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Search Results: 1 - 10 of 286455 matches for " R. W. Giannetta "
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Magnetic penetration depth in electron-doped cuprates - evidence for gap nodes
R. Prozorov,R. W. Giannetta,P. Fournier,R. L. Greene
Physics , 1999, DOI: 10.1016/S0921-4534(00)00945-X
Abstract: The in-plane penetration depth \lambda(T) is measured in electron-doped single crystals Nd1.85Ce0.15CuO4-x (NCCO) and Pr1.85Ce0.15CuO4-x (PCCO) using a 11 MHz LC resonator. In NCCO, \lambda(T) exhibits a minimum at 3.8 K and a pronounced upturn down to 0.4 K due to the paramagnetic contribution of Nd3+ ions. The London penetration depth contribution is linear in T. The paramagnetic contribution is absent in PCCO, where \lambda(T)~T^2 at low temperatures. Our results indicate the presence of nodes in the superconducting gap, i.e., non s-wave symmetry of the order parameter in electron-doped cuprates.
Evidence for nodal quasiparticles in electron-doped cuprates from penetration depth measurements
R. Prozorov,R. W. Giannetta,P. Fournier,R. L. Greene
Physics , 2000, DOI: 10.1103/PhysRevLett.85.3700
Abstract: The in-plane magnetic penetration depth, $\lambda (T)$, was measured down to 0.4 K in single crystals of electron-doped superconductors, Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ (PCCO) and Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ (NCCO). In PCCO, the superfluid density varies as $T^2$ from 0.025 up to roughly 0.3 $T/T_c$ suggestive of a d-wave state with impurities. In NCCO, $\lambda (T)$ shows a pronounced upturn for $T < 4$ K due to the paramagnetic contribution of Nd$^{3+}$ ions. Fits to an s-wave order parameter over the standard BCS range ($T/T_c$ = 0.32) limit any gap to less than $\Delta_{min} (0)/T_c = 0.57$ in NCCO. For PCCO, the absence of paramagnetism permits a lower temperature fit and yields an upper limit of $\Delta_{min} (0)/T_c = 0.2$.
Absence of non-linear Meissner effect in YBa2Cu3O6.95
A. Carrington,R. W. Giannetta,J. T. Kim,J. Giapintzakis
Physics , 1998, DOI: 10.1103/PhysRevB.59.R14173
Abstract: We present measurements the field and temperature dependence of the penetration depth (lambda) in high purity, untwinned single crystals of YBa2Cu3O6.95 in all three crystallographic directions. The temperature dependence of lambda is linear down to low temperatures, showing that our crystals are extremely clean. Both the magnitude and temperature dependence of the field dependent correction to lambda however, are considerably different from that predicted from the theory of the non-linear Meissner effect for a d-wave superconductor (Yip-Sauls theory). Our results suggest that the Yip-Sauls effect is either absent or is unobservably small in the Meissner state of YBa2Cu3O6.95.
Nodal Order Parameter in Electron-Doped Superconducting Films Pr(2-x)Ce(x)CuO(4-y) (x=0.13, 0.15 and 0.17)
A. Snezhko,R. Prozorov,D. D. Lawrie,R. W. Giannetta,J. Gauthier,J. Renaud,P. Fournier
Physics , 2003,
Abstract: The London penetration depth, lambda{ab}(T), is reported for thin films of the electron-doped superconductor Pr{2-x}Ce{x}CuO{4-y} at three doping levels (x = 0.13, 0.15 and 0.17). Measurements down to 0.35 K were carried out using a tunnel diode oscillator with excitation fields applied both perpendicular and parallel to the conducting planes. For all samples and both field orientations lambda{ab}(T) showed power law behavior implying a superconducting gap with nodes.
Low temperature vortex phase diagram of Bi2Sr2CaCu2O8 : a magnetic penetration depth study
R. Prozorov,R. W. Giannetta,T. Tamegai,P. Guptasarma,D. G. Hinks
Physics , 2000, DOI: 10.1016/S0921-4534(00)00814-5
Abstract: We report measurements of the magnetic penetration depth \lambda_m(T) in the presence of a DC magnetic field in optimally doped BSCCO-2212 single crystals. Warming, after magnetic field is applied to a zero-field cooled sample, results in a non-monotonic \lambda_m(T), which does not coincide with a curve obtained upon field cooling, thus exhibiting a hysteretic behaviour. We discuss the possible relation of our results to the vortex decoupling, unbinding, and dimensional crossover.
Meissner - London state in superconductors of rectangular cross-section in perpendicular magnetic field
R. Prozorov,R. W. Giannetta,A. Carrington,F. M. Araujo-Moreira
Physics , 2000, DOI: 10.1103/PhysRevB.62.115
Abstract: The distribution of magnetic induction in Meissner state with finite London penetration depth is analyzed for platelet samples of rectangular cross-section in a perpendicular magnetic field. The exact 2D numerical solution of the London equation is extended analytically to the realistic 3D case. Data obtained on Nb cylinders and foils as well as single crystals of YBCO and BSCCO are in a good agreement with the model. The results are particularly relevant for magnetic susceptibility, rf and microwave resonator measurements of the magnetic penetration depth in high-Tc superconductors.
Campbell Penetration Depth of a Superconductor in the Critical State
R. Prozorov,R. W. Giannetta,N. Kameda,T. Tamegai,J. A. Schlueter,P. Fournier
Physics , 2002, DOI: 10.1103/PhysRevB.67.184501
Abstract: The magnetic penetration depth $\lambda(T,H,j)$ was measured in the presence of a slowly relaxing supercurrent, $j$. In single crystal $\mathrm{Bi_2Sr_2CaCu_2O_8}$ below approximately 25 K, $\lambda(T,H,j)$ is strongly hysteretic. We propose that the irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. The Campbell length depends upon the ratio $j/j_{c}$ where $j_{c}$ is the critical current defined through the Labusch parameter. Similar effects were observed in other cuprates and in an organic superconductor.
Nonequilibrium Campbell length: probing the vortex pinning potential
R. Prozorov,R. W. Giannetta,T. Tamegai,P. Guptasarma,D. G. Hinks
Physics , 2000,
Abstract: The $AC$ magnetic penetration depth $\lambda (T,H,j)$ was measured in presence of a macroscopic $DC$ (Bean) supercurrent, $j$. In single crystal BSCCO below approximately 28 K, $\lambda (T,H,j)$ exhibits thermal hysteresis. The irreversibility arises from a shift of the vortex position within its pinning well as $j$ changes. It is demonstrated that below a new irreversibility temperature, the nonequilibrium Campbell length depends upon the ratio $j/j_c$. $\lambda (T,H,j)$ {\it increases} with $j/j_c$ as expected for a non-parabolic potential well whose curvature {\it decreases} with the displacement. Qualitatively similar results are observed in other high-$T_{c}$ and conventional superconductors.
Evidence for Surface Andreev Bound states in Cuprate Superconductors from Penetration Depth Measurements
A. Carrington,F. Manzano,R. Prozorov,R. W. Giannetta,N. Kameda,T. Tamegai
Physics , 2000, DOI: 10.1103/PhysRevLett.86.1074
Abstract: Tunneling and theoretical studies have suggested that Andreev bound states form at certain surfaces of unconventional superconductors. Through studies of the temperature and field dependence of the in-plane magnetic penetration depth lambda_ab at low temperature, we have found strong evidence for the presence of these states in clean single crystal YBCO and BSCCO. Crystals cut to expose a [110] interface show a strong upturn in lambda_ab at around 7K, when the field is oriented so that the supercurrents flow around this surface. In YBCO this upturn is completely suppressed by a field of ~0.1 T.
Energy gap and proximity effect in $MgB_2$ superconducting wires
R. Prozorov,R. W. Giannetta,S. L. Bud'ko,P. C. Canfield
Physics , 2001, DOI: 10.1103/PhysRevB.64.180501
Abstract: Measurements of the penetration depth $\lambda (T,H)$ in the presence of a DC magnetic field were performed in $MgB_2$ wires. In as-prepared wires $\lambda (T,H<130 Oe)$ shows a strong diamagnetic downturn below $\approx 10 K$. A DC magnetic field of $130 Oe$ completely suppressed the downturn. The data are consistent with proximity coupling to a surface $Mg$ layer left during synthesis. A theory for the proximity effect in the clean limit, together with an assumed distribution of the $Mg$ layer thickness, qualitatively explains the field and temperature dependence of the data. Removal of the $Mg$ by chemical etching results in an exponential temperature dependence for $\lambda (T)$ with an energy gap of $2 \Delta (0)/T_c\approx 1.54$ ($\Delta(0) \approx 2.61 meV$), in close agreement with recent measurements on commercial powders and single crystals. This minimum gap is only 44% of the BCS weak coupling value, implying substantial anisotropy.
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