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Register a new userCoherent c-axis transport in the underdoped cuprate superconductor YBCO
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The electrical resistivity rho_c of the underdoped cuprate superconductor YBCO was measured perpendicular to the CuO_2 planes on ultra-high quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of rho_c at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature. This crossover coincides with the emergence of the small electron pocket detected in the Fermi surface of YBCO via quantum oscillations, the Hall and Seebeck coefficients and with the detection of a unidirectional modulation of the charge density as seen by high-field NMR measurements. The low coherence temperature is quantitatively consistent with the small hopping integral t_perp inferred from the splitting of the quantum oscillation frequencies. We conclude that the Fermi-surface reconstruction in YBCO at dopings from p = 0.08 to at least p = 0.15, attributed to stripe order, produces a metallic state with 3D coherence deep in the underdoped regime.
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Quantum oscillations and negative Hall and Seebeck coefficients at low temperature and high magnetic field have shown the Fermi surface of underdoped cuprates to contain a small closed electron pocket. It is thought to result from a reconstruction by charge order, but whether it is the order seen by NMR and ultrasound above a threshold field or the short-range modulations seen by X-ray diffraction in zero field is unclear. Here we use measurements of the thermal Hall conductivity in YBCO to show that Fermi-surface reconstruction occurs only above a sharply defined onset field, equal to the transition field seen in ultrasound. This reveals that electrons do not experience long-range broken translational symmetry in the zero-field ground state, and hence in zero field there is no quantum critical point for the onset of charge order as a function of doping.
We report measurements of the phase of the conductivity, $phi_sigmaequiv arg(sigma)$, in the normal state of a $Bi_{2}Sr_{2}CaCu_{2}O_{8+delta}$ (BSCCO) thin film from 0.2-1.0 THz. From $phi_sigma$ we obtain the time delay of the current response, $tau_sigmaequivphi_sigma/omega$. After discovering a systematic error in the data analysis, the extracted $tau_sigma$ has changed from that reported earlier. The revised data is shown in the sole figure below. Analysis and discussion of these data will follow.
The observation of a reconstructed Fermi surface via quantum oscillations in hole-doped cuprates opened a path towards identifying broken symmetry states in the pseudogap regime. However, such an identification has remained inconclusive due to the multi-frequency quantum oscillation spectra and complications accounting for bilayer effects in most studies. We overcome these impediments with high resolution measurements on the structurally simpler cuprate HgBa2CuO4+d (Hg1201), which features one CuO2 plane per unit cell. We find only a single oscillatory component with no signatures of magnetic breakdown tunneling to additional orbits. Therefore, the Fermi surface comprises a single quasi-two-dimensional pocket. Quantitative modeling of these results indicates that biaxial charge-density-wave within each CuO2 plane is responsible for the reconstruction, and rules out criss-crossed charge stripes between layers as a viable alternative in Hg1201. Lastly, we determine that the characteristic gap between reconstructed pockets is a significant fraction of the pseudogap energy.
We report on a photo-induced transient state of YBa2Cu2O6+x in which transport perpendicular to the Cu-O planes becomes highly coherent. This effect is achieved by excitation with mid-infrared optical pulses, tuned to the resonant frequency of apical oxygen vibrations, which modulate both lattice and electronic properties. Below the superconducting transition temperature Tc, the equilibrium signatures of superconducting interlayer coupling are enhanced. Most strikingly, the optical excitation induces a new reflectivity edge at higher frequency than the equilibrium Josephson plasma resonance, with a concomitant enhancement of the low frequency imaginary conductivity. Above Tc, the incoherent equilibrium conductivity becomes highly coherent, with the appearance of a reflectivity edge and a positive imaginary conductivity that increases with decreasing frequency. These features are observed up to room temperature in YBa2Cu2O6.45 and YBa2Cu2O6.5. The data above Tc can be fitted by hypothesizing that the light re-establishes a transient superconducting state over only a fraction of the solid, with a lifetime of a few picoseconds. Non-superconducting transport could also explain these observations, although one would have to assume transient carrier mobilities near 10^4 cm^2/(V.sec) at 100 K, with a density of charge carriers similar to the below Tc superfluid density. Our results are indicative of highly unconventional non-equilibrium physics and open new prospects for optical control of complex solids.
Recent angle resolved photoemission cite{yang-nature-08} and scanning tunneling microscopy cite{kohsaka-nature-08} measurements on underdoped cuprates have yielded new spectroscopic information on quasiparticles in the pseudogap phase. New features of the normal state such as particle-hole asymmetry, maxima in the energy dispersion and accompanying drops in the spectral weight of quasiparticles agree with the ansatz of Yang textit{et al.} for the single particle propagator in the pseudogap phase. The coherent quasiparticle dispersion and reduced asymmetry in the tunneling density of states in the superconducting state can also be described by this propagator.
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