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Structural modulations in UPt3 single crystal whiskers
PHYSICA Physica B 205 (1995)346348
ELSEVIER
Structural modulations in UPt3 single crystal whiskers Brett Ellman*, Alicja Zaluska, Louis Taillefer Department of Physics, McGill University, 3600 University St., Montreal Que., Canada H3A 2T8
Received 16 June 1994; revised 15 August 1994
Abstract Recent transmission electron microscopy (TEM) data on the heavy fermion superconductor UPt3 find a complex periodic modulation in the basal plane, the nature of which is unclear. We present TEM data on single crystal whiskers of UPt3 grown spontaneously from the melt. We find a modulation in the a-c plane, consistent with a description in terms of a single q, allowing us to present the first accurate determination of the c* component of the modulation. This structure persists across a wide area of the whisker, making these samples candidates for X-ray studies of the modulation.
1. Introduction The observation of two zero field superconducting transitions in the heavy fermion superconductor UPt3 which merge into a single transition under applied pressure [1] constitutes strong evidence for the presence of a multiply degenerate order parameter at T~ in the absence of any phenomena that lowers the symmetry of the lattice. The two transitions observed at ambient pressure would then be a consequence of some additional internal field that breaks crystal symmetry. In most work to date [2], this symmetry breaking field is taken to be the weak in-plane antiferromagnetic order, which can also be suppressed by applied pressure [3], that sets in at T N = 5 K ( T c ~ 0 . S K ) . Recent T E M studies [4] of UPt3 bulk crystals have identified an additional possibility in the form of a long wavelength modulation of uncertain origin, incommensurate with the hexagonal crystal lattice. *Corresponding author.
In order to help elucidate the nature of these modulations, we have performed T E M observations of a single crystal whiskers of UPt3. Whiskers are small (millimeters long by tens of microns wide and thick) single crystals of UPt3 that spontaneously grow from the melt on cooling. We studied whiskers in the belief that they might, by virtue of their presumably simple growth mechanism and small size, possess higher crystalline quality, a belief which is consonant with their high residual resistivity ratios (as high as 3000). Furthermore, whiskers are sufficiently thin that preparation for the TEM is simplified and there are thus fewer opportunities for the introduction of artifacts. Finally, though small, our whiskers are sufficiently large to permit synchrotron X-ray scattering experiments on the nature of the symmetry breaking modulation.
2. Experiment The whiskers were prepared under ultra-high vacuum by induction melting of a high-purity UPt3
0921-4526/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSDI 0921-4526(94)00902-3
B. Ellman et al./Physica B 205 (1995) 346 348
Fig. 1. Bright field TEM image of the a c plane of UPt 3. The modulations discussed in the text appear as stripes spaced by approximately 60 ~,.
ingot. No post growth annealing was done. The bulk of the data presented here were obtained on a sample of approximate dimensions 2 × 0.05 x 0.03 mm 3 prepared for TEM by ion milling. Recent imaging of a second whisker from another growth gave results completely in accord with the first sample. No mechanical polishing, such as was done on the bulk sample in Ref. [4], was required. The microscope used was a 200kV instrument with a room temperature stage.
3. Discussion In Fig. 1 we present a bright field image with the modulation appearing as well defined (though somewhat irregular) bands. An electron diffraction pattern is shown in Fig. 2. The central result of this paper is the presence of a single satellite for each Bragg reflection. Fig. 3 emphasizes this fact, showing a lattice reflection with its satellite. This is in contrast to the multiple q vectors required to explain the images in Ref. [4]. The presence of a single satellite was confirmed in various areas in the ionmilled region of the whisker. There are various possibilities as to why the results are different. Since we are imaging in the a c plane while the results of Ref. [4] are for the a-b plane, one possibility is that
347
Fig. 2. Electron diffraction pattern of the a*-c* plane of UPt 3 with the c* axis running vertically. At this level of detail, the modulation evident in Fig. 1 manifests as a smearing or doubling of the Bragg peaks.
Fig. 3. Detail about a Bragg reflection of the diffraction pattern in Fig. 2.
our single modulation vector corresponds to one of those of the previous work with an appropriate a* projection and no b* component, e.g., 'q4' of Ref. [4]. We might also speculate that either the mechanical polishing or the growth method used in Ref. [4] introduced sufficient disorder or strain to account for the complex structure observed. Independent of the comparison of the two measurements, we conclude that the modulation has a sizable component along c*, of magnitude ~ twice that found in Ref. [4] by tilting the crystal. Using the data in Fig. 2, we estimate that the modulation seen in Fig. 1 is described by q ~ 0.1a* + 0.07 c*, where a * = 1 . 2 6 4 A and c* = 1.283 ,~ are the reciprocal lattice vectors for the SnNi3 hexagonal lattice of UPt3. This value of q corresponds to a real space modulation spacing of ~ 60 A, in agreement with Fig. 1. This value is in rough agreement with equivalent quantities extrac-
348
B. Ellman et al. / Physica B 205 (1995) 346 348
ted in Ref. [4] for a bright field image reconstructed from the ,qA, and ,qB, vectors (their Fig. l(d)). It is interesting that, while well defined diffraction spots were only observed for annealed specimens in Ref. [4], we see clear modulations for our unannealed whiskers. However, later observations by the Bristol group 1-5] find a multiplicity of possible modulation structures depending on the sample treatment, including some with well defined satellites in unannealed samples. We stress that we have essentially identical results from two independently grown samples, indicating that whiskers provide a high degree of reproducibility. Again, from our data, we may conclude that the a - c plane q-structure of the modulation is simple.
to the extremely small, disordered domains observed in many bulk samples, which smear out the X-ray satellites until they are too diffuse to observe. Whiskers, however, show extremely coherent modulations across macroscopic regions of the samples. Indeed, while we cannot rule out some domain structure in the whiskers, we have not observed any domain walls across the entire milled regions of two samples. This is consistent with a domain size at least of the order of fractions of a millimeter. We thus believe that whiskers present an excellent candidate for X-ray studies to establish the nature of the symmetry breaking modulations observed in electron microscopy, if indeed they are intrinsic to bulk UPt3.
4. Conclusions
Acknowledgements
We have observed a periodic structural modulation in TEM micrographs and electron diffraction patterns of single crystal whiskers of UPt3. Our data represent the first accurate determination of the c* component of the modulation and the first on samples not subjected to mechanical polishing. The modulations agree in period with those found in an earlier study of bulk crystals. The nature of these modulations is still unclear. In particular, it is possible that the structure is introduced during ion milling and is not intrinsic to the material. It is also possible that the modulation reflects properties of the surface (or at least the first several atomic planes) of UPt3 and not the bulk behavior. To this end, X-ray studies, which require no sample preparation and probe more deeply than TEM measurements, are required. Past attempts [6] on bulk samples have failed to discern any satellites to the Bragg spots. This failure may, however, be due
We thank Mark Sutton for very helpful suggestions. L.T. acknowledges the support of the Canadian Institute for Advanced Research and the Sloan Foundation. This work was supported by FCAR and NSERC.
References [1] H.V. L6hneysen, T. Trappmann and L. Taillefer, J. Magn. Magn. Mater. 108 (1992) 49. [2] For a review of several theories see R. Joynt, J. Magn. Magn. Mater. 108 (1992) 31. I-3] S.M. Hayden, L. Taillefer, C. Vettier and J. Flouquet, Phys. Rev. B 46 (1992) 8675. [4] P.A. Midgley, S.M. Hayden, L. Taillefer, B. Bogenberger and H.v. L6hneysen, Phys. Rev. Lett. 70 (1993) 678. 1-5] P.A. Midgley, S.M. Hayden and L. Taillefer, EMAG93 conference, Inst. Phys. Conf. Ser. No. 138, 345. [6] B. Lussier, M. Sutton, and L. Taillefer, unpublished.
ELSEVIER
Structural modulations in UPt3 single crystal whiskers Brett Ellman*, Alicja Zaluska, Louis Taillefer Department of Physics, McGill University, 3600 University St., Montreal Que., Canada H3A 2T8
Received 16 June 1994; revised 15 August 1994
Abstract Recent transmission electron microscopy (TEM) data on the heavy fermion superconductor UPt3 find a complex periodic modulation in the basal plane, the nature of which is unclear. We present TEM data on single crystal whiskers of UPt3 grown spontaneously from the melt. We find a modulation in the a-c plane, consistent with a description in terms of a single q, allowing us to present the first accurate determination of the c* component of the modulation. This structure persists across a wide area of the whisker, making these samples candidates for X-ray studies of the modulation.
1. Introduction The observation of two zero field superconducting transitions in the heavy fermion superconductor UPt3 which merge into a single transition under applied pressure [1] constitutes strong evidence for the presence of a multiply degenerate order parameter at T~ in the absence of any phenomena that lowers the symmetry of the lattice. The two transitions observed at ambient pressure would then be a consequence of some additional internal field that breaks crystal symmetry. In most work to date [2], this symmetry breaking field is taken to be the weak in-plane antiferromagnetic order, which can also be suppressed by applied pressure [3], that sets in at T N = 5 K ( T c ~ 0 . S K ) . Recent T E M studies [4] of UPt3 bulk crystals have identified an additional possibility in the form of a long wavelength modulation of uncertain origin, incommensurate with the hexagonal crystal lattice. *Corresponding author.
In order to help elucidate the nature of these modulations, we have performed T E M observations of a single crystal whiskers of UPt3. Whiskers are small (millimeters long by tens of microns wide and thick) single crystals of UPt3 that spontaneously grow from the melt on cooling. We studied whiskers in the belief that they might, by virtue of their presumably simple growth mechanism and small size, possess higher crystalline quality, a belief which is consonant with their high residual resistivity ratios (as high as 3000). Furthermore, whiskers are sufficiently thin that preparation for the TEM is simplified and there are thus fewer opportunities for the introduction of artifacts. Finally, though small, our whiskers are sufficiently large to permit synchrotron X-ray scattering experiments on the nature of the symmetry breaking modulation.
2. Experiment The whiskers were prepared under ultra-high vacuum by induction melting of a high-purity UPt3
0921-4526/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved SSDI 0921-4526(94)00902-3
B. Ellman et al./Physica B 205 (1995) 346 348
Fig. 1. Bright field TEM image of the a c plane of UPt 3. The modulations discussed in the text appear as stripes spaced by approximately 60 ~,.
ingot. No post growth annealing was done. The bulk of the data presented here were obtained on a sample of approximate dimensions 2 × 0.05 x 0.03 mm 3 prepared for TEM by ion milling. Recent imaging of a second whisker from another growth gave results completely in accord with the first sample. No mechanical polishing, such as was done on the bulk sample in Ref. [4], was required. The microscope used was a 200kV instrument with a room temperature stage.
3. Discussion In Fig. 1 we present a bright field image with the modulation appearing as well defined (though somewhat irregular) bands. An electron diffraction pattern is shown in Fig. 2. The central result of this paper is the presence of a single satellite for each Bragg reflection. Fig. 3 emphasizes this fact, showing a lattice reflection with its satellite. This is in contrast to the multiple q vectors required to explain the images in Ref. [4]. The presence of a single satellite was confirmed in various areas in the ionmilled region of the whisker. There are various possibilities as to why the results are different. Since we are imaging in the a c plane while the results of Ref. [4] are for the a-b plane, one possibility is that
347
Fig. 2. Electron diffraction pattern of the a*-c* plane of UPt 3 with the c* axis running vertically. At this level of detail, the modulation evident in Fig. 1 manifests as a smearing or doubling of the Bragg peaks.
Fig. 3. Detail about a Bragg reflection of the diffraction pattern in Fig. 2.
our single modulation vector corresponds to one of those of the previous work with an appropriate a* projection and no b* component, e.g., 'q4' of Ref. [4]. We might also speculate that either the mechanical polishing or the growth method used in Ref. [4] introduced sufficient disorder or strain to account for the complex structure observed. Independent of the comparison of the two measurements, we conclude that the modulation has a sizable component along c*, of magnitude ~ twice that found in Ref. [4] by tilting the crystal. Using the data in Fig. 2, we estimate that the modulation seen in Fig. 1 is described by q ~ 0.1a* + 0.07 c*, where a * = 1 . 2 6 4 A and c* = 1.283 ,~ are the reciprocal lattice vectors for the SnNi3 hexagonal lattice of UPt3. This value of q corresponds to a real space modulation spacing of ~ 60 A, in agreement with Fig. 1. This value is in rough agreement with equivalent quantities extrac-
348
B. Ellman et al. / Physica B 205 (1995) 346 348
ted in Ref. [4] for a bright field image reconstructed from the ,qA, and ,qB, vectors (their Fig. l(d)). It is interesting that, while well defined diffraction spots were only observed for annealed specimens in Ref. [4], we see clear modulations for our unannealed whiskers. However, later observations by the Bristol group 1-5] find a multiplicity of possible modulation structures depending on the sample treatment, including some with well defined satellites in unannealed samples. We stress that we have essentially identical results from two independently grown samples, indicating that whiskers provide a high degree of reproducibility. Again, from our data, we may conclude that the a - c plane q-structure of the modulation is simple.
to the extremely small, disordered domains observed in many bulk samples, which smear out the X-ray satellites until they are too diffuse to observe. Whiskers, however, show extremely coherent modulations across macroscopic regions of the samples. Indeed, while we cannot rule out some domain structure in the whiskers, we have not observed any domain walls across the entire milled regions of two samples. This is consistent with a domain size at least of the order of fractions of a millimeter. We thus believe that whiskers present an excellent candidate for X-ray studies to establish the nature of the symmetry breaking modulations observed in electron microscopy, if indeed they are intrinsic to bulk UPt3.
4. Conclusions
Acknowledgements
We have observed a periodic structural modulation in TEM micrographs and electron diffraction patterns of single crystal whiskers of UPt3. Our data represent the first accurate determination of the c* component of the modulation and the first on samples not subjected to mechanical polishing. The modulations agree in period with those found in an earlier study of bulk crystals. The nature of these modulations is still unclear. In particular, it is possible that the structure is introduced during ion milling and is not intrinsic to the material. It is also possible that the modulation reflects properties of the surface (or at least the first several atomic planes) of UPt3 and not the bulk behavior. To this end, X-ray studies, which require no sample preparation and probe more deeply than TEM measurements, are required. Past attempts [6] on bulk samples have failed to discern any satellites to the Bragg spots. This failure may, however, be due
We thank Mark Sutton for very helpful suggestions. L.T. acknowledges the support of the Canadian Institute for Advanced Research and the Sloan Foundation. This work was supported by FCAR and NSERC.
References [1] H.V. L6hneysen, T. Trappmann and L. Taillefer, J. Magn. Magn. Mater. 108 (1992) 49. [2] For a review of several theories see R. Joynt, J. Magn. Magn. Mater. 108 (1992) 31. I-3] S.M. Hayden, L. Taillefer, C. Vettier and J. Flouquet, Phys. Rev. B 46 (1992) 8675. [4] P.A. Midgley, S.M. Hayden, L. Taillefer, B. Bogenberger and H.v. L6hneysen, Phys. Rev. Lett. 70 (1993) 678. 1-5] P.A. Midgley, S.M. Hayden and L. Taillefer, EMAG93 conference, Inst. Phys. Conf. Ser. No. 138, 345. [6] B. Lussier, M. Sutton, and L. Taillefer, unpublished.