@article {702,
	title = {Effect of MA orientation and thickness on the bandgap in quasi-2D perovskites (PEA)$_{2}$(MA)ₙ$_{-}$$_{1}$PbₙI$_{3}$ₙ$_{+}$$_{1}$},
	journal = {J. Phys. Chem. C},
	volume = {128},
	year = {2024},
	pages = {17091},
	abstract = {Three-dimensional organic{\textendash}inorganic perovskite photovoltaic materials such as MAPbI$_{3}$ (MA is methylammonium) have rapidly advanced in their performance over the past decade, but their instability remains a problem. One of the methods to improve their stability is by reducing their dimensionality. This dimensional reduction is performed by adding a large organic cation as a spacer to separate the original bulk lattice, forming a new quasi-two-dimensional (quasi-2D) structure. In this study, we have investigated the electronic properties of the quasi-2D structures of (PEA)$_{2}$(MA)ₙ$_{-}$$_{1}$PbₙI$_{3}$ₙ$_{+}$$_{1}$ (PEA is phenylethylammonium) with various thicknesses (n) and MA orientations using first-principles calculations. Our results show that the bandgap decreased as the number of layers (n) increased for quasi-2D structures. The structures with in-plane MA orientations and mirror symmetry along the z-axis are found to be the most stable and exhibit a converging trend in the bulk bandgap when n > 6. Our calculations reveal how the orientations of the MA ions affect the electronic structure via symmetry breaking and electric dipole formation. These two factors change the electrostatic potential and band energies of the individual PbI$_{3}$ layers, which lead to enhanced band splitting and bandgap reduction. The results of this work would give some insights into the electronic structures and may provide directions for the crystal growth design of quasi-2D perovskite materials in photovoltaic applications.},
	doi = {10.1021/acs.jpcc.4c03540},
	author = {Yang-Hao Lee and Gennevieve M. Macam and Rovi Angelo B. Villaos and Liang-Ying Feng and Chia-Hsiu Hsu and Zhi-Quan Huang and Deng-Chi Wang and Hsin Lin and Feng-Chuan Chuang}
}
@article {701,
	title = {Electronic, magnetic, and topological properties of ferromagnetic 2D perovskite-type oxides},
	journal = {New J. Phys.},
	volume = {26},
	year = {2024},
	pages = {123031},
	abstract = {Two-dimensional (2D) materials within the hematene-type binary oxides and perovskites family have recently gathered huge research interest for nanoelectronic devices. However, the exploration of their fascinating topological properties remains limited. Herein, through first-principles calculations, we systematically examine the electronic, magnetic, and topological properties of substitutionally doped 2D ABO$_{3}$ (A = As, Sb, or Bi, and B = V, Nb, or Ta) perovskite structures at the B site of a B$_{2}$O$_{3}$ system. Interestingly, the atomic substitution makes the 2D ABO$_{3}$ structures dynamically stable. Our detailed calculations show the ferromagnetic (FM) and antiferromagnetic phases of these materials. The calculated Chern number (C) for the FM 2D ABO$_{3}$ (A = As, Sb, or Bi, B = Nb or Ta) suggests their topologically non-trivial phases. Furthermore, the computed nontrivial Berry curvature highlights the topological properties in AsNbO$_{3}$. These findings highlight opportunities in 2D-ABO$_{3}$ materials, for applications in spintronics.},
	doi = {10.1088/1367-2630/ad9be0},
	author = {Susaiammal Arokiasamy and Gennevieve M. Macam and Puthiya Covilakam Sreeparvathy and Rovi Angelo B. Villaos and Zhi-Quan Huang and Chia-Hsiu Hsu and Yoshinori Okada and Hsin Lin and Feng-Chuan Chuang}
}
@article {618,
	title = {Causal structure of interacting Weyl fermions in condensed matter systems},
	journal = {Nat. Commun.},
	volume = {14},
	year = {2023},
	pages = {2228},
	abstract = {The spacetime light cone is central to the definition of causality in the theory of relativity. Recently, links between relativistic and condensed matter physics have been uncovered, where relativistic particles can emerge as quasiparticles in the energy-momentum space of matter. Here, we unveil an energy-momentum analogue of the spacetime light cone by mapping time to energy, space to momentum, and the light cone to the Weyl cone. We show that two Weyl quasiparticles can only interact to open a global energy gap if they lie in each other{\textquoteright}s energy-momentum dispersion cones{\textendash}analogous to two events that can only have a causal connection if they lie in each other{\textquoteright}s light cones. Moreover, we demonstrate that the causality of surface chiral modes in quantum matter is entangled with the causality of bulk Weyl fermions. Furthermore, we identify a unique quantum horizon region and an associated {\textquoteright}thick horizon{\textquoteright} in the emergent causal structure.},
	doi = {10.1038/s41467-023-37931-w},
	url = {https://www.nature.com/articles/s41467-023-37931-w},
	author = {Wei-Chi Chiu and Guoqing Chang and Gennevieve M. Macam and Ilya Belopolski and Shin-Ming Huang and Robert Markiewicz and Jia-Xin Yin and Zi-Jia Cheng and Chi-Cheng Lee and Tay-Rong Chang and Feng-Chuan Chuang and Su-Yang Xu and Hsin Lin and M. Zahid Hasan and Arun Bansil}
}
@article {604,
	title = {Band engineering and van Hove singularity on HfX$_{2}$ thin films (X = S, Se, or Te)},
	journal = {ACS Appl. Electron. Mater.},
	volume = {3},
	year = {2021},
	pages = {1071},
	abstract = {Two-dimensional transition metal dichalcogenides (TMDs) have become well-known due to their versatile and tunable physical properties for potential applications, specifically on low-power and optical devices. Here, we explored the structural stability and electronic properties of bulk and thin-film (from 1 up to 6 layers) structures of hafnium dichalcogenides (HfX$_{2}$, X = S, Se, or Te) using first-principles calculations. Our calculations reveal that the most stable phase is 1T for both thin films and bulk. The bulk and thin-film structures of HfTe$_{2}$ are semimetallic, while those of HfS$_{2}$ and HfSe$_{2}$ are insulating. Both HfS$_{2}$ and HfSe$_{2}$ thin films exhibit a decreasing band gap with increasing thickness, while HfTe$_{2}$ thin films remain semimetallic with increasing number of layers. Moreover, van Hove singularity (vHs), due to the contribution of the pz orbital from S atoms, is observed in 3L-HfS$_{2}$ at the valence band maximum, which can be further enhanced by applying an in-plane biaxial strain, suggesting possible superconductivity. Finally, the bulk and monolayer band structures of HfTe$_{2}$, under HSE06 and GGA + U with the effective Hubbard U parameter of 4.6 eV, are in good agreement with the experimental ARPES data. Our results indeed show that HfX$_{2}$ have sensitive and tunable electronic properties through film thickness control and strain for future potential applications.},
	doi = {10.1021/acsaelm.0c00907},
	author = {Harvey N. Cruzado and John Symon C. Dizon and Gennevieve M. Macam and Rovi Angelo B. Villaos and Thi My Duyen Huynh and Liang-Ying Feng and Zhi-Quan Huang and Chia-Hsiu Hsu and Shin-Ming Huang and Hsin Lin and Feng-Chuan Chuang}
}
@article {605,
	title = {Theoretical prediction of topological insulators in two-dimensional ternary transition metal chalcogenides (MM{\textquoteright}X$_{4}$, M~=~Ta, Nb, or V; M{\textquoteright}= Ir, Rh, or Co; X~=~Se or Te)},
	journal = {Chinese J. Phys.},
	volume = {73},
	year = {2021},
	pages = {95},
	abstract = {Ternary transition metal chalcogenides (TTMCs) have attracted interest due to the discovery of their Weyl semimetallic property and the recent synthesis of layered TTMCs which are regarded as potential candidates for two-dimensional (2D) topological insulators. Here, employing first-principles calculations, we predicted the emergence of non-trivial band topologies in the monolayer MM{\textquoteright}X$_{4}$ family (M= V, Nb, or Ta; M{\textquoteright}~=~Co, Rh, or Ir; and X~=~Se or Te) within hybrid functional calculations. Five of eighteen 2D materials were found to be topological insulators, while four of them are magnetic thin films. The nontrivial topologies were verified via the calculated Z$_{2}$ topological invariant and topologically protected edge states. Further calculations showed a strain-induced phase transition in VCoTe$_{4}$ from a magnetic phase to a nonmagnetic topological insulating phase. Our comprehensive study revealed a diverse family of monolayer ternary transition metal chalcogenides adding new members to the current catalog of 2D topological insulators and 2D magnetic materials.},
	doi = {10.1016/j.cjph.2021.06.014},
	author = {Ali Sufyan and Gennevieve M. Macam and Chia-Hsiu Hsu and Zhi-Quan Huang and Shin-Ming Huang and Hsin Lin and Feng-Chuan Chuang}
}
@article {607,
	title = {Tuning topological phases and electronic properties of monolayer ternary transition metal chalcogenides (ABX$_{4}$, A/B = Zr, Hf, or Ti; X = S, Se, or Te)},
	journal = {Appl. Phys. Lett.},
	volume = {118},
	year = {2021},
	pages = {111901},
	abstract = {Topological materials are very promising materials for technological applications ranging from spintronics to quantum computation. Here, based on first-principles calculations, we predict a family of two-dimensional (2D) topological materials in nine ternary transition metal chalcogenides (TTMCs) ABX$_{4}$, where A/B = Zr, Hf, or Ti and X = S, Se, or Te. A total of three compounds (ZrTiTe$_{4}$, HfZrTe$_{4}$, and HfTiTe$_{4}$) are identified to be nontrivial within a hybrid functional calculation. The nontrivial phase originated from the p-d band inversion at the Г point with spin-orbit coupling. The structural stability of these monolayers is confirmed by phonon spectrum analysis, showing no negative phonon frequencies. The diversity of TTMCs will open a wide possibility for tuning the bandgap and will provide a variety of opportunities for 2D and topological materials research.},
	doi = {10.1063/5.0036838},
	author = {Gennevieve M. Macam and Ali Sufyan and Zhi-Quan Huang and Chia-Hsiu Hsu and Shin-Ming Huang and Hsin Lin and Feng-Chuan Chuang}
}
@article {608,
	title = {Quantum anomalous Hall insulator phases in Fe-doped GaBi honeycomb},
	journal = {Chinese J. Phys.},
	volume = {67},
	year = {2020},
	pages = {246},
	abstract = {We discuss electronic and magnetic properties of the Fe-doped GaBi honeycomb using first principles calculations. Our analysis shows that the pristine GaBi honeycomb transitions from being a two-dimensional quantum spin Hall (QSH) insulator to a quantum anomalous Hall (QAH) insulator when it is doped with one Fe atom in a 4 {\texttimes} 4 GaBi honeycomb. The QAH phase in Fe-doped GaBi is found to be robust in that it maintains its Chern number (C = 1) under fairly large strains (\~{} 4\%) and supports a gap as large as 112~meV at 2.21\% strain. The QAH phase is also retained when the Fe-doped GaBi is placed on a CdTe substrate, suggesting that Fe-doped GaBi films could be useful for spintronics applications.},
	doi = {10.1016/j.cjph.2020.07.007},
	url = {https://www.sciencedirect.com/science/article/pii/S0577907320301866},
	author = {Zhi-Quan Huang and Chia-Hsiu Hsu and Christian P. Crisostomo and Gennevieve M. Macam and Jing-Rong Su and Hsin Lin and Arun Bansil and Feng-Chuan Chuang}
}
@article {611,
	title = {Spin-orbit quantum impurity in a topological magnet},
	journal = {Nat. Commun.},
	volume = {11},
	year = {2020},
	pages = {4415},
	abstract = {Quantum states induced by single-atomic impurities are at the frontier of physics and material science. While such states have been reported in high-temperature superconductors and dilute magnetic semiconductors, they are unexplored in topological magnets which can feature spin-orbit tunability. Here we use spin-polarized scanning tunneling microscopy/spectroscopy (STM/S) to study the engineered quantum impurity in a topological magnet Co$_{3}$Sn$_{2}$S$_{2}$. We find that each substituted In impurity introduces a striking localized bound state. Our systematic magnetization-polarized probe reveals that this bound state is spin-down polarized, in lock with a negative orbital magnetization. Moreover, the magnetic bound states of neighboring impurities interact to form quantized orbitals, exhibiting an intriguing spin-orbit splitting, analogous to the splitting of the topological fermion line. Our work collectively demonstrates the strong spin-orbit effect of the single-atomic impurity at the quantum level, suggesting that a nonmagnetic impurity can introduce spin-orbit coupled magnetic resonance in topological magnets.},
	doi = {10.1038/s41467-020-18111-6},
	author = {Jia-Xin Yin and Nana Shumiya and Yuxiao Jiang and Huibin Zhou and Gennevieve M. Macam and Hano Omar Mohammad Sura and Songtian S. Zhang and Zi-Jia Cheng and Zurab Guguchia and Yangmu Li and Qi Wang and Maksim Litskevich and Ilya Belopolski and Xian P. Yang and Tyler A. Cochran and Guoqing Chang and Qi Zhang and Zhi-Quan Huang and Feng-Chuan Chuang and Hsin Lin and Hechang Lei and Brian M. Andersen and Ziqiang Wang and Shuang Ji and M. Zahid Hasan}
}
@article {596,
	title = {Atomically precise bottom-up synthesis of π-extended [5]triangulene},
	journal = {Sci. Adv.},
	volume = {5},
	year = {2019},
	pages = {eaav7717},
	abstract = {The zigzag-edged triangular graphene molecules (ZTGMs) have been predicted to host ferromagnetically coupled edge states with the net spin scaling with the molecular size, which affords large spin tunability crucial for next-generation molecular spintronics. However, the scalable synthesis of large ZTGMs and the direct observation of their edge states have been long-standing challenges because of the molecules{\textquoteright} high chemical instability. Here, we report the bottom-up synthesis of π-extended [5]triangulene with atomic precision via surface-assisted cyclodehydrogenation of a rationally designed molecular precursor on metallic surfaces. Atomic force microscopy measurements unambiguously resolve its ZTGM-like skeleton consisting of 15 fused benzene rings, while scanning tunneling spectroscopy measurements reveal edge-localized electronic states. Bolstered by density functional theory calculations, our results show that [5]triangulenes synthesized on Au(111) retain the open-shell π-conjugated character with magnetic ground states.},
	doi = {10.1126/sciadv.aav7717},
	author = {Jie Su and Mykola Telychko and Pan Hu and Gennevieve M. Macam and Pingo Mutombo and Hejian Zhang and Yang Bao and Fang Cheng and Zhi-Quan Huang and Zhizhan Qiu and Sherman J. R. Tan and Hsin Lin and Pavel Jel{\'\i}nek and Feng-Chuan Chuang and Jishan Wu and Jiong Lu}
}
@article {598,
	title = {Quantum phase transition of correlated iron-based superconductivity in LiFe$_{1}$$_{-}$ₓCoₓAs},
	journal = {Phys. Rev. Lett.},
	volume = {123},
	year = {2019},
	pages = {217004},
	abstract = {The interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the high-temperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy to image the electronic impact of Co atoms on the ground state of the LiFe$_{1}$$_{-}$ₓCoₓAs system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strong-coupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Born-limit scattering effect in an s{\textpm}-wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.},
	doi = {10.1103/PhysRevLett.123.217004},
	author = {Jia-Xin Yin and Songtian S. Zhang and Guangyang Dai and Yuanyuan Zhao and Andreas Kreisel and Gennevieve M. Macam and Xianxin Wu and Hu Miao and Zhi-Quan Huang and Johannes H. J. Martiny and Brian M. Andersen and Nana Shumiya and Daniel Multer and Maksim Litskevich and Zijia Cheng and Xian Yang and Tyler A. Cochran and Guoqing Chang and Ilya Belopolski and Lingyi Xing and Xiancheng Wang and Yi Gao and Feng-Chuan Chuang and Hsin Lin and Ziqiang Wang and Changqing Jin and Yunkyu Bang and M. Zahid Hasan}
}
@article {613,
	title = {Growth of a predicted two-dimensional topological insulator based on InBi-Si(111)-√7{\texttimes}√7},
	journal = {Phys. Rev. B},
	volume = {98},
	year = {2018},
	pages = {121404},
	abstract = {Using combined scanning tunneling microscopy (STM) measurements and first-principles electronic structure calculations, we extensively studied the atomic and electronic properties of a √7 -InBi overlayer on Si(111). We propose and demonstrate an effective experimental process to successfully form a large well-ordered √7 surface by depositing Bi atoms on the In-Si(111)4{\texttimes}1 substrate. The STM images exhibit a honeycomb pattern. After performing an exhaustive computational search, we identified the atomic structures of the surface at In and Bi coverages of 6/7 and 3/7 monolayers, respectively. We discovered a trimer model with a lower energy than the previously proposed model. The simulated STM images of trimer models confirm the presence of the honeycomb pattern in accord with our experimental STM images. Most importantly, we found that the surface is robust, preserving the topologically nontrivial phase. Our edge state calculations verify that the InBi overlayer on Si(111) is indeed a two-dimensional (2D) topological insulator (TI). Moreover, hybrid functional calculations result in band gaps up to 70 meV, which is high enough for room-temperature experiments. Our findings lay the foundation for the materials realization of 2D TIs by growing an InBi overlayer on a Si(111) substrate.},
	doi = {10.1103/PhysRevB.98.121404},
	author = {Chia-Hsiu Hsu and Zhi-Quan Huang and Cho-Ying Lin and Gennevieve M. Macam and Yu-Zhang Huang and Deng-Sung Lin and Tai Chang Chiang and Hsin Lin and Feng-Chuan Chuang and Li Huang}
}
@article {612,
	title = {Prediction of quantum anomalous hall effect in MBi and MSb (M: Ti, Zr, and Hf) honeycombs},
	journal = {Nanoscale Res. Lett.},
	volume = {13},
	year = {2018},
	pages = {43},
	abstract = {The abounding possibilities of discovering novel materials has driven enhanced research effort in the field of materials physics. Only recently, the quantum anomalous hall effect (QAHE) was realized in magnetic topological insulators (TIs) albeit existing at extremely low temperatures. Here, we predict that MPn (M =Ti, Zr, and Hf; Pn =Sb and Bi) honeycombs are capable of possessing QAH insulating phases based on first-principles electronic structure calculations. We found that HfBi, HfSb, TiBi, and TiSb honeycomb systems possess QAHE with the largest band gap of 15 meV under the effect of tensile strain. In low-buckled HfBi honeycomb, we demonstrated the change of Chern number with increasing lattice constant. The band crossings occurred at low symmetry points. We also found that by varying the buckling distance we can induce a phase transition such that the band crossing between two Hf d-orbitals occurs along high-symmetry point K2. Moreover, edge states are demonstrated in buckled HfBi zigzag nanoribbons. This study contributes additional novel materials to the current pool of predicted QAH insulators which have promising applications in spintronics.},
	doi = {10.1186/s11671-017-2424-y},
	author = {Zhi-Quan Huang and Wei-Chih Chen and Gennevieve M. Macam and Christian P. Crisostomo and Shin-Ming Huang and Rong-Bin Chen and Marvin A. Albao and Der-Jun Jang and Hsin Lin and Feng-Chuan Chuang}
}
