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Identifying ordered phases and their underlying symmetries in materials that exhibit high-temperature superconductivity is an important step towards understanding the mechanism of that phenomenon. Indeed, the critical behaviour related to phase transitions of those ordered phases is expected to be correlated with the superconductivity. In cuprate materials, efforts to find such ordered phases have mainly focused on symmetry breaking in the pseudogap region whereas the Fermi-liquid-like metallic region beyond the so-called critical doping at which the pseudogap disappears has been regarded as a trivial disordered state. Here, we uncover a broken mirror symmetry in the Fermi-liquid-like phase in (Bi,Pb)2Sr2CaCu2O8+δbeyond the critical doping. We do this by tracking the temperature dependence of the rotational-anisotropy of second-harmonic generation for two different dopings. We observe behaviour reminiscent of an order parameter with an onset temperature that coincides with the strange metal to Fermi-liquid-like metal crossover. Angle-resolved photoemission spectroscopy shows that the quasiparticle coherence between CuO2bilayers is enhanced in proportion to the symmetry-breaking response as a function of temperature, suggesting that the change in metallicity and symmetry breaking are linked. These observations contradict the conventional quantum disordered scenario for over-critical-doped cuprates.

The primordial ingredient of cuprate superconductivity is the CuO2unit cell. Theories usually concentrate on the intra-atom Coulombic interactions dominating the 3d9and 3d10configurations of each copper ion. However, if Coulombic interactions also occur between electrons of the 2p6orbitals of each planar oxygen atom, spontaneous orbital ordering may split their energy levels. This long-predicted intra-unit-cell symmetry breaking should generate an orbitally ordered phase, for which the charge transfer energy εseparating the 2p6and 3d10orbitals is distinct for the two oxygen atoms. Here we introduce sublattice-resolved ε(r) imaging to CuO2studies and discover intra-unit-cell rotational symmetry breaking of ε(r). Spatially, this state is arranged in disordered Ising domains of orthogonally oriented orbital order bounded by dopant ions, and within whose domain walls low-energy electronic quadrupolar two-level systems occur. Overall, these data reveal a Q= 0 orbitally ordered state that splits the oxygen energy levels by ~50 meV, in underdoped CuO2.

The fabrication of REBa2Cu3O7-δ (REBCO, RE = rare earth elements) bulks in single grain form and their flux pinning depend strongly on the initial conditions of the RE2BaCuO5 (RE211) phase used as precursor. We recently demonstrated a novel method of employing high-energy ultrasonication to create fine RE211 particles (RE211Ultra) with irregular shapes along with sharp edges and damaged surfaces. The developed method is convenient, free of contaminants, and low cost as compared to other methods currently utilized. Integrating these RE211Ultra in the top-seeded melt growth (MG) method, we improved the initial peritectic growth reactivity and thus, the superconducting properties of YBCO and GdBCO bulk single grains. Melt growth experiments were carried out on different REBCO (RE = Y and Gd) systems with the addition of 30 mol% of RE211Ultra. The RE211Ultra particles boosted the initial peritectic reaction i.e., growth rate during the MG process due to their sharp edges, and damaged surfaces in their nanometer size. The left-back fine-sized RE211 particles increased the interfacial defects and hence, the flux pinning. In the present work, we summarize the effect of ultrasonication of RE211 particles, their impact on peritectic reaction and improvement of superconducting properties of various REBCO bulks. Employing RE211Ultra powders for infiltration growth (IG) processing, we enabled a good control of the RE211-phase content and particle size in the final bulk YBCO single grains. The field-dependence properties of MG and IG-processed of YBCO superconductors are significantly improved without any additional chemical addition/doping.

The pair density wave (PDW) is a superconducting state in which Cooper pairs carry centre-of-mass momentum in equilibrium, leading to the breaking of translational symmetry1–4. Experimental evidence for such a state exists in high magnetic field5–8and in some materials that feature density-wave orders that explicitly break translational symmetry9–13. However, evidence for a zero-field PDW state that exists independent of other spatially ordered states has so far been elusive. Here we show that such a state exists in the iron pnictide superconductor EuRbFe4As4, a material that features co-existing superconductivity (superconducting transition temperature (Tc) ≈ 37 kelvin) and magnetism (magnetic transition temperature (Tm) ≈ 15 kelvin)14,15. Using spectroscopic imaging scanning tunnelling microscopy (SI-STM) measurements, we show that the superconducting gap at low temperature has long-range, unidirectional spatial modulations with an incommensurate period of about eight unit cells. Upon increasing the temperature above Tm, the modulated superconductor disappears, but a uniform superconducting gap survives to Tc. When an external magnetic field is applied, gap modulations disappear inside the vortex halo. The SI-STM and bulk measurements show the absence of other density-wave orders, indicating that the PDW state is a primary, zero-field superconducting state in this compound. Both four-fold rotational symmetry and translation symmetry are recovered above Tm, indicating that the PDW is a smectic order.

In conventional superconductors, the phase transition into a zero-resistance and perfectly diamagnetic state is accompanied by a jump in the specific heat and the opening of a spectral gap1. In the high-transition-temperature (high-Tc) cuprates, although the transport, magnetic and thermodynamic signatures of Tchave been known since the 1980s2, the spectroscopic singularity associated with the transition remains unknown. Here we resolve this long-standing puzzle with a high-precision angle-resolved photoemission spectroscopy (ARPES) study on overdoped (Bi,Pb)2Sr2CaCu2O8+δ(Bi2212). We first probe the momentum-resolved electronic specific heat via spectroscopy and reproduce the specific heat peak at Tc, completing the missing link for a holistic description of superconductivity. Then, by studying the full momentum, energy and temperature evolution of the spectra, we reveal that this thermodynamic anomaly arises from the singular growth of in-gap spectral intensity across Tc. Furthermore, we observe that the temperature evolution of in-gap intensity is highly anisotropic in the momentum space, and the gap itself obeys both the d-wave functional form and particle–hole symmetry. These findings support the scenario that the superconducting transition is driven by phase fluctuations. They also serve as an anchor point for understanding the Fermi arc and pseudogap phenomena in underdoped cuprates.

We performed a systematic study of the crystal structure, physical properties, and electronic structure of PbFCl-type intermetallic APX(A=Zr, Hf, X=S and Se) superconductor. We successfully synthesized single-phase polycrystalline samples for the Se substitution range of 0.4≤x≤0.8 in Zr(P2-xSex) using high pressure technique. On the other hand, S substitution range in Zr(P2-xSx) was narrow of 0.4≤x≤0.6, the S substitution range in Hf(P2-xSx) was narrow of x≈0.55, and the Se substitution range in Hf(P2-xSex) was also very narrow of x≈0.4. Zr(P2-xSex) exhibits a dome-like superconducting phase diagram for the substitution amount x. The superconducting transition temperature (Tc) is achieved at approximately x≈0.75 at which point the Tcis 6.3 K for Zr(P1.25Se0.75), 5.5 K for Hf(P1.30Se0.70), 5.0 K for Zr(P1.325S0.675), and 4.6 K for Hf(P1.50S0.50), respectively. Tcfor Zr(P1.25Se0.75) is increased from 6.3 K to 7.6 K by partially substituting a non-magnetic rare earth Sc atom for Zr. Single crystals of Zr(P1.25Se0.75) and partially substituted by Sc atom for Zr site of the ZrP2-xSexwere also grain grown using high pressure technique. Plate-like single crystal with approximate edge sizes of up to 500 × 300 × 20 μm3for (Zr0.50Sc0.50)PSe nominal composition was obtained. Tc= 8.36 K was reached and the Hc1//c-axis(0) and Hc1⊥c-axis(0) roughly determined are 0.0045 T and 0.0038 T, respectively. Assuming the type-II superconductivity in dirty limit, the Hc2//c-axis(0) value of 1.33 T was also obtained. In this presentation, the crystal growth and physical properties of this APX(A=Zr, Hf, X=S and Se) superconductor has reported.

It is generally difficult to quantify the amount of light elements in materials because of their low X-ray-scattering power, as this means that they cannot be easily estimated via X-ray analyses. Meanwhile, the recently reported layered superconductor, Sc20C8–xBxC20, requires a small amount of boron, which is a light element, for its structural stability. In this context, here, we quantitatively evaluate the optimal xvalue using both experimental and computational approaches. Using the high-pressure synthesis approach, which can maintain the starting composition even after sintering, we obtain the Sc20(C,B)8C20phase by the reaction of the previously reported Sc15C19and B (Sc15ByC19). Our experiments demonstrate that an increase in yvalues promotes the phase formation of the Sc20(C,B)8C20structure; however, there appears to be an upper limit to the nominal yvalue to form this phase. The maximum critical temperature (Tc= 7.6 K) is found to correspond with the actual xvalue of x≈ 5 under the assumption that the sample with the same Tcas the reported value (7.7 K) possesses the optimal xamount. Moreover, we construct the energy convex hull diagram by calculating the formation enthalpy based on first principles. Our computational results indicate that the composition of Sc20C4B4C20(x= 4) is the most thermodynamically stable, which is reasonably consistent with the experimentally obtained value.

In this study, we succeeded in synthesizing new antiperovskite phosphides MPd3P (M = Ca, Sr, Ba) and discovered the appearance of a superconducting phase (0.17 ≤ x≤ 0.55) in a solid solution (Ca1–xSrx)Pd3P. Three perovskite-related crystal structures were identified in (Ca1–xSrx)Pd3P, and a phase diagram was built on the basis of experimental results. The first phase transition from centrosymmetric (Pnma) to noncentrosymmetric orthorhombic (Aba2) occurred in CaPd3P near room temperature. The phase transition temperature decreased as Ca2+was replaced with a larger-sized isovalent Sr2+. Bulk superconductivity at a critical temperature (Tc) of approximately 3.5 K was observed in a range of x= 0.17–0.55; this was associated with the centrosymmetric orthorhombic phase. Thereafter, a noncentrosymmetric tetragonal phase (I41md) remained stable for 0.6 ≤ x≤ 1.0, and superconductivity was significantly suppressed as samples with x= 0.75 and 1.0 showed Tcvalues as low as 0.32 K and 57 mK, respectively. For further substitution with a larger-sized isovalent Ba2+, namely, (Sr1–yBay)Pd3P, the tetragonal phase continued throughout the composition range. BaPd3P no longer showed superconductivity down to 20 mK. Since the inversion symmetry of structure and superconductivity can be precisely controlled in (Ca1–xSrx)Pd3P, this material may offer a unique opportunity to study the relationship between inversion symmetry and superconductivity.

Lanthanides (Ln) are notoriously difficult to intercalate into graphite. We investigated the possibility of using Na to catalyze the formation of Ln-intercalated graphite and successfully synthesized LnC6(Ln= Sm, Eu, and Yb) significantly rapidly in high yields. The synthesis process involves the formation of the reaction intermediate NaCx, through the mixing of Na and C, which subsequently reacts with Lnupon heating to form LnC6. Well-sintered LnC6pellets with low residual Na concentrations (Ln:Na ≈ 98:2) were fabricated by the two-step method. The pellets enabled the evaluation of LnC6by powder X-ray diffraction and electrical resistivity measurements. This study highlights the versatility of the Na-catalyzed method and lays the foundation for the rapid mass production of LnC6, with potential applications in superconducting and rechargeable battery materials.

Atomically thin cuprates exhibiting a superconducting phase transition temperature similar to that of the bulk have recently been realized, although the device fabrication remains a challenge and limits the potential for many novel studies and applications. Here, we use an optical pump–probe approach to noninvasively study the unconventional superconductivity in atomically thin Bi2Sr2Ca0.92Y0.08Cu2O8+δ(Y-Bi2212). Apart from finding an optical response due to the superconducting phase transition that is similar to that of bulk Y-Bi2212, we observe that the sign and amplitude of the pump–probe signal in atomically thin flakes vary significantly in different dielectric environments depending on the nature of the optical excitation. By exploiting the spatial resolution of the optical probe, we uncover the exceptional sensitivity of monolayer Y-Bi2212 to the environment. Our results provide the first optical evidence for the intralayer nature of the superconducting condensate in Bi2212 and highlight the role of double-sided encapsulation in preserving superconductivity in atomically thin cuprates.

The discovery of nearly room-temperature superconductivity in superhydrides has motivated further materials research for conventional superconductors. To realize the moderately high critical temperature (Tc) in materials containing light elements, we explored new superconducting phases in a scandium borocarbide system. Here, we report the observation of superconductivity in a new ternary Sc–B–C compound. The crystal structure, which was determined through a Rietveld analysis, belongs to tetragonal space group P4/ncc. By complementarily using the density functional theory calculations, a chemical formula of the compound was found to be expressed as Sc20C8–xBxC20(x= 1 or 2). Interestingly, a small amount of B is essential to stabilize the present structure. Our experiments revealed the typical type-II superconductivity at Tc= 7.7 K. Additionally, we calculated the density of states within a first-principles approach and found that the contribution of the Sc-3d orbital was mainly responsible for the superconductivity.

We have grown single crystals of RFe2O4−δ(R= Y, Tm, Yb), proposed to be electronic ferroelectrics, by using the floating-zone melting technique under rather moderate conditions. For the crystal growth, a mixture of 95% N2+ 5% H2and CO2gases was used, instead of the mixture of CO and CO2usually utilized thus far, as an atmosphere. The full-width at half-maximum (fwhm) of the X-ray rocking curve was small enough to warrant that the quality of the resultant single crystals is rather high: 0.038, 0.024, and 0.041° for YFe2O4−δ, TmFe2O4−δ, and YbFe2O4−δsingle crystals, respectively. Single crystals of YbFe2O4−δwith a sufficient level of quality were grown at a growth rate as high as 18.80 mm/h. The magnetic properties have been examined to evaluate the nonstoichiometry of the present single crystals. Both magnetization and magnetic transition temperatures of single-crystalline YbFe2O4−δare increased by annealing in a CO2/CO atmosphere, suggesting that the annealing contributes to a decrease in the concentration of oxygen vacancies, which greatly affects the magnetic functionality.

Recently, we succeeded in synthesizing (La0.5–xNa0.5+x)Fe2As2((La,Na)122) with a solid solution range of 0 ≤ x≤ 0.35. Superconductivity was induced for 0.15 ≤ x≤ 0.35, with the highest transition temperature Tc= 27.0 K for x= 0.3. Here, we report the synthesis and physical properties of analogous compounds (Ln0.5–xNa0.5+x)Fe2As2((Ln,Na)122) (Ln= Ce, Pr). Samples were synthesized by precisely tuning the reaction temperature according to Lnand x. The solid solution ranges, 0.1 ≤ x≤ 0.3 (Ln= Ce) and 0.15 ≤ x≤ 0.25 (Ln= Pr), become narrower with increasing atomic number of Ln(which decreases the ionic radius of Ln3+). Bulk superconductivity emerged for 0.2 ≤ x≤ 0.3 and 0.15 ≤ x≤ 0.25 with the highest Tcof 25.6 K (x= 0.3) and 24.7 K (x= 0.25) for Ln= Ce and Pr, respectively. Crystal structures refined via the Rietveld analysis method showed that the (Ln,Na)122 compounds (Ln= La, Ce, Pr) with the highest Tchave almost the same As–Fe–As bond angles (∼107°) and As heights from Fe planes (∼1.43 Å). In addition to the solid solution ranges, the phases in the samples changed depending on the ionic radius of Ln3+. The (Ln,Na)122 phase competes with the non-superconducting CaFe4As3(143)-type phase of (Ln,Na)Fe4As3for Ln= Ce and Pr, whereas only the (Ln,Na)122 phase was stable for Ln= La. The 143-type phase alone was observed for Ln= Nd, and neither 122- nor 143-type phases were observed for Ln= Sm and Gd.

We have succeeded in crystallizing a new strontium digermanide (SrGe2−δ) with the ThSi2-type structure (tetragonal SrGe2), which is theoretically predicted to compete with the EuGe2-type one (trigonal SrGe2) under pressure. The tetragonal SrGe2appeared as a metastable phase in samples at approximately 900 °C under a pressure of 2 GPa. X-ray diffraction studies show that the tetragonal SrGe2is formed by the reaction between trigonal SrGe2and excess Sr. The composition of the tetragonal SrGe2was analyzed to be SrGe1.66(4). Lattice parameters for the tetragonal SrGe2are determined to be a= 4.559(4) Å and c= 14.42(1) Å. The tetragonal SrGe2shows metallic resistivity behavior and exhibits superconductivity with a critical temperature (Tc) of 7.3 K, which is the highest among compounds with the ThSi2-type structure. Superconducting properties of the tetragonal SrGe2, such as the upper critical field, and the effect of pressure on Tc, are presented and superconductivity is discussed on the basis of electronic band structure calculations.

We have investigated the elastic constant $C_{33}$ of Ba(Fe1-xCox)2As2with eight different Co concentrations by ultrasonic measurement. We found remarkable elastic anomalies near the quantum critical point. We have studied them by measuring the electrical resistivity, heat capacity, and ultrasonic attenuation in addition to the elastic constant. These results have revealed that the inter-layer three-dimensional properties appearing in $C_{33}$ to be possibly originated from the magnetic character of these materials. Our data about the elastic constant $C_{33}$ highlight the importance of controlling the $c$-axis length in the emergence of superconductivity in iron-based superconductors.

We study the effects of Zn substitution on single crystals of the low-dimensional multiferroic material Cu3Mo2O9. We report the magnetic field-temperature phase diagram obtained from the temperature and the magnetic field dependences of the specific heat, the dielectric constant, the differential magnetization, and the electric polarization-electric field curve in Cu2.85Zn0.15Mo2O9. We obtain three phases: a paraelectric and a ferroelectric phase, and a ferroelectric phase with a large coercive electric field. The origin of the last one is discussed based on the charge redistribution effect on a half-filled Mott insulator. In this phase, the Zn substitution induces a robust localized ferroelectric polarization that increases the coercive electric field.

We measured the electrical resistivity of a HgBa2Ca2Cu3O8+\delta(Hg-1223) polycrystalline sample under high pressure. Owing to the improved sample quality and the nearly hydrostatic pressure, we successfully observed zero resistivity, which is the unambiguous signature of superconductivity, up to the unprecedented temperature of 153.0 K at 15 GPa. This temperature surpasses the previous records by more than 10 K. Referring to the experimental results, we established the exact pressure ($P$) vs superconducting transition temperature ($T_{\text{c}}$) relationship. $T_{\text{c}}$ increases with $P$ up to 15 GPa, suggesting the possibility of further enhancement of $T_{\text{c}}$ at higher pressures.

Sizable single crystals of iron-based high-transition-temperature superconductors, BaFe2(As1-xPx)2, have been synthesized over an entire $x$ range from 0 to 1. The employment of a Ba2As3/Ba2P3self-flux method enables us to grow large single crystals (${\sim}7\times 7$ mm2) using a quartz tube, the P content of which is systematically controlled by tuning the As/P mixture ratio of the starting compositions. The post-annealing of the as-grown crystals results in sample quality improvement, evidenced by the increase in superconducting and antiferromagnetic-orthorhombic (AFO) transition temperatures as well as the significant reduction in residual resistivity in the AFO phase in the underdoped regime.

We have performed 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) meas urements on single-crystalline Ba1-xKxFe2As2for $x = 0.27{\mbox{--}}1$. 75As nuclear quadruple resonance frequency ($\nu_{\text{Q}}$) increases linearly with increasing $x$. The Knight shift $K$ in the normal state shows Pauli paramagnetic behavior with a weak temperature $T$ dependence. $K$ increases gradually with increasing $x$. By contrast, the nuclear spin--lattice relaxation rate $1/T_{1}$ in the normal state has a strong $T$ dependence, which indicates the existence of large antiferomagnetic (AF) spin fluctuations for all $x$'s. The $T$ dependence of $1/T_{1}$ shows a gaplike behavior below approximately 100 K for $0.6 < x < 0.9$. This behaviors is well explained by the change in the band structure with the expansion of hole Fermi surfaces and the shrinkage and disappearance of electron Fermi surfaces at the Brillouin zone (BZ) with increasing $x$. The anisotropy of $1/T_{1}$, represented by the ratio of $1/T_{1ab}$ to $1/T_{1c}$, is always larger than 1 for all $x$'s, which indicates that stripe-type AF fluctuations are dominant in this system. The $K$ in the superconducting (SC) state decreases, which corresponds to the appearance of spin-singlet superconductivity. The $T$ dependence of $1/T_{1}$ in the SC state indicates a multiple-SC-gap feature. A simple two-gap model analysis shows that the larger superconducting gap gradually decreases with increasing $x$ from 0.27 to 1 and a smaller gap decreases rapidly and nearly vanishes for $x > 0.6$ where electron pockets in BZ disappear.

One of the key motivations for the development of atomically resolved spectroscopic imaging scanning tunneling microscopy (SI-STM) has been to probe the electronic structure of cuprate high temperature superconductors. In both the $d$-wave superconducting (dSC) and the pseudogap (PG) phases of underdoped cuprates, two distinct classes of electronic states are observed using SI-STM. The first class consists of the dispersive Bogoliubov quasiparticles of a homogeneous $d$-wave superconductor. These are detected below a lower energy scale $|E|=\Delta_{0}$ and only upon a momentum space ($\mathbf{k}$-space) arc which terminates near the lines connecting $\mathbf{k}=\pm(\pi/a_{0},0)$ to $\mathbf{k}=\pm(0,\pi/a_{0})$. Below optimal doping, this “nodal” arc shrinks continuously with decreasing hole density. In both the dSC and PG phases, the only broken symmetries detected in the $|E|\leq\Delta_{0}$ states are those of a $d$-wave superconductor. The second class of states occurs at energies near the pseudogap energy scale $|E|\sim \Delta_{1}$ which is associated conventionally with the “antinodal” states near $\mathbf{k}=\pm(\pi/a_{0},0)$ and $\mathbf{k}=\pm(0,\pi/a_{0})$. We find that these states break the expected 90°-rotational ($C_{4}$) symmetry of electronic structure within CuO2unit cells, at least down to 180°-rotational ($C_{2}$) symmetry (nematic) but in a spatially disordered fashion. This intra-unit-cell $C_{4}$ symmetry breaking coexists at $|E|\sim\Delta_{1}$ with incommensurate conductance modulations locally breaking both rotational and translational symmetries (smectic). The characteristic wavevector $\mathbf{Q}$ of the latter is determined, empirically, by the $\mathbf{k}$-space points where Bogoliubov quasiparticle interference terminates, and therefore evolves continuously with doping. The properties of these two classes of $|E|\sim\Delta_{1}$ states are indistinguishable in the dSC and PG phases. To explain this segregation of $\mathbf{k}$-space into the two regimes distinguished by the symmetries of their electronic states and their energy scales $|E|\sim\Delta_{1}$ and $|E|\leq\Delta_{0}$, and to understand how this impacts the electronic phase diagram and the mechanism of high-$T_{\text{c}}$ superconductivity, represents one of a key challenges for cuprate studies.

We have performed structural analyses on iron-based superconductors, PrFeAsO1-yand PrFeAsO1-xFx, systematically, by means of Rietveld method on neutron powder diffraction data. The shifts of iron ion valence from +2, $\delta$, are accurately determined from the occupancies of O and O1-xFxsites obtained by the Rietveld analysis and F-concentration obtained by secondary ion-microprobe mass spectrometry. $T_{\text{c}}$--$\delta$ curve of PrFeAsO1-yis different from the curve of PrFeAsO1-xFx, indicating that $\delta$ is not a principal parameter for $T_{\text{c}}$ in so-called 1111 system. Structural parameters of the FeAs layers, for example, As--Fe--As bond angle and As-height from Fe layer, are different between both systems with similar $\delta$-values. Their parent compounds are also found to have different structural parameters, possibly due to the different synthetic conditions. These results suggest that the difference of structural parameters of FeAs layer is the origin of the discrepancy of $T_{\text{c}}$--$\delta$ curves of both systems and the $T_{\text{c}}$-value in the 1111 system is sensitive to the structural parameters. It may be attribute to an energy balance of the conducting bands contributing to the superconductivity.

We report an 75As-nuclear quadrupole resonance (NQR) study on heavily electron-doped LaFeAsO1-xFx(La1111) with $T_{\text{c}}=5$ K. The measurement of the nuclear spin relaxation rate ($1/T_{1}$) has revealed that a Hebel--Slichter (HS) peak partially recovers in heavily electron-overdoped regimes where the nesting condition of hole and electron Fermi surfaces (FSs) becomes significantly worse. This is in contrast to previous results reported for optimally doped La1111 with $T_{\text{c}}=28$ K that there is a lack of the HS peak. These results indicate that the interband scattering between the hole and electron FSs is strongly suppressed by an almost vanishing hole FS because of the heavy electron-overdoping. Our findings strongly suggest that the sign reversal of the gap functions on different FSs, that is, the $s_{\pm}$-wave state, is realized in La1111 compounds. We remark that interband scattering on well-nested FSs is essential for stabilizing the $s_{\pm}$-wave state and enhancing $T_{\text{c}}$ up to 28 K in LaFeAsO-based superconductors.

Enriching the material variation often contributes to the progress of materials science. We have discovered for the first time antiperovskite arsenide SrPd3As and revealed a hidden structural and superconducting phase in Sr(Pd1–xPtx)3As. The Pd-rich samples (0 ≤ x≤ 0.2) had the same noncentrosymmetric (NCS) tetragonal structure (a space group of I41md) as SrPd3P. For the samples with 0.3 ≤ x≤ 0.7, a centrosymmetric (CS) tetragonal structure (P4/nmm) identical to that of SrPt3P was found to appear, accompanied by superconductivity at a transition temperature (Tc) up to 3.7 K. In the samples synthesized with Pt-rich nominal compositions (0.8 ≤ x≤ 1.0), Sr2(Pd,Pt)8–yAs1+ywith an intergrowth structure (CS-orthorhombic with Cmcm) was crystallized. The phase diagram obtained for Sr(Pd,Pt)3As was analogous to that of (Ca,Sr)Pd3P in which superconductivity (Tc≥ 2 K) occurred in the CS phases induced by substitutions to the NCS phases. This study indicates the potential for further material variation expansion and the importance of elemental substitutions to reveal hidden phases in related antiperovskites.

Antiperovskites are a promising candidate structure for the exploration of new materials. We discovered an antiperovskite phosphide, LaPd3P, following our recent synthesis of APd3P (A= Ca, Sr, Ba). While APd3P and (Ca,Sr)Pd3P were found to be tetragonal or orthorhombic systems, LaPd3P is a new prototype cubic system (a= 9.0317(1) Å) with a noncentrosymmetric space group (I4̅3m). LaPd3P exhibited superconductivity with a transition temperature (Tc) of 0.28 K. The upper critical field, Debye temperature, and Sommerfeld constant (γ) were determined to be 0.305(8) kOe, 267(1) K, and 6.06(4) mJ mol–1K–2f.u.–1, respectively. We performed first-principles electronic band structure calculations for LaPd3P and compared the theoretical and experimental results. The calculated Sommerfeld constant (2.24 mJ mol–1K–2f.u.–1) was much smaller than the experimental value of γ because the Fermi energy (EF) was located slightly below the density of states (DOS) pseudogap. This difference was explained by the increase in the DOS at EFdue to the approximately 5 atom % La deficiency (hole doping) in the sample. The observed Tcvalue was much lower than that estimated using the Bardeen–Cooper–Schrieffer equation. To explain the discrepancy, we examined the possibility of an unconventional superconductivity in LaPd3P arising from the lack of space inversion symmetry.

We report the effects of low-temperature postannealing on the structural and superconducting properties of the recently discovered Ca-free double-layered cuprate, Sr2SrCu2O4+yF2–y. Although the as-synthesized sample prepared under high pressure has a tetragonal structure with a rock-salt-type blocking layer (the so-called T-phase), we found that the symmetry of the structure lowered to that of an orthorhombic system when annealed with CuF2. The structural refinements reveal that such a topochemical reaction leads not only to the removal of excess O2–from the apical site but also to the intercalation of extra F–into the interstitial site. The orthorhombic phase exhibits bulk superconductivity at a critical temperature of 107 K, which is significantly higher than that of the T-phase (∼50 K). Meanwhile, the T-phase turns into another structure possessing a fluorite-type blocking layer without apical fluorine (known as the T′-phase) by annealing without CuF2. Density functional theory calculations show that the T′-phase is more stable than the T-phase. This is the first report on the formation of a T′-type double-layered cuprate. Furthermore, the structural stability of the three phases of Sr2SrCu2O4+yF2–yis discussed in terms of lattice matching between the blocking and conducting layers.

We investigated the elastic properties of the iron-based superconductor Ba(Fe1-xCox)2As2with eight Co concentrations. The elastic constant $C_{66}$ shows a large elastic softening associated with structural phase transition. $C_{66}$ was analyzed on the basis of the localized and itinerant pictures of Fe-3d electrons, which shows a strong electron--lattice coupling and a possible mass enhancement in this system. The results are similar to those of unconventional superconductors, where the properties of the system are governed by quantum fluctuations associated with the zero-temperature critical point of long-range order, namely, the quantum critical point (QCP). In this system, the inverse of $C_{66}$ behaves just like the magnetic susceptibility in magnetic QCP systems. Although the QCPs of these existing superconductors are all ascribed to antiferromagnetism, our systematic studies on the canonical iron-based superconductor Ba(Fe1-xCox)2As2have revealed that there is a signature of “structural quantum criticality” in this material, which thus far has had no precedent. The elastic constant anomaly is suggested to concern with the emergence of superconductivity. These results highlight the strong electron--lattice coupling and effect of the band in this system, thus challenging the prevailing scenarios that focus on the role of iron 3d orbitals.

Zero-field and longitudinal-field muon spin relaxation ($\mu$SR) measurements have been performed in optimally doped and overdoped iron-based oxypnictide superconductors LaFeAsO1-xFxin order to investigate the spin dynamics over a wide temperature range. We have observed no sign of the slowing-down behavior of the spin fluctuations in all samples below and above the superconducting transition temperature. The results of the present study suggest that spins, which would play an important role in the appearance of superconductivity with the characteristics of the Fermi surface, fluctuate rapidly from the viewpoint of $\mu$SR and their fluctuation frequency is beyond the upper limit of the characteristic time window of $\mu$SR (${\sim}10^{-11}$ s) in optimally doped and overdoped LaFeAsO1-xFx.

Centimeter-sized platelet single crystals of KFe2As2were grown using a self-flux method. An encapsulation technique using a commercial stainless steel container allowed the stable crystal growth to last for more than 2 weeks. Ternary K--Fe--As systems with various starting compositions were examined to determine the optimal growth conditions. The employment of KAs flux led to the growth of large single crystals with a typical size of as large as $15 \times 10 \times 0.4$ mm3. The grown crystals exhibit a sharp superconducting transition at 3.4 K with the transition width 0.2 K, as well as the very large residual resistivity ratio exceeding 450, evidencing the good sample quality.

We report on a band structure calculation and de Haas--van Alphen measurements of KFe2As2. Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18$m_{\text{e}}$, $m_{\text{e}}$ being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (${\gtrsim}3$) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors.

We report our extensive 57Fe-, 75As-, 139La-, and 1H-NMR studies of La0.8Y0.2FeAsO1-y(La0.8Y0.21111) and LaFeAsO1-yHx(La1111H), where doping yttrium (Y) and hydrogen (H) into optimally doped LaFeAsO1-y[La1111(OPT)] increases $T_{\text{c}}=28$ to 34 and 32 K, respectively. In the superconducting (SC) state, the measurements of nuclear-spin lattice-relaxation rate $1/T_{1}$ have revealed in terms of a multiple fully gapped $s_{\pm}$-wave model that the SC gap and $T_{\text{c}}$ in La0.8Y0.21111 become larger than those in La1111(OPT) without any change in doping level. In La1111H, the SC gap and $T_{\text{c}}$ also increase slightly even though a decrease in carrier density and some disorders are significantly introduced. As a consequence, we suggest that the optimization of both the structural parameters and the carrier doping level to fill up the bands is crucial for increasing $T_{\text{c}}$ among these La1111-based compounds through the optimization of the Fermi surface topology.

The synthesis of a series of layered iron arsenides Ca1?xRExFeAs2(112) was attempted by heating at 1000 °C under a high pressure of 2 GPa. The 112 phase successfully forms with rare earth (RE) = La, Ce, Nd, Sm, Eu, and Gd, while RE-free samples do not contain the 112 phase. While a Ce-doped sample does not show superconductivity, La-, Pr-, Nd-, Sm-, Eu-, and Gd-doped samples show superconducting transition temperatures of 24.5, 13.2, 11.9, 11.6, 9.3, and 12.6 K, respectively, in magnetization measurements. Their Tcand Fe-Fe interlayer distances decreased with decreasing ionic radii of RE, but there are exceptions.

Polycrystalline samples of the title compounds are prepared from mixtures of CaO, Fe, Al, Fe2O3, and As or P (4.5 GPa, 1150 °C for Pn: As and 1300 °C for Pn: P, 1 h).

We clarify that the antiferromagnetic order in the distorted tetrahedral quasi-one-dimensional spin system induces electric polarization. In this system, the effects of low dimensionality and magnetic frustration are expected to appear simultaneously. We obtain the magnetic-field--temperature phase diagram of Cu3Mo2O9by studying the dielectric constant and spontaneous electric polarization. Around the tricritical point at 10 T and 8 K, the change in the direction of electric polarization causes a colossal magnetocapacitance. We calculate the charge redistribution in a small spin cluster consisting of two magnetic tetrahedra to demonstrate the electric polarization induced by the antiferromagnetism.

Low-energy spin fluctuations are studied for the electron-doped Fe-based superconductor LaFeAsO1-xFxby inelastic neutron scattering up to an energy transfer of $\omega = 15$ meV using polycrystalline samples. Superconducting samples ($x=0.057$, $T_{\text{c}}=25$ K and $x=0.082$, $T_{\text{c}}=29$ K) show a dynamical spin susceptibility $\chi”(\omega)$ comparable to that of the parent samples. However, $\chi”(\omega)$ almost vanishes in the $x=0.158$ sample, in which the superconductivity is highly suppressed. These results are compatible with the theoretical suggestions that spin fluctuation plays an important role in superconductivity.