Quantum Griffiths singularity (QGS) has been experimentally observed in a range of two-dimensional (2D) superconducting systems. Although it is theoretically suggested that the QGS also exists in three-dimensional (3D) superconductors, there is almost no experimental support to the theoretical prediction. In the present paper, we observe the occurrence of QGS in a series of $\sim$80-nm-thick Mo$_{0.8}$Ti$_{0.2}$N$_x$ ($0.84 \lesssim x \lesssim 1.12$) superconducting films near the field-driven superconductor-metal transition (SMT). These films have a NaCl structure and are 3D with respect to the superconductivity. For each film, the low-temperature magnetoresistance isotherms, measured at magnetic fields being perpendicular or parallel to the film plane, do not cross at a single point but at a clear wide region. The dynamical critical exponents $zν_{\perp}$ (for perpendicular field) and $zν_{\parallel}$ (for parallel field) obtained by analyzing the related magnetoresistance isotherms increase with decreasing temperature and tend to diverge as $T\rightarrow 0$ K. In addition, the effective resistivity data for the perpendicular and parallel field in the vicinity of the SMTs both obey an activated scaling based on the random transverse-field Ising model. We also fabricate a $\sim$80-nm-thick (Mo$_{0.8}$Ti$_{0.2}$)$_2$N$_{1.06}$ superconducting film with face-centered cubic structure at low nitrogen partial pressure. It is found that the low-temperature magnetoresistance isotherms for the perpendicular (parallel) field cross at a single point and the resistivity data for the perpendicular (parallel) field in the vicinity of the field-induced SMT obey the power-law scaling deduced from the dirty-boson model. Our results provide unambigous experimental evidence for the existence of QGS in 3D superconductors.
Robust Quantum Griffiths Singularity at above 1.5 Kelvin in Nitride Thin Films
量子格里菲思奇异性（QGS）与淬火无序密切相关，其特征是动力学临界指数的发散和激活标度行为的存在。通常这种量子现象是相当罕见的，只有在极低的温度下才能观察到。在这里，我们报告了一个强大的QGS在氮化物薄膜，NbN，它的生存在一个相当高的温度范围内的实验观察。在高达12 T的磁场下测量了其电输运性质。观察到了场致超导-金属相变，相变点随温度的降低而连续变化。基于传统幂律标度的动力学临界指数在接近低温极限时呈现发散趋势。此外，方块电阻的温度和场的依赖性可以描述的温度范围内高达1.6 K $\leq T \leq$ 4.0 K的激活标度分析。我们认为，QGS在本系统中的鲁棒性源于泡利顺磁效应，由于磁场和自旋自由度之间的强耦合。
Three-dimensional quantum Griffiths singularity in bulk iron-pnictide superconductors
Peking University（北京大学）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Dalian University of Technology（大连理工大学）；Southern University of Science and Technology（南方科技大学）；Hefei National Laboratory（合肥国家实验室）
Current distribution across type II superconducting films: a new vortex-free critical state
迈斯纳态中载流矩形薄膜厚度上的电流分布是很久以前由伦敦兄弟建立的。宽度上的分布更复杂，但后来被证明是高度不均匀的，在边缘发散。因此，II型超导体的标准观点是涡旋从边缘进入，随着电流的增加，涡旋被向内驱动，直到它们在中心自我湮灭，导致耗散。假定该条件定义临界电流。然而，我们已经表明，在自场（无外场）下，输运临界电流是伦敦表面电流，其中表面电流密度等于临界场除以λ，跨越整个宽度。因此，临界电流分布必须是均匀的。在这里，我们报告的商业YBa 2Cu 3 O 7导体的电流和场分布的研究，并确认在低电流接受的非均匀分布，但表现出激进的交叉在临界电流的均匀分布。这种交叉在奇点处不连续地结束，并且计算详细地定量地证实了这些结果。出乎意料的是，自场耗散的开始是热力学性质的，并且隐含的无涡旋临界状态似乎需要新的物理学。
