共查询到20条相似文献,搜索用时 15 毫秒
1.
A. Cuccoli A. Fort A. Rettori E. Adam J. Villain 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):39-46
We study the magnetic relaxation of a system of localized spins interacting through weak dipole interactions, at a temperature
large with respect to the ordering temperature but low with respect to the crystal field level splitting. The relaxation results
from quantum spin tunneling but is only allowed on sites where the dipole field is very small. At low times, the magnetization
decrease is proportional to as predicted by Prokofiev and Stamp, and at long times the relaxation can be described as an extension of a relaxed zone.
The results can be directly compared with very recent experimental data on Fe8 molecular clusters.
Received 9 February 1999 相似文献
2.
T. Ohm C. Sangregorio C. Paulsen 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):195-199
We observed non-exponential relaxation for a quantum tunneling molecular magnetic system at very low temperatures and argue
that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows
a square-root time dependence. A simple model is developed for the intermediate time range that is in good agreement with
the data over 4 decades in time. Detailed numerical calculations as well as measurements are presented which indicate unusual
correlation effects in these systems.
Received: 15 May 1998 / Revised: 10 July 1998 / Accepted: 11 July 1998 相似文献
3.
D.A. Garanin E.M. Chudnovsky 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(1):3-7
We compute the width and shape of the EPR and tunneling resonances due to dislocations in Mn12 acetate crystals. Uncorrelated dislocations produce the Gaussian shape of resonances while dislocations bound in pairs produce
the Lorentzian shape. We stress that the uniaxial spin Hamiltonian together with crystal defects can explain the totality
of experimental data on Mn12.
Received 2 August 2001 / Received in final form 15 March 2002 Published online 17 September 2002 相似文献
4.
R. Lü H. Pan J.-L. Zhu B.-L. Gu 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(4):479-491
The tunneling of a giant spin at excited levels is studied theoretically in mesoscopic magnets with a magnetic field at an
arbitrary angle in the easy plane. Different structures of the tunneling barriers can be generated by the magnetocrystalline
anisotropy, the magnitude and the orientation of the field. By calculating the nonvacuum instanton solution explicitly, we
obtain the tunnel splittings and the tunneling rates for different angle ranges of the external magnetic field ( θ
H = π/2 and π/2 < θ
H < π). The temperature dependences of the decay rates are clearly shown for each case. It is found that the tunneling rate
and the crossover temperature depend on the orientation of the external magnetic field. This feature can be tested with the
use of existing experimental techniques.
Received 12 March 2001 and Received in final form 18 October 2001 相似文献
5.
A. Honecker M. Kaulke K.D. Schotte 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(3):423-434
CsCuCl3 is a ferromagnetically stacked triangular spin-1/2 antiferromagnet. We discuss models for its zero-temperature magnetization
process. The models range from three antiferromagnetically coupled ferromagnetic chains to the full three-dimensional situation.
The situation with spin-1/2 is treated by expansions around the Ising limit and exact diagonalization. Further, weak-coupling
perturbation theory is used mainly for three coupled chains which are also investigated numerically using the density-matrix
renormalization group technique. We find that already the three-chain model gives rise to the plateau-like feature at one
third of the saturation magnetization which is observed in magnetization experiments on CsCuCl3 for a magnetic field perpendicular to the crystal axis. For a magnetic field parallel to the crystal axis, a jump is observed
in the experimental magnetization curve in the region of again about one third of the saturation magnetization. In contrast
to earlier spinwave computations, we do not find any evidence for such a jump with the model in the appropriate parameter
region.
Received 25 October 1999 and Received in final form 30 December 1999 相似文献
6.
R. Guirado-López D. Spanjaard M.-C. Desjonquères 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(4):459-466
The magnetic properties of small YN clusters are studied by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. Several
types of cluster geometries are considered in order to see the effects of the size and symmetry of the structures on the magnetic
properties. The average magnetic moments are found to be constant over large domains of variations in the interatomic distance, a fact that can be explained by the
existing closed shell electronic configurations at least for one spin direction in all our magnetic solutions. Small energy
gains upon the onset of magnetization are obtained, which reveals the low stability of the magnetic solutions. Our results
contradict the prediction of a magnetic-nonmagnetic transition at a large cluster size (about 90 atoms) for these kinds of
systems.
Received: 27 April 1998 / Received in final form: 23 June 1998 / Accepted: 17 July 1998 相似文献
7.
T.P. Bigioni L.E. Harrell W.G. Cullen D.K. Guthrie R.L. Whetten P.N. First 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,6(3):355-364
Scanning tunneling microscopy (STM) and spectroscopy (STS) have been used to determine the structural and electronic properties
of thiol-passivated 29000 amu gold nanocrystals, both individually and in spontaneously formed quasi-two-dimensional arrays.
Experiments were performed at temperatures of 300 K, 77 K, and 8 K. Even at room temperature, tunneling through these 1.7
nm nanocrystals is shown to give rise to a Coulomb blockade. At cryogenic temperatures, the spectroscopy of the nanocrystals
in arrays and in isolation shows an incremental charging effect (the Coulomb staircase) and evidence is found for quantization
of the electronic states.
Received: 10 September 1998 / Received in final form: 16 February 1999 相似文献
8.
J. Dalla Torre N. Barriquand M. Djafari Rouhani G. Landa 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):343-346
We have determined by computer simulations, some structural properties of Ge clusters embedded in a Si crystalline host matrix
for cluster sizes varying from 0.5 to 1.5 nm. In order to describe inter-atomic forces we have chosen a Valence Force Field (VFF) semi-empirical potential.
Next we have calculated the density of vibrational states by diagonalization of the dynamical matrix defined with the same
potential. The influence of the volume/interface ratio of Ge on the vibrational properties is discussed.
Received 10 December 1998 and Received in final form 7 April 1999 相似文献
9.
Rong Lü Yi Zhou Jia-Lin Zhu Bing-Lin Gu 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):703-712
The phenomenon of quantum nucleation is studied in a nanometer-scale antiferromagnet with biaxial symmetry in the presence
of a magnetic field at an arbitrary angle. Within the instanton approach, we calculate the dependence of the rate of quantum
nucleation and the crossover temperature on the orientation and strength of the field for bulk solids and two-dimensional
films of antiferromagnets, respectively. Our results show that the rate of quantum nucleation and the crossover temperature
from thermal-to-quantum transitions depend on the orientation and strength of the field distinctly, which can be tested with
the use of existing experimental techniques.
Received 13 June 2000 and Received in final form 24 October 2000 相似文献
10.
J.-L. Zhu R. Lü X.-B. Wang X. Chen L. Chang F.-C. Pu 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):223-231
The tunneling behaviors of the magnetization vector are studied in ferromagnetic systems with trigonal and hexagonal crystal
symmetries, respectively. The Euclidean transition amplitudes between the energetically degenerate easy directions are evaluated
with the help of the dilute instanton-gas approximation. By using the effective Hamiltonian method, the ground-state tunneling
level splittings are clearly shown for each kind of symmetry and are found to depend on the parity of the total spin of the
ferromagnetic particle. The effective Hamiltonian method is demonstrated to be equivalent to the dilute instanton-gas approximation.
Possible relevance to experiments is discussed.
Received: 18 November 1997 / Revised: 18 March 1998 / Accepted: 6 April 1998 相似文献
11.
A.M. Vichare D.G. Kanhere 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,4(1):89-94
Density based molecular dynamics has been used to investigate the ground state structures of heterogeneous binary clusters
Al13Lin, n
= 1, 2, 3, 4, 10, 19, 20, 21. Some of these structures have also been investigated by full Kohn-Sham based calculations. Our
earlier investigations have shown that in the Al-Li cluster, the ground state configurations are the ones where the Al atoms
form a core around which the Li atoms form a “cage”. In the present work, we have chosen the well-known Al13 icosahedron as the surface over which we study the evolution of the surface coverage as the number of Li atoms increases.
On the basis of the earlier work, we expect that the Al13Li20 cluster would be the most stable and indeed our simulations do yield such a novel fivefold symmetric stable structure formed
out of purely metal atoms. This icosahedral substrate is also used to study the coverage of the aluminum surface by lithium
atoms. For a small number of Li atoms, these studies suggest that the Li-Li dimerisation is not particularly favored.
Received: 24 October 1997 / Revised: 7 April 1998 / Accepted: 29 June 1998 相似文献
12.
D. Eberbeck 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(2):237-245
Four ferrofluids, distinct in size distribution and aggregate structure, were investigated. The relaxation time ,related to the temperature of susceptibility maximum, was fitted to a Vogel-Fulcher law. A mean ordering temperature, T0, was calculated using magnetic particle parameters derived from the structure. It is assumed that at T0 the particle moments of particle clusters correlate, leading to a spin glass-like transition. Hence, then dynamic slows down
considerably, as indicated by a strong broadening of relaxation-time distribution. T0 roughly agrees with the energy of competing interaction between particle moments, as calculated from the structure of particle
aggregates. Differences between particle arrangements clearly influence the dispersion and absorption, particularly within
the cluster phase.
Received 15 July 1998 相似文献
13.
R.M. Wießner A. Fledderjohann K.-H. Mütter M. Karbach 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(3):475-481
We study the magnetisation process of the one-dimensional spin-1/2 antiferromagnetic Heisenberg model with modulated couplings
over j=1,2,3sites. It turns out that the evolution of magnetisation plateaus depends on j and on the wave number q of the modulation according to the rule of Oshikawa et al. A mapping of two- and three-leg zig-zag ladders on one-dimensional systems with modulated couplings yields predictions for
the occurrence of magnetization plateaus. The latter are tested by numerical computations with the DMRG algorithm.
Received 14 October 1999 and Received in final form 6 January 2000 相似文献
14.
O. Diéguez R.C. Longo C. Rey L.J. Gallego 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,7(4):573-576
Using the noncentral embedded atom model potential recently proposed by Besson and Morillo for bulk alloys (), we performed computer simulations to predict the ground-state configurations of and clusters (). The computed structures of clusters are in general agreement with such theoretical results as have been obtained by density functional calculations
(i.e. for ). The results for Fe-rich clusters show surface segregation of Al, which is in keeping with the findings of a previous study of clusters.
Received 29 April 1999 相似文献
15.
N.T. Wilson R.L. Johnston 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(1):161-169
Molecular Dynamics Simulated Annealing has been used to probe the structure of small Au clusters consisting of between 2 and
40 atoms. The interatomic interactions within these clusters are described using an empirical Murrell-Mottram many-body potential
energy function. Four distinct structural motifs are present in the structures of the predicted global minima, based on octahedra,
decahedra, icosahedra and hexagonal prisms.
Received 30 September 1999 and Received in final form 23 March 2000 相似文献
16.
L. Henrard A. Loiseau C. Journet P. Bernier 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(4):661-669
Determining precisely the atomic structure of single-wall carbon nanotubes is essential since it tailors electronic properties
of this new carbon material. Here, we present a quantitative electron diffraction study of electric-arc produced single-wall
carbon nanotube bundles, combined with simulations based on the kinematic theory and with real-space images. We stress the
importance of the twist of the bundle in the interpretation of our data and we analyze both packing lattice parameters and
chirality distribution. We show that, within a given bundle, no chirality is favoured whereas SWNT diameters are almost uniform.
Received 5 February 1999 相似文献
17.
R. Popescu D. Macovei A. Devenyi R. Manaila P.B. Barna A. Kovacs J.L. Lábár 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(4):737-743
Structure of metal clusters and of the C60 matrix in Au/C60 and Cu/C60 nanosystems was investigated by X-ray diffraction. Results support a charge-transfer-type interaction at the metal-C60 interface, which affects the size distribution of metal clusters, favouring interstitial location of metal ions in the fullerite
lattice.
Received 5 February 1999 and Received in final form 7 July 1999 相似文献
18.
J. Valenta J. Dian K. Luterová P. Knápek I. Pelant M. Nikl D. Muller J. J. Grob J. -L. Rehspringer B. Hönerlage 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,8(3):395-398
Silicon nanocrystals were prepared by Si+-ion implantation and subsequent annealing of SiO2 films thermally grown on a c-Si wafer. Different implantation energies (20-150 keV) and doses - cm
-2
) were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration
of Si atoms of 5, 7, 10 and 15 atomic%. The presence of Si nanocrystals was verified by transmission electron microscopy.
The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of μs at room temperature.
The changes of PL in the range 70-300 K can be well explained by the exciton singlet-triplet splitting model. We show that
all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si+-implanted SiO2 films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials.
Received 1st September 1998 and Received in final form 7 September 1999 相似文献
19.
Memory of silica aggregates dispersed in smectic liquid crystals: Effect of the interface properties
A. Jákli L. Almásy S. Borbély L. Rosta 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):509-513
Previous studies on nematic liquid crystals containing silica particles indicated memory effects that might be due to hydrogen
bonds between the individual silica particles. We made smectic liquid crystal dispersions containing silica particles with
various surface properties. Using a neutron scattering technique we studied the interfacial surface effects on the structure
of the silica aggregates, and on the smectic layer alignment. Our observations indicate that the stability of the memory correlates
to the number of OH groups on the silica surfaces. The observations imply that, with fine-tuning of the OH content of the
silica surfaces, various types of memory devices can be designed. We considered three different effects that can memorise
the alignment of the liquid crystal. Measurements on tilted SmC phases indicate that surface effects become important only
after repeated heating-cooling cycles, as the average size of the aggregates decrease.
Received 31 July 1998 and Received in final form 17 December 1998 相似文献
20.
D.C. Cabra A. Honecker P. Pujol 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(1):55-73
We analyze the phase diagram of a system of spin-1/2 Heisenberg antiferromagnetic chains interacting through a zig-zag coupling,
also called zig-zag ladders. Using bosonization techniques we study how a spin-gap or more generally plateaux in magnetization
curves arise in different situations. While for coupled XXZchains, one has to deal with a recently discovered chiral perturbation, the coupling term which is present for normal ladders
is restored by an external magnetic field, dimerization or the presence of charge carriers. We then proceed with a numerical
investigation of the phase diagram of two coupled Heisenberg chains in the presence of a magnetic field. Unusual behaviour
is found for ferromagnetic coupled antiferromagnetic chains. Finally, for three (and more) legs one can choose different inequivalent
types of coupling between the chains. We find that the three-leg ladder can exhibit a spin-gap and/or non-trivial plateaux
in the magnetization curve whose appearance strongly depends on the choice of coupling.
Received 11 February 1999 and Received in final form 16 June 1999 相似文献