The Growth Optimization The Optimized Growth Process • Create a high-­‐quality precursor powder, Ba2CuO3, by grinding Ba(NO3)2 and CuO together and baking overnight at high temperatures. • Combine precursor powder with HgO and MgSO4 + 7H2O. •  Seal powders in a quartz tube under vacuum to prevent excess impurities. •  Place quartz tube in a furnace and heat to temperatures peaking at 1020°C for four days. Characterization The Meissner Effect was utilized to test the quality of the crystals. The Meissner Effect is a phenomenon in which superconducting materials expel magnetic >ield below their characteristic transition temperature. Superconductors are able to expel these >ields by creating an oppositely oriented magnetic moment, thus canceling magnetic >ields in their interior. To characterize the crystals, the crystals induced magnetic moment is measured as a function of temperature twice. The >irst measurement involves initially cooling the sample in a zero >ield zone (zero >ield cooled, ZFC). The second involves cooling the sample in a magnetic >ield (>ield cooled, FC). An ideal crystal would have identical FC and ZFC cooled measurements. However, for impure crystals, >ield cooling allows impurities to trap magnetic >ield vortices inside the crystal. These trapped vortices reduce the magnitude of the measured magnetic moment created by the sample. To gauge pureness, we considered the FC to ZFC ratio. Higher quality crystals have a larger ratio. Figure 1 (left): FC (top curve) and ZFC (bottom curve) measurements plotted together. 40
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Mass (mg) Figure 2: Two different growth methods are compared by observing the crystal quality (determined by the FC/ZFC ratio) over a range of crystal size. Several factors were adjusted in order to optimize the crystal growth process. To test the 70 Average FC/ZFC Ratio (%) effect of each factor, the quality of the resultant crystals was measured with methods described below. The most in>luential factor of the growth was found to be the impurity levels. Impurities are needed for the precursor and HgO to successfully mix and create large Hg1201 crystals. [1] Initially, the precursor powder was exposed to humid air for 6-­‐7 minutes. The water absorbed by the precursor during this time would serve as the impurity. However, other unwanted reactions would occur between the precursor and air that would make the sealed growths highly likely to explode, therefore rarely producing crystals. We applied a new method: sealing a hydrate, MgSO4 + 7H2O, inside the quartz tube with the precursor and HgO instead of exposing to air to drop the explosion rate. It was shown that using MgSO4 + 7H2O rather than exposing to the precursor to humid air was a cleaner way to add water to the growths. For each crystal size group, the MgSO4 method produced higher quality crystals than the the humid air method as shown in >igure 2. The MgSO4 production rate was also much higher than the humid air method. Adding MgSO4 reduces exposure time to ambient air, thus reducing unwanted reactions with the air and precursor that can cause the growths to explode and not produce crystals. Air Exposure 45
60 50 40 30 50 Average Mass per Crystal (mg) Methods Results Average FC/ZFC Ratio (%) Introduction HgBa2CuO4+δ (Hg1201) is of critical importance in the study of high-­‐temperature superconductivity as it possesses one of the highest Tc values and is considered a model compound with a relatively simple structure. Research on Hg1201, particularly in the area of neutron scattering, has been limited by the demanding crystal growth procedure. Neutron scattering has proven to be a useful tool as neutrons interact magnetically with crystals, allowing for the detection of magnetic excitations; magnetic properties in different phases of high-­‐Tc materials are thought to be linked to the mechanism for superconductivity. Preparing a sample suitable for neutron scattering involves an intensive growth effort, as the weak nature of the neutron scattering cross section requires large, high-­‐quality crystal samples. The growth process of Hg1201 was optimized, resulting in several crystal samples to be measured with neutron scattering. 45 40 35 30 25 20 15 10 5 0 20 10 15 20 25 30 35 40 45 50 MgSO4+7H2O added (mg) 10 0 10 15 20 25 30 35 40 45 50 MgSO4+ 7H2O added (mg) Figure 4: Crystal size is compared for crystals grown with different amounts of MgSO4. Figure 3: The crystal quality in terms of FC/ZFC ratio is compared for samples grown with different amounts of MgSO4. The effects of adding MgSO4 were closely studied. Higher amounts of MgSO4 produced larger crystals as seen in >igure 4. Although, as the amount of MgSO4 increased, the crystal quality decreased as shown in >igure 3. However, quality was found to be independent of crystal size; some large crystals still gave high FC/ZFC measurements if they were grown with a relatively small amount of MgSO4. Several neutron samples were created as a result of the improved growth method. A sample with a superconducting transition temperature of 45 K is shown to the right in >igure 5. Multiple samples at different doping levels have been measured with neutron scattering as a result of the new growth technique. One such measurement is shown in >igure 5. Figure 5: (left) The improved growth method produced crystals suitable for a neutron sample. Several crystals are mounted with their axes co-­aligned in order to imitate one large crystal. (right) Two magnetic excitations (near 30 and 55 meV) of a 95K sample are found with neutron scattering. Conclusion The crystal growth method has been improved by using MgSO4 + 7H2O to introduce water to the growths as opposed to exposure to humid air. This improved method successfully produced a suf>icient amount of suitable crystals for neutron scattering samples. Further improvements to the growth process could be made in >inding a way to consistently produce large crystals with small amounts of MgSO4, thereby producing large samples without sacri>icing quality. Future plans for the Hg1201 crystals involve measuring thermoelectric power, resistivity, and magnetic excitations with neutron scattering. This project was funded by the University of Minnesota’s UROP program and by the Department of Energy, under
my advisor, Martin Greven. [1] X. Zhao et al. Advanced Materials 18, 3243 (2006).