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- 摘要
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- 实验方案
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Thermal Fluctuations in the Phase Structure of the Excitonic Insulator Charge Density Wave State in the Extended Falicov–Kimball Model
摘要： Excitonic insulator (EI) instability accompanied by a charge density wave (CDW) state in the two-dimensional extended Falicov–Kimball model including electron–phonon interaction is investigated by applying the unrestricted Hartree–Fock approximation. Treating both the Coulomb attraction and the electron–phonon coupling on an equal footing, we have derived a set of self-consistent equations, which allows us to determine both the excitonic insulator order parameter and the lattice displacement. The phase structures show us that both the Coulomb interaction and the electron–phonon coupling act together in establishing the EI-CDW state, which typifies either a phononic-CDW or an excitonic-CDW state depending on the competition of the interactions. At a given low temperature, the EI-CDW state is found between two critical values of the Coulomb interaction. The window of the condensation state grows when increasing the electron–phonon coupling. Depending on the temperature and the Coulomb interaction the Bardeen–Cooper–Schrieffer–Bose–Einstein condensation (BCS–BEC) crossover of the EI-CDW state in the systems has been established. Under the effects of phonons, the BCS–BEC crossover deviates to a larger Coulomb interaction while the semimetal–semiconductor transition in the normal state remains.
关键词： semimetal–semiconductor transition,charge density wave,unrestricted Hartree–Fock approximation,Excitonic condensation
更新于2025-09-23 15:23:52
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Coherent photo-induced phonon emission in the charge-density-wave state of K<sub>0.3</sub>MoO<sub>3</sub>
摘要： We report on the observation of coherent terahertz (THz) emission from the quasi-one-dimensional charge-density wave (CDW) system, blue bronze (K0.3MoO3), upon photo-excitation with ultrashort near-infrared optical pulses. The emission contains a broadband, low-frequency component due to the photo-Dember effect, which is present over the whole temperature range studied (30-300 K), as well as a narrow-band doublet centered at 1.5 THz, which is only observed in the CDW state and results from the generation of coherent transverse-optical phonons polarized perpendicular to the incommensurate CDW b-axis. As K0.3MoO3 is centrosymmetric, the lowest-order generation mechanism which can account for the polarization dependence of the phonon emission involves either a static surface field or quadrupolar terms due to the optical field gradients at the surface. This phonon signature is also present in the ground-state conductivity, and decays in strength with increasing temperature to vanish above T ~100 K, i.e. significantly below the CDW transition temperature. The temporal behavior of the phonon emission can be well described by a simple model with two coupled modes, which initially oscillate with opposite polarity.
关键词： charge-density wave,terahertz emission,coherent phonons,K0.3MoO3,photo-Dember effect
更新于2025-09-23 15:23:52
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Universal renormalization group flow toward perfect Fermi-surface nesting driven by enhanced electron-electron correlations in monolayer vanadium diselenide
摘要： Reducing the thickness of three-dimensional samples on appropriate substrates is a promising way to control electron-electron interactions, responsible for so called electronic reconstruction phenomena. Although the electronic reconstruction has been investigated both extensively and intensively in oxide heterostructure interfaces, this paradigm is not well established in the van der Waals heterointerface system. In the present study, we examine the nature of a charge ordering transition in monolayer vanadium diselenide (VSe2). This two-dimensional phase transition would be distinguished from that of VSe2 bulk samples, driven by more enhanced electron-electron correlations. We recall that VSe2 bulk samples show a charge-density-wave (CDW) transition around TCDW ～ 105 K. This bulk phase transition results from Fermi-surface nesting properties, where the low-temperature CDW state coexists with itinerant electrons of residual Fermi surfaces. Recently, angle-resolved photoemission spectroscopy measurements [Nano Lett. 18, 5432 (2018)] uncovered that the Fermi-surface nesting becomes perfect, where the dynamics of hot electrons is dispersionless along the orthogonal direction of the nesting wave vector. In addition, scanning tunneling microscopy measurements [Nano Lett. 18, 5432 (2018)] confirmed that the resulting CDW state shows essentially the same modulation pattern as the three-dimensional system of VSe2. Here, we perform the renormalization group analysis based on an effective-field theory in terms of critical CDW fluctuations and hot electrons of imperfect Fermi-surface nesting. As a result, we reveal that the imperfect nesting universally flows into perfect nesting in two dimensions, where the Fermi velocity along the orthogonal direction of the nesting vector vanishes generically. We argue that this electronic reconstruction is responsible for the observation [Nano Lett. 18, 5432 (2018).] that the CDW transition temperature is much more enhanced to be around TCDW ～ 350 K than that of the bulk sample.
关键词： vanadium diselenide,electron-electron correlations,renormalization group,Fermi-surface nesting,charge-density-wave transition
更新于2025-09-23 15:23:52
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The Dependence of Resonance Frequency to Landing Angle in Reciprocal Scattering Phenomena of the Waves From an Elliptical Plasma Dielectric Antenna
摘要： Scattering of an electromagnetic plane wave in transverse electric mode which is obliquely incident to an elliptical plasma antenna with a dielectric rod is comprehensively investigated. The field solutions of the incident, transmitted, and the scattered waves will be derived in terms of Mathieu and modified Mathieu functions. Using asymptotic expansions of Mathieu functions, the scattering cross section per unit length is obtained. It will be shown that in such an elliptical plasma antenna, the backscattering cross section depends on the incident angle and the bistatic scattering cross section varies with scattering angle significantly. Such effects are not seen in antennas with circular cross section due to the circular symmetry in them. The validation of the presented formulations and numerical computation will be examined with the circular case that is an asymptotic well-known case. The graphs of backscattering cross section, scattering pattern, and their corresponding to the polarized charge densities are presented. The dependence of the backscattering cross section and the scattering pattern upon the density and geometrical dimension of the plasma and dielectric are investigated. In addition, it will be shown that the resonance frequencies, under which the plasma antenna response will be maximum, essentially depend on the incident angle of the incident wave.
关键词： dielectric rod,resonance frequency,polarized charge density,Backscattering cross section,scattering pattern,elliptical plasma antenna
更新于2025-09-23 15:22:29
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Polarization, inner and outer field and surface charge compensation of a molecular crystal
摘要： The polarization of molecules embedded in crystals is of fundamental importance for understand and modeling of dielectric properties of these materials. Typically, the simulations concern ideal crystals, meaning infinite objects lacking of any defect and of any truncation. But in real life, crystals of finite sizes, and containing defects, are used. In this work, we focus on the calculation of the polarization of crystals of 4-iodo-4’-nitrobiphenyl (belonging to the polar class mm2), by mimicking several finite sizes and analyzing the effects of surfaces. This simulation implies calculation of the inner electric field, starting from the dipole of each molecule or, for a better accuracy, of each atom in the crystal. Using the atomic/molecular polarizabilities, calculations are repeated iteratively until a self-consistency is achieved. These simulations enable calculation of the polarization of several kinds of crystal shapes, up to ca. 80 nm scale range. Needle-like crystals slightly enhance (10%) the average dipole moment in polar direction of the constituting molecules, compared to their values in vacuo, whereas plate-like crystals severely reduce it (ca. 50%). In real systems, the effects of screening charges on the crystal surfaces cannot be neglected. A simple model enables to calculate that the dipole of needle-like crystals, under the effect of compensating charges, can be reduced close to extinction. On the other hand, the situation of plate-like crystals does not differ significantly under the influence of screening charges.
关键词： polar crystals,compensation,outer electric field,polarization,surface charge density,inner electric field
更新于2025-09-23 15:21:21
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Study of materials structure physics of isomorphic LiNbO <sub/>3</sub> and LiTaO <sub/>3</sub> ferroelectrics by synchrotron radiation X-ray diffraction
摘要： Electron charge density studies of stoichiometric LiNbO3 and LiTaO3 ferroelectrics have been carried out by analyzing the synchrotron radiation X-ray powder di?raction data using a combination of the Rietveld and maximum entropy methods. The clear relationships between the Nb(Ta)–O bond length, the electron charge density on the Nb(Ta)–O bond, and the phase transition temperature TC are revealed for isomorphic structures. Nb(Ta)–O bonding plays an important role in the elevation of TC. The TC in LiNbO3 being higher than that in LiTaO3 is attributed to the larger lattice distortion of the Nb–O oxygen octahedron in LiNbO3. The validity of estimating TC for LiNbO3 family crystals from the degree of lattice distortion in the ferroelectric structure is discussed.
关键词： ferroelectric,LiTaO3,X-ray diffraction,synchrotron radiation,LiNbO3,phase transition temperature,electron charge density
更新于2025-09-23 15:21:21
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revealed by angle-resolved photoemission spectroscopy
摘要： We revisit charge density wave (CDW) behavior in the archetypal quasi-one-dimensional (quasi-1D) material NbSe3 by high-resolution angle-resolved photoemission spectroscopy measurements utilizing a microfocused laser with a photon energy of 6.3 eV. We present a detailed view of the electronic structure of this complex multiband system and unambiguously resolve CDW gaps at the Fermi level (EF). By employing a tight-binding model, we argue that these gaps are the result of interband coupling between electronic states that reside predominantly on distinct 1D chains within the material. Two such localized states are found to couple to an electronic state that extends across multiple 1D chains, highlighting the importance of a higher-dimensional interaction in stabilizing the CDW ordering in this material. In addition, the temperature evolution of intrachain gaps caused by the CDW periodicities far below EF deviate from the behavior expected for a Peierls-type mechanism driven by nesting; the upper and lower bands of the renormalized CDW dispersions maintain a fixed peak-to-peak distance while the gaps are gradually removed at higher temperatures. This points toward a gradual loss of long-range phase coherence as the dominant effect in reducing the CDW order parameter, which may correspond to the loss of coherence between the coupled chains. Furthermore, one of the gaps is observed above the bulk and surface CDW transition temperatures, implying the persistence of short-range incoherent CDW order. The influence of such higher-dimensional interactions likely plays an important role in a range of low-dimensional systems.
关键词： angle-resolved photoemission spectroscopy,tight-binding model,charge density wave,phase coherence,interband coupling,quasi-one-dimensional,NbSe3
更新于2025-09-23 15:21:01
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Charge Density Modulation on Asymmetric Fused-Ring Acceptors for High-Efficiency Photovoltaic Solar Cells
摘要： Charge density modulation on thieno[2'',3'':5',6']-s-indaceno[2',1':4,5]dithieno[3,2-b:2’,3’-d]pyrrole (IPT) core has been conducted for a systematic study of its impact on the electronic structure, molecular packing and photovoltaic performance of asymmetric fused-ring acceptors (FRAs). Herein, a series of IPT-based FRAs (ca. IN-4F, INO-4F, IPT-4F and IPCl-4F) are designed by adopting a corresponding side-chain of 2-ethylhexyl, 2-ethylhexyloxy, hydrogen or chloro onto IPT core. Enhanced electron-withdrawing side-chains contract the optical bandgap but lower the lowest unoccupied molecular orbital (LUMO) level, which yields a trade-off between JSC and VOC in organic solar cells (OSCs). Furthermore, the FRAs exhibit tuned miscibility and crystallinity, reflected on the FF and JSC values of the OSCs. By pairing with polymer donor PM6, IPT-4F based devices achieve the highest PCE of 14.62% with balanced VOC of 0.88 V and JSC of 22.15 mA cm-2 and high FF of 75.01%. Our research demonstrates that electronic density modulation on asymmetric FRAs is an effective way to systematically optimize the device parameters in pursuit of high performance OSCs.
关键词： asymmetric fused-ring acceptors,Charge density modulation,electron-withdrawing side-chains,organic solar cells,photovoltaic performance
更新于2025-09-23 15:21:01
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by angle-resolved photoelectron spectroscopy, scanning tunneling spectroscopy, and density functional theory
摘要： The electronic structure of 1T -TaS2 showing a metal-insulator transition and a sequence of different charge density wave (CDW) transformations was discussed in the frame of variable temperature angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. For the commensurate charge density wave phase (CCDW) the Mott gap was estimated to be 0.4 eV and energy gaps (cid:2)CCDW,1, (cid:2)CCDW,2, (cid:2)B3-HHB, (cid:2)B4-B3 were observed. For the nearly commensurate charge density wave phase (NCCDW), the reminiscent of higher and lower Hubbard bands and a very pronounced electronic state associated with the parabolic band at the ˉ(cid:3) point in the Brillouin zone were identi?ed. The incommensurate charge density wave phase (ICCDW) showed a high value of local density of states at the Fermi level and a very pronounced edge of the metallic surface state located in the range of 0.15–0.20 eV above the Fermi level. The obtained STS and ARPES results were consistent with our theoretical calculations performed within DFT formalism including spin-orbit coupling.
关键词： STS,1T -TaS2,charge density wave,ARPES,DFT,metal-insulator transition
更新于2025-09-23 15:21:01
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Exposes its Excitonic Character
摘要： Recent experiments suggest that excitonic degrees of freedom play an important role in precipitating the charge density wave (CDW) transition in 1T-TiSe2. Through systematic calculations of the electronic and phonon spectrum based on density functional perturbation theory, we show that the predicted critical doping of the CDW phase overshoots the experimental value by 1 order of magnitude. In contrast, an independent self-consistent many-body calculation of the excitonic order parameter and renormalized band structure is able to capture the experimental phase diagram in extremely good qualitative and quantitative agreement. This demonstrates that electron-electron interactions and the excitonic instability arising from direct electron-hole coupling are pivotal to accurately describe the nature of the CDW in this system. This has important implications to understand the emergence of superconductivity within the CDW phase of this and related systems.
关键词： charge density wave,1T-TiSe2,electron-electron interactions,superconductivity,excitonic instability
更新于2025-09-23 15:21:01