5,713 research outputs found
Low Temperature Magnetic Properties of the Double Exchange Model
We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong
coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener
to explain ferromagnetism has unexpected properties when there is more than one
itinerant electron. We find that, in general, the many-body ground state of the
DE model is {\it not} globally FM ordered (except for special filled-shell
cases). Also, the low energy excitations of this model are distinct from spin
wave excitations in usual Heisenberg ferromagnets, which will result in unusual
dynamic magnetic properties.Comment: 5 pages, RevTeX, 5 Postscript figures include
Interchain Coupling Effects and Solitons in CuGeO_3
The effects of interchain coupling on solitons and soliton lattice structures
in CuGeO3 are explored. It is shown that interchain coupling substantially
increases the soliton width and changes the soliton lattice structures in the
incommensurate phase. It is proposed that the experimentally observed large
soliton width in CuGeO3 is mainly due to interchain coupling effects.Comment: 4 pages, LaTex, one eps figure included. No essential changes except
forma
Persistent Current in the Ferromagnetic Kondo Lattice Model
In this paper, we study the zero temperature persistent current in a
ferromagnetic Kondo lattice model in the strong coupling limit. In this model,
there are spontaneous spin textures at some values of the external magnetic
flux. These spin textures contribute a geometric flux, which can induce an
additional spontaneous persistent current. Since this spin texture changes with
the external magnetic flux, we find that there is an anomalous persistent
current in some region of magnetic flux: near Phi/Phi_0=0 for an even number of
electrons and Phi/Phi_0=1/2 for an odd number of electrons.Comment: 6 RevTeX pages, 10 figures include
Spin Diffusion in Double-Exchange Manganites
The theoretical study of spin diffusion in double-exchange magnets by means
of dynamical mean-field theory is presented. We demonstrate that the
spin-diffusion coefficient becomes independent of the Hund's coupling JH in the
range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal
magnetoresistive manganites in the metallic part of their phase diagram. Our
study reveals a close correspondence as well as some counterintuitive
differences between the results on Bethe and hypercubic lattices. Our results
are in accord with neutron scattering data and with previous theoretical work
for high temperatures.Comment: 4.0 pages, 3 figures, RevTeX 4, replaced with the published versio
Exploring the Eastern Frontier: A First Look at Mobile App Tracking in China
Many mobile apps are integrated with mobile advertising and tracking services running in the background to collect information for tracking users. Considering China currently tops mobile traffic growth globally, this paper aims to take a first look at China’s mobile tracking patterns from a large 4G network. We observe the dominance of the top popular domestic trackers and the pervasive tracking on mobile apps. We also discover a very well-connected tracking community, where the non-popular trackers form many local communities with each community tracking a particular category of mobile apps. We further conclude that some trackers have a monopoly on specific groups of mobile users and 10% of users upload Personally Identifiable Information (PII) to trackers (with 90% of PII tracking flows local to China). Our results consistently show a distinctive mobile tracking market in China. We hope the results can inform users and stakeholders on the interplay between mobile tracking and potential security and privacy issues
Dynamic similarity design method for an aero-engine dualrotor test rig
This paper presents a dynamic similarity design method to design a scale dynamic similarity model (DSM) for a dual-rotor test rig of an aero-engine. Such a test rig is usually used to investigate the major dynamic characteristics of the full-size model (FSM) and to reduce the testing cost and time for experiments on practical aero engine structures. Firstly, the dynamic equivalent model (DEM) of a dual-rotor system is modelled based on its FSM using parametric modelling, and the first 10 frequencies and mode shapes of the DEM are updated to agree with the FSM by modifying the geometrical shapes of the DEM. Then, the scaling laws for the relative parameters (such as geometry sizes of the rotors, stiffness of the supports, inherent properties) between the DEM and its scale DSM were derived from their equations of motion, and the scaling factors of the above-mentioned parameters are determined by the theory of dimensional analyses. After that, the corresponding parameters of the scale DSM of the dual-rotor test rig can be determined by using the scaling factors. In addition, the scale DSM is further updated by considering the coupling effect between the disks and shafts. Finally, critical speed and unbalance response analysis of the FSM and the updated scale DSM are performed to validate the proposed method
P35. Intratumoral patterns of clonal evolution in meningiomas
Núm. a Art Públic: 1815Digitalitzat per Tecnodo
Composite Polarons in Ferromagnetic Narrow-band Metallic Manganese Oxides
A new mechanism is proposed to explain the colossal magnetoresistance and
related phenomena. Moving electrons accompanied by Jahn-Teller phonon and
spin-wave clouds may form composite polarons in ferromagnetic narrow-band
manganites. The ground-state and finite-temperature properties of such
composite polarons are studied in the present paper. By using a variational
method, it is shown that the energy of the system at zero temperature decreases
with the formation of composite polaron; the energy spectrum and effective mass
of the composite polaron at finite temperature is found to be strongly
renormalized by the temperature and the magnetic field. It is suggested that
the composite polaron contribute significantly to the transport and the
thermodynamic properties in ferromagnetic narrow-band metallic manganese
oxides.Comment: Latex, no figur
Charge Localization in Disordered Colossal-Magnetoresistance Manganites
The metallic or insulating nature of the paramagnetic phase of the
colossal-magnetoresistance manganites is investigated via a double exchange
Hamiltonian with diagonal disorder. Mobility edge trajectory is determined with
the transfer matrix method. Density of states calculations indicate that random
hopping alone is not sufficient to induce Anderson localization at the Fermi
level with 20-30% doping. We argue that the metal-insulator transtion is likely
due to the formation of localized polarons from nonuniform extended states as
the effective band width is reduced by random hoppings and electron-electron
interactions.Comment: 4 pages, RevTex. 4 Figures include
Molecular-field approach to the spin-Peierls transition in CuGeO_3
We present a theory for the spin-Peierls transition in CuGeO_3. We map the
elementary excitations of the dimerized chain (solitons) on an effective Ising
model. Inter-chain coupling (or phonons) then introduce a linear binding
potential between a pair of soliton and anti-soliton, leading to a finite
transition temperature. We evaluate, as a function of temperature, the order
parameter, the singlet-triplet gap, the specific heat, and the susceptibility
and compare with experimental data on CuGeO_3. We find that CuGeO_3 is close to
a first-order phase transition. We point out, that the famous scaling law
\sim\delta^{2/3} of the triplet gap is a simple consequence of the linear
binding potential between pairs of solitons and anti-solitons in dimerized spin
chains.Comment: 7.1 pages, figures include
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