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Recent studies have shown the potential for bipolar thermoelectricity in superconducting tunnel junctions with asymmetric energy gaps. The thermoelectric performance of these systems is significantly impacted by the inverse proximity effects present in the normal-superconducting bilayer, which is utilized to adjust the gap asymmetry in the junction. Here, we identify the most effective bilayer configurations, and we find that directly tunnel-coupling the normal metal side of the cold bilayer with the hot superconductor is more advantageous compared to the scheme used in experiments. By utilizing quasiclassical equations, we examined the nonlinear thermoelectric junction performance as a function of the normal metal film thickness and the quality of the normal-superconducting interface within the bilayer, thereby determining the optimal design to observe and maximize this nonequilibrium effect. Our results offer a roadmap to achieve improved thermoelectric performance in superconducting tunnel junctions, with promising implications for a number of applications., Comment: 6 pages, 3 figures

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 $\delta=\Delta_{10}/\Delta_{20}$, 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, $\delta=1$. 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., Comment: 12 pages, 9 figures

Asymmetric superconducting tunnel junctions with gaps $\Delta_1>\Delta_2$ have been proven to show a peculiar nonlinear bipolar thermoelectric effect. This arises due to the spontaneous breaking of electron-hole symmetry in the system, and it is maximized at the matching-peak bias $|V|=V_p=(\Delta_1-\Delta_2)/e$. In this paper, we investigate the interplay of photon-assisted tunneling (PAT) and bipolar thermoelectric generation. In particular, we show how thermoelectricity, at the matching peak, is supported by photon absorption/emission processes at the frequency-shifted sidebands $V=\pm V_p+n\hbar\omega$, $n \in \mathbb{Z}$. This represents a sort of microwave-assisted thermoelectricity. We show the existence of multiple stable solutions, being associated with different photon sidebands, when a load is connected to the junction. Finally, we discuss how the nonlinear cooling effects are modified by the PAT. The proposed device can detect millimeter wavelength signals by converting a temperature gradient into a thermoelectric current or voltage., Comment: 8 pages, 3 figures

Diodes are key elements for electronics, optics, and detection. The search for a material combination providing the best performances for the required application is continuously ongoing. Here, we present a superconducting spintronic tunnel diode based on the strong spin filtering and splitting generated by an EuS thin film between a superconducting Al and a normal metal Cu layer. The Cu/EuS/Al tunnel junction achieves a large rectification (up to $\sim40$\%) already for a small voltage bias ($\sim 200$ $\mu$V) thanks to the small energy scale of the system: the Al superconducting gap. With the help of an analytical theoretical model we can link the maximum rectification to the spin polarization of the barrier and describe the quasi-ideal Schottky-diode behavior of the junction. This cryogenic spintronic rectifier is promising for the application in highly-sensitive radiation detection for which two different configurations are evaluated. In addition, the superconducting diode may pave the way for future low-dissipation and fast superconducting electronics., Comment: 17 pages, 12 figures

In the linear regime, thermo-electric effects between two conductors are possible only in the presence of an explicit breaking of the electron-hole symmetry. We consider a tunnel junction between two electrodes and show that this condition is no longer required outside the linear regime. In particular, we demonstrate that a thermally-biased junction can display an absolute negative conductance (ANC), and hence thermo-electric power, at a small but finite voltage bias, provided that the density of states of one of the electrodes is gapped and the other is monotonically decreasing. We consider a prototype system that fulfills these requirements, namely a tunnel junction between two different superconductors where the Josephson contribution is suppressed. We discuss this nonlinear thermo-electric effect based on the spontaneous breaking of electron-hole symmetry in the system, characterize its main figures of merit and discuss some possible applications., Comment: biblio information updated

A superconductor with a spin-split excitation spectrum behaves as an ideal ferromagnetic spin-injector in a tunneling junction. It was theoretical predicted that the combination of two such spin-split superconductors with independently tunable magnetizations, may be used as an ideal $absolute$ spin-valve. Here we report on the first switchable superconducting spin-valve based on two EuS/Al bilayers coupled through an aluminum oxide tunnel barrier. The spin-valve shows a relative resistance change between the parallel and antiparallel configuration of the EuS layers up to 900% that demonstrates a highly spin-polarized currents through the junction. Our device may be pivotal for realization of thermoelectric radiation detectors, logical element for a memory cell in cryogenics superconductor-based computers and superconducting spintronics in general., Comment: 6 pages, 4 color figures, 1 table

Resumen del trabajo presentado al 1st Workshop Spain-Taiwan: "2D Materials and Interfaces for Spintronics", celebrado en Barcelona (España) del 23 al 25 de octubre de 2017., Spin-dependent fields acting on superconductivity lead to intriguing phenomena. Under equilibrium conditions these phenomena are attributed to the singlet-triplet conversion of the superconducting condensate. In this talk I first discuss the effect of spin-orbit coupling (SOC) and exchange fields on the properties of hybrid superconducting structures. I distinguish between intrinsic and extrinsic SOC and predict non-dissipative Spin-Hall, Anomalous-Hall, Edelstein and Spin-Galvanic effects in analogy to normal systems with different types of SOC. These predictions open new opportunities for the development of low-dissipation magneto-electronics. In the second part of the talk, I will focus on charge and heat currents in superconducting structures with ferromagnetic insulators (FI). The spin-splitting of the BCS density of the states induced in the superconductor leads to striking phenomena which I will illustrate by briefly discussing few examples: (i) a huge thermoelectric effect in a N-FI-S structure, (ii) the occurrence of a thermophase in S-FI-S structures, (iii) the possibility of using FI-S in high sensitive detectors and thermometers[6], and (iv) a heat valve based on a ferromagnetic Josephson junction.

Resumen del trabajo presentado al 13th International Workshop on Magnetism & Superconductivity at the Nanoscale, celebrado en Coma-Ruga, El Vendrell (España) del 2 al 7 de julio de 2017., A renewed interest in studying Ferromagnetic/superconductor structures came with the development of superconducting spintronics. Ferromagnetic insulators (FIs) attached to a superconductor are known to induce triplet superconducting pairing and an exchange energy splitting in the Bardeen-Cooper-Schrieffer (BCS) density of states proportional to the FI magnetization, and penetrating into the superconductor to a depth comparable with the superconducting coherence length. Experiments carried out at the end of 80's have demonstrated that the exchange field of FIs, such as EuS and EuO, can very effectively split the excitation spectrum of an adjacent superconductor (S), such as an Al thin film. Here we investigate this long-range magnetic proximity effect in EuS/Al bilayers tunnel coupled to an Al probing layer. Surprisingly the tunneling spectroscopy of our devices reveals that the exchange splitting of the BCS peaks is present even in the unpolarized state of the EuS, and is being further enhanced when magnetizing the sample by a magnetic field. The measurement data taken at the lowest temperatures (30 mK) feature a high contrast which has allowed us to relate the line shape of the split BCS conductance peaks to the characteristic magnetic domain structure of the EuS. By means of a microscopic model based on the quasiclassical Green's functions, we provided and exhaustive description of the magnetic proximity effect in EuS/Al bilayers strongly affected not only by the average magnetization but also by the domain structure and size of the FI. These results pave the way to engineering triplet superconductivity in EuS/Al bilayers. Furthermore, the hard gap and clear intrinsic spin-splitting observed in our tunneling spectroscopy measurements at zero field indicate that EuS/Al bilayers are an excellent platform for the development of devices requiring the coexistence of superconducting correlations and built-in spin-splitting exchange fields, as for example in the field of Majorana-based quantum computation.

Multi-terminal superconducting Josephson junctions based on the proximity effect offer the bright opportunity to tailor non trivial quantum states in nanoscale weak-links. These structures can realize exotic topologies in multidimensions as, for example, artificial topological superconductors able to support Majorana bound states, and pave the way to emerging quantum technologies and future quantum information schemes. Here, we report the first realization of a three-terminal Josephson interferometer based on a proximized nanosized weak-link. Our tunneling spectroscopy measurements reveal transitions between gapped (i.e., insulating) and gapless (i.e., conducting) states, those being controlled by the phase configuration of the three superconducting leads connected to the junction. We demonstrate the $topological$ nature of these transitions: a gapless state necessarily occurs between two gapped states of different topological index, very much like the interface between two insulators of different topology is necessarily conducting. The topological numbers characterizing such gapped states are given by superconducting phase windings over the two loops forming the Josephson interferometer. Since these gapped states cannot be transformed to one another continuously withouth passing through a gapless condition, these are topologically $protected$. Our observation of the gapless state is pivotal for enabling phase engineering of more sophisticated artificial topological materials realizing Weyl points or the anomalous Josephson effect., 10 pages, 8 color figures

We report the fabrication and characterization of an alternative design for a superconducting quantum interference proximity transistor (SQUIPT) based on a normal metal (N) probe. The absence of direct Josephson coupling between the proximized metal nanowire and the N probe allows us to observe the full modulation of the wire density of states around zero voltage and current \textit{via} the application of an external magnetic field. This results into a drastic suppression of power dissipation which can be as low as a few $\sim 10^{-17}$ W. In this context the interferometer allows an improvement of up to four orders of magnitude with respect to earlier SQUIPT designs, and makes it ideal for extra-low power cryogenic applications. In addition, the N-SQUIPT has been recently predicted to be the enabling candidate for the implementation of coherent caloritronic devices based on proximity effect., Comment: 5 pages, 4 color figures

The first evidence of the dc Josephson effect dates back to 1963 when J. S. Rowell measured the diffraction pattern of the critical current flowing through a single superconducting tunnel junction subjected to an in-plane magnetic field. Interference of Josephson currents through two tunnel junctions connected in parallel was achieved one year later leading to the first ever superconducting quantum interferometer. The latter, together with Rowell's observations, constituted the unequivocal demonstration of the Josephson supercurrent-phase relation. Yet, the Josephson effect has further profound implications going beyond electrical transport, as the interplay between the Cooper condensate and unpaired electrons provides thermal flow through the junction with phase coherence as well. Here we report the first demonstration of quantum diffraction of a heat flux showing that a temperature-biased single Josephson junction is exploited as a diffractor for thermal currents. Specifically, thermal diffraction manifests itself with a peculiar modulation of the electron temperature in a small metallic electrode nearby-contacted to the junction when sweeping the magnetic flux $\Phi$. Remarkably, the observed temperature dependence exhibits $\Phi$-symmetry and a clear reminiscence with a Fraunhofer-like modulation pattern, as expected fingerprints for a quantum diffraction phenomenon. Our results confirm a pristine prediction of quantum heat transport and, joined with double-junction heat interferometry demonstrated in Nature 492, 401 (2012), exemplify the complementary and conclusive proof of the existence of phase-dependent thermal currents in Josephson-coupled superconductors. This approach combined with well-known methods for phase-biasing superconducting circuits provides with a novel tool for mastering heat fluxes at the nanoscale., Comment: 5.5 pages, 4 color figures

Resonant transport is demonstrated in a hybrid superconductor-semiconductor heterostructure junction grown by molecular beam epitaxy on GaAs. This heterostructure realizes the model system introduced by de Gennes and Saint-James in 1963 [P. G. de Gennes and D. Saint-James, Phys. Lett. 4, 151 (1963)]. At low temperatures a single marked resonance peak is shown superimposed to the characteristic Andreev-dominated subgap conductance. The observed magnetotransport properties are successfully analyzed within the random matrix theory of quantum transport, and ballistic effects are included by directly solving the Bogoliubov–de Gennes equations.

We propose a heat valve based on the interplay between thermal transport and proximity-induced exchange splitting in Josephson tunnel junctions. We demonstrate that the junction heat conductance strongly depends on the relative alignment of the exchange fields induced in the superconductors. Colossal magneto-heat resistance ratios as large as 10^7% are predicted to occur under proper temperature and phase conditions, as well as suitable ferromagnet-superconductor combinations. Moreover, the quantum phase tailoring, intrinsic to the Josephson coupling, offers an additional degree of freedom for the control of the heat conductance. Our predictions for the phase-coherent and spin-dependent tuning of the thermal flux can provide a useful tool for heat management at the nanoscale., Comment: 4.5 pages, 3 color figures

The magnetic-field evolution of Coulomb blockade peaks in lateral In0.75Ga0.25As/In0.75Al0.25As quantum dots in the few-electron regime is reported. Quantum dots are defined by gates evaporated onto a 60-nm-thick hydrogen silsesquioxane insulating film. A gyromagnetic factor g(*)approximate to 4.4 is measured via zero-bias spin spectroscopy and a transition from singlet to triplet spin configuration is found at an in-plane magnetic field B=0.7 T. This observation opens the way to the manipulation of singlet and triplet states at moderate fields and its relevance for quantum information applications will be discussed.

The magnetic-field evolution of Coulomb blockade peaks in lateral InGaAs/InAlAs quantum dots in the few-electron regime is reported. Quantum dots are defined by gates evaporated onto a 60 nm-thick hydrogen silsesquioxane insulating film. A gyromagnetic factor of 4.4 is measured via zero-bias spin spectroscopy and a transition from singlet to triplet spin configuration is found at an in-plane magnetic field B = 0.7 T. This observation opens the way to the manipulation of singlet and triplet states at moderate fields and its relevance for quantum information applications will be discussed., Comment: 4 pages, 3 figures

At a superconductor (S)-normal metal (N) junction pairing correlations can "leak-out" into the N region. This proximity effect [1, 2] modifies the system transport properties and can lead to supercurrent flow in SNS junctions [3]. Recent experimental works showed the potential of semiconductor nanowires (NWs) as building blocks for nanometre-scale devices [4-7], also in combination with superconducting elements [8-12]. Here, we demonstrate an InAs NW Josephson transistor where supercurrent is controlled by hot-quasiparticle injection from normal-metal electrodes. Operational principle is based on the modification of NW electron-energy distribution [13-20] that can yield reduced dissipation and high-switching speed. We shall argue that exploitation of this principle with heterostructured semiconductor NWs opens the way to a host of out-of-equilibrium hybrid-nanodevice concepts [7, 21]., Comment: 6 pages, 6 color figures

We study theoretically the electronic entropy and specific heat in diffusive superconductor-normal metal-superconductor (SNS) Josephson junctions. In particular, we consider the influence of non-idealities occurring in an actual experiment, such as the presence of barriers at the NS interfaces, the spin-flip and inelastic scattering in the N region and quasiparticle subgap states in the superconductors. We find that spin-flip and inelastic scattering do not have, for typical parameters values, a large effect. On the contrary, the presence of barriers suppresses the superconducting correlations in the N region, with the consequence that the entropy and the specific heat get reduced eventually to those in the absence of superconductivity for opaque interfaces. Finally we suggest an experiment and check that it is possible, under realistic conditions, to measure the dependence of electronic specific heat on the phase difference between the superconductors., Comment: 8 pages, 10 color figures

Spin-polarized transport is investigated in normal metal-superconductor (NS) junctions as a function of interface transmissivity as well as temperature when the density of states of a superconductor is Zeeman-split in response to an exchange field (h_exc). Similarly to the "absolute spin-valve effect" predicted by D. Huertas-Hernando et al. [Phys. Rev. Lett. 88, 047003 (2002)] in superconducting proximity structures, we show that NS junctions can be used to generate highly spin-polarized currents, in alternative to half-metallic ferromagnets. In particular, the spin-polarized current obtained is largely tunable in magnitude and sign by acting on bias voltage and h_exc. While for tunnel contacts the current polarization can be as high as 100%, for transparent junctions it is dominated by the minority spin species. The effect can be enhanced by electron "cooling" provided by the superconducting gap., Comment: 4 pages, 4 color figures, published version

We analyze the broad range of spin-dependent nonequilibrium transport properties of hybrid systems composed of a normal region tunnel coupled to two superconductors with exchange fields induced by the proximity to thin ferromagnetic layers and highlight its functionalities. By calculating the quasiparticle distribution functions in the normal region, we find that they are spin dependent and strongly sensitive to the relative angle between exchange fields in the two superconductors. The impact of inelastic collisions on their properties is addressed. As a result, the electric current flowing through the system is found to be strongly dependent on the relative angle between exchange fields, giving rise to a huge value of magnetoresistance. Moreover, the current presents a complete spin polarization in a wide range of bias voltages, even in the quasiequilibrium case. In the nonequilibrium limit, we parametrize the distributions with an �effective� temperature, which turns out to be strongly spin dependent, though quite sensitive to inelastic collisions. By tunnel coupling the normal region to an additional superconducting electrode, we show that it is possible to implement a spin-polarized current source of both spin species, depending on the bias voltages applied.

The factors limiting cooling electrons by biased tunnel junctions have been considered. At low temperature range the deviation from equilibrium, superconductor overheating and states within the superconductor gap limit the achievable minimum temperature. Strong increase of electron-phonon coupling at higher temperature limits the temperature range of effective electron cooling to sub-Kelvin region. Control of Josephson coupling in superconductor-normal metal-superconductor weak link by electron cooling in normal electrode has been demonstrated experimentally.

Electron-phase modulation in magnetic and electric fields will be presented in In_(0.75)Ga_(0.25)As Aharonov-Bohm (AB) rings. The zero Schottky barrier of this material made it possible to nanofabricate devices with radii down to below 200 nm without carrier depletion. We shall present a fabrication scheme based on wet and dry etching that yielded excellent reproducibility, very high contrast of the oscillations and good electrical gating. The operation of these structures is compatible with closed-cycle refrigeration and suggests that this process can yield coherent electronic circuits that do not require cryogenic liquids. The InGaAs/AlInAs heterostructure was grown by MBE on a GaAs substrate [1], and in light of the large effective g-factor and the absence of the Schottky barrier is a material system of interest for the investigation of spin-related effects [2-4]} and the realization of hybrid superconductor/semiconductor devices [5]., Comment: 7 pages, 4 figures, to appear in Physica E

We report on the achievement of a two-dimensional electron gas in completely undoped In[0.75]Al[0.25]As/In[0.75]Ga[0.25]As metamorphic quantum wells. Using these structures we were able to reduce the carrier density, with respect to reported values in similar modulation-doped structures. We found experimentally that the electronic charge in the quantum well is likely due to a deep-level donor state in the In[0.75]Al[0.25]As barrier band gap, whose energy lies within the In[0.75]Ga[0.25]As/In[0.75]Al[0.25]As conduction band discontinuity. This result is further confirmed through a Poisson-Schroedinger simulation of the two-dimensional electron gas structure., Comment: 17 pages, 6 figures, to be published in J. Vac. Sci. Technol. B

The Italian institute for nuclear physics (INFN) has financed the SIMP project (2019-2021) in order to strengthen its skills and technologies in the field of meV detectors with the ultimate aim of developing a single microwave photon detector. This goal will be pursued by improving the sensitivity and the dark count rate of two types of photodetectors: current biased Josephson Junction (JJ) for the frequency range 10-50 GHz and Transition Edge Sensor (TES) for the frequency range 30-100 GHz. Preliminary results on materials and devices characterization are presented., Comment: 7 pages, 4 figures

We discuss the charge and the spin tunneling currents between two Bardeen-Cooper-Schrieffer (BCS) superconductors, where one density of states is spin-split by the proximity of a ferromagnetic insulator. In the presence of a large temperature bias across the junction, we predict the generation of a spin-polarized thermoelectric current. This thermospin effect is the result of a spontaneous particle-hole symmetry breaking in the absence of any polarizing tunnel barrier. The two spin components, which move in opposite directions, generate a spin current larger than the purely polarized case when the thermoactive component dominates over the dissipative one.

Electronic cooling in hybrid normal metal-insulator-superconductor junctions is a promising technology for the manipulation of thermal loads in solid state nanosystems. One of the main bottlenecks for efficient electronic cooling is the electron-phonon coupling, as it represents a thermal leakage channel to the phonon bath. Graphene is a two-dimensional material that exhibits a weaker electron-phonon coupling compared to standard metals. For this reason, we study the electron cooling in graphene-based systems consisting of a graphene sheet contacted by two insulator/superconductor junctions. We show that, by properly biasing the graphene, its electronic temperature can reach base values lower than those achieved in similar systems based on metallic ultra-thin films. Moreover, the lower electron-phonon coupling is mirrored in a lower heat power pumped into the superconducting leads, thus avoiding their overheating and preserving the cooling mechanisms. Finally, we analyze the possible application of cooled graphene as a bolometric radiation sensor. We study its main figures of merit, i.e. responsivity, noise equivalent power and response time. In particular, we show that the built-in electron refrigeration allows reaching a responsivity of the order of 50 nA/pW and a noise equivalent power of order of $\rm 10^{-18}\, W\, Hz^{-1/2}$ while the response speed is about 10 ns, corresponding to a thermal bandwidth in the order of 20MHz., Comment: 19 pages, 9 figures

We suggest a single-photon thermal detector based on the abrupt jump of the critical current of a temperature-biased tunnel Josephson junction formed by different superconductors, working in the dissipationless regime. The electrode with the lower critical temperature is used as radiation sensing element, so it is kept thermally floating and is connected to an antenna. The warming up resulting from the absorption of a photon can induce a drastic measurable enhancement of the critical current of the junction. We propose a detection scheme based on a threshold mechanism for single- or multi-photon detection. This Josephson threshold detector has indeed calorimetric capabilities being able to discriminate the energy of the incident photon. So, for the realistic setup that we discuss, our detector can efficiently work as a calorimeter for photons from the mid infrared, through the optical, into the ultraviolet, specifically, for photons with frequencies in the range $[30-9\times10^4]\;\text{THz}$. In the whole range of detectable frequencies a resolving power significantly larger than one results. In order to reveal the signal, we suggest the fast measurement of the Josephson kinetic inductance. Indeed, the photon-induced change in the critical current affects the Josephson kinetic inductance of the junction, which can be non-invasively read through a LC tank circuit, inductively coupled to the junction. Finally, this readout scheme shows remarkable multiplexing capabilities., Comment: 15 pages, 11 figures

Boolean algebra, the branch of mathematics in which variables can assume only true or false values, is the theoretical basis of classical computation. The analogy between Boolean operations and electronic switching circuits, highlighted by Shannon in 1938, paved the way for modern computation based on electronic devices. The growth in the computational power of such devices, after an exciting exponential - Moore's trend - is nowadays blocked by heat dissipation due to computational tasks, which are very demanding due to the miniaturization of chips. Heat is often a detrimental form of energy which increases the system's entropy, decreasing the efficiency of logic operations. Here, we propose a physical system that is able to perform thermal-logic operations by reversing the old heat-disorder epitome into a heat-order paradigm. We lay the foundations of heat computation by encoding logic state variables in temperature and introducing the thermal counterparts of electronic logic gates. Exploiting quantum effects in thermally biased Josephson junctions (JJs), we propound a possible realization of a functionally complete logic. Our architecture ensures high operational stability and robustness, with switching frequencies reaching the GHz range.

Superconducting field-effect transitor (SuFET) and Josephson field-effect transistor (JoFET) technologies take advantage of electric field induced control of charge carrier concentration in order to modulate the channel superconducting properties. Despite field-effect is believed to be unaffective for superconducting metals, recent experiments showed electric field dependent modulation of the critical current (IC) in a fully metallic transistor. Yet, the grounding mechanism of this phenomenon is not completely understood. Here, we show the experimental realization of Ti-based Dayem bridge field-effect transistors (DB-FETs) able to control IC of the superconducting channel. Our easy fabrication process DB-FETs show symmetric full suppression of IC for an applied critical gate voltage as low as VCG~+-8V at temperatures reaching about the 85% of the record critical temperature TC~550mK for titanium. The gate-independent TC and normal state resistance (RN) coupled with the increase of resistance in the supercoducting state (RS) for gate voltages close to the critical value (VCG) suggest the creation of field-effect induced metallic puddles in the superconducting sea. Our devices show extremely high values of transconductance (gMAXm~15uA/V at VG~+-6.5V) and variations of Josephson kinetic inductance (LK) with VG of two orders of magnitude. Therefore, the DB-FET appears as an ideal candidate for the realization of superconducting electronics, superconducting qubits, tunable interferometers as well as photon detectors., 9 pages, 4 figures

Since the the first studies of thermodynamics, heat transport has been a crucial element for the understanding of any thermal system. Quantum mechanics has introduced new appealing ingredients for the manipulation of heat currents, such as the long-range coherence of the superconducting condensate. The latter has been exploited by phase-coherent caloritronics, a young field of nanoscience, to realize Josephson heat interferometers, which can control electronic thermal currents as a function of the external magnetic flux. So far, only one output temperature has been modulated, while multi-terminal devices that allow to distribute the heat flux among different reservoirs are still missing. Here, we report the experimental realization of a phase-tunable thermal router able to control the heat transferred between two terminals residing at different temperatures. Thanks to the Josephson effect, our structure allows to regulate the thermal gradient between the output electrodes until reaching its inversion. Together with interferometers, heat diodes and thermal memories, the thermal router represents a fundamental step towards the thermal conversion of non-linear electronic devices, and the realization of caloritronic logic components., 9 pages, 5 figures

In the last 60 years conventional solid and electrolyte gating allowed sizable modulations of the surface carrier concentration in metallic superconductors resulting in tuning their conductivity and changing their critical temperature. Recent conventional gating experiments on superconducting metal nano-structures showed full suppression of the critical current without variations of the normal state resistance and the critical temperature. These results still miss a microscopic explanation. In this article, we show a complete set of gating experiments on Ti-based superconducting Dayem bridges and a suggested classical thermodynamic model which seems to account for several of our experimental findings. In particular, zero-bias resistance and critical current IC measurements highlight the following: the suppression of IC with both polarities of gate voltage, the surface nature of the effect, the critical temperature independence from the electric field and the gate-induced growth of a sub-gap dissipative component. In addition, the temperature dependence of the Josephson critical current seems to show the transition from the ballistic Kulik-Omelyanchuck behavior to the Ambegaokar-Baratoff tunnel-like characteristic by increasing the electric field. Furthermore, the IC suppression persists in the presence of sizeable perpendicular-to-plane magnetic fields. We propose a classical thermodynamic model able to describe some of the experimental observations of the present and previous works. Above all, the model grabs the bipolar electric field induced suppression of IC and the emergence of a sub-gap dissipative component near full suppression of the supercurrent. Finally, applications employing the discussed effect are proposed., Comment: 13 pages, 7 figures

We propose a single photon detector based on a superconducting quantum interference device (SQUID) with superconductor-normal metal-superconductor Josephson weak links. One of the two Josephson junctions is connected to an antenna, and is heated when a photon is absorbed. The increase of the weak link temperature exponentially suppresses the Josephson critical current thereby inducing an asymmetry in the SQUID. This generates a voltage pulse across the SQUID that can be measured with a threshold detector. Realized with realistic parameters the device can be used as a single photon detector, and as a calorimeter since it is able to discriminate photons frequency above $5~$THz with a signal-to-noise ratio larger than $20$. The detector performance are robust with respect to working temperatures between $0.1~$ K and $0.5~$K, and thermal noise perturbation., Comment: 8 pages, 6 figures. Updated version

The analysis of the performance and the very understanding of the physical mechanisms governing the behavior of nanostructured devices requires the detailed knowledge of all parameters of electronic systems confined in ever shrinking microdomains. The role of temperature and the investigation of heat transport in solidstate nanoscale systems are currently under the spotlight [1]. Electronic thermometers [2‐4], refrigerators [5‐8] and heat transistors [9] based on metallic and superconductor nanostructures were already demonstrated. Such devices are typically operated at subkelvin temperatures, where electron-phonon interaction is weak and, as a result, the relevant electronic temperature may significantly dier from that of the lattice (if at all defined). In this context, the ability to perform local measurements of the electronic temperature becomes highly desirable. While in metallic systems superconducting tunnel junctions are routinely employed for this purpose, for the case of two-dimensional electron gases (2DEGs) or semiconductor nanowires at thermal equilibrium, quantum dots (QDs) were shown to be suitable for absolute thermometry [10, 11], due to the fact that in these conditions the linewidth of zero-bias Coulomb blockade (CB) peaks in the weak-coupling regime is directly related to the electronic temperature of the leads. Here, we propose a method for the detection of the local electronic temperature in a 2DEG, based on the analysis of the CB-peak lineshape. We shall first discuss zero-bias transport through a weakly-coupled QD in the presence of a temperature bias and show that zerobias conductance measurements can be used to experimentally determine both temperature values. We shall then employ this technique to investigate energy relaxation mechanisms in an electronic microdomain defined electrostatically in a GaAs/AlGaAs heterostructure and heated by an externally-driven current. The domain is connected to the surrounding 2DEG regions through the QD and three quantum point contacts (QPCs). As a known heating power is delivered to the domain, conductance across the QD is probed and the steady-state temperature detected. The dependence of the measured electron temperature on heating power will also be studied for dierent values of the QPC resistances: as the coupling to the surrounding 2DEG regions is reduced, we observe the crossover from a regime where excess heat is carried away by hot quasiparticles tunneling through the QPCs to one where power exchange with lattice phonons dominates. We shall show that the latter mechanism follows the T 5 power law expected [12] for the screened electron-acoustic phonon piezoelectric interaction, and obtain a measurement of the relative coupling constant consistent with theoretical estimates [1, 13].

Reflectionless-tunneling was observed in Nb/GaAs superconductor/semiconductor junctions fabricated through a two-step procedure. First, periodic $\delta$-doped layers were grown by molecular beam epitaxy near the GaAs surface, followed by an As cap layer to protect the surface during {\it ex-situ} transfer. Second, Nb was deposited by dc-magnetron sputtering onto the GaAs(001) 2 $\times$ 4 surface {\it in-situ} after thermal desorption of the cap layer. The magnetotransport behavior of the resulting hybrid junctions was successfully analyzed within the random matrix theory of phase-coherent Andreev transport. The impact of junction morphology on reflectionless tunneling and the applicability of the fabrication technique to the realization of complex superconductor/semiconductor mesoscopic systems are discussed., Comment: 10 pages, 3 figures, to be published in Appl. Phys. Lett

We study the influence of superconducting correlations on the electronic specific heat in a diffusive superconductor-normal metal-superconductor Josephson junction. We present a description of this system in the framework of the diffusive-limit Green's function theory, taking into account finite temperatures, phase difference as well as junction parameters. We find that proximity effect may lead to a substantial deviation of the specific heat as compared to that in the normal state, and that it can be largely tuned in magnitude by changing the phase difference between the superconductors. A measurement setup to confirm these predictions is also suggested., Comment: 4+ pages, 4 figures

In this paper we review several aspects of mesoscopic hybrid superconducting systems. In particular we consider charge and heat transport properties in hybrid superconducting-metal structures and the effect of charging energy in superconducting nanostructures., Comment: 27 pages, 8 figures

Highly transmissive ballistic junctions are demonstrated between two superconducting Nb electrodes and the two-dimensional electron gas formed at an InAs/AISb heterojunction. A reproducible fabrication protocol is presented yielding good critical supercurrent values. Current–voltage characteristics were measured down to 0.4 K and the observed supercurrent behavior was analyzed within a ballistic model in the clean limit. This investigation allows us to demonstrate an intrinsic interface transmissivity exceeding 86%.

We report the realization of a hybrid superconductor-quantum dot device by means of top-down nanofabrication starting from a two dimensional electron gas in a InGaAs/InAlAs semiconductor heterostructure. The quantum dot is defined by electrostatic gates placed within the normal region of a planar Nb-InGaAs quantum well-Nb junction. Measurements in the regime of strong Coulomb blockade as well as cotunneling spectroscopy allow to directly probe the proximity-induced energy gap in a ballistic two-dimensional electron gas coupled to superconductors., 4 pages, 3 figures

We study theoretically the electronic entropy and specific heat in diffusive superconductor-normal metal-superconductor Josephson junctions. In particular, we consider the influence of nonidealities occurring in an actual experiment, such as the presence of barriers at the normal (N) metal- superconductor interfaces, the spin-flip, and inelastic scattering in the N-metal region and quasiparticle subgap states in the superconductors. We find that spin-flip and inelastic scattering do not have, for typical parameters values, a large effect. On the contrary, the presence of barriers suppresses the superconducting correlations in the N region, with the consequence that the entropy and the specific heat get reduced eventually to those in the absence of superconductivity for opaque interfaces. Finally we suggest an experiment and check that it is possible, under realistic conditions, to measure the dependence of electronic specific heat on the phase difference between the superconductors. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3121210]