The Distribution of the Energy Gap and Josephson IcRn Product in Bi2Sr2CaCu2O8+x by Tunneling Spectroscopy

We present direct measurements of the density of states by tunneling spectroscopy on slightly overdoped Bi₂Sr₂CaCu₂O_8+x (Bi2212) single crystals at low temperature using break-junction and point-contact techniques. We find that (i) the variation of the gap magnitude, Δ, between 20 and 36 meV is likely to be intrinsic to Bi2212, and (ii) there is a correlation between the maximum value of the Josephson I _c R _n product and the gap magnitude: I _c R _n decreases with the increase of Δ. The maximum I _c R _n value of 26 mV is observed at Δ = 20.5 meV. For Δ = 36.5 meV, the maximum measured value of I _c R _n is 7.3 mV. We conclude that (i) the distribution of the Josephson I _c R _n product as a function of gap magnitude cannot be explained by the presence of a single energy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the maximum Josephson strength.     

Correlation between Superconducting Gap and Pseudogap in High-T c Cuprates

We studied the low-temperature energy gap 2 ₀ on Bi ₂ Sr ₂ CaCu ₂ o ₈₊ (Bi2212) and La _2–x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity and magnetic susceptibility tend to be slightly suppressed. Similar suppression appears in and of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2 ₀ is closely related to T* in both Bi2212 and La214 systems; T* 2 ₀ /4.3k _B . It is also pointed out that 2 ₀ is in almost linear proportion to k _B T _max ( T*), where T _max is the temperature exhibiting a broad peak in –T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The factors in 2 ₀ k _B T _max J _eff are 1 for La214 and 2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with p ₀ except in highly doped samples, where T _c 2 ₀ .     

Tunneling measurements of the superconductor Nd1.85C0.15CuO4−δ

We report tunneling measurements of the electron-doped superconductor Nd_1.85Ce_0.15Cu0_4–gd using break junctions. The observed finest gap structure is well expressed by the BCS density of states with the energy gap of = 6.1 meV and a very small broadening parameter, having the ratio 2/k_BT_c = 8.2 with T_c = 17.3 K. These features are similar to our tunneling results for the hole-doped cuprate superconductors. Therefore we find no essential difference in the gap structure, in spite of a common believing of the pairingsymmetry difference between them.     

Investigation of the excitonic insulator phase in bismuth-antimony alloys

Pressure-induced metal-semiconductor transitions in bismuth-antimony alloys in a strong magnetic field (up to 70 kOe) at helium temperatures have been investigated. It is found that for values of the overlap-gap |G|1 meV the alloy forms an excitonic insulator (EI) in magnetic fields above a certain threshold (30–40 kOe). It is inferred that the EI energy gap increases with the magnetic field. The maximum gap observed in fields of 70 kOe turns out to be ₀₀7.5 K. An analysis of the results shows that transitions to the EI phase are observed from both the semimetal and the semiconducting states. The critical transition temperatureT _c is related to the EI gap by the expressionT _c0.7. Arguments are advanced in support of the fact that the formation of the EI phase involves the pairing of electrons at theL point with holes at theT point.     

Polarized Electronic Raman Scattering Studies of Underdoped and Overdoped High-T c Superconductors

We report on polarized Raman scattering from high-T _c superconductors in the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ (Bi2212) and HgBa ₂ CuO ₄₊ (Hg1201) classes, with an emphasis on the effect of superconductivity on the low frequency electronic continuum. The Hg-based materials confirm earlier observations from Bi2212 that the superconducting energy gap 2 peak is not present in underdoped samples, while its Raman shift is maximum in B _1g symmetry at optimal doping, and decreases dramatically with overdoping. These similarities despite the different numbers of bands crossing the Fermi surface argues against a multiple-band explanation for the anomalous doping dependence of the gap. We also discuss an anomalous B _1g feature observed at 600 cm ^–1 in Y underdoped Bi2212. This pseudogap-like feature has several properties which suggest it is of electronic - rather than phononic - origin.     

Magnetic field dependence of zero-sound attenuation close to the pair-breaking edge in3He-B due toJ=1−,J z =±1 collective modes

Our previous theory yielded for the Zeeman splitting of the imaginaryJ=1 collective mode in³He-B the result =2+0.25J _z ( is the effective Larmor frequency). In this paper we take into account the downward shift of the pair-breaking edge from 2 to 2₂– (₂ and ₁ are the longitudinal and transverse gap parameters). This leads to a complex Landé factor: the frequencies of theJ _z =±1 components become =2+0.39J _z , and the linewidths of these resonances become finite: =0.18. The coupling amplitudes of theJ _z =±1 components to density are found to be proportional to gap distortion, (₁–₂/(/)². Our results for the ultrasonic attenuation due to theJ _z =±1,J=1 modes are capable of explaining the field dependence of the attenuation close to the pair-breaking edge as observed by Dobbs, Saunders, et al. The observed peak is caused by theJ _z =–1 component: its height increases due to gap distortion as the field is increased, and the peak shifts downward in temperature and its width increases with the field due to the complex Landé factor. TheJ _z =+1 component gives rise to a corresponding dip relative to the continuum attenuation.     

Interaction between a dislocation and an impurity in KCl: Mg2+ single crystals

The interaction between a dislocation and the impurity in KCl: Mg²⁺ (0.035 mol% in the melt) was investigated at 77–178 K with respect to the two models: one is the Fleischer's model and the other the Fleischer's model taking account of the Friedel relation. The latter is termed the F-F. The dependence of strain-rate sensitivity due to the impurities on temperature for the specimen was appropriate to the Fleischer's model than the F-F. Furthermore, the activation enthalpy, H, for the Fleischer's model appeared to be nearly proportional to the temperature in comparison with the F-F. The Friedel relation between effective stress and average length of the dislocation segments is exact for most weak obstacles to dislocation motion. However, above-mentioned results mean that the Friedel relation is not suitable for the interaction between a dislocation and the impurity in the specimen. Then, the value of H(T _c) at the Fleischer's model was found to be 0.61 eV. H(T _c) corresponds to the activation enthalpy for overcoming of the strain field around the impurity by a dislocation at 0 K. In addition, the Gibbs free energy, G ₀, concerning the dislocation motion was determined to be between 0.42 and 0.48 eV on the basis of the following equation ln / = G ₀/(kT_p0)1 – (T/T _c)^{1/2 –1}(T/T _c)^1/2 + ln ₀/where k is the Boltzmann's constant, T the temperature, T _c the critical temperature at which the effective stress due to the impurities is zero, _p0 the effective shear stress without thermal activation, and ₀ the frequency factor.     

Singular Band Behavior of the Extended Emery Model for the Superconducting Cuprates

Mean field slave-boson approximation is performed on the extended Emery model for the CuO₂ conducting plane. The model is parameterized by Cu–O charge transfer energy _pd , copper–oxygen overlap t ₀, oxygen–oxygen overlap t', and Coulomb interaction U on the copper site taken as infinite. Special emphasis is placed on the role of t in the renormalization trends of the effective band parameters _pf and t, replacing _pd and t ₀, at small doping . It is shown that small, negative t expands the range of stability of the metallic phase, changing, in the second order of the perturbation theory, the nature of the metal–insulator transition point. In the nonperturbative limit, t modifies strongly the renormalization of _pf , making it saturate at the value of 4t. Finite doping suppresses the insulating state approximately symmetrically with respect to its sign. The regime _pf 4t fits very well the ARPES spectra of Y123, Bi2212, and LSCO and also explans, in the latter case, the evolution of the FS with doping accompanied by the spectral weight-transfer from the oxygen to the resonant band.     

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi₂Sr₂CaCu₂O₈₊ single crystals for different oxygen concentrations, from optimally doped (T_c=92.2 K) to highly overdoped (T_c=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (_p) is reduced and the dip structure beyond _p at negative sample bias¹ shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with T_c, and 2_p/k_BT_c stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of T_c     

YBCO Bicrystal Junctions on Sapphire: d-Wave Impact and Possible Applications

The current transport and Josephson effect have been investigated in YBCO junctions, fabricated on the sapphire bicrystal substrates. The YBCO film with thickness t150 nm was deposited by dc sputtering on the epitaxial CeO ₂ buffer layer made by rf magnetron sputtering. The junctions were characterized at dc and at mm waves. 5 m wide junctions have moderately high normal state resistance R _N = 5÷30 with critical currents I _c =50÷200 A which give I _c R _N product of order of 0.5÷2 mV. The tolerance of characteristic interface resistance (R _N S) was around 30% for the junctions on a chip. Experimental data, discussed in terms of d-wave symmetry, demonstrate possibility of design of small scale integrated Josephson microwave circuits.     

Effects of junction resistance and counterelectrode material on point-contact tunneling into single- and polycrystalline YBa2Cu3O7−y

The effects of junction resistance and Counterelectrode material on the results of point-contact tunneling studies into single- and polycrystalline YBa₂Cu₃O_7–y are illustrated. Although reasonably symmetric I(V) curves predominantly indicate energy gap values ~ 20 meV, large asymmetries are often found for high-resistance junctions. Very low-resistance junctions show the expected behavior of a pure metallic bridge and indicate ~ 2530 meV.     

Effect of metal electrode on thermoelectric power in bismuth telluride compounds

The measurements of apparent effective Seebeck coefficients S _a, thermoelectric powers E and I-V characteristics were made on a copper-semiconductor-metal contact junction, where the semiconductor is consisted of the p- and n-type bismuth telluride compounds. The S _a measured by heating either of copper and metal alternatively to produce the temperature differences of T = ±6 K changed slightly with the kind of metal electrodes, but it changed very little when the direction of the temperature gradient was reversed. The averaged S _a values over all kinds of metal electrodes agreed closely with the Seebeck coefficients S measured by the conventional technique using two alumel-chromel thermocouples as an electrode. The thermoelectric power E generated by imposing the temperature differences of T = ±6 K on a thermoelement tended to increase with increase of S _a and reached large values in noble metal electrodes of Ag and Au. The E was found to achieve enhancements of up to 10% or even more, when one end of a thermoelement contacts with Au electrode and the external electrical resistance is zero. Thus, the selection of the optimal metal electrode is necessary to make the thermoelectric conversion efficiency as high as possible.     

Experimental evaluation of relative mode parameters in the irregular earth-to-ionosphere waveguide

Conclusions A model for representing the propagation of superlong waves was selected. Certain relative mode parameters ()₁, ()₂, ()₃, (V)₁, (V)₂, (V)₃, C₁, C₂, and C₃ can be used in order to account for the time-measurement error produced as a resuit of the double-mode signal propagation, to determine in a semiempirical manner the ionosphere's effective reflection height at night and in the daytime, to calculate and apply signal-propagation corrections which take into account phase velocity changes due to variations in the ionosphere height from day to day, and to calculate the fields of transition paths.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 75–78, August, 1974.     

Subgap peak and Tomash-McMillan-Anderson oscillations in the density of states of SNS Bridges

The quasiparticle spectra and the densities of states of superconducting-normal-superconducting junctions are computed from the WKBJ transformed Bogoliubov-de Gennes Equations (BdGE), which are solved by Picard iteration and numerical integration. It is shown that the influence of the proximity effect on the bound states can be modeled by a rectangular pair potential well of effective normal layer thickness 2a*= _–L ^L [1–(z)/]dz, where (z) is the pair potential of the junction, is its asymptotic constant value, and 2L is the total length of the sample. The density of states exhibits a subgap peak at energies less than besides the BCS peak atE=; forE> there are geometrical resonances which are due to electron-hole interferences in finiteS layers of thicknessL-a*.     

Electron tunneling in oxide superconductors

Electron tunneling experiments in very well-characterized polycrystalline samples of Ba_1–xK_xBiO₃ and Bi₂Sr₂Ca₂Cu₃O_10– show very clear features of the energy gap. The energy gap 2 and the ratio 2/K _B T _c were determined. In the Ba_1–xK_xBiO₃ compound we also study the behavior of the tunneling conductance in the normal state.     

Thermodynamics for formation of each stable single phase in BSCCO thin films

High quality BSCCO thin films have been fabricated by means of an ion beam sputtering at various substrate temperatures, T_sub, and ozone gas pressures, PO₃. The correlation diagrams of the BSCCO phases appeared against T_sub and PO₃ are established in the 2212 and 2223 compositional films. In spite of 2212 compositional sputtering, Bi2201 and Bi2223 phases as well as Bi2212 one come out as stable phases depending on T_sub and PO₃. From these results, the thermodynamic evaluations of H and S, which are related with Gibbs' free energy change for single Bi2212 or Bi2223 phase, are performed.     

Flow-Equation Method for a Superconductor with Magnetic Correlations

The flow equation method has been used to calculate the energy of single impurity in a superconductor for the Anderson model with U0. We showed that the energy of the impurity depends only on the _R ² (renormalized order parameter), which depends on the renormalized Hubbard repulsion U ^R. For a strong Hubbard repulsion U ^R = U and ^R = ^I the effect of the s–d interactions are nonrelevant, a result that is expected for this model.     

Observation of subgap structures in high-quality Nb/Al-AlOx/Nb Josephson tunnel junctions

We have performedI-V measurements on high-quality Nb/Al-AlO_x/Nb Josephson junctions. All curves exhibit subgap structures. Some junctions exhibit in theI-V curves the (₁–₂)/e singularity, due to the presence of niobium films with different gaps. In these devices we observed sharply separated structures corresponding to the series 2₁/ne and 2₂/ne, with n3 corresponding to the tunneling of two and three electrons. The respective amplitudes of the two structures atn=2 allow one to conclude that the phenomenon is a multiparticle tunneling process.     

On mode I and mode II energy release rates of an interface crack

The opening (mode I) and sliding (mode II) components of the energy that is released during an incremental extension of an interface crack between two different elastic materials are evaluated by the Irwin's crack closure method. Each component of the energies (G _I and G _II ) is expressed in terms of the functions of the length of the incremental crack extension (a) and the real and imaginary part of the complex stress intensity factor defined by Malyshev and Salganik. It is found that values of G _I /a and G _II /a oscillate violently when a approaches zero and that, hence, in contrast with the case for homogeneous materials, each energy release rate should be defined as G _I /a and G _II /a for an actual crack growth step size.