In this article, we present an experimental setup for high-pressure resistivity measurements up to 3.2 GPa and its application to EuFe2As2, a mother compound of iron-based superconductors. Hydrostatic and nonhydrostatic pressures are produced by Daphne 7474 and Stycast 1266 pressure transmitting media, respectively, using a piston-cylinder-type NiCrAl/CuBe high-pressure apparatus. It is found that, in EuFe2As2, application of hydrostatic pressure is essential for the appearance of bulk superconductivity at Tc~30 K in the limited range of 2.5-3.0 GPa. Quality of pressure generated via Daphne 7373 and 7474 are also evaluated by use of two Manganin wires arranged parallel and vertical to the load axis.

A pressurizing trial was performed for a hybrid NiCrAl hybrid pressure cell and a pressure of 4.6 GPa was achieved under a steady load of 15.0 ton, which marks the highest pressure ever known for a piston-cylinder-type pressure cell. The pressure efficiency at 15.0 ton was 75% and the expansion of the inner diameter of the NiCrAl cylinder partially reached 5%. The pressure cell was applied to nuclear-magnetic-resonance experiments on cuprate and pnictide superconductors.

The measurements of the electrical resistivity ρ and the thermopower S of single crystal EuCo2P2 have been performed at temperatures from 1.5 to 300 K under hydrostatic pressures up to 3 GPa and in magnetic fields up to 10 T. The temperature dependences of ρ and S show drastic changes at a critical pressure Pc, which indicates a large modification of the electronic structure around the Fermi level due to the valence transition from Eu to Eu. In the low-pressure phase with magnetic Eu and nonmagnetic Co, the magnetic field dependences of ρ and S show sudden decrease at B ≈ 7 T, implying a metamagnetic transition. On the other hand, no drastic change in ρ(B) and S(B) curves is observed in the high-pressure phase with nonmagnetic Eu and magnetic Co.

We report measurements of ac magnetic susceptibility chi(ac) and de Haas-van Alphen (dHvA) oscillations in KFe2As2 under high pressure up to 24.7 kbar. The pressure dependence of the superconducting transition temperature T-c changes from negative to positive across P-c similar to 18 kbar, as previously reported. The ratio of the upper critical field to T-c, i.e., B-c2/T-c, is enhanced above P-c, and the shape of the chi(ac) versus field curves qualitatively changes across P-c. dHvA oscillations smoothly evolve across P-c, indicating no drastic change in the Fermi surface up to 24.7 kbar. Three dimensionality increases with pressure, while effective masses show decreasing trends. We suggest a crossover from a nodal to a full-gap s wave as a possible explanation.

Resistivity and Hall effect measurements of EuFe2As2 up to 3.2 GPa indicate no divergence of quasiparticle effective mass at the pressure P-c where the magnetic and structural transition disappears. This is corroborated by analysis of the temperature (T) dependence of the upper critical field. T-linear resistivity is observed at pressures slightly above P-c. The scattering rates for both electrons and holes are shown to be approximately T-linear. When a field is applied, a T-2 dependence is recovered, indicating that the origin of the T-linear dependence is spin fluctuations.

We describe our recent progress in high-pressure nuclear magnetic resonance (NMR) technique, which extends the range of practical NMR experiments to 10 GPa-class. The new opposed-anvil type cell has significantly large sample space, though the whole clamp cell size is small enough to put within superconducting magnets or fridges. We also show other new techniques on loading of argon pressure transmitting medium, pressure determination, and wiring of electrodes. The application on a pressure-induced superconductivity of SrFe2As2 has demonstrated a remarkable advantage brought by the combination of the clean crystal, pressure control, and spectroscopy.

We have carried out high-field resistivity measurements up to 27 T in EuFe2As2 at P = 2.5 GPa, a virtually optimal pressure for the P-induced superconductivity, where T-c = 30 K. The B-c2-T-c phase diagram has been constructed in a wide temperature range with a minimum temperature of 1.6 K (approximate to 0.05 x T-c), for both B parallel to ab (B-c2(ab)) and B parallel to c (B-c2(c)). The upper critical fields B-c2(ab)(0) and B-c2(c) (0), determined by the onset of resistive transitions, are 25 and 22 T, respectively, which are significantly smaller than those of other Fe-based superconductors with similar values of T-c. The small B-c2(0) values and the B-c2(T) curves with positive curvature around 20 K can be explained by a multiple pair-breaking model that includes the exchange field due to the magnetic Eu2+ moments. The anisotropy parameter, Gamma = B-c2(ab)/B-c2(c), in EuFe2As2 at low temperatures is comparable to that of other "122" Fe-based systems.

We have constructed a pressure$-$temperature ($P-T$) phase diagram of $P$-induced superconductivity in EuFe$_2$As$_2$ single crystals, via resistivity ($\rho$) measurements up to 3.2 GPa. As hydrostatic pressure is applied, an antiferromagnetic (AF) transition attributed to the FeAs layers at $T_\mathrm{0}$ shifts to lower temperatures, and the corresponding resistive anomaly becomes undetectable for $P$ $\ge$ 2.5 GPa. This suggests that the critical pressure $P_\mathrm{c}$ where $T_\mathrm{0}$ becomes zero is about 2.5 GPa. We have found that the AF order of the Eu$^{2+}$ moments survives up to 3.2 GPa without significant changes in the AF ordering temperature $T_\mathrm{N}$. The superconducting (SC) ground state with a sharp transition to zero resistivity at $T_\mathrm{c}$ $\sim$ 30 K, indicative of bulk superconductivity, emerges in a pressure range from $P_\mathrm{c}$ $\sim$ 2.5 GPa to $\sim$ 3.0 GPa. At pressures close to but outside the SC phase, the $\rho(T)$ curve shows a partial SC transition (i.e., zero resistivity is not attained) followed by a reentrant-like hump at approximately $T_\mathrm{N}$ with decreasing temperature. When nonhydrostatic pressure with a uniaxial-like strain component is applied using a solid pressure medium, the partial superconductivity is continuously observed in a wide pressure range from 1.1 GPa to 3.2 GPa.

We present the magnetic and superconducting phase diagram of EuFe2As2 for B‖c and B‖ab. The antiferromagnetic phase of the Eu2+ moments is completely enclosed in the superconducting phase. The upper critical field vs. temperature curves exhibit strong concave curvatures, which can be explained by the Jaccarino–Peter compensation effect due to the antiferromagnetic exchange interaction between the Eu2+ moments and conduction electrons.

We report resistivity ρ and Hall effect measurements on EuFe_2As_2 at ambient pressure and 28 kbar and magnetization measurements at ambient pressure. We analyze the temperature and magnetic-field dependence of ρ and the Hall effect using a molecular-field theory for magnetoresistance and an empirical formula for the anomalous Hall effect and find that electron scattering due to the Eu^2+ local moments plays only a minor role in determining electronic transport properties of EuFe_2As_2.

We have developed a new type of opposed-anvil high pressure cell With substantially improved space efficiency. The clamp cell and the gasket are made of non-magnetic Ni-Cr-Al alloy. Non-magnetic tungsten carbide (NMWC) is used for the anvils. The assembled cell with the dimension phi 29 mm x 41 mm is capable of generating pressure up to 9 GPa over a relatively large sample space to 7 mm(3). Our cell is particularly suitable for those experiments which require large sample space to achieve good signal-to-noise ratio, such as the nuclear magnetic resonance (NMR) experiment. Argon is used as the pressure transmitting medium to obtain good hydrostaticity. The pressure was calibrated ill situ by Measuring the fluorescence from ruby through a transparent moissanite (6H-SiC) window, We have Measured the pressure and temperature dependences of the Cu-63 nuclear-quadrupole-resonance (NQR) frequency of Cu2O, the in-plane Knight shift of metallic tin. and the Knight Shift of platinum. These quantities call be used as reliable manometers to determine the pressure Values ill situ during the NMR/NQR experiments up to 9 GPa.

We have performed the electrical resistivity measurement on single crystal heavy fermion compounds YbT2Zn20 (T = Co, Rh, Ir) under pressure up to 12 GPa. A pronounced resistivity maximum T-max, which scales with the Kondo temperature, develops with increasing pressure. In the case of YbCo2Zn20, T-max initially decreases with increasing pressure, reaches a minimum around 1 GPa, above which the pressure-induced magnetic ordering emerges at low temperature. For YbRh2Zn20 and YbIr2Zn20, T-max also exhibits a minimum at approximately 3 GPa, suggesting the appearance of pressure-induced magnetic transition below 2 K. Our results indicate the proximity to a magnetic instability in YbT2Zn20 systems at ambient pressure.

We report the ac magnetic susceptibility χ ac and resistivity ρ measurements of EuFe 2 As 2 under high pressure P . By observing nearly 100% superconducting shielding and zero resistivity at P =28 kbar, we establish that P -induced superconductivity occurs at T c ∼30 K in EuFe 2 As 2 . ρ shows an anomalous nearly linear temperature dependence from room temperature down to T c at the same P . χ ac indicates that an antiferromagnetic order of Eu 2+ moments with T N ∼20 K persists in the superconducting phase. The temperature dependence of the upper critical field is also determined.