Center Of Brillouin Zone Research Articles

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.

Interference of exciton-polariton waves in GaSe nanocrystals

Absorption, reflection, photoluminescence and wavelength modulation transmission spectra of GaSe single crystals were investigated in temperature rage 300 – 10 K. From fitting of contours of excitonic reflection spectra parameters of excitons as energy of transversal exciton ω0 = 2.1212 eV, longitudinal transversal splitting ωLT = 2.3 meV, effective mass M = 2.5m0, background permittivity εb= 5.2 and damping constant γ = 1.4 were determined. Effective masses of electrons (mC1 = 0.37m0) and holes (mV1 = 2.13m0) in the Brillouin zone center were estimated. An interference of waves of upper and lower branches of exciton-polaritons was found out in wavelength modulation transmission spectra of GaSe nanocrystals.

Dirac Fermions: Fermi Velocity Reduction of Dirac Fermions around the Brillouin Zone Center in In2Se3–Bilayer Graphene Heterostructures (Adv. Mater. 17/2021)

In article number 2007503, Zhenyu Wang, Jianfeng Wang, Chaoyu Chen, and co-workers develop a practical strategy to construct moiré Dirac cones around the Brillouin zone center by folding graphene Dirac cones in monolayer In2Se3/bilayer graphene heterostructures. The Fermi velocity of the moiré Dirac cones is reduced by ≈23%, and the reshaped Dirac point at the Γ point is elongated. Both features further confirm moiré Dirac fermion renormalization and interactions.

Fermi Velocity Reduction of Dirac Fermions around the Brillouin Zone Center in In2 Se3 -Bilayer Graphene Heterostructures.

Emergent phenomena such as unconventional superconductivity, Mott-like insulators, and the peculiar quantum Hall effect in graphene-based heterostructures are proposed to stem from the superlattice-induced renormalization of (moiré) Dirac fermions at the graphene Brillouin zone corners. Understanding the corresponding band structure commonly demands photoemission spectroscopy with both sub-meV resolution and large-momentum coverage, beyond the capability of the current state-of-the-art. Here the realization of moiré Dirac cones around the Brillouin zone center in monolayer In2 Se3 /bilayer graphene heterostructure is reported. The renormalization is evidenced by reduced Fermi velocity (≈23%) of the moiré Dirac cones and the reshaped Dirac point at the Γ point where they intersect. While there have been many theoretical predictions and much indirect experimental evidence, the findings here are the first direct observation of Fermi velocity reduction of the moiré Dirac cones. These features suggest strong In2 Se3 /graphene interlayer coupling, which is comparable with that in twisted bilayer graphene. The strategy expands the choice of materials in the heterostructure design and stimulates subsequent broad investigations of emergent physics at the sub-meV energy scale.

Identification of Excitons and Biexcitons in Sb2Se3 under High Photoluminescence Excitation Density

AbstractThe near‐band‐edge emission of Sb2Se3 microcrystals is studied in detail under high photoluminescence excitation density using a pulsed UV laser (λ = 266 nm, pulse width 0.6 ns). Based on the peak energy positions and the excitation power density and temperature dependencies (T = 3–110 K) of the photoluminescence spectra, the emission is interpreted as a recombination of two pairs (A and B) of free excitons and biexcitons appearing because of valence band splitting. The magnitude of the valence band splitting due to the crystal field (Δcr = 20–22 meV) is estimated in the Brillouin zone center. At T = 3 K, the A and B biexciton emission at 1.302 and 1.322 eV, respectively, and the A and B free exciton emission at 1.311 and 1.333 eV, respectively, are observed. The binding energy of A and B biexcitons is 9 and 11 meV, respectively, and the binding energy of A free exciton is 6 meV. The activation energy of thermal quenching for all observed peaks is 20 meV.

Light emission from direct band gap germanium containing split-interstitial defects

The lack of useful and cost-efficient group-IV direct band gap light emitters still presents the main bottleneck for complementary metal-oxide semiconductor-compatible short-distance data transmission, single-photon emission, and sensing based on silicon photonics. Germanium, a group-IV element like Si, is already widely used in silicon fabs. While the energy band gap of Ge is intrinsically indirect, we predict that the insertion of Ge-Ge split-[110] interstitials into crystalline Ge can open up a direct band gap transmission path. Here, we calculate from first principles the band structure and optical emission properties of Ge, Sb, and Sn split-[110] interstitials in bulk and low-dimensional Ge at different doping concentrations. Two types of electronic states provide the light-emission enhancement below the direct band gap of Ge: a hybridized L-$\mathrm{\ensuremath{\Gamma}}$ state at the Brillouin zone center and a conduction band of $\mathrm{\ensuremath{\Delta}}$ band character that couples to a raised valence band along the $\mathrm{\ensuremath{\Gamma}}$-X direction. Majority carrier introduced to the system through doping can enhance light emission by saturation of nonradiative paths. Ge-Sn split interstitials in Ge shift the top of the valence band towards the $\mathrm{\ensuremath{\Gamma}}$-X direction and increase the $\mathrm{\ensuremath{\Gamma}}$ character of the L-$\mathrm{\ensuremath{\Gamma}}$ state, which results in a shift to longer emission wavelengths. Key spectral regions for datacom and sensing applications can be covered by applying quantum confinement in defect-enhanced Ge quantum dots for an emission wavelength shift from the midinfrared to the telecom regime.

Quantization of the band at the surface of charge density wave material 2H-TaSe2 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774421, 11774424, 11574394, 11774423, 11822412, and 11874047), the National Key R&D Program of China (Grant Nos. 2016YFA0401002, 2018YFA0307000, 2016YFA0300504, and 2018FYA0305800), and the Fundamental Research Funds for the Central Universities, China (Grant No. 2042018kf-0030).

By using angle-resolved photoemission spectroscopy (ARPES) combined with the first-principles electronic structure calculations, we report the quantized states at the surface of a single crystal 2H-TaSe2. We have observed sub-bands of quantized states at the three-dimensional Brillouin zone center due to a highly dispersive band with light effective mass along kz direction. The quantized sub-bands shift upward towards E F while the bulk band at Γ shifts downward with the decrease of temperature across charge density wave (CDW) formation. The band shifts could be intimately related to the CDW. While neither the two-dimensional Fermi-surface nesting nor purely strong electron–phonon coupling can explain the mechanism of CDW in 2H-TaSe2, our experiment may ignite the interest in understanding the CDW mechanism in this family.

Predicting the impact sensitivities of energetic materials through zone-center phonon up-pumping.

The development of new energetic materials (EMs) is accompanied by significant hazards, prompting interest in their computational design. Before reliable in silico design strategies can be realized, however, approaches to understand and predict EM response to mechanical impact must be developed. We present here a fully ab initio model based on phonon up-pumping that successfully ranks the relative impact sensitivity of a series of organic EMs. The methodology depends only on the crystallographic unit cell and Brillouin zone center vibrational frequencies. We, therefore, expect this approach to become an integral tool in the large-scale screening of potential EMs.

Evidence of Weyl fermions in α−RuCl3

Layered honeycomb lattice antiferromagnetic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ has been studied intensively recently as a Kitaev spin liquid candidate. Here we present strong evidence that this material host realistic Weyl fermions derived from Cl $2p$ electrons. Our theoretical analysis suggests the existence of quadratic Weyl fermions without spin-orbit coupling and linear Weyl fermion with spin-orbit coupling. Angle-resolved photoemission spectroscopy with systematic photon energy-dependent measurements demonstrates that this Weyl cone possesses linear dispersion at bulk Brillouin zone center along all the three reciprocal directions and hyperbolic dispersion off center. Our work reveals the rich topological physics lying in the band structure of Kitaev QSL candidates and will stimulate further investigation not only limited to $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ but also honeycomb iridate family of materials.

Polarized Raman Spectra and Complex Raman Tensors of Antiferromagnetic Semiconductor CrPS4

Chromium thiophosphate (CrPS4), a monoclinic crystal of C_2^3 space group, is a ternary layered semiconductor with an optical bandgap of 1.4 eV and exhibits antiferromagnetism below 36 K. Despite its potential in optoelectronic and magnetic applications, the symmetry of its lattice vibrations has not been systematically studied. In this work, we performed a polarized Raman spectroscopy of bulk CrPS4 using three different excitation wavelengths of 457, 514, and 633 nm. High-quality crystals grown by the chemical vapor transport method were mechanically exfoliated or polished to expose three orthogonal crystallographic facets. Polarized Raman spectra were obtained in parallel and cross configurations by rotating samples about the surface normal to each of the facets. Among 33 Raman active modes (16 A and 17 B) at the Brillouin zone center, 19 (8) peaks observed in the parallel (cross) configuration were assigned as A (B) modes. Complex-valued Raman tensors were determined for 7 major A modes using the angle-dependent Raman spectra of the three facets. The results can also be used in determining the crystallographic orientations of CrPS4 unequivocally.

Determination of the surface states from the ultrafast electronic states in a thermoelectric material

We reveal the electronic structure in YbCd2Sb2, a thermoelectric material, by angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES (trARPES). Specifically, three bulk bands at the vicinity of the Fermi level are evidenced near the Brillouin zone center, consistent with the density functional theory (DFT) calculation. It is interesting that the spin-unpolarized bulk bands respond unexpectedly to right- and left-handed circularly polarized probe. In addition, a hole band of surface states, which is not sensitive to the polarization of the probe beam and is not expected from the DFT calculation, is identified. We find that the non-equilibrium quasiparticle recovery rate is much smaller in the surface states than that of the bulk states. Our results demonstrate that the surface states can be distinguished from the bulk ones from a view of time scale in the nonequilibrium physics.

Linear and nonlinear optical responses in the chiral multifold semimetal RhSi

Chiral topological semimetals are materials that break both inversion and mirror symmetries. They host interesting phenomena such as the quantized circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this work, we report a comprehensive theoretical and experimental analysis of the linear and nonlinear optical responses of the chiral topological semimetal RhSi, which is known to host multifold fermions. We show that the characteristic features of the optical conductivity, which display two distinct quasi-linear regimes above and below 0.4 eV, can be linked to excitations of different kinds of multifold fermions. The characteristic features of the CPGE, which displays a sign change at 0.4 eV and a large non-quantized response peak of around 160 μA/V2 at 0.7 eV, are explained by assuming that the chemical potential crosses a flat hole band at the Brillouin zone center. Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is necessary to increase the chemical potential as well as the quasiparticle lifetime. More broadly, our methodology, especially the development of the broadband terahertz emission spectroscopy, could be widely applied to study photogalvanic effects in noncentrosymmetric materials and in topological insulators in a contact-less way and accelerate the technological development of efficient infrared detectors based on topological semimetals.

Multiple Dirac points by high-order photonic bands in plasmonic-dielectric superlattices.

The emergence of Dirac points (DPs) characterizes the topological phase transition and the gapless interface states in composite metal-dielectric metamaterials. In this work, we study a kind of compound plasmonic-dielectric periodic structure (PDPS) which sustains both plasmonic modes and multiple photonic modes. The structure has primitive cell consisting of four layers made from triple constituent components. Due to the generalized Su-Schrieffer-Heeger, DPs can emerge at the Brillouin zone center. More specifically, in weak plasmonic-photonic mode interaction regime, multiple DPs would emerge at the Brillouin zone center and edge due to the band folding, from the perspective of general effective medium. From the rigorous field analysis, the origin of these DPs is clearly demonstrated. These interleaved DPs behave as the intermediate transitions of the surface impedance for the PDPS and raise fully spanned topological interface states originated from 0 to 2nd-order photonic bands in the PDPS. The cases of combining our PDPS with either a plasmonic or dielectric homogenous medium are presented.

Evaluation of Phonon Dispersion Relation for Bulk Silicon Germanium by Inelastic X-ray Scattering

1. Introduction Silicon germanium (SiGe) is one of the promising candidates as a next-generation material for thermoelectric devices as an ambient energy harvesting technique because it has low thermal conductivity. For a SiGe thermoelectric device, it is important to reveal the phonon scattering mechanism for the alloy material and the thermal transport in miniaturization to the nanoscale. However, the origin of the low thermal conductivity of the SiGe alloy has not been investigated in a sufficiently rigorous manner yet. In addition, the correlation among the thermal conductivity, the phonon dispersion curve, and the lifetime remains mostly unclear.From the above, we focused on direct phonon properties evaluation by inelastic X-ray scattering (IXS) with synchrotron radiation. IXS is a powerful technique that reveals the phonon dispersion curves in contrast to Raman spectroscopy, which can only give optical phonon modes at the Brillouin zone center (the Γ point). In this study, we evaluated the phonon dispersion curve of bulk SiGe by IXS. 2. Experimental method The single-crystalline SiGe samples for IXS were prepared by two different growth methods: the Czochralski (Cz) [1] and traveling liquidus zone (TLZ) methods [2]. The sample information is summarized in Table 1. Prior to the IXS measurements, the crystal orientation and Ge fraction were confirmed by X-ray diffraction (XRD) and electron backscattering pattern (EBSP) measurements, respectively.The IXS measurements were performed at the BL35XU beamline of the SPring-8 synchrotron facility. The measurements were conducted at room temperature using a reflection geometry. The incident X-ray energy was set to 17.795 keV, which corresponds to Si (9 9 9) reflection, and the overall energy resolution was around 3 meV. The incident X-ray beam size was approximately 60 x 75 μm2. The measurements were conducted along the Γ-X ([00q]) direction. 3. Results and Discussion As an example of phonon dispersion of bulk Si1-x Ge x , we show the IXS results of x = 0.45 in Fig. 1 obtained by peak positions of IXS spectra. As a result, all optical and acoustic phonon modes for bulk SiGe were observed as shown in Fig. 1. In addition, the energy of the optical phonon modes (Si-Si, Si-Ge, and Ge-Ge modes) obtained by IXS at the zone center (Γ point) was good agreement with results of Raman spectroscopy (λ = 532 nm). Both Raman and IXS indicate that, at around Γ point, the phonon energies of Si-Si and Ge-Ge modes tend to be lower than that of pure Si and Ge, respectively. These behaviors can be considered to be caused by random distribution of Si and Ge atoms in the alloy. On the other hand, it was revealed that the acoustic phonon (LA and TA modes) energies are located between those of pure Si [3] and Ge [4]. Based on the above, the origin of the dramatic thermal conductivity reduction of the SiGe alloy, which has been reported in previous study [5], may not be derived from the reduction of group velocity, but the low phonon lifetime reduction. Acknowledgements The IXS measurements were performed at SPring-8 with the approval of JASRI (Proposal Nos. 2016A1496, 2017B1630, 2019A1678, and 2019B1750). The authors thank Dr. Koji Usuda (KIOXIA Corp.) for his great support with analyzing the IXS results throughout the work.

Visualizing Half-Metallic Bulk Band Structure with Multiple Weyl Cones of the Heusler Ferromagnet.

Using a well-focused soft x-ray synchrotron radiation beam, angle-resolved photoelectron spectroscopy was applied to a full-Heusler-type Co_{2}MnGe alloy to elucidate its bulk band structure. A large parabolic band at the Brillouin zone center and several bands that cross the Fermi level near the Brillouin zone boundary were identified in line with the results from first-principles calculations. These Fermi-level crossings are ascribed to majority spin bands that are responsible for electron transport with extremely high spin polarization especially along the direction perpendicular to the interface of magnetoresistive devices. The spectroscopy confirms there is no contribution of the minority spin bands to the Fermi surface, signifying half-metallicity for the alloy. Furthermore, two topological Weyl cones with band crossing points were identified around the X point, yielding the conclusion that Co_{2}MnGe could exhibit topologically meaningful behavior such as large anomalous Hall and Nernst effects driven by the Berry flux in its half-metallic band structure.

Raman scattering from highly-stressed anvil diamond* *Project support by the National Natural Science Foundation of China (Grant No. 11774247).

The high-frequency edge of the first-order Raman mode of diamond reflects the stress state at the culet of anvil, and is often used for the pressure calibration in diamond anvil cell (DAC) experiments. Here we point out that the high-frequency edge of the diamond Raman phonon corresponds to the Brillouin zone (BZ) center Γ point as a function of pressure. The diamond Raman pressure gauge relies on the stability of crystal lattice of diamond under high stress. Upon the diamond anvil occurs failure under the uniaxial stress (197 GPa), the loss of intensity of the first-order Raman phonon and a stress-dependent broad Raman band centered at 600 cm−1 are observed, which is associated with a strain-induced local mode corresponding to the BZ edge phonon of the L 1 transverse acoustic phonon branch.

Magnetic anisotropy and exchange paths for octahedrally and tetrahedrally coordinated Mn2+ ions in the honeycomb multiferroic Mn2Mo3O8

We investigated the static and dynamic magnetic properties of the polar ferrimagnet Mn$_2$Mo$_3$O$_8$ in three magnetically ordered phases via magnetization, magnetic torque, and THz absorption spectroscopy measurements. The observed magnetic field dependence of the spin-wave resonances, including Brillouin zone-center and zone-boundary excitations, magnetization, and torque, are well described by an extended two-sublattice antiferromagnetic classical mean-field model. In this orbitally quenched system, the competing weak easy-plane and easy-axis single-ion anisotropies of the two crystallographic sites are determined from the model and assigned to the tetra- and octahedral sites, respectively, by ab initio calculations.

Effective medium theory for a photonic pseudospin- 12 system

Photonic pseudospin-1/2 systems, which exhibit Dirac cone dispersion at Brillouin zone corners in analogy to graphene, have been extensively studied in recent years. However, it is known that a linear band crossing of two bands cannot emerge at the center of Brillouin zone in a two-dimensional photonic system respecting time reversal symmetry. Using a square lattice of elliptical magneto-optical cylinders, we constructed an unpaired Dirac point at the Brillouin zone center as the intersection of the second and third bands corresponding to the monopole and dipole excitations. Effective medium theory can be applied to the two linearly crossed bands with the effective constitutive parameters numerically calculated using the boundary effective medium approach. It is shown that only the effective permittivity approaches zero while the determinant of the nonzero effective permeability vanishes at the Dirac point frequency, showing a different behavior from the double-zero index metamaterials obtained from the pseudospin-1 triply degenerate points for time reversal symmetric systems. Exotic phenomena, such as the Klein tunneling and Zitterbewegung, in the pseudospin-1/2 system can be well understood from the effective medium description. When the Dirac point is lifted, the edge state dispersion near the $\Gamma$ point can be accurately predicted by the effective constitutive parameters. We also further realized magneto-optical complex conjugate metamaterials for a wide frequency range by introducing a particular type of non-Hermittian perturbations which make the two linear bands coalescence to form exceptional points at the real frequency.