1212 X. Lü et al.
Within the context of Madelung fluid description,
the complex wave function (say Ψ ) is represented in
terms of modulus and phase, and submitted into the
Schrödinger equation to lead to the possible achieve-
ment of a pair of nonlinear fluid equations for the
“density” ρ =|Ψ |
2
and the “current velocity” v =
∇Arg(Ψ ): one is the continuity equation (taking into
account probability conservation) and the other one is
a Navier-Stokes-like equation of motion [6,7,15–20].
Substitution of Ψ(x, t) =
√
ρ(x, t) e
i
¯
h
Θ(x ,t)
into the
one-dimensional Schrödinger equation
i
¯
h
∂Ψ(x, t)
∂t
=−
¯
h
2
2 m
∂
2
Ψ(x, t)
∂x
2
+mU(x)Ψ (x, t), (1)
results in the following pair of coupled Madelung fluid
equations
∂ρ
∂t
+
∂
∂x
(ρ v) = 0, (2a)
∂v
∂t
+ v
∂v
∂x
−
¯
h
2
2 m
2
∂
∂x
1
√
ρ
∂
2
√
ρ
∂x
2
+
∂U
∂x
= 0,
(2b)
where ρ =|Ψ |
2
is the fluid density and v =
1
m
∂Θ
∂x
is the current velocity. Equation (2a) is a continuity
equation for the fluid density, while Eq. (2b) is an Euler
equation or equation of motion for the fluid velocity and
contains a force term proportional to the gradient of the
“quantum potential”,
¯
h
2
2 m
2
∂
∂x
1
√
ρ
∂
2
√
ρ
∂x
2
.
Madelung fluid description has been used to dis-
cuss families of generalized one-dimensional nonlinear
Schrödinger equations [15–19], as follows:
• In Ref. [15], the following nonlinear Schrödinger-
like equation
i μ
∂Ψ
∂t
=−
μ
2
2
∂
2
Ψ
∂x
2
+ U
|Ψ |
2
Ψ, (3)
where U
|Ψ |
2
and μ are as an arbitrary real func-
tional of the complex wave function Ψ and an arbi-
trary real dispersion/diffraction coefficient, respec-
tively, has been cast as
∂ρ
∂t
+
∂
∂x
(
ρv
)
= 0, (4a)
−ρ
∂v
∂t
+ v
∂ρ
∂t
+ 2
c
0
(t) −
∂v
∂t
dx
∂ρ
∂x
−
ρ
dU
dρ
+ 2U
∂ρ
∂x
+
μ
2
4
∂
3
ρ
∂x
3
= 0, (4b)
with c
0
(t) as an arbitrary function of t. Under the
hypothesis of stationary fluid, it is revealed that the
cubic nonlinear Schrödinger equation can be put
in correspondence with the standard Korteweg–de
Vries equation in such a way that the soliton solu-
tions of the latter are the squared modulus of the
envelope soliton solution of the former. The condi-
tions for different types of envelope solitons (bright,
dark or gray ones) are also discussed.
• In Ref. [16], a modified nonlinear Schrödinger
equation with a quartic nonlinear potential in the
modulus of the wave function has been studied
within the framework of Madelung fluid descrip-
tion, i.e., setting U
|Ψ |
2
= q
1
|Ψ |
2
+q
2
|Ψ |
4
with
q
1
and q
2
as real constants in Eq. (3). By consider-
ing different boundary conditions, up-shifted bright
soliton, upper-shifted bright soliton, gray soliton
and dark soliton are finally found and can be cast
into envelope solitons conversely.
• In Ref. [17], the existence of envelope soliton-like
solutions of a nonlinear Schrödinger equation con-
taining an anti-cubic nonlinearity (|Ψ |
−4
Ψ )plus
a ‘regular’ nonlinear part is investigated. In par-
ticular, in the case that the regular nonlinear part
consists of a sum of cubic and quintic nonlineari-
ties, i.e., setting U
|Ψ |
2
= Q
0
|Ψ |
−4
+q
1
|Ψ |
2
+
q
2
|Ψ |
4
with Q
0
, q
1
and q
2
as real constants in
Eq. (3), an upper-shifted bright envelope soliton-
like solution is explicitly found.
• In Ref. [18], a review is given on the results of inves-
tigations dealing with the connection between the
envelope soliton-like solutions of a wide family of
nonlinear Schrödinger equations and the soliton-
like solutions of a wide family of Korteweg–
de Vries equations. In two different fluid motion
regimes (uniform current velocity and stationary
profile current velocity variation, respectively),
bright- and gray-/dark-soliton-like solutions of
those equations are found.
• In Ref. [19], a similar discussion is given for a class
of derivative nonlinear Schrödinger-type equations.
For a motion with the stationary profile current
velocity, the fluid density satisfies a generalized sta-
tionary Gardner equation, and solitary wave solu-
tions are found. For the completely integrable cases,
these solutions are compared with existing solu-
tions in the literature.
In the present paper, we will investigate the
Gerdjikov–Ivanov envelope solitons with the frame-
work of Madelung fluid description. Various optical
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