Exchange interaction and Fano resonances in diatomic molecular systems

We propose a mechanism to use scanning tunneling microscopy (STM) for direct measurements of the two-electron singlet-triplet exchange splitting $J$ in diatomic molecular systems, unsing the coupling between the molecule and the substrate electrons. The different pathways for electrons lead to interference effects and generate kinks in the differential conductance at the energies for the singlet and triplet states. These features are related to Fano resonance due to the branched electron wave functions. The ratio between the tunneling amplitudes through the two atoms can be modulated by spatial movements of the tip along the surface.Comment: 4 pages, 2 figures, submitted - Changes in Fig. 1 (panel c) added), and minor modification in the main text - new version, as publishe

Impurities and Conductivity in a D-wave Superconductor

Impurity scattering in the unitary limit produces low energy quasiparticles with anisotropic spectrum in a two-dimensional $d$-wave superconductor. We describe a new {\em quasi-one-dimensional } limit of the quasiparticle scattering, which might occur in a superconductor with short coherence length and with {\em finite} impurity potential range. The dc conductivity in a $d$-wave superconductor is predicted to be proportional to the normal state scattering rate and is impurity-{\em dependent}. We show that {\em quasi-one-dimensional } regime might occur in high-$T_c$ superconductors with Zn impurities at low temp\ eratures $T\lesssim 10$~KComment: 6 pages , Revtex 3, Los Alamos Preprint LA-UR-94-9

Two energy scales in the magnetic resonance spectrum of electron and hole doped pnictide superconductors

We argue that a multiband superconductor with sign-changing gaps may have multiple spin resonances. We calculate the RPA-based spin resonance spectra of a pnictide superconductor by using the five band tight-binding model or angle-resolved photoemission spectroscopy (ARPES) Fermi surface (FS) and experimental values of superconducting (SC) gaps. The resonance spectra split in both energy and momenta due to the effects of multiband and multiple gaps in $s^{\pm}-$pairing; the higher energy peak appears around the commensurate momenta due to scattering between $\alpha-$FS to $\gamma/\delta-$FS pockets. The second resonance is incommensurate coming from $\beta-$FS to $\gamma/\delta-$FS scatterings and its $q-$vector is doping-dependent and hence on the FS topology. Energies of both resonances $\omega^{1,2}_{res}$ are strongly doping dependent and are proportional to the gap amplitudes at the contributing FSs. We also discuss the evolution of the spin excitation spectra with various other possible gap symmetries, which may be relevant when either both the electron pockets or both the hole pockets completely disappear.Comment: 4.1 pages, Accepted in Phys. Rev. Lett., Please refer to the publication link for supplementary materia

Engineering three-dimensional topological insulators in Rashba-type spin-orbit coupled heterostructures

Topological insulators represent a new class of quantum phase defined by invariant symmetries and spin-orbit coupling that guarantees metallic Dirac excitations at its surface. The discoveries of these states have sparked the hope of realizing nontrivial excitations and novel effects such as a magnetoelectric effect and topological Majorana excitations. Here we develop a theoretical formalism to show that a three dimensional topological insulator can be designed artificially via stacking bilayers of two-dimensional Fermi gases with opposite Rashba-type spin-orbit coupling on adjacent layers, and with inter-layer quantum tunneling. We demonstrate that in the stack of bilayers grown along a (001)-direction, a nontrivial topological phase transition occurs above a critical number of Rashba-bilayers. In the topological phase we find the formation of a single spin-polarized Dirac cone at the $\Gamma$-point. This approach offers an accessible way to design artificial topological insulators in a set up that takes full advantage of the atomic layer deposition approach. This design principle is tunable and also allows us to bypass limitations imposed by bulk crystal geometry.Comment: (v2): Two design principles for our proposals are included. Accepted for publication in Nature Communication

Stripes, spin resonance and dx2−y2−d_{x^2-y^2}-pairing symmetry in FeSe-based layered superconductors

We calculate RPA-BCS based spin resonance spectra of newly discovered iron-selenide superconductor using two orbitals tight-binding (TB) model. The slightly squarish electron pocket Fermi surfaces (FSs) at $(\pi,0)/(0,\pi)-$momenta produce leading interpocket nesting instability at incommensurate vector $q\sim(\pi,0.5\pi)$ in the normal state static susceptibility, pinning a strong stripe-like spin-density wave (SDW) or antiferromagnetic (AFM) order at some critical value of $U$. The same nesting also induces $d_{x^2-y^2}-$pairing. The superconducting (SC) gap is nodeless and isotropic on the FSs as they are concentric to the four-fold symmetry point of the $d-$wave gap maxima, in agreement with various measurements. This induces an slightly incommensurate spin resonance with `hour-glass'-like dispersion feature, in close agreement with neutron data of chalcogenides. We also calculate $T$ pendence of the SC gap solving BCS gap equations and find that the spin resonance follows the same $T$ evolution of $\Delta(T)$ both in energy and intensity, suggesting that an itinerant weak or intermediate pair coupling theory is relevant in this system.Comment: 4.5 pages, 4 figures; Submitted (v2): Some types are corrected (v3): Expanded and published versio