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The detection of individual quanta of light is important for quantum computation, fluorescence lifetime imaging, single-molecule detection, remote sensing, correlation spectroscopy, and more. Thanks to their broadband operation, high detection efficiency, exceptional signal-to-noise ratio, and fast recovery times, superconducting nanowire single-photon detectors (SNSPDs) have become a critical component in these applications. The operation of SNSPDs based on conventional superconductors, which have a low critical temperature ($T_c$), requires costly and bulky cryocoolers. This motivated exploration of other superconducting materials with higher $T_c$ that would enable single-photon detection at elevated temperatures, yet this task has proven exceedingly difficult. Here we show that with proper processing, high-$T_c$ cuprate superconductors can meet this challenge. We fabricated superconducting nanowires (SNWs) out of thin flakes of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and La$_{1.55}$Sr$_{0.45}$CuO$_4$/La$_2$CuO$_4$ (LSCO-LCO) bilayer films and demonstrated their single-photon response up to $25$ and $8$ K, respectively. The single-photon operation is revealed through the linear scaling of the photon count rate (PCR) on the radiation power. Both of our cuprate-based SNSPDs exhibited single-photon sensitivity at the technologically-important $1.5$ ${\mu}$m telecommunications wavelength. Our work expands the family of superconducting materials for SNSPD technology, opens the prospects of raising the temperature ceiling, and raises important questions about the underlying mechanisms of single-photon detection by unconventional superconductors., Comment: 8 pages, 3 figures

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at $T_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at $T_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to $T_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general., Comment: 21 pages, 15 figures; added more sample characterizations, results for another sample, more discussion; accepted version

The discovery of superconductivity in square-planar low valence nickelates has ignited a vigorous debate regarding their essential electronic properties: Do these materials have appreciable oxygen charge-transfer character akin to the cuprates, or are they in a distinct Mott-Hubbard regime where oxygen plays a minimal role? Here, we resolve this question using O $K$-edge resonant inelastic x-ray scattering (RIXS) measurements of the low valence nickelate La$_{4}$Ni$_{3}$O$_{8}$ and a prototypical cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x=0.35$). As expected, the cuprate lies deep in the charge-transfer regime of the Zaanen-Sawatzky-Allen scheme. The nickelate, however, is not well described by either limit of the ZSA scheme and is found to be of mixed charge-transfer/Mott-Hubbard character with the Coulomb repulsion $U$ of similar size to the charge-transfer energy $\Delta$. Nevertheless, the transition-metal-oxygen hopping is larger in La$_{4}$Ni$_{3}$O$_{8}$ than in La$_{2-x}$Sr$_{x}$CuO$_{4}$, leading to a significant superexchange interaction and an appreciable hole occupation of the ligand O orbitals in La$_{4}$Ni$_{3}$O$_{8}$ despite its larger $\Delta$. Our results clarify the essential characteristics of low valence nickelates and put strong constraints on theoretical interpretations of superconductivity in these materials., Comment: 13 pages; to appear in PRX

Electron density oscillations with acoustic dispersions and sustained at boundaries between different media provide information about surface and interface properties of hetero-structures. In ultra-thin metallic films these plasmonic excitations are heavily damped. Superconductivity is predicted to reduce dissipation allowing detection of these resonances. Emerging low-loss interface Cooper-pair waves have been studied before, however, the observation of surface-confined Josephson plasmons has remained elusive. Here, we report on generation and coupling to these excitations in an ultrathin single-crystal film of high-temperature superconductor La1.85Sr0.15CuO4. The film becomes brighter than Au below the critical temperature when probed with sub-gap THz photons. We show that the enhanced signal in the superconducting state, which can be visualized with a spatial resolution better than {\lambda}/3,000, originates from near-field coupling of light to surface Josephson plasmons. Our results open a path towards non-invasive investigation of enhanced superconductivity in artificial multilayers, buried interface states in topological hetero-structures, and non-linear phenomena in Josephson devices., Comment: references added; expanded discussion; journal reference

Using time-domain terahertz spectroscopy in pulsed magnetic fields up to 31 T, we measure the terahertz optical conductivity in an optimally-doped thin film of the high temperature superconducting cuprate La$_{1.84}$Sr$_{0.16}$CuO$_4$. We observe systematic changes in the circularly-polarized complex optical conductivity that are consistent with cyclotron absorption of p-type charge carriers characterized by a cyclotron mass of $4.9\pm 0.8$ $m_{\rm e}$, and a scattering rate that increases with magnetic field. These results open the door to studies aimed at characterizing the degree to which electron-electron interactions influence carrier masses in cuprate superconductors., Comment: 6 pages, 4 figures

In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level $p$. In most materials, $p$ cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, $T_\mathrm{c}$, using the assumption that $T_\mathrm{c}$ dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to $T_\mathrm{c}=91$ K. We show that $p$ can be determined from the underlying Fermi surfaces and that $in-situ$ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of $2\Delta_0=(4\pm1)k_\mathrm{B}T_\mathrm{c}$, Comment: 8 pages, 4 figures

Tungsten oxide and its associated bronzes (compounds of tungsten oxide and an alkali metal) are well known for their interesting optical and electrical characteristics. We have modified the transport properties of thin WO$_3$ films by electrolyte gating using both ionic liquids and polymer electrolytes. We are able to tune the resistivity of the gated film by more than five orders of magnitude, and a clear insulator-to-metal transition is observed. To clarify the doping mechanism, we have performed a series of incisive operando experiments, ruling out both a purely electronic effect (charge accumulation near the interface) and oxygen-related mechanisms. We propose instead that hydrogen intercalation is responsible for doping WO$_3$ into a highly conductive ground state and provide evidence that it can be described as a dense polaronic gas., Comment: Manuscript (27 pages, 6 figures) merged with Supplementary Material (12 pages, 9 figures)

The anomalous metallic state in high-temperature superconducting cuprates is masked by the onset of superconductivity near a quantum critical point. Use of high magnetic fields to suppress superconductivity has enabled a detailed study of the ground state in these systems. Yet, the direct effect of strong magnetic fields on the metallic behavior at low temperatures is poorly understood, especially near critical doping, $x=0.19$. Here we report a high-field magnetoresistance study of thin films of \LSCO cuprates in close vicinity to critical doping, $0.161\leq x\leq0.190$. We find that the metallic state exposed by suppressing superconductivity is characterized by a magnetoresistance that is linear in magnetic field up to the highest measured fields of $80$T. The slope of the linear-in-field resistivity is temperature-independent at very high fields. It mirrors the magnitude and doping evolution of the linear-in-temperature resistivity that has been ascribed to Planckian dissipation near a quantum critical point. This establishes true scale-invariant conductivity as the signature of the strange metal state in the high-temperature superconducting cuprates., Comment: 10 pages, 3 figures

The magnetic correlations within the cuprates have undergone intense scrutiny as part of efforts to understand high temperature superconductivity. We explore the evolution of the magnetic correlations along the nodal direction of the Brillouin zone in La2-xSrxCuO4, spanning the doping phase diagram from the anti-ferromagnetic Mott insulator at x = 0 to the metallic phase at x = 0.26. Magnetic excitations along this direction are found to be systematically softened and broadened with doping, at a higher rate than the excitations along the anti-nodal direction. This phenomenology is discussed in terms of the nature of the magnetism in the doped cuprates. Survival of the high energy magnetic excitations, even in the overdoped regime, indicates that these excitations are marginal to pairing, while the influence of the low energy excitations remains ambiguous., Comment: 8 pages, 6 figures