arXiv subjects

Yilin Han

Publications and source records attributed to Yilin Han.

16 recordsLinked to original sources

arXiv2026

Magnetic-Field Selection of Magnetic Order in Altermagnets and Noncollinear Antiferromagnets

Conventional field selection of magnetic order relies on the Zeeman coupling, which, however, vanishes in magnets without net magnetization, a rapidly growing class including altermagnets (AMs), noncollinear antiferromagnets (nc-AFMs), and PT-symmetric antiferromagnets (PT-AFMs). Here we show that the quantity that fundamentally couples a magnet to a uniform magnetic field is not the magnetization, but the binary order parameter eta that labels the two time-reversal-related minima of the Landau free energy. We develop a Landau theory of order selection based on eta under the constraints of magnetic point-group (MPG) symmetry, in which eta couples to odd-degree polynomials in the magnetic field. Within this framework, the linear term is the ferromagnetic Zeeman coupling, while higher-order couplings with leading degree n = 3, 5, 7, and 9 naturally appear in AMs and nc-AFMs. In contrast, combined PT symmetry forbids any such coupling. Consequently, it is the order-(n-1) magnetic susceptibility, rather than the net magnetization, that serves as the primary experimental observable for identifying the magnetic order of AMs and nc-AFMs. For all 122 MPGs, we classify the leading coupling degree and the corresponding polynomial forms. We demonstrate our framework in two representative materials: the AM MnF2 and the nc-AFM MnTe2. We further construct a symmetry-allowed spin model for an AM system to reveal the microscopic origin of the higher-order coupling and establish the coupling coefficient explicitly in terms of the spin-model parameters. Our work unifies the description of magnetic-order selection across magnets with and without net magnetization, offers a microscopic origin for this counterintuitive physics, and provides fingerprints for distinguishing intrinsic field selection from extrinsic switching.

cond-mat.mtrl-sci↗

arXiv2026

N\'eel-order-dependent transverse transport in noncoplanar antiferromagnet $\text{MnTe}_{2}$

Antiferromagnets hold appealing potential in next-generation spintronic devices with higher frequency and scalability, thanks to their alternating spin orientations that cancel out net magnetization. However, the lack of a nonzero magnetization makes the detection of the magnetic configuration of antiferromagnet difficult, hampering the applications of antiferromagnets. Here, we report a new transverse transport effect in noncoplanar antiferromagnet $\text{MnTe}_{2}$. This effect is antisymmetric in both magnetic field and N\'eel order, but symmetric in its two indices. It can be understood in terms of the contribution induced by both magnetic field and geometric quantities, as confirmed by our theoretical calculations. Our discovery of a new N\'eel-order-dependent transverse transport effect provides opportunities to the advancing antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2026

Fractional Spin Ferroelectric and Sliding Spin Current in Magnetic Sliding Ferroelectrics

We investigate the fractional spin ferroelectric (FSFE) in magnetic sliding ferroelectrics (SFEs), where ferroelectric switching is characterized not only by the reversal of the out-of-plane electric polarization but also by a variation of fractional in-plane spin electronic polarization. We show that interlayer sliding in FSFEs can naturally lead to a symmetry-protected pure spin current, termed the sliding spin current here. The underlying mechanism is that, during switching, the contributions of valence electrons and ions to the in-plane charge transfer cancel each other, whereas the in-plane spin transfer, which stems solely from valence electrons, persists, leading to a pure spin current. We demonstrate our ideas in various material candidates, including $H$-stacked bilayer CrI$_3$, whose few-layer form has been experimentally confirmed to be a magnetic SFE, and $R$-stacked bilayers $2H$-V$X_2$ ($X=$ S, Se, Te), which have been experimentally synthesised. For a typical switching time of about $1$ ns, the estimated spin-current densities for bilayer CrI$_3$ and V$X_2$ reach $10^9 (\hbar/2e)\mathrm{A/m^2}$ and $10^8 (\hbar/2e)\mathrm{A/m^2}$, respectively. This means that by applying a periodic out-of-plane electric field, a significant alternating spin current can be generated in magnetic SFEs. Thus, our findings propose a compelling new mechanism for the all-electrical generation of pure spin current, and predict concrete realistic materials for experimental verification.

cond-mat.mtrl-sci↗

arXiv2026

Unconventional Scaling of Electric Hall Effect in Magnetic Weyl Semimetals

Electric Hall Effect (EHE), a unique phenomenon in two-dimensional (2D) magnetic systems, refers to the generation of Hall current by an out-of-plane electric field $\Ez$. Here, we demonstrate that for 2D magnetic Weyl semimetals that host doubly degenerate nodal points, the EHE features multiple unconventional scaling laws. At zero temperature, the EHE exhibits a topological $E_F^{-1}$ Fermi-energy scaling. Remarkably, the prefactor of the scaling is determined by the global topological charge of the point without any dependence on the local parameters of the system, leading to a universal and significant enhancement of Hall response in any species of Weyl points as the Fermi energy approaches the Weyl point. This significant response enables a weak electric field to be directly converted into a measurable Hall signal. Surprisingly, this enhanced Hall response is not diminished by temperature, but evolves into an unconventional logarithmically corrected scaling at finite temperature $\sigma_{xy}\propto\Ez\ln(1/|\Ez|)$ for weak $\Ez$, still yielding a divergent electric-field susceptibility. Thus, our work not only unveils intriguing scaling laws resulting from the interaction between magnetism and topology, but also suggests a novel scaling-enhanced and temperature-robust mechanism that may enable weak electric-field sensing through a practical and all-electric route.

cond-mat.mes-hall↗

arXiv2026

Rotation topological states: theory and material realization

The conventional characterization of topological materials relies on topological invariants calculated from the entire set of occupied bands. However, when a system possesses rotational symmetry, the occupied Hilbert space can be decomposed into multiple subspaces labeled by distinct rotation eigenvalues. We show that this decomposition reveals hidden topological states characterized by a novel $\mathbb{Z}_2^n$ topological invariant, where $n$ is the number of subspaces, while the conventional $\mathbb{Z}_2$ invariant may fail to detect the topology hidden in the rotation subspaces. Remarkably, time-reversal symmetry pairs conjugate rotation eigenvalues and guarantees that the two subspaces have the same $\mathbb{Z}_2$ invariants, making the topology always hidden from the conventional global invariant. We formulate the theory of rotation-subspace topology and demonstrate its material realization in bulk CsCl. Using first-principles calculations and symmetry analysis, we show that bulk CsCl, which is diagnosed as topologically trivial by the conventional approach, features a nontrivial $\mathbb{Z}_2^3$ invariant along the $\Gamma$-R path and a nontrivial $\mathbb{Z}_2^4$ invariant along the $\Gamma$-Z and M-R paths, leading to double Weyl points on the (111) and (001) surfaces, respectively. The subspace $\mathbb{Z}_2^n$ invariant proposed here serves as a necessary refinement for symmetry-protected topological phases and will facilitate the identification of a large class of topological states overlooked by existing diagnostics.

cond-mat.mtrl-sci↗

arXiv2026

Topological Interstitial-Electron Conductor

Electron transport in solids arises primarily from two mechanisms: freely moving bulk electrons in metals, and gapless boundary states in topological insulators. Here, we report a new mechanism discovered in electrides. The topological interstitial-electron conductors (TIECs) proposed here are insulating electrides, but host interstitial electrons (IEs) distributed within crystal voids that traverse the entire unit cell. Without being tightly bound to real ions, the IEs generally experience low periodic potential barrier along the void channels. As a consequence, by applying a weak electric field sufficient to overcome the IE barriers but far below the system's dielectric breakdown threshold, one can expect that the TIECs would generate a persistent current contributed by the IEs and propagating along the void channels. We identify a family of realistic altermagnetic electrides, $A_5X_3$ ($A$ = Ca, Sr, Ba, Yb; $X$ = As, Sb), as TIECs. Remarkably, for $A_5X_3$ materials, the periodic potential barrier of the IEs along the void channels are ultralow, ranging from 13.43 to 67.96 meV per formula unit. This renders our proposal readily accessible to experimental verification. We further demonstrate that when the IEs of $A_5X_3$ undergo periodic motion along the channels, topological surface states will emerge at the boundary perpendicular to the channel direction, and continuously move across the bulk band gap. This pumping-like behaviour not only corroborates the topological nature of TIECs, but also rationalizes the finite-electric-field induced electronic transport within the band theory. Our findings expand the classification of electronic conductors, uncover unexplored transport properties of electrides, and establish a new material platform for low-power electronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Equivariant Space Group and Hamiltonian for Collinear Magnetic Systems

Condensed matter physics increasingly focuses on exploiting the magnetic order parameter orientation n as a tuning knob for properties of collinear magnetic materials, but a general method for constructing effective Hamiltonians with explicit n-dependence has been lacking. Here, we develop a symmetry-based framework, built on the equivariant space group, for constructing such Hamiltonians, termed equivariant magnetic Hamiltonians (EMHs). The resulting EMH lives in a higher-dimensional k-n space and exhibits unconventional symmetry actions and topological features. Using a 1D ferromagnetic chain and a 3D antiferromagnet as examples, we demonstrate that explicit n-dependence in EMHs enables the study of magnetic-dynamics-driven topological pumping, including even-integer charge pumping and a second-Chern-number-induced quantized pumping of surface anomalous Hall conductivity. Beyond model systems, we incorporate the framework into first-principles calculations to construct ab-initio EMHs that accurately capture the n-dependent band structures of real materials. The approach can also be generalized to non-collinear magnetic systems. Our work establishes a general framework for constructing EMHs and for exploring the rich physics arising from magnetic anisotropy and magnetic dynamics.

cond-mat.mtrl-sci↗

arXiv2026

Pressure-Induced Superconducting-like Transition in the $\it d$-wave Altermagnet Candidate CsV$_2$Se$_2$O

Altermagnetism generates exchange-type spin splitting without net magnetization and, in its $\it d$-wave form, resembles the angular symmetry of unconventional $\it d$-wave superconductivity. Whether this correspondence bears directly on superconducting instabilities in real correlated materials remains open. Here we study the quasi-two-dimensional vanadium oxychalcogenide CsV$_2$Se$_2$O (CVSO), a square-net $\it d$-wave altermagnet candidate, through combined experimental and theoretical investigation of its lattice structure, electronic structure and transport properties. At ambient pressure, CVSO is a weakly insulating parent state with a density-wave-like anomaly near 100 K, and its bulk properties are most consistent with a G-type compensated antiferromagnetic background. Under compression, the density-wave-like feature is suppressed, the magnetoresistance evolves from predominantly negative to positive, and a superconducting-like resistive downturn emerges below about 3 K. This low-temperature anomaly is reproducible across samples and pressure media, and is suppressed by magnetic field. Room-temperature X-ray diffraction reveals no symmetry lowering, whereas does show a pronounced compressibility anomaly over the same pressure range. CVSO thus reveals a pressure-tuned phase diagram in which a reconstructed weakly insulating parent state gives way to strange-metal-like transport and superconducting-like behavior, echoing broader phenomenology associated with unconventional superconductors, including cuprates and nickelates.

cond-mat.supr-con↗

arXiv2026

Probing Memes in LLMs: A Paradigm for the Entangled Evaluation World

Current evaluation paradigms for large language models (LLMs) characterize models and datasets separately, yielding coarse descriptions: items in datasets are treated as pre-labeled entries, and models are summarized by overall scores such as accuracy, together ignoring the diversity of population-level model behaviors across items with varying properties. To address this gap, this paper conceptualizes LLMs as composed of memes, a notion introduced by Dawkins as cultural genes that replicate knowledge and behavior. Building on this perspective, the Probing Memes paradigm reconceptualizes evaluation as an entangled world of models and data. It centers on a Perception Matrix that captures model-item interactions, enabling Probe Properties for characterizing items and Meme Scores for depicting model behavioral traits. Applied to 9 datasets and 4,507 LLMs, Probing Memes reveals hidden capability structures and quantifies phenomena invisible under traditional paradigms (e.g., elite models failing on problems that most models answer easily). It not only supports more informative and extensible benchmarks but also enables population-based evaluation of LLMs.

cs.CL↗

arXiv2025

Electric-Field Control of Terahertz Response via Spin-Corner-Layer Coupling in Altermagnetic Bilayers

Electric field control of electron charge and spin degrees of freedom is fundamental to modern semiconductor and spintronic devices. Yet controlling electromagnetic waves with an electric field, particularly in the terahertz (THz) band, remains a challenge. Here, we propose a spin-corner-layer coupling (SCLC) mechanism in second-order topological altermagnetic bilayers. By using an electric field to influence electrons between different layers, the SCLC mechanism enables simultaneous control over corner and spin degrees of freedom, thereby allowing electric-field tuning of the absorption, emission intensity, and even polarization of THz waves. Taking bilayer NiZrI$_6$ nanodisks as a prototype, we demonstrate that an ultralow electrostatic field can switch both the spin and the layer polarizations of corner states. This dual switching modulates transition dipole moments and oscillator strengths between different corner states, thereby enabling the manipulation of THz waves. This study establishes a mechanism for the electric-field control of spin and THz waves through SCLC, yielding important implications for the advancement of THz spintronics.

cond-mat.mtrl-sci↗

arXiv2025

Type-II Antiferroelectricity

Antiferroelectricity (AFE) is a fundamental concept in physics and materials science. Conventional AFEs have the picture of alternating local electric dipoles defined in real space. Here, we discover a new class of AFEs, termed type-II AFEs, which possess opposite polarizations defined in momentum space across a pair of symmetry decoupled subspaces. Unlike conventional AFEs, the order parameter of type-II AFEs is rigorously formulated through Berry-phase theory and can be quantitatively extracted from the electronic band structure. Focusing on a subclass of type-II AFEs that preserve spin-rotation symmetry, we establish the relevant symmetry constraints and identify all compatible spin point groups. Remarkably, we find that type-II AFE order intrinsically coexists with antiferromagnetism, revealing a robust form of magnetoelectric coupling. We construct an altermagnetic model and identify several concrete antiferromagnetic/altermagnetic materials, such as FeS, Cr2O3, MgMnO3, monolayer MoICl2 and bilayer CrI3, that exhibit this novel ordering. Furthermore, we uncover unique physical phenomena associated with type-II spin-AFE systems, including spin current generation upon AFE switching and localized spin polarization at boundaries and domain walls. Our findings reveal a previously hidden class of quantum materials with intertwined ferroic orders, offering exciting opportunities for both fundamental exploration and technological applications.

cond-mat.mtrl-sci↗

arXiv2024

Crossed real nodal-line phonons in gold monobromide

Spacetime inversion symmetry can generate intriguing types of spinless excitations in crystalline materials. Here, we propose a topological phase protected by spacetime inversion symmetry - the crossed real nodal line (RNL) in the phonon spectrum of gold monobromide (AuBr). In AuBr, there exist four straight nodal lines, which are linked by a crossed nodal line formed by two lower bands. Remarkably, each adjacent two of the four straight nodal lines is a pair, forming a crossed RNL with nontrivial real Chern number. Such configuration and pairing mode of RNL have never been reported. The crossed RNL exhibits unique surface and hinge states distinguished from that of the conventional RNLs. The symmetry protection and the transformation under the symmetry-preserving strain of the crossed RNL are also investigated. Our results open the door to a new class of topological states, and predict its realization in experimentally synthesized material.

cond-mat.mtrl-sci↗

arXiv2024

Electric Hall Effect and Quantum Electric Hall Effect

Exploring new Hall effect is always a fascinating research topic. The ordinary Hall effect and the quantum Hall effect, initially discovered in two-dimensional (2D) non-magnetic systems, are the phenomena that a transverse current is generated when a system carrying an electron current is placed in a magnetic field perpendicular to the currents. In this work, we propose the electric counterparts of these two Hall effects, termed as electric Hall effect (EHE) and quantum electric Hall effect (QEHE). The EHE and QEHE emerge in 2D magnetic systems, where the transverse current is generated by applying an electric gate-field instead of a magnetic field. We present a symmetry requirement for intrinsic EHE and QEHE. With a weak gate-field, we establish an analytical expression of the intrinsic EHE coefficient. We show that it is determined by intrinsic band geometric quantities: Berry curvature and its polarizability which consists of both intraband and interband layer polarization. Via first-principles calculations, we investigate the EHE in the monolayer Ca(FeN)$_2$, where significant EHE coefficient is observed around band crossings. Furthermore, we demonstrate that the QEHE can appear in the semiconductor monolayer $\rm BaMn_2S_3$, of which the Hall conductivity exhibits steps that take on the quantized values $0$ and $\pm1$ in the unit of $e^2/h$ by varying the gate-field within the experimentally achievable range. Due to the great tunability of the electric gate-field, the EHE and QEHE proposed here can be easily controlled and should have more potential applications.

cond-mat.mes-hall↗

arXiv2023

Hidden Real Topology and Unusual Magnetoelectric Responses in Monolayer Antiferromagnetic Cr$_2$Se$_2$O

Recently, the real topology has been attracting widespread interest in two dimensions (2D). Here, based on first-principles calculations and theoretical analysis, we reveal the monolayer Cr$_2$Se$_2$O (ML-CrSeO) as the first material example of a 2D antiferromagnetic (AFM) real Chern insulator (RCI) with topologically protected corner states. Unlike previous RCIs, we find that the real topology of the ML-CrSeO is rooted in one certain mirror subsystem of the two spin channels, and can not be directly obtained from all the valence bands in each spin channel as commonly believed. In particular, due to antiferromagnetism, the corner modes in ML-CrSeO exhibit strong corner-contrasted spin polarization, leading to spin-corner coupling (SCC). This SCC enables a direct connection between spin space and real space. Consequently, large and switchable net magnetization can be induced in the ML-CrSeO nanodisk by electrostatic means, such as potential step and in-plane electric field, and the corresponding magnetoelectric responses behave like a sign function, distinguished from that of the conventional multiferroic materials. Our work considerably broadens the candidate range of RCI materials, and opens up a new direction for topo-spintronics and 2D AFM materials research.

cond-mat.mtrl-sci↗

arXiv2023

Four-band tight-binding model of TiSiCO-family monolayers

The TiSiCO-family monolayers have recently been attracting significant attention due to their unique valley-layer coupling (VLC). In this work, we present a minimal, four-band tight-binding (TB) model to capture the low-energy physics of the TiSiCO-family monolayers $X_{2}Y$CO$_{2}$ ($X=$ Ti, Zr, Hf; $Y=$ Si, Ge) with strong VLC. These monolayers comprise two $X$ atom layers separated by approximately $4$ \AA ~in the out-of-plane direction. Around each valley ($X$ or $X'$), the conduction and valence bands are mainly dominated by the $A_{1}\{d_{z^{2}(x^{2}-y^{2})}\}$ and $B_{2}\{d_{yz}\}$ orbitals of the top $X$ atoms,and the $A_{1}\{d_{z^{2}(x^{2}-y^{2})}\}$ and $B_{1}\{d_{xz}\}$ orbitals of the bottom $X$ atoms. Using these four states as a basis, we construct a symmetry-allowed TB model. Through parameter fitting from first-principles calculations, the four-band TB model not only reproduces the electronic band structure, but also captures the strong VLC, high-order topology, and valley-contrasting linear dichroism of the monolayers. Furthermore, the TB model reveals that these monolayers may exhibit various intriguing topological phases under electric fields and biaxial strains. Hence, the TB model established here can serve as the starting point for future research exploring the physics related to VLC and the $X_{2}Y$CO$_{2}$ monolayers.

cond-mat.mtrl-sci↗

arXiv2023

Cornertronics in Two-Dimensional Second-Order Topological Insulators

Traditional electronic devices rely on the electron's intrinsic degrees of freedom (d.o.f.) to process information. However, additional d.o.f. like the valley, can emerge in the low-energy states of certain systems. Here, we show that the quantum dots (QDs) constructed from two-dimensional (2D) second-order topological insulators (SOTI) posses a new kind of d.o.f., namely corner freedom, related to the topological corner states that reside at different corners of the systems. Since the corner states are well separated in real space, they can be individually and intuitively manipulated, giving rise to the concept of cornertronics. Via symmetry analysis and material search, we identify the TiSiCO-family monolayers as the first prototype of cornertronics materials, where the corner states can be controlled by both electric and optical fields, due to novel corner-layer coupling (CLC) effect and corner-contrasted linear dichroism. Furthermore, we find that the band gap of the TiSiCO nanodisk lies in the terahertz region and is robust to size reduction. These results indicate that the TiSiCO nanodisks can be used to design terahertz devices with ultrasmall size and electric-field tunable band gap. Besides, the TiSiCO nanodisks are simultaneously sensitive to both the strength and polarization of the terahertz waves. Our findings not only pave the way for cornertronics, but also open a new direction for research in 2D SOTI, QD and terahertz electronics.

cond-mat.mes-hall↗