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Superconductivity is achieved in Nd1-xCexOBiS2 via electron doping using mixed-valence Ce ions. Single crystals with x = 0.2, 0.3, 0.4, and 0.5 are generated using a CsCl flux method. Plate-like single crystals with dimensions of 0.8 0.8 0.2 mm3 were obtained. The magnetic susceptibility chi(T) indicates large diamagnetism, and the electrical resistivity rho(T) indicates zero resistivity. The maximum value of Tc is observed at 4.7 K in Nd0.8Ce0.2OBiS2 from chi(T). From the rho(T) measurements taken in several magnetic fields, the upper critical field mu0Hc2(0) is estimated to be about 12 and 0.34 T for the ab- and c-planes, respectively. We redetermined mu0Hc2(0) of NdO0.7F0.3BiS2, as about 35 and 0.78 T for the ab- and c-planes, respectively. The anisotropic parameter is estimated to be about 35 for Nd0.7Ce0.3OBiS2 and 45 for NdO0.7F0.3BiS2. The mu0Hc2(0) of Nd0.7Ce0.3OBiS2 is approximately two times smaller than that of NdO0.7F0.3BiS2, although the difference of Gamma is approximately 10., 5 pages, 4 figures

We report the tunneling spectroscopy of an iron-based oxypnictide NdFeAsO0.7 with T C=51 K, measured by a mechanical point contact technique. Mainly two kinds of tunneling spectra have been observed reproducibly. One is tunneling conductance displaying sharp superconducting gap peaks at 6.0±1.0 mV, in which hump (or kink) structures are also observed at 20–30 mV. Another is that showing dominantly the larger superconducting gap Δ L with sharp conductance peaks at 14±1.0 meV, in which the trace of a smaller gap (Δ S=5–7 meV) is simultaneously observed. Our results give direct evidence for the existence of multiple gaps in the quasiparticle excitation spectrum of this multiband system, although the origin of the hump at 20–30 mV is still unclear.

The rapid increase in applications of terahertz waves requires new techniques to obtain continuous wave terahertz sources. Mesa structures fabricated from high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystal have been observed as an intense, coherent, continuous electromagnetic wave source in the terahertz (THz) frequency region. However, in order to produce coherent radiation with high applicable power, we need large mesa structures that enter a collective electromagnetic state in which their oscillations are largely synchronized in phase. On the other hand, large mesa structures cause a heating problem. In this study, we report on the critical current density dependence of mesa area and the crystal inhomogeneity to understand heating problems in large area mesas for terahertz radiation. Since the doping dependence of Bi2212 is an important parameter, the as-grown Bi2212 crystals were heat-treated at various temperatures under vacuum conditions. We have fabricated triple mesa structures from Bi2212 single crystal using e-beam lithography and argon ion beam etching techniques with same area and with different area on the same chip. We investigated and compared characteristics of triple mesas which are on the same chip and next to each other. In this way, we searched the crystal inhomogeneity in triple mesa structures and studied the critical current density dependence of mesa area to obtain high emission power for the THz radiation. Our experimental results clearly show that the Josephson critical current density is decreasing when the area of mesa is increasing. © 2015 Elsevier Ltd. All rights reserved., TUBITAK (110T248); SANTEZ (1386.STZ.2012-1)

Tunneling measurements are performed on Ca-rich single crystals of Bi2Sr2CaCu2O8+d (Bi2212), with various oxygen doping levels, using a novel point contact method. At 4.2 K, SIN and SIS tunnel junctions are obtained with well-defined quasiparticle peaks, robust dip and hump features and in some cases Josephson currents. The doping dependence of tunneling conductances of Ca-rich Bi2212 are analyzed and compared to stoichiometric Bi2212. A similar profile of energy gap vs. doping concentration is found although the Ca-rich samples have a slighly smaller optimum Tc and therefore smaller gap values for any doping level. The evolution of tunneling conductance peak height to background ratios with hole concentration are compared. For a given doping level, the Ca-rich spectra showed more broadened features compared to the stoichiometric counterparts, most likely due to increased disorder from the excess Ca. Comparison of the dip and hump features has provided some potential insights into their origins.

Proceedings of the Conference on Spectroscopies in Novel Superconductors, Break-junction tunneling data are reported in Bi2Sr2CaCu2O8+δ over a wide range of hole concentration from underdoped to overdoped. The strong conductance peaks in the superconducting state reveal a single gap consistent with d-wave symmetry. In addition, sharp dips are observed at a voltage, Ω/□, measured with respect to the gap edge. These features are shown to be reproduced in other junction types from the literature including atomically resolved STM and □-axis mesas, establishing their intrinsic character. Trends are observed with doping and temperature which link the dip to the resonance spin excitation and indicate that the quasiparticles are strongly coupled to this mode.

We report tunneling spectroscopy of superconductor-insulator-superconductor break junctions on heavily overdoped Bi2Sr2CaCu2O8 + δ with Tc = 56 K. At T Tc. Above Tc, the tunneling conductance shows a flat background without any indication of a pseudogap near the Fermi level.

Rapid increase in applications of the electromagnetic waves in the terahertz frequency range requires new techniques to obtain continues-wave terahertz radiation sources. It is shown that rectangular shaped mesa structure from Bi2Sr2CaCu2O8+d (Bi2212) is a good source for generation of terahertz radiation. In this study, we have fabricated triple mesa structures using e-beam lithography and argon ion beam etching techniques. Our aim is to find the critical current density dependence of mesa area to obtain maximum emission power for the terahertz radiation.

The evolution of tunneling spectra on Bi 2 Sr 2 CaCu 2 O 8+δ measured by SIN point contacts, SIS break junctions and STM/STS has been studied as a function of doping and temperature. The detailed examinations of the spectra show that the energy gap measured at low temperature appear to be due to superconducting pairing. The doping dependence of the superconducting gap at low temperature suggests a strong connection to the pseudogap temperature, T*, and this indicates that the pseudogap region is at least partly a consequence of some form of precursor superconductivity. In addition, dip/hump structures observed at high bias scale approximately with not only Δ but also superexchange interaction J over the entire doping range examined, indicating these features are linked to the underlying interaction responsible for superconductivity. We suggest that the hump structure may originate from short-range magnetic correlations.

It is demonstrated that tunneling spectra in various high T c cuprates display generic features. The principal conductance peaks in superconductor-insulator-normal metal (SIN) junctions indicate the superconducting gap in the density of states (DOS). Higher energy features include a dip and hump structure with a strength that is asymmetric in bias voltage. The dip and hump features follow the doping trends of the superconducting gap, Δ, with a rough scaling as ∼2Δ and ∼3Δ respectively. Tunneling spectra in underdoped Bi 2 Sr 2 CaCu 2 O 8+δ display a more pronounced hump feature suggestive of a second gap in the DOS. It is observed that the hump feature in the tunneling density of states is consistent with other experimental observations of the so-called high energy pseudogap which may have magnetic origins.

A theoretical model for tunneling spectroscopy employing tight-binding band structure, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ gap symmetry, group velocity, and tunneling directionality is studied. This is done to investigate if the model can exhibit the same wide range of characteristics observed in tunneling experiments on high-${T}_{c}$ superconductors. A band structure specific to optimally-doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ (Bi-2212) is used to calculate the tunneling density of states for a direct comparison to experimental tunneling conductance. A robust feature of the model is an asymmetric, decreasing conductance background, which is in agreement with experiment for Bi-2212. The model also produces generally good agreement with the tunneling data, especially in the gap region. In particular, the experimentally observed asymmetric conductance peaks can be understood with this model as a direct consequence of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ gap symmetry. Dip features observed at |eV|\ensuremath{\sim}2\ensuremath{\Delta} in the experimental data are not found for any range of parameters in this model, indicating that these features are caused by other physical mechanisms such as strong-coupling effects.

Reproducibility of phonon structures in the tunneling conductance of Bi 2 Sr 2 CaCu 2 O 8 is discussed in relation to their positions and intensities. As for the positions, correspondences with the phonon density of states are certainly observed. As for the intensity-ratios among structures corresponding to different phonon-modes, the reproducibility is not so certain as for the positions. As for the absolute intensity, which reflects the interaction strength, the apparent reproducibility is quite poor.

We report the temperature dependence of tunneling conductance from 5 to 190 K for overdoped TlBa2Ca2Cu3O8.5+δ (Tl1223) with TC ∼ 112 K. The tunneling conductances exhibit clear dips at a voltage, Vdip, below TC, suggesting that the dip is of superconducting origin. Moreover, we have found that the mode energy Ω is 38 meV and independent of temperature, where Ω is defined as the difference between eVdip and the superconducting gap Δ. In addition, the temperature dependence of the tunneling dip strength is similar to that of the magnetic resonance intensity at \((\pi ,\pi )\) observed in inelastic neutron scattering experiments. We found that the relationship between the average mode energy \(\bar{\Omega }\) and Δ for Tl1223 is a common feature of Bi2Sr2CaCu2O8+δ, where \(\bar{\Omega }/\Delta \) increases with decreasing Δ, but does not exceed 2. These characteristic features of the tunneling dips are consistent with not only those of resonance spin excitation but also pairing models in which the \((\pi ,\p...

We work on a frequency tunable solid state device to meet the needs of continuous, coherent, powerful terahertz emission sources that fill practically important terahertz gap. High temperature superconducting (HTS) coherently oscillating Josephson junctions in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) crystal make this approach very promising. Since doping dependence of Bi2212 is an important parameter, Bi2212 crystals are annealed in vacuum or purified argon gas flow at 425 °C. For further processing we pattern both single and double rectangular mesa structures by using electron beam lithography on the cleaved surface of the crystal. Resistance-temperature (R-T), and current-voltage behavior (I-V) measurements achieved.

Evidence for the validity of the pairing glue interpretation of high temperature superconductivity is presented using a modified Eliashberg analysis of experimental superconductor-insulator-superconductor (SIS) tunneling data in ${\mathrm{B}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ (Bi2212) over a wide range of doping. This is accomplished by extracting detailed information on the diagonal and anomalous contributions to the quasiparticle self-energy. In particular, a comparison of the imaginary part of the anomalous self-energy $\mathrm{Im}\ensuremath{\Phi}(\ensuremath{\omega})$ and the pairing glue spectral function ${\ensuremath{\alpha}}^{2}F(\ensuremath{\omega})$ used in the model is consistent with Hubbard model simulations in the literature. In addition, the real part of the diagonal self-energy for optimal doped Bi2212 bears a strong resemblance to that obtained from photoemission experiments.

Good correspondence between the structure in the tunneling conductance curve and the phonon density of states shows that they are really due to phonons at least in a single phonon frequency range. The structures disappeared at T c suggesting that they are due to an exchange of phonons and not due to the inelastic tunneling. From the phonon positions, the gap parameter was found to have the BCS like temperature dependence. 2Δ(0)/ k B T c =7.1. It is isotropic in the CuO 2 plane. A conventional method to determine the gap from the conductance peak-to-peak distance is not bad.

Tunneling spectroscopy measurements are performed on single crystals of single CuO2 layer Tl2Ba2CuO6+δ , double CuO2 layer Bi2Sr2CaCu2O8+δ (Bi2212) and polycrystal quadruple CuO2 layer CuBa2Ca3Cu4O12+δ using the point contact tunneling technique. IV and dI/dV-V characteristics are obtained at 4.2 K. In spite of different number of layers and Tc values, all three high-Tc superconductors exhibit similar spectral features including dip and hump features reminiscent of strong-coupling effects in conventional superconductors. The doping dependence of Bi2212 is studied and several effects of the hole concentration on spectral features are found. A novel effect is that the energy gap increases in the underdoped region even as Tc decreases. Combining the doping dependence of the energy gap and the dip energy provides additional information in order to understand the mechanism of high-Tc superconductivity. Point contact tunneling studies of the doping dependence of the energy gap in Bi2212 also helped to understand local variations of the gap magnitude observed by scanning tunneling microscopy, indicating that this type of spectroscopy is an integral part of the tunneling technique.

Phonon structures were observed in the tunneling conductance of Bi2Sr2CaCu2O8. The spectral function gives proper values for the critical temperature and the energy gap. There is no particular phonon-mode whose contribution to Tc is larger than 12 K. The resistivity and the nuclear-spin relaxation rate were calculated using the spectral function.

Tunneling spectra have been measured on high- T c cuprates including single crystals Bi 2 Sr 2− x La x CuO 6+ δ (Bi2201) and Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) using superconductor–insulator–normal metal point contact or superconductor–insulator–superconductor break junction methods. The doping dependence of the energy gap parameter is similar in both Bi2212 and Bi2201, increasing monotonically to very large values in the underdoped regime even as T c decreases. This doping dependence of superconducting gap is similar to that of pseudogap temperature, T * , indicating this is consistent with the scenario whereby the low-energy pseudogap is due to some type of precursor of superconductivity. The high-energy feature observed as the hump structure may be another kind of pseudogap whose energy scale is much larger than superconducting gap, and it may be magnetic in origin.

We present tunneling conductances on electron-doped Pr 0.82 LaCe 0.18 CuO 4 (PLCCO) single crystals with T c ∼ 16 K that are in overdoped regime. Our data exhibit reproducible superconducting-gap peaks at 4.44 ± 0.71 mV and the subgap-shape is cusplike which can be fitted well by using d -wave or nonmonotonic d -wave symmetry, showing that PLCCO is a superconductor with nodal gap. Furthermore, we found that the temperature dependence of the gap magnitude is BCS like and it disappeared at T c . We conclude that these characteristic features (such as d -wave order parameter and no pseudogap above T c ) of electron-doped cuprates in the overdoped regime are fundamentally similar to those of hole-doped cuprates in the overdoped regime.

Crystals of Bi 2 Sr 2 CaCu 2 O 8+δ with optimal T c =95 K have been underdoped using two different methods and the superconducting gaps have been obtained by tunneling. In some cases, three different tunneling geometries have been utilized: point contact, STM and break junctions. The first doping method involves control of the oxygen content by annealing in various partial pressures of oxygen. These crystals exhibit a narrow spread of gap values over a wide doping range from overdoped (T c =56 K) to underdoped with T c =70 K. However, for underdoped crystals with T c midpoints in the range 25 K − 63 K, there is a dramatic increase in the spread of gap values which may signal the development of static phase separation of either chemical or electronic origin. To avoid possible chemical phase separation, we have explored another doping procedure which incorporates Dy substitution on the Ca site. These crystals exhibit a relatively narrow superconducting transition width and some preliminary tunneling spectra will be presented.

Tunneling measurements are reported for superconductor-insulator-superconductor (SIS) break junctions on underdoped, optimally-doped, and overdoped single crystals of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8+/spl delta// (Bi-2212). The junction I-V characteristics exhibit well-defined quasiparticle current jumps at eV=2/spl Delta/ as well as hysteretic Josephson currents. The quasiparticle branch has been analyzed in the framework of d(x/sup 2/-y/sup 2/) (d-wave) superconductivity and indicates that there is preferential tunneling along the lobe directions of the d-wave gap. For overdoped Bi-2212 with T/sub c/=62 K, the Josephson current is measured as a function of junction resistance, R/sub n/, which varied by two orders of magnitude (1 k/spl Omega/ to 100 k/spl Omega/). I/sub c/R/sub n/ product is proportional to the 0.47 power of I/sub c/ and displays a maximum of 7.0 mV. When the hole doping is decreased from overdoped (T/sub c/=62 K) to the underdoped regime (T/sub c/=70 K), the average I/sub c/R/sub n/ product increases as does the quasiparticle gap. The maximum I/sub c/R/sub n/ is /spl sim/40% of the /spl Delta//e at each doping level, with a value as high as 25 mV in underdoped Bi-2212.

We present point contact tunneling conductances measured on single crystals of an electron-doped cuprate, Pr0.89LaCe0.11CuO4 (PLCCO) with Tc~26K, which is nearly optimally-doped regime. The tunneling conductances on PLCCO measured at 4.2K exhibit reproducible superconducting gap peaks at ~6.09 ? 0.47mV on the V-shaped background conductance, where the subgap-shape is cusplike, and the model with the nonmonotonic d-wave energy gap is in good agreement with the experimental tunneling conductance for PLCCO. Furthermore we observed a fine structure beyond the gap peak, which would reflect a bosonic mode for pairing, such like phonon for conventional superconductors.

The electron-phonon spectral function is determined by the tunneling spectroscopy for Bi2Sr2CaCu2O8/Au planar junctions, for which the phonon structures are observed on the second derivative resistance d 2V/dI2 curves though they are obscure on the dI/dV curves. The spectral function is similar to the generalized phonon density of states and gives proper value for the critical temperature in the Eliashberg framework.

The spectral function α2F of the electron-phonon interaction is determined from the experimental tunneling conductance of a Bi2Sr2CaCu2O8-GaAs junction. Correspondence with the phonon spectrum is satisfactory at 4.2 K. However, the experimental zero-frequency energy gap is a little smaller than the calculated one at 21 K and 40 K, and a fitted α2F is 90% and 85% of that at 4.2 K in height, respectively.

Phonon structures in the tunneling conductance must be properly normalized to evaluate the intensity of the electron-phonon interaction. However, this is very difficult because of the absence of the exact normal-state conductivity. Therefore we propose another method to judge whether or not the intensity becomes stronger with a critical temperature T c. This method compares the intensity of anharmonic structures, which are normalized to the single-phonon structures, between high- and low-T c substances. Our data for Bi-cuprates strongly suggest that the electron-phonon interaction is stronger in higher-T c substances.

A zero-bias resistance peak appeared in the tunneling resistance curve of Bi-cuprates, Na0.89WO3 and ReO3, contacted with GaAs or SnO2-x . Therefore the peak is not inherent to high-T c superconductors and may not be relevant to the electronic properties in the bulk.

Anisotropy in the energy gap of Bi2212 was investigated by using a point contact tunneling junction and it was noted that there was very little anisotropy. When a GaAs needle was contacted perpendicular to the c -plane, at least two kinds of conductance curves were obtained, one with a gap of 42 meV and the other with a gap of 64 meV. In the latter, phonon structures extended to a higher voltage and a three-phonon process was observed similar to our multiphonon exchange model for superconductivity.