14,485 research outputs found
Commensurate-Incommensurate Magnetic Phase Transition in Magnetoelectric Single Crystal LiNiPO
Neutron scattering studies of single-crystal LiNiPO reveal a spontaneous
first-order commensurate-incommensurate magnetic phase transition. Short- and
long-range incommensurate phases are intermediate between the high temperature
paramagnetic and the low temperature antiferromagnetic phases. The modulated
structure has a predominant antiferromagnetic component, giving rise to
satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg
reflection, and a ferromagnetic component giving rise to peaks at small
momentum-transfers around the origin at . The wavelength of the
modulated magnetic structure varies continuously with temperature. It is argued
that the incommensurate short- and long-range phases are due to
spin-dimensionality crossover from a continuous to the discrete Ising state.
These observations explain the anomalous first-order transition seen in the
magnetoelectric effect of this system
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Radiation induced zero-resistance states: a dressed electronic structure effect
Recent results on magnetoresistance in a two dimensional electron gas under
crossed magnetic and microwave fields show a new class of oscillations,
suggesting a new kind of zero-resistance states. A complete understanding of
the effect is still lacking. We consider the problem from the point of view of
the electronic structure dressed by photons due to a in plane linearly
polarized ac field. The dramatic changes in the dressed electronic structure
lead to a interpretation of the new magnetoresistance oscillations as a
persistent-current like effect, induced by the radiation field.Comment: 5 pages, 5 figures, revtex4, changes in introduction and added
reference
Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNbO in a transverse field: Geometric frustration and quantum renormalization effects
The quasi-one-dimensional (1D) Ising ferromagnet CoNbO has recently
been driven via applied transverse magnetic fields through a continuous quantum
phase transition from spontaneous magnetic order to a quantum paramagnet, and
dramatic changes were observed in the spin dynamics, characteristic of weakly
perturbed 1D Ising quantum criticality. We report here extensive single-crystal
inelastic neutron scattering measurements of the magnetic excitations
throughout the three-dimensional (3D) Brillouin zone in the quantum
paramagnetic phase just above the critical field to characterize the effects of
the finite interchain couplings. In this phase, we observe that excitations
have a sharp, resolution-limited line shape at low energies and over most of
the dispersion bandwidth, as expected for spin-flip quasiparticles. We map the
full bandwidth along the strongly dispersive chain direction and resolve clear
modulations of the dispersions in the plane normal to the chains,
characteristic of frustrated interchain couplings in an antiferromagnetic
isosceles triangular lattice. The dispersions can be well parametrized using a
linear spin-wave model that includes interchain couplings and further neighbor
exchanges. The observed dispersion bandwidth along the chain direction is
smaller than that predicted by a linear spin-wave model using exchange values
determined at zero field, and this effect is attributed to quantum
renormalization of the dispersion beyond the spin-wave approximation in fields
slightly above the critical field, where quantum fluctuations are still
significant.Comment: 11 pages, 6 figures. Updated references. Minor changes to text and
figure
Coasting cosmologies with time dependent cosmological constant
The effect of a time dependent cosmological constant is considered in a
family of scalar tensor theories. Friedmann-Robertson-Walker cosmological
models for vacumm and perfect fluid matter are found. They have a linear
expansion factor, the so called coasting cosmology, the gravitational
"constant" decreace inversely with time; this model satisfy the Dirac
hipotesis. The cosmological "constant" decreace inversely with the square of
time, therefore we can have a very small value for it at present time.Comment: 7 pages, latex file (ijmpal macro), accepted for publication in Int.
Mod. Phys.
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