Fermi surface and light quasi particles in hourglass nodal chain metalß-ReO2.

Quantum oscillations (QOs) in magnetic torque and electrical resistivity were measured to investigate the electronic structure ofß-ReO2, a candidate hourglass nodal chain (NC) metal (Dirac loop chain metal). All the de Haas-van Alphen oscillation branches measured at 30 mK in magnetic fields of up to 17.5 T were consistent with first-principles calculations predicting four Fermi surfaces (FSs). The small-electron FS of the four FSs exhibited a very small cyclotron mass, 0.059 times that of the free electrons, which is likely related to the linear dispersion of the energy band. The consistency between the QO results and band calculations indicates the presence of the hourglass NC predicted forß-ReO2in the vicinity of the Fermi energy.

Dirac semimetal phase and switching of band inversion in XMg2Bi2 (X = Ba and Sr).

Topological Dirac semimetals (TDSs) offer an excellent opportunity to realize outstanding physical properties distinct from those of topological insulators. Since TDSs verified so far have their own problems such as high reactivity in the atmosphere and difficulty in controlling topological phases via chemical substitution, it is highly desirable to find a new material platform of TDSs. By angle-resolved photoemission spectroscopy combined with first-principles band-structure calculations, we show that ternary compound BaMg2Bi2 is a TDS with a simple Dirac-band crossing around the Brillouin-zone center protected by the C3 symmetry of crystal. We also found that isostructural SrMg2Bi2 is an ordinary insulator characterized by the absence of band inversion due to the reduction of spin-orbit coupling. Thus, XMg2Bi2 (X = Sr, Ba, etc.) serves as a useful platform to study the interplay among crystal symmetry, spin-orbit coupling, and topological phase transition around the TDS phase.

Geometrical Spin Frustration and Monoclinic-Distortion-Induced Spin Canting in the Double Perovskites Ln2LiFeO6 (Ln = La, Nd, Sm, and Eu) with Unusually High Valence Fe5.

We discovered new B-site-ordered double perovskites Ln2LiFeO6 (Ln = La, Nd, Sm, and Eu) with most likely unusually high valence Fe5+, which was stabilized by strong oxidizing high-pressure synthesis. Despite large antiferromagnetic interactions between Fe5+ spins in these compounds, the magnetic ordering is strongly suppressed due to the geometrical frustration of Fe5+ located in a face-centered cubic lattice. In addition, canted magnetic structures are stabilized only in those with Ln = Sm and Eu, which is most likely due to significant Dzyaloshinskii Moriya interaction caused by large monoclinic structural distortion. These results provide a deep understanding of the structure-property relationships in geometrically frustrated B-site-ordered double perovskites.

Insight into the diffusion mechanism of sodium ion-polaron complexes in orthorhombic P2 layered cathode oxide NaxMnO2.

Using the density functional theory, we investigated the geometric, electronic structure, phase stability and electrochemical properties of a potential P2 layer orthorhombic cathode material NaxMnO2 (0 ≤ x ≤ 1) applied for sodium-ion batteries. Herein, we shed the light on the undeniable effect of the polaron formation and polaron migration on the diffusion of Na+ ions in the orthorhombic P2 layered oxides. Both GGA+U and HSE06 methods agree that, when a Na+ ion is removed from the fully charged state of NaMnO2, the accompanying polaron preferably forms at one of the third nearest Mn (3NN) octahedra to the Na vacancy, implying the oxidization of the Mn3+ ion at one of these 3NN sites to Mn4+. The positive polaron migrates simultaneously with the Na vacancy and would hinder the diffusion of Na ions. Two kinds of elementary diffusion processes, named parallel and crossing, have been explored which required almost same activation energy of about 423 meV (518 meV) by GGA+U (HSE06). In the fully discharged state, GGA+U and HSE06 methods indicate that the negative polaron forms at one of the second nearest Mn neighbours (2NN). The activation energy of 273 meV (327 meV) is needed for diffusion in a structure with a low Na concentration, which is much lower than that required for diffusion in the Na-rich regime. Consequently, Na+ ions can diffuse easier at lower Na concentrations. With the overall activation energy of 423 meV (518 meV), this material exhibits a faster ion diffusion in comparison with the prevailing lithium-based materials such as olivine phosphate.

First-Principles Study of Na-Ion Battery Performance and Reaction Mechanism of Tin Sulfide as Negative Electrode.

We study the Na-ion battery characteristics of SnS as a negative electrode by first-principles calculations. From energy analyses, we clarify the discharge reaction process of the Na/SnS half-cell system. We show a phase diagram of Na-Sn-S ternary systems by constructing convex-hull curves, and show a possible reaction route considering intermediate products in discharge reactions. Voltage-capacity curves are calculated based on the Na-SnS reaction path that is obtained from the ternary phase diagram. It is found that the conversion reactions and subsequently the alloying reactions proceed in the SnS electrode, contributing to its high capacity compared with the metallic Sn electrode, in which only the alloying reactions progresses stepwise. To verify the calculated reaction process, x-ray absorption spectra (XAS) are calculated and compared with experimental XAS at S K-edge, showing meaningful XAS changes associated with Na2 S and SnS in discharged and charged states, respectively.

Sparse modeling of chemical bonding in binary compounds.

A sparse model for quantifying energy difference between zinc-blende and rock-salt crystal structures in octet elemental and binary materials is constructed by using the linearly independent descriptor-generation method and exhaustive search, following the previous work by Ghiringhelli et al. [Phys Rev Lett. 2015;114:105503]. The obtained simplest model includes only atomic radius information of constituent atoms and its physical meaning is interpreted in relation to van Arkel-Ketelaar's triangle for classifying chemical bonding in binary compounds.

Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor Bi2Sr2CaCu2O8+δ as a Candidate of a Topological Superconductor.

One of the key challenges in condensed-matter physics is to establish a topological superconductor that hosts exotic Majorana fermions. Although various heterostructures consisting of conventional BCS (Bardeen-Cooper-Schrieffer) superconductors as well as doped topological insulators were intensively investigated, no conclusive evidence for Majorana fermions has been provided. This is mainly because of their very low superconducting transition temperatures ( Tc) and small superconducting-gap magnitude. Here, we report a possible realization of topological superconductivity at very high temperatures in a hybrid of Bi(110) ultrathin film and copper oxide superconductor Bi2Sr2CaCu2O8+δ (Bi2212). Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we found that three-bilayer-thick Bi(110) on Bi2212 exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to Tc of Bi2212 (85 K). The small Fermi energy and strong spin-orbit coupling of Bi(110), together with the large pairing gap and high Tc, make this system a prime candidate for exploring stable Majorana fermions at very high temperatures.

Ultrathin Bismuth Film on 1T-TaS2: Structural Transition and Charge-Density-Wave Proximity Effect.

We have fabricated bismuth (Bi) ultrathin films on a charge-density-wave (CDW) compound 1T-TaS2 and elucidated electronic states by angle-resolved photoemission spectroscopy and first-principles band-structure calculations. We found that the Bi film on 1T-TaS2 undergoes a structural transition from (111) to (110) upon reducing the film thickness, accompanied by a drastic change in the energy band structure. We also revealed that while two-bilayer-thick Bi(110) film on Si(111) is characterized by a dispersive band touching the Fermi level ( EF), the energy band of the same film on 1T-TaS2 exhibits holelike dispersion with a finite energy gap at EF. We discuss the origin of such intriguing differences in terms of the CDW proximity effect.

Strain Engineering for Anion Arrangement in Perovskite Oxynitrides.

Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca1-xSrxTaO2N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO4N2 octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LP-XANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable trans-type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements.

Selective and low temperature transition metal intercalation in layered tellurides.

Layered materials embrace rich intercalation reactions to accommodate high concentrations of foreign species within their structures, and find many applications spanning from energy storage, ion exchange to secondary batteries. Light alkali metals are generally most easily intercalated due to their light mass, high charge/volume ratio and in many cases strong reducing properties. An evolving area of materials chemistry, however, is to capture metals selectively, which is of technological and environmental significance but rather unexplored. Here we show that the layered telluride T2PTe2 (T=Ti, Zr) displays exclusive insertion of transition metals (for example, Cd, Zn) as opposed to alkali cations, with tetrahedral coordination preference to tellurium. Interestingly, the intercalation reactions proceed in solid state and at surprisingly low temperatures (for example, 80 °C for cadmium in Ti2PTe2). The current method of controlling selectivity provides opportunities in the search for new materials for various applications that used to be possible only in a liquid.

Coupling Ferroelectricity with Spin-Valley Physics in Oxide-Based Heterostructures.

The coupling of spin and valley physics is nowadays regarded as a promising route toward next-generation spintronic and valleytronic devices. In the aim of engineering functional properties for valleytronic applications, we focus on the ferroelectric heterostructure BiAlO3/BiIrO3, where the complex interplay among a trigonal crystal field, layer degrees of freedom, and spin-orbit coupling mediates a strong spin-valley coupling. Furthermore, we show that ferroelectricity provides a nonvolatile handle to manipulate and switch the emerging valley-contrasting spin polarization.

Signature of high Tc above 25 K in high quality superconducting diamond.

We have observed zero resistivity above 10 K and an onset of resistivity reduction at 25.2 K in a heavily B-doped diamond film. However, the effective carrier concentration is similar to that of superconducting diamond with a lower Tc. We found that the carrier has a longer mean free path and lifetime than in the previous report, indicating that this highest Tc diamond has better crystallinity compared to that of other superconducting diamond films. In addition, the susceptibility shows a small transition above 20 K in the high quality diamond, suggesting a signature of superconductivity above 20 K. These results strongly suggest that heavier carrier doped defect-free crystalline diamond could give rise to high Tc diamond.

Extremely large magnetoresistance in the nonmagnetic metal PdCoO2.

Extremely large magnetoresistance is realized in the nonmagnetic layered metal PdCoO(2). In spite of a highly conducting metallic behavior with a simple quasi-two-dimensional hexagonal Fermi surface, the interlayer resistance reaches up to 35,000% for the field along the [11[over ¯]0] direction. Furthermore, the temperature dependence of the resistance becomes nonmetallic for this field direction, while it remains metallic for fields along the [110] direction. Such severe and anisotropic destruction of the interlayer coherence by a magnetic field on a simple Fermi surface is ascribable to orbital motion of carriers on the Fermi surface driven by the Lorentz force, but seems to have been largely overlooked until now.

Influence of lone pair doping on the multiferroic property of orthorhombic HoMnO3: ab initio prediction.

Using first principles density functional theory, we predict new multiferroic compounds Ho1/2A1/2MnO3 (A = As, Sb, Bi) with enhanced polarization. We find that doping of lone pair cations with different ionic radii, at the A-site of orthorhombic HoMnO3, results in a marked increase of the electronic polarization and its development along the b-axis. This development of electronic polarization along the b-axis is attributed to the breaking of the two-fold rotational symmetry which leads to the emergence of a polar b-axis. Furthermore, this symmetry breaking leads to the emergence of two inequivalent Mn ions (Mn(0) and Mn(1)) and the variance in their octahedral (Mn(0)O6 and Mn(1)O6) distortions. We rationalize the observed trends in the total polarization in terms of disparate eg electron hopping along the two different Mn(0) and Mn(1) chains. We expect large ionic polarization in the doped compounds due to the presence of 4s(2) As, 5s(2) Sb and 6s(2) Bi lone pairs, but surprisingly the effect of the lone pairs seems to be inactive. This is attributed to the strong GdFeO3 distortions exhibited by the MnO6 octahedron which hinders polar displacement of the lone pair cations.

Tunable spin polarization in bismuth ultrathin film on Si(111).

We performed a spin- and angle-resolved photoemission spectroscopy of bismuth ultrathin film on Si(111) with various film thickness d. We found that the spin polarization of spin-split Rashba surface states near the Brillouin-zone boundary, which is high (0.7) at d = 40 BL (bilayers), is gradually reduced on decreasing d and almost vanishes at d = 8 BL. This finding provides a novel method to generate spin-polarized electrons with tunable spin-polarization.

Peculiar Rashba splitting originating from the two-dimensional symmetry of the surface.

A peculiar Rashba effect is found at a point in the Brillouin zone, where the time-reversal symmetry is broken, though this symmetry was believed to be a necessary condition for Rashba splitting. This finding obtained experimentally by photoemission measurements on a Bi/Si(111)-(sqrt(3) x sqrt(3)) surface is fully confirmed by a first-principles theoretical calculation. We found that the peculiar Rashba effect is simply understood by the two-dimensional symmetry of the surface, and that this effect leads to an unconventional nonvortical Rashba spin structure at a point with time-reversal invariance.

The surface Rashba effect: a k·p perturbation approach.

For surface systems, the Rashba effect is studied by using a k·p perturbation method. It is shown that the velocity-operator term in the perturbation gives the generalized Rashba Hamiltonian, of which a group-theoretical analysis is given to explain variations in the spin splitting and spin structure expected for typical surface symmetry. The matrix elements of the velocity and spin-angular-momentum operators play a key role in determining the characteristic features of the surface Rashba effect. Whether a surface system shows isotropic spin splitting and vortical spin structure as given by the original Rashba Hamiltonian or not depends on the group of k appearing in the corresponding two-dimensional Brillouin zone. It is especially emphasized that the ideal Rashba effect may be realized even for a wavevector k without time reversal, which is usually believed to be a necessary condition.

Ab initio study on the electronic structure and vibration modes of alkali and alkaline-earth amides and alanates.

We study the electronic structure and vibrational modes of several amides M(NH(2))(n) and alanates M(AlH(4))(n) (M = K, Na, Li, Ca and Mg), focusing on the role of cation states. Calculated breathing stretching vibration modes for these compounds are compared with measured infrared and Raman spectra. In the amides, we find a significant tendency such that the breathing mode frequencies and the structural parameters of NH(2) vary in accordance with the ionization energy of cation. The tendency may be explained by the strength in hybridization between cation orbitals and molecular orbitals of (NH(2))(-). The microscopic mechanism of correlations between the vibration frequencies and structural parameters is elucidated in relation to the electronic structure. A possible similar tendency in the alanates is also discussed.

Electronic structure of boron-doped diamond with B-H complex and B pair.

The electronic structure of boron-hydrogen complex and boron pair in diamond are studied by first-principles density-functional calculations with supercell models. The electronic structure calculated for the B-H complexes with C2v or C3v symmetry and the nearest-neighbor B pair is used to interpret recent experimental results such as B 1s x-ray photoemission spectroscopy, 11B nuclear quadruple resonance and B K-edge x-ray absorption spectroscopy, which cannot be explained solely by the isolated substitutional boron.

International conference on quantum simulators and design, hiroshima, Japan, 3-6 december 2006.

This special issue of Journal of Physics: Condensed Matter comprises selected papers from the 1st International Conference on Quantum Simulators and Design (QSD2006) held in Hiroshima, Japan, 3-6 December 2006. This conference was organized under the auspices of the Development of New Quantum Simulators and Quantum Design Grant-in-Aid for Scientific Research on Priority Areas, Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), and Hiroshima University Quantum design is a computational approach to the development of new materials with specified properties and functionalities. The basic ingredient is the use of quantum simulations to design a material that meets a given specification of properties and functionalities. For this to be successful, the quantum simulation should be highly reliable and be applicable to systems of realistic size. A central interest is, therefore, the development of new methods of quantum simulation and quantum design. This includes methods beyond the local density approximation of density functional theory (LDA), order-N methods, methods dealing with excitations and reactions, and so on, as well as the application of these methods to the design of new materials and devices. The field of quantum design has developed rapidly in the past few years and this conference provides an international forum for experimental and theoretical researchers to exchange ideas. A total of 183 delegates from 8 countries participated in the conference. There were 18 invited talks, 16 oral presentations and 100 posters. There were many new ideas and we foresee dramatic progress in the coming years. The 2nd International Conference on Quantum Simulators and Design will be held in Tokyo, Japan, 31 May-3 June 2008.