Simultaneous neutron scattering and thermal expansion measurements on the heavy-fermion superconductor URu 2 Si 2 under hydrostatic pressure of 0.67 GPa have been performed in order to detect the successive paramagnetic, hidden order, and large moment antiferromagnetic phases on cooling. The temperature dependence of the sharp low energy excitation at the wave vector Q 0 = ͑1,0,0͒ shows clearly that this resonance is a signature of the hidden order state. In the antiferromagnetic phase, this resonance disappears. The higher energy excitation at the incommensurate wave vector Q 1 = ͑1.4,0,0͒ persists in the antiferromagnetic phase but increases in energy.The elucidation of the nature of a hidden order in exotic materials, which belong often to the rich class of strongly correlated electronic systems, is a hot subject as it can lead to the discovery of unexpected new order parameters. Debates exist on quite different proposals such as orbital hidden order in the heavy fermion system URu 2 Si 2 , 1 multipolar ordering in rare earth skutterudites 2 or "spin order accompanying loop current" in cuprate superconductors. 3 Due to the dual character of the 5f electrons in URu 2 Si 2 between localized ͑leading to the possibility of multipolar ordering͒ and itinerant ͑possibility of large Fermi surface instabilities͒, this compound has been the subject of a large variety of experiments. 4 At zero pressure, a phase transition occurs from the paramagnetic ͑PM͒ phase to a so-called hidden order ͑HO͒ phase at a temperature T 0 ϳ 17.5 K. The hidden order label reflects the fact that this order may not be of dipolar origin. The order parameter is not yet determined: spin or charge density wave, 5-7 multipolar ordering, [8][9][10][11] orbital antiferromagnetism, 1 chiral spin state, 12 and helicity order 13 have been proposed. The long standing debate on the occurrence of a tiny ordered moment M 0 ϳ 0.02 B per U atom at T → 0 K for the antiferromagnetic ͑AF͒ wave vector Q AF = ͑0,0,1͒ seems to converge now toward an extrinsic origin directly related to the high sensitivity of URu 2 Si 2 to pressure and stress ͑low critical pressure P x ϳ 0.5 GPa͒. 4,[14][15][16] Pressure studies 4,17-19 reveal an interesting phase diagram ͑Fig. 1͒. At T → 0 K, neutron scattering experiments 4 show that the hidden-order ground state switches at P x to a large moment antiferromagnetic ͑AF͒ state of sublattice magnetization M 0 near 0.3 B / U with a propagation vector Q AF . The HO-AF boundary T x ͑P͒ meets the T 0 ͑P͒ line at the tricritical point ͑T ‫ء‬ ϳ 19.3 K, P ‫ء‬ ϳ 1.36 GPa͒; 19 above P ‫ء‬ , a unique ordered phase ͑AF͒ is established below T N ͑P͒. Previous nuclear magnetic resonance ͑NMR͒ experiments, 14,20 as well as transport measurements, 5,19 indicate clearly that nesting occurs at T 0 , as well as at T N , indicating also that the Fermi surface is not deeply modified through the transition line T x .The interest in URu 2 Si 2 is reinforced by the appearance of unconventional superconductivity at T sc ϳ 1.2 K for P =0 ͑Ref. 21͒, which d...

We report on the synthesis of superconducting single crystals of FeSe and their characterization by x-ray diffraction, magnetization and resistivity. We have performed ac susceptibility measurements under high pressure in a hydrostatic liquid argon medium up to 14 GPa and we find that T(C) increases up to 33-36 K in all samples, but with slightly different pressure dependences on different samples. Above 12 GPa no traces of superconductivity are found in any sample. We have also performed a room temperature high pressure x-ray diffraction study up to 12 GPa on a powder sample, and we find that, between 8.5 and 12 GPa, the tetragonal PbO structure undergoes a structural transition to a hexagonal structure. This transition results in a volume decrease of about 16% and is accompanied by the appearance of an intermediate, probably orthorhombic, phase.

Spirals and helices are common motifs of long-range order in magnetic solids, and they may also be organized into more complex emergent structures such as magnetic skyrmions and vortices. A new type of spiral state, the spiral spin-liquid, in which spins fluctuate collectively as spirals, has recently been predicted to exist. Here, using neutron scattering techniques, we experimentally prove the existence of a spiral spin-liquid in MnSc2S4 by directly observing the 'spiral surface' -a continuous surface of spiral propagation vectors in reciprocal space. We elucidate the multi-step ordering behavior of the spiral spin-liquid, and discover a vortex-like triple-q phase on application of a magnetic field. Our results prove the effectiveness of the J1-J2 Hamiltonian on the diamond lattice as a model for the spiral spin-liquid state in MnSc2S4, and also demonstrate a new way to realize a magnetic vortex lattice.Magnetic frustration, where magnetic moments (spins) are coupled through competing interactions that cannot be simultaneously satisfied 1 , usually leads to highly cooperative spin fluctuations 2,3 and unconventional longrange magnetic order 4,5 . An archetypal ordering in the presence of frustration is the spin spiral. Competing interactions and spiral orders give rise to many phenomena in magnetism, including the multitudinous magnetic phases of rare earth metals 6 , domains with multiferroic properties 7,8 , and topologically non-trivial structures such as the emergent skyrmion lattice 9,10 .Recently, a new spiral state -a spiral spin-liquid in which the ground states are a massively degenerate set of coplanar spin spirals -was predicted to exist in the J 1 -J 2 model on the diamond lattice (see Fig. 1a) [11][12][13] . Although the diamond lattice is bipartite, and therefore unfrustrated at the near-neighbour (J 1 ) level, the second-neighbour coupling (J 2 ) can generate strong competition. For classical spins, mean-field calculations show that when |J 2 /J 1 | > 0.125 the spiral spin-liquid appears, and that it is signified by an unusual continuous surface of propagation vectors q in reciprocal space (see Fig. 1b for the spiral surface of |J 2 /J 1 | = 0.85). At finite temperature, thermal fluctuations might select some specific q-vectors on the spiral surface 11 , resulting in an orderby-disorder transition 14,15 .Until now, several series of A-site spinels, in which the magnetic A ions form a diamond lattice, have been investigated, including: the cobaltates Co 3 O 4 and CoRh 2 O 4 16 ; the aluminates M Al 2 O 4 with M = Fe, Co, Mn 17-20 ; and the scandium thiospinels M Sc 2 S 4 with M = Fe, Mn 21 . For the spinels with Fe 2+ at the A-site, the e g orbital angular momentum of Fe 2+ is active, making the pure spin J 1 -J 2 model inadequate 22 . Among the other compounds, CoAl 2 O 4 and MnSc 2 S 4 manifest the strongest frustration. For CoAl 2 O 4 , the ratio of |J 2 /J 1 | has been identified as 0.109 19 , which is near, but still lower than, the 0.125 threshold for the spiral spin-liquid state. Many expe...