Department of Physics & Astronomy

2012 – 2013

*Where:
Brockman Hall 300When: Mondays at 4:00 p.m.*

Dr. Souleymane O. Diallo Oak Ridge National Laboratory Over the last half century, neutron scattering has emerged as a vital experimental probe for understanding materials properties in a wide range of fields such as physics, medicine, biology, chemistry, geology, engineering, archeology and so on. The nature of the neutron interaction with matter makes it a very powerful and yet unique technique for investigating the structure and the dynamics of materials on time and length scales that would not otherwise be accessible. In this seminar, I will give an overview on how neutron scattering can be used to provide key information in liquid and solid state physics. In particular, I will discuss recent results on the effects of applied electric field on the diffusion of water molecules in nano-confinement. |

Dr. Donghui Lu SLAC National Accelerator Laboratory The discovery of iron-based superconductors has opened up a new playground toward the ultimate understanding of high-temperature superconductivity. While this new family of high temperature superconductors shares many similarities with high-Tc cuprates, there are also important disparities in their physical properties and underlying electronic structures. One important difference is the multi-band and multi-orbital nature of the electronic states near the Fermi level in iron-based superconductors, which has been largely neglected in understanding the mechanism of superconductivity. In this talk, I will present two case studies of iron-based superconductors by angle-resolved photoemission spectroscopy (ARPES) to shed light on this issue. For the first part, I will discuss the observation of symmetry breaking orbital anisotropy in iron pnictides Ba(Fe1-xCox)2As2 and NaFeAs [1,2], which is manifested in the form of an anisotropic shift of the orthogonal dxz and dyz bands. The orbital anisotropy develops in such a way to enhance the nesting conditions along one direction, hence is intimately correlated with the long range collinear AFM order. Furthermore, the band splitting is observed to onset at slightly above TS for unstressed crystals, whereas for detwinned crystals it occurs at considerably higher temperatures, revealing the presence of large in-plane electronic nematic susceptibility. For the second part, I will report recent results on iron selenide superconductors AxFe2-ySe2 (A=K,Rb), in which a temperature-induced orbital-selective Mott transition was observed [3]. In the low temperature state, we observe an orbital-dependent renormalization for the bands near the Fermi level in which the dxy bands are heavily renormliazed compared to the dxz/dyz bands. Upon increasing temperature to above 150K, the spectral weight of the dxy bands diminishes while the dxz/dyz bands remain metallic. Combined with theoretical calculations [4], our observations can be consistently understood as a temperature induced crossover from a metallic state at low temperature to an orbital-selective Mott phase at high temperatures. These two examples suggest that the orbital degree of freedom plays an important role in determining the physical properties and possibly the mechanism of high temperature superconductivity in iron-based superconductors. [1] Ming Yi, Donghui Lu, Jiun-Haw Chu, James G. Analytis, Adam P. Sorini, Alexander F. Kemper, Brian Moritz, Sung-Kwan Mo, Rob G. Moore, Makoto Hashimoto, Wei-Sheng Lee, Zahid Hussain, Thomas P. Devereaux, Ian R. Fisher, and Zhi-Xun Shen, PNAS 108, 6878 (2011). [2] M. Yi, D. H. Lu, R. G. Moore, K. Kihou, C.-H. Lee, A. Iyo, H. Eisaki, T. Yoshida, A. Fujimori and Z.-X. Shen, New Journal of Physics 14, 073019 (2012). [3] M. Yi, D. H. Lu, R. Yu, S. C. Riggs, J.-H. Chu, B. Lv, Z. Liu, M. Lu, Y.-T. Cui, M. Hashimoto, S.-K. Mo, Z. Hussain, C. W. Chu, I. R. Fisher, Q. Si, and Z.-X. Shen, preprint at http://arxiv.org/abs/1208.5192. [4] Rong Yu, Qimiao Si, preprint at http://arxiv.org/abs/1208.5547. |

Dr. Pallab Goswami National High Magnetic Field Laboratory Conventional topological states have gapped spectrum, and their low energy universal properties are efficiently described by appropriate topological field theories, with quantized coefficients. Some well studied examples are the Chern-Simons theory of quantum Hall states, and the axion electrodynamics of (3+1)-dimensional strong topological insulators. In contrast, (3+1)-dimensional Weyl semi-metal is a gapless system, with non-trivial momentum space topology. I will first demonstrate that axionic field theory also describes the universal properties of Weyl semi-metal, but without a quantized coefficient. I will further show that axionic field theory properly encodes the bulk-boundary correspondence and provides correct description of the chiral surfaces states of the Weyl semi-metal. If time permits, I will show why in contrast to recent claims in the literature, the gravitational chiral anomaly fails to capture anomalous thermal Hall effect (quantized or non-quantized). In (2+1)-dimensions, I will consider a toy model of anyonic quantum Hall plateau transitions, and analyze certain aspects of topological phase transitions. Based on our recent results on relativistic Chern-Simons theory, the emergence of a semi-circle law for the conductivity tensor will be demonstrated. |

Prof. Pengcheng Dai University of Tennessee, Knoxville High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising since these undoped states display dramatically different antiferromagnetic (AF) spin arrangements and Neel temperatures. Although there is general consensus that magnetic interactions are important for superconductivity, much is still unknown concerning the microscopic origin of the magnetic states. In this talk, progress in this area is summarized, focusing on the evolution of spin dynamics in iron pnictides and discussing their microscopic implications. It is concluded that the parent compounds are in a state that is more complex than implied by a simple Fermi surface nesting scenario, and a dual description including both itinerant and localized degrees of freedom is needed to properly describe these fascinating materials. |

Prof. N. Peter Armitage Johns Hopkins University Topological insulators (TIs) are newly discovered states of matter characterized by an ��inverted�� band structure driven by strong spin-orbit coupling. One of their most touted properties is the existence of robust "topologically protected" surface states. I will discuss what topological protection means for transport experiments and how it can be probed using the technique of time-domain THz spectroscopy applied to thin films of Bi2Se3. By measuring the low frequency optical response, we can follow their transport lifetimes as we drive these materials through instabilities either by doping through a quantum phase transition into a topologically trivial regime or by reducing the film thickness. I'll also discuss our work on the magnetic field dependence of the Kerr rotation in Bi2Se3, where we find an unprecedentedly large value of the angle of rotation of reflected light, which is due to the cyclotron resonance of the 2D Dirac fermions. |

Dr. L. Andrew Wray Lawrence Berkeley National Laboratory Recent theoretical studies have predicted a number of topologically ordered states of matter that can appear in bulk crystalline materials. I will talk about X-ray photoemission measurements that have been used to identify the first known instances of topological states termed the topological insulator (TI), topological superconductor (TSC), and topological crystalline insulator (TCI). The presentation will review some current ideas about how these states of matter can potentially enable new technologies, how they are experimentally identified, and why they have hidden in plain sight in materials that were already widely studied. I will conclude by discussing possibilities enabled by new X-ray technologies, including an experimental frontier in which quantum topologies can be custom-created by manipulating materials with resonance-tuned X-rays. |

Dr. Rong Yu Rice University There is an emerging consensus that electron correlations play an important role in the iron-based superconductors. However, precisely how the correlation effects operate in these systems is still an open question. In this talk, I will addressed this issue in a set of multiorbital models pertinent to these materials. By using slave-spin/rotor methods, I show that the combination of Hubbard and Hund's couplings give rise to metal-to-Mott-inslator transitions as the strength of the interactions is varied in the parent materials. The calculations suggest that the parent compounds of iron pnictides and alkaline iron chalcogenides are both close to the Mott transition, but are located on two sides of the transition. This provides a unified picture in understanding the properties of the two families of iron-based superconductors. I also show that the correlation effects can be strongly orbital dependent, leading to a novel orbital-selective Mott phase in the alkaline iron chalcogenides. One of the direct experimental signatures of this orbital-selective Mott phase is the diminishing of the quasiparticle spectral weight of the 3d xy orbital, which has been observed in a recent ARPES measurement of the alkaline iron selenides. |

Dr. Pavlo Zolotavin University of Chicago TBA |

Prof. Takhee Lee Gwangju Institute of Science & Technology (GIST), Korea TBA |

Dr. Tzen Ong Rutgers University TBA |

Dr. Deepak Iyer Rutgers University There is sufficient evidence, both experimental and theoretical, that integrable models differ from nonintegrable ones in their time evolution following a quench. The difference is believed to be due to the large number of additional conserved quantities that such models possess. However, it is difficult to obtain even the long time behavior of interacting integrable models exactly for a variety of reasons. In this talk, I will describe a particular formalism based on the Bethe Ansatz that we have been working on. The formalism has been successful in understanding certain regimes of the time evolution of interacting bosons. Two interesting features we notice there are the fermionization of a repulsively interacting gas, and the appearance of bound states for the attractive gas. We have also recently extended the formalism to spin chains and I will present some preliminary results where again we can explicitly study the role of bound states. I'll end with related work that is underway, and chart out a possible path for the future. |

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