A spinel related oxide, Na4Ir3O8, was found to have a three dimensional network of corner shared Ir 4+ (t2g 5 ) triangles. This gives rise to an antiferromagnetically coupled S = 1/2 spin system formed on a geometrically frustrated hyperkagome lattice. Magnetization M and magnetic specific heat Cm data showed the absence of long range magnetic ordering at least down to 2 K. The large Cm at low temperatures is independent of applied magnetic field up to 12 T, in striking parallel to the behavior seen in triangular and kagome antiferromagnets reported to have a spin-liquid ground state. These results strongly suggest that the ground state of Na4Ir3O8 is a three dimensional manifestation of a spin liquid. PACS numbers: Valid PACS appear hereThe experimental realization of a quantum spin liquid in geometrically frustrated magnets has been one of the biggest challenges in the field of magnetism since Anderson proposed resonating valence bond theory [1] for antiferromagnetically coupled S = 1/2 spins on a triangular lattice. Geometrical frustration in magnets arises from the incompatibility of local spin-spin interactions, which gives rise to macroscopic degeneracy of the ground state. Possible playgrounds for this include triangular, kagome, pyrochlore and garnet lattices essentially consisting of networks of triangles. In real materials, however, it is not easy to prevent spin ordering at substantially lower temperatures than the Curie-Weiss temperature θ W . This is because the spin degeneracy can be lifted by coupling with the other degrees of freedom such as the orbitals, lattice and charges. Such an interplay between the frustrated spins, orbitals and lattice, for example, can be realized in the trimer singlet formation in the S = 1 triangular LiVO 2 [2, 3] with orbital ordering or the spin-Jahn-Teller transition in the S = 3/2 pyrochlore ZnCr 2 O 4 [4]. In addition, only a minute amount of disorder can strongly influence the spin-liquid state in geometrically frustrated magnets and may give rise to the formation of a glassy state of spins.The most likely candidate for the realization of a spinliquid ground state has been the two dimensional kagome antiferromagnet SrCr 9p Ga 12−9p O 19 (S = 3/2) [5,6]. It does not show any evidence for long range ordering down to 100 mK, and a large and field independent magnetic specific heat was observed which was ascribed to spinliquid contributions. Nevertheless, the strong spin glasslike behavior at low temperatures instills a certain ambiguity in identifying the spin-liquid state. Recently, a new generation of spin-liquid compounds has emerged, the S = 1/2 triangular magnet κ-(ET) 2 Cu 2 (CN) 3 [7], an organic Mott insulator, and the S = 1 triangular magnet NiGa 2 S 4 [8]. They were reported to have a spin-liquid ground state or at least a robust liquid phase down to 100 mK. Their magnetic and thermal properties are in striking parallel to those of SrCr 9p Ga 12−9p O 19 but the disorder effect appears to be much weaker.Here we report on a three dimensional analogue of thes...

One view of the cuprate high-transition temperature (high-T c ) superconductors is that they are conventional superconductors where the pairing occurs between weakly interacting quasiparticles, which stand in one-to-one correspondence with the electrons in ordinary metals -although the theory has to be pushed to its limit [1]. An alternative view is that the electrons organize into collective textures (e.g. charge and spin stripes) which cannot be mapped onto the electrons in ordinary metals. The phase diagram, a complex function of various parameters (temperature, doping and magnetic field), should then be approached using quantum field theories of objects such as textures and strings, rather than point-like electrons [2,3,4,5,6]. In an external magnetic field, magnetic flux penetrates type-II superconductors via vortices, each carrying one flux quantum [7]. The vortices form lattices of resistive material embedded in the non-resistive superconductor and can reveal the nature of the ground state -e.g. a conventional metal or an ordered, striped phase -which would have appeared had superconductivity not intervened. Knowledge of this ground state clearly provides the most appropriate starting point for a pairing theory. Here we report that for one high-T c superconductor, the applied field which imposes the vortex lattice, also induces antiferromagnetic order. Ordinary quasiparticle pictures cannot account for the nearly fieldindependent antiferromagnetic transition temperature revealed by our measurements.La 2-x Sr x CuO 4 , is the simplest high-T c superconductor. The undoped compound is an insulating antiferromagnet, where the spin moments on adjacent Cu 2+ ions are antiparallel [8]. Introduction of charge carriers via Sr doping reduces the ordered moment until it vanishes at x<0.13. In addition, for x>0.05 the commensurate antiferromagnetism is replaced by incommensurate order [2,3,9,10], where the repeat distance for the pattern of ordered moments is substantially larger than the spacing between neighbouring copper ions. La 2-x Sr x CuO 4 becomes a 2 superconductor for Sr dopings of 0.06

The presence or absence of a quantum critical point and its location in the phase diagram of high-temperature superconductors have been subjects of intense scrutiny. Clear evidence for quantum criticality, particularly in the transport properties, has proved elusive because the important low-temperature region is masked by the onset of superconductivity. We present measurements of the low-temperature in-plane resistivity of several highly doped La2-xSrxCuO4 single crystals in which the superconductivity had been stripped away by using high magnetic fields. In contrast to other quantum critical systems, the resistivity varies linearly with temperature over a wide doping range with a gradient that scales monotonically with the superconducting transition temperature. It is maximal at a critical doping level (pc) approximately 0.19 at which superconductivity is most robust. Moreover, its value at pc corresponds to the onset of quasi-particle incoherence along specific momentum directions, implying that the interaction that first promotes high-temperature superconductivity may ultimately destroy the very quasi-particle states involved in the superconducting pairing.

Neutron scattering is used to characterize the magnetism of the vortices for the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO4 (x = 0.163) in an applied magnetic field. As temperature is reduced, low-frequency spin fluctuations first disappear with the loss of vortex mobility, but then reappear. We find that the vortex state can be regarded as an inhomogeneous mixture of a superconducting spin fluid and a material containing a nearly ordered antiferromagnet. These experiments show that as for many other properties of cuprate superconductors, the important underlying microscopic forces are magnetic.

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 ...

High-resolution angle-resolved photoemission spectroscopy was used to study the superconducting energy gap and changes in the spectral function across the superconducting transition in the quasi-two-dimensional superconductor 2H-NbSe2. The momentum dependence of the superconducting gap was determined on different Fermi surface sheets. The results indicate Fermi surface sheet-dependent superconductivity in this low-transition temperature multiband system and provide a description consistent with thermodynamic measurements and the anomalous de Haas-van Alphen oscillations observed in the superconducting phase. The present data suggest the importance of Fermi surface sheet-dependent superconductivity in explaining exotic superconductivity in other multiband systems with complex Fermi surface topology, such as the borides and f-electron superconductors.

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Heat transport in the cuprate superconductors YBa2Cu3Oy and La2−xSrxCuO4 was measured at low temperatures as a function of doping. A residual linear term κ0/T is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from κ0/T is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component (ix) appearing at a putative quantum phase transition and argues against a non-superconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.