1,066 research outputs found
Quantum Oscillations in the Underdoped Cuprate YBa2Cu4O8
We report the observation of quantum oscillations in the underdoped cuprate
superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed
magnetic fields up to 85T. There is a clear signal, periodic in inverse field,
with frequency 660+/-15T and possible evidence for the presence of two
components of slightly different frequency. The quasiparticle mass is
m*=3.0+/-0.3m_e. In conjunction with the results of Doiron-Leyraud et al. for
YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a
general feature of underdoped copper oxide planes and provide information about
the doping dependence of the Fermi surface.Comment: Contains revisions addressing referees' comments including a
different Fig 1b. 4 pages, 4 figure
Ground State Spin Structure of Strongly Interacting Disordered 1D Hubbard Model
We study the influence of on-site disorder on the magnetic properties of the
ground state of the infinite U 1D Hubbard model. We find that the ground state
is not ferromagnetic. This is analyzed in terms of the algebraic structure of
the spin dependence of the Hamiltonian. A simple explanation is derived for the
1/N periodicity in the persistent current for this model.Comment: 3 pages, no figure
Calculating critical temperatures of superconductivity from a renormalized Hamiltonian
It is shown that one can obtain quantitatively accurate values for the
superconducting critical temperature within a Hamiltonian framework. This is
possible if one uses a renormalized Hamiltonian that contains an attractive
electron-electron interaction and renormalized single particle energies. It can
be obtained by similarity renormalization or using flow equations for
Hamiltonians. We calculate the critical temperature as a function of the
coupling using the standard BCS-theory. For small coupling we rederive the
McMillan formula for Tc. We compare our results with Eliashberg theory and with
experimental data from various materials. The theoretical results agree with
the experimental data within 10%. Renormalization theory of Hamiltonians
provides a promising way to investigate electron-phonon interactions in
strongly correlated systems.Comment: 6 pages, LaTeX, using EuroPhys.sty, one eps figure included, accepted
for publication in Europhys. Let
Passing to the Limit in a Wasserstein Gradient Flow: From Diffusion to Reaction
We study a singular-limit problem arising in the modelling of chemical
reactions. At finite {\epsilon} > 0, the system is described by a Fokker-Planck
convection-diffusion equation with a double-well convection potential. This
potential is scaled by 1/{\epsilon}, and in the limit {\epsilon} -> 0, the
solution concentrates onto the two wells, resulting into a limiting system that
is a pair of ordinary differential equations for the density at the two wells.
This convergence has been proved in Peletier, Savar\'e, and Veneroni, SIAM
Journal on Mathematical Analysis, 42(4):1805-1825, 2010, using the linear
structure of the equation. In this paper we re-prove the result by using solely
the Wasserstein gradient-flow structure of the system. In particular we make no
use of the linearity, nor of the fact that it is a second-order system. The
first key step in this approach is a reformulation of the equation as the
minimization of an action functional that captures the property of being a
curve of maximal slope in an integrated form. The second important step is a
rescaling of space. Using only the Wasserstein gradient-flow structure, we
prove that the sequence of rescaled solutions is pre-compact in an appropriate
topology. We then prove a Gamma-convergence result for the functional in this
topology, and we identify the limiting functional and the differential equation
that it represents. A consequence of these results is that solutions of the
{\epsilon}-problem converge to a solution of the limiting problem.Comment: Added two sections, corrected minor typos, updated reference
Electric field in 3D gravity with torsion
It is shown that in static and spherically symmetric configurations of the
system of Maxwell field coupled to 3D gravity with torsion, at least one of the
Maxwell field components has to vanish. Restricting our attention to the
electric sector of the theory, we find an interesting exact solution,
corresponding to the azimuthal electric field. Its geometric structure is to a
large extent influenced by the values of two different central charges,
associated to the asymptotic AdS structure of spacetime.Comment: LATEX, 15 pages, v2: minor correction
On the stability of travelling waves with vorticity obtained by minimisation
We modify the approach of Burton and Toland [Comm. Pure Appl. Math. (2011)]
to show the existence of periodic surface water waves with vorticity in order
that it becomes suited to a stability analysis. This is achieved by enlarging
the function space to a class of stream functions that do not correspond
necessarily to travelling profiles. In particular, for smooth profiles and
smooth stream functions, the normal component of the velocity field at the free
boundary is not required a priori to vanish in some Galilean coordinate system.
Travelling periodic waves are obtained by a direct minimisation of a functional
that corresponds to the total energy and that is therefore preserved by the
time-dependent evolutionary problem (this minimisation appears in Burton and
Toland after a first maximisation). In addition, we not only use the
circulation along the upper boundary as a constraint, but also the total
horizontal impulse (the velocity becoming a Lagrange multiplier). This allows
us to preclude parallel flows by choosing appropriately the values of these two
constraints and the sign of the vorticity. By stability, we mean conditional
energetic stability of the set of minimizers as a whole, the perturbations
being spatially periodic of given period.Comment: NoDEA Nonlinear Differential Equations and Applications, to appea
Flow equations for QED in the light front dynamics
The method of flow equations is applied to QED on the light front. Requiring
that the partical number conserving terms in the Hamiltonian are considered to
be diagonal and the other terms off-diagonal an effective Hamiltonian is
obtained which reduces the positronium problem to a two-particle problem, since
the particle number violating contributions are eliminated. No infrared
divergencies appear. The ultraviolet renormalization can be performed
simultaneously.Comment: 15 pages, Latex, 3 pictures, Submitted to Phys.Rev.
Flow equations for Hamiltonians: Contrasting different approaches by using a numerically solvable model
To contrast different generators for flow equations for Hamiltonians and to
discuss the dependence of physical quantities on unitarily equivalent, but
effectively different initial Hamiltonians, a numerically solvable model is
considered which is structurally similar to impurity models. By this we discuss
the question of optimization for the first time. A general truncation scheme is
established that produces good results for the Hamiltonian flow as well as for
the operator flow. Nevertheless, it is also pointed out that a systematic and
feasible scheme for the operator flow on the operator level is missing. For
this, an explicit analysis of the operator flow is given for the first time. We
observe that truncation of the series of the observable flow after the linear
or bilinear terms does not yield satisfactory results for the entire parameter
regime as - especially close to resonances - even high orders of the exact
series expansion carry considerable weight.Comment: 25 pages, 10 figure
Ferromagnetism in Correlated Electron Systems: Generalization of Nagaoka's Theorem
Nagaoka's theorem on ferromagnetism in the Hubbard model with one electron
less than half filling is generalized to the case where all possible
nearest-neighbor Coulomb interactions (the density-density interaction ,
bond-charge interaction , exchange interaction , and hopping of double
occupancies ) are included. It is shown that for ferromagnetic exchange
coupling () ground states with maximum spin are stable already at finite
Hubbard interaction . For non-bipartite lattices this requires a hopping
amplitude . For vanishing one obtains as in
Nagaoka's theorem. This shows that the exchange interaction is important
for stabilizing ferromagnetism at finite . Only in the special case
the ferromagnetic state is stable even for , provided the lattice allows
the hole to move around loops.Comment: 13 pages, uuencoded postscript, includes 1 table and 2 figure
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