We performed ultrasonic measurements on the unconventional superconductor Sr_2RuO_4 to investigate the dynamical properties of the electronic states near its superconducting transition temperature, T_c = 1.4 K. We observed an increase in the in-plane transverse ultrasonic attenuation coefficient as the temperature approached T_c. The ultrasonic attenuation exhibited a Landau-Khalatnikov-type ultrasonic frequency dependence with a typical relaxation time of approximately 10^-10 s. Under an applied magnetic field of 10 T, the superconducting transition was suppressed. However, the ultrasonic attenuation coefficient exhibited an increase down to low temperatures, indicating the slowing down of fluctuations associated with multipole degrees of freedom. Based on group-theoretical considerations, we propose that the electric hexadecapole plays a crucial role in the slowing down. Furthermore, we discuss the relationship between multi-component superconducting order parameters and multipole degrees of freedom.

We performed high-field ultrasonic measurements on LaTIn_5 (T = Co, Rh, Ir) to reveal the origin of the small Fermi surface that was recently observed in LaRhIn_5 with an oscillation frequency of 6.8 T. We observed quantum oscillations originating from this Fermi surface in LaRhIn_5. In addition, we revealed that LaCoIn_5 and LaIrIn_5 exhibit quantum osciilations with frequencies below 100 T, indicating hidden Fermi surfaces in these compounds. Furthermore, Co-substituted LaRhIn_5 exhibited quantum oscillations with a frequency of 10 T. Our results suggest that the small Fermi surface originates from bulk properties and that 3d electrons of the transition metal contribute to its formation.

We grew single crystals of an antiferromagnet GdAl(3 )with a hexagonal structure and antiferromagnets GdCu2Si2, GdGa6, and GdIrSi(3)with tetragonal structures. Their magnetic properties were investigated by measuring the electrical resistivity, specific heat, magnetic susceptibility, magnetization, and elastic constants, together with the Hall resistivity and magnetoresistance. Among these Gd antiferromagnets, GdAl(3 )was found to be a unique compound, exhibiting the fi first-order antiferromagnetic transition at the N & eacute;el temperature T-N = 17.6 K and the vertical phase boundary near T-N in the H H- H-T phase diagram. We also found an interesting magnetic phase in the antiferromagnetic state of GdAl(3 )for the magnetic field H || [1120]. This intermediate phase exists between two metamagnetic transitions, but the two transitions merge into a single metamagnetic transition with increasing temperature. A similar intermediate magnetic phase was also found for H||[100] of GdIrSi3 . For GdCu2Si2, we also confirmed that an intermediate magnetic phase exists between H Hm1 = 1.6 T and H Hm2 = 2.3 T at 2 K from peak structures in the Hall resistivity and magnetoresistance for H||[100]. These Gd compounds possess the magnetic easy-axes of H|| [ 1120] or [100], perpendicular to the c-axes ([0001] and [001]). In contrast, the magnetic easy-axis in GdGa(6 )is along the [001] direction, indicating only a single metamagnetic transition at H-m = 1.2 T at 1.4 K.

This study reveals a novel phenomenon demonstrating the softening of synthetic diamonds when cooled to very low temperatures below 1 K. Herein, we argue that this elastic softening can be attributed to the effect of electric-quadrupole degrees of freedom of the dangling bonds in the neutral single-atom vacancies of carbon. We present the results of ultrasonic investigations of single-crystalline synthetic diamonds, namely type-IIa (colorless) and Ib (yellow) diamonds grown by high-pressure-high-temperature synthesis as well as type-IIa diamond grown by chemical vapor deposition. We observe a magnetic-field-insensitive softening of the elastic constant C_44 in all samples at low temperatures below 1 K. Our results strongly suggest a ppb level concentration of neutral single-atomic vacancies in all investigated diamonds. Our findings open new avenues for the quantitative determination of single neutral vacancies in non-irradiated diamonds, an important information needed for their potential application for quantum technology and next-generation semiconductor devices.

FeTiO3 is an ilmenite antiferromagnetic insulator containing Fe2+, with two-dimensional ferromagnetic honeycomb layers antiferromagnetically stacked along the c-axis. The magnetic Bragg peak intensity is found to be enhanced under the application of ultrasound up to 300% in FeTiO3 crystals at low temperatures. The pronounced enhancement is attributed to strong spin-lattice coupling of Fe2+ in FeTiO3. This effect disappears above 35 K, suggesting that the energy splitting of Fe2+ levels induced by spin-orbit coupling is about 35 K. This finding suggests a promising pathway toward high efficiency acoustic spin pumping.

The magnon excitation by ultrasound injection in Y3Fe5O12 is studied by inelastic neutron scattering. Both longitudinal and transverse ultrasound injections enhanced the inelastic neutron scattering intensity. We analyzed the nonequilibrium magnon steady state using the effective magnon temperature model. The large deviation of the effective magnon temperature from the sample temperature is observed at the ultrasound longitudinal mode along [0, 0, 1] at ∼10 K and [1, 1, 1] at ∼140 K. This dependence suggests that the nonequilibrium steady state can be achieved only by strong spin–lattice coupling in Y3Fe5O12. The spin–lattice coupling exhibits a decrease largely above 100 K, indicating that it could be the main origin of the degradation of longitudinal spin Seebeck effect at the temperature range.

We have performed ultrasonic experiments on the iron pnictide superconductor Ba(Fe1-xCox)2As2 for x = 0.056 and 0.057 to investigate the criticality originating from degenerate orbitals at the ferro-quadrupole (FQ) quantum critical point (QCP). At this point, the FQ ordering that causes a structural phase transition disappears, and the superconducting transition temperature Tsc reaches its maximum. From analyzing the temperature dependence of the elastic soft mode C66, we found that the Jahn-Teller energy Delta Q and Weiss temperature Theta Q near the QCP are larger than those of samples away from the QCP. This indicates a significant increase in the quadrupole-strain interaction HQS, and in the quadrupole interaction HQQ obtained from the canonical transformation of HQS. Additionally, the ultrasonic attenuation coefficient alpha 66 of the hexadecapole susceptibility for x = 0.056 diverges toward the superconducting transition temperature Tsc = 24.8 K with decreasing temperature. We discovered that the temperature dependence of the relaxation time tau obtained from alpha 66 was fitted by tau proportional to divide T - Tc divide -z nu with the unconventional critical exponent z nu = 3 near the QCP. This result is significantly different from the critical exponent z nu = 1 in the mean-field approximation observed in x = 0.071. These observations of Delta Q, Theta Q, and z nu suggest that the quantum fluctuations originating from a non-Kramers doublet with two quadrupoles Ox,y, and Ox'2 _ y'2 and one orbital angular momentum lz can be enhanced near the QCP, where Tsc is at a maximum. The attractive interaction mediated by the order parameter fluctuations that restore the symmetry breaking of the structural phase transition probably plays a key role in the superconductivity.

Ultrasound injection effect on a magnetic Bragg peak of yttrium iron garnet has been studied by quasielastic neutron scattering. The magnetic Bragg peak is vastly enhanced with decreasing temperature. The energy width increases proportionally to the square root of the sample temperature increase induced by the ultrasound injection. Based on a liquid model, the estimated effective mass becomes light when magnetic domain walls are removed under a magnetic field. Because the magnetic Bragg peak is enhanced by the lattice vibration, the enhancement is expected to closely relate to the spin-lattice coupling. The sharp drop is observed above 100 K for the longitudinal mode, indicating the degradation of the spin-lattice coupling. It is consistent with the suppression of the spin Seebeck effect when the temperature rises above 100 K, demonstrating the spin-lattice coupling as the degradation mechanism.

Using inelastic neutron scattering, we investigate the spin wave excitations on the antiferromagnetic MnTiO3 (TN = 65 K), which has the stacked honeycomb structure. At T = 2 K, the spin wave energy...

Ultralow-lying magnon energy spectrum in yttrium iron garnet (YIG) has been studied by inelastic neutron scattering in an energy range from 10 to 45 mu eV. When a magnetic field of approximately 0.1 T was applied along [111] direction, ultralow-energy magnon anomaly was found at 10 K, suggesting the closure of the Zeeman energy gap. The anomaly was also observed in the temperature dependence of magnetization under a magnetic field along the [111] direction below 30 K. The specific heat capacity confirms the closure of the Zeeman energy gap at the magnetic field direction. All these anomalies strongly support the magnetic crossover below 30 K under the magnetic field along [111], in addition to a precursor anomaly below 150 K.

The nuclear and magnetic structure and full magnon dispersions of yttrium iron garnet Y3Fe5O12 have been studied using neutron scattering. The refined nuclear structure is distorted to a trigonal space group of R (3) over bar. The highest-energy dispersion extends up to 86 meV. The observed dispersions are reproduced by a simple model with three nearest-neighbor-exchange integrals between 16a (octahedral) and 24d (tetrahedral) sites, J(aa), J(ad), and J(dd), which are estimated to be 0.00 +/- 0.05, -2.90 +/- 0.07, and -0.35 +/- 0.08 meV, respectively. The lowest-energy dispersion below 14 meV exhibits a quadratic dispersion as expected from ferromagnetic magnons. The imaginary part of q-integrated dynamical spin susceptibility chi ''(E) exhibits a square-root energy dependence at low energies. The magnon density of state is estimated from chi ''(E) obtained on an absolute scale. The value is consistent with the single chirality mode for the magnon branch expected theoretically.

In Pr3Pd20Ge6, the Pr ions are located at two different crystallographic sites, 4a and 8c site. Antiferro-quadrupole ordering (AFQ) of the 8c site occurs at 250 mK. Ac susceptibility measurement indicated that antiferromagnetic ordering (AFM) of the 4a site and Hyperfine-enhanced Pr nuclear magnetic ordering of the 8c site occur at 77 and 9 mK, respectively. To clarify the magnetic and quadrupole properties of Pr3Pd20Ge6, thermal expansion and magnetostriction measurements on single crystal sample were carried out along the [001] direction up to 8 T down to 500 μK using a capacitive dilatometer. In zero field, relative length change ΔL/L in [001] direction had a dip at AFQ and abrupt decrease at AFM ordering. From thermal expansion and isothermal magnetostriction measurements, magnetic phase diagram of Pr3Pd20Ge6 along [001] direction was obtained.

Shin-ichi Shamoto, ∗ Takashi U. Ito, Hiroaki Onishi, Hiroki Yamauchi, Yasuhiro Inamura, Masato Matsuura, Mitsuhiro Akatsu, Katsuaki Kodama, Akiko Nakao, Taketo Moyoshi, Koji Munakata, Takashi Ohhara, Mitsutaka Nakamura, Seiko Ohira-Kawamura, Yuichi Nemoto, and Kaoru Shibata Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai, Naka, Ibaraki 319-1195, Japan Materials Sciences Research Center, Japan Atomic Energy Agency (JAEA), Tokai, Naka, Ibaraki 319-1195, Japan J-PARC Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society (CROSS), Tokai, Naka, Ibaraki 319-1106, Japan Dept. of Phys, Niigata Univ., Niigata, Niigata 950-2181, Japan Grad. Sch. of Sci. Tech. Niigata Univ., Niigata, Niigata 950-2181, Japan (Dated: September 8, 2018)

The nuclear and magnetic structure and full magnon dispersions of yttrium iron garnet Y$_3$Fe$_5$O$_{12}$ have been studied by neutron scattering. The refined nuclear structure is distorted to a trigonal space group of $R\bar{3}$. The highest-energy dispersion extends up to 86 meV. The observed dispersions are reproduced by a simple model with three nearest-neighbor-exchange integrals between 16$a$ (octahedral) and 24$d$ (tetrahedral) sites, $J_{aa}$, $J_{ad}$, and $J_{dd}$, which are estimated to be 0.00$\pm$0.05, $-$2.90$\pm$0.07, and $-$0.35$\pm$0.08 meV, respectively. The lowest-energy dispersion below 14 meV exhibits a quadratic dispersion as expected from ferromagnetic magnons. The imaginary part of $q$-integrated dynamical spin susceptibility $\chi$($E$) exhibits a square-root energy-dependence in the low energies. The magnon density of state is estimated from the $\chi$($E$) obtained on an absolute scale. The value is consistent with a single polarization mode for the magnon branch expected theoretically.