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 $120^{\circ}$ 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 R$_2$Mo$_{2}$O$_7$.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