arXiv subjects

C. -W. Chuang

Publications and source records attributed to C. -W. Chuang.

4 recordsLinked to original sources

arXiv2026

Growth of Altermagnetic α-MnTe Films: Substrate Variation and Surface Modification

Manganese telluride (MnTe) in its hexagonal α-MnTe crystal structure has evolved as one of the altermagnet workhorse materials. The synthesis of MnTe thin films is highly relevant for both fundamental science and device applications. Here, we report on the epitaxial growth of MnTe thin films and heterostructures. The films are studied by X-ray and electron diffraction as well as soft X-ray angle-resolved photoemission spectroscopy. We demonstrate the ability to grow high-quality α-MnTe on various substrates, ranging from transparent band insulators, over topological insulators, metallic transition metal chalcogenides, to the van der Waals ferromagnet Fe$_3$GeTe$_2$. While insulating substrates are useful for transport experiments or optical spectroscopy, metallic topological surface states may trigger spintronic interface effects, such as spin-orbit torques. Metallic substrates, in general, are highly relevant to avoid charging at the insulating MnTe films in electron spectroscopy or microscopy methods. Lastly, ferromagnetic substrates will be of interest to control magnetization across the interface. In addition, we discuss the formation of superstructures on the MnTe(0001) surfaces, that emerge directly after growth, upon subsequent tellurium evaporation and after thermal treatment. This will be relevant in further studying the surface magnetic and electronic structure in α-MnTe.

cond-mat.mtrl-sci↗

arXiv2026

Role of on-site Coulomb energy and negative-charge transfer in a Dirac semi-metal NiTe$_2$

Angle-resolved photoemission spectroscopy (ARPES) combined with band structure calculations have shown that the layered transition metal dichalcogenide(TMD) NiTe$_2$ is a type-II Dirac semimetal. However, conflicting conclusions were reported regarding the role of electron correlations in NiTe$_2$. We study core-levels and valence band electronic structure of single crystal NiTe$_2$ using soft and hard x-ray photoemission spectroscopy(SXPES, HAXPES), X-ray absorption spectroscopy(XAS) and Ni $2p-3d$ Resonant-PES to quantify electronic parameters in NiTe$_2$. The Ni $3d$ on-site Coulomb energy ($U_{dd}$) is quantified from measurements of the Ni $3d$ single particle density of states(DOS) and the two-hole correlation satellite. The Ni $2p$ core level and $L$-edge XAS spectra are analyzed by charge-transfer (CT) cluster model calculations using the experimental $U_{dd}$, and it shows that NiTe$_2$ exhibits a negative CT energy $Δ$. A comparative analysis of NiO $L$-edge XAS confirms its well-known strongly correlated CT insulator character, with a larger $U_{dd}$ and positive $Δ$. The $d$-$p$ hybridization strength $T_{eg}$ for NiTe$_2$$<$NiO, and shows that $T_{eg}$ is not responsible for reducing $U_{dd}$ in NiTe\textsubscript{2} compared to NiO. The negative-$Δ$ and a reduced $U_{dd}$ leads to the increase in $d^n$ count on the Ni site in NiTe$_{2}$ by nearly one electron. However, importantly, since $U_{dd}$$>$$|Δ|$, a finite repulsive $U_{dd}$ results in pushing $d$-states away from Fermi level and this is required to make NiTe$_{2}$ a moderately correlated Dirac semi-metal with band inversion in the $p$-$p$ type lowest energy excitations.

cond-mat.str-el↗

arXiv2026

The nexus between negative charge-transfer and reduced on-site Coulomb energy in a correlated topological metal CoTe$_2$

The layered $3d$ transition metal dichalcogenide (TMD) CoTe$_2$ is a topological Dirac Type-II metal. However, the Co $3d$-bands in CoTe$_2$ do not exhibit the expected correlation-induced band narrowing seen in CoO. We address this conundrum by studying the electronic structure of CoTe$_2$ using hard x-ray photoemission spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS) and Resonant-PES. We quantify the on-site Coulomb energy $U_{dd}$ via single-particle partial density of states and the two-hole correlation satellite using valence band Resonant-PES), and obtain $U_{dd}$ = 3.0 eV for CoTe$_2$. Charge-transfer (CT) cluster model simulations of the measured core-level Co $2p$ PES and $L$-edge XAS spectra of CoTe\textsubscript{2} and CoO validate their contrasting electronic parameters:$U_{dd}$ and CT energy $Δ$ are (3.0 eV, -2.0 eV) for CoTe\textsubscript{2}, and (5.0 eV, 4.0 eV) for CoO, respectively. The $d$-$p$ hybridization strength $T_{eg}$ for CoTe$_2$$<$CoO, and indicates that the reduced $U_{dd}$ in CoTe\textsubscript{2} is not due to $T_{eg}$. The increase in $d^n$-count$\sim$1 by CT from ligand to Co site in CoTe$_2$ is due to a negative-$Δ$ and reduced $U_{dd}$. Yet, only because $U_{dd}$$>$$\big|Δ\big|$, CoTe$_{2}$ becomes a topological metal with $p$$\rightarrow$$p$ type lowest energy excitations. The study reveals the nexus between negative-$Δ$ and reduced $U_{dd}$ required for setting up the electronic structure framework for achieving topological behavior via band inversion in the correlated metal CoTe$_2$.

cond-mat.str-el↗

arXiv2023

Unusual band evolution and persistence of topological surface states in high-T_C magnetic topological insulator

Understanding the mechanism of ferromagnetism in ferromagnetic topological insulators (TIs) is a key to realize exotic time-reversal-symmetry-broken quantum phases. However, electronic states relevant to the ferromagnetism are highly controversial. Here we report angle-resolved photoemission spectroscopy on (CrxSb1-x)2Te3 thin films, high-Curie-temperature (T_C) ferromagnetic TIs, spanning the non-doped (T_C=0 K) to highly-doped (T_C=192 K) region. We found that, upon Cr doping to Sb2Te3, the bulk valence-band valley exhibits filling-in behavior while retaining band inversion, leading to the formation of a nearly-flat band in high-T_C regime and evolution from a six-petal flower to a Star-of-David Fermi surface. Despite the weakening of spin-orbit coupling with Cr doping, the Dirac-cone state persists up to the highest-T_C sample, and shows a clear magnetic-gap opening below TC accompanied with an unexpected band shift, signifying its strong coupling with spontaneous ferromagnetism. The present result lays the foundation for understanding the interplay between band topology and ferromagnetism in TIs.

cond-mat.mes-hall↗