Biomedical subjects
Publications and source records attributed to D J Singh.
We show using density functional calculations that the small e'g Fermi surfaces in NaxCoO2 are destroyed by Na disorder. This provides a means to resolve the prediction of these sections in band structure calculations with their nonobservation in angle resolved photoemission experiments.
The electronic structure of the layered 4d transition metal oxide Sr2RhO4 is investigated by angle resolved photoemission. We find well-defined quasiparticle excitations with a highly anisotropic dispersion, suggesting a quasi-two-dimensional Fermi-liquid-like ground state. Markedly different from the isostructural Sr2RuO4, only two bands with dominant Rh 4dxz,zy character contribute to the Fermi surface. A quantitative analysis of the photoemission quasiparticle band structure is in excellent agreement with bulk data. In contrast, it is found that state-of-the-art density functional calculations in the local density approximation differ significantly from the experimental findings.
Density functional calculations for K(0.5)Li(0.5)NbO(3) show strong A-site driven ferroelectricity, even though the average tolerance factor is significantly smaller than unity and there is no stereochemically active A-site ion. This is due to the frustration of tilt instabilities by A-site disorder. There are very large off centerings of the Li ions, which contribute strongly to the anisotropy between the tetragonal and rhombohedral ferroelectric states, yielding a tetragonal ground state even without strain coupling.
Density functional calculations of the fermiology and magnetic properties of Ca3Ru2O7 reveal an unusual state: a bulk spin valve. The ground state consists of nearly half-metallic bilayers stacked antiferromagnetically with a weak coupling. Out of plane transport is very strongly suppressed by the antiferromagnetic alignment, which can be destroyed in favor of ferromagnetism at low energy cost. Furthermore, the spin transport in the ferromagnetic state is highly unusual; opposite sign spin polarizations are found for currents in plane and out of plane.
We report an electrical transport study in Ca2-xSrxRuO4 single crystals at high magnetic fields (B). For x=0.2, the Hall constant Rxy decreases sharply at an anisotropic metamagnetic transition, reaching its value for Sr2RuO4 at high fields. A sharp decrease in the coefficient of the resistivity T2 term and a change in the structure of the angular magnetoresistance oscillations for B rotating in the planes confirms the reconstruction of the Fermi surface. Our observations and local-density-approximation calculations indicate a strong dependence of the Fermi surface on Ca concentration and suggest the coexistence of itinerant and localized electronic states in single layered ruthenates.
Pyrochlore Y2Nb2O7 is studied with density functional calculations. In the ideal pyrochlore structure, no magnetism is found, consistent with experiments, but the band structure is metallic. The phonon dispersions show unstable modes corresponding to charge instabilities. These frustrated instabilities lead to a metal-insulator transition with the formation of "charge singlets". Partial substitution of Ti for Nb results in moment formation due to the occurrence of Ti3+.
We calculated the one-electron susceptibility of hydrated NaxCoO2 and find strong nesting, involving about 70% of all electrons at the Fermi level and nearly commensurate with a 2 x 2 superstructure. This nesting creates a tendency to a charge density wave compatible with the charge order often seen at x approximately 0.5 and usually ascribed to electrostatic repulsion of Na ions. In the spin channel, it leads to strong spin fluctuations, which should be important for superconductivity. The state most compatible with this nesting structure is an odd-gap triplet s-wave state.
Ni3Al and Ni3Ga are closely related materials on opposite sides of a ferromagnetic quantum critical point. The Stoner factor of Ni is virtually the same in both compounds and the density of states is larger in Ni3Ga. Thus in Stoner theory it should be more magnetic, and in local-density approximation (LDA) calculations it is. However, experimentally it is a paramagnet, while Ni3Al is an itinerant ferromagnet. We show that critical spin fluctuations are stronger in Ni3Ga, due to weaker q dependence of the susceptibility, and this effect is enough to reverse the trend. The approach combines LDA calculations with Landau theory and the fluctuation-dissipation theorem using the same momentum cutoff for both compounds. The calculations provide evidence for strong, beyond LDA, spin fluctuations associated with the critical point in both materials, but stronger in Ni3Ga than in Ni3Al.
A class of materials known as "ferroelectric metals" was discussed theoretically by Anderson and Blount in 1965 [Phys. Rev. Lett. 14, 217 (1965)]], but to date no examples of this class of materials have been reported. Here we present measurements of the elastic moduli of Cd2Re2O7 through the 200 K cubic-to-tetragonal phase transition. A Landau analysis of the moduli reveals that the transition is consistent with Cd2Re2O7 being classified as a ferroelectric metal in the weaker sense described by Anderson and Blount (loss of a center of symmetry). First-principles calculations of the lattice instabilities indicate that the dominant lattice instability corresponds to a twofold degenerate mode with E(u) symmetry and that motions of the O ions forming the O octahedra dominate the energetics of the transition.
It has long been suspected that spin fluctuations in ZrZn2 may lead to a triplet superconductivity. We point out another possibility, an inhomogeneous singlet (Fulde-Ferrell) state. We calculated the electronic structure, as well as the zone center phonons and their coupling with electrons. We find that the exchange splitting is nonuniform and the Fermi surface exhibits substantial nesting. Both factors favor a Fulde-Ferrell state at parts of the Fermi surface. We find a substantial coupling of Zr rattling modes with electrons, which can provide the necessary pairing in the s-channel.
Acute cor pulmonale is an uncommon manifestation of microscopic pulmonary tumor embolism. We describe the case of an 84-year-old man with a history of transitional cell carcinoma (TCC) of the urinary bladder who had acute cor pulmonale and died within a few hours after the onset of dyspnea. Autopsy showed that the right ventricle was dilated without hypertrophy. Microscopic examination of the lung showed that the small arteries, arterioles, and capillaries were filled with micrometastases of TCC. Microscopic pulmonary tumor embolism has rarely been reported with TCC, and to the best of our knowledge, such a fulminant course has not been previously described in the English language.
We undertook a study to determine the efficacy of stents in reducing restenosis in cardiac allograft vasculopathy. The result shows that coronary stenting significantly reduces restenosis in cardiac allograft vasculopathy compared with balloon angioplasty alone.
Cryotherapy is one of the available modalities for the management of primary haemorrhoids. It is a simple non-invasive procedure conducted without anaesthesia, ideal for surgery outpatients' department. Though practically given up in the west due to a distressing side-effect in the local population, it was evaluated for its efficacy and acceptability in the Indian population. The study of 174 patients revealed the procedure to be well accepted, extremely cost effective and with a low complication rate in the Indian population, in contrast to the western experience.