Nickel hydroxide electrode. 3: Thermogravimetric investigations of nickel (II) hydroxides

Water contained in Ni hydroxide influences its electrochemical reactivity. The water content of alpha and beta Ni hydroxides is different with respect to the amount and bond strength. Thermogravimetric experiments show that the water of the beta Ni hydroxides exceeding the stoichiometric composition is completely removed at 160 deg. The water contained in the interlayers of the beta hydroxide, however, is removed only at higher temperatures, together with the water originating from the decomposition of the hydroxide. These differences are attributed to the formation of II bonds within the interlayers and between interlayers and adjacent main layers. An attempt is made to explain the relations between water content and the oxidizability of the Ni hydroxides

Magnetic anomalies in single crystalline ErPd2Si2

Considering certain interesting features in the previously reported 166Er Moessbauer effect and neutron diffraction data on the polycrystalline form of ErPd2Si2 crystallizing in ThCr2Si2-type tetragonal structure, we have carried out magnetic measurements (1.8 to 300 K) on the single crystalline form of this compound. We observe significant anisotropy in the absolute values of magnetization (indicating that the easy axis is c-axis) as well as in the features due to magnetic ordering in the plot of magnetic susceptibility (chi) versus temperature (T) at low temperatures. The chi(T) data reveal that there is a pseudo-low dimensional magnetic order setting in at 4.8 K, with a three-dimensional antiferromagnetic ordering setting in at a lower temperature (3.8 K). A new finding in the chi(T) data is that, for H//, but not for H//, there is a broad shoulder in the range 8-20 K, indicative of the existence of magnetic correlations above 5 K as well, which could be related to the previously reported slow-relaxation-dominated Moessbauer spectra. Interestingly, the temperature coefficient of electrical resistivity is found to be isotropic; no feature due to magnetic ordering could be detected in the electrical resistivity data at low temperatures, which is attributed to magnetic Brillioun-zone boundary gap effects. The results reveal complex nature of the magnetism of this compound

Determining the underlying Fermi surface of strongly correlated superconductors

The notion of a Fermi surface (FS) is one of the most ingenious concepts developed by solid state physicists during the past century. It plays a central role in our understanding of interacting electron systems. Extraordinary efforts have been undertaken, both by experiment and by theory, to reveal the FS of the high temperature superconductors (HTSC), the most prominent strongly correlated superconductors. Here, we discuss some of the prevalent methods used to determine the FS and show that they lead generally to erroneous results close to half filling and at low temperatures, due to the large superconducting gap (pseudogap) below (above) the superconducting transition temperature. Our findings provide a perspective on the interplay between strong correlations and superconductivity and highlight the importance of strong coupling theories for the characterization as well as the determination of the underlying FS in ARPES experiments

Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study

Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The formation of magnetic clusters results in low-energy optical spectral weight shifts, which correspond to opening of the pseudogap in the conduction band of the itinerant charge carriers and development of the low- and high-spin intersite electronic transitions. This phenomenon, driven by self-trapping of electrons by magnetic fluctuations, could be common in correlated metals, including besides Kondo-lattice metals, Fe-based and cuprate superconductors.Comment: 30 pages, 6 Figure

Ni impurity induced enhancement of the pseudogap in cuprate high T_c superconductors

The influence of magnetic Ni and non-magnetic Zn impurities on the normal state pseudogap (PG) in the c-axis optical conductivity of NdBa$_{2}$\{Cu$_{1-y}$(Ni,Zn)$_{y}\}_{3}$O$_{7-\delta}$ crystals was studied by spectral ellipsometry. We find that these impurities which strongly suppress superconductivity have a profoundly different impact on the PG. Zn gives rise to a gradual and inhomogeneous PG suppression while Ni strongly enhances the PG. Our results challenge theories that relate the PG either to precursor superconductivity or to other phases with exotic order parameters, such as flux phase or d-density wave states, that should be suppressed by potential scattering. The apparent difference between magnetic and non-magnetic impurities instead points towards an important role of magnetic correlations in the PG state.Comment: 11 pages and 2 figure

Proposal for an Experiment to Test a Theory of High Temperature Superconductors

A theory for the phenomena observed in Copper-Oxide based high temperature superconducting materials derives an elusive time-reversal and rotational symmetry breaking order parameter for the observed pseudogap phase ending at a quantum-critical point near the composition for the highest $T_c$. An experiment is proposed to observe such a symmetry breaking. It is shown that Angle-resolved Photoemission yields a current density which is different for left and right circularly polarized photons. The magnitude of the effect and its momentum dependence is estimated. Barring the presence of domains of the predicted phase an asymmetry of about 0.1 is predicted at low temperatures in moderately underdoped samples.Comment: latex, 2 figure

Instability of a Landau Fermi liquid as the Mott insulator is approached

We examine a two-dimensional Fermi liquid with a Fermi surface which touches the Umklapp surface first at the 4 points $(\pm \pi/2, \pm \pi/2)$ as the electron density is increased. Umklapp processes at the 4 patches near $(\pm \pi/2, \pm\pi/2)$ lead the renormalization group equations to scale to strong coupling resembling the behavior of a 2-leg ladder at half-filling. The incompressible character of the fixed point causes a breakdown of Landau theory at these patches. A further increase in density spreads the incompressible regions so that the open Fermi surface shrinks to 4 disconnected segments. This non-Landau state, in which parts of the Fermi surface are truncated to form an insulating spin liquid, has many features in common with phenomenological models recently proposed for the cuprate superconductors.Comment: Minor changes. LaTeX2e, 12 pages, 5 figures. J. Phys. CM 10 (1998) L38

Transport properties in the d-density wave state: Wiedemann-Franz law

We study the Wiedemann-Franz (WF) law in the d-density wave (DDW) model. Even though the opening of the DDW gap $(W_{0})$ profoundly modifies the electronic density of states and makes it dependent on energy, the value of the WF ratio at zero temperature (T=0) remains unchanged. However, neither electrical nor thermal conductivity display universal behavior. For finite temperature, with T greater than the value of the impurity scattering rate at zero frequency $\gamma(0)$ i.e. $\gamma(0)<T\ll W_{0}$, the usual WF ratio is obtained only in the weak scattering limit. For strong scattering there are large violations of the WF law.Comment: 1 figur

High-Resolution Photoemission Study of MgB2

We have performed high-resolution photoemission spectroscopy on MgB2 and observed opening of a superconducting gap with a narrow coherent peak. We found that the superconducting gap is s-like with the gap value of 4.5 meV at 15 K. The temperature dependence (15 - 40 K) of gap value follows well the BCS form, suggesting that 2Delta/kBTc at T=0 is about 3. No pseudogap behavior is observed in the normal state. The present results strongly suggest that MgB2 is categorized into a phonon-mediated BCS superconductor in the weak-coupling regime.Comment: 3 pages, 3 figures, accepted in Physical Review Letter