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We compare the upper critical field of bulk single-crystalline samples of the two intrinsic transition metal dichalcogenide superconductors, 2H-NbSe _2 and 2H-NbS _2 , in high magnetic fields where their layer structure is aligned strictly parallel and perpendicular to the field, using magnetic torque experiments and a high-precision piezo-rotary positioner. While both superconductors show that orbital effects still have a significant impact when the layer structure is aligned parallel to the field, the upper critical field of NbS _2 rises above the Pauli limiting field and forms a Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state, while orbital effects suppress superconductivity in NbSe _2 just below the Pauli limit, which excludes the formation of the FFLO state. From the out-of-plane anisotropies, the coherence length perpendicular to the layers of 31 Å in NbSe _2 is much larger than the interlayer distance, leading to a significant orbital effect suppressing superconductivity before the Pauli limit is reached, in contrast to the more 2D NbS _2 .

In iron-based superconductors the interactions driving the nematic order that breaks the lattice four-fold rotational symmetry in the iron plane may also facilitate the Cooper pairing, but experimental determination of these interactions is challenging because the temperatures of the nematic order and the order of other electronic phases appear to match each other or to be close to each other. Here we performed field-dependent ^77 Se-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B _0 up to 16 T. The ^77 Se-NMR spectra and Knight shift split when the direction of B _0 is away from the direction perpendicular to the iron planes (i.e. B _0 ∥ c ) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the ^77 Se nucleus, but these do not happen when B _0 is perpendicular to the iron planes, thus demonstrating that there is an orbital ordering. Moreover, stripe-type antiferromagnetism is absent, while giant antiferromagnetic spin fluctuations measured by the NMR spin-lattice relaxation gradually developed starting at ∼40 K, which is far below the nematic order temperature T _nem = 89 K. These results provide direct evidence of orbital-driven nematic order in FeSe.

The ultrafast quasiparticle dynamics of Fe _1+ _y Se _1− _x Te _x single crystals were investigated by dual-color transient reflectivity measurements (Δ R / R ) from 4.3 to 290 K. The electron–phonon coupling strength λ (=0.16–0.01) and the temperature-dependent energy of longitudinal-acoustic phonons were, respectively, obtained from the relaxation time of a fast component and the period of an oscillation component in Δ R / R . Such a small λ demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent Δ R / R exhibits anomalous changes at T _c , 90 K and 230 K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.