Physical Characterization of the Heavy Fermion Superconducting Noncentrosymmetric CePt3Si System and its Variants T. L. Jones Department of Physics, University of Florida, Gainesville, Florida 32611-8440 (Dated: July 28, 2004) The CePt3Si system has recently been reported as the only heavy fermion system with a lack of inversion symmetry in the crystal structure to display superconducting properties. This discovery is in partial disagreement with the traditional Bardeen-Cooper-Schrieffer (BCS) theory of superconductors, which involves electron phonon mediated pairing. The previously published data, January 2004, reported a fairly broad phase transition into the superconducting region as well as at the Curie temperature. The data fit showed a finite value of C/T at temperature equal to zero Kelvin. Explored here are correlations between physical properties and stoichiometery, as well as doping effects on the system. The lack of inversion symmetry, at least with all prior experience, makes the CePt3Si system an unlikely candidate for superconductivity. The belief that the crystal has an inversion center has been assumed in almost all other previous studies, which in turn allows separate consideration of the even and odd components of the superconducting order parameter. Nonetheless CePt3Si has an antiferromagnetic phase which occurs at and below TN = 2.2 K and a superconducting phase which appears at Tc = 0.75 K. Since its discovery in January of 2004  there has been much theoretical work done to understand and model the superconducting behavior of this system. 2.0 CePt3Si Theoretical Structure CePt3B Structure Type Space Group : P4mm, No. 99 a = 4.0721 angstroms c = 5.4421 angstroms Intensity (Arb. Units) 1.8 1.6 1.4 1.2 1.0 20 40 60 80 100 120 2 θ (Degrees) FIGURE 1. Crystal structure of CePt3Si FIGURE 2. Theoretical X-Ray diffraction pattern The system forms the ternary boride compound CePt3B and has space group P4mm No.99. Using PowderCell we were able to generate the theoretical diffraction pattern to be used as a baseline comparison for the samples we produced (see Fig. 1 and 2, respectively). Unlike previous theoretical approaches, our experimentalist approach was to consider correlations between doping and stoichiometric variations in the mother compound to better understand its physical behavior. By slightly stressing the system, either with a smaller or bigger atom substitution, or by varying the mother sample, we hoped to discover correlations between TN, TC, Tc, phase transition widths, and suppressions or enhancements of all three. In the 12 week period we were able to synthesize and measure 32 different variations on the mother sample, which included some ten dopents: La, U, Pr, Th (all on the Ce site), Au, Ir, Os (all on the Pt site), Ge, Be, B (all on the Si site). In effect we also tried to create a purer, more ordered sample than what has previously been reported or create one with a small amount, up to total replacement, with a similar doping. Any significant increase, decrease or total suppression, due to stoichmetric effects of Tc, TC, or TN or some combination of all three would be considered a success as well as a C/T value equal to zero at temperature equal to zero Kelvin. Also we plan to report any other unexpected interesting physical attributes we might discover. Methods The samples for these investigations were prepared by arc melting the constituent elements, weighed out to a precision of 0.1 milligrams, on a water-cooled copper hearth under a high purity inert argon atmosphere. The samples were then flipped and re-melted several times to ensure homogeneity. The majority of samples experienced weight losses on the order of 0.1 % during the arc melting process. These samples were then split in half, using a mortar and pestle, and half of the sample was wrapped in tantalum foil, sealed in an evacuated quartz tube, and annealed at 1000o C for three weeks. The other half of the sample was used for X-Ray characterization (in order to confirm the correct lattice structure), magnetic properties measurement (in a Quantum Design MPMS system), and specific heat measurement. After the three week annealing period, the samples were taken out of the furnace, split in half once again, and half was re-wrapped in tantalum foil, sealed in evacuated quartz tube, and further annealed for three more weeks at 1000oC. The leftover annealed half was used for all the same measurements as the unannealed sample in order to determine if annealing helped to increase the ordering of the sample lattice. Some of the samples, that showed favorable results, were annealed at higher temperatures in order to determine the best annealing temperature in order to maximize lattice ordering while minimizing sample loss. Most commonly the samples were annealed at 1300oC in sealed BeO crucibles for a period of three weeks further. Results Since ten weeks for this type of investigation is not a long period of time, results and analysis of all the compounds synthesized are not yet available. Some of the samples are still annealing in the furnaces. Other samples that are needed to help understand results have not yet been produced. Within six months time, I hope to report a more complete picture of this study and a more in depth analysis of the observations. Although there is not enough space and time to completely discuss all results from the 32 different synthesized compounds and compare them with the 3, 6, and high temperature annealed versions, I would like to highlight some of the interesting samples we measured in hopes of giving a brief introduction to the correlations and effects so far seen. This results section will be used mainly to discuss the CePt3Si mother sample (to use as a basis for the other), and the CePt3Si1.02 sample. CePt3Si Sample CePt3Si Sample 4.0 CePt3Si Sample - High Angle X-Ray Scan annealed o 4.0 o o 3.5 3.0 annealed o o (1 week @ 950 C, 2 weeks @ 1000 C) 2.5 2.0 unannealed 1.5 1.0 CePt3Si Theoretical Structure CePt3B Structure type 0.5 Intensity (Arb. Units) Intensity (Arb. Units) o annealed (1 week @ 950 C, 5 weeks @ 1000 C) (1 week @ 950 C, 5 weeks @ 1000 C) 3.5 3.0 o o annealed (1 week @ 950 C, 2 weeks @ 1000 C) 2.5 2.0 unannealed 1.5 1.0 0.5 CePt3Si Theoretical Structure CePt3B Structure type 0.0 0.0 20 40 60 80 100 120 2 θ (Degrees) FIGURE 3. Full X-ray scan of CePt3Si sample 96 98 100 102 2 θ (Degrees) FIGURE 4. High angle X-ray scan of CePt3Si sample As shown in Fig. 3, the X-Ray diffraction pattern matched the theoretical produced pattern and improved only slightly with annealing. The higher angle lines also confirmed the correct structure (see Fig. 4). The effects of annealing, from bottom to top, can be observered as the line at ~98.2o starts to split from no annealing to the six-week annealed sample. This shoulder is expected due to the XRD two wavelengths it uses. The lack of the shoulder in the unannealed sample is due to some disorder in the crystal lattice. As the sample was annealed and the ordering increased, it became evident. In order to save space, the specific heat results for the CePt3Si sample will be superimposed in Fig. 5. CePt3Si1.02 CePt3Si1.02 Sample CePt3Si1.02 Sample 3.0 3.0 2.8 o annealed (3 weeks @ 1000 C) 2.6 2.8 2.4 2.2 Intensity (Arb. Units) Intensity (Arb. Units) o annealed (3 weeks @ 1000 C) 2.6 2.4 2.0 1.8 unannealed 1.6 1.4 1.2 1.0 0.8 CePt3Si Theoretical Structure CePt3B Structure Type 0.6 2.2 2.0 1.8 1.4 1.2 1.0 0.8 CePt3Si Theoretical Structure CePt3B Structure Type 0.6 0.4 0.4 0.2 0.2 0.0 unannealed 1.6 0.0 20 40 60 80 100 120 96 98 2 θ (Degrees) 100 102 2 θ (Degrees) FIGURE 5. Full X-ray scan of CePt3Si1.02 sample FIGURE 6. High angle X-ray scan of CePt3Si1.02 sample The 2% rich Si sample was made and formed into the correct crystal lattice structure (see Figs. 5 and 6), and as before, annealing seems to have had little effect on increasing ordering in the lattice. The specific heat results show a narrowing of the phase transition at TN = ~2.2K (see Fig. 7), as well as at Tc. A slight increase in the critical temperature, which is shown in Fig. 8, was achieved over the mother CePt3Si sample we first produced. By stressing the Si site, we have been able to increase Tc. This result brings hope that dopings on the Si site with have similar effects. 1200 CePt3Si (950C 1 week and 1000C 3 weeks) CePt3Si1.02 unannealed (C/T + 100) CePt3Si1.02 (1000C 3 weeks) (C/T + 200) CePt3Si (950C 1 week and 1000C 3 weeks) CePt3Si1.02 unannealed CePt3Si1.02 (1000C 3 weeks) previous published data 600 -2 C/T (mJ mole K ) -1 -1 -2 C/T (mJ mole K ) 1000 800 600 400 500 400 200 0 2 4 6 8 T (K) FIGURE 7. Specific heat data, 0-10K 10 0.2 0.4 0.6 0.8 1.0 1.2 T (K) FIGURE 8. Specific heat data, 0.2-1.4K 1.4 Conclusion In the preliminary results, we have been able to manipulate the onset of Tc, TC, and TN as well as narrow the phase transitions in some instances. But until we complete our study, any attempt to decisively state correlations would be unconvincing. For this reason much more work is still to be completed to verify that we can predict the outcomes of the changes we make. In the future we would like to dope onto the La site of LaPt3Si, since La, in contrast to Ce, has no f-electrons. The effects of the dopents f-electrons on the sample could be further studied. Our first attempt at making the completely substituted La sample has been a success, and we are beginning to prepare the doped samples for study. Acknowledgments I would like to warmly thank Drs. G. Stewart and J. Kim for all their insight, knowledge, patience, and help with this project; Drs. K. Ingersent and A. Dorsey, for allowing me the opportunity to participate in this wonderful program; and Ms. D. Balkcom and W. Bomstad for taking care of all the numerous details along the way.  E. Bauer et al. Phys. Rev. Lett. 92, 027003 (2004).