A.S. SGibbs 1 , J. Farrell 1 , J.-F. Mercure, R.S. Perry 2 , A.W. Rost 1 , A.P. Mackenzie 1

1. Summer Project 2007 Where Crystal Chemistry Meets Materials Physics Scottish Universities Physics Alliance University of St Andrews A.S. SGibbs1, J. Farrell1, J.-F. Mercure, R.S. Perry2, A.W. Rost1, A.P. Mackenzie1 1University of St Andrews, St. Andrews (Scotland) 2University of Edinburgh (CSEC), Edinburgh (Scotland) e- Introduction Impurity atom • One of the main research efforts in condensed matter physics is concentrating on transition metal oxides whose properties • are dominated by the d-level electrons of the transition metal ions. • In these materials the electrons are not independent of each other but highly correlated leading to many interesting novel • states such as strange metals, novel magnets and unconventional superconductivity. • These properties are highly sensitive to disorder and much research is done in optimising the growth processes for each • system making a comprehensive knowledge in chemistry indispensable in this field of physics. • The aim of this particular project was to grow high quality single crystals of Sr2RuO4 and investigate its physical properties. • In this material the electron correlations lead to a highly unusual superconducting state in which the electrons pair • with spins aligned parallel, something that in almost all other superconductors is energetically unfavourable. • The physical properties of particular interest were residual resistivity and the superconducting transition temperature, • both being important quantities for establishing the quality of the crystals. • Resistivity is the resistance, R, multiplied by a geometric factor for the sample. The residual resistivity ρo (at T=0K) is only dependent on the concentration of • defects and impurities. These atomic defects act as scattering centers for conduction electrons (see figure), impeding their path through the lattice and • thereforeincreasing the resistivity of the material. Therefore the lower the residual resistivity the better the quality of the crystal. • A superconducting state is one for which the material has no electrical resistance, so a current set up in such a state will circulate indefinitely without decaying. • Sr2RuO4 is a superconductor below a critical temperature, theoretically predicted to be ≈1.5K in an ideal crystal. Impurities lower this temperature dramatically. • The project overall therefore required both knowledge of crystal chemistry to allow preparation of high purity materials and estimate the effects of impurities • or dopants as well as knowledge in materials physics to be able to relate the low temperature measurement results to the microscopic physics of the crystal. Crystal Growth RuO2 octahedra • To grow the crystals, an infra-red image furnace was used (figure, • left). • This can be used to grow materials with melting temperatures • less than about 2300 °C that also absorb infra-red radiation. • The powder used to form the crystal is prepared by solid state • reaction (see equation below) and then compressed into a rod • and hung from wires at the top of the setup (this is the ‘feed’ rod). • A piece of pre-grown crystal is used as a seed crystal for the • molten zone. • The IR radiation is absorbed by the feed rod as it is lowered into • the hot zone (≈1cm3) and the rod melts and is joined to the seed • Both rods move downwards and with careful adjustment of • parameters during growth, the crystal grows over a few hours, • dependent on the material being grown SrO layer Gold plated ellipsoidal mirrors (focus radiation to ≈1cm3 hot zone) Feed Rod (polycrystalline) Above : The crystal structure of Sr2RuO4 [1] Below : Example of a single crystal grown in the project. Above : The infra-red Image Furnace used for growth (picture courtesy of N. Kikugawa) Below : Solid State reaction the initial materials undergo 1cm Quartz tube (pressures of up to ≈10atm) Seed Rod (single crystal) Results • We measured the residual resistivity with a 4-point measurement • (see figure 1) in a helium flow cryostat. • It was found to be ρo≈0.12μΩcm (figure 1) which means the • crystal is ‘ultra clean’ (has a residual resistivity of <1μΩcm). • The estimated mean free path is of the order of 104Å. • We furthermore investigated the superconducting transition by • using an AC susceptibility measurement (inset figure 3) in an • adiabatic demagnetization cryostat working between 100mK and • 1.5K. • The superconducting transition midpoint (midpoint of peak in • figure 2) was found to be 1.49K with the onset of superconductivity • being at 1.52K (figure 3). Both values are the same as the best • published ones [2] and confirm the ρo measurement (inset figure 2). • These excellent results allowed more complicated measurements • of the electronic structure of the material by de Haas van Alphen • experiments (figure 4). The very high amplitude of the oscillations • observed further confirmed the exceptional quality of the sample. The effect of impurities on conduction electrons: the impurity atom scatters the conduction electron, shortening the mean free path and therefore increasing the resistivity. Figure 1: The resistivity data for the Sr2RuO4 crystal grown. The fit was extrapolated to T=0 to allow determination of ρo. Figure 3: The real component of the AC susceptibility showing the onset of superconductivity. Inset: AC susceptibility coil setup (courtesy of J-F Mercure). Figure 2: The midpoint of the transition to superconductivity is measured as the peak in χ’’, the imaginary part of the AC susceptibility. Inset: The relationship between residual resistivity and critical temperature for Sr2RuO4 [3] Figure 4: The quantum oscillations seen in the de Haas van Alphen experiment. For further information please contact References Alexandra Gibbs (asg6@st-and.ac.uk) or Prof. A P Mackenzie (apm9@st-and.ac.uk) [1] S.I. Ikeda et al., Journal of Crystal Growth 237–239 (2002), 787–791 [2] Z Q Mao, Y Maeno, H Fukazawa, Materials Research Bulletin35 (2000), 1813-1824 [3]??????? Alexandra Gibbs 5th Year Masters Student Prof. Andy Mackenzie Supervisor