arXiv subjects

T. Jarlborg

Publications and source records attributed to T. Jarlborg.

49 records · Page 3Linked to original sources

arXiv2001

Spin-phonon interaction in doped high-T$_C$ superconductors from density functional calculations

Effects of coupling between phonon distortions and stripe-like spin waves in the CuO plane of HgBa$_2$CuO$_4$ are studied by band calculations. Local exchange enhancements depend sensitively on the local structure around Cu sites. Interactions where spin waves have twice as long wave length as phonon waves can lead to a 'dip' in the density of states (DOS) below the Fermi energy, $E_F$. This type of interaction is compatible with several seemingly contradictory observations among high-$T_C$ cuprates, like the isotope effect, anti-ferromagnetic fluctuations, stripes and pseudogaps. It can also account for a large $T_C$.

cond-mat.supr-con↗

arXiv2001

Longitudinal spin-fluctuations and superconductivity in ferromagnetic ZrZn$_{2}$ from ab initio calculations

The recent discovery of superconductivity coexisting with weak itinerant ferromagnetism in the $d$-electron intermetallic compound ZrZn$_{2}$ strongly suggests spin-fluctuation mediated superconductivity. {\it Ab initio} electronic structure calculations of the Fermi surface and generalized susceptibilities are performed to investigate the viability of longitudinal spin-fluctuation-induced spin-triplet superconductivity in the ferromagnetic state. The critical temperature is estimated to be of the order of 1 K. Additionally, it is shown that in spite of a strong electron-phonon coupling ($λ_{ph}=0.7$), conventional s-wave superconductivity is inhibited by the presence of strong spin-fluctuations.

cond-mat.supr-con↗

arXiv2001

Electronic structure, magnetism and superconductivity of MgCNi$_{3}$

The electronic structure of the newly discovered superconducting perovskite MgCNi$_3$ is calculated using the LMTO and KKR methods. The states near the Fermi energy are found to be dominated by Ni-d. The Stoner factor is low while the electron-phonon coupling constant is estimated to be about 0.7, which suggests that the material is a conventional type of superconductor where T$_C$ is not affected by magnetic interactions. However, the proximity of the Fermi energy to a large peak in the density of states in conjunction with the reported non-stoichiometry of the compound, has consequences for the stability of the results.

cond-mat.supr-con↗

arXiv2001

Electronic structure and magnetism in slightly doped SrB_6

Spin-polarized band calculations for supercells of SrB$_{6}$, where a La-, In- or Al- impurity or a vacancy is replacing one Sr, are performed within the local spin density approximation. Moderately large cells with 8 formula units (56 atoms) are studied for all dopings and large ones with 27 formula units (189 atoms) for the case of La-doping. The undoped system has a vanishing density-of-states (DOS) at the Fermi energy (E$_F$), while the additional La-d band makes E$_F$ to enter the bands above the gap. An Al (or In) impurity has the opposite effect, with a rigid-band like shift of E_F to below the gap. In the former case, the addition of a d-electron makes a local, impurity-like modification of the electronic structure close to the La atom. As has been shown in a previous publication, it can lead to a weak ferromagnetic state. This result is shown to persist at even lower doping, by the 189-atom cell calculations, with a moment of the order 0.1 $μ_B$ per La impurity. An addition of a p-electron, by an In-impurity, makes no similar effect. The magnetic state can be understood from a charge transfer and an additional gain in potential energy, as a spin-splitting is imposed. Different models with modified localization of the La-band are made in order to show the correlation between charge transfer and the size of the magnetic moment.

cond-mat.mtrl-sci↗

arXiv2001

Spinwaves and large electron-phonon coupling near the metal-insulator transition in hole-doped high-T$_C$ oxides

A theory for spin and lattice couplings to the electronic states in high-T$_C$ oxides is presented, with HgBa$_2$CuO$_4$ as an example. A simple analytical model suggests that the barrel band is sensitive to potential perturbations with long-range Fourier components, and that gaps can be opened at the Fermi energy. This is confirmed in self-consistent band calculations for modulations in elongated supercells, where spin waves and phonon coupling compete for equal q-vectors. It is argued that the wavelengths of spin-wave modulations and of the phonon modes with large coupling, depend on the doping. This mechanism supports the idea that pseudogaps are caused by stripe like spin-modulations in underdoped systems, while superconductivity is attributed to enhanced electron-phonon coupling for long wavelength phonons.

cond-mat.supr-con↗

arXiv2000

Ferromagnetism below the Stoner limit in La-doped SrB_6

Spin-polarized band calculations for LaSr_7B_{48} show a weak ferro-magnetic state. This is despite a low density-of-states (DOS) and a low Stoner factor. The reason for the magnetic state is found to be associated with a gain in potential energy in addition to the exchange energy, as a spin-splitting is imposed. An impurity like La DOS is essential for this effect. It makes a correction to the Stoner factor, and provides an explanation of the recently observed weak ferro-magnetism in doped hexaborides.

cond-mat.mtrl-sci↗

arXiv1999

Nesting properties and anisotropy of the Fermi surface of LuNi$_{2}$B$_{2}$C

The rare earth nickel borocarbides, with the generic formula $R$Ni$_{2}$B$_{2}$C, have recently been shown to display a rich variety of phenomena. Most striking has been the competition between, and even coexistence of, antiferromagnetism and superconductivity. We have measured the Fermi surface (FS) of LuNi$_{2}$B$_{2}$C, and shown that it possesses nesting features capable of explaining some of the phenomena experimentally observed. In particular, it had previously been conjectured that a particular sheet of FS is responsible for the modulated magnetic structures manifest in some of the series. We report the first direct experimental observation of this sheet.

cond-mat↗

arXiv1999

Electronic structure without exchange?

The correlation holes for densities of equal and opposite spin around a test electron are determined from the Schrödinger equation with proper boundary conditions. The traditional "exchange" term follows from the boundary condition which respects a spacial exclusion principle for equal spins. The resulting potential compares reasonably well with standard local density potentials and should simplify extensions towards non-local effects.

cond-mat↗

arXiv1999

Induced spin polarisation in a ferromagnetic gadolinium-yttrium alloy

The first direct evidence of an induced spin moment in Gd(62.4)Y(37.6) is presented. This additional moment, of 0.16 +/- 0.03 Bohr magnetons, arises from polarisation of Y electrons in the alloy. The moment was detected in a Compton scattering experiment via the measurement of the one dimensional projection of the momentum space electron spin density in Gd and in the alloy. The result is consistent with theoretical predictions calculated using the LMTO method within the local spin density approximation.

cond-mat↗

arXiv1998

Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}

Materials that exhibit colossal magnetoresistance (CMR) are currently the focus of an intense research effort, driven by the technological applications that their sensitivity lends them to. Using the angular correlation of photons from electron-positron annihilation, we present a first glimpse of the Fermi surface of a material that exhibits CMR, supported by ``virtual crystal'' electronic structure calculations. The Fermi surface is shown to be sufficiently cubic in nature that it is likely to support nesting.

cond-mat.str-el↗

arXiv1998

Electronic structure and properties of pure and doped $ε$-FeSi from ab-initio local density theory

Local density calculations of the electronic structure of FeSi, FeSi_{1-x}Al_x and Fe_{1-x}Ir_xSi systems in the B20 structure are presented. Pure FeSi has a semi-conducting gap of 6 mRy at 0 K. Effects of temperature (T) in terms of electronic and vibrational excitations are included. Various measurable properties, such as magnetic susceptibility chi(T), electronic specific heat C(T), thermoelectric power S(T), relative variations in resistivity rho(T), and peak positions in the density-of-states (DOS) are calculated. The feedback from vibrational disorder onto the electronic structure is found to be essential for a good description of most properties, although the results for S(T) in undoped FeSi can be described up to about 150 K without considerations of disorder. Above this T, only the filling of the gap due to disorder accompanied by exchange enhancement, can explain the large susceptiblity. The overall good agreement with experimental data for most properties in doped and pure FeSi suggests that this system is well described by LDA even at large T. Doped FeSi can be described quite well from rigid-band shifts of the Fermi energy on the DOS of pure FeSi. Spin-polarization in Ir doped FeSi leads to a semi-metallic magnetic state at low T. (Submitted to Phys. Rev. B)

cond-mat.mtrl-sci↗

arXiv1996

s and d-wave symmetries of the solutions of the Eliashberg equations

We examine the different possible symmetries of the superconducting gap obtained by solving the Eliashberg equations. We consider an electron-phonon interaction in a strong coupling scenario. The Coulomb pseudopotential plays the crucial role of providing the repulsion needed to favour the d-wave symmetry. But the key parameter that allows very anisotropic solutions even with very strong coupling is the small angular range of the interaction due to predominantly electron-phonon forward scattering that is found in the high-Tc superconductors. We find both s and d-wave solutions whose stability depends mainly on the angular range of the interaction.

supr-con↗

arXiv1996

Transport Properties, Thermodynamic Properties, and Electronic Structure of SrRuO3

SrRuO$_3$ is a metallic ferromagnet. Its electrical resistivity is reported for temperatures up to 1000K; its Hall coefficient for temperatures up to 300K; its specific heat for temperatures up to 230K. The energy bands have been calculated by self-consistent spin-density functional theory, which finds a ferromagnetic ordered moment of 1.45$μ_{\rm B}$ per Ru atom. The measured linear specific heat coefficient $γ$ is 30mJ/mole, which exceeds the theoretical value by a factor of 3.7. A transport mean free path at room temperature of $\approx 10 Å$ is found. The resistivity increases nearly linearly with temperature to 1000K in spite of such a short mean free path that resistivity saturation would be expected. The Hall coefficient is small and positive above the Curie temperature, and exhibits both a low-field and a high-field anomalous behavior below the Curie temperature.

cond-mat↗