Pressure-induced exotic states in rare earth hexaborides

Mixed-valence state in the dilute-impurity regime of La-substituted SmB6

Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of f-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the experimental determination of the limiting cases for which MV emerges. Here we address this question for SmB6, a prototypical MV system characterized by two nearly-degenerate Sm2+ and Sm3+ configurations. By combining angle-resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence, vSm, in the SmxLa1-xB6 series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at x = 0.2, with vSm ~ 2; surprisingly, by further reducing x, a re-entrant increase of vSm develops, approaching the value of vimp ~ 2.35 in the dilute-impurity limit. Such behaviour departs from a monotonic evolution of vSm across the whole series, as well as from the expectation of its convergence to an integer value for x → 0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the SmxLa1-xB6 series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime.

Chemical bonding dictates drastic critical temperature difference in two seemingly identical superconductors

Though YB6 and LaB6 share the same crystal structure, atomic valence electron configuration, and phonon modes, they exhibit drastically different phonon-mediated superconductivity. YB6 superconducts below 8.4 K, giving it the second-highest critical temperature of known borides, second only to MgB2. LaB6 does not superconduct until near-absolute zero temperatures (below 0.45 K), however. Though previous studies have quantified the canonical superconductivity descriptors of YB6's greater Fermi-level (Ef) density of states and higher electron-phonon coupling (EPC), the root of this difference has not been assessed with full detail of the electronic structure. Through chemical bonding, we determine low-lying, unoccupied 4f atomic orbitals in lanthanum to be the key difference between these superconductors. These orbitals, which are not accessible in YB6, hybridize with π B-B bonds and bring this π-system lower in energy than the σ B-B bonds otherwise at Ef. This inversion of bands is crucial: the optical phonon modes we show responsible for superconductivity cause the σ-orbitals of YB6 to change drastically in overlap, but couple weakly to the π-orbitals of LaB6. These phonons in YB6 even access a crossing of electronic states, indicating strong EPC. No such crossing in LaB6 is observed. Finally, a supercell (the M k-point) is shown to undergo Peierls-like effects in YB6, introducing additional EPC from both softened acoustic phonons and the same electron-coupled optical modes as in the unit cell. Overall, we find that LaB6 and YB6 have fundamentally different mechanisms of superconductivity, despite their otherwise near-identity.

Towards a Single Chemical Model for Understanding Lanthanide Hexaborides

Lanthanide hexaborides (LnB₆ ) have disparate and often anomalous properties, from structurally homogeneous mixed valency, to superconductivity, spectral anomalies, and unexplained phase transitions. It is unclear how such a diversity of properties may arise in the solids of identical crystal structures and seemingly very similar electronic structures. Building on our previous model for SmB₆ (mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB₆ that explains simultaneously EuB₆ (possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB₆ (mixed valent topological insulator), and rather ordinary LaB₆ . We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB₆ family, also easily access alternative bonding states through the electron-phonon coupling. The accessibility, relative energetics, and bonding nature of the states involved dictate the properties.

Observation of highly stable and symmetric lanthanide octa-boron inverse sandwich complexes

While boron forms a wide range of metal borides with important industrial applications, there has been relatively little attention devoted to lanthanide boride clusters. Here we report a joint photoelectron spectroscopy and quantum chemical study on two octa-boron di-lanthanide clusters, Ln₂B₈^- (Ln = La, Pr). We found that these clusters form highly stable inverse sandwich structures, [Ln-B₈-Ln]^-, with strong Ln and B₈ bonding via interactions between the Ln 5d orbitals and the delocalized σ and π orbitals on the B₈ ring. A (d-p)δ bond, involving the 5dδ and the antibonding π orbital of the B₈ ring, is observed to be important in the Ln-B₈ interactions. The highly symmetric inverse sandwich structures are overwhelmingly more stable than any other isomers. Upon electron detachment, the (d-p)δ orbitals become half-filled, giving rise to a triplet ground state for neutral La₂B₈ In addition to the two unpaired electrons in the (d-p)δ orbitals upon electron detachment, the neutral Pr₂B₈ complex also contains two unpaired 4f electrons on each Pr center. The six unpaired spins in Pr₂B₈ are ferromagnetically coupled to give rise to a septuplet ground state. The current work suggests that highly magnetic Ln…B₈…Ln inverse sandwiches or 1D Ln…B₈…Ln nanowires may be designed with novel electronic and magnetic properties.

Quantum phase transition and destruction of Kondo effect in pressurized SmB6

SmB₆ has been a well-known Kondo insulator for decades, but recently attracts extensive new attention as a candidate topological system. Studying SmB₆ under pressure provides an opportunity to acquire the much-needed understanding about the effect of electron correlations on both the metallic surface state and bulk insulating state. Here we do so by studying the evolution of two transport gaps (low temperature gap E_l and high temperature gap E_h) associated with the Kondo effect by measuring the electrical resistivity under high pressure and low temperature (0.3 K) conditions. We associate the gaps with the bulk Kondo hybridization, and from their evolution with pressure we demonstrate an insulator-to-metal transition at ∼4 GPa. At the transition pressure, a large change in the Hall number and a divergence tendency of the electron-electron scattering coefficient provide evidence for a destruction of the Kondo entanglement in the ground state. Our results raise the new prospect for studying topological electronic states in quantum critical materials settings.