arXiv subjects

Ulrich Zülicke

Publications and source records attributed to Ulrich Zülicke.

18 recordsLinked to original sources

arXiv2025

Multipole order in two-dimensional altermagnets

We theoretically investigate the magnetic-multipole orders in two-dimensional (2D) altermagnets, focusing on two representative models: a generic minimal three-site model, and a four-site model representative of monolayer FeSe. We construct low-energy effective Hamiltonians for both systems and calculate their respective multipole indicators to characterize the underlying magnetic order. Our analysis reveals an intriguing contrast between the two systems. We find that the generic minimal model exhibits the expected non-zero magnetic-octupole order. In the monolayer-FeSe model, however, the magnetic-octupole order vanishes globally, and a magnetic-hexadecapole order is present instead. The emergence of altermagnetic splitting in the band structure then arises via the interplay with a sublattice-isospin degree of freedom. Our work demonstrates how the classification and comprehensive understanding of 2D altermagnetic materials transcends bulk descriptions.

cond-mat.mes-hall↗

arXiv2025

Anisotropic light-tailored RKKY interaction in two-dimensional $d$-wave altermagnets

Altermagnets are known in spintronics for their intrinsic spin-splitting and unconventional magnetic responses, particularly to magnetic impurities. However, effectively controlling the magnetic exchange interactions in altermagnets is challenging for practical applications. Here, we propose using circularly polarized light to tune the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in two-dimensional $d$-wave altermagnets. Using the real-space retarded Green's functions approach, our results show that while the Heisenberg and Ising exchanges dominate, a notable Dzyaloshinskii-Moriya (DM) interaction also plays a key role. Furthermore, the inherent strength of altermagnetism imprints chirp-like signatures into the magnetic responses, which can be dynamically tuned via light. We mainly demonstrate that gate-induced Rashba spin-orbit coupling is essential in response to light -- light selectively and anisotropically adjusts the DM interaction without affecting the other exchanges. Our findings further indicate that rotating the altermagnet by $45^\circ$ relative to the light's polarization direction generates a Dirac-like dispersion and different DM interactions. We finally extract critical thresholds where light reverses DM interactions along one axis or balances both in-plane components. The anisotropic light-driven control of RKKY interactions in 2D altermagnets not only highlights their unique properties but also opens new avenues for engineering tailored magnetic characteristics in spintronic applications.

cond-mat.mes-hall↗

arXiv2025

Popularity and Innovation in Maven Central

Maven Central is a large popular repository of Java components that has evolved over the last 20 years. The distribution of dependencies indicates that the repository is dominated by a relatively small number of components other components depend on. The question is whether those elites are static, or change over time, and how this relates to innovation in the Maven ecosystem. We study those questions using several metrics. We find that elites are dynamic, and that the rate of innovation is slowing as the repository ages but remains healthy.

cs.SE↗

arXiv2023

Andreev bound states at boundaries of polarized 2D Fermi superfluids with s-wave pairing and spin-orbit coupling

A topological superfluid phase characterized by an emergent chiral-p-wave pair potential is expected to form in a two-dimensional Fermi superfluid subject to s-wave pairing, spin-orbit coupling and a large-enough Zeeman splitting. Andreev bound states appear at phase boundaries, including Majorana zero modes whose existence is assured by the bulk-boundary correspondence principle. Here we study the physical properties of these subgap-energy bound states at step-like interfaces using the spin-resolved Bogoliubov-deGennes mean-field formalism and assuming small spin-orbit coupling. Extending a recently developed spin-projection technique based on Feshbach partitioning [SciPost Phys. 5, 016 (2018)] combined with the Andreev approximation allows us to obtain remarkably simple analytical expressions for the bound-state energies as well as the majority and minority spin components of their wave functions. Besides the vacuum boundary, where a majority-spin Majorana excitation is encountered, we also consider the boundary between the topological and a nontopological superfluid phase that can appear in a coexistence scenario due to the first-order topological phase transition predicted for this system. At this superfluid-superfluid interface, we find a localized chiral Majorana mode hosted by the minority-spin sector. Our theory further predicts majority-spin subgap-energy bound states similar to those found at a Josephson junction between same-chirality p-wave superfluids. Their presence affects the Majorana mode due to a coupling of minority and majority spin sectors only in the small energy range where their spectra overlap. Our results may inform experimental efforts aimed at realizing and characterizing unconventional Majorana quasiparticles.

cond-mat.quant-gas↗

arXiv2022

Magnetoelectricity in two-dimensional materials

Since the initial isolation of few-layer graphene, a plethora of two-dimensional atomic crystals has become available, covering almost all known materials types including metals, semiconductors, superconductors, ferro- and antiferromagnets. These advances have augmented the already existing variety of two-dimensional materials that are routinely realized by quantum confinement in bulk-semiconductor heterostructures. This review focuses on the type of material for which two-dimensional realizations are still being actively sought: magnetoelectrics. We present an overview of current theoretical expectation and experimental progress towards fabricating low-dimensional versions of such materials that can be magnetized by electric charges and polarized electrically by an applied magnetic field - unusual electromagnetic properties that could be the basis for various useful applications. The interplay between spatial confinement and magnetoelectricity is illustrated using the paradigmatic example of magnetic-monopole fields generated by electric charges in or near magnetoelectric media. For the purpose of this discussion, the image-charge method familiar from electrostatics is extended to solve the boundary-value problem for a magnetoelectric medium in the finite-width slab geometry using image dyons, i.e., point objects having both electric and magnetic charges. We discuss salient features of the magnetoelectrically induced fields arising in the thin-width limit.

cond-mat.mes-hall↗

arXiv2022

Triplet character of 2D-fermion dimers arising from $s$-wave attraction via spin-orbit coupling and Zeeman splitting

We theoretically study spin-$1/2$ fermions confined to two spatial dimensions and experiencing isotropic short-range attraction in the presence of both spin-orbit coupling and Zeeman spin splitting - a prototypical system for developing topological superfluidity in the many-body sector. Exact solutions for two-particle bound states are found to have a triplet contribution that dominates over the singlet part in an extended region of parameter space where the combined Zeeman- and center-of-mass-motion-induced spin-splitting energy is large. The triplet character of dimers is purest in the regime of weak $s$-wave interaction strength. Center-of-mass momentum is one of the parameters determining the existence of bound states, which we map out for both two- and one-dimensional types of spin-orbit coupling. Distinctive features emerging in the orbital part of the bound-state wave function, including but not limited to its $p$-wave character, provide observable signatures of unconventional pairing.

cond-mat.quant-gas↗

arXiv2021

Reliable modeling of weak antilocalization for accurate spin-lifetime extraction

We examine models for the magneto-conductivity correction in 2DEGs with both Rashba and Dresselhaus spin-orbit coupling (SOC) for their applicability to experimental data fitting. In particular, we compare the Landau-quantized Cooperon approach, which is mostly only numerically treatable, and the quasi-classical approximation that was recently employed to obtain an explicit solution for arbitrary Rashba and Dresselhaus SOC [PRL 112, 156601 (2019)]. It is found that the quasi-classical approximation yields significantly different results even to lowest order in the magnetic field and appears unsuitable for reliable parameter fitting. The discrepancy emerges when a sum over Landau levels is replaced by an integral over wave vectors. Substantial improvement is achieved by supplementing the quasi-classical model with the first two corrections given by the Euler-MacLaurin formula. Corresponding modifications are, however, only feasible in special SOC parameter configurations where the mixing of Landau bands is negligible and a closed-form solution that accounts for Landau quantization is also available. Such a scenario appears in a parameter regime where a persistent spin helix emerges and a transition between weak anti- and weak localization takes place. Combining recent findings, we derive a generalized closed-form expression for the magneto-conductivity correction applicable to generic 2DEGs that are grown along a crystal direction with at least two growth-direction Miller indices equal in modulus. The result is a function of spin lifetimes of the long-lived spin textures and is valid close to the persistent-spin-helix regime. The accuracy of the derived formula is validated by comparing with results from numerical diagonalization of the multiband Cooperon as well as a recently established Monte-Carlo-based real-space simulation in exemplary (001)-, (113)-, and (110)-2DEGs.

cond-mat.mes-hall↗

arXiv2020

Signatures of the Higgs mode in transport through a normal-metal--superconductor junction

A superconductor subject to electromagnetic irradiation in the terahertz range can show amplitude oscillations of its order parameter. However, coupling this so-called Higgs mode to the charge current is notoriously difficult. We propose to achieve such a coupling in a particle-hole-asymmetric configuration using a DC-voltage-biased normal-metal--superconductor tunnel junction. Using the quasiclassical Green's function formalism, we demonstrate three characteristic signatures of the Higgs mode: (i) The AC charge current exhibits a pronounced resonant behavior and is maximal when the radiation frequency coincides with the order parameter. (ii) The AC charge current amplitude exhibits a characteristic nonmonotonic behavior with increasing voltage bias. (iii) At resonance for large voltage bias, the AC current vanishes inversely proportional to the bias. These signatures provide an electric detection scheme for the Higgs mode.

cond-mat.mes-hall↗

arXiv2020

Enhanced longevity of the spin helix in low-symmetry quantum wells

In a semiconductor, collective excitations of spin textures usually decay rather fast due to D'yakonov-Perel' spin relaxation. The latter arises from spin-orbit coupling, which induces wave-vector-dependent spin rotations that, in conjunction with random disorder scattering, generate spin decoherence. However, symmetries occurring under certain conditions can prevent the relaxation of particular homogeneous and inhomogeneous spin textures. The inhomogeneous spin texture, termed as persistent spin helix, is especially appealing as it enables us to manipulate the spin orientation while retaining a long spin lifetime. Recently, it was predicted that such symmetries can be realized in zinc-blende two-dimensional electron gases if at least two growth-direction Miller indices agree in modulus and the coefficients of the Rashba and linear Dresselhaus spin-orbit couplings are suitably matched [PRL 117, 236801 (2016)]. In the present paper, we systematically analyze the impact of the symmetry-breaking cubic Dresselhaus spin-orbit coupling, which generically coexists in these systems, on the stability of the emerging spin helices with respect to the growth direction. We find that, as an interplay between orientation and strength of the effective magnetic field induced by the cubic Dresselhaus terms, the spin relaxation is weakest for a low-symmetry growth direction that can be well approximated by a [225] lattice vector. These quantum wells yield a 30\% spin-helix lifetime enhancement compared to [001]-oriented electron gases and, remarkably, require a negligible Rashba coefficient. The rotation axis of the corresponding spin helix is only slightly tilted out of the quantum-well plane. This makes the experimental study of the spin-helix dynamics readily accessible for conventional optical spin orientation measurements where spins are excited and detected along the quantum-well growth direction.

cond-mat.mes-hall↗

arXiv2020

Finite-size effects in cylindrical topological insulators

We present a theoretical study of a nanowire made of a three-dimensional topological insulator. The bulk topological insulator is described by a continuum-model Hamiltonian, and the cylindrical-nanowire geometry is modelled by a hard-wall boundary condition. We provide the secular equation for the eigenergies of the systems (both for bulk and surface states) and the analytical form of the energy eigenfunctions. We describe how the surface states of the cylinder are modified by finite-size effects. In particular, we provide a $1/R$ expansion for the energy of the surface states up to second order. The knowledge of the analytical form for the wavefunctions enables the computation of matrix elements of any single-particle operators. In particular, we compute the matrix elements of the optical dipole operator, which describe optical absorption and emission, treating intra- and inter-band transition on the same footing. Selection rules for optical transitions require conservation of linear momentum parallel to the nanowire axis, and a change of $0$ or $\pm 1$ in the total-angular-momentum projection parallel to the nanowire axis. The magnitude of the optical-transition matrix elements is strongly affected by the finite radius of the nanowire.

cond-mat.mes-hall↗

arXiv2019

In-plane magnetoelectric response in bilayer graphene

A graphene bilayer shows an unusual magnetoelectric response whose magnitude is controlled by the valley-isospin density, making it possible to link magnetoelectric behavior to valleytronics. Complementary to previous studies, we consider the effect of static homogeneous electric and magnetic fields that are oriented parallel to the bilayer's plane. Starting from a tight-binding description and using quasi-degenerate perturbation theory, the low-energy Hamiltonian is derived including all relevant magnetoelectric terms whose prefactors are expressed in terms of tight-binding parameters. We confirm the existence of an expected axion-type pseudoscalar term, which turns out to have the same sign and about twice the magnitude of the previously obtained out-of-plane counterpart. Additionally, small anisotropic corrections to the magnetoelectric tensor are found that are fundamentally related to the skew interlayer hopping parameter $γ_4$. We discuss possible ways to identify magnetoelectric effects by distinctive features in the optical conductivity.

cond-mat.mes-hall↗

arXiv2015

Motion of an Impurity in a Bose-Einstein Condensate with Weyl Spin-Orbit Coupling: Non-collinear Drag Force and Anisotropic Critical Velocity

We consider the motion of a point-like impurity through a three-dimensional two-component Bose-Einstein condensate subject to Weyl spin-orbit coupling. Using linear-response theory, we calculate the drag force felt by the impurity and the associated anisotropic critical velocity from the spectrum of elementary excitations. The drag force is shown to be generally not collinear with the velocity of the impurity. This unusual behavior is a consequence of condensation into a finite-momentum state due to the spin-orbit coupling.

cond-mat.quant-gas↗

arXiv2009

Sign of tunnel coupling in barrier-separated Bose-Einstein condensates and stability of double-ring systems

We revisit recent claims about the instability of non-rotating tunnel coupled annular Bose-Einstein condensates leading to the emergence of angular-momentum Josephson oscillation [Phys. Rev. Lett. 98, 050401 (2007), arXiv:quant-ph/0609133v2]. It was predicted that all stationary states with uniform density become unstable in certain parameter regimes. By careful analysis, we arrive at a different conclusion. We show that there is a stable non-rotating and uniform ground state for any value of the tunnel coupling and repulsive interactions. The instability of an excited state with $π$ phase difference between the condensates can be interpreted in terms of the familiar snake instability. We further discuss the sign of the tunnel coupling through a separating barrier, which carries significance for the nature of the stationary states. It is found to always be negative for physical reasons.

quant-ph↗

arXiv2006

Rashba billiards

We study the energy levels of non-interacting electrons confined to move in two-dimensional billiard regions and having a spin-dependent dynamics due to a finite Rashba spin splitting. The Green's function for such Rashba billiards is constructed analytically and used to find the area and perimeter contributions to the density of states, as well as the smooth counting function. We show that, in contrast to systems with spin-rotational invariance, Rashba billiards always possess a negative energy spectrum. A semi-classical analysis is presented to interpret the singular behavior of the density of states at certain negative energies. Our detailed analysis of the spin structure of Rashba billiards reveals a finite out-of-plane spin projection for electron eigenstates.

cond-mat.mes-hall↗

arXiv2004

Rashba spin precession in quantum Hall edge channels

Quasi--one dimensional edge channels are formed at the boundary of a two-dimensional electron system subject to a strong perpendicular magnetic field. We consider the effect of Rashba spin--orbit coupling, induced by structural inversion asymmetry, on their electronic and transport properties. Both our analytical and numerical results show that spin--split quantum--Hall edge channels exhibit properties analogous to that of Rashba--split quantum wires. Suppressed backscattering and a long spin life time render these edge channels an ideal system for observing voltage--controlled spin precession. Based on the latter, we propose a magnet--less spin--dependent electron interferometer.

cond-mat.mes-hall↗

arXiv2004

Electronic and spin properties of Rashba billiards

Ballistic electrons confined to a billiard and subject to spin--orbit coupling of the Rashba type are investigated, using both approximate semiclassical and exact quantum--mechanical methods. We focus on the low--energy part of the spectrum that has negative eigenvalues. When the spin precession length is smaller than the radius of the billiard, the low--lying energy eigenvalues turn out to be well described semiclassically. Corresponding eigenspinors are found to have a finite spin polarization in the direction perpendicular to the billiard plane.

cond-mat.mes-hall↗

arXiv2004

Universal Rashba Spin Precession of Two-Dimensional Electrons and Holes

We study spin precession due to Rashba spin splitting of electrons and holes in semiconductor quantum wells. Based on a simple analytical expression that we derive for the current modulation in a broad class of experimental situations of ferromagnet/nonmagnetic semiconductor/ferromagnet hybrid structures, we conclude that the Datta-Das spin transistor (i) is feasible with holes and (ii) its functionality is not affected by integration over injection angles. The current modulation shows a universal oscillation period, irrespective of the different forms of the Rashba Hamiltonian for electrons and holes. The analytic formulas approximate extremely well exact numerical calculations of a more elaborate Kohn--Luttinger model.

cond-mat.mes-hall↗

arXiv2004

Two-dimensional hole precession in an all-semiconductor spin field effect transistor

We present a theoretical study of a spin field-effect transistor realized in a quantum well formed in a p--doped ferromagnetic-semiconductor- nonmagnetic-semiconductor-ferromagnetic-semiconductor hybrid structure. Based on an envelope-function approach for the hole bands in the various regions of the transistor, we derive the complete theory of coherent transport through the device, which includes both heavy- and light-hole subbands, proper modeling of the mode matching at interfaces, integration over injection angles, Rashba spin precession, interference effects due to multiple reflections, and gate-voltage dependences. Numerical results for the device current as a function of externally tunable parameters are in excellent agreement with approximate analytical formulae.

cond-mat.mes-hall↗