Soutenance de thèse de Haneen Abushhammala

Haneen Abushhammala, doctorants dans l'équipe Design et étude de nouveaux matériaux à propriétés remarquables (DEMARE) soutient sa thèse le jeudi 25 mars 2021 à 14 h.

IMPMC - Sorbonne Université - 4 place Jussieu, 75005 Paris, tour 23, 4e étage, couloir 22-23, salle 401

Tuning the multiorbital Mott transition of BaCoS₂

Abstract

The objective of the thesis is to tune the Matt insulating phase of BaCoS₂, a playground for the study of electronic correlations in a multiorbital system. To do so, we first investigated the phase diagram of BaCoS₂, which allowed us to develop successfully a navel self-flux technique for the growth of single crystals of improved quality and size, suitable for future inelastic neutron experiments. Second, we elucidated the controversy regarding the stability of the orthorhom­bic phase. Our results confirm that the stable phase at ambient conditions is orthorhombic Cmma and further indicate that the orthorhombic distortion is concomitant to the long-range antiferromagnetic (AFM) order of the Co ions at of T N, thus suggesting a scenario of orbital ordering that stabilise this order. Next, we explored the possibility of inducing a metallic state in BaCoS₂ using chemical pressure and electronic doping as control parameters. To reach this goal, we employed a high-pressure (HP) technique enabling the synthesis of high-quality BaCoS2 powder samples under high pressure. We found that the structural and physical properties of the recovered BaCoS₂ samples differ substantially from those of the samples synthesised under ambient conditions. Notable differences are a sizeable shrinking of the unit cell concomitant to a strong reduction of the orthorhombic distortion and a large reduction of T_N accompanied by a smearing of the specific heat jump. These results supports the scenario that the above distortion is required to stabilise the AFM order.

Finally, we successfully applied the above HP technique to the synthesis of Ba_1-xSr_xCoS₂ and Ba_1-xK_xCoS₂ samples with x up to 0.07, where the partial substitution of Ba for Sr or K is expected to produce chemical pressure or hole­doping, respectively, thus favouring a metallic phase in bath cases. We find that the Sr-substitution does induce a sizeable shrinking of the unit cell, as expected by taking into account the comparatively small size of the Sr²⁺ ion, and a full suppression of the orthorhombic distortion and of the AFM order. However, no indication of metallic properties is found in any of the Ba_1-xSr_xCoS₂ samples.

The K-substitution is found to induce similar structural changes and a simi­lar suppression of the AFM order as compared to the Sr-substitution. However, in this case, a sizeable value of Sommerfeld coefficient, 1 = 5.7 mJ mo1^-1 K^-2 suggests a metallic state induced by hole doping. Considering the compara­tively small density of holes introduced in the system by the K-substitution, the above sizeable value of 'Y is explained by a rather large renormalised mass, consistent with a picture of bad metal.

Further studies on Ba_1-xSr_xCoS₂ and Ba_1-xK_xCoS₂ single crystals synthe­sised under HP pressure may enable to determine the exchange coupling pa­rameters of the AFM structure and unveil whether the metallic state induced by hole doping displays Fermi liquid properties.

Jury

• Pierre Bordet - Institut Louis Néel – CNRS - Rapporteur 
• Véronique Brouet – LPS – Paris-Saclay université – Rapportrice
• Claudia Decorse  – ICMMO – Paris-Saclay université  – Examinatrice
• Franck Vidal  – INSP – Sorbonne université  – Examinateur
• Andrea Gauzzi  – IMPMC – Sorbonne université  – Directeur de thèse
• Yannick Klein  –  IMPMC – Sorbonne université  – Co-encadrant (invité)