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The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present here an inelastic neutron scattering study of the spin dynamics in this composition. Above Tf , the dynamical scattering function S(Q,E) is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below Tf , the spectral weight splits between a broad inelastic feature at 5.8 meV and toward the elastic line. The former feature can be ascribed to spin precessions within antiferromagnetic loops, lifted to finite energy by the effective biquadratic spin-lattice term in the spin Hamiltonian.

The electrical resistivity, magnetic susceptibility, and heat capacity of sintered samples of Ca2Ir2O7, in which pentavalent Ir atoms with 5d4 electron configuration form a pyrochlore structure, have been studied. The obtained experimental data strongly suggest that Ca2Ir2O7 is metallic below room temperature and exhibits no electronic or magnetic phase transitions above 0.12 K., 3 pages, 3 figures

Surface states of topological materials provide extreme electronic states for unconventional superconducting states. CaAg1−xPdxP is an ideal candidate for a nodal-line Dirac semimetal with drumhead surface states and no additional bulk bands. Here, we report that CaAg1−xPdxP has surface states that exhibit unconventional superconductivity (SC) around 1.5 K. Extremely sharp magnetoresistance, tuned by surface-sensitive gating, determines the surface origin of the ultrahigh-mobility “electrons.” The Pd-doping elevates the Fermi level towards the surface states, and as a result, the critical temperature (Tc) is increased up to 1.7 K from 1.2 K for undoped CaAgP. Furthermore, a soft point-contact study at the surface of Pd-doped CaAgP proved the emergence of unconventional SC on the surface. We observed the bell-shaped conductance spectra, a hallmark of the unconventional SC. Ultrahigh mobility carriers derived from the surface flat bands generate a new class of unconventional SC.

The effects of ball milling and subsequent annealing on the negative thermal expansion (NTE) properties of β-Cu1.8Zn0.2V2O7 were investigated. The NTE of β-Cu1.8Zn0.2V2O7 is generated by microstructural effects distinct to the ceramic body. Ball milling produces β-Cu1.8Zn0.2V2O7 particles about 1 μm in size, but the process degrades its NTE characteristics. Results show that subsequent annealing restores the NTE properties. By annealing at 500 °C, the absolute value of the negative coefficient of thermal expansion becomes greater than the crystallographic value, which indicates that the microstructures of β-Cu1.8Zn0.2V2O7 are not completely destroyed by ball milling. Ball milling used in combination with subsequent annealing may produce NTE β-Cu1.8Zn0.2V2O7 fine particles suitable for the control of thermal expansion in local regions.

Sintered samples of a spinel oxide LiFeCr4O8, where Cr3+ and Fe3+ ions have localized moments, were found to show a large magnetic-field-induced volume increase approaching 500 ppm by applying a magnetic field of 9 T. This large volume increase appeared only at around 30 K. At 30 K, a spin-reorientation transition from ferrimagnetic to conical order occurs, giving rise to this large volume increase. The coexistence of ferrimagnetic and conical phases at this transition was found to be important, suggesting that such a large magnetic-field-induced volume change can be realized at various magnetic transitions in localized magnets with strong spin-lattice coupling., 6 pages, 4 figures, J. Phys. Soc. Jpn., accepted

Rattling phenomena have been observed in materials characterized by a large cage structure but not in a simple ABO3-type perovskite because the size mismatch, if it exists, can be relieved by octahedral rotations. Here, we demonstrate that a stoichiometric perovskite oxide NaWO3, prepared under high pressure, exhibits anharmonic phonon modes associated with low-energy rattling vibrations, leading to suppressed thermal conductivity. The structural analysis and the comparison with the ideal perovskite KWO3 without rattling behavior reveal that the presence of two crystallographic Na1 (2 a) and Na2 (6 b) sites in NaWO3 (space group Im3) accompanied by three in-phase WO6 octahedral (a+a+a+) rotations generates an open space Δ ∼ 0.5 A for the latter site, which is comparable with those of well-known cage compounds of clathrates and filled skutterudites. The observed rattling in NaWO3 is distinct from a quadruple perovskite AA'3B4O12 (A, A': transition metals) where the A (2 a) site with lower multiplicity is the rattler. The present finding offers a general guide to induce rattling of atoms in pristine ABO3 perovskites.

We conducted magnetization measurements on an S = 3 / 2 “breathing” pyrochlore antiferromagnet LiInCr4O8 in magnetic fields of up to 130 T using a coaxial-type magnetization pickup coil in a horizontal single-turn coil megagauss generator. The magnetization process exhibits a phase transition to a 1/2-magnetization plateau phase accompanied by a sudden increase in the magnetization with a discernible hysteresis at approximately 100 T. In addition, we also observed a shoulder-like behavior in the magnetization process prior to the 1/2-plateau phase, implicating the existence of a novel magnetic phase, peculiar to the “breathing” pyrochlore antiferromagnet.

Stimulated by strong demand for thermal expansion control from advanced modern industries, various giant negative thermal expansion (NTE) materials have been developed during the last decade. Nevertheless, most such materials exhibit anisotropic thermal expansion in the crystal lattice. Therefore, strains and cracks induced during repeated thermal cycling degrade their performance as thermal-expansion compensators. Here we achieved giant isotropic NTE with volume change exceeding 3%, up to 4.1%, via control of the electronic configuration in Sm atoms of SmS, (4 f)6 or (4 f)5(5d)1, by partial replacement of Sm with Y. Contrary to NTE originating from cooperative phenomena such as magnetism, the present NTE attributable to the intra-atomic phenomenon avoids the size effect of NTE and therefore provides us with fine-grained thermal-expansion compensators, which are strongly desired to control thermal expansion of microregions such as underfill of a three-dimensional integrated circuit. Volume control of lanthanide monosulfides via tuning of the 4 f electronic configuration presents avenues for novel mechanical functions of a material, such as a volume-change driven actuator by an electrical field, which has a different drive principle from those of conventional strain-driven actuators such as piezostrictive or magnetostrictive materials.

Strain induced by a magnetic field is a common phenomenon for ferromagnets, but few antiferromagnets show large strain induced by a magnetic field. On the basis of linear strain measurements of sintered samples of triangular antiferromagnet ACrS2 (A = Cu, Ag, and Au) in magnetic fields up to 9 T, the AgCrS2 sample was found to show a large strain, yielding a large volume change over 700 ppm, which is one of the largest volume changes measured to date for an antiferromagnet. This large strain appeared only at the N\'eel temperature of 42 K and was not restored to its initial state when the applied magnetic field was decreased to zero; however, it was initialized by cooling the sample to far below the N\'eel temperature. These results suggest that the coexistence of magnetically ordered and paramagnetic phases at the first-order phase transition plays an important role. AuCrS2 showed a magnetic-field-induced strain with similar features, although it was smaller than that in AgCrS2., Comment: 6 pages, 7 figures, Applied Physics Letters, in press

We report the electrical resistivity, thermoelectric power, and thermal conductivity of single-crystalline and sintered samples of the 5d pyrochlore oxide CsW2O6. The electrical resistivity of the single crystal is 3 mohm cm at 295 K and gradually increases with decreasing temperature above 215 K (Phase I). The thermoelectric power of the single-crystalline and sintered samples shows a constant value of approximately -60 uV K-1 in Phase I. These results reflect that the electron conduction by W 5d electrons in Phase I is incoherent and in the hopping regime, although a band gap does not open at the Fermi level. The thermal conductivity in Phase I of both samples is considerably low, which might be due to the rattling of Cs+ ions. In Phase II below 215 K, the electrical resistivity and the absolute value of thermoelectric power of both samples strongly increase with decreasing temperature, corresponding to a transition to a semiconducting state with a band gap open at the Fermi level, while the thermal conductivity in Phase II is smaller than that in Phase I., 6 pages, 3 figures. Accepted for publication in J. Phys. Soc. Jpn

We report the electronic properties of single crystals of candidate nodal-line semimetal CaAgP. The transport properties of CaAgP are understood within the framework of a hole-doped nodal-line semimetal. In contrast, Pd-doped CaAgP shows a drastic increase of magnetoresistance at low magnetic fields and a strong decrease of electrical resistivity at low temperatures probably due to weak antilocalization. Hall conductivity data indicated that the Pd-doped CaAgP has not only hole carriers induced by the Pd doping, but also high-mobility electron carriers in proximity of the Dirac point. Electrical resistivity of Pd-doped CaAgP also showed a superconducting transition with onset temperature of 1.7-1.8 K., 6 pages, 3 figures, accepted for publication in Phys. Rev. B

We performed inelastic neutron scattering measurements on a polycrystalline sample of a classical kagome antiferromagnet ${\mathrm{NaBa}}_{2}{\mathrm{Mn}}_{3}{\mathrm{F}}_{11}$ to investigate the possibility of a dispersionless zero-energy excitation associated with rotation of spins along the chains. The observed spectra indeed exhibit such an excitation with strong intensity at low energy, as well as dispersive excitations with weak intensity at high energy. Combining the measurements with calculations from linear spin-wave theory reveals that ${\mathrm{NaBa}}_{2}{\mathrm{Mn}}_{3}{\mathrm{F}}_{11}$ is a good realization of the classical kagome antiferromagnet which exhibits a dispersionless mode lifted by the magnetic dipole-dipole interaction.

The local structure and thermal expansion properties of balled-milled manganese nitride fine particles with median diameters on the sub-micrometer scale were investigated. Negative thermal expansion (NTE) properties of the manganese nitrides are noticeably altered even with particle sizes of several micrometers. The present study indicates that the alteration of NTE properties is attributable to the structural disorder resulting from grinding. Moreover, the NTE properties were restored by annealing due to improved structural order. These achievements are expected to pave the way for the use of the giant NTE found in manganese nitrides to control thermal expansion in local regions.

The magnetization process of the breathing pyrochlore magnet ${\mathrm{CuInCr}}_{4}{\mathrm{S}}_{8}$ has been investigated in ultrahigh magnetic fields up to 150 T. Successive phase transitions characterized by a substantially wide 1/2-plateau from 65 T to 112 T are observed in this system, resembling those reported in chromium spinel oxides. In addition to the 1/2-plateau phase, the magnetization is found to exhibit two inherent behaviors: A slight change in the slope of the $M\text{\ensuremath{-}}H$ curve at $\ensuremath{\sim}85$ T and a shoulderlike shape at $\ensuremath{\sim}135$ T prior to the saturation. Both of them are accompanied by a hysteresis, suggesting first-order transitions. The theoretical calculation applicable to ${\mathrm{CuInCr}}_{4}{\mathrm{S}}_{8}$ is also shown, based on the microscopic model with the spin-lattice coupling. The calculation fairly well reproduces the main features of the experimentally observed magnetization process, including a relatively wide cant 2:1:1 phase clearly observed in the previous work [Y. Okamoto et al., J. Phys. Soc. Jpn. 87, 034709 (2018)]. The robust 1/2-plateau on ${\mathrm{CuInCr}}_{4}{\mathrm{S}}_{8}$ seems to be originated from the dominant antiferromagnetic exchange interactions and the strong spin-lattice coupling.

We present the topology of spin-split Fermi surface of CaAgAs as determined by de Haas-van Alphen (dHvA) effect measurements combined with ab initio calculations. We have determined the torus-shaped nodal-line Fermi surface from the dHvA oscillations of $\beta$ and $\gamma$ orbits. The former orbit encircles the nodal-line, while the latter does not. Nevertheless, a nontrivial Berry phase is found for both orbits. The nontrivial phase of $\beta$ arises from the orbital characters, which can be expressed as a pseudospin rotating around the nodal-line. On the other hand, the phase of $\gamma$ is attributed to the vortex of real spin texture induced by an antisymmetric spin-orbit interaction. Our result demonstrates that both the real- and pseudo-spin textures are indispensable in interpreting the electronic topology in noncentrosymmetric nodal-line semimetals., Comment: 11 pages, 7 figures

We report thermal expansion and magnetostriction in breathing pyrochlore magnets LiACr$_4$X$_8$ (A = Ga, In, X = O, S) measured by a dilatometric method on sintered samples. All four of these compounds showed a large volume contraction associated with antiferromagnetic order with decreasing temperature. Above the Ne\'el temperature, LiGaCr4S8 showed negative thermal expansion, LiInCr4O8 showed positive thermal expansion with concave-downward temperature dependence, and LiInCr4S8 showed positive forced volume magnetostriction. All these phenomena are likely caused by the complex structure-magnetism correlation within the breathing pyrochlore structure with J and J'. These results suggested that breathing pyrochlore magnets are promising for the realization of various volumetric phenomena related to their magnetism not only in the magnetically-ordered phase but also in the paramagnetic phase., Comment: 5 pages, 3 figures, Journal of the Physical Society of Japan, in press

One-dimensional tellurides Ta4SiTe4 and Nb4SiTe4 were found to show high thermoelectric performance below room temperature. This study reported the synthesis and thermoelectric properties of whisker crystals of Ta4SiTe4-Nb4SiTe4 solid solutions and Mo- or Ti-doped (Ta0.5Nb0.5)4SiTe4. Thermoelectric power of the solid solutions systematically increased with increasing Ta content, while their electrical resistivity was unexpectedly small. Mo- and Ti-doped (Ta0.5Nb0.5)4SiTe4 showed n- and p-type thermoelectric properties with large power factors exceeding 40 microW cm-1 K-2, respectively. The fact that not only Ta4SiTe4 and Nb4SiTe4 but also their solid solutions showed high performance indicated that this system is a promising candidate for thermoelectric applications at low temperatures., Comment: 5 pages, 4 figures, accepted for publication in Applied Physics Express

Negative thermal expansion (NTE) is exhibited over the entire x range for Cu1.8Zn0.2V2–x P x O7. In particular, dilatometric measurements using epoxy resin matrix composites containing the spray-dried powder demonstrated that the thermal expansion suppressive capability was almost unchanged for x ≤ 1.0. With increasing x, the X-ray diffraction peak position moves systematically, but some peaks are extremely broad and/or asymmetric, suggesting disorder in the internal structure. The crystallographic analysis confirmed NTE enhancement by microstructural effects at least for x = 0.2. Preliminary measurements suggest higher resistivity and lower dielectric constant than that of pure vanadate, which is suitable for application to electronic devices.

Since the discovery of the Verwey transition in magnetite, transition metal compounds with pyrochlore structures have been intensively studied as a platform for realizing remarkable electronic phase transitions. We report on a phase transition that preserves the cubic symmetry of the β-pyrochlore oxide CsW2O6, where each of W 5d electrons are confined in regular-triangle W3 trimers. This trimer formation represents the self-organization of 5d electrons, which can be resolved into a charge order satisfying the Anderson condition in a nontrivial way, orbital order caused by the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a regular-triangle trimer. An electronic instability due to the unusual three-dimensional nesting of Fermi surfaces and the strong correlations of the 5d electrons characteristic of the pyrochlore oxides are both likely to play important roles in this charge-orbital-spin coupled phenomenon., Many rules have been suggested to rationalize the behaviour of transition metal oxides but the complex underlying physics often leads to deviations from the expected behaviour. Here the authors show that the metal-insulator transition in CsW2O6 satisfies the Anderson condition in an unusual way.

By tuning the structural phase transition in Zn2–xMgxP2O7, large negative thermal expansion (NTE) was achieved at room temperature. An earlier report described that Zn2P2O7 undergoes a structural phase transition at 405 K, accompanied by volume contraction of 1.8% on heating. Results showed that as Mg doping proceeds, the transition temperature decreases. Also, the volume change becomes gradual with respect to temperature. Particularly, Zn1.6Mg0.4P2O7 has a large negative coefficient of linear thermal expansion αL of −60 ppm/K at 280–350 K. Structural analysis using synchrotron radiation revealed that this dilatometric NTE is almost identical to that of crystallographic unit cells, indicating less dominant structural effects on NTE. We also verified thermal expansion compensation capabilities of powdered Zn1.6Mg0.4P2O7 by evaluating the thermal expansion of the epoxy resin matrix composites. The present phosphates are promising for use as practical thermal expansion compensators because they are free of toxic or expensive elements and can be fabricated in air using the simple solid-state reaction method.

Discovery of giant negative thermal expansion (NTE) of Ti2O3 is reported herein. Ti2O3 undergoes a phase transition from a low-temperature (low-T) insulating state to a high-T metallic state gradually at temperatures of 400–600 K, accompanied by highly anisotropic thermal deformation of the crystallographic unit cell. This anisotropic deformation induces large bulk NTE in the sintered body, although the unit-cell volume estimated from diffraction experiments shows positive thermal expansion in this T range. Results of this study also demonstrate that partial replacement of Ti with Nb increases the total volume change related to bulk NTE and show that it lowers the operating-T range of NTE to include room temperature. The development of NTE materials particularly addressing such microstructural effects is effective and promising.

Large negative thermal expansion (NTE) has been discovered during the last decade in materials of various kinds, particularly materials associated with a magnetic, ferroelectric or charge-transfer phase transition. Such NTE materials have attracted considerable attention for use as thermal-expansion compensators. Here, we report the discovery of giant NTE for reduced layered ruthenate. The total volume change related to NTE reaches 6.7% in dilatometry, a value twice as large as the largest volume change reported to date. We observed a giant negative coefficient of linear thermal expansion α=−115 × 10−6 K−1 over 200 K interval below 345 K. This dilatometric NTE is too large to be attributable to the crystallographic unit-cell volume variation with temperature. The highly anisotropic thermal expansion of the crystal grains might underlie giant bulk NTE via microstructural effects consuming open spaces in the sintered body on heating., Materials displaying negative thermal expansion have received interest as thermal-expansion compensators. Here, authors report very large total volume change associated with this effect in reduced calcium ruthenates and propose a microstructural origin driven by highly anisotropic thermal lattice expansion.

Volborthite Cu3V2O7(OH)2·2H2O is a copper mineral that materializes a two-dimensional quantum magnet comprising a kagome net of spin-1/2 Cu2+ ions. We prepared single crystals of volborthite using hydrothermal conditions and investigated their crystal structures and magnetic properties. Unusual orbital "switching" and "flipping" transitions were observed: in the former type of transition (switching), the Cu 3d orbital occupied by an unpaired electron changes between the d(3z2-r2) and d(x2-y2) types, and in the latter type of transition (flipping), the d(x2-y2)-type orbitals change their directions. Their origin is ascribed to variations in the orientation of water molecules in the gap between the kagome layers and the accompanying changes of hydrogen bonding. These orbital transitions dramatically modify the magnetic interactions between Cu2+ spins, from the anisotropic kagome type to the formation of spin trimers over the kagome net. The effective spin 1/2 generated on the trimers exhibits a frustrated magnetism, resulting in a rich phase diagram in the magnetic fields. Volborthite is a unique compound showing an exceptional interplay between the orbital and spin degrees of freedom.

Rattling phenomena have been observed in materials characterized by a large cage structure but not in a simple ABO(3)-type perovskite because the size mismatch, if it exists, can be relieved by octahedral rotations. Here, we demonstrate that a stoichiometric perovskite oxide NaWO(3), prepared under high pressure, exhibits anharmonic phonon modes associated with low-energy rattling vibrations, leading to suppressed thermal conductivity. The structural analysis and the comparison with the ideal perovskite KWO(3) without rattling behavior reveal that the presence of two crystallographic Na1 (2a) and Na2 (6b) sites in NaWO(3) (space group Im [Formula: see text]) accompanied by three in-phase WO(6) octahedral (a(+)a(+)a(+)) rotations generates an open space Δ ~ 0.5 Å for the latter site, which is comparable with those of well-known cage compounds of clathrates and filled skutterudites. The observed rattling in NaWO(3) is distinct from a quadruple perovskite AA′(3)B(4)O(12) (A, A′: transition metals) where the A (2a) site with lower multiplicity is the rattler. The present finding offers a general guide to induce rattling of atoms in pristine ABO(3) perovskites.

We report the synthesis, electronic properties, and electronic structure of LaIrSi-type BaPtP with a noncentrosymmetric cubic crystal structure. Electrical resistivity and heat capacity data taken by using polycrystalline samples indicated that BaPtP is a metal, which was further supported by first principles calculations. A polycrystalline sample of BaPtP showed a zero resistivity below 0.2 K due to the superconducting transition. The first principles calculation results indicated that the spin splitting at around the Fermi energy is large in BaPtP. These results suggest that BaPtP is likely to exhibit interesting physical properties caused by a strong spin-orbit coupling of 5d electrons in the Pt atoms., Comment: 4 pages, 4 figures, JPS Conf. Proc., accepted

Solid-state thermoelectric cooling is expected to be widely used in various cryogenic applications such as local cooling of superconducting devices. At present, however, thermoelectric cooling using p- and n-type Bi2Te3-based materials has been put to practical use only at room temperature. Recently, M4SiTe4 (M = Ta, Nb) has been found to show excellent n-type thermoelectric properties down to 50 K. This paper reports on the synthesis of high-performance p-type M4SiTe4 by Ti doping, which can be combined with n-type M4SiTe4 in a cooling device at low temperatures. The thermoelectric power factor of p-type M4SiTe4 reaches a maximum value of approximately 60 uW cm-1 K-2 at 210 K and exceeds the practical level in a wide temperature range of 130-270 K. A finite temperature drop by Peltier cooling was also achieved in a cooling device made of p- and n-type Ta4SiTe4 whisker crystals. These results clearly indicate that M4SiTe4 is promising to realize a practical thermoelectric cooler for use at low temperatures, which are not covered by Bi2Te3-based materials., Comment: 5 pages, 4 figures

Volumetric and caloric effects associated with magnetic transition have been investigated systematically for a series of manganese nitrides based on Mn3GaN. The antiferromagnetic transition tempera...

We report the sol–gel synthesis of Cu2V2O7 fine particles, in which some of the constituent Cu is replaced with other elements. The sintered body of Zn substituted β-Cu1.8Zn0.2V2O7 shows a large negative thermal expansion (NTE) over a wide temperature range due to microstructural effects peculiar to a ceramic body. Using the sol–gel method, we successfully produced β-Cu1.8Zn0.2V2O7 ceramic fine particles that retain the same level of thermal expansion suppression capabilities as the bulk with a size of about 1 μm. We also succeeded in performing rare earth metal (Ce, Sm, Yb) substitutions, which might be a clue for improving NTE performance. These achievements provide particulate filler for thermal expansion control of a micrometer region, which has been earnestly sought in many fields of technology.

Since the discovery of the Verwey transition in magnetite, transition metal compounds with pyrochlore structures have been intensively studied as a platform for realizing remarkable electronic phase transitions. We report the discovery of a unique phase transition that preserves the cubic symmetry of the beta-pyrochlore oxide CsW$_2$O$_6$, where each of W 5d electrons are confined in regular-triangle W3 trimers. This trimer formation is an unprecedented self-organization of d electrons, which can be resolved into a charge order satisfying the Anderson condition in a nontrivial way, orbital order caused by the distortion of WO6 octahedra, and the formation of a spin-singlet pair in a regular-triangle trimer. Electronic instability due to the unusual three-dimensional nesting of Fermi surfaces and the localized nature of the 5d electrons characteristic of the pyrochlore oxides were found to play important roles in this unique charge-orbital-spin coupled phenomenon., Comment: 7 pages, 3 figures

We haved studied the ground state of the classical kagome antiferromagnet ${\mathrm{NaBa}}_{2}{\mathrm{Mn}}_{3}{\mathrm{F}}_{11}$. Strong magnetic Bragg peaks observed for $d$ spacings shorter than 6.0 \AA{} were indexed by the propagation vector of ${\mathbit{k}}_{0}=(0,0,0)$. Additional peaks with weak intensities in the $d$-spacing range above 8.0 \AA{} were indexed by the incommensurate vector of ${\mathbit{k}}_{1}=[0.3209(2),0.3209(2),0]$ and ${\mathbit{k}}_{2}=[0.3338(4),0.3338(4),0]$. Magnetic structure analysis unveils a ${120}^{\ensuremath{\circ}}$ structure with the tail-chase geometry having ${\mathbit{k}}_{0}$ modulated by the incommensurate vector. A classical calculation of the Heisenberg kagome antiferromagnet with antiferromagnetic second-neighbor interaction, for which the ground state a ${\mathbit{k}}_{0}\phantom{\rule{4pt}{0ex}}{120}^{\ensuremath{\circ}}$ degenerated structure, reveals that the magnetic dipole-dipole (MDD) interaction including up to the fourth neighbor terms selects the tail-chase structure. The observed modulation of the tail-chase structure is attributed to a small perturbation such as the long-range MDD interaction or the interlayer interaction.

One of key challenges in current material research is to search for new topological materials with inverted bulk-band structure. In topological insulators, the band inversion caused by strong spin-orbit coupling leads to opening of a band gap in the entire Brillouin zone, whereas an additional crystal symmetry such as point-group and nonsymmorphic symmetries sometimes prohibits the gap opening at/on specific points or line in momentum space, giving rise to topological semimetals. Despite many theoretical predictions of topological insulators/semimetals associated with such crystal symmetries, the experimental realization is still relatively scarce. Here, using angle-resolved photoemission spectroscopy with bulk-sensitive soft x-ray photons, we experimentally demonstrate that hexagonal pnictide CaAgAs belongs to a new family of topological insulators characterized by the inverted band structure and the mirror reflection symmetry of crystal. We have established the bulk valence-band structure in three-dimensional Brillouin zone, and observed the Dirac-like energy band and ring-torus Fermi surface associated with the line node, where bulk valence and conducting bands cross on a line in the momentum space under negligible spin-orbit coupling. Intriguingly, we found that no other bands cross the Fermi level and therefore the low-energy excitations are solely characterized by the Dirac-like band. CaAgAs provides an excellent platform to study the interplay among low-energy electron dynamics, crystal symmetry, and exotic topological properties., Comment: 23 pages, 3 figures

We report a comprehensive study on the magnetic and structural properties of the spinel sulfides LiInCr4S8, LiGaCr4S8, and CuInCr4S8, where Li^+/Cu^+ and Ga^3+/In^3+ ions form a zinc-blende-type order. On the basis of synchrotron X-ray diffraction and magnetization data obtained using polycrystalline samples, these three sulfides are suggested to be breathing pyrochlore magnets with alternating antiferromagnetic and ferromagnetic interactions on the small and large tetrahedra, respectively. The measured magnetization processes of the three sulfides up to 72 T are significantly different. The magnetization curves of LiInCr4S8 and CuInCr4S8 have large hysteresis loops with different shapes, while there is no hysteresis in that of LiGaCr4S8. Geometrical frustration of the small tetrahedron is likely to give rise to a wide variety of ground states, indicating the rich physics in these antiferromagnetic–ferromagnetic breathing pyrochlore magnets., ファイル公開：2019/03/15

We found that whisker crystals of Mo-doped Nb4SiTe4 show high thermoelectric performances at low temperatures, indicated by the largest power factor of 70 microW cm-1 K-2 at 230-300 K, much larger than those of Bi2Te3-based practical materials. This power factor is smaller than the maximum value in the 5d analogue Ta4SiTe4, but is comparable to that with a similar doping level. First principles calculation results suggest that the difference in thermoelectric performances between Nb and Ta compounds is caused by the much smaller band gap in Nb4SiTe4 than that in Ta4SiTe4, due to the weaker spin-orbit coupling in the former. We also demonstrated that the solid solution of Nb4SiTe4 and Ta4SiTe4 shows a large power factor, indicating that their combination is promising as a practical thermoelectric material, as in the case of Bi2Te3 and Sb2Te3. These results advance our understanding of the mechanism of high thermoelectric performances in this one-dimensional telluride system, as well as indicating the high potential of this system as a practical thermoelectric material for low temperature applications., Comment: 5 pages, 5 figures