We report an atomic-scale characterization of ${\mathrm{ZrTe}}_5$ by using scanning tunneling microscopy. We observe a bulk band gap of $\sim 80\mathrm{meV}$ with topological edge states at the step edge and, thus, demonstrate that ${\mathrm{ZrTe}}_5$ is a two-dimensional topological insulator. We also find that an applied magnetic field induces an energetic splitting of the topological edge states, which can be attributed to a strong link between the topological edge states and bulk topology. The relatively large band gap makes ${\mathrm{ZrTe}}_5$ a potential candidate for future fundamental studies and device applications.

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.

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.

Transition metal dichalcogenides (TMDs) WTe₂ and MoTe₂ with orthorhombic Td phase, being potential candidates as type-II Weyl semimetals, are attracted much attention recently. Here we synthesized a series of miscible Mo_1−xW_xTe₂ single crystals by bromine vapor transport method. Composition-dependent X-ray diffraction and Raman spectroscopy, as well as composition and temperature-dependent resistivity prove that the tunable crystal structure (from hexagonal (2H), monoclinic (β) to orthorhombic (Td) phase) can be realized by increasing W content in Mo_1−xW_xTe₂. Simultaneously the electrical property gradually evolves from semiconductor to semimetal behavior. Temperature-dependent Raman spectroscopy proves that temperature also can induce the structural phase transition from β to Td phase in Mo_1−xW_xTe₂ crystals. Based on aforementioned characterizations, we map out the temperature and composition dependent phase diagram of Mo_1−xW_xTe₂ system. In addition, a series of electrical parameters, such as carrier type, carrier concentration and mobility, have also been presented. This work offers a scheme to accurately control structural phase in Mo_1−xW_xTe₂ system, which can be used to explore type-II Weyl semimetal, as well as temperature/composition controlled topological phase transition therein.