Possible spin-orbit driven spin-liquid ground state in the double perovskite phase of Ba3YIr2O9

We report the structural transformation of hexagonal Ba3YIr2O9 to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8GPa at 1273K. While the ambient pressure (AP) synthesized sample undergoes long-range magnetic ordering at 4K, the high pressure(HP) synthesized sample does not order down to 2K as evidenced from our susceptibility, heat capacity and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form gamma*T+beta*T3 in the temperature (T) range of 2-10K with the Sommerfeld coefficient gamma=10mJ/mol-Ir K2. The 89Y NMR shift has no T-dependence in the range of 4-120K and its spin-lattice relaxation rate varies linearly with T in the range of 8-45K (above which it is T-independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a 5d based, gapless, quantum spin-liquid (QSL) in the cubic (HP) phase of Ba3YIr2O9. In this picture, the T term in the heat capacity and the linear variation of 89Y 1/T1 arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion [G. Chen, R. Pereira, and L. Balents, Phys. Rev. B 82, 174440 (2010)] that strong spin-orbit coupling can enhance quantum fluctuations and lead to a QSL state in the double perovskite lattice.Comment: 6 pages 5 figure

Correlated decay of triplet excitations in the Shastry-Sutherland compound SrCu2_2(BO3_3)2_2

The temperature dependence of the gapped triplet excitations (triplons) in the 2D Shastry-Sutherland quantum magnet SrCu$_2$(BO$_3$)$_2$ is studied by means of inelastic neutron scattering. The excitation amplitude rapidly decreases as a function of temperature while the integrated spectral weight can be explained by an isolated dimer model up to 10~K. Analyzing this anomalous spectral line-shape in terms of damped harmonic oscillators shows that the observed damping is due to a two-component process: one component remains sharp and resolution limited while the second broadens. We explain the underlying mechanism through a simple yet quantitatively accurate model of correlated decay of triplons: an excited triplon is long-lived if no thermally populated triplons are near-by but decays quickly if there are. The phenomenon is a direct consequence of frustration induced triplon localization in the Shastry--Sutherland lattice.Comment: 5 pages, 4 figure

Site dilution of quantum spins in the honeycomb lattice

We discuss the effect of site dilution on both the magnetization and the density of states of quantum spins in the honeycomb lattice, described by the antiferromagnetic Heisenberg spin-S model. For this purpose a real-space Bogoliubov-Valatin transformation is used. In this work we show that for the S>1/2 the system can be analyzed in terms of linear spin wave theory. For spin S=1/2, however, the linear spin wave approximation breaks down. In this case, we have studied the effect of dilution on the staggered magnetization using the Stochastic Series Expansion Monte Carlo method. Two main results are to be stressed from the Monte Carlo method: (i) a better value for the staggered magnetization of the undiluted system, m=0.2677(6); (ii) a finite value of the staggered magnetization of the percolating cluster at the classical percolation threshold, showing that there is no quantum critical transition driven by dilution in the Heisenberg model. In the solution of the problem using linear the spin wave method we pay special attention to the presence of zero energy modes. Using a combination of linear spin wave analysis and the recursion method we were able to obtain the thermodynamic limit behavior of the density of states for both the square and the honeycomb lattices. We have used both the staggered magnetization and the density of states to analyze neutron scattering experiments and Neel temperature measurements on quasi-two- -dimensional honeycomb systems. Our results are in quantitative agreement with experimental results on Mn_pZn_{1-p}PS_3 and on the Ba(Ni_pMg_{1-p})_2V_2O_8.Comment: 21 pages (REVTEX), 16 figure

Magnetic-field-induced spin excitations and renormalized spin gap of the underdoped superconductor La1.895_{1.895}Sr0.105_{0.105}CuO4_{4}

High-resolution neutron inelastic scattering experiments in applied magnetic fields have been performed on La$_{1.895}$Sr$_{0.105}$CuO$_{4}$ (LSCO). In zero field, the temperature dependence of the low-energy peak intensity at the incommensurate momentum-transfer $\mathbf{Q}^{\ }_{\mathrm{IC}}=(0.5,0.5\pm\delta,0),(0.5\pm\delta,0.5,0)$exhibits an anomaly at the superconducting$T^{\}_{c}$ which broadens and shifts to lower temperature upon the application of a magnetic field along the c-axis. A field-induced enhancement of the spectral weight is observed, but only at finite energy transfers and in an intermediate temperature range. These observations establish the opening of a strongly downward renormalized spin gap in the underdoped regime of LSCO. This behavior contrasts with the observed doping dependence of most electronic energy features.Comment: accepted for publication in Phys. Rev. Let

Orbital occupancies and the putative jeff = 1/2 groundstate in Ba2IrO4: a combined oxygen K edge XAS and RIXS study

The nature of the electronic groundstate of Ba2IrO4 has been addressed using soft X-ray absorption and inelastic scattering techniques in the vicinity of the oxygen K edge. From the polarization and angular dependence of XAS we deduce an approximately equal superposition of xy, yz and zx Ir4+ 5d orbitals. By combining the measured orbital occupancies, with the value of the spin-orbit coupling provided by RIXS, we estimate the crystal field splitting associated with the tetragonal distortion of the IrO6 octahedra to be small, \Delta=50(50) meV. We thus conclude definitively that Ba2IrO4 is a close realization of a spin-orbit Mott insulator with a jeff = 1/2 groundstate, thereby overcoming ambiguities in this assignment associated with the interpretation of X-ray resonant scattering experiments.Comment: 5 pages, 5 figure

Spin excitations in a single La2_2CuO4_4 layer

The dynamics of S=1/2 quantum spins on a 2D square lattice lie at the heart of the mystery of the cuprates \cite{Hayden2004,Vignolle2007,Li2010,LeTacon2011,Coldea2001,Headings2010,Braicovich2010}. In bulk cuprates such as \LCO{}, the presence of a weak interlayer coupling stabilizes 3D N\'{e}el order up to high temperatures. In a truly 2D system however, thermal spin fluctuations melt long range order at any finite temperature \cite{Mermin1966}. Further, quantum spin fluctuations transfer magnetic spectral weight out of a well-defined magnon excitation into a magnetic continuum, the nature of which remains controversial \cite{Sandvik2001,Ho2001,Christensen2007,Headings2010}. Here, we measure the spin response of \emph{isolated one-unit-cell thick layers} of \LCO{}. We show that coherent magnons persist even in a single layer of \LCO{} despite the loss of magnetic order, with no evidence for resonating valence bond (RVB)-like spin correlations \cite{Anderson1987,Hsu1990,Christensen2007}. Thus these excitations are well described by linear spin wave theory (LSWT). We also observe a high-energy magnetic continuum in the isotropic magnetic response. This high-energy continuum is not well described by 2 magnon LSWT, or indeed any existing theories.Comment: Revised version to appear in Nature Materials; 6 pages,4 figure

GUP1 and its close homologue GUP2, encoding multi-membrane-spanning proteins involved in active glycerol uptake in Saccharomyces cerevisiae

Many yeast species can utilise glycerol, both as sole carbon source and as an osmolyte. In Saccharomyces cerevisiae, physiological studies have previously shown the presence of an active uptake system driven by electrogenic proton symport. We have used transposon mutagenesis to isolate mutants affected in the transport of glycerol into the cell. Here we present the identification of YGL084c, encoding a multi-membrane-spanning protein, as being essential for proton symport of glycerol into Saccharomyces cerevisiae. The gene is named GUP1 (Glycerol UPtake) and is important for growth on glycerol as carbon and energy source, as well as for osmotic protection by added glycerol, of a strain deficient in glycerol production. Another ORF, YPL189w, presenting a high degree of homology to YGL084c, similarly appears to be involved in active glycerol uptake in salt-containing glucose-based media in strains deficient in glycerol production. Analogously, this gene is named GUP2. To our knowledge, this is the first report on a gene product involved in active transport of glycerol in yeasts. Mutations with the same phenotypes occurred in two other open reading frames of previously unknown function, YDL074c and YPL180w.Comunidade Europeia (CE) - contract BIO4-CT95-0161

Deficiency of Pkc1 activity affects glycerol metabolism in Saccharomyces cerevisiae

In pressProtein kinase C is apparently involved in the control of many cellular systems: the cell wall integrity pathway, the synthesis of ribosomes, the appropriated reallocation of transcription factors under specific stress conditions and also the regulation of N-glycosylation activity. All these observations suggest the existence of additional targets not yet identified. In the context of the control of carbon metabolism, previous data demonstrated that Pkc1 p might play a central role in the control of cellular growth and metabolism in yeast. In particular, it has been suggested that it might be involved in the derepression of genes under glucose-repression by driving an appropriated subcellular localization of transcriptional factors, such as Mig1 p. In this work, we show that pkc1∆ mutant is unable to grow on glycerol because it cannot perform the derepression of GUT1 gene that encodes for glycerol kinase. Additionally, active transport is also partially affected. Using this phenotype, we were able to isolate a new pkc1∆ revertant. We also isolated two transformants identified as the nuclear exportin Msn5 and the histone deacetylase Hos2 extragenic suppressors of this mutation. Based on these results, we postulate that Pkc1 p may be involved in the control of the cellular localization and/or regulation of the activity of nuclear proteins implicated in gene expression.Fundação Universidade Federal de Ouro Preto (FUFOP). Fundação de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG) - CBS-1875/95. Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) - 300998/89-9 to R.L.B., 301255/01-6 to L.G.F