Year

2013

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Gapless materials are one type of novel materials, in which their conduction and valence band edges touch and the band gap is zero. The well known gapless materials are 2D graphene, 2D and 3D topological insulators and recently discovered 3D Dirac semimetals. Such materials exhibit lots of unique properties including massless quasiparticles, extremely high electron mobility, linear magnetoresistances and so on. And they demonstrate great potential application in next generations of quantum computing, electronic, spintronic and optoelectronic devices.

Topological insulators are quantum materials with an insulating bulk state and a topologically protected metallic surface state with spin and momentum helical locking and a Dirac-like band structure. Antimony telluride (Sb2Te3) compounds are well known as typical three dimensional topological insulators. We have investigated the anisotropic in-plane and interlayer magneto-transport properties of Sb2Te3 single crystals over a broad range of temperatures, degrees and magnetic fields. Giant magnetoresistance (MR) of up to 400% was observed, which exhibits quadratic field dependences in low fields and becomes linear at high fields without any trend towards saturation. The giant MR also shows strong anisotropy in angle dependent measurements. The giant MR might result from strong inter-valley and intra-valley scattering of holes and the strong anisotropy is attributable to the anisotropy of hole mobility, relaxation time and effective mass in the Fermi surface. The observed giant anisotropic MR could find applications in Sb2Te3 based anisotropic magneto-electronic devices.

A flow of carriers along the c-axis is extremely sensitive to the orientation of an in-plane magnetic field due to in-plane mass anisotropy in layered compounds. Based on this mechanism, a rotatable in-plane magnetic field has been applied as a valley valve to tune the contribution of each valley in Sb2Te3 bulk single crystals to the total conductivity and interlayer MR. A valley-polarized current is generated, and the angular-dependent interlayer MR of up to 160% represents strong anisotropy. There are six inequivalent peaks over all temperature and magnetic field ranges. The giant MR results from the intra-valley and inter-valley hole Coulomb scattering in upper valence bands. And the interlayer MR anisotropy originates from field-induced polarization of valleys, Coulomb interaction induced valley distortion. The strong anisotropy of the angular-dependent interlayer MR reflects strong anisotropies of carrier scattering time and effective mass in the six valleys and their inequivalent contributions to total magnetoconductivity and interlayer MR in Sb2Te3.

Angular-dependences of in-plane and interlayer magnetotransport properties in topological insulator Bi2Te3 single crystals have been investigated over a broad range of temperatures and magnetic fields. Giant in-plane MR of up to 500% and interlayer MR of up to 200% were observed, respectively. The observed MR exhibits quadratic field dependences in low fields and linear field dependences in high fields. The angular dependences of the MR represent strong anisotropy and twofold oscillations. The observed angle-dependent, giant MR might result from the strong coulomb scattering of electrons as well as impurity scattering in the bulk conduction bands of Bi2Te3. The strong anisotropy of the MR may be attributable to the anisotropy of electron mobility, effective mass and relaxation time in the Fermi surface. The observed giant anisotropic MR in Bi2Te3 bulk single crystals paves the way for Bi2Te3 single crystals to be useful for practical applications in magnetoelectronic devices such as disk reading heads, anisotropic magnetic sensors, and other multifunctional electromagnetic applications.

Recently, theoretical calculation and transport measurements as well as Angle-resolved photoemission spectroscopy (ARPES) measurements demonstrate that such in-gap states actually are surface states, and SmB6 is a topological Kondo insulator (TKI). We report angular-dependence of out-of-plane MR oscillations in SmB6 single crystals with a rotated in-plane magnetic field. The four-fold degeneracy of Fermi surface electron pockets of SmB6 leads to a four-fold (C4) symmetry of out-of-plane MR oscillations. The C4 symmetric oscillations gradually lose and transit into nearly two-fold (C2) symmetry with decreasing temperature, which demonstrates a C4 rotational symmetry breaking of lattice. Such a symmetry breaking suggests profound reconstruction of Fermi surface and implies the possible emerging of electronic nematic states in Kondo insulator SmB6. These experimental observations shed new light on the 40-year old puzzle of the in-gap states in SmB6.

The thermotransport properties of SmB6 polycrystalline are investigated over a broad temperature ranging from 300 K to 2 K. An unexpected transition of temperature-dependent Seebeck coefficient S from S(T)∝ T-1 to S(T)∝T is observed around 12 K. Such a transition demonstrates a transformation of 3D bulk states to complete 2D metallic surface states. The figure of merit ZT of SmB6 displays a pronounced peak at 40 K, which is correlated to the Kondo gap opening. Our results solve a critical outstanding problem of Seebeck effect anomaly in topological insulators. And the results also suggest that the Seebeck effects can be a probe for surface states in topological insulators.

Graphene, a single layer carbon atoms, is a 2D gapless semiconductor. It demonstrates extraordinarily high electron mobility, thermal conductivity and mechanical strength, and has great potential for nanoelectronics, spintronics and optoelectronics. We investigated on the comparative study of magnetotransport properties of large-area vertical few-layer graphene networks with different morphology, measured in a strong (up to 10 T) magnetic field over a wide temperature range. The petal-like and tree-like graphene networks grown by plasma enhanced CVD process on a thin (500 nm) silicon oxide layer supported by a thick (500 nm) silicon wafer demonstrate a significant difference in the resistance – magnetic field dependencies at temperatures ranging from 2 to 200 K. This behaviour is explained in terms of the effect of electron scattering at ultra-long reactive edges and ultra-dense boundaries of the graphene nanowalls. Our results pave a way towards three-dimensional vertical graphene-based magnetoelectronic nanodevices with morphology-tuneable anisotropic magnetic properties.

We investigated different magnetization in vertical graphenes fabricated by plasma-enabled chemical conversion of organic precursors with various oxygen contents and bonding energies. The vertical graphenes grown from fat-like precursors exhibit magnetization reaching 8 emu/g, whereas the use of aromatic precursors results in much lower numbers. High Curie temperature was also demonstrated. The strong magnetism in vertical graphenes was achieved by satisfying 3 criteria of 1) defect control, 2) hydrogenation, 3) edge states. These multiple mechanisms was enabled during the plasma dry chemical conversion process.

Bi1−xSbx can be tuned from topologically trivial phase to topologically non-trivial phase through a critical point around x=0.04. At this point, Bi1−xSbx alloy becomes a semimetal with massless Dirac fermions. We investigated magnetic field induced metal-semiconductor transition and linear MR in Dirac semi-metal phase of Bi1-xSbx. A field dependent band gap is induced in Bi0.96Sb0.04 single crystals in high magnetic fields. Giant linear MR of up to 5000% were observed in 8 T and 5 K, which can be explained with the model of Abrikosov’s quantum MR. Additionally, low field linear MR has been observed in Bi0.96Sb0.04 single crystals with rough surface, which can be attributed to disorder effects.

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