https://repository.library.northeastern.edu/files/neu:331186/fulltext.pdf

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearTc, 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.

Specific heat of YBa2Cu3O7

Publisher Summary This chapter discusses the magnetic properties of lanthanide compounds.. Specific magnetic and magnetoelastic properties of the crystals are determined by the localized electronic magnetic moments of lanthanide ions, regularly distributed in a crystal lattice, and by their interactions with each other and the surrounding ions. Other sources of magnetism in these substances are the nuclear spins of lanthanide ions and diamagnetic ligands as well as effective magnetic moments of paramagnetic impurities. Practically all methods of physical experiments at low and ultralow temperatures—calorimetry, dilatometry, direct measurement of magnetic susceptibility and magnetization, gamma spectroscopy, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), resonance and nonresonance optical and acoustic spectroscopy, and spectroscopy of thermal neutrons—are used in investigations of magnetic properties of the lanthanide crystals. Severalinvestigations provide information not only of scientific but also significant applied value, particularly in problems of thermometry at low and ultralow temperatures and of magnetic cooling. The magnetic and magnetoelastic properties have been systematically investigated in rare-earth ethylsulfates (RES), tetrafluorides LiRF 4 with scheelite structure, RMO 4 crystals having zircon structure (M = V, P, As), trifluorides RF 3 , and lanthanide elpasolites.

Chapter 150 Magnetic properties of nonmetallic lanthanide compounds

The resistance of the nonstoichiometric compounds YBa2Cu3O6.8 (single crystals, ceramics) and YBa2Cu3O6.8 (Ce) (single crystal) to low-temperature decomposition (at 200°C) in air and an argon atmosphere is studied by X-ray diffraction and electron microscopy. At the first stage, the compounds are found to undergo oxygen separation into two phases, namely, oxygen-rich and oxygen-lean phases as compared to the initial state, that have different lattice parameters. After 20-to 35-h annealing, the disordering of the heavy Y, Ba, Ba, Y atoms along the c axis in ceramic pellets begins mainly because of the action of elastic stresses. This process occurs via the formation of the packets of numerous stacking faults on (001) planes. The disordering of the ceramic matrix ends in the formation of a CsCl-type cubic phase upon 100-h annealing, which is accompanied by a significant decrease in the diamagnetic response and an increase in Tc from 75 K in the initial state to 90 K. The retained superconductivity and the increase in Tc are caused by the presence of a large number of oxygenrich ortho-phase particles in a nonsuperconducting matrix; these particles are correlated with each other and form a multicoupled system of superconducting filaments.

Fine structure and the mechanism of the low-temperature decomposition of the nonstoichiometric compounds YBa2Cu3O6.8 and YBa2Cu3O6.8 doped with Ce

Alternative crystal field parameters for rare-earth ions obtained from various techniques I. Reanalysis of Mössbauer spectroscopy studies of Tm3+ ions in TmBa2Cu4O8 and TmBa2Cu3O7-δ high Tc superconductors

We present a detailed NQR, nuclear magnetic resonance (NMR), and $\ensuremath{\mu}\mathrm{SR}$ study of the magnetic phase obtained during a topotactic chemical reaction of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ high-temperature superconductor with low-pressure water vapor. $^{65}\mathrm{Cu}$-enriched samples have been used for NQR/NMR studies which allows to get a good resolution in the $\mathrm{Cu}(1)$ NQR and $\mathrm{Cu}(2)$ zero field NMR (ZFNMR) spectra. It is shown that the NQR spectrum of the starting material transforms progressively under insertion of water, and almost completely disappears when about one ${\mathrm{H}}_{2}\mathrm{O}$ molecule is inserted per unit cell. Similarly, a $^{65}\mathrm{Cu}$ ZFNMR signal characteristic of this water inserted material appears and grows with increasing water content, which indicates that the products of the reaction are nonsuperconducting antiferromagnetic phases in which the copper electronic magnetic moments in the ${\mathrm{CuO}}_{2}$ bilayers are ordered. The use of $^{65}\mathrm{Cu}$-enriched samples allowed us to reliably resolve three different copper resonances which correspond to different internal magnetic fields. The antiferromagnetic phases are also felt by proton NMR which reveals two sites with static internal fields of 150 and about 15 Gauss, respectively. $\ensuremath{\mu}\mathrm{SR}$ studies performed on a series of samples prepared in the same way as the $^{65}\mathrm{C}\mathrm{u}$-enriched ones reveal two muon sites with the same local fields as the proton sites, which vanish at $T\ensuremath{\approx}400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This indicates that muons preferentially occupy proton vacancy sites, and that the magnetic phases have similar N\'eel temperatures as the other bilayer undoped cuprate compounds. An analysis of the internal fields on the different spin probes suggests that they can be all assigned to a single magnetic phase at large water content in which the $\mathrm{Cu}(1)$ electron spins order with those of the $\mathrm{Cu}(2)$. The detailed evolution of the spectra with the progressive increase of water content is shown to be compatible with a coexistence of phases during the early stages ot the reaction. It appears that even samples packed in paraffin underwent a transformation of a substantial part of the sample after 6 years storage in atmosphere. Samples packed in Stycast epoxy resin heated moderately to a temperature $(200\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ undergo a reaction with epoxy decomposition products which yield the formation of the same final compound. It is clear that such effects should be considered quite seriously and avoided in experiments attempting to resolve tiny effects in these materials, such as those performed in some recent neutron scattering experiments.

Antiferromagnetic properties of a water-vapor-inserted YBa 2 Cu 3 O 6.5 compound studied by NMR, NQR, and μ SR

Many of presently known high-Tc superconductors contain rare-earth (RE) ions with an even number of electrons in an unfilled 4f-shell (Pr3+, Tb3+, Ho3+, Tm3+). If the ground state of 4f-electrons is non-degenerate and separated from excited states by high enough energy intervals, one can observe the so-called “enhanced NMR” of RE nuclei at low temperatures. In the present paper some aspects of the enhanced NMR are analyzed in applications to the crystal and electron structure of high-Tc superconductors.

NMR studies of singlet-ground-state rare-earth ions in high-Tc superconductors

The169Tm “enhanced” NMR in TmBa2Cu3O6+x (x=0.5, 0.6) at temperatures below 4.2K and the63Cu(1) NQR in YBa2Cu3O6.5 at temperatures above 4.2K are used to study properties of intrinsic paramagnetic centers incorporated into superconducting materials. The spin-lattice relaxation of thulium and copper nuclei reveals three types of paramagnetic centers to be present in oxygen-deficient 1-2-3 superconductors, those are (1) two-level ones with a spin S=1/2, localized outside CuO2 bilayers, (2) singlet-ground-state paramagnetic centers with an integer spin S≥1 in CuO2 bilayers, and (3) exchange copper-oxygen clusters with a half-integer spin S≥5/2, localized in a nearest neighborhood of CuOx basal plane at boundaries of superconducting OrthoII microdomains.

Intrinsic paramagnetic centers in 1-2-3 superconductors

The NMR spectra and nuclear relaxation of 141 Pr 3+ and 169 Tm 3+ ions in oriented powders of high-T c superconductors Pr 1.85 Ce 0.15 CuO 4−y ( T c =24 K ) and TmBa 2 Cu 3 O 7−y ( c =91 K ) at temperatures well below T c were investigated. The relaxation of Pr nuclear transverse magnetization at temperatures down to 0.05K gives an evidence for the trivalent state of Ce ions in the superconducting phase of the PrCeCuO compound. Two types of Tm centers inTmBaCuO were observed (of orthombic and tetragonal symmetry), exhibiting similar parameters of NMR spectra, but completely different relaxation rates. Anomalously slow nuclear transverse relaxation of tetragonal Tm centers at liquid helium temperatures allows one to conclude that those centers are formed by single Tm ions and/or small clusters.

141Pr and 169 Tm NMR study of superconducting PrCeCuO and TmBaCuO

The ground state of Tm3+ ion (4t2,3H6) in a crystal electric field (CEF) of TmBaCuO is a singlet, separated from the lowest excited states by an energy gap of 100÷130 cm−1. The pulsed NMR of169Tm nuclei (spinI=1/2) in a magnetically-oriented TmBa2Cu3O6.92 power is studied at temperatures 1.5÷4.2 K. Two sorts of Tm nuclei are observed: rapidly relaxing Tm1 (≥80% of total amount, having relaxation timesT1(1)≈35 ms,T2(1)=80÷250 μs) and slowly relaxing Tm2 nuclei (≦20%,T1(2)=300÷1000 μs). The169Tm NMR spectra are described by the spin-Hamiltonian ℋ1 = −ℏΣγiHiIi (i = x,y,z), where |γx(1)/2π| = 5.3(1), |γy(1)/2π| = 6.6(1), |γx(2)/2π| = |γy(2)/2π| = 5.1(1), |γz(1)/2π| = |γz(2)/2π| = 2.3(1) kHz/Oe, and axesx, y, z coincide with the crystal axesa, b, c. The Tm1 and Tm2 nuclei are identified as those belonging to orthorhombic (CEF of D2h symmetry) and tetragonal (D4h) phases of the TmBaCuO compound, respectively. Two-exponential and nearly temperature-independent nuclear relaxation is observed at temperatures 1.5÷4.2 K. The smallestT2(1)-values (≈80 μs) found for the orthorhombic (superconducting) phase in an external field H perpendicular to the crystalc-axis agree fairly well with those one could expect due to the dipole-dipole interaction of thulium nuclei. However, the slowing down of this rapid relaxation of the nuclear transverse magnetization in a field H‖c as well as the origin of unusually largeT2(2)-values remain unclear.

A. V. Egorov

Papers

Antiferromagnetic properties of a water-vapor-inserted YBa 2 Cu 3 O 6.5 compound studied by NMR, NQR, and μ SR

NMR studies of singlet-ground-state rare-earth ions in high-Tc superconductors

Intrinsic paramagnetic centers in 1-2-3 superconductors

141Pr and 169 Tm NMR study of superconducting PrCeCuO and TmBaCuO

169Tm NMR study of the high-Tc superconductor TmBa2Cu3O6.92