常压和高压合成MgB2的低温比热及两个超导能隙研究

测量了常压和高压合成MgB2超导样品6K—80K的比热,并在数据拟合中考虑了爱因斯坦振动模的贡献,得到其电子比热数据.解释了超声测量得到的德拜温度和低温比热拟合结果之间的巨大差异.并且在12K附近观察到了异常的电子比热,此异常来源于MgB2的第二个能隙.此外,对高压和常压合成对样品的影响做了初步分析. 关键词： 低温比热 MgB2 能隙     

La2-xBaxCuO4-y单晶样品的制备

本文采用熔融法成功地制备了La2-xBaxCuO4-y系列单晶,详细介绍了对单晶做的各项品质分析,样品的X衍射结果显示,该系列样品的晶体结构是典型的K2NiF4214T相结构,为四方相,且都无杂相峰,样品的单相性很好,样品的SEM结果显示,单晶的ab表面有少量缺陷,但表面大部分的品质还是很高的,样品的EDX组份分析实验结果显示,样品中有少量的Al进入,其含量估计大约为样品中Cu含量的2％。     

准一维Cr基超导体RbCr3As3的超导能隙

RbCr₃As₃是具有[（Cr₃As₃）^-]_∞线性链的准一维超导体,超导转变温度约为6.6 K.对RbCr₃As₃单晶进行了电输运和极低温热输运性质的研究.低温下,拟合了RbCr₃As₃正常态电阻率随温度的变化,发现其满足费米液体行为.通过拟合超导转变温度随磁场的关系,得到RbCr₃As₃单晶的上临界场约为25.6 T.对RbCr₃As₃进行了零场下的极低温热导率测量,得到其剩余线性项为7.5 μW·K^-2·cm^-1,占正常态热导率值的24%.测量不同磁场下RbCr₃As₃的热导率,发现与单带s波超导体相比较,RbCr₃As₃剩余线性项随磁场增加相对较快.这些结果表明RbCr₃As₃单晶很可能是有节点的非常规超导体.     

112型铁基化合物EuFeAs2的单晶生长与表征

铁基超导体中含有一类特殊的112型结构化合物,其层状结构中含有一层锯齿形的As链构型.本文报道了用CsCl助熔剂法生长新型铁基112型EuFeAs₂母体单晶的具体方法,以及对该单晶的结构和物性的详细表征.通过能量色散X射线能谱扫描对单晶样品进行的化学成分分析,以及单晶X射线衍射的结构解析,确定该单晶样品属于EuFeAs₂相,结构精修得到EuFeAs₂具有空间群为Imm2（No.44）的正交晶体结构,晶格常数分别是a=21.285（9）Å,b=3.9082（10）Å,c=3.9752（9）Å.通过低温电阻测量,发现在110 K附近和46 K附近存在两个异常电阻跳变.进一步分析表明,110 K附近存在两个邻近的相变,这两个相变与铁基母体材料中常见的结构相变和Fe²⁺的反铁磁相变相符合.结合磁化率测量分析,可知46 K附近的相变属于Eu²⁺的反铁磁相变.     

多晶MgC1-xNi3的微结构弛豫谱研究

我们测量了千赫兹范围内超导及非超导MgC1-xNi3样品从液氮温度至室温的微结构弛豫谱.对于超导样品,发现了两个内耗峰:P1(300K)和P2(125K).而对于非超导样品,P1峰位移到了250K,而P2峰几乎被完全抑制.变频实验发现随着测量频率升高,P2峰移向高温,计算所得激活能为0.13eV.我们认为P2峰可能与C原子在偏心位置的跳跃有关.进一步我们可以猜测C原子的这种行为与许多实验中观察到的超导样品在150K和50K正常态度越行为相对应.     

Tip-induced superconductivity commonly existing in the family of transition-metal dipnictides MPn_2

We report the tip-induced superconductivity on the topological semimetal NbSb₂, similar to the observation on TaAs₂ and NbAs₂. Belonging to the same family of MPn₂, all these materials possess similar band structures, indicating that the tip-induced superconductivity may be closely related to their topological nature and share a common mechanism. Further analysis suggests that a bulk band should play the dominant role in such local superconductivity most likely through interface coupling. In addition, the compatibility between the induced superconductivity and tips’ ferromagnetism gives an evidence for its unconventional nature. These results provide further clues to elucidate the mechanism of the tip-induced superconductivity observed in topological materials.     

Mg，C含量和Co替代对MgCNi3的结构和超导电性的影响

研究了MgCNi₃中Mg和C含量对其成相和超导电性的影响,以及Co掺杂的MgCNi_3-xCo_x体系的结构和超导电性质.发现初始配料中适当过量的Mg和C有利于获得单相样品并提高样品超导转变温度,最佳名义配比是MgC_1.45Ni₃且Mg过量20%(质量分数).掺Co的MgCNi_3-xCo_x体系可形成连续的固溶体,随着x增大,晶格常数缓慢变小,而T_c明显下降.Co（Mn）替代Ni会抑制超导电性,并且Co的抑制作用比Mn要小.     

Superconductivity in Sm-doped CaFe_2As_2 single crystals

In this article,the Sm-doping single crystals Ca_(1-x)Sm_xFe_2As_2(x = 0 ~0.2) were prepared by the Ca As flux method,and followed by a rapid quenching treatment after the high temperature growth.The samples were characterized by structural,resistive,and magnetic measurements.The successful Sm-substitution was revealed by the reduction of the lattice parameter c,due to the smaller ionic radius of Sm~(3+)than Ca~(2+).Superconductivity was observed in all samples with onset T_c varying from 27 K to 44 K upon Sm-doping.The coexistence of a collapsed phase transition and the superconducting transition was found for the lower Sm-doping samples.Zero resistivity and substantial superconducting volume fraction only happen in higher Sm-doping crystals with the nominal x 0.10.The doping dependences of the c-axis length and onset T_c were summarized.The high-T_c observed in these quenched crystals may be attributed to simultaneous tuning of electron carriers doping and strain effect caused by lattice reduction of Sm-substitution.     

Electric Field Induced Permanent Superconductivity in Layered Metal Nitride Chlorides HfNCl and ZrNCl

Devices of electric double-layer transistors(EDLTs) with ionic liquid have been employed as an effective way to dope carriers over a wide range. However, the induced electronic states can hardly survive in the materials after releasing the gate voltage V_G at temperatures higher than the melting point of the selected ionic liquid. Here we show that a permanent superconductivity with transition temperature T_c of 24 and 15 K is realized in single crystals and polycrystalline samples of HfNCl and ZrNCl upon applying proper V_G's at different temperatures.Reversible change between insulating and superconducting states can be obtained by applying positive and negative V_G at low temperature such as 220 K, whereas V_G's applied at 250 K induce the irreversible superconducting transition. The upper critical field H_(c2) of the superconducting states obtained at different gating temperatures shows similar temperature dependence. We propose a reasonable scenario that partial vacancy of Cl ions could be caused by applying proper V_G's at slightly higher processing temperatures, which consequently results in a permanent electron doping in the system. Such a technique shows great potential to systematically tune the bulk electronic state in the similar two-dimensional systems.     

Magnetic and Transport Properties of the Kondo Lattice Compound YbPtAs

We report the electrical resistivity, magnetic susceptibility, and heat capacity studies on a new intermetallic compound YbPtAs, which crystallizes in a modified AIB2 type structure. The Yb ions in YbPtAs are in a trivalent state and order antiferromagnetically around Neel temperatures T_(N1) = 6.5 K and T_(N2) = 2.2 K,respectively,deduced both from the magnetic susceptibility χ(T) and heat capacity C(T) measurements. The magnetic contribution in resistivity, ρ_m(T), exhibits a broad maximum at around 100 K and a logarithmic temperature dependence in the high-temperature region, indicative of the presence of the Kondo effect in YbPtAs.