Kondo State Research Articles

Paired Distortion and Anomalies in Kondo Lattice CeB6

Long-standing anomalies concerning hidden orderings in Ce x La 1- x B 6 competing with the Γ 8 quartet Kondo state are solved by introducing single-site dynamical and pair distorted Jahn-Teller effects. The latter is stronger and enhanced by an increased magnetic field. These characteristic Jahn-Teller effects are common in RB 6 and are related to the soft optical R phonon mode and begin at a temperature corresponding to about half of that of the optical R phonon, causing a long tail of specific heat and a strongly frustrated long-range ordering of paired systems, which is expected to be a [1/2, 1/2, 1/2] charge dipolar ordering accompanied by the same magnetic dipole ordering, which is particularly enhanced under a magnetic field. This model clearly explains long-standing anomalies in CeB 6 , in particular, the anomalous phase diagram and discrepancies between NMR and neutron scattering measurements.

Pressure-Induced Non Fermi Liquid Behavior near a Magnetic Instability.

Electrical resistivity of CePtSi2 (TN=1. 7 K), CePtGa (TN=3. 5 K) and Y1-x UxPd3 (x=0. 198) have been measured at high pressure. It is found that the antiferromagnetic ordering of the two Ce compounds is suppressed by pressure, i. e., Neel temperature TN becomes zero at a critical pressure Pc to show a non fermi liquid behavior above Pc. By pressurizing more above Pc, the electronic states show a crossover from non Fermi liquid (NFL) to Fermi liquid (FL) states or Kondo state. On the other hand for U-compound which shows NFL properties at ambient pressure a crossover from NFL to FL is also observed by applying pressure. These facts indicate that the NFL state becomes unstable at high pressure. The electronic states of these compounds show a general trends of magnetic ordering → NFL → FL as pressure increases.

Kondo State of CeAl2 under High Pressure.

The electrical resistivity of single-crystalline CeAl2 has been measured under high pressure up to 2 GPa in the temperature range 2-280 K. It is found that the magnetic contribution to the resistivity of CeAl2 has two peaks at T1 (=5 K) and Tm (=70 K) and these temperatures decrease with increasing pressure. The behavior of Tm at high pressure is attributed to the pressure dependence of the Kondo temperature and the splitting energy of 4f levels by the crystalline electric field. The Kondo state of CeAl2 under high pressure is briefly discussed on the basis of thermodynamics.

TOF neutron powder diffraction studies on f-electron systems

The occurrence of quadrupolar ordering in tetragonal TmAu 2 was investigated by neutron powder diffraction. The present work revealed that a structural transition to an orthorhombic form occurs simultaneously. CeCu 6 is a typical dense Kondo lattice compound. Appearance of a new Kondo state in CeInCu 5 was studied crystallographically. The structure parameters of CeInCu 5 show that only one of 4c sites of space group Pnma is fully occupied by In and that the rest of the four sites are completely by Cu. The decrease in one of the CeCu(5) distances on In doping was found, which might be related to the enhancement of T K.

The electrodynamic response of heavy-electron materials with magnetic phase transitions

We have investigated the electrodynamic response of the heavy-electron compounds U2Zn17, UPd2Al3, UCu5 and URu2Si2. Particular emphasis has been devoted to the optical evidence of the antiferromagnetic phase transitions at TN = 9.7 K, 14 K, 15 K and 17 K for U2Zn17, UPd2Al3, UCu5 and URu2Si2, respectively. In the excitation spectrum of UCu5 and URu2Si2, we found an absorption in the far-infrared, which develops below TN and is ascribed to the excitation across a spin-density-wave type gap, suggesting that the antiferromagnetic phase transition might be itinerant in nature, and invokes a Fermi surface instability. Since this gap-like feature is absent in U2Zn17 and UPd2Al3, we argue that these latter compounds belong to a characteristically different class of antiferromagnets, representative of the heavy-electron compounds with an ordering of essentially localized magnetic moments. The antiferromagnetic ordering then leads to a suppression of the spin-flip mechanism below TN. At low temperatures, we observe for all compounds the formation of a narrow Drude-like resonance in the optical conductivity, which is ascribed to the electrodynamic response of the heavy-quasiparticles, and is indicative of the progressive development of the manybody coherent Kondo state, coexisting with both types of magnetic ordering. In this review, we also present the evolution of the optical properties due to Ni- and Redoping in UCu5 and URu2Si2, respectively. The optical evidence of the itinerant antiferromagnetic ordering is suppressed in both compounds upon doping and particularly for the URu2Si2 compound this is consistent with a crossover to a ferromagnetic ground state upon Re-doping.

Ce L III-edge X-ray absorption and Ce resonant photoemission studies of new ternary compounds Ce 2Ni 2X and Ce 2Pd 2.04X 0.96 (X = In, Sn)

The electronic structure of Ce 2Ni 2X and Ce 2Pd 2.04X 0.96 (X = In, Sn) compounds has been studied by Ce L III X-ray absorption (XAS) and resonant photoemission spectroscopy. These compounds exhibit various physical properties ranging from an intermediate valence state to a magnetically ordered one. Based on the measured spectra and crystallographic data, the hybridization of 4f(Ce)-states with 3d(Ni), 4d(Pd) valence bands and 5p(In, Sn) states are discussed in connection with magnetic properties. A comparative study with CeMSn (M = Ni, Pd) stannides has given rise to an interpretation of the origin of Kondo states in terms of hybridization in the [Ce-M] and [M-X] networks.

Hidden Ordering and Heavy Mass in URu2Si2and Its Alloys

Various unusual properties in the anomalous superconductor URu 2 Si 2 in the normal state have been studied based on the standpoint of pair Jahn-Teller distortion. The pair distortion occurs well above the ordering temperature and thus the transition for the pair ordering is weak. Due to an inherent characteristic of pair ordering, the ordering is not perfect but includes unpaired U atoms. This is the origin of the weak antiferromagnetic ordering with the moment of 0.01–0.04 µ B /U and a heavy fermionlike behavior. No Kondo effect is observed. To confirm the above model, dilute alloy systems have also been studied and it is shown that all of them can be explained by a common model consistent with that of URu 2 Si 2 . The proposed two-channel Kondo state for dilute U atoms in the ThRu 2 Si 2 system was shown to be unacceptable.

Revisited Doniach diagram: Influence of short-range antiferromagnetic correlations in the Kondo lattice

The Kondo lattice model including nearest-neighbor magnetic exchange interactions is studied here in a mean-field approximation describing both the Kondo state and the intersite magnetic correlations. In the case of antiferromagnetic correlations, our model yields a decrease of the Kondo temperature compared to the one-impurity value, giving, therefore, a ``revisited'' version of the Doniach diagram with a rather flat Kondo temperature in the nonmagnetic case, as observed in several cerium compounds. Our model shows also that short-range magnetic correlations can appear at a temperature clearly larger than the Kondo temperature, as observed in compounds such as CeCu${}_{6}$ or CeRu${}_{2}$Si${}_{2}$.

Band ferromagnetism versus collective Kondo state in lattice fermion models

It is becoming widely recognized that, contrary to earlier expectations, the usual one-band Hubbard model does not give an explanation for itinerant ferromagnetism. After reviewing the status of magnetic ordering in the one-band model, we discuss the possibility of ferromagnetism in some recently introduced two-band Hubbard models, and in generalized Anderson lattices. It is argued that these two classes of models are closely related and that it is their common feature that the ferromagnetic phase has to compete with a collective Kondo state.

Overscreening of magnetic impurities indx2−y2-wave superconductors

We consider the screening of a magnetic impurity in a $d_{x^2-y^2}$ wave superconductor. The properties of the $d_{x^2-y^2}$ state lead to an unusual behavior in the impurity magnetic susceptibility, the impurity specific heat and in the quasiparticle phase shift which can be used to diagnose the nature of the condensed state. We construct an effective theory for this problem and show that it is equivalent to a multichannel (one per node) non-marginal Kondo problem with linear density of states and coupling constant J. There is a quantum phase transition from an unscreened impurity state to an overscreened Kondo state at a critical value J_c which varies with $\Delta_0$, the superconducting gap away from the nodes. In the overscreened phase, the impurity Fermi level $\epsilon_f$ and the amplitude $\Delta$ of the ground state singlet vanish at J_c like $\Delta_0 \exp(- const. / \Delta)$ and J-J_c respectively. We derive the scaling laws for the susceptibility and specific heat in the overscreened phase at low fields and temperatures.

The effect of pressure on the magnetic susceptibility of alloys

A transition from the Kondo state to an intermediate-valence state takes place in the quasibinary alloys with cubic -type structure. The effect of pressure P up to 2 kbar on the magnetic susceptibility of this system was measured at 78, 150, and 295 K. The value of obtained is well above those observed for similar RE compounds with stable valences. This derivative tends to decrease with temperature, and to increase (but not monotonically) with the tin content x. The experimental data on were analysed in terms of the changes in the spin-fluctuation temperature .

Kondo state of the mixed-valent compound CePd 3 under high pressure

The electrical resistivity ϱ( T) of the mixed-valent compound CePd 3 has been measured under high pressure up to 2.1 GPa. It is found that the temperature T max showing the maximum in the ϱ( T) curve increases with increasing pressure. By using the present results and the pressure dependence of volume, we estimated the value of Grüneisen parameter Λ K for CePd 3 to be about 6, which is the same order of magnitude as that of the typical mixed-valent compound.

Crystal fields in the valence-unstable CeNi-based compounds

The influence of a partial delocalization of cerium 4f electrons on the crystalline electric field (CEF) effects measured on Pr paramagnetic impurity ions have been investigated for the series of (Pr, Ce) x (La, Y) 1− x Ni compounds, which undergo the transition from the intermediate valence (IV) to Kondo state due to the chemical substitution. In the Kondo state the hybridization of cerium 4f electrons and conduction electrons results only in the simultaneous increase of all CEF parameters, while in the IV regime the partial delocalization of cerium 4f electrons is responsible for a charge redistribution that manifests itself in different scales of changes for different CEF parameters.

Spin-density wave in a Kondo lattice

An Anderson model with N-fold degeneracy in the Kondo regime is considered. It is presumed that the electron-electron correlations in the system of f electrons have their maximum strength. A criterion for instability against the formation of a weakly antiferromagnetic phase superposed on the Kondo state is obtained by the auxiliary-boson method using the 1/N expansion. An effective interaction leading to the formation of magnetic ordering appears because of the spin fluctuations in the system of localized electrons. The phase diagram of the system is constructed.

Ferromagnetic - paramagnetic phase diagram of the Kondo lattice

A theory which can easily be used to discuss the competition between ferromagnetic order and the Kondo effect is presented in this paper. The magnetic order parameters of conduction and localized electrons as well as the hybridization term describing the Kondo effect are introduced simultaneously in a Kondo lattice model using the functional integral method. At zero temperature, quite a complete phase diagram of ferromagnetic and paramagnetic (Kondo) states is obtained in the static saddle-point approximation. At finite temperatures, the dependences of the Curie temperature and the total magnetization on the exchange-coupling strength |J| for various concentrations of conduction electrons, are also given by the theory. The main results are in qualitative agreement with experimental observations in heavy-fermion ferromagnets.

Physical Mechanism in Kondo Insulator

Physical mechanism in Kondo insulator with nearly homogeneous energy gap is studied based on the model for the valence fluctuation regime in which the mixing band electron is of localized character and the first excited state is a s -wave exciton. This model is applied to new experimental studies by the inelastic neutron scattering in SmB 6 and YbB 12 , as well as their alloys. Therefore, the present model is based on the effective one site Kondo state model. The Luttinger theorem is also critically studied on this stand point. Detailed q -dependence of the s -wave exciton peak in SmB 6 is calculated based on the present model. Good agreement is obtained even though there is no adjustable parameter. In particular, two peak structure along both [111] and [011] directions with the intensity ratio of 3:1, as well as the sharp q dependences are explained very well. Anomalous behaviors in the alloy systems are also well explained based on the present model.

ESR studies of magnetic state evolution at the phase transition in Yb 0.5In 0.5Cu 2

The ESR of the Gd +3 ion was investigated in Yb 0.5In 0.5Cu 2 at temperatures below and above the first-order phase transition which occurs in this material at T p ∼ 50 K. We present arguments that this compound is in a Kondo state that varies with the temperature and that the phase transition is connected with the change in the Yb +3 volume due to the “Kondo-collapse” mechanism.

Crystal field in valence-fluctuating CeNi-based compounds

The effects of the crystal field (CF) on the paramagnetic Pr ion in a number of compounds of the type R1−x PrxNi (R = Ce, La, Y), in which a transition of the cerium ions from an intermediate-valence into a Kondo state occurs as La is substituted for Ce, are investigated. The level schemes of the Pr ion in the CF are reconstructed from inelastic neutron scattering spectra and the temperature dependence of the heat capacity in different magnetic fields (B=0–8 T). The parameters of the low-symmetry CF in the compounds RNi are determined from the experimental data. It is established that in the Kondo regime the hybridization of the f electrons with conduction electrons only gives a proportional increase in all the parameters of the CF potential. At the same time, partial delocalization of the f electrons in the intermediate-valence state results in charge redistribution, which is manifested in different scales for the changes in the different CF parameters.

Normal and Anomalous Hall Effect in CeSb and CeBi

Anomalous behavior of Hall constant and magnetoresistance in CeSb and CeBi under high magnetic field and high pressure is studied critically. At first, anomalous field dependence in the reference systems LaSb and LaBi is investigated based on the three band model in which two kinds of electrons are treated independently and it is concluded that the anomalous behavior is due to a small deviation from the stoicheometry, which is enhanced by a large mobility. In CeSb without pressure, no anomalous Hall effect is observed even in the ferromagnetic state. On the other hand, the normal Hall constant in CeSb and CeBi shows strong electron scattering in the whole temperature region confirming the model that the electrons also substantially contribute to the Γ 8 Kondo state. To study the anomalous Hall effect under high pressure and high magnetic field, general equations for anomalous Hall effect under high magnetic field are derived, and the anomalous Hall effect is identified to exit in the region in which a kin...

Physics in low carrier strongly correlated systems: Kondo insulator magnetic polaron and high Tc

It is shown that in low carrier density systems a strong tendency to form the Wigner crystal, or electron localization, is clearly seen through long-range Coulomb interaction. The Kondo insulator is one typical example, in which the Kondo electron is localized and an excitondashlike excitation is observed. The magnetic polaron liquid and lattice is another typical example and is seen most typically in Ce and Yb monopnictides. A detailed study on Cesb is shown. In this system, an enhanced Kondo state due to tendency of localization coexists. High T c CuO 2 layered materials are also another typical example belonging to the present category. A description by the magnetic polaron picture is given. It is shown that, due to strong antiferromagnetic short-range order, a modified magnetic polaron with a singlet core is formed at high temperature and at lower temperature a pair magnetic polaron is formed. The superconductivity occurs due to their Bose condensation.