Pressure effects on the magnetic phase transitions in Mn₃GaC and MnAs have been investigated microscopically by measuring zero field ⁵⁵Mn NMR under pressure up to about 19 kbar. It was confirmed that the pressure induces the transitions between ferro- and antiferromagnetic states in these compounds and remarkable changes in the electronic states of Mn atoms at the transitions.

View full abstract

The NQR technique was applied to investigate the valence phase transition of YbInCu₄ under high pressures. The Cu NQR frequency ν_Q increases with increasing pressure in both of low and high temperature phases. The transition temperature T_v decreases with increasing pressure at a rate of -2.2 K/kbar. The nuclear spin-lattice relaxation rate T₁^-1 shows no temperature and pressure dependence in the high temperature phase, suggesting the localized 4f state of Yb³⁺ ion. In the low temperature phase, (T₁T)^-1 are constant at various pressures and show a Korringa like behavior.

View full abstract

We have developed high-pressure NMR technique with an Indenter Cell using NiCrAl alloy. It was designed to set easily into a conventional dilution fridge and/or high field magnet. We have succeeded in ⁷Li-NMR measurement of a heavy fermion spinel LiV₂O₄ in T-range between 60 mK and 30 K under pressure up to 4.7 GPa. At ambient pressure, 1/T₁ shows a broad maximum around 50 K and is proportional to the temperature below 6 K. This is quite similar to the relaxation behavior in f-electron HF systems. However, 1/T₁T becomes larger with applying higher pressure below 10 K and does not obey T₁T = constant relation down to 1 K. Knight shift K, meanwhile, is independent of pressure above 2 GPa, indicating that the uniform component of susceptibility χ(0) does not change under high pressure. Some antiferromagnetic fluctuations with wave vector q ≠ 0 dominate relaxation rate in LiV₂O₄ near the boundary of the pressure induced insulating phase.

View full abstract

A high-temperature and high-pressure NMR method to investigate the structure and dynamics of supercritical water is reviewed. In this method, a high-temperature condition is realized by introducing hot air directly into the sample portion of a specially designed probe, and a high pressure is achieved by sealing the material of interest into a quartz capillary. The method allows a high-resolution measurement up to 400°C and 0.6 g/cm³ (corresponding to 55 MPa) of water. It is found from the proton chemical shift measurement that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one in the supercritical densities. The measurement of the spin-lattice relaxation time also shows that while the reorientational relaxation proceeds on the order of picosecond in ambient water, it does on the order of several tens of femtoseconds in supercritical water.

View full abstract

Although the conformation of a protein is considered largely fixed in crystal, it fluctuates amply in solution among ensembles of conformers differing in partial molar volume. Applying pressure causes a shift of equilibrium among conformers so that the system reaches a new equilibrium in which the population of a lower volume conformer is favored over a larger volume conformer. Because of the nearly parallel relationship between the conformational order of a protein and its partial molar volume, a less ordered (= higher energy) conformer has a lower volume than that of the stable, basic folded conformer. We apply variable pressure NMR to detect and analyze structures of these higher energy conformers in proteins. We find a number of alternate structures in various proteins, which have hitherto been overlooked spectroscopically. These structures differ from the stable, basic folded ones, often in regions of the molecule where the binding with a ligand or with other macromolecules takes place. These findings have led us to a new dynamic view of proteins such that proteins in solution fluctuates among multiple conformations, which are required for function. Furthermore, we have developed a notion that atom defects or cavities are major sources of conformational fluctuation leading to multiple conformations.

View full abstract

The temperature dependence of the ⁶³Cu nuclear spin-lattice relaxation rate (T₁T)^-1 , spin-spin relaxation rate T_2G^-1 and frequency spectra at the Cu(2) planar site in the normal state of YBa₂Cu₄O₈ have been measured under high pressure by the nuclear quadrupole resonance(NQR) technique. The pseudo spin gap temperature slightly decreased with increasing pressure. It seems that the pressure effects are resembled with carrier doping effects in the increase of superconducting transition temperature T_c and NQR frequency ν_Q(2) and decrease of pseudo spin gap temperature. If the increase of ν_Q(2) under pressure was caused by introducing carrier into Cu(2) planar site, the change of pseudo spin gap temperature is generally originated in carrier concentration at Cu(2) site.

View full abstract

Pressure-induced superconductivity in a spin-ladder cuprate Sr₂Ca₁₂Cu₂₄O₄₁ has not been studied on a microscopic level thus far although the superconductivity was already discovered in 1996. We have improved high-pressure technique with a large high-quality crystal, and succeeded in studying the superconductivity using ⁶³Cu nuclear magnetic resonance (NMR). We found that the superconductivity possesses an s-wave like character in the meaning that a finite gap exists in the quasi-particle excitation.

View full abstract

The recent progress of high pressure techniques, e.g. a development of nonmagnetic NiCrAl alloy, has enabled us to create pressure over 3 GPa by piston cylinder type pressure cells. Using these new techniques, the nuclear quadrupolar resonance (NQR) and ac-susceptibility (ac-χ) measurements were carried out on strongly correlated electron systems, CeRhIn₅ and YbInCu₄ under pressure. CeRhIn₅ shows pressure-induced phase transition from antiferromagnetic ordering to superconducting states. The present NQR and ac-χ studies revealed a homogeneous coexistence of these two types of orderings near the phase boundaries. Also the ac-χ and NQR investigations on YbInCu₄ showing the first-order valence transition at 42 K and at ambient pressure, indicated that ground state of this compound is a ferromagnetically ordered one after the valence transition is suppressed by pressure.

View full abstract

We have investigated the low-temperature phase appearing below T₀ = 1.75 K in URu₂Si₂ by ²⁹Si NMR in the pressure range from 0 to 1.75 GPa. At pressures below Pc = 1.5 GPa, we have observed ²⁹Si NMR lines arising from antiferromagnetic (AF) and paramagnetic (PM) regions in the sample, which provide evidence for a phase separated AF ordering below T₀. The AF region increases with increasing pressure up to Pc.

View full abstract

A large volume belt-type or girdle-type high pressure apparatus is generally used for materials synthesis at high pressure and high temperature conditions. In this article, pressure correction at high temperatures in these apparatuses was reviewed. Nominal pressures were determined at room temperature using the pressure-induced phase transitions such as Bi, Tl, Ba and Sn. Practical pressures at high temperatures were estimated from measured phase boundaries such as the melting curve of Ag and the equilibrium boundary of coesite-stishovite in a belt-type apparatus. It was clear from the correction that the pressure difference between the nominal pressure and the practical one was strongly dependent on pressure transmitting media. Also, various technical problems of high temperature generation and internal standard for pressure and temperature estimation are discussed.

View full abstract