55,068 research outputs found
Influence of rotational instability on the polarization structure of SrTiO3
The k-space polarization structure and its strain response in SrTiO3 with
rotational instability are studied using a combination of first-principles
density functional calculations, modern theory of polarization, and analytic
Wannier-function formulation. (1) As one outcome of this study, we rigorously
prove-both numerically and analytically-that folding effect exists in
polarization structure. (2) After eliminating the folding effect, we find that
the polarization structure for SrTiO3 with rotational instability is still
considerably different from that for non-rotational SrTiO3, revealing that
polarization structure is sensitive to structure distortion of oxygen-octahedra
rotation and promises to be an effective tool for studying material properties.
(3) Furthermore, from polarization structure we determine the microscopic
Wannier-function interactions in SrTiO3. These interactions are found to vary
significantly with and without oxygen-octahedra rotation.Comment: 25 pages, 7 figure
Observation of a (2X8) surface reconstruction on Si_(1-x)Ge_x alloys grown on (100) Si by molecular beam epitaxy
We present evidence supporting the formation of a new, (2Ă8) surface reconstruction on Si_(1âx)Ge_x alloys grown on (100) Si substrates by molecularâbeam epitaxy. Surfaces of Si_(1âx)Ge_x alloys were studied using reflection highâenergy electron diffraction (RHEED) and lowâenergy electron diffraction (LEED) techniques. RHEED patterns from samples with Ge concentrations, x, falling within the range 0.10â0.30 and grown at temperatures between 350 and 550â°C, exhibit n/8 fractionalâorder diffraction streaks in addition to the normal (2Ă1) pattern seen on (100) Si. The presence of fractionalâorder diffracted beams is indicative of an eightâfoldâperiodic modulation in electron scattering factor across the alloy surface. LEED patterns from surfaces of samples grown under similar conditions are entirely consistent with these results. In addition, the LEED patterns support the conclusion that the modulation is occurring in the direction of the dimer chains of a (2Ă1) reconstruction. We have examined the thermal stability of the (2Ă8) reconstruction and have found that it reverts to (2Ă1) after annealing to 700â°C and reappears after the sample temperature is allowed to cool below 600â°C. Such behavior suggests that the reconstruction is a stable, ordered phase for which the pairâcorrelation function of surface Ge atoms exhibits an eightfold periodicity in the "1" direction of a Siâlike (2Ă1) reconstruction. We also present a simulation in the kinematic approximation, confirming the validity of our interpretation of these finding
Local Spin Susceptibility of the S=1/2 Kagome Lattice in ZnCu3(OD)6Cl2
We report single-crystal 2-D NMR investigation of the nearly ideal spin S=1/2
kagome lattice ZnCu3(OD)6Cl2. We successfully identify 2-D NMR signals
originating from the nearest-neighbors of Cu2+ defects occupying Zn sites. From
the 2-D Knight shift measurements, we demonstrate that weakly interacting Cu2+
spins at these defects cause the large Curie-Weiss enhancement toward T=0
commonly observed in the bulk susceptibility data. We estimate the intrinsic
spin susceptibility of the kagome planes by subtracting defect contributions,
and explore several scenarios.Comment: 4 figures; published in PR-B Rapid Communication
Public-private partnerships in China's urban water sector
During the past decades, the traditional state monopoly in urban water management has been debated heavily, resulting in different forms and degrees of private sector involvement across the globe. Since the 1990s, China has also started experiments with new modes of urban water service management and governance in which the private sector is involved. It is premature to conclude whether the various forms of private sector involvement will successfully overcome the major problems (capital shortage, inefficient operation, and service quality) in ChinaÂżs water sector. But at the same time, private sector involvement in water provisioning and waste water treatments seems to have become mainstream in transitional China
Volume integrals associated with the inhomogeneous Helmholtz equation. Part 1: Ellipsoidal region
Problems of wave phenomena in fields of acoustics, electromagnetics and elasticity are often reduced to an integration of the inhomogeneous Helmholtz equation. Results are presented for volume integrals associated with the Helmholtz operator, nabla(2) to alpha(2), for the case of an ellipsoidal region. By using appropriate Taylor series expansions and multinomial theorem, these volume integrals are obtained in series form for regions r 4' and r r', where r and r' are distances from the origin to the point of observation and source, respectively. Derivatives of these integrals are easily evaluated. When the wave number approaches zero, the results reduce directly to the potentials of variable densities
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