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Academic Exchange

Time and place: 2022.1.11, 9:30 am, VooV Meeting

Presenter: Biao Lian

Title: Integrability and chaos of 1+1d chiral edge states

Abstract:I will talk about the integrability and chaos of 1+1d interacting chiral edge states, which may arise on the edge of 2+1d topological states of matter. We show that chiral Luttinger liquid is not always a good low energy description of the edge states, and marginal interactions can significantly affect their spectrum and integrability. We first study N identical chiral Majorana fermion modes with random 4-fermion interactions, where we show that the system undergoes a transition from integrable to quantum chaotic at N=7. The large N limit defines a chiral SYK model, where the Lyapunov exponent in the out-of-time-ordered correlation can be solved analytically. I will also present a chiral SY model which has Abelian anyon charge excitations and exhibits similar quantum chaos. Lastly, I will talk about the analytical and numerical study of the 4/3 FQH edge theory, which shows unusual behavior in its integrability.


Time and place: 2021.9.10, 16:00 pm, Wuhan National High Magnetic Field Center B206

Presenter: Huiqiu Yuan

Title: Magnetic Quantum Phase Transitions

Abstract:In correlated electron systems, a magnetic transition can be continuously suppressed to zero temperature upon applying a non-thermal parameter such as pressure, magnetic field or doping, giving rise to a quantum critical point (QCP), around which unconventional superconductivity and non-Fermi liquid behavior may appear. In this talk, I will briefly present our recent progresses on the studies of magnetic quantum criticality in the d- and f-electron compounds, with focus on the antiferromagnetic quantum criticality in CeRh6Ge4 and CdAs, tuned by multiple parameters; and the discovery of ferromagnetic quantum criticality and its associated strange metal behavior in a clean heavy-fermion compound CeRh6Ge4, which shows characteristics of localized magnetism and anisotropic hybridization .


Time and place: 2021.5.8, 10:00 am, Wuhan National High Magnetic Field Center C204

Presenter: Wei Zhu

Title: Doped Mott insulator and the implication for robust d-wave superconductivity in square lattice t-J model

Abstract:Unravelling competing orders emergent in doped Mott insulators and their interplay with unconventional superconductivity is one of the major challenges in condensed matter physics. To explore possible superconductivity state in the doped Mott insulator, we study a square-lattice t-J model with both the nearest and next-nearest-neighbor electron hoppings and spin Heisenberg interactions. By using the state-of-the-art density matrix renormalization group simulations with imposing charge U(1) and spin SU(2) symmetries on the large-scale six-leg cylinders, we establish a quantum phase diagram including three phases: a stripe charge density wave phase,a superconducting phase without static charge order, and a superconducting phase coexistent with a weak charge stripe order. Crucially, we demonstrate that the superconducting phase has a power-law pairing correlation decaying much slower than the charge density and spin correlations, which is a quasi-1D descendant or the uniform d-wave superconductor in two dimensions. These findings reveal that enhanced charge and spin fluctuations with optimal doping is able to produce robust d-wave superconductivity in doped Mott insulators, providing a foundation for connecting theories of superconductivity to models of strongly correlated systems.


Time and place: 2020.10.29, 19:00 pm, Skype Meeting

Presenter: Gregory S. Nusinovich

Title: Remote detection of excess ionizing radiation by using focused powerful terahertz wave beams

Abstract:A high-power source of sub-THz radiation allows one to remotely focus this radiation in a small volume where the RF electric field will concealed radioactive materials as a new application of gyrotrons.In this lecture, I am planning to start by describing the concept of remote detection of excess ionizing radiation by using high-power gyrotrons and some experiments with the 200kW, 0.67 THz Gyrotron. Then, I will focus on the working conditions of remote detection: formation of the breakdown-prone volume, production of free electrons by gamma rays and minimum detectable mass. Next step will be atmospheric conditions on the propagation and focusing of THz wave beams. The last part of this lecture will be devoted to plasma formation.


Time and place: 2020.10.15, 9:30 am, VooV Meeting

Presenter: Hu Miao

Title: Photon-based Spectroscopy Studies of Quantum Materials

Abstract:Correlated quantum materials possess remarkable physical properties, such as high-Tc superconductivity, anomalous Hall effect, novel topological orders etc. Yet, the underlining physics of these emergent quantum phenomena remain poorly understood. In recent few years, benefited by the fast development of new generation of x-ray light sources, ultra-high resolution inelastic x-ray scattering, fully coherent x-ray beam with nanometer x-ray spot, femtosecond time-resolution x-ray free electron laser (xFEL) become available to directly study the emerging low-energy excitations and their ultrafast processes. In this talk, I will present our works on various quantum materials and show how spin, charge and lattice excitations interact with each other and how these excitations conspire to determine the macroscopic physical properties.


Time and place: 2020.10.8, 16:00 pm, VooV Meeting

Presenter: Xiangang Wan

Title: Calculated Exchange Interactions and Competing S=0 and S=1 States in Doped NdNiO2

Abstract:Using density functional based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd1-xSrxNiO2. Our calculated nearest-neighbor exchange constant J1=82 meV is large, weakly affected by doping and is only slightly smaller than that found in the sister compound CaCuO2. We however find that the hole doping significantly enhances the inter-layer exchange coupling as it affects the magnetic moment of the Ni-3d{3z2-r2} orbital. This can be understood in terms of small hybridization of Ni-3d{3z2-r2} within the NiO2 plane which results in a flat band near the Fermi level, and its large overlap along z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level, do not affect the magnetic exchange interactions, and thus may not participate in the superconductivity of this compound. Whereas many previous works emphasize the importance of the Ni-3d{x2-y2} and Nd-5d orbitals, we instead propose that the material can be described by a Ni-3d{x2-y2}/Ni-3d{3z2-r2} two-band model. Its solution is studied here on the level of Dynamical Mean Field Theory and reveals an underlying Mott insulating state which, depending on precise values of the intra-atomic Hunds coupling, produces upon doping competing S=0 and S=1 two-hole states at low energies that lead to very different quasiparticle band structures. We propose that trends upon doping in spin excitational spectrum and quasiparticle density of state can be a way to probe Ni 3d8 configuration.


Time and place: 2020.9.17, 19:00 pm, Zoom Meeting

Presenter: Gregory S. Nusinovich

Title: Introduction to gyrotrons: principles and applications

Abstract:Gyrotrons are well known as high-power sources of coherent electromagnetic radiation. In the millimeter- and submillimeter-wavelength regions, the power that gyrotronscan radiate in continuous-wave and long-pulse regimes exceed the power of classical microwave tubes by many orders of magnitude. In this lecture, I will give an introduction to the history of the development of gyrotrons. I will start by describing three kinds of coherent electromagnetic radiation by electron beams: Cherenkov or Smith-Purcell radiation, transition radiation, and bremsstrahlung. Then, I will focus on bremsstrahlung, and move from that (describing the history of studies) to cyclotron resonance masers (CRMs). The next step will be the invention of gyrotrons. The last part of this lecture will be devoted to gyrotron applications.


Time and place: 2020.9.16, 9:00 am, VooV Meeting

Presenter: Jiaxin Yin

Title: Scanning tunneling microscopy of emergent topological matter

Abstract:The search for topological matter is evolving towards strongly interacting systems including topological magnets and superconductors, where novel effects and unusual phases emerge from the quantum level interplay between geometry, correlation, and topology. Equipped with unprecedented spatial resolution, electronic detection, and magnetic tunability, scanning tunneling microscopy has become an advanced tool to probe and discover the emergent topological matter. In this talk, I will review the proof-of-principle methodology to study the elusive quantum topology in this discipline, with particular attention on the studies under a vector magnetic field as the new direction, and project future perspectives in tunneling into other hitherto unknown topological matter.


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