arXiv subjects

G. S. Uhrig

Publications and source records attributed to G. S. Uhrig.

At least 19 recordsLinked to original sources

arXiv2026

Resonant spin mode from retarded relaxation

In ultrafast spin dynamics additional modes in the THz range appear which cannot be captured by conventional Landau-Lifshitz equations. Extending these equations by further time derivatives (inertial Landau-Lifshitz-Gilbert equation) solves the issue only partly. Here we suggest to explain these modes by retarded Lindbladian relaxation. In spite of the simplicity of this phenomenological approach the agreement with the experimental data is promising: physically plausible parameters describe the position and the large width of these resonances. The salient feature of several almost equidistant resonance peaks is in-line with experimental evidence as well.

cond-mat.str-el↗

arXiv2026

Pulsed Magnetophononics in Gapped Quantum Magnets

One route to the control of quantum magnetism at ultrafast timescales is magnetophononics, the modulation of magnetic interactions by coherently driven lattice excitations. Theoretical studies of a gapped quantum magnet subject to continuous, single-frequency driving of one strongly coupled phonon mode find intriguing phenomena including mutually repelling phonon-bitriplon excitations and global renormalization of the spin excitation spectrum. Because experiments are performed with ultrashort pulses that contain a wide range of driving frequencies, we investigate phonon-bitriplon physics under pulsed laser driving. We use the equations of motion to compute the transient response of the driven and dissipative spin-phonon system, which we characterize using the phonon displacement, phonon number, and triplon occupations. In the Fourier transforms of each quantity we discover a low-frequency energetic oscillation between the lattice and spin sectors, which is an intrinsically nonequilibrium collective mode, and demonstrate its origin as a beating between mutually repelling composite excitations. We introduce a phonon-bitriplon approximation that captures all the physics of hybridization, collective mode formation, and difference-frequency excitation, and show that sum-frequency phenomena also leave clear signatures in the repsonse. We model the appearance of such magnetophononic phenomena in the strongly-coupled spin-chain compound CuGeO$_3$, whose overlapping phonon and spin excitation spectra are well characterized, to deduce the criteria for their possible observation in quantum magnetic materials.

cond-mat.str-el↗

arXiv2024

Topological Properties of Single-Particle States Decaying into a Continuum due to Interaction

We investigate how topological Chern numbers can be defined when single-particle states hybridize with continua. We do so exemplarily in a bosonic Haldane model at zero temperature with an additional on-site decay of one boson into two and the conjugate fusion of two bosons into one. Restricting the Hilbert space to two bosons at maximum, the exact self-energy is accessible. We use the bilinear Hamiltonian $H_0$ corrected by the self-energy $Σ$ to compute Chern numbers by two different approaches. The results are gauged against a full many-body calculation in the Hilbert space where possible. We establish numerically and analytically that the effective Hamiltonian $H_\text{eff}=H_0(\vec k) +Σ(ω,\vec k)$ reproduces the correct many-body topology if the considered band does not overlap with the continuum. In case of overlaps, one can extend the definition of the Chern number to the non-Hermitian $H_\text{eff}$ and there is evidence that the Chern number changes at exceptional points. But the bulk-boundary correspondence appears to be no longer valid and edge modes delocalize.

cond-mat.mes-hall↗

arXiv2023

Strong-coupling magnetophononics: Self-blocking, phonon-bitriplons, and spin-band engineering

Magnetophononics, the modulation of magnetic interactions by driving infrared-active lattice excitations, is emerging as a key mechanism for the ultrafast dynamical control of both semiclassical and quantum spin systems by coherent light. We demonstrate that, in a quantum magnet with strong spin-phonon coupling, resonances between the driven phonon and the spin excitation frequencies exhibit an intrinsic self-blocking effect, whereby only a fraction of the available laser power is absorbed by the phonon. Using the quantum master equations governing the nonequilibrium steady states of the coupled spin-lattice system, we show how self-blocking arises from the self-consistent alteration of the resonance frequencies. We link this to the appearance of mutually repelling collective spin-phonon states, which in the regime of strong hybridization become composites of a phonon and two triplons. We then identify the mechanism and optimal phonon frequencies by which to control a global nonequilibrium renormalization of the lattice-driven spin excitation spectrum and demonstrate that this effect should be observable in ultrafast THz experiments on a number of known quantum magnetic materials.

cond-mat.str-el↗

arXiv2022

Nuclear magnetic resonance spectroscopy of nonequilibrium steady states in quantum dots

The optically induced polarization of localized electron spins in an ensemble of quantum dots (QDs) dephases due to the interaction with the surrounding nuclear spins. Despite this dephasing, the spins in the QDs can be controlled to respond coherently by applying periodic laser pulses, leading to a revival of the spin polarization before each pulse. This effect, known as spin mode locking, strongly depends on an emerging selection of certain polarizations of the nuclear spin bath which is driven to a steady state far from equilibrium. We investigate the influence of the nuclear composition in In$_x$Ga$_{1-x}$As QDs on this nonequilibrium behavior and demonstrate that nuclear magnetic resonances (NMR) appear as very sharp minima in the magnetic field dependence of the revival amplitude. This suggests a novel kind of NMR spectroscopy.

cond-mat.mes-hall↗

arXiv2022

Topological magnetic excitations

Topological properties play an increasingly important role in future research and technology. This also applies to the field of topological magnetic excitations which has recently become a very active and broad field. In this Perspective article, we give an insight into the current theoretical and experimental investigations and try an outlook on future lines of research.

cond-mat.mes-hall↗

arXiv2021

Femtosecond phononic coupling to both spins and charges in a room temperature antiferromagnetic semiconductor

Spintronics is postulated on the possibility to employ the magnetic degree of freedom of electrons for computation and couple it to charges. In this view, the combination of the high-frequency of spin manipulations offered by antiferromagnets, with the wide tunability of the electronic properties peculiar of semiconductors provides a promising and intriguing platform. Here we explore this scenario in $α$-MnTe, which is a semiconductor antiferromagnetically ordered at room temperature. Relying on a Raman mechanism and femtosecond laser pulses, we drive degenerate modes of coherent optical phonons, which modulate the chemical bonds involved in the super-exchange interaction. The spectrally-resolved measurements of the transient reflectivity reveal a coherent modulation of the band-gap at the frequency of 5.3 THz. The detection of the rotation of the polarisation, typically associated with magneto-optical effects, shows coherent and incoherent contributions. Modelling how the ionic motion induced by the phonons affects the exchange interaction in the material, we calculate the photoinduced THz spin dynamics: the results predict both a coherent and incoherent response, the latter of which is consistent with the experimental observation. Our work demonstrates that the same phonon modes modulate both the charge and magnetic degree of freedom, suggesting the resonant pumping of phonons as a viable way to link spin and charge dynamics even in nonlinear regimes.

physics.optics↗

arXiv2021

Giant Resonant Self-Blocking in Magnetophononically Driven Quantum Magnets

Magnetophononics, the modulation of magnetic interactions by driven infrared-active lattice excitations, is emerging as a key mechanism for the ultrafast dynamical control of both semiclassical and quantum spin systems by coherent light. We demonstrate that, in a quantum magnetic material with strong spin-phonon coupling, resonances between the driven phonon and the spin excitation frequencies exhibit a giant self-blocking effect. Instead of absorbing more energy, the spin system acts as a strong brake on the driven phonon, causing it to absorb only a tiny fraction of the power available from the laser. Using the quantum master equations governing the nonequilibrium steady states of the coupled spin-lattice system, we show how self-blocking dominates the dynamics, demonstrate the creation of mutually repelling hybrid spin-phonon states, and control the nonequilibrium renormalization of the lattice-driven spin excitation band.

cond-mat.str-el↗

arXiv2021

Resonant spin amplification in Faraday geometry

We demonstrate the realization of the resonant spin amplification (RSA) effect in Faraday geometry where a magnetic field is applied parallel to the optically induced spin polarization so that no RSA is expected. However, model considerations predict that it can be realized for a central spin interacting with a fluctuating spin environment. As a demonstrator, we choose an ensemble of singly-charged (In,Ga)As/GaAs quantum dots, where the resident electron spins interact with the surrounding nuclear spins. The observation of RSA in Faraday geometry requires intense pump pulses with a high repetition rate and can be enhanced by means of the spin-inertia effect. Potentially, it provides the most direct and reliable tool to measure the longitudinal $g$ factor of the charge carriers.

cond-mat.mes-hall↗

arXiv2021

Dynamical properties of a driven dissipative dimerized $S = 1/2$ chain

We consider the dynamical properties of a gapped quantum spin system coupled to the electric field of a laser, which drives the resonant excitation of specific phonon modes that modulate the magnetic interactions. We deduce the quantum master equations governing the time-evolution of both the lattice and spin sectors, by developing a Lindblad formalism with bath operators providing an explicit description of their respective phonon-mediated damping terms. We investigate the nonequilibrium steady states (NESS) of the spin system established by a continuous driving, delineating parameter regimes in driving frequency, damping, and spin-phonon coupling for the establishment of physically meaningful NESS and their related non-trivial properties. Focusing on the regime of generic weak spin-phonon coupling, we characterize the NESS by their frequency and wave-vector content, explore their transient and relaxation behavior, and discuss the energy flow, the system temperature, and the critical role of the type of bath adopted. Our study lays a foundation for the quantitative modelling of experiments currently being designed to control coherent many-body spin states in quantum magnetic materials.

cond-mat.str-el↗

arXiv2020

Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors

Recent findings of new Higgs modes in unconventional superconductors require a classification and characterization of the modes allowed by nontrivial gap symmetry. Here we develop a theory for a tailored nonequilibrium quantum quench to excite all possible oscillation symmetries of a superconducting condensate. We show that both a finite momentum transfer and quench symmetry allow for an identification of the resulting Higgs oscillations. These serve as a fingerprint for the ground state gap symmetry. We provide a classification scheme of these oscillations and the quench symmetry based on group theory for the underlying lattice point group. For characterization, analytic calculations as well as full scale numeric simulations of the transient optical response resulting from an excitation by a realistic laser pulse are performed. Our classification of Higgs oscillations allows us to distinguish between different symmetries of the superconducting condensate.

cond-mat.str-el↗

arXiv2019

Tunable signal velocity in the integer quantum Hall effect of tailored graphene

Topological properties in condensed matter physics are often claimed to be a fruitful resource for technical applications, but so far they only play a minor role in applications. Here we propose to put topological edge states to use in tailored graphene for Fermi velocity engineering. By tuning external control parameters such as gate voltages, the dispersions of the edge states regime are modified in a controllable way. This enables the realizations of devices such as tunable delay lines and interferometers with switchable delays.

cond-mat.mes-hall↗

arXiv2019

Delocalization of edge states in topological phases

The presence of a topologically non-trivial discrete invariants implies the existence of gapless modes in finite samples, but it does not necessarily imply their localization. The disappearance of the indirect energy gap in the bulk generically leads to the absence of localized edge states. We illustrate this behavior in two fundamental lattice models on the single-particle level. By tuning a hopping parameter the indirect gap is closed while maintaining the topological properties. The inverse participation ratio is used to measure the degree of localization.

cond-mat.mes-hall↗

arXiv2019

Topological magnon bands for magnonics

Topological excitations in magnetically ordered systems have attracted much attention lately. We report on topological magnon bands in ferromagnetic Shastry-Sutherland lattices whose edge modes can be put to use in magnonic devices. The synergy of Dzyaloshinskii-Moriya interactions and geometrical frustration is responsible for the topologically non-trivial character. Using exact spin wave theory we determine the finite Chern numbers of the magnon bands which give rise to chiral edge states. The quadratic band crossing point vanishes due the present anisotropies and the system enters a topological phase. We calculate the thermal Hall conductivity as an experimental signature of the topological phase. Different promising compounds are discussed as possible physical realizations of ferromagnetic Shastry-Sutherland lattices hosting the required antisymmetric Dzyaloshinskii-Moriya interactions. Routes to applications in magnonics are pointed out.

cond-mat.str-el↗

arXiv2017

Mutually attracting spin waves in the square-lattice quantum antiferromagnet

The Heisenberg model for S=1/2 describes the interacting spins of electrons localized on lattice sites due to strong repulsion. It is the simplest strong-coupling model in condensed matter physics with wide-spread applications. Its relevance has been boosted further by the discovery of curate high-temperature superconductors. In leading order, their undoped parent compounds realize the Heisenberg model on square-lattices. Much is known about the model, but mostly at small wave vectors, i.e., for long-range processes, where the physics is governed by spin waves (magnons), the Goldstone bosons of the long-range ordered antiferromagnetic phase. Much less, however, is known for short-range processes, i.e., at large wave vectors. Yet these processes are decisive for understanding high-temperature superconductivity. Recent reports suggest that one has to resort to qualitatively different fractional excitations, spinons. By contrast, we present a comprehensive picture in terms of dressed magnons with strong mutual attraction on short length scales. The resulting spectral signatures agree strikingly with experimental data

cond-mat.str-el↗

arXiv2017

Tunable edge states and their robustness towards disorder

The interest in the properties of edge states in Chern insulators and in $\mathbb{Z}_2$ topological insulator has increased rapidly in recent years. We present calculations on how to influence the transport properties of chiral and helical edge states by modifications of the edges in the Haldane and in the Kane-Mele model. The Fermi velocity of the chiral edge states becomes direction-dependent as does the spin-dependent Fermi velocity of the helical edge states. Moreover, it is possible to tune the Fermi velocity by orders of magnitude. Additionally, we explicitly investigate the robustness of edge states against local disorder. The edge states can be reconstructed in the Brillouin zone in presence of disorder. The influence of the width and of the length of the system is studied as well as the dependence on the strength of the disorder.

cond-mat.mes-hall↗

arXiv2016

Time-dependent correlations in quantum magnets at finite temperature

In this article we investigate the time dependence of the gap mode of copper nitrate at various temperatures. We combine state-of-the-art theoretical calculations with high precision neutron resonance spin-echo measurements to understand the anomalous decoherence effects found previously in this material. It is shown that the time domain offers a complementary view on this phenomenon, which allows us to directly compare experimental data and theoretical predictions without the need of further intensive data analysis, such as (de)convolution.

cond-mat.str-el↗

arXiv2016

Magnetic excitations in the S = 1/2 antiferromagnetic-ferromagnetic chain compound BaCu2V2O8 at zero and finite temperature

Unlike most quantum systems which rapidly become incoherent as temperature is raised, strong correlations persist at elevated temperatures in $S=1/2$ dimer magnets, as revealed by the unusual asymmetric lineshape of their excitations at finite temperatures. Here we quantitatively explore and parameterize the strongly correlated magnetic excitations at finite temperatures using the high resolution inelastic neutron scattering on the model compound BaCu$_2$V$_2$O$_8$ which we show to be an alternating antiferromagnetic-ferromagnetic spin$-1/2$ chain. Comparison to state of the art computational techniques shows excellent agreement over a wide temperature range. Our findings hence demonstrate the possibility to quantitatively predict coherent behavior at elevated temperatures in quantum magnets.

cond-mat.str-el↗