Showing papers by "Martin Gmitra published in 2023"
17 Jun 2023
TL;DR: In this paper , the spin-orbit coupling proximity effects in a bilayer heterostructure comprising phosphorene and WSe$_2$ monolayers were investigated, using first-principles methods and effective-model simulations.
Abstract: We investigate, using first-principles methods and effective-model simulations, the spin-orbit coupling proximity effects in a bilayer heterostructure comprising phosphorene and WSe$_2$ monolayers. We specifically analyze holes in phosphorene around the $\Gamma$ point, at which we find a significant increase of the spin-orbit coupling that can be attributed to the strong hybridization of phosphorene with the WSe$_2$ bands. We also propose an effective spin-orbit model based on the ${\bf C}_{1{\rm v}}$ symmetry of the studied heterostructure. The corresponding spin-orbit field can be divided into two parts: the in-plane field, present due to the broken nonsymmorphic horizontal glide mirror plane symmetry, and the dominant out-of-plane field triggered by breaking the out-of-plane rotational symmetry of the phosphorene monolayer. Furthermore, we also demonstrate that a heterostructure with 60$^\circ$ twist angle exhibits an opposite out-of-plane spin-orbit field, indicating that the coupling can effectively be tuned by twisting. The studied phosphorene/WSe$_2$ bilayer is a prototypical low common-symmetry heterostructure in which the proximity effect can be used to engineer the spin texture of the desired material.
06 Apr 2023
TL;DR: In this article , it was shown that the Ising protection can survive in bulk misfit compound superconductors, such as (LaSe)1.14(NbSe2) and LaSe) 1.14.
Abstract: Low-dimensional materials have remarkable properties that are distinct from their bulk counterparts. A paradigmatic example is Ising superconductivity that occurs in monolayer materials such as NbSe2 which show a strong violation of the Pauli limit. In monolayers, this occurs due to a combination of broken inversion symmetry and spin-orbit coupling that locks the spins of the electrons out-of-plane. Bulk NbSe2 is centrosymmetric and is therefore not an Ising superconductor. We show that bulk misfit compound superconductors, (LaSe)1.14(NbSe2) and (LaSe)1.14(NbSe2)2, comprised of monolayers and bilayers of NbSe2, exhibit unexpected Ising protection with a Pauli-limit violation comparable to monolayer NbSe2, despite formally having inversion symmetry. We study these misfit compounds using complementary experimental methods in combination with first-principles calculations. We propose theoretical mechanisms of how the Ising protection can survive in bulk materials. We show how some of these mechanisms operate in these bulk compounds due to a concerted effect of charge-transfer, defects, reduction of interlayer hopping, and stacking. This highlights how Ising superconductivity can, unexpectedly, arise in bulk materials, and possibly enable the design of bulk superconductors that are resilient to magnetic fields.
06 Feb 2023
TL;DR: In this paper , the direction of the intrinsic magnetic field in monolayer polytype 1T phase transition metal dichalcogenide superconductors PdTe, NbTe and TiSe was determined using the spin mixing parameter.
Abstract: The immunity of the Ising superconductors to external magnetic fields originates from a spin locking of the paired electrons to an intrinsic Zeeman-like field. The spin-momentum locking in non-centrosymmetric crystalline materials leads to type-I Ising pairing in which the direction of the intrinsic field can be deduced from the spin expectation values. In centrosymmetric crystals, all the states are spin degenerate due to time reversal symmetry, but split by the spin-orbit coupling to orbital doublets. The electron spins locked to the orbitals can form Ising type-II pairs. We present an efficient approach to determine the direction of the intrinsic field using the spin-mixing parameter $b^2$. By means of first principles calculations based on the density functional theory we study monolayer polytype 1T phase transition metal dichalcogenide superconductors PdTe$_2$, NbTe$_2$ and TiSe$_2$. We calculate $b^2$ for individual Fermi pockets crossing the Fermi energy and provide a general picture of possible Ising type-II pairing within the full Brillouin zone.
TL;DR: In this article , a detailed band structure calculation including spin-orbit coupling was performed to predict an unexpected behavior of the relaxation times of graphite, and it was shown that T1 is markedly different from T2.
Abstract: Graphite has been intensively studied, yet its electron spins dynamics remains an unresolved problem even 70 years after the first experiments. The central quantities, the longitudinal (T1) and transverse (T2) relaxation times were postulated to be equal, mirroring standard metals, but T1 has never been measured for graphite. Here, based on a detailed band structure calculation including spin-orbit coupling, we predict an unexpected behavior of the relaxation times. We find, based on saturation ESR measurements, that T1 is markedly different from T2. Spins injected with perpendicular polarization with respect to the graphene plane have an extraordinarily long lifetime of 100 ns at room temperature. This is ten times more than in the best graphene samples. The spin diffusion length across graphite planes is thus expected to be ultralong, on the scale of ~ 70 μm, suggesting that thin films of graphite - or multilayer AB graphene stacks - can be excellent platforms for spintronics applications compatible with 2D van der Waals technologies. Finally, we provide a qualitative account of the observed spin relaxation based on the anisotropic spin admixture of the Bloch states in graphite obtained from density functional theory calculations.
TL;DR: In this paper , the authors propose a density functional theory for 2-dimensional systems with transition metal dichalcogenides and van der Waals systems. But their method is limited to two dimensions.
Abstract: Received 5 June 2023DOI:https://doi.org/10.1103/PhysRevB.107.239905©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasSpintronicsPhysical Systems2-dimensional systemsGrapheneTransition metal dichalcogenidesVan der Waals systemsTechniquesDensity functional theoryFirst-principles calculationsCondensed Matter, Materials & Applied Physics