Electronic properties and phase transitions of strongly correlated systems : organic conductors and manganites

U fokusu ove doktorske disertacije su sustavi s bogatim faznim dijagramima, jakim elektronskim korelacijama i dimenzionalnosti od 3D do kvazi-1D: 3D manganiti La1−xCaxMnO3 (x > 0.5) kod kojih se pojavljuje faza uređenja naboja (CO), 2D organski vodiči α-(BEDT-TTF)2I3 koji pokazuje CO s nabojnim prugama i κ-(BEDT-TTF)2Cu2(CN)3 gdje faza CO vjerojatno ne postoji te kvazi-1D organski vodič TTF-TCNQ karakterističan po uzastopnim faznim prijelazima u stanje vala gustoće naboja. Uzorci su karakterizirani uglavnom magnetotransportnim tehnikama (otpornost, magnetootpor, anizotropija magnetootpora i Hallov efekt). U manganitima su detektirani fazni prijelazi iz paramagnetske u CO fazu, prisustvo mehanizma vodljivosti preskakanjem (VRH) ovisnog o spinu i koncentraciji x u CO fazi te smanjivanje magnetootpora i anizotropije magnetootpora s povećanjem x. Ovi rezultati su interpretirani pomoću koegzistencije feromagnetske i antiferomagnetske faze za 0, 5 0.5) with a charge order phase (CO), quasi-2D organic conductors α-(BEDT-TTF)2I3 which has a CO phase with charge stripes and κ-(BEDT-TTF)2Cu2(CN)3 where a CO phase probably does not exist and quasi-1D organic conductor TTF-TCNQ known for successive phase transitions to a charge density wave state. The samples were characterized by magnetotransport techniques (resistivity, magnetoresistance, anisotropy of magnetoresistance and Hall effect). In manganites we detected phase transitions from a paramagnetic to the CO phase, the presence of spin and concentration x dependent variable range hopping mechanism (VRH) and decreasing of magnetoresistance and anisotropy of magnetoresistance with increasing x. These results were interpreted by a coexistence of a ferromagnetic and an antiferromagnetic phase for 0, 5 < x ≤ 0, 58. Our results, like many others, show that there are no phase transitions in κ-(BEDT-TTF)2Cu2(CN)3. We proposed 2D VRH mechanism whose origin is ascribed to disorder in the anion network. In α-(BEDT-TTF)2I3 our results show the presence of the CO phase, the dominance of holes in the semimetalic state and the absence of VRH mechanism below the CO phase transition. Hall effect results in TTF-TCNQ were obtained for the first time at low temperatures and they showed the presence of all three phase transitions. We resolved open questions of old results and suggested that at high temperatures electrons and holes equally contribute to the conductivit

Electronic properties and phase transitions of strongly correlated systems : organic conductors and manganites

U fokusu ove doktorske disertacije su sustavi s bogatim faznim dijagramima, jakim elektronskim korelacijama i dimenzionalnosti od 3D do kvazi-1D: 3D manganiti La1−xCaxMnO3 (x > 0.5) kod kojih se pojavljuje faza uređenja naboja (CO), 2D organski vodiči α-(BEDT-TTF)2I3 koji pokazuje CO s nabojnim prugama i κ-(BEDT-TTF)2Cu2(CN)3 gdje faza CO vjerojatno ne postoji te kvazi-1D organski vodič TTF-TCNQ karakterističan po uzastopnim faznim prijelazima u stanje vala gustoće naboja. Uzorci su karakterizirani uglavnom magnetotransportnim tehnikama (otpornost, magnetootpor, anizotropija magnetootpora i Hallov efekt). U manganitima su detektirani fazni prijelazi iz paramagnetske u CO fazu, prisustvo mehanizma vodljivosti preskakanjem (VRH) ovisnog o spinu i koncentraciji x u CO fazi te smanjivanje magnetootpora i anizotropije magnetootpora s povećanjem x. Ovi rezultati su interpretirani pomoću koegzistencije feromagnetske i antiferomagnetske faze za 0, 5 0.5) with a charge order phase (CO), quasi-2D organic conductors α-(BEDT-TTF)2I3 which has a CO phase with charge stripes and κ-(BEDT-TTF)2Cu2(CN)3 where a CO phase probably does not exist and quasi-1D organic conductor TTF-TCNQ known for successive phase transitions to a charge density wave state. The samples were characterized by magnetotransport techniques (resistivity, magnetoresistance, anisotropy of magnetoresistance and Hall effect). In manganites we detected phase transitions from a paramagnetic to the CO phase, the presence of spin and concentration x dependent variable range hopping mechanism (VRH) and decreasing of magnetoresistance and anisotropy of magnetoresistance with increasing x. These results were interpreted by a coexistence of a ferromagnetic and an antiferromagnetic phase for 0, 5 < x ≤ 0, 58. Our results, like many others, show that there are no phase transitions in κ-(BEDT-TTF)2Cu2(CN)3. We proposed 2D VRH mechanism whose origin is ascribed to disorder in the anion network. In α-(BEDT-TTF)2I3 our results show the presence of the CO phase, the dominance of holes in the semimetalic state and the absence of VRH mechanism below the CO phase transition. Hall effect results in TTF-TCNQ were obtained for the first time at low temperatures and they showed the presence of all three phase transitions. We resolved open questions of old results and suggested that at high temperatures electrons and holes equally contribute to the conductivit

Grain-Size-Induced Collapse of Variable Range Hopping and Promotion of Ferromagnetism in Manganite La0.5Ca0.5MnO3

mong transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)- a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La1−xCaxMnO3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x = 0.5. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles