Epitaxial Superconducting Research Articles

Progress in Superconductor-Semiconductor Topological Josephson Junctions

Majorana bound states (MBSs) are quasiparticles that are their own antiparticles. They are predicted to emerge as zero-energy modes localized at the boundary of a topological superconductor. No intrinsic topological superconductor is known to date. However, by interfacing conventional superconductors and semiconductors with strong spin-orbit coupling, it is possible to create a system hosting topological states. Hence, epitaxial superconductors and semiconductors have emerged as an attractive material system with atomically sharp interfaces and broad flexibility in device fabrications incorporating Josephson junctions. We discuss the basics of topological superconductivity and provide insight into how to go beyond current state-of-the-art experiments. We argue that the ultimate success in realizing MBS physics requires the observation of non-Abelian braiding and fusion experiments. Published by the American Physical Society 2024

Role of a capping layer on the crystalline structure of Sn thin films grown at cryogenic temperatures on InSb substrates

Metal deposition with cryogenic cooling is a common technique in the condensed matter community for producing ultra-thin epitaxial superconducting layers on semiconductors. However, a significant challenge arises when these films return to room temperature, as they tend to undergo dewetting. This issue can be mitigated by capping the films with an amorphous layer. In this study, we investigate the influence of different in situ fabricated caps on the structural characteristics of Sn thin films deposited at 80 K on InSb substrates. Regardless of the type of capping, we consistently observe that the films remain smooth upon returning to room temperature and exhibit epitaxy on InSb in the cubic Sn (α-Sn) phase. Notably, we identify a correlation between alumina capping using an electron beam evaporator and an increased presence of tetragonal Sn (β-Sn) grains. This suggests that heating from the alumina source may induce a partial phase transition in the Sn layer. The existence of the β-Sn phase induces superconducting behavior of the films by percolation effect. This study highlights the potential for tailoring the structural properties of cryogenic Sn thin films through in situ capping. This development opens avenues for precise control in the production of superconducting Sn films, facilitating their integration into quantum computing platforms.

Large parametric amplification in kinetic inductance dominant resonators based on 3 nm-thick epitaxial superconductors

Ultra-thin superconducting aluminum films of 3-nm grown on sapphire by molecule-beam epitaxy show excellent superconductivity and large kinetic inductance. This results in a record high Kerr non-linearity of 33 kHz and 3.62 MHz per photon in notch-type and transmission-type resonators, respectively. 4-wave mixing leverages this non-linearity to achieve 12 dB parametric amplification in transmission type resonator, making the ultra-thin film ideal for photon detection and amplification applications.

Parallel InAs nanowires for Cooper pair splitters with Coulomb repulsion

Hybrid nanostructures consisting of two parallel InAs nanowires connected by an epitaxially grown superconductor (SC) shell recently became available. Due to the defect-free SC-semiconductor interface and the two quasi-one-dimensional channels being close by, these platforms can be utilized to spatially separate entangled pairs of electrons by using quantum dots (QD) in the so-called Cooper pair splitting (CPS) process. The minimized distance between the QDs overcomes the limitations of single-wire-based geometries and can boost the splitting efficiency. Here we investigate CPS in such a device where strong inter-dot Coulomb repulsion is also present and studied thoroughly. We analyze theoretically the slight reduction of the CPS efficiency imposed by the Coulomb interaction and compare it to the experiments. Despite the competition between crossed Andreev reflection (CAR) and inter-wire capacitance, a significant CPS signal is observed indicating the dominance of the superconducting coupling. Our results demonstrate that the application of parallel InAs nanowires with epitaxial SC is a promising route for the realization of parafermionic states relying on enhanced CAR between the wires.

Towards merged-element transmons using silicon fins: The FinMET

A merged-element transmon (MET) device based on silicon (Si) fins is proposed, and the first steps to form such a “FinMET” are demonstrated. This new application of fin technology capitalizes on the anisotropic etch of Si(111) relative to Si(110) to define atomically flat, high aspect ratio Si tunnel barriers with epitaxial superconductor contacts on parallel sidewall surfaces. This process circumvents the challenges associated with the growth of low-loss insulating barriers on lattice matched superconductors. By implementing low-loss, intrinsic float-zone Si as the barrier material rather than commonly used, potentially lossy AlOx, the FinMET is expected to overcome problems with standard transmons by (1) reducing dielectric losses; (2) minimizing the formation of two-level system spectral features; (3) exhibiting greater control over barrier thickness and qubit frequency spread, especially when combined with commercial fin fabrication and atomic-layer or digital etching; (4) potentially reducing the footprint by several orders of magnitude; and (5) allowing scalable fabrication. Here, as a first step to making such a device, the fabrication of Si fin capacitors on Si(110) substrates with shadow-deposited Al electrodes is demonstrated. These fin capacitors are then fabricated into lumped element resonator circuits and probed using low-temperature microwave measurements. Further thinning of silicon junctions toward the tunneling regime will enable the scalable fabrication of FinMET devices based on existing silicon technology while simultaneously avoiding lossy amorphous dielectrics for the tunnel barriers.

Tunable proximity effects and topological superconductivity in ferromagnetic hybrid nanowires

Hybrid semiconducting nanowire devices combining epitaxial superconductor and ferromagnetic insulator layers have been recently explored experimentally as an alternative platform for topological superconductivity at zero applied magnetic field. In this proof-of-principle work we show that the topological regime can be reached in actual devices depending on some geometrical constraints. To this end, we perform numerical simulations of InAs wires in which we explicitly include the superconducting Al and magnetic EuS shells, as well as the interaction with the electrostatic environment at a self-consistent mean-field level. Our calculations show that both the magnetic and the superconducting proximity effects on the nanowire can be tuned by nearby gates thanks to their ability to move the wavefunction across the wire section. We find that the topological phase is achieved in significant portions of the phase diagram only in configurations where the Al and EuS layers overlap on some wire facet, due to the rather local direct induced spin polarization and the appearance of an extra indirect exchange field through the superconductor. While of obvious relevance for the explanation of recent experiments, tunable proximity effects are of interest in the broader field of superconducting spintronics.

Topological superconductivity in tripartite superconductor-ferromagnet-semiconductor nanowires

Motivated by recent experiments searching for Majorana zero modes in tripartite semiconductor nanowires with epitaxial superconductor and ferromagnetic-insulator layers, we explore the emergence of topological superconductivity in such devices for paradigmatic arrangements of the three constituents. Accounting for the competition between magnetism and superconductivity, we treat superconductivity self consistently and describe the electronic properties, including the superconducting and ferromagnetic proximity effects, within a direct wave-function approach. We conclude that the most viable mechanism for topological superconductivity relies on a superconductor-semiconductor-ferromagnet arrangement of the constituents, in which spin splitting and superconductivity are independently induced in the semiconductor by proximity and superconductivity is only weakly affected by the ferromagnetic insulator.

Atomic-scale preparation and characterization ofhigh-temperature superconducting thin films

High-temperature ( T c) superconductors have been at the forefront of physics research for more than 30 years due to their rich physical properties and potential applications. Exploring and discovering new high- T c superconductors, as well as hereby clarifying the microscopic mechanism of unconventional superconductivity are among the long-desired goals of the world’s physicists. In this article, starting from the history and current challenges of high- T c superconductors, we review the atomic-scale understanding of conventional high- T c superconductivity based on heteroepitaxial films, especially focusing on the recent experimental advances on molecular beam epitaxy growth and quantum manipulation of high- T c superconducting thin films. We emphasize the atomic-scale exploration of epitaxial superconducting films by means of scanning tunneling microscopy, while a strategy of designing interfacial superconductors between two dissimilar materials is also introduced. We mainly devote to the experimental wisdom, methods, the microscopic understanding of high- T c superconductivity, and hope to inspire the related scientific researchers.

Elucidation of YBCO Growth Mechanism in KOH Flux Method

In this work, we present an extremely low-temperature (600 °C) liquid-phase synthesis method using KOH as flux to grow YBCO powder and epitaxial films. Oxides utilize solution medium KOH in which target materials are generated by cation’s physical and chemical transformations. Y123 and Y124 films could be synthesized by controlling configuration which is related to oxygen diffusion. Later, the process and mechanism of YBCO growth will be elucidated by DSC. In the presence of KOH, the decarboxylation of BaCO3 is kinetically favored with the formation of Ba (OH)2 at the first step, the same as Y2O3 and CuO. Above 350 °C, the formation of Y2Cu2O5 will take precedence over Ba2Cu3O5 due to the weak solubility of Y3+ and Cu2+ in molten KOH. With temperature increasing, liquid Ba(OH)2 begins to dehydrate, and Ba2+ subsequently reacts with Y3+ and Cu2+ to precipitate YBCO in the last step. Molten KOH provides a liquid chemical environment, which allows the synthesis of YBCO at low temperature. This low-temperature method is applicable to epitaxial superconductor and superconducting joint.

Enhanced ferroelectric polarization in epitaxial superconducting–ferroelectric heterostructure for non-volatile memory cell

We report the growth and polarization switching properties of epitaxial ferroelectric–superconducting heterostructure PbZr0.52Ti0.48O3 (PZT) (100 nm)/YBa2Cu3O7−δ (YBCO) (100 nm) thin films for non-volatile ferroelectric random access memory elements. The epitaxial nature of the heterostructure is verified using the reciprocal space mapping data with the superconducting phase transition temperature (Tc) of nearly 25 K far below the Tc of as-grown YBCO under the same condition. The significant remanent polarization (Pr) ∼ 45 µC/cm2 at 1 kHz can switch from one state to another using 1 μs pulse. The devices meet the basic criteria of memory elements, such as high resistance ∼10 GΩ at 8 V, a butterfly-like capacitance–voltage (C/V) loop, significant polarization, a sharp change in the displacement current, long-time charge retention, and small fatigue at room temperature.

Epitaxial superconducting δ-MoN and δ-NbN thin films by a chemical solution deposition

Epitaxial superconducting δ-MoN and δ-NbN thin films, exhibiting a superconducting transition temperature (Tconset) of 13 K and 10.3 K, respectively, were prepared by a simple chemical solution deposition on c-cut Al2O3 substrates. A sharp transition and the large residual resistivity ratio (RRR) ρ300K/ρonset of δ-MoN and δ-NbN thin films suggest high quality of the prepared epitaxial thin films. The critical current density (Jc) under 100 Oe at 5 K is of 1.37 and 0.74 MA/cm2 for the δ-MoN and δ-NbN thin films, respectively. The results will provide a facile route to prepare epitaxial superconducting metal nitride thin films.

Enhancing superconductivity in bulk β−Bi2Pd by negative pressure induced by quantum electronic stress

An effective way to enhance the critical temperature (${T}_{c}$) of bulk superconductivity (SC) is employing external pressure accompanied with or without phase transitions. However, the variations of ${T}_{c}$ are not always ``positively'' proportional to the pressure even in the same superconducting phase. There exist a class of bulk superconductors such as $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$, whose ${T}_{c}$ will be suppressed by pressure starting from the atmospheric pressure. Then the pressure approach of enhancing SC becomes invalid because it is impossible to apply a negative pressure. Here we demonstrate that quantum electronic stress (QES) induced by electronic doping affords a general approach to further enhance ${T}_{c}$ of such kinds of bulk superconductors without causing phase transition. Our first-principles calculations show that electron doping gives rise to a negative QES acting as an effective ``negative'' pressure, which expands the lattice of $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$. Consequently, it leads to an increase of electronic density of states at the Fermi level and softening of phonon vibration modes, which in turn enhances the strength of electron-phonon coupling. Conversely, the same concept also explains the experimental reports of pressure and hole-type substitution suppressed SC of bulk $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$, which unifies the effects of pressure and chemical substitution into the same theoretical framework of QES tuning SC. We also envision that surface QES induced by quantum confinement may play an important role in affecting the ${T}_{c}$ of epitaxial superconductors, such as the experimentally reported $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$ thin films.

Supercurrent in a Double Quantum Dot.

We demonstrate the Josephson effect in a serial double quantum dot defined in a nanowire with epitaxial superconducting leads. The supercurrent stability diagram adopts a honeycomb pattern. We observe sharp discontinuities in the magnitude of the critical current, I_{c}, as a function of dot occupation, related to doublet to singlet ground state transitions. Detuning of the energy levels offers a tuning knob for I_{c}, which attains a maximum at zero detuning. The consistency between experiment and theory indicates that our device is a faithful realization of the two-impurity Anderson model.

Scanning tunneling microscopic observation of enhanced superconductivity in epitaxial Sn islands grown on SrTiO3 substrate

Recent experimental and theoretical studies of single-layer FeSe film grown on SrTiO3 have revealed interface enhanced superconductivity, which opens up a pathway to promote the superconducting transition temperature. Here, to investigate the role of SrTiO3 substrate in epitaxial superconducting film, we grew a conventional superconductor β-Sn (bulk Tc ∼ 3.72 K) onto SrTiO3 substrate by molecular beam epitaxy. By employing scanning tunneling microscope and spectroscopic measurements, an enhanced Tc of 8.2 K is found for epitaxial β-Sn islands, deduced by fitting the temperature dependence of the gap values using the BCS formula. The observed interfacial charge injection and enhanced electron–phonon coupling are responsible for this Tc enhancement. Moreover, the critical field of 8.3 T exhibits a tremendous increase due to the suppression of the vortex formation. Therefore, the coexistence of enhanced superconductivity and high critical field of Sn islands demonstrates a feasible and effective route to improve the superconductivity by growing the islands of conventional superconductors on perovskite-type titanium oxide substrates.

GaN/NbN epitaxial semiconductor/superconductor heterostructures.

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors-silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor-an electronic gain element-to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance-a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.

Superconducting mesa-structures with a spin filtering manganite interlayer

We report on current transport and magnetism in hybrid superconducting mesa-structures prepared from epitaxial superconductor YBa2Cu3O7-δ and manganite LaMnO3 thin films. Superconducting Au-Nb bilayer was the top electrode. Measurements of magnetic resonance in heterostructure with the only Au top electrode (without Nb) showed ferromagnetic state close to Curie temperature of LaMnO3, 150 K. A spin filtering was revealed by measurements of tunneling magneto-resistance. Mesa-structures had no superconducting current even at reduced temperature T=300 mK and the smallest manganite interlayer thickness, dM=5.6 nm. Varying the external magnetic field |H|<10 Oe and dc bias current |I|<0.5 mA the microwave generation with power P <1 pW with a line-width of order 50 MHz was observed in GHz frequency band. The frequency of generation could be tuned by the biasing current with a rate 1013 Hz/A. The impact of spin polarized current on microwave generation is discussed taking into account the asymmetry of oscillation amplitudes over the applied magnetic field.

Generation of microwave oscillations in a superconducting tunnel mesa-structure with a ferromagnetic insulator interlayer

Spin-polarized current in thin-film tunnel mesa-structures formed by epitaxial cuprate superconducting (YBa2Cu3O7–δ) and manganite (LaMnO3) films and an upper superconducting Au–Nb bilayer is studied experimentally. Intrinsic narrow-band generation in the microwave range is reported. Its frequency is tuned by the bias voltage and an external magnetic field.

A Chemical Vapor Deposition Route to Epitaxial Superconducting NbTiN Thin Films

The deposition of epitaxial superconducting (Nb,Ti)N thin films is addressed with a new approach, using a chemical vapor deposition technique with in situ production of precursors. Both classic and reactive CVD process are optimized toward (i) the control of crystal structure of the deposited films and (ii) the control of stoichiometry and thus of superconducting properties. Films are chararcterized using thermodynamic, structural, and electrical characterization tools. Precession electron diffraction and electron backscatter diffraction techniques provide the phase and orientation mapping of the films down to the nanometer scale. We demonstrate that the cubic phase (structure with the most prominent superconducting properties) is the thermodynamically stable phase under classic CVD from 800 °C up to 1300 °C. The hexagonal (Nb,Ti)N is formed via a “nitridation like” process under reactive CVD, up to 1200 °C. The composition of the films is controlled by the chemistry of the gas phase, whereas the thicknes...

Transparent Semiconductor-Superconductor Interface and Induced Gap in an Epitaxial Heterostructure Josephson Junction

Measurement of multiple Andreev reflection (MAR) in a Josephson junction made from an InAs heterostructure with epitaxial aluminum is used to quantify the highly transparent semiconductor-superconductor interface, indicating near-unity transmission. The observed temperature dependence of MAR does not follow a conventional BCS form, but instead agrees with a model in which the density of states in the quantum well acquires an effective induced gap, in our case 180 {\mu}eV, close to that of the epitaxial superconductor. Carrier density dependence of MAR is investigated using a depletion gate, revealing the subband structure of the semiconductor quantum well, consistent with magnetotransport experiment of the bare InAs performed on the same wafer.

Epitaxial growth of insulating and superconducting monolayers of (BETS)2GaCl4on Ag(111)

The ability to fabricate crystalline monolayers of confined superconducting or magnetic condensate on surfaces is a key issue to realize new functionalities and understand the nature of competing orders in their phase diagram at the nanoscale. Herein, we outline a reliable method to pattern a monolayer of superconducting islands and Kagome lattice of (BETS)2GaCl4 (where BETS = bis(ethylenedithio) tetraselenafulvalene) on Ag(111). At a deposition temperature of 125 K, (BETS)2GaCl4 dimers form Kagome lattice with a pore size of 1.2 nm, making it possible to encapsulate small molecules within the nanoporous network. When deposited at 300 K, the molecules retain their superconducting structure and minimize substrate interaction by aligning their long molecular axis perpendicular to the substrate. These results provide guidelines for facile controllable fabrications of epitaxial superconducting and/or magnetic confined condensates on metal surfaces.