Kohn anomaly in MgB2 by inelastic X-ray scattering

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Physics

Background:

• Magnesium diboride (MgB2) is a superconductor with unique phonon properties.
• Understanding phonon behavior is crucial for explaining MgB2's superconducting mechanism.
• The E(2g) phonon mode and Kohn anomaly are central to MgB2's properties.

Purpose of the Study:

• To investigate phonon behavior in MgB2 using high-resolution inelastic X-ray scattering.
• To examine the E(2g) phonon mode and Kohn anomaly.
• To compare experimental findings with ab initio calculations and explore low-temperature behavior.

Main Methods:

• Inelastic X-ray scattering (IXS) with 1.6 meV and 6 meV resolution.
• Measurements performed along the Gamma-M and Gamma-A symmetry paths in the Brillouin zone.
• Variable temperature measurements, including near the superconducting transition temperature (Tc).

Main Results:

• Observed softening and broadening of the E(2g) phonon mode along Gamma-M, consistent with the Kohn anomaly and theoretical predictions.
• Found negligible changes in phonon modes at low temperatures (above and below Tc) for studied momentum transfers.
• Discovered a previously unpredicted longitudinal phonon mode along Gamma-A, close in energy to the E(2g) mode.

Conclusions:

• Experimental results largely validate theoretical models for MgB2 phonon dynamics, particularly the Kohn anomaly.
• Low-temperature measurements indicate minimal impact of superconductivity on the studied phonon modes.
• The identification of a novel longitudinal mode suggests potential limitations in current theoretical descriptions of MgB2.