A fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of two other instabilities. In addition to a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. Here, we report a very clear splitting of NMR resonance lines in FeSe at Tnem = 91 K, far above the superconducting Tc of 9.3 K. The splitting occurs for magnetic fields perpendicular to the Fe planes and has the temperature dependence of a Landau-type order parameter. Spin-lattice relaxation rates are not affected at Tnem, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.

We report a 35 Cl nuclear magnetic resonance study in the honeycomb lattice α-RuCl3, a material that has been suggested to potentially realize a Kitaev quantum spin liquid (QSL) ground state. Our results provide direct evidence that α-RuCl3 exhibits a magnetic-field-induced QSL. For fields larger than ∼ 10 T, a spin gap opens up while resonance lines remain sharp, evidencing that spins are quantum disordered and locally fluctuating. The spin gap increases linearly with an increasing magnetic field, reaching ∼ 50 K at 15 T, and is nearly isotropic with respect to the field direction. The unusual rapid increase of the spin gap with increasing field and its isotropic nature are incompatible with conventional magnetic ordering and, in particular, exclude that the ground state is a fully polarized ferromagnet. The presence of such a field-induced gapped QSL phase has indeed been predicted in the Kitaev model.When the interactions between magnetic spins are strongly frustrated, quantum fluctuations can cause spins to remain disordered even at very low temperatures [1]. The quantum spin liquid (QSL) state that ensues is conceptually very interesting -for instance, new fractionalized excitations appear that are very different from the ordinary spin-wave excitations in ordered magnets [2][3][4][5]. A QSL appears in the so-called Kitaev honeycomb model -a prototypical and mathematically wellunderstood model of strongly frustrated interacting spins [6,7]. In an external magnetic field the topological QSL state acquires a gap that, in the generic case grows linearly with field strength [8].This observation has motivated the search for the experimental realization of the Kitaev honeycomb model and its topological QSL phases. The quest was centered, until recently, mainly on honeycomb iridate materials [9, 10] of the type A 2 IrO 3 (A = Na or Li). However, in these iridates long-range magnetic order develops at low temperatures for all known different crystallographic phases [11][12][13][14][15]. Their QSL regime is most likely preempted by the presence of significant residual Heisenberg-type interactions, by longer-range interactions between the spins or by crystallographically distinct Ir-Ir bonds, if not by a combination of these factors [16][17][18][19]. More promising in this respect is ruthenium trichloride α-RuCl 3 in its honeycomb crystal phase, as numerous experimental and theoretical studies pointed the significance of the anisotropic Kitaev exchange in the material [20][21][22][23][24][25][26][27]. Neutron scattering studies have shown that the magnetic interactions in this material are closer to the Kitaev limit [28], although at low temperatures also this quasi-2D material exhibits long-range magnetic order.In this Letter, we show by means of nuclear magnetic resonance (NMR) that in α-RuCl 3 large magnetic fields larger than ∼ 10 T melt the magnetic order, and a spingap opens that scales linearly with the magnetic field, implying that the detrimental effects of residual magnetic interactions between the Ru moment...

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Heat capacity, magnetic susceptibility, NMR, and resistivity of SrNi2P2 single crystals are presented, illustrating a purely structural transition at 325 K with no magnetism. Bulk superconductivity is found at 1.4 K. The magnitude of the transition temperature Tc, fits to the heat capacity data, the small upper critical field Hc2 = 390 Oe, and Ginzburg-Landau parameter κ = 2.1 suggests a conventional fully gapped superconductor. With applied pressure a second structural phase transition occurs which results in an 8% reduction in the c/a ratio of lattice parameters. We find that superconductivity persists into this high pressure phase, although the transition temperature is monotonically suppressed with increasing pressure. Comparison of these Ni-P data as well as layered Fe-As and Ni-As superconductor indicates that reduced dimensionality can be a mechanism for increasing the transition temperature.