Origin of Unexpected Ir3+ in a Superconducting Candidate Sr2IrO4 System Analyzed by Photoelectron Holography.

The reason for the absence of superconductivity in Sr2IrO4 was estimated by photoelectron spectra and photoelectron holograms. The analysis of the La photoelectron hologram concluded that La atoms are substituted to Sr sites. Two O 1s peaks were observed and were identified as the oxygens in the IrO2 and SrO planes by photoelectron holography and density functional theory (DFT) calculations. In the Ir 4f spectrum of Sr2IrO4, an unexpected Ir3+ peak was observed as much as 50% of all of the Ir. The photoelectron hologram of Ir3+ showed a displacement of about 0.15 Å. This displacement is thought to be due to the oxygen vacancies in the IrO2 plane. These oxygen vacancies and the associated local displacement of the atoms might inhibit superconductivity in spite of sufficient electron doping.

Chiral phonons: circularly polarized Raman spectroscopy and ab initio calculations in a chiral crystal tellurium.

Recently, phonons with chirality (chiral phonons) have attracted significant attention. Chiral phonons exhibit angular and pseudoangular momenta. In circularly polarized Raman spectroscopy, the peak split of the Γ 3 mode is detectable along the principal axis of the chiral crystal in the backscattering configuration. In addition, peak splitting occurs when the pseudoangular momenta of the incident and scattered circularly polarized light are reversed. Until now, chiral phonons in binary crystals have been observed, whereas those in unary crystals have not been observed. Here, we observe chiral phonons in a chiral unary crystal Te. The pseudoangular momentum of the phonon is obtained in Te by an ab initio calculation. From this calculation, we verified the conservation law of pseudoangular momentum in Raman scattering. From this conservation law, we determined the handedness of the chiral crystals. We also evaluated the true chirality of the phonons using a measure with symmetry similar to that of an electric toroidal monopole.

Semiconductor-metal transition in Bi2Se3 caused by impurity doping.

Doping a typical topological insulator, Bi2Se3, with Ag impurity causes a semiconductor-metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi2Se3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi2Se3, extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the EF is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the EF at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the EF or the crossover between the bulk and the top surface transport.

Hole doping and chemical pressure effects on the strong coupling superconductor PdTe.

Chemical doping of known superconductors is a probate strategy to test and enhance our understanding of which parameters control the critical temperature Tc and the critical magnetic fields. The transition metal chalcogenide PdTe is considered a conventional type II superconductor but its resilience to magnetic Fe doping is noteworthy. Isoelectronic Ni doping has been performed, but the effects of doping charges into PdTe have been so far unexplored. We follow two strategies to introduce holes into PdTe and to exert chemical pressure on it: by pnictogen doping on the chalcogen site PdTe1-xSbx and by systematically introducing a Pd deficiency in Pd1-yTe. We find that the superconducting Tc is very sensitive to both kinds of doping. We employ density functional theory to rationalize the observations. We conclude that in PdTe, the effects of charge doping take the lead but we can also identify a structural parameter that correlates with Tc.

Superconductivity in Bi2-x Sb x Te3-y Se y (x = 1.0 and y = 2.0) under pressure.

The crystal structure of BiSbTeSe2 (Bi2-x Sb x Te3-y Se y (x = 1.0 and y = 2.0)) at 0-29 GPa is investigated through synchrotron x-ray diffraction (XRD) and two structural phase transitions are discovered. The stoichiometry of BiSbTeSe2 employed in this study is Bi1.19(4)Sb0.81(4)Te0.83(4)Se2.17(4), as determined from energy-dispersive x-ray spectroscopy. The sample demonstrated structural transitions, from a rhombohedral structure (space group no 166, R [Formula: see text] m) (phase I) to a monoclinic structure (space group no 12, C2/m) (phase II), and from phase II to a 9/10-fold monoclinic structure (space group no 12, C2/m) (phase III). The temperature dependence of resistance (R-T plot) exhibited a semiconducting behavior in a low pressure range and changed from semiconducting to metallic behavior with increasing pressure. Pressure-driven superconductivity is observed above 9.1 GPa in Bi1.19(4)Sb0.81(4)Te0.83(4)Se2.17(4). The pressure phase corresponds to phase II. The superconducting transition temperature, T c, increased with pressure. The maximum T c value is 8.3 K at 19.1 GPa. The magnetic field dependence of T c in phase II of Bi1.19(4)Sb0.81(4)Te0.83(4)Se2.17(4) is proceeded by a p-wave polar model, indicating topologically nontrivial superconductivity. In addition, the emergence of superconductivity and the change in superconducting behavior are closely associated with the structural transitions.

Two Linear Regimes in Optical Conductivity of a Type-I Weyl Semimetal: The Case of Elemental Tellurium.

Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa. This semimetallic Weyl phase can be tuned by increasing pressure further: a kink separates two linear regimes in the optical conductivity (at 3.7 GPa), a signature proposed for Type-II Weyl semimetals with tilted cones; this however reveals a different origin in trigonal tellurium. Our density-functional calculations do not reveal any significant tilting and suggest that Te-I remains in the Type-I Weyl phase, but with two valence bands in the vicinity of the Fermi level. Their interplay gives rise to the peculiar optical conductivity behavior with more than one linear regime. Pressure above 4.3 GPa stabilizes the more complex Te-II and Te-III polymorphs, which are robust metals.

Superconducting 3R-Ta1+xSe2 with Giant In-Plane Upper Critical Fields.

Molecular-beam epitaxy (MBE) enables the stabilization of a nonequilibrium material phase, providing a powerful approach to the exploration of emergent phenomena in condensed-matter research. Here we demonstrate that one of the metallic two-dimensional (2D) materials, TaSe2, grown by MBE crystallizes into the pure 3R phase with the self-intercalated Ta atoms, 3R-Ta1+xSe2, which is thermodynamically metastable and does not exist in nature as a pure material phase. Interestingly, the thick-enough 3R-Ta1+xSe2 film exhibits a superconducting (SC) critical temperature (Tc) of 3.0 K, which is the highest among all of the polymorphs in TaSe2. Thickness-dependence measurements reveal that Tc decreases with decreasing thickness, accompanied by the development of the charge-density wave phase. The 3R-Ta1+xSe2 films exhibit large in-plane upper critical fields (Hc2) in their SC states even in the thick-enough regime, most likely due to the suppression of the interlayer hopping associated with the unique 3R stacking. Moreover, the temperature dependence of the in-plane Hc2 evolves from linear to square-root behavior with decreasing thickness, indicating crossover behavior from anisotropic three-dimensional superconductivity to 2D superconductivity. Our results unveil intriguing SC properties of metastable 3R-Ta1+xSe2 distinct from those of thermodynamically stable 2H-TaSe2, demonstrating the essential importance of the MBE-based approach to the exploration of novel quantum phenomena in 2D materials research.

Superconducting behavior of a new metal iridate compound, SrIr2, under pressure.

Herein, we investigated the pressure dependence of electric transport in a new type of superconducting metal iridate compound, SrIr2, that exhibits a superconducting transition temperature, T c, as high as 6.6 K at ambient pressure, in order to complete the T c-pressure (p ) phase diagram. Very recently, this sample's superconductivity was discovered by our group, but the superconducting behavior has not yet been clarified under pressure. In this study, we fully investigated this sample's superconductivity in a wide pressure range. The T c value decreased with an increase in pressure, but the onset superconducting transition temperature, [Formula: see text], increased above a pressure of 8 GPa, indicating an unconventional superconductivity different from a BCS-type superconductor. The magnetic field dependence of electric resistance (R) against temperature (R - T plot) recorded at 7.94 and 11.3 GPa suggested an unconventional superconductivity, followed by a p -wave polar model, supporting the deviation from a simple s-wave pairing. Moreover, we fully investigated the pressure dependence of crystal structure in SrIr2 and discussed the correlation between superconductivity and crystal structure. This is the first systematic study on superconducting behavior of a new type of metal iridate compound, MIr2 (M: alkali-earth metal atom), under pressure.

Fermi level tuning of Ag-doped Bi2Se3 topological insulator.

The temperature dependence of the resistivity (ρ) of Ag-doped Bi2Se3 (AgxBi2-xSe3) shows insulating behavior above 35 K, but below 35 K, ρ suddenly decreases with decreasing temperature, in contrast to the metallic behavior for non-doped Bi2Se3 at 1.5-300 K. This significant change in transport properties from metallic behavior clearly shows that the Ag doping of Bi2Se3 can effectively tune the Fermi level downward. The Hall effect measurement shows that carrier is still electron in AgxBi2-xSe3 and the electron density changes with temperature to reasonably explain the transport properties. Furthermore, the positive gating of AgxBi2-xSe3 provides metallic behavior that is similar to that of non-doped Bi2Se3, indicating a successful upward tuning of the Fermi level.

Determination of the local structure of Sr2-xMxIrO4 (M = K, La) as a function of doping and temperature.

The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Néel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

The electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25 and 1.0).

We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-δ states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.

Observation of current-induced bulk magnetization in elemental tellurium.

The magnetoelectric effect in bulk matter is of growing interest both fundamentally and technologically. Since the beginning of the century, the magnetoelectric effect has been studied intensively in multiferroic materials. However, magnetoelectric phenomena in materials without any (anti-)ferroic order remain almost unexplored. Here we show the observation of a new class of bulk magnetoelectric effect, by revisiting elemental trigonal tellurium. We demonstrate that elemental tellurium, which is a nonmagnetic semiconductor, exhibits current-induced magnetization. This effect is attributed to spin splitting of the bulk band owing to the lack of inversion symmetry in trigonal tellurium. This finding highlights magnetoelectricity in bulk matter driven by moving electrons without any (anti-)ferroic order. Notably, current-induced magnetization generates a magnetic field that is not circular around but is parallel to the applied current; thus, this phenomenon opens a new area of magnetic field generation beyond Ampere's law that may lead to industrial applications.Electrical control of magnetic response in bulk material without electric or magnetic order is rare and potentially attractive for high efficient spintronics. Here, the authors report magnetization in elemental tellurium driven purely by current without any (anti-)ferroic order.

Observation of orbital resonance Hall effect in (TMTSF)2ClO4.

We report the observation of a Hall effect driven by orbital resonance in the quasi-1-dimensional (q1D) organic conductor (TMTSF)2ClO4. Although a conventional Hall effect is not expected in this class of materials due to their reduced dimensionality, we observed a prominent Hall response at certain orientations of the magnetic field B corresponding to lattice vectors of the constituent molecular chains, known as the magic angles (MAs). We show that this Hall effect can be understood as the response of conducting planes generated by an effective locking of the orbital motion of the charge carriers to the MA driven by an electron-trajectory resonance. This phenomenon supports a class of theories describing the rich behavior of MA phenomena in q1D materials based on altered dimensionality. Furthermore, we observed that the effective carrier density of the conducting planes is exponentially suppressed in large B, which indicates possible density wave formation.