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Highlights • In the cuprates an unusual superconducting state is discovered. • The critical temperature is determined by the superfluid density. • These finding defy the standard Bardeen–Cooper–Schrieffer theory. • The metallic state above T c is also anomalous ('nematic'). Abstract Over the course of three decades of intense study, apart from the exceptionally high critical temperature, many unusual properties of cuprates have been discovered, notably including resistivity linear in temperature, electronic Raman continuum and optical absorption extending throughout the infrared region, pseudogap, hour-glass spin excitation spectrum, etc. However, each of these features have been also observed in other materials, including some that are not even superconducting at all. Here, we describe an extensive experiment in which over 2000 films of the La 2 − x Sr x CuO 4 have been synthesized and studied in detail over the course of the last twelve years. We argue that, uniquely, in the cuprates an unusual superconducting state, that defies the standard BCS description, develops from an unusual metallic state, in which the rotational symmetry of the electron fluid is spontaneously broken. [ABSTRACT FROM AUTHOR]

Recently, we have released the results of a comprehensive study of a couple thousand single-crystal La2−xSrxCuO4 films, covering densely the entire overdoped side of the phase diagram (Božović et al. Nature 536, 309-311, 2016; Wu et al. Nature 547, 432-435, 2017). Here, we review the key experimental findings, place them in the context of other important well-established facts and observations, and discuss their implications for our understanding of high-temperature superconductivity in cuprates. We conclude that it involves some new physics that requires going beyond the standard Fermi liquid description of the normal state and Bardeen-Cooper-Schrieffer description of the superconducting state. [ABSTRACT FROM AUTHOR]

For overdoped cuprates, it is believed that the normal state behaves as an ordinary Fermi liquid while the superconducting state conforms to the BCS theory. We have put these beliefs to the test by a comprehensive experiment in which over two thousand cuprate films were synthesized by molecular beam epitaxy and studied in great detail and precision. Here, we compare our key experimental results to various proposed explanations based on BCS theory extended to dirty d-wave superconductors, including the cases of strong (unitary) and weak (Born) scattering on impurities. The discrepancies seem insurmountable, and point to the need to develop the theory further, likely beyond the canonical BCS paradigm. [ABSTRACT FROM AUTHOR]

Cuprate superconductors exhibit many features, but the ultimate question is why the critical temperature () is so high. The fundamental dichotomy is between the weak-pairing, Bardeen-Cooper-Schrieffer (BCS) scenario, and Bose-Einstein condensation (BEC) of strongly-bound pairs. While for underdoped cuprates it is hotly debated which of these pictures is appropriate, it is commonly believed that on the overdoped side strongly-correlated fermion physics evolves smoothly into the conventional BCS behavior. Here, we test this dogma by studying the dependence of key superconducting parameters on doping, temperature, and external fields, in thousands of cuprate samples. The findings do not conform to BCS predictions anywhere in the phase diagram. [ABSTRACT FROM AUTHOR]

We have synthesized LaSrCuO thin films using atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The films are high-quality-singe crystal, atomically smooth, and very homogeneous. The critical temperature ( T ) shows a very little (<1 K) variation within a film of 10×10 mm area. The large statistics (over 2000 films) is crucial to discern intrinsic properties. In these films, we have measured the absolute value of the magnetic penetration depth λ with the accuracy better than 1 % and mapped densely the entire overdoped side of the LaSrCuO phase diagram. A new scaling law is established accurately for the dependence of T on the superfluid density. The scaling we observe is incompatible with the standard Bardeen-Cooper-Schrieffer picture and points to local pairing. [ABSTRACT FROM AUTHOR]

We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La1.55Sr0.45CuO4) and a cuprate insulator (La2CuO4), in which each layer is just one unit cells thick. We have studied the magnetic field and temperature dependence of the complex sheet conductance, of these films. Experiments have been carried out at frequencies between 2 and 50 MHz using the single-spiral coil technique. We found that: (i) the inductive response starts at T = 3K lower temperatures than Re s(T), which in turn is characterized by a peak close to the transition, (ii) this shift is almost constant with magnetic field up to 14 mT; (iii) T increases sharply up to 4K at larger fields and becomes constant up to 8 T; (iv) the vortexdiffusion constant D(T) is not linear with T at low temperatures as in the case of free vortices, but is rather exponential due to pinning of vortex cores, and (v) the dynamic Berezinski-Kosterlitz-Thouless (BKT) transition temperature occurs at the point where Y = (1ω/ξ+)². Our experimental results can be described well by the extended dynamic theory of the BKT transition and dynamics of bound vortex-antivortex pairs with short separation lengths. [ABSTRACT FROM AUTHOR]

We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La1.55Sr0.45CuO4) and a cuprate insulator (La2CuO4) film, in which each layer is just one unit cell thick. We have studied the magnetic field and temperature dependences of the complex sheet conductance, σ(ω), of these films. Experiments have been carried out at frequencies between 2 and 20 MHz using the single-spiral coil technique. We found that (i) the inductive response,Lk-1(T), starts at 1.7 K lower temperatures than Reσ(T), which in turn is characterized by a peak close to the transition, (ii) this shift is almost constant with magnetic field up to 8 T; (iii) the vortex diffusion constant D(T) is not linear with T at low temperature, as in the case of free vortices, but rather is exponential due to pinning of vortex cores, and (iv) the Berezinski-Kosterlitz-Thouless (BKT) transition temperature occurs at the point where Y = (Ιω/ξ+)2 = 1. Our experimental results can be described well by the extended dynamic theory of the BKT transition and the dynamics of bound vortex-antivortex pairs with short separation lengths. [ABSTRACT FROM AUTHOR]

Observations of spontaneous breaking of the rotational symmetry in an electron fluid, the so-called "electronic nematicity," have been reported in several quantum materials. We have developed several different methods, based on angle-resolved transport measurements, to determine the amplitude and the director of the nematic order. We present methods that are applicable to thin films or single crystals, illustrate them with transport data obtained on copper oxide superconductors, and discuss their relative advantages and disadvantages. [ABSTRACT FROM AUTHOR]

The observation of nonzero voltage transverse to the current flow, in zero magnetic field, is a sensitive indication of the nematic order in cuprate superconductors. But this effect should be carefully identified and differentiated from various conceivable experimental artifacts. Here we discuss in detail the five types of artifacts commonly encountered in transport experiments and explain how we rule out these possibilities by decisive experimental evidence. [ABSTRACT FROM AUTHOR]

We describe an extensive experimental study of La 2- x Sr x CuO 4 films synthesized by molecular beam epitaxy and investigated by angle-resolved measurements of transverse resistivity (without applied magnetic field). The data show that an unusual metallic state, in which the rotational symmetry of the electron fluid is spontaneously broken, occurs in a large temperature and doping region. The superconducting state always emerges out of this nematic metal state. [ABSTRACT FROM AUTHOR]

Helium ion beams (HIB) focused to subnanometer scales have emerged as powerful tools for high-resolution imaging as well as nanoscale lithography, ion milling, or deposition. Quantifying irradiation effects is an essential step toward reliable device fabrication, but most of the depth profiling information is provided by computer simulations rather than the experiment. Here, we demonstrate the use of atomic force microscopy (AFM) combined with scanning near-field optical microscopy (SNOM) to provide three-dimensional (3D) dielectric characterization of high-temperature superconductor devices fabricated by HIB. By imaging the infrared dielectric response obtained from light demodulation at multiple harmonics of the AFM tapping frequency, we find that amorphization caused by the nominally 0.5 nm HIB extends throughout the entire 26.5 nm thickness of the cuprate film and by ∼500 nm laterally. This unexpectedly widespread damage in morphology and electronic structure can be attributed to a helium depth distribution substantially modified by the internal device interfaces. Our study introduces AFM-SNOM as a quantitative tomographic technique for noninvasive 3D characterization of irradiation damage in a wide variety of nanoscale devices. [ABSTRACT FROM AUTHOR]

Since the discovery of high-temperature superconductivity in cuprates, proposals have been made that pairing may be local, in particular in underdoped samples. Here, we briefly review evidence for local pairs from our experiments on thin films of La Sr CuO , synthesized by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). [ABSTRACT FROM AUTHOR]

The magnetic penetration depth (λ) in thin superconducting films is usually measured by the mutual inductance technique. The accuracy of this method has been limited by uncertainties in the geometry of the solenoids and in the film position and thickness, by parasitic coupling between the coils, etc. Here, we present several improvements in the apparatus and the method. To ensure the precise thickness of the superconducting layer, we engineer the films at atomic level using atomic-layerby- layer molecular beam epitaxy. In this way, we also eliminate secondary-phase precipitates, grain boundaries, and pinholes that are common with other deposition methods and that artificially increase the field transmission and thus the apparent λ. For better reproducibility, the thermal stability of our closed-cycle cryocooler used to control the temperature of the mutual inductance measurement has been significantly improved by inserting a custom-built thermal conductivity damper. Next, to minimize the uncertainties in the geometry, we fused a pair of small yet precisely wound coils into a single sapphire block machined to a high precision. The sample is spring-loaded to exactly the same position with respect to the solenoids. Altogether, we can measure the absolute value of λ with the accuracy better than ±1%. [ABSTRACT FROM AUTHOR]

A superconductivity research paper is presented on the topic scale-invariant magnetoresistance in superconducting cuprates.

La 2 − x Sr x CuO 4 nanowire devices have been fabricated and characterized using electrical transport measurements. Nanowires with widths down to 80 nm are patterned using high-resolution electron beam lithography. However, the narrowest nanowires show incomplete superconducting transitions with some residual resistance at T = 4 K. Here, we report on the refinement of the fabrication process to achieve narrower nanowire devices with complete superconducting transitions, opening the path to the study of novel physics arising from dimension-limited superconductivity on the nanoscale. [ABSTRACT FROM AUTHOR]

In an effort to scale down electronic devices to atomic dimensions, the use of transition-metal oxides may provide advantages over conventional semiconductors. Their high carrier densities and short electronic length scales are desirable for miniaturization, while strong interactions that mediate exotic phase diagrams open new avenues for engineering emergent properties. Nevertheless, understanding how their correlated electronic states can be manipulated at the nanoscale remains challenging. Here, we use angle-resolved photoemission spectroscopy to uncover an abrupt destruction of Fermi liquid-like quasiparticles in the correlated metal LaNiO3 when confined to a critical film thickness of two unit cells. This is accompanied by the onset of an insulating phase as measured by electrical transport. We show how this is driven by an instability to an incipient order of the underlying quantum many-body system, demonstrating the power of artificial confinement to harness control over competing phases in complex oxides with atomic-scale precision. [ABSTRACT FROM AUTHOR]

One of the most intensely studied scenarios of high-temperature superconductivity (HTS) postulates pairing by exchange of magnetic excitations. Indeed, such excitations have been observed up to optimal doping in the cuprates. In the heavily overdoped regime, neutron scattering measurements indicate that magnetic excitations have effectively disappeared, and this has been argued to cause the demise of HTS with overdoping. Here we use resonant inelastic X-ray scattering, which is sensitive to complementary parts of reciprocal space, to measure the evolution of the magnetic excitations in La2−xSrxCuO4 across the entire phase diagram, from a strongly correlated insulator (x = 0) to a non-superconducting metal (x = 0.40). For x = 0, well-defined magnon excitations are observed. These magnons broaden with doping, but they persist with a similar dispersion and comparable intensity all the way to the non-superconducting, heavily overdoped metallic phase. The destruction of HTS with overdoping is therefore caused neither by the general disappearance nor by the overall softening of magnetic excitations. Other factors, such as the redistribution of spectral weight, must be considered. [ABSTRACT FROM AUTHOR]

The recent discovery of superconductivity at the interface of two non-superconducting materials has received much attention. In cuprate bilayers, the critical temperature (Tc) can be significantly enhanced compared with single-phase samples. Several explanations have been proposed, invoking Sr interdiffusion, accumulation and depletion of mobile charge carriers, elongation of the copper-to-apical-oxygen bond length, or a beneficial crosstalk between a material with a high pairing energy and another with a large phase stiffness. From each of these models, one would predict Tc to depend strongly on the carrier density in the constituent materials. Here, we study combinatorial libraries of La2−xSrxCuO4-La2CuO4 bilayer samples-an unprecedentedly large set of more than 800 different compositions. The doping level x spans a wide range, 0.15 < x < 0.47, and the measured Hall coefficient varies by one order of magnitude. Nevertheless, across the entire sample set, Tc stays essentially constant at about 40 K. We infer that doping up to the optimum level does not shift the chemical potential, unlike in ordinary Fermi liquids. This result poses a new challenge to theory-cuprate superconductors have not run out of surprises. [ABSTRACT FROM AUTHOR]

The magnetic phase diagram of La2-xSrxCuO4 thin films grown on single-crystal LaSrAlO4 substrates has been determined by low-energy muon-spin rotation. The diagram shows the same features as the one of bulk La2-xSrxCuO4, but the transition temperatures between distinct magnetic states are significantly different. In the antiferromagnetic phase the Néel temperature TN is strongly reduced, and no hole spin freezing is observed at low temperatures. In the disordered magnetic phase (x ≳ 0.02) the transition temperature to the cluster spin-glass state Tg is enhanced. Possible reasons for the pronounced differences between the magnetic phase diagrams of thin-film and bulk samples are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The in-plane magnetoresistance (MR) in atomically smooth La2−x Sr x CuO4 thin films grown by molecular-beam-epitaxy was measured in magnetic fields B up to 9T over a wide range of temperatures T. The films, with x=0.03 and x=0.05, are insulating, and the positive MR emerges at . The positive MR exhibits glassy features, including history dependence and memory, for all orientations of B. The results show that this behavior, which reflects the onset of glassiness in the dynamics of doped holes, is a robust feature of the insulating state. [Copyright &y& Elsevier]

The nature of the underdoped pseudogap regime of the high-temperature copper oxide superconductors has been a matter of long-term debate. On quite general grounds, we expect that, owing 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. 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. Here we use THz time-domain spectroscopy to probe the temporal fluctuations of superconductivity above the critical temperature (Tc) in La2−xSrxCuO4 (LSCO) thin films over a doping range that spans almost the entire superconducting dome (x=0.09-0.25). Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, at most 16 K above Tc. 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 Tc. [ABSTRACT FROM AUTHOR]

Emission spectroscopy is performed to study the optical transitions between electronic bands of SWCNs formed in AlPO4−5 (AFI) crystal. Strong visible emission is observed under laser light illumination at room temperature. The emission is excited by the light polarized along the c-axis and its polarization is also along this direction. On the other hand, non-linear emission is observed under the resonant excitation of the lowest absorption band of SWCNs. The emission is excited by the light polarized along the c-axis, but it shows the polarization perpendicular to the c-axis. The polarization character is discussed on the basis of the selection rules of the optical transitions predicted by the full line-group symmetry of SWCNs. [Copyright &y& Elsevier]

High-temperature superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor-insulator transition is a key open question. The cleanest approach is to tune the carrier density using the electric field effect; for example, it was learned in this way that weak electron localization transforms superconducting SrTiO3 into a Fermi-glass insulator. But in the copper oxides this has been a long-standing technical challenge, because perfect ultrathin films and huge local fields (>109 V m−1) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration. Here we report synthesis of epitaxial films of La2− xSrxCuO4 that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor-insulator transition. The observed critical resistance is precisely the quantum resistance for pairs, RQ = h/(2e) = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization. [ABSTRACT FROM AUTHOR]

In copper-oxides that show high-temperature superconductivity (HTS), the critical temperature (Tc) has a dome-shaped doping dependence. The cause of demise of both Tc and superfluid density ns on the overdoped side is a major puzzle. A recent study of transport and diamagnetism in a large number of overdoped La2−xSrxCuO4 (LSCO) films shows that this cannot be accounted for by disorder within the conventional Bardeen-Cooper-Schrieffer theory. This brings to focus an alternative explanation - competition of HTS with ferromagnetic order, fluctuating in superconducting samples and static beyond the superconductor-to-metal transition. Here, we examine this proposal by growing single-crystal LSCO thin films with doping on both sides of the transition by molecular beam epitaxy, and using polarized neutron reflectometry to measure their magnetic moments. In a heavily overdoped, metallic but non-superconducting LSCO (x = 0.35) film, the spin asymmetry of reflectivity shows a very small static magnetic moment (~2 emu/cm3). Less-doped, superconducting LSCO films show no magnetic moment in neutron reflectivity, both above and below Tc. Therefore, the collapse of HTS with overdoping is not caused by competing ferromagnetic order. [ABSTRACT FROM AUTHOR]

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 antiferromagnetic 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 antinodal 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. [ABSTRACT FROM AUTHOR]

Heterostructures made of a layer of a cuprate insulator La2CuO4 on the top of a layer of a nonsuperconducting cuprate metal La1.55Sr0.45CuO4 show high-Tc interface superconductivity confined within a single CuO2 plane. Given this extreme quasi-two-dimensional quantum confinement, it is of interest to find out how interface superconductivity behaves when exposed to an external magnetic field. With this motivation, we have performed contactless tunnel-diode-oscillator-based measurements in pulsed magnetic fields up to 56 T as well as measurements of the complex mutual inductance between a spiral coil and the film in static fields up to 3 T. Remarkably, we observe that interface superconductivity survives up to very high perpendicular fields, in excess of 40 T. In addition, the critical magnetic field Hm(T) reveals an upward divergence with decreasing temperature, in line with vortex melting as in bulk superconducting cuprates. [ABSTRACT FROM AUTHOR]