arXiv subjects

Julia S. Meyer

Publications and source records attributed to Julia S. Meyer.

At least 19 recordsLinked to original sources

arXiv2026

Andreev bound states in a superconducting qubit at odd parity

The quantum mechanics of the Josephson effect is the core ingredient for quantum technologies with superconducting circuits. A new avenue was recently opened in this field by predicting that the Josephson quantum mechanics in the odd parity sector, when a quasiparticle in trapped in an Andreev bound state, is fundamentally different from the conventional one in the even sector. The focus was then on a Josephson junction surrounded by an electromagnetic environment formed of a collection of bosonic modes, including the case of an ohmic environment. Here we consider the distinct case of a superconducting qubit made of a single Josephson junction whose environment reduces to a capacitance. We find a novel structure for the low-lying discrete states in the odd sector, which is altogether different from the one that appears in the even sector. Our study of the bound-state spectrum ranges from the Coulomb-dominated (Cooper pair box) to the Josephson-dominated (transmon) regime. Our prediction could be tested in forthcoming experiments with superconductor/semiconductor/superconductor junctions, which have been studied intensively in recent years, both using nanowires as well as two-dimensional electron gases.

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arXiv2025

Multiple Quasiparticle Bound States in a Trap Created by a Local Superconducting Gap Variation

At low temperature, the concentration of quasiparticles observed in superconducting circuits far exceeds the predictions of microscopic BCS theory at equilibrium. As a source of dissipation, these excess quasiparticles degrade the performance of various devices. Therefore, understanding their dynamics, especially their recombination into Cooper pairs, is an active topic of current research. In disordered superconductors, spatial fluctuations in the superconducting gap can trap quasiparticles and modify their eigenspectrum. Since this spectrum plays a key role in quasiparticle dynamics, it must be carefully investigated. To this end, we introduce a toy model of a single trap. Specifically, we consider a shallow disk-shaped gap variation in a clean superconductor. Using a semiclassical approximation, we demonstrate the existence of multiple bound states and give the dependence of their number on the size and depth of the gap suppression. Extending our analysis beyond the semiclassical regime, in dimensions larger than one, we observe an infinite number of bound states very close to the gap edge, even for an arbitrarily small trap. These results deepen our understanding of trapped quasiparticles and may have important implications for their recombination in disordered superconductors.

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arXiv2024

Josephson diode effect in a ballistic single-channel nanowire

When time-reversal and inversion symmetry are broken, superconducting circuits may exhibit a so-called diode effect, where the critical currents for opposite directions of the current flow differ. In recent years, this effect has been observed in a multitude of systems and the different physical ingredients that may yield such an effect are well understood. On a microscopic level, the interplay between spin-orbit coupling and a Zeeman field may give rise to a diode effect in a single Josephson junction. However, so far there is no analytical description of the effect within a simple model. Here we study a single channel nanowire with Rashba spin-orbit coupling and in the presence of a Zeeman field. We show that the different Fermi velocities and spin projections of the two pseudo-spin bands lead to a diode effect. Simple analytical expressions for the diode efficiency can be obtained in limiting cases.

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arXiv2024

Paramagnetic limit of spin-triplet superconductors

We study the phase diagram of spin-triplet superconductors, considering the effect of the external magnetic field on the electrons' spins. For a given symmetry of the order parameter and a generic orientation of the field, we find that the paramagnetic limit for superconductivity diverges at low temperatures. Furthermore, we identify a range of temperatures where the transition between normal and superconducting phases becomes of the first order. When two tricritical points exist along the transition line, a first order phase transition between two superconducting phases may develop in vicinity of the tricritical point with lower temperature. We discuss the implications of our findings for the anisotropy of the upper critical field in UPt$_3$, a candidate material for triplet superconductivity, when both the paramagnetic and orbital effects are taken into account.

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arXiv2024

Theory of quasiparticle-induced errors in driven-dissipative Schrödinger cat qubits

Understanding the mechanisms of qubit decoherence is a crucial prerequisite for improving the qubit performance. In this work we discuss the effects of residual Bogolyubov quasiparticles in Schrödinger cat qubits, either of the dissipative or Kerr type. The major difference from previous studies of quasiparticles in superconducting qubits is that the Schrödinger cat qubits are operated under non-equilibrium conditions. Indeed, an external microwave drive is needed to stabilize "cat states", which are superpositions of coherent degenerate eigenstates of an effective stationary Lindbladian in the rotating frame. We present a microscopic derivation of the master equation for cat qubits and express the effect of the quasiparticles as dissipators acting on the density matrix of the cat qubit. This enables us to determine the conditions under which the quasiparticles give a substantial contribution to the qubit errors.

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arXiv2024

Opportunities for the direct manipulation of a phase-driven Andreev spin qubit

In a Josephson junction, the transfer of Cooper pairs from one superconductor to the other one can be associated with the formation of Andreev bound states. In a Josephson junction made with a semiconducting nanowire, the spin degeneracy of these Andreev states can be broken thanks to the presence of spin-orbit coupling and a finite phase difference between the two superconducting electrodes. The lifting of the spin degeneracy opened the way to the realization of Andreev spin qubits that do not require the application of a large magnetic field. So far the operation of these qubits relied on a Raman process involving two microwave tones and a third Andreev state [M. Hays et al., Science 373, 430 (2021)]. Still, time-reversal preserving impurities in the nanowire allow for spin-flip scattering processes. Here, using the formalism of scattering matrices, we show that these processes generically couple Andreev states with opposite spins. In particular, the non-vanishing current matrix element between them allows for the direct manipulation of phase-driven Andreev spin qubits, thereby circumventing the use of the above-mentioned Raman process.

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arXiv2023

Josephson quantum mechanics at odd parity

A Josephson junction may be in a stable odd parity state when a single quasiparticle is trapped in an Andreev bound state. Embedding such junction in an electromagnetic environment gives rise to a special quantum mechanics of superconducting phase that we investigate theoretically. Our analysis covers several representative cases, from the lifting of the supercurrent quench due to quasiparticle poisoning for a low ohmic impedance of the environment, to a Schmid transition in a current-biased junction that for odd parity occurs at four times bigger critical impedance. For intermediate impedances, the supercurrent in the odd state is higher than in the even one.

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arXiv2023

Spectral properties of disordered Ising superconductors with singlet and triplet pairing in in-plane magnetic fields

We study the spectral properties of disordered superconductors with Ising spin-orbit coupling (ISOC) subjected to in-plane magnetic fields. In addition to the conventional singlet pairing, we also consider the recently proposed equal-spin triplet pairing, which couples to the singlet at finite in-plane magnetic fields. While both singlet and triplet order parameters are immune to intravalley scattering, they are significantly affected by intervalley scattering. In the realistic regime of strong ISOC, we find that the properties of the superconductor are well described by a simple formula reminiscent of the well-known Abrikosov-Gor'kov theory, but with a modified self-consistency condition. Our results enable straightforward self-consistent calculation of singlet and triplet order parameters and the density of states of disordered Ising superconductors, which can be particularly useful for interpreting recent tunneling spectroscopy experiments in these systems. We also investigate the high-energy features in the density of states, the so-called mirage gaps, and discuss how they are modified by triplet pairing.

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arXiv2023

Geometrical effects on the downstream conductance in quantum-Hall--superconductor hybrid systems

We consider a quantum Hall (QH) region in contact with a superconductor (SC), i.e., a QH-SC junction. Due to successive Andreev reflections, the QH-SC interface hosts hybridized electron and hole edge states called chiral Andreev edge states (CAES). We theoretically study the transport properties of these CAES by using a microscopic, tight-binding model. We find that the transport properties strongly depend on the contact geometry and the value of the filling factor. We notice that it is necessary to add local barriers at the corners of the junction in order to reproduce such properties, when using effective one-dimensional models.

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arXiv2022

Spin susceptibility of nonunitary spin-triplet superconductors

The spin susceptibility is an important probe to characterize the symmetry of the order parameter in unconventional superconductors. Among them, nonunitary triplet superconductors have attracted a lot of attention recently in the context of the search for topological superconductivity. Here, we derive a general formula for the spin susceptibility of nonunitary triplet superconductors within a single-band model of non-magnetic, centrosymmetric materials with strong spin-orbit coupling. We use it to critically assess experimental claims of nonunitary triplet superconductivity in some materials.

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arXiv2022

Anomalous Josephson effect in planar noncentrosymmetric superconducting devices

In two-dimensional electron systems with broken inversion and time-reversal symmetries, a Josephson junction reveals an anomalous response: the supercurrent is nonzero even at zero phase difference between two superconductors. We consider details of this peculiar phenomenon in the planar double-barrier configurations of hybrid circuits, where the noncentrosymmetric normal region is described in terms of the paradigmatic Rashba model of spin-orbit coupling. We analyze this anomalous Josephson effect by means of both the Ginzburg-Landau formalism and the microscopic Green's functions approach in the clean limit. The magnitude of the critical current is calculated for an arbitrary in-plane magnetic field orientation, and anomalous phase shifts in the Josephson current-phase relation are determined in terms of the parameters of the model in several limiting cases.

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arXiv2022

Weyl singularities in polaritonic multi-terminal Josephson junctions

We study theoretically analog multi-terminal Josephson junctions formed by gapped superfluids created upon resonant pumping of cavity exciton-polaritons. We study the $p$-like bands of a 5-terminal junction in the 4D parameter space created by the superfluid phases acting as quasi-momenta. We find 4/6 Weyl points in 3D subspaces with preserved/broken time-reversal symmetry. We link the real space topology (vortices) to the parameter space one (Weyl points). We derive an effective Hamiltonian encoding the creation, motion, and annihilation of Weyl nodes in 4D. Our work paves the way to the study of exotic topological phases in a platform allowing direct measurement of eigenstates and band topology.

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arXiv2022

Josephson effect in superconductor-normal dot-superconductor junctions driven out of equilibrium by quasiparticle injection

We study theoretically the large variations of the supercurrent through a normal dot that are induced by a small quasiparticle injection current from normal leads connected to the dot. We find that the supercurrent decomposes into a subgap contribution, which depends on the voltages applied to the normal leads, as well as a contribution with opposite sign from energies outside the gap, which is insensitive to the voltages. As the voltages gradually suppress the subgap contribution, a critical voltage exists above which the contribution from energies outside the gap dominates, leading to a sign reversal of the current-phase relation, namely a transition to a so-called $π$-junction behavior. We determine the critical voltage and analyze the robustness of the effect with respect to temperature and inelastic relaxation in the dot.

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arXiv2021

Conductance quantization in topological Josephson trijunctions

The Josephson current flowing in a junction between two superconductors is a striking manifestation of macroscopic quantum coherence, with applications in metrology and quantum information. This equilibrium current is related with the formation of Andreev states localized in the junction, whose energy depends periodically on the superconducting phase difference. Topology emerged as a guide for predicting exotic properties of Andreev states. In particular, topological superconductors host Majorana modes at their ends. Then, in a junction with such leads, the hybridization of two Majorana modes results in an Andreev state with a period-doubling of its energy-phase dependence. Furthermore, topologically protected crossings between Andreev states in junctions with more than two leads may be revealed through a quantized transconductance. The prediction motivated recent efforts to fabricate multi-terminal junctions. Here we combine both topological effects to predict a robust non-vanishing quantized transconductance in trijunctions with topological leads. Such devices are envisioned to reveal the anyonic nature of Majorana states through their braiding. Our prediction can be used to assess that a given junction is indeed suitable to perform its braiding function.

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arXiv2020

Josephson radiation in a superconductor-quantum dot-superconductor junction

We investigate the Josephson radiation emitted by a junction made of a quantum dot coupled to two conventional superconductors. Close to resonance, the particle-hole symmetric Andreev states that form in the junction are detached from the continuum above the superconducting gap in the leads, while a gap between them opens near the Fermi level. Under voltage bias, we formulate a stochastic model that accounts for non-adiabatic processes, which change the occupations of the Andreev states. This model allows calculating the current noise spectrum and determining the Fano factor. Analyzing the finite-frequency noise, we find that the model may exhibit either an integer or a fractional AC Josephson effect, depending on the bias voltage and the size of the gaps in the Andreev spectrum. Our results assess the limitations in using the fractional Josephson radiation as a probe of topology.

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arXiv2019

Dynamical spin polarization of excess quasi-particles in superconductors

We show that the annihilation dynamics of excess quasi-particles in superconductors may result in the spontaneous formation of large spin-polarized clusters. This presents a novel scenario for spontaneous spin polarization. We estimate the relevant scales for aluminum, finding the feasibility of clusters with total spin $S \simeq 10^4 \hbar$ that could be spread over microns. The fluctuation dynamics of such large spins may be detected by measuring the flux noise in a loop hosting a cluster.

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arXiv2019

Weak localization in transition metal dichalcogenide monolayers and their heterostructures with graphene

We calculate the interference correction to the conductivity of doped transition metal dichalcogenide (TMDC) monolayers. Because of the interplay between valley structure and intrinsic spin-orbit coupling (SOC), these materials exhibit a rich weak localization (WL) behavior that is qualitatively different from conventional metals or similar two-dimensional materials such as graphene. Our results can also be used to describe graphene/TMDC heterostructures, where the SOC is induced in the graphene sheet. We discuss new parameter regimes that go beyond existing theories, and can be used to interpret recent experiments in order to assess the strength of SOC and disorder. Furthermore, we show that an in-plane Zeeman field can be used to distinguish the contributions of different kinds of SOC to the WL magnetoconductance.

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arXiv2018

Majorana-Weyl crossings in topological multi-terminal junctions

We analyze the Andreev spectrum in a four-terminal Josephson junction between one-dimensional topological superconductors in class D. We find that a topologically protected crossing in the space of three superconducting phase differences can occur between the two lowest Andreev bound states. This crossing can be detected through the transconductance quantization, in units of $2e^2/h$, between two voltage-biased terminals. Our prediction provides yet another example of topology in multi-terminal Josephson junctions. We discuss possible realizations of such junctions with semiconducting crossed nanowires and with quantum-spin Hall insulators.

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