Electric Field-Controlled Magneto-Optical Kerr Effect in A-Type Antiferromagnetic Fe2CX2 (X = F, Cl) and Its Janus Monolayer.

The magneto-optical Kerr effect (MOKE) is a powerful probe of magnetism and has recently gained new attention in antiferromagnetic (AFM) materials. Through extensive first-principles calculations and group theory analysis, we have identified Fe2CX2 (X = F, Cl) and Janus Fe2CFCl monolayers as ideal A-type collinear AFM materials with high magnetic anisotropy and Néel temperatures. By applying a vertical external electrical field (Ef) of 0.2 V/Å, the MOKE is activated for Fe2CF2 and Fe2CCl2 monolayers without changing their magnetic ground state, and the maximum Kerr rotation angles are 0.13 and 0.08°, respectively. Due to the out-of-plane spontaneous polarization, the intrinsic and nonvolatile MOKE is found in the Janus Fe2CFCl monolayer and the maximal Kerr rotation angle without external electronic field is 0.25°. Moreover, the intrinsic built-in electronic field also gives origin to more robust A-type AFM ordering and reversible Kerr angle against external Ef. Our study suggests that Ef is an effective tool for controlling MOKE in two-dimensional (2D) AFM materials. This research opens the possibility of related studies and applications in AFM spintronics.

Tunable Van Hove Singularity without Structural Instability in Kagome Metal CsTi_{3}Bi_{5}.

In kagome metal CsV_{3}Sb_{5}, multiple intertwined orders are accompanied by both electronic and structural instabilities. These exotic orders have attracted much recent attention, but their origins remain elusive. The newly discovered CsTi_{3}Bi_{5} is a Ti-based kagome metal to parallel CsV_{3}Sb_{5}. Here, we report angle-resolved photoemission experiments and first-principles calculations on pristine and Cs-doped CsTi_{3}Bi_{5} samples. Our results reveal that the van Hove singularity (vHS) in CsTi_{3}Bi_{5} can be tuned in a large energy range without structural instability, different from that in CsV_{3}Sb_{5}. As such, CsTi_{3}Bi_{5} provides a complementary platform to disentangle and investigate the electronic instability with a tunable vHS in kagome metals.

Straight and twisted open nodal-line phonon states in the CaI2 family of materials.

Based on first-principles calculations and symmetry analysis, we propose that trigonal CaI2 with the space group P3̄m1 possesses straight and twisted open nodal-line phonon states with linear dispersion. The symmetry analysis indicates that joint symmetry PT and rotational symmetry C3z protect the straight nodal lines along Γ-A and K-H while PT and mirror symmetry M010 (M110) maintain the twisted nodal lines that traverse Γ-M (Γ-K) and A-L (A-H). The calculated π Berry phase suggests that all the nodal lines are nontrivial and the corresponding drumhead-like surface states are clearly visible in the observation window, which is less than 6 THz, suggesting a significant chance for them to be measured using meV-resolution inelastic X-ray scattering. The distribution of the nodal lines in the Brillouin zone is also confirmed by the phononic tight-binding model. Furthermore, the isostructural compounds MgBr2 and MgI2 show similar phonon spectra and topological nontrivial surface states. This work provides promising candidates for investigating straight and twisted open nodal-line phonon states in a single material, which will facilitate future experimental observation.

A two-dimensional PtS2/BN heterostructure as an S-scheme photocatalyst with enhanced activity for overall water splitting.

Photocatalytic hydrogen production from water is a sustainable solution to the environmental pollution and energy crises. Encouraged by the successful synthesis of PtS2 and BN nanosheets and their suitable band edges, we have designed a PtS2/BN bilayer heterojunction and investigated its electronic and optical properties for the first time based on hybrid DFT calculations. In this system, the built-in electric field and band edge bending can retain useful electrons on the conduction band of BN and holes on the valence band of PtS2, which endow this system with a stronger redox ability. Meanwhile, this electric field can efficiently separate photoinduced electron-hole pairs and improve the photocatalytic efficiency. Compared with BN and PtS2 single layers, the PtS2/BN heterojunction with its smaller bandgap can make better use of visible and infrared light. Additionally, we have studied the effect of applied strain on the electronic and optical properties. This work aims to provide a method for constructing high-efficiency BN-based photocatalysts and illuminating the electron migration mechanism in step-scheme (S-scheme) heterostructures. We have found that the PtS2/BN bilayer heterojunction is a promising S-scheme photocatalyst for overall water decomposition.

Strain-induced quantum phase transition in the C3Sc4 monolayer: towards multiple gapless fermions.

Most two dimensional (2D) topological materials host only one kind of fermionic state. However, realizing multiple gapless fermions in a single 2D material is rarely reported. Furthermore, researchers face challenges in regulating various gapless fermion transitions using specific methods. Herein, we perform a study based on the first-principles calculation to investigate the electronic structures and the related fermionic states of strained 2D C3Sc4. C3Sc4 is an ideal system in the ground state with twelve Dirac points. The dynamical, mechanical, and thermal stabilities of the proposed C3Sc4 monolayer are demonstrated in detail. Interestingly, under the condition of 9.5% biaxial tensile strain, gapless and quadratic Weyl fermionic states are observed at the Γ point. A gapless and massless pseudospin-1 fermion appears at the Γ point in the 2D C3Sc4 system under 13% biaxial tensile strain and with hole doping. The Fermi velocity of this massless pseudospin-1 fermion is 2.1 × 105 m s-1, comparable to that of well-known 2D gapless topological materials. The results indicate that 2D C3Sc4 is an ideal playground to explore interesting behaviors of quantum phase transitions and rich gapless fermionic states and also reveal its potential applications in high-speed nano-devices.

Time-reversal-breaking Weyl nodal lines in two-dimensional A3C2 (A = Ti, Zr, and Hf) intrinsically ferromagnetic materials with high Curie temperature.

Most materials that feature nontrivial band topology are spin-degenerate and three dimensional, strongly restricting them from application in spintronic nanodevices. Hence, two-dimensional (2D) intrinsically spin-polarized systems with rich topological elements are still in extreme scarcity. Here, 2D A3C2 (A = Ti, Zr, and Hf) materials with the P6[combining macron]m2 type structure are reported as new ferromagnetic materials with intrinsic magnetism and good stability. Unlike the Weyl nodal lines existing in nonmagnetic 2D systems, A3C2 hosts time-reversal-breaking Weyl nodal rings (two Γ-centered, one K-centered, and one K'-centered) without spin-orbit coupling (SOC). These nodal rings still remained under SOC with magnetization along the z direction (easy magnetization axis). More interestingly, the Curie temperatures (TC) of A3C2 were determined based on the Monte Carlo simulation. Ti3C2 features an extraordinary TC (above 800 K), and those of Zr3C2 and Hf3C2 are above room temperature. Therefore, A3C2 materials are excellent platforms to study magnetic Weyl nodal lines in high TC ferromagnetic 2D materials.

First-principle investigation of all types of topological nodal lines in a realistic P63/mmc type titanium selenide.

Topological nodal line (TNL) materials with one-dimensional band-crossing points (BCPs) exhibit interesting electronic characteristics and have special applications in electronic devices. Normally, based on the slopes of the crossing bands, the BCPs can be divided into two types, i.e., type I and type II nodal points. Based on the combination of the different types of nodal points, the nodal lines (NLs) can be divided into three categories: (i) type I NL, type II NL, and hybrid NL, these being formed by type I nodal points, type II nodal points, and type I and II nodal points, respectively. Compared with the large number of predicted type I NL materials, there are less type II and hybrid NL materials. In this study, it is predicted that P63/mmc type TiSe metal is a topological material which exhibits all types of NL states. Furthermore, the dynamic stability as well as the effect of spin-orbit coupling on the topological signatures are examined. Also, the nontrivial surface states are shown to provide evidence for the occurrence of the NL states. This novel material can be seen as a good platform to use for further investigations on the three types of NLs and diverse fermions.

Phase Transition and Electronic Structures of All-d-Metal Heusler-Type X2MnTi Compounds (X = Pd, Pt, Ag, Au, Cu, and Ni).

In this work, we investigated the phase transition and electronic structures of some newly designed all-d-metal Heusler compounds, X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni), by means of the first principles. The competition between the XA and L21 structures of these materials was studied, and we found that X2MnTi favors to feature the L21-type structure, which is consistent with the well-known site-preference rule (SPR). Under the L21 structure, we have studied the most stable magnetic state of these materials, and we found that the ferromagnetic state is the most stable due to its lower energy. Through tetragonal deformation, we found that the L21 structure is no longer the most stable structure, and a more stable tetragonal L10 structure appeared. That is, under the tetragonal strain, the material enjoys a tetragonal phase transformation (i.e., from cubic L21 to tetragonal L10 structure). This mechanism of L21-L10 structure transition is discussed in detail based on the calculated density of states. Moreover, we found that the energy difference between the most stable phases of L10 and L21, defined as ΔE M (ΔE M = E Cubic-E Tetragonal), can be adjusted by the uniform strain. Finally, the phonon spectra of all tetragonal X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni) phases are exhibited, which provides a powerful evidence for the stability of the tetragonal L10 state. We hope that our research can provide a theoretical guidance for future experimental investigations.

Perfect Topological Metal CrB2: A One-Dimensional (1D) Nodal Line, a Zero-Dimensional (0D) Triply Degenerate Point, and a Large Linear Energy Range.

Topological materials with band-crossing points exhibit interesting electronic characteristics and have special applications in electronic devices. However, to further facilitate the experimental detection of the signatures of these band crossings, topological materials with a large linear energy range around the band-crossing points need to be found, which is challenging. Here, via first-principle approaches, we report that the previously prepared P6/mmm-type CrB2 material is a topological metal with one pair of 1D band-crossing points, that is, nodal lines, in the kz= 0 plane, and one pair of 0D band-crossing points, that is, triple points, along the A-Γ-A' paths. Remarkably, around these band-crossing points, a large linear energy range (larger than 1 eV) was found and the value was much larger than that found in previously studied materials with a similar linear crossing. The pair of nodal lines showed obvious surface states, which show promise for experimental detection. The effect of the spin-orbit coupling on the band-crossing points was examined and the gaps induced by spin-orbit coupling were found to be up to 69 meV. This material was shown to be phase stable in theory and was synthesized in experiments, and is therefore a potential material for use in investigating nodal lines and triple points.

Insight into the Topological Nodal Line Metal YB2 with Large Linear Energy Range: A First-Principles Study.

The presence of one-dimensional (1D) nodal lines, which are formed by band crossing points along a line in the momentum space of materials, is accompanied by several interesting features. However, in order to facilitate experimental detection of the band crossing point signatures, the materials must possess a large linear energy range around the band crossing points. In this work, we focused on a topological metal, YB2, with phase stability and a P6/mmm space group, and studied the phonon dispersion, electronic structure, and topological nodal line signatures via first principles. The computed results show that YB2 is a metallic material with one pair of closed nodal lines in the kz = 0 plane. Importantly, around the band crossing points, a large linear energy range in excess of 2 eV was observed, which was rarely reported in previous reports that focus on linear-crossing materials. Furthermore, YB2 has the following advantages: (1) An absence of a virtual frequency for phonon dispersion, (2) an obvious nontrivial surface state around the band crossing point, and (3) small spin-orbit coupling-induced gaps for the band crossing points.

Computational Insights Into the Electronic Structure and Magnetic Properties of Rhombohedral Type Half-Metal GdMnO3 With Multiple Dirac-Like Band Crossings.

In spintronics, half-metallic materials (HMMs) with Dirac-like cones exhibit interesting physical properties such as massless Dirac fermions and full spin polarization. We combined first-principles calculations with the quasi-harmonic Debye model, and we proposed that the rhombohedral GdMnO3 is an HMM with multiple linear band crossings. The physical properties of GdMnO3 were studied thoroughly. Moreover, the changes of multiple linear band crossings and 100% spin polarization under spin-orbit coupling as well as the electron and hole doping were also investigated. It is noted that such spin-polarized HMMs with linear band crossings are still very rare in two-dimensional and three-dimensional materials.

Strain tuning of closed topological nodal lines and opposite pockets in quasi-two-dimensional α-phase FeSi2.

Following topological nodal point semimetals, topological nodal line semimetals with one dimensional (1D) topological elements have recently aroused great interest worldwide in the fields of quantum chemistry and condensed matter physics. In this study, by means of first-principles, we predict that quasi-two-dimensional (2D) α-FeSi2 with a P4/mmm space group is a topological nodal line semimetal with two nodal lines close to the Fermi level, in the kz = 0 and kz = π planes. Usually, topological nodal line semimetals can be classified into type I, type II, and hybrid-type categories, each type with different physical properties. Importantly, for the first time, we find that type I, type II, and hybrid-type nodal lines can be realized in a realistic material, i.e., quasi-2D α-FeSi2, by strain switching. The realization of tunable nodal line types occurs because quasi-2D α-FeSi2 has special opposite-pocket-behaving bands around the Fermi level. The results presented herein reflect that α-FeSi2 is a valuable candidate for spintronics application by utilization of type I, type II, and hybrid-type topological nodal line semimetals in a single material tuned by mechanical strain.

Site preference and tetragonal distortion in palladium-rich Heusler alloys.

In this work, two kinds of competition between different Heusler structure types are considered, one is the competition between XA and L21 structures based on the cubic system of full-Heusler alloys, Pd2 YZ (Y = Co, Fe, Mn; Z = B, Al, Ga, In, Tl, Si, Ge, Sn, Pb, P, As, Sb). Most alloys prefer the L21 structure; that is, Pd atoms tend to occupy the a (0, 0, 0) and c (0.5, 0.5, 0.5) Wyckoff sites, the Y atom is generally located at site b (0.25, 0.25, 0.25), and the main group element Z has a preference for site d (0.75, 0.75, 0.75), meeting the well known site-preference rule. The difference between these two cubic structures in terms of their magnetic and electronic properties is illustrated further by their phonon dispersion and density-of-states curves. The second type of competition that was subjected to systematic study was the competitive mechanism between the L21 cubic system and its L10 tetragonal system. A series of potential tetragonal distortions in cubic full-Heusler alloys (Pd2 YZ) have been predicted in this work. The valley-and-peak structure at, or in the vicinity of, the Fermi level in both spin channels is mainly attributed to the tetragonal ground states according to the density-of-states analysis. ΔE M is defined as the difference between the most stable energy values of the cubic and tetragonal states; the larger the value, the easier the occurrence of tetragonal distortion, and the corresponding tetragonal structure is stable. Compared with the ΔE M values of classic Mn2-based tetragonal Heusler alloys, the ΔE M values of most Pd2CoZ alloys in this study indicate that they can overcome the energy barriers between cubic and tetragonal states, and possess possible tetragonal transformations. The uniform strain has also been taken into consideration to further investigate the tetragonal distortion of these alloys in detail. This work aims to provide guidance for researchers to further explore and study new magnetic functional tetragonal materials among the full-Heusler alloys.

Perovskite R{\bar 3}c phase AgCuF3: multiple Dirac cones, 100% spin polarization and its thermodynamic properties.

Very recently, experimentally synthesized R{\bar 3}c phase LaCuO3 was studied by Zhang, Jiao, Kou, Liao & Du [J. Mater. Chem. C (2018), 6, 6132-6137], and they found that this material exhibits multiple Dirac cones in its non-spin-polarized electronic structure. Motivated by this study, the focus here is on a new R{\bar 3}c phase material, AgCuF3, which has a combination of multiple Dirac cones and 100% spin polarization properties. Compared to the non-spin-polarized system LaCuO3, the spin-polarized Dirac behavior in AgCuF3 is intrinsic. The effects of on-site Coulomb interaction, uniform strain and spin-orbit coupling were added to examine the stability of its multiple Dirac cones and half-metallic behavior. Moreover, the thermodynamic properties under different temperatures and pressures were investigated, including the normalized volume, thermal volume expansion coefficient, heat capacity at constant volume and Debye temperature. The thermal stability and the phase stability of this material were also studied via ab initio molecular dynamic simulations and the formation energy of the material, respectively.

Investigations on the electron paramagnetic resonance parameters for Mn2+ in the ABF3 fluoroperovskites.

The electron paramagnetic resonance (EPR) parameters (g factor, the hyperfine structure constant A and the superhyperfine parameters A' and B') for Mn(2+) in the fluoroperovskites ABF(3) (A=K and Cs; B=Zn, Mg, Cd and Ca) are theoretically investigated from the perturbation formulas of these parameters for a 3d(5) ion under ideal octahedra. In the above treatments, not only the crystal-field mechanism but also the charge transfer mechanism is considered uniformly on the basis of the cluster approach. The theoretical EPR parameters are in good agreement with the experimental data. The charge transfer contribution to the g-shift Δg (≈g-g(s), where g(s)≈2.0023 is the spin-only value) is opposite (positive) in sign and comparable in magnitude to the crystal-field one. Nevertheless, the charge transfer contribution to the hyperfine structure constant shows the same sign and about 10% that of the crystal-field one. So, the conventional argument that the charge transfer contributions to the zero-field splittings are negligible for 3d(5) ions under low symmetrically distorted fluorine octahedra is proved no longer valid for the Δg analysis of ABF(3):Mn(2+) in view of the dominant second-order charge transfer perturbation terms. The unpaired spin densities of the fluorine 2s, 2p σ and 2p π orbitals are determined from the quantitative dependences upon the related molecular orbital coefficients, rather than obtained by fitting the observed superhyperfine parameters in the previous works.