Superconductivity In Ultrathin Films Research Articles

Vortex confinement through an unquantized magnetic flux

Geometrically confined superconductors often experience a breakdown in the quantization of magnetic flux owing to the incomplete screening of the supercurrent against field penetration. In this study, we report that magnetic field confinement occurs regardless of the dimensionality of the system, even extending to 1D linear potential systems. By using a vector-field magnetic force microscope, we successfully create a vortex‒antivortex pair connected by a 1D unquantized magnetic flux in ultrathin superconducting films. Through an investigation of the manipulation and thermal behavior of the vortex pair, we uncover a long-range interaction mediated by the unquantized magnetic flux. These findings suggest a universal phenomenon of unquantized magnetic flux formation, independent of the geometry of the system. Our results present an experimental route for investigating the impact of confinement on superconducting properties and order parameters in unconventional superconductors characterized by extremely low dimensionality.

Van der Waals Self-Epitaxial Growth of Inch-Sized Superconducting Niobium Diselenide Films.

Ultrathin superconducting films are the basis of superconductor devices. van der Waals (vdW) NbSe2 with noncentrosymmetry exhibits exotic superconductivity and shows promise in superconductor electronic devices. However, the growth of inch-scale NbSe2 films with layer regulation remains a challenge because vdW structural material growth is strongly dependent on the epitaxial guidance of the substrate. Herein, a vdW self-epitaxy strategy is developed to eliminate the substrate driving force in film growth and realize inch-sized NbSe2 film growth with thicknesses from 2.1 to 12.1 nm on arbitrary substrates. The superconducting transition temperature of 5.1 K and superconducting transition width of 0.30 K prove the top homogeneity and quality of superconductivity among all of the synthetic NbSe2 films. Coupled with a large area and substrate compatibility, this work paves the way for developing NbSe2 superconductor electronics.

Ultrathin epitaxial MgB2 on SiC: Substrate surface-polarity-dependent properties

High-quality, ultrathin superconducting films are required for advanced devices such as hot-electron bolometers, superconducting nanowire single-photon detectors, and quantum applications. Using hybrid physical-chemical vapor deposition, we show that ${\mathrm{MgB}}_{2}$ films as thin as 4 nm can be fabricated on the carbon-terminated $6H$-SiC (0001) surface with a superconducting transition temperature above 33 K and a rms roughness of 0.7 nm. Remarkably, the film quality is a function of the SiC surface termination, with the C-terminated surface preferred to the Si-terminated surface. To understand the ${\mathrm{MgB}}_{2}$ thin film/SiC substrate interactions giving rise to this difference, we characterized the interfacial structures using Rutherford backscattering spectroscopy/channeling, electron-energy-loss spectroscopy, and x-ray photoemission spectroscopy. The ${\mathrm{MgB}}_{2}$/SiC interface structure is complex and different for the two terminations. Both terminations incorporate substantial unintentional oxide layers influencing ${\mathrm{MgB}}_{2}$ growth and morphology, but with a different extent, intermixing, and interface chemistry. In this paper, we report measurements of transport, resistivity, and the critical superconducting temperature of ${\mathrm{MgB}}_{2}$/SiC that are different for the two terminations, and they link interfacial structure variations to observed differences. The result shows that the C face of SiC is a preferred substrate for the deposition of ultrathin superconducting ${\mathrm{MgB}}_{2}$ films.

Fabrication and transport properties of two dimensional Bi2Sr2Ca2Cu3O10+δ micro-bridge

Ultra-thin high-temperature superconducting films have attracted continuous interest due to their potential electronic applications, which also provide a unique platform of novel physics and properties in the two-dimensional limit. We, here, realized fabrication of two-unit-cell-thick micro-bridges from mechanically exfoliated ultra-thin Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals and systematically investigated their transport properties. The two-dimensional superconducting nature is verified by the existence of the Berezinskii–Kosterlitz–Thouless transition, which is simultaneously revealed by current-voltage properties and the zero-field temperature dependence of resistance. Comparing with Bi2223 bulk crystal, a Bi2223 micro-bridge shows a slight lower upper critical field but pronounced improvement in the critical current density. Our findings indicate that the ultra-thin Bi2223 single crystal is highly prospective for both scientific investigations of unconventional superconductivity and applications of high Tc superconducting devices.

Recent Advances in 2D Superconductors.

The emergence of superconductivity in 2D materials has attracted much attention and there has been rapid development in recent years because of their fruitful physical properties, such as high transition temperature (Tc ), continuous phase transition, and enhanced parallel critical magnetic field (Bc ). Tremendous efforts have been devoted to exploring different physical parameters to figure out the mechanisms behind the unexpected superconductivity phenomena, including adjusting the thickness of samples, fabricating various heterostructures, tuning the carrier density by electric field and chemical doping, and so on. Here, different types of 2D superconductivity with their unique characteristics are introduced, including the conventional Bardeen-Cooper-Schrieffer superconductivity in ultrathin films, high-Tc superconductivity in Fe-based and Cu-based 2D superconductors, unconventional superconductivity in newly discovered twist-angle bilayer graphene, superconductivity with enhanced Bc , and topological superconductivity. A perspective toward this field is then proposed based on academic knowledge from the recently reported literature. The aim is to provide researchers with a clear and comprehensive understanding about the newly developed 2D superconductivity and promote the development of this field much further.

Type-II Ising Superconductivity and Anomalous Metallic State in Macro-Size Ambient-Stable Ultrathin Crystalline Films.

Recent emergence of two-dimensional (2D) crystalline superconductors has provided a promising platform to investigate novel quantum physics and potential applications. To reveal essential quantum phenomena therein, ultralow temperature transport investigation on high-quality ultrathin superconducting films is critically required, although it has been quite challenging experimentally. Here, we report a systematic transport study on the ultrathin crystalline PdTe2 films grown by molecular beam epitaxy (MBE). Interestingly, a new type of Ising superconductivity in 2D centrosymmetric materials is revealed by the detection of large in-plane critical field more than 7 times the Pauli limit. Remarkably, in a perpendicular magnetic field, we provide solid evidence of an anomalous metallic state characterized by the resistance saturation at low temperatures with high-quality filters. The robust superconductivity with intriguing quantum phenomena in the macro-size ambient-stable ultrathin PdTe2 films remains almost the same for 20 months, showing great potentials in electronic and spintronic applications.

Mapping Dynamical Magnetic Responses of Ultrathin Micron-Size Superconducting Films Using Nitrogen-Vacancy Centers in Diamond.

Two-dimensional superconductors have attracted growing interest because of their scientific novelty, structural tunability, and useful properties. Studies of their magnetic responses, however, are often hampered by difficulties to grow large-size samples of high quality and uniformity. We report here an imaging method that employed NV- centers in diamond as a sensor capable of mapping out the microwave magnetic field distribution on an ultrathin superconducting film of micron size. Measurements on a 33 nm thick film and a 125 nm thick bulklike film of Bi2Sr2CaCu2O8+δ revealed that the alternating current (ac) Meissner effect (or repulsion of ac magnetic field) set in at 78 and 91 K, respectively; the latter was the superconducting transition temperature of both films. The unusual ac magnetic response of the thin film presumably was due to thermally excited vortex-antivortex diffusive motion in the film. Spatial resolution of our ac magnetometer was limited by optical diffraction and the noise level was at 14 μT/Hz1/2. The technique could be extended with better detection sensitivity to extract local ac conductivity/susceptibility of ultrathin or monolayer superconducting samples as well as ac magnetic responses of other two-dimensional exotic thin films of limited lateral size.

Diamagnetic response of a superconducting surface superstructure: Si(111)- √7×√3 -In

How does one detect the Meissner effect in ultrathin superconducting films that are usually extremely sensitive to air? Here, the authors succeed in observing the diamagnetic response of a superconducting surface superstructure by using an $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0.333em}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ two-coil mutual inductance technique embedded in a specially designed scanning tunneling microscope. The diamagnetic measurement allows for the calculation of the penetration depth, which provides invaluable information about the nature of superconductivity and the symmetry of the underlying order parameter.

Quantum fluctuations and density of states in low-dimensional superconductors

We investigate the influence of quantum phase fluctuations on electron density of states (DOS) in ultrathin superconducting films and nanowires. Using an effective action approach we derive a non-perturbative correction to DOS in such systems. The main effect of phase fluctuations in quasi-two-dimensional films is the appearance of electron states at subgap energies while in quasi-one-dimensional nanowires fluctuations also lead to smearing of the gap edge singularity in DOS.

Singular Temperature Dependence of the Equation of State of Superconductors with Spin-Orbit Interaction in Low-Temperature Region \u2013 II

We extend a previous study of the space and temperature dependence of the order parameter in an ultrathin superconducting film in a longitudinal magnetic field to the vicinity of the critical transition surface. In these films, the spin-orbit interaction can be sizeable. The calculations are restricted to the low-temperature regime. Special attention is given to the zero temperature limit where the state with the maximum value of the critical field is investigated for the given strength of the spin-orbit interaction.

Photon counting statistics of superconducting single-photon detectors made of a three-layer WSi film

Superconducting nanowire single-photon detectors (SNSPD) are used in quantum optics when record-breaking time resolution, high speed, and exceptionally low levels of dark counts (false readings) are required. Their detection efficiency is limited, however, by the absorption coefficient of the ultrathin superconducting film for the detected radiation. One possible way of increasing the detector absorption without limiting its broadband response is to make a detector in the form of several vertically stacked layers and connect them in parallel. For the first time we have studied single-photon detection in a multilayer structure consisting of three superconducting layers of amorphous tungsten silicide (WSi) separated by thin layers of amorphous silicon. Two operating modes of the detector are illustrated: an avalanche regime and an arm-trigger regime. A shift in these modes occurs at currents of ∼0.5–0.6 times the critical current of the detector.

Multifaceted impact of a surface step on superconductivity in atomically thin films

Recent experiments show that an atomic step on the surface of atomically thin metallic films can strongly affect electronic transport. Here we reveal multiple and versatile effects that such a surface step can have on superconductivity in ultrathin films. By solving the Bogoliubov-de Gennes equations self-consistently in this regime, where quantum confinement dominates the emergent physics, we show that the electronic structure is profoundly modified on the two sides of the step, as is the spatial distribution of the superconducting order parameter and its dependence on temperature and electronic gating. Furthermore, the surface step changes non-trivially the transport properties both in the proximity-induced superconducting order parameter and the Josephson effect, depending on the step height. These results offer a new route to tailor superconducting circuits and design atomically thin hetero-junctions made of one same material.

Critical Magnetic Field of Ultra-Thin Superconducting Films and Interfaces

We derive an analytic expression for the temperature dependent critical magnetic field parallel to ultrathin superconducting films with Rashba spin-orbit interaction. Thereby we cover the range from small to large spin-orbit interactions $\lambda$ compared with the gap parameter $\Delta_0$. We find that at a critical spin-orbit energy $\lambda_c$ a first-order phase transition takes place at which the pairing momentum of the Cooper pairs changes discontinuously. We speculate that this might give raise to new phenomena. With increasing $\lambda/\Delta_0$, the pair formation changes from interband to intraband pairing. For $\lambda>\lambda_c$, a dimensional cross-over of the critical field from two to one dimension is taking place.

As-Grown Versus Ion-Milled MgB2 Ultrathin Films for THz Sensor Applications

The exploration of superconductivity in ultrathin films has attracted much attention in the past decades for device applications such as hot electron bolometers (HEBs) and superconducting nanowire single photon detectors (SNSPDs). With a superconducting critical temperature ( Tc ) of 39 K, which is far above that of other conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, and its faster thermal response, ultrathin MgB 2 films present a promising alternative for the use in HEBs and SSNPDs. In this paper, we present the results on ultrathin MgB2 films grown by hybrid physical–chemical vapor deposition. The films less than 10 nm are difficult to achieve directly because of weak grain connectivity due to incomplete film coverage. To achieve high-quality films less than 10-nm Ar ion milling of thicker films was employed. The ultrathin films fabricated by ion milling show superior quality over as-grown films. The surface roughness of the films was significantly improved and the suppression of Tc from the bulk value is much slower in milled films than in as-grown films. The critical current density of the 5-nm milled film is 1 × 107 A·cm-2 at 4 K, which is the same as that of the 10-nm as-grown film.

Reentrant Resistive Behavior and Dimensional Crossover in Disordered Superconducting TiN Films

A reentrant temperature dependence of the normal state resistance often referred to as the N-shaped temperature dependence, is omnipresent in disordered superconductors – ranging from high-temperature cuprates to ultrathin superconducting films – that experience superconductor-to-insulator transition. Yet, despite the ubiquity of this phenomenon its origin still remains a subject of debate. Here we investigate strongly disordered superconducting TiN films and demonstrate universality of the reentrant behavior. We offer a quantitative description of the N-shaped resistance curve. We show that upon cooling down the resistance first decreases linearly with temperature and then passes through the minimum that marks the 3D–2D crossover in the system. In the 2D temperature range the resistance first grows with decreasing temperature due to quantum contributions and eventually drops to zero as the system falls into a superconducting state. Our findings demonstrate the prime importance of disorder in dimensional crossover effects.

Magnetic flux disorder and superconductor-insulator transition in nanohole thin films

We study the superconductor-insulator transition in nanohole ultrathin films in a transverse magnetic field by numerical simulation of a Josephson-junction array model. Geometrical disorder due to the random location of nanoholes in the film corresponds to random flux in the array model. Monte Carlo simulation in the path-integral representation is used to determine the critical behavior and the universal resistivity at the transition as a function of disorder and average number of flux quanta per cell, $f_o$. The resistivity increases with disorder for noninteger $ f_o$ while it decreases for integer $ f_o$, and reaches a common constant value in a vortex-glass regime above a critical value of the flux disorder $D_f^c$. The estimate of $D_f^c$ and the resistivity increase for noninteger $ f_o$ are consistent with recent experiments on ultrathin superconducting films with positional disordered nanoholes.

Superconductor-insulator transition of Josephson-junction arrays on a honeycomb lattice in a magnetic field

We study the superconductor to insulator transition at zero temperature in a Josephson-junction array model on a honeycomb lattice with $f$ flux quantum per plaquette. The path integral representation of the model corresponds to a (2+1)-dimensional classical model, which is used to investigate the critical behavior by extensive Monte Carlo simulations on large system sizes. In contrast to the model on a square lattice, the transition is found to be first order for $f=1/3$ and continuous for $f=1/2$ but in a different universality class. The correlation-length critical exponent is estimated from finite-size scaling of vortex correlations. The estimated universal conductivity at the transition is approximately four times its value for $f=0$. The results are compared with experimental observations on ultrathin superconducting films with a triangular lattice of nanoholes in a transverse magnetic field.

A conventional conundrum

High-temperature superconductivity in ultrathin films of iron selenide deposited on strontium titanate has been attributed to various exotic mechanisms. New experiments indicate that it may be conventional, with broader implications.

Motion of Magnetic Flux Quanta in Type-II Superconductors – Some Open Questions

AbstractThe motion of the magnetic flux quanta in the mixed state of a type-II superconductor, due to the Lorentz force of an electric transport current, represents the key mechanism for generating resistive losses. Whereas the most common result is Joule heating of the superconductor, also purely electronic non-equilibrium effects can play a dominant role in the case of effective cooling of the sample. The latter situation can be realized by means of very thin superconducting films (having a large surface/ volume ratio). In this paper we discuss experiments with Nd2−xCexCuOy (NCCO) films performed some time ago, yielding evidence for electronic non-equilibrium effects due to the energy dependence of the quasi-particle density of states in the mixed state of the films. The films were imbedded within super-fluid helium for cooling. The recent advances in the fabrication of epitaxial ultra-thin superconducting films promise to contribute to further clarification of these electronic non-equilibrium effects.

Characterization of the current-induced resistive spots in superconducting $$\hbox {YBa}_{2} \hbox {Cu}_{3} \hbox {O}_{7}$$ YBa 2 Cu 3 O 7 strips

For over a decade, ultrathin superconducting films have been developed for the detection of single photons at optical or near infrared frequencies, with competitive performances in terms of quantum efficiency, speed, and low dark count rate. In order to avoid the requirement of helium refrigeration, we consider here the use of high temperature materials, known to achieve very fast responsiveness to laser irradiation. We excite thin filaments of the cuprate $$\hbox {YBa}_{2} \hbox {Cu}_{3} \hbox {O}_{7}$$ by rectangular pulses of supercritical current so as to produce either a phase-slip centre (PSC) or a normal hot spot (HS), according to the temperature and the current amplitude selected. That procedure provides information about the maximum bias current to be used in a particle detector, about the return current back to the quiescent state after excitation, and about the rate of growth and decay of a HS. We also measure the time of PSC nucleation. A unique feature of that approach is to provide the rate of heat transfer between the film and its substrate at whatever temperature, in the superconducting state, in the practical conditions of operation.