Browsing by Author "Delaire, O"

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    Electron-phonon coupling and thermal transport in the thermoelectric compound Mo3Sb7-xTex
    (Physical Review B - Condensed Matter and Materials Physics, 2015-12-07) Bansal, D; Li, CW; Said, AH; Abernathy, DL; Yan, J; Delaire, O
    © 2015 American Physical Society.Phonon properties of Mo3Sb7-xTex (x=0,1.5,1.7), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic structure, local bonding, phonon density of states, dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening and a large overall stiffening of interatomic force constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering and resulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes nonuniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. Changes in phonon group velocities and phonon scattering rates are quantified, highlighting the large effect of electron-phonon coupling in this compound.
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    Itinerant Antiferromagnetism in RuO$_{2}$
    (PRL, 2017-02-23) Berlijn, T; Snijders, PC; Delaire, O; Zhou, H-D; Maier, TA; Cao, H-B; Chi, S-X; Matsuda, M; Wang, Y; Koehler, MR; Kent, PRC; Weitering, HH
    Bulk rutile RuO$_2$ has long been considered a Pauli paramagnet. Here we report that RuO$_2$ exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05 $\mu_B$ as evidenced by polarized neutron diffraction. Density functional theory plus $U$ (DFT+$U$) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard $U$ of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high N\'eel temperature and small itinerant moments make RuO$_2$ unique among ruthenate compounds and among oxide materials in general.
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    Phonon anharmonicity and negative thermal expansion in SnSe
    (Physical Review B - Condensed Matter and Materials Physics, 2016-08-09) Bansal, D; Hong, J; Li, CW; May, AF; Porter, W; Hu, MY; Abernathy, DL; Delaire, O
    © 2016 American Physical Society.The anharmonic phonon properties of SnSe in the Pnma phase were investigated with a combination of experiments and first-principles simulations. Using inelastic neutron scattering (INS) and nuclear resonant inelastic X-ray scattering (NRIXS), we have measured the phonon dispersions and density of states (DOS) and their temperature dependence, which revealed a strong, inhomogeneous shift and broadening of the spectrum on warming. First-principles simulations were performed to rationalize these measurements, and to explain the previously reported anisotropic thermal expansion, in particular the negative thermal expansion within the Sn-Se bilayers. Including the anisotropic strain dependence of the phonon free energy, in addition to the electronic ground state energy, is essential to reproduce the negative thermal expansion. From the phonon DOS obtained with INS and additional calorimetry measurements, we quantify the harmonic, dilational, and anharmonic components of the phonon entropy, heat capacity, and free energy. The origin of the anharmonic phonon thermodynamics is linked to the electronic structure.
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    The origin of incipient ferroelectricity in lead telluride.
    (Nat Commun, 2016-07-22) Jiang, MP; Trigo, M; Savić, I; Fahy, S; Murray, ÉD; Bray, C; Clark, J; Henighan, T; Kozina, M; Chollet, M; Glownia, JM; Hoffmann, MC; Zhu, D; Delaire, O; May, AF; Sales, BC; Lindenberg, AM; Zalden, P; Sato, T; Merlin, R; Reis, DA
    The interactions between electrons and lattice vibrations are fundamental to materials behaviour. In the case of group IV-VI, V and related materials, these interactions are strong, and the materials exist near electronic and structural phase transitions. The prototypical example is PbTe whose incipient ferroelectric behaviour has been recently associated with large phonon anharmonicity and thermoelectricity. Here we show that it is primarily electron-phonon coupling involving electron states near the band edges that leads to the ferroelectric instability in PbTe. Using a combination of nonequilibrium lattice dynamics measurements and first principles calculations, we find that photoexcitation reduces the Peierls-like electronic instability and reinforces the paraelectric state. This weakens the long-range forces along the cubic direction tied to resonant bonding and low lattice thermal conductivity. Our results demonstrate how free-electron-laser-based ultrafast X-ray scattering can be utilized to shed light on the microscopic mechanisms that determine materials properties.
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    Thermoelectric transport properties of CaMg 2Bi 2, EuMg 2Bi 2, and YbMg 2Bi 2
    (Physical Review B - Condensed Matter and Materials Physics, 2012-01-11) May, AF; McGuire, MA; Singh, DJ; Ma, J; Delaire, O; Huq, A; Cai, W; Wang, H
    The thermoelectric transport properties of CaMg 2Bi 2, EuMg 2Bi 2, and YbMg 2Bi 2 were characterized between 2 and 650 K. As synthesized, the polycrystalline samples are found to have lower p-type carrier concentrations than single-crystalline samples of the same empirical formula. These low carrier concentration samples possess the highest mobilities yet reported for materials with the CaAl 2Si 2 structure type, with a mobility of ∼740 cm2/V/s observed in EuMg 2Bi 2 at 50 K. Despite decreases in the Seebeck coefficient (α) and electrical resistivity (ρ) with increasing temperature, the power factor (α2ρ) increases for all temperatures examined. This behavior suggests a strong asymmetry in the conduction of electrons and holes. The highest figure of merit (zT) is observed in YbMg 2Bi 2, with zT approaching 0.4 at 600 K for two samples with carrier densities of approximately 2×1018cm -3 and 8×1018 cm -3 at room temperature. Refinements of neutron powder diffraction data yield similar behavior for the structures of CaMg 2Bi 2 and YbMg 2Bi 2, with smooth lattice expansion and relative expansion in c being ∼35% larger than relative expansion in a at 973 K. First-principles calculations reveal an increasing band gap as Bi is replaced by Sb and then As, and subsequent Boltzmann transport calculations predict an increase in α for a given n associated with an increased effective mass as the gap opens. The magnitude and temperature dependence of α suggests higher zT is likely to be achieved at larger carrier concentrations, roughly an order of magnitude higher than those in the current polycrystalline samples, which is also expected from the detailed calculations. © 2012 American Physical Society.
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    Weak coupling of pseudoacoustic phonons and magnon dynamics in the incommensurate spin-ladder compound S r14 C u24 O41
    (Physical Review B - Condensed Matter and Materials Physics, 2016-10-21) Chen, X; Bansal, D; Sullivan, S; Abernathy, DL; Aczel, AA; Zhou, J; Delaire, O; Shi, L
    © 2016 American Physical Society.Intriguing lattice dynamics have been predicted for aperiodic crystals that contain incommensurate substructures. Here we report inelastic neutron scattering measurements of phonon and magnon dispersions in Sr14Cu24O41, which contains incommensurate one-dimensional (1D) chain and two-dimensional (2D) ladder substructures. Two distinct pseudoacoustic phonon modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long wavelength limit, it is found that the sliding mode shows a remarkably small energy gap of 1.7-1.9 meV, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal a gapped and steep linear magnon dispersion of the ladder sublattice. The high group velocity of this magnon branch and weak coupling with acoustic and pseudoacoustic phonons can explain the large magnon thermal conductivity in Sr14Cu24O41 crystals. In addition, the magnon specific heat is determined from the measured total specific heat and phonon density of states and exhibits a Schottky anomaly due to gapped magnon modes of the spin chains. These findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.