Signatures of gate-driven out of equilibrium superconductivity in Ta/InAs nanowires (original) (raw)
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Switching dynamics in Al/InAs nanowire-based gate-controlled superconducting transistor
arXiv (Cornell University), 2023
The observation of the gate-controlled supercurrent (GCS) effect in superconducting nanostructures increased the hopes for realizing a superconducting equivalent of semiconductor field-effect transistors. However, recent works attribute this effect to various leakage-based scenarios, giving rise to a debate on its origin. A proper understanding of the microscopic process underlying the GCS effect and the relevant time scales would be beneficial to evaluate the possible applications. In this work, we observed gate-induced two-level fluctuations between the superconducting state and normal state in Al/InAs nanowires (NWs). Noise correlation measurements show a strong correlation with leakage current fluctuations. The time-domain measurements show that these fluctuations have Poissonian statistics. Our detailed analysis of the leakage current measurements reveals that it is consistent with the stress-induced leakage current (SILC), in which inelastic tunneling with phonon generation is the predominant transport mechanism. Our findings shed light on the microscopic origin of the GCS effect and give deeper insight into the switching dynamics of the superconducting NW under the influence of the strong gate voltage.
Gate-Controlled Supercurrent in Epitaxial Al/InAs Nanowires
Nano Letters
Gate-controlled supercurrent (GCS) in superconducting nanobridges has recently attracted attention as a means to create superconducting switches. Despite the clear advantages for applications, the microscopic mechanism of this effect is still under debate. In this work, we realize GCS for the first time in a highly crystalline superconductor epitaxially grown on an InAs nanowire. We show that the supercurrent in the epitaxial Al layer can be switched to the normal state by applying ≃±23 V on a bottom gate insulated from the nanowire by a crystalline hBN layer. Our extensive study of the temperature and magnetic field dependencies suggests that the electric field is unlikely to be the origin of GCS in our device. Though hot electron injection alone cannot explain our experimental findings, a very recent non-equilibrium phonons based picture is compatible with most of our results.
Gate-controlled supercurrent in an epitaxial Al/InAs nanowires
2021
Gate-controlled supercurrent (GCS) in superconductor nanobridges has recently attracted attention as a means to create superconducting field effect transistors. Despite the clear advantage for applications with low power consumption and high switching speeds, the microscopic mechanism of the field effect is still under debate. In this work, we realize GCS for the first time in an epitaxial superconductor, which is created as a shell on an InAs nanowire. We show that the supercurrent in the epitaxial Al layer can be switched to the normal state by applying ≃± 23V on a bottom gate insulated from the nanowire by a crystalline hBN layer. Our extensive study on the temperature and magnetic field dependencies of GCS suggests that hot electron injection alone cannot explain our experimental findings.
Superconducting Proximity Effect in InAs Nanowires
2014
First discovered by Holm and Meissner in 1932, the superconducting proximity effect has remained a subject of experimental and theoretical interest. In recent years, it has been proposed that proximity effect in a semiconductor with large gfactor and spin-orbit coupling could lead to exotic phases of superconductivity. This thesis focuses on proximity effect in one of the prime semiconductor candidates-InAs nanowires. The first set of experiments investigates the superconducting phase-dependent tunneling spectrum of a proximitized InAs quantum dot. We observe tunneling resonances of Andreev bound states in the Kondo regime, and induce quantum phase transitions of the quantum dot ground state with gate voltage and phase bias-the latter being the first experimental observation of its kind. An additional zero-bias peak of unknown origin is observed to coexist with the Andreev bounds states. The second set of experiments extends upon the first with sharper tunneling resonances and an increase in the device critical field. By applying an external magnetic field, we observe spin-resolved Andreev bound states in proximitized InAs quantum dots. From the linear splitting of the tunneling resonances, we extract g-factors of 5 iii Abstract and 10 in two different devices. The third set of experiments utilizes a novel type of epitaxial core-shell InAs-Al nanowire. We compare the induced gaps of these nanowires with control devices proximitized with evaporated Al films. Our results show that the epitaxial core-shell nanowires possess a much harder induced gap-up to two orders of magnitude in subgap conductance suppression as compared to a factor of five in evaporated control devices. This observation suggests that roughness in S-N interfaces plays a crucial role in the quality of the proximity effect. The fourth set of experiments investigates the gate-tunability of epitaxial halfshell nanowires. In a half-shell nanowire Josephson junction, we measure the normal state resistance, maximum supercurrent, and magnetic field-dependent supercurrent interference patterns. The gate dependences of these independent experimental parameters are consistent with one another and indicate that an InAs nanowire in good ohmic contact to a thin sliver of Al retains its proximity effect and is gate-tunable.
Switching Current Distributions of Superconducting Nanowires: Evidence of Quantum Phase Slip Events
2009
Phase slips are topological fluctuation events that carry the superconducting order-parameter field between distinct current carrying states. Owing to these phase slips low-dimensional superconductors acquire electrical resistance. In quasi-one-dimensional nanowires it is well known that at higher temperatures phase slips occur via the process of thermal barriercrossing by the order-parameter field. At low temperatures, the general expectation is that phase slips should proceed via quantum tunneling events, which are known as quantum phase slips (QPS). However, experimental observation of QPS is a subject of strong debate and no consensus has been reached so far about the conditions under which QPS occurs. In this study, strong evidence for individual quantum tunneling events undergone by the superconducting order-parameter field in homogeneous nanowires is reported. This is accomplished via measurements of the distribution of switching currents–the high-bias currents at which super...
Nano Letters, 2013
We report conductance and supercurrent of InAs nanowires coupled to Al-superconducting electrodes with short channel lengths and good Ohmic contacts. The nanowires are suspended 15 nm above a local gate electrode. The charge density in the nanowires can be controlled by a small change in the gate voltage. For large negative gate voltages, the number of conducting channels is reduced gradually and we observe a stepwise decrease of both conductance and critical current before the conductance vanishes completely.
2008
Phase slips are topological fluctuation events that carry the superconducting order-parameter field between distinct current carrying states. Owing to these phase slips low-dimensional superconductors acquire electrical resistance. In quasi-one-dimensional nanowires it is well known that at higher temperatures phase slips occur via the process of thermal barriercrossing by the order-parameter field. At low temperatures, the general expectation is that phase slips should proceed via quantum tunneling events, which are known as quantum phase slips (QPS). However, experimental observation of QPS is a subject of strong debate and no consensus has been reached so far about the conditions under which QPS occurs. In this study, strong evidence for individual quantum tunneling events undergone by the superconducting order-parameter field in homogeneous nanowires is reported. This is accomplished via measurements of the distribution of switching currents-the high-bias currents at which superconductivity gives way to resistive behavior-whose width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. A stochastic model of phase slip kinetics which relates the basic phase slip rates to switching rates is outlined. Comparison with this model indicates that the phase predominantly slips via thermal activation at high temperatures but at sufficiently low temperatures switching is caused by individual topological tunneling events of the order-parameter field, i.e., QPS. Importantly, measurements show that in nanowires having larger critical currents quantum fluctuations dominate thermal fluctuations up to higher temperatures. This fact provides strong support for the view that the anomalously high switching rates observed at low temperatures are indeed due to QPS, and not consequences of extraneous noise or hidden inhomogeneity of
Physical Review B
Competition between superconducting and ferromagnetic ordering at interfaces between ferromagnets (F) and superconductors (S) gives rise to several proximity effects such as odd-triplet superconductivity and spin-polarized supercurrents. A prominent example of an S/F proximity effect is the spin switch effect (SSE) observed in S/F/N/F superconducting spin valve multilayers, in which the superconducting transition temperature Tc is controlled by the angle φ between the magnetic moments of the F layers separated by a nonmagnetic metallic spacer N. Here we present an experimental study of SSE in Nb/Co/Cu/Co/CoOx nanowires measured as a function of bias current flowing in the plane of the layers. These measurements reveal an unexpected dependence of Tc(φ) on the bias current: Tc(π)-Tc(0) changes sign with increasing current bias. We attribute the origin of this bias dependence of the SSE to a spin Hall current flowing perpendicular to the plane of the multilayer, which suppresses Tc of the multilayer. The bias dependence of SSE can be important for hybrid F/S devices such as those used in cryogenic memory for superconducting computers as device dimensions are scaled down to the nanometer length scale.
Superconductor–insulator transition in nanowires and nanowire arrays
New Journal of Physics, 2015
Superconducting nanowires are the dual elements to Josephson junctions, with quantum phase-slip processes replacing the tunneling of Cooper pairs. When the quantum phaseslip amplitude E S is much smaller than the inductive energy E L , the nanowire responds as a superconducting inductor. When the inductive energy is small, the response is capacitive. The crossover at low temperatures as a function of E S /E L is discussed and compared with earlier experimental results. For one-dimensional and two-dimensional arrays of nanowires quantum phase transitions are expected as a function of E S /E L. They can be tuned by a homogeneous magnetic frustration.
arXiv (Cornell University), 2012
We measure quantum and thermal phase-slip rates using the standard deviation of the switching current in superconducting nanowires. Our rigorous quantitative analysis provides firm evidence for the presence of quantum phase slips (QPSs) in homogeneous nanowires at high bias currents. We observe that as temperature is lowered, thermal fluctuations freeze at a characteristic crossover temperature Tq, below which the dispersion of the switching current saturates to a constant value, indicating the presence of QPSs. The scaling of the crossover temperature Tq with the critical temperature Tc is linear, Tq ∝ Tc, which is consistent with the theory of macroscopic quantum tunneling. We can convert the wires from the initial amorphous phase to a single-crystal phase, in situ, by applying calibrated voltage pulses. This technique allows us to probe directly the effects of the wire resistance, critical temperature, and morphology on thermal and quantum phase slips.