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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

High-pressure control of optical nonlinearity in the polar Weyl semimetal TaAs

The transition metal monopnictide family of Weyl semimetals recently has been shown to exhibit anomalously strong second-order optical nonlinearity, which is theoretically attributed to a highly asymmetric polarization distribution induced by their polar structure. We experimentally test this hypothesis by measuring optical second harmonic generation (SHG) from TaAs across a pressure-tuned polar-to-nonpolar structural phase transition. Despite the high-pressure structure remaining noncentrosymmetric, the SHG yield is reduced by more than 60% by 20 GPa as compared to the ambient pressure value. By examining the pressure dependence of distinct groups of SHG susceptibility tensor elements, we find that the yield is primarily controlled by a single element that governs the response along the polar axis. Furthermore, our results confirm a connection between the polar axis and the giant optical nonlinearity of Weyl semimetals and demonstrate pressure as a means to tune this effect in situ.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Signature of nodal topology in nonlinear quantum transport across junctions in Weyl and multi-Weyl semimetals

We investigate quantum transport through a rectangular potential barrier in Weyl semimetals (WSMs) and multi-Weyl semimetals (MSMs), within the framework of Landauer-Büttiker formalism. Here, our study uncovers the role of nodal topology imprinted in the electric current and the shot noise. We find that, in contrast to the finite odd-order conductance and noise power, the even-order contributions vanish at the nodes. Additionally, depending on the topological charge ($J$), the linear conductance ($G_1$) scales with the Fermi energy ($E_F$) as $G^{E_F>U}_{1}$ $∝$ $E^{2/J}_{F}$, $U$ being the barrier height. We demonstrate that the $E_F$ dependence of the second-order conductance and shot noise power could quite remarkably distinguish an MSM from a WSM depending on the band topology, and may induce several smoking gun experiments in nanostructures made out of WSMs and MSMs. Analyzing shot noise and Fano factor, we show that the transport across the rectangular barrier follows the sub-Poissonian statistics. Interestingly, we obtain universal values of Fano factor at the nodes unique to their topological charges. The universality for a fixed $J$, however, indicates that only a fixed number of open channels participate in the transport through evanescent waves at the nodes. The proposed results can serve as a potential diagnostic tool to identify different topological systems in experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Absence of quantization in the circular photogalvanic effect in disordered chiral Weyl semimetals

The circularly polarized photogalvanic effect (CPGE) is studied in chiral Weyl semimetals with short-range quenched disorder. Without disorder, the topological properties of chiral Weyl semimetals lead to quantization of the CPGE, which is a second-order optical response. Furthermore, using a combination of diagrammatic perturbation theory in the continuum and exact numerical calculations via the kernel polynomial method on a lattice model, we show that disorder perturbatively destabilizes the quantization of the CPGE.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Topological features in the ferromagnetic Weyl semimetal CeAlSi: Role of domain walls

In the ferromagnetic (FM) Weyl semimetal CeAlSi both space-inversion and time-reversal symmetries are broken. Our quantum oscillation (QO) data indicate that the FM ordering modifies the Fermi-surface topology and also leads to an unusual drop in the QO amplitude. In the FM phase, we find a pressure-induced suppression of the anomalous and the loop Hall effects. This cannot be explained based on the electronic band structure or magnetic structure, both of which are nearly pressure independent. Instead, we show that a simplified model describing the scattering of Weyl fermions off FM domain walls can potentially explain the observed topological features. Our study highlights the importance of domain walls for understanding transport in FM Weyl semimetals.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Revealing Fermi surface evolution and Berry curvature in an ideal type-II Weyl semimetal

Abstract In type-II Weyl semimetals (WSMs), the tilting of the Weyl cones leads to the coexistence of electron and hole pockets that touch at the Weyl nodes. These electrons and holes experience the Berry curvature generated by the Weyl nodes, leading to an anomalous Hall effect that is highly sensitive to the Fermi level position. Here we have identified field-induced ferromagnetic MnBi 2-x Sb x Te 4 as an ideal type-II WSM with a single pair of Weyl nodes. By employing a combination of quantum oscillations and high-field Hall measurements, we have resolved the evolution of Fermi-surface sections as the Fermi level is tuned across the charge neutrality point, precisely matching the band structure of an ideal type-II WSM. Furthermore, the anomalous Hall conductivity exhibits a heartbeat-like behavior as the Fermi level is tuned across the Weyl nodes, a feature of type-II WSMs that was long predicted by theory. Our work uncovers a large free carrier contribution to the anomalous Hall effect resulting from the unique interplay between the Fermi surface and diverging Berry curvature in magnetic type-II WSMs.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Realization of a two-dimensional Weyl semimetal and topological Fermi strings

A two-dimensional (2D) Weyl semimetal, akin to a spinful variant of graphene, represents a topological matter characterized by Weyl fermion-like quasiparticles in low dimensions. The spinful linear band structure in two dimensions gives rise to distinctive topological properties, accompanied by the emergence of Fermi string edge states. We report the experimental realization of a 2D Weyl semimetal, bismuthene monolayer grown on SnS(Se) substrates. Using spin and angle-resolved photoemission and scanning tunneling spectroscopies, we directly observe spin-polarized Weyl cones, Weyl nodes, and Fermi strings, providing consistent evidence of their inherent topological characteristics. Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Epitaxial columnar growth of strain-free antiferromagnetic Weyl semimetal Mn 3 Sn on wurtzite c -plane GaN/Al 2 O 3 (0001)

Weyl semimetal thin films with excellent crystalline quality are of great interest for antiferromagnetic spintronics. Mn 3 Sn is one Weyl semimetal with great properties and promise for exciting science and applications. It has proven very challenging, however, to grow Mn 3 Sn thin films with smooth surfaces, negligible strain, and excellent crystallinity. In this work, we discuss the successful preparation of epitaxial Mn 3 Sn (0001)-oriented thin films via molecular beam epitaxial growth on c -plane wurtzite GaN which was grown by MBE on Al 2 O 3 (0001). We present the reflection high energy electron diffraction analysis along with x-ray diffraction in order to demonstrate the crystalline quality of the film, and we give atomic models to explain the epitaxial orientation relationships between the crystal lattices of the substrate, GaN layer, and Mn 3 Sn layer. Importantly, we discuss the film lattice parameters as compared to expected values, demonstrating negligible strain both in-plane and out-of-plane . Atomic force microscopy reveals an epitaxial columnar growth mode characterized by flat-top-mesa islands, while scanning tunneling microscopy shows the atomically smooth surfaces of the mesa-top structures. Finally, Rutherford backscattering informs the stoichiometry of the film as well as the layer thicknesses.

Chiral antiferromagnetic material↗
DOE OSTI2021

Nonlinear Hall effect in Weyl semimetals induced by chiral anomaly

In this work, we predict a nonlinear Hall effect in certain Weyl semimetals with broken inversion symmetry. When the energy dispersions about pairs of Weyl nodes are skewed-the Weyl cones are "tilted"-the concerted actions of the anomalous velocity and the chiral anomaly give rise to the nonlinear Hall effect. This Hall conductivity is linear in both electric and magnetic fields and depends critically on the tilting of the Weyl cones. We also show that this effect does not rely on a finite Berry curvature dipole, in contrast with the intrinsic quantum nonlinear Hall effect that was recently observed in type-II Weyl semimetals.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Visualization of bulk and edge photocurrent flow in anisotropic Weyl semimetals

Materials that rectify light into current in their bulk are desired for optoelectronic applications. In Weyl semimetals that break inversion symmetry, bulk photocurrents may arise due to nonlinear optical processes that are enhanced near the Weyl nodes. However, the photoresponse of these materials is commonly studied by scanning photocurrent microscopy, which convolves the effects of photocurrent generation and collection. Here we directly image the photocurrent flow inside the type-II Weyl semimetals WTe 2 and TaIrTe 4 using high-sensitivity quantum magnetometry with nitrogen-vacancy centre spins. We elucidate a mechanism for bulk photocurrent generation, which we call the anisotropic photothermoelectric effect, where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation. Using overlapping scanning photocurrent microscopy and magnetic imaging at the interior and edges of the sample, we visualize how the anisotropic photothermoelectric effect stimulates the long-range photocurrent collected in our WTe 2 and TaIrTe 4 devices through the Shockley–Ramo mechanism. Furthermore, our results highlight a widely relevant source of current flow and will inspire photodetectors that utilize bulk materials with thermoelectric anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetic and transport properties of Fe-doped Weyl semimetal Co 3 Sn 2 S 2

Ferromagnetic Weyl semimetal Co 3 Sn 2 S 2 has recently attracted a lot of interest on account of its large non-saturating magnetoresistance and the high carrier mobility common to topological semimetals. Moreover, the material also provides a platform for studying the interplay between the intrinsic magnetism and topological band structures, which generates a significant anomalous Hall effect. Chemical doping is the usual approach to adjust materials’ properties, and it can lead to fascinating results. Here, we present the influence of Fe-doping on the magnetic and transport properties of polycrystalline Co 3 Sn 2 S 2 . The Fe-doping suppresses the sample’s magnetic order, increases the electrical resistivity, results in a skew-scattering contribution to the anomalous Hall effect and Fe impurities cause the Kondo effect. In addition, we report on the impact of Fe-doping on the magnetoresistance of Co 3 Sn 2 S 2 , where Fe-doping leads to a dramatic decline in magnetoresistance. Furthermore, the hysteresis behavior in the low magnetic field region, detected previously in nanoflake samples only, is observed in both pure and Fe-doped Co 3 Sn 2 S 2 bulk samples. Finally, our findings serve as a reference and source of inspiration for future chemical doping research on topological semimetals.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Nonlinear optical control of chiral charge pumping in a topological Weyl semimetal

Solids with topologically robust electronic states exhibit unusual electronic and optical properties that do not exist in other materials. A particularly interesting example is chiral charge pumping, the so-called chiral anomaly, in recently discovered topological Weyl semimetals, where simultaneous application of parallel DC electric and magnetic fields creates an imbalance in the number of carriers of opposite topological charge (chirality). Here, using time-resolved terahertz measurements on the Weyl semimetal TaAs in a magnetic field, we optically interrogate the chiral anomaly by dynamically pumping the chiral charges and monitoring their subsequent relaxation of the nonequilibrium state. Theory based on Boltzmann transport shows that the observed effects originate from an optical nonlinearity in the chiral charge pumping process. Our measurements reveal that the nonequilibrium chiral excitation relaxation time is much greater than 1 ns. The observation of terahertz-controlled chiral carriers with long coherence times and topological protection suggests the application of Weyl semimetals for quantum optoelectronic technology.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Strong-coupling anisotropic s-wave superconductivity in the type-II Weyl semimetal TaIrTe 4

TaIrTe 4 is a recently discovered type-II Weyl semimetal, hosting only four Weyl points. In this work, we study the cleaved TaIrTe 4 crystal using scanning tunneling microscopy/spectroscopy and find that it also hosts a superconducting state with a transition temperature of 3.9 K. From Dynes function fitting, the superconducting phase is consistent with anisotropic s-wave pairing, with a superconducting gap of 1.31 meV. This value leads to a value of 2Δ max /k B T C = 7.81 , much larger than the 3.53 predicted by Bardeen-Cooper-Schrieffer theory for weak-coupling superconductors. The critical field is found to be 0.7 T based on the analysis of tunneling conductance as a function of magnetic field. Two types of nonmagnetic defects on the TaIrTe 4 surface are observed, neither of which induce bound states inside the superconducting gap, further supporting conventional s-wave superconductivity in the TaIrTe 4 system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Charge density waves in Weyl semimetals

Here, we present a theory of charge density wave (CDW) states in Weyl semimetals and their interplay with the chiral anomaly. In particular, we demonstrate a special nature of the shortest-period CDW state, which is obtained when the separation between the Weyl nodes equals exactly half a primitive reciprocal lattice vector. Its topological properties are shown to be distinct from all other Weyl CDW states. We make a connection between this observation and the three-dimensional fractional quantum Hall state, which was recently proposed to exist in magnetic Weyl semimetals.

Sehayek, Dan↗
DOE OSTI2024

Precise Fermi level engineering in a topological Weyl semimetal via fast ion implantation

The precise controllability of the Fermi level is a critical aspect of quantum materials. For topological Weyl semimetals, there is a pressing need to fine-tune the Fermi level to the Weyl nodes and unlock exotic electronic and optoelectronic effects associated with the divergent Berry curvature. However, in contrast to two-dimensional materials, where the Fermi level can be controlled through various techniques, the situation for bulk crystals beyond laborious chemical doping poses significant challenges. Here, we report the milli-electron-volt (meV) level ultra-fine-tuning of the Fermi level of bulk topological Weyl semimetal tantalum phosphide using accelerator-based high-energy hydrogen implantation and theory-driven planning. By calculating the desired carrier density and controlling the accelerator profiles, the Fermi level can be experimentally fine-tuned from 5 meV below, to 3.8 meV below, to 3.2 meV above the Weyl nodes. High-resolution transmission electron microscopy reveals the crystalline structure is largely maintained under irradiation, while electrical transport indicates that Weyl nodes are preserved and carrier mobility is also largely retained. Our work demonstrates the viability of this generic approach to tune the Fermi level in semimetal systems and could serve to achieve property fine-tuning for other bulk quantum materials with ultrahigh precision.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Incommensurate Magnetism Drives Singular Angular Magnetoresistance in the Magnetic Weyl Semimetal CeAlGe

Here, we demonstrate that a multi-𝐤 incommensurate magnetic state in the Weyl semimetal CeAlGe gives rise to a singular angular magnetoresistance (SAMR), which is an electrical transport signature capable of detecting magnetic field direction with exceptional precision. In contrast, its sister compound CeAlSi is devoid of both multi-𝐤 order and SAMR. We reveal that both phenomena appear upon 57% Ge substitution in CeAlSi 1−𝑥⁢ Ge 𝑥 and coincide with electronic structure changes that soften the single-ion in-plane anisotropy and enhance Weyl-mediated magnetic interactions. These results unveil a remarkable connection between band topology, electronic transport, and collective magnetism in Weyl semimetals.

Yao, Xiaohan [Boston College, Chestnut Hill, MA (U↗
DOE OSTI2026

Collapsing of spin-split Fermi surfaces in the magnetic Weyl semimetal candidate EuAuSb

EuAuSb is a magnetic Weyl semimetal candidate that exhibits in-plane ferromagnetic ordering and a helical magnetic order across the plane at temperatures below 3.9 K. These magnetic characteristics can drive an interaction-induced reconstruction of the Fermi surface. Here, we report a rare instance of Fermi surface splitting and collapsing driven by exchange coupling, as revealed by strongly temperature-dependent quantum oscillation frequencies in EuAuSb. Shubnikov–de Haas measurements reveal two distinct quantum oscillation frequencies (𝐹 𝛼 = 80 T and 𝐹 𝛽 = 165 T) at 2 K. As the temperature increases toward 40 K, the oscillation peaks at these two frequencies gradually merge into a single peak centered around 128 T. This behavior signifies the collapse of the spin-split Fermi surfaces, driven by the weakening of short-range magnetic correlations and the exchange coupling between conduction electrons and local magnetic moments as temperature increases. Furthermore, our results are further supported by density functional theory calculations, angle-resolved photoemission spectroscopy, and magnetization measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Effect of Pt vacancies on magnetotransport of Weyl semimetal candidate GdPtSb epitaxial films

Here, we examine the effects of Pt vacancies on the magnetotransport properties of Weyl semimetal candidate GdPtSb films, grown by molecular beam epitaxy on c-plane sapphire. Rutherford backscattering spectrometry and x-ray diffraction measurements suggest that phase-pure GdPt x ⁢Sb films can accommodate up to 15% of Pt vacancies (x = 0.85), which act as acceptors, as measured by the Hall effect. Two classes of electrical transport behavior are observed. Pt-deficient films display metallic temperature-dependent resistivity (dρ/dT > 0). The longitudinal magnetoresistance (LMR, magnetic field B parallel to electric field E) is more negative than transverse magnetoresistance (TMR, B⁢⊥⁢E), consistent with the expected chiral anomaly for a Weyl semimetal. The combination of Pt-vacancy disorder and doping away from the expected Weyl nodes, however, suggests that conductivity fluctuations may explain the negative LMR rather than chiral anomaly. Samples closer to stoichiometry display the opposite behavior: semiconductorlike resistivity (dρ/dT > 0) and more negative TMR than LMR. Hysteresis and other nonlinearities in the low-field Hall effect and magnetoresistance suggest that spin-disorder scattering and possible topological Hall effect may dominate the near-stoichiometric samples. Our findings highlight the complications of transport-based identification of Weyl nodes but point to possible topological spin textures in GdPtSb.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Highly Anisotropic Drude-Weight-Reduction and Enhanced Linear-Dichroism in van der Waals Weyl Semimetal T d -MoTe 2 with Coherent Interlayer Electronic Transport

Weyl semimetal (WSM) states can be achieved by breaking spatial-inversion symmetry or time reversal symmetry. However, the anisotropy of the energy reduction contributing to the emergence of WSM states has seldom been investigated by experiments. A van der Waals metal MoTe 2 exhibits a type-II WSM phase below the monoclinic-to-orthorhombic-phase-transition temperature T c ~ 250 K. Here, in this work, a combined linearly polarized optical-spectroscopy and electrical-transport study of MoTe 2 is reported at different temperatures. The Drude components in the a-axis, b-axis, and c-axis optical conductivity spectra, together with the metallic out-of-plane and in-plane electrical resistivities, indicate the coherent inter-layer and in-plane charge transports. Moreover, the Drude weight in σ 1a (ω), rather than the Drude weights in σ 1b (ω) and σ 1c (ω), decreases dramatically below T c , which exhibits a highly anisotropic decrease in its Drude weight and thus suggests a strongly anisotropic reduction of the electronic kinetic energy in the WSM phase. Furthermore, below T c , due to the in-plane anisotropic spectral-weight transfer from Drude component to high-energy region, the in-plane inter-band-absorption anisotropy increases remarkably ≈770 meV, and has the largest value (≈0.68) of normalized linear dichroism among the reported type-II WSMs. The work sheds light on seeking new WSMs and developing novel photonic devices based on WSMs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗