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U. Löw

Publications and source records attributed to U. Löw.

9 recordsLinked to original sources
arXiv2013

Magneto-elastic Effects in $\text{Tb}_{3}\text{Ga}_{5}\text{O}_{12}$

We report new results for the elastic constants studied in Faraday and Cotton-Mouton geometry in Tb$_3$Ga$_5$O$_{12}$ (TGG), a frustrated magnetic substance with strong spin-phonon interaction and remarkable crystal-electric-field (CEF) effects. We analyze the data in the framework of CEF theory taking into account the individual surroundings of the six inequivalent Tb$^{3+}$-ion positions. This theory describes both, elastic constants in the magnetic field and as a function of temperature. Moreover we present sound-attenuation data for the acoustic Cotton-Mouton effect in TGG.

cond-mat.mtrl-sci↗
arXiv2012

Magnetization, Magnetic Susceptibility and ESR in Tb3Ga5O12

We report on the measurement of the magnetic susceptibility and of ESR transitions in the garnet substance Tb$_3$Ga$_5$O$_{12}$ (TGG). The results are compared with a calculation in the framework of crystal field theory for the orthorhombic surroundings of the six inequivalent Tb ions of TGG. We also present a calculation of the magnetization for the three main crystal directions.

cond-mat.str-el↗
arXiv2003

Superstructures at low spin - high spin transitions

In many transition metal compounds, in particular those containing Fe^{2+} and Co^{3+}, there occur spin-state transitions between low-spin and high-spin (or intermediate-spin) states. We show that typical interactions between similar spin-state ions are short-range repulsion, and long-range interaction which can have different sign depending on the elastic anisotropy of the lattice and on the direction between respective ions. Due to such character of effective interactions at the spin-state transitions there may occur different superstructures -- ordered arrangement of different spin-states, which in particular may have the form of stripes. The properties of the system TlSr_2CoO_5 for which such a superstructure was recently observed experimentally, are discussed from this point of view.

cond-mat.str-el↗
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↗
arXiv1997

Magnetic Properties of the low dimensional spin system (VO)$_2$P$_2$O$_{7}$: ESR and susceptibility

Experimental results on magnetic resonance (ESR) and magnetic susceptibility are given for single crystalline (VO)$_2$P$_2$O$_{7}$. The crystal growth procedure is briefly discussed. The susceptibility is interpreted numerically using a model with alternating spin chains. We determine $J$=51 K and $δ$=0.2. Furthermore we find a spin gap of $\approx 6$meV from our ESR measurements. Using elastic constants no indication of a phase transition forcing the dimerization is seen below 300 K.

cond-mat↗
arXiv1997

Dynamical properties of the compounds CuGeO_3 and α'--NaV_2O_5 at temperatures $T\neq 0$

The dynamical structure factor of the frustrated spin 1/2 Heisenberg model for a series of frustration parameters at finite temperatures is presented. A sharp upper boundary of the spinon continuum is found. A simple method to extract the nearest neighbour coupling J from the spectral width is suggested. Signatures of frustration are discussed and a comparison with neutron inelastic scattering data is given.

cond-mat↗
arXiv1997

Reexamination of the microscopic couplings of the quasi one-dimensional antiferromagnet CuGeO_3

Experimental data for the magnetic susceptibility and magnetostriction of CuGeO_3 are analyzed within a one-dimensional antiferromagnetic model with nearest ({J_1}) and next-nearest neighbour interactions ({J_2}). We show that the ratio of the exchange constants in the antiferromagnetic chains of CuGeO_3 amounts to ${J_2}/{J_1}$ = 0.354(0.01), i.e. it is significantly larger than the critical value for the formation of a spontaneous gap in the magnetic excitation spectrum without lattice dimerization. The susceptibility data are reproduced by our numerical results over the temperature range from 20K to 950K to a high degree of accuracy for ${J_1} = 80.2 (3.0)$ and ${J_2} = 28.4 (1.8)$. The pressure dependence of the exchange constants is estimated from magnetostriction data. Furthermore, the specific heat data are checked on consistency against the calculated entropy of the above model.

cond-mat.str-el↗
arXiv1995

COMPLETE SOLUTION OF THE XXZ-MODEL ON FINITE RINGS. DYNAMICAL STRUCTURE FACTORS AT ZERO TEMPERATURE.

The finite size effects of the dynamical structure factors in the XXZ-model are studied in the euclidean time $(τ)$-representation. Away from the critical momentum $p=π$ finite size effects turn out to be small except for the large $τ$ limit. The large finite size effects at the critical momentum $p=π$ signal the emergence of infrared singularities in the spectral $(ω)$-representation of the dynamical structure factors.

cond-mat↗