Between order and disorder: Hamiltonians for Quasicrystals · Spectral properties of H(!) can be...
Transcript of Between order and disorder: Hamiltonians for Quasicrystals · Spectral properties of H(!) can be...
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Between order and disorder: Hamiltoniansfor Quasicrystals
Peter Stollmann
Chemnitz University of Technology
Kolloquium Regensburg, 27.10.2011
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Outline
I Quasicrystals?I Mathematical models of aperiodic orderI Hamiltonians
I DynamicsI Quantum transport.I Delone dynamical systems and Hamiltonians.
I Spectral properties:I The Wonderland theorem: generic results.I 1-d models with purely singular continuous spectrum.
Based on collaboration with S. Klassert and D. Lenz.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Quasicrystals?
What these creatures really are is not yet negotiated.However there’s enough evidence to speculate about thequestion ...
... as was done in the September 2005 issue of the Notices.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Quasicrystals?
This years Nobel prize in Chemistrygoes to Dany Schechtmanfor his discovery that was published as:
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Quasicrystals?
As a rule of thumb quasicrystals exhibit:
I Sharp diffraction peaks - usually coming with longrange order.
I Forbidden symmetries - excluding translationinvariance.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Mathematical models for aperiodic order
Aperiodic order can mathematically be described by tilings:
Figure: Original und Falschung: A real quasicrystal and thePenrose tiling
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Delone (Delaunay) sets
An alternative to tilings are Delone sets.
ω ⊂ Rd is called a Delone set, if there exist r ,R ∈ R suchthat
I ∀x , y ∈ ω, x 6= y : Ur (x) ∩ Ur (y) = ∅,I
⋃x∈ω BR(x) = Rd .
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Delone sets
By Dr ,R(Rd) = Dr ,R we denote the set of all (r ,R)-sets; itis a compact metric space in the natural topology.D(Rd) =
⋃0<r≤R Dr ,R(Rd) is the set of all Delone sets.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Hamiltonians: continuum models
The basic idea is very simple: at each point of a Delone setω an ion is sitting, whose potential is given by v . This leadsto the Hamiltonian
H(ω) := −∆ +∑x∈ω
v(· − x)
The potential
Vω =∑x∈ω
v(· − x)
is depicted below
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Hamiltonians: continuum models
If the Delone set ω is periodic, then H(ω) describes acrystal. If we choose the point set ω as the points of aPoisson process (typically no Delone set) then H(ω)describes a disordered solid. If ω is aperiodically ordered,then H(ω) can be used to describe electronic properties of aquasicrystal.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Hamiltonians
We are interested in the Schrodinger equation
ψ′(t) = −iH(ω)ψ(t) (SE )
it describes the time evolution of a wave function ψ(t).Spectral properties of H(ω) can be translated intoqualitative properties of solutions of (SE).The specific form of (dis-)order is encoded in H(ω).It will be very useful to consider a whole collection(H(ω), ω ∈ Ω) at the same time, for physical reasons andfor analytical reasons.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
The Schrodinger equation.
ψ(t) = −i(−∆ + V )ψ(t), ψ(0) = ψ0,
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Quantum dynamics.
Easy: the solution of the Schrodinger equation is
ψ(t) = e−iHtψ0.
But: what does it look like?To this end one studies the spectral resolution of H.
I If ψ0 is an eigenvector of H with eigenvalue E0 ⇒
ψ(t) = e−iE0tψ0.
One speaks of a bound state.
I If the spectral measure ρHψ0of ψ0 w.r.t H is continuous,
⇒ limT→∞
1
T
∫ T
−T‖χBψ(t)‖2dt = 0 for compact B.
One speaks of a scattering state.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Between order and disorder.
I Atomic models are described by a negative potential Vthat decays rapidely enough near infinity:one getsbound states for negative energies and scattering statesfor positive energies.
I Solid states with perfect order are described by aperiodic V ⇒ H = −∆ + V has only scattering states.
I P.W. Anderson proposed a new paradigm for disorderedsolids in dimension ≥ 3: dense pure point spectrum.
I The (dis-) order of quasicrystalline models suggests anintermediate spectral behaviour.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Delone dynamical systems
... simply consist of a translation invariant, compact setΩ ⊂ D(Rd), on which the group Tt : Rd → Rd(t ∈ Rd) oftranslations acts; we denote such a system by (Ω,T ).We interpret such a DDS (Ω,T ) as a model for a certaintype of (dis-)order. Ergodic properties of (Ω,T ) reflectcombinatorial properties of the elements ω ∈ Ω and viceversa. Moreover, spectral properties of the H(ω) aresometimes related to ergodic properties of the DDS. E.g.
(Ω,T ) minimal⇓
σ(H(ω)) = σ(H(ω′)) for all ω, ω′ ∈ Ω.
Minimality and unique ergodicity are equivalent to certaincombinatorial properties of the ω’s.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Spectral properties
For a DDS (Ω,T ) that describes aperiodic order one istempted to expect purely singular continuous spectrum andthis has been verified in some classes of discrete examples inone dimension (quasiperiodic Hamiltonians, substitutionpotentials) as well as continuum models in one dimension.However in higher dimensions there are only very fewrigorous results :-( .
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Spectral properties: generic results
A very modest step has been taken in showing thatgenerically in the topological sense singular continuousspectrum occurs.
Theorem (D. Lenz and P. S., Duke Math. J., 2006)
Let r ,R > 0 with 2r < R and v ≥ 0, v 6= 0. Then thereexists an open ∅ 6= U ⊂ R and a dense Gδ-set Ωsc ⊂ Dr ,R
such that for every ω ∈ Ωsc the spectrum of H(ω) containsU and is purely singular continuous in U.
This follows from a variant of Barry Simon’s WonderlandTheorem and uses heavily the spectral properties of periodicoperators.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Back to Wonderland
We use a variant of a result from B. Simon, Operatorswith singular continuous spectrum: I. General operators.Ann. Math. 141, 131 – 145 (1995)Fix S , a locally compact, σ-compact, separable metric space.Consider M+(S), the set of positive, regular Borel measures.µ ∈M+(S) diffusive or continuous:⇐⇒ µ(x) = 0 forevery x ∈ S .µ ⊥ ν, mutually singular:⇐⇒ ∃ C ⊂ S such thatµ(C ) = 0 = ν(S \ C ).
Theorem (D. Lenz and P. S., Duke Math. J., 2006)Let S be as above. Then(1) The set Mc(S) := µ ∈M+(S)| µ is diffusive is a Gδ-set inM+(S).(2) For any λ ∈M+(S), the set µ ∈M+(S)| µ ⊥ λ is a Gδ-setin M+(S).
The silent majority consists of strange creatures :-)
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
Conclusion of the Wonderland intrigue
I Mc(S) and Ms(S) are Gδ-sets, for polish S .
I This implies the Wonderland theorem and the fact thatgeneric measures are singular continuous in “nicespaces”.
I A particular example is given by “geometric disorder”(= Delone Hamiltonians) for which we can prove thatpurely singular continuous components turn up,generically.
QuasicrystalHamiltonians
Peter Stollmann
Quasicrystals?
Aperiodic order
Hamiltonians
Dynamical
Spectral
1-d models with purely singular continuousspectrum
Recall that Vω is given by translating a fixed potential v toall points of a Delone set ω. Now we are in 1-d, i.e. in R.?????? KlassertLS Theorem??????? Important difference to generic result??????? Even hard to find a single example