Time and place: 2017.12.18, 14:30 pm, Wuhan National High Magnetic Field Center C204
Presenter: Wenyu Shan
Title: Model and Transport Studies on Topological Materials with Dirac Valleys
Abstract：Topological materials have attracted much recent interest from both theoretical and experimental sides. The existence of Dirac valleys gives rise to exotic transport and optical properties. In this talk, I will focus on two types of systems with Dirac valleys: topological insulators and two-dimensional materials. For topological insulators, I will introduce the finite-size effect and the construction of effective model for thin film structure. For two-dimensional materials, I will discuss the recent puzzles on valley Hall experiments in graphene systems and our solutions. Furthermore I will talk about the generation and detection of pure valley current by optical approach in monolayer transition metal dichalcogenides.
Time and place: 2017.12.6, 14:30 pm, Wuhan National High Magnetic Field Center C204
Presenter: Jinglei Zhang
Title: Measuring Quantum Oscillations in Extreme Magnetic Fields
Abstract：Measurement of quantum oscillations is a very powerful way for studying the Fermi-surface topologies. Recently, several techniques for probing the quantum oscillations had been successfully constructed in our Water-Cooled magnets with the highest field up to 38.5T and lowest temperature down to 0.3K. In this presentation, I will first explain the basic principles of these techniques and demonstrate their applications on topological materials. In the second part, I will discuss the effect of hydrostatic pressure on the magnetotransport properties of zirconium pentatelluride. We find that the quasi-linear magnetoresistance decreases drastically under pressure. Besides, the change of the quantum oscillation phase from topological nontrivial to trivial is revealed around 2.0GPa. Both demonstrate that the accidental Dirac cone in ZrTe5 is violated under pressure.
Time and place: 2017.11.22, 9:30 am, Wuhan National High Magnetic Field Center C204
Presenter: Zhenbing Tan
Title: Toward quantum entanglement in solid state by Cooper pair splitting
Abstract：Quantum entanglement is at the heart of Einstein–Podolsky–Rosen (EPR) paradox, which is fundamental for quantum information. Quantum entanglement has been successfully realized in optics, where the experiment benefit from the easy generation of entangled photons. In solid state, however, the progress has been modest. One natural source for quantum entanglement in solid state is split Cooper pairs. A Cooper pair, split out from a superconductor into two different terminals, will form a non-local entangled spin pair. We report an experiment on a superconductor-graphene double quantum dot (QD) system, in which we observe Cooper pair splitting (CPS) up to a CPS efficiency of ~ 10%. Comparing to the previous Cooper pair splitters, we were able to independently tune the bias and the energy levels of the two graphene QDs. Benefit from that, for the first time, the energy levels of the two QDs were tuned to be asymmetric or symmetric with respect to Fermi level in the superconductor. And we observed CPS or elastic co-tunneling favored as predicted by current theories. The realization of CPS in graphene opens a door for graphene to be used for quantum information processing.
Time and place: 2017.11.9, 14:30 pm, Wuhan National High Magnetic Field Center C204
Presenter: Jia Zhang
Title: Recent progress of first-principles calculations on advanced magnetic materials interface
Abstract：Spintronics is an emergent developing research field in condensed matter physics in which the electron charge, spin interact with each other. Especially various interesting phenomenon will arise with different functional magnetic materials interface and lead to potential device applications.In this talk I will discuss our recent first-principles calculations results on some advanced magnetic interfaces aiming at the low power consumption spintronics applications.
Time and place: 2017.9.25, 14:30 pm, Wuhan National High Magnetic Field Center B206
Presenter: Kamran Behnia
Title: The challenge of efficient scientific communication
Abstract：A routine task of a contemporary scientist is to publish new findings in scientific journals. In general, one prefers to address the broadest possible audience by choosing a high-profile journal. On the other hand, the impact factor of a journal correlates with its rejection rate. This market-based process of scientific publication has become dominant in recent years.As a consequence, a considerable fraction of the time previously devoted to scientific research itself is nowadays spent in optimizing the quality of communicating the results of this research.
How to transform a scientific result to a scientific communication? How to manage to keep the quality of the initial ingredient during this transformation? I will try to address this issue with a focus on experimental condensed-matter physics. A few useful rules of thumb in addressing an ever-widening international scientific audience will be identified.
Time and place: 2017.9.11, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Zheng Wang
Title: Structural and dynamical evolution in La-based glasses and melts
Abstract：The limited plasticity before catastrophic failure is a long-standing conundrum hindering the wide application of metallic glasses (MGs). Recent study found that the local relaxation behavior can strongly affect the heterogeneous deformation in MGs, which can even govern their mechanical performance at room temperature. Furthermore, the local relaxation behavior is also considered to be related to regions in MGs exhibiting weak bonding and large amount of free volume. Therefore, tuning the local relaxation behavior may provide a practical option to optimize the mechanical performance of MGs. For La-(Ni,Co)-Al MGs, replacing transition metals (TM) Ni and Co with similar sized Cu results in a different local relaxation behavior, where a separate beta-relaxation peak in the glass state vanishes. The reason behind it is still unclear yet but is suspected to be related with chemical interaction and local packing effects.
In this study, we measured density and total structure factor of these La-based glass-forming melts using the newly developed containerless electrostatic levitation (ESL), trying to find out some clue rooted in their liquid structures. We found interestingly a positive excess volume in the binary La-Cu melts, in contrast to the negative excess volume in La-Ni/Co melts. The La60Cu40 liquid exhibits also a considerably different total structure factor S(q) compared to that of the La60(Ni/Co)40 melts, measured with the X-ray synchrotron diffraction. Both the specific volume and the reverse Monte Carlo simulation on measured S(q) suggest a looser packing in La-Cu-(Al) than in La-(Ni,Co)-(Al) melts, which might be related to the change of the relaxation behavior in the ternary La-TM-Al glasses.
Time and place: 2017.8.29, 10:30 am, Wuhan National High Magnetic Field Center B206
Presenter: Zhida Song
Title:Detecting the chiral magnetic effect by lattice dynamics in Weyl semimetals
Abstract：In the present work, we propose that the chiral magnetic effect, the direct consequence of the presence of Weyl points in the band structure, can be detected by its coupling to certain phonon modes, which behave like pseudo-scalars under point group transformations. Such coupling can generate resonance between intrinsic plasmon oscillation and the corresponding phonon modes, leading to dramatic modification of the optical response by the external magnetic field, which provides a way to study chiral magnetic effect by optical measurements.
Time and place: 2017.6.9, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Gang Cao
Title:The Challenge of Spin-Orbit-Tuned Ground States in Iridates
Abstract：Effects of spin-orbit interactions in condensed matter are an important and rapidly evolving topic. Strong competition between spin-orbit, on-site Coulomb and crystalline electric field interactions in iridates drives exotic quantum states that are unique to this group of materials. In particular, the “Jeff = ½” Mott state served as an early signal that the combined effect of strong spin-orbit and Coulomb interactions in iridates has unique, intriguing consequences. In this talk, we survey some current experimental studies of iridates. In essence, these materials tend to defy conventional wisdom: absence of conventional correlations between magnetic and insulating states, avoidance of metallization at high pressures, “S-shaped” I-V characteristic, emergence of an odd-parity hidden order, etc. It is particularly intriguing that there exist conspicuous discrepancies between current experimental results and theoretical proposals that address superconducting, topological and quantum spin liquid phases. This class of materials, in which the lattice degrees of freedom play a critical role seldom seen in other materials, evidently presents some profound intellectual challenges that call for more investigations both experimentally and theoretically. Physical properties unique to these materials may help unlock a world of possibilities for functional materials and devices.
Time and place: 2017.5.24, 10:00 am, Wuhan National High Magnetic Field Center B206
Presenter: Shuai Wei
Title:Glass Transition and Liquid-Liquid Transition in Phase-Change Materials and Bulk Metallic Glassformers
Abstract:Recent discoveries of three dimensional (3D) topological semimetals, including Dirac semimetals (DSM), Dirac nodal-line semimetals and Weyl semimetals, have generated immense interests since they represent new topological states of quantum matters. Both Dirac and Weyl semimetals feature relativistic fermions with linearly dispersing excitations. In this talk, I will first give a brief introduction to this emerging area and discuss how to characterize the properties of Dirac/Weyl fermions. Then I will present our recent studies on topological semimetals. I will show nearly massless Dirac fermions generated by 2D Sb layers coexists with ferromagnetism in Sr1-yMn1-zSb2, whichoffers a rare opportunity to investigatethe effect of time reversal symmetry breaking on the electronic band structure and explore a possible ferromagnetic Weyl state.I will also reportour recentdiscovery of two new nodal line semimetals – ZrSiSe and ZrSiTe. The nodal-line semimetals exhibit Dirac cones along a one dimensional line/loop in their electronic structure, contrasted with the Dirac/Weyl cones with discrete nodes in Dirac/Weyl semimetals. We have found signatures of nodal-line fermions from quantum oscillation experiments in these two materials and demonstrated that their atomically thin crystals are accessible via mechanical exfoliation, which raises the possibility of realizing the theoretically predicted 2D topological insulators in monolayers of ZrSiSe and zrSiTe. Finally I will discussexotic quantum transport behavior of Weyl semimetal YbMnBi2, which results from the zeroth Landau Level, a unique property of topological materials.
Time and place: 2017.5.24,2:30-3:30 pm, Wuhan National High Magnetic Field CenterC204
Presenter:Zhiqiang Mao
Title:Relativistic Fermions Generated by Square Lattices in Layered Compounds
Abstract:Phase-Change Materials such as Ge2Sb2Te5 and Ge1Sb2Te4 can be reversibly switched between amorphous and crystalline states by applying an external voltage pulse. The fast phase switching on nanoseconds timescales is of technological interests for chalcogenide-based phase-change non-volatile memory devices for next generation data storage applications. A favored phase-change material must possess a unique combination of physical properties such as ultrafast crystallization kinetics, strong electronic/optical contrast, and relatively stable amorphous state. The urgency of understanding the underlying physics in the context of technological applications should need no emphasis. Here we show a striking anomaly in the viscosity of chalcogenide glass-forming alloy Ge15Te85. Applying the Adam-Gibbs equation to calorimetric data, we find a fragile-to-strong liquid transition (FS-transition), and then predict the ‘strong’ liquid course of the viscosity down to Tg. The in-situ X-ray scattering data reveal that the transition is not only related to short-range-order (SRO) structural change, but also linked to a remarkable development of medium-range-order (MRO). The latter manifests as an emerging pre-peak in total structural factor S(Q) and an atomic pair correlation on the length scale of ~8 Å in the real-space G(r) function. The FS-transition is also associated with a semiconductor-metal transition and a density anomaly. By examining the electronic conductivity and, then, semiconductor-to-metal (SC-M) transitions in related alloys, we find a systematic tendency in SC-M transitions with the metallicity of alloy components and conclude that Ge-Sb-Te Phase-Change Materials possess SC-M (and liquid-liquid) transitions that have become submerged below the liquidus surface, which controls the liquid-state behavior and has significant impact on application-relevant phase-change material properties.In parallel, we compare the liquid-state behavior of Ge15Te85 with that in the bulk metallic glass-forming liquid, Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 (Vit.106a), where a liquid-liquid transition(LLT) in the deeply undercooled state at T/Tg∼1.2 was observed using high-energy synchrotron x-ray radiation combined with electrostatic levitation (ESL). In the case of bulk metallic glassforming liquid, a quantitative structure-fragility relation can be applied and a FS-transition can be derived from the structural data.
Time and place: 2017.4.14,10:00 am, Wuhan National High Magnetic Field CenterC204
Presenter:Jianming Lu
Title:The application of the field effect in low dimensional materials superconductors
Abstract:The characteristics of the material is closely related to the structure and height of Fermi surface, so It becomes an important means of material application and basic physics research to adjust Fermi surface or carrier concentration .Compared with the chemical doping, the traditional field effect transistors have less negative effects on impurities, avoiding the damage of sample eigen crystal structure; But the disadvantage is that the ability of adjusting Fermi surface is smaller, so often, only used in narrow band semiconductor metal insulator phase transition. In order to regulate the transformation of matter between various quantum states, such as the phase transformation from metal to superconducting state, we need a special kind of field effect of medium - ionic liquid: it inherits the traditional characteristics of the field effect that is not to introduce impurities, but the ability to adjust the carrier concentration is increased by an order of magnitude, which enables us to observe superconducting state in the semiconductor. Transition metal sulfide MoS2, for example, after entering the superconducting state, its unique structure of Fermi surface spin - valley "physical" - also be inherited to the superconductor, thus realized the Ising model superconducting state, having a significant impact on magnetic properties of the superconductor itself. In WS2 which has stronger spin orbit coupling , the superconducting state in single chemical vapor deposition of monolayer samples was observed, and the special insulators was found near superconducting phase space. Single-layer WS2 superconductor not only provide a ising superconductor paradigm, but also provide a new perspective for explaining the superconducting phase diagram induced by the widely observed multilayer samples and interfaces.
Time and place:2017.2.14, 3:00pm, Wuhan National High Magnetic Field Center C204
Presenter:Wenbo Wang
Title:Characterization of ferromagnetic domains in magnetic thin films by using magnetic force microscopy
Abstract:Magnetic force microscopy (MFM) is a powerful scanning probe technique to study the local magnetic properties of sample surface, by detecting the magnetic interaction between the magnetic tip and sample. Here we present cryogenic MFM studies of a 200 nm thick hexagonal (h) LuFeO3film grown by molecular-beam epitaxy (MBE). Labyrinth-like domains ~ 1.8um in size were observed after zero field cooling below the Neel temperature TN~ 147 K, where weak ferromagnetic order (P63cm) exist. At 6 K, MFM images of the magnetization reversal process reveal a typical domain behavior of a pinning-dominated hard magnet. The pinning strength is substantially reduced at elevated temperature. The temperature dependence of the domain contrast demonstrates that our MFM is able to detect the domain contrast of magnets with tiny magnetic moments (~ 0.002 µB/f.u.). Taking advantage of this powerful MFM technique with high sensitivity, we were able to study the ferromagnetism of a quantum anomalous Hall system, Cr,V co-doped (Bi,Sb)2Te3. The magnetization reversal process reveals a typical ferromagnetic domain behavior, via domain nucleation and domain wall propagation. The carrier density dependence suggests both RKKY and non-RKKY-like interaction play an important role in the ferromagnetism of the magnetic topological thin films. Our results provide microscopic evidence of the ferromagnetic nature of a QAH system.
Time and place:2016.12.19, 9:00am, Wuhan National High Magnetic Field Center B206
Presenter:Yixi Su
Title:monopoles and quantum dynamics in spin ice materials
Abstract:The realizations of magnetic monopoles as an emergent fractionalized quasi-particle in dipolar spin ice Ho2Ti2O7 and Dy2Ti2O7 pyrochlore compounds has been hailed as one of the major recent breakthroughs in condensed matter physics. Owing to the combination of a large Ising single-ion anisotropy and an effective ferromagnetic nearest-neighbor exchange interaction, the creation of magnetic monopoles as the consequence of the violation of the “2-in/2-out” ice rules becomes extremely energetically unfavorable below about 600 mK, thus leading to an almost completely suppressed spin dynamics and a frozen spin ice state possessing Pauling’s residual entropy. An intriguing question that thus arises and has already attracted tremendous amount of research efforts in the community is what may be the consequence if quantum fluctuation is invoked in the framework of the classical spin ice physics? Theories based on various forms of the quantum spin ice model have predicted the emergence of exotic ground state and collective excitations such as artificial photons. However, a convincing realization of the quantum spin ice state in real materials has not been achieved so far. In this talk, some of our recent neutron scattering studies of static and dynamics magnetic correlations in spin ice and quantum spin ice candidate materials based on frustrated pyrochlore oxides will be presented. Possible implication of our neutron scattering data within the context of the experimental signatures of magnetic monopoles and quantum dynamics will be discussed as well.
Time and place:2016.11.1, 10:30am, Wuhan National High Magnetic Field Center B206
Presenter:Suchitra Sebastian
Title:Exploring Materials Universes: the case of an exotic insulator that behaves like a meta
Abstract:Materials comprise trillions of electrons that interact with each other to create a diversity of physical behaviours. We owe much of modern technology - from powerful computing to the marvels of communication - to discoveries of new types of collective electron behaviours in materials. Such discoveries, however, are often serendipitous, given that materials can be thought of as complex universes teeming with vast numbers of electrons, making their behaviours challenging to understand or predict. A question we are often confronted with is how to make progress in discovering novel collective electron behaviours akin to new universes. I will discuss possible approaches to increasing the odds of making discoveries, with examples from cases such as new superconductors and new types of dual metal-insulating materials. In particular, I will discuss the surprising case of the Kondo insulator SmB6 in which we have used high magnetic fields to observe a Fermi surface characteristic of a metal. Potential models will be discussed in the context of our findings.
Time and place:2016.10.29, 9:00am, Wuhan National High Magnetic Field Center B206
Presenter:Hiroyuki Nojiri
Title:High Frequency EPR Studies for Low Dimensional Magnetic Systems
Abstract:Recent activities of the high frequency ESR on low dimensional magnetic systems will be introduced. In the first place, the ESR measuring systems in both pulsed and steady high magnetic fields will be presented. It will be followed by several examples on zero-dimensional magnetic systems such as hetero-metallic molecular magnets and high-spin giant magnetic clusters. In these compounds, ESR is used as the unique method to determine the magnetic levels and the short-range correlations. Moreover, the zero-field splitting and g-values are useful to identify the states of magnetic ions forming such clusters. For one-dimensional magnets, the development of the short-range correlation appears as the characteristic temperature dependence of the resonance field and the line width. The dimensionality also influence on the line shape of the resonance. A few examples will be introduced. Various examples of two-dimensional systems are also reviewed. The presentation includes the spin-ladder, the Shastry-Sutherland dimer system and perfectly frustrated two-dimensional dimer network. The spin gap excitation, triplets and the bound states are observed by the high frequency ESR. It will be summarized with a few comments on the expected developments of high frequency ESR.
Time and place:2016.10.28, 10:30am, Wuhan National High Magnetic Field Center B206
Presenter:Vivien Zapf
Title:High Magnetic Fields for Multiferroics and Complex Functional Spin Systems
Abstract:I will review the need for new multifunctional magnetic materials, in particular insulating magnetoelectrics for low-power sensing, computing, and frequency devices. Complex magnetic spin systems are a route to creating magnetoelectric behavior by tuning the symmetry of the magnetism and creating key magnetoelectric coupling mechanisms. Understanding complex magnetic spin systems and how they couple to ferroelectricity in turn requires high magnetic fields and fast magnetic fields. High magnetic fields allows us to tune and explore materials to the extreme limits of their phase diagram and thereby distinguish among multiple models. Fast magnetic fields provide record sensitivity in magnetoelectric measurements as well as access to dynamic behavior. The combination of neutron diffraction, high and fast magnetic fields and theory allow us to understand complex spin systems and their multifunctionality. I will also discuss new routes to multiferroic behavior in metal-organic systems that go beyond traditional ferromagnetism and ferroelectricity.
Time and place:2016.9.6, 9:00am, Wuhan National High Magnetic Field Center C204
Presenter:Rongjun Huang
Title:Proximity effect induced transport property between topological insulators (Bi2Se3, Sb doped Bi2Se3 ) and magnetic insulator CoFe2O4
Abstract:In this study, we mainly investigate the proximity effect in topological insulator and magnetic insulator bilayer system. In the experiment, Bi2Se3Sb/CoFe2O4 heterostructure was fabricated by using the molecule beam epitaxial technique and the pulsed laser deposition. A series of heterostructure magnetoresistances (MRs) are measured by physical properties measurement system (PPMS). The weak anti-localization (WAL) is strongly suppressed by the proximity effect in Bi2Se3Sb/CFO interface. The MR results are fitted by the modified HLN equation so that the size of surface state gap can be extracted successfully. The temperature dependent MR of the heterostructures at small and large perpendicular magnetic fields are also measured and analyzed. The results indicate that the phenomenon of surface band gap opening does not occur at small magnetic field because the CFO is in-plane magnetically polarized, but become manifest with increasing magnetic field. The approaches and results accommodated in this work show that CFO can effectively magnetizes Bi2Se3Sb and the heterostructures are promising for TI-based spintronic device applications.
Time and place:2016.6.23, 3:00pm, Wuhan National High Magnetic Field Center C204
Presenter:Weiwei Zhao
Title:Research on quantum effects in the magnetic topological insulators and the low dimensional superconducting
Abstract:Anomalous quantum hall effect has attracted much attention in the experimental and theoretical research after confirmed by the experiment for the first time in 2013.Here I will introduce that in V doped magnetic topological insulators, the first observed zero resistance anomalous quantum hall effect under zero magnetic field and experimental evidence with lossless topology boundary condition (Nature Materials, 14473, Phys. Rev. Lett., 115057, 206).Superconductors as another kind of material of loss-less resistor, its low dimensional system have the rich quantum behaviors, these effects can be used for the carrier of future quantum computing and storage devices. Here I will introduce a few low dimensional quantum behavior research in the superconducting system, including the macroscopic quantum tunneling in superconducting nanowires, single magnetic flux switch, as well as (KTB phase transformation of superconducting thin films, vortex phase transformation, Ising pair of observation, etc.
Time and place:2016.5.24.10am, Wuhan National High Magnetic Field Center B206
Presenter: Victor Pantsyrny
Title:High strength, high conductivity Cu matrix nanostructured composite wires processed by large plastic deformation
Abstract:New class of electrotechnical conductors on the base of nanostructured Cu-Nb alloy is under the consideration in this overview. It is known that anomalous increase of mechanical strength takes place in microcomposite Cu-Nb fine wires cold deformed by the drawing with large amount of deformation. The mechanisms of attaining of the extremely high mechanical strength at the level of 2000 MPa are discussed. The microstructure of heavily deformed Cu-Nb microcomposite consists of the ribbon like Nb filaments embedded in pure Cu matrix. The role of the specific structure of the interphases Cu-Nb boundaries is underlined. The interrelations of the mechanical strength and microstructure parameters such as the thickness of Nb ribbons are described. The decrease of the dislocation density during deformation was observed when the thickness of Nb ribbons and thickness of Cu matrix layers began to be comparable with the characteristic dimension of Frank-Read source of dislocation. It is also shown that the conductivity of nanostructured Cu-Nb materials is defined by the scattering of the electrons on the interphases Cu-Nb boundaries when the thickness of Cu matrix layers attain the mean free path of electrons in copper. The possibility to attain the high combination of conductivity in the range of 40% IACS to 80%IACS and Ultimate Tensile Strength in the range of 800 MPa to 1600 MPa in technical wires is presented. The examples of the high strength nanostructured technical Cu-Nb conductors with cross sections from 0.0007 mm2 (diameter of 30 µm) up to 120 mm2 are presented.
Time and place:2016.5.23, 3:00pm, Wuhan National High Magnetic Field Center C204
Presenter:Yaomin Dai
Title:Non-Fermi liquid behaviors in Fe-based superconductors
Abstract:The normal state of high-temperature (high-Tc) superconductors is very unusual, with the electrical resistivity (or quasiparticle scattering rate) varying with temperature in a peculiar way that deviates significantly from the quadratic T dependence expected from Landau’s Fermi liquid (FL) theory of metals. We study a series of LiFe1−xCoxAs compounds with different Co concentrations by transport, optical spectroscopy, angle-resolved photoemission spectroscopy, and nuclear magnetic resonance. We observe a Fermi-liquid to non-Fermi-liquid to Fermi-liquid (FL-NFL-FL) crossover alongside a monotonic suppression of the superconductivity with increasing Co content. In parallel to the FL-NFL-FL crossover, we find that both the low-energy spin fluctuations and Fermi surface nesting are enhanced and then diminished, strongly suggesting that the NFL behavior in LiFe1−xCoxAs is induced by low-energy spin fluctuations that are very likely tuned by Fermi surface nesting. Our study reveals a unique phase diagram of LiFe1−xCoxAs where the region of NFL is moved to the boundary of the superconducting phase, implying that they are probably governed by different mechanisms.
Time and place:2016.5.16, 3:00pm, Wuhan National High Magnetic Field Center B206
Presenter:Marcelo Jaime
Title:Magnetoelastic Correlations, Frustration, and Bose-Einstein Condensation in Quantum Magnets
Abstract:Quantum magnets are natural realizations of gases of interacting bosons whose relevant parameters such as dimensionality, lattice geometry, amount of disorder, nature of the interactions, and particle concentration can vary widely between different compounds. The particle concentration can be easily tuned by applying an external magnetic field which plays the role of a chemical potential. This rich spectrum of realizations offers a unique possibility for studying the different physical behaviors that emerge in interacting Bose gases from the interplay between their relevant parameters. The plethora of other bosonic phases that can emerge in quantum magnets, of which the Bose-Einstein condensate is the most basic ground state, is intriguing and not always easy to predict [1]. Here we review recent results with some attention paid to the strength of magnetoelastic correlations in quantum magnets as a smoking gun for frustration and broken symmetries crucial to determine the nature of the ground state.
Time and place:2016.5.10, 3:00pm ,Wuhan National High Magnetic Field Center C204
Presenter:David C. Larbalestier
Title:The National High Magnetic Field Laboratory HTS conductor and magnet R&D program
Abstract:The NHMFL has been encouraged by two National Research Council Reports, COHMAG in 2004 and MagSci in 2013, to develop new generations of high field magnets using HTS materials. For much of the first 5 or 6 years [1] the emphasis was primarily on conductor evaluation and development and on small coils [2] but this led to the construction of a 32 T all superconducting coil from REBCO coated conductor that is almost ready for users. However REBCO coated conductors have large magnetization currents that are less desirable for uniform field applications than a multifilament, round, twisted, isotropic conductor like round wire Bi-2212. Recent developments of Bi-2223 laminated with superalloy also bring it back into contention as a high field magnet conductor. I will describe some elements of our program, including the recent 40 T no insulation insert coil (9T in 31T).
Time and place:2016.5.10, 10:00am, Wuhan National High Magnetic Field Center C204
Presenter: Laura H. Greene
Title:High-Temperature Superconductivity: From History to Mystery
Abstract:At we pass the centenary of the discovery of superconductivity, the design of new and more useful superconductors remains as enigmatic as ever. As high-density current carriers with little or no power loss, high-temperature superconductors (HTS) offer unique solutions to fundamental grid challenges of the 21stcentury and hold great promise in addressing our global energy challenge in energy production, storage, and distribution. Traditionally guided by serendipity, our recent materials genome initiative is geared to develop predictive design of HTS. In this pursuit, we have chosen point contact spectroscopy (PCS) to aid in identifying promising candidates; as we have proved PCS to be an identifier of non-Fermi liquid (NFL) behavior above Tc, ubiquitous to all unconventional superconductors. We present a new definition of unconventional superconductivity; that the electronic fluid in the normal state is NFL, and that not necessarily the superconducting order parameter breaks the symmetry of the underlying lattice. We present how these studies will help to categorize and identify promising new HTS candidates.
Time and place:2016.5.10, 5:00pm, Wuhan National High Magnetic Field Center C204
Presenter:Faxian Xiu
Title:Field-driven phase transition and magneto-optical properties in topological Dirac semimetals
Abstract:In this talk, I will first report the Landau level splitting in TDS Cd3As2single crystals under high magnetic fields (up to 60T), which suggests the removal of spin degeneracy by breaking time reversal symmetry. The detected Berry phase develops an evident angular dependence and possesses a crossover from nontrivial to trivial state under high magnetic fields, a strong hint for a fierce competition between the orbit-coupling and the field-generated mass term. Then, I will briefly review our recent progress in chiral anomaly by showing exclusively new approaches to detect the chiral anomaly and the related valley transport in ultra-high mobility Cd3As2Dirac semimetal. Three independent evidences including the E∙B-generated magneto-optical Kerr effect, the negative MR, and the valley transport, are provided as a direct and convincing experimental identification for the chiral anomaly in crystals. Finally, I would like to talk about the magneto-optical measurements in Cd3As2/ZrTe5Dirac semimetals and some exciting transport measurements under ultra-high magnetic field, in which a striking topological phase transition takes place (around 30 T), i.e., the dynamical mass generation. The Dirac electron spontaneously acquires a Dirac mass due to electron-electron interactions. These transitions also manifest themselves as spin density waves in both first and zeroth Landau levels. Our study presents the very first example of the dynamical mass generation phenomenon occurring in three-dimensional massless Dirac fermions in condensed matter physics.
Time and place:2016.4.15, 2:30pm, Wuhan National High Magnetic Field Center C204
Presenter:Ke Zou
Title:Structural studies of high Tc superconductor FeSe/SrTiO3 grown by molecular beam epitaxy
Abstract:The marked enhancement of the superconducting critical temperature for FeSe grown on SrTiO3 (STO) is a notable recent discovery in the field of high temperature superconductivity. A complete understanding of the mechanism for this enhancement has not been elucidated and is thought to be due to how the electronic structure is modified by the interface. We determine the surface reconstruction of SrTiO3 that is used to achieve superconducting FeSe films in experiments. In particular, we observe the existence of a double TiO2 layer and identify the symmetry of the reconstruction at the FeSe/SrTiO3 interface. The double TiO2 layer plays two important roles. First, it facilitates epitaxial growth of FeSe films. Second, ab initio calculations reveal that electron transfer to the FeSe is enhanced by the double layer termination more strongly than by other surface structures of SrTiO3 . The enhanced electron transfer suppresses the hole pocket near the Γ point, leading to a band structure characteristic of superconducting samples. The characterization of the interface structure presented here is a key step towards understanding the electronic properties of this novel superconductor.
Time and place: 2016.4.1, 3:00pm, Wuhan National High Magnetic Field Center C204
Presenter: Hongtao Yuan
Title:Electronic Phase Control with an Electric Field
Abstract:Electric-field control of charge carrier density has attracted much attention since it is remarkably simple for modulating physical properties of condensed matters and for exploring new functionalities with a transistor configuration. To realize novel field-effect modulated electronic phenomena in solids, a broad range of attainable carrier density is always required. However, so far, owing to the limitation of dielectric breakdown in most solid dielectrics, the maximum carrier density accumulated in conventional field-effect transistors (FETs) is quite low (<< 1013 cm-2) and thus seriously limits the tunability of electronic states of solids, for example, not sufficient enough to induce insulator-to-superconductor transition. While, recently a new type of transistor, known as electric-double-layer transistor (EDLT), with ionic liquids (ILs) as gate dielectrics have been proved to be able to effectively attain a high carrier density up to levels of around 1015 cm-2 and to realize a large local electric field up to 50mV/cm at liquid/solid interfaces, which are attracting increasing interests because of their potential to greatly tune electronic states and even to create novel states of matter which are impossible or difficult to obtain in conventional methods. In this presentation, I will discuss the interfacial carrier accumulation within liquid gated EDLTs and their novel tunability of varied electronic phase transitions in oxides, chacogenides and Dirac materials like graphene and topological insulators. I will start with fundamentals of interface electrochemistry and charging mechanism in such fantastic liquid/solid EDL interfaces, for example the competation between electrochemistry charging and electrostatic charging, or the interfacial band alginment and band engineering confirmed with measuring working functions of different ILs by photoemmision spectroscopy (PES). Based on these investigations and by further taking great advantages of high carrier density at EDL interfaces (1015 cm-2), we successfully obtained electric-field-induced insulator-metal transition in ZnO and insulator-superconductor transition in SrTiO3, KTaO3 and ZrNCl. Also we achieved carrier-mediated room temperature ferromagnetism in diluted magnetic semiconductors and giant anomalous Hall effect in magnetically doped topological insulators through modulating the interfacial carrier accumulation. On the other hand, by taking the advantage of large local electric field (50MV/cm), spin-orbit interaction (SOI) and resulting spin splitting of energy band of 2D systems can be regulated with structure inversion asymmetry (SIA) originated from the interfacial band bending and large applied electric fields, and further be used for generating spin-polarized carriers in solids.
Time and place: 2016.3.24, 9:30am, Wuhan National High Magnetic Field Center C204
Presenter: Zhaoming Tian
Title:Exotic topological states and quantum metal-insulator transition in Nd2Ir2O7 pyrochlore iridates
Abstract:Pyrochlore iridates have attracted great interest as prime candidates to host topologically nontrivial states, quantum spin liquid state and quantum phase transition, in particular through the interplay between different degrees of freedom, such as local moments and mobile electrons. Based on our study using our high quality Nd2Ir2O7 single crystals, we will discuss such example, i.e. quadratic band touching state, Weyl semimetallic state and field-induced quantum phase transition in Nd2Ir2O7 pyrochlore iridates.
Time and place: 2015.12.28, 10:00am, Wuhan National High Magnetic Field CenterC204
Presenter: Feng Wang
Title:Exploiting lanthanide luminescence in core-shell nanoparticles
Abstract:In this talk, I systematically introduce our recent efforts on lanthanide-doped core-shell nanoparticles that are generally composed of a layer-by-layer structure with a set of lanthanide ions incorporated into separate layer. I discuss how to fabricate these nanostructures by wet-chemistry method and how to characterize the nanostructure by a combination of electron microscopy and luminescence spectroscopy. Examples will be given to demonstrate highly flexible luminescence processes across UV to NIR spectral region in single core-shell nanoparticles by controlling the dopant composition and distribution of dopant ions in the host lattice. The enhanced ability to manipulate photon energy opens up exciting new opportunities for technological applications.
Time and place:2015.10.29, 4:00pm, Wuhan National High Magnetic Field CenterB206
Presenter:Xiao Hu
Title:Questing after Majorana Particles in Topological Matte
Abstract:A Majorana particle is a fermion and equivalent to its antiparticle, which was proposed by E. Majorana in 1937 in order to explain the neutrino. While not yet confirmed as an elementary particle for about 80 years, physicists are excited recently by its presence in matters, since quasiparticle excitations in topological superconductors behave in the way with “particle being equivalent to antiparticle”, known as the Majorana condition. These Majorana quasiparticles (MQPs) are considered to be useful for realizing decoherence-free quantum computation. In this talk, I will introduce our recent work on search of MQPs in a topological superconductor formed by a topological insulator and a conventional s-wave superconductor. We reveal that, in the spin-resolved spatial-energy distribution of local density of states, there is a checkerboard-like pattern associated with MQP due to the intimate relation among the orbital and spin angular momenta and the phase winding of superconducting vortex. This, if measured successfully by the spin-resolved STM/STS, provides evidence for the long-searched-for Majorana particle.
Timeand place: 2015.10.28, 10:00am, Wuhan National High Magnetic Field CenterB206
Presenter: Cyril Proust
Title:Fermi surface reconstruction by charge order in the pseudogap phase of underdoped copper oxides
Abstract:Over the last years, quantum oscillation measurements have shown that the Fermi surface of underdoped cuprates suffer a drastic modification compare to the large hole-like cylinder observed in the overdoped side. Many studies such as NMR measurements, x-ray scattering point to a reconstruction of the Fermi surface due to charge density wave (CDW). Fermi-surface reconstruction and charge modulations are two universal signatures of underdoped cuprates, which begs the following questions: what is the impact of charge order on the electronic properties of underdoped cuprates? Is the Fermi surface seen by quantum oscillations compatible with a reconstruction by CDW modulations?
After an introduction starting from the observation of quantum oscillations to the discovery of charge order in cuprates, I will present thermodynamic and transport measurements in underdoped cuprates performed in high magnetic fields demonstrating that a phase transition takes place at low temperature. Compelling evidence that the Fermi surface of YBa2Cu3Oyis reconstructed by the CDW order detected by x-ray diffraction comes from the recent discovery of an additional small hole-like pocket in quantum oscillations measurements. The consistency between our quantum oscillation measurements and models of Fermi-surface reconstruction by the CDW order will be discussed.
Time and place:2015.10.26, 10:00am, Wuhan National High Magnetic Field CenterB206
Presenter:Amr S. Helmy
Title:Nano Photonic Architectures for Novel Communications and Sensing Technologies
Abstract:Plasmonic waveguides provide the unique ability to confine light within a few nanometers and allow for near perfect transmission through sharp bends as well as efficient light distribution between orthogonally intersecting waveguide junctions. However, due to free-carrier absorption in the metal, the enhanced mode confinement inevitably entails an increase in the waveguide loss. This lowers the device figure-of-merit achievable with passive plasmonic components and in turn hinders the performance of active plasmonic components. In order to mitigate the losses of these waveguides; a novel methodology for designing multi-layer hybrid plasmonic waveguides has been developed. This class of modes and the design methodology associated charts a route for obtaining modes with propagation loss values in the range of 5-10 dB/mm, while maintaining mode areas that are characteristic of plasmonics. This long range mode behavior empowers a new class of optical devices such as modulators, and detectors with record device sizes and parasitic capacitance values. With these structures as a building block, new levels of optoelectronic integration and performance metrics for athermal transceivers is achievable.
Time and place:2015.10.15, 8:30am, Wuhan National High Magnetic Field CenterB206
Presenter:Johan Vanacken
Title:High Temperature Superconductors in Pulsed High Magnetic Fields
Abstract:Solid-state physics is investigated in high pulsed magnetic fields of B=50 Tesla and above. High magnetic fields are a basic research tool in contemporary condensed matter physics. In many cases, higher magnetic fields improve the understanding of physical phenomena or are even the only way to reveal them.
In this presentation the work done at the high field laboratory in Leuven on high temperature superconducting cuprates and pnictides) will be reviewed. The high field results of hole and electron doped (collaboration IOP - Beijing) cuprates will be presented, as well as the results on the pnictides (Collaboration Nanjing University).
The presentation will show mainly experimental work, stimulating theoreticians to work on the understanding of the complex behavior of high temperature superconductors.
Time and place:2015.10.14, 8:30am, Wuhan National High Magnetic Field CenterB206
Presenter:Greg S. Boebinger
Title:Materials, Energy and Life: Entertaining Aspects of High Magnetic Field Research
Abstract:The MagLab exists to provide its international user community with unique magnets and expertise spanning condensed matter physics, materials research, chemistry, biochemistry, biology, and biomedicine. We generate magnetic fields exceeding two million times the Earth’s magnetic field. This talk seeks to answer the question, “Why would anyone want to do such a thing?” Illustrative examples from the portfolio of user research at the MagLab will include • MATERIALS:tweaking macroscopic quantum phenomena in two-dimensional square lattices of copper and oxygen to achieve high-temperature superconductivity or magnetic Bose-Einstein condensation;• ENERGY:analyzing nature’s most complex fluids, including petroleum, to improve utilization and mitigate pollution;and • LIFE:tracking sodium and gadolinium quantum dots to revolutionize magnetic resonance imaging.During the talk, we anticipate that jokes will very likely be told. The portion of the talk that surveys my own work on high-temperature superconductivity uses magnetic fields to suppress the superconductivity…with a goal of revealing the Wizard who pulls the strings behind the curtain. This work is a collaboration with Scott Riggs, Oskar Vafek and Jon Kemper of the MagLab branch at Florida State University; Jon Betts, Al Bert Migliori and Fedor Balakirev of the MagLab branch at Los Alamos National Laboratory; and W. N. Hardy, Ruixing Liang and Doug Bonn of the University of British Columbia.
