long-lived electronic quantum coherence. Here, we show that the optical 2D photon echo spectra of this complex at ambient temperature in aqueous solution do not provide evidence of any long-lived electronic quantum coherence, but confirm the orthodox view of rapidly decaying electronic quantum coherence on a time scale of 60 fs. Our results give no hint that electronic quantum coherence plays any biofunctional role in real photoactive biomolecular complexes. Since this natural energy transfer complex is rather small and has a structurally well defined protein with the distances between bacteriochlorophylls being comparable to other light-harvesting complexes, we anticipate that this finding is general and directly applies to even larger photoactive biomolecular complexes.

physics.bio-ph↗

arXiv2016

Dissipative dynamics of a quantum two-state system in presence of nonequilibrium quantum noise

We analyze the real-time dynamics of a quantum two-state system in the presence of nonequilibrium quantum fluctuations. The latter are generated by a coupling of the two-state system to a single electronic level of a quantum dot which carries a nonequilibrium tunneling current. We restrict to the sequential tunneling regime and calculate the dynamics of the two-state system, of the dot population, and of the nonequilibrium charge current on the basis of a diagrammatic perturbative method valid for a weak tunneling coupling. We find a nontrivial dependence of the relaxation and dephasing rates of the two-state system due to the nonequilibrium fluctuations which is directly linked to the structure of the unperturbed central system. In addition, a Heisenberg-Langevin-equation of motion allows us to calculate the correlation function of the nonequilibrium fluctuations. By this, we obtain a generalized nonequilibrium fluctuation relation which includes the equilibrium fluctuation-dissipation theorem. A straightforward extension to the case with a time-periodic ac voltage is shown.

cond-mat.mes-hall↗

arXiv2016

Dynamics of a quantum two-state system in a linearly driven quantum bath

When an open quantum system is driven by an external time-dependent force, the coupling of the driving to the central system is usually included whereas the impact of the driving field on the bath is neglected. We investigate the effect of a quantum bath of linearly driven harmonic oscillators on the relaxation dynamics of a quantum two-level system which itself is not directly driven. In particular, we calculate the frequency-dependent response of the system when the bath is subject to a Dirac and a Gaussian driving-pulse. We show that a time-retarded effective force on the system is induced by the driven bath which depends on the full history of the perturbation and the spectral characteristics of the underlying bath. In particular, when a structured Ohmic bath with a pronounced Lorentzian peak is considered, the dynamical response of the system to a driven bath is qualitatively different as compared to the undriven bath. Specifically, additional resonances appear which can be directly associated to a Jaynes-Cummings-like effective energy spectrum.

cond-mat.stat-mech↗

arXiv2015

Nonlocal spin torques in Rashba quantum wires with steep magnetic textures

We provide a general procedure to calculate the current-induced spin-transfer torque which acts on a general steep magnetic texture due to the exchange interaction with an applied spin-polarized current. As an example, we consider a one-dimensional ferromagnetic quantum wire and also include a Rashba spin-orbit interaction. The spin-transfer torque becomes generally spatially non-local. Likewise, the Rashba spin-orbit interaction induces a spatially nonlocal field-like nonequilibrium spin-transfer torque. We also find a spatially varying nonadiabaticity parameter and markedly different domain wall dynamics for very steep textures as compared to wide domain walls.

cond-mat.mes-hall↗

arXiv2015

An efficient tool to calculate two-dimensional optical spectra for photoactive molecular complexes

We combine the coherent modified Redfield theory (CMRT) with the equation of motion-phase matching approach (PMA) to calculate two-dimensional photon echo spectra for photoactive molecular complexes with an intermediate strength of the coupling to their environment. Both techniques are highly efficient, yet they involve approximations at different levels. By explicitly comparing with the numerically exact quasi-adiabatic path integral approach, we show for the Fenna-Matthews-Olson complex that the CMRT describes the decay rates in the population dynamics well, but final stationary populations and the oscillation frequencies differ slightly. In addition, we use the combined CMRT+PMA to calculate two-dimensional photon-echo spectra for a simple dimer model. We find excellent agreement with the exact path integral calculations at short waiting times where the dynamics is still coherent. For long waiting times, differences occur due to different final stationary states, specifically for strong system-bath coupling. For weak to intermediate system-bath couplings, which is most important for natural photosynthetic complexes, the combined CMRT+PMA gives reasonable results with acceptable computational efforts.

physics.chem-ph↗

arXiv2015

Nonequilibrium Phase Transition of Interacting Bosons in an Intra-Cavity Optical Lattice

We investigate the nonlinear light-matter interaction of a Bose-Einstein condensate trapped in an external periodic potential inside an optical cavity which is weakly coupled to vacuum radiation modes and driven by a transverse pump field. Based on a generalized Bose-Hubbard model which incorporates a single cavity mode, we include the collective backaction of the atoms on the cavity light field and determine the nonequilibrium quantum phases within the nonperturbative bosonic dynamical mean-field theory.With the system parameters adapted to recent experiments, we find a quantum phase transition from a normal phase to a self-organized superfluid phase, which is related to the Hepp-Lieb-Dicke superradiance phase transition. For even stronger pumping, a self-organized Mott insulator phase arises.

cond-mat.quant-gas↗

arXiv2014

Rashba induced chirality switching of domain walls and suppression of the Walker breakdown

In conventional domain wall systems the aim of a high domain wall velocity may be hindered by the occurrence of a Walker breakdown at comparably low current density. We show how a Rashba interaction can stabilize the domain wall dynamics and thereby shift the Walker breakdown to higher current densities. The Rashba interaction creates a field like spin torque, which breaks the symmetry of the system and modifies the internal structure of the domain wall. Besides a shift of the Walker breakdown it can additionally induce a chirality switch of the domain wall at sufficient Rashba fields. The preferred chirality may then be chosen by the direction of the current flow. Both, the suppression of the Walker breakdown and the chirality switching, affect the domain wall velocity. This is even more pronounced for short current pulses, where an additional domain wall movement after the pulse in either positive or negative direction can determine the final position of the domain wall.

cond-mat.mes-hall↗

arXiv2014

Photon-assisted confinement-induced resonances for ultracold atoms

We solve the two-particle s-wave scattering for an ultracold atom gas confined in a quasi-one-dimensional trapping potential which is periodically modulated. The interaction between the atoms is included in terms of Fermi's pseudopotential. For a modulated isotropic transverse harmonic confinement, the atomic center of mass and relative degrees of freedom decouple and an exact solution is possible. We use the Floquet approach to show that additional photon-assisted resonant scattering channels open up due to the harmonic modulation. Applying the Bethe-Peierls boundary condition, we obtain the general scattering solution of the time-dependent Schrödinger equation which is universal at low energies. The binding energies and the effective one-dimensional scattering length can be controlled by the external driving.

physics.atom-ph↗

arXiv2013

Nonequilibrium Rashba field driven domain wall motion in ferromagnetic nanowires

We study the effects of spin-orbit interaction (SOI) on the current-induced motion of a magnetic (Bloch) domain wall in ultrathin ferromagnetic nanowires. The conspiracy of spin relaxation and SOI is shown to generate a novel strong nonequilibrium Rashba field, which is dominant even for moderate SOI. This field causes intricate spin precession and a transition from translatory to oscillatory wall dynamics with increasing SOI. We show that current pulses of different lengths can efficiently be used to control the domain wall motion.

cond-mat.mes-hall↗

arXiv2012

Nonequilibrium quantum dynamics of the magnetic Anderson model

We study the non-equilibrium dynamics of a spinful single-orbital quantum dot with an incorporated quantum mechanical spin-1/2 magnetic impurity. Due to the spin degeneracy, double occupancy is allowed, and Coulomb interaction together with the exchange coupling of the magnetic impurity influence the dynamics. By extending the iterative summation of real-time path integrals (ISPI) to this coupled system, we monitor the time-dependent non-equilibrium current and the impurity spin polarization to determine features of the time-dependent non-equilibrium dynamics. We particulary focus on the deep quantum regime, where all time and energy scales are of the same order of magnitude and no small parameter is available. We observe a significant influence of the non-equilibrium decay of the impurity spin polarization both in the presence and in the absence of Coulomb interaction. The exponential relaxation is faster for larger bias voltages, electron-impurity interactions and temperatures. We show that the exact relaxation rate deviates from the corresponding perturbative result. In addition, we study in detail the impurity's back action on the charge current and find a reduction of the stationary current for increasing coupling to the impurity. Moreover, our approach allows us to systematically distinguish mean-field Coulomb and impurity effects from the influence of quantum fluctuations and flip-flop scattering, respectively. In fact, we find a local maximum of the current for a finite Coulomb interaction due to the presence of the impurity.

cond-mat.mes-hall↗

arXiv2011

Exciton dynamics and Quantumness of energy transfer in the Fenna-Matthews-Olson complex

We present numerically exact results for the quantum coherent energy transfer in the Fenna-Matthews-Olson molecular aggregate under realistic physiological conditions, including vibrational fluctuations of the protein and the pigments for an experimentally determined fluctuation spectrum. We find coherence times shorter than observed experimentally. Furthermore we determine the energy transfer current and quantify its "quantumness" as the distance of the density matrix to the classical pointer states for the energy current operator. Most importantly, we find that the energy transfer happens through a "Schrödinger-cat" like superposition of energy current pointer states.

cond-mat.mes-hall↗

arXiv2009

Landau Zener transitions in a dissipative environment: Numerically exact results

We study Landau-Zener transitions in a dissipative environment by means of the numerically exact quasiadiabatic propagator path-integral. It allows to cover the full range of the involved parameters. We discover a nonmonotonic dependence of the transition probability on the sweep velocity which is explained in terms of a simple phenomenological model. This feature, not captured by perturbative approaches, results from a nontrivial competition between relaxation and the external sweep.

cond-mat.mes-hall↗

arXiv2009

Coherent control of an effective two-level system in a non-Markovian biomolecular environment

We investigate the quantum coherent dynamics of an externally driven effective two-level system subjected to a slow Ohmic environment characteristic of biomolecular protein-solvent reservoirs in photosynthetic light harvesting complexes. By means of the numerically exact quasi adiabatic propagator path integral (QUAPI) method we are able to include non-Markovian features of the environment and show the dependence of the quantum coherence on the characteristic bath cut-off frequency omega_c as well as on the driving frequency omega_l and the field amplitude A. Our calculations extend from the weak coupling regime to the incoherent strong coupling regime. In the latter case, we find evidence for a resonant behaviour, beyond the expected behaviour, when the reorganization energy E_r coincides with the driving frequency. Moreover, we investigate how the coherent destruction of tunneling within the two-level system is influenced by the non-Markovian environment.

cond-mat.mes-hall↗

arXiv2009

Enhanced quantum entanglement in the non-Markovian dynamics of biomolecular excitons

We show that quantum coherence of biomolecular excitons is maintained over exceedingly long times due to the constructive role of their non-Markovian protein-solvent environment. Using a numerically exact approach, we demonstrate that a slow quantum bath helps to sustain quantum entanglement of two pairs of Forster coupled excitons, in contrast to a Markovian environment. We consider the crossover from a fast to a slow bath and from weak to strong dissipation and show that a slow bath can generate robust entanglement. This persists to surprisingly high temperatures, even higher than the excitonic gap and is absent for a Markovian bath.

cond-mat.mes-hall↗

arXiv2009

Molecular architectures based on pi-conjugated block copolymers for global quantum computation

We propose a molecular setup for the physical implementation of a barrier global quantum computation scheme based on the electron-doped pi-conjugated copolymer architecture of nine blocks PPP-PDA-PPP-PA-(CCH-acene)-PA-PPP-PDA-PPP (where each block is an oligomer). The physical carriers of information are electrons coupled through the Coulomb interaction, and the building block of the computing architecture is composed by three adjacent qubit systems in a quasi-linear arrangement, each of them allowing qubit storage, but with the central qubit exhibiting a third accessible state of electronic energy far away from that of the qubits' transition energy. The third state is reached from one of the computational states by means of an on-resonance coherent laser field, and acts as a barrier mechanism for the direct control of qubit entanglement. Initial estimations of the spontaneous emission decay rates associated to the energy level structure allow us to compute a damping rate of order 10^{-7} s, which suggest a not so strong coupling to the environment. Our results offer an all-optical, scalable, proposal for global quantum computing based on semiconducting pi-conjugated polymers.

cond-mat.mes-hall↗

arXiv2007

Spin-boson dynamics beyond conventional perturbation theories

A novel approximation scheme is proposed to describe the dynamics of the spin-boson problem. Being nonperturbative in the coupling strength nor in the tunneling frequency, it gives reliable results over a wide regime of temperatures and coupling strength to the thermal environment for a large class of bath spectral densities. We use a path-integral approach and start from the exact solution for the two-level system population difference in the form of a generalized master equation (GME). Then, we approximate inter-blip and blip-sojourns interactions up to linear order, while retaining all intra-blip correlations to find the kernels entering the GME in analytical form. Our approximation scheme, which we call Weakly-Interacting Blip Approximation (WIBA), fully agrees with conventional perturbative approximations in the tunneling matrix element (Non-Interacting Blip Approximation) or in the system-bath coupling strength.

cond-mat.stat-mech↗