4,036 research outputs found
Time Dependent Effects and Transport Evidence for Phase Separation in La_{0.5}Ca_{0.5}MnO_{3}
The ground state of La_{1-x}Ca_{x}MnO_{3} changes from a ferromagnetic
metallic to an antiferromagnetic charge-ordered state as a function of Ca
concentration at x ~ 0.50. We present evidence from transport measurements on a
sample with x = 0.50 that the two phases can coexist, in agreement with other
observations of phase separation in these materials. We also observe that, by
applying and then removing a magnetic field to the mainly charge-ordered state
at some temperatures, we can "magnetically anneal" the charge order, resulting
in a higher zero-field resistivity. We also observe logarithmic time dependence
in both resistivity and magnetization after a field sweep at low temperatures.Comment: 9 pages, LATEX, 3 postscript figure
Anomalous ferromagnetic spin fluctuations in an antiferromagnetic insulator Pr_{1-x}Ca_{x}MnO_{3}
The high temperature paramagnetic state in an antiferromagnetic (AFM)
insulator Pr_{1-x}Ca_{x}MnO_{3} is characterized by the ferromagnetic (FM) spin
fluctuations with an anomalously small energy scale. The FM fluctuations show a
precipitous decrease of the intensity at the charge ordering temperature
T_{CO}, but persist below T_{CO}, and vanish at the AFM transition temperature
T_{N}. These results demonstrate the importance of the spin ordering for the
complete switching of the FM fluctuation in doped manganites.Comment: REVTeX, 5 pages, 4 figures, submitted to Phys. Rev.
Anomalous elastic softening of SmRu_{4}P_{12} under high pressure
The filled skutterudite compound SmRu_4P_{12} undergoes a complex evolution
from a paramagnetic metal (phase I) to a probable multipolar ordering insulator
(phase II) at T_{MI} = 16.5 K, then to a magnetically ordered phase (phase III)
at T_{N} = 14 K. Elastic properties under hydrostatic pressures were
investigated to study the nature of the ordering phases. We found that distinct
elastic softening above T_{MI} is induced by pressure, giving evidence of
quadrupole degeneracy of the ground state in the crystalline electric field. It
also suggests that quadrupole moment may be one of the order parameters below
T_{MI} under pressure. Strangely, the largest degree of softening is found in
the transverse elastic constant C_{T} at around 0.5-0.6 GPa, presumably having
relevancy to the competing and very different Gruneisen parameters \Omega of
T_{MI} and T_{N}. Interplay between the two phase transitions is also verified
by the rapid increase of T_{MI} under pressure with a considerably large \Omega
of 9. Our results can be understood on the basis of the proposed octupole
scenario for SmRu_4P_{12}.Comment: 7 pages, 7 figure
Rearrangements and Dilatancy for Sheared Dense Materials
Constitutive equations are proposed for dense materials, based on the
identification of two types of free-volume activated rearrangements associated
to shear and compaction. Two situations are studied: the case of an amorphous
solid in a stress-strain test, and the case of a lubricant in tribology test.
Varying parameters, strain softening, shear thinning, and stick-slip motion can
be observed.Comment: 4 pages, 3 figure
Charge and spin ordering in Nd{1/3}Sr{2/3}FeO{3}
We have investigated the charge and spin ordering in Nd{1/3}Sr{2/3}FeO{3}
with neutron diffraction technique. This sample undergoes a charge ordering
transition accompanying charge disproportionation of 2Fe4+ -> Fe3+ + Fe5+. We
measured the superlattice reflections due to the charge and spin ordering, and
confirmed that charges and spins order simultaneously at Tco = 185 K. The
ordering pattern of charges and spins in this sample can be viewed as three
dimensional stripe order, and is compared with two dimensional stripe order
observed in other transition metal oxides.Comment: REVTeX, 4 pages, 3 figures, to be published in J. Phys. Chem. Solid
Double Exchange in a Magnetically Frustrated System
This work examines the magnetic order and spin dynamics of a double-exchange
model with competing ferromagnetic and antiferromagnetic Heisenberg
interactions between the local moments. The Heisenberg interactions are
periodically arranged in a Villain configuration in two dimensions with
nearest-neighbor, ferromagnetic coupling and antiferromagnetic coupling
. This model is solved at zero temperature by performing a
expansion in the rotated reference frame of each local moment.
When exceeds a critical value, the ground state is a magnetically
frustrated, canted antiferromagnet. With increasing hopping energy or
magnetic field , the local moments become aligned and the ferromagnetic
phase is stabilized above critical values of or . In the canted phase, a
charge-density wave forms because the electrons prefer to sit on lines of sites
that are coupled ferromagnetically. Due to a change in the topology of the
Fermi surface from closed to open, phase separation occurs in a narrow range of
parameters in the canted phase. In zero field, the long-wavelength spin waves
are isotropic in the region of phase separation. Whereas the average spin-wave
stiffness in the canted phase increases with or , it exhibits a more
complicated dependence on field. This work strongly suggests that the jump in
the spin-wave stiffness observed in PrCaMnO with at a field of 3 T is caused by the delocalization of the electrons rather
than by the alignment of the antiferromagnetic regions.Comment: 28 pages, 12 figure
- …