Session A53: Emergent Properties of Complex Materials Bulk I

Field-tunable toroidal moment in the chiral-lattice magnet BaCoSiO4

A toroidal dipole moment appears independent of the electric and magnetic dipole moment in the multipole expansion of electrodynamics. It arises naturally from vortex-like arrangements of spins. Observing and controlling spontaneous long-range orders of toroidal moments are highly promising for spintronics but remain challenging. In this talk, I will introduce our resent work on a chiral triangular-lattice magnet BaCoSiO₄ where a vortex-like spin configuration with a staggered arrangement of toroidal moments, a ferritoroidal state, is realized. Upon applying a magnetic field, we observe multi-stair toroidal transitions correlating directly with metamagnetic transitions. We establish a first-principles microscopic Hamiltonian that explains both the formation of toroidal states and the metamagnetic toroidal transition as a combined effect of the magnetic frustration and the Dzyaloshinskii-Moriya interactions allowed by the crystallographic chirality. I will also briefly touch on the remarkable dynamical response of this magnet where a strong resonance-like mode coexists with coninuous excitations.   

Neutron diffraction study of the fluctuation driven Colossal Magnetoresistance Mn3Si2Te6

Colossal magnetoresistance (CMR) is of great fundamental and technological significance and exists in a number of materials including the most studied manganites. Similar CMR effect was recently observed in the stoichiometric Mn₃Si₂Te₆ that is starkly different from other reported materials. The resistivity drops by orders of magnitude leading to an insulator-metal transition with an applied magnetic field above 9 T. The CMR occurs only when the field is applied along the hard axis, while such effect is conspicuously absent with the field applied in the basal plane, where magnetization is fully saturated. Neutron diffractions are used to characterize the ground state spin structure in zero and applied field along the c-axis. A prominent magnetic diffuse scattering is observed well above the magnetic transition. The relevance of the spin fluctuation is discussed in the context of magnetic polaron model where the CMR effect is directly associated with the suppression of the critical magnetic fluctuation.   

Cation Doping and Suppression of Charge Order in Sr1-xLaxCoO3

Charge ordering (CO) is a prominent feature of strongly correlated electron systems, however the exact origin of such phenomena and the consequences for physical properties are still being explored. Many doped Mott insulators show both electronic phase separation and CO. One example is SrCoO_3-y. As the oxygen concentration varies, the crystal structure evolves smoothly but magnetism in the compound shows two-phase behavior, separating into regions with differing ferromagnetic phases. The stable phases are associated with the special oxygen concentrations of SrCoO₃, SrCoO_2.88 and SrCoO_2.75 and support ferromagnetic phases with transition temperatures at 280K, 220K and 160K. Soft X-ray scattering revealed distinct charge ordering for SrCoO_2.75 ad SrCoO_2.88. The same overall Co-charge balance can be achieved by mixing La onto Sr-sites rather than introducing oxygen vacancies. Our recent study reveals that the cation doping suppresses both phase separation and charge ordering on these materials. We performed Resonant Elastic X-ray Scattering on La-doped SrCoO₃ thin films, after confirming the epitaxy and the presence of magnetic phases with XRD and SQUID. We deposited thin films of various thickness of Sr_0.75La_0.25CoO₃ on both STO and LAO substrates using Pulsed Laser Deposition.   

Persistent Mott state and vanishing Néel state in single-crystal Ca2Ru1-xRhxO4 (0≤ x ≤0.33)

The 4d-based Ca₂RuO₄ is a spin-orbit coupled Mott insulator with a first-order metal-insulator transition concurrent with a violent structural transition at T_MI = 357 K, followed by a well-separated antiferromagnetic state at T_N = 110 K stabilized by further structural distortions. Here we report the results of our recent study on Rh⁴⁺ (4d⁵) substituted Ca₂RuO₄ or Ca₂Ru_1-xRh_xO₄ single crystals. Rh substituting weakens the structural distortions and effectively removes the first-order metal-insulator transition at T_MI while unexpectedly retaining the Mott state. The magnetic properties, on the other hand, changes dramatically throughout the same Rh substitution range. Consequently, T_N drops linearly with increasing x, from 110 K for x = 0 down to 54 K for x = 0.33, accompanied by an elongated c-axis lattice parameter. The unparallel behavior of the transport and magnetic states indicates a disassociation between them in this highly distorted ruthenate.   

Exciton condensation in bilayer spin-orbit insulator

Nobel phenomena emerging from the cooperation between spin-orbit coupling and electron correlation, such as anomalous magnetoresistance, have caught great attention recently. In particular, 5d orbital electron systems, e.g., strontium iridates and associated thin films, are ideal platforms for studying the complex effect of entangled multiple degrees of freedom. Targetting the bilayer iridate Sr₃Ir₂O₇, we have investigated magnetic excitations of the bilayer Hubbard model in the random phase approximation. We clarified that the electron correlation creates preformed excitons and triggers an exciton Bose-Einstein condensation, which results in the antiferromagnetically ordered state observed in the real material. Here, the spin-orbit coupling induces the quantum critical point at which excitons condense, and the electron correlation drives the system from a paramagnetic band insulator to the Néel phase. Low-energy excitons in the paramagnetic phase re-emerge on the other side of the quantum critical point as the transverse and longitudinal modes of the antiferromagnetic state. A significant softening of the longitudinal mode also occurs in the vicinity of the finite-temperature phase transition point and can be detected in experimental probes, such as resonant inelastic X-ray scattering. These results indicate that the bilayer iridate Sr₃Ir₂O₇ is a realization of the long-sought excitonic insulator. We will show the theoretical calculation of the exciton condensation and discuss possible experimental identification of the antiferromagnetic excitonic insulator.   

Structure and Oxygen Storage Capabilities of Heavily Oxidized Y1-xTbxMnO3+d

Non-stoichiometric oxides have attracted tremendous interest for their promising potential for applications as materials with reversible oxygen storage/release capabilities. The development of oxygen storage materials is critical for the growth of new energy related technologies such as oxy-fuel and chemical looping combustion and reforming for clean coal energy and synthesis gas production.  We have recently reported superior oxygen storage/release performances displayed by rare-earth hexagonal manganites, namely RMnO_3+d,(where R is a rare earth elements or Y).  These materials are capable of storing as much as 2000 μmol-O/g when annealed in air.  Large changes of oxygen content occur on cycling at unusually low but desirable temperatures between 250 and 350° C.  Tb-substituted hexagonal Y_1-xTb_xMnO_3+d materials are singled out because of their capability to hold even more oxygen (up to 0.5 oxygen atom per unit formula) with the drastic enhancement causing significant distortions of the structural symmetry.  Neutron data enabled the determination of an elegant solution of a previously unseen superlattice which is four times larger than that of the pristine YMnO₃ parent material.  Detailed discussions of the structure and materials properties will be presented.   

The giant effect of dilute Bi doping on the ground state in the trimer lattice BaIrO3

The trimer lattice BaIrO₃ is a spin-orbit-coupled Mott insulator with a Néel temperature of T_N =183 K. We report structural, magnetic, transport, and thermal properties of single-crystal BaIrO₃ with dilute Bi doping for Ba or Ba_1-xBi_xIrO₃ with x ranging from 0.001 to 0.01. While retaining the native crystal structure, mere 0.3% Bi doping or x = 0.003 precipitates a wide range of drastic reductions in the resistivity by 5 orders of magnitude, and the magnetic moment (0.01 μ_B/Ir) and the Seebeck coefficient (~20 μV/K) by one order of magnitude. Furthermore, the Sommerfeld coefficient (e.g., 7 mJ/(mol*K²)) and the Wilson ratio (e.g., 2.4) consistently demonstrate an emergent metallic state that is both robust and correlated. The high susceptibility of the ground state to dilute Bi doping is largely because of the Ir₃O₁₂ trimers whose subtle alterations dictate the physical properties. The results will be discussed along with comparisons drawn with related iridates.   

Transformation from Metallic to Insulating Stripe-Ordered Phase in Trilayer Square-Planar Nickelates

Low-valent nickelates are intriguing for their structural and electronic features that are similar to superconducting cuprates, including superconductivity. Among them, the crystallographic isomorphs Pr₄Ni₃O₈ and La₄Ni₃O₈ with trilayer square-planar structural configuration have been demonstrated to possess the singularly different ground states: Pr₄Ni₃O₈ is metallic while La₄Ni₃O₈ exhibits insulating stripe-ordered phase. To explore and understand the evolution of the ground states from metallic Pr₄Ni₃O₈ to stripe-ordered La₄Ni₃O₈ in the R₄Ni₃O₈ family, we have successfully grown a series of isovalent-substituted single crystals (Pr_1-xLa_x)₄Ni₃O₈ (x = 0.1, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.9) using high oxygen pressure (pO₂) floating zone furnace. We performed a comprehensive study combining thermodynamics, electrical transport, magnetization as well as high-resolution synchrotron x-ray powder diffraction and synchrotron x-ray single crystal diffraction on this series of crystals. The results revealed that this series of nickelates undergo a systematic transformation from correlated metal to stripe-ordered insulator near x = 0.4, potentially hinting at the existence of a quantum critical point.   

Insights of cation ordering in double perovskites oxides from first-principles calculations and machine learning

In this work, we have employed first-principles density functional calculations and traditional machine learning (ML) techniques to explore insights of cation ordering in AA′BB′O₆ double perovskites. We have studied various possible oxidation states of A, A’, B and B’ maintaining the charge neutrality of the system and various B(3d)- B′(3d), B(3d)- B′(4d) and B(3d)- B′(5d) systems to construct a dataset consisting of a wide compositional space. A non-exclusive list of these features includes optimized structure parameters, bond length and angles, magnetic moments, orbital field-matrix and structural modes. Random Forest Classifier is then employed to build a decision-tree type ML model. Our model is successful in classifying various A-site cation orderings. It also identifies important features including structural modes relevant to predict A[Layer]B[Rock-salt] ordering which leads to polar space group (P2₁) if (a-a-c+) distortion is imposed. Based on the ML model combined with causal analyses, we aim to predict new compositions for stable A[Layer]B[Rock-salt] with functional properties.   

Tuning stability, reducibility, and properties of AA'BO3-δ perovskites by cations composition: A first-principles study

The parent compounds of high-T_c superconducting cuprates consists of layers of spin-1/2 cations forming a square lattice with a large in-plane antiferromagnetic coupling. Compounds with similar characteristics have long been searched for with little progress. Here, we use theoretical tools to explore the thermodynamic obstacles to reducing AA'BO₃ double perovskites into their infinite layered AA'BO₂ analogues. The formation enthalpy against decomposition into binary oxides and the oxygen vacancy formation energies computed from density functional theory calculations are presented as a measure of stability; providing insights into the synthesizability of these compounds. Furthermore, we use these calculations as the basis for examining the connection between structural parameters, such as the buckling of B-site layers and the simulated magnetic Neél temperature of promising AA'BO₂ compounds to gain potential insights into the possibility of superconductivity in these materials.