T. Jarlborg

Electronic structure, magnetism, and superconductivity of MgC x Ni 3

The electronic structure of the newly discovered superconducting perovskite MgCNi3 is calculated using the LMTO method. 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 0.5, which suggests that the material is a conventional type of superconductor where Tc 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 nonstoichiometry of the compound has consequences for the stability of the results.

Superconductivity above the lowest Earth temperature in pressurized sulfur hydride

A recent experiment has shown a macroscopic quantum coherent condensate at 203 K, about 19 degrees above the coldest temperature recorded on the Earth, 184 K, in pressurized sulfur hydride. This discovery is relevant not only in material science and condensed matter but also in other fields ranging from quantum computing to quantum physics of living matter. It has given the start to a gold rush looking for other macroscopic quantum coherent condensates in hydrides at the temperature range of living matter 200<Tc<400K. We present here a review of the experimental results and the theoretical works and we discuss the Fermiology of H3S focusing on Lifshitz transitions as a function of pressure. We discuss the possible role of the shape resonance near a neck disrupting Lifshitz transition, in the Bianconi-Perali Valletta (BPV) theory, for rising the critical temperature in a multigap superconductor, as the Feshbach resonance rises the critical temperature in Fermionic ultracold gases.

Effects of spin–phonon interaction within the CuO plane of high-TC superconductors

The mechanism of spin–phonon coupling (SPC) in high-TC copper oxides is explored from band calculations on La(2−x)SrxCuO4 and HgBa2CuO4 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-TC 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.

Breakdown of the Migdal approximation at Lifshitz transitions with giant zero-point motion in the H3S superconductor.

While 203 K high temperature superconductivity in H3S has been interpreted by BCS theory in the dirty limit here we focus on the effects of hydrogen zero-point-motion and the multiband electronic structure relevant for multigap superconductivity near Lifshitz transitions. We describe how the topology of the Fermi surfaces evolves with pressure giving different Lifshitz-transitions. A neck-disrupting Lifshitz-transition (type 2) occurs where the van Hove singularity, vHs, crosses the chemical potential at 210 GPa and new small 2D Fermi surface portions appear with slow Fermi velocity where the Migdal-approximation becomes questionable. We show that the neglected hydrogen zero-point motion ZPM, plays a key role at Lifshitz transitions. It induces an energy shift of about 600 meV of the vHs. The other Lifshitz-transition (of type 1) for the appearing of a new Fermi surface occurs at 130 GPa where new Fermi surfaces appear at the Γ point of the Brillouin zone here the Migdal-approximation breaks down and the zero-point-motion induces large fluctuations. The maximum Tc = 203 K occurs at 160 GPa where EF/ω0 = 1 in the small Fermi surface pocket at Γ. A Feshbach-like resonance between a possible BEC-BCS condensate at Γ and the BCS condensate in different k-space spots is proposed.

Effects of gradient corrections on electronic structure in metals

Gradient corrections to the local density (LD) potential proposed by Perdew and Wang (PW) and to some extent by Langreth, Mehl and Hu (LMH) have been used in self-consistent LMTO band calculatiom in order to detellnine groundstate and bands properties in some 3d, 4d and 5d transition metals, in the alkali metal Li and in Ce. The effect compared with LD is to expand and soften the lattice, which in general is too drastic for an improvement especially for 4d and 5d metals. Magnetic properties in Fe and Ni do not improve while the effect on some general band properties are only slightly better than in LD calculations. Despite some cases of improvement we conclude that the overall results obtained via gradient-corrected potentials are not yet sufficiently good to replace the LD potential.

Magnetic and structural properties of MnSi

The electronic structure of MnSi is calculated using the LMTO method. We show total energy results and densities of states for different lattice constants of both paramagnetic and spin-polarized calculations. Magnetic moments and exchange splittings are given as a function of pressure. The structural stability of the magnetic phase is discussed in terms of Stoners criterion and from total energy calculations. The ground state is found to be magnetic for all lattice constants ranging from 8.30 to 9.10 a.u.

Longitudinal Spin Fluctuations and Superconductivity in Ferromagnetic ZrZn2 from Ab Initio Calculations

The recent discovery of superconductivity coexisting with weak itinerant ferromagnetism in the d-electron intermetallic compound ZrZn2 strongly suggests spin-fluctuation mediated superconductivity. 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 ( lambda(ph) = 0.7), conventional s-wave superconductivity is inhibited by the presence of strong spin fluctuations.

Lifshitz transitions and zero point lattice fluctuations in sulfur hydride showing near room temperature superconductivity

Abstract Emerets’s experiments on pressurized sulfur hydride have shown that H3S metal has the highest known superconducting critical temperature Tc = 203 K. The Emerets data show pressure induced changes of the isotope coefficient between 0.25 and 0.5, in disagreement with Eliashberg theory which predicts a nearly constant isotope coefficient.We assign the pressure dependent isotope coefficient to Lifshitz transitions induced by pressure and zero point lattice fluctuations. It is known that pressure could induce changes of the topology of the Fermi surface, called Lifshitz transitions, but were neglected in previous papers on the H3S superconductivity issue. Here we propose thatH3S is a multi-gap superconductor with a first condensate in the BCS regime (located in the large Fermi surface with high Fermi energy) which coexists with second condensates in the BCS-BEC crossover regime (located on the Fermi surface spots with small Fermi energy) near the and Mpoints.We discuss the Bianconi-Perali-Valletta (BPV) superconductivity theory to understand superconductivity in H3S since the BPV theory includes the corrections of the chemical potential due to pairing and the configuration interaction between different condensates, neglected by the Eliashberg theory. These two terms in the BPV theory give the shape resonance in superconducting gaps, similar to Feshbach resonance in ultracold fermionic gases, which is known to amplify the critical temperature. Therefore this work provides some key tools useful in the search for new room temperature superconductors.

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,

Electronic structure and magnetism of modulated structures

{\mathrm{FeSi}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x},