Eur. Phys. J. C (2019) 79 :418 Page 3 of 11 418
which maps the region ξ>0ton ∈ R\[0, 1].
2
Note that
the particular value ξ =
1
4
which corresponds to the limit
n →∞will be treated separately. The potential U
b
(φ) and
the coupling ε
b
(φ) associated to the axionic fields depend
on a positive constant denoted by b whose range will be
conditioned by some reality conditions as shown below
U
b
(φ) =
1 − n
8n
1 − bφ
2
n−1
4
4b
3
(n − 3)(n −4)φ
2n+4
n−1
−3b
2
(n − 4)(3n −7)φ
2n+2
n−1
+ 6b(n − 3)
2
φ
2n
n−1
−(n − 3)(n −4)φ
2
− 6b
5−n
φ
8
n−1
, (2.3a)
ε
b
(φ) = 1 +
n − 1
8nκ
×
(n−2)φ
2
−(n−3)bφ
2n
n−1
− b
3−n
φ
4
n−1
(1−bφ
2
n−1
)
2
.
(2.3b)
It is also important to stress that in order to ensure a
mechanism of momentum dissipation as well as a cor-
rect sign for the axionic kinetic term, the minimal cou-
pling function ε
b
(φ) must be strictly positive everywhere.
Before proceeding, we would like to stress that the mini-
mal case ξ = 0 or equivalently n = 1 which has been
fully treated in [18] is clearly excluded from our anal-
ysis because of the form of the expressions U
b
(φ) and
ε
b
(φ). Also, we may note that for the conformal situation
ξ =
1
6
or equivalently n = 3, the potential vanishes iden-
tically and the axionic minimal coupling ε
b
(φ) = 1, and
this situation corresponds to the case already studied in
[17].
The field equations obtained by varying the action (2.1)
with respect to the metric, the Maxwell field A
μ
, the scalar
field φ and the axionic fields ψ
i
read respectively
κ(G
μν
+ g
μν
) =
1
2
T
φ
μν
+
1
2
T
ψ
μν
+
1
2
T
em
μν
,
∇
μ
F
μν
= 0,
φ =
n − 1
4n
Rφ +
dU
b
dφ
+
1
2
dε
b
dφ
2
i=1
g
μν
∂
μ
ψ
i
∂
ν
ψ
i
,
∇
α
ε
b
(φ)∇
α
ψ
i
= 0, i = 1, 2, (2.4)
where the different energy–momentum tensors are given by
T
φ
μν
= ∂
μ
φ∂
ν
φ − g
μν
1
2
∂
α
φ∂
α
φ +U
b
(φ)
+
n − 1
4n
g
μν
−∇
μ
∇
ν
+ G
μν
φ
2
,
2
A simple calculation shows that the range of values ξ<0 is not
compatible with the reality condition as defined by Eq. (2.6).
T
ψ
μν
= ε
b
(φ)
2
i=1
∂
μ
ψ
i
∂
ν
ψ
i
−
1
2
g
μν
∂
α
ψ
i
∂
α
ψ
i
,
T
em
μν
= F
μσ
F
σ
ν
−
1
4
g
μν
F
ρσ
F
ρσ
.
Dyonic black brane solutions of the field equations for
=−3 and with axionic fields homogeneously dis-
tributed along the two-dimensional planar base manifold can
be found, and are given by
ds
2
=−f (r)dt
2
+
dr
2
f (r)
+r
2
(dx
2
+ dy
2
),
f (r) =
1
r
2
r −
3ω
√
12κ
r +
ω
√
12κ
3
,
ψ
1
(x) = ωx,ψ
2
(y) = ωy
φ(r ) =
ω
b
√
12κr + ω
n−1
2
,
A
μ
dx
μ
=−
q
e
r
dt +
q
m
2
(xdy− ydx), (2.5)
where the intensity of the axionic fields ω is tied to the electric
and magnetic charges by the relation
ω =±
96 b
n−1
n κ
2
(q
2
e
+ q
2
m
)
n − 1 − 8 n κ b
n−1
1
4
. (2.6)
We will now be as exhaustive as possible to enumerate
the properties of these dyonic black brane solutions. For a
nonminimal coupling parameter ξ as parameterized by (2.2),
we have to distinguish between the case n > 1 corresponding
to ξ ∈]0,
1
4
[, and the case n < 0, i.e. ξ ∈]
1
4
, ∞[.Forn > 1,
in order to deal with a real constant ω as defined by (2.6),
the parameter b of the model must belong to the interval
b ∈]0, b
0
[ with
b
0
=
n − 1
8nκ
1
n−1
, (2.7)
while for n < 0, one must have b ∈]b
0
, ∞[. Also, the con-
straint (2.6) allows both sign for the intensity of the axionic
fields ω. This remark has an interesting consequence con-
cerning the location of the horizon r
+
since for ω>0,
we have r
+
=
3ω
√
12κ
while for ω<0, the event horizon
is located at r
+
=−
ω
√
12κ
. In this latter case, because of
the form of the metric function (2.5), the temperature of the
solution vanishes identically and the black brane configura-
tion can be interpreted as an extremal solution. It remains to
ensure that the scalar field (2.5) is well-defined in the region
outside the horizon, namely for r ∈[r
+
, ∞[.Forω>0,
the scalar field is regular everywhere in the region r > 0.
On the other hand, for a negative axionic intensity param-
eter ω<0, the reality condition on the scalar field on the
region r ∈[r
+
=−
ω
√
12κ
, ∞[ restricts the parameter n to
123