Electronic properties of alkali-metal loaded zeolites -- a "supercrystal" Mott insulator

First-principles band calculations are performed for the first time for an open-structured zeolite (LTA) with guest atoms (potassium) introduced in their cages. A surprisingly simple band structure emerges, which indicates that this system may be regarded as a "supercrystal", where each cluster of guest atoms with diameter $\sim$10\AA acts as a "superatom" with well-defined $s$- and $p$-like orbitals, which in turn form the bands around the Fermi energy. The calculated Coulomb and exchange energies for these states turn out to be in the strongly-correlated regime. With the dynamical mean-field theory we show the system should be on the Mott-insulator side, and, on a magnetic phase diagram for degenerate-orbital systems, around the ferromagnetic regime, in accord with experimental results. We envisage this class of systems can provide a new avenue for materials design.Comment: 4 pages, 4 figure

Magnetic Moments and Ordered States in Pyrochlore Iridates Nd2Ir2O7 and Sm2Ir2O7 Studied by Muon-Spin Relaxation

Magnetic-ordered states of the pyrochlore iridates Nd2Ir2O7 (Nd227) and Sm2Ir2O7 (Sm227), showing metal–insulator transitions at 33 and 117 K, respectively, were studied by both the muon-spin-relaxation (μSR) method and density functional theory (DFT) calculations. A long-range magnetic ordering of Ir moments appeared in conjunction with the metal insulator transition, and additional long-range-ordered states of Nd/Sm moments were confirmed at temperatures below about 10 K. We found that the all-in all-out spin structure most convincingly explained the present μSR results of both Nd227 and Sm227. Observed internal fields were compared with values derived from DFT calculations. The lower limits of the sizes of magnetic moments were estimated to be 0.12 μB and 0.2 μB for Ir and Nd moments in Nd227, and 0.3 μB and 0.1 μB for Ir and Sm moments in Sm227, respectively. Further analysis indicated that the spin coupling between Ir and Nd/Sm moments was ferromagnetic for Nd227 and antiferromagnetic for Sm227

Anomalous spiked structures in ESR signals from the chiral helimagnet CrNb3S6

We have performed X-band electron spin resonance (ESR) measurements on a single crystal of the metallic chiral helimagnet CrNb3S6 from 3.5 to 180 K and for the external magnetic fields Hext , up to 4 kOe, perpendicular to the c axis (the helical axis of CrNb3S6). This field-crystalline configuration is expected to provide the chiral soliton lattice (CSL) state in this system. The main resonance line can be fit with a Dysonian function above Tc = 127 K, but additional features in the spectra were observed below 105 K. Specifically, spiked anomalies superposed on the main signals were observed for magnetic fields between Hc1 and Hc2 that are the appearing and disappearing fields of the spiked anomalies, respectively. The resulting magnetic field vs temperature phase diagram possesses three regions, which are interpreted as different dynamical responses in the CSL phase. In addition, the values of Hc2 are close to those reported by the d2M/dH2 curve [Tsuruta et al., Phys. Rev. B 93, 104402 (2016)]. Furthermore, the field range between Hc1 and Hc2, where the spiked anomalies exist, depends on the field direction and shifts to higher fields when turning to the c axis, thereby providing additional evidence that these spiked anomalies must be related to the chiral soliton dynamics

Electrons of alkali metals in regular nanospaces of zeolites

The s-electrons of alkali metals loaded into regular nanospaces (nanocages) of zeolite crystals display novel electronic properties, such as a ferrimagnetism, a ferromagnetism, an antiferromagnetism, an insulator-to-metal transition, etc., depending on the kind of alkali metals, their loading density, and the structure type of zeolite frameworks. These properties are entirely different from those in bulk alkali metals of free-electrons. Alkali-metal clusters are stabilized in cages of zeolites, and new quantum states of s-electrons, such as 1s, 1p, and 1d states in the spherical quantum-well model, are formed. An electron correlation, a polaron effect, and an orbital degeneracy in the quantum states of s-electrons play critical roles in taking on the novel electronic properties. Electronic properties can be overviewed systematically by a coarse-grained model of localized s-electron states in cages based on the tight-binding approximation, followed by the t-U-S-n diagram of the correlated polaron system given by the so-called Holstein–Hubbard Hamiltonian: an electron transfer energy through windows of cages (t), a Coulomb repulsion energy between two s-electrons in the same cage (U), a short-range electron-phonon interaction energy due to the cation displacements (S), and an average number of s-electrons per cage (n). Beyond the jellium background model of alkali-metal clusters, a huge spin-orbit interaction has been observed in the 1p degenerate orbitals of clusters, similarly to the Rashba mechanism