Evolution Of Surface States Research Articles

Topological Surface State Evolution in Bi2Se3 via Surface Etching.

Topological insulators are materials that have an insulating bulk interior while maintaining gapless boundary states against back scattering. Bi2Se3 is a prototypical topological insulator with a Dirac-cone surface state around Γ. Here, we present a controlled methodology to gradually remove Se atoms from the surface Se-Bi-Se-Bi-Se quintuple layers, eventually forming bilayer-Bi on top of the quintuple bulk. Our method allows us to track the topological surface state and confirm its robustness throughout the surface modification. Importantly, we report a relocation of the topological Dirac cone in both real space and momentum space as the top surface layer transitions from quintuple Se-Bi-Se-Bi-Se to bilayer-Bi. Additionally, charge transfer among the different surface layers is identified. Our study provides a precise method to manipulate surface configurations, allowing for the fine-tuning of the topological surface states in Bi2Se3, which represents a significant advancement toward nanoengineering of topological states.

Understanding the Adiabatic Evolution of Surface States in Tetradymite Topological Insulators under Electrochemical Conditions.

Nontrivial surface states in topological materials have emerged as exciting targets for surface chemistry research. In particular, topological insulators have been used as electrodes in electrocatalytic reactions. Herein, we investigate the robustness of the topological surface states and band topology under electrochemical conditions, specifically in the presence of an electric double layer. First-principles band structure calculations are performed on the electrified (111) surfaces of Bi2Te3, Bi2Se3, and Sb2Te3 using an implicit electrolyte model. Our results demonstrate the adiabatic evolution of the surface states upon surface charging. Under oxidizing potentials, the surface states are shifted upward in energy, preserving the Dirac point on the surface and the band inversion in the bulk. Conversely, under reduced potentials, hybridization is observed between the surface and bulk states, suggesting a likely breakdown of topological protection. The position of the Fermi level, which dictates the working states in catalytic reactions, should ideally be confined within the bulk bandgap. This requirement defines a potential window for the effective application of topological electrocatalysis.

Mechanism of residue formation on Ge-rich germanium antimony tellurium alloys after plasma etching

In phase-change random access memory (PCRAM) applications, the germanium antimony tellurium alloy (GST) is patterned using halogen etching in inductively coupled plasma reactors. This paper focuses on the surface state evolution of an optimized Ge-rich GST material after plasma etching. Four hours after etching, big dome-shaped residues are observed on PCRAM structures. Their number and size increase with the time of air exposure. X-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy analyses reveal an important germanium oxidation on the surface with residues. Their formation is then investigated. Complementary analyses highlight that the moisture environment has a catalytic effect on the residue formation. Based on this study, a detailed mechanism responsible for residue formation is proposed.

Molecular insights into the growth and time evolution of surface states of CsPbBr3 nanoparticles synthesized using a scalable room temperature approach

Room temperature ligand-assisted reprecipitation syntheses of CsPbBr3 nanoparticles (NPs) under open air conditions and with non-polar solvents have recently emerged as viable strategies for large-scale production of highly emissive NPs.

Surface state evolution induced by magnetic order in axion insulator candidate EuIn2As2

Gapping of Dirac surface states through time reversal symmetry breaking may realize the axion insulator state in condensed matter. Despite tremendous efforts, only a few material systems fall in this category of intrinsic magnetic topological insulators (TI). Recent theoretical calculations proposed the antiferromagnetic EuIn$_2$As$_2$ to be a topologically non-trivial magnetic insulator with gapped surface states. Here we use scanning tunneling microscopy and spectroscopy (STM/STS) complemented with density-functional theory (DFT) calculations and modelling to probe the surface electronic states in EuIn$_2$As$_2$. We find a spin-orbit induced bulk gap of ~120 meV located only a few meV above the Fermi energy, within which topological surface states reside. Temperature dependent measurements provide evidence of the partial gapping (~40 meV) of the surface states at low temperatures below the AFM order, which decreases with increasing temperature but remains finite above $T_N$

Evolution of surface states of antiferromagnetic topological insulator MnBi2Te4 with tuning the surface magnetization

The interplay between magnetism and topologically non-trivial electronic states is an important subject in condensed matter physics. Recently, the stoichiometric intrinsic magnetic material MnBi2Te4 provides an ideal platform to study the magnetic topological phenomena, such as quantum anomalous Hall effect, axion insulator state, topological magnetoelectric effect. However, it is still controversial whether the topological surface state in the (111) plane is gapped or not. Here, we develop an effective method to study different surface magnetizations based on first-principles calculations. Then we investigate the band dispersions, the Fermi surfaces (FSs), the quasiparticle interferences (QPIs) and the spin texture of topological surface states of MnBi2Te4 with tuning the surface magnetization. We find that the surface magnetization has significant effects on the surface states. Our results also indicate that the symmetry breaking of FSs and QPIs may be a useful way to determine the possible surface magnetization of MnBi2Te4.

A vision-based approach for rolling contact fatigue evaluation in twin-disc tests on a railway wheel steel

ABSTRACT A vision-based experimental methodology was developed for monitoring the surface state evolution of specimens during twin-disc rolling contact tests, aimed at providing information for identifying the damage phenomena. The system is based on a high-speed camera and three laser pointers for illuminating the specimen surface. Images of the specimen surface are acquired and processed, allowing the definition of synthetic surface state indexes, as well as the section profiles of the surface. The vision system was applied to alternated dry–wet rolling–sliding contact tests on railway wheel steel specimens, highlighting its effectiveness in the damage evaluation. The potential of the section profile reconstruction as a tool for surface topology analysis was shown.

The performance and degradation process of a greenly synthesized transient heterojunction diode

Transient electronics are of great interest for applications in biomedical implants, hardware secure technologies and environmental friendly consumer devices. However, most transient devices having been reported require high temperature and high vacuum for fabrication, which is inconsistent with the ‘green’ idea of transient electronics. Besides, there are less reports on the degradation behavior of semiconducting metal oxides and organic semiconductors, particularly on the evolution of surface states. Herein, we demonstrated a transient diode based on zinc oxide and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, synthesized by a set of low-cost and green methods. The degradation behavior of the semiconducting films was addressed by electrochemical impedance spectroscopy, which simultaneously revealing the evolution of resistance, capacitance and surface states. The diode was able to operate properly in appropriate conditions and degraded easily in aqueous solution.

Evolution of the surface states during the in situ SiN layer formation on AlN/GaN heterostructures

The effect of a monolayer thick SiN film on the surface states of the AlN/GaN heterostructure grown by molecular beam epitaxy was investigated. It was revealed that the submonolayer SiN coating of the AlN surface leads to the formation of an ordered (√3 × √3)R30° structure. Further SiN film growth results in a Si-enriched SiN amorphous film formation. The Fermi level, pinned 1 eV above the valence band maximum of as-grown AlN, jumps 2.3 eV when (√3 × √3)R30° is formed, and then gradually goes to 3.1 eV with an increase in the thickness of the SiN film up to four monolayers. This is explained by the evolution of surface states, as well as the appearance of donor-like states in a Si-enriched SiN film. The presence of donor-like states was confirmed when studying the effect of current collapse in enhancement-mode high electron mobility transistors made of SiN/AlN/GaN heterostructures.

Hybridization of topological surface states with a flat band

We address the problem of hybridization between topological surface states and a non-topological flat bulk band. Our model, being a mixture of three-dimensional Bernevig–Hughes–Zhang and two-dimensional pseudospin-1 Hamiltonian, allows explicit treatment of the topological surface state evolution by continuously changing the hybridization between the inverted bands and an additional ‘parasitic’ flat band in the bulk. We show that the hybridization with a flat band lying below the edge of the conduction band converts the initial Dirac-like surface states into a branch below and one above the flat band. Our results univocally demonstrate that the upper branch of the topological surface states is formed by Dyakonov–Khaetskii surface states, known for HgTe since the 1980s. Additionally we explore an evolution of the surface states and the arising of Fermi arcs in Dirac semimetals when the flat band crosses the conduction band.

Time‐Dependent Surface Oxidation of Pd Nanocubes and its Role in Controlling Catalytic Performance

AbstractThe surface evolution of catalysts, which leads to the changes in physicochemical properties for various catalysts under different storage temperatures and amounts of time, is an important issue for altering catalytic performance. However, the influence of such a surface evolution under different storage conditions on the catalytic performance is often neglected, albeit enormous studies have been carried out in past decades. In this work, Pd nanocrystals are prepared through facile aqueous synthetic methods and employed as model catalysts for catalytic reactions. Our investigation reveals that the storage duration of Pd nanocubes has great influence on their catalytic performance. Generally, the catalytic performance of Pd nanocubes show a dramatically enhanced catalytic performance for Heck coupling reaction after a storage duration of 25 days. According to XPS characterization, this enhancement in catalytic performance is attributed to the presence of oxidized Pd on the surface of Pd nanocubes. Moreover, it is further discovered that the change in storage temperature and storage environment can also contribute to the surface state evolution of Pd nanocubes, affecting their catalytic performance. This work shows not only the storage condition‐dependent catalytic performance of the Pd nanocubes, but also the evidence that allows us to understand the structure sensitivity of Pd nanocubes over different storage condition.

Temperature-controlled spectral tuning of full-color carbon dots and their strongly fluorescent solid-state polymer composites for light-emitting diodes.

The development of full-color/white carbon-dot-based light-emitting diodes (LEDs) has been achieved, which show promising applications in full-color and flexible displays, backlights, and novel lighting sources. The gram-level synthesis of these full-color carbon dots (CDs) from citric acid by controlling the temperature has been achieved. By increasing the temperature from 120 to 180 °C, two, four, and six light-emitting CDs can be obtained, for which the emission wavelength shifts from 440 to 585 nm. This result reveals that temperature has a huge impact on the evolution of surface states, that is, increasing the temperature brings about enhanced surface functionalization and passivation, resulting in a red shift of the emission wavelength and enhancement of quantum yield. Then, full-color CDs/polymer composite phosphors are fabricated for efficient phosphor-based LED devices with quench-resistant solid-state fluorescence. By regulating the proportion of various CDs/polymer phosphors, white LEDs are realized with Commission Internationale de L'Eclairage coordinates of (0.32, 0.33) and a color rendering index of 82.7. The as-prepared CD-based full/white color LEDs can prove to be promising candidates for alternative light sources.

Topology and symmetry of surface Majorana arcs in cyclic superconductors

We study the topology and symmetry of surface Majorana arcs in superconductors with nonunitary "cyclic" pairing. Cyclic $p$-wave pairing may be realized in a cubic or tetrahedral crystal, while it is a candidate for the interior $^3P_2$ superfluids of neutron stars. The cyclic state is an admixture of full gap and nodal gap with eight Weyl points and the low-energy physics is governed by itinerant Majorana fermions. We here show the evolution of surface states from Majorana cone to Majorana arcs under rotation of surface orientation. The Majorana cone is protected solely by an accidental spin rotation symmetry and fragile against spin-orbit coupling, while the arcs are attributed to two topological invariants: the first Chern number and one-dimensional winding number. Lastly, we discuss how topologically protected surface states inherent to the nonunitary cyclic pairing can be captured from surface probes in candidate compounds, such as U$_{1-x}$Th$_{x}$Be$_{13}$. We examine tunneling conductance spectra for two competitive scenarios in U$_{1-x}$Th$_{x}$Be$_{13}$---the degenerate $E_u$ scenario and the accidental scenario.

Density wave instabilities and surface state evolution in interacting Weyl semimetals

We investigate the interplay of many-body and band structure effects of interacting Weyl semimetals (WSM). Attractive and repulsive Hubbard interactions are studied within a model for a time-reversal-breaking WSM with tetragonal symmetry, where we can approach the limit of weakly coupled planes and coupled chains by varying the hopping amplitudes. Using a slab geometry, we employ the variational cluster approach to describe the evolution of WSM Fermi arc surface states as a function of interaction strength. We find spin and charge density wave instabilities which can gap out Weyl nodes. We identify scenarios where the bulk Weyl nodes are gapped while the Fermi arcs still persist, hence realizing a quantum anomalous Hall state.

Chemical composition on the top of a surface determined with the evolution of surface states

The epitaxial growth of Cu on Ag(111) was explored on monitoring the evolution of its surface states with angle-resolved photoemission spectroscopy; the chemical element on the top of Cu islands on Ag(111) with a (9×9) reconstruction was determined to be not Cu but Ag. Our result demonstrates a technique with which the chemical property associated with a surface state is explorable and an identification of the chemical element in the topmost layer of the surface becomes feasible.

Measurement of Intrinsic Dirac Fermion Cooling on the Surface of the Topological InsulatorBi2Se3Using Time-Resolved and Angle-Resolved Photoemission Spectroscopy

We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator (TI) Bi(2)Se(3) to study the ultrafast dynamics of surface and bulk electrons after photoexcitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, the unique cooling of Dirac fermions in TI by acoustic phonons is manifested through a power law dependence of the surface temperature decay rate on carrier density.

Sputtering and surface state evolution of Bi under oblique incidence of 120 keV Ar + ions

The sputtering and surface state evolution of Bi/Si targets under oblique incidence of 120 keV Ar + ions have been investigated over the range of incidence angles 0° ⩽ θ i ⩽ 60°. Increasing erosion of irradiated samples (whose surface thickness reduced by ∼3% at normal incidence up to ∼8% at θ = 60°) and their surface smoothing with reducing grain sizing were pointed out using Rutherford backscattering (RBS), atomic force (AFM) and X-ray diffraction (XRD) techniques. Measured sputtering yield data versus θ i with fixed ion fluence to ∼1.5 × 10 15 cm −2 are well described by Yamamura et al. semi-empirical formula and Monte Carlo (MC) simulation using the SRIM-2008 computer code. The observed increase in sputter yield versus incidence angle is closely correlated to Bi surface topography and crystalline structure changes under ion irradiation.

Size-Dependent Time-Resolved Photoluminescence of Colloidal CdSe Nanocrystals

Monodisperse colloidal CdSe nanocrystals (NCs) with different sizes were synthesized, and the time-resolved photoluminescence (PL) spectra were measured to explore the evolution of surface states during the NC’s growth. The PL quantum yield, fluorescence lifetime, and the surface-related emission had the identical size dependence. The role of the nonradiative/radiative surface states in the PL carrier dynamics was discussed.

New experimental device for infrared spectral directional emissivity measurements in a controlled environment

A new experimental device for infrared spectral directional emissivity measurements in a controlled atmosphere is presented. The sample holder, which permits to measure spectral directional emissivity up to 1050K, is placed inside a stainless steel sample chamber that can be evacuated or filled with different gases. The signal detection is carried out by means of a Fourier transform infrared spectrometer. The experimental results focus on the capability of the device to perform emissivity measurements as a function of temperature, emission angle, and in situ surface state evolution. A careful study of the sample temperature homogeneity and the measurement method has been done, including the background radiation, the apparatus response function, and temperature differences between the sample and the blackbody radiator. As a consequence, a compact expression for the sample emissivity that generalizes those previously obtained for the direct radiometric measurement method is found. The error assessment shows that the main contribution to the emissivity uncertainty is related to the sample temperature. The overall uncertainty at intermediate temperature is estimated to be around 3% at short wavelengths. Emissivity measurements of Armco iron were used in order to check the accuracy of the experimental device. The experimental results show an excellent fit with direct emissivity data accessible in the literature, as well as with the theoretical emissivity obtained from the Hagen-Rubens relation.

Chemical etching of Gallium orthophosphate: Application to piezoelectric device manufacturing

This paper presents a study of the chemical etching of gallium orthophosphate in the way to design high frequency resonators. Four different orientations (X, Y, Z and AT-cuts) are used. The dissolution rate is of Arrhenius law type and depends of both the cut orientation and the concentration of the solvent. The study of the surface state evolution shows that there is no correlation between the surface state and the dissolution rate. Antimesa AT-plates with frequency up to 34MHz and QF factor of 2 x 10 11 are manufactured with this method.