Namrata Bansal

Thickness-Independent Transport Channels in Topological Insulator Bi2Se3 Thin Films

With high quality topological insulator Bi(2)Se(3) thin films, we report thickness-independent transport properties over wide thickness ranges. Conductance remained nominally constant as the sample thickness changed from 256 to ∼8  QL (where QL refers to quintuple layer, 1  QL≈1  nm). Two surface channels of very different behaviors were identified. The sheet carrier density of one channel remained constant at ∼3.0×10(13)  cm(-2) down to 2 QL, while the other, which exhibited quantum oscillations, remained constant at ∼8×10(12)  cm(-2) only down to ∼8  QL. The weak antilocalization parameters also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively.

Observation of Dirac plasmons in a topological insulator

Plasmons are quantized collective oscillations of electrons and have been observed in metals and doped semiconductors. The plasmons of ordinary, massive electrons have been the basic ingredients of research in plasmonics and in optical metamaterials for a long time. However, plasmons of massless Dirac electrons have only recently been observed in graphene, a purely two-dimensional electron system. Their properties are promising for novel tunable plasmonic metamaterials in the terahertz and mid-infrared frequency range. Dirac fermions also occur in the two-dimensional electron gas that forms at the surface of topological insulators as a result of the strong spin-orbit interaction existing in the insulating bulk phase. One may therefore look for their collective excitations using infrared spectroscopy. Here we report the first experimental evidence of plasmonic excitations in a topological insulator (Bi2Se3). The material was prepared in thin micro-ribbon arrays of different widths W and periods 2W to select suitable values of the plasmon wavevector k. The linewidth of the plasmon was found to remain nearly constant at temperatures between 6 K and 300 K, as expected when exciting topological carriers. Moreover, by changing W and measuring the plasmon frequency in the terahertz range versus k we show, without using any fitting parameter, that the dispersion curve agrees quantitatively with that predicted for Dirac plasmons.

Thickness-dependent bulk properties and weak antilocalization effect in topological insulator Bi2Se3

We show that a number of transport properties in topological insulator (TI) Bi

Epitaxial growth of topological insulator Bi2Se3 film on Si(111) with atomically sharp interface

{}_{2}

Terahertz Response and Colossal Kerr Rotation from the Surface States of the Topological Insulator Bi2Se3

Se

A sudden collapse in the transport lifetime across the topological phase transition in (Bi1-xInx)2Se3

{}_{3}

Topological-metal to band-insulator transition in (Bi(1-x)In(x))2Se3 thin films.

exhibit striking thickness dependences over a range of up to five orders of thickness (3 nm--170 \ensuremath{\mu}m). Volume carrier density decreased with thickness, presumably due to diffusion-limited formation of selenium vacancies. Mobility increased linearly with thickness in the thin film regime and saturated in the thick limit. The weak antilocalization effect was dominated by a single two-dimensional channel over two decades of thickness. The sublinear thickness-dependence of the phase coherence length suggests the presence of strong coupling between the surface and bulk states.

Emergence of Decoupled Surface Transport Channels in Bulk Insulating Bi2Se3 Thin Films

Abstract Atomically sharp epitaxial growth of Bi 2 Se 3 films is achieved on Si(111) substrate with molecular beam epitaxy. Two-step growth process is found to be a key to achieve interfacial-layer-free epitaxial Bi 2 Se 3 films on Si substrates. With a single-step high temperature growth, second phase clusters are formed at an early stage. On the other hand, with low temperature growth, the film tends to be disordered even in the absence of a second phase. With a low temperature initial growth followed by a high temperature growth, second-phase-free atomically sharp interface is obtained between Bi 2 Se 3 and Si substrate, as verified by reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and X-ray diffraction. The lattice constant of Bi 2 Se 3 is observed to relax to its bulk value during the first quintuple layer according to RHEED analysis, implying the absence of strain from the substrate. TEM shows a fully epitaxial structure of Bi 2 Se 3 film down to the first quintuple layer without any second phase or an amorphous layer.

Mapping the orbital wavefunction of the surface states in three-dimensional topological insulators

We report the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surface electrons. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in an applied magnetic field. This Kerr rotation can be as large as 65° and can be explained by a cyclotron resonance effect of the surface states.

Surface versus bulk state in topological insulator Bi2Se3 under environmental disorder

The quantum phase transition from a topological to a conventional insulator in In-doped Bi2Se3 occurs when the topological phase is destroyed by the hybridization of states on opposite surfaces. This is characterized by a sudden change in the transport lifetime, measured by means of optical spectroscopy.