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W. N. Hardy

Publications and source records attributed to W. N. Hardy.

At least 181 records · Page 10Linked to original sources
arXiv2002

Hole concentration and phonon renormalization in Ca-doped YBa_2Cu_3O_y (6.76 < y < 7.00)

In order to access the overdoped regime of the YBa_2Cu_3O_y phase diagram, 2% Ca is substituted for Y in YBa_2Cu_3O_y (y = 7.00,6.93,6.88,6.76). Raman scattering studies have been carried out on these four single crystals. Measurements of the superconductivity-induced renormalization in frequency (Delta ω) and linewidth (Δ2γ) of the 340 cm^{-1} B_{1g} phonon demonstrate that the magnitude of the renormalization is directly related to the hole concentration (p), and not simply the oxygen content. The changes in Δωwith p imply that the superconducting gap (Δ_{max}) decreases monotonically with increasing hole concentration in the overdoped regime, and Δωfalls to zero in the underdoped regime. The linewidth renormalization Δ2γis negative in the underdoped regime, crossing over at optimal doping to a positive value in the overdoped state.

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.

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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}.

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arXiv2002

The high-field phase diagram of the cuprates derived from the Nernst effect

Measurements of the Nernst signal in the vortex-liquid state of the cuprates to high fields (33 T) reveal that vorticity extends to very high fields even close to the zero-field critical temperature $T_{c0}$. In overdoped $\rm La_{2-x}Sr_xCuO_4$ (LSCO) we show that the upper critical field $H_{c2}(T)$ curve does not end at $T_{c0}$, but at a much higher temperature. These results imply that $T_{c0}$ corresponds to a loss in phase rigidity rather than a vanishing of the pairing amplitude. An intermediate field $H^*(T)\ll H_{c2}(T)$ is shown to be the field scale for the flux-flow resistivity.

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arXiv2002

Neutron scattering search for static magnetism in oxygen ordered YBa2Cu3O6.5

We present elastic and inelastic neutron scattering results on highly oxygen ordered YBa2Cu3O6.5 ortho-II. We find no evidence for the presence of ordered magnetic moments to a sensitivity of 0.003 Bohr magnetons, an order of magnitude smaller than has been suggested in theories of orbital or d-density-wave (DDW) currents. The absence of sharp elastic peaks, shows that the d-density-wave phase is not present, at least for the superconductor with the doping of 6.5 and the ordered ortho-II structure. We cannot exclude the possibility that a broad peak may exist with extremely short-range DDW correlations. For less ordered or more doped crystals it is possible that disorder may lead to static magnetism. We have also searched for the large normal state spin gap that is predicted to exist in an ordered DDW phase. Instead of a gap we find that the Q-correlated spin susceptibility persists to the lowest energies studied, 6 meV. Our results are compatible with the coexistence of superconductivity with orbital currents, but only if they are dynamic, and exclude a sharp phase transition to an ordered d-density-wave phase.

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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.

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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.

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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.

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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.

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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

Universal c-axis conductivity of high-$T_c$ oxides in the superconducting state

The anisotropy in the temperature dependence of the in-plane and c-axis conductivities of high-T_c cuprates in the superconducting state is shown to be consistent with a strong in-plane momentum dependence of both the quasiparticle scattering rate and the interlayer hopping integral. Applying the cold spot scattering model recently proposed by Ioffe and Millis (Phys. Rev. B {\bf 58}, 11631 (1998)) to the superconducting state, we find that the c-axis conductivity varies approximately as T^3 in an intermediate temperature regime, in good agreement with the experimental result for optimally doped YBa_2Cu_3O_{7-x} and Bi_2Sr_2CaCu_2O_{8+x}.

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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.

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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.

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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↗
arXiv1999

C-axis Penetration Depth and Inter-layer Conductivity in the Thallium Based Cuprate Superconductors

The c-axis Josephson plasmon in optimally doped single-layer and bi-layer high Tc cuprates Tl2201 and Tl2212 have been investigated using infrared spectroscopy. We observed the plasma frequencies for these two compounds at 27.8 and 25.6 cm-1 respectively, which we interpret as a Josephson resonance across the TlO blocking layers. No maximum in the temperature dependence of the c-axis conductivity was observed below Tc, indicating that even in the superconducting state a coherent quasi-particle contribution to the c-axis conductivity is absent or very weak, in contrast to the behaviour of the ab-plane conductivity.

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}.

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