99 research outputs found

### Emergence of quasi-one-dimensional physics in Mo$_3$S$_7$(dmit)$_3$, a nearly-isotropic three-dimensional molecular crystal

We report density functional theory calculations for Mo$_3$S$_7$(dmit)$_3$.
We derive an ab initio tight-binding model from overlaps of Wannier orbitals;
finding a layered model with interlayer hopping terms $\sim3/4$ the size of the
in-plane terms. The in-plane Hamiltonian interpolates the kagom\'e and
honeycomb lattices. It supports states localized to dodecahedral rings within
the plane, which populate one-dimensional (1D) bands and lead to a quasi-1D
spin-one model on a layered honeycomb lattice once interactions are included.
Two lines of Dirac cones also cross the Fermi energy.Comment: 5 pages, 3 figure

### Unified explanation of the Kadowaki-Woods ratio in strongly correlated materials

Discoveries of ratios whose values are constant within broad classes of
materials have led to many deep physical insights. The Kadowaki-Woods ratio
(KWR) compares the temperature dependence of a metal's resistivity to that of
its heat capacity; thereby probing the relationship between the
electron-electron scattering rate and the renormalisation of the electron mass.
However, the KWR takes very different values in different materials. Here we
introduce a ratio, closely related to the KWR, that includes the effects of
carrier density and spatial dimensionality and takes the same (predicted) value
in organic charge transfer salts, transition metal oxides, heavy fermions and
transition metals - despite the numerator and denominator varying by ten orders
of magnitude. Hence, in these materials, the same emergent physics is
responsible for the mass enhancement and the quadratic temperature dependence
of the resistivity and no exotic explanations of their KWRs are required.Comment: Final version accepted by Nature Phy

### Towards mechanomagnetics in elastic crystals: insights from [Cu(acac)$_2$]

We predict that the magnetic properties of \cuacac, an elastically flexible
crystal, change dramatically when the crystal is bent. We find that unbent
\cuacac\ is an almost perfect Tomonaga-Luttinger liquid. Broken-symmetry
density functional calculations reveal that the magnetic exchange interactions
along the chains is an order of magnitude larger than the interchain exchange.
The geometrically frustrated interchain interactions cannot magnetically order
the material at any experimentally accessible temperature. The ordering
temperature ($T_N$), calculated from the chain random phase approximation,
increases by approximately 24 orders of magnitude when the material is bent. We
demonstrate that geometric frustration both suppresses $T_N$ and enhances the
sensitivity of $T_N$ to bending. In \cuacac, $T_N$ is extremely sensitive to
bending, but remains too low for practical applications, even when bent.
Partially frustrated materials could achieve the balance of high $T_N$ and good
sensitivity to bending required for practical applications of mechanomagnetic
elastic crystals

### Sensitivity of the photo-physical 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
$\pi$ and $\pi^*$ orbitals of the ligands, and $\epsilon^*$, 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 (MLCT)
character of the lowest triplet state is strongly dependent on the ratio
$\epsilon^*/J$. $\epsilon^*$ 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 that small changes in
$\epsilon^*/J$ 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.Comment: Accepted for publication in J. Chem. Phys. 14 pages, 9 figures,
Supplementary Info: 15 pages, 17 figure

### Effects of anisotropy in spin molecular-orbital coupling on effective spin models of trinuclear organometallic complexes

We consider layered decorated honeycomb lattices at two-thirds filling, as
realized in some trinuclear organometallic complexes. Localized $S=1$ moments
with a single-spin anisotropy emerge from the interplay of Coulomb repulsion
and spin molecular-orbit coupling (SMOC). Magnetic anisotropies with bond
dependent exchange couplings occur in the honeycomb layers when the direct
intracluster exchange and the spin molecular-orbital coupling are both present.
We find that the effective spin exchange model within the layers is an XXZ +
120$^\circ$ honeycomb quantum compass model. The intrinsic non-spherical
symmetry of the multinuclear complexes leads to very different transverse and
longitudinal spin molecular-orbital couplings, which greatly enhances the
single-spin and exchange coupling anisotropies. The interlayer coupling is
described by a XXZ model with anisotropic biquadratic terms. As the correlation
strength increases the systems becomes increasingly one-dimensional. Thus, if
the ratio of SMOC to the interlayer hopping is small this stabilizes the
Haldane phase. However, as the ratio increases there is a quantum phase
transition to the topologically trivial `$D$-phase'. We also predict a quantum
phase transition from a Haldane phase to a magnetically ordered phase at
sufficiently strong external magnetic fields.Comment: 22 pages, 11 figures. Final version of paper to be published in PRB.
Important corrections to appendix

### Heisenberg and Dzyaloshinskii-Moriya interactions controlled by molecular packing in tri-nuclear organometallic clusters

Motivated by recent synthetic and theoretical progress we consider magnetism
in crystals of multi-nuclear organometallic complexes. We calculate the
Heisenberg symmetric exchange and the Dzyaloshinskii-Moriya antisymmetric
exchange. We show how, in the absence of spin-orbit coupling, the interplay of
electronic correlations and quantum interference leads to a quasi-one
dimensional effective spin model in a typical tri-nuclear complex,
Mo$_3$S$_7$(dmit)$_3$, despite its underlying three dimensional band structure.
We show that both intra- and inter-molecular spin-orbit coupling can cause an
effective Dzyaloshinskii-Moriya interaction. Furthermore, we show that, even
for an isolated pair of molecules the relative orientation of the molecules
controls the nature of the Dzyaloshinskii-Moriya coupling. We show that
interference effects also play a crucial role in determining the
Dzyaloshinskii-Moriya interaction. Thus, we argue, that multi-nuclear
organometallic complexes represent an ideal platform to investigate the effects
of Dzyaloshinskii-Moriya interactions on quantum magnets.Comment: This update incorporates the corrections described in a recently
submitted erratum. Changes are confined to sections IV.A and B. The
conclusions of the paper are unchanged. 12 + 4 pages, 9 figure

### Spin-orbit coupling in {Mo$_3$S$_7$(dmit)$_3$}

Spin-orbit coupling in crystals is known to lead to unusual direction
dependent exchange interactions, however understanding of the consequeces of
such effects in molecular crystals is incomplete. Here we perform four
component relativistic density functional theory computations on the
multi-nuclear molecular crystal {Mo$_3$S$_7$(dmit)$_3$} and show that both
intra- and inter-molecular spin-orbit coupling are significant. We determine a
long-range relativistic single electron Hamiltonian from first principles by
constructing Wannier spin-orbitals. We analyse the various contributions
through the lens of group theory. Intermolecular spin-orbit couplings like
those found here are known to lead to quantum spin-Hall and topological
insulator phases on the 2D lattice formed by the tight-binding model predicted
for a single layer of {Mo$_3$S$_7$(dmit)$_3$}

### Electronic properties of Fabre charge-transfer salts under various temperature and pressure conditions

Using density functional theory, we determine parameters of tight-binding
Hamiltonians for a variety of Fabre charge transfer salts, focusing in
particular on the effects of temperature and pressure. Besides relying on
previously published crystal structures, we experimentally determine two new
sets of structures; (TMTTF)$_2$SbF$_6$ at different temperatures and
(TMTTF)$_2$PF$_6$ at various pressures. We find that a few trends in the
electronic behavior can be connected to the complex phase diagram shown by
these materials. Decreasing temperature and increasing pressure cause the
systems to become more two-dimensional. We analyze the importance of
correlations by considering an extended Hubbard model parameterized using
Wannier orbital overlaps and show that while charge order is strongly activated
by the inter-site Coulomb interaction, the magnetic order is only weakly
enhanced. Both orders are suppressed when the effective pressure is increased.Comment: 12 pages, 16 figure

### When is the Kadowaki-Woods ratio universal?: Supplementary Material

The supplementary material contains details of the derivations.Sections:I. Scattering and the self-energy in arbitrary bandstructures systemsa. The Two-Band Caseb. The Spectral Density FunctionII. Interband scatteringa. Effects of significant interband scattering on the intraband self-energy componentsIII. Derivation of the conductivity formul

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