Octahedral and trigonal-prismatic coordination preferences in Nb-, Mo-, Ta-, and W-based ABX2 layered oxides, oxynitrides, and nitrides

Cd additive effect on self-flux growth of Cs-intercalated NbS2 superconducting single crystals

Single crystals of Cs-intercalated NbS2 (Cs x NbS2) were synthesized using a CsCl/KCl self-flux. The size and Cs content of Cs x NbS2 single crystals increased upon adding Cd metal into the starting materials. When 10–30 at% of Cd per Nb was provided in the starting materials, plate-like Cs x NbS2 (x ∼ 0.3) single crystals with 1–2 mm in size and 10–100 μm in thickness were obtained. The superconducting transition temperature of these Cs x NbS2 single crystals was 1.65 K.

p-type transparent superconductivity in a layered oxide

Development of p-type transparent conducting materials has been a challenging issue. The known p-type transparent conductors unsatisfy both of high transparency and high conductivity nor exhibit superconductivity. Here, we report on epitaxial synthesis, excellent p-type transparent conductivity, and two-dimensional superconductivity of Li_1-x NbO₂. The LiNbO₂ epitaxial films with NbO₂ sheets parallel to (111) plane of cubic MgAl₂O₄ substrates were stabilized by heating amorphous films. The hole doping associated with Li⁺ ion deintercalation triggered superconductivity below 4.2 kelvin. Optical measurements revealed that the averaged transmittance to the visible light of ~100-nanometer-thick Li_1-x NbO₂ was ~77%, despite the large number of hole carriers exceeding 10²² per cubic centimeter. These results indicate that Li_1-x NbO₂ is a previously unknown p-type transparent superconductor, in which strongly correlated electrons at the largely isolated Nb 4d _z2 band play an important role for the high transparency.

Intrinsic ferromagnetism and valley polarization in hydrogenated group V transition-metal dinitride (MN2H2, M = V/Nb/Ta) nanosheets: insights from first-principles

Due to the extraordinary electronic and magnetic properties, transition-metal dinitrides (TMDNs) and their derivatives are gaining importance in low-dimensional layered materials. In this work, through first-principles calculations, we have comprehensively investigated the structural and electronic properties of hydrogenated group V TMDN nanosheets. We find that surface hydrogenation can well stabilize the H-, T- and M-phase structures of group V TMDNs, for which the formed MN2H2 nanosheets have robust energetic, dynamical and thermal stabilities. Different from pristine MN2 systems, the H-phase system has become the most favorable structure of MN2H2 nanosheets. Intrinsic ferromagnetism is present in these H-MN2H2 nanosheets, which even exhibit bipolar magnetic semiconducting behaviors. More interestingly, large spontaneous valley polarization occurs in the H-MN2H2 nanosheets, and is attributed to the coexistence of remarkable spin-orbit coupling and magnetic exchange interactions according to the k·p model analysis. Among them, the H-NbN2H2 nanosheets are found to be a promising ferrovalley material, whose valley polarization value reaches as large as 0.11 eV and the Curie temperature is up to 225 K. Besides that, versatile electronic properties are obtained in the T- and M-phase structures of the MN2H2 nanosheets, which will be magnetic/nonmagnetic metals/semiconductors depending on the metal species and phase structures. Our study demonstrates that the hydrogenation can bring robust structural stabilities and unconventional electronic properties into the group V TMDN nanosheets, which enable many potential applications in spintronics and valleytronics.

Ternary nitride semiconductors in the rocksalt crystal structure

Inorganic nitrides with wurtzite crystal structures are well-known semiconductors used in optical and electronic devices. In contrast, rocksalt-structured nitrides are known for their superconducting and refractory properties. Breaking this dichotomy, here we report ternary nitride semiconductors with rocksalt crystal structures, remarkable electronic properties, and the general chemical formula Mgx TM 1-xN (TM = Ti, Zr, Hf, Nb). Our experiments show that these materials form over a broad metal composition range, and that Mg-rich compositions are nondegenerate semiconductors with visible-range optical absorption onsets (1.8 to 2.1 eV) and up to 100 cm2 V-1⋅s-1 electron mobility for MgZrN2 grown on MgO substrates. Complementary ab initio calculations reveal that these materials have disorder-tunable optical absorption, large dielectric constants, and electronic bandgaps that are relatively insensitive to disorder. These ternary Mgx TM 1-xN semiconductors are also structurally compatible both with binary TMN superconductors and main-group nitride semiconductors along certain crystallographic orientations. Overall, these results highlight Mgx TM 1-xN as a class of materials combining the semiconducting properties of main-group wurtzite nitrides and rocksalt structure of superconducting transition-metal nitrides.

Localized double phonon scattering and DOS induced thermoelectric enhancement of degenerate nonstoichiometric Li1−xNbO2 compounds

We report the synthesis and thermoelectric properties of a new p-type oxide thermoelectric material (Li_1−xNbO₂, with x = 0–0.6), in which Li-vacancies play a significant role in the enhancement of the thermoelectric performance.