dung-hai lee u.c. berkeley
DESCRIPTION
Quantum state that never condenses. Dung-Hai Lee U.C. Berkeley. Condense = develop some kind of order. As a solid develops order, some symmetry is broken. Spin rotational symmetry is broken !. Ice crystal. Superfluid. Neutron star. Expanding universe. Examples of order. - PowerPoint PPT PresentationTRANSCRIPT
Ice crystal Superfluid
Neutron star Expanding universe
Metals are characterized by the Fermi surface
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.
Different types of Fermi surface instability lead to different order.
Cooper instability superconductivity
Fermi surface nesting instability spin density wave, or charge density wave
Metal
Superconductivity
Charge density wave
Spin density wave
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.
Insulators with integer filling factor are good candidates
Fermion band insulator Boson Mott insulator
Fermion Mott insulator
Mott insulator
Boson Mott insulator
Insulating due to repulsion between particles.
YBa2Cu3O6 – the parent compound of high temperature superconductor
CuO2 sheet
An example of electron Mott insulator
Why are we interested in insulators ?Doping make them very useful !
Most of the time, doping make the particle mobile, hence can conduct.
Doping Mott insulators has produced many materials with interesting properties.
High Tc superconductors Colossal magneto-resistive materials
Doped YBa2Cu3O6 Doped LaMnO3
Doped Mott insulators
An interesting fact: all insulators with fractional filling factor break some kind of symmetry hence exhibit some kind of order.
Antiferromagnet Dimmerization
fermion boson
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.
Why is uncondensed insulator so rare at fractional filling ?
Can a fractional filled insulator exist without symmetry breaking ?
Oshikawa PRL 2000
It is generally believed that featureless insulators will have very unusual
properties.
Such as fractional-charge excitations …
Anderson’s spin liquid idea
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.
Resonating singlet patterns
Anderson, Science 1987
Condensed matter physicists have searched for such insulators for 20
years.
The usual search guide line is “frustration”.
?
Melts crystal order but never changes the C-M position preserve 3-fold degeneracy.
A new idea: symmetry protected uncondensed quantum state
Filling factor =1/3
All existing models in the literature that exhibit uncondensed quantum state conserve the center-of-mass position and momentum.