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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]

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]

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]

The molecular beam epitaxy (MBE) technique is well known for producing atomically smooth thin films as well as impeccable interfaces in multilayers of many different materials. In particular, molecular beam epitaxy is well suited to the growth of complex oxides, materials that hold promise for many applications. Rapid synthesis and high throughput characterization techniques are needed to tap into that potential most efficiently. We discuss our approach to doing that, leaving behind the traditional one-growth-one-compound scheme and instead implementing combinatorial oxide molecular beam epitaxy in a custom built system. [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]

Chemical doping provides an effective method to control the electric properties of complex oxides. However, the state-of-art accuracy in controlling doping is limited to about 1%. This hampers elucidation of the precise doping dependences of physical properties and phenomena of interest, such as quantum phase transitions. Using the combinatorial molecular beam epitaxy, we improve the accuracy in tuning the doping level by two orders of magnitude. We illustrate this novel method by two examples: a systematic investigation of the doping dependence of interface superconductivity, and a study of the competing ground states in the vicinity of the insulator-to-superconductor transition. [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]

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]