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Register a new userEmergence of decoupled surface transport channels in bulk insulating Bi2Se3 thin films
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In ideal topological insulator (TI) films the bulk state, which is supposed to be insulating, should not provide any electric coupling between the two metallic surfaces. However, transport studies on existing TI films show that the topological states on opposite surfaces are electrically tied to each other at thicknesses far greater than the direct coupling limit where the surface wavefunctions overlap. Here, we show that as the conducting bulk channels are suppressed, the parasitic coupling effect diminishes and the decoupled surface channels emerge as expected for ideal TIs. In Bi2Se3 thin films with fully suppressed bulk states, the two surfaces, which are directly coupled below ~10 QL, become gradually isolated with increasing thickness and are completely decoupled beyond ~20 QL. On such a platform, it is now feasible to implement transport devices whose functionality relies on accessing the individual surface layers without any deleterious coupling effects.
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With high quality topological insulator (TI) Bi2Se3 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 (QL: 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 x 10^13 cm^-2 down to 2 QL, while the other, which exhibited quantum oscillations, remained constant at ~8 x 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.
The microstructure of Bi2Se3 topological-insulator thin films grown by molecular beam epitaxy on InP(111)A and InP(111)B substrates that have different surface roughnesses has been studied in detail using X-ray diffraction, X-ray reflectivity, atomic force microscopy and probe-corrected scanning transmission electron microscopy. The use of a rough Fe-doped InP(111)B substrate results in complete suppression of twin formation in the Bi2Se3 thin films and a perfect interface between the films and their substrates. The only type of structural defects that persist in the twin-free films is an antiphase domain boundary, which is associated with variations in substrate height. It is also shown that the substrate surface termination determines which family of twin domains dominates.
We present measurements of ferromagnetic-resonance - driven spin pumping and inverse spin-Hall effect in NbN/Y3Fe5O12 (YIG) bilayers. A clear enhancement of the (effective) Gilbert damping constant of the thin-film YIG was observed due to the presence of the NbN spin sink. By varying the NbN thickness and employing spin-diffusion theory, we have estimated the room temperature values of the spin diffusion length and the spin Hall angle in NbN to be 14 nm and -1.1 10-2, respectively. Furthermore, we have determined the spin-mixing conductance of the NbN/YIG interface to be 10 nm-2. The experimental quantification of these spin transport parameters is an important step towards the development of superconducting spintronic devices involving NbN thin films.
Segregation at surfaces of metal-covalent binary liquids is often non-classical and in extreme cases such as AuSi, the surface crystallizes above the melting point. In this study, we employ atomic-scale computational frameworks to study the surface crystallization of AuSi films and droplets as a function of composition, temperature and size. For temperatures in the range $T_s^ast=765-780$K above the melting point $(T_s^astapprox1.3,T_m)$, both thin film and droplet surfaces undergo a first order transition, from a 2D Au$_2$Si crystalline phase to a laterally disordered yet stratified layer. The thin film surfaces exhibit an effective surface tension that increases with temperature and decreases with Si concentration. On the other hand, for droplets in the size range $10-30$ nm, the bulk Laplace pressure alters the surface segregation as it occurs with respect to a strained bulk. Above $T_s^ast$ the size effect on the surface tension is small, while for $T<T_s^ast$ the surface layer is strained and composed of 2D crystallites separated by extended grain boundary scars that lead to large fluctuations in its energetics. As a specific application, all-atom simulations of AuSi droplets on Si(111) substrate subject to Si surface flux show that the supersaturation dependent surface tension destabilizes the contact line via formation of a precursor wetting film on the solid-vapor interface, and has ramifications for size selection during VLS-based routes for nanowire growth. Our study sheds light on the interplay between stability and energetics of surfaces in these unique class of binary alloys and offers pathways for exploiting their surface structure for varied applications such as catalytic nanocrystal growth, dealloying, and polymer crystallization.
In a recent work by Ji Seop Oh et al., BaBiO3(001) thin films were grown on SrTiO3 by Pulsed Laser Deposition. It was argued that the films are BiO2-terminated from the modelling of angle-resolved photoemission spectroscopy experiments. The authors claim, in opposition to previous theoretical predictions, that there are no metallic surface states on their films. In this short comment we question that the authors have enough evidence to make such a claim, as we consider that the large mismatch between SrTiO3 and BaBiO3 and the lack of control of their fabrication process with reflection high energy electron difraction could unlikely give high quality films with a single BiO2- termination, which is one of the requisites for the stabilization of these surface metallic states.
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