arXiv subjects

Wolfgang Belzig

Publications and source records attributed to Wolfgang Belzig.

At least 37 records · Page 2Linked to original sources

arXiv2025

Sextets in four-terminal Josephson junctions

Multiterminal superconducting junctions have revitalized the investigation of the Josephson effect. One of the most interesting aspects of these hybrid systems is the occurrence of multi-Cooper pair tunneling processes that have no analog in two-terminal devices. Such correlated tunneling events are also intimately connected to the Andreev bound states (ABSs) supported by these structures. Josephson junctions with four superconducting terminals have attracted special attention because they are predicted to support ABSs with nontrivial topological properties. Here, we present a theoretical study of sextets, which are correlated tunneling processes involving three Cooper pairs and four different superconducting terminals. We investigate how sextets can be identified from the analysis of the current-phase relation, we show how sextets are connected to the hybridization of ABSs, and we discuss their existence in recent experiments on four-terminal devices realized in hybrid Al/InAs heterostructures.

cond-mat.supr-con↗

arXiv2025

Nonclassical Photon-Assisted Transport in Superconducting Tunnel Junctions

Advances in circuit quantum electrodynamics have enabled the generation of arbitrary nonclassical microwave states and paved the way for addressing novel physics questions. Here, we present a theoretical study of the electrical current in a Josephson tunnel junction interacting with a nonclassical electromagnetic environment. This allows us to generalize classical transport phenomena like photon-assisted tunneling and Shapiro steps to the quantum regime. We predict that the analysis of the supercurrent in such a setup enables the complete reconstruction of quantum states of the electromagnetic environment, something that is not possible with normal tunnel junctions.

cond-mat.mes-hall↗

arXiv2025

Hidden quantum correlations in the ground states of quasiclassical spin systems

Frustrated spin models may lead to the formation of both classical non-collinear spin structures and unique quantum phases including highly entangled quantum spin liquids. Here, we study the entanglement and spatial quantum correlations in linear spin-wave theory around a classical spin-spiral ground state. We find that the entanglement between pairs of sites is short-ranged, and is completely absent in certain cases. In contrast, the entanglement hidden in multi-site clusters is peaked close to phase transitions and shows an asymptotic behavior modulated by the period of the magnetic structure. These findings motivate further exploring the connection in the entanglement properties of fully quantum and of quasiclassical spin models.

cond-mat.str-el↗

arXiv2025

Specular-Andreev reflection and Andreev interference in an Ising superconductor junction

Being resilient to magnetic field, Ising superconductor serves as an exceptional platform for studying the interplay between superconductivity and magnetism. In this Letter, we first explore the transport properties of a two-terminal graphene-Ising superconductor junction where mirage gaps are induced in the superconductor by an exchange field due to magnetic proximity effect. We demonstrate that the chemical potential range of graphene supporting specular-Andreev reflection at the interface is between the two mirage gaps and about twice the Ising spin-orbit coupling strength. This enhances the resilience of observing specular-Andreev reflection against graphene potential fluctuations in experiments. We further study the Andreev interference effect based on a four-terminal junction of which two terminals consist of Ising superconductors in the presence of exchange fields. Due to the finite contribution from the spin-triplet pairing, the interference can be modulated by tuning the relative orientation of the exchange fields in addition to the traditional scheme by changing superconducting phase difference and the chemical potential of the normal region.

cond-mat.mes-hall↗

arXiv2025

Spin pumping into quantum spin nematic states

We theoretically study spin pumping into a spin-nematic state in a junction system composed of a ferromagnetic insulator and a spin-nematic insulator, described by using the spin-1 bilinear-biquadratic model. We analyze an increase of the Gilbert damping in ferromagnetic resonance (FMR) due to an interfacial exchange coupling within a mean-field theory based on the Schwinger boson method. We find that the two Schwinger bosons contribute in distinct ways to spin pumping. We report a detailed dependence of the spin pumping on a resonant frequency, a magnetic field, and an interface type.

cond-mat.mes-hall↗

arXiv2025

Multiterminal Josephson junctions with tunable topological properties

Since the discovery of the Andreev reflection process at normal-metal/superconductor junctions and the corresponding Andreev bound states in superconductor/normal-metal/superconductor junctions, various multiterminal Josephson junctions have been studied to explore many exotic phases of quantum matter, where the formation of Andreev bound states in the normal region account for dissipationless supercurrent and play a central role in determining exotic properties. Recently, an intriguing aspect of the multiterminal Josephson junctions has been proposed to study the topological properties, wherein the Andreev bound states acquire topological characteristics upon tuning the phase differences of superconducting terminals. In this work, we investigate topologically non-trivial phases in four-terminal Josephson junctions based on square and graphene lattices. Additionally, we apply a gating potential that smoothly drives the Andreev bound states from a topologically non-trivial state to a trivial state. Furthermore, we observe that the gating potential in our setup produces the similar physics of the topological Andreev bound states of the double (single) quantum-dot multiterminal Josephson junctions when the gating potential is small (large) compared to the superconducting gap.

cond-mat.supr-con↗

arXiv2025

Zero-temperature phase-flip rate in a biased parametric oscillator

A parametrically driven oscillator has two stable vibrational states at half the modulation frequency. The states have opposite phase and equal amplitudes. An extra drive at half the modulation frequency provides an effective bias that lifts the state symmetry. Quantum fluctuations lead to switching between the states, i.e., to phase-flip transitions. We develop a semiclassical approach that allows us to find the dependence of the switching rates on the amplitude of the bias and the parameters of the modulating field. We find that the rate of switching from a ''shallow'' state can become anomalously small at certain parameter values, leading to an efficient localization in this state. This is a consequence of the change of the topology of the oscillator phase trajectories. The results pave the way for implementing nonreciprocal quantum Ising systems based on parametric oscillators.

quant-ph↗

arXiv2025

Exploring the energy spectrum of a four-terminal Josephson junction: Towards topological Andreev band structures

Hybrid multiterminal Josephson junctions (JJs) are expected to harbor a novel class of Andreev bound states (ABSs), including topologically nontrivial states in four-terminal devices. In these systems, topological phases emerge when ABSs depend on at least three superconducting phase differences, resulting in a three-dimensional (3D) energy spectrum characterized by Weyl nodes at zero energy. Here, we realize a four-terminal JJ in a hybrid Al/InAs heterostructure, where ABSs form a synthetic 3D band structure. We probe the energy spectrum using tunneling spectroscopy and identify spectral features associated with the formation of a tri-Andreev molecule, a bound state whose energy depends on three superconducting phases and, therefore, is able to host topological ABSs. The experimental observations are well described by a numerical model. The calculations predict the appearance of four Weyl nodes at zero energy within a gap smaller than the experimental resolution. These topological states are theoretically predicted to remain stable within an extended region of the parameter space, well accessible by our device. These findings establish an experimental foundation to study high-dimensional synthetic band structures in multiterminal JJs, and to realize topological Andreev bands.

cond-mat.mes-hall↗

arXiv2024

Theory of three-terminal Andreev spin qubits

In this paper, we introduce a concise theoretical framework for the equilibrium three-terminal Josephson effect in spin-orbit-interacting systems, inspired by recent experiments on an InAs/Al heterostructure [Phys. Rev. X 14, 031024 (2024)]. We develop an analytical model to capture the essential low-energy physics of the system and examine its potential as an Andreev spin qubit, while also reconciling some findings of Ref. [Phys. Rev. B 90, 155450 (2014)]. Our analysis of the transitions between the Andreev levels in the junction shows that, in an idealized scenario, the transition between the lowest pair of pseudo-spin-split Andreev levels is blocked by pseudo-spin conservation. We demonstrate that to operate the system as an Andreev spin qubit, leveraging the significant spin splitting observed experimentally, additional ingredients such as external magnetic filed or magnetic impurities are required. Finally, we apply our model to investigate the coupling between two such qubits, mediated by supercurrent.

cond-mat.mes-hall↗

arXiv2024

The Andreev-Ising-Josephson Diode

The transition-metal dichalcogenides featuring Ising spin-orbit coupling in so-called Ising superconductors offer a unique system to study the interplay of singlet and triplet superconductivity. The presence of high critical fields, spectral properties such as the mirage gap, and field-tunable charge and spin currents in Ising-superconductor Josephson junctions are some of the important features. In this work, we study Ising-superconductor Josephson junction with a transparent interface and show that Andreev bound states are spin-split due to a relative misorientation of in-plane fields in the superconducting contacts. Correspondingly, supercurrent-phase relations display a strongly non-sinusoidal behavior. Introducing additional spin-polarized channels with low transmission results in a nonreciprocal current-phase relation with a diode effect that can be tuned by the in-plane exchange fields. The diode efficiency reaches high values of the order of 40% and is not sensitive to disorder in the junction. Such structures can be realized in van-der-Waals heterostructures of two dimensional superconductors and magnets.

cond-mat.mes-hall↗

arXiv2024

Dynamical renormalization of the magnetic excitation spectrum via high-momentum nonlinear magnonics

Controlling macroscopic properties of quantum materials requires the ability to induce and manipulate excited states. The set of collective excitations of a solid is encoded in its dispersion relations. We find that the spectra of the low-momentum eigenmodes are renormalized by resonantly driving the high-momentum excitations. Our experimental data rule out laser-induced thermal processes as an origin of the renormalization. The photoinduced changes of the amplitudes and frequencies can be explained theoretically and numerically with a resonant light-scattering mechanism that couples high- and low-momentum eigenmodes in the dispersion relation across momentum space. While we demonstrate the renormalization of the magnetic spectrum in a quantum material, our experimental approach can be further generalized to lattice modes in semiconductors.

cond-mat.str-el↗

arXiv2024

Gate control of superconducting current: Mechanisms, parameters and technological potential

In conventional metal-oxide semiconductor (CMOS) electronics, the logic state of a device is set by a gate voltage (VG). The superconducting equivalent of such effect had remained unknown until it was recently shown that a VG can tune the superconducting current (supercurrent) flowing through a nanoconstriction in a superconductor. This gate-controlled supercurrent (GCS) effect can lead to superconducting logics like CMOS logics, but with lower energy dissipation. The physical mechanism underlying the GCS effect, however, remains under debate. In this review article, we illustrate the main mechanisms proposed for the GCS effect, and the material and device parameters that mostly affect it based on the evidence reported. We will come to the conclusion that different mechanisms are at play in the different studies reported so far. We then outline studies that can help answer open questions on the effect and achieve control over it, which is key for applications. We finally give insights into the impact that the GCS effect can have towards high-performance computing with low-energy dissipation and quantum technologies.

cond-mat.supr-con↗

arXiv2024

Fractional positional jumps in stochastic systems with tilted periodic double-well potentials

We present a theoretical investigation of the stochastic dynamics of a damped particle in a tilted periodic potential with a double well per period. By applying the matrix continued fraction technique to the Fokker-Planck equation in conjunction with the full counting statistics and master equation approaches, we determine the rates of specific processes contributing to the system's overall dynamics. At low temperatures, the system can exhibit one running state and two distinct locked metastable states. We focus primarily on two aspects: the dynamics of positional jumps, which are rare thermally induced particle jumps over potential maxima, and their impact on the overall velocity noise; and the retrapping process, involving the transition from the running to the locked metastable states. We demonstrate the existence of fractional (in units of $2π$) positional slips that differ qualitatively from conventional $2π$ jumps observed in single-well systems. Fractional positional slips significantly influence the system dynamics even in regimes dominated by dichotomous-like switching between running and locked states. Furthermore, we introduce a simple master equation approach that proves effective in analyzing various stages of the retrapping process. Interestingly, our analysis shows that even for a system featuring a well-developed double-well periodic potential, there exists a broad parameter range where the stochastic dynamics can be accurately described by an effective single-well periodic model. The techniques introduced here allow for valuable insights into the complex behavior of the system, offering avenues for understanding and controlling its steady-state and transient dynamics, which go beyond or can be complementary to direct stochastic simulations.

cond-mat.stat-mech↗

arXiv2024

Quantum Magnetic Skyrmion Operator

We propose a variational wave function to represent quantum skyrmions as bosonic operators. The operator faithfully reproduces two fundamental features of quantum skyrmions: their classical magnetic order and a "quantum cloud" of local spin-flip excitations. Using exact numerical simulations of the ground states of a 2D chiral magnetic model, we find two regions in the single-skyrmion state diagram distinguished by their leading quantum corrections. We use matrix product state simulations of the adiabatic braiding of two skyrmions to verify that the operator representation of skyrmions is valid at large inter-skyrmion distances. Our work demonstrates that skyrmions can be approximately coarse-grained and represented by bosonic quasiparticles, which paves the way toward a field theory of many-skyrmion quantum phases and, unlike other approaches, incorporates the microscopic quantum fluctuations of individual skyrmions.

cond-mat.str-el↗

arXiv2024

Signature of long-ranged spin triplets across a two-dimensional superconductor/helimagnet van der Waals interface

The combination of a superconductor with a magnetically inhomogeneous material has been established as an efficient mechanism for the generation of long-ranged spin-polarized (spin-triplet) Cooper pairs. Evidence for this mechanism, however, has been established based on studies done on three-dimensional systems, where the strong bonds existing at the interface between the superconductor and the magnetic material should in principle enhance proximity effects and strengthen any electronic correlations. Here, we fabricate devices based on van der Waals stacks of flakes of the two-dimensional superconductor $NbS_2$ combined with flakes of $Cr_{1/3}NbS_2$, which has a built-in magnetic inhomogeneity due to its helimagnetic spin texture at low temperatures. We find that the critical temperature of these vdW bilayers is strongly dependent on the magnetic state of $Cr_{1/3}NbS_2$, whose degree of magnetic inhomogeneity can be controlled via an applied magnetic field. Our results demonstrate evidence for the generation of long-ranged spin-triplet pairs across the $Cr_{1/3}NbS_2$/$NbS_2$ vdW interface.

cond-mat.supr-con↗

arXiv2024

Influence of disorder at Insulator-Metal interface on spin transport

Motivated by experimental work showing enhancement of spin transport between Yttrium Iron Garnet and Platinum by a thin antiferromagnetic insulator between them, we consider spin transport through the interface of a non-magnetic metal and compensated antiferromagnetically ordered insulator and focus on the significance of the interface itself. The spin transport is carried by spin-polarized electrons in the metal and by magnons in the insulator. We compute the spin current in the presence of a spin accumulation in the metal, cause by the spin Hall effect, and a thermal gradient using Fermi's Golden Rule in the presence of interfacial disorder. For a perfectly clean interface, the in-plane momentum is conserved by the electron-magnon scattering events that govern the spin transport through the interface. We calculate how disorder-induced broadening of scattering matrix elements with respect to the in-plane momentum influences the spin current. As a general result, we observe that for many experimental setups, specifically for high temperatures, one should expect a rather small effect of interface disorder on the measured spin current, while for small temperatures there is a significant reduction of a spin current with increasing disorder.

cond-mat.mes-hall↗

arXiv2024

Dipole-dipole-interaction-induced entanglement between two-dimensional ferromagnets

We investigate the viability of dipole-dipole interaction as a means of entangling two distant ferromagnets. To this end we make use of the Bogoliubov transformation as a symplectic transformation. We show that the coupling of the uniform magnon modes can be expressed using four squeezing parameters which we interpret in terms of hybridization, one-mode and two-mode squeezing. We utilize the expansion in terms of the squeezing parameters to obtain an analytic formula for the entanglement in the magnon ground state using the logarithmic negativity as entanglement measure. Our investigation predicts that for infinitely large two-dimensional ferromagnets, the dipole-dipole interaction does not lead to significant long-range entanglement. However, in the case of finite ferromagnets, finite entanglement can be expected

quant-ph↗

arXiv2024

Thermodynamic properties of a superconductor interfaced with an altermagnet

Recently introduced magnetic materials called altermagnets (AM) feature zero net magnetization but a momentum dependent magnetic exchange field, which can have intriguing implications when combined with superconductivity. In our work, we use the quasiclassical framework to study the effects of such a material on a conventional superconductor (S) in an AM/S bilayer. We discuss the superconducting phase diagram and heat capacity of AM/S while making a comparison with a ferromagnet-superconductor bilayer. Furthermore, we examine the density of states and analyze the system's response to an external magnetic field. We illustrate the anisotropy of spin-susceptibility and magnetization of AM/S by considering an external field in the in-plane and out-of-plane direction, thereby facilitating the scope of experimental detection and characterization of an AM in an AM/S hybrid system.

cond-mat.supr-con↗