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Phononic crystal band gaps (BGs), which are realized by Bragg scattering, have a central frequency and width related to the unit cell's size and the impedance mismatch between material phases. BG tuning has generally been performed by either trial and error or by computational tools such as topology optimization. In either case, understanding how to systematically change the design for a particular band structure is missing. This paper addresses this by closely studying the displacement modes within the wavebands that are responsible for the BG. We look at the variation in different displacement modes due to the changes in the geometry and correlate these changes to their corresponding band structures. We then use this insight to design the unit cell for a particular application, for instance, for generating partial BGs.

A novel photonics-based technique of microwave frequency measurement (MFM) is proposed and verified. The method is implemented by introducing a parameter called Brillouin phase-gain ratio (BPGR) which combines Brillouin phase shift and Brillouin gain, where BPGR features a linear response to the unknown signal frequency. Moreover, it enables to enlarge the channel bandwidth of frequency measurement relative to the method of applying only the phase shift spectrum slope or gain spectrum slope, and the important problem of pump-power-dependency affecting system performance can be alleviated. The experiment is established to verify its feasibility and the results show that this method is competent for frequency measurement with very high accuracy. In the end, within two frequency bands of linear slope 0.12-0.17 and 10.12-10.17 GHz, the measurement error below 5 MHz is successfully achieved.

Floquet-Bloch lattices are systems in which wave packets are subjet to periodic modulations both in time and space, showing rich dynamics. While this type of lattice is difficult to implement in solid-state physics, optical systems have provided excellent platforms to probe their physics: among other effects, they have revealed genuine phenomena such as the anomalous Floquet topological insulator and the funnelling of light into localised interface modes. Despite the crucial importance of the band dispersion in the photon dynamics and the topological properties of the lattice, the direct experimental measurement of the Floquet-Bloch bands has remained elusive. Here we report the direct measurement of the Floquet-Bloch bands of a photonic lattice with a single shot method. We use a system of two coupled fibre rings that implements a time-multiplexed Floquet-Bloch lattice. By Fourier transforming the impulse response of the lattice we obtain the band structure together with an accurate characterization of the lattice eigenmodes, i. e. the amplitudes and the phases of the Floquet-Bloch eigenvectors over the entire Brillouin zone. Our results open promising perspectives for the observation of topological effects in the linear and nonlinear regime in Floquet systems. Floquet-Bloch lattices are systems that are relatively difficult to implement in solid-state physics. Here, using a photonic lattice as an alternative, the authors experimentally observe Floquet-Bloch bands using a single-shot method which allows them to fully characterise the lattice eigenmode structure.

This article re-examines the Brillouin flow solutions in crossed-field diodes, with applications to magnetrons, magnetically insulated line oscillators (MILOs), and magnetically insulated transmission lines (MITLs). The Brillouin flow solutions are constructed for various geometries, including planar magnetrons, MILOs, and MITLs, cylindrical magnetrons with electrons flowing in the azimuthal direction, cylindrical MITLs and MILOs with electrons flowing in the axial direction, and radial MITLs and MILOs with electrons flowing in the radial direction. A common theme of this analysis is that two main external parameters are used to characterize the Brillouin flow: the anode–cathode voltage ( $V_{a}$ ) and the total magnetic flux within the crossed-field diodes ( $A_{a}$ ). These two parameters are equivalent to the gap voltage and a specification of the degree of magnetic insulation, which is approximately equal to the ratio of the magnetic field to the Hull cutoff (HC) magnetic field. The magnetic flux may be provided externally by a magnet (as in a magnetron) or by the wall currents without an external magnet (as in a MILO or MITL), or by some combination of the two, as in the intermediate case of a magnetron–MILO hybrid. Once these two parameters are specified, the electron flow speed at the top of the Brillouin hub is uniquely determined. This immediately yields the Buneman–Hartree (BH) condition according to the Brillouin flow model, whether it be a planar magnetron or a cylindrical MILO. In so doing, we have obtained, for the first time using the Brillouin flow model, the BH condition for a cylindrical MILO, and we show that the same condition is obtained from the single-particle orbit model. We also found that, in general, the electron current within the Brillouin hub contributes only to a very small fraction of the magnetic flux $A_{a}$ , regardless of the gap voltage $V_{a}$ , thereby correcting an erroneous notion that the electron flow within the crossed-field gap could be responsible for the magnetic insulation. Another counter-intuitive finding is that, for a given degree of magnetic insulation, the Brillouin hub height decreases as the gap voltage $V_{a}$ increases. These conclusions, and other results, were based on the simple, explicit analytic expressions that we have obtained for the Brillouin flow profiles, including the velocity, electron density, as well as the self-magnetic field and the self-electric field profiles due to the Brillouin hub electrons. From these analytical expressions, we deduce useful scaling laws that are applicable to the prevailing cases where the magnetic field exceeds 1.5 times the HC magnetic field, and they are valid in both relativistic and non-relativistic regimes. Thus, these scaling laws show the contrast between magnetrons and MILOs, and a ready assessment of the viability of building a moderate-current MILO, a low-voltage MILO, and a magnetron–MILO hybrid which might combine the advantages of a magnetron and a MILO. The Brillouin flow profiles in a radial MITL are explicitly calculated. Additional issues are addressed.

The influence of a high (~1 V/nm) external electric field on biological soft matter systems has been widely studied for a long time. However, a detailed understanding of the behavior of density fluctuations in such systems under a high electric field is still an open problem. Here, we develop a statistical model of short-wavelength density fluctuations in the hydrophobic region of a lipid bilayer in an external quasistatic uniform electric field. Using this model, we derive the Brillouin components of the dynamic structure factor of the system from first principles. We also compare the results of the developed theory with new results of molecular dynamics simulations on the initiation of lipid bilayer electropores to show a nearly perfect agreement. We show that the presented theory could be applied to handle a broad range of similar soft matter systems as well as nonpolar dielectric fluids.

The spectral properties of an ensemble of spin-polaron quasiparticles have been studied within the spin–fermion model of cuprate superconductors using the method combining the Feynman diagram technique and the diagram technique for spin operators. It has been shown that strong spin–charge coupling results in the formation of the lower spin-polaron band separated by a wide energy gap from the band of bare holes. It has been shown that the spin-polaron band has a local minimum near the (π/2, π/2) point of the Brillouin zone. A class of diagrams for the self-energy part that have a fundamental significance for the description of the main features of the spin-polaron spectrum has been determined.

The section of the Bilbao Crystallographic Server (https://www.cryst.ehu.es/) dedicated to subperiodic groups contains crystallographic and Brillouin‐zone databases for the layer groups. The crystallographic databases include the generators/general positions (GENPOS), Wyckoff positions (WYCKPOS) and maximal subgroups (MAXSUB). The Brillouin‐zone database (LKVEC) offers k‐vector tables and Brillouin‐zone figures of all 80 layer groups which form the background of the classification of their irreducible representations. The symmetry properties of the wavevectors are described applying the so‐called reciprocal‐space‐group approach and this classification scheme is compared with that of Litvin & Wike [(1991), Character Tables and Compatibility Relations of the Eighty Layer Groups and Seventeen Plane Groups. New York: Plenum Press]. The specification of independent parameter ranges of k vectors in the representation domains of the Brillouin zones provides a solution to the problems of uniqueness and completeness of layer‐group representations. The Brillouin‐zone figures and k‐vector tables are described in detail and illustrated by several examples., The set of databases of the Bilbao Crystallographic Server (https://www.cryst.ehu.es/) including crystallographic data on generators, general positions, Wyckoff positions, maximal subgroups and Brillouin‐zone figures and k‐vector tables for all 80 layer groups are discussed in detail and illustrated.

Temperature-dependent angle-resolved photoemission spectroscopy (ARPES) was carried out on single-crystalline EuB6 samples. By measuring ARPES spectra in an extended Brillouin zone, a B 2p hole pocket centered at the X point is clearly observed, thus proving the semimetallic nature of EuB6. Below the Curie temperature TC, ARPES spectra show two B 2p bands of which separation is due to an exchange interaction between local Eu 4f and itinerant B 2p electrons. The exchange splitting becomes smaller as the temperature increases and disappears well above TC. Additionally, a diffuse structure near the Fermi level survives just above TC. Such behavior is well described by Monte Carlo simulations of a Kondo lattice model, thus supporting the formation of magnetic polarons in EuB6, which accounts for the resistivity upturn near above TC when lowering the temperature., Comment: 6 pages, 5 figures

Twisted bilayer graphene is a fascinating system due to the possibility of tuning the electronic and optical properties by controlling the twisting angle $$\theta$$ θ between the layers. The coupling between the Dirac cones of the two graphene layers gives rise to van Hove singularities (vHs) in the density of electronic states, whose energies vary with $$\theta$$ θ . Raman spectroscopy is a fundamental tool to study twisted bilayer graphene (TBG) systems since the Raman response is hugely enhanced when the photons are in resonance with transition between vHs and new peaks appear in the Raman spectra due to phonons within the interior of the Brillouin zone of graphene that are activated by the Moiré superlattice. It was recently shown that these new peaks can be activated by the intralayer and the interlayer electron–phonon processes. In this work we study how each one of these processes enhances the intensities of the peaks coming from the acoustic and optical phonon branches of graphene. Resonance Raman measurements, performed in many different TBG samples with $$\theta$$ θ between $$4^{\circ }$$ 4 ∘ and $$16^{\circ }$$ 16 ∘ and using several different laser excitation energies in the near-infrared (NIR) and visible ranges (1.39–2.71 eV), reveal the distinct enhancement of the different phonons of graphene by the intralayer and interlayer processes. Experimental results are nicely explained by theoretical calculations of the double-resonance Raman intensity in graphene by imposing the momentum conservation rules for the intralayer and the interlayer electron–phonon resonant conditions in TBGs. Our results show that the resonant enhancement of the Raman response in all cases is affected by the quantum interference effect and the symmetry requirements of the double resonance Raman process in graphene.

Spin waves can transfer information free of electron transport and are promising for wave-based computing technologies with low-power consumption as a solution to severe energy losses in modern electronics. Logic circuits based on the spin-wave interference have been proposed for more than a decade, while it has yet been realized at the nanoscale. Here, we demonstrate the interference of spin waves with wavelengths down to 50 nm in a low-damping magnetic insulator. The constructive and destructive interference of spin waves is detected in the frequency domain using propagating spin-wave spectroscopy, which is further confirmed by the Brillouin light scattering. The interference pattern is found to be highly sensitive to the distance between two magnetic nanowires acting as spin-wave emitters. By controlling the magnetic configurations, one can switch the spin-wave interferometer on and off. Our demonstrations are thus key to the realization of spin-wave computing system based on nonvolatile nanomagnets.

We present results of the analysis of Brillouin light-scattering (BLS) measurements of spin waves performed on ultrathin single and multirepeat CoFeB layers with adjacent heavy metal layers. From a detailed study of the spin-wave dispersion relation, we independently extract the Heisenberg exchange interaction (also referred to as symmetric exchange interaction), the Dzyaloshinskii–Moriya interaction (DMI, also referred to as antisymmetric exchange interaction), and the anisotropy field. We find a large DMI in CoFeB thin films adjacent to a Pt layer and nearly vanishing DMI for CoFeB films adjacent to a W layer. Furthermore, the influence of the dipolar interaction on the dispersion relation and on the evaluation of the Heisenberg exchange parameter is demonstrated. Eventually, an experimental analysis of the impact of the DMI on the spin-wave lifetime is presented.

Low-temperature (5 K) microphotoluminescence has been measured in order to study individual luminescent centers formed by the cores of partial dislocations in a CdTe/Si film and a ZnSe quantum well. It has been shown that these centers are characterized by a noticeable interaction with longitudinal optical (LO) phonons, whose quasimomentum corresponds to the center of the Brillouin zone. At the same time, the relative intensities of LO- and 2LO-phonon replicas differ by an order of magnitude from the values predicted by the Huang–Rhys formula, which describes the intensities of multi-LO-phonon replicas for impurity luminescent centers. The nonstandard intensity distribution of phonon replicas is attributed to a change in the curvature of the adiabatic potential (frequency defect), which is due to the instability of the bond structure in the dislocation core with respect to the state of the electronic subsystem.

Electronic structure in the normal state and properties in the superconducting state of the orthorhombic phase in HTSC cuprates are studied within the multielectron generalized tight-binding (GTB) approach. The joint effect of variation of average Cu-O distance and orthorhombic distortion on the Fermi contour, band structure, and concentration dependence of $${T_c}$$ is studied. Quasiparticle excitations were constructed within the framework of the five-band p-d model for the layer of the CuO $$_6$$ octahedra. The electronic structure of quasiparticle excitations in the effective Hubbard model is calculated using the equation of motion for Green’s functions in the generalized mean-field approximation. Orthorhombic distortion leads to asymmetric with respect to the nodal direction of the Brillouin zone dispersion surface of quasiparticle excitations and to splitting of each van Hove singularity into two peaks. Transformation of the Fermi contour from hole pockets to the large hole and electron pockets occurs as a result of two quantum-phase transitions at dopings $${x_{c1}}$$ and $${x_{c2}}$$ . Simultaneous average Cu-O distance elongation and orthorhombic distortion decreasing result in $$T_c$$ decrease. $$T_c$$ dependence on average Cu-O distance in the orthorhombic phase is in agreement with the behavior of experimental $${T_{c\max }}$$ values when the DOS effect on $$T_c$$ prevails over the effect of the exchange parameter. There are two $$T_c$$ maxima in the orthorhombic system with the suppressed exchange parameter, and these maxima appear at concentrations $${x_{c1}}$$ and $${x_{c2}}$$ .

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.

Many of graphene’s remarkable properties arise from its linear dispersion of the electronic states, forming a Dirac cone at the K points of the Brillouin zone. Silicene, the 2D allotrope of silicon, is also predicted to show a similar electronic band structure, with the addition of a tunable bandgap, induced by spin–orbit coupling. Because of these outstanding electronic properties, silicene is considered as a promising building block for next-generation electronic devices. Recently, it has been shown that silicene grown on Au(111) still possesses a Dirac cone, despite the interaction with the substrate. Here, to fully characterize the structure of this 2D material, we investigate the vibrational spectrum of a monolayer silicene grown on Au(111) by polarized Raman spectroscopy. To enable a detailed ex situ investigation, we passivated the silicene on Au(111) by encapsulating it under few layers hBN or graphene flakes. The observed spectrum is characterized by vibrational modes that are strongly red-shifted with respect to the ones expected for freestanding silicene. By comparing low-energy electron diffraction (LEED) patterns and Raman results with first-principles calculations, we show that the vibrational modes indicate a highly (>7%) biaxially strained silicene phase., This work was funded by the Fonds zur Förderung der Wissenschaftlichen Forschung (FWF), Austria (Project P29244-N27). We also acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under Grant FEDER-MAT2017-90024-P and the Severo Ochoa Centres of Excellence Program under Grant CEX2019-000917-S, and by the Generalitat de Catalunya under Grant 2017 SGR 1506. The project i-LINK action LINKA20047 funded by CSIC is also acknowledged for financial support. R.R. acknowledges useful discussions with Mariusz Krawiec. CzechNanoLab Project LM2018110 funded by MEYS CR is gratefully acknowledged for the financial support of the measurements at CEITEC Nano Research Infrastructure. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant JPMXP0112101001, JSPS KAKENHI Grant JP20H00354, and the CREST(JPMJCR15F3), JST.

The different translational symmetry of the cuprite crystal sublattices is taken into account by unfolding the phonon spectrum from the Brillouin zone of the crystal into the Brillouin zones of the anion and metal. Such a representation of the spectrum is reasonable because 6 optical branches are essentially caused by the vibrations of anions and 12 acoustic branches are due to the vibrations of metal owing to the great value difference between their masses. The volumes of the Brillouin zones of the anion and metal sublattices are two and four times, respectively, as large as those of the crystal resulting in the qualitative differences in the corresponding unfolded spectra because of the metal tetrahedron vibrations of the distortion type in the acoustic part of the spectrum. These vibrations located on the Х′(1,0,0)–W′(1,0,1/2) and Х′(0,1,0)– W′(0,1,1/2) lines of the metal Brillouin zone are responsible for the negative thermal expansion. The unfolded spectra have only three branches in the corresponding Brillouin zones and this simplifies the selection rules for the physical processes with the participation of the sublattice particle vibrations.

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr3 crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties. Neutron and X-ray scattering measurements provide further insight into the anharmonic behaviour of lead halide perovskites, revealing that rotations of PbBr6 octahedra in CsPbBr3 crystals occur in a correlated fashion along two-dimensional planes.

Metamaterial with various degrees of bandgap tunability is an emerging area in manipulating elastic wave transmission characteristics for next generation phononic devices. Although several attempts are made employing multi-fields such as magnetic, electric and thermal etc., mechanical deformation based tunable bandgap is a major interest. However, achieving tunability in terms of broadening or terminating the bandgap in locally resonant acoustic metamaterial (LRAM) remains as challenging tasks. In this work, we propose LRAM by integrating a two-dimensionally periodically arranged square unit cells consisting of soft coated hard inclusion embedded in a polymer scaffold. We develop computational framework to simulate the low frequency bandgap of unit cell under external deformation. At first quasi-static analysis of unit cell has been performed to find static displacement field under prescribed deformation. Further, utilizing Bloch-Floquet wave analysis for the periodic unit cell and finite element based numerical scheme, an eigenvalue problem is formulated to predict dispersion response within irreducible Brillouin zone (IBZ). Analyzing the symmetry group of the unit cell under deformed state for a series of loading, we extract the bandgap responses. Moreover, we illustrate tunable bandgap at low frequency regimes for different inclusion shapes and sizes. To correlate extension and suppression of bandgap under the applied deformation, evolution of local resonances are described with vibration mode shapes.

The coupled nonequilibrium dynamics of electrons and phonons in monolayer MoS2 is investigated by combining first-principles calculations of the electron-phonon and phonon-phonon interactions with the time-dependent Boltzmann equation. Strict phase-space constraints in the electron-phonon scattering are found to influence profoundly the decay path of excited electrons and holes, restricting the emission of phonons to crystal momenta close to a few high-symmetry points in the Brillouin zone. As a result of momentum selectivity in the phonon emission, the nonequilibrium lattice dynamics is characterized by the emergence of a highly anisotropic population of phonons in reciprocal space, which persists for up to 10 ps until thermal equilibrium is restored by phonon-phonon scattering. Achieving control of the nonequilibrium dynamics of the lattice may provide unexplored opportunities to selectively enhance the phonon population of two-dimensional crystals and, thereby, transiently tailor electron-phonon interactions over subpicosecond time scales.

Moire superlattices in transition metal dichalcogenide (TMD) heterostructures can host novel correlated quantum phenomena due to the interplay of narrow moire flat bands and strong, long-range Coulomb interactions1–9. However, microscopic knowledge of the atomically reconstructed moire superlattice and resulting flat bands is still lacking, which is critical for fundamental understanding and control of the correlated moire phenomena. Here we quantitatively study the moire flat bands in three-dimensional (3D) reconstructed WSe2/WS2 moire superlattices by comparing scanning tunnelling spectroscopy (STS) of high-quality exfoliated TMD heterostructure devices with ab initio simulations of TMD moire superlattices. A strong 3D buckling reconstruction accompanied by large in-plane strain redistribution is identified in our WSe2/WS2 moire heterostructures. STS imaging demonstrates that this results in a remarkably narrow and highly localized K-point moire flat band at the valence band edge of the heterostructure. A series of moire flat bands are observed at different energies that exhibit varying degrees of localization. Our observations contradict previous simplified theoretical models but agree quantitatively with ab initio simulations that fully capture the 3D structural reconstruction. Our results reveal that the strain redistribution and 3D buckling in TMD heterostructures dominate the effective moire potential and the corresponding moire flat bands at the Brillouin zone K points. Scanning tunnelling spectroscopy and ab initio simulations reveal buckling reconstruction and in-plane strain redistribution in WSe2/WS2 moire heterostructures.

The results of detection of resonant amplification of light scattering intensity by optical phonons in a homoepitaxial nanoscale 70 nm thick n-GaP layer grown by gas-phase epitaxy from organometallic compounds on a conducting strongly doped n-GaP crystal substrate oriented along the (001) axis have been presented. It has been shown that it is possible to detect rather narrow bands of second-order light scattering lines in the frequency range from 600 to 800 cm–1 at room temperature in the Raman light scattering spectrum of such a (001) n-GaP nanolayer in the n-GaP/n-GaP (001) sample in comparison with those in the spectrum of a high-resistance (001) si-GaP crystal sample. It has been established that such bands are caused by the total combinations and overtones of transverse TO(Γ) and longitudinal LO(Γ) optical phonons with wave vectors corresponding to the points Σ, K, X, L, and Γ of the Brillouin zone of a GaP crystal. It has been shown that the light scattering is resonant and caused by the presence of impurities because of the exciton-phonon interaction.

Band crossings observed in a wide range of condensed matter systems are recognized as a key to understand low-energy fermionic excitations that behave as massless Dirac particles. Despite rapid progress in this field, the exploration of non-equilibrium topological states remains scarce and it has potential ability of providing a new platform to create unexpected massless Dirac states. Here we show that in a semiconductor quantum-well driven by a cw-laser with linear polarization, the optical Stark effect conducts bulk-band crossing, and the resulting Floquet-Dirac semimetallic phase supports an unconventional edge state in the projected one-dimensional Brillouin zone under a boundary condition that an electron is confined in the direction perpendicular to that of the laser polarization. Further, we reveal that this edge state mediates a transition between topological and non-topological edge states that is caused by tuning the laser intensity. We also show that the properties of the edge states are strikingly changed under a different boundary condition. It is found that such difference originates from that nearly fourfold-degenerate points exist in a certain intermediate region of the bulk Brillouin zone between high-symmetry points.

The thermal transport behavior of single crystal LiNbO3 and LiTaO3 is crucially important in the application of laser devices, however, the underlying sciences of thermal conductivity of both crystals are still unknown. In present study, we explore the origin of different thermal conductivity between LiNbO3 and LiTaO3 based on first-principle calculation. We find that the remarkable contribution of optical phonon branches to the total thermal conductivity stems from the high phonon group velocity at high frequency range (500-900 cm-1), and the anisotropic behavior of thermal conductivity can be regarded as the different value of phonon velocity along c- and a-axis at the low frequency range (0-400 cm-1). As the decisive factor in thermal conductivity, the results of phonon lifetime indicate that large value of acoustic phonon relaxation time in LiTaO3 are responsible for the higher thermal conductivity compared to LiNbO3. The phonon-phonon scattering rates of three acoustic phonon branches reveal that more scattering channels are existed throughout the Brillouin zone for LiNbO3 than that of LiTaO3. Additionally, the unusually large phonon scattering rate of high-lying LA branch along the Γ-F direction indicates more emission channels of LiNbO3, which is considered as another origin of the different phonon anharmonicity and thermal conductivity between LiNbO3 and LiTaO3.

Electron transport in graphene is dominated by its Dirac-like charge carriers. Grain boundaries add a geometric aspect to the transport behavior by coupling differently oriented grains. In the phase coherent limit this aspect allows to relate the transport properties to two factors: the electronic structure of individual grains around the Dirac points and the orientation relation of the Dirac cones within the grain boundary Brillouin zone. Based on this picture it is possible to quantify the size and strain modulation of transport gaps without the need for explicit transport calculations within the non-equilibrium Green functions formalism. In this work we present a semi-analytical method that exploits this picture. Our method can explore arbitrary grain misorientations in the presence of an external strain providing valuable information about the electronic properties of individual grain boundaries.

The mechanical properties of organic molecular solids have progressed well beyond simple brittle fracture to encompass a broad spectrum of unique behaviors: flexibility in one- and two-dimensions, twisting, bending and even jumping. Design models are under construction to help navigate property expression through crystal engineering, though no clear consensus is available on how structural and energetic (interaction anisotropy) factors combine to generate a specific mechanical response. Subtle differences in intermolecular strength or spatial organization have significant impact on material mechanics, and while diffraction and computational data have been instrumental to recent success, a need has developed for new tools that better quantify the interaction anisotropy. Herein we introduce a unique application of powder Brillouin light scattering (p-BLS) to assess the elastic anisotropy of molecular solids. We hypothesize that the width of the longitudinal frequency distribution directly correlates with a material's elastic anisotropy. We introduce a new characteristic frequency (νL,peak) and define our elastic anisotropy index (EAI) as the ratio of νL,max/νL,peak. This EAI is validated against a library of 24 molecular solids with experimentally determined stiffness tensors (Cij) used to calculate previously reported EAIs. Using energy frameworks and sound velocity polar plots, the elastic anisotropy from p-BLS is extended as an experimental window into the interaction anisotropy in organic molecular solids. We conclude with an application of p-BLS to mechanically responsive solids – 3,4-dichlorophenol and 4-bromo-3-chlorophenol – to discriminate elastic flexibility and plastic bending from the perspective of interaction anisotropy.

Ideal topological materials are those stable materials with less nontrivial band crossing near the Fermi surface and a long Fermi arc. By means of first-principles calculations, here we present that the 3D monochalcogenide molybdenum telluride (Pm-MoTe) without an inversion center shows a type-II Weyl semimetal (WSM) phase which cannot checked by symmetry index method. A total of eight Weyl points (WPs) are found in different quadrants of the Brillouin zone (BZ) of Pm-MoTe, which guarantee a long Fermi arc. The WSM phase is robust against the spin–orbit coupling (SOC) effect because of mirror symmetry and time reversal symmetry. It is also found that a topological phase transition can be tuned by strain. For different types of strain, the number of WPs can be effectively modulated to a minimum number, and their energies could be closer to Fermi level. These findings propose a promising material candidate that partly satisfies the ideal WSM criteria and extends the potential applications of the tunable topological phase.

The exceptional thermoelectric performance of PbTe is partially due to the high valley degeneracy, NV, of the band edges. In this paper, we specifically look at the valence band (VB) structure of PbTe, which has been described using a two-band model consisting of a light valence band with a maximum at the L-point in the first Brillouin zone and a heavy band with its maximum along the Σ-line. The light and heavy band extrema are located at low symmetry points, resulting in valley degeneracies of NV = 4 and NV = 12, respectively. The relative energy positions of the light and heavy valence bands can be tuned with temperature and defect concentration to make them effectively converged. In this study, analytical solutions from the tight-binding method are employed to understand the possible orbital interactions that lead to this VB convergence. Both temperature and alloying based methods are explained as means to tailor the VB structure to achieve high valley degeneracy through band convergence. We predict five strategies for tuning valence band convergence: (i) alloying to introduce unfilled cation-s (s0) defect states above valence band edge, (ii) increasing the lattice parameter through alloying or temperature, (iii) alloying to decrease the energy difference between anion-p and anion-s states, (iv) alloying to replace filled Pb-s (s2) states with filled cation-s states that are lower in energy, and (v) decreasing the strength of anion spin–orbit interactions. Furthermore, we find that when the VBs are in a highly converged state, there is a topological transition in the electronic band structure to two-dimensional (2D) and potentially to one-dimensional (1D) character. Assuming acoustic-phonon scattering, we use the Boltzmann transport equation (BTE) to predict a significant enhancement in thermoelectric performance for the 2D and 1D band model, with a theoretical zT > 10 predicted for the 1D topology, ∼20 times greater than that predicted for the 3D topology. Although we specifically discuss PbTe, the qualitative results of this work can be applied to all the IV–VI (IV = Ge, Sn, Pb; VI = S, Se, Te) rock salt compounds.

Nontrivial topological behaviors in superconducting materials provide resourceful ground for the emergence and study of unconventional quantum states. Charge doping by the controlled intercalation of donor atoms is an efficient route for enhancing/inducement of superconducting and topological behaviors in layered topological insulators and semimetals. Herein, we enhanced the superconducting temperature of TaSe2 by 20-folds (∼ 3 K) through Sn atoms intercalation. Using first-principles calculations, we demonstrated the existence of nontrivial topological features. Sn0.5TaSe2 displays topological nodal lines around the K high symmetry point in the Brillouin zone, with drumhead-like shaped surface states protected by inversion symmetry. Altogether, the coexistence of these properties makes Sn0.5TaSe2 a potential candidate for topological superconductivity.

We improve the linear tetrahedron method to overcome systematic errors due to overestimations (underestimations) in integrals for convex (concave) functions, respectively. Our method is applicable to various types of calculations such as the total energy, the harge (spin) density, response functions, and the phonon frequency, in contrast with the Bl\"ochl correction, which is applicable to only the first two. We demonstrate the ability of our method by calculating phonons in MgB$_2$ and fcc lithium., Comment: 9pages, 8 figures

The interaction between different types of wave excitation in hybrid systems is usually anisotropic. Magnetoelastic coupling between surface acoustic waves and spin waves strongly depends on the direction of the external magnetic field. However, in the present study we observe that even if the orientation of the field is supportive for the coupling, the magnetoelastic interaction can be significantly reduced for surface acoustic waves with a particular profile in the direction normal to the surface at distances much smaller than the wavelength. We use Brillouin light scattering for the investigation of thermally excited phonons and magnons in a magnetostrictive CoFeB/Au multilayer deposited on a Si substrate. The experimental data are interpreted on the basis of a linearized model of interaction between surface acoustic waves and spin waves.

Thermoelectric materials (TMs) can uniquely convert waste heat into electricity, which provides a potential solution for the global energy crisis that is increasingly severe. Bulk Cu2Se, with ionic conductivity of Cu ions, exhibits a significant enhancement of its thermoelectric figure of merit zT by a factor of ~3 near its structural transition around 400 K. Here, we show a systematic study of the electronic structure of Cu2Se and its temperature evolution using high-resolution angle-resolved photoemission spectroscopy. Upon heating across the structural transition, the electronic states near the corner of the Brillouin zone gradually disappear, while the bands near the centre of Brillouin zone shift abruptly towards high binding energies and develop an energy gap. Interestingly, the observed band reconstruction well reproduces the temperature evolution of the Seebeck coefficient of Cu2Se, providing an electronic origin for the drastic enhancement of the thermoelectric performance near 400 K. The current results not only bridge among structural phase transition, electronic structures, and thermoelectric properties in a condensed matter system, but also provide valuable insights into the search and design of new generation of thermoelectric materials., Comment: To appear in Science Bulletin

Summary Persistent spin textures (PSTs) in solid-state materials arise from a unidirectional spin-orbit field in momentum space and offer a route to deliver the necessary long carrier spin lifetimes utilized in future quantum microelectronic devices. Nonetheless, few bulk materials host PSTs owing to crystal symmetry and chemical requirements, with even fewer experimentally demonstrated examples. This scarcity makes both PST materials discovery and performance assessment challenging. Here, we demonstrate that bulk PSTs exist in the family of layered A3B2O7 oxides and that a persistent spin helix (PSH) occurs over a large region of the Brillouin zone—a feature essential to experimental realization. By solving the spin-diffusion equations in the strong coupling limit and using the PST performance criteria we formulate, we find that the spin lifetime of the PSH is ∼63 ns in Sr3Hf2O7—substantially larger than that predicted (∼10 ns) and demonstrated experimentally (∼600 ps) in GaAs/AlGaAs quantum wells.

The results of studies of the spectral and kinetic characteristics of the photoluminescence of photonic crystals formed on the basis of structures with self-assembled Ge(Si) nanoislands are reported. The experimentally observed enhancement of the photoluminescence-signal intensity of the nanoislands in the spectral range 1.1–1.6 μm due to interaction with the radiative modes of photonic crystals in the vicinity of the Γ point of the Brillouin zone and the effect of such interaction on the probability of radiative recombination in Ge(Si) nanoislands are considered.

The results of studies of the luminescence properties of two-dimensional photonic crystals formed on the basis of silicon structures with self-assembled Ge(Si) nanoislands are reported. The possibilities of substantially enhancing the luminescence response of the active medium (Ge(Si) nanoislands) in the wavelength range 1.2–1.6 μm are shown for such structures. The specific features of the luminescence response of a photonic crystal in the vicinity of the Γ point of its Brillouin zone are studied. It is shown that, along with the broadband response characteristic of the radiative modes of photonic crystals, high-Q resonances, for which the Q factor exceeds 103, can also be observed in such structures. The last-mentioned resonances are observed in a certain range of lattice parameters of photonic crystals.

Layered kagome-lattice 3d transition metals are emerging as an exciting platform to explore the frustrated lattice geometry and quantum topology. However, the typical kagome electronic bands, characterized by sets of the Dirac-like band capped by a phase-destructive flat band, have not been clearly observed, and their orbital physics are even less well investigated. Here, we present close-to-textbook kagome bands with orbital differentiation physics in CoSn, which can be well described by a minimal tight-binding model with single-orbital hopping in Co kagome lattice. The capping flat bands with bandwidth less than 0.2 eV run through the whole Brillouin zone, especially the bandwidth of the flat band of out-of-plane orbitals is less than 0.02 eV along Γ−M. The energy gap induced by spin-orbit interaction at the Dirac cone of out-of-plane orbitals is much smaller than that of in-plane orbitals, suggesting orbital-selective character of the Dirac fermions., The understanding of kagome bands, which are characterized by Dirac-like bands capped by a flat band, remains largely elusive. Here, Liu et al. report the observation of a flat band and Dirac bands as ideal features of kagome bands in CoSn, revealing orbital-selective character.

We predict a new family of two-dimensional (2D) rare earth monochalcogenide materials MX (M = Sc, Y; X = S, Se, Te). Based on first-principles calculations, we confirm their stability and systematically investigate their mechanical properties. We find that these materials are metallic and interestingly, they possess nodal lines in the low-energy band structure surrounding the whole Brillouin zone, protected by nonsymmorphic crystal symmetries in the absence of spin-orbit coupling (SOC). SOC opens small energy gaps at the nodal line, except for two high-symmetry points, at which fourfold degenerate 2D spin-orbit Dirac points are obtained. We show that these topological band features are robust under uniaxial and biaxial strains, but can be lifted by the shear strain. We also investigate the optical conductivities of these materials, and show that the transformation of the band structure under strain can be inferred from the optical absorption spectrum. Our work reveals a new family of 2D topological metal materials with interesting mechanical and electronic properties, which will facilitate the study of nonsymmorphic symmetry enabled nodal features in 2D., Comment: 15 pages, 4 figures

A one-dimensional steady-state model for stimulated Raman backscatter (SRS) and stimulated Brillouin backscatter (SBS) processes in laser-irradiated plasmas is presented. Based on a novel “predictor-corrector” method, the model is capable to deal with broadband scattered light and inhomogeneous plasmas, exhibiting robustness and high efficiency. Influences of the electron density and temperature on the linear gains of both SRS and SBS are investigated, which indicates that the SRS gain is more sensitive to the electron density and temperature than that of the SBS. For the low-density case, the SBS dominates the scattering process, while the SRS exhibits much higher reflectivity in the high-density case. The nonlinear saturation mechanisms and competition between SRS and SBS are included in our model by a phenomenological method. The typical anti-correlation between SRS and SBS versus electron density is reproduced in the model. Calculations of the reflectivities are qualitatively in agreement with the typical results of experiments and simulations.

A two-dimensional phononic crystals based hypersonic-wave switch is theoretically investigated. The temperature effect is considered for a two-dimensional solid–solid phononic crystal composed of a square array of Epoxy rods embedded in a Tungsten matrix. Using finite element method, temperature dependence of the band gap is examined. The numerical results show that the bandgap position and width strongly depend on the temperature and they can be modified when the temperature increases from 10 to 45 °C. Due to its good contrast with Tungsten and Epoxy in elastic properties and its high temperature dependence, Barium Strontium Titanate [Ba0.7Sr0.3TiO3] (BST) is used as the defect material. Defect modes could be tuned by modifying the radius of BST filled rods. The strong temperature dependence and phononic crystals periodicity prove the structure desirable for thermal switching. A hypersonic-wave 1 × 3 switch is designed for the specific frequency f = 1.50 GHz at room temperature.

The prediction of non-trivial topological electronic states hosted by half-Heusler compounds makes them prime candidates for discovering new physics and devices as they harbor a variety of electronic ground states including superconductivity, magnetism, and heavy fermion behavior. Here we report normal state electronic properties of a superconducting half-Heusler compound YPtBi using angle-resolved photoemission spectroscopy (ARPES). Our data reveal the presence of a Dirac state at the zone center of the Brillouin zone at 500 meV below the chemical potential. We observe the presence of multiple Fermi surface pockets including two concentric hexagonal and six half oval shaped pockets at the gamma and K points of the Brillouin zone, respectively. Furthermore, our measurements show Rashba-split bands and multiple surface states crossing the chemical potential which are supported by the first-principles calculations. Our finding of a Dirac state in YPtBi plays a significant role in establishing half-Heusler compounds as a new potential platform for novel topological phases and explore their connection with superconductivity., 6 pages, 4 figures

Antiferromagnetic spin waves have been predicted to offer substantial functionalities for magnonic applications due to the existence of two distinct polarizations, the right-handed and left-handed modes, as well as their ultrafast dynamics. However, experimental investigations have been hampered by the field-immunity of antiferromagnets. Ferrimagnets have been shown to be an alternative platform to study antiferromagnetic spin dynamics. Here we investigate thermally excited spin waves in ferrimagnets across the magnetization compensation and angular momentum compensation temperatures using Brillouin light scattering. Our results show that right-handed and left-handed modes intersect at the angular momentum compensation temperature where pure antiferromagnetic spin waves are expected. A field-induced shift of the mode-crossing point from the angular momentum compensation temperature and the gyromagnetic reversal reveal hitherto unrecognized properties of ferrimagnetic dynamics. We also provide a theoretical understanding of our experimental results. Our work demonstrates important aspects of the physics of ferrimagnetic spin waves and opens up the attractive possibility of ferrimagnet-based magnonic devices. Right- and left-handed spin-wave modes are identified in ferrimagnets, and their dynamics are revealed.

In this Letter, we study the structural, electronic, and elastic properties of single-layer striped borophene stacked on top of single-layer graphene. Through DFT calculation, we show that both the properties of striped borophene and graphene are not fully preserved in the novel heterostructure, which obviously depends on the nature of the chemical bond between the layers. The obtained phonon spectrum confirms the stability of this compound. The divergence of branches in the band structure appears below the Fermi level in the K point in the first Brillouin zone. Moreover, this heterostructure possesses excellent elastic properties and can be considered for use in the field of 2D acousto- and optoelectronics.

Environmental heat-to-electric energy conversion provides a promising solution to power sensors used for wearable and portable devices. Yet the near-room-temperature thermoelectric materials are extremely rare. The natural heterostructure [Bi2]m[Bi2Q3]n family provides an important platform to search and develop the cheaper and less toxic of such materials. However, the bottleneck problem in this family is how to enhance the interlayer electrical conductivity (σ). Herein, we uncover for the first time that the delocalized π-bond interaction between the stacking layers in the [Bi2]m[Bi2Se3]n family effectively increases the interlayer carrier mobility (μH) and σ. Moreover, we propose an empirical index, F = Dpx,py(Bi0)/Dpx,py(Bi3+) along the kz direction in the Brillouin zone to evaluate the strength of the interlayer delocalized π-bond. F is optimized at a value of 1, under which μH is maximized. Interestingly, Bi8Se7 possessing an optimal F = 1.06 is predicted to have the best μH in the [Bi2]m[Bi2Q3]n family. Our subsequent experiments confirm the as-synthesized Bi8Se7 exhibiting n-type behavior with a μH value (33.08 cm2/(V s) at 300 K) that is higher than that of BiSe (26.19 cm2/(V s) at 300 K) and an enhanced σ value. Furthermore, the Te/Sb codoping, via varying the top of the valence band, significantly increases the Seebeck coefficient and eventually enhances the ZT value to ∼0.7 in Bi5.6Sb2.4Se5Te2 at 425 K along the hot-pressing direction, which is comparable to the optimized value of BiSe. According to the single parabolic band model prediction, the ZT of Bi5.6Sb2.4Se5Te2 may reach ∼1.2 at 425 K, suggesting a novel and promising n-type thermoelectric material near room temperature.

A Talbot effect is observed in a Kagome lattice photonic crystal (KPC) at the Dirac-like cone. The Dirac-like cone and Dirac-like point are achieved at the center of the Brillouin zone (BZ) with triply degenerate bands with two linear bands and one flat band intersecting the cone at the same frequency. Using band-structure calculations, it is shown that the Dirac-like cone is formed in a relatively high-frequency region and composed of interchangeable but opposite cones which are formed by two linear dispersion curves of the KPC in the vicinity of the Dirac-like point. Near the point of degeneracy, in addition to Talbot effect, unique wave propagation phenomena such as defect-insensitive propagation characteristics and cloaking of objects are observed.

The spectroscopic ellipsometry method is used to investigate the optical properties of PbSe nanostructured thin films formed by the chemical deposition method. Function d2e/dω2 formed by numerical differentiation of experimental data of the dielectric function e/ω is used for better structural resolution of the interband transitions and the determination of critical points. Theoretical fitting is performed using the “Graphical Analysis” program. The best fit is found for the two-dimensional (2D) shape of the critical point (m = 0) for the energy region E = 2–3 eV, and one critical point corresponding to Eg = 2.5 eV is determined. This value is attributed to the L4 → L6 transition of the Brillouin zone.

Topological nodal-line semimetals (TNLSMs) are materials whose conduction and valence bands cross each other, meeting a topologically protected closed loop rather than discrete points in the Brillouin zone (BZ). The anticipated properties for TNLSMs, include drumhead-like nearly flat surface states, unique Landau energy levels, special collective modes, long-range Coulomb interactions, or the possibility of realizing high-temperature superconductivity. Recently, SrAs3 has been theoretically proposed and then experimentally confirmed to be a TNLSM. Here, we report high-pressure experiments on SrAs3, identifying a Lifshitz transition below 1 GPa and a superconducting transition accompanied by a structural phase transition above 20 GPa. A topological crystalline insulator (TCI) state is revealed by means of density functional theory (DFT) calculations on the emergent high-pressure phase. As the counterpart of topological insulators, TCIs possess metallic boundary states protected by crystal symmetry, rather than time reversal. In consideration of topological surface states (TSSs) and helical spin texture observed in the high-pressure state of SrAs3, the superconducting state may be induced in the surface states, and is most likely topologically nontrivial, making pressurized SrAs3 a strong candidate for topological superconductor.

Band nesting occurs when conduction and valence bands are approximately equispaced over regions in the Brillouin zone. In two-dimensional materials, band nesting results in singularities of the joint density of states and thus in a strongly enhanced optical response at resonant frequencies. We exploit the high sensitivity of such resonances to small changes in the band structure to sensitively probe strain in semiconducting transition metal dichalcogenides (TMDs). We measure and calculate the polarization-resolved optical second harmonic generation (SHG) at the band nesting energies and present the first measurements of the energy-dependent nonlinear photoelastic effect in atomically thin TMDs (MoS2, MoSe2, WS2, and WSe2) combined with a theoretical analysis of the underlying processes. Experiment and theory are found to be in good qualitative agreement displaying a strong energy dependence of the SHG, which can be exploited to achieve exceptionally strong modulation of the SHG under strain. We attribute this sensitivity to a redistribution of the joint density of states for the optical response in the band nesting region. We predict that this exceptional strain sensitivity is a general property of all 2D materials with band nesting.

This article presents an efficient reduced formulation of the Bloch Operator Finite Element method to calculate complex band structures of periodic waveguides. The use of a Bloch operator formulation allows building and solving a Bloch eigenvalue problem along a generic wave direction, thus being not limited to the unit cell Irreducible Brillouin Zone (IBZ) edges, so that band gap directionality and material absorption in elastic and damped waveguides can be fully disclosed. The proposed Reduced-Order Modeling (ROM) exploits a small set of Bloch modes, extracted at relevant frequency locations along one or more wave directions and post-processed with a Singular Value Decomposition, to reduce the dimensions of the eigenvalue problem. The performances of the proposed numerical technique are evaluated in terms of accuracy and computational saving by analyzing a linear elastic and a damped bi-periodic stubbed plate. Results demonstrate that the reduced formulation yields accurate predictions of propagative, evanescent and complex wave solutions with a reduction in computational time of more than one order of magnitude with respect to the full model calculations. Complex band structures can thus be efficiently computed over the whole IBZ.

The Rashba splitting are found in the buckled square lattice. Here, by applying fully relativistic density-functional theory (DFT) calculation, we confirm the existence of the Rashba splitting in the conduction band minimum of various two-dimensional MX monochalcogenides (M = Ge, Sn and X = S, Se, Te) exhibiting a pair inplane Rashba rotation of the spin textures. A strong correlation has also been found between the size of the Rashba parameter and the atomic number of chalcogen atom for Γ and M point in the first Brillouin zone. Our investigation clarifies that the buckled square lattice are promising for inducing the substantial Rashba splitting suggesting that the present system is promising for spintronics device.