arXiv subjects

Chaowei Hu

Publications and source records attributed to Chaowei Hu.

53 records · Page 3Linked to original sources

arXiv2021

Tuning the magnetism and band topology through antisite defects in Sb doped MnBi4Te7

The fine control of magnetism and electronic structure is crucial since the interplay between magnetism and band topology can lead to various novel magnetic topological states including axion insulators, magnetic Weyl semimetals and Chern insulators etc. Through crystal growth, transport, thermodynamic, neutron diffraction measurements, we show that with Sb-doping, the newly-discovered intrinsic antiferromagnetic topological insulator MnBi4Te7 evolves from antiferro-magnetic to ferromagnetic and then ferrimagnetic. We attribute this to the formation of Mn(Bi,Sb) antisites upon doping, which result in additional Mn sublattices that modify the delicate interlayer magnetic interactions and cause the dominant Mn sublattice to go from antiferromagnetic to ferro-magnetic. We further investigate the effect of antisites on the band topology using the first-principles calculations. Without considering antisites, the series evolves from antiferromagnetic topological insulator (x = 0) to ferromagnetic axion insulators. In the exaggerated case of 16.7% of periodic antisites, the band topology is modified and 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.

cond-mat.str-el↗

arXiv2021

Phonon modes and Raman signatures of MnBi2nTe3n+1 (n=1,2,3,4) magnetic topological heterostructures

An intrinsic antiferromagnetic topological insulator $\mathrm{MnBi_2Te_4}$ can be realized by intercalating Mn-Te bilayer chain in a topological insulator, $\mathrm{Bi_2Te_3}$. $\mathrm{MnBi_2Te_4}$ provides not only a stable platform to demonstrate exotic physical phenomena, but also easy tunability of the physical properties. For example, inserting more $\mathrm{Bi_2Te_3}$ layers in between two adjacent $\mathrm{MnBi_2Te_4}$ weakens the interlayer magnetic interactions between the $\mathrm{MnBi_2Te_4}$ layers. Here we present the first observations on the inter- and intra-layer phonon modes of $\mathrm{MnBi_{2n}Te_{3n+1}}$ (n=1,2,3,4) using cryogenic low-frequency Raman spectroscopy. We experimentally and theoretically distinguish the Raman vibrational modes using various polarization configurations. The two peaks at 66 cm$^{-1}$ and 112 cm$^{-1}$ show an abnormal perturbation in the Raman linewidths below the magnetic transition temperature due to spin-phonon coupling. In $\mathrm{MnBi_4Te_7}$, the $\mathrm{Bi_2Te_3}$ layers induce Davydov splitting of the A$_{1g}$ mode around 137 cm$^{-1}$ at 5 K. Using the linear chain model, we estimate the out-of-plane interlayer force constant to be $(3.98 \pm 0.14) \times 10^{19}$ N/m$^3$ at 5 K, three times weaker than that of $\mathrm{Bi_2Te_3}$. Our work discovers the dynamics of phonon modes of the $\mathrm{MnBi_2Te_4}$ and the effect of the additional $\mathrm{Bi_2Te_3}$ layers, providing the first-principles guidance to tailor the physical properties of layered heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

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 (~50%$\frac{e^2}{h}$). Such a layer Hall effect uncovers a highly rare layer-locked Berry curvature, which serves as a unique character of the space-time $\mathcal{PT}$-symmetric AFM topological insulator state. Moreover, we found that the layer-locked Berry curvature can be manipulated by the Axion field, E$\cdot$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.

cond-mat.mtrl-sci↗

arXiv2021

Revealing the competition between charge-density wave and superconductivity in CsV$_3$Sb$_5$ through uniaxial strain

In this paper we report the impact of uniaxial strain $\varepsilon$ applied along the crystalline $a$ axis on the newly discovered kagome superconductor CsV$_3$Sb$_5$. At ambient conditions, CsV$_3$Sb$_5$ shows a charge-density wave (CDW) transition at $T_{\rm CDW}=94.5$ K and superconducts below $T_C = 3.34$ K. In our study, when the uniaxial strain $\varepsilon$ is varied from $-0.90\%$ to $0.90\%$, $T_C$ monotonically increases by $\sim 33\%$ from 3.0 K to 4.0 K, giving rise to the empirical relation $T_C (\varepsilon)=3.4+0.56\varepsilon+0.12\varepsilon^2$. On the other hand, for $\varepsilon$ changing from $-0.76\%$ to $1.26\%$, $T_{\rm CDW}$ decreases monotonically by $\sim 10\%$ from 97.5 K to 87.5 K with $T_{\rm CDW}(\varepsilon)=94.5-4.72\varepsilon-0.60\varepsilon^2$. The opposite response of $T_C$ and $T_{\rm CDW}$ to the uniaxial strain suggests strong competition between these two orders. Comparison with hydrostatic pressure measurements indicate that it is the change in the $c$-axis that is responsible for these behaviors of the CDW and superconducting transitions, and that the explicit breaking of the sixfold rotational symmetry by strain has a negligible effect. Combined with our first-principles calculations and phenomenological analysis, we conclude that the enhancement in $T_C$ with decreasing $c$ is caused primarily by the suppression of $T_{\rm CDW}$, rather than strain-induced modifications in the bare superconducting parameters. We propose that the sensitivity of $T_{\rm CDW}$ with respect to the changes in the $c$-axis arises from the impact of the latter on the trilinear coupling between the $M_1^+$ and $L_2^-$ phonon modes associated with the CDW. Overall, our work reveals that the $c$-axis lattice parameter, which can be controlled by both pressure and uniaxial strain, is a powerful tuning knob for the phase diagram of CsV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2021

Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi8Te13 and Sb-doped MnBi2nTe3n+1 (n=2, 3)

MnBi2nTe3n+1(MBT) is the first intrinsic magnetic topological insulator and is promising to hostemergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n >= 4 or with Sb doping. We studied the magnetic dynamics in a few selected ferromag-netic (FM) MBT compounds, including MnBi8Te13and Sb doped MnBi2nTe3n+1(n= 2,3) usingAC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all threecompounds, suggesting its universality among FM MBT. We attribute the origin of the relaxationbehavior to the irreversible domain movements since they only appear below the saturation fieldswhen ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by thelow-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imagedvia our magneto-optical measurements. Finally, we ascribe the rare "double-peak" behavior ob-served in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domainformations.

cond-mat.mtrl-sci↗

arXiv2021

Comprehensive Anisotropic Linear Optical Properties of Weyl Semimetals, TaAs and NbAs

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 points: determining the complete anisotropic dielectric response, and to explore if and how they can reveal essential Weyl physics. For the first time, 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 reported to reveal Weyl physics. We suggest that strong optical resonances from trivial bands are the likely origin of the large optical second harmonic generation previously reported at these energies. Furthermore, by comparing the contribution of a small k-space centered around the Weyl cones to 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; above 1eV, 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.

cond-mat.mtrl-sci↗

arXiv2021

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

Two-dimensional van der Waals MnBi$_{2n}$Te$_{3n+1}$ (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$_{2n}$Te$_{3n+1}$ (n = 1, 2, 3, 4) while Mn is no detectable 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.

cond-mat.str-el↗

arXiv2020

Structural distortion and incommensurate noncollinear magnetism in EuAg4As2

Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. Here 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 $q_1=\pm$(0, 0.25, 0.5) and $q_2=\pm$(0.25, 0, 1) emerge. Between 9 K to 15 K, the hexagonal Eu$^{2+}$ sub-lattice enters an unpinned state, with magnetic Bragg reflections pictured as circular-sectors. Below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12), where the magnetic structure is helical along the $c$ axis and cycloidal along the $b$ axis with a moment of 6.4 $μ_B$/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.

cond-mat.str-el↗

arXiv2020

A Néel-type antiferromagnetic order in the spin 1/2 rare-earth honeycomb YbCl$_3$

Most of the searches for Kitaev materials deal with $4d/5d$ 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 Kiteav physics. We perform thermodynamic, $ac$ 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 ${J_{\text{eff}}=1/2}$ local moment. 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 $ac$ plane with an angle of 16(11)$^{\circ}$ 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 non-monotonically under the out-of-plane magnetic fields. The in-plane magnetic anisotropy and the reduced order moment 0.8(1) $μ_B$ at 0.25 K indicate that YbCl$_3$ could be a two-dimensional spin system to proximate the Kitaev physics.

cond-mat.str-el↗

arXiv2020

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 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 $R_I$-${3c}$ while MnBi$_{4}$Te$_{7}$ is antiferromagnetic below 13 K with a magnetic space group $P_c$-${3c1}$. 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$_{2n}$Te$_{3n+1}$ (n$\geq$1) with the building unit of [Bi$_2$Te$_3$]. A comparison of magnetic properties between MnBi$_{2}$Te$_{4}$ and MnBi$_{4}$Te$_{7}$, together with the recent density-functional theory calculations, enables us to draw that a two-dimensional magnetism limit might be realized in the derivatives. Our work may promote the theoretical studies of topological magnetic states in the series of MnBi$_{2n}$Te$_{3n+1}$.

cond-mat.str-el↗

arXiv2020

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 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 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 dependence of the upper structural transition temperature as well as the high and low temperature in-plane resistivities suggest that a change in the electronic structure may occur in this pressure range. The results are compared with the literature data for SrAg$_4$As$_2$.

cond-mat.mtrl-sci↗

arXiv2019

Strongly Gapped Topological Surface States on Protected Surfaces of Antiferromagnetic MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$

The search for materials to support the Quantum Anomalous Hall Effect (QAHE) have recently centered on intrinsic magnetic topological insulators (MTIs) including MnBi$_2$Te$_4$ or heterostructures made up of MnBi$_2$Te$_4$ and Bi$_2$Te$_3$. While MnBi$_2$Te$_4$ is itself a MTI, most recent ARPES experiments indicate that the surface states on this material lack the mass gap that is expected from the magnetism-induced time-reversal symmetry breaking (TRSB), with the absence of this mass gap likely due to surface magnetic disorder. Here, utilizing small-spot ARPES scanned across the surfaces of MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$, we show the presence of large mass gaps (~ 100 meV scale) on both of these materials when the MnBi$_2$Te$_4$ surfaces are buried below one layer of Bi$_2$Te$_3$ that apparently protects the magnetic order, but not when the MnBi$_2$Te$_4$ surfaces are exposed at the surface or are buried below two Bi$_2$Te$_3$ layers. This makes both MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$ excellent candidates for supporting the QAHE, especially if bulk devices can be fabricated with a single continuous Bi$_2$Te$_3$ layer at the surface.

cond-mat.str-el↗

arXiv2019

A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling

Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall (QAH) effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~ 0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2019

Gapless surface Dirac cone in antiferromagnetic topological insulator MnBi$_2$Te$_4$

The recent discovered antiferromagnetic topological insulators in Mn-Bi-Te family with intrinsic magnetic ordering have rapidly drawn broad interest since its cleaved surface state is believed to be gapped, hosting the unprecedented axion states with half-integer quantum Hall effect. Here, however, we show unambiguously by using high-resolution angle-resolved photoemission spectroscopy that a gapless Dirac cone at the (0001) surface of MnBi$_2$Te$_4$ exists between the bulk band gap. Such unexpected surface state remains unchanged across the bulk Néel temperature, and is even robust against severe surface degradation, indicating additional topological protection. Through symmetry analysis and $\textit{ab}$-$\textit{initio}$ calculations we consider different types of surface reconstruction of the magnetic moments as possible origins giving rise to such linear dispersion. Our results reveal that the intrinsic magnetic topological insulator hosts a rich platform to realize various topological phases such as topological crystalline insulator and time-reversal-preserved topological insulator, by tuning the magnetic configurations.

cond-mat.mtrl-sci↗

arXiv2019

Superfluid Onset and Compressibility of $^4$He Films Adsorbed on Carbon Nanotubes

Third sound measurements of superfluid $^4$He thin films adsorbed on 10 nm diameter multiwall carbon nanotubes are used to probe the superfluid onset temperature as a function of the film thickness, and to study the temperature dependence of the film compressibility. The nanotubes provide a highly ordered carbon surface, with layer-by-layer growth of the adsorbed film as shown by oscillation peaks in the third sound velocity at the completion of the third, fourth, and fifth atomic layers, arising from oscillations in the compressibility. In temperature sweeps the third sound velocity at very low temperatures is found to be linear with temperature, but oscillating between positive and negative slope depending on the film thickness. Analysis shows that this can be attributed to a linearly decreasing compressibility of the film with temperature that appears to hold even near zero temperature. The superfluid onset temperature is found to be linear in the film thickness, as predicted by the Kosterlitz-Thouless theory, but the slope is anomalous, a factor of three smaller than the predicted universal value.

cond-mat.other↗

arXiv2017

Charge ordering and ferrimagnetism in the strongly correlated $β$-V$_2$PO$_5$ single crystal

A combined study of transport, thermodynamic, neutron diffraction, nuclear magnetic resonance measurements and first principles calculation were performed for $β$-V$_2$PO$_5$ single crystal. It was shown to be a semiconductor with a band gap of 0.48 eV, undergoing a charge ordering (unusual V$^{2+}$ and V$^{3+}$) phase transition accompanied by a tetragonal to monoclinic structural distortion at 610 K and a paramagnetic to ferrimagnetic phase transition at 128 K with a propagation vector of $\textbf{k} = 0$. The easy axis is in the monoclinic $ac$ plane pointing 47(9)$^\circ$ away from the monoclinic $a$ axis. This collinear ferrimagnetic structure and anisotropic isothermal magnetization measurements suggest weak magnetic anisotropy in this compound. The first principles calculations indicate that the intra-chain interactions in the face-sharing VO$_6$ chains dominate the magnetic hamiltonian and identify the $Γ_5^+$ normal mode of the lattice vibration to be responsible for the charge ordering and thus the structural phase transition.

cond-mat.str-el↗

arXiv2017

Magnetic order induces symmetry breaking in the single crystalline orthorhombic CuMnAs semimetal

Recently, orthorhombic CuMnAs has been proposed to be a magnetic material where topological fermions exist around the Fermi level. Here we report the magnetic structure of the orthorhombic Cu0.95MnAs and Cu0.98Mn0.96As single crystals. While Cu0.95MnAs is a commensurate antiferromagnet (C-AFM) below 360 K with a propagation vector of k = 0, Cu0.98Mn0.96As undergoes a second-order paramagnetic to incommensurate antiferromagnetic (IC-AFM) phase transition at 320 K with k = (0.1,0,0), followed by a second-order IC-AFM to C-AFM phase transition at 230 K. In the C-AFM state, the Mn spins order parallel to the b-axis but antiparallel to their nearest-neighbors with the easy axis along the b axis. This magnetic order breaks Ry gliding and S2z rotational symmetries, the two crucial for symmetry analysis, resulting in finite band gaps at the crossing point and the disappearance of the massless topological fermions. However, the spin-polarized surface states and signature induced by non-trivial topology still can be observed in this system, which makes orthorhombic CuMnAs promising in antiferromagnetic spintronics.

cond-mat.mtrl-sci↗