Effect of disorder on the quantum spin liquid candidate Na4_4Ir3_3O8_8

We report on the effects of introducing magnetic and non-magnetic disorder in the hyperkagome iridate quantum spin liquid (QSL) candidate Na$_4$Ir$_3$O$_8$ by partially replacing Ir$^{4+}$ ($S = 1/2$) with Ru$^{4+}$ ($S = 1$) or Ti$^{4+}$ ($S = 0$). Specifically, we synthesized Na$_4$(Ir$_{1-x}$Ru$_x$)$_3$O$_8 (x = 0.05, 0.10, 0.2, 0.3)$ and Na$_4$Ir$_{2.7}$Ti$_{0.3}$O$_8$ samples and measured electrical transport, AC and DC magnetization, and heat capacity down to $T = 1.8$ K. Na$_4$Ir$_3$O$_8$ is associated with a large Weiss temperature $\theta = -650$ K, a broad anomaly in magnetic heat capacity C$_{mag}$ at T $\approx25$ K, low temperature power-law heat capacity, and spin glass freezing below $T_f \approx 6$ K. We track the change in these characteristic features as Ir is partially substituted by Ru or Ti. We find that for Ru substitution, $\theta$ increases and stays negative, the anomaly in C$_{mag}$ is suppressed in magnitude and pushed to lower temperatures, low temperature $C \sim T^\alpha$ with $\alpha$ between $2$ and $3$ and decreasing towards $2$ with increasing $x$, and $T_f$ increases with increase in Ru concentration $x$. For Ti substitution we find that $\theta$ and T$_f$ become smaller and the anomaly in $C_{mag}$ is completely suppressed. In addition, introducing non-magnetic Ti leads to the creation of orphan spins which show up in the low temperature magnetic susceptibility.Comment: 6 pages, 6 figures, 2 Table

Antiferromagnetic Mott insulating state in single crystals of the hexagonal lattice material Na2IrO3

We have synthesized single crystals of Na_2IrO_3 and studied their structure, transport, magnetic, and thermal properties using powder x-ray diffraction (PXRD), electrical resistivity, isothermal magnetization M versus magnetic field H, magnetic susceptibility \chi versus temperature T, and heat capacity C versus T measurements. Na_2IrO_3 crystallizes in the monoclinic \emph{C2/c} (No. 15) type structure which is made up of Na and NaIr_2O_6 layers alternately stacked along the c axis. The \chi(T) data show Curie-Weiss behavior at high T > 200K with an effective moment \mu_eff = 1.82(1) \mu_B indicating an effective spin S_eff = 1/2 on the Ir^4+ moments. A large Weiss temperature \theta = - 116(3)K indicates substantial antiferromagnetic interactions between these S_eff = 1/2, Ir^4+ moments. Sharp anomalies in \chi(T) and C(T) data indicate that Na_2IrO_3 undergoes a transition into a long-range antiferromagnetically ordered state below T_N = 15 K. The magnetic entropy at T_N is only about 20% of what is expected for S_eff = 1/2 moment ordering. The reduced entropy and the small ratio T_N/\theta \approx 0.13 suggest geometrical magnetic frustration and/or low-dimensional magnetic interactions in Na_2IrO_3. In plane resistivity measurements show insulating behavior. This together with the local moment magnetism indicates that bulk Na_2IrO_3 is a Mott insulator.Comment: 6 figures, 1 table. This preprint replaces arXiv:1003.1055 which has been withdraw

Fragile magnetic order in the honeycomb lattice Iridate Na2_{2}IrO3_3 revealed by magnetic impurity doping

We report the structure, magnetic, and thermal property measurements on single crystalline and polycrystalline samples of Ru substituted honeycomb lattice iridate Na$_2$Ir$_x$Ru$_{1-x}$O$_3$ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5). The evolution of magnetism in Na$_2$Ir$_x$Ru$_{1-x}$O$_3$ has been studied using dc and ac magnetic susceptibility, and heat capacity measurements. The parent compound Na$_2$IrO$_3$ is a spin-orbit driven Mott insulator with magnetic order of reduced moments below $T_N = 15$~K\@. In the Ru substituted samples the antiferromagnetic long range state is replaced by a spin glass like state even for the smallest substitution suggesting that the magnetic order in Na$_2$IrO$_3$ is extremely fragile. We argue that these behaviors indicate the importance of nearest-neighbor magnetic exchange in the parent Na$_2$IrO$_3$. Additionally, all samples show insulating electrical transport.Comment: 6 pages, 6 figures, 3 table

Crystal growth and magnetic anisotropy in the spin-chain ruthenate Na2_2RuO4_4

We report single crystal growth, electrical resistivity $\rho$, anisotropic magnetic susceptibiltiy $\chi$, and heat capacity $C_p$ measurements on the one-dimensional spin-chain ruthenate Na$_2$RuO$_4$. We observe variable range hopping (VRH) behaviour in $\rho(T)$. The magnetic susceptibility with magnetic field perpendicular ($\chi_\perp$) and parallel ($\chi_\parallel$) to the spin-chains is reported. The magnetic properties are anisotropic with $\chi_\perp > \chi_\parallel$ in the temperature range of measurements T $\approx 2$ to $305$ K with $\chi_\perp / \chi_\parallel$ $\approx 1.4$ at $305$ K. Analysis of the $\chi(T)$ data reveals an anisotropy in the $g$-factor and Van-Vleck paramagnetic contribution. An anomaly in $\chi(T)$ and a corresponding lambda-like anomaly in $C_p$ at $T_N = 37$ K confirms long-range antiferromagnetic ordering. This temperature is an order of magnitude smaller than the Weiss temperature $\theta \sim -250$ K and points to suppression of long range magnetic order due to low dimensionality. However, we were unable to get a satisfactory fit of the experimental $\chi(T)$ by an isolated one-dimensional spin-chain model, suggesting the importance of inter-chain interactions in Na$_2$RuO$_4$.Comment: 5 pages, 5 figures, 1 Tabl

First order density wave like transitions in surface doped Na2_2IrO3_3

We demonstrate that the surface of the honeycomb lattice iridate Na$_2$IrO$_3$ is extremely tunable by plasma etching. We have succeeded in turning the surface of Na$_2$IrO$_3$ metallic by Ar plasma etching which leads to the removal of Na from the surface. The surface structure does not change in this process as revealed by grazing incidence small angle x-ray scattering (GISAXS). The sheet resistance $R_s$ can be reduced by several orders of magnitude by varying the etching duration. Temperature dependent $R_s(T)$ for the metallic samples show signatures of spin or charge density wave transitions with abrupt changes in $R_s$. Thermal hysteresis between cooling and warming measurements across the transition indicates a first order transition. For the most metallic sample $R_s(T)$ data at low temperatures follow a $T^2$ behaviour suggesting normal Fermi liquid behaviour.Comment: 6 pages, 2 Tables, 5 figure

Evolution of Superconductivity and Charge Density Wave Ordering in the Lu5_5Ir4_4(Si1−x_{1-x}Gex_x)10_{10} Alloy System

The compounds Lu$_5$Ir$_4$Si$_{10}$ and Lu$_5$Ir$_4$Ge$_{10}$ crystallize in the tetragonal Sc$_5$Co$_4$Si$_{10}$ type structure. Lu$_5$Ir$_4$Si$_{10}$ is known to superconduct below 3.9 K and it also exhibits a strongly coupled charge density wave (CDW) transition below 83 K. Lu$_5$Ir$_4$Ge$_{10}$ undergoes a transition into the superconducting state below about 2.4 K without any CDW transition at higher temperatures. Recent Si NMR measurements on polycrystalline samples of Lu$_5$Ir$_4$Si$_{10}$ suggest that there is no energy gap at the Si site across the CDW transition. Thus it is of interest to study the evolution of the superconductivity and the CDW transition when we dope at the Si site with small quantities of Ge. Here we present the evolution of T$_C$ and T$_{CDW}$ with concentration x of Ge in the alloy system Lu$_5$Ir$_4$(Si$_{1-x}$Ge$_x$)$_{10}$ (x=0.0, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.4, 1.0) as estimated from dc susceptibility measurements. We find that the CDW is strongly suppressed with increasing x and there is a simultaneous enhancement of the superconducting transition temperature T$_C$ from 3.9 K for the undoped sample to almost 6.6 K for only 10% concentration of Ge in the alloy.Comment: 8 pages, 2 figure

Density wave like transport anomalies in surface doped Na2_{2}IrO3_3

We report that the surface conductivity of Na$_{2}$IrO$_3$ crystal is extremely tunable by high energy Ar plasma etching and can be tuned from insulating to metallic with increasing etching time. Temperature dependent electrical transport for the metallic samples show signatures of first order phase transitions which are consistent with charge or spin density wave like phase transitions recently predicted theoretically. Additionally, grazing-incidence small-angle x-ray scattering (GISAXS) reveal that the room temperature surface structure of Na$_{2}$IrO$_3$ does not change after plasma etching.Comment: 4 pages, 3 fugures, 2 Table

Dilepton emission at temperature dependent baryonic quark-gluon plasma

A fireball of QGP is evoluted at temperature dependent chemical potential by a statistical model in the pionic medium. We study the dilepton emission rate at temperature dependent chemical potential (TDCP) from such a fireball of QGP. In this model, we take the dynamical quark mass as a finite value dependence on temparature and parametrization factor of the QGP evolution. The temperature and factor in quark mass enhance in the growth of the droplets as well as in the dilepton emission rates. The emission rate from the plasma shows dilepton spectrum in the intermediate mass region (IMR) of (1.0-4.0) GeV and its rate is observed to be a strong increasing function of the temperature dependent chemical potential for quark and antiquark annihilation.Comment: Pdf file 8 pages 8 figures. It will be appeared in proceeding of conference series of NN2012, San Antonio, US

Low-dimensional magnetism in the mixed Cr valence spin-chain compound Sr_4Cr_3O_9

Sr_4Cr_3O_9 is the n = 2 member of a family of quasi-one-dimensional compounds A_{n+2}T_{n+1}O_{3n+3} (A = Ca, Sr, or Ba, T = transition metal, and n = 1 to \infty) having a crystal structure which consists of chains of n TO_6 octahedra alternated by one TO_6 trigonal prism running along the c-axis. The chains are arranged on a triangular lattice in the ab-plane. We report the synthesis, structure, magnetization M versus magnetic field H, magnetic susceptibility \chi versus temperature T and specific heat C versus T measurements on sintered and arc-melted polycrystalline samples of Sr_4Cr_3O_9. The \chi data have a T dependence which is typical of low-dimensional magnetic systems with dominant antiferromagnetic (AF) exchange interactions. Specifically, \chi(T) shows a broad maximum at T_{max} \approx 200K for the sintered pellet and T_{max}\approx 265K for the arc-melted sample, indicating the onset of short-range magnetic order. Below T = 15K we observe 2 anomalies in the \chi(T) data for both samples suggesting the onset of long-range magnetic ordering. The corresponding anomalies in the C(T) data however are weak indicating that only a small amount of the expected magnetic entropy is recovered at the magnetic transitions and that strong short-range AF order exists above these temperatures.Comment: 6 pages, 7 figure

Acceptance sampling plans for inverse Weibull distribution based on truncated life test

In this paper, we develop double acceptance sampling plan and group acceptance sampling plan for an inverse Weibull distribution based on a truncated life test. We consider the median lifetime of the test units as a quality parameter and obtain the design parameters such as sample size and acceptance number. These plans are obtained under the consumer's risk and the producer's risk simultaneously involved at a certain confidence level. We present a simulation study to support the proposed methods and a comparison between single and double acceptance sampling plans is made. A real data set is also analyzed to illustrate the implementation of the proposed sampling plans. Further, the situation under which the proposed samplings plans can also be used for other percentiles points is discussed. Finally a conclusion is presented.Comment: 14 pages, 8 tables, 1 real data se