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.

In high-quality c-axis-oriented MgB2 thin films, we observed high critical current densities (Jc) of ∼ 16 MA/cm 2 at 15 K under self fields comparable to, and exceeding, those of cuprate hightemperature superconductors. The extrapolated value of Jc at 5 K was estimated to be ∼ 40 MA/cm 2 . At a magnetic field of 5 T, a Jc of ∼ 0.1 MA/cm 2 was detected at 15 K, suggesting that this compound would be a very promising candidate for practical applications at high temperature and lower power consumption. The vortex-glass phase is considered to be a possible explanation for the observed high current carrying capability.The recent discovery of the binary metallic MgB 2 superconductor [1] with a remarkably high transition temperature T c = 39 K has attracted great interest in both basic scientific [2][3][4][5][6] and practical applications [7][8][9][10][11][12][13][14]. This new compound is expected to be useful for superconducting magnets and microelectronic devices at low cost because its transition temperature is 2 − 4 times higher than those of conventional metallic superconductors such as Nb 3 Sn and Nb-Ti alloy. The strongly linked nature of the intergrains [7] with a high charge carrier density [6] in this material is a further indication of its possible use in technological applications. Recently, an upper critical field, H c2 (0), of 29 ∼ 39 T [8,9], which was much higher than previously reported, was observed, suggesting that MgB 2 should be of considerable use for practical application in superconducting solenoids using mechanical cryocoolers, such as closed-cycle refrigerator. In addition to the higher T c and H c2 in MgB 2 , the magnitude of the critical current density is a very important factor for practical applications. For example, if a superconducting wire carries a high electric power, the size of the cryogenic system can be reduced considerably so that the system can operate with lower power consumption. Indeed, the successful fabrication of Fe-clad MgB 2 tape has been reported [10]. This tape showed a J c of 1.6 × 10 4 A/cm 2 at 29.5 K under 1 T, which is encouraging for practical application of MgB 2 .In order to explain the nature of the vortex state in strong magnetic field for cuprate high-T c superconductors (HTS), Fisher et al. [15] proposed the theory of vortex-glass superconductivity by considering both the pinning and the collective effects of vortex lines. According to this theory, a diverging vortex glass correlation length (ξ) near the vortex-glass transition (T g ) can be described by ξ ∼ |T − T g | −ν and a correlation time scale ξ z , where ν is a static exponent and z is a dynamic exponent; thus, I − V curves can be expressed by universal scaling functions. For HTS, experimental evidence of a vortex glass phase has been reported [16]. Moreover, a vortex-glass transition was observed in an untwinned single crystal of YBa 2 Cu 3 O 7 after inducing a sufficiently high density of pinning centers, suggesting that a vortexglass phase may be one origin of the high J c [17].In this Letter,...

We report measurements of the inverse squared magnetic penetration depth, λ −2 (T ), in Pr2−xCexCuO 4−δ (0.115 ≤ x ≤ 0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. λ −2 (0), Tc and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, 0.124 ≤ x ≤ 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 ≤ ∆min/kBTc ≤ 1.01.It is still a puzzle whether pairing symmetry in n-type cuprates is d wave or not [1,2,3,4,5,6,7,8,9]. Recently, novel concepts on pairing symmetry of n-and p-type cuprates have come forward: a possible transition in pairing symmetry [10,11] and/or a mixed symmetry order parameter [12,13,14]. Our previous work [10] involved La 2−x Ce x CuO 4−δ (LCCO) and Pr 2−x Ce x CuO 4−δ (PCCO) films grown directly on SrTiO 3 substrates. We found that at low Ce doping levels, λ −2 (T ) at low T was quadratic in T , but at higher dopings, λ −2 (T ) showed activated behavior. These results suggested a d-to s-wave pairing transition near optimal doping, as was also suggested by tunneling experiments[11] on PCCO films. We have subsequently improved film quality by eliminating the interface between the film and substrate, by growing PCCO films onto Pr 2 CuO 4 (PCO)/SrTiO 3 instead of directly onto SrTiO 3 . The insulating PCO layer is thought to lessen lattice mismatch between PCCO film and SrTiO 3 substrate, so that these films should be more homogeneous through their thickness. In fact, their normal state resistivities are somewhat lower than those of unbuffered PCCO films for the same doping, x. T c 's at optimal doping in the two film families are the same, T c ≃ 24 K.Films were prepared by molecular-beam epitaxy (MBE) on 10 mm × 10 mm × 0.35 mm SrTiO 3 substrates as detailed elsewhere [15]. The same growth procedures and parameters were used for all films. For all films, PCCO and PCO layers are 750Å and 250Å thick, respectively. Ce concentrations, x, are measured to better than ±0.005 by inductively coupled plasma spectroscopy. X-ray rocking curves show full-width at half maximum of (006) reflection for all films to be less than 0.4• , which implies that the films are highly c-axis oriented.The penetration depth, λ(T ), was measured down to T ≃ 0.5 K using a mutual inductance apparatus, described in detail elsewhere [16,17], in a He 3 refrigerator. The system temperature was measured with a Cernox resistor (LakeShore Inc.) and its reliability, below 1 K, was confirmed by measuring the superconducting transition temperature of a Zn plate, T c = 0.875 K.Each film was centered between drive and pick-up coils with diameters of ∼ 1 mm. A small current at 50 kHz in the drive coil induced diamagnetic screening currents in the film, i.e., parallel to the CuO 2 planes. The time d...

We report the temperature-and magnetic-field-dependent resistivity of MgB2 sintered at high temperature and high pressure condition. The superconducting transition width for the resistivity measurement was about 0.4 K, and the low-field magnetization showed a sharp superconducting transition with a transition width of about 1 K. The resistivity in the normal state roughly followed T 2 behavior with smaller residual resistivity ratio (RRR) of 3 over broad temperature region above 100 K rather than reported T 3 behavior with larger RRR value of ∼ 20 in the samples made at lower pressures. Also, the resistivity did not change appreciably with the applied magnetic field, which was different from previous report. These differences were discussed with the microscopic and structural change due to the high-pressure sintering.

The magnetic penetration depth, λ(T ), in the basal plane of a magnesium diboride (MgB2) film was measured using a two-coil mutual inductance technique at 50 kHz. This film has Tc ≃ 38 K, ∆Tc ≤ 1 K, and λ(0) ∼ 1500Å. 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: ∆S(0) = 2.61 ± 0.41 meV and ∆L(0) = 6.50 ± 0.33 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 gap anisotropy does not play a significant role in determining the temperature dependence of the penetration depth.