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1.
In the past different models for the magnetic salt vanadyl pyrophosphate (VO)2P2O7 were discussed. Neither a spin ladder nor an alternating chain are capable to describe recently measured magnetic excitations.
In this paper we propose a 2D model that fits better to experimental observations.
Received: 29 May 1998 / Received in final form: 25 September 1998 / Accepted: 30 September 1998 相似文献
2.
C. Kohl S.M. El-Gammal F. Calvayrac E. Suraud P.-G. Reinhard 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,5(2):271-274
We investigate spin modes in the ground state and the polarized first isomer of the Na12 cluster describing the valence electrons in time-dependent local-spin-density approximation (TDLSDA) and the detailed ionic
background using local pseudopotentials. The spin modes show a collective redshift compared to the unperturbed particle-hole
excitations. They are strongly fragmented and the average energy of the modes along the principal axes are related to the
underlying geometry (triaxial or axially symmetric). For the polarized isomer, we find significant cross talk between the
spin modes and the dipole plasmon, which hints at a possible spectroscopic identification.
Received: 22 June 1998 / Accepted: 29 July 1998 相似文献
3.
Y. Piskunov D. Jérome P. Auban-Senzier P. Wzietek U. Ammerahl G. Dhalenne A. Revcolevschi 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):417-420
63Cu-NMR experiments of Knight shift and relaxation time T1 have been performed on the two-leg spin ladders of a Sr2Ca12Cu24O41 single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of
the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed
to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction.
Received 21 October 1999 相似文献
4.
A.V. Prokofiev F. Büllesfeld W. Assmus H. Schwenk D. Wichert U. Löw B. Lüthi 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):313-316
Experimental results on magnetic resonance (ESR) and magnetic susceptibility are given for single crystalline (VO)2P2O7. The crystal growth procedure is briefly discussed. The susceptibility is interpreted numerically using a model with alternating
spin chains. We determine J
=51 K and . Furthermore we find a spin gap of meV from our ESR measurements. Using elastic constants no indication of a phase transition forcing the dimerization is seen
below 300 K.
Received: 22 December 1997 / Accepted: 17 March 1998 相似文献
5.
S. Petit A.H. Moudden B. Hennion A. Vietkin A. Revcolevschi 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(2):163-167
Inelastic neutron scattering measurements have been performed on high quality single crystals of in order to study the spin dynamics of this compound. In addition to the well-established incommensurate magnetic response,
we show the existence of a new set of low energy excitations present in the whole superconducting region of the phase diagram.
This new feature of the dynamical cross section is characterized, below about 10 K, by very short range ( lattice spacing) antiferromagnetic correlations and by a low energy scale of meV. At higher temperatures these fluctuations become nearly Q-independent. Different possible origins of these new spin correlations are discussed.
Received: 3 September 1997 / Revised: 21 November 1997 / Accepted: 26 January 1998 相似文献
6.
N. Cavadini Ch. Rüegg W. Henggeler A. Furrer H.-U. Güdel K. Krämer H. Mutka 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):565-571
A complete temperature characterization of the spin dynamics in the unconventional S =1/2 antiferromagnet KCuCl3 is presented from single crystal inelastic neutron scattering studies. KCuCl3 features a quantum disordered singlet ground state with a finite spin gap to triplet excitations of dimer origin. Three dimensional magnetic correlations support the dispersive propagation of the excitations in the whole reciprocal space. Upon increasing the temperature, a renormalization in the energy, in the intensity and in the damping rate of the triplet modes is reported. The experimental observations can be described within the framework of a selfconsistent dimer RPA theory, with no free parameters. The driving mechanism behind the model is the thermally activated decrease of the occupation difference n 0 - n 1 between singlet and triplet dimer states. This is the expression of kinematic constraints which are of minor importance for classical magnons in Néel ordered antiferromagnets. Implications for the temperature dependence of macroscopic quantities are discussed. 相似文献
7.
R. Calemczuk J. Riera D. Poilblanc J.-P. Boucher G. Chaboussant L. Lévy O. Piovesana 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):171-174
Specific heat (CV) measurements in the spin-1/2 Cu2(C2H12N2)2Cl4 system under a magnetic field up to H
=8.25 T are reported and compared to the results of numerical calculations based on the 2-leg antiferromagnetic Heisenberg ladder.
While the temperature dependences of both the susceptibility and the low-field specific heat are accurately reproduced by
this model, deviations are observed above the critical field HC1 at which the spin gap closes. In this Quantum High Field phase, the contribution of the low-energy quantum fluctuations are
stronger than in the Heisenberg ladder model. We argue that this enhancement can be attributed to dynamical lattice fluctuations.
Finally, we show that such a Heisenberg ladder, for H
>
H
C1, is unstable, when coupled to the 3D lattice, against a lattice distortion. These results provide an alternative explanation
for the observed low temperature ( K-0.8 K) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state.
Received: 23 July 1998 / Accepted: 24 August 1998 相似文献
8.
P. Schieffer C. Krembel M.-C. Hanf M.-H. Tuilier P. Wetzel G. Gewinner K. Hricovini 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):165-168
We studied the magnetic properties of ultra-thin Mn films deposited on Ag (001) held at 80 K with soft X-ray absorption and
magnetic circular dichroism. The observed shape and branching ratio of the Mn 2p absorption edge as a function of Mn coverage demonstrate that, up to , the Mn adopts a stable high spin state similar to the Mn atom Hund's rule 6
S
5/2
ground state. Above this coverage a rapid transition from localized high spin to itinerant low spin behavior of the Mn 3d electrons is evidenced. Magnetic circular dichroism shows no sign of long range ferromagnetic order in these films at 80
K. The data, first confirm the large atomic-like local magnetic moment, and second are in line with the in-plane antiferromagnetic order, reported recently (Phys. Rev. B 57, 1141 (1998)), for Mn in the nearly ideal on-top Mn monolayer formed by 0.9 ML deposited at 80 K.
Received: 4 May 1998 相似文献
9.
M. Sassetti B. Kramer 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(3):357-367
The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band
models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified,
and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the
polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the
signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency
of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes.
The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies
are characteristic for a non-Fermi liquid.
Received: 25 March 1998 / Accepted: 3 June 1998 相似文献
10.
D. Augier D. Poilblanc 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):19-28
Properties of low-dimensional spin-Peierls systems are described by using a one-dimensional S
=1/2 antiferromagnetic Heisenberg chain linearly coupled to a single phonon mode of wave vector (whose contribution is expected to be dominant). By exact diagonalizations of small rings with up to 24 sites supplemented by a finite size scaling analysis, static and dynamical properties are investigated. Numerical evidences
are given for a spontaneous discrete symmetry breaking towards a spin gapped phase with a frozen lattice dimerization. Special
emphasis is put on the comparative study of the two inorganic spin-Peierls compounds CuGeO3 and NaV2O5 and the model parameters are determined from a fit of the experimental spin gaps. We predict that the spin-phonon coupling
is 2 or 3 times larger in NaV2O5 than in CuGeO3. Inelastic neutron scattering spectra are calculated and similar results are found in the single phonon mode approximation
and in the model including a static dimerization. In particular, the magnon S
=1 branch is clearly separated from the continuum of triplet excitations by a finite gap.
Received: 30 July 1997 / Revised: 16 September 1997 / Accepted: 10 October 1997 相似文献
11.
Y.S. Lee M. Greven B.O. Wells R.J. Birgeneau G. Shirane 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):15-22
We report a neutron scattering study of the instantaneous spin correlations in the two-dimensional spin S
=5/2 square-lattice Heisenberg antiferromagnet Rb2MnF4. The measured correlation lengths are quantitatively described, with no adjustable parameters, by high-temperature series
expansion results and by a theory based on the quantum self-consistent harmonic approximation. Conversely, we find that the
data, which cover the range from about 1 to 50 lattice constants, are outside of the regime corresponding to renormalized
classical behavior of the quantum non-linear model. In addition, we observe a crossover from Heisenberg to Ising critical behavior near the Néel temperature; this crossover
is well described by a mean-field model with no adjustable parameters.
Received: 3 March 1998 / Received in final form: 4 May 1998 /
Accepted: 19 May 1998 相似文献
12.
E. Ercolessi G. Morandi M. Roncaglia 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,32(4):489-493
Recent neutron scattering experiments on CsNiCl3 reveal some features that are not well described by the standard nonlinear σ model, nor by numerical simulations, for isolated S
= 1 spin chains. In particular, in real systems at the antiferromagnetic point of the Brillouin zone, the intensity of the continuum
of multiparticle excitations, at T
= 6 K, is about 5 times greater than predicted. Also, the spin gap is higher and the correlation length is smaller than predicted.
We propose a theoretical scenario where the interchain interaction is approximated by an effective staggered magnetic field,
and that yields a correct prediction for the observed quantities.
Received 2 October 2002 / Received in final form 19 March 2003 Published online 7 May 2003 相似文献
13.
C. Waldtmann H.-U. Everts B. Bernu C. Lhuillier P. Sindzingre P. Lecheminant L. Pierre 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):501-507
We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis
of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather
unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless
continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these
states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg
antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably
they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets.
Received: 27 November 1997 / Accepted: 29 January 1998 相似文献
14.
R. Guirado-López D. Spanjaard M.-C. Desjonquères A.M. Oleś 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(4):437-446
We analyze the stability of magnetic states obtained within the tight-binding model for cubooctahedral (Oh) and icosahedral (Ih) clusters of early 4d (Y, Zr, Nb, Mo, and Tc) transition metals. Several metastable magnetic clusters are identified which suggests the existence
of multiple magnetic solutions in realistic systems. A bulk-like parabolic behavior is observed for the binding energy of
Oh and Ih clusters as a function of the atomic number along the 4
d-series. The charge transfer on the central atom changes sign, while the average magnetic moments present an oscillatory behavior
as a function of the number of d electrons in the cluster. Our results are in agreement with other theoretical calculations.
Received: 20 November 1997 / Received in final form: 9 March 1998 / Accepted: 30 March 1998 相似文献
15.
P. Vonlanthen J.L. Gavilano B. Ambrosini H.R. Ott Y. Aoki H. Sugawara H. Sato 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):15-20
We report the results of 69Ga- and 71Ga-NMR measurements on NdGa2 at temperatures between 0.1 and and in applied magnetic fields between zero and 74 kOe. NdGa2 orders antiferromagnetically below and undergoes several metamagnetic transitions in external magnetic fields. In zero applied magnetic field and below the temperature dependence of the spin-lattice relaxation rate T1
-1
(
T
) shows a large linear-in-T term, about two orders of magnitude higher than for the reference compound LaGa2. This strong enhancement confirms the presence of low-energy excitations in the antiferromagnetic phase of NdGa2 as was previously indicated by specific heat data. Above , T1
-1
(
T
) is dominated by an exponential term, which we associate with excitations between the lowest energy levels of the f-electron system. The separation of these energy levels is determined by exchange, crystal-field and Zeeman interactions.
Received 3 September 1998 and Received in final form 3 November 1998 相似文献
16.
M. Vojta K.W. Becker 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(4):427-435
We study frequency- and wave-vector dependent charge correlations in weakly doped antiferromagnets using Mori-Zwanzig projection
technique. The system is described by the two-dimensional t-J model. The ground state is expressed within a cumulant formalism which has been successfully applied to study magnetic properties
of the weakly doped system. Within this approach the ground state contains independent spin-bag quasiparticles (magnetic polarons).
We present results for the charge-density response function and for the optical conductivity at zero temperature for different
values of t
/
J. They agree well with numerical results calculated by exact diagonalization techniques. The density response function for
intermediate and large momenta shows a broad continuum on energy scales of order of several t whereas the optical conductivity for is dominated by low energy excitations (at 1.5-2J). We show that these weak-doping properties can be well understood by transitions between excited states of spin-bag quasiparticles.
Received: 10 July 1997 / Revised: 19 March 1998 / Accepted: 3 April 1998 相似文献
17.
J.-C. Lasjaunias K. Biljaković D. Starešinić P. Monceau S. Takasaki J. Yamada S.-I. Nakatsuji H. Anzai 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(4):541-550
We report on specific heat measurements of the quasi-one-dimensional organic salt (TMTSF)2AsF6 in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)2PF6, we find discontinuities in the lattice contribution at 1.9 K an d 3.5 K ascribed to sub-spin density wave phases. Time-dependent
effects due to dynamics of low-energy excitations in metastable states occur only below 0.2 K which yields an activation energy
for the equilibrium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)2PF6. Finally the reduction of the low-energy excitations contribution to the specific heat in comparison to PF6 reveals an intermediate cubic-like regime between 0.25 and 0.5 K that we tentatively describe as the phason contribution
of the incommensurate spin density wave modulation.
Received: 17 March 1998 / Revised: 27 July 1998 / Accepted: 22 September 1998 相似文献
18.
Y. Piskunov D. Jérome P. Auban-Senzier P. Wzietek C. Bourbonnais U. Ammerhal G. Dhalenne A. Revcolevschi 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):443-456
63Cu-NMR measurements have been performed on two-leg hole-doped spin ladders Sr14-xCaxCu24O41 single crystals 0 ?
x
? 12 at several pressures up to the pressure domain where the stabilization of a superconducting ground state can be achieved.
The data reveal a marked decrease of the spin gap derived from Knight shift measurements upon Ca substitution and also under
pressure and confirm the onset of low lying spin excitations around P
c
as previously reported. The spin gap in Sr
2
Ca
12
Cu
24
O
41
is strongly reduced above 20 kbar. However, the data of an experiment performed at P
= 36 kbar where superconductivity has been detected at 6.7 K by an inductive technique have shown that a significant amount of
spin excitations remains gapped at 80 K when superconductivity sets in. The standard relaxation model with two and three-magnon
modes explains fairly well the activated relaxation data in the intermediate temperature regime corresponding to gapped spin
excitations using the spin gap data derived from Knight shift experiments. The data of Gaussian relaxation rates of heavily
doped samples support the limitation of the coherence length at low temperature by the average distance between doped holes.
We discuss the interplay between superconductivity and the spin gap and suggest that these new results support the exciting
prospect of superconductivity induced by the interladder tunneling of preformed pairs as long as the pressure remains lower
than the pressure corresponding to the maximum of the superconducting critical temperature.
Received 8 March 2001 and Received in final form 27 July 2001 相似文献
19.
F. Tedoldi A. Campana P. Carretta 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,7(2):219-223
μSR measurements in the antiferromagnetic (AF) phase of Ni1-xLixO for are reported. While in pure NiO the muon longitudinal depolarization rate is found almost temperature independent, in the Li-doped compounds broad maxima around 130 K are observed. These maxima are
associated with the progressive freezing of the spin fluctuations of S
=1/2 defects induced by the localization of the extra-holes. From the temperature dependence of and the stretched exponential form of the depolarization, insights on the distribution of correlation times for the fluctuating
field at the muon site are derived.
Received: 27 April 1998 / Received in final form and Accepted: 21 August 1998 相似文献
20.
A. Furrer H.-U. Güdel 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(1):81-84
The inelastic neutron scattering technique was employed to study the magnetic excitation spectra in the diluted one-dimensional
Heisenberg antiferromagnet CsMn1-xMgxBr3 (x
=0, 0.05, 0.10, 0.25, 0.50). The spectral response is interpreted in terms of spin-wave excitations in finite chain segments
of Mn2+ ions, which are found to exist as long as the chain length exceeds twice the wavelength of the spin excitation. This limit
determines the crossover into the mesoscopic regime.
Received 31 December 1999 相似文献