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By taking the spatial and temporal Fourier transforms of the coordinates of the atoms in molecular dynamics simulations conducted using an embedded-atom-method potential, we calculate the inelastic scattering of x-rays from copper singlecrystals shocked along [001] to pressures of up to 70 GPa. Above the Hugoniot elastic limit (HEL), we find that the copious stacking faults generated at the shock front introduce strong quasi-elastic scattering (QES) that competes with the inelastic scattering signal, which remains discernible within the first Brillouin zone; for specific directions in reciprocal space outside the first zone, the QES dominates the inelastic signal overwhelmingly. The synthetic scattering spectra we generate from our Fourier transforms suggest that energy resolutions of order 10 meV would be required to distinguish inelastic from quasi-elastic scattering within the first Brillouin zone of shock-loaded copper. We further note that high-resolution inelastic scattering also affords the possibility of directly measuring particle velocities via the Doppler shift. These simulations are of relevance to future planned inelastic scattering experiments at x-ray Free Electron Laser (FEL) facilities.

We present measurements of electron momentum density (EMD) of V2O5 employing a lowest intensity 100 mCi 241Am Compton spectrometer. The experimental Compton profile is compared with those deduced using density functional theory with local density approximation (LDA) and revised functional of Perdew-Burke-Ernzerhof for solids (PBEsol) as facilitated in the linear combination of atomic orbitals (LCAO) method. On the basis of goodness of fitting, we observed that the theoretical EMDs derived using PBEsol scheme shows a close agreement with the experimental EMD than that of LDA. Going above EMDs, we have also computed the energy bands and density of states of V2O5 using PBEsol scheme. An analysis of energy bands shows the valence band maximum at T point and conduction band minimum at Γ point of the Brillouin zone. Calculated indirect energy band gaps of V2O5 are 1.63 and 1.58 eV using PBEsol and LDA schemes, respectively. Therefore, the band gap derived using PBEsol scheme is close to experimental band gap value 2.3 eV, confirming the use of applicability of PBEsol model for V2O5 type of materials.

Here we report on strain mediated control of the dipolar coupling of spin waves propagating in bilateral three channel yttrium iron garnet stripes. The numerical model based on nearest-neighbor coupling was proposed. The multimode coupling regime was revealed. It was shown that the strain mediated control of the width mode coupling length leads to the simultaneous frequency and spatial separation of the propagated modes of confined structures. The micromagnetic numerical simulation was performed to show the efficient operational regime of tunable coupler with the Gilbert damping model taking into account. Reconfigurable spatial patterns within magnonic waveguides were obtained with Brillouin light scattering spectroscopy. The demonstrated voltage induced control of spin-wave propagation opens the promising alternative of the magnon straintronic coupler in the way of energy efficient beyond-CMOS functional units.

Half-Heusler (HH) compounds are novel materials known for their diverse applications such as, magnetic semi-conductors, thermo-electrics and spintronic devices. Generally, HH systems are semi-conducting but, spin-locked surface states can be realized by application of stress/strain driving the system into Topological Phase Transition (TPT). In the current study, we use first-principles method to predict that, HH compound LiMgSb undergoes a TPT under the application of Volume Expansive Pressure (VEP). We find that, at 0% VEP the system exhibits a semi-conducting nature with indirect band gap. Whereas, at a considerably higher VEP (∼ 17 %) a Dirac Cone forms along the high symmetry point Γ in the Brillouin Zone (BZ). The existence of corresponding surface states have been confirmed by calculating, computational Angle Resolved Photo Emission Spectroscopy (ARPES). On further increment, the band reopens indicating Topological Insulator (TI) nature. We quantify the TI nature by calculating Z2 indices to conclude that, LiMgSb is a TI characterized by the Z2 indices as (0, 0 0 0). The predicted HH can thus have multi-functional applications in the field of spintronics and quantum computation.

Ab initio calculations showing strain induced bandgap variation in 2D graphene like structure of CdO monolayer is presented in this work. The density functional theory (DFT) calculations were performed using pseudopotential and numerical nano orbitals basis set as implemented in SIESTA. The electron exchange correlation is treated within the local density approximation (CA-LDA). KS-orbitals were expanded based on the double zeta polarization (DZP) with mesh cutoff 250 Ry. The first Brillouin zone was integrated by using 8 x 8 x 1 k-mesh within Monkhorst pack scheme. The optimized lattice parameter was found to be 3.66 □, which is comparable with reported data. Within the compressive strain limit chosen in this work (- 8%), the bandgap remains direct. In the tensile strain, transition from direct to indirect bandgap is observed at 5 % and when the bond length is stretched further, metallic like behavior is observed at 14%, beyond which the conduction band minima falls below the Fermi level. The nature of variation of effective masses of electrons, heavy holes and light holes are calculated from energy bands close to the gamma point, where the band lines are parabolic. While the electrons and light holes effective masses remain almost constant in the range of our calculations, the heavy holes effective masses significantly decreased in the compressive strain and in the tensile strain it significantly increases from its value at the relaxed lattice. Our theoretical predictions in this work will provide valuable information for fabrication of CdO monolayer in nano-electromechanical systems (NEMS) and nano-optomechanical systems (NOMS) devices.

Optical bound states in the continuum (BICs) provide a way to engineer resonant response in planar photonic structures with ultimately high-quality factors. Here, we employ optical BIC in a photonic crystal slab (PCS) to realize the regime of strong coupling between light and excitons in a monolayer MoSe2 semiconductor. We experimentally demonstrate BIC-like behavior of the lower polariton branch, with strong suppression of radiation into far-field at Γ-point of the Brillouin zone. With the decrease of the in-plane Bloch wavevector, polariton lifetime rapidly increases, while the group velocity decays. These effects balance each other granting a nearly constant polariton propagation length of less than 10 μm over a broad range of k-vectors. This result suggests the possibilities for the development of ultracompact BIC-based planar polaritonic devices for sensing, lasing, and nonlinear optics.

The isotropic experimental Compton profile of Ag2MoO4 is measured at an intermediate resolution using a 740 GBq137Cs Compton spectrometer. To compare the experimental momentum densities, theoretical Compton profiles have been deduced using Hartree-Fock (HF) and density functional theory (DFT) within linear combination of atomic orbitals (LCAO) method. It is seen that DFT-WC1LYP (hybrid HF and DFT functionals) theory based Compton line shape is in better agreement with the measurement than other model calculations considered here. In addition, we have also measured the band structure and density of states (DOS) using presently employed LCAO method. The present data show direct band gap of 3.18 eV in Ag2MoO4 at Γ point of Brillouin zone.

We report a general method to design a unique type of a non-Hermitian electromagnetic double-near-zero index medium by a two-dimensional photonic crystal. The synergy of a nonsymmorphic glide symmetry of the lattice, a period-doubling of the unit cell, and the non-Hermitian perturbation of the photonic crystal induces a phase transition in the eigenvalue spectrum. Near the Brillouin zone center, such a photonic crystal is effectively an anisotropic double-near-zero index medium. Along the direction of interest, the real parts of its effective permittivity and permeability are simultaneously near zero, while the imaginary parts of the effective parameters are nonzero values with opposite signs, leading to a real and positive effective refractive index. This medium enables many fascinating applications such as an angular sensor, a coherent perfect absorber, and a laser.

We report unconventional thermoelectric power (Seebeck coefficient, S) in L10 structured FePt films. The temperature dependence of S can be well fitted by a phenomenological expression consisting of electron diffusion and magnon-drag contributions. Interestingly, the magnon drag coefficient carries an opposite sign to that of electron diffusion, revealing a dominant contribution from the elusive electron-magnon Umklapp scattering. Density-functional theory calculations identify several bands crossing the Brillouin zone boundaries, facilitating the Umklapp process. The large spin–orbit coupling in FePt results in strong mixing of majority and minority spins among some of those bands, greatly enhancing the electron-magnon scattering.

Spin waves in micrometer-sized, patterned Y-shaped Permalloy structures were studied using micro-focus Brillouin light scattering (BLS) with a magnetic field applied in-plane. For in-plane magnetized thin films and microstrips, the dispersion relations depend on the angle of the magnetization with respect to the microstrip axis. BLS measurements show that spin waves generated in the two arms that form the top of the Y structure can be channeled into a longer magnetic microstrip that forms the base when the applied field is oriented perpendicular to the long axis of the base. In this configuration, the base supports surface spin waves. A comparison of the BLS data with micromagnetic simulations reveals that low- k spin waves generated by a microstrip antenna in the arms are converted to higher- k spin waves in the base, which may be useful for nanomagnonic applications.

Two-dimensional, honeycomb, and sandwich-structured transition metal dichalcogenides (TMDs) have two nonequivalent energy valleys at the six corners of the hexagonal first Brillouin zone, resulting in promising applications in valleytronics. Here, based on the WSe2/CrSe2 heterojunction, biaxial and uniaxial tensile strains with magnitudes of 0%–6% are demonstrated to have a similar effect on magnetism-induced valley splitting in the lowest conduction band of WSe2. However, at larger magnitudes of 6%–10%, uniaxial strain dramatically decreases the valley splitting. This decrease in valley splitting can be understood by the spin-orbit coupling induced different spin splitting between the two valleys. The findings provide valuable guidance for the valleytronic applications of TMDs.

Acoustic media with Willis coupling possess intrinsic directionality owing to the vector nature of coupling coefficients in constitutive relations. Here, we report a type of sonic topological insulator that exhibits the valley Hall effect by using an acoustic fluid of the Willis type. We find that the valley Hall phase transition can be triggered by tuning the coupling vector. In addition, the Dirac cones or valley position are displaced away from high symmetry points in the Brillouin zone. The tunability of the valley offset offered by Willis coupling helps to realize equally robust one-way transport for both zigzag and armchair domain walls and for more tortuous wave channels.

We demonstrate that properties of spin-wave propagation in the adjacent magnonic crystal stripes with one of them in contact with a piezoelectric layer can be controlled by an external electric field. We perform microwave spectroscopy and employ a theoretical approach based on the analysis of the set of coupled wave equations. By considering incident and reflected waves in the first Brillouin zone, we calculate the reflection coefficients of the magnonic structure. Two narrow magnon bands are observed in the experiment, and their behavior with the variation of the electric field applied to the piezoelectric layer was shown. The finite-element calculations of the self-consistent eigenvalue problem elucidate how the influence of the piezoelectric layer can be modeled as a localized strain-induced internal magnetic field and its variation affects the spin-wave dispersion. Both the frequency shift and closing of a magnon band are detected in our measurements and confirmed by the simulations and the analytical approach. Therefore, we demonstrate the electric field control of the magnonic bands. Our results reveal the mechanism of the spin-wave spectra control in the coupled magnonic crystals. The results pave the way for the implementation of frequency selective magnonic devices based on a straintronic approach.

We investigated the crystalline direction dependent Dzyaloshinskii–Moriya interaction (DMI) in single crystal CoFeB(211)/Pt(110) films utilizing the Brillouin light scattering technique. The in-plane magnetic anisotropy and the DMI coefficient show a clear 1/tCFB relation. The quantified DMI strength is −2.5 pJ/m along Pt [ 001 ] and −1.2 pJ/m along Pt [ 1 1 ¯ 0 ] with a large anisotropy ratio of 2.1. Such a strong anisotropy of DMI is attributed to the C2v crystal symmetry at the CoFeB/Pt(110) interface. Our results provide a route toward designing topological magnetic textures by tailoring the DMI strength along the specific crystal orientation in ferromagnet/Pt(110) systems.

A broad, bell-shaped intensity component is observed in low-energy electron diffraction from high-quality epitaxial 2D-systems. Three 2D- systems, graphene on Ir(111), graphene on SiC(0001), and hexagonal boron nitride on Ir(111), have been prepared in situ under ultra- high vacuum conditions. In all three systems— independent of substrate material—similar strong diffuse intensity is observed, exhibiting a width as large as 50% of the Brillouin zone and an integrated intensity more than 10 times the intensity of the Bragg spots. The presented experimental results provide evidence for a common origin of such diffuse diffraction intensity in different atomically thin 2D- materials.

We experimentally demonstrate the enhanced four-wave mixing (FWM) by harnessing the forward stimulated Brillouin scattering (FSBS) within a silicon-based cascaded racetrack microring resonator (MRR). The frequency spacing of the split resonant peaks is precisely designed to match the Brillouin frequency shift (BFS). The cooperative interaction of the FSBS resonance and cascaded MRR resonance achieves an FWM enhancement of 2.97 and 2.43 dB for anti-Stokes and Stokes sidebands under the launched pump power of 35.48 mW and probe power of 17.78 mW. The BFS from 3.34 to 7.13 GHz is demonstrated by changing the waveguide width. Moreover, we show that this same system behaves as a single-sideband modulator, providing more than the 17 dB single-sideband rejection ratio under the condition that the probe light and Stokes are resonant in the cascaded MRR. Building on these results, this device opens the door to new types of all-silicon Brillouin laser, amplifier, isolator, and single-sideband modulator.

We perform analytical and numerical analysis of the electronic and phonon spectrum evolution of graphene during formation of a boundary with a “zigzag” chirality. It is determined, that the excited gap wave has a relativistic dispersion near the Fermi level that propagates along the boundary and decays with distance from it. Both properties and formation of the wave is considered. It is shown that the wave propagation occurs only along the atoms of the sub-lattice, which contains atoms with bonds broken during the boundary formation. The gap wave forms narrow resonance peaks in the local density of states of the sublattice atoms. It is shown, that the boundary formation on a graphene layer with this chirality similarly affects the phonon modes polarized normal to the layer, forming narrow maxima with frequencies nearing that of the quasiflexural phonons with the quasiwave vector at the K-point of the first Brillouin zone. This way, the formation of the “zigzag”-boundary increases both the number of charge...

The Brillouin theorem has been generalized for the coupling strength parameter (a) extended non-relativistic electronic Hamiltonian (H∇+Hne+aHee). The mathematical case a=0 generates an orto-normalized set of single Slater determinants which can be used as basis set for configuration interactions (CI) calculations for the physical case a=1, removing the known restriction by the original Brillouin theorem and opening a new way to practice.

Double-zero-index properties of electromagnetic and acoustic waveguides have been recently realized based on Dirac/Diraclike 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. Published under license by AIP Publishing. K. T. Tan acknowledges the Faculty Start-Up Grant support from The University of Akron. J.C. gratefully acknowledges financial support from the MINECO through a Ramón y Cajal Grant (No. RYC-2015-17156). Z. Li acknowledges financial support from the National Natural Science Foundation of China under Grant No. 11672004.

Molybdenum dichalcogenides are probably the most studied single layer TMDCs by virtue of being appealing for sundry possible applications suchlike transistors, diodes, solar cells or more fundamental studies of spin or valley pseudospin and their interactions. Tungsten-based counterparts are on the other hand evincing much stronger spin-orbit coupling due to which all the spin-related effects are more stable at room temperature and thus more feasible for application. WTe2, a type-II Weyl semimetal is in particular interesting due to having two pairs of spin-differentiated Weyl points above Fermi energy. We have conducted several experiments following the evolution of the band dispersion in the vicinity of X and Y points of the Brillouin zone of WTe2 which is substantial for understanding the fundamental properties of the structure-property relation of the system. Ab-initio set of photoemission calculations was performed using SPR-KKR package and compared to experimental results.

In this work, we report on structural and electronic properties of monolayer structure of WSe2 has been elucidated using full potential linear augmented plane wave (FP-LAPW) method. The calculated equilibrium lattice parameters, bulk modulus, bulk pressure derivatives and equilibrium energy of this compound were presented. From the density of states, it was observed that at valence band (VB), hybridization occur interaction between W-d states and Se-p states near the Fermi level. From the other side of conduction band (CB) near Fermi level, the band formations occur due to W-d states. The direct band gap nature can be clearly depicted from the band structure as observed form the valence band maxima (VBM) and conduction band minima (CBM) at K direction of Brillouin Zone. The calculated spin splitting and band gap values are 478 meV and 1.523 eV for WSe2 compound respectively, which can be used for this compound further in favor of optoelectronic and spintronic applications.In this work, we report on structural and electronic properties of monolayer structure of WSe2 has been elucidated using full potential linear augmented plane wave (FP-LAPW) method. The calculated equilibrium lattice parameters, bulk modulus, bulk pressure derivatives and equilibrium energy of this compound were presented. From the density of states, it was observed that at valence band (VB), hybridization occur interaction between W-d states and Se-p states near the Fermi level. From the other side of conduction band (CB) near Fermi level, the band formations occur due to W-d states. The direct band gap nature can be clearly depicted from the band structure as observed form the valence band maxima (VBM) and conduction band minima (CBM) at K direction of Brillouin Zone. The calculated spin splitting and band gap values are 478 meV and 1.523 eV for WSe2 compound respectively, which can be used for this compound further in favor of optoelectronic and spintronic applications.

We have unveiled a magnetic exchange-induced topological phase transition in a bulk natural van der Waals crystal MnSb2Te4, based on magnetization and magnetotransport measurements and first principles calculations. At the A-type antiferromagnetic ground state, MnSb2Te4 is a topologically trivial insulator with a bandgap of ∼ 42 meV at the Γ point of the Brillouin zone. A small magnetic field less than 1.4 T along the c axis can drive the system into a spin fully polarized state, which hosts only a single pair of Weyl points setting near the Γ point at the Fermi level without other band mixing, supported by both the first principles calculations and the measured anomalous Hall effect. The results would setup an excellent paradigm for the study of interplay between magnetism and nontrivial topology of the electronic band structure.

We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75 eV) into the vacuum ultraviolet (8.5 eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Γ-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24 me) at the bottom of the Γ-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24 me). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Γ-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier–Mott type excitons with an effective Rydberg energy of 14.8 meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements ( e ∞ = 1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)].

The electrical switching of magnetization through spin–orbit torque (SOT) has potential applications for energy-efficient spintronic devices. Previous studies focused mostly on 5d heavy metals with strong spin–orbit coupling (SOC) to generate a spin current or a nonequilibrium spin accumulation and exert SOTs on the magnetization of a neighboring ferromagnetic layer. Recent theoretical and experimental studies indicated that 4d metals with weak SOC may also generate a sizable torque and realize the current-induced magnetization switching. In this work, we studied the current-induced SOTs in 4d metal Rh-based magnetic heterostructures with a perpendicular magnetic anisotropy. The damping-like SOT efficiency ξDL of [Ni/Co]3/Rh multilayers increases with the Rh thickness tRh and becomes saturated at tRh = 5 nm. Although the spin-Hall angle of Rh is rather small about 0.028 ± 0.005, a reversible current-induced SOT switching can still be achieved. In addition, the interfacial Dzyaloshinskii-Moriya interaction (iDMI) in Rh/Co heterostructures was quantitatively characterized by using Brillouin light scattering. The iDMI constant D increases with tRh and reaches 224 ± 39 μJ/m2 at tRh = 5 nm. Our results indicated that even for a weak SOC 4d metal Rh, it is still possible to obtain a current-induced magnetization switching and observe an obvious iDMI effect in the Rh-based magnetic heterostructures, which may broaden the scope of spintronic materials used for SOT devices.

Thoria (ThO2) has lately gained attention due to its potential for use as a nuclear fuel. From a physics standpoint, ThO2 is an actinide-bearing material with no 5f electrons and is thus ideally suited as a baseline material for future studies of the physical properties of actinide systems with correlated electrons. Current investigations of ThO2 as a nuclear fuel focus on the influence of radiation-induced lattice defects on its thermal properties, especially the conductivity. This work presents a first investigation of the impact of point defect disorder on phonon thermal conductivity of ThO2 by solving the Boltzmann transport equation within the single-mode relaxation time approximation. The relaxation times of intrinsic, three-phonon scattering are calculated by a rigorous sampling of k-points within the irreducible Brillouin zone of the face-centered cubic crystal structure. The effect of point defects on the thermal conductivity of ThO2 is predicted using the classic model by Klemens for phonon relaxation times that result from the change in mass and induced lattice strain associated with point defects. Within this model, the change in force constants and atomic radii are computed using input from an atomistic model of ThO2. The defects considered are uranium substitution at a thorium site, oxygen vacancies and interstitials, and thorium vacancies and interstitials. The results show that the conductivity of ThO2 is highly sensitive to intrinsic point defects and less sensitive to U substitution on the cation sublattice.

It is reported for the electronic properties of an in-plane twisted bilayer phosphorene, known as the 2O-tαP phase, and the only dynamically stable phase beyond the AB stacking. This was achieved using first-principles calculations, a generalized empirical tight-binding model inclusive of electric field effects, and a two-parameter low energy effective model, the latter two providing an efficient scheme for nanoelectronics related applications. The tight-binding model reproduces a global fit to the first-principles dispersion, and the low energy model provides more accurate near-gap bands. Both are orders-of-magnitude faster and less memory-intensive than performing first-principles calculations. The twisted 2O-tαP structure possesses a direct bandgap of 1.27 eV, larger than that of the shifted AB structure (1.03 eV). The hole and electron polar effective mass anisotropy ratios are 27.34 and 1.95, respectively. An important observation is that the layer twisting results in the removal of Dirac cones as a reflection of a different band topology compared to the AB one, while the twofold degeneracy at the Brillouin zone boundary and the symmetry of the energy surface are both broken by an external vertical electric field. With an increasing electric field strength, a decreasing bandgap and an increasing energy difference between the valence band maximum and the twisted band point are both predicted by the tight-binding model and the low energy model.

Recently, bulk and multi-layer Cr2Ge2Te6 (CGT) has garnered much attention because of its interesting physical properties and promising technological aspects, including information storage and spintronic application. Here, using density functional theory calculations, we investigate the spin-wave spectrum and magnon-phonon interaction (MPI) in the CGT monolayer. The strength of MPI and the magnon relaxation time are estimated by the broadening in the spin-wave spectrum. We find that the MPI in the CGT monolayer exhibits weak in-plane isotropy and a strong wave vector dependence. The broadening of optical magnons is constant near the center of the Brillouin zone, while that for acoustic magnons shows a quadratic wave-vector dependence. Moreover, this wave-vector dependence is independent of temperature. As the temperature increases from 5 K to 55 K, the magnon relaxation time decreases 4 times, revealing the importance of MPI in spin dynamics of the ferromagnetic CGT monolayer. Our work provides an in-depth understanding of the magnon-phonon interaction in the ferromagnetic Cr2Ge2Te6 monolayer.

This article presents a method for improving the amplification performance of strong-coupling stimulated Brillouin scattering by adding an external static magnetic field longitudinal to the laser propagation direction. Simulation results show that when the laser intensity is low, the output seed intensity can be increased by applying an external magnetic field and a right-hand circularly polarized laser pulse. When the seed intensity is higher, the amplification enters the regime of relativistic electron nonlinearity, the amplification oscillates, and the portion of the seed that is amplified moves from the first peak to the second and then the third. Under these circumstances, a reversed magnetic field helps to obtain a higher output intensity and a smoother output pulse profile. Adjusting the magnetic field dynamically, such as by using a double-pass scheme, combines these two advantages and maximizes the intensity enlarging and profile smoothing of the output pulse.

Magnetoresistance Δρ(H,T) of several heavy-fermion compounds, CeAl2, CeAl3 and CeCu6, substitutional solid solutions with quantum critical behavior CeCu6–xAux (x = 0.1, 0.2) and alloys with magnetic ground state Ce(Al1–xMx)2 (M = Co, Ni, x ≤ 0.8) was studied in a wide range of temperatures (1.8–40 K) in magnetic fields up to 80 kOe. It was shown that a consistent interpretation of the field dependences of the resistance for both non-magnetic and magnetically ordered cerium-based intermetallic compounds with strong electron correlations can be achieved within the framework of an approach that accounts for scattering of charge carriers by localized magnetic moments in a metal matrix. Within this approach, three different components of the magnetoresistance of cerium intermetallic compounds were identified: the negative Brillouin contribution proportional to the local magnetization ( −Δρ/ρ∼Mloc2), the alternating linear contribution ( Δρ/ρ∼H) and the magnetic component, saturating in magnetic fields below 15...

The acoustic properties of tetragonal PbxBa1−xNb2O6 (PBN-x) single crystals with x = 0.5 were studied by Brillouin spectroscopy over a wide temperature range from room temperature to 600 °C. Anomalous changes in the Brillouin frequency shift and the half width of acoustic waves were observed at ∼350 °C that corresponds to the ferroelectric–paraelectric phase transition temperature of PBN-0.50. The longitudinal sound velocity and its associated elastic constant (C33) showed substantial softening in the paraelectric phase when the phonon propagated along the polar c axis, while their degree of softening for the phonon along the a axis was small, revealing clear acoustic anisotropy. It was associated with the strong polarization dependence of the quasi-elastic central peak. The relaxation time derived from the central peak seemed to follow the critical slowing-down behavior indicating order–disorder nature of the phase transition. Among C44 and C66, the former displayed substantial softening in the ferroelectric phase resembling the order parameter behavior. The longitudinal sound velocity and the related acoustic damping showed significant anomalies in the ferroelectric phase of unpoled PBN-0.5, which disappeared completely under poled condition. The possible origin for these unexpected acoustic anomalies was suggested.

Despite similar chemical compositions, the CuMO2 (M = H, Li, Na, K, Rb) compounds show remarkably distinct structural, electronic, dynamical, and optical properties. Different alkali atoms have a significant influence on their electronic, dynamical, and optical behavior. By means of first principles based density functional theory calculations, we explore the universality of electronic characteristics, dynamical stability, and optical properties of these compounds. The electronic band structures, vibrational frequencies, and optical properties are deeply connected with the atomic radius of the alkali atoms. The electronic bandgap of CuMO2 (M = H, Li, Na, K, Rb) lies within the range of 0.5–1.0 eV bringing them in the group of low bandgap p-type semiconductors. We found a significant increase in the bandgap and p–d hybridization as going from H to Rb. Partial density of states revealed strong metal–oxygen (Cu–O) overlap due to the strong p–d hybridization. The phonon dispersion curves obtained for these compounds confirm the dynamical stability as there is no imaginary frequency throughout the Brillouin zone. The static dielectric constants and refractive index fall within the range of 8.0–12.91 and 1.98–3.55, respectively, suggesting the usefulness of scrutinized compounds in non-linear optical devices. The optical properties depict that the alkali atoms based delafossites can serve as promising candidates for highly efficient optical devices within a broad range from visible to ultraviolet light of electromagnetic spectra.

Nickel oxide (NiO) has a cubic rock salt crystal structure at room temperature. Raman scattering of the transverse optical (TO) and longitudinal optical (LO) phonon in NiO is Raman inactive. Thus, it is difficult to employ the Raman scattering technique to study the lattice vibration dynamics and spin–phonon coupling in NiO. In this work, crystalline stoichiometry of NiO nanoparticles with different nanocrystalline sizes was tuned to make the Raman scattering selection rules dramatically relaxed. Well-defined Raman scattering peaks of the two zone-boundary folded modes TO(Δ) and LO(Δ) were observed. These two modes are situated at the midpoint along the Γ–Δ–X direction in the Brillouin zone. The Raman scattering of these two modes are induced by magnetostriction and nonstoichiometric Ni–O stretching, respectively. The well-defined Raman peaks of TO(Δ) and LO(Δ) allow us to study the spin–phonon coupling effect in NiO. It is found that spin–phonon coupling is responsible for the Raman scattering anomalies, namely, the relatively large Raman shift hardening and peak width narrowing below the Neel temperature for LO and its overtone 2LO phonons.

Both backward and forward directed Stimulated Brillouin Light Scattering (SBLS) are observed in an aqueous suspension of SiO2 nanoparticles with diameters of 100 and 200 nm by using for excitation a second harmonic (λ = 532 nm, 10 ns) of a single (∼0.005 cm − 1) mode YAG : N d 3 + laser. Both backward and forward directed SBLS are characterized by a low beam divergence and typically for a water Stokes shift of ≈0.25 cm − 1. The microscopic explanation of forward directed SBLS in an aqueous suspension of SiO2 spherical nanoparticles is proposed.

Exploring the metastable magnetic nanostructures of Mn substituted α″-Fe16N2 with large saturation magnetization μ0MS, high Curie temperature TC and giant magnetic anisotropy are of technological merit as promising candidates for non-rare-earth based permanent magnets. Here, we present in-depth analysis for the structural and magnetic properties of Fe16 − xMnxN2 using first-principles calculations. We predict a large magnetocrystalline anisotropy energy (MAE) constant of K1 = 2.02 MJ/m3 for the Fe14Mn2N2 alloy, which is more than twice that of pristine Fe16N2. The underlying mechanism associated with boosting K1 is attributed to the local distortion of orbitals induced by Mn substitution. The MAE is also carefully analyzed in terms of reciprocal space analysis by employing the magnetic force theorem, revealing the regions in the Brillouin zone that are prominent for giving rise to MAE.

Epitaxial metal/semiconductor superlattice heterostructures with lattice-matched abrupt interfaces and suitable Schottky barrier heights are attractive for thermionic energy conversion, hot electron-based solar energy conversion, and optical hyperbolic metamaterials. HfN/ScN is one of the earliest demonstrations of epitaxial single-crystalline metal/semiconductor heterostructures and has attracted significant interest in recent years to harness its excellent properties in device applications. Although the understanding of the mechanism of thermal transport in HfN/ScN superlattices is extremely important for their practical applications, not much attention has been devoted to measuring their phonon dispersion and related properties. In this Letter, we employ non-resonant meV-resolution inelastic x-ray scattering to determine the momentum-dependent phonon modes in epitaxial metallic HfN and lattice-matched HfN/ScN metal/semiconductor superlattices. HfN exhibits a large phononic bandgap (∼40 meV) and Kohn anomaly in the longitudinal and transverse acoustic phonon modes at q ∼ 0.73 along the [100] and [110] directions of the Brillouin zone due to the nesting of the Fermi surface by the wave vector ( q). The in-plane [100] acoustic phonon dispersion of the HfN/ScN superlattices is found to be dominated by the HfN phonons, while the optical phonons exhibit both ScN and HfN characteristics. First-principles density functional perturbation theory modeling is performed to explain the experimental phonon spectra, and temperature-dependent thermal conductivity is measured using a pump-probe spectroscopic technique. These results will help understand the phonons in HfN and HfN/ScN metal/semiconductor superlattices for thermionic energy conversion.

We theoretically study the generation of microwave radiation at frequency 2 ω 1 − ω 2, produced by two microwaves, of frequencies ω 1 and ω 2, normally incident on monolayer graphene grown epitaxially on a SiC substrate and doped by electrostatic gating. Two mechanisms responsible for this third-order nonlinear effect are considered: (i) the graphene’s conduction band nonparabolicity arising from the substrate-induced bandgap opening at the Dirac points of the graphene’s Brillouin zone and (ii) the energy dependence of the electron-momentum relaxation time. Both mechanisms are incorporated in our Boltzmann-equation-based treatment of the graphene’s third-order nonlinear response to the applied microwave radiation field. Within the framework of the approach, we evaluate the output power P 2 ω 1 − ω 2 of the ( 2 ω 1 − ω 2 ) microwave radiation as a function of the graphene’s carrier density n S controlled by the applied gate voltage. Our formulation predicts an unexpected nonmonotonous behavior of this function: there is a pronounced minimum in the P 2 ω 1 − ω 2 ( n S ), which is followed by a maximum. This finding may serve to provide a rational explanation for this type of oscillation as observed by Dragoman et al. [Appl. Phys. Lett. 97, 093101 (2010)] in the output power of the microwave harmonic generation in gated graphene, which are not properly construed so far.

Gated molybdenum disulphide (MoS2) exhibits a rich phase diagram upon increasing electron doping, including a superconducting phase, a polaronic reconstruction of the bandstructure, and structural transitions away from the 2H polytype. The average time between two charge-carrier scattering events - the scattering lifetime - is a key parameter to describe charge transport and obtain physical insight in the behavior of such a complex system. In this work, we combine the solution of the Boltzmann transport equation (based on ab-initio density functional theory calculations of the electronic bandstructure) with the experimental results concerning the charge-carrier mobility, in order to determine the scattering lifetime in gated MoS2 nanolayers as a function of electron doping and temperature. From these dependencies, we assess the major sources of charge-carrier scattering upon increasing band filling, and discover two narrow ranges of electron doping where the scattering lifetime is strongly suppressed. We indentify the opening of additional intervalley scattering channels connecting the simultaneously-filled K/K' and Q/Q' valleys in the Brillouin zone as the source of these reductions, which are triggered by the two Lifshitz transitions induced by the filling of the high-energy Q/Q' valleys upon increasing electron doping., 10 pages, 6 figures

FePS$_3$ is a van der Waals compound with a honeycomb lattice that is a good example of a two-dimensional antiferromagnet with Ising-like anisotropy. Neutron spectroscopy data from FePS3 were previously analysed using a straight-forward Heisenberg Hamiltonian with a single-ion anisotropy. The analysis captured most of the elements of the data, however some significant discrepancies remained. The discrepancies were most obvious at the Brillouin zone boundaries. The data are subsequently reanalysed allowing for unequal exchange between nominally equivalent nearest-neighbours, which resolves the discrepancies. The source of the unequal exchange is attributed to a biquadratic exchange term in the Hamiltonian which most probably arises from a strong magnetolattice coupling. The new parameters show that there are features consistent with Dirac magnon nodal lines along certain Brillouin zone boundaries., 8 pages, 4 figures. The following article has been accepted by the Journal of Applied Physics. After it is published, it will be found at (https://publishing.aip.org/resources/librarians/products/journals/). The article was submitted as part of a special topic edition (https://publishing.aip.org/publications/journals/special-topics/jap/2d-quantum-materials-magnetism-and-superconductivity/)

Running a Brillouin laser in diamond without guided wave optics affords a wide range of output power, which could benefit a plethora of potential applications.

Through an alternative paradigm, a predictive design of a Dirac-like point is introduced in a linear periodic metamaterial for the spatial guidance of acoustic waves. Dirac conelike dispersion at the Г point (for k → = 0) in a Brillouin zone is called a “Dirac-like cone,” which seldom occurs due to accidental degeneracy. However, a deaf band-based predictive model shows incredible potential to achieve an engineered Dirac cone at a predictive pivoted frequency. A targeted Dirac cone at a higher frequency is carried out in this article validating the orthogonal energy transport in a spiral pattern. The dominance of asymmetric deaf band modes triggers total internal reflection and guiding of acoustic waves inside phononic crystals. To elucidate the versatility of this methodology, experimental validation of orthogonal wave transport is presented.

Auxetic metamaterials have received wide attention due to their idiosyncratic physical properties. In this paper, experiments and simulations were performed to study the mechanical and acoustic properties of a 3D anti-chiral auxetic metamaterial subjected to quasi-static loading. The results shows that Poisson's ratio of the designed metamaterial can be changed from −0.45 to 0.35 with different cell parameters. Acoustic tests show that the designed metamaterial provides an ultra-wide phononic bandgap and exhibits outstanding wave attenuation properties. By applying rational tensile/compression force, the bandgap is widened/narrowed, which indicates the tunability of the bandgap through deformation. Moreover, the bandgap tuning is through all high symmetry points in the Brillouin zone due to the iso-auxeticity of the metamaterial. This study provides a possible way for controlling the bandgap with the deformable metamaterial.

The theory of phononic friction attributes that the multiphonon processes are the main cause of the mechanical energy dissipation in a wear-free friction process. Unfortunately, it is still impossible to set up a direct relationship between the phonons and the frictional force. In this study, a classical molecular dynamics simulation model is used to mimic a piece of graphene sliding over a supported graphene substrate. It is found that the lifetime of some phonons, especially the modes around the Γ point of the first Brillouin zone, gradually decreases with the increase of the sliding velocity. A phonon lifetime-based model is proposed to explain the variation of the frictional force as a function of the sliding velocity, i.e., the shorter phonon lifetime corresponding to a higher friction force under the same temperature. This model is consistent with the traditional Prandtl-Tomlinson model at a low sliding velocity range, which predicts that the friction force increases logarithmically with the sliding velocity. Once the sliding velocity exceeds a critical value, the lifetime of the excited phonons is far longer than the time for the tip sweeping a lattice constant. In this case, the excited phonons do not have enough time to dissipate the mechanical energy, which leads to the reduced friction force with the increase of the sliding velocity.

The ternary chalcopyrite compounds and related structures are well known for their noteworthy electronic and optical properties. The interaction between monovalent and trivalent atoms has a significant influence on their electronic as well as optical behavior. In the present work, a density functional theory based first-principles calculation is performed to investigate the structural, electronic, lattice dynamical, and optical properties of rhombohedral CuRhX2 (X = S, Se, Te) compounds. The electronic band structure of these compounds depicts semiconducting nature with an indirect bandgap of 1.8, 1.17, and 0.75 eV for CuRhS2, CuRhSe2, and CuRhTe2, respectively. There is a greater hole mobility and p-type conductivity in these compounds due to strong p-d hybridization. The phonon dispersion curves of these compounds confirm their dynamical stability as there is no imaginary frequency for any of the phonon modes in the entire Brillouin zone (BZ). Furthermore, we discuss mode compatibility at the zone center of the BZ and other high symmetry points of the BZ. The Raman spectra of CuRhX2 demonstrate two Raman active modes, namely, the Eg and A1g. The frequency of Raman active modes Eg and A1g decreases due to the increase in Rh–X bond length. The static dielectric constants fall in the range of 8.7–10.4. The absorption coefficient of these compounds is in the range of 1.5–2.0 eV depending upon the ionic radii of chalcogen atoms. Thus, it can be deduced that these systems can be efficiently used in solar energy converters in the UV as well as in the visible region.

Field emission properties of zigzag graphenenanoribbons terminated with C-O-C ether groups (including cyclic and alternative ether groups at edge, denoted as ZGNR-CE and ZGNR-AE) are studied by adopting a self-consistent method based on density functional theory calculation. The results show that the field emissions of these two nanoribbons are dominated by states around Brillouin zone center and close to Fermi level. Because of lower work function, the ZGNR-CE can produce much stronger emission current than reconstructed zigzag graphenenanoribbon. The ZGNR-AE has nearly completely spin-polarized emission current, although its emission current is not strong enough. It is also found that under the lower E-field, the uniaxial strain can effectively modulate their emission currents but the spin polarization of ZGNR-AE keeps unchanged with the varied strain. The underlying mechanisms are revealed by combining the analyses of their work functions and band structures with edge dipole model.

Thermalization of a parametrically driven magnon gas leading to the formation of a Bose–Einstein condensate at the bottom of a spin-wave spectrum was studied by time- and wavevector-resolved Brillouin light scattering spectroscopy. Two distinct channels of the thermalization process related on dipolar and exchange parts of a magnon gas spectrum are clearly determined. It has been found that the magnon population in these thermalization channels strongly depends on applied microwave pumping power. The observed magnon redistribution between the channels is caused by the downward frequency shift of the magnon gas spectrum due to the decrease of the saturation magnetization in the course of injection of parametrically pumped magnons.

Atomic intercalation can be utilized to engineer the electronic structure of two dimensional layered materials at the atomic scale, thereby governing distinctive properties in comparison with the pristine ones. Herein, a minute amount of Rhenium (Re) atoms (∼1.3% wt.) were controllably intercalated inside the layers of semiconducting Zirconium diselenide (ZrSe2) single crystal. Our angle-resolved photoemission spectroscopy revealed that Re intercalation could move down the bottom of the ZrSe2 conduction band without band dispersion changes, resulting in a small electronic pocket at the Brillouin zone boundary at the M point. The subsequent low-temperature transport results further confirmed the anomalous metallic characteristics in the semiconducting ZrSe2 after low-level Re intercalation.

We analytically investigate properties of magnetostatic surface spin wave propagation in irregular narrow ferromagnetic waveguides that are important elements of magnonic logic. The developed mathematical model is based on the Galerkin method. Theoretical investigations are proved by Brillouin light scattering spectroscopy. We demonstrate that the confinement effect in the narrow waveguide leads to multimode regime propagation, wave beats, and energy redistribution. These processes can be controlled by tuning the structure and excitation parameters. A gradual change in the waveguide width can be used to vary the spin wave energy density. Our results show that the impact of the width effect and the irregularity of the waveguide on the spin wave propagation are crucial.

This work presents the results of study of the behavior of longitudinal (LA), quasilongitudinal (QLA), and transverse (TA) acoustic phonons in single crystals of Na1/2Bi1/2TiO3 (NBT) within a temperature range from 110 K to 850 K. Measurements were carried out by means of Brillouin light scattering on phonons with a wave vector qph || [100], qph || [010], and qph || [110], regarding the cubic phase of NBT. Temperature dependences of sound velocity, attenuation, and intensity of LA, QLA, and TA phonons have been plotted. Anomalies in the behavior of sound velocity and attenuation of LA phonons in the vicinity of antiferrodistortive structural phase transition from the cubic to the tetragonal phases have been found. It is shown that broad anomalies in the sound velocity and attenuation of acoustic phonons typical to diffuse phase transition are not connected directly with the anomaly in the dielectric permittivity of the crystal. Elastic C11 and C44 modules in the cubic phase of NBT have been defined.

[Pt(1.5 nm)/Co(tCo)/W(1.5 nm)]N multilayers of different Co thicknesses (tCo) and number of repeats (N) have been grown by sputtering on Si substrates, and their magnetic properties have been studied. The x-ray reflectivity has been used to measure thicknesses of each layer as well as their roughness. The dependence of the magnetic moment on tCo and N (as determined by vibrating sample magnetometry) indicates the existence of a magnetic dead layer, which increases with N and reaches its maximum values for N ≥ 3. A similar N dependence of the magnetization at saturation is found. Ferromagnetic resonance and Brillouin light scattering have been used to investigate perpendicular magnetic anisotropy, damping, and interfacial Dzyaloshinskii-Moriya interaction (iDMI) vs Co thickness and the number of Pt/Co/W sequence repeats. We show that these parameters result from interface contributions that vary in a similar way with N, confirming that the first two Pt/Co/W trilayers are of lower quality. We thus conclude that for these systems, the increase of N improves the quality of interfaces and the volume of the stack, leading to the enhancement of the magnetic properties. Moreover, the measured weak iDMI constant, even for the higher N values, suggests that most probably, this iDMI results mainly from the Pt/Co interfaces.

Based on the Maxwell duality, we consider nonuniform magnetic fields produced by nonuniform magnetic charge densities and nonuniform electric fields produced by uniform and nonuniform magnetic current densities. Then, we analyze the interaction of these fields with the magnetic quadrupole moment of a neutral particle from a semiclassical point of view by applying the Wentzel, Kramers, and Brillouin approximation.