X-ray Diffraction Study of MgB2 at Low Temperatures

Xue, Yun; Asada, Seiji; Hosomichi, Akiko; Naher, S.; Xue, J. S.; Kaneko, Hiroshi; Suzuki, Haruhiko; Muranaka, Toshiya; Akimitsu, Jun

doi:10.1007/s10909-004-2903-2

Cited by 9 publications

(11 citation statements)

References 17 publications

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“…Our lattice parameters at room temperature were a RT = 3.0856Å and c RT = 3.5239Å, in a good agreement with a 297K = 3.08489Å from 34 and c RT = 3.52196Å from 35 . The calculated thermal expansions are in good agreement with experiment to 900 K and, like in earlier reports 18,[34][35][36][37][38] , are about twice as large along c than along a. The neutron-weighted phonon densities of states (NWDOS) of MgB 2 and Mg 0.75 Al 0.25 B 2 ( Figs.…”

Section: Computationalsupporting

confidence: 88%

Nonharmonic phonons in MgB2at elevated temperatures

et al. 2011

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Inelastic neutron scattering was used to measure phonon spectra on MgB2 and Mg0.75Al0.25B2 from 7 K to 750 K to investigate anharmonicity and adiabatic electron-phonon coupling. First principles calculations of phonons with a linear response method were performed at multiple unit cell volumes, and the Helmholtz free energy was minimized to obtain the lattice parameter and phonon dynamics at elevated temperature in the quasiharmonic approximation. Most of the temperaturedependence of the phonon DOS could be understood with the quasiharmonic approximation, although there was also significant thermal broadening of the phonon spectra. In comparison to Mg0.75Al0.25B2, in the energy range of 60 to 80 meV the experimental phonon spectra from MgB2 showed a non-monotonic change with temperature around 500 K. This may originate from a change with temperature of the adiabatic electron-phonon coupling.2

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Section: Computationalsupporting

confidence: 88%

Nonharmonic phonons in MgB2at elevated temperatures

et al. 2011

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“…In this Letter, high-resolution thermal expansion measurements of polycrystalline MgB 2 are presented. The results reveal a change in sign of α at T c , with negative thermal expansion below T c ; these data agree with diffraction investigations [9,10], but offer exceedingly greater detail. Analysis of the bulk Grüneisen function reveals anomalous behavior due to dominant low-energy phonon modes.…”

supporting

confidence: 84%

“…Behavior in ∆L/L 6K similar to that of Fig. 2 was revealed in diffraction measurements [9,10], but the present data offer far more detail.…”

supporting

confidence: 81%

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Negative thermal expansion ofMgB2in the superconducting state and anomalous behavior of the bulk Grüneisen function

et al. 2005

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The thermal expansion coefficient α of MgB2 is revealed to change from positive to negative on cooling through the superconducting transition temperature Tc. The Grüneisen function also becomes negative at Tc followed by a dramatic increase to large positive values at low temperature. The results suggest anomalous coupling between superconducting electrons and low-energy phonons.PACS numbers: 74.70.Ad, 74.62.Fj Superconductivity in the binary compound MgB 2 near 39 K is a fascinating development. Over the last few years, scientists have argued that a lattice instability [1] and/or anomalous phonon behavior [2] might be responsible for the high transition temperature. Specific attention has focused on the E 2g phonon, a bond-stretching phonon within the plane of the hexagonal crystal structure [2,3,4]. In-plane tensile strain, induced by lattice mismatch through thin-film growth, increases the superconducting transition temperature T c to 41.8 K; this enhancement was attributed to a decrease in the E 2g phonon frequency [5]. Phonons can be studied with techniques such as Raman spectroscopy [3,4] and heat capacity [6,7,8]. Often neglected in the study of phonons is thermal expansion, partly because of the exceptional resolution needed to resolve the transition at T c .Thermal expansion from powder diffraction measurements [9, 10] of MgB 2 have revealed an anomalous volume expansion on cooling below T c . However, highresolution thermal expansion measurements (dilatometry) with a relative sensitivity approximately four orders of magnitude better than powder diffraction are required for meaningful thermodynamic analysis. Such measurements of polycrystalline MgB 2 were reported [11], but discrepancies with the diffraction data, such as the temperature at which the thermal expansion coefficient α changes from positive to negative, are apparent.In this Letter, high-resolution thermal expansion measurements of polycrystalline MgB 2 are presented. The results reveal a change in sign of α at T c , with negative thermal expansion below T c ; these data agree with diffraction investigations [9,10], but offer exceedingly greater detail. Analysis of the bulk Grüneisen function reveals anomalous behavior due to dominant low-energy phonon modes. The change in sign of α at precisely T c suggests a connection between these phonon modes and superconductivity.MgB 2 , synthesized with 11 B as described previously [12], was pelletized (diameter = 4.6 mm), placed in a boron nitride crucible and heated to 800• C for 30 min at 3 GPa using a cubic multi-anvil press. A very thin black layer, impurities from surface reaction with boron nitride, was removed, leaving behind a brilliant goldcolored MgB 2 sample with density 2.56 g/cm 3 (100% of theoretical density). Heat capacity was measured with a thermal-relaxation technique. T c versus pressure was determined inductively (0.12 Oe rms field at 1023 Hz) to 0.63 GPa using a helium pressure medium; a manganin sensor at room temperature served as a manometer [13].Thermal expansion was meas...

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“…The thermal expansion coefficient of MgB 2 satisfies well with Grüneisen relationship (Neumeier et al, 2005;Jorgensen et al, 2001;Xue et al, 2005):…”

Section: Conductivity Vs Temperature Of Mgb 2 Compositementioning

confidence: 54%