arXiv subjects

D. M. Broun

Publications and source records attributed to D. M. Broun.

32 records · Page 2Linked to original sources

arXiv2013

Microwave spectroscopy of vortex dynamics in ortho-II YBa2Cu3O6.52

We present measurements of the vortex-state surface impedance, Z_s = R_s + i X_s, of a high quality, ortho-II-ordered single crystal of the cuprate high temperature superconductor YBa2Cu3O6.52 (T_c = 59K). Measurements have been made at four microwave frequencies (ω/2π= 2.64, 4.51, 9.12 and 13.97 GHz), for magnetic fields ranging from 0 to 7 T. From these data we obtain the field, frequency and temperature dependence of the vortex viscosity, pinning constant, depinning frequency and flux-flow resistivity. The vortex viscosity, η(ω,T), has a surprisingly strong frequency dependence and bears a striking resemblance to the zero-field quasiparticle conductivity, σ_qp(ω,T), suggesting that the dominant dissipative mechanism for the flux lines is induced electric fields coupling to bulk, long-lived d-wave quasiparticles outside the vortex cores. This is in sharp contrast to the conventional Bardeen-Stephen picture, in which dissipation takes place inside quasi-normal vortex cores. The strong frequency dependence of the vortex viscosity in the microwave range requires us to treat it as a complex response function, with an imaginary part that is predicted to contribute to the apparent pinning force on the vortices. Measurements of the frequency dependence of the pinning force confirm that this term is present, and in a form consistent with the requirements of causality. At low temperatures the flux-flow resistivity, ρ_ff \propto 1/η, has the form ρ_ff(T) = ρ_0 + ρ_1 \ln(1/T), reminiscent of the DC resistivity of cuprates in the pseudogap regime.

cond-mat.supr-con↗

arXiv2008

Stability of nodal quasiparticles in underdoped YBa2Cu3O6+y probed by penetration depth and microwave spectroscopy

High resolution measurements of superfluid density and broadband quasiparticle conductivity have been used to probe the low energy excitation spectrum of nodal quasiparticles in underdoped YBCO. Penetration depth is measured to temperatures as low as 0.05 K. Microwave conductivity is measured from 0.1 to 20 GHz and is a direct probe of zero-energy quasiparticles. The data are compared with predictions for a number of theoretical scenarios that compete with or otherwise modify pure d-wave superconductivity, in particular commensurate and incommensurate spin and charge density waves; d + i s and d + i d superconductivity; circulating current phases; and the BCS--BEC crossover. We conclude that the data are consistent with a pure d-wave state in the presence of a small amount of strong scattering disorder, and are able to rule out most candidate competing states either completely, or to a level set by the energy scale of the disorder, ~ 4 K. Commensurate spin and charge density orders, however, are not expected to alter the nodal spectrum and therefore cannot be excluded.

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arXiv2008

Effective magnetic penetration depth in superconducting cylinders and spheres with highly anisotropic electrodynamics

Effective magnetic penetration depth and microwave surface impedance are derived for anisotropic layered superconductors in the shape of spheres and long cylinders, where the external magnetic field is applied in the plane of the highly conducting layers to induce out-of-plane screening currents. The results are extended by analytic continuation to highly anisotropic conductors and to lossy superconductors at high frequency. The electrodynamics for the general case of a superconductor or metal with arbitrary anisotropy are presented. The treatment is then specialized to layered materials with unixaxial anisotropy, in which the penetration depth for currents flowing perpendicular to the layers, lambda_c, is much greater than that for in-plane currents, lambda_a. Exact solutions are found in the limit lambda_a goes to zero, and are expected to provide an accurate representation of many experiments on cuprates and other layered superconductors, particularly on grain-aligned powders.

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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.

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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.

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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.

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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.

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arXiv2005

Electrical transport measurements in the superconducting state of Bi2212 and Tl2201

Precise measurements of the in-plane microwave surface impedance of high-quality single crystals of Bi2212 and Tl2201 are used to probe the relaxation time of nodal quasiparticles in the d-wave superconducting state through a two-fluid analysis of the microwave conductivity. While this analysis requires us to posit a form for the frequency-dependent quasiparticle conductivity, we clearly demonstrate that the extraction of the relaxation rate is quite insensitive to the assumed shape of the quasiparticle spectrum. The robustness of the analysis is rooted in the oscillator-strength sum rule and the fact that we simultaneously measure the real and imaginary parts of the conductivity. In both Bi2212 and Tl2201 we infer a linear temperature dependence of the transport relaxation rate 1/tau and a small but finite zero-temperature intercept. The linear temperature dependence of 1/tau is in accord with expectations for weak elastic scattering in an unconventional superconductor with line nodes and a small residual density of states. The same analysis reveals an onset of inelastic scattering at higher temperatures similar to that seen in the YBCO superconductors. Finally we extrapolate the two-fluid model over a range of frequencies up to five times the measurement frequency, where the extrapolation predicts behaviour that is qualitatively similar to terahertz conductivity data on Bi2212 thin films. While relaxation rates in Bi2212 and Tl2201 are substantially higher than in YBCO there are qualitative similarities between all three materials, and the differences can likely be attributed to varying levels of static disorder. We therefore conclude that a universal picture of quasiparticle scattering in the cuprates is emerging.

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.

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arXiv2003

Microwave Spectroscopy of PrOs_4Sb_12: Josephson-Coupled Two-Band Superconductivity and Itinerant f Electrons

We report microwave surface impedance measurements made on a single crystal of PrOs_4Sb_12 across the frequency range 0.5 to 21 GHz. The penetration depth data provide clear thermodynamic evidence for superconductivity arising in two bands coupled by Josephson pair-tunneling, indicating two order parameters with the same symmetry. Detailed conductivity spectra obtained using high-resolution bolometry confirm this picture and extend it, revealing the itinerant nature of the Pr f electrons through the observation of quasiparticles with heavily renormalized mass.

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arXiv2003

Bolometric technique for high-resolution broadband microwave spectroscopy of ultra-low-loss samples

A novel low temperature bolometric method has been devised and implemented for high-precision measurements of the microwave surface resistance of small single-crystal platelet samples having very low absorption, as a continuous function of frequency. The key to the success of this non-resonant method is the in-situ use of a normal metal reference sample that calibrates the absolute rf field strength. The sample temperature can be controlled independently of the 1.2 K liquid helium bath, allowing for measurements of the temperature evolution of the absorption. However, the instrument's sensitivity decreases at higher temperatures, placing a limit on the useful temperature range. Using this method, the minimum detectable power at 1.3 K is 1.5 pW, corresponding to a surface resistance sensitivity of $\approx$1 $μΩ$ for a typical 1 mm$\times$1 mm platelet sample.

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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

CeCoIn5 - a quantum critical superfluid

We have made the first complete measurements of the London penetration depth $λ(T)$ of CeCoIn5, a quantum-critical metal where superconductivity arises from a non-Fermi-liquid normal state. Using a novel tunnel diode oscillator designed to avoid spurious contributions to $λ(T)$, we have established the existence of intrinsic and anomalous power-law behaviour at low temperature. A systematic analysis raises the possibility that the unusual observations are due to an extension of quantum criticality into the superconducting state.

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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.

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