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Achim Rosch

Publications and source records attributed to Achim Rosch.

At least 91 records · Page 5Linked to original sources
arXiv2014

Quench dynamics and statistics of measurements for a line of quantum spins in two dimensions

Motivated by recent experiments, we investigate the dynamics of a line of spin-down spins embedded in the ferromagnetic spin-up ground state of a two-dimensional xxz model close to the Ising limit. In a situation where the couplings in x and y direction are different, the quench dynamics of this system is governed by the interplay of one-dimensional excitations (kinks and holes) moving along the line and single-spin excitations evaporating into the two-dimensional background. A semiclassical approximation can be used to calculate the dynamics of this complex quantum system. Recently, it became possible to perform projective quantum measurements on such spin systems, allowing to determine, e.g., the z-component of each individual spin. We predict the statistical properties of such measurements which contain much more information than correlation functions.

cond-mat.quant-gas↗
arXiv2014

Directed motion of doublons and holes in periodically driven Mott insulators

Periodically driven systems can lead to a directed motion of particles. We investigate this ratchet effect for a bosonic Mott insulator where both a staggered hopping and a staggered local potential vary periodically in time. If driving frequencies are smaller than the interaction strength and the density of excitations is small, one obtains effectively a one-particle quantum ratchet describing the motion of doubly occupied sites (doublons) and empty sites (holes). Such a simple quantum machine can be used to manipulate the excitations of the Mott insulator. For suitably chosen parameters, for example, holes and doublons move in opposite direction. To investigate whether the periodic driving can be used to move particles "uphill", i.e., against an external force, we study the influence of a linear potential $- g x$. For long times, transport is only possible when the driving frequency $ω$ and the external force $g$ are commensurate, $n_0 g = m_0 ω$, with $\frac{n_0}{2},m_0 \in \mathbb{Z}$.

cond-mat.quant-gas↗
arXiv2014

Quench dynamics of one-dimensional interacting bosons in a disordered potential: Elastic dephasing and critical speeding-up of thermalization

The dynamics of interacting bosons in one dimension following the sudden switching on of a weak disordered potential is investigated. On time scales before quasiparticles scatter (prethermalized regime), the dephasing from random elastic forward scattering causes all correlations to decay exponentially fast, but the system remains far from thermal equilibrium. For longer times, the combined effect of disorder and interactions gives rise to inelastic scattering and to thermalization. A novel quantum kinetic equation accounting for both disorder and interactions is employed to study the dynamics. Thermalization turns out to be most effective close to the superfluid-Bose glass critical point where nonlinearities become more and more important. The numerically obtained thermalization times are found to agree well with analytic estimates.

cond-mat.str-el↗
arXiv2014

Dynamics and energetics of emergent magnetic monopoles in chiral magnets

The formation and destruction of topologically quantized magnetic whirls, so-called skyrmions, in chiral magnets is driven by the creation and motion of singular hedgehog defects. These can be identified with emergent magnetic monopoles and antimonopoles. We investigate how the energetics of and forces between monopoles and antimonopoles influence their creation rate and dynamics. We study a single skyrmion line defect in the helical phase using both micromagnetic simulations and a Ginzburg-Landau analysis. Monopole-antimonople pairs are created in a thermally activated process, largely controlled by the (core) energy of the monopole. The force between monopoles and antimonopoles is linear in distance and described by a string tension. The sign and size of the string tension determines the stability of the phases and the velocity of the monopoles.

cond-mat.str-el↗
arXiv2013

Hydrodynamic long-time tails after a quantum quench

After a quantum quench, a sudden change of parameters, generic many particle quantum systems are expected to equilibrate. A few collisions of quasiparticles are usually sufficient to establish approximately local equilibrium. Reaching global equilibrium is, however, much more difficult as conserved quantities have to be transported for long distances to build up a pattern of fluctuations characteristic for equilibrium. Here we investigate the quantum quench of the one-dimensional bosonic Hubbard model from infinite to finite interaction strength U using semiclassical methods for weak, and exact diagonalization for strong quenches. Equilibrium is approached only slowly, as t^{-1/2} with subleading corrections proportional to t^{-3/4}, consistent with predictions from hydrodynamics. We show that these long-time tails determine the relaxation of a wide range of physical observables.

cond-mat.quant-gas↗
arXiv2013

Interaction effects on almost flat surface bands in topological insulators

We consider ferromagnetic instabilities of two-dimensional helical Dirac fermions hosted on the surface of three-dimensional topological insulators. We investigate ways to increase the role of interactions by means of modifying the bulk properties which in turn changes both the surface Dirac theory and the screening of interactions. We discuss both the long-ranged part of the Coulomb interactions controlled by the dimensionless coupling constant $α= e^{2}/(\hbar εv_{F}^{\mathrm{surf}})$ as well as the effects of local interactions parametrized by the ratio $U_{\mathrm{surf}}/D_{\mathrm{surf}}$ of a local interaction on the surface, $U_{\mathrm{surf}}$, and the surface bandwidth, $D_{\mathrm{surf}}$. If large compared to 1, both mechanisms can induce spontaneously surface ferromagnetism, thereby gapping the surface Dirac metal and inducing an anomalous quantum Hall effect. We investigate two mechanisms which can naturally lead to small Fermi velocities $v_{F}^{\mathrm{surf}}$ and a corresponding small bandwidth $D_{\mathrm{surf}}$ at the surface when the bulk band gap is reduced. The same mechanisms can, however, also lead to an enhanced screening of surface interactions. While in all considered cases the long-ranged part of the Coulomb interaction is screened efficiently, $α\lesssim 1$, we discuss situations, where $U_{\mathrm{surf}}/D_{\mathrm{surf}}$ becomes \emph{parametrically}\ large compared to 1, thus inducing surface magnetism.

cond-mat.str-el↗
arXiv2013

Condensation of Photons coupled to a Dicke Field in an Optical Microcavity

Motivated by recent experiments reporting Bose-Einstein condensation (BEC) of light coupled to incoherent dye molecules in a microcavity, we show that due to a dimensionality mismatch between the 2D cavity-photons and the 3D arrangement of molecules, the relevant molecular degrees of freedom are collective Dicke states rather than individual excitations. For sufficiently high dye concentration the coupling of the Dicke states with light will dominate over local decoherence. This system also shows Mott criticality despite the absence of an underlying lattice in the limit when all dye molecules become excited.

cond-mat.quant-gas↗
arXiv2013

Phase-Space Berry Phases in Chiral Magnets: Dzyaloshinskii-Moriya Interaction and the Charge of Skyrmions

The semiclassical motion of electrons in phase space, x=(R, k), is influenced by Berry phases described by a 6-component vector potential, A=(A^R, A^k). In chiral magnets Dzyaloshinskii-Moriya (DM) interactions induce slowly varying magnetic textures (helices and skyrmion lattices) for which all components of A are important inducing effectively a curvature in mixed position and momentum space. We show that for smooth textures and weak spin-orbit coupling phase space Berry curvatures determine the DM interactions and give important contributions to the charge. Using ab initio methods we calculate the strength of DM interactions in MnSi in good agreement with experiment and estimate the charge of skyrmions.

cond-mat.str-el↗
arXiv2012

Unwinding of a one-dimensional topological superconductor

We show that a topological superconductor made of four chains of superconducting spinless fermions characterized by four Majorana edge states can adiabatically be deformed into a trivial band insulator. To unwind this time-reversal invariant topological superconductor, interactions to spinful fermions are switched on along an adiabatic path. Thereby, we couple modes which belong to two different representations of the time-reversal symmetry operator T with T^2 = 1 and T^2 = -1, respectively. This observation can be understood by investigating how the relevant symmetries act on the entanglement spectrum giving rise to four instead of eight different topological phases with Majorana edge modes. We also show that a simple level crossing of doubly and singly degenerate states occurs in the entanglement spectrum upon deforming the quantum state.

cond-mat.str-el↗
arXiv2012

Quantum criticality with multiple dynamics

Quantum critical systems with multiple dynamics possess not only one but several time scales, tau_i ~ xi^(z_i), which diverge with the correlation length xi. We investigate how scaling predictions are modified for the simplest case of multiple dynamics characterized by two dynamical critical exponents, z_> and z_<. We argue that one should distinguish the case of coupled and decoupled multiple dynamic scaling depending on whether there exists a scaling exponent which depends on both z_i or not. As an example, we study generalized Phi^4-theories with multiple dynamics below their upper critical dimension, d+z_<<4. We identify under which condition coupled scaling is generated. In this case the interaction of quantum and classical fluctuations leads to an emergent dynamical exponent, z_e=z_>/(nu (z_>-z_<)+1).

cond-mat.str-el↗
arXiv2012

Mott criticality and pseudogap in Bose-Fermi mixtures

We study the Mott transition of a mixed Bose-Fermi system of ultracold atoms in an optical lattice, where the number of (spinless) fermions and bosons adds up to one atom per lattice, n_F+n_B=1. For weak interactions, a Fermi surface coexists with a Bose-Einstein condensate while for strong interaction the system is incompressible but still characterized by a Fermi surface of composite fermions. At the critical point, the spectral function of the fermions, A(k,w), exhibits a pseudo-gapped behavior, rising as |w| at the Fermi momentum, while in the Mott phase it is fully gapped. Taking into account the interaction between the critical modes leads at very low temperatures either to p-wave pairing or the transition is driven weakly first order. The same mechanism should also be important in antiferromagnetic metals with a small Fermi surface.

cond-mat.quant-gas↗
arXiv2012

Fermionic transport in a homogeneous Hubbard model: Out-of-equilibrium dynamics with ultracold atoms

Transport properties are among the defining characteristics of many important phases in condensed matter physics. In the presence of strong correlations they are difficult to predict even for model systems like the Hubbard model. In real materials they are in general obscured by additional complications including impurities, lattice defects or multi-band effects. Ultracold atoms in contrast offer the possibility to study transport and out-of-equilibrium phenomena in a clean and well-controlled environment and can therefore act as a quantum simulator for condensed matter systems. Here we studied the expansion of an initially confined fermionic quantum gas in the lowest band of a homogeneous optical lattice. While we observe ballistic transport for non-interacting atoms, even small interactions render the expansion almost bimodal with a dramatically reduced expansion velocity. The dynamics is independent of the sign of the interaction, revealing a novel, dynamic symmetry of the Hubbard model.

cond-mat.quant-gas↗
arXiv2012

Rotating skyrmion lattices by spin torques and field or temperature gradients

Chiral magnets like MnSi form lattices of skyrmions, i.e. magnetic whirls, which react sensitively to small electric currents j above a critical current density jc. The interplay of these currents with tiny gradients of either the magnetic field or the temperature can induce a rotation of the magnetic pattern for j>jc. Either a rotation by a finite angle of up to 15 degree or -- for larger gradients -- a continuous rotation with a finite angular velocity is induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping terms in combination with a phenomenological treatment of pinning forces to develop a theory of the relevant rotational torques. Experimental neutron scattering data on the angular distribution of skyrmion lattices suggests that continuously rotating domains are easy to obtain in the presence of remarkably small currents and temperature gradients.

cond-mat.str-el↗
arXiv2011

Wilson chains are not thermal reservoirs

Wilson chains, based on a logarithmic discretization of a continuous spectrum, are widely used to model an electronic (or bosonic) bath for Kondo spins and other quantum impurities within the numerical renormalization group method and other numerical approaches. In this short note we point out that Wilson chains can not serve as thermal reservoirs as their temperature changes by a number of order Delta E when a finite amount of energy Delta E is added. This proves that for a large class of non-equilibrium problems they cannot be used to predict the long-time behavior.

cond-mat.str-el↗
arXiv2011

Topological insulators in magnetic fields: Quantum Hall effect and edge channels with non-quantized θ-term

We investigate how a magnetic field induces one-dimensional edge channels when the two-dimensional surface states of three-dimensional topological insulators become gapped. The Hall effect, measured by contacting those channels, remains quantized even in situations where the θ-term in the bulk and the associated surface Hall conductivities, σ_{xy}^S, are not quantized due to the breaking of time-reversal symmetry. The quantization arises as the θ-term changes by \pm 2 πn along a loop around n edge channels. Model calculations show how an interplay of orbital and Zeeman effects leads to quantum Hall transitions, where channels get redistributed along the edges of the crystal. The network of edges opens new possibilities to investigate the coupling of edge channels.

cond-mat.mes-hall↗
arXiv2011

Current induced rotational torques in the skyrmion lattice phase of chiral magnets

In chiral magnets without inversion symmetry, the magnetic structure can form a lattice of magnetic whirl lines, a two-dimensional skyrmion lattice, stabilized by spin-orbit interactions in a small range of temperatures and magnetic fields. The twist of the magnetization within this phase gives rise to an efficient coupling of macroscopic magnetic domains to spin currents. We analyze the resulting spin-transfer effects, and, in particular, focus on the current induced rotation of the magnetic texture by an angle. Such a rotation can arise from macroscopic temperature gradients in the system as has recently been shown experimentally and theoretically. Here we investigate an alternative mechanism, where small distortions of the skyrmion lattice and the transfer of angular momentum to the underlying atomic lattice play the key role. We employ the Landau-Lifshitz-Gilbert equation and adapt the Thiele method to derive an effective equation of motion for the rotational degree of freedom. We discuss the dependence of the rotation angle on the orientation of the applied magnetic field and the distance to the phase transition.

cond-mat.str-el↗
arXiv2011

Majorana Fermions in Strongly Interacting Helical Liquids

Majorana fermions were proposed to occur at edges and interfaces of gapped one-dimensional systems where phases with different topological character meet due to an interplay of spin-orbit coupling, proximity-induced superconductivity and external magnetic fields. Here we investigate the effect of strong particle interactions, and show that the helical liquid offers a mechanism that protects the very existence of Majorana edge states: whereas moderate interactions close the proximity gap which supports the edge states, in helical liquids the gap re-opens due to two-particle processes. However, gapless fermionic excitations occur at spatial proximity to the Majorana states at interfaces and may jeopardize their long term Majorana coherence.

cond-mat.str-el↗