arXiv subjects

Ruixing Liang

Publications and source records attributed to Ruixing Liang.

123 records · Page 7Linked to original sources

arXiv2003

Sum rules and energy scales in the high-temperature superconductor YBa2Cu3O6+x

The Ferrell-Glover-Tinkham (FGT) sum rule has been applied to the temperature dependence of the in-plane optical conductivity of optimally-doped YBa_2Cu_3O_{6.95} and underdoped YBa_2Cu_3O_{6.60}. Within the accuracy of the experiment, the sum rule is obeyed in both materials. However, the energy scale ω_c required to recover the full strength of the superfluid ρ_s in the two materials is dramatically different; ω_c \simeq 800 cm^{-1} in the optimally doped system (close to twice the maximum of the superconducting gap, 2Δ_0), but ω_c \gtrsim 5000 cm^{-1} in the underdoped system. In both materials, the normal-state scattering rate close to the critical temperature is small, Γ< 2Δ_0, so that the materials are not in the dirty limit and the relevant energy scale for ρ_s in a BCS material should be twice the energy gap. The FGT sum rule in the optimally-doped material suggests that the majority of the spectral weight of the condensate comes from energies below 2Δ_0, which is consistent with a BCS material in which the condensate originates from a Fermi liquid normal state. In the underdoped material the larger energy scale may be a result of the non-Fermi liquid nature of the normal state. The dramatically different energy scales suggest that the nature of the normal state creates specific conditions for observing the different aspects of what is presumably a central mechanism for superconductivity in these materials.

cond-mat.supr-con↗

arXiv2003

Transport in Ultraclean YBa$_2$Cu$_3$O$_7$: neither Unitary nor Born Impurity Scattering

The thermal conductivity of ultraclean YBa$_2$Cu$_3$O$_7$ was measured at very low temperature in magnetic fields up to 13 T. The temperature and field dependence of the electronic heat conductivity show that two widespread assumptions of transport theory applied to unconventional superconductors fail for clean cuprates: impurity scattering cannot be treated in the usual unitary limit (nor indeed in the Born limit), and scattering of quasiparticles off vortices cannot be neglected. Our study also sheds light on the long-standing puzzle of a sudden onset of a "plateau" in the thermal conductivity of Bi-2212 versus field.

cond-mat.supr-con↗

arXiv2003

Superconductivity and Field-Induced Magnetism in Pr$_{2-x}$Ce$_x$CuO$_4$ Single Crystals

We report muon-spin rotation/relaxation (muSR) measurements on single crystals of the electron-doped high-T_c superconductor Pr$_{2-x}$Ce$_x$CuO$_4$. In zero external magnetic field, superconductivity is found to coexist with Cu spins that are static on the muSR time scale. In an applied field, we observe a Knight shift that is primarily due to the magnetic moment induced on the Pr ions. Below the superconducting transition temperature T_c, an additional source of static magnetic order appears throughout the sample. This finding is consistent with antiferromagnetic ordering of the Cu spins in the presence of vortices. We also find that the temperature dependence of the in-plane magnetic penetration depth in the vortex state resembles that of the hole-doped cuprates at temperatures above ~ 0.2 T_c.

cond-mat.supr-con↗

arXiv2003

Thermal Conductivity across the Phase Diagram of Cuprates: Low-Energy Quasiparticles and Doping Dependence of the Superconducting Gap

Heat transport in the cuprate superconductors YBa$_2$Cu$_3$O$_{y}$ and La$_{2-x}$Sr$_x$CuO$_4$ was measured at low temperatures as a function of doping. A residual linear term kappa_{0}/T is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from kappa_{0}/T is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component (ix) appearing at a putative quantum phase transition and argues against a non-superconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.

cond-mat.supr-con↗

arXiv2002

Top-Seeded Melt-Growth of YBa_2Cu_3O_x Crystals for Neutron Diffraction Studies

We have grown cubic centimetre-size crystals of YBa2Cu3Ox suitable for neutron studies, by a top-seeded melt-growth technique. Growth conditions were optimized with an eye toward maximizing phase purity. It was found that the addition of 2% Y2BaCuO5 and 0.5% Pt (by mass) were required to prevent melt loss and to obtain the highest crystallinity. A neutron diffraction study on a mosaic of six such crystals found that the final Y2BaCuO5 concentration was 5%, while other impurity phases comprised less than 1% by volume. The oxygen content was set to x=6.5, the crystals were detwinned and then carefully annealed to give the well-ordered ortho-II phase. The neutron study determined that 70% of the mosaic's volume was in the majority orthorhomic domain. The neutron (0,0,6) and (1,1,0) rocking curve widths were ~1 degree per crystal and \~2.2 degrees for the mosaic, and the oxygen chain correlation lengths were >100 A in the a- and b-directions and ~50 A in the c-direction.

cond-mat.supr-con↗

arXiv2002

Observation of Weak-Limit Quasiparticle Scattering via Broadband Microwave Spectroscopy of a d-Wave Superconductor

There has long been a discrepancy between microwave conductivity measurements in high temperature superconductors and the conductivity spectrum expected in the simplest models for impurity scattering in a d-wave superconductor. Here we present a new type of broadband measurement of microwave surface resistance that finally shows some of the spectral features expected for a d_{x^2-y^2} pairing state. Cusp-shaped conductivity spectra, consistent with weak impurity scattering of nodal quasiparticles, were obtained in the 0.6-21 GHz frequency range in highly ordered crystals of YBa_2Cu_3O_{6.50} and YBa_2Cu_3O_{6.99}.

cond-mat.supr-con↗

arXiv2001

Preparation and Characterization of Homogeneous YBCO Single Crystals with Doping Level near the SC-AFM Boundary

High-purity and homogeneous YBa2Cu3Oy single crystals with carrier doping level near the AFM-SC boundary have been obtained in the oxygen content range between y = 6.340 and 6.370. The crystals are ortho-II phase at room temperature and undergo the orthorhombic to tetragonal transition at about 140_Degree_C. They show sharp superconducting transitions, with Tc between 4 and 20 K. Tc changes by 0.8 K when the oxygen content y is changed by 0.001, and is also sensitive to annealing conditions near room temperature, due to the dependence of doping on oxygen ordering correlation lengths. Crystals with oxygen content y lower than 6.345 are non-superconducting.

cond-mat.supr-con↗

arXiv2001

Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)

We report measurements of the photoinduced change in reflectivity of an untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay rate of the transient change in reflectivity is found to decrease rapidly with decreasing temperature and, below Tc, with decreasing laser intensity. We interpret the decay as a process of thermalization of antinodal quasiparticles, whose rate is determined by an inelastic scattering rate of quasiparticle pairs.

cond-mat.supr-con↗

arXiv2001

Effects of Vortex Pinning and Thermal Fluctuations on the Josephson Plasma Resonance in Tl2Ba2CaCu2O8 and YBa2Cu3O6.5

Infrared spectroscopy has been used to investigate the temperature dependence and c-axis magnetic field dependence of the Josephson plasma resonance in optimally doped Tl2Ba2CaCu2O8 thin films, and underdoped YBa2Cu3O6.5 ortho - II single crystals. The resonances are directly related to the c-axis penetration depths, yielding low temperature, zero fields values of about 20 micrometer, and 7 micrometer respectively. While the temperature dependencies of the resonances in the two compounds are very similar, the magnetic field dependence in YBCO is much weaker. We attribute this weak magnetic field dependence to the lower anisotropy of YBCO, and discuss the observed behaviours in terms of thermal fluctuations and pinning of pancake vortices, valid for highly anisotropic layered superconductors.

cond-mat.supr-con↗

arXiv1999

Giant Peak Effect Observed in Ultrapure YBCO Crystal

A giant peak in the temperature dependence of the screening current is observed in the ac magnetic response of an ultra-pure YBa$_2$Cu$_3$O$_{6.993}$ crystal in a magnetic field. At H = 2.0 T ({\bf $H$}$||${\bf c}), the screening current density $J_c(T)$ exhibits a 35-fold rise with 0.5 K increase in temperature, indicating an abrupt $10^3$-fold collapse in the characteristic volume of ordered regions in the vortex array. The peak-effect anomaly is most pronounced for $H<$ 4.0 T, but detectable up to 7.0 T. The temperature dependence of the equilibrium magnetization exhibits a small discontinuous jump (for high fields) inside the peak-effect regime, suggesting that the underlying phase transition is a weak first-order vortex-lattice melting transition.

cond-mat.supr-con↗

arXiv1998

Microwave Spectroscopy of Thermally Excited Quasiparticles in YBa_2Cu_3O_{6.99}

We present here the microwave surface impedance of a high purity crystal of $YBa_2Cu_3O_{6.99}$ measured at 5 frequencies between 1 and 75 GHz. This data set reveals the main features of the conductivity spectrum of the thermally excited quasiparticles in the superconducting state. Below 20 K there is a regime of extremely long quasiparticle lifetimes, due to both the collapse of inelastic scattering below $T_c$ and the very weak impurity scattering in the high purity $BaZrO_3$-grown crystal used in this study. Above 20 K, the scattering increases dramatically, initially at least as fast as $T^4$.

cond-mat.supr-con↗

arXiv1998

Measurements of the Magnetic Field Dependence of Lambda in YBa_2Cu_3O_6.95: Results as a Function of Temperature and Field Orientation

We present measurements of the magnetic field dependence of the penetration depth Lambda(H) for untwinned YBa_2Cu_3O_6.95 for temperatures from 1.2 to 70 K in dc fields up to 42 gauss and directions 0, 45 and 90 degrees with respect to the crystal b-axis. The experiment uses an ac susceptometer with fields applied parallel to the ab-plane of thin platelet samples. The resolution is about 0.15 Angstroms in zero dc field, degrading to 0.2 or 0.3 Angstroms at the higher fields. At low temperatures the field dependencies are essentially linear in H, ranging from 0.04 Angstroms/gauss for Delta-Lambda_a to 0.10 Angstroms/gauss for Delta-Lambda_b, values comparable to the T=0 Yip and Sauls prediction for a d-wave superconductor. However, the systematics versus temperature and orientation do not agree with the d-wave scenario probably due, in part, to residual sample problems.

cond-mat.supr-con↗

arXiv1998

Magnetic Penetration Depth and Surface Resistance in YBa_2Cu_3O_{7-delta}: New Results for Ultra High Purity Crystals

We have succeeded in growing very high purity (99.995%) YBa_2Cu_3O_{7-delta} crystals in BaZrO_3 crucibles and have measured Delta-lambda(T) and R_s(T) at 1 GHz in crystals with various oxygen treatments. For an oxygen vacancy level of delta=0.007, Delta-lambda and R_s essentially reproduce our previous results and show no sign of the existence of the two order parameter components as recently reported by Srikanth et al. on BaZrO_3-grown crystals. For other oxygen concentrations, we have in some cases observed deviations from the linear low T dependence of Delta-lambda, but never any sign of a second transition.

cond-mat.supr-con↗

arXiv1997

Muon spin rotation measurement of the fundamental length scales in the vortex state of YBa2Cu3O6.60

The internal field distribution in the vortex state of YBa2Cu3O6.60 is shown to be a sensitive measure of both the magnetic penetration depth and the vortex-core radius. The temperature dependence of the vortex core radius is found to be weaker than in the conventional superconductor NbSe2 and much weaker than theoretical predictions for an isolated vortex. The effective vortex-core radius decreases sharply with increasing H, whereas the penetration depth is found to be much stronger than in NbSe2.

cond-mat.supr-con↗

arXiv1995

Specific Heat of YBa$_2$Cu$_3$O$_{7 - {\rm δ}}$ Single Crystals: Implications for the Vortex Structure

The anisotropy of the magnetic field dependence of the specific heat of YBa$_2$Cu$_3$O$_{7 - {\rm δ}}$ can be used to identify different low-energy excitations, which include phonons, spin-$ \frac{1}{2}$ particles, and electronic contributions. With a magnetic field H applied perpendicular to the copper oxide planes, we find that the specific heat includes a linear-T term proportional to $\sqrt{H}$. The nonlinear field dependence of the density of states at the Fermi level suggests that there are quasiparticle excitations throughout the entire vortex, not just in the vortex core. The $\sqrt{H} T$ term agrees quantitatively with G. Volovik's prediction for a superconductor with lines of nodes in the gap. A similar, but much smaller, effect is predicted for fields parallel to the planes, and sensitive measurements of the in-plane anisotropic magnetic field dependence of the specific heat could be used to map out the nodes.

cond-mat↗