Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99


Any time

2019 ( 17 )

2018 ( 31 )

2017 ( 28 )

2016 ( 32 )

Custom range...

Search Results: 1 - 10 of 10563 matches for " Matthew Brahlek "
All listed articles are free for downloading (OA Articles)
Page 1 /10563
Display every page Item
Emergence of decoupled surface transport channels in bulk insulating Bi2Se3 thin films
Matthew Brahlek,Nikesh Koirala,Maryam Salehi,Namrata Bansal,Seongshik Oh
Physics , 2014, DOI: 10.1103/PhysRevLett.113.026801
Abstract: 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.
Transport Properties of Topological Insulators: Band Bending, Bulk Metal-to-Insulator Transition, and Weak Anti-Localization
Matthew Brahlek,Nikesh Koirala,Namrata Bansal,Seongshik Oh
Physics , 2014, DOI: 10.1016/j.ssc.2014.10.021
Abstract: We reanalyze some of the critical transport experiments and provide a coherent understanding of the current generation of topological insulators (TIs). Currently TI transport studies abound with widely varying claims of the surface and bulk states, often times contradicting each other, and a proper understanding of TI transport properties is lacking. According to the simple criteria given by Mott and Ioffe-Regel, even the best TIs are not true insulators in the Mott sense, and at best, are weakly-insulating bad metals. However, band-bending effects contribute significantly to the TI transport properties including Shubnikov de-Haas oscillations, and we show that utilization of this band-bending effect can lead to a Mott insulating bulk state in the thin regime. In addition, by reconsidering previous results on the weak anti-localization (WAL) effect with additional new data, we correct a misunderstanding in the literature and generate a coherent picture of the WAL effect in TIs.
Restoring Pristine Bi2Se3 Surface with an Effective Se Decapping Process
Jixia Dai,Wenbo Wang,Matthew Brahlek,Nikesh Koirala,Maryam Salehi,Seongshik Oh,Weida Wu
Physics , 2014, DOI: 10.1007/s12274-014-0607-8
Abstract: High quality thin films of topological insulators (TI) such as Bi2Se3 have been successfully synthesized by molecular beam epitaxy (MBE). Although the surface of MBE films can be protected by capping with inert materials such as amorphous Se, restoring an atomically clean pristine surface after decapping has never been demonstrated, which prevents in-depth investigations of the intrinsic properties of TI thin films with ex-situ tools. Using high resolution scanning tunneling microscopy/spectroscopy (STM/STS), we demonstrate a simple and highly reproducible Se decapping method that allows recovery of the pristine surface of extremely high quality Bi2Se3 thin films grown and capped with Se in a separate MBE system then exposed to atmosphere during transfer into the STM system. The crucial step of our decapping process is the removal of the surface contaminants on top of amorphous Se before thermal desorption of Se at a mild temperature (~210 {\deg}C). This effective Se decapping process opens up the possibility of ex-situ characterizations of pristine surfaces of interesting selenide materials and beyond using cutting-edge techniques.
Surface versus bulk state in topological insulator Bi2Se3 under environmental disorder
Matthew Brahlek,Yong Seung Kim,Namrata Bansal,Eliav Edrey,Seongshik Oh
Physics , 2011, DOI: 10.1063/1.3607484
Abstract: Topological insulators (TIs) are predicted to be composed of an insulating bulk state along with conducting channels on the boundary of the material. In Bi2Se3, however, the Fermi level naturally resides in the conduction band due to intrinsic doping by selenium vacancies, leading to metallic bulk states. In such non-ideal TIs it is not well understood how the surface and bulk states behave under environmental disorder. In this letter, based on transport measurements of Bi2Se3 thin films, we show that the bulk states are sensitive to environmental disorder but the surface states remain robust.
Topological Surface States Originated Spin-Orbit Torques in Bi2Se3
Yi Wang,Praveen Deorani,Karan Banerjee,Nikesh Koirala,Matthew Brahlek,Seongshik Oh,Hyunsoo Yang
Physics , 2015,
Abstract: Three dimensional topological insulator bismuth selenide (Bi2Se3) is expected to possess strong spin-orbit coupling and spin-textured topological surface states, and thus exhibit a high charge to spin current conversion efficiency. We evaluate spin-orbit torques in Bi2Se3/Co40Fe40B20 devices at different temperatures by spin torque ferromagnetic resonance measurements. As temperature decreases, the spin-orbit torque ratio increases from ~ 0.047 at 300 K to ~ 0.42 below 50 K. Moreover, we observe a significant out-of-plane torque at low temperatures. Detailed analysis indicates that the origin of the observed spin-orbit torques is topological surface states in Bi2Se3. Our results suggest that topological insulators with strong spin-orbit coupling could be promising candidates as highly efficient spin current sources for exploring next generation of spintronic applications.
Stability of low-carrier-density topological-insulator Bi$_2$Se$_3$ thin films and effect of capping layers
Maryam Salehi,Matthew Brahlek,Nikesh Koirala,Jisoo Moon,Liang Wu,N. P. Armitage,Seongshik Oh
Physics , 2015, DOI: 10.1063/1.4931767
Abstract: Although over the past number of years there have been many advances in the materials aspects of topological insulators (TI), one of the ongoing challenges with these materials is the protection of them against aging. In particular, the recent development of low-carrier-density bulk-insulating Bi$_2$Se$_3$ thin films and their sensitivity to air demands reliable capping layers to stabilize their electronic properties. Here, we study the stability of the low-carrier-density Bi$_2$Se$_3$ thin films in air with and without various capping layers using DC and THz probes. Without any capping layers, the carrier density increases by ~150% over a week and by ~280% over 9 months. In situ-deposited Se and ex situ-deposited Poly(methyl methacrylate) (PMMA) suppresses the aging effect to ~27% and ~88% respectively over 9 months. The combination of effective capping layers and low-carrier-density TI films will open up new opportunities in topological insulators.
Thickness-independent transport channels in topological insulator Bi2Se3 thin films
Namrata Bansal,Yong Seung Kim,Matthew Brahlek,Eliav Edrey,Seongshik Oh
Physics , 2011, DOI: 10.1103/PhysRevLett.109.116804
Abstract: 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.
Observation of inverse spin Hall effect in bismuth selenide
Praveen Deorani,Jaesung Son,Karan Banerjee,Nikesh Koirala,Matthew Brahlek,Seongshik Oh,Hyunsoo Yang
Physics , 2014, DOI: 10.1103/PhysRevB.90.094403
Abstract: Bismuth Selenide (Bi2Se3) is a topological insulator exhibiting helical spin polarization and strong spin-orbit coupling. The spin-orbit coupling links the charge current to spin current via the spin Hall effect (SHE). We demonstrate a Bi2Se3 spin detector by injecting the pure spin current from a magnetic permalloy layer to a Bi2Se3 thin film and detect the inverse SHE in Bi2Se3. The spin Hall angle of Bi2Se3 is found to be 0.0093 and the spin diffusion length in Bi2Se3 to be 6.2 nm at room temperature. Our results suggest that topological insulators with strong spin-orbit coupling can be used in functional spintronic devices.
Transferring MBE-grown topological insulator films to arbitrary substrates and Metal-insulator transition via Dirac gap
Namrata Bansal,Myung Rae Cho,Matthew Brahlek,Nikesh Koirala,Yoichi Horibe,Jing Chen,Weida Wu,Yun Daniel Park,Seongshik Oh
Physics , 2014, DOI: 10.1021/nl404363b
Abstract: Mechanical exfoliation of bulk crystals has been widely used to obtain thin topological insulator (TI) flakes for device fabrication. However, such a process produces only micro-sized flakes that are highly irregular in shape and thickness. In this work, we developed a process to transfer the entire area of TI Bi2Se3 thin films grown epitaxially on Al2O3 and SiO2 to arbitrary substrates, maintaining their pristine morphology and crystallinity. Transport measurements show that these transferred films have lower carrier concentrations and comparable or higher mobilities than before the transfer. Furthermore, using this process we demonstrated a clear metal-insulator transition in an ultrathin Bi2Se3 film by gate-tuning its Fermi level into the hybridization gap formed at the Dirac point. The ability to transfer large area TI films to any substrate will facilitate fabrication of TI heterostructure devices, which will help explore exotic phenomena such as Majorana fermions and topological magnetoelectricity.
Topological-Metal to Band-Insulator Transition in (Bi1-xInx)2Se3 Thin Films
Matthew Brahlek,Namrata Bansal,Nikesh Koirala,Su-Yang Xu,Madhab Neupane,Chang Liu,M. Zahid Hasan,Seongshik Oh
Physics , 2012, DOI: 10.1103/PhysRevLett.109.186403
Abstract: By combining transport and photo emission measurements on (Bi1-xInx)2Se3 thin films, we report that this system transforms from a topologically non-trivial metal into a topologically trivial band insulator through three quantum phase transitions. At x = 3-7%, there is a transition from a topologically non-trivial metal to a trivial metal. At x = 15%, the metal becomes a variable-range-hopping insulator. Finally, above x = 25%, the system becomes a true band insulator with its resistance immeasurably large even at room temperature. This material provides a new venue to investigate topologically tunable physics and devices with seamless gating/tunneling insulators.
Page 1 /10563
Display every page Item

Copyright © 2008-2017 Open Access Library. All rights reserved.