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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.

Antiferromagnetism in ${\mathrm{EuCo}}_{2}{\mathrm{P}}_{2}$ ( ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure) is due to ordering of the ${\mathrm{Eu}}^{2+}(^{8}S_{7/2})$ sublattice moments $({T}_{N}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}66.5\mathrm{K})$ while the $\mathrm{Co}(3d)$ sublattice does not carry any magnetic moment. ${\mathrm{EuCo}}_{2}{\mathrm{P}}_{2}$ undergoes a pressure-induced isostructural phase transition at ${p}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3.1\mathrm{GPa}$. ${}^{151}$Eu high-pressure M\"ossbauer experiments $(0\ensuremath{\le}p\ensuremath{\le}5\mathrm{GPa})$ reveal a valence transition of Eu from ${\mathrm{Eu}}^{2+}$ to nonmagnetic ${\mathrm{Eu}}^{3+}(^{7}F_{0})$ at $pg{p}_{c}$ with the consequence that the disappearance of the $\mathrm{Eu}(4f)$ sublattice magnetism is accompanied by a simultaneous appearance of $\mathrm{Co}(3d)$ sublattice magnetism $({T}_{N}^{*}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}260\mathrm{K})$. The latter is explained by the filling of the $3d$ states.

Antiferromagnetism in EuCo2P2 (ThCr2Si2-type structure) is due to ordering of the Eu2+(S-8(7/2)) sublattice moments (T-N = 66.5 K) while the Co(Sd) sublattice does not carry any magnetic moment. EuCo2P2 undergoes a pressure-induced isostructural phase transition at p(c) = 3.1 GPa. Eu-151 high-pressure Mossbauer experiments (0 less than or equal to p less than or equal to 5 GPa) reveal a valence transition of Eu from Eu2+ to nonmagnetic Eu3+(F-7(0)) at p > p(c) with the consequence that the disappearance of the Eu(4f) sublattice magnetism is accompanied by a simultaneous appearance of Co(3d) sublattice magnetism (T-N* = 260 K). The latter is explained by the filling of the 3d states.

We present a detailed study of the resistivity and the thermopower in the stripe ordered non superconducting low temperature tetragonal phase of rare earth (RE=Nd,Eu) doped La2−xSrxCuO4. The anomalies of the thermopower due to the structural low temperature transition are maximum at x=1/8, whereas the increase of the resistivity is minimum at x≤1/8. We discuss our data within the framework of static stripes of charge and spins. In particular our experimental findings for the resistivity are only compatible with this model if one assumes a rather large conductivity in the quasi one-dimensional charge stripes.

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Measurements of the DC susceptibility and ESR of Gd spin probes in Eu-doped La 2 − xSrxCuO4 are reported. The data show that magnetism of the low-temperature tetragonal phase of this compound differs qualitatively from that in the orthorhombic phase but only if the former phase is nonsuperconducting. We discuss the obtained results in connection with stripe correlations and critical buckling of the CuO2 planes.

Point contacts between the heavy-fermion superconductor URu(2)S(i)2 and Nb are studied. A finite de Josephson current is found in contacts aligned parallel to the a-b directions of URu2Si2 whereas it is absent in contacts aligned along the c direction. We attribute this extreme anisotropy; of the Josephson current to an unconventional superconducting order parameter in URu2Si2, with a symmetry leading to destructive interference for Josephson currents along the c direction.

Four compounds ARh(2)P(2) (A = Ca, Sr, Eu, Ba) were prepared by heating mixtures of the elements and investigated by means of single crystal X-ray methods. They crystallize in the ThCr2Si2 type structure (I4/mmm; Z = 2) with P-P distances along [001] reaching from 2.26 Angstrom (CaRh2P2) to 3.74 Angstrom (BaRh2P2). With increasing temperature (EuRh2P2) or increasing pressure (SrRh2P2) a first order phase transition occurs with strong changes of the P-P distances. Substitution of the atoms changes the bond lengths of the compounds too.

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 .

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 PP distance in the framework of band-structure calculations at the first-order phase transition the PP state changes from a ‘no-bond’ to a ‘single-bond’ state.

SrNi2P2 and BaNi2P2 were prepared by heating mixtures of the elements and investigated by single crystal X-ray methods. The Sr compound at room temperature crystallizes in a superstructure of the ThCr2Si2 type (NT-phase; Immm; Z = 6; a = 3.951(2), b = 11.853(2), c = 10.432(2)Angstrom), which is caused by displacements of the atoms from the ideal positions; the P-P distances are 2.45 and 3.28 Angstrom. With increasing temperature at 45 degrees C (ambient pressure) and increasing pressure at 4 kbar (room temperature) respectively the compound undergoes first order phase transitions and crystallizes after that in the undistorted ThCr2Si2 type (I4/mmm; Z = 2). While the P atoms of the high temperature phase (HT-SrNi2P2: a = 3.948(1), c = 10.677(3) Angstrom; 100 degrees C) are isolated from each other (d(P-P): 3.12 Angstrom) they most probably form pairs in the high pressure phase (HD-SrNi2P2: a = 4.003(1), c = 9.761(2)Angstrom; ca. 4 kbar). This will be discussed on the basis of band structure calculations. BaNi2P2 (a = 3.947(1), c = 11.820(1)Angstrom) also crystallizes in the ThCr2Si2 type structure, the P-P distance is extended to 3.71 Angstrom.

We outline the possibility to study europium valence fluctuations with the μSR method and report on μSR experiments on the intermetallic compounds EuPdAs and NdPdAs. Above a magnetic transition at 15 K the temperature dependence of the relaxation rate in the trivalent neodymium system behaves like a typical localized moment system. In the valence fluctuating europium compound the zero field relaxation rate levels off at 1.0\ μs-1 above 40 K. Furthermore, the relaxation enhancement in transverse field experiments is much smaller than expected for a pure dipolar coupling. Therefore an isotropic hyperfine coupling of typical strength is assumed and a valence fluctuation rate of 0.8 μs-1 at 200 K is derived. Below the magnetic transition at 5 K a disordered spin freezing is concluded in EuPdAs.

Among rare-earth and actinide Be-13 intermetallic compounds, YbBe13 is anomalous, with evidence for mixed-valence and heavy-fermion behavior. The resistivity is comparatively low and the resulting large mean free path enables point-contact spectroscopy in the ballistic regime. In the d(2)V/dI(2) characteristics of point contacts between YbBe13 and normal metals, measured at temperatures between 0.2 and 40 K, we find structures due to crystal-field and Kondo scattering. The characteristics show a pronounced asymmetry, which we explain by the enhanced effective electronic mass in YbBe13 and by a shift of the chemical potential in the contact region due to the nonthermal occupation of excited crystal-field levels.

First-order phase transitions with strong and extremely anisotropic changes of the lattice parameters were observed in the ThCr${}_{2}$Si${}_{2}$ structure-type compounds EuCo${}_{2}$P${}_{2}$ and SrNi${}_{2}$P${}_{2}$. At room temperature, with increasing pressure the phase transition occurs in SrNi${}_{2}$P${}_{2}$ at 4 kbar and in EuCo${}_{2}$P${}_{2}$ at 30 kbar which is in the latter probably accompanied by a valence change of Eu. On the basis of single-crystal data of $A$Co${}_{2}$P${}_{2}$ $(A$ = Ca, Sr, La, Ce, Pr, Nd, Eu) at ambient pressure and temperature we discuss the pressure dependence of the bond lengths in these compounds.

The antiferromagnetic ordering temperatures of the rare earth (RE) moments in RE Ba2 Cu3O7-gd, obtained from specific heat measurements, are roughly in agreement with the de Gennes factors, i.e., the ordering mechanism can be mainly seen as an indirect spin-spin exchange. However, the oxygen dependence of T N is found to be reversed for the light rare earths compared to the heavy rare earths. As origin for this systematic observation an indirect interaction between the 3d-moments of copper and the 4f-moments of rare earths is discussed as a second order effect. Such an interaction is supported by measurements of the 4f relaxation behavior on the Nd 1: 2: 3 cuprates by inelastic magnetic neutron scattering. Here, the usual thermally driven increase of the magnetic relaxation rate is suppressed up to about 80 K. This correlates with the appearance of a spin gap found by Rossat- Mignod in YBa2Cu3O7 and therefore the 3d-4f coupling can be understood as an interaction of the 4f moments with a spin-fluctuation exchange in the CuO2 planes. Furthermore, the quasielastic magnetic response has a Gaussian contribution at temperatures below 100 K, i.e., much above the long ranged ordering temperatures T N. Magnon-like excitations appear already at slightly larger temperatures than T N. In addition the paramagnetic inelastic spectra show only little dependence of the crystal field scheme on the oxygen concentration.

Metallic point-contacts betweenU Be 13 and conventional superconductors (Ta, Nb, NbTi) behave likeS−N−S junctions without Josephson coupling, instead, the superconductivity in each electrode appears to be destroyed individually by different critical currents. In particulary, the part of theI–V characteristics due to theU Be 13-superconductivity is almost indistinguishable from that of contacts against normal metals. In magnetic fields of a few Tesla the junctions show an anomalous behaviour: theU Be 13 critical current increases with the field. This effect is absent in point-contacts with a normal metal counterelectrode.

The coefficients of the thermal expansion parallel (αa,b) and perpendicular (αc) to the CuO2 planes as well as the specific heat (cp) of a Bi2Sr2CaCu2O8+δ single crystal have been measured. A clear anomaly at Tc is present in αa,b whereas no corresponding anomaly could be observed in αc. The anomalies of cp as well as αa,b show pronounced fluctuations of the superconducting order parameter and in addition a small mean-field-like increase of αa,b below Tc can be resolved, where the relative size of the anomaly is much larger than in the specific heat. Comparing the anomalies of αa,b and cp an uniaxial pressure dependence for pressure parallel to the CuO2 planes of dTc/dpa,b = 0.9 (1) K/GPa is determined for Bi2212, which seems to represent the “intrinsic” pressure dependence of the high-Tc cuprates. Besides the anomaly at Tc we observe a jump-like increase of the thermal expansion parallel to the c-axis at about 120 K indicating a structural instability of Bi2Sr2CaCu2O8+δ at this temperature.