An intriguing phenomenon in topological semimetals and topological insulators is the negative magnetoresistance (MR) observed when a magnetic field is applied along the current direction. A prevailing understanding to the negative MR in topological semimetals is the chiral anomaly, which, however, is not well defined in topological insulators. We calculate the MR of a threedimensional topological insulator, by using the semiclassical equations of motion, in which the Berry curvature explicitly induces an anomalous velocity and orbital moment. Our theoretical results are in quantitative agreement with the experiments. The negative MR is not sensitive to temperature and increases as the Fermi energy approaches the band edge. The orbital moment and g factors also play important roles in the negative MR. Our results give a reasonable explanation to the negative MR in 3D topological insulators and will be helpful in understanding the anomalous quantum transport in topological states of matter.

Topological Weyl semimetals can host Weyl nodes with monopole charges in momentum space. How to detect the signature of the monopole charges in quantum transport remains a challenging topic. Here, we reveal the connection between the parity of monopole charge in topological semimetals and the quantum interference corrections to the conductivity. We show that the parity of monopole charge determines the sign of the quantum interference correction, with odd and even parity yielding the weak antilocalization and weak localization effects, respectively. This is attributed to the Berry phase difference between time-reversed trajectories circulating the Fermi sphere that encloses the monopole charges. From standard Feynman diagram calculations, we further show that the weak-field magnetoconductivity at low temperatures is proportional to + √ B in double-Weyl semimetals and − √ B in single-Weyl semimetals, respectively, which could be verified experimentally.

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Many key features of higher dimensional Sachdev-Ye-Kitaev (SYK) model at finite N remain unknown. Here we study the SYK chain consisting of N (N ≥2) fermions per site with random interactions and hoppings between neighboring sites. In the limit of vanishing SYK interactions, from both supersymmetric field theory analysis and numerical calculations we find that the randomhopping model exhibits Anderson localization at finite N , irrespective of the parity of N . Moreover, the localization length scales linearly with N , implying no Anderson localization only at N = ∞. For finite SYK interaction J, from the exact diagonalization we show that there is a dynamic phase transition between many-body localization and thermal diffusion as J exceeds a critical value Jc. In addition, we find that the critical value Jc decreases with the increase of N , qualitatively consistent with the analytical result of Jc/t ∝ 1 N 5/2 log N derived from the weakly interacting limit. Introduction:The seminal Sachdev-Ye-Kitaev (SYK) model [1, 2] presents a zero-dimensional (0D) cluster consisting of N Majorana fermions with random all-to-all interactions. In the large-N limit it is exactly solvable, exhibiting maximal quantum chaos [2-4], emergent SL(2, R) symmetry as well as a holographic dual to dilaton gravity theory in nearly AdS 2 geometry [2,3]. Owing to its solvability and intriguing properties, it has stimulated enormous excitement . In particular, the large-N limit of the SYK model, after properly generalized to higher dimensions [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49], could provide an insightful and promising avenue to investigate the spectral and transport properties of non-Fermi liquid states. Nonetheless, features of the higher-dimensional SYK models with finite N remain largely unknown. As the case of finite N is directly relevant to possible experimental realizations [50-53] of SYK models, it is desired to understand the characterizing properties of the higher dimensional SYK models at finite N .Here we consider a generic SYK chain model of Majorana fermions respecting time-reversal symmetry, which includes four-fermion random interactions and random hoppings between neighboring sites as shown in Fig. 1 [see Eq. (1) below]. Note that the neighboring fermion hopping on a bipartite lattice respects the time-reversal symmetry defined as γ j,x → (−) x γ j,x where γ j,x represents the Majorana fermion with flavor j = 1, · · ·, N on site x. Both the random hoppings and the random interactions are characterized by Gaussian random variables with zero-mean; and their variances are given by t 2 /N and 3!J 2 /N 3 , respectively. We first consider the noninteracting limit, namely J = 0, for which the model in Eq. (1) reduces to a one-dimensional (1D) random-hopping model [54]. The presence of time-reversal symmetry renders the Majorana system in the BDI class [55,56]. In particular, when the system size L is odd, there will be N zero-energy singleparticle modes in the band center due to the particle-hole symmetry. ...

Composite materials have increasingly become a high proportion of the structural weight of aircraft due to their excellent performances. Different types of damages may occur in the aircraft service period, which will bring potential safety risks to aircrafts. To investigate the defect damage detection and its spectral characteristics and imaging of carbon-fiber-reinforced polymer composite laminates, defects from the low-velocity impact damage in composites were measured by the THz time-domain reflection imaging system. Results show that there exists obvious THz spectral differences between the impact damaged defects and nondefect. The effective detection frequency band for the low-speed impact damaged defect is 0.12–2.0 THz. In the time domain, there are attenuations and delays in the spectra of defects relative to those of nondefects. In the frequency domain, with the increase of frequency, the power spectral density of the defect first increases and then decreases, and the absorption coefficient increases slowly. In general, the imaging results in time-domain imaging are better than those from the frequency-domain imaging, which not only is suitable for the qualitative detection of defects but also has great potential and application prospects in quantitative detection. This work shows an important guide for the application of THz technology to detect the composite material defects in civil aircraft.