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有效拉格朗日算子和质量算子是在2 + 1维量子电动力学的一环近似中,在恒定的外部均匀磁场中计算平面带电的无质量和无质量费米子的(QED $$ _ {2 + 1} $ 2 + 1)。 我们获得质量为m的带电荷费米子的可重整化有效拉格朗日算子和费米子质量算子,然后针对无质量情况计算这些量。 对于纯QED $$ _ {2 + 1} $ 2 + 1以及所谓的降低QED $$的情况,发现了带电荷的无质量费米子处于最低Landau水平时的辐射校正。 _ {3 + 1} $$ 3 + 1的2头牛。 如果带电的费米子具有较小的裸质量$$ m_0 $$ m0,并且在简化的QED中,发现费米子质量可以在纯QED $ _ {_ {2 + 1} $$ 2 + 1的外部磁场中动态生成。 即使在$$ m_0 = 0 $$ m0 = 0的情况下,也可以在2头上的$$ _ {3 + 1} $$ 3 + 1。 在恒定的均匀磁场(垂直于石墨烯样品)存在的情况下,单层石墨烯中似乎可能显示出动态质量。
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Eur. Phys. J. C (2019) 79:196
https://doi.org/10.1140/epjc/s10052-019-6717-4
Regular Article - Theoretical Physics
Radiative dynamical mass of planar charged fermion in a constant
homogeneous magnetic field
V.R. Khalilov
a
Faculty of Physics, M.V. Lomonosov Moscow State University, 119991 Moscow, Russia
Received: 29 November 2018 / Accepted: 22 February 2019 / Published online: 5 March 2019
© The Author(s) 2019
Abstract The effective Lagrangian and mass operator are
calculated for planar charged massive and massless fermions
in a constant external homogeneous magnetic field in the
one-loop approximation of the 2+1 dimensional quantum
electrodynamics (QED
2+1
). We obtain the renormalizable
effective Lagrangian and the fermion mass operator for a
charged fermion of mass m and then calculate these quan-
tities for the massless case. The radiative corrections to the
mass of charged massless fermion when it occupies the low-
est Landau level are found for the cases of the pure QED
2+1
as well as the so-called reduced QED
3+1
on a 2-brane. The
fermion masses were found can be generated dynamically in
an external magnetic field in the pure QED
2+1
if the charged
fermion has small bare mass m
0
and in the reduced QED
3+1
on a 2-brane even at m
0
= 0. The dynamical mass seems to
be likely to be revealed in monolayer graphene in the pres-
ence of constant homogeneous magnetic field (normal to the
graphene sample).
1 Introduction
Quantum systems of planar charged fermions in external
electromagnetic fields are interesting in view of possible
applications of the corresponding field-theory models to a
number of condensed-matter quantum effects such as, for
example, the quantum Hall effect [1] and high-temperature
superconductivity [2] as well as in connection with problems
of graphene (see, [3–6]). In graphene, the electron dynam-
ics near the Fermi surface can be described by the Dirac
equation in 2+1 dimensions for a zero-mass charged fermion
[4] though the case of massive charged fermions is also of
interest [7]. The field-theory models applied for study are the
pure 2+1 dimensional quantum electrodynamics (QED
2+1
)
as well as the so-called reduced QED
3+1
on a 2-brane. In
the latter model, fermions are confined to a plane, neverthe-
a
e-mail: khalilov@phys.msu.ru
less the electromagnetic interaction between them is three-
dimensional [8,9].
The radiative one-loop shift of an electron energy in the
ground state in a constant homogeneous magnetic field in
QED
2+1
was calculated in [10] and the one-loop electron
self-energy in the topologically massive QED
2+1
at finite
temperature and density was obtained in [11]. The effective
Lagrangian, the electron mass operator and the density of
vacuum electrons (induced by the background field) in an
external constant homogeneous magnetic field were derived
in the one-loop QED
2+1
approximation in [12].
Since the effective fine structure constant in graphene
is large, the QED
2+1
effects can be significant already in
the one-loop approximation. The polarization operator in
graphene in a strong constant homogeneous magnetic field
perpendicular to the graphene membrane has been obtained
in the one-loop approximation of the QED
2+1
in [9,13–15].
The effective potential and vacuum current in graphene in
a superposition of a constant homogeneous magnetic field
and an Aharonov–Bohm vortex was studied in [16]. We
also note that the induced vacuum current in the field of a
solenoid perpendicular to the graphene sample was investi-
gated in [17], and the vacuum polarization of massive and
massless fermions in an Aharonov–Bohm vortex in the one-
loop approximation of the QED
2+1
was studied in [18]. The
one-loop self-energy of a Dirac electron of mass m in a thin
medium simulating graphene in the presence of external mag-
netic field was investigated in the reduced QED
3+1
on a 2-
brane in [19], in which it was shown that the radiative mass
correction in the lowest Landau level does not vanish at the
limit m → 0.
In this work, we calculate the effective Lagrangian and
the mass operator of planar charged fermions in the pres-
ence of an external constant homogeneous magnetic field in
the one-loop approximation of the QED
2+1
. We also calcu-
late the radiative corrections to the mass of charged fermion
when it occupies the lowest Landau level for the cases of the
123
196 Page 2 of 7 Eur. Phys. J. C (2019) 79 :196
pure QED
2+1
as well as the so-called reduced QED
3+1
on
a 2-brane. The fermion masses were found can be generated
dynamically in an external magnetic field in the pure QED
2+1
if the charged fermion has small bare mass m
0
and in the
reduced QED
3+1
on a 2-brane even at m
0
= 0. The dynam-
ical mass seems to be likely to be revealed in monolayer
graphene in the presence of constant homogeneous magnetic
field (normal to the graphene sample) though we also see that
an one-loop result is not accurate certainly when the coupling
constant is large (see, [19]).
We shall adopt the units where c =
¯
h = 1.
2 Eigenfunctions, the Green’s function for the Dirac
equation in a constant magnetic field in 2+1
dimensions. Effective Lagrangian
As far as the charged fermions are supposed to move in a
plane the exact solutions and the Green’s function for the
Dirac equation in a constant homogeneous magnetic field can
be found in 2+1 dimensions. The Dirac equation for a fermion
in an external electromagnetic field in 2+1 dimensions is
written just as in 3+1 dimensions nevertheless the Dirac γ
μ
-
matrix algebra in 2+1 dimensions is known to be represented
in terms of the two-dimensional Pauli matrices σ
j
γ
0
= σ
3
,γ
1
= i ζσ
1
,γ
2
= i σ
2
, (1)
where the parameter ζ =±1 can label two types of fermions
in accordance with the signature of the two-dimensional
Dirac matrices [20]; it can be applied to characterize two
states of the fermion spin (spin “up” and “down”) [21].
Then, the Dirac equation for a fermion of mass m and
charge e =−e
0
< 0 “minimally” interacting with the back-
ground electromagnetic field is written in the covariant form
as
(γ
μ
P
μ
− m)(x ) = 0, (2)
where P
μ
= p
μ
− eA
μ
≡ (P
0
, P
1
, P
2
) is the gener-
alized fermion momentum operator (a three-vector) and
p
μ
= (i ∂
t
, −i∂
x
, −i∂
y
) ≡ (p
0
, p
1
, p
2
). We take the vector
potential in the Cartesian coordinates in the Landau gauge
A
0
= 0, A
1
= 0, A
2
= Bx, then the magnetic field
is defined as B = ∂
1
A
2
− ∂
2
A
1
≡ F
21
, where F
μν
is the
electromagnetic field tensor.
The squared Dirac operator is
(γ P)
2
= P
2
0
− P
2
1
− P
2
2
− eσ
3
B. (3)
The matrix function E
p
, introduced by Ritus [22] for the case
of 3+1 dimensions, satisfies the equation
(γ P)
2
E
p
= p
2
E
p
, (4)
where the eigenvalue p
2
can be any real number and in the
magnetic field considered E
p
also is an eigenfunction of the
operators
i∂
t
E
p
= p
0
E
p
, −i∂
y
E
p
= p
2
E
p
,
(P
2
1
+ P
2
2
− eσ
3
B)E
p
= 2|eB|kE
p
, k = 0, 1 ... .
(5)
It is obvious that p
0
, p
2
and 2|eB|n label the solutions of
Dirac Eq. (2)aswellastheE
p
eigenfunctions of operators
(4) and (5) can also classifies by the eigenvalues ζ =±1of
σ
3
. The eigenfunctions E
pζ
(t, r) are given by
E
pζ
(t, r) =
1
2
3/2
π
e
−ip
0
t+ip
2
y
U
n
(X)w
ζ
, (6)
where the normalized functions U
n
(X) are expressed through
the Hermite polynomials H
n
(X) as
U
n
(X) =
|eB|
1/4
(2
n
n!π
1/2
)
1/2
e
−X
2
/2
H
n
(X),
X =
|eB|(x − p
2
/eB), (7)
n = k + sign(eB)ζ/2 − 1/2, n = 0, 1,..., and w
ζ
are
eigenvectors of σ
3
.
The eigenfunctions of the Dirac equation Hamiltonian
H
D
= σ
1
P
2
− σ
2
P
1
+ σ
3
m (8)
are
(t, r) =
1
√
2E
n
√
E
n
+ ζ mU
n
(X)
−sign(eB)
√
E
n
− ζ mU
n−1
(X)
×exp(−iE
n
t + ip
2
y), (9)
where
E
n
=
m
2
+ 2n|eB| (10)
is the energy eigenvalues (the Landau levels). All the energy
levels except the lowest level (n = 0) with ζ = 1foreB > 0
and ζ =−1foreB < 0 are doubly degenerate on spin
ζ =±1. This means that the eigenvalues of the fermion
energy except the lowest level are actually spin-independent
in the configuration under investigation. For definiteness, we
consider the case where eB < 0. It should be also emphasized
that (t, r) are not eigenfunctions of σ
3
.
The electron propagator in an external constant homoge-
neous magnetic field in 3+1 dimensions was found for the
first time by Schwinger [23]. In the 2+1 dimensions it was
obtained in [24] in the momentum representation (see, also,
[15]) and in [12] in the coordinate (proper-time) representa-
tionintheform
123
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