24,987 research outputs found

### 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, $T_1$, 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 $T_1$ 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.Comment: 8 pages, 6 figs + sup info (20 pages, 19 figures - to view pdf
download the source files

### The origin of the difference in the superconducting critical temperatures of the beta_H and beta_L phases of (BEDT-TTF)_2I_3

Incommensurate lattice fluctuations are present in the beta_L phase (T_c =
1.5 K) of ET_2I_3 (where ET is BEDT-TTF -
bis(ethylenedithio)tetrathiafulvalene) but are absent in the beta_H phase (T_c
= 7 K). We propose that the disorder in the conformational degrees of freedom
of the terminal ethylene groups of the ET molecules, which is required to
stabilise the lattice fluctuations, increases the quasiparticle scattering rate
and that this leads to the observed difference in the superconducting critical
temperatures, T_c, of the two phases. We calculate the dependence of T_c on the
interlayer residual resistivity. Our theory has no free parameters. Our
predictions are shown to be consistent with experiment. We describe experiments
to conclusively test our hypothesis.Comment: 4 pages, 1 figur

### Spin-state ice in geometrically frustrated spin-crossover materials

Spin crossover materials contain metal ions that can access two spin-states:
one low-spin (LS), the other high-spin (HS). We propose that frustrated elastic
interactions can give rise to spin-state ices -- phases of matter without
long-range order, characterized by a local constraint or `ice rule'. The
low-energy physics of spin-state ices is described by an emergent
divergence-less gauge field with a gap to topological excitations that are
deconfined quasi-particles with spin fractionalized midway between the spins of
the LS and HS states.Comment: Major edit, some new results; 8+2 page

### Geometrical frustration in the spin liquid beta'-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 (DFT) 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 0.9, consistent
with predictions for the Heisenberg model. All salts with 0.5 < J'/J < 0.9,
where many-body theories find a number of competing ground states, are known,
experimentally, to be charge ordered, valence bond solids or spin liquids.Comment: Accepted for publication in Phys. Rev. Lett. 4+11 pages, 3+15
figures, major rewrite, added calculations of Hubbard

### Nuclear Magnetic Resonance in Low-Symmetry Superconductors

We consider the nuclear spin-lattice relaxation rate, $1/T_1T$ in
superconductors with accidental nodes. We show that a Hebel-Slichter-like peak
occurs even in the absence of an isotropic component of the superconducting
gap. The logarithmic divergence found in clean, non-interacting models is
controlled by both disorder and electron-electron interactions. However, for
reasonable parameters, neither of these effects removes the peak altogether.Comment: 10 pages, 5 figure

### Quantum frustration in organic Mott insulators: from spin liquids to unconventional superconductors

We review the interplay of frustration and strong electronic correlations in
quasi-two-dimensional organic charge transfer salts, such as k-(BEDT-TTF)_2X
and Et_nMe_{4-n}Pn[Pd(dmit)2]2. These two forces drive a range of exotic phases
including spin liquids, valence bond crystals, pseudogapped metals, and
unconventional superconductivity. Of particular interest is that in several
materials there is a direct transition as a function of pressure from a spin
liquid Mott insulating state to a superconducting state. Experiments on these
materials raise a number of profound questions about the quantum behaviour of
frustrated systems, particularly the intimate connection between spin liquids
and superconductivity. Insights into these questions have come from a wide
range of theoretical techniques including first principles electronic
structure, quantum many-body theory and quantum field theory. In this review we
introduce the basic ideas of the field by discussing a simple frustrated
Heisenberg model with four spins. We then describe the key experimental
results, emphasizing that for two materials, k-(BEDT-TTF)_2Cu_2(CN)_3 and
EtMe_3Sb[Pd(dmit)_2]_2, there is strong evidence for a spin liquid ground
state, and for EtMe_3P[Pd(dmit)_2]_2, a valence bond solid ground state. We
review theoretical attempts to explain these phenomena, arguing that this can
be captured by a Hubbard model on the anisotropic triangular lattice at half
filling, and that resonating valence bond wavefunctions can capture most of the
essential physics. We review evidence that this model can have a spin liquid
ground state for a range of parameters that are realistic for the relevant
materials. We conclude by summarising the progress made thus far and
identifying some of the key questions still to be answered.Comment: Major rewrite. New material added and many typos corrected. 67 pages,
41 figures. Thanks to those who commented on the previous versio

### Symmetry of the superconducting order parameter in frustrated systems determined by the spatial anisotropy of spin correlations

We study the resonating valence bond (RVB) theory of the Hubbard-Heisenberg
model on the half-filled anisotropic triangular lattice. Varying the
frustration changes the wavevector of maximum spin correlation in the Mott
insulating phase. This, in turn, changes the symmetry of the superconducting
state, that occurs at the boundary of the Mott insulating phase. We propose
that this physics is realised in several families of quasi-two-dimensional
organic superconductors.Comment: To appear in Phys. Rev. Lett. - 5 pages, 4 fig

### Space infrared telescope pointing control system. Infrared telescope tracking in the presence of target motion

The use of charge-coupled-devices, or CCD's, has been documented by a number of sources as an effective means of providing a measurement of spacecraft attitude with respect to the stars. A method exists of defocussing and interpolation of the resulting shape of a star image over a small subsection of a large CCD array. This yields an increase in the accuracy of the device by better than an order of magnitude over the case when the star image is focussed upon a single CCD pixel. This research examines the effect that image motion has upon the overall precision of this star sensor when applied to an orbiting infrared observatory. While CCD's collect energy within the visible spectrum of light, the targets of scientific interest may well have no appreciable visible emissions. Image motion has the effect of smearing the image of the star in the direction of motion during a particular sampling interval. The presence of image motion is incorporated into a Kalman filter for the system, and it is shown that the addition of a gyro command term is adequate to compensate for the effect of image motion in the measurement. The updated gyro model is included in this analysis, but has natural frequencies faster than the projected star tracker sample rate for dim stars. The system state equations are reduced by modelling gyro drift as a white noise process. There exists a tradeoff in selected star tracker sample time between the CCD, which has improved noise characteristics as sample time increases, and the gyro, which will potentially drift further between long attitude updates. A sample time which minimizes pointing estimation error exists for the random drift gyro model as well as for a random walk gyro model

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