arXiv subjects

J. C. Baglo

Publications and source records attributed to J. C. Baglo.

5 recordsLinked to original sources

arXiv2024

Thermoelectric power of overdoped Tl2201 crystals: Charge density waves and $T^1$ and $T^2$ resistivities

We report measurements of the in-plane thermoelectric power (TEP) for an overdoped (OD) crystal of the single layer cuprate superconductor Tl$_2$Ba$_2$CuO$_{6+x}$ (Tl2201) at several hole concentrations ($p$), from 300 or 400 K to below the superconducting transition temperature ($T_c$). For $p$ = 0.192 and 0.220, small upturns in the TEP below 150 K are attributed to the presence of charge density waves (CDW) detected by resonant inelastic X-ray scattering studies. This suggests that measurement of the TEP could provide a simple and effective guide to the presence of a CDW. Over a certain temperature range, often strongly restricted by the CDW, the TEP is consistent with the Nordheim-Gorter rule and the $T^1$ and $T^2$ terms in the in-plane resistivity of similar crystals observed below 160 K. Two scenarios in which the $T^1$ scattering term is uniform or non-uniform around the Fermi surface are discussed. As found previously by others, for uniform scattering the $T^1$ terms give scattering rates ($τ^{-1}$) at lower $p$ that are somewhat larger than the Planckian value $k_B T/\hbar$ and fall to zero for heavily OD crystals. Near 160 K, $τ^{-1}$ from the $T^2$ terms corresponds to the Planckian value.

cond-mat.supr-con↗

arXiv2016

Wiedemann-Franz law in the underdoped cuprate superconductor YBa2Cu3Oy

The recent detection of charge-density modulations in YBa2Cu3Oy and other cuprate superconductors raises new questions about the normal state of underdoped cuprates. In one class of theories, the modulations are intertwined with pairing in a dual state, expected to persist up to high magnetic fields as a vortex liquid. In support of such a state, specific heat and magnetisation data on YBa2Cu3Oy have been interpreted in terms of a vortex liquid persisting above the vortex-melting field Hvs at T = 0. Here we report high-field measurements of the electrical and thermal Hall conductivities in YBa2Cu3O6.54 that allow us to probe the Wiedemann-Franz law, a sensitive test of the presence of superconductivity in a metal. In the T = 0 limit, we find that the law is satisfied for fields immediately above Hvs. This rules out the existence of a vortex liquid and it places strict constraints on the nature of the normal state in underdoped cuprates.

cond-mat.supr-con↗

arXiv2015

Onset field for Fermi-surface reconstruction in the cuprate superconductor YBCO

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.

cond-mat.supr-con↗

arXiv2015

Separation of magnetic and superconducting behaviour in YBCO6.33 (Tc=8.4 K)

Neutron scattering from high-quality YBa2Cu3O6.33 (YBCO6.33) single crystals with a Tc of 8.4 K shows no evidence of a coexistence of superconductivity with long-range antiferromagnetic order at this very low, near-critical doping of p~0.055. However, we find short-range three dimensional spin correlations that develop at temperatures much higher than Tc. Their intensity increases smoothly on cooling and shows no anomaly that might signify a Neel transition. The system remains subcritical with spins correlated over only one and a half unit cells normal to the planes. At low energies the short-range spin response is static on the microvolt scale. The excitations out of this ground state give rise to an overdamped spectrum with a relaxation rate of 3 meV. The transition to the superconducting state below Tc has no effect on the spin correlations. The elastic interplanar spin response extends over a length that grows weakly but fails to diverge as doping is moved towards the superconducting critical point. Any antiferromagnetic critical point likely lies outside the superconducting dome. The observations suggest that conversion from Neel long-range order to a spin glass texture is a prerequisite to formation of paired superconducting charges. We show that while pc =0.052 is a critical doping for superconducting pairing, it is not for spin order.

cond-mat.supr-con↗

arXiv2010

Precision Microwave Electrodynamic Measurements of K- and Co-doped BaFe$_2$As$_2$

We have studied the microwave electrodynamics of single crystal iron-based superconductors Ba$_{0.72}$K$_{0.28}$Fe$_2$As$_2$ (hole-doped, $T_\mathrm{c}$ $\approx $30 K) and Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$ (electron-doped, $T_\mathrm{c}$ $\approx$20 K), by cavity perturbation and broadband spectroscopy. SQUID magnetometry was used to confirm the quality and homogeneity of the samples under study. Through cavity perturbation techniques, the temperature dependence of the in-plane London penetration depth $Δλ(T)$, and therefore the superfluid phase stiffness $λ^2(0)/λ^2(T)$ was measured. Down to 0.4 K, the data do not show the exponential saturation at low temperatures expected from a singly-, fully-gapped superconductor. Rather, both the electron- and the hole-doped systems seem to be best described by a power law behavior, with $λ^2(0)/λ^2(T)$ $\sim$ $T^n$ and \emph{n} $\approx$ 2.5. In the three samples we studied, a weak feature near the sensitivity limit of our measurements appears near $T/T_\mathrm{c}$ =~0.04, hinting at a corresponding low energy feature in the superconducting density of states. The data can also be relatively well-described by a simple two-gap s-wave model of the order parameter, but this yields parameters which seem unrealistic and dependent on the fit range. Broadband surface resistance measurements reveal a sample dependent residual loss whose origin is unclear. The data from the \FeAs samples can be made to scale as $ω^2$ if the extrinsic loss is treated as an additive component, indicating large scattering rates. Finally, the temperature dependence of the surface resistance at 13 GHz obeys a power law very similar to those observed for $Δλ(T)$.

cond-mat.supr-con↗