arXiv subjects

Francesco Giazotto

Publications and source records attributed to Francesco Giazotto.

At least 73 records · Page 4Linked to original sources

arXiv2023

Thermoelectric signatures of order-parameter symmetries in iron-based superconducting tunnel junctions

Thermoelectrical properties are frequently used to characterize the materials and endow the free energy from wasted heat for useful purposes. Here, we show that linear thermoelectric effects in tunnel junctions with Fe-based superconductors, not only address the dominance between particle and hole states, but even provide information about the superconducting order parameter symmetry. In particular, we observe that nodal order parameters present a maximal thermoelectric effect at lower temperatures than for nodeless cases. Furthermore, we show also that superconducting tunnel junctions between Fe-based and BCS superconductors could provide a thermoelectric efficiency ZT exceeding 6 with a linear Seebeck coefficient around $S\approx 800\;μ\text{V/K}$ at a few Kelvin. These results pave the way to novel thermoelectric machines based on multi-band superconductors.

cond-mat.supr-con↗

arXiv2023

Josephson diode effect in monolithic dc-SQUIDs based on 3D Dayem nanobridges

It was recently experimentally proved that the superconducting counterpart of a diode, i.e., a device that realizes nonreciprocal Cooper pairs transport, can be realized by breaking the spatial and time-reversal symmetry of a system simultaneously. Here we report the theory, fabrication, and operation of a monolithic dc superconducting quantum interference device (dc-SQUID) that embedding three-dimensional (3D) Dayem nanobridges as weak links realizes an efficient and magnetic flux-tunable supercurrent diode. The device is entirely realized in Al and achieves a maximum rectification efficiency of $\sim 20\%$, which stems from the high harmonic content of its current-to-phase relation only without the need of any sizable screening current caused by a finite loop inductance. Our interferometer can be easily integrated with state-of-the-art superconducting electronics, and since it does not require a finite loop inductance to provide large rectification its downsizing is not limited by the geometrical constraints of the superconducting ring.

cond-mat.supr-con↗

arXiv2023

Bipolar thermoelectric superconducting single-electron transistor

Thermoelectric effects in normal metals and superconductors are usually very small due to the presence of electron-hole symmetry. Here, we show that superconducting junctions brought out of equilibrium manifest a sizable bipolar thermoelectric effect that stems from a strong violation of the detailed balance. To fully control the effect, we consider a thermally biased SIS'IS junction where the capacitance of the central S' region is small enough to establish a Coulomb blockade regime. By exploiting charging effects we are able to tune the Seebeck voltage, the thermocurrent, and thereby the power output of this structure, via an external gate. We then analyse the main figures of merit of bipolar thermoelectricity and we prospect for possible applications.

cond-mat.supr-con↗

arXiv2023

Bipolar Thermoelectricity in Bilayer-Graphene--Superconductor Tunnel Junctions

We investigate the thermoelectric properties of a hybrid nanodevice composed by a 2D carbon based material and a superconductor. This system presents nonlinear bipolar thermoelectricity as induced by the spontaneous breaking of the Particle-Hole (PH) symmetry in a tunnel junction between a BiLayer Graphene (BLG) and a Bardeen-Cooper-Schrieffer (BCS) superconductor. In this scheme, the nonlinear thermoelectric effect, predicted and observed in SIS' junctions is not affected by the competitive effect of the Josephson coupling. From a fundamental perspective, the most intriguing feature of this effect is its bipolarity. The capability to open and control the BLG gap guarantees improved thermoelectric performances, that reach up to 1 mV/K regarding the Seebeck coefficient and a power density of 1 nW/$μ$m$^2$ for temperature gradients of tens of Kelvins. Furthermore, the externally controlled gating can also dope the BLG, which is otherwise intrinsically PH symmetric, giving us the opportunity to investigate the bipolar thermoelectricity even in presence of a controlled suppression of the PH symmetry. The predicted robustness of this system could foster further experimental investigations and applications in the near future, thanks to the available techniques of nano-fabrication.

cond-mat.mes-hall↗

arXiv2023

Half-integer Shapiro steps in highly transmissive InSb nanoflag Josephson junctions

We investigate a ballistic InSb nanoflag-based Josephson junction with Nb superconducting contacts. The high transparency of the superconductor-semiconductor interfaces enables the exploration of quantum transport with parallel short and long conducting channels. Under microwave irradiation, we observe half-integer Shapiro steps that are robust to temperature, suggesting their possible non-equilibrium origin. Our results demonstrate the potential of ballistic InSb nanoflags Josephson junctions as a valuable platform for understanding the physics of hybrid devices and investigating their non-equilibrium dynamics.

cond-mat.supr-con↗

arXiv2023

A highly-sensitive broadband superconducting thermoelectric single-photon detector

We propose a passive single-photon detector based on the bipolar thermoelectric effect occurring in tunnel junctions between two different superconductors thanks to spontaneous electron-hole symmetry breaking. Our thermoelectric detector (TED) converts a finite temperature difference caused by the absorption of a single photon into an open circuit thermovoltage. Designed with feasible parameters, our TED is able to reveal single-photons of frequency ranging from about 15 GHz to about 150 PHz depending on the chosen design and materials. In particular, this detector is expected to show values of signal-to-noise ratio SNR about 15 at ν = 50 GHz when operated at a temperature of 10 mK. Interestingly, this device can be viewed as a digital single-photon detector, since it generates an almost constant voltage VS for the full operation energies. Our TED can reveal single photons in a frequency range wider than 4 decades with the possibility to discern the energy of the incident photon by measuring the time persistence of the generated thermovoltage. Its broadband operation suggests that our TED could find practical applications in several fields of quantum science and technology, such as quantum computing, telecommunications, optoelectronics, THz spectroscopy and astro-particle physics.

cond-mat.mes-hall↗

arXiv2022

A gate- and flux-controlled supercurrent diode

Non-reciprocal charge transport in supercurrent diodes (SDs) polarized growing interest in the last few years for its potential applications in superconducting electronics (SCE). So far, SD effects have been reported in complex hybrid superconductor/semiconductor structures or metallic systems subject to moderate magnetic fields, thus showing a limited potentiality for practical applications in SCE. Here, we report the design and the realization of a monolithic SD by exploiting a Dayem bridge-based superconducting quantum interference device (SQUID). Our structure allows reaching rectification efficiencies ($η$) up to about 6%. Moreover, the absolute value and the polarity of $η$ can be selected on demand by the modulation of an external magnetic flux or by a gate voltage, thereby guaranteeing high versatility and improved switching speed. Furthermore, our SD operates in a wide range of temperatures up to about the 70% of the superconducting critical temperature of the titanium film composing the interferometer. Our SD can find extended applications in SCE by operating in synergy with widespread superconducting technologies, such as nanocryotrons, rapid single flux quanta (RSFQs) and memories.

cond-mat.mes-hall↗

arXiv2022

Hybrid normal-superconducting Aharonov-Bohm quantum thermal device

We propose and theoretically investigate the behavior of a ballistic Aharonov-Bohm (AB) ring when embedded in a N-S two-terminal setup, consisting of a normal metal (N) and superconducting (S) leads. This device is based on available current technologie and we show in this work that it constitutes a promising hybrid quantum thermal device, as quantum heat engine and quantum thermal rectifier. Remarkably, we evidence the interplay of single-particle quantum interferences in the AB ring and of the superconducting properties of the structure to achieve the hybrid operating mode for this quantum device. Its efficiency as a quantum heat engine reaches $55\%$ of the Carnot efficiency, and we predict thermal rectification factor attaining $350\%$. These predictions make this device highly promising for future phase-coherent caloritronic nanodevices.

cond-mat.mes-hall↗

arXiv2022

Phase-control of bipolar thermoelectricity in Josephson tunnel junctions

Not so long ago, thermoelectricity in superconductors was believed to be possible only by breaking explicitly the particle-hole symmetry. Recently, it has been theoretically predicted that a superconducting tunnel junction can develop bipolar thermoelectric phenomena in the presence of a large thermal gradient owing to non-equilibrium spontaneous PH symmetry breaking. The experimental realization of the first thermoelectric Josephson engine then followed. Here, we give a more extended discussion and focus on the impact of the Josephson contribution on thermoelectricity modulating the Cooper pairs transport in a double-loop SQUID. When the Cooper pairs current prevails on the quasiparticle one, the Josephson contribution short-circuits the junction thereby screening the thermoelectric effect. We demonstrate that the thermoelectric generation due to the pure quasiparticle transport is phase-independent, once Josephson contribution is appropriately removed from the net current measured. At the same time, we investigate an additional metastable state at V\simeq0 determined by the presence of the Josephson coupling, which peculiarly modifies the hysteretic behavior of our thermoelectric engine realized. At the end, we also discuss how the current-voltage characteristics are affected by the presence of multiple thermoelectric elements, which improve the generated output power.

cond-mat.supr-con↗

arXiv2022

Temperature-biased double-loop Josephson flux transducer

We theoretically study the behavior of the critical current of a thermally-biased tunnel Josephson junction with a particular design, in which the electrodes of the junction are enclosed in two different superconducting loops pierced by independent magnetic fluxes. In this setup, the superconducting gaps can be modified independently through the magnetic fluxes threading the loops. We investigate the response of the device as a function of the magnetic fluxes, by changing the asymmetry parameter, i.e., the ratio between the zero-temperature superconducting gaps $δ=Δ_{10}/Δ_{20}$, and the temperatures of the two rings. We demonstrate a magnetically controllable step-like response of the critical current, which emerges even in a symmetric junction, $δ=1$. Finally, we discuss the optimal working conditions and the high response of the critical current to small changes in the magnetic flux, reporting good performances of the transducer, with a high transfer function that depends on the operating point and the quality of the junction.

cond-mat.supr-con↗

arXiv2022

Josephson Diode Effect in High Mobility InSb Nanoflags

We report evidence of non-reciprocal dissipation-less transport in single ballistic InSb nanoflag Josephson junctions, owing to a strong spin-orbit coupling. Applying an in-plane magnetic field, we observe an inequality in supercurrent for the two opposite current propagation directions. This demonstrates that these devices can work as Josephson diodes, with dissipation-less current flowing in only one direction. For small fields, the supercurrent asymmetry increases linearly with the external field, then it saturates as the Zeeman energy becomes relevant, before it finally decreases to zero at higher fields. We show that the effect is maximum when the in-plane field is perpendicular to the current vector, which identifies Rashba spin-orbit coupling as the main symmetry-breaking mechanism. While a variation in carrier concentration in these high-quality InSb nanoflags does not significantly influence the diode effect, it is instead strongly suppressed by an increase in temperature. Our experimental findings are consistent with a model for ballistic short junctions and show that the diode effect is intrinsic to this material. Our results establish InSb Josephson diodes as a useful element in superconducting electronics.

cond-mat.supr-con↗

arXiv2022

Ultra linear magnetic flux-to-voltage conversion in superconducting quantum interference proximity transistors

Superconducting interferometers are quantum devices able to transduce a magnetic flux into an electrical output with excellent sensitivity, integrability and power consumption. Yet, their voltage response is intrinsically non-linear, a limitation which is conventionally circumvented through the introduction of compensation inductances or by the construction of complex device arrays. Here we propose an intrinsically-linear flux-to-voltage mesoscopic transducer, called bi-SQUIPT, based on the superconducting quantum interference proximity transistor as fundamental building block. The bi-SQUIPT provides a voltage-noise spectral density as low as $\sim10^{-16}$ V/Hz$^{1/2}$ and, more interestingly, under a proper operation parameter selection, exhibits a spur-free dynamic range as large as $\sim60$ dB, a value on par with that obtained with state-of-the-art SQUID-based linear flux-to-voltage superconducting transducers. Furthermore, thanks to its peculiar measurement configuration, the bi-SQUIPT is tolerant to imperfections and non-idealities in general. For the above reasons, we believe that the bi-SQUIPT could provide a relevant step-beyond in the field of low-dissipation and low-noise current amplification with a special emphasis on applications in cryogenic quantum electronics.

cond-mat.supr-con↗

arXiv2022

Single charge transport in a fully superconducting SQUISET locally tuned by self-inductance effects

We present a single-electron device for the manipulation of charge states via quantum interference in nanostructured electrodes. Via self-inductance effects, we induce two independent magnetic fluxes in the electrodes and we demonstrate sensitivity to single charge states and magnetic field at variable temperature. Moreover, our approach allows us to demonstrate local and independent control of the single-particle conductance between nano-engineered tunnel junctions in a fully-Superconducting Quantum Interference Single-Electron Transistor (SQUISET), thereby increasing the flexibility of our single-electron transistors. Our devices show a robust modulation of the current-to-flux transfer function via control currents, while exploiting the single-electron filling of a mesoscopic superconducting island. Further applications of the device concept to single-charge manipulation and magnetic-flux sensing are also discussed.

cond-mat.supr-con↗

arXiv2022

Thermal superconducting quantum interference proximity transistor

Superconductors are known to be excellent thermal insulators at low temperature owing to the presence of the energy gap in their density of states (DOS). In this context, the superconducting \textit{proximity effect} allows to tune the local DOS of a metallic wire by controlling the phase bias ($φ$) imposed across it. As a result, the wire thermal conductance can be tuned over several orders of magnitude by phase manipulation. Despite strong implications in nanoscale heat management, experimental proofs of phase-driven control of thermal transport in superconducting proximitized nanostructures are still very limited. Here, we report the experimental demonstration of efficient heat current control by phase tuning the superconducting proximity effect. This is achieved by exploiting the magnetic flux-driven manipulation of the DOS of a quasi one-dimensional aluminum nanowire forming a weal-link embedded in a superconducting ring. Our thermal superconducting quantum interference transistor (T-SQUIPT) shows temperature modulations up to $\sim 16$ mK yielding a temperature-to-flux transfer function as large as $\sim 60$ mK/$Φ_0$. Yet, phase-slip transitions occurring in the nanowire Josephson junction induce a hysteretic dependence of its local DOS on the direction of the applied magnetic field. Thus, we also prove the operation of the T-SQUIPT as a phase-tunable \textit{thermal memory}, where the information is encoded in the temperature of the metallic mesoscopic island. Besides their relevance in quantum physics, our results are pivotal for the design of innovative coherent caloritronics devices such as heat valves and temperature amplifiers suitable for thermal logic architectures.

cond-mat.mes-hall↗

arXiv2022

Frustration driven Josephson phase dynamics

The Josephson equations predict remarkable effects concerning the phase state of a superconducting junction with an oscillating current induced by a static voltage. Whether the paradigm can be twisted by yielding an oscillating voltage without making use of harmonic drives is a fundamentally relevant problem yet not fully settled. Here, we demonstrate that a dynamical regime with an oscillating phase evolution is a general hallmark of driven Josephson systems exhibiting sign competition in the Josephson couplings. We show that in frustrated Josephson systems an oscillating phase dynamics gets switched on by driving the changeover among different ground states, which can be induced by varying the parameters that set the phase state. Remarkably, the character of the transitions in the Josephson phase space allows different types of dynamics, with few or several harmonics. This result sets out a characteristic mark of any superconducting system with frustrated Josephson couplings and can be exploited to disentangle the complexity of the underlying phases.

cond-mat.supr-con↗

arXiv2022

Materials and devices for fundamental quantum science and quantum technologies

Technologies operating on the basis of quantum mechanical laws and resources such as phase coherence and entanglement are expected to revolutionize our future. Quantum technologies are often divided into four main pillars: computing, simulation, communication, and sensing & metrology. Moreover, a great deal of interest is currently also nucleating around energy-related quantum technologies. In this Perspective, we focus on advanced superconducting materials, van der Waals materials, and moiré quantum matter, summarizing recent exciting developments and highlighting a wealth of potential applications, ranging from high-energy experimental and theoretical physics to quantum materials science and energy storage.

quant-ph↗

arXiv2021

Ultra-Highly Linear Magnetic Flux-to-Voltage response in Proximity-based Mesoscopic bi-SQUIDs

Superconducting double-loop interferometers (bi-SQUIDs) have been introduced to produce magnetic flux sensors specifically designed to exhibit ultra-highly linear voltage response as a function of the magnetic flux. These devices are very important for the quantum sensing and for signal processing of signals oscillating at the radio-frequencies range of the electromagnetic spectrum. Here, we report an Al double-loop bi-SQUIDs based on proximitized mesoscopic Cu Josephson junctions. Such a scheme provides an alternative fabrication approach to conventional tunnel junction-based interferometers, where the junction characteristics and, consequently, the magnetic flux-to-voltage and magnetic flux-to-critical current device response can be largely and easily tailored by the geometry of the metallic weak-links. We discuss the performance of such sensors by showing a full characterization of the device switching current and voltage drop \textit{vs.} magnetic flux for temperatures of operation ranging from 30 mK to $\sim 1$ K. The figure of merit of the transfer function and of the total harmonic distortion are also discussed. The latter provides an estimate of the linearity of the flux-to-voltage device response, which obtained values as large as 45 dB. Such a result let us foresee a performance already on pair with that achieved in conventional tunnel junction-based bi-SQUIDs arrays composed of hundreds of interferometers.

cond-mat.supr-con↗

arXiv2021

Zero magnetic-field orbital vortices in s-wave spin-singlet superconductors

Breaking of time-reversal and point-group spatial symmetries can have a profound impact on superconductivity. One of the most extraordinary effects, due to the application of a magnetic field, is represented by the Abrikosov vortices with charged supercurrents circulating around their cores. Whether a similar phenomenon can be obtained by exploiting spatial symmetry breaking, e.g. through electric fields or mechanical strain, is a fundamentally relevant but not yet fully settled problem. Here, we show that in two-dimensional spin-singlet superconductors with unusually low degree of spatial symmetry content, vortices with supercurrents carrying angular momentum around the core can form and be energetically stable. The vortex has zero net magnetic flux since it is made up of counter-propagating Cooper pairs with opposite orbital moments. By solving self-consistently the Bogoliubov - de Gennes equations in real space, we demonstrate that the orbital vortex is stable and we unveil the spatial distribution of the superconducting order parameter around its core. The resulting amplitude has a characteristic pattern with a pronounced angular anisotropy that deviates from the profile of conventional magnetic vortices. These hallmarks guide predictions and proposals for the experimental detection.

cond-mat.supr-con↗