454 research outputs found
Frustration-induced insulating chiral spin state in itinerant triangular-lattice magnets
We study the double-exchange model at half-filling with competing
superexchange interactions on a triangular lattice, combining exact
diagonalization and Monte-Carlo methods. We find that in between the expected
itinerant ferromagnetic and Yafet-Kittel phases a robust
scalar-chiral, insulating spin state emerges. At finite temperatures the
ferromagnet - scalar-chiral quantum critical point is characterized by
anomalous bad-metal behavior in charge transport as observed in frustrated
itinerant magnets RMoO.Comment: 4+ pages, published versio
Spontaneous Symmetry Breaking and Decoherence in Superconductors
We show that superconductors have a thin spectrum associated with spontaneous
symmetry breaking similar to that of antiferromagnets, while still being in
full agreement with Elitzur's theorem, which forbids the spontaneous breaking
of local (gauge) symmetries. This thin spectrum in the superconductors consists
of in-gap states that are associated with the spontaneous breaking of a global
phase symmetry. In qubits based on mesoscopic superconducting devices, the
presence of the thin spectrum implies a maximum coherence time which is
proportional to the number of Cooper pairs in the device. Here we present the
detailed calculations leading up to these results and discuss the relation
between spontaneous symmetry breaking in superconductors and the Meissner
effect, the Anderson-Higgs mechanism and the Josephson effect. Whereas for the
Meissner effect a symmetry breaking of the phase of the superconductor is not
required, it is essential for the Josephson effect.Comment: 11 pages, 4 figures; corrected typo
Effect of Curvature on the Electronic Structure and Bound State Formation in Rolled-up Nanotubes
We analyze the electronic properties of a two-dimensional electron gas
rolled-up into a nanotube by both numerical and analytical techniques. The
nature and the energy dispersion of the electronic quantum states strongly
depend upon the geometric parameters of the nanotube: the typical radius of
curvature and the number of windings. The effect of the curvature results in
the appearance of atomic-like bound states localized near the points of maximum
curvature. For a two-dimensional sheet rolled up into an Archimedean spiral we
find that the number of bound states is equal to the number of windings of the
spiral.Comment: 5 pages, 5 figure
Spontaneous Symmetry Breaking in Quantum Mechanics
We present a clear and mathematically simple procedure explaining spontaneous
symmetry breaking in quantum mechanical systems. The procedure is applicable to
a wide range of models and can be easily used to explain the existence of a
symmetry broken state in crystals, antiferromagnets and even superconductors.
It has the advantage that it automatically brings to the fore the main players
in spontaneous symmetry breaking: the symmetry breaking field, the
thermodynamic limit, and the global excitations of the thin spectrum.Comment: 4 pages, 0 figure
Thin Spectrum States in Superconductors
We show that finite size superconductors have a spectrum of states at
extremely low energy, i.e. inside the superconducting gap. The presence of this
{\it thin spectrum} is a generic feature and related to the fact that in a
superconductor the global phase rotation symmetry is spontaneously broken. For
a strong coupling superconductor we find the spectrum by exactly solving the
Lieb-Mattis type Hamiltonian onto which the problem maps. One of the physical
consequences of the presence of thin states is that they cause quantum
decoherence in superconducting qubits of finite extent.Comment: 4 pages, 2 figure
Absence of magnetic monopoles in Maxwellian magnetoelectrics
The electromagnetic response of topological insulators is governed by axion
electrodynamics, which features a topological magnetoelectric term in the
Maxwell equations. As a consequence magnetic fields become the source of
electric fields and vice-versa, a phenomenon that is general for any material
exhibiting a linear magnetoelectric effect. Axion electrodynamics has been
associated with the possibility to create magnetic monopoles, in particular by
a electrical charge that is screened above the surface of a magnetoelectric
material. Here we present the exact solution for the electromagnetic fields in
this geometry and show that while vortex-like magnetic screening fields are
generated by the electrical charge their divergence is identically zero at
every point in space which implies a strict absence of magnetic monopoles.
Although magnetic image charges can be made explicit in the problem, no bound
state with electric charges yielding a dyon arises. A dyon-like angular
momentum follows from our analysis, but is quantized in a universal way,
because of its dependence on the dielectric constant. This is consistent with a
general argument that precludes magnetic monopoles to be generated in Maxwell
magnetoelectrics.Comment: v2: 9 pages, 3 figures; improved presentation and more detailed
appendices; added calculation of angular momentum; appendix is made more
pedagogical and now includes the detailed solution for a point charge in the
presence of a topological dielectric sphere; several references are adde
Topological Edge States with Zero Hall Conductivity in a Dimerized Hofstadter Model
The Hofstadter model is a simple yet powerful Hamiltonian to study quantum
Hall physics in a lattice system, manifesting its essential topological states.
Lattice dimerization in the Hofstadter model opens an energy gap at half
filling. Here we show that even if the ensuing insulator has a Chern number
equal to zero, concomitantly a doublet of edge states appear that are pinned at
specific momenta. We demonstrate that these states are topologically protected
by inversion symmetry in specific one-dimensional cuts in momentum space,
define and calculate the corresponding invariants and identify a platform for
the experimental detection of these novel topological states.Comment: 5 pages, 3 figures + Supplemental Material (6 pages
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