arXiv subjects

B. Wolf

Publications and source records attributed to B. Wolf.

28 records · Page 2Linked to original sources

arXiv2007

Anomalous Lattice Response at the Mott Transition in a Quasi-2D Organic Conductor

Discontinuous changes of the lattice parameters at the Mott metal-insulator transition are detected by high-resolution dilatometry on deuterated crystals of the layered organic conductor $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. The uniaxial expansivities uncover a striking and unexpected anisotropy, notably a zero-effect along the in-plane c-axis along which the electronic interactions are relatively strong. A huge thermal expansion anomaly is observed near the end-point of the first-order transition line enabling to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent $\tildeα \simeq$ 0.8 $\pm$ 0.15 at odds with the novel criticality recently proposed for these materials [Kagawa \textit{et al.}, Nature \textbf{436}, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.

cond-mat.str-el↗

arXiv2007

Nature of the spin dynamics and 1/3 magnetization plateau in azurite

We present a specific heat and inelastic neutron scattering study in magnetic fields up into the 1/3 magnetization plateau phase of the diamond chain compound azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$. We establish that the magnetization plateau is a dimer-monomer state, {\it i.e.}, consisting of a chain of $S = 1/2$ monomers, which are separated by $S = 0$ dimers on the diamond chain backbone. The effective spin couplings $J_{mono}/k_B = 10.1(2)$ K and $J_{dimer}/k_B = 1.8(1)$ K are derived from the monomer and dimer dispersions. They are associated to microscopic couplings $J_1/k_B = 1(2)$ K, $J_2/k_B = 55(5)$ K and a ferromagnetic $J_3/k_B = -20(5)$ K, possibly as result of $d_{z^2}$ orbitals in the Cu-O bonds providing the superexchange pathways.

cond-mat.stat-mech↗

arXiv2004

Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-2D organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br: quest for intrinsic scattering contributions

Resistivity measurements have been performed on a low (LR)- and high (HR)-resistance variant of the kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br superconductor. While the HR sample was synthesized following the standard procedure, the LR crystal is a result of a somewhat modified synthesis route. According to their residual resistivities and residual resistivity ratios, the LR crystal is of distinctly superior quality. He-gas pressure was used to study the effect of hydrostatic pressure on the different transport regimes for both variants. The main results of these comparative investigations are (i) a significant part of the inelastic-scattering contribution, which causes the anomalous rho(T) maximum in standard HR crystals around 90 K, is sample dependent, i.e. extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a strongly temperature-dependent behavior at T > T* to an only weakly T-dependent rho(T) at T < T* is unaffected by this scattering contribution and thus marks an independent property, most likely a second-order phase transition, (iii) both variants reveal a rho(T) proportional to AT^2 dependence at low temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent coefficients A and upper bounds for the T^2 behavior measured by T_0. The latter result is inconsistent with the T^2 dependence originating from coherent Fermi-liquid excitations.

cond-mat.str-el↗

arXiv2002

Pristine CNO abundances from Magellanic Cloud B stars I. The LMC cluster NGC 2004 with UVES

We present chemical abundances for four main sequence B stars in the young cluster NGC 2004 in the Large Magellanic Cloud (LMC). Apart from H II regions, unevolved OB-type stars are currently the only accessible source of present-day CNO abundances for the MCs not altered by stellar evolution. Using UVES on the VLT, we obtained spectra of sufficient resolution (R = 20 000) and signal-to-noise (S/N > 100) to derive abundances for a variety of elements (He, C, N, O, Mg and Si) with NLTE line formation. This study doubles the number of main sequence B stars in the LMC with detailed chemical abundances. More importantly and in contrast to previous studies, we find no CNO abundance anomalies brought on by e.g. binary interaction or rotational mixing. Thus, this is the first time that abundances from H II regions in the LMC can sensibly be cross-checked against those from B stars by excluding evolutionary effects. We confirm the H II-region CNO abundances to within the errors, in particular the extraordinarily low nitrogen abundance of epsilon(N) around 7.0. Taken at face value, the nebular carbon abundance is 0.16 dex below the B-star value which could be interpreted in terms of interstellar dust depletion. Oxygen abundances from the two sources agree to within 0.03 dex. In comparison with the Galactic thin disk at MC metallicities, the Magellanic Clouds are clearly nitrogen-poor environments.

astro-ph↗

arXiv2000

Soundwave Anomalies in SrCu_{2}(BO_{3})_{2}

The temperature and high magnetic field dependence of the longitudinal soundwave mode c_{11} in the two dimensional dimer system SrCu_{2}(BO_{3})_{2} is presented. c_{11}(T) shows anomalies due to strong interdimer spin-strain coupling. We can quantitatively interpret the temperature dependence of c_{11}(T) together with the magnetic susceptibility $χ_{m}(T)$ with a molecular field approximation of coupled dimer triplets. The sound velocity up to 50T shows very sharp softening between the magnetization plateaus at low temperature. We argue that these pronounced effects arise from a resonant interaction between the phonons and the magnetic excitations which show softening between the plateaus.

cond-mat.str-el↗

arXiv1999

Electron renormalization of sound interaction with two-level systems in superconducting metglasses

The crossing of temperature dependencies of sound velocity in the normal and the superconducting state of metallic glasses indicates renormalization of the intensity of sound interaction with two-level systems (TLS) caused by their coupling with electrons. In this paper we have analyzed different approaches to a quantitative description of renormalization using the results of low-temperature ultrasonic investigation of ZrTiCuNiBe amorphous alloy. It is shown that the adiabatic renormalization of the coherent tunneling amplitude can explain only part of the whole effect observed in the experiment. There exists another mechanism of renormalization affecting only nearly symmetric TLS.

cond-mat↗

arXiv1999

Sound attenuation in the superconducting amorphous alloy ZrTiCuNiBe

The superconducting energy gap and the parameter of the intensity of electron scattering at two-level systems in amorphous ZrTiCuNiBe are determined from the results of measurements of sound attenuation. The mechanism of adiabatic renormalization of the amplitude of coherent tunneling is used for a quantitative description of the peculiarities of sound absorption in the vicinity of critical temperature.

cond-mat↗

arXiv1999

High Magnetic Field ESR in the Haldane Spin Chains NENP and NINO

We present electron spin resonance experiments in the one-dimensional antiferromagnetic S=1 spin chains NENP and NINO in pulsed magnetic fields up to 50T. The measured field dependence of the quantum energy gap for B||b is analyzed using the exact diagonalization method and the density matrix renormalization group method (DMRG). A staggered anisotropy term (-1)^i d(S_i^x S_i^z + S_i^z S_i^x) was considered for the first time in addition to a staggered field term (-1)^i S_i^x B_st. We show that the spin dynamics in high magnetic fields strongly depends on the orthorhombic anisotropy E.

cond-mat.str-el↗

arXiv1998

ESR Modes in CsCuCl3 in Pulsed Magnetic Fields

We present ESR results for 35-134GHz in the antiferromagnet CsCuCl3 at T=1.5K. The external field is applied perpendicular to the hexagonal c-axis. With our pulsed field facility we reach 50T an unprecedented field for low temperature ESR. We observe strong resonances up to fields close to the ferromagnetic region of ~30T. These results are discussed in a model for antiferromagnetic modes in a two-dimensional frustrated triangular spin system.

cond-mat.str-el↗

arXiv1998

Electron renormalization of sound interaction with two-level systems in superconducting metglasses

The crossing of temperature dependences of the sound velocity in normal and superconducting state of metallic glasses points out unambiguously the renormalization of the intensity of sound interaction with two-level systems (TLS), caused by their coupling with electrons. We propose a simple scenario which allows us to describe qualitatively the influence of the superconducting transition on the sound velocity and attenuation in superconducting metglasses and to make quantative estimations of renormalization parameters.

cond-mat↗