arXiv subjects

M. Naito

Publications and source records attributed to M. Naito.

43 records · Page 3Linked to original sources

arXiv2001

Interplay of Cooper and Kondo singlet formations in high-TC cuprates

We performed high-field magnetotransport experiments up to 50 T on strained La1.85Sr0.15CuO4 films. Two films with different strain, one expansively strained with Tc ~ 26 K and the other compressively strained with Tc ~ 44 K, show a striking contrast in the normal-state transport: the former has a prominent low-temperature upturn while the latter has not. Identifying the low-temperature upturn as due to the Kondo effect, our experimental observations indicate that Cooper and Kondo singlet formations are competing in high- Tc cuprates: Kondo singlet formation hinders Cooper pair formation. Based on these results, we propose a new scenario for the electronic phase diagram of La2-xSrxCuO4. According to our scenario, underdoped insulating La2-xSrxCuO4 can be regarded as a "Kondo insulator".

cond-mat.supr-con↗

arXiv2001

Vortex-Glass Melting Transition and Scaling Behavior in the Cuprate Superconductor Nd2-xCexCuOy at Low Temperatures

We have investigated the vortex-glass transition for the cuprate superconductor Nd2-xCexCuOy (NCCO) in the region of low temperatures (T < 4.4 K) and moderate up to high magnetic fields (3 T < B < 5 T) compared to the upper critical field Bc2 (Bc2 approx. 6 T for NCCO). We analyzed our data using the vortex-glass theory and found an excellent scaling behavior of the current-voltage characteristics in this temperature region. The critical exponents have been calculated and were compared to the few available investigations for NCCO and to the results for other materials. Additionally we determined the melting line for low temperatures and compared it to the available literature data. In this way we could draw a complete picture of the mixed-state phase diagram for this material. The knowledge about the phase diagram is crucial for the understanding of the transport properties of the high-Tc cuprates in the mixed state. It is found that for NCCO, thermal fluctuations play an important role even at temperatures below 4.2 K. This is possibly due to the relatively high anisotropy of this material. An extrapolation procedure is proposed in order to overcome our experimental limitation of the temperature range and to obtain the melting line for T > 4.4 K. A kink in the extrapolated portion of the melting line is found and explained in terms of a loss of universality of the critical exponents for temperatures T > 6 K.

cond-mat.supr-con↗

arXiv2001

Optical Cooper Pair Breaking Spectroscopy of Cuprate Superconductors

The photon energy dependence of optical Cooper pair breaking rate (CPBR) is studied for compressibly strained La1.85Sr0.15CuO4 (LSCO) films and YBa2Cu2.92Zn0.08O7-delta (YBCZO) thin films, and compared to that in YBa2Cu3O7-delta (YBCO). Unlike in the case of YBCO, the CPBR for LSCO does not show obvious photon energy dependence. In YBCZO the CPBR shows strong photon energy dependence similar to YBCO, but with a red shift in the peak position. Analysis of these results strongly favours a physical picture based on electronic phase separation in high Tc superconductivity.

cond-mat.supr-con↗

arXiv2001

Possible pseudogap behavior of electron doped high-temperature superconductors

We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y) and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature Tc. In contrast, below Tc for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below Tc. Possible interpretations of this observation are the existence of a normal state pseudogap in the electron doped HTS or the existence of a spatially non-uniform superconducting state.

cond-mat.supr-con↗

arXiv1999

Anomalous Low Temperature Behavior of Superconducting Nd(1.85)Ce(0.15)CuO(4-y)

We have measured the temperature dependence of the in-plane London penetration depth lambda(T) and the maximum Josephson current Ic(T) using bicrystal grain boundary Josephson junctions of the electron-doped cuprate superconductor Nd(1.85)Ce(0.15)CuO(4-y). Both quantities reveal an anomalous temperature dependence below about 4 K. In contrast to the usual monotonous decrease (increase) of lambda(T) (Ic(T)) with decreasing temperature, lambda(T) and Ic(T) are found to increase and decrease, respectively, with decreasing temperature below 4 K resulting in a non-monotonous overall temperature dependence. This anomalous behavior was found to be absent in analogous measurements performed on Pr(1.85)Ce(0.15)CuO(4-y). From this we conclude that the anomalous behavior of Nd(1.85)Ce(0.15)CuO(4-y) is caused by the presence of the Nd3+ paramagnetic moments. Correcting the measured lambda(T) dependence of Nd(1.85)Ce(0.15)CuO(4-y) for the temperature dependent susceptibility due to the Nd moments, an exponential dependence is obtained indicating isotropic s-wave pairing. This result is fully consistent with the lambda(T) dependence measured for Pr(1.85)Ce(0.15)CuO(4-y).

cond-mat.supr-con↗

arXiv1998

Observation of Bound Surface States in Grain Boundary Junctions of High Temperature Superconductors

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.

cond-mat.supr-con↗

arXiv1998

Superconducting NdCeCuO Bicrystal Grain Boundary Josephson Junctions

We have studied the electric transport properties of symmetrical [001] tilt NdCeCuO bicrystal grain boundary Josephson junctions (GBJs) fabricated on SrTiO bicrystal substrates with misorientation angles of 24 and 36.8 degree. The superconducting properties of the NdCeCuO-GBJs 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 (about 0.1 mV at 4.2 K) are small compared to the gap voltage and fit well to the universal scaling law (IcRn is proportional to the square root of 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.

cond-mat.supr-con↗