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 [1] 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.
[1] E. Bauer et al. Phys. Rev. Lett. 92, 027003 (2004).