Fractionnally charged excitations in the charge density wave state of a quarter-filled t-J chain with quantum phonons

Elementary excitations of the 4k$_F$ charge density wave state of a quarter-filled strongly correlated electronic one-dimensional chain are investigated in the presence of dispersionless quantum optical phonons using Density Matrix Renormalization Group techniques. Such excitations are shown to be topological unbound solitons carrying charge $e/2$. Relevance to the 4k$_F$ charge density wave instability in $\rm (DI-DCNQI)_2Ag$ or recently discovered in (TMTTF)$_2$X (X=PF$_6$, AsF$_6$) is discussed.Comment: 4 pages, 4 figure

Perfect absorption of water waves by linear or nonlinear critical coupling

We report on experiments of perfect absorption for surface gravity waves impinging a wall structured by a subwavelength resonator. By tuning the geometry of the resonator, a balance is achieved between the radiation damping and the intrinsic viscous damping, resulting in perfect absorption by critical coupling. Besides, it is shown that the resistance of the resonator, hence the intrinsic damping, can be controlled by the wave amplitude, which provides a way for perfect absorption tuned by nonlinear mechanisms. The perfect absorber that we propose, without moving parts or added material, is simple, robust and it presents a deeply subwavelength ratio wavelength/size $\simeq 18$

Similarities between the t−Jt-J and Hubbard models in weakly correlated regimes

We present a comparative study of the Hubbard and $t-J$ models far away from half-filling. We show that, at such fillings the $t-J$ Hamiltonian can be seen as an effective model of the repulsive Hubbard Hamiltonian over the whole range of correlation strength. Indeed, the $|t/U| \in [0,+\infty [$ range of the Hubbard model can be mapped onto the finite range $|J/t| \in [1, 0 ]$ of the $t-J$ model, provided that the effective exchange parameter $J$ is defined variationally as the local singlet-triplet excitation energy. In this picture the uncorrelated limit U=0 is associated with the super-symmetric point $J=-2|t|$ and the infinitely correlated $U=+\infty$ limit with the usual J=0 limit. A numerical comparison between the two models is presented using different macroscopic and microscopic properties such as energies, charge gaps and bond orders on a quarter-filled infinite chain. The usage of the $t-J$ Hamiltonian in low-filled systems can therefore be a good alternative to the Hubbard model in large time-consuming calculations.Comment: To be published in EPJB. 6 pages. 5 figure

Observatoires et gouvernance territoriale : une approche par la co-construction de modèles

International audienceObservatories, defined as sociotechnical devices for observation, analysis and debate, tend increasingly to play a key role in territorial governance. Political, semantical and organizational difficulties arise then, and the risks of failure are many. After analyzing these difficulties, we describe two models and explain how then can remediate: the " model of action " and the " observation model ". We then articulate the co-construction of these models and the more technical tasks of information system building within the Co-Obs method.Les observatoires, définis comme des dispositifs sociotechniques d'observation, d'analyse et de mise en débat, tendent de plus en plus à devenir des supports majeurs de la gouvernance au sein des territoires. Des difficultés politiques, sémantiques et organisationnelles surgissent alors, et les risques d'échec sont nombreux. Après avoir analysé ces difficultés, nous décrivons deux modèles pour les aborder fructueusement lors de la construction de l'observatoire : le « modèle de l'action » et le « modèle de l'observation ». Nous articulons ensuite, au sein de la méthode originale Co-Obs, la co-construction de ces modèles avec les tâches plus techniques de développement du système d'information

Stress-assisted versus strain-induced martensites formed by cryogenic ultrasonic shot peening in austenitic stainless steels

International audienceIntroduction Severe plastic deformation (SPD) is used to create nanocrystalline metallic materials resulting in high strength but associated, generally and unfortunately, with a reduced ductility [1]. On one side, the cryogenic temperature that improves the grain refinement by preventing dynamic recrystallization or self-annealing, has been used during SPD processes such as equal channel angular extrusion (ECAE) or high pressure torsion (HPT), effectively producing significant extra grain refinement down to the nanometer scale [2-4]. On the other side, numerous research works have been done to improve the low ductility by creating multi-length scale structures [5] or grain size gradients [6]. In steels, other mechanisms can be active and lead to a significant improvement of the strength/ductility balance such as TRIP (Transformation Induce Plasticity) [7] or the TWIP (TWinning Induced Plasticity) [8] effects. In the case of the metastable austenitic stainless steel, the TRIP effect is produced through the martensitic phase transformation. The martensitic transformation requires an activation energy to be triggered which can be produced either thermally or by a mechanical loading. Two temperatures, the Ms and Md30, are used to evaluate the occurrence of the martensitic transformation. The Ms temperature represents the temperature at which the martensitic phase transformation can be triggered spontaneously without an external loading. By applying a loading, the transformation can take place at higher temperatures than Ms and the stress or strain required to activate the process will vary with the temperature [9]. The Md30 temperature, higher than the Ms, reflects the temperature at which a martensitic fraction of 50% can be formed under a true strain of 30 %. When the martensitic phase transformation is triggered slightly higher than the material Ms temperature, elastic stresses in the microstructure are enough to activate the transformation and the elastic energy induced in the material is enough to compensate the missing chemical driving force at this temperature [11]. On the other hand, when the deformation is applied close to the material Md30 temperature, the transformation will be mainly controlled by plastic deformation and the role of deformation defects will control the transformation process [10]. The so-formed martensites can then be considered as different and called Stress-Assisted Martensite (SAM) and Strain-Induced Martensite (SIM), respectively. On the other hand, TWIP can happen when Stacking Fault Energies (SFE) is in the range 18-45 m.Jm-2 for austenitic structures. Deformation twinning is especially promoted by high strain rate. The ' martensite can be produced at the intersection of mechanical twins as this volume is double-sheared, resulting in the nucleation of the phase:    (twins)  '. In the case of lower SFE (<18 mJm-2), martensitic transformation can involve the formation of a transient phase named -martensite. The formation of the -martensite is driven by the insertion of Shockley partial dislocations in every two successive {111} plans [13]. The face-centered cubic austenite is consequently transformed in the hexagonal close-packed -martensite as they share their same atomic packing factor. Thus, under increasing loading, the -martensite will act as a transient phase to produce the more stable ' martensite as follows:     '