K S Liang | PubMed Health

PubMed2006

Low-energy charge-density excitations in MgB2: Striking interplay between single-particle and collective behavior for large momenta.

A sharp feature in the charge-density excitation spectra of single-crystal MgB2, displaying a remarkable cosinelike, periodic energy dispersion with momentum transfer (q) along the c* axis, has been observed for the first time by high-resolution nonresonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-q collective mode residing in the single-particle excitation gap of the B pi bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB2.

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PubMed2006

Orbitally driven spin-singlet dimerization in S=1 La4Ru2O10.

Using x-ray absorption spectroscopy at the Ru-L2,3 edge we reveal that the Ru4+ ions remain in the S=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U band structure calculations that the crystal fields in the low-temperature phase are not strong enough to stabilize the S=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La4Ru2O10 appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi-two-dimensional S=1 system.

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PubMed2005

Hyperhomocysteinemia is associated with volumetric white matter change in patients with small vessel disease.

BACKGROUND: Hyperhomocysteinemia is associated with cerebral small vessel disease (SVD). We examined the relationship between homocysteine and 1) volumetric measure of white matter change (WMC), 2) silent brain infarcts, 3) cerebral atrophy on MRI and 4) cognition on a consecutive cohort of patients with stroke associated with SVD. SUBJECTS AND METHODS: Fifty-seven patients consecutively admitted to the Acute Stroke Unit in a university hospital due to stroke associated with SVD were recruited and assessed three months after the stroke. Non-fasting homocysteine was obtained. Using MRI, the number of infarcts, volume of WMC and cerebral atrophy were measured. General cognitive functions were assessed using the Mini Mental State Examination and Alzheimer's disease Assessment Scale. Mattis Dementia Rating Scale - Initiation/Perseveration subset was used to assess executive cognitive functions. RESULTS: Hyperhomocysteinemia (> or = 14.88 micromol/L) significantly accounted for the volume of WMC on MRI in a multivariate stepwise regression model (adjusted R(2)=0.058, p <0.05) after adjustment for age and folate level. Patients in the highest quartile of WMC volume had significantly higher levels of homocysteine than those in lowest quartile (p <0.001). No significant relationship was found between homocysteine and silent brain infarcts, cerebral atrophy and performance on psychometric tests. CONCLUSION: Hyperhomocysteinemia is associated with volumetric measure of WMC among patients with SVD. The role of homocysteine in the development of silent brain infarcts and cerebral atrophy as previously reported cannot be ascertained in this study. No direct relationship was found between homocysteine and cognitive functions.

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