June 11
1. 14:00-15:30
Venue：E10-222
Speaker: Prof. Wei Li, Institute of Theoretical Physics, Chinese Academy of Sciences
Language: Chinese
Title: Tensor Network Approaches for Hubbard Model
Abstract: Accurate calculations of the two-dimensional (2D) Hubbard model constitute one of the most challenging problems in condensed matter and quantum physics. Thermal tensor network provides a viable approach for 2D Hubbard model at finite temperature. In this talk, I will introduce the finite-temperature tensor network methods we developed in the past years, including the exponential tensor renormalization group (XTRG) and the tangent space tensor renormalization group (tanTRG) for the calculations of the 2D Hubbard and t-J models. Applications of zero- and finite-temperature tensor network approaches to single-layer model for cuprates and bilayer model for 327 nickelates will also be discussed.
2. 15:45-17:15
Venue：E10-222
Speaker: Prof. Xiaoquan Yu, China Academy of Engineering Physics
Language: Chinese
Title: Ferrodark solitons in a spinor superfluid: exact solutions, novel speed limit and anomalous dynamics
Abstract: Exact propagating topological solitons are found in the easy-plane phase of ferromagnetic spin-1 Bose-Einstein condensates, manifesting themselves as kinks in the transverse magnetization. Such topological defects are referred to as ferrodark solitons (FDSs). Propagation is only possible when the symmetry-breaking longitudinal magnetic field is applied. Ferrodark solitons have two types: a low energy branch (type-I) with positive inertial mass and a higher energy branch (type-II) with negative inertial mass. Both types become identical at the maximum speed, a new speed bound that is different from speed limits set by the elementary excitations. In a finite one-dimensional system subject to a linear potential, the soliton undergoes oscillations caused by transitions between the two types occurring at the maximum speed. In two dimensions, a line type-II FDS exhibits a snake instability and a spin-twist instability, with the latter involving the generation of out of plane spin winding. Distinct from the normal dynamics of negative mass solitons under long wave length transverse perturbations, the snake instability does not lead to the type-II FDS breaking down. Instead, segments of the type-II FDS convert to type-I and mass vortex dipoles are produced. The resulting hybridized-chain of the two soliton types and vortices exhibits complex 2D soliton dynamics at long times while the vortices remain confined and the topological structure of a magnetic domain wall is preserved.
June 12
1. 9:30-11:00
Venue：E10-405
Speaker: Prof. Wang Yang, Nankai University
Language: Chinese
Title: Characterization of computational power in measurement-based quantum computation
Abstract: An important progress in the field of measurement-based quantum computation (MBQC) in the past decade is the discovery of one-dimensional (1D) symmetry protected topological states as MBQC resource states. In a recent work [Quantum 7, 1215 (2023)] by Robert Raussendorf et. al., a deep connection between string order parameters in condensed matter physics and a computational order parameter for MBQC is found, where the computational order parameter can be used to quantitatively characterize the computational power of a many-body spin-1/2 system. In this talk, I will discuss the generalization of the framework to qudit systems, and show that the computational order parameter exactly matches with the conventional string order parameter.
2. 15:30-17:00
Venue：E10-405
Speaker: Prof. Lin Jiao, Zhejiang University
Language: Chinese
Title: High-Tc superconductivity with zero resistance and strange metal behavior in La3Ni2O7
Abstract: Recently, signatures of superconductivity were observed close to 80 K in La3Ni2O7 under pressure. This discovery positions La3Ni2O7 as the first bulk nickelate with high-temperature superconductivity, but the lack of zero resistance presents a significant drawback for validating the findings. In this talk, I will report transport measurements under pressure up to 30 GPa using a liquid pressure medium. We observed clear evidence for superconductivity in single crystals of La3Ni2O7 with zero resistance. Analysis of the normal state T-linear resistance suggests an intricate link between this strange metal behaviour and superconductivity, whereby at high pressures both the linear resistance coefficient and superconducting transition are slowly suppressed by pressure, while at intermediate pressures both the superconductivity and strange metal behaviour appear disrupted, possibly due to a nearby structural instability. These results provide a modified p-T phase diagram for La3Ni2O7. I will also discuss the connections between strange metal behaviour and high-temperature superconductivity as observed in many other unconventional superconductors.
Ref. Y.N. Zhang et al., arxiv:2307.14819.
June 14
1. 14:00-15:30
Venue：E10-304
Speaker: Prof. Wei Wu, Sun Yat-Sen University
Language: Chinese
Title: On the Superconducting Instabilities in the Bilayer Two-Orbital Hubbard- Kanamori Model of La3Ni2O7.
Abstract: The mechanism behind the high-temperature superconductivity in the recently discovered nickelate La3Ni2O7 is currently under heated debate. In this work, by employing cluster dynamical mean-field theory, we systematically investigate superconductivities in the bilayer two-orbital Hubbard- Kanamori model of La3Ni2O7. We show that for moderate hole concentration levels of the d_{z^2} orbital, a two-component superconductivity driven by d_{z^2} spin correlations predominates, which remains robust against changing various physical parameters including Hund's coupling J, d_{x^2-y^2}-d_{z^2} orbital hybridization V, and the value of Hubbard U. As the d_{z^2} orbital approaches half-filling and Hund's coupling increases, a Hund's superconductivity that originates from spin correlations transferred from interlayer d_{z^2} to d_{x^2-y^2} orbitals via Hund's coupling, emerges. These two supercondutivies have comparable maximum superconducting transition temperatures and are both S+/- wave, while they exhibit distinct dependences on d_{z^2} hole concentration and d_{x^2-y^2}-d_{z^2} hybridization strength V. We find no dominant d-wave pairing instablity in the La3Ni2O7 parameter regime. Increasing the splitting between the two d_{x^2-y^2} bands does not lead to significant enhancenment of d-wave pairing in our study. Systematic superconducting phase diagrams are established in our study.
2. 15:45-17:15
Venue：E10-304
Speaker: Prof. Yuefeng Nie, Nanjing University
Language: English
Title: Exploring superconductivity in nickelate films by molecular beam epitaxy
Abstract: The observation of unconventional superconductivity in layered nickelates with infinite-layer and Ruddlesden-Popper (RP) structures has attracted significant attention, as it provides new opportunities to uncover the underlying mechanisms of unconventional high-temperature superconductivity. In this talk, I will present our recent study of infinite-layer nickelate films synthesized by molecular beam epitaxy (MBE) and in situ atomic-hydrogen reduction. The crystalline structure, dimensionality, superconducting gap, and momentum-resolved electronic structure were explored, revealing some similarities and distinct differences between cuprates and infinite-layer nickelates. If time permits, I will also briefly discuss our exploration of RP nickelate films by MBE.
