595 research outputs found
- and - Relaxation Dynamics of a fragile plastic crystal
We present a thorough dielectric investigation of the relaxation dynamics of
plastic crystalline Freon112, which exhibits freezing of the orientational
degrees of freedom into a glassy crystal below 90 K. Among other plastic
crystals, Freon112 stands out by being relatively fragile within Angell's
classification scheme and by showing an unusually strong -relaxation.
Comparing the results to those on Freon112a, having only a single molecular
conformation, points to the importance of the presence of two molecular
conformations in Freon112 for the explanation of its unusual properties.Comment: 17 pages, 6 figure
Low temperature dielectric relaxation study of aqueous solutions of diethylsulfoxide
In the present work, dielectric spectra of mixtures of diethylsulfoxide
(DESO) and water are presented, covering a concentration range of 0.2 - 0.3
molar fraction of DESO. The measurements were performed at frequencies between
1 Hz and 10 MHz and for temperatures between 150 and 300 K. It is shown that
DESO/water mixtures have strong glass-forming abilities. The permittivity
spectra in these mixtures reveal a single relaxation process. It can be
described by the Havriliak-Negami relaxation function and its relaxation times
follow the Vogel-Fulcher-Tammann law, thus showing the typical signatures of
glassy dynamics. The concentration dependence of the relaxation parameters,
like fragility, broadening, and glass temperature, are discussed in detail.Comment: 20 pages, 5 figure
Evolution of magnetic states in frustrated diamond lattice antiferromagnetic Co(Al1-xCox)2O4 spinels
Using neutron powder diffraction and Monte-Carlo simulations we show that a
spin-liquid regime emerges at $all compositions in the diamond-lattice
antiferromagnets Co(Al1-xCox)2O4. This spin-liquid regime induced by
frustration due to the second-neighbour exchange coupling J2, is gradually
superseded by antiferromagnetic collinear long-range order (k=0) at low
temperatures. Upon substitution of Al3+ by Co3+ in the octahedral B-site the
temperature range occupied by the spin-liquid regime narrows and TN increases.
To explain the experimental observations we considered magnetic anisotropy D or
third-neighbour exchange coupling J3 as degeneracy-breaking perturbations. We
conclude that Co(Al1-xCox)2O4 is below the theoretical critical point
J2/J1=1/8, and that magnetic anisotropy assists in selecting a collinear
long-range ordered ground state, which becomes more stable with increasing x
due to a higher efficiency of O-Co3+-O as an interaction path compared to
O-Al3+-O
Plastic-crystalline solid-state electrolytes: Ionic conductivity and orientational dynamics in nitrile mixtures
Many plastic crystals, molecular solids with long-range, center-of-mass
crystalline order but dynamic disorder of the molecular orientations, are known
to exhibit exceptionally high ionic conductivity. This makes them promising
candidates for applications as solid-state electrolytes, e.g., in batteries.
Interestingly, it was found that the mixing of two different
plastic-crystalline materials can considerably enhance the ionic dc
conductivity, an important benchmark quantity for electrochemical applications.
An example is the admixture of different nitriles to succinonitrile, the latter
being one of the most prominent plastic-crystalline ionic conductors. However,
until now only few such mixtures were studied. In the present work, we
investigate succinonitrile mixed with malononitrile, adiponitrile, and
pimelonitrile, to which 1 mol% of Li ions were added. Using differential
scanning calorimetry and dielectric spectroscopy, we examine the phase behavior
and the dipolar and ionic dynamics of these systems. We especially address the
mixing-induced enhancement of the ionic conductivity and the coupling of the
translational ionic mobility to the molecular reorientational dynamics,
probably arising via a "revolving-door" mechanism.Comment: 9 pages, 7 figures; revised version as accepted for publication in J.
Chem. Phy
Spin-orbiton and quantum criticality in FeSc2S4
In FeSc2S4 spin-orbital exchange competes with strong spin-orbit coupling,
suppressing long-range spin and orbital order and, hence, this material
represents one of the rare examples of a spin-orbital liquid ground state.
Moreover, it is close to a quantum-critical point separating the ordered and
disordered regimes. Using THz and FIR spectroscopy we study low-lying
excitations in FeSc2S4 and provide clear evidence for a spin-orbiton, an
excitation of strongly entangled spins and orbitals. It becomes particularly
well pronounced upon cooling, when advancing deep into the quantum-critical
regime. Moreover, indications of an underlying structureless excitation
continuum are found, a possible signature of quantum criticality.Comment: 11 pages, 11 figures (including 4 pages and 5 figures in Supplemental
Material). Revised version according to suggestions of referees. Phys. Rev. B
(in press
Characteristics of ferroelectric-ferroelastic domains in N{\'e}el-type skyrmion host GaVS
GaVS is a multiferroic semiconductor hosting N{\'e}el-type magnetic
skyrmions dressed with electric polarization. At T = 42K, the compound
undergoes a structural phase transition of weakly first-order, from a
non-centrosymmetric cubic phase at high temperatures to a polar rhombohedral
structure at low temperatures. Below T, ferroelectric domains are formed
with the electric polarization pointing along any of the four axes. Although in this material the size and the shape of the
ferroelectric-ferroelastic domains may act as important limiting factors in the
formation of the N{\'e}el-type skyrmion lattice emerging below T=13\:K, the
characteristics of polar domains in GaVS have not been studied yet.
Here, we report on the inspection of the local-scale ferroelectric domain
distribution in rhombohedral GaVS using low-temperature piezoresponse
force microscopy. We observed mechanically and electrically compatible lamellar
domain patterns, where the lamellae are aligned parallel to the (100)-type
planes with a typical spacing between 100 nm-1.2 m. We expect that the
control of ferroelectric domain size in polar skyrmion hosts can be exploited
for the spatial confinement and manupulation of N{\'e}el-type skyrmions
Weak ferromagnetism and glassy state in kappa-(BEDT-TTF)2Hg(SCN)2Br
Since the first observation of weak ferromagnetism in the charge-transfer
salt kappa-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840
(1992)], no further evidence of ferromagnetism in this class of organic
materials has been reported. Here we present static and dynamic spin
susceptibility measurements on kappa-(BEDT-TTF)2Hg(SCN)2Br revealing weak
ferromagnetism below about TWF = 20 K. We suggest that frustrated spins in the
molecular dimers suppress long-range order, forming a spin-glass ground state
in the insulating phase
Magnetic ground state and 2D behavior in pseudo-Kagome layered system Cu3Bi(SeO3)2O2Br
Anisotropic magnetic properties of a layered kagome-like system
Cu3Bi(SeO3)2O2Br have been studied by bulk magnetization and magnetic
susceptibility measurements as well as powder and single-crystal neutron
diffraction. At T_N = 27.4 K the system develops an alternating
antiferromagnetic order of (ab) layers, which individually exhibit canted
ferrimagnetic moment arrangement, resulting from the competing ferro- and
antiferro-magnetic intralayer exchange interactions. A magnetic field B_C ~ 0.8
T applied along the c axis (perpendicular to the layers) triggers a
metamagnetic transition, when every second layer flips, i.e., resulting in a
ferrimagnetic structure. Significantly higher fields are required to rotate the
ferromagnetic component towards the b axis (~7 T) or towards the a axis (~15
T). The estimates of the exchange coupling constants and features indicative of
an XY character of this quasi-2D system are presented.Comment: 7 pages, 6 figures, final versio
Anisotropic Exchange in LiCuVO probed by ESR
We investigated the paramagnetic resonance in single crystals of LiCuVO
with special attention to the angular variation of the absorption spectrum. To
explain the large resonance linewidth of the order of 1 kOe, we analyzed the
anisotropic exchange interaction in the chains of edge-sharing CuO
octahedra, taking into account the ring-exchange geometry of the
nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The
exchange parameters, which can be estimated from theoretical considerations,
nicely agree with the parameters obtained from the angular dependence of the
linewidth. The anisotropy of this magnetic ring exchange is found to be much
larger than it is usually expected from conventional estimations which neglect
the bonding geometry. Hence, the data yield the evidence that in copper oxides
with edge-sharing structures the role of the orbital degrees of freedom is
strongly enhanced. These findings establish LiCuVO as one-dimensional
compound at high temperatures.
PACS: 76.30.-v, 76.30.Fc, 75.30.EtComment: 18 pages, 6 figure
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