NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprate Sr2_2Ca12_{12}Cu24_{24}O41_{41}

Pressure-induced superconductivity was studied for a spin-ladder cuprate Sr$_2$Ca$_{12}$Cu$_{24}$O$_{41}$ using nuclear magnetic resonance (NMR) under pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a transitional change from a spin-gapped state to a Fermi-liquid state at temperatures higher than $T_c$. The relaxation rate $1/T_1$ shows activated-type behavior at an onset pressure, whereas Korringa-like behavior becomes predominant at the optimal pressure, suggesting that an increase in the density of states (DOS) at the Fermi energy leads to enhancement of $T_c$. Nuclear quadrupole resonance (NQR) spectra suggest that pressure application causes transfer of holes from the chain to the ladder sites. The transfer of holes increases DOS below the optimal pressure. A dome-shaped $T_c$ versus pressure curve arises from naive balance between the transfer of holes and broadening of the band width

Kondo effect in CeXc_{c} (Xc_{c}=S, Se, Te) studied by electrical resistivity under high pressure

We have measured the electrical resistivity of cerium monochalcogenices, CeS, CeSe, and CeTe, under high pressures up to 8 GPa. Pressure dependences of the antiferromagnetic ordering temperature $T_{N}$, crystal field splitting, and the $\ln T$ anomaly of the Kondo effect have been studied to cover the whole region from the magnetic ordering regime at low pressure to the Fermi liquid regime at high pressure. $T_{N}$ initially increases with increasing pressure, and starts to decrease at high pressure as expected from the Doniach's diagram. Simultaneously, the $\ln T$ behavior in the resistivity is enhanced, indicating the enhancement of the Kondo effect by pressure. It is also characteristic in CeX$_{c}$ that the crystal field splitting rapidly decreases at a common rate of $-12.2$ K/GPa. This leads to the increase in the degeneracy of the $f$ state and further enhancement of the Kondo effect. It is shown that the pressure dependent degeneracy of the $f$ state is a key factor to understand the pressure dependence of $T_{N}$, Kondo effect, magnetoresistance, and the peak structure in the temperature dependence of resistivity.Comment: 9 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp

Novel phase transition and the pressure effect in YbFe2Al10-type CeT2Al10 (T=Fe, Ru, Os)

We have succeeded in growing single crystals of orthorhombic CeT2Al10 (T=Fe, Ru, Os) by Al self-flux method for the first time, and measured the electrical resistivity at pressures up to 8 GPa, the magnetic susceptibility and specific heat at ambient pressure. These results indicate that CeT2Al10 belongs to the heavy fermion compounds. CeRu2Al10 and CeOs2Al10 show a similar phase transition at T0 = 27.3 and 28.7 K, respectively. The temperature dependences in the ordered phases are well described by the thermally activated form, suggesting that partial gap opens over the Fermi surfaces below T0. When pressure is applied to CeRu2Al10, T0 disappears suddenly between 3 and 4 GPa, and CeRu2Al10 turns into a Kondo insulator, followed by a metal. The similarity of CeT2Al10 under respective pressures suggests a scaling relation by some parameter controlling the unusual physics in these compounds.Comment: 9 pages, 5 figure

High pressure effects on the electrical resistivity behavior of the Kondo lattice, YbPd2Si2

We report the influence of external pressure (P= up to 8 GPa) on the temperature dependence of electrical resistivity of a Yb-based Kondo lattice, YbPd2Si2, which does not undergo magnetic ordering under ambient pressure condition. There are qualitative changes in the temperature dependence of electrical resistivity due to the application of external pressure. While the resistivity is found to vary quadratically below 15 K (down to 45 mK) characteristic of Fermi-liquids, a drop is observed below 0.5 K for P= 1 GPa. Since the resistance values do not attain zero, we are attempted to attribute this drop to magnetic ordering, rather than to superconductivity. The temperature at which this fall occurs goes through a peak as a function of P (8 K for2 GPa and about 5 K at higher pressures). mimicking Doniach's magnetic phase diagram. We conclude that this is one of the few Yb-based stoichiometric materials, in which one can travers from valence fluctuation to magnetic ordering by the application of external pressure.Comment: 3 figure

Softening of Magnetic Excitations Leading to Pressure-Induced Quantum Phase Transition in Gapped Spin System KCuCl3_3

KCuCl$_3$ is a three dimensionally coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at a critical pressure of $P_{\rm c} \simeq 8.2$ kbar. Magnetic excitations in KCuCl$_3$ at a hydrostatic pressure of 4.7 kbar have been investigated by conducting neutron inelastic scattering experiments using a newly designed cylindrical high-pressure clamp cell. A well-defined single excitation mode is observed. The softening of the excitation mode due to the applied pressure is clearly observed. From the analysis of the dispersion relations, it is found that an intradimer interaction decreases under hydrostatic pressure, while most interdimer interactions increase.Comment: 4 pages, 5 figures, 1 table, jpsj2.cls, to be published in J. Phys. Soc. Jpn. Vol.76 (2007), the graphic problem of Fig.2 was fixe

Electrical resistivity and tunneling anomalies in CeCuAs2

The compound CeCuAs2 is found to exhibit negative temperature (T) coefficient of electrical resistivity (rho) under ambient pressure conditions in the entire T-range of investigation (45 mK to 300 K), even in the presence of high magnetic fields. Preliminary tunneling spectroscopic measurements indicate the existence of a psuedo-gap at least at low temperatures, thereby implying that this compound could be classified as a Kondo semi-conductor, though rho(T) interestingly is not found to be of an activated type.Comment: To appear in the proceedings of SCES200