Broken particle-hole symmetry at atomically flat a-axis YBa2Cu3O7-delta interfaces

We have studied quasiparticle tunneling into atomically flat a-axis films of YBa(2)Cu(3)O(7-delta) and DyBa(2)Cu(3)O(7-delta) through epitaxial CaTiO3 barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor-insulator interface. Below T(c), the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zero-bias. This is evidence that superconducting YBCO crystals, atomically truncated along the lobe direction with a titanate layer, have intrinsically broken particle-hole symmetry over macroscopically large areas.     

Collective and plastic vortex motion probed by STM

The motion of flux line lattices (VL) in various single crystals of NbSe₂ has been observed using fast scanning tunneling microscopy. In crystals with intrinsic weak point disorder the VL creeps coherently as a large bundle along one of the principal axes of the VL. However, we observe striking and for the creep regime unanticipated velocity modulations in the direction of motion, related to the VL periodicity, i.e. with the “washboard” frequency. In the transverse direction the fluctuations of the center-of-mass displacement are random. In crystals with strong line disorder (correlated pinning) caused by columnar defects due to swift ion irradiation collective pinning is observed in fields up to almost double the dose-equivalent field Bφ while interesting plastic flow patterns are detected at 2Bφ.     

外磁场中单畴反铁磁颗粒的宏观量子效应

用瞬子方法研究了外加磁场对单畴双轴反铁磁颗粒宏观量子效应的影响. 当外磁场沿易磁化方向时,简并基态中的一个能量抬高,变为亚稳态,其隧穿衰变率随外磁场增大;当外磁场沿中间磁化方向时,能级的隧穿劈裂随外磁场的变化而振荡.从而提出一种观察单畴反铁磁颗粒中宏观量子相干和隧穿现象的实验方法.     

Step-like growth of Bi2Sr2CaCu2O8 films on off axis oriented (001) SrTiO3

High-quality Bi₂Sr₂CaCu₂O₈ (BSCCO) films were grown by means of pulsed-laser deposition on 10° off-axis oriented (001) SrTiO₃ substrates. The films exhibit a step-like morphology and they are properly oriented along the (001) direction. Their normal-state resistivity measured along the tilt (off-axis (100) direction of the substrate) was, typically, 260 times higher than that along the (010) direction. Such films reveal a transverse Seebeck-effect which significantly exceeds that observed with YBa₂Cu₃O₇ (YBCO).     

Anisotropic conductance of Pb–induced chain structures on Si(557) in the monolayer regime

We present data of a study of four-point conductance of adsorbed Pb films on Si(557) in the thickness range between 0.6 up to several monolayers (ML) at various annealing stages. These measurements are combined with tunneling microscopy (STM). Onset of conductance is found close to the percolation limit. Pb layers annealed to room temperature are characterized by activated contributions to conductance up to 3 ML, a purely metallic temperature dependence at thicker layers, and an anisotropy of at most a factor of 2. On the contrary, annealing to 640 K, leaving only the first monolayer on the Si(557) surface results in extremely high surface state conductance which is quasi one–dimensional below a critical temperature of Tc=78 K, associated with an order–disorder phase transition. Induced by a 10-fold superperiodicity along the Pb chains and their lateral ordering, the system switches from low to high conductance anisotropy, with a metal–insulator transition in the direction perpendicular to the chain structure, while in the direction along the chains conductance with a 1/T + const. temperature dependence was found.     

Evidence for surface Andreev bound states in cuprate superconductors from penetration depth measurements

Tunneling and theoretical studies have suggested that Andreev bound states form at certain surfaces of unconventional superconductors. Through studies of the temperature and field dependence of the in-plane magnetic penetration depth lambda(ab) at low temperature, we have found strong evidence for the presence of these states in clean single crystal YBCO and BSCCO. Crystals cut to expose (110) surfaces show a strong upturn in lambda(ab) at around 7 K, when the field is oriented along the c axis. In YBCO this upturn is completely suppressed by a field of approximately 0.1 T.     

Collective excitations and quantum size effects on the surfaces of Pb(111) films: An experimental study

Pb(111) film is a special system that exhibits strong quantum size effects in many electronic properties. The collective excitations, i.e., plasmons, in Pb(111) films are also expected to show signatures of the quantum size effect. Here, using high-resolution electron energy loss spectroscopy, we measured the plasmons on the surface of Pb(111) films with different film thicknesses and analyzed the plasmon dispersions. One surface plasmon branch exhibits prominent damping in the small momentum range, which can be attributed to the interaction between the top and bottom interfaces of the Pb(111)films. With the film thickness increasing, the critical momentum characterizing the damping in Pb(111) films decays not only much slower in Pb(111) films than in other metal films, and even in films with the thickness up to 40 monolayers the damping still exists. The slow decay of the surface plasmon damping, manifesting the strong quantum size effect in Pb(111) films, might be related to the strong nesting of the Fermi surface along the(111) direction.     

Magneto-optical study of the superconducting gap of MgB(2) single crystals

We present magneto-optical reflectivity results in the basal plane of the hexagonal MgB(2). The data were collected on a mosaic of MgB(2) single crystals with T(c)=38 K from the ultraviolet down to the far infrared as a function of temperature and magnetic field oriented along the c axis. In the far infrared, there is a clear signature of the superconducting gap with a gap ratio 2 Delta/k(B)T(c) approximately 1.2, well below the weak-coupling value. The gap is suppressed in an external magnetic field, which is a function of temperature. We extract the upper critical field H(c2) along the c axis. The temperature dependence of H(c2) is compatible with the Helfand-Werthamer behavior.     

Andreev Bound States at the Onset of Phase Coherence in Bi(2)Sr(2)CaCu(2)O(8)

A new technique of planar tunneling spectroscopy has been developed to access the in-plane density of states of optimally doped Bi(2)Sr(2)CaCu(2)O(8) single crystals. The low energy spectrum is observed to depend on crystallographic orientation. When tunnel current is injected nominally along the Cu-Cu bond direction, a zero-bias conductance peak is observed to appear simultaneously with the onset of bulk superconductivity. These data demonstrate the existence of surface-induced states in this system and confirm the d-wave symmetry of the superconducting order parameter.     

Investigation of the anisotropy of the transverse magnetoresistance in single-crystal ferromagnetic films

The transverse magnetoresistance in films of nickel, iron, and several alloys is investigated when there is a current along the [100] and [110] directions as a function of the angle between the direction of the field (16 kG) and the plane of the film. The results obtained are compared with experimental data and with theory for bulk single crystals. An explanation of the anomalies observed in the film is given.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 7–10, December, 1973.     

Impurity states and interlayer tunneling in high temperature superconductors

We argue that the scanning tunneling microscope (STM) images of resonant states generated by doping Zn or Ni impurities into Cu-O planes of BSCCO are the result of quantum interference of the impurity signal coming from several distinct paths. The impurity image seen on the surface is greatly affected by interlayer tunneling matrix elements. We find that the optimal tunneling path between the STM tip and the metal (Cu, Zn, or Ni) d(x(2)-y(2)) orbitals in the Cu-O plane involves intermediate excited states. This tunneling path leads to the fourfold nonlocal filter of the impurity state in Cu-O plane that explains the experimental impurity spectra. Applications of the tunneling filter to the Cu vacancy defects and "direct" tunneling into Cu-O planes are also discussed.     

Zero bias conductance peak enhancement in Bi2Sr2CaCu2O8/Pb tunneling junctions

Recently, there has been tremendous interest on a tunneling feature called zero bias conductance peak (ZBCP) commonly found in high T_csuperconductor tunnel junctions. The interest stemmed from the feature’s root in the d-wave symmetry of high T_csuperconductors. In this paper we will review results of our study on ZBCP within a Pb/Bi₂Sr₂CaCu₂O₈(BSCCO) tunneling junction. This allows us to investigate the intricate interactions between s- and d-wave superconductivity. The ZBCP indeed demonstrates interesting properties when Pb becomes superconducting. We attribute these results to the suppression of the d-wave phase on the surface of BSCCO by the s-wave superconductivity from Pb through a proximity effect. We will also report some recent results on the magnetic field dependence of the ZBCP.     

Centimeter-size BiFeO3 single crystals grown by flux method

A narrow part of Fe₂O₃–Bi₂O₃ phase diagram was re-investigated in order to elaborate single crystals of the multiferroic BiFeO₃ (BFO). Centimeter-size single crystals were successfully obtained by flux method, and present a preferred growth direction. X-ray diffraction studied have highlighted that the growth direction is along the polar axis [111] _r of the structure. The stability of BFO versus temperature (reversible ferroelectric transition followed by multiple irreversible decompositions) is discussed in the light of Differential Scanning Calorimetry (DSC) analysis performed between 25 and 1400°C.     

Ultrasound tunneling through 3D phononic crystals

We report the study of ultrasound tunneling in 3D phononic crystals, consisting of fcc arrays of close-packed tungsten carbide beads in water. The transmission coefficient, phase velocity, and group velocity were measured along the [111] direction, allowing us to systematically investigate the tunneling of ultrasound at frequencies in the lowest band gap. Our experimental data are interpreted using multiple scattering theory, which provides a good explanation of our results. The effect of absorption and the difference between the tunneling of classical waves and quantum waves are discussed.     

Vortex imaging in the pi band of magnesium diboride

We report scanning tunneling spectroscopy imaging of the vortex lattice in single crystalline MgB2. By tunneling parallel to the c axis, a single superconducting gap (Delta=2.2 meV) associated with the pi band is observed. The vortices in the pi band have a large core size compared to estimates based on H(c2) and show an absence of localized states in the core. Furthermore, superconductivity between the vortices is rapidly suppressed by an applied field. These results suggest that superconductivity in the pi band is, at least partially, induced by the intrinsically superconducting sigma band.     

Bi2Sr2CaCu2O8超导单晶中的电阻反常行为

测量了Bi2 Sr2 CaCu2 O8单晶的ab面和c轴方向电阻 ,在其超导转变温度附近发现了反常的电阻峰出现 .其随外磁场 (>10 0Gs)和电流的增加而逐渐消失 .文章认为这个反常的电阻峰是由于单晶中超导相的不均匀分布而导致的准再进入行为 .     

Biaxial Orientation Mechanism of Drawn Natural Rubber. II. Quantitative Analysis of the Biaxial Orientation

Natural rubber film was prepared from triple centrifuged latex. The film was uniaxially drawn at room temperature and the induced crystalline orientation was quantitatively studied by wide angle X‐ray diffraction. The intensity distribution of 200 and 120 reflections confirmed that the induced crystals have a biaxial orientation with the c‐axis parallel to the draw direction. The orientation of the a‐axis was evaluated by using an orientation function of the (200) plane. The function (F ₂₀₀) indicated that the crystalline a‐axis is parallel to the film surface depending on the draw ratio and on positions in the film. The experimental results are possible to explain by shish‐kebab‐like crystallization developed from a highly oriented fibril. The secondary crystallization grows perpendicularly to the draw direction along the a‐axis. The population of the secondary crystals is controlled by an ellipsoidal free space. The shape of the ellipsoid is changed by the fibril distribution depending on width and thickness of the sample film. In this study, the quantitative biaxial orientation is consistently explained by the shish‐kebab‐like crystallization and the ellipsoidal free space.     

Structural and Dielectric Properties of Organic Ferroelectric 2-Methylbenzimidazole

2-methylbenzimidazole single crystals and films are obtained by evaporation from an ethanol solution. The films have a texture with a predominant orientation of the pseudotetragonal axis in the film plane. The blocks are split crystals of the spherulite type, in which the pseudotetragonal axis rotates around the block center. Dielectric hysteresis loops are investigated in the crystals and films in the temperature range of 290–350 K. In the films, loops are observed for both in-plane and out-of-plane electric-field orientation. The maximum value of polarization in the films is close to that observed in single crystals P _s ~ 5 μC/cm². The difference in the temperature behavior of the hysteresis loops in the crystals and films is associated with the specific structure of the blocks.     

LiKSO4单晶体内体位相光栅引起的光衍射现象

本文报道光通过LiKSO₄单晶体产生的有规律性的衍射现象,衍射环和衍射带。观察了不同实验条件对衍射现象的影响,可以肯定这些现象是波矢k垂直于c轴的体位相光栅所产生。伴随衍射环内外各出现副环的来源尚未能理解。 关键词：     

Crystal growth and surface morphology of Pb,Bi, Sr,Ca, Cu,O, high T c superconductors

Two types of Pb, Bi, Sr, Ca, Cu, O superconducting single crystals: plate with a flat surface and bulk mica-like were successfully grown by a slow cooling method. As-grown crystal dimensions up to 1 cm² have been obtained. Magnetization and resistivity measurements indicate that the as-grown single crystals have a superconducting transition temperature at 85–90 K. Surface morphology observations by means of differential interference contrast microscope (DICM) and scanning tunneling microscope (STM) are reported.