1,421 research outputs found
Magneto-elastic interaction in cubic helimagnets with B20 structure
The magneto-elastic interaction in cubic helimagnets with B20 symmetry is
considered. It is shown that this interaction is responsible for negative
contribution to the square of the spin-wave gap which is alone has to
disrupt assumed helical structure. It is suggested that competition between
positive part of which stems from magnon-magnon interaction and
its negative magneto-elastic part leads to the quantum phase transition
observed at high pressure in and . This transition has to occur
when . For from rough estimations at ambient pressure both
parts and are comparable with the experimentally
observed gap. The magneto-elastic interaction is responsible also for 2\m k
modulation of the lattice where \m k is the helix wave-vector and
contribution to the magnetic anisotropy.
Experimental observation by -ray and neutron scattering the lattice
modulation allows determine the strength of anisotropic part of the
magneto-elastic interaction responsible for above phenomena and the lattice
helicity
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
Coupled quantum wires
We study a set of crossed 1D systems, which are coupled with each other via
tunnelling at the crossings. We begin with the simplest case with no
electron-electron interactions and find that besides the expected level
splitting, bound states can emerge. Next, we include an external potential and
electron-electron interactions, which are treated within the Hartree
approximation. Then, we write down a formal general solution to the problem,
giving additional details for the case of a symmetric external potential.
Concentrating on the case of a single crossing, we were able to explain recent
experinents on crossed metallic and semiconducting nanotubes [J. W. Janssen, S.
G. Lemay, L. P. Kouwenhoven, and C. Dekker, Phys. Rev. B 65, 115423 (2002)],
which showed the presence of localized states in the region of crossing.Comment: 11 pages, 10 figure
Ferroelectrically induced weak-ferromagnetism in a single-phase multiferroic by design
We present a strategy to design structures for which a polar lattice
distortion induces weak ferromagnetism. We identify a large class of
multiferroic oxides as potential realizations and use density-functional theory
to screen several promising candidates. By elucidating the interplay between
the polarization and the Dzyaloshinskii-Moriya vector, we show how the
direction of the magnetization can be switched between 180 symmetry
equivalent states with an applied electric field.Comment: Significantly revised for clarit
The Casimir zero-point radiation pressure
We analyze some consequences of the Casimir-type zero-point radiation
pressure. These include macroscopic "vacuum" forces on a metallic layer
in-between a dielectric medium and an inert () one. Ways
to control the sign of these forces, based on dielectric properties of the
media, are thus suggested. Finally, the large positive Casimir pressure, due to
surface plasmons on thin metallic layers, is evaluated and discussed.Comment: 4 2-column pages, LATE
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
Electromagnon excitations in modulated multiferroics
The phenomenological theory of ferroelectricity in spiral magnets presented
in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe
consistently states with both uniform and modulated-in-space ferroelectric
polarizations. A key point in this description is the symmetric part of the
magnetoelectric coupling since, although being irrelevant for the uniform
component, it plays an essential role for the non-uniform part of the
polarization. We illustrate this importance in generic examples of modulated
magnetic systems: longitudinal and transverse spin-density wave states and
planar cycloidal phase. We show that even in the cases with no uniform
ferroelectricity induced, polarization correlation functions follow to the soft
magnetic behavior of the system due to the magnetoelectric effect. Our results
can be easily generalized for more complicated types of magnetic ordering, and
the applications may concern various natural and artificial systems in
condensed matter physics (e.g., magnon properties could be extracted from
dynamic dielectric response measurements).Comment: 5 page
Dynamical magnetoelectric effects in multiferroic oxides
Multiferroics with coexistent ferroelectric and magnetic orders can provide
an interesting laboratory to test unprecedented magnetoelectric responses and
their possible applications. One such example is the dynamical and/or resonant
coupling between magnetic and electric dipoles in a solid. As the examples of
such dynamical magnetoelectric effects, (1) the multiferroic domain wall
dynamics and (2) the electric-dipole active magnetic responses are discussed
with the overview of recent experimental observations.Comment: 15 pages including 6 figures; Accepted for publication in Phil.
Trans. A Roy. Soc. (Special issue, Spin on Electronics
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
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