W. Schlabitz

High-magnetic-field and high-pressure effects in monocrystalline URu2Si2

Abstract High-field magnetisation and high-pressure resistivity measurements have been performed on monocrystalline samples of the superconducting compound URu2Si2 just above Tc. At 1.4 K, well below the magnetic ordering temperature of about 17.5 K, two sharp transitions at 36 and 40 T have been observed in the magnetisation versus field curves along the tetragonal c-axis, whereas the a-axis magnetisation curve remains linear up to 40 T. These transitions point to a complex magnetic behaviour. Above the second transition the magnetic moment per uranium atom exceeds a value of 1.3 μB. The resistivity versus temperature curve is distinguished by two characteristic temperatures of 75 and 17.4 K, corresponding with the onset of coherent scattering and the magnetic transition, respectively. Both characteristic temperatures increase with pressure.

Final-state effects in divalent Eu pnictides

From Mossbauer effect, lattice constant and susceptibility measurements we show that Eu is nearly divalent in Eu(Cu,Ag,Au)(P,As,Sb). However, the LIII X-ray absorption spectra consist of a pronounced double structure, suggesting a non-integral mean Eu valence. The high-energy structure in the LIII-edge spectra is interpreted as a final-state (shake-up) effect. This finding strongly supports the existence of final-state effects in the LIII X-ray absorption spectra of divalent Eu compounds as previously reported by Sampathkumaran et al (1985) in EuPd2P2.

First- and second-order phase transitions in ternary europium phosphides with ThCr2Si2-type structure

Abstract The ThCr2Si2- type compounds EuFe2P2 and EuRu2P2 exhibit as a function of pressure continuous phase transitions which are accompanied by extremely strong changes of the lattice parameters and particularly of the P–P distance dP−P along the tetragonal c-axis. In the isostructural integralvalent LaT2P2 compounds similar phase transitions of second (T=Fe) and first order (T=Co) occur as a function of pressure indicating that the T element essentially determines the nature of the phase transition. The different types of phase transitions are explained phenomenologically.

First-order phase transitions in the ThCr2Si2-type phosphides ARh2P2 (A = Sr, Eu)

Abstract We observed for the first time a first-order phase transition with strong and extremely anisotropic changes of the lattice parameters in compounds crystallizing in the ThCr2Si2 structure type. The phase transition occurs SrRh2P2 with increasing pressure at 6 GPa (300 K) and in EuRh2P2 with increasing temperature at 810 K (ambient pressure). On the basis of single-crystal data of ARh2P2 (A = Ca, Sr, Ba, Eu) at ambient pressure and temperature we discuss the Pue5f8P distance in the framework of band-structure calculations at the first-order phase transition the Pue5f8P state changes from a ‘no-bond’ to a ‘single-bond’ state.

Low-temperature observation of localized uranium-moments in dilute (La, U)Al2 alloys

Abstract Dilute alloys of U in LaAl 2 show low-temperature anomalies in both their normal state properties (susceptibility, thermopower) and superconducting properties (transition temperature, specific heat jump), which are phenomenologically related to the Kondo anomalies of Ce in LaAl 2 . This is the first observation of the Kondo effect in a dilute actinide alloy and demonstrates that in a favorable metallic host U-ions can carry localized magnetic moments.

Pressure-driven valence change in ternary Eu pnictides

We observed a structural phase transition with extremely anisotropic changes of the lattice parameters as a function of pressure at 2.6 GPa in EuPdP, which crystallizes in the hexagonal layered structure type. On the basis of the results of pressure-dependent x-ray diffraction experiments on the isostructural series APdP and APdAs (A = Sr or a trivalent rare-earth element) we show that the phase transition in EuPdP is accompanied by a valence change of the Eu. Strong but continuous changes of the lattice parameters with increasing pressure, which are due to increase of the Eu valence, were observed in EuNiP, EuPtP and EuPdAs, too. An estimation of the average Eu valence in these compounds leads to preferred values of the order of .

Temperature and pressure driven valence change in ternary Eu-pnictides

Abstract We report on measurements of lattice constants as a function of pressure and temperature on EuPdAs and EuPtP (EuTX), which crystallize in the hexagonal layered Ni2In structure. Below room temperature the valence changes dramatically (Δν ≅ 0.16) at one (EuPdAs) or two (EuPtP) valence phase transitions accompanied by a giant collapse of the c-axis, while the a-axis increases slightly. The mean Eu-valence, which is temperature independent above the phase transitions, is ν ≅ 2.16. Pressure dependent measurements on EuPtP and LaPdAs show larger compressibilities in c- than in a-direction due to the anisotropic, layered structure of the compounds. A strong decrease of the volume (5%) from 0 to 1.5 GPa indicates (at least) one phase transition below 1.5 GPa. Furthermore we present a model of electrostatically charged layers that may explain the observed, unusual behaviour of the lattice constants phenomenologically.

Instabilities of ternary Eu pnictides

Abstract Ternary Eu pnictides EuTX (T = Ni, Pd, Pt; X = P, As) are mixed valent and undergo one or two first-order phase transitions in dependence of temperature. At the phase transitions the Eu valence changes by about 1 6 . The phase transitions are connected with an unusual behaviour of the lattice constants. The observations are explained within a model of electrostatically charged layers.

Magnetic 3d-4f coupling in REBa2Cu3O7−δ

Abstract We present specific heat and inelastic magnetic neutron scattering data on REBa2Cu3O7−δ (Re = rare earth). Independent of the oxygen content drastic anomalies are observed in the temperature behaviour of the magnetic neutron scattering response. The quasielastic linewidth is nearly independent of temperature up to about 100 K and then increases linearly. Below T ≈ 100 K the quasielastic scattering changes continuously from Lorentzian to Gaussian character. Together with the systematic variation of the 4f ordering temperatures found by specific heat measurements this suggests an increase of an indirect 3d-4f coupling with decreasing atomic number of RE and could therefore also explain the anomalies in PrBa2CuO7−δ