The realization of high-transition-temperature (high-T(c)) superconductivity confined to nanometre-sized interfaces has been a long-standing goal because of potential applications and the opportunity to study quantum phenomena in reduced dimensions. This has been, however, a challenging target: in conventional metals, the high electron density restricts interface effects (such as carrier depletion or accumulation) to a region much narrower than the coherence length, which is the scale necessary for superconductivity to occur. By contrast, in copper oxides the carrier density is low whereas T(c) is high and the coherence length very short, which provides an opportunity-but at a price: the interface must be atomically perfect. Here we report superconductivity in bilayers consisting of an insulator (La(2)CuO(4)) and a metal (La(1.55)Sr(0.45)CuO(4)), neither of which is superconducting in isolation. In these bilayers, T(c) is either approximately 15 K or approximately 30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm of the interface. If such a bilayer is exposed to ozone, T(c) exceeds 50 K, and this enhanced superconductivity is also shown to originate from an interface layer about 1-2 unit cells thick. Enhancement of T(c) in bilayer systems was observed previously but the essential role of the interface was not recognized at the time.

The question of how thin cuprate layers can be while still retaining high-temperature superconductivity (HTS) has been challenging to address, in part because experimental studies require the synthesis of near-perfect ultrathin HTS layers and ways to profile the superconducting properties such as the critical temperature and the superfluid density across interfaces with atomic resolution. We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La(1.65)Sr(0.45)CuO4) and a cuprate insulator (La2CuO4) in which each layer is just three unit cells thick. We selectively doped layers with isovalent Zn atoms, which suppress superconductivity and act as markers, to show that this interface HTS occurs within a single CuO2 plane. This approach may also be useful in fabricating HTS devices.

The nature of the underdoped pseudogap regime of the high-temperature superconductors has been a matter of long-term debate [1][2][3]. On quite general grounds, one expects that due to their low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties in this part of the phase diagram [4,5]. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap, and their significance for understanding high-temperature superconductivity [6][7][8][9][10]. In this work we use THz time-domain spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity above the critical temperature (T c ) in La 2−x Sr x CuO 4 thin films over a doping range that spans almost the entire superconducting dome (x = 0.09 to 0.25). Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, at most 16 K above T c . Our measurements show that superconducting correlations do not make an appreciable contribution to the charge transport anomalies of the pseudogap in LSCO at temperatures well above T c .In general, continuous phase transitions are typified by fluctuations with correlation length and time scales that diverge near T c . Dynamical measurements like TTDS are a sensitive probe of the onset of superconductivity [11] and measure its temporal correlations on the time scales of interest. In the presence of superconducting vortices such highfrequency measurements are not affected by effects like vortex pinning, creep, and edge barriers that often complicate interpretation of low frequency and DC results. In this study, we investigate the fluctuation superconductivity in thin films of LSCO grown by molecular beam epitaxy (MBE). This synthesis technique provides exquisite control of the thickness and chemical composition of the films; the intrinsic chemical tunability of LSCO allows us to investigate essentially the entire phase diagram. For details on the films, see the 'Supplementary Information' (SI).In Figs. 1a and b, we show the real (σ 1 ) and imaginary (σ 2 ) parts of the THz conductivity measured at a number of different temperatures for optimally doped LSCO (x = 0.16) with T c =41 K. We obtain similar data at other doping levels. The spectra are easily understood in the limiting cases of high and low temperatures. Well above the onset of superconductivity, the real part of the conductivity is almost frequency independent and the imaginary part is small, consistent with the expectation for the behavior of a metal at frequencies well below the normal state scattering rate. At the lowest temperature the conductivity is consistent with that expected for a long-range ordered superconductor; σ 1 is small as most of the low frequency spectral weight has condensed into the ω = 0 delta function, and the frequency dependence of σ 2 is very close to 1/ω. Our principal interest...

The report that T(c) was doubled in underdoped La2-xSrxCuO4 films under compressive epitaxial strain has stirred great interest. We show that such films are extremely sensitive to oxygen intake, even at very low temperature, with startling consequences including colossal lattice expansion and a crossover from semiconductor to metallic behavior. We can bring T(c) up to 40 K in La2CuO4 films on SrTiO3 substrates-without any Sr doping and under tensile strain. On LaSrAlO4 substrates, we reached T(c)=51.5 K, the highest so far in La2-xSrxCuO4.

Nonequilibrium phase transitions, which are defined by the formation of macroscopic transient domains, are optically dark and cannot be observed through conventional temperature- or pressure-change studies. We have directly determined the structural dynamics of such a nonequilibrium phase transition in a cuprate superconductor. Ultrafast electron crystallography with the use of a tilted optical geometry technique afforded the necessary atomic-scale spatial and temporal resolutions. The observed transient behavior displays a notable "structural isosbestic" point and a threshold effect for the dependence of c-axis expansion (Deltac) on fluence (F), with Deltac/F = 0.02 angstrom/(millijoule per square centimeter). This threshold for photon doping occurs at approximately 0.12 photons per copper site, which is unexpectedly close to the density (per site) of chemically doped carriers needed to induce superconductivity.