Moiré-Assisted Realization of Octahedral MoTe2 Monolayer.

A current key challenge in 2D materials is the realization of emergent quantum phenomena in hetero structures via controlling the moiré potential created by the periodicity mismatch between adjacent layers, as highlighted by the discovery of superconductivity in twisted bilayer graphene. Generally, the lattice structure of the original host material remains unchanged even after the moiré superlattice is formed. However, much less attention is paid for the possibility that the moiré potential can also modify the original crystal structure itself. Here, it is demonstrated that octahedral MoTe2 which is unstable in bulk is stabilized in a commensurate MoTe2 /graphene hetero-bilayer due to the moiré potential created between the two layers. It is found that the reconstruction of electronic states via the moiré potential is responsible for this stabilization, as evidenced by the energy-gap opening at the Fermi level observed by angle-resolved photoemission and scanning tunneling spectroscopies. The present results provide a fresh approach to realize novel 2D quantum phases by utilizing the moiré potential.

Thermally Reentrant Crystalline Phase Change in Perovskite-Derivative Nickelate Enabling Reversible Switching of Room-Temperature Electrical Resistivity.

Reversible switching of room-temperature electrical resistivity due to crystal-amorphous transition is demonstrated in various chalcogenides for development of non-volatile phase change memory. However, such reversible thermal switching of room-temperature electrical resistivity has not reported in transition metal oxides so far, despite their enormous studies on the electrical conduction like metal-insulator transition and colossal magnetoresistance effect. In this study, a thermally reversible switching of room-temperature electrical resistivity is reported with gigantic variation in a layered nickelate Sr2.5 Bi0.5 NiO5 (1201-SBNO) composed of (Sr1.5 Bi0.5 )O2 rock-salt and SrNiO3 perovskite layers via unique crystalline phase changes between the conducting 1201-SBNO with ordered (O-1201), disordered Sr/Bi arrangements in the (Sr1.5 Bi0.5 )O2 layer (D-1201), and insulating oxygen-deficient double perovskite Sr2 BiNiO4.5 (d-perovskite). The O-1201 is reentrant by high-temperature annealing of ≈1000 °C through crystalline phase change into the D-1201 and d-perovskite, resulting in the thermally reversible switching of room-temperature electrical resistivity with 102 - and 109 -fold variation, respectively. The 1201-SBNO is the first oxide to show the thermally reversible switching of room-temperature electrical resistivity via the crystalline phase changes, providing a new perspective on the electrical conduction for transition metal oxides.

Slow magnetic relaxation in a ferromagnetic CuII chain complex, induced by a phonon bottleneck effect.

The first slow magnetic relaxation in a ferromagnetic Cu(II) chain compound, Cu(dipic)(OH2)2 (dipicH2 = pyridine-2,6-dicarboxylic acid), induced by a phonon bottleneck effect under a magnetic field of 0.6 T, with a relaxation time of 2.2 s at 2.8 K, was observed.

Near-Ultraviolet Light-Driven Photocathodic Activity for (001)-Oriented BiOCl Thin Films Synthesized by Mist Chemical Vapor Deposition.

Semitransparent and homogeneous bismuth oxychloride (BiOCl) thin films with (001) preferred orientation were synthesized on polycrystalline Sn:In2O3-glass substrates by mist chemical vapor deposition. The films showed photocathodic activity even under near-ultraviolet light within the band gap due to the in-gap states induced by oxygen vacancies. Higher synthesis temperatures resulted in a significant increase of photocurrent density under ultraviolet light. While the longer lifetime of photocarriers led to an increase of internal quantum efficiency, the larger band-edge absorption significantly contributed to the higher external quantum efficiency.

A high-TC heavy rare earth monoxide semiconductor TbO with a more than half-filled 4f orbital.

Terbium monoxide, TbO, with an unusual valence state of Tb2+ ([Xe]4f85d1) has been known to exist as a gas phase. In this study, we report a solid phase rock-salt structured TbO in the form of an epitaxial thin film. This TbO is a narrow gap ferromagnetic semiconductor with a bandgap of ∼0.1 eV and high Curie temperature of 231 K, and exhibited negative magnetoresistance of -14% at 9 T and 2 K. Its magnetization steeply increased with the appearance of a wasp-waisted hysteresis curve significantly below the Curie temperature at about 20 K. These results suggested that both 4f and 5d magnetic sublattices were formed at a single magnetic Tb2+ site in TbO.

Superconductivity and improved electrical conduction in anti-ThCr2Si2-type RE 2O2Sb and RE 2O2Bi with pnictogen square net.

Layered compounds have shown rich physical properties such as high-temperature superconductivity. Recently, monatomic honeycomb lattice systems, such as graphene, have been studied extensively, whereas monatomic pnictogen square nets in layered compounds also exhibit interesting electronic properties owing to their unusual negative valence states. Among them, anti-ThCr2Si2-type RE 2O2 Pn (RE = rare earth, Pn = Sb, Bi) with monoatomic Pn square nets were recently found to exhibit interesting electronic properties such as superconductivity and high carrier mobility. In this article, we review recent studies on crystal structures, electronic properties, and thin-film growth of RE 2O2 Pn.

Versatile control of the superconducting transition temperature in anti-ThCr2Si2-type Y2O2Bi via H, Li, or F doping.

In Y2O2Bi with Bi square net, H substitution and Li intercalation led to higher superconducting transition temperature (Tc), while F substitution led to lower Tc, where Tc is universally scaled by unit cell tetragonality c/a. Li intercalated Y2O2Bi showed a higher Tc than previously reported Y2O2Bi even for the similar c/a.

Heterospin frustration in a metal-fullerene-bonded semiconductive antiferromagnet.

Lithium-ion-encapsulated fullerenes (Li+@C60) are 3D superatoms with rich oxidative states. Here we show a conductive and magnetically frustrated metal-fullerene-bonded framework {[Cu4(Li@C60)(L)(py)4](NTf2)(hexane)}n (1) (L = 1,2,4,5-tetrakis(methanesulfonamido)benzene, py = pyridine, NTf2- = bis(trifluoromethane)sulfonamide anion) prepared from redox-active dinuclear metal complex Cu2(L)(py)4 and lithium-ion-encapsulated fullerene salt (Li+@C60)(NTf2-). Electron donor Cu2(L)(py)2 bonds to acceptor Li+@C60 via eight Cu‒C bonds. Cu-C bond formation stems from spontaneous charge transfer (CT) between Cu2(L)(py)4 and (Li+@C60)(NTf2-) by removing the two-terminal py molecules, yielding triplet ground state [Cu2(L)(py)2]+(Li+@C60•-), evidenced by absorption and electron paramagnetic resonance (EPR) spectra, magnetic properties and quantum chemical calculations. Moreover, Li+@C60•- radicals (S = ½) and Cu2+ ions (S = ½) interact antiferromagnetically in triangular spin lattices in the absence of long-range magnetic ordering to 1.8 K. The low-temperature heat capacity indicated that compound 1 is a potential candidate for an S = ½ quantum spin liquid (QSL).

Low-temperature topotactic oxidation using the solid-state oxidant Zr-doped CeO2.

A new topotactic oxidation was developed using the solid-state oxidant Zr-doped CeO2. For the anti-ThCr2Si2 type Y2O2Bi, which became superconducting via oxygen intercalation during solid-state oxidation at 1000 °C, the topotactic oxidation enabled not only the oxygen intercalation at a much lower temperature of 200 °C, hampering the segregation of impurity phases, but also the highest superconducting transition temperature for Y2O2Bi.

Robust charge-density wave strengthened by electron correlations in monolayer 1T-TaSe2 and 1T-NbSe2.

Combination of low-dimensionality and electron correlation is vital for exotic quantum phenomena such as the Mott-insulating phase and high-temperature superconductivity. Transition-metal dichalcogenide (TMD) 1T-TaS2 has evoked great interest owing to its unique nonmagnetic Mott-insulator nature coupled with a charge-density-wave (CDW). To functionalize such a complex phase, it is essential to enhance the CDW-Mott transition temperature TCDW-Mott, whereas this was difficult for bulk TMDs with TCDW-Mott < 200 K. Here we report a strong-coupling 2D CDW-Mott phase with a transition temperature onset of ~530 K in monolayer 1T-TaSe2. Furthermore, the electron correlation derived lower Hubbard band survives under external perturbations such as carrier doping and photoexcitation, in contrast to the bulk counterpart. The enhanced Mott-Hubbard and CDW gaps for monolayer TaSe2 compared to NbSe2, originating in the lattice distortion assisted by strengthened correlations and disappearance of interlayer hopping, suggest stabilization of a likely nonmagnetic CDW-Mott insulator phase well above the room temperature. The present result lays the foundation for realizing monolayer CDW-Mott insulator based devices operating at room temperature.

Increased hole mobility in anti-ThCr2Si2-type La2O2Bi co-sintered with alkaline earth metal oxides for oxygen intercalation and hole carrier doping.

Metallic anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with Bi square nets show superconductivity while insulating La2O2Bi shows high hole mobility, by expanding the c-axis length through oxygen intercalation. In this study, alkaline earth metal oxides (CaO, SrO and BaO) were co-sintered with La2O2Bi. CaO and BaO served as oxygen intercalants without the incorporation of Ca and Ba in La2O2Bi. On the other hand, SrO served not only as an oxygen intercalant but also as a hole dopant via Sr substitution for La in La2O2Bi. The oxygen intercalation and hole doping resulted in the expansion of the c-axis length, contributing to the improved electrical conduction. In addition, the hole mobility was enhanced up to 150 cm2 V-1 s-1 in La2O2Bi, which is almost double the mobility in a previous study.

Synthesis of Stoichiometric SrTiO3 and Its Carrier Doping from Air-Stable Bimetallic Complexes.

We demonstrate the synthesis of perovskite oxide SrTiO3 with ideal cation stoichiometry and homogeneous La doping using air-stable Sr-Ti and La-Ti bimetallic peroxo complexes with a 1:1 cation ratio. Phase-pure SrTiO3, La2Ti2O7, and LaTiO3 were successfully synthesized by thermal decomposition of those complexes. La-doped SrTiO3 was obtained by mixing the Sr-Ti and La-Ti complexes in an acid solution, followed by thermal decomposition. La-doped SrTiO3 showed systematic chemical trends in the lattice constant and electrical conduction. Not only those bulk polycrystals but also cation-stoichiometric SrTiO3 epitaxial thin films were grown with an atomically flat surface from the Sr-Ti complex.

Epitaxial growth of bismuth oxyhalide thin films using mist CVD at atmospheric pressure.

We report the epitaxial growth of bismuth oxyhalide BiOX (X = Cl, Br, and I) thin films using mist chemical vapour deposition at atmospheric pressure. The thin films grown under optimum conditions possessed atomically flat surfaces and high crystallinity, where the lattice constants of BiOX were controlled by epitaxial strain.

Tetragonality induced superconductivity in anti-ThCr2Si2-type RE2O2Bi (RE = rare earth) with Bi square nets.

We report a series of layered superconductors, anti-ThCr2Si2-type RE2O2Bi (RE = rare earth), composed of electrically conductive Bi square nets and magnetic insulating RE2O2 layers. Superconductivity was induced by separating the Bi square nets as a result of excess oxygen incorporation, irrespective of the presence of magnetic ordering in RE2O2 layers. Intriguingly, the transition temperature of all RE2O2Bi including nonmagnetic Y2O2Bi was approximately scaled by unit cell tetragonality (c/a), implying a key role in the relative separation of the Bi square nets to induce superconductivity.

Anion-Substitution-Induced Nonrigid Variation of Band Structure in SrNbO3- xN x (0 ≤ x ≤ 1) Epitaxial Thin Films.

Pervoskite oxynitrides exhibit rich functionalities such as colossal magnetoresistance and high photocatalytic activity. The wide tunability of physical properties by the N/O ratio makes perovskite oxynitrides promising as optical and electrical materials. However, composition-dependent variation of the band structure, especially under partially substituted composition, is not yet well understood. In this study, we quantitatively analyzed the composition-dependent variation of band structure of a series of SrNbO3- xN x (0 ≤ x ≤ 1.02) epitaxial thin films. Electrical conductivity decreased along with the increase of N content x as a result of an increase in Nb valence from 4+ to 5+. Optical measurements revealed that the N 2p band is formed at a critical composition between 0.07 < x < 0.38, which induces charge-transfer transition (CTT) in the visible-light region. These variations in the band structure were explained by first-principles calculations. However, the CTT energy slightly increased at higher N contents (i.e., lower carrier density) on contrary to the expectation based on the rigid-band-like shift of the Fermi level, which suggests a complex combination of the following band-shifting effects induced by N-substitution: whereas (1) reduction of the Burstein-Moss effect causes CTT energy reduction, (2) enhancement of hybridization between Nb 4d and N 2p orbitals and/or (3) suppression of many-body effects enlarge the band gap energy at larger N content. The band structure variation in perovskite oxynitride as presently elucidated would be a guidepost for future material design.

Superconductivity in Anti-ThCr2Si2-type Er2O2Bi Induced by Incorporation of Excess Oxygen with CaO Oxidant.

Recently, superconductivity was induced by expanding interlayer distance between Bi square nets in anti-ThCr2Si2-type Y2O2Bi through incorporation of excess oxygen with increased nominal amount of oxygen. However, such oxygen incorporation was applicable to only Y2O2Bi among R2O2Bi ( R = rare earth metal), probably due to a larger amount of oxygen incorporation for Y2O2Bi. In this study, the interlayer distance in Er2O2Bi was increased by cosintering with CaO, which served as an oxidant, indicating that excess oxygen was incorporated in Er2O2Bi. As a result, superconductivity was induced in Er2O2Bi at 2.2 K.

Superconductivity of Rock-Salt Structure LaO Epitaxial Thin Film.

We report a superconducting transition in a LaO epitaxial thin film with the superconducting transition onset temperature ( Tc) at around 5 K. This Tc is higher than those of other lanthanum monochalcogenides and opposite to their chemical trend: Tc = 0.84, 1.02, and 1.48 K for LaX (X = S, Se, Te), respectively. The carrier control resulted in a dome-shaped Tc as a function of electron carrier density. In addition, the Tc was significantly sensitive to epitaxial strain in spite of the highly symmetric crystal structure. This rock-salt superconducting LaO could be a building block to design novel superlattice superconductors.

New Lutetium Oxide: Electrically Conducting Rock-Salt LuO Epitaxial Thin Film.

C-rare earth structure lutetium sesquioxide Lu2O3 has been recognized as a high-k widegap insulator with closed shell Lu3+ ions. In this study, rock-salt structure lutetium monoxide LuO with unusual valence of Lu2+ (4f145d1), previously known as the gaseous phase, was synthesized as an epitaxial thin film by the pulsed laser deposition method. In contrast with transparent and highly insulating Lu2O3, LuO possessed a dark-brown color and high electrical conductivity concomitant with strong spin-orbit coupling as a manifestation of Lu 5d electron carriers.

Two-Dimensional Superconductivity Emerged at Monatomic Bi(2-) Square Net in Layered Y2O2Bi via Oxygen Incorporation.

Discovery of layered superconductors such as cuprates and iron-based compounds has unveiled new science and compounds. In these superconductors, quasi-two-dimensional layers including transition metal cations play principal role in the superconductivity via carrier doping by means of aliovalent-ion substitution. Here, we report on a two-dimensional superconductivity at 2 K in ThCr2Si2-type layered oxide Y2O2Bi possessing conducting monatomic Bi(2-) square net, possibly associated with an exotic superconductivity. The superconductivity emerges only in excessively oxygen-incorporated Y2O2Bi with expanded inter-net distance, in stark contrast to nonsuperconducting pristine Y2O2Bi reported previously. This result suggests that the element incorporation into hidden interstitial site could be an alternative approach to conventional substitution and intercalation methods for search of novel superconductors.

2D Electronic Transport with Strong Spin-Orbit Coupling in Bi(2-) Square Net of Y2O2Bi Thin Film Grown by Multilayer Solid-Phase Epitaxy.

Highly crystalline Y2O2Bi epitaxial thin film with monatomic Bi(2-) square net layer was grown by newly developed multilayer solid phase epitaxy. High reactivity of the nanometer-scale multilayered precursor enabled efficient formation of single crystalline Y2O2Bi phase with one-step heating. The reductive state of Bi(2-) square net was observed by X-ray photoemission spectroscopy. The electrical resistivity was one order lower than that of polycrystalline powder in previous study. The magnetotransport showed weak antilocalization effect well fitted by the Hikami-Larkin-Nagaoka model, exhibiting two-dimensional electronic nature with strong spin-orbit coupling in the Bi(2-) square net.