Tunneling conductance in a gapped graphene-based superconducting structure: Case of massive Dirac electrons

The tunneling conductance in a NG/SG graphene junction in which the graphene was grown on a SiC substrate is simulated. The carriers in the normal graphene (NG) and the superconducting graphene (SG) are treated as massive relativistic particles. It is assumed that the Fermi energy in the NG and SG are E_FN400 meV and E_FS400 meV+U, respectively. Here U is the electrostatic potential from the superconducting gate electrode. It is seen that the Klein tunneling disappears in the case where a gap exist in the energy spectrum. As U→∞, the zero bias normalized conductance becomes persistent at a minimal value of G/G₀1.2. The normalized conductance G/G₀ is found to depend linearly on U with constant slope of , where is the size of the gap Δ opening up in the energy spectrum of the graphene grown on the SiC substrate. It is found that G/G₀2+αU for potentials in the range −270 meV<U<0 meV and G=0 for potentials U<−270 meV. As α→∞, the conductance for eV=Δ (V is the bias voltage placed across the NG/SG junction) can be approximated by a unit step function G(eV=Δ,U)/G₀2Θ(U). This last behavior indicates that a NG/SG junction made with gapped graphene could be used as a nano switch having excellent characteristics.     

REBCO厚膜的液相外延生长研究

本文从基片和种膜、LPE的初始阶段研究和超导性能的控制这三个方面回顾总结了液相外延生长REBCO(RE=稀土元素)厚膜的研究进展.我课题组发现的高温超导体氧化物YBCO的过热现象直接证实了采用低包晶熔点的YBCO种膜可以生长包晶温度较高的REB-CO,丰富了种膜的选择范围,此现象尤为值得关注.在LPE生长的初始阶段存在种膜的部分溶解、夹杂的形成和与稀土元素种类有关的择优生长等现象,用包括曲率效应和界面能影响的粗化机制可以解释以上现象.因而生长高结晶性的LPE厚膜时选用高品质的种膜甚为重要.为优化生长工艺,须考虑种膜的品质、熔剂中的Ba/Cu比和气氛对Mg的污染和结晶取向,RE离子对Ba的替代等方面问题,以达到获得高质量的REBCO LPE厚膜的目的.     

一种宽带高温超导微带天线的研究

为提高辐射效率和扩展工作带宽,对一种口径耦合馈电的多层介质高温超导微带天线进行了设计.设计时首先利用高温超导体低表面电阻的特性,通过在馈电网络部分引入高温超导材料的方法提高了天线的辐射效率,然后针对高温超导微带天线衬底介电常数过高,导致天线带宽过窄的问题,在天线的结构上增加了一层真空衬底,降低了天线的平均介电常数,同时,为进一步扩展天线的工作带宽,设计时在天线的馈电部分还采用了口径耦合馈电的方式.仿真结果表明了文中设计思想的可行性和有效性.     

高温超导体时效与记忆效应实验研究

熔融织构的钇钡铜氧化物(YBCO)高温超导块材具有较高的临界电流密度和较好的力学性能,在制作高场磁体、悬浮列车等方面具有重要的应用。本文首先在恒定的液氮温度下,通过对不同等待时间, 场冷和零场冷条件下悬浮力特征的测试, 发现高温超导块材在液氮温度表现出较强的时效性;接着,对于不同温度循环引起的悬浮系统悬浮力时间弛豫特征研究显示出高温超导块材在低温下具有鲜明的记忆特性,随着温度的升高,其记忆效应逐渐消失,且正、负温度循环对其记忆效应影响表现出不一致性。所有这些特性表明,高温超导块材具有和自旋玻璃类似的性质,其根源为超导体内部复杂的微观结构。     

Structural,magnetic and electronic properties of the iron–chalcogenide AxFe2-ySe2 (A=K,Cs, Rb,and Tl,etc.) superconductors

The latest discovery of a new iron–chalcogenide superconductor A_xFe_2-ySe₂ (A = K, Cs, Rb, and Tl, etc.) has attracted much attention due to a number of its unique characteristics, such as the possible insulating state of the parent compound, the existence of Fe-vacancy and its ordering, a new form of magnetic structure and its interplay with superconductivity, and the peculiar electronic structures that are distinct from other Fe-based superconductors. In this paper, we present a brief review on the structural, magnetic and electronic properties of this new superconductor, with an emphasis on the electronic structure and superconducting gap. Issues and future perspectives are discussed at the end of the paper.     

Anisotropy of p-wave Josephson junction

<正>The anisotropy of the dc Josephson current in the superconducting junctions of the p-wave equal spin pairing symmetry is theoretically investigated by the Furusaki-Tsukada-like formula.The current phase relations exhibit different oscillation periods and different phase shifts for the current along different directions,respectively.     

Band structure,Fermi surface,and superconducting gap in FeAs-based superconductors revealed by angle-resolved photoemission spectroscopy

In this paper, we present a brief review on our angle-resolved photoemission measurements on the band structure, Fermi surface, and superconducting gap of the newly-discovered FeAs-based high temperature superconductors. (1) The Fermi surface of the FeAs-based compounds are characterized by the hole-like Fermi surface sheets near Γ (0, 0) and the existence of singular Fermi spots near M(π,     

Theory for superconductivity in iron pnictides at large coulomb U limit

Superconductivity in iron pnictides is studied by using a two-orbital Hubbard model in the large U limit. The Coulomb repulsion induces an orbital-dependent pairing between charge carriers. The pairing is found mainly from the scattering within the same Fermi pocket where usually one single orbital dominates. The inter-pocket pair scatterings determine the symmetry of the singlet superconductivity, which is an extended s-wave at small Hund’s coupling, and d-wave at large Hund’s coupling and large U. The former is consistent with recent experiments of ARPES and Andreev reflection spectroscopy. Spin triplet states only become important at large exchange interaction J.     

电子型掺杂高温超导体La2-xCexCuO4飞秒时间分辨动力学研究

利用飞秒时间分辨光抽运探测技术研究了电子型掺杂La_2-xCe_xCuO₄(LCCO)高温超导材料的准粒子超快动力学过程.得到低温(T<0.7T_c)、转变温度附近(0.7T_c≤T≤T_c)和高温(T>T_c)三个温区内的动力学行为.研究发 关键词： 电子型掺杂高温超导体 飞秒时间分辨 准粒子 声子瓶颈