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The crystal structure of the double-layered Ca 3 Ru 2 O 7 has been studied by convergent beam electron diffraction and powder neutron diffraction. The temperature dependence of the diffraction pattern reveals that all the lattice constants jump at the first-order metal-nonmetal transition at 48 K without a change of the space group symmetry of Bb2 1 m. In the neutron diffraction experiment, an additional magnetic reflection emerges below the Néel temperature, 56 K. A possible model for this antiferromagnetic ordering is proposed, in which model magnetic moments align ferromagnetically within the double layer and antiferromagnetically between the double layers. This model reasonably explains the characteristic field dependence of the magnetoresistance observed at around 6 T.

We have measured the electrical resistivity of a single crystal of YbCo 2 Zn 20 at pressures up to 2.37 GPa and at temperatures from 50 mK to 300 K. Above a critical pressure P c ($ 1 GPa), we have found a resistivity anomaly at T M ($ 0:15 K at 1 GPa) that increases with the pressure. At the ambient pressure, the system shows a nonmagnetic ground state described by the Fermi-liquid model. The T 2 coefficient of the electrical resistivity A strongly increases with the pressure upon approaching P c . However, in the vicinity of P c , the temperature dependence of the resistivity deviates from the Fermiliquid description. These observations suggest that the application of hydrostatic pressure induces a magnetically ordered state for P ! P c and T T M .In intermetallic compounds, including Ce and Yb, the hybridization between the 4f and itinerant conduction-band electrons induces the instability of magnetic moments and charge configurations. In recent years, one of the most interesting topics is the ground state properties of heavy fermion metals located at or close to a magnetic quantum critical point (QCP). 1) The application of external pressure is one of the important tools for controlling the electronic configurations as well as chemical pressure. In the case of heavy fermion Ce compounds exhibiting antiferromagnetic order, such as CeIn 3 and CePd 2 Si 2 , 2) the magnetic order is suppressed by applying pressure. Interestingly, unconventional superconductivity appears in the vicinity of the QCP at which the magnetic ordering temperature is decreased to zero.So far, pressure-induced magnetic transitions have been observed in some Yb-based compounds. [3][4][5][6][7] The key point here is that Yb ions fluctuate between the nonmagnetic Yb 2þ (J ¼ 0) and the magnetic Yb 3þ (J ¼ 7=2) states. Since the ionic volume of the magnetic Yb 3þ state is smaller than that of the nonmagnetic Yb 2þ one, applying pressure stabilizes the magnetic Yb 3þ configuration and induces the appearance of a magnetically ordered state in contrast to the Ce case. However, the critical pressure for these compounds is as high as or higher than 6 GPa, which prevents us from understanding the physics in the vicinity of the magnetic QCP because of difficulties in high-pressure experiments.The series of compounds YbT 2 Zn 20 (T = Fe, Co, Ru, Rh, Or, Ir) belongs to a new heavy fermion system crystallizing in the cubic CeCr 2 Al 20 structure. 8,9) All these compounds show an enhanced Sommerfeld coefficient of the specific heat exceeding 400 mJ/(mol K 2 ). The high-temperature magnetic susceptibility of these compounds follows the Curie-Weiss law with the effective moments close to the value for the free Yb 3þ ion ( eff ¼ 4:54 B ), although there is no indication of magnetic order down to 20 mK. In the YbT 2 Zn 20 family, YbCo 2 Zn 20 exhibits some notable features as follows. The low temperature electrical resistivity and specific heat can be described by the formulas ¼ 0 þ AT 2 and C=T ¼ constant, as expected from the Fermiliquid behavior; the value...

This paper presents an inelastic neutron scattering study of the proper ferroelectric and elastic neutron scattering results on the satellite diffraction pattern which characterizes the modulated phase. The temperature dependences of the satellite intensities and modulation wavevector are in fair agreement with results from previous x-ray experiments. Close to the incommensurate-to-ferroelectric transition temperature , an unexpected intensity overshoot is observed, similar to that seen in birefringence and dilatation experiments. The relationship between the lattice dynamics and the observed phase transition sequence is examined. The dispersion of the ferroelectric soft optic phonon (-polarization) and of the acoustic phonons is followed along the and -directions. In the ferroelectric phase, the TO mode shows a considerable softening as the incommensurate phase is approached from below. In the paraelectric and incommensurate phases, the response from the TO (-polarization) and TA ( strain) branches has been investigated via a series of constant-q scans in the -direction (approximately the modulation wavevector direction). The combined inelastic line-shapes, as observed in a number of non-equivalent Brillouin zones, could all be analysed in terms of a coupled-mode damped harmonic oscillator model. In addition, a diverging, resolution-limited, central peak is observed close to . It is suggested that the TO-TA coupling lies at the origin of the incommensurate instability. A phenomenological free energy is developed, in the continuum approximation, in which the TO-TA interaction is included via a pseudo-Lifshitz term of the type .

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