The coupling between spin and charge degrees of freedom is one of the key ingredients to induce the rich novel electronic phases in strongly correlated electron systems, especially in correlated heterostructures. In these systems, novel electronic phases are easily stabilized due to the competing in...
The coupling between spin and charge degrees of freedom is one of the key ingredients to induce the rich novel electronic phases in strongly correlated electron systems, especially in correlated heterostructures. In these systems, novel electronic phases are easily stabilized due to the competing interactions of spin and charge degree of freedoms. Therefore, exploring and understanding the various electronic phases in strongly spin-charge coupled systems can be considered as attractive research topic. In this respect, the correlated heterostructures provide an opportunity to manipulate the spin and charge degrees of freedom, and to find new electronic ground states. In this thesis, various novel electronic phases are investigated in three types of spin-charge coupled compounds, ranging from a simple layered compounds to complex heterostructure such as FeSe, PdCrO2 and Sr2VO3FeAs. The exotic superconductivity in FeSe, where itinerant electrons are coupled with its spins as well as orbital degrees of freedom, will be discussed in the first part. We provide experimental evidences for the multi-band Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in FeSe stabilized at high magnetic fields. Our experimental findings are well explained by the competition between the two FFLO phases with different modulation length scales, set by multiple Fermi surfaces. These results strongly suggest that FeSe, atypical iron-based superconductor, represents a unique model system for studying the multiband FFLO states. In the second part of the thesis, unusual transport and magnetic phenomena due to the interaction between the itinerant electrons and localized spins in PdCrO2 will be given. PdCrO2 has a simple heterostructure consisting of the conducting layers and localized magnetic layers. The strong coupling between the itinerant electrons in the Pd layers and localized spins in the CrO layers induces Fermi surface reconstruction which is confirmed by magnetic quantum oscillations. This also introduces the anomalous Hall effects above TN and unusual 1/9-magnetization plateau in PdCrO2. Our results demonstrate that PdCrO2 is a rare example of 2D metallic frustrated magnets, where exotic transport/magnetic properties induced by the strong coupling between itinerant electrons and underlying frustrated magnetic structure. In the last part of this thesis, the complex electronic phases in Sr2VO3FeAs will be studied. Sr2VO3FeAs is one of the complex correlated heterostructure of spin-orbital-entangled metallic FeAs layers and the Mott-insulating SrVO3 layers. Thus new emergent ground state is expected to be stabilized. In fact, we observed an exotic phase with non-nematic C4 orbital order and gapped spin excitation, which has never been observed in either iron-based superconductors or vanadium oxides. Therefore, the physics of spin-orbital-coupled Fe pnictieds becomes even richer in the correlated heterostructure based on the iron-based superconductor offering a versatile platform to explore unprecedented exotic electronic phase transitions. Our studies presented in this thesis coherently points out that the diverse and intriguing electronic phases can be stabilized by the interplay between the spin and charge degrees of freedom across the layers in heterostructure. Hetero-structured correlated compounds, in which spin and charge are strongly coupled, can thus be excellent playground to explore new electronic ground states.
The coupling between spin and charge degrees of freedom is one of the key ingredients to induce the rich novel electronic phases in strongly correlated electron systems, especially in correlated heterostructures. In these systems, novel electronic phases are easily stabilized due to the competing interactions of spin and charge degree of freedoms. Therefore, exploring and understanding the various electronic phases in strongly spin-charge coupled systems can be considered as attractive research topic. In this respect, the correlated heterostructures provide an opportunity to manipulate the spin and charge degrees of freedom, and to find new electronic ground states. In this thesis, various novel electronic phases are investigated in three types of spin-charge coupled compounds, ranging from a simple layered compounds to complex heterostructure such as FeSe, PdCrO2 and Sr2VO3FeAs. The exotic superconductivity in FeSe, where itinerant electrons are coupled with its spins as well as orbital degrees of freedom, will be discussed in the first part. We provide experimental evidences for the multi-band Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in FeSe stabilized at high magnetic fields. Our experimental findings are well explained by the competition between the two FFLO phases with different modulation length scales, set by multiple Fermi surfaces. These results strongly suggest that FeSe, atypical iron-based superconductor, represents a unique model system for studying the multiband FFLO states. In the second part of the thesis, unusual transport and magnetic phenomena due to the interaction between the itinerant electrons and localized spins in PdCrO2 will be given. PdCrO2 has a simple heterostructure consisting of the conducting layers and localized magnetic layers. The strong coupling between the itinerant electrons in the Pd layers and localized spins in the CrO layers induces Fermi surface reconstruction which is confirmed by magnetic quantum oscillations. This also introduces the anomalous Hall effects above TN and unusual 1/9-magnetization plateau in PdCrO2. Our results demonstrate that PdCrO2 is a rare example of 2D metallic frustrated magnets, where exotic transport/magnetic properties induced by the strong coupling between itinerant electrons and underlying frustrated magnetic structure. In the last part of this thesis, the complex electronic phases in Sr2VO3FeAs will be studied. Sr2VO3FeAs is one of the complex correlated heterostructure of spin-orbital-entangled metallic FeAs layers and the Mott-insulating SrVO3 layers. Thus new emergent ground state is expected to be stabilized. In fact, we observed an exotic phase with non-nematic C4 orbital order and gapped spin excitation, which has never been observed in either iron-based superconductors or vanadium oxides. Therefore, the physics of spin-orbital-coupled Fe pnictieds becomes even richer in the correlated heterostructure based on the iron-based superconductor offering a versatile platform to explore unprecedented exotic electronic phase transitions. Our studies presented in this thesis coherently points out that the diverse and intriguing electronic phases can be stabilized by the interplay between the spin and charge degrees of freedom across the layers in heterostructure. Hetero-structured correlated compounds, in which spin and charge are strongly coupled, can thus be excellent playground to explore new electronic ground states.
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