arXiv subjects

Yugui Yao

Publications and source records attributed to Yugui Yao.

At least 19 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

Quantized Spin Hall Effect in Three-Dimensional Nodal-Ring Semimetal: Geometric Scaling and Symmetry-Engineered Spin Response

The anomalous Hall conductivity in magnetic Weyl semimetals scales linearly with the momentum separation between Weyl nodes, establishing a geometric paradigm for three-dimensional Hall responses. Here we discover an analogous phenomenon in the spin Hall effect: a quantized spin Hall conductivity (SHC) in nodal-ring semimetals that scales linearly with the nodal-ring radius $R$. From an ideal model with a single nodal ring, we derive analytically that the SHC inside the spin-orbit-coupled gap obeys $\sigma_{\alpha \beta}^{S, 3D}=\sigma_0^{S,2D} \cdot (\pi R/2 \pi)$, where $\sigma_0^{S,2D}=(e^2/h) \cdot (\hbar/2 e)$ is the two-dimensional quantum spin Hall conductance. Crucially, the symmetry of the spin-orbit coupling acts as an independent switch: Rashba coupling generates purely conventional SHC components, while Weyl coupling additionally activates unconventional ones, providing separate control over response magnitude and tensor symmetry. We validate this principle in yttrium nitride, where strain tunes $R$ and symmetry breaking toggles between response types. Our work establishes a new paradigm for engineering quantized geometric responses in three dimensions, opening pathways to tailored spin-orbit functionalities.

cond-mat.mtrl-sci↗

arXiv2026

Net and Hidden Spin-Valley Locking Enable Ultrahigh Hole Mobility in Covalent Bulk WN$_2$

High carrier mobility at room temperature underpins high-performance electronics, yet high hole mobility remains rare in bulk semiconductors. Spin-valley locking can suppress intervalley scattering and enhance mobility, but it is limited to materials with broken inversion symmetry. Hidden spin polarization offers a possible route beyond this constraint, although whether its compensated spin textures could protect charge transport remains unclear. Using ab initio electron-phonon and transport calculations, we show that the two hexagonal phases of bulk WN$_2$ realize net and hidden spin-valley locking and exhibit ultrahigh room-temperature hole mobilities. In non-centrosymmetric $\alpha$-WN$_2$, a large valley spin splitting produces net spin-valley locking that nearly eliminates phonon-mediated intervalley scattering. In centrosymmetric $\beta$-WN$_2$, hidden Zeeman-type spin polarization yields a compensated, sector-resolved spin texture that reverses between valleys and suppresses intervalley scattering as effectively as the net locking does. The stiff W-N/N-N covalent network further keeps the remaining intravalley scattering weak. Our results establish hidden spin polarization as an effective transport-protection mechanism and extend spin-valley engineering to centrosymmetric bulk semiconductors.

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

Generalized Space Groups from Internal Configuration Spaces

We develop a unified construction of generalized space groups for crystals with unconventional internal degrees of freedom. Starting from the full group $G_P$ of allowed internal transformations and the stabilizer $P$ of a reference object, we determine the pointwise and setwise symmetries, $J$ and $K$, of the allowed configuration set. Goursat's lemma then couples the internal quotient $K/J$ to a spatial quotient. The framework includes ordinary, magnetic, spin, and color space groups as special cases. As an example, we consider a dodecahedral object with $P=I\simeq A_5$, for which we obtain the nontrivial pair $T\triangleleft O$ with $O/T\simeq\mathbb Z_2$. The resulting generalized space group hosts a point node with topological charge $|C|=12$.

cond-mat.mtrl-sci↗

arXiv2026

3D Topologically Polarized Elastic Metamaterials Enable Asymmetric Energy Isolation at Low Frequencies

Topologically polarized elasticity has been extensively studied in lower-dimensions, yet its three-dimensional (3D) counterpart remains largely unexplored. Here, we demonstrate omnidirectional topological elasticity in 3D structures that incorporate bending stiffness, which elevates zero-frequency topological mechanical states into finite-frequency phononic modes. These modes are localized at a single boundary, creating a pronounced stiffness contrast in both static and finite-frequency dynamic regimes. This three-dimensional structure exhibits highly polarized mechanical behavior across all spatial dimensions, establishing omnidirectional asymmetric topological elasticity. Experimental and numerical results confirm robust, asymmetric energy isolation, arising from the interplay between bulk topological polarization and boundary-localized surface modes. Our findings establish a paradigm for 3D metamaterials, with promising applications in vibration shielding and directional wave manipulation.

cond-mat.soft↗

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

Theory of In-Plane Orbital Magnetization with Layer Hybridization

The modern theory of orbital magnetization successfully describes the response of Bloch electrons to magnetic fields in fully periodic crystals, but it does not directly address the distinct regime of an in-plane field in multilayer systems with layer hybridization. Coherent interlayer tunneling allows electrons to form circulating current loops, producing an in-plane orbital response that is absent in a strictly two-dimensional limit and qualitatively different from the conventional three-dimensional one. Here we develop a theory of in-plane orbital magnetization for this {\it transdimensional} regime, where the layer thickness is comparable to the vertical mean free path. Starting from the current-loop picture, we construct the in-plane orbital angular momentum operator and derive exact expressions for the orbital magnetic moment and the in-plane orbital magnetic susceptibility. As an application, we predict a gate-tunable in-plane orbital magnetoelectric effect in layered materials. Our framework establishes a general foundation for in-plane orbital responses and suggests new opportunities for orbitronics in layer-hybridized quantum materials.

cond-mat.mes-hall↗

arXiv2026

Distinguishing Majorana zero modes from trivial defect states in an iron-based superconductor

Majorana zero modes, which obey non-Abelian exchange statistics, are promising candidates for topological quantum computation due to their robustness against environmental perturbations. The iron-based superconductor Fe(Te,Se) has been identified as an intrinsic topological superconductor, possibly hosting Majorana zero modes. In this paper, we report the observation of near-zero-energy localized states at multiple structural defects on the Fe(Te,Se) surface, which could be misidentified as Majorana zero modes without additional verification. By using spin-polarized scanning tunneling spectroscopy, we demonstrate that the near-zero-energy localized states on step edges and line defects originate from topologically trivial Yu-Shiba-Rusinov states. In addition, zero-energy bound states are also observed for regions without surface defects. A combined spatial and magnetic field dependent analysis of the spin-resolved tunneling spectra in these regions reveals that this type of zero-energy states cannot be attributed to the presence of Majorana bound states. These findings emphasize the importance of spin-dependent studies of low-energy states for pursuing Majorana zero modes.

cond-mat.supr-con↗

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

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

Charge density wave in a band insulator

Charge density wave (CDW) implies a periodic modulation of the charge density. Typically observed in metallic systems, CDWs arise from Fermi surface instabilities, resulting in the total or partial gapping of the Fermi surface. Here, we present experimental evidence for a CDW state emerging in a band insulator which has no Fermi surface. The bulk and surface of our material platform, Bi4Br4, is gapped over the entire Brillouin zone. Through topographic and spectroscopic imaging at low temperatures, we unveil an unexpected unidirectional charge modulation in Bi4Br4, breaking the lattice translation symmetry. The CDW develops at temperatures below 40 K and adds an energy gap atop the existing insulating gap of Bi4Br4. Furthermore, our transport measurements reveal nonlinear electrical conduction, a phenomenon conventionally associated with the sliding or phason mode of incommensurate CDWs. These highly unusual observations represent a new type of CDW and demand a new theoretical framework for CDWs.

cond-mat.str-el↗

arXiv2026

Unconventional Magnetism: Symmetry Classification, Hybrid-parity and Unconstrained-parity Classes

Unconventional magnetism has emerged as a transformative frontier in condensed matter physics. Such phases are characterized by substantial non-relativistic spin splitting (NSS) in symmetry-compensated magnets. They have been classified by the parity of their spin textures under momentum inversion, leading to the paradigms of altermagnets (even-parity) and odd-parity magnets. However, the full symmetry landscape remains largely unexplored. In this Letter, we present a systematic classification framework for unconventional magnetism based on the representation theory of the spin textures and the associated parity properties. Within this framework, we predict two previously unidentified classes beyond the established pure-parity categories: hybrid-parity magnets (HPMs) and unconstrained-parity magnets (UPMs), where the spin textures exhibit contrasting parities among their Cartesian components and the parity of the spin textures is ill-defined, respectively. We derive universal symmetry criteria that categorize HPMs into three distinct types. Importantly, by combining the spin splitting characteristics of altermagnets and odd-parity magnets, HPMs can enable the coexistence of the spin current and Edelstein effects. Taking FePO4 as an example, we perform first-principles calculations to demonstrate this coexistence. Finally, we discuss the potential applications of HPMs in spintronic devices. Our work provides a comprehensive symmetry classification of unconventional magnetism and establishes HPMs as a promising platform for multi-functional spintronics.

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

Nonlinear Hall quantum oscillations to probe topological Brown-Zak fermions in graphene moir\'e systems

Due to the deep connection with the quantum geometry of electronic Bloch wavefunctions, the second-order nonlinear Hall effect (NLHE) has been an attractive topic since its proposal. However, studies on NLHE under a magnetic field have been lacking. Given that quantum oscillations in the linear response regime have been proven to be useful tools in investigating electronic systems, searching for quantum oscillations in NLHE is of great interest and is expected to provide new avenues to unveil rich quantum geometric properties of novel quasiparticles. Here, we propose a new type of NLHE quantum oscillations and experimentally probe it in graphene moir\'e systems. It stems from the alternation of the dominant NLHE mechanisms with recurring Bloch states under magnetic field, which enables sensitive detection of Brown-Zak fermions, giving an onset field as low as 0.5 T. Most importantly, when the commensurability condition is satisfied, the nonlinear transport of Brown-Zak fermions is mainly governed by quantum geometric contributions. Our findings not only establish a new type of quantum oscillations, but also demonstrate the first experimental detection of the topological nature of Brown-Zak fermions, shedding light on the exploration of novel topological quasiparticles.

cond-mat.mes-hall↗

arXiv2026

Theory of quantum decoherence in macroscopic topological insulators

Quantum decoherence-the loss of quantum coherence due to interactions with an environment-plays a central role in quantum transport, and controlling this ubiquitous yet inevitable phenomenon is essential for practical quantum technologies. Despite its importance, the microscopic mechanisms of decoherence in infinite-size topological insulators remain poorly understood. Here, we develop a comprehensive theory that quantitatively investigates how quantum decoherence shapes the quantum spin Hall effect in macroscopic topological insulators, and reveal that decoherence-induced corrections scale quadratically with impurity density. Besides, we uncover a previously unidentified mechanism of the extrinsic spin Hall effect: a second-order skew-scattering process intrinsically tied to quantum decoherence-fundamentally distinct from, yet substantially stronger than, the conventional third-order skew-scattering mechanism. Furthermore, we predict a new scaling law in which the decoherence-induced spin Hall conductivity scales quadratically with the longitudinal conductivity, providing a clear experimental signature of decoherence effects. Our results establish the essential role of decoherence in quantum transport of topological insulators and reveal that macroscopic topological insulators offer a promising platform for next-generation spintronic applications.

cond-mat.mes-hall↗