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Abstract Surface states of topological materials provide extreme electronic states for unconventional superconducting states. CaAg1−x Pd x P is an ideal candidate for a nodal-line Dirac semimetal with drumhead surface states and no additional bulk bands. Here, we report that CaAg1−x Pd x P 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 (T c) 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.

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.

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.

In this review, I present the electronic properties of the β-pyrochlore oxide CsW2O6 and other related materials. At 215 K, CsW2O6 exhibits an electronic phase transition to a nonmagnetic insulating state, which exhibits a complex selforganization of 5d electrons. In this phase transition, various factors, such as geometrical frustration of the pyrochlore structure, moderately strong electron correlation, Jahn-Teller-like distortion, phase transition to the insulating state, and formation of chemical bonds in solids, which are powerful driving forces for achieving a wide variety of electronic properties in transition metal oxides, play important roles. In CsW2O6, the interplay of these factors has led to the emergence of an electronic phase transition that preserves the cubic symmetry, three-dimensional nesting of Fermi surfaces, a possible charge order with a fractional valence satisfying the Anderson condition, and an equilateraltriangular trimer formation by a three-centered-two-electron bond. In addition to the introduction of these unique features of CsW2O6, each of them has been compared to those of other materials to provide an overview of the electronic properties of a wide variety of related materials, which can contribute to a complete understanding of the vast and infinite electronic phenomena in transition metal oxides. [ABSTRACT FROM AUTHOR]

The electronic properties of ScPdGe and ScPdSi, crystallizing in the hexagonal ZrNiAl and orthorhombic TiNiSi structures, respectively, are investigated. ScPdGe and ScPdSi are found to show bulk superconductivity below 0.9 and 1.7 K, respectively, based on electrical resistivity and heat capacity data measured using synthesized polycrystalline samples. First-principles calculations indicate the presence of large contributions of Sc 3d and Pd 4d electrons at the Fermi energy in both materials. The electronic properties and electronic states of these materials are discussed in comparison with those of several superconductors containing scandium and a 4d transition metal element. [ABSTRACT FROM AUTHOR]

We report the discovery of bulk superconductivity in Sc6MTe2 with seven kinds of transition-metal elements M. The critical temperatures for M = 3d elements are higher than those for 4d and 5d elements and increase in the order of M = Ni, Co, and Fe with the highest Tc of 4.7K in Sc6FeTe2. First principles calculations indicate the presence of significant contribution of Fe 3d orbitals at the Fermi energy, which most likely enhance the Tc of Sc6FeTe2. The upper critical field for M = Os is considerably enhanced by the strong spin-orbit coupling. These results show Sc6MTe2 to constitute a unique family of d-electron superconductors, in which d electrons of 3d and 5d M atoms strongly influence the superconducting properties. [ABSTRACT FROM AUTHOR]

NaPrTe2, NaNdTe2, and NaTbTe2 are found to be triangular lattice magnets with the α-NaFeO2 structure, where lanthanoid atoms with 4 f electrons form a triangular lattice, based on the structural analysis and physical property measurements of synthesized polycrystalline samples. The α-NaFeO2 structure is a new polymorph of NaPrTe2, which has been reported to crystallize in the cubic LiTiO2 structure. Polytypism in NaPrTe2 is discussed based on the structural parameters determined by Rietveld analysis. NaPrTe2 is suggested to be in the proximity of the phase boundary between the LiTiO2 and α-NaFeO2 types, compared to NaNdTe2 and NaTbTe2, indicating that this compound may be interesting from the perspective of the dimensional control of geometrically frustrated lattices. The magnetic susceptibility and heat capacity data indicate that NaPrTe2 do not exhibit a long-range magnetic order or spin-glass transition above 2 K. [ABSTRACT FROM AUTHOR]

The mutual coupling of spin and lattice degrees of freedom is ubiquitous in magnetic materials and potentially creates exotic magnetic states in response to the external magnetic field. Particularly, geometrically frustrated magnets serve as a fertile playground for realizing magnetic superstructure phases. Here, we observe an unconventional two-step magnetostructural transition prior to a half-magnetization plateau in a breathing pyrochlore chromium spinel by means of state-of-the-art magnetization and magnetostriction measurements in ultrahigh magnetic fields available up to 600 T. Considering a microscopic magnetoelastic theory, the intermediate-field phase can be assigned to a magnetic superstructure with a three-dimensional periodic array of 3-up-1-down and canted 2-up-2-down spin molecules. We attribute the emergence of the magnetic superstructure to a unique combination of the strong spin--lattice coupling and large breathing anisotropy. [ABSTRACT FROM AUTHOR]

For Mg-doped Zn2P2O7, this systematic investigation of co-doping onto Zn sites has elucidated specific effects on negative thermal expansion (NTE). The low-cost and low-environmental-impact NTE material Zn2-xMgxP2O7 shows large NTE in a temperature range including room temperature for x = 0.4. Although Mg doping broadens the operating-temperature window, it remains several dozen degrees wide. Moreover, the total volume change related to NTE becomes less than that of the Zn2P2O7 parent material. Findings obtained from this study demonstrate that co-doping of Mg and of another element onto the Zn site is effective for achieving simultaneous expansion of the operatingtemperature window and maintenance of the volume change related to NTE. One illustrative case is that Zn1.64Mg0.30Al0.06P2O7 has a large negative coefficient of linear thermal expansion of about -65 ppm/K at temperatures of 300375 K. In fact, at temperatures high above room temperature, Zn1.64Mg0.30Al0.06P2O7 powder shows better thermal expansion compensation capability than the composition without Al. The Aldoped phosphates are expected to have broad practical application because of their performance, cost, and environmental load characteristics. [ABSTRACT FROM AUTHOR]

Antiperovskite manganese nitride Mn3.1Zn0.5Sn0.4N was composited with Al by compressive torsion processing (CTP). This class of nitrides is anticipated for use as a thermal-expansion compensator because of giant negative thermal expansion (NTE). It is difficult to form a composite with molten metal that causes a chemical reaction. To date, only limited success has been achieved. Composites formed by CTP at 573K exhibit lower thermal expansion and electrical resistivity than those produced by electric-current sintering. That achievement is attributable to suppressed chemical erosion of the filler by treatment at a lower temperature while the strong mechanical energy strengthens the interface between the filler and the matrix and reduces pores in the material. Compressive torsion processing is a promising method for forming metal matrix composites containing giant NTE fillers. [ABSTRACT FROM AUTHOR]

We report the synthesis, electronic properties, and electronic structure of ullmannite-type PtSbS, which has a cubic crystal structure without space inversion symmetry. Electrical resistivity and magnetization measured at low temperatures suggested that this compound is a bulk superconductor with a superconducting transition temperature of Tc = 0.15 K. First principles calculations indicated that Fermi surfaces of PtSbS include strongly nested hole pockets, which can make this compound interesting if they contribute to the emergence of superconductivity. [ABSTRACT FROM AUTHOR]

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 = 11K, 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 SðQ; EÞ is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below Tf, the spectral weight splits between a broad inelastic feature at 5.8meV 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. [ABSTRACT FROM AUTHOR]

We report the discovery of a very large thermoelectric power over -400 µV K-1 in the whisker crystals of a one-dimensional telluride Ta4SiTe4, while maintaining a low electrical resistivity of ρ = 2 mΩ cm, yielding a very large power factor of P=80 µW cm-1 K-2 at an optimum temperature of 130 K. This temperature is widely controlled from the cryogenic temperature of 50K to room temperature by chemical doping, resulting in the largest P of 170 µW cm-1K-2 at 220-280 K. These P values far exceed those of the Bi2Te3-Sb2Te3 alloys at around room temperature, offering an avenue for realizing the practical-level thermoelectric cooling at low temperatures. The coexistence of a one-dimensional electronic structure and a very small band gap appearing in the vicinity of the Dirac semimetals probably causes the very large power factors in Ta4SiTe4, indicating that the "one-dimensional Dirac semimetal" is a promising way to find high-performance thermoelectric materials for the low temperature applications. [ABSTRACT FROM AUTHOR]

We report the discovery of a very large thermoelectric power over -400 µV K-1 in the whisker crystals of a one-dimensional telluride Ta4SiTe4, while maintaining a low electrical resistivity of ρ = 2 mΩ cm, yielding a very large power factor of P=80 µW cm-1 K-2 at an optimum temperature of 130 K. This temperature is widely controlled from the cryogenic temperature of 50K to room temperature by chemical doping, resulting in the largest P of 170 µW cm-1K-2 at 220-280 K. These P values far exceed those of the Bi2Te3-Sb2Te3 alloys at around room temperature, offering an avenue for realizing the practical-level thermoelectric cooling at low temperatures. The coexistence of a one-dimensional electronic structure and a very small band gap appearing in the vicinity of the Dirac semimetals probably causes the very large power factors in Ta4SiTe4, indicating that the "one-dimensional Dirac semimetal" is a promising way to find high-performance thermoelectric materials for the low temperature applications. [ABSTRACT FROM AUTHOR]

We study polycrystalline samples of the hexagonal pnictides, CaAgP and CaAgAs, both of which are ideal candidates for line-node Dirac semimetals. The polycrystalline samples of CaAgP and CaAgAs obtained in this study are low-carrier metals, where hole carriers are dominant. By combining the hole carrier densities estimated from Hall coefficients and the electronic structures calculated by first principles calculations, both samples are found to have a ring-torus Fermi surface, derived from a ring-shaped Dirac line node. In the phosphide sample, the Fermi energy EF is located at around the end of the linear dispersion region of the electronic bands, while the EF in the arsenide sample exists in the middle of this region, suggesting that the arsenide is a more promising system for uncovering the physics of line-node Dirac semimetals. [ABSTRACT FROM AUTHOR]

Two noncentrosymmetric ternary pnictides, CaAgP and CaAgAs, are reported as topological line-node semimetals protected solely by mirror-reflection symmetry. The band gap vanishes on a circle in momentum space, and surface states emerge within the circle. Extending this study to spin-orbit coupled systems reveals that, compared with CaAgP, a substantial band gap is induced in CaAgAs by large spin-orbit interaction. The resulting states are a topological insulator, in which the Z2 topological invariant is given by 1; 000. To clarify the Z2 topological invariants for timereversal-invariant systems without spatial-inversion symmetry, we introduce an alternative way to calculate the invariants characterizing a line node and topological insulator for mirror-reflection-invariant systems. [ABSTRACT FROM AUTHOR]

We report the discovery of a bulk superconducting transition at 3.4K in the ternary phosphide, ScIrP, which crystallizes in a hexagonal ZrNiAl-type structure without spatial inversion symmetry. On the basis of heat capacity data in a zero magnetic field, ScIrP is suggested to be a weakly-coupled Bardeen-Cooper-Schrieffer superconductor. Alternatively, experimental results under magnetic fields indicate that this material is a type-II superconductor with an upper critical field Hc2 at magnetic fields above 5 T at zero temperature. This moderately high Hc2 does not violate the Pauli limit, but it does imply that there is a significant effect from the strong spin-orbit interaction of Ir 5d electrons in the noncentrosymmetric crystal structure. Electronic structure calculations show an interesting feature of ScIrP, where both the Sc 3d and Ir 5d orbitals contribute to the electronic density of states at the Fermi level. [ABSTRACT FROM AUTHOR]

The large diamagnetic susceptibility of LaV2Al20 is studied by magnetization and de Haas-van Alphen (dHvA) oscillation measurements on single crystals as well as by Ti-for-V substitution (hole doping) experiments. Its origin is ascribed to a tiny holelike Fermi surface (FS) with a low Fermi temperature of 140K and a small dHvA frequency of 19 T. The FS has a characteristic anisotropy that is approximated by six spheroidal hole pockets elongated along the cubic ‹001› directions with a minimum effective mass of 0.067 times the free electron mass. This characteristic FS can generate an unusually large Landau-Peierls diamagnetic susceptibility as observed experimentally in LaV2Al20. [ABSTRACT FROM AUTHOR]

We present a neutron scattering study of the chromate spinel LiInCr4O8, which realizes a "breathing" pyrochlore lattice of S = 3/2 spins. High-resolution neutron diffraction reveals a phase separation into a mixture of minority cubic and majority tetragonal phases below 18 K, followed by apparent magnetic order in a complex two-up two-down pattern in the latter around 12 K. The spin-spin correlation length appears to remain small down to 1.7 K. Above both transitions, time-of-flight neutron spectroscopy shows that the response is inelastic rather than quasielastic, consistent with the relatively large alternation in sizes of adjacent Cr tetrahedra. Finally, a remarkable softening of the inelastic mode is observed preceding the structural transition, implying proximity to a critical point. [ABSTRACT FROM AUTHOR]

The spinel oxides LiGaCr4O8 and LiInCr4O8 contain size-alternating pyrochlore lattices of spin-3/2 Cr3+ tetrahedra with different magnitudes of alternation. We show here that the solid solutions LiGa1-xInxCr4O8 between these two “breathing” pyrochlore compounds display (i) rapid suppression of magnetic and structural transitions upon doping the end members, (ii) spin-glass-like freezing above 2 K in the range 0.1≲x≲0.6, and (iii) apparent spin-gap behavior for x≳0.7. Furthermore, no transitions are observed above 2 K at x ∼ 0.9, where magnetic susceptibility remains finite at 2 K and magnetic heat capacity shows a quadratic temperature dependence at 1-5 K. Our work shows that breathing pyrochlore compounds provide a unique opportunity for studying both geometrical frustration and bond alternation. [ABSTRACT FROM AUTHOR]

In a frustrated J1-J2 chain with the nearest-neighbor ferromagnetic interaction J1 and the next-nearest-neighbor antiferromagnetic interaction J2, novel magnetic states such as a spin-nematic state are theoretically expected. However, they have been rarely examined in experiments because of the difficulty in obtaining suitable model compounds. We show here that the quasi-one-dimensional antiferromagnet NaCuMoO4(OH), which comprises edge-sharing CuO2 chains, is a good candidate J1-J2 chain antiferromagnet. The exchange interactions are estimated as J1 = -51K and J2 = 36K by comparing the magnetic susceptibility, heat capacity, and magnetization data with the data obtained using calculations by the exact diagonalization method. High-field magnetization measurements at 1.3K show a saturation above 26 T with little evidence of a spin nematic state expected just below the saturation field, which is probably due to smearing effects caused by thermal fluctuations and the polycrystalline nature of the sample. [ABSTRACT FROM AUTHOR]

A strategy to improve the thermoelectric properties of LaCoO3 by the double substitution of Sr and Mg in La1-xSrxCo1-xMgxO3 (x=0, 0.025, 0.05, 0.1) is presented. Effects of either Sr or Mg substitution are also investigated separately in the series of La1-xSrxCoO3 and LaCo1-xMgxO3 (x=0, 0.02, 0.05, 0.1, 0.2). All the samples were successfully synthesized by the citrate sol-gel method. Replacing La by a small amount of Sr increases electronic conductivity but decreases the Seebeck coefficient; the decrease in the Seebeck coefficient is reduced when Mg is simultaneously substituted for Co. The employment of both Sr and Mg substitutions benefit to the enhancement in thermoelectric efficiency since both electrical conductivity and Seebeck coefficient are enhanced while thermal conductivity is suppressed. With this strategy, a sample with a small amount of double substitution, which is typically La0.975Sr0.025Co0.0975Mg0.025O3, exhibits a ZT of 0.047 at room temperature. [ABSTRACT FROM AUTHOR]

A new type of magnetically frustrated lattice is found in the layered fluoride NaBa2Mn3F11. A kagome-type array of regular triangles composed of Mn2+ ions (spin 5/2) deforms so as to generate the next-nearest-neighbor interaction J2 between three out of six spins in the hexagon of a normal kagome lattice, in addition to the nearest-neighbor interaction J1. As a function of |J2/J1|, this lattice can interconnect the kagome (J2 = 0) and the triangular (J2 = J1) lattices and thus is called the kagome-triangular (KT) lattice. Magnetic susceptibility and heat capacity measurements performed on a polycrystalline sample of NaBa2Mn3F11 show an intensive short-range antiferromagnetic correlation below 14K probably due to the specific magnetic frustration of the KT lattice. In addition, a long-range order at 2.0K is observed, which is significantly low compared with the antiferromagnetic Weiss temperature of 32.3 K. Theoretical considerations of Heisenberg spins in the KT lattice reveal unique non-coplanar magnetic orders in the case of ferromagnetic J1 and antiferromagnetic J2. NaBa2Mn3F11 may actually exist in this regime according to the results of our analysis based on classical Monte-Carlo simulation. [ABSTRACT FROM AUTHOR]

A new type of magnetically frustrated lattice is found in the layered fluoride NaBa2Mn3F11. A kagome-type array of regular triangles composed of Mn2+ ions (spin 5/2) deforms so as to generate the next-nearest-neighbor interaction J2 between three out of six spins in the hexagon of a normal kagome lattice, in addition to the nearest-neighbor interaction J1. As a function of |J2/J1|, this lattice can interconnect the kagome (J2 = 0) and the triangular (J2 = J1) lattices and thus is called the kagome–triangular (KT) lattice. Magnetic susceptibility and heat capacity measurements performed on a polycrystalline sample of NaBa2Mn3F11 show an intensive short-range antiferromagnetic correlation below 14K probably due to the specific magnetic frustration of the KT lattice. In addition, a long-range order at 2.0K is observed, which is significantly low compared with the antiferromagnetic Weiss temperature of 32.3 K. Theoretical considerations of Heisenberg spins in the KT lattice reveal unique non-coplanar magnetic orders in the case of ferromagnetic J1 and antiferromagnetic J2. NaBa2Mn3F11 may actually exist in this regime according to the results of our analysis based on classical Monte-Carlo simulation. [ABSTRACT FROM AUTHOR]

The effects of iron doping on the giant negative thermal expansion (NTE) of Ca2RuO4−y ceramics are investigated. Fe-doped ruthenate ceramics exhibit a large NTE even though the crystallographic unit-cell volume shows no NTE. The anisotropic thermal strain of crystal grains consumes open spaces in the sintered body and causes giant bulk volume contraction on heating. This giant NTE is reproducible against repeated thermal cycling, indicating a tough microstructure. The thermal expansion of epoxy resin is fully suppressed below 400 K by the 56 vol % loading of the Fe-doped ruthenate ceramic powder, demonstrating tough microstructures and the capability of thermal expansion compensation of the ruthenate sintered body. [ABSTRACT FROM AUTHOR]

The magnetization processes of the spin-3/2 antiferromagnet LiInCr4O8 comprising a "breathing" pyrochlore lattice, which is an alternating array of small and large tetrahedra, are studied under ultrahigh magnetic fields of up to 130 T using state-of-the-art pulsed magnets. A half magnetization plateau is observed above 90 T to 130 T, suggesting that LiInCr4O8 has a strong spin-lattice coupling, similar to conventional chromium spinel oxides. The magnetization of LiGa0.125In0.875Cr4O8, in which the structural and magnetic transitions at low temperatures have been completely suppressed, shows a sudden increase above 13 T, indicating that a spin gap of 2.2 meV exists between a tetramer singlet ground state and an excited state with total spin 1, with the latter being stabilized by the application of a magnetic field. The breathing pyrochlore antiferromagnet is found to be a unique frustrated system with strong spin-lattice coupling and bond alternation. [ABSTRACT FROM AUTHOR]

We report 7Li-NMR studies on LiInCr4O8 and LiGaCr4O8, in which Cr3+ ions with spin 3/2 form a breathing pyrochlore lattice, a network of tetrahedra with alternating sizes. In LiInCr4O8 with large alternation, the nuclear relaxation rate 1 /T1 shows an activated temperature (T) dependence down to 18 K, indicating a singlet ground state with a spin gap. This behavior, however, is disrupted by an antiferromagnetic transition at 13 K, which is preceded by another, most likely structural, transition at 16 K. In contrast, LiGaCr4O8 with a small alternation shows no spin gap but exhibits a first-order antiferromagnetic transition over a distributed T range 13-20 K. Nevertheless, 1 /T1 of the paramagnetic phase diverges toward 13 K, indicating proximity to a second-order transition. The results indicate that LiGaCr4O8 is located in the vicinity of a tricritical point in the phase diagram. [ABSTRACT FROM AUTHOR]

The phase diagram of the quasi-one-dimensional magnet NaCuMoO4(OH) is established through single-crystal NMR and heat-capacity measurements. The 23Na and 1H NMR experiments indicate a spiral and a collinear spin density wave (SDW) order below and above Bc = 1.5–1.8 T, respectively. Moreover, in the paramagnetic state above the SDW transition temperature, the nuclear spin-lattice relaxation rate 1/T1 indicates anisotropic spin fluctuations that have gapped excitations in the transverse spectrum but gapless ones in the longitudinal spectrum. These static and dynamic properties are well described by a theoretical model assuming quasi-one-dimensional chains with competing ferromagnetic nearest-neighbor interactions J1 and antiferromagnetic next-nearest-neighbor interactions J2 (J1−J2 chains). Because of the excellent crystal quality and good one-dimensionality, NaCuMoO4(OH) is a promising compound to elucidate the unique physics of the frustrated J1−J2 chain. [ABSTRACT FROM AUTHOR]

Multiple line nodes in energy-band gaps are found in semimetals preserving mirror-reflection symmetry. We classify possible configurations of multiple line nodes with crossing points (crossing line nodes) under point-group symmetry. Taking the spin-orbit interaction (SOI) into account, we also classify topological phase transitions from crossing-line-node Dirac semimetals to other topological phases, e.g., topological insulators and point-node semimetals. This study enables one to find crossing-line-node semimetal materials and their behavior in the presence of SOI from the band structure in the absence of SOI without detailed calculations. As an example, the theory applies to hexagonal rare-earth trihydrides with the HoD3 structure and clarifies that it is a crossing-line-node Dirac semimetal hosting three line nodes. [ABSTRACT FROM AUTHOR]

The layered compound KCu3 As2 7(OD)3, comprising distorted kagome planes of S = 1/2 Cu2+ ions, is a recent addition to the family of type-II multiferroics. Previous zero-field neutron diffraction work has found two helically ordered regimes in KCu3 As2 7(OD)3, each showing a distinct coupling between the magnetic and ferroelectric order parameters. Here, we extend this work to magnetic fields up to 20 T using neutron powder diffraction, capacitance, polarization, and high-field magnetization measurements, hence determining the H - T phase diagram. We find metamagnetic transitions in both low-temperature phases around μ0Hc ~ 3.7 T, which neutron powder diffraction reveals to correspond to rotations of the helix plane away from the easy plane, as well as a small change in the propagation vector. Furthermore, we show that the sign of the ferroelectric polarization is reversible in a magnetic field, although no change is observed (or expected on the basis of the magnetic structure) due to the transition at 3.7 T. We finally justify the temperature dependence of the polarization in both zero-field ordered phases by a symmetry analysis of the free energy expansion, and attempt to account for the metamagnetic transition by adding anisotropic exchange interactions to our existing model for KCu3 As2 7(OD)3. [ABSTRACT FROM AUTHOR]