Center Of Brillouin Zone Research Articles

Revealing light-induced charge density wave melting and carrier redistribution in 1T-TiSe2

Abstract Capturing the ultrafast dynamics over a large momentum space is critical for revealing the relationship between the electronic and structural modulation in quantum materials. Here, by performing time- and angle-resolved photoemission spectroscopy measurements at Synergetic Extreme Condition User Faclity (SECUF) equipped with extreme ultraviolet light source, we reveal the ultrafast dynamics of a charge density wave (CDW) material 1T-TiSe2 upon photo-excitation. The pump-induced CDW melting is revealed from two aspects, a gap closing of CDW at the Brillouin zone (BZ) center and a weakening of the CDW folded band at the BZ boundary. By comparing the transient electronic structure and spectral weight over a large momentum space, we further reveal the carrier redistribution involving excitation of electrons from the Γ point to the M point. Our work provides a comprehensive picture on the physics and ultrafast dynamics of a CDW material across the entire BZ.

Electronic, optical, elastic, piezoelectric and vibrational properties of P63 KLiSO4

Abstract KLiSO4 of the P63 symmetry is a well refined crystal at room temperature, which is a pyroelectric material with a large second harmonic generation response. However, its fundamental physical properties are still not well studied. In this work, first principles calculations are performed to study its electronic, optical, elastic, piezoelectric and vibrational properties. The results indicate that it is an ionic crystal with a large indirect band gap. Calculated optical properties imply that P63 KLiSO4 has little optical anisotropy at low frequencies. Obtained elastic constants reveal that it is mechanically stable but anisotropic, as illustrated by the directional bulk and shear moduli. Piezoelectric coefficients, dielectric constants, and Born effective charges (BECs) are computed using the density functional perturbation method. Studies disclose that it has a greater piezoelectric coefficient along the c axis. The ions have more contribution to the total dielectric constants than the electrons. The S atoms have the largest BECs. The phonon vibrational modes at the Brillouin zone center are analyzed by the factor group theory. Its infrared and Raman spectra are simulated. The causation for the vanishment of some infrared peaks in the computed infrared spectrum is uncovered. Additionally, elastic related moduli, hardness, melting point and electromechanical coupling coefficients of P63 KLiSO4 are also predicated.

Strain-Induced Antiferroelectric-Ferroelectric-Antiferroelectric Phase Transitions in Epitaxial LaTaO4 Films.

We employed first-principles to delve into the strain-induced structural phase transitions in epitaxial LaTaO4 films. The ground state of bulk LaTaO4 adopts a monoclinic antiferroelectric phase, characterized by the antiphase rotation of two adjacent oxygen octahedra layers. Under epitaxial tensile strain, LaTaO4 thin films undergo a consecutive phase transition, namely, antiferroelectric-ferroelectric-antiferroelectric phase transitions. The ferroelectricity in the orthorhombic phase under tensile strain originates from the in-phase rotation of two adjacent oxygen octahedra layers, in contrast to the case of perovskite systems, in which octahedra rotation suppresses the conventional proper ferroelectricity. With spin-orbit coupling effects, a pronounced Rashba effect at the Brillouin zone center naturally leads to the formation of opposite directions of spin texture. Moreover, the tensile strain also synergistically enhances the corresponding Rashba parameters. Our findings may provide novel insights for controlling oxygen octahedra rotation to achieve control over the ferroelectricity and Rashba effect, which holds promising implications for applications in electronics and spintronics.

Photoluminiscence and absorption of emission by phonons of indirect transitions in layered GaS single crystals

Absorption in E||b and E⊥c polarizations and photoluminescence at the absorption edge were studied at temperatures range 10–300 K. It was found that the smallest indirect transitions take place from the minimum of conduction band C1 (point M) to the maximum of valence band V1 in the Brillouin zone center (point Γ or nearby). A distance between these extrema is 2.4484 eV at 10 K. In addition, the magnitude of indirect transitions from the second band C2 (point M) to valence band V1 (Brillouin zone center) is 2.4912 eV at 10 K. The splitting of C1–C2 bands in M point of the Brillouin zone is equal to ∼43 meV. The indirect transitions from third conduction band C3 (point K) to valence band V1 in the nearby of Γ point are equal to 2.7683 eV at 10 K. The splitting of conduction bands C2 (point M) and C3 (point K) is higher than one for the splitting C1–C2. It was observed bands associated with self-absorption of photoluminescence emission by phonons with symmetries E1g1+A1g2, A1g2+A1g1 and A1g2+E2g2 involved in indirect transitions.

On the absorption coefficient of GaP1-xNx layers and its potential application for silicon photovoltaics

The absorption coefficient and the energy gap of GaP1-xNx layers has been obtained by spectroscopic ellipsometry for samples grown on Si(001) substrates by chemical beam epitaxy with N mole fractions in the range 0 ≤ x ≤ 0.081. The resulting absorption spectra exhibit a direct band-like behavior near the absorption edge. The absorption coefficient values increase with the N content, reaching values in the range α ∼ 1-2x104 cm−1 in the vicinity of the absorption edge below the original GaP direct bandgap, which are comparable to those obtained for high efficiency solar cell materials. Furthermore, dependence of the absorption coefficient with increasing N content points to a strong GaP Γ-like character of the conduction-band wave function of GaP1-xNx alloys near the Brillouin zone center at k = 0, as predicted by the band anticrossing model. Bandgap energy values obtained by spectroscopic ellipsometry are compared with previous values obtained by photoluminescence measurements on the same samples, observing a shift of about 50–100 meV. Finally, the value of the band anticrossing parameter coupling the N level and the host GaP conduction band has been obtained from the dependence of both, the bandgap and the absorption coefficient, with the N content (2.1 and 3.3 eV respectively).

First principles study on the dielectric and infrared properties of single crystal Al3BC3

The dielectric and infrared vibrational properties of single crystal Al3BC3 were computed using density functional perturbation theory. Utilizing a group theory approach, the frequencies and vibrational modes of all the infrared active modes at the Brillouin zone center were determined. The study explored the dielectric function, infrared reflectivity, and Born effective charge of Al3BC3 in directions both parallel and perpendicular to the c-axis. The analysis of the Born effective charge confirmed the existence of strong covalent bonds between B and C, as well as ionic bonds between Al and C in Al3BC3 ceramics.

The optical properties of nano-structural α-Fe2O3 dependence on the shape.

Three different crystal morphologies of α-Fe2O3, including uniform hexagonal, square, and rhombic shapes, were prepared according to the aqueous-thermal reaction. The hexagonal-shaped α-Fe2O3 was enclosed by the 104 plane, while the square and rhombic structures were enclosed by the 110 plane. Two absorption peaks at 455 and 532 cm-1 were found for the perpendicular (⊥) modes, and one absorption peak at 650 cm-1 appeared for the parallel (||) mode for hexagon-shaped α-Fe2O3 during analysis by Fourier-transform infrared spectroscopy. However, the peaks of square- and rhombic-shaped α-Fe2O3 for perpendicular (⊥) mode blueshifted, and the former two peaks merged together forming a broad band at approximately 480 cm-1. For Raman spectra determination, the peaks arose from the Brillouin zone center, and two additional peaks were observed at 660 and 1320 cm-1, belonging to 1 longitudinal optical (1LO) and 2 longitudinal optical (2LO) modes. All three materials exhibited higher intensities when excited at a wavelength of 633 cm-1. Furthermore, in the polarization state, the centers of all peak positions slightly shifted for hexagon-shaped α-Fe2O3, but all peak positions for square-shaped and rhombic-shaped α-Fe2O3 exhibited a significant blueshift. The structure of hexagon-shaped α-Fe2O3 was relatively tolerant regarding the polarization properties of vibration modes; however, the symmetry of crystal square-shaped and rhombic-shaped α-Fe2O3 changed, subsequently revealing different optical properties. RESEARCH HIGHLIGHTS: The hexagon-shaped, square-shaped, and rhombic-shaped α-Fe2O3 enclosed by different planes were synthesized. The Fourier Transform Infrared spectrometer peaks of α-Fe2O3 depended on their hexagon, square and rhombic shapes. Compared with hexagon-shaped α-Fe2O3, the Raman peaks for square and rhombi ones significantly shifted. The hexagon-shaped α-Fe2O3 is relatively tolerant regarding the polarization properties.

Phase transitions in typical fluorite-type ferroelectrics

While ferroelectric hafnia (HfO2) has become a technically important material for microelectronics, the physical origin of its ferroelectricity remains poorly understood. The tetragonal P42/nmc phase is commonly assigned as its paraelectric mother phase but has no soft mode at the Brillouin zone center. In this work, we propose that the paraelectric—ferroelectric transition in the fluorite-type Pca21 ferroelectric family can be described by a Pcca—Pca21 transition, where the Pcca mother phase will evolve into either the Pca21 ferroelectric phase or the centrosymmetric P21/c monoclinic phase, depending on the strain conditions. The Pcca phase is directly linked to both phases in the context of continuous phase transition. Hafnia is regarded as a special case of this family in that it has accidental atomic degeneracy because all anions are oxygen. The theory is also correlated with the seven-coordination theory that explains the ferroelectricity in hafnia from a chemical perspective. In addition, the strain conditions to promote the ferroelectric phase in hafnia are discussed.

Three-dimensional topological crystalline insulator without spin–orbit coupling in nonsymmorphic photonic metacrystal

Abstract By including certain point group symmetry in the classification of band topology, Fu proposed a class of three-dimensional topological crystalline insulators (TCIs) without spin–orbit coupling in 2011. In Fu’s model, surface states (if present) doubly degenerate at Γ ¯ and M ¯ when time-reversal and C 4 symmetries are preserved. The analogs of Fu’s model with surface states quadratically degenerate at M ¯ are widely studied, while surface states with quadratic degeneracy at Γ ¯ are rarely reported. In this study, we propose a three-dimensional TCI without spin–orbit coupling in a judiciously designed nonsymmorphic photonic metacrystal. The surface states of photonic TCIs exhibit quadratic band degeneracy in the (001) surface Brillouin zone (BZ) center ( Γ ¯ point). The gapless surface states and their quadratic dispersion are protected by C 4 and time-reversal symmetries, which correspond to the nontrivial band topology characterized by Z 2 topological invariant. Moreover, the surface states along lines from Γ ¯ to the (001) surface BZ boundary exhibit zigzag feature, which is interpreted from symmetry perspective by building composite operators constructed by the product of glide symmetries with time-reversal symmetry. The metacrystal array surrounded with air possesses high order hinge states with electric fields highly localized at the hinge that may apply to optical sensors. The gapless surface states and hinge states reside in a clean frequency bandgap. The topological surface states emerge at the boundary of the metacrystal and perfect electric conductor (PEC), which provide a pathway for topologically manipulating light propagation in photonic devices.

Ultrafast Polarization-Resolved Phonon Dynamics in Monolayer Semiconductors.

Monolayer transition metal dichalcogenide semiconductors exhibit unique valleytronic properties interacting strongly with chiral phonons that break time-reversal symmetry. Here, we observed the ultrafast dynamics of linearly and circularly polarized E'(Γ) phonons at the Brillouin zone center in single-crystalline monolayer WS2, excited by intense, resonant, and polarization-tunable terahertz pulses and probed by time-resolved anti-Stokes Raman spectroscopy. We separated the coherent phonons producing directional sum-frequency generation from the incoherent phonon population emitting scattered photons. The longer incoherent population lifetime than what was expected from coherence lifetime indicates that inhomogeneous broadening and momentum scattering play important roles in phonon decoherence at room temperature. Meanwhile, the faster depolarization rate in circular bases than in linear bases suggests that the eigenstates are linearly polarized due to lattice anisotropy. Our results provide crucial information for improving the lifetime of chiral phonons in two-dimensional materials and potentially facilitate dynamic control of spin-orbital polarizations in quantum materials.

Hidden symmetry-induced effective moving double-zero-index metamaterials.

Materials possessing an effective zero refractive index are often associated with Dirac-like cone dispersion at the center of the Brillouin zone (BZ). It has been reported the presence of hidden symmetry-enforced triply degenerate points [nexus points (NP)] away from the Brillouin zone center with the stacked dielectric photonic crystals. The spin-1 Dirac-like dispersion in the xy plane near the nexus point suggests a method for achieving zero refractive index materials. The stacked photonic crystals at the nexus points can be deemed as an effective moving double-zero-index medium (MDZIM) traveling with a speed relative to the laboratory reference. The ability of this moving double-zero-index medium to enable perfect wave tunneling across barriers without reflection has been demonstrated, dependent on the incident waves' specific angular orientations.

L -gap surface resonance at Pt(111): Influence of atomic structure, d bands, and spin-orbit interaction

Pt(111) hosts a surface resonance with peculiar properties concerning energy vs momentum dispersion and spin texture. At variance with the free-electron-like behavior of the L-gap Shockley-type surface states on the fcc(111) surfaces of Au, Ag, and Cu, it splits into several branches with distinct spin polarization around the center of the surface Brillouin zone Γ¯. Theoretical predictions based on density-functional theory vary depending on the particular functionals used. To clarify this issue, we investigate the atomic structure of Pt(111) by low-energy electron diffraction and the unoccupied electronic structure by spin- and angle-resolved inverse photoemission. The experimental results are backed by theoretical studies using different functionals, which show that the characteristics of the surface band depend critically on the lattice constant. From the analysis of the energy-dependent low-energy electron diffraction intensities, we derive structural parameters of the Pt(111) surface relaxation with high accuracy. In addition, we give an unambiguous definition of the nonequivalent mirror-plane directions Γ¯M¯ and Γ¯M¯′ at fcc(111) surfaces, which is consistent with band-structure calculations and inverse-photoemission data. Concerning the surface resonance at the bottom of the L gap, we identified a delicate interplay of several contributions. Lattice constant, hybridization with d bands, and the influence of spin-orbit interaction are critical ingredients for understanding the peculiar energy dispersion and spin character of the unoccupied surface resonance. Published by the American Physical Society 2024

Light emission from ion-implanted SiGe quantum dots grown on Si substrates

We report on electroluminescence spectroscopy experiments demonstrating room-temperature light emission from heavily alloyed SiGe quantum dots, for which the light emission properties are enhanced by incorporated split-[110] self-interstitials. The quantum dots are formed during molecular beam epitaxy deposition of Si0.6Ge0.4 alloys on n-doped silicon-on-insulator substrates. To create the split-[110] self-interstitials the quantum dots were co-implantated in-situ using Si and Ge ions. The hybrid emitters were further embedded into the intrinsic region of a p-i-n diode structure to enable electrical pumping. Similar to previous theoretical results on unstrained Ge-based quantum dots containing these defects, radiative direct transitions are possible for these SiGe light emitters. However, in SiGe dots these transitions are not at the Brillouin zone center. Instead, first-principles calculations indicate that the presence of the split-[110] self-interstitial defect in strained and unstrained SiGe can lead to optically direct transitions in momentum space in the X-direction of the Brillouin zone.

Bound states in the continuum in divided triangular hole metasurfaces

We investigate the bound states in the continuum (BICs) in dielectric metasurfaces consisting of a two-part divided triangular hole in the unit cell of a square lattice, with emphasis on the generation, splitting, and merging of BICs. At the smallest height ratio between the upper triangular and the lower trapezoidal holes, the accidental BIC with an extremely large quality factor emerges on an isolated dispersion band at the Brillouin zone center, which is recognized as a polarization singularity (V point) with an integer topological charge. As the height ratio increases, the accidental BIC is split into a pair of circularly polarized states, which are polarization singularities (C points) with half-integer topological charges. The two states depart from each other to a maximum distance, and then approach each other as the height ratio continues to change. They finally merge to another polarization singularity (V point) with an integer topological charge, which is identified as the Friedrich-Wintgen BIC that occurs near the avoided crossing between two interacting dispersion bands.

Mechanical and thermodynamic stability of the high melting point and hardness metallic anti-perovskite Cr[formula omitted]GeN

Utilizing density functional and perturbation theory, combined with ab initio molecular dynamics methods, we investigate the phonon and vibrational properties, mechanical and thermodynamic characteristics, as well as melting points of the tetragonal and cubic phases of anti-perovskite nitride Cr3GeN. Our findings demonstrate that the cubic phase is stable at high temperatures but exhibits the softening of the optical phonon mode at the Brillouin zone center at low temperatures, thereby transforming into the tetragonal phase. Both the tetragonal and cubic phases exhibit mechanical robustness and satisfy the conditions for mechanical stability. As a metallic compound, the anti-perovskite Cr3GeN additionally possesses high hardness and melting point, thus stimulating high hardness and temperature applications.

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Semiconductors have a valence band consisting of multi-subbands. Inter-subband interaction causes multiple degenerate points near the Brillouin zone center Γ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Gamma$$\\end{document} even when the system exhibits space-reversal asymmetry. In the present study, we analyzed the effect of the existence of multiple degenerate points on topological features using Berry parameter investigation. For reasonable generalization, we propose a model Hamiltonian that represents multiple degeneracies in a two-dimensional quantum-well system. We also explored “topological” modifications through numerical calculations of the effective magnetic field and Berry’s connection and curvature. We found that the energy dependence of the Berry phase is affected by the interaction between degenerate points.Graphical abstract

Plasmon dispersion in bilayer cuprate superconductors

The essential building blocks of cuprate superconductors are two-dimensional CuO2 sheets interspersed with charge reservoir layers. In bilayer cuprates, two closely spaced CuO2 sheets are separated by a larger distance from the subsequent pair in the next unit cell. In contrast to single-layer cuprates, prior theoretical work on bilayer systems has predicted two distinct acoustic plasmon bands for a given out-of-plane momentum transfer. Here we report random phase approximation (RPA) calculations for bilayer systems which corroborate the existence of two distinct plasmon bands. We find that the intensity of the lower-energy band is negligibly small in most parts of the Brillouin zone, whereas the higher-energy band carries significant spectral weight. We also present resonant inelastic x-ray scattering (RIXS) experiments at the O K-edge on the bilayer cuprate Y0.85Ca0.15Ba2Cu3O7 (Ca-YBCO), which show only one dispersive plasmon branch, in agreement with the RPA calculations. In addition, the RPA results indicate that the dispersion of the higher-energy plasmon band in Ca-YBCO is not strictly acoustic but exhibits a substantial energy gap of approximately 250 meV at the two-dimensional Brillouin zone center. Published by the American Physical Society 2024

Acoustic and optical phonons in quasi-two-dimensional MPS3 antiferromagnetic semiconductors

We report the results of the investigation of the acoustic and optical phonons in quasi-two-dimensional antiferromagnetic semiconductors of the transition metal phosphorus trisulfide family with Mn, Fe, Co, Ni, and Cd as metal atoms. The Brillouin–Mandelstam and Raman light scattering spectroscopies were conducted at room temperature to measure the acoustic and optical phonon frequencies close to the Brillouin zone center and the Γ−A high symmetry direction. The absorption and index of refraction were measured in the visible and infrared ranges using the reflectometry technique. We found an intriguing large variation, over ∼28%, in the acoustic phonon group velocities in this group of materials with similar crystal structures. Our data indicate that the full-width-at-half-maximum of the acoustic phonon peaks is strongly affected by the optical properties and the electronic bandgap. The acoustic phonon lifetime extracted for some of the materials was correlated with their thermal properties. The results are important for understanding the layered van der Waals semiconductors and assessing their potential for optoelectronic and spintronic device applications.

Moiré Pattern Controlled Phonon Polarizer Based on Twisted Graphene.

Twisted van der Waals materials featuring Moiré patterns present new design possibilities and demonstrate unconventional behaviors in electrical, optical, spintronic, and superconducting properties. However, experimental exploration of thermal transport across Moiré patterns has not been as extensive, despite its critical role in nanoelectronics, thermal management, and energy technologies. Here, thefirst experimental study is conducted on thermal transport across twisted graphene, demonstrating a phonon polarizer concept from the rotational misalignment between stacked layers. The direct thermal and acoustic measurements,structural characterizations, and atomistic modeling, reveal a modulation up to 631% in thermal conductance with various Moiré angles, while maintaining a high acoustic transmission. By comparing experiments with density functional theory and molecular dynamics simulations, mode-dependent phonon transmissions are quantified based on the angle alignment of graphene band structures and attributed to the coupling among flexural phonon modes. The agreement confirms the dominant tuning mechanisms in adjusting phonon transmission from high-frequency thermal modes while having negligible effects on low-frequency acoustic modes near Brillouin zone center. This study offers crucial insights into the fundamental thermal transport in Moiré structures, opening avenues for the invention of quantum thermal devices and new design methodologies based on manipulations of vibrational band structures and phonon spectra.

Evolution of structural and electronic properties standardized description in rhenium disulfide at the bulk-monolayer transition

The structural and electronic properties of ReS2 different forms — three-dimensional bulk and two-dimensional monolayer — were studied within density functional theory and pseudopotentials. A method for standardizing the description of bulk unit cells and "artificial" slab unit cells for DFT research has been proposed. The preference of this method for studying zone dispersion has been shown. The influence of the vacuum layer thickness on specified special high-symmetry points is discussed. Electron band dispersion in both classical 3D Brillouin zones and transition to 2D Brillouin zones in the proposed two-dimensional approach using the Niggli form of the unit cell was compared. The proposed two-dimensional approach is preferable for low-symmetry layered crystals such as ReS2. It was established that the bulk ReS2 is a direct gap semiconductor (band gap of 1.20 eV), with the direct transition lying in the X point of the first Brillouin zone, and it is in good agreement with published experimental data. The reduction in material dimension from bulk to monolayer was conducted with an increasing band gap up to 1.45 eV, with a moving direct transition towards the Brillouin zone center. The monolayer of ReS2 is a direct-gap semiconductor in a wide range of temperatures, excluding only a narrow range at low temperatures, where it comes as a quasi-direct gap semiconductor. The transition, situated directly in the Γ-point, lies 3.3 meV below the first direct transition located near this point. The electronic density of states of ReS2 in the bulk and monolayer cases of ReS2 were analyzed. The molecular orbitals were built for both types of ReS2 structures as well as the electron difference density maps. For all types of ReS2 structures, an analysis of populations according to Mulliken and Voronoi was carried out. All calculated data is discussed in the context of weak quantum confinement in the 2D case.