He began with an introduction about the puzzle of the pseudogap in High Tc. It is he said some kind of state in which we see a change of state - seen in measurements. Initial knight shift and T1 measurements suggested a spin gap appearing at T* >> Tc. Later, ARPES showed a pseudogap - a gap in the DOS that survives above Tc, but without a coherence peak. This was also found in STM measurement - Aharon showed nice data from his own group - noting the presence of a non-dispersing charge modulation that persists up to some temperature, called T*. (He will use T* as a general term for a crossover temperature where something happens in the measurement...)
Two types of T* discussed by theorists:
(1) representing a crossover into a state with preformed pairs and a d-wave gap.
(2) marks a true transition into a phase with broken symmetry which ends at a QCP, such as loop currents in the d-density wave (Charavarty et al Q > 0) or Chandra Varma's theory (Q=0).
The T* phase may have various different configurations - (a) T* line merges with Tc line at large doping (preformed pairs) (b) T* crosses into the dome (c) where it ends at the dome. Various experiments invented to look for signatures of the various scenarios.
Key questions posed in review by Norman, Pines and Kallin (Advances in Physics, 54, 715 (2005).
- Is there LRO associated with T*?
- Is charge ordering central to the pseudogap
YBCO and Bisco as examples - Pseudogap is found to be above the peak of the SC dome. On the other hand, for 221, looks like T* line appears to suggest a convergence of T* to the top of the dome.
Aharon wants to concentrate next on expts that do suggest T* is a real phase transition, by looking at a particular Bragg diffraction peak. YBACuO and HgBa2CuO. Onset of magnetic scattering in Bragg peak on a T* line that seem to follow one-ahother, extrapolating to x=0.2. On the other hand, a similar type of exploration on Lr2-xSrxCuO4, eg for x= 0.13, T* is about 450K from optical, but neutron saw development of short range order at 120K. Seemed not to be a unique quantity.
Question from Andriy - since they claimed to see real phase transitions - did anyone try to study specific heat around these points.
A - yes people tried, but no feature seen in S-Heat - its very difficult to see electronic parts to the s-heat. i know of any good high resolution sheat that shows these features. There is perhaps some unpublished data from Greg Boebinger's group, but I can't talk about it. I'll come back to it data.
Anyway - in LSCO, there is a big discrepancy.
In LaBCO, near 1/8th seen CO followed by Spin order - a feature at around 50-40K, butfrom susceptibility, the T* is around 400-500K. Big discrepancy between the phase transition and T* from susceptibility /knight shift.
Observations:
- Some material systems, different probes indicate different T*
- Most probes extract T* from some change in the systems behavior. Generally crossover
- Only expts to indicate true broken symmetry below T* are neturon scattering, also Kerr and possibly Nernst.
- In seceral systems, BLCO, charge ordering seems to be the most proninent phenomenon below T*.
Insight from Kerr measurements
Magneto optics have advantages probes bulk, highest quatlity samples and can probe SC and normal states. In addition, polar Kerr effect using the loopless Sagnac Interferometer provide high sensitivity.
Kerr effect measures the difference of nR and nL. Different to Faraday, which is transmission, Kerr effect is in reflection. Kerr effect depends on the off-diagonal component of the conductivity, which when non-zero indicates time reversal symmetry breaking.
Theta_K = - Im ((nL-nR)/(nL.nr-1)]
really measuring the imaginary part of sigma _xy.
Results on YBCO Method really a descendent of an earlier machine used to test for anyonic superconductivity. Method motivated by current loop or scenario with staggered AFM, expected to get a Kerr effect in the case of orthorhombic distortion with AFM.
YBCO, 1/8th doping, Tc = 65K, A signal produced by trapped flux in SC, but small signal above Tc from 65 to 155K. Cooling in zero field eliminates the vortex effect, see a signal that disappears at a higher than Tc.
All the data together, get a Kerr effect line at Ts(x). Kerr effect, consistent with muSR and elastic neutron scattering .
Optimally doped YBCuO, Ts ~ 105K, Tc=89K,
Summary of single xtal/thin filme results. Conclusion - some kind of charge ordering at 35-60K
near optimal doping (below Tc).
Beyond the thin-film YBCO LBCO - 1/8th doping - see large signal developing at 54K, max at 41K where there is spin ordering, and reduced at 19K. 54 - charge, 41- spin, 19K - sc. Canfield says - doesn't this mean that the charge ordered state has a FM moment? Aharon says - no - lets assume that I mean, that it could be, but in the most general case, suppose I have a piece of magnetic material that is not magnetic, coupled to the charge ordered system, OK - I can couple it in several ways - I can couple it in several says - an impurity phase - some kind of squushing - all the way to they are linked together.
Canfield said - I did not say it was causal - Aharon ultimately agreed. FM is something else - and may have been so in the earlier data.
Laura Greene asks - what kind of length scale do you need for these results?
Aharon answers - can only measure a global FM moment
The blogger is left very confused about how charge ordering can induce FM, unless close to a FM instability or canted AFM - or some other thing that has a bit of FM order.
Another material - found Kerr effect turns on at the charge ordering temperature. (bisco 2201 at optimally doped).
Finally if I go back to Bourges LSCO - here T* 450, but transition at 120K -
Summary -
- The Kerr effect seems to be an indirect probe of charge ordering - some coupling
- Charge ordering is a pronounced effect in all hi Tc sc.
- Charge, followed by spin ordering is the last effect before sc
- There are several magnetic effects, some may occur through phase transitions above Tc.
- This motivates, he said, studying the gneeral problem of charge order coupled to other order parameters.
Question from Andriy Nevidomskyy in Lanthanum Barium. Dont' you see a cusp in Kerr at the point it goes sc?
A: Yes - of course I do.
Q - what is the reason for the cusp?
Q: its vortices.
Q - from Sang Cheong. I'd like to clarify the connection between charge ordering and Ferromagnetism. wouldn't it be fun to look at other charge ordered systems?
A - yes
Q Blogger - don't you need to have quadratic couplings of the charge order to the FM order, that is close to criticality.
A- yes it would need to be close to criticality.
There followed a discussion about whether one needs quadratic couplings between the charge order and
finally tuned ferromagnetism. Another suggestion was made by ZT followed up by Sang Cheong, and AK seemed to be in broad agreement.
Zlatko Tesanovic:- "this issue of close to FM criticality" - everything is - if there are spins around- charge ordering will produce a FM order. But Sc - this a singlet state - and this diminishes such a response.
Aharon K. Might not be quadratic coupling but - because of the smallness of the effect, a piece of stuff is squshed, and this might make it FM. Its a kind of indicator of charge ordering - a possibility that I can not rule out.
Cheung - if you buy the best quality - best you can get is 5 -9s - you know how much impurity you have - if the background is metallic you don't see it - but its very possible that it responds to charge order.
No comments:
Post a Comment