arXiv subjects

Run-Wu Zhang

Publications and source records attributed to Run-Wu Zhang.

At least 19 recordsLinked to original sources

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

Ridge-Spin-Layer Coupling and Emergent Ridgetronics in 2D Altermagnets

Extending valleytronics from discrete points to continuous lines in momentum space transforms dispersionless bands into a controllable degree of freedom. Here we introduce ridge--spin--layer coupling (RSLC) in two-dimensional (2D) altermagnets, where a one-dimensional continuous line of dispersionless electronic states (a ridge) in momentum space locks to both spin polarization and atomic sublayer. This ridge-induced quenching of kinetic energy mimics flat-band physics, yet crucially, RSLC grants external control, allowing for layer-selective switching of ridge orientation in reciprocal space, spin-filtered transport in real space, and a distinct electric Hall response. Guided by collinear spin layer group symmetry, we identify three 2D candidate materials, namely Mg$_2$Mo$_2$(PO$_5$)$_2$, Ca(FeP)$_2$, and Mg$_2$V$_2$(SO$_5$)$_2$, each featuring a crossed-ridge structure with two ridges, one per spin channel and sublayer. Our work establishes ridgetronics as a controllable platform for direction-discriminating currents, bridging dispersionless bands with multifunctional device operation.

cond-mat.mtrl-sci↗

arXiv2026

Spin layer groups and their corepresentations

Spin layer groups are the crystallographic symmetry groups with a periodic plane, and their symmetry operations are inherited from three-dimensional (3D) spin space groups. However, the direct application of 3D symmetry groups to two-dimensional systems is often inadequate due to anisotropic axes and dimensional reduction. In this work, we systematically classify inequivalent spin layer groups and analytically derive their irreducible corepresentations. This classification establishes a foundational framework for investigating symmetry-protected properties and novel quantum states in low-dimensional magnetic materials.

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↗

arXiv2025

Nonvolatile Electrical Control of Spin via Sliding Fractional Quantum Multiferroics

We propose a fractionally quantized polarization induced by interlayer sliding in bilayer altermagnets, unveiling a previously unrecognized multiferroic phase termed sliding fractional quantum multiferroicity (SFQM). This unconventional magnetic phase uniquely integrates sliding ferroelectricity with fractional quantum ferroelectricity, enabling highly efficient switching and nonvolatile electrical control of spin.~Unlike conventional multiferroics, SFQM simultaneously exhibits lattice-scale atomic displacements, ultralow switching barriers, and spin splitting, giving rise to a large fractionally quantized polarization and strong magnetoelectric coupling. Through symmetry analysis and first-principles calculations, we identify bilayer altermagnet Ca(CoN)$_2$ and its family materials as promising candidates hosting SFQM. In contrast to gate-controlled schemes, the spin-layer coupling in SFQM is intrinsically induced by spontaneous electrical and layer polarization, requiring no sustained gate field and exhibiting nonvolatile character. This mechanism enables nonvolatile electrical control of spin through biaxial sliding, where displacements along the \textit{x}- and \textit{y}-axes generate opposite polarization directions in the layer-dependent electrical polarization. Furthermore, SFQM exhibits a fully switchable anomalous Hall effect and a pronounced magneto-optical response, which can be utilized for its detection and distinction. These findings highlight the promising role of sliding-mediated couplings among unconventional magnetism, fractional quantum ferroelectricity, and stacking order in realizing electrically controllable two-dimensional multiferroics.

cond-mat.mtrl-sci↗

arXiv2025

Switchable Giant Spin Injection Current in Janus Altermagnet Fe$_2$SSeO

Generating and controlling spin current in miniaturized magnetic quantum devices remains a central objective of spintronics, due to its potential to enable future energy-efficient information technologies. Among the existing magnetic phases, altermagnetism have recently emerged as a highly promising platform for spin current generation and control, going beyond ferromagnetism and antiferromagnetism. Here, we propose a symmetry-allowed spin photovoltaic effect in two-dimensional (2D) altermagnetic semiconductors that enables predictable control of giant spin injection currents. Distinct from parity-time ($\mathcal{PT}$)-antiferromagnets, Janus altermagnetic semiconductors generate not only shift current but also a unique injection current with spin momentum locked in a specific direction under linearly polarized light -- a mechanism absent in $\mathcal{PT}$-antiferromagnets. Through symmetry analysis and first-principles calculations, we identify Janus Fe$_2$SSeO as a promising candidate. Specifically, the monolayer Fe$_2$SSeO exhibits a polarization-dependent injection conductivity reaching $\sim$1,200~$\mu$A/V$^{2}\!\cdot\!\hbar/2e$, and the giant spin injection current can be effectively switched by rotating the magnetization direction and engineering strains. These findings underscore the potential of 2D altermagnets in spin photovoltaics and open avenues for innovative quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

Symmetry Classification of Altermagnetism and Emergence of Type-IV Magnetism in Two Dimensions

Two-dimensional (2D) magnetism, particularly 2D altermagnetism (AM), has attracted considerable interest due to its exceptional physical properties and broad application potential. However, the classification of AM undergoes a fundamental paradigm shift when transitioning from three-dimensional (3D) to 2D symmetry-enforced fully compensated collinear magnetism$-$a shift that has remained largely overlooked. Here, by extending unconventional magnetism to 2D collinear systems, we identify the symmetry conditions and electronic band characteristics of a distinct magnetic phase: type-IV magnetism. This new class lies beyond the established descriptions of ferromagnetism, conventional antiferromagnetism, and AM. Type-IV magnetism supports the successive emergence of both nonrelativistic spin-degenerate and relativistic spin-splitting phenomena, belonging strictly to neither conventional antiferromagnetism nor standard AM. We further establish a universal symmetry classification framework for 2D type-IV magnets via a mapping from the collinear spin layer group to the magnetic layer group. Monolayer MgCr$_2$O$_3$ and monolayer BaMn$_2$Ch$_3$ (Ch=Se, Te) are showcased as representative materials, exhibiting gate-tunable reversible spin textures and the quantum electric Hall effect, respectively. Our work underscores the rich functional prospects of type-IV magnets, offering a new route toward spin manipulation and anomalous transport that promises innovative designs for high-performance spintronic devices.

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↗

arXiv2025

Type-III Weyl Semi-Half-Metal in an Ultralight Monolayer Li$_2$N

The interplay between magnetic ordering and band topology has emerged as a fertile ground for discovering novel quantum states with profound implications for fundamental physics and next-generation electronics. Here, we theoretically predict a new type-III Weyl semi-half-metal (SHM) state in monolayer Li$_2$N, uniquely combining magnetic half-metallicity and type-III Weyl semimetal characteristics. First-principles calculations reveal a fully spin-polarized and critically tilted Weyl cone around the Fermi level in monolayer Li$_2$N, driven by $p$-orbital ferromagnetism. This arises from the symmetry-protected band crossing between a flat valence band and a highly dispersive conduction band, leading to type-III Weyl fermions with strong transport anisotropy. A low-energy $k{\cdot}p$ Hamiltonian is constructed and corresponding nontrivial edge states are uncovered to capture the topological nature of Li$_2$N. Notably, this Weyl SHM phase remains robust under biaxial strain ranging from -2$\%$ to $4\%$, with an ideal type-III Weyl fermion emerging alongside a line-like ergodic surface emerging at 3.7$\%$ strain, offering a promising platform for exploring correlated electronic phenomena. Our results establish Li$_2$N as a viable candidate for realizing exotic type-III Weyl SHM states and open a new avenue for exploring the intricate interplay among magnetism, topology, and flat-band physics.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Hall Effect in Type IV 2D Collinear Magnets

We identify a previously unrecognized class of two-dimensional (2D) collinear magnetic phase that extends beyond the established categories of ferromagnets, antiferromagnets, and altermagnets. These type IV 2D collinear magnets exhibit spin-degenerate bands in the nonrelativistic limit, yet support time-reversal symmetry-breaking responses, such as the anomalous Hall effect (AHE), despite having zero net magnetization. Based on spin layer group analysis, we derive the symmetry criteria for this phase and perform first-principles calculations to screen viable candidate materials from 2D databases. Using monolayer Hf2S as a prototype, we demonstrate that in the absence of spin-orbit coupling, the bands are spin degenerate, while its inclusion induce an AHE driven by spin-polarized and even spin-neutral currents, accompanied by a symmetry-protected, truly full-space persistent spin texture. These findings expand the classification of magnetic phases and broaden avenues for realizing unconventional spintronic functionalities in two dimensions.

cond-mat.mes-hall↗

arXiv2025

Quantized Spin-Hall Conductivity in Altermagnet Fe$_2$Te$_2$O with Mirror-Spin Coupling

Due to spin-orbit coupling (SOC), crucial for the quantum spin Hall (QSH) effect, a quantized spin-Hall conductivity has not yet been reported in QSH insulators and other realistic materials. Here, we tackle this challenge by predicting robust quantized spin-Hall conductivity in monolayer Fe$_2$Te$_2$O. The underlying physics originates from the unrecognized mirror-spin coupling (MSC), which couples spin-up and spin-down states into two orthogonal mirror eigenstates. We show that the MSC can naturally emerge in the two-dimensional altermagnets with out-of-plane N\'eel vector and horizontal mirror. A remarkable consequence of the MSC is that it can dramatically weaken the spin hybridization of the altermagnetic materials when SOC is included. When SOC is neglected, Fe$_2$Te$_2$O is an altermagnetic Weyl semimetal with MSC. With SOC, it evolves into the first material candidate for magnetic mirror Chern insulator. Remarkably, under the protection of MSC, the spin hybridization of both bulk and topological edge states in Fe$_2$Te$_2$O with SOC at low energy is negligible. As a consequence, a quantized spin-Hall conductivity emerges within the bulk band gap of the system. By unveiling a novel effect, our findings represent a significant advancement in spin Hall transport, and broaden the material candidates hosting intriguing altermagnetic phenomena.

cond-mat.mes-hall↗

arXiv2024

A Theory of Anisotropic Magnetoresistance in Altermagnets and Its Applications

Altermagnets, a newly discovered class of magnets, integrate the advantages of both ferromagnets and antiferromagnets, such as enabling anomalous transport without stray fields and supporting ultrafast spin dynamics, offering exciting opportunities for spintronics. A key challenge in altermagnetic spintronics is the efficient reading and writing of information by switching the Neel vector orientations to represent binary 0 and 1. Here, we develop a microscopic theory of the magnetoresistance effect in altermagnets and propose that magnetoresistance anisotropy can serve as an effective mechanism for the electrical readout of the Neel vector. Our theory describes a two-step charge-spin-charge conversion process governed by the interplay between spin splitting and spin Hall effects: a longitudinal electric field induces transverse drift spin currents, which induce significant spin accumulation at the boundaries, generating a diffusive spin current that is converted back into a longitudinal charge current. By switching the Neel vector, a substantial change in magnetoresistance, akin to giant magnetoresistance in ferromagnets, is realized, enabling an electrically readable altermagnetic memory. Our microscopic theory provides deeper insights into the fundamental physics of the magnetoresistance effect in altermagnets and offers valuable guidance for designing next-generation ultradense and ultrafast spintronic devices based on altermagnetism.

cond-mat.mes-hall↗

arXiv2024

Planar Hall Plateau in Magnetic Weyl Semimetals

Despite the rapid progress in the study of planar Hall effect (PHE) in recent years, all the previous works only showed that the PHE is connected to local geometric quantities, such as Berry curvature. Here, for the first time, we point out that the PHE in magnetic Weyl semimetals is directly related to a global quantity, namely, the Chern number of the Weyl point. This leads to a remarkable consequence that the PHE observation predicted here is robust against many system details, including the Fermi energy. The main difference between non-magnetic and magnetic Weyl points is that the latter breaks time-reversal symmetry T, thus generally possessing an energy tilt. Via semiclassical Boltzmann theory, we investigate the PHE in generic magnetic Weyl models with energy tilt and arbitrary Chern number. We find that by aligning the magnetic and electric fields in the same direction, the trace of the PHE conductivity contributed from Berry curvature and orbital moment is proportional to the Chern number and the energy tilt of the Weyl points, resulting in previously undiscovered quantized PHE plateau by varying Fermi energy. We further confirm the existence of PHE plateaus in a more realistic lattice model without T symmetry. By proposing a new quantized physical quantity, our work not only provides a new tool for extracting the topological character of the Weyl points but also suggests that the interplay between topology and magnetism can give rise to intriguing physics.

cond-mat.mes-hall↗

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↗

arXiv2024

Mirror real Chern insulator in two and three dimensions

A real Chern insulator (RCI) featuring a real Chern number and a second-order boundary mode appears in a two-dimensional (2D) system with the space-time inversion symmetry (PT ). Here, we propose a kind of RCI: mirror real Chern insulator (MRCI) which emerges from the system having additional horizontal mirror symmetry Mz. The MRCI generally is characterized by two independent real Chern numbers, respectively defined in the two mirror subsystems of the system. Hence, the MRCI may host the second-order boundary modes different from the conventional RCI. We show that for spinless systems, the definition of the MRCI is straightforward, as PT keeps each mirror subsystem invariant. For the spinful systems with both PT and Mz, the real Chern number for the total system remain well defined, as MzPT = C2zT , and (C2zT )2= 1. However, since C2zT exchanges the two mirror subsystems, the definition of the MRCI in spinful systems requires the help of projective symmetry algebra. We also discuss the MRCIs in 3D systems, where the MRCI is defined on certain mirror-invariant 2D planes. Compared with its 2D counterpart, the 3D MRCI can exhibit more abundant physics when the systems have additional nonsymmorphic operators. Several concrete MRCI models including 2D and 3D, spinless and spinful models are constructed to further demonstrate our ideas.

cond-mat.mtrl-sci↗

arXiv2024

Quasi-one-dimensional spin transport in altermagnetic $Z^3$ nodal net metals

In three dimensions, quasi-one-dimensional (Q1D) transport has traditionally been associated with systems featuring a Q1D chain structure. Here, based on first-principle calculations, we go beyond this understanding to show that the Q1D transport can also be realized in certain three-dimensional (3D) altermagnetic (AM) metals with a topological nodal net in momentum space but lacking Q1D chain structure in real space, including the existing compounds $\beta$-Fe$_2$(PO$_4$)O, Co$_2$(PO$_4$)O, and LiTi$_2$O$_4$. These materials exhibit an AM ground state and feature an ideal crossed $Z^3$ Weyl nodal line in each spin channel around Fermi level, formed by three straight and flat nodal lines traversing the entire Brillouin zone. These nodal lines eventually lead to an AM $Z^3$ nodal net. Surprisingly, the electronic conductivity $\sigma_{xx}$ in these topological nodal net metals is dozens of times larger than $\sigma_{yy}$ and $\sigma_{zz}$ in the up-spin channel, while $\sigma_{yy}$ dominates transport in the down-spin channel. This suggests a distinctive Q1D transport signature in each spin channel, and the principal moving directions for the two spin channels are orthogonal, resulting in Q1D direction-dependent spin transport. This novel phenomenon cannot be found in both conventional 3D bulk materials and Q1D chain materials. In particular, the Q1D spin transport gradually disappears as the Fermi energy moves away from the nodal net, further confirming its topological origin. Our work not only enhances the comprehension of topological physics in altermagnets but also opens a new direction for the exploration of topological spintronics.

cond-mat.mtrl-sci↗

arXiv2024

Crystal Thermal Transport in Altermagnetic RuO2

We demonstrate the emergence of a pronounced thermal transport in the recently discovered class of magnetic materials-altermagnets. From symmetry arguments and first-principles calculations performed for the showcase altermagnet, RuO2, we uncover that crystal Nernst and crystal thermal Hall effects in this material are very large and strongly anisotropic with respect to the Neel vector. We find the large crystal thermal transport to originate from three sources of Berry's curvature in momentum space: the Weyl fermions due to crossings between well-separated bands, the strong spin-flip pseudonodal surfaces, and the weak spin-flip ladder transitions, defined by transitions among very weakly spin-split states of similar dispersion crossing the Fermi surface. Moreover, we reveal that the anomalous thermal and electrical transport coefficients in RuO2 are linked by an extended Wiedemann-Franz law in a temperature range much wider than expected for conventional magnets. Our results suggest that altermagnets may assume a leading role in realizing concepts in spin caloritronics not achievable with ferromagnets or antiferromagnets.

cond-mat.mtrl-sci↗