Thomas R. Lemberger

Evidence for a transition in the pairing symmetry of the electron-doped cuprates La(2-x)Ce(x)CuO(4-y) and Pr(2-x)Ce(x)CuO(4-y).

We present measurements of the magnetic penetration depth, lambda(-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, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-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 nominal optimally doped samples, our results are evidence for a transition from d- to s-wave pairing near optimal doping.

Quantum critical behaviour in the superfluid density of strongly underdoped ultrathin copper oxide films

A central challenge in the physics of high-temperature superconductors is to understand superconductivity within a single copper oxide layer or bilayer, the fundamental structural unit, and how superconductivity is lost with underdoping of charge carriers. A seminal property of crystals and thick films1,2,3,4 is that when mobile holes are removed from optimally doped CuO2 planes, the transition temperature, Tc, and superfluid density, ns(0), decrease in a surprisingly correlated fashion. We elucidate the essential physics of strongly underdoped bilayers by studying two-dimensional (2D) samples near the critical doping level where superconductivity disappears. We report measurements of ns(T) in films of Y1−xCaxBa2Cu3O7−δ as thin as two copper oxide bilayers with Tc values as low as 3 K. In addition to seeing the 2D Kosterlitz–Thouless–Berezinski transition5,6 at Tc, we observe a remarkable scaling of Tc with ns(0), which indicates that the disappearance of superconductivity with underdoping is due to quantum fluctuations near a 2D quantum critical point.

Numerical modeling of a two-coil apparatus for measuring the magnetic penetration depth in superconducting films and arrays

We determine the magnetic penetration depth λ(T) in superconducting films by measuring the mutual inductance of two coils located on opposite sides of the films. The apparatus is designed to produce an accurate value for λ(0) without any assumptions about the dependence of λ(T/Tc)/λ(0) on T/Tc. In a typical configuration of coils and a 100±10 nm thick by 6 mm radius film, λ can be measured to better than ±12% as long as λ≳1500 A. The noise level is typically ±2 A. If two films are made the same way, so that they have the same thickness d, and they are measured in the same apparatus, then the relative uncertainty in λ between the two films is only ±9%, because uncertainties in d, the coil dimensions, etc., are eliminated. This article describes the apparatus and a detailed numerical model which illustrates the induced current density in the film and establishes the sources of uncertainties. The accuracy of the model is demonstrated through comparison with measurements on 0.15 mm thick circular Pb disks.

Numerical modeling and experimental considerations for a two-coil apparatus to measure the complex conductivity of superconducting films

We consider the accuracy of measurements of the complex conductivity of superconducting films with a two-coil mutual inductance technique. We present a numerical analysis of the procedure by which we deduce the real and imaginary parts of the conductivity, σ=σ1−iσ2, of thin films from the in-phase and out-of-phase components of the mutual inductance of coaxial coils located on opposite sides of the film. The accuracy of the procedure is verified for the full ranges of film radii, thicknesses, and conductivities that are encountered for typical films of a wide variety of cuprate superconductors. We determine both experimentally and theoretically what effect flaws in the film would have on the accuracy of the measurement by examining the effects of holes located at various places in a superconducting film. The effect of capacitive coupling between the coils is measured and shown to be negligible when care is taken in grounding the drive and pickup coil circuits. The mutual inductance of the coils changes wi...

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, lambda(T), in five optimally doped Pr(1.855)Ce(0.145)CuO(4-y) films for 1.6 K< or =T < or =T(c) approximately 24 K. Low resistivities, high superfluid densities n(s)(T) proportional, variant lambda(-2)(T), high T(c)s, and small transition widths are reproducible and indicative of excellent film quality. For all five films, lambda(-2)(T)/lambda(-2)(0) at low T is well fitted by an exponential temperature dependence with a gap, Delta(min), of 0.85k(B)T(c). This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.

Magnetic penetration depth measurements of Pr2-xCexCuO4-delta films on buffered substrates: evidence for a nodeless gap.

We report measurements of the inverse squared magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-delta) (0.115< or =x < or =0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. lambda(-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< or =x < or =0.144, lambda(-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 lambda(-2)(T) produce values of the minimum superconducting gap in the range of 0.29< or =Delta(min)/k(B)T(c)< or =1.01.

Correlation between Superfluid Density and T~C of Underdoped YBa~2Cu~3O~6~+~x Near the Superconductor-Insulator Transition

We report measurements of the ab-plane superfluid density n(s) (magnetic penetration depth lambda) of heavily underdoped films of YBa2Cu3O6+x, with T(C)s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO2 layers. At the lowest dopings, T(C) is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO2 bilayers. Our main finding is that T(C) is not proportional to n(s)(0); instead, we find T(C) proportional to ns(1/2.3+/-0.4). This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.

Penetration depth λ (T) of YBa2Cu3O7-δ films determined from the kinetic inductance

We examine the temperature dependence of the magnetic penetration depth λ(T) of YBa2Cu3O7−δ, determined from the kinetic inductance of a film patterned into a long meander line. This technique has sufficient sensitivity to study λ(T) to lower temperatures than have been examined previously. A numerical model which includes both the magnetic and kinetic inductances of the samples extracts λ(T) from the measured voltage. In reasonable agreement with other measurements, λ(0)≊2100 A is deduced from fitting λ(0)2/λ(T)2 to the function 1−(T/Tc)2 for T/Tc≥0.4. We find λ(T)/λ(0)−1 is proportional to (T/Tc)2 for 0.06 ≤T/Tc≤0.4.

Superconductor-to-metal quantum phase transition in overdoped La2−xSrxCuO4

We investigate

Self-assembled multilayers and enhanced superconductivity in (YBa2Cu3O7−x)0.5:(BaZrO3)0.5 nanocomposite films

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