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Ruixing Liang

Publications and source records attributed to Ruixing Liang.

At least 109 records · Page 6Linked to original sources
arXiv2007

Superfluid Density in a Highly Underdoped YBCO Superconductor

The superfluid density rho_s(T) = 1/lambda^2(T) has been measured at 2.64 GHz in highly underdoped YBCO, at 37 dopings with T_c between 3 K and 17 K. Within limits set by the transition width Delta T_c ~ 0.4 K, rho_s(T) shows no evidence of critical fluctuations as T goes to T_c, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_s displays the behaviour expected for a quantum phase transition in the (3 + 1)-dimensional XY universality class, with rho_s0 ~ (p - p_c), T_c ~ (p - p_c)^1/2 and rho_s(T) ~ (T_c - T)^1 as T goes to T_c.

cond-mat.supr-con↗
arXiv2007

Doping dependence of the superconducting gap in Tl_2Ba_2CuO_{6+delta} from heat transport

We present low-temperature thermal conductivity measurements on the cuprate Tl_2Ba_2CuO_{6+delta} throughout the overdoped regime. In the T -> 0 limit, the thermal conductivity due to d-wave nodal quasiparticles provides a bulk measurement of the superconducting gap, Delta. We find Delta to decrease with increasing doping, with a magnitude consistent with spectroscopic measurements (photoemission and tunneling). This argues for a pure and simple d-wave superconducting state in the overdoped region of the phase diagram, which appears to extend into the underdoped regime down to a hole concentration of 0.1 hole/Cu. As hole concentration is decreased, the gap-to-Tc ratio increases, showing that the suppression of the superconducting transition temperature Tc (relative to the gap) begins in the overdoped regime.

cond-mat.str-el↗
arXiv2006

Oxygen Chain Disorder as the Weak Scattering Source in YBa$_2$Cu$_3$O$_{6.50}$

The microwave conductivity of an ultra-pure single crystal of YBa$_2$Cu$_3$O$_{6.50}$ has been measured deep in the superconducting state as a continuous function of frequency from $0.5\to20$ GHz. Conductivity spectra were first measured at four temperatures below 10 K after having prepared the crystal in the so called ortho-II phase in which the CuO chain oxygen are ordered into alternating full and empty chains. These spectra exhibit features expected for quasiparticle scattering from dilute weak impurities (small scattering phaseshift) in an otherwise clean d-wave superconductor. The measurements were repeated on the same crystal after heating and then rapidly quenching the sample to reduce the degree of oxygen order in the CuO chains. With the increased disorder, the conductivity spectra retain the distinctive weak-limit scattering features, but have increased widths reflecting an increase in quasiparticle scattering. These measurements unambiguously establish that CuO chain oxygen disorder is the dominant source of in-plane quasiparticle scattering in high purity YBCO.

cond-mat.supr-con↗
arXiv2006

Onset of a boson mode at superconducting critical point of underdoped YBa2Cu3Oy

The thermal conductivity $κ$ of underdoped \Y was measured in the $T \to 0$ limit as a function of hole concentration $p$ across the superconducting critical point at $p_{SC}$ = 5.0%. ``Time doping'' was used to resolve the evolution of bosonic and fermionic contributions with high accuracy. For $p \leqslant p_{SC}$, we observe an additional $T^3$ contribution to $κ$ which we attribute to the boson excitations of a phase with long-range spin or charge order. Fermionic transport, manifest as a linear term in $κ$, is seen to persist unaltered through $p_{SC}$, showing that the state just below $p_{SC}$ is a thermal metal. In this state, the electrical resistivity varies as log$(1/T)$ and the Wiedemann-Franz law is violated.

cond-mat.supr-con↗
arXiv2006

NMR evidence for Friedel-like oscillations in the CuO chains of ortho-II YBa$_2$Cu$_3$O$_{6.5}$

Nuclear magnetic resonance (NMR) measurements of CuO chains of detwinned Ortho-II YBa$_2$Cu$_3$O$_{6.5}$ (YBCO6.5) single crystals reveal unusual and remarkable properties. The chain Cu resonance broadens significantly, but gradually, on cooling from room temperature. The lineshape and its temperature dependence are substantially different from that of a conventional spin/charge density wave (S/CDW) phase transition. Instead, the line broadening is attributed to small amplitude static spin and charge density oscillations with spatially varying amplitudes connected with the ends of the finite length chains. The influence of this CuO chain phenomenon is also clearly manifested in the plane Cu NMR.

cond-mat.supr-con↗
arXiv2006

Phenomenology of a-axis and b-axis charge dynamics from microwave spectroscopy of highly ordered YBa2Cu3O6.50 and YBa2Cu3O6.993

Extensive measurements of the microwave conductivity of highly pure and oxygen-ordered \YBCO single crystals have been performed as a means of exploring the intrinsic charge dynamics of a d-wave superconductor. Broadband and fixed-frequency microwave apparatus together provide a very clear picture of the electrodynamics of the superconducting condensate and its thermally excited nodal quasiparticles. The measurements reveal the existence of very long-lived excitations deep in the superconducting state, as evidenced by sharp cusp-like conductivity spectra with widths that fall well within our experimental bandwidth. We present a phenomenological model of the microwave conductivity that captures the physics of energy-dependent quasiparticle dynamics in a d-wave superconductor which, in turn, allows us to examine the scattering rate and oscillator strength of the thermally excited quasiparticles as functions of temperature. Our results are in close agreement with the Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and the phenomenological model. Separate experiments for currents along the $\hat a$ and $\hat b$ directions of detwinned crystals allow us to isolate the role of the CuO chain layers in \YBCO, and a model is presented that incorporates both one-dimensional conduction from the chain electrons and two-dimensional transport associated with the \cuplane plane layers.

cond-mat.supr-con↗
arXiv2005

Insulating transition in the flux-flow resistivity of a high temperature superconductor

Measurements of the DC resistivity of under-doped cuprate superconductors have revealed a metal--insulator transition at low temperatures when superconductivity is suppressed by a very large magnetic field, with the resistivity growing logarithmically in the low temperature limit. This insulating behaviour has been associated not only with the large magnetic fields, but also with the under-doped composition and intrinsic sample inhomogeneity, and it is important to establish whether these factors are essential to it. Here we report high resolution microwave measurements of the flux-flow resistivity of optimally doped YBa_(2)Cu_(3)O_(6+x) in the mixed state at temperatures down to 1.2 K. We find that the effective resistivity of the vortex cores exhibits a metal-insulator transition, with a minimum at 13 K and a logarithmically growing form below 5 K. The transition is seen in samples of the highest quality and in magnetic fields as low as 1 T. Our work is the first report of a metal-insulator transition in optimally doped YBa_(2)Cu_(3)O_(6+x), and the first such transition to be seen in a system in which superconductivity has not been globally suppressed.

cond-mat.supr-con↗
arXiv2005

Evaluation of CuO2 plane hole doping in YBa2Cu3O6+x single crystals

The electron hole concentration p in the CuO2 planes of YBa2Cu3O6+x (YBCO) has been evaluated using the unit cell length in the c-direction together with a simple estimation for p in the chain oxygen ordered phases. The empirical relationship between p and c obtained allows calculation of p from the c-axis lattice parameter for the entire doping range of YBCO. It is also suggested that the empirical parabola describing the relationship between Tc and p in La2-dSrdCuO4 is reasonably correct for YBCO except for the region around p =1/8, where the Tc of YBCO is suppressed by as much as 17 K. A correction of the parabola for the p = 1/8 effect is presented.

cond-mat.supr-con↗
arXiv2005

Apparatus for high resolution microwave spectroscopy in strong magnetic fields

We have developed a low temperature, high-resolution microwave surface impedance probe that is able to operate in high static magnetic fields. Surface impedance is measured by cavity perturbation of dielectric resonators, with sufficient sensitivity to resolve the microwave absorption of sub-mm-sized superconducting samples. The resonators are constructed from high permittivity single-crystal rutile (TiO2) and have quality factors in excess of 10^6. Resonators with such high performance have traditionally required the use of superconducting materials, making them incompatible with large magnetic fields and subject to problems associated with aging and power-dependent response. Rutile resonators avoid these problems while retaining comparable sensitivity to surface impedance. Our cylindrical rutile resonators have a hollow bore and are excited in TE_01(n-d) modes, providing homogeneous microwave fields at the center of the resonator where the sample is positioned. Using a sapphire hot-finger technique, measurements can be made at sample temperatures in the range 1.1 K to 200 K, while the probe itself remains immersed in a liquid helium bath at 4.2 K. The novel apparatus described in this article is an extremely robust and versatile system for microwave spectroscopy, integrating several important features into a single system. These include: operation at high magnetic fields; multiple measurement frequencies between 2.64 GHz and 14.0 GHz in a single resonator; excellent frequency stability, with typical drifts < 1 Hz per hour; the ability to withdraw the sample from the resonator for background calibration; and a small pot of liquid helium separate from the external bath that provides a sample base temperature of 1.1 K.

cond-mat.supr-con↗
arXiv2005

Fermi Surface and Quasiparticle Excitations of overdoped Tl2Ba2CuO6+d by ARPES

The electronic structure of the high-T_c superconductor Tl2Ba2CuO6+d is studied by ARPES. For a very overdoped Tc=30K sample, the Fermi surface consists of a single large hole pocket centered at (pi,pi) and is approaching a topological transition. Although a superconducting gap with d_x^2-y^2 symmetry is tentatively identified, the quasiparticle evolution with momentum and binding energy exhibits a marked departure from the behavior observed in under and optimally doped cuprates. The relevance of these findings to scattering, many-body, and quantum-critical phenomena is discussed.

cond-mat.supr-con↗
arXiv2005

The a-axis optical conductivity of detwinned ortho-II YBa_2Cu_3O_6.50

The a-axis optical properties of a detwinned single crystal of YBa_2Cu_3O_6.50 in the ortho II phase (Ortho II Y123, T_c= 59 K) were determined from reflectance data over a wide frequency range (70 - 42 000 cm^-1) for nine temperature values between 28 and 295 K. Above 200 K the spectra are dominated by a broad background of scattering that extends to 1 eV. Below 200 K a shoulder in the reflectance appears and signals the onset of scattering at 400 cm^-1. In this temperature range we also observe a peak in the optical conductivity at 177 cm^-1. Below 59 K, the superconducting transition temperature, the spectra change dramatically with the appearance of the superconducting condensate. Its spectral weight is consistent, to within experimental error, with the Ferrell-Glover-Tinkham (FGT) sum rule. We also compare our data with magnetic neutron scattering on samples from the same source that show a strong resonance at 31 meV. We find that the scattering rates can be modeled as the combined effect of the neutron resonance and a bosonic background in the presence of a density of states with a pseudogap. The model shows that the decreasing amplitude of the neutron resonance with temperature is compensated for by an increasing of the bosonic background yielding a net temperature independent scattering rate at high frequencies. This is in agreement with the experiments.

cond-mat.supr-con↗
arXiv2005

Delocalized fermions in underdoped cuprates

Low temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly-doped cuprate YBa$_{2}$Cu$_{3}$O$_{6.33}$. Samples were measured on either side of the superconducting phase boundary, in both zero and applied magnetic field. We report the observation of delocalized fermionic excitations at zero energy in the non-superconducting state, which shows that the ground state of underdoped cuprates is metallic. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e. nodal. The insulating ground state observed in underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ is attributed to the competing spin-density-wave order present in that system.

cond-mat.supr-con↗
arXiv2004

Universal scaling relation in high-temperature superconductors

Scaling laws express a systematic and universal simplicity among complex systems in nature. For example, such laws are of enormous significance in biology. Scaling relations are also important in the physical sciences. The seminal 1986 discovery of high transition-temperature (high-T_c) superconductivity in cuprate materials has sparked an intensive investigation of these and related complex oxides, yet the mechanism for superconductivity is still not agreed upon. In addition, no universal scaling law involving such fundamental properties as T_c and the superfluid density ρ_s, a quantity indicative of the number of charge carriers in the superconducting state, has been discovered. Here we demonstrate that the scaling relation ρ_s \propto σ_{dc} T_c, where the conductivity σ_{dc} characterizes the unidirectional, constant flow of electric charge carriers just above T_c, universally holds for a wide variety of materials and doping levels. This surprising unifying observation is likely to have important consequences for theories of high-T_c superconductivity.

cond-mat.supr-con↗
arXiv2004

Absolute values of the London penetration depth in YBa2Cu3O6+y measured by zero field ESR spectroscopy on Gd doped single crystals

Zero-field electron spin resonance (ESR) of dilute Gd ions substituted for Y in the cuprate superconductor YBa$_2$Cu$_3$O$_{\rm 6+y}$ is used as a novel technique for measuring the absolute value of the low temperature magnetic penetration depth $λ(T\to 0)$. The Gd ESR spectrum of samples with $\approx 1%$ substitution was obtained with a broadband microwave technique that measures power absorption bolometrically from 0.5 GHz to 21 GHz. This ESR spectrum is determined by the crystal field that lifts the level degeneracy of the spin 7/2 Gd$^{3+}$ ion and details of this spectrum provide information concerning oxygen ordering in the samples. The magnetic penetration depth is obtained by relating the number of Gd ions exposed to the microwave magnetic field to the frequency-integrated intensity of the observed ESR transitions. This technique has allowed us to determine precise values of $λ$ for screening currents flowing in the three crystallographic orientations ($\hat a$, $\hat b$ and $\hat c$) in samples of Gd$_{\rm x}$Y$_{\rm 1-x}$Ba$_2$Cu$_3$O$_{6+{\rm y}}$ of three different oxygen contents ${\rm y}=0.993$ ($T_c = 89$ K), ${\rm y}=0.77$ ($T_c=75$ K) and ${\rm y}=0.52$ ($T_c=56$ K). The in-plane values are found to depart substantially from the widely reported relation $T_c\propto 1/λ^2$.

cond-mat.supr-con↗
arXiv2003

Survival of the d-wave superconducting state near the edge of antiferromagnetism in the cuprate phase diagram

In the cuprate superconductor $YBa_2Cu_3O_{6+x}$, hole doping in the $CuO_2$ layers is controlled by both oxygen content and the degree of oxygen-ordering. At the composition $\rm YBa_2Cu_3O_{6.35}$, the ordering can occur at room temperature, thereby tuning the hole doping so that the superconducting critical temperature gradually rises from zero to 20 K. Here we exploit this to study the c-axis penetration depth as a function of temperature and doping. The temperature dependence shows the d-wave superconductor surviving to very low doping, with no sign of another ordered phase interfering with the nodal quasiparticles. The only apparent doping dependence is a smooth decline of superfluid density as Tc decreases.

cond-mat.supr-con↗
arXiv2003

Diffusion of Nonequilibrium Quasiparticles in a Cuprate Superconductor

We report a transport study of nonequilibrium quasiparticles in a high-Tc cuprate superconductor using the transient grating technique. Low-intensity laser excitation (at photon energy 1.5 eV) was used to introduce a spatially periodic density of quasiparticles into a high-quality untwinned single crystal of YBa2Cu3O6.5. Probing the evolution of the initial density through space and time yielded the quasiparticle diffusion coefficient, and both inelastic and elastic scattering rates. The technique reported here is potentially applicable to precision measurement of quasiparticle dynamics, not only in cuprate superconductors, but in other electronic systems as well.

cond-mat.supr-con↗
arXiv2003

Single-quasiparticle stability and quasiparticle-pair decay in YBa2Cu3O6.5 (Ortho II)

We report results and analysis of time-resolved photoinduced reflectivity experiments on the cuprate superconductor YBa2Cu3O6.5. The sample, which has Tc=45 K, was characterized by a high degree of purity and Ortho II ordering. The change in reflectivity (Delta R) was induced and probed using pulses of 100 femtosecond duration and photon energy 1.55 eV from a Ti:Sapphire laser. We provide a detailed picture of the decay rate (gamma) of Delta R as a function of temperature T and pump intensity I. At low T, gamma decreases linearly with decreasing I, extrapolating to nearly zero in the limit that I tends to zero. At higher temperatures gamma has the same linear dependence, but with nonzero limit as I goes to zero. In the interpretation of these results we assume that Delta R is proportional to the nonequilibrium quasiparticle density created by the laser. From an analysis of the gamma vs. I we estimate beta, the coefficient of proportionality relating the quasiparticle decay rate to the density. The intercept of gamma vs. I yields the thermal equilibrium quasiparticle decay rate. In a discussion section, we argue that the quasiparticles induced by the laser occupy primarily states near the antinodal regions of the Brillouin zone. We explain the divergence of the lifetime as T and I both tend to zero as a consequence of momentum and energy conservation in electron-electron scattering. We discuss the significance of the measured value of beta (approximately 0.1 cm2/s) in terms of the electron-phonon interactions.

cond-mat.supr-con↗