[Show abstract][Hide abstract] ABSTRACT: Tris-sarcosine calcium chloride (TSCC) is a highly uniaxial ferroelectric
with a Curie temperature of approximately 130K. By suppressing ferroelectricity
with bromine substitution on the chlorine sites, pure single crystals were
tuned through a ferroelectric quantum phase transition. The resulting quantum
critical regime was investigated in detail - the first time for a uniaxial
ferroelectric and for an organic ferroelectric - and was found to persist up to
temperatures of at least 30K to 40K. The nature of long-range dipole
interactions in uniaxial materials, which lead to non-analytical terms in the
free-energy expansion in the polarization, predict a dielectric susceptibility
varying as $1/T^3$ close to the quantum critical point. Rather than this, we
find that the dielectric susceptibility varies as $1/T^2$ as expected and
observed in better known multi-axial systems. We explain this result by
identifying the ultra-weak nature of the dipoles in the TSCC family of
crystals. Interestingly we observe a shallow minimum in the inverse dielectric
function at low temperatures close to the quantum critical point in
paraelectric samples that may be attributed to the coupling of quantum
polarization and strain fields. Finally we present results of the heat capacity
and electro-caloric effect and explain how the time dependence of the
polarization in ferroelectrics and paraelectrics should be considered when
making quantitative estimates of temperature changes induced by applied
electric fields.
[Show abstract][Hide abstract] ABSTRACT: The thermal conductivity of uranium dioxide has been studied for over half a century, as uranium dioxide is the fuel used in a majority of operating nuclear reactors and thermal conductivity controls the conversion of heat produced by fission events to electricity. Because uranium dioxide is a cubic compound and thermal conductivity is a second-rank tensor, it has always been assumed to be isotropic. We report thermal conductivity measurements on oriented uranium dioxide single crystals that show anisotropy from 4 K to above 300 K. Our results indicate that phonon-spin scattering is important for understanding the general thermal conductivity behaviour, and also explains the anisotropy by coupling to the applied temperature gradient and breaking cubic symmetry.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that the Curie temperature, Tc , of tris-sarcosine calcium chloride can be lowered from 130 K to near 0 K by chemical doping of the anion. In the low temperature limit, quantum fluctuations persist over a range of chemical doping concentrations. Below TC the temperature dependence of the fractional length change, ΔLLo, scales as (ΔLLo)-1=a+a1tλ (red line) where t is the reduced temperature, and λ = -1.5. The behavior of linear coefficient of thermal expansion near TC is shown in the inset. Three additional phase transitions near 43 K, 50 K, and 61 K are resolved.
[Show abstract][Hide abstract] ABSTRACT: The scientifically fascinating question of the spatial extent and bonding of the 5f orbitals of Pu and its six different phases extends to its delta-retained alloys and the mechanism by which Ga and a number of other unrelated elements stabilize its lowdensity face-centered-cubic (fcc) structure. This issue of phase stability is also important technologically because of its significance to Science-Based Stockpile Stewardship. Answering these questions requires information on the local order and structure around the Ga and its effects on the Pu. We have addressed this by characterizing the structures of a large number of Pu-Ga and two Pu-In and one Pu-Ce delta alloys, including a set of high purity delta Pu1-xGax materials with 1.7 <= x <= 6.4 at. % Ga that span the low [Ga] portion of the d region of the phase diagram across the similar to 3.3 at. % Ga metastability boundary, with extended x-ray absorption fine structure (EXAFS) spectroscopy that probes the element specific local structure, supplemented by x-ray pair distribution function analysis that gives the total local structure to longer distances, and x-ray diffraction that gives the long-range average structure of the periodic component of the materials. Detailed analyses indicate that the alloys at and below a nominal composition of similar to 3.3 at. % Ga are heterogeneous and in addition to the delta phase also contain up to similar to 20% of a novel, coexisting "sigma" structure for Pu that forms in nanometer scale domains that are locally depleted in Ga. The invariance of the Ga EXAFS with composition indicates that this sigma structure forms in Ga-depleted domains that result from the Ga atoms in the delta phase self-organizing into a quasi-intermetallic with a stoichiometry of Pu25-35Ga so that delta Pu-Ga is neither a random solid solution nor the more stable Pu3Ga + alpha. Above this similar to 3.3 at. % Ga nominal composition, the delta Pu-Ga alloy is homogeneous, and no s phase is present. These results that demonstrate that collective and cooperative behavior in the interactions between the alloy elements as well as local elastic forces are crucial in determining the properties of complex materials and contradict the conventional mechanism for martensitic transformations, in this case indicating that nucleation is not the rate limiting step.
Physical Review B 06/2014; 89(22):224102. DOI:10.1103/PhysRevB.89.224102 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: δ Pu–Ga alloys and their response to self-irradiation are important scientifically because of the unique complexity of Pu and technologically because of their importance in Science Based Stockpile Stewardship. The local order and structure and the role of the Ga are crucial to understanding the phase stability and the aging effects. X-ray diffraction that gives the long-range average structure of the periodic component of the materials and pair distribution functions analysis and X-ray absorption fine structure that give the overall and the element specific local structure have been used to examine a variety of new and aged materials, including a set of high purity δ Pu1–xGax alloys with 1.7 ≤ x ≤ 6.4 atom % Ga that span the low [Ga] portion of the δ region of the phase diagram across the 3.3 atom % Ga metastability boundary, a 1.7 atom % Ga alloy that was enriched with Pu238 to accelerate the aging process, and others. We find that metastable alloys contain tens of percents of a novel, “σ”, Pu structure that we attribute to rearrangement of the Ga-depleted regions after the self-organization of the Ga to form quasi-intermetallic Pu25–35Ga. This collective and cooperative behavior involving the Ga and other defects in terms of a tendency to aggregate into domains with structures that differ from the δ host and the resulting nanoscale heterogeneity also appears to play an important role in the observation of analogous locally ordered structures in aged materials. This description of these materials and their aging is radically different from current conceptual basis derived from other experiments that are insensitive to ordering on the angstrom–nanometer length scale.
The Journal of Physical Chemistry C 04/2014; 118(16):8541–8563. DOI:10.1021/jp5004038 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the mechanism of the B2 → R martensitic transformation in the shape memory alloy AuZn by
means of first-principles theory. Phonon anomalies in the TA2 acoustic branch along the -M [ξ,ξ,0] direction
associated with a structural transformation are observed. The calculated Fermi surface of the B2 phase of AuZn
reveals large portions nested with each other by a translation through a vector q = 1 [1,1,0] associated with the 3
soft mode. In addition, we find that the B2 phase can be stabilized by pressure in the low-temperature limit. The energetic barrier for the B2 → R transition is 2 mRy and appears to be near a critical point.
Physical Review B 03/2014; 89(10):104101. DOI:10.1103/PhysRevB.89.104101 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We probe the volume collapse transition (ΔV/Vo ∼ 15%) between the isostructural γ and α phases (T ∼ 100 K) of Ce0.9Th0.1 using the Hall effect, three-terminal capacitive dilatometry, and electrical resistivity measurements. Hall effect measurements confirm the itinerant ground state as the carrier concentration increases by a factor of 7 in the α phase, γ phase (nH = 5.28 × 10(26) m(-3)), and the α phase (nH = 3.76 × 10(27) m(-3)). We were able to detect a noise spectrum consisting of avalanches while slowly varying the temperature through the hysteretic region. We surmise that the avalanches originate from intergranular stresses at the interfaces between partially transformed high-volume and low-volume phases. The statistical distribution of avalanches obey power laws with energy exponent ϵ ≃ 1.5. Hall effect measurements, combined with universal critical exponents, point to short electron mean-free percolation pathways and carrier localization at phase interfaces. Carrier localization was predicted many years ago for elemental cerium by Johansson (1974 Phil. Mag. 30 469).
[Show abstract][Hide abstract] ABSTRACT: Valence band photoemission measurements have been made on crystalline and supercooled liquid gallium, and across the liquid and solid phases of bismuth and indium. Measurements are angle integrated and made using photon excitations of 21.21 and 40.81 eV. In all cases the Bloch states are destroyed upon melting and the free electron gas is constrained by a charge-neutral liquid. The spectra of indium show little change upon solidification, indicating a common electronic structure for crystalline and liquid phases. In contrast, the energy distribution curves for supercooled gallium and bismuth show large changes in the electronic structure from solid to liquid phases, giving rise to the formation of pseudogaps in the density of states at the Fermi energy, EF. Observations of this kind enable us to distinguish normal or anomalous melting from photoemission measurements.
[Show abstract][Hide abstract] ABSTRACT: We present a new analysis of the heat capacity of δ-phase Pu-5 at.% Al with a fit using a single-ion Kondo term and a low-temperature Schottky anomaly in addition to the Debye and linear terms. The Kondo and Schottky terms together contribute 1.2 R to the entropy at 300 K. We show how the extra entropy could affect the alloy phase diagrams of δ-phase Pu.
[Show abstract][Hide abstract] ABSTRACT: Polyisoprene,
(-CH2C(CH3)CH=CH2-)n,
consists of isoprene (2--methyl--1,3--butadiene) molecules that are
linked end--to--end to form long chains. There are two different
isomers, cis and trans, with respect to configuration about the C=C
double bond. Cis--polyisoprene, the main component in natural rubber
derived originally from the Pará rubber tree, is a
non--crystalline elastomer at room temperature. In contrast,
trans--polyisoprene, derived originally from the gutta percha tree, is a
crystalline solid that is rigid and tough. More interestingly, the trans
isomer exhibits shape--memory properties, whereas the cis isomer does
not. As for shape--memory transformations in alloys, trans--polyisoprene
exhibits clear Af and Mf temperatures, with values
338 K and 300 K, respectively. Here we report thermodynamic and
structural measurements of the shape--memory effect in
trans--polyisoprene. We discuss our results in terms of the mechanism of
shape--memory.
[Show abstract][Hide abstract] ABSTRACT: For decades UO2 has been the most widely studied actinide
oxide because of its technological importance as fuel material for
nuclear reactors. Therefore there is a large interest in understanding
its thermal, transport and thermodynamic properties. We present recent
experimental results for the thermal conductivity and thermal expansion
of high quality UO2 single crystal, obtained for different
crystallographic directions, and compare with results of molecular
dynamics simulations. We will discuss the implications of this study.
[Show abstract][Hide abstract] ABSTRACT: Despite large experimental and theoretical efforts the structure of the
superconducting gap and the origin of the pairing mechanism in
iron-based superconductors in still unresolved. Measurements of the low
temperature specific heat and its magnetic response inside the
superconducting state give important information about the symmetry of
the gap. Here, we present results of our studies of codoped
Ba0.55K0.45Fe1.95Co0.05As2
with a Tc of 32.5 K. The high quality of the material is
marked by a pronounced peak at Tc as well as by a low
residual specific heat γ0 = 2.4 mJ/mol K^2. We will
discuss the implications of the new specific heat results on the
symmetry of the order parameter in this system.
Physical Review B 06/2012; 85:224504. · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Low-temperature specific heat and thermal expansion measurements are used to study the hydrogen-based ferroelectric lawsonite over the temperature range 1.8 K ≤ T ≤ 300 K. The second-order phase transition near 125 K is detected in the experiments, and the low-temperature phase is determined to be improper ferroelectric and co-elastic. In the ferroelectric phase T ≤ 125 K, the spontaneous polarization P(s) is proportional to (1) the volume strain e(s), and (2) the excess entropy ΔS(e). These proportionalities confirm the improper character of the ferroelectric phase transition. We develop a structural model that allows the off-centering of hydrogen positions to generate the spontaneous polarization. In the low-temperature limit we detect a Schottky anomaly (two-level system) with an energy gap of Δ ∼ 0.5 meV.
[Show abstract][Hide abstract] ABSTRACT: We address the error resulting from application of the harmonic Debye-Waller factor to anharmonic vibrations. The mean-square atomic displacement 〈u2〉 determined from the harmonic analysis is compared to values obtained from an exact anharmonic analysis. In the case of strong anharmonicity, we find that the harmonic approximation introduces at most a ∼25% error. The temperature dependence determined from the harmonic analysis follows that found from the exact anharmonic analysis. Errors introduced by the harmonic approximation are comparable in magnitude to the usual systematic errors associated with diffraction experiments and Rietveld refinements.
[Show abstract][Hide abstract] ABSTRACT: Quantized waves obeying Bose-Einstein statistics will contribute a T^3/2
term to the specific heat if the dispersion relation goes as q2. We
measure the magnetic and electric field dependence of the specific heat
on the ferroelectric material tri-glycine sulphate (TGS) over the
temperature range 0.05 K < T < 350 K. We detect a T^3/2 term in
the specific heat in the low-temperature limit, which is taken to be the
dielectric analog to magnetic spin wave. Near the Curie temperature
(TC = 320 K), the shape of the specific-heat anomaly is
thermally broadened. However, the anomaly changes to the characteristic
sharp lambda-shape expected for a continuous transition with the
application of either a magnetic field or electric field, giving the
expected entropy change at TC of Rln2. These results are
explained on the basis that the frequencies of optical dipole
oscillations are split by the magnetic field, and the resulting gas of
excitations are paramagnetic. Consequently they contribute to the
specific heat near TC, which increases with magnetic field.
[Show abstract][Hide abstract] ABSTRACT: The gamma to alpha isostructural transition in the
Ce(0.9-x)La(x)Th(0.10) system is measured as a function of La alloying
and external pressure up to 1 Mbar using magnetic susceptibility. We
probe a line of discontinuous transitions, as indicated by the change in
volume, decreasing exponentially from 118 K to close to 0 K with
increasing La doping, and the transition changes from being first order
to continuous at a critical concentration close to x = 0.14. At the
tricritical point, the magnetic susceptibility increase rapidly near the
critical concentration and approaches large values at x= 0.35 signifying
that a heavy Fermi-liquid state evolves at large doping near the
critical concentration. The Wilson ratio reaches a value above two for a
narrow range of concentrations where the specific heat and
susceptibility vary most rapidly with the doping concentration.
[Show abstract][Hide abstract] ABSTRACT: The Debye-Waller factor relates the intensities of the Bragg peaks to
the mean square displacements of the atoms. In the structural refinement
of diffraction data it is standard practice to use the harmonic
expression for Debye-Waller factor. For most materials and conditions
the phonons are only mildly anharmonic, thus the harmonic assumption is
a good one. For some materials and conditions, however, the phonons can
be strongly anharmonic, and thus the harmonic assumption is physically
unrealistic. As examples we cite the rattling atoms in clathrates and
skutterudites, and atoms participating in displacive phase transitions.
In the present study we investigate the error associated with using the
harmonic Debye--Waller factor to analyze anharmonic vibrations. We find
that even for strongly anharmonic potentials, such as a double well, the
mean square displacements deduced using the harmonic approximation are
at most 15% larger than those deduced using a full anharmonic analysis.
Furthermore, the quasi-harmonic and anharmonic values have nearly the
same temperature dependences. We conclude that the error introduced by
using the harmonic approximation is comparable to or smaller than the
usual errors associated with measurement and refinement of diffraction
patterns.
[Show abstract][Hide abstract] ABSTRACT: We have used a suite of diffraction, thermodynamic, and transport measurements to study the localized rattling of Al guest atoms in VAl10.1. The mean-square displacement of the rattling atom shows a concave-down temperature dependence. This is characteristic of an anharmonic vibration, the frequency of which increases with amplitude, akin to a particle in a box. We find that the rattling is best described in terms of a sixth-order interatomic potential, with negligible contributions from harmonic and quartic terms. The rattler has a characteristic temperature of θRM=21 K and couples strongly to both the acoustic phonons and conduction electrons. The coupling to the phonons is evident from the large value of the Grüneisen parameter, which increases with decreasing temperature to Γ≈43 at 5 K. Below 6 K, the electrical resistivity varies as T3, which can be explained in terms of electron scattering from damped sixth-order vibrations. VAl10.1 is a superconductor below Tc=1.53 K, with an upper critical field of ∼1 kG.
[Show abstract][Hide abstract] ABSTRACT: We report an experimental determination of the dispersion of the soft phonon
mode along [1,0,0] in uranium as a function of pressure. The energies of these
phonons increase rapidly, with conventional behavior found by 20 GPa, as
predicted by recent theory. New calculations demonstrate the strong pressure
(and momentum) dependence of the electron-phonon coupling, whereas the
Fermi-surface nesting is surprisingly independent of pressure. This allows a
full understanding of the complex phase diagram of uranium, and the interplay
between the charge-density wave and superconductivity.