Bi Bilayer Research Articles

Orbit- and atom-resolved spin textures of intrinsic, extrinsic, and hybridized Dirac cone states

Combining first-principles calculations and spin- and angle-resolved photoemission spectroscopy measurements, we identify the helical spin textures for three different Dirac cone states in the interfaced systems of a two-dimensional (2D) topological insulator (TI) of a Bi(111) bilayer and a three-dimensional (3D) TI Bi${}_{2}$Se${}_{3}$ or Bi${}_{2}$Te${}_{3}$. The spin texture is found to be the same for the intrinsic Dirac cone of Bi${}_{2}$Se${}_{3}$ or Bi${}_{2}$Te${}_{3}$ surface states, the extrinsic Dirac cone of Bi bilayer induced by the Rashba effect, and the hybridized Dirac cone between the former two states. Further orbit- and atom-resolved analysis shows that $s$ and ${p}_{z}$ orbits have a clockwise (counterclockwise) spin rotation tangent to the iso-energy contour of the upper (lower) Dirac cone, while ${p}_{x}$ and ${p}_{y}$ orbits have radial spin components. The Dirac cone states may reside on different atomic layers, but have the same spin texture. Our results suggest that the unique spin texture of Dirac cone states is a signature property of spin-orbit coupling, independent of topology.

Homologous series of layered structures in binary and ternary Bi-Sb-Te-Se systems:Ab initiostudy

In order to account explicitly for the existence of long-periodic layered structures and the strong structural relaxations in the most common binary and ternary alloys of the Bi-Sb-Te-Se system, we have developed a one-dimensional cluster expansion (CE) based on first-principles electronic structure calculations, which accounts for the Bi and Sb bilayer formation. Excellent interlayer distances are obtained with a van der Waals density functional. It is shown that a CE solely based on pair interactions is sufficient to provide an accurate description of the ground-state energies of Bi-Sb-Te-Se binary and ternary systems without making the data set of ab initio calculated structures unreasonably large. For the binary alloys ${A}_{1\ensuremath{-}x}{Q}_{x}$ ($A=\text{Sb}$, Bi; $Q=\text{Te}$, Se), a ternary CE yields an almost continuous series of (meta)stable structures consisting of consecutive $A$ bilayers next to consecutive ${A}_{2}{Q}_{3}$ for $0<x<0.6$. For $x>0.6$, the binary alloy segregates into pure $Q$ and ${A}_{2}{Q}_{3}$. The Bi-Sb system is described by a quaternary CE and is found to be an ideal solid solution stabilized by entropic effects at $T\ensuremath{\ne}0$ K but with an ordered structure of alternating Bi and Sb layers for $x=0.5$ at $T=0$ K. A quintuple CE is used for the ternary Bi-Sb-Te system, where stable ternary layered compounds with an arbitrary stacking of ${\text{Sb}}_{2}$${\text{Te}}_{3}$, ${\text{Bi}}_{2}$${\text{Te}}_{3}$, and Te-Bi-Te-Sb-Te quintuple units are found, optionally separated by mixed Bi/Sb bilayers. Electronic properties of the stable compounds were studied taking spin-orbit coupling into account.

Nontrivial topological electronic structures in a single Bi(111) bilayer on different substrates: A first-principles study

Electronic structures, minimum energy configurations, and band topology of strained Bi(111) single bilayers placed on a variety of semiconducting and insulating substrates are investigated using first-principles calculations. A topological phase diagram of a free-standing Bi bilayer is presented to help guide the selection of suitable substrates. The insulating hexagonal-BN is identified as the best candidate substrate material for supporting nontrivial topological insulating phase of Bi bilayer thin films. A planar hexagonal Bi layer is predicted under tensile strain, which we show could be realized on a SiC substrate. The Bi bilayer becomes metallic under the compressive strain induced by Si and Ge substrates.

Electronic Structures and Topological Properties of Bi(111) Ultrathin Films

Based on the first-principles calculations, we systematically study the electronic structures and topological properties of bismuth (Bi) ultrathin films in (111) orientation. Considering the spin–orbit coupling, the Bi(111) films show the thickness dependent behavior of their band structures and topological properties. The single and two bilayers Bi(111) films are confirmed to be nontrivial two-dimensional (2D) topological insulator system, while for the films from three to five bilayers, a transition from semiconductor to semimetal is found. Moreover, the single bilayer Bi(111) thin film is very robust against external electric field, but the two bilayers film is not robust.

Origin of the Bismuth-Induced Decohesion of Nickel and Copper Grain Boundaries

Ductile metals such as Ni and Cu can become brittle when certain impurities (e.g., Bi) diffuse and segregate into their grain boundaries (GBs). Using first-principles calculations, we investigate the microscopic origin of the Bi-induced loss of cohesion of Ni and Cu GBs. We find that the Bi bilayer interfacial phase is the most stable impurity phase under the Bi-rich condition, while the Bi monolayer phase is a metastable phase regardless of the value of the Bi chemical potential. Our finding is consistent with the recent experimental observation for Ni GBs [Luo et al. Science 333, 1730 (2011)]. The electric polarization effect of the Bi bilayer substantially enhances the strength of the Bi-metal interfacial bonds, stabilizing the bilayer phase over other phases. The Bi-Bi interlayer bonding is significantly weakened in the GBs, leading to a factor of 20 to 50 decrease in the GB cohesion, which has strong implications for the understanding of Bi-induced intergranular fracture of Ni and Cu polycrystals.

Engineering quantum anomalous Hall phases with orbital and spin degrees of freedom

Combining tight-binding models and first principles calculations, we investigate the quantum anomalous Hall (QAH) effect induced by intrinsic spin-orbit coupling (SOC) in buckled honeycomb lattice with sp orbitals in an external exchange field. Detailed analysis reveals that nontrivial topological properties can arise utilizing not only spin but also orbital degrees of freedom in the strong SOC limit, when the bands acquire non-zero Chern numbers upon undergoing the so-called orbital purification. As a prototype of a buckled honeycomb lattice with strong SOC we choose the Bi(111) bilayer, analyzing its topological properties in detail. In particular, we show the emergence of several QAH phases upon spin exchange of the Chern numbers as a function of SOC strength and magnitude of the exchange field. Interestingly, we observe that in one of such phases, namely, in the quantum spin Chern insulator phase, the quantized charge and spin Hall conductivities co-exist. We consider the possibility of tuning the SOC strength in Bi bilayer via alloying with isoelectronic Sb, and speculate that exotic properties could be expected in such an alloyed system owing to the competition of the topological properties of its constituents. Finally, we demonstrate that 3d dopants can be used to induce a sizeable exchange field in Bi(111) bilayer, resulting in non-trivial Chern insulator properties.

Topologically Protected Quantum Transport in Locally Exfoliated Bismuth at Room Temperature

We report electrical conductance measurements of Bi nanocontacts created by repeated tip-surface indentation using a scanning tunneling microscope at temperatures of 4 and 300 K. As a function of the elongation of the nanocontact, we measure robust, tens of nanometers long plateaus of conductance G0 = 2e2/h at room temperature. This observation can be accounted for by the mechanical exfoliation of a Bi(111) bilayer, a predicted quantum spin Hall (QSH) insulator, in the retracing process following a tip-surface contact. The formation of the bilayer is further supported by the additional observation of conductance steps below G0 before breakup at both temperatures. Our finding provides the first experimental evidence of the possibility of mechanical exfoliation of Bi bilayers, the existence of the QSH phase in a two-dimensional crystal, and, most importantly, the observation of the QSH phase at room temperature.

Surface termination of cleaved Bi2Se3 investigated by low energy ion scattering.

It has been widely assumed that Se terminates the surface of the topological insulator, bismuth selenide. Although some Se is initially at the surface after cleaving at 80K, low energy ion scattering reveals a complete Bi termination at room temperature. Density functional theory shows that a Bi bilayer atop the bulk-terminated structure is energetically favorable. It is thus proposed that a thermally activated process induces a spontaneous termination change after cleaving. This has profound implications on the electrical transport and long-term stability of such materials and devices.

Quasiparticle dynamics in reshaped helical Dirac cone of topological insulators

Topological insulators and graphene present two unique classes of materials, which are characterized by spin-polarized (helical) and nonpolarized Dirac cone band structures, respectively. The importance of many-body interactions that renormalize the linear bands near Dirac point in graphene has been well recognized and attracted much recent attention. However, renormalization of the helical Dirac point has not been observed in topological insulators. Here, we report the experimental observation of the renormalized quasiparticle spectrum with a skewed Dirac cone in a single Bi bilayer grown on Bi(2)Te(3) substrate from angle-resolved photoemission spectroscopy. First-principles band calculations indicate that the quasiparticle spectra are likely associated with the hybridization between the extrinsic substrate-induced Dirac states of Bi bilayer and the intrinsic surface Dirac states of Bi(2)Te(3) film at close energy proximity. Without such hybridization, only single-particle Dirac spectra are observed in a single Bi bilayer grown on Bi(2)Se(3), where the extrinsic Dirac states Bi bilayer and the intrinsic Dirac states of Bi(2)Se(3) are well separated in energy. The possible origins of many-body interactions are discussed. Our findings provide a means to manipulate topological surface states.

Creation of helical Dirac fermions by interfacing two gapped systems of ordinary fermions

Topological insulators are a unique class of materials characterized by a Dirac cone state of helical Dirac fermions in the middle of a bulk gap. When the thickness of a three-dimensional topological insulator is reduced, however, the interaction between opposing surface states opens a gap that removes the helical Dirac cone, converting the material back to a normal system of ordinary fermions. Here we demonstrate, using density function theory calculations and experiments, that it is possible to create helical Dirac fermion state by interfacing two gapped films-a single bilayer Bi grown on a single quintuple layer Bi(2)Se(3) or Bi(2)Te(3). These extrinsic helical Dirac fermions emerge in predominantly Bi bilayer states, which are created by a giant Rashba effect with a coupling constant of ~4 eV·Å due to interfacial charge transfer. Our results suggest that this approach is a promising means to engineer topological insulator states on non-metallic surfaces.

Identification of a bilayer grain boundary complexion in Bi-doped Cu

Using aberration-corrected scanning transmission electron microscopy, we have directly observed a bilayer grain boundary complexion in Bi-doped Cu, akin to that observed in Ni–Bi [Science, 333: 1730 (2011)]. In comparison with the Ni–Bi bilayer, the Cu–Bi bilayer appears to exist in a much narrower chemical potential window attributable to the fact that Cu–Bi and Ni–Bi have different pair-interaction potentials. Furthermore, these bilayers often form in conjunction with nanoscale faceting. This study demonstrates that direct imaging of the atom columns provides a more accurate understanding of the structure, chemistry and distribution of the adsorbates in a grain boundary and their role in embrittlement.

Molecular beam epitaxy of bilayer Bi(111) films on topological insulator Bi2Te3: A scanning tunneling microscopy study

We report on molecular beam epitaxy growth of bilayer Bi(111) films on topological insulator Bi2Te3. In situ scanning tunneling microscopy/spectroscopy shows that Bi growth mode changes from quasi bilayer-by-bilayer to step-flow with increasing substrate temperature. Bilayer Bi(111) exhibits an electron donor behavior, causing an 80 meV downshift of the Dirac point of Bi2Te3. Local work function difference between the bilayer films and Bi2Te3 films is measured to be 390 meV. Based on the observations, we propose a schematic energy-band diagram which reveals band bending effect at the Bi/Bi2Te3 interface. Our work paves a way to explore the exotic topological properties of bilayer islands and thin films of Bi.

Spatial and Energy Distribution of Topological Edge States in Single Bi(111) Bilayer

By combining scanning tunneling microscopy and spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory band calculations, we directly observe and resolve the one-dimensional edge states of single bilayer (BL) Bi(111) islands on clean Bi(2)Te(3) and Bi(111)-covered Bi(2)Te(3) substrates. The edge states are localized in the vicinity of step edges having an ∼2 nm wide spatial distribution in real space and reside in the energy gap of the Bi(111) BL. Our results demonstrate the existence of nontrivial topological edge states of single Bi(111) bilayer as a two-dimensional topological insulator.

Topological phases of Bi(111) bilayer in an external exchange field

Using first-principles methods, we investigate topological phase transitions as a function of exchange field in a Bi(111) bilayer. Evaluation of the spin Chern number for different magnitudes of the exchange field reveals that when the time-reversal symmetry is broken by a small exchange field, the system enters the time-reversal broken topological insulator phase, introduced by Yang et al. [Phys. Rev. Lett. 107, 066602 (2011)]. After a metallic phase in the intermediate region, the quantum anomalous Hall phase with a nonzero Chern number emerges at a sufficiently large exchange field. We analyze the phase diagram from the viewpoint of the evolution of the electronic structure, edge states, and transport properties and demonstrate that different topological phases can be distinguished by the spin polarization of the edge states as well as spin or charge transverse conductivity.

Surface relaxation of topological insulators: Influence on the electronic structure

The surface structure of topological insulators Bi${}_{2}$Te${}_{3}$(111) and a single bilayer bismuth on it was studied by low-energy electron-diffraction analysis. The topmost quintuple layer of Bi${}_{2}$Te${}_{3}$ showed only a slight relaxation ($\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}1%$ contraction). On the other hand, the bilayer Bi was strongly distorted compared to bulk Bi (3.5 $\phantom{\rule{-0.16em}{0ex}}%$ in-plane contraction and $\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}7%$ out-of-plane expansion). First-principles calculation reveals that this distortion has a large influence on the electronic structure and can enlarge the band gap.

The Coexistence of Superconductivity and Topological Order in the Bi 2 Se 3 Thin Films

Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi(2)Se(3)) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi(2)Se(3) surface in the regime of Bi(2)Se(3) film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.

Interfacing 2D and 3D Topological Insulators: Bi(111) Bilayer onBi2Te3

We report the formation of a bilayer Bi(111) ultrathin film, which is theoretically predicted to be in a two-dimensional quantum spin Hall state, on a Bi(2)Te(3) substrate. From angle-resolved photoemission spectroscopy measurements and ab initio calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and Bi(2)Te(3). Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface.

High-performance bilayered electrolyte intermediate temperature solid oxide fuel cells

The ESB/GDC bilayer electrolyte concept has been proved to improve open circuit voltage and reduce the effective area specific resistance of SOFCs utilizing a conventional single-layer GDC electrolyte. However, high performance from such bilayer cells had not yet been demonstrated. The main obstacles toward this end have been fabrication of anode-supported thin-film electrolytes and the reactivity of ESB with conventional cathodes. Recently, an ESB-compatible low area specific resistance cathode was developed: microstructurally optimized Bi 2Ru 2O 7-ESB composites. In addition, we recently developed a novel anode functional layer which can significantly enhance the performance of SOFC utilizing GDC electrolytes. This study combines these recent achievements in SOFC studies and shows that exceptionally high performance of SOFC is possible using ESB/GDC bilayer electrolytes and Bi 2Ru 2O 7-ESB composite cathodes. The result confirms that the bilayer electrolyte and the Bi 2Ru 2O 7-ESB cathode can increase the open circuit potential and reduce the total area specific resistance. The maximum power density of the bilayered SOFC was improved to 1.95 W cm −2 with 0.079 Ω cm 2 total cell area specific resistance at 650 °C. This is the highest power yet achieved in the IT range and we believe redefines the expectation level for maximum power under IT-SOFC operating conditions.

Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations

The surface structure of Bi(111) was investigated by low-energy electron diffraction (LEED) intensity analysis for temperatures between 140 and $313\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and by first-principles calculations. The diffraction pattern reveals a $(1\ifmmode\times\else\texttimes\fi{}1)$ surface structure and LEED intensity versus energy simulations confirm that the crystal is terminated with a Bi bilayer. Excellent agreement is obtained between the calculated and measured diffraction intensities in the whole temperature range. The first interlayer spacing shows no significant relaxation at any temperature while the second interlayer spacing expands slightly. The Debye temperatures deduced from the optimized atomic vibrational amplitudes for the two topmost layers are found to be significantly lower than in the bulk. The experimental results for the relaxations agree well with those of our first-principles calculation.

TUNNELING SPECTROSCOPY OF TRILAYER HIGH-TC CUPRATE, TlBa2Ca2Cu2O10-δ

We present point contact tunneling spectroscopy measured on TlBa 2 Ca 2 Cu 3 O 10-δ ( Tl 1223) with three CuO 2 planes in a unit cell. Our samples of Tl 1223 with Tc~91 K are heavily overdoped at the outer CuO 2 planes (OP), but slightly underdoped at the inner CuO 2 planes (IP). The tunneling conductances on Tl 1223 exhibit two kinds of gaps that originate from crystallographically inequivalent IP and OP. The overall spectral shape of tunneling conductance for Tl 1223 is consistent with that observed on a bilayer Bi 2 Sr 2 CaCu 2 O 8+δ, that is, unusual peak-dip-hump (PDH) structures are observed. The origin of PDH structures is controversial, but these results, especially, the observation of PDH for trilayer Tl 1223 in which interlayer effects should be quite different from that in bilayer, put severe constraints on any theoretical model for the origin of these structures.