Akira Oyamada

Observation of Successive Phase Transitions in Kagomé Lattice Antiferromagnets RFe3(OH)6(SO4)2[R=NH4, Na, K]

Successive magnetic phase transitions were found in the Kagome lattice antiferromagnets RFe 3 (OH) 6 (SO 4 ) 2 [ R=NH 4 , Na, K]. The temperature dependence of the susceptibility has two cusps around 60 K for each compound. The X-ray diffraction patterns and the quadrupole resonance spectra show that the transitions are not caused by a structural change. The presence of these transitions was confirmed by NMR experiments. The NMR spectra show the development of magnetic ordering below the upper transition temperature. In the low-temperature phase the nuclear magnetization does not relax exponentially, which may suggest a domain structure of 120° spin structure due to degeneracy on the Kagome lattice. The successive transitions may be caused by a small Ising-like anisotropy in the Heisenberg antiferromagnet.

Heavy-electron behavior in a low-carrier-concentration compound YbAs

Low-temperature specific heat of YbAs has been measured down to about 50 mK. In the antiferromagnetic ordered state ( T N ∼0.5 K), the nuclear contribution is observed below about 150 mK. By subtraction of this contribution from the measured specific heat, the linear coefficient of the electronic specific heat is estimated to be about 270 mJ/K 2 mole. This value is extraordinarily large because the carrier concentration in YbAs is of the order of 0.01 per formula unit. This large mass enhancement may be ascribed to a mutual 4 f -spin compensation mechanism instead of the usual Kondo spin fluctuation. We also show that the magnetic field effect on the specific heat is rather small.

Spin-liquid state in an organic spin-1/2 system on a triangular lattice, EtMe3Sb[Pd(dmit)2]2

The series of [Pd(dmit)2] salts (dmit = 1,3-dithiole-2-thione-4,5-dithiolate) is a Mott insulator on an anisotropic triangular lattice and has an antiferromagnetic exchange constant J~ 200 K. While it has been revealed that most of them undergo antiferromagnetic transitions, the nature of the spin state of EtMe3Sb[Pd(dmit)2]2 which has a nearly isotropic triangular lattice is still unknown. We investigated the spin state by means of 13C NMR measurements down to 1.37 K for polycrystalline samples in which the carbon atoms in both ends of the Pd(dmit)2 molecule were selectively enriched with the 13C isotope. Both behaviours of the spin–lattice relaxation rate and the spectra indicate no spin ordering/freezing at least down to 1.37 K, which is lower than one-hundredth of the exchange interaction. Furthermore, the spin–lattice relaxation rate does not show an exponential temperature dependence and seems to retain a finite value at low temperatures. Thus, the ground state of this system is possibly the almost-gapless spin-liquid state.

BIS and XPS study of YbP, YbAs and Yb4As3

X-ray bremsstrahlung isochromat spectra (BIS) as well as photoemission spectra (XPS) are measured for YbP, YbAs and Yb 4 As 3 . The BIS spectra have revealed clear differences in the Yb 4f and 5d states between the Yb-monopnictides and Yb 4 As 3 . Yb 4p and 5p core XPS spectra have shown the mixed valence of Yb 4 As 3 compared with the predominance of Yb 3+ in Yb-monopnictides.

de Haas-van Alphen Studies of YbAs above and below Néel Temperature

We have studied the Fermi surface and the cyclotron mass of a semimetallic heavy fermion compound YbAs both in the antiferromagnetic state and the paramagnetic state. We have found the Fermi surface of three ellipsoidal electron surfaces and one spherical hole surface, and that the shape of these Fermi surfaces is the same in both states. In addition, we have found that the temperature dependence of the dHvA oscillatory amplitudes change anomalously at the Neel temperature. This anomaly is ascribed to the change of the cyclotron mass and the Dingle temperature at the Neel temperature. To explain the large electronic specific heat coefficient, the extremely heavy hole surface with about 10 3 m 0 should exist.

Spin Gap State of S = 1/2 Heisenberg Antiferromagnet YbAl3C3

YbAl 3 C 3 has been investigated in detail, which was previously reported to show an antiferroquadrupolar (AFQ) ordering at an extremely high ordering temperature of 80 K. The present study has revealed that the specific heat, magnetic susceptibility, and electrical resistivity of the single crystalline YbAl 3 C 3 clearly exhibit the characteristic features of the first order transition around 77 K. In addition, we have discovered that LuAl 3 C 3 also exhibits a sharp peak in the specific heat around 110 K, which most likely corresponds to the 77 K transition of YbAl 3 C 3 . Therefore, the 77 K transition of YbAl 3 C 3 is considered to be irrelevant to the nature of the 4 f -electrons such as the AFQ ordering but is ascribed to a structural one. Low temperature physical properties of YbAl 3 C 3 below 77 K is indicative of a spin gap and a nonmagnetic ground state. A new model is proposed, in which the adjacent spins on the triangular lattice of Yb 3+ ions are antiferromagnetically coupled to form a dimer ...

Mixing-Typed Antiferroquadrupolar Ordering in YbSb

Specific heat and magnetization measurements have been performed with a low carrier heavy fermion compound YbSb. Successive phase transitions at 0.5 K and 5 K were observed by specific heat measurements under external magnetic fields up to 12 T. The transition temperature at 5 K increases with increasing external magnetic field. The magnetization measurements indicate the existence of field induced magnetic ordered moments at 5 K. These magnetic properties of the phase transition at 5 K are discussed in terms of a mixing-typed antiferroquadrupolar ordering model with a Γ 6 Kramers doublet as the crystalline electric field ground state.

Study of physical properties of Yb-monopnictides

Abstract Studies of magnetic and thermal properties have been carried out in ytterbium monopnictides YbX (X  N, P, As and Sb). Combining the new experimental results with other measurements, many thermal and magnetic properties were found to change regularly from YbAs to YbP and to YbN. This suggests the strength of magnetic exchange interaction increases and the strength of Kondo interaction (or the effect of magnetic polaron) decreases from YbAs to YbP and to YbN. YbSb is a very exceptional case.

Spin dynamics in classical and quantum Kagome lattice magnets studied by NMR

Jarosite, KFe 3 (OH) 6 (SO 4 ) 2 , is a typical example of a classical frustrated antiferromagnet with s = 5/2 Fe 3+ ions on the Kagome lattice, and a metallic complex, [Cu 3 (titmb) 2 (CH 3 CO 2 ) 6 ]·H 2 O, abbreviated as Cu-titmb, is considered to be a candidate material for a quantum frustrated magnet with s = 1/2 on the Kagome lattice. Jarosite has a Weiss temperature of -530 K and shows magnetic ordering below 65 K due to weak anisotropy. The magnetic structure in the ordered phase has been determined, using neutron scattering and NMR, to be a q = 0 type 120° spin structure with + chirality. The nuclear spin-lattice relaxation rate T -1 1 and the spontaneous magnetization indicate the existence of the spin wave in the ordered phase. A small number of substitutions or deficiencies induce successive phase transitions, which represent two-dimensional ordering on the Kagome planes and three-dimensional ordering between the planes. For the quantum spin magnet, Cu-titmb, the nuclear spin relaxation at low temperatures shows the existence of anomalous low-lying excited states in the magnetic field. The relaxation rates at high temperatures give much smaller exchange interaction compared to the value estimated from the specific heat. Several interesting properties, such as double peaks of the specific heat, a 1/3 magnetization plateau for the pulse field and a ferromagnetic transition at 56 mK have been reported. We review the experimental results of Cu-titmb.

Competition between Kondo and magnetic polaron states in Ce and Yb monopnictides

Abstract Recently, it has been demonstrated that the long-range Coulomb interaction is really important in the low carrier density strongly correlated systems. This is particularly clear in Ce and Yb monopnictides from both the experimental and the theoretical standpoints. As a result, a Wigner crystal liquid enhanced by a weak magnetic polaron can exist at high temperature competing with the Kondo state, but it changes to a strong magnetic polaron lattice through various steps at low temperature. Recent developments are reviewed.