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

Copper magnetic centers in oxygen deficient RBa2Cu3O6+x (R=Nd, Sm): An EPR and magnetic study.

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

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

The [sup 169]Tm nuclear spin-lattice relaxation in oxygen-deficient TmBa[sub 2]Cu[sub 3]O[sub 6+[ital x]] compounds, as quenched and room-temperature annealed, has been measured at low temperatures. The results are consistent with the existence of paramagnetic centers in the CuO[sub 2] double layer, which have a nonmagnetic (singlet) ground state separated from an excited magnetic state by an energy gap of the order of 1 K.

Singlet-ground-state paramagnetic centers in CuO2 layers as seen from 169Tm NMR in TmBa2Cu3O6+x 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

It is widely believed that the long-range antiferromagnetic order in the RBa2Cu3O6+x compounds (R=Y and rare earths except of Ce, Pr, Tb) is totally suppressed for the oxygen index x>0.4 (AFM insulator-metal transition). We present the results of the copper NQR/NMR studies of aged RBa2Cu3O6+x (R=Tm,Y) samples showing that a magnetic order can still be present at oxygen contents x up to at least 0.7 and at temperatures as high as 77K.

Cu(2) nuclear resonance evidence for a magnetic phase in aged 60-K superconductors RBa 2 Cu 3 O 6+x (R=Tm, Y)

Crystal electric field at Tm3+ sites in TmBa2Cu4O8

The copper NQR and ZFNMR and thulium NMR are studied in a series of 123 superconductors It is found that in the aged 60-K superconductors RBa2Cu3O6+x the well known conducting (nonmagnetic) phase co-exists with the magnetically-ordered phase Both microphases contain two types of inequivalent Cu(2) sites in CuO2 planes In the conducting phase, these sites differ only in the Cu(2) NQR frequency, whereas in the magnetic one they differ also in the value of hyperfine magnetic fields at Cu(2) nuclei These differences and the occupation numbers of the two sites are found to be independent of x (in the range x=05÷06) and of the volume fraction of the magnetic phase At low temperatures, the nuclear spin-lattice relaxation in both microphases proceeds via magnetic centers formed by hole-doped Cu–O–Cu chain fragments

O. N. Bakharev

Papers

Singlet-ground-state paramagnetic centers in CuO2 layers as seen from 169Tm NMR in TmBa2Cu3O6+x superconductors.

Intrinsic paramagnetic centers in 1-2-3 superconductors

Cu(2) nuclear resonance evidence for a magnetic phase in aged 60-K superconductors RBa 2 Cu 3 O 6+x (R=Tm, Y)

Crystal electric field at Tm3+ sites in TmBa2Cu4O8

Inhomogeneity of Charge and Spin Distribution in CuO2 Layers of High-Tc Superconductors: NQR/NMR Studies of 1-2-3 Compounds