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Thermoelectric Properties of Metallic Materials
Published in D.M. Rowe, CRC Handbook of Thermoelectrics, 2018
At very low temperatures impurity scattering dominates all other scattering processes. One of the most interesting effects of the electron-impurity interaction is the Kondo effect. This will be briefly discussed in the last section of the chapter. In general, at the lowest temperatures the thermoelectric power tends to zero with decreasing temperature.
Unconventional Superconductivity in Heavy Fermion and Ruthenate Materials
Published in David A. Cardwell, David C. Larbalestier, I. Braginski Aleksander, Handbook of Superconductivity, 2023
At high temperature (above 10 to 50 K typically), the partially filled f-orbitals in heavy fermion systems produce Curie–Weiss susceptibility, showing that they are behaving as independently fluctuating local moments. Moreover, the electrical resistivity is high (typically >100μΩcm) due to strong scattering of the conduction electrons by these incoherently fluctuating local moments. As T→0, the kBln2 entropy of the local moments must fall to zero. In the vast majority of f-electron materials this happens through a phase transition to static antiferromagnetic or ferromagnetic long-range order, but this tendency to magnetic order is weakened if the f-shell is nearly empty (as in cerium or uranium) or nearly full (as in ytterbium). In heavy fermion systems magnetic order either doesn't set in, or else it is very weak. Instead, the fluctuations of the magnetic moments somehow become coherent, achieving a low entropy state without static order, and the resistivity falls dramatically. Historically, this has been ascribed to the Kondo effect, by which isolated magnetic moments in metals become screened at low temperature by forming a spin-singlet state with an electron from the conduction electron sea. The true picture is probably more complex than this, and recently other models, such as the two-fluid Kondo lattice model of Pines and collaborators [105], have been under development.
Magnetism and Transport in DMS Quantum Dots
Published in Evgeny Y. Tsymbal, Igor Žutić, Spintronics Handbook: Spin Transport and Magnetism, Second Edition, 2019
Joaquín Fernández‑ Rossier, Ramón Aguado
As the tunneling to the reservoirs becomes larger, namely as the resistance of the barriers approaches the quantum of resistance, higher-order tunneling events become relevant. In this situation, quantum fluctuations dominate transport and electrons are allowed to tunnel via intermediate virtual states where first-order tunneling would be suppressed. Thus, the intrinsic width of the energy levels of the quantum dot does not only include contributions from direct elastic tunneling but also tunneling via virtual states. These higher-order tunneling events are referred to as co-tunneling processes. These higher-order tunneling events lead to spectacular effects when the spin of the electrons is also involved. Importantly, a quantum dot with a net spin coupled to electron reservoirs resembles a magnetic impurity coupled to itinerant electrons in a metal and, thus, can exhibit the Kondo effect. In this regime, quantum fluctuations induce an effective exchange coupling J≈ΓEc|Δε||Δε+Ec| between the quantum dot spin and the reservoir ones. The spin and the accompaning exchange result in a complete screening of the quantum dot singly, the ground state becomes a singlet between confined and itinerant carriers. This mechanism gives rise to a new many-body resonance in the density of states of the quantum dot around the Fermi energy of the reservoirs. The most remarkable manifestation of this new “Kondo” resonance is the transition from near-zero conductance due to Coulomb Blockade to perfect transmission as the temperature is lowered well below the so-called Kondo temperature TK∼e−π|Δε||Δε+EC|2ΓEC. Clear experimental signatures of Kondo physics have been reported in various types of quantum dots as well as other systems which can be understood in terms of a SET setup. The latter includes molecules, break junctions, etc. For a short review on the Kondo effect in quantum dots [86].
Possible observation of Kondo screening cloud in Yb14MnSb11 *
Published in Philosophical Magazine, 2020
Brian C. Sales, V. O. Garlea, M.B. Stone, M. D. Lumsden, S. E. Nagler, D. Mandrus, M. A. McGuire
For over 50 years there has been sustained interest in how conduction electrons screen local magnetic moments, a process normally referred to as the Kondo effect [1,2]. In metals with a dilute concentration of magnetic impurities the problem is well understood and models that capture the essence of the many body physics have been solved exactly [3–5]. The Kondo effect is at the heart of heavy fermion physics and is related to many problems in the physics of strongly correlated materials. Recent interest has focussed on combining Kondo physics and topology. There have been proposals of a topological Kondo effect with Majorana fermions [6] and examples of using spin–orbit coupling to tune a Kondo insulator into a correlated Weyl semimetal [7,8].
Magnetic, transport and thermal properties of δ-phase UZr2
Published in Philosophical Magazine Letters, 2021
Xiaxin Ding, Tiankai Yao, Lyuwen Fu, Zilong Hua, Jason Harp, Chris Marianetti, Madhab Neupane, Michael E. Manley, David Hurley, Krzysztof Gofryk
The temperature dependence of the electrical resistivity of δ-UZr is shown in Figure 3a. The overall shape and magnitude of is typical for uranium intermetallic compounds [22, 45]. The residual-resistivity ratio (RRR), defined as is low and estimated to be ∼1.05. This indicates that δ-UZr is an electronically disordered system, consistent with the disorder in its crystal structure. In general, the low-temperature electron scattering on defects and dislocations results in just a shift in the electrical resistivity towards higher value and hence lowering the RRR value. It will not, however, affect the temperature dependence of resistivity. Besides the s-shaped [22], there is an upturn at low temperatures with the resistivity minimum at 15 K. The low-temperature resistivity upturn, observed in 4f- and 5f-electron materials is usually associated with Kondo effect [46, 47]. However, in δ-UZr, this seems to be unlikely because the magnetic susceptibility shows no signatures of localised 5f-electrons and the magnetoresistance (MR) is small and positive (see below). Interestingly, the low-temperature resistivity upturn and positive MR have also been observed in ThAsSe [48] and M-As-Se (M = Zr, Hf, Th) phases [49]. Such behaviour has been interpreted as a signature of the non-magnetic Kondo effect. However, to draw any firm conclusions on the nature of the low temperature behaviour in this material, more studies are required. The inset of Figure 3a shows the magnetic field dependence of MR, defined as , where is the resistivity under zero magnetic field. The value of MR exhibits a very weak field dependence and, at 2 K and 8 T, it reaches only 0.2%. The red line is a fit of to the experimental data, where A and b are fitting parameters. The analysis gives b = 1.2 which is smaller than the value of 2 that is observed in normal metals.