arXiv subjects

C. Geibel

Publications and source records attributed to C. Geibel.

175 records · Page 10Linked to original sources

arXiv1998

NaV_2O_5 as a quarter-filled ladder compound

A new X-ray diffraction study of the one-dimensional spin-Peierls compound α-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.

cond-mat↗

arXiv1997

Magnetic Resonance in the Spin-Peierls compound $α'-NaV_2O_5$

We present results from magnetic resonance measurements for 75-350 GHz in $α$'-NaV$_{2}$O$_{5}$. The temperature dependence of the integrated intensity indicates that we observe transitions in the excited state. A quantitative description gives resonances in the triplet state at high symmetry points of the excitation spectrum of this Spin-Peierls compound. This energy has the same temperature dependence as the Spin-Peierls gap. Similarities and differences with the other inorganic compound CuGeO$_{3}$ are discussed.

cond-mat↗

arXiv1997

Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline alpha'-NaV2O5. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K slightly below the onset of a second-order transition at T2onset around (34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T1 induced by the incipient spin-Peierls (SP) order parameter above T2=TSP.

cond-mat↗

arXiv1997

Normal-State Hall Effect and the Insulating Resistivity of High-T_c Cuprates at Low Temperatures

The normal-state Hall coefficient R_H and the in-plane resistivity ρ_{ab} are measured in La-doped Bi_{2}Sr_{2}CuO_{y} (T_c \simeq 13 K) single crystals and La_{2-x}Sr_{x}CuO_{4} thin films by suppressing superconductivity with 61-T pulsed magnetic fields. In contrast to data above T_c, the R_H below \sim 10 K shows little temperature dependence in all the samples measured, irrespective of whether ρ_{ab} exhibits insulating or metallic behavior. Thus, whatever physical mechanism gives rise to insulating behavior in the low-temperature normal state, it leaves the Hall conductivity relatively unchanged.

cond-mat.supr-con↗

arXiv1997

Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3

The effect of in-chain and off-chain substitutions on 1D spin fluctuations in the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in order to understand the interplay between defect induced states, enhanced spin-spin correlations and the ground state of low dimensional systems. In-chain and off-chain substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D gap-induced mode as well as a constant intensity of a spinon continuum are observed at low temperatures. A 3D two-magnon density of states now gradually extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This effect is more pronounced in the case of off-chain substitutions (Si) for which a Néel state occurs over a larger substitution range, starting at very low concentrations. Besides, additional low energy excitations are induced. These effects, i.e. the shift of a dimensional crossover to higher temperatures are due to an enhancement of the spin-spin correlations induced by a small amount of substitutions. The results are compared with recent Monte Carlo studies on substituted spin ladders, pointing to a similar instability of coupled, dimerized spin chains and spin ladders upon substitution.

cond-mat.str-el↗

arXiv1997

Experimental evidence for a spin-Peierls-transition in $α'-NaV_2O_5$

We present the first measurements of magnetisation and Raman light scattering on $α'-NaV_2O_5$ single crystals. Below 34 K, we observe a pronounced isotropic decrease of the susceptibility indicating the opening of a spin gap. The transition temperature is slightly field dependent. Raman experiments reveal a crystallographic distortion at the transition. Our results clearly establish $α'-NaV_2O_5$ to be the second inorganic spin-Peierls system.

cond-mat.str-el↗

arXiv1996

Magnon-magnon interactions in the Spin-Peierls compound CuGeO_3

In a magnetic substance the gap in the Raman spectrum, Delta_R, is approximatively twice the value of the neutron scattering gap, Delta_S, if the the magnetic excitations (magnons) are only weakly interacting. But for CuGeO_3 the experimentally observed ratio Delta_R/Delta_S is approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in the quasi-1D Spin-Peierls compound CuGe_3. We present numerical estimates for Delta_R/Delta_S from exact diagonalization studies for finite chains and find agreement with experiment for intermediate values of the frustration parameter alpha. An analysis of the numerical Raman intensity leads us to postulate a continuum of two-magnon bound states in the Spin-Peierls phase. We discuss in detail the numerical method used, the dependence of the results on the model parameters and a novel matrix-element effect due to the dimerization of the Raman-operator in the Spin-Peierls phase.

cond-mat.str-el↗

arXiv1996

Break junctions of the heavy-fermion superconductors

Mechanical-controllable break junctions of the heavy-fermion superconductors can show Josephson-like superconducting anomalies. But a systematic study on the contact size demonstrates that these anomalies are mainly due to Maxwell's resistance being suppressed in the superconducting heavy-fermion phase. Up to day, we could not find any superconducting features by vacuum-tunnelling spectroscopy, providing further evidence for the pair-breaking effect of the heavy-fermion interfaces.

cond-mat.supr-con↗

arXiv1996

Substitution effects on spin fluctuations in the spin-Peierls compound CuGeO_3

Using Raman scattering we studied the effect of substitutions on 1D spin fluctuations in CuGeO_3 observed as a spinon continuum in frustration induced exchange scattering. For temperatures below the spin-Peierls transition (T_{SP}=14K) the intensity of this continuum at 120-500 cm^{-1} is exponentially suppressed and transferred into a 3D two-magnon density of states. Besides a spin-Peierls gap-induced mode at 30 cm^{-1} and additional modes at 105 and 370 cm^{-1} are observed. Substitution of Zn on the Cu-site and Si on the Ge-site of CuGeO_3 quenches easily the spin-Peierls state. Consequently a suppression of the spin-Peierls gap observable below T_{SP}=14K as well as a change of the temperature dependence of the spinon continuum are observed. These effects are discussed in the context of a dimensional crossover of this compound below T_{SP} and strong spin-lattice interaction.

cond-mat.str-el↗

arXiv1996

The J_1-J_2 model revisited : Phenomenology of CuGeO_3

We present a mean field solution of the antiferromagnetic Heisenberg chain with nearest (J_1) and next to nearest neighbor (J_2) interactions. This solution provides a way to estimate the effects of frustration. We calculate the temperature-dependent spin-wave velocity, v_s(T) and discuss the possibility to determine the magnitude of frustration J_2/J_1 present in quasi 1D compounds from measurements of v_s(T). We compute the thermodynamic susceptibility at finite temperatures and compare it with the observed susceptibility of the spin-Peierls compound CuGeO_3. We also use the method to study the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls transition.

cond-mat.str-el↗

arXiv1996

Frustration induced Raman scattering in CuGeO_3

We present experimental data for the Raman intensity in the spin-Peierls compound CuGeO_3 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 chains.

cond-mat↗

arXiv1996

Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering

Raman experiments on the spin-Peierls compound CuGeO$_3$ and the substituted (Cu$_{1- x}$,Zn$_x$)GeO$_3$ and Cu(Ge$_{1-x}$,Ga$_x$)O$_3$ compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-1/2 chain to spin anisotropies and substitution-induced disorder. In pure CuGeO$_3$, 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 T$_{SP}$=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 T$_{SP}$ and approximately 100 K a broad intensity maximum centered at 300 cm$^{-1}$ is observed.

cond-mat↗

arXiv1994

Magnetic properites of the Heavy Fermion Antiferromagnets YbNiAl and YbPtAl

Measurements of electrical resistivity and Hall effect as a function of magnetic field on the Heavy Fermion Systems YbNiAl and YbPtAl are presented. Both compounds order antiferro magnetically and show metamagnetic behavior in a magnetic field. Scaling behavior of the magnetoresistance above TN suggests that the paramagnetic regime for YbNiAl can be described in terms of a single ion Kondo effect.

cond-mat↗