arXiv subjects

Yuanchang Li

Publications and source records attributed to Yuanchang Li.

At least 19 recordsLinked to original sources

arXiv2026

Hyperspin Altermagnets

The behavior of spin quantum in k-space is key to identifying altermagnets (AMs) as the third kind of fundamental collinear magnetism. In contrast, non-collinear magnets,though abundant in nature,lack well-defined spin quantum numbers, and the resulting spin textures are often highly complex, which limits their potential for next-generation spintronic applications. Here we propose hyperspin, which lives in a higher-dimensional space, to address these drawbacks. Through analyzing the commutation relations between spin and Hamiltonian for a class of non-collinear magnets, we reveal it is a hyperspin, rather than the usual spin, that commutes with Hamiltonian. Unexpectedly, these non-collinear magnets should also show collinear spin-split bands in k-space like collinear AMs. We therefore classify such non-collinear magnets as hyperspin altermagnets (HAMs), as opposed to the usual collinear AMs. Our theory elucidates the fundamental physics of AMs and HAMs and provides a framework for exploring the wide range of non-collinear magnets that may possess other kinds of conserved quantities.

cond-mat.mtrl-sci↗

arXiv2026

Minimizing propagated density errors of atomic core-electron for simultaneously accurate bandgaps and lattice constants in closed-shell Copper semiconductors

Density functional theory struggles to accurately determine electron density of atoms, whose error is inevitably encoded into the pseudopotential and propagated into solid-state calculations. However, little is known about how this affects accuracy nor how to remedy it. In this work, through a systematic study of the effect of Cu atomic density on bandgap and lattice constants of over 50 Cu-containing simple closed-shell semiconductors, we find that core-electron density can drastically affect nuclear attraction to valence electrons and subsequent charge distribution and energy position of Cu 3$d$ electrons. The error can be eliminated at its source by employing modified Hartree-Fock pseudopotentials for Cu core while retaining (semi-)local functionals for valence electrons. This real-space partitioning approach leads to simultaneous high-accuracy in bandgap and lattice constants across the entire material class.

cond-mat.mtrl-sci↗

arXiv2025

Magnetoelectric effect in multiferroic metals via a direct spin-charge interaction

Much is known about the magnetoelectric effect of multiferroic insulators, yet little is understood about multiferroic metals. In this work, we propose a stacking engineering strategy based on monolayer magnets to construct multiferroic metals, with validation through first-principles calculations on six experimentally synthesized materials. Such multiferroic metals exhibit predominantly linear magnetoelectric response, originating from direct spin-charge interactions as a result of external field-modulated Fermi energy. This fundamentally differs from spin-charge-lattice or spin-orbit coupling mechanisms in multiferroic insulators, offering application advantages in the field of high-speed response. We derive a universal formula for understanding the magnetoelectric coupling in these multiferroic metals. Our work provides insights for exploring magnetoelectric coupling mechanisms and designing functional materials with strong magnetoelectric coupling.

cond-mat.mtrl-sci↗

arXiv2025

Pseudopotentials, an overlooked source and remedy of DFT errors

First-principles calculations rely heavily on pseudopotentials, yet their impact on accuracy is hardly addressed. In this work, we show that most pseudopotentials to date introduce errors, which manifest themselves as errors of atomic energy levels, leading to a $de facto$ deviation from the Hohenberg-Kohn theorem. We consider the atomic-level adjusted pseudopotentials, whose interplay with exchange-correlation functional provides a pragmatic correction that balances accuracy and efficiency. We benchmark our theory with bandgap calculation for 54 semiconductors containing monovalent Cu. The results, compared to those from conventional studies, not only remove all erroneous metal predictions for 11 compounds, but also reduce the mean relative error from 80\% to 20\%. Overall accuracy even exceeds those of standard hybrid functionals and GW methods.

cond-mat.mtrl-sci↗

arXiv2025

Are there type-III multiferroics?

Multiferroics are known to be classified into two types. However, type-I lacks sufficient magnetoelectric coupling and type-II lacks sufficient electric polarization, making both practically difficult. In this work, we explore the possibility of type-III multiferroics, where the origins of ferroelectricity and magnetism are highly intertwined but not causally related, with a combination of strong magnetoelectric coupling and large polarization. Our first-principles calculations predict that monolayer TiCdO$_{4}$ is such a type-III ferroelectric-ferromagnetic multiferroics with both electronic and magnetic orders originating from competing electron populations on oxygen atoms. It shows an electric polarization of 50 $\mu$C/m$^{2}$ while the maximum linear and quadratic magnetoelectric response are as high as 35000 ps/m and 1.59 $\times$ 10$^{-14}$ s/A, respectively. Our study opens up new perspectives for the discovery and design of much-anticipated multiferroics that can be used for cross-modulation.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and Optical Properties of Ta$_2$NiSe$_5$ Monolayer: A First-principles Study

The crystal structure, stability, electronic and optical properties of the Ta$_2$NiSe$_5$ monolayer have been investigated using first-principles calculations in combination with the Bethe-Salpeter equation. The results show that it is feasible to directly exfoliate a Ta$_2$NiSe$_5$ monolayer from the low-temperature monoclinic phase. The monolayer is stable and behaves as a normal narrow-gap semiconductor with neither spontaneous excitons nor non-trivial topology. Despite the quasi-particle and optical gaps of only 266 and 200 meV, respectively, its optically-active exciton has a binding energy up to 66 meV and can exist at room temperature. This makes it valuable for applications in infrared photodetection, especially its inherent in-plane anisotropy adds to its value in polarization sensing. It is also found that the inclusion of spin-orbit coupling is theoretically necessary to properly elucidate the optical and excitonic properties of monolayer.

cond-mat.mtrl-sci↗

arXiv2024

Identifying topological excitonic insulators via bulk-edge correspondence

Excitonic insulator remains elusive and there has been a lack of reliable identification methods. In this work, we demonstrate the promise of topological excitonic insulators for identification due to their unique bulk-edge correspondence, as illustrated by the LiFe$X$ ($X$ = S, Se, and Te) family. First-principles Bethe-Salpeter equation calculations reveal excitonic instabilities in these spin-orbit coupling quantum anomalous Hall insulators. Effective Hamiltonian analyses indicate that spontaneous exciton condensation does not disrupt the gapless edge state but reconstructs the bulk-gap to be almost independent of the spin-orbit coupling strength. This change in the bulk-edge correspondence can be experimentally inspected by angle-resolved photoelectron spectroscopy or electron compressibility measurements, providing observational evidence for the identification of topological excitonic insulators. Moreover, exciton condensation raises the critical temperature of the topological nontrivial phase above room temperature.

cond-mat.str-el↗

arXiv2024

One-Dimensional Magnetic Excitonic Insulators

Dimensionality significantly affects exciton production and condensation. Despite the report of excitonic instability in one-dimensional materials, it remains unclear whether these spontaneously produced excitons can form Bose-Einstein condensates. In this work, we first prove statistically that one-dimensional condensation exists when the spontaneously generated excitons are thought of as an ideal neutral Bose gas, which is quite different from the inability of free bosons to condense. We then derive a general expression for the critical temperature in different dimensions and find that the critical temperature increases with decreasing dimension. We finally predict by first-principles $GW$-BSE calculations that experimentally accessible single-chain staircase Scandocene and Chromocene wires are an antiferromagnetic spin-triplet excitonic insulator and a ferromagnetic half-excitonic insulator, respectively.

cond-mat.str-el↗

arXiv2024

A way to identify whether a DFT gap is from right reasons or error cancellations: The case of copper chalcogenides

Gap opening remains elusive in copper chalcogenides (Cu$_{2}X$, $X$ = S, Se and Te), not least because Hubbard + $U$, hybrid functional and ${GW}$ methods have also failed. In this work, we elucidate that their failure originates from a severe underestimation of the 4$s$-3$d$ orbital splitting of the Cu atom, which leads to a band-order inversion in the presence of an anionic crystal field. As a result, the Fermi energy is pinned due to symmetry, yielding an invariant zero gap. Utilizing the hybrid pseudopotentials to correct the underestimation on the atomic side opens up gaps of experimental magnitude in Cu$_{2}X$, suggesting their predominantly electronic nature. Our work not only clarifies the debate about the Cu$_{2}X$ gap, but also provides a way to identify which of the different methods really captures the physical essence and which is the result of error cancellation.

cond-mat.mtrl-sci↗

arXiv2024

Stable phases of freestanding monolayer TiO$_2$: The emergence of out-of-plane ferroelectricity

Despite being successfully synthesized [Zhang $et$ $al.$, Nat. Mater. \textbf{20}, 1073 (2021)], the monolayer structure of stable hexagonal TiO$_2$ is unknown, and it is not even clear whether it can exist in a freestanding form. Through first-principles calculations, we have identified two previously uncharted stable structures, namely, distorted 1$\times$$\sqrt{3}$ 1T-TiO$_2$ and $\sqrt{3}$$\times$$\sqrt{3}$ 1T-TiO$_2$, both of which are energetically more favourable than commonly adopted 1H and 1T phases. Here structural distortions are characterized by the out-of-plane shifts of Ti atoms due to the pseudo-Jahn-Teller interactions, which break one and all two inversion symmetries of 1T configuration. As a consequence, the 1$\times$$\sqrt{3}$ 1T remains centrosymmetric while the $\sqrt{3}$$\times$$\sqrt{3}$ 1T exhibits out-of-plane ferroelectricity. Electronic structure calculations show that both two are wide-bandgap semiconductors with bandgaps larger than their bulk counterparts. Our study not only deepens the understanding of structural instability in wide-gap semiconductors but also adds a new member to the rare family of two-dimensional out-of-plane ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2023

Giant enhancement of exciton radiative lifetime by ferroelectric polarization: The case of monolayer TiOCl$_2$

Exciton binding energy and lifetime are the two most important parameters controlling exciton dynamics, and the general consensus is that the larger the former the larger the latter. However our first-principles study of monolayer ferroelectric TiOCl$_2$ shows that this is not always the case. We find that ferroelectric polarization tends to weaken exciton binding but enhance exciton lifetime. This stems from the different effects of the induced built-in electric field and structural distortion by the spontaneous polarization: the former always destabilizes or even dissociates the exciton while the latter leads to a relaxation of the selection rule and activates excitons that are otherwise not optically active. Their combined effect leads to a halving of the exciton binding energy but a substantial increase in lifetime by 40 times. Our results deepen the understanding of the interaction of light with ferroelectric materials and provide new insights into the use of ferroelectricity to control exciton dynamics.

cond-mat.mtrl-sci↗

arXiv2023

Robust high-temperature topological excitonic insulator of transition-metal carbides (MXenes)

Topological excitonic insulators combine topological edge states and spontaneous exciton condensation, with dual functionality of topological insulators and excitonic insulators. Yet, they are very rare and little is known about their formation. In this work, we find that a mechanism dubbed as parity frustration prevents excitonic instability in usual topological insulators, and those whose band inversion is independent of spin-orbit coupling are possible candidates. We verify this by first-principles calculations on monolayer transition-metal carbides (MXenes), which show a robust thermal-equilibrium exciton condensation, being sufficient for topological applications at room temperature. Such a state can be identified by angle-resolved photoemission spectroscopy and transport measurement. Our work provides not only a guide for finding more topological excitonic insulators, but also a new platform for studying the interplay between non-trivial band topology and quantum many-body effects.

cond-mat.mes-hall↗

arXiv2022

Optical signature for distinguishing between Mott-Hubbard, intermediate and charge-transfer insulators

Determining the nature of band gaps in transition-metal compounds remains challenging. We present a first-principles study on electronic and optical properties of CoO using hybrid functional pseudopotentials. We show that optical absorption spectrum can provide a clear fingerprint to distinguish between Mott-Hubbard, intermediate and charge-transfer insulators. This discrimination is reflected by the qualitative difference in peak satellites due to unique interplay between $d$-$d$ and $p$-$d$ excitations, thus allowing identification from experimental data alone, unlike the existing methods that require additional theoretical interpretation. We conclude that the CoO is an intermediate, rather than a Mott-Hubbard insulator as is initially believed.

cond-mat.str-el↗

arXiv2022

First-principles perspective on full-spectrum infrared photodetectors from doping an excitonic insulator

Innovations in imaging technology involves finding strategies and materials suitable for detection applications over the entire infrared range. Herein, we propose a new design concept based on the unique feature of an excitonic insulator, namely, negative exciton transition energy ($E_t$). We demonstrate this concept using first-principles $GW$-BSE calculations on one-dimensional organometallic wire (CrBz)$_\infty$. The pristine (CrBz)$_\infty$ exhibits an excitonic instability due to a negative $E_t$ for the lowest exciton. Substitutional doping can continuously tune the $E_t$ from $\sim$0 to $\sim$0.6 eV, which shows the ability of photon detection from terahertz to near-infrared. This type of detectors have advantages of outstanding wavelength selectivity, reduced thermal disturbance and elevated working temperature. Our work not only adds another member in the family of rare one-dimensional excitonic insulators, but also opens a new avenue for the development of high-performance infrared photodetectors in the future.

cond-mat.mtrl-sci↗

arXiv2021

Excitonic Instability and Electronic Property of Two-dimensional AlSb Limit

Motivated by the recent synthesis of two-dimensional monolayer AlSb, we theoretically investigate its ground state and electronic properties using the first-principles calculations coupled with Bethe-Salpeter equation. An excitonic instability is revealed as a result of larger exciton binding energy than the corresponding one-electron energy gap by $\sim$0.1 eV, which is an indicative of a many-body ground state accompanied by spontaneous exciton generation. Including the spin-orbit coupling is proven to be a must to correctly predict the ground state. At room temperature, the two-dimensional monolayer AlSb is a direct gap semiconductor with phonon-limited electron and hole mobilities both around 1700 cm$^2$/V$\cdot$s. These results show that monolayer AlSb may provide a promising platform for realization of the excitonic insulator and for applications in the next-generation electronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Transition from band insulator to excitonic insulator via alloying Se into Monolayer TiS$_3$: A Computational Study

First-principles density functional theory plus Bethe-Salpeter equation calculations are employed to investigate the electronic and excitonic properties of monolayer titanium trichalcogenide alloys TiS$_{3-x}$Se$_x$ ($x$=1 and 2). It is found that bandgap and exciton binding energy display asymmetric dependence on the substitution of Se for S. While the bandgap can be significantly decreased as compared to that of pristine TiS$_3$, the exciton binding energy just varies a little, regardless of position and concentration of the Se substitution. A negative exciton formation energy is found when the central S atoms are replaced by Se atoms, suggesting a many-body ground state with the spontaneous exciton condensation. Our work thus offers a new insight for engineering an excitonic insulator.

cond-mat.mtrl-sci↗

arXiv2020

Observation of the Topologically Originated Edge States in large-gap Quasi-One-Dimensional a-Bi$_4$Br$_4$

Two-dimensional topological insulator features time-reversal-invariant spin-momentum-locked one-dimensional (1D) edge states with a linear energy dispersion. However, experimental access to 1D edge states is still of great challenge and only limited to few techniques to date. Here, by using infrared absorption spectroscopy, we observed robust topologically originated edge states in a-Bi4Br4 belts with definitive signature of strong infrared absorption at belt sides and distinct anisotropy with respect to light polarizations, which is further supported by first-principles calculations. Our work demonstrates for the first time that the infrared spectroscopy can offer a power-efficient approach in experimentally probing 1D edge states of topological materials.

cond-mat.mtrl-sci↗

arXiv2020

Spin-Triplet Excitonic Insulator: The Case of Graphone

While various excitonic insulators have been studied in the literature, due to the perceived too-small spin splitting, spin-triplet excitonic insulator is rare. In two-dimensional systems such as a graphone, however, it is possible, as revealed by first-principles calculations coupled with Bethe-Salpeter equation. The critical temperature, given by an effective Hamiltonian, is 11.5 K. While detecting excitonic insulators is still a daunting challenge, the condensation of triplet excitons will result in spin superfluidity, which can be directly measured by a transport experiment. Nonlocal dielectric screening also leads to an unexpected phenomenon, namely, an indirect-to-direct transition crossover between single-particle band and exciton dispersion in graphone, which offers yet another test by experiment.

cond-mat.mtrl-sci↗