Michelle Johannes

Unconventional Superconductivity with a Sign Reversal in the Order Parameter of LaFeAsO1-xFx

We argue that the newly discovered superconductivity in a nearly magnetic, Fe-based layered compound is unconventional and mediated by antiferromagnetic spin fluctuations, though different from the usual superexchange and specific to this compound. This resulting state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets. The main role of doping in this scenario is to lower the density of states and suppress the pair-breaking ferromagnetic fluctuations.

Problems with reconciling density functional theory calculations with experiment in ferropnictides

First principles calculations of magnetic and, to a lesser extent, electronic properties of the novel LaFeAsO-based superconductors show substantial apparent controversy, as opposed to most weakly or strongly correlated materials. Not only do different reports disagree about quantitative values, there is also a schism in terms of interpreting the basic physics of the magnetic interactions in this system. In this paper, we present a systematic analysis using four different first principles methods and show that while there is an unusual sensitivity to computational details, well-converged full-potential all-electron results are fully consistent among themselves. What makes results so sensitive and the system so different from simple local magnetic moments interacting via basic superexchange mechanisms is the itinerant character of the calculated magnetic ground state, where very soft magnetic moments and long-range interactions are characterized by a particular structure in the reciprocal (as opposed to real) space. Therefore, unravelling the magnetic interactions in their full richness remains a challenging, but utterly important task.

Fermi surface nesting and the origin of charge density waves in metals

The concept of a charge density wave (CDW), which is induced by Fermi-surface nesting, originated from the Peierls idea of electronic instabilities in purely one-dimensional metals and is now often applied to charge ordering in real low-dimensional materials. The idea is that if Fermi surface contours coincide when shifted along the observed CDW wave vector, then the CDW is considered to be nesting derived. We show that, in most cases, this procedure has no predictive power, since Fermi surfaces either do not nest at the right wave vector or nest more strongly at the wrong vector. We argue that only a tiny fraction, if any, of the observed charge ordering phase transitions are true analogs of the Peierls instability because electronic instabilities are easily destroyed by even small deviations from perfect nesting conditions. By using prototypical CDW materials

What superconducts in sulfur hydrides under pressure and why

{\text{NbSe}}_{2}

Tailoring Native Defects in LiFePO4: Insights from First-Principles Calculations

,

Tetragonal vs. cubic phase stability in Al – free Ta doped Li7La3Zr2O12 (LLZO)

{\text{TaSe}}_{2}

Origin Of The Structural Phase Transition In Li7La3Zr2O12

, and

Fermi surface nesting and the origin of the charge density wave in NbSe2

{\text{CeTe}}_{3}

Unconventional Fermi surface in an insulating state

, we show that such conditions are hardly ever fulfilled and that the CDW phases are actually structural phase transitions driven by the concerted action of electronic and ionic subsystems, i.e., a

Microscopic origin of magnetism and magnetic interactions in ferropnictides

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