Ames W.F., Harrel E.M., Herod J.V. (editors). Differential Equations with Applications to Mathematical Physics
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20
J.
Bellissard,
A.
Bovier
and
J.-M.
Ghez
under
a
simple supplementary hypothesis, which can be verified al-
gorithmically, the second general result, that is the singularity of the
spectrum of
Hv,
has been very recently generalized by two of us
[22].
This is achieved by extending the analysis of the stable set of
the trace map performed for the period-doubling sequence.
We start with
a
primitive substitution
(
defined on
a
finite alpha-
bet
A.
For
w
€
AN,
let
&"(W)
be the trace of the transfer matrix
associated to
w.
By construction, there is
a
finite alphabet
B,
in-
cluding
A,
such that the trace map of
(,
that is the map
(
fpi)illBl
defined by
dn+')(pj)
=
fpi
(d")(pj),
...,
dn)(ppl)),
is
a dynamicd
system on
RIBI
[27].
It is clear that the essential role in the van-
ishing of the Lyapunov exponent is played by the dominant terms
in the
fpi.
Therefore its crucial property is the existence for each
i
of
a
unique monomial of highest degree
Ypi,
called the
reduced truce
map,
and of the associated substitution
@
on
B.
Actually, defining
a
semi-primitive
substitution as
a
substitution satisfying:
i)
37
C
B
s.t.
@lc
is
a
primitive substitution
from
C
to
C";
ii)
3k
s.t.
Vp
E
B,
&(p)
contains at least one letter from
C,
we can prove:
THEOREM
4.1
:
Let
Hv
be
a
1D
discrete Schrijdinger operator gen-
erated
by
a primitive substitution
t
on a finite alphabet. Assume that
there is a trace map such that the substitution
iD
associated to its
reduced truce map is semi-primitive and also that there
is
a
finite
k
s.t.
('(0)
contains the
word
pp
for
some
p
E
B.
Then the spectrum
of
Hv
is singular and supported
on
a set
of
zero Lebesgue measure.
The proof of theorem
4.1
can be summarized
as
follows: Let
6
C
U
be the open "generalized" unstable set of
(
(see
[22]
for
a
precise
definition). Generalizing the proof of theorem
3.1,
we use the crucial
fact that, for primitive
6,
the lengths of the words
Itnal
(a
E
A)
grow with
n
exponentially fast with the same rate
On,
where
8
is
the
Perron-Frobenius eigenvalue of the substitution matrix
[2], [17],
to
show that, for semi-primitive
iD,f(6)'
C
Ov.
As
in sect.
2,
this implies the following sequence of inclusions:
E(6)'
c
OV
c
a(&)
c
(Int(E(U))'
c
E(fi)'
(6)
Schrdinger Operators Generated
by
Substitutions
21
and thus
a(Hv)
=
Ov,
which concludes the proof of theorem
4.1
(see Remark
1
after the proof of theorem
3.1).
Remark
4:
If
we assume that
('(0)
begins with the word
pp,
we can
prove that
Hv
has no eigenvalues and therefore that the spectrum
of
Hv
is
singular continuous and supported on
a
Cantor set
of
zero
Lebesgue measure.
Bibliography
[l]
D.
Shechtman,
I.
Blech,
D.
Gratias and J.V. Cahn, Phys. Rev.
Lett.
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QuefElec,
Substitution dynarnical systems. Spectral analysis,
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1294,
Berlin, Heidelberg,
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1987.
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Sut6,
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Stat. Phys.
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Number theory and physics,
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J.
Bellissard, A.
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1870-1872.
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E.D. Siggia, Phys. Rev. Lett.
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1983,
p.
1873-1876.
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Bellissard and B. Simon,
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408-419.
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de Physique
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1989,
p.
3447-3461.
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Sire and R. Mosseri,
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de Physique
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Allouche and
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Peyribre,
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Acad. Sci. Paris
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4,
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J.
Bellissard, R. Lima and
D.
Testard, in
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Streit, Ed., Singapore,
Philadelphia, World Scientific
1985,
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1-64.
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J.
Bellissard, in
Statistical mechanics and field theory,
T.C. Dor-
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M.N. Hugenholtz and M. Winnink, Eds., Lecture Notes in
Physics, vol.
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Berlin, Heidelberg, New York, Springer
1986,
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99-156.
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Bellissard, in
From
number theory to physics,
M. Wald-
Schmidt,
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Heidelberg,
New
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538-630.
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5834-5849.
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A. Bovier and J.-M. Ghez,
Spectml properties
of
one dimensional
Schr6dinger opemtors with potentials generated
by
substitutions,
Preprint
CPT-92/2705.
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Martinelli and
E.
Scoppola, Rivista del Nuovo Cimento
10,
1987.
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Kotani, Rev. Math. Phys.
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Schriidinger Operators Generated
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23
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Wave Packets Localized
on
Closed
Classical
Trajectories
S.
De
Bikvre,
J.C.
Houard and
M.
Irac-Astaud
L.P.T.M.
Universitb Paris
7
Abstract
In the classical limit eigenfunctions
of
Hamiltonians tend to lo-
calize in phase spa.ce on energy surfaces
if
the system is ergodic,
or
on invariant tori for completely integrable systems. In cases when
the energy levels are highly degenerate, one may hope to construct
eigenstates that localize
on
lower dimensional flow invariant mani-
folds such as closed orbits. This
is
known to be true for the Kepler
problem. We establish the same result for n-dimensional harmonic
oscillators. The construction generalizes to yield states well-localized
on closed orbits of more general Hamiltonians.
1
Introduction
Let
HO
be
a
C" IIamiltonian on phase space
R2"
=
T*R".
Let
y
:
t
E
[O,T]
--f
y(2)
E
(q(t),p(t))
E
R2"
be
a
periodic solution
(y(0)
=
y(T))
of the corresponding Hamiltonian equations
of
motion.
We shall write
EO
=
Ho(q(t),p(t)).
We then consider
< >,:
fo
E
Crn(IR2")
+<
fo
>+
This defines
a
classical state, i.e.
a
probability measure on phase
space, which is concentrated on
y
and flow invariant in the sense
Differential Equations
with
Copyright
@
1993
by Academic Press,
Inc.
Applications to Mathematical
All
rights
of
reproduction in any form reserved.
Physics
ISBN
0-12-056740-7
25
26
S.
De
Bikvre,
J.C.
Houard,
M.
Irac-Astaud
that
<
fo
0
4t
>y=<
fo
>y,
Qt
E
R,
(1.2)
where we wrote
Consider then self-adjoint operators
H(h),
F(h)
on
L2(IR"),
which
have
Ho,
respectively
fo,
as
their principal Weyl symbols. The eigen-
states
$a
of
H(h)
satisfy the quantum equivalent of
(1.2),
i.e.
for the flow defined by
Ho.
It is then natural to
ask
whether it is possible to construct a family
of eigenstates
H(fi)$h
=
W+bh,
(1.4a)
with
E(h)-+Eo
as
h+O
(1.4b)
and such that
for
all
In
of the
F(h)
as
above.
general, this is impossible. Indeed, as
a
first example, think
double symmetric potential well.
In
that case,
all
eigenstates
satisfy
I
$h(z)
12=1
+h(-z)
12.
Hence they can never concentrate on
a classical trajectory in one of the two wells in the limit
h
+
0.
More
generally, consider the case when
HO
is completely integrable. The
classical limit of energy eigenstates for such systems has been studied
extensively in the literature
[8]
[l].
Let
T'R"
=
IR"
x
R"*
be the
classical phase space and
:
T'R"
-+
R"
n
commuting constants
of the motion for the Hamiltonian
Ho,
i.e.
{Pi,
P,"}
=
0
(1.6a)
and
Ho
=
PJ.
(1.6b)
In
the corresponding quantum system, one has self-adjoint opera-
tors
Pi(h)
having
P:
as their principal Weyl symbol. They form
a
Wave Packets Localised
on
Clased Classical llajectories
27
complete set of commuting observables on the Hilbert space
L2(R").
As
a
result, fixing their eigenvalues
&(ti)
determines
a
unique eigen-
state of the quantum Hamiltonian
H(h)
and one expects that,
as
h
+
0,
this eigenstate concentrates
-
in phase space
-
uniformly on
the corresponding classical torus
?-'(&).
This is indeed established
in
[l],
under suitable conditions on
Ho.
The results in
[l]
lead one to
conclude that non-degenerate eigenstates of
H(h),
which are auto-
matically eigenstates of all the
Pi(h),
cannot in general be expected
to satisfy (1.5). In fact, one expects that (1.4)-(1.5) can only be
satisfied if
H(h)
admits highly degenerate eigenspaces
so
that one
can construct many eigenstates of
H(h)
that are not simultaneously
eigenstates of the other
P;(h).
There are
two
known examples where (1.4)-(1.5) can be satisfied
for
all
the classical closed trajectories. They are the hydrogen atom
[3]
and the isotropic harmonic oscillator [2]. In both cases the method
of construction is based on group-theoretical arguments using the
hidden symmetries of the problem.
In section 2, we construct eigenstates of the anisotropic harmonic
oscillator satisfying (1.5). Symmetry arguments cannot be used in
this case, but instead we propose
a
very natural construction using
coherent states.
Since the requirement that
$Jh
is an eigenstate
is
in general in-
compatible with
(1.5),
it is customary
to
replace it by the weaker
condition
II
(m4
-
Jw))$J(h)
II=
WN)
(1.7)
for some
N
E
IN.
One then says that
$JA
is
a
quasimode. Quasimodes
localized on closed classical trajectories were constructed by Ralston
[6]
for
a
class of partial differential operators under certain natural
stability conditions on
7
which determine
N
and supposing
q(t)
#
In section
3
we show how our construction of section 2 can be
generalized very simply to construct states satisfying (1.5) and hence
(1.7) with
N
=
1,
without any stability conditions on
7.
In the
absence of stability requirements, one can probably not hope to do
better than this. While this work was in progress, we learned of
recent results
of
Paul and Uribe [5], who use the same construction
0,Vt
E
[O,T].
28
S.
De
BiBvre,
J.C.
Houard,
M.
Irac-Astaud
to prove
(1.7)
for
all
N
in the case where
n
=
1
and
H(h)
is an
ordinary differential operator with polynomial coefficients.
2
The
Anisotropic Oscillator
Let
"P?
1
22
H
=
-
+
-m;wjQj
2mi
2
i=l
be the usual harmonic oscillator Hamiltonian on
L2(lR").
Its spec-
trum is given by
1
2
Em
=
h(wlm1
+
w2m2
+
...
+
w,m,
+
-(w1
+
...
+
w,)).
(2.2)
The corresponding classical system, with Hamiltonian function
"1 1
2m;
2
Ho(q,p)
=
-p:
+
-m;w;q;
i=l
always admits closed classical trajectories.
If
all
w;
are two by two
incommensurate, the only such trajectories are the ones in which
only one mode
of
the oscillator is excited.
If,
on the other hand,
wjl,
w;,
,
...,
w;,
(k
5
n)
are two by two commensurate, the others being
incommensurate, then all trajectories in which only the degrees of
freedom
il,
...,
ik
are excited, will be periodic.
They then have
a
common period, which is the least common multiple of the
Ti,
=
$.
>
Let us now
fix
a
closed trajectory
of the Hamiltonian in
(2.3).
We shall write
for the corresponding energy. In the rest
of
this section, we construct
an h-dependent sequence
of
eigenfunctions
of
H,
all
with energy
Eo,
concentrating on
7
as
ti
-+
0
in the sense explained in section
1.
Wave Packets Localised
on
Closed Classical Trajectories
29
First, we briefly recall the definition
of
coherent states. We define
where
K
is
the matrix
It is then well known that
where we introduced the notation
n
I
w
I=
cw;.
(2.76)
i=
1
The coherent state
I
q,p
>
being optimally localized around the
phase space point
(q,p),
it is natural to construct
a
state
which is
a
superposition
of
coherent states localized on points
of
the
trajectory
7.
Taking
40)
=
Eot
(2.9~)
.IWP
a(t)
=
exp
-2-
2
it is easily verified that,
Vs
E
R,
provided
3n
E
Z
so
that
10
I
27rnh
[EO
-
h-]
=
-
2
T.
(2.9b)
(2.10a)
(2.1
Ob)
Furthermore one verifies readily that
I
7
>
in
(2.11)
is identically
zero, unless
hj1,
...,
mik
E
N
so
that
(2.11)