Properties Of Topological Surface States Research Articles

Extraordinary Thermoelectric Properties of Topological Surface States in Quantum-Confined Cd3As2 Thin Films.

Topological insulators and semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport associated with the Fermi arc topological surface states on topological Dirac semimetals remains less explored. This work systematically examines thermoelectric transport in a series of topological Dirac semimetal Cd3As2 thin films grown by molecular beam epitaxy. Surprisingly, significantly enhanced Seebeck effect and anomalous Nernst effect are found at cryogenic temperatures when the Cd3As2 layer is thin. In particular, a peak Seebeck coefficient of nearly 500µVK-1 and a corresponding thermoelectric power factor over 30mWK-2m-1 are observed at 5K in a 25-nm-thick sample. Combining angle-dependent quantum oscillation analysis, magnetothermoelectric measurement, transport modeling, and first-principles simulation, the contributions from bulk and surface conducting channels are isolated and the unusual thermoelectric properties are attributed to the topological surface states. The analysis showcases the rich thermoelectric transport physics in quantum-confined topological Dirac semimetal thin films and suggests new routes to achieving high thermoelectric performance at cryogenictemperatures.

Distinct Quantum States in Topological Insulator Surfaces of Nanowires and Nanoribbons of Bismuth Selenide (Bi2Se3)

AbstractTopological insulators (TIs) exhibit unconventional quantum phases that can be tuned by external quantum confinements. The geometry of the surface of 3D TIs plays a crucial role. For example, the geometrical crossover from 2D surfaces to a 1D cylinder results in a novel state with a Spin‐Berry Phase (SBP). Surface‐Enhanced Raman Scattering (SERS) with a sub‐micron spatial resolution is utilized to study the quantum‐confinement effects of quasi‐relativistic electrons along the perimeter of the circular bismuth selenide (Bi2Se3) nanowires. The presence of diameter‐dependent SERS in nanowires can be attributed to the self‐interference effect of the electronic wave‐function along the circumferential direction of the TI nanowires. Nanoribbons with rectangular cross‐section do not show this effect. Further gold nanoparticles are applied as plasmonic SERS sensors attached to the distinct topological surface states to manipulate quasi‐relativistic surface states of nanoribbons and nanowires. This technique enables to discriminate between different geometries of TI surface states and also opens a novel pathway to probe the quantum properties of topological surface states.

Investigation of magnetotransport properties of topological surface states in SnBi4Te7 single crystal

Investigation of magnetotransport properties of topological surface states in SnBi4Te7 single crystal

Quantum interference effects in a 3D topological insulator with high-temperature bulk-insulating behavior

The Bi2Se3-family of 3D topological insulators (3DTI) exhibit insulating bulk states and surface states presenting a Dirac cone. At low temperatures, the conduction channels through the bulk of the material are fully gapped, making 3DTIs perfect systems to study the 2D transport behavior of Dirac fermions. Here, we report a 3DTI Bi1.1Sb0.9STe2 with a reduced level of defects, and thus, high-temperature insulating behavior in its bulk states. The insulator-to-metal transition occurs at ∼250 K, below which the bulk contributions are negligible. Even at room temperature, the conductivity contribution from the bulk channel is less than 20%. Quantum transport properties of topological surface states are observed in the Bi1.1Sb0.9STe2 nanoflake devices, e.g., high Hall mobility (∼1150 cm2/V s at 3 K), strong Shubnikov–de Haas oscillations with π Berry phase, weak antilocalization, and electron–electron interaction. Notably, additional oscillation patterns with quasi-periodicity-in-B and field-independent amplitude features are observed. The surface dominant transport behavior up to room temperature suggests that Bi1.1Sb0.9STe2 is a room temperature topological insulator for electronic/spintronic applications.

Multiple channel transport properties of topological surface states in SmB6 nanoribbons

SmB6, a topological Kondo insulator, possesses topologically protected surface states, in which carrier mobility is supposed to be high but is significantly varied in orders of magnitude dependent upon characterization methods. Herein, we performed magnetoresistance measurements on a Hall-bar device constructed on a highly uniform SmB6 nanoribbon with (001) surface grown by chemical vapor deposition. Unusual non-linear Hall resistance was observed at low temperature and explained with a two-carrier model. The two types of carriers were attributed to two types of Fermi pockets on the Brillouin zone of the SmB6 (001) surface. The Fermi pocket on Γ point provided high hole mobility up to 1.4×103 cm2/V s but low hole density at a temperature of 2.2 K. On the other hand, the Fermi pockets on X points supplied low electron mobility down to 1.8 cm2/V s but high electron density. The identification of the two types of pockets laid the foundation for the understanding of the transport via the topological surface states of the SmB6 (001) surface.

High-Mobility Ambipolar Magnetotransport in Topological Insulator Bi2Se3 Nanoribbons

Nanoribbons of topological insulators (TIs) have been suggested for a variety of applications exploiting the properties of the topologically protected surface Dirac states. In these proposals it is crucial to achieve a high tunability of the Fermi energy, through the Dirac point while preserving a high mobility of the involved carriers. Tunable transport in TI nanoribbons has been achieved by chemical doping of the materials so to reduce the bulk carriers' concentration, however at the expense of the mobility of the surface Dirac electrons, which is substantially reduced. Here we study bare ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ nanoribbons transferred on a variety of oxide substrates and demonstrate that the use of a large relative permittivity ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ substrate enables the Fermi energy to be tuned through the Dirac point and an ambipolar field effect to be obtained. Through magnetotransport and Hall conductance measurements, performed on single ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ nanoribbons, we demonstrate that electron and hole carriers are exclusively high-mobility Dirac electrons, without any bulk contribution. The use of ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ allows therefore an easy field effect gating in TI nanostructures providing an ideal platform to take advantage of the properties of topological surface states.

Super-Resonant Transport of Topological Surface States Subjected to In-Plane Magnetic Fields.

Magnetic oscillations of Dirac surface states of topological insulators are typically expected to be associated with the formation of Landau levels or the Aharonov-Bohm effect. We instead study the conductance of Dirac surface states subjected to an in-plane magnetic field in the presence of a barrier potential. Strikingly, we find that, in the case of large barrier potentials, the surface states exhibit pronounced oscillations in the conductance when varying the magnetic field, in the absence of Landau levels or the Aharonov-Bohm effect. These novel magnetic oscillations are attributed to the emergence of super-resonant transport by tuning the magnetic field, in which many propagating modes cross the barrier with perfect transmission. In the case of small and moderate barrier potentials, we identify a positive magnetoconductance due to the increase of the Fermi surface by tilting the surface Dirac cone. Moreover, we show that for weak magnetic fields, the conductance displays a shifted sinusoidal dependence on the field direction with period π and phase shift determined by the tilting direction with respect to the field direction. Our predictions can be applied to various topological insulators, such as HgTe and Bi_{2}Se_{3}, and provide important insights into exploring and understanding exotic magnetotransport properties of topological surface states.

Quantum Transport in Nanostructures of 3D Topological Insulators

Quantum transport measurement is an efficient tool to unveil properties of topological surface states in 3D topological insulators. Herein, experimental and theoretical results are reviewed, presenting first some methods for the growth of nanostructures. The effect of the disorder and the band bending is discussed in details both experimentally and theoretically. Then, the focus is put on disorder and quantum confinement effect in topological surface states of 3D topological insulators narrow nanostructures. Such effect can be revealed by investigating quantum interferences at very low temperature such as Aharonov–Bohm oscillations or universal conductance fluctuations.

Suppression of the spectral weight of topological surface states on the nanoscale via local symmetry breaking

In topological crystalline insulators the topological conducting surface states are protected by crystal symmetry, in principle making it possible to pattern nanoscale insulating and conductive motifs solely by breaking local symmetries on an otherwise homogenous, single-phase material. We show using scanning tunneling microscopy/spectroscopy that defects that break local symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations. Complementary ab initio calculations show how local symmetry breaking obstructs topological surface states as shown by a threefold reduction of the spectral weight of the topological surface states. The finding highlights the potential benefits of manipulating the surface morphology to create devices that take advantage of the unique properties of topological surface states and can operate at practical temperatures.

Tunable unconventional Kondo effect on topological insulator surfaces

We study Kondo physics of a spin-$\frac{1}{2}$ impurity in electronic matter with strong spin-orbit interaction, which can be realized by depositing magnetic adatoms on the surface of a three-dimensional topological insulator. We show that magnetic properties of topological surface states and the very existence of Kondo screening strongly depend on details of the bulk material, and specifics of surface preparation encoded in time-reversal preserving boundary conditions for electronic wavefunctions. When this tunable Kondo effect occurs, the impurity spin is screened by purely orbital motion of surface electrons. This mechanism gives rise to a transverse magnetic response of the surface metal, and spin textures that can be used to experimentally probe signatures of a Kondo resonance. Our predictions are particularly relevant for STM measurements in ${\rm Pb Te}$-class crystalline topological insulators, but we also discuss implications for other classes of topological materials.

Surface states of charge carriers in epitaxial films of the topological insulator Bi2Te3

The galvanomagnetic properties of p-type bismuth telluride heteroepitaxial films grown by the hot wall epitaxy method on oriented muscovite mica substrates have been investigated. Quantum oscillations of the magnetoresistance associated with surface electronic states in three-dimensional topological insulators have been studied in strong magnetic fields ranging from 6 to 14 T at low temperatures. The cyclotron effective mass, charge carrier mobility, and parameters of the Fermi surface have been determined based on the results of analyzing the magnetoresistance oscillations. The dependences of the cross-sectional area of the Fermi surface S(kF), the wave vector kF, and the surface concentration of charge carriers ns on the frequency of magnetoresistance oscillations in p-type Bi2Te3 heteroepitaxial films have been obtained. The experimentally observed shift of the Landau level index is consistent with the value of the Berry phase, which is characteristic of topological surface states of Dirac fermions in the films. The properties of topological surface states of charge carriers in p-type Bi2Te3 films obtained by analyzing the magnetoresistance oscillations significantly expand fields of practical application and stimulate the investigation of transport properties of chalcogenide films.

Effects of surface modification on the properties of topological surface states in Bi2Se3

The effects of surface modification on the topological surface states in Bi2Se3 have been studied using first-principles calculations. Although surface modification cannot change the topological order, however, the spin polarization, dispersion of topological surface bands and the position of Dirac points are altered. All the studied surface modifications can push Dirac cone upwards effectively. Bi-capped surface can attract the quantum well states into bulk band gap, forming new Dirac cone at M¯ point. Both Bi-capped and Se-removed surface can slightly enhance spin polarization of topological surface states, while flatten the energy dispersion of the topological surface bands above Dirac point.