Nonsinusoidal current-phase relationship in Josephson junctions from the 3D topological insulator HgTe

Limitations of the Current-Phase Relation Measurements by an Asymmetric dc-SQUID

Exotic quantum transport phenomena established in Josephson junctions (JJs) are reflected by a nonsinusoidal current-phase relation (CPR). The solidified approach to measuring the CPR is via an asymmetric dc-SQUID with a reference JJ that has a high critical current. We probed this method by measuring CPRs of hybrid JJs based on the 3D topological insulator (TI) Bi₂Te₂Se with a nanobridge acting as a reference JJ. We captured both highly skewed and sinusoidal critical current oscillations within single devices which contradict the uniqueness of the CPR. This implies that the widely used method provides inaccurate CPR measurement and leads to misinterpretation. It was shown that the accuracy of the CPR measurement is mediated by the asymmetry in derivatives of the CPRs but not in critical currents, as was previously thought. Finally, we provided considerations for an accurate CPR measurement via the most commonly used reference JJs.

Contribution of Processes in SN Electrodes to the Transport Properties of SN-N-NS Josephson Junctions

In this paper, we present a theoretical study of electronic transport in planar Josephson Superconductor-Normal Metal-Superconductor (SN-N-NS) bridges with arbitrary transparency of the SN interfaces. We formulate and solve the two-dimensional problem of finding the spatial distribution of the supercurrent in the SN electrodes. This allows us to determine the scale of the weak coupling region in the SN-N-NS bridges, i.e., to describe this structure as a serial connection between the Josephson contact and the linear inductance of the current-carrying electrodes. We show that the presence of a two-dimensional spatial current distribution in the SN electrodes leads to a modification of the current-phase relation and the critical current magnitude of the bridges. In particular, the critical current decreases as the overlap area of the SN parts of the electrodes decreases. We show that this is accompanied by a transformation of the SN-N-NS structure from an SNS-type weak link to a double-barrier SINIS contact. In addition, we find the range of interface transparency in order to optimise device performance. The features we have discovered should have a significant impact on the operation of small-scale superconducting electronic devices, and should be taken into account in their design.

Josephson Diode Effect in Supercurrent Interferometers

A Josephson diode is a nonreciprocal circuit element that supports a larger dissipationless supercurrent in one direction than in the other. In this Letter, we propose a class of Josephson diodes based on supercurrent interferometers composed of Andreev bound state Josephson junctions or interacting quantum dot Josephson junctions, which are not diodes themselves but possess nonsinusoidal current-phase relations. We show that such Josephson diodes have several important advantages, like being electrically tunable and requiring only time-reversal breaking by a magnetic flux. We also show that our diodes have a characteristic ac response, revealed by the Shapiro steps. Even the simplest realization of our Josephson diode paradigm that relies on only two junctions can achieve efficiencies of up to ∼40% and, interestingly, far greater efficiencies are achievable by concatenating interferometer loops. We hope that our Letter will stimulate the search for highly tunable Josephson diode effects in Josephson devices based semiconductor-superconductor hybrids, 2d materials, and topological insulators, where nonsinusoidal current-phase relations were recently observed.

Josephson quantum spin thermodynamics

A 1D Josephson junction (JJ) loop, doped with a spin-flipper and attached to two thermal reservoirs is shown to operate as a heat engine, or a refrigerator, or a Joule pump or even as a cold pump. When operating as a quantum heat engine, the efficiency of this device exceeds that of some recent Josephson heat engine proposals. Further, as a quantum refrigerator, the coefficient of performance of this device is much higher than previously proposed JJ based refrigerators. In addition, this device can be tuned from engine mode to refrigerator mode or to any other mode, i.e., Joule pump or cold pump by either tuning the temperature of reservoirs, or via the flux enclosed in the JJ loop. In presence of spin flip scattering we can tune our device from engine mode to other operating modes by only changing the enclosed flux in JJ loop without changing the temperatures of the reservoirs. This is potentially an advantage with respect to other proposals. This makes the proposed device much more versatile as regards possible applications.

Pressure-Induced Superconductivity in HgTe Single-Crystal Film

HgTe film is widely used for quantum Hall well studies and devices, as it has unique properties, like band gap inversion, carrier-type switch, and topological evolution depending on the film thickness modulation near the so-called critical thickness (63.5 Å), while its counterpart bulk materials do not hold these nontrivial properties at ambient pressure. Here, much richer transport properties emerging in bulk HgTe crystal through pressure-tuning are reported. Not only the above-mentioned abnormal properties can be realized in a 400 nm thick bulk HgTe single crystal, but superconductivity is also discovered in a series of high-pressure phases. Combining crystal structure, electrical transport, and Hall coefficient measurements, a p-n carrier type switching is observed in the first high-pressure cinnabar phase. Superconductivity emerges after the semiconductor-to-metal transition at 3.9 GPa and persists up to 54 GPa, crossing four high-pressure phases with an increased upper critical field. Density functional theory calculations confirm that a surface-dominated topologic band structure contributes these exotic properties under high pressure. This discovery presents broad and efficient tuning effects by pressure on the lattice structure and electronic modulations compared to the thickness-dependent critical properties in 2D and 3D topologic insulators and semimetals.

Colossal Magnetoresistance without Mixed Valence in a Layered Phosphide Crystal

Materials with strong magnetoresistive responses are the backbone of spintronic technology, magnetic sensors, and hard drives. Among them, manganese oxides with a mixed valence and a cubic perovskite structure stand out due to their colossal magnetoresistance (CMR). A double exchange interaction underlies the CMR in manganates, whereby charge transport is enhanced when the spins on neighboring Mn³⁺ and Mn⁴⁺ ions are parallel. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here an enormous CMR at low temperatures in EuCd₂ P₂ without manganese, oxygen, mixed valence, or cubic perovskite structure is shown. EuCd₂ P₂ has a layered trigonal lattice and exhibits antiferromagnetic ordering at 11 K. The magnitude of CMR (10⁴ %) in as-grown crystals of EuCd₂ P₂ rivals the magnitude in optimized thin films of manganates. The magnetization, transport, and synchrotron X-ray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics.

One-Dimensional Edge Transport in Few-Layer WTe2

WTe₂ is a layered transitional-metal dichalcogenide (TMD) with a number of intriguing topological properties. Recently, WTe₂ has been predicted to be a higher-order topological insulator (HOTI) with topologically protected hinge states along the edges. The gapless nature of WTe₂ complicates the observation of one-dimensional (1D) topological states in transport due to their small contribution relative to the bulk. Here, we study the behavior of the Josephson effect in magnetic field to distinguish edge from bulk transport. The Josephson effect in few-layer WTe₂ reveals 1D states residing on the edges and steps. Moreover, our data demonstrates a combination of Josephson transport properties observed solely in another HOTI-bismuth, including Josephson transport over micrometer distances, extreme robustness in a magnetic field, and nonsinusoidal current-phase relation (CPR). Our observations strongly suggest the topological origin of the 1D states and that few-layer WTe₂ is a HOTI.

Isolation solution for extreme environmental vibrations for quantum-enabling cryogenic setups installed on raised frames

Cryogenic quantum sensing techniques are developing alongside the ever-increasing requirements for noiseless experimental environments. For instance, several groups have isolated internal system vibrations from cold heads in closed-cycle dilution refrigerators. However, these solutions often do not account for external vibrations, necessitating novel strategies to isolate the entire cryogenic systems from their environments in a particular set of raised cryostats. Here, we introduce a dual-stage external active vibration-isolation solution in conjunction with a closed-cycle dilution refrigerator that isolates it from the environment. This dual stage includes two sets of active attenuators and a customized steel tower for supporting experimental probes at heights of 3 m from the floor. Both stages achieve 20-40 dB of attenuation with the active systems engaged, corresponding to levels of vibration in the VC-G range (a standardized Vibration Criterion appropriate for extremely quiet research spaces) on the cryostat's room temperature baseplate and the steel tower. Our unique vibration isolation solution therefore expands the applications of modern cryogenic equipment beyond exclusively quiet specialty buildings, rendering such equipment suitable for interdisciplinary, open-floor research centers.

Cryogen-free variable temperature scanning SQUID microscope

Scanning Superconducting QUantum Interference Device (SQUID) microscopy is a powerful tool for imaging local magnetic properties of materials and devices, but it requires a low-vibration cryogenic environment, traditionally achieved by thermal contact with a bath of liquid helium or the mixing chamber of a "wet" dilution refrigerator. We mount a SQUID microscope on the 3 K plate of a Bluefors cryocooler and characterize its vibration spectrum by measuring SQUID noise in a region of sharp flux gradient. By implementing passive vibration isolation, we reduce relative sensor-sample vibrations to 20 nm in-plane and 15 nm out-of-plane. A variable-temperature sample stage that is thermally isolated from the SQUID sensor enables the measurement at sample temperatures from 2.8 K to 110 K. We demonstrate these advances by imaging inhomogeneous diamagnetic susceptibility and vortex pinning in optimally doped yttrium barium copper oxide above 90 K.

Probing the magnetoelectric effect in noncentrosymmetric superconductors by equal-spin Andreev tunneling

In noncentrosymmetric superconductors (NCSs), the conversion of a charge current into spin magnetization-the so called magnetoelectric effect-is the direct indicator of the unconventional, mixed-parity order parameter. This paper proposes a scheme to detect the magnetoelectric effect by anomalous, equal-spin Andreev tunneling in NCS/ferromagnet contacts. The proposal relies on the ability to generate spin-polarized triplet pairing by passing an electric current through an NCS. Such an induced triplet pairing bears a similarity to the paradigmatic nonunitary pairing in triplet superfluids with a complex vector order parameter [Formula: see text]. The qualitative difference is that the induced nonunitary state can be realised in NCSs with a purely real [Formula: see text] by breaking the time-reversal symmetry in current-biased setups. This offers a possibility to access the unconventional superconductivity in NCSs through electrical transport measurements.

Chirality Josephson Current Due to a Novel Quantum Anomaly in Inversion-Asymmetric Weyl Semimetals

We study Josephson junctions based on inversion-asymmetric but time-reversal symmetric Weyl semimetals under the influence of Zeeman fields. We find that, due to distinct spin textures, the Weyl nodes of opposite chirality respond differently to an external magnetic field. Remarkably, a Zeeman field perpendicular to the junction direction results in a phase shift of opposite sign in the current-phase relations of opposite chirality. This leads to a finite chirality Josephson current (CJC) even in the absence of a phase difference across the junction. This feature could allow for applications in chiralitytronics. In the long junction and zero temperature limit, the CJC embodies a novel quantum anomaly of Goldstone bosons at π phase difference which is associated with a Z_{2} symmetry at low energies. It can be detected experimentally via an anomalous Fraunhofer pattern.

Second-Harmonic Current-Phase Relation in Josephson Junctions with Ferromagnetic Barriers

We report the observation of a current-phase relation dominated by the second Josephson harmonic in superconductor-ferromagnet-superconductor junctions. The exotic current-phase relation is realized in the vicinity of a temperature-controlled 0-to-π junction transition, at which the first Josephson harmonic vanishes. Direct current-phase relation measurements, as well as Josephson interferometry, nonvanishing supercurrent and half-integer Shapiro steps at the 0-π transition self-consistently point to an intrinsic second harmonic term, making it possible to rule out common alternative origins of half-periodic behavior. While surprising for diffusive multimode junctions, the large second harmonic is in agreement with theory predictions for thin ferromagnetic interlayers.

Weak-link Josephson Junctions Made from Topological Crystalline Insulators

We report on the fabrication of Josephson junctions using the topological crystalline insulator Pb_{0.5}Sn_{0.5}Te as the weak link. The properties of these junctions are characterized and compared to those fabricated with weak links of PbTe, a similar material yet topologically trivial. Most striking is the difference in the ac Josephson effect: junctions made with Pb_{0.5}Sn_{0.5}Te exhibit a rich subharmonic structure consistent with a skewed current-phase relation. This structure is absent in junctions fabricated from PbTe. A discussion is given on the origin of this effect as an indication of novel behavior arising from the topologically nontrivial surface state.

Creation of Spin-Triplet Cooper Pairs in the Absence of Magnetic Ordering

In superconducting spintronics, it is essential to generate spin-triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction-the Dirac surface states of a strong topological insulator (TI)-out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin-triplet filter across the SC, generating equal-spin Cooper pairs via crossed Andreev reflection.

Induced unconventional superconductivity on the surface states of Bi2Te3 topological insulator

Topological superconductivity is central to a variety of novel phenomena involving the interplay between topologically ordered phases and broken-symmetry states. The key ingredient is an unconventional order parameter, with an orbital component containing a chiral p x  + ip y wave term. Here we present phase-sensitive measurements, based on the quantum interference in nanoscale Josephson junctions, realized by using Bi2Te3 topological insulator. We demonstrate that the induced superconductivity is unconventional and consistent with a sign-changing order parameter, such as a chiral p x  + ip y component. The magnetic field pattern of the junctions shows a dip at zero externally applied magnetic field, which is an incontrovertible signature of the simultaneous existence of 0 and π coupling within the junction, inherent to a non trivial order parameter phase. The nano-textured morphology of the Bi2Te3 flakes, and the dramatic role played by thermal strain are the surprising key factors for the display of an unconventional induced order parameter.

Inducing superconductivity in Weyl semimetal microstructures by selective ion sputtering

By introducing a superconducting gap in Weyl or Dirac semimetals, the superconducting state inherits the nontrivial topology of their electronic structure. As a result, Weyl superconductors are expected to host exotic phenomena, such as nonzero-momentum pairing due to their chiral node structure, or zero-energy Majorana modes at the surface. These are of fundamental interest to improve our understanding of correlated topological systems, and, moreover, practical applications in phase-coherent devices and quantum applications have been proposed. Proximity-induced superconductivity promises to allow these experiments on nonsuperconducting Weyl semimetals. We show a new route to reliably fabricate superconducting microstructures from the nonsuperconducting Weyl semimetal NbAs under ion irradiation. The significant difference in the surface binding energy of Nb and As leads to a natural enrichment of Nb at the surface during ion milling, forming a superconducting surface layer (T_c ~ 3.5 K). Being formed from the target crystal itself, the ideal contact between the superconductor and the bulk may enable an effective gapping of the Weyl nodes in the bulk because of the proximity effect. Simple ion irradiation may thus serve as a powerful tool for the fabrication of topological quantum devices from monoarsenides, even on an industrial scale.

Magnetically-driven colossal supercurrent enhancement in InAs nanowire Josephson junctions

The Josephson effect is a fundamental quantum phenomenon where a dissipationless supercurrent is introduced in a weak link between two superconducting electrodes by Andreev reflections. The physical details and topology of the junction drastically modify the properties of the supercurrent and a strong enhancement of the critical supercurrent is expected to occur when the topology of the junction allows an emergence of Majorana bound states. Here we report charge transport measurements in mesoscopic Josephson junctions formed by InAs nanowires and Ti/Al superconducting leads. Our main observation is a colossal enhancement of the critical supercurrent induced by an external magnetic field applied perpendicular to the substrate. This striking and anomalous supercurrent enhancement cannot be described by any known conventional phenomenon of Josephson junctions. We consider these results in the context of topological superconductivity, and show that the observed critical supercurrent enhancement is compatible with a magnetic field-induced topological transition.

Physical details of a Josephson junction may drastically modify the properties of supercurrent. Here, the authors observe a colossal enhancement of the critical supercurrent in a Josephson junction subject to a perpendicular magnetic field, indicating topological phase transitions.

Electrically Tunable Multiterminal SQUID-on-Tip

We present a new nanoscale superconducting quantum interference device (SQUID) whose interference pattern can be shifted electrically in situ. The device consists of a nanoscale four-terminal-four-junction SQUID fabricated at the apex of a sharp pipet using a self-aligned three-step deposition of Pb. In contrast to conventional two-terminal-two-junction SQUIDs that display optimal sensitivity when flux biased to about a quarter of the flux quantum, the additional terminals and junctions allow optimal sensitivity at arbitrary applied flux, thus eliminating the magnetic field "blind spots". We demonstrate spin sensitivity of 5 to 8 μ_B/Hz^1/2 over a continuous field range of 0 to 0.5 T with promising applications for nanoscale scanning magnetic imaging.

Probing Quantum Capacitance in a 3D Topological Insulator

We measure the quantum capacitance and probe thus directly the electronic density of states of the high mobility, Dirac type two-dimensional electron system, which forms on the surface of strained HgTe. Here we show that observed magnetocapacitance oscillations probe-in contrast to magnetotransport-primarily the top surface. Capacitance measurements constitute thus a powerful tool to probe only one topological surface and to reconstruct its Landau level spectrum for different positions of the Fermi energy.

4π-periodic Josephson supercurrent in HgTe-based topological Josephson junctions

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4π-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction made of a HgTe weak link. The response is understood as due to a 4π-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.

Realization of Microwave Quantum Circuits Using Hybrid Superconducting-Semiconducting Nanowire Josephson Elements

We report the realization of quantum microwave circuits using hybrid superconductor-semiconductor Josephson elements comprised of InAs nanowires contacted by NbTiN. Capacitively shunted single elements behave as transmon circuits with electrically tunable transition frequencies. Two-element circuits also exhibit transmonlike behavior near zero applied flux but behave as flux qubits at half the flux quantum, where nonsinusoidal current-phase relations in the elements produce a double-well Josephson potential. These hybrid Josephson elements are promising for applications requiring microwave superconducting circuits operating in a magnetic field.