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R. Liang

Publications and source records attributed to R. Liang.

84 records · Page 5Linked to original sources
arXiv2001

Correlations Between Charge Ordering and Local Magnetic Fields in Overdoped YBa$_2$Cu$_3$O$_{6+x}$

Zero-field muon spin relaxation (ZF-$μ$SR) measurements were undertaken on under- and overdoped samples of superconducting YBa$_2$Cu$_3$O$_{6+x}$ to determine the origin of the weak static magnetism recently reported in this system. The temperature dependence of the muon spin relaxation rate in overdoped crystals displays an unusual behavior in the superconducting state. A comparison to the results of NQR and lattice structure experiments on highly doped samples provides compelling evidence for strong coupling of charge, spin and structural inhomogeneities.

cond-mat.supr-con↗
arXiv2001

Evidence for Static Magnetism in the Vortex Cores of Ortho-II YBa$_2$Cu$_3$O$_{6.50}$

Evidence for static alternating magnetic fields in the vortex cores of underdoped YBa$_2$Cu$_3$O$_{6+x}$ is reported. Muon spin rotation measurements of the internal magnetic field distribution of the vortex state of YBa$_2$Cu$_3$O$_{6.50}$ in applied fields of $H = 1$ T and $H = 4$ T reveal a feature in the high-field tail of the field distribution which is not present in optimally doped YBa$_2$Cu$_3$O$_{6.95}$ and which fits well to a model with static magnetic fields in the vortex cores. The magnitude of the fields is estimated to be 18(2) G and decreases above $T = 10$ K. We discuss possible origins of the additional vortex core magnetism within the context of existing theories.

cond-mat.supr-con↗
arXiv2001

Superconducting Gap and Strong In-Plane Anisotropy in Untwinned YBa2Cu3O7-d

With significantly improved sample quality and instrumental resolution, we clearly identify in the (pi,0) ARPES spectra from YBa2Cu3O6.993, in the superconducting state, the long-sought `peak-dip-hump' structure. This advance allows us to investigate the large a-b anisotropy of the in-plane electronic structure including, in particular, a 50% difference in the magnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the CuO chains, raises serious questions about attempts to quantitatively explain the YBa2Cu3O7-d data from various experiments using models based on a perfectly square lattice.

cond-mat.supr-con↗
arXiv2001

Observation of the Transverse Optical Plasmon in SmLa0.8Sr0.2CuO4-d

We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-d, which are direct evidence for the existence of a transverse optical plasma mode, observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a (La,Sr)O block layer. From the frequencies and the intensities of the collective modes we determine the value of the compressibility of the two dimensional electron fluid in the copper oxygen planes.

cond-mat.supr-con↗
arXiv2000

Giant enhancement of the thermal Hall conductivity κ_{xy} in the superconductor YBa_2Cu_3O_7

In high-purity crystals of YBa_2Cu_3O_7, the quasiparticle (qp) lifetime τand the (weak-field) thermal Hall conductivity κ_{xy} undergo dramatic increases below 90K. We present a detailed picture of the behavior of κ_{xy} at low temperature, in particular its scaling properties, which are directly relevant to the issue of whether Landau quantization of the qp states occurs.

cond-mat.supr-con↗
arXiv1999

Field Induced Reduction of the Low Temperature Superfluid Density in YBa2Cu3O6.95

A novel high magnetic field (8 T) spectrometer for muon spin rotation has been used to measure the temperature dependence of the in-plane magnetic penetration depth in YBa2Cu3O6.95. At low H and low T, the penetration depth exhibits the characteristic linear T dependence associated with the energy gap of a d_x^2-y^2-wave superconductor. However, at higher fields the penetration depth is essentially temperature independent at low T. We discuss possible interpretations of this surprising new feature in the low-energy excitation spectrum.

cond-mat.supr-con↗
arXiv1999

Melting and Dimensionality of the Vortex Lattice in Underdoped YBa2Cu3O6.60

Muon spin rotation measurements of the magnetic field distribution in the vortex state of the oxygen deficient high-Tc superconductor YBa{2}Cu{3}O{6.60} reveal a vortex-lattice melting transition at much lower temperature than that in the fully oxygenated material. The transition is best described by a model in which adjacent layers of ``pancake'' vortices decouple in the liquid phase. Evidence is also found for a pinning-induced crossover from a solid 3D to quasi-2D vortex lattice, similar to that observed in the highly anisotropic superconductor Bi{2+x}Sr{2-x}CaCu{2}O{8+y}.

cond-mat.supr-con↗
arXiv1999

Oxygen Ordering Superstructures and Structural Phase Diagram of YBa_2Cu_3O_{6+x} Studied by Hard X-ray Diffraction

High energy x-ray diffraction is used to investigate the bulk oxygen ordering properties of YBa_2Cu_3O_{6+x}. Superstructures of Cu-O chains aligned along the b axis and ordered with periodicity ma, along the a axis have been observed. For $x < 0.62$ the only observed superstructure is ortho-II with m=2. At room temperature we find ortho-III (m=3) for $0.72\le x\le 0.82$, ortho-V (m=5) in a mixed state with ortho-II at x=0.62, and ortho-VIII (m=8) at x=0.67. Ortho-II is a 3D ordered structural phase, the remaining ones are essentially 2D. None of the superstructures develops long range ordering. Studies of the ortho-II ordering properties in samples prepared with x=0.5 but by different methods show that finite size domains with internal thermodynamic equilibrium are formed. The crystal perfection as well as the thermal annealing history determine the domain size. The temperature dependence of the observed superstructure ordering is investigated explicitly and a structural phase diagram is presented.

cond-mat.supr-con↗
arXiv1998

Expansion of the Vortex Cores in YBa2Cu3O6.95 at Low Magnetic Fields

Muon spin rotation spectroscopy has been used to measure the effective size of the vortex cores in optimally doped YBa2Cu3O6.95 as a function of temperature and magnetic field deep in the superconducting state. While the core size at H=2T is close to 20 angstroms and consistent with that measured by STM at 6T, we find a striking increase in the core size at lower magnetic fields, where it approaches an extraordinarily large value of about 100 angstroms. This suggests that the average value of the superconducting coherence length in cuprate superconductors may be larger than previously thought at low magnetic fields.

cond-mat.supr-con↗
arXiv1998

c-axis electrodynamics of ybco

New measurements of surface impedance in ybco show that the c-axis penetration depth and conductivity below Tc exhibit behaviour different from that observed in the planes. The c-axis penetration depth never has the linear temperature dependence seen in the ab-plane. Instead of the conductivity peak seen in the planes, the c-axis microwave conductivity falls to low values in the superconducting state, then rises slightly below 20K. These results show that c-axis transport remains incoherent below Tc, even though this is one of the least anisotropic cuprate superconductors.

cond-mat.supr-con↗
arXiv1995

Optical properties along the c axis of YBa_2Cu_3O_{6+x}, for x=0.50 -> 0.95: evolution of the pseudogap

The reflectance of high-quality, unpolished single crystals of YBa_2Cu_3O_{6+x}, for the doping range x=0.50 -> 0.95, has been measured with radiation polarized along the c axis from approx 50 cm^{-1} to 5000 cm^{-1} between 10 K and 300 K. In highly doped (x=0.95) material, the normal-state conductivity shows a metallic response. For intermediate dopings (x=0.85 -> 0.90) the conductivity is no longer metallic, increasing with temperature, and for low dopings (x=0.50 -> 0.80) this behavior is clearly seen to be caused by a pseudogap at approx 280 cm^{-1}, which develops well above T_c. In the superconducting state, in the optimally-doped material, a gap-like depression develops below T_c but there is residual conductivity to very low frequency indicating either an anisotropic gap, or a gap with nodes. In the underdoped materials the superconductivity-induced changes to the conductivity are harder to see, but it appears that the weight of the condensate originates at frequencies much higher than the conventional weak-coupling value of 3.5k_BT_c.

cond-mat↗