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Flipping electron pairs instead of magnetic spins:

Jülich physicists explain the charge Kondo effect

The charge Kondo effect, a phenomenon with unusual properties, was predicted theoretically in the 1970's and 1980's. One unusual property is that impurities exhibiting the charge Kondo effect induce superconductivity in a nominally non-superconducting host. This contrasts with the usual spin Kondo effect, which inhibits superconductivity. Theoretical physicists from the Peter Grünberg Institute/Institute for Advanced Simulation now propose that the charge Kondo effect can be convincingly demonstrated in semiconducting lead telluride (PbTe) doped with thallium (Tl) acceptor impurities. By combining ab-initio electronic structure information for lead telluride with a numerical renormalization group treatment of the thallium impurities, the scientists explain the measured anomalous normal state properties of this system: the doping and temperature dependence of the carrier density, the resistivity, and the local thallium-impurity spectral function. Their results are published in the current issue of the science magazine “Physical Review Letters”. Their findings could also help in the development of more efficient thermoelectric materials, as addressed in a 2011 Rapid Communication in “Physical Review B”.

In conventional spin Kondo systems, such as iron impurities in gold, the impurities have a spin magnetic moment. This spin magnetic moment can flip when an electron scatters from the impurity, and this in turn leads to an anomalous increase in the resistivity with decreasing temperature, a phenomenon known as the Kondo effect. The same phenomenon has been measured in lead telluride doped with thallium; however thallium impurities are non-magnetic, so what is flipping to cause the Kondo effect?  “We found, that thallium impurities in lead telluride act in a unique way”, explains Dr. Theodoulos Costi. “Namely, at low temperatures the outer 6s electrons of thallium impurities, instead of repelling each other, attract each other. Consequently, thallium impurities prefer to have either a pair of electrons in their outer 6s valence shell or no electrons at all. These empty and doubly occupied states are analogous to the up and down spin states of a magnetic impurity. If these two charge or valence states can be made degenerate in energy, fluctuations between them induce a novel charge analogue to the usual spin Kondo effect.”

The unique aspect of thallium impurities in lead telluride which allows this to occur, is the fact that the system self-tunes to the degeneracy point when the concentration of thallium impurities reaches or exceeds 0.3 %. Once the degeneracy point is reached, dynamic fluctuations of pairs of electrons on the thallium impurities produce the non-magnetic charge Kondo effect and hence the exotic properties associated with this new phenomenon. “One such property is that the charge Kondo effect can provide a mechanism for enhancing the thermoelectric performance of materials with charge Kondo impurities via a giant enhancement of the Seebeck coefficient”, says Costi. “This could have applications in terms of low temperature cooling or in the recovery of waste heat. In future, we aim to investigate further the charge Kondo effect in PbTe(Tl) and in molecular transistors for the purpose of improving the thermoelectric performance of such systems.”

 

Original publications:

T. A. Costi and V. Zlatic, Phys. Rev. Lett. 108, 036402 (2012), DOI: 10.1103/PhysRevLett.108.036402

S. Andergassen, T. A. Costi, V. Zlatic, Phys. Rev. B 84, 241107 (2011), DOI: 10.1103/PhysRevB.84.241107

 

More information:

 Y. Matsushita, H. Bluhm, T. H. Geballe, and I. R. Fisher, Phys. Rev. Lett. 94, 157002 (2005)

M. Dzero and J. Schmalian, Phys. Rev. Lett. 94, 157003 (2005)

 

Peter Grünberg Institut-2: Theoretische Nanoelektronik

Research group “Strong correlations and mesoscopic physics“ an der RWTH Aachen

 

 


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