Search

Polarons--electronic charge carriers 'dressed' by a local polarization of the background environment--are among the most fundamental quasiparticles in interacting many-body systems, and emerge even at the level of a single dopant.sup.1. In the context of the two-dimensional Fermi-Hubbard model, polarons are predicted to form around charged dopants in an antiferromagnetic background in the low-doping regime, close to the Mott insulating state.sup.2-7; this prediction is supported by macroscopic transport and spectroscopy measurements in materials related to high-temperature superconductivity.sup.8. Nonetheless, a direct experimental observation of the internal structure of magnetic polarons is lacking. Here we report the microscopic real-space characterization of magnetic polarons in a doped Fermi-Hubbard system, enabled by the single-site spin and density resolution of our ultracold-atom quantum simulator. We reveal the dressing of doublons by a local reduction--and even sign reversal--of magnetic correlations, which originates from the competition between kinetic and magnetic energy in the system. The experimentally observed polaron signatures are found to be consistent with an effective string model at finite temperature.sup.7. We demonstrate that delocalization of the doublon is a necessary condition for polaron formation, by comparing this setting with a scenario in which a doublon is pinned to a lattice site. Our work could facilitate the study of interactions between polarons, which may lead to collective behaviour, such as stripe formation, as well as the microscopic exploration of the fate of polarons in the pseudogap and 'bad metal' phases. Magnetic polarons are imaged with single-site spin and density resolution in the low-doping regime of the atomic Fermi-Hubbard model, showing that mobile delocalized doublons are necessary for polaron formation., Author(s): Joannis Koepsell [sup.1] , Jayadev Vijayan [sup.1] , Pimonpan Sompet [sup.1] , Fabian Grusdt [sup.2] [sup.3] , Timon A. Hilker [sup.1] [sup.5] , Eugene Demler [sup.2] , Guillaume Salomon [...]

One of the issues in applying the generalized gradient approximation-1/2 (GGA-1/2) quasiparticle approximation to correct the excited-state properties in defective semiconductors is determining the cutoff to the self-energy function at the point defect. Pure and large supercells of N-doped rutile Ti[O.sub.2] were studied using this method to correct excited states and find the correct position of the defect states. A relation between the Bader charge at the defect and the self-energy cutoff parameter of the GGA-1/2 method is shown, with the cutoff value in its p-orbital, which optimizes the position of the defect levels. It was found that, upon nitrogen substitution and associated oxygen vacancy formation, an impurity level at 0.40-0.52 eV above the valence band maximum appears. The same result was obtained upon nitrogen substitution and associated background charge. Finally, the method was also applied to other p-orbital defect systems like Si:X (X = N, P, B) to validate the method. PACS Nos.: 71.55.-i, 71.15.Qe. Une des questions concernant l'utilisation de l'approximation quasiparticule GGA-1/2 pour corriger les proprietes des etats excites de semi-conducteurs avec defauts est la determination de la coupure (cutoff) de la fonction d'auto-energie au defaut ponctuel. Nous etudions des super-cellules pures et grandes de rutile Ti[O.sub.2] a dopage N en utilisant cette methodologie pour corriger les etats excites et trouver la bonne position des etats defauts. Nous montrons une relation entre la charge partielle de Bader et le parametre de coupure de la methode GGA-1/2, avec la valeur de coupure dans l'orbitale p, qui optimise la position des niveaux defauts. Nous trouvons que, sous substitution de l'azote et la formation de lacunes oxygenes associees, apparait un niveau d'impurete a 0.4-0.52 eV au dessus du sommet de labande de valence. Le meme resultat est obtenu sous substitution de l'azote et de la charge de fond associee. Finalement, nous appliquons la meme technique fi d'autres defauts dans l'orbitale p de systemes comme Si:X (X = N, P, B) afin de valider la methode. [Traduit par le Redaction], 1. Introduction Photocatalytic materials have emerged as a potential clean replacement for fossil fuels in hydrogen production processes [1, 2]. In particular, Ti[O.sub.2] has been studied most, both theoretically and [...]