112 research outputs found
Power-law spin correlations in pyrochlore antiferromagnets
The ground state ensemble of the highly frustrated pyrochlore-lattice
antiferromagnet can be mapped to a coarse-grained ``polarization'' field
satisfying a zero-divergence condition From this it follows that the
correlations of this field, as well as the actual spin correlations, decay with
separation like a dipole-dipole interaction (). Furthermore, a lattice
version of the derivation gives an approximate formula for spin correlations,
with several features that agree well with simulations and neutron-diffraction
measurements of diffuse scattering, in particular the pinch-point
(pseudo-dipolar) singularities at reciprocal lattice vectors. This system is
compared to others in which constraints also imply diffraction singularities,
and other possible applications of the coarse-grained polarization are
discussed.Comment: 13 pp, revtex, two figure
Static Correlation and Dynamical Properties of Tb3+-moments in Tb2Ti2O7 -Neutron Scattering Study-
Static and dynamical properties of the magnetic moment system of pyrochlore
compound Tb2Ti2O7 with strong magnetic frustration, have been investigated down
to the temperature T=0.4 K by neutron scattering on a single crystal sample.
The scattering vector (Q)-dependence of the magnetic scattering intensity
becomes appreciable with decreasing T at around 30 K, indicating the
development of the magnetic correlation. From the observed energy profiles, the
elastic, quasi elastic and inelastic components have been separately obtained.
The quasi elastic component corresponds to the diffusive motion of the magnetic
moments within the lowest states, which are formed of the lowest energy levels
of Tb3+ ions. Magnetic correlation pattern which can roughly reproduce the
Q-dependence of the scattering intensities of the elastic and quasi elastic
component is discussed based on the trial calculations for clusters of 7
moments belonging to two corner-sharing tetrahedra. A possible origin of the
glassy state, which develops at around 1.5 K with decreasing T is discussed.Comment: 10 pages, 12 figures, to be published in J. Phys. Soc. Jpn.
71(2002)No.2 59
The order parameter-entropy relation in some universal classes: experimental evidence
The asymptotic behaviour near phase transitions can be suitably characterized
by the scaling of with , where is
the excess entropy and is the order parameter. As is obtained by
integration of the experimental excess specific heat of the transition , it displays little experimental noise so that the curve versus is better constrained than, say,
versus . The behaviour of for different
universality classes is presented and compared. In all cases, it clearly
deviates from being a constant. The determination of this function can then be
an effective method to distinguish asymptotic critical behaviour. For
comparison, experimental data for three very different systems, Rb2CoF4,
Rb2ZnCl4 and SrTiO3, are analysed under this approach. In SrTiO3, the function
does not deviate within experimental resolution from a straight
line so that, although Q can be fitted with a non mean-field exponent, the data
can be explained by a classical Landau mean-field behaviour. In contrast, the
behaviour of for the antiferromagnetic transition in Rb2CoF4 and
the normal-incommensurate phase transition in Rb2ZCl4 is fully consistent with
the asymptotic critical behaviour of the universality class corresponding to
each case. This analysis supports, therefore, the claim that incommensurate
phase transitions in general, and the ABX compounds in particular, in
contrast with most structural phase transitions, have critical regions large
enough to be observable.Comment: 13 pp. 9 ff. 2 tab. RevTeX. Submitted to J. Phys.: Cond. Matte
Magnetic susceptibility of diluted pyrochlore and SCGO antiferromagnets
We investigate the magnetic susceptibility of the classical Heisenberg
antiferromagnet with nearest-neighbour interactions on the geometrically
frustrated pyrochlore lattice, for a pure system and in the presence of
dilution with nonmagnetic ions. Using the fact that the correlation length in
this system for small dilution is always short, we obtain an approximate but
accurate expression for the magnetic susceptibility at all temperatures. We
extend this theory to the compound SrCr_{9-9x}Ga_{3+9x}O_{19} (SCGO) and
provide an explanation of the phenomenological model recently proposed by
Schiffer and Daruka [Phys. Rev. B56, 13712 (1997)].Comment: 4 pages, Latex, 4 postscript figures automatically include
Визначення тенденцій розвитку екранопланів як високоефективних сучасних носіїв засобів океанотехніки різноманітного призначення
Сліжевський, С. М. Визначення тенденцій розвитку екранопланів як високоефективних сучасних носіїв засобів океанотехніки різноманітного призначення = Outlining the development trends of wig crafts as high-performance advanced carriers of oceanic engineering equipment of various purpose / С. М. Сліжевський, В. М. Зінкін, І. С. Абляєв // Зб. наук. пр. НУК. – Миколаїв : НУК, 2018. – № 1–2 (473). – С. 3–6.Анотація. Виконано огляд та аналіз основних параметрів екранопланів – сучасних високоефективних транспортних засобів широкого загальногосподарського і військово-прикладного призначення.Abstract. Application of WIG crafts while performing various operations and general economic and applied military purposes has been considered. The aim of the study is to identify the trends in the development of advanced highspeed transport systems, WIG crafts, and expand their scope of application in the role of carriers of different objects and underwater ocean engineering equipment at field work. There has been performed a comparative analysis of the operation of WIG crafts and other vehicles similar in their weight category. The study results can be taken into account when creating a high-performance platform WIG carrier of universal modules for different purposes depending on their task.Аннотация. Выполнен обзор и анализ основных параметров экранопланов – современных высокоэффективных транспортных средств широкого общехозяйственного и военно-прикладного назначения
Transition to Long Range Magnetic Order in the Highly Frustrated Insulating Pyrochlore Antiferromagnet Gd_2Ti_2O_7
Experimental evidence from measurements of the a.c. and d.c. susceptibility,
and heat capacity data show that the pyrochlore structure oxide, Gd_2Ti_2O_7,
exhibits short range order that starts developing at 30K, as well as long range
magnetic order at K. The Curie-Weiss temperature, =
-9.6K, is largely due to exchange interactions. Deviations from the Curie-Weiss
law occur below 10K while magnetic heat capacity contributions are found
at temperatures above 20K. A sharp maximum in the heat capacity at K
signals a transition to a long range ordered state, with the magnetic specific
accounting for only 50% of the magnetic entropy. The heat capacity above
the phase transition can be modeled by assuming that a distribution of random
fields acts on the ground state for Gd. There is no
frequency dependence to the a.c. susceptibility in either the short range or
long range ordered regimes, hence suggesting the absence of any spin-glassy
behavior. Mean field theoretical calculations show that no long range ordered
ground state exists for the conditions of nearest-neighbor antiferromagnetic
exchange and long range dipolar couplings. At the mean-field level, long range
order at various commensurate or incommensurate wave vectors is found only upon
inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar
coupling. The properties of Gd$_2Ti_2O_7 are compared with other geometrically
frustrated antiferromagnets such as the Gd_3Ga_5O_{12} gadolinium gallium
garnet, RE_2Ti_2O_7 pyrochlores where RE = Tb, Ho and Tm, and Heisenberg-type
pyrochlore such as Y_2Mo_2O_7, Tb_2Mo_2O_7, and spinels such as ZnFe_2O_4Comment: Letter, 6 POSTSCRIPT figures included. (NOTE: Figure 5 is not
included --) To appear in Physical Review B. Contact:
[email protected]
Low-temperature properties of classical, geometrically frustrated antiferromagnets
We study the ground-state and low-energy properties of classical vector spin
models with nearest-neighbour antiferromagnetic interactions on a class of
geometrically frustrated lattices which includes the kagome and pyrochlore
lattices. We explore the behaviour of these magnets that results from their
large ground-state degeneracies, emphasising universal features and systematic
differences between individual models. We investigate the circumstances under
which thermal fluctuations select a particular subset of the ground states, and
find that this happens only for the models with the smallest ground-state
degeneracies. For the pyrochlore magnets, we give an explicit construction of
all ground states, and show that they are not separated by internal energy
barriers. We study the precessional spin dynamics of the Heisenberg pyrochlore
antiferromagnet. There is no freezing transition or selection of preferred
states. Instead, the relaxation time at low temperature, T, is of order
hbar/(k_B T). We argue that this behaviour can also be expected in some other
systems, including the Heisenberg model for the compound SrCr_8Ga_4O_{19}.Comment: to appear in Phys. Rev.
Forgetting, Reminding, and Remembering: The Retrieval of Lost Spatial Memory
Retrograde amnesia can occur after brain damage because this disrupts sites of storage, interrupts memory consolidation, or interferes with memory retrieval. While the retrieval failure account has been considered in several animal studies, recent work has focused mainly on memory consolidation, and the neural mechanisms responsible for reactivating memory from stored traces remain poorly understood. We now describe a new retrieval phenomenon in which rats' memory for a spatial location in a watermaze was first weakened by partial lesions of the hippocampus to a level at which it could not be detected. The animals were then reminded by the provision of incomplete and potentially misleading information—an escape platform in a novel location. Paradoxically, both incorrect and correct place information reactivated dormant memory traces equally, such that the previously trained spatial memory was now expressed. It was also established that the reminding procedure could not itself generate new learning in either the original environment, or in a new training situation. The key finding is the development of a protocol that definitively distinguishes reminding from new place learning and thereby reveals that a failure of memory during watermaze testing can arise, at least in part, from a disruption of memory retrieval
Serum MicroRNAs as Biomarkers for Hepatocellular Carcinoma in Chinese Patients with Chronic Hepatitis B Virus Infection
BACKGROUND: MicroRNAs (miRNAs) have been shown to anticipate great cancer diagnostic potential. Recently, circulating miRNAs have been reported as promising biomarkers for various pathologic conditions. The objective of this study was to investigate the potential of serum miRNAs as novel biomarkers for hepatocellular carcinoma (HCC). METHODOLOGY/PRINCIPAL FINDINGS: This study was divided into four phases: (I) Ten candidate serum miRNAs were detected by using real-time RT-PCR, corresponding 10 HCC patients with chronic hepatitis B virus (HBV) infection and 10 age- and sex-matched healthy subjects. (II) Marker validation by real-time RT-PCR on HBV patients with (n = 48) or without HCC (n = 48), and healthy subjects (n = 24). (III) Marker detection by real-time RT-PCR in sera from another 14 HCC patients before and 1 month after surgical resection. (IV) We examined the correlation between the expressions of candidate serum miRNAs with clinical parameters of HCC patients. Although miR-222, miR-223 or miR-21 were significantly up- or down-regulated between HCC patients and healthy controls, no significant difference was observed in the levels of these miRNAs between HBV patients without and with HCC. MiR-122 in serum was significantly higher in HCC patients than healthy controls (p<0.001). More importantly, it was found that the levels of miR-122 were significantly reduced in the post-operative serum samples when compared to the pre-operative samples. Although serum miR-122 was also elevated in HBV patients with HCC comparing with those without HCC, the difference was at the border line (p = 0.043). CONCLUSIONS/SIGNIFICANCE: Our results suggest that serum miR-122 might serve as a novel and potential noninvasive biomarker for detection of HCC in healthy subjects, moreover, it might serve as a novel biomarker for liver injury but not specifically for detection of HCC in chronic HBV infection patients
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