arXiv subjects

Alfredo Levy Yeyati

Publications and source records attributed to Alfredo Levy Yeyati.

At least 19 recordsLinked to original sources

arXiv2026

Visualizing impurity-driven scattering phase textures in EuCd2As2

Understanding how disorder modifies electronic states in magnetic semiconductors is important for controlling spin-dependent transport and topological responses. Here we use scanning tunneling microscopy to visualize scattering phase textures in EuCd2As2. By isolating a single surface wavevector we reconstruct spatial phase maps of the local density of states and identify phase dislocations characterized by 2pi winding around impurity sites. These phase singularities emerge systematically within charge puddles generated by Eu interstitials and their positions evolve with bias voltage. We show that their spatial structure is consistent with interference between multiple scattering channels, including contributions from spin-orbit coupling. We provide a model which reproduces phase dislocations and relates the decay of the phase gradient to the relative strength of spin-orbit and scalar scattering. Our results establish a route to access the phase of electronic scattering in real space and study the role of local disorder and spin-orbit interactions in shaping electronic states in quantum materials.

cond-mat.mtrl-sci↗

arXiv2025

Coherent subgap transport in spin-split Josephson junctions

We report the first experimental observation of subgap transport in ferromagnetic insulator/superconductor/insulator/superconductor junctions realized in EuS/Al/AlOx/Al vertical stacks. Differential conductance measurements reveal multiple Andreev reflection peaks, with odd-order peaks split by the spin-splitting induced in the superconductor adjacent to EuS, while even-order peaks remain unaffected. Combining experiments with quasiclassical transport modeling, we extract the spin-splitting and the distribution of transmission channels, finding that a significant fraction ($\sim 23\%$) of highly transparent channels ($τ\approx 0.9$) dominates transport. The observation of a Josephson current further confirms strong superconducting coupling through these channels. Our results demonstrate that a single spin-split superconductor is sufficient to observe the even-odd MAR effect. Our work establishes EuS/Al junctions as a versatile platform to study subgap transport, Josephson coupling, and spin-polarized superconducting phenomena.

cond-mat.supr-con↗

arXiv2025

Effective conduction-band model for zincblende III-V semiconductors in the presence of strain: tuning the properties of bulk crystals and nanostructures

Strain provides a powerful knob to tailor the electronic properties of semiconductors. Simple yet accurate approximations that capture strain effects in demanding simulations of mesoscopic nanostructures are therefore highly desirable. However, for III-V compounds, key materials for quantum applications, such approaches remain comparatively underdeveloped. In this work, we derive a compact, effective Hamiltonian that describes the conduction band of zincblende III-V semiconductors incorporating strain effects. Starting from the eight-band k$\cdot$p model with Bir-Pikus corrections, we perform a folding-down procedure to obtain analytical expressions for conduction-band strain-renormalized parameters, including the effective mass, chemical potential, spin-orbit coupling, and $g$-factor. The model reproduces full multiband results under small to moderate strain, while retaining a form suitable for device-scale calculations. We benchmark the model for bulk deformations and apply it to representative nanostructures, such as core.shell nanowires and planar heterostructures. Our results provide a practical and versatile tool for incorporating strain into the design of III-V semiconductor devices, enabling reliable predictions of their properties with direct implications for spintronic, straintronic, optoelectronic, and topological quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

The feedback driven atomic scale Josephson microscope

The ultimate spatial limit to establish a Josephson coupling between two superconducting electrodes is an atomic-scale junction. The Josephson effect in such ultrasmall junctions has been used to unveil new switching dynamics, study coupling close to superconducting bound states or reveal non-reciprocal effects. However, the Josephson coupling is weak and the sensitivity to temperature reduces the Cooper pair current magnitude. Here we show that a feedback element induces a time-dependent bistable regime which consists of spontaneous periodic oscillations between two different Cooper pair tunneling states (corresponding to the DC and AC Josephson regimes respectively). The amplitude of the time-averaged current within the bistable regime is almost independent of temperature. By tracing the periodic oscillations in the new bistable regime as a function of the position in a Scanning Tunneling Microscope, we obtain atomic scale maps of the critical current in 2H-NbSe$_2$ and find spatial modulations due to a pair density wave. Our results fundamentally improve our understanding of atomic size Josephson junctions including a feedback element in the circuit and provide a promising new route to study superconducting materials through atomic scale maps of the Josephson coupling.

cond-mat.supr-con↗

arXiv2025

Emergent topology by Landau level mixing in quantum Hall-superconductor nanostructures

We demonstrate the emergence of novel topological phases in quantum Hall-superconductor hybrid systems driven by Landau level mixing and spin-orbit interactions. Focusing on a narrow superconducting stripe atop a two-dimensional electron gas, we identify regimes where the hybridization of the chiral Andreev states at each side of the stripe leads to different phases beyond the long sought $p$-wave superconducting one. These topological phases exhibit distinctive transport signatures, including quantized nonlocal conductance arising from electron cotunneling at filling factor $ν=1$, which can coexist with quantized crossed Andreev reflection at $ν=2$. A combination of numerical simulations and effective modelling reveals the role of spin-orbit coupling and stripe geometry in controlling these transitions. Our findings suggest new strategies for realizing and detecting topology in proximized quantum Hall devices.

cond-mat.mes-hall↗

arXiv2025

YSR Bond Qubit in a Double Quantum Dot with cQED Operation

Connecting two half-filled quantum dots to two superconducting leads induces a competition of bonds, with the dots forming either an interdot exchange bond or two individual Yu-Shiba-Rusinov (YSR) screening bonds with the leads. Defining a qubit using these singlet parity bonding states provides dot charge noise protection, attributed to the chargeless nature of the screening quasiparticles, and magnetic noise protection, as the bonds guard against magnetic polarization. In this paper, we propose embedding a Double Quantum Dot (DQD) Josephson junction in parallel with a transmon to enable circuit Quantum Electrodynamics (cQED) measurements and operation of a YSR bond qubit. We demonstrate that, under realistic parameters, two-tone spectroscopy of the DQD can be performed, revealing a significant parameter regime suitable for qubit operation. Additionally, coherent manipulations of the bond states can be achieved through dot gates, and single-shot readout is enabled by measurements of a capacitively coupled resonator. Finally, we analyze noise sources and estimate gate noise on couplings as the primary source of qubit decoherence. Since this qubit is protected against nuclear Overhauser fields and does not rely on spin-orbit interactions for operation, a broader range of material platforms becomes available compared to current Andreev spin qubits.

cond-mat.mes-hall↗

arXiv2025

Revealing inter-band electron pairing in a superconductor with spin-orbit coupling

Most superconducting mechanisms pair electrons within the same band, forming spin singlets. However, the discovery of multi-band superconductivity has opened new scenarios for pairing, particularly in systems with strong spin-orbit coupling. Here, we reveal inter-band pairing in the superconductor \bipd\ by mapping the amplitude of sub-gap Yu-Shiba-Rusinov (YSR) states around Vanadium adatoms deposited on its surface. The surface of \bipd\ is characterized by spin-helical-like bands near the Fermi level. Scanning tunneling spectroscopy reveals anisotropic YSR amplitude oscillations around the impurity, driven by spin-conserving Bogoliubov quasiparticle interference (BQPI). Analysis of the BQPI patterns at the YSR energy exposes inter-band pairing in this material. Interestingly, only a small subset of all possible inter-band scattering processes observed in the normal state contribute to the BQPI patterns. Combining experimental data and theory, we demonstrate that the observed band selectivity results from the hybridization of the band coupled with the impurity with other bands. Our findings reveal unconventional pairing mechanisms in \bipd\ and highlight the crucial role of spin-orbit interactions in their formation.

cond-mat.mes-hall↗

arXiv2024

Error threshold in active steering protocols for few-qubit systems

We study active steering protocols for weakly measured qubits in the presence of error channels due to amplitude and phase noise. If the error rate is sufficiently small, the protocol approaches and stabilizes a predesignated pure target state with high fidelity and high purity, and thus implements autonomous state stabilization. We present numerical simulation results for one and two qubits, taking Andreev qubit circuits as example. As function of the error rate, a sharp threshold separates an error-correcting weak-damping regime from a strong-damping regime where the target state cannot be reached anymore. At the threshold, the purity gap closes.

quant-ph↗

arXiv2024

Interplay between Majorana and Shiba states in a minimal Kitaev chain coupled to a superconductor

Two semiconducting quantum dots (QDs) coupled through a superconductor constitute a minimal realisation of a Kitaev chain with Majorana zero modes (MZMs). Such MZMs can be detected by e.g., tunneling conductance between each QD and normal leads [Dvir et al, Nature 614, 445 (2023)]. We here discuss how the seemingly trivial substitution of one of the normal leads by a superconducting (SC) one gives rise to a plethora of new effects. In particular, the coupling to the SC lead induces non-local Majorana effects upon variations of the QDs' energies. Furthermore, the lowest excitation of the chain is no longer determined by the bulk gap but rather by the energy of an emergent subgap Yu-Shiba-Rusinov (YSR) state coexisting with the MZMs. The YSR state hybridizes with the MZMs when the coupling between the SC and the QD is larger than the spin splitting, spoiling the Majorana properties, including the quantized conductance.

cond-mat.mes-hall↗

arXiv2024

Signatures of triplet superconductivity in nu=2-chiral Andreev states

We study the behavior of the conductance and the current-noise in three-terminal configurations of edge modes of a quantum Hall system in the nu=2 filling factor with normal and s-wave superconducting contacts. We discuss the impact of spin-orbit coupling in the quantum Hall system and the possibility of effectively inducing triplet pairing in the egde states. We show that the presence of these correlations imprints very clear signatures in both the non-linear conductance and noise in these type of devices.

cond-mat.mes-hall↗

arXiv2024

Supercurrent through an Andreev trimer

Detection and control of Andreev Bound States (ABSs) localized at semiconductor-superconductor interfaces are essential for their use in quantum applications. Here we investigate the impact of ABSs on the supercurrent through a Josephson junction containing a quantum dot (QD). Additional normal-metal tunneling probes on both sides of the junction unveil the ABSs residing at the semi-superconductor interfaces. Such knowledge provides an ingredient missing in previous studies, improving the connection between theory and experimental data. By varying the ABS energies using electrostatic gates, we show control of the switching current, with the ability to alter it by more than an order of magnitude. Finally, the large degree of ABS tunability allows us to realize a three-site ABS-QD-ABS molecule (Andreev trimer) in which the central QD is screened by both ABSs. This system is studied simultaneously using both supercurrent and spectroscopy.

cond-mat.mes-hall↗

arXiv2024

Bolometric detection of Josephson radiation

A Josephson junction (JJ) has been under intensive study ever since 1960's. Yet even in the present era of building quantum information processing devices based on many JJs, open questions regarding a single junction remain unsolved, such as quantum phase transitions, coupling of the JJ to an environment and improving coherence of a superconducting qubit. Here we design and build an engineered on-chip reservoir that acts as an efficient bolometer for detecting the Josephson radiation under non-equilibrium (biased) conditions. The bolometer converts ac Josephson current at microwave frequencies, up to about $100\,$GHz, into a measurable dc temperature rise. The present experiment demonstrates an efficient, wide-band, thermal detection scheme of microwave photons and provides a sensitive detector of Josephson dynamics beyond the standard conductance measurements. Using a circuit model, we capture both the current-voltage characteristics and the measured power quantitatively.

cond-mat.mes-hall↗

arXiv2023

Heat dissipation mechanisms in hybrid superconductor-semiconductor devices revealed by Joule spectroscopy

Understanding heating and cooling mechanisms in mesoscopic superconductor-semiconductor hybrid devices is crucial for their application in quantum technologies. Owing to the poor thermal conductivity of typical devices, heating effects can drive superconducting-to-normal phase transitions even at low applied bias, observed as sharp conductance dips through the loss of Andreev excess currents. Tracking such dips across magnetic field, cryostat temperature, and applied microwave power, which constitutes Joule spectroscopy, allows to uncover the underlying cooling bottlenecks in different parts of a device. By applying this technique, we analyze heat dissipation in devices based on full-shell InAs-Al nanowires and reveal that superconducting islands are strongly susceptible to heating as their cooling is limited by the rather inefficient electron-phonon coupling, as opposed to grounded superconductors that primarily cool by quasiparticle diffusion. Our measurements indicate that powers as low as 50-150 pW are able to fully suprpress the superconductivity of an island. Finally, we show that applied microwaves lead to similar heating effects as DC signals, and explore the interplay of the microwave frequency and the effective electron-phonon relaxation time.

cond-mat.mes-hall↗

arXiv2023

Bolometric detection of Josephson inductance in a highly resistive environment

The Josephson junction is a building block of quantum circuits. Its behavior, well understood when treated as an isolated entity, is strongly affected by coupling to an electromagnetic environment. In 1983, Schmid predicted that a Josephson junction shunted by a resistance exceeding the resistance quantum $\mathbf{\textit{R}}_\mathrm{Q} = h/4e^2 \approx 6.45$ k$\mathbfΩ$ for Cooper pairs would become insulating since the phase fluctuations would destroy the coherent Josephson coupling. However, recent microwave measurements have questioned this interpretation. Here, we insert a small Josephson junction in a Johnson-Nyquist-type setup where it is driven by weak current noise arising from thermal fluctuations. Our heat probe minimally perturbs the junction's equilibrium, shedding light on features not visible in charge transport. We find that the Josephson critical current completely vanishes in DC charge transport measurement, and the junction demonstrates Coulomb blockade in agreement with the theory. Surprisingly, thermal transport measurements show that the Josephson junction acts as an inductor at high frequencies, unambiguously demonstrating that a supercurrent survives despite the Coulomb blockade observed in DC measurements. The discrepancy between these two measurements highlights the difference between the low and the high frequency response of a junction and calls for further theoretical and experimental inputs on the dynamics of Josephson junctions \textcolor{black}{operating at high frequencies in highly resistive environments.

cond-mat.mes-hall↗

arXiv2023

Intrinsic non-magnetic $ϕ_0$ Josephson junctions in twisted bilayer graphene

Recent experiments have demonstrated the possibility to design highly controllable junctions on magic angle twisted bilayer graphene, enabling the test of its superconducting transport properties. We show that the presence of chiral pairing in such devices manifests in the appearance of an anomalous Josephson effect ($ϕ_0$ behavior) even in the case of symmetric junctions and without requiring any magnetic materials or fields. Such behavior arises from the combination of chiral pairing and nontrivial topology of the twisted bilayer graphene band structure that can effectively break inversion symmetry. Moreover, we show that the $ϕ_0$ effect could be experimentally enhanced and controlled by electrostatic tuning of the junction transmission properties.

cond-mat.mes-hall↗

arXiv2023

Transport features of a topological superconducting nanowire with a quantum dot: conductance and noise

We study two-terminal configurations in junctions between a topological superconducting wire with spin-orbit coupling and magnetic field, and an ordinary conductor with an embedded quantum dot. One of the signatures of the Majorana zero modes in the topological phase is a quantization of the zero-bias conductance at $G(V=0)=2e^2/h$. However, the finite size of the wires and the presence of the quantum dot in the junction generate more complicated features which lead to deviations from this simple picture. Here, we analyze the behavior of the conductance at zero and finite bias, $G(V)$, as a function of a gate voltage applied at the quantum dot in the case of a finite-length wire. We analyze the effect of the angle between the magnetic field and the orientation associated to the spin-orbit coupling. We provide a detailed description of the spectral features of the quantum wire weakly and also strongly coupled to the quantum dot and describe the conditions to have zero-energy states in these two regimes for both the topological and non-topological phases. We also analyze the concomitant behavior of the noise. We identify qualitative features that are useful to distinguish between the topological and non-topological phases. We show that in a strongly coupled quantum dot the simultaneous hybridization with the topological modes and the supragap states of the wire mask the signatures of the Majorana bound states in both the conductance and the Fano factor.

cond-mat.supr-con↗

arXiv2023

Dynamical parity selection in superconducting weak links

Excess quasiparticles play a crucial role in superconducting quantum devices ranging from qubits to quantum sensors. In this work we analyze their dynamics for phase-biased finite-length weak links with several Andreev subgap states, where the coupling to a microwave resonator allows for parity state (even/odd) readout. Our theory shows that almost perfect dynamical polarization in a given parity sector is achievable by applying a microwave pulse matching a transition in the opposite parity sector. Our results qualitatively explain key features of recent experiments on hybrid semiconducting nanowire Josephson junctions and provide theoretical guidelines for efficiently controlling the parity state of Andreev qubits.

cond-mat.mes-hall↗

arXiv2022

Joule spectroscopy of hybrid superconductor-semiconductor nanodevices

Hybrid superconductor-semiconductor devices offer highly tunable platforms, potentially suitable for quantum technology applications, that have been intensively studied in the past decade. Here we establish that measurements of the superconductor-to-normal transition originating from Joule heating provide a powerful spectroscopical tool to characterize such hybrid devices. Concretely, we apply this technique to junctions in full-shell Al-InAs nanowires in the Little-Parks regime and obtain detailed information of each lead independently and in a single measurement, including differences in the superconducting coherence lengths of the leads, inhomogeneous covering of the epitaxial shell, and the inverse superconducting proximity effect; all-in-all constituting a unique fingerprint of each device and highlighting the large variability present in these systems. Besides the practical uses, our work also underscores the importance of heating in hybrid devices, an effect that is often overlooked.

cond-mat.mes-hall↗