Hybridization gap and Fano resonance in SmB₆

Abstract

Hybridization between conduction electrons and the strongly interacting f-electrons in rare earth or actinide compounds may result in new states of matter. Depending on the exact location of the concomitant hybridization gap with respect to the Fermi energy, a heavy fermion or an insulating ground state ensues. To study this entanglement locally, we conducted scanning tunneling microscopy and spectroscopy (STS) measurements on the "Kondo insulator" SmB 6 . The vast majority of surface areas investigated were reconstructed, but infrequently, patches of varying sizes of nonreconstructed Smor B-terminated surfaces also were found. On the smallest patches, clear indications for the hybridization gap with logarithmic temperature dependence (as expected for a Kondo system) and for intermultiplet transitions were observed. On nonreconstructed surface areas large enough for coherent cotunneling, we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure, not proving but leaving open the possibility of the existence of a topologically protected surface state.M aterials with strong electron correlations continue to draw enormous attention, not only because they may give rise to fundamentally new states of matter or new phenomena but also because of the hope for advanced technological applications. Heavy fermion (HF) materials, i.e., intermetallics of certain rare earths (REs), such as Ce, Sm, and Yb, are model systems to study strong electronic correlations (1). Here, the RE-derived localized 4f states are covalently mixed with the conduction-band states and, thus, acquire a finite lifetime. The associated decay rate in relation to the energy of the localized 4f state corresponds to the valency. In an Sm-based HF system, the valence lies between 3+ (4f 5 ) and 2+ (4f 6 ), which implies a considerable amount of charge fluctuations. This usually is referred to as intermediate valence (2). In addition to the abovementioned mixing of 4f states and the conduction band, which is well described within the framework of one-electron models, a manybody interaction is operating between the 4f and conduction electrons. This "Kondo effect" (3) eventually leads to a screening of the local moments as a result of particle-hole excitations that are manifested by a narrow Abrikosov-Suhl, or Kondo, resonance at E F , the width of which is given by the single-ion Kondo temperature T K .Because of the periodic arrangement of REs in an HF intermetallic, the Kondo resonances form a weakly dispersive HF or "coherent 4f−" band, resulting in a heavy Fermi liquid state well below T K . The band interaction between the renormalized 4f and the conduction band generates a so-called hybridization gap, which opens at around T K . Under certain conditions, E F may reside inside this gap, characterizing a so-called Kondo insulator (4).SmB 6 is such a Kondo insulator, with a valence ν ∼ 2:6 (5), ν being slightly temperature dependent (6, 7). A sharp de...

Topological surface states interacting with bulk excitations in the Kondo insulator SmB 6 revealed via planar tunneling spectroscopy

“…This discrepancy may result from the chemical potential being sensitive to the surface chemistry. More specifically, while the cleaved surfaces measured by ARPES are likely to be electrically nonneutral due to dangling bonds, as shown experimentally (16,20,21), we argue that they are pacified in our junctions via the surface oxidation process to form a tunnel barrier (SI Appendix, section 2). That the surface states are still detected after the harsh process of polishing and oxidation attests to their robustness and thus, their topological origin.…”

“…In particular, samarium hexaboride (SmB 6 ) has drawn great attention owing to its telltale resistivity behavior saturating below 4 K (5). Various experiments have been implemented to investigate this possibility (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24), establishing the robustness of the surface states and the Kondo hybridization leading to a formation of the bulk gap, but their topological origin and nature has not been unambiguously confirmed. Factors contributing to this situation include their inherently complex nature due to strong correlations, nontrivial surface chemistry, and insufficient energy resolution.…”

Planar tunneling spectroscopy of the topological Kondo insulator SmB6

“…Although several photoemission measurements on SmB 6 30-36 have revealed the formation of a hybridization gap below ∼50 K and in-gap states, the exact topological nature is not unveiled yet. Also, clear signatures for the surface Dirac fermions such as linear conductance are lacking in several STS [37][38][39] and point-contact spectroscopy 40 measurements. The challenges encountered in studying SmB 6 , or TKIs in general, are manifold.…”

In-gap collective mode spectrum of the topological Kondo insulatorSmB6

“…As a putative strong topological insulator (TI), SmB 6 is expected to have a metallic crystal boundary protected against any source of backscattering that respects the time-reversal (TR) symmetry. Indeed, several transport [23][24][25][26][27][28] , quantum oscillation 29 and surface spectroscopy [30][31][32][33][34] experiments have directly probed this metallic boundary and generally provided evidence for its existance. At the same time, SmB 6 has been heralded as the first true TI with a fully insulating bulk 35 , unlike the original bismuth-based TIs whose bulks remain weakly conducting due to statistically unavoidable crystal defects [36][37][38] .…”