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We report small-angle neutron scattering measurements of the vortex lattice (VL) structure in single crystals of the lightly underdoped cuprate superconductor YBa2Cu3O6.85. At 2 K, and for fields of up to 16 T applied parallel to the crystal c-axis, we observe a sequence of field-driven and first-order transitions between different VL structures. By rotating the field away from the c-axis, we observe each structure transition to shift to either higher or lower field dependent on whether the field is rotated towards the [100] or [010] direction. We use this latter observation to argue that the Fermi surface morphology must play a key role in the mechanisms that drive the VL structure transitions. Furthermore, we show this interpretation is compatible with analogous results obtained previously on lightly overdoped YBa2Cu3O7. In that material, it has long-been suggested that the high field VL structure transition is driven by the nodal gap anisotropy. In contrast, the results and discussion presented here bring into question the role, if any, of a nodal gap anisotropy on the VL structure transitions in both YBa2Cu3O6.85 and YBa2Cu3O7.

Using the small angle neutron scattering (SANS) technique we investigated the vortex lattice (VL) in the mixed state of the stannide superconductor Yb3Rh4Sn13. We find a single domain VL of slightly distorted hexagonal geometry for field strengths between 350 and 18 500Gand temperatures between T = 0.05 and 6.5 K.We observe a clear in plane rotation of the VL for different magnetic field directions relative to the crystallographic axes. We also find that the hexagonal symmetry of the VL is energetically favorable in Yb3Rh4Sn13 for external fields oriented along axes of different symmetries: twofold [110] threefold [111] and fourfold [100]. The observed behavior is different from other conventional and unconventional superconductors. The superconducting state is characterized by an isotropically gapped order parameter with an amplitude of Delta(0) = 1.57 ± 0.05 meV. At the lowest temperatures the field dependence of the magnetic form factor in our material reveals a London penetration depth of ?L = 2508 ± 17 Angstrom and a Ginzburg coherence length of ? = 100 ± 1.3 Angstrom i.e. it is a strongly type II superconductor ? = ?L/? = 25.

We report small-angle neutron scattering measurements of the vortex lattice (VL) structure in single crystals of the lightly underdoped cuprate superconductor YBa2Cu3O6.85. At 2 K, and for fields of up to 16 T applied parallel to the crystal c-axis, we observe a sequence of field-driven and first-order transitions between different VL structures. By rotating the field away from the c-axis, we observe each structure transition to shift to either higher or lower field dependent on whether the field is rotated towards the [100] or [010] direction. We use this latter observation to argue that the Fermi surface morphology must play a key role in the mechanisms that drive the VL structure transitions. Furthermore, we show this interpretation is compatible with analogous results obtained previously on lightly overdoped YBa2Cu3O7. In that material, it has long-been suggested that the high field VL structure transition is driven by the nodal gap anisotropy. In contrast, the results and discussion presented here bring into question the role, if any, of a nodal gap anisotropy on the VL structure transitions in both YBa2Cu3O6.85 and YBa2Cu3O7.

Frustrated systems exhibit remarkable properties due to the high degeneracy of their ground states. Stabilised by competing interactions, a rich diversity of typically nanometre-sized phase structures appear in polymer and colloidal systems, while the surface of ice pre-melts due to geometrically frustrated interactions. Atomic spin systems where magnetic interactions are frustrated by lattice geometry provide a fruitful source of emergent phenomena, such as fractionalised excitations analogous to magnetic monopoles. The degeneracy inherent in frustrated systems may prevail all the way down to absolute zero temperature, or it may be lifted by small perturbations or entropic effects. In the geometrically frustrated Ising--like magnet Ca3Co2O6, we follow the temporal and spatial evolution of nanoscale magnetic fluctuations firmly embedded inside the spin--density--wave magnetic structure. These fluctuations are a signature of a competing ferrimagnetic phase with an incommensurability that is different from, but determined by the host. As the temperature is lowered, the fluctuations slow down into a super-paramagnetic regime of stable spatiotemporal nano-structures.

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.

Strongly correlated electron systems are quantum materials that reveal a deep intertwining between different electronic charge, orbital, spin and lattice degrees of freedom. The interaction among them can stabilize ground states that feature novel collective phenomena and that potentially contribute to the development of future technical applications, if they are understood on a microscopic level. Particular complex quantum phenomena occur in systems containing rare earth elements, where the conduction electrons either screen or couple the magnetic moments of partially filled electronic $f$-states. The subtle balance between these energy scales yields strong electronic fluctuations that trigger a rich diversity of ground states, including unconventional superconductivity, antiferromagnetism or correlated insulating, metallic and topological protected states. CeCoIn$_5$ is a model heavy-fermion $d$-wave superconductor that is believed to be mediated by magnetic fluctuations. Superconductivity is Pauli limited and features an additional phase at very low temperatures and large magnetic fields. This so-called Q-phase reveals magnetic order that only survives inside the superconducting condensate and directly couples to it. Here, we show that the substitution of the local-moment element Nd for Ce in CeCoIn$_5$ tunes the hybridization between the 4$f$-electrons and the conduction band, such that the system is driven into an antiferromagnetic state arising from a small Fermi surface. We demonstrate that the Q-phase is stable under a small perturbation represented by a Nd doping of 5%. The high-field phase is separated from a low-field antiferromagnetic state via a magnetic instability that may origin from a field-induced quantum phase transition. Intriguingly, both phases display an identical magnetic symmetry, which prevents the emergence of a primary order parameter of magnetic nature in the Q-phase. The detailed investigation of the magnetic order in the two phases shows that the spin-density modulation directions are affected differently by a rotation of the magnetic field inside the tetragonal plane. While the anisotropic spin susceptibility in both phases arises from intertwined spin and orbital degrees of freedom, the coupling between superconductivity and magnetism is altered in the high-field state. These results suggest that the field-induced quantum phase transition triggers the emergence of an auxiliary superconducting order parameter in the Q-phase that couples magnetic order with $d$-wave superconductivity. In contrast, we suggest that magnetism and superconductivity are decoupled in the low-field phase. This conclusion is based on the investigation of the low-energy excitation spectrum of Nd$_{0.05}$Ce$_{0.95}$CoIn$_5$ at zero field. We observe magnetic fluctuations that are related to the superconducting condensate, but which are not affected by magnetic order. We suggest that the superconducting resonance consists of Ising-like fluctuations along the direction of static magnetic order.