量子宇宙物理和超统一场论—吴岳良|本周物理学术讲座

1量子宇宙物理和超统一场论

报告人：吴岳良，中国科学院理论物理研究所

时间：8月29日（周三）15:00

单位：中科院物理研究所

地点：M楼234报告厅

探索极小量子粒子与极大膨胀宇宙的内在联系，理解由引力效应发现的暗物质和暗能量以及与时空结构相关的引力本质和宇宙起源，建立引力与其它三种基本作用力的统一理论被认为是21世纪基础科学最具挑战的悬而未决的科学问题。统一场论的建立必将对时空观念、几何观念、宇宙观念以及物质观念和能量观念提出新的挑战。引力波的精确测量和引力宇宙的深入研究将对人类认知宇宙的起源和演化，物质的起源和运动，甚至生命的起源和进化起到重要的作用。

2Quantum anomalies of time-reversal and mirror-reflection symmetries in topological orders

报告人：Chenjie Wang，City University of Hong Kong

时间：8月28日（周二）14:00

单位：清华大学物理系

地点：物理系理科楼C302

In quantum field theories, anomalous symmetries are those that are preserved classically but violated in the full quantum-mechanical theory. One way to avoid the violation is to put the theory on the boundary of a bulk theory in one higher dimensions. In condensed matter physics, these bulk systems are known as Symmetry-Protected Topological (SPT) phases (famous examples include 3D topological insulators/superconductors). Because of the recent development of SPT phases, we now have a better understanding of anomalies associated with various symmetries. In this talk, I will discuss quantum anomalies of time-reversal and mirror-reflection symmetries in 2D gapped topologically ordered systems, where excitations carry exotic properties such as fractional statistics and fractional charges.

3Why holes are necessary for superconductivity

报告人：Jorge E. Hirsch，UCSD

时间：8月28日（周二）15:00

单位：中科院物理研究所

地点：M楼238报告厅

The empirical observation that superconductivity is favored in materials that have hole carriers was made already in the 1930's, but no explanation for it was found at the time. BCS theory, developed in 1957, makes no distinction between electrons and holes. The discovery of high Tc cuprates in 1986 highlighted the important role of hole carriers in many materials, yet it is still generally believed today that both electron and hole carriers can give rise to superconductivity. Instead, we have proposed since 1989 that superconductivity cannot exist without hole carriers. I will review the vast empirical evidence indicating that holes are necessary for superconductivity including the elements, transition metal alloys, A-15 materials, doped semiconductors, hole- and electron-doped cuprates, MgB2, iron pnictides and chalcogenides, and elements under high pressure. 

4Inhomogeneous phases in QCD & Introduction to neutron star physics：Inhomogeneous phases in QCD

报告人：Abuki Hiroaki，Aichi University of Education

时间：8月28日（周二）15:00

单位：中科院高能物理研究所

地点：312 Theoretical Physics Division

5μSR studies of unconventional Fe and Cr-based superconductors

报告人：Devashibhai Adroja，Rutherford Appleton Laboratory

时间：8月29日（周三）10:00

单位：中科院物理研究所

地点：M234会议室 

Muon spin relaxation/rotation (μSR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remains a puzzle. In this talk, I will present a brief review of a few selected Fe- and Cr-based superconducting materials and highlight some of the major outstanding problems with an emphasis on the superconducting pairing symmetries revealed by μSR technique

6 Structural and Magnetic Evolution of Co Substituted BiFeO3

报告人：Hajime Hojo，Kyushu University

时间：8月29日（周三）15:00

单位：中科院物理所

地点：物理所M楼238

BiFeO3 is attracting keen interest because of large electric polarization and possible multiferroic applications. However, the piezoelectric constant is as small as 60 pm/V and the presence of cycloidal spin modulation prohibits the appearance of spontaneous magnetization. The effect of Co substitution for Fe on crystal and magnetic structures of BiFe1-xCoxO3 will be discussed. The crystal structure changes from rhombohedral one to giant tetragonal BiCoO3 type at x~0.35. A monoclinic phase with 2a 2a a unit cell and Cm space group, essentially the same as those of MPB phase of PZT, was found at x~0.3. Synchrotron X-ray powder diffraction revealed the occurrence of polarization rotation as functions of composition and temperature. Moreover, the enhancement of piezoelectric response was confirmed in the epitaxial thin films fabricated by PLD method. At lower Co substitution, the rhombohedral crystal structure is preserved while the cycloidal spin modulation disappears at a room temperature.Spontaneous magnetization generated by Dzyaloshinsky-Moriya interaction appears. It is demonstrated that the ferroelectric domain switching is accompanied by the magnetization reversal.

7 Orbital-Selective Electrons in Iron-Based Superconductors

报告人：Andreas Kreisel，University of Leipzig

时间：8月30日（周四）10:30

单位：清华大学物理系

地点：物理系理科楼C302

Iron-based superconductors offer an opportunity to study the physical processes that are responsible for high-temperature superconductivity. The quantum-mechanical processes in these materials lead to a large number of effects and allows for various stable phases. Theoretically, it seems that investigations are stuck because of a fundamental issue. It is unknown whether the electronic states should be treated in an itinerant approach or using the opposite picture where electrons are almost localized. Guided by recent experiments in the superconducting phase of iron-based superconductors, we discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. 

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