Emergent non-Fermi-liquid phenomena in multipolar quantum impurity systems

Abstract

Discovery of novel spin-orbital entangled quantum ground states paves an important avenue for controllable quantum materials via unique couplings to the lattice and other external perturbations. In this work, motivated by recent experiments on cubic heavy fermion materials with multipolar local moments, we theoretically investigate strongly-interacting spin-orbital entangled quantum ground states in multipolar quantum impurity systems. Here itinerant electrons are interacting with the local moments carrying quadrupolar and octupolar moments, in contrast to the conventional Kondo problem with dipolar local moment. Using perturbative renormalization group methods, we uncover a number of non-Fermi liquid ground states, which are characterized by an absence of well-defined quasiparticles and singular power-law behaviours in physical properties. We show that the non-Fermi liquid states found here are outside the known categories of non-Fermi liquid states in the conventional multi-channel Kondo problem. This work lays a novel ground for the identification of unexpected non-Fermi liquid phases in many strongly spin-orbital-coupled quantum materials.

Unconventional Superconductivity arising from Multipolar Kondo Interactions

“…The nature of the interaction between the multipolar moments and conduction electrons, and the subsequent many-body ground state, is strongly dependent on the available conduction electron spin and orbitals [42][43][44][45]. As a representative example, we consider conduction electrons, characterized by orbital p x, y,z (l = 1) and spin−1/2 degrees of freedom, forming a Fermi surface well localized about the high-symmetry zone-centre of the Brillouin zone [66,67].…”

“…The phenomenological order parameters transform as their microscopic counterparts under the generating elements of the T d point group (improper rotation S 4z and a C 3 rotation along the [111] axis), namely [43,53]:…”

“…As a consequence of their non-trivial transformations under lattice symmetries, conduction electrons may interact with them in atypical manners. For instance, in the single-impurity limit, so-called multipolar Kondo interactions lead to the development of both multi-channel as well as exotic non-Fermi liquid states, where both the conduction electron spin and orbital degrees of freedom become intertwined under scattering events with the moment [42][43][44][45]. In the generalized lattice setting, this Kondo effect competes with RKKY interactions between the moments leading to a rich phase diagram of exotic phenomena including hidden multipolar-ordered phases [46][47][48][49][50][50][51][52][53], emergent non-Fermi liquids [54][55][56], and unconventional superconductivity [57][58][59][60][61] in the neighbourhood of a putative quantum critical point [62,63].…”

Quantum criticality and sequential destruction of spin-orbital Kondo entanglement

“…Because they couple with each other and with conduction electrons in new ways, these multipolar systems not only provide a setting to demonstrate the robustness of the notion that local degrees of freedom influence metallic quantum criticality, but also allow for the realization of even richer forms of QCPs. For the Kondo effect per se, various kinds of local degrees of freedom have led to a variety of Kondo states relevant to multipolar heavy fermion metals [14][15][16][17][18][19][20][21] , multiorbital iron-based compounds [22][23][24][25] , synthetic systems such as ultracold atoms 26 and mesoscopic devices [27][28][29] , and other correlated settings [30][31][32][33] .…”

Thermal and field-induced transitions in ferroquadrupolar Kondo systems

“…From this viewpoint, we expect multispin interactions to also emerge naturally in Kondo materials if we view the large-to-small FS transition as an orbital-selective Mott transition of the f electrons [42,43]. The impact of such couplings has only recently been investigated in multipolar Kondo systems [44][45][46][47], although there has been some suggestive previous work in dipolar Kondo materials [48,49]. Given this, we ask the following questions.…”