Underscreened Kondo Lattice Model Research Articles

Heavy quasiparticle bands in the underscreened quasiquartet Kondo lattice

We study the quasiparticle spectrum in an underscreened Kondo-lattice (KL) model that involves a single spin degenerate conduction band and two crystalline-electric-field (CEF) split Kramers doublets coupled by both orbital-diagonal and non-diagonal exchange interactions. We find the three quasiparticle bands of the model using a constrained fermionic mean field approach. While two bands are similar to the one-orbital model a new genuinely heavy band inside the main hybridization gap appears in the quasiquartet model. Its dispersion is due to effective hybridization with conduction states but the bandwidth is controled by the size of the CEF splitting. Furthermore several new indirect and direct hybridztion gaps may be identified. By solving the selfconsistency equation we calculate the CEF- splitting and exchange dependence of effective Kondo low energy scale, hybridization gaps and band widths. We also derive the quasiparticle spectral densities and their partial orbital contributions. We suggest that the two-orbital KL model can exhibit mixed CEF/Kondo excitonic magnetism.

Ferromagnetic behavior of the Kondo lattice compoundNp2PtGa3

Here we report on a study of the ternary ${\text{Np}}_{2}{\text{PtGa}}_{3}$ compound. The x-ray-powder diffraction analysis reveals that the compound crystallizes in the orthorhombic CeCu${}_{2}$-type crystal structure (space group $Imma$) with lattice parameters $a=0.4409(2)$ nm, $b=0.7077(3)$ nm, and $c=0.7683(3)$ nm at room temperature. The measurements of dc magnetization, specific heat, and electron transport properties in the temperature range 1.7--300 K and in magnetic fields up to 9 T imply that this intermetallic compound belongs to a class of ferromagnetic Kondo systems. The Curie temperature of ${T}_{C}\ensuremath{\sim}$ 26 K is determined from the magnetization and specific-heat data. An enhanced coefficient of the electronic specific heat $\ensuremath{\gamma}$ = 180 mJ/(mol at. Np K${}^{2}$) and a $\ensuremath{-}$lnT dependence of the electrical resistivity indicate the presence of a Kondo effect, which can be described in terms of the $S=1$ underscreened Kondo-lattice model. The estimated Kondo temperature ${T}_{K}\ensuremath{\sim}24$ K, Hall mobility of $\ensuremath{\sim}$16.8 cm${}^{2}$/V s, and effective mass of $\ensuremath{\sim}$83${m}_{e}$ are consistent with an assumption that the heavy-fermion state develops in ${\text{Np}}_{2}{\text{PtGa}}_{3}$ at low temperatures. We compare the observed properties of ${\text{Np}}_{2}{\text{PtGa}}_{3}$ to that found in ${\text{Np}}_{2}{\text{PdGa}}_{3}$ and discuss their difference in regard to change in the exchange interaction between the conduction and localized 5$f$ electrons. We have used the Fermi wave vector ${k}_{F}$ to evaluate the Rudermann-Kittel-Kasuya-Yosida (RKKY) exchange. Based on experimental data of the (U, Np)${}_{2}$(Pd, Pt)Ga${}_{3}$ compounds we suggest that the evolution of the magnetic ground states in these actinide compounds can be explained within the RKKY formalism.

Application of the underscreened Kondo lattice model to neptunium compounds

The coexistence of Kondo effect and ferromagnetic order has been observed inmany uranium and neptunium compounds such as UTe or Np2PdGa3. This coexistence canbe described within the underscreened Anderson lattice model with two f-electrons and S = 1spins on each site. After performing the Schrieffer-Wolff transformation on this model, we haveobtained an effective Hamiltonian with a f-band term in addition to the Kondo interactionfor S = 1 spins. The results indicate a coexistence of Kondo effect and ferromagnetic order,with different relative values of the Kondo TK and Curie TC temperatures. We emphasize hereespecially the case TK < TC where there is a Kondo behavior below TC and a clear decrease ofthe magnetization below TK. Such a behavior has been observed in the magnetization curvesof NpNiSi2 at low temperatures.

ChemInform Abstract: The Kondo Lattice Models and Magnetism for Cerium and Uranium Systems

AbstractReview: 58 refs.

Interplay between ferromagnetism, SDW order, and underscreened Kondo lattice in UCu2Si2

Electrical resistivity and low temperature magnetoresistivity (MR) measurements made on a single crystal of UCu${}_{2}$Si${}_{2}$ are reported. By using as a phonon reference the temperature dependence of the electrical resistivity of ThCu${}_{2}$Si${}_{2}$, we could establish that UCu${}_{2}$Si${}_{2}$ has both ferromagnetic (FM) and Kondo behaviors, which can be described by the underscreened Kondo lattice model. Our MR measurements revealed the presence of magnetic fluctuations within the FM order as it had been reported before for UGe${}_{2}$. Also, one of the calculated Fermi surface sheets exhibited nesting properties, being in perfect agreement with the previous neutron diffraction data, which supports the possibility of the presence of the spin-density wave (SDW) phase. In the ternary silicide considered here in the region of temperatures where the strong FM order exists, this phase is signalized by magnetic fluctuations. The latter may emerge as an ordered antiferromagnetic (AFM) phase of an SDW-like character when the local ferromagnetism disappears at a slightly lower temperature than its ${T}_{\mathrm{N}}$. The differences in the lack or observation of these two transitions in the temperature run were found to be strongly sample dependent.

Kondo Lattice Models for Rare-Earth and Actinide Systems

We present here some works on the strong competition between the Kondo e ect, magnetic order and eventually spin glass or frustration e ect in anomalous rare-earth and actinide systems. First, we develop an underscreened Kondo lattice model with Sf = 1 spins for the 5f -electrons and we have recently improved it by deriving, by the Schrie er Wol transformation, a 5f -band with a nite bandwidth. The underscreened Kondo lattice model can account for properties of some uranium and neptunium compounds, like UTe, Np2PdGa3 or UCu2Si2 which have a large Curie temperature Tc of order 100 K and present also a Kondo behavior. In particular, we can account for the observed maximum of Tc under pressure in UTe and the magnetization curves of NpNiSi2 showing the occurrence of the Kondo e ect at low temperatures below Tc. Second, we have studied the properties of disordered cerium alloys like CeCuxNi1−x or CeRhxPd1−x by considering the Kondo e ect, a ferromagnetic order and a spin glass behavior described by several approaches. The van Hemmen approach gives a good explanation of the properties of cerium alloys and we are describing the magnetic glass clusters which occur in both spin glass and ferromagnetic phases. Third, we present a new description of a frustrated Kondo lattice model, which can account for the behavior under pressure or doping of some ytterbium compounds like Yb2Pd2Sn and YbAgGe.

Kondo Effect in the Presence of Ferromagnetism in U1-xThxNiSi2

Isostructural solid solutions of ferromagnetic UNiSi 2 and non-magnetic ThNiSi 2 were studied in a polycrystalline form in wide temperature and magnetic field ranges by means of magnetization, specific heat and electrical resistivity measurements. The experiments revealed that the value of the effective magnetic moment of uranium in U 1- x Th x NiSi 2 (0 \leqslant x < 1) does not change significantly upon expansion of the unit cell volume. Clear anomalies in temperature dependencies of the magnetization, the specific heat and the electrical resistivity of the alloys indicate suppression of the Curie temperature with decreasing U content from 95 K in UNiSi 2 down to about 3 K in U 0.1 Th 0.9 NiSi 2 . The electrical resistivity of all the samples exhibits in the paramagnetic range a negative logarithmic temperature dependence, characteristic of the Kondo effect. The observed behavior of the system U 1- x Th x NiSi 2 is in line with predictions of the underscreened Kondo lattice model.

The Kondo Lattice Models and Magnetism for Cerium and Uranium Systems

The interplay between the Kondo effect and magnetism is important in anomalous rare-earth and actinide systems and a brief review of the different Kondo lattice models is presented here. The first studied case is the classical Kondo lattice model with S = 1/2 spins for the 4f localized electrons, which describes the strong competition between the Kondo effect and magnetic ordering leading to small ordering temperatures less than roughly 10 K. The second reviewed case describes the underscreened Kondo lattice model with S = 1 spins for the 5f localized electrons, which can account for the coexistence of both the Kondo effect and the ferromagnetic order and, therefore, the large Curie temperatures observed in actinide compounds like UTe and NpNiSi2; the question of the delocalization of the 5f electrons is also discussed here. The last case describes the competition between the “spin glass” behaviour, the Kondo effect and the magnetic ordering in disordered cerium alloys like CeNixCu1−x or CeRhxPd1−x and several models used for the spin glass state are reviewed, as well as the occurrence of magnetic glass clusters.

Underscreened Kondo lattice model versus underscreened Anderson lattice model: Application to uranium compounds

Abstract We present here two theoretical models, the underscreened Kondo lattice (UKL) model and the underscreened Anderson lattice (UAL) model, which are appropriate models for a description of the coexistence of Kondo effect and ferromagnetic order experimentally observed in various uranium compounds. First we discuss a Kondo-ferromagnetic diagram (KFD), obtained in the framework of the UKL model with S = 1 [N.B. Perkins, M.D. Nunez-Regueiro, J.R. Iglesias, B. Coqblin, Phys. Rev. B 76 (2007) 125101]. The KFD differs significantly from the analogous Doniach diagram for the regular Kondo lattice with S = 1 2 . While both the Kondo and Curie temperatures in KFD increase with an increase of the intrasite Kondo coupling, J K , the Neel temperature in the Doniach diagram decreases with increase of J K . Ferromagnetism is not suppressed by the Kondo interaction, but instead the coexistence between the two phenomena takes place. Then, we compare the obtained phase diagram with available experimental data. Indeed, the predicted behavior has been observed in some uranium compounds. However, there are certain difficulties in describing the pressure dependence of the Curie temperature of UTe, which firstly passes through a maximum and then decreases at higher pressures. This pressure dependence of the Curie temperature clearly indicates that the character of 5f electrons changes with pressure, becoming more and more itinerant. Then, the UKL model cannot capture the change of localization of the 5f electrons. In this case, the underscreened Anderson lattice (UAL) model, taking explicitly into account the possibility of the formation of a f-band, becomes more appropriate. It allows to treat the case of a non-integer number of f electrons and a finite f bandwidth. We present here the two-orbital UAL model and discuss some preliminary results.

The Schrieffer–Wolff transformation for the underscreened Anderson lattice

Abstract We present a derivation of the Schrieffer–Wolff transformation for the Anderson Lattice Hamiltonian with a two-fold degenerate f-level in each site. The degeneracy of the f-electrons has been taken into account in order to describe uranium and other actinide magnetic compounds with a spin larger than 1 2 , for example a total S = 1 spin for the f-electrons. The transformed Hamiltonian has several terms as in the s = 1 2 classical case, but we have obtained here both an exchange (Kondo) interaction between the S = 1 f-spins and the spins of the conduction electrons, and also an effective f-band term. This f-band term describes better the underscreened Kondo lattice model which has been recently developed to explain the Kondo-ferromagnetism coexistence observed in uranium compounds such as UTe [N.B. Perkins, M.D. Nunez-Regueiro, J. R. Iglesias, B. Coqblin, Phys. Rev. B 76 (2007) 125101].

Kondo Lattice and Magnetic Properties in Strongly Correlated Electron Systems

We review here the magnetic properties of some strongly correlated electron systems and especially cerium and uranium compounds. The normal Kondo lattice model with a localized S = 1=2 spin can account for the Kondo-magnetism competition observed in cerium or ytterbium systems, while the underscreened Kondo lattice model with a localized S = 1 spin can well account for the ferromagnetism | Kondo coexistence observed in some uranium compounds such as UTe. Then, we discuss the spin glass-Kondo competition and present the resulting phase diagrams showing spin glass, Kondo and magnetically ordered phases observed in disordered heavy fermion cerium alloys such as CeNixCu1ix alloys.

Magnetic properties of NpNiSi2

NpNiSi2 represents the first transuranium-based system in the isostructural family withrare earths and uranium homologues that crystallizes into the orthorhombicCmcm structure. It exhibits strong anisotropic ferromagnetic ordering belowTC = 51.5 K with the easy magnetization axis along the shortest(c) axis. The ordered magnetic moment inferred from237Np Mössbauer spectroscopyamounts to μNp = 1.4 μB. Theisomer shift value of 6.8 mm s−1 versus NpAl2 suggests that the neptunium ions are in theNp3+ charge state withelectronic configuration 5f4 (J = 4). From the values of the quadrupolar interaction parameters, thedirection of the main principal axis of the electric field gradient(VZZ) is deduced to bealong the b axis. TheSommerfeld coefficient γ = 133 mJ mol−1 K−2 obtained from the low temperature behaviour of specific heat qualifiesNpNiSi2 as a moderately heavy fermion. Finally, the logarithmic decrease of thehigh temperature resistivity and the reduced (compared to the freeNp3+ ion) magnetic moments (both ordered and effective) point to the occurrence of a Kondo effect.NpNiSi2 hence appears as a good candidate for being described with the underscreened Kondolattice model of Perkins et al (2007 Phys. Rev. B 76 125101) which accounts for thecoexistence of the Kondo effect and ferromagnetic order.

Ferromagnetic Doniach diagram for the underscreened Kondo lattice

A coexistence between the Kondo effect and the ferromagnetic order has been observed in some Uranium compounds such as UTe or UCu 0.9Sb 2, which order ferromagnetically at roughly 100 K and present a Kondo resistivity decrease above it. Recently we have developed the underscreened Kondo lattice (UKL) model, where the competition between the ferromagnetic order and the underscreened Kondo effect is studied by considering a Kondo coupling J K of localized S f = 1 spins with a conduction band and an inter-site f–f ferromagnetic exchange interaction J H . Here we present a diagram for the UKL model, which appears to be significantly different from the Doniach diagram for the Kondo lattice model with localized spins S = 1 2 . We obtain that the ferromagnetic order exists for all values of the ratio J K / J H , while the Kondo-ferromagnetism coexistence takes place only for sufficiently large values of the ratio J K / J H . This new ferromagnetic diagram for the UKL model can, therefore, account for the strong Kondo-ferromagnetism coexistence observed in some Uranium compounds.

Underscreened Kondo lattice model applied to heavy fermion uranium compounds

We present theoretical results for the underscreened Kondo lattice model with localized $S=1$ spins coupled to a conduction band through a Kondo coupling, ${J}_{K}$, and interacting among them ferromagnetically. We use a fermionic representation for the spin operators and expand the Hamiltonian in terms of bosonic fields. For large values of ${J}_{K}$, we obtain a ferromagnetically ordered solution and a Kondo regime with a Kondo temperature, ${T}_{K}$, larger than the Curie temperature, ${T}_{C}$. This finding suggests a scenario for a coexistence of Kondo effect and ferromagnetic order. In some uranium compounds, such as UTe or $\mathrm{U}{\mathrm{Cu}}_{0.9}{\mathrm{Sb}}_{2}$, this kind of coexistence has been experimentally observed: they order ferromagnetically with a Curie temperature of order ${T}_{C}\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and exhibit a Kondo behavior for $T&gt;{T}_{C}$. The proposed underscreened Kondo lattice model accounts well for the coexistence between magnetic order and Kondo behavior and yields to a new ``ferromagnetic Doniach diagram.''

Underscreened Kondo lattice model at finite temperature

We have developed an underscreened Kondo lattice model with an intra-site Kondo exchange interaction J K between the conduction band and localized 5f electrons—described by S = 1 spins in the f 2 configuration—and an inter-site ferromagnetic exchange f–f interaction J H . Using a fermionic representation for the spin operators and a mean-field approximation we obtain, from T = 0 up to a critical temperature, a Kondo-ferromagnetic coexistence. This is in agreement with the experimental situation of some Uranium compounds, such as UTe or UCu 0.9 Sb 2 , which order ferromagnetically with a Curie temperature of order 100 K and present also a Kondo behavior. The Curie and Kondo temperatures are also determined.

A [formula omitted] underscreened Kondo lattice model

The underscreened Kondo lattice model presented here includes both an intra-site Kondo exchange interaction J K between the conduction band and localized 5 f electrons described by S = 1 spins, and an inter-site exchange f – f interaction J H . We write both localized and itinerant spins in a Fermionic representation, and then use a mean-field approximation. We obtain a coexistence of Kondo effect and magnetism which can account for the behavior of some Uranium compounds.

Singular dynamics and pseudogap formation in the underscreened Kondo impurity and Kondo lattice models

We study a generalization of the Kondo model in which the impurity spin is represented by Abrikosov fermions in a rotation group SU(P) larger than the SU(N) group associated to the spin of the conduction electrons, thereby forcing the single electronic bath to underscreen the localized moment. We demonstrate how to formulate a controlled large N limit preserving the property of underscreening, and which can be seen as a ``dual'' theory of the multichannel large N equations usually associated to overscreening. Due to the anomalous scattering on the uncompensated degrees of freedom, the Fermi liquid description of the electronic fluid is invalidated, with the logarithmic singularities known to occur in the S=1 SU(2) Kondo impurity model being replaced by continuous power laws at N=\infty. The present technique can be extended to tackle the related underscreened Kondo lattice model in the large N limit. We discover the occurence of an insulating pseudogap regime in place of the expected renormalized metallic phase of the fully screened case, preventing the establishement of coherence over the lattice. This work and the recent observation of a similar weakly insulating behavior on transport in CeCuAs_2 should give momentum for further studies of underscreened impurity models on the lattice.

The underscreened Kondo lattice model: application to uranium compounds

Abstract We study here the underscreened Kondo-lattice model, where the localized spins S=1 are not completely screened by the s= 1 2 conduction electron spins. It results a coexistence of a 5f magnetic order and a Kondo effect which can qualitatively account for the behavior of some uranium compounds.