Masaki Sugishima

Effects of magnetic field and pressure on the intermediate valence state of YbPd

High field magnetization, magnetoresistance and pressure effects of magnetic susceptibility, thermal expansion and electrical resistivity were examined for the intermediate valence system YbPd, which undergoes two first-order transitions at T(1) = 125 K and T(2) = 105 K. Analyses of high field magnetization suggest that half of the Yb atoms have magnetic moments below 100 K up to 55 T. The Yb valence state and the first-order transitions are stable up to 55 T. On the other hand, T(1) and T(2) decrease with increasing pressure and the first-order transitions disappear at around 4 GPa. Above the critical pressure, the experimental results suggest that the intermediate valence state persists down to the lowest temperature or a heavy fermion state is formed. We will show that most experimental results are explained reasonably by assuming the first-order transitions as a valence order transition of Yb. The magnetic ordering temperature is decreased with increasing pressure, in contrast to other Yb intermediate valence or Kondo systems. This may be correlated with the instability of the valence ordered state in this compound.

Origins of Phase Transitions in Valence Fluctuating YbPd

The cubic YbPd compound undergoes two first-order phase transitions at \(T_{1}=125\) K and \(T_{2}=105\) K. We report the experimental results of the X-ray diffraction (XRD) and electron diffraction of single crystals of YbPd at low temperatures. Below \(T_{1}\), the \((h00)\) Bragg peaks (\(h\): an integer) are split into two subpeaks. Below \(T_{2}\), the superlattice reflections of \((n/2,0,0)\) (\(n\): an odd number) in the XRD patterns and the superlattice spots of \((1/2,0,0)\), \((0,1/2,0)\), and \((1/2,1/2,0)\) in the electron diffraction profile appear, which suggests the valence order of YbPd. We discuss two possible structures, assuming a tetragonal symmetry below \(T_{1}\). The origins of the transitions at \(T_{1}\) and \(T_{2}\) are probably the band Jahn–Teller effect and the valence order, respectively.

Detailed Investigation of Elastic Properties of YbPd Single Crystal

We have investigated elastic properties of the mixed-valent intermetallic compound YbPd by means of ultrasonic measurements on the single-crystals. A pronounced elastic anomaly (elastic softening) was observed at around the transition temperature T 1 ≈130 K with a significant hysteresis, being indicative of a first-order phase transition. Furthermore, it was found that the elastic anomaly showing up around T 1 was hardly affected by application of a magnetic field, suggesting the non-magnetic origin. We can analyze quantitatively the temperature dependence of all measured elastic modes C 11 and C 44 with a Jahn–Teller formula. Our findings suggest that the pronounced elastic softening toward T 1 will enable us to predict the possible charge ordering origin.

Raman Scattering Investigation on YbPd

Raman spectra of the single crystal of YbPd have been measured in the temperature range from 4 to 300 K. In this temperature range, YbPd undergoes two transitions at T 1 = 132 K and T 2 = 110 K. At 4 K, two new peaks have been observed, and the peak P2 at 91 cm -1 have been found up to 120 K. This result indicates the structural distortion below T 1 . The energy and linewidth of P2 have been examined below 120 K. Its energy shows the minimum around 90 K and increases with increasing temperature up to T 2 . The anomalous energy change below 90 K can be explained by the temperature dependence of the volume change. The steep change of the energy around T 2 is due to the structural change through the first order transition at T 2 . The anomalous increase of the linewidth has been also observed below 20 K.

Valence-lattice interaction on YbPd

An intermediate valence compound YbPd undergoes successive structural transitions at 105 K and 125 K. The recent inelastic X-ray scattering measurement have revealed softening of longitudinal acoustic (LA) mode around X point, q = (0, 0, 0.5). We have attempted to reproduce this softening from first-principles calculations. The calculated results with fixed valence Yb do not show any anomaly in the LA mode around X. To include valence change in the lattice vibrations, the nearest-neighbor valence-lattice interaction has been added. The interaction reduces the energy of the LA mode around X as observed by the experiment. This soft mode leads to the structure below 105 K. On the other hand, the lowest energy is found at the slightly different wavevector, q = (0.1, 0.1, 0.5). This wavevector corresponds to the superlattice reflection observed by the experiment in the intermediate incommensurate phase between 105 K and 125 K. Therefore, the successive structural transitions of YbPd can be correctly reproduced by including this valence-lattice interaction.

Inelastic X-ray Scattering of Valence Fluctuating YbPd

Inelastic X-ray scattering (IXS) of cubic YbPd is carried out at 300 K to examine lattice instabilities associated with the valence ordering. The IXS spectra in the longitudinal -X (-X(L)) direction show that the phonon energy decreases from the middle of the zone to the zone boundary, which indicates the presence of a lattice instability at the X(L) point. On the other hand, normal IXS spectra are observed in the -X(T) and -M directions, which implies absense of any instabilities at the X(T) and M points. Although our previous diffraction measurements suggested a superlattice structure of either a a 2a or 2a 2a a below T = 105 K, the IXS results indicate that the a a 2a structure is much more convincing. The broad linewidth of the IXS spectra observed at around the X(L) point is possibly due to strong electron-phonon coupling, not to the Q-resolution in the IXS measurements.

Effects of magnetic field, pressure and dilution of Yb on phase transitions in valence fluctuating compound YbPd

High field magnetization, pressure effect on electrical resistivity and dilution of Yb with Lu were examined for YbPd. The high field magnetization suggests that half of Yb ions have magnetic moments between 50 K and 115 K up to 55 T. The phase transition temperatures, T 1 and T 2 , which are observed under ambient pressure at 125 and 105 K, respectively, decrease with increasing pressure and the transitions disappear at around 4 GPa. This indicates that the valence fluctuating state above T 1 and T 2 becomes stable down to zero temperature above 4 GPa. Substitution of Lu for Yb also leads to depression of T 1 and T 2 , which is possibly caused by disturbing the periodicity of Yb ions. These results suggest that the valence ordered state of Yb ions is realized in YbPd below T 1 or T 2 .

Low temperature phase of YbPd investigated by Raman scattering

The possibility of the structural change in YbPd has been investigated by Raman scattering in the temperature range from 4 K to 300 K. One peak was clearly observed around 91 cm−1 below 132 K. This is a clear evidence of a structural change at the transition temperature 132 K from the CsCl structure. The polarization dependence of the 91 cm−1 peak shows that the cubic and tetragonal symmetries are ruled out for the crystal structure at 4 K.