Surface States Of Bi Research Articles

Transient THz Study of Phonon-Assisted Carrier Scattering between Bulk and Surface States in Bi2Te3

Transient THz Study of Phonon-Assisted Carrier Scattering between Bulk and Surface States in Bi₂Te₃

Three-Dimensional Charge Density Wave and Surface-Dependent Vortex-Core States in a Kagome Superconductor CsV3Sb5

The transition-metal-based kagome metals provide a versatile platform for correlated topological phases hosting various electronic instabilities. While superconductivity is rare in layered kagome compounds, its interplay with nontrivial topology could offer an engaging space to realize exotic excitations of quasiparticles. Here, we use scanning tunneling microscopy (STM) to study a newly discovered Z$_2$ topological kagome metal CsV$_3$Sb$_5$ with a superconducting ground state. We observe charge modulation associated with the opening of an energy gap near the Fermi level. When across single-unit-cell surface step edges, the intensity of this charge modulation exhibits a {\pi}-phase shift, suggesting a three-dimensional 2$\times$2$\times$2 charge density wave ordering. Interestingly, a robust zero-bias conductance peak is observed inside the superconducting vortex core on the Cs 2$\times$2 surfaces that does not split in a large distance when moving away from the vortex center, resembling the Majorana bound states arising from the superconducting Dirac surface states in Bi$_2$Te$_3$/NbSe$_2$ heterostructures. Our findings establish CsV$_3$Sb$_5$ as a promising candidate for realizing exotic excitations at the confluence of nontrivial lattice geometry, topology and multiple electronic orders.

Persistence of the Topological Surface States in Bi2Se3 against Ag Intercalation at Room Temperature

The electronic and atomic structures of topological insulator Bi2Se3, upon Ag atom deposition, have been investigated by combined experimental methods of scanning tunneling microscopy (STM), photoe...

Topological Phase Control of Surface States in Bi2Se3 via Spin-Orbit Coupling Modulation through Interface Engineering between HfO2-X.

The direct control of topological surface states in topological insulators is an important prerequisite for the application of these materials. Conventional attempts to utilize magnetic doping, mechanical tuning, structural engineering, external bias, and external magnetic fields suffer from a lack of reversible switching and have limited tunability. We demonstrate the direct control of topological phases in a bismuth selenide (Bi2Se3) topological insulator in 3 nm molecular beam epitaxy-grown films through the hybridization of the topological surface states with the hafnium (Hf) d-orbitals in the topmost layer of an underlying oxygen-deficient hafnium oxide (HfO2) substrate. The higher angular momentum of the d-orbitals of Hf is hybridized strongly by topological insulators, thereby enhancing the spin-orbit coupling and perturbing the topological surface states asymmetry in Bi2Se3. As the oxygen defect is cured or generated reversibly by external electric fields, our research facilitates the complete electrical control of the topological phases of topological insulators by controlling the defect density in the adjacent transition metal oxide. In addition, this mechanism can be applied in other related topological materials such as Weyl and Dirac semimetals in future endeavors to facilitate practical applications in unit-element devices for quantum computing and quantum communication.

Composition-dependent structural transition in epitaxial Bi1−xSbx thin films on Si(111)

Bismuth-antimony alloys (${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$) are topological insulators between 7 and 22% Sb in bulk crystals, with an unusually high conductivity suitable for spin-orbit torque applications. Reducing the thickness of epitaxial ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ films is expected to increase the maximum band gap through quantum confinement, which may improve isolation of topological surface-state transport. Like Bi(001) on Si(111), ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ has been predicted to form a black phosphoruslike allotrope with unique electronic properties in nanoscale films; however, the impact of Sb alloying on both the bulklike and nanoscale crystal structures on Si(111) is currently unknown. Here we demonstrate that the allotropic transition in ultrathin epitaxial ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ films on Si(111) is suppressed above 8--9% Sb, resulting in an unexpected (012) orientation within the topologically insulating regime. The metallic temperature-dependent conductivity associated with surface states in Bi(001) was not observed in the ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}(012)$ films, suggesting that the (012) orientation may significantly reduce surface-state transport. Growth on a Bi(001) buffer layer may prevent this orientation transition. Finally, we demonstrate that Sb alloying improves the continuity and quality of nanoscale ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}(012)$ films in the thickness regime expected for the black phosphorus allotrope, suggesting a promising route to large-area growth of puckered-layer two-dimensional ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$, which will be necessary to harness its unique electronic properties in practical applications.

Onset of a Two-Dimensional Superconducting Phase in a Topological-Insulator-Normal-Metal Bi_{1-x}Sb_{x}/Pt Junction Fabricated by Ion-Beam Techniques.

Inducing superconductivity in a topological insulator can lead to novel quantum effects. However, experimental approaches to turn a topological insulator into a superconductor are limited. Here, we report on superconductivity in topological insulator Bi_{0.91}Sb_{0.09} induced via focused ion-beam deposition of a Pt thin film. The superconducting phase exhibits a Berezinski-Kosterlitz-Thouless transition, demonstrative of its two-dimensional character. From the in-plane upper critical field measurements, we estimate the superconducting thickness to be ∼17 nm for a 5.5-μm-thick sample. Our results provide evidence that the interface superconductivity could originate from the surface states of Bi_{0.91}Sb_{0.09}.

Analytical Solutions for the Surface States of Bi1−xSbx (0 ≤ x \( \lesssim \) 0.1)

Analytical solutions for the surface state (SS) of an extended Wolff Hamiltonian, which is a common Hamiltonian for strongly spin-orbit coupled systems, are obtained both for semi-infinite and finite-thickness boundary conditions. For the semi-infinite system, there are three types of SS solutions: (I-a) linearly crossing SSs in the direct bulk band gap, (I-b) SSs with linear dispersions entering the bulk conduction or valence bands away from the band edge, and (II) SSs with nearly flat dispersions entering the bulk state at the band edge. For the finite-thickness system, a gap opens in the SS of solution I-a. Numerical solutions for the SS are also obtained based on the tight-binding model of Liu and Allen [Phys. Rev. B, 52, 1566 (1995)] for Bi$_{1-x}$Sb$_x$ ($0\le x \le 0.1$). A perfect correspondence between the analytic and numerical solutions is obtained around the $\bar{M}$ point including their thickness dependence. This is the first time that the character of the SS numerically obtained is identified with the help of analytical solutions. The size of the gap for I-a SS can be larger than that of bulk band gap even for a "thick" films ($\lesssim 200$ bilayers $\simeq 80$ nm) of pure bismuth. Consequently, in such a film of Bi$_{1-x}$Sb$_x$, there is no apparent change in the SSs through the band inversion at $x\simeq 0.04$, even though the nature of the SS is changed from solution I-a to I-b. Based on our theoretical results, the experimental results on the SS of Bi$_{1-x}$Sb$_x$ ($0\le x \lesssim 0.1$) are discussed.

Spin-dependent quantum interference in photoemission process from spin-orbit coupled states

Spin–orbit interaction entangles the orbitals with the different spins. The spin–orbital-entangled states were discovered in surface states of topological insulators. However, the spin–orbital-entanglement is not specialized in the topological surface states. Here, we show the spin–orbital texture in a surface state of Bi(111) by laser-based spin- and angle-resolved photoelectron spectroscopy (laser-SARPES) and describe three-dimensional spin-rotation effect in photoemission resulting from spin-dependent quantum interference. Our model reveals that, in the spin–orbit-coupled systems, the spins pointing to the mutually opposite directions are independently locked to the orbital symmetries. Furthermore, direct detection of coherent spin phenomena by laser-SARPES enables us to clarify the phase of the dipole transition matrix element responsible for the spin direction in photoexcited states. These results permit the tuning of the spin polarization of optically excited electrons in solids with strong spin–orbit interaction.

Impurity effect on surface states of Bi (111) ultrathin films**Project supported by the National Basic Research Program of China (Grants Nos. 2015CB921400 and 2011CB921802) and the National Natural Science Foundation of China (Grants Nos. 11374057, 11434003, and 11421404).

The surface impurity effect on the surface-state conductivity and weak antilocalization (WAL) effect has been investigated in epitaxial Bi (111) films by magnetotransport measurements at low temperatures. The surface-state conductivity is significantly reduced by the surface impurities of Cu, Fe, and Co. The magnetotransport data demonstrate that the observed WAL is robust against deposition of nonmagnetic impurities, but it is quenched by the deposition of magnetic impurities which break the time reversal symmetry. Our results help to shed light on the effect of surface impurities on the electron and spin transport properties of a 2D surface electron systems.

Gigantic surface lifetime of an intrinsic topological insulator.

The interaction between light and novel two-dimensional electronic states holds promise to realize new fundamental physics and optical devices. Here, we use pump-probe photoemission spectroscopy to study the optically excited Dirac surface states in the bulk-insulating topological insulator Bi_{2}Te_{2}Se and reveal optical properties that are in sharp contrast to those of bulk-metallic topological insulators. We observe a gigantic optical lifetime exceeding 4 μs (1 μs=10^{-6} s) for the surface states in Bi_{2}Te_{2}Se, whereas the lifetime in most topological insulators, such as Bi_{2}Se_{3}, has been limited to a few picoseconds (1 ps=10^{-12} s). Moreover, we discover a surface photovoltage, a shift of the chemical potential of the Dirac surface states, as large as 100mV. Our results demonstrate a rare platform to study charge excitation and relaxation in energy and momentum space in a two-dimensional system.

Realizing Majorana zero modes by proximity effect between topological insulators andd-wave high-temperature superconductors

We theoretically study superconducting proximity effect between a topological insulator (TI) and a high-temperature d-wave superconductor (dSC). When the TI-dSC heterostructure violates 90 degree-rotation and certain reflection symmetries, we show that a sizable s-wave pairing, coexisting with a d-wave one, emerges in the proximity-induced superconductivity in the TI's top surface states. Weak disorder further suppresses d-wave pairing but not s-wave one in the TI's surface states. More importantly, the pairing gap in surface states is found to be nodeless and nearly-isotropic when the Fermi pocket of surface states is relatively small. Our theoretical results qualitatively explain recent experimental evidences of a nearly-isotropic pairing gap on surface states of Bi_2Se_3 induced by proximity with high-T_c cuprate Bi_2Ca_2Cu_2O_{8+\delta}. We also demonstrate convincing evidences of Majorana zero modes in a magnetic hc/2e vortex core, which may be detectable in future experiments.

Topological Surface States Originated Spin-Orbit Torques in Bi(2)Se(3).

The three dimensional topological insulator bismuth selenide (Bi(2)Se(3)) is expected to possess strong spin-orbit coupling and spin-textured topological surface states and, thus, exhibit a high charge to spin current conversion efficiency. We evaluate spin-orbit torques in Bi(2)Se(3)/Co(40)Fe(40)B(20) devices at different temperatures by spin torque ferromagnetic resonance measurements. As the temperature decreases, the spin-orbit torque ratio increases from ∼0.047 at 300 K to ∼0.42 below 50 K. Moreover, we observe a significant out-of-plane torque at low temperatures. Detailed analysis indicates that the origin of the observed spin-orbit torques is topological surface states in Bi(2)Se(3). Our results suggest that topological insulators with strong spin-orbit coupling could be promising candidates as highly efficient spin current sources for exploring the next generation of spintronic applications.

Quantum size effect on Shubnikov-de Haas oscillations in 100 nm diameter single-crystalline bismuth nanowire

Quantum size effect (QSE) in Bi nanowire is theoretically predicted to decrease band overlap energy resulting in semimetal-to-semiconductor transition. However, this effect has been rarely demonstrated on transport properties because of carrier-surface scattering and charge carriers induced from surface states of Bi. We report QSE on Shubnikov-de Haas (SdH) oscillations in a single-crystalline Bi nanowire with a diameter of 100 nm. The variation of intrinsic properties estimated using SdH oscillations indicates that the subband energy shift due to QSE. The enhanced effective mass of the electrons is consistent with the theoretical prediction pertaining to strong electron-hole coupling of Bi.

Magnetothermoelectric properties of Bi2Se3

We present a study of entropy transport in Bi${}_{2}$Se${}_{3}$ at low temperatures and high magnetic fields. In the zero-temperature limit, the magnitude of the Seebeck coefficient quantitatively tracks the Fermi temperature of the three-dimensional Fermi surface at the $\ensuremath{\Gamma}$ point as the carrier concentration changes by two orders of magnitude (10${}^{17}$ to 10${}^{19}$ cm${}^{\ensuremath{-}3}$). In high magnetic fields, the Nernst response displays giant quantum oscillations indicating that this feature is not exclusive to compensated semimetals. A comprehensive analysis of the Landau level spectrum firmly establishes a large $g$ factor in this material and a substantial decrease of the Fermi energy with increasing magnetic field across the quantum limit. Thus, the presence of bulk carriers significantly affects the spectrum of the intensively debated surface states in Bi${}_{2}$Se${}_{3}$ and related materials.

Unoccupied electronic band structure of the semi-metallic Bi(111) surface probed with two-photon photoemission

While many photoemission studies have dealt with both the bulk band structure and various surface states and resonances, the unoccupied electronic structure above the Fermi level of the Bi(111) surface has not yet been measured directly although understanding of this model semi-metal is of great interest for topological insulators, spintronics, and related fields. We use angle-resolved two-photon photoemission to directly investigate the occupied and unoccupied $p$ bands of Bi, including the bulk hole pocket at the $T$ point, as well as the image potential states and surface states of Bi(111).

Manipulation of Electronic Transport in the Bi(111) Surface State

We demonstrate the controlled manipulation of the 2D-electronic transport in the surface state of Bi(111) through the deposition of small amounts of Bi to generate adatoms and 2D islands as additional scatterers. The corresponding increase in resistance is recorded insitu and in real time. Model calculations based on mean-field nucleation theory reveal a constant scattering efficiency of adatoms and of small 2D Bi islands, independent of their size. This finding is supported by a detailed scanning tunneling microscopy and spectroscopy study at 5K which shows a highly anisotropic scattering pattern surrounding each surface protrusion.

Interplay between topological insulators and superconductors

Topological insulators are insulating in the bulk but possess metallic surface states protected by time-reversal symmetry. Here, we report on a detailed electronic transport study in high-quality Bi${}_{2}$Se${}_{3}$ topological insulator thin films contacted by superconducting (In, Al, and W) electrodes. The resistance of the film shows an abrupt and significant upturn when the electrodes become superconducting. In turn, the Bi${}_{2}$Se${}_{3}$ film greatly weakens the superconductivity of the electrodes, significantly reducing both their transition temperatures and their critical fields. A possible interpretation of these results is that the superconducting electrodes are accessing the surface states and the experimental results are consequences of the interplay between the Cooper pairs of the electrodes and the spin-polarized current of the surface states in Bi${}_{2}$Se${}_{3}$.

Symmetry-constrained reorganization of Dirac cones in topological insulators by surface modification

Topological insulators possess a symmetry-protected surface spin current, a property highly relevant to spintronics applications. This current arises from gapless spin-polarized surface states. Calculations show that these states can undergo drastic transformations with surface treatment, despite the overall symmetry-based constraints. Specifically, the single Dirac cone at the zone center for the surface states in Bi${}_{2}$Se${}_{3}$ (Bi${}_{2}$Te${}_{3}$) is transformed, upon hydrogen termination, into three Dirac cones at the zone boundary. This trifurcation enhances the surface spin polarization, but leaves the overall topological order invariant.

Robust topological surface state against direct surface contamination

Angle resolved photoemission spectroscopy (ARPES) study were performed on the (111) surface of topological insulators Bi2Te3, Bi2Se3 and semimetals Bi and Sb. In all four materials, we observed clear surface states centered at the Γ point of the surface Brillouin zone. We further studied the evolution of these surface states under controlled surface contamination of CO and O2 gas molecules, and found that the surface states in Bi2Te3 and Bi2Se3 were both robust against the direct influence of these gas molecules, while the surface states of Bi and Sb could be easily destroyed under the same condition. Our finding provides direct experimental evidence confirming the unusual robustness of the topological surface state of topological insulators with the presence of the time reversal symmetry.

Structural defects and electronic properties of the Cu-doped topological insulator Bi2Se3

Motivated by the occurrence of superconductivity transition in Cu-doped topological insulator Bi${}_{2}$Se${}_{3}$, we perform a combined study of low temperature scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy experiments and of ab initio density functional theory to clarify the doping nature of Cu atoms in Bi${}_{2}$Se${}_{3}$ films. By measuring the structural and electronic properties of the Cu-doped Bi${}_{2}$Se${}_{3}$ films at different doping temperatures, we find that Cu atoms behave as donors at intercalated and interstitial sites in Bi${}_{2}$Se${}_{3}$ films. Only the interstitial defect density plays an important role in the observation of Landau quantization of the topological surface states in Bi${}_{2}$Se${}_{3}$.