arXiv subjects

Haifeng Lv

Publications and source records attributed to Haifeng Lv.

8 recordsLinked to original sources

arXiv2026

Geometry-Driven Magnetoelectric Coupling in Two-Dimensional Compensated Ferrimagnets

The magnetoelectric coupling in compensated magnets enables stray-field-free manipulation of spin-splitting, holding great promise for spintronics, but inherently hindered by the symmetry mismatch between spatial-inversion-broken ferroelectricity and time-reversal-broken spin states. Here, based on a symmetry-decoupled analysis of magnetoelectric coupling in compensated magnets, we establish a geometry-driven spin-ferroelectric coupling mechanism in bilayer breathing kagome lattices. Within this geometric framework interlocking the out-of-plane electric polarization with cooperative intralayer structural distortions, we demonstrate that polarization switching drives a deterministic reversal of the global spin splitting. First-principles calculations on a prototype bilayer Nb3Cl8 successfully validate this mechanism, demonstrating the switching of spin-splitting states through an energetically feasible, asynchronous layer-by-layer transition pathway. Our proposed coupling originates from lattice geometry and structural symmetry, establishing a unique route toward switchable spin splitting in compensated ferrimagnets.

cond-mat.mtrl-sci↗

arXiv2026

Harmonic Hierarchy of Altermagnetic Spin Splitting from Symmetry-Adapted Wavefunctions

Altermagnets combine magnetic compensation with spin-momentum-locked splitting in the absence of spin-orbit coupling, yet existing descriptions, formulated primarily in terms of spin symmetry and lattice geometry, provide limited insight into the electric-structure perspective of its angular harmonic form. Here, we identify a wavefunction-level framework for altermagnetism in two-dimensional square lattices. Using symmetry-adapted polynomial wavefunctions, we show that the harmonic structure of momentum-space spin splitting is inherited from the geometry of the electronic wavefunctions which can be selected by crystal fields. Identical orbital sectors preserve conventional antiferromagnetic degeneracy, whereas intertwined linear and quadratic wavefunctions generate d-wave and g-wave altermagnetic anisotropies, respectively. Tight-binding analysis connects this hierarchy to inequivalent same-spin hopping channels. First-principles calculations on the g-wave mcm-type reticular material platforms confirm high-symmetry-linear degeneracy together with finite generic-k splitting. Our results establish a hierarchy linking wavefunction geometry, orbital realization, microscopic hopping anisotropy, and altermagnetic electronic structure.

cond-mat.mtrl-sci↗

arXiv2023

Many-body hybrid Excitons in Organic-Inorganic van der Waals Heterostructures

The coherent many-body interaction at the organic-inorganic interface can give rise to intriguing hybrid excitons that combine the advantages of the Wannier-Mott and Frenkel excitons simultaneously. Unlike the 2D inorganic heterostructures that suffer from moment mismatch, the hybrid excitons formed at the organic-inorganic interface have a momentum-direct nature, which have yet to be explored. Here, we report hybrid excitons at the copper phthalocyanine/molybdenum diselenide (CuPc/MoSe2) interface with strong molecular orientation dependence using low-temperature photoluminescence spectroscopy. The new emission peaks observed in the CuPc/MoSe2 heterostructure indicate the formation of interfacial hybrid excitons. The density functional theory (DFT) calculation confirms the strong hybridization between the lowest unoccupied molecular orbital (LUMO) of CuPc and the conduction band minimum (CBM) of MoSe2, suggesting that the hybrid excitons consist of electrons extended in both layers and holes confined in individual layers. The temperature-dependent measurements show that the hybrid excitons can gain the signatures of the Frenkel excitons of CuPc and the Wannier-Mott excitons of MoSe2 simultaneously. The out-of-plane molecular orientation is used to tailor the interfacial hybrid exciton states. Our results reveal the hybrid excitons at the CuPc/MoSe2 interface with tunability by molecular orientation, which suggests that the emerging organic-inorganic heterostructure can be a promising platform for many-body exciton physics.

physics.optics↗

arXiv2022

Interfacial Charge-transfer Excitonic Insulator in a Two-dimensional Organic-inorganic Superlattice

Excitonic insulators are long-sought-after quantum materials predicted to spontaneously open a gap by the Bose condensation of bound electron-hole pairs, namely, excitons, in their ground state. Since the theoretical conjecture, extensive efforts have been devoted to pursuing excitonic insulator platforms for exploring macroscopic quantum phenomena in real materials. Reliable evidences of excitonic character have been obtained in layered chalcogenides as promising candidates. However, owing to the interference of intrinsic lattice instabilities, it is still debatable whether those features, such as charge density wave and gap opening, are primarily driven by the excitonic effect or by the lattice transition. Herein, we develop a novel charge-transfer excitonic insulator in organic-inorganic superlattice interfaces, which serves as an ideal platform to decouple the excitonic effect from the lattice effect. In this system, we observe the narrow gap opening and the formation of a charge density wave without periodic lattice distortion, providing visualized evidence of exciton condensation occurring in thermal equilibrium. Our findings identify spontaneous interfacial charge transfer as a new strategy for developing novel excitonic insulators and investigating their correlated many-body physics.

cond-mat.mtrl-sci↗

arXiv2021

Two-Dimensional Bipolar Magnetic Semiconductor with High Curie Temperature and Electrically Controllable Spin Polarization Realized in Exfoliated Cr(pyrazine)$_2$ Monolayer

Exploring two-dimensional (2D) magnetic semiconductors with room temperature magnetic ordering and electrically controllable spin polarization is a highly desirable but challenging task for nanospintronics. Here, through first principles calculations, we propose to realize such a material by exfoliating the recently synthesized organometallic layered crystal Li$_{0.7}$[Cr(pyz)$_2$]Cl$_{0.7}$0.25$\cdot$(THF) (pyz = pyrazine, THF = tetrahydrofuran) [Science 370, 587 (2020)]. The feasibility of exfoliation is confirmed by the rather low exfoliation energy of 0.27 J/m$^2$, even smaller than that of graphite. In exfoliated Cr(pyz)$_2$ monolayer, each pyrazine ring grabs one electron from the Cr atom to become a radical anion, then a strong $d$-$p$ direct exchange magnetic interaction emerges between Cr cations and pyrazine radicals, resulting in room temperature ferrimagnetism with a Curie temperature of 342 K. Moreover, Cr(pyz)$_2$ monolayer is revealed to be an intrinsic bipolar magnetic semiconductor where electrical doping can induce half-metallic conduction with controllable spin-polarization direction.

cond-mat.mtrl-sci↗

arXiv2021

Mechanical Properties of Atomically Thin Tungsten Dichalcogenides: WS$_2$, WSe$_2$ and WTe$_2$

Two-dimensional (2D) tungsten disulfide (WS$_2$), tungsten diselenide (WSe$_2$), and tungsten ditelluride (WTe$_2$) draw increasing attention due to their attractive properties deriving from the heavy tungsten and chalcogenide atoms, but their mechanical properties are still mostly unknown. Here, we determine the intrinsic and air-aged mechanical properties of mono-, bi-, and trilayer (1-3L) WS$_2$, WSe$_2$ and WTe$_2$ using a complementary suite of experiments and theoretical calculations. High-quality 1L WS$_2$ has the highest Young's modulus (302.4+-24.1 GPa) and strength (47.0+-8.6 GPa) of the entire family, overpassing those of 1L WSe$_2$ (258.6+-38.3 and 38.0+-6.0 GPa, respectively) and WTe$_2$ (149.1+-9.4 and 6.4+-3.3 GPa, respectively). However, the elasticity and strength of WS$_2$ decrease most dramatically with increased thickness among the three materials. We interpret the phenomenon by the different tendencies for interlayer sliding in equilibrium state and under in-plane strain and out-of-plane compression conditions in the indentation process, revealed by finite element method (FEM) and density functional theory (DFT) calculations including van der Waals (vdW) interactions. We also demonstrate that the mechanical properties of the high-quality 1-3L WS$_2$ and WSe$_2$ are largely stable in the air for up to 20 weeks. Intriguingly, the 1-3L WSe$_2$ shows increased modulus and strength values with aging in the air. This is ascribed to oxygen doping, which reinforces the structure. The present study will facilitate the design and use of 2D tungsten dichalcogenides in applications, such as strain engineering and flexible field-effect transistors (FETs).

cond-mat.mtrl-sci↗

arXiv2018

Electron-Doping Induced Semiconductor to Metal Transitions in ZrSe2 Layers via Copper Atomic Intercalation

Atomic intercalation in two dimensional (2D) layered materials can engineer the electronic structure at the atomic scale, bringing out tunable physical and chemical properties which are quite distinct in comparison with pristine one. Among them, electron-doped engineering induced by intercalation is an efficient route to modulate electronic states in 2D layers. Herein, we demonstrate a semiconducting to the metallic phase transition in zirconium diselenide (ZrSe2) single crystal via controllable incorporation of copper (Cu) atoms. Combined with first-principles density functional theory (DFT) calculations, our angle resolved photoemission spectroscopy (ARPES) characterizations clearly revealed the emergence of conduction band dispersion at M/L point of Brillouin zone due to Cu-induced electron doping in ZrSe2 interlayers. Moreover, the field-effect transistor (FET) fabricated on ZrSe2 displayed a n-type semiconducting transport behavior, while the Cu-intercalated ZrSe2 posed linear Ids vs Vds curves with metallic character shows n-type doping. The atomic intercalation approach has high potential for realizing transparent electron-doping systems for many specific 2D-based nano-electronics.

cond-mat.str-el↗

arXiv2015

Quantum fluctuations in the BCS-BEC crossover of two-dimensional Fermi gases

We present a theoretical study of the ground state of the BCS-BEC crossover in dilute two-dimensional Fermi gases. While the mean-field theory provides a simple and analytical equation of state, the pressure is equal to that of a noninteracting Fermi gas in the entire BCS-BEC crossover, which is not consistent with the features of a weakly interacting Bose condensate in the BEC limit and a weakly interacting Fermi liquid in the BCS limit. The inadequacy of the 2D mean-field theory indicates that the quantum fluctuations are much more pronounced than those in 3D. In this work, we show that the inclusion of the Gaussian quantum fluctuations naturally recovers the above features in both the BEC and the BCS limits. In the BEC limit, the missing logarithmic dependence on the boson chemical potential is recovered by the quantum fluctuations. Near the quantum phase transition from the vacuum to the BEC phase, we compare our equation of state with the known grand canonical equation of state of 2D Bose gases and determine the ratio of the composite boson scattering length $a_{\rm B}$ to the fermion scattering length $a_{\rm 2D}$. We find $a_{\rm B}\simeq 0.56 a_{\rm 2D}$, in good agreement with the exact four-body calculation. We compare our equation of state in the BCS-BEC crossover with recent results from the quantum Monte Carlo simulations and the experimental measurements and find good agreements.

cond-mat.quant-gas↗