We consider properties of a fully superconducting variant of the superconducting quantum interference proximity transistor magnetic flux sensor. We study the density of states in a finite-size superconducting metal wire in the diffusive limit, and how it depends on the phase gradient of the order parameter. We describe the dependence on the junction length and interface transparency, and discuss properties relevant for using the structure in magnetic flux detection applications.
We present the realization and characterization of a novel-concept interferometer, the superconducting quantum interference proximity transistor (SQUIPT). Its operation relies on the modulation with the magnetic field of the density of states of a proximized metallic wire embedded in a superconducting ring. Flux sensitivities down to $\sim 10^{-5} Φ_0$Hz$^{-1/2}$ can be achieved even for a non-optimized design, with an intrinsic dissipation ($\sim 100$ fW) which is several orders of magnitude smaller than in conventional superconducting interferometers. Our results are in agreement with the th…
In this article we review the basic theoretical properties of a hybrid superconducting interferometer which exploits the phase dependence of the density of states of a metallic nanowire placed in good electric contact with a superconductor to achieve high sensitivity to magnetic flux. The operation of a prototype structure based on this principle, the superconducting quantum interference proximity transistor (SQUIPT), has been recently reported. Limited power dissipation joined with the opportunity to access single-spin detection make this interferometer attractive for the investigation of the…
The design for an inductive superconducting quantum interference proximity transistor with enhanced performance, the L-SQUIPT, is presented and analyzed. The interferometer is based on a double-loop structure, where each ring comprises a superconductor-normal metal-superconductor mesoscopic Josephson weak-link and the read-out electrode is implemented in the form of a superconducting tunnel probe. Our design allows both to improve the coupling of the transistor to the external magnetic field and to increase the characteristic magnetic flux transfer functions, thereby leading to an improved ult…
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 improvem…
Here we report the fabrication and characterization of fully superconducting quantum interference proximity transistors (SQUIPTs) based on the implementation of vanadium (V) in the superconducting loop. At low temperature, the devices show high flux-to-voltage (up to 0.52$\ \textrm{mV}/Φ_0$) and flux-to-current (above 12$\ \textrm{nA}/Φ_0$) transfer functions, with the best estimated flux sensitivity $\sim$2.6$\ μΦ_0/\sqrt{\textrm{Hz}}$ reached under fixed voltage bias, where $Φ_0$ is the flux quantum. The interferometers operate up to $T_\textrm{bath}\simeq$ 2 $ \textrm{K}$, with an improveme…
We demonstrate simultaneous measurements of DC transport properties and flux noise of a hybrid superconducting magnetometer based on the proximity effect (superconducting quantum interference proximity transistor, SQUIPT). The noise is probed by a cryogenic amplifier operating in the frequency range of a few MHz. In our non-optimized device, we achieve minimum flux noise $\sim 4\;μΦ_0/Hz^{1/2}$, set by the shot noise of the probe tunnel junction. The flux noise performance can be improved by further optimization of the SQUIPT parameters, primarily minimization of the proximity junction length …
We propose a device based on a topological Josephson junction where the helical edge states of a two-dimensional topological insulator are in close proximity to two superconducting leads. The presence of a magnetic flux through the junction leads to a Doppler shift in the spectrum of Andreev bound states, and affects the quantum interference between proximized edge states. We inspect the emergent features, accessing the density of states through a tunnel-coupled metallic probe, thus realizing a Topological Superconducting Quantum Interference Proximity Transistor (TSQUIPT). We calculate the ex…
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 tran…
This letter presents fabrication and characterization of an optimized SQUIPT (superconducting quantum interference proximity transistor). The present device, characterized with reduced tunnel junction area and shortened normal-metal section, demonstrates no hysteresis at low temperatures as we increased the Josephson inductance of the weak link by decreasing its cross section. It has consequently almost an order of magnitude improved magnetic field responsivity as compared to the earlier design. The modulation of both the current and the voltage across the junction have been measured as a func…
We present an exhaustive theoretical analysis of a double-loop Josephson proximity interferometer, as the one recently realized by Strambini et al. for the control of the Andreev spectrum via an external magnetic field. This system, called $ω$-SQUIPT, consists of a T-shaped diffusive normal metal (N) attached to three superconductors (S) forming a double loop configuration. By using the quasiclassical Green function formalism, we calculate the local normalized density of states, the Josephson currents through the device and the dependence of the former on the length of the junction arms, the a…
We report the design and implementation of a high-performance superconducting quantum interference proximity transistor (SQUIPT) based on aluminum-copper (Al-Cu) technology. With the adoption of a thin and short copper nanowire we demostrate full phase-driven modulation of the proximity-induced minigap in the normal metal density of states. Under optimal bias we record unprecedently high flux-to-voltage (up to 3 mV/$Φ_0$) and flux-to-current (exceeding 100 nA/$Φ_0$) transfer function values at sub-Kelvin temperatures, where $Φ_0$ is the flux quantum. The best magnetic flux resolution (as low a…
We theoretically investigate the temperature-to-phase conversion (TPC) process occurring in dc superconducting quantum interferometers based on superconductor--normal metal--superconductor (SNS) mesoscopic Josephson junctions. In particular, we predict the temperature-driven rearrangement of the phase gradients in the interferometer under the fixed constraints of fluxoid quantization and supercurrent conservation. This allows sizeable phase variations across the junctions for suitable structure parameters and temperatures. We show that the TPC can be a basis for sensitive single-photon sensors…
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 s…
We consider the superconducting vortex solid-to-liquid transition in heavy ion-irradiated untwinned YBa2Cu3O7-d single crystals in the case where the magnetic field direction does not coincide with that of the irradiation-induced linear columnar defects. For a certain range of angles, the resistivities measured in three orthogonal spatial directions vanish at the transition as three clearly different powers of reduced temperature. At previously known second-order phase transitions, scaling of physical quantities has either been isotropic or anisotropic in one direction. Thus, our findings yiel…
The superconducting state in a fully frustrated wire network with the dice lattice geometry is investigated in the vicinity of the transition temperature. Using Abrikosov's variational procedure, we write the Ginzburg-Landau free energy functional projected on its unstable supspace as an effective model on the triangular lattice of sixfold coordinated sites. For this latter model, we obtain a large class of degenerate equilibrium configurations in one to one correspondence with those previously constructed for the pure XY model on the maximally frustrated dice lattice. The entropy of these sta…
Due to the large spin-orbital coupling in the layered 5d-transition metal chalcogenides compound, the occurrence of superconductivity in Ir2-xPdxTe2 offers a good chance to search for possible topological superconducting states in this system. We did comprehensive studies on the superconducting properties and electronic structures of single crystalline Ir0.95Pd0.05Te2 samples. The superconducting gap size, critical fields and coherence length along different directions were experimentally determined. Macroscopic bulk measurements and microscopic low temperature scanning tunneling spectroscopy …
We report the specific heat (SH) measurements on single crystals of hole doped $FeAs$-based superconductor $Ba_{0.6}K_{0.4}Fe_2As_2$. It is found that the electronic SH coefficient $γ_e(T)$ is not temperature dependent and increases almost linearly with the magnetic field in low temperature region. These point to a fully gapped superconducting state. Surprisingly the sharp SH anomaly $ΔC/T|_{T_c}$ reaches a value of 98 $mJ/mol K^2$ suggesting a very high normal state quasiparticle density of states ($γ_n \approx 63 mJ/mol K^2$). A detailed analysis reveals that the $γ_e(T)$ cannot be fitted wi…
We are developing a superconductor electronics fabrication process with up to nine planarized superconducting layers, stackable stud vias, self-shunted Nb/AlOx-Al/Nb Josephson junctions, and one layer of MoNx kinetic inductors. The minimum feature size of resistors and inductors in the process is 250 nm. We present data on the mutual inductance of Nb stripline and microstrip inductors with linewidth and spacing from 250 nm to 1 μm made on the same or adjacent Nb layers, as well as the data on the linewidth and resistance uniformity.
We report measurements of the in-plane magnetic penetration depth $λ$ in single crystals of SmFeAsO$_{1-x}$F$_y$ ($x\simeq y \simeq 0.2$) with $T_c \simeq 45$ K. We find that at low temperature $λ$ has an exponential temperature dependence which suggests that the Fermi surface is fully gapped. The magnitude of the minimum energy gap, $Δ_1=1.1\pm 0.1 k_BT_c$ at T=0 K, is significantly smaller than the BCS weak-coupling value suggesting that the gap is either strongly anisotropic or varies significantly between the different Fermi surface sheets. Our data fits well a two gap model with an additi…
We derive a microscopic effective action for superconducting contacts with arbitrary transmission distribution of conducting channels. Provided fluctuations of the Josephson phase remain sufficiently small our formalism allows to fully describe fluctuation and interaction effects in such systems. As compared to the well studied tunneling limit our analysis yields a number of qualitatively new features which occur due to the presence of subgap Andreev bound states in the system. We investigate the equilibrium supercurrent noise and evaluate the electron-electron interaction correction to the Jo…
The noncentrosymmetric superconductor Re$_6$Zr has attracted much interest due to the observation of broken time-reversal symmetry in the superconducting state. Here we report an investigation of the superconducting gap structure of Re$_6$Zr single crystals by measuring the magnetic penetration depth shift $Δλ(T)$ and electronic specific heat $C_e(T)$. $Δλ(T)$ exhibits an exponential temperature dependence behavior for $T~\ll~T_c$, which indicates a fully-open superconducting gap. Our analysis shows that a single gap $s$-wave model is sufficient to describe both the superfluid density $ρ_s(T)$…
High-resolution specific-heat measurements of the organic superconductor kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br in the superconducting (B = 0) and normal (B = 14 T) state show a clearly resolvable anomaly at Tc = 11.5 K and an electronic contribution, Ces, which can be reasonably well described by strong-coupling BCS theory. Most importantly, Ces vanishes exponentially in the superconducting state which gives evidence for a fully gapped order parameter.
The structure and the degenracy of the ground state of a fully-frustrated XY-model are investigated for the case of a dice lattice geometry. The results are applicable for the description of Josephson junction arrays and thin superconducting wire networks in the external magnetic field providing half-integer number of flux quanta per plaquette. The mechanisms of disordering of vortex pattern in such systems are briefly discussed.
The recent discovery of the topologically protected surface states in the beta-phase of PdBi2 has reignited the research interest in this class of superconductors. Here, we show results of our muon spin relaxation and rotation (muSR) measurements carried out to investigate the superconducting and magnetic properties and the topological effect in the superconducting ground state of beta-PdBi2. Zero-field muSR data reveal that no sizeable spontaneous magnetization arises with the onset of superconductivity implying that the time reversal symmetry is preserved in the superconducting state of beta…
Unconventional superconductivity in the Chevrel phase offers a wide structural aspect to understand the superconducting ground state. A detailed investigation on the superconducting properties of Re based pseudobinary telluride Chevrel phase Mo$_4$Re$_2$Te$_8$ is reported. It crystallizes in a trigonal structure with the space group $R\bar{3}H$ having superconducting transition temperature at $T_C$ = 3.26(3) K. Specific heat measurements suggests a fully gapped superconducting state; however, the proximity of upper critical field value from the Pauli limiting field can be attributed to unconve…
Superconducting properties of hypothetical simple hexagonal CaB2 are studied using the fully anisotropic Eliashberg formalism based on electronic and phononic structures and electron-phonon interactions which are obtained from ab initio pseudopotential density functional calculations. The superconducting transition temperature Tc, the superconducting energy gap Delta(k) on the Fermi surface, and the specific heat are obtained and compared with corresponding properties of MgB2. Our results suggest that CaB2 will have a higher Tc and a stronger two-gap nature, with a larger Delta(k) in the sigma…
We show results on the vortex core dissipation through current-voltage measurements under applied pressure and magnetic field in the superconducting phase of CeCoIn$_5$. We find that as soon as the system becomes superconducting, the vortex core resistivity increases sharply as the temperature and magnetic field decrease. The sharp increase in flux flow resistivity is due to quasiparticle scattering on critical antiferromagnetic fluctuations. The strength of magnetic fluctuations below the superconducting transition suggests that magnetism is complimentary to superconductivity and therefore mu…
We have investigated the vortex state in a superconducting dice network using the Bitter decoration technique at several magnetic frustrations f=1/2 and 1/3. In contrast to other regular network geometries where the existence of a commensurate state was previouly demonstrated, no ordered state was observed in the dice network at f=1/2 and the observed vortex-vortex correlation length is close to one lattice cell.
Polarized and unpolarized triple-axis spectrometry has been used to study an underdoped YBa_2Cu3O6.6. Our results indicate that spin excitations in the superconducting state are essentially the same as those in the fully doped material except that the unusual 41 meV resonance is observed at 34.8 meV. The normal state spin excitations are characterized by a weakly energy-dependent continuum whose temperature dependence shows the clear signature of a superconducting gap at Tc. The enhancement at the resonance is accompanied by a suppression of magnetic fluctuations at both higher and lower energ…
It is now well established that at a superconductor/ferromagnet (S/F) interface an unconventional superconducting state arises in which the pairing state is odd-frequency. The hallmark signature of this superconducting state is generally understood to be an enhancement of the electronic density of states (DoS) at subgap energies close to the S/F interface. However, here we show that an odd frequency state can be present even if the DoS is fully gapped. As an example, we show that this is the case in the pioneering S/FI (where FI is a insulating ferromagnet) tunneling experiments of Meservey an…
We study the interplay of spin and charge coherence in a single-level quantum dot. A tunnel coupling to a superconducting lead induces superconducting correlations in the dot. With full spin symmetry retained, only even-singlet superconducting correlations are generated. An applied magnetic field or attached ferromagnetic leads partially or fully reduce the spin symmetry, and odd-triplet superconducting correlations are generated as well. For single-level quantum dots, no other superconducting correlations are possible. We analyze, with the help of a diagrammatic real-time technique, the inter…
In this letter we describe the observation of a magnetic field dependent electronic gap, suggestive of local superconductivity, in the point-contact spectrum of micro-crystalline graphite. Magnetic field dependent point-contact spectroscopy was carried out at a temperature of $1.8\,\mathrm{K}$ using an etched aluminium tip. At zero field a gap structure in the differential conductance is observed, showing a gap of $Δ= 4.2\,\mathrm{meV}$. On applying magnetic fields of up to $500\,\mathrm{mT}$, this gap gradually closes, following the theoretical prediction by Ginzburg and Landau for a fully fl…
The Bloch-oscillating transistor (BOT) is an amplifier that utilizes semiclassical dynamics of states in energy bands under traveling quasimomentum. In a BOT, a single quasiparticle tunneling across a base tunnel junction switches the state of a superconducting tunnel junction to a lower Bloch band, triggering a series of resonant Cooper-pair tunnelings through an emitter Josephson junction (Bloch oscillations). Here, we investigate experimentally and theoretically an alternative realization of this device, based only on superconducting tunnel junctions. We discover new amplification schemes, …
We present electrical transport, magnetization, and specific heat measurements on bulk MgB2 samples (T_{c} = 38.5 K) synthesized under 200 MPa pressure using a process based on hot isostatic pressing with cooling under pressure. The samples are fully dense and display excellent superconducting properties, including a narrow superconducting transition width (ΔT_{c} = 0.75 K), a high upper critical field H_{c2} (H_{c2}(0) ~ 155 kOe), and a critical current density J_{c} that is the largest yet measured for bulk samples of MgB2 (J_{c}(0) ~ 1.4 MA/cm^{2}). Specific heat measurements yielded a jump…
Superconductivity in intercalated graphite CaC6 and H under extreme pressure, in the framework of superconducting density functional theory, is discussed. A detailed analysis on how the electron-phonon and electron-electron interactions combine together to determine the superconducting gap and critical temperature of these systems is presented. In particular, we discuss the effect on the calculated Tc of the anisotropy of the electron-phonon interaction and of the different approximations for screening the Coulomb repulsion. These results contribute to the understanding of multigap and anisotr…
We report crystallographic, specific heat, transport, and magnetic properties of the recently discovered noncentrosymmetric 5d-electron superconductors CaIrSi3 (Tc = 3.6 K) and CaPtSi3 (Tc = 2.3 K). The specific heat suggests that these superconductors are fully gapped. The upper critical fields are less than 1 T, consistent with limitation by conventional orbital depairing. High, non-Pauli-limited μ0 Hc2 values, often taken as a key signature of novel noncentrosymmetric physics, are not observed in these materials because the high carrier masses required to suppress orbital depairing and reve…
Oxide heterostructures have repeatedly been shown to display apical properties at the interfaces, some of which favorable to the formation of two-dimensional electron systems, as well as high transition temperature superconductivity. In this study, we propose a novel heterostructure to potentially achieve near room-temperature superconductivity, via the carrier injection in cuprate interfaces with ferro-electrics. Using a digital design approach guided by density-functional theory, the systems of XTiO3/XCuO3/XTiO3 are thoroughly examined, confirming the formation of a two-dimensional electron …
We report on the superconducting gap and pairing symmetry in layered superconductor Bi4O4S3. The measurement of temperature dependence of magnetic penetration depth was carried out using tunnel diode oscillator technique. It is observed that Bi4O4S3 is a conventional s-wave type superconductor with fully developed gap. The zero-temperature value of the superconducting energy gap Δ0 was found to be 1.54 meV, corresponding to the ratio 2Δ0/kBTc=7.2 which is much higher than the BCS value of 3.53. In the superconducting range, superfluid density is very well described by single gap s- wave model.
With the aid of the Keldysh technique we develop a microscopic theory of non-local electron transport in three-terminal NSN structures consisting of a chaotic superconducting quantum dot attached to one superconducting and two normal electrodes. Our theory fully accounts for non-equilibrium effects and disorder in a superconducting terminal. We go beyond perturbation theory in tunneling and derive a general expression for the system conductance matrix which remains valid in both weak and strong tunneling limits. We demonstrate that the proximity effect yields a decrease of crossed Andreev refl…
We present the results of fully-relaxed, full-potential electronic structure calculations for the new superconductor MgB2, and BeB2, NaB2, and AlB2, using density-functional-based methods. Our results described in terms of (i) density of states (DOS), (ii) band-structure, and (iii) the DOS and the charge density around the Fermi energy EF, clearly show the importance of B p-band for superconductivity. In particular, we show that around EF, the charge density in MgB2, BeB2 and NaB2 is planar and is associated with the B plane. For BeB2 and NaB2, our results indicate qualitative similarities but…
Recently, the niobium (Nb)-doped topological insulator Bi_2Se_3, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi_2Se_3 layers, has been shown to exhibit both superconductivity with T_c = 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature-dependent of effective magnetic penetration depth Lambda_eff(T) in the layered topological superconductor candidate Nb_0.25Bi_2Se_3. The exponential temperature dependence of lambda_(eff)^(-2)(T) at low temperatures suggests a fully gapped su…
We measured the ultra-low-temperature specific heat and thermal conductivity of Au$_2$Pb single crystal, a possible three-dimensional Dirac semimetal with a superconducting transition temperature $T_c \approx$ 1.05 K. The electronic specific heat can be fitted by a two-band s-wave model, which gives the gap amplitudes $Δ_1$(0)/$k_BT_c$ = 1.38 and $Δ_2$(0)/$k_BT_c$ = 5.25. From the thermal conductivity measurements, a negligible residual linear term $κ_0/T$ in zero field and a slow field dependence of $κ_0/T$ at low field are obtained. These results suggest that Au$_2$Pb has a fully gapped supe…
The origin and the evolution of the universe are concealed in the evanescent diffuse extragalactic background radiation (DEBRA). To reveal these signals, the development of innovative ultra-sensitive bolometers operating in the gigahertz band is required. Here, we review the design and experimental realization of two bias-current-tunable sensors based on one dimensional fully superconducting Josephson junctions: the nanoscale transition edge sensor (nano-TES) and the Josephson escape sensor (JES). In particular, we cover the theoretical basis of the sensors operation, the device fabrication, t…
We report on tunneling spectroscopy measurements using a Scanning Tunneling Microscope (STM) on the spin triplet superconductor Sr2RuO4. We find a negligible density of states close to the Fermi level and a fully opened gap with a value of $Δ$=0.28 meV, which disappears at T$_c$ = 1.5 K. $Δ$ is close to the result expected from weak coupling BCS theory ($Δ_0$=1.76kBT$_c$ = 0.229 meV). Odd parity superconductivity is associated with a fully isotropic gap without nodes over a significant part of the Fermi surface.
The search for topological superconductors (TSCs) is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exist the zero energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. So far, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at Fermi level combined with fully gapped bulk …
We investigated the superconducting gap structure of SrNi$_2$P$_{2}$ ($T_c$=1.4 K) via low-temperature magneto-thermal conductivity $κ(T,H)$ measurements. Zero field thermal conductivity $κ$ decreases exponentially $κ\propto$ exp($-aT_c/T$) with $a$=1.5, in accord with the BCS theory, and rolls over to a phonon-like $κ\propto T^3$ behavior at low temperature, similar to a number of conventional s-wave superconductors. In addition, we observed a "concave" field dependence of the residual linear term $κ_0(H)/T$. These facts strongly rule out the presence of nodes in the superconducting energy ga…
We investigate the superconducting gap function of topological superconductor PbTaSe$_2$. Temperature, magnetic field, and three-dimensional (3D) field-angle dependences of the specific heat prove that the superconductivity of PbTaSe$_2$ is fully-gapped, with two isotropic $s$-wave gaps. The pair-breaking effect is probed by systematically increasing non-magnetic disorders through H$^+$-irradiations. The superconducting transition temperature, $T_{\rm{c}}$, is found to be robust against disorders, which suggests that the pairing should be sign-preserved rather than sign-reversed.
We report a comprehensive study of the Zr$_5$Pt$_3$C$_x$ superconductors, with interstitial carbon comprised between 0 and 0.3. At a macroscopic level, their superconductivity, with $T_c$ ranging from 4.5 to 6.3 K, was investigated via electrical-resistivity-, magnetic-susceptibility-, and specific-heat measurements. The upper critical fields $μ_0H_\mathrm{c2}$ $\sim$ 7 T were determined mostly from measurements of the electrical resistivity in applied magnetic fields. The microscopic electronic properties were investigated by means of muon-spin rotation and relaxation ($μ$SR) and nuclear magn…
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 …
