Valence Regime Research Articles

Transport properties and μSR spectroscopy of Yb(Ni xCu 1− x) 2Si 2

We report on transport measurements and μSR data for the solid solution Yb(Ni x Cu 1− x ) 2Si 2 (0< x<1). By increasing x, a crossover from an intermediate valence regime to a magnetic Kondo lattice regime is observed. The magnetic order develops above x=0.125.

Ferromagnetism in the periodic Anderson model

Within a recently introduced approximation scheme for the periodic Anderson model, one finds ferromagnetic ordering in a large area of the parameter space away from the symmetric point. Whereas in the Kondo regime, the ferromagnetic phase shows signatures of a local moment magnet, close to the intermediate valence regime, a more band-like magnetism is found.

Electronic configuration of Ce in Ce/Fe(1 0 0) and Ce/Pd(1 0 0)

The electronic state of cerium has been investigated by photoemission spectroscopy in ultra thin films of cerium deposited on monocrystalline Fe and Pd. In both systems, we demonstrated the reduced Ce configuration admixture in the surface layer. We were also able to obtain core level spectra without any surface contribution. In the Ce/Pd system, the “bulk” spectra clearly show an exceptional configuration mixture suggesting that Ce is in the intermediate valence regime.

Hybridization model for a pair of intermediate valence manganese impurities

Manganese ions in a mixed-valent state of two magnetic configurations, and , play an important role in the magnetoresistance of -based systems. We describe each Mn impurity with the represented by a spin S = 3/2 (three localized d electrons in the orbitals with their spins ferromagnetically coupled) and the configuration having an additional localized d electron in one of the orbitals to form a total spin (S + 1/2). The electron hybridizes with the conduction electrons and the multiple occupancy of the level is excluded by a large Coulomb energy at each site. This gives rise to a quadrupolar Kondo effect, which compensates the orbital degrees of freedom into a quadrupolar singlet, and interferes with the usual spin Kondo effect. We consider a pair of such manganese ions and allow the electrons to hop between the two sites. Hence, bonding and antibonding levels are formed giving rise to the ferromagnetic double-exchange mechanism. We study the interaction between the impurities in the integer-valent and the mixed-valent regimes. In the integer-valent limit we renormalize the interactions using the vertex function in the leading logarithmic approximation. Two neighbouring impurities with the same integer valence interact ferromagnetically. ions have in addition a quadrupolar Kondo effect. In the intermediate valence regime we calculate the ground state energy, the valence, the population difference between the bonding and antibonding states, the charge susceptibility, the quadrupolar susceptibility and the response to a charge imbalance between the two sites as a function of the energy of the level in zero magnetic field and for the spin-polarized limit (ferromagnetic lattice) using a mean-field slave-boson formulation. The results indicate that the intersite hopping suppresses charge order and lattice distortions.

Transition from Kondo to intermediate valence regime in : an ESR study

We report on electron spin resonance (ESR) results for the series of compounds doped with Gd. We show that from the Gd g-value and linewidth one can determine the concentration where the system goes from Kondo single impurity to Kondo lattice (and intermediate valence) regimes. The observed behaviour in both the g-value and the slope of the linewidth-T curves is explained using the indirect interaction between Gd and Ce moments. The calculation leads to the determination of the Ce-Gd exchange parameter meV and the Ce-conduction electron exchange parameter meV, for x = 1.

Ferromagnetism within the Periodic Anderson Model: A New Approximation Scheme

We introduce a new approach to the periodic Anderson model (PAM) that allows a detailed investigation of the magnetic properties in the Kondo as well as the intermediate valence regime. Our method is based on an exact mapping of the PAM onto an effective medium strong-coupling Hubbard model. For the latter, the so-called spectral density approach (SDA) is rather well motivated since it is based on exact results in the strong coupling limit. Besides the T=0 phase diagram, magnetization curves and Curie temperatures are presented and discussed with help of temperature-dependent quasiparticle densities of state. In the intermediate valence regime, the hybridization gap plays a major role in determining the magnetic behaviour. Furthermore, our results indicate that ferromagnetism in this parameter regime is not induced by an effective spin-spin interaction between the localized levels mediated by conduction electrons as it is the case in the Kondo regime. The magnetic ordering is rather a single band effect within an effective f-band.

Deuteron structure functions in the context of few-body physics

Few-body systems offer a unique challenge to those interested in deep-inelastic scattering. Using the deuteron as the simplest and most easily solvable nuclear system we review the main physics issues in the valence and sea quark regimes. For the former the key issue is the understanding of Fermi motion and binding corrections, and our ability to extract the ratio F 2 n F 2 p as x → 1. The most recent analysis of deuteron data for F 2 n F 2 p is consistent with perturbative QCD, but inconsistent with the assumptions common to all standard parton distribution fits. This should be taken into account in calculating event rates at HERA — especially at large x and Q 2. The extraction of g 1 n also presents an opportunity to refine the description of nuclear effects in spin-dependent structure functions. In the region of small x one must correct for shadowing and meson exchange current effects to explore the flavor structure of see, which is, in turn, deeply related to the chiral structure of the nucleon.

Exact solution for a one-dimensional multichannel model of correlated electrons with an Anderson-like impurity

We study a one-dimensional n-channel highly correlated electron model (based on the supersymmetric t-J model) with a magnetic impurity of arbitrary spin S that can hybridize with up to one itinerant electron per channel. The model extends the multichannel Kondo problem to correlations in the host and to the mixed valence regime. The Bethe ansatz equations are derived and solved for the ground state and for low temperatures. We conclude that the critical non-Fermi-liquid (overscreened) behaviour cannot be reached for this interacting electron system. Possible applications of the model are discussed.

High-pressure magnetic ordering of YbSi and YbCu 2Si 2

We report high-pressure resistivity measurements of YbSi and YbCu 2Si 2. At ambient pressure, YbSi is a reduced-moment antiferromagnetically ordered Kondo compound, while YbCu 2Si 2 is a non-magnetic system not far from its intermediate valence regime. With increasing pressure, the Kondo temperature T K decreases, magnetic ordering appears for YbCu 2Si 2 and the magnetic ordering temperature T M rises. For YbSi above 5 GPa, the resistivity shows an exponential variation in a narrow temperature range which could be the signature of gap opening.

Toulouse points and non-Fermi-liquid states in the mixed-valence regime of the generalized Anderson model.

We study the mixed valence regime of a generalized Anderson impurity model using the bosonization approach. This single impurity problem is defined by the $U=\infty$ Anderson model with an additional density-density interaction, as well as an explicit exchange interaction, between the impurity and conduction electrons. We find three points in the interaction parameter space at which all the correlation functions can be calculated explicitly. These points represent the mixed valence counterparts of the ususal Toulouse point for the Kondo problem, and are appropriately named the Toulouse points of the mixed valence problem. Two of the Toulouse points exhibit the strong coupling, Fermi liquid behavior. The third one shows spin-charge separation; here, the spin-spin correlation functions are Fermi-liquid-like, the charge-charge correlation functions and the single particle Green function have non-Fermi-liquid behaviors, and a pairing correlation function is enhanced compared to the Fermi liquid case. This third Toulouse point describes the novel intermediate mixed valence phase we have previously identified. In deriving these results, we emphasize the importance of keeping track of the anticommutation relation between the fermion fields when the bosonization method is applied to quantum impurity problems.

Investigation of HgSe/HgSe: Fe quantum wells and super lattices

High-field magnetospectroscopy measurements on HgSe,/HgSe: Fe quantum wells and superlattices are reported. Fe 2+ substitution of Hg 2+ manifests itself in a donor about 210 meV above the conduction band edge and thus for sufficiently high carrier concentration as a Fermi level pinned system in the mixed valence regime. The MBE-grown samples on ZnTe/GaAs substrates exhibit 2D effects for characteristic well or superlattice dimensions < 20 nm. Strong anisotropy effects due to strain are observed. By variation of the size quantization the predicted transition from a 2D to 3D QHE for Fermi level pinned systems is demonstrated.

Asymptotically exact solution of a local copper-oxide model.

We present an asymptotically exact solution of a local copper-oxide model abstracted from the multiband models. The phase diagram is obtained through the renormalization-group analysis of the partition function. In the strong coupling regime, we find an exactly solved line, which crosses the quantum critical point of the mixed valence regime separating two different Fermi-liquid (FL) phases. At this critical point, a many-particle resonance is formed near the chemical potential, and a marginal-FL spectrum can be derived for the spin and charge susceptibilities.

Transport properties of Hg1-xZnxSe and Hg1-xMnxSe doped with Fe resonant donors.

We present here a study of the influence of iron doping on transport and magnetotransport properties of HgSe-based narrow-gap semiconducting mixed crystals, namely ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se (x\ensuremath{\le}0.072) and ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se (x\ensuremath{\le}0.13). The crystals were grown by the Bridgman method. Measurements were performed in the temperature range 1.7--300 K in magnetic fields up to 6 T. For both systems, the electron concentration as a function of increasing concentration of Fe dopants changes in a similar way: it rises proportionally below a certain Fe donor concentration ${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$ (which depends on the mole fraction x of mixed crystal) and then reaches a constant value (which also depends on x). The electron mobility in samples with ${\mathit{n}}_{\mathrm{Fe}}$&gt;${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$ shows anomalously high values, similarly to the case of HgSe:Fe studied earlier. We interpret the observed phenomena by assuming that iron dopants create, in the compounds investigated, a resonant donor state with an energy level degenerate with the conduction band continuum. For ${\mathit{n}}_{\mathrm{Fe}}$&gt;${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$ the system of Fe donors is only partially occupied by electrons, i.e., ${\mathrm{Fe}}^{2+}$ neutral donors and ${\mathrm{Fe}}^{3+}$ singly ionized donors coexist, resulting in a mixed valence regime.The Coulomb repulsion between the electrons localized on donors leads to a correlation of their positions and results in a dramatic reduction of the scattering rate by ionized impurity potentials. The analysis of the mobility (in the case of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se also of the Dingle temperature) in terms of the scattering from ionized centers (with the possible spatial correlation of impurity charges taken into account) and the alloy scattering (in the case of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se also spin-dependent scattering) leads to a fair agreement between the measured data and the theoretical description. The lack of any dependence of the electron concentration upon the level of iron doping in the mixed valence regime (when the Fermi level is pinned to the iron state) enabled us to determine the position of the Fe level as a function of the Zn or Mn mole fraction x. This, in turn, made it possible to estimate the ${\mathrm{\ensuremath{\Gamma}}}_{8}$ and ${\mathrm{\ensuremath{\Gamma}}}_{6}$ band offsets (denoted W and V, respectively) between HgSe and MnSe and between HgSe and ZnSe. The values determined by us are, in the ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se system W=-0.5 eV and V=2.6 eV, and in the case of the ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se system W=-1.2 eV and V=3 eV. Moreover, it is noted that even weak Fe doping (corresponding to ${\mathit{n}}_{\mathrm{Fe}}$${\mathit{n}}_{\mathrm{Fe}}^{\mathrm{*}}$) can lead to a substantial improvement of electrical properties of the compounds studied by us. This is particularly clearly seen for ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Se.

Coulomb versus Madelung Gap: Ordering in a System of Point Charges

Spatial correlations of impurity charges in a mixed valence regime are studied with the use of Monte Carlo simulations. The influence of various kinds of disorder on the one-particle density of states is determined. A continuous transitions from a soft Coulomb gapa (density of states vanishing only at the chemical potential) to a hard gap (with a finite range of energies with vanishing density of states) is found, driven by decreasing amount of built-in disorder in the system. The hard Coulomb gap resembles the Madelung gap, found in crystalline arrangements of charges. The similarity reflects the fact that both the Coulomb and Madelung gaps are manifestations of the same phenomenon, resulting from ordering of the positions of point charges, the only difference being the range of correlations. PACS numbers: 71.55.-i, 72.20.Fr

FIR and transport properties of II–VI narrow gap semiconductors in high magnetic fields

In HgSe:Fe the Fe ++-state acts as a donor degenerate with the conduction band. For a threshold concentration of n Fe =5×10 18 cm −3 the Fermi level gets pinned to the donor energy and spatial correlation effects of the ionized donors become important. Different experimental methods are presented to probe the density of states near the Fermi surface in the mixed valence regime. No final model exists so far to explain the imaging of the density of states at the Fermi energy in all relevant experimental data. For further studies epitaxial layers of Q2D are desirable. As preliminary result for this issue epitaxial layers of pure HgSe have been successfully grown and checked by magnetospectroscopy.

Quantum chemistry, anomalous dimensions, and the breakdown of Fermi liquid theory in strongly correlated systems

We formulate a local picture of strongly correlated systems as a Feynman sum over atomic configurations. The hopping amplitudes between these atomic configurations are identified as the renormalization group charges, which describe the local physics at different energy scales. For a metallic system away from half-filling, the fixed point local Hamiltonian is a generalized Anderson impurity model in the mixed valence regime. There are three types of fixed points: a coherent Fermi liquid (FL) and two classes of self-similar (scale invariant) phases which we denote incoherent metallic states (IMS). When the transitions between the atomic configurations proceed coherently at low energies, the system is a Fermi liquid. Incoherent transitions between the low energy atomic configurations characterize the incoherent metallic states. The initial conditions for the renormalization group flow are determined by the physics at rather high energy scales. This is the domain of local quantum chemistry. We use simple quantum chemistry estimates to specify the basin of attraction of the IMS fixed points.

Local approach to the magnetic susceptibility and specific heat of the Anderson lattice

Green's functions relevant to the periodic Anderson Hamiltonian are evaluated via perturbation theory around the atomic limit, within an approximation that reproduces exact results in three different limits: zero band width, zero hybridization and zero Coulomb correlation. The magnetic susceptibility and the electronic specific heat are calculated for both Kondo and intermediate valence regimes. The results are in qualitative agreement with experiments and can be related to other theoretical calculations.

Correlation of donor electrons in diluted magnetic semiconductors with iron

Substitutional iron in HgSe forms a resonant donor state whose energy is superimposed on the conduction band continuum. Above a certain doping level the system of Fe donors is only partially occupied by electrons, i.e. two charge states, Fe3+ and Fe2+, coexist, corresponding to the inhomogeneous mixed valence regime. Under these conditions the Coulomb repulsion between the donor electrons tends to keep them apart, leading to a correlation of their positions. The existence of the correlation results in a dramatic reduction of the rate of scattering by ionised impurity potentials. The same Coulomb interactions are responsible for the formation of the Coulomb gap in the one-particle density of impurity states, which in turn suppresses (otherwise very efficient) resonant scattering. As a result, the mobility and the Dingle temperature in HgSe:Fe can exceed those in HgSe doped with Ga to similar doping levels. A review is given of relevant experimental results obtained in HgSe:Fe, Hg1-xMnxSe:Fe and HgSe1-xTex:Fe by means of the Shubnikov-de Haas effect, resistivity and Hall voltage studies. Experimental findings are compared with results of numerical simulations and with a simple model calculation which accounts for short-range correlations.

Magnetic properties of the Anderson model with particle-hole attraction

The periodic Anderson model (PAM) has been proved to be very rich, capable of describing diverse phenomena such as the moment localization, mixed valence, heavy fermions etc. In this model the number of holes in the localized states increases as the mixed valence regime is approached. Therefore, the magnetic properties of the model are studied after supplementing PAM with an attractive interaction between the localized holes and electrons in the extended states. The model is made tractable by making the mean-field approximation. This results in the renormalization of the model parameters which become temperature dependent. The equations are derived for the average occupation number of the localized states, of the band states and for the excitonic correlation function for both spin directions. A self-consistent solution of these equations leads to the determination of the average valence n f, the spontaneous magnetization m f and the Curie temperature T C. The influence of various model parameters, with particular emphasis on the particle-hole attraction strength, on n f, m f and T C has been studied. The results are summarised in a phase diagram.

Non-linear magnetic response of intermediate valence state

The linear and non-linear magnetic responses of intermediate valent state have been analysed based on the periodic Anderson model. Within the intermediate valence regime the non-linear magnetic susceptibility shows a sharp dip as a function of temperature and in the limit of zero field, its magnitude is nearly divergent at a temperature T 0 where the zero field linear magnetic susceptibility shows a maximum. The magnetization in the presence of field passes through a broad maximum as a function of temperature. Around the temperature T 0 , the dependence of the total non-linear response on the square of the magnetic field is similar to that of the magnetization on the field in a paramagnet. Such a behaviour of the magnetic response resembles that observed in convential spin-glass state. The excitonic correlation decreases the non-linear response.