Dung-Hai Lee U.C. Berkeley

1. Quantum state that never condenses Dung-Hai LeeU.C. Berkeley Condense = develop some kind of order

2. As a solid develops order, some symmetry is broken. Spin rotational symmetry is broken !

3. Ice crystal Superfluid Neutron star Expanding universe

4. Examples of order

5. Metals do break any symmetry, but they are not stable at zero temperature. Metals always turn into some ordered states with symmetry breaking as T  0. Metals are characterized by the Fermi surface

6. Different types of Fermi surface instability lead to different order. Fermi surface nesting instability  spin density wave, or charge density wave Cooper instability  superconductivity

7. Landau’s paradigm • Ordered state is characterized by the symmetry that is broken. • All ordered states originate from the metallic state due to Fermi surface instability. Metal Spin density wave Superconductivity Charge density wave

8. Is it possible for a solid notto develop any order at zero temperature ?

9. Insulators with integer filling factor are good candidates Fermion band insulator Boson Mott insulator

10. Mott insulator Boson Mott insulator Fermion Mott insulator Insulating due to repulsion between particles.

11. Examples of electron band insulator C, Si, Ge, GaAs, …

12. An example of electron Mott insulator YBa2Cu3O6 – the parent compound of high temperature superconductor CuO2 sheet

13. An example of boson Mott insulator: optical lattice of neutral atoms Greiner et al, Nature 02

14. Why are we interested in insulators ? Doping make them very useful ! Most of the time, doping make the particle mobile, hence can conduct.

15. Doped band insulator A Silicon chip

16. Doped Mott insulators Doping Mott insulators has produced many materials with interesting properties. Doped YBa2Cu3O6 Doped LaMnO3 Colossal magneto-resistive materials High Tc superconductors

17. Is it possible that a solid remains insulating after doping ? Yes

18. An interesting fact: all insulators with fractional filling factor break some kind of symmetry hence exhibit some kind of order. fermion boson Antiferromagnet Dimmerization

19. Why is uncondensed insulator so rare at fractional filling ? Oshikawa’s theorem If the system is insulating, and if the filling factor = p/q, the ground state is q-fold degenerate. Usually the required degeneracy is achieved by long range order. Can a fractional filled insulator exist without symmetry breaking ? Oshikawa PRL 2000

20. It is generally believed that featureless insulators will have very unusual properties. Such as fractional-charge excitations …

21. Anderson’s spin liquid idea Resonating singlet patterns + + . . . Spin liquid is a featureless insulator (at half filling) with no long range order ! It has S=1/2 excitations (spinons). It exists in the parent state of high-temperature superconductors. Anderson, Science 1987

22. Condensed matter physicists have searched for such insulators for 20 years. The usual search guide line is “frustration”. ?

23. A new idea: symmetry protected uncondensed quantum state Filling factor =1/3 Melts crystal order but never changes the C-M position  preserve 3-fold degeneracy.

24. The Quantum Hall effect The fractional quantum Hall effect Rxx = VL /I; Rxy = VH /I

25. One example of this type of state is the fractional quantum Hall liquid Lee & Leinaas, PRL 2004

26. Another example is the quantum dimer liquid Moessner & Sondhi, PRL, 2001

27. All existing models in the literature that exhibit uncondensed quantum state conserve the center-of-mass position and momentum.