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T Lorenz

Publications and source records attributed to T Lorenz.

At least 19 recordsLinked to original sources
PubMed2006

Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism.

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L(2,3) edge, we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.

Journal Article↗
PubMed2006

Raman scattering in the Mott insulators LaTiO3 and YTiO3: evidence for orbital excitations.

Raman scattering is used to observe pronounced electronic excitations around 230 meV--well above the two-phonon range--in the Mott insulators LaTiO3 and YTiO3. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.

Journal Article↗
PubMed2006

Spin thermal conductivity of the haldane chain compound Y(2)BaNiO(5).

We have measured the thermal conductivity of the spin S=1 chain compound Y(2)BaNiO(5). Analyzing the anisotropy of the thermal transport allows us to identify a definite spin-mediated thermal conductivity kappa(s) along the chain direction. The calculated spin-related energy diffusion constant D(E)(T) shows a broad peak around 120 K. Close to room temperature, D(E)(T) approaches the theoretically predicted high-temperature value, while scattering of spin excitations by magnetic impurities seems to be the major limiting factor of kappa(s) at low temperature.

Journal Article↗
PubMed2005

Structural aspects of metamagnetism in Ca2-xSrxRuO4: evidence for field tuning of orbital occupation.

The crystal structure of Ca(2-x)Sr(x)RuO(4) with 0.2 < or = x < or = 1.0 has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the c direction and elongate in the a and b planes, whereas the opposite occurs upon cooling at high field (x = 0.2 and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.

Journal Article↗
PubMed2005

Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl.

We report on the magnetic, thermodynamic, and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g., the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.

Journal Article↗
PubMed2005

Magnetoelastic coupling in the spin-dimer system TlCuCl3.

Based on high-resolution measurements of the thermal expansion and the magnetostriction we analyze the magnetoelastic coupling of TlCuCl3. We find that the phase boundary between the low-field spin liquid phase and the magnetic-field-induced Néel phase is extremely sensitive to pressure and would change by about +/-185%/GPa depending on the direction of uniaxial pressure. This drastic effect can unambiguously be traced back to the pressure dependencies of the intradimer coupling, whereas changes of the interdimer couplings under pressure play a minor role. The observed strong decrease of the interdimer coupling, when Tl is substituted by K, is therefore not a consequence of chemical pressure.

Journal Article↗
PubMed2005

Determination of the orbital moment and crystal-field splitting in LaTiO3.

Utilizing a sum rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO3 is strongly reduced from its ionic value, both below and above the Ne el temperature. Using Ti L2,3 x-ray absorption spectroscopy as a local probe, we found that the crystal-field splitting in the t2g subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.

Journal Article↗
PubMed2004

Different look at the spin state of Co(3+) ions in a CoO(5) pyramidal coordination.

Using soft-x-ray absorption spectroscopy at the Co L(2,3) and O K edges, we demonstrate that the Co3+ ions with the CoO5 pyramidal coordination in the layered Sr2CoO3Cl compound are unambiguously in the high spin state. Our result questions the reliability of the spin state assignments made so far for the recently synthesized layered cobalt perovskites and calls for a reexamination of the modeling for the complex and fascinating properties of these new materials.

Journal Article↗
PubMed2002

Evidence for spin-charge separation in quasi-one-dimensional organic conductors.

Interacting conduction electrons are usually described within Fermi-liquid theory, which states that, in spite of strong interactions, the low-energy excitations are electron-like quasiparticles with charge and spin. In recent years there has been tremendous interest in conducting systems that are not Fermi liquids, motivated by the observation of highly anomalous metallic states in various materials, most notably the copper oxide superconductors. Non-Fermi-liquid behaviour is generic to one-dimensional interacting electron systems, which are predicted to be Luttinger liquids. One of their key properties is spin-charge separation: instead of quasiparticles, collective excitations of charge (with no spin) and spin (with no charge) are formed, which move independently and at different velocities. However, experimental confirmation of spin-charge separation remains a challenge. Here we report experiments probing the charge and heat current in quasi-one-dimensional conductors--the organic Bechgaard salts. It was found that the charge and spin excitations have distinctly different thermal conductivities, which gives strong evidence for spin-charge separation. The spin excitations have a much larger thermal conductivity than the charge excitations, which indicates that the coupling of the charge excitations to the lattice is important.

Journal Article↗
PubMed2002

Experimental quest for orbital waves.

One challenge in condensed-matter physics is the experimental confirmation of a new kind of elementary excitation orbital waves, or orbitons, which are predicted to exist in an orbitally ordered state. Saitoh et al. have observed three peaks at 160, 144 and 126 meV in the Raman scattering of orbitally ordered lanthanum manganate (LaMnO(3)), and interpret these as evidence of orbitons. However, we find similar peaks in the optical conductivity, sigma(omega), of LaMnO(3) and point out that the direct observation of orbitons in sigma(omega) is prohibited by a selection rule. This suggests that the Raman peaks observed by Saitoh et al. arise from multiphonons, and that the existence of orbitons has yet to be experimentally confirmed.

Journal Article↗
PubMed2001

Strong damping of phononic heat current by magnetic excitations in SrCu2(BO3)(2).

Measurements of the thermal conductivity as a function of temperature and magnetic field in the 2D dimer spin system SrCu2(BO3)(2) are presented. In zero magnetic field the thermal conductivity along and perpendicular to the magnetic planes shows a pronounced double-peak structure as a function of temperature. The low-temperature maximum is drastically suppressed with increasing magnetic field. Our quantitative analysis reveals that the heat current is due to phonons and that the double-peak structure arises from pronounced resonant scattering of phonons by magnetic excitations.

Journal Article↗
PubMed2001

Differential effects of citrate versus PPACK anticoagulation on measured platelet inhibition by abciximab, eptifibatide and tirofiban.

BACKGROUND: High levels of glycoprotein (GP) IIb/IIIa receptor inhibition are required to prevent arterial thrombosis following percutaneous coronary intervention. Ex-vivo turbidometric platelet aggregation in citrate anticoagulated blood samples has been the primary method previously utilized to derive dose regimens for administering platelet GP IIb/IIIa inhibitors. Enhanced GP IIb/IIIa binding and inhibition of platelet aggregation for eptifibatide secondary to citrate induced reduction of ionized plasma calcium concentrations has been reported. METHODS/RESULTS: We evaluated the differential effects of citrate versus PPACK anticoagulation on turbidometric platelet inhibition in normal volunteers by eptifibatide, tirofiban or abciximab. The decrease in ionized calcium afforded by citrate was associated with enhanced in vitro platelet inhibition for all three GP IIb/IIIa inhibitors, including abciximab. The magnitude of citrate effect was greatest for eptifibatide. Both tirofiban and abciximab have similar citrate calcium chelation associated enhancement of measured platelet inhibition. CONCLUSION: Accurate assessment and comparison of platelet inhibition by GP IIb/IIIa inhibitors may require avoidance of calcium chelating anticoagulants.

Abciximab↗
PubMed1997

Differential regulation of glomerular gelatinase B (MMP-9) and tissue inhibitor of metalloproteinase-1 (TIMP-1) in obese Zucker rats.

The obese Zucker rat represents a model of obesity combined with insulin resistance and hyperlipidaemia, which over a period of several months develops spontaneous glomerulosclerosis. The present study addressed the question as to whether glomerular sclerosis was associated with alterations in the degradation of matrix components. In the early phase (up to 6 months) glomeruli from obese rats displayed increased total collagen content (+64%) and decreased gelatinolytic activity (-34%) as compared to lean control animals. This decline in glomerular gelatinolytic activity was due to a reduction in gelatinase B [matrix metalloproteinase (MMP)-9]. Glomerular MMP-9 mRNA was reduced 4.6 +/- 0.6-fold (n = 3; p < 0.05), MMP-9 protein was not detectable by Western blotting and MMP-9 activity was considerably suppressed in gelatin zymograms. MMP-2, in terms of mRNA expression and activity, was unchanged. Tissue inhibitor of metalloproteinases (TIMP)-1 mRNA expression, TIMP-1 protein (immunohistochemistry) and TIMP-1 activity (reverse zymography) were enhanced in glomeruli from obese rats, while TIMP-2 mRNA remained unchanged. Moreover, mRNA for the alpha 1 IV collagen chain was 2.1 +/- 0.8-fold higher in glomeruli isolated from obese animals (n = 3; p < 0.05). These findings indicate that matrix expansion in glomeruli from obese Zucker rats is due to both enhanced synthesis of matrix components as well as reduced degradation by matrix metalloproteinases. Apparently the latter effect is based on a reduction in MMP-9 and up-regulation of its inhibitor TIMP-1.

Albuminuria↗
PubMed1996

[Polyneuropathy, diarrhea].

A 44-year-old patient with 24 kg reduction of body weight within three years due to recurrent diarrhea was admitted to the hospital. Physical findings of the patient were not remarkable, except for hypesthesia and hypalgesia of the front of the tibia and of the lower legs. Laboratory as well as endoscopic and functional examinations of the gastrointestinal tract did not reveal any remarkable finding. Infectious origins of the disease were excluded. A histological examination of a rectum specimen yielded amyloid deposits in the submucosa of the rectum. In the family history of the patient, relatives with liver and gastrointestinal diseases due to amyloidosis were detected. Amyloid deposits and the positive family history of the patient led to the diagnosis of a gastrointestinal manifestation of familial amyloidosis.

Adult↗