Magnetic quantum-critical phenomena
(For a PhD PROJECT
on pressure-tuning of electronic matter with focus on magnetism and superconductivity click here
Many metals show striking quantum-critical behaviour in a wide temperature range above magnetic quantum phase transitions. Strong deviations from normal metallic (Fermi liquid) behaviour as well as novel phases (including unconventional superconductivity) have been observed (figure).
Our strategy has been to study systems with relatively simple crystal and electronic structures and bring them into the quantum-critical regime using the highly controlled methods of high-pressure and magnetic-field tuning.
In the past we studied NiS2, which becomes an antiferromagnetic metal at high pressure. At the antiferromagnetic quantum phase transition, NiS2 shows an intriguing temperature dependence in the resistivity exponent, which indicates short-circuiting of so-called “hot regions“ (susceptible to antiferromagnetic spin-fluctuations) of the Fermi surface by “cold regions”.
The weakly ferromagnetic metal Ni3Al reveals a dramatically extended region of a non-Fermi-liquid state in the pressure-temperature phase diagram, which resembles a Marginal Fermi liquid state, an electronic state of great interest predicted in the context of high-temperature superconductivity and in other fields.
In experiments on antiferromagnetic stoichiometric YbAgGe we have revealed that strong non-Fermi liquid behaviour can be induced by the application of an external magnetic field. Curious frustration effects in the quasi-elastic dispersion, seen in neutron scattering experiments at zero field, suggest that localisation and destruction of heavy quasiparticles might play an important role in this compound.
We are testing further aspects for weakly magnetic metals, e.g. the predicted instability of continuous ferromagnetic quantum phase transitions against modulated order.
in collaboration with U Cambridge, TU Munich CEA Grenoble, Iowa State University, UCL
 PG Niklowitz, MJ Steiner, GG Lonzarich, D Braithwaite, G Knebel, J Flouquet, JA Wilson, ”Unconventional resistivity at the border of metallic antiferromagnetism in NiS2”, Phys.Rev.B 77 (2008) 115135.
 PG Niklowitz, F Beckers, GG Lonzarich, G Knebel, B Salce, J Thomasson, N Bernhoeft, D Braithwaite, J Flouquet, “Spin-fluctuation dominated electrical transport of Ni3Al at high pressure”, Phys.Rev.B 72 (2005) 24424.
 PG Niklowitz, G Knebel, J Flouquet, SL Bud’ko, PC Canfield, “Field-induced non-Fermi-liquid resistivity of stoichiometric YbAgGe single crystals”, Phys.Rev.B 73 (2006) 125101.
 B Fåk, DF McMorrow, PG Niklowitz, S Raymond, E Ressouche, J Flouquet, PC Canfield, SL Budko, Y Janssen, MJ Gutmann, “An inelastic neutron scattering study of single-crystal heavy-fermion YbAgGe”, J. Phys.: Condens. Matter 17 (2005) 301-311.