Superconducting Point Contacts Research Articles

Quantized conductance in hybrid split-gate arrays of superconducting quantum point contacts with semiconducting two-dimensional electron systems

A quantum point contact (QPC)—a constriction in a semiconducting two-dimensional electron system with a quantized conductance—is a building block of novel spintronic and topological electronic circuits. QPCs can also be used as readout electronics, charge sensors, or switches in quantum nanocircuits. A short and impurity-free constriction with superconducting contacts is a Cooper-pair QPC analogue known as a superconducting quantum point contact (SQPC). The technological development of such quantum devices has been prolonged due to the challenges of maintaining their geometrical requirement and near-unity superconductor-semiconductor interface transparency. Here, we develop advanced nanofabrication, material and device engineering techniques and report on an innovative realization of nanoscale hybrid SQPC arrays with split gate technology in semiconducting 2D electron systems. We exploit the special gate tunability of the quantum wells, and demonstrate the first experimental observation of conductance quantization in hybrid InGaAs-Nb SQPCs. We observe reproducible quantized conductance at zero magnetic fields in multiple quantum nanodevices fabricated in a single chip and systematically investigate the quantum transport of SQPCs at low and high magnetic fields for their potential applications in quantum metrology, for extremely accurate voltage standards, and fault-tolerant quantum technologies. Published by the American Physical Society 2024

Microwave sensing of Andreev bound states in a gate-defined superconducting quantum point contact

We use a superconducting microresonator as a cavity to sense absorption of microwaves by a superconducting quantum point contact defined by surface gates over a proximitized two-dimensional electron gas. Renormalization of the cavity frequency with phase difference across the point contact is consistent with coupling to Andreev bound states. Near \ensuremath{\pi} phase difference, we observe random fluctuations in absorption with gate voltage, related to quantum interference-induced modulations in the electron transmission. Close to pinch-off, we identify features consistent with the presence of a single Andreev bound state and describe the Andreev-cavity interaction using a Jaynes-Cummings model. By fitting the weak Andreev-cavity coupling, we extract \ensuremath{\sim}GHz decoherence consistent with charge noise and the transmission dispersion associated with a localized state.

Nonballistic transport characteristics of superconducting point contacts

In the "ballistic" regime, the transport across a normal metal (N)/superconductor (S) point-contact is dominated by a quantum process called Andreev reflection. Andreev reflection causes an enhancement of the conductance below the superconducting energy gap, and the ratio of the low-bias and the high-bias conductance cannot be greater than 2 when the superconductor is conventional in nature. In this regime, the features associated with Andreev reflection also provide energy and momentum-resolved spectroscopic information about the superconducting phase. Here we theoretically consider various types of N/S point contacts, away from the ballistic regime, and show that even when the superconductor under investigation is simple conventional in nature, depending on the shape, size and anatomy of the point contacts, a wide variety of spectral features may appear in the conductance spectra. Such features may misleadingly mimic theoretically expected signatures of exotic physical phenomena like Klein tunneling in topological superconductors, Andreev bound states in unconventional superconductors, multiband superconductivity and Majorana zero modes.

Above-gap differential conductance dips in superconducting point contacts

Point-contact Andreev spectroscopy is one of the most powerful techniques for obtaining quantitative energy and momentum-resolved information on the electronic properties of a superconductor. However, appropriate measurements often reveal unexpected conductance features that cannot be adequately modeled using conventional theory. In particular, these include conductance dips at voltage biases slightly exceeding the expected energy gap value for a superconductor. Guided by recent point-contact measurements of proximity-induced superconductivity in topological materials, we explain these features by inhomogeneous superconducting state in the studied material, conditionally divided into two parts with different gap values and a semitransparent potential barrier between them. To test this assumption, we designed a model experiment that mimics the main features of the model. The heterostructure under study is formed by an almost ideal point contact, formed using a new technology of self-textured nanofilaments based on the resistive-switching effect in a TiO2 film, a Pt counter-electrode, and a proximized Al/Ni0.5Cu0.5/NbN system, where superconducting correlations from the NbN superconductor are penetrating into a weak ferromagnetic Ni0.5Cu0.5 alloy in contact with an Al film. For a quantitative description of the conductance dips observed in all such devices and their evolution with temperature, we propose an extension of the conventional theory, the main element of which is a non-equilibrium-driven superconducting-to-normal transition and a crossover from coherent to sequential transmission in the Al/NiCu bilayer. Understanding the origin of the conductance dips imposes some restrictions on the explanation of induced superconductivity and paves the way for discovering novel superconducting materials with advanced properties.

Levitons in superconducting point contacts

We investigate the transport properties of a superconducting quantum point contact in the presence of an arbitrary periodic drive. In particular, we calculate the dc current and noise in the tunnel limit, obtaining general expressions in terms of photoassisted probabilities. Interesting features can be observed when the frequency is comparable to the gap. Here, we show that quantized Lorentzian pulses minimize the excess noise, further strengthening the hierarchy among different periodic drives observed in the electron quantum optics domain. In this regime, the excess noise is directly connected to the overlap between electron and hole energy distributions driven out of equilibrium by the applied voltage. In the adiabatic limit, where the frequency of the drive is very small compared to the superconducting gap, we recover the conventional Shapiro-spikes physics in the supercurrent.

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.

Thermopower and thermal conductance of a superconducting quantum point contact

We find the charge and heat currents caused by a temperature difference applied to a superconducting point contact or to a quantum point contact between a superconducting and normal conductors. The results are formulated in terms of the properties of the electron scattering matrix of the quantum point contact in its normal state, and are valid at any transmission coefficient. In the low-transmission limit, the new theory provides reliable results, setting the limits for the use of the popular method of tunnel Hamiltonian.

Transport regimes of a split gate superconducting quantum point contact in the two-dimensional LaAlO3/SrTiO3 superfluid

One of the hallmark experiments of quantum transport is the observation of the quantized resistance in a point contact in GaAs/AlGaAs heterostructures. Being formed with split gate technology, these structures represent in an ideal manner equilibrium reservoirs which are connected only through a few electron mode channel. It has been a long standing goal to achieve similar experimental conditions also in superconductors. Here we demonstrate the formation of a superconducting quantum point contact (SQPC) with split gate technology in a two-dimensional superconductor, utilizing the unique gate tunability of the superfluid at the LaAlO3/SrTiO3 interface. When the constriction is tuned through the action of metallic split gates we identify three regimes of transport: First, SQPC for which the supercurrent is carried only by a few quantum transport channels. Second, superconducting island strongly coupled to the equilibrium reservoirs. Third, charge island with a discrete spectrum weakly coupled to the reservoirs.

Discovery of highly spin-polarized conducting surface states in the strong spin-orbit coupling semiconductor Sb2Se3

Majority of the A$_2$B$_3$ type chalcogenide systems with strong spin-orbit coupling, like Bi$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$ etc., are topological insulators. One important exception is Sb$_2$Se$_3$, where a topological non-trivial phase was argued to be possible under ambient conditions, but such a phase could be detected to exist only under pressure. In this Letter, we show that like Bi$_2$Se$_3$, Sb$_2$Se$_3$, displays generation of highly spin-polarized current under mesoscopic superconducting point contacts as measured by point contact Andreev reflection spectroscopy. In addition, we observe a large negative and anisotropic magnetoresistance in Sb$_2$Se$_3$, when the field is rotated in the basal plane. However, unlike in Bi$_2$Se$_3$, in case of Sb$_2$Se$_3$ a prominent quasiparticle interference (QPI) pattern around the defects could be obtained in STM conductance imaging. Thus, our experiments indicate that Sb$_2$Se$_3$ is a regular band insulator under ambient conditions, but due to it's high spin-orbit coupling, non-trivial spin-texture exists on the surface and the system could be on the verge of a topological insulator phase.

Inelastic electron-phonon scattering and excess current in superconducting point contacts with a short coherence length

Nonlinear electrical effects in superconducting S-c-S contacts, including the spectroscopy of electron-phonon interactions (EPI) in these systems, and the recovery of the EPI function from experimental data are discussed. The effect of a magnetic field on the current-voltage characteristics (I-V curves) and their derivatives for ErNi2B2C point contacts (PC) with d ≥ ξ (where d is the diameter of the PC and ξ is the coherence length) is studied. It is found that in zero magnetic fields and in near-critical fields, when the size of the superconducting gap can be neglected, the position of the peaks in dV/dI coincides with the peaks in the Yanson EPI spectra. In low fields the peaks are shifted toward lower energies and in intermediate fields, the peaks split. For PC with diameters greater than or on the order of the coherence length, the relative size of the negative phonon contribution to the excess current is considerably greater than in ballistic contacts. This leads to substantial suppression of the high-frequency peaks in the spectra for the superconducting state. In order to recover the EPI function from these spectra it is necessary to correct their intensities at high energies. For “dirty” NbSe2 and Nb point-contacts with d ≥ ξ, which have no phonon features in the second derivative of the I-V curve in the normal state, the EPI can be reconstructed from the superconducting state.

Magnetic and Superconducting Properties of Vanadium Nanoconstrictions

We study the magnetic and superconducting properties in a paramagnetic vanadium (V) nanoconstriction with changing its size using a mechanically controllable break junction technique. In the normal state the magnetoresistance effect is observed below the diameter d ~ 8 nm. Moreover, a Fano resonance appears around zero-bias voltage in the differential conductance for the atomic-size contacts and changes shape as the size of the constriction changes. On the other hand, below the superconducting critical temperature the superconducting gap features in V contacts are largely different from those in Pb contacts which exhibit typical features expected in superconducting point contacts. Only a single Andreev anomaly at 2Δ, where Δ is the superconducting energy gap, is observed in the spectra of the V contacts, while two anomalies at Δ and 2Δ appear in the case of the Pb contacts. In the tunnel conductance regime, the structure of the superconducting quasiparticle tunneling density of states is not seen in the V spectra in contrast to the Pb spectra. The origin of these features is discussed.

Spin-polarized Shapiro steps and spin-precession-assisted multiple Andreev reflection

We investigate the charge and spin transport of a voltage-biased superconducting point contact coupled to a nanomagnet. The magnetization of the nanomagnet is assumed to precess with the Larmor frequency, $\omega_L$, due to ferromagnetic resonance. The interplay between the ac Josephson current and the magnetization dynamics leads to spin-polarized Shapiro steps at voltages $|V|=\hbar \omega_L/2en$ for $n=1,2,...$ and the subharmonic steps with $n>1$ are a consequence of multiple Andreev reflection (MAR). Moreover, the spin-precession-assisted MAR generates quasiparticle scattering amplitudes that, due to interference, lead to current-voltage characteristics of the dc charge and spin currents with subharmonic gap structures displaying an even-odd effect.

Josephson coupling through one-dimensional ballistic channel in semiconductor-superconductor hybrid quantum point contacts

We study a superconducting quantum point contact made of a narrow In$_{0.75}% $Ga$_{0.25}$As channel with Nb proximity electrodes. The narrow channel is formed in a gate-fitted constriction of InGaAs/InAlAs/InP heterostructure hosting a two-dimensional electron gas. When the channel opening is varied with the gate, the Josephson critical current exhibits a discretized variation that arises from the quantization of the transverse momentum in the channel. The quantization of Josephson critical current persists down to the single-channel regime, providing an unambiguous demonstration of a semiconductor--superconductor hybrid Josephson junction involving only a single ballistic channel.

Spin-precession-assisted supercurrent in a superconducting quantum point contact coupled to a single-molecule magnet

The supercurrent of a quantum point contact coupled to a nanomagnet strongly depends on the dynamics of the nanomagnet's spin. We employ a fully microscopic model to calculate the transport properties of a junction coupled to a spin whose dynamics is modeled as Larmor precession brought about by an external magnetic field and find that the dynamics affects the charge and spin currents by inducing transitions between the continuum states below the superconducting gap edge and the Andreev levels. This redistribution of the quasiparticles leads to a non-equilibrium population of the Andreev levels and an enhancement of the supercurrent which is visible as a modified current-phase relation as well as a non-monotonous critical current as function of temperature. The non-monotonous behavior is accompanied by a corresponding change in spin-transfer torques acting on the precessing spin and leads to the possibility of using temperature as a means to tune the back-action on the spin.

Circuit quantum electrodynamics with a superconducting quantum point contact

We consider a superconducting quantum point contact in a circuit quantum electrodynamics setup. We study three different configurations, attainable with current technology, where a quantum point contact is coupled galvanically to a coplanar waveguide resonator. Furthermore, we demonstrate that the strong and ultrastrong coupling regimes can be achieved with realistic parameters, allowing the coherent exchange between a superconducting quantum point contact and a quantized intracavity field.

Sensing with Superconducting Point Contacts

Superconducting point contacts have been used for measuring magnetic polarizations, identifying magnetic impurities, electronic structures, and even the vibrational modes of small molecules. Due to intrinsically small energy scale in the subgap structures of the supercurrent determined by the size of the superconducting energy gap, superconductors provide ultrahigh sensitivities for high resolution spectroscopies. The so-called Andreev reflection process between normal metal and superconductor carries complex and rich information which can be utilized as powerful sensor when fully exploited. In this review, we would discuss recent experimental and theoretical developments in the supercurrent transport through superconducting point contacts and their relevance to sensing applications, and we would highlight their current issues and potentials. A true utilization of the method based on Andreev reflection analysis opens up possibilities for a new class of ultrasensitive sensors.

Detection of ultrafast oscillations in superconducting point contacts by means of supercurrent measurements

We present a microscopic calculation of the nondissipative current through a superconducting quantum point contact coupled to a mechanical oscillator. Using the non-equilibrium Keldysh Green function approach, we determine the current-phase relation. The latter shows that at certain phases, the current is sharply suppressed. These dips in the current-phase relation provide information about the oscillating frequency and coupling strength of the mechanical oscillator. We also present an effective two-level model from which we obtain analytical expressions describing the position and width of the dips. Our findings are of relevance for nanomechanical resonators based on superconducting materials.

Supercurrent and Andreev bound state dynamics in superconducting quantum point contacts under microwave irradiation

We present here an extensive theoretical analysis of the supercurrent of a superconducting point contact of arbitrary transparency in the presence of a microwave field. Our study is mainly based on two different approaches: a two-level model that describes the dynamics of the Andreev bound states in these systems and a fully microscopic method based on the Keldysh-Green function technique. This combination provides both a deep insight into the physics of irradiated Josephson junctions and quantitative predictions for arbitrary range of parameters. The main predictions of our analysis are: (i) for weak fields and low temperatures, the microwaves can induce transitions between the Andreev states leading to a large suppression of the supercurrent at certain values of the phase, (ii) at strong fields, the current-phase relation is strongly distorted and the corresponding critical current does not follow a simple Bessel-function-like behavior, and (iii) at finite temperature, the microwave field can enhance the critical current by means of transitions connecting the continuum of states outside the gap region and the Andreev states inside the gap. Our study is of relevance for a large variety of superconducting weak links as well as for the proposals of using the Andreev bound states of a point contact for quantum computing applications.

Evidence for Long-Lived Quasiparticles Trapped in Superconducting Point Contacts

We have observed that the supercurrent across phase-biased, highly transmitting atomic size contacts is strongly reduced within a broad phase interval around π. We attribute this effect to quasiparticle trapping in one of the discrete subgap Andreev bound states formed at the contact. Trapping occurs essentially when the Andreev energy is smaller than half the superconducting gap Δ, a situation in which the lifetime of trapped quasiparticles is found to exceed 100 μs. The origin of this sharp energy threshold is presently not understood.

Spin-dependent scattering and the spin polarization of a diffusive current in partly disordered L10 epitaxial FePd

We report magnetic domain wall (DW) resistance in epitaxial films of FePd. When equal numbers of Fe and Pd atoms are present, this material forms an ordered structure with alternating crystal planes of Fe and Pd. We prepared films enriched with Pd to varying degrees, gradually degrading this structure. As might be expected, this increased the electrical resistivity of the films by introducing extra defects that can scatter electrons. However, unexpectedly, the additional resistance arising from the ∼10 nm thick DWs rose as a proportion of the overall resistivity, roughly doubling when halving the degree of chemical ordering—as determined from x-ray diffraction measurements—within the films. These data can be used to infer a rise in the spin polarization of the current flowing in the layers when extra Pd atoms are introduced. On the other hand, a separate measurement of spin polarization using a superconducting point contact technique that is insensitive to electron scattering revealed no changes as extra Pd was introduced. We conclude that Pd atoms scatter electrons of one spin far more strongly than the other, suggesting a possible means of producing highly spin-polarized currents for use in spintronic devices.