arXiv subjects

T. Jarlborg

Publications and source records attributed to T. Jarlborg.

At least 37 records · Page 2Linked to original sources

arXiv2009

Mechanisms for higher $T_C$ in copper oxide superconductors; Ideas from band calculations

Band calculations for the hole doped La$_2$CuO$_4$ system show that artificial periodicities of Ba dopants can give the material different properties than from a uniform distribution of dopants. A periodicity within the planes make static pseudogaps which could be tuned to raise the density-of-states (DOS) at $E_F$ and the superconducting $T_C$. A periodic doping dependence perpendicular to the CuO planes can increase the matrix element for spin fluctuations.

cond-mat.supr-con↗

arXiv2009

Properties of high-$T_c$ copper oxides from band models of spin-phonon coupling

The mechanism of spin-phonon coupling (SPC) and possible consequencies for the properties of high-$T_C$ copper oxides are presented. The results are based on ab-initio LMTO band calculations and a nearly free-electron (NFE) model of the band near $E_F$. Many observed properties are compatible with SPC, as for the relation between doping and $\vec{q}$ for spin excitations and their energy dependence. The main pseudogap is caused by SPC and waves along [1,0,0], but it is suggested that secondary waves, generated along [1,1,0], contribute to a 'waterfall' structure. Conditions for optimal $T_C$, and the possiblities for spin enhancement at the surface are discussed.

cond-mat.supr-con↗

arXiv2008

The importance of local band effects for ferromagnetism in hole doped La$_2$CuO$_4$

Band calculations for supercells of La$_{(2-x)}$Ba$_x$CuO$_4$ show that the rigid band model for doping is less adequate than what is commonly assumed. In particular, weak ferromagnetism (FM) can appear locally around clusters of high Ba concentration. The clustering is important at large dilution and averaged models for magnetism, such as the virtual crystal approximation, are unable to stabilize magnetic moments. These results give a support to the idea that weak FM can be the cause of the destruction of superconductivity at high hole doping.

cond-mat.supr-con↗

arXiv2008

Spin-phonon coupling, q-dependence of spin excitations and high-T$_C$ superconductivity from band models

An understanding of spin excitations in cuprates is essential since the mechanism of high-T$_C$ superconductivity might be linked to spin fluctuations. Band calculations for long "1-dimensional" unit cells of La$_2$CuO$_4$ show that the coupling between antiferromagnetic spin waves and phonons is larger for distortions of oxygens than for Cu or La. When this result is applied to a 2-dimensional, free-electron like band, it leads to a q-dependent spin excitation spectrum in good agreement with recent experiments. It is argued that important parameters for spin-phonon coupling, which comes out from the comparison between experiment and theory, are relevant for the mechanism of superconductivity, and are large enough to explain a high T$_C$.

cond-mat.supr-con↗

arXiv2007

Properties of high-T$_C$ copper oxides from the nearly-free electron model

The generic band structure of high-T$_C$ copper oxides is simulated by the nearly free-electron model (NFE) in two dimensions (2-D) with parameters from band calculations. Interaction between phonons and spin waves will cause potential modulations and pseudogaps, and the strength of the modulations, the wave lengths and the doping, are all related. A Fermi-surface "arc" is found for dynamic spin/phonon waves. The confinement of superconductivity between two limiting dopings can be a result of competition with the pseudogap at low doping and weak coupling at high doping.

cond-mat.supr-con↗

arXiv2007

The importance of thermal disorder and electronic occupation for the T-dependence of the optical conductivity in FeSi and MnSi

The spectral weight (SW) for optical transitions in FeSi and MnSi are calculated as function of temperature by means of LMTO-LDA band calculations. The main effects, caused by structural disorder and electronic Fermi-Dirac distribution, act oppositely on the T-dependence of the SW, while the variation of the magnetic moment in MnSi has only a minor effect. The calculations agree with the experimental findings of an increasing SW in FeSi and a decreasing SW in MnSi as function of T. The results can be understood from the change of the bandstructure with disorder.

cond-mat.mtrl-sci↗

arXiv2007

Optical evidence for heavy charge carriers in FeGe

The optical spectrum of the cubic helimagnetic metal FeGe has been investigated in the frequency range from 0.01 - 3.1 eV for different temperatures from 30 K to 296 K. The optical conductivity shows the evolution of a low energy (0.22 eV) interband transition and the development of a narrow free carrier response with a strong energy and temperature dependence. The frequency dependent effective mass and scattering rate derived from the optical data indicate the formation of dressed quasi-particles with a mass renormalization factor of 12. Similar to FeSi the spectral weight in FeGe is not recovered over a broad frequency range, an effect usually attributed to the influence of the on-site Coulomb interaction.

cond-mat.str-el↗

arXiv2006

Varying Cu-Ti hybridization near the Fermi energy in Cu$_{x}$TiSe$_{2}$: Results from supercell calculations

The properties of Cu$_{x}$TiSe$_{2}$ are studied by band structure calculation based on the density functional theory for supercells. The density-of-states (DOS) for $x$=0 has a sharply raising shoulder in the neighborhood of the Fermi energy, $E_F$, which can be favorable for spacial charge modulations. The Cu impurity adds electrons and brings the DOS shoulder below $E_F$. Hybridization makes the Ti-d DOS at $E_F$, the electron-phonon coupling and the Stoner factor very large. Strong pressure dependent properties are predicted from the calculations, since the DOS shoulder is pushed to higher energy at a reduced lattice constant. Effects of disorder are also expected to be important because of the rapidly varying DOS near $E_F$.

cond-mat.mtrl-sci↗

arXiv2006

Metallic State in Cubic FeGe beyond its Quantum Phase Transition

We report on results of electrical resistivity and structural investigations on the cubic modification of FeGe under high pressure. The long-wavelength helical order ($T_C=280$ K) is suppressed at a critical pressure $p_c\approx 19$ GPa. An anomaly in the resistivity data at $T_X(p)$ and strong deviations from a Fermi-liquid behavior in a wide pressure range above $p_c$ suggest that the suppression of $T_C$ disagrees with the standard notion of a quantum critical phase transition. The metallic ground state persisting at high pressure can be described by band-structure calculations if structural disorder due to zero-point motion is included. Discontinuous changes in the pressure dependence of the shortest Fe-Ge interatomic distance occurring close to the $T_C(p)$ phase line could be interpreted as a symmetry-conserving transition of first order.

cond-mat.str-el↗

arXiv2006

Effects of spin-phonon interaction on the properties of in high-T$_C$ superconductors

The mechanism of spin-phonon coupling in high-T$_C$ copper oxides is explored from band calculations on La$_{(2-x)}$Sr$_x$CuO$_4$ and HgBa$_2$CuO$_4$ systems. The LMTO band calculations, based on the local density approximation, are made for cells containing frozen phonon displacements and/or spin waves within the CuO plane. The virtual crystal approximation is used for studies of hole doped systems. The main result is that phonons are favorable for spin waves and vice-versa, and that pseudogaps appear naturally in the band structures of striped materials with strong SPC. The qualitative results are compatible with many observations showing that the properties of high-T$_C$ superconductors depend both on lattice interactions and magnetic fluctuations. The band results are used to model various properties, mainly of the normal state, such as isotope effects, pseudogaps, Fermi surface broadening, T-dependence of the pseudogap, phonon softening and some aspects of superconductivity. The possibility of perpendicular SPC is investigated, partly by the use of a nearly free electron model.

cond-mat.mtrl-sci↗

arXiv2005

Spin-Phonon Coupling in High-T_c Copper Oxides

Band calculations on HgBa$_2$CuO$_4$ and La$_{(2-x)}$Sr$_x$CuO$_4$ with phonon and spin-waves within the CuO planes show that partial gaps are created at various energies depending on wavelengths. Spin and phonon gaps appear at different energies when the modulations are along [1,1,0], while they are at the same energy for modulations along [1,0,0]. It is shown that the ability to form gaps and antiferromagnetic waves is correlated with the strength of the interaction parameter $λ_{sf}$ for spin fluctuations. Many unusual properties of the high-T$_C$ oxides can be understood from spin-phonon coupling.

cond-mat.supr-con↗

arXiv2004

Electronic structure and magnetism for FeSi$_{(1-x)}$Ge$_x$ from supercell calculations

Recent studies of FeSi$_{(1-x)}$Ge$_x$, which found a transition from an insulating to a magnetic metallic state near $x$=0.25, have revived the discussion about the role of strong correlation in these systems. Here are spin polarized band calculations made for 64-atom supercells of FeSi$_{(1-x)}$Ge$_x$ for different $x$ and different volumes for large $x$. The results show that the small band gap in FeSi is closed for $x \geq 0.3$, because of both substitutional disorder and increased volume. Ferromagnetism appears near this composition and becomes enforced for increasing $x$. The $x$-dependence of the electronic specific heat can be understood from the exchange splitting of the density-of-states near the gap. Strong volume dependencies for the properties of FeGe suggest experiments using pressure instead of $x$ for investigations of the gap.

cond-mat.mtrl-sci↗

arXiv2004

Anti-ferromagnetism, spin-phonon interaction and the local-density approximation in high-T$_C$ superconductors

Results from different sets of band calculations for undoped and doped HgBa$_2$CuO$_4$ show that small changes in localization can lead to very different ground states. The normal LDA results are compared with 'modified' LDA results, in which different linearization energies make the O-p band more localized. The ground states in the normal calculations are far from the anti-ferromagnetic ones, while nearly AFM states are found in the modified calculations. The proximity of an AFM state in the doped system leads to increased $λ_{sf}$, and the modified band structure has favorable conditions for spin-phonon coupling and superconductivity mediated by spin fluctuations.

cond-mat.supr-con↗

arXiv2003

Spin-phonon interaction and band effects in the high-T_C superconductor HgBa_2CuO_4

Band calculations show that a stripe-like anti-ferromagnetic spin wave is enforced by a 'half-breathing' phonon distortion within the CuO plane of HgBa_2CuO_4. This spin-phonon coupling is increased further by shear distortion and by increased distance between Cu and apical oxygens. The effects from spin-phonon coupling are consistent with many observations in high-T_C materials. Spin-phonon coupling can be important for the mechanism of spin fluctuations and superconductivity, although the effects are quantitatively weak when using the local density potential.

cond-mat.supr-con↗

arXiv2003

Temperature dependence of magnetism near defects in SrB_6

The T-dependence of magnetic moments in SrB_6 is studied through spin-polarized band calculations for a supercell of Sr_{27}B_{156} containing a B_6 vacancy. The magnetic moment decays rather quickly with T despite the fact that only electronic Fermi-Dirac effects are included. This result and the T-dependence of moments near a La impurity can hardly explain the reports of a very high Curie temperature in hexaborides, but suggest that the magnetism is caused by some other type of impurity.

cond-mat.mtrl-sci↗

arXiv2003

Electronic structure and magnetism in X_xW_{1-x}O_3 (X=Nb,V,Re) from supercell calculations

Some doped semiconductors have recently been shown to display superconductivity or weak ferromagnetism. Here we investigate the electronic structure and conditions for magnetism in a supercells of cubic XW_{26}O_{81}, where X=Nb,V and Re. The undoped material is an insulator, and although the slightly doped material is a metal, it is far from the Stoner criterion of magnetism. The conditions of a localized density-of-states which varies rapidly with the energy, resemble those of doped hexaborides. The virtual crystal approximation is used to vary the doping level. A small moment appears if the Fermi energy coincides with a large derivative of the DOS.

cond-mat.mtrl-sci↗

arXiv2002

Spin fluctuations, electron-phonon coupling and superconductivity in near-magnetic elementary metals; Fe,Co,Ni and Pd

An investigation of possibilities for superconductivity mediated by spin fluctuations in some elementary metals is motivated by the recent discovery of superconductivity in the hcp high-pressure phase of iron. The electronic structure, the electron-phonon coupling ($λ_{ph}$) and the coupling due to spin-fluctuations ($λ_{sf}$) are calculated for different phases and different volumes for four elementary metals. The results show that such possibilities are best for systems near, but on the non-magnetic side of, a magnetic instability. Fcc Ni, which show stable magnetism over a wide pressure range, is not interesting in this respect. Ferro- and antiferro-magnetic fluctuations in hcp Fe contribute to a relatively strong coupling in the pressure range where superconductivity is observed. The absence of fluctuations at large q-vectors makes fcc Pd only moderately interesting despite its large exchange enhancement for q=0. Fcc Co at high pressure ($\sim$ 0.5 Mbar) behaves as an improved version of Pd, where the fluctuations extend to larger q. The estimations of T$_C$, which reproduce the experimental situation in Fe quite well, suggest a measurable T$_C$ for the high-pressure phase of fcc Co, while the estimate is lower for the ambient-pressure phase of fcc Pd.

cond-mat.supr-con↗

arXiv2001

Ferromagnetic and antiferromagnetic spin fluctuations and superconductivity in the hcp-phase of Fe

High purity iron, which transforms into the hcp phase under pressure, has recently been reported to be superconducing in the pressure range 150-300 kBar [shim]. The electronic structure and the electron-phonon coupling ($λ_{ph}$) are calculated for hcp iron at different volumes. A parameter-free theory for calculating the coupling constants $λ_{sf}$ from ferromagnetic (FM) and antiferromagnetic (AFM) spin fluctuations is developed. The calculated $λ_{sf}$ are sufficiently large to explain superconductivity especially from FM fluctuations. The results indicate that superconductivity mediated by spin fluctuations is more likely than from electron-phonon interaction.

cond-mat.supr-con↗