Supraleitung in Seltenerdmetall-Carbidhalogeniden des Typs SE2X2C2

Superconductivity in Rare Earth Metal Carbide Halides of the Type SE₂X₂C₂ The metallic nature of the carbide halides Y₂X₂C₂ is due to Y? C covalency. The superconductivity of the compounds is attributed to a pairwise attraction of conduction electrons by C₂-π* states at the Fermi level. The hypothesis is followed by experiments and band structure calculations. – Neutron powder diffraction reveals d(C? C) = 128(1) pm for Y₂Br₂C₂. X-ray single crystal investigations on Y₂Br₂C₂ and Y₂I_1.5Br_0.5C₂ show a characteristic variation of the coordination of the C₂ unit. Systematic changes of the average halide radius in Y₂(X,X′)₂C₂ (X,X′ = Br, Cl I, Cl and I, Br) lead to a monotonic increase of T_c = 2.3 K (X = Cl) via T_c = 5.05 K (X = Br) to a maximum T_c = 11.2 K for Y₂I_1.6Br_0.4C₂. No isotope effect for ¹²C/¹³C could be detected. Photoelectron spectra of Y₂Br₂C₂ (excitation energies between 40 and 140 eV) are compared with the results of band structure calculations (LMTO, E.H.). The electronic structure reveals two bands crossing the Fermi level. One of them has C₂-π*-Y-d_xz,yz character and exhibits a saddle-point at E_F. The other intersects the Fermi level with large dispersion and has exclusively Y-d character at the crossing point. The results are discussed with respect to theoretical models (van Hove singularity, local pairs and itinerant electrons).     

Vortices in weakly coupled layered superconductors

The vortex system in high-temperature layered superconductors exhibits a rich phase diagram with many proposals of phase transitions modifying the correlations both within and between the layers. We focus on the limit where the magnetic coupling between “pancake” vortices dominates over the interlayer Josephson coupling. The weak, but long-ranged nature of this magnetic interaction allows for an accurate “mean-field” treatment where the pancakes in each layer move independently in a self-consistent substrate potential. We calculate the form of the two relevant phase transitions in this system. First, we determine when the substrate potential is too weak to stabilize the two-dimensional (2D) fluctuations and the lattice evaporates to a pancake gas. Second, within the lattice we find a Kosterlitz–Thouless unbinding transition of vacancies and interstitials. For a small but finite Josephson term, this is identified with the phase-decoupling transition.     

Looking at the superconducting gap of iron pnictides

THz and infrared spectroscopies are widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron-phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides.     

超导磁储能装置在风电场中控制策略的研究

超导磁储能装置(SMES)是将超导技术、电力电子技术、控制理论和能量管理技术相结合的一种新型储能装置,它从及时补偿系统中由于各种原因产生的不平衡功率这一新的角度出发,考虑提高电力系统稳定性的问题。理论研究表明:这是一种提高电力系统稳定性有效的新措施。为了促使这一理论的广泛应用,同时进一步提高SMES的可靠性,研究了将超导磁储能装置应用于风电场,可稳定系统输出;并在此基础上,对风电场中超导磁储能装置的关键技术之一——系统的信号选取和控制策略做了研究。最后对其发展趋势进行了介绍。     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

The emergence of bound states in a superconducting gap at the topological insulator edge

We consider the edge of a superconducting topological insulator with the impurity in the presence of the Zeeman field. We analytically prove that in the trivial phase two Andreev bound states (ABSs) arise with energies moving from the superconducting gap edges to zero forming two Majorana-like bound states, as the impurity strength varies from 0 to ±2. When the Zeeman field is locally perturbed, ABSs arise both in the trivial and topological phases, but in the topological phase ABSs with energy near the gap edges cannot transform into Majorana bound states and vice versa.     

第二类拓扑外尔半金属WP2高压超导相的常压截获

第二类拓扑外尔半金属WP₂高压下发生超导转变,然而,高压超导相能否在常压截获还没有相关报道.本文对密封于金刚石对顶砧中的WP₂高压超导相在室温条件下快速卸压至不同压强,原位测量电阻-温度曲线,验证能否在常压截获高压超导相.研究表明：WP₂的高压超导相可以在常压截获,从而提供了一个通过角分辨光电子能谱和扫描隧道谱探寻马约拉纳零能模的拓扑超导体材料平台.     

Vortex analysis of the periodic Ginzburg–Landau model

We study the vortices of energy minimizers in the London limit for the Ginzburg–Landau model with periodic boundary conditions. For applied fields well below the second critical field we are able to describe the location and number of vortices. Many of the results presented appeared in [H. Aydi, Doctoral Dissertation, Université Paris-XII, 2004], others are new.