Studies of spin dynamics in 122 transition metal arsenides using inelastic neutron scattering technique

The diverse physical properties and interrelationship between various ground states offer a rich physics to explore in 122 $A$TM$_2$As$_2$ ($A =$Ca, Sr, Ba and TM = 3d transition metals). This thesis discusses three examples of the 122 $A$TM$_2$As$_2$ compounds, TM = Fe, Co and Mn, where the focus is to understand their diverse magnetic properties and interplay between the magnetism and electronic properties including superconductivity. Therefore, the spin dynamics (spin fluctuations) of these compounds were studied using inelastic neutron scattering technique. CaFe$_2$As$_2$ and the derived compounds are the first system discussed in this work. In this com- pound, correlation between the magnetism and superconductivity was studied. CaFe$_2$As$_2$ belongs to the 122 $A$Fe$_2$As$_2$ family of high–Tc iron-based superconductors, where previous studies suggest that the overdamped spin dynamics are necessary for superconductivity. The CaFe$_2$As$_2$ compound has some unique features compared to other members of $A$Fe$_2$As$_2$ compounds, therefore presents a different scenario to study this relationship. A study performed using the inelastic neutron scattering on various Co substituted CaFe$_2$As$_2$ compounds are discussed. The results verify the link between the overdamped spin dynamics and superconductivity. In addition, the results also indicate that some peculiar features are present in the spin fluctuations of Ca(Fe$_{1−x}$Co$_x$)$_2$As$_2$ compounds. These peculiar features seem to be consistent with the unique magnetostructural properties of Ca(Fe$_{1−x}$Co$_x$)$_2$As$_2$ compounds. Another compound discussed in this thesis is CaCo$_{1.86}$As$_2$, which is in the collapsed tetragonal phase and exhibits magnetic ground state, unlike its $A$Fe$_2$As$_2$ counterparts. Inelastic neutron scattering measurements performed on this compound revealed extremely anisotropic spin fluctuations along the two directions of the reciprocal space. The result suggests that the CaCo$_{1.86}$As$_2$ is a unique example of highly–frustrated square–lattice system. The final compound discussed is the K substituted BaMn$_2$As$_2$, i.e. Ba$_{1−x}$K$_x$Mn$_2$As$_2$ compounds. K substitution (hole doping) in BaMn$_2$As$_2$ changes the insulating ground state of the parent BaMn$_2$As$_2$ compound to the metallic state. Previous measurements of these compounds indicate that the hole doping induces itinerant ferromagnetism that coexists with the local moment antiferromagnetism of the parent BaMn$_2$As$_2$ compound. To further understand the effects of hole doping on magnetism, inelastic neutron scattering measurements were performed on Ba$_{1−x}$K$_x$Mn$_2$As$_2$ compounds with x = 0, 0.125 and 0.25. The results suggest minor changes in the spin fluctuations with the hole doping, i.e. minor changes up to hole carrier concentrations of 12.5% per Mn ion (i.e. x = 0.25). This is consistent with the idea that the charge transport and antiferromagnetism are decoupled in the Ba$_{1−x}$K$_x$Mn$_2$As$_2$ compounds, i.e. the doped holes have small effects on the antiferromagnetism

Pressure induced half-collapsed-tetragonal phase in CaKFe4_4As4_4

We report the temperature-pressure phase diagram of CaKFe$_4$As$_4$ established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe$_4$As$_4$ is suppressed and then disappears at $p$ $\gtrsim$ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe$_4$As$_4$ under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for $p$ $\geq$ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe$_4$As$_4$: a half-collapsed-tetragonal phase

Reduction of the ordered magnetic moment and its relationship to Kondo coherence in

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series Ce1-&ITx&ITLa&ITx&ITCu2Ge2 due to nonmagnetic dilution by La are revealed through neutron diffraction results for x = 0.20, 0.40, 0.75, and 0.85. Magnetic Bragg peaks are found for 0.20 \u3c= x \u3c= 0.75, and both the Ned temperature T-N and the ordered magnetic moment per Ce mu linearly decrease with increasing x. The reduction in mu points to strong hybridization of the increasingly diluted Ce 4f electrons, and we find a remarkable quadratic dependence of mu on the Kondo-coherence temperature. We discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory and show that Ce1-&ITx&ITLa&ITx&ITCu2Ge2 provides an exceptional opportunity to quantitatively study the multiple magnetic interactions in a Kondo lattice

Unconventional nodal superconductivity in miassite Rh17_{17}S15_{15}

Unconventional superconductivity has long been believed to arise from a lab-grown correlated electronic system. Here we report compelling evidence of unconventional nodal superconductivity in a mineral superconductor \rhs. We investigated the temperature-dependent London penetration depth $\Delta\lambda(T)$ and disorder evolution of the critical temperature $T_c$ and upper critical field $H_{c2}(T)$ in synthetic miassite \rhs. We found a power-law behavior of $\Delta\lambda(T)\sim T^n$ with $n\approx 1.1$ at low temperatures below $0.3T_c$ ($T_c$ = 5.4 K), which is consistent with the presence of lines of the node in the superconducting gap of \rhs. The nodal character of the superconducting state in \rhs~was supported by the observed pairbreaking effect in $T_c$ and $H_{c2}(T)$ in samples with the controlled disorder that was introduced by low-temperature electron irradiation. We propose a nodal sign-changing superconducting gap in the $A_{1g}$ irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the superfluid density, $\lambda^2(0)/\lambda^2(T)$

Hedgehog spin-vortex crystal stabilized in a hole-doped iron-based superconductor

Magnetism is widely considered to be a key ingredient of unconventional superconductivity. In contrast to cuprate high-temperature superconductors, antiferromagnetism in most Fe-based superconductors (FeSCs) is characterized by a pair of magnetic propagation vectors, (π,0) and (0,π). Consequently, three different types of magnetic order are possible. Of these, only stripe-type spin-density wave (SSDW) and spin-charge-density wave (SCDW) orders have been observed. A realization of the proposed spin-vortex crystal (SVC) order is noticeably absent. We report a magnetic phase consistent with the hedgehog variation of SVC order in Ni-doped and Co-doped CaKFe 4As 4 based on thermodynamic, transport, structural and local magnetic probes combined with symmetry analysis. The exotic SVC phase is stabilized by the reduced symmetry of the CaKFe 4As 4 structure. Thus, our results suggest that the possible magnetic ground states in FeSCs have very similar energies, providing an enlarged configuration space for magnetic fluctuations to promote high-temperature superconductivity

Distinct pressure evolution of coupled nematic and magnetic orders in FeSe

We present a microscopic study of nematicity and magnetism in FeSe over a wide temperature and pressure range using high-energy x-ray diffraction and time-domain Mössbauer spectroscopy. The low-temperature magnetic hyperfine field increases monotonically up to ∼ 6 GPa. The orthorhombic distortion initially decreases under increasing pressure but is stabilized at intermediate pressures by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature at p = 6.8 GPa , the orthorhombic distortion suddenly disappears and a new tetragonal magnetic phase occurs. The pressure and temperature evolution of the structural and magnetic order parameters suggests that they have distinct origins

Studies of spin dynamics in 122 transition metal arsenides using inelastic neutron scattering technique

The diverse physical properties and interrelationship between various ground states offer a rich physics to explore in 122 $A$TM$_2$As$_2$ ($A =$Ca, Sr, Ba and TM = 3d transition metals). This thesis discusses three examples of the 122 $A$TM$_2$As$_2$ compounds, TM = Fe, Co and Mn, where the focus is to understand their diverse magnetic properties and interplay between the magnetism and electronic properties including superconductivity. Therefore, the spin dynamics (spin fluctuations) of these compounds were studied using inelastic neutron scattering technique. CaFe$_2$As$_2$ and the derived compounds are the first system discussed in this work. In this com- pound, correlation between the magnetism and superconductivity was studied. CaFe$_2$As$_2$ belongs to the 122 $A$Fe$_2$As$_2$ family of high–Tc iron-based superconductors, where previous studies suggest that the overdamped spin dynamics are necessary for superconductivity. The CaFe$_2$As$_2$ compound has some unique features compared to other members of $A$Fe$_2$As$_2$ compounds, therefore presents a different scenario to study this relationship. A study performed using the inelastic neutron scattering on various Co substituted CaFe$_2$As$_2$ compounds are discussed. The results verify the link between the overdamped spin dynamics and superconductivity. In addition, the results also indicate that some peculiar features are present in the spin fluctuations of Ca(Fe$_{1−x}$Co$_x$)$_2$As$_2$ compounds. These peculiar features seem to be consistent with the unique magnetostructural properties of Ca(Fe$_{1−x}$Co$_x$)$_2$As$_2$ compounds. Another compound discussed in this thesis is CaCo$_{1.86}$As$_2$, which is in the collapsed tetragonal phase and exhibits magnetic ground state, unlike its $A$Fe$_2$As$_2$ counterparts. Inelastic neutron scattering measurements performed on this compound revealed extremely anisotropic spin fluctuations along the two directions of the reciprocal space. The result suggests that the CaCo$_{1.86}$As$_2$ is a unique example of highly–frustrated square–lattice system. The final compound discussed is the K substituted BaMn$_2$As$_2$, i.e. Ba$_{1−x}$K$_x$Mn$_2$As$_2$ compounds. K substitution (hole doping) in BaMn$_2$As$_2$ changes the insulating ground state of the parent BaMn$_2$As$_2$ compound to the metallic state. Previous measurements of these compounds indicate that the hole doping induces itinerant ferromagnetism that coexists with the local moment antiferromagnetism of the parent BaMn$_2$As$_2$ compound. To further understand the effects of hole doping on magnetism, inelastic neutron scattering measurements were performed on Ba$_{1−x}$K$_x$Mn$_2$As$_2$ compounds with x = 0, 0.125 and 0.25. The results suggest minor changes in the spin fluctuations with the hole doping, i.e. minor changes up to hole carrier concentrations of 12.5% per Mn ion (i.e. x = 0.25). This is consistent with the idea that the charge transport and antiferromagnetism are decoupled in the Ba$_{1−x}$K$_x$Mn$_2$As$_2$ compounds, i.e. the doped holes have small effects on the antiferromagnetism.</p