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Hu, Chaowei

Publications and source records attributed to Hu, Chaowei.

29 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Fragile 3D Order in V 1 - x Mo x O 2

The metal-to-insulator transition in rutile VO2 has proven uniquely difficult to characterize because of the complex interplay between electron correlations and atomic structure. Here, we report the discovery of the sudden collapse of three-dimensional order in the low-temperature phase of V1-xMoxO2 at x = 0.17 and the emergence of a novel frustrated two-dimensional order at x = 0.19, with only a slight change in electronic properties. Single crystal diffuse x-ray scattering reveals that this transition from the 3D M1 phase to a 2D variant of the M2 phase results in long-range structural correlations along symmetry-equivalent (11L) planes of the tetragonal rutile structure, yet extremely short-range correlations transverse to these planes. These findings suggest that this two dimensionality results from a novel form of geometric frustration that is essentially structural in origin.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Tuning magnetism and band topology through antisite defects in Sb-doped MnBi 4 ⁢Te 7

The fine control of magnetism and electronic structure in a magnetic topological insulator is crucial in order to realize various novel magnetic topological states including axion insulators, magnetic Weyl semimetals, Chern insulators, etc. Through crystal growth, transport, thermodynamic, neutron diffraction measurements, we show that under Sb doping the newly discovered intrinsic antiferromagnetic (AFM) topological insulator MnBi 4 Te 7 evolves from AFM to ferromagnetic (FM) and then ferrimagnetic. We attribute this to the formation of Mn(Bi,Sb) antisites upon doping, which results in additional Mn sublattices that modify the delicate interlayer magnetic interactions and cause the dominant Mn sublattice to go from AFM to FM. We further investigate the effect of antisites on the band topology using the first-principles calculations. Without considering antisites, the series evolves from AFM topological insulator (x=0) to FM axion insulators. In the exaggerated case of 16.7% of periodic antisites, the band topology is modified and a type-I magnetic Weyl semimetal phase can be realized at intermediate dopings. Therefore, this doping series provides a fruitful platform with continuously tunable magnetism and topology for investigating emergent phenomena, including quantum anomalous Hall effect, Fermi arc states, etc.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Layer Hall effect in a 2D topological axion antiferromagnet

While ferromagnets have been known and exploited for millennia, antiferromagnets (AFMs) were only discovered in the 1930s. The elusive nature indicates AFMs' unique properties: At large scale, due to the absence of global magnetization, AFMs may appear to behave like any non-magnetic material; However, such a seemingly mundane macroscopic magnetic property is highly nontrivial at microscopic level, where opposite spin alignment within the AFM unit cell forms a rich internal structure. In topological AFMs, such an internal structure leads to a new possibility, where topology and Berry phase can acquire distinct spatial textures. Here, we study this exciting possibility in an AFM Axion insulator, even-layered MnBi 2 Te 4 flakes, where spatial degrees of freedom correspond to different layers. Remarkably, we report the observation of a new type of Hall effect, the layer Hall effect, where electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under no net electric field, even-layered MnBi 2 Te 4 shows no anomalous Hall effect (AHE); However, applying an electric field isolates the response from one layer and leads to the surprising emergence of a large layer-polarized AHE (~50e 2 /h). Such a layer Hall effect uncovers a highly rare layer-locked Berry curvature, which serves as a unique character of the space-time PT-symmetric AFM topological insulator state. Moreover, we found that the layer-locked Berry curvature can be manipulated by the Axion field, E∙B, which drives the system between the opposite AFM states. Our results achieve previously unavailable pathways to detect and manipulate the rich internal spatial structure of fully-compensated topological AFMs. The layer-locked Berry curvature represents a first step towards spatial engineering of Berry phase, such as through layer-specific moiré potential.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Comprehensive anisotropic linear optical properties of the Weyl semimetals TaAs and NbAs

We report that TaAs and NbAs are two of the earliest identified Weyl semimetals that possess many intriguing optical properties, such as chirality-dependent optical excitations and giant second harmonic generation (SHG). Linear and nonlinear optics have been employed as tools to probe the Weyl physics in these crystals. Here, we extend these studies to address two important aspects: determining the complete anisotropic dielectric response and exploring if and how they can reveal essential Weyl physics. We determine the complete anisotropic dielectric functions of TaAs and NbAs by combining spectroscopic ellipsometry and density functional theory (DFT). Parameterized Lorentz oscillators are reported from 1.2 - 6 eV (experiment) and 0 - 6 eV (DFT), and good agreement is shown between them. Both linear and nonlinear optical properties have been previously reported to reveal Weyl physics. We suggest that strong optical resonances from trivial bands are the likely origin of the large optical SHG previously reported at these energies. Furthermore, by comparing the contribution of a small k -space centered around the Weyl cones with the total linear dielectric function, we find that these contributions are highly anisotropic and are < 25% of the total dielectric function below 0.5 eV; abov1 eV, these contributions are negligible. Thus, the study of Weyl physics using optical techniques requires very low energies, and even there, a careful assessment is required in distinguishing the much smaller contributions of the Weyl bands from the dominant contributions of the trivial bands and Drude response to the total dielectric function.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Neutron diffraction study of magnetism in van der Waals layered MnBi 2n Te 3n+1

Two-dimensional van der Waals MnBi 2 n Te 3 n +1 ( n = 1, 2, 3, 4) compounds have been recently found to be intrinsic magnetic topological insulators rendering quantum anomalous Hall effect and diverse topological states. Here, we summarize and compare the crystal and magnetic structures of this family, and discuss the effects of chemical composition on their magnetism. We found that a considerable fraction of Bi occupies at the Mn sites in MnBi 2 n Te 3 n +1 ( n = 1, 2, 3, 4) while there is no detectable Mn at the non-magnetic atomic sites within the resolution of neutron diffraction experiments. The occupancy of Mn monotonically decreases with the increase of n . The polarized neutron diffraction on the representative MnBi 4 Te 7 reveals that its magnetization density is exclusively accumulated at the Mn site, in good agreement with the results from the unpolarized neutron diffraction. The defects of Bi at the Mn site naturally explain the continuously reduced saturated magnetic moments from n = 1 to n = 4. The experimentally estimated critical exponents of all the compounds generally suggest a three-dimensional character of magnetism. Our work provides material-specified structural parameters that may be useful for band structure calculations to understand the observed topological surface states and for designing quantum magnetic materials through chemical doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Realization of an intrinsic ferromagnetic topological state in MnBi 8 Te 13

Novel magnetic topological materials pave the way for studying the interplay between band topology and magnetism. However, an intrinsically ferromagnetic topological material with only topological bands at the charge neutrality energy has so far remained elusive. Using rational design, we synthesized MnBi 8 Te 13 , a natural heterostructure with [MnBi 2 Te 4 ] and [Bi 2 Te 3 ] layers. Thermodynamic, transport, and neutron diffraction measurements show that despite the adjacent [MnBi 2 Te 4 ] being 44.1 Å apart, MnBi 8 Te 13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. First-principles calculations and angle-resolved photoemission spectroscopy measurements reveal it is an axion insulator with sizable surface hybridization gaps. Our calculations further demonstrate the hybridization gap persists in the two-dimensional limit with a nontrivial Chern number. Therefore, as an intrinsic ferromagnetic axion insulator with clean low-energy band structures, MnBi 8 Te 13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized magnetoelectric effect.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Néel-type antiferromagnetic order and magnetic field–temperature phase diagram in the spin-1/2 rare-earth honeycomb compound YbCl 3

Most of the searches for Kitaev materials deal with 4 d / 5 d magnets with spin-orbit-coupled J = 1 / 2 local moments such as iridates and α - RuCl 3 . Here we propose the monoclinic YbCl 3 with a Yb 3 + honeycomb lattice for the exploration of Kitaev physics. We perform thermodynamic, a c susceptibility, angle-dependent magnetic torque, and neutron diffraction measurements on YbCl 3 single crystal. We find that the Yb 3 + ion exhibits a Kramers doublet ground state that gives rise to an effective spin J eff = 1 / 2 local moment. Additionally, the compound exhibits short-range magnetic order below 1.20 K, followed by a long-range Néel-type antiferromagnetic order at 0.60 K, below which the ordered Yb 3 + spins lie in the a c plane with an angle of 16(11) ° away from the a axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the Néel temperature varies nonmonotonically under the out-of-plane magnetic fields, suggesting a reduced spin dimensionality. Finally, together with the strong in-plane magnetic anisotropy and the reduced order moment 0.8(1) μ B at 0.25 K, all indicate that YbCl 3 could be a two-dimensional spin system to proximate the Kitaev physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Structural distortion and incommensurate noncollinear magnetism in EuAg 4 As 2

Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. In this work, we report the observation of structural distortion and noncollinear magnetism in layered pnictide EuAg 4 As 2 via transport, magnetization, single crystal x-ray, and neutron diffraction data.EuAg 4 As 2 single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of q 1 =±(0, 0.25, 0.5) and q 2 =±(0.25, 0, 1) emerge. Between 9–15 K, the hexagonal Eu 2+ sublattice enters an unpinned incommensurate magnetic state, with magnetic Bragg reflections pictured as circular sectors. Furthermore, below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12) and a very rare magnetic structure, which is helical along the c axis and cycloidal along the b axis with a moment of 6.4 μB/Eu 2+ . Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in EuAg 4 As 2 are revealed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Pressure tuning of structural and magnetic transitions in EuAg 4 As 2

We report temperature-dependent measurements of ambient-pressure specific heat, magnetic susceptibility, anisotropic resistivity, and thermal expansion as well as in-plane resistivity under pressure up to 20.8 kbar on single crystals of EuAg 4 As 2 . Based on thermal expansion and in-plane electrical transport measurements at ambient pressure this compound has two, first-order, structural transitions in the 80–120 K temperature range. Ambient-pressure specific heat, magnetization, and thermal expansion measurements show a cascade of up to seven transitions between 8 and 16 K associated with the ordering of the $Eu^{2+}$ moments. In-plane electrical transport is able to detect the more prominent of these transitions, at 15.5, 9.9, and 8.7 K, as well as a weak feature at 11.8 K at ambient pressure. Pressure-dependent electrical transport data show that the magnetic transitions shift to higher temperatures under pressure, as does the upper structural transition, whereas the lower structural transition is suppressed and ultimately vanishes. A jump in resistivity, associated with the upper structural transition, decreases under pressure with an extrapolated disappearance (or a change of sign) by 30–35 kbar. In the 10–15 kbar range a kink in the pressure dependency of the upper structural transition temperature as well as the high- and low-temperature in-plane resistivities suggests that a change in the electronic structure may occur in this pressure range. The results are compared with the literature data for EuAg 4 As 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Crystal and magnetic structures of magnetic topological insulators MnBi 2 Te 4 and MnBi 4 Te 7

Using single-crystal neutron diffraction, we present a systematic investigation of the crystal structure and magnetism of the van der Waals topological insulators MnBi 2 Te 4 and MnBi 4 Te 7 , where rich topological quantum states have been recently predicted and observed. Structural refinements reveal that considerable Bi atoms occupied on the Mn sites in both materials, distinct from the previously reported antisite disorder. We show unambiguously that MnBi 2 Te 4 orders antiferromagnetically below 24 K, featured by a magnetic symmetry R I - 3 c , while MnBi 4 Te 7 is antiferromagnetic below 13 K with a magnetic space group P c - 3 c 1 . They both present antiferromagnetically coupled ferromagnetic layers with spins along the c axis. We put forward a stacking rule for the crystal structure of an infinitely adaptive series MnBi 2 n Te 3 n + 1 ( n ≥ 1 ) with a building unit of [ Bi 2 Te 3 ] . Overall, by comparing the magnetic properties between MnBi 2 Te 4 and MnBi 4 Te 7 , together with recent density-functional theory calculations, we concluded that a two-dimensional magnetism limit might be realized in the derivatives. Our work may promote theoretical studies of topological magnetic states in the series of MnBi 2 n Te 3 n + 1 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗