E. Walker

Inelastic neutron scattering study of cerium heavy fermion compounds

We will review inelastic neutron scattering experiments performed on single crystals of the heavy fermion compounds CeRu2Si2 and CeCu6. At high temperatures, the magnetic scattering can be described by a single quasi-elastic Lorentzian peak. At low temperatures antiferro and incommensurate magnetic correlations develop below 70 and 10 K in CeRu2Si2 and CeCu6, respectively; the associated wave vectors are k1 = (0.3, 0, 0) and k2 = (0.3, 0.3, 0)for CeRu2Si2 and k1 = (0, 0, 1) and k1 = (0, 0, 1) andk2 = (0.85, 0, 0) for CeCu6. These magnetic correlations are destroyed by a magnetic field applied along the easy axis (Hc ≈ 25 kOe for CeCu6, Hc ≈ 83 kOe for CeRu2Si2). These high field experiments allow us to establish that in both compounds, at low T and H = 0, the magnetic scattering is the superposition of two contributions: i) a q-independent (single site) quasi-elastic contribution of Lorentzian type, slowly decreasing at high field, ii) a strongly peaked inelastic contribution associated with magnetic correlations, centered at a finite energy ħω0 with characteristic energy width Γ ≈ ħω0 ≈ 1.2 meV and 0.2 meV for CeRu2Si2 and CeCu6, respectively.

BaZrO3: the solution for the crucible corrosion problem during the single crystal growth of high-Tc superconductors REBa2Cu3O7−δ; RE = Y, Pr

We report on the solution growth of single crystals of YBa2Cu3O7−δ and PrBa2Cu3O7−δ in a new type of home-made crucible, namely made of BaZrO3, which has the advantage that it does not react with the occurring melts which are commonly used as a flux. These new crucibles have several advantages: not only the crystals obtained after the growth experiment contain no impurities from the crucible material, but also an important problem is solved, e.g. the corrosion of the crucible does no longer change the composition of the melt during the growth experiment, thus avoiding uncontrollable growth conditions. Hence, due to this novel inert crucible material the use of more sophisticated crystal growth techniques such as top seeded solution growth (TSSG) are now possible.

The use of BaZrO3 crucibles in crystal growth of the high-Tc superconductors Progress in crystal growth as well as in sample quality

Home-made BaZrO3 crucibles have been used as a crucible material for solution growth of single crystals of REBa2Cu3−δ (RE - Y, Er, Dy, Pr) and of Y1−xPrxBa2Cu3O7−δ single crystals (0 ≤ x ≤ 1). As we recently reported [Physica C 245 (1995) 245], this new crucible material does not react with the melts commonly used as flux [ A. Erb, PhD. thesis, Universitat Karlsruhe (1994) ]. As a consequence, the composition of the melt is not altered during crystal growth and the impurity content in the single crystals is much smaller than for crystals grown in other crucible materials. The absence of corrosion also makes the crystal growth far more efficient; thus it was worthwhile to grow crystals using high-purity (5N) starting materials, in order to further enhance their quality. The high quality of the 123 crystals grown in BaZrO3 crucibles is reflected in their physical properties. As an example, the first observation of the flux lattice with STM experiments on the 123 superconductors has been performed on an uncleaved crystal grown in BaZrO3 without any surface conditioning, thus revealing superconducting behaviour up to the uppermost layer of the crystal. Another example is furnished by low-temperature specific heat measurements: besides a very narrow superconducting transition, these measurements show the absence of the upturn usually attributed to flux inclusions and hence to magnetic ordering of the flux component BaCuO2 These results prove that the quality of 123 single crystals has been considerably improved by using BaZrO3 crucibles.

Raman spectra of single crystal CuO

Polarized Raman measurements on single crystals of CuO yield the symmetry assignments of the three predicted Raman active lattice modes : 297cm−1 (Ag, 344cm−1 (Bg) and 629cm−1 (Bg). These results are compared to literature data, including IR spectra. Our measurements confirm at low temperature the appearance of an additional Raman band around 240 cm−1. The temperature dependence of the linewidth of the Ag mode presents an anomalous behavior near the magnetic phase transition, suggesting the possible presence of magnon-phonon couplings in the antiferromagnetic phase.

Carrier relaxation, pseudogap, and superconducting gap in high-Tc cuprates: A Raman scattering study

We describe results of electronic Raman-scattering experiments in differently doped single crystals of Y-123 and Bi-2212. The comparison of AF insulating and metallic samples suggests that at least the low-energy part of the spectra originates predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach. Dynamical scattering rates and mass-enhancement factors for the carriers are obtained. In B2g symmetry the Raman data compare well to the results obtained from ordinary and optical transport. For underdoped materials the dc scattering rates in B1g symmetry become temperature independent and considerably larger than in B2g symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in B2g symmetry in the range between the superconducting transition at Tc and a characteristic temperature T* of order room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity related features in the spectra can be observed only in B2g symmetry. The peak frequencies scale with Tc. We do not find a direct relation between the pseudogap and the superconducting gap.

The distribution of the deuterium atoms in the deuterated cubic laves-phase ZrV2D4·5

Abstract A neutron diffraction analysis at room temperature of ZrV 2 D 4·5 has been made. The deuterium atoms are found to occupy two types of tetrahedral interstices in the metal atom host structure. One is formed by 1 Zr and 3 V atoms and has an occupancy of 39%, whereas the other one is formed by 2 Zr and 2 V atoms, having an occupancy of 25%. Together these interstices can be considered as forming a three-dimensional infinite network of diffusion paths which is similar to that observed in the deuterated hexagonal Laves-phase ZrMn 2 D 3 . The adjacent D atom sites in this network are about 1.3 aA apart and the average Zr-D and V-D distances are 2.10 and 1.77 aA, respectively.

Specific heat up to 14 tesla and magnetization of a Bi2Sr2CaCu2O8 single crystal thermodynamics of a 2D superconductor

Abstract A 0.25 g single crystal of Bi 2 Sr 2 CaCu 2 O 8 with a sharp superconducting transition at 85 K is characterized by AC susceptibility, X-ray diffraction, Meissner effect, normal-state susceptibility, magnetization in the mixed state, and specific heat from 1 to 300 K. The residual linear term γ ∗ T in the specific heat at low T is small and field independent. High-resolution ( t c show that: (a) there is no mean-field specific heat jump at T c ; (b) a symmetrical logarithmic ‘divergence’ occurs at the transition; (c) its amplitude decreases with B -2 3 ; (d) its width scales with B 1 2 ; (e) T c does not depend on the field, and coincides with the crossing point of the magnetization curves M ∗ ( H, T ∗ ); (f) a crossing point C x (H, T x ) is found in the specific heat 10 K below T c ; (g) the effect of a field along the CuO planes is at least 50 times smaller than that of a field along the c axis; (h) no transition is observed at the irreversibility line. An empirical formula for ϑ( C/T )/ϑ H that accounts for C ( H, T ) and M ( H, T ) data over a wide temperature and field range is proposed.

Reversible suppression of the so-called fishtail effect in ultra pure single crystals of YBa2Cu3O7−δ achieved by proper oxygenation

We report on the absence of the so-called fishtail or peak effect in the magnetisation curves of properly oxygenated high purity (5N) single crystals of YBa2Cu3O7−δ. As we recently reported [1] high pressure oxygenation procedures are suitable to produce a monotonous dependence of the critical current density with the magnetic field. In this article we report furthermore how to reversibly produce or remove this effect on the same crystal by proper oxygenation methods. Along with the high purity of the samples, — due to crystal growth in non reactive BaZrO3 crucibles —, this proves that at least in pure samples the fishtail effect is due to different oxygenation states and/or oxygen distributions. In less pure samples, however, other sources for this anomaly like metallic impurities can act as an additional source for the fishtail anomaly.

Transport properties of CeCu6 single crystals

The electrical resistivity ϱ and the thermoelectric power of CeCu6 single crystals are strongly anisotropic. The inverse of the temperature of the Kondo resistivity maximum (Tmax) roughly scales the linear temperature coefficient B of ϱ as well as the residual value (ϱ0 ÷ B ÷ 1/Tmax). Along the [1 0 0] direction ϱ follows a T2 Fermi-liquid law between 30 and 90 mK. The thermoelectric power is positive over the investigated temperature range (1–300 K) and shows two contributions.

D-WAVE SCALING RELATIONS IN THE MIXED-STATE SPECIFIC HEAT OF YBA2CU3O7

The low temperature specific heat C(B,T) of an YBa2Cu3O7.00 single crystal is measured from 1.2 to 10 K in magnetic fields up to 14 T. The anisotropic component Caniso(T,B)=C(T,B//c)-C(T,B//ab) is a pure vortex quantity obtained directly from experiment. It follows a scaling relation predicted recently for line nodes characteristic of d-wave vortices. Our experimental field and temperature range corresponds to a crossover region where the limit Caniso(T,B)is proportional to T*sqrt(B) does not strictly apply. The variation of the entropy caused by the magnetic field at low T is thermodynamically compatible with measurements near Tc.