Tuesday, July 31, 2007

Open Discussion and Short Presentations

Tuesday, 31st July.

Open Discussion and short presentations.
10.30am-1.pm Bethe Meeting Place.

The group met to discuss the structure of the workshop. Five speakers
gave their perspective on some of the open questions in this field.

1. Andre Marie Tremblay, Sherbrooke.

"Antiferromagnetism vs d-wave superconductivity: Insights
from the organics"

2. Gabi Kotliar, Rutgers.

"Superconductivity near the Mott transition."



3. Qimiao Si, Rice.

"Quantum Criticality and Superconductivity in Heavy Fermions"


4. Doug Scalapino, UCSB.

"Some issues motivated by the cuprate problem".


5. Subir Sachdev, Harvard.

" Fractionalization on the route from Neel order to d-wave superconductivity"


Summary of Discussions

1. Andrey Marie Tremblay:
"Antiferromagnetism vs d-wave superconductivity: Insights
from the organics"

Andrey emphasized that an ultimate test of our understanding of unconventional superconductivity, is to see how successful the theory is when applied to a diverse set of componds. The Organic superconductors display many aspects in common with the 115
and the high temperature superconductors - proximity to antiferromagnetism, frustration and Mott physics.

Andrey discussed the K- (ET)2 X layered organics. The physics of these systems is believed to be described by a 2D Hubbard model on a triangular lattice, with hopping t and t'. You can think of them like the cuprates, but with only one t' cross-link per square plaquet. Unlike the cuprates, by changing the anion X that separates the layers, you can tune t'/t

U ~ 400 meV
t ~ 30 meV
t'/t - (0.6 - 1.1 variation).

Andrey presented the results of a cluster dynamical mean field theory calculation on this model. Without pairing it has a 1st order Mott Transition line. For larger U/t, the system develops antiferromagnetism, or spin liquid. For smaller U/t, it enters a d-wave sc phase and then a metal.

Questions: At large frustration, does the d-wave sc become unstable to new symmetries - e.g p-wave symmetry?

Gabi Kotliar questioned the similarity with the cuprates. Here, the superfluid stiffness is a maximum near the Mott transition, whereas it goes to zero at the Mott transition in the cuprates. Others questioned whether this is due to the difference between doping and U/t tuning.

Also - here the temperature dependence of the superfluid stiffness has an anomalous T^3/2 variation.

2. Gabi Kotliar, Rutgers.

"Superconductivity near the Mott transition."

Gabi discussed the phase diagram of the t-J model of high temperature superconductors, contrasting the predictions of slave boson theory, with that of cluster dynamical field theory
(CDMFT). CDMFT predicts significant anisotropies in k-space that are absent from a slave boson theory. In particular


  • The rate at which the quasiparticle renormalization constant Z goes to zero in the approach to the Mott transition, is much faster in the antinodal regions. (Measured in the superconducting phase).
  • The v_Delta - the component of the qp velocity coming from momentum dependence of the gap, decreases with the doping (linearly? ), whereas v_F remains doping independent.
This last point has some important consequences. In particular, the T coefficient of the
superfluid spin stiffness, "a" in

rho = rho_0 - a T

becomes doping independent, rather than proportional to doping squared, as in RVB theory.
A similar feature is seen in the omega coefficient of chi''(omega) in Raman spectroscopy, which is predicted to be doping independent.

Gabi's questions:

  • Are these features observed experimentally?
  • What is the origin of this doping dependence?
  • Can we understand the solutions to the CDMFT in a simple language, perhaps analytically?

3. Qimiao Si, Rice.

"Quantum Criticality and Superconductivity in Heavy Fermions"

Qimiao started his presentation with the remark

" Most of the really interesting questions about heavy fermion superconductivity have not been deeply explored.

What are these questions? Qimiao began his talk with a summary of the key properties of heavy electron quantum critical points, taking as examples, CeCu_6-x Au_x (doping tuned), CePd_2Si_2 (pressure tuned) and YbRh_2Si_2 (field tuned).

He later turned to discuss the case of the 115 materials, showing a phase diagram that was quite similar to that presented by Andre-Marie Tremblay. 115 materials have the chemical formula

CeX In_5

where X= Co, Rh, Ir. They are layered heavy electron systems that display antiferromagnetism and superconductivity. Application of pressure to CeRhIn5 leads to a transition from antiferromagnetism, to a region of co-existent superconductivity, then into a purely superconducting phase. However, if you apply a magnetic field to remove the sc, there is a single QCP between the antiferromagnet and paramagnet, where the Fermi surface volume appears to "jump". This jump is associated with the delocalization of f-electrons.

Qimiao asked:

  • Can superconductivity co-exist with a state that appears to undergo "fermi surface fluctuations" ?
  • How should one characterize superconductivity that forms a dome above a second order QCP, particularly one where the Fermi surface jumps?
4. Doug Scalapino, UCSB.

"Some issues motivated by the cuprate problem".

Doug Scalapino asked the question:

What is the (mother) phase that underlies superconductivity?


As an example of the importance of this question, he discussed two different models of how superconductivity emerges from an antiferromagnetic Mott insulator:

Antiferromagnetically mediated pairing, in which the "mother state" of the superconductor is
a nearly antiferromagnetic metal. In this scenario, the omega dependence of the gap function
Delta(k,omega) should reflect the underlying spectral function of the spin fluctuations.

RVB model of superconductivity, in which the "mother state" of the superconductor is a spin liquid - a Mott insulator without Neel order. In this case, one might expect a disconnect between chi(q,omega) and the frequency dependence of the gap function.

But beyond this, Doug listed all sorts of possible "mother states" of high temperature superconductors:

Stripes
d-symmetry CDWs
d-symmetry SDWs
2-leg ladders
Orbitally ordered states

"Now you may not agree of the starting point, or the mother state, but the important point is that there are a lot of different materials, and room enough for everyone!" (paraphrased).

Doug also mentioned the challenge of the s-wave superconductor Barium Potassium Bismuthate (Ba_1-xKx BiO_3) where T_c ~ 40K. He asked whether the underlying mechanism here is solely phonons, or does charge disproportionation

2Ba(4+) <----> Ba (3+) + Ba(5+)

play an important role?

There was a lot of discussion.

Andy Millis raised the issue of the Taillefer group measurements, that reveal a small Fermi surface in the high field state of underdoped high temperature superconductors. He asked - should we regard this as a "mother state" of the high temperature superconductor, or is it just a competing state?

Thomas Vojta felt that if the transition between competing states was second order, then they would still influence one another.

Shankar asked whether we should consider Grandmother states?!

Sudip Chakravarty pointed out that in simple systems like Pb, there was no ambiguity about the mother state - it was a Ferm liquid with e-phonon interactions, but already once one gets to V3Si, the underlying state might already be a CDW.

There was a philosophical discussion about why it was, when physicists manage to link
Arpes data with spin fluctuation data, it is no widely accepted..... Doug said something about Physicists being quite artful at fitting selected data....




5. Subir Sachdev, Harvard.

" Fractionalization on the route from Neel order to d-wave superconductivity"

Subir posed his questions at the beginning of his talk. They are:

  • Is the low doping limit of cuprate superconductors a BCS state (+ some other unconventional order), or is it exotic or "fractionalized"?
  • If there is an exotic state at low doping - can this state be obtained by doping a Neel state (and not a non-existent spin liquid?)
Subir then discussed some work he has recently done with Senthil, Levin and others,
in which they discuss the effect of doping the deconfined quantum critical point that is thought
to separate a Neel state from a valence bond solid. The key point about this, is that the quasiparticles of the state that emerges are "fractional", in the sense that they carry a non-trivial gauge charge. These particles, they believe, are possibly the origin of the pockets seen in the Taillefer experiment. Subir also described how, when they pair, they form a conformally invariant fluid, in which the superfluid stiffness has a T-dependence that is independent of doping - rather like the results of the Kotliar work

rho_s(x,T) = constant - R T

where R is universal.






8 comments:

Subir said...

A (somewhat) non-technical summary of our work is at http://sachdev.physics.harvard.edu/p160.pdf

More details on earlier work is at
http://sachdev,physics.harvard.edu/p154.pdf

Subir

Anonymous said...

My response to the question of why linking ARPES data with neutron measurements of the spin-fluctuation spectrum didn't settle the issue of what are the important excitations which couple to the electrons is: it is possible to fit a given piece of data in various ways and the peak-dip-hump feature in the ARPES data has been fit based on spin-fluctuations and the pi-resonance, B1g phonons , as well as other approaches. While I happen to favor the approach using the neutron scattering spin-fluctuation spectrum, to get wide spread agreement in the physics community one will need to show that a given approach fits "all" of the what is seen as the relevant cuprate data.
Doug Scalapino

André-Marie said...

A "slides" version of the talk can be found at
http://www.physique.usherbrooke.ca/tremblay/ conferences/0707_Aspen_Organics.pdf

Refs: Quantum cluster calculations on the organics:
Kyung-Tremblay
http://link.aip.org/link?prl/97/046402
Sahebsara-Sénéchal
http://link.aip.org/link?prl/97/257004

Comments:
The most interesting results are:
- For t'/t>0.5, the maximum d-wave order parameter occurs at the first order transition with the insulating phases, and it decreases with frustration, correlating with observed Tc in the family. So d-wave is more robust near AFM phase than near spin-liquid phase, as observed.
- New sequence of transition, AFM -Spin liquid- d-Sc with increasing pressure is predicted.
- The first order Mott transition (t'/t > 0.5) is rather insensitive to the presence of LRO.

Qimiao said...

The following articles:

* Physica B 378, 23-27 (2006)

* PRL 99, 016401 (2007) [w/ S. J. Yamamoto]

contain some recent theoretical discussions about the Fermi surfaces across heavy fermion QCPs, as well as relevant references.

Qimiao Si

Rotary Washing Lines said...

There are some really complex isssues here, it is bio-superconductors that will be the next main step of the electronics industry, the stalemate of size and functionality of electronic components has pushed a lot of funding and experimentation into biological superconductors and their possible use as circuits nad carriers, there is always someone that is willing to think outside the normal channels that most chemists and phycisists are afraid to do so, it is here in the greay areas where major breakthroughs occur.

Frank Sargent said...

I build electronics and as we advance it helps to make the circuit designs smaller and more reliable, we design for tv, camera, mp3 and often see our work with argos lcd tvs and halfords sat nav system that we built and tested.

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