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: the lattice is tetragonal (C4, i.e. 90° rotationally symmetric), but considerable evidence has accumulated that the phase is C2 symmetric (180° rotationally symmetric). Through careful strain-tuning experiments to artificially lift the native symmetry of the lattice, supplemented with vector magnetic field experiments, we found that the phase is in fact C4 symmetric, consisting of perpendicular components that co-exist microscopically. This study is highly relevant to other compounds, including the cuprate superconductors, which have C2-symmetric electronic phases (or probably have C2-symmetric electronic phases), and where the electronically-induced reduction in symmetry from C4 to C2 is thought to be important to their properties. In Sr3Ru2O7, we found that the phase is C4-symmetric, but we also found that 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.

Sr<sub>3</sub>Ru<sub>2</sub>O<sub>7</sub>: resistivity against strain and magnetic field Zoom Image
Sr3Ru2O7: resistivity against strain and magnetic field

Strain- and Vector-Magnetic-Field Tuning of the Anomalous Phase in Sr3Ru2O7 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, in preparation.

Valley-Dependent Density of States in Bismuth 

Using the high-resolution dilatometer described above, we measured the angle-resolved magnetostriction of the elemental metal bismuth at very low temperatures, and magnetic fields as high as 18 T. With these measurements we could quantify the carrier number in each energy structure minimum, so-called valley. We discovered that the density of states in bulk bismuth is valley-dependent. In the figure below, we show the field-dependent change in carrier density, ΔN, calculated from the measured magnetostriction along the crystallographic trigonal axis, ε33: ε33 = ΔLz/Lz = cΔN/B. We compare this quantity measured at opposite angles, +3.5° and -3.5°. Shown in the inset is the derivative of ΔN with field, which emphasises the differences. Bismuth has three identical valleys in which electrons collect. While their spectra are identical, we found that they differ substantially in their densities of states at the Fermi level. Thus, at low temperature and high field the electron fluid does not keep the symmetry of the lattice.

Variation in the density of states of bismuth with fields Zoom Image
Variation in the density of states of bismuth with fields

Thermodynamic evidence for valley-dependent density of states in bulk bismuthR. Küchler, L. Steinke, R. Daou, M. Brando, K. Behnia, and F. Steglich, Nature Materials 13, 461 (2014). MPG.PuRe 

Press Release:

http://www.cpfs.mpg.de/2353272/news_publication_8242301?c=2332

In collaboration with:

  • L. Steinke, Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
  • K. Behnia, UPMC CNRS ESPCI, Lab Phys & Etude Mat, F-75005 Paris, France
  • R. Daou, CNRS Ensicaen UCBN, CRISMAT UMR6508, 6 Blvd Maréchal Juin, F-14050, Caen 4, France

Strong Increase of Tc of Sr2RuO4 Under Both Compressive and Tensile Strain 

Our first result using our piezoelectric-based strain device: 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. The original aim of the experiment was to test the proposed px±ipy superconducting order in Sr2RuO4: by lifting the native tetragonal symmetry of the lattice, the Tc’s of the px and py components ought to have been split. We did not find this result, although it may be that the splitting was too small for us to resolve. We did, however, find the striking increase in Tc under strain, and what it indicates is that the van Hove singularities are important to the superconductivity of Sr2RuO4: Sr2RuO4 has a Fermi surface that passes very close to the Brillouin zone boundary, and small distortions of the lattice induce strikingly large shifts in the Fermi surface in these regions, close to the van Hove singularities.

Strong Increase in Tc of Sr2RuO4 Under Both Compressive and Tensile Strain; 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, Science 344 283 (2014). MPG.PuRe

In collaboration with:

  • Y. Maeno and S. Yonezawa, Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
 
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