Kondo Singlet Research Articles

F–f Magnetic polaron Wigner glass and anomalous superconductivity in U 1− xTh xBe 13

Mechanisms of the anomalous properties in the heavy fermion superconductor UBe 13 and its alloys, in particular for the Th dopings, are studied in detail based on the fundamental electronic states to be consistent with all the crucial experimental results. As the reference systems for the magnetic polaron formation, Ce monopnictides, as well as USb and UTe, are mentioned. From detailed systematic studies of the dilute alloy systems, it is postulated that the 5f states in UBe 13 split into the well-localized core 5fΓ 2 7 singlet state and other delocalized 5f states situated around the Fermi energy forming the f–f magnetic polarons through the strong intra-atomic ferromagnetic f–f exchange interaction. The accompanied lattice polarons are also shown to play important roles. In the p–d band states, the f–f exchange interaction and the intersite p–f mixing interactions for the p–f Kondo state are of nearly equal strengths causing a rich variety of delicately balanced states. For the p–f Kondo state, the polarization of the core Γ 2 7 state through the f–f exchange interaction causes novel Kondo effect different from that in Ce compounds. At higher temperatures above 50 K, the conventional Kondo singlet through the p–f mixing dominates. Below 5 K, the f–f magnetic polarons form amorphous types of Wigner crystallization, or Wigner glass, and the p–d band states are strongly scattered forming pseudolocalized states, in particular below the Fermi energy. The complicated unusual anomalies in UBe 13 and its alloys originate fundamentally in the above delicate balance. The anomalous superconducting state was identified as the s-wave pairing through the strong charge fluctuation of the Wigner glass and occurs in competition with the magnetic polaron and the pseudolocalized states with a strongly reduced T c due to the large resistivity at T c causing a double peak structure of T c as a function of Th doping, as well as unusual characteristics of the temperature dependence of H c2. In general, the Wigner glass state persists even in the superconducting state staying in the normal state causing unusual bulk superconducting state.

Transport Properties in Phase IV of CexLa1-xB6

We have performed the detailed studies of the magnetoresistance of Ce x La 1- x B 6 ( x =0.75, 0.7, 0.65, 0.6, 0.5, 0.47, 0.38, 0.36, 0.25) in detail and obtained the magnetic phase diagram of Ce x La 1- x B 6 . The results show that recently discovered new phase IV exists between 0.3 ≤ x ≤0.8 and the Kondo singlet ground state is realized for x < 0.3. All the long-range orders seem to disappear at x ∼0.3. As for the nature of phase IV, judging from the results of the electrical resistivity, the specific heat, magnetic susceptibility, magnetic phase diagram and the similarities between the natures of two phases of PrB 6 and ones of phase III and IV of Ce 0.75 La 0.25 B 6 , we conjecture that phase IV is the incommesurate AF magnetic oredred state.

Negative Josephson coupling in the Kondo strong-coupling limit

We show that pair hopping through a Kondo singlet gives rise to a negative Josephson coupling. Thus our calculation supports the existence of staggered pair correlations in the strong-coupling limit of a one-dimensional Kondo lattice.

Co-operative Kondo effect in the two-channel Kondo lattice

We discuss the possibility of a co-operative Kondo effect driven by channel interference in a Kondo lattice where local moments are coupled to a single Fermi sea via two orthogonal scattering channels. In this situation, the channel quantum number is not conserved. We argue that the absence of channel conservation causes the Kondo effect in the two channels to constructively interfere, giving rise to a superconducting condensate of composite pairs, formed between the local moments and the conduction electrons. Our arguments are based on the observation that a heavy Fermi surface gives rise to zero modes for Kondo singlets to fluctuate between screening channels of different symmetry, producing a divergent composite pair susceptibility. Secondary screening channels couple to these divergent fluctuations, promoting an instability into a state with long-range composite order. We present detailed a detailed mean-field theory for this superconducting phase, and discuss the possible implications for heavy fermion physics.

Magnetic and Thermodynamic Properties of the Three-Dimensional Periodic Anderson Hamiltonian

The three-dimensional periodic Anderson model is studied with the quantum Monte Carlo method. We find that the crossover to the Kondo singlet regime is remarkably sharp at low temperatures, and that the behavior of magnetic correlations is consistently reflected in both the thermodynamics and the density of states. The abruptness of the transition suggests that energy changes associated with the screening of local moments by conduction electrons might be sufficient to drive large volume changes in systems where applied pressure tunes the ratio of interband hybridization to correlation energy.

Spin, Charge and Quasi-Particle Gaps in the One-Dimensional Kondo Lattice withf2Configuration

The ground state properties of the one-dimensional Kondo lattice with an f^2 configuration at each site are studied by the density matrix renormalization group method. At half-filling, competition between the Kondo exchange J and the antiferromagnetic intra f-shell exchange I leads to reduction of energy gaps for spin, quasi-particle and charge excitations. The attractive force among conduction electrons is induced by the competition and the bound state is formed. As J/I increases the f^2 singlet gives way to the Kondo singlet as the dominant local correlation. The remarkable change of the quasi-particle gap is driven by the change of the spin-1/2 excitation character from the itinerant one to the localized one. Possible metal-insulator transition is discussed which may occur as the ratio J/I is varied by reference to mean-field results in the f^2 lattice system and the two impurity Kondo system.

Ferromagnetic Polarons in the Kondo Lattice Model

Using a bosonization technique we describe the ferromagnetic polaron formation in one-dimensional Kondo lattice models with partial conduction band filling. We show that at low electron concentration large ferromagnetic polarons are formed. The width of these polarons will gradually decrease as the electron concentration increases and finally local Kondo singlets will dominate. Bipolarons or multipolarons will not form in the lattice.

Small moments in Kondo-lattice systems

The Kondo-lattice model is studied by a self-consistent incremental method. The ground-state energy and the magnetic moment are calculated. An explanation is offered for the small magnetic moments. It is based on the interplay between the formation of Kondo singlets and the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction. However, one must go beyond a simple superposition of these two competing physical phenomena in order to derive the small moments.

Quadrupolar Kondo effect in U 3Ni 3Sb 4− xSn x alloys

In the pseudobinary series U 3Ni 3Sb 4− x Sn x there is a cross-over from a localised non-magnetic state to a many-body Kondo singlet state, accompanied by a transition from semiconducting to metallic behaviour. Our analysis of the crystal field spectra of U 3Ni 3Sb 4 has shown that its non-magnetic ground state is a doublet, so that the system meets the criteria for the quadrupolar Kondo effect. For intermediate compositions the susceptibility shows weak power law divergences at low temperature, consistent with non-Fermi liquid behaviour.

Magnetic excitations observed by means of inelastic neutron scattering in polycrystalline

Neutron scattering measurements have been performed with time-of-flight spectrometers on an polycrystal. The spectra show clearly the presence of magnetic excitations and a gap at around 14 meV. At low temperature, two broad signals , one asymmetric, the other one of Lorentzian shape, are well defined. After subtracting the phonon contribution, the former signal is further decomposed into a sharp 15.5 meV peak and a broader 20 meV peak. The 15.5 meV peak behaves similarly to that of the exciton at 14.5 meV for as regards both the Q- and T-dependencies. The 20 meV peak was identified as manifesting a transition from a localized Kondo singlet with a gap to a magnetic state. With decreasing temperature, the gap disappears and the scattering becomes quasi-elastic, like for the metallic Kondo system. The second signal, centred at 37 meV, was assigned previously to a crystal-field excitation. However, the observed behaviour is anomalous, indicating a shift of the intensity from the lower peaks to the higher peak, suggesting a change in the -value due to lattice distortion.

Positron Lifetime Studies in the Kondo Insulator, FeSi

Positron lifetime measurements have been carried out in the Kondo insulator FeSi in the temperature range of 5 K to 700 K. In the high temperature range of 500 K to 700 K, the lifetime is seen to increase rapidly from ~133±2 ps to ~141±2 ps, and is associated with positron trapping at thermally generated vacancies. At temperatures below 200 K, the positron lifetime shows an unusual decrease to 122 ps at 5 K, and interestingly it is seen to correlate with the measured dc conductivity. With the help of positron density distribution and lifetime calculations, the observed decrease in the positron lifetime at low temperatures is inferred to arise due to the delocalization of the Fe d electron, an electronic structure change arising as a consequence of the Kondo singlet formation.

Unusual Magnetic Phase Diagram of CexLa1-xB6(x=0.5, 0.75) Studied by the Magnetoresistance

We have studied the magnetoresistance of Ce x La 1- x B 6 ( x =0.25, 0.5, 0.75) single crystals under the magnetic fields up to 15 T and in a temperature range between 0.4 K and 15 K. In Phase I (paramagnetic), in all the samples the negative magnetoresistance is observed, whose magnitude becomes larger with decreasing temperature. However, in Ce 0.5 La 0.5 B 6 , the magnetoresistance below T ∼1 K becomes very small neverthless the negative magnetoresistance is observed above ∼1 K. This is completely different from the magnetoresistance in Ce 0.25 La 0.75 B 6 where the magnitude of the negative magnetoresistance becomes larger with decreasing temperature. The weak magnetic field dependence of the electrical resistivity is observed also in Phase IV in Ce 0.75 La 0.25 B 6 . These results indicate that the ground state of Ce 0.5 La 0.5 B 6 is not the Kondo singlet but the AF (antiferro) magnetic state called Phase VI ′ is realized below ∼1 K. Phase IV ′ is similar to Phase IV in Ce 0.75 La 0.25 B 6 . The anisotropy of the phase boundaries between I, II (AFQ (antiferroquadrupolar) ordered), III (antiferromagnetic ordered) and IV ′ in Ce 0.5 La 0.5 B 6 has been studied. The phase boundary I–IV ′ does not show the anisotropy but that of III–II shows the large anisotropy depending on the applied field direction. These are discussed from the stand point that the magnetic anisotropy and the intereference effect between the AFQ and AF interaction are important. We propose the mechanism of the phase transition from IV ′ to III or II.

Positron-lifetime studies in the Kondo insulator FeSi

Positron lifetime measurements have been carried out in the Kondo insulator FeSi in the temperature range of 5 K to 700 K. In the high temperature range of 500 K to 700 K, the lifetime is seen to increase rapidly from ~133±2 ps to ~141±2 ps, and is associated with positron trapping at thermally generated vacancies. At temperatures below 200 K, the positron lifetime shows an unusual decrease to 122 ps at 5 K, and interestingly it is seen to correlate with the measured dc conductivity. With the help of positron density distribution and lifetime calculations, the observed decrease in the positron lifetime at low temperatures is inferred to arise due to the delocalization of the Fe d electron, an electronic structure change arising as a consequence of the Kondo singlet formation.

Low-Temperature Properties of the Dense Kondo System Ce0.5La0.5B6

Low-temperature specific heat and electric resistivity of the dense Kondo compound Ce 0.5 La 0.5 B 6 have been measured. The specific heat for H =0 shows a broad peak at around 0.9 K corresponding to the Kondo effect. No indication of magnetic or quadrupolar phase transition is observed down to 100 mK for H =0. Very large specific heat coefficients of γ=1.8 J/K 2 mol and β=2.9 J/K 4 mol for H =0 indicate the Kondo singlet ground state without long-range ordering.

Quadrupolar Response of Kondo Compound CePd2Al3

The elastic constants of Kondo compound CePd 2 Al 3 with a hexagonal crystal structure have been investigated by the ultrasonic measurement. In the present single crystal we have observed absence of long range order down to 0.5 K in applying field up to 120 kOe. The elastic softening of the transverse C 66 mode is ascribed to the quadrupolar response for the crystalline field state of CePd 2 Al 3 . The low temperature softenings of the longitudinal C 33 and C 11 modes below the Kondo temperature T K ∼20 K are dominated by the Kondo singlet state.

Ultrasonic investigation of quadrupolar response in Kondo system Ce xLa 1− xB 6

We have studied the magnetic phase diagrams of the dense Kondo compounds CeB 6 and Ce 0.5La 0.5B 6 by the use of low temperature ultrasonic measurements. In CeB 6 long-range ordered states were observed. On the contrary, a crossover from the ordinary Γ 8 ground state to the Kondo singlet state has been found in Ce 0.5La 0.5B 6.

Magnetic Phase Diagrams of the Dense Kondo Compounds CeB6and Ce0.5La0.5B6

We have studied the magnetic phase diagrams of the dense Kondo compounds CeB 6 and Ce 0.5 La 0.5 B 6 by use of low temperature ultrasonic measurements. A transition associated with a quadrupolar reorientation has clearly been observed around 1 kOe in the antiferroquadrupolar phase II of CeB 6 . In the paramagnetic phase I of Ce 0.5 La 0.5 B 6 a crossover from the ordinaly Γ 8 ground state to the Kondo singlet state has been found. The field induced transition from the Kondo singlet state to the antiferromagnetic phase III or the phase II has been found at a critical field 17 kOe. The field angle dependence of the phase diagram of Ce 0.5 La 0.5 B 6 is also presented.

Static and dynamical properties of a magnetic impurity in a strongly correlated electronic system.

Using numerical techniques we study the zero temperature properties of a substitutional magnetic impurity in a one-dimensional correlated system described by the Hubbard model with repulsive interaction $U$. We find that, while the static spin correlation maintains a $2k_F$ periodicity, the charge correlations show Friedel oscillations with a $4k_F$ periodicity for large values of $U$. The static on-site susceptibility decreases as the interactions in the chain are turned on and we interpret this as an increase of the Kondo temperature. For large values of $U$ the short range antiferromagnetic correlations dominate over the collective Kondo singlet formation.

Quadrupolar response in Kondo compound Ce xLa 1− xB 6

We have performed ultrasonic measurements of the Kondo compound Ce x La 1− x B 6 at low temperatures. A quenching of the quadrupolar moment of the Λ 8 ground state in Ce 0.03La 0.97B 6. A remarkable increase of the ultrasonic attenuation due to the quadrupolar fluctuations in Ce 0.5La 0.5B 6 has been found. The effect of the Kondo singlet formation at very low temperatures is discussed.

Néel orders, Haldane gap, and Kondo singlet phase in the anisotropic Kondo chain.

The ground-state phase diagram of the Kondo chain with an anisotropic exchange is investigated. Four different phases are identified, two types of N\'eel phases, the Haldane phase, and the Kondo singlet phase. The boundaries between these phases in the strong coupling region are determined by mapping to effective models: the spin-1 Heisenberg model with single-ion anisotropy and the Ising model in a transverse magnetic field. It is shown that these transition lines extend to the weak-coupling limit and they are estimated by using the exact numerical diagonalization for a finite system.