A SYSTEMATIC STUDY OF THERMODYNAMIC AND TRANSPORT PROPERTIES OF LAYERED 4D AND 5D CORRELATED ELECTRON SYSTEMS

Correlated electron materials have been at the forefront of condensed matter research in the past couple of decades. Correlation in materials, especially, with open d and f electronic shells often lead to very exciting and intriguing phenomenon like high temperature superconductivity, Mott metal-insulator transition, colossal magnetoresistance (CMR). This thesis focuses on triple-layered Sr4Ru3O10, Sr substituted double layered (Ca1-- xAx)3Ru2O7 (A = Ba, Sr) and 5d system Sr2IrO4 and Sr3Ir2O7. Triple-layered Sr4Ru3O10 displays interesting phenomena ranging from quantum oscillations, tunneling magnetoresistance, unusual low temperature specific heat, strong spin-lattice coupling to switching behavior. The central feature, however, is the unique borderline magnetism: along the c-axis. Sr4Ru3O10 shows spontaneous ferromagnetism, indicating a strong Coulomb exchange interaction, U and a large density of states at the Fermi surface, g(EF ), hence Ug(EF ) ≥ 1 (Stoner criterion). But within the ab-plane it features a pronounced peak in magnetization and a first-order metamagnetic transition. The coexistence of the interlayer ferromagnetism and the intralayer metamagnetism makes Sr4Ru3O10 a really unique system. Also, in this thesis the spin-valve behavior exhibited by impurity doping at the Ca site by Ba and Sr in the double layered Ca3Ru2O7 is reported. Spin valve effect is a phenomenon only realized in multilayer thin films. Here, spin valve is observed in bulk single crystals of impurity dopedCa3Ru2O7, Ca3(Ru1-xCrx)2O7 and (Ca1- xAx)3Ru2O7 (A = Ba, Sr). 5d Iridates are expected to be more metallic and less magnetic than their 3d and 4f counterparts because of the extended 5d orbitals. In marked contrast, many iridates are magnetic insulators with exotic properties. The focus in this thesis is on Sr2IrO4 which diplays a novel Jeff = 1/2 Mott state. Magnetic, electrical, and thermal measurements on single-crystals of Sr2IrO4, reveal a novel giant magneto-electric effect (GME) arising from a frustrated magnetic/ferroelectric state. The GME and electric polarization hinge on a spin-orbit gapping of 5d-bands, rather than the magnitude and spatial dependence of magnetization, as traditionally accepted

Decoupling of the Antiferromagnetic and Insulating States in Tb doped Sr2IrO4

Sr2IrO4 is a spin-orbit coupled insulator with an antiferromagnetic (AFM) transition at TN=240 K. We report results of a comprehensive study of single-crystal Sr2Ir1-xTbxO4. This study found that mere 3% (x=0.03) tetravalent Tb4+(4f7) substituting for Ir4+ (rather than Sr2+) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x=0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0, 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund's rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. This work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.Comment: 8 figure

Enhanced Critical Field of Superconductivity at an Oxide Interface

The nature of superconductivity and its interplay with strong spin-orbit coupling at the KTaO3(111) interfaces remains a subject of debate. To address this problem, we grew epitaxial LaMnO3/KTaO3(111) heterostructures. We show that superconductivity is robust against the in-plane magnetic field, with the critical field of superconductivity reaching 25 T in optimally doped heterostructures. The superconducting order parameter is highly sensitive to carrier density. We argue that spin-orbit coupling drives the formation of anomalous quasiparticles with vanishing magnetic moment, providing the condensate significant immunity against magnetic fields beyond the Pauli paramagnetic limit. These results offer design opportunities for superconductors with extreme resilience against magnetic field

Fermi-surface topologies and low-temperature phases of the filled skutterudite compounds CeOs4Sb12 and NdOs4Sb12

MHz conductivity, torque magnetometer and magnetization measurements are reported on single crystals of CeOs4Sb12 and NdOs4Sb12 using temperatures down to 0.5 K and magnetic fields of up to 60 tesla. The field-orientation dependence of the de Haas-van Alphen and Shubnikov-de Haas oscillations is deduced by rotating the samples about the [010] and [0¯11] directions. The results indicate that NdOs4Sb12 has a similar Fermi surface topology to that of the unusual superconductor PrOs4Sb12, but with significantly smaller effective masses, supporting the importance of local phonon modes in contributing to the low-temperature heat capacity of NdOs4Sb12. By contrast, CeOs4Sb12 undergoes a field-induced transition from an unusual semimetal into a high-field, hightemperature state characterized by a single, almost spherical Fermi-surface section. The behavior of the phase boundary and comparisons with models of the bandstructure lead us to propose that the field-induced phase transition in CeOs4Sb12 is similar in origin to the well-known α − γ transition in Ce and its alloys

Decoupling of the Antiferromagnetic and Insulating States in Tb-Doped Sr\u3csub\u3e2\u3c/sub\u3eIrO\u3csub\u3e4\u3c/sub\u3e

Sr2IrO4 is a spin-orbit-coupled insulator with an antiferromagnetic (AFM) transition at TN=240K. We report results of a comprehensive study of single-crystal Sr2Ir1−xTbxO4(0≤x≤0.03). This study found that a mere 3% (x=0.03) of tetravalent Tb4+(4f7) substituting for Ir4+ (rather than Sr2+) completely suppresses the long-range collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of the magnetic interactions and charge gap. The insulating state at x=0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95,0,0) and (0,0.95,0) in the neutron diffraction, suggesting a spiral or spin-density-wave order. It is apparent that Tb doping effectively changes the relative strength of the spin-orbit interaction (SOI) and the tetragonal crystal electric field and enhances the Hund\u27s rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM is accompanied by no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and causes a persistent insulating state. This work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate

Sr\u3csub\u3e2\u3c/sub\u3eIr\u3csub\u3e1−\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eRh\u3csub\u3e\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e(x\u3c0.5): An Inhomogeneous \u3cem\u3ej\u3c/em\u3e\u3csub\u3eeff\u3c/sub\u3e=1/2 Hubbard system

In a combined experimental and theoretical study, we investigate the properties of Sr2Ir1−xRhxO4. From the branching ratios of the L-edge isotropic x-ray absorption spectra, we determine that the spin-orbit coupling is remarkably independent of x for both iridium and rhodium sites. DFT+U calculations show that the doping is close to isoelectronic and introduces impurity bands of predominantly rhodium character close to the lower Hubbard band. Overlap of these two bands leads to metallic behavior. Since the low-energy states for xjeff=1/2 character, we suggest that the electronic properties of this material can be described by an inhomogeneous Hubbard model, where the on-site energies change due to local variations in the spin-orbit interaction strength combined with additional changes in binding energy

Combining microscopic and macroscopic probes to untangle the single-ion anisotropy and exchange energies in an S=1 quantum antiferromagnet

The magnetic ground state of the quasi-one-dimensional spin-1 antiferromagnetic chain is sensitive to the relative sizes of the single-ion anisotropy (D) and the intrachain (J) and interchain (J') exchange interactions. The ratios D/J and J'/J dictate the material's placement in one of three competing phases: a Haldane gapped phase, a quantum paramagnet and an XY-ordered state, with a quantum critical point at their junction. We have identified [Ni(HF)2(pyz)_2]SbF6, where pyz = pyrazine, as a rare candidate in which this behavior can be explored in detail. Combining neutron scattering (elastic and inelastic) in applied magnetic fields of up to 10~tesla and magnetization measurements in fields of up to 60~tesla with numerical modeling of experimental observables, we are able to obtain accurate values of all of the parameters of the Hamiltonian [D = 13.3(1)~K, J = 10.4(3)~K and J' = 1.4(2)~K], despite the polycrystalline nature of the sample. Density-functional theory calculations result in similar couplings (J = 9.2~K, J' = 1.8~K) and predict that the majority of the total spin population resides on the Ni(II) ion, while the remaining spin density is delocalized over both ligand types. The general procedures outlined in this paper permit phase boundaries and quantum-critical points to be explored in anisotropic systems for which single crystals are as yet unavailable

Revealing quantum Hall states in epitaxial topological half-Heusler semimetal

Prediction of topological surface states (TSS) in half-Heusler compounds raises exciting possibilities to realize exotic electronic states and novel devices by exploiting their multifunctional nature. However, an important prerequisite is identification of macroscopic physical observables of the TSS, which has been difficult in these semi-metallic systems due to prohibitively large number of bulk carriers. Here, we introduce compensation alloying in epitaxial thin films as an effective route to tune the chemical potential and simultaneously reduce the bulk carrier concentration by more than two orders of magnitude compared to the parent compound. Linear magnetoresistance is shown to appear as a precursor phase that transmutes into a TSS induced quantum Hall phase on further reduction of the coupling between the surface states and the bulk carriers. Our approach paves the way to reveal and manipulate exotic properties of topological phases in Heusler compounds.Comment: 8 pages, 4 figures. Supplementary Infromation contains 7 sections and 17 figure

Stress‐induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin‐state ordered phase of a spin crossover complex. Single crystal X‐ray diffraction and resonant ultrasonic spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mn 3+ compound [Mn(3,5‐diBr‐sal 2 (323))]BPh 4 , 1. The first at 250 K, involves the space group change Cc → Pc and is thermodynamically continuous, while the second, Pc → P1 at 85 K, is discontinuous and related to spin crossover and spin‐state ordering. Stress‐induced domain wall mobility was detected as softening of the phonon modes at the Pc → P1 transition