Intra-atomic Spin-orbit Coupling Research Articles

Enhancing the Rashba spin splitting at X/Au/Si(111) surface by induced intrinsic spin-polarization

Adsorbed/modified surface is usually treated as a prototype to study Rashba spin splitting. Using first-principles calculations and analyses, we first explain the Rashba spin splitting of monovalent alkali metal adsorbed on Au/Si(111) surface as realized in experiment. We clearly identify that Na adatom donates one electron to pristine Au/Si(111) surface, leading to the Fermi level shift upward to the parabolic range of Au d and Si p hybridized states and thus leading to large Rashba spin splitting in Na/Au/Si(111) system. However, for the larger intra-atomic spin-orbit coupling Cs adsorbate, it not only plays the identical role to Na adatom, but induces intrinsic spin-polarization surprisingly, which further enhances the Rashba spin splitting on the Cs/Au/Si(111) surface up to ∼200 meV. Our proposed mechanism that intrinsic spin-polarization significantly enhancing the Rashba spin splitting is generally applicable and can be extended readily to alkali-earth metal as epitaxial layer grown on Au/Si(111) surface.

Density functional theory calculations for magnetic properties of Co3W systems

Cheaper permanent magnetic nanostructures with magnetic properties equivalent to those of noble-metal or rare-earth nanomagnets have been experimentally developed for their potential applications in ultrahigh storage densities in magnetic memory. To date, their intrinsic magnetic properties are not well understood under the micro-level of local atomic arrangements and electronic structures. In this work, we performed theoretical investigations on the Co3W bulk, the clean surface, nanoclusters, and the Co|Co3W bilayers and superlattices for their geometrical structures, magnetic moments, and magnetic anisotropy energies (MAEs). We found that the Co3W nanostructures we constructed are stable and have the local minima in the energetic landscape, whose stabilities increase with increasing proportion of W and cluster size. The Co and W atoms in clusters are antiferromagnetically coupled, and their local magnetic moments decrease with increasing proportion of W. The breakdown of the Hund's third rule in W atoms observed in experiment can be interpreted as the competition between the intra-atomic spin-orbit coupling in W atoms and interatomic Co-W hybridizations. The highest MAE of about a few tens of meV is obtained in small cluster sizes, whereas it is an order of magnitude reduction in large cluster sizes. The magnetic systems of Co3W clean surface, Co|Co3W bilayer and superlattice can present large MAEs, and their easy-axes of magnetization are perpendicular to the (001) surface. Our calculated MAEs are of the same order of magnitude as that of the experimental measurements, and the electronic origin is revealed through the second-order perturbation method.

Quadrupole-driven non-Fermi-liquid and magnetic-field-induced heavy fermion states in a non-Kramers doublet system

Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in the electrical resistivity and the 4f contribution to the specific heat, C_4f, in the paramagnetic state at T > TQ. Moreover, in magnetic fields below 6 T, all data set of the electrical resistivity and C_4f(T) are well scaled with characteristic temperatures T0's. This is the first observation of the NFL state in the nonmagnetic quadrupole-active system, whose origin is intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons. Below 0.13 K, the electrical resistivity and C_4f(T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.

Interatomic versus intraatomic Ru interactions in perovskites

The competition between interatomic Ru–O–Ru interactions responsible for itinerant-electron ferromagnetism in SrRuO 3 and intraatomic spin–orbit coupling that suppresses some Ru–O–Ru magnetic interactions in CaRuO 3 so as to prevent long-range magnetic order has been further tested by investigating the double perovskites La 2RuZnO 6, La 2RuCoO 6, La 2TiCoO 6, and the solid solution Ca 2−2 x La 2 x Ru 2− x Co x O 6 (0.0⩽ x⩽1.0). The paramagnetic state of the low-spin, localized Ru 4+:t 4e 0 configuration of LaRuZnO 6 undergoes a progressive transition on lowering the temperature to a non-magnetic state ( J=0) as a result of spin–orbit coupling, λ L· S. Comparison of the susceptibilities of La 2RuCoO 6 and La 2TiCoO 6 shows a similar behavior with long-range antiferromagnetic cobalt ordering via non-magnetic RuO 6/2 and TiO 6/2 bridges. Comparison is also made of the effect of substituting Ga for Ru in CaRuO 3 with the effect of other dopants M in CaRu 1− x M x O 3. The evolution of transport properties in Ca 2−2 x La 2 x Ru 2− x Co x O 6 is also presented.

Spin-orbit coupling in curved graphene, fullerenes, nanotubes, and nanotube caps

A continuum model for the effective spin orbit interaction in graphene is derived from a tight-binding model which includes the $\pi$ and $\sigma$ bands. We analyze the combined effects of the intra-atomic spin-orbit coupling, curvature, and applied electric field, using perturbation theory. We recover the effective spin-orbit Hamiltonian derived recently from group theoretical arguments by Kane and Mele. We find, for flat graphene, that the intrinsic spin-orbit coupling $\Hi \propto \Delta^ 2$ and the Rashba coupling due to a perpendicular electric field ${\cal E}$, $\Delta_{\cal E} \propto \Delta$, where $\Delta$ is the intra-atomic spin-orbit coupling constant for carbon. Moreover we show that local curvature of the graphene sheet induces an extra spin-orbit coupling term $\Delta_{\rm curv} \propto \Delta$. For the values of $\cal E$ and curvature profile reported in actual samples of graphene, we find that $\Hi < \Delta_{\cal E} \lesssim \Delta_{\rm curv}$. The effect of spin-orbit coupling on derived materials of graphene, like fullerenes, nanotubes, and nanotube caps, is also studied. For fullerenes, only $\Hi$ is important. Both for nanotubes and nanotube caps $\Delta_{\rm curv}$ is in the order of a few Kelvins. We reproduce the known appearance of a gap and spin-splitting in the energy spectrum of nanotubes due to the spin-orbit coupling. For nanotube caps, spin-orbit coupling causes spin-splitting of the localized states at the cap, which could allow spin-dependent field-effect emission.

Spin-orbit Coupling Origin of Large Reduction of the Effective Moment in Na2V3O7

We have shown that the observed large reduction of the effective moment in Na2V3O7 may be caused by conventional crystal-field interactions and the intra-atomic spin-orbit coupling of the V4+ ion. The fine discrete electronic structure of the 3d1 configuration with the weakly-magnetic Kramers-doublet ground state, caused by the large orbital moment, is the reason for anomalous properties of Na2V3O7. Moreover, according to the Quantum Atomistic Solid-State Theory (QUASST), Na2V3O7 is expected to exhibit pronounced heavy-fermion phenomena at low temperatures. PACS No: 71.70.E; 75.10.D Keywords: crystal-field interactions, spin-orbit coupling, orbital moment. Na2V3O7

Ground state, electronic structure and magnetism of LaMnO3

We have calculated the discrete low-energy electronic structure in LaMnO3 originating from the atomic-like states of the strongly correlated 3d4 electronic system occurring in the Mn3+ ion. We take into account very strong intra-atomic correlations, crystal field interactions and the intra-atomic spin-orbit coupling. We calculated magnetic and paramagnetic state of LaMnO3 within the consistent description given by Quantum Atomistic Solid State Theory (QUASST). Our studies indicate that the intra-atomic spin-orbit coupling and the orbital magnetism are indispensable for the physically adequate description of electronic and magnetic properties of LaMnO3. Keywords: 3d oxides, crystal field, spin-orbit coupling, LaMnO3 PACS: 71.70Ej, 75.10Dg

Orbital moment in CoO and in NiO

The total, orbital and spin moment of the Co 2+ ion in CoO have been calculated within the quasi-atomic approach taking into account strong correlations, crystal-field interactions and the intra-atomic spin–orbit coupling. The orbital moment of 1.39 μ B amounts at 0 K , in the magnetically-ordered state, to more than 34% of the total moment (4.01 μ B) and yields the L/ S ratio of 1.05, close to the experimental value. The same calculations yield for NiO the orbital and total moment of 0.46 μ B and 2.45 μ B, respectively.

Disappearance of the Co magnetic moment in LaCoO3

In this contribution we discuss the origin of the intriguing non-magnetic ground state observed in LaCoO3. We show that the non-magnetic state of the Co3+ ion can be understood within the Quantum Atomistic Solid-State theory (QUASST) and is caused by the off-cubic distortion of the local crystal field related to the distortion of the local octahedron. In formation of the non-magnetic state the relatively small intra-atomic spin-orbit coupling is essentially important. We point out that in LaCoO3 exists the fine meV-scale electronic structure associated with the atomic-like discrete energy states.

The Jahn–Teller-effect formation of the non-magnetic state of the Co3+ ion in LaCoO3

It is shown that the Co3+ ion can have a non-magnetic ground state as the realization of the Jahn–Teller effect. This non-magnetic localized singlet ground state with a highly magnetic excited doublet is formed by the trigonal distortion of the local octahedral surrounding. Such structure is exactly accounted for by crystal-field interactions if the intra-atomic spin–orbit coupling is correctly taken into account. The presented crystal-field-based approach can be applied to other Co3+ and Fe2+ ion compounds.