Unusual behavior of pseudogap in high-temperature superconductors under the influence of external factors. Part II (Review article)

A considerable part of the theoretical and experimental works reflecting the current status of research on high-temperature superconductivity and the unusual phenomenon of a pseudogap in high-temperature superconductors (HTSCs) is reviewed. The concept of local pairs in systems with low and intermediate charge-carrier density, which can include HTSCs, is examined. The experimental part is primarily based on the study of excess and fluctuation conductivity (FC) in YBa2Cu3O7−y (TBCO) and Y1−xPrxBa2Cu3O7−y (YPrBCO) thin epitaxial films. A new approach to analyzing FC and the pseudogap in such high-temperature systems is proposed and checked experimentally. The approach is based on the idea that excess conductivity σ′(T) forms in HTSCs at temperatures substantially above critical temperature Tc as a result of the formation of pair states in the form of noninteracting strongly bound bosons, demonstrating with decreasing temperature a transition from a regime with localized pairs conforming to the Bose–Einstein condensation theory into a regime with fluctuation Cooper pairs which conform to the BCS theory.

The paper investigates the effect of high hydrostatic pressure on the conductivity, s(T), in the basic ab-plane of the HTSC single crystal Y0.66Pr0.34Ba2Cu3O7–δ. The YBa2Cu3O7–δ single crystal was grown using the well-known solution-melt technology. Y2O3, BaCO3, CuО and Pr5O11 compounds were used in the appropriate percentage ratio as initial components for growing Y0.66Pr0.34Ba2Cu3O7–δ single crystal. The modes of growth and oxygen saturation of the Y1–xPrxBa2Cu3O7–δ crystal were the same as for undoped single crystals. Electrical resistance in the ab-plane was measured according to the standard 4-pin method. Hydrostatic pressure was created in a piston-cylinder multiplier. The temperature dependences of the specific electrical resistance, r(Т), of the studied single crystals in the ab-plane in the temperature range Tc–300 K at pressures of 0–10 kbar were obtained experimentally. At all applied pressures, the experimental curves r(T) contain linear sections at temperatures T > T*. When T < T*, the r(T) curves deviate downward from their linear extrapolation, i.e., excess conductivity, Ds(T), appears in the studied samples. It was established that Ds(T) of the samples in a wide temperature range Tf < T < T* are characterized by an exponential temperature dependence Δs ~ (1–Т/Т*)exp(Δ*ab/T), where T* is the mean-field temperature of the superconducting transition, and can be interpreted in terms of the BCS-BEC crossover theory. Using this ratio, the temperature dependence of the pseudogap, Δ*ab(T), from T* to the temperatures corresponding to the maximum value of the pseudogap was constructed. An increase in the applied pressure leads to the effect of narrowing the temperature interval of realization of the pseudogap (PG) mode and, as a result, to the expansion of the interval of the linear dependence of the specific electrical resistance in the ab-plane. As the pressure increases, the temperature dependence of the pseudogap shows a crossover from the dependence of the BCS type to the dependence of the BEK type.

The effects of high-energy (100keV) electron irradiation have been observed in a heavy-metal fluoride glass using in situ time-resolved electron energy loss spectroscopy in a scanning transmission electron microscope. Formation of F2 has been observed. The irradiation-induced changes of configuration and composition are discussed based on the evolution of the near-edge fine structures of F K edge. We suggest that the high sensitivity to electron irradiation is due to the existence of nonbridging F in heavy-metal fluoride glasses so that is there is a tendency of the electron irradiation to eliminate nonbridging F in the irradiated region.

The effect of high-energy (150 eV) electron irradiation in an electron microscope column on crystals of fluorides of alkaline earth elements CaF2, SrF2, and BaF2 is studied. During structural investigations by electron diffraction and electron microscopy, the electron irradiation causes chemical changes in MF2 crystals such as the desorption of fluorine and the accumulation of oxygen in the irradiated area with the formation of oxide MO. The fluorine desorption rate increases significantly when the electron-beam density exceeds the threshold value of ∼2 × 103 pA/cm2). In BaF2 samples, the transformation of BaO into Ba(OH)2 was observed when irradiation stopped. The renewal of irradiation is accompanied by the inverse transformation of Ba(OH)2 into BaO. In the initial stage of irradiation of all MF2 compounds, the oxide phase is in the single-crystal state with a lattice highly matched with the MF2 matrix. When the irradiation dose is increased, the oxide phase passes to the polycrystalline phase. Gaseous products of MF2 destruction (in the form of bubbles several nanometers in diameter) form a rectangular array with a period of ∼20 nm in the sample.

The effect of high-energy electron irradiation on the temperature dependences of the resistivity ρ(), fluctuation conductivity (FLC), and pseudogap (PG) Δ*() of YBa2Cu3O7−δ (YBCO) single crystals containing virtually no twins was studied. A linear increase in the resistivity and a linear decrease in the superconducting (SC) transition temperature Tc with increasing irradiation doses φ were observed. For relatively small φ, the linear Tc can be described by the Abrikosov-Gorkov (AG) pair breaking theory, and for large φ, by the Emery-Kivelson (EK) theory, which takes into account the suppression of Tc by quantum phase fluctuations caused by irradiation defects. As FLC shows, at the average value of φ3=2.5×1019e/cm2, which corresponds to the AG-EK crossover, the distance between the conducting CuO2 planes, d01, as well as the coherence lengths along the c axis, ξc(0), and the region of SC fluctuations, ΔTfl, increase sharply, and the two-dimensional contribution of the Maki-Thompson fluctuations (2D-MT) unexpectedly changes to the two-dimensional contribution of the Aslamazov-Larkin (2D-AL). Surprisingly, no features in ρ(φ) and Tc(φ) indicating the AG-EK crossover are observed. At the same time, at φ3, a sharp increase in the opening temperature of PG, *, as well as the value of PG, Δ*, is observed, which indicates a possible decrease in DOS under the influence of defects. With a further increase in φ, all the parameters of PG and its dimensions are greatly reduced, and an unusual shape of Δ*() is found. However, quite unexpectedly, at φ5=5.6×1019e/cm2 the temperature dependences of both FLC and PG demonstrate curves typical for well-structured YBCO, regardless of the number of defects.

We report here the high-energy (1 MeV) electron irradiation effects on C60 films. Fourier transform infrared spectroscopy, Raman spectroscopy, and x-ray diffraction results are consistent and provide clear evidence of the nondestruction of C60 molecules upon irradiation. The presence of C60 signature peaks in the spectra of all three techniques indicates that the C60 films have not been transformed into a hitherto proposed structure of amorphous carbon for low-energy irradiation. A very short collision time mechanism has been proposed to explain the observation.

The physical properties of hole-doped high temperature superconductors (HTCS) are characterized by a ‘normal’ state (NS) for temperatures T > T*, and a pseudogap (PG) in the electronic spectrum, for T* > T > T c . Strikingly similar behavior occurs in the charge-density wave (CDW) state, T 0 > T > T c , in the transition metal dichalcogenides (TMD) 2H-MX2, where M = Ta, Nb, and X = S, Se, both in the NS (T > T 0) and in the incommensurate charge-density wave (T ICDW > T > T c ) states. Such strikingly similar behavior has also been seen in the organic layered superconductors (OLS) κ-(ET)2X, where ET is bis(ethylenedithio)tetrathiafulvalene, and X = Cu[N(CN)2]Cl, Cu[N(CN)2]Br, and Cu(SCN)2, both in the NS, T > T SDW > T c , and in the spin-density wave regime, T SDW > T > T c . In all three materials classes, the anomalous transport and thermodynamic properties associated with the pseudogap or density-wave regime are completely independent of the applied magnetic field strength, whereas the same properties below T c are all strongly field dependent. Hence, we propose that the pseudogap in the HTSC arises from charge- and/or spin-density waves, but not from either superconducting fluctuations or charged quasiparticle “preformed pairs”.

The effect of high hydrostatic pressure on the electrical conductivity, Пѓ(Рў), in the basal ab plane of the high-temperature superconductor (HTSC) Y0.66Pr0.34Ba2Cu3O7-Оґ single crystals was investigated. It was determined that excess conductivity О”Пѓ(T) of the studied samples in a certain temperature range Tf < T < T* are characterized by a modified exponential temperature dependence О”Пѓ ~ ( 1-T/T*)exp(О”*ab/T), (T* is the mean field temperature of the superconducting transition), which is interpreted in terms of the BCS-BEC crossover theory. An increase in external pressure leads to a narrowing of the temperature range for the existence of a pseudogap (PG) regime, resulting an expansion of the linear temperature dependence of electrical resistivity in the basal ab plane.

The effect of high-energy electron irradiation on poly(vinylidene fluoride-trifluoroethylene) 80/20 mol % copolymers has been studied in a broad dose ranging from 0 to 120 Mrad. It was found that the copolymers transformed from a normal ferroelectric to a relaxor ferroelectric phase (RFE) at high electron doses. For copolymers irradiated at the dose above 90 Mrad, they exhibited typical dielectric behavior of RFE, e.g., whose dielectric constant peaks show strong frequency dispersion. X-ray diffraction patterns showed that the irradiation induced a coexistence of the polar and nonpolar phase in the crystalline regions, leading to nanosized coherent regions in the irradiated copolymer films. Differential scanning calorimetery and polarization hysteresis loops also revealed the reduction in the crystallinity and stability of the ferroelectric state, reflecting significant changes in the ferroelectric-to-paraelectric phase transition due to the effect of high-energy electron irradiation.

The mysterious pseudogap in high temperature superconductors: an infrared view

The effect of 21-MeV electron irradiation on the optical absorption characteristics of Czochralski-grown forsterite (Mg2SiO4) single crystals (both undoped and chromium-doped) has been investigated. The irradiation is found to induce additional optical absorption (AOA) in the crystals in the range of 225–1200 nm due to the formation of color centers based on intrinsic host point defects and the change in the oxidation state of chromium ions. The AOA spectra have been decomposed into elementary bands. The influence of the chromium concentration in crystals, the oxygen content in the growth atmosphere, and additional doping with lithium on the behavior of these bands has been analyzed. A possible structure of the color centers responsible for the AOA is discussed. It is shown that the electron irradiation somewhat decreases the intensity of the characteristic absorption bands of tri- and tetravalent chromium ions and gives rise to a new absorption band in Mg2SiO4:Cr and Mg2SiO4:Cr,Li crystals heavily doped with chromium.

Pseudogap in high-temperature superconductors studied by neutron-crystal field spectroscopy

The Role of Water in the Dissociation of Enolase, a Dimeric Enzyme

An investigation into the influence of hydrostatic pressure on the ultimate capacity of tubular joints is presented. Finite element studies of cross (DT) joints subjected to axial compression and in-plane bending are used to assess the effect that varying amounts of external pressure have on joint capacity. The matrix of geometries evaluated include variations in branch to chord diameter ratio (?), chord radius to thickness ratio (?), and branch to chord thickness ratio (?). Variations in yield strength were also considered, but the same value was always assumed for both the branch and chord. Findings can be divided into two categories. First, longitudinal loads resulting from capped end pressures, as deduced from consideration of isolated members or a frame analysis, may be treated merely as additional axial loads. For deeper water, the magnitude of the branch axial loads from hydrostatic pressure can be substantial and, thus can increase the joint strength utilization ratio. The effect of capped end forces on the chord can be estimated from the normal chord stress effect term, Qf, of the joint capacity equations. However, the impact is expected to be trivial in most instances. The second category of results concerns the effect of hydrostatic radial pressure alone on joint capacity. The analyses show that the capacity can be reduced by as much as 25%, depending on joint parameters and the type of longitudinal load in the branch. Parameter ? is the most important one in relating nondimensionalized branch load and nondimensionalized hydrostatic pressure. Based on a limited number of T-joint cases studied, it appears that the DT-joint interaction equation adjustments, suggested in this study, are a conservative representation T-joint behavior. Also, it seems likely that capacities of other joint configurations will be affected by hydrostatic pressure and should be investigated. INTRODUCTION The offshore oil industry is exploring in progressively deeper water in order to locate economic oil reserves. The deeper waters confront the designers of offshore structures with challenging problems. The member and joint dimensions for shallow water structures are designed according to the expected loading and component capacities, which are provided in various design equations. For deep water structures, an important additional source of loading can be hydrostatic pressure. Overall member behavior in the presence of hydrostatic pressure has received some attention latelyl. In fact, the draft version of API RP2A-LRFD2 reflects these recent studies. However, the effect of hydrostatic pressure on joint capacity has received little or no attention to date. The intent of this analysis has been to provide some design guidance by determining correction factors that can be used to adjust existing joint interaction equations to account for the effect of hydrostatic pressure on joint capacity. The following are interaction equation provided by Hoadley3 and the API2 for tubular joints. Mathematical equation (Available in full paper) The out-of-plane bending terms are not included in Eq. 1 or 2. The results presented below will permit the designer to replace the PU and MU terms in the above equations by new terms that include the effect of hydrostatic pressure.

A systematic study of radiation effects on the major parameters of ohmic and Schottky contacts based on n-ZnO is introduced. Al and Au metals were used as contact elements in order to fabricate the ohmic and Schottky structures, respectively. The transmission line method (TLM) measurements on Al/n-ZnO have revealed that high-energy (6, 9, 12 MeV) and relatively low-dose (3 × 1012 e− cm−2) electron irradiation produced lower specific ohmic contact resistivity values as compared with the reference sample. The current–voltage (I–V) and capacitance–voltage (C–V) measurements on the Au/n-ZnO structures are shown to increase in ideality and to decrease in the Schottky barrier heights with increasing electron energy. These findings have been interpreted based on the assumption that the atoms of the contact elements diffused into the semiconductor material, thus turning the rectifying character to ohmic behaviour with the influence of radiation–matter interaction and subsequent annealing effects.