Thermoelectric signals of state transition in polycrystalline SmB6

Topological Kondo insulator SmB6 has attracted quite a lot of attentions from condensed matter physics community. A number of unique electronic properties, including low- temperature resistivity anomaly, 1D electronic transport and 2D Fermi surfaces have been observed in SmB6. Here, we report on thermoelectric transport properties of polycrystalline SmB6 over a broad temperature from 300 K to 2 K. An anomalous transition in the temperature-dependent Seebeck coefficient S from S(T) ~ T-1 to S(T) ~ T was observed around 12 K. Such a transition demonstrates a transition of conductivity from 3D metallic bulk states to 2D metallic surface states with insulating bulk states. Our results suggest that the thermotransport measurements could be used for the characterization of state transition in topological insulators.Comment: 9 pages, 3 figures, Journal of Physics: Condensed Matter 201

Spin gapless semiconductors

Spin gapless semiconductors (SGSs) are a new class of zero gap materials which have a fully spin polarised electrons and holes. They bridge zero gap materials and half-metals. The band structures of the SGSs can have two types of energy dispersions: Dirac linear dispersion and parabolic dispersion. The Dirac type SGSs exhibit fully spin polarized Dirac cones, and offer a platform for massless and fully spin polarized spintronics as well as dissipationless edge state via quantum anomalous Hall effect. Due to its fascinating spin and charge states, they hold great potential application in spintronics. There have been tremendous efforts worldwide on searching for suitable candidates of SGSs. In particularly, there is an increasing interest in searching for Dirac type SGSs. In the past decade, a large number of Dirac or parabolic type SGSs have been predicted by density functional theory and some of parabolic SGSs have been experimentally demonstrated. The SGSs hold great potential for high speed and low-energy consumption spintronics, electronics and optoelectronics. Here, we review both Dirac and parabolic types of SGSs in different materials systems and outline the concepts of SGSs, novel spin and charge states, and potential applications of SGSs in next generation spintronic devices.Comment: Small in press. Submitted on September 10, 2019, Accepted on May 6, 202

Intrinsically core-shell plasmonic dielectric nanostructures with ultrahigh refractive index

Topological insulators are a new class of quantum material s with metallic (edge) surface states and insulating bulk states. They demonstrate a variety of novel electronic and optical properties, which make them highly promising electronic, spintronic, and optoelectronic materials. We report on a novel conic plasmonic nanostructure that is made of bulk-insulating topological insulators and has an intrinsic core-shell formation. The insulating (dielectric) core of the nanocone displays an ultrahigh refractive index of up to 5.5 in the near-infrared frequency range. On the metallic shell, plasmonic response and strong backward light scattering were observed in the visible frequency range. Through in- tegrating the nanocone arrays into a-Si thin film solar cel ls, up to 15% enhancement of light absorption was predicted in the ultraviolet and visible ranges. With these unique features, the intrinsically core-shell plasmonic nanostructure paves a new way for designing low-loss and high-performance visible to infrared optical devices

Modulation of crystal and electronic structures in topological insulators by rare-earth doping

We study magnetotransport in a rare earth doped topological insulator, Sm0.1Sb1.9Te3 single crystals, under magnetic fields up to 14 T. It is found that that the crystals exhibit Shubnikov de Haas oscillations in their magneto-transport behaviour at low temperatures and high magnetic fields. The SdH oscillations result from the mixed contributions of bulk and surface states. We also investigate the SdH oscillations in different orientations of the magnetic field, which reveals a three dimensional Fermi surface topology. By fitting the oscillatory resistance with the Lifshitz Kosevich theory, we draw a Landau-level fan diagram that displays the expected nontrivial phase. In addition, the density functional theory calculations shows that Sm doping changes the crystal structure and electronic structure compared with pure Sb2Te3. This work demonstrates that rare earth doping is an effective way to manipulate the Fermi surface of topological insulators. Our results hold potential for the realization of exotic topological effects in magnetic topological insulators.Comment: 16 pages, 5 figure

Quantum oscillations of robust topological surface states up to 50 K in thick bulk-insulating topological insulator

As personal electronic devices increasingly rely on cloud computing for energy-intensive calculations, the power consumption associated with the information revolution is rapidly becoming an important environmental issue. Several approaches have been proposed to construct electronic devices with low-energy consumption. Among these, the low-dissipation surface states of topological insulators (TIs) are widely employed. To develop TI-based devices, a key factor is the maximum temperature at which the Dirac surface states dominate the transport behavior. Here, we employ Shubnikov-de Haas oscillations (SdH) as a means to study the surface state survival temperature in a high-quality vanadium doped Bi1.08Sn0.02Sb0.9Te2S single crystal system. The temperature and angle dependence of the SdH show that: (1) crystals with different vanadium (V) doping levels are insulating in the 3-300 K region; (2) the SdH oscillations show two-dimensional behavior, indicating that the oscillations arise from the pure surface states; and (3) at 50 K, the V0.04 single crystals (Vx:Bi1.08-xSn0.02Sb0.9Te2S, where x = 0.04) still show clear sign of SdH oscillations, which demonstrate that the surface dominant transport behavior can survive above 50 K. The robust surface states in our V doped single crystal systems provide an ideal platform to study the Dirac fermions and their interaction with other materials above 50 K

Electronic transport, magnetic and thermal properties of gapless materials

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

Giant interlayer magnetoresistances and strong anisotropy in p-type Sb2Te3 single crystals

Anisotropic interlayer magneto-transport properties have been studied over a broad range of temperatures and magnetic fields in p-type Sb2Te3 single crystals. Giant interlayer 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 interlayer MR displays strong anisotropy and is attributable to the anisotropy of the Fermi surface. The observed giant anisotropic interlayer MR could find applications in p-type Sb2Te3 based magneto-electronic devices