Abstract: Frequency spectra of quantum beats (QB) in nuclear forward scattering (NFS) are analysed and compared to Mössbauer spectra. Lineshape, number of lines, sensitivity to minor sites, and other specific properties of the frequency spectra are discussed. The most characteristic case of combined magnetic and quadrupole interactions is considered in detail for 57Fe. Pure magnetic Zeeman splitting corresponds to a eight-line spectrum of QB, six of which show the same energy separation as the six lines in Mössbauer spectra. Two other lines (called 2 and 3) are the lower-energy satellites of the lines 2 and 3. As the quadrupole interaction EQ appears, the satellites remain unsplit in the quantum beat frequency spectra, as well as the first (zero-frequency) and the 6th (largest frequency) lines. Each of the lines 3 and 5 generates a doublet split by 2EQ, and the lines 2 and 4 generate triplets. In QB frequency spectra (QBFS) of thin absorbers of GdFeO3 we demonstrate the enhanced spectral resolution compared to Mössbauer spectra. Small particle size in an antiferromagnet (Fe2O3) was found to affect the QBFS via enhancement of the intensity around zero-frequencies. An asymmetric hyperfine field distribution mixes up into the hybridization with dynamical beats, which enlarges the frequencies of the low-lying QBFS lines and makes their shifts relatively large compared to the shift of the highest-frequency line.

Abstract: The effect of strong magnetic fields (11 T) on superconductivity in YBa2Cu3O(7-delta) is investigated using high-resolution thermal expansion. We show that the field-induced broadening of the superconducting transition is due to a finite-size effect resulting from the field-induced vortex-vortex length scale. The physics of this broadening has recently been elucidated for the closely related case of rotating superfluid 4He [Phys. Rev. B 60, 12 373 (1999)]]. Our results imply that the primary effect of magnetic fields of the order of 10 T is to destroy the phase coherence; the pairing, on the other hand, appears to be quite insensitive to these fields.

Abstract: With significantly improved sample quality and instrumental resolution, we clearly identify in the ( pi,0) photoemission spectra from YBa(2)Cu(3)O(6.993), in the superconducting state, the long-sought "peak-dip-hump" structure. This advance allows us to investigate the large a-b anisotropy of the in-plane electronic structure including, in particular, a 50% difference in the magnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the CuO chains, raises serious questions about attempts to quantitatively explain the YBa(2)Cu(3)O(7-delta) data from various experiments using models based on a perfectly square lattice.

Abstract: We present high-resolution thermal expansion data from 5-500 K of untwinned YBa2Cu3Ox (Y123) single crystals for x approximately 6.95 and x = 7.0. Large contributions to the thermal expansivities due to O ordering are found for x approximately 6.95, which disappear below a kinetic glasslike transition near 280 K. The kinetics at this glass transition is governed by an energy barrier of 0.98+/-0.07 eV, in very good agreement with other O-ordering studies. Using thermodynamic arguments, we show that O ordering in the Y123 system is particularly sensitive to uniaxial stress along the chain axis and that the lack of well-ordered chains in Nd123 and La123 is most likely a consequence of a chemical-pressure effect.

Abstract: We compare the angle-resolved photoemission spectra of the hole-doped Cu-O chains in PrBa2Cu3O7 (Pr123) and in PrBa2Cu4O8 (Pr124). While, in Pr123, a dispersive feature from the chain takes a band maximum at k(b) (momentum along the chain) approximately pi/4 and loses its spectral weight around the Fermi level, it reaches the Fermi level at k(b) approximately pi/4 in Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4 filled, they show contrasting behaviors: While the chains in Pr123 have an instability to charge ordering, those in Pr124 avoid it and show an interesting spectral feature of a metallic coupled-chain system.

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Abstract: M6ssbauer spectra using ion-57 and tin-119 nuclei in YBa2Cu307 oxides are reported, using samples prepared under different conditions of heat and gas treatment. The separation ofcopper charge states between different structural positions is supported by these studies. Keyword

Abstract: Considerable progress was recently made in understanding the rich and varied vortex phase behavior of high-Tc superconductors. The important role played by thermal fluctuations near Tc enriches the vortex phase diagram with a novel vortex liquid state. The liquid and solid phases are separated by the melting line Tm(H ) on which the thermal energy is strongly reduced compared to that on the mean-field line Tc2(H ). The melting line Tm(H) appears to be the 1st order phase boundary located far below Tc2(H) exactly as it was predicted for the thermal fluctuations in two dimensions (D = 2) [1]. Recent experiments on the clean crystals of Bi2Sr2CaCu2O8 (BSCCO) and YBa2Cu3O7-δ (YBCO) proved that the transition at the low-field branch of the Tm-line is indeed of 1st order [2-6]. Magnetization of a clean superconductor undergoes a jump at crossing the melting line. This was first demonstrated in BSCCO in which the melting line was traced by the (H,T)-location of the jump in the local magnetic induction [2]. In YBCO, the δ-spikes observed in specific heat cH(T)=δ(T-Tm ) leave no doubts [5] that the internal energy and entropy, associated with the vortex matter, changes discontinuously between the delineated aggregate states. The jumps in entropy ΔS and in magnetization ΔM are linked via Clapeyron-Clausius relation [6]: ΔS = - μ0ΔM (dH m /dTm) (1) The evidences of jumps ΔS and ΔM are of fundamental importance for understanding the nature of the phase transition. In the liquid state, the entropy of vortex disordering is added to the entropy of vortex solid, however, a major contribution to ΔS near Tc comes from additional degrees of freedom related to divergence of penetration depth at T → Tc [7,8]. The sign of ΔM is that of densifying vortex matter at melting alike to the melting of ice into water. Most likely, the flux density increases at melting because the intervortex repulsion become screened with the addition of thermally excited vorticity[9]. On the other hand, for a high-fields branch of melting line ΔS = 0.