arXiv subjects

Bang-Gui Liu

Publications and source records attributed to Bang-Gui Liu.

At least 55 records · Page 3Linked to original sources

arXiv2016

Electronic structure, magnetism, and optical properties of orthorhombic GdFeO3 from first principles

Orthorhombic GdFeO$_3$ has attracted considerable attention in recent years because its magnetic structure is similar to that in the well-known BiFeO$_3$ material. We investigate electronic structure, magnetism, and optical properties of the orthorhombic GdFeO$_3$ in terms of density-functional-theory calculations. The modified Becke-Johnson (mBJ) exchange potential is adopted to improve on the description of the electronic structure. Our calculation show that the G-type antiferromagnetic (G-AFM ordering of Fe spins) phase of orthorhombic GdFeO$_3$ is stable compared to other magnetic phases. The semiconductor gap calculated with mBJ, substantially larger than that with GGA, is in good agreement with recent experimental values. Besides, we also investigate effect of the spin-orbit coupling on the electronic structure, and calculate with mBJ the complex dielectric functions and other optical functions of photon energy. The magnetic exchange interactions are also investigated, which gives a Neel temperature close to experimental observation. For comparison towards supporting our results, we study the electronic structure of rhombohedral (R3c) BiFeO$_3$ with mBJ. These lead to a satisfactory theoretical understanding of the electronic structure, magnetism, and optical properties of orthorhombic GdFeO$_3$ and can help elucidate electronic structures and optical properties of other similar materials.

cond-mat.mtrl-sci↗

arXiv2016

Biaxial-stress-driven full spin polarization in ferromagnetic hexagonal chromium telluride

It is important to spintronics to achieve fully-spin-polarized magnetic materials that are stable and can be easily fabricated. Here, through systematical density-functional-theory investigations, we achieve high and even full spin polarization for carriers in the ground-state phase of CrTe by applying tensile biaxial stress. The resulting strain is tensile in the xy plane and compressive in the z axis. With the in-plane tensile strain increasing, the ferromagnetic order is stable against antiferromagnetic fluctuations, and a half-metallic ferromagnetism is achieved at an in-plane strain of 4.8%. With the spin-orbit coupling taken into account, the spin polarization is equivalent to 97% at the electronic phase transition point, and then becomes 100.0% at the in-plane strain of 6.0%. These make us believe that the full-spin-polarized ferromagnetism in this stable and easily-realizable hexagonal phase could be realized soon, and applied in spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Ferroelectricity-induced asymmetrical two-dimensional electron gas in superlattices consisteing of insulating GdTiO3 and ferroelectric BaTiO3

Two-dimensional electron gas due to semiconductor interfaces can have high mobility and exhibits superconductivity, magnetism, and other exotic properties that are unexpected in constituent bulk materials. We study crystal structures, electronic states, and magnetism of short-period (BTO)$_m$/(GTO)$_2$ ($m$=2 and 4) superlattices consisting of ferroelectric BaTiO$_3$ (BTO) and ferrimagnetic insulating polar GdTiO$_3$ (GTO) by first principles calculations. Our investigation shows that the middle Ti-O monolayer in the GTO layer becomes metallic because the ferroelectricity in the insulating BTO layer induces an inhomogeneous electric field against the polarity-produced electric field in the GTO layer and thus differentially changes the d energy levels of the three Ti-O monolayers related with the GTO layer. Through avoiding electron reconstruction, the ferroelectric polarization also makes the electronic states and magnetism of two interfacial Ti-O monolayers become substantially different from those in the GTO/SrTiO$_3$ superlattices without ferroelectricity. Such superlattices are interesting for potential spintronics applications because of their unique asymmetrical two-dimensional electron-gas properties and possible useful spin-orbit effects.

cond-mat.mtrl-sci↗

arXiv2016

Two-Dimensional Wide-Band-Gap II-V Semiconductors with a Dilated Graphene-like Structure

Since the advent of graphene, two-dimensional (2D) materials become very attractive and there is growing interest to explore new 2D beyond graphene. Here, through density functional theory (DFT) calculations, we predict 2D wide-band-gap II-V semiconductor materials of M$_3$X$_2$ (M=Zn, Cd and X=N, P, As) with a dilated graphene-like honeycomb structure. The structure features that the group-V X atoms form two X-atomic planes symmetrically astride the centering group-IIB M atomic plane. The 2D Zn$_3$N$_2$, Zn$_3$P$_2$, and Zn$_3$As$_2$ are shown to have direct band gaps of 2.87, 3.81, and 3.55 eV, respectively, and the 2D Cd$_3$N$_2$, Cd$_3$P$_2$, and Cd$_3$As$_2$ exhibit indirect band gaps of 2.74, 3.51, and 3.29 eV, respectively. Each of the six 2D materials is shown to have effective carrier (either hole or electron) masses down to $0.03\sim 0.05$ $m_0$. The structural stability and feasibility of experimental realization of these 2D materials has been shown in terms of DFT phonon spectra and total energy comparison with related existing bulk materials. On the experimental side, there already are many similar two-coordinate structures of Zn and other transition metals in various organic materials, which can be considered to support our DFT prediction. Therefore, these 2D semiconductors can enrich the family of 2D electronic materials and may have promising potential for achieving novel transistors and optoelectronic devices.

cond-mat.mes-hall↗

arXiv2016

Floquet Weyl fermions in circularly-polarised-light-irradiated three-dimensional stacked graphene systems

Using Floquet theory, we illustrate that Floquet Weyl fermions can be created in circularly-polarised-light-irradiated three-dimensional stacked graphene systems. One or two semi-Dirac points can be formed due to overlapping of Floquet sub-bands. Each pair of Weyl points have a two-component semi-Dirac point parent, instead of a four-component Dirac point parent. Decreasing the light frequency will make the Weyl points move in the momentum space, and the Weyl points can approach to the Dirac points when the frequency becomes very small. The frequency-amplitude phase diagram is worked out. It is shown that there exist Fermi arcs in the surface Brillouin zones in circularly-polarised-light-irradiated semi-infinitely-stacked and finitely-multilayered graphene systems. The Floquet Weyl points emerging due to the overlap of Floquet sub-bands provide a new platform to study Weyl fermions.

cond-mat.mes-hall↗

arXiv2015

Exact magnetic field control of nitrogen-vacancy center spin for realizing fast quantum logic gates

The negatively charged nitrogen-vacancy (NV) center spin in diamond can be used to realize quantum computation and to sense magnetic fields. Its spin triplet consists of three levels labeled with its spin z-components of +1, 0, and -1. Without external field, the +1 and -1 states are degenerate and higher than the 0 state due to the zero-field splitting. By taking the symmetrical and anti-symmetrical superpositions of the +1 and -1 states as our qubit basis, we obtain exact evolution operator of the NV center spin under time-dependent magnetic field by mapping the three-level system on time-dependent quantum two-level systems with exact analytical solutions. With our exact evolution operator of the NV center spin including three levels, we show that arbitrary qubits can be prepared from the starting 0 state and arbitrary rapid quantum logic gates of these qubits can be realized with magnetic fields. In addition, it is made clear that the typical quantum logic gates can be accomplished within a few nanoseconds and the fidelity can be very high because only magnetic field strength needs to be controlled in this approach. These results should be useful to realizing quantum computing with the NV center spin systems in diamond and exploring other effects and applications.

cond-mat.mes-hall↗

arXiv2015

Long-lived Neel states in antiferromagnetic quantum spin chains with strong uniaxial anisotropy for atomic-scale antiferromagnetic spintronics

It has been experimentally established that magnetic adatoms on surfaces can be arranged to form antiferromagnetic quantum spin chains with strong uniaxial anisotropy and Neel states in such spin systems can be used to realize information storage. Here, we investigate eigen states, quantum spin dynamics, and life times of Neel states in short antiferromagnetic quantum spin chains with strong uniaxial anisotropy on the basis of numerical exact diagonalization method. We show rigorously that as long as the uniaxial anisotropy is very strong, the ground state and the first excitation state, being nearly degenerate, are safely separated from the other states and thus dominate the quantum dynamics of the Neel states. Through further numerical analysis, we achieve a powerful life-time expression of the Neel states for arbitrary spin and model parameters. It is interesting that for the famous Fe adatom chains on Cu$_2$N surface, 14 or 16 Fe adatoms are enough to obtain a practical long life-time for Neel state storage of information. These should be applicable to other similar antiferromagnetic spin systems for atomic-scale antiferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2015

Hidden chiral symmetry protected $\mathbb{Z}\oplus\mathbb{Z}$ topological insulators in a ladder dimer model

We construct a two-leg ladder dimer model by using two orbitals instead of one in Su-Schrieffer-Heeger (SSH) dimer chain and find out a chiral symmetry in it. In this model, the otherwise-hidden additional chiral symmetry allows us to define two chiral massless fermions and show that there exist interesting topological states characterized by $\mathbb{Z}\oplus\mathbb{Z}$ and corresponding zero mode edge states. Our complete phase diagram reveals that there is an anomalous topologically nontrivial region in addition to the normal one similar to that of SSH model. The anomalous region features that the inter-cell hopping constants can be much smaller than the intra-cell ones, being opposite to the normal region, and the zero mode edge states do not have well-defined parity each. Finally, we suggest that this ladder dimer model can be realized in double-well optical lattices, ladder polymer systems, and adatom double chains on semiconductor surfaces.

cond-mat.str-el↗

arXiv2015

Intrinsic life-time and external manipulation of Neel states in antiferromagnetic adatom spins on semiconductor surfaces

It has been proposed that antiferromagnetic Fe adatom spins on semiconductor Cu-N surfaces can be used to store information [S. Loth {\it et al}, Science \textbf{335}, 196 (2012)]. Here, we investigate spin dynamics of such antiferromagnetic systems through Monte Carlo simulations. We find out the temperature and size laws of switching rates of Néel states and show that the Néel states can become stable enough for the information storage when the number of spins reaches to one or two dozens of the Fe spins. We also explore promising methods for manipulating the Néel states. These could help realize information storage with such antiferromagnetic spin systems.

cond-mat.mtrl-sci↗

arXiv2014

Monte Carlo simulated dynamical magnetization of single-chain magnets

Here, a dynamical Monte-Carlo (DMC) method is used to study temperature-dependent dynamical magnetization of famous Mn2Ni system as typical example of single-chain magnets with strong magnetic anisotropy. Simulated magnetization curves are in good agreement with experimental results under typical temperatures and sweeping rates, and simulated coercive fields as functions of temperature are also consistent with experimental curves. Further analysis indicates that the magnetization reversal is determined by both thermal-activated effects and quantum spin tunnelings. These can help explore basic properties and applications of such important magnetic systems.

cond-mat.mes-hall↗

arXiv2014

High-Curie-temperature ferrimagnetism and ferroelectricity in Bi2FeMoO6

BiFeO3 is the most famous multiferroic material, but its G-type antiferromagnetism is highly desirable to be replaced by strong macroscopic magnetism beyond room temperature. Here we obtain double perovskite Bi2FeMoO6 with R3 (#146) space group by substituting Mo for 50% Fe in BiFeO3. Our first-principles calculated results show that it is a semiconductor with gap reaching to 0.725 eV, its net magnetic moment is 2μ_B per formula unit, and its ferroelectric polarization is 85μC/cm^2. This ferroelctricity is comparable with that of BiFeO3, but here the magnetism is a strong ferrimagnetism with Curie temperature of 650 K. Our first-principles phonon spectra establishes that this R3 phase is stable. Electric polarization and magnetic easy axis are shown to be in pseudo-cubic [111] axis. Our further analysis shows that the multiferroic mechanism is similar to that in BiFeO3. Therefore, this Bi2FeMoO6 can be used to achieve strong macroscopic magnetism and ferroelectricity well above room temperature, being useful for designing new multifunctional materials and devices.

cond-mat.mtrl-sci↗

arXiv2014

Re-t2g-splitting-driven semiconductor gaps in ferrimagnetic double perovskite Ca2MReO6 (M=Cr,Fe) from first principles

Motivated by the observation of nonmetallic nature in double perovskite Ca2CrReO6 and Ca2FeReO6 with high magnetic Curie temperatures of 360 and 522 K, we systematically investigate the structural, electronic, and magnetic properties of Ca2MReO6 (M=Cr,Fe) using the full-potential linear augmented plane wave (FP-LAPW) method within the density functional theory. Our full optimization confirms the stable ground-state structure with $P2_1/n$ symmetry. The modified Becke-Johnson (mBJ) exchange potential is used for investigating electronic structures. Our mBJ calculation shows that they are both ferrimagnetic semiconductors with semiconductor gaps of 0.38 eV and 0.05 eV, respectively, in contrast with wrong metallic phases from the generalized gradient approximation (GGA). The origin of semiconductor gap is due to the further distortion of ReO$_6$ octahedra caused by John-Teller effect, which drives the three partially-occupied Re $t_{2g}$ bands split into two fully-filled bands and one empty band in the minority-spin channel. With the spin-orbit coupling (SOC) taken into account, the Ca2MReO6 (M=Cr,Fe) shows high magneto-crystalline anisotropy (MCA) with the magnetic easy axis along pseudocubic [010] direction, and the total magnetic moments increase by 0.209$μ_B$ and 0.258$μ_B$ per formula unit, respectively, due to the strong SOC effect on Re ion. Although reducing to 0.31 and 0.03 eV, the semiconductor gaps remain open in spite of the SOC broadening of the Re $t_{2g}$-related bands. Therefore, our DFT investigation with mBJ has established the correct ferrimagnetic semiconductor ground state for the double perovskites Ca2MReO6 (M=Cr,Fe). This mechanism, different from that in double perovskite Sr2CrOsO6, can help understand physical properties of other similar compounds.

cond-mat.mtrl-sci↗

arXiv2014

Electrical switching effect of a single-unit-cell CrO2 layer on rutile TiO2 surface

Rutile CrO2 is the most important half-metallic material with nearly 100% spin polarization at the Fermi level, and rutile TiO2 is a wide-gap semiconductor with many applications. Here we show through first-principles investigation that a single-unit-cell CrO2 layer on rutile TiO2 (001) surface is ferromagnetic and semiconductive with a gap of 0.54 eV, and its electronic state transits abruptly to a typical metallic state when an electrical field is applied. Consequently, this makes an interesting electrical switching effect which may be useful in designing spintronic devices.

cond-mat.mes-hall↗

arXiv2012

Promising ferrimagnetic double perovskite oxides towards high spin polarization at high temperature

We predict through our first-principles calculations that four double perovskite oxides of Bi2ABO6 (AB = FeMo, MnMo, MnOs, CrOs) are half-metallic ferrimagnets. Our calculated results shows that the four optimized structures have negative formation energy, from -0.42 to -0.26 eV per formula unit, which implies that they could probably be realized. In the case of Bi2FeMoO6, the half-metallic gap and Curie temperature are predicted to reach to 0.71 eV and 650 K, respectively, which indicates that high spin polarization could be kept at high temperatures far beyond room temperature. It is believed that some of them could be synthesized soon and would prove useful for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2012

Electronic structure and magnetic and optical properties of double perovskite Bi2FeCrO6 from first-principles investigation

Double perovskite Bi$_2$FeCrO$_6$, related with BiFeO$_3$, is very interesting because strong ferroelectricity and high magnetic Curie temperature beyond room temperature are observed in it. However, existing density-functional-theory (DFT) studies, using pseudo-potentials, produce metallic ground state under the local density approximation (LDA) and need LDA+U method to yield needed nonmetallic ground state, resulting in low magnetic Curie temperature (below 130 K). Here, we optimize its crystal structure and then investigate its electronic structure and magnetic and optical properties by combining the full-potential augmented plane wave method with Monte Carlo simulation. Our optimized structure is a robust ferrimagnetic semiconductor. This nonmetallic phase is formed due to crystal field splitting and spin exchange splitting, in contrast to Mott-Hubbard states in previous DFT studies. Spin exchange constants and optical properties are calculated. Our ab initio magnetic Curie temperature is 450 K, much higher than previous DFT-based value and consistent with experimental results. Our study and analysis reveals that the main magnetic mechanism is an antiferromagnetic superexchange between Fe and Cr over the intermediate O atom. These results are useful to understanding such perovskite materials and exploring their potential applications.

cond-mat.mtrl-sci↗

arXiv2010

Dynamical Monte Carlo investigation of spin reversals and nonequilibrium magnetization of single-molecule magnets

In this paper, we combine thermal effects with Landau-Zener (LZ) quantum tunneling effects in a dynamical Monte Carlo (DMC) framework to produce satisfactory magnetization curves of single-molecule magnet (SMM) systems. We use the giant spin approximation for SMM spins and consider regular lattices of SMMs with magnetic dipolar interactions (MDI). We calculate spin reversal probabilities from thermal-activated barrier hurdling, direct LZ tunneling, and thermal-assisted LZ tunnelings in the presence of sweeping magnetic fields. We do systematical DMC simulations for Mn$_{12}$ systems with various temperatures and sweeping rates. Our simulations produce clear step structures in low-temperature magnetization curves, and our results show that the thermally activated barrier hurdling becomes dominating at high temperature near 3K and the thermal-assisted tunnelings play important roles at intermediate temperature. These are consistent with corresponding experimental results on good Mn$_{12}$ samples (with less disorders) in the presence of little misalignments between the easy axis and applied magnetic fields, and therefore our magnetization curves are satisfactory. Furthermore, our DMC results show that the MDI, with the thermal effects, have important effects on the LZ tunneling processes, but both the MDI and the LZ tunneling give place to the thermal-activated barrier hurdling effect in determining the magnetization curves when the temperature is near 3K. This DMC approach can be applicable to other SMM systems, and could be used to study other properties of SMM systems.

cond-mat.mes-hall↗

arXiv2010

Improved half-metallic ferromagnetism of transition-metal pnictides and chalcogenides calculated with a modified Becke-Johnson exchange potential

We use a density-functional-theory (DFT) approach with a modified Becke-Johnson exchange plus local density approximation (LDA) correlation potential (mBJLDA) [semi-local, orbital-independent, producing accurate semiconductor gaps. see F. Tran and P. Blaha, Phys. Rev. Lett. 102, 226401 (2009)] to investigate the electronic structures of zincblende transition-metal (TM) pnictides and chalcogenides akin to semiconductors. Our results show that this potential does not yield visible changes in wide TM d-t_{2g} bands near the Fermi level, but makes the occupied minority-spin p-bands lower by 0.25~0.35 eV and the empty (or nearly empty) minority-spin e_g bands across the Fermi level higher by 0.33~0.73 eV. Consequently, mBJLDA, having no atom-dependent parameters, makes zincblende MnAs become a truly half-metallic (HM) ferromagnet with a HM gap (the key parameter) 0.318eV, being consistent with experiment. For zincblende MnSb, CrAs, CrSb, CrSe, or CrTe, the HM gap is enhanced by 19~56% compared to LDA and generalized gradient approximation results. The improved HM ferromagnetism can be understood in terms of the mBJLDA-enhanced spin exchange splitting.

cond-mat.mtrl-sci↗

arXiv2010

Robust half-metallic ferromagnet of cubic VGe3Si4

To seek half-metallic ferromagnets compatible with the GeSi semiconductor, we use state-of-the-art density-functional-theory methods to study cubic $\mathrm{VGe_3Si_4}$. After optimizing its crystal structure with various magnetic orders considered, we find the ferromagnetic structure is the ground state phase and investigate its stability, and then study its spin-resolved electronic structure with an improved method. Our calculated results show that the cubic $\mathrm{VGe_3Si_4}$ is a half-metallic ferromagnet with half-metallic gap 350 meV, and it is structurally stable against deformations and magnetically robust against antiferromagnetic fluctuations. These results make us believe that the cubic $\mathrm{VGe_3Si_4}$ could be synthesized as good thin films soon and used in practical spintronic devices.

cond-mat.mtrl-sci↗