High-temperature Superconductor La1 Research Articles

Surface Josephson plasma waves in a high-temperature superconductor

Electron density oscillations with acoustic dispersions and sustained at boundaries between different media provide information about surface and interface properties of heterostructures. In ultrathin 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 in highly anisotropic superconductors 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 λ/3000, 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 heterostructures, and non-linear phenomena in Josephson devices.

Two-dimensional type-II Dirac fermions in layered oxides

Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear also as low-energy quasi-particle excitations in electronic band structures. In condensed matter systems, their massless nature can be protected by crystal symmetries. Classification of such symmetry-protected relativistic band degeneracies has been fruitful, although many of the predicted quasi-particles still await their experimental discovery. Here we reveal, using angle-resolved photoemission spectroscopy, the existence of two-dimensional type-II Dirac fermions in the high-temperature superconductor La1.77Sr0.23CuO4. The Dirac point, constituting the crossing of d_{x^2 - y^2} and d_{z^2} bands, is found approximately one electronvolt below the Fermi level (EF) and is protected by mirror symmetry. If spin-orbit coupling is considered, the Dirac point degeneracy is lifted and the bands acquire a topologically non-trivial character. In certain nickelate systems, band structure calculations suggest that the same type-II Dirac fermions can be realised near EF.

Field-induced interplanar magnetic correlations in the high-temperature superconductorLa1.88Sr0.12CuO4

We present neutron scattering studies of the inter-planar correlations in the high-temperature superconductor La1.88Sr0.12CuO4 (T_c=27 K). The correlations are studied both in a magnetic field applied perpendicular to the CuO2 planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field is to increase the magnetic scattering signal at all values of the out-of-plane wave vector L, indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only L~0 is probed. Interestingly, the results of quench-cooling the sample are similar to those obtained by applying a magnetic field. Finally, a small variation of the incommensurate peak position as a function of L provides evidence that the incommensurate signal is twinned with the dominating and sub-dominant twin displaying peaks at even or odd L, respectively.

Glassy low-energy spin fluctuations and anisotropy gap in La1.88Sr0.12CuO4

We present high-resolution triple-axis neutron scattering studies of the high-temperature superconductor La1.88Sr0.12CuO4 (Tc=27 K). The temperature dependence of the low-energy incommensurate magnetic fluctuations reveals distinctly glassy features. The glassiness is confirmed by the difference between the ordering temperature TN ~ Tc inferred from elastic neutron scattering and the freezing temperature Tf ~ 11 K obtained from muon spin rotation studies. The magnetic field independence of the observed excitation spectrum as well as the observation of a partial suppression of magnetic spectral weight below 0.75 meV for temperatures smaller than Tf, indicate that the stripe frozen state is capable of supporting a spin anisotropy gap, of a magnitude similar to that observed in the spin and charge stripe ordered ground state of La1.875Ba0.125CuO4. The difference between TN and Tf implies that the significant enhancement in a magnetic field of nominally elastic incommensurate scattering is caused by strictly in-elastic scattering -- at least in the temperature range between Tf and Tc -- which is not resolved in the present experiment. Combining the results obtained from our study of La1.88Sr0.12CuO4 with a critical reappraisal of published neutron scattering work on samples with chemical composition close to p=0.12, where local probes indicate a sharp maximum in Tf(p), we arrive at the view that the low-energy fluctuations are strongly dependent on composition in this regime, with anisotropy gaps dominating only sufficiently close to p=0.12 and superconducting spin gaps dominating elsewhere.

Magnetic Field-Induced Closure of the Spin Excitation Gap near Optimal Doping in La2-xSrxCuO4

We have investigated how a magnetic field applied perpendicular to the CuO2 planes of the near-optimally hole-doped high-temperature superconductor La1.855Sr0.145CuO4 (Tc ≈36 K) influences the low-energy magnetic excitation spectrum. Our detailed single-crystal neutron scattering experiments reveal that the gap to magnetic excitations falls off linearly with increasing field and reaches zero at the magnetic field µ0Hs=7±1 T required to induce long-range incommensurate magnetic order. A comparison with the electron-doped cuprate Nd1.85Ce0.15CuO4 is made and the possible link between field-induced magnetic order and Fermi surface reconstruction in cuprates is discussed.

Magnetic-Field-Induced Soft-Mode Quantum Phase Transition in the High-Temperature SuperconductorLa1.855Sr0.145CuO4: An Inelastic Neutron-Scattering Study

Inelastic neutron-scattering experiments on the high-temperature superconductor La1.855Sr0.145CuO4 reveal a magnetic excitation gap Delta that decreases continuously upon application of a magnetic field perpendicular to the CuO2 planes. The gap vanishes at the critical field required to induce long-range incommensurate antiferromagnetic order, providing compelling evidence for a field-induced soft-mode driven quantum phase transition.

Photoemission kinks and phonons in cuprates

Arising from: F. Giustino, M. L. Cohen & S. G. Louie , 975–978 (2008)10.1038/nature06874 One of the possible mechanisms of high transition temperature (Tc) superconductivity is Cooper pairing with the help of bosons, which change the slope of the electronic dispersion as observed by photoemission. Giustino et al.1 calculated that in the high temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations (phonons) should have a negligible effect on photoemission spectra and concluded that phonons do not have an important role. Here we show that the calculations used by Giustino et al.1 do not reproduce the huge influence of electron–phonon coupling on important phonons observed in experiments. Thus, we would similarly expect that these calculations do not explain the role of electron–phonon coupling for the electronic dispersion.

A clamp-type pressure cell for high energy x-ray diffraction

We present a clamp-type pressure cell for high energy x-ray diffraction. The pressure cell was specifically designed for studies of weak superstructure reflections at low temperatures in transition metal oxides, resulting from, e.g., charge density modulations. Using a photon energy of E=100 keV, the bulk properties of single crystals with a volume of typically 2-5 mm3 can be studied in transmission geometry. To demonstrate the performance of the pressure cell, we present data on the charge stripe order in the high-temperature superconductor La1.875Ba0.125CuO4.

43Ca NMR study of the doping effects in the high temperature superconductor (La1–xCax)(Ba1.75–xLa0.25+x)Cu3Oy

Nous mesurons la resonance magnetique nucleaire du 43 Ca en fonction de la temperature dans l'etat normal du supraconducteur a haute temperature (Ca x La 1-x )(Ba 1.75-x La 0,25+x )Cu 3 O y . Les echantillons sont choisis de maniere a comparer l'effet d'une modification des teneurs en calcium et en oxygene dans le regime sous-dope. Nous determinons les parametres quadripolaires et le deplacement de Knight (KS). Enfin, nous mesurons la susceptibilite magnetique macroscopique et l'utilisons pour estimer le champ hyperfin du 43 Ca. La variation de KS lors d'une augmentation de la teneur en calcium ne montre pas la signature d'une augmentation du niveau de dopage, contrairement a ce que suggere la variation des proprietes macroscopiques.

Low field magnetic response of the granular superconductor La1.8Sr0.2CuO4

The properties of the low excitation field magnetic response of the granular high-temperature (HT c) superconductor La1.8Sr0.2CuO4 have been analyzed at low temperatures. The response of the Josephson currents has been derived from the data. It is shown that intergrain current response is fully irreversible, producing a shielding response, but that it does not carry Meissner magnetization. Analysis of the data shows that the system of Josephson currents freezes into a glassy state even in the absence of external magnetic field, which is argued to be a consequence of the d-wave nature of superconductivity in La1.8Sr0.2CuO4. The macroscopic diamagnetic response to very weak variations of the magnetic field is shown to be strongly irreversible, but still qualitatively different from any previously known kind of critical-state behavior in superconductors. A phenomenological description of these data is given in terms of a newly proposed fractal model of irreversibility in superconductors.

A study of the microstructure and the critical current density of La1.85Sr0.15CuO4 superconductor

The irreversible magnetization, the critical current density and the microstructure of the high-temperature superconductor La1.85Sr0.15CuO4 are compared. Direct observations by transmission electron microscopy (TEM) revealed two orthogonal arrays of extended screw-type dislocations (2 to 5 × 1011 lines per cm2) parallel to the crystallographic directions 〈100〉 and 〈010〉, respectively. These dislocations are dissociated into partial bound stacking faults. Microanalysis showed that Sr concentration around the dislocation cores is about 10% lower than that of the matrix. Measurements of the hysteresis cycle, M(H,T), at various angular orientations (θ.∅) of the field H with respect to the crystalline directions show no detectable correlation between the directions of the dislocations and the critical current density. This suggests that the anisotropy of the critical current density is first of all controlled by the intrinsic properties of the material rather than by the anisotropy of the dislocation structure.

Investigation of energy gap and observation of electron–phonon interaction in high-temperature superconductor La1.8Sr0.2CuO4–normal metal point contacts

Electrical characteristics [I(eV), dV/dI(eV), d2V/dI2(eV)] are studied comprehensively for pressed submicron point contacts between fresh fractures of LaSrCuO ceramic pellets and a normal metal (Cu). Point contacts (PC) with the direct-type conductivity are investigated, which are characterized by a considerable excess current due to Andreev reflection of quasiparticles at an NS boundary of high transparency. Basic requirements for point contacts suitable for PC measurements are formulated. The models of investigated point contacts permitting the interpretation of the observed characteristics are considered. The results of measurement of the energy gap reveal that LaSrCuO is a superconductor with a strong coupling since the maximum value of the ratio 2Δ/kTc is about 11. It is found that superconducting properties may vary appreciably over distances of several tens of angstroms, resulting in some cases in the simultaneous emergence of two gaps corresponding to two critical temperatures. The dV/dI(eV) and d2V/dI2(eV) curves display some high-intensity singularities whose position on the eV-axis is practically independent of temperature and correlates with the phonon spectrum obtained in neutron scattering experiments. This points to a strong electron-phonon interaction in LaSrCuO.

Inverse photoemission spectrometer for interface studies

An inverse photoemission spectrometer is described which combines momentum resolution for measurements done with low energy incident electrons (photon energy range 12-44 eV) and momentum integration for high energy studies in the isochromat mode (photon energy 14896.6 eV). This unique combination makes it possible to maximise surface sensitivity, enhance bulk sensitivity, and identify the orbital angular momentum character of specific unoccupied states through variations in photoionisation cross section. Experimental capabilities are included for characterisation of ordered, disordered and reactive overlayers deposited in situ on metal and semiconductor surfaces. Representative results are presented for interfaces formed by depositing Fe onto cleaved surfaces of the high temperature superconductor La1.85Sr0.15CuO4.

Evidence against bulk superconductivity in the high temperature superconductor La1.85Sr0.15CuO4- y

From μSR investigations of the high temperature superconductor La 1.85 Sr 0.15 CuO 4- y it is shown that there is a very small part of the sample that exhibits a well developed Meissner effect. This indicates that the sample, although it is showing the characteristic drop in resistivity and change in susceptibility at the superconducting transition, does not contain “bulk” superconducting regions. The penetration depth of the externally applied magnetic field can be estimated from the change in line width of the μSR signal to be about 2300 Å and its temperature dependence can be interpreted in terms of the BCS theory.