F. Holtzberg

Growth of YBa2Cu3Ox single crystals

We report a set of conditions for crystal growth of the high‐temperature superconductor YBa2Cu3Ox. The as‐grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.We report a set of conditions for crystal growth of the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub x/. The as-grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.

New ferromagnetic 5:4 compounds in the rare earth silicon and germanium systems

Abstract The rare earth elements Gd, Tb, Dy, Ho, and Er form compounds with Ge and Si at the 5 : 4 stoichiometry which have orthorhombic crystal structures. The suicides and germanides are in the same space group and have very similar lattice constants, but incomplete solid solubility in the system Gd 5 Si 4 Gd 5 Ge 4 indicates that they are not isostructural. Magnetic measurements show that the suicides are ferromagnetic with relatively high ordering temperatures. The germanides are antiferromagnetic, but replacement of a small amount of Ge by Si in Gd 5 Ge 4 produces solid solutions which are ferromagnetic at low temperatures and have intermediate transitions to antiferromagnetism before becoming paramagnetic.

Growth of YBa/sub 2/Cu/sub 3/O/sub x/ single crystals

We report a set of conditions for crystal growth of the high‐temperature superconductor YBa2Cu3Ox. The as‐grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.We report a set of conditions for crystal growth of the high-temperature superconductor YBa/sub 2/Cu/sub 3/O/sub x/. The as-grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.

M‐edge x‐ray absorption spectroscopy of 4f instabilities in rare‐earth systems (invited)

Soft x‐ray absorption spectroscopy in the M‐edge region is explored as a tool for studying 4f instabilities. The quasiatomic MIV,V spectra, recorded by total‐electron yield under UHV conditions, carry information on the initial‐state 4f occupancy. For compounds of the heavy rare earths, even at high dilution (e.g., Tm0.05Y0.95Se), mean valences can be derived, which agree with those from lattice‐constant systematics (e.g., for TmSe and Sm0.3Y0.7S). A moderate surface sensitivity allows the observation of surface‐induced valence changes on EuPd3, TmS, and SmAl2. The MIV,V spectra studied for 10 Ce compounds fall in two categories: pure 3d94f1 final‐state multiplets for γ‐like compounds, and additional peaks at about 5 eV higher energies for α‐like and intermediate systems. With increasing intensity of the satellites, which are assigned to 3d94f1 final states, the 3d94f2 multiplet structure gets washed out. An interpretation of the spectra with the recent Anderson‐impurity theory of Gunnarsson and Schonhamm...

Multiplet Structure in the Absorption Spectrum of Eu2

The Eu2+ ion in alkaline earth fluorides produces two broad absorption bands arising from 4f−5d transitions. The band at 3 eV has a structure with six resolved peaks. This structure is independent of Eu concentration. This band is attributed to transitions from the 4f 7(8 S) ground state to 4f 6(7 FJ ) 5eg , the peaks being associated with the different J values in f 6(7 F). The higher energy band in the fluorides and the absorption band in the europous chalcognides are correspondingly attributed to 4f 6(7 FJ ) 5t 2g states. With this model, with 7 FJ intervals obtained from the resolved structure in the EuF2 data, and from the experimental oscillator strengths of the observed absorption bands, the 4f−5d radial integral is calculated and a consistent set of values of 0.23±0.01 A obtained. The Faraday and Voigt magneto‐optic effects are calculated for EuF2 at 4330 A. The room‐temperature Verdet constant is −7.1 min/Oe·cm as compared with the experimental value of −6.6 min/Oe·cm. The Voigt birefringence at 15°K and 20 kOe is n∥−‐n⊥=−2.0×10−4 while experiment gives −1.7×10−4.

Ferromagnetism in Rare‐Earth Group VA and VIA Compounds with Th3P4 Structure

The bcc structure I43d−Td6(Th3P4) occurs as a defect structure in 2:3 compounds of rare‐earth elements with S, Se, and Te, or as an inverted structure in 4:3 compounds with group VA elements. The structural, electric, and magnetic properties of the metallic 4:3 compounds and the 2:3 rare‐earth semiconductors have been investigated with specific reference to gadolinium, for which the S ground state gives the minimum crystal field effect.Gd4Bi3 and Gd4Sb3 are both ferromagnetic with a saturation magnetization obeying the spin‐wave T32 law up to 0.8 of the Curie temperature. The Curie temperature Tc varies in the solid solution system Gd4Sb3– Gd4Bi3 with composition from 260° to 340°K. The semiconducting compound Gd2Se3(ρRT=3 Ω cm) has been found to be antiferromagnetic below TN=6°K. Solution of Gd in the holes of the defect Th3P4 structure decreases the electrical resistivity without a measurable variation of lattice constant (a0=8.718 A). With increasing conductivity the material changes from antiferromag...

The anisotropic nature of the superconducting properties of single crystal Y1Ba2Cu3O7 − x

Abstract A series of noncontact magnetic measurements on high-quality single crystals of Y1Ba2Cu3O7 − x have enabled us to demonstrate that the superconducting-state properties of Y1Ba2Cu3O7 − x are those of a conventional, anisotropic superconductor with the anisotropy being associated with the highly conducting Cu O sheets in the Y1Ba2Cu3O7 − x crystal structure. The anisotropy in the superconducting state is reflected most strongly in the critical current anisotropy, which is as large as 20 to 1 at low temperatures and low fields and gets arbitrarily large at higher temperatures and higher fields. The upper critical field HC2 shows an anisotropy that varies from 6:1 to 10:1 in different crystals of high quality. Along the copper-oxygen sheets the upper critical field is enormous, extrapolating to ∼60 Tesla by 77 K and implying a low-temperature Ginzburg-Landau coherence length normal to the layers of 2A− 4A. Strong upward curvature is evident in the HC2 data in both orientations of applied field. This curvature can be interpreted in terms of thermodynamic fluctuations, and, if this explanation is correct, the critical region is enormous, extending more than 25 K below TC.

Core and valence XPS spectra of clean, cleaved single crystals of YBa2Cu3O7

Abstract Small single crystals of YBa2Cu3O7 with sharp superconducting transitions at 91 K were cleaved at 300 K in UHV (10−9 torr) and their XPS spectra were measured using small spot (300 μm dia.) Al Kα radiation. The cleaved surfaces showed no initial contamination, as determined by a lack of C(1s) intensity. Only one Ba core level feature is found to be characteristic of the bulk superconductor (Ba(5p 3 2 ), Ba(4d 5 2 ), and Ba(3d 5 2 ) at 12.0 eV, 87.0 eV, and 777.4 eV respectively). A second Ba feature at ~1 eV higher binding energy, found on some cleaved surfaces, represents a surface Ba phase. For the intrinsic surface the O(1s) spectrum is dominated by a peak at 528.0 eV and contains at least two unresolved components. Any large intensity at higher binding energy (530 to 533 eV are often reported) represents reactions at the surface. A relatively large intensity is observed at EF for high quality single crystals. This intensity and the Ba, Y and Cu core level features are stable for a few hours following the cleave at 300 K. There no evidence for oxygen loss at 300 K as judged by the constant O(1s) intensity. These results are compared to results in the literature, where it is often claimed that the higher binding energy feature relates to the intrinsic superconductor. We believe this claim is erroneous, as is the claim that oxygen is lost rapidly from the superconductor in vacuum if the sample is not maintained at 20 K. Finally, the Cu(2p 3 2 ) spectrum of the intrinsic superconductors surface is very similar to that of Cu2+ in CuO, whereas data reported in literature often show a strong Cu1+ component.

New mechanisms for irreversibility in high‐Tc superconductors (invited)

We discuss the zero‐field cooled (ZFC), field‐cooled (FC), and remanent (rem) magnetization of YBaCuO crystals in the context of an extended Bean critical‐state model. We introduce a novel way to include the reversible magnetization in this theory and show how this can explain the concave‐downwards shape of the ZFC magnetization near Tc. We explain the persistence of remanence in a temperature range where FC and ZFC magnetizations merge in terms of a residual irreversibility arising from a step in the reversible B(H) curve. We also introduce the concept of a flux‐trapping depth which helps explain the field dependence of the Meissner effect.

Resistive transition of crystalline Y1Ba2Cu3O7−x in magnetic fields

Abstract Transport and a.c. susceptibility data on a crystalline sample of Y 1 Ba 2 Cu 3 O 7− x in fields up to 9 T along the c-axis are presented. Careful study of the I–V curves and the frequency dependent a.c. susceptibility indicate that the critical current develops suddenly at a line in the H–T phase diagram that coincides with the irreversibility line as measured by a.c. susceptibility. Above this line, which is interpreted as the melting of a pinned vortex regime, the I–V curves are linear and the resistance has a flux flow like character. However, the R versus T behaviour in the melted region supports the interpretation of two different regions of flux flow viscosity.