Superconductivity In Ultrathin Films Research Articles

Discovery of superconductivity in KTaO3 by electrostatic carrier doping

Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 1960(1). So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO(3), a material in which superconductivity has not been observed before. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO(3) (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO(3) at 50 mK for two-dimensional carrier densities in the range 2.3 × 10(14) to 3.7 × 10(14) cm(-2). The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.

Superconducting nano-striplines as quantum detectors

The recent progress in the nanofabrication of superconducting films opens the road toward detectors with highly improved performances. This is the case for superconducting nano-striplines where the thickness and the width are pushed down to the extreme limits to realize detectors with unprecedented sensitivity and ultra fast response time. In this way quantum detectors for single photons at telecommunication wavelengths and for macromolecules such as proteins can be realized. As is often the case in applied nanotechnology, it is a challenge to make devices with the necessary macroscopic dimensions that are needed to interface present technologies, while maintaining the performance improvements. For nano-stripline detectors, both the fast temporal response and the device sensitivity is generally degraded when the area is increased. Here, we present how such detectors can be scaled up to macroscopic dimensions without losing the performance of the nano-structured active elements by using an innovative configuration. In order to realize ultrathin superconducting film the nano-layer is growth with a careful setup of the deposition technique which guarantees high quality and thickness uniformity at the nano-scale size. The active nano-strips are defined with the state-of-the-art electron beam nanolithography to achieve a highly uniform linewidth. We present working detectors based on nano-strips with thicknesses 9–40 nm and widths of 100–1000 nm which exhibit unprecedented speed and area coverage (40 × 40 μm2 for single photon detectors and 1 × 1 mm2 for single molecule detectors) based on niobium nitride thus enabling practical use of this nanotechnology.

Single photon response of superconducting nanowire single photon detector

Single photon detection is one of the key technologies for quantum key distribution in quantum communication. As a novel single photon detection technology, superconducting nanowire single photon detector (SNSPD) surpasses conventional semiconducting single photon detectors with high count rate and low dark count rate. In this article, we introduce SNSPD fabricated from NbN ultrathin superconducting film and lab-based SNSPD system. The characteristics of single photon response pulse of SNSPD are analyzed in detail. Also discussed is the relationship between waveform of single photon response and system bandwidth. Circuit model is made to analyze the performance of SNSPD. The simulation result agrees well with the experimental data. Those results are valuable for understanding the mechanism of SNSPD and building future SNSPD system for quantum communication.

Focused-Ion-Beam Direct-Writing of Ultra-Thin Superconducting Tungsten Composite Films

Tungsten composite films of thickness as low as 19 nm have been deposited using a 30 keV Ga <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> focused-ion-beam with tungsten carboxyl (W(CO) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) as the gas precursor. Films of thickness 25 nm or more are superconducting with a transition temperature exceeding 5 K. Films in the thickness range 25 nm to 50 nm show an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">increasing</i> T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">decreasing</i> film thickness. This correlates well with the measured dependence of the normal state resistivity upon film thickness. We attribute this behavior to an increase in the BCS electron-phonon interaction potential resulting from a reduction in the electron mean-free-path as the film thickness is reduced. In the light of these data we discuss the applicability of FIB-deposited tungsten for devices requiring ultra-thin superconducting films, including photon detectors and phase-slip qubits.

Evidence of local superconductivity in granular Bi nanowires fabricated by electrodeposition

An unusual enhancement of resistance (i.e., a ``superresistivity'') below a certain characteristic temperature ${T}_{\text{sr}}$ was observed in granular Bi nanowires. This ``superresistive'' state was found to be dependent on the applied magnetic field as well as the excitation current. The suppression of ${T}_{\text{sr}}$ by magnetic field resembles that of a superconductor. The observed superresistivity appears to be related to the nucleation of local superconductivity inside the granular nanowire without long-range phase coherence. The phenomenon is reminiscent of the ``Cooper-pair insulator'' observed previously in ultrathin two-dimensional superconducting films and three-dimensional percolative superconducting films.

Spectroscopy With Nanostructured Superconducting Single Photon Detectors

Superconducting single-photon detectors (SSPDs) are nanostructured devices made from ultrathin superconducting films. They are typically operated at liquid helium temperature and exhibit high detection efficiency, in combination with very low dark counts, fast response time, and extremely low timing jitter, within a broad wavelength range from ultraviolet to mid-infrared (up to 6 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mu</i> m). SSPDs are very attractive for applications such as fiber-based telecommunication, where single-photon sensitivity and high photon-counting rates are required. We review the current state-of-the-art in the SSPD research and development, and compare the SSPD performance to the best semiconducting avalanche photodiodes and other superconducting photon detectors. Furthermore, we demonstrate that SSPDs can also be successfully implemented in photon-energy-resolving experiments. Our approach is based on the fact that the size of the hotspot, a nonsuperconducting region generated upon photon absorption, is linearly dependent on the photon energy. We introduce a statistical method, where, by measuring the SSPD system detection efficiency at different bias currents, we are able to resolve the wavelength of the incident photons with a resolution of 50 nm.

Dispersionless modes and the superconductivity of ultrathin films

A self-consistent analytical solution of the problem of the superconductivity of ultrathin metal films is found within the tight-binding model for normal-metal electrons with a simple example of a film of three atomic layers. Superconductivity is not destroyed in atomically thin films if the energies of the electron subsystem lie near the Fermi surface at least for some values of the quasimomentum component along the film. A substantial increase in the critical temperature of an ultrathin metal film as compared to its bulk value is possible if the electron excitation spectrum contains low-energy modes with an anomalously weak dispersion.

Direct measurement of penetration length in ultrathin and/or mesoscopic superconducting structures

As the dimensions of thin superconducting structures become comparable with or less than the penetration length of magnetic flux into the structures, it becomes increasingly necessary to devise experimental tests of available theoretical models. One approach which we shall describe enables penetration lengths to be derived from the measurement of the effective area of planar, thin-film structures with linear dimensions in the range of 1–100μm. The effective area is defined by measurement of the inductive coupling of the structures to dc or low-frequency magnetic fields. The structures described consist of two parts: (1) an ultrathin annular superconducting film with transition temperature Tca (“washer”) and (2) surrounding the washer is a superconducting ring with transition temperature Tcs. Because the films are prepared in such a way that Tca&amp;lt;Tcs, the ring-washer combination acts as a dc superconducting quantum interference device up to and beyond Tca, enabling the effective area of the washer to be measured over a wide temperature range. Results for the temperature dependence of the Pearl penetration length Λ(T), derived directly from measurements of the effective area, are compared both with theory and with other experimental data. Whereas alternative methods may be restricted to narrow-band, high-frequency fields and require sample dimensions of order of 10mm or greater, the method is inherently broadband and is applicable to dimensions ⩾1μm.

Magnetic-flux periodic response of nanoperforated ultrathin superconducting films

We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K $<T_{co} < $5 K. These regular perforations give the films a phase-sensitive periodic response to an applied magnetic field. By measuring this response in their resistive transitions, $R(T)$, we are able to distinguish regimes in which fluctuations of the amplitude, both the amplitude and phase, and the phase of the superconducting order parameter dominate the transport. The portion of $R(T)$ dominated by amplitude fluctuations is larger in lower $T_{co}$ films and thus, grows with proximity to the superconductor to insulator transition.

Modulation of superconductivity by ferroelectric polarization in confined ferroelectric-superconductor-ferroelectric films

We show that the electric polarization at the interface with ultrathin superconducting (S) films sandwiched between ferroelectric (FE) layers allows achievement of substantially stronger modulation of inner carrier density and superconducting transition temperature as compared to FE-S bilayers typically used in superconducting FETs. We find that not only the larger penetration depths but also the pairing symmetry should be responsible for the fact that the electric field effect in high temperature superconductors is much stronger than in conventional systems. Discussing the advantages of multilayers, we propose a novel design concept for superconducting electric field-effect transistors based on ferroelectric films.

Evidence for a quantum-vortex-liquid regime in ultrathin superconducting films.

We report the observation of activated dissipation in ultrathin superconducting films near the superconductor-to-insulator transition (SIT) in magnetic fields. The activation energy ${T}_{0}$ scales with the mean field transition temperature, film thickness, and logarithm of the applied field, in qualitative agreement with the activation of defects in a vortex solid. We find ${T}_{0}$ extrapolates to zero at ${H}_{0}$, a field well below the upper critical field implying the existence of a quantum-vortex-liquid regime. We discuss the implications of this result for the magnetic field tuned SIT in this system.

Pair breaking by magnetic impurities in ultrathin superconducting films: Tc degradation mechanisms in disordered superconductors.

The transition temperature, ${\mathit{T}}_{\mathit{c}}$, of ultrathin superconducting films depends strongly on their thickness. We doped quench condensed films of ${\mathrm{Pb}}_{0.9}$${\mathrm{Bi}}_{0.1}$ on Ge with dilute concentrations of the magnetic impurity Gd to investigate how \ensuremath{\alpha}', the pair-breaking rate per magnetic impurity, varies with ${\mathit{T}}_{\mathit{c}}$. \ensuremath{\alpha}' varies by less than 15% and decreases with decreasing ${\mathit{T}}_{\mathit{c}}$ for 4 K${\mathit{T}}_{\mathit{c}}$6 K in agreement with a theory of perturbative disorder effects. For ${\mathit{T}}_{\mathit{c}}$4 K, \ensuremath{\alpha}' increases slightly. Within a generalized Eliashberg theory, the data suggests that disorder reduces ${\mathit{T}}_{\mathit{c}}$ in these films by enhancing repulsive Coulomb interactions in the Cooper channel.

The two-dimensional superconductor-insulator transition

Abstract Superconductor-to-insulator transitions in ultrathin films can be brought about either by the variation of film properties such as sheet resistance, thickness, and carrier concentration in zero magnetic field, or by the application of a magnetic field. Interest in this problem has been heightened by the prospect that the transitions are zero-temperature quantum phase transitions, which can be described by a Boson-Hubbard model. Variants of this model are relevant to aspects of systems as diverse as helium films, quantum Hall systems, and high temperature superconductors with columnar defects. Ultrathin superconducting films, depending on their properties or the value of the applied magnetic field, exhibit either insulating or superconducting behavior in the limit of zero temperature. However, for films described by certain parameters, or at critical values of the magnetic field, there appears to be a finite, nonzero resistance in the zero temperature limit which has been identified as the resistance at the quantum critical point. The Boson-Hubbard model predicts this resistance to be universal. In the case of the zero-field transition the value of the limiting resistance may indeed be universal and close to the value h 4e 2 , or 6450 Ω.

Evidence for quantum tunneling of vortices in ultrathin superconducting films (abstract)

The creep and flow of vortices have been subjects of study because they give rise to nonzero electrical resistance even below the superconducting transition temperature. In the classical model of thermally activated creep of vortices the resistance vanishes exponentially at low temperatures with a dependence given by an Arrhenius form. Non-Arrhenius behavior at low temperatures can result from an alternative to the thermal processes involving macroscopic quantum tunneling. Unusual temperature dependence of electrical resistance has been observed in ultrathin superconducting films of Pb, Al, and Bi in small magnetic fields. At low temperatures, film sheet resistances varied with temperatures as R≊R0 exp(T/T0), where T0 and R0 are constants. The parameter T0 is independent of the sheet resistance at high temperatures. These results are not expected in conventional models of vortex motion involving thermal activation, but may be explained using a quantum tunneling picture.1

Evidence for quantum tunneling of vortices in superconductors

Flux creep in disordered superconductors may be governed by quantum tunneling of Abrikosov vortices rather than by thermal activation processes. The expectation is that in the quantum tunneling regime the creep rate would be temperature independent. This assumes that the parameters describing the pinning potential and other aspects of the superconducting films are temperature independent. In the case of extremely thin superconducting films the coherence length retains its temperature dependence well into the quantum tunneling regime, leading to an unusual temperature dependence of the electrical resistance in this regime. This has been observed in ultrathin superconducting films of Pb, Al, and Bi. In low magnetic fields, at low temperatures, sheet resistances vary with temperature as R≈R0 exp(T/T0), where T0 and R0 are constants.

A theoretical analysis of the thickness dependence of the localization effect on the normal-state resistivities in high-T c Y1Ba2Cu3O7−δ thin films

We have demonstrated that the normal-state resistivity of sufficiently thick Y1Ba2Cu3O7−δ thin films can be fitted very well by the Bloch–Gruneisen equation. As the film thickness decreases, the localization effect becomes predominant presuming the manifestation of surface defects and vacancies over the bulk region. The evaluated Debye temperatures span over a range which is consistent with experimental values. Below some critical thicknesses, the transport behavior in thin Y1Ba2Cu3O7−δ films deviates from the Boltzmann conductivity and the temperature coefficient of the electric resistivity becomes negative, which can be explained by adopting quantum corrections for localization and interaction effects. The origin of nonmetallic behavior can be understood quantitatively in terms of the competition between the quantum corrections and the conventional Boltzmann conductivity. In these ultrathin superconducting films, the elastic mean-free path arising from defect and heavy impurity scattering is very short and much smaller than the inelastic mean-free path, even at room temperature.

Resistive transitions in ultrathin superconducting films: Possible evidence for quantum tunneling of vortices.

Unusual temperature dependence of electrical resistance has been observed in ultrathin superconducting films of Pb, Al, and Bi. In small magnetic fields, at low temperatures, sheet resistances varied with temperature as R\ensuremath{\approxeq}${\mathit{R}}_{0}$ exp(T/${\mathit{T}}_{0}$), where ${\mathit{T}}_{0}$ and ${\mathit{R}}_{0}$ are constants. This result is not expected in conventional models of vortex motion involving thermal activation. On the other hand, it may be explained in a quantum tunneling picture.

Anisotropy of the pinning force density and the resistive transitions in YBa 2Cu 3O 7/PrBa 2Cu 3O 7 superlattices

Finely modulated ( YBa 2 Cu 3 O 7) nY /( PrBa 2 Cu 3 O 7) nPr superlattices (SL) with sharp X-ray satellite lines (up to the fifth order) were prepared by sequential dc-sputtering. The high degree of structural order is also reflected in high superconducting critical temperatures T c and critical current densities j c measured for SL with nY=1 for the first time. The angular dependence of the pinning force density, i.e. j c ab ( B, T, ϕ), and the resistive transitions ϱ( B, T, ϕ), where ϕ is the angle between the external field B and the c -axis, show a strong enhancement of the intrinsic anisotropy due to the superlattice structure. For B parallel to the CuO 2-layers ϱ( B, T) and j c ( B, T) of SL with nY≤2 are nearly independent on the magnetic field. Since the superconducting order parameter is zero in the PrBa 2Cu 3O 7 layers, we conclude that these SLs act like a stack of ultrathin superconducting film. Consequently, they exhibit a 2-dimensional behavior at low temperatures with j c ( B, ϕ)= j c (Bcosϕ,0°).

Onset of superconductivity in ultrathin granular metal films.

The evolution of superconductivity in ultrathin films of Sn, Pb, Ga, Al, and In has been examined as a function of thickness and temperature. The films were grown in increments by condensation from the vapor onto substrates held at temperatures below 18 K. For each metal, global superconductivity or zero electrical resistance was found when the normal-state sheet resistance ${R}_{N}$ fell below a value close to h/4${e}^{2}$, or 6.45 k\ensuremath{\Omega}/\ensuremath{\square}, an observation uncorrelated with either structural or material parameters such as thickness or transition temperature. Prior evidence of superconductivity with nonzero resistance, local superconductivity, was found at earlier stages of film growth. All evidences of superconducting behavior were observed at temperatures close to the bulk transition temperature beginning in the range of thicknesses for which normal-state resistivities were greater than 200 \ensuremath{\mu}\ensuremath{\Omega}-cm and were rapidly changing with thickness. This implies that the films consisted of fully superconducting grains connected by tunneling junctions. The strong disorder represented by a broad distribution of junction parameters can be renormalized into weak disorder. Thus theoretical calculations based on regular arrays of superconducting sites coupled by (Josephson) junctions appear to be relevant. The extreme thinness of the films implies very small junction capacitances leading to large quantum fluctuations of the phase differences of their superconducting order parameters. Two classes of theories explaining a nearly universal resistance threshold for superconductivity have emerged. Both classes involve the quenching of these quantum fluctuations. In the limit of very small junction capacitances the threshold occurs at resistance values near h/4${e}^{2}$, and is essentially independent of the capacitance and the energy gap, in good agreement with the experimental data. Not contained in any of the models is an explanation of the observed regular variation of the low-temperature resistances of the films with the normal-state sheet resistance for values just above the resistance threshold.

Quantum fluctuations in continuous superconducting films

Motivated by recent experiments on ultrathin continuous superconducting films, we study the dependence ofT c and Δ (for zero temperatures) on the film thicknessd. Using field-theoretical methods, we express the Coulomb interaction in terms of a fluctuating potential, and the fluctuation correction to the free energy (in one-loop approximation) is determined. Based on the standard dirty-limit expressions for the response functions, we findT c and Δ by a numerical investigation of the gap equation. Generally, we find that the decrease ofT c and Δ vs.d −1 is quite similar, but depends sensitively on both the large-wave-vector cut-off and the strength of the interaction. In particular, however, for a strong interaction (Coulomb interaction), the order parameter is more strongly suppressed than the critical temperature, which is due to long-wavelength fluctuations of the phase and the potential.