As a quantum material, Weyl semimetal has a series of electronic-band-structure features, including Weyl points with left and right chirality and corresponding Berry curvature, which have been observed in experiments. These band-structure features also lead to some unique nonlinear properties, especially high-order harmonic generation (HHG) due to the dynamic process of electrons under strong laser excitation, which has remained unexplored previously. Herein, we obtain effective HHG in type-II Weyl semimetal β-WP₂ crystals, where both odd and even orders are observed, with spectra extending into the vacuum ultraviolet region (190 nm, 10th order), even under fairly low femtosecond laser intensity. In-depth studies have interpreted that odd-order harmonics come from the Bloch electron oscillation, while even orders are attributed to Bloch oscillations under the “spike-like” Berry curvature at Weyl points. With crystallographic orientation-dependent HHG spectra, we further quantitatively retrieved the electronic band structure and Berry curvature of β-WP₂. These findings may open the door for exploiting metallic/semimetallic states as solid platforms for deep ultraviolet radiation and offer an all-optical and pragmatic solution to characterize the complicated multiband electronic structure and Berry curvature of quantum topological materials.

The asymmetric electron dispersion in type-II Weyl semimetal theoretically hosts anisotropic transport properties. Here, we observe the significant anisotropic Adler-Bell-Jackiw (ABJ) anomaly in the Fermi-level delicately adjusted ${\mathrm{WTe}}_{1.98}$ crystals. Quantitatively, $C_W$, a coefficient representing the intensity of the ABJ anomaly along the $a$ and $b$ axis of ${\mathrm{WTe}}_{1.98}$ are 0.030 and $0.051{\mathrm{T}}^{-2}$ at 2 K, respectively. We found that the temperature-sensitive ABJ anomaly is attributed to a topological phase transition from a type-II Weyl semimetal to a trivial semimetal, which is verified by a first-principles calculation using experimentally determined lattice parameters at different temperatures. Theoretical electrical transport study reveals that the observation of an anisotropic ABJ along both the $a$ and $b$ axes in ${\mathrm{WTe}}_{1.98}$ is attributed to electrical transport in the quasiclassical regime. Our work may suggest that electron-doped ${\mathrm{WTe}}_2$ is an ideal playground to explore the novel properties in type-II Weyl semimetals.

ZrTe₅ and HfTe₅ have attracted increasingly attention recently since the theoretical prediction of being topological insulators (TIs). However, subsequent works show many contradictions about their topolog-ical nature. Three possible phases, i.e. strong TI, weak TI, and Dirac semi-metal, have been observed in different experiments until now. Essentially whether ZrTe₅ or HfTe₅ has a band gap or not is still a question. Here, we present detailed first-principles calculations on the electronic and topological prop-erties of ZrTe₅ and HfTe₅ on variant volumes and clearly demonstrate the topological phase transition from a strong TI, going through an intermediate Dirac semi-metal state, then to a weak TI when the crystal expands. Our work might give a unified explain about the divergent experimental results and propose the crucial clue to further experiments to elucidate the topological nature of these materials.

Recently, the extremely large magnetoresistance (MR) observed in transition metal telluride, like WTe2, attracted much attention because of the potential applications in magnetic sensor. Here, we report the observation of extremely large magnetoresistance as 3.0 × 104% measured at 2 K and 9 T magnetic field aligned along [001]-ZrSiS. The significant magnetoresistance change (∼1.4 × 104%) can be obtained when the magnetic field is titled from [001] to [011]-ZrSiS. These abnormal magnetoresistance behaviors in ZrSiS can be understood by electron-hole compensation and the open orbital of Fermi surface. Because of these superior MR properties, ZrSiS may be used in the magnetic sensors.

Very recently, a novel two-dimension (2D) MXene, MoSi₂N₄, was successfully synthesized with excellent ambient stability, high carrier mobility, and moderate band gap (2020 Science 369 670). In this work, the intrinsic lattice thermal conductivity of monolayer MoSi₂N₄ is predicted by solving the phonon Boltzmann transport equation based on the first-principles calculations. Despite the heavy atomic mass of Mo and complex crystal structure, the monolayer MoSi₂N₄ unexpectedly exhibits a quite high lattice thermal conductivity over a wide temperature range between 300 to 800 K. At 300 K, its in-plane lattice thermal conductivity is 224 Wm⁻¹ K⁻¹. The detailed analysis indicates that the large group velocities and small anharmonicity are the main reasons for its high lattice thermal conductivity. We also calculate the lattice thermal conductivity of monolayer WSi₂N₄, which is only a little smaller than that of MoSi₂N₄. Our findings suggest that monolayer MoSi₂N₄ and WSi₂N₄ are potential 2D materials for thermal transport in future nano-electronic devices.

Ion migration in halide perovskite materials usually brings an intractable problem in the working stability of solar cells and photoelectrical detectors. The mechanism of ion migration and its impact on physical properties are still open questions. In this work, the ion migration behavior in solution-grown CsPbBr₃ crystals was observed by the hysteresis in current–voltage curves and the temperature dependent reversed current–time measurements. Defect proliferation phenomena (new defects of [V_Cs]⁻ and [Pb_Br]²⁺) originating from ion migration were verified by thermally stimulated current spectroscopy. Our results also give evidence that Cs⁺ ions also participate in the process of ion migration except the widely considered Br⁻ ions. Furthermore, the photoelectric properties of the CsPbBr₃ device were found to be seriously deteriorated after the ion migration. Our work demonstrates the strong correlation between the ion migration and physical properties in halide perovskites.