arXiv subjects

N. Hollmann

Publications and source records attributed to N. Hollmann.

23 records · Page 2Linked to original sources

arXiv2009

Spin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4

Using Co-L_(2,3) and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La_(1.5)Sr_(0.5)CoO4 involves high spin (S=3/2) Co^2+ and low spin (S=0) Co^3+ ions. This provides evidence for the spin blockade phenomenon as a source for the extremely insulating nature of the La_(2-x)Sr_(x)CoO4 series. The associated e_g^2 and e_g^0 orbital occupation accounts for the large contrast in the Co-O bond lengths, and in turn, the high charge ordering temperature. Yet, the low magnetic ordering temperature is naturally explained by the presence of the non-magnetic (S=0) Co^3+ ions. From the identification of the bands we infer that La_(1.5)Sr_(0.5)CoO4 is a narrow band material.

cond-mat.str-el↗

arXiv2008

Anisotropic Susceptibility of La_2-xSr_xCoO_4 related to the Spin States of Cobalt

We present a study of the magnetic susceptibility of La_2-xSr_xCoO_4 single crystals in a doping range 0.3<=x<=0.8. Our data shows a pronounced magnetic anisotropy for all compounds. This anisotropy is in agreement with a low-spin ground state (S=0) of Co^3+ for x>=0.4 and a high-spin ground state (S=3/2) of Co^2+. We compare our data with a crystal-field model calculation assuming local moments and find a good description of the magnetic behavior for x>=0.5. This includes the pronounced kinks observed in the inverse magnetic susceptibility, which result from the anisotropy and low-energy excited states of Co^2+ and are not related to magnetic ordering or temperature-dependent spin-state transitions.

cond-mat.str-el↗

arXiv2007

Insulator to semiconductor transition and magnetic properties of the one-dimensional S = 1/2 system In_2VO_5

We report structural, magnetization, electrical resistivity and nuclear- and electron spin resonance data of the complex transition metal oxide In_2VO_5 in which structurally well-defined V-O chains are realized. An itinerant character of the vanadium d-electrons and ferromagnetic correlations, revealed at high temperatures, are contrasted with the insulating behavior and predominantly antiferromagnetic exchange between the localized V^{4+} S = 1/2-magnetic moments which develop below a certain characteristic temperature T* ~ 120 K. Eventually the compound exhibits short-range magnetic order at $T_SRO ~ 20 K. We attribute this crossover occurring around T* to the unusual anisotropic thermal contraction of the lattice which changes significantly the overlap integrals and the character of magnetic intra- and interchain interactions.

cond-mat.str-el↗

arXiv2006

Thermodynamics of the coupled spin-dimer system TlCuCl3 close to a quantum phase transition

We present thermal expansion alpha, magnetostriction and specific heat C measurements of \tal, which shows a quantum phase transition from a spin-gap phase to a Neel-ordered ground state as a function of magnetic field around H_{C0}->4.8T. Using Ehrenfest's relation, we find huge pressure dependencies of the spin gap for uniaxial as well as for hydrostatic pressure. For T->0 and H->H_{C0} we observe a diverging Grueneisen parameter Gamma(T)=alpha/C, in qualitative agreement with theoretical predictions. However, the predicted individual temperature dependencies alpha(T) and C(T) are not reproduced by our experimental data.

cond-mat.str-el↗

arXiv2006

A New Multiferroic Material: MnWO4

We report the multiferroic behaviour of MnWO$_4$, a magnetic oxide with monoclinic crystal structure and spiral long-range magnetic order. Based upon recent theoretical predictions MnWO$_4$ should exhibit ferroelectric polarization coexisting with the proper magnetic structure. We have confirmed the multiferroic state below 13 K by observing a finite electrical polarization in the magnetically ordered state via pyroelectric current measurements.

cond-mat.str-el↗