arXiv subjects

Yugui Yao

Publications and source records attributed to Yugui Yao.

At least 253 records · Page 14Linked to original sources

arXiv2017

Topological, Valleytronic, and Optical Properties of Monolayer PbS

We systematically examine the topological, valleytronic, and optical properties of experimentally accessible PbS (001) few-layers, with a focus on the monolayer. With even-odd layer-dependent oscillations and without band inversions, the band gaps cover a wide spectrum from infrared to visible, making the few-layers promising for optoelectronics. Intriguingly, the uniaxial (biaxial) compressive strain can tune the trivial monolayer into a $\mathcal{Z}_2$-topological (topological crystalline) insulator, which may be utilized for controllable low-power electronic devices. Although elliptical dichroism vanishes in the monolayer due to inversion symmetry, optical pumping provides an efficient tool to characterize the three phases and to realize charge, spin, and valley Hall effects in optoelectronic transport that are tunable by external strain and light ellipticity.

cond-mat.mes-hall↗

arXiv2017

Tunable Hyperbolic Dispersion and Negative Refraction in Natural Electride Materials

Hyperbolic (or indefinite) materials have attracted significant attention due to their unique capabilities for engineering electromagnetic space and controlling light propagation. A current challenge is to find a hyperbolic material with wide working frequency window, low energy loss, and easy controllability. Here, we propose that naturally existing electride materials could serve as high-performance hyperbolic medium. Taking the electride Ca$_2$N as a concrete example and using first-principles calculations, we show that the material is hyperbolic over a wide frequency window from short-wavelength to near infrared. More importantly, it is almost lossless in the window. We clarify the physical origin of these remarkable properties, and show its all-angle negative refraction effect. Moreover, we find that the optical properties can be effectively tuned by strain. With moderate strain, the material can even be switched between elliptic and hyperbolic for a particular frequency. Our result points out a new route toward high-performance natural hyperbolic materials, and it offers realistic materials and novel methods to achieve controllable hyperbolic dispersion with great potential for applications.

cond-mat.mtrl-sci↗

arXiv2016

Theoretical prediction of MoN2 monolayer as a high capacity electrode material for metal ion batteries

Benefited from the advantages on environmental benign, easy purification, and high thermal stability, the recently synthesized two-dimensional (2D) material MoN2 shows great potential for clean and renewable energy applications. Here, through first-principles calculations, we show that the monolayered MoN2 is promising to be a high capacity electrode material for metal ion batteries. Firstly, identified by phonon dispersion and exfoliation energy calculations, MoN2 monolayer is proved to be structurally stable and could be exfoliated from its bulk counterpart in experiments. Secondly, all the studied metal atoms (Li, Na and K) can be adsorbed on MoN2 monolayer, with both pristine and doped MoN2 being metallic. Thirdly, the metal atoms possess moderate/low migration barriers on MoN2, which ensures excellent cycling performance as a battery electrode. In addition, the calculated average voltages suggest that MoN2 monolayer is suitable to be a cathode for Li-ion battery and anodes for Na-ion and K-ion batteries. Most importantly, as a cathode for Li-ion battery, MoN2 possesses a comparable average voltage but a 1-2 times larger capacity (432 mA h g-1) than usual commercial cathode materials; as an anode for Na-ion battery, the theoretical capacity (864 mA h g-1) of MoN2 is 2-5 times larger than typical 2D anode materials such as MoS2 and most MXenes. Finally we also provide an estimation of capacities for other transition-metal dinitrides materials. Our work suggests that the transition-metal dinitrides MoN2 is an appealing 2D electrode materials with high storage capacities.

cond-mat.mtrl-sci↗

arXiv2016

Predicted Unusual Magnetoresponse in Type-II Weyl Semimetals

We show several distinct signatures in the magneto-response of type-II Weyl semimetals. The energy tilt tends to squeeze the Landau levels (LLs), and for a type-II Weyl node, there always exists a critical angle between the B-fileld and the tilt, at which the LL spectrum collapses, regardless of the fileld strength. Before collapse, signatures also appear in the magneto-optical spectrum, including the invariable presence of intraband peaks, the absence of absorption tails, and the special anisotropic fileld dependence.

cond-mat.mes-hall↗

arXiv2016

Borophene as an extremely high capacity electrode material for Li-ion and Na-ion batteries

Two-dimensional (2D) materials as electrodes is believed to be the trend for future Li-ion and Na-ion batteries technologies. Here, by using first-principles methods, we predict that the recently reported borophene (2D born sheet) can serve as an ideal electrode material with high electrochemical performance for both Li-ion and Na-ion batteries. The calculations are performed on the two experimentally stable borophene structures, namely \b{eta}12 and \c{hi}3 structures. The optimized Li and Na adsorption sites are identified, and the host materials are found to maintain good electric conductivity before and after adsorption. Besides advantages including small diffusion barriers and low average open-circuit voltages, most remarkably, the storage capacity can be as high as 1984 mA h g-1 in \b{eta}12 borophene and 1240 mA h g-1 in \c{hi}3 borophene for both Li and Na, which is several times higher than the commercial graphite electrode and is the highest among all the 2D materials discovered to date. Our results highly support that borophenes can be appealing anode materials for both Li-ion and Na-ion batteries with extremely high power density.

cond-mat.mes-hall↗

arXiv2016

Possible Superconductivity Approaching Ice Point

Recently BCS superconductivity at 203 K has been discovery in a highly compressed hydrogen sulfide. We use first-principles calculations to systematically examine the effects of partially substituting the chalcogen atoms on the superconductivity of hydrogen chalcogenides under high pressures. We find detailed trends of how the critical temperature changes with increasing the V-, VI- or VII-substitution rate, which highlight the key roles played by low atomic mass and by strong covalent metallicity. In particular, a possible record high critical temperature of 280 K is predicted in a stable H3S0.925P0.075 with the Im-3m structure under 250 GPa.

cond-mat.supr-con↗

arXiv2016

van der Waals Heterostructures of Germanene, Stanene and Silicene with Hexagonal Boron Nitride and Their Topological Domain Walls

We investigate van der Waals (vdW) heterostructures made of germanene, stanene or silicene with hexagonal Boron Nitride (h-BN). The intriguing topological properties of these buckled honeycomb materials can be maintained and further engineered in the heterostructures, where the competition between the substrate effect and external electric fields can be used to control the tunable topological phase transitions. Using such heterostructures as building blocks, various vdW topological domain walls (DW) are designed, along which there exist valley polarized quantum spin Hall edge states or valley-contrasting edge states which are protected by valley(spin)- resolved topological charges and can be tailored by the patterning of the heterojunctions and by external fields.

cond-mat.mes-hall↗

arXiv2015

Electronic structures and theoretical modelling of two-dimensional group-VIB transition metal dichalcogenides

Atomically thin group-VIB transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional (2D) semiconductors with extraordinary properties including the direct band gap in the visible frequency range, the pronounced spin-orbit coupling, the ultra-strong Coulomb interaction, and the rich physics associated with the valley degree of freedom. These 2D TMDs exhibit great potentials for device applications and have attracted vast interest for the exploration of new physics. 2D TMDs have complex electronic structures which underlie their physical properties. Here we review the bulk electronic structures in these new 2D materials as well as the theoretical models developed at different levels, along which we sort out the understandings on the origins of a variety of properties observed or predicted.

cond-mat.mes-hall↗

arXiv2015

Topological magnetic phase in LaMnO$_3$ (111) bilayer

Candidates for correlated topological insulators, originated from the spin-orbit coupling as well as Hubbard type correlation, are expected in the ($111$) bilayer of perovskite-structural transition-metal oxides. Based on the first-principles calculation and tight-binding model, the electronic structure of a LaMnO$_3$ ($111$) bilayer sandwiched in LaScO$_3$ barriers has been investigated. For the ideal undistorted perovskite structure, the Fermi energy of LaMnO$_3$ ($111$) bilayer just stays at the Dirac point, rendering a semi-metal (graphene-like) which is also a half-metal (different from graphene nor previous studied LaNiO$_3$ ($111$) bilayer). The Dirac cone can be opened by the spin-orbit coupling, giving rise to nontrivial topological bands corresponding to the (quantized) anomalous Hall effect. For the realistic orthorhombic distorted lattice, the Dirac point moves with increasing Hubbard repulsion (or equivalent Jahn-Teller distortion). Finally, a Mott gap opens, establishing a phase boundary between the Mott insulator and topological magnetic insulator. Our calculation finds that the gap opened by spin-orbit coupling is much smaller in the orthorhombic distorted lattice ($\sim$$1.7$ meV) than the undistorted one ($\sim$$11$ meV). Therefore, to suppress the lattice distortion can be helpful to enhance the robustness of topological phase in perovskite ($111$) bilayers.

cond-mat.mtrl-sci↗

arXiv2015

Topological p+ip superconductivity in doped graphene-like single-sheet materials BC$_3$

We theoretically study exotic superconducting phases in graphenelike single-sheet material BC$_3$ doped to its type-II Van Hove singularity whose saddle point momenta are {\it not} time-reversal-invariant. From combined renormalization group analysis and RPA calculations, we show that the dominant superconducting instability induced by weak repulsive interactions is in the time-reversal-invariant $p+ip$ pairing channel because of the interplay among dominant ferromagnetic fluctuations, subleading spin fluctuations at finite momentum, and spin-orbit coupling. Such time-reversal-invariant $p+ip$ superconductivity has nontrivial $\mathbb{Z}_2$ topological invariant. Our results show that doped BC$_3$ provides a promising route to realize a genuine 2D helical $p+ip$ superconductor.

cond-mat.str-el↗

arXiv2015

Electric field controlled spin- and valley-polarized edge states in silicene with extrinsic Rashba effect

In the presence of extrinsic Rashba spin-orbit coupling, we find that silicene can host a new quantum anomalous Hall state with spin- and valley-polarized edge states, which can be effectively controlled by the exchange field and electric field. In this new state, the pair of nontrivial edge states reside in one specific valley and have a strong but opposite spin polarization. A distinctive feature of this new state is that both of the spin and valley index of the edge states can be switched by reversing the electric field. We also present a microscopic mechanism for the origination of the new state. Our findings provide an efficient way to control the topologically protected spin- and valley-polarized edge states, which is crucial for spintronics and valleytronics.

cond-mat.mes-hall↗

arXiv2015

Strain effects on electronic and optic properties of monolayer C$_2$N holey two-dimensional crystals

A new two-dimensional material, the C$_2$N holey 2D (C$_2$N-$h$2D) crystal, has recently been synthesized. Here we investigate the strain effects on the properties of this new material by first-principles calculations. We show that the material is quite soft with a small stiffness constant and can sustain large strains $\geq 12\%$. It remains a direct gap semiconductor under strain and the bandgap size can be tuned in a wide range as large as 1 eV. Interestingly, for biaxial strain, a band crossing effect occurs at the valence band maximum close to a 8\% strain, leading to a dramatic increase of the hole effective mass. Strong optical absorption can be achieved by strain tuning with absorption coefficient $\sim10^6$ cm$^{-1}$ covering a wide spectrum. Our findings suggest the great potential of strain-engineered C$_2$N-$h$2D in electronic and optoelectronic device applications.

cond-mat.mtrl-sci↗

arXiv2015

Weak Topological Insulators and Composite Weyl Semimetals: $β$-Bi$_{4}$X$_{4}$ (X=Br, I)

While strong topological insulators (STI) have been experimentally realized soon after their theoretical predictions, a weak topological insulator (WTI) has yet to be unambiguously confirmed. A major obstacle is the lack of distinct natural cleavage surfaces to test the surface selective hallmark of WTI. With a new scheme, we discover that Bi$_{4}$X$_{4}$ (X=Br, I), stable or synthesized before, can be WTI with two natural cleavage surfaces, where two anisotropic Dirac cones stabilize and annihilate, respectively. We further find four surface state Lifshitz transitions under charge doping and two bulk topological phase transitions under uniaxial strain. Near the WTI-STI transition, there emerges a novel Weyl semimetal phase, in which the Fermi arcs generically appear at both cleavage surfaces whereas the Fermi circle only appears at one selected surface.

cond-mat.mes-hall↗

arXiv2015

Large magneto-optical Kerr effect in noncollinear antiferromagnets Mn$_{3}X$ ($X$ = Rh, Ir, or Pt)

Magneto-optical Kerr effect, normally found in magnetic materials with nonzero magnetization such as ferromagnets and ferrimagnets, has been known for more than a century. Here, using first-principles density functional theory, we demonstrate large magneto-optical Kerr effect in high temperature noncollinear antiferromagnets Mn$_{3}X$ ($X$ = Rh, Ir, or Pt), in contrast to usual wisdom. The calculated Kerr rotation angles are large, being comparable to that of transition metal magnets such as bcc Fe. The large Kerr rotation angles and ellipticities are found to originate from the lifting of the band double-degeneracy due to the absence of spatial symmetry in the Mn$_{3}X$ noncollinear antiferromagnets which together with the time-reversal symmetry would preserve the Kramers theorem. Our results indicate that Mn$_{3}X$ would provide a rare material platform for exploration of subtle magneto-optical phenomena in noncollinear magnetic materials without net magnetization.

cond-mat.mtrl-sci↗

arXiv2015

Large-Gap Quantum Spin Hall Insulator in single layer bismuth monobromide Bi$_{4}$Br$_{4}$

Quantum spin Hall (QSH) insulators have gapless topological edge states inside the bulk band gap, which can serve as dissipationless spin current channels protected by the time-reversal symmetry. The major challenge currently is to find suitable materials for this topological state. Here, we predict a new large-gap QSH insulator with bulk direct band gap of $\sim$0.18 eV, in single-layer Bi$_{4}$Br$_{4}$, which could be exfoliated from its three-dimensional bulk material due to the weakly-bonded layered structure. The band gap of single-layer Bi$_{4}$Br$_{4}$ is tunable via strain engineering, and the QSH phase is robust against external strain. In particular, because this material consists of special one-dimensional molecular chain as its basic building block, the single layer Bi$_{4}$Br$_{4}$ could be easily torn to ribbons with clean and atomically sharp edges, which are much desired for the observation and application of topological edge states. Our work thus provides a new promising material for experimental studies and practical applications of QSH effect.

cond-mat.mtrl-sci↗

arXiv2015

Itinerant Ferromagnetism and $p+ip'$ Superconductivity in Doped Bilayer Silicene

We study the electronic instabilities of doped bilayer silicene using the random phase approximation. In contrast to the singlet $d+id'$ superconductivity at the low doping region, we find that the system is an itinerant ferromagnet in the narrow doping regions around the Van Hove singularities, and a triplet $p+ip'$ superconductor in the vicinity of these regions. Adding a weak Kane-Mele spin-orbit coupling to the system further singles out the time-reversal invariant equal-spin helical $p+ip'$ pairing as the leading instability. The triplet pairing identified here is driven by the ferromagnetic fluctuations, which become strong and enhance the superconducting critical temperature remarkably near the phase boundaries between ferromagnetism and superconductivity.

cond-mat.supr-con↗

arXiv2015

Valley-polarized quantum anomalous Hall phases and tunable topological phase transitions in half-hydrogenated Bi honeycomb monolayers

Based on first-principles calculations, we find novel valley-polarized quantum anomalous Hall (VP-QAH) phases with a large gap-0.19 eV at an appropriate buckled angle and tunable topological phase transitions driven by the spontaneous magnetization within a half-hydrogenated Bi honeycomb monolayer. Depending on the magnetization orientation, four different phases can emerge, i.e., two VP-QAH phases, ferromagnetic insulating and metallic states. When the magnetization is reversed from the +$\mathbf{z}$ to -$\mathbf{z}$ directions, accompanying with a sign change in the Chern number (from -1 to +1), the chiral edge state is moved from valley $K$ to $K'$. Our findings provide a platform for designing dissipationless electronics and valleytronics in a more robust manner through the tuning of the magnetization orientation.

cond-mat.mtrl-sci↗

arXiv2015

Time-reversal-invariant topological superconductivity in n-doped BiH

Intrinsic and symmetry protected topological states have attracted lots of interest in condensed matter physics recently. In particular, time reversal symmetry protected fermion topological insulators have been theoretically predicted and experimentally verified. However despite considerable experimental and theoretical works, definitive evidence for time reversal invariant topological superconductivity is still lacking. Here we propose that upon electron doping the hydrogenated single bilayer Bi, namely BiH, will exhibit time reversal invariant topological superconductivity. If confirmed experimentally this material will constitute the first example of TRI topological superconductor.

cond-mat.str-el↗