In this theoretical investigation, we analyze light-induced nonlinear spin Hall currents in a gated single-layer 1T ′-WTe2, flowing transversely to the incident laser polarization direction. Our ...study encompasses the exploration of the second and third-order rectified spin Hall currents using an effective low-energy Hamiltonian and employing the Kubo’s formalism. We extend our analysis to a wide frequency range spanning both transparent and absorbing regimes, investigating the influence of light frequency below and above the optical band gap. Additionally, we investigate the influence of an out-of-plane gate potential on the system, disrupting inversion symmetry and effectively manipulating both the strength and sign of nonlinear spin Hall responses. We predict a pronounced third-order spin Hall current relative to its second-order counterpart. The predicted nonlinear spin currents show strong anisotropic dependence on the laser polarization angle. The outcomes of our study contribute to a generalized framework for nonlinear response theory within the spin channel will impact the development of emerging field of opto-spintronic.
We study the second-order nonlinear optical response of a Weyl semimetal (WSM), i.e., a three-dimensional metal with linear band touchings acting as pointlike sources of Berry curvature in momentum ...space, termed “Weyl-Berry monopoles.” We first show that the anomalous second-order photocurrent of WSMs can be elegantly parametrized in terms of Weyl-Berry dipole and quadrupole moments. We then calculate the corresponding charge and node conductivities of WSMs with either broken time-reversal invariance or inversion symmetry. In particular, we predict a dissipationless second-order anomalous node conductivity for WSMs belonging to the TaAs family.
Interaction-driven symmetry breaking in a metallic (doped) Dirac system can manifest in the spontaneous gap generation at the nodal point buried below the Fermi level. Across this transition linear ...conductivity remains finite, making its direct observation difficult in linear transport. We propose the nonlinear Hall effect as a direct probe of this transition when inversion symmetry is broken. Specifically, for a two-dimensional Dirac material with a tilted low-energy dispersion, we predict a transformation of the characteristic interband resonance peak into a non-Lorentzian form in the collisionless regime. Furthermore, we show that inversion-symmetry-breaking quantum phase transition is controlled by an exotic tilt-dependent line of critical points. As this line is approached from the ordered side, the nonlinear Hall conductivity is suppressed owing to the scattering between the strongly coupled incoherent fermionic and bosonic excitations. Our results should motivate further studies of nonlinear responses in strongly interacting Dirac materials.
Deep Convolutional Neural Networks (DCNNs) contain a high level of complexity and nonlinearity, so it is not clear based on what features DCNN models make decisions and how they can reach such ...promising results. There are two types of visualization techniques to interpret and explain the deep models: Backpropagation-based and Perturbation-based algorithms. The most notable drawback of the backpropagation-based visualization is that they cannot be applied for all architectures, whereas Perturbation-based visualizations are totally independent of the architectures. These methods, however, take a lot of computation and memory resources which make them slow and expensive, thereby unsuitable for many real-world applications. To cope with these problems, in this paper, a perturbation-based visualization method called Fast Multi-resolution Occlusion (FMO) are presented which is efficient in terms of time and resource consumption and can be considered in real-world applications. In order to compare the FMO with five well-known Perturbation-based visualizations methods such as Occlusion Test, Super-pixel perturbation (LIME), Randomized Input Sampling (RISE), Meaningful Perturbation and Extremal Perturbation, different experiments are designed in terms of time-consumption, visualization quality and localization accuracy. All methods are applied on 5 well-known DCNNs DenseNet121, InceptionV3, InceptionResnetV2, MobileNet and ResNet50 using common benchmark datasets ImageNet, PASCAL VOC07 and COCO14. According to the experimental results, FMO is averagely 2.32 times faster than LIME on five models DenseNet121, InceptionResnetV2, InceptionV3, MobileNet and ResNet50 with images of ILSVRC2012 dataset as well as 24.84 times faster than Occlusion Test, 11.87 times faster than RISE, 8.72 times faster than Meaningful Perturbation and 10.03 times faster than Extremal Perturbation on all of the five used models with images of common dataset ImageNet without scarifying visualization quality. Moreover, the methods are evaluated in terms of localization accuracy on two hard common datasets of PASCAL VOC07 and COCO14. The results show that FMO outperforms the compared relevant methods in terms of localization accuracy. Also, FMO extends the superimposing process of the Occlusion Test method, which yields a heatmap with more visualization quality than the Occlusion Test on many colorful images.
Abstract
Nonlinear optical processes, such as harmonic generation, are of great interest for various applications, e.g., microscopy, therapy, and frequency conversion. However, high-order harmonic ...conversion is typically much less efficient than low-order, due to the weak intrinsic response of the higher-order nonlinear processes. Here we report ultra-strong optical nonlinearities in monolayer MoS
2
(1L-MoS
2
): the third harmonic is 30 times stronger than the second, and the fourth is comparable to the second. The third harmonic generation efficiency for 1L-MoS
2
is approximately three times higher than that for graphene, which was reported to have a large
χ
(3)
. We explain this by calculating the nonlinear response functions of 1L-MoS
2
with a continuum-model Hamiltonian and quantum mechanical diagrammatic perturbation theory, highlighting the role of trigonal warping. A similar effect is expected in all other transition-metal dichalcogenides. Our results pave the way for efficient harmonic generation based on layered materials for applications such as microscopy and imaging.
Abstract
We show that the standard concepts of nonlinear response to electromagnetic fields break down in two-dimensional Dirac systems, like graphene, in the quantum regime close to the Dirac point. ...We present a compelling many-body theory for nonlinear transport focusing on disorder scattering as a benchmark example. We show that, although the diamagnetic two-photon vertex is absent at the non-interacting level, disorder effects give rise to a self-generation of such two-photon vertex surviving even in the clean limit. We predict that the two-photon vertex self-generation is present only in two dimensions. The impact of such a striking scenario on the nonlinear quantum transport is discussed, predicting a huge enhancement of third-order dc conductivity comparing to the common models.
Abstract
Conversion of mechanical forces to electric signal is possible in non-centrosymmetric materials due to linear piezoelectricity. The extraordinary mechanical properties of two-dimensional ...materials and their high crystallinity make them exceptional platforms to study and exploit the piezoelectric effect. Here, the piezoelectric response of non-centrosymmetric hexagonal two-dimensional crystals is studied using the modern theory of polarization and
k
·
p
model Hamiltonians. An analytical expression for the piezoelectric constant is obtained in terms of topological quantities, such as the
valley Chern number
. The theory is applied to semiconducting transition metal dichalcogenides and hexagonal Boron Nitride. We find good agreement with available experimental measurements for MoS
2
. We further generalize the theory to study the polarization of samples subjected to inhomogeneous strain (e.g., nanobubbles). We obtain a simple expression in terms of the strain tensor, and show that charge densities ≳10
11
cm
−2
can be induced by realistic inhomogeneous strains,
ϵ
≈ 0.01–0.03.
The low-energy (intraband) range of the third harmonic generation of graphene in the terahertz regime is governed by the damping terms induced by the interactions. A controlled many-body description ...of the scattering processes is thus a compelling and desirable requirement. In this paper, using a Kadanoff-Baym approach, we systematically investigate the impact of many-body interaction on the third harmonic generation of graphene, taking elastic impurity scattering as a benchmark example. We predict the onset in the mixed inter- and intraband regime of incoherent features driven by the interaction at four- and five-photon transition frequencies in the third harmonic optical conductivity with a spectral weight proportional to the scattering rate. We also show that in spite of the complex many-body physics, the purely intraband term governing the limit ω → 0 resembles the constraints of the phenomenological model. We ascribe this agreement to the fulfilling of the conservation laws enforced by the conserving approach. However, the overlap with incoherent features and the impact of many-body-driven multiphoton vertex couplings severely limit the validity of phenomenological description.
We develop a microscopic large-N theory of electron-electron interaction corrections to multilegged Feynman diagrams describing second- and third-order non-linear-response functions. Our theory, ...which reduces to the well-known random-phase approximation in the linear-response limit, is completely general and is useful to understand all second- and third-order nonlinear effects, including harmonic generation, wave mixing, and photon drag. We apply our theoretical framework to the case of graphene, by carrying out microscopic calculations of the second- and third-order non-linear-response functions of an interacting two-dimensional (2D) gas of massless Dirac fermions. We compare our results with recent measurements, where all-optical launching of graphene plasmons has been achieved by virtue of the finiteness of the quasihomogeneous second-order nonlinear response of this inversion-symmetric 2D material.
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