574 research outputs found
Electronic instabilities of a Hubbard model approached as a large array of coupled chains: competition between d-wave superconductivity and pseudogap phase
We study the electronic instabilities in a 2D Hubbard model where one of the
dimensions has a finite width, so that it can be considered as a large array of
coupled chains. The finite transverse size of the system gives rise to a
discrete string of Fermi points, with respective electron fields that, due to
their mutual interaction, acquire anomalous scaling dimensions depending on the
point of the string. Using bosonization methods, we show that the anomalous
scaling dimensions vanish when the number of coupled chains goes to infinity,
implying the Fermi liquid behavior of a 2D system in that limit. However, when
the Fermi level is at the Van Hove singularity arising from the saddle points
of the 2D dispersion, backscattering and Cooper-pair scattering lead to the
breakdown of the metallic behavior at low energies. These interactions are
taken into account through their renormalization group scaling, studying in
turn their influence on the nonperturbative bosonization of the model. We show
that, at a certain low-energy scale, the anomalous electron dimension diverges
at the Fermi points closer to the saddle points of the 2D dispersion. The
d-wave superconducting correlations become also large at low energies, but
their growth is cut off as the suppression of fermion excitations takes place
first, extending progressively along the Fermi points towards the diagonals of
the 2D Brillouin zone. We stress that this effect arises from the vanishing of
the charge stiffness at the Fermi points, characterizing a critical behavior
that is well captured within our nonperturbative approach.Comment: 13 pages, 7 figure
Why and when the Minkowski's stress tensor can be used in the problem of Casimir force acting on bodies embedded in media
It is shown that the criticism by Raabe and Welsch of the
Dzyaloshinskii-Lifshitz-Pitaevskii theory of the van der Waals-Casimir forces
inside a medium is based on misunderstandings. It is explained why and at which
conditions one can use the ''Minkowski-like '' stress tensor for calculations
of the forces. The reason, why approach of Raabe and Welsch is incorrect, is
discussed.Comment: Comment, 2 pages. 2 misprints were correcte
RPAE versus RPA for the Tomonaga model with quadratic energy dispersion
Recently the damping of the collective charge (and spin) modes of interacting
fermions in one spatial dimension was studied. It results from the nonlinear
correction to the energy dispersion in the vicinity of the Fermi points. To
investigate the damping one has to replace the random phase approximation (RPA)
bare bubble by a sum of more complicated diagrams. It is shown here that a
better starting point than the bare RPA is to use the (conserving) linearized
time dependent Hartree-Fock equations, i.e. to perform a random phase
approximation (with) exchange
(RPAE) calculation. It is shown that the RPAE equation can be solved
analytically for the special form of the two-body interaction often used in the
Luttinger liquid framework. While (bare) RPA and RPAE agree for the case of a
strictly linear disperson there are qualitative differences for the case of the
usual nonrelativistic quadratic dispersion.Comment: 6 pages, 3 figures, misprints corrected; to appear in PRB7
Electrically driven magnetism on a Pd thin film
Using first-principles density functional calculations we demonstrate that
ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd
metal thin-film surface through application of an external electric field. As
free charges are either accumulated or depleted at the Pd surface to screen the
applied electric field there is a corresponding change in the surface density
of states. This change can be made sufficient for the Fermi-level density of
states to satisfy the Stoner criterion, driving a transition locally at the
surface from a paramagnetic state to an itinerant ferromagnetic state above a
critical applied electric field, Ec. Furthermore, due to the second-order
nature of this transition, the surface magnetization of the ferromagnetic state
just above the transition exhibits a substantial dependence on electric field,
as the result of an enhanced magnetoelectric susceptibility. Using a linearized
Stoner model we explain the occurrence of the itinerant ferromagnetism and
demonstrate that the magnetic moment on the Pd surface follows a square-root
variation with electric field consistent with our first-principles
calculations.Comment: 8 pages, 7 figure
Deformation of anisotropic Fermi surfaces due to electron-electron interactions
We analyze the deformations of the Fermi surface induced by electron-electron
interactions in anisotropic two dimensional systems. We use perturbation theory
to treat, on the same footing, the regular and singular regions of the Fermi
surface. It is shown that, even for weak local coupling, the self-energy
presents a nontrivial behavior showing momentum dependence and interplay with
the Fermi surface shape. Our scheme gives simple analytical expressions based
on local features of the Fermi surface.Comment: 7 pages, 3 figure
Magnetoelectric effects in single crystals of the cubic ferrimagnetic helimagnet Cu2OSeO3
We present magnetodielectric measurements in single crystals of the cubic
spin-1/2 compound CuOSeO. A magnetic field-induced electric
polarization () and a finite magnetocapacitance (MC) is observed at
the onset of the magnetically ordered state ( K). Both and
MC are explored in considerable detail as a function of temperature (T),
applied field , and relative field orientations with respect to the
crystallographic axes. The magnetodielectric data show a number of anomalies
which signal magnetic phase transitions, and allow to map out the phase diagram
of the system in the -T plane. Below the 3up-1down collinear ferrimagnetic
phase, we find two additional magnetic phases. We demonstrate that these are
related to the field-driven evolution of a long-period helical phase, which is
stabilized by the chiral Dzyalozinskii-Moriya term D \vec{M}
\cdot(\bs{\nabla}\times\vec{M}) that is present in this non-centrosymmetric
compound. We also present a phenomenological Landau-Ginzburg theory for the
ME effect, which is in excellent agreement with experimental data, and
shows three novel features: (i) the polarization has a uniform as
well as a long-wavelength spatial component that is given by the pitch of the
magnetic helices, (ii) the uniform component of points along the
vector , and (iii) its strength is proportional to
, where is the longitudinal
and is the transverse (and spiraling) component of the magnetic
ordering. Hence, the field dependence of P provides a clear signature of the
evolution of a conical helix under a magnetic field. A similar phenomenological
theory is discussed for the MC
Broken parity and a chiral ground state in the frustrated magnet CdCr2O4
We present a model describing the lattice distortion and incommensurate
magnetic order in the spinel CdCr2O4, a good realization of the Heisenberg
"pyrochlore" antiferromagnet. The magnetic frustration is relieved through the
spin-Peierls distortion of the lattice involving a phonon doublet with odd
parity. The distortion stablizes a collinear magnetic order with the
propagation wavevector q=2\pi(0,0,1). The lack of inversion symmetry makes the
crystal structure chiral. The handedness is transferred to magnetic order by
the relativistic spin-orbit coupling: the collinear state is twisted into a
long spiral with the spins in the ac plane and q shifted to 2\pi(0,\delta,1).Comment: Incremental changes in response to referee report
Functional renormalization group for Luttinger liquids with impurities
We improve the recently developed functional renormalization group (fRG) for
impurities and boundaries in Luttinger liquids by including renormalization of
the two-particle interaction, in addition to renormalization of the impurity
potential. Explicit flow-equations are derived for spinless lattice fermions
with nearest neighbor interaction at zero temperature, and a fast algorithm for
solving these equations for very large systems is presented. We compute
spectral properties of single-particle excitations, and the oscillations in the
density profile induced by impurities or boundaries for chains with up to
1000000 lattice sites. The expected asymptotic power-laws at low energy or long
distance are fully captured by the fRG. Results on the relevant energy scales
and crossover phenomena at intermediate scales are also obtained. A comparison
with numerical density matrix renormalization results for systems with up to
1000 sites shows that the fRG with the inclusion of vertex renormalization is
remarkably accurate even for intermediate interaction strengths.Comment: 35 pages, 16 figures, revised version as publishe
Anisotropic Fermi surfaces and Kohn-Luttinger superconductivity in two dimensions
The instabilities induced on a two-dimensional system of correlated electrons
by the anisotropies of its Fermi line are analyzed on general grounds. Simple
scaling arguments allow to predict the opening of a superconducting gap with a
well-defined symmetry prescribed by the geometry of the Fermi line. The same
arguments predict a critical dimension of 3/2 for the transition of the
two-dimensional system to non-Fermi liquid behavior. The methods are applied to
the t-t' Hubbard model in a wide range of dopings.Comment: 25 pages, 13 postscript figure
Kinematics of electrons near a Van Hove singularity
A two dimensional electronic system, where the Fermi surface is close to a
Van Hove singularity, shows a variety of weak coupling instabilities, and it is
a convenient model to study the interplay between antiferromagnetism and
anisotropic superconductivity. We present a detailed analysis of the kinematics
of the electron scattering in this model. The similitudes, and differences,
between a standard Renormalization Group approach and previous work based on
parquet summations of log divergences are analyzed, with emphasis on the
underlying physical processes. General properties of the phase diagram are
discussed.Comment: 5 pages, 3 postscript figure
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