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Using angle-resolved photoemission spectroscopy and band structure calculation, we have investigated the three-dimensional nature and the orbital character of the low-energy electronic structure in the layered ternary boride ${\mathrm{AlMn}}_{2}{\mathrm{B}}_{2}$, in which ferromagnetic Mn $ab$ layers couple antiferromagnetically along the $c$ axis (N\'eel temperature ${\mathrm{T}}_{\mathrm{N}}\ensuremath{\sim}320\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). The calculation indicates that electronic bands in the vicinity of the Fermi energy ${E}_{\mathrm{F}}$ are dominated by the Mn $3d$ orbitals. The ${e}_{g}$ orbitals especially contribute high density of states (DOS) with large effective electron masses near ${E}_{\mathrm{F}}$. The calculated bands are renormalized by a factor of $\ensuremath{\sim}1.5$ to match the overall experimental observations, indicating moderate electronic correlation. Considering that the neighbor compound ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, with one more $3d$ electron, is ferromagnetic, the high DOS near ${E}_{\mathrm{F}}$ from the different $3d$ orbitals should be associated with the different magnetic orders along the special directions in these layered ternary borides. On the other hand, our studies of electronic structure of ${\mathrm{AlMn}}_{2}{\mathrm{B}}_{2}$ would further prompt its potential applications as a layered room-temperature antiferromagnetic material.

${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, a layered ferromagnet with a Curie temperature near room temperature (${T}_{c}\ensuremath{\sim}270$ K), has attracted growing attention recently in condensed-matter physics and materials science. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide evidence of a linear band and a saddlelike flat band in the vicinity of the Fermi energy (${E}_{F}$) in ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$, consisting of slabs of ${\mathrm{Fe}}_{2}{\mathrm{B}}_{2}$ atoms separated by layers of Al. The anomalous scattering rate of the linear band varies linearly with the kinetic energy up to 120 meV, showing non-Fermi-liquid behavior consistent with T-linear dependence of resistivity. The flat band with ${d}_{xy}$ orbital character and with strong ${k}_{y}$ dispersion may be associated with spin orientation along the $a$ axis in layered ${\mathrm{AlFe}}_{2}{\mathrm{B}}_{2}$. Electronic and magnetic correlation can be modulated by tuning the flat band near ${E}_{F}$ in layered ternary borides. Our findings have important implications to exploit emergent physics and quantum phases in $3d$ transition metals.

We report on the Andreev spectroscopy and specific heat of high-quality single crystals of ${\mathrm{BaFe}}_{1.9}{\mathrm{Ni}}_{0.1}{\mathrm{As}}_{2}$. The intrinsic multiple Andreev reflection spectroscopy reveals two anisotropic superconducting gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\ensuremath{-}4.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.2\ensuremath{-}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ (the ranges correspond to the minimum and maximum value of the coupling energy in the ${k}_{x}{k}_{y}$ plane). The $25%\ensuremath{-}30%$ anisotropy shows the absence of nodes in the superconducting gaps. Using a two-band model with $s$-wave-like gaps ${\mathrm{\ensuremath{\Delta}}}_{L}\ensuremath{\approx}3.2\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{S}\ensuremath{\approx}1.6\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, the temperature dependence of the electronic specific heat can be well described. A linear magnetic field dependence of the low-temperature specific heat offers further support of $s$-wave type of the order parameter. We find that a $d$-wave or single-gap BCS theory under the weak-coupling approach cannot describe our experiments.

We report on local magnetization, tunnel diode oscillators, and specific-heat measurements in a series of Ba(NixFe1−x)2As2 single crystals (0.26≤x≤0.74). We show that the London penetration depth λ(T)=λ(0)+Δλ(T) scales as λ(0)∝1/Tc^0.85±0.2, Δλ(T)∝T^2.3±0.3 (for T≤T_c/3) and $\partial\Delta\lambda/\partial T^2\propto 1/T_c^{2.8\pm0.3}$ in both underdoped and overdoped samples. Moreover, the slope of the upper critical field ($H'_{c2}=-(dH_{c2}/dT)_{|T\rightarrow T_c}$) decreases with $T_c$ in overdoped samples but increases with decreasing $T_c$ in underdoped samples. The remarkable variation of $\lambda(0)$ with $T_c$ and the non exponential temperature dependence of $\Delta\lambda$ clearly indicates that pair breaking effects are important in this system. We show that the observed scalings strongly suggest that those pair breaking effects could be associated with quantum fluctuations near 3D superconducting critical points.

Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$. For the insulating Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$, neutron polarization analysis and single-crystal neutron diffraction unambiguously confirm the earlier proposed $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ block antiferromagnetic structure. For superconducting samples (${T}_{c}=30$ K), we find that in addition to the tetragonal $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structure transition at 513 K, the material develops a separate $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ superlattice structure at a lower temperature of 480 K. These results suggest that superconducting Rb${}_{y}$Fe${}_{1.6+x}$Se${}_{2}$ is phase separated with coexisting $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ and $\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5}$ superlattice structures.