Global Existence of the Solution for a Reduced Model of the Vectorial Quantum Zakharov System ()
1. Introduction
In this paper, we consider the vectorial quantum Zakharov system
(1)
where
is the slowly varying envelope of the rapidly oscillating electric field, and
is the deviation of the ion density from its mean value.
denotes the ionic speed of sound, the parameter
defined as the square ratio of the light speed and the electron Fermi velocity is usually large, and the coefficient Γ that measures the influence of quantum effects is usually very small. This model describes the nonlinear interaction between high-frequency quantum Langmuir waves and low-frequency quantum ion-acoustic waves, we refer to [1] for more physical background.
Most of the known results are concerned on the scalar quantum Zakharov system which reads
(2)
For example, the references [2] [3] proved the local or global well-posedness results for (2), and for scattering results, we refer to [4] [5] . When
, we can obtain the classical Zakharov system ( [6] )
(3)
which has been extensively studied for the local and global well-posedness [7] - [15] .
By isolating the light dispersive term and the quantum dispersive term for E, the linear part of the first equation of (1) can be transformed into the form
(4)
The detailed computations are given in the appendix. Then we are interested in the following reduced model of the vectorial quantum Zakharov system
(5)
Here, we have set
for simplicity. Yang-Zhang-Jiang [16] proved local existence of the solution and the limit behavior for this system. In this work, our aim is to show the global existence of (5) in a two-dimensional case.
Before stating the main result, we first introduce some notations that will be used in the paper. For
, we denote
the usual inhomogeneous Sobolev space. If
, we define its norm to be
or
where ess sup
denotes the essential supremum of a set of functions.
The homogeneous Sobolev space
is defined as
with norm
where
is the Fourier transform of u.
We denote the product space
as
and
The main result is stated in the following theorem.
Theorem 1 Let
and
is a positive integer. Then, the system (5) has a unique global solution
satisfying
Theorem 1 gives the global existence result without any size restriction under the quantum effect. This is quite different from the classical Zakharov system where a global solution exists with small initial data.
2. Preliminaries
In this section, we give the conserved quantities and a basic
type estimate.
Lemma 2 (Young inequality) [16] Let
,
,
, then
Lemma 3 (Hölder inequality) [16] Let
,
,
, if there has
,
, then
Lemma 4 (Gagliardo-Nirenberg inequality) [16] Let
,
,
,
,
, Then, there are
, satisfying
with
,
Lemma 5 Let
,
,
,
,
, Then, there holds
with C depending on
.
The proof of this lemma can be found in [17] [18] . When
,
,
,
, we have
(6)
for
and
, where C is a constant depending only on K.
Proposition 6 For smooth solutions of (5), there hold two conserved quantities:
where
and
with
(7)
Proof. We first derive the L2 bound of F1. Taking the imaginary part of the inner product in L2 between the first equation of (5) and F1, we have
. Therefore,
Similarly, we have
Thus, we get
Next, we multiply the first equation of (5) by
and consider the real part. This leads to
(8)
Similarly, we obtain
(9)
On the other hand, we take the inner product of the third equation of (5) with
, then we can obtain
(10)
From (8)-(10), we obtain
3. Proof of Theorem 1
Now we are going to prove Theorem 1.
Proof of Theorem 1. According to the local existence theory, it is sufficient to show the a-priori bound of the solution. From Proposition 6, we know
which implies
(11)
Here, for the nonlinear integral term, we have used the Gagliardo-Nirenberg inequality and Young’s inequality to obtain
(12)
Therefore, we get
(13)
and the inequality (6) implies
(14)
Next, we estimate the higher-order norms for F1, F2 and n. We perform
energy estimate for F1, we get
(15)
Recalling (5), we see
. Hence, we deduce
(16)
Similarly, we can obtain
(17)
For
, it is easy to see (by (13))
(18)
As to
, we have
(19)
Taking inner product of both sides of the third equation of (5) with
, there is
Thus, we have
(20)
The Equation (5) indicates that
(21)
(22)
Now using (21)-(22), the integral term
can be estimated as
(23)
For
, it can be estimated by
(24)
For
, we have
(25)
Then it follows from (20) and (23)-(25) that
(26)
Now, collecting the estimates (16)-(19) and (26) yield
(27)
The nonlinear part in the left-hand side of (27) can be estimated by
(28)
And using inequality (21)-(22), we get
(29)
For
, we define
then the estimates (27)-(29) give
By Gronwall’s inequality and (13), one deduces from the above inequality that
for any
. Following the same argument as above, we can obtain
Since the proof is similar, we omit further details. The proof of Theorem 1 is then completed.
4. Appendix
According to the equality
, the linear system (1) is equivalent to
(30)
we take the Fourier transform for (30) to obtain
(31)
(31) can be rewritten as in the matrix form
where
.
Let
then there is
(32)
Therefore, the determinant (32) implies
For
, the corresponding eigenvector is
For
, the corresponding eigenvector is
After unitization, we attain
(33)
Then from (33), we can obtain the orthogonal matrix
Since Q satisfies
and
If we set
then
(34)
Now we take the inverse Fourier transform for (34) to derive an equivalent form of the linear part of (1)
(35)