English 多重極限物質系 of ltlab

Hiroshima University
Low Temperature Physics Laboratory

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Last up dated at 2015-12-22

Multi-extreme conditions

Strongly correlated electron systems

Multiferroics

Research for a novel electronic state originating from strongly correlated electron systems and a large amplitude atomic motion

　We focus on a novel electronic state due to strongly correlated electron systems in f-electron and d-electron compounds and a large amplitude atomic motion, which is the so-called rattling motion, under multi-extreme conditions. Our research is performed by measuring the elastic modulus, ultrasonic attenuation, specific heat, magnetization, magnetic susceptibility, and many thermodynamic properties.

Ultrasonic measurement

EN_図1.jpg　The ultrasonic technique (measuring elastic modulus and ultrasonic attenuation) is a powerful tool for studying the novel electronic state arising from strongly correlated electron systems and the rattling motion. The elastic modulus and ultrasonic attenuation can be obtained from the ultrasonic velocity and dissipation of ultrasound, respectively, when ultrasound propagates through the sample. The elastic modulus is a coefficient of a strain to a stress and corresponds to the spring coefficient in the Hooke's law. Generally, the modulus increases monotonically with decreasing temperature in case of no phase transition and no anomalous interaction. Figure 1 shows a simplified block diagram of the elastic modulus measurement system (phase-comparison pulse-echo method). EN_図2.jpgA relative change of the sound velocity in the sample is measured as a relative change of ultrasonic frequency. An example of signal of ultrasonic pulse echoes is shown in Fig. 2.
　Frequency is physical quantity which can be measured with high accuracy. In our laboratory, we have achieved measurement accuracy of 10-7 by improving the feedback system from a conventional analog system to a digital system. In addition, we developed a ultrasonic measurement system: ORPHEUS (Orthogonal Phase-detection Experimental method for Ultra Sound) in collaboration with Yoshizawa laboratory at Iwate University.
　On the other hand, it is possible to investigate dynamic properties of the sample, such as the relaxation time, by ultrasonic attenuation. Here, a sound wave has longitudinal or transverse wave characteristic, and the crystal possesses specific symmetry. By a combination of the crystal axis and sound wave, a variety of elastic modes exist. We can assess symmetry of an order parameter by the difference of elastic anomalies among each mode.

Research objects and recent achievements

1. Strongly correlated electron systems
EN_図3.jpg　In lanthanoid and actinoid compounds with high crystal symmetry, localized f-electrons have the charge, spin, and orbital (electric quadrupole) degrees of freedom. The multipolar degrees of freedom often play an important role for the understanding of their physical properties. The electric quadrupole is a second-rank tensor, as with a strain. The strain induced by ultrasound bilinearly couples to a corresponding quadrupole moment. The elastic modulus is quadrupole susceptibility as is the case with magnetic susceptibility which is susceptibility of magnetic dipoles, as shown in Fig. 3. In our laboratory, we are studying f-electron compounds, such as PrIr2Zn20, PrRh2Zn20, TbB4, and UCu2Sn.
　Recent results are as follows. The praseodymium compounds PrTr2Zn20 (Tr=Ir, Rh) undergo a superconducting phase transition at 0.05 K and 0.06 K, respectively. Both compounds also show a phase transition at TQ = 0.11 K (Tr=Ir) and 0.06 K (Tr=Rh) other than the superconducting transition. It was suggested that the transition at TQ is caused by a non-magnetic doublet under a crystal electric field (CEF) in a cubic symmetry. The modulus (C11-C12)/2 is the linear response to the strain corresponding to the non-magnetic doublet. If the non-magnetic doublet is the ground state, remarkable elastic softening of (C11-C12)/2 is expected at low temperatures. We performed ultrasonic measurements in order to clarify the origin of the phase transition at TQ.
EN_図4.jpg　Figure 4 shows the temperature T dependence of the transverse modulus (C11-C12)/2. At high temperatures, (C11-C12)/2 increases monotonically with decreasing T. Below 7 K, elastic softening due to a quadrupole interaction is observed. As shown in the inset of Fig. 4, the softening stops at TQ, and then (C11-C12)/2 turns into increase below TQ, suggesting that most of the quadrupole degrees of freedom disappears at TQ. We performed a theoretical fitting using the CEF effect with a strain-quadrupole coupling and a quadrupole–quadrupole coupling. The red solid curve in Fig. 4 is the best fit, and (C11-C12)/2 is well reproduced. The negative quadrupole–quadrupole coupling constant (g’=-0.13 K) and disappearance of the quadrupole degrees of freedom at TQ indicate that the origin of the transition at TQ is antiferroquadrupolar (AFQ) ordering. In the magnetic field H-temperature phase diagram obtained by ultrasonic measurements in H, TQ increases with increasing H, and then TQ decreases gradually with further increasing H. The re-entrant behavior of TQ also indicates that the transition at TQ is AFQ ordering. We experimentally determined that the phase transition at TQ is AFQ ordering in PrIr2Zn20. Similarly, we also clarified that PrRh2Zn20 undergoes AFQ ordering at TQ.
　Meanwhile, the rare earth tetraboride TbB4 shows two phase transitions at TN1 = 42.1 K and TN2 = 21.7 K. It was reported that the transition at TN1 is magnetic ordering, however, the origin of the transition at TN2 is unclear. We carried out ultrasonic measurements on TbB4 and found a significant softening of (C11-C12)/2 with 45 % reduction of the stiffness at TN2. By the theoretical analysis using the strain susceptibility, the softening is well reproduced and the constant g’ is positive. We experimentally determined that the phase transition at TN2 is ferroquadrupolar ordering in TbB4.

2. The rattling motion
EN_図5.jpg　Cage compounds have attracted much attention because of the so-called rattling motion, which is a large-amplitude atomic motion of the guest atom accommodated in polyhedral cages, as shown in Fig. 5. Thermal conductivity in cage compounds is very low due to phonon scattering by the rattling motion. The cage compounds are expected to be thermoelectric materials which effectively convert waste heat to electricity in terms of application. From the viewpoint of basic study, it is reported that the rattling motion contributes to a novel superconductivity and a metal-insulator phase transition. The physical properties involving the ionic degrees of freedom of the rattling motion, in addition to the charge, spin, and orbital degrees of freedom, have been intensively studied. In our laboratory, we are investigating cage compounds, such as the filled skutterudites RFe4Sb12 (R: rare earth) and intermetallic clathrates A8Ga16C30 (A=Ba, Sr, Eu; C=Ge, Sn).
EN_図6.jpg　In ultrasonic measurements on cage compounds with the rattling motion, ultrasonic dispersion (UD), which is the ultrasonic frequency dependences of elastic modulus and ultrasonic attenuation, and elastic softening at low temperatures are characteristically observed, as shown in Fig. 6. In previous ultrasonic experiments, UD is reported in a particular elastic modulus, suggesting that it possesses ultrasonic mode-selectivity. However, we found UD in all elastic moduli measured in the filled skutterudite LaFe4Sb12. This is the first report on no-mode-selective UD in cage compounds. As for the origin of UD, we semi-quantitatively assessed the strength of electron–phonon coupling using experimental results, the calculated density of states, and the Hattori-Miyake theory. We pointed out that no-mode-selective UD is caused by the coupling between an acoustic phonon and low-lying optical phonon interacting with electrons. The tendency of UD to appear is directly related to the strength of electron-phonon coupling. LaFe4Sb12 has a stronger electron–phonon coupling in all elastic modes.
EN_図7.jpg　On the other hand, the type-I clathrate Ba8Ga16Sn30 also has a caged structure. The guest atom locates in four off-center sites from the cage center. Here, the off-center rattling is reported only in type-I clathrates. We carried out ultrasonic measurements on many type-I clathrates and found elastic softening of C44 among the off-center rattling compounds, as shown in Fig. 7. We clarified that the elastic softening originates from the lattice instability by the off-center rattling motion.

　In addition to above researches, we are investigating magnetism, a quadrupole interaction, and the heavy-fermion state in f-electron compounds, such as RT2Al10 (R: rare earth, T=Fe, Ru, Os) and YbMGe (M=Ir, Pt, Rh, Pd, Ge). The rattling motion in the tetrahedrites Cu12-xTMxSb4S13 (TM=Mn, Fe, Co, Ni, Zn), which is expected to be thermoelectric materials consisting of mainly environmentally friendly elements, is studied. We also have an interest in chiral magnetic compounds, such as the green needle and RNi3Ga9.