Recent progress and perspectives in topological insulators: quantum Hall effects, ballistic vs. diffusive regimes and Anderson transitions

1Shamoon College of Engineering Universal conductance fluctuations (UCF) in topological insulators and quantum Hall systems V. Kagalovsky 1Shamoon College of Engineering Beer-Sheva, Israel

Context UCF in metals UCF in the mesoscopic samples at the Integer quantum Hall regime UCF in 3D topological insulators UCF in 2D topological insulators?

REVIEW B VOLUME 35, NUMBER 3 15 JANUARY 1987-II Lee, Stone, Fukuyama Universal conductance fluctuations in metals: Effects of finite temperature, interactions, and magnetic field UCF in metals Theoretical investigations of these phenomena have shown that they are indeed a quantum interference effect characteristic of metals, and the fluctuations have a universal T =0 amplitude of order e2/h, independent of sample size and degree of disorder (as long as the sample is metallic). The universality of the amplitude of the fluctuations at low temperature has been confirmed in a variety of quite different metallic systems (see Fig. I). A key insight leading to the latter result was the realization that these sample-specific fluctuation phenomena in conductance versus magnetic field or chemical potential could be viewed as manifestations of the statistical fluctuations which would occur in the conductance of an ensemble of metallic samples which differed only in their microscopic impurity configurations. This idea made it possible to make a connection between analytic theoretical results, which typically average over a statistical ensemble, and experimental results which typically involve measurements on one or a small number of samples.

REVIEW B VOLUME 35, NUMBER 3 15 JANUARY 1987-II Lee, Stone, Fukuyama Universal conductance fluctuations in metals: Effects of finite temperature, interactions, and magnetic field Comparison of aperiodic magnetoconductance fluctuations in three different systems. (a) g (B) in O.s-pm-diam gold ring, analysis of data from Refs. 3 and 4, reprinted with the permission of Webb et aI. (the rapid Aharonov-Bohm oscillations have been filtered out). (b) g (B) for a quasi-1D silicon MOSFET, data from Ref. 9, reprinted with the permission of Skocpol et al. (c) Numerical calculation of g(B) for an Anderson model using the technique of Ref. 11. Conductance is measured in units of e /h, magnetic field in tesla. Note the 3 order-of-magnitude variation in the background conductance while the fluctuations remain order unity. Comparison of sample-to-sample fluctuations and fluctuations in g(B) and g(E) in a single sample. Data are from nurnerical simulations on a 100x10 site Anderson model with disorder W=1 in units of the hopping matrix element, using the technique of Ref. 11. (a) g for 20 samples differing only in their impurity configurations. (b) g(B) over a range of approximately 10 times the field correlation range. (c) g(E) over a range of approximately 10 times the energy correlation range. Note that the size of the fluctuations is roughly the same in all three cases, lending qualitative support for our ergodic hypothesis; some quantitative support was reported in Fig. 2 of Ref. 13.

PH YSICA L R EV I EW L ET T ERS VOLUME 91, NUMBER 23 Peled et.al. UCF in QHE Near-Perfect Correlation of the Resistance Components of Mesoscopic Samples at the Quantum Hall Regime E. Peled,1 D. Shahar,1 Y. Chen,2 E. Diez,2,* D. L. Sivco,3 and A.Y. Cho3 FIG. 1 (color). (a) Geometry of the Hall-bar samples. The black areas represent Au-Ge-Ni contacts. The separation of the current and voltage contacts are 12 x W and 2 x W, respec- tively. (b)RL and RH vs B of the 2mm Hall bar in the vicinity of the n=2-1 transition, T=10mK.

Topological insulators Insulator in the bulk Conducting states on the surface Realized as 2D and 3D systems Physical mechanism — quantum spin Hall effect Spin-orbit interaction (Rashba) effective magnetic field depends on spin-projection 𝐻 𝑆𝑂 = 𝛼 ∙[𝒑× 𝑠 ]

SCIENTIFIC REPORTS | 2 : 595 | DOI: 10.1038/srep00595 Li et. al. Two-dimensional universal conductance fluctuations and the electron-phonon interaction of surface states in Bi2Te2Se microflakes Figure 1 | The UCF and its temperature dependence. (a) the schematic diagram of the measurement configuration. (b) Temperature dependence of the resistance and resistivity of a Bi2Te2Se microflake. The left inset shows its AFM image with the scale bar of 4 mm. The right inset shows the Arrhenius fitting of r(T) with the result of a 4.9 meV band gap. (c) Conductance fluctuations plotted against B at various temperatures (h50). The aperiodic dG-B patterns appear repeatedly. For clarity, adjacent curves are displaced vertically. (d) dGrms and its temperature dependence. The inset shows the data in a linear scale. The solid curve is fit by the traditional UCF theory.

UCF on the surface of 3D TI Figure 2 | The 2D UCFs demonstrated by the field-tilting measurement. (a) The schematic diagram showing the 2D UCF solely depends on the perpendicular component of the magnetic field (BH). (b) The B-tilting dG-B data of a Bi2Te2Se microflake measured at 2 K. The black, red and blue circle marked lines respectively show the similar features, namely p1, p2 and p3, in all the dG-B curves. For clarity, adjacent curves are displaced vertically. (c) The positions of the UCF features plotted against h. The black, red and blue data are from those of p1, p2 and p3 in (b), respectively. The solid curves are the 1/cosh fitting. (d) h dependent dGrms. The dashed curve is for eye guiding. The UCF measured at h590u is interpreted as the contribution from the bulk carriers in TIs.

2D Topological Insulator (TI) Insulator in the bulk, conductor at the edge Experimentally CdTe-HgTe-CdTe heterostructure 𝛼 (𝑝, ↑) (−𝑝,↓) Two counterpropagating spin-polarized states at the edge

Topological Insulator (TI) in quantum spin Hall (QSH) state Integer quantum Hall effect (IQHE) Each spin projection feels ist own effective magnetic field Strong external magnetic field

In addition, edge transport in InAs/GaSb has so far only been indirectly assessed in ballistic samples [9,11].

UCF on the surface of 2D TI?

UCF on the surface of 2D TI? TI QSH “In addition, fluctuations on the conductance plateaus in traces are reproducible and do not stem from, e.g., electrical noise.”