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101.
Microscopic phonon theory of semiconductor nanocrystals (NCs) is reviewed in this paper. Phonon modes of Si and Ge NCs with various sizes of up to 7 nm are investigated by valence force field theory. Phonon modes in spherical SiGe alloy NCs approximately 3.6 nm (containing 1147 atoms) in size have been investigated as a function of the Si concentration. Phonon density-of-states, quantum confinement effects, as well as Raman intensities are discussed.   相似文献   
102.
Deuterium NMR relaxation experiments, low temperature deuterium NMR lineshape analysis, and FTIR spectra are consistent with a new model for solid state jump dynamics of water in (2)H(2)O-synthesized kanemite and (2)H(2)O-hydrated Na(+)-Zeolite A. Exchange occurs between two populations of water: one in which water molecules are directly coordinated to sodium ions and experience C(2) symmetry jumps of their OH bonds, and a population of interstitial water molecules outside the sodium ion coordination sphere that experience tetrahedral jumps of their OH bonds. For both samples the C(2) jump rate is much faster than the tetrahedral jump rate. (2)H NMR relaxation experiments match well with the fast exchange regime of the model over a wide range of temperatures, including room temperature and above. For hydrated Zeolite A, the kinetic activation parameters for the tetrahedral and C(2) symmetry jumps are Delta H tet++=+17 kJ/mol, Delta S tet++=-109 J/(mol K), Delta H C2++=+19 kJ/mol, and Delta S C2++=-20 J/(mol K). For kanemite, Delta H tet++ =+23 kJ/mol, Delta S tet++=-69 J/(mol K), Delta H C2++ =+23 kJ/mol, and Delta S C2++ =-11 J/(mol K).  相似文献   
103.
Optical detection of the frequency-dependent magnetic relaxation signal is used to monitor the binding of biological molecules to magnetic nanoparticles in a ferrofluid. Biological binding reactions cause changes in the magnetic relaxation signal due to an increase in the average hydrodynamic diameter of the nanoparticles. To allow the relaxation signal to be detected in dilute ferrofluids, measurements are made using a balanced photodetector, resulting in a 25 μV/√Hz noise floor, within 50% of the theoretical limit imposed by photon shot noise. Measurements of a ferrofluid composed of magnetite nanoparticles coated with anti-IgG antibodies show that the average hydrodynamic diameter increases from 115.2 to 125.4 nm after reaction with IgG.  相似文献   
104.
We demonstrate the growth of self-assembled nanowires of cobalt silicide on Si(111), (100), and (110) substrates during deposition of Co onto a heated Si substrate. Silicide islands form via an endotaxial mechanism, growing into the substrate along inclined Si{111} planes, which breaks the symmetry of the surface and leads to a long, thin nanowire shape. During growth, both the length and width of the islands increase with time in a fixed proportion that varies strongly with growth temperature, which shows that the nanowire shape is kinetically determined. It is expected that nanowires could form in many other overlayer/substrate systems via this mechanism.  相似文献   
105.
We present experimental evidence for the spontaneous formation of a macroscopic spin polarization in overlapping regions of two independent Bose-Einstein condensates produced in different hyperfine states of 87Rb. The condensates are independent in the sense that we do not explicitly introduce a relative phase between them. A single "spin-tip" pulse maps the transverse spin polarization into longitudinal spin polarization, and the atomic density distributions are measured with a Stern-Gerlach imaging method. The resulting matter-wave interference patterns are anticorrelated.  相似文献   
106.
We prove a global Birkhoff decomposition for almost split real forms of loop groups, when an underlying finite dimensional Lie group is compact. Among applications, this shows that the dressing action–by the whole subgroup of loops which extend holomorphically to the exterior disc–on the UU-hierarchy of the ZS-AKNS systems, on curved flats and on various other integrable systems, is global for compact cases. It also implies a global infinite dimensional Weierstrass-type representation for Lorentzian harmonic maps (1+11+1 wave maps) from surfaces into compact symmetric spaces. An “Iwasawa-type” decomposition of the same type of real form, with respect to a fixed point subgroup of an involution of the second kind, is also proved, and an application given.  相似文献   
107.
Geometric phase may enable inherently fault-tolerant quantum computation. However, due to potential decoherence effects, it is important to understand how such phases arise for mixed input states. We report the first experiment to measure mixed-state geometric phases in optics, using a Mach-Zehnder interferometer, and polarization mixed states that are produced in two different ways: decohering pure states with birefringent elements; and producing a nonmaximally entangled state of two photons and tracing over one of them, a form of remote state preparation.  相似文献   
108.
Surface-stabilized orthoconic antiferroelectric liquid crystals (OAFLCs) have a director tilt of theta = 45 degrees and are, with no field applied, negatively uniaxial with the optic axis perpendicular to the cell substrates. We demonstrate that OAFLCs can be utilized to achieve lossless phase modulation with three almost equidistant phase levels. This turns out to be true also for polymer-stabilized OAFLCs, where the polymer network increases the switching speed of the device without affecting the phase modulation appreciably.  相似文献   
109.
The electronic band gap of SrSe, in the CsCl-stuctured phase, was measured to 42 GPa via optical absorption studies. The indirect electronic band gap was found to close monotonically with pressure for the range of pressures studied. The change in band gap with respect to pressure, dEgap/dP, was determined to be −6.1(5)×10−3 eV/GPa. By extrapolation of our line fit, we estimate band gap closure to occur at 180(20) GPa.  相似文献   
110.
We used a high-throughput method to screen for direct methanol fuel cell anode electrocatalysts in the Pt-Bi-Pb system. Previous studies showed that PtBi and PtPb (both NiAs structure type) were active electrocatalysts for the oxidation of formic acid, but only PtPb was active in oxidizing methanol. We synthesized thin films with continuous composition spreads of the three elements by magnetron sputtering at deposition temperatures from ambient to 510 °C. A fluorescence method was then used to identify compositions that were active toward methanol oxidation. Only films deposited between temperatures of 160 and 400 °C showed electrocatalytic activity. The areas that were active for methanol oxidation showed predominantly the NiAs structure type according to XRD, with optimal activity for compositions near PtBi0.01Pb0.53.  相似文献   
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