Showing papers on "Brillouin zone published in 2020"
TL;DR: In this article, the generalized Brillouin zone (GBZ) is calculated analytically in one-dimensional non-Hermitian systems, which helps us to understand the non-hermitian bulk-boundary correspondence.
Abstract: We provide a systematic and self-consistent method to calculate the generalized Brillouin zone (GBZ) analytically in one-dimensional non-Hermitian systems, which helps us to understand the non-Hermitian bulk-boundary correspondence. In general, a $n$-band non-Hermitian Hamiltonian is constituted by $n$ distinct sub-GBZs, each of which is a piecewise analytic closed loop. Based on the concept of resultant, we can show that all the analytic properties of the GBZ can be characterized by an algebraic equation, the solution of which in the complex plane is dubbed as auxiliary GBZ (aGBZ). We also provide a systematic method to obtain the GBZ from aGBZ. Two physical applications are also discussed. Our method provides an analytic approach to the spectral problem of open boundary non-Hermitian systems in the thermodynamic limit.
196 citations
TL;DR: Topological flat bands are observed near the Fermi level in a kagome metal CoSn, with flat bands as well as Dirac bands originating from 3d orbitals in a frustrated kagom geometry demonstrated to be frustration-driven.
Abstract: Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize strongly-correlated phenomena. Theoretically, the flat bands can naturally arise in certain geometrically frustrated lattices, often with nontrivial topology if combined with spin-orbit coupling. Here, we report the observation of topological flat bands in frustrated kagome metal CoSn, using angle-resolved photoemission spectroscopy and band structure calculations. Throughout the entire Brillouin zone, the bandwidth of the flat band is suppressed by an order of magnitude compared to the Dirac bands originating from the same orbitals. The frustration-driven nature of the flat band is directly confirmed by the chiral d-orbital texture of the corresponding real-space Wannier functions. Spin-orbit coupling opens a large gap of 80 meV at the quadratic touching point between the Dirac and flat bands, endowing a nonzero Z2 invariant to the flat band. These findings demonstrate that kagome-derived flat bands are a promising platform for novel emergent phases of matter at the confluence of strong correlation and topology. The experimental realization of lattice-born flat bands with nontrivial topology has been elusive. Here, the authors observe topological flat bands near the Fermi level in a kagome metal CoSn, with flat bands as well as Dirac bands originating from 3d orbitals in a frustrated kagome geometry.
139 citations
TL;DR: In this article, the flat band was observed to have a dispersionless electronic excitation along the G-M high symmetry direction, with an order of magnitude lower bandwidth (below 150 meV) compared to the Dirac bands originating from the same orbitals.
Abstract: Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize strong correlation phenomena. In certain lattices with built-in geometrical frustration, electronic confinement and flat bands can naturally arise from the destructive interference of electronic hopping pathways. Such lattice-borne flat bands are often endowed with nontrivial topology if combined with spin-orbit coupling, while their experimental realization in condensed matter system has been elusive so far. Here, we report the direct observation of topological flat bands in the vicinity of the Fermi level in frustrated kagome system CoSn, using angle-resolved photoemission spectroscopy and band structure calculations. The flat band manifests itself as a dispersionless electronic excitation along the G-M high symmetry direction, with an order of magnitude lower bandwidth (below 150 meV) compared to the Dirac bands originating from the same orbitals. The frustration-driven nature of the flat band is directly confirmed by the real-space chiral d-orbital texture of the corresponding effective Wannier wave functions. Spin-orbit coupling opens a large gap of 80 meV at the quadratic band touching point between the Dirac and flat bands, endowing a nonzero Z2 topological invariant to the flat band in the two-dimensional Brillouin zone. Our observation of lattice-driven topological flat band opens a promising route to engineer novel emergent phases of matter at the crossroad between strong correlation physics and electronic topology.
124 citations
TL;DR: In this article, it was shown that valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation, which leads to a series of photoluminescence peaks as valley phonon replicas of dark trions.
Abstract: The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects. Here, we report the observation of multiple valley phonons – phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone – and the resulting exciton complexes in the monolayer semiconductor WSe2. We find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncover an intervalley exciton near charge neutrality. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons. In monolayer semiconductors phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone couple strongly to carriers’ spin and valley degree of freedom. Here, the authors report the observation of multiple valley phonons and the resulting exciton complexes in the monolayer semiconductor WSe2.
114 citations
TL;DR: The observation of a flat band and Dirac bands as ideal features of kagome bands in CoSn, revealing orbital-selective character of the Dirac fermions.
Abstract: Layered kagome-lattice 3d transition metals are emerging as an exciting platform to explore the frustrated lattice geometry and quantum topology. However, the typical kagome electronic bands, characterized by sets of the Dirac-like band capped by a phase-destructive flat band, have not been clearly observed, and their orbital physics are even less well investigated. Here, we present close-to-textbook kagome bands with orbital differentiation physics in CoSn, which can be well described by a minimal tight-binding model with single-orbital hopping in Co kagome lattice. The capping flat bands with bandwidth less than 0.2 eV run through the whole Brillouin zone, especially the bandwidth of the flat band of out-of-plane orbitals is less than 0.02 eV along Γ−M. The energy gap induced by spin-orbit interaction at the Dirac cone of out-of-plane orbitals is much smaller than that of in-plane orbitals, suggesting orbital-selective character of the Dirac fermions. The understanding of kagome bands, which are characterized by Dirac-like bands capped by a flat band, remains largely elusive. Here, Liu et al. report the observation of a flat band and Dirac bands as ideal features of kagome bands in CoSn, revealing orbital-selective character.
104 citations
TL;DR: It is demonstrated that the tTLG system exhibits a wide range of magic angles at which VHS merge and that the density of states has a sharp peak at the charge-neutrality point through two distinct mechanisms: the incommensurate perturbation of twisted bilayer graphene's flatbands or the equal hybridization between two bilayer moiré superlattices.
Abstract: We introduce twisted trilayer graphene (tTLG) with two independent twist angles as an ideal system for the precise tuning of the electronic interlayer coupling to maximize the effect of correlated behaviors. As established by experiment and theory in the related twisted bilayer graphene system, van Hove singularities (VHS) in the density of states can be used as a proxy of the tendency for correlated behaviors. To explore the evolution of VHS in the twist-angle phase space of tTLG, we present a general low-energy electronic structure model for any pair of twist angles. We show that the basis of the model has infinite dimensions even at a finite energy cutoff and that no Brillouin zone exists even in the continuum limit. Using this model, we demonstrate that the tTLG system exhibits a wide range of magic angles at which VHS merge and that the density of states has a sharp peak at the charge-neutrality point through two distinct mechanisms: the incommensurate perturbation of twisted bilayer graphene's flatbands or the equal hybridization between two bilayer moire superlattices.
91 citations
30 Oct 2020
TL;DR: In this article, the authors demonstrate the existence of helical damping and dynamical critical skin effect in quantum open systems and show that the change of generalized Brillouin zone equation is the origin of critical skin effects.
Abstract: The authors demonstrate the existence of helical damping and dynamical critical skin effect in quantum open systems and show that the change of generalized Brillouin zone equation is the origin of critical skin effect.
88 citations
TL;DR: In this paper, the authors used neutron scattering to show that the phase transition in the 2D honeycomb lattice is a weakly first order transition and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy.
Abstract: We use neutron scattering to show that ferromagnetic (FM) phase transition in the two-dimensional (2D) honeycomb lattice ${\mathrm{CrI}}_{3}$ is a weakly first order transition and controlled by spin-orbit coupling (SOC) induced magnetic anisotropy, instead of magnetic exchange coupling as in a conventional ferromagnet. With increasing temperature, the magnitude of magnetic anisotropy, seen as a spin gap at the Brillouin zone center, decreases in a power law fashion and vanishes at ${T}_{C}$, while the in-plane and $c$-axis spin-wave stiffnesses associated with magnetic exchange couplings remain robust at ${T}_{C}$. We also compare parameter regimes where spin waves in ${\mathrm{CrI}}_{3}$ can be described by a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction or a Heisenberg-Kitaev Hamiltonian. These results suggest that the SOC induced magnetic anisotropy plays a dominant role in stabilizing the FM order in single layer 2D van der Waals ferromagnets.
72 citations
TL;DR: This work identifies a number of previously unknown 2D excitonic species and shows that monolayer WSe 2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.
Abstract: The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects because they possess unique spin-valley locking physics for hosting rich excitonic species and the reduced screening for strong Coulomb interactions. Here, we report the observation of multiple valley phonons, phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone, and the resulting exciton complexes in the monolayer semiconductor WSe2. From Lande g-factor and polarization analyses of photoluminescence peaks, we find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncovered an intervalley exciton near charge neutrality, and extract its short-range electron-hole exchange interaction to be about 10 meV. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.
65 citations
TL;DR: High resolution angle-resolved photoemission measurements covering multiple planes in the 3D Brillouin zone of UTe_{2}, revealing distinct Fermi-level features from two orthogonal quasi-one-dimensional light electron bands and one heavy band are presented.
Abstract: The compound UTe_{2} has recently been shown to realize spin triplet superconductivity from a nonmagnetic normal state. This has sparked intense research activity, including theoretical analyses that suggest the superconducting order parameter to be topologically nontrivial. However, the underlying electronic band structure is a critical factor for these analyses, and remains poorly understood. Here, we present high resolution angle-resolved photoemission measurements covering multiple planes in the 3D Brillouin zone of UTe_{2}, revealing distinct Fermi-level features from two orthogonal quasi-one-dimensional light electron bands and one heavy band. The electronic symmetries are evaluated in comparison with numerical simulations, and the resulting picture is discussed as a platform for unconventional many-body order.
65 citations
TL;DR: In this paper, the spin and electronic structure in a p-type semiconductor, elemental tellurium, with the simplest chiral structure was investigated by using spin and angle-resolved photoemission spectroscopy.
Abstract: The chiral crystal is characterized by a lack of mirror symmetry and inversion center, resulting in the inequivalent right- and left-handed structures. In the noncentrosymmetric crystal structure, the spin and momentum of electrons are expected to be locked in the reciprocal space with the help of the spin-orbit interaction. To reveal the spin textures of chiral crystals, we investigate the spin and electronic structure in a p-type semiconductor, elemental tellurium, with the simplest chiral structure by using spin- and angle-resolved photoemission spectroscopy. Our data demonstrate that the highest valence band crossing the Fermi level has a spin component parallel to the electron momentum around the Brillouin zone corners. Significantly, we have also confirmed that the spin polarization is reversed in the crystal with the opposite chirality. The results indicate that the spin textures of the right- and left-handed chiral crystals are hedgehoglike, leading to unconventional magnetoelectric effects and nonreciprocal phenomena.
TL;DR: In this article, the authors present close-to-textbook kagome bands with orbital differentiation physics in CoSn, which can be well described by a minimal tight-binding model with single-orbital hopping in Co kagomes.
Abstract: Layered kagome-lattice 3d transition metals are emerging as an exciting platform to explore the frustrated lattice geometry and quantum topology. However, the typical kagome electronic bands, characterized by sets of the Dirac-like band capped by a phase-destructive flat band, have not been clearly observed, and their orbital physics are even less well investigated. Here, we present close-to-textbook kagome bands with orbital differentiation physics in CoSn, which can be well described by a minimal tight-binding model with single-orbital hopping in Co kagome lattice. The capping flat bands with bandwidth less than 0.2 eV run through the whole Brillouin zone, especially the bandwidth of the flat band of out-of-plane orbitals is less than 0.02 eV along G-M. The energy gap induced by spin-orbit interaction at the Dirac cone of out-of-plane orbitals is much smaller than that of in-plane orbitals, suggesting orbital-selective character of the Dirac fermions.
Journal Article•
TL;DR: In this paper, angle-resolved photo-emission spectroscopy was used to study the anomalous metallic state in high-temperature superconductors and showed that it is a prerequisite for the pseudogap.
Abstract: In normal metals, macroscopic properties are understood using the concept of quasiparticles. In the cuprate high-temperature superconductors, the metallic state above the highest transition temperature is anomalous and is known as the “strange metal.” We studied this state using angle-resolved photoemission spectroscopy. With increasing doping across a temperature-independent critical value pc ~ 0.19, we observed that near the Brillouin zone boundary, the strange metal, characterized by an incoherent spectral function, abruptly reconstructs into a more conventional metal with quasiparticles. Above the temperature of superconducting fluctuations, we found that the pseudogap also discontinuously collapses at the very same value of pc. These observations suggest that the incoherent strange metal is a distinct state and a prerequisite for the pseudogap; such findings are incompatible with existing pseudogap quantum critical point scenarios.
TL;DR: Stimulated Brillouin scattering microscopy overcomes this trade-off and enables the cross-sectional imaging of live Caenorhabditis elegans at the organ and subcellular levels as discussed by the authors.
Abstract: Label-free, non-contact imaging with mechanical contrast and optical sectioning is a substantial challenge in microscopy. Spontaneous Brillouin scattering microscopy meets this challenge, but encounters a trade-off between acquisition speed and the specificity for biomechanical constituents with overlapping Brillouin bands. Stimulated Brillouin scattering microscopy overcomes this trade-off and enables the cross-sectional imaging of live Caenorhabditis elegans at the organ and subcellular levels, with both elasticity and viscosity contrasts at high specificity and with practical recording times.
TL;DR: In this article, the electron-phonon self-energy and carrier mobilities in semiconductors are computed on dense meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles.
Abstract: We describe a new approach to compute the electron-phonon self-energy and carrier mobilities in semiconductors. Our implementation does not require a localized basis set to interpolate the electron-phonon matrix elements, with the advantage that computations can be easily automated. Scattering potentials are interpolated on dense $\mathbf{q}$ meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles. To reduce significantly the computational cost, we take advantage of crystal symmetries and employ the linear tetrahedron method and double-grid integration schemes, in conjunction with filtering techniques in the Brillouin zone. We report results for the electron mobility in Si, GaAs, and GaP obtained with this new methodology.
TL;DR: In this article, a periodic lattice of acoustic black holes (ABHs) is proposed to support the formation and propagation of quasi-unidirectional elastic guided modes that are topologically protected against backscattering due to impurities or defects in the lattice.
TL;DR: A strong Brillouin amplification per unit length is used to realize a low-threshold continuous-wave single-frequency laser that can in principle operate at any wavelength and to demonstrate distributed temperature sensing with no strain cross-sensitivity.
Abstract: Among all the nonlinear effects stimulated Brillouin scattering offers the highest gain in solid materials and has demonstrated advanced photonics functionalities in waveguides. The large compressibility of gases suggests that stimulated Brillouin scattering may gain in efficiency with respect to condensed materials. Here, by using a gas-filled hollow-core fibre at high pressure, we achieve a strong Brillouin amplification per unit length, exceeding by six times the gain observed in fibres with a solid silica core. This large amplification benefits from a higher molecular density and a lower acoustic attenuation at higher pressure, combined with a tight light confinement. Using this approach, we demonstrate the capability to perform large optical amplifications in hollow-core waveguides. The implementations of a low-threshold gas Brillouin fibre laser and a high-performance distributed temperature sensor, intrinsically free of strain cross-sensitivity, illustrate the potential for hollow-core fibres, paving the way to their integration into lasing, sensing and signal processing. A strong Brillouin amplification per unit length, observed in a gas-filled hollow-core fibre, is used to realize a low-threshold continuous-wave single-frequency laser that can in principle operate at any wavelength and to demonstrate distributed temperature sensing with no strain cross-sensitivity.
TL;DR: How the direction of quantum dot (QD) lasing can be engineered by exploiting high-symmetry points in plasmonic nanoparticle (NP) lattices is reported, which expands the selection of cavity modes for any desired lasing emission angle.
Abstract: We report how the direction of quantum dot (QD) lasing can be engineered by exploiting high-symmetry points in plasmonic nanoparticle (NP) lattices. The nanolaser architecture consists of CdSe-CdS core-shell QD layers conformally coated on two-dimensional square arrays of Ag NPs. Using waveguide-surface lattice resonances (W-SLRs) near the Δ point in the Brillouin zone as optical feedback, we achieved lasing from the gain in CdS shells at off-normal emission angles. Changing the periodicity of the plasmonic lattices enables other high-symmetry points (Γ or M) of the lattice to overlap with the QD shell emission, which facilitates tuning of the lasing direction. We also increased the thickness of the QD layer to introduce higher-order W-SLR modes with additional avoided crossings in the band structure, which expands the selection of cavity modes for any desired lasing emission angle.
TL;DR: High-resolution angle-resolved photoemission measurements on single crystals of Pt_{2}HgSe_{3} grown by high-pressure synthesis reveal a gapped Dirac nodal line whose (001) projection separates the surface Brillouin zone in topological and trivial areas.
Abstract: We report high-resolution angle-resolved photoemission measurements on single crystals of Pt_{2}HgSe_{3} grown by high-pressure synthesis. Our data reveal a gapped Dirac nodal line whose (001) projection separates the surface Brillouin zone in topological and trivial areas. In the nontrivial k-space range, we find surface states with multiple saddle points in the dispersion, resulting in two van Hove singularities in the surface density of states. Based on density-functional theory calculations, we identify these surface states as signatures of a topological crystalline state, which coexists with a weak topological phase.
TL;DR: In this article, the authors presented a systematic study on the low-energy bulk and surface electronic structure of pyrite-structured PdSb2 and verified the existence of sixfold fermions in this compound, which are formed by three doubly degenerate bands centered at the R point in the Brillouin zone.
Abstract: Pyrite-structured PdSb2 with the nonsymmorphic crystalline symmetry, has long been predicted to host sixfold-degenerate exotic fermions beyond Dirac and Weyl. Although magnetotransport measurements on PdSb2 have suggested its topologically nontrivial character, the direct spectroscopic evidence still remains absent. By utilizing high-resolution angle-resolved photoemission spectroscopy, we present a systematic study on the low-energy bulk and surface electronic structure of pyrite-structured PdSb2. Through careful comparison with first-principles calculations, we verify the existence of sixfold fermions in this compound, which are formed by three doubly degenerate bands centered at the R point in the Brillouin zone. These bands exhibit parabolic dispersion close to sixfold fermion nodes, in sharp contrast to previously reported sixfold double spin-1 chiral fermions. Furthermore, our findings reveal no protected Fermi arcs in PdSb2, which is compatible with its achiral structure. Our findings suggest that pyrite-structured PdSb2 provides an ideal platform for the investigation of fermions and their potential applications.
03 Mar 2020
TL;DR: In this article, a Brillouin laser based on free-space laser action in an extreme optical material is demonstrated, achieving high-power output power of 11 W with a linewidth of 12 MHz.
Abstract: Brillouin lasers providing extremely narrow-linewidth are emerging as a powerful tool for microwave photonics, coherent communications, quantum processors, and spectroscopy. So far, laser performance and applications have been investigated for a handful of select materials and using guided-wave structures such as micro-resonators, optical fibers, and chip-based waveguides. Here, we report a Brillouin laser based on free-space laser action in an extreme optical material. Continuous-wave lasing 167 GHz from a 532 nm pump is demonstrated in diamond using a doubly resonant ring cavity, generating a pump-limited output power of 11 W. The Brillouin gain coefficient is measured to be 79 cm GW−1 with a linewidth of 12 MHz. These properties, along with an exceptionally high Brillouin frequency and wide transmission range, make diamond Brillouin lasers a promising high-power source of narrow-linewidth output and mm-wave beat notes.
19 Mar 2020
TL;DR: In this article, the phonon spectra of twisted bilayer graphene (tBLG) were analyzed for a series of 692 twisting angle values in the $[0,30]$ range.
Abstract: The phonon spectra of twisted bilayer graphene (tBLG) are analyzed for a series of 692 twisting angle values in the $[0,30{\degree}]$ range. The evolution of the phonon bandstructure as a function of twist angle is examined using a band unfolding scheme where the large number of phonon modes computed at the $\Gamma$ point for the large moir\'e tBLG supercells are unfolded onto the Brillouin Zone (BZ) of one of the two constituent layers. In addition to changes to the low-frequency breathing and shear modes, a series of well-defined side-bands around high-symmetry points of the extended BZ emerge due to the twist angle-dependent structural relaxation. The results are rationalized by introducing a nearly-free-phonon model that highlights the central role played by solitons in the description of the new phonon branches, which are particularly pronounced for structures with small twist angles, below a buckling angle $\theta_{\rm B}\sim {3.75}{\degree}$.
TL;DR: In this paper, a 3D acoustic double-zero-index medium (DZIM) made of a cubic lattice of metal rods has been realized, where a fourfold degenerate point with conical dispersion can be induced at the Brillouin zone center, such that the material becomes a three-dimensional DZIM with the effective mass density and compressibility simultaneously acquiring near zero values.
Abstract: We report the first realization of a three-dimensional (3D) acoustic double-zero-index medium (DZIM) made of a cubic lattice of metal rods. While the past decade has seen several realizations of 2D DZIM, achieving such a medium in 3D has remained an elusive challenge. Here, we show how a fourfold degenerate point with conical dispersion can be induced at the Brillouin zone center, such that the material becomes a 3D DZIM with the effective mass density and compressibility simultaneously acquiring near-zero values. To demonstrate the functionalities of this new medium, we have fabricated an acoustic waveguide of 3D DZIM in form of a ``periscope'' with two 90\ifmmode^\circ\else\textdegree\fi{} turns and observed tunneling of a normally incident planar wave through the waveguide yielding undistorted planar wave front at the waveguide exit. Our findings establish a practical route to realize 3D DZIM as an effective acoustic ``void space'' that offers unprecedented control over acoustic wave propagation.
TL;DR: The present study shows that the nonsymmorphic compound PdSb2 hosts six-component fermions or sextuplets and proves that under weak spin-orbit interaction, a band inversion occurs.
Abstract: Multifold degenerate points in the electronic structure of metals lead to exotic behaviors. These range from twofold and fourfold degenerate Weyl and Dirac points, respectively, to sixfold and eightfold degenerate points that are predicted to give rise, under modest magnetic fields or strain, to topological semimetallic behaviors. The present study shows that the nonsymmorphic compound PdSb2 hosts six-component fermions or sextuplets. Using angle-resolved photoemission spectroscopy, crossing points formed by three twofold degenerate parabolic bands are directly observed at the corner of the Brillouin zone. The group theory analysis proves that under weak spin-orbit interaction, a band inversion occurs.
TL;DR: Li et al. as mentioned in this paper proposed a BFS extraction convolutional neural network (BFSCNN) to retrieve the distributed BFS directly from the measured two-dimensional data, which can effectively improve the performances of existing Brillouin sensors.
Abstract: Distributed optical fiber Brillouin sensors detect the temperature and strain along a fiber according to the local Brillouin frequency shift (BFS), which is usually calculated by the measured Brillouin spectrum using Lorentzian curve fitting. In addition, cross-correlation, principal component analysis, and machine learning methods have been proposed for the more efficient extraction of BFS. However, existing methods only process the Brillouin spectrum individually, ignoring the correlation in the time domain, indicating that there is still room for improvement. Here, we propose and experimentally demonstrate a BFS extraction convolutional neural network (BFSCNN) to retrieve the distributed BFS directly from the measured two-dimensional data. Simulated ideal Brillouin spectra with various parameters are used to train the BFSCNN. Both the simulation and experimental results show that the extraction accuracy of the BFSCNN is better than that of the traditional curve fitting algorithm with a much shorter processing time. The BFSCNN has good universality and robustness and can effectively improve the performances of existing Brillouin sensors.
TL;DR: Scanning tunneling microscopy/spectroscopy is used to investigate quasiparticle interference at various terminations of a CoSi single crystal and provides strong evidence that CoSi hosts the unconventional chiral fermions and the extensive Fermi arc states.
Abstract: Chiral fermions in solid state feature "Fermi arc" states, connecting the surface projections of the bulk chiral nodes. The surface Fermi arc is a signature of nontrivial bulk topology. Unconventional chiral fermions with an extensive Fermi arc traversing the whole Brillouin zone have been theoretically proposed in CoSi. Here, we use scanning tunneling microscopy / spectroscopy to investigate quasiparticle interference at various terminations of a CoSi single crystal. The observed surface states exhibit chiral fermion-originated characteristics. These reside on (001) and (011) but not (111) surfaces with pi-rotation symmetry, spiral with energy, and disperse in a wide energy range from ~-200 to ~+400 mV. Owing to the high-energy and high-space resolution, a spin-orbit coupling-induced splitting of up to ~80 mV is identified. Our observations are corroborated by density functional theory and provide strong evidence that CoSi hosts the unconventional chiral fermions and the extensive Fermi arc states.
TL;DR: In this paper, the authors investigate the potential weak coupling instabilities in twisted bilayer graphene, particularly at the higher-order Van Hove singularity, and uncover a host of unconventional correlated phases including chiral and helical superconductivities, ferromagnetism, and valley-polarized order with potential rotation symmetry breaking.
Abstract: The discovery of unconventional superconductivity and correlated insulators in twisted bilayer graphene has triggered enormous interest in modern condensed matter research. Here, the authors investigate the potential weak-coupling instabilities in such a system, particularly at the higher-order Van Hove singularity. An asymptotically exact renormalization group analysis uncovers a host of unconventional correlated phases. These include chiral and helical superconductivities, ferromagnetism, and valley-polarized order with potential rotation symmetry breaking. The results may serve as interesting indications for the experimentally observed correlated phases in twisted bilayer graphene.
TL;DR: It is demonstrated that Brillouin spectroscopy is able to reveal both the elastic and viscous properties of biopolymers that are central to the structure and function of biological tissues.
Abstract: Many problems in mechanobiology urgently require characterization of the micromechanical properties of cells and tissues. Brillouin light scattering has been proposed as an emerging optical elastography technique to meet this need. However, the information contained in the Brillouin spectrum is still a matter of debate because of fundamental problems in understanding the role of water in biomechanics and in relating the Brillouin data to low-frequency macroscopic mechanical parameters. Here, we investigate this question using gelatin as a model system in which the macroscopic physical properties can be manipulated to mimic all the relevant biological states of matter, ranging from the liquid to the gel and the glassy phase. We demonstrate that Brillouin spectroscopy is able to reveal both the elastic and viscous properties of biopolymers that are central to the structure and function of biological tissues.
Abstract: The FeSe nematic phase has been the focus of recent research on iron-based superconductors (IBSs) due to its unusual properties, which are distinct from those of the pnictides. A series of theoretical/experimental studies were performed to determine the origin of the nematic phase. However, they yielded conflicting results and caused additional controversies. Here, we report the results of angle-resolved photoemission and X-ray absorption spectroscopy studies on FeSe detwinned by a piezo stack. We fully resolved band dispersions with orbital characters near the Brillouin zone (BZ) corner, and revealed an absence of any Fermi pocket at the Y point in the 1-Fe BZ. In addition, the occupation imbalance between $${d}_{{xz}}$$
and $${d}_{{yz}}$$
orbitals was the opposite of that of iron pnictides, consistent with the identified band characters. These results resolve issues associated with the FeSe nematic phase and shed light on the origin of the nematic phase in IBSs. Twinning is a phenomenon which describes the intergrowth between two identical crystals, and can complicate the analysis of their intrinsic properties both from a physical and structural perspective. Here, the authors use a piezo stack to detwin FeSe crystals and then investigate the band dispersions using photoelectron and X-ray absorption spectroscopies in order to further clarify the origins of the nematic phase in FeSe.
TL;DR: It is shown that higher-order effects are comparable in magnitude to the leading order in polar semiconductors, and that including the 2ph processes is necessary to accurately predict the electron mobility in GaAs from first principles.
Abstract: Electron-phonon (e–ph) interactions are usually treated in the lowest order of perturbation theory. Here we derive next-to-leading order e–ph interactions, and compute from first principles the associated electron-two-phonon (2ph) scattering rates. The derivations involve Matsubara sums of two-loop Feynman diagrams, and the numerical calculations are challenging as they involve Brillouin zone integrals over two crystal momenta and depend critically on the intermediate state lifetimes. Using Monte Carlo integration together with a self-consistent update of the intermediate state lifetimes, we compute and converge the 2ph scattering rates, and analyze their energy and temperature dependence. We apply our method to GaAs, a weakly polar semiconductor with dominant optical-mode long-range e–ph interactions. We find that the 2ph scattering rates are as large as nearly half the value of the one-phonon rates, and that including the 2ph processes is necessary to accurately predict the electron mobility in GaAs from first principles. Electron-phonon scattering plays a decisive role in electron transport and is taken into account in ab initio calculations by leading-order perturbations involving scattering events with one phonon. Here, the authors show that higher-order effects are comparable in magnitude to the leading order in polar semiconductors.