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

Abstract

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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“…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.…”

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