Influence of Alkaline-Earth-Metal Substitutions on the Bismuth Ruthenate Structure: Bi2-xA'xRu2O6O'1-y (A' = Mg, Ca, Sr).

Solid solutions with the formula of Bi2-xA'xRu2O7-y (A' = Mg, Ca, Sr; 0 ≤ x ≤ 0.2 for Mg, 0 ≤ x ≤ 1 for Ca, and 0 ≤ x ≤ 0.5 for Sr) have been synthesized and characterized. The crystal structures for these phases are found to be in the pyrochlore family, crystallizing in the cubic space group Fd3̅m with complex A/A' cation coordination environments. The Bi cation is found to be off-center from the ideal position because of a lone-pair distortion, while the positions of the substituted A' cations vary based on the size and ionicity. The neutron structure refinements reveal a similar propensity to off-center regarding Ca and Sr, while Mg features the largest static displacement of up to 0.48 Å. Interestingly, this is one of only two known pyrochlores with Mg2+ located in an 8-coordinated site. The average Ru oxidation state for each substitution is found to increase, and charge compensates for the lower divalent A' substitution. The solid solutions show temperature-independent resistance across the series with small changes in magnitude that scale with the amount of substitution, while displaying Pauli paramagnetic behavior throughout.

Os4+ Instability in the Pyrochlore Structure: Tl2-xBixOs2O7-y.

Osmium-containing oxides are rare due to the difficulty in stabilizing complex structures with a fixed stoichiometry and metastability of the phases. Bismuth-substituted thallium osmate pyrochlore samples, Tl2-xBixOs2O7-y, were synthesized using solid-state reactions where the solubility limit was found to be approximately x = 1.4. Members of this solid solution were characterized by their structural, electronic, magnetic, and thermal properties to understand the influence of Bi3+ substitution on the ground state. The Os-containing pyrochlores crystallize in the ideal cubic pyrochlore structure (Fd3m), and the lattice parameter a was found to slightly increase as a function of Bi content. A possible interplay between structure and cation valence states was explored using both neutron powder diffraction and X-ray absorption spectroscopy, suggesting that a combination of Os4+/Os5+ and Tl1+/Tl3+ mixed valency throughout the solid solution allows for the stabilization of the pyrochlore structure. The system is metallic for the entire solid solution and predominantly exhibits temperature-independent paramagnetism. Specific heat measurements show an enhanced Sommerfeld coefficient, a possible flat-band signature. This system gave insight into the bonding preferences of Os, indicating a dependence on high oxidation states and mixed valence for the stability of complex structures.

A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling.

Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena.

Degeneracy of the 1/8 Plateau and Antiferromagnetic Phases in the Shastry-Sutherland Magnet TmB_{4}.

The 1/8 fractional plateau phase (1/8 FPP) in Shastry-Sutherland lattice (SSL) spin systems has been viewed an exemplar of emergence on an Archimedean lattice. Here we explore this phase in the Ising magnet TmB_{4} using high-resolution specific heat (C) and magnetization (M) in the field-temperature plane. We show that the 1/8 FPP is smoothly connected to the antiferromagnetic phase on ramping the field from H=0. Thus, the 1/8 FPP is not a distinct thermodynamic ground state of TmB_{4}. The implication of these results for Heisenberg spins on the SSL is discussed.

Bi2-xCaxIr2O6+y Pyrochlore Phases: Structure and Properties with Varied Ir Oxidation State from 3.9+ to 4.3.

Pyrochlore phases Bi2-xCaxIr2O6Oy' with x from 0.0 to 1.0 have been evaluated based on Rietveld analysis of neutron diffraction data, electrical resistivity and thermopower data from 3 to 756 K, and magnetic susceptibility data from 3 to 298 K. The average Ir oxidation state is less than 4+ at low x, above 4+ for high x, and is very close to 4+ at x = 0.5. All samples show metallic properties with an electrical resistivity of ∼10-3 Ω·cm at room temperature. For low x, the sign of the Seebeck coefficient is negative at low temperature but becomes positive at high temperature. For high x, the sign of the Seebeck coefficient is positive from 3 to 756 K. Magnetic measurements indicate no magnetic ordering down to 3 K for all values of x. All Bi is in its ideal position for all values of x, but much of the Ca is strongly displaced from the ideal A cation site. This displacement of Ca apparently only occurs when there is an adjacent vacancy at the O' site.

Thermodynamic Measurement of Angular Anisotropy at the Hidden Order Transition of URu_{2}Si_{2}.

The heavy fermion compound URu_{2}Si_{2} continues to attract great interest due to the unidentified hidden order it develops below 17.5 K. The unique Ising character of the spin fluctuations and low-temperature quasiparticles is well established. We present detailed measurements of the angular anisotropy of the nonlinear magnetization that reveal a cos^{4}θ Ising anisotropy both at and above the ordering transition. With Landau theory, we show this implies a strongly Ising character of the itinerant hidden order parameter.

Structure and Properties of Ir-Containing Oxides with Large Spin-Orbit Coupling: Ba2In(2-x)Ir(x)O(5+δ).

In this work, the solid solution series Ba2In(2-x)Ir(x)O5+δ (x = 0-1.4, 2) was synthesized, and its structural, magnetic, and charge-transport properties were measured. With increasing Ir content, three transitions in the room-temperature structure were observed: orthorhombic to tetragonal to cubic to a monoclinic distortion of a hexagonal BaTiO3 structure. Neutron diffraction shows Ba2In(1.6)Ir(0.4)O5.4 to be cubic and Ba2InIrO6 to be monoclinic, the latter contrary to previously published X-ray diffraction refinements. Magnetization measurements show Curie-Weiss behavior for x = 0.2-0.6, which arises from nearly 50:50 ratio of Ir(V) and Ir(VI). To our knowledge, this is the first time Ir(VI) has been stabilized with standard solid-state methods under ambient conditions. The electrical resistivity measurements show all the compounds studied are semiconducting and that resistivity decreases with increasing Ir content, suggesting the proximity to a metal-insulator transition. A sign reversal in the high-temperature Seebeck coefficient is observed indicating both electron and hole charge transport.

Magnetic phase evolution in the spinel compounds Zn(1-x)Co(x)Cr(2)O(4).

We present the magnetic properties of complete solid solutions of ZnCr(2)O(4) and CoCr(2)O(4): two well studied oxide spinels with very different magnetic ground states. ZnCr(2)O(4), with non-magnetic d(10) cations occupying the A site and magnetic d(3) cations on the B site, is a highly frustrated antiferromagnet. CoCr(2)O(4), with magnetic d(7) cations (three unpaired electrons) on the A site as well, exhibits Néel ferrimagnetism as well as commensurate and incommensurate non-collinear magnetic order. More recently, CoCr(2)O(4) has been studied extensively because of its polar behavior which arises from conical magnetic ordering. Gradually introducing magnetism on the A site of ZnCr(2)O(4) results in a transition from frustrated antiferromagnetism to glassy magnetism at low concentrations of Co, and eventually to ferrimagnetic and conical ground states at higher concentrations. Real-space Monte Carlo simulations of the magnetic susceptibility suggest that the first magnetic ordering transition and features of the susceptibility across x are captured by near-neighbor self-couplings and cross-couplings between the magnetic A and B atoms. We present, as a part of this study, a method for displaying the temperature dependence of magnetic susceptibility in a manner which helps distinguish between compounds possessing purely antiferromagnetic interactions from compounds where other kinds of ordering are present.

Oxygen-related band gap state in single crystal rubrene.

A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Two-photon excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene.

Mechanism of carrier photogeneration and carrier transport in molecular crystal tetracene.

Models for the carrier photoexcitation mechanism in molecular crystals have been established initially on the bases of measurements on oligoacenes and later applied to conjugated polymers as well. These models emphasize the localized nature of photoexcitations and describe carrier generation as a secondary process involving exciton dissociation. The results of our photoconductivity studies of single crystal tetracene are at variance with these widely accepted models, and in fact indicate that the photocarrier quantum efficiency appears independent of temperature, photon energy, and light intensity, thus featuring the hallmarks of direct interband carrier photogeneration and coherent carrier transport at band states.

Unusual low-energy phonon dynamics in the negative thermal expansion compound ZrW2O8.

An infrared study of the phonon spectra of ZrW2O8 as a function of temperature which includes the low-energy (2-10 meV) region relevant to negative thermal expansion is reported and discussed in the context of specific heat and neutron density of state results. The prevalence of infrared active phonons at low energy and their observed temperature dependence are highly unusual and indicative of exotic low-energy lattice dynamics. Eigenvector calculations indicate a mixing of librational and translational motion within each low-frequency IR mode. The role of the underconstrained structure in establishing the nature of these modes and the relationship between the IR spectra and the large negative thermal expansion in ZrW2O8 are discussed.

Weak-localization-like temperature-dependent conductivity of a dilute two-dimensional hole gas in a parallel magnetic field.

We have studied the magnetotransport properties of a high mobility two-dimensional hole gas (2DHG) in a 10 nm GaAs quantum well with densities in the range of (0.7-1.6) x 10(10) cm(-2) on the metallic side of the zero-field "metal-insulator transition." In a parallel field well above B(c) that suppresses the metallic conductivity, the 2DHG exhibits a conductivity Delta(g)(T) approximately (1/pi) (e(2)/h)lnT reminiscent of weak localization for Fermi liquids. The experiments are consistent with the coexistence of two phases in our system: a metallic phase and a weakly insulating Fermi liquid phase.

Two-dimensional metal in a parallel magnetic field.

We have investigated the effect of an in-plane parallel magnetic field (B(axially) on two high mobility metallic-like dilute two-dimensional hole gas systems in GaAs quantum wells. The experiments reveal that, while suppressing the magnitude of the low temperature resistance drop, B(axially) does not affect E(a), the characteristic energy scale of the metallic resistance drop. The field B(c) at which the metallic-like resistance drop vanishes is dependent on both the width of the quantum well and the orientation of B(axially). It is unexpected that E(a) is unaffected by B(axially) up to B(c) despite the fact that the Zeeman energy at B(c) is roughly equal to E(a).