Surface superconductivity in the type II Weyl semimetal TaIrTe4

Superconductor Vortex Spectrum Including Fermi Arc States in Time-Reversal Symmetric Weyl Semimetals

Using semiclassics to surmount the hurdle of bulk-surface inseparability, we derive the superconductor vortex spectrum in nonmagnetic Weyl semimetals and show that it stems from the Berry phase of orbits made of Fermi arcs on opposite surfaces and bulk chiral modes. Tilting the vortex transmutes it between bosonic, fermionic, and supersymmetric, produces periodic peaks in the density of states that signify novel nonlocal Majorana modes, and yields a thickness-independent spectrum at magic "magic angles." We propose (Nb,Ta)P as candidate materials and tunneling spectroscopy as the ideal experiment.

Surface Superconductivity with High Transition Temperatures in Layered CanBn+1Cn+1 Films

Proposed by Ginzberg nearly 60 years ago, surface superconductivity refers to the emergent phenomenon that the electrons on or near the surface of a material becomes superconducting despite its bulk is nonsuperconducting. Here, based on first-principles calculations within density functional theory, we predict that the superconducting transition temperature T_c at the surfaces of Ca_nB_n+1C_n+1 (n = 1, 2, 3, ...) films can be drastically enhanced to ∼90 K from 8 K for bulk CaBC. Our detailed analyses reveal that structural symmetry reduction at surfaces induces pronounced carrier self-doping into the surface B-C layer of the films and shifts the σ-bonding states toward the Fermi level; furthermore, the in-plane stretching modes of the surface layers experience significant softening. These two effects work collaboratively to strongly enhance the electron-phonon coupling, which in turn results in much higher T_c values than the McMillian limit. These findings point to new material platforms for realizing unusually high-T_c surface superconductivity.

Strain modulation of photocurrent in Weyl semimetal TaIrTe4

We study the effect of the strain on the energy bands of a TaIrTe₄ sheet and the photocurrent in the Cu-TaIrTe₄-Cu heterojunction by using the quantum transport simulations. It is found that the Weyl points can be completely broken with an increase of the strain along the z direction. One can obtain a large photocurrent in the Cu-TaIrTe₄-Cu heterojunction in the absence of the strain; while the photocurrent can be sharply enhanced by the strain and reach a large value. Accordingly, the maximum values of the photocurrent can be explained in terms of the transitions between peaks of density of states and band structures. The strain-induced energy bands and photocurrent exhibit anisotropic behaviors. Our results provide a novel, to the best of our knkowledge, route to effectively modulate the energy bands and the photocurrent by utilizing mechanical methods for TaIrTe₄-based devices.

Room-temperature third-order nonlinear Hall effect in Weyl semimetal TaIrTe4

The second-order nonlinear Hall effect observed in the time-reversal symmetric system has not only shown abundant physical content, but also exhibited potential application prospects. Recently, a third-order nonlinear Hall effect has been observed in MoTe₂ and WTe₂. However, few-layer MoTe₂ and WTe₂ are usually unstable in air and the observed third-order nonlinear Hall effect can be measured only at low temperature, which hinders further investigation as well as potential application. Thus, exploring new air-stable material systems with a sizable third-order nonlinear Hall effect at room temperature is an urgent task. Here, in type-II Weyl semimetal TaIrTe₄, we observed a pronounced third-order nonlinear Hall effect, which can exist at room temperature and remain stable for months. The third-order nonlinear Hall effect is connected to the Berry-connection polarizability tensor instead of the Berry curvature. The possible mechanism of the observation of the third-order nonlinear Hall effect in TaIrTe₄ at room temperature has been discussed. Our findings will open an avenue towards exploring room-temperature nonlinear devices in new quantum materials.

A Polarization-Sensitive Self-Powered Photodetector Based on a p-WSe2/TaIrTe4/n-MoS2 van der Waals Heterojunction

Polarization-sensitive photodetection is highly appealing considering its great important applications. However, the inherent in-plane symmetry of a material and the single structure of a detector hinder the further development of polarization detectors with high anisotropic ratios. Herein, we design a p-WSe₂/TaIrTe₄/n-MoS₂ (p-Ta-n) heterojunction. As a type-II Weyl semimetal, TaIrTe₄ with an orthorhombic structure has strong in-plane asymmetry, which is confirmed by angle-resolved polarized Raman spectroscopy and second-harmonic generation. Due to the specific structure of the p-Ta-n junction with two vertical built-in electric fields, the device obtains a broadband self-powered photodetection ranging from visible (405 nm) to telecommunication wavelength (1550 nm) regions. Further, an optimized device containing 50-70 nm-thick layered TaIrTe₄ has been realized. What is more, high-resolution imaging of "T" based on the device with clear borders illustrates excellent stability of the device. Significantly, the photocurrent anisotropic ratio of the p-Ta-n detector can reach 9.1 under 635 nm light, which is more than eight times that of the best known TaIrTe₄-based photodetector reported before. This p-Ta-n junction containing a type-II Weyl fermion semimetal can provide an effective approach toward highly polarization-sensitive and high-performance integrated broadband photodetectors.

Layered van der Waals Topological Metals of TaTMTe4 (TM = Ir, Rh, Ru) Family

Layered van der Waals materials of the family TaTMTe₄ (TM = Ir, Rh, Ru) are showing interesting electronic properties. We report the growth and characterization of TaIrTe₄, TaRhTe₄, TaIr_1-xRh_xTe₄ (x = 0.06, 0.14, 0.78, 0.92), Ta_1+xRu_1-xTe₄ single crystals. X-ray powder diffraction confirms that TaRhTe₄ is isostructural to TaIrTe₄. All these compounds are metallic with diamagnetic behavior. Below T ≈ 4 K we observed signatures of the superconductivity in the TaIr_1-xRh_xTe₄ compounds for x = 0.92. All samples show weak quadratic-in-field magnetoresistance (MR). However, for TaIr_1-xRh_xTe₄ with x ≈ 0.78, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe₄ series the MR is almost isotropic. Electronic structure calculations for TaIrTe₄ and TaRhTe₄ reveal appearance of the Rh band close to the Fermi level.

Superconductivity in quasi-2D InTaX2(X = S, Se) type-II Weyl semimetals

Herein, first-principles calculations were employed to study the electronic, topological, and superconducting properties of InTaX₂(X = S, Se). InTaX₂exhibits nodal lines in the absence of spin-orbit coupling (SOC); on SOC inclusion, the nodal lines form Weyl rings with the Weyl points classified as a type-II Weyl semimetal (WSM) with tilted cones. Using Green functions method calculations, surface states distinguishable from the bulk states, and Fermi arcs surface states were visualized on the (001) easily cleavable indium terminated surface of both materials. The electron-phonon calculations using the Allen-Dynes relations predict InTaSe₂and InTaS₂to be superconducting around 2.38 K and 3.25 K. The prediction of these exotic properties in InTaX₂(X = S, Se) makes them suitable for experimental validation of topological superconductivity in type-II WSMs.

Anisotropic gapping of topological Weyl rings in the charge-density-wave superconductor InxTaSe2

Topological materials and topological phases have recently become a hot topic in condensed matter physics. In this work, we report an In-intercalated transition-metal dichalcogenide In_xTaSe₂ (named 112 system), a topological nodal-line semimetal in the presence of both charge density wave (CDW) and superconductivity. In the x = 0.58 sample, the 2×3 commensurate CDW (CCDW) and the 2×2 CCDW are observed below 116 and 77 K, respectively. Consistent with theoretical calculations, the spin-orbital coupling gives rise to two twofold-degenerate nodal rings (Weyl rings) connected by drumhead surface states, confirmed by angle-resolved photoemission spectroscopy. Our results suggest that the 2×2 CCDW ordering gaps out one Weyl ring in accordance with the CDW band folding, while the other Weyl ring remains gapless with intact surface states. In addition, superconductivity emerges at 0.91 K, with the upper critical field deviating from the s-wave behavior at low temperature, implying possibly unconventional superconductivity. Therefore, we think this type of the 112 system may possess abundant physical states and offer a platform to investigate the interplay between CDW, nontrivial band topology and superconductivity.