A novel cuprate Volborthite, Cu 3 V 2 O 7 (OH) 2 ·2H 2 O, containing an S-1/2 (Cu 2+ spin) kagomé-like lattice is studied by magnetic susceptibility, specific heat, and 51 V NMR measurements. Signs for neither long-range order nor spin-gapped singlet ground states are detected down to 1.8 K, in spite of large antiferromagnetic couplings of ~ 100 K between Cu spins forming a two-dimensional kagomé-like network. It is suggested that Volborthite represents a system close to a quantum critical point between classical long-range ordered and quantum disordered phases. *E-mail: hiroi@issp.u-tokyo.ac.jp §1. Introduction G e o m e t r i c a l f r u s t r a t i o n i n q u a n t u m antiferromagnets (AFMs) tends to stabilize unusual ground states such as a spin glass and a spin liquid instead of classical Néel order. It occurs on various triangle-based lattices like one-dimensional (1D) trestle lattice, two-dimensional (2D) triangular and kagomé lattices, and three-dimensional B-site spinel and pyrochlore lattices.1) In order to reduce total magnetic energy for antiferromagnetically interacting Heisenberg spins on triangles, the compromise arrangement, the so-called 120º state, is realized for the 2D triangular lattice.2) In contrast, such a compromise arrangement is not stabilized for the more frustrating kagomé lattice, because there still remains a degeneracy in propagating the 120º state on a triangle plaquette to neighboring triangles due to corner-sharing.1) This local degeneracy results in a finite entropy for the classical ground state, and should be lifted by quantum fluctuations. Most theoretical studies have focused on S-1/2 Heisenberg antiferromagnets on the kagomé lattice, and it has been believed that the ground state is a spin liquid with a finite excitation energy gap ∆.3-7) However, the physical picture of the ground state as well as the nature of low-lying excitations are still questions under debate. For example, Elstner and Young 5)suggested a spin liquid consisting of short-range singlet dimer pairs with ∆ ~ 0.25 J, where J is the magnitude of pairwise antiferromagnetic (AF) couplings, while Waldtmann et al. 7) claimed a much smaller gap of 0.025 J < ∆ < 0.1 J, implying dominant longer-range correlations. They also insisted that the singlet-triplet gap is filled with nonmagnetic excitations, the origin of which is possibly related to the ground state degeneracy of the classical model.To clarify the essential feature of the kagomé AFMs, we need a real-life material on which a quasi-2D kagomé lattice is realized. Unfortunately, however, we have not yet been given such an ideal kagomé compound suitable for detailed experimental characterizations. So far well studied are a garnet compound SrCr 9-x Ga 3+x O 19 with Cr 3+ (S = 3/2) 8, 9) and the Jarosite family of minerals KM 3 (OH) 6 (SO 4 ) 2 with M = Cr 3+ or Fe 3+. [10][11][12][13] In both of them Heisenberg spins form a kagomé lattice with strong AF interactions: the Curie-Weiss constant Θ is -500 K for the former, and -67.5 K (Cr 3+ ) or -600 ...

We report the first pyrochlore oxide superconductor Cd 2 Re 2 O 7 . Resistivity, magnetic susceptibility, and specific heat measurements on single crystals evidence a bulk superconductivity at 1 K. Another phase transition found at 200 K suggests that a peculiar electronic structure lies behind the superconductivity.

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We have investigated the crystal structure and superconducting properties of thin films of FeSe 0.5 Te 0.5 grown on eight different substrates. Superconductivity is not correlated with the lattice mismatch, but rather it is correlated with the degree of in-plane orientation and with the lattice parameter ratio c/a. The best superconducting properties are observed in films on MgO and LaAlO 3 (T c zero of 9.5 K). TEM observation shows that the presence or absence of the amorphous-like layer at the substrate surface plays a key role in determining the structural and superconducting properties of the grown films.* E-mail address: imai@maeda1.c.u-tokyo.ac.jpAfter the discovery of superconductivity in F-doped LaFeAsO, 1) numerous studies on iron-based superconductors have been carried out. One common iron-based superconductor is FeSe with a superconducting transition temperature T c of 8 K, 2) and the partial substitution of Te for Se raises T c to a maximum of 14 K.3)This material has the tetragonal PbO-type structure, which is the simplest structure of all the iron-based superconductors. Thus, FeSe and related materials are considered the most suitable systems to investigate how superconductivity correlates to the crystal structures.Many studies on the film growth of FeSe 1-x Te x have already been reported. 4-11)However, the question of what substrates are suitable for the growth of thin FeSe 1-x Te x films remains controversial. For example, Kumary et al. 6) reported that the T c value of the film on SrTiO 3 (STO) was higher than on LaAlO 3 (LAO). In contrast, Han et al. 7)reported an opposite result; the T c value on LAO was higher than that on STO. In addition, Bellingeri et al. thickness were fixed at 573 K and at approximately 50 nm, respectively. The crystal structure and the orientation of the films were characterized by a θ-2θ and a 4-circle X-ray diffraction (XRD) using Cu Kα radiation at room temperature. We also performed a transmission electron microscopy (TEM) observation. The electrical resistivity (ρ) was measured by a four-terminal method from 2 to 300 K. Figure 1 shows the XRD patterns of the eight films. Except for a few unidentified peaks, only the 00l reflections of a tetragonal PbO-type structure are observed, which shows that the out-of-plane alignment is excellent. It should be noted that the c-axis orientation is observed even in the film prepared on the (0001) plane in hexagonal Al 2 O 3 , as shown in Fig. 1 (h). This indicates that the FeSe 1-x Te x films intrinsically favor two-dimensional growth. The temperature dependence of ρ is summarized in Fig. 2. As can be easily seen, the eight films exhibit a variety of ρ(T)behavior. Except for the film on Al 2 O 3 , the ρ value of these films at T = 300 K is