71 research outputs found
Thickness-dependent magnetic structure of ultrathin Fe/Ir(001) films: from spin-spiral states towards ferromagnetic order
We present a detailed study of the ground-state magnetic structure of
ultrathin Fe films on the surface of fcc Ir(001). We use the spin-cluster
expansion technique in combination with the relativistic disordered local
moment scheme to obtain parameters of spin models and then determine the
favored magnetic structure of the system by means of a mean field approach and
atomistic spin dynamics simulations. For the case of a single monolayer of Fe
we find that layer relaxations very strongly influence the ground-state spin
configurations, whereas Dzyaloshinskii-Moriya (DM) interactions and biquadratic
couplings also have remarkable effects. To characterize the latter effect we
introduce and analyze spin collinearity maps of the system. While for two
monolayers of Fe we find a single-q spin spiral as ground state due to DM
interactions, for the case of four monolayers the system shows a noncollinear
spin structure with nonzero net magnetization. These findings are consistent
with experimental measurements indicating ferromagnetic order in films of four
monolayers and thicker.Comment: 9 pages, 7 figure
Spin-correlations and magnetic structure in an Fe monolayer on 5d transition metal surfaces
We present a detailed first principles study on the magnetic structure of an
Fe monolayer on different surfaces of 5d transition metals. We use the
spin-cluster expansion technique to obtain parameters of a spin model, and
predict the possible magnetic ground state of the studied systems by employing
the mean field approach and in certain cases by spin dynamics calculations. We
point out that the number of shells considered for the isotropic exchange
interactions plays a crucial role in the determination of the magnetic ground
state. In the case of Ta substrate we demonstrate that the out-of-plane
relaxation of the Fe monolayer causes a transition from ferromagnetic to
antiferromagnetic ground state. We examine the relative magnitude of nearest
neighbour Dzyaloshinskii-Moriya (D) and isotropic (J) exchange interactions in
order to get insight into the nature of magnetic pattern formations. For the
Fe/Os(0001) system we calculate a very large D/J ratio, correspondingly, a spin
spiral ground state. We find that, mainly through the leading isotropic
exchange and Dzyaloshinskii-Moriya interactions, the inward layer relaxation
substantially influences the magnetic ordering of the Fe monolayer. For the
Fe/Re(0001) system characterized by large antiferromagnetic interactions we
also determine the chirality of the N\'eel-type ground state.Comment: 15 pages, 8 figures, 2 table
Theory of anisotropic Rashba splitting of surface states
We investigate the surface Rashba effect for a surface of reduced in-plane
symmetry. Formulating a k.p perturbation theory, we show that the Rashba
splitting is anisotropic, in agreement with symmetry-based considerations. We
show that the anisotropic Rashba splitting is due to the admixture of bulk
states of different symmetry to the surface state, and it cannot be explained
within the standard theoretical picture supposing just a normal-to-surface
variation of the crystal potential. Performing relativistic ab initio
calculations we find a remarkably large Rashba anisotropy for an
unreconstructed Au(110) surface that is in the experimentally accessible range.Comment: 4 pages, 5 figure
Exchange interaction between magnetic adatoms on surfaces of noble metals
We present first-principles calculations of the exchange interactions between magnetic impurities deposited on (001), (110), and (111) surfaces of Cu and Au and analyze them, in particular, in the asymptotic regime. For the (110) and the (111) surfaces, we demonstrate that the interaction shows an oscillatory behavior as a function of the distance, R, of the impurities and that the amplitude of the oscillations decays as 1/R(2). Furthermore, the frequency of the oscillations is closely related to the length of the Fermi vector of the surface states existing on these surfaces. Due to the asymmetry of the surface-states dispersion, the frequency of the oscillations becomes also asymmetric on the (110) surfaces, while on the Au(111) surface two distinct frequencies are found in the oscillations as a consequence of the Bychkov-Rashba splitting of the surface states. Remarkably, no long-range oscillations of the exchange interaction are observed for the (001) surfaces where the surface states are unoccupied. When burying the impurities beneath the surface layer, oscillations mediated by the bulk states become visible
Nonunitary triplet pairing in the noncentrosymmetric superconductor LaNiC2
We present a first-principles based semiphenomenological approach to study the superconductivity in the noncentrosymmetric superconductor LaNiC2 with spin–orbit coupling. Based on group theoretical considerations, it was already shown that the breaking of time-reversal symmetry is only compatible with nonunitary triplet pairing states. To investigate the pairing mechanism by which this comes about and the possible role of the lack of inversion symmetry, we have combined the relativistic spin-polarized version of Korringa–Kohn–Rostoker method for the solution of the Dirac–Bogoliubov–de Gennes equations with a semiphenomenological parametrization of the pairing interaction. This made possible to study different orbital specific pairing models in quantitative details. We compare our predictions for the temperature dependence of the specific heat and it is found that it can be described by an interorbital equal-spin pairing on the nickel which breaks the time-reversal symmetry
Higher-order contributions to the Rashba-Bychkov effect with application to Bi/Ag(111) surface alloy
In order to explain the anisotropic Rashba-Bychkov effect observed in several
metallic surface-state systems, we use k.p perturbation theory with a simple
group-theoretical analysis and construct effective Rashba Hamiltonians for
different point groups up to third order in the wavenumber. We perform
relativistic ab initio calculations for the Bi/Ag(111) ordered surface alloy
and from the calculated splitting of the band dispersion we find evidence of
the predicted third-order terms. Furthermore, we derive expressions for the
corresponding third-order Rashba parameters to provide a simple explanation to
the qualitative difference concerning the Rashba-Bychkov splitting of the
surface states at Au(111) and Bi/Ag(111).Comment: 7 pages, 3 figure
Local Charge Excesses in Metallic Alloys: a Local Field Coherent Potential Approximation Theory
Electronic structure calculations performed on very large supercells have
shown that the local charge excesses in metallic alloys are related through
simple linear relations to the local electrostatic field resulting from
distribution of charges in the whole crystal.
By including local external fields in the single site Coherent Potential
Approximation theory, we develop a novel theoretical scheme in which the local
charge excesses for random alloys can be obtained as the responses to local
external fields. Our model maintains all the computational advantages of a
single site theory but allows for full charge relaxation at the impurity sites.
Through applications to CuPd and CuZn alloys, we find that, as a general rule,
non linear charge rearrangements occur at the impurity site as a consequence of
the complex phenomena related with the electronic screening of the external
potential. This nothwithstanding, we observe that linear relations hold between
charge excesses and external potentials, in quantitative agreement with the
mentioned supercell calculations, and well beyond the limits of linearity for
any other site property.Comment: 11 pages, 1 table, 7 figure
Screened Coulomb interactions in metallic alloys: I. Universal screening in the atomic sphere approximation
We have used the locally self-consistent Green's function (LSGF) method in
supercell calculations to establish the distribution of the net charges
assigned to the atomic spheres of the alloy components in metallic alloys with
different compositions and degrees of order. This allows us to determine the
Madelung potential energy of a random alloy in the single-site mean field
approximation which makes the conventional single-site density-functional-
theory coherent potential approximation (SS-DFT-CPA) method practically
identical to the supercell LSGF method with a single-site local interaction
zone that yields an exact solution of the DFT problem. We demonstrate that the
basic mechanism which governs the charge distribution is the screening of the
net charges of the alloy components that makes the direct Coulomb interactions
short-ranged. In the atomic sphere approximation, this screening appears to be
almost independent of the alloy composition, lattice spacing, and crystal
structure. A formalism which allows a consistent treatment of the screened
Coulomb interactions within the single-site mean-filed approximation is
outlined. We also derive the contribution of the screened Coulomb interactions
to the S2 formalism and the generalized perturbation method.Comment: 28 pages, 8 figure
- …