Emilian Nica

Kondo Destruction and Quantum Criticality in Kondo Lattice Systems

Considerable efforts have been made in recent years to theoretically understand quantum phase transitions in Kondo lattice systems. A particular focus is on Kondo destruction, which leads to quantum criticality that goes beyond the Landau framework of order-parameter fluctuations. This unconventional quantum criticality has provided an understanding of the unusual dynamical scaling observed experimentally. It also predicted a sudden jump of the Fermi surface and an extra (Kondo destruction) energy scale, both of which have been verified by systematic experiments. Considerations of Kondo destruction have in addition yielded a global phase diagram, which has motivated the current interest in heavy fermion materials with variable dimensionality or geometrical frustration. Here we summarize these developments, and discuss some of the ongoing work and open issues. We also consider the implications of these results for superconductivity. Finally, we address the effect of spin–orbit coupling on the global phase ...

Orbital-selective pairing and superconductivity in iron selenides

An important challenge in condensed matter physics is understanding iron-based superconductors. Among these systems, the iron selenides hold the record for highest superconducting transition temperature and pose especially striking puzzles regarding the nature of superconductivity. The pairing state of the alkaline iron selenides appears to be of d-wave type based on the observation of a resonance mode in neutron scattering, while it seems to be of s-wave type from the nodeless gaps observed everywhere on the Fermi surface. Here we propose an orbital-selective pairing state, dubbed sτ3, as a natural explanation of these disparate properties. The pairing function, containing a matrix τ3 in the basis of 3d-electron orbitals, does not commute with the kinetic part of the Hamiltonian. This dictates the existence of both intraband and interband pairing terms in the band basis. A spin resonance arises from a d-wave-type sign change in the intraband pairing component, whereas the quasiparticle excitation is fully gapped on the FS due to an s-wave-like form factor associated with the addition in quadrature of the intraband and interband pairing terms. We demonstrate that this pairing state is energetically favored when the electron correlation effects are orbitally selective. More generally, our results illustrate how the multiband nature of correlated electrons affords unusual types of superconducting states, thereby shedding new light not only on the iron-based materials but also on a broad range of other unconventional superconductors such as heavy fermion and organic systems.Unconventional superconductivity: Orbital selective pairing in iron selenidesOrbital-selective pairing could explain the unusual properties observed in the unconventional superconductor iron selenide. Conventional superconductivity arises when electrons form Cooper pairs due to electron-phonon coupling. In some materials, however, unconventional superconductivity can arise, which is driven by electron-electron rather than electron-phonon couplings. The detailed mechanism that facilitates electron pairing in unconventional systems remains elusive but iron selenide systems could help to provide insights as they exhibit both relatively high temperature superconductivity, and also strong electron correlations. With different experiments suggesting different pairing mechanisms, however, these systems are somewhat puzzling. An international team of researchers led by Qimiao Si from Rice University now theoretically demonstrate that an orbital-selective pairing state could explain this unusual behaviour, which may also be at play in other unconventional superconductors such as heavy fermion and organic systems.A major puzzle about the nature of the iron-based superconductivity appears in the case of the alkaline iron selenides. Compared to the iron pnictides, these systems have only electron Fermi pockets (i.e. no hole Fermi pockets) but comparable superconducting transition temperatures. The challenge lies in reconciling the two basic experimental features of their superconducting state: a node-less gap and the existence of a resonance in the spin excitation spectrum. We propose a mechanism based on reconstructing two quasi-degenerate pairing states, one in an

Fully gapped d-wave superconductivity in CeCu2Si2

s

Antiferroquadrupolar Order and Rotational Symmetry Breaking in a Generalized Bilinear-Biquadratic Model on a Square Lattice

-wave

Glide reflection symmetry, Brillouin zone folding, and superconducting pairing for the P4/nmm space group

A_{1g}

Quantum critical Kondo destruction in the Bose-Fermi Kondo model with a local transverse field

channel that is fully gapped, and the other in a

Charge transport in graphene-based mesoscopic realizations of Sachdev-Ye-Kitaev models.

d

Orbital-selective superconductivity in the nematic phase of FeSe

-wave

Landau levels from neutral Bogoliubov particles in two-dimensional nodal superconductors under strain and doping gradients

B_{1g}

Kondo destruction in multipolar order: Implications for heavy-fermion quantum criticality

channel whose pairing function changes sign across the electron Fermi pockets at the Brillouin-zone boundary. The resulting intermediate pairing state, which we call an orbital-selective