Magnetic Penetration Depth Measurements ofPr2−xCexC

Abstract

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...

Magnetic penetration depth in unconventional superconductors

“…Later mutual inductance measurements on PCCO thin films have shown a variety of temperature dependencies ranging from 3 T to T and, more recently, exponential, depending upon the method of film growth and existence of a buffer layer. [109] Our own measurements on laser ablated thin films of PCCO continue to show power law behavior that depends upon the oxygen doping level. [193] Figure 7 shows data for optimally doped PCCO film and the fit to the disordered d-wave behavior, Eq.…”

Two-band model of Raman scattering on electron-doped high-Tcsuperconductors

“…Pairing symmetry of electron-doped high-T c cuprate superconductors such as Nd 2−x Ce x CuO 4 and Pr 2−x Ce x CuO 4 is a long standing problem [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. Although no consensus has been reached yet, more and more recent experimental results have suggested that the order parameter of electron-doped cuprates is likely to have d x 2 −y 2 -wave pairing symmetry [1,2,3,5,9,10,14,16,17], in close resemblance to that of hole-doped materials.…”

Distinct pairing symmetries inNd1.85Ce0.15CuO4−yandLa1.89

“…In addition, Raman scattering [10] and recent ARPES [11] experiments show a nonmonotonic d-wave order parameter. However, this has been contrasted by tunnelling [12,13] and some other specific heat [14] and penetration depth measurements [15,16,17]. In particular, there may be a crossover from d-wave to s-wave symmetries by changing the doped electron concentration [18,19] or decreasing temperature [20].…”