Search

The quantum limit in a Fermi liquid, realized when a single Landau level is occupied in strong magnetic fields, gives rise to unconventional states, including the fractional quantum Hall effect and excitonic insulators. Stronger interactions in metals with nearly localized f-electron degrees of freedom increase the likelihood of these unconventional states. However, access to the quantum limit is typically impeded by the tendency of f-electrons to polarize in a strong magnetic field, consequently weakening the interactions. In this study, we propose that the quantum limit in such systems must be approached in reverse, starting from an insulating state at zero magnetic field. In this scenario, Landau levels fill in the reverse order compared to regular metals and are closely linked to a field-induced insulator-to-metal transition. We identify YbB12 as a prime candidate for observing this effect and propose the presence of an excitonic insulator state near this transition. Metallic systems in magnetic fields enter the quantum limit when the cyclotron energy exceeds the Fermi energy. Here the authors introduce the analogue of the quantum limit for insulators, where the Zeeman energy exceeds the cyclotron energy, and show that it explains key features of the Kondo insulator YbB12. [ABSTRACT FROM AUTHOR]

As a promising candidate for solid-state electrolytes in Li-ion batteries, the garnet-type Li-ion conductor series Li5+xLa3Nb2−xZrxO12 (LLNZO) (0 ≤ x ≤ 2) exhibits high ionic conductivity at room temperature. However, no previous single-crystal growth or characterization has been reported for LLNZO compositions 0 ≤ x ≤ 1. To obtain a complete understanding of the trend in the structure–property relationship in this class of materials, we used the floating zone (FZ) method to grow a single crystal of Li5.5La3Nb1.5Zr0.5O12 that was 4 mm in diameter and 10 mm in length. Using Laue neutron single-crystal diffraction, two distinct Li sites were observed: tetrahedral 24d and octahedral 96h sites. The maximum entropy method (MEM) based on neutron single-crystal diffraction data was used to map Li nuclear density and estimate that the bottleneck of Li transport exists between neighboring tetrahedral and octahedral sites, and that Li is delocalized between split octahedral sites. Room-temperature Li-ion conductivity in Li5.5La3Nb1.5Zr0.5O12 measured with electrochemical impedance spectroscopy (EIS) was 1.37 × 10−4 S cm−1. The Li migration activation energy was estimated to be 0.50 eV from EIS and 0.47 eV from dielectric relaxation measurements. The Li-ion jump attempt rate was estimated to be 1.47 × 1012 Hz while the time scale of successful migration is 10−7 to 10−6 s. [ABSTRACT FROM AUTHOR]

High‐entropy oxides (HEOs) composed of multiple metal elements have attracted great attention as anode materials for lithium‐ion batteries (LIBs) due to the synergistic effects of various metal species. However, the practical applications of HEOs are still plagued by poor conductivity, unstable solid electrolyte interphase (SEI) and poor cycling stability. Herein, nanosized (FeCoNiCrMn)3O4 HEO (NHEO) is prepared successfully by the NaCl‐assisted mechanical ball‐milling strategy. Novelly, polyacrylonitrile (PAN) is used as the binder and then in situ thermochemically cyclized to construct a cyclized PAN (cPAN) outer layer onto NHEO (NHEO‐cPAN). The in situ formed cPAN coating not only improves the electrical conductivity, but also reinforces the structural and interfacial stability, and thereby, the resulted NHEO‐cPAN electrode exhibits significantly enhanced rate and cyclic performance. Specifically, NHEO‐PAN500 electrode delivers a high reversible capacity of 560 mAh g−1 at 5 A g−1 and a high‐capacity retention of 83% over 800 cycles at 3 A g−1. Furthermore, the structural evolution and electrochemical behavior of NHEO‐PAN electrode during discharge/charge is systematically investigated by operando X‐ray diffraction, in situ impedance spectroscopy and ex situ high‐resolution transmission electron microscopy. Therefore, this work provides new insights into the engineering of electrode and interphase for high‐performance HEO electrode materials, potentially enlightening the practical applications of HEO‐based LIBs. [ABSTRACT FROM AUTHOR]