Quantum Criticality in metals with magnetic and charge order

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• Dr. Sven Friedemann, University of Bristol
• Friday 06 June 2014, 14:00-15:00
• Physics East 217.

If you have a question about this talk, please contact Elizabeth Blackburn.

A quantum critical point (QCP) emerges as a continuous phase transition at zero temperature. Experimentally, a QCP is accessed by suppressing finite temperature transitions with the help of external tuning parameters like pressure, magnetic field, or composition. Surprisingly, the quantum fluctuations associated with the QCP can dominate the physical properties at finite temperature of the order of 10s of kelvins as clearly demonstrated for magnetic and charge ordered insulating and metallic systems. In fact, in some cases these fluctuations seem to promote the occurrence of unconventional superconductors as shown in heavy-fermion materials and discussed in cuprate high-temperature superconductors.

Whilst in metals antiferromagnetic QCPs are very common, theory suggests that ferromagnetic QCPs are forbidden above 2 dimensions. Here, I will present results on the metallic ferromagnets NbFe2 and YbNi4P2.

NbFe2 is a rather three dimensional metal with ferromagnetic ground state for Fe-rich samples. Approaching stoichiometric samples we find the avoidance of the ferromagnetic-to-paramagnetic QCP with the system escaping into a secondary instability, a AFM ground state. We trace the disappearance of the FM instability inside the AFM dome. The relation of the Fermi-liquid breakdown to the two magnetic orders is discussed.

YbNi4P2 is a heavy-fermion system with ferromagnetic ground state. The divergence of the Gruneisen parameter presents clear evidence of a QCP in seeming contradiction to the theoretical expectations. We suggest a resolution of this contradiction by the quasi one-dimensional electronic structure of YbNi4P2 arising from the presence of rather isolated Yb-chains. We present band structure calculations together with first quantum oscillation measurements.

Finally, time permitting I will present ongoing research on the charge density wave quantum critical point in LaAgSb2 and the absence of mass divergence and superconductivity in this system.

This talk is part of the Condensed Matter Physics Seminars series.

This talk is included in these lists:

• Bham Talks
• Cond. Mat. seminar
• Condensed Matter Physics Seminars
• Physics East 217
• What's on in Physics?

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