arXiv subjects

Qikun Tian

Publications and source records attributed to Qikun Tian.

5 recordsLinked to original sources

arXiv2025

Two-dimensional Rashba semiconductors and inversion-asymmetric topological insulators in monolayer Janus MAA'ZxZ'(4-x) family

The Rashba effect in Janus structures, accompanied by nontrivial topology, plays an important role in spintronics and even photovoltaic applications. Herein, through first-principles calculations, we systematically investigate the geometric stability and electronic structures of 135 kinds of Janus MAA'ZxZ'(4-x) family derived from two-dimensional MA2Z4 (M=Mg, Ga, Sr; A=Al, Ga; Z=S, Se, Te) monolayers, and design numerous Rashba semiconductors and inversion-asymmetric topological insulators. Specifically, there are a total of 26 Rashba semiconductors with isolated spin splitting bands contributed by Se/Te-pz orbitals at conduction band minimum, and the magnitude of the Rashba constant correlates strongly with both the intrinsic electric field and the strength of spin-orbit coupling (SOC). As the atomic number increases, the bandgap of Janus MAA'ZxZ'(4-x) continually decreases until it shrinks to a point where, when SOC is considered, band inversion occurs, leading to a reopening of the bandgap with nontrivial topological phases. In conjunction with band inversion, pz orbitals near the Fermi level can introduce double Rashba splitting featuring a distinctive hybrid spin texture, which can be further effectively adjusted through small biaxial strains and show a continuous evolution of topological to non-topological accompanied by different spin textures. This work provides significant insights into Rashba and topology physics and further presents indispensable inversion asymmetry materials for the development of nonlinear optoelectronics.

physics.app-ph↗

arXiv2025

Rashba effect modulation in two-dimensional A2B2Te6 (A = Sb, Bi; B = Si, Ge) materials via charge transfer

Designing two-dimensional (2D) Rashba semiconductors, exploring the underlying mechanism of Rashba effect, and further proposing efficient and controllable approaches are crucial for the development of spintronics. On the basis of first-principles calculations, we here theoretically design all possible types (common, inverse, and composite) of Janus structures and successfully achieve numerous ideal 2D Rashba semiconductors from a series of five atomic-layer A2B2Te6 (A = Sb, Bi; B = Si, Ge) materials. Considering the different Rashba constant αR and its modulation trend under external electric field, we comprehensively analyze the intrinsic electric field Ein in terms of work function, electrostatic potential, dipole moment, and inner charge transfer. Inspired by the quantitative relationship between charge transfer and the strength of Ein and even the αR, we propose a straightforward strategy of introducing a single adatom onto the surface of 2D monolayer to introduce and modulate the Rashba effect. Lastly, we also examine the growth feasibility and electronic structures of the Janus Sb2Ge2Se3Te3 system and Janus-adsorbed systems on a 2D BN substrate. Our work not only conducts a detailed analysis of A2B2Te6-based Rashba systems, but also proposes a new strategy for efficiently and controllably modulating the αR through the reconfiguration of charge transfer.

cond-mat.mes-hall↗

arXiv2024

Asymmetric electron distribution induced intrinsically strong anisotropy of thermal transport in bulk CrOCl

Anisotropic heat transfer offers promising solutions to the efficient heat dissipation in the realm of electronic device thermal management. However, the fundamental origin of the anisotropy of thermal transport remains mysterious. In this paper, by combining frequency domain thermoreflectance (FDTR) technique and first-principles-based multiscale simulations, we report the intrinsic anisotropy of thermal transport in bulk CrOCl, and further trace the origin of the anisotropy back to the fundamental electronic structures. The in-plane and cross-plane thermal conductivities ($κ$) at 300 K are found to be 21.6 and 2.18 Wm$^{-1}$K$^{-1}$, respectively, showcasing a strong $κ_\mathrm{in-plane}/κ_\mathrm{cross-plane}$ ratio of $\sim$10. Deep analysis of orbital-resolved electronic structures reveals that electrons are mainly distributed along the in-plane direction with limited interlayer distribution along the cross-plane direction, fundamentally leading to the intrinsic anisotropy of thermal transport in bulk CrOCl. The insight gained in this work sheds light on the design of advanced thermal functional materials.

physics.app-ph↗

arXiv2023

Biaxial strain modulated electronic structures of layered two-dimensional MoSiGeN4 Rashba systems

The two-dimensional (2D) MA2Z4 family has received extensive attention in manipulating its electronic structure and achieving intriguing physical properties. However, engineering the electronic properties remains a challenge. Herein, based on first-principles calculations, we systematically investigate the effect of biaxial strains on the electronic structures of 2D Rashba MoSiGeN4 (MSGN), and further explore how the interlayer interactions affect the Rashba spin splitting in such strained layered MSGNs. After applying biaxial strains, the band gap decreases monotonically with increasing tensile strains but increases when the compressive strains are applied. An indirect-direct-indirect band gap transition is induced by applying a moderate compressive strain (< 5%) in the MSGNs. Due to the symmetry breaking and moderate spin-orbit coupling (SOC), the monolayer MSGN possess an isolated Rashba spin splitting (R) near the Fermi level, which could be effectively regulated to the Lifshitz transition (L) by biaxial strain. For instance, a L-R-L transformation of Fermi surface is presented in monolayer and a more complex and changeable L-R-L-R evolution is observed in bilayer and trilayer MSGNs as the biaxial strain vary from -8% to 12%, which actually depend on the appearance, variation, and vanish of the Mexican hat band in the absence of SOC under different strains. The contribution of Mo-dz2 orbital hybridized with N-pz orbital in the highest valence band plays a dominant role on the band evolution under biaxial strains, where the R-L evolution corresponds to the decreased Mo-dz2 orbital contribution. Our study highlights the biaxial strain controllable Rashba spin splitting, in particular the introduction and even the evolution of Lifshitz transition near Fermi surface, which makes the strained MSGNs as promising candidates for future applications in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

The record low thermal conductivity of monolayer Cuprous Iodide (CuI) with direct wide bandgap

Two-dimensional materials have attracted lots of research interests due to the fantastic properties that are unique to the bulk counterparts. In this paper, from the state-of-the-art first-principles, we predicted the stable structure of monolayer counterpart of the γ-CuI (Cuprous Iodide), which is a p-type wide bandgap semiconductor. The monolayer CuI presents multifunctional superiority in terms of electronic, optical, and thermal transport properties. Specifically, the ultralow thermal conductivity of 0.116 Wm-1K-1 is predicted for monolayer CuI, which is much lower than γ-CuI (0.997 Wm-1K-1) and other typical semiconductors. Moreover, an ultrawide direct bandgap of 3.57 eV is found in monolayer CuI, which is larger than γ-CuI (2.95-3.1 eV), promoting the applications in nano-/optoelectronics with better optical performance. The ultralow thermal conductivity and direct wide bandgap of monolayer CuI as reported in this study would promise its potential applications in transparent and wearable electronics.

cond-mat.mtrl-sci↗