报告人：李涛，中国人民大学

摘要：We discuss the origin of the pseudogap phenomena in the cuprate superconductors by analyzing three recent ARPES observations from the perspective of the spin-Fermion model. Firstly, we show that the recent observation of the vanishing of the pseudogap around (π,0) in the electron-doped cuprate Pr1.3−xLa0.7CexCuO4 is consistent with the AF band folding picture of the pseudogap in the electron-doped cuprates, provided that we assume a strongly momentum dependent quasi-particle scattering rate on the Fermi surface [1,2]. Secondly, we argue that the pseudogap in the hole-doped cuprates is unlikely an AF band folding gap, since the spin fluctuation in the hole-doped cuprates is much more short-ranged and dynamical in nature than that in the electron-doped cuprates. In particular, we show that electron pairing is indispensable to eliminate the Fermi level crossing along (π,0)-(π,π) in a way that is consistent with the ARPES observation in the under-doped Bi-2201 system around T* [3,4]. Nevertheless, we find that the AF spin fluctuation in the hole-doped cuprates is responsible for the emergence of the broad high energy hump structure in the anti-nodal region, and the mismatch between the back-bending momentum of the hump maximum and the underlying Fermi momentum along (π,0)-(π,π), and in particular, the extremely flatness of the anti-nodal quasi-particle dispersion in the superconducting state [5,6]. We argue that the particle-hole symmetry breaking should better be studied in the energy space, rather than in the momentum space [3]. Lastly, we show that the recent observation of the critical behavior at the pseudogap end point is consistent with the spin-Fermion picture [7,8], even though the local spin in the cuprates is far from critical at such a high doping level, if one note the singular AF response of the quasi-particle system at the VHS doping.
[1] Hario et al, arXiv: 1801.04147.
[2] Tao Li and Da-Wei Yao, arXiv: 1803.08226 and EPL(2018)。
[3] Hashimoto et al, Nat. Phys. 6, 414 (2010).
[4] Tao Li and Da-Wei Yao, arXiv: 1805.05530.
[5] Rui-Hua He et al Science 331, 1579 (2011).
[6] Tao Li and Da-Wei Yao, arXiv: 1805.04883.
[7] Michon et al, arXiv: 1804.08502.
[8] Tao Li, arXiv: 1805.06395.

报告人简介：李涛，中国科学技术大学物理系低温物理专业本科、硕士，北京大学物理系凝聚态物理专业博士，清华大学高等研究院博士后、副研究员，2005年起担任中国人民大学物理系教授。主要从事高温超导机理和量子磁性的基础理论研究，研究方法包括低能有效理论、变分蒙特卡洛模拟以及唯象理论分析，对于量子自旋液体的变分搜索和低能有效理论构造、强关联电子系统动力学谱的变分构造和算法发展、以及高温超导体电子能谱的唯象理论分析有长期和较深入的研究。

时间：2019年6月13日（周四）下午2:00
地点：M830会议室
邀请人：杨义峰

报告人： Marcus Kollar，University of Augsburg

摘要：When an isolated quantum many-body system is suddenly forced out of equilibrium, it is expected to relax to the thermal state predicted by statistical mechanics, which depends only on energy and particle number. This behavior is indeed observed, e.g., for intermediate interaction quenches in Hubbard models. On the other hand, integrable systems usually relax instead to a nonthermal state, because a detailed memory on the initial conditions persists due to the many constants of motion. Weakly interacting systems can at first be trapped in a so-called prethermalized state due to the proximity of an integrable, noninteracting Hamiltonian. I will discuss examples and concepts for these phenomena.

时间：2019年4月11日（周四）上午10:00
地点：M830会议室
邀请人：杨义峰

报告人： 石弢，中国科学院理论物理研究所

摘要：We present a new variational method for investigating the ground state and out of equilibrium dynamics of quantum many-body bosonic and fermionic systems. Our approach is based on constructing variational wavefunctions which extend Gaussian states by including generalized canonical transformations between the fields. The key advantage of such states compared to simple Gaussian states is presence of non-factorizable correlations and the possibility of describing states with strong entanglement between particles. In contrast to the commonly used canonical transformations, such as the polaron or Lang-Firsov transformations, we allow parameters of the transformations to be time dependent, which extends their regions of applicability. We derive equations of motion for the parameters characterizing the states both in real and imaginary time using the differential structure of the variational manifold. The ground state can be found by following the imaginary time evolution until it converges to a steady state. Collective excitations in the system can be obtained by linearizing the real-time equations of motion in the vicinity of the imaginary time steady-state solution. Our formalism allows us not only to determine the energy spectrum of quasiparticles and their lifetime, but to obtain the complete spectral functions and to explore far out of equilibrium dynamics such as coherent evolution following a quantum quench. We illustrate and benchmark this framework with several examples: Kondo models and lattice gauge theory.

时间：2018年9月3日（周一）上午10:00
地点：M830会议室
邀请人：杨义峰

报告人： 全海涛，北京大学物理学院

摘要：By introducing a novel concept of work functional along individual "path", we reformulate the two point measurement scheme in quantum thermodynamics based on the path integral formulation of quantum mechanics. We can also apply this formalism to an open quantum system in the strong coupling regime described by the quantum Brownian motion model. Using the work functional, we derive a path-integral expression for the work statistics. This formalism provides an effective way to calculate the work in open quantum systems by utilizing various path integral techniques. By performing the \hbar expansion, we analytically prove the quantum-classical correspondence of the work statistics. In addition, we obtain the n-th order quantum correction to the classical work.

时间：2017年10月23日（周一）下午 2:00
地点：M830会议室
邀请人：杨义峰

报告人： Dr. Stefan Kirchner，Center for Correlated Matter, Zhejiang University

摘要： We address the origin of the magnetic-field independent -|A|T^1/2-term observed in the low-temperature resistivity of several As-based metallic systems of the PbFCl structure type. For the layered compound ZrAs_1.58Se_0.39, we show that vacancies in the square nets of As give rise to the low-temperature transport anomaly over a wide temperature regime of almost two decades in temperature. This low-temperature behavior is in line with the non-magnetic version of the two-channel Kondo effect. I show that the two-channel Kondo effect can arise due to a dynamic Jahn-Teller effect at the vacancy-carrying As layer and that it is symmetry-protected against level splitting. The pair-breaking nature of these dynamical defects in the square nets of As explains the low superconducting transition temperature Tc ≈ 0.14K of ZrAs_1.58Se_0.39, as compared to the free-of-vacancies homologue ZrP_1.54S_0.46 (Tc ≈ 3.7K). Our findings should be relevant to a wide class of metals with disordered pnictogen layers.

时间：2016年12月14日（周三） 14：00
地点：M830会议室
邀请人：李永庆、杨义峰

报告人： Dr. Hanoh Lee，Zhejiang University

摘要：Ce₃Au₃Sb₄ has diverging specific heat coefficient at low temperature in a semiconducting state, a property which can be approached differently within the Kondo and band insulator viewpoints. Sample sensitivity here presents difficulty for determining the intrinsic behavior of this system. I will present physical properties under pressure and magnetic fields down to low temperature to discuss its unique physical properties which still need more understanding on its nature.
   Newly synthesized single crystal of YbCo₆Ge₆ displays a large enhancement of the specific heat coefficient below 10K, followed by a broad peak-like feature below 1K, which reaches a maximum around 0.55K, indicating a possible enhancement of fluctuations near a transition that is close to zero temperature. However, the temperature dependence of the resistivity shows good metallic behavior with a moderate Kondo effect, which is typical behavior of a localized Kondo lattice system. The origin of the large specific heat coefficient and broad peak around 0.55K will be discussed as well as the magnetic, thermal, and transport properties.

 报告人简介：

   Hanoh Lee graduated UC Davis under supervision of Prof. Zachary Fisk. First postoctorial work at LANL with Dr. Joe D. Thompson; Second postdoctorial work at Stanford with Prof. Ian Fisher. She worked at Global Foundries at a senior engineer position in Technical development department for 1 year. Currently, She is in Zhejiang university as a tenure track in Center for Correlated matter.

时间：2016年 9月27日（周二） 14：00
地点：M253会议室
邀请人：杨义峰 （9448）

报告人： Dr. Lin Jiao，Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany

摘要：SmB₆ has been recently proposed as a topological Kondo insulator, which possesses topological nontrivial surface states in the Kondo band gap. However, the experimental results are still controversial among different reports after such intensive investigations. Scanning tunneling microscopy (STM) has the unique capability of providing combined topographic and spectroscopic information. We conducted STM on numerous samples cleaved in {001} plane at low temperature. Various surface structures have been observed on this sample, including non-reconstructed surface, 2x1 reconstructed surface and disordered surface. For non-reconstructed surface the dI/dV-curves can be well described by a Fano resonance above 5K. However, a pronounced in-gap state around -2.5mV was detected at low temperature. On the other hand, both the hybridization gap and the in-gap state are very robust against external magnetic field, with no change up to 12T. All types of surfaces, reconstructed and non-reconstructed, displayed a finite zero-bias conductance of considerable magnitude. This finding, in spite of different surface topologies, confirms the robustness of the metallic states.

 报告人简介：Dr. Lin Jiao is now a postdoctoral research fellow in Max Planck Institute for Chemical Physics of Solids at Dresden. Starting from 2015, his research is supported by the Humboldt Research Fellowship for Postdoctoral Researchers. He got his Ph.D. degree in physics from Zhejiang University at 2014. His Ph.D. thesis is focused on the transport properties under extreme conditions (e.g. low temperature, strong magnetic field and high pressure) of strongly correlated materials, such as heavy fermions, iron pnictides, as well as noncentrosymmetric superconductors. Now, he focuses on the study of some novel materials by utilizing STM/STS, combining with low temperature and high magnetic field.

2015年 时间：7月20日（周一） 10：00
地点：M253会议室
邀请人：杨义峰 （9448）
联系人：孙培杰 （8081）