Center Of Brillouin Zone Research Articles

Rashba-split surface state and spin-dependent photon emission from Re(0001) at Γ¯

The unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission, experimentally and theoretically. The work is focused on the states around the center of the surface Brillouin zone $\overline{\mathrm{\ensuremath{\Gamma}}}$, where a hole pocket of a surface state with Rashba-type spin splitting is detected. Furthermore, we observe spin-dependent photon emission from unpolarized surface and bulk states at $\overline{\mathrm{\ensuremath{\Gamma}}}$. The size and sign of the spin asymmetry depends on experimental parameters such as the direction of the electron spin polarization and the photon detection angle. Maximum (zero) spin asymmetry is detected if the electron spin polarization and the plane of photon emission are perpendicular (parallel). The effect is traced back to spin-orbit-induced hybridization of the involved states.

DFT pressured BaTiO3 calculation using WC and HSE functionals and ONCV pseudopotentials

Theoretical BaTiO3 (BTO) single crystal with tetragonal phase was studied as a function of tensile and compressive pressure using Wu-Cohen (WC) and Heyd–Scuseria–Ernzerhof (HSE) exchange-correlational functional and the optimized norm-conserving Vanderbilt (ONCV) pseudopotentials. The results show different physical property anomalies associated with possible phase transitions. The lattice parameters evidence an anomaly at −7 GPa corresponding to the known super-tetragonality transition reported by Tinte et al. and Kvasov et al. [21]. The tetragonality coefficient indicates an apparent cubic phase at P= +2.17 GPa, however, the different Ti-O bond lengths and the phonons associated with the optical modes in the Brillouin zone center suggest that symmetric order of cubic phase is not reached in this pressure. While an additional anomaly at +7 GPa seems to be related to a well-ordered cubic structure. Furthermore, Löwdin atomic population analysis with variation in hydrostatic pressure provided information on the behavior of chemical bonds in BTO. Finally, the theoretical results are compared with those experimentally reported in the literature.

Dirac semimetal phase and switching of band inversion in XMg2Bi2 (X\u2009=\u2009Ba and Sr)

Topological Dirac semimetals (TDSs) offer an excellent opportunity to realize outstanding physical properties distinct from those of topological insulators. Since TDSs verified so far have their own problems such as high reactivity in the atmosphere and difficulty in controlling topological phases via chemical substitution, it is highly desirable to find a new material platform of TDSs. By angle-resolved photoemission spectroscopy combined with first-principles band-structure calculations, we show that ternary compound BaMg2Bi2 is a TDS with a simple Dirac-band crossing around the Brillouin-zone center protected by the C3 symmetry of crystal. We also found that isostructural SrMg2Bi2 is an ordinary insulator characterized by the absence of band inversion due to the reduction of spin–orbit coupling. Thus, XMg2Bi2 (X = Sr, Ba, etc.) serves as a useful platform to study the interplay among crystal symmetry, spin–orbit coupling, and topological phase transition around the TDS phase.

Incipient geometric lattice instability of cubic fluoroperovskites

Inorganic metal halide perovskites are promising materials for next-generation technologies due to a plethora of unique physical properties, many of which cannot be observed in the oxide perovskites. On the other hand, the search for ferroelectricity and multiferroicity in lead-free inorganic halide perovskites remains a challenging research topic. Here, we experimentally show that cubic fluoroperovskites exhibit proximity to incipient ferroelectrics, which manifested in the softening of the low-frequency polar phonons in the Brillouin zone center at cooling. Furthermore, we reveal the coupling between harmonic and anharmonic force constants of the softening phonons and their correlation with the perovskite tolerance factor. Next, using first-principles calculations, we examine the lattice dynamics of the cubic fluoroperovskites and disclose the incipient lattice instability at which the harmonic force constants of low-lying phonons tend to decrease with a reduction of tolerance factor at all high-symmetry points of the Brillouin zone. The correlations with the tolerance factor indicate the geometric origin of observed incipient lattice instability in the cubic fluoroperovskites caused by the steric effect due to the volume filling of the unit cell by different radius ions. These results provide insights into the lattice dynamics and potential ferroelectric properties of inorganic lead-free metal halide perovskites, relevant to further design and synthesis of new multifunctional materials.

Electronic Structure and Stacking Arrangement of Tungsten Disulfide at the Gold Contact.

There is an intensive effort to control the nature of attractive interactions between ultrathin semiconductors and metals and to understand its impact on the electronic properties at the junction. Here, we present a photoelectron spectroscopy study on the interface between WS2 films and gold, with a focus on the occupied electronic states near the Brillouin zone center (i.e., the Γ point). To delineate the spectra of WS2 supported on crystalline Au from the suspended WS2, we employ a microscopy approach and a tailored sample structure, in which the WS2/Au junction forms a semi-epitaxial relationship and is adjacent to suspended WS2 regions. The photoelectron spectra, as a function of WS2 thickness, display the expected splitting of the highest occupied states at the Γ point. In multilayer WS2, we discovered variations in the electronic states that spatially align with the crystalline grains of underlying Au. Corroborated by density functional theory calculations, we attribute the electronic structure variations to stacking variations within the WS2 films. We propose that strong interactions exerted by Au grains cause slippage of the interfacing WS2 layer with respect to the rest of the WS2 film. Our findings illustrate that the electronic properties of transition metal dichalcogenides, and more generally 2D layered materials, are physically altered by the interactions with the interfacing materials, in addition to the electron screening and defects that have been widely considered.

Topological Anderson Insulator in Cation-Disordered Cu2ZnSnS4.

We present the first candidate for the realization of a disorder-induced Topological Anderson Insulator in a real material system. High-energy reactive mechanical alloying produces a polymorph of Cu2ZnSnS4 with high cation disorder. Density functional theory calculations show an inverted ordering of bands at the Brillouin zone center for this polymorph, which is in contrast to its ordered phase. Adiabatic continuity arguments establish that this disordered Cu2ZnSnS4 can be connected to the closely related Cu2ZnSnSe4, which was previously predicted to be a 3D topological insulator, while band structure calculations with a slab geometry reveal the presence of robust surface states. This evidence makes a strong case in favor of a novel topological phase. As such, the study opens up a window to understanding and potentially exploiting topological behavior in a rich class of easily-synthesized multinary, disordered compounds.

Direct Observation of Global Elastic Intervalley Scattering Induced by Impurities on Graphene.

The scattering process induced by impurities in graphene plays a key role in transport properties. Especially, the disorder impurities can drive the ordered state with a hexagonal superlattice on graphene by electron-mediated interaction at a transition temperature. Using angle-resolved photoemission spectroscopy (ARPES), we reveal that the epitaxial monolayer and bilayer graphene with various impurities display global elastic intervalley scattering and quantum interference below the critical temperature (34 K), which leads to a set of new folded Dirac cones at the Brillouin-zone center by mixing two inequivalent Dirac cones. The Dirac electrons generated from intervalley scattering without chirality can be due to the breaking of the sublattice symmetry. In addition, the temperature-dependent ARPES measurements indicate the thermal damping of quantum interference patterns from Dirac electron scattering on impurities. Our results demonstrate that the electron scattering and interference induced by impurities can completely modulate the Dirac bands of graphene.

Dual Topological Features of Weyl Semimetallic Phases in Tetradymite BiSbTe3 Supported by the National Natural Science Foundation of China (Grant Nos. 11604032, 11674040, and 51672270), and the Fundamental Research Funds for the Central Universities (Grant No. 106112016CDJZR308808).

Based on first-principles calculations and symmetry arguments, we reveal that the non-centrosymmetric ternary tetradymite BiSbTe3 possesses exotic dual topological features of Weyl semimetallic phases with Z 2 index (1:000). The results show that the helical Dirac-type surface states protected by the time-reversal symmetry are present in the vicinity of the Brillouin zone center, which is consistent with the experimental report. Furthermore, we show that four pairs of Weyl points reside exactly at the Fermi level, which are guaranteed to be located on high-symmetry planes due to mirror symmetries. The helical surface states and the projected Weyl nodes are well separated in the momentum space, facilitating their observations in experiments. This work not only uncovers a unique quantum phenomenon with dual topological features in the tetradymite family but also paves a fascinating avenue for exploring the coexistence of multi-topological states with wide applications.

Evolution of the electronic structure in Ta2NiSe5 across the structural transition revealed by resonant inelastic x-ray scattering

We utilized high-energy-resolution resonant inelastic X-ray scattering (RIXS) at both the Ta and Ni $L_3$-edges to map out element-specific particle-hole excitations in Ta$_2$NiSe$_5$ across the phase transition. Our results reveal a momentum dependent gap-like feature in the low energy spectrum, which agrees well with the band gap in element-specific joint density of states calculations based on ab initio estimates of the electronic structure in both the low temperature monoclinic and the high temperature orthorhombic structure. Below $T_c$, the RIXS energy-momentum map shows a minimal gap at the Brillouin zone center ($\sim$ 0.16 eV), confirming that Ta$_2$NiSe$_5$ possesses a direct band gap in its low temperature ground state. However, inside the gap, no signature of anticipated collective modes with an energy scale comparable to the gap size can be identified. Upon increasing the temperature to above $T_c$, whereas the gap at the zone center closes, the RIXS map at finite momenta still possesses the gross features of the low temperature map, suggesting a substantial mixing between the Ta and Ni orbits in the conduction and valence bands, which does not change substantially across the phase transition. Our experimental observations and comparison to the theoretical calculations lend further support that the phase transition and the corresponding gap opening in Ta$_2$NiSe$_5$ is largely structural by nature with possible minor contribution from the putative exciton condensate.

First principles study of the electronic, elastic and vibrational properties of BiOI

BiOI is reported to be a promising photocatalytic material for splitting water and decomposing pollutants in the visible range. However, previous investigations focus mainly on its photocatalytic property and other properties are not well studied. Here we put our effort to study the electronic, elastic and vibrational properties of BiOI. The results indicate that BiOI is an indirect narrow band gap semiconductor. Bi–O bonds have both covalent and ionic characteristics while Bi–I bonds may have more ionic nature. BiOI is mechanically and dynamically stable. However, calculated directional bulk and Young's moduli reveal that it is seriously mechanically anisotropic, originating from its layered structure. Its infrared absorption spectrum is simulated and four peaks are found. The origins of these peaks are explored based on the calculated Born effective charges and vibrational eigenvectors at the Brillouin zone center. In addition, the dielectric constants of BiOI are computed and discussed. We hope our study will enrich the knowledge of BiOI and other similar materials.

First-principles study of the electronic structure and elastic properties of SrGa2 under pressure

Determination of detailed electronic and lattice dynamical properties of hexagonal SrGa2 binary system under external pressure with the aid of first-principles calculation methods is of great importance in terms of the basic characteristics of binary system. On this basis, the phonon dispersion curves are calculated by the high-symmetry directions and related phonon frequencies parameters at the Brillouin zone center using density functional perturbation theory when the elastic constants are computed through the metric-tensor formulation. Besides, the fundamental quantities such as the electronic band structures, Fermi surface topologies, and detailed phonon dispersions are sensitively examined. At the same time, it is examined whether there is a close relationship between the determined properties and superconducting properties of SrGa2 material. However, the calculations clearly show that the information obtained directly from the electronic band structure and Fermi surfaces is insufficient to explain the superconductivity phenomenon of such materials.

Spin polarized density functional theory\xa0calculations of the electronic structure and magnetism of the 112 type iron pnictide compound hbox {EuFeAs}_2

Using density-functional theory, we investigate the electronic, magnetic, and hyperfine-interaction properties of the 112-type iron-pnictide compound {hbox {EuFeAs}}_2, which is isostructural to the high-temperature iron-based superconductor {hbox {Ca}}_{1-x}{hbox {La}}_x{hbox {FeAs}}_2. We show that the band structure of {hbox {EuFeAs}}_2 is similar to that of the 112-type compounds’ family, with hole-like and electron-like bands at the Brillouin-zone center and corners, respectively. We demonstrate that the bands near the Fermi level originate mainly from the Fe atoms. The presence of a mixture of ionic and covalent bonding is predicted from the charge-density and atom-resolved density-of-states calculations. There is good agreement between the calculated hyperfine-interaction parameters with those obtained from the ^{57}Fe and ^{151}Eu Mössbauer measurements. The spatial distribution of atoms in {hbox {EuFeAs}}_2 leads to an in-plane 2D magnetism. Moreover, ab-initio calculations predict the compound’s magnetic moment and the magnetic moments of each constituent atom. Also, the density of states profile provides insight into the relative magnitude of these moments. Electronic structure calculations and Fermi surface topology reveal various physical and chemical properties of {hbox {EuFeAs}}_2. Valence electron density maps indicate the co-existence of a wide range of chemical bonds in this system, and based on structural properties, the transport characteristics are deduced and discussed. A thorough analysis of the atomic structure of {hbox {EuFeAs}}_2 and its role in the bond formation is presented.

Magnetic excitations and exchange interactions in the substituted multiferroics (Nd,Tb)Fe3(BO3)4 revealed by inelastic neutron scattering

Inelastic neutron scattering spectra in the antiferromagnetic ferroborates ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}{\mathrm{Fe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ ($x$ = 0, 0.1, 0.2, and 1) reveal various magnetic excitations of the interacting iron and rare-earth subsystems. We observe an evolution of the magnetic system from ``easy-plane'', in the Tb-free ($x$ = 0) case, to ``easy-axis'' anisotropy for samples substituted with Tb. The spectra show hybridized Fe and Nd branches, which are determined by the Fe-Nd exchange splitting of the ground-state ${\mathrm{Nd}}^{3+}$ doublet. In the easy-plane configuration, near the Brillouin zone center, there are two different pairs of anticrossing quasiacoustic Fe and Nd modes in contrast to the easy-axis state, where the two corresponding pairs of the branches are degenerated. The high-energy (exchange) branches are similar in both spin configurations. The Ising-type anisotropy of the Tb ion prevents the magnetic moment from precession. The increasing of the Tb content changes the effective magnetic anisotropy and stabilizes the easy-axis state. The spin-wave dispersion in the substituted and pure $\mathrm{Tb}{\mathrm{Fe}}_{3}{({\mathrm{BO}}_{3})}_{4}$ compounds, which have the same, easy-axis magnetic structure, but different crystal symmetry, strongly differ. The observed spectra were analysed in the frame of linear spin-wave theory and the exchange parameters were determined.

Miniband engineering and topological phase transitions in topological-insulator–normal-insulator superlattices

Periodic stacking of topologically trivial and non-trivial layers with opposite symmetry of the valence and conduction bands induces topological interface states that, in the strong coupling limit, hybridize both across the topological and normal insulator layers. Using band structure engineering, such superlattices can be effectively realized using the IV-VI lead tin chalcogenides. This leads to emergent minibands with a tunable topology as demonstrated both by theory and experiments. The topological minibands are proven by magneto-optical spectroscopy, revealing Landau level transitions both at the center and edges of the artificial superlattice mini Brillouin zone. Their topological character is identified by the topological phase transitions within the minibands observed as a function of temperature. The critical temperature of this transition as well as the miniband gap and miniband width can be precisely controlled by the layer thicknesses and compositions. This witnesses the generation of a new fully tunable quasi-3D topological state that provides a template for realization of magnetic Weyl semimetals and other strongly interacting topological phases.

Strain tuned low thermal conductivity in Indium Antimonide (InSb) through increase in anharmonic phonon scattering - A first-principles study

InSb is a promising thermoelectric material and a decrease in its thermal conductivity (k) can further improve its thermoelectric performance. Using first-principles computations, we report a 29% decrease in k of InSb through strain. k value decreases from 18.8 W/mK for unstrained InSb to 13.4 W/mK for 5% biaxially compressed InSb at 300 K. This decrease in k is found to be due to a large increase in anharmonic scattering of longitudinal acoustic (LA) phonons in strained InSb by almost 330% in Brillouin-zone center, driven by shift of transverse acoustic (TA) phonons to lower frequencies in strained InSb. This shift leads to a simultaneous increase in number of decay scattering channels and a two-fold increase in the magnitude of three-phonon coupling elements, leading to the observed large increase in scattering rates of LA phonons. Scattering of transverse acoustic (TA) phonons is also found to increase by ~61% at Brillouin-zone edge, facilitated by an increase in number of absorption scattering channels. First-principles computations based on deriving interatomic force interactions from density-functional theory are used to provide detailed understanding of increase in phonon scattering rates in strained InSb.

Genuine electronic structure and superconducting gap structure in (Ba0.6K0.4)Fe2As2 superconductor

The electronic structure and superconducting gap structure are prerequisites to establish microscopic theories in understanding the superconductivity mechanism of iron-based superconductors. However, even for the most extensively studied optimally-doped (Ba0.6K0.4)Fe2As2, there remain outstanding controversies on its electronic structure and superconducting gap structure. Here we resolve these issues by carrying out high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements on the optimally-doped (Ba0.6K0.4)Fe2As2 superconductor using both Helium lamp and laser light sources. Our results indicate the “flat band” feature observed around the Brillouin zone center in the superconducting state originates from the combined effect of the superconductivity-induced band back-bending and the folding of a band from the zone corner to the center. We found direct evidence of the band folding between the zone corner and the center in both the normal and superconducting state. Our resolution of the origin of the flat band makes it possible to assign the three hole-like bands around the zone center and determine their superconducting gap correctly. Around the zone corner, we observe a tiny electron-like band and an M-shaped band simultaneously in both the normal and superconducting states. The obtained gap size for the bands around the zone corner (~5.5 meV) is significantly smaller than all the previous ARPES measurements. Our results establish a new superconducting gap structure around the zone corner and resolve a number of prominent controversies concerning the electronic structure and superconducting gap structure in the optimally-doped (Ba0.6K0.4)Fe2As2. They provide new insights in examining and establishing theories in understanding superconductivity mechanism in iron-based superconductors.

Transport in the non-Fermi liquid phase of isotropic Luttinger semimetals

Luttinger semimetals have quadratic band crossings at the Brillouin zone-center in three spatial dimensions. Coulomb interactions in a model that describes these systems stabilize a non-trivial fixed point associated with a non-Fermi liquid state, also known as the Luttinger-Abrikosov-Beneslavskii phase. We calculate the optical conductivity $\sigma (\omega) $ and the dc conductivity $\sigma_{dc} (T) $ of this phase, by means of the Kubo formula and the Mori-Zwanzig memory matrix method, respectively. Interestingly, we find that $\sigma (\omega) $, as a function of the frequency $\omega$ of an applied ac electric field, is characterized by a small violation of the hyperscaling property in the clean limit, which is in marked contrast to the low-energy effective theories that possess Dirac quasiparticles in the excitation spectrum and obey hyperscaling. Furthermore, the effects of weak short-ranged disorder on the temperature-dependence of $\sigma_{dc} (T)$ give rise to a much stronger power-law suppression at low temperatures compared to the clean limit. Our findings demonstrate that these disordered systems are actually power-law insulators. Our theoretical results agree qualitatively with the data from recent experiments performed on Luttinger semimetal compounds like the pyrochlore iridates [ (Y$_{1-x}$Pr$_x$)$_2$Ir$_2$O$_7$ ].

Rashba valleys and quantum Hall states in few-layer black arsenic.

Exciting phenomena may emerge in non-centrosymmetric two-dimensional electronic systems when spin-orbit coupling (SOC)1 interplays dynamically with Coulomb interactions2,3, band topology4,5 and external modulating forces6-8. Here we report synergetic effects between SOC and the Stark effect in centrosymmetric few-layer black arsenic, which manifest as particle-hole asymmetric Rashba valley formation and exotic quantum Hall states that are reversibly controlled by electrostatic gating. The unusual findings are rooted in the puckering square lattice of black arsenic, in which heavy 4p orbitals form a Brillouin zone-centred Γ valley with pz symmetry, coexisting with doubly degenerate D valleys of px origin near the time-reversal-invariant momenta of the X points. When a perpendicular electric field breaks the structure inversion symmetry, strong Rashba SOC is activated for the px bands, which produces spin-valley-flavoured D± valleys paired by time-reversal symmetry, whereas Rashba splitting of the Γ valley is constrained by the pz symmetry. Intriguingly, the giant Stark effect shows the same px-orbital selectiveness, collectively shifting the valence band maximum of the D± Rashba valleys to exceed the Γ Rashba top. Such an orchestrating effect allows us to realize gate-tunable Rashba valley manipulations for two-dimensional hole gases, hallmarked by unconventional even-to-odd transitions in quantum Hall states due to the formation of a flavour-dependent Landau level spectrum. For two-dimensional electron gases, the quantization of the Γ Rashba valley is characterized by peculiar density-dependent transitions in the band topology from trivial parabolic pockets to helical Dirac fermions.

Common (π,π) Band Folding and Surface Reconstruction in FeAs-Based SuperconductorsSupported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0300300, 2017YFA0302900, 2018YFA0704200 and 2019YFA0308000), the National Natural Science Foundation of China (Grant Nos. 11888101, 11922414 and 11874405), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB25000000), the Youth Innovation Promotion Association

High resolution angle-resolved photoemission spectroscopy (ARPES) measurements are carried out on CaKFe4As4, KCa2Fe4As4F2 and (Ba0.6K0.4)Fe2As2 superconductors. Clear evidence of band folding between the Brillouin zone center and corners with a (π,π) wave vector has been found from the measured Fermi surface and band structures in all the three kinds of superconductors. A dominant surface reconstruction is observed on the cleaved surface of CaKFe4As4 by scanning tunneling microscopy (STM) measurements. We propose that the commonly observed reconstruction in the FeAs-based superconductors provides a general scenario to understand the origin of the (π,π) band folding. Our observations provide new insights in understanding the electronic structure and superconductivity mechanism in iron-based superconductors.

Charge-four Weyl phonons

By using ab initio calculations and symmetry analysis, we define a class of Weyl phonons: charge-four Weyl phonons (CFWPs) characterized by Chern number $\ifmmode\pm\else\textpm\fi{}4$ in their acoustic phonon spectra due to chiral point-group symmetries. Some high-symmetry points in the chiral space groups (SGs) of phononic systems behaving as quadratic Weyl points tend to form CFWPs. As enumerated in this paper, the CFWPs are located at the boundaries or the center of the three-dimensional Brillouin zone and are protected by the time-reversal symmetry $\mathcal{T}$ and the corresponding point-group symmetries. Moreover, in a realistic chiral crystal example of BiIrSe in SG 198, a monopole CFWP is confirmed at point $\mathrm{\ensuremath{\Gamma}}$ with quadratic twofold degeneracies, and in another example of ${\mathrm{Li}}_{3}{\mathrm{CuS}}_{2}$ in SG 214, the CFWPs are found at the high-symmetry points $\mathrm{\ensuremath{\Gamma}}$ and $H$. Our theoretical results not only uncover a class of Weyl phonons but also put forward an effective way to search for CFWPs in spinless systems.