Kondo temperature for the two-channel Kondo models of tunneling centers

A two-channel Kondo (2CK) non-Fermi liquid state in a metal resulting from the interaction between electrons and structural defects modeled by double-well potentials (DWP) is revisited. Account only of the two lowest states in DWP is known to lead to rather low Kondo temperature, T(K). We prove that the contribution of higher excited states reduces T(K), if all of the intermediate states are taken into account. Prefactor in T(K) is shown to be determined by the spacing between the second and the third levels epsilon(3) in DWP rather than by the electron Fermi energy epsilon(F). Since epsilon(3)相似文献   

Physical properties and crystal chemistry of Ce2Ga12Pt

Single crystals of the new ternary compound Ce(2)Ga(12)Pt were prepared by the self-flux technique. The crystal structure with the space group P4/nbm was established from single crystal x-ray diffraction data and presents a derivative of the LaGa(6)Ni(0.6) prototype. Magnetic susceptibility measurements show Curie-Weiss behaviour due to local Ce(3+) moments. At high temperatures, the magnetic anisotropy is dominated by the crystal-electric-field (CEF) effect with the easy axis along the crystallographic c direction. Ce(2)Ga(12)Pt undergoes two antiferromagnetic phase transitions at T(N,1) = 7.3 K and T(N,2) = 5.5 K and presents several metamagnetic transitions for the magnetic field along c. Specific-heat measurements prove the bulk nature of these magnetic transitions and reveal a doublet CEF ground state. The 4f contribution to the resistivity shows a broad maximum at T(max) ≈ 85 K due to Kondo scattering off the CEF ground state and excited levels.     

Interplay between crystal field splitting and Kondo effect in CeNi(9)Ge(4-x)Si(x)

The pseudo-ternary solid solution CeNi(9)Ge(4-x)Si(x) (0?≤?x?≤?4) has been investigated by means of x-ray diffraction, magnetic susceptibility, specific heat, electrical resistivity, thermopower and inelastic neutron scattering studies. The isoelectronic substitution of germanium by silicon atoms causes a dramatic change of the relative strength of competing Kondo, RKKY and crystal field (CF) energy scales. The strongest effect is the continuous elevation of the Kondo temperature T(K) from approximately 3.5?K for CeNi(9)Ge(4) to about 70?K for CeNi(9)Si(4). This increase of the Kondo temperature is attended by a change of the CF level scheme of the Ce ions. The interplay of the different energy scales results in an incipient reduction of the ground state degeneracy from an effectively fourfold degenerate non-magnetic Kondo ground state with unusual non-Fermi-liquid features of CeNi(9)Ge(4) to a lower one, followed by an increase towards a sixfold, fully degenerate ground state multiplet in CeNi(9)Si(4) (T(K)?～?Δ(CF)).     

New spectroscopy solves an old puzzle: the Kondo scale in heavy fermions

Resonant inelastic x-ray scattering (RIXS) yields clear evidence of spectroscopic Kondo scales in heavy fermions. In YbInCu4 and YbAgCu4 RIXS probes the Yb2+ component of the hybrid ground state and the temperature dependence of the Yb 4f occupation. We report a sudden valence change at a phase transition in YbInCu4, but a continuous temperature dependence in YbAgCu4, consistent with the predictions of the Anderson impurity model, for a Kondo temperature T(K) = 70 K. These results solve a long-standing controversy and establish RIXS as a quantitative probe of the electronic structure of strongly correlated electron systems.     

Kondo state for a compact Cr trimer on a metallic surface

The ground state of a Cr trimer supported on the Au(111) surface is investigated by means of a variational approach to the Coqblin-Schrieffer Hamiltonian. The temperature of Kondo-resonance formation (T(K)) for equilateral trimers increases drastically as compared to T(K) for a single Cr adatom. The Kondo state of a Cr trimer proves to be very sensitive to geometry and a small shift of any atom from the symmetrical position leads to a rapid decrease in T(K). These results are in good agreement with recent observations of the Kondo response of a single antiferromagnetic chromium trimer [T. Jamneala, Phys. Rev. Lett. 87, 256804 (2001)]].     

Magnetic-field probing of an SU(4) Kondo resonance in a single-atom transistor

Semiconductor devices have been scaled to the point that transport can be dominated by only a single dopant atom. As a result, in a Si fin-type field effect transistor Kondo physics can govern transport when one electron is bound to the single dopant. Orbital (valley) degrees of freedom, apart from the standard spin, strongly modify the Kondo effect in such systems. Owing to the small size and the s-like orbital symmetry of the ground state of the dopant, these orbital degrees of freedom do not couple to external magnetic fields which allows us to tune the symmetry of the Kondo effect. Here we study this tunable Kondo effect and demonstrate experimentally a symmetry crossover from an SU(4) ground state to a pure orbital SU(2) ground state as a function of magnetic field. Our claim is supported by theoretical calculations that unambiguously show that the SU(2) symmetric case corresponds to a pure valley Kondo effect of fully polarized electrons.     

Intersite coupling effects in a kondo lattice

The La dilution of the Kondo lattice CeCoIn5 is studied. The scaling laws found for the magnetic susceptibility and the specific heat reveal two well-separated energy scales, corresponding to the single-impurity Kondo temperature T(K) and an intersite spin-liquid temperature T(*). The Ce-dilute alloy has the expected Fermi liquid ground state, while the specific heat and resistivity in the dense Kondo regime exhibit non-Fermi-liquid behavior, which scales with T(*). These observations indicate that the screening of the magnetic moments in the lattice involves antiferromagnetic intersite correlations with a larger energy scale in comparison with the Kondo impurity case.     

Order parameter statistics in the critical quantum Ising chain

The probability distribution of the order parameter is expected to take a universal scaling form at a phase transition. In a spin system at a quantum critical point, this corresponds to universal statistics in the distribution of the total magnetization in the low-lying states. We obtain this scaling function exactly for the ground state and first excited state of the critical quantum Ising spin chain. This is achieved through a remarkable relation to the partition function of the anisotropic Kondo problem, which can be computed by exploiting the integrability of the system.     

CePd2Ga3: A new ferromagnetic Kondo lattice

Various temperature-, pressure- and field dependent investigations on CePd₂Ga₃ indicate this ternary compound as belonging to the group of ferromagnetically ordered Kondo lattices, with the Curie temperatureT _C =6K and the Kondo temperatureT _K =4K. The first excited crystal field level of this hexagonal compound is about 40 K above the crystal field ground state, while the overall splitting is much larger.     

Kondo model for the "0.7 anomaly" in transport through a quantum point contact

Experiments on quantum point contacts have highlighted an anomalous conductance plateau around 0.7(2e(2)/h), with features suggestive of the Kondo effect. Here, an Anderson model for transport through a point contact analyzed in the Kondo limit. Hybridization to the band increases abruptly with energy but decreases with valence, so that the background conductance and the Kondo temperature T(K) are dominated by different valence transitions. This accounts for the high residual conductance above T(K). The model explains the observed gate-voltage, temperature, magnetic field, and bias-voltage dependences. A spin-polarized current is predicted even for low magnetic fields.     

Low temperature electron spin resonance of the Kondo ion in a heavy fermion metal: YbRh2Si2

We report an electron spin resonance (ESR) study on single crystals of the heavy fermion metal YbRh2Si2 which shows pronounced non-Fermi liquid behavior related to a close antiferromagnetic quantum critical point. It is shown that the observed ESR spectra can be ascribed to a bulk Yb3+ resonance. This is the first observation of ESR of the Kondo ion itself in a dense Kondo lattice system. The ESR signal occurs below the Kondo temperature (T(K)) which thus indicates the existence of large unscreened Yb3+ moments below T(K). We observe the spin dynamics as well as the static magnetic properties of the Yb3+ spins to be consistent with the results of nuclear magnetic resonance and magnetic susceptibility.     

Conduction through a quantum dot near a singlet-triplet transition

Kondo effect in the vicinity of a singlet-triplet transition in a vertical quantum dot is considered. This system is shown to map onto a special version of the two-impurity Kondo model. At any value of the control parameter, the system has a Fermi-liquid ground state. Explicit expressions for the linear conductance as a function of the control parameter and temperature T are obtained. At T = 0, the conductance reaches the unitary limit approximately 4e(2)/h at the triplet side of the transition, and decreases with the increasing distance to the transition at the singlet side. At finite temperature, the conductance exhibits a peak near the transition point.     

Zero-field kondo splitting and quantum-critical transition in double quantum dots

Double quantum dots offer unique possibilities for the study of many-body correlations. A system containing one Kondo dot and one effectively noninteracting dot maps onto a single-impurity Anderson model with a structured (nonconstant) density of states. Numerical renormalization-group calculations show that, while band filtering through the resonant dot splits the Kondo resonance, the singlet ground state is robust. The system can also be continuously tuned to create a pseudogapped density of states and access a quantum-critical point separating Kondo and non-Kondo phases.     

Fermi-surface reconstruction without breakdown of Kondo screening at the quantum critical point

Motivated by recent Hall-effect experiment in YbRh(2)Si(2), we study ground state properties of a Kondo lattice model in a two-dimensional square lattice using variational Monte Carlo method. We show that there are two types of phase transition, an antiferromagnetic transition and a topological one (Fermi-surface reconstruction). In a wide region of parameters, these two transitions occur simultaneously without the breakdown of Kondo screening, accompanied by a discontinuous change of the Hall coefficient. This result is consistent with the experiment and gives a novel theoretical picture for the quantum critical point in heavy-fermion systems.     

Molecular-field theory of moment reduction in Kondo systems with crystal-field effects

Several dense Kondo compounds have a low-temperature ordered phase in which the magnetic moments are reduced with respect to the values expected for the crystal-field (CEF) ground state. In the present work the phenomenon of moment reduction is studied within a molecular-field theory combined with a variational solution of the one-impurity Anderson model with CEF effects. The calculated zero-temperature magnetization and susceptibility agree well with available exact results; the present method is easily applied to systems of any symmetry. We first study the f ¹ configuration in cubic symmetry, for small values of the ratio T _K/Δ between Kondo temperature and CEF splitting. With a Γ _∞ ground state and a field along a 〈100〉 direction, an inflection point occurs in the magnetization curve, which gives rise to a first order transition in the zero-temperature phase diagram. This feature is not found for a field along 〈110〉 or 〈111〉, for which the transition is second order. For a Γ ₇ ground state and small values of T _K/Δ, the magnetic-nonmagnetic transition is second order for all field directions. On increasing T _K/Δ an inflection point in the magnetization curve appears first for a field along 〈111〉. The theory is applied to a study of cubic CeAg, CeAl₂, CePb₃, CeIn₃, CeTe, and hexagonal CePd₂Ga₃. The bare value of the moment is found to be strongly increased by exchange coupling to excited CEF states, and the amount of Kondo reduction is found to be substantial, confirming the importance of the Kondo effect in these compounds.     

Kondo tunneling through real and artificial molecules

When an asymmetric double dot is hybridized with itinerant electrons, its singlet ground state and lowly excited triplet state cross, leading to a competition between the Zhang-Rice mechanism of singlet-triplet splitting in a confined cluster and the Kondo effect (which accompanies the tunneling through quantum dot under a Coulomb blockade restriction). The rich physics of an underscreened S = 1 Kondo impurity in the presence of low-lying triplet-singlet excitations is exposed and estimates of the magnetic susceptibility and the electric conductance are presented, together with applications for molecule chemisorption on metallic substrates.     

Chiral splitting of Kondo peak in triangular triple quantum dot

New characteristics of the Kondo effect, arising from spin chirality induced by the Berry phase in the equilibrium state, are investigated. The analysis is based on the hierarchical equations of motion (HEOM) approach in a triangular triple quantum-dot (TTQD) structure. In the absence of magnetic field, TTQD has four-fold degenerate chiral ground states with degenerate spin chirality. When a perpendicular magnetic field is applied, the chiral interaction is induced by the magnetic flux threading through TTQD and the four-fold degenerate states split into two chiral state pairs. The chiral excited states manifest as chiral splitting of the Kondo peak in the spectral function. The theoretical analysis is confirmed by the numerical computations. Furthermore, under a Zeeman magnetic field B, the chiral Kondo peak splits into four peaks, owing to the splitting of spin freedom. The influence of spin chirality on the Kondo effect signifies an important role of the phase factor. This work provides insight into the quantum transport of strongly correlated electronic systems.     

Two-stage Kondo effect in a quantum dot at a high magnetic field

We report a strong Kondo effect (Kondo temperature approximately 4 K) at high magnetic field in a selective area growth semiconductor quantum dot. The Kondo effect is ascribed to a singlet-triplet transition in the ground state of the dot. At the transition, the low-temperature conductance approaches the unitary limit. Away from the transition, for low bias voltages and temperatures, the conductance is sharply reduced. The observed behavior is compared to predictions for a two-stage Kondo effect in quantum dots coupled to single-channel leads.     

Low temperature properties of a new compound based on (Yb,Zr)B<Subscript>12</Subscript> substitution

The thermodynamic, kinetic, and magnetic characteristics of (Yb, Zr)B₁₂ have been investigated in detail in the temperature range from 4 to 300 K to reveal the effect of the band structure on the properties of the YbB₁₂ Kondo insulator ground state. It is found that electron doping due to 20% substitution in the Yb sublattice significantly changes the properties of the low-temperature ground state of YbB₁₂ Kondo insulator and only slightly affects the high-temperature spin-fluctuating state forming at T > 50 K.