19 research outputs found
A topological Dirac insulator in a quantum spin Hall phase : Experimental observation of first strong topological insulator
When electrons are subject to a large external magnetic field, the
conventional charge quantum Hall effect \cite{Klitzing,Tsui} dictates that an
electronic excitation gap is generated in the sample bulk, but metallic
conduction is permitted at the boundary. Recent theoretical models suggest that
certain bulk insulators with large spin-orbit interactions may also naturally
support conducting topological boundary states in the extreme quantum limit,
which opens up the possibility for studying unusual quantum Hall-like phenomena
in zero external magnetic field. Bulk BiSb single crystals are
expected to be prime candidates for one such unusual Hall phase of matter known
as the topological insulator. The hallmark of a topological insulator is the
existence of metallic surface states that are higher dimensional analogues of
the edge states that characterize a spin Hall insulator. In addition to its
interesting boundary states, the bulk of BiSb is predicted to
exhibit three-dimensional Dirac particles, another topic of heightened current
interest. Here, using incident-photon-energy-modulated (IPEM-ARPES), we report
the first direct observation of massive Dirac particles in the bulk of
BiSb, locate the Kramers' points at the sample's boundary and
provide a comprehensive mapping of the topological Dirac insulator's gapless
surface modes. These findings taken together suggest that the observed surface
state on the boundary of the bulk insulator is a realization of the much sought
exotic "topological metal". They also suggest that this material has potential
application in developing next-generation quantum computing devices.Comment: 16 pages, 3 Figures. Submitted to NATURE on 25th November(2007
The credibility of digital identity information on the social web: a user study
A theory of the dynamical conductance of mesoscopic conductors is presented.
It is applied to mesoscopic capacitors, resonant double barriers, ballistic
wires, metallic diffusive wires, and to the Corbino disk and the Hall bar in
quantizing magnetic fields. Central to this approach is a discussion of the
charge and potential distribution in mesoscopic conductors. It is necessary to
take into account the implications of the long-range Coulomb interaction in
order to obtain a charge and current conserving theory. We emphasize the
low-frequency response. This has the advantage that the approach is of
considerable generality. The theory can be used to discuss the self-consistency
of the dc-conductance formula. The theory can also be applied to discuss the
rectifying (nonlinear) behavior of mesoscopic conductors.Comment: 29 pages, figures not included (preprints with figures can be
obtained by conventional mail on request from T.Christen
[email protected]
Admittance and Nonlinear Transport in Quantum Wires, Point Contacts, and Resonant Tunneling Barriers
We present a discussion of the admittance (ac-conductance) and nonlinear I-V-characteristic for a number of mesoscopic conductors. Our approach is based on a generalization of the scattering approach which now includes the effects of the (long-range) Coulomb interaction. We discuss the admittance of a wire with an impurity and with a nearby gate. We extend a discussion of the low-frequency admittance of a quantum point contact to investigate the effects of the gates used to form the contact. We discuss the nonlinear I-V characteristic of a resonant double barrier structure and discuss the admittance for the double barrier for a large range of frequencies. Our approach emphasizes the overall conservation of charge (gauge invariance) and current conservation and the resulting sum rules for the admittance matrix and nonlinear transport coefficients