Contact Geometry in Superconductors and New Massive Gravity
Daniel Flores-Alfonso and Marco Maceda
Departamento de Fı́sica, Universidad Autónoma Metropolitana - Iztapalapa,
Avenida San Rafael Atlixco 186, A.P. 55534, C.P. 09340, Ciudad de México, Mexico
Cesar S. Lopez-Monsalvo
arXiv:2011.13499v2 [gr-qc] 2 Dec 2020
Conacyt-Universidad Autónoma Metropolitana Azcapotzalco, Avenida San Pablo Xalpa 180,
Azcapotzalco, Reynosa Tamaulipas, C.P. 02200, Ciudad de México, Mexico
The defining property of every three-dimensional ε-contact manifold is shown to be equivalent
to requiring the fulfillment of London’s equation in 2+1 electromagnetism. To illustrate this point,
we consider S 3 equipped with a contact structure together with an associated metric tensor such
that the canonical generators of the contact distribution are null. The resulting Lorentzian metric is
shown to be a vacuum solution of three-dimensional massive gravity. Moreover, by coupling the New
Massive Gravity action to Maxwell-Chern-Simons we obtain a class of charged solutions stemming
directly from the contact metric structure. Finally, we repeat the exercise for the Abelian Higgs
theory.
PACS numbers: 04.60.Kz, 11.10.Kk, 11.15.Wx
INTRODUCTION
There is a long tradition on applications of geometrical
methods in physics; over the years branches as Hamiltonian dynamics, geometric optics, fluid dynamics and
General Relativity have benefited from the techniques
developed with a geometric perspective [1–6]. In the
case of pseudo-Riemannian manifolds, this becomes even
more true with the use of para-contact geometry [7–12].
More specifically, para-Sasakian geometry has paved its
way into General Relativity as a tool to analyze Ricci
solitons, lightlike hypersurfaces, Killing vectors and associated horizons [13–18]. In this Letter, we discuss further implications of para-Sasakian structures within the
realm of New Massive Gravity (NMG) [19]. In particular,
we show that the structure gives rise to a distinguished
Trkalian flow; a special type of Beltrami flow.
Beltrami fields where originally introduced in the realm
of hydrodynamics to describe flows whose stream lines
are parallel to their vorticity. In the case of electromagnetism, these have received the name of force-free magnetic fields. That is, magnetic fields such that the induced currents in a conducting medium experience a vanishing Lorentz force. This feature is, indeed, a property
of the medium and it is described by London’s constitutive relations. In this letter we show that such relations
are completely captured by the geometry of a class of
three dimensional metric contact manifolds whose structural elements give rise to propagating force-free fields.
In particular, we study the case of S 3 endowed with a
contact structure together with an associated metric such
that the generators of the contact distribution together
with the Reeb vector field generate a null triad for a
(2+1) spacetime. We explicitly obtain the conditions for
the metric to be a solution to New Massive Gravity for
the vacuum, Maxwell-Chern-Simons and Abelian-Higgs
cases.
BELTRAMI FIELDS AND SUPERCONDUCTORS
Let us begin by considering a 3-dimensional manifold
endowed with a ε-contact metric structure [20]. That is,
a contact metric manifold such that the contact 1-form
η and the metric g satisfy the relation
⋆η = −ℓdη,
(1)
where ⋆ is the Hodge star associated with a metric g̃
in the conformal class defined by g̃ = ℓ2 g with ℓ 6= 0.
Here, without loss of generality, we have written (1) for
Lorentzian para-Sasakian geometries such as the one in
equation (13).
To each contact 1-form η there is a distinguish vector
field ξ defined by the conditions η(ξ) = 1 and ι̇ξ dη = 0.
If the metric g satisfies the condition g(ξ) = η, then the
field ξ is a Beltrami field, namely ξ = ℓ curl(ξ).
It is straightforward to verify that
(δd − 1/ℓ2 )η = 0,
and δη = 0.
(2)
The first equation is reminiscent of the sourceless Proca
field equation, in which case A = η. Then, the second
of the two equations above is the Lorenz gauge. This interpretation means that equation (1) is a sort of “square
root” of the Proca field equation. The first time this
concept was explored was in reference [21]. Therein, the
concept of self-duality for gauge fields was extended to
odd dimensions. In three dimensions self-duality is defined by
⋆F =
1
A,
ℓ
(3)
2
but this is merely equation (1) under our gauge field interpretation. Moreover, equations (2) allow us to write
− 1/ℓ2 ⋆ F = 0,
(4)
which is exactly the force-free field equation associated
with superconducting media [22, 23]. In our case, an effective material medium is described by (13). In particular, this provides us with an interpretation of the conformal parameter ℓ as the penetration depth in the medium.
Additionally, the inhomogeneous Maxwell equation together with (1) yields the relation
d⋆F = J =
1
F,
ℓ
(5)
where J is the induced current 2-form in the medium. In
this sense, equation (1) corresponds to a constitutive relation for a conducting medium, that is, a relation between
the induced current and the field strength characterizing
the response of the medium to electromagnetic stimuli.
Indeed, (1) is a metric relation between the contact form
and its exterior derivative for the para-Sasakian class of
contact metric manifolds. Moreover, at this point, we are
ready to rewrite equation (1) as
1
d ⋆ J = 2 F,
ℓ
(6)
which is no other than London’s constitutive relation for
superconducting media [24]. This is a remarkable result,
since we have not yet used any particular form of the
metric. That is, an ε-contact 3-manifold represents a
superconducting medium.
CONTACT METRIC STRUCTURE
Let us consider a topological three-dimensional
sphere endowed with its standard contact structure,
parametrized by the one-form
η=
1
(dψ + cos θdφ) ,
2
(7)
where we have used Euler angles 0 ≤ ψ ≤ 4π, 0 ≤ θ ≤ π
and 0 ≤ φ ≤ 2π to coordinate the manifold. Notice that
for the contact sphere w = ψ/2, q = φ/2 and p = − cos θ
are the set of local coordinates in the Darboux theorem [25]. This is not surprising when one keeps in mind
the Hopf fibration of the hypersphere. A contact form η
defines a unique vector field ξ satisfying the conditions
ι̇ξ dη = 0 and ι̇ξ η = 1 known as the Reeb vector field.
Indeed, in the case of S 3 equipped with the contact form
(7) the Reeb field is tangent to the Hopf circle fiber.
A three-dimensional contact structure is a completely
non-integrable distribution of two-dimensional planes in
the tangent bundle. The generators of the contact distribution are vector fields annihilated by the contact 1-form.
Thus, in the present case, these are given by
1 ∂
∂
∂
, and P =
− cos θ
,
Q=2
∂φ
∂ψ
sin θ ∂θ
(8)
which, in particular, show that the contact distribution
given by (7) is bracket-generating [26] as we have
[P, Q] = ξ,
[ξ, Q] = 0,
and [ξ, P ] = 0.
(9)
This is to say, P and Q together with their iterated
Lie brackets generate a basis for the tangent bundle.
Moreover, equation (9) exhibits the fact that the noncoordinate basis {P, Q, ξ} satisfies the Heisenberg algebra.
There is a certain freedom in choosing a metric associated with a contact structure [27]. In the present work,
we consider a metric satisfying
g(ξ, ξ) = 1 and g(ξ, P ) = g(ξ, Q) = 0,
(10)
together with
g(Q, Q) = g(P, P ) = 0.
(11)
Conditions (10) merely state that the metric is compatible with the contact 1-form, that is, the Reeb vector field
is normalized and is orthogonal to the generators of the
contact distribution; conditions (11) imply that the metric is Lorentzian. These are the defining properties of an
associated metric to the almost para-contact structure
ϕ(ξ) = 0,
ϕ(Q) = Q,
and ϕ(P ) = −P,
(12)
representing a reflection in the θ direction of the contact
distribution. Therefore, (S 3 , η, ξ, ϕ, g), is a para-contact
manifold. Here, the metric g = η ⊗ η − dη ◦ (ϕ ⊗ 1) on the
contact sphere, whose line element in local coordinates is
given by
1
dψ 2 + 2 cos θdψdφ − 4 sin θdθdφ + cos2 θdφ2 ,
4
(13)
defines a (2+1) spacetime where (P, Q, ξ) is its
Newmann-Penrose null triad. Moreover, the congruences associated with the null vector fields P and Q are
geodesics and have no expansion, shear nor twist, therefore, g defines a Kundt spacetime. Notice that ξ corresponds to the spacelike vector field m whilst P and Q to
the null vector fields l and n, respectively. From this, it
is straightforward to construct the orthonormal triad
ds2 =
√
∂
1 ∂
∂
2e0 = P + Q = −2 cos θ
+
+2 ,
∂ψ sin θ ∂θ
∂φ
√
1 ∂
∂
∂
+
−2 ,
2e1 = P − Q = 2 cos θ
∂ψ
sin θ ∂θ
∂φ
∂
e2 = ξ = 2
,
∂ψ
(14)
(15)
(16)
3
so that, using the coframe {e0 , e1 , e2 }, the line element
of the geometry is expressed as ds2 = −e0 e0 + e1 e1 +
e2 e2 where the Lorentz signature is manifest. Hence, the
geometry is not given by the standard round Riemannian
metric. Nor is equation (13) the canonical Lorentz metric
on the sphere. Even though we have chosen the standard
contact structure on the hypersphere.
The present contact metric sphere is not Einstein, however, it is η-Einstein, that is, the Ricci tensor satisfies
Ric =
1
g − η ⊗ η.
2
(17)
It has been established that three-dimensional η-Einstein
Sasakian manifolds must have constant sectional curvature when restricted to planes in the contact distribution
[28]. Furthermore, if the value of this constant is -3 the
metric is nil (a.k.a. Heisenberg) [28, 29]. For metric (13)
this constant is equal to 3 (when scaled for compatibility with [28]). This suggests to us that the metric is nil
and we attribute this difference in signs to the geometry’s
para-Sasakian nature.
The metric admits four solutions to the Killing vector
field equations, which we write in Euler angles and in
Darboux coordinates with Heisenberg basis
1
∂
= ξ,
∂ψ
2
1
1
∂
= Q − pξ,
ξ2 =
∂φ
2
2
1 ∂
∂
+
= P + qξ,
ξ3 = φ
∂ψ
sin θ ∂θ
cos θ ∂
∂
+
= qQ − pP − pqξ.
ξ4 = φ
∂φ
sin θ ∂θ
ξ1 =
[ξ1 , ξ2 ] = 0,
and [ξ1 , ξ3 ] = 0,
[ξ4 , ξ3 ] = ξ3 ,
Higher dimensional para-Sasakian geometries analogous to (13) have been found to be Einstein-GaussBonnet vacuua [35]. However, in three dimensions
the quadratic-curvature Gauss-Bonnet term vanishes.
For this reason we consider instead the most general
quadratic-curvature theory in three dimensions
Z
√
S[g] = d3 x −g R − 2Λ + β1 R2 + β2 Rµν Rµν . (24)
We find that the metric is a solution of the theory whenever
ℓ2 =
(19)
(20)
(21)
(22)
while the fourth vector field acts on the former by Lie
bracket as
[ξ4 , ξ1 ] = 0,
MASSIVE GRAVITY
(18)
The first three vector fields form a notable subalgebra
[ξ2 , ξ3 ] = ξ1 ,
of ξ3 and ξ4 which is incompatible with the identification
φ ∼ φ+2π of the three-sphere, as they would not be single
valued. Hence, the only Killing fields are (18) and (19)
which yield a compact isometry group, U(1)×U(1), as required. This distinction between local and global structures is analogous to the renowned Bañados-TeitelboimZanelli (BTZ) black hole [34] which is locally diffeomorphic to Anti-de Sitter spacetime (AdS) but not globally.
The present contact sphere is only locally equivalent to
a Lorentz-Heisenberg spacetime.
and [ξ4 , ξ2 ] = −ξ2 . (23)
We emphasize that, to find Killing fields one must solve
equations of local nature, nevertheless, Killing vector
fields are global entities . Moreover, equations (22) and
(23) imply that the geometry (13) is one of only three
possible Lorentzian left-invariant Heisenberg metrics [30]
— one of which is Minkowski spacetime. This is, indeed,
what was suggested to us above when we examined the
metric’s constant ϕ-holomorphic sectional curvature [31].
However, it was established in [32] that closed simply connected Lorentzian manifolds must have compact
isometry groups. Thus, in the spirit of [33] we inspect the
Killing vector fields searching for incompatibilities with
the defining identifications of spacetime. From equations
(20) and (21) we see that it is precisely the φ-dependence
1
,
8Λ
and β1 = −
1
− 3β2 ,
8Λ
(25)
In other words, the cosmological constant determines the
characteristic length scale and the quadratic couplings
are restricted. However, Gauss-Bonnet is a ghost-free
theory, thus a closer analogy between theories is provided
by New Massive Gravity, which is also ghost-free [19].
The action is given by
Z
√
(26)
S[g] = d3 x −gLNMG ,
with
LNMG =
1
1
3 2
2
R
−
2Λ
−
|Ric|
−
.
R
2κ2
m2
8
(27)
Here Λ is the cosmological constant and m is the mass of
the propagating degrees of freedom. It is a famous result
that this theory is equivalent at the linearized level to
the (unitary) Fierz-Pauli action for a massive field with
spin two. The equations of motion are
1
1
Ric − Rg + Λg −
K =0
2
2m2
(28)
where K is a tensor with components
1
1
Kµν = 2Rµν − ∇µ ∇ν R − Rgµν + 4Rµανβ Rαβ
2
2
3
3
αβ
(29)
− RRµν − Rαβ R gµν + R2 gµν .
2
8
4
It can be directly verified that the metric (13) of our
para-Sasakian 3-sphere is a solution to these equations
provided that
ℓ2 =
1
,
8Λ
and m2 = 21Λ.
(30)
One might also wonder if the metric is a solution of the
theory when the action is additionally coupled to the
Cotton tensor, see for example [36]. We find the answer
to be positive. Moreover, we mention that the Jordan
normal form of the metric’s Cotton tensor reveals the
spacetime to be of “Petrov” type D [36, 37].
The η-Einstein nature of the metric greatly simplifies
the equations of motion, as second order quantities, e.g.,
the Ricci tensor, become zero-order cf. (17). This also
occurs for fourth order quantities such as
Ric =
3
1
g − 2 η ⊗ η.
4
2ℓ
2ℓ
(31)
Now, equations (30) tell us that the Heisenberg group
with one of its left-invariant metrics is a solution of NMG.
Moreover, we have checked that the Euclidean metric is
also a solution to the equations of motion (28). This is
one of the eight Thurston geometries. These geometries
have been vastly studied in mathematicis and physics. In
string theory, these geometries have been studied in the
framework of string dualities [38].Therein, the geometries
have been found to be dual amongst themselves with one
exception, the sol geometry. They have also been studied in string-inspired three dimensional gravity [39]. In
this light, it is a natural question if all Thurston geometries are NMG vacua [40]. The answer is positive, all
eight Thurston geometries are solutions to NMG. Furthermore, when considering Lorentzian signature there
are, instead of eight, four relevant geometries [41], two
of which have constant sectional curvature: Minkowski
and AdS. The remaining two are the Lorentzian versions
of the nil and sol geometries. We intend to report the
full details concerning these solutions in a forthcoming
article.
Coupling to the Maxwell-Chern-Simons and Abelian
Higgs fields
Drawing from our previous examination of equation
(1) where the Hodge-star operator is associated to the
metric (13), it is straightforward to verify that (S 3 , η, g)
is an ε-contact structure. In addition, since every every
contact form on a three-dimensional manifold represents
a solution of the Maxwell’s equations [42], let us consider
a gauge potential given by A = −2qη so that the field
strenth is given by
F = dA = q sin θdθ ∧ dφ,
(32)
the standard homogeneous field strength on a two-sphere.
To understand how a field like (32) is supported by
the para-Sasakian 3-sphere and what is the nature of the
corresponding induced current, we consider two obvious
choices, namely, NMG coupled to Maxwell-Chern-Simons
theory and alternatively coupled to the Abelian Higgs
model.
Consider the NMG action functional (26) coupled to
Maxwell-Chern-Simons theory (MCS)
Z
√
(33)
S[g, A] = d3 x −gLNMG + SMCS ,
where
SMCS
1
=
2
Z
−F ∧ ⋆F + µA ∧ F.
(34)
Note that the firstterm of the MCS action is the helicity
integral of the field, that is, a measure of the degree in
which the field lines are linked [43]. Thus, the Maxwell
equation (5) is satisfied provided ℓ = 1/µ, yielding
( − µ2 ) ⋆ F = 0,
(35)
for equation (4). As usual, the Chern-Simons coupling
constant µ determines the mass of the gauge field. In the
present case it also fixes the characteristic length scale ℓ
of spacetime. Since the field is Trkalian, cf. (35), then
the topologically massive gauge theory (34) is gauge invariant.
For this NMG-MCS theory the gauge field is supported
by the ε-contact provided
q2 =
µ2 − 8Λ
,
4κ2 µ4
and m2 =
21µ4
,
16(µ2 − 4Λ)
(36)
hold. Since q 2 ≥ 0 this provides us with a the restriction µ2 − 8Λ ≥ 0. When these inequalities are saturated
we recover (30). Hence, this charged solution smoothly
connects with the vacuum case.
We now move on to the Abelian Higgs theory, which
generalizes the Ginzburg-Landau theory where superconductors were originally described by Abrikosov [44]. We
also refer the reader to [45] for a closer analogue of the
following configuration and to [46] for recent work on
gravitating superconducting configurations.
We now couple the Abelian Higgs theory to New Massive Gravity
Z
√
S[g, A, Φ] = d3 x −gLNMG + SAH ,
(37)
with
SAH =
Z
1
1
− F ∧ ⋆F + DΦ ∧ ⋆DΦ† − ⋆V (|Φ|2 ). (38)
2
2
The Higgs field Φ is in general complex-valued and its
covariant derivative is given by DΦ = dΦ − iAΦ. The
Higgs field satisfies the equation of motion
⋆D ⋆ DΦ = V ′ .
(39)
5
Here, we consider a contribution from the scalar field to
the energy momentum tensor, so that
1
Φ
Tµν
= −Dµ ΦDν Φ + gµν (Dα ΦDα Φ − V ) .
2
(40)
Additionally, the Higgs field’s electric current is given by
⋆J = −iΦDΦ† .
(41)
By considering a constant real-valued Higgs field Φ = h
the previous equation becomes a London equation ⋆J =
h2 A which when compared to (6) shows that the value
of the Higgs field plays the role of µ in the MCS case.
Indeed, considering ℓ = 1/h is sufficient for the Maxwell
equations to hold. Moreover, the Higgs equation (39)
fixes the self-interaction potential to
V (|Φ|) =
λ 4
|Φ| .
4
(42)
The scalar field’s equation of motion also fixes the charge
of the Maxwell field (32) through 4q 2 = λ. When compared to the MCS configuration above, it possesses a rigid
Maxwell field which compensates the extra degree of freedom coming from the Higgs field.
The Maxwell and Higgs fields self-gravitate on the
background whenever
√
1 ± 1 − 32κ2 λΛ
21h2
2
2
h =
,
and
m
=
.
2κ2 λ
8(1 + 2κ2 λh2 )
(43)
Notice that the limit λ → 0 turns off all the field content
simultaneously. Only the negative branch of h2 in the
previous equation is well defined. This branch smoothly
connects to the vacuum solution, given by (30).
CLOSING REMARKS
In this manuscript, we showed that the metric relation (1) – which holds for every 3-dimensional manifold
equipped with an ε-contact strucutre – serves as a constitutive relation for electromagnetic fields such that their
potential 1-form is proportional to the contact form of
the manifold. This result does not rely on the particular
case explored in this manuscript and constitutes a general result for 3-dimensional electromagnetic fields. That
is, a material medium described by a metric constitutive relation is a superconductor whenever the manifold
is a 3-dimensiona ε-contact metric structure. Motivated
by this result, we studied the self gravitating MaxwellChern-Simons and the Abelian-Higgs model coupled to
New Massive Gravity in the case of (S 3 , g) and found the
conditions this particular spacetime must satisfy in order
to be a solution. In every case the fields were found to
be rigid, completely fixed by the couplings. Moreover, all
but one of the coupling constants was found to be free
in each case. To us, this indicates a rather high degree
of naturalness. The solutions with matter content where
found to smoothly connect with the vacuum case in the
appropriate limits.
We studied a non-standard metric structure for S 3 . In
particular, by considering the 3-sphere as a contact manifold, we obtained a locally homogeneous, left invariant,
Lorentzian Heisenberg metric g, cf. equation (13). Considering the pair (S 3 , g) as a 3-dimensional spacetime, we
verified that it is a Petrov D, Kundt spacetime. Furthermore, to the best of our knowledge, this is a new vacuum
solution for New Massive Gravity. In addition, when coupled to the Maxwell-Chern-Simons action, written solely
in terms of geometric objects directly linked to the contact strucutre of S 3 , we obtained a new charged solution
to NMG-MCS. These result appears to be natural in the
sense that we considered S 3 as the Hopf-fibration, where
the Reeb vector field associated with the contact form is,
in fact, a Beltrami field. Thus, it is not surprising that
it corresponds to a solution to the helicity integral of the
field, which provides a measure of the degree in which the
field lines are linked (cf. Chapter 5 in [43]). The helicity
integral is in turned defined in terms of the Hodge dual
of the metric (13), which is itself associated with the contact structure. Therefore, as one might have expected,
the self gravitating solutions are completely determined
from the value of the cosmological constant Λ or, equivalently, from the penetration depth ℓ in the corresponding
analogue superconducting material.
This exercise has shown us that metric contact manifolds might play an important role in the exploration
of 3-dimensional field theories. Moreover, it has open
new ways to understand 3-dimensional superconductors
in terms of Beltrami fields on a ε-contact manifold. This
constitutes a completely geometric picture of the macroscopic phenomenon of superconductivity which may shed
some light on its higher dimensional counterpart.
Acknowledgments.— Discussions with Eloy Ayón-Beato
and Fabrizio Canfora are gratefully acknowledged. DFA
would like to thank the Mexican Secretariat of Public
Education (Secretarı́a de Educación Pública) for support
under grant PRODEP 12313509.
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