95,257 research outputs found
Current instability and diamagnetism in small-diameter carbon nanotubes
We investigate the electronic instabilities in carbon nanotubes of short
radius, looking for the breakdown of the Luttinger liquid regime from the
singular behavior of the charge stiffnesses at low energies. We show that such
a breakdown is realized in the undoped (3,3) nanotubes through the onset of
phase separation into regions with opposite electronic current. The
phenomenology derived from this regime is consistent with the formation of a
pseudogap in the single-particle spectrum as well as with a divergent
diamagnetic susceptibility, as observed in the experiments carried out in
carbon nanotubes of small diameter.Comment: 5 pages, submitted to PR
Magnetic and Kohn-Luttinger instabilities near a Van Hove singularity: monolayer versus twisted bilayer graphene
We investigate the many-body instabilities of electrons interacting near Van
Hove singularities arising in monolayer and twisted bilayer graphene. We show
that a pairing instability must be dominant over the tendency to magnetic order
as the Fermi level is tuned to the Van Hove singularity in the conduction band
of graphene. As a result of the extended character of the saddle points in the
dispersion, we find that the pairing of the electrons takes place
preferentially in a channel of f-wave symmetry, with an order parameter
vanishing at the position of the saddle points along the Fermi line. In the
case of the twisted bilayers, the dispersion has instead its symmetry reduced
down to the C_{3v} group and, most importantly, it leads to susceptibilities
that diverge at the saddle points but are integrable along the Fermi line. This
implies that a ferromagnetic instability becomes dominant in the twisted
graphene bilayers near the Van Hove singularity, with a strength which is
amplified as the lowest subband of the electron system becomes flatter for
decreasing twist angle.Comment: 16 pages, 10 figure
Rippling transition from electron-induced condensation of curvature field in graphene
A quantum field theory approach is applied to investigate the dynamics of
flexural phonons in a metallic membrane like graphene, looking for the effects
deriving from the strong interaction between the electronic excitations and
elastic deformations. Relying on a self-consistent screening approximation to
the phonon self-energy, we show that the theory has a critical point
characterized by the vanishing of the effective bending rigidity of the
membrane at low momentum. We also check that the instability in the sector of
flexural phonons takes place without the development of an in-plane static
distortion, which is avoided due to the significant reduction of the
electron-phonon couplings for in-plane phonons at large momenta. Furthermore,
we analyze the scaling properties of the many-body theory to identify the order
parameter that opens up at the point of the transition. We find that the
vanishing of the effective bending rigidity and the onset of a nonzero
expectation value of the mean curvature are concurrent manifestations of the
critical behavior. The results presented here imply that, even in the absence
of tension, the theory has a critical point at which the flat geometry becomes
an unstable configuration of the metallic membrane, with a condensation of the
mean curvature field that may well reproduce the smooth distribution of ripples
observed in free-standing graphene.Comment: 15 pages, 6 figure
Phase diagram of the Quantum Electrodynamics of 2D and 3D Dirac semimetals
We study the Quantum Electrodynamics of 2D and 3D Dirac semimetals by means
of a self-consistent resolution of the Schwinger-Dyson equations, aiming to
obtain the respective phase diagrams in terms of the relative strength of the
Coulomb interaction and the number N of Dirac fermions. In this framework, 2D
Dirac semimetals have just a strong-coupling instability characterized by
exciton condensation (and dynamical generation of mass) that we find at a
critical coupling well above previous theoretical estimates, thus explaining
the absence of that instability in free-standing graphene samples. On the other
hand, we show that 3D Dirac semimetals have a richer phase diagram, with a
strong-coupling instability leading to dynamical mass generation up to N = 4
and a line of critical points for larger values of N characterized by the
vanishing of the electron quasiparticle weight in the low-energy limit. Such a
critical behavior signals the transition to a strongly correlated liquid,
characterized by noninteger scaling dimensions that imply the absence of a pole
in the electron propagator and are the signature of non-Fermi liquid behavior
with no stable electron quasiparticles.Comment: 11 pages, 11 figures, extended physical discussio
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