| 1. |
- Aleksandrovskii, A. N., et al.
(författare)
-
Thermal expansion of single-crystal fullerite C60 at helium temperatures
- 2000
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 26:1, s. 75-80
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Tidskriftsartikel (refereegranskat)abstract
- The thermal expansion of single-crystal fullerite C60 has been studied in the range of liquid-helium temperatures (2–10 K). At temperatures below ~4.5 K the thermal expansion of fullerite C60 becomes negative, in agreement with the previous results on polycrystalline materials. A qualitative explanation of the results is proposed.
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| 2. |
- Andersson, Ove, et al.
(författare)
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Reorientational relaxation in C60 following a pressure induced change in the pentagon/hexagon equilibrium ratio
- 1995
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Ingår i: Physics Letters A. - : Elsevier Science B.V.. - 0375-9601 .- 1873-2429. ; 206:3-4, s. 260-264
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Tidskriftsartikel (refereegranskat)abstract
- The orientational structure of C60 depends on pressure and temperature. Pressurization below the glass transition temperature Tg can freeze in non-equilibrium orientational structures. The relaxation of such structures on heating through Tg has been studied through thermal conductivity measurements and the effects observed are explained in a simple model.
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| 3. |
- Lundin, Anders, et al.
(författare)
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Compressibility and Structure of C70
- 1995
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 30:8, s. 469-474
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Tidskriftsartikel (refereegranskat)abstract
- The compressibility of C70 has been investigated by means of direct piston and cylinder measurements in the temperature range 150 to 365 K and up to 1 GPa. At 296 and 343 K we find a very rapid change of volume with pressure below 0.15 GPa. We tentatively interpret this as a continuous molecular reorientation with pressure, probably resulting in a transformation from a mixture of face-centred cubic (f.c.c.) and rhombohedral (r.h.) phases to mainly pure r.h. phase. At 365 K as well as at 236 K and below we see no anomalies indicating any structural or rotational anomalies in the pressure range investigated. The zero-pressure bulk modulus decreases with increasing temperature, from 13.1 GPa at 185 K to 7.9 GPa at 365 K. Our volumetric measurements confirm a recently proposed phase diagram for C70.
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| 4. |
- Lundin, Anders, et al.
(författare)
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Compressibility of C61D2 up to 1 GPa in the temperature range 175 - 345 K
- 1996
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Ingår i: Carbon. - : Elsevier Science Ltd.. - 0008-6223 .- 1873-3891. ; 34:9, s. 1119-1121
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Tidskriftsartikel (refereegranskat)abstract
- We have measured the bulk modulus K for C61D2 up to 1 GPa in the temperature range 175–343 K. For face-centered cubic C61D2 above 290 K, we find an anomalously low value for K below about 0.15 GPa, possibly indicating pressure-induced changes in the structure. The (extrapolated) zero-p bulk modulus K(0) decreases with increasing T from 6.7 GPa at 175 K to 5.2 GPa at 343 K. A comparison with hypothetical expanded f.c.c. C60 with the same lattice constant shows that K(0) values are similar, indicating that the main intermolecular interactions are still between molecular bellies, not the sidegroups.
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| 5. |
- Nagel, Peter, et al.
(författare)
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C60 one- and two-dimensional polymers, dimers, and hard fullerite: Thermal expansion, anharmonicity, and kinetics of depolymerization
- 1999
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 60:24, s. 16920-16927
-
Tidskriftsartikel (refereegranskat)abstract
- We report on high-resolution thermal expansion measurements of high-temperature-pressure treated C60 [one-dimensional (1D) and (2D) polymers and “hard fullerite”], as well as of C60 dimers and single crystal monomer C60 between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric C60 due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the “hard” material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of 1.9±0.1 eV (1D and 2D polymers) and 1.75±0.05 eV (dimer). Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of C70 polymers are presented as well.
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| 6. |
- Persson, Per-Axel, et al.
(författare)
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NMR and Raman characterization of pressure polymerized C-60
- 1996
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 258:5-6, s. 540-546
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Tidskriftsartikel (refereegranskat)abstract
- Bulk C60 has been treated at 1.1 GPa and 550–585 K, producing a dense insoluble material which on heating to above 600 K reverts to normal C60. Raman and IR studies on modified material show a large number of new lines, and the Raman pentagon pinch mode shifts from 1469 to 1458 cm−1 as on photopolymerization. MAS NMR shows one broadened line at the original C60 shift 144 ppm and a small peak at about 77 ppm due to the bridging carbons. None of the new resonances observed for C60 polymerized by other methods were observed. The results verify previously suggested polymeric structures where the fullerence cages are connected by four-membered rings.
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| 7. |
- Persson, Per-Axel, et al.
(författare)
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The physical properties of high-pressure polymerized C60
- 1997
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Ingår i: Journal of Physics and Chemistry of Solids, volume 58, issue 11. - : Elsevier B.V.. ; 58:11, s. 1881-1885
-
Konferensbidrag (refereegranskat)abstract
- We have studied the structural, thermophysical, and spectroscopic properties of polymeric C60 obtained by high pressure treatment at pressures and temperatures near 1 GPa and 600 K. We present here a brief overview of our results for the structural and thermophysical properties and a more detailed report on recent results obtained by Raman spectroscopy on both thin films, polycrystalline, and single crystal material. The results presented include a comparison between Raman results for photopolymerized and pressure polymerized thin films and a preliminary estimate of the binding energy of polymeric C60.
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| 8. |
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| 9. |
- Sing, M., et al.
(författare)
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Electronic structure studies of pressure-polymerized C60
- 1999
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Ingår i: Synthetic Metals vol. 103. - : Elsevier B.V.. ; 103:1-3, s. 2454-2455
-
Konferensbidrag (refereegranskat)abstract
- We have investigated the electronic structure of one and two-dimensional C60 polymers with regard to both their low-lying excitations and C 1s excitation spectra by means of electron energy-loss spectroscopy in transmission. We compare the results with those for pristine C60. In general, the spectra for the polymers resemble those for pristine C60 but show a broadening due to both the lowering of the symmetry and the increased intermolecular overlap. This is also reflected by a reduction of the optical gap in comparison with pristine C60.
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| 10. |
- Soldatov, Alexander, et al.
(författare)
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Molecular rotation in C70 at high pressures: a thermal conductivity study
- 1996
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier Science Ltd. - 0022-3697 .- 1879-2553. ; 57:9, s. 1371-1375
-
Tidskriftsartikel (refereegranskat)abstract
- We have measured the thermal conductivity ε and the heat capacity per unit volume varrhocp of highly pure C70 in the temperature interval 100–450 K under pressures up to 1 GPa. Anomalies indicating freezing of uniaxial molecular rotation were observed in λ and varrhocp upon both cooling and increasing pressure. The phase boundary for this transition has an approximate slope dT/dp = 70 K Gpa−1. The temperature and pressure dependence of λ indicate a substantial amount of structural defects in the sample and strong metastability effects.
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| 11. |
- Soldatov, Alexander, et al.
(författare)
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Polymeric fullerenes: from C60 to C70
- 1999
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Ingår i: AIP Conference Proceedings vol. 486. - Melville, NY : American Institute of Physics (AIP). - 1563969009 ; , s. 12-15
-
Konferensbidrag (refereegranskat)abstract
- For the first time polymerization of both powder and single crystals of C70 fullerene was established after their subjection to high pressure (1.1 - 2 GPa) at elevated temperature (500 - 580 K). High-resolution capacitance dilatometry, FTIR/Raman spectroscopy and thermal conductivity were employed to characterise the polymeric phase of C70. The results demonstrate drastic changes in the physical properties of C70 on polymerization. We report on a reverse transformation to the monomeric state on heating the polymer to 500 K at ambient pressure. The activation energy of depolymerization was determined to be 1.8(1) eV. We discuss our results in terms of existing structural models for polymerization of C70 and compare the physical properties of C70 and C60 polymers.
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| 12. |
- Soldatov, Alexander, et al.
(författare)
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Transport and vibrational properties of pressure polymerized C60
- 1998
-
Ingår i: Molecular Materials (Molecular Crystals and Liquid Crystals, Section C), vol. 11. - Amsterdam : Overseas Publishers Association. ; 11:1-2, s. 1-6
-
Konferensbidrag (refereegranskat)abstract
- The vibrational spectrum of polycrystalline C60 polymerized at 1.1 GPa and 585 K was studied by inelastic neutron scattering. We find drastic changes in the spectrum compared to the vibrational spectrum of pristine C60: the appearance of a new, broad vibrational band at low energies, and splitting and significant changes in the peak positions of various modes. The thermal conductivity lambda of polymerized C60 was measured in the temperature range 150-320 K and was found to increase with a rise in temperature, which reflects strong phonon scattering. A high degree of structural disorder in the crystalline lattice of the polymeric phase is most probably responsible for the glass-like beahviour of lambda(T).
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| 13. |
- Sundqvist, Bertil, et al.
(författare)
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Phase diagram, structure, and disorder in C60 below 300 K and 1 GPa
- 1995
-
Ingår i: Solid State Communications. - : Elsevier Science Ltd. - 0038-1098 .- 1879-2766. ; 93:2, s. 109-112
-
Tidskriftsartikel (refereegranskat)abstract
- Earlier structural studies have shown that the pentagon-to-hexagon orientation ratio in the orientationally ordered simple cubic phase of C60 decreases under pressure. From anomalies observed in the compressibility and thermal conductivity of C60 under pressure we have deduced a pressure-temperature phase diagram for this substance in the range below 300 K and 1 GPa (10 kbar). We conclude that C60 forms a new, completely “hexagon” ordered structural phase above about 0.6 GPa at 150 K (1.2 GPa at 300 K), and that the glass transition shifts upwards in T under pressure by 54 K GPa−1. However, above 0.1 GPa, pentagon-to-hexagon orientation relaxation seems to occur on heating at an almost pressure independent temperature near 100 K.
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| 14. |
- Sundqvist, Bertil, et al.
(författare)
-
Structural and physical properties of pressure polymerized C60
- 1998
-
Ingår i: Carbon, vol. 36 issue 5-6. - : Elsevier B.V.. ; 36:5-6, s. 657-660
-
Konferensbidrag (refereegranskat)abstract
- We discuss the structural and dynamic properties of C60 polymerized under low-P, low-T conditions, and suggest that the disordered mixed orthorhombic-tetragonal-rhombohedral phases produced under these conditions arise from nucleation of molecular chains in random directions because of the quasi-free molecular rotation under standard reaction conditions in the fcc phase of C60. Polymerization in He gives results qualitatively different from those obtained in other media.
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| 15. |
- Wågberg, Thomas, et al.
(författare)
-
2D polymerization and doping of fullerenes under pressure
- 2000
-
Ingår i: High Pressure Research vol. 18. - : Gordon & Breach. ; 18:1-6, s. 139-143
-
Konferensbidrag (refereegranskat)abstract
- Tetragonal polymeric C60 has been studied by Raman spectroscopy and other methods. Attempts have been made to transform samples from the tetragonal to the orthorhom-bic phase and vice versa. The results suggest that the transformation is direct with no intermediate stage with free molecules. Tetragonal C60 has also been intercalated by potassium metal.
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| 16. |
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| 17. |
- Larson, Jonas, et al.
(författare)
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Spin-orbit-coupled Bose-Einstein condensate in a tilted optical lattice
- 2010
-
Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - : APS. - 1050-2947 .- 1094-1622. ; 82:4, s. 043620-
-
Tidskriftsartikel (refereegranskat)abstract
- Bloch oscillations appear for a particle in a weakly tilted periodic potential. The intrinsic spin Hall effect is an outcome of a spin-orbit coupling. We demonstrate that both of these phenomena can be realized simultaneously in a gas of weakly interacting ultracold atoms exposed to a tilted optical lattice and to a set of spatially dependent light fields inducing an effective spin-orbit coupling. It is found that both the spin Hall and the Bloch oscillation effects may coexist, showing, however, a strong correlation between the two. These correlations are manifested as a transverse spin current oscillating in-phase with the Bloch oscillations. On top of the oscillations originating from the periodicity of the model, a trembling motion is found which is believed to be atomic Zitterbewegung. It is argued that the damping of these Zitterbewegung oscillations may to a large extent be prevented in the present setup considering a periodic optical lattice potential.
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| 18. |
- Lebegue, Sebastien, et al.
(författare)
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Ab initio study of the electronic properties and Fermi surface of the uranium dipnictides
- 2006
-
Ingår i: PHYSICAL REVIEW B. - 1098-0121. ; 73:4, s. 045119-
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of the uranium dipnictides UX2 (X=As, Sb, and Bi) is investigated by means of ab initio calculations based on density functional theory. The calculated Fermi surfaces are presented and compared to available experimental models obtained from de Haas-van Alphen experiments. In agreement with experiments they are found to have a significant two-dimensional character. Also, the change of the electronic properties through the series is discussed.
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| 19. |
- Aleksandrovskii, A. N., et al.
(författare)
-
Negative thermal expansion of fullerite C60 at helium temperatures
- 1997
-
Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 23:11, s. 943-946
-
Tidskriftsartikel (refereegranskat)abstract
- The thermal expansion of fullerite C60 has been measured in the temperature range 2–9 K. A compacted fullerite sample with a diameter of about 6 mm and height of 2.4 mm was used. It was found that at temperatures below ~ 3.4 K the linear thermal expansion coefficient becomes negative. At temperatures above 5 K our results are in good agreement with the available literature data. A qualitative explanation of the results is proposed
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| 20. |
- Andersson, Ove, et al.
(författare)
-
Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K
- 1996
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54:5, s. 3093-3100
-
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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| 21. |
- Azimi Mousolou, Vahid, 1982-
(författare)
-
Quantum Holonomy for Many-Body Systems and Quantum Computation
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The research of this Ph. D. thesis is in the field of Quantum Computation and Quantum Information. A key problem in this field is the fragile nature of quantum states. This be comes increasingly acute when the number of quantum bits (qubits) grows in order to perform large quantum computations. It has been proposed that geometric (Berry) phases may be a useful tool to overcome this problem, because of the inherent robustness of such phases to random noise. In the thesis we investigate geometric phases and quantum holonomies (matrix-valued geometric phases) in many-body quantum systems, and elucidate the relationship between these phases and the quantum correlations present in the systems. An overall goal of the project is to assess the feasibility of using geometric phases and quantum holonomies to build robust quantum gates, and investigate their behavior when the size of a quantum system grows, thereby gaining insights into large-scale quantum computation. In a first project we study the Uhlmann holonomy of quantum states for hydrogen-like atoms. We try to get into a physical interpretation of this geometric concept by analyzing its relation with quantum correlations in the system, as well as by comparing it with different types of geometric phases such as the standard pure state geometric phase, Wilczek-Zee holonomy, Lévay geometric phase and mixed-state geometric phases. In a second project we establish a unifying connection between the geometric phase and the geometric measure of entanglement in a generic many-body system, which provides a universal approach to the study of quantum critical phenomena. This approach can be tested experimentally in an interferometry setup, where the geometric measure of entanglement yields the visibility of the interference fringes, whereas the geometric phase describes the phase shifts. In a third project we propose a scheme to implement universal non-adiabatic holonomic quantum gates, which can be realized in novel nano-engineered systems such as quantum dots, molecular magnets, optical lattices and topological insulators. In a fourth project we propose an experimentally feasible approach based on “orange slice” shaped paths to realize non- Abelian geometric phases, which can be used particularly for geometric manipulation of qubits. Finally, we provide a physical setting for realizing non-Abelian off-diagonal geometric phases. The proposed setting can be implemented in a cyclic chain of four qubits with controllable nearest-neighbor interactions. Our proposal seems to be within reach in various nano-engineered systems and therefore opens up for first experimental test of the non-Abelian off-diagonal geometric phase.
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| 22. |
- Castelain, Mickaël, et al.
(författare)
-
Fast uncoiling kinetics of F1C pili expressed by uropathogenic Escherichia coli are revealed on a single pilus level using force-measuring optical tweezers
- 2011
-
Ingår i: European Biophysics Journal. - : Springer Science and Business Media LLC. - 0175-7571 .- 1432-1017. ; 40:3, s. 305-316
-
Tidskriftsartikel (refereegranskat)abstract
- Uropathogenic Escherichia coli (UPEC) expressvarious kinds of organelles, so-called pili or fimbriae, thatmediate adhesion to host tissue in the urinary tract throughspecific receptor-adhesin interactions. The biomechanicalproperties of these pili have been considered important forthe ability of bacteria to withstand shear forces from rinsingurine flows. Force-measuring optical tweezers have beenused to characterize individual organelles of F1C typeexpressed by UPEC bacteria with respect to such properties.Qualitatively, the force-versus-elongation response wasfound to be similar to that of other types of helix-like piliexpressed by UPEC, i.e., type 1, P, and S, with force-inducedelongation in three regions, one of which represents theimportant uncoiling mechanism of the helix-like quaternarystructure. Quantitatively, the steady-state uncoiling forcewas assessed as 26.4 ±1.4 pN, which is similar to those ofother pili (which range from 21 pN for SI to 30 pN for type 1).The corner velocity for dynamic response (1,400 nm/s) wasfound to be larger than those of the other pili (400–700 nm/sfor S and P pili, and 6 nm/s for type 1). The kinetics werefound to be faster, with a thermal opening rate of 17 Hz, afew times higher than S and P pili, and three orders ofmagnitude higher than type 1. These data suggest that F1Cpili are, like P and S pili, evolutionarily selected to primarilywithstand the conditions expressed in the upper urinary tract.
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| 23. |
- Persson, Per-Axel, et al.
(författare)
-
High-pressure polymerized C60
- 1996
-
Ingår i: Extended Abstracts of Carbon´96, European Carbon Conference, Newcastle-upon-Tyne 1996, volume 2. - : The British Carbon Group. ; , s. 746-747, s. 746-747
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 24. |
- Soldatov, Alexander, et al.
(författare)
-
Topochemical polymerization of C70 controlled by monomer crystal packing
- 2001
-
Ingår i: Science. - : American Association for the Advanement of Science. - 0036-8075 .- 1095-9203. ; 293:5530, s. 680-683
-
Tidskriftsartikel (refereegranskat)abstract
- Polymeric forms of C60 are now well known, but numerous attempts to obtain C70 in a polymeric state have yielded only dimers. Polymeric C70 has now been synthesized by treatment of hexagonally packed C70 single crystals under moderate hydrostatic pressure (2 gigapascals) at elevated temperature (300°C), which confirms predictions from our modeling of polymeric structures of C70. Single-crystal x-ray diffraction shows that the molecules are bridged into polymeric zigzag chains that extend along the c axis of the parent structure. Solid-state nuclear magnetic resonance and Raman data provide evidence for covalent chemical bonding between the C70 cages.
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| 25. |
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| 26. |
- Winter, Johannes, et al.
(författare)
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Charge transfer in alkali-metal-doped polymeric fullerenes
- 1996
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54, s. 17486-17492
-
Tidskriftsartikel (refereegranskat)abstract
- We present room-temperature Raman measurements of pressure-polymerized C60 and compare them with the spectra of RbC60 in the orthorhombic phase. Although both materials were prepared according to two completely different routes the spectra show a surprising similarity with respect to mode positions and line splitting. We concluded from this that both materials, the uncharged pressure-polymerized C60 and the rubidium-doped orthorhombic compound, have the same overall structure and the AC60 compounds can be considered as the doped species of the C60, polymerized using moderate low pressure and high temperatures. From a detailed comparison between both spectra mode shifting and line broadening as a consequence of the charge transfer was determined and electron-phonon coupling constants were estimated for the high-frequency Hg(7) and Hg(8) modes. The low values for the coupling constants compared to the ones in the K3C60 can explain the lack of superconductivity in the AC60 compounds.
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| 27. |
- Wågberg, Thomas, et al.
(författare)
-
Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers
- 2006
-
Ingår i: The European Physical Journal B: Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 49:1, s. 59-65
-
Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C60. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.
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| 28. |
- Sjöqvist, Erik, et al.
(författare)
-
Conceptual aspects of geometric quantum computation
- 2016
-
Ingår i: Quantum Information Processing. - : Springer Science and Business Media LLC. - 1570-0755 .- 1573-1332. ; 15:10, s. 3995-4011
-
Tidskriftsartikel (refereegranskat)abstract
- Geometric quantum computation is the idea that geometric phases can be used to implement quantum gates, i.e., the basic elements of the Boolean network that forms a quantum computer. Although originally thought to be limited to adiabatic evolution, controlled by slowly changing parameters, this form of quantum computation can as well be realized at high speed by using nonadiabatic schemes. Recent advances in quantum gate technology have allowed for experimental demonstrations of different types of geometric gates in adiabatic and nonadiabatic evolution. Here, we address some conceptual issues that arise in the realizations of geometric gates. We examine the appearance of dynamical phases in quantum evolution and point out that not all dynamical phases need to be compensated for in geometric quantum computation. We delineate the relation between Abelian and non-Abelian geometric gates, and find an explicit physical example where the two types of gates coincide. We identify differencies and similarities between adiabatic and nonadiabatic realizations of quantum computation based on non-Abelian geometric phases.
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| 29. |
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| 30. |
- G.E. Grechnev,, et al.
(författare)
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Electronic structure and magnetic properties of Lithium manganese spinels
- 2003
-
Ingår i: J.Magn. Magn. Matt.. ; 258, s. 287-289
-
Tidskriftsartikel (refereegranskat)abstract
- Electronic and magnetic structures of the spinel-type lithium–manganese oxides LixMn2O4, x=0,0.5,1, are studied ab initio by employing a full-potential LMTO method. The effect of the orthorhombic distortion on electronic structure and magnetism of LiMn2O4 was investigated, and our calculations do not show a substantial charge ordering at the structural transition from the cubic spinel to the orthorhombic structure.
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| 31. |
- Joyce, J.J., et al.
(författare)
-
Photoemission and the electronic structure of PuGaCo$_5$
- 2003
-
Ingår i: Physical Review Letters. ; 91, s. 176401-
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of the first Pu-based superconductor PuCoGa5 is explored using photoelectron spectroscopy and a novel theoretical scheme. Exceptional agreement between calculation and experiment defines a path forward for understanding the electronic structure aspects of Pu-based materials. The photoemission results show two separate regions of 5f electron spectral intensity, one at the Fermi energy and another centered 1.2 eV below the Fermi level. The results for PuCoGa5 clearly indicate 5f electron behavior on the threshold between localized and itinerant. Comparisons to delta phase Pu metal show a broader framework for understanding the fundamental electronic properties of the Pu 5f levels in general within two configurations, one localized and one itinerant.
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| 32. |
- Talyzin, Alexandr, 1969-, et al.
(författare)
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Brodie vs Hummers graphite oxides for preparation of multi-layered materials
- 2017
-
Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 115, s. 430-440
-
Tidskriftsartikel (refereegranskat)abstract
- Graphite oxides synthesized by one and two step Brodie oxidation (BGO) and Hummers (HGO) methods were analyzed by a variety of characterization methods in order to evaluate the reasons behind the difference in their properties. It is found that the Brodie method results in a higher relative amount of hydroxyl groups and a more homogeneous overall distribution of functional groups over the planar surface of the graphene oxide flakes. The higher number of carbonyl and carboxyl groups in HGO, detected by several methods, including XPS, NMR and FTIR, unavoidably results in defects of the graphene "skeleton", holes and overall disruption of the carbon-carbon bond network, stronger deviation from planar flake shape and poor ordering of the graphene oxide layers. It is also suggested that functional groups in HGO are less homogeneously distributed over the flake surface, forming some nanometer-sized graphene areas. The presence of differently oxidized areas on the GO surface results in inhomogeneous solvation and hydration of HGO and effects of inter- and intra-stratification. The proposed interpretation of the data explains the higher mechanical strength of multi-layered BGO membranes/papers, which are also less affected by humidity changes, thus providing an example of a membrane property superior to that of HGO. (C) 2016 Published by Elsevier Ltd.
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| 33. |
- Vidya, R., et al.
(författare)
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Tailor-made electronic and magnetic properties in one-dimensional perovskite-like oxides
- 2003
-
Ingår i: Physical Review Letters. ; 91, s. 186404-
-
Tidskriftsartikel (refereegranskat)abstract
- Full-potential density-functional calculations show that the electronic structure of one-dimensional ferrimagnetic Ca3Co2O6 varies from metal to half metal to insulator as its magnetic ordering changes from the ferrimagnetic through the ferromagnetic to the paramagnetic state. The present Letter is the first to establish the occurrence of half metallicity in one-dimensional oxides. Moreover, the electronic and magnetic properties of this material can be tuned by substitution of Y for Ca, as shown by our detailed study on Ca3–xYxCo2O6 (x = 0, 0.3, 0.75, and 1). The Co ions are in two different valence states [Co4+ (low-spin) and Co2+ (high-spin)], and hence the occurrence of charge ordering in addition to spin ordering is established. For specific Y concentrations we predict a rarely seen combination of ferromagnetic and insulating behavior.
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| 34. |
- Du, Yi-Hsien, et al.
(författare)
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Noncommutative field theory of the Tkachenko mode : Symmetries and decay rate
- 2024
-
Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 6:1
-
Tidskriftsartikel (refereegranskat)abstract
- We construct an effective field theory describing the collective Tkachenko oscillation mode of a vortex lattice in a two-dimensional rotating Bose-Einstein condensate in the long-wavelength regime. The theory has the form of a noncommutative field theory of a Nambu-Goldstone boson, which exhibits a noncommutative version of dipole symmetry. From the effective field theory, we show that, at zero temperature, the decay width Γ of the Tkachenko mode scales with its energy E as Γ∼E3 in the low-energy limit. We also discuss the width of the Tkachenko mode at a small temperature.
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| 35. |
- Dick Thelander, Kimberly
(författare)
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Epitaxial Growth and Design of Nanowires and Complex Nanostructures
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes the epitaxial growth of III-V semiconductor nanowires using Au seed particles, and the design of more complex three-dimensional branched structures from these wires. Growth was performed by metallorganic vapour phase epitaxy, in which precursor molecules for the semiconductor material components are introduced in a low-pressure vapour. Nanowires grow epitaxially (with controlled crystal orientation) on a semiconductor substrate; diameter control is achieved via the Au particles, while length is controlled by growth parameters. Particle-assisted nanowire growth is used extensively today to achieve well-controlled structures. The current understanding of this growth mechanism was developed over forty years ago, and is known as the Vapour-Liquid-Solid (VLS) mechanism. This model indicates that a liquid alloy is formed between the seed particle and the growth material(s), and growth proceeds by precipitation from a supersaturated particle. The enhanced growth rate compared to the bulk growth from the vapour is typically attributed to preferential decomposition of precursor materials at or near the particle surface. Recently, however, several inconsistencies have been observed between this model, which was developed for Au-assisted Si whiskers (micro-scale wires), and particle-assisted growth of other materials. In particular, there have been many reports of nanowire growth at temperatures too low for a liquid alloy to form. As well, nanowire growth has been reported for systems where no precursor molecules are used, and thus growth enhancement cannot be explained by preferential decomposition. Other reports have given evidence that such preferential decomposition does not necessarily occur when precursors are used. The first part of this thesis presents the current understanding of particle-assisted growth, both generally and for the specific materials and growth systems used here. Semiconductor nanowires present the possibility for numerous applications, and many simple device components have been demonstrated. The development of practical applications of such prototypes may rely on the ability to assemble nanowires into more complex structures. The second part of this thesis presents techniques for the production of three-dimensional branched nanowire structures, including methods to achieve controlled structure and morphology. The assembly of branched structures into large-scale interconnected networks is also presented.
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| 36. |
- Fan, Qunping, 1989, et al.
(författare)
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Over 14% efficiency all-polymer solar cells enabled by a low bandgap polymer acceptor with low energy loss and efficient charge separation
- 2020
-
Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry. - 1754-5692 .- 1754-5706. ; 13:12, s. 5017-5027
-
Tidskriftsartikel (refereegranskat)abstract
- Obtaining both high open-circuit voltage (V-oc) and short-circuit current density (J(sc)) has been a major challenge for efficient all-polymer solar cells (all-PSCs). Herein, we developed a polymer acceptor PF5-Y5 with excellent optical absorption capability (onset extending to similar to 880 nm and maximum absorption coefficient exceeding 105 cm(-1) in a film), high electron mobility (3.18 x 10(3) cm(2) V-1 s(-1)) and high LUMO level (-3.84 eV) to address such a challenge. As a result, the PBDB-T:PF5-Y5-based all-PSCs achieved a high power conversion efficiency of up to 14.45% with both a high Voc (0.946 V) and a high Jsc (20.65 mA cm(-2)), due to the high and broad absorption coverage, small energy loss (0.57 eV) and efficient charge separation and transport in the device, which are among the best values in the all-PSC field. In addition, the all-PSC shows a similar to 15% improvement in PCE compared to its counterpart small molecule acceptor (Y5)-based device. Our results suggest that PF5-Y5 is a very promising polymer acceptor candidate for applications in efficient all-PSCs.
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| 37. |
- Ulfat, I., et al.
(författare)
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Effects of nonuniform Mn distribution in (Ga,Mn)As
- 2014
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Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 89:4, s. art no 045312-
-
Tidskriftsartikel (refereegranskat)abstract
- Resonant in situ photoemission from Mn 3d states in Ga(1−x)MnxAs is reported for Mn concentrations down to the very dilute level of 0.1%. Concentration-dependent spectral features are analyzed on the basis of first-principles calculations for systems with selected impurity positions as well as for random alloys. Effects of direct Mn-Mn interaction are found for concentrations as low as 2.5%, and are ascribed to statistical (nonuniform) distribution of Mn atoms.
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| 38. |
- Bergkvist Sylvan, Sara, 1977-
(författare)
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Numerical studies of spin chains and cold atoms in optical lattices
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An important, but also difficult, research field in condensed matter physics is that of strongly correlated systems. This thesis considers two topics in this field. The first topic is disorder and frustration in spin models. The introduction of disorder into quantum spin chains creates a complex problem. The ground state of the random-bond spin-1 Heisenberg chain is studied by means of stochastic series expansion quantum Monte Carlo simulation, applying the concept of directed loops. It is found that this system undergoes a phase transition to the random-singlet phase if the bond disorder is strong enough. Further a frustrated spin system is investigated. The frustration is introduced by having spins positioned on a triangular lattice. Performing a quantum Monte Carlo simulation for such a frustrated lattice leads to the occurrence of the infamous sign problem. This problem is investigated and it is shown that it is possible to use a meron cluster approach to reduce its effect for some specific models. The second topic concerns atomic condensates in optical lattices. A system of trapped bosonic atoms in such a lattice is described by a Bose-Hubbard model with an external confining potential. Using quantum Monte Carlo simulations it is demonstrated that the local density approximation that relates the observables of the unconfined and the confined models yields quantitatively correct results in most of the interesting parameter range of the model. Further, the same model with the addition that the atoms carry spin-1 is analyzed using density matrix renormalization group calculations. The anticipated phase diagram, with Mott insulating regions of dimerized spin-1 chains for odd particle density, and on-site singlets for even density is confirmed. Also an ultracold gas of bosonic atoms in an anisotropic two dimensional optical lattice is studied. It is found that if the system is finite in one direction it exhibits a quantum phase transition. The Monte Carlo simulations performed show that the transition is of Kosterlitz-Thouless type.
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| 39. |
- Colarieti Tosti, Massimiliano
(författare)
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Theory of Crystal Fields and Magnetism of f-electron Systems
- 2004
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A parameter free approach for the calculation of the crystal field splitting of the lowest Russel-Saunders J-multiplet in f-electron systems has been developed and applied to selected compounds. The developed theory is applicable to general symmetries and is based on symmetry constrained density functional theory calculations in the local density or in the generalised gradient approximation.The magnetocrystalline anisotropy of Gd has been analysed. It has been shown that the peculiar orientation of the easy axis of magnetisation is consistent with an S-ground state. Further, the temperature dependence of the easy axis of magnetisation has been investigated and it has been shown that the temperature driven reduction of the effective magnetisation is the principal mechanism responsible for it.A new method has been developed that allows for theoretical studies of the electronic structure and total energy of elements and compounds in an intermediate valence regime. The method combines model and first-principles band structure calculations, therefore being accurate and computationally efficient. It has been applied to Yb metal under pressure obtaining a remarkable agreement with experimental observations for the equation of state and the x-ray absorption spectroscopy.
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| 40. |
- Seung Ki, Baek, 1979-, et al.
(författare)
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Phase transition of q-state clock models on heptagonal lattices
- 2009
-
Ingår i: Physical Review E - Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755. ; 80:1, s. 011133-
-
Tidskriftsartikel (refereegranskat)abstract
- We study the q-state clock models on heptagonal lattices assigned on a negatively curved surface. We show that the system exhibits three classes of equilibrium phases; in between ordered and disordered phases, an intermediate phase characterized by a diverging susceptibility with no magnetic order is observed at every q >= 2. The persistence of the third phase for all q is in contrast with the disappearance of the counterpart phase in a planar system for small q, which indicates the significance of nonvanishing surface-volume ratio that is peculiar in the heptagonal lattice. Analytic arguments based on Ginzburg-Landau theory and generalized Cayley trees make clear that the two-stage transition in the present system is attributed to an energy gap of spin-wave excitations and strong boundary-spin contributions. We further demonstrate that boundary effects break the mean-field character in the bulk region, which establishes the consistency with results of clock models on boundary-free hyperbolic lattices.
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| 41. |
- Manzetti, Sergio, et al.
(författare)
-
A Korteweg-DeVries type model for helical soliton solutions for quantum and continuum phenomena
- 2021
-
Ingår i: International Journal of Modern Physics C. - : World Scientific. - 0129-1831. ; 32:03
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum mechanical states are normally described by the Schrödinger equation, which generates real eigenvalues and quantizable solutions which form a basis for the estimation of quantum mechanical observables, such as momentum and kinetic energy. Studying transition in the realm of quantum physics and continuum physics is however more difficult and requires different models. We present here a new equation which bears similarities to the Korteweg–DeVries (KdV) equation and we generate a description of transitions in physics. We describe here the two- and three-dimensional form of the KdV like model dependent on the Plank constant ℏ and generate soliton solutions. The results suggest that transitions are represented by soliton solutions which arrange in a spiral-fashion. By helicity, we propose a conserved pattern of transition at all levels of physics, from quantum physics to macroscopic continuum physics.
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| 42. |
- Bessarab, Pavel F., et al.
(författare)
-
Lifetime of racetrack skyrmions
- 2018
-
Ingår i: Scientific Reports. - : Springer. - 2045-2322. ; 8
-
Tidskriftsartikel (refereegranskat)abstract
- The skyrmion racetrack is a promising concept for future information technology. There, binary bits are carried by nanoscale spin swirls-skyrmions-driven along magnetic strips. Stability of the skyrmions is a critical issue for realising this technology. Here we demonstrate that the racetrack skyrmion lifetime can be calculated from first principles as a function of temperature, magnetic field and track width. Our method combines harmonic transition state theory extended to include Goldstone modes, with an atomistic spin Hamiltonian parametrized from density functional theory calculations. We demonstrate that two annihilation mechanisms contribute to the skyrmion stability: At low external magnetic field, escape through the track boundary prevails, but a crossover field exists, above which the collapse in the interior becomes dominant. Considering a Pd/Fe bilayer on an Ir(111) substrate as a well-established model system, the calculated skyrmion lifetime is found to be consistent with reported experimental measurements. Our simulations also show that the Arrhenius pre-exponential factor of escape depends only weakly on the external magnetic field, whereas the pre-exponential factor for collapse is strongly field dependent. Our results open the door for predictive simulations, free from empirical parameters, to aid the design of skyrmion-based information technology.
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| 43. |
- Laraña Aragón, Jorge, 1993-
(författare)
-
Linear response theory : from black hole thermalization to Weyl semimetals
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Linear response theory is an incredibly powerful calculation tool. We apply this framework in quantum field theory to a variety of models originated from distinct areas in theoretical physics and for different reasons. In the context of black hole holography, we consider a quench model where we investigate effective thermalization as well as the boundary signal of the so called evanescent modes which indicate the presence of a black hole like object in the bulk. The problem of quantum thermalization plays a central role within the holographic duality between thermal states in the boundary field theory and black hole like objects in the bulk. However, quantum thermalization is also an interesting question in itself from a fundamental point of view and with that motivation we continue to explore this phenomenon further. Inspired by recent progress in understanding how operators in quantum field theories thermalize, which occurs even when considering integrable models, we investigate the so called operator thermalization hypothesis. We focus on gauge theories at finite temperature with a large number of fields which present a phase transition between the low-temperature and high-temperature regimes. In particular, these theories are the so called vector model and the adjoint matrix model. Last, within the common background of linear response theory we investigate transport properties in a family of Weyl semimetal systems. Concretely, we develop a general analytic method to compute the magneto-optical conductivity of these systems in the presence of an external magnetic field aligned with the tilt of the spectrum.
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| 44. |
- Rusz, Ján, et al.
(författare)
-
Influence of plural scattering on the quantitative determination of spin and orbital moments in electron magnetic chiral dichroism measurements
- 2011
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 83:13, s. 132402-
-
Tidskriftsartikel (refereegranskat)abstract
- Recent quantitative measurements of the orbital to spin magnetic moment ratio m(L)/m(S) in electron magnetic circular dichroism (EMCD) experiments have given a m(L)/m(S) ratio that is larger than commonly accepted values. We demonstrate here that plural scattering may noticeably influence the m(L)/m(S) ratio. An equation is derived which describes its influence as a function of the spectral integrals of the plasmon scattering region and zero-loss peak. The influence of the electron-plasmon scattering can be removed when electron energy-loss spectra of the ionization edge are deconvoluted by the low-loss signal. For a bcc-Fe sample we obtain m(L)/m(S) = 0.04 after plasmon removal. We conclude that the plural scattering should be considered when extracting quantitative information from EMCD measurements.
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| 45. |
- Frigerio, Jacopo, et al.
(författare)
-
Mid-infrared second harmonic generation in Ge/SiGe coupled quantum wells
- 2020
-
Ingår i: 2020 IEEE Photonics Conference, IPC 2020 - Proceedings. ; September 2020
-
Konferensbidrag (refereegranskat)abstract
- We present the theoretical investigation and the experimental demonstration of second harmonic generation in the mid-infrared by hole-doped Ge/SiGe asymmetric quantum wells.
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| 46. |
- Negi, Devendra Singh, et al.
(författare)
-
Proposal for Measuring Magnetism with Patterned Apertures in a Transmission Electron Microscope
- 2019
-
Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 122:3
-
Tidskriftsartikel (refereegranskat)abstract
- We propose a magnetic measurement method utilizing a patterned post-sample aperture in a transmission electron microscope. While utilizing electron magnetic circular dichroism, the method circumvents previous needs to shape the electron probe to an electron vortex beam or astigmatic beam. The method can be implemented in standard scanning transmission electron microscopes by replacing the spectrometer entrance aperture with a specially shaped aperture, hereafter called ventilator aperture. The proposed setup is expected to work across the whole range of beam sizes -- from wide parallel beams down to atomic resolution magnetic spectrum imaging.
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| 47. |
- Waltersson, Erik, 1979-, et al.
(författare)
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Controlled operations in a strongly correlated two-electron quantum ring
- 2009
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 79:11, s. 115318-
-
Tidskriftsartikel (refereegranskat)abstract
- We have analyzed the electronic spectrum and wave-function characteristics of a strongly correlated two-electron quantum ring with model parameters close to those observed in experiments. The analysis is based on an exact diagonalization of the Hamiltonian in a large B-spline basis. We propose a qubit pair for storing quantum information, where one component is stored in the total electron spin and one multivalued “quMbit” is represented by the total angular momentum. In this scheme the controlled-NOT quantum gate is demonstrated with near 100% fidelity for a realistic far-infrared electromagnetic pulse.
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| 48. |
- Wersäll, Martin, 1985, et al.
(författare)
-
Directional Nanoplasmonic Antennas for Self-Referenced Refractometric Molecular Analysis
- 2014
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:36, s. 21075-21080
-
Tidskriftsartikel (refereegranskat)abstract
- Localized surface-plasmon resonance (LSPR) sensors are typically based on tracing resonance peak shifts that precisely follow changes in the local refractive index. Such measurements usually require a spectrometer, a stable light source, and an accurate LSPR position tracing technique. As a simple but efficient alternative, we investigated a self-referenced single-wavelength sensing scheme based on angle-dependent and highly directional radiation patterns originating from a monolayer of asymmetric gold nanodimers. We found that one could easily trace a model biotinneutravidin recognition reaction as well as minute bulk refractive index changes, by measuring the intensity ratio between the light scattered in two different directions with respect to the dimers. The refractometric resolution of the methodology was estimated to be on the order of Delta n approximate to 10(-5) RIU. These results may be particularly useful for label-free biosensing applications that require a combination of simple and cost-effective optical readout with a reasonable sensitivity.
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| 49. |
- Zhang, Jiang, et al.
(författare)
-
Fast non-Abelian geometric gates via transitionless quantum driving
- 2015
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 5
-
Tidskriftsartikel (refereegranskat)abstract
- A practical quantum computer must be capable of performing high fidelity quantum gates on a set of quantum bits (qubits). In the presence of noise, the realization of such gates poses daunting challenges. Geometric phases, which possess intrinsic noise-tolerant features, hold the promise for performing robust quantum computation. In particular, quantum holonomies, i.e., non-Abelian geometric phases, naturally lead to universal quantum computation due to their non-commutativity. Although quantum gates based on adiabatic holonomies have already been proposed, the slow evolution eventually compromises qubit coher- ence and computational power. Here, we propose a general approach to speed up an implementation of adiabatic holonomic gates by using transitionless driving techniques and show how such a universal set of fast geometric quantum gates in a superconducting circuit architecture can be obtained in an all geometric approach. Compared with standard non-adiabatic holonomic quantum computation, the holonomies ob- tained in our approach tends asymptotically to those of the adiabatic approach in the long run-time limit and thus might open up a new horizon for realizing a practical quantum computer.
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| 50. |
- Baggioli, Matteo, et al.
(författare)
-
Holographic plasmon relaxation with and without broken translations
- 2019
-
Ingår i: Journal of High Energy Physics (JHEP). - : SPRINGER. - 1126-6708 .- 1029-8479. ; :9
-
Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics and the relaxation of bulk plasmons in strongly coupled and quantum critical systems using the holographic framework. We analyze the dispersion relation of the plasmonic modes in detail for an illustrative class of holographic bottom-up models. Comparing to a simple hydrodynamic formula, we entangle the complicated interplay between the three least damped modes and shed light on the underlying physical processes. Such as the dependence of the plasma frequency and the effective relaxation time in terms of the electromagnetic coupling, the charge and the temperature of the system. Introducing momentum dissipation, we then identify its additional contribution to the damping. Finally, we consider the spontaneous symmetry breaking (SSB) of translational invariance. Upon dialing the strength of the SSB, we observe an increase of the longitudinal sound speed controlled by the elastic moduli and a decrease in the plasma frequency of the gapped plasmon. We comment on the condensed matter interpretation of this mechanism.
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