52 research outputs found
Magnon Exchange Mechanism of Ferromagnetic Superconductivity
The magnon exchange mechanism of ferromagnetic superconductivity
(FM-superconductivity) was developed to explain in a natural way the fact that
the superconductivity in , and is confined to the
ferromagnetic phase.The order parameter is a spin anti-parallel component of a
spin-1 triplet with zero spin projection. The transverse spin fluctuations are
pair forming and the longitudinal ones are pair breaking. In the present paper,
a superconducting solution, based on the magnon exchange mechanism, is obtained
which closely matches the experiments with and . The onset of
superconductivity leads to the appearance of complicated Fermi surfaces in the
spin up and spin down momentum distribution functions. Each of them consist of
two pieces, but they are simple-connected and can be made very small by varying
the microscopic parameters. As a result, it is obtained that the specific heat
depends on the temperature linearly, at low temperature, and the coefficient
is smaller in the superconducting phase than in the
ferromagnetic one. The absence of a quantum transition from ferromagnetism to
ferromagnetic superconductivity in a weak ferromagnets and is
explained accounting for the contribution of magnon self-interaction to the
spin fluctuations' parameters. It is shown that in the presence of an external
magnetic field the system undergoes a first order quantum phase transition.Comment: 9 pages, 7 figures, accepted for publication in Phys.Rev.
Ferromagnetic phases in spin-Fermion systems
Spin-Fermion systems which obtain their magnetic properties from a system of
localized magnetic moments being coupled to conducting electrons are
considered. The dynamical degrees of freedom are spin- operators of
localized spins and spin-1/2 Fermi operators of itinerant electrons.
Renormalized spin-wave theory, which accounts for the magnon-magnon
interaction, and its extension are developed to describe the two ferrimagnetic
phases in the system: low temperature phase , where all electrons
contribute the ordered ferromagnetic moment, and high temperature phase
, where only localized spins form magnetic moment. The
magnetization as a function of temperature is calculated. The theoretical
predictions are utilize to interpret the experimentally measured
magnetization-temperature curves of ..Comment: 9 pages, 5 figure
Magnon-Paramagnon Effective Theory of Itinerant Ferromagnets
The present work is devoted to the derivation of an effective
magnon-paramagnon theory starting from a microscopic lattice model of
ferromagnetic metals. For some values of the microscopic parameters it
reproduces the Heisenberg theory of localized spins. For small magnetization
the effective model describes the physics of weak ferromagnets in accordance
with the experimental results. It is written in a way which keeps O(3) symmetry
manifest,and describes both the order and disordered phases of the system.
Analytical expression for the Curie temperature,which takes the magnon
fluctuations into account exactly, is obtained. For weak ferromagnets is
well below the Stoner's critical temperature and the critical temperature
obtained within Moriya's theory.Comment: 14 pages, changed content,new result
Spin-Wave Theory of the Spiral Phase of the t-J Model
A graded H.P,realization of the SU(2|1) algebra is proposed.A spin-wave
theory with a condition that the sublattice magnetization is zero is
discussed.The long-range spiral phase is investigated.The spin-spin correlator
is calculated.Comment: 17 page
Hidden Quantum Critical Point in a Ferromagnetic Superconductor
We consider a coexistence phase of both Ferromagnetism and superconductivity
and solve the self-consistent mean-field equations at zero temperature. The
superconducting gap is shown to vanish at the Stoner point whereas the
magnetization doesn't. This indicates that the para-Ferro quantum critical
point becomes a hidden critical point. The effective mass in such a phase gets
enhanced whereas the spin wave stiffness is reduced as compared to the pure FM
phase. The spin wave stiffness remains finite even at the para-Ferro quantum
critical point.Comment: 4 pages, Phys. Rev. B (Rapid) accepte
Dimensional structural constants from chiral and conformal bosonization of QCD
We derive the dimensional non-perturbative part of the QCD effective action
for scalar and pseudoscalar meson fields by means of chiral and conformal
bosonization. The related structural coupling constants L_5 and L_8 of the
chiral lagrangian are estimated using general relations which are valid in a
variety of chiral bosonization models without explicit reference to model
parameters. The asymptotics for large scalar fields in QCD is elaborated, and
model-independent constraints on dimensional coupling constants of the
effective meson lagrangian are evaluated. We determine also the interaction
between scalar quarkonium and the gluon density and obtain the scalar
glueball-quarkonium potential.Comment: 21 pages, LaTe
Phase diagrams of in Double Exchange Model with added antiferromagnetic and Jahn-Teller interaction
The phase diagram of the multivalent manganites , in
space of temperature and doping , is a challenge for the theoretical
physics. It is an important test for the model used to study these compounds
and the method of calculation. To obtain theoretically this diagram for
, we consider the two-band Double Exchange Model for manganites with
added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to
calculate Curie and N\'{e}el temperatures we derive an effective Heisenberg
model for a vector which describes the local orientation of the total
magnetization of the system. The exchange constants of this model are different
for different space directions and depend on the density of electrons,
antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well
known phase transitions from A-type antiferromagnetism to ferromagnetism at low
and C-type antiferromagnetism to G-type antiferromagnetism at large , we
argue that the antiferromagnetic exchange constants should depend on the
lattice direction. We show that ferromagnetic to A-type antiferromagnetic
transition results from the Jahn-Teller distortion. Accounting adequately for
the magnon-magnon interaction, Curie and N\'{e}el temperatures are calculated.
The results are in very good agreement with the experiment and provide values
for the model parameters, which best describe the behavior of the critical
temperature for .Comment: 13 pages, 5 figure
Zero temperature phases of the frustrated J1-J2 antiferromagnetic spin-1/2 Heisenberg model on a simple cubic lattice
At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg
antiferromagnet on a simple cubic lattice with competing first and second
neighbor exchanges (J1 and J2) is investigated using the non-linear spin wave
theory. We find existence of two phases: a two sublattice Neel phase for small
J2 (AF), and a collinear antiferromagnetic phase at large J2 (CAF). We obtain
the sublattice magnetizations and ground state energies for the two phases and
find that there exists a first order phase transition from the AF-phase to the
CAF-phase at the critical transition point, pc = 0.28. Our results for the
value of pc are in excellent agreement with results from Monte-Carlo
simulations and variational spin wave theory. We also show that the quartic 1/S
corrections due spin-wave interactions enhance the sublattice magnetization in
both the phases which causes the intermediate paramagnetic phase predicted from
linear spin wave theory to disappear.Comment: 19 pages, 4 figures, Fig. 1b modified, Appendix B text modifie
The -boson-fermion realizations of quantum suprealgebra
We show that our construction of realizations for Lie algebras and quantum
algebras can be generalized to quantum superalgebras, too. We study an example
of quantum superalgebra and give the boson-fermion realization
with respect to one pair od q-deformed boson operator and 2 pairs of fermions.Comment: 8 page
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