Discovery of a colossal thermomagnetic effect caused by superconducting fluctuation effect
Suggests possible application to thermoelectric conversion materials
A group of researchers discovered that thermomagnetic effects in some uranium compounds become several orders of magnitude more powerful than those of conventional superconducting materials.
• Doctor's course at the Graduate School of Science, Kyoto University -- YAMASHITA Takuya , graduate student, SUMIYOSHI Hiroaki , graduate student, MATSUDA Yuji , Professor
• Graduate School of Frontier Sciences, The University of Tokyo -- SHIBAUCHI Takasada , Professor; Visiting Professor Graduate School of Science, Kyoto University
• Graduate School of Engineering Science, Osaka University -- FUJIMOTO Satoshi , Professor
• Advanced Science Research Center, Sector of Nuclear Science Research, Japan Atomic Energy Agency -- HAGA Yoshinori , Chief Researcher
Thermomagnetic effect is the conversion of temperature differences directly into electricity in a magnetic field. The colossal thermomagnetic effect obtained by this group was caused by superconducting fluctuation effect formed at a little higher temperatures than superconductive transition temperature, a critical temperature at which matter becomes a superconductive state. This colossal thermomagnetic effect can be explained by the electron scattering process in an unusual superconducting state demonstrated by superconducting fluctuation effects in uranium compounds. The thermomagnetic effects observed by this group suggest possible application to thermoelectric conversion materials based on new superconducting fluctuations.
Comment by Prof. Fujimoto
When a superconductor is heated above its critical temperature Tc, macroscopic coherence vanishes, leaving behind droplets of thermally fluctuating Cooper pair. This superconducting fluctuation effect above Tc has been investigated for many decades and its influence on the transport, thermoelectric and thermodynamic quantities in most superconductors is well understood by the standard Gaussian fluctuation theories. The transverse thermoelectric (Nernst) effect is particularly sensitive to the fluctuations, and the large Nernst signal found in the pseudogap regime of the underdoped high-Tc cuprates has raised much debate on its connection to the origin of superconductivity. Here we report on the observation of a colossal Nernst signal due to the superconducting fluctuations in the heavy-fermion superconductor URu2Si2. The Nernst coefficient is enhanced by as large as one million times over the theoretically expected value within the standard framework of superconducting fluctuations. This, for the first time in any known material, results in a sizable thermomagnetic figure of merit approaching unity. Moreover, contrary to the conventional wisdom, the enhancement in the Nernst signal is more significant with the reduction of the impurity scattering rate. This anomalous Nernst effect intimately reflects the highly unusual superconducting state embedded in the so-called hidden-order phase of URu2Si2. The results invoke possible chiral or Berry-phase fluctuations originated from the topological aspect of this superconductor, which are associated with the effective magnetic field intrinsically induced by broken time-reversal symmetry of the superconducting order parameter.
The superconducting fluctuation effect, due to preformed Cooper pairs above the critical temperature T c , has been generally understood by the standard Gaussian fluctuation theories in most superconductors. The transverse thermoelectric (Nernst) effect is particularly sensitive to the fluctuations, and the large Nernst signal found in the pseudogap regime of the underdoped cuprates has raised much debate. Here we report on the observation of a colossal Nernst signal due to the superconducting fluctuations in the heavy-fermion superconductor URu 2 Si 2 . The Nernst coefficient is anomalously enhanced (by a factor of ~10 6 ) as compared with the theoretically expected value of the Gaussian fluctuations. Moreover, contrary to the conventional wisdom, the enhancement is more significant with a reduction of the impurity scattering rate. This unconventional Nernst effect intimately reflects the highly unusual superconducting state of URu 2 Si 2 . The results invoke possible chiral or Berry-phase fluctuations associated with the broken time-reversal symmetry of the superconducting order parameter.
Figure 1. Schematic view of novel mechanism of thermoelectric effect.
Temperature gradient along the horizontal direction generates voltage along the vertical direction.Conduction electrons are scattered asymmetrically by bubbles of Cooper pairs raised by superconducting fluctuations, which leads to the transverse voltage.
To learn more about this research, please view the full research report entitled " Colossal thermomagnetic response in the exotic superconductor UR u2 Si 2 " at this page of the Nature Physics website.