Showing papers by "Hongming Weng published in 2023"

Optical spectroscopy and band structure calculations of the structural phase transition in the vanadium-based kagome metal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>ScV</mml:mi><mml:mn>6</mml:mn></mml:msub><mml:msub><mml:mi>Sn</mml:mi><mml:mn>6</mml:mn></m

Abstract: In condensed matter physics, materials with kagome lattice display a range of exotic quantum states, including charge density wave (CDW), superconductivity, and magnetism. Recently, the intermetallic kagome metal ${\mathrm{ScV}}_{6}{\mathrm{Sn}}_{6}$ was discovered to undergo a first-order structural phase transition with the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ifmmode\times\else\texttimes\fi{}3$ CDW at around 92 K. The bulk electronic band properties are crucial to understanding the origin of the structural phase transition. Here, we conducted an optical spectroscopy study in combination with band structure calculations across the structural transition. Our findings showed abrupt changes in the optical reflectivity/conductivity spectra as a result of the structural transition, without any observable gap formation behavior. The optical measurements and band calculations actually reveal a sudden change of the band structure after transition. It is important to note that this phase transition is of the first-order type, which distinguishes it from conventional density-wave type condensations. Our results provide an insight into the origin of the structural phase transition in this new and unique kagome lattice intermetallic material.

Physical properties, electronic structure, and strain-tuned monolayer of the weak topological insulator RbTi3Bi5 with Kagome lattice

Abstract: Kagome metals AV 3 Sb 5 (A = K, Rb, and Cs) with a V-Kagome lattice acting as a fertile platform to investigate geometric frustration, electron correlation, superconductivity, and nontrivial band topology, have attracted tremendous attention. Here we reported the structure and properties of ATi 3 Bi 5 (A = Rb, Cs) family with a Ti-Kagome lattice, specifically focusing on the electronic structure and nontrivial band topology of RbTi 3 Bi 5 . ATi 3 Bi 5 (A = Rb, Cs) is found to be non-superconducting metal with strong quasi-two-dimensional feature, moderate electron correlation, and small Pauli paramagnetism. Based on first principles calculations, RbTi 3 Bi 5 is determined to be a weak topological insulator with gapless surface states along (100) plane, and the electronic band structure along (001) plane is in great agreement with experimentally observed one. In particular, the electronic properties of the RbTi 3 Bi 5 monolayer can be efficiently tuned by a biaxial strain according to calculation, with its lower saddle points coming from Kagome lattice approaching the Fermi level. These results highlight ATi 3 Bi 5 (A = Rb, Cs) with Ti-Kagome lattice is a new Kagome metal to explore nontrivial band topology and exotic phases

Structure, physical properties, and magnetically tunable topological phases in topological semimetal EuCuBi

Abstract: A single material achieving multiple topological phases can provide potential application for topological spintronics, whereas the candidate materials are very limited. Here, we report the structure, physical properties, and possible emergence of multiple topological phases in the newly discovered, air-stable EuCuBi single crystal. EuCuBi crystallizes in a hexagonal space group P63/mmc (No. 194) in ZrBeSi-type structure with an antiferromagnetic (AFM) ground state below TN = 11.2 K. There is a competition between AFM and ferromagnetic (FM) interactions below TN revealed by electrical resistivity and magnetic susceptibility measurements. With the increasing magnetic field, EuCuBi evolves from the AFM ground state with a small amount of FM component, going through two possible metamagnetic phases, finally reaches the field-induced FM phase. Based on the first-principles calculations, we demonstrate that the Dirac, Weyl, and possible mirror Chern insulator can be achieved in EuCuBi by tuning the temperature and applying magnetic field, making EuCuBi a promising candidate for exploring multiple topological phases.

Spatial symmetry modulation of planar Hall effect in Weyl semimetals

Abstract: The planar Hall effect in Weyl semimetals has been proposed as a key feature of chiral anomaly and nontrivial Berry curvature, while the effect of fundamental spatial symmetry remains to be explored. Here, we show with general symmetry analysis and lattice model calculations that the planar Hall effect can be significantly modulated by a certain class of spatial symmetry operations. The odd terms of conductivity with respect to magnetic field, including the linear terms, vanish in the plane perpendicular to the axis of twofold rotation (${C}_{2}$) or the ${C}_{2}$ combined symmetry operations (mirror, twofold screw rotation, glide plane). When applying the magnetic field perpendicular to the glide plane, we find that the planar Hall conductivity in bulk forbidden by glide symmetry becomes nonzero in its quasi-two-dimensional counterpart with broken glide symmetry. It is further clarified that the nonzero planar Hall conductivity comes from surface states. This signature can be used to distinguish bulk and surface transport in Weyl semimetals.

Gate-Tunable Multiband Transport in ZrTe5 Thin Devices.

Abstract: Interest in ZrTe5 has been reinvigorated in recent years owing to its potential for hosting versatile topological electronic states and intriguing experimental discoveries. However, the mechanism of many of its unusual transport behaviors remains controversial: for example, the characteristic peak in the temperature-dependent resistivity and the anomalous Hall effect. Here, through employing a clean dry-transfer fabrication method in an inert environment, we successfully obtain high-quality ZrTe5 thin devices that exhibit clear dual-gate tunability and ambipolar field effects. Such devices allow us to systematically study the resistance peak as well as the Hall effect at various doping densities and temperatures, revealing the contribution from electron-hole asymmetry and multiple-carrier transport. By comparing with theoretical calculations, we suggest a simplified semiclassical two-band model to explain the experimental observations. Our work helps to resolve the longstanding puzzles on ZrTe5 and could potentially pave the way for realizing novel topological states in the two-dimensional limit.

Magnetic bulk photovoltaic effect as a probe of magnetic structures of $EuSn_2As_2$

Abstract: The bulk photovoltaic effect (BPVE) is a second-order optical process in noncentrosymmetric materials that converts the light into DC currents. BPVE is classified into shift current and injection current according to the generation mechanisms, whose dependence on the polarization of light is sensitive to the spatial and time-reversal symmetry of materials. In this work, we present a comprehensive study on the BPVE response of $EuSn_2As_2$ with different magnetic structures through symmetry analysis and first-principles calculation. We demonstrate that the interlayer antiferromagnetic (AFM) $EuSn_2As_2$ of even-layer breaks the inversion symmetry and has the second-order optical responses. Moreover, the bilayer AFM $EuSn_2As_2$ not only displays distinct BPVE responses when magnetic moments align in different directions, but also shows symmetry-related responses in two phases which have mutually perpendicular in-plane magnetic moments. Due to the dependence of BPVE responses on the polarization of light and magnetic symmetry, these magnetic structures can be distinguished by the circular polarized light with well-designed experiments. Our work demonstrates the feasibility of the BPVE response as a tool to probe the magnetic structure.

Basis-set-error-free RPA correlation energies for atoms based on the Sternheimer equation

Abstract: The finite basis set errors for all-electron random-phase approximation (RPA) correlation energy calculations are analyzed for isolated atomic systems. We show that, within the resolution-of-identity (RI) RPA framework, the major source of the basis set errors is the incompleteness of the single-particle atomic orbitals used to expand the Kohn-Sham eigenstates, instead of the auxiliary basis set (ABS) to represent the density response function $\chi^0$ and the bare Coulomb operator $v$. By solving the Sternheimer equation for the first-order wave function on a dense radial grid, we are able to eliminate the major error -- the incompleteness error of the single-particle atomic basis set -- for atomic RPA calculations. The error stemming from a finite ABS can be readily rendered vanishingly small by increasing the size of the ABS, or by iteratively determining the eigenmodes of the $\chi^0 v$ operator. The variational property of the RI-RPA correlation energy can be further exploited to optimize the ABS in order to achieve a fast convergence of the RI-RPA correlation energy. These numerical techniques enable us to obtain basis-set-error-free RPA correlation energies for atoms, and in this work such energies for atoms from H to Kr are presented. The implications of the numerical techniques developed in the present work for addressing the basis set issue for molecules and solids are discussed.

Tunable magnetism and electron correlation in Titanium-based Kagome metals RETi3Bi4 (RE = Yb, Pr, and Nd) by rare-earth engineering

Abstract: Rare-earth engineering is an effective way to introduce and tune the magnetism in topological Kagome magnets, which has been acting as a fertile platform to investigate the quantum interactions between geometry, topology, spin, and correlation. Here we report the structure and properties of three newly discovered Titanium-based Kagome metals RETi3Bi4 (RE = Yb, Pr, and Nd) with various magnetic states. They crystalize in the orthogonal space group Fmmm (No.69), where slightly distorted Ti Kagome lattice, RE triangular lattice, Bi honeycomb and triangular lattices stack along the a axis. By changing the rare earth atoms on RE zag-zig chains, the magnetism can be tuned from nonmagnetic YbTi3Bi4 to short-range ordered PrTi3Bi4 (Tanomaly ~ 8.2 K), and finally to ferromagnetic NdTi3Bi4 (Tc ~ 8.5 K). The measurements of resistivity and specific heat capacity demonstrate an evolution of electron correlation and density of states near the Fermi level with different rare earth atoms. In-situ resistance measurements of NdTi3Bi4 under high pressure further reveal a potential relationship between the electron correlation and ferromagnetic ordering temperature. These results highlight RETi3Bi4 as another family of topological Kagome magnets to explore nontrivial band topology and exotic phases in Kagome materials.

Electronic Structure of the Weak Topological Insulator Candidate Zintl Ba3Cd2Sb4

Abstract: One of the greatest triumph of condensed matter physics in the past ten years is the classification of materials by the principle of topology. The existence of topological protected dissipationless surface state makes topological insulators great potential for applications and hotly studied. However, compared with the prosperity of strong topological insulators, theoretical predicted candidate materials and experimental confirmation of weak topological insulators (WTI) are both extremely rare. In this work, by combining systematic first principles calculation and angle-resolved photoemission spectroscopy (ARPES) measurements, we have studied the electronic structure of the dark surface of the WTI candidate Zintl Ba3Cd2Sb4 and another related material Ba3Cd2As4. The existence of two Dirac surface states (SSs) on specific side surfaces predicted by theoretical calculations and the observed two band inversions in the Brillouin zone give strong evidence to prove that the Ba3Cd2Sb4 is a WTI. The spectroscopic characterization of this Zintl Ba3Cd2N4 (N = As and Sb) family materials will facilitate the application of their novel topological properties.

Superconductivity in unconventional metals

Abstract: In this work, we define unconventional metals, where a set of low-energy bands belong to an elementary band representation (EBR) on an empty site. The characteristic could give rise to soft phonon modes and strong electron-phonon coupling, yielding charge-density wave (CDW) or/and superconductivity (SC). Based on the first-principles calculations, we demonstrate that 1H/2H-phase transition metal dichalcogenides $MX_2~(M={\rm Nb,Ta}; X={\rm S,Se,Te})$ are unconventional metals, which have a single band of $A_1'@1e$ EBR at the Fermi level ($E_F$). The computed phonon dispersions indicate the stability of the system at high temperatures, while the presence of soft phonon modes suggests a phase transition to the CDW state at low temperatures. Based on the Bardeen-Cooper-Schrieffer (BCS) theory and computed electron-phonon coupling, our calculations show that the SC in NbSe$_2$ is mainly attributed to the soft phonon modes. In the end, the SC has been predicted in unconventional metal 2H-TaN$_2$ with estimated $T_C= 26$ K. These results demonstrate that the unconventional metals offer an attractive platform for studying the interplay between the empty-site EBR and correlated states.

On the Theory of Solid-State Harmonic Generation Governed by Crystal Symmetry

Abstract: The solid-state harmonic generation (SHG) derives from photocurrent coherence. The crystal symmetry, including spatial-inversion, mirror, rotational symmetries and time-reversal symmetry, constrains the amplitude and phase of the photocurrent, thus manipulates the coherent processes in SHG. We revisit the expression of photocurrent under the electric dipole approximation and give a picture of non-equilibrium dynamical process of photoelectron on laser-dressed effective bands. We reveal the indispensable role of shift vector and transition dipole phase in the photocurrent coherence in addition to the dynamical phase. Microscopic mechanism of the selection rule, orientation dependence, polarization characteristics, time-frequency analysis and ellipticity dependence of harmonics governed by crystal symmetry is clarified analytically and numerically. The theory in this paper integrates non-equilibrium electronic dynamics of condensed matter in strong laser fields, and paves a way to explore more nonlinear optical phenomena induced by the crystal symmetry.

Mottness in two-dimensional van der Waals <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Nb</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi>X</mml:mi><mml:mn>8</mml:mn></mml:msub></mml:mrow></mml:math> monolayers <mml:math xmlns:mm

Abstract: We investigate strong electron-electron correlation effects on 2-dimensional van der Waals materials Nb$_3$X$_8$ (X=Cl, Br, I). We find that the monolayers Nb$_3$X$_8$ are ideal systems close to the strong correlation limit. They can be described by a half-filled single band Hubbard model in which the ratio between the Hubbard, U, and the bandwidth, W, U/W $\approx$ 5 $\sim$ 10. Both Mott and magnetic transitions of the material are calculated by the slave boson mean field theory. Doping the Mott state, a $d_{x^2-y^2}+id_{xy}$ superconducting pairing instability is found. We also construct a tunable bilayer Hubbard system for two sliding Nb$_3$X$_8$ layers. The bilayer system displays a crossover between the band insulator and Mott insulator.

Twist-Induced Modification in the Electronic Structure of Bilayer WSe2.

Abstract: The recent discovery of strongly correlated phases in twisted transition-metal dichalcogenides (TMDs) highlights the significant impact of twist-induced modifications on electronic structures. In this study, we employed angle-resolved photoemission spectroscopy with submicrometer spatial resolution (μ-ARPES) to investigate these modifications by comparing valence band structures of twisted (5.3°) and nontwisted (AB-stacked) bilayer regions within the same WSe2 device. Relative to the nontwisted region, the twisted area exhibits pronounced moiré bands and ∼90 meV renormalization at the Γ-valley, substantial momentum separation between different layers, and an absence of flat bands at the K-valley. We further simulated the effects of lattice relaxation, which can flatten the Γ-valley edge but not the K-valley edge. Our results provide a direct visualization of twist-induced modifications in the electronic structures of twisted TMDs and elucidate their valley-dependent responses to lattice relaxation.

Large anomalous Nernst effects at room temperature in Fe3 Pt thin films.

Abstract: Heat current in ferromagnets can generate a transverse electric voltage perpendicular to magnetization, which is known as anomalous Nernst effect (ANE). ANE originates intrinsically from the combination of large Berry curvature and density of states near the Fermi energy EF . It shows technical advantages over conventional longitudinal Seebeck effect in converting waste heat to electricity due to its unique transverse geometry. However, materials showing giant ANE especially at zero magnetic field, remain to be explored. Herein, we report the discovery of a large ANE thermopower of Syx ≈ 2 µV K-1 at room temperature in ferromagnetic Fe3 Pt epitaxial films, which also shows a giant transverse thermoelectric conductivity of αyx ≈ 4 A K-1 m-1 and a remarkable coercive field of about 1300 Oe. Besides the consequent strong magnetic anisotropy, our theoretical analysis reveals that the strong spin-orbit interaction in addition to the hybridization between Pt 5d and Fe 3d electrons leads to a series of distinct energy gaps and large Berry curvature in the Brillouin zone, which is the key for the large ANE. These results highlight the important roles of both Berry curvature and spin-orbit coupling in achieving large ANE at zero magnetic field, providing pathways to explore materials with giant transverse thermoelectric effect without external magnetic field. This article is protected by copyright. All rights reserved.

Magnetic bulk photovoltaic effect as a probe of magnetic structures of $\mathrm{{EuSn_{2}As_{2}}}$

Abstract: Abstract The bulk photovoltaic effect (BPVE) is a second-order optical process in noncentrosymmetric materials that converts the light into DC currents. BPVE is classified into shift current and injection current according to the generation mechanisms and their dependence on the polarization of light is sensitive to the spatial and time-reversal symmetry of materials. In this work, we present a comprehensive study on the BPVE response of $\mathrm{EuSn_{2}As_{2}}$ E u S n 2 A s 2 with different magnetic structures through symmetry analysis and first-principles calculation. We demonstrate that the interlayer antiferromagnetic (AFM) $\mathrm{EuSn_{2}As_{2}}$ E u S n 2 A s 2 of even-layer breaks the inversion symmetry and has the second-order optical responses. Moreover, the bilayer AFM $\mathrm{EuSn_{2}As_{2}}$ E u S n 2 A s 2 not only displays distinct BPVE responses when magnetic moments align in different directions, but also shows symmetry-related responses in two phases which have mutually perpendicular in-plane magnetic moments. Due to the dependence of BPVE responses on the polarization of light and magnetic symmetry, these magnetic structures can be distinguished by the circular polarized light with well-designed experiments. Our work demonstrates the feasibility of the BPVE response as a tool to probe the magnetic structure.

Enumeration of spin-space groups: Towards a complete description of symmetries of magnetic orders

Abstract: Symmetries of three-dimensional periodic scalar fields are described by 230 space groups (SGs). Symmetries of three-dimensional periodic (pseudo-) vector fields, however, are described by the spin-space groups (SSGs), which were initially used to describe the symmetries of magnetic orders. In SSGs, the real-space and spin degrees of freedom are unlocked in the sense that an operation could have different spacial and spin rotations. SSGs gives a complete symmetry description of magnetic structures, and have natural applications in the band theory of itinerary electrons in magnetically ordered systems with weak spin-orbit coupling.\textit{Altermagnetism}, a concept raised recently that belongs to the symmetry-compensated collinear magnetic orders but has non-relativistic spin splitting, is well described by SSGs. Due to the vast number and complicated group structures, SSGs have not yet been systematically enumerated. In this work, we exhaust SSGs based on the invariant subgroups of SGs, with spin operations constructed from three-dimensional (3D) real representations of the quotient groups for the invariant subgroups. For collinear and coplanar magnetic orders, the spin operations can be reduced into lower dimensional real representations. As the number of SSGs is infinite, we only consider SSGs that describe magnetic unit cells up to 12 times crystal unit cells. We obtain 157,289 non-coplanar, 24,788 coplanar-non-collinear, and 1,421 collinear SSGs. The enumerated SSGs are stored in an online database at \url{https://cmpdc.iphy.ac.cn/ssg} with a user-friendly interface. We also develop an algorithm to identify SSG for realistic materials and find SSGs for 1,626 magnetic materials. Our results serve as a solid starting point for further studies of symmetry and topology in magnetically ordered materials.

Flat optical conductivity in the topological kagome magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mi>TbMn</mml:mi></mml:mrow><mml:mn>6</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>Sn</mml:mi></mml:mrow><mml:mn>6</mml:mn></mml:msub></mml:math>

Abstract: The kagome magnet ${\mathrm{TbMn}}_{6}{\mathrm{Sn}}_{6}$ is a new type of topological material that is known to support exotic quantum magnetic states. Experimental work has identified that ${\mathrm{TbMn}}_{6}{\mathrm{Sn}}_{6}$ hosts Dirac electronic states that could lead to topological and Chern quantum phases, but the optical response of the Dirac fermions of ${\mathrm{TbMn}}_{6}{\mathrm{Sn}}_{6}$ and its properties remain to be explored. Here, we perform an optical spectroscopy measurement combined with first-principles calculations on a single-crystal sample of ${\mathrm{TbMn}}_{6}{\mathrm{Sn}}_{6}$ to investigate the associated exotic phenomena. ${\mathrm{TbMn}}_{6}{\mathrm{Sn}}_{6}$ exhibits frequency-independent optical conductivity spectra in a broad range from 1800 to 3000 ${\mathrm{cm}}^{\ensuremath{-}1}$ (220--370 meV) in experiments. The theoretical band structures and optical conductivity spectra are calculated with several shifted Fermi energies to compare with the experiment. The theoretical spectra with a 0.56 eV shift for Fermi energy are well consistent with our experimental results. In addition, massive quasi-two-dimensional (quasi-2D) Dirac bands, which have a linear band dispersion in the ${k}_{x}\text{\ensuremath{-}}{k}_{y}$ plane and no band dispersion along the ${k}_{z}$ direction, exist close to the shifted Fermi energy. According to a tight-binding model analysis, the quasi-2D Dirac bands give rise to a flat optical conductivity, while its value is smaller than (about one tenth of) that from the calculations and experiments. It indicates that the other trivial bands also contribute to the flat optical conductivity.

The destiny of obstructed atomic insulator under correlation

Abstract: The obstructed atomic insulators are insulators with both atomic limits and boundary states. In this work, we study the obstructed atomic insulators under correlation. We use the symmetry indicators by constructing many-body wavefunctions in momentum space to prove the obstruction properties in di ff erent models including the SSH chain, anisotropic square lattice model, the quadrupole insulator model and etc. We demonstrate that the obstruction properties with boundary modes persist at large U where the charge freedom is well-gapped, namely, this insulator phase can smoothly connect to its Mott phase without Mott transition.