3,759 research outputs found
Heat transport study of the spin liquid candidate 1T-TaS2
We present the ultra-low-temperature thermal conductivity measurements on
single crystals of the prototypical charge-density-wave material 1-TaS,
which was recently argued to be a candidate for quantum spin liquid. Our
experiments show that the residual linear term of thermal conductivity at zero
field is essentially zero, within the experimental accuracy. Furthermore, the
thermal conductivity is found to be insensitive to the magnetic field up to 9
T. These results clearly demonstrate the absence of itinerant magnetic
excitations with fermionic statistics in bulk 1-TaS and, thus, put a
strong constraint on the theories of the ground state of this material.Comment: 5 pages, 3 figure
Equivalent Circuit Modeling of the Dielectric Loaded Microwave Biosensor
This article describes the modeling of biological tissues at microwave frequency using equivalent lumped elements. A microwave biosensor based on microstrip ring resonator (MRR), that has been utilized previously for meat quality evaluation is used for this purpose. For the first time, the ring-resonator loaded with the lossy and high permittivity dielectric material, such as; biological tissue, in a partial overlay configuration is analyzed. The equivalent circuit modeling of the structure is then performed to identify the effect of overlay thickness on the resonance frequency. Finally, the relationship of an overlay thickness with the corresponding RC values of the meat equivalent circuit is established. Simulated, calculated and measured results are then compared for validation. Results are well agreed while the observed discrepancy is in acceptable limit
Origin of Electric Field Induced Magnetization in Multiferroic HoMnO3
We have performed polarized and unpolarized small angle neutron scattering
experiments on single crystals of HoMnO3 and have found that an increase in
magnetic scattering at low momentum transfers begins upon cooling through
temperatures close to the spin reorientation transition at TSR ~ 40 K. We
attribute the increase to an uncompensated magnetization arising within
antiferromagnetic domain walls. Polarized neutron scattering experiments
performed while applying an electric field show that the field suppresses
magnetic scattering below T ~ 50 K, indicating that the electric field affects
the magnetization via the antiferromagnetic domain walls rather than through a
change to the bulk magnetic order
Evidences of a consolute critical point in the Phase Separation regime of La(5/8-y)Pr(y)Ca(3/8)MnO(3) (y = 0.4) single crystals
We report on DC and pulsed electric field sensitivity of the resistance of
mixed valent Mn oxide based La(5/8-y)Pr(y)Ca(3/8)MnO(3) (y = 0.4) single
crystals as a function of temperature. The low temperature regime of the
resistivity is highly current and voltage dependent. An irreversible transition
from high (HR) to a low resistivity (LR) is obtained upon the increase of the
electric field up to a temperature dependent critical value (V_c). The
current-voltage characteristics in the LR regime as well as the lack of a
variation in the magnetization response when V_c is reached indicate the
formation of a non-single connected filamentary conducting path. The
temperature dependence of V_c indicates the existence of a consolute point
where the conducting and insulating phases produce a critical behavior as a
consequence of their separation.Comment: 5 pages, 6 figures, corresponding author: C. Acha ([email protected]
Relationship between macroscopic physical properties and local distortions of low doping La{1-x}Ca{x}MnO3: an EXAFS study
A temperature-dependent EXAFS investigation of La{1-x}Ca{x}MnO3 is presented
for the concentration range that spans the ferromagnetic-insulator (FMI) to
ferromagnetic-metal (FMM) transition region, x = 0.16-0.22. The samples are
insulating for x = 0.16-0.2 and show a metal/insulator transition for x = 0.22.
All samples are ferromagnetic although the saturation magnetization for the 16%
Ca sample is only ~ 70% of the expected value at 0.4T. We find that the FMI
samples have similar correlations between changes in the local Mn-O distortions
and the magnetization as observed previously for the colossal magnetoresistance
(CMR) samples (0.2 < x < 0.5) - except that the FMI samples never become fully
magnetized. The data show that there are at least two distinct types of
distortions. The initial distortions removed as the insulating sample becomes
magnetized are small and provides direct evidence that roughly 50% of the Mn
sites have a small distortion/site and are magnetized first. The large
remaining Mn-O distortions at low T are attributed to a small fraction of
Jahn-Teller-distorted Mn sites that are either antiferromagnetically ordered or
unmagnetized. Thus the insulating samples are very similar to the behavior of
the CMR samples up to the point at which the M/I transition occurs for the CMR
materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of
the sample is magnetic, implies that there must be preferred magnetized Mn
sites and that such sites do not percolate at these concentrations.Comment: 27 pages, 8 figures, to be submitted to Phys. Rev.
Effect of impurities on quasi-2D quantum antiferromagnet
We have studied the static and dynamic properties of quasi-two-dimensional
quantum antiferromagnets (AF) diluted with spinless impurities using spin-wave
theory and -matrix approximation. We show that the spectrum of a 2D AF at
long wavelengths is overdamped at arbitrary concentration of spinless
impurities. The scattering leads to a new length scale , being impurity concentration and the lattice spacing,
beyond which the influence of impurities on the spectrum is dominant. Although
the dynamical properties are significantly modified we show that 2D is not the
lower critical dimension for this problem. Thus, in low-dimensional systems
with disorder the connection between static and dynamic quantities is not
straightforward. Our results are in quantitative agreement with the recent
Monte Carlo simulations and experimental data for , S=1, and . We
have also proposed experiments which can further test the results of our
theory.Comment: RevTeX, 3 pages, 3 figures, Proceedings of the 46 Conference on
Magnetism and Magnetic Materials, Seattle 2001, to be published in J. Appl.
Phy
Colossal magnon-phonon coupling in multiferroic EuYMnO
We report the spectra of magnetically induced electric dipole absorption in
EuYMnO from temperature dependent far infrared
spectroscopy (10-250 cm). These spectra, which occur only in the
polarization, consist of two relatively narrow electromagnon features that
onset at K and a broad absorption band that persists to
temperatures well above K. The observed excitations account for the
step up of the static dielectric constant in the ferroelectric phase. The
electromagnon at 80 cm is observed to be strongly coupled to the nearby
lowest optical phonon which transfers more than 1/2 of its spectral weight to
the magnon. We attribute the origin of the broad background absorption to the
two magnon emission decay process of the phonon.Comment: 4 pages, 3 figure
Outbreaks of Bagworms and their Natural Enemies in an Oil Palm, Elaeis Guineensis, Plantation at Hutan Melintang, Perak, Malaysia
A field study on population of bagworms was carried out in oil palm, Elaeis guineensis, plantation at Hutan Melintang, Perak, Malaysia from October 2005 to April 2006. Sampling was conducted at 14 days interval. The result suggests bagworm infestation was not correlated with amount and distribution of rainfall as being claimed by many planters. The result also shows that Pteroma pendula (Lepidoptera: Psychidae) was the most dominant bagworm species among all of the bagworms recorded. Natural enemies, predators, parasitoids and some entomopathogenic fungi, contributed to mortality of the bagworms. Larvae of Callimerus arcufur (Coleoptera: Cleridae) were the most common predator attacking the bagworms. Cosmelestes picticeps (Hemiptera: Reduviidae) was also observed attacking P. pendula. Among three species of parasitoids, Dolichodenidea metasae (Hymenoptera: Braconidae) was the most significant parasitoid of bagworm and this parasitoid commonly attacked by a hyperparasitoid, Pediobius imbrues (Hymenoptera: Braconidae). Two species of entomopathogenic fungi, Peacilomyces fumosoroseus and Metarhizium ansopliae, were isolated from fungal-infected bagworms. Even though natural enemies were affecting the bagworm populations in the field and their resultant impact in controlling the bagworm populations in the field was still far from desirable
First-order nature of the ferromagnetic phase transition in (La-Ca)MnO_3 near optimal doping
Neutron scattering has been used to study the nature of the ferromagnetic
transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and
polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the
naturally occurring O-16 can be replaced with the O-18 isotope. Small angle
neutron scattering on the x=0.3 single crystal reveals a discontinuous change
in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1.
Strong relaxation effects are observed for this domain scattering, for the
magnetic order parameter, and for the quasielastic scattering, demonstrating
that the transition is not continuous in nature. There is a large oxygen
isotope effect observed for the T_C in the polycrystalline samples. For the
optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5
K at 90% O-18 substitution. The temperature dependence of the spin-wave
stiffness is found to be identical for the two samples despite changes in T_C.
Hence, T_C is not solely determined by the magnetic subsystem, but instead the
ferromagnetic phase is truncated by the formation of polarons which cause an
abrupt transition to the paramagnetic, insulating state. Application of
uniaxial stress in the x=0.3 single crystal sharply enhances the polaron
scattering at room temperature. Measurements of the phonon density-of-states
show only modest differences above and below T_C and between the two different
isotopic samples.Comment: 13 pages, 16 figures, submitted to Phys. Rev.
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