arXiv subjects

Hongming Weng

Publications and source records attributed to Hongming Weng.

At least 109 records · Page 6Linked to original sources

arXiv2022

Quantized and half-quantized Anomalous Hall effect induced by in-plane magnetic field

In this paper we propose that, quantized and nearly half-quantized intrinsic anomalous Hall effect can be induced by in-plane external magnetic field through the Zeeman coupling in non-magnetic 2D systems with sizeable spin-orbital coupling but without two-fold rotational symmetry. An analytical result is derived for 2D electron gas model with $C_{3v}$ symmetry. Based on the $\boldsymbol{k\cdot p}$ Hamiltonian derived from first principle calculations, we find that quantized and nearly half-quantized conductance can be observed in $\mathrm{Sb_2Te_3}$ thin film in the clean limit with strong in-plane magnetic field $B>20\ \mathrm{T}$ and low temperature $T<100\ \mathrm{mK}$.

cond-mat.mes-hall↗

arXiv2022

Switchable Topological Phase Transition and Novel Nonlinear Optical Properties in ReC2H Monolayer

Extensive investigations on topological phase transition (TPT) in three-dimensional compounds have been done. whereas, rare in two-dimensional systems, let alone noncentrosymmetric materials. In this work, based on first-principles calculations, we explore an inversion symmetry broken structural ReC2H monolayer. We reveal that it undergoes two TPTs, namely from normal insulator to Z2 topological insulator and back to normal insulator, at the critical biaxial strain of 2.3% and 7.8%, respectively. The band inversion occurs at the generic momentum in the first TPT, while at high symmetric K point in the second one. Usually, band inversion is identified by the exchange in the components or irreducible representations of the wavefunctions. These quantities can be easily obtained in theoretical calculation but hard to be detected in experimental techniques like Angle-resolved photoemission spectroscopy. It is well known that nonlinear optical (NLO) response is very sensitive to the components and symmetries of the engaged bands, which also incorporates information of band topology. Therefore, we study the shift current, one of the widely explored NLO responses in noncentrosymmetric systems, during the two TPTs. We find that in both cases band inversion leads to the sign change of shift vectors around the momenta where the bandgap closes and reopens. Whereas the shift current, as the overall contribution of shift vectors weighted by the absorption rate in the whole Brillouin zone, may keep its direction. This work offers insight that a scrutinized examination is highly demanded in utilizing shift current to detect TPT.

cond-mat.mtrl-sci↗

arXiv2022

Physical realization of topological Roman surface by spin-induced ferroelectric polarization in cubic lattice

Topology, a mathematical concept in geometry, has become an ideal theoretical tool for describing topological states and phase transitions. Many topological concepts have found their physical entities in real or reciprocal spaces identified by topological/geometrical invariants, which are usually defined on orientable surfaces such as torus and sphere. It is natural to quest whether it is possible to find the physical realization of more intriguing non-orientable surfaces. Herein, we show that the set of spin-induced ferroelectric polarizations in cubic perovskite oxides AMn3Cr4O12 (A = La and Tb) resides on the topological Roman surface, a non-orientable two-dimensional manifold formed by sewing a Mobius strip edge to that of a disc. The induced polarization may travel in a loop along the non-orientable Mobius strip or orientable disc depending on how the spin evolves as controlled by external magnetic field. Experimentally, the periodicity of polarization can be the same or the twice of the rotating magnetic field, being well consistent with the orientability of disc and Mobius strip, respectively. This path dependent topological magnetoelectric effect presents a way to detect the global geometry of the surface and deepens our understanding of topology in both mathematics and physics

cond-mat.str-el↗

arXiv2022

Unconventional Materials: the mismatch between electronic charge centers and atomic positions

The complete band representations (BRs) have been constructed in the work of topological quantum chemistry. Each BR is expressed by either a localized orbital at a Wyckoff site in real space, or by a set of irreducible representations in momentum space. In this work, we define unconventional materials with a common feature of the mismatch between average electronic centers and atomic positions. They can be effectively diagnosed as whose occupied bands can be expressed as a sum of elementary BRs (eBRs), but not a sum of atomic-orbital-induced BRs (aBRs). The existence of an essential BR at an empty site is described by nonzero real-space invariants (RSIs). The "valence" states can be derived by the aBR decomposition, and unconventional materials are supposed to have an uncompensated total "valence" state. The high-throughput screening for unconventional materials has been performed through the first-principles calculations. We have discovered 423 unconventional compounds, including thermoelectronic materials, higher-order topological insulators, electrides, hydrogen storage materials, hydrogen evolution reaction electrocatalysts, electrodes, and superconductors. The diversity of these interesting properties and applications would be widely studied in the future.

cond-mat.mtrl-sci↗

arXiv2022

Topological states and topological phase transition in Cu2SnS3 and Cu2SnSe3

Based on the first-principles calculations within local density approximation and model analysis, we propose that the iso-structural compounds Cu2SnS3 and Cu2SnSe3 are both the simplest nodal line semimetals with only one nodal line in their crystal momentum space when spin-orbit coupling (SOC) is ignored. The including of SOC drives Cu2SnS3 into a Weyl semimetal (WSM) state with only two pairs of Weyl nodes, the minimum number required for WSM with time reversal symmetry. In contrast, SOC leads Cu2SnSe3 to strong topological insulator (TI) state. This difference can be well understood as there is a topological phase transition (TPT). In it, the Weyl nodes are driven by tunable SOC and annihilate in a mirror plane, resulting in a TI. This TPT, together with the evolution of Weyl nodes, the changing of mirror Chern numbers of mirror plane and the Z2 indices protected by time-reversal symmetry has been demonstrated by the calculation of Cu2SnS3-xSex within virtual crystal approximation and an effective k $\cdot$ p model analysis. Though our first-principles calculations have overestimated the topological states in both compounds, we believe that the theoretical demonstration of controlling TPT and evolution of Weyl nodes will stimulate further efforts in exploring them.

cond-mat.mtrl-sci↗

arXiv2022

The Role of Shift Vector in High-Harmonic Generation from Non-Centrosymmetric Topological Insulators under Strong Laser Fields

As a promising avenue to obtain new extreme ultraviolet light source and detect electronic properties, high-harmonic generation (HHG) has been actively developed in both theory and experiment. In solids lacking inversion symmetry, when electrons undergo a nonadiabatic transition, a directional charge shift occurs and is characterized by shift vector, which measures the real-space shift of the photoexcited electron and hole. For the first time, we have revealed that shift vector plays prominent roles in the real-space tunneling mechanism of three-step model for electrons under strong laser fields. Since shift vector is determined by the topological properties of related wave functions, we expect HHG with its contribution can provide direct knowledge on the band topology in noncentrosymmetric topological insulators (TIs). In both Kane-Mele model and realistic material BiTeI, we have found that the shift vector reverses when band inversion happens during the topological phase transition between normal and topological insulators. Under oscillating strong laser fields, the reversal of shift vector leads to completely opposite radiation time of high-order harmonics. This makes HHG a feasible all-optical strong-field method to directly identify the band inversion in non-centrosymmetric TIs.

physics.optics↗

arXiv2021

Topological phases in pyrochlore thallium niobate Tl$_2$Nb$_2$O$_{6+x}$

The discovery of new topological electronic materials brings a chance to uncover novel physics and plays a key role in observing and controlling various intriguing phenomena. Up to now, many materials have been theoretically proposed and experimentally proved to host different kinds of topological states. Unfortunately, there is little convincing experimental evidence for the existence of topological oxides. The reason is that oxidation of oxygen leads to ionic crystal in general and makes band inversion unlikely. In addition, the realization of different topological states in a single material is quite difficult, but strongly needed for exploring topological phase transitions. In this work, using first-principles calculations and symmetry analysis, we propose that the experimentally tunable continuous solid solution of oxygen in pyrochlore Tl$_2$Nb$_2$O$_{6+x}$ ($ 0\leq x \leq 1.0$) leads to various topological states. Topological insulator, Dirac semimetal and triply degenerate nodal point semimetal can be realized in it via changing the oxygen content and/or tuning the crystalline symmetries. When $x = 1$, it is a semimetal with quadratic band touching point at Fermi level. It transits into a Dirac semimetal or a topological insulator depending on the in-plane strain. When $x = 0.5$, the inversion symmetry is spontaneously broken in Tl$_2$Nb$_2$O$_{6.5}$, leading to triply degenerate nodal points. When $x = 0$, Tl$_2$Nb$_2$O$_6$ becomes a trivial insulator with a narrow band gap. These topological phase transitions driven by solid solution of oxygen are unique and physically plausible due to the variation of valence state of $Tl^{1+}$ and $Tl^{3+}$. This topological oxide will be promising for studying correlation induced topological states and potential applications.

cond-mat.mtrl-sci↗

arXiv2021

The evolution of Weyl nodes in Ni doped thallium niobate pyrochlore Tl$_{2-x}$Ni$_x$Nb$_2$O$_7$

Magnetic Weyl semimetal (WSM) is of great importance both for fundamental physics and potential applications due to its spontaneous magnetism, robust band topology, and enhanced Berry curvature. It possesses many unique quantum effects, including large intrinsic anomalous Hall effect, Fermi arcs, and chiral anomaly. In this work, using ab initio calculations, we propose that Ni doped pyrochlore Tl$_2$Nb$_2$O$_7$ is a magnetic WSM caused by exchange field splitting on bands around its quadratic band crossing point. The exchange field tuned by Ni 3d on-site Coulomb interaction parameter $U$ drives the evolution of Weyl nodes and the resulting topological phase transition. Since Weyl nodes can exist at generic point in Brillouin zone and are hard to be exactly identified, the creation and annihilation of them, i.e., the change in their number, chirality and distribution, have been consistently confirmed with a combined theoretical approach, which employs parity criterion, symmetry indicator analysis and the Wilson loop of Wannier center. We find that Weyl nodes remain in a quite large range of $U$ and are close to Fermi level, which makes the experimental observation very possible. We think this method and our proposal of magnetic WSM will be useful in finding more WSMs and adding our understanding on the topological phase transition.

cond-mat.mtrl-sci↗

arXiv2021

RTGW2020: A powerful implementation of DFT + Gutzwiller method

In the present paper, we propose an efficient numerical scheme for Gutzwiller method for multi-band Hubbard models with general onsite Coulomb interaction. Following the basic idea of Deng et al. [Phys. Rev. B 79, 075114 (2009)] and extensions by Lanata et al. [Phys. Rev. B 85, 035133 (2012)], the ground state is variationally determined through optimizing the total energy with respect to the variational single particle density matrix (n0), which is called "outer loop". In the corresponding "inner loop" where n0 is fixed, the non-interacting wave function and the parameters contained in the Gutzwiller projector are determined by a two-step iterative approach. All derivatives of the implementation process have been analytically derived, which allows us to apply some advanced minimization or root-searching algorithms for both the inner and outer loops leading to the highly efficient convergence. In addition, an atomic diagonalization method taking the point group symmetry into account has been developed for the customized design of the Gutzwiller projector, making it convenient to explore many interesting orders at a lower cost of computation. As benchmarks, several different types of correlated models have been studied utilizing the proposed method, which are in perfect agreement with the previous results by DMFT and multi-orbital slave-boson mean field method. Compared with the linear mixing method, the newton method with analytical derivatives shows much faster convergence for the inner loop. As for the outer loop, the minimization using analytical derivatives also shows much better stability and efficiency compared with that using numerical derivatives.

cond-mat.str-el↗

arXiv2021

Charge instability of topological Fermi arcs in chiral crystal CoSi

Topological boundary states, emerged at the spatial boundary between topological non-trivial and trivial phases, are usually gapless, or commonly referred as metallic states. For example, the surface state of a topological insulator is a gapless Dirac state. These metallic topological boundary states are typically well described by non-interacting fermions. However, the behavior of topological boundary states with significant electron-electron interactions, which could turn the gapless boundary states into gapped ordered states, e.g., density wave states or superconducting states, is of great interest theoretically, but is still lacking evidence experimentally. Here, we report the observation of incommensurable charge density wave (CDW) formed on the topological boundary states driven by the electron-electron interactions on the (001) surface of CoSi. The wavevector of CDW varies as the temperature changes, which coincides with the evolution of topological surface Fermi arcs with temperature. The orientation of CDW phase is determined by the chirality of the Fermi arcs, which indicates direct association between CDW and Fermi arcs. Our finding will stimulate the search of more interactions-driven ordered states, such as superconductivity and magnetism, on the boundaries of topological materials.

cond-mat.str-el↗

arXiv2021

Unusual Electronic Structure of Dirac Material BaMnSb$_2$ Revealed by Angle-Resolved Photoemission Spectroscopy

High resolution angle resolved photoemission measurements and band structure calculations are carried out to study the electronic structure of BaMnSb$_2$. All the observed bands are nearly linear that extend to a wide energy range. The measured Fermi surface mainly consists of one hole pocket around $Γ$ and a strong spot at Y which are formed from the crossing points of the linear bands. The measured electronic structure of BaMnSb$_2$ is unusual and deviates strongly from the band structure calculations. These results will stimulate further efforts to theoretically understand the electronic structure of BaMnSb$_2$ and search for novel properties in this Dirac material.

cond-mat.mtrl-sci↗

arXiv2021

Application of topological quantum chemistry in electrides

The recently developed theory of topological quantum chemistry (TQC) has built a close connection between band representations in momentum space and orbital characters in real space. It provides an effective way to diagnose topological materials, leading to the discovery of lots of topological materials after the screening of all known nonmagnetic compounds. On the other hand, it can also efficiently reveal spacial orbital characters, including average charge centers and site-symmetry characters. By using TQC theory with the computed irreducible representations in the first-principles calculations, we demonstrate that the electrides with excess electrons serving as anions at vacancies can be well identified by analyzing band representations (BRs), which cannot be expressed as a sum of atomic-orbital-induced band representations (aBRs). In fact, the floating bands (formed by the excess electrons) belong to the BRs induced from the "pseudo-orbitals" centered at vacancies. In other words, the electrides are proved to be unconventional ionic crystals, where a set of occupied bands is not a sum of aBRs but necessarily contains a BR from vacancies. The TQC theory provides a promising avenue to pursue more electride candidates in ionic crystals.

cond-mat.mtrl-sci↗

arXiv2021

Observation of nearly identical superconducting transition temperatures in pressurized Weyl semimetal MIrTe4 (M=Nb and Ta)

Here we report the observation of pressure-induced superconductivity in type-II Weyl semimetal (WSM) candidate NbIrTe4 and the evolution of its Hall coefficient (RH), magnetoresistance (MR), and lattice with increasing pressure to ~57 GPa. These results provide a significant opportunity to investigate the universal high-pressure behavior of ternary WSMs, including the sister compound TaIrTe4 that has been known through our previous studies. We find that the pressure-tuned evolution from the WSM to the superconducting (SC) state in these two compounds exhibit the same trend, i.e., a pressure-induced SC state emerges from the matrix of the non-superconducting WSM state at ~ 27 GPa, and then the WSM state and the SC state coexist up to 40 GPa. Above this pressure, an identical high-pressure behavior, characterized by almost the same value of RH and MR in its normal state and the same value of Tc in its SC state, appears in both compounds. Our results not only reveal a universal connection between the WSM state and SC state, but also demonstrate that NbIrTe4 and TaIrTe4 can make the same contribution to the normal and SC states that inhabit in the high-pressure phase, although these two compounds have dramatically different topological band structure at ambient pressure.

cond-mat.supr-con↗

arXiv2021

Electronic Structure Examination on the Topological Properties of CaMnSb$_{2}$ by Angle-Resolved Photoemission Spectroscopy

We have carried out detailed high resolution ARPES measurements and band structure calculations to study the electronic structure of CaMnSb$_{2}$. The observed Fermi surface mainly consists of one hole pocket around $Γ$ point and one tiny hole pocket at Y point. Strong spectral weight accumulation along the $Γ$-X direction is observed on the hole-like Fermi surface around $Γ$ point, suggesting strong anisotropy of the density of states along the Fermi surface. The tiny hole pocket at Y point originates from an anisotropic Dirac-like band with the crossing point of the linear bands lying $\sim$ 10 meV above the Fermi level. These observations are in a good agreement with the band structure calculations. In addition, we observe additional features along the $Γ$-Y line that cannot be accounted for by the band structure calculations. Our results provide important information in understanding and exploration of novel properties in CaMnSb$_{2}$ and related materials.

cond-mat.mtrl-sci↗

arXiv2021

Giant Nonlinear Hall Effect in Twisted Bilayer WTe2

In a system with inversion symmetry broken, a second-order nonlinear Hall effect can survive even in the presence of time-reversal symmetry. In this work, we show that a giant nonlinear Hall effect can exist in twisted bilayer WTe2 system. The Berry curvature dipole of twisted bilayer WTe2 (θ = 29.4°) can reach up to ~1400 Å, which is much larger than that in previously reported nonlinear Hall systems. In twisted bilayer WTe2 system, there exists abundant band anticrossings and band inversions around the Fermi level, which brings a complicated distribution of Berry curvature, and leading to the nonlinear Hall signals exhibit dramatically oscillating behavior in this system. Its large amplitude and high tunability indicate that the twisted bilayer WTe2 can be an excellent platform for studying the nonlinear Hall effect.

cond-mat.mtrl-sci↗

arXiv2021

Non-Abelian reciprocal braiding of Weyl points and its manifestation in $\textrm{ZrTe}$

Weyl semimetals in three-dimensional crystals provide the paradigm example of topologically protected band nodes. It is usually taken for granted that a pair of colliding Weyl points annihilate whenever they carry opposite chiral charge. In a stark contrast, here we report that Weyl points in systems symmetric under the composition of time-reversal with a $π$-rotation are characterized by a non-Abelian topological invariant. The topological charges of the Weyl points are transformed via braid phase factors which arise upon exchange inside symmetric planes of the reciprocal momentum space. We elucidate this process with an elementary two-dimensional tight-binding model implementable in cold-atoms setups and in photonic systems. In three dimensions, interplay of the non-Abelian topology with point-group symmetry is shown to enable topological phase transitions in which pairs of Weyl points may scatter or convert into nodal-line rings. By combining our theoretical arguments with first-principles calculations, we predict that Weyl points occurring near the Fermi level of zirconium telluride ($\textrm{ZrTe}$) carry non-trivial values of the non-Abelian charge, and that uniaxial compression strain drives a non-trivial conversion of the Weyl points into nodal lines.

cond-mat.mes-hall↗

arXiv2021

Topological insulators in the NaCaBi family with large SOC gaps

By means of first-principles calculations and crystal structure searching techniques, we predict that a new NaCaBi family crystallized into the ZrBeSi-type structure (\ie $P6_{3}/mmc$) are strong topological insulators (STIs). Taking $P6_{3}/mmc$ NaCaBi as an example, the calculated band structure indicates that there is a band inversion between two opposite-parity bands at the $Γ$ point. In contrast to the well-known Bi$_2$Se$_3$ family, the band inversion in the NaCaBi family has already occured even without spin-orbit coupling (SOC), giving rise to a nodal ring surrounding $Γ$ in the $k_z=0$ plane (protected by $M_z$ symmetry). With time reversal symmetry $(\cal T)$ and inversion symmetry $(\cal I)$, the spinless nodal-line metallic phase protected by $[{\cal TI}]^2=1$ is the weak-SOC limit of the spinful topological insulating phase. Upon including SOC, the nodal ring is gapped, driving the system into a STI. Besides inversion symmetry, the nontrivial topology of NaCaBi can also be indicated by $\bar{6}$ symmetry. More surprisingly, the SOC-induced band gap in NaCaBi is about 0.34 eV, which is larger than the energy scale of room temperature. Four other compounds (KBaBi, KSrBi, RbBaBi and RbSrBi) in the family are stable at ambient pressure, both in thermodynamics and lattice dynamics, even though the gaps of them are smaller than that of NaCaBi. Thus, they provide good platforms to study topological states both in theory and experiments.

cond-mat.mtrl-sci↗

arXiv2021

Spectroscopic Evidence on Realization of a Genuine Topological Nodal Line Semimetal in LaSbTe

The nodal line semimetals have attracted much attention due to their unique topological electronic structure and exotic physical properties. A genuine nodal line semimetal is qualified by the presence of Dirac nodes along a line in the momentum space that are protected against the spin-orbit coupling. In addition, it requires that the Dirac points lie close to the Fermi level allowing to dictate the macroscopic physical properties. Although the material realization of nodal line semimetals have been theoretically predicted in numerous compounds, only a few of them have been experimentally verified and the realization of a genuine nodal line semimetal is particularly rare. Here we report the realization of a genuine nodal line semimetal in LaSbTe. We investigated the electronic structure of LaSbTe by band structure calculations and angle-resolved photoemission (ARPES) measurements. Taking spin-orbit coupling into account, our band structure calculations predict that a nodal line is formed in the boundary surface of the Brillouin zone which is robust and lies close to the Fermi level. The Dirac nodes along the X-R line in momentum space are directly observed in our ARPES measurements and the energies of these Dirac nodes are all close to the Fermi level. These results constitute clear evidence that LaSbTe is a genuine nodal line semimetal,providing a new platform to explore for novel phenomena and possible applications associated with the nodal line semimetals.

cond-mat.mtrl-sci↗