Kondo Alloys Research Articles

Spin-current Kondo effect: Kondo effect in the presence of spin accumulation

We present a detailed theoretical description of the influence of the spin accumulation in metallic Fermi leads on the Kondo effect in systems such as quantum dots and Kondo alloys. We discuss an interplay of the spin accumulation, magnetic field, and ferromagnetic leads spin polarization on the Kondo spin-dependent densities of states, conductance, and resistance. It has been shown that the presence of the above-mentioned factors by breaking the spin symmetry leads to the suppression of the Kondo effect. However, for appropriately selected parameter values, these effects can compensate each other, which may lead to the restoration of the Kondo effect in the analyzed systems. We also address some recent experiments related to the spin current in the Kondo alloys.

Photo-emission signatures of coherence breakdown in Kondo alloys: dynamical mean-field theory approach

We study the Kondo alloy model on a square lattice using dynamical mean-field theory for Kondo substitution and disorder effects, together with static mean-field approximations. We computed and analyzed photoemission properties as a function of electronic filling n c , Kondo impurity concentration x, and strength of Kondo temperature T K. We provide a complete description of the angle resolved photoemission spectroscopy (ARPES) signals expected in the paramagnetic (PM) Kondo phases. By analyzing the Fermi surface (FS), we observe the Lifshitz-like transition predicted previously for strong T K at x = n c and we discuss the evolution of the dispersion from the dense coherent to the dilute Kondo regimes. At smaller T K, we find that this transition marking the breakdown of coherence at x = n c becomes a crossover. However, we identify another transition at a smaller concentration x ⋆ where the effective mass continuously vanishes. x ⋆ separates the one-branch and the two-branches ARPES dispersions characterizing respectively dilute and dense Kondo PM regimes. The x − T K phase diagrams are also described, suggesting that the transition at x ⋆ might be experimentally observable since magnetically ordered phases are stabilized at much lower T K. FS reconstructions in antiferromagnetic and ferromagnetic phases are also discussed.

Phase diagrams of Kondo alloys

We analyze the ground state phase diagrams of the Kondo alloy model (KAM) as a function of Kondo coupling (JK), electronic band filling (nc) and magnetic impurity concentration (x). We use DMFT/CPA method to treat the disorder and the inter-site electronic exchange correlations along with mean-field (MF) approximations to decouple the Kondo interaction. Here, we generalize “Doniach-like” phase diagrams of the periodic Kondo lattice (x=1) to Kondo alloys. We consider the competition between the paramagnetic Kondo phase (K) and purely ferromagnetic (F) or staggered antiferromagnetic (AF) ordered phases. Our phase diagrams are consistent with various experimentally obtained phase diagrams for alloys like CexLa1-xPt2Si2, CexLa1-xNi2Ge2, CexLa1-xRu2Si2, CexLa1-xPd2Si2, or CexLa1-xCu2Ge2.

Heavy fermion behavior in Pr0.5Ir4Sb10.2Sn1.8

We report on the properties of single crystals of Pr0.5Ir4Sb10.2Sn1.8, a new partially-filled rare earth skutterudite which exhibits Kondo alloy and heavy fermion behavior. Using magnetic susceptibility, specific heat, and resistivity measurements we find evidence of Kondo-screening of the f-moments. This material shows a Kadowaki-Woods ratio consistent with heavy fermion behavior. Susceptibility measurements predict a Kondo temperature TK≈4K, however the minimum found in the low temperature resistivity appears at a higher temperature T≈10K. A small fraction of the material (<0.5%) becomes superconducting with a critical temperature Tc≈4.2K.

Breakdown of coherence in Kondo alloys: crucial role of concentration versus band filling

We study the low energy states of the Kondo alloy model (KAM) as function of the magnetic impurity concentration per site, x, and the conduction electron average site occupation, nc. In previous works, two different Fermi liquid regimes had been identified at strong Kondo coupling , that may be separated by a transition at x = nc. Here, we analyze the KAM for finite on a Bethe lattice structure. First, using the mean-field coherent potential approximation (DMFT-CPA) which is exact at lattice coordination , we show that the real part of the local potential scattering may be located outside the conduction electron band, revealing a possible breakdown of Luttinger theorem for intermediate values of impurity concentration x. Unusual physical signatures are expected, e.g. in ARPES experiments. In order to take into account fluctuations associated with finite dimensionality, i.e. finite Z, we extend this analysis by studying the KAM with an adaptation of the statistical-DMFT method that was developped elsewhere. We review the distributions of local potential scattering and their evolution with model parameters: concentration, strength of Kondo coupling, coordination number, local site neighborhood, connection with percolation issue. Relevence for Kondo alloys material with f -electrons is also discussed.

Magnetic susceptibility of Dirac fermions, Bi-Sb alloys, interacting Bloch fermions, dilute nonmagnetic alloys, and Kondo alloys

Wide ranging interest in Dirac Hamiltonian is due to the emergence of novel materials, namely, graphene, topological insulators and superconductors, the newly-discovered Weyl semimetals, and still actively-sought after Majorana fermions in real materials. We give a brief review of the relativistic Dirac quantum mechanics and its impact in the developments of modern physics. The quantum band dynamics of Dirac Hamiltonian is crucial in resolving the giant diamagnetism of bismuth and Bi-Sb alloys. Quantitative agreement of the theory with the experiments on Bi-Sb alloys has been achieved, and physically meaningful contributions to the diamagnetism has been identified. We also treat relativistic Dirac fermion as an interband dynamics in uniform magnetic fields. For the interacting Bloch electrons, the role of translation symmetry for calculating the magnetic susceptibility avoids any approximation to second order in the field. The expressions for magnetic susceptibility of dilute nonmagnetic alloys give a firm theoretical foundation of the empirical formulas used in fitting experimental results. The unified treatment of all the above calculations is based on the lattice Weyl-Wigner formulation of discrete phase-space quantum mechanics. For completeness, the magnetic susceptibility of Kondo alloys is also given since Dirac fermions in conduction band and magnetic impurities exhibit Kondo effect.

Direct evidence for suppression of the Kondo effect due to pure spin current

We study the effect of a pure spin current on the Kondo singlet in a diluted magnetic alloy using non-local lateral spin valve structures with highly spin polarized Co2FeSi electrodes. Temperature dependence of the non-local spin signals shows a sharp reduction with decreasing temperature, followed by a plateau corresponding to the low temperature Fermi liquid regime below the Kondo temperature (TK). The spin diffusion length of the Kondo alloy is found to increase with the evolution of spin accumulation. The results are in agreement with the intuitive description that the Kondo singlet cannot survive any more in sufficiently large spin accumulation even below TK.

Hidden Spin Scattering Behavior in [Ni1−xCux(tmdt)2] System with Unprecedentedly Strong π–d Interaction

We have measured the electrical resistivity and magnetoresistance of [Ni1−xCux(tmdt)2] (x = 0 and x ≈ 0.10) single crystals to investigate the role of the π–d exchange interaction, Jπ–d, where tmdt stands for trimethylenetetrathiafulvalenedithiolate. At 0 T, the difference in the resistivity between x ≈ 0.10 and x = 0 as a function of temperature, \(\rho _{\text{mag}}(T)\), increases logarithmically with decreasing temperature expected for a Kondo alloy. However, it does not show a logarithmic divergence but tends to saturate at a constant value. For H > 0, \(\rho _{\text{mag}}(T,H)\) is quite field-dependent, which is very similar to that observed in other Kondo compounds. Assuming that the effect of magnetic field on \(\rho _{\text{mag}}(T)\) is attributed to the suppression of the Kondo effect, the Kondo magnetic field \(H_{\text{K}} \sim 7\) T and the Kondo temperature \(T_{\text{K}} \sim 5\) K are found, which is consistent with the previous report on the magnetic susceptibility measurements. The π–d interaction of \(J_{\pi {\text{--}}d} \approx - 30\) meV is obtained using the relation \(k_{\text{B}}T_{\text{K}} \approx E_{\text{F}}\exp( - 1/|J_{\pi {\text{--}}d}|N(E_{\text{F}}))\), where \(N(E_{\text{F}})\) is the density of states at the Fermi energy EF.

Thermal conductivity and Lorenz number of the Ce1−xLaxNiAl4 Kondo alloys

The temperature dependence of the thermal conductivity (κ) for Ce1−xLaxNiAl4 alloys has been studied. In comparison to LaNiAl4, the substitution of La by Ce reduces the κ values. The behaviour of the Lorenz number L in the Kondo alloys Ce1−xLaxNiAl4 differs considerably both in magnitude and in temperature dependence from L characteristic of pure metals. The Lorenz number of Ce1−xLaxNiAl4 exhibits positive values and peaks at low temperatures. The obtained values of L at low temperatures are higher than the theoretical value of L0. The reason for the increased Lorenz number may be the contribution of phonons, Kondo and crystal electric field (CEF) effects. The values of L/L0 are close to 1 at high temperatures, which is in agreement with the conclusion that the electronic part κe(T) dominates at high temperatures. Such a temperature dependence of L/L0 is commonly observed in Kondo compounds. Applying an external magnetic field has no significant effect on the thermal conductivity below 30K. Above this temperature a gentle changes of κ can be observed.

Phenomenon of Maximum and Minimum in the Resistivity of Cu-Mn Kondo Alloys at Low Temperatures

A New analysis of the electrical resistivity has been studied for Cu-Mn Kondo alloys, and it was found a solution like a logarithm power series between resistivity and temperature, which cover all ranges of maximum and minimum in the resistivity and all kinds of interactions at low temperatures. The s-d exchange integral Jsd acting the main role in Kondo effect Phenomenon, Jsd is negative or positive, according to a kind of interaction between conduction electron spins of the host normal metal and localized spins of the transition element. PACS: 75: 50, 75:10, 75: 40, 75: 30.

Lifshitz Transition in Kondo Alloys

We study the low-energy states of Kondo alloys as a function of the magnetic impurity concentration per site x and the conduction electron average site occupation n(c). Using two complementary approaches, the mean-field coherent potential approximation and the strong-coupling limit, we identify and characterize two different Fermi-liquid regimes. We propose that both regimes are separated by a Lifshitz transition at x=n(c). Indeed, we predict a discontinuity of the number of quasiparticles that are enclosed in the Fermi surface. This feature could provide a scenario for the non-Fermi liquid properties that were recently observed in Kondo alloy systems around x=n(c).

Heavy-fermion superconductivity in the Kondo-lattice system CeCu2Si2

The formation of “heavy fermions” (HFs), i.e., charge carriers with largely enhanced effective mass, in Ce-based Kondo lattice (KL) systems is recalled. In contrast to the disastrous pair-breaking effect of magnetic Ce3+ impurities in certain superconducting Kondo alloys, a periodic dense array of Ce3+ ions is a prerequisite for the generation of HF superconductivity (SC) below Tc = 0.6 K in the KL system CeCu2Si2. Subsequent to this discovery in 1979, more than 35 other Ce-, Pr-, Yb-, U-, Np- and Pu-based HF superconductors (with maximum Tc = 18.5 K) have been found. These materials exhibit a variety of fascinating properties, e.g., even- or odd-parity Cooper pairs, multi-phase diagrams or the coexistence with magnetic order. In particular, the interplay between HF SC and quantum criticality is of high timely interest. CeCu2Si2 has emerged as a prototype system showing a three-dimensional (3D) spindensity-wave (SDW) quantum-critical point. Recent results of inelastic-neutron-scattering experiments on this compound are briefly discussed. They lend strong support to the perception of Cooper pairs mediated by nearly quantum-critical 3D-SDW fluctuations covering a wide frequency range.

How Are Heavy and Itinerant Electrons Born in a Dilute Kondo Alloy?

We report a continuous evolution of Fermi surface properties with temperature in a dilute Kondo alloy of Ce 0.02 La 0.98 Ru 2 Si 2 via measurements of the de Haas–van Alphen (dHvA) effect. The temperature variation of resistivity in this alloy exhibits a typical impurity Kondo behavior, i.e., the resistivity increases with decreasing temperature proportionally to -log T and becomes nearly constant below about 0.3 K. The Kondo temperature of this alloy is estimated to be about 1.3 K. The frequency of the dHvA oscillation from the main hole surface decreases with decreasing temperature implying that the hole surface shrinks with decreasing temperature. The temperature dependence of the frequency change is found to be similar to that of resistivity. The temperature dependence of this dHvA oscillation amplitude deviates largely from that expected from the Lifshitz–Kosevich formula conventionally employed to determine the effective mass from the temperature dependence. By assuming that the scattering of the co...

Thermal conductivity of Ce1−xLaxCu4Al Kondo alloys

Experimental results of thermal conductivity (κ) for Ce1−xLaxCu4Al alloy series are reported. The hexagonal CeCu5-type structure was confirmed by the powder X-ray diffraction technique. The measured thermal conductivity increases with increasing temperature. The reduced Lorentz number L/L0 of Ce1−xLaxCu4Al increases with cooling and reaches a maximum value at low temperatures. The magnetic contribution to the thermal resistivity Wmag was separated, and it was shown that the product WmagT has a logT-behavior, suggesting of Kondo effect. The log T-behavior for the WmagT is quite similar to that observed as for magnetic resistivity ρmag.

How Are Heavy and Itinerant Electrons Born in a Dilute Kondo Alloy?

How Are Heavy and Itinerant Electrons Born in a Dilute Kondo Alloy?

Thermopower coefficient of pine-wood biocarbon preforms and the biocarbon-preform/copper composites

The thermopower S(T) of the composites prepared by filling empty sap channels in high-porosity biocarbon preforms of white pine wood by melted copper in vacuum and the thermopower S(T) of these preforms have been measured in the temperature range 5–300 K. The biocarbon preforms have been obtained by pyrolysis of pine wood in an argon flow at two carbonization temperatures, 1000 and 2400°C. An analysis of the experimental data has demonstrated that the thermopower of the composites is determined by the contribution related to the copper filling the channels of the biocarbon preform and exhibits a characteristic temperature dependence with a deep minimum close to 20 K. This suggests that copper in the preform channels is essentially a Kondo alloy with the iron and manganese impurities entering into it from the carbon preform in the course of infiltration of melted copper.

Why Y b xLu 1− xAl 3 and Y b xY 1− xInCu 4 have quite opposite concentration dependence of the Vickers microhardness

We report on the results of the Vickers microhardness measurements of the Y b x Lu 1− x Al 3 alloy system. It is shown that the Mott–Nabarro theory is not able to explain the concentration dependence of the measured microhardness, as is possible in some similar Kondo alloy systems containing Ce and U. Interestingly enough, while microhardness increases with the Yb content in Y b x Lu 1− x Al 3, it decreases with Yb in the Y b x Y 1− x InCu 4 alloy system. Therefore, we try to explain the experimental data, taking into account the difference of the electronic structure and the change of the electronic structure with alloying in both systems.

Dilution and non-Fermi-liquid effects in the CePtIn Kondo lattice

Measurements of electrical resistivity(ρ(T)), magnetoresistivity (MR), magnetic susceptibility(χ(T)) and heatcapacity (CP(T)) arepresented for the (Ce1−xLax)PtIn alloy system of which the CePtIn parent is a known dense Kondocompound that does not order magnetically down to 50 mK.χ(T) for alloys0≤x≤0.8 exhibits Curie–Weissbehaviour. ρ(T) results indicate a transition from a dense Kondo behaviour for0≤x≤0.2 to a single-ion Kondoregion (0.3≤x≤0.8). The Kondoenergy scale as given by TK values calculated from MR studies and by the temperatureTmaxρ(mag) where the magneticcontribution to ρ(T) exhibits a maximum value, is compared with theoretical models. It is shown that theexperimental results not only depend on a volume effect as given by the compressibleKondo lattice model of Lavagna but in addition confirm the more complex behaviourrecently presented by Burdin and Fulde for a Kondo alloy system in which the magnetic(Ce) and non-magnetic (La) atoms are distributed randomly. Non-Fermi-liquidbehaviour is predicted by Burdin and Fulde at certain critical concentrationsof the alloy system and experimental evidence for this is presented throughχ(T),ρ(T) and CP(T) measurements.

Random Kondo alloys investigated with the coherent potential approximation

The interplay between the Kondo effect and disorder is studied. This is done by applying a matrix coherent potential approximation and treating the Kondo interaction on a mean-field level. The resulting equations are shown to agree with those derived by the dynamical mean-field theory. By applying the formalism to a Bethe tree structure with infinite coordination, the effects of diagonal and off-diagonal disorder are studied. Special attention is paid to the behavior of the Kondo and the Fermi-liquid temperature as function of disorder and concentration of the Kondo ions. The nonmonotonous dependence of these quantities is discussed.

From Kondo impurities to heavy-fermion superconductivity and quantum critical points

Early results on the dilute Kondo alloy (La, Ce)Al 2 as well as on the Kondo-lattice system CeAl 2 are discussed, before previous and more recent investigations on CeCu 2Si 2, the first heavy-fermion superconductor, are addressed. After a brief review of the highly unusual properties of the non-Fermi-liquid metal YbRh 2Si 2, a short remark concerning the interplay between quantum criticality and heavy-fermion superconductivity concludes the paper.