Publications

2004

• Vortex Phase Diagram in Bi2Sr2CaCu2O8+$\delta$ with Damage Tracks Created by 30 MeV Fullerene Irradiation
  
  • Ishikawa N.
  • van der Beek C. J.
  • Dunlop A.
  • Jaskierowicz G.
  • Li Ming
  • Kes P. H.
  • Della-Negra S.
  Journal of the Physical Society of Japan, Physical Society of Japan 日本物理学会, 2004, 73, pp.2813-2821. Using 30 MeV C60 fullerene irradiation, we have produced latent tracks of diameter 20 nm and length 200 nm, near the surface of single crystalline Bi2Sr2CaCu2O8+δ. A preliminary transmission electron microscopy study shows evidence for a very high density of deposited energy, and the ejection of material from the track core in very thin specimens. The latent tracks reveal themselves to be exceptionally strong pinning centers for vortices in the superconducting mixed state. Both the critical current density and magnetic irreversibility line are significantly enhanced. The irradiated crystals present salient features of the (B,T) phase diagram of vortex matter both of pristine crystals, such as the first order vortex phase transition, and the exponential Bose-glass line characteristic of heavy ion-irradiated crystals. We show that the latter is manifestly independent of the pinning potential (10.1143/JPSJ.73.2813)
  DOI : 10.1143/JPSJ.73.2813
• Raman investigation of structural changes induced by e-beam irradiation in Ge doped silica MCVD glasses
  
  • S. Jacqueline A.
  • Poumellec B.
  • C. Chervin J.
  • Garcia-Blanco S.
  • Esnouf S.
  Materials Science and Engineering: B, Elsevier, 2004, 107, pp.46. Microstructural changes of germanium (Ge)-doped silica glasses irradiated with electron beam (e-beam) have been investigated by Raman spectroscopy. Two mechanisms correlated to the specific volume changes induced by e-beam irradiation were observed by surface topography measurement. As we have previously demonstrated, they depend on the Ge content and the irradiation dose. The compaction of the structure (which decreases with the Ge concentration) is attributed to an increase of the three and four-membered ring species which is maximal at the surface and decreases with depth. By means of Monte-Carlo simulations, we have correlated the energy deposition with the compaction process. Some specific Raman components are attributed to the expansion of the structure observed for high Ge content and high irradiation dose. In this case, an increase of the anti-symmetric stretching modes of the bridging oxygen in the Si---O---Ge and Si---O---Si groups are observed at the surface
• Long- and short-range magnetic order charge transfer, and superconductivity in Yba2Cu3 -xCOxO7
  
  • Maury F.
  • Mirebeau I.
  • A. Hodges J.
  • Bourges P.
  • Sidis Yvan
  • Forget A.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2004, 69, pp.094506. We have performed powder-neutron-diffraction measurements in YBa2Cu3–xCoxO7 for a large range of Co concentrations (x = 0.04–0.72) and determined the magnetic phase diagram. As previously observed, superconductivity disappears for x0.42 and antiferromagnetic long-range order emerges above this value. The antiferromagnetic order shows noticeable differences compared to that in unsubstituted YBa2Cu3O6 + and surprisingly survives to a much higher hole doping. In contrast to recent results in YBa2Cu3–xCoxO7 single crystals, there is no evidence of any long-range antiferromagnetic order in the superconducting phase. The Co valence evolves from 4+ to 3+ with increasing Co concentration. We studied the short-range chemical and magnetic correlations between the Co ions in superconducting (x0.36) and antiferromagnetic (x0.42) samples. The Co ions predominantly occupy the Cu1 sites and their moments remain mostly disordered, forming random antiferromagnetic next nearest neighbor pairs, which compete with the long-range order. This can explain the low value for the ordered Cu1 site moment
• Local crystallization induced in Fe-based amorphous alloys by swift heavy projectiles
  
  • Rizza G.
  • Dunlop A.
  • Jaskierowicz G.
  • Kopcewicz M.
  Nuclear Instruments and Methods in Physics Research, North-Holland ; Elsevier [1981-1984], 2004, 226, pp.609-621. We report the effect of the high density of electronic excitation deposited along the path of swift heavy ions in various amorphous alloys. The samples were irradiated at low temperature with GeV Pb ions up to fluences of 1 × 1011 to 2.3 × 1013 cm−2. The formation of nanocrystalline phases was observed by transmission electron microscopy in the FINEMET-like Fe73.5Nb4.5Cr5Cu1B16 alloy and in the NANOPERM ternary Fe90Zr7B3 glassy systems but not in the Fe80.5Nb7B12.5 alloy. The crystallization phenomenon was interpreted in terms of the relaxation of the high level of energy deposited in electronic excitations along the path of Pb ions in the target, which induce radial pressure waves in the vicinity of the projectile trajectories. These mechanical stresses destabilize the amorphous structure and induce nanocrystallization
• Applications of Time-Dependent Density Functional Theory
  
  • Botti Silvana
  Physica Scripta, IOP Publishing, 2004, T109, pp.54. The progress of experimental techniques makes more and more precise measurements available. Their interpretation requires improved theoretical tools. The time-dependent density functional theory (TDDFT) allows to study electronic excitations involved in spectroscopic experiments, possibly conserving a computational effort comparable to that of ground-state density functional theory. I will present some applications of TDDFT to the calculation of absorption and electron energy loss spectra in extended and finite systems, discussing the advantages and the limits of different approximation schemes. (10.1238/Physica.Topical.109a00054)
  DOI : 10.1238/Physica.Topical.109a00054
• Guest displacement in silicon clathrates
  
  • Tournus Florent
  • Masenelli Bruno
  • Mélinon Patrice
  • Connétable Damien
  • Blase Xavier
  • Flank Anne-Marie
  • Lagarde Pierre
  • Cros Christian
  • Pouchard Michel
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2004, 69 (3), pp.035208 (6 p.). We study both theoretically and experimentally the structure of the doped silicon clathrate II NaxSi34. We find that contrary to published works, the sodium atoms do not retain the Td symmetry inside the Si28 cages and move about 1 Å away from the center of the cage. This displacement, in conjunction with that of a sodium atom in an adjacent Si28 cage, leads to a “dimerization” of sodium atoms. As a consequence, Rietveld refinements of x-ray diffraction spectra and transport, vibrational, and electronic properties must be revisited. (10.1103/PhysRevB.69.035208)
  DOI : 10.1103/PhysRevB.69.035208
• Origin of linear extended defects in single crystalline Bi2Sr2CaCu2O8
  
  • Li M.
  • H. Kes P.
  • F. W. R. Rycroft S.
  • J. van Der Beek C.
  • Konczykowski M.
  Physica C: Superconductivity and its Applications, Elsevier, 2004, 408, pp.25. Magneto-optical imaging and direction-dependent resistivity measurements are cross-correlated to provide evidence that the extended line defects in Bi2Sr2CaCu2O8 single crystals are needle-like Bi2Sr2Ca2Cu3O10 intergrowths that form along the crystal growth direction
• Vortex-lattice melting front in thin superconductors with pinning
  
  • V. Indenbom M.
  • H. Brandt E.
  • J. van Der Beek C.
  • Konczykowski M.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2004, 7014, pp.4525. Magneto-optical observations of a second flux front, which occurs at the second peak in the magnetization of Bi2Sr2CaCu2Ox single crystals related to the known first-order "vortex-lattice melting," are reconsidered. We show that, in thin samples, electrodynamics necessarily leads to an extended region in which the magnetic induction adopts a nearly constant value close to that at which the phase transition occurs at thermal equilibrium. In this region a dynamical phase mixture of vortex "solid" and "liquid" should exist. Interestingly, the observed second flux front does not mark the "melting" front, as it was naively interpreted earlier, but indicates the disappearance of the last "solid droplets".
• Ab initio study of the optical absorption and wave-vector-dependent dielectric response of graphite
  
  • G. Marinopoulos A.
  • Reining L.
  • Rubio A.
  • Olevano Valério
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2004, 6924, pp.5419. We performed ab initio calculations of the optical absorption spectrum and the wave-vector-dependent dielectric and energy-loss functions of graphite in the framework of the random-phase approximation. In the absorption spectrum, the most prominent peaks were analyzed in terms of interband transitions from specific regions of the Brillouin zone. The inclusion of the crystal local-field effects (LFE) in the response had an important influence on the absorption spectrum for light polarization parallel to the c axis. The calculated electron energy-loss spectra, even without LFE, were in very good agreement with existing momentum-dependent energy-loss experiments concerning the peak positions of the two valence-electron plasmons. Important aspects of the line shape and anisotropy of the energy-loss function at large momentum transfer q were also well described: the splitting of the total (pi+sigma) plasmon and the appearance of peaks originating from interband transitions. Finally, the role of the interlayer interaction was studied, in particular with regard to its effect on the absorption spectrum for light polarization parallel to c, and to the position of the higher-frequency pi+sigma plasmon
• Long range contribution to the exchange-correlation kernel of time-dependent density functional theory
  
  • Botti Silvana
  • Sottile F.
  • Vast N.
  • Olevano Valério
  • Reining L.
  • Weissker H-Ch.
  • Rubio A.
  • Onida G.
  • del Sole R.
  • Godby R.W.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2004, 69, pp.155112. We discuss the effects of a static long-range contribution -α/q2 to the exchange-correlation kernel fxc(q) of time-dependent density functional theory. We show that the optical absorption spectrum of solids exhibiting a strong continuum excitonic effect is considerably improved with respect to calculations where the adiabatic local-density approximation is used. We discuss the limitations of this simple approach, and in particular that the same improvement cannot be found for the whole spectral range including the valence plasmons and bound excitons. On the other hand, we also show that within the range of validity of the method, the parameter α depends linearly on the inverse of the dielectric constant, and we demonstrate that this fact can be used to predict continuum excitonic effects in semiconductors. Results are shown for the real and imaginary part of the dielectric function of Si, GaAs, AlAs, diamond, MgO, SiC and Ge, and for the loss function of Si. (10.1103/PhysRevB.69.155112)
  DOI : 10.1103/PhysRevB.69.155112
• Ferromagnetic and twin domains in LCMO manganites
  
  • Jung G.
  • Markovich V.
  • Mogilyanski D.
  • van Der Beek C.
  • Mukovskii Y.M.
  Journal of Magnetism and Magnetic Materials, Elsevier, 2004, 290-291 (Part 2), pp.902-905. Ferromagnetic and twin domains in lightly Ca-doped La1-xCaxMnO3 single crystals have been visualized and investigated by means of the magneto-optical technique. Both types of domains became visible below the Curie temperature. The dominant structures seen in applied magnetic field are associated with magneto-crystalline anisotropy and twin domains. In a marked difference to the twin domains which appear only in applied magnetic field, ferromagnetic domains show up in zero applied field and are characterized by oppositely oriented spontaneous magnetization in adjacent domains. Ferromagnetic domains take form of almost periodic, corrugated strip-like structures. The corrugation of the ferromagnetic domain pattern is enforced by the underlying twin domains
• Colour center production in yttria-stabilizing zirconia by swift charge particle irradiations.
  
  • Costantini Jean-Marc
  • Beuneu François
  • Gourier Didier
  • Trautmann Christina
  • Calas Georges
  • Toulemonde Marcel
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2004, 16, pp.3971. We have studied the colour centre production by swift electron and heavy ion irradiations of yttria-stabilized zirconia (YSZ), i.e. ZrO2:Y with 9.5 mol% Y2O3. For this purpose, we performed irradiations of \langle 100\rangle - or \langle 110\rangle -oriented YSZ single crystals with 2.5 MeV electrons, 145 MeV 13C, 180 MeV 32S, 200 MeV 58Ni, 230 MeV 79Br, 120 MeV 127I, 200 MeV 127I, 200 MeV 197Au, and 2.6 GeV 238U ions. X-band electron paramagnetic resonance (EPR) and UV–visible optical absorption measurements were used to monitor the point defect formation. The EPR line saturations were measured between 6 and 150 K, in order to obtain the spin–lattice relaxation time (T1). Electron and ion beams produce the same two colour centres: (i) the first one is identified as an F+-type centre (singly ionized oxygen vacancy) with an axial \langle 100\rangle symmetry, a small g-factor anisotropy (g_{\bot }=1.972 and g_{\parallel }=1.996 ) and long T1 values, (ii) the second one is similar to the well known T-centre (Zr3+ in a trigonal oxygen environment) with an axial \langle 111\rangle symmetry and a large g-factor anisotropy (g_{\bot }=1.855 and g_{\parallel }=1.986 ), which is also produced by photon irradiations. A broad optical absorption band centred at a wavelength near 500 nm is observed with an absorption coefficient proportional to the volume density of the F+-type centre deduced from the room temperature EPR spectra. Since no change of this band occurs between 10 and 300 K, it indicates that the electron–phonon coupling of this colour centre must be strong, in agreement with an F+-type centre. Owing to the axial \langle 100\rangle symmetry and lack of hyperfine structure of the EPR lines of this defect, it is suggested that the first coordination shell must contain one native oxygen vacancy. The plots of the volume density of this centre versus fluence are on the whole rescaled as functions of the number of displacements per atom induced by elastic collisions. (10.1088/0953-8984/16/23/014)
  DOI : 10.1088/0953-8984/16/23/014
• Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach
  
  • G. Marinopoulos A.
  • Wirtz L.
  • Marini A.
  • Olevano Valério
  • Rubio A.
  • Reining L.
  Applied physics. A, Materials science & processing, Springer Verlag, 2004, 78, pp.1157. We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy
• Ab initio and semiempirical dielectric response of superlattices
  
  • Botti Silvana
  • Vast Nathalie
  • Reining Lucia
  • Olevano Valério
  • Claudio Andreani Lucio
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2004, 70, pp.045301. We present a study of the dielectric response of (GaAs)p∕(AlAs)p (001) superlattices in a wide range of barrier and well widths. We applied density functional theory and a semiempirical method to obtain the superlattice band structures. These were then used as a starting point to evaluate the optical spectra and macroscopic dielectric constants using time-dependent density functional theory. In this context, we investigated the role of crystal local field effects in determining the anisotropy of the dielectric constants. Furthermore, we calculated absorption spectra including the strong continuum excitonic effect through the use of an appropriate model exchange-correlation kernel. We analyzed in detail the complementarity of the ab initio and semiempirical approaches and we compared the successes and limitations of the different approximation schemes. (10.1103/PhysRevB.70.045301)
  DOI : 10.1103/PhysRevB.70.045301
• Heating of Conduction Band Electrons by Intense Femtosecond Laser Pulses
  
  • Belsky Andrei
  • Martin Patrick
  • Bachau Henri
  • Vassiliev A.N.
  • Yatsenko Boris
  • Guizard Stéphane
  • Geoffroy Ghita
  • Petite Guillaume
  EPL - Europhysics Letters, European Physical Society / EDP Sciences / Società Italiana di Fisica / IOP Publishing, 2004, 67, pp.301-306.
• 111. Observation of high energy photoelectrons from solids at moderate laser intensity
  
  • Belsky Andrei
  • Bachau Henri
  • Gaudin Jérôme
  • Geoffroy Ghita
  • Guizard Stéphane
  • Martin Patrick
  • Petite Guillaume
  • Philippov A.
  • Vassil'Ev Andrei N.
  • Yatsenko Boris
  Applied Physics B - Laser and Optics, Springer Verlag, 2004, 78, pp.989-994.
• A structural approach by MAS NMR spectroscopy of mechanisms occurring under β-irradiation in mixed alkali aluminoborosilicate glasses
  
  • Ollier Nadège
  • Charpentier Thibault
  • Boizot Bruno
  • Petite Guillaume
  Journal of Physics: Condensed Matter, IOP Publishing [1989-....], 2004, 16, pp.7625-7635. Two series of mixed Na/K and Na/Li aluminoborosilicate glasses have been irradiated with electrons of 1.8 MeV at doses close to 109 Gy. Si, B and Al glass former environment changes under irradiation have been probed by MAS NMR spectroscopy. It was shown that the mixed alkali effect acts on the middle range compositions by operating with a selective blockage in irradiated glasses, depending on the alkali nature and concerning either modifier or charge compensator alkalis.
• Detailed analysis of the segregation driving forces for Ni(Ag) and Ag(Ni) in the sigma = 11{113} and sigma = 11{332} grain boundaries
  
  • Lezzar B.
  • Khalfallah O.
  • Larere A.
  • Paidar V.
  • Hardouin Duparc Olivier
  Acta Materialia, Elsevier, 2004, 52, pp.2809. The behaviour with respect to segregation is studied for two qualitatively different symmetrical tilt grain boundaries (GBs): a low energy boundary with mirror-symmetric structure on the {1 1 3} plane and the high energy {3 3 2} GB with a "zigzag" structural pattern. These two structures have been confirmed by high resolution transmission electronic microscopy observations in nickel. Intergranular segregation is studied in the limit of infinitely diluted solution for Ni(Ag) and Ag(Ni) systems modelled with Finnis–Sinclair like potentials. The atomic sites are characterized by a geometrical parameter defined by their exact Voronoy volumes and the pressure locally exerted. The segregation energies and driving forces are compared with these parameters in order to characterize a segregation "susceptibility" of intergranular atomic sites. The size effect calculated by the method of virtual impurity represents the main segregation driving force for the considered systems
• Transport properties and magnetic domain structure in low-doped LaCaMnO manganite single crystals
  
  • Jung G.
  • Markovich V.
  • Yuzhelevski Y.
  • Indenbom M.
  • J. van Der Beek C.
  • Mogilyanski D.
  • M. Mukovskii Y.
  Journal of Magnetism and Magnetic Materials, Elsevier, 2004, 272, pp.1800. The low temperature resistivity up-turn in lightly Ca doped LaCaMnO single crystals has been attributed to the domination of the tunneling mechanism in low temperature transport. The intrinsic tunnel barriers have been associated with spontaneous ferromagnetic domains walls. The magneto-optical technique has been employed to observe the relevant domain structure. A regular network of spontaneous ferromagnetic domains undergoes significant changes resulting in apparent rotation of the domain walls in the temperature range coinciding with the resistivity up-turn
• Flux lenses in the crossing lattices regime of layered superconductors
  
  • Cole D.
  • Crisan A.
  • J. Bending S.
  • Tamegai T.
  • van Der Beek K.
  • Konczykowski M.
  Physica C: Superconductivity and its Applications, Elsevier, 2004, 404, pp.99. We report here measurements of flux lensing behaviour which have been performed on Bi2Sr2CaCu2O8+delta single crystals placed on top of linear Hall probe arrays. In the crossing lattices regime we observe significant flux focussing as the in-plane field is swept up with a small constant density of pancake vortices. We also observe a sharp discontinous lock-in feature in local magnetic induction when the net field is close to the a-b plane
• Dynamics of Femtosecond Laser Interactions with Dielectrics
  
  • Mao S.S.
  • Quéré Fabien
  • Guizard Stéphane
  • Mao X. L.
  • Russo R. E.
  • Petite Guillaume
  • Martin Philippe
  Applied physics. A, Materials science & processing, Springer Verlag, 2004, 79 (7), pp.1695. Femtosecond laser pulses appear as an emerging and promising tool for processing wide band-gap dielectric materials for a variety of applications. This article aims to provide an overview of recent progress in understanding the fundamental physics of femtosecond laser interactions with dielectrics that may have the potential for innovative materials applications. The focus of the overview is the dynamics of femtosecond laser-excited carriers and the propagation of femtosecond laser pulses inside dielectric materials.
• Exactly solvable models : a solution to different problems of laser matter interaction
  
  • Petite Guillaume
  • Shvartsburg Alexander
  , 2004, pp.11-20. With the increasing use of ultrashort laser pulses and nanoscale-materials, one is regularly confronted to situations in which the properties of the media supporting propagation are not varying slowly with time (or space). Hence, the usual WKB-type approximations fail, and one has to resort to numerical treatments of the problems, with a considerable loss in our insight into the physics of laser-matter interaction. We will present a new approach which allows a fully analytical solution of such problems, based on a transformation of the propagation equations into a new space where phase accumulation is linear with either time or space, which greatly simplifies their treatment. Though this method is restricted to some special models of the time or space varying dielectric constant, those are however general enough to encompass practically all experimental situations. It allows to introduce the concept of "non-stationarity induced" (or "inhomogeneity induced") dispersion. We will analyse the problem of reflection and propagation in two types of media whose dielectric constant vary rapidly at either the laser period or the laser wavelength scale. Extension of such techniques to the case of arbitrarily high non linearities will be considered too.
• Electron acceleration by surface plasma waves in the interaction between femtosecond laser pulses and sharp-edged overdense plasmas
  
  • Kupersztych J
  • Raynaud M
  • Riconda C
  Physics of Plasmas, American Institute of Physics, 2004. The relativistic acceleration of electrons by the field of surface plasma waves created in the interaction between ultrashort high-intensity laser pulses with sharp-edged overdense plasmas has been investigated. It is shown that the initial phase of the wave experienced by the electrons play a leading part by yielding a well-defined peaked structure in the energy distribution function. This study suggests that resonant excitation of surface plasma waves could result in quasi-monokinetic energetic electron bunches. When the space charge field becomes too strong, this mechanism can evolve toward a true absorption process of the surface wave energy via an enhanced ''vacuum heating'' mechanism generalized to the case of surface plasma waves. (10.1063/1.1650353͔)
  DOI : 10.1063/1.1650353͔
• Optical anisotropy induced by cesium adsorption on the As-rich c(2X8) reconstruction of GaAs(001)
  
  • Hogan C.
  • Paget D.
  • E. Tereshchenko O.
  • Reining L.
  • Onida G.
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2004, 6912, pp.5332. Upon adsorption of Cs, the As-rich c(2 X 8)/(2 X 4) reconstruction of GaAs(001) is found to exhibit an intense negative signal between 3 eV and 5 eV in the reflectance anisotropy spectrum. This signal has a universal character in that similar features also appear on the Ga-rich surface. The mechanism of this signal is interpreted using ab initio calculations of Cs adsorption at As and Ga sites of the As-rich surface. The calculations succeed in explaining the universality of the signal. In the vicinity of the E-0' bulk critical point at 4.5 eV, the signal arises from perturbation of bulk states terminating at the surface. At lower energies, the signal arises from the creation of new surface resonances induced by the Cs adatom
• Spin-dependent magnetoresistance in multiwall carbon nanotubes
  
  • Hoffer X.
  • Klinke C.
  • M. Bonard J.
  • Gravier L.
  • E. Wegrowe J.
  EPL - Europhysics Letters, European Physical Society / EDP Sciences / Società Italiana di Fisica / IOP Publishing, 2004, 67, pp.103. The spin-dependent transport in multiwall carbon nanotubes, obtained by chemical vapour deposition in porous alumina membranes, has been investigated. The zero-bias anomaly is shown to follow the typical power law GT(-alpha) (eV/kT). In the nanotubes contacted with Ni pads, the magnetoresistance due to the spin-polarised current (SD-MR) is destroyed. In the case of those contacted with Co pads, however, the SD-MR is observed. These measurements show that both the observation of a typical scaling law of the conductance (for nanotubes tip to 1.5 mum) and a short spin diffusion length (less than 0.15 mum with nickel contacts and 0.7 mum with cobalt contacts) coexist through the nanotube. This observation may be interpreted in terms of a reminiscence of the Luttinger-liquid behaviour with spin-charge separation