2,135 research outputs found
Lattice effects in the quasi-two-dimensional valence-bond-solid Mott insulator EtMeP[Pd(dmit)]
The organic charge-transfer salt EtMeP[Pd(dmit)] is a
quasi-two-dimensional Mott insulator with localized spins = 1/2 residing on
a distorted triangular lattice. Here we report measurements of the uniaxial
thermal expansion coefficients along the in-plane = - and
-axis as well as along the out-of-plane -axis for temperatures 1.4\,K
T 200\,K. Particular attention is paid to the lattice effects
around the phase transition at = 25\,K into a low-temperature
valence-bond-solid phase and the paramagnetic regime above where effects of
short-range antiferromagnetic correlations can be expected. The salient results
of our study include (i) the observation of strongly anisotropic lattice
distortions accompanying the formation of the valence-bond-solid, and (ii) a
distinct maximum in the thermal expansion coefficients in the paramagnetic
regime around 40\,K. Our results demonstrate that upon cooling through
the in-plane -axis, along which the valence bonds form, contracts
while the second in-plane -axis elongates by the same relative amount.
Surprisingly, the dominant effect is observed for the out-of-plane -axis
which shrinks significantly upon cooling through . The pronounced
anomaly in around 40\,K is attributed to short-range magnetic
correlations. It is argued that the position of this maximum, relative to that
in the magnetic susceptibility around 70\,K, speaks in favor of a more
anisotropic triangular-lattice scenario for this compound than previously
thought.Comment: 6 pages, 4 figures, submitted to Phys. Rev.
Field-induced length changes in the spin-liquid candidate -(BEDT-TTF)Cu(CN)
Measurements of the coefficient of thermal expansion on the spin-liquid
candidate -(BEDT-TTF)Cu(CN) have revealed distinct and
strongly anisotropic lattice effects around 6 K - a possible spin-liquid
instability. In order to study the effects of a magnetic field on the
low-temperature spin-liquid state, dilatometric measurements have been
conducted both as a function of temperature at \emph{B} = const. and as a
function of field at \emph{T} = const. While the 6 K anomaly is found to be
insensitive to magnetic fields \emph{B} 10 T, the maximum field applied,
surprisingly strong \emph{B}-induced effects are observed for magnetic fields
applied along the in-plane \emph{b}-axis. Above a threshold field of 0.5 T <
\emph{B} 1 T, a jump-like anomaly is observed in the \emph{b}-axis
lattice parameter. This anomaly, which is located at 8.7 K at \emph{B} = 1 T,
grows in size and shifts to lower temperatures with increasing the magnetic
field. Although the anomaly bears resemblance to a first-order phase
transition, the lack of hysteresis suggests otherwise.Comment: 3 pages, 3 figures, proceedings of ISCOM 2011, physica status solidi
(c)(in press
Magnetic Field-Induced Lattice Effects in a Quasi-2D Organic Conductor Close to the Mott Metal-Insulator Transition
We present ultra-high-resolution dilatometric studies in magnetic fields on a
quasi-two-dimensional organic conductor
-(D8-BEDT-TTF)Cu[N(CN)]Br, which is located close to the
Mott metal-insulator (MI) transition. The obtained thermal expansion
coefficient, , reveals two remarkable features: (i) the Mott MI
transition temperature = (13.6 0.6)\,K is insensitive to fields
up to 10\,T, the highest applied field; (ii) for fields along the interlayer
\emph{b}-axis, a magnetic-field-induced (FI) phase transition at =
(9.5 0.5)\,K is observed above a threshold field 1 T,
indicative of a spin reorientation with strong magneto-elastic coupling.Comment: 5 pages, 4 figure
Probing the Ionic Dielectric Constant Contribution in the Ferroelectric Phase of the Fabre-Salts
In strongly correlated organic materials it has been pointed out that
charge-ordering could also achieve electronic ferroelectricity at the same
critical temperature . A prototype of such phenomenon are the quasi-one
dimensional (TMTTF) Fabre-salts. However, the stabilization of a
long-range ferroelectric ground-state below requires the break of
inversion symmetry, which should be accompanied by a lattice deformation. In
this work we investigate the role of the monovalent counter-anion in such
mechanism. For this purpose, we measured the quasi-static dielectric constant
along the -axis direction, where layers formed by donors and anions
alternate. Our findings show that the ionic charge contribution is three orders
of magnitude lower than the intra-stack electronic response. The
dielectric constant () probes directly the charge response of
the monovalent anion , since the anion mobility in the structure should help
to stabilize the ferroelectric ground-state. Furthermore, our
measurements %conjugated with earlier investigations of the lattice
thermal expansion, show that the dielectric response is thermally broaden below
if the ferroelectric transition occurs in the temperature range where
the anion movement begin to freeze in their methyl groups cavity. In the
extreme case of the PF-H salt, where occurs at the freezing
point, a relaxor-type ferroelectricity is observed. Also, because of the slow
kinetics of the anion sub-lattice, global hysteresis effects and reduction of
the charge response upon successive cycling are observed. In this context, we
propose that anions control the order-disorder or relaxation character of the
ferroelectric transition of the Fabre-salts.Comment: 8 pages, 7 figures. To appear in Physical Review
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