1st Brillouin Zone Research Articles

Investigating structural, optoelectronic, and mechanical properties of novel Tungsten-based oxides double-perovskites compounds Sr2XWO6 (X= Mn, Fe): A DFT approach

We employ PF-LAPW (full-potential linear augmented plane-wave) integrated within DFT (density functional theory) for a comprehensive exploration of the structural, optoelectronic, and mechanical properties of novel Tungsten-based oxides double-perovskites compounds Sr2XWO6 (X= Mn, Fe) through the quantum mechanical WIEN2K simulation package. All the computations are done by considering TB-mBJ (Tran-Blaha modified Becke-Johnson potential) and GGA (generalized gradient approximation) as the exchange-correlation potential. The stability and formation of the Sr2XWO6 (X= Mn, Fe) compounds in a cubic structure are validated through structural optimization and the tolerance factor. The analysis of elastic constants and Born-Huang stability criteria predict that the interested oxide double perovskites are ductile, mechanically stable, hard to scratch, anisotropic, and possess a dominant covalent bonding. Both Sr2MnWO6 and Sr2FeWO6 materials are indirect semiconductors with a band gap of 2.05 eV for Sr2FeWO6 and 1.98 eV for Sr2MnWO6 from W-L symmetry points within the 1st Brillouin zone. The small values of band gap and various parameters of optical properties, specifically the broad monotonically increasing absorption spectra, reveal that novel Tungsten-based oxides double-perovskites compounds Sr2XWO6 (X= Mn, Fe) exhibit great potential for optoelectronic properties.

Strain to shine: stretching-induced three-dimensional symmetries in nanoparticle-assembled photonic crystals

Stretching elastic materials containing nanoparticle lattices is common in research and industrial settings, yet our knowledge of the deformation process remains limited. Understanding how such lattices reconfigure is critically important, as changes in microstructure lead to significant alterations in their performance. This understanding has been extremely difficult to achieve due to a lack of fundamental rules governing the rearrangements. Our study elucidates the physical processes and underlying mechanisms of three-dimensional lattice transformations in a polymeric photonic crystal from 0% to over 200% strain during uniaxial stretching. Corroborated by comprehensive experimental characterizations, we present analytical models that precisely predict both the three-dimensional lattice structures and the macroscale deformations throughout the stretching process. These models reveal how the nanoparticle lattice and matrix polymer jointly determine the resultant structures, which breaks the original structural symmetry and profoundly changes the dispersion of photonic bandgaps. Stretching induces shifting of the main pseudogap structure out from the 1st Brillouin zone and the merging of different symmetry points. Evolutions of multiple photonic bandgaps reveal potential optical singularities shifting with strain. This work sets a new benchmark for the reconfiguration of soft material structures and may lay the groundwork for the study of stretchable three-dimensional topological photonic crystals.

In-plane Fermi surface mapping of ZrSiS and HfSiS by de Haas-van Alphen oscillations

We have conducted a comprehensive experimental investigation of the in-plane de Haas van Alphen quantum oscillations in ZrSiS and HfSiS and compared the results to the ab-initio calculations. We found a moderately good agreement for a set of frequencies located between 100 T and 200 T. These could be associated with extremal orbitals of the electron pockets located around the k_z=\pm\pi/2ckz=±π/2c planes at the edge of the 1st Brillouin zone. In contrast, the experimentally detected low-frequency set of oscillations found around 20 T could not be reproduced by the calculations. Further refinement of the ab-initio calculation has to be conducted to fully capture all the observed features of the Fermi surface in ZrSiS and HfSiS.

Anisotropic Phonon Bands in H-Bonded Molecular Crystals: The Instructive Case of α-Quinacridone

Phonons play a crucial role in the thermodynamic and transport properties of solid materials. Nevertheless, rather little is known about phonons in organic semiconductors. Thus, we employ highly reliable quantum mechanical calculations for studying the phonons in the α-polymorph of quinacridone. This material is particularly interesting, as it has highly anisotropic properties with distinctly different bonding types (H-bonding, π-stacking, and dispersion interactions) in different spatial directions. By calculating the overlaps of modes in molecular quinacridone and the α-polymorph, we associate Γ-point phonons with molecular vibrations to get a first impression of the impact of the crystalline environment. The situation becomes considerably more complex when analyzing phonons in the entire 1st Brillouin zone, where, due to the low symmetry of α-quinacridone, a multitude of avoided band crossings occur. At these, the character of the phonon modes typically switches, as can be inferred from mode participation ratios and mode longitudinalities. Notably, avoided crossings are observed not only as a function of the length but also as a function of the direction of the phonon wave vector. Analyzing these avoided crossings reveals how it is possible that the highest frequency acoustic band is always the one with the largest longitudinality, although longitudinal phonons in different crystalline directions are characterized by fundamentally different molecular displacements. The multiple avoided crossings also give rise to a particularly complex angular dependence of the group velocities, but combining the insights from the various studied quantities still allows drawing general conclusions, e.g., on the relative energetics of longitudinal vs transverse deformations (i.e., compressions and expansions vs slips of neighboring molecules). They also reveal how phonon transport in α-quinacridone is impacted by the reinforcing H-bonds and by π-stacking interactions (resulting from a complex superposition of van der Waals, charge penetration, and exchange repulsion).

(Invited) Spin-Valley Coupling in Monolayer Transition Metal Dichalcogenides

The monolayers of group VI transition metal dichalcogenides feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. In this talk we discuss the spin-valley coupling in monolayers and multilayers WS2. We explored the interplay of spin and valley degrees of freedom with photocurrent experiments. We experimentally demonstrate that this giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.

Dirac points with giant spin-orbit splitting in the electronic structure of two-dimensional transition-metal carbides

Two-dimensional (2D) materials, especially their most prominent member, graphene, have greatly influenced many scientific areas. Moreover, they have become a base for investigating the relativistic properties of condensed matter within the emerging field of Dirac physics. This has ignited an intense search for new materials where charge carriers behave as massless or massive Dirac fermions. Here, we theoretically show the existence of Dirac electrons in a series of 2D transition-metal carbides, known as MXenes. They possess twelve conical crossings in the 1st Brillouin zone with giant spin-orbit splitting. Our findings indicate that the 2D band structure of MXenes is protected against external perturbations and preserved even in multilayer phases. These results, together with the broad possibilities to engineer the properties of these materials phases, make Dirac MXenes a potential candidate for studying and developing novel Dirac-physics-based technologies.

The Study of Spin‐Valley Coupling in Atomically Thin Group VI Transition Metal Dichalcogenides

In the hunt for ultimately thin electronic devices, atomically thin layers of group VI transition metal dichalcogenides (TMDCs) are recognized as ideal 2D materials after the success of graphene. Monolayer TMDCs feature nonzero but contrasting Berry curvatures and valence-band spin splitting with opposite sign at inequivalent K and K' valleys located at the corners of the 1st Brillouin zone. These features raise the possibility of manipulating electrons' valley and spin degrees of freedom by optical and electric means, which subsequently makes monolayer TMDCs promising candidates for spintronics and valleytronics applications.

Abandoned Functionality of Thin‐Film Opal Photonic Crystals

Angle‐resolved polarized transmission spectra of thin‐film opals are studied experimentally and theoretically as a function of the azimuthal rotation of the plane of incidence at different angles of light incidence. In such 3‐dimensional lattices, the refraction acquires the form of diffraction orders, each with a distinct spectrum, that propagate simultaneously along different directions. The corresponding continuous and patchy stop‐bands in the photonic energy band structure of the opal photonic crystal are determined numerically. For diffraction at high Miller index planes, the diffraction pattern is often distorted due to multiple‐wave diffraction. The finite stacking of hexagonally close‐packed layers leads to an intrinsic 3‐fold rotation symmetry of these spectra, which is particularly pronounced in transmission for frequencies that correspond to wave vectors within the 1st Brillouin zone. Despite the cubic symmetry of the opal lattice, cross‐polarization coupling effects occur in the volume of the opal crystal. These effects are associated with the fact that, the opal eigenmodes are Bloch waves whose electromagnetic field distributions and dispersion relations are markedly different from plane waves.

Impacts of uniaxial elongation on the bandstructures of two-dimensional sonic crystals and associated applications

Influences of uniaxial elongation along the [11] direction of triangular and [10] direction of square sonic crystals under the constraint of conserved unit cell area are investigated by examining band structures and equi-frequency contours. Lowest-lying band gap of the triangular lattice observed at high filling fractions diminishes for negative elongation (compression), whereas another band gap develops at lower frequencies whose width reaches appreciable values for moderate elongation. The band gap of the square lattice, which appears at high filling fractions, is modified slightly with elongation. Frequency ranges of the bands, and thus the group velocities along the high-symmetry directions, vary with elongation which may be useful in applications like slow sound propagation. Elongation is observed to modify the equi-frequency contours significantly through reducing the lattice symmetry. The most prominent impact is the transformation of closed contours into open ones, whereas the rest are stretched either along or normal to the elongation axis of the 1st Brillouin Zone. This observation is utilized to implement wide-band all-angle self-collimation and superprism effect, which are demonstrated through Finite-Element computations.

Laser‐generated surface acoustic waves in copper line arrays on silicon

Laser‐generated acoustics waves are widely used to control the fabrication on metal interconnect structures in the semiconductor industry. On the other hand, acoustic properties of such structures, typically comprised of periodic arrays of metal lines embedded in a dielectric film on a silicon substrate, pose some interesting wave propagation problems. We will report measurements of surface acoustic waves (SAWs) performed using laser‐induced transient grating technique on periodic arrays of micron‐wide copper lines. As expected, SAW dispersion curves yield small band gaps at the Brillouin zone boundary. However, much larger band gaps are observed within the 1st Brillouin zone. We believe that these giant band gaps are formed due to avoided crossing of the 1st and 2nd order surface acoustic modes of the film/substrate structure when the 1st mode dispersion curve becomes folded due to periodicity. Another observed phenomenon is a spectacularly sharp increase in the acoustic attenuation occurring when the folded surface mode becomes coupled to a bulk wave.

Incoherent modulation instability in a nonlinear photonic lattice

We study modulation instability (MI) of random-phase waves in nonlinear photonic lattices. We find that an incoherent superposition of extended nonlinear eigenstates of the system, that is, an incoherent extended stationary beam, may become unstable due to nonlinearity. The instability process depends on the nonlinearity, on the structure of the diffraction curves of the lattice, as well as on the properties of the beam, whose spectrum can be comprised of Bloch modes from different bands, and from different regions of diffraction (normal/anomalous). This interplay among diffraction, incoherence, and nonlinearity leads to a variety of phenomena, including the possibility of tailoring the diffraction curve of the lattice, or the coherence properties of the beam, to enhance or suppress the instability. We present several examples of such phenomena, including a case where increasing the lattice depth flattens the diffraction curve thereby enhancing the instability, "locking" the most unstable mode to the edge of the 1st Brillouin zone for large nonlinearity, and incoherent MI in self-defocusing media.

Studying disorder in graphite-based systems by Raman spectroscopy

Raman spectroscopy has historically played an important role in the structural characterization of graphitic materials, in particular providing valuable information about defects, stacking of the graphene layers and the finite sizes of the crystallites parallel and perpendicular to the hexagonal axis. Here we review the defect-induced Raman spectra of graphitic materials from both experimental and theoretical standpoints and we present recent Raman results on nanographites and graphenes. The disorder-induced D and D' Raman features, as well as the G'-band (the overtone of the D-band which is always observed in defect-free samples), are discussed in terms of the double-resonance (DR) Raman process, involving phonons within the interior of the 1st Brillouin zone of graphite and defects. In this review, experimental results for the D, D' and G' bands obtained with different laser lines, and in samples with different crystallite sizes and different types of defects are presented and discussed. We also present recent advances that made possible the development of Raman scattering as a tool for very accurate structural analysis of nano-graphite, with the establishment of an empirical formula for the in- and out-of-plane crystalline size and even fancier Raman-based information, such as for the atomic structure at graphite edges, and the identification of single versus multi-graphene layers. Once established, this knowledge provides a powerful machinery to understand newer forms of sp(2) carbon materials, such as the recently developed pitch-based graphitic foams. Results for the calculated Raman intensity of the disorder-induced D-band in graphitic materials as a function of both the excitation laser energy (E(laser)) and the in-plane size (L(a)) of nano-graphites are presented and compared with experimental results. The status of this research area is assessed, and opportunities for future work are identified.

De Haas–van Alphen Effect in LaB2C2

Fermi surfaces of LaB 2 C 2 were investigated by the measurement of the de Haas–van Alphen (dHvA) oscillations. The angular dependence of the dHvA frequency indicates the presence of the multiple three-dimensional closed Fermi surfaces, that is, the 5d orbitals in lanthanum strongly hybridize with the 2p orbitals in boron-carbon sheets. The angular dependence of the dHvA frequency coincides very well with that predicted by the band calculations. The branch having the highest frequency larger than cross-section of the 1st Brillouin zone corresponds to the large orbit originating from magnetic breakdown.

Angle-resolved photoemission from Cu(001): Line widths at highest angular resolution

A detailed experimental study of the effect of angular resolution on the line widths of transitions from Cu(001) is presented. We find a dramatic narrowing of the direct transition lines from the volume sp band when reducing the angular acceptance cone much below 1° to an ultimate value of a few tenths of a degree. This effect occurs for transitions located in the 2nd Brillouin zone (BZ) and for the d band surface state near the M point of the surface BZ, whereas such a strong effect is not observed for transitions from sp bands in the first BZ. In the 2nd BZ we obtain widths of 0.25eV as close as 1.0eV from the Fermi edge, while previously published widths for transitions from the 1st BZ are of the order of 0.7eV or more for the Cu sp transitions. We find a more or less linear increase of line width as the Fermi level is approached.

De Haas-van Alphen effect in κ-(DMET)2AuBr2

We have performed magnetization measurements of the title compound up to 12 tesla and down to 0.5 K in order to examine the Fermi-surface topology. The de Haas-van Alphen effect has been clearly observed. This is the first report on the quantum oscillation in κ-phase compound other than the BEDT-TTF salts. The oscillation has two components with different frequencies. The faster one corresponds just to the cross-sectional area of the 1st Brillouin zone and the other to 21 % of that. Such a cross-sectional profile of the Fermi surface is essentially the same as in the κ-phase of BEDT-TTF compounds. However, the simultaneous oscillation even at low field in this system is different from the result for κ-(BEDT-TTF)2Cu(NCS)2, where the faster oscillation is reported to occur only at higher field as a magnetic breakdown effect. The present result implies that κ-(DMET)2AuBr2 has no gap against the crossing of the Fermi surface at the zone boundary unlikely κ-(BEDT-TTF)2Cu(NCS)2. This comes from the difference in the space group of the two crystals. The effective mass for each orbit is estimated.