Destabilization of Sn2+ 5s2 Lone-Pair States of SnO through Dimensional Crossover.

Materials exhibiting unique electronic properties arising from a characteristic crystal structure have physical properties that are sensitive to structural dimensionality. This study involves the destabilization of Sn 5s2 lone-pair states of SnO films by decreasing their structural dimensionality in the out-of-plane direction. The inherent dispersive band structure of the SnO films remained unchanged between 80 and 11 nm. Below 11 nm, their dispersive band structure disappeared, the O/Sn ratio increased, and the carrier type changed from the p type to the n type, whereas the Sn valency remained constant at +2. These unconventional changes arose from the electronic separation corresponding to the Debye length, which is proportional to permittivity, and were attributed to weakened interactions between Sn 5s2 lone-pair electrons. Therefore, designing low-permittivity materials is beneficial for reducing the crystallite size required for stabilizing lone-pair states. These results are essential for designing emergent p-type oxides and improving their semiconducting properties and performance in transparent or high-power electronics.

Characteristic Electronic Structure of SnO Film Showing High Hole Mobility.

Actual knowledge of the intrinsic electronic characteristics of p-type oxide semiconductors should help guide the design of innovative electronic devices. The electronic characteristics of oxide semiconductors in thin-film form potentially differ from those in the bulk form owing to lattice strain. In this Letter, we report on the empirical band structure of stannous oxide (SnO) film, which has been shown to have a higher hole mobility than the theoretically expected values for SnO in the bulk form. In vacuo angle-resolved photoemission spectroscopy measurements reveal that the uppermost valence band is anisotropic between the out-of-plane and in-plane directions, and more dispersive than the theoretical predictions. Our findings unveil the underlying mechanism of the semiconductor properties of SnO films and suggest a suitable device structure based on the electronic characteristics.

Antiperovskite Superconductor LaPd3P with Noncentrosymmetric Cubic Structure.

Antiperovskites are a promising candidate structure for the exploration of new materials. We discovered an antiperovskite phosphide, LaPd3P, following our recent synthesis of APd3P (A = Ca, Sr, Ba). While APd3P and (Ca,Sr)Pd3P were found to be tetragonal or orthorhombic systems, LaPd3P is a new prototype cubic system (a = 9.0317(1) Å) with a noncentrosymmetric space group (I4̅3m). LaPd3P exhibited superconductivity with a transition temperature (Tc) of 0.28 K. The upper critical field, Debye temperature, and Sommerfeld constant (γ) were determined to be 0.305(8) kOe, 267(1) K, and 6.06(4) mJ mol-1 K-2 f.u.-1, respectively. We performed first-principles electronic band structure calculations for LaPd3P and compared the theoretical and experimental results. The calculated Sommerfeld constant (2.24 mJ mol-1 K-2 f.u.-1) was much smaller than the experimental value of γ because the Fermi energy (EF) was located slightly below the density of states (DOS) pseudogap. This difference was explained by the increase in the DOS at EF due to the approximately 5 atom % La deficiency (hole doping) in the sample. The observed Tc value was much lower than that estimated using the Bardeen-Cooper-Schrieffer equation. To explain the discrepancy, we examined the possibility of an unconventional superconductivity in LaPd3P arising from the lack of space inversion symmetry.

Bipolar Semiconducting Properties in α-SnWO4 Based on the Characteristic Defect Structure.

Diodes, memories, logic circuits, and most other current information technologies rely on the combined use of p- and n-type semiconductors. Although oxide semiconductors have many technologically attractive functionalities, such as transparency and high dopability to enable their use as conducting films, they typically lack bipolar conductivity. In particular, the absence of p-type semiconducting properties owing to the innate electronic structures of oxides represents a bottleneck for the development of practical devices. Here, bipolar semiconducting properties are demonstrated in α-SnWO4 within a 100 °C temperature window after appropriate thermal treatment. Comprehensive spectroscopic observations reveal that Sn4+ is present in p-type α-SnWO4 in a notably greater quantity than in n-type. This result strongly suggests that the Sn4+ substitutional defects on the W6+ sites contribute to hole-carrier generation in α-SnWO4. We also find that oxygen vacancies are initially formed in Sn-O-W bonds and migrate to W-O-W bonds with changes in semiconducting properties from p-type to n-type. These findings suggest useful strategies for exploring p-type oxide semiconductors and controlling their carrier type by utilizing the octahedral structure.

Experimental and Computational Determination of Optimal Boron Content in Layered Superconductor Sc20C8-xBxC20.

It is generally difficult to quantify the amount of light elements in materials because of their low X-ray-scattering power, as this means that they cannot be easily estimated via X-ray analyses. Meanwhile, the recently reported layered superconductor, Sc20C8-xBxC20, requires a small amount of boron, which is a light element, for its structural stability. In this context, here, we quantitatively evaluate the optimal x value using both experimental and computational approaches. Using the high-pressure synthesis approach, which can maintain the starting composition even after sintering, we obtain the Sc20(C,B)8C20 phase by the reaction of the previously reported Sc15C19 and B (Sc15ByC19). Our experiments demonstrate that an increase in y values promotes the phase formation of the Sc20(C,B)8C20 structure; however, there appears to be an upper limit to the nominal y value to form this phase. The maximum critical temperature (Tc = 7.6 K) is found to correspond with the actual x value of x ≈ 5 under the assumption that the sample with the same Tc as the reported value (7.7 K) possesses the optimal x amount. Moreover, we construct the energy convex hull diagram by calculating the formation enthalpy based on first principles. Our computational results indicate that the composition of Sc20C4B4C20 (x = 4) is the most thermodynamically stable, which is reasonably consistent with the experimentally obtained value.

Superconductivity in a Scandium Borocarbide with a Layered Crystal Structure.

The discovery of nearly room-temperature superconductivity in superhydrides has motivated further materials research for conventional superconductors. To realize the moderately high critical temperature (Tc) in materials containing light elements, we explored new superconducting phases in a scandium borocarbide system. Here, we report the observation of superconductivity in a new ternary Sc-B-C compound. The crystal structure, which was determined through a Rietveld analysis, belongs to tetragonal space group P4/ncc. By complementarily using the density functional theory calculations, a chemical formula of the compound was found to be expressed as Sc20C8-xBxC20 (x = 1 or 2). Interestingly, a small amount of B is essential to stabilize the present structure. Our experiments revealed the typical type-II superconductivity at Tc = 7.7 K. Additionally, we calculated the density of states within a first-principles approach and found that the contribution of the Sc-3d orbital was mainly responsible for the superconductivity.

Flat-Band in Pyrochlore Oxides: A First-Principles Study.

Using a first-principles electronic band calculation, we obtained a quasi flat-band near the Fermi level for the six pyrochlore oxides, A2B2O7. These quasi flat-bands are mostly characterized by the s-orbitals of the A-site. The band structures of these oxides are well described by the non-interacting Mielke model. Spin-polarized calculations showed that the ground state of these compounds was ferromagnetic after appropriate carrier doping, despite the absence of the magnetic element.

Superconductivity in Uncollapsed Tetragonal LaFe2As2.

We report synthesis, crystal structure, and superconductivity in ThCr2Si2-type LaFe2As2 (La122). La122 was synthesized at 960 °C for 1.5 h under a pressure of 3.4 GPa. An as-synthesized La122 (nonsuperconductor) had a collapsed tetragonal structure with a short c-axis length of 11.0144(4) Å as observed in CaFe2As2 under pressure. The collapsed tetragonal structure transformed into an uncollapsed tetragonal structure by annealing the as-synthesized La122 at 500 °C. The c-axis length remarkably extended to 11.7317(4) Å, and superconductivity emerged at 12.1 K in the uncollapsed tetragonal La122. A cylindrical hole-like Fermi surface around the Γ point that plays an important role for an s± wave pairing in iron-based superconductors was missing in the uncollapsed tetragonal La122 because of heavy electron doping. Superconductivity in La122 may be closely related to that induced in CaFe2As2 under pressure.

Synthesis and Superconductivity of a Strontium Digermanide SrGe2-δ with ThSi2 Structure.

We have succeeded in crystallizing a new strontium digermanide (SrGe2-δ) with the ThSi2-type structure (tetragonal SrGe2), which is theoretically predicted to compete with the EuGe2-type one (trigonal SrGe2) under pressure. The tetragonal SrGe2 appeared as a metastable phase in samples at approximately 900 °C under a pressure of 2 GPa. X-ray diffraction studies show that the tetragonal SrGe2 is formed by the reaction between trigonal SrGe2 and excess Sr. The composition of the tetragonal SrGe2 was analyzed to be SrGe1.66(4). Lattice parameters for the tetragonal SrGe2 are determined to be a = 4.559(4) Å and c = 14.42(1) Å. The tetragonal SrGe2 shows metallic resistivity behavior and exhibits superconductivity with a critical temperature (Tc) of 7.3 K, which is the highest among compounds with the ThSi2-type structure. Superconducting properties of the tetragonal SrGe2, such as the upper critical field, and the effect of pressure on Tc, are presented and superconductivity is discussed on the basis of electronic band structure calculations.

Large enhancement of superconducting transition temperature of SrBi3 induced by Na substitution for Sr.

The Matthias rule, which is an empirical correlation between the superconducting transition temperature (Tc) and the average number of valence electrons per atom (n) in alloys and intermetallic compounds, has been used in the past as a guiding principle to search for new superconductors with higher Tc. The intermetallic compound SrBi3 (AuCu3 structure) exhibits a Tc of 5.6 K. An ab-initio electronic band structure calculation for SrBi3 predicted that Tc increases on decreasing the Fermi energy, i.e., on decreasing n, because of a steep increase in the density of states. In this study, we demonstrated that high-pressure (~ 3 GPa) and low-temperature ( < 350 °C) synthesis conditions enables the substitution of Na for about 40 at.% of Sr. With a consequent decrease in n, the Tc of (Sr,Na)Bi3 increases to 9.0 K. A new high-Tc peak is observed in the oscillatory dependence of Tc on n in compounds with the AuCu3 structure. We have shown that the oscillatory dependence of Tc is in good agreement with the band structure calculation. Our experiments reaffirm the importance of controlling the number of electrons in intermetallic compounds.

Crystal structure and superconductivity of BaIr2Ge7 and Ba3Ir4Ge16 with two-dimensional Ba-Ge networks.

The Ba-Ir-Ge ternary compounds BaIr2Ge7 and Ba3Ir4Ge16 exhibit superconductivity (SC) at 2.5 and 5.2 K, respectively. Detailed single-crystal structural analysis revealed that these compounds share unique quasi-two-dimensional networks composed of crown-shaped Ge rings that accommodate Ba atoms at the center, referred to as "edge-shared crown-shaped BaGe16 polyhedra". The layered Ba-Ge network yielded a modest anisotropy of 1.3-1.4 in the upper critical field, which is in good agreement with the band structure calculations. The Ba-Ge structural unit is similar to cage structures seen in various clathrates in which the anharmonic vibration of the central atoms, the so-called "rattling" behavior, brings about strong-coupling SC. However, each Ba-Ge unit is relatively small compared to these materials, which likely excludes the possibility of unconventional SC.

'True' bosonic coupling strength in strongly correlated superconductors.

Clarifying the coupling between electrons and bosonic excitations (phonons or magnetic fluctuations) that mediate the formation of Cooper pairs is pivotal to understand superconductivity. Such coupling effects are contained in the electron self-energy, which is experimentally accessible via angle-resolved photoemission spectroscopy (ARPES). However, in unconventional superconductors, identifying the nature of the electron-boson coupling remains elusive partly because of the significant band renormalization due to electron correlation. Until now, to quantify the electron-boson coupling, the self-energy is most often determined by assuming a phenomenological 'bare' band. Here, we demonstrate that the conventional procedure underestimates the electron-boson coupling depending on the electron-electron coupling, even if the self-energy appears to be self-consistent via the Kramers-Kronig relation. Our refined method explains well the electron-boson and electron-electron coupling strength in ruthenate superconductor Sr2RuO4, calling for a critical revision of the bosonic coupling strength from ARPES self-energy in strongly correlated electron systems.