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Bo, Liu

Publications and source records attributed to Bo, Liu.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Effects of germanium and nitrogen incorporation on crystallization of N-doped Ge{sub 2+x}Sb{sub 2}Te{sub 5} (x = 0,1) thin films for phase-change memory

The phase-change behavior and microstructure changes of N-doped Ge{sub 3}Sb{sub 2}Te{sub 5}[N-GST(3/2/5)] and Ge{sub 2}Sb{sub 2}Te{sub 5}[GST(2/2/5)] films during the phase transition from an amorphous to a crystalline phase were studied using in situ temperature-dependent sheet resistance measurements, X-ray diffraction, and transmission electron microscopy. The optical band gaps of N-GST(3/2/5) films are higher than that of GST(2/2/5) film in both the amorphous and face-centered-cubic (fcc) phases. Ge nitride formation by X-ray photoelectron spectroscopy analysis increased the optical band gap and suppressed crystalline grain growth, resulting in an increase in the crystallization temperature and resistance in the fcc phase. As a result, the Ge- and N-doped GST(2/2/5) composite films can be considered as a promising material for phase-change memory application because of improved thermal stability and reduced power consumption.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Correlation energy of a homogeneous dipolar Fermi gas

We study the normal state of a 3d homogeneous dipolar Fermi gas beyond the Hartree-Fock approximation. The correlation energy is found of the same order as the Fock energy, unusually strong for a Fermi-liquid system. As a result, the critical density of mechanical collapse is smaller than that estimated in the Hartree-Fock approximation. With the correlation energy included, an energy functional is proposed for the trapped system, and its property is explored.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Antiferromagnetism and superfluidity of a dipolar Fermi gas in a two-dimensional optical lattice

In a dipolar Fermi gas, the dipole-dipole interaction between fermions can be turned into a dipolar Ising interaction between pseudospins in the presence of an ac electric field. When trapped in a two-dimensional optical lattice, this dipolar Fermi gas has a very rich phase diagram at zero temperature, due to the competition between antiferromagnetism and superfluidity. At half-filling, the antiferromagnetic state is the favored ground state. The superfluid state appears as the ground state at a smaller filling factor. In between there is a phase-separated region. The order parameter of the superfluid state can display different symmetries depending on the filling factor and interaction strength, including the d-wave (d), the extended s-wave (xs), or their linear combination (xs+id). Implications for the current experiment are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

COEXISTENCE OF GRAVITATIONALLY-BOUND AND RADIATION-DRIVEN C IV EMISSION LINE REGIONS IN ACTIVE GALACTIC NUCLEI

There are mutually contradictory views in the literature of the kinematics and structure of high-ionization line (e.g., C IV) emitting regions in active galactic nuclei (AGNs). Two kinds of broad emission line region (BELR) models have been proposed, outflow and gravitationally-bound BELR, which are supported, respectively, by blueshift of the C IV line and reverberation mapping observations. To reconcile these two apparently different models, we present a detailed comparison study between the C IV and Mg II lines using a sample of AGNs selected from the Sloan Digital Sky Survey. We find that the kinematics of the C IV region is different from that of Mg II, which is thought to be controlled by gravity. A strong correlation is found between the blueshift and asymmetry of the C IV profile and the Eddington ratio. This provides strong observational support for the postulation that the outflow is driven by radiation pressure. In particular, we find robust evidence that the C IV line region is largely dominated by outflow at high Eddington ratios, while it is primarily gravitationally-bounded at low Eddington ratios. Our results indicate that these two emitting regions coexist in most AGNs. The emission strength from these two gases varies smoothly with Eddington ratio in opposite ways. This explanation naturally reconciles the apparently contradictory views proposed in previous studies. Finally, candidate models are discussed which can account for both the enhancement of outflow emission and suppression of normal BEL in AGNs with high Eddington ratios.

79 ASTRONOMY AND ASTROPHYSICS↗