153 research outputs found
Magnetic Anisotropy of Co2+ as Signature of Intrinsic Ferromagnetism in ZnO:Co
We report on the magnetic properties of thoroughly characterized Zn1-xCoxO
epitaxial thin films, with low Co concentration, x=0.003-0.005. Magnetic and
EPR measurements, combined with crystal field theory, reveal that isolated Co2+
ions in ZnO possess a strong single ion anisotropy which leads to an "easy
plane" ferromagnetic state when the ferromagnetic Co-Co interaction is
considered. We suggest that the peculiarities of the magnetization process of
this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co
materials.Comment: 4 pages, 4 figure
Integer Quantum Hall Effect in Trilayer Graphene
The Integer Quantum Hall Effect (IQHE) is a distinctive phase of
two-dimensional electronic systems subjected to a perpendicular magnetic field.
Thus far, the IQHE has been observed in semiconductor heterostructures and in
mono- and bi-layer graphene. Here we report on the IQHE in a new system:
trilayer graphene. Experimental data are compared with self-consistent Hartree
calculations of the Landau levels for the gated trilayer. The plateau structure
in the Hall resistivity determines the stacking order (ABA versus ABC). We find
that the IQHE in ABC trilayer graphene is similar to that in the monolayer,
except for the absence of a plateau at filling factor v=2. At very low filling
factor, the Hall resistance vanishes due to the presence of mixed electron and
hole carriers induced by disorder.Comment: 5 pages, 4 figure
An unusual occurrence of Nautilus macromphalus in a cenote in the Loyalty Islands (New Caledonia)
Exploration of a landlocked cenote on Lifou (Loyalty Islands) revealed 37 shells of the cephalopod Nautilus macromphalus Sowerby, 1849, in saltwater on the cenote floor, approximately 40 m below the water surface. The occurrence of these shells is unusual because N. macromphalus is restricted to the open marine waters surrounding the island. All of the shells are mature, and nearly all of them are unbroken, with faded red-brown color stripes. We analyzed seven shells to determine their age. Radiocarbon dating yielded ages of 6380¡30 to 7095¡30 y BP. The 238U-series radionuclides 210Pb (half-life 522.3 y) and 226Ra (half-life 51600 y) also were measured. Two of the samples showed radioactive equilibrium between the nuclides, consistent with the old radiocarbon dates, but the other five samples showed excess 210Pb. When corrected for radioactive decay, the 226Ra activities were much greater than those found in living Nautilus. We conclude that exposure to high activities of 222Rn and 226Ra in the salty groundwater of the cenote altered the activities originally incorporated into the shells. Human placement of the shells in the cavity is rejected based on their radiocarbon age and the geometry of the cenote. The most probable explanation is that the animals entered the flooded karstic system through a connection on the seaward side at approximately 7,000 y BP, during an interval of slowly rising sea level. Unable to find an exit and/or due to anoxic bottom waters, the animals were trapped and died inside. The open connection with the sea persisted for ,700 y, but after ,6400 y BP, the connection was lost, probably due to a roof collapse. This is a rare example of Nautilus in a karstic coastal basin and provides a minimum age for the appearance of N. macromphalus in the Loyalty Islands
Tuning the magnetic ground state of a novel tetranuclear Nickel(II) molecular complex by high magnetic fields
Electron spin resonance and magnetization data in magnetic fields up to 55 T
of a novel multicenter paramagnetic molecular complex [L_2Ni_4(N_3)(O_2C
Ada)_4](Cl O_4) are reported. In this compound, four Ni centers each having a
spin S = 1 are coupled in a single molecule via bridging ligands (including a
\mu_4-azide) which provide paths for magnetic exchange. Analysis of the
frequency and temperature dependence of the ESR signals yields the relevant
parameters of the spin Hamiltonian, in particular the single ion anisotropy gap
and the g factor, which enables the calculation of the complex energy spectrum
of the spin states in a magnetic field. The experimental results give
compelling evidence for tuning the ground state of the molecule by magnetic
field from a nonmagnetic state at small fields to a magnetic one in strong
fields owing to the spin level crossing at a field of ~25 T.Comment: revised version, accepted for publication in Physical Review
Antiferromagnetic Dimers of Ni(II) in the S=1 Spin-Ladder Na_2Ni_2(C_2O_4)_3(H_2O)_2
We report the synthesis, crystal structure and magnetic properties of the S=1
2-leg spin-ladder compound Na_2Ni_2(C_2O_4)_3(H_2O)_2. The magnetic properties
were examined by magnetic susceptibility and pulsed high field magnetization
measurements. The magnetic excitations have been measured in high field high
frequency ESR. Although the Ni(II) ions form structurally a 2-leg ladder, an
isolated dimer model consistently describes the observations very well. The
analysis of the temperature dependent magnetization data leads to a magnetic
exchange constant of J=43 K along the rungs of the ladder and an average value
of the g-factor of 2.25. From the ESR measurements, we determined the single
ion anisotropy to D=11.5 K. The validity of the isolated dimer model is
supported by Quantum Monte Carlo calculations, performed for several ratios of
interdimer and intradimer magnetic exchange and taking into account the
experimentally determined single ion anisotropy. The results can be understood
in terms of the different coordination and superexchange angles of the oxalate
ligands along the rungs and legs of the 2-leg spin ladder.Comment: 8 pages, 10 figure
Possible Fractional Quantum Hall Effect in Graphite
Measurements of basal plane longitudinal rho_b(B) and Hall rho_H(B)
resistivities were performed on highly oriented pyrolytic graphite (HOPG)
samples in pulsed magnetic field up to B = 50 T applied perpendicular to
graphene planes, and temperatures 1.5 K 30 T and for all
studied samples, we observed a sign change in rho_H(B) from electron- to
hole-like. For our best quality sample, the measurements revealed the
enhancement in rho_b(B) for B > 34 T (T = 1.8 K), presumably associated with
the field-driven charge density wave or Wigner crystallization transition.
Besides, well defined plateaus in rho_H(B) were detected in the ultra-quantum
limit revealing the signatures of fractional quantum Hall effect in graphite.Comment: 15 pages, including 4 figure
Quantum Size Effect transition in percolating nanocomposite films
We report on unique electronic properties in Fe-SiO2 nanocomposite thin films
in the vicinity of the percolation threshold. The electronic transport is
dominated by quantum corrections to the metallic conduction of the Infinite
Cluster (IC). At low temperature, mesoscopic effects revealed on the
conductivity, Hall effect experiments and low frequency electrical noise
(random telegraph noise and 1/f noise) strongly support the existence of a
temperature-induced Quantum Size Effect (QSE) transition in the metallic
conduction path. Below a critical temperature related to the geometrical
constriction sizes of the IC, the electronic conductivity is mainly governed by
active tunnel conductance across barriers in the metallic network. The high 1/f
noise level and the random telegraph noise are consistently explained by random
potential modulation of the barriers transmittance due to local Coulomb
charges. Our results provide evidence that a lowering of the temperature is
somehow equivalent to a decrease of the metal fraction in the vicinity of the
percolation limit.Comment: 21 pages, 8 figure
Puddle-Induced Resistance Oscillations in the Breakdown of the Graphene Quantum Hall Effect
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