Curvature-induced Rashba spin-orbit interaction in strain-driven nanostructures

We derive the effective dimensionally reduced Schr\"odinger equation with spin-orbit interaction in low-dimensional electronic strain driven nanostructures. A method of adiabatic separation among fast normal quantum degrees of freedom and slow tangential quantum degrees of freedom is used to show the emergence of a strain-induced Rashba-like spin-orbit interaction (SOI). By applying this analysis to one-dimensional curved quantum wires we demonstrate that the curvature-induced Rashba SOI leads to enhanced spin-orbit effects.Comment: 5 pages, 3 figures, to be published in SPIN (World Scientific) as Topical Issue on Functional Nanomembrane

Phenomenological model for magnetotransport in a multi-orbital system

By means of the Boltzmann equation, we have calculated some magnetotransport quantities for the layered multi-orbital compound Sr$_2$RuO$_4$. The Hall coefficient, the magnetoresistance and the in-plane resistivity have been determined taking into account the Fermi surface curvature and different time collisions for the electrons in the $t_{2g}$ bands. A consistent explanation of the experimental results has been obtained assuming different relaxation rates for the in-plane transport with and without an applied magnetic field, respectively.Comment: 4 pages, 3 Figure; to appear in Phys. Rev.

Magnetic-Field-Induced Topological Reorganization of a P-wave Superconductor

In this work we illustrate the detrimental impact of the Cooper pair's spin-structure on the thermodynamic and topological properties of a spin-triplet superconductor in an applied Zeeman field. We particularly focus on the paradigmatic one-dimensional case (Kitaev chain) for which we self-consistently retrieve the energetically preferred Cooper pair spin-state in terms of the corresponding spin d-vector. The latter undergoes a substantial angular and amplitude reorganization upon the variation of the strength and the orientation of the field and results to a modification of the bulk topological phase diagram. Markedly, when addressing the open chain we find that the orientation of the d-vector varies spatially near the boundary, affecting in this manner the appearance of Majorana fermions at the edge or even altering the properties of the bulk region. Our analysis reveals the limitations and breakdown of the bulk-boundary correspondence in interacting topological systems.Comment: 5 pages, 3 panels of figures; Minor corrections in the new version, which will appear in Phys. Rev. B as a Rapid Communicatio

Spin-Orbital Order Modified by Orbital Dilution in Transition Metal Oxides: From Spin Defects to Frustrated Spins Polarizing Host Orbitals

We study the $3d$ substitution in $4d$ transition metal oxides in the cases of $3d^3$ doping at either $3d^2$ or $4d^4$ sites which realize orbital dilution. We derive the effective $3d-4d$ (or $3d-3d$) superexchange in a Mott insulator with different ionic valencies, underlining the emerging structure of the spin-orbital coupling between the impurity and the host sites and demonstrate that it is qualitatively different from that encountered in the host itself. This derivation shows that the interaction between the host and the impurity depends in a crucial way on the type of doubly occupied $t_{2g}$ orbital. One finds that in some cases, due to the quench of the orbital degree of freedom at the $3d$ impurity, the spin and orbital order within the host is drastically modified by doping. The impurity acts either as a spin defect accompanied by an orbital vacancy in the spin-orbital structure when the host-impurity coupling is weak, or it favors doubly occupied active orbitals (orbital polarons) along the $3d-4d$ bond leading to antiferromagnetic or ferromagnetic spin coupling. This competition between different magnetic couplings leads to quite different ground states. We find that magnetic frustration and spin degeneracy can be lifted by the quantum orbital flips of the host but they are robust in special regions of the incommensurate phase diagram. The spin-orbit coupling can lead to anisotropic spin and orbital patterns along the symmetry directions and cause a radical modification of the order imposed by the spin-orbital superexchange. Our findings are expected to be of importance for future theoretical understanding of experimental results for doped $4d$ transition metal oxides doped with $3d^3$ ions. We suggest how the local or global changes of the spin-orbital order induced by such impurities could be detected experimentally.Comment: 27 pages, 16 figures, submitte

Novel Spin-Orbital Phases Induced by Orbital Dilution

We demonstrate that magnetic $3d$ impurities with $S=3/2$ spins and no orbital degree of freedom induce changes of spin-orbital order in a $4d^4$ Mott insulator with $S=1$ spins. Impurities act either as spin defects which decouple from the surrounding ions, or trigger orbital polarons along $3d$-$4d$ bonds. The $4d$-$4d$ superexchange in the host $J_{\rm host}$ competes with $3d$-$4d$ superexchange $J_{\rm imp}$ --- it depends on which orbital is doubly occupied. The spin-orbital order within the host is totally modified at doping $x=1/4$. Our findings provide new perspective for future theoretical and experimental studies of doped transition-metal oxides.Comment: 4 pages, 3 figure