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Quantum Phase Transition at 0K is clarified in Uranium-based compound
- New mechanism of superconductivity is suggested -
Jan. 21, 2009
Summary
Usual phase transitions take place at a finite phase transition temperature Ttr (Ttr>0K). In contrast, when Ttr approaches 0K, the property of phase transition becomes particular since quantum fluctuations enhance compared with thermal fluctuations. This particular phase transition at 0K is called �Quantum phase transition (QPT)� which attracts strong interests. Recently, two different theoretical models (spin density wave and Kondo screening models, see below) have been proposed to describe the magnetic QPT, however, no definitive experimental evidence has been obtained to determine the proper model so far. In this work, we have obtained a clear experimental evidence which supports the spin density in an uranium compound USn3, using the nuclear magnetic resonance (NMR) method. Since the unconventional superconductivity appears at the vicinity of QPT in uranium compounds, the origin of such superconductivity is believed to relate magnetic fluctuations around QPT, thus the understanding of QPT is an important issue in the condensed matter physics from this point of view. New mechanism of superconductivity is suggested from this work.
Two models for Quantum phase transition
In f-electron systems such as uranium compounds, local magnetic moments due to f-electrons interact trough the magnetic exchange interactions each other, at the same time, interact with conduction electrons trough the Kondo interaction. As described in figure 1, if the magnetic exchange interaction is dominant, the local magnetic moments have a tendency to be polarized to deduce the magnetic exchange energy (spin density wave state). On the other hand, the local magnetic moments would be screened by conduction electron spins if the Kondo interaction is dominant (Kondo screening state). Because of the competition between these two interactions, the quantum phase transition appears easily in uranium compounds e.g. USn3.
In the present work, temperature dependence of magnetic coherence length is determined in USn3 from NMR spin-spin and spin-lattice relaxation rates measurements. Obtained experimental results (red circles) are well coincided with expected ones (green blue lines) from the spin density wave model as clearly seen in figure 2, indicating that the spin density wave is relevant to the QPT in uranium compounds.
Unconventional superconductivity and Quantum phase transition
In the superconducting state, two electrons form a pair (Cooper pair) due to an attractive force. For conventional superconductors, lattice vibration (phonon) is the origin of attractive force, in contrast, for unconventional superconductors, magnetic fluctuation around quantum phase transition is believed to induce the Cooper pairs. The present study suggests that the unconventional superconductivity in uranium compounds is caused by the spin density wave fluctuation as described in figure 3.
A future new superconductor with higher Tc is considered to be realized in magnetic-fluctuation-mediated superconducting system, since Tc of phonon-mediated superconductors reaches its peak already. This study suggests that a search for materials possessing enhanced spin density wave fluctuation is a guideline to find new high Tc superconductors.
This work has been published in Physical Review Letters 102, 037208(2009).
•Quantum Phase Transition (QPT) at 0K is clarified in Uranium-based compound -New mechanism of superconductivity is suggested-
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