6,812 research outputs found
Non-Abelian toplogical superconductors from topological semimetals and related systems under superconducting proximity effect
Non-Abelian toplogical superconductors are characterized by the existence of
{zero-energy} Majorana fermions bound in the quantized vortices. This is a
consequence of the nontrivial bulk topology characterized by an {\em odd} Chern
number. It is found that in topological semimetals with a single two-bands
crossing point all the gapped superconductors are non-Abelian ones. Such a
property is generalized to related but more generic systems which will be
useful in the search of non-Abelian superconductors and Majorana fermions
Superconductivity at the Border of Electron Localization and Itinerancy
The superconducting state of iron pnictides and chalcogenides exists at the
border of antiferromagnetic order. Consequently, these materials could provide
clues about the relationship between magnetism and unconventional
superconductivity. One explanation, motivated by the so-called bad-metal
behaviour of these materials, proposes that magnetism and superconductivity
develop out of quasi-localized magnetic moments which are generated by strong
electron-electron correlations. Another suggests that these phenomena are the
result of weakly interacting electron states that lie on nested Fermi surfaces.
Here we address the issue by comparing the newly discovered alkaline iron
selenide superconductors, which exhibit no Fermi-surface nesting, to their iron
pnictide counterparts. We show that the strong-coupling approach leads to
similar pairing amplitudes in these materials, despite their different Fermi
surfaces. We also find that the pairing amplitudes are largest at the boundary
between electronic localization and itinerancy, suggesting that new
superconductors might be found in materials with similar characteristics.Comment: Version of the published manuscript prior to final journal-editting.
Main text (23 pages, 4 figures) + Supplementary Information (14 pages, 7
figures, 3 tables). Calculation on the single-layer FeSe is added.
Enhancement of the pairing amplitude in the vicinity of the Mott transition
is highlighted. Published version is at
http://www.nature.com/ncomms/2013/131115/ncomms3783/full/ncomms3783.htm
Non-Abelian anyonic interferometry with a multi-photon spin lattice simulator
Recently a pair of experiments demonstrated a simulation of Abelian anyons in
a spin network of single photons. The experiments were based on an Abelian
discrete gauge theory spin lattice model of Kitaev. Here we describe how to use
linear optics and single photons to simulate non-Abelian anyons. The scheme
makes use of joint qutrit-qubit encoding of the spins and the resources
required are three pairs of parametric down converted photons and 14 beam
splitters.Comment: 13 pages, 5 figures. Several references added in v
Thermally driven spin injection from a ferromagnet into a non-magnetic metal
Creating, manipulating and detecting spin polarized carriers are the key
elements of spin based electronics. Most practical devices use a perpendicular
geometry in which the spin currents, describing the transport of spin angular
momentum, are accompanied by charge currents. In recent years, new sources of
pure spin currents, i.e., without charge currents, have been demonstrated and
applied. In this paper, we demonstrate a conceptually new source of pure spin
current driven by the flow of heat across a ferromagnetic/non-magnetic metal
(FM/NM) interface. This spin current is generated because the Seebeck
coefficient, which describes the generation of a voltage as a result of a
temperature gradient, is spin dependent in a ferromagnet. For a detailed study
of this new source of spins, it is measured in a non-local lateral geometry. We
developed a 3D model that describes the heat, charge and spin transport in this
geometry which allows us to quantify this process. We obtain a spin Seebeck
coefficient for Permalloy of -3.8 microvolt/Kelvin demonstrating that thermally
driven spin injection is a feasible alternative for electrical spin injection
in, for example, spin transfer torque experiments
Kaleidoscope of topological phases with multiple Majorana species
Exactly solvable lattice models for spins and non-interacting fermions
provide fascinating examples of topological phases, some of them exhibiting the
localized Majorana fermions that feature in proposals for topological quantum
computing. The Chern invariant is one important characterization of such
phases. Here we look at the square-octagon variant of Kitaev's honeycomb model.
It maps to spinful paired fermions and enjoys a rich phase diagram featuring
distinct abelian and nonabelian phases with and . The and phases all support localized Majorana
modes and are examples of Ising and anyon theories respectively.Comment: 6 pages, 5 figures. The second version has a new title, reflecting a
change of focus of the presentation in this version. The third version
contains minor changes and is essentially the one published in New Journal of
Physic
Weak magnetism and non-Fermi liquids near heavy-fermion critical points
This paper is concerned with the weak-moment magnetism in heavy-fermion
materials and its relation to the non-Fermi liquid physics observed near the
transition to the Fermi liquid. We explore the hypothesis that the primary
fluctuations responsible for the non-Fermi liquid physics are those associated
with the destruction of the large Fermi surface of the Fermi liquid. Magnetism
is suggested to be a low-energy instability of the resulting small Fermi
surface state. A concrete realization of this picture is provided by a
fractionalized Fermi liquid state which has a small Fermi surface of conduction
electrons, but also has other exotic excitations with interactions described by
a gauge theory in its deconfined phase. Of particular interest is a
three-dimensional fractionalized Fermi liquid with a spinon Fermi surface and a
U(1) gauge structure. A direct second-order transition from this state to the
conventional Fermi liquid is possible and involves a jump in the electron Fermi
surface volume. The critical point displays non-Fermi liquid behavior. A
magnetic phase may develop from a spin density wave instability of the spinon
Fermi surface. This exotic magnetic metal may have a weak ordered moment
although the local moments do not participate in the Fermi surface.
Experimental signatures of this phase and implications for heavy-fermion
systems are discussed.Comment: 20 pages, 8 figures; (v2) includes expanded discussion and solution
of quantum Boltzmann equatio
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