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We have investigated the low-temperature magnetism of sodium superoxide (NaO2), in which spin, orbital, and lattice degrees of freedom are closely entangled. The magnetic susceptibility shows anomalies at T1 = 220 K and T2 = 190 K, which correspond well to the structural phase transition temperatures, and a sudden decrease below T3 = 34 K. At 4.2 K, the magnetization shows a clear stepwise anomaly around 30 T with a large hysteresis. In addition, the muon spin relaxation experiments indicate no magnetic phase transition down to T = 0.3 K. The inelastic neutron scattering spectrum exhibits magnetic excitation with a finite energy gap. These results confirm that the ground state of NaO2 is a spin-singlet state. To understand this ground state in NaO2, we performed Raman scattering experiments. All the Raman-active libration modes expected for the marcasite phase below T2 are observed. Furthermore, we find that several new peaks appear below T3. This directly evidences the low crystal symmetry, namely, the presence of the phase transition at T3.We conclude that the singlet ground state of NaO2 is due to the spin-Peierls instability.

High magnetic field x-ray diffraction experiments of solid oxygen have been performed at 10 K using DC magnetic fields of up to 5 T as well as pulsed magnetic fields of up to 25 T. The $\ensuremath{\alpha}$ phase of oxygen exhibits no magnetostriction greater than $\mathrm{\ensuremath{\Delta}}d/d={10}^{\ensuremath{-}4}$ at 5 T, where $d$ and $\mathrm{\ensuremath{\Delta}}d$ denote a lattice plane spacing and its magnetic field variation, respectively. The $\mathrm{\ensuremath{\Delta}}d/d$ at higher fields of up to 25 T is found to be smaller than $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. These results contradict the previously reported giant magnetostriction in the $\ensuremath{\alpha}$ phase where the volume magnetostriction $\mathrm{\ensuremath{\Delta}}V/V$ reaches ${10}^{\ensuremath{-}2}$ at 7.5 T [K. Katsumata et al., J. Phys.: Condens. Matter 17, L235 (2005)].

We comprehensively studied the magnetotransport properties of LaAgSb$_2$ under high pressure up to 4 GPa, which showed unique successive charge density wave (CDW) transitions at $T_{CDW1}\sim 210$ K and $T_{CDW2}\sim 190$ K at ambient pressure. With the application of pressure, both $T_{CDW1}$ and $T_{CDW2}$ were suppressed and disappeared at the critical pressures of $P_{CDW1}=3.0$--3.4 GPa and $P_{CDW2}=1.5$--1.9 GPa, respectively. At $P_{CDW1}$, the Hall conductivity showed a step-like increase, which is consistently understood by the emergence of two-dimensional hollow Fermi surface at $P_{CDW1}$. We also observed a significant negative magnetoresistance effect when the magnetic field and current were applied parallel to the $c$ axis. Shubnikov--de Haas (SdH) oscillation measurements under pressure directly showed the changes in the Fermi surface across the CDW phase boundaries. In $PP_{CDW1}$, we observed a single frequency of $\sim 48$ T with a cyclotron effective mass of 0.066 $m_0$, whose cross section in the reciprocal space corresponded to only 0.22\% of the first Brillouin zone. Besides, we observed another oscillation component with frequency of $\sim 9.2$ T, which is significantly enhanced in the limited pressure range of $P_{CDW2}, Comment: 15pages, 13 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

The magnetic-field-temperature phase diagram of solid oxygen is investigated by the adiabatic magnetocaloric effect (MCE) measurement with pulsed magnetic fields. Relatively large temperature decrease with hysteresis is observed at just below the $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ and $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ phase-transition temperatures owing to the field-induced transitions. The magnetic field dependence of these phase boundaries are obtained as ${T}_{\ensuremath{\beta}\ensuremath{\gamma}}(H)=43.8\ensuremath{-}1.55\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{H}^{2}$ K and ${T}_{\ensuremath{\alpha}\ensuremath{\beta}}(H)=23.9\ensuremath{-}0.73\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{H}^{2}$ K. The magnetic Clausius-Clapeyron equation quantitatively explains the $H$ dependence of ${T}_{\ensuremath{\beta}\ensuremath{\gamma}}$, but does not ${T}_{\ensuremath{\alpha}\ensuremath{\beta}}$. The MCE curve at ${T}_{\ensuremath{\beta}\ensuremath{\gamma}}$ is of typical first order, while the curve at ${T}_{\ensuremath{\alpha}\ensuremath{\beta}}$ seems to have both characteristics of first- and second-order transitions. We discuss the order of the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ phase transition and propose possible reasons for the unusual behaviors.

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.

The phase diagram of solid oxygen in the magnetic-field-temperature ($B\ensuremath{-}T$) plane is revealed by magnetization and magnetotransmission measurements. The high-field phase of solid oxygen, which we term the $\ensuremath{\theta}$ phase, is induced at the temperature below 42 K. The transition fields at the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\theta}$ and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\theta}$ transitions are almost temperature independent, with the phase boundary being very steep. This phase boundary indicates that the entropy change is very small at the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\theta}$ and $\ensuremath{\beta}\ensuremath{-}\ensuremath{\theta}$ transitions, in contrast to the entropy-driven $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ transition at zero field. We argue that the $\ensuremath{\theta}$ phase differs from the high-temperature $\ensuremath{\gamma}$ phase in terms of entropy; the molecular rotation is inhibited in the $\ensuremath{\theta}$ phase, whereas permitted in the $\ensuremath{\gamma}$ phase. This finding consolidates the novelty of the $\ensuremath{\theta}$ phase.

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 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.

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.

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.

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 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.

The heat-capacity and magnetization measurements under high pressure have been carried out in a ferromagnetic superconductor ${\mathrm{UGe}}_{2}.$ Both measurements were done using a same pressure cell in order to obtain both data for one pressure. Contrary to the heat capacity at ambient pressure, an anomaly is found in the heat capacity at the characteristic temperature ${T}^{*}$ where the magnetization shows an anomalous enhancement under high pressure where the superconductivity appears. This suggests that a thermodynamic phase transition takes place at ${T}^{*}$ at least under high pressure slightly below ${P}_{c}^{*}$ where ${T}^{*}$ becomes zero. The heat-capacity anomaly associated with the superconducting transition is also investigated, where a clear peak of $C/T$ is observed in a narrow pressure region $(\ensuremath{\Delta}P\ensuremath{\sim}0.1\mathrm{GPa})$ around ${P}_{c}^{*}$ contrary to the previous results of the resistivity measurement. Present results suggest the importance of the thermodynamic critical point ${P}_{c}^{*}$ for the appearance of the superconductivity.

Evolution of the pressure-induced heavy fermion state and superconductivity in ${\mathrm{CeRhIn}}_{5}$ have been established by high-pressure Fermi surface studies using dHvA technique. The effective mass of the conduction electrons named branch ${\ensuremath{\beta}}_{2}$ increases from 5.5 ${m}_{0}$ at ambient pressure to 20 ${m}_{0}$ at 1.6 GPa where superconductivity sets in. This heavy electron mass continues to increase steeply up to 2.1 GPa, 45 ${m}_{0}$ at 2.1 GPa, with a concomitant increase of the superconducting transition temperature. However, the underlying Fermi surface remains approximately same even at 2.1 GPa, strongly suggesting that antiferromagnetic fluctuations of Ce moments do not contribute to the volume of the Fermi surface, but enhance the cyclotron mass of the conduction electrons.

Magnetoresistance measurements on the quasi-one-dimensional organic conductor $(\mathrm{TMTSF}{)}_{2}{\mathrm{PF}}_{6}$ performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K and under pressures P up to 14 kbar have revealed new phases on its $P\ensuremath{-}B\ensuremath{-}T$ phase diagram. We found a new boundary which subdivides the field induced spin density wave (FISDW) phase diagram into two regions. Whereas a low-temperature region of the FISDW diagram is characterized by a hysteresis behavior typical for the first-order transitions, in a high-temperature region of the FISDW phase diagram, the hysteresis was found to disappear. We also found that the temperature dependence of the resistance (at a constant $B)$ changes sign at about the same boundary. We compare these results with recent theoretical models.

Using scanning tunneling microscopy observations, it has been found that the atomically flat 1-ML Sb/ Si(100) interface is metastable at a temperature of about 750°C and becomes rough upon brief annealing, although the Sb-dimer-row structure of the top layer is completely preserved. The roughening is explained by displacive adsorption of Sb (i.e., by substitution of the top-layer Si atoms by Sb atoms) and this phenomenon is suggested to be a common one for group-V adsorbates on Si(100) and Ge(100) surfaces.

We report the pressure $(P)$-temperature $(T)$ phase diagram of antiferromagnetism and superconductivity in ${\mathrm{CeRhIn}}_{5}$ and ${\mathrm{CeIn}}_{3}$ revealed by the ${}^{115}\mathrm{In}$ nuclear-spin-lattice-relaxation ${(T}_{1})$ measurement. In the itinerant magnet ${\mathrm{CeRhIn}}_{5},$ we found that the N\'eel temperature ${T}_{N}$ is reduced at $Pg~1.23\mathrm{GPa}$ with an emergent pseudogap behavior. In ${\mathrm{CeIn}}_{3},$ the localized magnetic character is robust against the application of pressure up to $P\ensuremath{\sim}1.9\mathrm{GPa},$ beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.

That the $4f$ electrons in the rare-earth antiferromagnetic compound ${\mathrm{CeRh}}_{2}{\mathrm{Si}}_{2}$ should be considered as localized electrons at cerium sites has been confirmed by comparing a de Haas--van Alphen experiment to the result of energy band calculations for ${\mathrm{LaRh}}_{2}{\mathrm{Si}}_{2}.$ When pressure p is applied to the compound, the N\'eel temperature ${T}_{\mathrm{N}1}=36 \mathrm{K}$ decreases and finally becomes zero at ${p}_{\mathrm{c}}\ensuremath{\simeq}1.0$--1.1 GPa. The topology of the Fermi surface is found to be almost unchanged up to 1.0 GPa, but the cyclotron effective mass increases with increasing pressure. Above 1.1 GPa, the topology of the Fermi surface changes abruptly. A new Fermi surface is explained by the $4f$-itinerant-band model. The corresponding cyclotron mass is large, being about ${22m}_{0}.$

High-energy spin excitations up to above 8000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were measured by polarized Raman scattering in the insulating phases of S=1/2 copper oxides, ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.2}$, ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{0.5}$${\mathrm{Y}}_{0.5}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8+\mathit{y}}$, ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, and ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$, and an S=1 nickel oxide ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$. All the copper oxides commonly show anomalous spectra, especially the secondary scattering peaks near 4J (J is the two-spin superexchange energy). Those anomalies are caused by the large four-spin cyclic exchange interactions. The present experiments demonstrated that the four-spin exchanges show distinctive characteristics in cuprates as compared to other materials.

To elucidate the ground state of the pressure-stabilized high-temperature (HT) phase of ${\mathrm{YbInCu}}_{4},$ we have carried out electrical resistivity $\ensuremath{\rho}$ and ac-susceptibility ${\ensuremath{\chi}}_{\mathrm{ac}}$ measurements at high pressures. For pressures above 2.49 GPa, the first-order valence transition is completely suppressed (below $\ensuremath{\sim}80\mathrm{mK}).$ Separately, above 2.39 GPa, a clear peak appears in ${\ensuremath{\chi}}_{\mathrm{ac}}$ with a small kink in $\ensuremath{\rho}$ at around ${T}_{M}=2.4\mathrm{K}.$ The ${\ensuremath{\chi}}_{\mathrm{ac}}$ peak is easily diminished by applying low magnetic fields and disappears above $\ensuremath{\sim}500\mathrm{Oe}.$ The characteristic behavior of ${\ensuremath{\chi}}_{\mathrm{ac}}$ at ${T}_{M}$ can generally be ascribed to the onset of long-range ferromagnetic ordering and, therefore, the ground state of the pressure-stabilized HT phase is most probably a ferromagnetically ordered state. This result is compatible with the occurrence of weak ferromagnetism recently reported for the Y-substituted compound ${\mathrm{Yb}}_{0.8}{\mathrm{Y}}_{0.2}{\mathrm{InCu}}_{4}$ under pressure of 1.2 GPa.

A new phenomenological model describing nonequilibrium superconductivity is proposed which is characterized by the introduction of a quasithermal distribution of phonons with energies of 0--infinity. The model exhibits ..mu..(-model-like and T(-model-like ..delta.. vs n and ..mu..( vs n characteristics when the excess quasiparticle density n is low and high, respectively. The estimated critical density n/sub c/ for the onset of instability is much lower than that of the ..mu..( model. These results are in fairly good agreement with the experimental results reported by Willemsen and Gray, and Akoh and Kajimura.