Quasiparticle Injection Research Articles

Nonequilibrium Phenomena in Planar Mesoscopic Josephson SNS Structures Based on Superconducting Nb

Nonequilibrium phenomena in planar Josephson SNS nanostructures, where the superconductor (S) is Nb and the normal metal (N) is Cu or Au, have been studied experimentally. Using additional N electrodes attached to the S banks of the Josephson SNS junction, transport measurements have been performed at low temperatures with the injection of quasiparticles with the use of local and nonlocal connection schemes. The charge-imbalance relaxation length in niobium at temperatures much lower than the superconducting transition temperature has been determined experimentally for the first time.

Spin-dependent coupling of supercurrent and nonequilibrium quasiparticles in high-field superconductors

We report on an experimental investigation of the combined effect of nonequilibrium quasiparticle injection and supercurrent in superconducting aluminum wires. At low temperature, we observe the supercurrent-induced coupling of energy and charge imbalance with spectral resolution. At high magnetic fields, in the presence of a Zeeman splitting of the density of states, we find evidence for an additional spin-dependent coupling which has been recently predicted theoretically.

Josephson effect in superconductor/normal-dot/superconductor junctions driven out of equilibrium by quasiparticle injection

We study theoretically the large variations of the supercurrent through a normal dot that are induced by a small quasiparticle injection current from normal leads connected to the dot. We find that the supercurrent decomposes into a subgap contribution, which depends on the voltages applied to the normal leads, as well as a contribution with opposite sign from energies outside the gap, which is insensitive to the voltages. As the voltages gradually suppress the subgap contribution, a critical voltage exists above which the contribution from energies outside the gap dominates, leading to a sign reversal of the current-phase relation, namely a transition to a so-called $\pi$-junction behavior. We determine the critical voltage and analyze the robustness of the effect with respect to temperature and inelastic relaxation in the dot.

Electrostatic Field-Driven Supercurrent Suppression in Ionic-Gated Metallic Superconducting Nanotransistors.

Recent experiments have shown the possibility of tuning the transport properties of metallic nanosized superconductors through a gate voltage. These results renewed the longstanding debate on the interaction between electrostatic fields and superconductivity. Indeed, different works suggested competing mechanisms as the cause of the effect: an unconventional electric field-effect or quasiparticle injection. Here, we provide conclusive evidence for the electrostatic-field-driven control of the supercurrent in metallic nanosized superconductors, by realizing ionic-gated superconducting field-effect nanotransistors (ISFETs) where electron injection is impossible. Our Nb ISFETs show giant suppression of the superconducting critical current of up to ∼45%. Moreover, the bipolar supercurrent suppression observed in different ISFETs, together with invariant critical temperature and normal-state resistance, also excludes conventional charge accumulation/depletion. Therefore, the microscopic explanation of this effect calls upon a novel theory able to describe the nontrivial interaction of static electric fields with conventional superconductivity.

High-energy quasiparticle injection into mesoscopic superconductors.

At non-zero temperatures, superconductors contain excitations known as Bogoliubov quasiparticles (QPs). The mesoscopic dynamics of QPs inform the design of quantum information processors, among other devices. Knowledge of these dynamics stems from experiments in which QPs are injected in a controlled fashion, typically at energies comparable to the pairing energy1-5. Here we perform tunnel spectroscopy of a mesoscopic superconductor under high electric fields. We observe QP injection due to field-emitted electrons with 106 times the pairing energy, an unexplored regime of QP dynamics. Upon application of a gate voltage, the QP injection decreases the critical current and, at sufficiently high electric field, a field-emission current (<0.1 nA in our device) switches the mesoscopic superconductor into the normal state, consistent with earlier observations6. We expect that high-energy injection will be useful for developing QP-tolerant quantum information processors, will allow rapid control of resonator quality factors and will enable the design of electric-field-controlled superconducting devices with new functionality.

Gate-Controlled Suspended Titanium Nanobridge Supercurrent Transistor.

Under standard conditions, the electrostatic field-effect is negligible in conventional metals and was expected to be completely ineffective also in superconducting metals. This common belief was recently put under question by a family of experiments that displayed full gate-voltage-induced suppression of critical current in superconducting all-metallic gated nanotransistors. To date, the microscopic origin of this phenomenon is under debate, and trivial explanations based on heating effects given by the negligible electron leakage from the gates should be excluded. Here, we demonstrate the control of the supercurrent in fully suspended superconducting nanobridges. Our advanced nanofabrication methods allow us to build suspended superconducting Ti-based supercurrent transistors which show ambipolar and monotonic full suppression of the critical current for gate voltages of VGC ≃ 18 V and for temperatures up to ∼80% of the critical temperature. The suspended device architecture minimizes the electron-phonon interaction between the superconducting nanobridge and the substrate, and therefore, it rules out any possible contribution stemming from charge injection into the insulating substrate. Besides, our finite element method simulations of vacuum electron tunneling from the gate to the bridge and thermal considerations rule out the cold-electron field emission as a possible driving mechanism for the observed phenomenology. Our findings promise a better understanding of the field effect in superconducting metals.

Characterization of Amplification Properties of the Superconducting-Ferromagnetic Transistor

We report on the measurement results of the superconducting-ferromagnetic transistors (SFTs). The devices were made at Northwestern University and Hypres (SeeQC), Inc. (Nevirkovets et al., 2014; 2015). SFT is a multiterminal device with the SIS'FIFS (or SFIFS'IS) structure (where S, I, and F denote a superconductor, an insulator, and a ferromagnetic material, respectively) exploiting intense quasiparticle injection in order to modify the nonlinear I-V curve of a superconducting tunnel junction. SFT is capable of providing voltage, current, and power amplification while having good input/output isolation. We characterized the devices using different measurement techniques. We measured S parameters of the single- and double-acceptor devices at frequencies up to 5 MHz. Importantly, we confirmed that the isolation between the input and output of the device is quite good. However, the techniques typically employed to characterize semiconductor devices do not allow for revealing the full potential of our low-resistive SFT devices, especially those having two acceptors. In the latter case, we also tested the devices using the battery-powered current sources with floating grounds. Analyzing double-acceptor I-V curves recorded at different levels of injection currents, for an optimal load, we deduced a small-signal voltage gain of 33 and a power gain of 2.4. We suggest that further improvement of the SFT device parameters is possible in optimized devices, so that the device potentially may serve as a preamplifier for readout of output signals of cryogenic detectors and be useful as an element of other superconductor-based circuits. In addition, we used scanning transmission electron microscopy to identify some problems in the fabrication of the devices without any planarization.

Phonon-mediated quasiparticle poisoning of superconducting microwave resonators

Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices subjected to local quasiparticle injection. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the timescale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order of magnitude reduction in quasiparticle loss for a given injected quasiparticle power.

Anomalous relaxation kinetics and charge-density-wave correlations in underdoped BaPb1−xBixO3

Superconductivity often emerges in proximity of other symmetry-breaking ground states, such as antiferromagnetism or charge-density-wave (CDW) order. However, the subtle interrelation of these phases remains poorly understood, and in some cases even the existence of short-range correlations for superconducting compositions is uncertain. In such circumstances, ultrafast experiments can provide new insights by tracking the relaxation kinetics following excitation at frequencies related to the broken-symmetry state. Here, we investigate the transient terahertz conductivity of BaPb1-x Bi x O3--a material for which superconductivity is "adjacent" to a competing CDW phase--after optical excitation tuned to the CDW absorption band. In insulating BaBiO3 we observed an increase in conductivity and a subsequent relaxation, which are consistent with quasiparticles injection across a rigid semiconducting gap. In the doped compound BaPb0.72Bi0.28O3 (superconducting below TC = 7 K), a similar response was also found immediately above TC This observation evidences the presence of a robust gap up to T [Formula: see text] 40 K, which is presumably associated with short-range CDW correlations. A qualitatively different behavior was observed in the same material for [Formula: see text] 40 K. Here, the photoconductivity was dominated by an enhancement in carrier mobility at constant density, suggestive of melting of the CDW correlations rather than excitation across an optical gap. The relaxation displayed a temperature-dependent, Arrhenius-like kinetics, suggestive of the crossing of a free-energy barrier between two phases. These results support the existence of short-range CDW correlations above TC in underdoped BaPb1-x Bi x O3, and provide information on the dynamical interplay between superconductivity and charge order.

Investigation of Current Gain in Superconducting-Ferromagnetic Transistors With High-j ${}_{\rm c}$ Acceptor

We report the results of a study of the current gain in high Josephson critical current density ( j ${}_{\rm c}$ ) superconducting-ferromagnetic transistors with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator (AlO ${}_{x}$ ), and a ferromagnetic material (permalloy, Py), respectively]. The Nb/AlO ${}_{x}$ /Nb trilayer, which serves as the acceptor (SIS) junction, is estimated to have Josephson critical current density j ${}_{\rm c}$ of 19 kA/cm2. The Al/Py/Al/AlO ${}_{x}$ /Py/Al/Nb multilayer is deposited in a separate vacuum run after in situ ion milling of about 8 nm of the top Nb layer. The devices are patterned using optical lithography and tested at 4.2 K. We have observed a small-signal current gain in the range of 5–9. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection.

Dynamical Stability Limit for the Charge Density Wave in K_{0.3}MoO_{3}.

We study the response of the one-dimensional charge density wave in K_{0.3}MoO_{3} to different types of excitation with femtosecond optical pulses. We compare direct excitation of the lattice at midinfrared frequencies with injection of quasiparticles across the low energy charge density wave gap and with charge transfer excitation in the near infrared. For all three cases, we observe a fluence threshold above which the amplitude-mode oscillation frequency is softened and the mode becomes increasingly damped. We show that all the data can be collapsed onto a universal curve in which the melting of the charge density wave occurs abruptly at a critical lattice excursion. These data highlight the existence of a universal stability limit for a charge density wave, reminiscent of the Lindemann criterion for the melting of a crystal lattice.

Nonlinear terahertz spectroscopy of Higgs mode in s-wave superconductors

We review our recent experiments of ultrafast dynamics in s-wave superconductors Nb1-xTixN by using nonlinear terahertz (THz) spectroscopy. The free oscillation of the Higgs mode, i.e. the amplitude mode of the superconducting order parameter, is observed after instantaneous injection of quasiparticles at the superconducting gap edge by an intense monocycle THz pulse. The ultrafast nonequilibrium dynamics of the order parameter under the strong AC driving field with the photon energy tuned below the superconducting gap is also investigated. A resonant nonlinear interaction between the Higgs mode and the electromagnetic field is revealed, as manifested by an efficient THz third-harmonic generation from the superconductor.

Critical Current Gain in High-jc Superconducting-Ferromagnetic Transistors

We report experimental and theoretical results on the current gain in superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator ( ${\rm{AlO}}_{x}$ ), and a ferromagnetic material (Ni), respectively]. The Josephson critical current density $j_{{\rm{ca}}}$ of the acceptor (SISF) junction in the devices is above 9 kA/cm2, which is higher than that in our previous devices. The critical current gain is defined as $ \vert \delta I_{{\rm{ca}}}\vert / \vert \delta I_{i}\vert $ , where a change $\vert \delta I_{i}\vert $ in the injector (SFIFS junction) current produces a change $\vert \delta I_{{\rm{ca}}}\vert $ in the acceptor maximum Josephson current. We observed a small-signal gain as high as 7.8 and a large-signal current gain of about 2.2. In addition, we found that the Josephson current of the acceptor junction is sensitive to the state of the ferromagnetic layers. A theoretical model is proposed to describe the nonequilibrium processes in the SFT devices, which agrees with the experimental observations. The calculation shows that, in the present device configuration, the dominant contribution to the gap suppression in the middle Nb electrode is due to the quasiparticle injection; spin injection plays a secondary role. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection. We conclude that SFT devices can be used as input/output isolators and amplifiers for memory, digital, and RF applications.

Injection of nonequilibrium quasiparticles into Zeeman-split superconductors: A way to create long-range spin imbalance

A theory of spin transport and spin detection in Zeeman-split superconducting films at low temperatures is developed. It is shown that an injection of spin-unpolarized quasiparticles into a Zeeman-split superconductor gives rise to a spin imbalance. The relaxation length of such a spin signal is determined by the energy relaxation length and can be extremely large as compared to the usual spin relaxation length. There can exist two types of signals: due to nonthermalized quasiparticle distribution and due to thermalized overheated electron distribution. They have different decay lengths and can be distinguished by their different dependencies on the applied voltage. The decay length of the nonthermalized signal is determined by the electron-electron scattering rate, renormalized due to superconductivity. The decay length of the thermalized signal is determined by the length on which energy leaves the electronic subsystem and can be very large under special conditions. Applications of the theory to recent experimental data on spin relaxation in Zeeman-split and exchange-split superconductors are discussed. In particular, it can explain the extremely high spin relaxation lengths, experimentally observed in Zeeman-split superconductors, and their growth with the magnetic field and with the applied voltage.

Control of Supercurrent in Hybrid Superconducting–Ferromagnetic Transistors

We report experimental results on characteristics of SFIFS junctions and multiterminal SFIFSIS and SISFIS devices (where S, I, and F denote a superconductor (Nb), an insulator (AlOx), and a ferromagnetic material (Ni), respectively). The SFIFS (SFIS) junctions serve as injectors in the SFIFSIS (SISFIS) devices, which have transistorlike properties; for this reason, we call them superconducting-ferromagnetic transistors. We have found the F (Ni) thickness at which the SFIFS current-voltage characteristic becomes linear. For three-terminal SFIFSIS devices, we focused on studying the influence of the injection current through the SFIFS junction on the maximum Josephson current of the SIS acceptor. For four-terminal SISFIS devices, we studied dependence of the transport current through the middle Nb electrode on injection current level through the SFIS junction. In both cases, we found that the output superconducting current (either Josephson or transport) can be efficiently modulated by the quasiparticle injection from SFIFS or SFIS input junction. The experiments indicate that, after optimization of the device parameters, they can be used as input/output isolators and amplifiers for memory, digital, and RF applications.

Superconducting spintronics

Traditional studies that combine spintronics and superconductivity have mainly focused on the injection of spin-polarized quasiparticles into superconducting materials. However, a complete synergy between superconducting and magnetic orders turns out to be possible through the creation of spin-triplet Cooper pairs, which are generated at carefully engineered superconductor interfaces with ferromagnetic materials. Currently, there is intense activity focused on identifying materials combinations that merge superconductivity and spintronics to enhance device functionality and performance. The results look promising: it has been shown, for example, that superconducting order can greatly enhance central effects in spintronics such as spin injection and magnetoresistance. Here, we review the experimental and theoretical advances in this field and provide an outlook for upcoming challenges in superconducting spintronics.

Influence of spin injection on the in-plane and out-of-plane transport properties of BSCCO single crystal

We have studied the influence of spin injection on the in-plane and out-of-plane transport properties of $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ . Two different types of samples have been fabricated. One is a mesa composed of $\hbox{Co/Au/Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ for the out-of-plane transport measurement, and another is $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ single crystal bridge for the in-plane transport measurement. For the former, the hysteretic field dependence of $I_{c} $ of intrinsic Josephson junctions (IJJs) in the mesa, due to the spin injection effect, was observed at 77 K. Moreover, it was observed that the hysteresis in the $I_{c}$ – $B$ curves decreased with increasing the number of IJJs in mesas. This suggests that the contribution of the Josephson effect to the $c$ -axis transport property becomes larger than that of the spin injection effect with the increase in the number of junction. On the other hand, for the latter, hysteretic field dependence of the magnetoresistance was observed. These results give us the evidence for the superconductivity suppression of $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ due to the injection of spin-polarized quasiparticles.

Effect of injected spins with different polarized orientations on the superconductivity of La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 thin films

The influence of spin-polarized quasi-particle injection on the superconductivity of epitaxial La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 composite thin films is systematically investigated. Different electrode configurations on the same specimen are designed to study the role of spin injection. It is found that the upper critical field and flux pinning potential are reduced by the spin injection, especially for the field parallel to ab-plane. The manipulation of superconductivity by the spin polarization orientations of injected quasi-particles possibly originates from the different suppressions on the superconducting pairing strength, which is considered to be related to the local antiferromagnetic spin fluctuations in La1.85Sr0.15CuO4.

Instability of Nonequilibrium Superconducting State and the Proximity Effect in MoRe/Al Sandwiches

The proximity effect in MoRe/Al sandwiches with different critical temperatures T c of MoRe films has been studied. The estimation of the proximity effect parameters has been made and their dependence on the Al thickness built. Features associated with instability of nonequilibrium superconducting state have been observed in the current-voltage characteristics of MoRe/Al–I–Pb tunnel junctions as a result of quasiparticle tunnel injection into the MoRe/Al sandwich. The estimation of the parameter of tunnel injection has been made and its impact on instabilities emergence studied. MoRe–I–Pb tunnel junctions have been produced and their current-voltage characteristics studied to be compared with the results obtained.

The impact of quasiparticle tunnel injection on the proximity effect in Pb/Sn sandwich

The proximity effect in Pb/Sn sandwich under the impact of quasiparticle tunnel injection has been studied. The estimation of the γ and γBN proximity effect parameters has been made; the dependence of the γ and γBN parameters on the Sn thickness has been built. It has been shown by studying the current-voltage characteristics of the double-barrier tunnel junction with a Pb/Sn intermediate proximity electrode that an increase of the γm parameter of the strength of the proximity effect occurs as a result of quasiparticle tunnel injection into the Pb/Sn proximity electrode.