UVSOR 5U

A topological metal at the surface of an ultrathin BiSb alloy film
T. Hirahara1, Y. Sakamoto1, Y. Saisyu1, H. Miyazaki2, S. Kimura2,T. Okuda3,4, I. Matsuda4, S. Murakami5, and S. Hasegawa1
1
Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Hiroshima Synchrotron Radiation Center, Hiroshima University,
Higashi-Hiroshima 739-0046, Japan
4
Synchrotron Radiation Laboratory, ISSP University of Tokyo, Kashiwa 277-8581, Japan
5
Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
Recently there has been growing interest in
topological insulators or the quantum spin Hall(QSH) phase, which are insulating materials withbulk band gaps but have metallic edge states that areformed topologically and robust against any
non-magnetic impurity [1]. In a three-dimensional
material, the two-dimensional surface statescorrespond to the edge states (topological metal) andtheir intriguing nature in terms of electronic and spin
structures have been experimentally observed in bulk
Bi1-xSbx single crystals [2,3,4]. However, if we wantto know the transport properties of these topologicalmetals, high purity samples as well as very low
temperature will be needed because of thecontribution from bulk states or impurity effects. In arecent report, it was also shown that an intriguing
coupling between the surface and bulk states will
occur [5]. A simple solution to this bothersome
problem is to prepare a topological metal on an
ultrathin film, in which the surface-to-bulk ratio isdrastically increased.
Therefore in the present study, we haveinvestigated if there is a method to make an ultrathin
Bi1-xSbx film on a semiconductor substrate. Fromreflection
high-energy
electrondiffraction
observation, it was found that single crystal Bi1-xSbx
films (0<x<0.25) as thin as ~30 Å can be prepared on
Si(111)-7x7. The transport properties of such films
were characterized by in situ monolithic microfour-point probes [6]. The temperature dependence ofthe resistivity for the x=0.1 samples was insulatingwhen the film thickness was 240 Å. However, it
became metallic as the thickness was reduced down
to 30 Å, indicating surface-state dominant electricalconduction. Figure 1 shows the Fermi surface of 40 Å
thick Bi0.92Sb0.08 (a) and Bi0.84Sb0.16 (b) films mappedby angle-resolved photoemission spectroscopy. The
basic features of the electronic structure of thesesurface states were shown to be the same as thosefound on bulk surfaces, meaning that topologicalmetals can be prepared at the surface of an ultrathinfilm. The details will be given in the presentation.

(a) Bi0.92Sb0.08
0.2(c)
0-0.2(b) Bi0.84Sb0.16

0.2(d)
0-0.2

00.2

0.4

0.6

0.8

1.0k Γ-M(Å-1)

Fig. 1. The Fermi surface mapped by ARPES at 10
K for 40 Å thick Bi0.92Sb0.08 (a) and Bi0.84Sb0.16 (b)films, respectively. A circularly polarized light wasused at the photon energy of 28 eV.
References:[1] L. Fu and C. L. Kane, Phys. Rev. B 76, 045302(2007).
[2] D. Hsieh et al., Nature 452, 970 (2008).
[3] D. Hsieh et al., Science 323, 919 (2009).
[4] A. Nishide et al., cond-mat/09022251, submitted
to Phys. Rev. Lett. (2009)[5] A. A. Taskin and Y. Ando, Phys. Rev. B 80,085303 (2009).
[6] T. Tanikawa et al., e-Journal of Surface Scienceand Nanotechnology, 1, 50 (2003).