arXiv subjects

Cheng-Cheng Liu

Publications and source records attributed to Cheng-Cheng Liu.

At least 19 recordsLinked to original sources

arXiv2026

Complete Hierarchy of Nonrelativistic Odd-Parity Spin Splitting in Collinear Magnets

Momentum-dependent nonrelativistic spin splitting provides a symmetry fingerprint of collinear magnets and can govern unconventional electronic, magnonic, and transport phenomena. Whereas even-parity $s$-, $d$-, $g$-, and $i$-wave splittings in collinear magnets have been extensively studied, odd-parity counterparts remain unexplored beyond the $p$-wave and $f$-wave classes. Here, using group theory, we establish the complete classification of odd-parity spin splitting in collinear magnets. We show that, in addition to the $p$-wave and $f$-wave forms, $h$- and $k$-wave splittings with $\ell=5$ and $7$ are allowed, while $m$-wave splitting with $\ell=9$ constitutes the upper bound. We derive a complete mapping from crystallographic point-group irreducible representations to the lowest-order odd-parity basis functions and formulate the coupling rule between a symmetry-breaking axial field and the parent N\'eel order that selects the induced odd-parity class. We further construct minimal lattice models that realize $h$-, $k$-, and $m$-wave splitting. Guided by this classification, we screen the MAGNDATA database and show that circularly polarized light can drive the $\mathcal{PT}$-symmetric antiferromagnets Fe$_2$TeO$_6$ and MgFe$_6$Ge$_6$ into $h$-wave and $k$-wave phases, respectively, exhibiting the hallmark spin splittings in both electronic bands and magnon spectra. Symmetry analysis and Berry-curvature calculations show that collinear odd-parity magnets of both $h$- and $k$-wave allow an anomalous Hall response, whereas the $m$-wave class forbids it. Together, these results complete the partial-wave hierarchy of odd-parity spin splitting in collinear magnets and establish symmetry criteria for anomalous transport in the high-partial-wave classes.

cond-mat.mtrl-sci↗

arXiv2026

Odd-Parity Magnons

Magnons, as charge-neutral spin excitations, can transport spin information without Joule heating and therefore offer a promising platform for low-power spintronics. However, in collinear magnets, the effective time-reversal symmetry forbids odd-parity magnon band splitting. Here we propose odd-parity magnons and establish a general mechanism for realizing them in collinear antiferromagnets. We provide a complete spin-point-group classification of odd-parity magnon splitting in two-dimensional collinear antiferromagnets by identifying the leading splitting types and their symmetry-allowed basis functions. This classification serves as a practical guide for searching for odd-parity magnons. We show that breaking effective time-reversal symmetry, for example by circularly polarized light or loop currents, can induce highly tunable $p$- and $f$-wave magnon splitting. In bilayer systems, the dynamical modulation can drive a topological magnon phase transition, accompanied by chiral edge modes and an abrupt jump in the magnon thermal Hall conductivity. Material-specific first-principles calculations further demonstrate the feasibility of this mechanism in real van der Waals antiferromagnets. Our study identifies the odd-parity magnons as a new class of spin excitations and provides a theoretical foundation for odd-parity magnons and ultrafast optically controlled topological magnonic devices.

cond-mat.mtrl-sci↗

arXiv2026

The Effect of Planetary Rotation Period on Clouds in a Global Climate Model with a Bin Microphysics Scheme

Clouds are the largest source of uncertainty in climate simulations. For exoplanets, cloud simulation is particularly challenging because of the lack of observational data to tune parameterized cloud models. Here we apply Community Aerosol and Radiation Model for Atmospheres (CARMA), a size-resolved bin cloud microphysics model, to the atmospheric global climate model Community Atmosphere Model (CAM6) and simulate exoplanets with a range of planetary rotation rates. CARMA produces fewer liquid clouds than the native CAM6 parameterized cloud microphysics scheme (Morrison-Gettelman two-moment microphysics, MG), more ice clouds, and a significantly different ice cloud size distribution. Overall, this leads to a decrease in the magnitude of the net CRE by 4-10 $W/m^2$, which is unlikely to change the determination of habitability from a climate perspective in most cases. The difference in ice cloud size distribution is likely to strongly affect transmission spectral retrievals. Our work confirms that the MG parameterized cloud microphysics scheme can produce reasonable climate simulation when extrapolated to some exoplanet contexts and highlights the value of resolved cloud microphysics for evaluating parameterized schemes and for interpreting observations.

astro-ph.EP↗

arXiv2026

Spontaneous Fully Compensated Ferrimagnetism

We propose a general mechanism for the spontaneous emergence of filling-enforced fully compensated ferrimagnetism (fFIM), characterized by zero net magnetization yet ferromagnetic-like spin-split band structures. Using Hartree-Fock mean-field calculations of the Hubbard model, we map out the stability regime of spontaneous fFIM over a broad parameter space of interaction strength and staggered potential. We show the unique quantum-geometry-governed optical selection rules and the abundant valley- and spin-related physics of electronics and optics arising from the emergence of fFIM order, with tunable spin-polarized and valley-contrasting charge and spin currents. Furthermore, based on our theory, we demonstrate that spontaneous fFIM can be realized in nominally nonmagnetic graphene via defect engineering. Our results establish a unified framework for the mechanism, emergent properties, and materials realization of spontaneous fFIM, opening new opportunities for spintronic, valleytronic, and optoelectronic applications.

cond-mat.str-el↗

arXiv2026

Antiferroaxial altermagnetism

The antiferroaxial state is emerging as an important ferroic order in condensed matter systems. Here, we establish antiferroaxial altermagnetism as a broadly prevalent, generic, and microscopically grounded multiferroic mechanism, in which antiferroaxial counter-rotating distortions both induce altermagnetism and enable its deterministic and reversible switching. Within a unified Landau-theory and symmetry framework, we identify a symmetry-allowed trilinear invariant coupling the antiferroaxial order, the N\'{e}el vector, and the altermagnetic order, and derive general symmetry criteria for its occurrence. This coupling locks the induced altermagnetism to the antiferroaxial order, so that reversing the latter reverses the spin splitting and associated time-reversal-odd responses, such as anomalous Hall conductivity. We provide a practical spin group dictionary mapping N\'{e}el-vector representations to the resulting $d$-, $g$-, and $i$-wave antiferroaxial altermagnetism, validate the mechanism with ligand-rotation tight-binding models and first-principles calculations, and identify many candidate materials by screening the MAGNDATA and C2DB databases. Our results elevate antiferroaxiality to a universal ferroic control knob for structurally programmable altermagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2026

MoireStudio: A Universal Twisted Electronic Structure Calculation Package

Twistronics is an emerging and captivating field in condensed matter physics and material science. However, accurately and efficiently calculating the electronic structures of twisted systems remains a significant challenge. To address this, we have developed MoireStudio, a universal Python-based computational package for twisted electronic structures. Its functionalities include commensurate structure search, structure generation, parameterization, and construction for tight-binding models and continuum models, and the precise incorporation of full relaxation effects. The package is applicable to arbitrary combinations of two-dimensional materials, including rectangular lattices and heterostructures. User-friendly and easy to use, MoireStudio supports parallel large-scale computations, provides visualization capabilities, and offers interfaces with third-party software. It is poised to become a convenient and powerful tool for researchers in twistronics fields.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional Magneto-Optical Effects in Altermagnets

The ideal altermagnets are a class of collinear, crystal-symmetry-enforced fully compensated magnets with nonrelativistic spin-split bands, in which contributions from Berry curvature to magneto-optical effects (MOEs) are strictly forbidden by an effective time-reversal symmetry. Here we show that, in such systems, MOEs are exclusively induced by the quantum metric and, in realistic altermagnets, are typically dominated by it. We refer to Berry-curvature-induced MOEs as conventional MOEs and to quantum-metric-dominated MOEs as unconventional MOEs. We derive general formulas that incorporate both Berry curvature and quantum metric for unconventional MOEs in altermagnets, enabling a quantitative evaluation of their respective contributions. Through symmetry analysis, we prove that ideal altermagnets are constrained to exhibit only unconventional MOEs. Using the three-dimensional canonical altermagnet MnTe and the emerging two-dimensional bilayer twisted altermagnet CrSBr as illustrative examples, we demonstrate that unconventional MOEs are prevalent in altermagnets. Our results establish altermagnets as a natural platform for quantum-metric-driven optical phenomena, substantially broadening the scope of MOEs and providing concrete predictions that can be tested in future experimental studies.

cond-mat.mtrl-sci↗

arXiv2025

Relaxation and Its Effects on Electronic Structure in Twisted Systems: An Analytical Perspective

Lattice relaxation profoundly reshapes electronic structures in twisted materials. Prevailing treatments, however, typically rely on large-scale density functional theory (DFT), which is computationally costly and mechanistically opaque. Here, we develop a unified analytical framework to overcome these limitations. From continuum elastic theory, we derive closed-form solutions for both in-plane and out-of-plane relaxation fields. We further introduce an analytical phase factor expansion theory that maps relaxation into the electronic Hamiltonian. By applying this framework, the relaxation-mediated single-particle and many-body topological phase transitions in twisted MoTe$_{2}$ is accurately captured, and the evolution of flat bands in magic-angle graphene is quantitatively reproduced. Our work transforms the research of moir\'e relaxation from black-box numerical fitting to an analytical paradigm, offering fundamental insights, exceptional efficiency, and general applicability to a wide range of twisted materials.

cond-mat.mtrl-sci↗

arXiv2025

Quantum metric-based optical selection rules

The optical selection rules dictate symmetry-allowed/forbidden transitions, playing a decisive role in engineering exciton quantum states and designing optoelectronic devices. While both the real (quantum metric) and imaginary (Berry curvature) parts of quantum geometry contribute to optical transitions, the conventional theory of optical selection rules in solids incorporates only Berry curvature. Here, we propose quantum metric-based optical selection rules. We unveil a universal quantum metric-oscillator strength correspondence for linear polarization of light and establish valley-contrasted optical selection rules that lock orthogonal linear polarizations to distinct valleys. Tight-binding and first-principles calculations confirm our theory in two models (altermagnet and Kane-Mele) and monolayer $d$-wave altermagnet $\mathrm{V_2SeSO}$. This work provides a quantum metric paradigm for valley-based spintronic and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Tunable Topological Superconductivity by Fully Compensated Ferrimagnets

We propose a platform based on a fully compensated ferrimagnet (fFIM) for realizing and controlling topological superconductivity with Majorana bound states across multiple dimensions. Through symmetry analysis and microscopic modeling, we demonstrate that fFIM-based heterostructures host (i) Majorana zero modes localized at the ends of one-dimensional nanowires, (ii) chiral Majorana edge states along two-dimensional boundaries, and (iii) tunable Majorana corner modes in higher-order topological phases. The unique properties of fFIMs enable an electric field to drive topological superconductivity phase transitions and N\'eel vector orientation to control the spatial distribution of Majorana modes, without external magnetic fields. Crucially, the absence of net magnetization in fFIM-based heterostructures preserves superconductivity, circumventing the usual trade-off between tunability and superconducting coherence in magnetized systems. Our results establish fFIM-based heterostructures as a versatile platform for tunable topological superconductivity.

cond-mat.supr-con↗

arXiv2025

Pressure-Tunable Generalized Wigner Crystal and Fractional Chern Insulator in twisted MoTe$_2$

Due to the forming of low-energy flat bands, the moir\'e superlattices of the transition metal dichalcogenides are fascinating platforms for studying novel correlated states when such flat bands are fractionally filled, with the Coulomb interaction dominating. Here, we demonstrate that pressure can efficiently tune the flatness and quantum geometry of the single-particle bands in twisted bilayer MoTe$_2$ ($\textit{t}$MoTe$_2$). By fractionally filling the topmost valence band, we find that pressure can act as a flexible means to modulate the fractional Chern insulator (FCI) and the generalized Wigner crystal (GWC) and control their many-body topological phase transitions. Moreover, our results indicate a remarkable correspondence between the single-particle band geometry and the formation of FCI and GWC. As the recent experiments report the presence of FCI phases in $\textit{t}$MoTe$_2$, our predictions could be readily implemented experimentally.

cond-mat.mes-hall↗

arXiv2025

Quantum metric-induced generalized magneto-optical effects in $\mathcal{PT}$-symmetric antiferromagnets

The magneto-optical effects (MOEs), as fundamental physical phenomena, can reveal the electronic structures of materials. The related probing methods are widely used in the study of magnetic materials. The conventional MOEs are understood to arise from the Berry curvature (the imaginary part of the quantum geometry). Within the framework of conventional MOEs, space-time inversion ($\mathcal{PT}$) symmetric antiferromagnets are magneto-optically inactive. Here, we propose quantum metric (the real part of the quantum geometry) induced generalized MOEs and build generic formulas with quantum metric for Kerr and Faraday angles in three-dimensional and two-dimensional $\mathcal{PT}$-symmetric antiferromagnets. Combining the tight-binding model and first-principles calculations, we demonstrate the quantum metric-induced generalized MOEs in the $\mathcal{PT}$-symmetric antiferromagnets. Our theory broadens the research on MOEs and also provides a microscopic understanding of experimentally observed Kerr rotations in $\mathcal{PT}$-symmetric antiferromagnets. Our theory overcomes the zero-net-moment limitation preventing (conventional) MOEs from detecting magnetic phase transitions and spin orderings in $\mathcal{PT}$-symmetric antiferromagnets -- enabling non-destructive spin-state tomography in $\mathcal{PT}$-symmetric antiferromagnets and creating new quantum metric-based pathways toward ultrafast magneto-optical applications, such as memories and sensors.

cond-mat.mtrl-sci↗

arXiv2025

Two-dimensional fully-compensated Ferrimagnetism

Antiferromagnetic spintronics has long been a subject of intense research interest, and the recent introduction of altermagnetism has further ignited enthusiasm in the field. However, fully-compensated ferrimagnetism, which exhibits band spin splitting but zero net magnetization, has yet to receive enough attention. Since the experimental preparation of two-dimensional (2D) magnetic van der Waals (vdW) materials in 2017, 2D magnetic materials, thanks to their super tunability, have quickly become an important playground for spintronics. Here, we extend the concept of fully-compensated ferrimagnetism (fFIM) to two dimensions and propose 2D \textit{filling-enforced} fFIM, demonstrate its stability and ease of manipulation, and present three feasible realization schemes with respective exemplary candidate materials. A simple model for 2D fully-compensated ferrimagnets (fFIMs) is developed. Further investigation of 2D fFIMs' physical properties reveals that they not only exhibit significant magneto-optical response but also show fully spin-polarized currents and the anomalous Hall effect in the half-metallic states, displaying characteristics previously almost exclusive to ferromagnetic materials, greatly broadening the research and application prospects of spintronic materials.

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

General Electronic Structure Calculation Method for Twisted Systems

In recent years, two-dimensional twisted systems have gained increasing attention. However, the calculation of electronic structures in twisted material has remained a challenge. To address this, we have developed a general computational methodology that can generate twisted geometries starting from monolayer structure and obtain the precisely relaxed twisted structure through a machine learning-based method. Then the electronic structure properties of the twisted material are calculated using tight-Binding (TB) and continuum model methods, thus the entire process requires minimal computational resources. In this paper, we first introduce the theoretical methods for generating twisted structures and computing their electronic properties. We then provide calculations and brief analyses of the electronic structure properties for several typical two-dimensional materials with different characteristics. This work serves as a solid foundation for researchers interested in studying twisted systems.

cond-mat.mes-hall↗

arXiv2024

Valley polarization in twisted altermagnetism

The combination of altermagnetism, twistronics and valleytronics is of great significance for potential applications in advanced electronic devices. Twisted magnetic van der Waals bilayers have been identified as an ideal platform for altermagnetism of any type, such as $d$-wave, $g$-wave, and $i$-wave, by choosing the constituent monolayer with specific symmetry [arXiv:2404.17146 (2024)]. Here, we propose a way for achieving valley polarization in twisted altermagnetism by applying out-of-plane external electric field. Since the out-of-plane electric field creates a layer-dependent electrostatic potential, the valleys form different layers will stagger, producing valley polarization. We also demonstrate the effectiveness of our proposed way using the twisted tight-binding model. It is found that the applied electric field can also induce valley/spin-gapless semiconductor and half metal besides valley polarization. Based on first-principles calculations, our proposed way to achieve valley polarization can be verified in twisted bilayer VOBr and monolayer $\mathrm{Ca(CoN)_2}$ as a special twisted altermagnet. These findings provide new opportunities for innovative spintronics, twistronics and valleytronics applications.

cond-mat.mtrl-sci↗

arXiv2024

Twisted Magnetic Van der Waals Bilayers: An Ideal Platform for Altermagnetism

We introduce a universal methodology for generating and manipulating altermagnetism in two-dimensional (2D) magnetic van der Waals (MvdW) materials through twisting. We find that a key in-plane 2-fold rotational operation can be achieved in a twisted bilayer of any 2D MvdW material, which takes one of all five 2D Bravais lattices, thereby inducing altermagnetism. By choosing the constituent MvdW monolayer with specific symmetry, our approach can tailor altermagnetism of any type, such as $d$-wave, $g$-wave, and $i$-wave. Furthermore, the properties of our twisted altermagnetic materials can be easily engineered. Taking a transition-metal oxyhalide VOBr as an example, we find that by tuning the twist angle and Fermi level a giant spin Hall angle can be obtained, much larger than the experimentally reported. This approach establishes a general, robust, and adjustable platform to explore altermagnetism, and provides a new efficient way to generate and manipulate the spin current.

cond-mat.mtrl-sci↗

arXiv2024

Creation and Manipulation of Higher-Order Topological States by Altermagnets

We propose to implement tunable higher-order topological states in a heterojunction consisting of a two-dimensional (2D) topological insulator and the recently discovered altermagnets, whose unique spin-polarization in both real and reciprocal space and null magnetization are in contrast to conventional ferromagnets and antiferromagnets. Based on symmetry analysis and effective edge theory, we show that the special spin splitting in altermagnets with different symmetries, such as $d$-wave, can introduce Dirac mass terms with opposite signs on the adjacent boundaries of the topological insulator, resulting in the higher-order topological state with mass-domain bound corner states. Moreover, by adjusting the direction of the N\'{e}el vector, we can manipulate such topological corner states by moving their positions. By first-principles calculations, taking a 2D topological insulator bismuthene with a square lattice on an altermagnet MnF$_2$ as an example, we demonstrate the feasibility of creating and manipulating the higher-order topological states through altermagnets. Finally, we discuss the experimental implementation and detection of the tunable topological corner states, as well as the potential non-Abelian braiding of the Dirac corner fermions.

cond-mat.mes-hall↗