Normal-state Conductivity Research Articles

Electronic and superconducting properties of CoSi[formula omitted] films on silicon — An unconventional superconductor with technological potential

We report the observation of unusual normal-state electronic conduction properties and superconducting characteristics of high-quality CoSi2/Si films grown on silicon Si(100) and Si(111) substrates. A good understanding of these features shall help to address the underlying physics of the unconventional pairing symmetry recently discovered in transparent CoSi2/TiSi2 heterojunctions (Chiu et al., 2021; Chiu et al., 2023), where CoSi2/Si is a superconductor with a superconducting transition temperature Tc≃ (1.1–1.5) K, dependent on its dimensions, and TiSi2 is a normal metal. In CoSi2/Si films, we find a pronounced positive magnetoresistance caused by the weak-antilocalization effect, indicating a strong Rashba spin–orbit coupling (SOC). This SOC generates two-component superconductivity in CoSi2/TiSi2 heterojunctions. The CoSi2/Si films are stable under ambient conditions and have ultralow 1/f noise. Moreover, they can be patterned via the standard lithography techniques, which might be of considerable practical value for future scalable superconducting and quantum device fabrication.

Disentangling Photodoping, Photoconductivity, and Photosuperconductivity in the Cuprates.

The normal-state conductivity and superconducting critical temperature of oxygen-deficient YBa_{2}Cu_{3}O_{7-δ} can be persistently enhanced by illumination. Strongly debated for years, the origin of those effects-termed persistent photoconductivity and photosuperconductivity (PPS)-has remained an unsolved critical problem, whose comprehension may provide key insights to harness the origin of high-temperature superconductivity itself. Here, we make essential steps toward understanding PPS. While the models proposed so far assume that it is caused by a carrier-density increase (photodoping) observed concomitantly, our experiments contradict such conventional belief: we demonstrate that it is instead linked to a photo-induced decrease of the electronic scattering rate. Furthermore, we find that the latter effect and photodoping are completely disconnected and originate from different microscopic mechanisms, since they present different wavelength and oxygen-content dependences as well as strikingly different relaxation dynamics. Besides helping disentangle photodoping, persistent photoconductivity, and PPS, our results provide new evidence for the intimate relation between critical temperature and scattering rate, a key ingredient in modern theories on high-temperature superconductivity.

Superfluid response of an atomically thin gate-tuned van der Waals superconductor

A growing number of two-dimensional superconductors are being discovered in the family of exfoliated van der Waals materials. Due to small sample volume, the superfluid response of these materials has not been characterized. Here, we use a local magnetic probe to directly measure this key property of the tunable, gate-induced superconducting state in MoS2. We find that the backgate changes the transition temperature non-monotonically whereas the superfluid stiffness at low temperature and the normal state conductivity monotonically increase. In some devices, we find direct signatures in agreement with a Berezinskii-Kosterlitz-Thouless transition, whereas in others we find a broadened onset of the superfluid response. We show that the observed behavior is consistent with disorder playing an important role in determining the properties of superconducting MoS2. Our work demonstrates that magnetic property measurements are within reach for superconducting devices based on exfoliated sheets and reveals that the superfluid response significantly deviates from simple BCS-like behavior.

Quality factor for zero-bias conductance peaks in Majorana nanowire

Despite recent experimental progress towards observing large zero-bias conductance peaks (ZBCPs) as signatures of Majorana modes, confusion remains about whether Majorana modes have been observed. This is in part due to the theoretical prediction of fine-tuned trivial (i.e., non-topological) zero-bias peaks that occur because of uncontrolled quantum dots or disorder potentials. While many aspects of the topological phase can be somewhat fine-tuned because the topological phase space is often small, the quantized height of the ZBCP associated with a Majorana mode is known to be robust at sufficiently low temperatures even as the tunnel barrier is pinched off to vanishingly small normal-state conductance. The key shortcoming of the existing experimental works is an acute lack of stability of the putative Majorana mode features, indicating the probable absence of a topological phase, and the current paper suggests specific experimentally accessible measures for a careful quantitative analysis of the measured ZBCP stability. In this paper, we study how the counter-intuitive robustness of the ZBCP height to the tunnel barrier strength can be used to distinguish Majorana modes from non-topological ZBCPs. To this end, we introduce a dimensionless quality factor $F$ to quantify the robustness of the ZBCP height based on the range of normal-state (i.e. above-gap) conductance (which depends crucially on the tunnel barrier height) over which the ZBCP height remains within a pre-specified range of quantization. By computing this quality factor $F$ together with the topological characteristics for a wide range of models and parameters, we find that Majoranas are significantly more robust (i.e., have a higher value of $F$) compared with non-topological ZBCPs in the ideal low-temperature limit. Even at a temperature as high as the experimentally used $20$ mK, we find that we can set a threshold value of $F\sim 2.5$ (for $\epsilon=0.1$) so that ZBCPs associated with a quality factor $F>2.5$ are likely topological and $F\ll 2.5$ are topologically trivial. More precisely, the value of $F$ is operationally related to the degree of separation of the Majorana modes in the system, although $F$ uses only the experimentally measured tunnel conductance properties. Finally, we discuss how the quality factor $F$ measured in a transport setup can help estimate the quality of topological qubits made from Majorana modes. In particular, we show that if the induced gap can be enhanced somehow beyond the currently available $\sim 30$ $\mu$eV in InAs/Al samples, large (small) values of $F$ could easily distinguish between stable topological (unstable trivial) ZBCPs with the quantum dot induced quasi-Majorana bound states occasionally behaving similar to topological Majorana modes in short wires.

Gate-Tunable Kinetic Inductance in Proximitized Nanowires

We report the detection of a gate-tunable kinetic inductance in a hybrid InAs/Al nanowire. For this purpose, we have embedded the nanowire into a quarter-wave coplanar waveguide resonator and measured the resonance frequency of the circuit. We find that the resonance frequency can be changed via the gate voltage that controls the electron density of the proximitized semiconductor and thus the nanowire inductance. Applying Mattis-Bardeen theory, we extract the gate dependence of the normal state conductivity of the nanowire, as well as its superconducting gap. Our measurements complement existing characterization methods for hybrid nanowires and provide a new and useful tool for gate-controlled superconducting electronics.

Characterization of the Parameters of Superconducting NbN and NbTiN Films Using Parallel Plate Resonator

This work is dedicated to the determination of parameters of superconducting NbN and NbTiN films such as critical temperature, normal state conductivity, ratio between the gap and critical temperature and parameter incorporating the quasiparticle states inside the gap that emerge due to strong-coupling effects. The measurements were performed using a parallel plate resonator method. From the experiment, the dependence of London penetration depth on temperature was obtained. The experimental data were fitted using the expressions of Mattis-Bardeen theory extended by the consideration of strong-coupling effects. In order to design the experimental setup and visualize the results a modeling in Ansys HFSS was performed.

Epitaxial Bi2Sr2CuOu y thin films as p-type transparent conductors

Development of p-type transparent conducting thin films is tireless due to the trade-off issue between optical transparency and conductivity. The rarely concerned low normal state resistance makes Bi-based superconducting cuprates the potential hole-type transparent conductors, which have been realized in Bi2Sr2CaCu2O y thin films. In this study, epitaxial superconducting Bi2Sr2CuO y and Bi2Sr1.8Nd0.2CuO y thin films with superior normal state conductivity are proposed as p-type transparent conductors. It is found that the Bi2Sr1.8Nd0.2CuO y thin film with thickness 15 nm shows an average visible transmittance of 65% and room-temperature sheet resistance of 650 Ω/sq. The results further demonstrate that Bi-based cuprate superconductors can be regarded as potential p-type transparent conductors for future optoelectronic applications.

Two-Dimensional Superconductivity of Ca-Intercalated Graphene on SiC: Vital Role of the Interface between Monolayer Graphene and the Substrate.

Ca-intercalation has enabled superconductivity in graphene on SiC. However, the atomic and electronic structures that are critical for superconductivity are still under discussion. We find an essential role of the interface between monolayer graphene and the SiC substrate for superconductivity. In the Ca-intercalation process, at the interface a carbon layer terminating SiC changes to graphene by Ca-termination of SiC (monolayer graphene becomes a bilayer), inducing more electrons than a free-standing model. Then, Ca is intercalated in between the graphene layers, which shows superconductivity with the updated critical temperature (TC) of up to 5.7 K. In addition, the relation between TC and the normal-state conductivity is unusual, "dome-shaped". These findings are beyond the simple C6CaC6 model in which s-wave BCS superconductivity is theoretically predicted. This work proposes a general picture of the intercalation-induced superconductivity in graphene on SiC and indicates the potential for superconductivity induced by other intercalants.

Resistance of two-dimensional superconducting films

We consider the problem of finite resistance $R$ in superconducting films with geometry of a strip of width $W$ near zero temperature. The resistance is generated by vortex configurations of the phase field. In the first type of process, quantum phase slip, the vortex worldline in 2+1 dimensional space-time is space-like (i.e., the superconducting phase winds in time and space). In the second type, vortex tunneling, the worldline is time-like (i.e., the phase winds in the two spatial directions) and connects opposite edges of the film. For moderately disordered samples, processes of second type favor a train of vortices, each of which tunnels only across a fraction of the sample. Optimization with respect to the number of vortices yields a tunneling distance of the order of the coherence length $\xi$, and the train of vortices becomes equivalent to a quantum phase slip. Based on this theory, we find the resistance $\ln R \sim -g W/\xi$, where $g$ is the dimensionless normal-state conductance. Incorporation of quantum fluctuations indicates a quantum phase transition to an insulating state for $g \lesssim 1$.

Nb -Based Nanoscale Superconducting Quantum Interference Devices Tuned by Electroannealing

In this work, we show that targeted and controlled modifications of the Josephson-junction properties of a bridge-type $\mathrm{Nb}$ nanoSQUID can be achieved by an electroannealing process allowing us to tune and tailor the response of a single device. The electroannealing consists in substantial Joule heating produced by large current densities followed by a rapid temperature quench. We report on a highly nontrivial evolution of the material properties when performing subsequent electroannealing steps. As the current density is increased, an initial stage characterized by a modest improvement of the superconducting critical temperature and normal-state conductivity of the bridges, is observed. This is followed by a rapid deterioration of the junction properties, i.e., decrease of critical temperature and conductivity. Strikingly, further electroannealing leads to a noteworthy recovery before irreversible damage is produced. Within the electroannealing regime where this remarkable resurrection of the superconducting properties are observed, the nanoSQUID can be operated in nonhysteretic mode in the whole temperature range and without compromising the critical temperature of the device. The proposed postprocessing is particularly appealing in view of its simplicity and robustness.

Epitaxial Al-InAs Heterostructures as Platform for Josephson Junction Field-Effect Transistor Logic Devices

We fabricate Josephson junction field-effect transistors (JJ-FETs) using InAs quantum well heterostructure as the channel, epitaxial Al as superconducting electrodes, and scaled-down Al2O3 gate dielectrics. A systematic investigation of the gate voltage ( ${V}_{{G}}$ ) dependence of the critical current, normal state conductance, and characteristic voltage of the JJ-FETs coupled with capacitance measurements reveals different ${V}_{{G}}$ regimes in the proximity effect characterization of JJ-FETs. Self-consistent Poisson–Schrodinger simulations allow us to associate these ${V}_{{G}}$ regimes with the carriers populating one or more subbands at different vertical locations in the heterostructure. We also discuss the importance of oxide/channel interface quality and its impact on the practical implementation of JJ-FET Boolean logic gates with signal restoration.

On the Intersecting Temperature in Presence of Several Normal State Conduction Mechanisms in High-Tc Superconductors

An intersection temperature of the normal state conduction and a state which follows the three-dimensional Aslamazov-Larkin (3DAL) has been studied in several high-temperature superconductors (HTS). Extraction of the coherence length by using the 3DAL equation depends on the existing normal state conduction mechanism. We have used bulk HTS having normal states of different types which can be explained either by using the metallic behavior or the variable range hopping (VRH) conduction. In presence of the several coherence lengths, the shifting of the intersecting point has been studied.

Zero-bias conductance peak in Dirac semimetal-superconductor devices

Majorana zero modes (MZMs), fundamental building blocks for realizing topological quantum computers, can appear at the interface between a superconductor and a topological material. One of the experimental signatures that has been widely pursued to confirm the existence of MZMs is the observation of a large, quantized zero-bias conductance peak (ZBCP) in the differential conductance measurements. In this Letter, we report observation of such a large ZBCP in junction structures of normal metal (titanium/gold Ti/Au) + Dirac semimetal (cadmium arsenide Cd3As2) + conventional superconductor (aluminum Al), with a value close to four times that of the normal state conductance. Our detailed analyses suggest that this large ZBCP is most likely not caused by MZMs. We attribute the ZBCP, instead, to the existence of a supercurrent between two far-separated superconducting Al electrodes, which shows up as a zero-bias peak because of the circuitry and thermal fluctuations of the supercurrent phase, a mechanism conceived by Ivanchenko and Zil'berman more than 50 years ago [JETP 28, 1272 (1969)]. Our results thus call for extreme caution when assigning the origin of a large ZBCP to MZMs in a multiterminal semiconductor or topological insulator/semimetal setup. We thus provide criteria for identifying when the ZBCP is definitely not caused by an MZM. Furthermore, we present several remarkable experimental results of a supercurrent effect occurring over unusually long distances and clean perfect Andreev reflection features.

Tunneling spectroscopy of c -axis epitaxial cuprate junctions

Atomically precise epitaxial structures are unique systems for tunneling spectroscopy that minimize extrinsic effects of disorder. We present a systematic tunneling spectroscopy study, over a broad doping, temperature, and bias range, in epitaxial c-axis La$_{2-x}$Sr$_{x}$CuO$_{4}$/La$_{2}$CuO$_{4}$/La$_{2-x}$Sr$_{x}$CuO$_{4}$ heterostructures. The behavior of these superconductor/insulator/superconductor (SIS) devices is unusual. Down to 20 mK there is complete suppression of c-axis Josephson critical current with a barrier of only 2 nm of La$_{2}$CuO$_{4}$, and the zero-bias conductance remains at 20-30% of the normal-state conductance, implying a substantial population of in-gap states. Tunneling spectra show greatly suppressed coherence peaks. As the temperature is raised, the superconducting gap fills in rather than closing at $T_{c}$. For all doping levels, the spectra show an inelastic tunneling feature at $\sim$ 80 meV, suppressed as $T$ exceeds $T_{c}$. These nominally simple epitaxial cuprate junctions deviate markedly from expectations based on the standard Bardeen-Cooper-Schrieffer (BCS) theory.

The effect of superconducting fluctuations on the ac conductivity of a 2D electron system in the diffusive regime

We report a complete analytical expression for the one-loop correction to the ac conductivity σ(ω) of a disordered two-dimensional electron system in the diffusive regime. The obtained expression includes the weak localization and Altshuler–Aronov corrections as well as the corrections dueto superconducting fluctuations above superconducting transition temperature. The derived expression has no 1∕(iω) divergency in the static limit, ω→0, in agreement with general expectations for the normal state conductivity of a disordered electron system.

Fermi-arc supercurrent oscillations in Dirac semimetal Josephson junctions

One prominent hallmark of topological semimetals is the existence of unusual topological surface states known as Fermi arcs. Nevertheless, the Fermi-arc superconductivity remains elusive. Here, we report the critical current oscillations from surface Fermi arcs in Nb-Dirac semimetal Cd3As2-Nb Josephson junctions. The supercurrent from bulk states are suppressed under an in-plane magnetic field ~0.1 T, while the supercurrent from the topological surface states survives up to 0.5 T. Contrary to the minimum normal-state conductance, the Fermi-arc carried supercurrent shows a maximum critical value near the Dirac point, which is consistent with the fact that the Fermi arcs have maximum density of state at the Dirac point. Moreover, the critical current exhibits periodic oscillations with a parallel magnetic field, which is well understood by considering the in-plane orbital effect from the surface states. Our results suggest the Dirac semimetal combined with superconductivity should be promising for topological quantum devices.

Andreev reflection in ballistic normal metal/graphene/superconductor junctions

We report the study of ballistic transport in normal metal/graphene/superconductor junctions in edge-contact geometry. While in the normal state, we have observed Fabry-P\'{e}rot resonances suggesting that charge carriers travel ballistically, the superconducting state shows that the Andreev reflection at the graphene/superconductor interface is affected by these interferences. Our experimental results in the superconducting state have been analyzed and explained with a modified Octavio-Tinkham-Blonder-Klapwijk model taking into account the magnetic pair-breaking effects and the two different interface transparencies, \textit{i.e.}\,between the normal metal and graphene, and between graphene and the superconductor. We show that the transparency of the normal metal/graphene interface strongly varies with doping at large scale, while it undergoes weaker changes at the graphene/superconductor interface. When a cavity is formed by the charge transfer occurring in the vicinity of the contacts, we see that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.

Landauer Formula for a Superconducting Quantum Point Contact.

We generalize the Landauer formula to describe the dissipative electron transport through a superconducting point contact. The finite-temperature, linear-in-bias, dissipative dc conductance is expressed in terms of the phase- and energy-dependent scattering matrix of the Bogoliubov quasiparticles in the quantum point contact. The derived formula is also applicable to hybrid superconducting-normal structures and normal contacts, where it agrees with the known limits of Andreev reflection and normal-state conductance, respectively.

Influence of Pb doping on superconductivity of α-BiPd and β-Bi2Pd alloys

A series of Pb-doped Bi-based alloys, α-Bi1-xPbxPd (0 ≤ x ≤ 0.5) and β-Bi2-xPbxPd (0 ≤ x ≤ 0.6), were synthesized by facile solid-state reaction method. The powder X-ray diffraction confirmed that the crystal structure of samples remained the same after Pb-doping and the cell volume decreased with increasing Pb content. The influence of Pb-doping effect on superconductivity of BiPd and Bi2Pd alloys has been systematically examined by the magnetic susceptibility and resistivity measurements. Both alloy systems have shown bulk superconductivity and metallic conductivity in normal state. Furthermore, the superconducting onset temperature (Tc) gradually decreased with higher Pb content for both systems. The Tc dropped from 3.7 K for BiPd to 2.7 K for Bi0.8Pb0.2Pd, whereas the Tc dropped from 5.3 K for Bi2Pd to 2.4 K for Bi1.5Pb0.5Pd.

СВЧ-импеданс тонкопленочных гибридных структур сверхпроводник--нормальный металл с большим отношением проводимостей

Temperature dependence of linear electrodynamic response of thin-film hybrid structures superconductor (MoN) — normal metal (Al) with large ratio of normal-state conductivities was studied theoretically and experimentally. Low-frequency measurements of the mutual inductance of two coils with a sample placed between them indicated an increase in magnetic screening ability of the superconductor – normal metal (SN) hybrid structures at liquid helium temperatures as $d_Al$ increases, where $d_Al$ is the thickness of the Al layer. Measurements of the frequency shift $\delta f$ of the microwave dielectric resonator, which was in contact with the SN samples as a function of temperature and $d_Al$ demonstrated that (i) type of the $\delta f(T)$ dependence depends significantly on $d_Al$ and (ii) the shift of resonant frequency of the SN structures at temperatures close to the critical temperature Tc cannot be approximated by a functional dependence $const/(1–T/T_c)$, which is typical for thin superconducting films. Numerical calculations performed within the Usadel model describe the observed effects quite well. Thus, the mentioned anomalies of the electrodynamic properties of the SN hybrid structures can be explained by an appearance of a mini-gap in the spectrum of quasi-particle excitation caused by the proximity effect in the normal metal layer, which depends on $d_Al$ as well as by the high conductivity of the Al layer.