Optical conductivity of a moderately disordered superconducting NbN film was investigated by terahertz time-domain spectroscopy in external magnetic field applied along the film plane. The film thickness of about 5 nm was comparable with the coherence length, so vortices should not form. This was confirmed by the fact that no marked difference between the spectra with terahertz electric field set perpendicular and parallel to the external magnetic field was observed. Simultaneous use of Maxwell-Garnett effective medium theory and the model of optical conductivity by Herman and Hlubina proved to correctly reproduce the terahertz spectra obtained experimentally in a magnetic field of up to 7 T. This let us conclude that the magnetic field tends to suppress the superconductivity, resulting in an inhomogeneous state where superconducting domains are enclosed within a normal-state matrix. The scattering rate due to pair-breaking effects was found to linearly increase with magnetic field.
Ultrathin $\rm Bi_2Se_3$-NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick $\rm Bi_2Se_3$ film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a …
Giant second-harmonic generation (SHG) in the terahertz (THz) frequency range is observed in a thin film of an s-wave superconductor NbN, where the time-reversal ($\mathcal{T}$-) and space-inversion ($\mathcal{P}$-) symmetries are simultaneously broken by supercurrent injection. We demonstrate that the phase of the second-harmonic (SH) signal flips when the direction of supercurrent is inverted, i.e., the signal is ascribed to the nonreciprocal response that occurs under broken $\mathcal{P}$- and $\mathcal{T}$-symmetries. The temperature dependence of the SH signal exhibits a sharp resonance, …
Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscop…
Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point (QCP). This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the QCP, the vortex-core energy, an important energy scale in the BKT transition, scales with the superconducting gap, …
Using time-domain Terahertz spectroscopy we performed direct studies of the photoinduced suppression and recovery of the superconducting gap in a conventional BCS superconductor NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the bare quasiparticle recombination rate, the Cooper pair-breaking rate and the electron-phonon coupling constant, λ= 1.1 +/- 0.1, which is in excellent agreement with theoretical estimates.
We studied a thin superconducting NbN film in magnetic fields up to 8 T above the zero- temperature limit by means of time-domain terahertz and scanning tunneling spectroscopies in order to understand the vortex response. Scanning tunneling spectroscopy was used to determine the optical gap and the upper critical field of the sample. The obtained values were subsequently used to fit the terahertz complex conductivity spectra in the magnetic field in the Faraday geometry above the zero temperature limit. These spectra are best described in terms of the Coffey-Clem self-consistent solution of a …
Using scanning tunneling spectroscopy (STS), we address the problem of the superconductor-insulator phase transition (SIT) in homogeneously disordered ultrathin (2-15 nm) films of NbN. Samples thicker than 8 nm, for which the Ioffe-Regel parameter $k_F l \geq 5.6$, manifest a conventional superconductivity : A spatially homogeneous BCS-like gap, vanishing at the critical temperature, and a vortex lattice in magnetic field. Upon thickness reduction, however, while $k_F l$ lowers, the STS revealed striking deviations from the BCS scenario, among which a progressive decrease of the coherence peak…
Transmission of terahertz waves through a thin layer of the superconductor NbN deposited on an anisotropic R-cut sapphire substrate is studied as a function of temperature in a magnetic field oriented parallel with the sample. A significant difference is found between transmitted intensities of beams linearly polarized parallel with and perpendicular to the direction of applied magnetic field.
Disorder has different profound effects on superconducting thin films. For a large variety of materials, increasing disorder reduces electronic screening which enhances electron-electron repulsion. These fermionic effects lead to a mechanism described by Finkelstein: when disorder combined to electron-electron interactions increases, there is a global decrease of the superconducting energy gap $Δ$ and of the critical temperature $T_c$, the ratio $Δ$/$k_BT_c$ remaining roughly constant. In addition, in most films an emergent granularity develops with increasing disorder and results in the forma…
We present results from an extensive study of fluctuation phenomena in superconducting nanowires made from sputtered NbN. Nanoscale wires were fabricated in form of a meander and operated at a constant temperature T~0.4Tc(0). The superconducting state is driven close to the electronic phase transition by a high bias current near the critical one. Fluctuations of sufficient strength temporarily drive a section of the meander structure into the normal conducting state, which can be registered as a voltage pulse of nanosecond duration. We considered three different models (vortex-antivortex pairs…
Ionic gating is a powerful technique for tuning the physical properties of a material via electric field-induced charge doping, but is prone to introduce extrinsic disorder and undesired electrochemical modifications in the gated material beyond pure electrostatics. Conversely, reversible, volatile and electrostatic modulation is pivotal in the reliable design and operation of novel device concepts enabled by the ultrahigh induced charge densities attainable via ionic gating. Here we demonstrate a simple and effective method to achieve reversible and volatile gating of surface-sensitive ultrat…
We report on experimental study of the effect of disorder on electronic parameters and inelastic scattering mechanisms in ultrathin superconducting NbN films, which are commonly used in single-photon detectors. An increase in disorder in the studied 2.5 nm thick NbN films characterized by Ioffe-Regel parameter from 6.3 to 1.6 is accompanied by a decrease in the critical temperature $T_c$ from 11.5 K to 3.4 K. By measuring magnetoconductance in the range from $T_c$ to $\sim3T_c$, we extract the inelastic scattering rates of electrons, including electron-phonon (e-ph) scattering rates $τ_{e-ph}^…
Here we report an optical investigation in the terahertz region of a 40 nm ultrathin BaFe$_{1.84}$Co$_{0.16}$As$_2$ superconducting film with superconducting transition temperature T$_c$ = 17.5 K. A detailed analysis of the combined reflectance and transmittance measurements showed that the optical properties of the superconducting system can be described in terms of a two-band, two-gap model. The zero temperature value of the large gap $Δ_B$, which seems to follow a BCS-like behavior, results to be $Δ_B$(0) = 17 cm$^{-1}$. For the small gap, for which $Δ_A$(0) = 8 cm$^{-1}$, the temperature d…
The electronic structure of heterointerfaces play a pivotal role in their device functionality. Recently, highly crystalline ultrathin films of superconducting NbN have been integrated by molecular beam epitaxy with the semiconducting GaN. We use soft X-ray angle-resolved photoelectron spectroscopy to directly measure the momentum-resolved electronic band structures for both NbN and GaN constituents of this Schottky heterointerface, and determine their momentum-dependent interfacial band offset as well as the band-bending profile into GaN. We find, in particular, that the Fermi states in NbN a…
We report on the growth and characterization of ultrathin YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have been also successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films, and critical current densities, which are only limited by vortex dynamics. The 10 nm thick YBCO nanowires without the Au capping present hysteretic current voltage characteristics, characteri…
We investigate the interplay between gap oscillations and damping in the dynamics of superconductors taken out of equilibrium by strong optical pulses with sub-gap Terahertz frequencies. A semi-phenomenological formalism is developed to include the damping within the electronic subsystem that arises from effects beyond BCS, such as interactions between Bogoliubov quasiparticles and decay of the Higgs mode. Such processes are conveniently expressed as $T_{1}$ and $T_{2}$ times in the standard pseudospin language for superconductors. Comparing with data on NbN that we report here, we argue that …
Signatures of Dynes superconductivity in the THz response of ALD-grown NbN thin films
Universität Stuttgart（斯图加特大学）；Center for Integrated Quantum Science and Technology (IQST), Universität Stuttgart（斯图加特大学集成量子科学与技术中心）；Universität Ulm（乌尔姆大学）；Institute for Complex Quantum Systems and IQST, Universität Ulm（乌尔姆大学复杂量子系统与集成量子科学与技术中心）；Leibniz Institute of Photonic Technology（莱布尼茨光子技术研究所）；Friedrich-Schiller Universität Jena（耶拿弗里德里希·席勒大学）；Institute for Solid State Physics, Friedrich-Schiller Universität Jena（耶拿弗里德里希·席勒大学固体物理研究所）
We study the nonlinear optical response in a superconducting NbN thin film with strong terahertz (THz)wave. Besides the expected third harmonic generation, we observe a new transient oscillation which softensin frequency with temperature increasing towards superconducting transition temperatureTc. We identify thisnew mode as the Higgs transient oscillation. To verify this proposal, we introduce a time-frequency resolvedtechnique, named spectrogram for visualizing THz spectrum. The dynamic decaying behavior of the mode isobserved, which is consistent with theoretical expectation about intrinsic…
Power-Law Suppression of Superfluid Stiffness in High-Kinetic-Inductance NbN Films
Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体与超导研究所）；Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden)（德累斯顿莱布尼茨固体与材料研究所）；Technische Universität Dresden（德累斯顿工业大学）；CSIR - National Physical Laboratory（印度科学工业研究理事会-国家物理实验室）
Anomalous terahertz nonlinearity in disordered s-wave superconductor close to the superconductor-insulator transition
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料国际中心）；School of Electronic Science and Engineering, Nanjing University（南京大学电子科学与工程学院）；Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）
Finite-momentum Cooper plasmons in superconducting terahertz microcavities
Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质结构与动力学研究所）；Center for Free-Electron Laser Science (CFEL)（自由电子激光科学中心）；Department of Physics, Columbia University（哥伦比亚大学物理系）；Department of Physics, University of California Santa Barbara（加州大学圣塔芭芭拉分校物理系）
The lack of energy dissipation and abrupt electrical phase transition of superconductors favorite them for nanoscale technologies, including radiation detectors, and quantum technologies. Moreover, understanding the nanoscale behavior of superconductivity is significant for revealing the onset of collective-electron behavior in nature. Nevertheless, the limited number of accessible superconductors restricts availability of the superconducting properties, encumbering the realization of their potential. Superconducting nanowire single photon detectors (SNSPDs) sense single-IR photons faster and …
We report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in which we measure charge carrier dynamics and excitations of thin-film superconducting systems at low temperatures in the THz spectral range. The characteristics of the set-up and the experimental procedures are described comprehensively. We discuss the single-particle density of states and a theory of electrodynamic absorption and optical conductivity of conventional superconductors. We present the experimental performance of the setup at low temperatures for a broad spectral range from 0.1 - 1.1 THz by the example o…
The microscopic nature of an insulating state in the vicinity of a superconducting state, in the presence of disorder, is a hotly debated question. While the simplest scenario proposes that Coulomb interactions destroy the Cooper pairs at the transition, leading to localization of single electrons, an alternate possibility supported by experimental observations suggests that Cooper pairs instead directly localize. The question of the homogeneity, granularity, or possibly glassiness of the material on the verge of this transition is intimately related to this fundamental issue. Here, by combini…
The concept of mass-generation via the Higgs mechanism was strongly inspired by earlier works on the Meissner-Ochsenfeld effect in superconductors. In quantum field theory, the excitations of longitudinal components of the Higgs field manifest as massive Higgs bosons. The analogous Higgs mode in superconductors has not yet been observed due to its rapid decay into particle-hole pairs. Following recent theories, however, the Higgs mode should decrease below the pairing gap $2Δ$ and become visible in two-dimensional systems close to the superconductor-insulator transition (SIT). For experimental…
Superconductivity in disordered systems close to an incipient localization transition has been an area of investigation for many years. It has been noted that in such highly disordered superconductors, anomalous spectral weight develops in their conductivity near and below the superconducting gap energy. In this work we investigate the low frequency conductivity in disordered superconducting NbN thin films close to the localization transition with time-domain terahertz spectroscopy. In the normal state, strong deviations from the Drude form due to incipient localization are found. In the super…
Collective modes manifest themselves in a variety of different physical systems ranging from superconductors to superfluid $^{3}$He. The collective modes are generated via the Higgs-Anderson mechanism that is based on the symmetry breaking double well potential. Recently collective modes were explored in superconducting NbN and InO in the presence of a strong terahertz laser field. In both cases a single collective mode that oscillates with twice the frequency of the superconducting energy gap $Δ$ was discovered. Superfluid $^{3}$He is the host for a whole variety of collective modes. In parti…
Imaging superconducting weak spots through vortex-assisted THz near-field photovoltage
ICFO – Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology（光子科学研究所，巴塞罗那科学技术研究院）；University of Antwerp（安特卫普大学）；Stavropoulos Center for Complex Quantum Matter, Department of Physics and Astronomy, University of Notre Dame（圣母大学物理与天文学系斯塔夫罗普洛斯复杂量子物质中心）；Center for Nanoscale Materials, Argonne National Laboratory（阿贡国家实验室纳米材料中心）；Geballe Laboratory for Advanced Materials, Stanford University（斯坦福大学盖巴尔高级材料实验室）；Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（SLAC国家加速器实验室斯坦福材料与能源科学研究所）；Department of Applied Physics, Stanford University（斯坦福大学应用物理系）；Department of Physics, Stanford University（斯坦福大学物理系）；ICREA – Institució Catalana de Recerca i Estudis Avançats（加泰罗尼亚研究高级学院）
