Band conductivity oscillations in a gate-tunable graphene superlattice.

Electrons exposed to a two-dimensional (2D) periodic potential and a uniform, perpendicular magnetic field exhibit a fractal, self-similar energy spectrum known as the Hofstadter butterfly. Recently, related high-temperature quantum oscillations (Brown-Zak oscillations) were discovered in graphene moiré systems, whose origin lies in the repetitive occurrence of extended minibands/magnetic Bloch states at rational fractions of magnetic flux per unit cell giving rise to an increase in band conductivity. In this work, we report on the experimental observation of band conductivity oscillations in an electrostatically defined and gate-tunable graphene superlattice, which are governed both by the internal structure of the Hofstadter butterfly (Brown-Zak oscillations) and by a commensurability relation between the cyclotron radius of electrons and the superlattice period (Weiss oscillations). We obtain a complete, unified description of band conductivity oscillations in two-dimensional superlattices, yielding a detailed match between theory and experiment.

Ultrafast ab Initio Quantum Chemistry Using Matrix Product States.

Ultrafast dynamics in chemical systems provide a unique access to fundamental processes at the molecular scale. A proper description of such systems is often very challenging because of the quantum nature of the problem. The concept of matrix product states (MPS), however, has proven its performance in describing such correlated quantum systems in recent years for a wide range of applications. In this work, we continue the development of the MPS approach to study ultrafast electron dynamics in quantum chemical systems. The method combines time evolution schemes, such as fourth-order Runge-Kutta and Krylov space time evolution, with MPS, in order to solve the time-dependent Schrödinger equation efficiently. This allows for describing electron dynamics in molecules on a full configurational interaction (CI) level for a few femtoseconds after excitation. As a benchmark, we compare MPS-based calculations to full CI calculations for a chain of hydrogen atoms and for the water molecule. Krylov space time evolution is in particular suited for the MPS approach, as it provides a wide range of opportunities to be adjusted to the reduced MPS dimension case. Finally, we apply the MPS approach to describe charge migration effects in iodoacetylene and find direct agreement between our results and experimental observations.

Current-driven spin dynamics of artificially constructed quantum magnets.

The future of nanoscale spin-based technologies hinges on a fundamental understanding and dynamic control of atomic-scale magnets. The role of the substrate conduction electrons on the dynamics of supported atomic magnets is still a question of interest lacking experimental insight. We characterized the temperature-dependent dynamical response of artificially constructed magnets, composed of a few exchange-coupled atomic spins adsorbed on a metallic substrate, to spin-polarized currents driven and read out by a magnetic scanning tunneling microscope tip. The dynamics, reflected by two-state spin noise, is quantified by a model that considers the interplay between quantum tunneling and sequential spin transitions driven by electron spin-flip processes and accounts for an observed spin-transfer torque effect.

Proposal of a robust measurement scheme for the nonadiabatic spin torque using the displacement of magnetic vortices.

A spin-polarized current traversing a ferromagnet with continuously varying magnetization exerts a torque on the magnetization. The nonadiabatic contribution to this spin-transfer torque is currently under strong debate, as its value differs by orders of magnitude in theoretical predictions and in measurements. Here, a measurement scheme is presented that allows us to determine the strength of the nonadiabatic spin torque accurately and directly. Analytical and numerical calculations show that the scheme is robust against the uncertainties of the exact current direction and Oersted fields.

Width dependence of the nonadiabatic spin-transfer torque in narrow domain walls.

We analytically determine the spatially varying spin-transfer torque within a domain wall. In the case of ballistic spin and diffusive charge transport, the spin-transfer torque as well as the local degree of nonadiabaticity oscillate within a domain wall. In narrow domain walls, the degree of nonadiabaticity ceases to be a constant material parameter but depends on the domain-wall width including a possible sign change, which is crucial for experiments and the technological utilization in spin-transfer-torque-based storage devices.

Dependence of magnetic domain-wall motion on a fast changing current.

A dependence of current-induced domain-wall motion in nanowires on the temporal shape of current pulses is observed. The results show that the motion of the wall is amplified for alterations of the current on a time scale smaller than the intrinsic time scale of the domain wall which is a few nanoseconds in permalloy. This effect arises from an additional force on the wall by the spin-transfer torque due to a fast changing current and improves the efficiency of domain-wall motion. The observations provide an understanding for the efficient domain-wall motion with nanosecond current pulses.

Self-trapping of bosons and fermions in optical lattices.

We theoretically investigate the enhanced localization of bosonic atoms by fermionic atoms in three-dimensional optical lattices and find a self-trapping of the bosons for attractive boson-fermion interaction. Because of this mutual interaction, the fermion orbitals are substantially squeezed, which results in a strong deformation of the effective potential for bosons. This effect is enhanced by an increasing bosonic filling factor leading to a large shift of the transition between the superfluid and the Mott-insulator phase. We find a nonlinear dependency of the critical potential depth on the boson-fermion interaction strength. The results, in general, demonstrate the important role of higher Bloch bands for the physics of attractively interacting quantum gas mixtures in optical lattices and are of direct relevance to recent experiments with 87Rb-40K mixtures, where a large shift of the critical point has been found.