共查询到20条相似文献,搜索用时 31 毫秒
1.
L. Brossard R. Clerac C. Coulon M. Tokumoto T. Ziman D.K. Petrov V.N. Laukhin M.J. Naughton A. Audouard F. Goze A. Kobayashi H. Kobayashi P. Cassoux 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(4):439-452
In order to understand the magnetic field-induced restoration of a highly conductive state in , static (SQUID) and dynamic (ESR and AFR) magnetization measurements were performed on polycrystalline samples and single
crystals, respectively. In addition, cantilever and resistivity measurements under steady fields were performed. While the
metal-insulator transition curve of the () phase diagram exhibits a first order character, a “spin-flop” transition line divides the insulating state when the magnetic
field is applied along the easy axis of magnetization. The effects of a RKKY-type indirect exchange and of applied magnetic
field are described within the framework of a generalized Kondo lattice, namely two chains of localised spins coupled through the itinerant spins of the 2D sheets of BETS. The calculations, which can incorporate intramolecular
electron correlations within a mean field theory, are in qualitative agreement with the field induced transition from the
antiferromagnetic insulating ground state to a canted one, i.e. a not fully oriented paramagnetic, but metallic state.
Received: 6 August 1997 / Received: 5 November 1997 / Accepted: 10 November 1997 相似文献
2.
We study the forced aspiration of small ( mm) and large ( cm) liquid drops, deposited on prewetted porous membranes, and pumped mechanically with a constant current J. Two kinds of membranes are used where the pores are i) disconnected, cylindrical and calibrated or ii) interconnected “sponge-like”.
Whatever the size of the drops and the intensity J of the current, two suction regimes are observed versus time: 1) a “locked” regime, when the drop is pinned, with a dynamic contact angle decreasing from advancing () to finite receding () contact angle; 2) an “unlocked” regime, where the contour line recedes with a constant contact angle closed to . In both regimes, the shape of the drop remains quasistatic, during the suction process, i.e. a spherical cap for small drops and a flat “gravity pancake” for large ones.
Received 19 January 2000 相似文献
3.
The Mössbauer effect in129I in conjunction with Diamond-Anvil-Cell high pressure techniques was applied to investigate the high pressure phase(s) of the molecular crystal GeI4. The129IQuadrupole Interaction was the main probe for characterizing theintermolecular structural transformation with pressure. With increasing pressure, at about 15 GPa, the onset of a partial molecular-association phase (HP1) is first observed. In HP1 two out of the four iodines strongly overlap to form linear chains of GeI4. The HP1 phase coexists with the low pressure (LP) molecular phase, but its population increases with increasing pressure. AtP=20 GPa a second high pressure phase (HP2) is identified where all four iodines strongly overlap to form a three dimensional, fully molecular-associated structure. With increasing pressure and atP>20 GPa, HP2 is the only phase up toP=34 GPa, the highest pressure used. A significant hysteresis of the relative abundances with pressure is observed. The isomer shift of the HP2 and HP1 structures is considerably larger than that of the LP one. 相似文献
4.
A.A. Zvyagin A. Klümper J. Zittartz 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(1):25-36
The exactly solvable model of supersymmetric t - J chains (STJC) of correlated electrons with next-nearest-neighbour (NNN) interactions is proposed and studied. The model with
interactions between nearest neighbours and NNN interactions in one chain can also be considered as a two-chain model with
zigzag-like coupling between the chains. The NNN interaction (coupling between chains) causes the onset of additional Dirac
seas for low-lying charge and/or spin excitations. These Dirac seas change the low-energy (conformal) behavior of the model.
The filling of those seas depends on the values of the NNN coupling (interactions between chains), external magnetic field
and applied voltage. We identify the new ground state phases which appear due to the NNN as incommensurate ones. The NNN coupling
in the incommensurate phases induces spontaneous magnetization and/or spontaneous filling of the Dirac sea for charge excitations
(“spontaneous charge ordering”). The onset of this order implies a first order quantum phase transition driven by the field
with hysteresis phenomena.
Received 13 September 2000 相似文献
5.
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
6.
R. Schorr H. Rieger 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,33(3):347-354
We consider the optimal paths in a d-dimensional lattice, where the bonds have isotropically correlated random weights. These paths can be interpreted as the ground state configuration of a simplified polymer model in a random
potential. We study how the universal scaling exponents, the roughness and the energy fluctuation exponent, depend on the
strength of the disorder correlations. Our numerical results using Dijkstra's algorithm to determine the optimal path in directed
as well as undirected lattices indicate that the correlations become relevant if they decay with distance slower than 1/r in d = 2 and 3. We show that the exponent relation 2ν - ω = 1 holds at least in d = 2 even in case of correlations. Both in two and three dimensions, overhangs turn out to be irrelevant even in the presence
of strong disorder correlations.
Received 20 December 2002 / Received in final form 10 April 2003 Published online 20 June 2003
RID="a"
ID="a"e-mail: schorr@lusi.uni-sb.de 相似文献
7.
D. Stauffer A. Aharony L. da Fontoura Costa J. Adler 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(3):395-399
Neural networks are supposed to recognise blurred images (or patterns) of N pixels (bits) each. Application of the network to an initial blurred version of one of P pre-assigned patterns should converge to the correct pattern. In the “standard" Hopfield model, the N “neurons” are connected to each other via N2 bonds which contain the information on the stored patterns. Thus computer time and memory in general grow with N2. The Hebb rule assigns synaptic coupling strengths proportional to the overlap of the stored patterns at the two coupled
neurons. Here we simulate the Hopfield model on the Barabási-Albert scale-free network, in which each newly added neuron is
connected to only m other neurons, and at the end the number of neurons with q neighbours decays as 1/q
3. Although the quality of retrieval decreases for small m, we find good associative memory for 1 ≪ m ≪ N. Hence, these networks gain a factor N/m ≫ 1 in the computer memory and time.
Received 12 January 2003 Published online 11 April 2003
RID="a"
ID="a"e-mail: stauffer@thp.uni-koeln.de 相似文献
8.
Using the path integral method, we calculate the partition function and the generating functional (of the field strengths)
on nonlocal generalized 2D Yang–Mills theories (nlgYM's), which are nonlocal in the auxiliary field. This has been considered before by Saaidi and Khorrami. Our calculations are
done for general surfaces. We find a general expression for the free energy of in nlgYM theories at the strong coupling phase (SCP) regime () for large groups. In the specific model, we show that the theory has a third order phase transition.
Received: 24 June 2000 / Published online: 23 January 2001 相似文献
9.
Ngai KL 《The European physical journal. E, Soft matter》2002,8(2):225-235
Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as
evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including
the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended
to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the
lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce
the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel
to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a
polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass
transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced
almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of
the local segmental relaxation time found experimentally.
Received 1 August 2001 and Received in final form 1 December 2001 相似文献
10.
E. Leber S. Barsotti I.I. Fabrikant J.M. Weber M.-W. Ruf H. Hotop 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(1):125-131
Using a high resolution ( meV) laser photoelectron attachment method, we have studied the formation of (CO 2)
q
− ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO2)
N
() clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists
of several narrow vibrational Feshbach resonances of the type [(CO 2)
N
−1CO which involve a vibrationally-excited molecular constituent ( denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at
energies below those of the vibrational excitation energies of the neutral clusters [(CO 2)
N
−1CO ]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies.
The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects
the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster.
Received 11 January 2000 and Received in final form 10 April 2000 相似文献
11.
Single- and double-stranded DNA and many biological and synthetic polyelectrolytes undergo two structural transitions upon
increasing the concentration of multivalent salt or molecules. First, the expanded-stretched chains in low monovalent salt
solutions collapse into nearly neutral compact structures when the density of multivalent salt approaches that of the monomers.
With further addition of multivalent salt the chains redissolve acquiring expanded-coiled conformations. We study the redissolution
transition using a two-state model (F.J. Solis, M. Olvera de la Cruz, J. Chem. Phys. 112, 2030 (2000)). The redissolution occurs when there is a high degree of screening of the electrostatic interactions between
monomers, thus reducing the energy of the expanded state. The transition is determined by the chemical potential of the multivalent
ions in the solution, μ and the inverse screening length, κ. The transition point also depends on the charge distribution
along the chain but is nearly independent of the molecular weight and degree of flexibility of the polyelectrolytes. We generate
a diagram of μversusκ2 where we find two regions of expanded conformations, one with charged chains and the other with overcharged (inverted charge)
chains, separated by a collapsed nearly neutral conformation region. The collapse and redissolution transitions occur when
the trajectory of the properties of the salt crosses the boundaries between these regions. We find that in most cases the
redissolution occurs within the same expanded branch from which the chain precipitates.
Received 15 May 2000 and Received in final form 28 June 2000 相似文献
12.
G.I. Japaridze S. Sarkar 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(2):139-144
A one-dimensional model of interacting electrons with on-site U, nearest-neighbor V, and pair-hopping interaction W is studied at half-filling using the continuum limit field theory approach. The ground state phase diagram is obtained for
a wide range of coupling constants. In addition to the insulating spin-density wave (SDW) and charge-density wave (CDW) phases
for large U and V, respectively, we identify a bond-charge-density-wave (BCDW) phase W < 0, | U - 2V| < | 2W| and a bond-spin-density-wave (BSDW) for W > 0, | U - 2V| < W. The possibility of bond-located ordering results from the site-off-diagonal nature of the pair-hopping term and is a special
feature of the half-filled band case. The BCDW phase corresponding to an enhanced Peierls instability in the system. The BdSDW
is an unconventional insulating magnetic phase, characterized by a gapless spin excitation spectrum and a staggered magnetization
located on bonds between sites. The general ground state phase diagram including insulating, metallic, and superconducting
phases is discussed. A transition to the η-superconducting phase at | U - 2V| ≪ 2t?W is briefly discussed.
Received 20 February 2002 / Received in final form 11 April 2002 Published online 19 July 2002 相似文献
13.
M. Pinterić T. Vuletić M. Lončarić S. Tomić J.U. von Schütz 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(3):487-493
We report a detailed characterization of an unique 3-fold commensurate insulating state in single crystals of the organic
– inorganic d hybrid Cu(DMe-DCNQI)2 systems with deuterated and partially deuterated DCNQI ring, by means of low-frequency dielectric spectroscopy. A broad relaxation
mode of strength centred at kHz is observed in the hysteresis temperature region in which the insulating phase coexists with metallic islands. At lower
temperatures, outside the nucleation range, the relaxation narrows, approaching a Debye-like form for an overdamped response
of a system with a single degree of freedom. Both, the relaxation strength and the mean relaxation time () are much larger than that expected for single-particle excitations. These features suggest the origin of the dielectric
relaxation as an intrinsic property of the N = 3 charge density wave state.
Received 1 December 1999 and Received in final form 5 April 2000 相似文献
14.
L. Proville S. Aubry 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(1):41-58
The spatially localized bound states of two electrons in the adiabatic two-dimensional Holstein-Hubbard model on a square
lattice are investigated both numerically and analytically. The interplay between the electron-phonon coupling g, which tends to form bipolarons and the repulsive Hubbard interaction , which tends to break them, generates many different ground-states. There are four domains in the phase diagram delimited by first order transition lines. Except for the domain at weak electron-phonon coupling (small g) where the electrons remain free, the electrons form bipolarons which can 1) be mostly located on a single site (small , large g); 2) be an anisotropic pair of polarons lying on two neighboring sites in the magnetic singlet state (large , large g); or 3) be a “quadrisinglet state” which is the superposition of 4 electronic singlets with a common central site. This quadrisinglet
bipolaron is the most stable in a small central domain in between the three other phases. The pinning modes and the Peierls-Nabarro
barrier of each of these bipolarons are calculated and the barrier is found to be strongly depressed in the region of stability
of the quadrisinglet bipolaron.
Received 10 December 1998 相似文献
15.
C.M. Marques D. Izzo T. Charitat E. Mendes 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):353-358
We study the scattering intensity of dilute and semi-dilute solutions of star polymers. The star conformation is described
by a model introduced by Daoud and Cotton. In this model, a single star is regarded as a spherical region of a semi-dilute
polymer solution with a local, position dependent screening length. For high enough concentrations, the outer sections of
the arms overlap and build a semi-dilute solution (a sea of blobs) where the inner parts of the actual stars are embedded.
The scattering function is evaluated following a method introduced by Auvray and de Gennes. In the dilute regime there are
three regions in the scattering function: the Guinier region (low wave vectors, ) from where the radius of the star can be extracted; the intermediate region () that carries the signature of the form factor of a star with f arms: ; and a high wavevector zone () where the local swollen structure of the polymers gives rise to the usual q
-5/3
decay. In the semi-dilute regime the different stars interact strongly, and the scattered intensity acquires two new features:
a liquid peak that develops at a reciprocal position corresponding to the star-star distances; and a new large wavevector
contribution of the form q
-5/3
originating from the sea of blobs.
Received: 3 September 1997 / Revised: 13 January 1988 / Accepted: 31 March 1998 相似文献
16.
The sodium salt of polyacrylic acid (NaPA) precipitates in the presence of Ca2+-ions. This phase behaviour can be represented by a phase diagram where the critical NaPA concentration is plotted versus the critical Ca2+ concentration resulting in a straight line as a phase boundary. The location of this phase boundary is influenced by the
presence of an inert monovalent salt like NaCl. The present contribution focuses on the coil dimensions of NaPA chains in
dilute aqueous solution corresponding to the one phase region of such a phase diagram. A variety of parameters with which
the size and shape of the polyelectrolyte chains can be modulated are revealed. Approaching the phase boundary by decreasing
the NaPA concentration at a constant Ca2+ content leads to a collapse of the NaPA chains. Combined static and dynamic light scattering suggests a compact spherical
shape as the final state of this transition, both in 0.1 M NaCl and in 0.01 M NaCl. In the lower NaCl concentration, indication
is presented for the existence of a cigar or pearl necklace like intermediate. Most strikingly, the collapsed chains can be
reexpanded by increasing the concentration of inert NaCl at constant content of NaPA and Ca2+. Clearly, excessive Na+-ions displace the Ca2+-ions from the NaPA chains.
Received 18 July 2000 and Received in final form 24 August 2000 相似文献
17.
A.K. Ray J.C. Boettger 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(3):429-433
The linear combinations of Gaussian type orbitals fitting function (LCGTO-FF) method is used to study the electronic and geometrical
properties of plutonium monolayers with square and hexagonal symmetry. The effects of several common approximations are examined:
(1) scalar-relativity vs. full-relativity (i.e., with spin-orbit coupling included); (2) paramagnetic vs. spin-polarized; and (3) local-density approximation (LDA) vs. generalized- gradient approximation (GGA). The results indicate that spin-orbit coupling has a much stronger effect on the
monolayer properties compared to the effects of spin-polarization. In general, the GGA is found to predict a larger lattice
constant and a smaller cohesive energy compared to LDA predictions. We also find a significant compression of the monolayers
compared to the bulk, contradicting the only other published result on a Pu monolayer. The current result supports the existence
of a δ-like surface on α-Pu.
Received 17 October 2001 Published online 6 June 2002 相似文献
18.
F. Schönfeld G. Bouzerar G.S. Uhrig E. Müller-Hartmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):521-528
Using the density matrix renormalization group method (DMRG) we calculate the magnetization of frustrated S=1/2 Heisenberg chains for various modulation patterns of the nearest neighbour coupling: commensurate, incommensurate with
sinusoidal modulation and incommensurate with solitonic modulation. We focus on the order of the phase transition from the
commensurate dimerized phase (D) to the incommensurate phase (I). It is shown that the order of the phase transition depends
sensitively on the model. For the solitonic model in particular, a k-dependent elastic energy modifies the order of the transition. Furthermore, we calculate gaps in the incommensurate phase
in adiabatic approximation.
Received: 9 March 1998 / Accepted: 17 April 1998 相似文献
19.
Lagerwall JP Heppke G Giesselmann F 《The European physical journal. E, Soft matter》2005,18(1):113-121
We study the effects of mixing ferroelectric and antiferroelectric liquid-crystal compounds (FLCs and AFLCs) when the former
are strictly synclinic and the latter strictly anticlinic, i.e. one mixture component exhibits only SmC* and the other only SmC a* as tilted phase. Three different paths between syn- and anticlinicity were detected: transition directly between SmC* and
SmC a*, transition via the SmCβ* and SmCγ* subphases, or by “escaping” the clinicity frustration by reducing the tilt to zero, i.e. the SmA* phase is extended downwards in temperature, separating SmC* from SmC a* in the phase diagram. The most common path is the one via the subphases, demonstrating that these phases appear as a result
of frustration between syn- and anticlinic and, consequently, between syn- and antipolar order. For assessing the role of
chirality, we also replaced the FLC with non-chiral synclinics. With one of the AFLCs, the route via supbhases was detected
even in this case, suggesting that chirality --although necessary-- does not have quite the importance that has previously
been attributed to the appearance of the subphases. The path chosen in the mixture study seemed to be determined mainly by
the synclinic component, the subphase induction occurring only when the SmA*-SmC* transition was second order. 相似文献
20.
H. Nienhaus V. van Elsbergen W. Mönch 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(2):179-182
Si-terminated 3C-SiC(001) surfaces with () and ()reconstructions were investigated by high-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction
(LEED) and Auger electron spectroscopy. The surfaces were prepared by subsequent annealing steps after cleaning by heating
in a Si flux. At ()-reconstructed surfaces, the HREELS intensity increases while the widths of the loss lines decrease with proceeding preparation.
Eventually, weak loss structures at 380 and 700 cm-1 are detected besides the strong Fuchs-Kliewer phonon loss peaks. They are attributed to surface-localized vibrations, i.e., to stretching modes of on-top Si dimers and of C-Si-C groups, respectively. The weak features vanish after exposure to atomic
deuterium, but reappear after subsequent annealing. At () reconstructed surfaces the HREELS lines are broadened and no surface-localized modes were resolved.
Received 18 January 1999 相似文献