arXiv subjects

John A. Skinta

Publications and source records attributed to John A. Skinta.

5 recordsLinked to original sources

arXiv2004

The Role of Thermal Phase Fluctuations in Underdoped YBCO Films

The effect of thermal phase fluctuations (TPF's) on the ab-plane penetration depth, lambda(T), of thin YBa2Cu3O(7-delta) (YBCO) films is found to be much smaller than expected from the paradigm of cuprates as weakly-coupled 2D superconducting layers. A 2D vortex-pair-unbinding transition is observed, but the effective thickness for fluctuations is the film thickness, not a CuO bilayer thickness. In a strongly underdoped YBCO film, Tc= 34 K, TPF's suppress Tc by only about 3 K. They cannot be a significant factor in the suppression of Tc and emergence of the pseudogap with underdoping.

cond-mat.supr-con↗

arXiv2003

Magnetic Penetration Depth Measurements of Pr$_{2-x}$Ce$_x$CuO$_{4-δ}$ Films on Buffered Substrates: Evidence for a Nodeless Gap

We report measurements of the inverse squared magnetic penetration depth, $λ^{-2}(T)$, in Pr$_{2-x}$Ce$_{x}$CuO$_{4-δ}$ ($0.115 \leq x \leq 0.152$) superconducting films grown on SrTiO$_3$ (001) substrates coated with a buffer layer of insulating Pr$_{2}$CuO$_{4}$. $λ^{-2}(0)$, $T_c$ and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, $0.124\leq x \leq 0.144$, $λ^{-2}(T)$ at low $T$ is flat: it changes by less than 0.15% over a factor of 3 change in $T$, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in $λ^{-2}(T)$ produce values of the minimum superconducting gap in the range of $0.29\leqΔ_{\rm min}/k_BT_c\leq1.01$.

cond-mat.supr-con↗

arXiv2002

Reflection of two-gap nature in penetration depth measurements of MgB$_2$ film

The magnetic penetration depth, $λ^{-2}(T)$, in the basal plane of a magnesium diboride (MgB$_2$) film was measured using a two-coil mutual inductance technique at 50 kHz. This film has $T_c\simeq 38$ K, $ΔT_c \leq 1$ K, and $λ(0)\sim 1500$ Å. At low temperatures, $λ^{-2}(T)$ shows a clear exponential temperature dependence, indicating s-wave superconducting order parameter symmetry. However, the data are not quantitatively well described by theory assuming a single gap. From the data fit by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: $Δ_S(0)=2.61\pm0.41$ meV and $Δ_L(0)=6.50\pm0.33$ meV. The contributions of the small and the large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we consider the effect of gap anisotropy on the penetration depth measurements, and find that the anisotropy does not play a significant role in determining the temperature dependence of the penetration depth.

cond-mat.supr-con↗

arXiv2002

Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr_{1.855}Ce_{0.145}CuO_{4-y} Films

We present measurements of the ab-plane magnetic penetration depth, λ(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6 K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities n_s(T)\propto λ^{-2}(T), high T_c's, and small transition widths are reproducible and indicative of excellent film quality. For all five films, λ^{-2}(T)/λ^{-2}(0) at low T is well fitted by an exponential temperature dependence with a gap, Δ_{min}, of 0.85 k_B T_c. This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

cond-mat.supr-con↗

arXiv2002

Evidence for a Transition in the Pairing Symmetry of the Electron-Doped Cuprates La_{2-x}Ce_xCuO_{4-y} and Pr_{2-x}Ce_xCuO_{4-y}

We present measurements of the magnetic penetration depth, λ^{-2}(T), in Pr_{2-x}Ce_{x}CuO_{4-y} and La_{2-x}Ce_{x}CuO_{4-y} films at three Ce doping levels, x, near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, λ^{-2}(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, λ^{-2}(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d- and s-wave pairing symmetry in nominally optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

cond-mat.supr-con↗