arXiv subjects

Yuanchang Li

Publications and source records attributed to Yuanchang Li.

30 records · Page 2Linked to original sources

arXiv2019

Understanding the origin of bandgap problem in transition and post-transition metal oxides

Improving electronic structure calculations for practical and technologically-important materials has been a never-ending pursue. This is especially true for transition and post-transition metal oxides for which the current first-principles approaches still suffer various drawbacks. Here we present a hierarchical-hybrid functional approach built on the use of pseudopotentials. The key is to introduce a discontinuity in the exchange functional between core and valence electrons. It allows for treating the localization errors of sp and d electrons differently, which have been known to be an important source of error for the band gap. Using ZnO as a prototype, we show the approach is successful in simultaneously reproducing the band gap and d-band position. Remarkably, the same approach, without having to change the hybrid mixing parameters from those of Zn, works reasonably well for other binary 3d transition and post-transition metal oxides across board. Our findings point to a new direction of systematically improving the exchange functional in first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2019

Half Excitonic Insulator: A Single-Spin Bose-Einstein Condensate

First-principles calculations reveal an unusual electronic state (dubbed as half excitonic insulator) in monolayer 1T-MX2 (M = Co, Ni and X = Cl, Br). Its one spin channel has a many-body ground state due to excitonic instability, while the other is characterized by a conventional band insulator gap. This disparity arises from a competition between the band gap and exciton binding energy, which exhibits a spin-dependence due to different orbital occupations. Such a state can be identified by optical absorption measurements and angle-resolved photoemission spectroscopy. Our theory not only provides new insights for the study of exciton condensation in magnetic materials but also suggests that strongly-correlated materials could be fertile candidates for excitonic insulators.

cond-mat.mtrl-sci↗

arXiv2019

Two-dimensional ferromagnetic-ferroelectric multiferroics in violation of the d0 rule

Contribution of d-electron to ferroelectricity of type-II multiferroics causes strong magneto-electric coupling and distinguishes them from the conventional type-I multiferroics. However, their therein polarization is too small because the ferroelectricity is merely a derivative from the magnetic order. Here we report a new class of multiferroic materials, monolayer VOX2 (X = Cl, Br, and I), which combine the advantages of type-I and type-II multiferroics. Both ferroelectricity and magnetism arise from the same V cation, where the filled d-orbital is perpendicular to an a priori ferroelectric polarization and thus poses no hindrance to ferroelectricity, indicating a violation of the usual d0 rule. This makes the combination of large polarizations and strong magneto-electric coupling possible. Our findings not only add new ferromagnetic-ferroelectric multiferroics, but also point to a unique mechanism to engineer multiferroics.

cond-mat.mtrl-sci↗

arXiv2018

Realizing intrinsic excitonic insulator by decoupling exciton binding energy from the minimum band gap

Direct-gap materials hold promises for excitonic insulator. In contrast to indirect-gap materials, here the difficulty to distinguish from a Peierls charge density wave is circumvented. However, direct-gap materials still suffer from the divergence of polarizability when the band gap approaches zero, leading to diminishing exciton binding energy. We propose that one can decouple the exciton binding energy from the band gap in materials where band-edge states have the same parity. First-principles calculations of two-dimensional GaAs and experimentally mechanically exfoliated single-layer TiS 3 lend solid supports to the new principle.

cond-mat.mtrl-sci↗

arXiv2018

Effect of Hartree-Fock Pseudopotential on the First-principles Electronic Structure

Density functional theory (DFT) can run into serious difficulties with localized states in elements such as transition metals with occupied-d states and oxygen. In contrast, Hartree-Fock (HF) method can be a better approach for such localized states. Here, we develop HF pseudopotentials to be used alongside with DFT for solids. The computation cost is on par with standard DFT. Calculations for a range of II-VI, III-V and group-IV semiconductors with diverse physical properties show observably improved band gap for systems containing d-electrons, whereby pointing to a new direction in electronic theory.

cond-mat.mtrl-sci↗

arXiv2016

Gate-independent energy gap in non-covalently intercalated bilayer graphene on SiC(0001)

Our first-principles calculations show that an energy gap around 0.12-0.25 eV can be engineered in epitaxial graphene on SiC(0001) through the non-covalent intercalation of transition- or alkali-metals, yet originated from the distinct mechanisms. The former is attributed to the combined effects of metal induced perpendicular electric field and interaction while the latter is solely attributed to the built-in electric field. A great advantage of this scheme is that the gap size is almost independent of the gate voltage up to 1 V/nm, thus reserving the electric means to tune the Fermi level of graphene when configured as field-effect transistors. Given the recent progress in experimental techniques on the intercalated graphene, our findings provide a practical way to incorporate the graphene with the current semiconductor industry.

cond-mat.mes-hall↗

arXiv2016

Unconventional quantized edge transport in the presence of inter-edge coupling in intercalated graphene

It is generally believed that the inter-edge coupling destroys the quantum spin Hall (QSH) effect along with the gap opening at the Dirac points. Using first-principles calculations, we find that the quantized edge transport persists in the presence of inter-edge coupling in Ta intercalated epitaxial graphene on SiC(0001), being a QSH insulator with the non-trivial gap of 81 meV. In this case, the band is characterized by two perfect Dirac cones with different Fermi velocities, yet only one maintains the edge state feature. We attribute such an anomalous behavior to the orbital-dependent decay of edge states into the bulk, which allows the inter-edge coupling just between one pair of edge states rather than two.

cond-mat.mes-hall↗

arXiv2015

Theory of the Dirac Half Metal and Quantum Anomalous Hall Effect in Mn Intercalated Epitaxial Graphene

The prospect of a Dirac half metal, a material which is characterized by a bandstructure with a gap in one spin channel but a Dirac cone in the other, is of both fundamental interest and a natural candidate for use in spin-polarized current applications. However, while the possibility of such a material has been reported based on model calculations[H. Ishizuka and Y. Motome, Phys. Rev. Lett. 109, 237207 (2012)], it remains unclear what material system might realize such an exotic state. Using first-principles calculations, we show that the experimentally accessible Mn intercalated epitaxial graphene on SiC(0001) transits to a Dirac half metal when the coverage is > 1/3 monolayer. This transition results from an orbital-selective breaking of quasi-2D inversion symmetry, leading to symmetry breaking in a single spin channel which is robust against randomness in the distribution of Mn intercalates. Furthermore, the inclusion of spin-orbit interaction naturally drives the system into the quantum anomalous Hall (QAH) state. Our results thus not only demonstrate the practicality of realizing the Dirac half metal beyond a toy model but also open up a new avenue to the realization of the QAH effect.

cond-mat.str-el↗

arXiv2015

High-$T_c$ $d^{0}$ ferromagnetism in a doped Mott insulator: the case of hydrogenated epitaxial graphene on SiC(0001)

We show that the $d^{0}$ ferromagnetism with high Curie temperature ($T_c$) can be achieved in the electron doped hydrogenated epitaxial graphene on some certain SiC substrates through first-principles calculations. The pristine systems are found to be a Mott insulator independent of SiC polytypes (2$H$, 4$H$ or 6$H$) which, however, play a significant role in the modulation of magnetic interaction. Carrier doping enhances the ferromagnetic coupling due to the double exchange mechanism and thus realizes the phase transition from antiferromagnetism to ferromagnetism. A $T_c$ of around 400 K is predicted on the 2$H$-SiC. We employ a non-degenerate Hubbard model to demonstrate how the SiC affects the interfacial magnetism in intra-atomic Coulomb repulsion and intersite hopping interactions.

cond-mat.mes-hall↗

arXiv2014

Dirac Fermions in Blue-Phosphorus

We propose that Dirac cones can be engineered in phosphorene with fourfold-coordinated phosphorus atom. The key is to separate in energy the in-plane ($s$, $p_x$ and $p_y$) and out-of-plane ($p_z$) oribtals through the $sp^2$ configuration, yielding respective $σ$- and $π$-character Dirac cones, and then quench the latter. As a proof-of-principle study, we realize $σ$-character Dirac cone in hydrogenated/fluorinated phosphorene with the honeycomb lattice. The obtained Dirac cones are at $K$-points, slightly anisotropic, with Fermi velocities of 0.91/1.23 times that of graphene along $Γ$K/KM direction, and maintain a good linearity up to $\sim$2 eV for holes. One substantive advantage of $σ$-character Dirac cone is its convenience to tune the Dirac gap via in-plane strain. Our findings pave a new way for development of high performance electronic devices based on Dirac materials.

cond-mat.mes-hall↗

arXiv2014

Quasi-freestanding monolayer heterostructure of graphene and hexagonal boron nitride on Ir(111) with a chiral boundary

Monolayer lateral heterostructure of graphene and hexagonal boron nitride (h-BNC) has attracted a growing attention mainly due to its tunable band-gap character and unique physical properties at interface. Hereby, we reported the first-time synthesis of a nearly freestanding h-BNC hybrid on a weakly coupled substrate of Ir (111), where graphene and h-BN possessing different surface heights and corrugations formed a perfect monolayer hybrid. With the aid of scanning tunneling microscopy/spectroscopy (STM/STS), we demonstrated that h-BN can patch alongside the boundary of pre-deposited graphene domains and vice versa to form a seamless monolayer hybrid, with the realization of predominant zigzag type chiral boundaries at the interface. Density-functional theory calculations and STM/STS measurements aided us to reveal that this interface between graphene and h-BN were atomically sharp in aspects of the chemical bonding as well as the local electronic property from both theoretical and experimental points of view.

physics.chem-ph↗

arXiv2012

Dirac Fermion in Strongly-Bound Graphene Systems

It is highly desirable to integrate graphene into existing semiconductor technology, where the combined system is thermodynamically stable yet maintain a Dirac cone at the Fermi level. Firstprinciples calculations reveal that a certain transition metal (TM) intercalated graphene/SiC(0001), such as the strongly-bound graphene/intercalated-Mn/SiC, could be such a system. Different from free-standing graphene, the hybridization between graphene and Mn/SiC leads to the formation of a dispersive Dirac cone of primarily TM d characters. The corresponding Dirac spectrum is still isotropic, and the transport behavior is nearly identical to that of free-standing graphene for a bias as large as 0.6 V, except that the Fermi velocity is half that of graphene. A simple model Hamiltonian is developed to qualitatively account for the physics of the transfer of the Dirac cone from a dispersive system (e.g., graphene) to an originally non-dispersive system (e.g., TM).

cond-mat.mes-hall↗