[超导国家重点实验室学术报告] Resonant Inelastic X-ray Scattering study of Magnetic Excitations in high-Tc cuprate superconductors

Resonant Inelastic X-ray Scattering (RIXS) is a photon-in and photon-out technique which becomes popular in recent years due to the fast progress in the instrumentation and by importanttheoretical achievements. RIXS can be used in a very effective way to study the charge and spin excitations in strongly correlated 3d transition-metal systems.

[超导国家重点实验室学术报告] Thin film deposition and high-field transport properties of Fe-based superconductors

The Fe-based superconductors (FBS) are very interesting materials both for basic research as well as in view of potential applications. FBS are unconventional, anisotropic, partially quantum-critical, multi-band superconductors, and all of these characteristics reflect to some extend in the electrical transport properties. They usually show very high upper critical fields and concurrent high irreversibility fields at low temperatures. This opens the possibility of high in-field critical current densities Jc, interesting especially for high-field applications.

[超导国家重点实验室学术报告] Thermoelectric Materials and Fast Ionic Conductors with Structural Instabilities

Copper selenide attracted interest due to high ionic conductivity and efficient thermoelectric properties. I will present results of neutron scattering investigation of the crystal structure, diffuse scattering, quasi-elastic neutron scattering and BWS calculations in the Cu-Se superionic compounds.

[超导国家重点实验室学术报告]Current-induced crossover between two distinct quantum coherence effects in superconducting Nb nano-ring

   I will report on our new magnetoresistance measurements in a granular Nb nano-ring revealing current-induced crossover between two distinct quantum coherence effects. At low bias currents, Cooper-pairs coherence is manifested by Little-Parks oscillations with flux periodicity of h/2e. At high bias currents, magnetoresistance oscillations with flux period of h/e are observed and interpreted as Aharonov-Bohm oscillations, reflecting phase coherence of individual quasi-particles. The model explaining these data views the ring as a chain of superconducting grains weakly coupled by tunnel junctions. Low bias currents allow coherent tunneling of Cooper pairs between the grains. Increasing the current above the critical current of all the junctions creates a quasi-particles conduction channel along the ring, allowing for quantum interference of quasi-particles.

[超导国家重点实验室学术报告] Combining superconductors and ferromagnets :  from p-junctions to spin-triplets

  The study of superconductor (S) / ferromagnet (F) hybrids is a by now a well-established field. One of the early interests was formed by the realization that a thin F layer in an SFS structure could lead to a phase change of π between the condensates in the two S contacts. Such π-junctions are still investigated for use in devices such as superconducting memory elements. More recently it became clear that also spin triplet pair correlations can be induced in ferromagnets and lead to long-range supercurrents. These triplets are the centrepiece of the newly emerging field of superconducting spintronics. They are generated by engineering magnetic inhomogeneities at the S/F interface, for instance by using a sandwich of different layers, in order to mix the two spin channels and rotate the quantization axis. Using ferromagnets also allows control over the supercurrents, and two examples will be discussed. One concerns CrO2, which is a fully spin-polarized ferromagnet, in which it is possible to induce supercurrents with very high current density (above 109 A/m2) over distances approaching a micron. The other example demonstrates a device which also allows control over the supercurrent distribution based on a disk-shaped Josephson junction with a Co layer containing a magnetic vortex. With a magnetic field  we can change the position of the vortex, and thereby the supercurrent pathways. This device exemplifies a novel platform where adaptable supercurrent paths can be customized for a given application.

[超导国家重点实验室学术报告]Go beyond the performance of Nb cavity: potential of alternative materials in SRF

Superconducting radio-frequency (SRF) resonant cavities for particle accelerators regularly achieve a very high qualityfactor, Q0~1010, and high accelerating electric field Eacc~30-40MV/m. Improving these two parameters even more has been the main challenge for SRF researchers. Cavities must be operated in Meissner state to keep Q0 very high, so naively, the maximum field inside the cavity is limited by Hc1 of the cavity material. Thus, the modern SRF cavities are made from pure Nb with Hc1~170mT at 2K, which is the largest Hc1 among materials available.This corresponds to 40MV/mof the present standard cavity shape for electron accelerators. To achieve much higher Eacc, a simple bulk Nb is not enough, but Nblooks the best because of its Hc1. Then, how to go beyond Nb cavity?In this talk, the superconductor–insulator–superconductor (S’IS) multilayer structure forapplication in SRF cavities for higher Eaccis reviewed. S’IS multilayer consists of a thin superconducting layer with a higher thermodynamic critical field formed on a bulk superconductor with a higher Hc1 (i.e., Nb).The theoretical field limit,optimum layer thicknesses and material combination are discussed for theS’IS structure and are also reviewed for the superconductor–superconductor (S’S) bilayer structure.

[超导国家重点实验室学术报告] Statistics of thermomagnetic breakdown in Nb superconducting films

As dissimilar as they might appear, lightning during a dielectric breakdown, failure ofinterconnects in integrated circuits, snow avalanches on mountain slopes, or popcornexplosion share a common physics ground corresponding to the triggering of acatastrophic event when a threshold stress is exceeded. Interestingly, even if the sameexperience is repeated under identical environmental conditions, the event will nothappen at exactly the same threshold stress. The question as to whether the statisticaldistribution of this threshold stress follows a universal law for all phenomena isexperimentally difficult to answer, since exceeding the threshold point brings alongdramatic consequences leading to irreversible changes in the system. Remarkably,superconducting materials offer a unique opportunity to investigate the statisticaldistribution of similar catastrophic events consisting of non-destructive magnetic fluxavalanches in the same sample and thus ruling out the spreading factors associated tounavoidable uncontrollable variations in the replicas of the system.

[超导国家重点实验室学术报告] Controlling Electronic Properties of Correlated Two-Dimensional Materials

Vast opportunities exist in correlated Two-dimensional (2D) materials: they feature a number of exotic phenomena such as charge density waves (CDWs), high-temperature superconductivity (HTS) and magnetism. However, experimentally extracting monolayers from bulk correlated materials and control their electronic properties turned out to be extremely challenging. In this talk, I will introduce the fabrication and control technique we developed in the past years and intriguing results we obtained in several 2D systems. In the model CDW material 1T-TaS2, we are able to control the phase transitions by thinning it down to the 2D limit or electrochemical gating. In itinerant magnet Fe3GeTe2, we observe intrinsic ferromagnetism in the 2D limit and the Curie temperature can be turned up to room temperature. In monolayer high-temperature superconductor Bi2Sr2CaCu2O8+d, we obtain a phase diagram lack of dimensionality effect, which indicates that monolayer cuprate contains all essential physics of HTS.

[超导国家重点实验室学术报告] Interplay between CDW and superconducting fluctuations in high-Tc cuprates

The phase diagram of underdoped superconducting cuprates is known to be extremely rich and complex. In addition to a transition from an antiferromagnetic insulator at half-doping to a strange metal at optimal doping, and the opening of a pseudogap in the density of states, the normal state is also the location for a charge ordering and a Fermi surface reconstruction. By analyzing La1-xSrxCuO4 thin films resistivity under high pulsed magnetic field for various doping, I will carefully inspect the behavior of superconducting thermal and quantum fluctuations and the existence of quantum critical points. I will extract a systematic (H,T) phase diagram and discuss the interplay between charge ordering and superconductivity. In particular I will relate our findings to a theoretical model proposed by J. Lorenzana and A. Attanasi, which enables to demonstrate that disorder can induce a charge-density-wave to become superconducting. This paradoxical effect occurs close to the phase boundary between CDW and SC and is analogous to the occurrence of supersolid effects in 4He. It originates from the topological stabilization of filamentary superconductivity at the interfaces of different CDW domains at low temperature and intermediate/large magnetic fields. By comparing experimental and theoretical phase diagrams I will show that the previously observed two-step phase transition in under-doped films is related to the interplay between superconducting fluctuations and CDW. I will then analyze the doping dependence of this diagram and show that p=1/8 is specific for this CDW/SC competition at least in LSCO, since the coexistence takes place at zero magnetic field for this specific doping.

[超导国家重点实验室学术报告] Quasi-periodic magnetic flux avalanches in doubly-connected superconductors

     Magneto-optical imaging (MOI) has been established as a powerful tool for studying magnetic flux distributions in superconductors. In particular, this technique has been extensively utilized in the study of flux penetration in form of dendritic avalanches in a variety of superconducting films. it was found that with increasing field above a certain threshold, , dendrites penetrate the samples at random fields and propagate along paths that terminate inside the sample. In this talk I will focus on our recent findings, revealing a dramatic effect of the sample topology on the dendritic phenomenon. Specifically, MOI of Nb rings, open rings and strips, reveal that that dendrites in a ring can cross the entire width of the sample. Such dendrites appear when the difference between the applied field and the average field inside the central hole reaches a certain threshold level, . With increasing applied field, this condition is reached quasi-periodically as a crossing dendrite creates momentarily a hot channel through which flux flows into the central hole, balancing the field inside the hole with that outside the ring. The threshold  differs in magnitude from the onset field  of magnetic instability, and exhibits qualitatively different dependence on the rim width.