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We report a study of the basal-plane anisotropy of the superfluid density in underdoped YBa2Cu4O8 (Y124), showing the effects of both the CuO2 planes and the fully occupied CuO chains. From small-angle neutron scattering measurements of the vortex lattice, we can infer the superconducting (SC) properties for a temperature (T) range T = 1.5 K to T-c and magnetic induction B from 0.1 to 6 T. We find that the superfluid density along a has a simple d-wave T dependence. However, along b (the chain direction) the superfluid density falls much more rapidly with T and also with increasing field. This strongly suggests the suppression of proximity-effect-induced superconductivity in the CuO chains. In addition, our results do not support a common framework for the low-field in-plane SC response in Y124 and related YBa2Cu3O7, and also indicate that any magnetic field induced charge-density-wave order in Y124 exists only for fields above 6 T.

Using small angle neutron scattering, the anisotropy of the magnetic vortex lattice (VL), in the heavily hole-doped pnictide superconductor KFe2As2, was studied. Well-ordered VL scattering patterns were measured with fields applied in directions between B parallel to c and the basal plane, rotating either towards [100] or [110]. Slightly distorted hexagonal patterns were observed when B parallel to c. However, the scattering pattern distorted more strongly as the field was rotated away from the c axis. At low field, the arrangement of vortices is affected by the anisotropy of penetration depth in the plane perpendicular to the field. By fitting the distortion with the anisotropic London model, we obtain an estimate of similar to 3.4 for the anisotropy factor gamma between the in-plane and c-axis penetration depths at the lowest temperature studied. The results further reveal VL phase transitions as a function of field direction. We discuss these transitions using the "hairy ball" theorem.

The vortex lattice (VL) in the high-κ superconductor YBa2Cu3O7, at 2 K and with the magnetic field parallel to the crystal c axis, undergoes a sequence of transitions between different structures as a function of applied magnetic field. However, from structural studies alone, it is not possible to determine precisely the system anisotropy that governs the transitions between different structures. To address this question, here we report new small-angle neutron scattering measurements of both the VL structure at higher temperatures and the field and temperature dependence of the VL form factor. Our measurements demonstrate how the influence of anisotropy on the VL, which in theory can be parameterized as nonlocal corrections, becomes progressively important with increasing magnetic field, and suppressed by increasing the temperature toward Tc. The data indicate that nonlocality due to different anisotropies plays an important role in determining the VL properties.

By neutron scattering, we have observed a well-ordered magnetic vortex lattice (VL) in KFe${}_{2}$As${}_{2}$ single crystals. With the field along the $c$ axis, a nearly isotropic hexagonal VL is formed, with no symmetry transitions up to high fields, indicating a small anisotropy of the superconducting state around this axis. This rules out line nodes parallel to the $c$ axis, and thus $d$-wave or anisotropic $s$-wave pairing. However, the strong temperature dependence of the signal at $T\ensuremath{\ll}{T}_{c}$ shows that extremely small gap values exist; these may arise from nodal lines perpendicular to the $c$ axis.

We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.

We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3O7 at 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twin-domain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.

SUMMARY 1. Rabbit-skin homografts transplanted to recipients that are between 20 and 24 days pregnant survive for about twice as long as do homografts transplanted to pregnant rabbits early or late in pregnancy or to normal males or females. 2. Pregnancy does not interfere with the manifestation of a pre-existing immunity to a donor's skin. 3. The results are taken to provide evidence that the production of corticosteroids is increased during pregnancy. Possible sources of such increased production are discussed. 4. It is suggested that adrenocortical hormones may help to protect the foetus from the immunological consequences of its genetic incompatibility with the mother.