Altermagnetic surface states: towards the observation and utilization of altermagnetism in thin films, interfaces and topological materials

The altermagnetism influences the electronic states allowing the presence of non-relativistic spinsplittings. Since altermagnetic spin-splitting is present along specific k-paths of the 3D Brillouin zone, we expect that the altermagnetic surface states will be present on specific surface orientations. We unveil the properties of the altermagnetic surface states considering three representative space groups: tetragonal, orthorhombic and hexagonal. We calculate the 2D projected Brillouin zone from the 3D Brillouin zone. We study the surfaces with their respective 2D Brillouin zones establishing where the spin-splittings with opposite sign merge annihilating the altermagnetic properties and on which surfaces the altermagnetism is preserved. Looking at the three principal surface orientations, we find that for several cases two surfaces are blind to the altermagnetism, while the altermagnetism survives for one surface orientation. Which surface preserves the altermagnetism depends also on the magnetic order. We show that an electric field orthogonal to the blind surface can activate the altermagnetism. Our results predict which surfaces to cleave in order to preserve altermagnetism in surfaces or interfaces and this paves the way to observe non-relativistic altermagnetic spin-splitting in thin films via spin-resolved ARPES and to interface the altermagnetism with other collective modes. We open future perspectives for the study of altermagnetic effects on the trivial and topological surface states.Comment: 8 pages, 10 figure

Orbital-selective altermagnetism and correlation-enhanced spin-splitting in transition metal oxides

We investigate the altermagnetic properties of strongly-correlated transition metal oxides considering the family of the quasi two-dimensional A2BO4 and three-dimensional ABO3. As a test study, we analyze the Mott insulators Ca2RuO4 and YVO3. In both cases, the orbital physics is extremely relevant in the t2g subsector with the presence of an orbital-selective Mott physics in the first case and of a robust orbital-order in the second case. Using first-principles calculations, we show the presence of an orbital-selective altermagnetism in the case of Ca2RuO4. In the case of YVO3, we study the altermagnetism as a function of the magnetic ordering and of the Coulomb repulsion U. We find that the altermagnetism is present in all magnetic orders with the symmetries of the Brillouin zone depending on the magnetic order. Finally, the Coulomb repulsion enhances the non-relativistic spin-splitting making the strongly-correlated systems an exciting playground for the study of the altermagnetism.Comment: 7 pages, 7 figure

Interplay between altermagnetism and nonsymmorphic symmetries generating large anomalous Hall conductivity by semi-Dirac points induced anticrossings

We investigate the interplay between altermagnetic spin-splitting and nonsymmorphic symmetries using the space group no. 62 as a testbed. Studying different magnetic orders by means of first-principles calculations, we find that the altermagnetism (AM) is present in the C-type magnetic configuration while it is absent for the G-type and A-type configurations due to different magnetic space group types. The nonsymmorphic symmetries constrain the system to a four-fold degeneracy at the border of the Brillouin zone with semi-Dirac dispersion. In the case of large hybridization as for transition metal pnictides, the interplay between AM and nonsymmorphic symmetries generates an intricate network of several crossings and anticrossings that we describe in terms of semi-Dirac points and glide symmetries. When we add the spin-orbit coupling (SOC), we find a Neel-vector dependent spin-orbit splitting at the time-reversal invariant momenta points since the magnetic space groups depend on the Neel vector. The magnetic space group type-I produces antiferromagnetic hourglass electrons that disappear in the type-III. When the Neel vector is along x, we observe a glide-protected crossing that could generate a nodal-line in the altermagnetic phase. The SOC splits the remaining band crossings and band anticrossings producing a large anomalous Hall effect in all directions excluding the Neel-vector directio

Emergence of Rashba splitting and spin-valley properties in Janus MoGeSiP2As2 and WGeSiP2As2 monolayers

First-principles calculations are performed to study the structural stability and spintronics properties of Janus MoGeSiP2As2 and WGeSiP2As2 monolayers. The high cohesive energies and the stable phonon modes confirm that both these structures are experimentally accessible. In contrast to pristine MoSi2P4, the Janus monolayers demonstrate reduced direct bandgaps and large spin-split states at K/-K. In addition, their spin textures exposed that breaking the mirror symmetry brings Rashba-type spin splitting in the systems which can be increased by using higher atomic spin-orbit coupling. The large valley spin splitting together with the Rashba splitting in these Janus monolayer structures can make a remarkable contribution to semiconductor valleytronics and spintronics

Topological transition in Pb1-xSnxSe using Meta-GGA

We calculate the mirror Chern number (MCN) and the band gap for the alloy Pb1-xSnxSe as a function of the concentration x by using virtual crystalline approximation. We use the electronic structure from the relativistic density functional theory calculations in the Generalized-Gradient- Approximation (GGA) and meta-GGA approximation. Using the modified Becke-Johnson meta- GGA functional, our results are comparable with the available experimental data for the MCN as well as for the band gap. We advise to use modified Becke-Johnson approximation with the parameter c=1.10 to describe the transition from trivial to topological phase for this class of compounds.Comment: 5 pages, 5 figures, 1 table. arXiv admin note: text overlap with arXiv:2302.0621

Tuning interchain ferromagnetic instability in A2Cr3As3 ternary arsenides by chemical pressure and uniaxial strain

We analyze the effects of chemical pressure induced by alkali metal substitution and uniaxial strain on magnetism in the A2Cr3As3 (A = Na, K, Rb, Cs) family of ternary arsenides with quasi-one dimensional structure. Within the framework of the density functional theory, we predict that the non-magnetic phase is very close to a 3D collinear ferrimagnetic state, which realizes in the regime of moderate correlations, such tendency being common to all the members of the family with very small variations due to the different interchain ferromagnetic coupling. We uncover that the stability of such interchain ferromagnetic coupling has a non-monotonic behavior with increasing the cation size, being critically related to the degree of structural distortions which is parametrized by the Cr-As-Cr bonding angles along the chain direction. In particular, we demonstrate that it is boosted in the case of the Rb, in agreement with recent experiments. We also show that uniaxial strain is a viable tool to tune the non-magnetic phase towards an interchain ferromagnetic instability. The modifcation of the shape of the Cr triangles within the unit cell favors the formation of a net magnetization within the chain and of a ferromagnetic coupling among the chains. This study can provide relevant insights about the interplay between superconductivity and magnetism in this class of materials.Comment: Accepted in Phys. Rev. Materials as a regular article. 13 pages, 23 figures, 3 table

Intra-chain collinear magnetism and inter-chain magnetic phases in Cr3As3-K-based materials

We perform a comparative study of the KCr3As3 and the K2Cr3As3 quasi 1D compounds, and show that the strong interplay between the lattice and the spin degrees of freedom promotes a new collinear ferrimagnetic ground state within the chains in presence of intrachain antiferromagnetic couplings. We propose that the interchain antiferromagnetic coupling in KCr3As3 plays a crucial role for the experimentally observed spin-glass phase with low critical temperature. In the same region of the parameter space, we predict K2Cr3As3 to be non-magnetic but on the verge of the magnetism, sustaining interchain ferromagnetic spin fluctuations while the intrachain spin fluctuations are antiferromagnetic.Comment: Accepted in Phys. Rev. B as a regular articl

Dirac surface states, multiorbital dimerization and superconductivity in Nb- and Ta-based A15 compounds

Using first-principle calculations, we investigate the electronic, topological and superconducting properties of Nb$_3$X (X = Ge, Sn, Sb) and Ta$_3$Y (Y = As, Sb, Bi) A15 compounds. We demonstrate that these compounds host Dirac surface states which are related to a nontrivial Z$_2$ topological value. The spin-orbit coupling (SOC) splits the eightfold degenerate R point close to the Fermi level enhancing the amplitude of the spin Hall conductance. Indeed, despite the moderate spin-orbit of the Nb-compounds, a large spin Hall effect is also obtained in Nb$_3$Ge and Nb$_3$Sn compounds. We show that the Coulomb interaction opens the gap at the R point thus making more evident the occurrence of Dirac surface states. We then investigate the superconducting properties by determining the strength of the electron-phonon BCS coupling. The evolution of the critical temperature is tracked down to the 2D limit indicating a reduction of the transition temperature which mainly arises from the suppression of the density of states at the Fermi level. Finally, we propose a minimal tight-binding model based on three coupled Su-Schrieffer-Heeger chains with t$_{2g}$ Ta- and Nb-orbitals reproducing the spin-orbit splittings at the R point among the $\pi$-bond bands in this class of compounds. We separate the kinetic parameters in $\pi$ and $\delta$-bonds, in intradimer and interdimer hoppings and discuss their relevance for the topological electronic structure. We point out that Nb$_3$Ge might represent a Z$_2$ topological metal with the highest superconducting temperature ever recorded.Comment: 16 pages, 12 figures in main text, 3 figures in appendix, Paper submitted to Physical Review