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Two new compounds, Zn2FeSbO6 and Zn2MnSbO6, have been synthesized under high-pressure and high-temperature conditions. The synthesis, single-crystal and powder X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES), optical second harmonic generation (SHG), and magnetic and heat capacity measurements were carried out for both compounds and are described. The lattice parameters are a = 5.17754(6) Å and c = 13.80045(16) Å for Zn2FeSbO6 and a = 5.1889(10) Å and c = 14.0418(3) Å for Zn2MnSbO6. Single-crystal X-ray diffraction analyses indicate that Zn2FeSbO6 consists of a cocrystal of superimposed Ni3TeO6 (NTO) and ordered ilmenite (OIL) components with a ratio of approximately 2:1 and Zn2MnSbO6 contains two nearly identical, but noncrystallographically related, OIL components in a ratio of approximately 6:1. 

Iron-chalcogenide superconductors FeSe_1−xS_xpossess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (μSR) measurements in FeSe_1−xS_xsuperconductors for0≤x≤0.22covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperatureT_cfor all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-fieldμSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x>0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe_1−xS_x, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity. 

The local structure of the highly “overdoped” 95 K superconductor Sr₂CuO_3.3determined by Cu K X-ray absorption fine structure (XAFS) at 62 K in magnetically oriented samples shows that 1) the magnetization is perpendicular to thecaxis; 2) at these levels of precision the Cu sublattice is tetragonal in agreement with the crystal structure; the O sublattice has 3) continuous -Cu-O- chains that orient perpendicular to an applied magnetic field; 4) approximately half-filled -Cu-O- chains that orient parallel to this field; 5) a substantial number of apical O vacancies; 6) O ions at some apical positions with expanded Cu-O distances; and 7) interstitial positions that imply highly displaced Sr ions. These results contradict the universally accepted features of cuprates that require intact CuO₂planes, magnetization along thecaxis, and a termination of the superconductivity when the excess charge on the CuO₂Cu ions exceeds 0.27. These radical differences in charge and structure demonstrate that this compound constitutes a separate class of Cu-O–based superconductors in which the superconductivity originates in a different, more complicated structural unit than CuO₂planes while retaining exceptionally high transition temperatures. 

A common characteristic of many “overdoped” cuprates prepared with high-pressure oxygen isT_cvalues ≥ 50 K that often exceed that of optimally doped parent compounds, despite O stoichiometries that place the materials at the edge or outside of the conventional boundary between superconducting and normal Fermi liquid states. X-ray absorption fine-structure (XAFS) measurements at 52 K on samples of high-pressure oxygen (HPO) YSr₂Cu_2.75Mo_0.25O_7.54,T_c= 84 K show that the Mo is in the (VI) valence in an unusually undistorted octahedral geometry with predominantly Mo neighbors that is consistent with its assigned substitution for Cu in the chain sites of the structure. Perturbations of the Cu environments are minimal, although the Cu X-ray absorption near-edge structure (XANES) differs from that in other cuprates. The primary deviation from the crystal structure is therefore nanophase separation into Mo- and Cu-enriched domains. There are, however, indications that the dynamical attributes of the structure are altered relative to YBa₂Cu₃O₇, including a shift of the Cu-apical O two-site distribution from the chain to the plane Cu sites. Another effect that would influenceT_cis the possibility of multiple bands at the Fermi surface caused by the presence of the second phase and the lowering of the Fermi level. 

Significance The quasi–1-dimensional bismuth bromide, α-Bi₄Br₄, has been predicted to be a rotational symmetry-protected topological crystalline insulator. The structural study under high pressure indicates that the α-Bi₄Br₄phase is stable up to 4.3 GPa. There is a rich phase diagram of physical properties under high pressure in the α-Bi₄Br₄phase (i.e., a pressure-induced insulator–metal transition and, most importantly, a superconductive phase near the boundary of the insulator–metal transition). These findings help to answer questions, such as whether it is possible for the symmetry-protected electrons to form Cooper pairs. The α-Bi₄Br₄undergoes a pressure-induced structural transition above 4.3 GPa to a triclinicP-1 phase, which is another superconductive phase.