| 2001. |
- Wood, RF, et al.
(författare)
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Acoustic-phonon anomaly in MgB2
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 66:1
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Tidskriftsartikel (refereegranskat)abstract
- Recent first-principles calculations of the phonon dispersion curves in MgB2 have suggested the presence of anomalies in some of the curves, particularly in the longitudinal acoustical (LA) branch in the Gamma to A direction. Similar behavior has been observed in numerous other superconductors with T-c's higher than those of standard electron-phonon BCS superconductors. Phenomenological calculations of the Gamma-->A LA dispersion based on both an acoustical plasmon and a "resonant polarization" mechanism are given here to emphasize the importance of these similarities.
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| 2002. |
- Wortelen, H., et al.
(författare)
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Spin-orbit influence on dz2 -type surface state at Ta(110)
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:16
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Tidskriftsartikel (refereegranskat)abstract
- The influence of spin-orbit interaction on an occupied surface state at Ta(110) is investigated with spin- and angle-resolved photoemission and electronic structure calculations. The surface state appears in a symmetry gap at a binding energy of 0.45 eV at Γ¯ and exhibits a free-electron-like E(k) dispersion with an effective mass m∗/me of about -1.35 along Γ¯H¯. Photoemission results for excitation with s- and p-polarized light confirm the predicted dz2-type symmetry of the state close to Γ¯. Spin-resolved data for finite k reveal a pure Rashba-type spin texture with a Rashba parameter of 0.063±0.007eVÅ. These findings clearly prove a sizable impact of spin-orbit coupling on the dz2 surface state and resolve a longstanding disagreement on this issue.
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| 2003. |
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| 2004. |
- Wu, Phillip, et al.
(författare)
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Evidence for the formation of quasibound states in an asymmetrical quantum point contact
- 2012
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 85:8
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Tidskriftsartikel (refereegranskat)abstract
- Features below the first conductance plateau in ballistic quantum point contacts (QPCs) are often ascribed to electron interaction and spin effects within the single mode limit. In QPCs with a highly asymmetric geometry, we observe sharp resonance peaks when the point contacts are gated to the single mode regime, and surprisingly, under certain gating conditions, a complete destruction of the 2e(2)/h first quantum plateau. The temperature evolution of the resonances suggest non-Fermi-liquid behavior, while the overall nonlinear characterizations reveal features reminiscent of the 0.7 effect. We attribute these unusual behaviors to the formation of a quasibound state, which is stabilized by a momentum mismatch accentuated by asymmetry.
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| 2005. |
- Wågberg, Thomas, et al.
(författare)
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Comparative Raman study of photopolymerised and pressure-polymerised C60 films
- 1999
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 60:7, s. 4535-4538
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Tidskriftsartikel (refereegranskat)abstract
- We have made a Raman study of low-pressure polymerized and photopolymerized C60 films, polymerized in different ways, but otherwise initially identical. Although the structures developing in the films during polymerization are initially very similar, an analysis of characteristic features in the Raman spectra shows that photopolymerized material forms a more complex polymeric structure when the polymerization approaches saturation. We suggest that this complex structure contains branched polymeric chains and in particular that a mode near 1454 cm-1 in photopolymerized C60 is characteristic for this type of structure.
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| 2006. |
- Wågberg, Thomas, et al.
(författare)
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Raman study of the two-dimensional polymers Na4C60 and tetragonal C60.
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 65:15, s. 155421-
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Tidskriftsartikel (refereegranskat)abstract
- We present a Raman investigation of the monoclinic two-dimensionally polymerized phase Na4C60 and a comparison with the pure tetragonal C60 polymer and the doped K3C60 structure. The Raman spectrum of Na4C60 depends on the probing laser wavelength, and high wave-number modes are hard to observe with a low-energy laser excitation. The spectrum for Na4C60 is very rich with a pronounced splitting of the original Hg modes and a large number of new modes. Several modes at low wave numbers support the presence of intermolecular bonds and the line broadening observed for low-frequency Hg modes is typical for electron-phonon coupling in metallic fullerene systems. From the shift of the pentagonal pinch mode we deduce an unexpectedly low charge transfer of approximately three electrons per C60 molecule. The presence of a distinct mode around 980 cm-1 throws doubts on earlier assumptions that modes in this range are connected to vibrations in the intermolecular cyclobutane rings found in C60 polymers. No superconductivity is observed in Na4C60, although the electron-phonon interaction derived from the Raman spectrum is similar to that in K3C60.
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| 2007. |
- Xia, Chao, et al.
(författare)
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Si intercalation/deintercalation of graphene on 6H-SiC(0001)
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 85:4, s. 045418-
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Tidskriftsartikel (refereegranskat)abstract
- The intercalation and deintercalation mechanisms of Si deposited on monolayer graphene grown on SiC(0001) substrates and after subsequent annealing steps are investigated using low-energy electron microscopy (LEEM), photoelectron spectroscopy (PES), and micro-low-energy electron diffraction (mu-LEED). After Si deposition on samples kept at room temperature, small Si droplets are observed on the surface, but no intercalation can be detected. Intercalation is revealed to occur at an elevated temperature of about 800. C. The Si is found to migrate to the interface region via defects and domain boundaries. This observation may provide an answer to the problem of controlling homogeneous bi-/multilayer graphene growth on nearly perfect monolayer graphene samples prepared on SiC(0001). Likewise, Si penetrates more easily small monolayer graphene domains because of the higher density of domain boundaries. Upon annealing at 1000-1100 degrees C, formation of SiC on the surface is revealed by the appearance of a characteristic surface state located at about 1.5 eV below the Fermi level. A streaked mu-LEED pattern is also observed at this stage. The SiC formed on the surface is found to decompose again after annealing at temperatures higher than 1200 degrees C.
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| 2008. |
- Xiao, D., et al.
(författare)
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Parametric autoexcitation of magnetic droplet soliton perimeter modes
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 95:2
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Tidskriftsartikel (refereegranskat)abstract
- Recent experiments performed in current-driven nanocontacts with strong perpendicular anisotropy have shown that spin-transfer torque can drive self-localized spin waves [W. H. Rippard, A. M. Deac, M. R. Pufall, J. M. Shaw, M. W. Keller, S. E. Russek, G. E. W. Bauer, and C. Serpico, Phys. Rev. B 81, 014426 (2010); S. M. Mohseni, S. R. Sani, J. Persson, T. N. A. Nguyen, S. Chung, Y. Pogoryelov, and J. Akerman, Phys. Status Solidi RRL, 5, 432 (2011)], that above a certain intensity threshold can condense into a nanosized and highly nonlinear dynamic state known as a magnetic droplet soliton [S. M. Mohseni, S. R. Sani, J. Persson, T. N. A. Nguyen, S. Chung, Y. Pogoryelov, P. K. Muduli, E. Iacocca, A. Eklund, R. K. Dumas, S. Bonetti, A. Deac, M. A. Hoefer, and J. Akerman, Science 339, 1295 (2013)]. Here we demonstrate analytically, numerically, and experimentally that at sufficiently large driving currents and for a spin polarization direction tilted away from the normal to a nanocontact plane, the circular droplet soliton can become unstable against the excitations in the form of periodic deformations of its perimeter. We also show that these perimeter excitation modes (PEMs) can be excited parametrically when the fundamental droplet soliton precession frequency is close to the double frequency of one of the PEMs. As a consequence, with increasing magnitude of a bias magnetic field the PEMs with progressively higher indices and frequencies can be excited. Full qualitative and partly quantitative agreement with experiment confirm the presented theoretical picture.
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| 2009. |
- Xiao, Ruijuan, et al.
(författare)
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Density functional investigation of rhombohedral stacks of graphene: Topological surface states, nonlinear dielectric response, and bulk limit
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:16, s. 165404-
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive density-functional theory (DFT)-based investigation of rhombohedral (ABC)-type graphene stacks with finite and infinite layer numbers and zero or finite static electric fields applied perpendicular to the surface is presented. Electronic band structures and field-induced charge densities are critically compared with related literature data including tight-binding and DFT approaches as well as with our own results on (AB) stacks. It is found that the undoped AB bilayer has a tiny Fermi line consisting of one electron pocket around the K point and one hole pocket on the line K-Gamma. In contrast to (AB) stacks, the breaking of translational symmetry by the surface of finite (ABC) stacks produces a gap in the bulklike states for slabs up to a yet unknown critical thickness N(semimet) andgt;andgt; 10, while ideal (ABC) bulk (beta graphite) is a semimetal. Unlike in (AB) stacks, the ground state of (ABC) stacks is shown to be topologically nontrivial in the absence of an external electric field. Consequently, surface states crossing the Fermi level must unavoidably exist in the case of (ABC)-type stacking, which is not the case in (AB)-type stacks. These surface states in conjunction with the mentioned gap in the bulklike states have two major implications. First, electronic transport parallel to the slab is confined to a surface region up to the critical layer number N(semimet). Related implications are expected for stacking domain walls and grain boundaries. Second, the electronic properties of (ABC) stacks are highly tunable by an external electric field. In particular, the dielectric response is found to be strongly nonlinear and can, e. g., be used to discriminate slabs with different layer numbers. Thus, (ABC) stacks rather than (AB) stacks with more than two layers should be of potential interest for applications relying on the tunability by an electric field.
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| 2010. |
- Xie, Mengyao, et al.
(författare)
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Assessing structural, free-charge carrier, and phonon properties of mixed-phase epitaxial films: The case of InN
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 90:19, s. 195306-
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Tidskriftsartikel (refereegranskat)abstract
- We develop and discuss appropriate methods based on x-ray diffraction and generalized infrared spectroscopic ellipsometry to identify wurtizte and zinc-blende polymorphs, and quantify their volume fractions in mixed-phase epitaxial films taking InN as an example. The spectral signatures occurring in the azimuth polarization (Muller matrix) maps of mixed-phase epitaxial InN films are discussed and explained in view of polymorphism (zinc-blende versus wurtzite), volume fraction of different polymorphs and their crystallographic orientation, and azimuth angle. A comprehensive study of the structural, phonon and free electron properties of zinc-blende InN films containing inclusions of wurtzite InN is also presented. Thorough analysis on the formation of the zinc-blende and wurtzite phases is given and the structural evolution with film thickness is discussed in detail. The phonon properties of the two phases are determined and discussed together with the determination of the bulk free-charge carrier concentration, and electron accumulation at the mixed-phase InN film surfaces.
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