Session H31: Focus Session: Topological Insulators: Synthesis and Characterization: Bulk Crystals

Low carrier concentration crystals of the topological insulator Bi$_2$Te$_2$Se

We report the characterization of Bi$_2$Te$_2$Se crystals obtained by the modified Bridgeman and Bridgeman-Stockbarger crystal growth techniques. X-ray diffraction study confirms an ordered Se-Te distribution in the inner and outer chalcogen layers, respectively, with a small amount of mixing. The crystals displaying high resistivity ($> 1~ \mathrm{\Omega cm}$) and low carrier concentration ($\sim 5\times 10^{16}$/cm$^3$) at 4 K were found in the central region of the long Bridgeman-Stockbarger crystal, which we attribute to very small differences in defect density along the length of the crystal rod. Analysis of the temperature dependent resistivities and Hall coefficients reveals the possible underlying origins of the donors and acceptors in this phase.   

Structural study of topological insulator Bi$_{2}$(Se$_{3-x}$Te$_{x})$

With neutron diffraction, we systematically investigated the local and average structures of topological insulator Bi$_{2}$(Se$_{3-x}$Te$_{x})$ (x=0, 1, 1.5, 2, and 3) from 5 to 500 K. The average crystal structure of Bi$_{2}$Se$_{3}$ is rhombohedral R-3m symmetry with 2 unique chalcogen sites. This gives rise to 2 types of Bi-(Se/Te) bonds in quintuple layer composed of Se1-Bi-Se2 --Bi-Se1 layers, covalent Se1-Bi bond with the bonding length of 2.85 {\AA} and ionic Se2-Bi bond with the bonding length of 3.07 {\AA} at 5 K. The Se1-Se1 bonds between quintuple layers are governed by Van der Walls of the length of 3.47 {\AA} at 5 K. With increasing temperature, it is observed that the quintuple layer unit contracts from the anti-parallel motion of the Bi-(Se1)$_{3}$ tetrahedra toward the Se2-center layer. Also such a contraction implies the Van der Waals bonding between quintuple layers weakens with temperature. A similar temperature dependence of atomic structure is observed on Bi$_{2}$Se$_{3}$.   

Hole doping of p-type thin topological insulators

Recent studies of intrinsically n-type topological insulator (TI) Bi$_{2}$Se$_{3}$ demonstrated that doping with Cu introduces electrons into this system, and that in the narrow range of doping Cu$_{x}$Bi$_{2}$Se$_{3}$ becomes a superconductor below Tc 5 K. It is presumed that Cu intercalates into Van der Waals gaps in the tetradymite structure of layered Bi$_{2}$Se$_{3}$, although this remains to be confirmed. We report on hole doping experiments on the naturally p-type Te-based TIs: thin films of Bi$_{2}$Te$_{3}$ and nanocrystalline plates of Sb$_{2}$Te$_{3}$. The samples were iodine doped either during the growth or by the exposure to iodine vapor. Transport measurements on films indicate a very unusual T$^{3}$ temperature dependence of longitudinal resistivity. It drops significantly below 30 K with decreasing temperature, although the drop appears arrested by a metal-insulator transition in the 300 mK range. Magnetization measurements on nanoplates indicate the development of large diamagnetic signal. These results will be discussed in comparison with the superconductivity obtained by Cu doping in Bi$_{2}$Se$_{3}$. * Supported in part by NSF-DMR-1122594.   

Synthesis and Characterization of New Topological Insulators

In this talk, I will show detailed information on synthesizing process and characterization results of new topological insulator (TI) materials with interesting properties. Among the synthesized materials, TlBiSe$_2$ was the first ternary TI and has the largest bulk band gap [1], TlBi(S$_{1-x}$,Se$_x$)$_2$ presents a topological phase transition with unexpected Dirac mass [2], BiTe$_2$Se presents a large bulk resistivity [3], and Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ has finally achieved the surface-dominated transport in bulk single crystals [4]. It is essentially easy to grow single crystals of all the chalcogenides above, because those compounds melt congruently at relatively low temperatures. Therefore, the melt-growth method is applicable if the raw materials are in a sealed condition, e.g., in a quartz tube. However, crucial techniques for obtaining high-quality samples vary between the systems. Besides the growth method, characterizations of the transport properties, ARPES, the X-ray diffraction, and quantitative chemical analysis will also be presented. \\[4pt] This work is in collaboration with A. A. Taskin, S. Sasaki, Zhi Ren, K. Eto, T. Minami, and Y. Ando (Osaka Univ.), and T. Sato, S. Souma, H. Guo, K. Sugawara, K. Kosaka and K. Nakayama, and T. Takahashi (Tohoku Univ.). \\[4pt] \noindent [1] T. Sato, Kouji Segawa, T. Takahashi, Y. Ando {\it et al.}, Phys. Rev. Lett. {\bf 105}, 136802 (2010). \\ \noindent [2] T. Sato, Kouji Segawa, Y. Ando, T. Takahashi {\it et al.}, Nature Physics, {\bf 7}, 840 (2011). \\ \noindent [3] Zhi Ren, Kouji Segawa, Y. Ando {\it et al.}, Phys. Rev. B (Rapid Comm.) {\bf 82}, 241306(R) (2010). \\ \noindent [4] A. A. Taskin, Kouji Segawa, and Y. Ando {\it et al.}, Phys. Rev. Lett. {\bf 107}, 016801 (2011).   

Transport properties of new Pb-based Topological Insulators

A topological insulator (TI) has a gapped insulating bulk and a gapless metallic surface. So far, such materials as Bi$_{1-x}$Sb$_{x}$, Bi$_{2}$Se$_{3}$, and TlBiSe$_{2}$ are known to be TIs. Recently, several theoretical predictions have been made for new TI materials. In this work, we focus on the Pb-based ternary chalcogenides as new candidate TIs. We have grown a number of single crystals in the systems of Pb-Bi-Se, Pb-Bi-Te, Pb-Sb-Te and Pb-(Sb,Bi)-Te. After selecting single-phase samples, we measured the transport properties to check for their bulk-insulating nature. It was found that Pb(Sb$_{x}$Bi$_{1-x}$)$_{2}$Te$_{4}$ shows a change in the carrier type at around x= 0.55 as inferred both by the themopower and by the Hall effect, but the temperature dependences of the resistivity remained metallic in all the samples studied. We discuss the prospect of making a bulk-insulating material in the Pb-based TIs.   

Millikelvin transport of high quality Bi2Se3 crystals at high pressure

In this study we present electronic transport in Bi2Se3 single crystals in a diamond anvil pressure cell. Reaching temperatures down to 20 mK and pressures exceeding 30 GPa we measure changes in electrical resistivity, magnetoresistance, and the Hall effect and discuss the results as they pertain to topologically interesting behavior.   

Entropy transport in Bi$_{2}$Se$_{3}$

Bi$_2$Se$_3$ and Bi$_2$Te$_3$ are well known compounds in the thermoelectricity community as they present a high figure of merit [1]. Although the thermoelectric power of Bi$_2$Se$_3$ has been extensively studied at high temperature, little is known about its behaviour at the low temperature limit. In this presentation, we will report the results of our entropy measurement of Bi$_{2}$Se$_{3}$ at low temperature and high magnetic field for a bulk carrier concentration from 10$^{17}$cm$^3$ to 10$^{19}$cm$^3$. In all compounds we show significant quantum oscillations in the Seebeck and Nernst responses. Based on the bulk Fermi surface, we propose a simple description of the entropy transport measurement in Bi$_2$Se$_3$ (in the range of concentrations studied). Indeed, Bi$_2$Se$_3$ (non compensated system) appears as a complementary system of bismuth [2] and graphite [3] (compensated systems) to understand the entropy transport in the low carrier concentration limit. \\[4pt] [1] G.S. Nolas et al, Thermoelectrics Basic Principles and New Materials Developments, Springer.\\[0pt] [2] K.Behnia et al, PRL, 98, 166602 (2007)\\[0pt] [3] Z.Zhu et al, Nature Physics, 6, 26 (2009)   

Search and Design of Topological Insulators by High-throughput Method

Topological insulators (TIs) have attracted enormous interest because of their novel surface conduction effects which are protected by time-reversal symmetry. A high-purity TI with a highly insulating bulk is necessary to realize the potential practical applications of this class of materials. Therefore, numerous attempts are being made to search for TIs with desired properties (e.g., a large band gap and amenability to high-quality crystal growth). In this presentation, we will introduce an effective high-throughput approach to search and design TIs from a vast electronic structure database such as Inorganic Crystal Structure Database (ICSD) and Heusler alloys.   

Effects of Intrinsic Defects on Topological Insulator Behavior: Theory and Experiments on Ternary Tetradymite Compounds

Ternary tetradymites Bi$_{2}$Te$_{x}$Se$_{(3-x)}$ are predicted to be topological insulators via density functional theory (DFT) surface band structure calculations. Experimentally, we find that Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ form a continuous solid solution at the two non-equivalent group VI sites with different site preferences for Se and Te. The DFT formation energies for ordered and partially ordered compounds agree well with experimental data. Importantly, we calculated the intrinsic defect formation energies of binary and ternary tetradymites, and find they correlate well with the bulk conductivity measurement. Angle resolved photoemission spectroscopy confirms the existence of the Dirac cone in the surface band of Bi$_{2}$Te$_{2}$Se.   

Transport properties of chalcogenide and related topological insulator bulk crystals

Three-dimensional (3D) topological insulators (TIs) have attracted strong theoretical and experimental interest in the condensed matter community. We have used the Bridgman method to synthesize various chalcogenide and related 3D TI crystals. We present an experimental survey of a group of binary, tertiary, and quaternary bulk crystals of various compositions of Bi, Sb, Ge, Se, Te, and S. Our survey includes systems without intentional doping, as well as systems doped with magnetic and non-magnetic impurities. We present magnetotransport data over a range of temperatures. We also measure thin films exfoliated from these bulk crystals to examine efficacy of carrier modulation through an applied gate voltage. We discuss the results of these measurements in the context of TI properties, with the ultimate goal of identifying systems that display electronic transport properties consistent with an insulating bulk and spin-helical Dirac fermion surface states.   

Bulk Excitations in single crystal Bi$_{2}$Se$_{3}$: Electron Energy Loss Spectroscopy Study

Bi$_{2}$Se$_{3}$ with larger figure of merit has been used in room-temperature thermoelectric applications. Furthermore, it is also one of a handful known topological insulators. Most of recent studies were focused on the topological surface states of Bi$_{2}$Se$_{3}$ while few of them studied the electronic excitations of bulk Bi$_{2}$Se$_{3}$. Here, we report studies of electronic excitations of single-crystal Bi$_{2}$Se$_{3}$ by electron energy-loss spectroscopy (EELS). EELS spectrum in bulk Bi$_{2}$Se$_{3}$ reveals several spectral features at $\sim $7, $\sim $16.8, $\sim $26.4 and $\sim $28.4 eV. The peaks at $\sim $26.4 and $\sim $28.4 eV are due to excitations from Bi 5$d$ electrons. The $\sim $7 and $\sim $16.8 eV peaks are easily identified as bulk-plasmon excitations according to the frequency-dependent complex dielectric function derived from experimental spectrum with Kramers-Kr\"onig analysis. Furthermore, momentum ($q)$-dependent EELS spectra along [110], [300] and [001] directions were also performed in this study. When momentum transfer $q$ is parallel [110] and [300] directions, the 16.8 eV-peak (bulk plasmon) significantly shift to higher energy (up to 23 eV) with increasing $q$ values, while this peak shifts less than 1 eV when momentum transfer $q$ is parallel to [001] direction, revealing the distinct anisotropy of bulk plasmon dispersions. Detailed characteristics of this anisotropic behavior will also be discussed.   

Insights on the electronic and vibrational properties of Bi(111) from first principles

Bi(111) is known to have surface electron carriers close to $\Gamma $ as well as hole carriers at $\Gamma $ and along the $\Gamma $M directions. The lattice dynamics of Bi(111) is however largely unknown. We investigate both the electronic structure and lattice dynamics of Bi(111) films via density-functional-theory and density-functional-perturbation-theory calculations taking into account the spin-orbit coupling (SOC). While the splitting of the branches is dominated by the SOC almost everywhere along the $\Gamma $M direction, around the zone boundary (M), the delocalized character of this state plays an important role. Reducing the thickness of a film decreases the band gap progressively. At $\sim $3-nm thickness, the highest valence band re-crosses the Fermi level and creates extra electron pockets. We find, however, that the lattice dynamics of Bi(111) is robust with respect to film thickness. Bi(111) has a number of ``high-lying'' surface modes in the optical band almost everywhere along the $\Gamma $KM and $\Gamma $M directions, most notably, a vertical mode slightly above the bulk band. Surface acoustic modes are also present as well as some ``low frequency'' optical modes in small regions of the zone. A comparison with recent measurements will be presented, as well as the possible implications on the electron-phonon coupling.   

Electron beam irradiation effect on Bi$_{2}$Se$_{3}$ topological insulator nanodevices

Nanofabrication is found to introduce excess charge carriers and results in a strong metallic state in Bi$_{2}$Se$_{3}$. The uncontrolled carrier density causes the Fermi level to rise to the conduction band. To verify the effect of the electron beam lithography (EBL), we have measured the carrier density of Bi$_{2}$Se$_{3}$ nanodevices before and after EBL with a range of electron beam energies and doses and find that the Fermi level rises in both n- and p-type devices. To effectively control the position of the Fermi level, we have developed a nanofabrication-free technique for Bi$_{2}$Se$_{3}$ nanodevices, with which the initial state of the bulk materials can be well preserved. We deliberately choose p-type Ca-doped Bi$_{2}$Se$_{3}$ devices and systematically introduce more electrons using successive EBI. Resistiviy temperature dependence shows that the Fermi level position is gradually tuned from the valence band into the band gap. Further fine tuning of the Fermi level is accomplished by applying a gate voltage to the devices. An increase of a factor of 10 in mobility has been observed in the device as the Fermi level is brought into the band gap, which is consistent with the suppressed backscattering of the surface states in topological insulators.