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 Physics , 2010, DOI: 10.1166/jnn.2011.3907 Abstract: The nano-scale intrinsic Josephson junctions in highly anisotropic cuprate superconductors have potential for generation of terahertz electromagnetic waves. When the thickness of a superconductor sample is much smaller than the wavelength of electromagnetic waves in vacuum, the superconductor renders itself as a cavity. Unlike conventional lasers, the presence of the cavity does not guarantee a coherent emission because of the internal degree of freedom of the superconductivity phase in long junctions. We study the excitation of terahertz wave by solitons in a stack of intrinsic Josephson junctions, especially for relatively short junctions. Coherent emission requires a rectangular configuration of solitons. However such a configuration is unstable against weak fluctuations, contrarily solitons favor a triangular lattice corresponding to an out phase oscillation of electromagnetic waves. To utilize the cavity, we propose to use an array of stacks of short intrinsic Josephson junctions to generate powerful terahertz electromagnetic waves. The cavity synchronizes the plasma oscillation in different stacks and the emission intensity is predicted to be proportional to the number of stacks squared.
 Physics , 1999, DOI: 10.1103/PhysRevLett.83.2644 Abstract: 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).
 Shi-Zeng Lin Physics , 2014, DOI: 10.1063/1.4874677 Abstract: Certain high-$T_c$ cuprate superconductors, which naturally realize a stack of Josephson junctions, thus can be used to generate electromagnetic waves in the terahertz region. A plate-like single crystal with $10^4$ junctions without cavity resonance was proposed to achieve strong radiation. For this purpose, it is required to synchronize the Josephson plasma oscillation in all junctions. In this work, we propose to use two stacks of junctions shunted in parallel to achieve synchronization. The two stacks are mutually synchronized in the whole IV curve, and there is a phase shift between the plasma oscillation in the two stacks. The phase shift is nonzero when the number of junctions in different stacks is the same, while it can be arbitrary when the number of junctions is different. This phase shift can be tuned continuously by applying a magnetic field when all the junctions are connected by superconducting wires.
 Physics , 2004, DOI: 10.1103/PhysRevB.71.104504 Abstract: We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle tunneling spectra of thin film grain-boundary Josephson junctions made of the electron doped cuprate superconductor LaCeCuO. An applied magnetic field reduces the spectral weight around zero energy and shifts it non-linearly to higher energies consistent with a Doppler shift of the Andreev bound states (ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the onset of superconductivity. These observations strongly suggest that the ZBCP results from the formation of ABS at the junction interfaces, and, consequently, that there is a sign change in the symmetry of the superconducting order parameter of this compound consistent with a d-wave symmetry.
 Physics , 2007, DOI: 10.1103/PhysRevB.76.052508 Abstract: We study theoretically the Josephson effect in d-wave superconductor / diffusive normal metal /insulator/ diffusive normal metal/ d-wave superconductor (D/DN/I/DN/D) junctions. This model is aimed to describe practical junctions in high-$T_C$ cuprate superconductors, in which the product of the critical Josephson current ($I_C$) and the normal state resistance ($R$) (the so-called $I_{\rm C}R$ product) is very small compared to the prediction of the standard theory. We show that the $I_{\rm C}R$ product in D/DN/I/DN/D junctions can be much smaller than that in d-wave superconductor / insulator / d-wave superconductor junctions and formulate the conditions necessary to achieve large $I_{\rm C}R$ product in D/DN/I/DN/D junctions. The proposed theory describes the behavior of $I_{\rm C}R$ products quantitatively in high-$T_{\rm C}$ cuprate junctions.
 Physics , 2008, DOI: 10.1088/0953-8984/21/3/035701 Abstract: With simple but exactly solvable model, we investigate the supercurrent transferring through the c-axis cuprate superconductor-normal metal-superconductor junctions with the clean normal metal much thicker than its coherence length. It is shown that the supercurrent as a function of thickness of the normal metal decreases much slower than the exponential decaying expected by the proximity effect. The present result may account for the giant proximity effect observed in the c-axis cuprate SNS junctions.
 Physics , 2001, DOI: 10.1103/PhysRevLett.89.057003 Abstract: We compute the frequency and magnetic field dependencies of the reflectivity $R(\omega)$ in layered superconductors with two alternating intrinsic Josephson junctions with different critical current densities and quasiparticle conductivities for the electric field polarized along the c-axis. The parameter $\alpha$ describing the electronic compressibility of the layers and the charge coupling of neighboring junctions was extracted for the SmLa$_{1-x}$Sr$_{x}$CuO$_{4-\delta}$ superconductor from two independent optical measurements, the fit of the loss function $L(\omega)$ at zero magnetic field and the magnetic field dependence of the peak positions in $L(\omega)$. The experiments are consistent with a free electron value for $\alpha$ near the Josephson plasma frequencies.
 Physics , 2004, DOI: 10.1080/14786430600622427 Abstract: We calculate the critical current $I_c$ across in-plane (001) tilt grain boundary junctions of high temperature superconductors. We solve for the electronic states corresponding to the electron-doped cuprates, two slightly different hole-doped cuprates, and an extremely underdoped hole-doped cuprate in each half-space, and weakly connect the two half-spaces by either specular or random quasiparticle tunneling. We treat symmetric, straight, and fully asymmetric junctions with s-, extended-s-, or d$_{x^2-y^2}$-wave order parameters. For symmetric junctions with random grain boundary tunneling, our results are generally in agreement with the Sigrist-Rice form for ideal junctions that has been used to interpret phase-sensitive'' experiments consisting of such in-plane grain boundary junctions. For specular grain boundary tunneling across symmetric juncitons, our results depend upon the Fermi surface topology, but are usually rather consistent with the random facet model of Tsuei {\it et al.} [Phys. Rev. Lett. {\bf 73}, 593 (1994)]. Our results for asymmetric junctions of electron-doped cuparates are in agreement with the Sigrist-Rice form. However, ou resutls for asymmetric junctions of hole-doped cuprates show that the details of the Fermi surface topology and of the tunneling processes are both very important, so that the phase-sensitive'' experiments based upon the in-plane Josephson junctions are less definitive than has generally been thought.
 Physics , 1998, DOI: 10.1063/1.121449 Abstract: 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.
 Physics , 2000, DOI: 10.1103/PhysRevB.62.15200 Abstract: Andreev reflection is investigated in layered anisotropic normal metal / superconductor (N/S) systems in the case of an energy gap \Delta in S not negligible with respect to the Fermi energy E_F, as it probably occurs with high critical temperature superconductors (HTS). We find that in these limits retro-reflectivity, which is a fundamental feature of Andreev reflection, is broken modifying sensitively transport across S/N interfaces. We discuss the consequences for supercurrents in HTS Josephson junctions and for the midgap states in S-N contacts
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