The Investigation of the Stripe Orbital Order in Pnictide and Chalcogenide

DoctorIn solid state physics, it is important to investigate into the interaction between various degree of freedoms, like spin, charge, lattice and orbital. Because any ordered degree of freedom has to influence the others, then their interaction will change the ground state of the system. In this thesis, I will focus on the orbital ordering in Pnictide and Chalcogenide system. Especially, in these compounds, the orbitals are ordered, having a stripe arrangement. In first part of this thesis, we studied the transport properties of Ru doped-BaFe2As2. The effects of isovalent Ru substitution at the Fe sites of BaFe2-xRuxAs2 are investigated by measuring resistivity and Hall coefficient(RH) on high-quality single crystals in a wide range of doping (0< x <1.4). Ru substitution weakens the antiferromagnetic (AFM) order, inducing superconductivity for relatively high doping level of 0.4< x < 0.9. Near the AFM phase boundary, the transport properties show non-Fermi-liquid-like behaviour with a linear-temperature dependence of ρ and a strong temperature dependence of RH with a sign change. Upon higher doping, however, both ρ and RH recover conventional Fermi-liquid behaviour. Strong doping dependence of RH together with a small magneto-resistance suggest that the anomalous transport properties can be explained in terms of anisotropic charge carrier scattering due to inter-band AFM fluctuations rather than a conventional multiband scenario. Also, we observed the nematic fluctuation related to the sign-reversal of resistivity anisotropy using precise technique, in-situ tunable strain. We attribute such behaviour to the spin-fluctuation which predict a sign-reversal of resistivity anisotropy as the band structure change. Our observation serves as a fingerprint for the predominance of electronic scattering due to spin fluctuations in the normal state of these superconductors. At next part, we focused on the dimer status in IrTe2. Firstly, I investigated a de Haas?van Alphen (dHvA) oscillation study on IrTe2 single crystals showing complex dimer formations. By comparing the angle dependence of dHvA oscillations with band structure calculations, I observed distinct Fermi surface reconstruction induced by a 1/5-type and a 1/8-type dimerizations. This verifies that an intriguing quasi-two-dimensional conducting plane across the layers is induced by dimerization in both cases. A phase transition to the 1/8 phase with higher dimer density reveals that local instabilities associated with intraand inter-dimer couplings are the main driving force for complex dimer formations in IrTe2. Continuously, we identify the conduction carrier’s behaviour through the magnetoelectrical transports measurements and two band analysis. Despite of the resistivity anomaly in both of two phase transition, the dominant carrier show the contrary behaviour near phase transition. We suggest that the Fermi surface reconstruction to 2D states with two band character and the dimer density increase through the phase transition are important role on the electrical transports for each phase transition temperature regime. But still, the low temperature behaviour of conduction carrier related to the severe non-linear Hall curves requires to be more studies in the near future

Modulation of Metal-Insulator Transitions by Electric-Field-Controlled Strain in NdNiO3/SrTiO3/PMN-PT(001) Heterostructures

1

Modulation of metal-insulator transitions by electric-field-induced strain in NdNiO3/SrTiO3/PMN-PT (001) heterostructures

1

Layer-by-layer Switching of Magnetization in a (GaMnAs/GaAs:Be)10 Superlattice

2