Tight-Binding Approach to Pyrazine-Mediated Superexchange in Copper-Pyrazine Antiferromagnets.

We investigate the cause of spatial superexchange anisotropy in a family of copper-based, quasi-two-dimensional materials with very similar geometries. The compounds in this family differ mainly in their inter-layer separation but they have very different magnetic interactions, even within the basal plane. We use density functional theory and Wannier functions to parameterize two complimentary tight-binding models and show that the superexchange between the Cu2+ ions is dominated by a σ-mediated interaction between hybrid Cu-pyrazine orbitals centered on the copper atoms. We find no correlations between the strength of this exchange interaction and homologous geometric features across the compounds, such as Cu and pyrazine bond lengths and orientations of nearby counterions. We find that the pyrazine tilt angles do not affect the Cu-pyrazine-Cu exchange because the lowest unoccupied molecular orbital on pyrazine is at a very high energy (relative to the frontier orbitals, which are Cu-based). We conclude that careful control of the entire crystal structure, including non-homologous geometric features such as the inter-layer organic ligands, is vital for engineering magnetic properties.

Spin-State Ice in Elastically Frustrated Spin-Crossover Materials.

Molecules with bistable spin states are widely studied because of their importance to the natural world and their potential applications as molecular scale switches. In molecular crystals and framework materials, elastic interactions between molecules lead to collective phenomena including hysteresis, multistep transitions, and antiferroelastic order of spin states. Elastic frustration, the inability to simultaneously minimize competing elastic interactions, plays a key role in many of the most important phenomena in spin crossover materials. Here we use an elastic model to predict that a new phase of matter occurs for bistable molecules on the kagome lattice, which is intrinsically frustrated as it is composed of equilateral triangles. In this phase, which we call "spin-state ice" in analogy to water and spin ices, there is no long-range order of spin-states; instead they follow a local "ice rule" that each triangle must contain two metal centers in one spin state and one in the other. We show that spin-state ice supports mobile collective excitations that carry a spin midway between the two spin states of a single metal center but no electrical charge. We show that there are distinctive signatures of spin-state ice in neutron scattering, electron paramagnetic resonance, and thermodynamic experiments.

Erratum: Dynamical Reduction of the Dimensionality of Exchange Interactions and the "Spin-Liquid" Phase of κ-(BEDT-TTF)_{2}X [Phys. Rev. Lett. 119, 087204 (2017)].

This corrects the article DOI: 10.1103/PhysRevLett.119.087204.

Dynamical Reduction of the Dimensionality of Exchange Interactions and the "Spin-Liquid" Phase of κ-(BEDT-TTF)_{2}X.

We show that the anisotropy of the effective spin model for the dimer Mott insulator phase of κ-(BEDT-TTF)_{2}X salts is dramatically different from that of the underlying tight-binding model. Intradimer quantum interference results in a model of coupled spin chains, where frustrated interchain interactions suppress long-range magnetic order. Thus, we argue, the "spin liquid" phase observed in some of these materials is a remnant of the Tomonaga-Luttinger physics of a single chain. This is consistent with previous experiments and resolves some outstanding puzzles.

Haldane phase in the Hubbard model at 2/3-filling for the organic molecular compound Mo3S7(dmit)3.

We report the discovery of a correlated insulator with a bulk gap at 2/3 filling in a geometrically frustrated Hubbard model that describes the low-energy physics of Mo3S7(dmit)3. This is very different from the Mott insulator expected at half-filling. We show that the insulating phase, which persists even for very weak electron-electron interactions (U), is adiabatically connected to the Haldane phase and is consistent with experiments on Mo3S7(dmit)3.

Bone characteristics and femoral strength in commercial toms: the effect of protein and energy restriction.

Selection for rapid growth in turkeys has resulted in skeletal problems such as femoral fractures. Slowing growth rate has improved bone structure, but the effect on mechanical properties of the bone is unclear. The current study's hypothesis was that slowing the growth of turkeys by reducing energy and CP in the diet would result in increased femur integrity. Commercial turkeys were fed 1 of 3 diets: control with 100% of NRC energy and CP levels, as well as a diet feeding 80 or 60% of NRC energy and CP levels. All other nutrients met or exceeded NRC requirements. Control birds were grown to 20 wk of age, whereas the 80 and 60% NRC birds were sampled when BW matched that of control birds at wk 4, 8, 12, 16, and 20. Both femurs were extracted, with one being measured and ashed and the other twisted to failure to evaluate mechanical properties. Total bone length, diameter, cortical thickness, and cortical density were measured. The total femur length was longer in the 60% NRC birds at 5 and 10 kg of BW compared with control (P < 0.05); this significance was lost by the time birds reached 16 kg of BW. At 5 and 10 kg of BW, ash content was higher in the control birds than in the 60% NRC birds (P < 0.05). At 16 kg of BW, the 60% NRC birds had the highest femur ash (P < 0.05). The mechanical testing parameters were failure torque, shear strength, and shear modulus of the bones. The 60% diet produced the highest failure torque (P < 0.05), at 16 kg of BW and onward. The shear strength was greater (P = 0.01) once the birds reached 5 kg of BW for the 60% diet than other diets. In conclusion, reducing the energy and protein in the diet to 60% of NRC recommendations, thus slowing growth, improved bone strength, as measured by failure torque, and bone quality, as measured by shear strength, without altering bone length or ash content by the time birds reached market weight.

Geometrical frustration in the spin liquid ß'-Me3EtSb[Pd(dmit)2]2 and the valence-bond solid Me3EtP[Pd(dmit)2]2.

We show that the electronic structures of the title compounds predicted by density functional theory are well described by tight binding models. We determine the frustration ratio, J'/J, of the Heisenberg model on the anisotropic triangular lattice, which describes the spin degrees of freedom in the Mott insulating phase for a range of Pd(dmit)2 salts. All of the antiferromagnetic materials studied have J'/J is < or approximately equal to 0.5 or J'/J > or approximately equal to 0.9, and all salts with 0.5 < or approximately equal to J'/J < or approximately equal to 0.9 are known, experimentally, to be charge ordered valence-bond solids or spin liquids.

Effects of fluorination on iridium(III) complex phosphorescence: magnetic circular dichroism and relativistic time-dependent density functional theory.

We use a combination of low temperature, high field magnetic circular dichroism, absorption, and emission spectroscopy with relativistic time-dependent density functional calculations to reveal a subtle interplay between the effects of chemical substitution and spin-orbit coupling (SOC) in a family of iridium(III) complexes. Fluorination at the ortho and para positions of the phenyl group of fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) cause changes that are independent of whether the other position is fluorinated or protonated. This is demonstrated by a simple linear relationship found for a range of measured and calculated properties of these complexes. Further, we show that the phosphorescent radiative rate, k(r), is determined by the degree to which SOC is able to hybridize T(1) to S(3) and that k(r) is proportional to the inverse fourth power of the energy gap between these excitations. We show that fluorination in the para position leads to a much larger increase of the energy gap than fluorination at the ortho position. Theory is used to trace this back to the fact that fluorination at the para position increases the difference in electron density between the phenyl and triazolyl groups, which distorts the complex further from octahedral symmetry, and increases the energy separation between the highest occupied molecular orbital (HOMO) and the HOMO-1. This provides a new design criterion for phosphorescent iridium(III) complexes for organic optoelectronic applications. In contrast, the nonradiative rate is greatly enhanced by fluorination at the ortho position. This may be connected to a significant redistribution of spectral weight. We also show that the lowest energy excitation, 1A, has almost no oscillator strength; therefore, the second lowest excitation, 2E, is the dominant emissive state at room temperature. Nevertheless the mirror image rule between absorption and emission is obeyed, as 2E is responsible for both absorption and emission at all but very low (<10 K) temperatures.

Sensitivity of the photophysical properties of organometallic complexes to small chemical changes.

We investigate an effective model Hamiltonian for organometallic complexes that are widely used in optoelectronic devices. The two most important parameters in the model are J, the effective exchange interaction between the π and π* orbitals of the ligands, and ε*, the renormalized energy gap between the highest occupied orbitals on the metal and on the ligand. We find that the degree of metal-to-ligand charge transfer character of the lowest triplet state is strongly dependent on the ratio ε*/J. ε* is purely a property of the complex and can be changed significantly by even small variations in the complex's chemistry, such as replacing substituents on the ligands. We find that small changes in ε*/J can cause large changes in the properties of the complex, including the lifetime of the triplet state and the probability of injected charges (electrons and holes) forming triplet excitations. These results give some insight into the observed large changes in the photophysical properties of organometallic complexes caused by small changes in the ligands.

Toward the parametrization of the Hubbard model for salts of bis(ethylenedithio)tetrathiafulvalene: a density functional study of isolated molecules.

We calculate the effective Coulomb repulsion between electrons/holes U(m) ((v)) and site energy for an isolated bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecule in vacuo. U(m) ((v))=4.2+/-0.1 eV for 44 experimental geometries taken from a broad range of conformations, polymorphs, anions, temperatures, and pressures (the quoted "error" is one standard deviation). Hence we conclude that U(m) ((v)) is essentially the same for all of the compounds studied. This shows that the strong (hydrostatic and chemical) pressure dependence observed in the phase diagrams of the BEDT-TTF salts is not due to U(m) ((v)). Therefore, if the Hubbard model is sufficient to describe the phase diagram of the BEDT-TTF salts, there must be significant pressure dependence on the intramolecular terms in the Hamiltonian and/or the reduction in the Hubbard U due to the interaction of the molecule with the polarizable crystal environment. The renormalized value of U(m) ((v)) is significantly smaller than the bare value of the Coulomb integral, F(0)=5.2+/-0.1 eV, across the same set of geometries, emphasizing the importance of using the renormalized value of U(m) ((v)). The site energy (for holes), xi(m)=5.0+/-0.2 eV, varies only a little more than U(m) ((v)) across the same set of geometries. However, we argue that this variation in the site energy plays a key role in understanding the role of disorder in bis(ethylenedithio)tetrathiafulvalene salts. We explain the differences between the beta(L) and beta(H) phases of (BEDT-TTF)(2)I(3) on the basis of calculations of the effects of disorder.

Vertex corrections and the Korringa ratio in strongly correlated electron materials.

We show that the Korringa ratio, associated with nuclear magnetic resonance in metals, is unity if vertex corrections to the dynamic spin susceptibility are negligible, the hyperfine coupling is momentum independent, and there exists an energy scale below which the density of states is constant. In the absence of vertex corrections we also find a Korringa behaviour for T(1), the nuclear spin relaxation rate, i.e., [Formula: see text], and a temperature independent Knight shift. These results are independent of the form and magnitude of the self-energy (so far as is consistent with neglecting vertex corrections) and of the dimensionality of the system.

Transition dipole strength of eumelanin.

We report the transition dipole strength of eumelanin (the principal human photoprotective pigment) in the ultraviolet and visible. We have used both theoretical (density functional) and experimental methods to show that eumelanin is not an unusually strong absorber amongst organic chromophores. This is somewhat surprising given its role as a photoprotectant, and suggests that the dark coloring in vivo (and in vitro) of the eumelanin pigment is a concentration effect. Furthermore, by observing the polymerization of a principle precursor (5,6-dihydroxyindole-2-carboxylic acid) into the full pigment, we observe that eumelanin exhibits a small amount (approximately 20%) of hyperchromism (i.e., the reaction process enhances the light absorption ability of the resultant macromolecule relative to its monomeric precursor). These results have significant implications for our understanding of the photophysics of these important functional biomolecules. In particular, they appear to be consistent with the recently proposed chemical disorder secondary structure model of eumelanins.

Mixed order parameters, accidental nodes and broken time reversal symmetry in organic superconductors: a group theoretical analysis.

We present a group theoretical analysis of several classes of organic superconductor. We predict that highly frustrated organic superconductors, such as κ-(ET)(2)Cu(2)(CN)(3) (where ET is BEDT-TTF, bis(ethylene-dithio)tetrathiafulvalene) and ß'-[Pd(dmit)(2)](2)X, undergo two superconducting phase transitions, the first from the normal state to a d-wave superconductor and the second to a d+id state. We show that the monoclinic distortion of κ-(ET)(2)Cu(NCS)(2) means that the symmetry of its superconducting order parameter is different from that of orthorhombic κ-(ET)(2)Cu[N(CN)(2)]Br. We propose that ß'' and θ phase organic superconductors have d(xy)+s order parameters.

Conservation laws, radiative decay rates, and excited state localization in organometallic complexes with strong spin-orbit coupling.

There is longstanding fundamental interest in 6-fold coordinated d(6) (t(2g)(6)) transition metal complexes such as [Ru(bpy)3](2+) and Ir(ppy)3, particularly their phosphorescence. This interest has increased with the growing realisation that many of these complexes have potential uses in applications including photovoltaics, imaging, sensing, and light-emitting diodes. In order to design new complexes with properties tailored for specific applications a detailed understanding of the low-energy excited states, particularly the lowest energy triplet state, T1, is required. Here we describe a model of pseudo-octahedral complexes based on a pseudo-angular momentum representation and show that the predictions of this model are in excellent agreement with experiment - even when the deviations from octahedral symmetry are large. This model gives a natural explanation of zero-field splitting of T1 and of the relative radiative rates of the three sublevels in terms of the conservation of time-reversal parity and total angular momentum modulo two. We show that the broad parameter regime consistent with the experimental data implies significant localization of the excited state.

The discriminate validity of the Psychiatric Diagnostic Interview.

The discriminate validity of the Psychiatric Diagnostic Interview (PDI), a criterion-referenced, structured diagnostic instrument, is discussed. Data presented indicate that the PDI has an acceptable level of discriminate ability, making it a useful screening tool for detecting a variety of well-defined psychiatric disorders.

Primary and secondary depression in alcoholic men: an important distinction?

The primary-secondary depression distinction was investigated in male alcoholic patients from five Veterans Administration Medical Centers. The Psychiatric Diagnostic Interview, a DSM-III-compatible, criterion-referenced, structured interview, was administered to 565 patients admitted to the Alcoholism and Drug Treatment Units. Seventy-eight patients (13.8%) who exhibited only alcoholism and depression were divided into three subgroups based on the temporal onset of depression relative to the onset of alcoholism. Although few statistical differences were found, observed trends suggested more impairment in alcoholic patients with primary depression than in those with concurrent or secondary depression. The findings indicate that the primary-secondary depression distinction may have important clinical relevance and should be made whenever possible.

Prevalence of additional psychiatric syndromes among male alcoholics.

The prevalence of psychiatric syndromes among 565 male alcoholic VA inpatients was examined using the structured Psychiatric Diagnostic Interview (PDI). Almost two-thirds fulfilled criteria for one or more additional psychiatric syndromes. Subjects positive only for alcoholism reported a significantly later onset of problem drinking and fewer first-degree relatives who abused alcohol than those positive for one or more additional syndromes. The most frequent additional syndromes were affective disorders and antisocial personality. The possible significance of subtyping alcoholics by the presence or absence of coexisting psychiatric syndromes is discussed.

Clinical practice guidelines for bipolar disorder from the Department of Veterans Affairs.

BACKGROUND: For the last several years, the Department of Veterans Affairs (VA) has been involved in the development of practice guidelines for major medical, surgical, and mental disorders. This article describes the development and content of the VA-Clinical Practice Guidelines for Bipolar Disorder, which are available in their entirety on the Journal Web site (http://www. psychiatrist.com). METHOD: A multidisciplinary work group composed of content experts in the field of bipolar disorder and practitioners in general clinical practice was convened by the VA's Office of Performance and Quality and the Mental Health Strategic Health Group. The work group was instructed in algorithm development and methods of evidence evaluation. Draft guidelines were developed over the course of 6 months of meetings and conference calls, and that draft was then sent to nationally prominent content experts for final critique. RESULTS: The Bipolar Guidelines are part of the family of the VA Clinical Guidelines for Management of Persons with Psychosis and consist of explicit algorithms supplemented by annotations that explain the specific decision points and their basis in the scientific literature. The guidelines are organized into 5 modules: a Core Module for diagnosis and assignment to mood state plus 4 treatment modules (Manic/Hypomanic/Mixed Episode, Bipolar Depressive Episode, Rapid Cycling, and Bipolar Disorder With Psychotic Features). The modules specify particular diagnostic and treatment tasks at each step, including both somatotherapeutic and psychotherapeutic interventions. CONCLUSION: The VA Bipolar Guidelines are designed for easy clinical reference in decision making with individual patients, as well as for use as a scholarly reference tool. They also have utility in training activities and quality improvement programs.

Acquired preference for morphine but no d-amphetamine as a result of saccharine adulteration.

Consumption of morphine sulfate and d-amphetamine was studied in two groups of rats. In a choice situation, preference for both drugs remained low after 46 days of drinking. In two additional groups morphine and d-amphetamine solutions were prepared with 1% saccharine. Morphine drinking was significantly increased by saccharine adulteration, whereas drinking of amphetamine solutions decreased. Addition of saccharine to morphine solutions increased drinking in more than a simple additive way. Saccharine facilitates the acquisition of drug-directed behavior. The slope of the acquisition trials for the morphine-saccharine group was significantly different from horizontal (O-slope) and significantly different from the slope found for the morphine without saccharine group.