arXiv subjects

Khuong Kim Huynh

Publications and source records attributed to Khuong Kim Huynh.

9 recordsLinked to original sources

arXiv2018

Ultrathin film of 3D topological insulators by vapor-phase epitaxy: Surface dominant transport in wide temperature revealed by Seebeck measurement

Realization of intrinsic surface dominant transport in a wide temperature region for a topological insulators (TIs) is an important frontier research in order to promote the progresses of TIs towards the future electronics. We report here systematic measurements of longitudinal electrical transport, Shubnikov-de-Haas (SdH) quantum oscillations, Hall coefficient (R_H^2D), and Seebeck coefficient as a function of film thickness (d) and temperature using high quality Bi2-xSbxTe3-ySey (BSTS) single crystal thin films grown by physical vapor-phase deposition. The thickness dependence of sheet conductance and Seebeck coefficient clearly show the suppression of semiconducting hole carriers of bulk states by reducing film thickness, reaching to the surface dominant transport at below dc=14 nm. Quantitative arguments are made as to how the contribution of itinerant carrier number (n) can be suppressed, using both R_H^2D (n_Hall^2D) and SdH (n_SdH). Intriguingly, the value of n_Hall^2D approaches to be twice of n_SdH below dc. While R_H^2D shows a negative sign in whole temperature region, a change from negative to positive polarity is clearly observed for S at high temperatures when d is thick. We point out that this inconsistency observed between R_H^2D and S is intrinsic in 3D-TIs and its origin is the large difference in carrier mobility between the bulk and the topological surface. We propose that Seebeck coefficient can become a convenient and powerful tool to evaluate the intrinsic carrier concentration for the topological surface in 3D-TIs even in the absence of magnetic field.

cond-mat.mtrl-sci↗

arXiv2017

Large-Area and Transferred High-Quality Three-Dimensional Topological Insulator Bi2-xSbxTe3-ySey Ultrathin Film by Catalyst-Free Physical Vapor Deposition

Uniform and large area synthesis of bulk insulating ultrathin films is an important subject toward applications of a surface of three dimensional topological insulators (3D-TIs) in various electronic devices. Here we report epitaxial growth of bulk insulating three dimensional topological insulator (3D-TI) Bi2-xSbxTe3-ySey (BSTS) ultrathin films, ranging from a few quintuple to several hundreds of layaers, on mica in a large-area (1 cm2) via catalyst free physical vapor deposition. These films can nondestructively be exfoliated using deionized water and transferred to various kinds of substrates as desired. The transferred BSTS thin films show good ambipolar characteristics as well as well defined quantum oscillations arising from the topological surface states. Carrier mobility of 2500-5100 cm2(Vs)-1 is comparable to the high quality bulk BSTS single crystal. Moreover, tunable electronic states from the massless to the massive Dirac fermion were observed with a decrease in the film thickness. Both the feasible large area synthesis and the reliable film transfer process can promise that BSTS ultrathin films will pave a route to many applications of 3D-TIs.

cond-mat.mes-hall↗

arXiv2017

Thermoelectric properties of 3D topological insulator: Direct observation of topological surface and its gap opened states

We report thermoelectric (TE) properties of topological surface Dirac states (TSDS) in three-dimensional topological insulators (3D-TIs) purely isolated from the bulk by employing single crystal Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ films epitaxially grown in the ultrathin limit. Two intrinsic nontrivial topological surface states, a metallic TSDS (m-TSDS) and a gap-opened semiconducting topological state (g-TSDS), are successfully observed by electrical transport, and important TE parameters (electrical conductivity ($σ$), thermal conductivity ($κ$), and thermopower ($S$)) are accurately determined. Pure m-TSDS gives $S$=-44 μVK$^{-1}$, which is an order of magnitude higher than those of the conventional metals and the value is enhanced to -212 μVK$^{-1}$ for g-TSDS. It is clearly shown that the semi-classical Boltzmann transport equation (SBTE) in the framework of constant relaxation time ($τ$) most frequently used for conventional analysis cannot be valid in 3D-TIs and strong energy dependent relaxation time $τ(E)$ beyond the Born approximation is essential for making intrinsic interpretations. Although $σ$ is protected on the m-TSDS, $κ$ is greatly influenced by the disorder on the topological surface, giving a dissimilar effect between topologically protected electronic conduction and phonon transport.

cond-mat.mtrl-sci↗

arXiv2016

Electric Properties of Dirac Fermions Captured into 3D Nanoporous Graphene Networks

Graphene, as a promising material of post-silicon electronics, opens a new paradigm for the novel electronic properties and device applications. On the other hand, the 2D feature of graphene makes it technically challenging to be integrated into 3D transistors with a sufficient processor capacity. Although there are many attempts to assemble 2D graphene into 3D structures, the characteristics of massless Dirac fermions cannot be well preserved in these materials for transistor applications. Here we report a high-performance graphene transistor by utilizing 3D nanoporous graphene which is comprised of an interconnected single graphene sheet and a commodious open porosity to infuse an ionic liquid for a tunable electronic state by applying electric fields. The 3D nanoporous graphene transistor, with high carrier mobility of 5000-7500 cm$^2$V$^{-1}$s$^{-1}$, exhibits two to three orders of magnitude higher electric conductance and capacitance than those of 2D graphene devices, along with preserved ambipolor electronic nature of Dirac cones. Moreover, the 3D graphene networks with Dirac fermions turn out to exhibit a unique nonlinear Hall resistance in a wide range of the gate voltages. The high quality 3D nanoporous graphene EDLT may open a new field for utilizing Dirac fermions in 3D network structures for various fundamental and practical applications.

cond-mat.mtrl-sci↗

arXiv2016

In-plane Topological p-n Junction in the Three-Dimensional Topological Insulator Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$

A topological p-n junction (TPNJ) is an important concept to control spin and charge transport on a surface of three dimensional topological insulators (3D-TIs). Here we report successful fabrication of such TPNJ on a surface of 3D-TI Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ thin films and experimental observation of the electrical transport. By tuning the chemical potential of n-type topological Dirac surface of BSTS on its top half by employing tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a half surface can be converted to p-type with leaving the other half side as the opposite n-type, and consequently TPNJ can be created. By sweeping the back-gate voltage in the field effect transistor structure, the TPNJ was controlled both on the bottom and the top surfaces. A dramatic change in electrical transport observed at the TPNJ on 3D-TI thin films promises novel spin and charge transport of 3D-TIs for future spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Non-Fermi liquid behavior of electrical resistivity close to the nematic critical point in Fe$_{1-x}$Co$_x$Se and FeSe$_{1-y}$S$_y$

Temperature dependence of resistivity of single crystals of Fe$_{1-x}$Co$_x$Se and FeSe$_{1-y}$S$_y$ is studied in detail under zero and high magnetic field (magnetoresistance), the latter of which enables to monitor the temperature ($T$) evolution of resistivity below the onset of superconducting transition temperature ($T_{\rm c}$). In FeSe$_{1-y}$S$_y$, $T$-linear dependence of resistivity is prominent in $y$ = 0.160 below 40 K, whereas it changes to a Fermi-liquid(FL)-like $T^2$ one below 10 K in $y$ = 0.212. These suggest that the quantum critical point (QCP) originating from the electronic nematicity resides around $y$ = 0.160 and the fluctuation in QCP gives rise anomalous $T$-linear dependence in resistivity in a wide $T$ range. In Fe$_{1-x}$Co$_x$Se, resistivity gradually changes from linear- to quadratic- $T$-dependent one at low temperatures in the range between $x$ = 0.036 and 0.075. These could be interpreted by scenarios of both the nematic QCP and the crossover in the ground states between the orthorhombic nematic phase and the tetragonal phase. The anomalies found as $T$-linear resistivity are discussed in terms of orbital and spin fluctuation arising from the nematic QCP.

cond-mat.supr-con↗

arXiv2014

Van der Waals epitaxial growth of topological insulator Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ ultrathin nanoplate on electrically insulating fluorophlogopite mica

We report the growth of high quality Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ ultrathin nanoplates (BSTS-NPs) on an electrically insulating fluorophlogopite mica substrate using a catalyst-free vapor solid method. Under an optimized pressure and suitable Ar gas flow rate, we control the thickness, the size and the composition of BSTS-NPs. Raman spectra showing systematic change indicate that the thicknesses and compositions of BSTS-NPs are indeed accurately controlled. Electrical transport demonstrates that a robust Dirac cone carrier transport in BSTS-NPs. Since BSTS-NPs provide superior dominant surface transport of the tunable Dirac cone surface states with negligible contribution of the conduction of the bulk states, BSTS-NPs provide an ideal platform to explore intrinsic physical phenomena as well as technological applications of 3-dimensional topological insulators in the future.

cond-mat.mtrl-sci↗

arXiv2014

Mobility spectrum analytical approach for intrinsic band picture of Ba(FeAs)$_2$

Unconventional high temperature superconductivity as well as three-dimensional bulk Dirac cone quantum states arising from the unique d-orbital topology has been a recent priority research area in physics. In iron pnictide compounds, although transport phenomena arisen from this multiple band Fermi surface are intriguing and scientifically important, they still do not give an adequate matching to neither experimental observations on the band picture nor theoretical calculations and a debate continues. Here we describe a new analytical approach of mobility spectrum, in which the carrier number is conveniently described as a function of mobility without any hypothesis about the number of carriers, on both longitudinal and transverse transport of high quality single crystal Ba(FeAs)$_2$ in a wide range of magnetic field. We show that the major numbers of carriers reside in large parabolic hole and electron pockets with very different topology as well as remarkably different mobility spectra, while the minor number of Dirac carriers resides in both hole- and electron- Dirac quantum states with the largest mobility as high as 70,000 cm$^2$(Vs)$^{-1}$.

cond-mat.supr-con↗

arXiv2014

Electron and Hole Injection via Charge Transfer at the Topological-Insulator $Bi_{2-x}Sb_xTe_{3-y}Se_y$/Organic-Molecule Interface

As a methodology for controlling the carrier transport of topological insulators (TI's), a flexible tuning in carrier number on the surface states (SS's) of three dimensional TI's by surface modifications using organic molecules is described. The principle of the carrier tuning and its type conversion of TI's presented in this research are based on the charge transfer of holes or electrons at the TI/organic molecule interface. By employing 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) as an electron acceptor or tetracyanoquinodimethane (TCNQ) as a donor for n- and p- Bi2-xSbxTe3-ySey (BSTS) single crystals, successful carrier conversion from n to p and its reverse mode is demonstrated depending on the electron affinities of the molecules. The present method provides a nondestructive and efficient method for local tuning in carrier density of TI's, and is useful for future applications.

cond-mat.mtrl-sci↗