77 research outputs found
Existence and topological stability of Fermi points in multilayered graphene
We study the existence and topological stability of Fermi points in a
graphene layer and stacks with many layers. We show that the discrete
symmetries (spacetime inversion) stabilize the Fermi points in monolayer,
bilayer and multilayer graphene with orthorhombic stacking. The bands near
and in multilayers with the Bernal stacking depend on the
parity of the number of layers, and Fermi points are unstable when the number
of layers is odd. The low energy changes in the electronic structure induced by
commensurate perturbations which mix the two Dirac points are also
investigated.Comment: 6 pages, 6 figures. Expanded version as will appear in PR
Symmetry-based approach to electron-phonon interactions in graphene
We use the symmetries of monolayer graphene to write a set of constraints
that must be satisfied by any electron-phonon interaction hamiltonian. The
explicit solution as a series expansion in the momenta gives the most general,
model-independent couplings between electrons and long wavelength acoustic and
optical phonons. As an application, the possibility of describing elastic
strains in terms of effective electromagnetic fields is considered in detail,
with an emphasis on group theory conditions and the role of time reversal
symmetry.Comment: 11 pages, 1 figure. Treatment of ripples in suspended graphene sheets
included. Revised journal version with improved presentation and two new
appendice
Topological phonon analysis of the 2D buckled honeycomb lattice: an application to real materials
By means of group theory, topological quantum chemistry, first-principles and
Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb
lattice phonon spectra. Taking the pure crystal structure as an input, we show
that eleven distinct phases are possible, five of which necessarily have
non-trivial topology according to topological quantum chemistry. Another four
of them are also identified as topological using Wilson loops in an analytical
model that includes all the symmetry allowed force constants up to third
nearest neighbors, making a total of nine topological phases. We then compute
the ab initio phonon spectra for the two-dimensional crystals of Si, Ge, P, As
and Sb in this structure and construct its phase diagram. Despite the large
proportion of topological phases found in the analytical model, all of the
crystals lie in a trivial phase. By analyzing the force constants space using
Monte Carlo calculations, we elucidate why topological phonon phases are
physically difficult to realize in real materials with this crystal structure
Non-Abelian anomalies and hadronic fluids
By using differential geometry methods, we study the role of non-Abelian anomalies
in relativistic fluids. We obtain closed expressions for the covariant currents derived from the
Chern-Simons effective action. Our results are also applied to the Wess-Zumino-Witten action
that accounts for the interaction of Goldstone bosons with external electromagnetic fields. We
particularize these results to QCD with two light flavors.Plan Nacional de Altas Energias Spanish MINECO
FPA2015-64041-C2-1-P
FPA2015-64041-C2-2-PBasque Government IT979-16Spanish MINEICOEuropean Union (EU)
FIS2017-85053-C2-1-PJunta de Andalucia
FQM-225Spanish MINEICO Ramon y Cajal Program
RYC-2016-20678Universidad del Pais Vasco UPV/EHU, Bilbao, Spai
Effective theory for the Goldstone field in the BCS-BEC crossover at T=0
We perform a detailed study of the effective Lagrangian for the Goldstone
mode of a superfluid Fermi gas at zero temperature in the whole BCS-BEC
crossover. By using a derivative expansion of the response functions, we derive
the most general form of this Lagrangian at the next to leading order in the
momentum expansion in terms of four coefficient functions. This involves the
elimination of all the higher order time derivatives by careful use of the
leading order field equations. In the infinite scattering length limit where
conformal invariance is realized, we show that the effective Lagrangian must
contain an unnoticed invariant combination of higher spatial gradients of the
Goldstone mode, while explicit couplings to spatial gradients of the trapping
potential are absent. Across the whole crossover, we determine all the
coefficient functions at the one-loop level, taking into account the dependence
of the gap parameter on the chemical potential in the mean-field approximation.
These results are analytically expressed in terms of elliptic integrals of the
first and second kind. We discuss the form of these coefficients in the extreme
BCS and BEC regimes and around the unitary limit, and compare with recent work
by other authors.Comment: 27 pages. 4 references added, typos corrected, expanded Section III
Non-Abelian anomalous (super)fluids in thermal equilibrium from differential geometry
We apply differential geometry methods to the computation of the anomalyinduced
hydrodynamic equilibrium partition function. Implementing the imaginary-time
prescription on the Chern-Simons effective action on a stationary background, we obtain
general closed expressions for both the invariant and anomalous part of the partition function.
This is applied to the Wess-Zumino-Witten action for Goldstone modes, giving the
equilibrium partition function of superfluids. In all cases, we also study the anomalyinduced
gauge currents and energy-momentum tensor, providing explicit expressions for
them.This work has been supported by Plan Nacional de Altas Energías Spanish MINECO
grants FPA2015-64041-C2-1-P, FPA2015-64041-C2-2-P, and by Basque Government grant
IT979-16. The research of E.M. is also supported by Spanish MINEICO and European
FEDER funds grant FIS2017-85053-C2-1-P, Junta de Andalucía grant FQM-225, as well
as by Universidad del País Vasco UPV/EHU through a Visiting Professor appointment
and by Spanish MINEICO Ramón y Cajal Progra
Anomalies and WZW-term of two-flavour QCD
The U(2)_R x U(2)_L symmetry of QCD with two massless flavours is subject to
anomalies which affect correlation functions involving the singlet currents
A^0_\mu or V^0_\mu. These are relevant for pion-photon interactions, because -
for two flavours - the electromagnetic current contains a singlet piece. We
give the effective Lagrangian required for the corresponding low energy
analysis to next-to-leading order, without invoking an expansion in the mass of
the strange quark. In particular, the Wess-Zumino-Witten term that accounts for
the two-flavour anomalies within the effective theory is written down in closed
form.Comment: 17 pages, 1 figur
Local Anomalies, Local Equivariant Cohomology and the Variational Bicomplex
The locality conditions for the vanishing of local anomalies in field theory
are shown to admit a geometrical interpretation in terms of local equivariant
cohomology, thus providing a method to deal with the problem of locality in the
geometrical approaches to the study of local anomalies based on the
Atiyah-Singer index theorem. The local cohomology is shown to be related to the
cohomology of jet bundles by means of the variational bicomplex theory. Using
these results and the techniques for the computation of the cohomology of
invariant variational bicomplexes in terms of relative Gel'fand-Fuks cohomology
introduced in [6], we obtain necessary and sufficient conditions for the
cancellation of local gravitational and mixed anomalies.Comment: 36 pages. The paper is divided in two part
Temperature dependence of the anomalous effective action of fermions in two and four dimensions
The temperature dependence of the anomalous sector of the effective action of
fermions coupled to external gauge and pseudo-scalar fields is computed at
leading order in an expansion in the number of Lorentz indices in two and four
dimensions. The calculation preserves chiral symmetry and confirms that a
temperature dependence is compatible with axial anomaly saturation. The result
checks soft-pions theorems at zero temperature as well as recent results in the
literature for the pionic decay amplitude into static photons in the chirally
symmetric phase. The case of chiral fermions is also considered.Comment: RevTex, 19 pages, no figures. References adde
Strain-induced partially flat band, helical snake states, and interface superconductivity in topological crystalline insulators
Topological crystalline insulators in IV-VI compounds host novel topological
surface states consisting of multi-valley massless Dirac fermions at low
energy. Here we show that strain generically acts as an effective gauge field
on these Dirac fermions and creates pseudo-Landau orbitals without breaking
time-reversal symmetry. We predict the realization of this phenomenon in IV-VI
semiconductor heterostructures, due to a naturally occurring misfit dislocation
array at the interface that produces a periodically varying strain field.
Remarkably, the zero-energy Landau orbitals form a flat band in the vicinity of
the Dirac point, and coexist with a network of snake states at higher energy.
We propose that the high density of states of this flat band gives rise to
interface superconductivity observed in IV-VI semiconductor multilayers at
unusually high temperatures, with non-BCS behavior. Our work demonstrates a new
route to altering macroscopic electronic properties to achieve a partially flat
band, and paves the way for realizing novel correlated states of matter.Comment: Accepted by Nature Physic
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