Search Results - (Author, Cooperation:C. Shih)

2018-04-13

Nature Publishing Group (NPG)

2041-1723

Biology

Chemistry and Pharmacology

Natural Sciences in General

Physics

2014-10-16

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Animals ; Antibodies/immunology/metabolism ; Antibody Formation ; B-Lymphocytes/*cytology/*immunology/metabolism/secretion ; Bone Marrow Cells/cytology/immunology ; Feedback ; Germinal Center/*cytology/*immunology ; Inducible T-Cell Co-Stimulator Ligand/immunology/*metabolism ; Mice ; Plasma Cells/cytology/immunology/secretion ; Signal Transduction/immunology ; T-Lymphocytes, Helper-Inducer/*cytology/*immunology/metabolism

2011-04-23

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Animals ; Embryo, Mammalian/metabolism ; Female ; Fetus/embryology/*metabolism ; Humans ; In Vitro Techniques ; Maternal-Fetal Exchange/*physiology ; Mice ; Placenta/*metabolism ; Pregnancy ; Prenatal Exposure Delayed Effects ; Prosencephalon/*embryology/*metabolism ; Raphe Nuclei/cytology ; Rhombencephalon/embryology/metabolism ; Serotonin/analysis/*biosynthesis/metabolism ; Time Factors ; Transcription Factors/deficiency/genetics

2018-05-02

The American Society for Microbiology (ASM)

0099-2240

1098-5336

Biology

2018-04-26

Royal Society

2054-5703

Natural Sciences in General

acoustics, medical physics, biomedical engineering

2018-10-04

Genetics Society of America (GSA)

2160-1836

Biology

2018-08-11

The American Society for Pharmacology and Experimental Therapeutics

0022-3565

1521-0103

Medicine

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We describe the design and characterization of a new variable low-temperature scanning tunneling microscope (STM) which has been proven to give atomic resolution at temperatures between 77 K and room temperature but which is also capable of performing experiments as low as 4 K. The STM "head'' itself consists of a unique cold dewar made up of an upper and lower reservoir connected together by two tubes which pass through an 8 in. conflat flange. The STM stage is suspended in between the two reservoirs by three long springs which pass through holes in the upper reservoir and also attach to the top flange. An adjustable cold shroud surrounds the STM stage for radiation shielding while allowing sample and tip transfer when raised. It has the additional advantage of providing a controllable heat leak. By raising the shroud, the temperature can be increased; by lowering it, the temperature can be decreased. The cold reservoirs can be filled with either liquid nitrogen or liquid helium. In the case of liquid helium, an additional liquid nitrogen "jacket'' surrounds the STM head. Everything is operated inside a customized ultrahigh vacuum chamber containing low-energy electron diffraction and various sample preparation facilities. A detailed description of the STM is presented together with performance results. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We present a cross-sectional scanning tunneling microscopy (XSTM) study of the spontaneous ordering of Ga0.48In0.52P and Ga0.52In0.48P grown on (001) GaAs substrates by molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE), respectively. The (111)-type alloy ordering could be seen clearly in the OMVPE-grown alloy region. On the other hand, the MBE-grown region shows a very small degree of ordering as revealed by the STM. Most of the ordered region shows (InP)1(GaP)1-type ordering: alternating InP- and GaP-like (1¯11) planes. In addition to this type of ordering, we also observe another type of ordering consisting of two InP-like (1¯11) planes and one GaP-like (1¯11) plane that we call (InP)2(GaP)1-type ordering. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A scanning near-field microscope design using the reflected light intensity as the feedback mechanism is described. Multiple fibers with high numerical apertures provide a high collection efficiency in a reflection geometry. The performance with regard to its response to large spatial variations has been tested by using a Si-grating sample and with regard to variations of local indices of refraction by using GaAs/AlGaAs heterostructure samples. In addition, spatially resolved spectroscopy on GaAs/AlGaAs heterostructures has been obtained. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The valence-band structures of 50-A(ring)-thick layers of GaAs(001) in tension and In0.2Ga0.8As(001) in compression have been determined using angle-resolved photoemission spectroscopy. Our studies show that the Δ3+Δ4 bands and the Δ1 band respond differently to the strain perturbation. For strained GaAs(001), the Δ3+Δ4 bands (Px+Py-like) are shifted up in energy by a maximum amount of 0.3 eV, while in contrast, the Δ1 band (Pz-like) is shifted down by about 0.1 eV. For strained In0.2Ga0.8As(001), the band shifts are in the opposite direction, consistent with the opposite strain conditions. For both materials, the strain-induced changes cannot be characterized simply by rigid band shifts, but rather exhibit significant wave vector dependence. This results in a reduction of the effective mass of the Δ3+Δ4 bands for both GaAs and In0.2Ga0.8As.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Granular flow of comminuted ceramics governs the resistance for penetration of ceramic armor under impact. To understand the mechanism of the granular flow, silicon carbide was subjected to high-strain, high-strain-rate deformation by radial symmetric collapse of a thick-walled cylinder by explosive. The deformation, under compressive stresses, was carried out in two stages: the first stage prefractured the ceramic, while a large deformation was accomplished in the second stage. The total tangential strain (−0.23) was accommodated by both homogeneous deformation (−0.10) and shear localization (−0.13). Three microstructures, produced by different processing methods, were investigated. The microstructural differences affected the microcrack propagation: either intergranular or transgranular fracture was observed, depending on the processing conditions. Nevertheless, the spacing between shear bands and the shear displacement within the shear bands were not significantly affected by the microstructure. Within the shear bands, the phenomenon of comminution occurred, and the thickness of the shear bands increased gradually with the shear strain. A bimodal distribution of fragments developed inside the shear bands. The comminution proceeded through the incorporation of fragments from the shear-band interfaces and the erosion of fragments inside the shear band. Outside the shear bands, an additional comminution mechanism was identified: localized bending generated comminution fronts, which transformed the fractured material into the comminuted material. The observed features of high-strain-rate deformation of comminuted SiC can be used for validation of computer models for penetration process. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The natural valence-band offset (NVBO) between semiconductors in a common anion alloy system can be determined through photoemission core level measurements. In this work, we tested this method in the InxGa1−xAs system. The NVBO between GaAs and InAs is measured to be 0.11±0.05 eV. This result is in approximate agreement with the experimental value of 0.17±0.07 eV determined by x-ray photoemission spectroscopy measurements.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Mono- and polycrystalline Ni films, ranging in thickness from 19 to 382 nm, have been produced by vacuum deposition on (001) monocrystalline Ag/NaCl and on glass substrates, respectively. Subsequently, saturation magnetization Ms, magnetogyric factor γ/2π, and effective uniaxial anisotropy Hk were measured by a combination of vibrating sample magnetometer and ferromagnetic resonance (FMR), and corresponding internal stress σ was measured by a levered optical beam reflection technique. In addition, the microstructure of both mono- and polycrystalline films has been characterized by conventional and cross-section transmission electron microscopy. Saturation magnetization can be interpreted by the presence of a magnetic dead layer of about 9 nm and a constant value of 434 emu/cm3, thereafter. Results from FMR can be described by contributions from shape and stress-induced anisotropy. Corresponding values of σ, as computed from Hk, are in satisfactory agreement with values measured by the optical beam reflection technique and yield values of about 1010 dyn/cm2; σ generally decreases with increasing film thickness and annealing time and temperature. Finally, monocrystalline films are characterized by a single FMR peak; polycrystalline films thicker than about 200 nm are characterized by multiple peaks. Occurrence of these multiple peaks is attributed to variation in the effective magnetoelastic constant as a result of a change from equiaxed to columnar grain morphology.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

We report on the development of a new two-dimensional micropositioning device, or walker, which is capable of moving across very large distances (in principle unlimited) and with a very small step size (as small as 100 A(ring)/step) in both directions. Based on a unique tracking design, the motion is extremely orthogonal with very little crosstalk between the two directions. Additionally, there is no detectable backlash in either direction. The walker performance has been extensively tested by using a position-sensitive proximitor probe. Tests have been done between 77 and 300 K. However, we project that the walker will be able to operate at temperatures as low as 4 K. This walker system has shown extremely reliable performance in a UHV environment for use with scanning tunneling microscopy and has been especially useful for cross-sectional scanning tunneling microscopy and spectroscopy studies of semiconductor hetero- and homostructures. We show one example of results on the (AlGa)As/GaAs heterostructure system.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Scanning tunneling microscopy (STM) was used to study the (NH4)2S-passivated (110) cross-sectional surfaces of both doped and undoped Al0.3Ga0.7As/GaAs heterostructures on n+-substrates. The ex situ (NH4)2S treatment of the cross-sectional surfaces of heterostructures was found to be very stable against oxidation. STM images showed no appreciable deterioration of surface quality in vacuum after more than 40 days. The spectroscopic results on the undoped epilayer showed diodelike behavior, confirming that an undoped large band gap region can be imaged by STM through carrier injection from the conductive regions.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Thin films of YBa2Cu3O7−δ were sequentially grown on both sides of 3 in. diameter LaAlO3 substrates by a single-source MOCVD technique to facilitate the fabrication of double-sided high temperature superconductor microwave devices. YBCO films on both sides of the LaAlO3 substrates had thicknesses greater than 4000 A(ring), Tc's of 87–88.5 K, and microwave surface resistances as low as 23 mΩ at 94 GHz and 77 K (scaling to 260 μΩ at 10 GHz). It was found that the substrate temperature, brought to the desired value by radiant heating, played a critical role in the deposition of very low surface resistance YBCO films. Microwave bandpass filters with a 19-pole forward coupled configuration were fabricated from these films. The filters showed excellent performance with dissipation losses as low as 0.5 dB. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We report studies of GaAs/AlAs short period superlattices using cross-sectional scanning tunneling microscopy. In particular, we investigate the role of growth interrupt time on the resulting interfacial structure. Superlattices with repeated periods of four layers of GaAs and two layers of AlAs are resolved atom by atom. Superlattices grown using a 30 s growth interrupt time are observed while those grown with a 5 s growth interrupt time are not. We also discuss residual effects of the growth interrupt process on layers grown on top of the short-period superlattice. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Scanning tunneling microscopy and spectroscopy is used to study GaAs multiple pn junction samples cleaved in ultrahigh vacuum. Direct topographic contrast over the pn junctions can be observed in the constant current imaging mode. The topographic height in the p-type regions appears much lower (by about 5 A(ring)) than that in the n-type regions. Tunneling spectroscopy measurements show consistency with the assignment of the p- and n-type regions. We discuss a possible mechanism for the observed contrast.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Cross-sectional scanning tunneling microscopy is used to study dilute AlxGa1−xAs with x=0.05 to investigate the bonding configurations within this ternary alloy. Atomically resolved scanning tunneling microscopy images combined with symmetry considerations provide the assignment of first and second layer aluminum atoms. The Al–Al pair distribution function based on the experimental data is compared with the theoretical pair distribution function of a random alloy. While there exists a qualitative agreement, small deviations from the ideal random distribution are also found. © 1996 American Institute of Physics.

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