39,097 research outputs found
Angular-dependent Magnetoresistance Oscillations in NaCoO Single Crystal
We report measurements of the c-axis angular-dependent magnetoresistance
(AMR) for a NaCoO single crystal, with a magnetic field of 10 T
rotating within Co-O planes. Below the metal-insulator transition temperature
induced by the charge ordering, the oscillation of the AMR is dominated by a
two-fold rotational symmetry. The amplitudes of the oscillation corresponding
to the four- and six-fold rotational symmetries are distinctive in low
temperatures, but they merge into the background simultaneously at about 25 K.
The six-fold oscillation originates naturally from the lattice symmetry. The
observation of the four-fold rotational symmetry is consistent with the picture
proposed by Choy, et al., that the Co lattice in the charge ordered state will
split into two orthorhombic sublattice with one occupied by Co ions and
the other by Co ions. We have also measured the c-axis AMR for
NaCoO and NaCoO single crystals, and found no
evidence for the existence of two- and four-fold symmetries.Comment: 4 pages, 6 figures. Submitted to PR
Metamagnetic Transition in NaCoO Single Crystals
We report the magnetization, specific heat and transport measurements of high
quality NaCoO single crystals in applied magnetic fields up to
14T. In high temperatures, the system is in a paramagnetic phase. It undergoes
a magnetic phase transition below about 20K. When the field is applied along
the c-axis, the measurement data of magnetization, specific heat and
magnetoresistance reveal a metamagnetic transition from an antiferromagnetic
state to a quasi-ferromagnetic state at about 8T in low temperatures. However,
no transition is observed in the magnetization measurements up to 14T when the
field is applied perpendicular to the c-axis. The low temperature magnetic
phase diagram of NaCoO is determined.Comment: 4 pages, 5 figure
Evidence for a full energy gap for nickel-pnictide LaNiAsO_{1-x}F_x superconductors by ^{75}As nuclear quadrupole resonance
We report systematic ^{75}As-NQR and ^{139}La-NMR studies on nickel-pnictide
superconductors LaNiAsO_{1-x}F_x (x=0, 0.06, 0.10 and 0.12). The spin lattice
relaxation rate 1/T_1 decreases below T_c with a well-defined coherence peak
and follows an exponential decay at low temperatures. This result indicates
that the superconducting gap is fully opened, and is strikingly different from
that observed in iron-pnictide analogs. In the normal state, 1/T_1T is constant
in the temperature range T_c \sim 4 K < T <10 K for all compounds and up to
T=250 K for x=0 and 0.06, which indicates weak electron correlations and is
also different from the iron analog. We argue that the differences between the
iron and nickel pnictides arise from the different electronic band structure.
Our results highlight the importance of the peculiar Fermi-surface topology in
iron-pnictides.Comment: 4 pages, 5 figure
Atomic-scale compensation phenomena at polar interfaces
The interfacial screening charge that arises to compensate electric fields of
dielectric or ferroelectric thin films is now recognized as the most important
factor in determining the capacitance or polarization of ultrathin
ferroelectrics. Here we investigate using aberration-corrected electron
microscopy and density functional theory how interfaces cope with the need to
terminate ferroelectric polarization. In one case, we show evidence for ionic
screening, which has been predicted by theory but never observed. For a
ferroelectric film on an insulating substrate, we found that compensation can
be mediated by interfacial charge generated, for example, by oxygen vacancies.Comment: 3 figure
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