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

By Dawn's Early Light: CMB Polarization Impact on Cosmological Constraints

Cosmic microwave background polarization encodes information not only on the early universe but also dark energy, neutrino mass, and gravity in the late universe through CMB lensing. Ground based surveys such as ACTpol, PolarBear, SPTpol significantly complement cosmological constraints from the Planck satellite, strengthening the CMB dark energy figure of merit and neutrino mass constraints by factors of 3-4. This changes the dark energy probe landscape. We evaluate the state of knowledge in 2017 from ongoing experiments including dark energy surveys (supernovae, weak lensing, galaxy clustering), fitting for dynamical dark energy, neutrino mass, and a modified gravitational growth index. Adding a modest strong lensing time delay survey improves those dark energy constraints by a further 32%, and an enhanced low redshift supernova program improves them by 26%.Comment: 8 pages; 8 figures; some references update

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

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

Effective Actions for 0+1 Dimensional Scalar QED and its SUSY Generalization at T≠0T\neq 0

We compute the effective actions for the 0+1 dimensional scalar field interacting with an Abelian gauge background, as well as for its supersymmetric generalization at finite temperature.Comment: 5 pages, Latex fil

Ground-state of graphene in the presence of random charged impurities

We calculate the carrier density dependent ground state properties of graphene in the presence of random charged impurities in the substrate taking into account disorder and interaction effects non-perturbatively on an equal footing in a self-consistent theoretical formalism. We provide detailed quantitative results on the dependence of the disorder-induced spatially inhomogeneous two-dimensional carrier density distribution on the external gate bias, the impurity density, and the impurity location. We find that the interplay between disorder and interaction is strong, particularly at lower impurity densities. We show that for the currently available typical graphene samples, inhomogeneity dominates graphene physics at low ($\lesssim 10^{12}$ cm$^{-2}$) carrier density with the density fluctuations becoming larger than the average density.Comment: Final version, accepted for publication in Phys. Rev. Let