Search

We have measured the temperature dependence of resistivity in single-crystalline CeNiGe$_{3}$ under hydrostatic pressure in order to establish the characteristic pressure-temperature phase diagram. The transition temperature to AFM-I phase $T_{\rm N1}$ = 5.5 K at ambient pressure initially increases with increasing pressure and has a maximum at $\sim$ 3.0 GPa. Above 2.3 GPa, a clear zero-resistivity is observed (SC-I phase) and this superconducting (SC) state coexists with AFM-I phase. The SC-I phase suddenly disappears at 3.7 GPa simultaneously with the appearance of an additional kink anomaly corresponding to the phase transition to AFM-II phase. The AFM-II phase is continuously suppressed with further increasing pressure and disappears at $\sim$ 6.5 GPa. In the narrow range near the critical pressure, an SC phase reappears (SC-II phase). A large initial slope of upper critical field $\mu_0H_{\rm c2}$ and non-Fermi liquid behavior indicate that the SC-II phase is mediated by antiferromagnetic fluctuations. On the other hand, the robust coexistence of the SC-I phase and AFM-I phase is unusual on the contrary to superconductivity near a quantum critical point on most of heavy-fermion compounds., Comment: 5 pages, 6 figures

This study investigates the transport properties of a chiral elemental semiconductor tellurium (Te) under magnetic fields and pressure. Application of hydrostatic pressure reduces the resistivity of Te, while its temperature dependence remains semiconducting up to 4 GPa, contrary to recent theoretical and experimental studies. Application of higher pressure causes structural as well as semiconductor--metal transitions. The resulting metallic phase above 4 GPa exhibits superconductivity at 2 K along with a noticeable linear magnetoresistance effect. On the other hand, at ambient pressure, we identified metallic surface states on the as-cleaved (10$\bar{1}$0) surfaces of Te. The nature of these metallic surface states has been systematically studied by analyzing quantum oscillations observed in high magnetic fields. We clarify that a well-defined metallic surface state exists not only on chemically etched samples that were previously reported, but also on as-cleaved ones., 6 pages, 5 figures

We performed $^{185/187}$Re nuclear quadrupole resonance (NQR) measurements under pressure to investigate the superconducting properties of noncentrosymmetric superconductor Cd$_{2}$Re$_{2}$O$_{7}$ under various crystal structures. The pressure dependence of superconducting transition temperature $T_{\rm c}$ determined through ac susceptibility measurements is consistent with the results of previous resistivity measurements [T. C. Kobayashi $et al$., J. Phys. Soc. Jpn. 80, 023715 (2011).]. Below 2.2 GPa, in the nuclear spin-lattice relaxation rate $1/T_{1}$, a clear coherence peak was observed just below $T_{\rm c}$, indicating conventional $s$-wave superconductivity. In contrast, the coherence peak disappears at 3.1 GPa, suggesting a change in superconducting symmetry to the $p$-wave dominant state against pressure., Comment: 5 pages, 5 figures

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$. In YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\gtrsim~10$ mK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure - induced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-$T$ unconventional heavy - fermion superconductors and other families of unconventional superconductors with higher $T_c$s, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon., 30 pages, 7 Figures, Accepted for publication in Philosophical Magazine

The $5d$-transition-metal pyrochlore oxide $\mathrm{C}{\mathrm{d}}_{2}\mathrm{R}{\mathrm{e}}_{2}{\mathrm{O}}_{7}$, which was recently suggested to be a prototype of the spin-orbit-coupled metal [Phys. Rev. Lett. 115, 026401 (2015)], exhibits an inversion-symmetry-breaking (ISB) transition at 200 K and a subsequent superconductivity below 1 K at ambient pressure. We study the crystal structure at high pressures up to 5 GPa by means of synchrotron x-ray powder diffraction. A rich structural phase diagram is obtained, which contains at least seven phases, and is almost consistent with the electronic phase diagram determined by previous resistivity measurements. Interestingly, the ISB transition vanishes at $\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ where the enhancement of the upper critical field was observed in resistivity. Moreover, it is shown that the point groups at 8 K, probably kept in the superconducting phases, sequentially transform into piezoelectric, ferroelectric, and centrosymmetric structures on the application of pressure.

An overview of the crystal structures and physical properties of the recently discovered iron-platinum-arsenide superconductors, Ca10(PtnAs8)(Fe2-xPtxAs2)5 (n = 3 and 4), which have a superconducting transition temperature up to 38 K, is provided. The crystal structure consists of superconducting Fe2As2 layers alternating with platinum-arsenic layers, PtnAs8. The upper critical field Hc2, hydrostatic pressure dependence of superconducting transition temperature Tc, and normal-state magnetic susceptibility are reported., 7 pages, 6 figures

We report pressure-induced superconductivity in antiferromagnetic cerium compounds and a ferromagnetic uranium compound, focusing on the relation between the superconducting state and the Fermi surface instability in CeRhIn5 and the huge upper critical eld Hc2 in CeIrSi3 without inversion symmetry in the crystal structure. The pairing interaction is well correlated with ferromagnetism, as well as antiferromagnetism or antiferromagnetic uctuations. In the recently discovered superconductor NpPd5Al2, superconductivity is found to be located in the vicinity of an antiferromagnetically ordered state. The upper critical eld Hc2 is strongly supressed by the large paramagneic e ect, and the corresponding magnetization curve indicates a step-like increase at Hc2, producing a rst-order phase transition.

High-pressure AC-susceptibility measurement has been carried out in UIr. Three series of peaks corresponding to the successive ferromagnetic transitions indicate the existence of three ferromagnetic phases FM1-3. The phase boundary between FM2 and FM3 was not realized in the resistivity measurement. The Meissner effect is observed successfully in the pressure range of 2.63–2.74 GPa where the peak of AC-susceptibility survives. This result indicates that the superconducting phase is embedded in the FM3 phase with the ordered moment less than 0.05 μ B /U and adjoins the quantum critical point.

Pressure-induced superconductivity is found in UIr without inversion symmetry. The pressure–temperature phase diagram has been investigated by means of the electrical resistivity and magnetization ...

We have measured the electrical resistivity of an antiferromagnetic Kondo compound CeNiGe3 under pressure. The Neel temperature initially increases with pressure P up to 3 GPa, then decreases rather steeply with further increasing pressure, and becomes zero at a critical pressure GPa. The A and ρ0 values of the resistivity ρ = ρ0+AT2 in the Fermi liquid relation become maximum around Pc, the A value attaining an extremely large value which is comparable with that in a heavy fermion superconductor CeCu2Si2. Superconductivity is found below 0.48 K in a wide pressure region from 4 to 10 GPa. The upper critical field Hc 2(0) is about 2 T, indicating heavy fermion superconductivity.

We present recent advances in the magnetism and superconductivity of rare earth and uranium compounds. Heavy fermions are formed on the basis of the competition between the RKKY interaction and the Kondo effect in these compounds. The application of pressure to these compounds is useful to control the electronic states in the antiferromagnets CeRh 2 Si 2 and CeRhIn 5 , and the ferromagnet UGe 2 . The quadrupole interaction or the quadrupole Kondo effect is found to be responsible for heavy fermions in PrFe 4 P 12 . A rich variety of superconducting properties are demonstrated: superconductivity of CeCoIn 5 and CeRhIn 5 in the vicinity of a quantum critical point, coexistence of ferromagnetism and superconductivity in UGe 2 , and superconductivity based on the quadrupole fluctuations in PrOs 4 Sb 12 , together with the relation between the superconducting transition temperature and the quasi-two dimensionality of the electronic states.

We report the discovery of pressure-induced superconductivity in ferromagnetic UIr, which lacks inversion symmetry in the crystal structure. The Curie temperature TC1 = 46 K at ambient pressure decreases with increasing pressure, reaching a value of 11 K at 1.5 GPa. It presumably decreases further up to about Pc1 = 1.7 GPa. The ferromagnetic region named 'F1' exists up to Pc1. A second magnetic phase named 'F2' with a low ferromagnetic moment appears in the pressure range from 1.9 to 2.4 GPa. In the 'F2' phase, the magnetic transition temperature TC2 decreases with pressure, from 18 K at 1.9 GPa to approximately zero at PC2 = 2.6–2.7 GPa. In this critical pressure region, superconductivity appears below Tsc = 0.14 K.

We have measured the electrical resistivity under pressure up to 2 GPa for an antiferromagnet, CeRhGe. The resistivity at ambient pressure decreases monotonically below a Neel temperature TN = 10 K. With increasing pressure P, the resistivity has a peak around 10 K, which is anomalously enhanced up to 1.2 GPa. With further increasing pressure, the resistivity peak is diminished; the Neel temperature becomes zero at a critical pressure Pc = 1.9 GPa. The A-value of the resistivity ρ in the Fermi liquid relation ρ = ρ0 + AT2 is also found to increase with increasing pressure and has a maximum at Pc which corresponds to the value for the heavy-fermion superconductor CeCu2Si2.

When pressure is applied to cerium and uranium compounds, the magnetic ordering temperature is suppressed and becomes zero at a critical pressure P c . Around P c , non-Fermi liquid and/or superconductivity are observed. We clarified a change of the electronic state via a de Haas–van Alphen (dHvA) experiment when pressure crosses P c . The dHvA experiment under pressure was done for the antiferromagnets CeRh 2 Si 2 and CeRhIn 5 , and for a ferromagnet UGe 2 . We found an abrupt change of the Fermi surface for CeRh 2 Si 2 and UGe2 when crossing P c , indicating a first-order like phase transition. For CeRhIn 5 , the topology of the Fermi surface was unchanged, but the cyclotron mass increased steeply above 1.6 Gpa, where superconductivity sets in.

In part 1, we review techniques developed in our laboratory for producing the complex extreme condition of very low temperature and ultra-high pressure and those for measuring electrical resistance and magnetization of the sample confined in the extremely small space of the used pressure cell. In part 2, we review our experimental results in search for pressure-induced superconductivity, which have been obtained by the use of developed techniques. Typical examples are shown in the case of simple inorganic and organic molecular crystals, ionic crystals, and magnetic metals.

Thermal properties of a ferromagnetic superconductor UGe2 were investigated by the heat capacity measurements. The bulk nature of the superconductivity was confimied by the observation of a peak in the heat capacity at the superconducting transition temperature TSC, From the pressure dependence of the linear heat capacity coefficient γ, it is suggested that the low energy magnetic excitation near the critical point Pc*, where T* becomes OK, might play an important role on the superconductivity. The pressure dependence of the superconducting peak and the temperature dependence of CIT above TSC suggest that the order parameter of the superconducting state might be different between the phases below and above PC*.

The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular emphasis on the sample-quality issue. The compound has drawn attention as the only superconductor (Tc = 1.0 K) that has been found in the family of {\alpha}-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits two characteristic structural transitions from the cubic pyrochlore structure, with the inversion symmetry broken at the first one at 200 K. Recently, it has attracted increasing attention as a candidate spin-orbit coupled metal (SOCM), in which specific Fermi liquid instability is expected to lead to an odd-parity order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu: Phys. Rev. Lett. 115 (2015) 207002; Y. Wang et al., Phys. Rev. B 93 (2016) 134512]. We review our previous experimental results in comparison with those of other groups in the light of the theoretical prediction of the SOCM, which we consider meaningful and helpful for future progress in understanding this unique compound., Comment: 15 pages, 11 figures, J. Phys. Soc. Jpn, in press

The effects of pressure on antiferromagnetic (AFM) and superconducting phase transitions of 112-type Ca$_{1-x}$La$_{x}$FeAs$_{2}$ were studied, and the in-plane electrical resistivity $\rho_{ab}$ was measured with an indenter-type pressure cell. The AFM phase transition temperatures of $T_{\rm N}$ = 47, 63, and 63 K at ambient pressure for $x$ = 0.18, 0.21, and 0.26 was suppressed by applying pressure $P$, with superconductivity emerging at critical pressures of $P_{\rm c}$ $\simeq$ 0, 1.5, and 3.4 GPa, respectively. Correspondingly, the slope of $T_{\rm N}$ against $P$ decreased as $dT_{\rm N}/P$ $\simeq$ $-$15 and $-$2 K/GPa for $x$ = 0.21 and 0.26, respectively. Thus, although the AFM phase was stabilized with La doping $x$, the AFM phase was suppressed by pressure, and superconductivity eventually emerged., Comment: 4 pages, 2 figures, Note added after publication: We noticed a paper by Yazhou Zhou et al. [Science Bulletin, 62, 857(2017)] reporting the pressure-induced superconductivity in the 112 system

We measured the pressure dependence of in-plane resistivity ${\ensuremath{\rho}}_{ab}$ in the recently discovered iron-based superconductor ${\mathrm{Ca}}_{10}({\mathrm{Ir}}_{4}{\mathrm{As}}_{8}){({\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{As}}_{2})}_{5}$, which shows a unique structural phase transition in the absence of magnetic ordering, with a superconducting transition temperature ${T}_{\mathrm{c}}=16$ K and a structural phase transition temperature ${T}_{\mathrm{s}}\ensuremath{\simeq}100$ K at ambient pressure. ${T}_{\mathrm{c}}$ and ${T}_{\mathrm{s}}$ are suppressed on applying pressure and disappear at approximately 0.5 GPa, suggesting a relationship between superconductivity and structure. ${\mathrm{Ca}}_{10}({\mathrm{Ir}}_{4}{\mathrm{As}}_{8}){({\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{As}}_{2})}_{5}$ is a rather rare example in which the superconductivity appears only in a low-temperature ordered phase. The fact that the change in the crystal structure is directly linked with superconductivity suggests that the crystal structure as well as magnetism are important factors governing superconductivity in iron pnictides.

The pressure dependence of the superconducting transition temperature (Tc) and unit cell metrics of tetragonal (NH3)yCs0.4FeSe were investigated in high pressures up to 41 GPa. The Tc decreases with increasing pressure up to 13 GPa, which can be clearly correlated with the pressure dependence of c (or FeSe layer spacing). The Tcvs. c plot is compared with those of various (NH3)yMxFeSe (M: metal atoms) materials exhibiting different Tc and c, showing that the Tc is universally related to c. This behaviour means that a decrease in two-dimensionality lowers the Tc. No superconductivity was observed down to 4.3 K in (NH3)yCs0.4FeSe at 11 and 13 GPa. Surprisingly, superconductivity re-appeared rapidly above 13 GPa, with the Tc reaching 49 K at 21 GPa. The appearance of a new superconducting phase is not accompanied by a structural transition, as evidenced by pressure-dependent XRD. Furthermore, Tc slowly decreased with increasing pressure above 21 GPa and at 41 GPa superconductivity disappeared entirely at temperatures above 4.9 K. The observation of a double-dome superconducting phase may provide a hint for pursuing the superconducting coupling-mechanism of ammoniated/non-ammoniated metal-doped FeSe.

The isovalent-substituted iron-pnictide superconductor ${\mathrm{SrFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$ ($x=0.35$) has a slightly higher optimum critical temperature than the similar system ${\mathrm{BaFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$, and its parent compound ${\mathrm{SrFe}}_{2}$${\mathrm{As}}_{2}$ has a much higher N\'eel temperature than ${\mathrm{BaFe}}_{2}$${\mathrm{As}}_{2}$. We have studied the band structure and the Fermi surfaces of optimally doped ${\mathrm{SrFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$ by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around $(0,0)$ and two electronlike FSs around $(\ensuremath{\pi},\ensuremath{\pi})$ have been observed as in the case of ${\mathrm{BaFe}}_{2}$(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$. Measurements with different photon energies have revealed that the outermost hole FS is more strongly warped along the ${k}_{z}$ direction than the corresponding one in BaFe(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$, and that the innermost one is an ellipsoidal pocket. The electron FSs are almost cylindrical, unlike the corrugated ones in BaFe(${\mathrm{As}}_{1\text{\ensuremath{-}}x}$${\mathrm{P}}_{x}$)${}_{2}$. A comparison of the ARPES data with a first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character, but also between the hole and electron FSs of the same orbital character. By examining the nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the ${d}_{xy}$ and/or ${d}_{xz/yz}$ orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

The pressure dependence of transport properties in CeRhIn5 was studied up to 8.5 GPa by using a diamond-anvil cell. The electrical resistivity does not follow the T 2 -dependecne of a Fermi liquid nature under any pressure. In this non-Fermi liquid state, superconductivity was observed in a wide pressure range of 1:5 GPa

We performed the DC-magnetization and neutron scattering experiments under pressure {\it P} for a pressure-induced superconductor UGe$_2$. We found that the magnetic moment is enhanced at a characteristic temperature {\it T}$^{*}$ in the ferromagnetic state, where {\it T}$^{*}$ is smaller than a Curie temperature {\it T}$_{\rm C}$. This enhancement becomes remarkable in the vicinity of {\it P}$_{\rm C}^{*}$ = 1.20 GPa, where {\it T}$^{*}$ becomes 0 K and the superconducting transition temperature {\it T}$_{\rm SC}$ shows a maximum. The characteristic temperature {\it T}$^{*}$, which decreases with increasing pressure, also depends on the magnetic field., Comment: To be published in J.Phys.Soc.Jpn

NdBa 2 Cu 3 O y (Nd123) thin films are deposited on ( 1 0 0 )MgO, ( 1 0 0 )Y-stablized ZrO (YSZ) and ( 1 0 0 )SrTiO 3 (STO) at the low temperature of 650°C by pulsed laser ablation with a shadow mask (eclipsed pulsed laser deposition (PLD)). X-ray diffraction and reflection high-energy electron diffraction patterns show that all thin films are oriented in ( 0 0 1 ) direction. Compared with the conventional PLD samples, the eclipsed PLD samples exhibit a narrower superconducting transition width ( ΔT c ). The suppression of ΔT c is believed to result from the elimination of droplets and the stoichiometric deviation of Nd specie. The film on STO shows better superconducting properties than those on YSZ and MgO. Its onset T c , ΔT c and J c reaches 93.5 K, 3.5 K and 6×10 6 A/cm 2 , respectively. A Nd123/MgO thin film step-edge junction is prepared as well. A typical RSJ-like (Josephson) current–voltage curve is observed.

We have measured nuclear quadrupole resonance (NQR) for 63 Cu nuclei under high pressure up to 4.8 GPa. A linear increase in NQR frequency 63 v Q of 63 Cu nuclei was observed below 3 GPa. However, 63 v Q deviates gradually from the linear relation above 3.5 GPa. The downward deviation is associated with a change of valence and consistent with the results of band calculations with the structural parameters under pressure. Antiferromagnetic (AFM) spin fluctuations are drastically suppressed with increasing pressure, whereas T c increases from 0.7 K at ambient pressure to 1.64 K at 4.2 GPa. We have found that bulk superconductivity suddenly disappears at 4.8 GPa accompanied with a rapid decrease in the density of states at the Fermi level.

We confirmed bulk-superconductivity of a ferromagnet UGe2 by the specific heat measurement, together with the measurements of the electrical resistivity and ac susceptibility, in a pressure range from p = 1.0 to 1.5 GPa, where the Curie temperature TC( = 22-36 K) is still high, but another characteristic temperature T* is close to zero. In this pressure range, the heavy fermion state is found to be formed at low temperatures.

Elastic neutron-scattering and ac-susceptibility measurements have been performed on a 2 φ single crystal of URu 2 Si 2 under hydrostatic pressure up to ∼ 1.1 GPa using Fluorinert 70 & 77 and Daphne oil 7373 as pressure-transmitting media. Daphne oil produces better hydrostaticity of the applied pressure than Fluorinert, resulting in a slight shift of the phase boundary between hidden order and antiferromagnetic order to a lower pressure. The superconducting phase is also influenced, but never overlaps with the antiferromagnetic phase.

We report 123 Sb nuclear quadrupole resonance (NQR) measurements of the filled skutterudite heavy-fermion superconductor PrOs 4 Sb 12 under high pressure. The temperature dependence of NQR frequency and spin–lattice relaxation rate 1/ T 1 indicate that the crystal electric field splitting Δ CEF between the ground state Γ 1 singlet and the first excited state Γ 4 (2) triplet decreases with increasing pressure. However, it is found Δ CEF and T c stay constant above P ∼2 GPa revealed by Sb-NQR up to P = 3.82 GPa for the first time. Remarkably, the temperature dependence of 1/ T 1 below T c = 1.55 K at P = 1.91 GPa shows no coherence peak just below T c and power-law behavior well below T c in contrast to the exponential decrease observed at P = 0. Present results indicate the superconductivity in PrOs 4 Sb 12 under pressure has nodes in the superconducting gap function.

We report 73 Ge-nuclear-quadrupole resonance (NQR) study of heavy-fermion compound CeNi 2 Ge 2 . The temperature dependence of the 73 Ge nuclear-spin-lattice-relaxation rate 1 / T 1 indicates the development of magnetic correlations and the formation of a Fermi-liquid state at temperatures lower than T FL = 0.4 K , where 1 / T 1 T is constant. The 1 / T 1 T decrease below T c NQR = 0.1 K , whereas resistance decreases below T c onset = 0.2 K and does not become zero. These results indicate CeNi 2 Ge 2 closely locates to a superconducting quantum critical point.

UIr crystallizes in the monoclinic crystal structure without inversion symmetry. The magnetization measurement indicated that UIr is an Ising-type ferromagnet with the Curie temperature T C 1 = 46 K at ambient pressure. We grew a high-quality single crystal by the Czochralski method in a tetra-arc furnace. The ingot was annealed under high vacuum by the solid state electrotransport method. Superconductivity with a transition temperature T sc = 0.14 K was found under pressure.

A review of recent developments in high-pressure techniques and their application in the search for superconductivity at low temperature is presented. We have focused the discussion mainly on the experimental results obtained by our group for simple molecular systems.

The pressure range for electrical transport measurements has been dramatically increased up to 220 GPa in the 0.05-300 K temperature region and at magnetic fields of up to 10 T. Metallization of caesium iodide was found at a pressure of 115 GPa. A superconducting transition with was observed above 180 GPa. decreases with increasing pressure. The superconducting transition shifts to lower temperatures and disappears under magnetic field above 0.3 Tesla.

Enhancement of superconducting transition temperature T c is confirmed at around 16 GPa in the pressure-induced superconductor CeCu 2 Ge 2 . The superconducting H c - T c phase diagrams at the low- T c and high- T c pressure regions indicates that the heavy quasi particles form the singlet Cooper pairs. At both low- T c and high- T c pressure regions, Fermi liquid behavior of ρ∼ A T 2 is observed only at low temperatures below 1 K in the normal state induced by the external field above H c 2 . At 21 GPa where the superconductivity disappears down to 50 mK, the temperature region following ρ∼ A T 2 increases drastically up to 40 K and the coefficient A decreases rapidly. These results indicate that the strongly correlated electrons are responsible essentially for the appearance of superconductivity in this system.

A zero-temperature magnetic-to-nonmagnetic transition has been investigated in the heavy-fermion ferromagnet ${\mathrm{CeRu}}_{2}{\mathrm{Ge}}_{2}$ from electrical-resistance measurements at pressures up to 15 GPa and temperatures down to 60 mK. We found that the magnetic-to-nonmagnetic phase boundary exists at around ${P}_{c}\ensuremath{\sim}10 \mathrm{GPa}$. The resistivity shows the quadratic temperature variation expected for the conventional Fermi liquid at low temperatures near ${P}_{c}.$ In the present pressure range, neither the metamagnetic nor the superconducting transition was observed at least down to 60 mK.

Experimental techniques on production of ultra-high pressure at very low temperature, electrical resistance and magnetization measurement are developed for the purpose of searching for pressure-induced superconductivity in some simple systems. Previous results of pressure induced superconductivity of I, Br, Ca, SnI4 and C6I4O2 are reviewed and remaining problems are discussed shortly. Recent findings of metallization and superconductivity of O and S are also reported.

Anisotropic resistivity in UIr has been investigated under high pressure. A huge enhancement of residual resistivity was found under high pressure for the measuring current parallel to the [0 1 0] direction while any anomalies were not observed in the resistivity for [1 0 1 ]. The residual resistivity for [0 1 0] reduced to the original value in the pressure range where superconductivity appears. This anomalous behavior indicates that the ground state in the high-ρ 0 state is not simple ferromagnet and the application of pressure induces the first-order transition to slightly different electronic state.

Pressure dependence of the Ce valence in ${\mathrm{CeCu}}_{2}{\mathrm{Ge}}_{2}$ has been measured up to 24 GPa at 300 K and to 17 GPa at 18--20 K using x-ray absorption spectroscopy in the partial fluorescence yield. A smooth increase of the Ce valence with pressure is observed across the two superconducting (SC) regions without any noticeable irregularity. The chemical pressure dependence of the Ce valence was also measured in ${\mathrm{Ce}({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{Si}}_{2}$ at 20 K. A very weak, monotonic increase of the valence with $x$ was observed, without any significant change in the two SC regions. Within experimental uncertainties, our results show no evidence for the valence transition with an abrupt change in the valence state near the SC II region, challenging the valence-fluctuation mediated superconductivity model in these compounds at high pressure and low temperature.

We have measured the electrical resistivity of antiferromagnetic Kondo compounds CeNiGe 3 and CeNi 2 Al 5 under pressure. In CeNiGe 3 , Neel temperature T N initially increases with increasing pressure P up to 3 GPa, then decreases rather steeply with further increasing pressure, and becomes zero at a critical pressure P c ≃ 5.5 GPa . The A and ρ 0 values of the resistivity ρ = ρ 0 + AT 2 in the Fermi liquid relation become maximum around P c . Superconductivity is found below 0.48 K in a wide pressure region from 4 to 10 GPa. In CeNi 2 Al 5 , the similar pressure dependence of T N is found, has a maximum at 3.9 GPa, and becomes zero around 6 GPa.

Pressure-induced superconductivity is found in UIr without inversion symmetry. The pressure–temperature phase diagram has been investigated by means of electrical resistivity and magnetization. The phase diagram consists of two ferromagnetic phases. The ferromagnetic phase FM1 with T C 1 ∼ 46 K at ambient pressure disappears around 1.7 GPa. With further increase of pressure, the phase (FM2) with small ferromagnetic moment appears in the pressure range of P C 1 P 2.6 GPa . The superconducting phase exists near the disappearance point of the FM2 phase, indicating that the superconductivity is associated with the ferromagnetic critical fluctuation around P C 2 .

Magnetization measurements of URu 2 Si 2 have been performed under hydrostatic pressures. The superconducting transition temperature decreases linearly with pressure up to 1.1 GPa and disappears above 1.5 GPa. The hysteretic magnetization shows characteristic behaviors at high pressures, suggesting that the pinning properties are changed due to the switch of the normal state to the antiferromagnetic state.

Alkali-metal-intercalated FeSe materials, (NH3)(y)M0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)(y)Cs0.4FeSe shows a superconducting transition temperature (T-c) as high as 31.2 K, which is higher by 3.8 K than the T-c of nonammoniated Cs0.4FeSe. The T(c)s of (NH3)(y)K0.4FeSe and (NH3)(y)Rb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The T-c of (NH3)(y)Cs0.4FeSe shows a negative pressure dependence. A clear correlation between T-c and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity.

Potassium-doped picene (K3.0picene) with a superconducting transition temperature (T C) as high as 14 K at ambient pressure has been prepared using an annealing technique. The shielding fraction of this sample was 5.4% at 0 GPa. The T C showed a positive pressure-dependence and reached 19 K at 1.13 GPa. The shielding fraction also reached 18.5%. To investigate the chemical composition and the state of the picene skeleton in the superconducting sample, we used energy-dispersive x-ray (EDX) spectroscopy, MALDI-time-of-flight (MALDI-TOF) mass spectroscopy and x-ray diffraction (XRD). Both EDX and MALDI-TOF indicated no contamination with materials other than K-doped picene or K-doped picene fragments, and supported the preservation of the picene skeleton. However, it was also found that a magnetic K-doped picene sample consisted mainly of picene fragments or K-doped picene fragments. Thus, removal of the component contributing the magnetic quality to a superconducting sample should enhance the volume fraction.

Various types of ground states in strongly correlated electron systems have been systematically investigated by means of NMR/NQR at low temperatures under high magnetic field and pressure. We focus on two well-known heavy-electron families, CeCu 2 X 2 (X = Si and Ge) (Ce(122)) and UM 2 Al 3 (M = Ni and Pd) (U(123)). The Cu NQR experiments on CeCu 2 X 2 under high pressure indicate that the physical property of CeCu 2 Ge 2 at high pressure, i.e. above the transition at 7.6 GPa from antiferromagnetic (AF) to superconductivity, are clearly related to tha CeCu 2 Si 2 at ambient pressure. In addition to the H-T phase diagram established below 7 T, NMR and specific heat experiments on polycrystal CeCu 2.05 Si 2 have revealed the presence of a new phase above 7 T. In a high-quality polycrystal of UPd 2 Al 3 with a record high- T c of 2 K at ambient pressure and the narrowest Al NQR line width, the nuclear-spin lattice relaxation rate, 27 (1/ T 1 ) measured in zero field has been found to obey the T 3 law down to 0.13 K, giving strong evidence that the energy gap vanishes along lines on the Fermi surface. Thus it seems that all heavy-electron superconductors exhibit lines of zero gap, regardless of their different magnetic properties.

The effects of Ni substitution on Ce(Cu1-xNix)2Si2 have been studied by specific heat and electrical resistivity measurements. The specific heat measurement has revealed that the enhanced quantum fluctuations around an antiferromagnetic quantum critical point are markedly suppressed by Ni substitution, and that the Fermi liquid state recovers in the Nirich region (x > 0.12). The characteristic T-linear dependence of the resistivity has been observed at approximately x ~ 0.10 together with a sign of superconductivity. The variation of n in the form of rho - rho0 = aT^n against Tmax^1, at which the resistivity peaks, coincides with the case of high-pressure experiments on pure CeCu2Si2. The anomalous T-linear behavior appears to occur in the crossover region from the Kondo regime to the valence fluctuation regime rather than in the conventional antiferromagnetic quantum critical region., 11 pages, 5 figures, to be published (J. Phys. Soc. Jpn)

Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ undergoes successive phase transitions with increasing Rh doping in the $T$ $=$ 0 limit. The antiferromagnetic-metal phase with orthorhombic structure at 0.00 $\le$ $x$ $\le$ 0.020 is driven to a superconducting phase with uncollapsed-tetragonal (ucT) structure at 0.020 $\le$ $x$ $\le$ 0.024; a non-superconducting collapsed-tetragonal (cT) phase takes over at $x$ $\geq$ 0.024. The breakdown of Fermi-liquid transport is observed in the ucT phase above $T_{\rm c}$. In the adjacent cT phase, Fermi-liquid transport is restored along with a disappearance of superconductivity. This interplay of superconductivity and Fermi-liquid transport suggests the essential role of magnetic fluctuations in the emergence of superconductivity in doped CaFe$_2$As$_2$., 11 pages, 4 figures

Strain and resistivity measurements in UIr showed the occurrence of two structural phase transitions under high pressure, indicating the existence of three phases I--III, with slightly different crystal structures. It is suggested that ferromagnetic phases FM1--FM3 emerge in phases I--III, respectively. The pressure-temperature phase diagram was sensitive to the hydrostaticity of pressure. FM3 and superconductivity are observed in the measurements in which Daphne 7373 is used as a pressure-transmitting medium; however they are both absent in the case where petroleum ether is used as a pressure-transmitting medium. These results indicate a close relationship between ferromagnetism and superconductivity.

We succeeded in growing a high-quality single crystal of CeRh2Si2 with a residual resistivity ratio of 110. Under a pressure of 1.1 GPa, superconductivity is found below Tc=0.38 K, where a Neel temperature of TN=36 K is suppressed down to almost zero.

We present evidence for unconventional superconducting fluctuations in a heavy-fermion compound CeNi$_2$Ge$_2$. The temperature dependence of the $^{73}$Ge nuclear-spin-lattice-relaxation rate $1/T_1$ indicates the development of magnetic correlations and the formation of a Fermi-liquid state at temperatures lower than $T_{\rm FL}=0.4$ K, where $1/T_1T$ is constant. The resistance and $1/T_1T$ measured on an as-grown sample decrease below $T_{\rm c}^{\rm onset} = 0.2$ K and $T_{\rm c}^{\rm NQR} = 0.1$ K, respectively; these are indicative of the onset of superconductivity. However, after annealing the sample to improve its quality, these superconducting signatures disappear. These results are consistent with the emergence of unconventional superconducting fluctuations in close proximity to a quantum critical point from the superconducting to the normal phase in CeNi$_2$Ge$_2$., 4pages,5figures,to appear in J. Phys. Soc. Jpn

We have studied the electrical resistivity of the filled skutterudite compound $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ under high pressure. The antiferroquadrupolar ordering temperature ${T}_{Q}$ decreases monotonically with increasing pressure up to $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Above $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, we have found that a metal-insulator $(M\text{\ensuremath{-}}I)$ transition appears. The insulating state is easily suppressed by a magnetic field. The observed Kondo effect and a field-induced heavy-fermion state at high pressure suggest that the quadrupolar interactions survive in the insulating region. The quadrupolar interactions might play an essential role in the $M\text{\ensuremath{-}}I$ transition. $\mathrm{Pr}{\mathrm{Fe}}_{4}{\mathrm{P}}_{12}$ is an unusual compound in that it shows a pressure-induced transition from a metal to an insulator.