Superconductivity of barium with highest transition temperatures in metallic materials at ambient pressure.

Pressure-induced superconductivity often occurs following structural transition under hydrostatic pressure (PHP) but disappears after the pressure is released. In the alkali-earth metal barium, superconductivity appears after structural transformation from body-centered cubic structure to hexagonal-close-packed (hcp) structure at PHP = 5 GPa, and the superconducting transition temperature (Tc) reaches a maximum of 5 K at PHP = 18 GPa. Furthermore, by stabilizing the low-temperature phase at PHP ~ 30 GPa, Tc reached a higher level of 8 K. Herein, we demonstrate a significantly higher Tc superconductivity in Ba even at ambient pressure. This was made possible through severe plastic deformation of high-pressure torsion (HPT). In this HPT-processed Ba, we observed superconductivity at Tc = 3 K and Tc = 24 K in the quasi-stabilized hcp and orthorhombic structures, respectively. In particular, the latter Tc represents the highest value achieved at ambient pressure among single-element superconducting metals, including intermetallics. The phenomenon is attributed to a strained high-pressure phase, stabilized by residual strains generated from lattice defects such as dislocations and grain boundaries. Significantly, the observed Tc far exceeds predictions from DFT calculations under normal hydrostatic compressions. The study demonstrates the importance of utilizing high-pressure strained phases as quasi-stable superconducting states at ambient pressure.

Ab initio calculation for electronic structure and optical property of tungsten carbide in a TiCN-based cermet for solar thermal applications.

We present an ab initio calculation to understand electronic structures and optical properties of a tungsten carbide WC being a major component of a TiCN-based cermet. The TiCN-based cermet is widely used as a cutting tool, and is discarded as usual after use. On the other hand, cermet itself is also a famous ingredient of a solar absorption film. We found that the WC has a fairly low-energy plasma excitation [Formula: see text] 0.6 eV (2 [Formula: see text]m) and therefore can be a good constituent of a solar selective absorber. The evaluated figure of merit for photothermal conversion is prominently high compared to those of the other materials included in the TiCN-based cermet. The imaginary part of the dielectric function is considerably small around the zero point of the real part of the dielectric function, corresponding to the plasma excitation energy. Therefore, a clear plasma edge appeared, ensuring the high performance of the WC as the solar absorber. This is a fascinating aspect, because the wasted TiCN-based cermet cutting tool can be recycled as the solar absorption film after proper treatments and modifications.

Electronic Correlation Strength of Inorganic Electrides from First Principles.

Strongly correlated electron systems, generally recognized as d- and f-electron systems, have attracted attention as a platform for the emergence of exotic properties such as high-Tc superconductivity. However, correlated electron behaviors have been recently observed in a group of novel materials, electrides, in which s-electrons are confined in subnanometer-sized spaces. Here, we present a trend of electronic correlation of electrides by evaluating the electronic correlation strength obtained from model parameters characterizing effective Hamiltonians of 19 electrides from first principles. The calculated strengths vary in the order 0D ≫ 1D > 2D ∼ 3D electrides, which corresponds to experimental trends, and exceed 10 (a measure for the emergence of exotic properties) in all of the 0D and some of the 1D electrides. We also found the electronic correlation depends on the cation species surrounding the s-electrons. The results indicate that low-dimensional electrides will be new research targets for studies of strongly correlated electron systems.

Large enhancement of superconducting transition temperature in single-element superconducting rhenium by shear strain.

Finding a physical approach for increasing the superconducting transition temperature (Tc) is a challenge in the field of material science. Shear strain effects on the superconductivity of rhenium were investigated using magnetic measurements, X-ray diffraction, transmission electron microscopy, and first-principles calculations. A large shear strain reduces the grain size and simultaneously expands the unit cells, resulting in an increase in Tc. Here we show that this shear strain approach is a new method for enhancing Tc and differs from that using hydrostatic strain. The enhancement of Tc is explained by an increase in net electron-electron coupling rather than a change in the density of states near the Fermi level. The shear strain effect in rhenium could be a successful example of manipulating Bardeen-Cooper-Schrieffer-type Cooper pairing, in which the unit cell volumes are indeed a key parameter.

Effect of electron-phonon interactions on orbital fluctuations in iron-based superconductors.

To investigate the possibility of whether electron-phonon coupling can enhance orbital fluctuations in iron-based superconductors, we develop an ab initio method to construct the effective low-energy models including the phonon-related terms. With the derived effective electron-phonon interactions and phonon frequencies, we estimate the static part (ω=0) of the phonon-mediated effective on site intra- or interorbital electron-electron attractions as ∼-0.4 eV and exchange or pair-hopping terms as ∼-0.02 eV. We analyze the model with the derived interactions together with the Coulomb repulsions within the random phase approximation. We find that the enhancement of the orbital fluctuations due to the electron-phonon interactions is small, and that the spin fluctuations enhanced by the Coulomb repulsions dominate. It leads to the superconducting state with the sign reversal in the gap functions (s± wave).

Ab initio evidence for strong correlation associated with Mott proximity in iron-based superconductors.

We predict that iron-based superconductors discovered near d(6) configuration (5 Fe 3d orbitals filled by 6 electrons) is located on the foot of an unexpectedly large dome of correlated electron matter centered at the Mott insulator at d(5) (namely, half filling). This is based on the many-variable variational Monte Carlo results for ab initio low-energy models derived by the downfolding. The d(5) Mott proximity extends to subsequent emergence of incoherent metals, orbital differentiations due to the Mott physics, and Hund's rule coupling, followed by antiferromagnetic quantum criticality, in quantitative accordance with available experiments.

Effect of through-space electron transfer on infrared spectrum of amorphous selenium.

In this paper we present theoretical analyses on an infrared (IR) spectrum of amorphous selenium. The system is described by a 216-atom-chain model, and a set of molecular-dynamics simulations is performed to generate vitreous structures and vibrational modes. To describe an electronic structure of the system we employ a complete neglect of differential overlap model parametrized by ab initio cluster calculations. An IR intensity is evaluated with the Berry-phase formula for an electronic polarization. The effect of the through-space electron transfer on the IR spectrum is studied by artificially changing the magnitude of matrix elements associated with the electron transfer between nonbonded atoms in the chain. We find that the through-space electron transfer leads to (i) the enhancement of the bending IR peak at 135 cm(-1) and (ii) the appearance of a new low-frequency peak around 50 cm(-1), thus resulting in a good agreement with the experiment. The mechanism is discussed by a simple dipole model.