The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO₃) and the wide-gap insulator lanthanum aluminate (LaAlO₃) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO₃ as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO₃ layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB${}_6$ may be a realization of a topological Kondo insulator. We have performed an angle-resolved photoemission spectroscopy study on SmB${}_6$ in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like $5d$ bulk band centered at the $X$ point of the bulk Brillouin zone that is hybridized with strongly correlated $f$ electrons, as well as the opening of a Kondo band gap ($\Delta$ ${}_B$ $\sim$ 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the center $\overline{\Gamma }$ point and boundary $\overline{X}$ point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB${}_6$ is likely to be related to the pronounced surface states sitting inside the band hybridization gap and/or the presence of a topological Kondo insulating state.

Altermagnets are a novel class of magnetic materials, where magnetic order is staggered both in coordinate and momentum space. The metallic rutile oxide RuO₂, long believed to be a textbook Pauli paramagnet, recently emerged as a putative workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While some experiments seem consistent with altermagnetism, magnetic order in RuO₂remains controversial. We show that RuO₂is nonmagnetic, both in bulk and thin film. Muon spectroscopy complemented by density-functional theory finds at most 1.14 × 10⁻⁴ μ_B/Ru in bulk and at most 7.5 × 10⁻⁴ μ_B/Ru in 11 nm epitaxial films, at our spectrometers’ detection limit, and dramatically smaller than previously reported neutron results that were used to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO₂single crystals identify multiple scattering as the source for the false signal in earlier studies.

In an ideal bulk topological insulator (TI) conducting surface states protected by time-reversal symmetry enfold an insulating crystal. However, the archetypical TI, Bi${}_{2}$Se${}_{3}$, is actually never insulating; it is in fact a relatively good metal. Nevertheless, it is the most studied system among all the TIs, mainly due to its simple band structure and large spin-orbit gap. Recently, it was shown that copper intercalated Bi${}_{2}$Se${}_{3}$ becomes superconducting and it was suggested as a realization of a topological superconductor. Here we use a combination of techniques that are sensitive to the shape of the Fermi surface (FS): the Shubnikov-de Haas effect and angle-resolved photoemission spectroscopy to study the evolution of the FS shape with carrier concentration, $n$. We find that as $n$ increases, the FS becomes two-dimensional-like. These results are of crucial importance for understanding the superconducting properties of Cu${}_{x}$Bi${}_{2}$Se${}_{3}$.

$1T\text{−}{\mathrm{TaS}}_2$ is a layered transition-metal dichalcogenide with a very rich phase diagram. At $T=180$ K it undergoes a metal to Mott insulator transition. Mott insulators usually display antiferromagnetic ordering in the insulating phase but $1T\text{−}{\mathrm{TaS}}_2$ was never shown to order magnetically. In this paper we show that $1T\text{−}{\mathrm{TaS}}_2$ has a large paramagnetic contribution to the magnetic susceptibility but it does not show any sign of magnetic ordering or freezing down to 20 mK, as probed by muon spin relaxation, possibly indicating a quantum spin liquid ground state. Although $1T\text{−}{\mathrm{TaS}}_2$ exhibits a strong resistive behavior both in and out of plane at low temperatures we find a linear term in the heat capacity suggesting the existence of a Fermi surface, which has an anomalously strong magnetic field dependence.