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.
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.
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 measurements of the ab-plane penetration depth, λ(T ), in YBa2Cu3O 7−δ films at various δ. At optimal doping, critical fluctuation effects are absent, and 1/λ 2 (T ) from 4 K to 0.99 TC is that of a clean, strong-coupling d-wave superconductor with ∆0(0)/kBTC ≃ 3.3. As in crystals, underdoping reduces the superfluid density, ns(0) ∝ 1/λ 2 (0), without affecting the low-T slope of 1/λ 2 (T ). These results, as well as electronic heat capacity data, are well described by an ad hoc model in which contributions to the superfluid and entropy are lost from regions of the Fermi surface occupied by the pseudogap.PACS Nos. 74.25.Fy, 74.25.Nf, 74.40.+k, 74.76.Bz A large body of experimental evidence indicates the opening of a k-dependent gap, or pseudogap, at a temperature, T * , above the superconducting transition temperature, T C , in underdoped cuprates [1]. The pseudogap competes with the superconducting gap: T * and the fraction of the Fermi surface (FS) occupied by the pseudogap increase with underdoping, while T C , the superfluid density in the ab-plane, n S (0), and the peak value of the electronic specific heat coefficient, γ(T ), at T C decrease. A great deal of effort currently focuses on understanding the coexistence of these two gaps. Lee and coworkers propose that the fundamental physics lies in spin-charge separation, a key element being the segmentation of the FS into regions either occupied or unoccupied by the pseudogap in the normal state [2]. With this in mind, we construct a simple model to describe our measurements of n S (T ) and literature results for γ(T ), in which only portions of the FS unoccupied by the pseudogap contribute to the superfluid and entropy in the superconducting state.We present new measurements of 1/λ 2 (T ) ∝ n S (T ) in YBa 2 Cu 3 O 7−δ (YBCO) films at various δ. We show that 1/λ 2 (T ) in optimally-doped films is that of a clean, strong-coupling d-wave superconductor with a full FS. Underdoped films are well described by a Fermi liquidlike model, in which electronic properties are expressed as integrals over the FS, but the integrals extend only over sections of the FS not occupied by the pseudogap in the normal state. The fraction of the FS that survives is equal to the ratio of n S (0) of the underdoped film to n S,opt. (0) of the same film at optimal doping.The absence of critical fluctuations in optimally-doped YBCO films [3] and measurements of the effect of thermal phase fluctuations on 2D films of a conventional superconductor [4] lead us to conclude that fluctuation effects are weak in the underdoped films. However, the relative importance of thermal phase fluctuations (TPF's) to single particle excitations is controversial. Carlson et al. [5] have shown numerically that a fluctuation driven superfluid density in Josephson junction (JJ) arrays displays features similar to some very clean YBCO crystals, namely, T -linear behavior at low-T [6], T C roughly proportional to n S (0), and a wide critical region [7,8]. Terahertz measurements of the sheet condu...
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.