Evolution from a localized to an intermediate valence regime in Ce2Cu(2-x)Ni(x)In

Polycrystalline samples of the solid solution Ce2Cu(2-x)Ni(x)In were studied by means of x-ray powder diffraction, magnetic susceptibility and electrical resistivity measurements performed in a wide temperature range. Partial substitution of copper atoms by nickel atoms results in a quasi-linear decrease of the lattice parameters and the unit cell volume of the system. The lattice compression leads to an increase in the exchange integral and yields a reversal in the order of the magnetic 4f(1) and nonmagnetic 4f(0) states, being in line with the Doniach phase diagram. In the localized regime, where an interplay of the Kondo scattering and the crystalline electric field effect occurs, the rise in the hybridization strength is accompanied by a relative reduction in the scattering conduction electrons on excited crystal field levels.     

Transition from an antiferromagnetic behaviour to an intermediate valence state in the Ce(Rh1-x Ir x )Ge system

In order to study the transition from an antiferromagnetic behaviour (CeRhGe) to an intermediate valence state (CeIrGe), the Ce(Rh_1-x Ir _x )Ge system has been investigated using various techniques: X-ray diffraction, magnetic susceptibility, electrical resistivity and CeL_III absorption edge. All alloys crystallize in the orthorhombic TiNiSi-type structure with a sharp decrease of the unit cell volume forx=0.25–0.30 corresponding to a cerium valence change.     

First-order transition from a Kondo insulator to a ferromagnetic metal in single crystalline FeSi(1-x)Ge(x)

The phase diagram of FeSi(1-x)Ge(x), obtained from magnetic, thermal, and transport measurements on single crystals, shows a discontinuous transition from Kondo insulator to ferromagnetic metal with x at a critical concentration, x(c) approximately 0.25. The gap of the insulating phase strongly decreases with x. The specific heat gamma coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with an insulator-metal transition induced by a reduction of the hybridization with x in conjunction with disorder on the Si/Ge ligand site.     

Single-Ion Kondo scaling of the coherent Fermi liquid regime in Ce(1-x)La(x)Ni2Ge2

Thermodynamic and transport properties of the La-diluted Kondo lattice CeNi(2)Ge(2) were studied in a wide temperature range. The Ce-rich alloys Ce(1-x)La(x)Ni(2)Ge(2) were found to exhibit distinct features of the coherent heavy Fermi liquid. At intermediate compositions (0.7≤x≤0.9), non-Fermi liquid properties have been observed, followed by the local Fermi liquid behavior in the dilute limit. The 4f-electron contribution to the specific heat was found to follow the predictions of the Kondo-impurity model in both the local as well as the coherent regimes, with the characteristic Kondo temperature decreasing rapidly from about 30 K for the parent compound CeNi(2)Ge(2) to about 1 K in the most dilute samples. The specific heat does not show any evidence for the emergence of a new characteristic energy scale related to the formation of the coherent Kondo lattice.     

Evolution of Kondo resonance from a single impurity molecule to the two-dimensional lattice

The successive spectral evolution of the Kondo resonance state was investigated from a single iron(II) phthalocyanine molecule to the two-dimensional lattice on Au(111) by interrogating the individual molecules with a scanning tunneling microscope. A sharp Kondo peak appears in the single-impurity regime, which broadens and splits as the lattice builds up. The origin of spectral evolution together with the electronic ground state of the lattice are discussed based on the competition of the Kondo effect and Rudermann-Kittel-Kasuya-Yosida coupling between the molecular spins.     

Magnetic excitations in EuCu(2)(Si(x)Ge(1-x))(2): from mixed valence towards magnetism

We report the inelastic neutron scattering study of spin dynamics in EuCu(2)(Si(x)Ge(1-x))(2) (x?=?1, 0.9, 0.75, 0.6), performed in a wide temperature range. At x?=?1 the magnetic excitation spectrum was found to be represented by the double-peak structure well below the energy range of the Eu(3+) spin-orbit (SO) excitation (7)F(0)→(7)F(1), so that at least the high-energy spectral component can be assigned to the renormalized SO transition. Change of the Eu valence towards 2?+ with increased temperature and/or Ge concentration results in further renormalization (lowering the energy) and gradual suppression of both inelastic peaks in the spectrum, along with developing sizeable quasielastic signal. The origin of the spectral structure and its evolution is discussed in terms of excitonic model for the mixed valence state.     

Exciton effects in a scaling theory of intermediate valence and Kondo systems

An interplay of the Kondo scattering and exciton effects (d-f Coulomb interaction) in intermediate valence systems and Kondo lattices is demonstrated to lead to an essential change of the scaling behavior in comparison with the standard Anderson model. In particular, a marginal regime can occur where characteristic fluctuation rate is proportional to flow cutoff parameter. In this regime, the “Kondo temperature” itself is strongly temperature-dependent, which may give a key to the interpretation of controversial experimental data for heavy fermions and related systems.     

Correlated electron state in Ce(1-x)Yb(x)CoIn5 stabilized by cooperative valence fluctuations

X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce(1-x)Yb(x)CoIn5 (0≤x≤1) reveal that many of the characteristic features of the x=0 correlated electron state are stable for x≤0.775 and that phase separation occurs for x>0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T(c) decreases linearly with x towards 0 K as x→1, in contrast with other HF superconductors where T(c) scales with T(coh).     

Pressure induced transition from intermediate valence to metallic behaviour in collapsed SmS

The resistivity ? and Hall coefficient R_h of nominally stoichiometric pressure collapsed SmS single crystals were measured as a function of temperature T (1.5 K< T < 300 K) at several pressures P from 4 to 22 kbar. With increasing pressure we observed a change in the ? (T) and R_h (T) variations from an intermediate valence behaviour (P < 20 kbar) to a metallic behaviour with Kondo like scattering (P > 20 kbar).     

Intermediate valence: Phase diagram and Kondo behaviour in a simple model

We have studied an infinitely narrow band version of the Anderson Hamiltonian. Including a Coulomb repulsion between localized and band states in the mean field approximation, we have been able to describe metal-insulator phase transitions and different properties characteristic of intermediate valence in both phases. These include: non-integer occupation, specific heat, saturating static magnetic succeptibility and dynamical properties such as Mössbauer spectra and spin flip neutron spectra. We discuss for what range of values of the parameters some results can be reproduced by a narrow band version of the Kondo Hamiltonian.     

Quantum critical behavior in an antiferromagnetic heavy-fermion Kondo lattice system(Ce_(1-x)La_x)_2Ir_3Ge_5

The measurements on temperature dependences of magnetic susceptibility χ(T), specific heat C(T), and electrical resistivity ρ(T) were carried out for the antiferromagnetic(AFM)(Ce_(1-x)La_x)_2Ir_3Ge_5(0 ≤ x ≤ 0.66) system. It was found that the Neel temperature TNdecreases with increasing La content x, and reaches 0 K near a critical content xcr =0.6. A new phase diagram was constructed based on these measurements. A non-Fermi liquid behavior in ρ(T) and a log T relationship in C(T) were found in the samples near xcr, indicating them to be near an AFM quantum critical point(QCP) with strong spin fluctuation. Our finding indicates that(Ce_(1-x)La_x)_2Ir_3Ge_5 may be a new platform to search for unconventional superconductivity.     

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)).     

Persistent medium-range order and anomalous liquid properties of Al(1-x)Cu(x) alloys

The development of short-to-medium-range order in atomic arrangements has generally been observed in noncrystalline solid systems such as metallic glasses. Whether such medium-range order (MRO) can exist in materials at well above their melting or glass-transition temperature has been a long-standing important scientific issue. Here, using ab initio molecular dynamics simulations, we show that a novel, persistent MRO exists in liquid Al-Cu alloys near the composition of CuAl3. The correlated atomic motions associated with the MRO give rise to a substantially enhanced viscosity in the vicinity of the composition. The component of the MRO liquid state gradually decreases with increasing temperature, and it disappears above a crossover temperature T(LLC). The continuous liquid-liquid crossover through a percolationlike transition leads to a pronounced heat capacity peak at T(LLC).     

Possible observation of the quadrupolar Kondo effect in dilute quadrupolar system Pr(x)La(1-x)Pb3 for x <or= 0.05

We have studied non-Fermi-liquid (NFL) behavior in Pr(x)La(1-x)Pb3 with Gamma3 quadrupolar moments in the crystalline-electric-field ground state. The specific heat C/T shows NFL behavior in the very dilute region for x 相似文献   

Magnetic behaviour and site occupation of YFe12-x V x (x=2,3,4)

The effects of vanadium substitution on the iron magnetism in YFe_12-x V _x compounds (x～2–4) have been studied by⁵⁷Fe Mössbauer spectroscopy. The preference of vanadium atoms for the 8i site of these ThMn₁₂-structure type compounds is confirmed. The rapid collapse of the iron sublattice magnetization with increasing vanadium concentration is analysed in terms of the ‘magnetic valence’ model.     

Unusual evolution of the conduction-electron state in CexLa1-xB6 from non-Fermi liquid to Fermi liquid

We report unusual evolution of the conduction-electron state in the localized f electron system CexLa1-xB6 from normal electron state to heavy Fermi liquid (FL) state through local FL and non-FL states with increasing Ce concentration and/or with increasing magnetic field. The effective mass of quasiparticle or the coefficient A of T2 term of resistivity is found to increase divergently near the boundary between FL state and non-FL state. The features of the non-FL state are also different from those of the typical non-FL systems previously observed or theoretically predicted.     

From stable divalent to valence-fluctuating behaviour in Eu(Rh(1-x)Ir(x))2Si2 single crystals

We have succeeded in growing high-quality single crystals of the valence-fluctuating system EuIr(2)Si(2), the divalent Eu system EuRh(2)Si(2) and the substitutional alloy Eu(Rh(1-x)Ir(x))(2)Si(2) across the range 0 < x < 1, which we characterized by means of x-ray diffraction, energy-dispersive x-ray spectroscopy, specific heat, magnetization and resistivity measurements. On increasing x, the divalent Eu ground state subsists up to x = 0.25 with a slight increase in Néel temperature, while for 0.3≤x < 0.7 a sharp hysteretic change in susceptibility and resistivity marks the first-order valence transition. For x?0.7 the broad feature observed in the physical properties is characteristic of the continuous valence evolution beyond the critical end point of the valence transition line, and the resistivity is reminiscent of Kondo-like behaviour while the Sommerfeld coefficient indicates a mass renormalization of at least a factor of 8. The resulting phase diagram is similar to those reported for polycrystalline Eu(Pd(1-x)Au(x))(2)Si(2) and EuNi(2)(Si(1-x)Ge(x))(2), confirming its generic character for Eu systems, and markedly different to those of homologue Ce and Yb systems, which present a continuous suppression of the antiferromagnetism accompanied by a very smooth evolution of the valence. We discuss these differences and suggest them to be related to the large polarization energy of the Eu half-filled 4f shell. We further argue that the changes in the rare earth valence between RRh(2)Si(2) and RIr(2)Si(2) (R = Ce, Eu, Yb) are governed by a purely electronic effect and not by a volume effect.