563 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
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
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
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
How does a quadratic term in the energy dispersion modify the single-particle Green's function of the Tomonaga-Luttinger model?
We calculate the effect of a quadratic term in the energy dispersion on the
low-energy behavior of the Green's function of the spinless Tomonaga-Luttinger
model (TLM). Assuming that for small wave-vectors q = k - k_F the fermionic
excitation energy relative to the Fermi energy is v_F q + q^2 / (2m), we
explicitly calculate the single-particle Green's function for finite but small
values of lambda = q_c /(2k_F). Here k_F is the Fermi wave-vector, q_c is the
maximal momentum transfered by the interaction, and v_F = k_F / m is the Fermi
velocity. Assuming equal forward scattering couplings g_2 = g_4, we find that
the dominant effect of the quadratic term in the energy dispersion is a
renormalization of the anomalous dimension. In particular, at weak coupling the
anomalous dimension is tilde{gamma} = gamma (1 - 2 lambda^2 gamma), where gamma
is the anomalous dimension of the TLM. We also show how to treat the change of
the chemical potential due to the interactions within the functional
bosonization approach in arbitrary dimensions.Comment: 17 pages, 1 figur
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
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
Luttinger liquids with curvature: Density correlations and Coulomb drag effect
We consider the effect of the curvature in fermionic dispersion on the
observable properties of Luttinger liquid (LL). We use the bosonization
technique where the curvature is irrelevant perturbation, describing the decay
of LL bosons (plasmon modes). When possible, we establish the correspondence
between the bosonization and the fermionic approach. We analyze modifications
in density correlation functions due to curvature at finite temperatures, T.
The most important application of our approach is the analysis of the Coulomb
drag by small momentum transfer between two LL, which is only possible due to
curvature. Analyzing the a.c. transconductivity in the one-dimensional drag
setup, we confirm the results by Pustilnik et al. for T-dependence of drag
resistivity, R_{12} ~ T^2 at high and R_{12} ~ T^5 at low temperatures. The
bosonization allows for treating both intra- and inter-wire electron-electron
interactions in all orders, and we calculate exact prefactors in low-T drag
regime. The crossover temperature between the two regimes is T_1 ~ E_F \Delta,
with \Delta relative difference in plasmon velocities. We show that \Delta \neq
0 even for identical wires, due to lifting of degeneracy by interwire
interaction, U_{12}, leading to crossover from R_{12} ~ U_{12}^2 T^2 to R_{12}
\~ T^5/U_{12} at T ~ U_{12}.Comment: 16 pages, 10 figures, REVTE
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
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
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