arXiv subjects

Michael Thorwart

Publications and source records attributed to Michael Thorwart.

At least 37 records · Page 2Linked to original sources

arXiv2021

Bath-induced decoherence in finite-size Majorana wires at non-zero temperature

Braiding Majorana zero-modes around each other is a promising route towards topological quantum computing. Yet, two competing maxims emerge when implementing Majorana braiding in real systems: On the one hand, perfect braiding should be conducted adiabatically slowly to avoid non-topological errors. On the other hand, braiding must be conducted fast such that decoherence effects introduced by the environment are negligible, which are generally unavoidable in finite-size systems. This competition results in an intermediate time scale for Majorana braiding that is optimal, but generally not error-free. Here, we calculate this intermediate time scale for a T-junction of short one-dimensional topological superconductors coupled to a bosonic bath that generates fluctuations in the local electric potential, which stem from, e.g., environmental photons or phonons of the substrate. We thereby obtain boundaries for the speed of Majorana braiding with a predetermined gate fidelity. Our results emphasize the general susceptibility of Majorana-based information storage in finite-size systems and can serve as a guide for determining the optimal braiding times in future experiments.

cond-mat.mes-hall↗

arXiv2021

Disentangling Dynamical Quantum Coherences in the Fenna-Matthews-Olson Complex

In the primary step of light-harvesting, the energy of a photon is captured in antenna chlorophyll as an exciton. Its efficient conversion to stored chemical potential occurs in the special pair reaction center, which has to be reached by down-hill ultrafast excited state energy transport. The interaction between the chromophores leads to spatial delocalization and quantum coherence effects, the importance of which depends on the coupling between the chlorophylls in relation to the intensity of the fluctuations and reorganization dynamics of the protein matrix, or bath. The latter induce uncorrelated modulations of the site energies, and thus quantum decoherence, and localization of the exciton. Current consensus is that under physiological conditions quantum decoherence occurs on the 10 fs time scale, and quantum coherence plays little role for the observed picosecond energy transfer dynamics. In this work, we reaffirm this from a different point of view by finding that the true onset of electronic quantum coherence only occurs at extremely low temperatures of ~20 K. We have directly determined the exciton coherence times by two-dimensional electronic spectroscopy of the Fenna-Matthew-Olson complex over an extensive temperature range with a supporting theoretical modelling. At 20 K, electronic coherences persist out to 200 fs (close to the antenna) and marginally up to 500 fs at the reaction-center side. It decays markedly faster with modest increases in temperature to become irrelevant above 150 K. This temperature dependence also allows disentangling the previously reported long-lived beatings. We show that they result from mixing vibrational coherences in the electronic ground state ...

cond-mat.other↗

arXiv2020

Rotating edge-field driven processing of chiral spin textures in racetrack devices

Topologically distinct magnetic structures like skyrmions, domain walls, and the uniformly magnetized state have multiple applications in logic devices, sensors, and as bits of information. One of the most promising concepts for applying these bits is the racetrack architecture controlled by electric currents or magnetic driving fields. In state-of-the-art racetracks, these fields or currents are applied to the whole circuit. Here, we employ micromagnetic and atomistic simulations to establish a concept for racetrack memories free of global driving forces. Surprisingly, we realize that mixed sequences of topologically distinct objects can be created and propagated over far distances exclusively by local rotation of magnetization at the sample boundaries. We reveal the dependence between the chirality of the rotation and the direction of propagation and define the phase space where the proposed procedure can be realized. The advantages of this approach are the exclusion of high current and field densities as well as its compatibility with an energy-efficient three-dimensional design.

cond-mat.mes-hall↗

arXiv2020

Time-Resolved Probing of the Nonequilibrium Structural Solvation Dynamics by the Time-Dependent Stokes Shift

The time-dependent fluorescence Stokes shifts monitors the relaxation of the polarization of a polar solvent in the surroundings of a photoexcited solute molecule, but also the structural variation of the solute following photoexcitation and the subsequent molecular charge redistribution. Here, we formulate a simple nonequilibrium quantum theory of solvation for an explicitly time-dependent continuous solvent. The time-dependent solvent induces nonequilibrium fluctuations on the solvent dynamics which are directly reflected in different time components in the time-dependent Stokes shift. We illustrate the structural dynamics in the presence of an explicitly time-dependent solvent by the example of a dynamically shrinking solute which leads to a bi-modal Stokes shift. Interestingly, both contributions are mutually coupled. Furthermore, we can explain a prominent long-tail decay of the Stokes shift associated to slow structural dynamical variations.

physics.chem-ph↗

arXiv2020

Photoinduced Vibrations Drive Ultrafast Structural Distortion in Lead Halide Perovskite

Organic-inorganic perovskites have shown great promise towards their application in optoelectronics. The success of this class of material is dictated by the complex interplay between various underlying microscopic phenomena. The structural dynamics of organic cations and the inorganic sublattice after photoexcitation is hypothesized to have a direct effect on the material properties, thereby affecting the overall device performance. Here, we use two-dimensional (2D) electronic spectroscopy to reveal impulsively excited vibrational modes of methylammonium (MA) lead iodide perovskite, which drive the structural distortion after photoexcitation. The vibrational analysis of the measured data allows us to directly monitor the time evolution of the librational motion of the MA cation along with the vibrational coherences of inorganic sublattice. Wavelet analysis of the observed vibrational coherences uncovers the interplay between these two types of phonons. It reveals the coherent generation of the librational motion of the MA cation within ~300 fs, which is complemented by the coherent evolution of the skeletal motion of the inorganic sublattice. We have employed time-dependent density functional theory (TDDFT) to study the atomic motion of the MA cation and the inorganic sublattice during the process of photoexcitation. The TDDFT calculations support our experimental observations of the coherent generation of librational motions in the MA cation and highlight the importance of the anharmonic interaction between the MA cation and the inorganic sublattice. Our calculations predict the transfer of the photoinduced vibrational coherence from the MA cation to the inorganic sublattice, which drives the skeleton motion to form a polaronic state leading to long lifetimes of the charge carriers. This work may lead to novel design principles for next generation of solar cell materials.

physics.chem-ph↗

arXiv2020

Vortex Majorana braiding in a finite time

Abrikosov vortices in Fe-based superconductors are a promising platform for hosting Majorana zero modes. Their adiabatic exchange is a key ingredient for Majorana-based quantum computing. However, the adiabatic braiding process can not be realized in state-of-the-art experiments. We propose to replace the infinitely slow, long-path braiding by only slightly moving vortices in a special geometry without actually physically exchanging the Majoranas, like a Majorana carousel. Although the resulting finite-time gate is not topologically protected, it is robust against variations in material specific parameters and in the braiding-speed. We prove this analytically. Our results carry over to Y-junctions of Majorana wires.

cond-mat.supr-con↗

arXiv2020

Charge Transfer Through Redox Molecular Junctions in Non-Equilibrated Solvents

Molecular conduction operating in dielectric solvent environments are often described using kinetic rates based on Marcus theory of electron transfer at a molecule-metal electrode interface. However, the successive nature of charge transfer in such system implies that the solvent does not necessarily reach equilibrium in such process. Here we generalize the theory to account for solvent nonequilibrium and consider a molecular junction consisting of an electronic donor-acceptor system coupled to two metallic electrodes and placed in a polarizable solvent. We determine the nonequilbrium distribution of the solvent by solving diffusion equations in the strong- and weak-friction limits and calculate the charge current and its fluctuating behavior. In extreme limits: the absence of the solvent or fast solvent relaxation, the charge transfer statistics is Poissonian, while it becomes correlated by the dynamic solvent in between these limits. A Kramers-like turnover of the nonequilibrium current as a function of the solvent damping is found. Finally, we propose a way to tune the solvent-induced damping using geometrical control of the solvent dielectric response in nanostructured solvent channels.

physics.chem-ph↗

arXiv2019

Tuning the Order of the Nonequilibrium Quantum Phase Transition in a Hybrid Atom-Optomechanical System

We show that a hybrid atom-optomechanical quantum many-body system with two internal atom states undergoes both first- and second-order nonequilibrium quantum phase transitions. A nanomembrane is placed in a pumped optical cavity, whose outcoupled light forms a lattice for an ultracold Bose gas. By changing the pump strength, the effective membrane-atom coupling can be tuned. Above a critical intensity, a symmetry-broken phase emerges which is characterized by a sizeable occupation of the high-energy internal states and a displaced membrane. The order of this nonequilibrium quantum phase transition can be changed by tuning the transition frequency. For a symmetric coupling, the transition is continuous below a certain transition frequency and discontinuous above. For an asymmetric coupling, a first-order phase transition occurs.

quant-ph↗

arXiv2019

Intermolecular Vibrations Drive Ultrafast Singlet Fission

Singlet fission is a spin-allowed exciton multiplication process in organic semiconductors that converts one spin-singlet exciton to two triplet excitons. It offers the potential to enhance solar energy conversion by circumventing the Shockley-Queisser limit on efficiency. Recently, the mechanism of the primary singlet fission process in pentacene and its derivatives have been extensively investigated, however, the nature of the primary ultrafast process in singlet fission is still a matter of debate. Here, we study the singlet fission process in a pentacene film by employing a combination of transient-grating (TG) and two-dimensional (2D) electronic spectroscopy complemented by quantum chemical and nonadiabatic dynamics calculations. The high sensitivity of heterodyne detected TG spectroscopy enabled us to capture the vibrational coherence and to show that it mediates the transition from the singlet excited electronic state to the triplet-pair state. This coherent process is further examined by 2D electronic spectroscopy. Detailed analysis of the experimental data reveals that significant vibronic couplings of a few key modes in the low- and high-frequency region connect the excited singlet and triplet-pair states. Based on quantum chemical calculations, we identify these key intermolecular rocking modes along the longitudinal molecular axis between the pentacene molecules. They play the essential role of an electronic bridge between the singlet and triplet-pair states. Along with high-frequency local vibrations acting as tuning modes, these rocking motions drive the ultrafast dynamics at the multidimensional conical intersection in the singlet fission process.

physics.chem-ph↗

arXiv2019

Does electronic coherence enhance anticorrelated pigment vibrations under realistic conditions?

The light-harvesting efficiency of a photoactive molecular complex is largely determined by the properties of its electronic quantum states. Those, in turn, are influenced by molecular vibrational states of the nuclear degrees of freedom. Here, we reexamine two recently formulated concepts that a coherent vibronic coupling between molecular states would either extend the electronic coherence lifetime or enhance the amplitude of the anticorrelated vibrational mode at longer times. For this, we study a vibronically coupled dimer and calculate the nonlinear two-dimensional (2D) electronic spectra which directly reveal electronic coherence. The timescale of electronic coherence is initially extracted by measuring the anti-diagonal bandwidth of the central peak in the 2D spectrum at zero waiting time. Based on the residual analysis, we identify small-amplitude long-lived oscillations in the cross-peaks, which, however, are solely due to groundstate vibrational coherence, regardless of having resonant or off-resonant conditions. Our studies neither show an enhancement of the electronic quantum coherence nor an enhancement of the anticorrelated vibrational mode by the vibronic coupling under ambient conditions.

physics.bio-ph↗

arXiv2019

Winding up quantum spin helices: How avoided level crossings exile classical topological protection

A magnetic helix can be wound into a classical Heisenberg chain by fixing one end while rotating the other one. We show that in quantum Heisenberg chains \new{of finite length}, the magnetization slips back to the trivial state beyond a finite turning angle. Avoided level crossings thus undermine classical topological protection. Yet, for special values of the axial Heisenberg anisotropy, stable spin helices form again, which are non-locally entangled. Away from these sweet spots, spin helices can be stabilized dynamically or by dissipation. For half-integer spin chains of odd length, a spin slippage state and its Kramers partner define a qubit with a non-trivial Berry connection.

cond-mat.mes-hall↗

arXiv2018

Enhancing nanomechanical squeezing by atomic interactions in a hybrid atom-optomechanical system

In a hybrid atom-optomechanical system, the optical coupling of a mechanical mode of a nanomembrane in an optical cavity with a distant interacting atom gas permits highly non-classical quantum many-body states. We show that the mechanical mode can be squeezed by the back-action of internal excitations of the atoms in the gas. A Bogoliubov approach reveals that these internal excitations form a fluctuating environment of quasi-particle excitations for the mechanical mode with a gaped spectral density. Nanomechanical squeezing arises due to quasi-particle excitations in the interacting atom gas when the mechanical frequency is close to resonance with the internal atomic transitions. Interestingly, nanomechanical squeezing is enhanced by atom-atom interactions.

cond-mat.quant-gas↗

arXiv2018

Dragging of magnetic domain walls by interlayer exchange

Magnetic domain walls can be moved by spin-polarized currents due to spin-transfer torques. This opens the possibility to use them in spintronic memory devices as, e.g., in racetrack storage. Naturally, in miniaturized devices domain walls can get very close to each other and affect each others dynamics. In this work we consider two separated domain walls in different layers which interact via an interlayer exchange coupling. One of these walls is moved by a spin-polarized current. Depending on several parameters as the current density, the interlayer coupling or the pinning potential, the combined dynamics of the two domain walls can change very strongly allowing, e.g., for a correlated motion of the walls. In addition, more subtle effect appear as a suppression of the Walker breakdown accompanied by an increase of the domain wall velocity.

cond-mat.mes-hall↗

arXiv2018

Quantum mechanical response to a driven Caldeira-Leggett bath

We determine the frequency-dependent response characteristics of a quantum system to a driven Caldeira-Leggett bath. The bath degrees of freedom are explicitly driven by an external time-dependent force, in addition to the direct time-dependent forcing of the system itself. After general considerations of driven Caldeira-Leggett baths, we consider the Rubin model of a chain of quantum particles coupled by linear springs as an important model of a quantum dissipative system. We show that in the presence of time-dependent driving of the chain, this model can be mapped to a quantum system which couples to a driven Caldeira-Leggett bath. The effect of the bath driving is captured by a time-dependent force on the central system, which is, in principle, non-Markovian in nature. We study two specific examples, the exactly solvable case of a harmonic potential, and a quantum two-state system for which we assume a weak system-bath coupling. We evaluate the dynamical response to a periodic driving of the system and the bath. The dynamic susceptibility is shown to be altered qualitatively by the bath drive: The dispersive part is enhanced at low frequencies and acquires a maximum at zero frequency. The absorptive part develops a shoulder-like behavior in this frequency regime. These features seem to be generic for quantum systems in a driven Caldeira-Leggett bath.

cond-mat.stat-mech↗

arXiv2018

Antiresonant quantum transport in ac driven molecular nanojunctions

(Dated: July 17, 2017) We calculate the electric charge current flowing through a vibrating molecular nanojunction, which is driven by an ac voltage, in its regime of nonlinear oscillations. Without loss of generality, we model the junction by a vibrating molecule which is doubly clamped to two metallic leads which are biased by time-periodic ac voltages. Dressed-electron tunneling between the leads and the molecule drives the mechanical degree of freedom out of equilibrium. In the deep quantum regime, where only a few vibrational quanta are excited, the formation of coherent vibrational resonances affects the dressed-electron tunneling. In turn, back action modifies the electronic ac current passing through the junction. The concert of nonlinear vibrations and ac driving induces quantum transport currents which are antiresonant to the applied ac voltage. Quantum back action on the flowing nonequilibriun current allows us to obtain rather sharp spectroscopic information on the population of the mechanical vibrational states.

cond-mat.mes-hall↗

arXiv2018

Nonequilibrium quantum solvation with a time-dependent Onsager cavity

We formulate a theory of nonequilibrium quantum solvation in which parameters of the solvent are explicitly depending on time. We assume in a simplest approach a spherical molecular Onsager cavity with a time-dependent radius. We analyze the relaxation properties of a test molecular point dipole in a dielectric solvent and consider two cases: (i) a shrinking Onsager sphere, and, (ii) a breathing Onsager sphere. Due to the time-dependent solvent, the frequency-dependent response function of the dipole becomes time-dependent. For a shrinking Onsager sphere, the dipole relaxation is in general enhanced. This is reflected in a temporally increasing line width of the absorptive part of the response. Furthermore, the effective frequency-dependent response function shows two peaks in the absorptive part which are symmetrically shifted around the eigenfrequency. In contrast, a breathing sphere reduces damping as compared to the static sphere. Interestingly, we find a non-monotonous dependence of the relaxation rate on the breathing rate and a resonant suppression of damping when both rates are comparable. Moreover, the line width of the absorptive part of the response function is strongly reduced for times when the breathing sphere reaches its maximal extension.

physics.chem-ph↗

arXiv2018

Nonequilibrium Quantum Phase Transition in a Hybrid Atom-Optomechanical System

We consider a hybrid quantum many-body system formed by both a vibrational mode of a nanomembrane, which interacts optomechanically with light in a cavity, and an ultracold atom gas in the optical lattice of the out-coupled light. After integrating over the light field, an effective Hamiltonian reveals a competition between the localizing potential force and the membrane displacement force. For increasing atom-membrane interaction we find a nonequilibrium quantum phase transition from a localized non-motional phase of the atom cloud to a phase of collective motion. Near the quantum critical point, the energy of the lowest collective excitation vanishes, while the order parameter of the condensate becomes non-zero in the symmetry-broken state. The effect occurs when the atoms and the membrane are non-resonantly coupled.

quant-ph↗

arXiv2017

Implications of a temperature-dependent magnetic anisotropy for superparamagnetic switching

The macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier in order to switch its direction. This barrier is formed by magnetic anisotropies in the material and may be surmounted typically after 10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the associated switching rate may be described by a Neel-Brown-Arrhenius law, in which the energy barrier is assumed as constant or a given temperature. Yet, magnetic anisotropies in general depend on temperature themselves which is known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the implications of a temperature-dependent anisotropy on the switching rate and in particular for the interpretation of the prefactor as an attempt frequency. In particular, we show that realistic numbers for the attempt frequency are obtained when the temperature dependence of the anisotropy is taken into account.

cond-mat.mes-hall↗