23 research outputs found
Long-range modulation of a composite crystal in a five-dimensional superspace
Citation: Guerin, L., Mariette, C., Rabiller, P., Huard, M., Ravy, S., Fertey, P., . . . Toudic, B. (2015). Long-range modulation of a composite crystal in a five-dimensional superspace. Physical Review B, 91(18), 7. doi:10.1103/PhysRevB.91.184101The intergrowth crystal of n-tetracosane/urea presents a misfit parameter, defined by the ratio gamma = c(h)/c(g) (c(host)/c(guest)), that is very close to a commensurate value (gamma congruent to 1/3). High-resolution diffraction studies presented here reveal an aperiodic misfit parameter of gamma = 0.3369, which is found to be constant at all temperatures studied. A complex sequence of structural phases is reported. The high temperature phase (phase I) exists in the four-dimensional superspace group P6(1)22(00 gamma). At T-c1 = 179(1) K, a ferroelastic phase transition increases the dimension of the crystallographic superspace. This orthorhombic phase (phase II) is characterized by the five-dimensional (5D) superspace group C222(1)(00 gamma)(10 delta) with a modulation vector a(o)* + c(m)* = a(o)* + delta . c(h)*, in which the supplementary misfit parameter is delta = 0.025(1) in host reciprocal units. This corresponds to the appearance of a modulation of very long period (about 440 +/- 16 angstrom). At T-c2 = 163.0(5) K, a 5D to 5D phase transition leads to the crystallographic superspace group P2(1)2(1)2(1)(00 gamma)(00 delta) with a very similar value of delta. This phase transition reveals a significant hysteresis effect
High frequency ESR investigation on dynamical charge disproportionation and spin gap excitation in NaV_2O_5
A significant frequency dependence of the ESR line width is found in NaV_2O_5
between 34-100 K and the line width increases as the resonance frequency is
increased from 95 GHz to 760 GHz. The observed frequency dependence is
qualitatively explained in terms of the dynamical charge disproportionation.
The present results show the essential role of the internal charge degree of
freedom in a V-O-V bond. We have also proposed the existence of the
Dzyaloshinsky-Moriya interaction in the low temperature charge ordered phase
considering the breaking of the selection rule of ESR realized as the direct
observation of the spin gap excitation.Comment: 9 figures submitted to J. Phys.Soc. Jp
Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation
Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independentlyfrom the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscopeallows for local implantation and patterning down to the nanometer resolution, which is of interest for deviceapplications. We present here a study of bismuth ferrite (BiFeO3) films where strain was patterned locally byhelium implantation. Our combined Raman, x-ray diffraction, and transmission electron microscopy (TEM)study shows that the implantation causes an elongation of the BiFeO3unit cell and ultimately a transition towardsthe so-called supertetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset ofamorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phasetransition from the R-like to T-like BiFeO3appears as first-order in character, with regions of phase coexistenceand abrupt changes in lattice parameters
Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in -NaVO studied by far infrared spectroscopy
We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3
and 200cm-1 in polarizations of incident light parallel to a, b, and c
crystallographic axes in magnetic fields up to 33T. The triplet origin of an
excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The
magnitude of the spin gap at low temperatures is found to be magnetic field
independent at least up to 33T. All other infrared-active transitions appearing
below Tc are ascribed to zone-folded phonons. Two different dynamic
Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to
the oscillator strength of the otherwise forbidden singlet to triplet
transition. 1. The strongest singlet to triplet transition is an electric
dipole transition where the polarization of the incident light's electric field
is parallel to the ladder rungs, and is allowed by the dynamic DM interaction
created by a high frequency optical a-axis phonon. 2. In the incident light
polarization perpendicular to the ladder planes an enhancement of the singlet
to triplet transition is observed when the applied magnetic field shifts the
singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon
energy. The origin of this mechanism is the dynamic DM interaction created by
the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated
for both mechanisms using the presented theory.Comment: 21 pages, 22 figures. Version 2 with replaced fig. 18 were labels had
been los
Using electronic structure changes to map the H-T phase diagram of alpha'-NaV2O5
We report polarized optical reflectance studies of \alpha'-NaV2O5 as a
function of temperature (4-45 K) and magnetic field (0-60 T). Rung directed
electronic structure changes, as measured by near-infrared reflectance ratios
\Delta R(H)=R(H)/R(H=0 T), are especially sensitive to the phase boundaries. We
employ these changes to map out an H-T phase diagram. Topological highlights
include the observation of two phase boundaries slightly below T_{SG}, enhanced
curvature of the 34 K phase boundary above 35 T, and, surprisingly, strong
hysteresis effects of both transitions with applied field.Comment: 4 pages, 3 figures, PRB accepte
Elastic Constants and Charge Ordering in a'-NaV2O5
We present the temperature dependence of shear and longitudinal elastic
constants in a'-NaV2O5. For the longitudinal c22 and c33 modes we find
anomalies at Tc in contrast to the Spin Peierls substance CuGeO3 where only the
longitudinal mode along the chain shows a pronounced effect at TSP. The c66
shear mode (propagation along the chain in b-direction polarization in
a-direction) shows strong softening of 12%. Such a large effect is absent for
all shear modes in CuGeO3. We can interpret this softening with a coupling of
the exy symmetry strain to the charge fluctuation of B1g symmetry. We give the
possible low temperature charge distribution.Comment: PDF file, 4 pages, 4 figures include
Thermodynamics of Spin S = 1/2 Antiferromagnetic Uniform and Alternating-Exchange Heisenberg Chains
The magnetic susceptibility chi and specific heat C versus temperature T of
the spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenberg
chain are studied for the entire range 0 \leq alpha \leq 1 of the alternation
parameter alpha = J2/J1. For the uniform chain (alpha = 1), detailed
comparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemper
and Johnston are made with the asymptotically exact low-T field theory
predictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T)
are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 is
extracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the above
numerical data are obtained. We examine in detail the theoretical predictions
of Bulaevskii for chi(alpha,T) and compare them with our results. Our
experimental chi(T) and C(T) data for NaV2O5 single crystals are modeled in
detail. The chi(T) data above the spin dimerization temperature Tc = 34 K are
not in agreement with the prediction for the uniform Heisenberg chain, but can
be explained if there is a moderate ferromagnetic interchain coupling and/or if
J changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2)
K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) and
average exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) are
derived from the data. A spin pseudogap with a large magnitude \approx 0.4
Delta(0) is consistently found just above Tc, which decreases with increasing
T. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropy
change due to the transition at Tc and associated order parameter fluctuations
arise from the lattice and/or charge degrees of freedom and less than 23% from
the spin degrees of freedom.Comment: 53 two-column REVTeX pages, 50 embedded figures, 7 tables. Revisions
required due to incorrect Eq. (39) in Ref. 51 which gives the low-T
approximation for the specific heat of a S = 1/2 1D system with a spin gap;
no conclusions were changed. Additional minor revisions made. Phys. Rev. B
(in press
X-ray Investigation of the Magneto-elastic Instability of alpha'-NaV2O5
We present an X-ray diffuse scattering study of the pretransitional
structural fluctuations of the magneto-elastic transition in alpha'-NaV2O5.
This transition is characterized by the appearance below Tsp~35K of satellite
reflections at the reduced wave vector (1/2,1/2,1/4). A large regime of
structural fluctuations is measured up to 90 K. These fluctuations are three
dimensional between Tsp and ~50K and quasi-one dimensional above ~60K. At 40 K
the anisotropy ratio is found to be (xib :xia :xic)= (3.8 : 1.8 : 1), which
reveals the importance of transverse interactions in the stabilization of the
low temperature phase. We discuss our results within the framework of recent
theories dealing with the simultaneous occurrence of a charge ordering, a spin
gap and a lattice distortion in this intriguing compound.Comment: Accepted in PRB Rapid.comm. Corrected typos, references added,
figures improve
A microscopic model for the structural transition and spin gap formation in alpha'-NaV2O5
We present a microscopic model for alpha'-NaV2O5. Using an extended Hubbard
model for the vanadium layers we derive an effective low-energy model
consisting of pseudospin Ising chains and Heisenberg chains coupled to each
other. We find a ``spin-Peierls-Ising'' phase transition which causes charge
ordering on every second ladder and superexchange alternation on the other
ladders. This transition can be identified with the first transition of the two
closeby transitions observed in experiment. Due to charge ordering the
effective coupling between the lattice and the superexchange is enhanced. This
is demonstrated within a Slater-Koster approximation. It leads to a second
instability with superexchange alternation on the charge-ordered ladders due to
an alternating shift of the O sites on the rungs of that ladder. We can explain
within our model the observed spin gap, the anomalous BCS ratio, and the
anomalous shift of the critical temperature of the first transition in a
magnetic field. To test the calculated superstructure we determine the
low-energy magnon dispersion and find agreement with experiment.Comment: 32 pages, 12 figures include