Clues Obtained from the Oxygen Isotope Effect on NMR/NQR Parameters Observed in YBa2Cu4O8

Nuclear magnetic and nuclear quadrupole resonance (NMR, NQR) techniques have a precision allowing one to determine rather small isotope effects. Well-defined oxygen stoichiometry and negligible oxygen diffusion makes YBa₂Cu₄O₈ an ideal compound for studies of small isotope effects that require experimental results not hampered by reproducibility problems. We report on high-precision measurements of the temperature dependence of plane-^63,65Cu NMR/NQR parameters such as Knight shift, spin–lattice relaxation rate R = 1/T ₁, NQR line frequency _Q and NQR linewidth _Q, as well as ⁸⁹Y Knight shift, performed in normal and superconducting ¹⁶O and ¹⁸O exchanged YBa₂Cu₄O₈.     

Oxygen isotope shifts of the Raman modes of YBa2Cu3O7−x

Raman scattering experiments have been carried out on sintered pellets of YBa₂Cu₃ ¹⁶O_7−x and YBa₂Cu₃ ¹⁸O_7−x obtained both by gas exchange and by growth with substituted oxides. The frequencies of the modes at 340, 435 and 502 cm⁻¹, which involve motion of the oxygen atoms and which shift significantly upon oxygen isotope substitution, have been measured for several sets of samples. The measured frequency shifts indicate that the isotope exchange on the O(2) and O(3) sites is more complete than the exchange on the O(4) site. The 502 cm⁻¹ line of the¹⁸O samples is observed to be broadened and this is attributed to¹⁸O-¹⁶O disorder on the O(4) sites. The results are discussed with reference to previous measurements of site activation energies and models for the exchange mechanism.     

Doping and Isotope Dependence of the Pseudogap in High-Tc Cuprates Observed by Neutron Crystal-Field Spectroscopy: A Fast Local Probe

The temperature dependence of the relaxation rate of crystal-field excitations in the high-T _c superconductors HoBa₂Cu₄O₈ and HoBa₂Cu₃O _x (6.4 < x < 7) was investigated by inelastic neutron scattering techniques. The data show clear evidence for the opening of an electronic gap in the normal state at T* > T _c in the underdoped regime as well as for slightly overdoped samples. For HoBa₂Cu₄O₈ T* increases from 170 to 220 K upon oxygen isotope substitution (¹⁶O vs ¹⁸O). This huge isotope shift (which is absent in NMR and NQR experiments) suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range 10^–8 > 10^–13 s.     

NMR studies of Nd2−x Ce x CuO4

Copper NMR has been studied as a function of temperature in a number of superconducting Nd_2−x Ce _x CuO₄ samples. The electric field gradient is very small and the Knight shift is 2380 ppm at room temperature, both of these implying that the copper is in a Cu⁺ state. The Knight shift decreases with temperature particularly belowT _c . The spin contribution to the Knight shift is estimated to be ∼ 200 ppm (about a factor ten smaller than in YBa₂Cu₃O₇) indicating thatN _s (E _F ) at the copper sites is small in this material.     

NMR studies of spin dynamics in cuprates

We report recent NMR results in cuprates. The oxygen Knight shift and the Cu nuclear spin-lattice relaxation rate in Bi_2.1Sr_1.94Ca_0.88Cu_2.07O₈₊ single crystals revealed a gapless superconducting state, which can be most naturally explained by a d-wave pairing state and the intrinsic disorder in this material. The Cu nuclear spin-spin relaxation rate in underdoped YBa₂Cu₃O_6.63 shows distinct temperature dependence from the spin-lattice relaxation rate, providing direct evidence for a pseudo spin-gap near the antiferromagnetic wave vector.     

Sintering studies on submicrometre-sized Y-Ba-Cu-oxide powder

The isothermal sintering behaviour of submicrometre-sized (<50 nm) powders of single-phase YBa₂Cu₃O _x (123) and unreacted stoichiometric mixture of submicrometre-sized (<50 nm) powders of BaCO₃, Y₂O₃ and CuO (which on calcination at 1173 K gives YBa₂Cu₃O _x ) was investigated through dilatometry under different sintering atmospheres. The sintering rate of the powder compacts was impeded by the presence of oxygen. The activation energies,Q, of sintering were determined to be 1218 kJ mol^–1 in argon, 1593 kJ mor^–1 in air and 2142 kJ mol^–1 in oxygen. A decrease in the apparent sintered density with increasing oxygen partial pressure was also observed. X-ray diffraction and thermal analyses (thermogravimetry and differential thermal analysis) showed no reaction during sintering of the single-phase product. Pellets fabricated from uncalcined powder exhibit two stages of sintering, one between 1073 and 1173 K having an activation energyQ=627kJ mol^–1, and a second one above 1173 K withQ=383.7 kJ mol^–1. A.c. susceptibility, resistivity and critical current density were determined as a function of the temperature of the sintered samples.     

Tilting Mode Relaxation and Oxygen Isotope Effect in La2−xSrxCuO4 Studied by Electron Paramagnetic Resonance

The electron paramegnetic resonance (EPR) of Mn²⁺ doped into La_2?x Sr _x CuO₄ was used to probe the copper spin relaxation via the bottleneck effect for oxygen isotope (¹⁶O and ¹⁸O)–substituted samples. It was found that the EPR linewidth is larger for the ¹⁸O isotope samples than for the ¹⁶O samples. For x = 0.03, the linewidth for the ¹⁸O sample is larger than for ¹⁶O sample by a factor of 2. The isotope effect is pronounced at low temperatures and decreases with increasing Sr concentration. This effect is quantitatively explained by the Cu²⁺ S = 1/2 spin relaxation to the lattice via Dzyaloshinski terms coupled linearly to the local Q ₄/Q ₅ tilting modes of the CuO₆ octahedra as proposed by Kochelaev et al. [1] The Q ₄/Q ₅ modes are coupled sterically to the Q ₂ Jahn–Teller modes considered to be relevant for the (bi)-polaron formation and thus for the high-temperature superconductivity (HTSC).     

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.     

Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films

The evolution of Raman spectra with illumination has been studied in YBa₂Cu₃O_6.4 films at temperatures between 5–300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm^?1, O(2)-O(3) at 338 cm^?1, and O(4) at 488 cm^?1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequenciesω, which is in agreement with the enhancement of the static conductivityσ(ω→0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7−x

The effects of addition to YBa₂Cu₃O_7?x of lithium halides (YBa₂Cu₃O_7?x (LiF) _y , and YBa₂Cu₃O_7?x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7?x (LiF) _y and 91.30 K in YBa₂Cu₃O_7?x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _c∝B ^?α, with a lithium-concentration-dependent α. The voltage–current characteristics follow a law typical for granular superconductors, V∝(I?I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Superconducting gap in Pr x Y1−x Ba2Cu4O8 and YBa2−y Sr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1?x Ba₂Cu₄O₈ and YBa_2?y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω～300 cm^?1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.     

Nd substitution effects in YBa2Cu4O8 prepared using a conversion reaction from Y1−xNdxBa2Cu3O7−δ and CuO

Synthesis, crystal structure and magnetic properties of samples with partial substitution of Nd for Y in YBa₂Cu₄O₈ are considered. The samples with nominal composition Y_1−xNd_xBa₂Cu₄O₈ (x=0.0–0.75) were prepared using a solid state conversion reaction from Y_1−xNd_xBa₂Cu₃O_7−δ and CuO using LiF as a flux. The annealing was performed at 790°C in a flowing oxygen atmosphere. The products with x=0.0–0.5 showed thermal stability up to 660°C in N₂(g) atmosphere revealing a single phase composition. The success of the synthesis is considered in relation to the effective ionic size of the Nd³⁺ ion. The crystal structures of the compounds are described on the basis of powder neutron diffraction measurements. The T_c values of the Nd substituted samples were noticeably lower than for the pristine YBa₂Cu₄O₈. Possible reasons for the decrease in T_c are discussed on the basis of calculated values for hole concentrations in the Cu–O₂ planes obtained via the BVS (bond valence sum) method, and on the basis of some Nd³⁺ ions being introduced at Ba²⁺ ion sites in the lattice.     

Infrared and Structural Properties of Y1−xNdxBa2Cu3O7−δ (0 ≤ x ≤ 1)

Infrared and structural properties of Y_1?x Nd _x Ba₂Cu₃O_7?δ (0 ≤ x ≤ 1) were investigated using infrared absorption spectroscopy and X-ray powder diffraction. The unit cell parameters of the samples were defined using X-ray diffraction data. The resistance measurements showed that the samples revealed superconductivity in the temperature range of 80–100 K. It was observed that by the substitution of Nd to Y in YBa₂Cu₃O_{7 ? δ} IR band at 573 cm^?1 that is assigned as Cu–O axial antisymmetric stretching mode shifts to 533 cm^?1 while the band at 620 cm^?1 that is due to Cu–O symmetric stretching mode in YBa₂Cu₃O_7?δ shifts to 588 cm^?1.     

Unusual oxygen doping behavior in Hg cuprates and its pressure dependence

Self-consistent electronic structure studies show that the extra oxygen in the O(4) site in Hg cuprates is unlike oxygen ions in other cuprates that have been studied: it is singly charged O^1–rather than being near O^2–. This behavior is necesary to make a consistent interpretation of the doping behavior by oxygen in Hg cuprates. We find that the change in doping level at 9.2 GPa in HgBa₂Ca₂Cu₃O₈₊ is far too small to account for the observed rise inT _c . We comment on strain effects on the electronic structure in YBa₂Cu₃O₇.     

A microscopic model for the different Tc(x) dependence in REBa2Cu3O6+x HTc-superconductors containing different RE-ions

Replacing Y in YBa₂Cu₃O_6+x by the larger ions Nd or La increases the critical oxygen content for the transition from antiferromagnetic to superconducting behavior from x=0.4 to 0.6. From the analysis of the NQR spectra of the Cu(1) sites at different x follows a change of the interaction in the oxygen pairs connected by a Cu(1) ion from attractive for Y to repulsive for La.     

Influence of Ba content and Ce doping on the structural features of YBa2Cu4O8 superconductor—a neutron study

The structures of A: YBa₂Cu₄O₈ (T _c=80 K), B: Y_0.9Ce_0.1Ba₂Cu₄O₈ (T _c=5K), and C: YBa_2.2Cu₄O₈ (T _c=80 K) have been investigated using the powder neutron technique with a view to assessing the influence of the bridging Cu(2)-O(4) and Cu(1)-O(4) bonds on the superconducting transition temperature. The Cu(2)-O(4) bond for samples A and C (withT _c=80 K) appears to be compressed as compared to the reported data. On the other hand, the substitution of Ce at 10% Y sites elongates the Cu(2)-O(4) bond substantially and also brings theT _cdown to 55 K. The role of Cu(2)-O(4) bond length as well as other parameters expected to influence theT _cbehavior of Y-124 samples are discussed in the light of the present data. The study also shows that the ambient oxygen pressure route, if tried with the starting composition YBa_2.2Cu₄O _y , leads to a quite homogeneous 124-phase.     

Synthesis of Y-Ba-Cu-O superconductors from Y2O3 BaO2, and Cu2O: The optional oxygen treatment

YBa₂Cu₃O_x superconductors were synthesized from mixed powders of Y₂O₃, BaO₂, and Cu₂O under oxygen, air, and argon atmospheres. All samples were superconductive at the boiling point of liquid nitrogen, indicating the oxygen treatment is optional for this powder synthesis method.     

A new high-T c superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-x Bi x )1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+δ (123 phase) and Bi₄Sr₄Ca₂Cu₄O_16+δ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+δ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+δ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Anisotropic uniaxial pressure effects in YBa2Cu3O7−δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7?δ crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2?x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

Variations of internal friction in YBa2Cu3Ox superconductors

The internal friction (Q ^–1) spectrum of YBa₂Cu₃O_x superconducting ceramic exhibits several peaks. It has been confirmed that the high-temperature peak (around 240 K) depends on structural changes and varies during subsequent cycles of cooling and heating.Q ^–1 conductivity, X-ray spectra and the shielding effect have been measured on several samples having different superconducting properties obtained by various thermal treatments. Splitting is a characteristic feature of the high-temperature internal friction peak of the sample which exhibits good superconducting properties. In the case of the specimen exhibiting the worst properties the peaks decrease and overlap. In both cases an increase can be observed of this peak with the number of thermal cycles. After ageing at 470 K, the high-temperature peak disappears. Subsequent thermal cycles slightly recover it. Hysteresis of the Young modulus is also observed. The results are interpreted as transition of the 04 oxygen atom between two energy minima in the O4-Cu-O4 chain.