751 research outputs found
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\{Cu(Ni,Zn)O 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 . 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 as the
electron density is increased. Umklapp processes at the 4 patches near 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 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
i.e. , 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
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