Center Of Brillouin Zone Research Articles

Refractive index from negative to positive with frequencies at the Dirac-like cone in a photonic crystal

In this paper, we numerically study the threefold accidental degeneracy conical dispersion (Driac-like cone) at the Brillouin zone center of the two-dimensional photonic crystals, which are composed of silicon pillars arranging in a triangular lattice. The effective permittivity and permeability near the Dirac-like point evolve from negative to positive by using the method of eigen-field averaging. Also, the isotropic behaviour of the Dirac-like cone is revealed by analysing the isofrequency contours. Moreover, we carry out numerical simulations including the reverse Snell’s law effect, negative Goos–Hanchen shifts and superfocusing lens to verify the negative refractive index characteristics of the designed structure. The proposed structure might find significant applications in the on-chip photonic interconnect and the photonic integrated circuit techniques.

Discrete modulational instability and bright localized spin wave modes in easy-axis weak ferromagnetic spin chains involving the next-nearest-neighbor coupling**Project supported by the National Natural Science Foundation of China (Grant Nos. 11604121 and 11875126), the Natural Science Fund Project of Hunan Province, China (Grant No. 2017JJ3255), the National College Students’ Innovation Entrepreneurship Training Program, China (Grant No. 201810531014), and the Scientific

We report a theoretical work on the properties of modulational instability and bright type nonlinear localized modes in one-dimensional easy-axis weak ferromagnetic spin lattices involving next-nearest-neighbor couplings. With a linear stability analysis, we calculate the growth rates of the modulational instability, and plot the instability regions. When the strength of the next-nearest-neighbor coupling is large enough, two new asymmetric modulational instability regions appear near the boundary of the first Brillouin zone. Furthermore, analytical forms of the bright nonlinear localized modes are constructed by means of a quasi-discreteness approach. The influence of the next-nearest-neighbor coupling on the Brillouin zone center mode and boundary mode are discussed. In particular, we discover a reversal phenomenon of the propagation direction of the Brillouin zone boundary mode.

Thermal Disorder and Bond Anharmonicity in Cesium Lead Iodide Studied by Neutron Total Scattering and the Reverse Monte Carlo Method

Cesium lead iodide, CsPbI3, combines promising optoelectronic applications with long-term chemical stability. It has a cubic perovskite crystal structure at temperatures above 600 K. Previous experimental work showed that the I atoms move substantially in directions perpendicular to the Pb–I–Pb linkages, while computational modeling shows that there are soft, anharmonic distortion modes for wave vectors in both the Brillouin zone center and zone boundary. However, there has been no direct experimental probe of the energy profile associated with these distortions. We report here total neutron scattering data from a powder sample of this material as a function of temperature. We use reverse Monte Carlo modeling to construct configurations of atoms consistent with both the local and long-range structure. Our key finding is that the Pb–I bonds are extremely anharmonic, and thus it follows that all near-neighbor bond distributions are significantly asymmetric. The distribution of nearest-neighbor Pb–I distance...

Ab initio study of the electronic, elastic, vibrational and dielectric properties of KBaBi

KBaBi is predicted to be a topological insulator recently, but most fundamental properties of it have not been studied. In this work, first-principles calculations were carried out to study its electronic, bonding, elastic, vibrational and dielectric properties. The results indicate that KBaBi has a band gap of 0.22 eV with the presence of spin orbital coupling. Weak p-d hybridization exists between the Ba 5d and the Bi 6p orbitals, so KBaBi has some covalent property, as revealed by the electron density difference. Studies imply that KBaBi is mechanically and dynamically stable, but it is mechanically anisotropic. KBaBi has a theoretical bulk modulus of 20.8 GPa and a shear modulus of 11.7 GPa. Group theory analysis indicates that there are six infrared- and four Raman-active modes at the Brillouin zone center of KBaBi. The vibrational frequencies and the corresponding eigenvectors were calculated and discussed. Calculation unveils that the electrons have more dielectric contribution to the total dielectric constants than the ions in the ab plane, but it is revised along the c axis. Investigations of the band structure, the Debye temperature and the phonon dispersion curves indicate that KBaBi is a promising thermalelectric material.

Soft-phonon driven hexagonal-orthorhombic phase transition in BaVS3

The lattice dynamics of low-dimensional BaVS3 is reported accross the hexagonal-orthorhombic phase transition occuring at TS = 250 K using a combination of Thermal Diffuse Scattering of X-rays, Inelastic X-ray Scattering and DFT calculations. We observed the occurence of strongly temperature-dependent diffuse scattering upon approaching the transition, centered at the Γ points associated with weak Bragg reflections with odd L. Inelastic scattering experiment related this observation to the condensation of a low-lying overdamped optical phonon at the Brillouin zone center, evidenced by significant variations of quasi-elastic scattering. These results unravel the dynamical origin of the zigzag distortion of the V chains at TS, which is probably a prerequisite of the Peierls anomaly driving the metal-insulator transition at T MI = 70 K.

Electronic, elastic and lattice dynamic properties of the topological Dirac semimetal Na3Bi

Na3Bi is a topological Dirac semimetal, which has a three-dimensional topological Dirac node at its Brillouin zone center. Here we performed a systematic study on its electronic, elastic and lattice dynamic properties by first principles calculation to enrich our knowledge on it. Studies confirmed the semimetallic nature of Na3Bi. Calculated elastic properties indicate that Na3Bi is mechanically stable but anistropic. Na3Bi is a brittle crystal. Shear deformation is easier to occur in its (00 ± 1)[0 ± 10] slip system. The infrared- and Raman-active modes at its Brillouin zone center were identified and detailedly discussed. All the computed vibrational frequencies of the phonons are positive, indicating the dynamical stability of Na3Bi. The Born effective charges and dielectric constants of Na3Bi were also calculated.

Strain-induced spontaneous Hall effect in an epitaxial thin film of a Luttinger semimetal

Pyrochlore iridates have provided a plethora of novel phenomena owing to the combination of topology and correlation. Among them, much attention has been paid to [Formula: see text], as it is known as a Luttinger semimetal characterized by quadratic band touching at the Brillouin zone center, suggesting that the topology of its electronic states can be tuned by a moderate lattice strain and external magnetic field. Here, we report that our epitaxial [Formula: see text] thin films grown by solid-state epitaxy exhibit a spontaneous Hall effect that persists up to 50 K without having spontaneous magnetization within our experimental accuracy. This indicates that the system breaks the time reversal symmetry at a temperature scale that is too high for the magnetism to be due to Pr 4f moments and must be related to magnetic order of the iridium 5d electrons. Moreover, our analysis finds that the chiral anomaly induces the negative contribution to the magnetoresistance only when a magnetic field and the electric current are parallel to each other. Our results indicate that the strained part of the thin film forms a magnetic Weyl semimetal state.

Tensile-strain induced phonon splitting in diamond**Project supported by the National Natural Science Foundation of China (Grant No. 11774247).

The first-order Raman spectroscopy of diamond exhibits splitting and redshift after the burst of high-pressure (160–200 GPa) and high-temperature (∼2000 K). The observed longitudinal optical (LO) and the transverse optical (TO) splitting of Raman phonon is related to the tensile-strain induced activation of the forbidden or silent Raman modes that arise in the proximity of the Brillouin zone center.

Modulation of terahertz radiation from graphene surface plasmon polaritons via surface acoustic wave.

We present a theoretical study of terahertz (THz) radiation induced by surface plasmon polaritons (SPPs) on a graphene layer under modulation by a surface acoustic wave (SAW). In our gedanken experiment, SPPs are excited by an electron beam moving on a graphene layer situated on a piezoelectric MoS2 flake. Under modulation by the SAW field, charge carriers are periodically distributed over the MoS2 flake, and this causes periodically distributed permittivity. The periodic permittivity structure of the MoS2 flake folds the SPP dispersion curve back into the center of the first Brillouin zone, in a manner analogous to a crystal, leading to THz radiation emission with conservation of the wavevectors between the SPPs and the electromagnetic waves. Both the frequency and the intensity of the THz radiation are tuned by adjusting the chemical potential of the graphene layer, the MoS2 flake doping density, and the wavelength and period of the external SAW field. A maximum energy conversion efficiency as high as ninety percent was obtained from our model calculations. These results indicate an opportunity to develop highly tunable and integratable THz sources based on graphene devices.

Magnon and Phonon Excitations in Nanosized NiO

Abstract Single-crystal, microcrystalline and nanocrystalline nickel oxides (NiO) have been studied by Raman spectroscopy. A new band at ~200 cm−1 and TO-LO splitting of the band at 350–650 cm−1 have been found in the spectra of single-crystals NiO(100), NiO(110) and NiO(111). The Raman spectra of microcrystalline (1500 nm) and nanocrystalline (13–100 nm) NiO resemble those of the single crystals. They all contain the two-magnon band at 1500 cm−1, indicating that the oxides remain at room temperature in the antiferromagnetic phase. Besides, a new sharp Raman band has been observed at 500 cm−1 in nanocrystalline NiO. Its temperature dependence suggests the magnetic origin of the band, possibly associated with the one-phonon–one-magnon excitation at the Brillouin zone centre.

Structure of surface electronic states in strained mercury telluride

We present the theory describing the various surface electronic states arisen from the mixing of conduction and valence bands in a strained mercury telluride (HgTe) bulk material. We demonstrate that the strain-induced band gap in the Brillouin zone center of HgTe results in the surface states of two different kinds. Surface states of the first kind exist in the small region of electron wave vectors near the center of the Brillouin zone and have the Dirac linear electron dispersion characteristic for topological states. The surface states of the second kind exist only far from the center of the Brillouin zone and have the parabolic dispersion for large wave vectors. The structure of these surface electronic states is studied both analytically and numerically in the broad range of their parameters, aiming to develop its systematic understanding for the relevant model Hamiltonian. The results bring attention to the rich surface physics relevant for topological systems.

Particle–Hole Transformation in Strongly-Doped Iron-Based Superconductors

An exact particle–hole transformation is discovered in a local-moment model for a single layer of heavily electron-doped FeSe. The model harbors hidden magnetic order between the iron d x z and d y z orbitals at the wavenumber ( π , π ) . It potentially is tied to the magnetic resonances about the very same Néel ordering vector that have been recently discovered in intercalated FeSe. Upon electron doping, the local-moment model successfully accounts for the electron-pocket Fermi surfaces observed experimentally at the corner of the two-iron Brillouin zone in electron-doped FeSe, as well as for isotropic Cooper pairs. Application of the particle–hole transformation predicts a surface-layer iron-based superconductor at strong hole doping that exhibits high T c, and that shows hole-type Fermi-surface pockets at the center of the two-iron Brillouin zone.

Observation of unconventional chiral fermions with long Fermi arcs in CoSi.

Chirality-the geometric property of objects that do not coincide with their mirror image-is found in nature, for example, in molecules, crystals, galaxies and life forms. In quantum field theory, the chirality of a massless particle is defined by whether the directions of its spin and motion are parallel or antiparallel. Although massless chiral fermions-Weyl fermions-were predicted 90 years ago, their existence as fundamental particles has not been experimentally confirmed. However, their analogues have been observed as quasiparticles in condensed matter systems. In addition to Weyl fermions1-4, theorists have proposed a number of unconventional (that is, beyond the standard model) chiral fermions in condensed matter systems5-8, but direct experimental evidence of their existence is still lacking. Here, by using angle-resolved photoemission spectroscopy, we reveal two types of unconventional chiral fermion-spin-1 and charge-2 fermions-at the band-crossing points near the Fermi level in CoSi. The projections of these chiral fermions on the (001) surface are connected by giant Fermi arcs traversing the entire surface Brillouin zone. These chiral fermions are enforced at the centre or corner of the bulk Brillouin zone by the crystal symmetries, making CoSi a system with only one pair of chiral nodes with large separation in momentum space and extremely long surface Fermi arcs, in sharp contrast to Weyl semimetals, which have multiple pairs of Weyl nodes with small separation. Our results confirm the existence of unconventional chiral fermions and provide a platform for exploring the physical properties associated with chiral fermions.

Anomalous variations of spectral linewidth in internal excitonic quantum transitions of ultrafast resonantly excited single-walled carbon nanotubes

How the exciton fine structure and spectral linewidth in single-walled carbon nanotubes (SWNTs) evolve after ultrafast photoexcitation is critical for both understanding the fundamental optoelectronic properties and the development of nanophotonic functional devices. Yet, studies so far have mostly detected a subset of excitons near the Brillouin-zone center due to symmetry and momentum restrictions of the interband optical probes used, such as photoluminescence and absorption. Here we use ultrafast terahertz spectroscopy to probe the excitonic spectral linewidth associated with internal quantum transition across the entire momentum $K$ space in resonantly excited semiconducting SWNTs. The lowest-lying intraexcitonic transition surprisingly sharpens at both high pump fluence and initial times immediately following the photoexcitation. We attribute these anomalous variations in the spectral linewidth to the role of the dark state $1s(g)$ that influences the lifetime broadening of the bright state $1s(u)$, consistent with the temperature-dependent dephasing time with a crossover temperature set by the internal quantum level spacing of excitons.

Dual Dirac cones in elastic Lieb-like lattice metamaterials

Double-zero-index properties of electromagnetic and acoustic waveguides have been recently realized based on Dirac/Dirac-like cones at the Brillouin zone (BZ) center. However, very limited research has been devoted to double-zero-index structural systems of elastic waveguides, and almost no lattice system has been able to achieve multiple separated Dirac cones generated around different frequencies at the BZ center. Here, we report two separated elastic-wave Dirac-like cones, which are simultaneously achieved around different Dirac points at the BZ center, due to the accidental degeneracy and frequency repulsion effect in a Lieb-like lattice metamaterial. Using the proposed elastic medium, the double-zero-index properties of various elastic wave modes are theoretically analyzed, numerically computed, and experimentally observed at the neighborhood of both Dirac-like points. The performance of near total transmission without the phase change and the ability of wave-front shaping are unambiguously verified by numerical simulation and experimental measurements.

Observation of Chiral Fermions with a Large Topological Charge and Associated Fermi-Arc Surface States in CoSi.

Topological semimetals materialize a new state of quantum matter where massless fermions protected by a specific crystal symmetry host exotic quantum phenomena. Distinct from well-known Dirac and Weyl fermions, structurally chiral topological semimetals are predicted to host new types of massless fermions characterized by a large topological charge, whereas such exotic fermions are yet to be experimentally established. Here, by using angle-resolved photoemission spectroscopy, we experimentally demonstrate that a transition-metal silicide CoSi hosts two types of chiral topological fermions, a spin-1 chiral fermion and a double Weyl fermion, in the center and corner of the bulk Brillouin zone, respectively. Intriguingly, we found that the bulk Fermi surfaces are purely composed of the energy bands related to these fermions. We also find the surface states connecting the Fermi surfaces associated with these fermions, suggesting the existence of the predicted Fermi-arc surface states. Our result provides the first experimental evidence for the chiral topological fermions beyond Dirac and Weyl fermions in condensed-matter systems, and paves the pathway toward realizing exotic electronic properties associated with unconventional chiral fermions.

Modification of Pb quantum well states by the adsorption of organic molecules

The successful implementation of nanoscale materials in next generation optoelectronic devices crucially depends on our ability to functionalize and design low dimensional materials according to the desired field of application. Recently, organic adsorbates have revealed an enormous potential to alter the occupied surface band structure of tunable materials by the formation of tailored molecule-surface bonds. Here, we extend this concept of adsorption-induced surface band structure engineering to the unoccupied part of the surface band structure. This is achieved by our comprehensive investigation of the unoccupied band structure of a lead (Pb) monolayer film on the Ag(1 1 1) surface prior and after the adsorption of one monolayer of the aromatic molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using two-photon momentum microscopy, we show that the unoccupied states of the Pb/Ag(1 1 1) bilayer system are dominated by a parabolic quantum well state (QWS) in the center of the surface Brillouin zone with Pb p orbital character and a side band with almost linear dispersion showing Pb p orbital character. After the adsorption of PTCDA, the Pb side band remains completely unaffected while the signal of the Pb QWS is fully suppressed. This adsorption induced change in the unoccupied Pb band structure coincides with an interfacial charge transfer from the Pb layer into the PTCDA molecule. We propose that this charge transfer and the correspondingly vertical (partially chemical) interaction across the PTCDA/Pb interface suppresses the existence of the QWS in the Pb layer. Our results hence unveil a new possibility to orbital selectively tune and control the entire surface band structure of low dimensional systems by the adsorption of organic molecules.

Observation of intercalation-driven zone folding in quasi-free-standing graphene energy bands

Two-photon photoemission measurements reveal a near-zero-dispersion empty electronic state, approximately 2.6 eV above the Fermi energy and near the Brillouin zone center, induced by oxygen intercalation at the graphene-Ir(111) interface. While oxygen intercalation leads to quasi-free-standing graphene, electron diffraction shows $2\ifmmode\times\else\texttimes\fi{}2$ periodicity due to the patterned intercalant. Near the zone center, large-wave-vector zone folding, driven by this $2\ifmmode\times\else\texttimes\fi{}2$ periodicity, replicates states from near the Dirac cone that have little dispersion due to trigonal warping, explaining the nearly flat band. The zone-folding mechanism is supported by results from angle-resolved photoemission measurements and from density-functional-theory-based calculations of the unfolded energy bands. These results demonstrate zone-folding effects in graphene on a wave vector and energy scale that has largely been unexplored, and may open new opportunities to engineer the graphene electronic states.

Acoustic phonon confinement by band inversion

Abstract We study the formation of a topological interface mode between two nanometric acoustic superlattices presenting inverted band structures. That is, the two lattices share a common band gap but present opposite spatial mode symmetries of the two Bloch modes bounding the gap from below and above. We first show that by tuning the acoustic-thickness ratio between the two materials in a unit cell of a GaAs/AlAs superlattice a band structure inversion can be induced at the Brillouin zone center. Secondly, we show that two concatenated superlattices with inverted band structures form a topological interface mode with its strain spatially confined.

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

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