1. Localized Perturbations of Integrable Billiards Saar Rahav Technion, Haifa, May 2004

2. Outline Motivation: spectral statistics and universality What is a perturbation at a point Two ways for spectral statistics Connection to star graphs Periodic orbits and spectral statistics Dependence on location of perturbation Conclusions

3. Spectral statistics of dynamical systems Dynamical systems exhibit spectral statistics of “random matrices” Time reversal symmetry  statistics of random real symmetric matrices (GOE) No time reversal symmetry  random Hermitian matrices (GUE) Integrable systems  Poissonian statistics (uncorrelated levels) Spectral statistics do not depend on details: Universality How to explain this universality? Some properties of the dynamical system itself must be universal (sum rules).

4. Examples for different dynamical systems: Sinai billiard - chaotic Ray splitting – pseudointegrable, non universal Šeba billiard – singular billiard Non universal

5. The statistical measures Nearest neighbor spacing distribution Spectral form factor

6. Semiclassical analysis of spectral statistics Trace formulae: Gutzwiller, Berry and Tabor Relates the density of states and periodic orbits For large energies ( semiclassical limit ) Contributions with random phases in the cosine The sum is dominated by almost equal actions: The semiclassical Form factor Berry

7. The diagonal approximation: Take only pairs with the same action Berry Evaluated using the Hannay & Ozorio de Almeida sum rule The correct short time asymptotics ! Validity of RMTSum rules for periodic orbits Higher order terms? corrections GOE were calculated by Sieber and Richter, see also Muller, Haake, et. al.

8. What are singular billiards? “Physical” point of view: Integrable Quantum systems, with local perturbation The diffraction constant is proportional to the scattering amplitude and satisfy the optical theorem The perturbation can be described be means of scattering theory: (without the boundary) Geometrical theory of diffraction, Keller.

9. “Mathematical” point of view: The self-adjoint extension of aHamiltonian One can define a family of extensions, with a simple Green function: Zorbas is related to the scattering strength The new eigenvalues are the poles of For closed systems: A quantization condition for new eigenvalues

10. Why singular billiards? Dynamics intermediate between integrable and chaotic Important diffraction effects Simple system What is the spectral statistics? New universality class for spectral statistics? A new ‘test’ for periodic orbit theory

11. Two approaches for spectral statistics of singular billiards 1. Periodic orbits Simple scattering without a boundary The boundary can be added using an integral equation In the semiclassical limit, The integrals over the boundary are dominated by contributions that perform specular reflections Rahav, Fishman Bogomolny, Giraud

12. The integrals lead to two types of orbits: Periodic orbits - do not hit the scatterer Orbits with segments which start and end at r 0 – diffracting orbits leads to a modified trace formula: With More diffractions: more powers of Higher powers of more segments Non diagonal contributions

13. 2. Ensemble averaging of the quantization condition Approximately: Bogomolny, Gerland, Giraud, Schmit Properties:LHS has poles at ‘unperturbed’ energy levels LHS monotonically decreasing with z Exactly one solution in

14. The density of states is: Integrable system are independent random variables The distribution of is uncorrelated with One can build statistical measures, e.g. And average over the unperturbed energies and wavefunction values A kind of ensemble average Advantage – the integrals separate into independent farctors Results: (simplified) Level repulsion Exponential falloff Intermediate statistics

15. Connection to star graphs Quantum graphs: Kottos, Smilansky Free motion on bonds, boundary conditions on vertices Star Graphs: Berkolaiko, Bogomolny, Keating For star graphs, the quantization condition is In the limit of infinite number of bonds with random bond lengths The spectral statistics of star graphs are those of Seba billiard with

16. Periodic orbit calculation of spectral statistics Reminder: Where the lengths may be composed of several diffracting segments What types of contributions may survive? For the rectangular billiard: Diagonal contributions: The periodic orbits contribute Diffracting orbits with n segments Sieber Can one find diffracting orbit with the same length of a periodic orbit?

17. Yes. A forward diffracting orbit! A ‘kind’ of diagonal contribution: Non diagonal contributions: For The difference in phase is small for There are many (~k) such contributions

18. Results: Scatterer at the center Typical location of scatterer: All form factors start at 1 and exhibit a dip before going back to 1. Intermediate statistics

19. Dependence on location: For the rectangular billiard the spectral statistics depend in a complicated manner on the location of the perturbation: Complementary explanations: 1.Degeneracies in lengths of diffracting orbits 2.The distribution of values of wavefunctions: Differs ifare rational or not Is such behavior typical? The Circle billiard: Angular momentum conservation Quantum wave functions are exponentially small for

20. So for exponentially small wavefunction the eigenvalues are almost unchanged The spectrum of the singular circle billiard can be (approximately) divided into two components: 1.Almost unperturbed spectrum, composed of wavefunctions localized on r>r 0. 2.Strongly perturbed spectrum. How many levels are unperturbed? Superposing the two spectra: The statistics depend on the location of the scatterer. Partial level repulsion?

21. Conclusions The spectral statistics differ from known universality classes – Intermediate statistics Strong contribution due to diffraction – non classical Statistics depend on location of perturbation – non universal However, the statistics of different singular billiards show similarities The wavefunctions are not ergodic ( Berkolaiko, Keating, Marklof, Winn )

22. Interesting open problems Understanding pseudointegrable systems, where the diffraction contributions are non uniform Resummation of the series for the form factor Understanding singularities of form factors Better understanding of wavefunctions Dependence on number of scatterers