M. Weiden

NaV2O5 as a Quarter-Filled Ladder Compound

A new X-ray diffraction study of the one-dimensional spin-Peierls compound \alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of NaV_2O_5 and its magnetic properties.

Magnetic properties of new CeTMg compounds (T Ni, Pd)

Abstract We present results of the first investigation of ternary Ce-compounds containing a transition metal and Mg. The structure, resistivity and susceptibility of Ce 2 Ni 2 Mg, CeNi 4 Mg and CePdMg are reported and discussed. These compounds crystallise in the same structure as their respective In-homologues and show very similar structural parameters, but weaker strength of the Ce-4f-hybridization.

Transport and magnetic properties of new ternary Ce2T3X9-compounds (T = Rh, Ir, X = Al, Ga)

Abstract Measurements of thermoelectric power, electrical resistivity, and magnetic susceptibility on new Ce 2 T 3 X 9 -compounds (T=Rh, Ir; X=Al, Ga) are presented. These compounds crystallize in the orthorhombic Y 2 Co 3 Ga 9 -structure. The transport data show for each compound two characteristic anomalies, around 100 K and around 20 K, respectively. In contrast to many Ce-systems, where such anomalies can be ascribed to crystalline-field and Kondo effects, respectively, for these systems the temperature dependence of the susceptibility and the specific heat suggest an intermediate valent Ce-state which shows additional low energy interactions.

Investigation of the Spin-Peierls transition in CuGeO3 by Raman scattering

Abstract Raman experiments on the Spin-Peierls compound CuGeO3 and the substituted (Cu2−x,Znx)GeO3 and Cu(Ge1−x,GaxO3 compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-chain to spin anisotropies and substitution-induced disorder. In pure CuGeO3, in addition to normal phonon scattering which is not affected at all by the spin-peierls transition, four types of magnetic scattering features were observed. Below TSP = 14 K a singlet-triplet excitation at 30 cm−1, two-magnon scattering from 30 to 227 cm−1 and folded phonon modes at 369 and 819 cm−1 were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-peierls transition TSP and approsimately 100 K a broad intensity maximum centered at 300 cm−1 is observed. The temperature dependence of this intensity is similar to the behavior of lattice fluctuations recently observed in an electron diffraction study. This scattering is attributed to dynamical spin-peierls fluctuations of the weakly or non-static dimerized spin chain.

Doping effects in CuGeO3

Abstract We investigated the effect of doping on the crystallographic, dynamic and magnetic properties of (Cu1−zZnz)GeO3 as well as Cu(Ge1−xAx)O3 with A  Al, Si, Ga, As, In, Sn, Sb and Ti. A large solubility was only observed for Zn, Si and Ti. For those elements, the lattice parameters show a linear dependence on doping within the whole solubility range. The spin-Peierls transition temperature TSP decreases linearly with doping, though the slope is strongly dependent on the dopant. For Zn-doping, a Neel-like ground state which exhibits spin-flop behavior appears for higher doping after the complete disappearance of the spin-Peierls ground state. Si- and Ti-doped samples exhibit a similar phase in a coexistence with the spin-Peierls ground state. The analysis of the susceptibility in the spin-Peierls phase for low Zn content indicates a reduction of the dimerization within the Cu chains and the energy gap upon doping. Whereas for Si and Ti all phonon frequencies changed linearly with the doping level, we observed in Zn-doped samples for some phonons a clear minimum at the composition z = 0.02 where the spin-Peierls transition disappears, suggesting a strong spin-phonon coupling for these modes. For higher Zn concentrations, no lattice dimerization can be observed by Raman scattering.

Yb2Ni2Al: A prototypical Yb-based heavy-fermion system

Abstract We have investigated the properties of Yb2Ni2Al by means of resistivity, susceptibility and specific heat measurements. Our results suggest that it is the first magnetically non-ordered Yb heavy-fermion system with a low characteristic energy and clear coherence effects in the resistivity. A logarithmic temperature dependence of C/T indicates that it is close to the transition to a magnetic ordered ground state. We further found a simple systematic for composition dependence of the Yb-valence in binary and ternary YbTAl compounds (T: Ni, Pd, Pt).

Frustration-induced Raman scattering in CuGeO3

We present experimental data for the Raman intensity in the spin-Peierls compound CuGeO3 and theoretical calculations from a one-dimensional frustrated spin model. The theory is based on (a) exact diagonalization and (b) a recently developed solitonic mean field theory. We find good agreement between the 1D-theory in the homogeneous phase and evidence for a novel dimerization of the Raman operator in the spin-Peierls state. Finally we present evidence for a coupling between the interchain exchange, the spin-Peierls order parameter and the magnetic excitations along the

The zero-gap Ce3Au3Sb4-x system: a transport, thermodynamic and optical study

Abstract We report our resistivity, thermoelectric power and optical investigations on the Ce3Au3Sb4−x narrow-gap insulator. The resistivity for x=0.12 and 0.17 is characterized by a semiconducting-like behaviour at low temperatures, which contrasts with the metallic-like temperature dependence of the thermoelectric power. This apparent contradiction can be reconciled by a so-called “zero-gap” scenario, as supported by the optical investigation.

J1-J2 model revisited: Phenomenology of CuGeO3

We present a mean field solution of the antiferromagnetic Heisenberg chain with nearest (J1) and next to nearest neighbor (J2) interactions. This solution provides a way to estimate the effects of frustration. We calculate the temperature-dependent spin-wave velocity, vs(T) and discuss the possibility to determine the magnitude of frustration J2/J1 present in quasi 1D compounds from measurements of vs(T). We compute the thermodynamic susceptibility at finite temperatures and compare it with the observed susceptibility of the spin-Peierls compound CuGeO3. We also use the method to study the two-magnon Raman continuum observed in CuGeO3 above the spin-Peierls transition.

Magnetic order in Zr-doped CeNiSn

The effects of Zr doping on the low-temperature properties of CeNiSn were studied by means of measurements of the thermoelectric power, the electrical resistivity, the magnetic susceptibility and the specific heat. Even Zr concentrations as small as 3% were found to dramatically increase the low-temperature metallic character in this otherwise gapped material and to stabilize antiferromagnetic ordering below 5 K.