Phonon Dispersion Measurements of YBa2Cu3O6.15 and YBa2Cu3O6.95 by Time-of-Flight Neutron Spectroscopy

We measured the phonon dispersions of YBa₂Cu₃O_6.15 and YBa₂Cu₃O_6.95 by time-of-flight inelastic neutron scattering. The in-plane bond-stretching modes in the metallic phase showed a distinct a-b plane anisotropy beyond what is expected for structural origin. Such anisotropy in the longitudinal optical modes, which is absent in the TO, suggests strong in-plane anisotropy in the underlying electronic structure. Apical oxygen bond-stretching modes showed a large frequency change between the insulating and the metallic phases. This large softening also is beyond structural origin, and suggests the effect of local electronic environment.     

In plane anisotropy in optimally doped YBa2Cu3O6.95

Single crystals untwinned optimally doped YBa₂Cu₃O_9.65 show a large degree of anisotropy in a-b plane properties such as resistivity, infrared conductivity and value of zero temperature penetration depth. This source of orthorhombicity must ultimately reside in the CuO chains. Using a three plane model; 2 CuO₂ and one CuO (chain plane), we study solutions of three coupled BCS equations which include inter- as well as intra-plane coupling. The gaps are found to contain s- as well as d-wave symmetries. From our solutions, we calculate the penetration depth and compare with experiments.     

YBa2 Cu3 O7-x单晶的制备

介绍了制备高温超导体YBa2Cu3O7-x单晶的主要方法,分析了在制备过程中存在的问题,指出液相的性质是影响生长速率的重要因素.在此基础上,着重探讨了通过控制氧压和元素替代等方法来改变液相的性质,从而达到提高YBCO单晶生长速率的目的.     

A Polarized Neutron Scattering Study of the Normal Phase in YBa2Cu3O6.95

Polarized neutron scattering with polarization analysis has revealed negligible magnetic scattering up to 30 meV in the normal phase of YBa₂Cu₃O_6.95 indicating the absence of local moments.     

A Model for the Crystal Growth of YBa2Cu3O y

The crystal growth and crystal morphology are very important properties of YBa₂Cu₃O _y superconductors. In this work, we approach these problems with polycrystals. Several typical crystal morphologies were observed by scanning electron microscopy, and the terraced crystals with a layer-by-layer structure were found to be the major form for the well-grown YBa₂Cu₃O _y . To interpret the results, a step-growth model was proposed. In this model, each crystal terrace is close-packed with crystal particles in order. And the particles, each consisting of tens of crystal cells, are movable during crystal growth. Particles at edges or margins of small crystals can move to their larger neighboring crystals through diffusion. With this particle diffusion, small crystals vanish and large crystals grow larger. This proposed model is confirmed by experimental observations.     

Resistivity Studies by Multiterminal Transport Measurements on Single Crystal YBa2Cu3O7−δ

Multiterminal transport studies provide a powerful technique to investigate vortex structure and correlations in the vicinity of the vortex solid-to-liquid transition in single crystals of high temperature superconductors. Recent results are described, including new contact configurations, allowing both longitudinal and transverse vortex correlations to be studied in one crystal during a single temperature sweep at constant field.     

Thermal Conductivity of the Four-Leg Spin-Ladder System La2Cu2O5 Single Crystal

We have investigated the magnetic susceptibility, , and the thermal conductivity, , in magnetic fields for the four-leg spin-ladder system La₂Cu₂O₅ single crystal. The in a magnetic field parallel to the ladder exhibits a kink at 130 K in correspondence to the magnetic ordering. The along the ladder exhibits a peak at 25 K and a shoulder at 14 K, which are probably related to the thermal conductivity due to magnons, _magnon, and that due to phonons, _phonon, respectively. The perpendicular to the ladder, on the other hand, exhibits only one broad peak related to _phonon. The observed large anisotropy of has been explained based upon the anisotropy of _magnon.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Glassy behavior of low-temperature energy relaxation in YBa2Cu3O7 and YBa2Cu3O6

Measuring the power release after rapid cooling a YBa₂Cu₃O₇ sample (m=42.85 g, T_c=91 K) from the equilibrium temperature T₁ (2.35 KT₁15.1 K) to T₀=1.5 K, we observed a time dependence typical of a glass: is proportional to t^–1. The results allow us to determine the linear term of the heat capacity (0.8 mJ/mole · K²) due to the two-level systems. While the low-temperature heat capacity anomaly noticeably decreases, the power release is essentially unchanged after oxygen reduction of the sample.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Electronic structure of YBa2Cu3O7 and YBa2Cu3O6—charge-self-consistent band structure calculations

Charge self-consistent LCAO band structure (CSCBS) calculations are reported for orthorhombic YBa₂Cu₃O₇ and tetragonal YBa₂Cu₃O₆ assuming ordered vacancy models. The effective atomic charges are used to study the charge transfer. In YBa₂Cu(1)₁Cu(2)₂O₇, the two types of copper atoms have their energy bands almost overlapping with effective valency of each copper as 7/3 (or effective valency of each oxygen as approximately — 13/7), so that electron hopping can take place without any loss or gain of energy while in YBa₂Cu(1)₁Cu(2)₂O₆, Cu(1)₁ is monovalent and Cu(2)₂ are divalent with significant difference in their bands. Therefore, YBa₂Cu₃O₇ should conduct much better compared to YBa₂Cu₃O₆. This corroborates the experimental observations that YBa₂Cu₃O₇ is a (super)conductor while YBa₂Cu₃O₆ is not. The calculated effective charges and DOS support the above view.     

Charge-transfer in YBa2Cu3O x

The charge-transfer hypothesis is shown to be inconsistent with data for YBa₂Cu₃O_x: (i) The two-step behavior ofT _c(x) (with jumps from zero to ≈60 K and then to ≈90 K) is not reflected as a similar, statistically significant two-step behavior in the bond-valence-sum charge of cuprate-plane Cu ions (as once believed), (ii) as a consequence of the law of conservation of charge, the derivatives of the layer charges with respect to oxygen contentx for both the Ba-O layers and the charge-reservoir Cu-O chains have the opposite signs to those predicted, and (iii) the charge-transfer observed for superconducting YBa₂Cu₃O_x is not sufficient to produce superconductivity, as demonstrated by insulating PrBa₂Cu₃O_x, which has virtually the same layer charges.     

Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10≥nY≥1; 18>nPr≥ 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.     

Oxygen-enrichment of YBa2Cu3 YBa2Cu3O7-δ using the fluidization techniqueusing the fluidization technique

The oxygen-deficient phase of the highT _c superconductor, YBa₂Cu₃O₇, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa₂Cu₃O₇ powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0–90, 90–180 and 180–420 μm. The fluidized particles were crystalline with orthorhombic distortion.T _c ^onset , estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7–85·3%, one of the highest values reported so far for YBa₂Cu₃O₇.     

Vortex Phase Transition and Oxygen Vacancy in YBa2Cu3O y Single Crystals

The atomic images of YBa₂Cu₃O _y (YBCO, y6.94) single crystals are measured by low-temperature scanning tunneling microscopy (LT-STM). We find that the oxygen vacancies form clusters along the b-axis in the CuO chain layer. Near the clusters, the strong atomic distortion is observed along the a-axis. The vortex phase diagram is very sensitive to the amount of oxygen vacancies in the region between optimal and overdoped regions. For optimally doped untwinned YBCO (y6.92), we find a first-order transition T _L(H) in the vortex liquid above the terminal point H _mcp (7 T) of both the vortex glass line T _g (H) and the field-driven disordering transition line H*(T). The obtained small entropy change (0.02 kB/vortex/layer) and the critical endpoint H _cep (11 T) of T _L(H) indicate that the vortex liquid undergoes the vortex slush regime before the solidification into the vortex glass phase. Below H _mcp, the vortex liquid phase shows the first-order melting transition into the Bragg glass phase.     

Theory of electron distributions and63Cu and17O nuclear quadrupole interactions in YBa2Cu3O7 and YBa2Cu3O6

Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa₂Cu₃O₇ and YBa₂Cu₃O₆ systems. The results of these investigations provide satisfactory explanations of available⁶³Cu and¹⁷O nuclear quadrupole interaction data. The electron distributions obtained rule out the presence of Cu³⁺ ions and are supportive of the presence of Cu²⁺, Cu¹⁺, O¹⁻ ions in the O₇ system and Cu²⁺, Cu¹⁺ and O²⁻ in the O₆ system with actual charges departing significantly from the formal charges, especially in the O₇ system, indicating the importance of covalency effects. Suggestions will be made regarding possible sources that can bridge the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters.     

Optical study of localization in theab-plane conductivity of single crystals of YBa2Cu3O6.95 induced by ion damage

We studied the changes in the dc resistivity, Raman scattering, and IR conductivity of single crystals of YBa₂Cu₃O_6.95 induced by damage from low-energy He⁺ ion bombardment. It appears thatT _c , transport properties, and optical conductivity are strongly affected by modest irradiation doses whereas the chemical composition of the sample is not modified. Carrier localization is evidenced by the frequency dependence of the optical conductivity in the strongly damaged crystal and is used to explain the suppression of both the superfluid density andT _c upon irradiation. The temperature dependence of the superfluid density is in agreement with the theoretical predictions for ad-wave superconductor. We also show that the intrinsic residual losses in the FIR are dramatically reduced in the disordered crystal.