1. Research Expertise
  2. Publications
  3. Biography
Research Expertise -

Current Research Interests

Sketch of a phase diagram where a magnetic phase boundary line approaches zero temperature as the control parameter d, like pressure, chemical composition or magnetic field, reaches a critical value dc. Here, at the quantum critical point (QCP), quantum critical fluctuations – instead of thermal fluctuations – dominate the physical properties at finite temperatures and novel phases like unconventional superconductivity or unusual metallic states can emerge.
Sketch of a phase diagram where a magnetic phase boundary line approaches zero temperature as the control parameter d, like pressure, chemical composition or magnetic field, reaches a critical value dc. Here, at the quantum critical point (QCP), quantum critical fluctuations – instead of thermal fluctuations – dominate the physical properties at finite temperatures and novel phases like unconventional superconductivity or unusual metallic states can emerge.

My research interests are in the investigation of quantum phase transitions (QPT), defined as phase transitions that are driven by quantum rather than thermal fluctuations. This area has attracted a great deal of interest in recent years due to the fact that quantum fluctuations dominate the physical properties of systems in the vicinity of a quantum critical point (QCP). This can result in non-Fermi-Liquid (NFL) behaviour, i.e. a strongly enhanced specific heat coefficient, non-quadratic temperature dependence of the electrical resistivity or in the formation of new ground states including unconventional superconductivity.

The ground state properties can be tuned across a QCP by a control parameter such as external pressure or chemical substitution. The vast majority of the compounds studied so far order antiferromagnetically. Only a few ferromagnetic compounds showing a QPT have been found. I recently focused on one of these, the binary compound FeGe, a itinerant helimagnet below a temperature TC = 280 K. External pressure suppresses the long-range spiral ordering temperature at 19 GPa. The metallic state, although with unusual properties, persists up to higher pressure. This is interpreted as a breakdown of the standard scenario of a quantum critical phase transition.
 
Powder diffraction data on FeGe at low temperature yield a symmetry-retaining phase transition with no volume change. It occurs in close vicinity to the TC(p) phase boundary line determined from transport measurements. Thus, FeGe might be another example of the interaction of electronic/magnetic properties and the crystal structure like the well-known cases of Ce or SmS, where large volume changes signaled the valence of change of the 4f element. Studying the structural properties upon crossing a phase boundary as function of temperature and pressure at high-brilliance synchrotron sources opens a possibility for understanding the interplay of structural changes and unusual transport and thermodynamic properties.
Publications - +

Selected Publications

1) Scaling study and thermodynamic properties of the cubic helimagnet FeGe; Phys. Rev. B 94, 144424 (2016)
DOI: 10.1103/PhysRevB.94.144424

2) Phase diagram and magnetic relaxation phenomena in Cu2OSeO3, Phys. Rev. B 94, 064418 (2016)
DOI: 10.1103/PhysRevB.94.064418

3) Pressure-Induced Inhomogeneous Chiral-Spin Ground State in FeGe; Phys. Rev. Lett. 114, 016803 (2015)

4) Complex Chiral Modulations in FeGe Close to Magnetic Ordering; Phys. Rev. Lett. 110, 077207 (2013)
DOI: 10.1103/PhysRevLett.110.077207

5) Confinement of chiral magnetic modulations in the precursor region of FeGe; J. Phys. Cond. Matter 24, 294204 (2012)
DOI: 10.1088/0953-8984/24/29/294204

6) Precursor Phenomena at the Magnetic Ordering of the Cubic Helimagnet FeGe; Phys. Rev. Lett. 107, 127203 (2011)
DOI: 10.1103/PhysRevLett.107.127203

7) Metallic State in Cubic FeGe Beyond Its Quantum Phase Transition; Phys. Rev. Lett. 98, 047204 (2007)
DOI: 10.1103/PhysRevLett.98.047204

Biography - +

Biography

Heribert Wilhelm is Principal Beamline Scientist on the Extreme Conditions beamline I15 and Privat Docent in the Department of Condensed Matter Physics at the University of Geneva, Switzerland. His main interests are in the physics of strongly correlated electron systems and quantum phase transitions induced by pressure, chemical substitution or magnetic fields. The main challenge in this research is the combination of very sensitive measuring techniques and high pressure to study phase transitions at several hundred thousand atmospheres in the milli-Kelvin region.

Heribert Wilhelm

profilephoto

Heribert Wilhelm is Principal Beamline Scientist for I15 and I15-1.

Email: Heribert. Wilhelm@diamond.ac.uk
Tel: +44 (0) 1235 778581

Key Research Area

Other Specialist Areas

  • Diffraction
  • Magnetism

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