We report magnetization and ac susceptibility of single crystals of the spin-ice compound Dy 2 Ti 2 O 7. Saturated moments at 1.8 K along the characteristic axes ͓100͔ and ͓110͔ agree with the expected values for an effective ferromagnetic nearest-neighbor Ising pyrochlore with local ͗111͘ anisotropy, where each magnetic moment is constrained to obey the ''ice rule.'' At high enough magnetic fields along the ͓111͔ axis, the saturated moment exhibits a breaking of the ice rule; it agrees with the value expected for a three-in, one-out spin configuration. Assuming the realistic magnetic interaction between Dy ions given by the dipolar spin ice model, we completely reproduce the results at 2 K by Monte Carlo calculations. However, down to at least 60 mK, we have not found any experimental evidence of the long-range magnetic ordering predicted by this model to occur at around 180 mK. Instead, we confirm the spin freezing of the system below 0.5 K.

We report the specific heat and entropy of single crystals of the spin ice compound Dy 2 Ti 2 O 7 at temperatures down to 0.35 K. We apply magnetic fields along the four characteristic directions ͓100͔, ͓110͔, ͓111͔, and ͓112͔. Because of Ising anisotropy, we observe anisotropic release of the residual zero-point entropy, attributable to the difference in frustration dimensionality. In the high magnetic field along these four directions, the residual entropy is almost fully released and the activation entropy reaches R ln2. However, in the intermediate-field region, the entropy in the fields along the ͓111͔ direction is different from those for the other three field directions. For the ͓111͔ direction, the frustration structure changes from that of the three-dimensional pyrochlore to that of a two-dimensional Kagome-like lattice with constraint due to the ice rule, leading to different values of the zero-point entropy.Recently, there has been increasing attention in the physics of geometrical frustration in pyrochlore oxides A 2 B 2 O 7 . The spin ice behavior 1 has been observed in Ho 2 Ti 2 O 7 , 2-6 Dy 2 Ti 2 O 7 , 7-11 and Ho 2 Sn 2 O 7 . 12 Since there is no long-range magnetic ordering of the rare-earth moments in these materials at least down to 50 mK, 2,8 the ground state is believed to be macroscopically degenerate. In fact, the observed residual entropy 7,13 shows an excellent agreement with the expected zero-point entropy of (1/2)R ln(3/2). 14 In these materials, the A-site ions constitute a threedimensional ͑3D͒ network of corner-shared tetrahedra ͑the pyrochlore lattice͒. Because of the crystal-field effect, the magnetic moments of the A-site ions, such as Dy 3ϩ and Ho 3ϩ , have Ising anisotropy along the local ͗111͘ direction, which points to the center of the tetrahedron from a vertex. Owing to this Ising anisotropy, the spin responses to magnetic fields are very anisotropic. In measurements on polycrystalline samples, there are specific-heat peaks at fieldindependent temperatures of 0.34 K, 0.47 K, and 1.12 K; it was speculated that these peaks may be due to ordering of spins with their Ising axes perpendicular to the field. 7 Moreover, owing to the difference in the spin configurations in fields along different directions, the process of releasing the residual zero-point entropy should be qualitatively different reflecting the structure of frustration. However, to date there are few studies of the entropy release using single crystals. 13,15,16 In this paper, we report the specific heat and entropy of single crystalline Dy 2 Ti 2 O 7 in magnetic fields along four characteristic directions ͓100͔, ͓110͔, ͓111͔, and ͓112͔. To the best of our knowledge, relevant data for the ͓110͔ and ͓112͔ directions have not been published previously. We indeed observed an anisotropic process of releasing residual entropy for fields along different directions. We will discuss this anisotropic behavior in terms of a qualitative difference in the dimensionality of frustration structures.The single crystals of Dy 2 Ti...

We report the magnetodielectric response of single crystals of the spin-ice compound Dy 2 Ti 2 O 7 down to 0.26 K. The dielectric constant under zero magnetic field exhibits a clear decrease reflecting the development of the local two-spins-in, two-spins-out structure below about 1.2 K. Both the real and imaginary parts of the dielectric constant under magnetic fields sensitively respond to various changes in the spin structures. We found that the real part can be described in terms of local spin correlations among the moments of tetrahedra, rather than among individual Dy 3+ moments. Using the peaks in the imaginary part, we have constructed a precise field-temperature phase diagram in the ͓111͔ field direction. We thus demonstrate that the magnetodielectric response can be a high-sensitivity local probe of the spin state of geometrically frustrated systems.

We report the specific heat of single crystals of the spin ice compound Dy 2 Ti 2 O 7 at temperatures down to 100 mK in the so-called Kagome ice state. In our previous paper, we showed the anisotropic release of residual entropy in different magnetic field directions and reported new residual entropy associated with spin frustration in the Kagome slab for field in the [111] direction. In this paper, we confirm the first-order phase transition line in the field-temperature phase diagram and the presence of a critical point at (0.98 T, 400 mK), previously reported from the magnetization and specific-heat data. We newly found another peak in the specific heat at 1.25 T below 0.3 K. One possible explanation for the state between 1 T and 1.25 T is the coexistence of states with different spin configurations including the 2-in 2-out one (Kagome ice state), the 1-in 3-out state (ordered state) and paramagnetic one (free-spin state).

We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.Since the discovery of Bi4O4S3 and REO1-xFxBiS2 (RE: rare earth) superconductors, BiCh2-based (Ch: chalcogen) superconductors have been drawing much attention as a new class of layered superconductors [1][2][3]. Since the crystal structure composed of alternate stacks of the electrically conducting BiCh2 layers and the insulating (blocking) layers resembles those of the Cuprate and the FeAs-based high-transition-temperature (high-Tc) superconductors [4,5], many experimental and theoretical studies have been performed to clarify the superconductivity mechanisms of this system and to increase Tc. However, the mechanisms have not been understood completely. Recently, Morice et al. proposed that a 2 weak-coupling electron-phonon mechanism cannot explain the emerging Tc, as high as 11 K, in the BiCh2-based systems, from first principle calculations [6]. Therefore, full understandings of the basic characteristics of the superconductivity in the BiCh2-based system are crucial.The parent phase of the BiCh2-based superconductor is a band insulator [1,7,8]. On the basis of the calculated band structure, electrons carriers are doped into the bands mainly composed of Bi-6px and Bi-6py components. Electron-doped BiCh2-based compounds are expected to become metallic. Indeed, superconductivity is experimentally observed in electron-doped compounds [1][2][3]. However, the real situations are not simple as expected from the band structure. Although the superconductivity in BiCh2-based compounds is emerged by carrier doping, metallic transport is sometimes absent, and weakly localized behavior (semiconducting-like behavior) is observed in electrical resistivity measurements; a good example would be optimally doped LaO0.5F0.5BiS2. In semiconducting-like samples of LaO0.5F0.5BiS2, bulk superconductivity is not observed, while weak (filamentary) superconductivity is observed. To induce bulk superconductivity in the LaO0.5F0.5BiS2 system, external pressure effects [9][10][11][12][13][14][15][16] and/or element substitution at the La site [17][18][19][20], which optimize the crystal structure, are available. Namely, both electron carrier doping and crystal structure optimization are required for the e...