We present resistivity and thermal-conductivity measurements of superconducting FeSe in intense magnetic fields up to 35 T applied parallel to the $ab$ plane. At low temperatures, the upper critical field ${\mu }_0{H}_{c2}^{ab}$ shows an anomalous upturn, while thermal conductivity exhibits a discontinuous jump at ${\mu }_0{H}^{*}\approx 24\mathrm{T}$ well below ${\mu }_0{H}_{c2}^{ab}$, indicating a first-order phase transition in the superconducting state. This demonstrates the emergence of a distinct field-induced superconducting phase. Moreover, the broad resistive transition at high temperatures abruptly becomes sharp upon entering the high-field phase, indicating a dramatic change of the magnetic-flux properties. We attribute the high-field phase to the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state, where the formation of planar nodes gives rise to a segmentation of the flux-line lattice. We point out that strongly orbital-dependent pairing as well as spin-orbit interactions, the multiband nature, and the extremely small Fermi energy are important for the formation of the FFLO state in FeSe.

We have in detail characterized the anisotropic charge response of the dimer Mott insulator $\kappa \text{−}\left(\text{BEDT-TTF}{)}_2{\mathrm{Cu}}_2\right(\text{CN}{)}_3$ by dc conductivity, Hall effect, and dielectric spectroscopy. At room temperature, the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity $\rho \left(T\right)$. Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K, we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also, the relaxation levels off. The observed behavior bears features of relaxorlike ferroelectricity. Because heterogeneities impede its long-range development, only a weak-tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

The recent discovery of a nonmagnetic nematic quantum critical point (QCP) in the iron chalcogenide family ${\mathrm{FeSe}}_{1\text{−}x}{\mathrm{S}}_x$ has raised the prospect of investigating, in isolation, the role of nematicity on the electronic properties of correlated metals. Here we report a detailed study of the normal state transverse magnetoresistance (MR) in ${\mathrm{FeSe}}_{1\text{−}x}{\mathrm{S}}_x$ for a series of S concentrations spanning the nematic QCP. For all temperatures and $x$ values studied, the MR can be decomposed into two distinct components: one that varies quadratically in magnetic field strength ${\mu }_0\mathit{H}$ and one that follows precisely the quadrature scaling form recently reported in metals at or close to a QCP and characterized by a $H$-linear MR over an extended field range. The two components evolve systematically with both temperature and S substitution in a manner that is determined by their proximity to the nematic QCP. This study thus reveals the coexistence of two independent charge sectors in a quantum critical system. Moreover, the quantum critical component of the MR is found to be less sensitive to disorder than the quadratic (orbital) MR, suggesting that detection of the latter in previous MR studies of metals near a QCP may have been obscured.

The Mott insulator $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{Ag}}_2{\left(\mathrm{CN}\right)}_3$ forms a highly-frustrated triangular lattice of $S=1/2$ dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response, and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasidegenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound $\kappa -(\text{BEDT}-\text{TTF}){}_2{\mathrm{Cu}}_2\left(\mathrm{CN}\right){}_3$.

The organic charge-transfer salt $\kappa -\left(\text{BEDT-TTF}{)}_2\mathrm{Hg}\right(\mathrm{SCN}{)}_2\mathrm{Br}$ is a quasi-two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below $T=80\mathrm{K}$, it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition, we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized $\kappa -\left(\text{BEDT-TTF}\right)$ salts, in particular the Cl analog, where a charge-order transition takes place at $T_{\mathrm{CO}}=30\mathrm{K}$.

The quest for high-temperature superconductivity at ambient pressure is a central issue in physics. In this regard, the relationship between unconventional superconductivity and the quantum critical point (QCP) associated with the suppression of some form of symmetry-breaking order to zero temperature has received particular attention. The key question is how the strength of the electron pairs changes near the QCP, and this can be verified by high-field experiments. However, such studies are limited mainly to superconductors with magnetic QCPs, and the possibility of unconventional mechanisms by which nonmagnetic QCP promotes strong pairing remains a nontrivial issue. Here, we report systematic measurements of the upper critical field $H_{c2}$ in nonmagnetic ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ superconductors, which exhibit a pure QCP of electronic nematicity characterized by spontaneous rotational-symmetry breaking. As the magnetic field increases, the superconducting phase of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ shrinks to a narrower dome surrounding the nematic QCP. The analysis of $H_{c2}$ reveals that the Pauli-limiting field is enhanced toward the QCP, implying that the pairing interaction is significantly strengthened via nematic fluctuations emanating from the QCP. Remarkably, this nematic QCP is not accompanied by a divergent effective mass, distinct from the magnetically mediated pairing. Our observation opens up a nematic route to high-temperature superconductivity.