高气压反射式高能电子衍射仪监控脉冲激光外延氧化物薄膜

在超高真空分子束外延（MBE）生长技术中，反射式高能电子衍射仪（RHEED）能实时显示半导体和金属外延生长过程，给出薄膜表面结构和平整度的信息，成为MBE必备的原位表面分 析仪.为了研究氧化物薄膜如高温超导（YBa₂Cu₃O₇） 、铁电薄膜（Sr_1-xBa_x TiO₃）及它们的同质和异质外延结构的生长机理，获得高质量的符合各种应用 需要的氧化 物多层薄膜结构，在常规的制备氧化 关键词： 高温超导薄膜 RHEED     

Superconducting anisotropy and vortex pinning in CaKFe_4As_4 and KCa_2Fe_4As_4F_2

The vortex pinning determining the current carrying capacity of a superconductor is an important property to the applications of superconducting materials.For layered superconductors,the vortex pinning can be enhanced by a strong interlayer interaction in accompany with a suppression of superconducting anisotropy,which remains to be investigated in iron based superconductors(FeSCs)with the layered structure.Here,based on the transport and magnetic torque measurements,we experimentally investigate the vortex pinning in two bilayer FeSCs,CaKFe₄As₄(Fe1144)and KCa₂Fe₄As4F₂(Fe12442),and compare their superconducting anisotropyγ.While the anisotropyγ≈3 for Fe1144 is much smaller thanγ≈15 in Fe12442 around Tc,a higher flux pinning energy as evidenced by a higher critical current density is found in Fe1144,as compared with the case of Fe12442.In combination with the literature data of Ba_0.72K_0.28Fe2As₂ and Nd Fe As_O0.82F_0.18,we reveal an anti-correlation between the pinning energy and the superconducting anisotropy in these Fe SCs.Our results thus suggest that the interlayer interaction can not be neglected when considering the vortex pinning in Fe SCs.     

对比生物体系以及合成体系中单核铜氧物种对碳氢键和氧氢键活化的理论研究展望

由于具有较好的催化性能,含过渡金属的酶一直备受研究者的关注.其中,铜作为生物体中含量仅次于铁和锌的过渡金属,在新陈代谢过程中发挥着重要作用.铜酶广泛存在于自然界中,该类生物大分子涉及电子转移、氧化还原、氧气的运输与活化等生物化学过程.多种铜酶在氧气活化方面表现出引人注目的性质,例如:颗粒状甲烷单加氧酶(pMMO)、多糖单加氧酶(LPMO)、双铜单加氧酶肽基甘氨酸α-羟基化酶(PHM)和多巴胺β-单加氧酶(DβM),它们均可活化氧气,并生成相应的铜-氧活性物种.铜酶或铜配合物活化氧气可生成多种铜-氧活性物种,包括Cu(II)-O₂^?,Cu(II)-OOH(R),Cu(II)-O^?及Cu(III)-OH等.这些活性物种通常具有不同强度的攫氢能力,可能是铜酶C-H/O-H活化的活性氧物种.近几十年来,为了深入揭示铜酶的催化活性和反应机理,阐明关键铜-氧中间体的结构和性质,研究人员合成了众多含铜配合物体系,并模拟探索了铜-氧活性物种的反应活性尤其是对底物C-H/O-H键活化能力.本文聚焦于含有单铜活性位点的酶及其配合物体系.结合密度泛函理论计算,深入比较了铜酶及配合物体系中铜-氧活性物种反应活性,总结得到以下普遍规律:(1)MN15泛函可准确描述单核铜氧配合物的热动力学性质,对各类铜氧物种催化的C-H/O-H键活化计算中,得到的热动力学性质都能与实验结果很好地吻合;(2)Cu(II)-O₂^?在生物体系和人工合成体系中表现出一致的反应活性:它对O-H键活化表现出高反应活性,但对C-H键反应活性较差,因此不太可能是铜酶中C-H活化的活性中间体;(3)Cu(II)-OOH没有活化C-H键的能力,但其近端氧原子上的自由基特性使其能够攫取中等强度O-H键上的氢原子,另外该物种还可以与另一分子Cu(I)偶联形成双铜物种.因此,在生物体系和合成体系中Cu(II)-OOH均可作为氧气活化路径上的关键中间体,但并不能作为C-H键活化的氧化剂;(4)Cu(II)-O^?对C-H键活化具有高反应活性,因此该类物种可能在单铜单加氧酶的C-H键活化中起关键作用;(5)尽管许多实验证明Cu(III)-OH对C-H键的活化有较高的反应活性,但是在单加氧酶中,生成这一物种在热力学上是非常不利的,因此不太可能是单加氧酶中的活性物种.综上,本文的观点可以为生物体系及合成体系中的铜氧物种的反应活性提供新的视角与思考.     

Development of series SQUID array with on-chip filter for TES detector

A cold preamplifier based on superconducting quantum interference devices(SQUIDs)is currently the preferred readout technology for the low-noise transition edge sensor(TES).In this work,we have designed and fabricated a series SQUID array(SSA)amplifier for the TES detector readout circuit.In this SSA amplifier,each SQUID cell is composed of a first-order gradiometer formed using two equally large square washers,and an on-chip low pass filter(LPF)as a radiofrequency(RF)choke has been developed to reduce the Josephson oscillation interference between individual SQUID cells.In addition,a highly symmetric layout has been designed carefully to provide a fully consistent embedded electromagnetic environment and achieve coherent flux operation.The measured results show smooth V-Φcharacteristics and a swing voltage that increases linearly with increasing SQUID cell number N.A white flux noise level as low as 0.28μφ;/Hz;is achieved at 0.1 K,corresponding to a low current noise level of 7 pA/Hz;.We analyze the measured noise contribution at mK-scale temperatures and find that the dominant noise derives from a combination of the SSA intrinsic noise and the equivalent current noise of the room temperature electronics.     

YBa2Cu3O7-δ薄膜厚度在多层异质结构(Ag/Ba0.1Sr0.9TiO3/YBa2Cu3O7-δ/LaAlO3)中对Ba0.1Sr0.9TiO3介电性能的影响研究

铁电/超导(Ba, Sr)TiO3/YBa2Cu3O7-δ异质结在可调谐微波器件方面具有非常好的应用前景.我们采用1.2°斜切LaAlO3基片,以脉冲激光沉积法(PLD)制备出性能较好的Ba0.1Sr0.9TiO3/YBa2Cu3O7-δ (BST/YBCO)异质薄膜.并进一步研究了YBCO薄膜厚度对BST性能的影响.研究发现当YBCO薄膜厚度增加到180nm附近时,其生长模式由二维step-flow转变为三维岛状模式,严重损害了在YBCO上面生长的BST薄膜的介电性能.具体表现在:BST薄膜的介电常数和可调谐率明显降低,介电损耗和漏电流却大幅度上升.通过测量电容与温度的关系,以应力效应模型对这一实验现象作出解释,认为YBCO薄膜厚度超过临界值,生长模式的转变促使晶格失配应力在YBCO和BST薄膜中得到释放,这导致BST/YBCO界面粗糙,以及BST薄膜中产生了大量的位错和缺陷,BST薄膜的性能因而大为降低.此外,通过对完全相同条件生长的单层YBCO薄膜的表面形貌进行了AFM研究,测试结果进一步验证了YBCO薄膜厚度增加到180nm时,其表面变得异常粗糙,均方根粗糙度(RMS)从120nm厚度时的3nm增加到180nm厚度时的9nm.因此,我们提出:通过严格控制底层YBCO薄膜的厚度,进而控制它的生长模式,能够非常有效地提高BST薄膜的介电性能.     

带平板封头的双层爆炸容器动力响应的实验研究

对采用平封头的圆柱形双层爆炸容器在内部装药爆炸所产生的载荷作用下的动力响应进行了实验研究。研究了容器内部抽真空和内层容器结构形式等因素对爆炸容器弹塑性响应的影响,探讨了平封头应用的可行性。     

The 0–π Phase Transition in Epitaxial NbN/Ni_(60)Cu_(40)/NbN Josephson Junctions

We fabricate high quality superconductor/ferromagnet/superconductor(SFS) Josephson junctions using epitaxial NbN/Ni_(60)Cu_(40)/NbN trilayer heterostructures. Both experimental measurements and theoretical calculations of the ferromagnet layer thickness dependence of the Josephson critical current are performed. We observe the damped oscillation behavior of the critical current as a function of the ferromagnetic layer thickness at 4.2 K,which shows a 0–π phase transition in this type of magnetic Josephson junction. Clear 0– and reverse –0 phase transitions occur around the Ni_(60)Cu_(40) thicknesses of 3.2 and 6.7 nm. Numerical calculations based on the quasi-classical Usadel equation and the Green function fit well with the experimental results. Compared with the dirty limit, the intermediate regime without the dead layer gives better fit for our SFS Josephson junctions because of the epitaxial structure. Both of the 0-and -phase junctions show the ideal magnetic field dependence with a Fraunhofer-like pattern at 4.2 K.     

Fabrication and characterization of all-Nb lumped-element Josephson parametric amplifiers

Josephson parametric amplifiers (JPAs) with nearly quantum-limited noise performance have become indispensable devices for the measurements of superconducting quantum information. We have developed an all-Nb lumped-element flux-driven JPA operating in the three-wave mixing mode. Our Nb-based JPA comprises Nb/Al-AlO_x/Nb Josephson junctions, a parallel-plate capacitor with SiO₂ dielectric sandwiched between two Nb layers, a bottom coplanar waveguides layer, and a top Nb wiring layer. We experimentally demonstrate a 20 dB gain over a 190 MHz bandwidth, a mean 1 dB compression of -123 dBm, and near quantum-limited noise performance. This fabrication process can be further used to design impedance transformed parametric amplifiers for multiple-qubit readout.