55,696 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
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
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
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
Temperature 1 Self-Assembly: Deterministic Assembly in 3D and Probabilistic Assembly in 2D
We investigate the power of the Wang tile self-assembly model at temperature
1, a threshold value that permits attachment between any two tiles that share
even a single bond. When restricted to deterministic assembly in the plane, no
temperature 1 assembly system has been shown to build a shape with a tile
complexity smaller than the diameter of the shape. In contrast, we show that
temperature 1 self-assembly in 3 dimensions, even when growth is restricted to
at most 1 step into the third dimension, is capable of simulating a large class
of temperature 2 systems, in turn permitting the simulation of arbitrary Turing
machines and the assembly of squares in near optimal
tile complexity. Further, we consider temperature 1 probabilistic assembly in
2D, and show that with a logarithmic scale up of tile complexity and shape
scale, the same general class of temperature systems can be simulated
with high probability, yielding Turing machine simulation and
assembly of squares with high probability. Our results show a sharp
contrast in achievable tile complexity at temperature 1 if either growth into
the third dimension or a small probability of error are permitted. Motivated by
applications in nanotechnology and molecular computing, and the plausibility of
implementing 3 dimensional self-assembly systems, our techniques may provide
the needed power of temperature 2 systems, while at the same time avoiding the
experimental challenges faced by those systems
- âŚ