Surface Resonance States Research Articles

Surface spectroscopy and surface–bulk hybridization of Weyl semimetals

Weyl semimetal showing open-arc surface states is a prominent example of topological quantum matter in three dimensions. With the bulk-boundary correspondence present, nontrivial surface-bulk hybridization is inevitable but less understood. Spectroscopies have been often limited to verifying the existence of surface Fermi arcs, whereas its spectral shape related to the hybridization profile in energy-momentum space is not well studied. We present an exactly solvable formalism at the surface for a wide range of prototypical Weyl semimetals. The resonant surface state and the bulk influence coexist as a surface-bulk hybrid and are treated in a unified manner. Directly accessible to angle-resolved photoemission spectroscopy, we analytically reveal universal information about the system obtained from the spectroscopy of resonant topological states. We systematically find inhomogeneous and anisotropic singular responses around the surface-bulk merging borderline crossing Weyl points, highlighting its critical role in the Weyl topology. The response in scanning tunneling spectroscopy is also discussed. The results will provide much-needed insight into the surface-bulk-coupled physical properties and guide in-depth spectroscopic investigation of the nontrivial hybrid in many topological semimetal materials.

A New Method of AFM‐Based Nanolithography Using Frequency Enhanced Electrochemical Pressure Solution Etching

AbstractA new method of direct‐write nanolithography that is able to rapidly etch silica surfaces under a scanning atomic force microscopy (AFM) probe in tapping mode (TM) is reported. In this lithography technique, silica surfaces are etched using a recently described electro‐chemo‐mechanical phenomenon of frequency enhanced electrochemical pressure solution (FEEPS). In FEEPS, the tapping of the AFM tip generates oscillations of the Stern potential at the silica‐water interface that can accelerate the silica dissolution kinetics by more than 5–6 orders of magnitude when surface resonance states are achieved; i.e., when the oscillation frequency is in phase with the dynamics of interfacial chemical reaction steps. By scanning silica surfaces in TM, silica is selectively dissolved below the tapping tip as it is scanned. The FEEPS accelerated silica dissolution rates can generate etched features with depths of more than 60 nm in a single AFM tip pass. The rate of etching can be controlled easily by varying the scanning rate or by modulating the tapping frequency. This fine control over the silica etching process and because material is removed (dissolved) rather than displaced as with nanoscratching, the FEEPS process lends itself to gray‐scale nanolithography which is demonstrated.

Observation of Gapped Topological Surface States and Isolated Surface Resonances in PdTe2 Ultrathin Films.

The superconductor PdTe2 is known to host bulk Dirac bands and topological surface states. The coexistence of superconductivity and topological surface states makes PdTe2 a promising platform for exploring topological superconductivity and Majorana bound states. In this work, we report the spectroscopic characterization of ultrathin PdTe2 films with thickness down to three monolayers (ML). In the 3 ML PdTe2 film, we observed spin-polarized surface resonance states, which are isolated from the bulk bands due to the quantum size effects. In addition, the hybridization of surface states on opposite faces leads to a thickness-dependent gap in the topological surface Dirac bands. Our photoemission results show clearly that the size of the hybridization gap increases as the film thickness is reduced. The observation of isolated surface resonances and gapped topological surface states sheds light on the applications of PdTe2 quantum films in spintronics and topological quantum computation.

Topologically protected surface states in TaPdTe5

By combining angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations, we have studied the electronic band structure of a topological material candidate TaPdTe5.Two inverted bulk gaps determined through the parity analysis were observed in TaPdTe5 near the Fermi level, which causes the appearance of topologically protected surface states on the (010) surface. Beside the surface state, we also observed strong surface resonance state. The hybridization between the surface resonance state and the surface state changes the quasi-one-dimensional like Fermi surface sheets of the surface resonance state into two-dimensional Fermi pockets. Our findings provide comprehensive understanding about the interesting surface states in TaPdTe5.

Near Fermi Superatom State Stabilized by Surface State Resonances in a Multiporous Molecular Network.

Two-dimensional honeycomb molecular networks confine a substrate’s surface electrons within their pores, providing an ideal playground to investigate the quantum electron scattering phenomena. Besides surface state confinement, laterally protruding organic states can collectively hybridize at the smallest pores into superatom molecular orbitals. Although both types of pore states could be simultaneously hosted within nanocavities, their coexistence and possible interaction are unexplored. Here, we show that these two types of pore states do coexist within the smallest nanocavities of a two-dimensional halogen-bonding multiporous network grown on Ag(111) studied using a combination of scanning tunneling microscopy and spectroscopy, density functional theory calculations, and electron plane wave expansion simulations. We find that superatom molecular orbitals undergo an important stabilization when hybridizing with the confined surface state, following the significant lowering of its free-standing energy. These findings provide further control over the surface electronic structure exerted by two-dimensional nanoporous systems.

Ultrafast evolution of bulk, surface and surface resonance states in photoexcited hbox {Bi}_{2}hbox {Te}_{3}

We use circular dichroism (CD) in time- and angle-resolved photoemission spectroscopy (trARPES) to measure the femtosecond charge dynamics in the topological insulator (TI) hbox {Bi}_{2}hbox {Te}_{3}. We detect clear CD signatures from topological surface states (TSS) and surface resonance (SR) states. In time-resolved measurements, independently from the pump polarization or intensity, the CD shows a dynamics which provides access to the unexplored electronic evolution in unoccupied states of hbox {Bi}_{2}hbox {Te}_{3}. In particular, we are able to disentangle the unpolarized electron dynamics in the bulk states from the spin-textured TSS and SR states on the femtosecond timescale. Our study demonstrates that photoexcitation mainly involves the bulk states and is followed by sub-picosecond transport to the surface. This provides essential details on intra- and interband scattering in the relaxation process of TSS and SR states. Our results reveal the significant role of SRs in the subtle ultrafast interaction between bulk and surface states of TIs.

Experimental verification of a temperature-induced topological phase transition in TlBiS2 and TlBiSe2

Temperature dependence of the band structure, as well as the spin-polarization on the topologically trivial insulator ${\mathrm{TlBiS}}_{2}$ and nontrivial insulator $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$, have been investigated by spin- and angle-resolved photoelectron spectroscopy. Despite the recent theoretical prediction [Phys. Rev. Lett. 117, 246401 (2016).], topological phase transition (trivial to nontrivial for ${\mathrm{TlBiS}}_{2}$, nontrivial to trivial for $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$) does not occur in the observed temperature ranges (from 50 to 400 K for ${\mathrm{TlBiS}}_{2}$, from 50 to 475 K for $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$). Our results indicate that the discrepancy between the theory and the experiment can be understood by considering the overestimation of lattice parameters and the effect of lattice expansion that is neglected in the previous theory. Although some spin-polarized states are observed in ${\mathrm{TlBiS}}_{2}$, it is probably due to the Rashba-like surface (resonant) states and not the topological surface states. On the other hand, the topological surface states in $\mathrm{Tl}\mathrm{Bi}{\mathrm{Se}}_{2}$ is very robust even at 400 K and the spin-polarized states can be used for the real spintronic devices in the large temperature range.

Surface States on Photonic Crystals As Hybrid Dielectric Metasurface Bound States of the Termination Layer

Dark resonant surface states on photonic crystals are of great technological interest for providing easily accessible, very high Q-factor concentration of light into subwavelength volumes and for t...

Engineering zero modes, Fano resonance, and Tamm surface states in the waveguide-array realization of the modified Su-Schrieffer-Heeger model.

In this article, we developed a generalized coupled-mode theory for mixing an isolated state with a continuum having an intrinsic energy gap, which dubbed as "the bound states in the gapped continuum" (BIGC). We investigated the mixture interaction by mimicking the Su-Schrieffer-Heeger model in an optical coupled waveguide array (WA), and presented a unified engineering mechanism for topologically-protected zero modes, Fano resonance, and Tamm surface states, even though those phenomena are diverse in topological insulators, atomic physics and semiconductors, respectively. By tuning the on-site potential and coupling strength of the isolated state, we found the unified operating characteristics for zero modes, Fano resonance, and Tamm states, with demonstrating their localization, transmission spectra, and distinct evolution dynamics explicitly. As an extension for triple-modes coupling, two special sandwich-like configurations are studied: the isolated-continuous-isolated and continuous-isolated-continuous configurations lead to adiabatic eliminations and domain walls, respectively, revealing possible applications and wide connections in many fields of physics and optics.

Surface state transport in double-gated and magnetized topological insulators with hexagonal warping effects

We explore the scattering of Dirac electrons in a double-gated topological insulator in the presence of magnetic proximity effects and warped surface states. It is found that a magnetic field can shift the Dirac cone in momentum space and deform the constant-energy contour, or opens up a band gap at the Dirac point, depending on the magnetization orientation. The double gate voltage induces quantum wells and/or quantum barriers on the surface of topological insulators, generating surface resonant tunnelling states. It is found that the hexagonal warping effect can increase the electronic transport at high energies when the constant-energy contour exhibits a snowflake shape. The energy-dependent conductances in the parallel and antiparallel magnetic configurations exhibit out-of-phase oscillations due to the quantum interference of propagating waves in the region between the two magnetized segments. Although the conductance spectrum of the double-well structure is higher than that of the double-barrier structure, the magnetoresistance ratio versus the separation distance between the two magnetized barriers exhibits pronounced oscillations due to the resonant tunnelling states. We show that the surface state transport can be controlled by the exchange field and gate voltage without breaking time reversal symmetry, suggesting that the double gated and magnetized topological insulators can be utilized to achieve a large magnetoresistance ratio with a tunable sign.

First-principles characterization of Mg low-index surfaces: Structure, reconstructions, and surface core-level shifts

In this paper, first-principles calculations provide structural characterization of three low-index Mg surfaces - Mg(0001), Mg(1010), and Mg(1120) - and their respective surface core-level shifts (SCLSs). Inspired by the close similarities between Be and Mg surfaces, we also explore the reconstruction of Mg(1120). Through the calculation of surface energies and the use of the angular-component decomposed density of states, we show that reconstructions are likely to occur at the Mg(1120) surface, similarly to what was found earlier for Be(1120). Indeed, the surface energy of some of the explored reconstructions is slightly lower than that of the unreconstructed surface. In addition, because of lattice symmetry, the morphology of the unreconstructed surface (1120) results in a steplike zig-zag chain packing, with topmost chains supporting a resonant, quasi-one-dimensional (1D), partially filled electronic state. As the presence of partially filled quasi-1D bands is a necessary condition for Peierls-like dimerization, we verify that the undimerized surface chain remains stable with respect to it. Some of the reconstructions, namely, the 2×1 and 3×1 added row reconstructions, induce a stronger relaxation of the topmost chains, increasing the coupling with lower layers and thus significantly damping the quasi-1D character of this state. The original approach followed offers a common and general framework to identify quasi-1D bands - even in the case of resonant electronic surface states - and to meaningfully compare calculated and measured SCLSs even in the presence of multicomponent peak contributions. (Less)

Interface resonance in Fe/Pt/MgO multilayer structure with large voltage controlled magnetic anisotropy change

Electric control of magnetism has been a topic of interest for various spintronic applications. It is known that monoatomic Pt layer insertion at the Fe/MgO interface increases voltage-controlled magnetic anisotropy (VCMA). However, the reason for the optimality of this thickness has not been explained thus far. In this study, we observed the changes in the electronic states at the Fe/MgO interface using tunneling spectroscopy on an epitaxial Fe(001)/Pt/MgO(001) structure to characterize the density of states around the Fermi level. We found that a surface resonant state is formed at the Fermi level by the insertion of a monoatomic Pt layer, which is consistent with our first principles study. In addition, the VCMA enhancement owing to the formation of this surface resonance state agrees with the recently proposed microscopic theory.

Spin-polarized resonant surface state in (1 1 1) Sm1−xGdxAl2, a zero-magnetization ferromagnet

The electronic structure of (1 1 1) Sm1−xGdxAl2, a zero-magnetization ferromagnet, is investigated by angle- and spin- resolved photoemission spectroscopy. An intense electron pocket strongly localized around and close to the Fermi level is observed and analyzed in detail. Its various characteristics, combined with electronic structure calculations, reveal a resonant surface state of 5d character and Λ1 symmetry, likely built on bulk states developing around L points. It exhibits moreover a low temperature positive spin polarization at the Fermi level, of strong interest for spin-dependent transport properties in Sm1−xGdxAl2-based spintronic devices.

Ultrafast dynamics of an unoccupied surface resonance state in Bi2Te2Se

Electronic structure and electron dynamics in the ternary topological insulator $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{2}\mathrm{Se}$ are studied with time- and angle-resolved photoemission spectroscopy using optical pumping. An unoccupied surface resonance split off from the bulk conduction band previously indirectly observed in scanning tunneling measurements is spectroscopically identified. Furthermore, an unoccupied topological surface state (TSS) is found, which is serendipitously located at about 1.5 eV above the occupied TSS, thereby facilitating direct optical transitions between the two surface states at $\ensuremath{\hbar}\ensuremath{\omega}=1.5\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ in an $n$-type topological insulator. An appreciable nonequilibrium population of the bottom of the bulk conduction band is observed for longer than 15 ps after the pump pulse. This leads to a long recovery time of the lower TSS, which is constantly populated by the electrons coming from the bulk conduction band. Our results demonstrate $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{2}\mathrm{Se}$ to be an ideal platform for designing future optoelectronic devices based on topological insulators.

Weak antilocalization effect in exfoliated black phosphorus revealed by temperature- and angle-dependent magnetoconductivity

Recently, there have increasingly been debates on whether there exists a surface resonance state (SRS) in black phosphorus (BP), as suggested by recent angle-resolved photoemission spectroscopy results. To resolve this issue, we have performed temperature- and angle-dependent magnetoconductivity measurements on exfoliated, high-quality BP single crystals. A pronounced weak-antilocalization (WAL) effect was observed within a narrow temperature range of 8–16 K, with the electrical current flowing parallel to the cleaved ac-plane (along the a- or c-axis) and the magnetic field along the b-axis. The angle-dependent magnetoconductivity and the Hikami–Larkin–Nagaoka model-fitted results have revealed that the observed WAL effect shows surface-bulk coherent features, which supports the existence of SRS in BP.

Direct observation of a surface resonance state and surface band inversion control in black phosphorus

We report a Cs-doping-induced band inversion and the direct observation of a surface resonance state with an elliptical Fermi surface in black phosphorus (BP) using angle-resolved photoemission spectroscopy. By selectively inducing a higher electron concentration (1.7 × 1014 cm−2) in the topmost layer, the changes in the Coulomb potential are sufficiently large to cause surface band inversion between the parabolic valence band of BP and a parabolic surface state around the point of the BP Brillouin zone. Tight-binding calculations reveal that band gap openings at the crossing points in the two high-symmetry directions of the Brillouin zone require out-of-plane hopping and breaking of the glide mirror symmetry. Ab initio calculations are in very good agreement with the experiment if a stacking fault on the BP surface is taken into account. The demonstrated level of control over the band structure suggests the potential application of few-layer phosphorene in topological field-effect transistors.

Interplay between quantum confinement and surface effects in thickness selective stability of thin Ag and Eu films

Using first-principles calculations, we show that both face-centered cubic (fcc) Ag (1 1 0) ultrathin films and body-centered cubic (bcc) Eu(1 1 0) ultrathin films exhibit thickness selective stability. However, the origin of such thickness selection is different. While the thickness selective stability in fcc Ag(1 1 0) films is mainly due to the well-known quantum well states ascribed to the quantum confinement effects in free-electron-like metal films, the thickness selection in bcc Eu(1 1 0) films is more complex and also strongly correlated with the occupation of the surface and surface resonance states.

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

Using time-, spin- and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi$_2$Te$_3$ following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in about $\sim$2 ps. One state is located below $\sim$0.4 eV just above the bulk continuum, the other one at $\sim$0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within $\sim$0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically-locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with fs resolution to fully understand the sub-ps dynamics of transient electrons on the surface of topological insulators.

Phonon-Enabled Carrier Transport of Localized States at Non-Polar Semiconductor Surfaces: A First-Principles-Based Prediction.

Electron-phonon coupling can hamper carrier transport either by scattering or by the formation of mass-enhanced polarons. Here, we use time-dependent density functional theory-molecular dynamics simulations to show that phonons can also promote the transport of excited carriers. Using nonpolar InAs (110) surface as an example, we identify phonon-mediated coupling between electronic states close in energy as the origin for the enhanced transport. In particular, the coupling causes localized excitons in the resonant surface states to propagate into bulk with velocities as high as 10(6) cm/s. The theory also predicts temperature enhanced carrier transport, which may be observable in ultrathin nanostructures.

Interband interaction between bulk and surface resonance bands of a Pb-adsorbed Ge(001) surface

We investigated the valence band structure of a Pb-adsorbed Ge(001) surface by angle-resolved photoelectron spectroscopy. Three Ge bands, G1, G2, and G3, were observed in a Ge(001) 2 × 1 clean surface. In addition to these three bands, a fourth band (R band) is found on the surface with 2 ML of Pb. The R band continuously appeared even when the surface superstructure was changed. The position of the R band does not depend on Pb coverage. These results indicate that the R band derives from Ge subsurface states, known as surface resonance states. Furthermore, the effective mass of G3 is significantly reduced when the R band exists. We found that this reduction of G3 effective mass was explained by the interaction of the G3 and R bands. Consequently, the surface resonance band is considered to penetrate into the Ge subsurface region affecting the Ge bulk states. We determine the hybridization energy to be 0.068 eV by fitting the observed bands.