Corresponding Author

Pascal Gehring

Max Planck Institute for Solid State Research

References

1.
Pesin, D.; MacDonald, M.H.
Spintronics and pseudospintronics in graphene and topological insulators
2.
Gehring, P.; Benia, H.M.; Weng, Y.; Dinnebier, R.; Ast, C.R.; Burghard, M.; Kern, K.
A natural topological insulator
3.
Hikami, S.; Larkin, A.I.; Nagaoka, Y.
Spin-orbit interaction and magnetoresistance in the 2-dimensional random system

Department "Nanoscale Science"

Kawazulite – A natural topological insulator

Authors

P. Gehring, H.M. Benia, R. Dinnebier, Y. Weng, C.R. Ast, M. Burghard, and K. Kern

Departments

Nanoscale Science (Klaus Kern)

Topological insulators represent a new state of matter that is topologically distinct from conventional band insulators. The interlocking of spin and momentum in their surface states opens up promising perspectives for applications in spintronics. Using angle-resolved photoemission spectroscopy and magnetotransport experiments, we demonstrate that Kawazulite, a mineral occurring in gold mines, is a natural topological insulator. Owing to its relatively low bulk doping level and good carrier mobility, Kawazulite readily competes with its synthetic counterparts.

While topological insulators (TIs) are electrical insulators in the bulk, they comprise conducting helical states at their boundaries. These states originate from strong spin-orbit coupling without any external magnetic field, such that time reversal symmetry is conserved and accordingly backscattering is forbidden. Moreover, they exhibit spin polarization due to spin-momentum locking, which renders them interesting for spintronic applications [1]. We discovered that the mineral Kawazulite represents a naturally occurring three-dimensional (3D) topological insulator whose electronic properties compete well with those of its synthetic counterparts [2]. Kawazulite thus joins a range of natural materials that have been thoroughly investigated with the prospect of applications in various fields like optics, electronics, or mechanics. Examples include diamond that is formed in the lithosphere at considerable depth, supplying the required high pressure and temperature. Furthermore, quasi-crystals were very recently detected in meteorites, although they were hitherto believed to be only synthetically accessible. Another notable natural compound is graphite as a source of graphene, which has emerged as one of the most promising components of future electronic devices.

The crystal structure of Kawazulite, which received its name from the Kawazu mine in Japan, was first reported in 1961. It belongs to the Tetradymite group (rhombohedral, space group 3R̅m) featuring a crystal structure composed of quintuple layers (VI(1)−V−VI(2)−V−VI(1), where VI=(Se, Te, S) and V=Bi) that are held together through van der Waals bonds. We investigated Kawazulite samples (see Fig. 1(a)) from a former gold mine in Jílové u Prahy, Czech Republic. In gold veins, chalcogenides have been shaped over billions of years via hydrothermal deposition at high temperatures, rendering them well-suited to host TIs. Some of these compounds contain heavy elements like bismuth or antimony which are a necessary ingredient for the strong spin-orbit coupling in TIs.

<p><strong>Fig. 1:</strong> (a) Photographic image of the investigated Kawazulite specimens with a size between 3 and 10mm. (b) Rietveld plot for Kawazulite comprising two phases. (c) Magnified plot of the X-ray diffraction (XRD) data within the range of 11 to 13.4. (d) Sketch of the unit cell of the mineral, as derived from the XRD analysis.</p> Zoom Image

Fig. 1: (a) Photographic image of the investigated Kawazulite specimens with a size between 3 and 10mm. (b) Rietveld plot for Kawazulite comprising two phases. (c) Magnified plot of the X-ray diffraction (XRD) data within the range of 11 to 13.4. (d) Sketch of the unit cell of the mineral, as derived from the XRD analysis.

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Inductively coupled plasma mass spectrometry analysis of mechanically extracted Kawazulite pieces revealed a composition of (Bi2.12Sb0.06)Te2(Se0.14S0.32). In addition to these major elements, fifteen trace elements could be identified, including Au, As and Co. Powder diffractometry (Fig. 1(b)) indicated that the Kawazulite is composed of two different crystalline phases, which both crystallize in rhombohedral (3R̅m) structure and exhibit slightly different lattice constants. While the major phase (89.6wt%) is characterized by the lattice parameters a=4.25Å and c=29.70Å (Se rich), with a stoichiometry of Bi2(Se0.2S0.74)(Te0.61Se0.39)2, values of a=4.37Å and c=30.42Å (Te rich) and a stoichiometry of Bi2(Se0.59Te0.41)Te2 were found for the minor phase (≈10wt%). These lattice parameters are smaller than for Bi2Te3, consistent with the fact that S and Se are smaller than Te atoms. Moreover, the observation of (107) and (00.12) peaks (Fig. 1(c)) testifies a sizable extent of chalcogen ordering and correspondingly the presence of an ordered layer structure (see sketch of the unit cell in Fig. 1(d)). In addition to the two Kawazulite phases, we observed also peaks originating from the surrounding matrix of the mineral, which could be identified as Clinochlore (Mg,Fe,Al)6(Si,Al)4O10(OH)8.

<p><strong>Fig. 2:</strong> (a) ARPES spectrum of Kawazulite, and (b) sketch of the electronic band structure derived from the measurement.</p> Zoom Image

Fig. 2: (a) ARPES spectrum of Kawazulite, and (b) sketch of the electronic band structure derived from the measurement.

Direct proof that Kawazulite is a TI could be gained by angle-resolved photoemission spectroscopy (ARPES). The electronic structure of the (111)-surface near the Γ-point (k=0), as determined from a tiny (0.7×0.7 mm2) crystal, features a surface state with the typical Dirac-like conical dispersion in the center (see Fig. 2(a)). The corresponding electronic band structure is schematically illustrated in Fig. 2(b). Evaluation of the ARPES spectrum indicates a lower bound of 0.25eV for the band gap in Kawazulite, as well as a surface electron density of ns=kF2/4π=6.45×1016m-2 (assuming a circular Fermi surface). This density is consistent with values reported for synthetic bismuth chalcogenide-based TIs.

Furthermore, electrical characterization of individual thin flakes prepared by micromechanical cleavage of the extracted crystals, provided hints that charge transport occurs through the two-dimensional surface states in Kawazulite. In particular, low temperature (T=1.5K) Hall measurements on a range of different flakes yielded a Hall mobility between 300 and 1300cm2/(Vs), and an average electron density of n=1×1017m−2 (normalized by sample thickness). The latter value is only slightly higher than the value gained from the ARPES data, pointing toward dominant contribution of the two-dimensional (2D) surface state to the total charge transport. Further support for this conclusions stems from low temperature magnetoresistance of the samples. Figure 3(a) exemplifies the emergence of a pronounced weak antilocalization (WAL) peak in the sheet conductance Δσ=σ(B)–σ(B=0). Such peak arises from the Berry phase difference of π between the electron wave functions belonging to time reversed paths in a diffusive conductor with strong spin orbit coupling.

<p><strong>Fig. 3:</strong> (a) Magnetoconductance of a thin Kawazulite sheet at different temperatures, featuring weak antilocalization and universal conductance fluctuations. (b) Low <em>B</em>-field range of the data in panel (a), plotted as a function of the <em>B</em>-field component normal to the surface.</p> Zoom Image

Fig. 3: (a) Magnetoconductance of a thin Kawazulite sheet at different temperatures, featuring weak antilocalization and universal conductance fluctuations. (b) Low B-field range of the data in panel (a), plotted as a function of the B-field component normal to the surface.

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The resulting destructive interference along these paths prevents carrier localization, leading to a positive correction to the total conductivity. However, when the time reversal symmetry is broken by applying a magnetic field, antilocalization is no longer effective, and correspondingly the conductivity decreases. Importantly, when plotting Δσ as a function of Bcos(θ), i.e., the B-field component normal to the surface, all curves coincide (see Fig. 3(b)), testifying the 2D character of the effect. Moreover, the low-field magnetoconductance data can be well fitted by the Hikami-Larkin-Nagaoka (HLN) model for 2D localization [3]. From such fits at different temperatures, we obtained a phase coherence length lϕ of 200nm at 1.5K, and 60nm at 30K. Furthermore, lϕ was observed to significantly decrease with increasing temperature, with lϕT-1/2 dependence indicative of predominant electron-electron scattering in a 2D system. Another noteworthy observation is the emergence of pronounced universal conductance fluctuations (UCFs) in the magnetoconductance curves (Fig. 3(a)). Analysis of the UCF amplitude as a function of both, temperature and the B-field orientation provided further support of the 2D character of the charge transport.

The discovery of Kawazulite as a natural TI whose electrical properties are comparable to those of state-of-the-art synthetic compounds encourages the search for further minerals belonging to this fascinating class of materials in nature. Prospective candidates are the members of the Tetradymite and Aleksite group which together comprise more than 20 compounds. In general, compounds with high defect formation energies seem particularly promising, as due to their geological age, their crystal structure should have reached thermodynamic equilibrium and hence an ultimately low defect concentration. Thus, it may be possible to spot natural TIs which display further reduced bulk doping and accordingly even more accessible surface state transport, as compared to Kawazulite.


 
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