Physical properties of strongly correlated low-charge-carrier-density materials
Final Report Abstract
The advancement of man’s technological environment places ever-rising demands on innovative technologies, ingenious new materials, and more efficient use of energy. This project was founded in a search for new materials that would draw upon a sophisticated new approach towards clean energies. Thermoelectricity is a crafty use of the interplay between temperature and electrical properties in materials. It offers the possibility of turning wasteful energies in modern-day life, such as heat generated by the internal combustion engine in motor vehicles, into electricity. Conversely, materials of high thermoelectric merit may equally profitably be used to turn the electricity generated in materials of high thermoelectric merit into benevolent forms of energy. Our project was conceived to search for materials in which a certain intriguing electronic property, namely the thermoelectric power, can be tuned in order to create conditions favorable for the electrons in metals to become correlated. The correlated state of electrons is based upon a very fundamental property of the electron, namely its electrical charge and the way in which electrons interact with one another. In highly correlated states of matter, electrons may acquire effective masses that are a thousand times higher than in ordinary metals, and this special class of materials was planned to be the concourse of our research. Cooperative forms of behaviour among highly correlated electrons is a study field at the frontier of condensed matter physics. We started this projects with the fascinating correlated semi metal CeRu4Sn6 and finally we where able to describe to low temperature properties in the frame of a new “correlated in-gap states” model. Unexpected, CeRu4Sn6 earned now a revived very strong interest in the community because of the topological aspect of these states (“topological Kondo Insulator”) like in the other correlated semi-metal SmB6. Furthermore during this project, we have uncovered three exciting new types of materials. They all belong to the class of strongly correlated materials, but with the additional classification of a cage type architecture in their crystal structures. This was the goal of the materials research part of the project. Atomic cage structures are framework formations in crystalline solids with oversized sphere-like cages that may be empty, or with an atom captured inside the cage. Under certain circumstances such a captured atom may exhibit thermal properties that are surprisingly uncoupled from the remainder of the framework. It was one of the unexpected milestones of this project to have discovered a magnetic phase transition of the wellknown antiferromagnetic type, in an atomic crystal arrangement where it would least be expected to occur, but moreover at a temperature more than 300 times higher than what our well-established understanding of magnetic phase transitions would predict in such a compound. The compound CeRu2Al10 forms in a cage-like crystal structure with strong semi-conducting electronic correlations prevalent near room temperature, before the electrons surprisingly succumb to magnetic interactions at a temperature as high as 27 K. Detail of this cooperative phase transition remain elusive and this compound and related materials continue to attract efforts in the community devoted to understand this exceptional behaviour. Cubic crystal systems offer a special attraction in solid state physics due to their high spatial symmetry and its consequences to physical and thermal properties. In the first of two cubic compounds that we explored in pursuit of electronic correlations in cage-type structures, we have discovered the surprising phenomenon of charge ordering in the compound Yb3Ir4Ge13. The single atomic site provided for the Yb atom in this lattice does not intuitively permit such a periodic arrangement of two different valencies. This correlated state appears to be a singular occurrence in this crystal system and work is on-going to study properties of this phase transition in more detail. Finally, in the cubic complex compound Ce3Pd20Si6 our studies identified the existence in electrical transport of two different types of electronic order at very low temperatures, namely one of magnetic type (spin ordering) and the other of electric type (charge arrangement). This branch of our research has attracted much interest in the community and the physics exposed in our project has shone a light on the current topic of quantum criticality, which is the study field connected with quantum phase transitions and their astonishing behaviour at the edge of cooperative behaviour among electrons.
Publications
- Physica B 403 (2008) 746-748. R3Ir4Ge13 (R=Yb, Lu): Thermal and magnetic properties
A.M. Strydom, N. Oeschler, and F. Steglich
- Journal of Alloys and Compounds 480 (2009) 150-151. Physical phenomena of the cage compounds RE3Pd20Si (RE=Lu, Yb)
A.M. Strydom
- Kernspinresonanzuntersuchungen in Cer-basierten Kondo-Gitter-Systemen. Berlin ; mbv, Mensch-und-Buch-Verl. ; 2009 ; 150 Seiten ISBN: 978-3-86664-740-4
Eva Maria Brüning
- Physica B 404 (2009) 2981-2984. Thermal and electronic transport in CeRu2Al10: evidence for a metal-insulator transition
A.M. Strydom
- Physical Review B 80 (2009) 235107-1-235107-8. Crystal growth and composition-property relationship of Ce3Pd20Si6 single crystals
A. Prokoev, J. Custers, M. Kriegisch, S. Laumann, M. Müller, H. Sassik, R. Svagera, M. Waas, K. Neumaier, A.M. Strydom, and S. Paschen
- Physical Review B 82 (2010) 125115. Low-temperature properties of CeRu4Sn6 from NMR and specific heat measurements: Heavy fermions emerging from a Kondo-insulating state
E.M. Brüning, M. Brando, M. Baenitz, A. Bentien, A.M. Strydom, R.E. Walstedt, and F. Steglich
- Thermoelectric Transport in Correlated Electron Systems. Berlin ; mbv, Mensch-und-Buch-Verl. ; 2010 ; 158 Seiten ISBN: 978-3866648340
Stefanie Hartmann
- Physical Review B 83 (2011) 054411. Hysteretic behaviour and magnetic ordering in CeRuSn
J.A. Mydosh, A.M. Strydom, M. Baenitz, B. Chevalier, W. Hermes, R. Pöttgen
- Journal of the Physical Society of Japan 81 (2012) SB18/1-5. Magnetism and superconductivity in LnRh2Sn2 (Ln=La, Ce)
A.M. Strydom, and D. Brits
- Nature Materials 11 (2012) 189-194. Destruction of the Kondo effect in cubic heavy fermion compound Ce3Pd20Si6
J. Custers, K.-A. Lorenzer, M. Müller, A. Prokoev, A. Sidorenko, H. Winkler, A.M. Strydom, Y. Shimura, T. Sakakibara, R. Yu, Q. Si, and S. Paschen
- Physical Review B 86 (2012) 220401. Field tuned critical fluctuations in YFe2Al10: Evidence from magnetization and 27Al NMR and NQR investigation
P. Khuntia, A. Strydom, L.S. Wu, M.C. Aronson, F. Steglich, and M. Baenitz
(See online at https://doi.org/10.1103/PhysRevB.86.220401) - Z. Naturforsch. 67b (2012) 473. Sn NMR and Mössbauer Spectroscopic Studies of the Intermediate-valent Stannide CeRuSn
F. M. Schappacher, P. Khuntia, A.K. Rajarajan, M. Baenitz, J. A. Mydosh, B. Chevalier, S.F. Matar, R. Pöttgen
- Physica Status Solidi (b) 250 (2013) 630-633. Fluctuations and the ferromagnetic instability in YFe2Al10: the role of Fe Stoichiometry
A.M. Strydom, P. Khuntia, M. Baenitz, P. Peratheepan, and F. Steglich
(See online at https://doi.org/10.1002/pssb.201200823) - Physica Status Solidi B 250 (2013) 525-528. Quenching of spin fluctuations in 3d- and 4f aluminides YFe2Al10 and YbFe2Al10: a comparative 27AL-NMR and specific heat study
P. Khuntia, A.M. Strydom, F. Steglich, and M. Baenitz
(See online at https://doi.org/10.1002/pssb.201200758) - Physical Review B 87 (2013) 115129/1-4. Anisotropic optical conductivity of the putative Kondo insulator CeRu4Sn6
V. Guritanu, P. Wissgott, T. Weig, H. Winkler, J. Sichelschmidt, M. Scheffler, A. Prokoev, S. Kimura, T. Iizuku, A.M. Strydom, M. Dressel, F. Steglich, K. Held, and S. Paschen
(See online at https://doi.org/10.1103/PhysRevB.87.115129)
