10,694 research outputs found
Effective potential for composite operators and for an auxiliary scalar field in a Nambu-Jona-Lasinio model
We derive the effective potentials for composite operators in a
Nambu-Jona-Lasinio (NJL) model at zero and finite temperature and show that in
each case they are equivalent to the corresponding effective potentials based
on an auxiliary scalar field. The both effective potentials could lead to the
same possible spontaneous breaking and restoration of symmetries including
chiral symmetry if the momentum cutoff in the loop integrals is large enough,
and can be transformed to each other when the Schwinger-Dyson (SD) equation of
the dynamical fermion mass from the fermion-antifermion vacuum (or thermal)
condensates is used. The results also generally indicate that two effective
potentials with the same single order parameter but rather different
mathematical expressions can still be considered physically equivalent if the
SD equation corresponding to the extreme value conditions of the two potentials
have the same form.Comment: 7 pages, no figur
Spin Current and Current-Induced Spin Transfer Torque in Ferromagnet-Quantum Dot-Ferromagnet Coupled Systems
Based on Keldysh's nonequilibrium Green function method, the spin-dependent
transport properties in a ferromagnet-quantum dot (QD)-ferromagnet coupled
system are investigated. It is shown the spin current shows quite different
characteristics from its electrical counterpart, and by changing the relative
orientation of both magnetizations, it can change its magnitude even sign. The
current-induced spin transfer torque (CISTT) is uncovered to be greatly
enhanced when the bias voltage meets with the discrete levels of the QD at
resonant positions. The relationship between the CISTT, the electrical current
and the spin current is also addressed.Comment: 21 pages, 8 figure
Eliashberg theory of excitonic insulating transition in graphene
A sufficiently strong Coulomb interaction may open an excitonic fermion gap
and thus drive a semimetal-insulator transition in graphene. In this paper, we
study the Eliashberg theory of excitonic transition by coupling the fermion gap
equation self-consistently to the equation of vacuum polarization function.
Including the fermion gap into polarization function increases the effective
strength of Coulomb interaction because it reduces the screening effects due to
the collective particle-hole excitations. Although this procedure does not
change the critical point, it leads to a significant enhancement of the
dynamical fermion gap in the excitonic insulating phase. The validity of the
Eliashberg theory is justified by showing that the vertex corrections are
suppressed at large limit.Comment: 8 pages, 6 figure
Fate of non-Fermi liquid behavior in QED at finite chemical potential
The damping rate of two-dimensional massless Dirac fermions exhibit non-Fermi
liquid behavior, , due to gauge field at zero
temperature and zero chemical potential. We study the fate of this behavior at
finite chemical potential. We fist calculate explicitly the temporal and
spatial components of vacuum polarization functions. The analytical expressions
imply that the temporal component of gauge field develops a static screening
length at finite chemical potential while the transverse component remains
long-ranged owing to gauge invariance. We then calculate the fermion damping
rate and show that the temporal gauge field leads to normal Fermi liquid
behavior but the transverse gauge field leads to non-Fermi liquid behavior
at zero temperature. This energy-dependence is more
regular than and does not change as chemical potential
varies.Comment: 12 pages, 1 figur
Dark-bright gap solitons in coupled-mode one-dimensional saturable waveguide arrays
In the present work, we consider the dynamics of dark solitons as one mode of
a defocusing photorefractive lattice coupled with bright solitons as a second
mode of the lattice. Our investigation is motivated by an experiment which
illustrates that such coupled states can exist with both components in the
first gap of the linear band spectrum. This finding is further extended by the
examination of different possibilities from a theoretical perspective, such as
symbiotic ones where the bright component is supported by states of the dark
component in the first or second gap, or non-symbiotic ones where the bright
soliton is also a first-gap state coupled to a first or second gap state of the
dark component. While the obtained states are generally unstable, these
instabilities typically bear fairly small growth rates which enable their
observation for experimentally relevant propagation distances
Thermodynamic Curvature of the BTZ Black Hole
Some thermodynamic properties of the Ba\~nados-Teitelboim-Zanelli (BTZ) black
hole are studied to get the effective dimension of its corresponding
statistical model. For this purpose, we make use of the geometrical approach to
the thermodynamics: Considering the black hole as a thermodynamic system with
two thermodynamic variables (the mass and the angular momemtum ), we
obtain two-dimensional Riemannian thermodynamic geometry described by positive
definite Ruppeiner metric. From the thermodynamic curvature we find that the
extremal limit is the critical point. The effective spatial dimension of the
statistical system corresponding to the near-extremal BTZ black holes is one.
Far from the extremal point, the effective dimension becomes less than one,
which leads to one possible speculation on the underlying structure for the
corresponding statistical model.Comment: 19 pages, LaTeX with revtex macro, 4 figures in eps file
Spin-Polarized Transport in Ferromagnet-Marginal Fermi Liquid Systems
Spin-polarized transport through a marginal Fermi liquid (MFL) which is
connected to two noncollinear ferromagnets via tunnel junctions is discussed in
terms of the nonequilibrium Green function approach. It is found that the
current-voltage characteristics deviate obviously from the ohmic behavior, and
the tunnel current increases slightly with temperature, in contrast to those of
the system with a Fermi liquid. The tunnel magnetoresistance (TMR) is observed
to decay exponentially with increasing the bias voltage, and to decrease slowly
with increasing temperature. With increasing the coupling constant of the MFL,
the current is shown to increase linearly, while the TMR is found to decay
slowly. The spin-valve effect is observed.Comment: 5 pages, 6 figures, Phys. Rev. B 71, 064412 (2005
Tunable Fano effect in parallel-coupled double quantum dot system
With the help of the Green function technique and the equation of motion
approach, the electronic transport through a parallel-coupled double quantum
dot(DQD) is theoretically studied. Owing to the inter-dot coupling, the bonding
and antibonding states of the artificial quantum-dot-molecule may constitute an
appropriate basis set. Based on this picture, the Fano interference in the
conductance spectra of the DQD system is readily explained. The possibility of
manipulating the Fano lineshape in the tunnelling spectra of the DQD system is
explored by tuning the dot-lead coupling, the inter-dot coupling, the magnetic
flux threading the ring connecting dots and leads, and the flux difference
between two sub-rings. It has been found that by making use of various tuning,
the direction of the asymmetric tail of Fano lineshape may be flipped by
external fields, and the continuous conductance spectra may be magnetically
manipulated with lineshape retained. More importantly, by adjusting the
magnetic flux, the function of two molecular states can be exchanged, giving
rise to a swap effect, which might play a role as a qubit in the quantum
computation.Comment: 9 pages, 10 figure
First- and Second-Order Phase Transitions, Fulde-Ferrel Inhomogeneous State and Quantum Criticality in Ferromagnet/Superconductor Double Tunnel Junctions
First- and second-order phase transitions, Fulde-Ferrel (FF) inhomogeneous
superconducting (SC) state and quantum criticality in
ferromagnet/superconductor/ferromagnet double tunnel junctions are
investigated. For the antiparallel alignment of magnetizations, it is shown
that a first-order phase transition from the homogeneous BCS state to the
inhomogeneous FF state occurs at a certain bias voltage ; while the
transitions from the BCS state and the FF state to the normal state at are of the second-order. A phase diagram for the central superconductor
is presented. In addition, a quantum critical point (QCP), , is
identified. It is uncovered that near the QCP, the SC gap, the chemical
potential shift induced by the spin accumulation, and the difference of free
energies between the SC and normal states vanish as with
the quantum critical exponents , 1 and 2, respectively. The tunnel
conductance and magnetoresistance are also discussed.Comment: 5 pages, 4 figures, Phys. Rev. B 71, 144514 (2005
Dual Actions for Born-Infeld and Dp-Brane Theories
Dual actions with respect to U(1) gauge fields for Born-Infeld and -brane
theories are reexamined. Taking into account an additional condition, i.e. a
corollary to the field equation of the auxiliary metric, one obtains an
alternative dual action that does not involve the infinite power series in the
auxiliary metric given by ref. \cite{s14}, but just picks out the first term
from the series formally. New effective interactions of the theories are
revealed. That is, the new dual action gives rise to an effective interaction
in terms of one interaction term rather than infinite terms of different
(higher) orders of interactions physically. However, the price paid for
eliminating the infinite power series is that the new action is not quadratic
but highly nonlinear in the Hodge dual of a -form field strength. This
non-linearity is inevitable to the requirement the two dual actions are
equivalent.Comment: v1: 11 pages, no figures; v2: explanation of effective interactions
added; v3: concision made; v4: minor modification mad
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