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| 3. |
- Fleck, M., et al.
(författare)
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Magnetism near the Van Hove singularity
- 1998
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Ingår i: Journal of Magnetism and Magnetic Materials. - 0304-8853. ; 177-181, s. 599-601
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Tidskriftsartikel (refereegranskat)abstract
- We study the phase diagrams of the s-band and d-band Hubbard model in two dimensions including electron correlations. While the ferromagnetic instability in the s-band case occurs only near the Van Hove singularity, ferromagnetism is more stable in the d-band model, and the values of the Stoner parameters in transition metals are reduced below their LDA values.(14 refs)
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| 4. |
- Fleck, M., et al.
(författare)
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Quasiparticle broadening in insulating copper oxides
- 1999
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Ingår i: Physica B. ; 259-261, s. 729-731
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Tidskriftsartikel (refereegranskat)abstract
- We calculate the spectral function of the half-filled two-dimensional Hubbard model with realistic hopping integrals for Sr2CuO2Cl2 within the dynamical mean-field theory at finite temperatures. It is shown that precursor effects of incommensurate spin dynamics lead to an incoherent broadening of the quasiparticle peaks on an energy scale that depends on local spin order. This might serve as an indirect method of detecting changes of local spin order under doping.
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| 5. |
- Arct, Aneta, et al.
(författare)
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Effects of elevated nest box temperature on incubation behaviour and offspring fitness-related traits in the Collared Flycatcher Ficedula albicollis
- 2022
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Ingår i: Journal of Ornithology. - : Springer Nature. - 2193-7192 .- 2193-7206. ; 163, s. 263-272
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Tidskriftsartikel (refereegranskat)abstract
- Ambient temperature experienced by an animal during development or subsequently as an adult can affect many aspects of its behaviour and life-history traits. In birds, egg incubation is a vital component of reproduction and parental care. Several studies have suggested that environmental factors (such as nest microclimate) can influence the ability of incubating parents to maintain suitable conditions for embryo development. Here, we manipulated the developmental conditions of embryos through a modification of nest box thermal microclimate to investigate female incubation behaviour and its impact on offspring fitness-related traits in a wild population of the Collared Flycatcher (Ficedula albicollis). The temperature in experimental nests was increased using a heat-pack placed under the roof of a nest box, resulting in an average temperature increase of 2.5 degrees C, which corresponds to projected climate change scenarios. We demonstrated that females from nests with elevated temperature spent less time in the nest box during egg incubation and had more off-bouts than females from control nests. Moreover, we found that offspring from the experimentally heated nests had larger body mass at fledging in comparison to the control ones. Our study indicates that nest microclimate during the incubation period affects female incubation behaviour and offspring quality, indicating that environmental variation in nest temperature early in ontogeny can have important and long-lasting fitness consequences.
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- Basak, Surajit, et al.
(författare)
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Electronic and dynamical properties of cobalt monogermanide CoGe phases under pressure
- 2024
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 244
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Tidskriftsartikel (refereegranskat)abstract
- We present the pressure dependence of the electronic and dynamical properties of six different CoGe phases with orthorhombic Cmmm, hexagonal P6/mmm and P (6) over bar 2m, monoclinic C2/m, cubic P213, and orthorhombic Pnma symmetries. Using first-principles DFT calculations and the direct force-constants method, we study the dynamical stability of individual phases under external pressure. We show that the orthorhombic (Cmmm) and hexagonal (P6/mmm) structures are unstable over a broad pressure range and most pronounced imaginary phonon soft mode in both cases leads to a stable hexagonal (P6(over bar)2m) structure of the lowest ground-state energy of all studied phases at ambient and low (below ∼ 3 GPa) external pressure. Under these conditions, the cubic structure has the highest energy, however, together with monoclinic and orthorhombic phases, it is dynamically stable and all these three structures can potentially coexist as meta-stable phases. Above ∼ 3 GPa, the cubic phase becomes the most energetically favorable. Fitting the Birch-Murnaghan equation of state, we derive bulk modulus for all mentioned phases. The results indicate relatively high resistance of CoGe to compression. Such conclusions are confirmed by band structure calculations. Additionally, we show that electronic bands of the hexagonal (P6(over bar)2m) phase reveal characteristic features of the kagome-like structure, while in the cubic phase the electronic bands contain spin-1 and double Weyl fermions. In both cases, the external pressure induces the Lifshitz transition, related to the modification of the Fermi surface topology.
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| 7. |
- Fleck, Marcus, et al.
(författare)
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Dynamical Mean-Field Theory for Doped Antiferromagnets
- 1998
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Ingår i: Physical Review Letters. - 1079-7114. ; 80, s. 2393-2393
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Tidskriftsartikel (refereegranskat)abstract
- We have generalized the dynamical mean-field theory to study the doping dependence of the crossover from antiferromagnetic to short-range order modeled by an incommensurate spin density wave in the Hubbard model. The local self-energy which includes spin fluctuations gives quasiparticle weights and spectral properties in good agreement with quantum Monte Carlo and exact diagonalization data in two dimensions. The spectra at finite doping are characterized by a Mott-Hubbard “gap” accompanied by a pseudogap induced by the local spin order.
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| 8. |
- Fleck, Marcus, et al.
(författare)
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Magnetic phases near the Van Hove singularity in s- and d-band Hubbard models
- 1997
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Ingår i: Physical Review B. - 1550-235X. ; 56:6, s. 3159-3159
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the magnetic instabilities of the nondegenerate (s-band) and a degenerate (d-band) Hubbard model in two dimensions using many-body effects due to particle-particle diagrams and Hund’s rule local correlations. The density of states and the position of the Van Hove singularity change depending on the value of next-nearest-neighbor hopping t′. The Stoner parameter is strongly reduced in the s-band case, and ferromagnetism survives only if the electron density is small and the band has flat regions. Due to next-nearest-neighbor hopping there are flat regions in Γ−X and Γ−Y directions. In contrast, for the d-band case the reduction of the Stoner parameter which follows from particle-particle correlations is much smaller and ferromagnetism survives to a large extent. Inclusion of local spin-spin correlations has a limited destabilizing effect on the magnetic states.
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| 9. |
- Fleck, Marcus, et al.
(författare)
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Optical properties of doped antiferromagnets
- 1999
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Ingår i: Journal of Superconductivity. - 0896-1107. ; 12:1, s. 147-149
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the consequences of an incommensurate magnetic order in doped La2-xSrxCuO4 using dynamical mean-field theory for the effective single-band model. The high-energy optical transitions are due to high-energy excitations across large Mott-Hubbard 'gap,' while low-energy excitations involve a pseudogap induced by the local spin order. The latter lead to a strong drop in the scattering rate at low ω, in qualitative agreement with the experimental data.(16 refs)
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