Belzig, Wolfgang

Lade...
Profilbild
E-Mail-Adresse
Geburtsdatum
Forschungsvorhaben
Organisationseinheiten
Berufsbeschreibung
Nachname
Belzig
Vorname
Wolfgang
Name
Weiterer Name

Suchergebnisse Publikationen

Gerade angezeigt 1 - 10 von 277
Vorschaubild nicht verfügbar
Veröffentlichung

Interplay of Andreev Reflection and Coulomb Blockade in Hybrid Superconducting Single-Electron Transistors

2024-02-02, Sobral Rey, Laura, Ohnmacht, David, Winkelmann, Clemens B., Siewert, Jens, Belzig, Wolfgang, Scheer, Elke

We study the interplay between Coulomb blockade and superconductivity in a tunable superconductor–superconductor–normal-metal single-electron transistor. The device is realized by connecting the superconducting island via an oxide barrier to the normal-metal lead and with a break junction to the superconducting lead. The latter enables Cooper pair transport and (multiple) Andreev reflection. We show that these processes are relevant also far above the superconducting gap and that signatures of Coulomb blockade may reoccur at high bias while they are absent for small bias in the strong-coupling regime. Our experimental findings agree with simulations using a rate equation approach in combination with the full counting statistics of multiple Andreev reflection.

Vorschaubild nicht verfügbar
Veröffentlichung

Quartet Tomography in Multiterminal Josephson Junctions

2023-11-30, Ohnmacht, David, Coraiola, Marco, Garcia Esteban, Juan José, Sabonis, Deividas, Nichele, Fabrizio, Belzig, Wolfgang, Cuevas, Juan Carlos

We investigate the detection of quartets in hybrid multiterminal Josephson junctions. Using simple models of quantum dots coupled to superconducting leads, we find that quartets are ubiquitous and show how to rigorously extract their contribution to the current-phase relation (CPR). We also demonstrate that quartets are closely related to the hybridization of Andreev bound states (ABSs) in these systems and propose a method to identify quartets directly in ABS spectra. We illustrate our method by analyzing the spectroscopic measurements of the ABS spectrum in a three-terminal Josephson junction realized in an InAs/Al heterostructure. Our analysis strongly suggests the existence of quartets in the studied hybrid system.

Lade...
Vorschaubild
Veröffentlichung

Magnon squeezing in conical spin spirals

2023-11-06, Wuhrer, Dennis, Rózsa, Levente, Nowak, Ulrich, Belzig, Wolfgang

We investigate squeezing of magnons in a conical spin spiral configuration. We find that while the energy of magnons propagating along the k and the −k directions can be different due to the non-reciprocal dispersion, these two modes are connected by the squeezing, hence can be described by the same squeezing parameter. The squeezing parameter diverges at the center of the Brillouin zone due to the translational Goldstone mode of the system, but the squeezing also vanishes for certain wave vectors. We discuss possible ways of detecting the squeezing.

Lade...
Vorschaubild
Veröffentlichung

Phase-engineering the Andreev band structure of a three-terminal Josephson junction

2023-10-25, Coraiola, Marco, Haxell, Daniel Z., Sabonis, Deividas, Weisbrich, Hannes, Svetogorov, Aleksandr, Hinderling, Manuel, ten Kate, Sofieke C., Cheah, Erik, Krizek, Filip, Schott, Rüdiger, Wegscheider, Werner, Cuevas, Juan Carlos, Belzig, Wolfgang, Nichele, Fabrizio

In hybrid Josephson junctions with three or more superconducting terminals coupled to a semiconducting region, Andreev bound states may form unconventional energy band structures, or Andreev matter, which are engineered by controlling superconducting phase differences. Here we report tunnelling spectroscopy measurements of three-terminal Josephson junctions realised in an InAs/Al heterostructure. The three terminals are connected to form two loops, enabling independent control over two phase differences and access to a synthetic Andreev band structure in the two-dimensional phase space. Our results demonstrate a phase-controlled Andreev molecule, originating from two discrete Andreev levels that spatially overlap and hybridise. Signatures of hybridisation are observed in the form of avoided crossings in the spectrum and band structure anisotropies in the phase space, all explained by a numerical model. Future extensions of this work could focus on addressing spin-resolved energy levels, ground state fermion parity transitions and Weyl bands in multiterminal geometries.

Vorschaubild nicht verfügbar
Veröffentlichung

On-demand population of Andreev levels by their ionization in the presence of Coulomb blockade

2023-12-12, Kurilovich, Pavel D., Kurilovich, Vladislav D., Svetogorov, Aleksandr, Belzig, Wolfgang, Devoret, Michel H., Glazman, Leonid I.

A mechanism to deterministically prepare a nanowire Josephson junction in an odd parity state is proposed. The mechanism involves population of two Andreev levels by a resonant microwave drive breaking a Cooper pair, and a subsequent ionization of one of the levels by the same drive. Robust preparation of the odd state is allowed by a residual Coulomb repulsion in the junction. A similar resonant process can also be used to prepare the junction in the even state. Our theory explains a recent experiment [J. J. Wesdorp, et al., Phys. Rev. Lett. 131, 117001 (2023)].

Vorschaubild nicht verfügbar
Veröffentlichung

Andreev and normal reflections in gapped bilayer graphene–superconductor junctions

2023-11-14, Ram, Panch, Beckmann, Detlef, Danneau, Romain, Belzig, Wolfgang

We study the Andreev and normal reflection processes—retro as well as specular—in a bilayer graphene–superconductor junction where equal and opposite displacement fields are applied for the top and bottom layers to induce a band gap. By employing the Dirac-Bogoliubov–de Gennes equation for the gapped bilayer graphene–superconductor junction, we calculate the reflection probabilities within the scattering theory approach. The subgap conductance, calculated in the framework of Blonder-Tinkham-Klapwijk formalism, shows the contribution from the Andreev retro reflection (specular reflection) when the applied bias voltage is below (above) the Fermi energy. Notably, both retro and specular reflections are modified in the presence of the displacement field, and the retro-to-specular crossover gets amplified when the displacement field is relatively small. They can be further tuned to either specular or retro Andreev reflection by adjusting the Fermi energy. Furthermore, our study reveals the simultaneous existence of double Andreev reflections and double normal reflections when the displacement field becomes comparable to the interlayer coupling strength. The existence of the normal retro-reflection process in a bilayer graphene–superconductor junction is a finding which shows a distinctive feature in the conductance that can be experimentally verified.

Vorschaubild nicht verfügbar
Veröffentlichung

Dynamical renormalisation of a spin Hamiltonian via high-order nonlinear magnonics

2023-10-30, Schönfeld, Christoph, Feuerer, Lennart, Wuhrer, Dennis, Belzig, Wolfgang, Leitenstorfer, Alfred, Juraschek, Dominik, Bossini, Davide

The macroscopic magnetic order in the ground state of solids is determined by the spin-dependent Hamiltonian of the system. In the absence of external magnetic fields, this Hamiltonian contains the exchange interaction, which is of electrostatic origin, and the spin-orbit coupling, whose magnitude depends on the atomic charge. Spin-wave theory provides a representation of the entire spectrum of collective magnetic excitations, called magnons, assuming the interactions to be constant and the number of magnons in the system negligible. However, the electric field component of light is able to perturb electrostatic interactions, charge distributions and, at the same time, can create a magnon population. A fundamental open question therefore concerns the possibility to optically renormalise the spin Hamiltonian. Here, we test this hypothesis by using femtosecond laser pulses to resonantly pump electric-dipole-active pairs of high-energy magnons near the edges of the Brillouin zone. The transient spin dynamics reveals the activation and a surprising amplification of coherent low-energy zone-centre magnons, which are not directly driven. Strikingly, the spectrum of these low-energy magnons differs from the one observed in thermal equilibrium, the latter being consistent with spin-wave theory. The light-spin interaction thus results in a room-temperature renormalisation of the magnetic Hamiltonian, with an estimated modification of the magnetic interactions by 10% of their ground-state values. We rationalise the observation in terms of a novel resonant scattering mechanism, in which zone-edge magnons couple nonlinearly to the zone-centre modes. In a quantum mechanical model, we analytically derive the corrections to the spectrum due to the photo-induced magnon population, which are consistent with our experiments. Our results present a milestone for an all-optical engineering of Hamiltonians.

Vorschaubild nicht verfügbar
Veröffentlichung

Flux-Tunable Josephson Diode Effect in a Hybrid Four-Terminal Josephson Junction

2023-12-07, Coraiola, Marco, Svetogorov, Aleksandr, Haxell, Daniel Z., Sabonis, Deividas, Hinderling, M., ten Kate, Sofieke C., Cheah, Erik, Krizek, Filip, Schott, Rüdiger, Wegscheider, Werner, Cuevas, Juan Carlos, Belzig, Wolfgang, Nichele, Fabrizio

We investigate the direction-dependent switching current in a flux-tunable four-terminal Josephson junction defined in an InAs/Al two-dimensional heterostructure. The device exhibits the Josephson diode effect, with switching currents that depend on the sign of the bias current. The superconducting diode efficiency, reaching a maximum of |η|≈34%, is widely tunable - both in amplitude and sign - as a function of magnetic fluxes and gate voltages. Our observations are supported by a circuit model of three parallel Josephson junctions with non-sinusoidal current-phase relation. With respect to conventional Josephson interferometers, phase-tunable multiterminal Josephson junctions enable large diode efficiencies in structurally symmetric devices, where local magnetic fluxes break both time-reversal and spatial-inversion symmetries. Our work establishes a pathway to develop Josephson diodes with wide-range tunability and that do not rely on exotic materials or externally applied magnetic fields.

Vorschaubild nicht verfügbar
Veröffentlichung

Microscopic theory of supercurrent suppression by gate-controlled surface depairing

2023-11-13, Chakraborty, Subrata, Nikolić, Danilo, Cuevas, Juan Carlos, Giazotto, Francesco, Di Bernardo, Angelo, Scheer, Elke, Cuoco, Mario, Belzig, Wolfgang

Recently gate-mediated supercurrent suppression in superconducting nano-bridges has been reported in many experiments. This could be either a direct or an indirect gate effect. The microscopic understanding of this observation is not clear till now. Using the quasiclassical Green's function method, we show that a small concentration of magnetic impurities at the surface of the bridges can significantly help to suppress superconductivity and hence the supercurrent inside the systems while applying a gate field. This is because the gate field can enhance the depairing through the exchange interaction between the magnetic impurities at the surface and the superconductor. We also obtain a \emph{symmetric} suppression of the supercurrent with respect to the gate field, a signature of a direct gate effect. Future experiments can verify our predictions by modifying the surface with magnetic impurities.

Lade...
Vorschaubild
Veröffentlichung

Microwave-induced conductance replicas in hybrid Josephson junctions without Floquet—Andreev states

2023-10-26, Haxell, Daniel Z., Coraiola, Marco, Sabonis, Deividas, Hinderling, Manuel, ten Kate, Sofieke C., Cheah, Erik, Krizek, Filip, Schott, Rüdiger, Wegscheider, Werner, Belzig, Wolfgang, Cuevas, Juan Carlos, Nichele, Fabrizio

Light–matter coupling allows control and engineering of complex quantum states. Here we investigate a hybrid superconducting–semiconducting Josephson junction subject to microwave irradiation by means of tunnelling spectroscopy of the Andreev bound state spectrum and measurements of the current–phase relation. For increasing microwave power, discrete levels in the tunnelling conductance develop into a series of equally spaced replicas, while the current–phase relation changes amplitude and skewness, and develops dips. Quantitative analysis of our results indicates that conductance replicas originate from photon assisted tunnelling of quasiparticles into Andreev bound states through the tunnelling barrier. Despite strong qualitative similarities with proposed signatures of Floquet–Andreev states, our study rules out this scenario. The distortion of the current–phase relation is explained by the interaction of Andreev bound states with microwave photons, including a non-equilibrium Andreev bound state occupation. The techniques outlined here establish a baseline to study light–matter coupling in hybrid nanostructures and distinguish photon assisted tunnelling from Floquet–Andreev states in mesoscopic devices.