Using angle-resolved photoemission spectroscopy, we report on the direct observation of the energy gap in $2H\mathrm{\text{−}}{\mathrm{NbSe}}_2$ caused by the charge-density waves (CDW). The gap opens in the regions of the momentum space connected by the CDW vectors, which implies a nesting mechanism of CDW formation. In remarkable analogy with the pseudogap in cuprates, the detected energy gap also exists in the normal state ($T>T_0$) where it breaks the Fermi surface into “arcs,” it is nonmonotonic as a function of temperature with a local minimum at the CDW transition temperature ($T_0$), and it forestalls the superconducting gap by excluding the nested portions of the Fermi surface from participating in superconductivity.

Platelike high-quality ${\mathrm{NaYbS}}_2$ rhombohedral single crystals with lateral dimensions of a few mm have been grown and investigated in great detail by bulk methods such as magnetization and specific heat, but also by local probes such as nuclear magnetic resonance (NMR), electron-spin resonance (ESR), muon-spin relaxation ($\mu \mathrm{SR}$), and inelastic neutron scattering over a wide field and temperature range. Our single-crystal studies clearly evidence a strongly anisotropic quasi-two-dimensional magnetism and an emerging spin-orbit entangled $J_{\mathrm{eff}}=\frac{1}{2}$ state of Yb towards low temperatures together with an absence of long-range magnetic order down to 260 mK. In particular, the clear and narrow Yb ESR lines together with narrow ${}^{23}\mathrm{Na}$ NMR lines evidence an absence of inherent structural distortions in the system, which is in strong contrast to the related spin-liquid candidate ${\mathrm{YbMgGaO}}_4$ falling within the same space group $R\overline{3}m$. This identifies ${\mathrm{NaYbS}}_2$ as a rather pure spin-$\frac{1}{2}$ triangular-lattice magnet and a putative quantum spin liquid.

Small-angle neutron scattering experiments were performed on a bulk single crystal of chiral-lattice multiferroic insulator Cu${}_2$OSeO${}_3$. In the absence of an external magnetic field, helical spin order with magnetic modulation vector $q\parallel \langle 001\rangle$ was identified. When a magnetic field is applied, a triple-$q$ magnetic structure emerges normal to the field in the A phase just below the magnetic ordering temperature $T_c$, which suggests the formation of a triangular lattice of skyrmions. Notably, the favorable $q$ direction in the A phase changes from $q\parallel \langle 110\rangle$ to $q\parallel \langle 001\rangle$ upon approaching $T_c$. Near the phase boundary between these two states, the external magnetic field induces a 30${}^{\circ }$ rotation of the skyrmion lattice. This suggests a delicate balance between the magnetic anisotropy and the spin texture near $T_c$, such that even a small perturbation significantly affects the ordering pattern of the skyrmions.

Using angle-resolved photoemission spectroscopy we demonstrate that a normal-state pseudogap exists above $T_{\mathrm{N}\mathrm{\text{−}}\mathrm{IC}}$ in one of the most studied two-dimensional charge-density wave (CDW) dichalcogenides $2H\mathrm{\text{−}}{\mathrm{TaSe}}_2$. The initial formation of the incommensurate CDW is confirmed as being driven by a conventional nesting instability, which is marked by a pseudogap. The magnitude, character, and anisotropy of the 2D-CDW pseudogap bear considerable resemblance to those seen in superconducting cuprates.

Here we present a combined study of the slightly underdoped novel pnictide superconductor ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ by means of x-ray powder diffraction, neutron scattering, muon-spin rotation ($\mu \mathrm{SR}$), and magnetic force microscopy (MFM). Static antiferromagnetic order sets in below $T_m\approx 70\mathrm{K}$ as inferred from the neutron scattering and zero-field-$\mu \mathrm{SR}$ data. Transverse-field $\mu \mathrm{SR}$ below $T_c$ shows a coexistence of magnetically ordered and nonmagnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting- or normal-state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors.