arXiv subjects

Alessandro Braggio

Publications and source records attributed to Alessandro Braggio.

At least 19 recordsLinked to original sources

arXiv2026

Subgap Linear Thermoelectricity in Superconducting Quantum Hall Systems

We show that an integer quantum Hall setup proximized by superconductors can exhibit subgap thermoelectric effects in the linear-response regime when triplet superconducting correlations are present. We devise a minimal setup that enables a nonzero Seebeck effect mediated by Andreev processes and predict that the corresponding Seebeck coefficient can reach values on the order of $k_B/e$ in the middle of the quantum Hall plateau. We analytically show that both triplet correlations and spin polarization are essential for the emergence of the thermoelectric effect, which arises despite the linear band dispersion of the edge states. We characterize the dependence of the thermoelectric response on the Hamiltonian parameters and the system's temperature regime.

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arXiv2026

Nonequilibrium Cooper quartet generation in superconducting devices

Cooper quartets are aggregates of four electrons that generalize the concept of Cooper pairs, and their study can unfold unexplored perspectives in correlated matter and many-body physics. We propose a method to isolate them in a double-quantum-dot system coupled to conventional superconducting and normal leads. By driving the system out of equilibrium, we show that a resonance between the vacuum $|0\rangle$ and the four-electron state $|4e\rangle$ emerges in the high bias voltage regime, which involves a two-Cooper pair exchange process and is characterized by finite quartet correlations. We study the transport properties of the system and show that a peak in the Andreev current at high bias voltage has a width that scales with the magnitude of the quartet coupling $\Gamma_{4e}$, which can be tuned by the phase of additional superconducting leads, yielding distinctive signatures. By further studying the current-current correlations and the Fano factor, we establish a regime characterized by equal auto- and cross-correlations, which we interpret as a definitive signature of fast coherent two-Cooper-pair oscillations between the dots and the superconducting leads. The proposed platform, experimentally accessible in a quantum solid-state laboratory, enables exploration of quartet correlations and multifermion-correlated states of matter.

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arXiv2026

Magnetically Induced Switching-Current Jumps in InAs/Al Josephson Junctions

We report Barkhausen-like switching at millitesla fields in an $n$-doped InAs/Al nanowire Josephson junction, which serves as an interferometric probe of intrinsic magnetic reconfigurations, as evidenced by discrete switching-current jumps. At $T=30$~mK the device displays a Fraunhofer-like modulation with $I_{\mathrm{sw}}(0)\approx 0.24~\mu\mathrm{A}$ and an abrupt transition at $|B|\approx 3~\mathrm{mT}$ between two branches differing by $\Delta I_{\mathrm{sw}}\approx 0.13~\mu\mathrm{A}$. By tracking the characteristic field scales from $30$ to $900$~mK, we find that the jump field is essentially temperature-independent, whereas the superconducting critical field decreases with temperature, as expected for thin Al films. The sharp discontinuity, sweep-direction asymmetry, and reproducibility across repeated scans point to avalanche-like switching between metastable magnetic configurations of the local magnetic texture, which are directly coupled to the weak link. Within an effective-field framework, each reconfiguration modifies a local field offset, thereby reshaping the interference response and leading to an abrupt reorganization of the switching-current pattern.

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arXiv2025

Enhanced Thermoelectricity in Nanowires with inhomogeneous Helical states

Semiconductor nanowires (NWs) with strong Rashba spin-orbit coupling (RSOC), when exposed to a suitably applied Zeeman field, exhibit one-dimensional helical channels with a spin orientation locked to the propagation direction within the magnetic energy gap. Here, by adopting a scattering-matrix approach applied to a tight-binding model of the NW, we demonstrate that the thermoelectric (TE) properties can be widely controlled by tuning the misalignment angle $\phi$ between the spin-orbit directions of two NW segments. In particular, when the RSOC vectors are antiparallel (Dirac paradox configuration) we predict a significant violation of the Wiedemann-Franz law, and a strong enhancement of the Seebeck coefficient and the $ZT$ figure of merit. We also show that the Zeeman gap determines the optimal energy window for doping and temperatures. These results suggest that controlling the spin-orbit field direction, which can be achieved with suitably applied wrap gates, is a promising alternative for tuning and optimizing the TE response in quantum-coherent semiconducting NW devices.

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arXiv2025

Giant thermal modulation via a semiconductor-superconductor photonic field-effect heat transistor

We present a groundbreaking demonstration of thermal modulation in a field-effect-controllable semiconductor-superconductor hybrid structure, wherein the heating mechanism is exclusively radiative. The architecture comprises two reservoirs separated by $\sim 1$ mm and interconnected via a completely non-galvanic electrical circuit, enabling the transfer of black-body radiation from the hot to the cold reservoir. Our device utilizes a superconducting Josephson field-effect transistor to achieve magnetic-field-free gate-tunable regulation of heat currents within the circuit. While prior studies have indicated the potential for electrostatic modulation of thermal transport properties, our framework demonstrates a temperature modulation of up to $\sim 45$ mK, exceeding prior findings by more than an order of magnitude. Furthermore, it proves a thermal transimpedance of $\sim 20$ mK/V at a bath temperature of $30$ mK. The development of such systems holds substantial promise for advancing heat management and routing in quantum chips and radiation sensors, as it enables precise nonlocal control of heat flow towards a designated structure, even when the heat source is distant and non-galvanically coupled.

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arXiv2025

Quantum Bipolar Thermoelectricity

Thermoelectricity is generally understood as a classical effect emerging from energy-dependent transport asymmetries. Here we uncover a purely quantum mechanism, where a superconducting S-I-S' tunnel junction in thermal equilibrium develops a nonlinear bipolar thermoelectric response owing to the dynamical Coulomb blockade and the emission-absorption imbalance of a cold electromagnetic bath. Two representative environments are analysed, revealing Seebeck coefficients up to 100 $\mu$V/K for realistic junction parameters. Because the response directly reflects the spectral properties of the surrounding environment, our results suggest that bipolar quantum thermoelectricity could provide a new route for spectroscopic sensing of electromagnetic modes and for designing low-temperature thermoelectric devices with environmentally engineered performance.

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arXiv2025

Streamline controlled rectification of supercurrent in thin-film asymmetric weak links

In this study, we examined the supercurrent diode effect (SDE) in mesoscopic superconducting weak links formed by asymmetric Dayem bridges. These planar metallic constrictions, which naturally exhibit Josephsonlike behavior, offer a fundamental platform for investigating nonreciprocal transport phenomena in a regime where the bridge width aligns with the superconducting coherence length. The foundational concept is inspired by the Tesla valve, a classical fluidic device that achieves flow rectification through interference and turbulence between fluid streams enabled by geometric asymmetry. Analogously, we demonstrate that spatial asymmetry within superconducting structures can result in rectification due to the polarity-dependent interaction between transport and screening currents. By implementing controlled geometric defects at the junction between the constriction and superconducting leads, we induce current crowding and disrupt spatial inversion symmetry, thus facilitating directional switching behavior. Experimental results indicate a linear-in-field rectification regime at low magnetic fields, driven by the interaction between transport and screening currents, which is succeeded by complex vortex dynamics within the superconducting banks at elevated fields. Time-dependent Ginzburg-Landau simulations replicate significant features of the experimental observations and substantiate the influence of both screening currents and rearrangements of Abrikosov vortices. A comparative study across various geometries highlights the crucial role of defect shape and spatial confinement in determining the rectification efficiency, revealing a minimum threshold in bridge width below which crowding-induced SDE is significantly reduced. Our findings advocate for mesoscopic Dayem bridges as a flexible platform for designing and controlling superconducting diode functionalities.

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arXiv2025

Enhanced thermoelectric effects in a driven one-dimensional system

We investigate the thermoelectric properties of a one-dimensional quantum system in the presence of an external driving. We employ Floquet scattering theory to calculate linear-response stationary thermoelectric figures of merit in a single-channel conductor subjected to a periodically varying delta-like potential barrier. We also include a step barrier in one of the leads as a model of a nanoscale inhomogeneous semiconducting system. In the absence of a step barrier, we found that external driving can significantly enhance the Seebeck coefficient, particularly at low temperatures, with a relative increase of up to 200% at high frequencies compared to the static case. In the presence of a step barrier, we found that the thermoelectric Onsager coefficient for the driven case is also enhanced compared to the static case, with a significant photon-assisted effect at low temperatures when the chemical potential is within the semiconductor's gap. Our results demonstrate that external driving can be used to tune and enhance the thermoelectric capabilities of low-electron-density nanodevices.

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arXiv2025

Eliashberg theory prediction of critical currents in superconducting thin films under DC electric fields

Superconducting thin metallic films, functioning as supercurrent gate-tunable transistors, have considerable potential for future quantum electronic devices. Despite extensive research, a comprehensive microscopic quantitative mechanism that elucidates the control or suppression of supercurrents in thin films remains elusive. Focusing on NbN, a prototypical material, and starting from a phenomenological ansatz that links the critical electric field with the kinetic energy parameter needed to break Cooper pairs, we provide a quantitative analysis of the critical current using Eliashberg theory in the dirty limit without adjustable parameters. The critical kinetic energy value is identified, corresponding to the maximum supercurrent that can flow in the thin film. The peak in supercurrent density as a function of the Cooper pairs' kinetic energy arises from the interplay between the increase in supercurrent due to increased kinetic energy and the depairing effect when the kinetic energy becomes sufficiently large. The critical value of the pair's kinetic energy is subsequently employed to estimate the critical value of an external electric field required to suppress superconductivity in the sample. This estimation is in parameter-free agreement with the experimental observations. Although the disorder reduces the temperature dependence of the gating effect on the critical current, at the same time, it increases the unscreened critical electric field needed to suppress superconductivity. This enables the proposal of methods to control and reduce the critical field value necessary to suppress superconductivity in superconducting electronics.

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arXiv2025

Microscopic mechanism of electric field-induced superconductivity suppression in metallic thin films

Supercurrent field-effect transistors made from thin metallic films are a promising option for next-generation high-performance computation platforms. Despite extensive research, there is still no complete quantitative microscopic explanation for how an external DC electric field suppresses superconductivity in thin films. This study aims to provide a quantitative description of superconductivity as a function of film thickness based on Eliashberg's theory. The calculation considers the electrostatics of the electric field, its realistic penetration depth in the film, and its effect on the Cooper pair, which is described as a standard s-wave bound state according to BCS theory. The estimation suggests that an external electric field of approximately $10^8$ V/m is required to suppress superconductivity in 10-30-nm-thick films, which aligns with experimental observations. Ultimately, the study offers "materials by design" guidelines for suppressing supercurrent when an external electric field is applied to the film surface. Furthermore, the proposed framework can be easily extended to investigate the same effects for ultrathin films.

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arXiv2025

Photonic heat amplifiers based on a disordered semiconductor

A photonic heat amplifier (PHA) designed for cryogenic operations is introduced and analyzed. This device comprises two variable-range-hopping reservoirs connected by lossless lines, which allow them to exchange heat through photonic modes. This configuration enables negative differential thermal conductance (NDTC), which can be harnessed to amplify thermal signals. To achieve this, one reservoir is maintained at a high temperature, serving as the source terminal of a thermal transistor. Concurrently, in the other one, we establish tunnel contacts to metallic reservoirs, which function as the gate and drain terminals. With this arrangement, it is possible to control the heat flux exchange between the source and drain by adjusting the gate temperature. We present two different parameter choices that yield different performances: the first emphasizes modulating the source-drain heat current, while the second focuses on the modulation of the colder temperature variable range hopping reservoir. Lastly, we present a potential design variation in which all electronic reservoirs are thermally connected through only photonic modes, allowing interactions between distant elements. The proposal of the PHA addresses the lack of thermal transistors and amplifiers in the mK range while being compatible with the rich toolbox of circuit quantum electrodynamics. It can be adapted to various applications, including sensing and developing thermal circuits and control devices at sub-Kelvin temperatures, which are relevant to quantum technologies.

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arXiv2024

$\Delta T$-noise in Multiterminal Hybrid Systems

The study of charge current fluctuations (noise) can give useful insights into the properties of nanoscale systems. In this work, the peculiar properties of noise in multiterminal hybrid normal-superconducting systems are explored in the thermal out-of-equilibrium regime, i.e., when temperature biases are present ($\Delta T$-noise). Using the Landauer-B\"uttiker approach, we identify two contributions: background noise and excess noise, analyzing them when both electrical and thermal biases are applied. When temperature biases are present, and superconducting terminals are grounded, we find that the first contribution depends not only on the electrical conductance, as the Johnson-Nyquist at equilibrium, but also on a quantity strictly related to the heat conductance. This is our first main result. On the other hand, the second contribution shows, as expected, additional terms originating from the partitioning of currents into different transport channels, including the ones associated with Andreev reflection processes. However, noise induced by the temperature differences unveil also interference terms that cannot be present either in voltage bias or in the absence of any Andreev processes. Finally, we apply the results obtained to two different specific physical situations. The first is a generic three-terminal normal-superconductor-normal system, and the second is a device based on spin-resolved co-propagating chiral channels in the integer quantum Hall regime with a superconducting region. In these example setups, we investigate mainly the shot-noise regimes, when high-voltage or high-temperature biases are applied. We find remarkable differences between the two limits, which ultimately show the different nature of electrically and thermally induced charge current fluctuations.

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arXiv2024

Extremely weak sub-kelvin electron-phonon coupling in InAs On Insulator

We are proposing, as an ideal candidate for caloritronic devices operating at subKelvin temperatures, a hybrid superconductor-semiconductor platform named InAs on insulator (InAsOI). This heterostructure is made by doped InAs grown on an insulating buffer of InAlAs on a GaAs substrate. Caloritronic devices aim to heat or cool electrons out of equilibrium with respect to the phonon degree of freedom. However, their performances are usually limited by the strength of the electron-phonon (e-ph) coupling and the associated power loss. Our work discusses the advantages of the InAsOI platform, which are based on the significantly low e-ph coupling measured compared to all-metallic state-of-the-art caloritronic devices. Our structure demonstrates values of the e-ph coupling constant up to two orders of magnitude smaller than typical values in metallic structures.

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arXiv2024

Giant Thermoelectric Response of Fluxons in Superconductors

Thermoelectric devices that operate on quantum principles have been under extensive investigation in the past decades. These devices are at the fundamental limits of miniaturized heat engines and refrigerators, advancing the field of quantum thermodynamics. Most research in this area concerns the use of conduction electrons and holes as charge and heat carriers, and only very recently have superconductors been considered as thermal engines and thermoelectric devices. Here, we investigate the thermoelectric response of an Abrikosov vortex in type-II superconductors in the deep quantum limit. We consider two thermoelectric geometries, a type-II SIN junction and a local Scanning Tunneling Microscope (STM)-tip normal metal probe over the superconductor. We exploit the strong breaking of particle-hole symmetry in bound states at sub-gap energies within the superconducting vortex to realize a giant thermoelectric response in the presence of fluxons. We predict a thermovoltage of a few mV/K at sub-Kelvin temperatures using both semi-analytic and numerical self-consistent solutions of the Bogoliubov-de Gennes equations. Relevant thermoelectric coefficients and figures of merit are found within our model, both in linear and nonlinear regimes. The ZT of the SIN junction is around 1, rising to above 3 for the STM junction centered at the vortex core. We also discuss how this system can be used as a sensitive thermocouple, diode, or localized bolometer to detect low-energy single photons.

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arXiv2024

Nonlocal thermoelectricity in quantum wires as a signature of Bogoliubov-Fermi points

We study nonlocal thermoelectricity in a superconducting wire subject to spin-orbit coupling and a magnetic field with a relative orientation $\theta$ between them. We calculate the current flowing in a normal probe attached to the bulk of a superconducting wire, as a result of a temperature difference applied at the ends of the wire. We focus on the linear response regime, corresponding to a small temperature bias. We find that the nonlocal thermoelectric response is strongly dependent on the angle $\theta$ and occurs in ranges which correspond to the emergence of Bogoliubov Fermi points in the energy spectrum of the superconducting wire.

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arXiv2024

Cooper quartets in interacting hybrid superconducting systems

Cooper quartets represent exotic fermion aggregates describing correlated matter at the basis of charge-$4e$ superconductivity and offer a platform for studying four-body interactions, of interest for topologically protected quantum computing, nuclear matter simulations, and more general strongly correlated matter. Focusing on solid-state systems, we show how to quantum design Cooper quartets in a double-dot system coupled to ordinary superconducting leads through the introduction of an attractive interdot interaction. A fundamentally novel, maximally correlated double-dot ground state, in the form of a superposition of vacuum $|0\rangle$ and four-electron state $|4e\rangle$, emerges as a narrow resonance in a many-body quartet correlator that is accompanied by negligible pair correlations and features a rich phenomenology. The system represents an instance of correlated Andreev matter and the results open the way to the exploration of interaction effects in hybrid superconducting devices, and the study of novel correlated states of matter with ingredients available in a quantum solid-state laboratory.

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arXiv2023

Thermoelectric single-photon detection through superconducting tunnel junctions

Bipolar thermoelectricity in tunnel junctions between superconductors of different energy gap has been recently predicted and experimentally demonstrated. This effect showed thermovoltages up to $\pm150\;μ$V at milliKelvin temperatures. Thus, superconducting tunnel junctions can be exploited to realize a passive single-photon thermoelectric detector $TED$ operating in the broadband range 15 GHz - 50 PHz. In particular, this detector is expected to show a signal-to-noise ratio of about 15 down to $ν=50$ GHz and a operating window of more than 4 decades. Therefore, the $TED$ might find applications in quantum computing, telecommunications, optoelectronics, spectroscopy and astro-particle physics.

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arXiv2023

Bipolar thermoelectrical SQUIPT (BTSQUIPT)

We theoretically study the quasiparticle current behaviour of a thermally-biased bipolar thermoelectrical superconducting quantum interference proximity transistor, formed by a normal metal wire embedded in a superconducting ring and tunnel-coupled to a superconducting probe. In this configuration, the superconducting gap of the wire can be modified through an applied magnetic flux. We analyse the thermoelectric response as a function of magnetic flux, at fixed temperatures, in the case of a device made of the same superconductor. We demonstrate magnetically controllable, bipolar thermoelectric behaviour and discuss optimal working conditions by looking at the thermoelectric power and other figures of merit of the device.

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