Probing the enigmatic Fermi surface of SmB6 by low-energy electrodynamics

• Applicant: Dr. Marc Scheffler
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 419885010

In the field of correlated electron physics, samarium hexaboride (SmB6) is one of the most studied and yet most puzzling materials. Traditionally considered a Kondo insulator, SmB6 was more recently interpreted as the first topological insulator with electronic correlations. This notion calls for a two-dimensional Fermi surface caused by the charges of the topological surface states, but recent high-profile quantum-oscillation results on SmB6 are gravely conflicting: one study reported signatures for a two-dimensional Fermi surface whereas another indicated a three-dimensional Fermi surface. The latter result cannot be reconciled with SmB6 being truly insulating in the bulk at low temperature, and it has sparked a plethora of new theoretical concepts, some of them invoking exotic quasiparticles quite different from regular electrons in solids.In this project we want to elucidate the nature of the Fermi surface of SmB6 by means of low-energy electrodynamics. In particular, we want to detect and characterize the cyclotron resonance of SmB6. This will be an independent experimental access to the properties of the Fermi surface, regardless of whether the latter is two- or three-dimensional. In conventional solids, cyclotron resonance is probed by applying a microwave electric field to a sample in a static magnetic field. For the case of SmB6, there is no consensus on the values of effective mass and scattering rate of the relevant mobile quasiparticles, and it is not even clear whether they are charge carriers or neutral. Therefore, at this stage, we cannot predict at which combination of magnetic field and excitation frequency and below which temperature the cyclotron resonance of SmB6 will be observable. Hence we will use a variety of experimental techniques (at frequencies between 45 MHz and 1.3 THz, at magnetic fields up to 8 T, at temperatures down to 20 mK, and with option to selectively excite with either electric or magnetic high-frequency field) to unriddle the electrodynamic response of SmB6. These experiments in extremely wide parameter ranges will reveal key information on the Fermi surface via detection of cyclotron resonance, and in addition they will help us to answer another puzzle in the properties of SmB6, namely whether the previously reported THz conductivity of SmB6, which is much higher than expected from the established dc conductivity, is caused by the same enigmatic quasiparticles that possibly generate the three-dimensional Fermi surface of SmB6.

• DFG Programme: Research Grants