We calculate the Hall transport in a multiband system with a dominant interband interaction between carriers having electron and hole character. We show that this situation gives rise to an ...unconventional scenario, beyond the Boltzmann theory, where the quasiparticle currents dressed by vertex corrections acquire the character of the majority carriers. This leads to a larger (positive or negative) Hall coefficient than what may be expected on the basis of the carrier balance, with a marked temperature dependence. Our results explain the puzzling measurements in pnictides and provide a more general framework for transport properties in multiband materials.
The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments call for a prominent role of ...local correlations and place iron superconductors at the entrance of a Hund's metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hund's coupling. We show that Hund's physics strongly affects the nematic properties of the system. It severely constrains the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the zx/yz orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes overlooked so far in the interpretation of experiments. As notable examples the splittings between zx and yz bands at Γ and M points are modified, with important consequences for angle-resolved photoemission spectroscopy measurements.
Electronic correlations in Hund metals Fanfarillo, L.; Bascones, E.
Physical review. B, Condensed matter and materials physics,
08/2015, Letnik:
92, Številka:
7
Journal Article
Odprti dostop
To clarify the nature of correlations in Hund metals and its relationship with Mott physics we analyze the electronic correlations in multiorbital systems as a function of intraorbital interaction U, ...Hund's coupling J sub(H) and electronic filling n. We show that the main process behind the enhancement of correlations in Hund metals is the suppression of the double occupancy of a given orbital, as it also happens in the Mott insulator at half-filling. However, contrary to what happens in Mott correlated states the reduction of the quasiparticle weight Z with J sub(H)can happen in spite of increasing charge fluctuations. Therefore, in Hund metals the quasiparticle weight and the mass enhancement are not good measurements of the charge localization. Using simple energetic arguments we explain why the spin polarization induced by Hund's coupling produces orbital decoupling. We also discuss how the behavior at moderate interactions, with correlations controlled by the atomic spin polarization, changes at large U and J sub(H)due to the proximity to a Mott insulating state.
Abstract
The Eliashberg theory of superconductivity accounts for the fundamental physics of conventional superconductors, including the retardation of the interaction and the Coulomb pseudopotential, ...to predict the critical temperature
T
c
. McMillan, Allen, and Dynes derived approximate closed-form expressions for the critical temperature within this theory, which depends on the electron–phonon spectral function
α
2
F
(
ω
). Here we show that modern machine-learning techniques can substantially improve these formulae, accounting for more general shapes of the
α
2
F
function. Using symbolic regression and the SISSO framework, together with a database of artificially generated
α
2
F
functions and numerical solutions of the Eliashberg equations, we derive a formula for
T
c
that performs as well as Allen–Dynes for low-
T
c
superconductors and substantially better for higher-
T
c
ones. This corrects the systematic underestimation of
T
c
while reproducing the physical constraints originally outlined by Allen and Dynes. This equation should replace the Allen–Dynes formula for the prediction of higher-temperature superconductors.
With Tc ∼ 9.6 K, Be22Re exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound ...to 30 GPa. The data show that the critical temperature Tc is suppressed gradually at a rate of dTc/dP = −0.05K/GPa. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in Be22Re overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in Tc. High-pressure x-ray diffraction measurements show that the ambient pressure crystal structure of Be22Re persists to at least 154 GPa.
Since their discovery, it has been suggested that pairing in pnictides can be mediated by spin fluctuations between hole and electron bands. In this view, multiband superconductivity would ...substantially differ from other systems like MgB sub(2), where pairing is predominantly intraband. Indeed, interband-dominated pairing leads to the coexistence of bonding and antibonding superconducting channels. Here, we show that this has profound consequences on the nature of the low-energy superconducting collective modes. In particular, the so-called Leggett mode for phase fluctuations is absent in the usual two-band description of pnictides. On the other hand, when also the repulsion between the hole bands is taken into account, a more general three-band description should be used, and a Leggett mode is then allowed. Such a model, which has been proposed for strongly hole-doped 122 compounds, can also admit a low-temperature s + is phase that breaks the time-reversal symmetry. We show that the (quantum and thermal) transition from the ordinary superconductor to the s +is state is accompanied by the vanishing of the mass of Leggett-like phase fluctuations, regardless the specific values of the interaction parameters. This general result can be obtained by means of a generalized construction of the effective action for the collective degrees of freedom that allows us also to deal with the nontrivial case of dominant interband pairing.
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