| 1811. |
- Sundqvist, Bertil
(författare)
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Point defects and thermal conductivity of C60
- 1993
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 48:19, s. 14712-14713
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Tidskriftsartikel (refereegranskat)abstract
- Recent literature data for the thermal conductivity κ of single-crystal C60 are analyzed using a standard model. It is shown that in the intermediate range 100–250 K, below the rotational transition, κ can be described extremely well by a model taking into account phonon and point defect scattering only. The magnitude derived for the phonon-phonon scattering term agrees very well with an estimate from a simple model.
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| 1812. |
- Sundqvist, Bertil, et al.
(författare)
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Pressure dependence of the electron-phonon interaction and Fermi-surface properties of Al, Au, bcc Li, Pb, and Pd
- 1985
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 32:4, s. 2200-2212
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Tidskriftsartikel (refereegranskat)abstract
- The electrical resistance of Li, Pd, Au, and Pb is measured as a function of temperature and pressure in the region -20 °C to +30 °C and 0–1.3 GPa. Self-consistent linear muffin-tin orbital band-structure calculations of these elements and of Al are performed at normal and reduced volumes. Results are obtained for the density of states N(EF), the average Fermi ve- locity, the optical mass, the plasma frequency ω, and the volume dependence of these parameters. The pressure dependence of the electron-phonon interaction λ(p) is obtained from these measurements and the calculated ω(p). For the superconducting elements there is good agreement with the measured superconducting Tc(p). Results from our previous measurements and calculations on Al, V, Nb, and La and published results for ω(p) are included in this comparison. λ(p) increases with pressure for bcc Li and decreases for Pd and Au. Pressure is expected to suppress spin fluctuations much faster than λ(p) in Pd. The possibility of inducing superconductivity by pressure in Pd and bcc Li is discussed. The electronic Grüneisen parameter γe is obtained from λ(p) and the volume dependence of N(EF). Comparison with other results for γe generally shows good agreement.
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| 1813. |
- Sundqvist, Bertil, et al.
(författare)
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Pressure-temperature phase boundaries in KC24: Evidence for a kinetically-hindered low-temperature staging transition
- 1986
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 34:5, s. 3532-3535
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Tidskriftsartikel (refereegranskat)abstract
- The pressure-temperature phase diagram of KC24 was mapped out using in-plane resistivity anomalies to identify phase boundaries. Surprisingly, the 300-K staging transitions at 3.4 and 6.4 kbar are found to connect to the 1-bar ordering-stacking transitions at Tu=123 K and Tl=96 K, respectively, whereas the loci of P, T anomalies cannot be equilibrium phase boundaries since the low-P, low-T transitions do not involve stage transformations. We suggest that this unusual behavior is due to kinetic hindering of the low-T staging transitions, predicted by simple equilibrium models and observed in the analogous Li compound. A possible origin of the slow staging kinetics at low T in KC24 is the pinning of discommensurations at Daumas-Herold boundaries.
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| 1814. |
- Sundqvist, Bertil, et al.
(författare)
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Pressure-temperature phase diagrams of stage-2-4 potassium-graphite intercalation compounds deduced from anomalies in the basal plane resistivity
- 1987
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 35:15, s. 8231-8242
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Tidskriftsartikel (refereegranskat)abstract
- The basal-plane resistivities ρ of stage-2–4 potassium-graphite intercalation compounds (K-GIC’s) have been measured as functions of temperature T and pressure p using a contactless method. The pressure range investigated was 0–1.4 GPa. For stage-2 K-GIC (KC24) measurements were made in the temperature range 90–300 K, while for the higher-stage materials the range was mainly limited to 245–300 K. In all cases ρ increases with p through the anomalous regime near the phase transitions, whereas dρ/dp is always negative in the stable high-p (high-stage) phases. Approximate p-T phase diagrams are deduced from anomalies in ρ corresponding to the staging and in-plane order-disorder transitions, respectively. In all cases the slopes dp/dT of the phase boundaries for the high-pressure staging transitions are such that the transitions extrapolate to positive T at atmospheric p, although no such transitions have been reported. The low-p staging-ordering phase boundary of KC24 is particularly complex, involving segments with both dp/dT→0 (125<200 K) and dp/dT→∞ (T≊125 K). The p-T diagrams are compared with recent structural experiments and models. We suggest that the complexity of the KC24 phase diagram is attributable to the weakly incommensurate in-plane structure at low p and T.
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| 1815. |
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| 1816. |
- Sundqvist, PA, et al.
(författare)
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Phase transitions of a few-electron system in a spherical quantum dot
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 66:7
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Tidskriftsartikel (refereegranskat)abstract
- The spin configurations of a spherical quantum dot, defined by a three-dimensional (3D) harmonic confinement potential, containing a few Coulomb Fermi particles (electrons or holes) are studied. Quantum transitions involving a spin transformation and a "cold melting" (from a Wigner crystal-like state, i.e., from regime of strongly correlated electrons, to a Fermi-liquid-like phase) is driven by the dimensionless quantum control parameter q (which is connected with steepness of the confinement potential) is demonstrated. The pair correlation and radial distribution functions which characterize electronic quantum delocalization are analyzed. The calculations using the unrestricted variational Hartree-Fock method (for the ground state at T=0 K) and the more computer intensive quantum path integral Monte Carlo method (for Tnot equal0 K) are performed and compared. For small q, the ground state of the three electron system is crystal-like and has C-3 symmetry, i.e., the maxima of electron density are located at the nodes of an equilateral triangle. The preferable spin configuration for small q is "ferromagnetic," with total spin S=3/2. As q rises, the widths of the one-electron wave functions grow and become overlapping. At a critical value q(1) the ground state changes from S=3/2 to S=1/2 and at the same time, asymmetry appears in the triangle (i.e., spontaneous breaking of the C-3 symmetry to C-2 symmetry). At a second critical value q(2) the electron distribution undergoes a symmetry phase transition, from trianglelike (with C-2 symmetry) to axial symmetric (with C-infinity symmetry). As q grows further, we obtain a Fermi-liquid-like (non-interacting) electron configuration in the ground state (S=1/2). In addition, the S=3/2 state, at a critical q value (which is slightly larger than q(1)) undergoes a dramatic charge redistribution.
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| 1817. |
- Sundqvist, PA, et al.
(författare)
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Polarization of an exciton in a ZnO layer using a split gate potential
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 68:15
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we focus on structural and optical transitions of an exciton in a Zinc Oxide (ZnO) layer, which could be widely controlled by a split gate potential. We have solved the exciton problem by a self-consistent Schrodinger-Poisson technique, where the Hamiltonian includes the boundary conditions for the split gate structure. The gate voltage creates a paraboliclike potential, which at a typical threshold voltage separates or polarizes the exciton strongly. This sharp structural transition brings the exciton from being strongly correlated with a large overlap to a regime where the correlation is very small (with small overlap). The resulting structure for the exciton at negative gate voltages is a structure where the hole is located like a ring around a dotlike electron. For positive values of the gate voltage the situation is opposite. We have especially studied the ground-state binding energy and the optical transitions of the exciton. We found that the ground-state energy for ZnO could be tuned and the decrease of the ground-state energy can be as large as the double of the bulk exciton energy (60 meV for ZnO) with a gate voltage of -5 V. The ground-state energy is almost constant for small values of the gate voltage but at a typical threshold voltage (approximately -2 V) the energy suddenly changes and becomes linear with the gate voltage. We also analyze the lifetime for the exciton, which is shown to increase from nanoseconds to beyond milliseconds. This was shown to be an effect of the small overlap between the hole and the electron when the gate voltage increased above the threshold voltage. Stimulated by the long lifetime of the ground state of the exciton we also calculated the optical transition frequency and the corresponding oscillator strength for the transition between the ground state and the dominating excited (self-consistent) exciton states. The transition frequency was found to occur in the THz region and the oscillator strength in the range of 0.3-0.4 for gate voltages between -2 V and -5 V. In addition, we have also analytically described polarization and especially total charge densities for excitons in small linear electric fields.
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| 1818. |
- Sundqvist, PA, et al.
(författare)
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Self-consistent drift-diffusion model of nanoscale impurity profiles in semiconductor layers, quantum wires, and quantum dots
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:16
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Tidskriftsartikel (refereegranskat)abstract
- We propose and model an experiment where impurity profiles in low dimensional structures can be controlled (during heat treatment) by an external parabolic potential defined by a variety of gate arrangements. At high temperatures the impurities are ionized and are able to move relatively quickly. After a realistic equilibrium time of typically one hour, the profiles are rapidly cooled such that the impurities are frozen in place. The model, which takes the electronic distribution as well as the mobile impurities into account results in a nonlinear Poisson equation. Similar models are widely used in semiconductor device theory where doping profiles are fixed. A parabolic potential in one, two, and three dimensions is applied to a semiconductor layer, a cylindrical quantum wire, and a spherical quantum dot, respectively. The impurity profiles are typically Gaussian shaped, where the distribution broadens with increasing temperature. The results demonstrate that the profile can be widely altered by changing the temperature, the average doping density, the size (radius), and the parabolic potential constant. The effect of parabolic confinement dimensionality on the diffusion is also studied. The temperature effect is studied up to a theoretical zero-temperature limit for which an analytic solution for the impurity profile is derived. The impurity profiles are sharper as the parabolic constant increases and the processing temperature is lowered. The processing time, however, increases exponentially as the temperature is lowered, and this must be considered in the practical situation.
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| 1819. |
- Suorsa, J., et al.
(författare)
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Quasihole condensates in quantum Hall liquids
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:23, s. 235130-
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Tidskriftsartikel (refereegranskat)abstract
- We develop a formalism to describe quasihole condensates in quantum Hall liquids and thereby extend the conformal field theory approach to the full hierarchy of spin-polarized Abelian states and to several classes of non-Abelian hierarchical states. Most previously proposed spin-polarized quantum Hall wave functions appear as special cases. In this paper we explain the physical motivations for the approach, and exemplify it by explicitly constructing the level-two quasihole condensate state at filling fraction 2/3, and the two level-three states at 5/13 and 5/7 which are built from combinations of quasielectron and quasihole condensates.
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| 1820. |
- Suzuki, M.-T., et al.
(författare)
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Dynamical mean-field theory of a correlated gap formation in plutonium monochalcogenides
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 80:16, s. 161103-
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Tidskriftsartikel (refereegranskat)abstract
- The correlated Kondo insulator state of the plutonium monochalcogenides is investigated using the dynamical mean-field theory (DMFT) in fluctuating-exchange approximation and the local-density approximation +U (LDA+U). The DMFT-dynamical fluctuations lead to a correlated insulator state at elevated temperature, in sharp contrast to the static LDA+U approach that fails to reproduce both the insulating behavior and anomalous lattice constant. The DMFT conversely predicts the experimentally observed anomalous increase of the gap with pressure and explains the lattice constant very well.
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