Correlation of tunneling spectra with surface nanomorphology and doping in thin YBa 2 Cu 3 O 7 − δ films (original) (raw)
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Europhysics Letters (EPL), 2003
Tunneling spectroscopy of epitaxial (100) oriented YBa2Cu3O 7−δ films was performed using an STM at 4.2 K. On atomically smooth areas, tunneling spectra revealing clear U-shaped gaps with relatively low zero bias conductance were measured. These spectra can be well fitted to the tunneling theory into a d-wave superconductor only when introducing a strong dependence of the tunneling probability on the wave vector k. Possible origins for this k-selectivity in STM measurements will be discussed. On other areas, V-shaped gaps as well as zero bias conductance peaks are observed, indicating relaxation of k-selectivity and the effect of nanofaceting, respectively.
2000
We present a systematic study of the charge transport and quasiparticle tunneling properties of Y 1Ϫx Pr x Ba 2 Cu 3 O 7 thin films. Pr doping increases the resistivity along the copper oxide planes and suppresses the superconducting critical temperature T c , ultimately inducing a superconductor-insulator transition. The tunneling conductance is reproducible and correlated with the crystallographic film orientation. The crystallographic dependence can be divided into two distinct categories: tunneling into ͑001͒-oriented (c-axis͒ films and tunneling into ͑100͒-, ͑110͒-, and ͑103͒-oriented (ab-oriented͒ films. c-axis tunneling data exhibit a conductance dip at zero bias and a broad temperature-dependent peak over ϳ15-40 mV that decreases in magnitude but stays fixed in energy for increasing Pr doping levels. ab-plane tunneling data exhibit a zero-bias conductance peak and a gaplike feature at an energy that scales roughly linearly with T c for xϭ0.0, 0.2, and 0.4. When xϭ0.5, the resistivity is not linear in temperature and a zero-bias conductance dip is observed. The background conductance that ensures conservation of states in the low-temperature ab-plane data exhibits temperature-and doping-dependent structure over ϳ15-40 mV that is very similar to the peak observed in c-axis tunneling. Finally, analysis of the temperature and magnetic field dependence of the zero bias conductance peak indicates that states are conserved to within ϳ20%, supporting its interpretation as a feature of a superconducting density of states.
Phys Rev Lett, 2005
We report on an ab anisotropy of Jc∥b/Jc∥a≅1.8 and IcRn∥b/IcRn∥a≅1.2 in ramp-edge junctions between untwinned YBa2Cu3O7 and s-wave Nb. For these junctions, the angle θ with the YBa2Cu3O7 crystal b axis is varied as a single parameter. The RnA(θ) dependence presents twofold symmetry. The minima in IcRn at θ≅50° suggest a real s-wave subdominant component and negligible dxy-wave or imaginary s-wave admixtures. The IcRn(θ) dependence is well fitted by 83% dx2-y2-, 15% isotropic s-, and 2% anisotropic s-wave order parameter symmetry, consistent with Δb/Δa≅1.5.
Field-induced nodal order parameter in the tunneling spectrum of YBa2Cu3O7−x superconductor
Physical Review B, 2008
We report planar tunneling measurements on thin films of YBa2Cu3O7−x at various doping levels under magnetic fields. By choosing a special setup configuration, we have probed a field induced energy scale that dominates in the vicinity of a node of the d-wave superconducting order parameter. We found a high doping sensitivity for this energy scale. At Optimum doping this energy scale is in agreement with an induced idxy order parameter. We found that it can be followed down to low fields at optimum doping, but not away from it.
Physical Review B, 2002
Tunneling spectroscopy of epitaxial ͑110͒ Y 1Ϫx Ca x Ba 2 Cu 3 O 7Ϫ␦ films reveals a doping-dependent transition from a pure d x 2 Ϫy 2 to d x 2 Ϫy 2ϩ is or d x 2 Ϫy 2ϩ id xy order parameter. The subdominant (is or id xy ) component manifests itself in a splitting of the zero-bias conductance peak and the appearance of subgap structures. The splitting is seen in the overdoped samples, increases systematically with doping, and is found to be an inherent property of the overdoped films. It was observed in both local tunnel junctions, using scanning tunneling microscopy ͑STM͒, and in macroscopic planar junctions, for films prepared by either sputtering or laser ablation. The STM measurements exhibit a fairly uniform splitting size in ͓110͔ oriented areas on the order of 10 nm 2 but vary from area to area, indicating some doping inhomogeneity. U-and V-shaped gaps were also observed, with good correspondence to the local faceting, a manifestation of the dominant d-wave order parameter.
Transition in the tunneling conductance of YBa2Cu3O7−δ films in magnetic fields up to 32.4T
Physical Review B, 2005
We studied the tunneling density of states in YBCO films under strong currents flowing along node directions. The currents were induced by fields of up to 32.4T parallel to the film surface and perpendicular to the CuO2 planes. We observed a remarkable change in the tunneling conductance at high fields where the gap-like feature shifts discontinuously from 15meV to a lower bias of 11meV, becoming more pronounced as the field increases. The effect takes place in increasing fields around 9T and the transition back to the initial state occurs around 5T in decreasing fields.
Low Temperature Physics, 2010
The tunneling spectra of YBa2Cu3O 7−δ break-junctions have been investigated for the tunneling direction close to the node one. The zero-bias conductance peak (ZBCP) and Josephson current have been studied with temperature and magnetic field. The observed deep splitting of ZBCP which starts at TS<20-30 K is in agreement with the theory for the d x 2 −y 2 ±is order parameter [Y. Tanuma, Y. Tanaka, and S. Kashiwaya, Phys. Rev. B 64, 214519 (2001)]. The low (0.04 T) magnetic field depresses significantly such splitting. The 1/T temperature dependence of maximum Josephson current goes to saturation at T <TS also confirming the mixed order parameter formation. PACS numbers: 74.20.Rp, 74.50.+r, 74.72Bk For the d-wave superconductors, theory predicts specific quasiparticle bound states (Andreev bound states) near scattering structures such as surfaces, interfaces and other defects 1 . In these areas an order parameter (OP) may change significantly and subdominant OP may appear leading to a mixed OP (such as d x 2 −y 2 ±is or d x 2 −y 2 ±id xy with the spontaneous breaking of time reversal symmetry (BTRS) 2,3,4,5 .
Tunneling spectroscopy in thin films YBCO/Pb tunnel structures
IEEE Transactions on Magnetics, 1989
Experimental r e s u l t s on electron tunneling i n t o t h i n superconducting f i l m s YBa2Cu30x i n YBCO/Pb tunnel junct i o n s are reported. Thin superconducting f i l m s YBCO (thickness 0.7(um) were prepared by codeposition of Y, BaO, and Cu onto single c r y s t a l substratessaphire, MgO, SrTi03. Maximum c r i t i c a l temperature a t zero resis-
Local tunneling probe of (110) Y_{0.95}Ca_{0.05}Ba_{2}Cu_{3}O_{7−δ} thin films in a magnetic field
Physical Review B, 2010
Scanning tunneling spectroscopy was performed on (110)-oriented thin films of Ca-overdoped Y0.95Ca0.05Ba2Cu3O 7−δ at 4.2K, to probe the local evolution of Andreev−Saint-James surface states in a c-axis magnetic field. In zero field, we observed conductance spectra with spontaneously-split peaks and spectra with unsplit zero-bias peaks. The former showed enhanced splitting with field, and the latter showed threshold splitting above finite fields. Although both field evolutions can be described in terms of screening and orbital supercurrents, within the framework of d ± iα pairing (d=d x 2 −y 2 ; α=dxy,s), the enhanced splitting is consistent with only the d+iα state. Our results have direct implications on the stability of broken time-reversal symmetry in cuprate superconductors.