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The detection of individual quanta of light is important for quantum communication, fluorescence lifetime imaging, remote sensing and more. Due to their 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. However, the operation of conventional SNSPDs requires costly cryocoolers. Here we report the fabrication of two types of high-temperature superconducting nanowires. We observe linear scaling of the photon count rate on the radiation power at the telecommunications wavelength of 1.5 μm and thereby reveal single-photon operation. SNSPDs made from thin flakes of Bi2Sr2CaCu2O8+δexhibit a single-photon response up to 25 K, and for SNSPDs from La1.55Sr0.45CuO4/La2CuO4bilayer films, this response is observed up to 8 K. While the underlying detection mechanism is not fully understood yet, our work expands the family of materials for SNSPD technology beyond the liquid helium temperature limit and suggests that even higher operation temperatures may be reached using other high-temperature superconductors.

A brief account is presented of an extensive experiment performed at Brookhaven National Laboratory, aimed at understanding the nature of high-temperature superconductivity in cuprates. Over the course of the last 12 years, over 2000 films of the prototypical high-$$T_\mathrm{c}$$ Tc superconductor, $$\hbox {La}_{2-{x}}\hbox {Sr}_{{x}}\hbox {CuO}_{4}$$ La2-xSrxCuO4 , have been synthesized using atomic-layer-by-layer molecular beam epitaxy (ALL-MBE), characterized by a range of techniques, and patterned into devices. These were used to measure accurately the key physical parameters in both the superconducting and the normal states, and establish their precise dependence on doping, temperature, and external fields. The results bring in some great surprises, challenge the commonly held beliefs, rule out many theoretical models, and point to a new path for raising $$T_\mathrm{c}$$ Tc even further.

The origin of high-temperature superconductivity in copper oxides and the nature of the ‘normal’ state above the critical temperature are widely debated. In underdoped copper oxides, this normal state hosts a pseudogap and other anomalous features; and in the overdoped materials, the standard Bardeen–Cooper–Schrieffer description fails, challenging the idea that the normal state is a simple Fermi liquid. To investigate these questions, we have studied the behaviour of single-crystal La2–xSrxCuO4films through which an electrical current is being passed. Here we report that a spontaneous voltage develops across the sample, transverse (orthogonal) to the electrical current. The dependence of this voltage on probe current, temperature, in-plane device orientation and doping shows that this behaviour is intrinsic, substantial, robust and present over a broad range of temperature and doping. If the current direction is rotated in-plane by an angle ϕ, the transverse voltage oscillates as sin(2ϕ), breaking the four-fold rotational symmetry of the crystal. The amplitude of the oscillations is strongly peaked near the critical temperature for superconductivity and decreases with increasing doping. We find that these phenomena are manifestations of unexpected in-plane anisotropy in the electronic transport. The films are very thin and epitaxially constrained to be tetragonal (that is, with four-fold symmetry), so one expects a constant resistivity and zero transverse voltage, for every ϕ. The origin of this anisotropy is purely electronic—the so-called electronic nematicity. Unusually, the nematic director is not aligned with the crystal axes, unless a substantial orthorhombic distortion is imposed. The fact that this anisotropy occurs in a material that exhibits high-temperature superconductivity may not be a coincidence.

We summarize previous field effect studies in high-Tccuprates and then discuss our method to smoothly tune the carrier concentration of a cuprate film over a wide range using an applied electric field. We synthesized epitaxial one-unit-cell thick films of La2−xSrxCuO4and from them fabricated electric double layer transistor devices utilizing various gate electrolytes. We were able to vary the carrier density by about 0.08 carriers per Cu atom, with the resulting change in Tcof 30 K. The superconductor-insulator transition occurred at the critical resistance very close to the quantum resistance for pairs, RQ= h/(2e)2= 6.5 kΩ. This is suggestive of a quantum phase transition, possibly driven by quantum phase fluctuations, between a “Bose insulator” and a high-Tcsuperconductor state.

We summarize previous field effect studies in high-Tccuprates and then discuss our method to smoothly tune the carrier concentration of a cuprate film over a wide range using an applied electric field. We synthesized epitaxial one-unit-cell thick films of La2−xSrxCuO4and from them fabricated electric double layer transistor devices utilizing various gate electrolytes. We were able to vary the carrier density by about 0.08 carriers per Cu atom, with the resulting change in Tcof 30 K. The superconductor-insulator transition occurred at the critical resistance very close to the quantum resistance for pairs, RQ= h/(2e)2= 6.5 kΩ. This is suggestive of a quantum phase transition, possibly driven by quantum phase fluctuations, between a “Bose insulator” and a high-Tcsuperconductor state.

The physics of underdoped copper oxide superconductors, including the pseudogap, spin and charge ordering and their relation to superconductivity, is intensely debated. The overdoped copper oxides are perceived as simpler, with strongly correlated fermion physics evolving smoothly into the conventional Bardeen–Cooper–Schrieffer behaviour. Pioneering studies on a few overdoped samples indicated that the superfluid density was much lower than expected, but this was attributed to pair-breaking, disorder and phase separation. Here we report the way in which the magnetic penetration depth and the phase stiffness depend on temperature and doping by investigating the entire overdoped side of the La2−xSrxCuO4phase diagram. We measured the absolute values of the magnetic penetration depth and the phase stiffness to an accuracy of one per cent in thousands of samples; the large statistics reveal clear trends and intrinsic properties. The films are homogeneous; variations in the critical superconducting temperature within a film are very small (less than one kelvin). At every level of doping the phase stiffness decreases linearly with temperature. The dependence of the zero-temperature phase stiffness on the critical superconducting temperature is generally linear, but with an offset; however, close to the origin this dependence becomes parabolic. This scaling law is incompatible with the standard Bardeen–Cooper–Schrieffer description.

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On 4 October, 2007, the scientific session of the Division of Physical Sciences of the Russian Academy of Sciences on room-temperature superconductivity was held at the conference hall of the Lebedev Physics Institute, Russian Academy of Sciences. The following reports were made at this session: