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Distinguishing from traditional working environments, we propose the harsh gamma radiation method to study the stability and reliability of the emerging two-dimensional (2D) quantum material tungsten diselenide (WSe2). Transmission electron microscopy studies showed clear chemical modulation with an atomically sharp interface, indicating that the selenium vacancy content increased with the irradiation dose. The WSe2 crystal could be transitioned into an n-doped semiconductor due to the anion vacancies created by radiation. Changes in the lattice vibrational modes induced by the passivation of oxygen was captured via Raman spectroscopy. The frequency shifts in both in-plane and out-of-plane modes are dependent linearly on the selenium vacancy content. The friction of WSe2 increases with the irradiation dose. Electrostatic properties were investigated by measuring the surface potential via Kelvin probe force microscopy. Due to different environments, molecular collisions lead to an increase in the concentration of vacancy defects, which made our results different from those previously reported. The first principles calculation suggests that an increase in the selenium vacancy population is generally accompanied by a transition from a direct gap material to an indirect one. This opens up a new venue to engineer the optical, frictional and electronic properties of transition metal dichalcogenides using irradiation.

Low-dimensional ferroelectricity and Dirac materials with protected band crossings are fascinating research subjects. Based on first-principles calculations, we predict the coexistence of spontaneous in-plane polarization and novel 2D emergent fermions in dynamically stable quadruple-layer (QL) XSbO$_2$ (X= Li, Na). Depending on the different polarization configurations, QL-XSbO$_2$ can exhibit unconventional inner-QL ferroelectricity and antiferroelectricity. Both ground states harbor robust ferroelectricity with enhanced spontaneous polarization of 0.56 nC/m and 0.39 nC/m for QL-LiSbO$_2$ and QL-NaSbO$_2$, respectively. Interestingly, the QL-LiSbO$_2$ possesses two other metastable ferroelectric (FE) phases, demonstrating the first 2D example with multiple FE orders. The ground FE phase can be flexibly driven into one of the two metastable FE phases and then into the antiferroelectric (AFE) phase. During this phase transition, several types of 2D fermions emerge, for instance, hourglass hybrid and type-II Weyl loops in the ground FE phase, type-II Weyl fermions in the metastable FE phase, and type-II Dirac fermions in the AFE phase. These 2D fermions are robust under spin-orbit coupling. Notably, two of these fermions, e.g., an hourglass hybrid or type-II Weyl loop, have not been observed before. Our findings identify QL-XSbO$_2$ as a unique platform for studying 2D ferroelectricity relating to 2D emergent fermions., 5 figures

We investigate the transport properties of bulk Ca2YZ (Y = Au, Hg; Z = As, Sb, Bi, Sn and Pb) by a combination method of first-principles and Boltzmann transport theory. The focus of this article is the systematic study of the thermoelectric properties under the effect of a spin–orbit coupling. The highest dimensionless figure of merit (ZT) of Ca2AuAs at optimum carrier concentration are 1.23 at 700 K. Interestingly enough, for n-type Ca2HgPb, the maximum ZT are close to each other from 500 K to 900 K and these values are close to 1, which suggests that semimetallic material can also be used as an excellent candidate for thermoelectric materials. From another viewpoint, at room temperature, the maximum PF for Ca2YZ are greater than 3 mW m−1 K−2, which is very close to that of ∼3 mW m−1 K−2 for Bi2Te3 and ∼4 mW m−1 K−2 for Fe2VAl. However, the room temperature theoretical κl of Ca2YZ is only about 0.85–1.6 W m−1 K−1, which is comparing to 1.4 W m−1 K−1 for Bi2Te3 and remarkably lower than 28 W m−1 K−1 for Fe2VAl at same temperature. So Ca2YZ should be a new type of promising thermoelectric material at room temperature.

The scarcity of materials with coexisting ferroelectric polarization and magnetization has hindered the development of multiferroic-based spintronic applications. Therefore, the generation of polarization in magnetic compounds is an important task in an effort to make single-phase multiferroic materials. Magnetic domain walls with strong discontinuities in spin ordering provide an ideal platform for the generation of polarization. In this work, we investigate spin-induced ferroelectric polarization at antiferromagnetic domain walls in the rare-earth orthoferrites AFeO3 (A = Lu, Y, Gd). We find that the ferroelectric polarization depends on the atomic radius of the rare-earth element, with the largest polarization of 0.092 μC cm−2 being realized in LuFeO3, which has the smallest rare-earth radius and the highest domain wall density along the b direction. Based on the octahedral distortion and the unified polarization model, we also perform a mechanistic analysis of the ferroelectric polarization at domain walls along the b direction. The different octahedral tilt angles and the different unit cell volumes caused by A-site ions are the main reasons for the polarization difference in the three compounds. Our results improve the theoretical understanding of magnetically induced ferroelectric polarization at domain walls and provide a basis for experimental application in spintronic devices based on domain wall multiferroicity.

SrTiO3-based transition-metal oxide heterostructures with superconducting, ferromagnetic, ferroelectric, and ferroelastic properties exhibit high application potential in the fields of energy storage, energy conversion, and spintronic devices. Meanwhile, high effective (charge)-Seebeck coefficient materials composed of a ferromagnetic layer and SrTiO3 insulator layer have been achieved but we still have blocks to pursuing high spin-Seebeck coefficient materials. Here, we use first-principles calculations combined with spin-resolved Boltzmann transport theory to investigate the spin- and effective-Seebeck coefficients in the LaX(X = Mn and Co)O3/SrTiO3 superlattice. Compared with the LaMnO3/SrTiO3 superlattice, LaCoO3/SrTiO3 with ferromagnetic ordering has high spin polarization, relatively low valence valley degeneracy but high effective mass. Utilizing these characteristics, the maximum spin-Seebeck coefficient of LaMnO3/SrTiO3 is −152 μV K−1 at 450 K along the cross-plane direction, while LaCoO3/SrTiO3 reaches −247 μV K−1 under the same conditions. Interestingly, the spin- and effective-Seebeck coefficients are amazingly consistent with each other below 200 K, which indicates that one spin channel (spin-up or spin-down) dominates the carrier transport, and the other one (spin-down or spin-up) is filtered out. These characteristics are mainly associated with the magnetic MnO2/CoO2 layers with distinct dxy and dz2 orbitals near the Fermi level. Our results clarify the relationship of spin- and effective-Seebeck coefficients and indicate that SrTiO3-based transition metal oxide heterointerfaces are a key candidate for spin caloritronics.

Perovskite oxide interfaces have been used recently as platforms for demonstrating rich physical properties that are not possessed by their bulk constituents. In this work, we will not only demonstrate rich physical properties at polar perovskite oxide interfaces, but also the tuning of such properties by switching the polarization in PbTiO3/tetragonal BiFeO3 (BFO) two-dimensional ferroelectric heterostructures (HSs). Our first-principles calculations reveal that both magnetism and conductivity can be induced by the discontinuity of the ferroelectric (FE) polarization and the valence states at the interface, and they can also be robustly manipulated by reversing polarization. These properties include the coupling between an interfacial two-dimensional electron gas and magnetism, a transition from half-metal to band insulator or Mott insulator, and the conversion from G-type antiferromagnetic order to local ferromagnetic (FM) ordering, and finally, to the local C-type antiferromagnetic order (C-AFM). Furthermore, by setting different terminations, we can artificially control the local FM ordering that occurs arbitrarily in any one of the polarization directions, and we can even control whether the local FM ordering appears at the interface or at the surface. This novel control can be applied in the fabrication of memory devices for information storage where bits can be written by an electric field (to switch ferroelectric polarization) while reading magnetically. Such a device will drastically reduce energy consumption.

The magneto-Seebeck effect has recently attracted considerable attention because of its novel fundamental physics and future potential application in spintronics. Herein, employing first-principles calculations and the spin-resolved Boltzmann transport theory, we have systematically investigated the electronic structures and spin-related transport properties of Co2FeAl/MgO/Co2FeAl multilayers with parallel (P) and anti-parallel (AP) magnetic alignment. Our results indicate that the sign of tunneling magneto-Seebeck (TMS) value with Co2/O termination is consistent with that of the measured experimental result although its value (-221%) at room temperature is smaller than the experimental one (-95%). The calculated spin-Seebeck coefficients of the Co2/O termination with P and AP states and the FeAl/O termination with the AP state are all larger than other typical Co2MnSi/MgO/Co2MnSi heterostructures. By analyzing the geometries, electronic structures, and magnetic behaviors of two different terminations (Co2/O and FeAl/O terminations), we find that the two terminations in the interface region form anti-bonding and bonding states, reconstructing the energy gap, changing the magnetic moment of O atoms, and improving the spin-polarization (-82%). This phenomenon can be ascribed to the charge transfer and hybridization between Co/Fe 3d and O 2p states, which also results in a bowknot orbital shape of Co atoms with Co2/O termination and an ankle shape of Co atoms with FeAl/O termination far away from the interface. Moreover, there are spin-splitting transmission gaps with the Co2/O-termination around the Fermi level, while the transmission gaps with the FeAl/O-termination are closed and thus show a typical metallic character. Our findings will guide the experimental design of magneto-Seebeck devices for future spintronic applications.

Magnetism in two-dimensional (2D) materials, that is, a 2D version of the magnetism of three-dimensional bulk materials, and the associated novel physics have recently been the focus of many spintronics researchers. Here we investigate the manipulation of 2D magnetism at the interfaces of ferromagnetic/ferroelectric hematene/BaTiO3(001) heterostructures (HSs) fabricated via a precisely chosen sequence. By introducing four types of interfaces of 2D hematene and three-dimensional BaTiO3 that induce different oxygen environments, the control of magnetism is directly demonstrated from first-principles. An obvious 2D electron gas originates from the Fe-3d and O-2p hybridization; the electron gas is sensitive to the interfacial atomic displacements. Robust control of both the direction and magnitude of the net magnetization has been realized for an Fe/TiO2 terminated bilayer HS. The electron occupancies of the dxy and dxz orbitals and changes to the Fe-O bond play a key role in determining the magnetism of our systems. Our work not only demonstrates the technique's potential for manipulating magnetism in 2D hematene, but also sheds light on the underlying mechanism and the fundamental properties of hematene HSs.

We have investigated the structural, electronic, and magnetic properties of A-site-ordered double-perovskite-structured oxides, AA'3B4O12 (A = Na, Ca, and La) with Mn and V at A' and B sites, respectively, using first-principle calculations based on the density functional theory. Our calculation results show that the antiferromagnetic phase is the ground state for all the compounds. By changing the A-site ions from Na(+) to Ca(2+) and then to La(3+), the transfer of charge between Mn and O ions was changed from 1.56 to 1.55 and then to 1.50, and that between the V and O ions changed from 2.01 to 1.95 and then to 1.93, revealing the cause for the unusual site-selective doping effect. Mn 3d electrons dominate the magnetic moment and are localized, with an intense hybridization with O 2p orbitals, which indicates that the magnetic exchange interaction between Mn ions is mediated through O and that the super exchange mechanism will take effect. These materials have a large one-electron bandwidth W, and the ratio of the on-site Coulomb repulsion U to W is less than the critical value (U/W)c, which leads to metallic behavior of AMn3V4O12. This is further evidenced by the large number of free electrons contributed by V at the Fermi surface. These calculations, in combination with the reported experimental data, prove that these double perovskites belong to the rare antiferromagnetic metallic oxides.

BiCuSeO is an attractive material for its high temperature stability, high Seebeck coefficient, and low lattice thermal conductivity. However, its electrical conductivity is low. To enhance the thermoelectric properties of BiCuSeO further, we investigated the material's electronic structure and transport property using first-principles calculations. We determine that the low electrical conductivity may originate from strong ionic bonding and without a Cu–Se conductive pathway forming near the Fermi level. Moreover, although p-type BiCuSeO has a high thermopower due to the high density of states near the Fermi level, its electrical conductivity is relatively low because no conductive pathway for carrier transport exists along the c axis. In contrast, a conductive pathway is formed between Bi and Cu atoms at the conduction-band minimum along the c axis. This feature would lead to high electrical conductivity for n-type BiCuSeO. With relatively high electrical conductivity and slightly decreased thermopower, n-type BiCuSeO exhibits a higher power factor than that of p-type BiCuSeO. The maximum ZT value could be improved by 32% for n-type BiCuSeO compared with the prevailing p-type doping approach at 920 K.

Ca5Ga2As6 gives us a chance to achieve a high thermoelectric performance with a normal bulk system. This is mainly due to the fact that its complex chemical structure can not only supply an ultra-low lattice thermal conductivity but also show a high Seebeck coefficient. In this paper, we have discussed the lattice, electronic, and transport properties of Ca5Ga2As6 for elucidating its high thermoelectric performance. In our calculations, density-functional theory and Boltzmann transport theory were used. For intrinsic Ca5Ga2As6, the valence band effective mass is larger than the conduction band effective mass, which leads to its positive sign of S over the whole temperature range. By studying the anisotropy of its transport properties, we find that the transport properties of p-type Ca5Ga2As6 are better than that of the n-type one, which mainly comes from the large anisotropy of its band structure. Moreover its transport properties along the z direction are much better than those along the x and y directions, which can be attributed to its anisotropic one-dimensional lattice structure. At different temperatures, the peak value of ZeT appears at different carrier concentrations, which makes it possible to obtain a highest efficiency under large temperature gradients. The carrier concentrations corresponding to large ZeT are in the range of 0.033–0.5 e per uc, which is equal to 5.19 × 1019 cm−3 to 7.87 × 1020 cm−3. The largest value of ZeT is equal to 0.95.