Research Unit FOR538

Doping Dependence of Phase Transitions and Ordering Phenomena in Copper-Oxygen Superconductors

P4 Crystal Growth of Superconducting Cuprates

Andreas Erb, Michael Lambacher, Astrid Habel and Rudolf Gross

In 2004 the DFG (Deutsche Forschungsgemeinschaft) founded the research unit FOR 538: ” Doping Dependence of Phase Transitions and Ordering Phenomena in Cuprate Superconductors ”, which consisted of 7 different projects, one of which is ”Crystal Growth of p and n-doped cuprate superconductors”. Goal of this project is the growth and characterization of high purity single crystals of the high temperature superconductors to provide samples for different spectroscopic experiments within the ”Research Unit High Temperature Superconductivity”. By the end of the year 2006 the renewed proposal for the Research united was not only reviewed positively again, but was even enlarged and now consists of 9 groups. The project crystal growth within the research unit is of particular importance in that sense, that it provides the common sample basis to the different experimental projects within the research unit. Only such a common sample basis of high quality single crystals of the cuprates makes the comparison of the results obtained by the various experimental techniques reliable.

Sample Space

Goal of this project is the growth and characterization of high purity single crystals of the high temperature superconductors to provide samples for different spectroscopic experiments within the "Forschergruppe High Temperature Superconductivity". In order to cover the whole phase diagram of the high temperature superconductors we want to grow crystals with well defined and homogeneous doping of both, the p- and n-doped compounds.

In order to cover the whole phase diagram of the high temperature superconductors, which is one of the central ideas of this research unit, crystals with well defined and homogeneous doping of both, the p and n-doped compounds need to be grown. On the hole doped side of the phase diagram we focused on the systems of YBa₂Cu₃O_7-δ and Bi₂Sr₂CaCu₂O₈. There are restrictions in the accessible doping regime for the YBa₂Cu₃O_7-δ even when co-doping with Ca is applied (see Fig. 1).

 

Fig 1: Accessible regions of the phase diagram for the different compounds of the high temperature superconductors
 

For the Bi₂Sr₂CaCu₂O₈ system one is actually limited to a relatively small region around optimal doping. For this reason we have started to grow single crystals of the La_2-xSr_xCuO₄ system, since they can be used to access the reminder part of the phase diagram as well as it is possible to access the whole doping range of the hole doped side by using one compound only. Moreover, they allow a more direct comparison of the n- and p-doped side of the phase diagram, due to the similarity of both the critical temperature and the structure with the electron doped 214-compounds. For the electron doped side of the phase diagram only 214-compounds are available to probe the phase diagram. We concentrated our efforts on the crystal growth of Pr_2-xCe_xCuO₄, Nd_2-xCe_xCuO₄, Sm_2-xCe_xCuO₄ or mixtures such as LaPr_1-xCe_xCuO₄.
 

 

Fig 2: Single crystal of La_2-xSr_xCuO₄ grown in the mirror furnace at WMI. The last about 6 cm on the left end of the sample are formed by a single crystallite.

 

Fig 3: Transition curves for single crystals of the  La_2-xSr_xCuO₄ series  grown in the mirror furnace at WMI.

Fig 4: Transition curves for the electron doped 214 systems  single crystals of the  Nd_2-xCe_xCuO₄ and Pr_2-xCe_xCuO₄

 

For more information you may read the annual report of the Crystal Growth group of the WMI.
 

 

Selected Papers: