Kondo Screening Effect Research Articles

Quantum oscillations of Kondo screening phases in strong magnetic fields

We generalize the iterative diagonalization procedure adopted in method of numerical renormalization group to analyze the Kondo effect in strong magnetic fields, where the density of states for itinerary electrons at the chemical potential varies discontinuously as the magnetic field changes. We first examine phases of many-body ground states in the presence of single impurity. By investigating change of $z$-component of total spin, $\Delta S_z$, and spin-spin correlation between the impurity and conduction electrons, we find that there are three states competing for the ground state when Zeeman splitting is present. One of the states is doublet in which the impurity spin is unscreened. The other two states are Kondo screening states with $\Delta S_z=1/2$ and $\Delta S_z=1$, in which the impurity spin is partially screened and completely screened respectively. For Kondo systems with two-impurities in strong magnetic fields, we find that the interplay between the Kondo screening effect, RKKY interaction, and quantum oscillations due to Landau levels determines the ground state of the system. Combination of these three factors results in different screening scenarios for different phases in which spins of two impurities can form spin-0 or spin-1 states, while impurity spins in these phases can be either screened, partially screened, or unscreened by conduction electrons. The emergence of the ground state from these competing states oscillates with the change of magnetic field, chemical potential or inter-impurity distance. This leads to quantum oscillations in magnetization and conductivity. In particular, we find extra peak structures in longitudinal conductivity that reflect changes of Kondo screening phases and are important features to be observed in experiments. Our results provide a complete characterization of phases for Kondo effect in strong magnetic fields.

Few-body solutions under spin-exchange interaction: Magnetic bound state and the Kondo screening effect

Motivated by recent progresses on ultracold alkaline-earth atoms towards the goal of simulating Kondo physics, in this work we exactly solve the few-body problem of one and two trapped fermions in one dimension interacting with a localized impurity under tunable spin-exchange interaction. It is found that depending on the sign of the spin-exchange coupling, ferromagnetic(FM) or anti-ferromagnetic(AFM), the attractive and repulsive branches can hold different magnetic structures. For the two fermions case, we demonstrate the Kondo screening effect for the attractive branch with AFM coupling, and show that such screening is absent for the ground state with FM coupling. Moreover, we find a sequence of FM upper branches in the AFM coupling side. These FM states are orthogonal to all other attractive branches and their wave functions feature a full spin-charge separation. The effect of an additional contact interaction and the extension of our results to many particles are also discussed. This work reveals the intriguing physics uniquely associated with the spin-exchange interaction in the few-body point of view, which are promisingly to be explored in the experiment of ultracold alkaline-earth atoms.

Possible orbital crossover in the ferromagnetic Kondo lattice compound CeAgSb2

Utilizing femtosecond time-resolved pump probe spectroscopy, we investigated the ultrafast quasiparticle dynamics of the ferromagnetic Kondo lattice compound ${\mathrm{CeAgSb}}_{2}$. The photoinduced reflectivity demonstrates a positive relaxation process connected with simple metal physics at room temperature, while an additional negative decay channel associated with Kondo screening effect can be recognized immediately with temperature decreasing. The temperature-dependent amplitude of the latter exhibits a kink at ${T}^{*}\ensuremath{\approx}$ 45 K, which is close to the energy scale of the first excited crystal field splitting Ce $4f$ level. We propose that an orbital crossover is responsible for this intriguing feature. The dominating $4f$ orbitals that participated in the Kodno $c\ensuremath{-}f$ hybridization varies from the ground state below ${T}^{*}$ to multiple crystal field splitting levels. In the meantime, the hybridization gap changes from 12.6 to 220.4 K correspondingly, which is also in agreement with the fluence-dependent measurements.

Finite temperature physics of 1D topological Kondo insulator: Stable Haldane phase, emergent energy scale and beyond

We have studied the one-dimensional $p$-wave periodic Anderson model at finite temperature with the help of the numerically exact determinant quantum Monte Carlo simulation. It is found that the topological Haldane phase established for ground-state is still stable against small thermal fluctuation and its characteristic edge magnetization develops at low temperature. Moreover, the saturated low-$T$ spin structure factor and the $\frac{1}{T}$-law of susceptibility are useful to detect the free edge spin moment, which may be relevant for experimental explorations. We have also comparatively studied the conventional $s$-wave periodic Anderson model, which helps us identify an emergent energy scale $T_{cr}$. $T_{cr}$ signals a crossover into interesting low-$T$ regime and seems to be the expected Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling. Finally, the collective Kondo screening effect has been examined and it is heavily reduced at boundary, which may give a fruitful playground for novel physics beyond the well-established Haldane state and topological band insulators.

Phase diagram of the Kondo-Heisenberg model on honeycomb lattice with geometrical frustration

We calculated the phase diagram of the Kondo-Heisenberg model on a two-dimensional honeycomb lattice with both nearest-neighbor and next-nearest-neighbor antiferromagnetic spin exchanges, to investigate the interplay between RKKY and Kondo interactions in the presence of magnetic frustration. Within a mean-field decoupling technology in slave-fermion representation, we derived the zero-temperature phase diagram as a function of Kondo coupling Jk and frustration strength Q. The geometrical frustration can destroy the magnetic order, driving the original antiferromagnetic (AF) phase to non-magnetic valence bond solids (VBS). In addition, we found two distinct VBS. As Jk is increased, a phase transition from AF to Kondo paramagnetic (KP) phase occurs, without the intermediate phase coexisting AF order with Kondo screening found in square lattice systems. In the KP phase, the enhancement of frustration weakens the Kondo screening effect, resulting in a phase transition from KP to VBS. We also found a process to recover the AF order from VBS by increasing Jk in a wide range of frustration strength. Our work may provide predictions for future experimental observation of new processes of quantum phase transitions in frustrated heavy-fermion compounds.

Anisotropic magnetic properties and magnetic structure of YbPdSi

YbPdSi with orthorhombic crystal structure (space group Pmmn) exhibits a magnetic transition at K, below which a ferromagnetic moment develops with an enhanced electronic specific-heat coefficient mJ K−2 mol−1. We have investigated the magnetization, electrical resistivity, and specific heat of YbPdSi using single crystalline samples as functions of temperature and magnetic field. It has been found that the ferromagnetic moment points to the c-direction, although the magnetic moments have an Ising-like anisotropy along the b-direction above the magnetic-transition temperature. Field dependence of the magnetization and electrical resistivity shows a metamagnetic-like transition at T when field is applied along the b-axis below T = 3 K, suggesting the existence of an antiferromagnetic component along this direction. The magnetic structure has been investigated by neutron diffraction using powder samples. The magnetic unit cell is identical to the crystal unit cell. The Rietveld fitting has revealed that Yb at the 2a and 2b positions exhibit a collinear ferromagnetic order along the c-axis, whereas Yb at the 4e position undergoes a non-collinear order, involving the ferromagnetic moment along the c-axis and an antiferromagnetic component along the b-axis. The ferromagnetic moments determined by the neutron diffraction are 0.26, 1.3, and 0.15 for Yb at the 4e, 2b, and 2a sites, respectively. The reduced moments for the 4e and the 2a sites suggest that the Kondo screening effect is important in YbPdSi.

Phase evolution of the two-dimensional Kondo lattice model near half-filling

Within a mean-field approximation, the ground state and finite temperature phase diagrams of the two-dimensional Kondo lattice model have been carefully studied as functions of the Kondo coupling J and the conduction electron concentration . In addition to the conventional hybridization between local moments and itinerant electrons, a staggered hybridization is proposed to characterize the interplay between the antiferromagnetism and the Kondo screening effect. As a result, a heavy fermion antiferromagnetic phase is obtained and separated from the pure antiferromagnetic ordered phase by a first-order Lifshitz phase transition, while a continuous phase transition exists between the heavy fermion antiferromagnetic phase and the Kondo paramagnetic phase. We have developed an efficient theory to calculate these phase boundaries. As decreases from the half-filling, the region of the heavy fermion antiferromagnetic phase shrinks and finally disappears at a critical point , leaving a first-order critical line between the pure antiferromagnetic phase and the Kondo paramagnetic phase for . At half-filling limit, a finite temperature phase diagram is also determined on the Kondo coupling and temperature (J-T) plane. Notably, as the temperature is increased, the region of the heavy fermion antiferromagnetic phase is reduced continuously, and finally converges to a single point, together with the pure antiferromagnetic phase and the Kondo paramagnetic phase. The phase diagrams with such triple point may account for the observed phase transitions in related heavy fermion materials.

Cooperative magnetic behaviour in the new valence fluctuating compound Ce2Rh3Ge

In this study we report the physical properties of the new ternary compound Ce2Rh3Ge that crystallizes in the rhombohedral, triple hexagonal MgCu2-type of structure. The electronic ground state properties of Ce2Rh3Ge were characterized by magnetic susceptibility, specific heat, electrical resistivity and thermal transport measurements. The results indicate the presence of short range magnetic interaction, probably of ferromagnetic origin below TC = 4 K. The shape of χ−1(T) deviates from the Curie–Weiss behavior with a broad minimum at about = 450 K reminiscent of valence fluctuating cerium systems. At T = 10 K, the magnetic part of the resistivity ρ4 f (T) exhibits a shallow minimum followed by increase of resistivity ρ(T) −lnT, which hints at a substantial Kondo screening effect. Ce2Rh3Ge belongs to a small group of strongly correlated cerium compounds in which the two competing effects of Kondo and RKKY interactions produce long-range magnetic order from strongly hybridized and intermediate-valent 4 f spins. At sufficiently low temperatures Ce2Rh3Ge scales well with the Kadowaki–Woods ratio A/γ2 and the value of the Wilson ratio χ(T → 0)/γ found for this compound classifies it as a mixed-valence compound. The presence of valence fluctuation and magnetic order it is rare for these attributes to be found simultaneously in same compound, in same temperature range. In our opinion a novelty of presented results of Ce2Rh3Ge is that this compound adds a new member to a small but growing class of systems bearing a strongly mixed- or intermediate-valent 4 f magnetic moment, but in which the lattice of spins nevertheless end up finding it possible to order magnetically.

Magnetic structure of the ferromagnetic Kondo-lattice compounds YbPtGe and YbPdGe

YbPtGe and YbPdGe exhibit ferromagnetic ordering below and 11.4 K with enhanced electronic specific heat coefficients of and 150 mJ K−2 mol, respectively. In order to shed light on the origin of the coexistence of a ferromagnetic state and heavy-fermion behavior, we studied the powder neutron diffraction of YbPtGe and YbPdGe at low temperatures. Weak reflections due to magnetic ordering have been resolved. The data were analyzed using the Rietveld method together with group theory analysis. It has been found that YbPtGe exhibits a non-collinear ferromagnetic structure, with a spontaneous moment along the c-axis and a weak antiferromagnetic component along the a-axis. The presence of this antiferromagnetic component explains the origin of the observed metamagnetic-like behavior. In the case of YbPdGe, magnetization measurements confirmed the ferromagnetic moment along the b-axis and revealed a metamagnetic transition at 0.2 T for a field parallel to the c-axis. The neutron diffraction results indicate that the magnetic structure of YbPdGe is also of a non-collinear type, with ferromagnetic moments parallel to the b-axis and weak antiferromagnetic components along the c-axis, which is consistent with the magnetization data. A comparison of the results for YbPtGe and YbPdGe has been made. It is suggested that both the Kondo screening effect of ferromagnetic moments and the fluctuation of antiferromagnetic components can contribute to the enhanced mass in the ferromagnetic state.

μSRand inelastic neutron scattering investigations of the noncentrosymmetric antiferromagnetCeNiC2

The magnetic state of the noncentrosymmetric antiferromagnet ${\mathrm{CeNiC}}_{2}$ has been studied by magnetic susceptibility, heat capacity, muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$, and inelastic neutron scattering (INS) measurements. ${\mathrm{CeNiC}}_{2}$ exhibits three magnetic phase transitions at ${T}_{{N}_{1}}=19.5$ K, ${T}_{{N}_{2}}=10$ K, and ${T}_{{N}_{3}}=2.5$ K. The presence of long-range magnetic order below 20 K is confirmed by the observation of oscillations in the $\ensuremath{\mu}\mathrm{SR}$ spectra between 10 and 20 K and a sharp increase in the muon depolarization rate. INS studies reveal two well-defined crystal electric field (CEF) excitations around 8 and 30 meV. INS data have been analyzed using a CEF model and the wave functions were evaluated. We also calculated the direction and magnitude of the ground state moment using CEF wave functions and compare the results with that proposed from the neutron diffraction. Our CEF model correctly predicts that the moments order along the $b$ axis (or $y$ axis) and the estimated magnetic moment is $0.687(5){\ensuremath{\mu}}_{B}$, which is higher than the moment observed from the neutron diffraction $(0.25{\ensuremath{\mu}}_{B}/\mathrm{Ce})$. We attribute the observed reduced moment due to the Kondo screening effect.

Pairing Symmetry of Heavy Fermion Superconductivity in the Two-Dimensional Kondo—Heisenberg Lattice Model

In the two-dimensional Kondo—Heisenberg lattice model away from half-filled, the local antiferromagnetic exchange coupling can provide the pairing mechanism of quasiparticles via the Kondo screening effect, leading to the heavy fermion superconductivity. We find that the pairing symmetry strongly depends on the Fermi surface (FS) structure in the normal metallic state. When JH/JK is very small, the FS is a small hole-like circle around the corner of the Brillouin zone, and the s-wave pairing symmetry has a lower ground state energy. For the intermediate coupling values of JH/JK, the extended s-wave pairing symmetry gives the favored ground state. However, when JH/JK is larger than a critical value, the FS transforms into four small hole pockets crossing the boundary of the magnetic Brillouin zone, and the d-wave pairing symmetry becomes more favorable. In that regime, the resulting superconducting state is characterized by either a nodal d-wave or nodeless d-wave state, depending on the conduction electron filling factor as well. A continuous phase transition exists between these two states. This result may be related to the phase transition of the nodal d-wave state to a fully gapped state, which has recently been observed in Yb-doped CeCoIn5.

Weak ferromagnetism with the Kondo screening effect in the Kondo lattice systems

We carefully consider the interplay between ferromagnetism and the Kondo screening effect in the conventional Kondo lattice systems at finite temperatures. Within an effective mean-field theory for small conduction electron densities, a complete phase diagram has been determined. In the ferromagnetic ordered phase, there is a characteristic temperature scale to indicate the presence of the Kondo screening effect. We further find two distinct ferromagnetic long-range ordered phases coexisting with the Kondo screening effect: spin fully polarized and partially polarized states. A continuous phase transition exists to separate the partially polarized ferromagnetic ordered phase from the paramagnetic heavy Fermi liquid phase. These results may be used to explain the weak ferromagnetism observed recently in the Kondo lattice materials.

D-wave superconductivity induced by short-range antiferromagnetic correlations in the two-dimensional Kondo lattice model

The possible heavy fermion superconductivity is carefully reexamined in the two-dimensional Kondo lattice model with an antiferromagnetic Heisenberg superexchange between local magnetic moments. In order to establish an effective mean field theory in the limit of the paramagnetic heavy Fermi liquid and near the half-filling case, we find that the spinon singlet pairing from the local antiferromagnetic short-range correlations can reduce the ground state energy substantially. In the presence of the Kondo screening effect, the Cooper pairs between the conduction electrons is induced. Depending on the ratio of the Heisenberg and the Kondo exchange couplings, the resulting superconducting state is characterized by either a d-wave nodal or d-wave nodeless state, and a continuous phase transition exists between these two states. These results are related to some quasi-two dimensional heary fermion superconductors.

Kondo Screening Effect and Ferromagnetic Order in UCu2Si2

Kondo Screening Effect and Ferromagnetic Order in UCu₂Si₂

Unique properties of CuZrAl bulk metallic glasses induced by microalloying

We studied the glass forming abilities (GFA), mechanical, and physical properties of (CuZr)92.5Al7X0.5 (X = La, Sm, Ce, Gd, Ho, Y, and Co) bulk metallic glasses (BMGs). We find that the GFA, mechanical, and physical properties can be markedly changed and modulated by the minor rare earth addition. The Kondo screening effect is found to exist in (CuZr)92.5Al7Ce0.5 BMG at low temperatures and the Schottky effect exists in all the rare earth element doped BMGs. Our results indicate that the minor addition is an effective way for modulating and getting desirable properties of the BMGs. The mechanisms of the effects of the addition are discussed. The results have implications for the exploration of metallic glasses and for improving the mechanical and low temperature physical properties of BMGs.

Application of theS=1underscreened Anderson lattice model to Kondo uranium and neptunium compounds

Magnetic properties of uranium and neptunium compounds showing the coexistence of the Kondo screening effect and ferromagnetic order are investigated within the Anderson lattice Hamiltonian with a two-fold degenerate $f$ level in each site, corresponding to $5{f}^{2}$ electronic configuration with $S=1$ spins. A derivation of the Schrieffer-Wolff transformation is presented and the resulting Hamiltonian has an effective $f$-band term, in addition to the regular exchange Kondo interaction between the $S=1$ $f$ spins and the $s=1/2$ spins of the conduction electrons. The resulting effective Kondo lattice model can describe both the Kondo regime and a weak delocalization of the $5f$ electrons. Within this model we compute the Kondo and Curie temperatures as a function of model parameters, namely the Kondo exchange interaction constant ${J}_{K}$, the magnetic intersite exchange interaction ${J}_{H}$, and the effective $f$ bandwidth. We deduce, therefore, a phase diagram of the model which yields the coexistence of the Kondo effect and ferromagnetic ordering and also accounts for the pressure dependence of the Curie temperature of uranium compounds such as UTe.

Kondo screening coexisting with ferromagnetic order as a possible ground state for Kondo lattice systems

We consider the competition between the Kondo screening effect and ferromagnetic long-range order (FLRO) within a mean-field theory of the Kondo lattice model for low conduction electron densities $n_{c}$. Depending on the parameter values, several types of FLRO ground states are found. When $n_{c}<0.16$, a polarized FLRO phase is dominant. For $0.16<n_{c}<0.82$, a non-polarized FLRO phase appears in the weak Kondo coupling region; while in the intermediate coupling region the ground state corresponds to the polarized and non-polarized FLRO phases, respectively, coexisting with the Kondo screening. For a strong Kondo coupling, the product of pure Kondo singlets is the ground state. Moreover, we also find that a weak magnetic field makes the pure Kondo singlet phase vanish, while the non-polarized FLRO state with the Kondo screening spans a large area in the phase diagram.

Rattling in Clathrate Compounds of ROs4Sb12and R3Pd20Ge6

We have performed ultrasonic experiments on clathrate compounds ROs 4 Sb 12 and R 3 Pd 20 Ge 6 to explore rattling and tunneling of a guest rattler's motion. We have found frequency dependence of the elastic constants and ultrasonic attenuations, which is called as ultrasonic dispersion, due to thermally activated rattling. At low temperatures, Curie-type elastic softening obeying 1/ T with antiferro-type intersite interaction, which corresponds to quantum tunneling, appears and continues even to the lowest 20 mK in LaOs 4 Sb 12 and La 3 Pd 20 Ge 6 . The understanding of the local charge fluctuation, rattling and tunneling, being coupled to conduction electrons in metals is a basic issue in the aspect of electron–phonon interaction in strongly correlated phenomena such as heavy fermion state, superconductivity and multi-channel Kondo screening effect.

Kondo effect in the quantum dot embedded in the Aharonov-Bohm ring

The properties of the ground state of a closed dot-ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the Slave-Boson mean-field theory. It is shown that at T＝０, a suppressed Kondo effect exists in this system even when the mean level spacing of electrons in the ring is larger than the bulk Kondo temperature; the physical quantities of this system depend sensitively on both the parity of the system and the size of the ring; the rich physical behaviors of this system can attribute to the coexistence of both the f inite-size effect and the Kondo screening effect in this system. It is also poss ible to detect the Kondo screening cloud by measuring the persistent current or the zero_field impurity susceptibility χimp directly in future exp eriments.

Finite-size effect and Kondo screening effect in an A-B ring with a quantum dot

The properties of the ground state of a closed dot–ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that at T=0, a suppressed Kondo effect exists in this system even when the mean level spacing of electrons in the ring is larger than the bulk Kondo temperature. The physical quantities depend sensitively on both the parity of the system and the size of the ring; the rich physical behaviour can be attributed to the coexistence of both the finite-size effect and the Kondo screening effect. It is also possible to detect the Kondo screening cloud by measuring the persistent current or the zero field impurity susceptibility χimp directly in future experiments.