| 1. |
- da Silva, AF, et al.
(författare)
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Impurity resistivity of the double-donor system Si : P,Bi
- 1999
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 60:23, s. 15824-15828
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Tidskriftsartikel (refereegranskat)abstract
- The electrical resistivity of the shallow double-donor system Si:P,Bi, prepared by ion implantation, was investigated in the temperature range from 1.7 to 300 K. Good agreement was obtained between the measured resistivities and resistivities calculated by a generalized Drude approach for the same temperatures and dopant concentrations. The critical impurity concentration for the metal-nonmetal transition for the double-doped Si:P,Bi system was found to lie between the critical concentrations of the two single-doped systems, Si:P and Si:Bi. [S0163-1829(99)11747-8].
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| 2. |
- Andersson, Ove, et al.
(författare)
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Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K
- 1996
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54:5, s. 3093-3100
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Tidskriftsartikel (refereegranskat)abstract
- The thermal conductivity λ of C60 shows anomalies near 260 K and 90 K which are associated with the well-established phase transition and glass transition, respectively. Both transition temperatures increase with pressure, at the rates 120 K GPa-1 and 62 K GPa-1, respectively. With increasing temperature, λ of the simple cubic (sc) phase increased below 170 K (glasslike behavior) but decreased above. The glasslike behavior of λ is probably due to a substantial amount of lattice defects. Possible reasons for the change of sign of dλ/dT near 170 K are discussed. In the face centered cubic (fcc) phase (T≳260 K at atmospheric pressure) λ was almost independent of temperature, a behavior which is far from that of an ordered crystal (λ∝T-1 for T≳Debye temperature). This result can be attributed to the molecular orientational disorder of the fcc phase. The relaxation behavior associated with the glassy state and its unusually strong dependence on thermal history are discussed briefly, and data which support a previously reported relaxation model are presented. At room temperature, the density dependencies of λ, (∂ lnλ/∂ lnρ)T, were 5.5 and 9.5 for the fcc and sc phases, which are values typical for an orientationally disordered phase and a normal crystal phase, respectively.
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| 3. |
- Andersson, Per H., et al.
(författare)
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Effect of hydrogenation on the magnetic state in cubic Pd3Mn
- 1999
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 60:9, s. 6765-6769
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Tidskriftsartikel (refereegranskat)abstract
- The effect of hydrogenation on the magnetic and electronic state of cubic Pd3Mn has been calculated using a full-potential linear augmented plane-wave method in the local spin-density approximation. A complex magnetic structure is found in very good agreement with experiment. For the nonhydrogenated system we observe a noncommensurate spin spiral although we cannot identify the ground state of the spin structure as the total energy of the trigonal spin structure is almost degenerate with the total energy of the tetragonal spin structure. When hydrogenated the direction of the spiral is aligned along the (001) axis. The spin structures agree with experiment and are connected to features on the calculated Fermi surfaces.
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| 4. |
- Andersson, Per H., et al.
(författare)
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Theoretical study of structural and electronic properties of VHx
- 1998
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 58:9, s. 5230-5235
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Tidskriftsartikel (refereegranskat)abstract
- We have calculated the change in the electronic structure and the distortion of the lattice in vanadium upon hydrogenation from first principles using the full-potential linear muffin-tin-orbital method and the linear augmented plane-wave method in the local-density approximation. The calculated hydrogen induced volume expansions agree with experiment and the change in the c/a ratio is also in good agreement with observations where such are available for single phase VHx. Among several changes in the electronic structure, we note a hybridization of the d band of vanadium with the hydrogen 1s band. We also observe an antiferromagnetically ordered moment at V/Vexp=1.08. The possibility of producing magnetic V by means of hydrogenation in combination with epitaxial growth is suggested.
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| 5. |
- Ao, Ping, et al.
(författare)
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Microscopic theory of vortex dynamics in homogeneous superconductors
- 1999
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 60:9, s. 6850-6877
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Tidskriftsartikel (refereegranskat)abstract
- Vortex dynamics in a type-II superconductor is systematically investigated by the influence functional method. The irrelevant fermionic degrees of freedom are integrated out and their effects on the dynamics are treated in terms of the vortex coordinate. When an isolated vortex is moving against its background, forces proportional to the first order of vortex velocity on the vortex are calculated within the present formulation. The total transverse force on the moving vortex is explicitly shown to be proportional to the superfluid number density and insensitive to impurities. Its equivalent expressions in terms of the Berry phase and the various summations of transitions between quasiparticle (hole) states are discussed. At finite temperatures, due to the finite population of quasiparticle (hole) excitations above (below) the energy gap, there is a friction against vortex motion which diverges logarithmically in the low-frequency limit. Nonmagnetic impurities give rise to an additional friction from the core states which saturates to a value independent of the normal-state resistivity in the dirty limit. In this limit, the coupling to the electromagnetic field does not change the conclusions if charge neutrality in the superconductor is maintained. Macroscopic constraints on vortex dynamics by the second law of thermodynamics and by the fluctuation-dissipation theorems are also discussed.
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| 6. |
- Aryasetiawan, F., et al.
(författare)
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GW spectral functions of Gd and NiO
- 1996
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 54:8, s. 5353-5357
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Tidskriftsartikel (refereegranskat)abstract
- The spectral functions of highly correlated systems Gd and NiO have been calculated within the GW approximation. In the case of Gd, satellites are found at the positions of the experimental main peaks. However, some quasiparticle weight remains at the local density eigenvalue but its weight is unusually small (0.3). In NiO, satellite structure is observed at around -10 eV when starting from the local density Hamiltonian. It is found, however, that the satellite intensity is reduced towards self-consistency and almost disappears. © 1996 The American Physical Society.
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| 7. |
- Bobbert, PA, et al.
(författare)
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Exchange-correlation energy of a hole gas including valence band coupling
- 1997
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 56:7, s. 3664-3671
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Tidskriftsartikel (refereegranskat)abstract
- We have calculated an accurate exchange-correlation energy of a hole gas, including the complexities related to the valence band coupling as occurring in semiconductors like GaAs, but excluding the band warping. A parametrization for the dependence on the density and the ratio between light-and heavy-hole masses is given. We apply our results to a hole gas in an AlxGa1-xAs/GaAs/AlxGa1-xAs quantum well and calculate the two-dimensional band structure and the band-gap renormalization. The inclusion of the valence band coupling in the calculation of the exchange-correlation potentials for holes and electrons leads to a much better agreement between theoretical and experimental data than when it is omitted.
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| 8. |
- Brauer, H.E., et al.
(författare)
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Na intercalation of VSe2 studied by photoemission and scanning tunneling microscopy
- 1997
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 55:15, s. 10022-10026
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Tidskriftsartikel (refereegranskat)abstract
- In situ Na intercalation of the layered compound VSe2 has been studied with photoemission and scanning tunneling microscopy. Core-level spectroscopy proves that Na deposited in UHV onto the VSe2 surface rapidly intercalates, leaving only small amounts at the surface. The scanning tunneling microscopy measurements show that the intercalated Na is not uniformly distributed between the VSe2 layers, but preferentially in two-dimensional islands. Thus the surface region is divided into intercalated and nonintercalated areas. Hole-like features in the intercalated areas are interpreted as locally missing Na.
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| 9. |
- Breuer, S. J., et al.
(författare)
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Interstitial hydrogen and enhanced dissociation of C-H complexes in GaAs
- 1996
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 53:24, s. 16289-16296
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Tidskriftsartikel (refereegranskat)abstract
- Local-density-functional-based ab initio calculations are used to investigate hydrogen and carbon-hydrogen defects in GaAs. The equilibrium structure for both the C-H and C-H- complexes are shown to be similar, with the hydrogen located at a C-Ga bond-centered site. The dissociation of these complexes is investigated and it is found that the energy barrier of 1.84 eV for the process C-H → C- + H+ is substantially lowered to 0.88 eV in the presence of an electron resonantly bound to the defect. This is in good quantitative agreement with recent experiments. Isolated interstitial hydrogen is found to lie at a Ga-As bond-centered site for both H+ and H0 and at an antibonding site relative to a Ga atom for H-. It is also found that the stable form of the hydrogen dimer is a H2 molecule, the dissociation energy of which is 1.64 eV, and that interstitial hydrogen is a negative-U defect. Finally, a mechanism for minority-carrier-induced device degradation is proposed.
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| 10. |
- Budde, M., et al.
(författare)
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Local modes of the H2* dimer in germanium
- 1996
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 54:8, s. 5485-5494
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Tidskriftsartikel (refereegranskat)abstract
- Local vibrational modes of the H2* defect in crystalline germanium are identified by a combination of infrared-absorption spectroscopy, uniaxial stress measurements, and ab initio theory. Germanium crystals are implanted with protons and/or deuterons at 30 K, and subsequently annealed at room temperature. A number of local vibrational modes of hydrogen are revealed by infrared-absorption spectroscopy. In particular, modes at 765, 1499, 1774, and 1989 cm-1 originate from the same defect which has trigonal symmetry according to the uniaxial stress measurements. The 765-cm-1 mode is two dimensional, while the 1774- and 1989-cm-1 modes are one dimensional. Measurements on samples coimplanted with protons and deuterons show that the defect contains a pair of weakly coupled and inequivalent hydrogen atoms. The 765-, 1499-, 1774-, and 1989-cm-1 modes are ascribed to the H2* defect. The 765-cm-1 mode is a Ge-H bend mode with an overtone at 1499 cm-1 and the modes at 1774 and 1989 cm-1 are Ge-H stretch modes. An excellent fit to the stretch frequencies is obtained with a simple model based on two coupled Morse-potential oscillators. In addition, the model gives intensity ratios in fair agreement with those observed. The structure, the local-mode frequencies, and the isotope shifts of H2* are calculated with ab initio local-density-functional cluster theory. The theoretical frequencies are consistently 5-10 % too high, as expected from the theory which often leads to overbinding. The isotope shifts, however, are in fair agreement with observations. These results provide additional support for our assignments, and show that the 765- and 1774-cm-1 modes primarily involve the hydrogen at the antibonding site, while the 1989-cm-1 mode is related mainly to vibration of the hydrogen near the bond-center site.
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