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**Condensed Matter Theory Group Seminar**

**Realistic quantum critical points**

**Date:**

**Monday, January 31st**

**Time:**4:00 pm

**Place:**5280 Chamberlin

**Speaker:**Munehisa Matsumoto, University of California-Davis

**Abstract:**Quantum criticality has been discussed to play a key role in interesting phenomena in strongly-correlated systems, such as high-

*T*superconductivity in cuprates (here

_{c}*T*is the superconducting transition temperature), recently-discovered iron pnictides/chalcogenides, and heavy-fermion materials. In the main part of the talk I will show how the magnetic quantum critical point (QCP) in heavy-fermion materials can be quantitatively predicted by combining electronic-stricture calculations based on local-density approximation (LDA) and dynamical-mean field theory (DMFT) for the LDA-derived effective low-energy Hamiltonian. We utilize state-of-the-art continuous-time quantum Monte Carlo method to solve the impurity problem in DMFT formulated on the basis of localized f-electrons, which enables us to obtain numerically-exact solutions at low temperatures down to

_{c}*O*(1) [K] within DMFT. Thus we reach at a good position to address the quantum critical point quantitatively and we find the followings: 1) striking multiple quantum critical points are found in a realistic phase diagram for Plutonium-based compounds, which is attributed to the strong-coupling nature of the effective Kondo-lattice model. PuCoGa

_{5}, with the highest

*T*= 18.5 [K] among f-electron based materials, is found be located in the proximity to the third QCP [1]. 2) CeCoIn

_{c}_{5}, which has the highest

*T*= 2.3 [K] among Cerium-based heavy-fermion compounds, its parent material CeIn

_{c}_{3}, and its new two-dimensional (2D) analogue CePt

_{2}In

_{7}are concentrated around a QCP where CeCoIn

_{5}is found to be right on top of QCP. The reason the most 2D one does not come closest to QCP is attributed to the subtlety in the competition between the dimensionality and hybridization effects along the c-axis [2]. In the final part of the talk I will discuss the possible subtle nature of what has been called QCP, which still challenges realistic numerics but careful numerical analyses of an effective field theory [3] tells us QCP might not truly be critical. Possible consequence for having the resonating valence bond state around what has been QCP [4] is revisited.

References

[1] MM, Q. Yin, J. Otsuki, S. Y. Savrasov, preprint [arXiv:1101.1582].

[2] MM, M. J. Han, J. Otsuki, S. Y. Savrasov, Phys. Rev. B 82, 180515(R) (2010) [arXiv:1004.5457].

[3] A. B. Kuklov, MM, N. V. Prokof'ev, B. V. Svistunov, M. Troyer, Phys. Rev. Lett. 101, 050405 (2008) [arXiv:0805.4334].

[4] P. Coleman and N. Andrei, J. Phys.: Condens. Matter 1, 4057 (1990).

**Host:**Robert Joynt