- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
Diagrammatic approach to nonlinear optical response with application to Weyl semimetals
摘要： Nonlinear optical responses are a crucial probe of physical systems including periodic solids. In the absence of electron-electron interactions, they are calculable with standard perturbation theory starting from the band structure of Bloch electrons, but the resulting formulas are often large and unwieldy, involving many energy denominators from intermediate states. This work gives a Feynman diagram approach to calculating nonlinear responses. This diagrammatic method is a systematic way to perform perturbation theory, which often offers shorter derivations and also provides a natural interpretation of nonlinear responses in terms of physical processes. Applying this method to second-order responses concisely reproduces formulas for the second-order-harmonic shift current. We then apply this method to third-order responses and derive formulas for third-order-harmonic generation and self-focusing of light, which can be directly applied to tight-binding models. Third-order responses in the semiclassical regime include a Berry curvature quadrupole term, whose importance is discussed including symmetry considerations and when the Berry curvature quadrupole becomes the leading contribution. The method is applied to compute third-order optical responses for a model Weyl semimetal, where we find a new topological contribution that diverges in a clean material, as well as resonances with a peculiar linear character.
关键词： Weyl semimetals,perturbation theory,Berry curvature,nonlinear optical responses,Feynman diagrams
更新于2025-09-23 15:23:52
-
Semimetallicity and Negative Differential Resistance from Hybrid Halide Perovskite Nanowires
摘要： In the rapidly progressing field of organometal halide perovskites, the dimensional reduction can open up new opportunities for device applications. Herein, taking the recently synthesized trimethylsulfonium lead triiodide (CH3)3SPbI3 perovskite as a representative example, first-principles calculations are carried out and the nanostructuring and device application of halide perovskite nanowires are studied. It is found that the 1D (CH3)3SPbI3 structure is structurally stable, and the electronic structures of higher-dimensional forms are robustly determined at the 1D level. Remarkably, due to the face-sharing [PbI6] octahedral atomic structure, the organic ligand-removed 1D PbI3 frameworks are also found to be stable. Moreover, the PbI3 columns avoid the Peierls distortion and assume a semimetallic character, contradicting the conventional assumption of semiconducting metal-halogen inorganic frameworks. Adopting the bundled nanowire junctions consisting of (CH3)3SPbI3 channels with sub-5 nm dimensions sandwiched between PbI3 electrodes, high current densities and large room-temperature negative differential resistance (NDR) are finally obtained. It will be emphasized that the NDR originates from the combination of the near-Ohmic character of PbI3-(CH3)3SPbI3 contacts and a novel NDR mechanism that involves the quantum-mechanical hybridization between channel and electrode states. This work demonstrates the great potential of low-dimensional hybrid perovskites toward advanced electronic devices beyond actively pursued photonic applications.
关键词： semimetals,halide perovskite nanowires,first-principles calculations,negative differential resistance
更新于2025-09-23 15:22:29
-
Two-dimensional Kagome Lattices Made of Hetero Triangulenes are Dirac Semimetals or Single-Band Semiconductors
摘要： Here we discuss, based on first-principles calculations, two-dimensional (2D) kagome lattices composed of polymerized hetero-triangulene units, planar molecules with D3h point group containing a B, C or N center atom and CH2, O or CO bridges. We explore the design principles for a functional lattice made of 2D polymers, which involves control of π-conjugation and electronic structure of the knots. The former is achieved by the chemical potential of the bridge groups, while the latter is controlled by the heteroatom. The resulting 2D kagome polymers have a characteristic electronic structure with a Dirac band sandwiched by two flat bands and are either Dirac semimetals (C center), or single-band semiconductors - materials with either exclusively electrons (B center) or holes (N center) as charge carriers of very high mobility, reaching values of up to ~8×103 cm2V-1s-1, which is comparable to crystalline silicon.
关键词： high mobility,charge carriers,single-band semiconductors,hetero-triangulene units,Dirac semimetals,two-dimensional kagome lattices
更新于2025-09-23 15:21:21
-
Photoinduced tunable anomalous Hall and Nernst effects in tilted Weyl semimetals using Floquet theory
摘要： In this paper, we discuss the effect of a periodically driving circularly polarized laser beam in the high-frequency limit, on the band structure and thermal transport properties of type-I and type-II Weyl semimetals (WSMs). We develop the notion of an effective Fermi surface stemming from the time-averaged Floquet Hamiltonian and discuss its effects on the steady-state occupation numbers of electrons and holes in the linearized model. In order to compute the transport coef?cients averaged over a period of the incident laser source, we employ the Kubo formalism for Floquet states and show that the Kubo formula for the conductivity tensor retains its well-known form with the difference that the eigenstates and energies are replaced by the Floquet states and their quasienergies. We ?nd that for type-I WSMs the anomalous thermal Hall conductivity grows quadratically with the amplitude A0 of the U(1) gauge ?eld for low tilt, while the Nernst conductivity remains unaffected. For type-II WSMs, the Hall conductivity decreases nonlinearly with A0 due to the contribution from the physical momentum cutoff, required to keep ?nite electron and hole pocket sizes, and the Nernst conductivity falls off logarithmically with A2 0. These results may serve as a diagnostic for material characterization and transport parameter tunability in WSMs, which are currently the subject of a wide range of experiments.
关键词： Nernst effect,Floquet theory,anomalous Hall effect,thermal transport,Weyl semimetals
更新于2025-09-23 15:21:01
-
Axion-Field-Enabled Nonreciprocal Thermal Radiation in Weyl Semimetals
摘要： Objects around us constantly emit and absorb thermal radiation. The emission and absorption processes are governed by two fundamental radiative properties: emissivity and absorptivity. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff’s law of thermal radiation. This restriction limits the degree of freedom to control thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems. Existing approaches to violate Kirchhoff’s law typically utilize magneto-optical effects with an external magnetic field. However, these approaches require either a strong magnetic field (～3T), or narrow-band resonances under a moderate magnetic field (～0.3T), because the non-reciprocity in conventional magneto-optical effects is weak in the thermal wavelength range. Here, we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters that near completely violate Kirchhoff’s law over broad angular and frequency ranges, without requiring any external magnetic field.
关键词： topological materials,Weyl semimetals,non-reciprocity,magneto-optical effects,thermal radiation
更新于2025-09-23 15:19:57
-
Two-dimensional honeycomb borophene oxide: strong anisotropy and nodal loop transformation
摘要： The search for topological semimetals is mainly focused on heavy-element compounds by following the footsteps of previous research on topological insulators, with less attention on light-element materials. However, the negligible spin orbit coupling with light elements may turn out to be beneficial for realizing topological band features. Here, using first-principles calculations, we propose a new two-dimensional light-element material—the honeycomb borophene oxide (h-B2O), which has nontrivial topological properties. The proposed structure is based on the recently synthesized honeycomb borophene on an Al (111) substrate [W. Li, L. Kong, C. Chen, J. Gou, S. Sheng, W. Zhang, H. Li, L. Chen, P. Cheng and K. Wu, Sci. Bull., 2018, 63, 282–286]. The h-B2O monolayer is completely flat, unlike the oxides of graphene or silicene. We systematically investigate the structural properties of h-B2O, and find that it has very good stability and exhibits significant mechanical anisotropy. Interestingly, the electronic band structure of h-B2O hosts a nodal loop centered around the Y point in the Brillouin zone, protected by the mirror symmetry. Furthermore, under moderate lattice strain, the single nodal loop can be transformed into two loops, each penetrating through the Brillouin zone. The loops before and after the transition are characterized by different Z × Z topological indices. Our work not only predicts a new two-dimensional material with interesting physical properties, but also offers an alternative approach to search for new topological phases in 2D light-element systems.
关键词： borophene oxide,light-element materials,topological semimetals,nodal loop,mechanical anisotropy
更新于2025-09-19 17:15:36
-
High responsivity and fast UV-Vis-SWIR photodetector based on Cd3As2/MoS2 heterojunction
摘要： High responsivity, fast response time, ultra-wide detection spectrum are pursuing goals for state-of-art photodetectors. Cd3As2, as a three-dimensional (3D) Dirac semimetal, has zero-bandgap, high light absorption rate in broad spectral region, and higher mobility than graphene at room temperature. However, photoconductive detectors based Cd3As2 suffer low quantum efficiency due to the absence of high built-in field. Here, Cd3As2 nanoplate/multilayer MoS2 heterojunction photodetector was fabricated and achieved a quite high responsivity of 2.7×103 A/W at room temperature. The photodetector exhibits a short response time of in broad spectra region from ultraviolet (365 nm) to short-wavelength-infrared (1550 nm) and reached 65 μs at 650 nm. This work provides a great potential solution for high-performance photodetector and broadband imaging by combining 3D Dirac semi-metal materials with semiconductor materials.
关键词： 3D Dirac semimetals,broadband,photodetectors,responsivity,heterojunction
更新于2025-09-11 14:15:04
-
Giant tunable nonreciprocity of light in Weyl semimetals
摘要： The propagation of light in Weyl semimetal films is analyzed. The magnetic family of these materials is known by anomalous Hall effect, which, being enhanced by the large Berry curvature, allows one to create strong gyrotropic and nonreciprocity effects without external magnetic field. The existence of nonreciprocal waveguide electromagnetic modes in ferromagnetic Weyl semimetal films in the Voigt configuration is predicted. Thanks to the strong dielectric response caused by the gapless Weyl spectrum and the large Berry curvature, ferromagnetic Weyl semimetals combine the best waveguide properties of magnetic dielectrics or semiconductors with strong anomalous Hall effect in ferromagnets. The magnitude of the nonreciprocity depends both on the internal Weyl semimetal properties, the separation of Weyl nodes, and the external factor, the optical contrast between the media surrounding the film. By tuning the Fermi level in Weyl semimetals, one can vary the operation frequencies of the waveguide modes in THz and mid-IR ranges. Our findings pave the way to the design of compact, tunable, and effective nonreciprocal optical elements.
关键词： Weyl semimetals,optical elements,anomalous Hall effect,nonreciprocity,waveguide modes
更新于2025-09-11 14:15:04
-
Free-carrier dynamics in Au <sub/>2</sub> Pb probed by optical conductivity measurements
摘要： We measured the optical reflectivity of the Dirac material Au2Pb in a broad frequency range (30–48 000 cm?1) for temperatures between 9 and 300 K. The optical conductivity, computed from the reflectivity, is dominated by free-carrier contributions from topologically trivial bulk bands at all temperatures. The temperature-independent total plasma frequency of these carriers is 3.9 ± 0.2 eV. Overall, optical response of Au2Pb is typically metallic with no signs of localization and bad-metal behavior.
关键词： topological semimetals,optical conductivity,Dirac materials
更新于2025-09-10 09:29:36
-
<i>Ab initio</i> optical and energy loss spectra of transition metal monopnictides TaAs, TaP, NbAs, and NbP
摘要： Transition metal monopnictides represent a new class of topological semimetals with low-energy excitations, namely, Weyl fermions. We report optical properties across a wide spectral energy range for TaAs, TaP, NbAs, and NbP, calculated within density functional theory. Spectra are found to be somewhat independent of the anion and the light polarization. Their features are explained in terms of the upper s, p, d, and f electrons. Characteristic absorption features are related to the frequency dependence of the Fresnel re?ectivity. While the lower part of the energy loss spectra is dominated by plasmonic features, the high-energy structures are explained by interband transitions.
关键词： Topological semimetals,Energy loss spectra,Optical properties,Transition metal monopnictides,Weyl fermions
更新于2025-09-09 09:28:46