Time and place: 2019.12.20, 13:30 pm, Wuhan National High Magnetic Field Center C204
Presenter: Jian Shi
Title: Phase Transitions of Correlated Oxides and Neuromorphic Electronics
Abstract：As the finest computing system in our planet, human brain outperforms IBM Blue Gene at many aspects (energy efficiency, memory capacity and computational speed). Inspired by biological neural systems, neuromorphic systems open up new computing paradigms to explore cognition, learning and limits of parallel computation. In our brains, synapse is believed to be responsible for the learning and memory behaviors. The success of synapse concept at software-level artificial neural networks in the applications of voice and image recognitions has been driving the search for their hardware counterpart – synaptic transistor. Here we emulate the synapse by using a strongly correlated oxide – SmNiO3.
Time and place: 2019.11.22, 15:00 pm, Wuhan National High Magnetic Field Center B206
Presenter: Marc Janoschek
Title: Dissecting Metallic Quantum Matter with Neutrons
Abstract：When many quantum particles interact in a solid frequently unexpected collective behavior emerges that is extremely difficult to predict. A prime example are metals near magnetic quantum phase transitions, notably magnetic instabilities in the zero-temperature limit that are accessed by non-thermal tuning parameters such as pressures and magnetic fields, at which strong electronic correlations manifest via new energy scales, as well as the emergence of novel quantum matter states. Examples include charge stripe and loop order, electronic nematic order, “hidden” order, multipolar order, and unconventional superconductivity. The SINQ neutron source at PSI provides a unique toolset for high-precision studies of such quantum matter states with access to the underlying energy scales and the associated collective behavior. I will provide an overview of current frontiers and developments in the field. I will summarize the progress made by recent neutron scattering experiments carried out by us and others and discuss how they offer the potential for exciting break throughs in the understanding of metallic quantum matter, notably in the extreme of high magnetic fields.
Time and place: 2019.11.14, 14:30 pm, Wuhan National High Magnetic Field Center C204
Presenter: Zhi Wang
Title: Intrinsic hysteresis of topological Josephson junctions with Majorana zero modes
Abstract：Topological Josephson junctions are distinct from conventional ones in that they host Majorana bound states which form a two-level quantum system, the so-called Majorana qubit. The energies of the two levels are depending on the Josephson phase across the junction，while reversely the effective potential for the Josephson phase is controlled by the wave function of the Majorana qubit. We build a quantum resistively shunted junction model to describe the correlation between the Majorana qubit and the classical Josephson phase. Within this model, we study the I-V characteristics of the junction, and demonstrate an intrinsic hysteric behavior which is independent of the parameters of the junction. Our results are quantitatively in agreement with recent experiments.
Time and place: 2019.11.13, 15:00 pm, Wuhan National High Magnetic Field Center B206
Presenter: Ke He
Title: The road to high temperature quantum anomalous Hall effect in magnetic topological insulators
Abstract：The quantum anomalous Hall effect is a quantum Hall effect that can occur without applied magnetic field. The effect has been experimentally realized in magnetically doped topologically insulators, but usually an ultralow temperature below 100mK is required to observe full quantization of the anomalous Hall resistance and vanishing longitudinal resistance. Elevating the temperature to realize the quantum anomalous Hall effect to above 77K or higher is crucial for its practical applications in electronics. In this talk, I will review the progresses in the past years on searching for higher temperature quantum anomalous Hall systems. Based on our understandings on these results, especially recent ones on magnetically intercalated topological insulators, we will give the principles and a roadmap for designing and constructing materials to realize high temperature quantum anomalous Hall effect.
Time and place: 2019.11.4, 10:30 am, Wuhan National High Magnetic Field Center B206
Presenter: Xuerong Liu
Title: Delocalized local orbitals in iridates: dimerization and molecularization
Abstract：Besides the charge and spin degrees of freedom, in transition metal compounds the orbital degree of freedom could lead to nontrivial consequences. The relativistic spin-orbit coupling (SOC) promotes the significance of the orbital degree of freedom, and has led to the so called SOC assisted Mott insulator concept in the iridates. If the strong SOC is embedded into special crystal geometry, and consider equal footing competition from hopping, more interesting quantum objects, such as dimer and molecular orbitals, could be realized. Combined with resonant inelastic X-ray scattering (RIXS) measurements and theoretical calculation, we demonstrate the dimer states in Ba5Allr2O11.
Time and place: 2019.9.23, 16:00 pm, Wuhan National High Magnetic Field Center C204
Presenter: Yong Xu
Title: Intrinsic magnetic topological insulator MnBi2Te4
Abstract：The interplay of magnetism and topology is a key research subject in condensed matter physics and material science, which offers great opportunities to explore emerging new physics, like the quantum anomalous Hall (QAH) effect, axion electrodynamics and Majorana fermions. However, these exotic physical effects have rarely been realized in experiment, due to the lacking of suitable working materials. In this talk, we will present our recent findings of intrinsic magnetic topological insulators in van der Waals layered MnBi2Te4-family materials. The materials intrinsically show two-dimensional (2D) ferromagnetism in the single layer and three-dimensional (3D) A-type antiferromagnetism in the bulk, which could serve as a next-generation material platform for the state-of-art research. We predict extremely rich topological quantum effects with outstanding features in an experimentally available material MnBi2Te4, including a 3D antiferromagnetic topological insulator with the long-sought topological axion states, the type-II magnetic Weyl semimetal (WSM) with one pair of Weyl points, and intrinsic QAH insulators and axion insulators. Recent theoretical and experimental progresses will be introduced. As important progresses, we experimentally observed two exotic topological phases (i.e., axion insulator and Chern insulator) and quantum phase transition between them in one stoichiometric crystal MnBi2Te4 at relatively high temperature.
Time and place: 2019.9.16, 9:30 am, Wuhan National High Magnetic Field Center B206
Presenter: Youguang Guo
Title: Some Research Activities on Electric Motors and Drives at UTS CEMPE
Abstract：Electric motors and drive systems play a key role in modern industry and domestic applications. This talk aims to present some research activities and ideas at the Center for Electrical Machines and Power Electronics (CEMPE), University of Technology Sydney (UTS). The focus will be the advanced material application and system-level multi-discipline optimization, which are considered as the crucial issues for developing high-performance electric drive systems. The talk also tries to explain the significance, innovation and urgency of these topics.
Time and place: 2019.7.4, 14:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Congjun Wu
Title: Orbital-active honeycomb materials
Abstract：We provide a unified view for a class of orbital-active honeycomb materials, including bis-muthene, stanene, exciton-polarition lattice, and the recent focus of the twisted bilayer graphene. These materials are orbital-active possessing a pair of degenerate px and py orbitals on each site, which are unified under the E-representation of the C6v group symmetry. We started the research on orbital-active honeycomb systems in ultra-cold atom optical lattices, and found that similar physics also applies to the above solid state materials unified under the same symmetry structure. For solid state applications, we propose a new mechanism to boost the topological gap to the full scale of the atomic spin-orbit coupling, i.e., the order of 1eV. This mechanism has been recently realized in experiments of bismuthene on SiC, which shows the evidence of the gap up to 0.67eV. Mech-anism for unconventional superconductivity assisted by the orbital structure in this class of mate-rials is also studied. The flat band structure arises in 2D orbital-active honeycomb materials, such as organic metal frameworks and transition metal oxide films, in which strong correlation phe-nomena including Wigner crystallization and ferromagnetism appear.
Time and place: 2019.5.21, 9:30 am, Wuhan National High Magnetic Field Center C204
Presenter: Haifeng Li
Title: Neutron/synchrotron X-ray scattering studies of strongly-correlated electron systems
Abstract：The first question we pose while studying any new family of correlated electron materials is the question concerning the internal structure: what are the crystalline and magnetic building blocks and how are they arranged? The second question concerns the microscopic dynamics: how do these building blocks move and what are their internal degrees of freedom? These herculean tasks can be accomplished by modern neutron and synchrotron X-ray scattering. Especially, neutron scattering is a unique powerful tool to solve magnetic structures and determine magnetic excitations and fluctuations. In this talk, I will present our work on iron-based superconductors & intermetallic compounds, focusing on the studies of crystalline and magnetic structures as well as spin dynamics, investigated mainly by neutron scattering technique.
Time and place: 2019.5.17, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Yi Zhou
Title: Hidden SU(2) Symmetries, the Symmetry Hierarchy and the Emergent Eight-Fold Way in Spin-1 Quantum Magnets
Abstract：The largest allowed symmetry in a spin-1 quantum system is an SU(3) symmetry rather than the SO(3) spin rotation. We reveal some SU(2) symmetries as subgroups of SU(3) that, to the best of our knowledge, have not previously been recognized. Then, we construct SU(2) symmetric Hamiltonians and explore the ground-state phase diagram in accordance with the SU(3)⊃SU(2)x(1) symmetry hierarchy. It is natural to treat the eight generators of the SU(3) symmetry on an equal footing; this approach is called the eight-fold way. We find that the spin spectral functions and spin quadrupole spectral functions share the same structure，provided that the elementary excitations are flavor waves at low energies, which serves as a clue to the eight-fold way. An emergent S=l/2 gapless quantum spin liquid is found to coexist with spin nematic order in one of the ground states.
Time and place: 2019.4.30, 14:00 pm, Wuhan National High Magnetic Field Center B206
Presenter: Shula Chen
Title: Dilute Nitride Semiconductors and Its Application in Nanophotonics and Optospintronics
Abstract：Dilute nitride semiconductors, e.g. Ga(In)NAs and GaNP, have been another versatile III-V material system which receives extensive research interest. This is driven by their unique giant bowing effect, i.e. a small incorporation of N into GaAs and GaP enables not only a significant reduction of bandgap, but also decreased lattice constant to integrate with well-established Si-based nanotechnology, therefore holding promise in visible and infrared-ranged nanophotonic applications. I will present our recent results on the GaNAs nanowire lasers, where we for the first time demonstrated the near-IR lasing from such nanostructured dilute nitride materials. Via both alloy and cavity engineering, we achieved wide lasing wavelength tunability and modal switching, which presents GaNAs nanowires as promising candidate for future IR nanolasers. Next, the opto-spintronic properties of GaNAs will be introduced. We demonstrated a room temperature spin-photon interface based on the GaNAs nanopillar array, where a defect-mediated spin filtering mechanism leads to an efficient electron spin amplification by 1200% and achieves a high spin polarization degree up to 60% in single nanopillar at room temperature. These results provide an exciting alternative to realize room temperature nanosized spin laser for future quantum information technology.
Time and place: 2019.4.24, 15:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Walter Escoffier
Title: Aperiodic quantum oscillations in the two-dimensional electron gas at the LaAlO3/SrTiO3 interface
Abstract：We investigate the transport properties of a high-mobility quasi-two-dimensional electron gas at the interface between two insulators LaAlO3 and SrTiO3. Since its first discovery in 2004, the electronic band structure derived from magnetotransport studies in different experimental conditions of temperature and magnetic field provided equivocal conclusions. Recently, the use of high magnetic field clearly revealed non 1/B-periodic magneto-resistance oscillations, as evidenced by a highly non-linear Landau plot (Landau level index versus 1/B). Among various scenarios, the Roth-Gao-Niu model provides a natural explanation for this effect, in relation with the magnetic response functions of the system. The experimental magneto-transport results are discussed jointly with dedicated theoretical DFT band-structure calculations, as well as with structural analysis of the interface using high-resolution scanning transmission electron microscopy.
Time and place: 2019.4.17, 14:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Jian-Qiao Sun
Title: Multi-Objective Optimization of Structures
Abstract：In this talk, we discuss conflicting objectives of various structures in engineering and in nature. The multi-objective optimization problem (MOP) is then formulated. The solution methods for MOP in the literature are reviewed. In particular, we introduce a hybrid evolutionary algorithm and cell mapping method for the global solution of MOPs. Several examples are presented including a nonlinear control example, a case study of multi-objective structural-acoustic optimization, and an MOP design of airfoils.
Time and place: 2019.4.16, 14:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Jian-Qiao Sun
Title: Data-Driven Modeling for Analysis, Fault Detection, Optimization and Control of Dynamic Systems
Abstract：In this talk, we review efforts of the past few decades in developing mathematical models for analysis, fault detection, optimization and control of various civil, mechanical and biological systems. Examples with “small” number of data are first discussed. These include the data driven modeling of acoustic materials, fatigue life prediction of metallic structures, modeling, fault detection and control of HVAC systems in office buildings, and surgical outcome prediction. The methods for data-driven research are discussed including statistical analysis, principal component analysis, correlation analysis and neural networks. We then discuss the implications of the availability of “big” data. We then review the recent advances of methods in artificial intelligence, and discuss their potentials and challenges for applications to civil and mechanical systems operating in complex environment. An example of our preliminary study of fault detection of rotor dynamic systems with deep learning is then presented to conclude the talk. It is hoped that this talk will stimulate the interests in artificial intelligence and its application to traditional engineering disciplines.
Time and place: 2019.3.29, 15:00 pm, Wuhan National High Magnetic Field Center B206
Presenter: Enke Liu
Title: Topologically Enhanced Transverse Electronic Transport Effects in Magnetic Weyl Semimetal Co3Sn2S2
Abstract：Topological physical states originating from non-trivial band characters in momentum space have motivated increasing interests in the condense matter physics. The magnetic Weyl semimetals with magnetic monopoles of Berry curvature is expected to generate the gaint, intrinsic transverse transport effects. In this talk, I will present a quasi-two-dimensional, kagome-lattice magnetic Weyl semimetal Co3Sn2S2. The chiral-anomaly-induced negative magnetoresistance, ARPES and STM observations provide signatures of Weyl fermions in Co3Sn2S2. The intrinsic properties including anomalous Hall conductivity, anomalous Hall angle, anomalous Nernst effect, and transverse thermoelectric conductivity are observed experimentally to reach up to an order of magnitudes larger than common magnetic systems. Our study establishes kagome-lattice magnetic Weyl semimetal with an out-of-plane ferromagnetic order as a key class of materials for fundamental research and device applications under the connection of topological physics and spintronics/spin-caloritronics.
Time and place: 2019.3.29, 9:30 am, Wuhan National High Magnetic Field Center B206
Presenter: Haihu Wen
Title: Topological Superconductivity in SrxBi2Se3 and Bi2Te3/FeTe0.55Se0.45 heterostructures
Abstract：Topological superconductor is a timely and frontier topic in condensed matter physics. Here we report superconductivity with twofold symmetry in some related materials. In Sr doped Bi2Se3, we find the nematic superconductivity from the c-axis transport measurements. Furthermore, we show the systematic study of scanning tunneling microscope/spectroscopy on SrxBi2Se3. We find that the surface Dirac electrons will simultaneously condense into the superconducting state within the bulk superconducting gap. We deposit Bi2Se3 thin film on the FeTe0.55Se0.45 substrate and get the proximity induced superconductivity. Within the vortex cores of FeTe0.55Se0.45, we observed the long sought discrete Caroli-de Gennes-Matricon bound states. By using the quasiparticle interference technique, we demonstrate clear evidence of twofold symmetry of the superconducting gap. The gap minimum is along one of the main crystalline axis following the so-called Δ4y notation. This is also accompanied by the elongated vortex shape. Our results reveal the direct evidence of superconductivity with two-fold symmetry in Bi2Se3 thin film.
Time and place: 2019.3.28, 16:00 pm, Wuhan National High Magnetic Field Center B206
Presenter: Haihu Wen
Title: Iron-based superconductors
Abstract：The iron based superconductors are expected to have strong potential in electricity transport, high magnetic field and sensitive magnetic detection. One of the on-going frontier studies on iron based superconductors is about the pairing mechanism. The Fermi surfaces in different systems are extremely distinct: some have both electron and hole pockets, but others have only the electron pockets. Therefore there is no consensus yet about the pairing gap structure and pairing mechanism. We have conducted extensive STM/STS studies on many different iron based superconductors and try to explore a unified understanding of the pairing mechanism in all iron based superconductors.
Time and place: 2019.1.4, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Feng Liu
Title: Topological dipoles and quadrupoles
Abstract：Topology offers us a unique dimension of designing solid-state materials. One famous example is the Quantum spin Hall effect (QSHE) where electrons of opposite spins propagate oppositely. The origin of QSHE comes from a geometric field strength in momentum space that is the so-called Berry curvature. Besides QSHE, the geometric vector potential whose curl yields the Berry curvature – the Berry connection, can induce an electric dipole even without Berry curvature. The topological electric dipoles result fractional surface charges that manifest as topological edge states, which are robust to defects and edge roughness. Furthermore, a pair of such the topological dipoles can form a high order multipole – quadrupole, which corresponds to the topological corner states. In this talk we will discuss a simple tight-binding model that possesses topological dipoles and quadrupoles in zero Berry curvature. Experimental realizations based on solid-state material and dielectric photonic crystal are proposed. Furthermore, we show that in a pure quadrupole phase, topological edge state is pseudo-spin polarized in general.
Time and place: 2018.12.10, 16:00 pm, Wuhan National High Magnetic Field Center C204
Presenter: Zenghui Wang
Title: A Vibrant New World—Exploring New Physics and Materials with Nanoelectromechanical Systems (NEMS)
Abstract：The advent of low-dimensional nanostructures has enabled a plethora of new devices and systems. Among them, nanoelectromechanical systems (NEMS) offers the unique capability of coupling the exquisite material properties found in these atomically-defined nanostructures with their mechanical degree of freedom, opening new opportunities for exploring exotic phenomena at the nanoscale. In this talk I will discuss two such examples: 1) using an individual carbon nanotube as a nanoscale balance to study low-dimensional phase transition, 2) using multimode resonance in black phosphorus NEMS resonator to resolve the intrinsic anisotropy in these nanoscale crystals, and 3) studying Brownian resonance and nonlinear motion in MoS2 NEMS.
Time and place: 2018.12.4, 9:30 am, Wuhan National High Magnetic Field Center B206
Presenter: Masayuki Hagiwara
Title: High Magnetic Field Sciences at AHMF in Osaka University
Abstract：I will talk about two topics studied in high magnetic fields at the Center for Advanced High Magnetic Field Science (AHMF) after brief introduction of our high magnetic field facilities and experimental apparatus.The first topic is “the spin multipolar excitations observed in åkermanite compounds Sr2CoB2O7 (B=Ge and Si)”. In these compounds, Co2+ ions have magnetic moments (S=3/2). These are magneto-electric multiferroic materials in which spontaneous electric polarization appears due to the d-p hybridization mechanism. The Sr2CoGe2O7, abbreviated as SCGO, has small magnetic anisotropy, while the Sr2CoSi2O7, abbreviated as SCSO, has large magnetic anisotropy. In SCGO, we observed a two-magnon spin excitation appearing above the saturation magnetic field (Hsat) in the electron spin resonance spectra. Owing to small anisotropy, this excitation has purely spin-quadrupolar nature, and hence its observation is possible by electric component of the light (electro-magnon) due to the spin nematic interaction in SCGO. On the other hand, we observed not only two-magnon but also three-magnon spin excitations above Hsat in SCSO. The three-magnon (spin-octupolar) excitation was observed due to large magnetic anisotropy which mixes magnetic dipole and quadrupole terms, resulting in making this excitation possible by magnetic component of the light. We discuss the similarity and difference in spin excitations between SCGO and SCSO.The second topic is “the high-field magnetization of the S=1/2 honeycomb-lattice antiferromagnet Cu2(pymca)3(ClO4) where pymca stands for pyrimidine-2-carboxylate”. Recently, a honeycomb-lattice magnet with bond-dependent exchange interactions has been extensively studied as the Kitaev model [3], but in this talk, I will present the studies on a simple Heisenberg honeycomb-lattice antiferromagnet. The magnetic susceptibility shows a broad peak near 25 K, and no long range order is observed down to 0.6 K in the specific heat measurements. The magnetization curve up to 70 T at 1.4 K shows three step-like increases. Assuming three different exchange bonds based on the structure, the calculated magnetization curve reproduces the observed one except for the magnetization near 70 T, where the observed magnetization indicates another step while the calculated magnetization becomes saturated. The relationship between the magnetization plateaus and exchange bonds is discussed based on the numerical calculations.
Time and place: 2018.11.26, 14:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Olgerts Dumbrajs
Title: History and presentstate of gyrotrons
Abstract：Gyrotron is well recognized as a promising high-power millimeter-wave and terahertz source. They are capable of providing hundreds of kilowatts of power at microwave and millimetric wavelengths. I will start the talk with an introduction to the electromagnetic spectrum and the operating principle of gyrotron oscillator. In the electron cyclotron maser (ECM), the effective frequency is directly related to the magnetic field, namely, the strength of the magnetic field determines the radiation frequency. So, in the next part, I will explain the magnetic field required for a gyrotronto generate higher frequency waves. In the second part, I will introduce the development path of gyrotron. The conventional gyrotron oscillator was first built and is now still widely used in different laboratories. The coaxial cavity gyrotrons show many advantages, but the technical complexity is increased. To achieve higher operating frequency at a given magnetic field, gyrotrons can operate at harmonics modes, with an increased risk of mode competition. The advantages and challenges of such types of gyrotron will be illustrated. In the last, I will present the present theoretical work on gyrotrons at University of Latvia.
Time and place: 2018.11.15, 15:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Ulrich Zeitler
Title: Quantum Matter in High Magnetic Fields
Abstract：High magnetic fields are an extremely powerful tool to investigate, to control and to manipulate the quantum properties of matter. In this talk I will present some recent results in this domain obtained at the High Field Magnet Laboratory in Nijmegen, the Netherlands.I will start the talk with a general presentation of HFML-EMFL, the high-field facility in Nijmegen with static magnetic fields up to 38 T (45 T in the near future). More specifically, I will present the status of our current and future magnets and the wide range of state-of-the art experiments offered to external users. In the second part I will present in some more detail two examples where we show how electron-electron interaction can spectacularly influence the high field magneto-transport properties of new low-dimensional material systems: Transition-metal dichalcogenides (TMDs) and oxide-based heterostructures.
Time and place: 2018.11.7, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Glenn S. Daehn
Title: Impulse Welding and Manufacturing: Methods and Opportunities
Abstract：Explosive-like impulse can do remarkable things. Impact welding, for example can produce very strong welds between wildly dissimilar metals in a solid-state process, avoiding the intermetallic compounds that often cause brittleness, and melting that can destroy microstructure and strength. Explosive forming can dramatically extend forming limits and can avoid presses and fixed dies. This presentation will discuss many ways that explosive-like methods can be used in conventional lab or factory environments. While these methods can offer vastly different pressure-time profiles with pressures ranging to GPa and time scales to tens of nanoseconds, all these methods offer opportunities very light equipment and enable new phenomena that can enable new methods of joining, forming, cutting and surface treatment.
After a brief, but broad introduction to impulse manufacturing, we focus on the use of the new methods of the vaporizing foil technique and laser impulse methods to enable solid-state welding. Modeling and simulation will be synthesized with experimental macroscopic and microstructural information. Efforts to further commercialize these methods will also be discussed.
Time and place: 2018.9.3, 16:00 pm, Wuhan National High Magnetic Field Center C204
Presenter: Devashibhai Adroja
Title: Introduction to ISIS pulsed Neutron and Muon facility and simple science examples
Abstract：The ISIS pulsed neutron and muon source at the Rutherford Appleton Laboratory in Oxfordshire UK is a world-leading centre for research in the physical and life sciences. ISIS produces beams of neutrons and muons that allow scientists to study materials at the atomic level using a suite of instruments, often described as ‘super-microscopes’. It supports a national and international community of more than 2000 scientists who use neutrons and muons for research in physics, chemistry, materials science, geology, engineering and biology. From the original vision over 30 years ago, ISIS has become one of the UK’s major scientific achievements. As the world’s leading pulsed neutron and muon source. ISIS has changed the way the world views neutron scattering and muon spin rotation and relaxation, which provide microscopic and fundamental information on the complex many body systems. I will briefly introduce the neutron and muon techniques and discussed the production of neutrons and muons at ISIS facility. I will discussed neutron and muon instruments, data analysis software and present some recent science examples. The science examples will be focussed on the investigations of spin excitations in Fe-based superconductors and heavy fermion superconductors, spin wave investigation in mutliferroics materials, spin gap and spin wave investigation in Kondo insulators and magnetic excitations in molecular magnets. Further, I will discuss investigations of novel crystal structure using the high resolution neutron powder diffraction and magnetic structures using a cold neutron diffractometer and diffuse scattering investigations using a single crystal diffractometer. I will also show how muon technique is used to investigate elemental analysis. I will also present upgrade plan for excitations instruments. Further I will show new funding mechanism for Chinese user community to use ISIS Facility.
Time and place: 2018.9.3, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Dmitry Smirnov
Title: When high magnetic fields meet optical spectroscopy: probing electronic structure and interaction effects in novel electronic and magnetic materials
Abstract：A magnetic field is one of the few fundamental thermodynamic parameters (alongside with temperature or pressure, for example) widely used in experimental condensed matter physics to probe or induce new states of matter. The magnetic field can be applied in situ in a highly controllable manner, reversibly and with perfect tunability. It lifts degeneracy of electronic orbital and spin states, breaks time reversal symmetry, introduces energy and length scales of quantum states that may result in establishing new field-induced order and quantum effects.
A review of recent experimental results obtained at the US National High Magnetic Filed Lab is given to demonstrate the aptitude of high-field magneto-spectroscopy methods in studying novel electronic and magnetic materials. Illustrative examples from the portfolio of our research will include: (i) Probing and controlling excitons in monolayer transition metal dichalcogenides (MoSe2, WSe2) with high magnetic fields, (ii) infrared magneto-spectroscopy of Dirac (ZrTe5) and Weyl (NbP) semimetals, (iii) magneto-Raman probe of magneto-elastic coupling in a quasi-2D, frustrated quantum antiferromagnet SrCu2(BO3)2.
Time and place: 2018.8.24, 16:00 pm, Wuhan National High Magnetic Field Center A319
Presenter: Ryuichi Shindou
Title: Theories of topological spin-nematic excitonic insulators in graphite under high magnetic field and transport scaling in disordered semimetals
Abstract：In the first part of my talk, I will argue that three-dimensional topological excitonic insulator is realized in graphite under high magnetic field. Graphite under high magnetic field exhibits consecutive metal-insulator (MI) transitions as well as re-entrant insulator-metal (IM) transition at low temperature. A part of the experiment was discovered more than 30 years ago, while the identities of the low-temperature insulating phases are still unclear by now. We identify these enigmatic insulator phases with excitonic insulator phases, where electron and hole pocket(s) form spin-triplet excitonic pairings. We show that the re-entrant IM transition in the graphite experiment can be naturally explained by an enhanced quantum spin fluctuation in the presence of smaller electron and hole pocket(s). We further argue that the odd-parity spin-triplet excitonic pairing reconstructs chiral surface Fermi arc state of electron and that of hole into a 2+1 massless surface Dirac fermion (topological excitonic insulator).
In the second part of my talk, I will talk about transport scaling theories in disordered Weyl semimetal. In electronic band structure of solid state material, two band touching points with linear dispersion (called as `Weyl node') appear in pair in the momentum space. When they annihilate with each other, the system undergoes a quantum phase transition from Weyl semimetal (WSM) phase to a band insulator (BI) phase. The continuous phase transition is recently discovered in solid state materials. The phase transition is described by a critical theory with a `magnetic dipole' like object in the momentum space. I will argue that the critical theory hosts a new disorder-driven quantum multicritical phenomena. Based on the renormalization group argument, we clarify transport scaling properties around the Weyl node around the quantum multicritical point as well as the direct phase line between BI and WSM phases.
Time and place: 2018.7.11, 15:00 pm, Wuhan National High Magnetic Field Center C204
Presenter: Chenhao Jin
Title: Generation, transport and detection of pure valley currents in two-dimensional heterostructures
Abstract：Two-dimensional (2D) hexagonal materials provide a promising platform for valleytronics devices, owing to the convenient generation and manipulation of valley qubits. However, efficient generation of valley information carriers with long valley lifetime is difficult to achieve in single material due to intrinsic valley relaxation channels. Here we show that, such intrinsic limit can be overcome through combining two materials into a van der Waals heterostructure; and report both near-perfect generation efficiency of valley information carriers, as well as record-high valley lifetime. Furthermore, we demonstrate generation, transport, and spatial-temporal imaging of the valley currents in a single device, which opens up new exciting opportunities to realize novel spintronic and valleytronic applications.
Time and place: 2018.7.4, 9:30 am, Wuhan National High Magnetic Field Center C204
Presenter: Jing Wang
Title: Chiral Majorana fermions from quantum anomalous Hall state
Abstract：The chiral Majorana fermion, a massless fermionic particle being its own antiparticle, could arise as a one-dimensional quasiparticle edge state of a two-dimensional topological states of quantum matter. The propagation of chiral Majorana fermions could lead to non-abelian braiding and may be useful in topological quantum computation. Despite intensive searches, the chiral Majorana fermion is still have to achieve.Recently, we propose to realize a two-dimensional chiral topological superconducting state from the quantum anomalous Hall plateau transition in a magnetic topological insulator thin film through the proximity effect to a conventional s-wave superconductor. Furthermore, we predicted the half-quantized plateau as smoking gun signature of chiral Majorana fermion. Remarkably, such half-quantized plateau has been observed in experiments as an evidence for chiral Majorana fermion edge state in the quantum anomalous Hall-superconductor system.
Time and place: 2018.7.3, 9:30 am, Wuhan National High Magnetic Field Center B206
Presenter: Congjun Wu
Title: Quantum Dynamics – Space-time Crystal and Bethe String States
Abstract：We present recent works on two different aspects of quantum dynamics – symmetry and strong correlations. For the symmetry aspect, we propose a new concept of “space-time” crystal as a general framework for studying intertwined space-time periodic structures, which include both the static crystal and the Floquet lattice as special cases. A new mathematic structure of “space-time” group is constructed to describe the symmetries of a space-time crystal, which augments the space group with non-symmorphic operations involving fractional translations along the time domain: “time-screw rotation” and “time-glide reflection”. Classifications for the 1+1 D and 2+1D space-time crystals (groups) are completed, and their consequences on dynamic band structures will be discussed. For the strong correlation aspect, we have studied the real frequency response at high energy which is a hard problem of condensed matter physics. We studied the role of Bethe-string states in the quantum spin dynamics in antiferromagnetic spin chains in high magnetic fields based on algebraic Bethe ansatz via the form-factor formulae. Close to quantum criticality, the string excitations govern the quantum spin dynamics, whereas the fractional excitations, which are dominant at low energies, reflect the antiferromagnetic quantum fluctuations. These states have been recently observed in the electron-spin-resonance spectroscopy measurement on SrCo2V2O8. This work is helpful for experimental studies on spin dynamics in both condensed matter and cold atom systems beyond the low energy effective Luttinger liquid theory.
Time and place: 2018.6.29, 14:30 pm, Wuhan National High Magnetic Field Center C204
Presenter: Fan Zhang
Title: Quantum Hall Effects in Transition Metal Dichalcogenides
Abstract：The family of atomically thin transition metal dichalcogenides (TMDs) is a rapidly rising star on the horizon of condensed matter physics and has already revealed a number of spin-valley-helicity coupled optical phenomena. More fascinatingly, our magnetotransport measurements have provided compelling evidence for the presence of a three-fold Q-valley degeneracy and a heavily massive G-valley in few-layer n-type and p-type TMDs, respectively. In this talk, I will discuss our recent experimental progress on quantum transport in few-layer TMDs, guided by our earlier theoretical predictions. Particularly, I will introduce that the Q valleys offer an unprecedented opportunity to realize the solid-state version of flavor SU(3) symmetry that has been rare in electron systems to date and to explore novel electromagnetic phenomena that can arise from the interplay between the emergent symmetry and electron-electron interactions. Surprisingly, electrons carry flavor-dependent electric dipole moments even at zero magnetic field, rendering a ferroelectric nematic phase, allowing electric-field manipulation of the flavors, and leading to the concept of flavortronics.
Time and place: 2018.6.25, 10:00 am, Wuhan National High Magnetic Field Center B204
Presenter: Xiwen Guan
Title: Quantum liquids and quantum dynamics with interacting spins
Abstract：Spins, intrinsic degrees freedom carried by elementary particles, atomicnuclei and quasiparticles, are the main theme in the study of modern physics. The quantum systems of interacting electrons, bosons and fermions essen-tially involve in interacting spins, forming rich quantum many-body phenom-ena, such as quantum liquid, criticality and dynamics etc. In this talk, using exactly solvable models, I shall discuss the unique phenomena of spin chargeseparation, quantum collapse and revival in ultracold atoms and quantum devices of interacting spins. Our exact results provide precise understanding of fundamental physics at a many-body level.
Time and place: 2018.6.22, 10:00 am, Wuhan National High Magnetic Field Center C204
Presenter: Haizhou Lu
Title: 3D Quantum Hall effect
Abstract：The discoveries of the quantum Hall effect have led to three Nobel prizes and the booming field of topological states of quantum matter. So far, the quantum Hall effect is observed only in 2D systems. We show that the Fermi arcs can give rise to a distinctive quantum Hall effect in topological semimetals in three dimensions. Topological semimetals are 3D topological states of matter, in which the energy bands touch at a finite number of Weyl nodes. They host topologically-protected surface states, known as the Fermi arcs. Via a “wormhole” tunnelling assisted by the Weyl nodes, the Fermi arcs at two opposite surfaces can form a single 2D electron gas and support a quantum Hall effect in three dimensions. Possible signatures of the 3D quantum Hall effect have been observed in several experiments on the topological Dirac semimetal Cd3As2. Our discovery of this 3D quantum Hall give an example of (d-2)-dimensional boundary states, a promising direction in topological states of quantum matter.
Time and place: 2018.5.21, 10:00 am, Wuhan National High Magnetic Field Center C204
Presenter: Likai Song
Title: EPR Applications in Biology: Examples from HIV Membrane Protein gp41 and an Antimicrobial AApeptide
Abstract：EPR is a powerful tool for biological research. The focus of this work is to develop EPR methods for characterizing biological samples, with specific aims to investigate HIV surface protein gp41 and an antibacterial peptide (AA1). 1). HIV gp41-antibody interaction at the viral membrane interface. The membrane proximal ectodomain region (MPER) of gp41 plays a critical role during the viral fusion process and is a major target of anti-gp41 antibodies and vaccine design. In this study, EPR and NMR techniques were used to define MPER structure on the membrane, MPER-lipid interaction, and how anti-HIV antibodies recognize their membrane-immersed epitopes. The analyses revealed a structurally conserved pair of helices immersed in the viral membrane separated by a flexible hinge. Neutralizing anti-gp41 antibodies disrupt the MPER hinge function by perturbing MPER hinge orientation, and/or extracting part of the MPER from the membrane. These findings have revealed important features of gp41-antibody interaction at the viral membrane interface. 2). Selective membrane disruption mechanism of an antibacterial AApeptide. AApeptides are a new class of antibacterial peptidomimetics that are not prone to antibiotic resistance and are highly resistant to protease degradation. We have characterized the membrane interaction of a lipo-cyclic-gamma-AApeptide (AA1). The analyses revealed that AA1 binding increases the membrane permeability of POPC/POPG liposomes, which mimic negatively charged bacterial membranes. AA1 binding also induces significant lipid lateral-ordering and membrane thinning. In contrast, minimal membrane property changes were observed upon AA1 binding for liposomes mimicking mammalian cell membranes, which consist of neutral lipids and cholesterol. Our findings suggest that AA1 interacts and disrupts bacterial membranes through a “carpet-like” mechanism.
Time and place: 2018.5.10, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Jurek Krzystek
Title: Terahertz Electron Paramagnetic Resonance
Abstract：The recent increase of interest in single-ion molecular magnets based on transition metal and lanthanides coordination complexes has necessitated developing techniques that reliably measure the parameter of importance for characterizing their properties, namely magnetic anisotropy, also known as zero-field splitting (zfs) for transition metals, or crystal-field splitting (cfs) for lanthanides and actinides. Whereas for many metal complexes High-Frequency and-Field EPR (HFEPR) has been fully adequate to determine zfs lying in the range of approximately 1-20 cm-1，many of those that display single-ion magnet properties are characterized by so-called “giant anisotropy”, which means zfs or cfs exceeding, often significantly so, these numbers, sometimes reaching 100 cm-1, and more. In such a case, increasing the EPR operating frequency into terahertz range is necessary. An overview of available and prospective techniques that enable it will be presented together with applications.
Time and place: 2018.5.9, 15:00 pm, Wuhan National High Magnetic Field Center C204
Presenter: Zaiyao Fei
Title: Edge conduction and gate induced superconductivity in monolayer WTe2
Abstract：Topology and correlation are two essential concepts in modern condensed matter physics. They give rise to a lot of interesting phenomena and exotic ground states, among which the quantum spin Hall (QSH) effect and superconductivity are two famous examples. Monolayer WTe2, a topological nontrivial semimetal in the bulk form, has recently been predicted to be a QSH insulator if a bulk gap opens. In experiment, we find that at temperatures below about 100 K a gap appears and the 2D bulk becomes insulating near zero doping, while the edges remain conducting. At lower temperatures, the edge conduction is strongly suppressed by in-plane magnetic field. Most of the observations are consistent with monolayer WTe2 being a quantum spin Hall insulator. Surprisingly, when the monolayer is electrostatically doped away from the insulating state, the 2D bulk turns superconducting below about 1 K. Bilayer and trilayer WTe2, on the other hand, do not show any signature of edge conduction or superconductivity. Monolayer WTe2 thus provides a unique platform for studying different electronic ground states and the interplay among them.
Time and place: 2018.4.27, 15:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Plamen Stanislavov Stamenov
Title: Point Contact Andreev Reflection and the Measurement of Spin Polarization - High Fields and Novel Materials
Abstract：Following a very brief introduction to the research activities within the Magnetism and Spin Electronics Group at TCD, one topic of current development will be presented, as an example.
Point Contact Andreev Reflection (PCAR) is one of the few available methods for the determination of the Fermi level spin polarisation in metals and degenerate semiconductors. It has traditionally been applied at fixed (liquid He) temperatures, using pure niobium as the superconductor, and at essentially zero applied magnetic fields, all of which limit the amount of information that it can provide – i.e. do not allow for the extraction of the sign of the spin polarisation and make the assignment of the transport regime to ballistic or diffusive almost impossible.
Here a series of experiments is described, aimed at the expansion of this parameter space to higher magnetic fields and to higher temperatures. These require redesigned experimental setups and the use of higher performance superconductors. Demonstrations are described of the determination of the sign of the spin polarisation, at fields of more than 5-7 Tesla using a low-Z superconductor, as well as operations beyond 9.2 K. Doubts about the practical reliability of the PCAR technique are dispersed using systematic series of samples – the heavy rare-earths and comparisons with alternatives, such as spin-polarised field emission, photo-emission and Tedrow-Meservey tunnelling.
The specific material examples presented include 3d-metals, order-disorder transition alloys and zero-moment half-metals – Fe, FeAl and MnRuGa, alternative low-Z and high-Z superconductors – MgB2 and NbTi, and magnetic topological insulators, such as Cr- and V-doped (Bi1-xSbx)2Te3. Finally, remarks will be made towards the possibilities of using Andreev reflection in very high DC and pulsed magnetic fields.
Time and place: 2018.4.4, 10:00 am, Wuhan National High Magnetic Field Center C204
Presenter: Jie Liu
Title: Fractional Josephson effect in semiconductor superconducting wires
Abstract：The 4π-periodic Josephson effect is a distinguishing feature of a topological Josephson junction. However, stringent conditions make it hard to observe in experiments. In this work, we study the transient transport properties in a topological Josephson junction numerically. We show that the 4π Josephson current can be sustained under nonequilibrium conditions. The properties of the Josephson current are analyzed for different conditions and three main regimes are identified. First, when both the superconducting wires of the Josephson junction lie in the topologically nontrivial region, a 4π Josephson current can appear upon suddenly applying a dc voltage. Second, when one superconducting wire lies in the trivial region, while the other wire lies in the nontrivial region, the Josephson current is 2π periodic but the component of the higher-order Josephson current increases. Third, when both wires lie in the trivial region, a stable 2π Josephson current is observed. Most importantly, the fractional Josephson effect is fragile in the presence of disorder. Hence experiments should be designed carefully to eliminate the effect of disorder. These results could be helpful to optimize fine-tuning of the experimental parameters to observe the 4π-periodic Josephson current in a topological Josephson junction.
Time and place: 2018.3.29, 10:00 am, Wuhan National High Magnetic Field Center C204
Presenter: Simin Nie
Title: Topological nodal-line semimetals in ferromagnetic rare-earth-metal monohalides
Abstract：Topological semimetals, extending the topological classification from insulators to metals, have greatly enriched our understanding of topological states in condensed matter. This is particularly true for topological nodal-line semimetals (TNLSs). In the present paper, we identify layered materials as promising candidates for hosting TNLSs. Based on first-principles calculations and effective model analysis, we propose that layered ferromagnetic rare-earth-metal monohalides LnX (Ln=La, Gd; X=Cl, Br) exhibit long pursued topological phases. Specifically, single-layer LaX and single-layer GdX are ideal two-dimensional (2D) Weyl semimetals and large-gap 2D quantum anomalous Hall insulators (QAHIs), with band gaps up to 61 meV, respectively. In addition, 3D LaX and 3D GdX are TNLSs with a pair of mirror-symmetry protected nodal lines and 3D weak QAHIs, respectively. The nodal lines in 3D LaX extending through the whole Brillouin zone (BZ) are fairly robust against strong spin-orbit coupling (SOC) and located close to the Fermi level, providing a novel platform toward exploring the exotic properties in nodal-line fermions as well as related device designs.
Time and place: 2018.1.24, 10:00 am, Wuhan National High Magnetic Field Center C204
Presenter: Xiaoxiang Xi
Title: Superconductivity and charge-density-wave order in NbSe2
Abstract：Atomically thin van der Waals materials have emerged as a frontier for both fundamental physics and device applications. Although novel single-particle and excitonic properties have been extensively studied, the collective electron phenomena in these materials remain less well understood. In this talk, we will discuss superconductivity and charge-density-wave (CDW) order in atomically thin group-V transition metal dichalcogenide NbSe2 down to the monolayer limit. Electrical transport measurements show that the superconducting transition temperature decreases monotonically with reducing the layer thickness. The temperature dependent Raman scattering, on the other hand, shows enhanced CDW order as the sample thickness reduces. Magneto-transport measurements further reveal that the in-plane upper critical fields in atomically thin NbSe2 superconductors significantly exceed the Pauli paramagnetic limit. We will discuss possible mechanisms for these phenomena and electrical tuning of the superconductivity and CDW in this 2D metal.
Time and place: 2018.1.15, 9:30 am, Wuhan National High Magnetic Field Center C204
Presenter: Tiantian Zhang
Title: Double-Weyl Phonons in Transition-Metal Monosilicides
Abstract：We employed ab initio calculations to identify a class of crystalline materials of MSi(M=Fe, Co, Mn, Re, Ru) having double-Weyl points in both their acoustic and optical phonon spectra. They exhibit novel topological points termed “spin-1 Weyl point” at the Brillouin zone center and “charge-2 Dirac point” at the zone corner. The corresponding gapless surface phonon dispersions are two helicoidal sheets whose isofrequency contours form a single noncontractible loop in the surface Brillouin zone. In addition, the global structure of the surface bands can be analytically expressed as double-periodic Weierstrass elliptic functions.
