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Showing papers on "Brillouin zone published in 2022"


Journal ArticleDOI
TL;DR: In this paper , the electronic nature of the charge density wave (CDW) phase in high-resolution angle-resolved photoemission measurements on KV3Sb5 superconductors was revealed.
Abstract: The Kagome superconductors AV3Sb5 (A=K, Rb, Cs) have received enormous attention due to their nontrivial topological electronic structure, anomalous physical properties and superconductivity. Unconventional charge density wave (CDW) has been detected in AV3Sb5. High-precision electronic structure determination is essential to understand its origin. Here we unveil electronic nature of the CDW phase in our high-resolution angle-resolved photoemission measurements on KV3Sb5. We have observed CDW-induced Fermi surface reconstruction and the associated band folding. The CDW-induced band splitting and the associated gap opening have been revealed at the boundary of the pristine and reconstructed Brillouin zones. The Fermi surface- and momentum-dependent CDW gap is measured and the strongly anisotropic CDW gap is observed for all the V-derived Fermi surface. In particular, we have observed signatures of the electron-phonon coupling in KV3Sb5. These results provide key insights in understanding the nature of the CDW state and its interplay with superconductivity in AV3Sb5 superconductors.

82 citations


Journal ArticleDOI
TL;DR: In this article , the authors proposed a new material with a charge-two Weyl point phonons (WPPs) and two charge-one WPPs at high-symmetry points, which can support double-helicoid phonon surface states.
Abstract: In recent years, charge-two Weyl point phonons (WPPs) have attracted increasing attention. Charge-two WPPs not only provide a new platform for realizing phonon-based unconventional Weyl points (WPs) but also help in realizing specific phonon-based transport behaviors. Herein, based on the first-principles calculations and symmetry analysis, we propose a realistic material with the $P{3}_{1}21$ space group, ${\mathrm{BaZnO}}_{2}$, which has a charge-two WPP and two charge-one WPPs at high-symmetry points. These three unpaired WPPs form a unique triangular Weyl complex. Unlike previously reported material candidates with type-I or type-II charge-two WPPs, the proposed ${\mathrm{BaZnO}}_{2}$ has a type-III charge-two WPP, which has a constant frequency surface that contains two electronlike or holelike states connected at the charge-two WP. ${\mathrm{BaZnO}}_{2}$ can support double-helicoid phonon surface states that cover the entire (001) surface Brillouin zone. The clean type-III WPP, unique triangular Weyl complex, and clear and long surface states in ${\mathrm{BaZnO}}_{2}$ suggest that it is an excellent platform for further research into the physics and applications of type-III charge-two WPPs. Furthermore, we pointed out that charge-two WPPs with type-III band dispersion may appear at high-symmetry points in space groups 75--80, 89--98, 143--146, 149--155, 168--173, 177--182, 196, 207--210. Besides trigonal ${\mathrm{BaZnO}}_{2}$, some material candidates, including tetragonal ${\mathrm{MgTiO}}_{4}$, trigonal ${\mathrm{Li}}_{2}{\mathrm{GeF}}_{6}$, hexagonal ${\mathrm{CaSO}}_{4}$, and cubic ${\mathrm{Li}}_{10}{\mathrm{B}}_{14}{\mathrm{Cl}}_{2}{\mathrm{O}}_{25}$, are also shown to be the hosts of type-III charge-two WPPs.

34 citations


Journal ArticleDOI
TL;DR: In this article , the design and development of a cascaded Brillouin laser based utilizing a free-space optical layout with a diamond crystal as the gain medium was reported, and a quasi-continuous-wave, 1'μm laser was used as the pump laser.
Abstract: Cascaded Brillouin lasers based on guided-wave structures are applied across a range of important fields such as optical communications, microwave photonics, and sensing. However, restricted by the volume and available transmission range of the gain medium, the power output and wavelength diversity of guided-wave Brillouin devices are somewhat limited. In this work, we report the design and development of a cascaded Brillouin laser based utilizing a free-space optical layout with a diamond crystal as the Brillouin gain medium. A quasi-continuous-wave, 1 μm laser was used as the pump laser, and Raman wavelength conversion is used as an intermediate process to facilitate stimulated Brillouin scattering with a low threshold. When the output transmission of the diamond cavity is 0.37% and the incident pump power is 220 W, cascading of the Brillouin–Stokes field to the eighth Stokes and the seventh anti-Stokes orders was observed. By adjusting the cavity length, the order of the cascaded Brillouin laser output is controlled. A comprehensive analysis of the Brillouin generation process and the cascade of Stokes orders is undertaken for different incident pump powers and cavity lengths. This work is expected to enable practical applications of high-power Brillouin lasers and Brillouin frequency combs.

28 citations


Journal ArticleDOI
TL;DR: In this article , the authors proposed a series of materials with two-nodal phonons in their phonon dispersions, which are dominated by twofold screw symmetry and time-reversal symmetry.
Abstract: This year, researchers have been on the lookout for real materials with one-nodal, two-nodal (two-NS), and three-nodal surface phonons. However, materials with two-NS phonons have been scarce until recently. This paper contributes to the understanding of the symmetry conditions of two-NS phonons. Two-NS phonons have NS states that are localized on two of three ${k}_{i}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ $(i=x,y,z)$ planes in the three-dimensional Brillouin zone). They are dominated by twofold screw symmetry and time-reversal symmetry. This paper also contributes the prediction of a series of materials with two-NS states in their phonon dispersions. First, by screening all 230 space groups (SGs), we discovered 19 SG candidates (with Nos. 18, 55--60, 90, 94, 113, 114, 127--130, and 135--138) that have two-NS phonons. Second, based on first-principles calculations, we proposed 19 realistic material candidates hosting two-NS phonons: $P{2}_{1}{2}_{1}2$-type ${\mathrm{ZnTeMoO}}_{6}$ (with SG No. 18), $Pbma$-type ${\mathrm{Cs}}_{2}{\mathrm{Te}}_{2}$ (with SG No. 55), $Pccn$-type ${\mathrm{Sr}}_{2}{\mathrm{SnO}}_{4}$ (with SG No. 56), $Pbcm$-type CaAlPd (with SG No. 57), $Pnnm$-type ${\mathrm{PtO}}_{2}$ (with SG No. 58), $Pmmn$-type ${\mathrm{KLi}}_{2}\mathrm{As}$ (with SG No. 59), $Pbcn$-type ${\mathrm{CV}}_{2}$ (with SG No. 60), $P{42}_{1}2$-type ${\mathrm{BaVCu}}_{4}{\mathrm{P}}_{4}{\mathrm{O}}_{17}$ (with SG No. 90), $P{4}_{2}{2}_{1}2$-type ${\mathrm{Na}}_{5}{\mathrm{Fe}}_{3}{\mathrm{F}}_{14}$ (with SG No. 94), $P\overline{4}{2}_{1}m$-type ${\mathrm{BaS}}_{3}$ (with SG No. 113), $P\overline{4}{2}_{1}c$-type ${\mathrm{Na}}_{4}{\mathrm{SnS}}_{4}$ (with SG No. 114), $P4/mbm$-type ${\mathrm{ReO}}_{3}$ (with SG No. 127), $P4/mnc$-type ${\mathrm{Sr}}_{4}{\mathrm{Li}}_{2}{\mathrm{Si}}_{4}{\mathrm{N}}_{8}\mathrm{O}$ (with SG No. 128), $P4/nmm$-type ${\mathrm{BaHfN}}_{2}$ (with SG No. 129), $P4/ncc$-type ${\mathrm{Bi}}_{2}{\mathrm{CuO}}_{4}$ (with SG No. 130), $P{4}_{2}/mbc$-type ${\mathrm{YB}}_{2}\mathrm{C}$ (with SG No. 135), $P{4}_{2}/mnm$-type ${\mathrm{MgF}}_{2}$ (with SG No. 136), $P{4}_{2}/nmc$-type ${\mathrm{YB}}_{4}{\mathrm{Rh}}_{4}$ (with SG No. 137), and $P{4}_{2}/ncm$-type ${\mathrm{LiClO}}_{2}$ (with SG No. 138). Third, we discovered 622 (out of 10 037) materials with two-NS phonons by checking the phonon database at Kyoto University. Our present paper provides a better understanding of the two-NS states in phonon systems (or even other bosonic systems) and suggests a huge number of material candidates with two-NS phonons.

25 citations


Journal ArticleDOI
TL;DR: In this article , the primary and secondary order parameters, as well as their interplay, in the charge density wave (CDW) state of the kagome metal AV3Sb5 were investigated.
Abstract: We employ polarization-resolved electronic Raman spectroscopy and density functional theory to study the primary and secondary order parameters, as well as their interplay, in the charge density wave (CDW) state of the kagome metal AV3Sb5. Previous x-ray diffraction data at 15K established that the CDW order in CsV3Sb5 comprises of a 2x2x4 structure: one layer of inverse-star-of-David and three consecutive layers of star-of-David pattern. We analyze the lattice distortions based the 2x2x4 structure at 15K, and find that U lattice distortion is the primary order parameter while M and L distortions are secondary order parameters for Vanadium displacements. This conclusion is confirmed by the calculation of bare susceptibility that shows a broad peak at around qz=0.25 along the hexagonal Brillouin zone face central line (U-line). We also identify several phonon modes emerging in the CDW state, which are lattice vibration modes related to V and Sb atoms as well as alkali atoms. The detailed temperature evolution of these modes' frequencies, HWHM, and integrated intensities support a phase diagram with two successive structural phase transitions in CsV3Sb5: the first one with a primary order parameter appearing at TS=94K and the second isostructural one appearing at around 70K.

24 citations


Journal ArticleDOI
TL;DR: In this paper , the authors demonstrate a pathway towards an oscillator-based IM using arrays of nanoconstriction spin Hall nano-oscillators (SHNOs) and show how SHNOs can be readily phase binarized and how their resulting microwave power corresponds to well defined global phase states.
Abstract: Ising machines (IMs) are physical systems designed to find solutions to combinatorial optimization (CO) problems mapped onto the IM via the coupling strengths between its binary spins. Using its intrinsic dynamics and different annealing schemes, the IM relaxes over time to its lowest-energy state, which is the solution to the CO problem. IMs have been implemented on different platforms, and interacting nonlinear oscillators are particularly promising candidates. Here we demonstrate a pathway towards an oscillator-based IM using arrays of nanoconstriction spin Hall nano-oscillators (SHNOs). We show how SHNOs can be readily phase binarized and how their resulting microwave power corresponds to well-defined global phase states. To distinguish between degenerate states, we use phase-resolved Brillouin-light-scattering microscopy and directly observe the individual phase of each nanoconstriction. Micromagnetic simulations corroborate our experiments and confirm that our proposed IM platform can solve CO problems, showcased by how the phase states of a $2\ifmmode\times\else\texttimes\fi{}2$ SHNO array are solutions to a modified max-cut problem. Compared with the commercially available D-Wave ${\mathrm{Advantage}}^{\mathrm{TM}}$, our architecture holds significant promise for faster sampling, substantially reduced power consumption, and a dramatically smaller footprint.

24 citations


Journal ArticleDOI
10 Jan 2022-eLife
TL;DR: In this paper , a method combining optical diffraction tomography and epifluorescence imaging for explicitly measuring the Brillouin shift, refractive index (RI), and absolute density with specificity to fluorescently labeled structures is presented.
Abstract: Quantitative measurements of physical parameters become increasingly important for understanding biological processes. Brillouin microscopy (BM) has recently emerged as one technique providing the 3D distribution of viscoelastic properties inside biological samples - so far relying on the implicit assumption that refractive index (RI) and density can be neglected. Here, we present a novel method (FOB microscopy) combining BM with optical diffraction tomography and epifluorescence imaging for explicitly measuring the Brillouin shift, RI, and absolute density with specificity to fluorescently labeled structures. We show that neglecting the RI and density might lead to erroneous conclusions. Investigating the nucleoplasm of wild-type HeLa cells, we find that it has lower density but higher longitudinal modulus than the cytoplasm. Thus, the longitudinal modulus is not merely sensitive to the water content of the sample - a postulate vividly discussed in the field. We demonstrate the further utility of FOB on various biological systems including adipocytes and intracellular membraneless compartments. FOB microscopy can provide unexpected scientific discoveries and shed quantitative light on processes such as phase separation and transition inside living cells.

23 citations


Journal ArticleDOI
TL;DR: In this article , the authors demonstrate that the electronic structure of a material can be deformed into Floquet pseudobands with arbitrarily tailored shapes with a combination of quantum optimal control theory and Floquet engineering.
Abstract: We demonstrate that the electronic structure of a material can be deformed into Floquet pseudobands with arbitrarily tailored shapes. We achieve this goal with a combination of quantum optimal control theory and Floquet engineering. The power and versatility of this framework is demonstrated here by utilizing the independent-electron tight-binding description of the π electronic system of graphene. We show several prototype examples focusing on the region around the K (Dirac) point of the Brillouin zone: creation of a gap with opposing flat valence and conduction bands, creation of a gap with opposing concave symmetric valence and conduction bands (which would correspond to a material with an effective negative electron-hole mass), and closure of the gap when departing from a modified graphene model with a nonzero field-free gap. We employ time-periodic drives with several frequency components and polarizations, in contrast to the usual monochromatic fields, and use control theory to find the amplitudes of each component that optimize the shape of the bands as desired. In addition, we use quantum control methods to find realistic switch-on pulses that bring the material into the predefined stationary Floquet band structure, i.e., into a state in which the desired Floquet modes of the target bands are fully occupied, so that they should remain stroboscopically stationary, with long lifetimes, when the weak periodic drives are started. Finally, we note that although we have focused on solid state materials, the technique that we propose could be equally used for the Floquet engineering of ultracold atoms in optical lattices and for other nonequilibrium dynamical and correlated systems.

21 citations


Journal ArticleDOI
TL;DR: In this paper , the fundamental relationships among the COF electronic structures, the symmetries of their 2D lattices, and the frontier molecular orbitals (MOs) of their core and linker components are discussed.
Abstract: Two-dimensional covalent organic frameworks (2D-COFs), also referred to as 2D polymer networks, display unusual electronic-structure characteristics, which can significantly enrich and broaden the fields of electronics and spintronics. In this Focus article, our objective is to lay the groundwork for the conceptual description of the fundamental relationships among the COF electronic structures, the symmetries of their 2D lattices, and the frontier molecular orbitals (MOs) of their core and linker components. We focus on monolayers of hexagonal COFs and use tight-binding model analyses to highlight the critical role of the frontier-MO symmetry, in addition to lattice symmetry, in determining the nature of the electronic bands near the Fermi level. We rationalize the intriguing feature that, when the core unit has degenerate highest occupied MOs [or lowest unoccupied MOs], the COF highest valence band [or lowest conduction band] is flat but degenerate with a dispersive band at a high-symmetry point of the Brillouin zone; the consequences of having such band characteristics are briefly described. Multi-layer and bulk 2D COFs are found to maintain the salient features of the monolayer electronic structures albeit with a reduced bandgap due to the interlayer coupling. This Focus article is thus meant to provide an effective framework for the engineering of flat and Dirac bands in 2D polymer networks.

19 citations


Journal ArticleDOI
TL;DR: In this article , the authors prove rigorous Bloch theorems for hyperbolic lattices through the identification of appropriate periodic boundary conditions, which is generally nonabelian in nature and involves infinitely many Brillouin zones for a single lattice.
Abstract: Significance Recent experiments in circuit quantum electrodynamics and electric circuit networks have demonstrated the coherent propagation of wave-like excitations on hyperbolic lattices. The negative curvature of space that underlies such lattices invalidates the familiar Bloch theorem of ordinary solid-state physics. In this work, we prove rigorous Bloch theorems for hyperbolic lattices through the identification of appropriate periodic boundary conditions. Unlike the ordinary Bloch theorem for crystalline lattices, our hyperbolic Bloch theorem is generally nonabelian in nature and involves infinitely many Brillouin zones for a single lattice. Our work initiates a chapter in band theory and establishes deep connections between condensed matter physics and algebraic geometry.

18 citations


Journal ArticleDOI
TL;DR: In this paper , femtosecond photoemission momentum microscopy is used to obtain energy-momentum fingerprints of the interlayer excitons by mapping their spectral signatures within the mini Brillouin zone that is built up by the twisted heterostructure.
Abstract: Moir\'e superlattices in atomically thin van-der-Waals heterostructures hold great promise for an extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moir\'e lattices, and the formation of novel exotic quantum phases. Such moir\'e-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. In order to exploit the full potential of correlated moir\'e and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement in the moir\'e potential is indispensable. However, direct experimental access to these parameters is limited since most excitonic quasiparticles are optically dark. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moir\'e interlayer excitons. We find that interlayer excitons are dominantly formed on the sub-50~fs timescale via interlayer tunneling at the K valleys of the Brillouin zones. In addition, we directly measure energy-momentum fingerprints of the moir\'e interlayer excitons by mapping their spectral signatures within the mini Brillouin zone that is built up by the twisted heterostructure. From these momentum-fingerprints, we gain quantitative access to the modulation of the exciton wavefunction within the moir\'e potential in real-space. Our work provides the first direct access to the interlayer moir\'e exciton formation dynamics in space and time and reveals new opportunities to study correlated moir\'e and exciton physics for the future realization of exotic quantum phases of matter.

Journal ArticleDOI
TL;DR: In this article , the authors argue that the nonsymmorphic symmetry of the crystal lattice enables the spin polarization of the Bloch states at the Brillouin zone edges to be parallel to the edges, which ensures a near-degeneracy between even and odd-parity superconducting channels.
Abstract: CeRh${}_{2}$As${}_{2}$ has attracted attention for a field-induced transition between two distinct superconducting states. The authors argue that the nonsymmorphic symmetry of the crystal lattice enables this. This symmetry enforces the spin polarization of the Bloch states at the Brillouin zone edges to be parallel to the edges. Such a spin polarization ensures a near-degeneracy between even- and odd-parity superconducting channels, provided the Fermi surface sits near these edges. First-principles calculations incorporating electronic correlations reveal a Fermi surface consistent with this picture.

Journal ArticleDOI
12 Jan 2022-Optica
TL;DR: In this article , a series of lasing modes that experience Brillouin amplification at discrete spatial locations in the test fiber is introduced, which enables precise measurements of this lasing frequency.
Abstract: Brillouin fiber sensors can provide distributed strain and temperature measurements over long distances in standard off-the-shelf fiber by measuring the Brillouin frequency shift as a function of position along a fiber. The primary drawback of these systems is their limited sensitivity, which results from the challenge in identifying the Brillouin frequency shift to within a small fraction of the Brillouin linewidth. In this work, we introduce a technique that overcomes this fundamental limitation by establishing a series of lasing modes that experience Brillouin amplification at discrete spatial locations in the test fiber. The linewidth narrowing and high intensity associated with the lasing transition enable precise measurements of this lasing frequency. As an initial demonstration, we present a sensor that simultaneously excites 40 lasing modes in a 400 m fiber, providing a measurement of the strain at 40 discrete locations with a spatial resolution of 4 m. Each sensor exhibits a minimum detectable strain as low as 4 n ε / H z 1 / 2 with a dynamic range of > 5 m ε and a bandwidth of 10 k H z . As the first demonstration that Brillouin lasing can be used for distributed fiber sensing, this work establishes an approach that could enable ultrahigh strain sensitivity using off-the-shelf fiber.

Journal ArticleDOI
TL;DR: In this paper , the authors analyze the band topology of acoustic phonons in 2D materials by considering the interplay of spatial and internal symmetries with additional constraints that arise from the physical context.
Abstract: We analyze the band topology of acoustic phonons in 2D materials by considering the interplay of spatial and internal symmetries with additional constraints that arise from the physical context. These supplemental constraints trace back to the Nambu-Goldstone theorem and the requirements of structural stability. We show that this interplay can give rise to previously unaddressed non-trivial nodal charges that are associated with the crossing of the acoustic phonon branches at the center ($\Gamma$-point) of the phononic Brillouin zone. We moreover apply our perspective to the concrete context of graphene, where we demonstrate that the phonon spectrum harbors these kinds of non-trivial nodal charges. Apart from its fundamental appeal, this analysis is physically consequential and dictates how the phonon dispersion is affected when graphene is grown on a substrate. Given the generality of our framework, we anticipate that our strategy that thrives on combining physical context with insights from topology should be widely applicable in characterizing systems beyond electronic band theory.

Journal ArticleDOI
TL;DR: In this article , the authors report an unusual linking-number invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet and show that each loop links each other loop twice.
Abstract: Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state1-13. Over the past decades, these invariants have come to play a central role in describing matter, providing the foundation for understanding superfluids5, magnets6,7, the quantum Hall effect3,8, topological insulators9,10, Weyl semimetals11-13 and other phenomena. Here we report an unusual linking-number (knot theory) invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet14-20. Using state-of-the-art spectroscopic methods, we directly observe three intertwined degeneracy loops in the material's three-torus, T3, bulk Brillouin zone. We find that each loop links each other loop twice. Through systematic spectroscopic investigation of this linked-loop quantum state, we explicitly draw its link diagram and conclude, in analogy with knot theory, that it exhibits the linking number (2, 2, 2), providing a direct determination of the invariant structure from the experimental data. We further predict and observe, on the surface of our samples, Seifert boundary states protected by the bulk linked loops, suggestive of a remarkable Seifert bulk-boundary correspondence. Our observation of a quantum loop link motivates the application of knot theory to the exploration of magnetic and superconducting quantum matter.

Journal ArticleDOI
TL;DR: In this article , a diamond Brillouin laser (DBL) employing doubly resonant technology at 1064nm was reported, achieving an output power of 22.5 W with a linewidth of 46.9 kHz.
Abstract: Stimulated Brillouin scattering (SBS), with its advantages of low quantum defect and narrow gain bandwidth, has recently enabled an exciting path toward narrow-linewidth and low-noise lasers. Whereas almost all work to date has been in guided-wave configurations, adaptation to unguided Brillouin lasers (BLs) offers a greater capacity for power scaling, cascaded Stokes control, and greater flexibility for expanding wavelength range. Here, we report a diamond Brillouin laser (DBL) employing doubly resonant technology at 1064 nm. Brillouin output power of 22.5 W with a linewidth of 46.9 kHz is achieved. The background noise from the pump amplified spontaneous emission (ASE) is suppressed by 35 dB. The work represents a significant step toward realizing Brillouin oscillators that simultaneously have high power (tens-of-watts+) and kHz-linewidths.

Journal ArticleDOI
TL;DR: In this article , the authors show that a sizable amplitude of DMI can be achieved in CoPt or FePt films with BMA and strong spin-orbit coupling.
Abstract: A broken interfacial inversion symmetry in ultrathin ferromagnet/heavy metal (FM/HM) bilayers is generally believed to be a prerequisite for accommodating the Dzyaloshinskii-Moriya interaction (DMI) and for stabilizing chiral spin textures. In these bilayers, the strength of the DMI decays as the thickness of the FM layer increases and vanishes around a few nanometers. In the present study, through synthesizing relatively thick films of compositions CoPt or FePt, CoCu or FeCu, FeGd and FeNi, contributions to DMI from the composition gradient-induced bulk magnetic asymmetry (BMA) and spin-orbit coupling (SOC) are systematically examined. Using Brillouin light scattering spectroscopy, both the sign and amplitude of DMI in films with controllable direction and strength of BMA, in the presence and absence of SOC, are experimentally studied. In particular, we show that a sizable amplitude of DMI (±0.15 mJ/m^{2}) can be realized in CoPt or FePt films with BMA and strong SOC, whereas negligible DMI strengths are observed in other thick films with BMA but without significant SOC. The pivotal roles of BMA and SOC are further examined based on the three-site Fert-Lévy model and first-principles calculations. It is expected that our findings may help to further understand the origin of chiral magnetism and to design novel noncollinear spin textures.

Journal ArticleDOI
TL;DR: In this paper , the authors connect the topological geometry of the open-boundary spectrum with the Brillouin zone (GBZ) and provide an efficient numerical method capable of calculating them accurately.
Abstract: Periodic-boundary spectrum, open-boundary spectrum, as well as the generalized Brillouin zone (GBZ) are three essential properties of a one-dimensional non-Hermitian system. In this paper we illustrate that the deep connections between them can be revealed by a series of special similar transformations. This viewpoint closely connects the topological geometry of the open-boundary spectrum with the GBZ and provides an efficient numerical method capable of calculating them accurately. We further extend these connections to non-Hermitian systems in the symplectic symmetry class. We show that if just the open-boundary features of a non-Hermitian system such as the spectrum and the GBZ, are concerned, the relevant symmetry we should consider is not that of the original system itself, but that of one that has higher symmetry and is related to the original system by a similarity transformation.

Journal ArticleDOI
TL;DR: In this article , the stiffness tensor for a lamellar network of collagen fibrils and angle-resolved Brillouin measurements were used to determine the longitudinal stiffness coefficients describing the ex vivo porcine cornea as a transverse isotropic material.
Abstract: Abstract Load-bearing tissues are typically fortified by networks of protein fibers, often with preferential orientations. This fiber structure imparts the tissues with direction-dependent mechanical properties optimized to support specific external loads. To accurately model and predict tissues’ mechanical response, it is essential to characterize the anisotropy on a microstructural scale. Previously, it has been difficult to measure the mechanical properties of intact tissues noninvasively. Here, we use Brillouin optical microscopy to visualize and quantify the anisotropic mechanical properties of corneal tissues at different length scales. We derive the stiffness tensor for a lamellar network of collagen fibrils and use angle-resolved Brillouin measurements to determine the longitudinal stiffness coefficients (longitudinal moduli) describing the ex vivo porcine cornea as a transverse isotropic material. Lastly, we observe significant mechanical anisotropy of the human cornea in vivo, highlighting the potential for clinical applications of off-axis Brillouin microscopy.

Journal ArticleDOI
TL;DR: In this paper , the skin effect for flexural waves in a non-Hermitian piezoelectric phononic beam with feedback control between a sensor and an actuator in each unit cell is realized.
Abstract: Non-Hermitian systems have gained a great deal of interest in various wave problems due their ability of exhibiting unprecedented phenomena such as invisibility, cloaking, enhanced sensing, or the skin effect. The latter manifests itself by the localization of all bulk modes in a specific frequency range at a given boundary, with an unconventional bulk-boundary correspondence. In this work, we propose to realize the skin effect for flexural waves in a non-Hermitian piezoelectric phononic beam with feedback control between a sensor and an actuator in each unit cell. By implementing a non-Hermitian parameter, effective gain and loss can be achieved in the phononic beam characterized by complex eigen frequencies, and non-reciprocal pass bands are obtained. We highlight that the split point separating the gain and loss areas can occur not only at the edges of the Brillouin zones but also inside the same Brillouin zone. We further analyze the influence of the geometric and non-Hermitian parameters on the complex dispersions and the split point. The topology of the complex bands is characterized by the winding number, which supports the skin effect together with the non-reciprocity. The localization degree of the skin mode manifested by the enhanced beam's vibration energy at one boundary is related to the strength of the non-reciprocity, and the skin mode can be always excited regardless of the source position. Our results provide a potential platform to introduce non-Hermiticity into phononic or metamaterial systems with novel functions for elastic waves such as topological insulators, vibration attenuation or amplification, and energy harvesting.

Journal ArticleDOI
TL;DR: In this paper , angle-resolved photoemission spectroscopy of KV3Sb5 and demonstrate a substantial reconstruction of Fermi surface in the CDW state that accompanies the formation of small three-dimensional pockets.
Abstract: Kagome lattices offer a fertile ground to explore exotic quantum phenomena associated with electron correlation and band topology. The recent discovery of superconductivity coexisting with charge-density wave (CDW) in the kagome metals KV3Sb5, RbV3Sb5, and CsV3Sb5 suggests an intriguing entanglement of electronic order and superconductivity. However, the microscopic origin of CDW, a key to understanding the superconducting mechanism and its possible topological nature, remains elusive. Here, we report angle-resolved photoemission spectroscopy of KV3Sb5 and demonstrate a substantial reconstruction of Fermi surface in the CDW state that accompanies the formation of small three-dimensional pockets. The CDW gap exhibits a periodicity of undistorted Brillouin zone along the out-of-plane wave vector, signifying a dominant role of the in-plane inter-saddle-point scattering to the mechanism of CDW. The characteristics of experimental band dispersion can be captured by first-principles calculations with the inverse star-of-David structural distortion. The present result indicates a direct link between the low-energy excitations and CDW, and puts constraints on the microscopic theory of superconductivity in alkali-metal kagome lattices.

Journal ArticleDOI
TL;DR: In this paper , the appearance of ideal nodal-net, nodal chain, and nodal cage phonons in the momentum space of phonons has been demonstrated based on first-principle calculations.
Abstract: The topological interpretation of phonons provides a new platform for new concepts in phonon physics. While reports on topological electronic excitations are plenty, the number of reports on phonons is extremely limited. In this paper, we present some realistic materials as examples and demonstrate the appearance of ideal nodal-net, nodal-chain, and nodal-cage phonons in these materials based on first-principle calculations; Pna21 LiAlSe2 has a nodal-net phonon, which is composed of two nodal-chain phonons along the U-R and U-Z directions; Pnma NaMgH3 has a nodal-chain phonon in the extended Brillouin zone (BZ), which comprises two Weyl nodal-ring phonons in the kx-kz and kx-ky planes. Moreover, P42/ncm AuBr has a nodal-cage phonon, which is composed of nodal-line phonons in the kz = 0, kz = π, kx = 0, and ky = 0 planes. Because the presented phonon bands and phonon surface states in these materials are quite clean, they can be easily detected in experiments. Our research results provide an ideal platform for realising complex geometric shapes (formed by nodal lines) in the momentum space of phonons.

Journal ArticleDOI
TL;DR: In this article , 2D Dirac states in antimony atomic layers with a phosphorene structure were observed and shown to be isotropic due to the constraints of crystal symmetries.
Abstract: Two-dimensional (2D) Dirac states with linear dispersion have been observed in graphene and on the surface of topological insulators. 2D Dirac states discovered so far are exclusively pinned at high-symmetry points of the Brillouin zone, for example, surface Dirac states at [Formula: see text] in topological insulators Bi2Se(Te)3 and Dirac cones at K and [Formula: see text] points in graphene. The low-energy dispersion of those Dirac states are isotropic due to the constraints of crystal symmetries. In this work, we report the observation of novel 2D Dirac states in antimony atomic layers with phosphorene structure. The Dirac states in the antimony films are located at generic momentum points. This unpinned nature enables versatile ways such as lattice strains to control the locations of the Dirac points in momentum space. In addition, dispersions around the unpinned Dirac points are highly anisotropic due to the reduced symmetry of generic momentum points. The exotic properties of unpinned Dirac states make antimony atomic layers a new type of 2D Dirac semimetals that are distinct from graphene.

Journal ArticleDOI
TL;DR: In this article , two conduction bands with very different effective masses are converged at the X point in the half-Heusler Brillouin zone for thermoelectric enhancement.
Abstract: Two conduction bands with very different effective masses are usually at the X point in the half-Heusler Brillouin zone. Our orbital phase diagram provides feasible strategies to converge these two bands for thermoelectric enhancement.

Journal ArticleDOI
16 Mar 2022
TL;DR: In this article , the photoreflectance method was used to investigate the temperature dependencies of optical transitions at the high-symmetry point of the Brillouin zone and band nesting.
Abstract: Following the rise of interest in the properties of transition metal dichalcogenides, many experimental techniques were employed to research them. However, the temperature dependencies of optical transitions, especially those related to band nesting, were not analyzed in detail for many of them. Here, we present successful studies utilizing the photoreflectance method, which, due to its derivative and absorption-like character, allows investigating direct optical transitions at the high-symmetry point of the Brillouin zone and band nesting. By studying the mentioned optical transitions with temperature from 20 to 300 K, we tracked changes in the electronic band structure for the common transition metal dichalcogenides (TMDs), namely, MoS2, MoSe2, MoTe2, WS2, and WSe2. Moreover, transmission and photoacoustic spectroscopies were also employed to investigate the indirect gap in these crystals. For all observed optical transitions assigned to specific k-points of the Brillouin zone, their temperature dependencies were analyzed using the Varshni relation and Bose–Einstein expression. It was shown that the temperature energy shift for the transition associated with band nesting is smaller when compared with the one at high-symmetry point, revealing reduced average electron–phonon interaction strength.

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TL;DR: In this article , the temperature dependence of low-energy magnetic excitations in the spin-triplet superconductor UTe 2 was measured via inelastic neutron scattering in the normal and superconducting states.
Abstract: Abstract The temperature dependence of the low-energy magnetic excitations in the spin-triplet superconductor UTe 2 was measured via inelastic neutron scattering in the normal and superconducting states. These excitations have a peak instensity at 4 meV, follow the Brillouin zone edges near the crystallographic b-axis, obey the paramagnetic structural symmetry, and track the temperature evolution of the heavy fermion bulk magnetic susceptibility. Thus, the imaginary part of the dynamic susceptibility follows the behavior of interband correlations in a hybridized Kondo lattice with an appropriate characteristic energy. These excitations are a lower-dimensional analog of phenomena observed in other Kondo lattice materials, such that their presence is not necessarily due to dominance of ferromagnetic or antiferromagnetic correlations. The onset of superconductivity alters the magnetic excitations noticeably on the same energy scales, suggesting that these changes originate from additional electronic structure modification.

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TL;DR: In this paper , the authors proposed a new optical state in a twisted bilayer photonic crystal slab, which is called as moiré quasi-BIC, and numerically demonstrated that such an exotic optical state possesses dual characteristics of the magic-angle flat bands and BICs.
Abstract: The novel physics of twisted bilayer graphene has motivated extensive studies of magic-angle flat bands hosted by moiré structures in electronic, photonic, and acoustic systems. On the other hand, bound states in the continuum (BICs) have also attracted great attention in recent years because of their potential applications in the field of designing superior optical devices. Here, we combine these two independent concepts to construct a new optical state in a twisted bilayer photonic crystal slab, which is called as moiré quasi-BIC, and numerically demonstrate that such an exotic optical state possesses dual characteristics of moiré flat bands and quasi-BICs. To illustrate the mechanism for the formation of moiré flat bands, we develop an effective model at the center of the Brillouin zone and show that moiré flat bands could be fulfilled by balancing the interlayer coupling strength and the twist angle around the band edge above the light line. Moreover, by decreasing the twist angle of moiré photonic crystal slabs with flat bands, it is shown that the moiré flat-band mode at the Brillouin center gradually approaches a perfect BIC, where the total radiation loss from all diffraction channels is significantly suppressed. To clarify the advantage of moiré quasi-BICs, enhanced second-harmonic generation (SHG) is numerically proven with a wide-angle optical source. The efficiency of SHG assisted by designed moiré quasi-BICs can be greatly improved compared with that based on dispersive quasi-BICs with similar quality factors.

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TL;DR: In this paper , the experimental realization of a 2D weak topological insulator (WTI) in spinless Su-Schrieffer-Heeger circuits with parity-time and chiral symmetries is reported.
Abstract: We report the experimental realization of a two-dimensional (2D) weak topological insulator (WTI) in spinless Su-Schrieffer-Heeger circuits with parity-time and chiral symmetries. Strong and weak Z2 topological indexes are adopted to explain the experimental findings that a Dirac semimetal (DSM) phase and four WTI phases emerge in turn when we modulate the centrosymmetric circuit deformations. In the DSM phase, it is found that the Dirac cone is highly anisotropic and that it is not pinned to any high-symmetry points but can widely move within the Brillouin zone, which eventually leads to the phase transition between WTIs. In addition, we observe a pair of flat-band domain wall states by designing spatially inhomogeneous node connections. Our work provides the first experimental evidence for 2D WTIs, which significantly advances our understanding of the strong and weak nature of topological insulators, the robustness of flat bands, and the itinerant and anisotropic features of Dirac cones.

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TL;DR: In this article , the authors showed that nonmagnetic spinless systems can host a class of single-pair-Weyl-point (SP-WP) states, where the two Weyl points are located at two high-symmetry time-reversal-invariant momenta.
Abstract: Topological semimetal states having the minimal number, i.e., only a single pair, of Weyl points are desirable for the study of effects associated with chiral topological charges. So far, the search for such states is focused on magnetic spinful systems. Here, we find that nonmagnetic spinless systems can host a class of single-pair-Weyl-point (SP-WP) states, where the two Weyl points are located at two high-symmetry time-reversal-invariant momenta. We identify 32 candidate space groups that host such states, and we show that the chiral charge of each Weyl point in the SP-WP state must be an even integer. Besides achieving the minimal number, Weyl points in SP-WP states are far separated in momentum space, making the physics of each individual point better exposed. The large separation combined with the even topological charge lead to extended surface Fermi loops with a non-contractible winding topology on the surface Brillouin zone torus, distinct from conventional Weyl semimetals. We confirm our proposal in the phonon spectra of two concrete materials TlBO$_2$ and KNiIO$_6$. Our finding applies to a wide range of systems, including electronic, phononic, and various artificial systems. It offers a new direction for the search of ideal platforms to study chiral particles.

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TL;DR: In this article , the authors demonstrate that multiple tapers in optical fibers allow for quasi-distributed refractive index sensing via a high spatial resolution Brillouin Optical Frequency-Domain Analysis (BOFDA) configuration.
Abstract: In this paper, we demonstrate that multiple tapers in optical fibers allow for quasi-distributed refractive index sensing via a high spatial resolution Brillouin Optical Frequency-Domain Analysis (BOFDA) configuration. We first characterize, theoretically and experimentally, the variation of the Brillouin frequency shift (BFS) with the diameter of the tapered fiber. Then, we characterize the dependence of the BFS from the outer refractive index in an optical fiber taper with a waist diameter of 10 µm. Finally, we show that more tapers can be realized along the same fiber, in order to provide multi-point refractive index sensing.