Investigation of Superconductivity in Ce-Doped (La,Pr)OBiS2 Single Crystals.

Single crystals of Ce-doped (La,Pr)OBiS2 superconductors, the multinary rare-earth elements substituted ROBiS2, were successfully grown. The grown crystals typically had a size of 1-2 mm and a plate-like shape with a well-developed c-plane. The c-axis lattice constants of the obtained (La,Ce,Pr)OBiS2 single crystals were approximately 13.6-13.7 Å, and the superconducting transition temperature was 1.23-2.18 K. Valence fluctuations of Ce and Pr were detected through X-ray absorption spectroscopy analysis. In contrast to (Ce,Pr)OBiS2 and (La,Ce)OBiS2, the superconducting transition temperature of (La,Ce,Pr)OBiS2 increased with the increasing concentrations of the tetravalent state at the R-site.

Growth and Characterization of ROBiS2 High-Entropy Superconducting Single Crystals.

A series of high-entropy superconductors, ROBiS2 (R = La + Ce + Pr + Nd + Sm), have been successfully grown in the form of single crystals using CsCl flux. The obtained single crystals have a platelike shape with a size of 0.5-2.0 mm and a thickness of 70-450 µm, and they are cleavable along the c-plane. The c-axis lattice constants of the obtained ROBiS2 single crystals have similar values of 13.47-13.57 Å. The Ce in the obtained ROBiS2 single crystals was in a mixed-valence state, comprising both Ce3+ and Ce4+. On the other hand, Pr and Sm showed only the trivalent state. The superconducting transition temperatures of ROBiS2 single crystals were approximately 2-4 K. The superconducting transition temperature and superconducting anisotropies of R-site mixed high-entropy samples increased with a decrease in the mean ionic radius of the R-site. Moreover, a deviation in the tendency to exhibit superconducting properties was observed based on the difference in the R-site mixed entropy. R-site mixed entropy in ROBiS2 superconductors may affect their superconducting properties.

Flux Growth and Superconducting Properties of (Ce,Pr)OBiS2 Single Crystals.

Ce1-x Pr x OBiS2 (0. 1 ≤ x ≤ 0.9) single crystals were grown using a CsCl flux method. Their structural and physical properties were examined by X-ray diffraction, X-ray absorption, transmission electron microscopy, and electrical resistivity. All of the Ce1-x Pr x OBiS2 single crystals with 0.1 ≤ x ≤ 0.9 exhibited tetragonal phase. With increasing Pr content, the a-axis and c-axis lattice parameters decreased and increased, respectively. Transmission electron microscope analysis of Ce0.1Pr0.9OBiS2 (x = 0.9) single crystal showed no stacking faults. Atomic-resolution energy dispersive X-ray spectrometry mapping revealed that Bi, Ce/Pr, O, and S occupied different crystallographic sites, while Ce and Pr randomly occupied the same sites. X-ray absorption spectra showed that an increase of the Pr ratio increased the ratio of Ce4+/Ce3+. All of the Ce1-x Pr x OBiS2 crystals showed superconducting transition, with a maximum transition temperature of ~4 K at x = 0.9.

Crystal Structure and Superconductivity of Tetragonal and Monoclinic Ce1- xPr xOBiS2.

Ce1- xPr xOBiS2 powders and Ce0.5Pr0.5OBiS2 single crystals were synthesized and their structure and superconductive properties were examined by X-ray diffraction, X-ray absorption, electronic resistivity, and magnetization. While PrOBiS2 was found to be in a monoclinic phase with one-dimensional Bi-S zigzag chains showing no superconductive transition above 0.1 K, CeOBiS2 was in a tetragonal phase with two-dimensional Bi-S planes showing zero resistivity below 1.3 K. In the range x = 0.3-0.9 in Ce1- xPr xOBiS2, both monoclinic and tetragonal phases were formed together with zero resistivity up to a maximum temperature of 2.2 K. A Ce0.5Pr0.5OBiS2 single crystal, which showed both zero resistivity and a decrease in magnetization at ∼2.4 K, presented a tetragonal structure. Short Bi-S bonding in flat two-dimensional Bi-S planes and mixed Ce3+/Ce4+ were characteristic features of the Ce0.5Pr0.5OBiS2 single crystal, which presumably triggered its superconductivity.

Crystal growth of La2/3-x Li3x TiO3 by the TSFZ method.

Double-perovskite-type La2/3-x Li3x TiO3 (LLT) crystals were grown by the travelling solvent floating zone (TSFZ) method. When the floating zone (FZ) crystal growth method was applied, the La2Ti2O7 phase was deposited as an inclusion in the initial growth region. Using the TSFZ crystal growth method, however, inclusion-free LLT crystals were obtained for a 10 mol% La2Ti2O7-poor composition solvent relative to the stoichiometric LLT crystals. The molten zone was initially unstable as a result of habit plane formation during the crystal growth; however, the molten zone was stably maintained for a long period of time by decreasing the feed rate compared with the growth rate. Hence, LLT crystals of approximately 5 mmφ and 37 mm in length were obtained. The anisotropic ionic conductivity of the crystals annealed in air was σ[110]/σ[001] ≈ 3, with σ[110] = 1.64 × 10-3 S cm-1 and σ[001] = 5.26 × 10-4 S cm-1. LLT single crystals are candidates for high-performance solid-state electrolytes in all-solid-state Li ion batteries.

Direct evidence of hidden local spin polarization in a centrosymmetric superconductor LaO0.55 F0.45BiS2.

Conventional Rashba spin polarization is caused by the combination of strong spin-orbit interaction and spatial inversion asymmetry. However, Rashba-Dresselhaus-type spin-split states are predicted in the centrosymmetric LaOBiS2 system by recent theory, which stem from the local inversion asymmetry of active BiS2 layer. By performing high-resolution spin- and angle-resolved photoemission spectroscopy, we have investigated the electronic band structure and spin texture of superconductor LaO0.55F0.45BiS2. Here we present direct spectroscopic evidence for the local spin polarization of both the valence band and the conduction band. In particular, the coexistence of Rashba-like and Dresselhaus-like spin textures has been observed in the conduction band. The finding is of key importance for fabrication of proposed dual-gated spin-field effect transistor. Moreover, the spin-split band leads to a spin-momentum locking Fermi surface from which superconductivity emerges. Our demonstration not only expands the scope of spintronic materials but also enhances the understanding of spin-orbit interaction-related superconductivity.

Unconventional Superconductivity in the BiS_{2}-Based Layered Superconductor NdO_{0.71}F_{0.29}BiS_{2}.

We investigate the superconducting-gap anisotropy in one of the recently discovered BiS_{2}-based superconductors, NdO_{0.71}F_{0.29}BiS_{2} (T_{c}∼5 K), using laser-based angle-resolved photoemission spectroscopy. Whereas the previously discovered high-T_{c} superconductors such as copper oxides and iron-based superconductors, which are believed to have unconventional superconducting mechanisms, have 3d electrons in their conduction bands, the conduction band of BiS_{2}-based superconductors mainly consists of Bi 6p electrons, and, hence, the conventional superconducting mechanism might be expected. Contrary to this expectation, we observe a strongly anisotropic superconducting gap. This result strongly suggests that the pairing mechanism for NdO_{0.71}F_{0.29}BiS_{2} is an unconventional one and we attribute the observed anisotropy to competitive or cooperative multiple paring interactions.

Coexistence of superconductivity and charge-density wave in the quasi-one-dimensional material HfTe3.

We present the first experimental evidence for metallicity, superconductivity (SC) and the co-existence of charge density waves (CDW) in the quasi-one-dimensional material HfTe3. The existence of such phenomena is a typical characteristic of the transition metal chalcogenides however, without the application of hydrostatic pressure/chemical doping, it is rare for a material to exhibit the co-existence of both states. Materials such as HfTe3 can therefore provide us with a unique insight into the relationship between these multiple ordered states. By improving on the original synthesis conditions, we have successfully synthesised single phase HfTe3 and confirmed the resultant structure by performing Rietveld refinement. Using low temperature resistivity measurements, we provide the first experimental evidence of SC at ~1.4 K as well as a resistive anomaly indicative of a CDW formation at ~82 K. By the application of hydrostatic-pressure, the resistivity anomaly shifts to higher temperature. The results show that HfTe3 is a promising new material to help study the relationship between SC and CDW.

Erratum: Floating Zone Growth and Characterization of (Ca1-x Nd x )12Al14O33+6x (x ∼ 0.001) Single Crystals.

[This corrects the article DOI: 10.1021/acsomega.6b00409.].

Floating Zone Growth and Characterization of (Ca1-x Nd x )12Al14O33+6x (x ∼ 0.001) Single Crystals.

Rare earth Nd3+ ion-doped Ca12Al14O33 single crystals were grown at a rate of 1 mm/h by the floating zone technique in an oxygen atmosphere. The Nd concentration in the central area of the as-grown crystals was higher than that in the peripheral area due to the highly concave growth interface shape. The average Nd concentration in the as-prepared parallelepiped slices of ∼6 × 3 × 2 mm3 for electride fabrication was ∼0.093 ± 0.011 atom % Nd in contrast to the nominal 0.1 atom % Nd concentration. The Nd-doped C12A7:Nd single crystalline electride prepared by thermal treatment with a Ti metal at 1100 °C for 48 h induced an insulator-metal transition with the highest conductivity of approximately 1165 S/cm. These annealing conditions provide an approach to completely remove clathrated oxide ions from the crystallographic cages, leading to the formation of C12A7:Nd(e -) and C12A7:e - electrides with high-density electrons (∼2.4 ± 0.2 × 1021 and ∼1.8 ± 0.1 × 1021 cm-3) localized in the cages. The carrier mobility at room temperature in C12A7:Nd(e -) was very similar to that in C12A7:e -, even though it was very high at temperatures below ∼150 K.

Superconducting double perovskite bismuth oxide prepared by a low-temperature hydrothermal reaction.

Perovskite-type structures (ABO3) have received significant attention because of their crystallographic aspects and physical properties, but there has been no clear evidence of a superconductor with a double-perovskite-type structure, whose different elements occupy A and/or B sites in ordered ways. In this report, hydrothermal synthesis at 220 °C produced a new superconductor with an A-site-ordered double perovskite structure, (Na(0.25)K(0.45))(Ba(1.00))3(Bi(1.00))4O12, with a maximum T(c) of about 27 K.

Nanoporous crystal 12CaO.7Al2O3: a playground for studies of ultraviolet optical absorption of negative ions.

A novel nanoporous material 12CaO.7Al2O3 (C12A7) offers a possibility of incorporating large concentrations (>1021 cm-3) of a wide range of extraframework anions inside its nanopores. We have investigated, both experimentally and theoretically, optical absorption associated with several types of such anions, including F-, OH-, O-, O2-, O2-, and O22-, and assigned their optical absorption bands. It is demonstrated that the chemical identity and concentration of extraframework anions can be controlled by an appropriate treatment of "as grown" C12A7. We also show that the position of the adsorption edge is, in turn, determined by the chemical identity of the extraframework species and can be varied in the range of approximately 4-6 eV. We suggest that C12A7 is a unique host material, which can be used as a playground for studying negatively charged species that are unstable in other environments.

Charge ordering, commensurability, and metallicity in the phase diagram of the layered NaxCoO2.

The phase diagram of nonhydrated NaxCoO2 has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x=1/2), then to a "Curie-Weiss metal" (around 0.70), and finally to a weak-moment magnetically ordered state (x>0.75). The unusual properties of the state at 1/2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.

Dissipationless anomalous Hall current in the ferromagnetic spinel CuCr2Se4-xBrx.

In a ferromagnet, an applied electric field E invariably produces an anomalous Hall current JH that flows perpendicular to the plane defined by E and M (the magnetization). For decades, the question of whether JH is dissipationless (independent of the scattering rate) has been debated without experimental resolution. In the ferromagnetic spinel CuCr2Se4-xBrx, the resistivity rho (at low temperature) may be increased by several decades by varying x (Br) without degrading M. We show that JH/E (normalized per carrier, at 5 kelvin) remains unchanged throughout. In addition to confirming the dissipationless nature of JH, our finding has implications for the generation and study of spin-Hall currents in bulk samples.