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Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB2.
Jan Langmann1, Hasan Kepenci1, Georg Eickerling1
1CPM, Institut für Physik, Universität Augsburg, 86159Augsburg, Germany.
The Journal of Physical Chemistry. A
|November 5, 2022
Summary
High-resolution X-ray diffraction (XRD) reveals that temperature changes, not magnesium vacancies, alter electron density in MgB2 below its superconducting transition. This study clarifies MgB2
Area of Science:
- Solid State Physics
- Materials Science
- Crystallography
Background:
- X-ray diffraction (XRD) effectively tracks structural transitions linked to superconductivity in systems like 1T-TiSe2 and YBa2Cu3O7-δ.
- The application of XRD to superconductors lacking symmetry-breaking transitions, such as MgB2, is less straightforward.
- Previous studies suggested electron density variations in MgB2 below its superconducting transition temperature (Tc ≈ 39 K) using powder XRD and maximum entropy methods.
Purpose of the Study:
- To investigate the utility of high-resolution single-crystal XRD and multipolar refinements in understanding MgB2's electronic structure.
- To clarify the origin of observed temperature-dependent electron density changes in MgB2.
- To critically assess the presence and impact of magnesium vacancies on MgB2's superconducting properties.
Main Methods:
- High-resolution single-crystal X-ray diffraction experiments on MgB2.
- Multipolar refinements using an Extended Hansen-Coppens model.
- Analysis of atomic displacement parameters and electron density distributions.
Main Results:
- Observed temperature-dependent electron density changes in MgB2 are primarily attributed to reduced thermal vibrations (atomic displacement parameters) at lower temperatures.
- No significant evidence for magnesium vacancies (Mg1-xB2 with x ≈ 0.05) was found in MgB2 samples synthesized at high temperatures.
- Advanced refinement models and high-resolution data challenge previous claims of non-stoichiometric phase formation.
Conclusions:
- The superconducting properties of MgB2 are not significantly controlled by magnesium vacancies as previously suggested.
- High-resolution XRD with advanced multipolar refinements provides a more accurate picture of MgB2's electronic structure and defect-free nature.
- Thermal vibrations, not stoichiometry defects, are the main drivers of electron density changes with temperature in MgB2.
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