Direct observation of surface-state thermal oscillations in SmB6 oscillators

Abstract

SmB6 is a mixed valence Kondo insulator that exhibits a sharp increase in resistance following an activated behavior that levels off and saturates below 4K. This behavior can be explained by the proposal of SmB6 representing a new state of matter, a Topological Kondo insulator, in which a Kondo gap is developed and topologically protected surface conduction dominates low temeprarture transport. Exploiting its non-linear dynamics, a tunable SmB6 oscillator device was recently demonstrated, where a small DC current generates large oscillating voltages at frequencies from a few Hz to hundreds of MHz. This behavior was explained by a theoretical model describing the thermal and electronic dynamics of coupled surface and bulk states. However, a crucial aspect of this model, the predicted temperature oscillation in the surface state, hasn't been experimentally observed to date. This is largely due to the technical difficulty of detecting an oscillating temperature of the very thin surface state. Here we report direct measurements of the time-dependent surface state temperature in SmB6 with a RuO2 micro-thermometer. Our results agree quantitatively with the theoretically simulated temperature waveform, and hence support the validity of the oscillator model, which will provide accurate theoretical guidance for developing future SmB6 oscillators at higher frequencies.PACS numbers: 72.15. Qm, 71.20.Eh, 72.20.Ht Samarium Hexaboride (SmB 6 ), a strongly correlated mixed valence compound [1,2], has garnered much attention recently due to the discovery of a low temperature conducting surface state [3][4][5][6][7][8]. It is proposed to form a strongly interacting version of a Topological Insulator [9][10][11][12][13], with a Kondo Gap in the bulk and a gapless states on the surface that seems to be protected by time-reversal symmetry [7]. The lack of impurity conduction in the bulk has allowed the realization of a radio frequency oscillator device [15] based on SmB 6 micro-crystals, where a small DC current drives the SmB 6 crystal into an oscillating state and generates large oscillating voltages. This behavior was explained by a theoretical model [15], which utilized a coupling between two conduction channels, namely, metallic surface conduction and thermally activated bulk conduction which result in significant differences in sample heating under an applied current. While this Letter will focus on the verification of this model in a proposed topological Kondo insulator, a general seiconducting sample with metallic channels with electrical and thermal coupling can be described equally well. The proposed model can be written as follows:(1)Here I S and I 0 are the surface and applied currents through the sample, respectively; C is the total capacitance representing the combined internal and external capacitance ; G = (R S + R B )/R B where R S is the surface resistance and R B = R Solving this simultaneous set of equations yields oscillatory solutions of both temperature and crystal voltage which has a constant phase shift ...

An STM perspective on hexaborides: Surface states of the Kondo insulator SmB$_6$

“…Reducing the thickness of the SmB 6 plate requires a smaller external capacitor until the device spontaneously starts oscillating with a frequency of the order of 10 7 Hz [88]. The behaviors of the oscillators agree well with a theoretical model describing the thermal Joule heating and electronic dynamics of coupled surface and bulk states [88,89]. An impedance study on SmB 6 single crystals, performed at low T and over a wide frequency range revealed the transition from surface to bulk dominated electrical conduction between 2 and 10 K [90].…”

Impedance Spectroscopy of SmB$_6$ single crystals

“…19 A model which involves both surface and bulk states, in addition to a strong coupling between the thermal and the electrical properties, has recently been put forward to explain this response. 20,21 Nevertheless, there are only few reports on the frequency-dependent transport properties of SmB 6 .…”

Dielectric and transient electrical response of SmB6 single crystals

“…This ratio is about or less than 80 in the crystals of Refs. [16,17,24], whereas it is larger than 10 4 for SmB 6 we have studied. As a result, raising the sample temperature by 1 K in the former case (from a base temperature of 2 K) is sufficient to drive SmB 6 crystals into a bulk conduction regime but our samples must be heated at least 5 K in order to reach the same conduction regime.…”