Seminars And Colloquia Next Week

ics High Energy
Wednesday, March 28, 2018
3:30 PM
Physics Building, Room 204

ics Condensed Matter
Thursday, March 29, 2018
11:00 AM
Physics Building, Room 313
Adriana Moreo [Host: Despina Louca]
University of Tennessee
"New Directions in Theoretical Studies of High Tc Superconductors"

The discovery of high critical temperature superconductivity in iron-based pnictides and chalcogenides brought to the  forefront the need to develop efficient theoretical procedures to treat multiorbital models of interacting electrons. Among the many challenges, we need to clarify the role that the orbital degree of freedom plays in pairing and how its interaction with magnetic and lattice degrees of freedom leads to the stabilization of exotic phases such as the nematic state. Theoretical studies in the strong and weak coupling limits cannot address the physically relevant intermediate regime, with a mixture of itinerant and localized degrees of freedom. Traditional numerical methods, such as Lanczos or quantum Monte Carlo, have either a too rapidly growing Hilbert space with increasing size or sign problems. For this reason, it is necessary to develop new models and techniques, and also better focus on systems where both experiments and accurate theory can be used in combination to reach a real understandingof iron pairing tendencies. Examples of recent advances along these directions that will be discussed in this talk include:
i) The development of spin-fermion models [1] that allow studies in the difficult nematic regime with a finite
short-range antiferromagnetic correlation length above the ordering critical temperatures. This type of studies
also allow the inclussion of doping, quenched disorder, and the study of transport and real-frequency responses;
ii) The application of the Density Matrix Renormalization Group (DMRG) approach to multi-orbital Hubbard
models in chain and ladder structures [2] triggered by the discovery of superconductivity at high pressure in ladder
iron-based compounds such as BaFe2S3 and BaFe2Se3. In this context,
the recently reported [2] pairing tendencies unveiled at intermediate Hubbard U will be discussed;
iii) Results for a newly developed multi-orbital spin-fermion model for the CuO2 planes in high Tc cuprates.[3]

 [1] S.Liang {\it et al.}, Phys.Rev.Lett.{\bf 109}, 047001 (2012) and Phys. Rev. Lett. {\bf 111} 047004 (2013); Phys. Rev. B{\bf 92} 104512 (2015); C. Bishop {\it et al.}, Phys. Rev. Lett. {\bf 117} 117201 (2016); Phys. Rev. B{\bf 96} 035144 (2017). [2] N.D. Patel {\it et al.}, Phys. Rev. B{\bf 96}, 024520(2017). See also  N.D. Patel {\it et al.}, Phys. Rev. B{\bf 94}, 075119(2016). [3] Mostafa Hussein et al., in preparation.

ics Colloquium
Friday, March 30, 2018
3:30 PM
Physics Building, Room 204
Adam Kaminski [Host: Utpal Chatterjee]
Iowa State and Ames Lab.
"Chasing Relativistic Electrons in Topological Quantum Materials"

The discovery of Dirac fermions in graphene has inspired a search for Dirac and Weyl semimetals in three dimensions thereby making it possible to realize exotic phases of matter first proposed in particle physics. Such materials are characterized by the presence of nontrivial quantum electronic states, where the electron’s spin is coupled with its momentum and Fermi surfaces are no longer closed contours in the momentum space, but instead consist of disconnected arcs. This opens up the possibility for developing new devices in which information is stored and processed using spin rather than charge. Such platforms may significantly enhance the speed and energy efficiency of information storage and processing. In this talk we will discuss the electronic properties of several of newly discovered tellurium based topological quantum materials. In WTe2 we have observed a topological transition involving a change of the Fermi surface topology (known as a Lifshitz transition) driven by temperature. The strong temperature-dependence of the chemical potential that is at the heart of this phenomenon is also important for understanding the thermoelectric properties of such semimetals. In a close cousin, MoTe2, by using high-resolution laser based Angle Resolved Photoemission Spectroscopy (ARPES) we identify Weyl points and Fermi surface arcs, showing a new type of topological Weyl semimetal with electron and hole pockets that touch at a Weyl point. I will also present evidence for a new topological state in PtSn4, that manifests itself by presence of set of extended arcs rather than Dirac points, and so far is not yet understood theoretically. These results open up new directions for research aimed at enhancing topological responsiveness of new quantum materials.

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