Flux line lattice reorientation in the borocarbide superconductors with H parallel a

Small angle neutron scattering studies of the flux line lattice in LuNi2B2C and ErNi2B2C induced by a field parallel to the a axis reveal a first order flux line lattice reorientation transition. Below the transition the flux line lattice nearest neighbor direction is parallel to the b axis, and above the transition it is parallel to the c axis. This transition cannot be explained using nonlocal corrections to the London model. In addition, the anisotropy of the penetration depth lambda and the coherence length xi change at the transition.     

Temperature dependence of the flux line lattice transition into square symmetry in superconducting LuNi2B2C

We have investigated the temperature dependence of the H parallel to c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi2B2C ( T(c) = 16.6 K). At temperatures below 10 K the transition onset field, H2(T), is only weakly temperature dependent. Above 10 K, H2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H2(T) merging with the upper critical field and suggests that just below the H(c2)(T) curve the flux line lattice might be hexagonal.     

Gap function with point nodes in borocarbide superconductor YNi2B2C

To determine the superconducting gap function of YNi2B2C, the c-axis thermal conductivity kappa(zz) was measured in H rotated in various directions. The angular variation of kappa(zz) in H rotated within the ab plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when H is rotated conically around the c axis with a tilt angle of 45 degrees. These results provide the first compelling evidence that the gap function has point nodes located along the a and b axes. This unprecedented gap structure challenges the current view on the pairing mechanism.     

Anisotropy of the superconducting gap of the borocarbide superconductor YNi2B2C with ultrasonic attenuation

The ultrasonic attenuation alpha of the highly anisotropic s-wave superconductor YNi2B2C has been measured for all the symmetrically independent elastic modes to explore the location of the zero superconducting gap region on the Fermi surface. The attenuation of the longitudinal mode shows a pronounced anisotropy in the superconducting state: While alpha shows a thermally activated behavior along [110] and [001] directions, it shows T-linear dependence along [100]. These results together with those for the transverse modes demonstrate the presence of point nodes or zero-gap regions along [100] and [010] directions. This is a clear demonstration of ultrasonic attenuation as a powerful probe for the structure of the anisotropic superconducting gap.     

Evidence for the coexistence of an anisotropic superconducting gap and nonlocal effects in the nonmagnetic superconductor LuNi2B2C

A study of the dependence of the heat capacity C(p)(alpha) on the field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, C(p)(alpha) exhibits a fourfold variation as the field H (alpha=0). A slightly disordered sample, however, develops anomalous secondary minima along <110> for mu(0)H>1 T, leading to an eightfold pattern at 2 K and 1.5 T. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and nonlocal effects.     

Anisotropic multiband many-body interactions in LuNi2B2C

We present a comprehensive de Haas-van Alphen study on the nonmagnetic borocarbide superconductor LuNi2B2C. The analysis of the angular-dependent effective masses for different bands in combination with full-potential density functional calculations allowed us to determine the mass-enhancement factors, lambda, for the different electronic bands and their wave-vector dependences. Our data clearly show the anisotropic multiband character of the superconductivity in LuNi2B2C.     

Flux line lattice melting and the formation of a coherent quasiparticle Bloch state in the ultraclean URu2Si2 superconductor

We find that in the ultraclean heavy-fermion superconductor URu(2)Si(2) (T_{c0}=1.45 K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number of carriers with heavy mass in this system results in exceptionally large thermal fluctuations even at sub-Kelvin temperatures, which are witnessed by a sizable region of the flux line liquid phase. The uniqueness is further highlighted by an enhancement of the quasiparticle mean free path below the melting transition, implying a possible formation of a quasiparticle Bloch state in the periodic flux line lattice.     

Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C

We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron diffraction. The ordering vector is Q(A) = (0.48,0,0) and the phase appears above a critical in-plane magnetic field of 0.9 T. The field was applied in order to test the assumption that the zero-field magnetic structure at Q(F) = (0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity were destroyed. We present theoretical calculations which show that two effects are important: a suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase and a reduction of the superconducting condensation energy due to the periodic modulation of the moments at Q(A).     

Vortex lattice transitions in YNi2B2C

We have performed extensive small-angle neutron scattering (SANS) diffraction studies of the vortex lattice in single crystal YNi₂B₂C for B‖c. High-resolution SANS, combined with a field-oscillation vortex lattice preparation technique, allows us to separate Bragg scattered intensities from two orthogonal domains and accurately determine the unit cell angle, β. The data suggest that upon increasing field there is a finite transition width where both low- and high-field distorted hexagonal vortex lattice phases, mutually rotated by 45°, coexist. The smooth variation of diffracted intensity from each phase through the transition corresponds to a redistribution of populations between the two types of domains.     

Vortex phase diagram studies in the weakly pinned single crystals of YNi2B2C and LuNi2B2C

We present a study of magnetization measurements performed on the single crystals of YNi₂B₂C and LuNi₂B₂C. For both the compounds, we find flux jumps in magnetisation values in the respective field regions, where the structural transitions in the flux line lattice symmetry have been reported in these systems via the small angle neutron scattering experiments. The magnetisation hysteresis loops and the AC susceptibility measurements show pronounced peak effect as well as second magnetisation peak anomaly for both YNi₂B₂C and LuNi₂B₂C. Based on these results, a vortex phase diagram has been constructed for YNi₂B₂C forH∥c depicting different glassy phases of the vortex matter.     

无序超导体磁通运动的两次退钉扎效应和重新进入超导相

考虑平面内和不同平面磁通之间的相互作用力，计算了无序各向异性超导体中磁通运动的平均速度、微分电阻随驱动力F_l的变化规律，用层间关联函数C_z的值来判断2D塑性流动和3D关联流动的运动图像.观察到随着外驱动力的增大微分电阻出现两个尖峰，它对应着磁通运动存在两次退钉扎现象.在一定层间耦合条件下，在微分电阻双峰之间，可观察到重新进入微分电阻为零的钉扎相.这与最近实验上新发现的无序弱钉扎超导体有重新进入超导相的巨大峰值效应相吻合.同时，也可发现随着驱动电流的增大，磁通运动出现由2D塑性流动到3D弹性流动的相变，这一维度的变化对应着微分电阻dV/dI曲线中的二次峰位置. 并证明当层间耦合(即代表磁场的大小)在一定范围时，3D-2D相变对应的临界电流随磁场的增大而增大, 反映了第二磁化峰附近的磁通格子软硬度改变的微观图像. 关键词： 第Ⅱ类超导体 磁通线格子 钉扎 峰值效应     

Colored Flux Tube in Euclidean Spacetime

No Heading The flux tube solution in the Euclidean spacetime with the color longitudinal electric field in the SU(2) Yang-Mills-Higgs theory with broken gauge symmetry is found. Some arguments are given that this flux tube is a pure quantum object in the SU(3) quantum theory reduced to the SU(2) Yang-Mills-Higgs theory.     

Low energy quasiparticle excitation in the vortex state of borocarbide superconductor yni2b2c

We measured the heat capacity C(p) and microwave surface impedance Z(s) in the vortex state of YNi2B2C. In contrast to conventional s-wave superconductors, C(p) shows a square root[H] dependence. This square root[H] dependence persists even after the introduction of the columnar defects which change the electronic structure of the vortex core regime and destroy the regular vortex lattice. On the other hand, flux flow resistivity is nearly proportional to H. These results indicate that the vortex state of YNi2B2C is fundamentally different from the conventional s-wave counterparts, in that the delocalized quasiparticle states around the vortex core are important, similar to d-wave superconductors.     

Atomic resolution and vortex lattice studies of magnetic superconductors: A first approach in the nickel borocarbide TmNi2B2C

We present new scanning tunneling microscopy measurements in the superconductor TmNi₂B₂C. The topography shows in some areas flat surfaces, where atomic size modulations can be identified. We find a hexagonal vortex lattice between 0.15 T and 1.4 T, when the magnetic field is applied along the basal plane of the tetragonal crystal structure (B⊥c)$(B\perp c)$, and a hexagonal to square transition around 0.15 T when the field is applied along the c  -axis (B‖c)$(B\Vert c)$. Measured intervortex distance are smaller than expected at high field, due to the internal field being larger than the applied field.     

Spin fluctuations and unconventional pairing on the Lieb lattice

The spin fluctuations and superconducting pairing symmetries in the dispersive band of Lieb lattice are studied by fluctuation exchange approximation. The antiferromagnetic spin density wave is found to exist on the A sublattice (the lattice sites with four nearest neighbors) at half filling. When slightly doped away from half filling, a balance between the combined effects of the (π,π$\pi ,\pi$) and (0.4π,0$0.4\pi ,0$) spin fluctuations and the gaining of the condensation energy leads to the nearly degenerate _dx2−y2$d_{x^2-y^2}$- and _gxy(x2−y2)$g_{xy(x^2-y^2)}$-wave pairing states. After further doped, the _dxy$d_{xy}$-wave state is favored via the intra-sublattice spin fluctuations with a wave vector (π,0$\pi ,0$). We emphasize that the sublattices' contribution and the renormalization of the spectral function play a crucial role on the spin fluctuations and the pairing symmetry. The effect of the imbalance of the on-site energy at different sublattices is also discussed.     

晶格不具备五度及六度以上对称轴的双轴证明是正确的

指出晶格不具备五度及六度以上对称轴的双轴证明是正确的     

Simulation study for the orientation of the driven vortex lattice in an amorphous superconductor

We investigate the orientation of the vortex lattice driven by an applied current by means of numerical simulations based on the time-dependent Ginzburg–Landau (TDGL) theory. A lattice order is restored by a current driving of vortices under the influence of random vortex pinnings. The orientation of the moving vortex lattice is different between the presence and the absence of vortex pinnings. We show results of TDGL simulations for these phenomena.     

Multiple flux difference effect in the lattice hydrodynamic model

Considering the effect of multiple flux difference, an extended lattice model is proposed to improve the stability of traffic flow. The stability condition of the new model is obtained by using linear stability theory. The theoretical analysis result shows that considering the flux difference effect ahead can stabilize traffic flow. The nonlinear analysis is also conducted by using a reductive perturbation method. The modified KdV (mKdV) equation near the critical point is derived and the kink-antikink solution is obtained from the mKdV equation. Numerical simulation results show that the multiple flux difference effect can suppress the traffic jam considerably, which is in line with the analytical result.