arXiv subjects

Bei Deng

Publications and source records attributed to Bei Deng.

8 recordsLinked to original sources

arXiv2022

Parity-breaking in single-element phases: Ferroelectric-like elemental polar metals

Polar metals based on binary and ternary compounds have been demonstrated in literature. Here, we propose a design principle for ferroelectric-like elemental polar metals and relate it to real materials. The design principle is that, to be an elemental polar metal, atoms should occupy at least two inequivalent Wyckoff positions in a crystal with a polar space group, where inversion symmetry is spontaneously broken. According to this rule, we propose the first class of potential ferroelectric-like elemental polar metals in a distorted {\alpha}-La-like structure with a polar space group P63mc in which two inequivalent Wyckoff positions 2a (0, 0, z) and 2b (1/3, 2/3, z) are occupied by group-V elements (phosphorus, arsenic, antimony, and bismuth). Analyses based on first-principles calculations indicate that the dynamically stable polar phase results from a lone pair driven polar distortion of the nonploar phase in P63/mmc symmetry where two inequivalent Wyckoff positions 2a (0, 0, 0) and 2c (1/3, 2/3, 1/4) are occupied. This ferroelectric-like transition involves a transition from a metallic state to a semimetallic state. These predicted polar phases are metastable with respect to their corresponding ground phases. Moreover, ionic bonding characters are found due to the inequivalence in Wyckoff positions between group-V atoms. Our work opens a route to single-element parity-breaking phases.

cond-mat.mtrl-sci↗

arXiv2019

Polarity and spin-orbit coupling induced large interfacial exchange coupling an asymmetric charge transfer in iridate-manganite heterostructure

Charge transfer is of particular importance in manipulating the interface physics in transition-metal oxide heterostructures. In this work, we have fabricated epitaxial bilayers composed of polar 3d LaMnO3 and nonpolar 5d SrIrO3. Systematic magnetic measurements reveal an unexpectedly large exchange bias effect in the bilayer, together with a dramatic enhancement of the coercivity of LaMnO3. Based on first-principles calculations and x-ray absorption spectroscopy measurements, such a strong interfacial magnetic coupling is found closely associated with the polar nature of LaMnO3 and the strong spin-orbit interaction in SrIrO3, which collectively drives an asymmetric interfacial charge transfer and leads to the emergence of an interfacial spin glass state. Our study provides new insight into the charge transfer in transition-metal oxide heterostructures and offers a novel means to tune the interfacial exchange coupling for a variety of device applications.

cond-mat.mes-hall↗

arXiv2017

Robust and Pristine Top ological Dirac Semimetal Phase in Pressured Two-Dimensional Black Phosphorous

Very recently, in spite of various efforts in searching for two dimensional topological Dirac semimetals (2D TDSMs) in phosphorene, there remains a lack of experimentally efficient way to activate such phase transition and the underlying mechanism for the topological phase acquisition is still controversial. Here, from first-principles calculations in combination with a band-sorting technique based on k.p theory, a layer-pressure topological phase diagram is obtained and some of the controversies are clarified. We demonstrate that, compared with tuning by external electric-fields, strain or doping by adsorption, hydrostatic pressure can be an experimentally more feasible way to activate the topological phase transition for 2D TDSM acquisition in phosphorene. More importantly, the resultant TDSM state is a pristine phase possessing a single pair of symmetry-protected Dirac cones right at the Fermi level, in startling contrast to the pressured bulk black phosphorous where only a carrier-mixed Dirac state can be obtained. We corroborate that the Dirac points are robust under external perturbation as long as the glide-plane symmetry preserves. Our findings provide a means to realize 2D pristine TDSM in a more achievable manner, which could be crucial in the realization of controllable TDSM states in phosphorene and related 2D materials.

cond-mat.mes-hall↗

arXiv2017

Step Stone Effect: A sp anti-bonding Mediated Long-Range Ferromagnetism in Cr-doped Carrier-Free Bi2Te3

Despite the recent success in the realization of the quantum anomalous Hall effect, the underlying physical mechanism of the long range Ferromagnetism is still unclear. Based on our density functional theory calculations, we discovered an intriguing long-range ferromagnetic order in Cr-doped, carrier-free Bi2Te3, with the separation between dopants more than 8 Å. We found that this magnetic coupling is facilitated by an anti-bonding state originated from the lone pair of a Te 5p state and a Bi 6s state, despite this anti-bonding state lies below the valence band maximum. Such a state serves as a hidden step stone merged in the electron sea and mediates the long-range magnetic order. An effective electron hopping model is proposed to explain this mechanism. This novel mechanism sheds light on the understanding of long-range ferromagnetism in insulators and may lead to the realization of the long-range magnetic order in dilute magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

Giant atomic magnetocrystalline anisotropy from degenerate orbitals around Fermi level

Nano-structures with giant magnetocrystalline anisotropy energies (MAE) are desired in designing miniaturized magnetic storage and quantum computing devices. Through ab initio and model calculations, we propose that special p-element dimers and single-adatom on symmetry-matched substrates possess giant atomic MAE of 72-200 meV with room temperature structural stability. The huge MAE originates from degenerate orbitals around Fermi level. More importantly, we developed a simplified quantum mechanical model to understand the principle on how to obtain giant MAE for supported magnetic structures. These discoveries and mechanisms provide a paradigm to design giant atomic MAE in nanostructures.

cond-mat.mtrl-sci↗

arXiv2015

Realization of Stable Ferromagnetic Order in Topological Insulator: Codoping Enhanced Magnetism in 4f Transition Metal Doped Bi2Se3

The realization of long range and insulating ferromagnetic states in topological insulator (TI) has been a pressing issue since its discovery. Only recently, such state was achieved in Cr-doped Bi2-xSbxTe3, leading to the discovery of quantum anomalous Hall effect (QAHE). However, the effect is only observed at extremely low temperatures mainly due to the limited magnetism. To fully understand the mechanism of the ferromagnetic ordering whereby improving the ferromagnetism, we investigated 4f transition metal-doped Bi2Se3, using density-functional-theory approaches. We found that Eu and Sm prefer the Bi substitutional sites with large magnetic moments to ensure stable long-range ferromagnetic states. Additionally, codoping can be a novel strategy to preserve the insulating property of the host material as well as improving the incorporation of magnetic dopants. Our findings thus offer the critical step in facilitating the realization of QAHE in TI systems.

cond-mat.mtrl-sci↗

arXiv2015

Surface energy calculations from Zinc blende (111)/(-1-1-1) to Wurtzite (0001)/(000-1):a study of ZnO and GaN

The accurate absolute surface energies of (0001)/(000-1) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating dangling bond energy of asymmetrically bonded surface atoms. In this study, we used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches. And the surface energies of (0001)/(000-1) surfaces of wurtzite ZnO and GaN we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group I and group VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces, and the result were also compared with the above method. These calculations and comparisons may provide important insights to crystal growths of the above materials, thereby leading to significant performance enhancements of semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2015

Pseudo-Hydrogen Passivation_A Novel Way to Calculate Absolute Surface Energy of Zinc Blende (111) Surface

Determining accurate absolute surface energies for polar surfaces of semiconductors has been a great challenge in decades. Here, we propose pseudo-hydrogen passivation to calculate them, using density functional theory approaches. By calculating the energy contribution from pseudo-hydrogen using either a pseudo molecule method or a tetrahedral cluster method, we obtained (111) surfaces energies of Si, GaP, and ZnS with high self-consistency. This method quantitatively confirms that surface energy is determined by the number and the energy of dangling bonds of surface atoms. Our findings may greatly enhance the basic understandings of different surfaces and lead to novel strategies in the crystal growth.

cond-mat.mtrl-sci↗