Band structure of topological insulators from noise measurements in tunnel junctions

Abstract

The unique properties of spin-polarized surface or edge states in topological insulators (TIs) make these quantum coherent systems interesting from the point of view of both fundamental physics and their implementation in low power spintronic devices. Here we present such a study in TIs, through tunneling and noise spectroscopy utilizing TI/Al 2 O 3 /Co tunnel junctions with bottom TI electrodes of either Bi 2 Te 3 or Bi 2 Se 3 . We demonstrate that features related to the band structure of the TI materials show up in the tunneling conductance and even more clearly through low frequency noise measurements. The bias dependence of 1/f noise reveals peaks at specific energies corresponding to band structure features of the TI. TI tunnel junctions could thus simplify the study of the properties of such quantum coherent systems that can further lead to the manipulation of their spin-polarized properties for technological purposes. A topological insulator (TI) is a material which is insulating in the bulk but presents spin-dependent conducting edge or surface states which are protected by time-reversal symmetry.1-3 A 2D or 3D TI presents edge or surface states, respectively, which are spin-polarized in-plane, and locked at right angles to the carrier momentum, so that electrons with spin-up/down propagate in opposite directions. The edge or surface states of a TI consist of an odd number of massless Dirac cones. These properties along with their high mobility make TI materials interesting for next generation, low dissipation, and spintronic applications 4,5 in which the electron spins are manipulated even without any magnetic fields. The experimental surge regarding these materials occurred with the prediction of Bi-based TIs 6 and their posterior experimental realization.7 Bi 2 Se 3 and Bi 2 Te 3 , in particular, became the prototypical TI materials that were studied most heavily.To date, the experimental verification of the band structure of TI materials has been predominantly carried out by angle-resolved photoemission spectroscopy (ARPES), which yields energy-momentum graphs of band dispersion for probing depths of under a few nm. 7,8 Also, the use of spin-ARPES has allowed the determination of the spin dependence of the topological surface states. 9 On the other hand, the use of scanning tunneling microscope (STM) allows obtaining information regarding the local density of states (DOS) and the topography of surfaces. By the study of quasiparticle scattering with STM, bands can be mapped very close to the Fermi surface, with a considerably lower energy range and at a smaller scale than with ARPES. [10][11][12][13] Although immensely useful, these techniques are usually cumbersome and the conditions of study are far from a practical application of TIs. A versatile and relatively simple technique to determine the DOS of TI materials could be the study of electronic transport and noise through planar tunneling devices. So far, individual or heterostructure devices with TI layers have mainly dealt with lateral electro...

Bulk-impurity induced noise in large-area epitaxial thin films of topological insulators

“…However, sensitivity of these surface states to different types of disorder is still a matter of active debate [4][5][6][7][8][9][10]. Recently, flicker noise or 1/f -noise in bulk TI systems has been established as a powerful tool not only as a performance-marker for electronic applications but also for defect-spectroscopy and even band structure determination [8,11,12]. Noise measurements in exfoliated TI devices from bulk crystals have revealed that in the thickness range of 50 nm to 80 µm charge number fluctuations in the bulk can give rise to the resistance fluctuations in the surface states [8].…”

Layered Topological Insulators and Semimetals for Magnetoresistance Type Sensors

“…In addition to the ratio of the MR and its anisotropy, another key factor that determines the device performance (accuracy and sensitivity) is the level of low‐frequency flicker noise (or the 1/ f noise) present in the device. 1/ f noise has been explored in TI systems . TRS protection can lead to noise reduction in TI surfaces.…”

“…Conventional transport measurements cannot provide a clear signature of the crystal defects and impurity bands. 1/ f noise has been used to demonstrate the band structure of the TI electrodes in perpendicular tunneling junctions through an Al 2 O 3 barrier, which could pave the way for the manipulation of their spin‐polarized currents for technological purposes . 1/ f noise has also been explored in magnetically doped Cr x (Bi,Sb) 2‐ x Te 3 to estimate whether the samples are highly disordered …”

“…1/f noise has been explored in TI systems. [104][105][106][107] TRS protection can lead to noise reduction in TI surfaces. Thus, at a small channel thickness, the noise magnitude of BSTS-based TI devices can be extremely small, corresponding to a Hooge parameter of 10 −4 .…”

1 / f noise in van der Waals materials and hybrids

“…It is remarkable that while the insulating bulk of the TI does not directly contribute to electrical conduction, it appears to be the dominant source of 1/f noise in TI-based devices. The effect of bandstructure of TIs has also been investigated by measuring 1/f spectra [126] in perpendicular tunnel junctions. In Figure 5(e), the device geometry consisting of a bottom electrode of MBE grown Bi 2 Se 3 /Bi 2 Te 3 and Co with Al 2 O 3 as a tunnel barrier is shown.…”