Orientation dependence of tunneling spectra in YBCO and NCCO

Alff, Lambert; Takashima, Hiroshi; Kashiwaya, Satoshi; Terada, Norio; Ito, T.; Oka, Kunihiko; Tanaka, Yukio; Koyanagi, M.

doi:10.1016/s0921-4534(97)00849-6

Cited by 25 publications

(32 citation statements)

References 8 publications

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“…Such direct splitting has not been observed in our experiments down to temperatures of 100 mK for LSCO and down to 4.2 K for YBCO. Our observations are in agreement to the recent findings of other groups [21,22]. Although the detailed reason for the presence or absence of a zero field splitting of the ZBCO is not clear, one can speculate that the junction property preventing the observation of a direct splitting of the zero bias conductance peak by an applied magnetic field also may result in the absence of any direct splitting in zero applied field.…”

Section: Andreev Bound States In D-wave Superconductorssupporting

confidence: 93%

“…Hence, Andreev bound states are expected only for hole doped HTS, which are believed to have a dominating dwave symmetry of the order parameter, but should be absent for NCCO. As we shall see below, this is indeed the case [21,22].…”

Section: Introductionmentioning

confidence: 71%

“…A low temperature scanning tunneling spectroscopy study of different HTS materials with different orientations has been given recently by Alff et al [21,98]. Details on the low temperature scanning tunneling spectroscopy technique can be found in [21,98] and in the references cited therein. Typical conductance versus voltage curves obtained by low temperature scanning tunneling spectroscopy are shown in Fig.…”

Section: Comparison To Low Temperature Scanning Tunneling Spectroscopmentioning

confidence: 99%

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Andreev bound states in high temperature superconductors

et al. 1998

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Abstract. Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa2Cu3O 7−δ (YBCO), Bi2Sr2CaCu2O 8+δ (BSCCO), and La1.85Sr0.15CuO4 (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd1.85Ce0.15CuO4−y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.

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Section: Andreev Bound States In D-wave Superconductorssupporting

confidence: 93%

Section: Introductionmentioning

confidence: 71%

Section: Comparison To Low Temperature Scanning Tunneling Spectroscopmentioning

confidence: 99%

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Andreev bound states in high temperature superconductors

et al. 1998

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“…The normal resistance times junction area, ρ n , of the 24 • tilt GBJs ranged between 1.5 and 8 × 10 −6 Ωcm 2 and was more than an order of magnitude larger for the 36.8 • tilt GBJs. [11,12,19,20]. The observation of a clear gap structure and the temperature independent conductance well above V g strongly suggest that the dominating transport mechanism in the NCCO-GBJs is elastic tunneling.…”

mentioning

confidence: 80%

Superconducting Nd1.85Ce0.15CuO4−y bicrystal grain boundary Josephson junctions

Kleefisch

Alff

Schoop

et al. 1998

Applied Physics Letters

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We have studied the electric transport properties of symmetrical [001] tilt Nd1.85Ce0.15CuO4−y (NCCO) bicrystal grain boundary Josephson junctions (GBJs) fabricated on SrTiO3 bicrystal substrates with misorientation angles of 24 • and 36.8 • . The superconducting properties of the NCCOGBJs are similar to those of GBJs fabricated from the hole doped high temperature superconductors (HTS). The critical current density Jc decreases strongly with increasing misorientation angle. The products of the critical current Ic and the normal resistance Rn (∼ 100 µV at 4.2 K) are small compared to the gap voltage and fit well to the universal scaling law IcRn ∝ √ Jc found for GBJs fabricated from the hole doped HTS. This suggests that the symmetry of the order parameter, which most likely is different for the electron and the hole doped HTS has little influence on the characteristic properties of symmetrical [001] tilt GBJs. PACS: 74.25.Fy, 74.50.+r Applied Physics Letters to be published June 1, 1998Bicrystal grain boundary Josephson junctions (GBJs) have been studied intensively using epitaxial thin films of the various hole doped high temperature superconductors (HTS) [1][2][3]. However, there is very limited information on GBJs fabricated from the electron doped material Nd 1.85 Ce 0.15 CuO 4−y (NCCO) [4]. For the vast majority of GBJs the transport properties can be well described within the intrinsically shunted junction (ISJ) model [2,5,6]. In this model a continuous, but spatially inhomogeneous insulating grain boundary barrier is assumed that contains a high density of localized defect states. The localized states allow for resonant tunneling of quasiparticles providing an intrinsic resistive shunt, whereas resonant tunneling of Cooper pairs is prevented by Coulomb repulsion. This results in reduced I c R n products and a scaling behavior I c R n ∝ √ J c observed in many experiments [2,[5][6][7]. Meanwhile, it is well established that most hole doped HTS have a dominant d-wave component of the order parameter (OP) [8]. The influence of the d-wave symmetry of the OP on the magnetic field dependence of I c has been shown for asymmetric 45 • tilt YBCO-GBJs [9]. So far, in the ISJ-model the likely d-wave pairing state of the hole doped HTS has not been taken into account and the relevance of the d-wave symmetry of the OP for the characteristic properties of symmetrical [001] tilt GBJs such as the small value and the scaling behavior of the I c R n product is still a point of controversy. To clarify this issue we have studied [001] tilt NCCO-GBJs. Since for the electron doped material NCCO there is convincing experimental evidence for a s-wave symmetry of the OP [10-13], NCCO-GBJs represent an interesting model system to test the influence of the OP symmetry on the transport properties of GBJs.The NCCO-GBJs were fabricated by molecular beam epitaxy (MBE) of c-axis oriented NCCO thin films on SrTiO 3 bicrystal substrates. The substrate temperature during growth was about 730 • C and ozone was used as oxidation gas. For ...

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“…Some ASJ data indicating a possible change of symmetry near optimum doping are also available on LSCO (Achsaf et al, 1996; and on the electron doped PCCO (Qazilbash et al, 2003). The electron doped cuprates had long been considered an exception to the d-wave symmetry (Fournier et al, 1998), inter alia because of the absence of a ZBCP (Alff et al, 1997). However, more recent tri-crystal experiments , (Tsuei and Kirtley, 2000a,b), indicated a d-wave symmetry.…”

Section: A Asj Bound States Under Applied Fieldsmentioning

confidence: 99%

Andreev–Saint-James reflections: A probe of cuprate superconductors

2005

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Electrical transport through a normal metal / superconductor contact at biases smaller than the energy gap can occur via the reflection of an electron as a hole of opposite wave vector. The same mechanism of electron-hole reflection gives rise to low energy states at the surface of unconventional superconductors having nodes in their order parameter. The occurrence of electron-hole reflections at normal metal / superconductor interfaces was predicted independently by Saint James and de Gennes and by Andreev, and their spectroscopic features discussed in detail by Saint James in the early sixties. They are generally called Andreev reflections but, for that reason, we call them Andreev -Saint James (ASJ) reflections. We present a historical review of ASJ reflections and spectroscopy in conventional superconductors, and review their application to the High Tc cuprates. The occurrence of ASJ reflections in all studied cuprates is well documented for a broad range of doping levels, implying that there is no large asymmetry between electrons and holes near the Fermi level in the superconducting state. In the underdoped regime, where the pseudo-gap phenomenon has been observed by other methods such as NMR, ARPES and Giaever tunneling, gap values obtained from ASJ spectroscopy are smaller than pseudo-gap values, indicating a lack of coherence in the pseudo-gap energy range. Low energy surface bound states have been observed in all studied hole doped cuprates, in agreement with a dominant d-wave symmetry order parameter. Results are mixed for electron doped cuprates. In overdoped Y Ba 2 Cu 3 O 7−δ (δ < 0.08) and La 2−x SrxCuO 4 , ASJ spectroscopy is consistent with the presence of an additional imaginary component of the order parameter. Results of ASJ spectroscopy under applied magnetic fields are also reviewed. A short section at the end is devoted to some recent results on spin effects. * Electronic address: guyde@tau.ac.il 2

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Orientation dependence of tunneling spectra in YBCO and NCCO

Cited by 25 publications

References 8 publications

Andreev bound states in high temperature superconductors

Andreev bound states in high temperature superconductors

Superconducting Nd1.85Ce0.15CuO4−y bicrystal grain boundary Josephson junctions

Andreev–Saint-James reflections: A probe of cuprate superconductors

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