Hicks, Clifford
Clifford Hicks
Group leader
Phone: +49 351 4646-3227
Fax: +49 351 4646-3232
Mackenzie, Andrew
Andrew Mackenzie
Phone: +49 351 4646-5900
Fax: +49 351 4646-5902
Steppke, Alexander
Alexander Steppke
Post-doctoral research scientist
Phone: +49 351 4646-3219

Research groups

Precision Instrumentation and Correlated Electron Materials


We perform challenging measurements on correlated electron materials. Our goal is to measure as directly and precisely as possible the quantities one wishes to know to answer open scientific questions, developing the necessary tools along the way. Our most important tool at present is controlled lattice distortion, using piezoelectric-based uniaxial pressure cells that were developed in-house. Other techniques under development include controlled-stress techniques, mesoscopic sample preparation, and scanning magnetic susceptometry.

 By focusing on scientific goals and possibilities we push ourselves to technical achievements that we did not think possible. There is a strong emphasis in our group on the engineering required to achieve our scientific goals, and I hope that students in my group will develop not only scientific knowledge but also strong applied and organisational skills. The measurements we take on are challenging; our goal is to make them feel easy through a high degree of teamwork and careful preparation.

 Our current research topics include:

  • the superconductivity of Sr2RuO4
  • quantum criticality in Sr3Ru2O7
  • the effect on Tc and transport properties of lattice deformation in cuprate superconductors
  • ultra-high conductivity in delafossite compounds
  • magnetic properties of iridates under controlled lattice strain
  • the interaction of nematicity and superconductivity

Group members

  • Clifford Hicks, Group Leader
  • Alexander Steppke, Postdoctoral Research Assistant (joint with Mackenzie group)
  • Joonbum Park, Postdoctoral Research Assistant
  • Lishan Zhao, Doctoral Student (joint with Mackenzie group)
  • Mark Barber, Doctoral Student (joint with Mackenzie group)
  • You-Sheng Li, Doctoral Student (joint with Mackenzie and Nicklas groups)
  • Nabhanila Nandi, Doctoral Student (joint with Mackenzie group)
  • Jack Bartlett (joint with Mackenzie group)
  • Veronika Sunko, Doctoral Student (joint with Mackenzie group and King group, U. of St Andrews)


Selected Results


Strain-tuning of Sr3Ru2O7

Sr3Ru2O7 has an anomalous phase around a metamagnetic quantum critical point. What has brought this phase considerable attention is the possibility that, through the effects of electronic interactions alone, it has a lower symmetry than the lattice. Through strain-tuning experiments, we found that this is not the case. But the phase shows a very strong response to strain, and a high saturation resistivity, which is not explained and will help us learn about quantum criticality in metallic systems.

D.O. Brodsky, M.E. Barber, R.A. Borzi, J.A.N. Bruin, S.A. Grigera, R.S. Perry, A.P. Mackenzie, and C.W. Hicks, “Strain- and Vector-Magnetic-Field Tuning of the Anomalous Phase in Sr3Ru2O7”, in preparation.


Response of Sr2RuO4 to orthorhombic lattice distortion

We developed a piezoelectric-based apparatus that can both compress and tension samples. Using this, we discovered that the superconducting Tc of Sr2RuO4 increases strongly under both uniaxial compression and uniaxial tension. In other words, Tc responds much more strongly to orthorhombic distortion than to volume change.

C.W. Hicks, D.O. Brodsky, E.A. Yelland, A.S. Gibbs, J.A.N. Bruin, M.E. Barber, S.D. Edkins, K. Nishimura, S. Yonezawa, Y. Maeno, and A.P. Mackenzie, “Strong Increase in Tc of  Sr2RuO4 Under Both Compressive and Tensile Strain,” Science 344  283 (2014).


Magnetic reconstruction in PdCrO2

PdCrO2 is a metallic triangular antiferromagnet. It has a nonmagnetic analogue, PdCoO2. Through quantum oscillation measurements, we determined that the Fermi surfaces of PdCrO2 result, to very high precision, from magnetic reconstruction of the nonmagnetic PdCoO2 Fermi surface.

C.W. Hicks, A.S. Gibbs, L. Zhao, P. Kushwaha, H. Borrmann, A.P. Mackenzie, H. Takatsu, S. Yonezawa, Y. Maeno, and E.A. Yelland, “Quantum oscillations and magnetic reconstruction in the delafossite PdCrO2 ,” Phys. Rev. B  92 014425 (2015).


Phonon drag in PdCoO2



PdCoO2 is one of the most conductive oxides known. It has a single, large, and closed Fermi surface. The existence of a large Umklapp gap means that small-angle electron-phonon scattering events cannot relax the momentum of the combined electron-phonon system. The phonons are “dragged” by the electrons, and do not contribute to resistivity. So the resistivity is temperature-independent at low temperatures, rather than following the usual electron-phonon T5 form.

C.W. Hicks, A.S. Gibbs, A.P. Mackenzie, H. Takatsu, Y. Maeno, and E.A. Yelland, “Quantum oscillations and high carrier mobility in PdCoO2 ,” Phys. Rev. Lett. 109 116401 (2012).


Penetration depth of LaFePO

By using a scanning SQUID, we were able to perform the first local measurements of the magnetic penetration depth of a superconductor. Traditional techniques such as cavity perturbation effectively measure over the entire sample surface, resulting in large uncertainties due to irregular sample geometry and sample inhomogeneity.

C.W. Hicks, T.M. Lippman, M.E. Huber, J.G. Analytis, J.-H. Chu, A.S. Erickson, I.R. Fisher, and K.A. Moler, Phys. Rev. Lett. 103 127003 (2009).

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