Hamlin Laboratory
Science of materials under extreme conditions
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Research
Our goup's research is directed towards utilizing a combination of materials synthesis, ambient pressure characterization, and high pressure and high magnetic field measurements to advance our understanding of novel and potentially useful electronic and magnetic materials. In particular, we study the formation of unconventional and/or high temperature superconducting states in the vicinity of quantum phase transitions, although in the course of characterizing new materials, other phenomena are also investigated.
Prior Research Highlights
Iron-based superconductors: The past several years have been an exciting time for superconductivity research, as a new class of high temperature superconductors (Tc < 55 K) containing iron-pnictide/chalcogenide planes was discovered. The first studies of these materials were based on polycrystalline samples. We synthesized single crystals of LaFePO, which allowed us to examine the anisotropy of the superconducting properties.1 Infrared measurements, carried out on our LaFePO crystals, showed the experimental electron kinetic energy to be only 50% of that determined via band theory, demonstrating the presence of significant electronic correlations.2 In the arsenic series LnFeAsO1-d and LnFeAsO1-xFx (Ln = lanthanide), Tc is nearly doubled upon substituting magnetic lanthanide elements for non-magnetic lanthanum, an observation at odds with conventional superconductivity. This motivated our substitution of magnetic lanthanides into LaFePO, leading us to uncover superconductivity in two additional compounds, NdFePO and PrFePO.3 The Tc values of these compounds turned out to be lower than in LaFePO, hinting at some fundamental difference between the phosphides and arsenides. Recently, I have been involved in an effort to characterize single crystals of LnFeAsO compounds that we synthesized.
| 1 |
Superconductivity in single crystals of LaFePO J. J. Hamlin, R. E. Baumbach, D. A. Zocco, T. A. Sayles, and M. B. Maple J. Phys.: Condens. Matter 20, 365220 (2008) |
| 2 |
Electronic correlations in the iron pnictides M. M. Qazilbash, J. J. Hamlin, R. E. Baumbach, L. Zhang, D. J. Singh, M. B. Maple, and D. N. Basov Nature Phys. 5, 647 (2009) |
| 3 |
Superconductivity in LnFePO (Ln = La, Pr, and Nd) single crystals R. E. Baumbach, J. J. Hamlin, L. Shu, D. A. Zocco, N. M. Crisosto, and M. B. Maple New J. Phys. 11, 025018 (2009) |
Rare-earth tritellurides: The quasi-2D rare-earth tritelluride compounds RTe3 (R = La-Nd, Sm, and Gd-Tm) have lately received significant attention as the first system in which nominal square-planar symmetry is broken by the formation of a unidirectional charge density wave. In TbTe3, we discovered a highly unusual pressure-temperature phase diagram which involves two orthogonal charge density waves, three closely spaced magnetic transitions, and superconductivity.4 This result points to the rare earth tritellurides as a possible model system to explore the interplay of superconductivity with various types of charge and spin order.
| 4 |
Pressure induced superconducting phase in the charge density wave compound terbium tritelluride J. J. Hamlin, D. A. Zocco, T. A. Sayles, M. B. Maple, J. -H. Chu, and I. R. Fisher Phys. Rev. Lett. 102, 177002 (2009) |
Pressure effects on topological insulator Bi2Se3: We recently studied the effect of pressure on the transport and structural properties of the topological insulator Bi2Se3.5 Initially, pressure drives Bi2Se3 towards increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance that is positive at low fields and becomes negative at higher fields. These results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3.
| 5 |
High pressure transport properties of the topological insulator Bi2Se3 J. J. Hamlin, J. R. Jeffries, N. P. Butch, P. Syers, D. A. Zocco, S. T. Weir, Y. K. Vohra, J. Paglione, M. B. Maple J. Phys.: Condens. Matter 24, 035602 (2012) |
Non-centrosymmetric Zr2Fe12P7-type compounds: Many years ago, Jeitschko and co-workers reported the synthesis of a variety of noncentrosymmetric Zr2Fe12P7-type compounds with the general formula R2T12Pn7, where R = rare earth, T = transition metal, and Pn = P or As. In fact, these compounds are only the most explored subset of a much larger class of noncentrosymmetric materials with the general formula Rn(n-1)T(n+1)(n+2)Pnn(n+1)+1. We have been involved in a systematic effort to characterize members of the so-called "2-12-7" family of compounds. In Yb2Fe12P7, we reported an unusual temperature-magnetic field phase diagram that displays a cross over from one non-Fermi-liquid regime at low field to another distinctly different non-Fermi-liquid regime at higher field.6 Interestingly, the crossover appears to be disconnected from the extrapolated location of a magnetic quantum critical point and the high field NFL state is remarkably robust over a wide range of applied fields. Sm2Fe12P7 appears to be a rare example of a Sm-based heavy-fermion ferromagnet.7 In Yb2Co12P7 we found evidence for short range correlations well above the ferromagnetic ordering temperature of 5 K.8 It appears that 2-12-7s may represent a largely unexplored reservoir of strongly correlated electron phenomena.
| 6 |
Unconventional T-H phase diagram in the noncentrosymmetric compound Yb2Fe12P7 R. E. Baumbach, J. J. Hamlin, L. Shu, D. A. Zocco, J. R. O'Brien, P.-C. Ho, and M. B. Maple Phys. Rev. Lett. 105, 106403 (2010) |
| 7 |
The noncentrosymmetric heavy fermion ferromagnet Sm2Fe12P7 M. Janoschek, R. E. Baumbach, J. J. Hamlin, I. K. Lum, and M. B. Maple J. Phys.: Condens. Matter 23, 094221 (2011) |
| 8 |
Transport, magnetic, and thermal properties of non-centrosymmetric Yb2Co12P7 J. J. Hamlin, M. Janoschek, R. E. Baumbach, B. D. White, and M. B. Maple Phil. Mag. 92, 647 (2012) |
Elemental Superconductors
A focal point of my graduate research was a systematic effort to induce superconductivity at record high temperatures for a single element. This resulted in the discovery that the Tc of pure yttrium metal climbs all the way to 20 K at pressures near one megabar.1,2 It took roughly half a century of research to find a compound with a Tc value near 20 K, but by utilizing high pressure, similarly high critical temperatures can be induced in a single element. This is an illustration of the substantial extent to which pressure is capable of tuning the properties of a material. Upon increasing the pressure from ambient to one megabar, the density of yttrium is more than doubled. It is not possible to tune the atomic density to anywhere near this amount by chemical substitution.
In the analysis of our results on yttrium metal, we pointed out a possible correlation in the non-magnetic, trivalent rare-earths between Tc and the ratio of the ion core radius to the average inter-atomic separation, which is related to the d-band occupancy. This correlation hinted that high transition temperatures might appear in scandium and lutetium under extreme pressure. Indeed, we found that the Tc of scandium also surged to 20 K near one megabar. In lutetium, we found a somewhat lower Tc = 12 K, despite pushing the pressure all the way to 1.7 Mbar.3,4 To my knowledge this remains the highest pressure at which a superconducting transition has been detected using the ac magnetic susceptibility technique. Later, this correlation motivated us to search for, and discover, pressure-induced superconductivity in europium metal, which may undergo a valence transition from divalent (4f7, J=7/2) with a local magnetic moment to trivalent (4f6, J=0), exhibiting weak Van Vleck paramagnetism.5
| 1 |
Superconductivity at 17 K in Yttrium Metal under Nearly Hydrostatic Pressures to 89 GPa J. J. Hamlin, V. G. Tissen and J. S. Schilling Phys. Rev. B 73, 094522 (2006) |
| 2 |
Superconductivity at 20 K in Yttrium Metal at Pressures Exceeding 1 Mbar J. J. Hamlin, V. G. Tissen, and J. S. Schilling Physica C 451, 82 (2007) |
| 3 |
Pressure-Induced Superconductivity in Sc to 74 GPa J. J. Hamlin and J. S. Schilling Phys. Rev. B 76, 012505 (2007) |
| 4 |
Comparison of the pressure dependences of Tc in the trivalent d-electron superconductors Sc, Y, La, and Lu up to megabar pressures.
M. Debessai, J. J. Hamlin, and J. S. Schilling Phys. Rev. B 78, 064519 (2008) |
| 5 |
Pressure-induced superconducting state of europium metal at low temperatures M. Debessai, T. Matsuoka, J. J. Hamlin, J. S. Schilling, and K. Shimizu Phys. Rev. Lett. 102, 197002 (2009) |