July 14
1. 14:00-15:30
Venue：E10-222
Speaker: Prof. Lexian Yang, Tsinghua University
Language: Chinese
Title: Electronic structure and ultrafast dynamics of bilayer and trilayer Nickel-based superconductors
Abstract: High-temperature superconductivity (HTSC) remains one of the most challenging and fascinating mysteries in condensed matter physics. Recently, superconductivity with transition temperature exceeding liquid-nitrogen temperature is discovered in La3Ni2O7 at high pressure, which provides a new platform to explore the unconventional HTSC. In this work, using high-resolution angle-resolved photoemission spectroscopy and ultrafast reflectivity spectroscopy, we systematically investigate the electronic structures and ultrafast dynamics of La3Ni2O7 and La4Ni3O10 at ambient pressure. The measured electronic structures agree with ab-initio calculations after considering an orbital-dependent band renormalization effect. The strong electron correlation effect pushes a flat band of orbital component below the Fermi level (EF), which is predicted to locate right at EF under high pressure. Moreover, the band shows a pseudogap-like behavior with suppressed spectral weight and diminished quasiparticle peak near EF. While the electronic structures of La3Ni2O7 and La4Ni3O10 are very similar, their ultrafast dynamics are radically different. We will also discuss the different electron-phonon coupling effects in the two compounds. Our findings provide important insights into the electronic behaviors of La3Ni2O7 and La4Ni3O10, which will shed light on the understanding of the unconventional superconductivity in nickelates.
2. 15:45-17:15
Venue：E10-222
Speaker: Prof. Liling Sun, Center for High Pressure Science & Technology Advanced Research
Language: Chinese
Title: Investigations of key issues on the reproducibility of high-Tc superconductivity emerging from compressed La3Ni2O7
Abstract: Recently, the signatures of superconductivity near 80 K have been discovered in the single crystal of La3Ni2O7 under pressure1, which makes it a new candidate of the high-temperature superconductors dominated by 3d transition elements after the cuprate and iron-pnictide superconductors2-4, and thus has attracted significant attention. However, there are several critical issues that have been perplexing the scientific community. These include (1) what factors contribute to the poor reproducibility of the experimental results, (2) what the intrinsic nature of the pressure-induced superconductivity is, bulk or filamentary, (3) where the superconducting phase locates within the sample if it is indeed filamentary, and (4) what the oxygen content is necessary for the development and stabilization of superconductivity. In this study, we employ comprehensive high-pressure measurements to address these crucial issues. By demonstrating both zero resistance and diamagnetism, we are the first to confirm the existence of high-temperature superconductivity in La3Ni2O7. Through our sensitive ac susceptibility measurements, we are the first to quantify the superconducting volume fraction in La3Ni2O7 at the level of 1%. In tandem with our observation of the anisotropic zero-resistance state only in some of the samples, we suggest that the superconductivity in this nickelate is filamentary-like. By our scanning transmission electron microscopy (STEM) investigations, we propose that the filamentary superconductivity most likely emerges at the interface between the La3Ni2O7 and La4Ni3O10 phases. Further, the upper and lower bounds of the oxygen content required for the presence of superconductivity were determined to be 7.35 and 6.89, respectively. Our results provide not only new insights into understanding the puzzling issues in this material, but also significant information for achieving a better understanding on the superconductivity of this material.
July 15
1. 14:00-15:30
Venue：E10-222
Speaker: Prof. Weiqiang Chen, Southern University of Science and Technology
Language: Chinese
Title: RPA study of pairing mechanism of La3Ni2O7 and La4Ni3O10 materials
Abstract: Recently, evidence of superconductivity (SC) has been reported in La3Ni2O7 and La4Ni3O10 under a moderate pressure. Here we study the possible pairing mechanism and pairing symmetry in these materials. We perform a random-phase approximation based study, which reveal s±-wave pairing symmetry in both materials. We also discuss the role of apical-oxygen deficiencies in La3Ni2O7 material. Our results show that the Tc may be enhanced by eliminating oxygen deficiencies during the synthesis of the samples.
2. 15:45-17:15
Venue：E10-222
Speaker: Prof. Rong Yu, Renmin University of China
Language: Chinese
Title: Electron correlations in La3Ni2O7 under pressure tuning
Abstract: We address the recently discovered high temperature superconductivity in La3Ni2O7, by anchoring our description via the spectroscopic evidence of strong correlations in the system. We discuss the electron correlation effects in a bilayer Hubbard model including the Ni 3d x2-y2 and z2 orbitals. The system is found to exhibit strong orbital selectivity over a wide crossover regime. Our results provide a theoretical understanding of the spectroscopic and tranport experimental data.
July 16
1. 9:30-11:00
Venue：E10-304
Speaker: Prof. Dao-Xin Yao, Sun Yat-sen University
Language: Chinese
Title: Multiorbital models and superconducting properties of newly found nickelate superconductors
Abstract: The recently discovered Ruddlesden-Popper bilayer superconductor La3Ni2O7 has attracted widespread attention due to its superconducting transition temperature exceeding the boiling point of liquid nitrogen, reaching approximately 80 K under a pressure. The superconducting phase under high pressure has an orthorhombic structure of Fmmm space group with the 3dx2−y2 and 3dz2 orbitals of Ni cations strongly mixing with oxygen 2p orbitals. Our density functional theory calculations indicate that the superconductivity emerges coincidently with the metallization of the σ-bonding bands under the Fermi level, consisting of the 3dz2 orbitals with the apical oxygen ions connecting the Ni–O bilayers. Here we propose a bilayer two-orbital model for La3Ni2O7 under high pressure, primarily based on the 3dx2−y2 and 3dz2 orbitals of Ni. Through Wannier downfolding and symmetry analysis, we obtain parameters such as electron hopping and site-energy, which provide an excellent description of the electronic band structure and Fermi surface. We find that the Fermi surface of the high-pressure phase consists of three pockets, with α and β being electron pockets, and γ being a hole pocket (mainly originating from the 3dz2 orbital). To explicitly consider the physics of O-p orbitals, we introduce a higher energy model (eleven-orbital model). Based on these models, we study the charge transfer, Zhang-Rice singlet bands, pairing symmetry, and superconducting transition temperature in La3Ni2O7. We obtain a comprehensive superconducting phase diagram in the doping plane and find that the La3Ni2O7 under pressure is situated roughly in the optimal doping regime of the phase diagram. Recently, the discovery of superconductivity in Ruddlesden-Popper (RP) La4Ni3O10 under pressure has further expanded the realm of nickelate-based superconductor family. Based on the DFT results, we further propose a trilayer two-orbital model by performing Wannier downfolding on Ni-eg orbitals. Our model reveals four Fermi surface sheets with α, β, β′, γ pockets, bearing resemblance to that of bilayer La3Ni2O7. According to the model, our calculated spin susceptibility under random phase approximation predicts an analogous magnetic signal at q = (π/2, π/2), which is more associated with nesting within β, β′ pocktes. Finally, a high energy sixteen-orbital model with direct dp, pp hoppings is proposed, which implies that La4Ni3O10 also lies in charge-transfer picture within Zaanen-Sawatzky-Allen scheme. Our exposition of electronic reconstructions and multi-orbital models shed light on theoretical electronic correlation study and experimental exploration for lower pressure in RP series. References: Nature 621, 493 (2023)；Phys. Rev. Lett. 131, 126001 (2023)；Nature Communications 15，4373 (2024)；SCPMA 67, 117403 (2024)；SCPMA 67, 117402 (2024)；arXiv:2308.16564 (NPJ Quantum Materials accepted)；Phys. Rev. B 110, 014503 (2024); Chin. Phys. Lett. 41, 077402 (2024); arXiv:2405.19161.
2. 14:00-15:30
Venue：E10-304
Speaker: Prof. Wenxin Ding, Anhui University
Language: Chinese
Title: Algebraic-Dynamical Theory and Strong Coupling Perturbation for AFM Mott State of Hubbard Models and implication for nicklates parent states
Abstract: In this work, we present an analytical framework for studying antiferromagnetic (AFM) Mott insulating states in the Hubbard model. We first derive an analytical solution for the single-particle Green's functions in the atomic limit. Within a second-order perturbation approach, we compute the ground state energy and show that the ground state is antiferromagnetically ordered. Then we derive an analytical solution for single-particle Green's functions when effects of the hopping term are considered in the Neel state. With the analytical solution, we compute and explain various properties of antiferromagnetic Mott insulators observed both experimentally and numerically: i) magnetic blueshift of the Mott gap; ii) spectral functions with features comparable to observations by angle-resolved photoemission spectroscopy on parental compounds of cuprate high Tc superconductors. This work comprehends the electronic properties of antiferromagnetic Mott states analytically and provides a foundation for future investigations of doped antiferromagnetic Mott insulators, aiming for the mechanism of cuprates high-Tc superconductivity.
3. 15:45-17:15
Venue：E10-304
Speaker: Prof. Jiawei Mei, Southern University of Science and Technology
Language: Chinese
Title: Correlation effects and spin fluctuations in nickelate superconductors
Abstract: In this talk, I will explore the intriguing correlation effects and spin fluctuations in two compounds: Nd1-xSrxNiO2 and La3Ni2O7. The presentation will begin with an in-depth examination of Mott physics and the application of the effective t-J model to Nd1-xSrxNiO2. I will then investigate the phenomenon of s+/--wave pairing and the role of apical-oxygen deficiencies in pressurized La3Ni2O7, using calculations based on the Random Phase Approximation (RPA) in the context of Fermi surface nesting. Additionally, the local spin moments induced by oxygen vacancies will be further supported by DFT simulations.
July 17
1. 9:30-11:00
Venue：E10-222
Speaker: Prof. Wei Ku, Shanghai Jiao Tong University
Language: English
Title: Pressure driven fractionalization of ionic spins results in cuprate-like high-Tc superconductivity in La3Ni2O7
Abstract: Beyond 14GPa of pressure, bi-layered La3Ni2O7 was recently found to develop strong superconductivity above the liquid nitrogen boiling temperature. An immediate essential question is the pressure-induced qualitative change of electronic structure that enables the exciting high-temperature superconductivity. We investigate this timely question via a numerical multi-scale derivation of effective many-body physics. At the atomic scale, we first clarify that the system has a strong charge transfer nature with itinerant carriers residing mainly in the in-plane oxygen between spin-1 Ni2+ ions. We then elucidate in eV- and sub-eV-scale the key physical effect of the applied pressure: It induces a cuprate-like electronic structure via fractionalizing the Ni ionic spin from 1 to 1/2. This suggests a high-temperature superconductivity in La3Ni2O7 with microscopic mechanism and (d-wave) symmetry similar to that in the cuprates.
2. 14:00-15:30
Venue：E10-222
Speaker: Prof. Meng Wang, Sun Yat-sen University
Language: Chinese
Title: Experimental investigations on the nickelate high-Tc superconductors
Abstract: Since the discovery of superconductivity at 80 K in single crystals of La3Ni2O7 at pressures above 14.0 GPa [1-5], extensive efforts have been made to understand the properties of the bilayer nickelate system at both ambient and high pressures. CDW, SDW, structural transition, strange metal behavior, orbital dependent correlations, etc. which are profound in copper oxide and iron-based superconductors also present in the pressure-dependent phase diagram of La3Ni2O7. They may be related or irrelevant to the superconductivity of nickelates under pressure. Currently, many questions are open. In this talk, I will briefly introduce the discovery of the superconductivity in La3Ni2O7 and discuss the research progress in nickelate superconductors [6-13].
Reference:
[1] Z. Liu, H. L. Sun, M. W. Huo, et al., Sci. China-Phys. Mech. Astron. 66, 217411(2023).
[2] H. L. Sun, M. W. Huo, X. W. Hu et al., Nature 621, 493-498(2023)
[3] Y. N. Zhang, M. Wang, H. Q. Yuan et al., arXiv:2307.14819(Nature Physics, 2024)
[4] J. Hou, M. Wang, J. G. Cheng et al., Chinese Physics Letters 40, 117302(2023)
[5] Y. Z. Zhou, J. Guo, S. Cai et al., arXiv: 2311.12361 (2023)
[6] L. H. Wang, Y. Li, S. Y. Xie et al., JACS 146, 7506(2024)
[7] Z. Liu, Y. M. Dai, M. Wang, H. H. Wen et al., arXiv:2307.02950(2023)
[8] J. Yang, L. Zhao, M. Wang, X. J. Zhou et al., Nat. Commnu. 15, 4373(2024)
[9] J. Li, C. Chen, C. Huang et al., Sci. China-Phys. Mech. Astron. 67, 117403 (2024)
[10] Z. H. Dong et al., Nature 630, 847 (2024)
[11] T. Xie M. Wang et al., arXiv:2401.12635(2024)
[12] D. L. Feng, K. J. Zhou et al., arXiv:2401.12657(2024)
[13] D. Zhao, T. Wu, X. H. Chen at al., arXiv:2402.03952(2024)
3. 15:45-17:15
Venue：E10-222
Speaker: Prof. Huiqiu Yuan, Zhejiang University
Language: Chinese
Title: Superconductivity, strange metal behavior and quantum criticality
Abstract: In strongly correlated electron systems, the electron interactions can be efficiently tuned by applying either pressure, magnetic field or doping, giving rise to emergent quantum phases or critical behaviors near a quantum critical point. In this talk, I will overview our recent progresses on the studies of heavy fermions and nickelate superconductors [1-5], including unconventional superconductivity, strange metal behavior and quantum phase transitions. A brief comparison of the strange metal behavior among various correlated electron systems will be provided.
References:
[1] Z. F. Weng et al., Rep. Prog. Phys. 79, 094503 (2016).
[2] M. Smidman et al., Rev. Mod. Phys. 95, 031002 (2022).
[3] L. Jiao et al., PNAS 112, 673 (2015).
[4] B. Shen et al., Nature 579, 51 (2020).
[5] Y. N. Zhang et al., arXiv: 2307.14819; Nature Physics (2024).
July 18
1. 9:30-11:00
Venue：E10-222
Speaker: Prof. Fa Wang, Peking University
Language: Chinese
Title: Gutzwiller approximation study of magnetism and superconductivity in the t-J model of La3Ni2O7
Abstract: 我们假设高温超导体La3Ni2O7的母体为完全由高自旋S=1的Ni2+构成的双层正方晶格，在此基础上导出了对dx2-y2轨道进行空穴掺杂得到的有效t-J模型，并利用多带Gutzwiller近似方法研究了这个模型中可能的超导和磁性基态，发现在低掺杂区域可以存在G型反铁磁和s波超导的共存，在高掺杂区域基态为A型反铁磁。我们的工作首次使用统一的量子多体理论方法处理了 La3Ni2O7相关模型中可能的超导和磁性的竞争与共存，突显了在这类模型中考虑磁性的必要性。
2. 14:00-15:30
Venue：E10-222
Speaker: Prof. Yi-feng Yang, Institute of Physics, Chinese Academy of Sciences
Language: Chinese
Title: Theory of nickelate superconductors and new route to high-temperature superconductivity
Abstract: 我将简要介绍我们针对无限层、双层、三层镍氧化物超导提出的理论图像，包括无限层镍氧化物的Mott-Kondo理论，双层镍氧化物超导的两分量理论，和三层镍氧化物的阻挫超导，说明镍氧化物中Mott、Hund、Kondo物理共存和相互影响，是导致其独特超导性质的根源。其中两分量理论是一种新型高温超导机制，由接近半满的dz2轨道通过层间超交换相互作用（J）形成局域的层间自旋单态配对，再与dx2-y2金属能带杂化诱导层内相位相干而发生超导。我们的理论预言了与实验一致的超导相图和最大Tc，并解释了Tc从双层到三层发生抑制的原因。我们进而对一类较为广泛的模型进行了模拟，并系统比较了镍氧化物、铜氧化物、铁基、重费米子等多个非常规超导体系的实验数据，发现最大Tc会受到强耦合极限的限制，可能不超过0.04-0.07J的量级，从而对室温超导探索提出了新的要求。综合上述考虑，我们提出一种基于两分量图像的从实空间出发构建新型高温超导体的新思路，和大家讨论其可行性。
3. 15:45-17:15
Venue：E10-222
Speaker: Prof. Zhen Chen, Institute of Physics and University of Chinese Academy of Sciences
Language: Chinese
Title: Nanoscale inhomogeneities of oxygen vacancies and ligand holes in La3Ni2O7 superconductors
Abstract: La3Ni2O7 superconductor is one of the typical Ruddlesden-Popper phases, which can easily exhibit structural disorders such as stacking faults and chemical off-stoichiometry. Such structural inhomogeneities are still the major obstacle in obtaining high quality samples for studies of the mechanism of high Tc in these nickelates. In this talk, I will present a comprehensive structural investigation of La3Ni2O7 by further developing our new record-breaking technique, electron ptychography in electron microscopy. We find that oxygen vacancies are mainly located in the inner apical oxygen sites between the two NiO2 layers. We quantify the concentration of oxygen vacancies at the atomic level for the first time and find that oxygen vacancies vary significantly at the sub-100 nm length scale, even in what are generally considered to be high quality single crystal samples. This may be the main reason for the low superconducting volume estimated from magnetic susceptibility. We also directly correlate the structural inhomogeneity with the ligand holes from O-K edge electron energy loss spectroscopy. Atomic-plane-resolved O-K spectra show that the ligand holes are preferentially distributed in the inner apical oxygen and NiO2 planes. These results provide important guidelines for further improving sample quality and facilitate the understanding of superconductivity in nickelates.
