Two-dimensional heavy fermions on the strongly correlated boundaries of Kondo topological insulators

Abstract

Samarium hexaboride (SmB6), a representative Kondo insulator, has been characterized recently as a likely topological insulator. It is also a material with strong electron correlations, evident by the temperature dependence of its bandgap and the existence of a nearly flat collective mode whose energy lies within the bandgap. Similar strong correlations can affect or even destabilize the two-dimensional metallic state of topological origin at the crystal boundary. Here we construct the minimal lattice model of the correlated boundary of topological Kondo insulators, and make phenomenological predictions for its possible ground states. Depending on the microscopic properties of the interface between the topological Kondo material and a conventional insulator, the boundary metal can exhibit a varied degree of hybridization between the d and f orbitals of the rare earth element, yielding a rich two-dimensional heavy fermion phenomenology. A pronounced participation of the f orbitals is expected to create a heavy fermion Dirac metal, possibly unstable to a spin density wave, electron localization or even superconductivity. The opposite limit of "localized magnetic moments" helped by the partial Kondo screening on the crystal boundary can bring about a non-Fermi liquid of d electrons that exhibits two-dimensional quantum electrodynamics, or other unconventional states. In addition, ultra-thin films made from topological Kondo insulators could open the possibility of creating exotic incompressible quantum liquids with non-Abelian fractional excitations, whose dynamics shaped by the strong Rashba spin-orbit coupling resembles that of fractional quantum Hall systems.

Radio Frequency Tunable Oscillator Device Based on aSmB6Microcrystal

“…The bulk of SmB 6 is extremely insulating [6,7] and free from impurity conductions, unless under extremely high pressure [2] or magnetic field [14], when the density of states of the bulk Fermi surface starts to emerge. The combination of a truly insulating bulk, a robust surface state, and strong electronic correlation [15][16][17][18] makes SmB 6 a promising candidate to search for useful properties [5]. Up to date, most research has been focused on the equilibrium properties of SmB 6 ; its transient dynamics could also be interesting.When biased with a few mA of DC current, self-heating in SmB 6 causes a large nonlinear resistance [19], which could lead to oscillation behavior.…”

QCD Kondo excitons

“…Inverse-propagators for the fluctuations of heavy-light modes in the momentum space are derived by taking a second derivative with respect toΦ in Eq. (26). When we take the direction of the QCD Kondo exciton's propaga-tions along the third-axis, we find that four independent channels of (σ a , V 3 a ), (π a , A 3 a ), (V 1 a .A 2 a ), and (V 2 a , A 1 a ) exist due to the parity invariance.…”

“…Kondo insulators (Kondo lattices) are composed of bound states (quasi-particles) as superpositions of itinerant electrons and spin-impurities, whose interaction is dynamically enhanced by the Kondo effect [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Inside the Kondo insulators, there exist the Kondo excitons which are bound states of quasi-electrons and quasi-holes as excited states upon the interactioninduced ground state [24][25][26][27][28][29]. The Kondo excitons affect the transport phenomena, such as sound waves and heat conduction accompanied by the number and energy fluctuations, respectively, while they are usually irrelevant to electric conductivity due to the neutral charge [30].…”

Fractionalized Fermi liquid on the surface of a topological Kondo insulator

“…However, as the TKI phase is well-described within a mean field framework [10], its topological properties are not expected to be markedly different from what has already been observed in its uncorrelated cousins. More intriguing is the potential the topologically protected surface states present for new interesting phases [22][23][24][25]. In SmB 6 , this expectation is motivated experimentally by ARPES measurements which find light surface quasiparticles [17][18][19] in contradiction to current theories which predict heavy particles at the surface [8,9,26,27].…”