学术报告

Variational Study of Fermionic and Bosonic Systems with Non-Gaussian States: Theory and Applications

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

Path Integral Approach to Quantum Thermodynamics

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

Fermi Arcs and Bulk Transport in Weyl Semimetals

Two-channel Kondo physics due to As vacancies in the layered compound ZrAs

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_{1.58}Se_{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_{1.58}Se_{0.39}, as compared to the free-of-vacancies homologue ZrP_{1.54}S_{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

Fermi Arcs and Bulk Transport in Weyl Semimetals

Unusual physical properties of Ce

报告人： Dr. Hanoh Lee

Zhejiang University

摘要：

Ce_{3}Au_{3}Sb_{4} 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 YbCo_{6}Ge_{6} 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.

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）

老引力，新引用

从引力波闲谈科普与研究

Cd-Doping in Heavy Fermion Critical Superconductors CeCoIn

Surface structure of SmB

报告人： Dr. Lin Jiao

Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany

摘要：

SmB_{6} 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.

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）

Quantum criticality and superconductivity in heavy fermion metals

Applications of Muon Spin Relaxation/Rotation for Superconductivity

A Family of Heavy-Fermion Compounds Built from CeIn

Yi-feng Yang, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, China

Tel: (86)-10-82649448, (86)-18911600270； Fax: (86)-10-62553698；Email: yifeng@iphy.ac.cn