Center Of Surface Brillouin Zone Research Articles

Anisotropic surface state in a topological semimetal candidate Ta3SiTe6

Using high-resolution angle-resolved photoemission spectroscopy, we studied the surface state of Ta3SiTe6, a candidate of topological semimetal protected by nonsymmorphic symmetry. Through photon-energy dependent measurements, the surface state near the Fermi level was detected. By determining the band dispersions, we found that the surface state is topological trivial. Around the surface Brillouin zone center, the surface bands disperse linear-like, forming a band crossing (zero energy gap) point at about 0.2 eV below the Fermi level. In Ta3SiTe6's rectangular surface Brillouin zone, the surface bands only cross the Fermi level along one axis, forming two hole-like Fermi pockets near surface Brillouin zone boundaries. Although the observed surface state is not topological, we proposed that its anisotropic Fermi surface topography could be potentially useful for future applications.

Spin-orbit splitting of quantum well states in n -monolayer Ir/Au(111) heterostructures

The effect of spin-orbit coupling on quantum well states (QWSs) in atomically thin Ir adlayers deposited on the Au(111) substrate is studied in the framework of the density functional theory. Varying the Ir film thickness from 1 to 3 atomic layers, we find numerous Ir-derived QWSs, which are mainly of $d$ character. The resulting band dispersion of QWSs appearing around the surface Brillouin zone center in a wide Au(111) energy gap is analyzed in the framework of the Rashba model. In all such QWSs, the fitted values of the Rashba parameter exceed $2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$. The maximal value of $6.4\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$ was obtained for the 1-monolayer-Ir/Au(111) system. We explain such large spin splitting by hybridization between different QWSs. Strong enhancement is observed in the density of electronic states at the surface in the energy region around the Fermi level caused by these QWSs.

Modification of a Shockley-Type Surface State on Pt(111) upon Deposition of Gold Thin Layers.

We present a first-principles fully-relativistic study of surface and interface states in the n one monolayer (ML) Au/Pt(111) heterostructures. The modification of an unoccupied -type surface state existing on a Pt(111) surface at the surface Brillouin zone center upon deposition of a few atomic Au layers is investigated. In particular, we find that the transformation process of such a surface state upon variation of the Au adlayer thickness crucially depends on the nature of the relevant quantum state in the adsorbate. When the Au adlayer consists of one or two monolayers and this relevant state has energy above the Pt(111) surface state position, the latter shifts downward upon approaching the Au adlayer. As a result, in the 1 ML Au/Pt(111) and 2 ML Au/Pt(111) heterostructures at the equilibrium adlayer position, the Pt-derived surface state experiences strong hybridization with the bulk electronic states and becomes a strong occupied resonance. In contrast, when the number n of atomic layers in the Au films increases to three or more, the Pt(111) surface state shifts upward upon reduction of the distance between the Pt(111) surface and the Au adlayer. At equilibrium, the Pt-derived surface state transforms into an unoccupied quantum-well state of the Au adlayer. This change is explained by the fact that the relevant electronic state in free-standing Au films with has lower energy in comparison to the Pt(111) surface state.

Engineering Weyl Superfluid in Ultracold Fermionic Gases by One-Dimensional Optical Superlattices

In this paper, we theoretically demonstrate by using one-dimensional superlattices to couple two-dimensional time-reversal-breaking gapped topological superfluid models, an anomalous Weyl superfluid (WS) can be obtained. This new phase features its unique Fermi arc states (FAS) on the surfaces. In the conventional WS, FAS exist only for a part of the line connecting the projections of Weyl points and extending to the border and/or center of surface Brillouin zone. But for the anomalous WS, FAS exist for the whole line. As a proof of principle, we self-consistently at the mean-field level claim the achievement of the anomalous WS in the model with a dichromatic superlattice. In addition, inversion symmetry and band inversion in this model are analyzed to provide the unique features of identifying the anomalous WS experimentally by the momentum-resolved radio-frequency spectroscopy.

Topological phase transition and two-dimensional topological insulators in Ge-based thin films

We discuss possible topological phase transitions in Ge-based thin films of Ge(Bi$_x$Sb$_{1-x}$)$_2$Te$_4$ as a function of layer thickness and Bi concentration $x$ using the first principles density functional theory framework. The bulk material is a topological insulator at $x$ = 1.0 with a single Dirac cone surface state at the surface Brillouin zone center, whereas it is a trivial insulator at $x$ = 0. Through a systematic examination of the band topologies we predict that thin films of Ge(Bi$_x$Sb$_{1-x}$)$_2$Te$_4$ with $x$ = 0.6, 0.8 and 1.0 are candidates for two-dimensional (2D) topological insulators, which would undergo a 2D topological phase transition as a function of $x$. A topological phase diagram for Ge(Bi$_x$Sb$_{1-x}$)$_2$Te$_4$ thin films is presented to help guide their experimental exploration.

Study of Surface Rashba Effect by Spin- and Angle-Resolved Photoelectron Spectroscopy

Spin-split energy band structure appears in the surface state due to a broken space inversion symmetry at the surface boundary as well as a strong spin-orbit coupling. Spin- and angle-resolved photoemission spectroscopy (spin-resolved ARPES) has been applied to the study of the spin-split energy band structures of non-magnetic solid surfaces. First of all, some explanation of the spin-splitting mechanism for free electrons, so-called Rashba effect, will be given. Next, our spin-resolved ARPES system equipped with the compact Mott-type spin polarimeter will be introduced. We will show how the surface Rashba effect is observed from the spin-resolved ARPES with the recent result for the surface states of Bi(111). Here, we have observed strong and anti-symmetric spin polarizations with respect to the center of surface Brillouin zone in the surface states of Bi(111).

Spin injection from Fe into Si(001):Ab initiocalculations and role of the Si complex band structure

We study the possibility of spin injection from Fe into Si(001), using the Schottky barrier at the Fe/Si contact as tunneling barrier. Our calculations are based on density-functional theory for the description of the electronic structure and on a Landauer-B\"uttiker approach for the current. The current-carrying states correspond to the six conduction-band minima (pockets) of Si, which, when projected on the (001) surface Brillouin zone (SBZ), form five conductance hot spots: one at the SBZ center and four symmetric satellites. The satellites yield a current polarization of about 50%, while the SBZ center can, under very low gate voltage, yield up to almost 100%, showing a zero-gate anomaly. This extremely high polarization is traced back to the symmetry mismatch of the minority-spin Fe wave functions to the conduction-band wave functions of Si at the SBZ center. The tunneling current is determined by the complex band structure of Si in the [001] direction, which shows qualitative differences compared to that of direct-gap semiconductors. Depending on the Fermi level position and Schottky barrier thickness, the complex band structure can cause the contribution of the satellites to be orders of magnitude higher or lower than the central contribution. Thus, by appropriate tuning of the interface properties, there is a possibility to cut off the satellite contribution and to reach high injection efficiency. Also, we find that a moderate strain of 0.5% along the [001] direction is sufficient to lift the degeneracy of the pockets so that only states at the zone center can carry current.

D-band quantum well states in Ag(111) monolayer films; substrate-induced shifts

We report a study of 4d electronic states in monolayer silver films grown on Pd(111),Ni(111), Mo(110) and Cu(100) surfaces studied by means of high-resolution angle-resolvedphotoemission spectroscopy (ARPES). The Ag-4d states, when measured in the surfaceBrillouin zone centre (SBZ), show substrate-dependent shifts. Density functional theory(DFT) calculations for a free-standing silver monolayer provide evidence that theobserved shifts are not induced by lateral expansion, compression or distortion of thesilver unit cell. Using the phase accumulation model we show that 4d-derivedelectronic states in silver monolayers can be described in terms of quantum well statesand that the matching of the electron wavefunctions at the interface with thesubstrate is one of the important mechanisms that generates the Ag-4d energy shifts.The dispersion of the states around the SBZ centre is measured and discussed.

Calculations of energy gap and optical transitions in (001) quantum films of diamond: Quantum-confinement and surface effects

The electronic structure of diamond (001) ultrathin quantum films terminated by monohydrogenated dimers on both surfaces with 2×1 periodicity is studied by the extended Hückel-type nonorthogonal tight-binding method. In all diamond films studied, an unoccupied surface-related state occurs just below the conduction-band minimum of bulk diamond at the center of the (001) surface Brillouin zone (SBZ). It is shown that, although the band gap in bulk diamond is indirect, a direct energy gap is created between the film valence-band maximum and the lowest unoccupied surface state at the SBZ center by both quantum-confinement and surface effects. It is also demonstrated that optical transitions across the energy gap are dipole allowed and exhibit significantly high oscillator strength.

Strong dispersion of the surface optical phonon of silicon carbide in the near vicinity of the surface Brillouin zone center

The surface optical or Fuchs–Kliewer phonons of the (0 0 1) surface of 3C-SiC and the Si-terminated (0 0 0 1) surfaces of 4H- and 6H-SiC have been investigated with high resolution electron energy loss spectroscopy (HREELS). For each of the SiC polytypes the frequency of the surface optical phonon changes with surface reconstruction, indicating subtle differences in the static polarization at differently reconstructed surfaces. Due to their anisotropy, hexagonal surfaces exhibit a second, much weaker Fuchs–Kliewer mode. For all surfaces under examination, a linear dispersion of the Fuchs–Kliewer mode frequency has been found for wave vectors close to the Γ ¯ -point. This dispersion can be explained by dynamical dipole coupling between atomic oscillators at the surface of the highly polar silicon carbide.

Hybridization schemes for Ag films on V(1 0 0)

The electronic structure of ultra thin silver films deposited on a V(1 0 0) substrate is investigated by means of angle resolved photoemission spectroscopy. We focus on the details of the electronic structure related to the hybridization between the quantized film states of s–p origin and substrate electronic states. Two distinct regions of the surface Brillouin zone (SBZ) are found to be of particular importance: the SBZ center and the region that is typically more than 0.2 Å −1 outside the zone center. In relation to these SBZ parts two different hybridization schemes are identified and discussed. The first one influences the parabolic curvature of the quantum well state dispersion in the proximity of SBZ center, i.e., the effective mass. The second one relates to prominent changes of the quantum well state dispersion due to the interaction with the particular substrate bulk band and dramatically affects the photoemission intensity.

Femtosecond evolution of electronic interactions at the Ni(111) surface

We used time-resolved photoemission spectroscopy to measure the changes in electron self energy for the occupied surface resonance near the surface Brillouin zone center of Ni/W(110) single crystalline films following optical excitations with a fs laser pulse. Binding energy and lifetime display pronounced variations on a 100 fs time scale. We provide evidence that this is directly linked to the laser induced population of electronic levels.

Spin-polarized surface state of MnSb(0 0 0 1)

Knowledge of the spin-dependent electronic structure at surfaces and interfaces plays an increasingly important role when assessing possible use of novel magnetic materials for spintronic applications. It is shown that spin- and angle-resolved photoelectron spectroscopy together with ab initio electronic structure methods provides a full characterization of the surface electronic structure of ferromagnetic MnSb(0 0 0 1). Two different surface reconstructions have been compared in spin- and angle-resolved valence-band photoemission. For annealing at elevated temperatures, the (1 × 1)-structure transforms into 2 × 2 and a majority-spin peak appears at −1.7 eV inside a majority-spin bulk band gap at the surface Brillouin zone centre. Its sensitivity to oxygen supports an interpretation as magnetic compound surface state. Local spin density calculations predict at the same energy (−1.75 eV) a prominent d surface state of majority spin for (1 × 1)-Mn terminated MnSb(0 0 0 1) but no such feature for (1 × 1)-Sb termination. The calculation shows that neither the bulk nor the surface is half-metallic, in agreement with the expectation for the hexagonal NiAs structure.

Photoemission studies of Mg and Rb layers on Zn(0 0 0 1)

The electronic structure of the clean Zn(0 0 0 1) surface is studied by angle resolved photoemission. An earlier detected surface state at the surface Brillouin zone centre is confirmed and a new surface state is found at the surface Brilluoin zone boundary. The surface electronic structure of Zn is found to be similar to the that of Cd. Evaporation of thin films of Mg and Rb onto the Zn(0 0 0 1) surface quenches the emission from both surface states and reduces the intensity of the bulk related structures.

Band structure symmetry effects in Gd multilayers grown on W(1 1 2)

We present evidence of coverage-dependent band structure symmetry effects in scanning tunneling microscopy. We find that the symmetry of the Gd induced band located at about 2 eV below the Fermi energy, at the surface Brillouin zone center, appears to be strongly dependent on the atomic structure in the bilayer regime for Gd on W(1 1 2). Light polarization dependent photoemission from this band indicates more d z 2 symmetry character for rectangular Gd structures and rather more d xz d yz character for the quasi hexagonal bilayer structure. Band symmetry changes accompanying the structural phase transition from rectangular to quasi-hexagonal Gd bilayer structures are consistent with arguments used to explain the apparent STM corrugation observed for Gd on W(1 1 2) (Surf Sci 520(2002)43).

The electronic structure of gadolinium grown on Mo(112)

The electronic structure of strained ultra-thin and thin films of Gd grown on a corrugated Mo(112) surface are described. Gadolinium overlayers order at a coverage of monolayers forming a LEED pattern. At this coverage an interface state of symmetry is formed at a binding energy of at the surface Brillouin zone centre . The effective mass of this interface state was determined to be , in both orthogonal directions along the nearly square reduced Brillouin zone. For thicker Gd films of approximately 3 to 10 ML thickness, the corresponding gadolinium state switches symmetry to the representation (Gd or 6s character) and has a much larger effective mass. The overlayer forms a rectangular surface Brillouin zone resembling the hcp surface. Gadolinium films thicker than approximately 10 ML form strained hexagonal ordered films also with substantial misfit dislocations. The strain of the thin hexagonal ordered Gd films is reflected by a reduced Brillouin zone size along by approximately 4% with respect to the less strained Gd overlayers on W(110) and Gd(0001) single crystals. The induced strain severely alters the band structure of the Gd 5d/6s bulk bands, which disperse in the opposite direction relative to the corresponding bands of the relaxed Gd(0001) structure. The surface of the strained hexagonal fcc (111) or hcp (0001) Gd films forms a localized state of symmetry (Gd or 6s character) at approximately 0.7 eV binding energy. There is little observed strain relief within the Gd films up to approximately 150 Å film thickness.

Lattice vibrations at the Be(101̄0) surface

We have measured the surface phonon dispersion of Be(101̄0) by electron energy loss spectroscopy (EELS) and found three surface resonances at the surface Brillouin zone centre as well as two surface phonons at the Ā and M̄ points. The experimental data do not agree with a calculation for the truncated bulk. In particular, the predicted loss associated with the Rayleigh wave at the zone boundaries is not observed. The character of the surface vibrations suggests that the bonding is more complicated than on Be(0001) where the qualitative features of the surface phonon dispersion can be described within a simple central force model.

Photoemission with picosecond lasers: fast electron dynamics at the GaAs [110] surface

Subpicosecond pulses of 10.7-eV radiation from a tunable laser-based source are used to carry out photoemission investigations of the electronic dynamics on the laser-excited surface of GaAs [110]. Angle-resolved studies have revealed a rapid surface intervalley scattering process that transfers electrons between the directly excited, surface Brillouin zone center to a previously unobserved valley within the bandgap at the surface zone edge. The scattering time has been determined to be 0.4+or-0.1 ps. A model that describes the scattering dynamics is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-resolution photoemission study of the electronic structure of the noble-metal (111) surfaces.

High-resolution angle-resolved photoemission studies of the (111) surfaces of copper, silver, and gold are reported which investigate in detail the properties of the intrinsic surface states located in the projected sp-band gaps at the center of the surface Brillouin zones. Accurate two-dimensional energy dispersion relations are reported for each surface state and are quantified in terms of effective masses at the surface Brillouin-zone center. The masses for the three metals are found to be remarkably similar when normalized to the effective mass of the lower edge of the bulk continuum. The decay length of the surface state wave function into the surface was determined for all three surfaces. These results are expressed in terms of an effective mass of the complex dispersion relation within the projected band gap. In accord with our previous results on the copper state, these effective masses are found to be anomalously large by approximately a factor of 2 relative to expectations based on effective mass theory coupled to first-principles bulk band calculations. An explanation of this anomaly involving the nonorthogonality of effective-mass-theory-derived states is explored. All experimental results are compared to the predictions of recent self-consistent surface electronic structure calculations for these surfaces.

An angle resolved photoemission study of the interaction between bromine and a copper (001) surface

The dissociative adsorption of bromine on a Cu(001) single crystal surface has been studied at room temperature by angle-resolved photoemission. There are two distinct stages in the bromine uptake before a c(2 × 2) chemisorbed layer is formed following a total exposure of ca. 2.5 L. Further exposure leads to the growth of a cuprous bromide layer at the surface. The c(2 × 2) bromine overlayer gives rise to adsorbate-adsorbate interactions which are strong enough to produce dispersion in adsorbate derived bands. An adsorbate-induced feature appearing at the Surface Brillouin Zone centre and 1.8 eV below the Fermi level is shown to have E( \\ ̄ gG 5) symmetry. The main bonding contribution seems to result from the interaction between Cu 4s and 3d z 2 bands and the bromine 4p z orbital.