arXiv subjects

Xiangang Wan

Publications and source records attributed to Xiangang Wan.

At least 37 records · Page 2Linked to original sources

arXiv2024

Topological state evolution by symmetry-breaking

Previous symmetry-based database searches have already revealed ubiquitous band topology in nature, while the destiny of band topology under symmetry-breaking is yet to be studied comprehensively. Here we first develop a framework allowing systematically ascertaining topological state evolution as expressed via a tree-like graph for magnetic/non-magnetic crystalline material belonging to any of the 1651 magnetic space groups. Interestingly, we find that specifying different ways of realizing symmetry-breaking leads to various contractions of the tree-like graph, as a new angle of comprehensively characterizing the correlation between a spontaneous symmetry-breaking and any symmetry-group-indicated physics consequence. We also perform a high-throughput investigation on the 1267 stoichiometric magnetic materials ever-experimentally synthesized to reveal a hierarchy of topological states along all continuous paths of symmetry-breaking (preserving the translation symmetry) from the parent magnetic space group to P1. The results in this work are expected to aid experimentalists in selecting feasible and appropriate means to tune band topology towards realistic applications and promote further studies on using tree-like graph to explore the interconnection between topology and other intriguing orderings.

cond-mat.mtrl-sci↗

arXiv2024

Realization of Hopf-link structure in phonon spectra: Symmetry guidance and High-throughput investigation

The realization of Hopf-link structure in the Brillouin zone is rather rare hindering the comprehensive exploration and understanding of such exotic nodal loop geometry. Here we first tabulate 141 space groups hosting Hopf-link structure and then investigate Phonon Database at Kyoto University consisting of 10034 materials to search for phonon realization of the Hopf-link nodal structure. It is found that almost all the investigated materials own nodal loops or nodal chains while only 113 materials can host Hopf-link structure in phonon spectra, among which 8 representative materials are manually selected to showcase relatively clean Hopf-link structure including LiGaS$_2$, LiInSe$_2$, Ca$_2$Al$_2$Si(HO$_4$)$_2$, Ca$_7$GeN$_6$, Al(HO)$_3$, NaNd(GaS$_2$)$_4$, Ga$_5$(PS)$_3$ and RbTh$_3$F$_{13}$. The visible phonon drumhead surface states corresponding to the nodal loops in the Hopf-link structure are further demonstrated using Ga$_5$(PS)$_3$ as an example.The listed 113 crystalline materials provide a good platform for experimentalists to further explore the interesting properties related to Hopf-link structure.

cond-mat.mtrl-sci↗

arXiv2024

ToMSGKpoint: A user-friendly package for computing symmetry transformation properties of electronic eigenstates of nonmagnetic and magnetic crystalline materials

The calculation of (co)irreducible representations of energy bands at high-symmetry points (HSPs) is essential for high-throughput research on topological materials based on symmetry-indicators or topological quantum chemistry. However, existing computational packages usually require transforming crystal structures into specific conventions, thus hindering extensive application, especially to materials whose symmetries are yet to be identified. To address this issue, we developed a Mathematica package, \texttt{ToMSGKpoint}, capable of determining the little groups and (co)irreducible representations of little groups of HSPs, high-symmetry lines (HSLs), and high-symmetry planes (HSPLs) for any nonmagnetic and magnetic crystalline materials in two and three dimensions, with or without considering spin-orbit coupling. To the best of our knowledge, this is the first package to achieve such functionality. The package also provides magnetic space group operations, supports the analysis of (co)irreducible representations of energy bands at HSPs, HSLs, and HSPLs using electronic wavefunctions obtained from \textit{ab initio} calculations interfaced with VASP. Designed for user convenience, the package generates results in a few simple steps and presents all relevant information in clear tabular format. Its versatility is demonstrated through applications to nonmagnetic topological insulator Bi$_2$Se$_3$ and Dirac semimetal Na$_3$Bi, as well as the antiferromagnetic topological material MnBi$_2$Te$_4$. Suitable for any crystal structure, this package can be conveniently applied in a streamlined study once magnetic space group varies with various symmetry-breakings caused by phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

Catalog of phonon emergent particles

The outcome of conventional topological materials prediction scheme could sensitively depend on first-principles calculations parameters. Symmetry, as a powerful tool, has been exploited to enhance the reliability of predictions. Here, we establish the relationship between the Wyckoff positions (WYPOs) and the phonon wavefunctions at each high-symmetry point (HSP) in all 230 space groups (SGs). Based on this, on one hand, we obtain a complete mapping from WYPO to the occurrence of emergent particles (EMPs) at each HSP in 230 SGs, and establish several rules of enforcing EMPs for phonons; on the other hand, we determine the contribution of the WYPO to the phonon angular momentum. Then we unambiguously identify 20,516,167 phonon EMPs in 111,872 materials in two databases. The purely symmetry-determined wavefunctions generalize the conventional Bloch theorem, could find a wide scope of application to physical properties related with basis functions of irreducible representations.

cond-mat.mtrl-sci↗

arXiv2024

The influence of magnon renormalization and interband coupling on the spin Seebeck effect in YIG

With exceptionally low magnetic damping, YIG has been extensively applied in the realm of magnetism, encompassing the researches into the spin Seebeck effect. YIG has 20 magnon bands, including 8 higher-energy bands denoted as $α_{1\sim8}$, and 12 lower-energy bands denoted as $β_{1\sim12}$. Here, we study the impact of the complex intraband and interband magnon couplings on the transport coefficients of YIG. Four-magnon processes in YIG are considered, and a self-consistent mean-field approximation is made for these interaction terms. We find that the $β$ bands exhibit minimal variation with increasing temperature, whereas the $α$ bands undergo a noticeable decline as the temperature rises. These counterintuitive results agree well with the observation of earlier inelastic neutron scattering experiments and the results of the theoretical calculations in recent years. We also find it sufficient to include only the contribution of magnons on the acoustic band $β_1$ when studying the spin conductivity ($σ_m$). However, when calculating the spin Seebeck coefficient ($S_m$) and the magnon thermal conductivity ($κ_m$), the results calculated using only $β_{1}$ show noticeable deviations over a large temperature range compared to the full band calculations. These deviations are well mitigated when the $β_{2}$ and $β_{3}$ bands are considered. Finally, we find our results satisfy the Wiedemann-Franz law for magnon transport.

cond-mat.mtrl-sci↗

arXiv2024

Symmetry-driven anisotropic coupling effect in antiferromagnetic topological insulator: Mechanism for high-Chern-number quantum anomalous Hall state

Antiferromagnetic (AFM) topological insulators (TIs), which host magnetically gapped Dirac-cone surface states and exhibit many exotic physical phenomena, have attracted great attention. Here, we find that the coupled surface states can be intertwined to give birth to a set of $2n$ unique new Dirac cones, dubbed intertwined Dirac cones, through the anisotropic coupling enforced by crystalline $n$-fold ($n=2, 3, 4, 6$) rotation symmetry $C_{nz}$ in the presence of a $PT$-symmetry breaking potential, for example, an electric field. Interestingly, we also find that the warping effect further drives the intertwined Dirac-cone state into a quantum anomalous Hall phase with a high Chern number ($C=n$). Then, based on first-principles calculations, we have explicitly demonstrated six intertwined Dirac cones and a Chern insulating phase with a high Chern number ($C=3$) in MnBi$_2$Te$_4$$/$(Bi$_2$Te$_3$)$_{\mathrm{m}}/$MnBi$_2$Te$_4$ heterostructures, as well as the $C=2$ and $C=4$ phases in HgS and $α$-Ag$_2$Te films, respectively. This work discovers the intertwined Dirac-cone state in AFM TI thin films, which reveals a mechanism for designing the quantum anomalous Hall state with a high Chern number and also paves a way for studying highly tunable high-Chen-number flat bands of twistronics.

cond-mat.mes-hall↗

arXiv2024

Evidence for Unfolded Fermi Surfaces in the Charge-Density-Wave State of Kagome Metal FeGe Revealed by de Haas-van Alphen Effect

The antiferromagnetic kagome lattice compound FeGe has been revealed to host an emergent charge-density-wave (CDW) state which manifests complex interplay between the spin, charge and lattice degrees of freedom. Here, we present a comprehensive study of the de Haas-van Alphen effect by measuring torque magnetometry under magnetic fields up to 45.2 T to map Fermi surfaces in this unusual CDW state. For field along the $c$ direction, we resolve four cyclotron orbits; the largest one roughly corresponding to the area of the 2$\times$2 folded Brillouin zone. Three smaller orbits are characterized by light effective cyclotron masses range from (0.18-0.30) $m_e$. Angle-resolved measurements identify one Fermi surface segment with weak anisotropy. Combined with band structure calculations, our results suggest that features of unfolded Fermi surfaces are robust against CDW reconstruction, corroborating the novel effect of a short-ranged CDW on the electronic structure.

cond-mat.str-el↗

arXiv2023

Absence of backscattering in Fermi-arc-mediated conductivity of topological Dirac semimetal Cd$_{3}$As$_{2}$

Having previously been the subject of decades of semiconductor research, cadmium arsenide has now reemerged as a topological material, realizing ideal three-dimensional Dirac points at the Fermi level. These topological Dirac points lead to a number of extraordinary transport phenomena, including strong quantum oscillations, large magnetoresistance, ultrahigh mobilities, and Fermi velocities exceeding graphene. The large mobilities persist even in thin films and nanowires of cadmium arsenide, suggesting the involvement of topological surface states. However, computational studies of the surface states in this material are lacking, in part due to the large 80-atom unit cell. Here we present the computed Fermi arc surface states of a cadmium arsenide thin film, based on a tight-binding model derived directly from the electronic structure. We show that despite the close proximity of the Dirac points, the Fermi arcs are very long and straight, extending through nearly the entire Brillouin zone. The shape and spin properties of the Fermi arcs suppress both back- and side- scattering at the surface, which we show by explicit integrals over the phase space. The introduction of a small symmetry-breaking term, expected in a strong electric field, gaps the electronic structure, creating a weak topological insulator phase that exhibits similar transport properties. Crucially, the mechanisms suppressing scattering in this material differ from those in other topological materials such as Weyl semimetals and topological insulators, suggesting a new route for engineering high-mobility devices based on Dirac semimetal surface states.

cond-mat.str-el↗

arXiv2023

Magnon-magnon interaction in monolayer MnBi$_2$Te$_4$

MnBi$_2$Te$_4$, the first confirmed intrinsic antiferromagnetic topological insulator, has garnered increasing attention in recent years. Here we investigate the energy correction and lifetime of magnons in MnBi$_2$Te$_4$ caused by magnon-magnon interaction. First, a calculation based on the density functional theory was performed to get the parameters of the magnetic Hamiltonian of MnBi$_2$Te$_4$. Subsequently, the perturbation method of many-body Green's function was employed and the first-order self-energy [$Σ^{(1)}(\bm k)$] and second-order self-energy [$Σ^{(2)}(\bm k,\varepsilon_{\bm k})$] of magnon were obtained. Numerical computations reveal that the corrections from both $Σ^{(1)}(\bm k)$ and $Σ^{(2)}(\bm k,\varepsilon_{\bm k})$ strongly rely on momentum and temperature, with the energy renormalization near the Brillouin zone (BZ) boundary being significantly more pronounced than that near the BZ center. Furthermore, our findings indicate the occurrence of dip structures in the renormalized magnon spectrum near the $\rm K$ and $\rm M$ points. These dip structures are determined to be attributed to the influence of $Σ^{(2)}(\bm k,\varepsilon_{\bm k})$.

cond-mat.str-el↗

arXiv2023

Antiferromagnetic $\mathbb{Z}_2$ topological metal near the metal-insulator transition in MnS$_2$

Antiferromagnetic (AFM) semiconductor MnS$_2$ possesses both high-spin and low-spin magnetic phases that can be reversibly switched by applying pressure. With increasing pressure, the high-spin state undergoes pressure-induced metalization before transforming into a low-spin configuration, which is then closely followed by a volume collapse and structural transition. We show that the pressure driven band inversion is in fact topological, resulting in an antiferromagnetic $\mathbb{Z}_2$ topological metal (Z2AFTM) phase with a small gap and a Weyl metal phase at higher pressures, both of which precede the spin-state crossover and volume collapse. In the Z2AFTM phase, the magnetic order results in a doubling of the periodic unit cell, and the resulting folding of the Brillouin zone leads to a $\mathbb{Z}_2$ topological invariant protected by the persisting combined time-reversal and half-translation symmetries. Such a topological phase was proposed theoretically by Mong, Essin, and Moore in 2010 for a system with AFM order on a face-centered cubic (FCC) lattice, which until now has not been found in the pool of real materials. MnS$_2$ represents a realization of this original proposal through AFM order on the Mn FCC sublattice. A rich phase diagram of topological and magnetic phases tunable by pressure, establishes MnS$_2$ as a candidate material for exploring magnetic topological phase transitions and for potential applications in AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Origin of the exotic electronic states in antiferromagnetic NdSb

Using angle resolved photoemission spectroscopy measurements and first principle calculations, we report that the possible unconventional 2q antiferromagnetic (AFM) order in NdSb can induce unusual modulation on its electronic structure. The obvious extra bands observed in the AFM phase of NdSb are well reproduced by theoretical calculations, in which the Fermi-arc-like structures and sharp extra bands are originated from the in-gap surface states. However, they are demonstrated to be topological trivial. By tuning the chemical potential, the AFM phase of NdSb would go through a topological phase transition, realizing a magnetic topological insulator phase. Hence, our study sheds new light on the rare earth monopnictides for searching unusual AFM structure and the potential of intrinsic magnetic topological materials.

cond-mat.mtrl-sci↗

arXiv2023

Novel three-dimensional Fermi surface and electron-correlation-induced charge density wave in FeGe

As the first magnetic kagome material to exhibit the charge density wave (CDW) order, FeGe has attracted much attention in recent studies. Similar to AV$_{3}$Sb$_{5}$ (A = K, Cs, Rb), FeGe exhibits the CDW pattern with an in-plane 2$\times $2 structure and the existence of van Hove singularities (vHSs) near the Fermi level. However, sharply different from AV$_{3}$Sb$_{5}$ which has phonon instability at $M$ point, all the theoretically calculated phonon frequencies in FeGe remain positive. Here, we perform a comprehensive study of the band structures, Fermi surfaces and nesting function of FeGe through first-principles calculations. Surprisingly, we find that the maximum of nesting function is at $K$ point instead of $M$ point. Two Fermi pockets with Fe-$d_{xz}$ and Fe-$d_{x^{2}-y^{2}}$/$d_{xy}$ orbital characters have large contribution to the Fermi nesting, which evolve significantly with $k_{z}$, indicating the highly three-dimensional (3D) feature of FeGe in contrast to AV$_{3}$Sb$_{5}$. Meanwhile, the vHSs are close to the Fermi surface only in a small $k_{z}$ range, and does not play a leading role in nesting function. Considering the effect of local Coulomb interaction, we reveal that the Fermi level eigenstates nested by vector $K$ are mainly distributed from unequal sublattice occupancy, thus the instability at $K$ point is significantly suppressed. Meanwhile, the wave functions nested by vector $M$ have many ingredients located at the same Fe site, thus the instability at $M$ point is enhanced. This indicates that the electron correlation, rather than electron-phonon interaction, plays a key role in the CDW transition at $M$ point.

cond-mat.str-el↗

arXiv2022

First-principles study of spin orbit coupling contribution to anisotropic magnetic interaction

Anisotropic magnetic exchange interactions lead to a surprisingly rich variety of the magnetic properties. Considering the spin orbit coupling (SOC) as perturbation, we extract the general expression of a bilinear spin Hamiltonian, including isotropic exchange interaction, antisymmetric Dzyaloshinskii-Moriya (DM) interaction and symmetric $Γ$ term. Though it is commonly believed that the magnitude of the DM and $Γ$ interaction correspond to the first and second order of SOC strength $% λ$ respectively, we clarify that the term proportional to $λ^{2}$ also has contribution to DM interaction. Based on combining magnetic force theorem and linear-response approach, we have presented the method of calculating anisotropic magnetic interactions, which now has been implemented in the open source software WienJ. Furthermore, we introduce another method which could calculate the first and second order SOC contribution to the DM interaction separately, and overcome some shortcomings of previous methods. Our methods are successfully applied to several typical weak ferromagnets for $3d$, $4d$ and $5d$ transition metal oxides. We also predict the conditions where the DM interactions proportional to $λ$ are symmetrically forbidden while the DM interactions proportional to $λ^{2}$ are nonzero, and believe that it is widespread in certain magnetic materials.

cond-mat.mtrl-sci↗

arXiv2022

Discovery of a metallic oxide with ultralow thermal conductivity

A compound with metallic electrical conductivity usually has a considerable total thermal conductivity because both electrons and photons contribute to thermal transport. Here, we show an exceptional example of iridium oxide, Bi3Ir3O11, that concurrently displays metallic electrical conductivity and ultralow thermal conductivity approaching 0.61 W m-1 K-1 at 300 K. The compound crystallizes into a cubic structural framework with space group Pn-3. The edge- and corner-sharing IrO6 octahedra with a mixed Ir4.33+ charge state favor metallic electrical transport. Bi3Ir3O11 exhibits an extremely low lattice thermal conductivity close to the minimum limit in theory owing to its tunnel-like structure with filled heavy atoms Bi rattling inside. Theoretical calculations reveal the underlying mechanisms for the extraordinary compatibility between metallic electrical conductivity and ultralow thermal conductivity. This study may establish a new avenue for designing and developing unprecedented heat-insulation metals.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic interactions and possible structural distortion in kagome FeGe from first-principles study and symmetry analysis

Based on density functional theory and symmetry analysis, we present a comprehensive investigation of electronic structure, magnetic properties and possible structural distortion of magnetic kagome metal FeGe. We estimate the magnetic parameters including Heisenberg and Dzyaloshinskii-Moriya (DM) interactions, and find that the ferromagnetic nearest-neighbor $J_{1}$ dominates over the others, while the magnetic interactions between nearest kagome layers favors antiferromagnetic. The Néel temperature $T_{N}$ and Curie-Weiss temperature $θ_{CW}$ are successfully reproduced, and the calculated magnetic anisotropy energy is also in consistent with the experiment. However, these reasonable Heisenberg interactions and magnetic anisotropy cannot explain the double cone magnetic transition, and the DM interactions, which even exist in the centrosymmetric materials, can result in this small magnetic cone angle. Unfortunately, due to the crystal symmetry of the high-temperature structure, the net contribution of DM interactions to double cone magnetic structure is absent. Based on the experimental $2\times 2\times 2$ supercell, we thus explore the subgroups of the parent phase. Group theoretical analysis reveals that there are 68 different distortions, and only four of them (space group $P622$ or $P6_{3}22$) without inversion and mirror symmetry thus can explain the low-temperature magnetic structure. Furthermore, we suggest that these four proposed CDW phases can be identified by using Raman spectroscopy. Since DM interactions are very sensitive to small atomic displacements and symmetry restrictions, we believe that symmetry analysis is an effective method to reveal the interplay of delicate structural distortions and complex magnetic configurations.

cond-mat.str-el↗

arXiv2022

Continuous Electrical Manipulation of Magnetic Anisotropy and Spin Flopping in van der Waals Ferromagnetic Devices

Controlling the magnetic anisotropy of ferromagnetic materials plays a key role in magnetic switching devices and spintronic applications. Examples of spin-orbit torque devices with different magnetic anisotropy geometries (in-plane or out-of-plane directions) have been demonstrated with novel magnetization switching mechanisms for extended device functionalities. Normally, the intrinsic magnetic anisotropy in ferromagnetic materials is unchanged within a fixed direction, and thus, it is difficult to realize multifunctionality devices. Therefore, continuous modulation of magnetic anisotropy in ferromagnetic materials is highly desired but remains challenging. Here, we demonstrate a gate-tunable magnetic anisotropy transition from out-of-plane to canted and finally to in-plane in layered Fe$_5$GeTe$_2$ by combining the measurements of the angle-dependent anomalous Hall effect and magneto-optical Kerr effect with quantitative Stoner-Wohlfarth analysis. The magnetic easy axis continuously rotates in a spin-flop pathway by gating or temperature modulation. Such observations offer a new avenue for exploring magnetization switching mechanisms and realizing new spintronic functionalities.

cond-mat.mtrl-sci↗

arXiv2022

Gate-tunable Lifshitz transition of Fermi arcs and its nonlocal transport signatures

One hallmark of the Weyl semimetal is the emergence of Fermi arcs (FAs) in the surface Brillouin zone that connect the projected Weyl nodes of opposite chirality. The unclosed FAs can give rise to various exotic effects that have attracted tremendous research interest. The configurations of the FAs are usually thought to be determined fully by the band topology of the bulk states, which seems impossible to manipulate. Here, we show that the FAs can be simply modified by a surface gate voltage. Because the penetration length of the surface states depends on the in-plane momentum, a surface gate voltage induces an effective energy dispersion. As a result, a continuous deformation of the surface band can be implemented by tuning the surface gate voltage. In particular, as the saddle point of the surface band meets the Fermi energy, the topological Lifshitz transition takes place for the FAs, during which the Weyl nodes switch their partners connected by the FAs. Accordingly, the magnetic Weyl orbits composed of the FAs on opposite surfaces and chiral Landau bands inside the bulk change its configurations. We show that such an effect can be probed by the nonlocal transport measurements in a magnetic field, in which the switch on and off of the nonlocal conductance by the surface gate voltage signals the Lifshitz transition. Our work opens a new route for manipulating the FAs by surface gates and exploring novel transport phenomena associated with the topological Lifshitz transition.

cond-mat.mes-hall↗

arXiv2022

All hourglass bosonic excitations in the 1651 magnetic space groups and 528 magnetic layer groups

The band connectivity as imposed by the compatibility relations between the irreducible representations of little groups can give rise to the exotic hourglass-like shape composed of four branches of bands and five band crossings (BCs). Such an hourglass band connectivity could enforce the emergence of nontrivial excitations like Weyl fermion, Dirac fermion or even beyond them. On the other hand, the bosons, like phonons, magnons, and photons, were also shown to possess nontrivial topology and a comprehensive symmetry classification of the hourglass bosonic excitations would be of great significance to both materials design and device applications. Here we firstly list all concrete positions and representations of little groups in the Brillouin zone (BZ) related with the hourglass bosonic excitations in all the 1651 magnetic space groups and 528 magnetic layer groups, applicable to three dimensional (3D) and two dimensional (2D) systems, respectively. 255 (42) MSGs (MLGs) are found to essentially host such hourglass BCs: Here ``essentially'' means that the bosonic hourglass BC exists definitely as long as the studied system is crystallized in the corresponding MSG/MLG. We also perform first-principles calculations on hundreds of 3D nonmagnetic materials essentially hosting hourglass phonons and propose that the 2D material AlI can host hourglass phonons. We choose AuX (X=Br and I) as illustrative examples to demonstrate that two essential hourglass band structures can coexist in the phonon spectra for both materials while for AuBr, an accidental band crossing sticking two hourglasses is found interestingly. Our results of symmetry conditions for hourglass bosonic excitations can provide a useful guide of designing artificial structures with hourglass bosonic excitations.

cond-mat.mtrl-sci↗