LiV2O4: frustration induced heavy fermion metal

We propose a two-stage spin-quenching scenario for the unusual heavy fermion state realized in the mixed valent metal LiV2O4. In this theory, local valence fluctuations are responsible for the formation of partially quenched, spin-1 / 2 moments below room temperature. Frustration of the intersite spin couplings then drives the system to produce the heavy Fermi liquid seen at low temperatures. The anomalous resistivity and the sign change of the Hall constant can be understood naturally within this model, which also predicts a unique symmetry for the heavy quasiparticle bands that may be observed in de Haas-van Alphen experiments.     

Antiferromagnetic correlations in the cubic heavy fermion compound CeInCu2

CeInCu₂ is a heavy fermion compound close to magnetic instability. The electrical resistivity has aT ² behaviour between 1 and 2.5 K. and the AC field susceptibility has a faint maximum at 0.9 K, indicating the onset of coherence near 1 K for this compound. The magnetoresistance keeps a negative sign down to 0.3 K. Neutron inelastic scattering give a crystal field splitting of 90 K, with a doublet ground state, and a residual quasielastic linewidth of 0.3 meV.We have studied the resistivity, susceptibility and specific heat of some dilute solutions La_1–xCe_xInCu₂ and Y_1–xCe_xInCu₂. The spin-dependent part of the resistivity may be decomposed into a single impurity term plus a pair interaction term, the magnitude of which folows a Curie-Weiss law, as in classical spin glassesCuMn orAuFe.The magnetisation and susceptibility at low temperatures may be represented within the resonant level model, taking into account antiferromagnetic interactions. Finally, the specific heat of CeInCu₂ shows a bump near 2.3 K, absent for dilute solutions, which may also be interpreted by introducing a magnetic interaction term.     

Magnetic correlations in heavy fermion CeAl3 compound

To study the effect of hybridization on inter-ion correlations the measurements of the intensity of the quasielastic scattering on momentum transfer in Ce_0.9Y _0.1Al₃, Ce_0.9La_0.1Al₃ and CeAl₃ have been performed. Some hints that quasielastic magnetic scattering more sensitive to regularity in rare-earth sublattice than to the hybridization have been obtained. We also get some indications on the presence of ferromagnetic inter-ion correlations. In a contrast to quasielastic scattering the inelastic part of spectra is very sensitive to the hybridization.     

Enhanced Landau-Placzek ratio in a heavy fermion metal observed by Brillouin scattering

Brillouin spectroscopy of the heavy fermion metal UPt₃ shows strong quasielastic scattering, which has no analogue in ordinary metals. An enhanced Landau-Placzek ratio relating the quasielastic to the inelastic scattering intensity is deduced from the line shape analysis. The quasielastic line is of nonmagnetic origin and is attributed to electron density fluctuations as predicted by a recent theory of hydrodynamic fluctuations in heavy fermion systems.     

Pair correlations in a ferromagnetic colloid

We discuss some properties of the equation of state and of the static correlations for spherical ferromagnetic grains, at thermal equilibrium, in a passive fluid. At high external fieldsH, low concentrations, and not too high temperatures, the grains tend to formchains along the directions ofH. In zero field (but otherwise identical conditions), some chains are still present but oriented at random and in competition with closed rings and clusters. Various experimental methods which could give information on these chain structures are listed.     

YbNi2: A heavy fermion ferromagnet

Measurements of the magnetization and specific heat of YbNi₂ binary alloy are reported. The DC magnetic susceptibility displays a ferromagnetic behavior with a Curie temperature T_C=10.5 K, one of the highest found in Yb compounds. Moreover, the temperature dependence of the specific heat exhibits a lambda anomaly with a peak of 5.12 J/mol K at 9.4 K. The analysis also shows an additional magnetic contribution around 32 K stemming from the crystalline electric field of a quartet at ${\Delta }_1=72\mathrm{K}$ and a doublet at ${\Delta }_2=126\mathrm{K}$, according to the splitting of the Yb³⁺ ion in cubic symmetry. From the magnetic contribution to the specific heat, a relatively high Kondo temperature $T_K=27\mathrm{K}$ is estimated. Below the magnetic transition, the specific heat shows a huge value of the electronic coefficient ${\gamma }_{\mathit{LT}}=573\mathrm{mJ}/\mathrm{mol}\mathrm{K}$, which is a signature of a heavy fermion behavior. Therefore, this alloy is a fine example of enhanced ferromagnetism and heavy fermion behavior among Yb compounds.     

Universality in heavy fermion systems with general degeneracy

We discuss the relation between the T2 coefficient of electrical resistivity A and the T-linear specific-heat coefficient gamma for heavy-fermion systems with general N, where N is the degeneracy of quasiparticles. A set of experimental data reveals that the Kadowaki-Woods relation, A/gamma2=1 x 10(-5) muOmega cm(K mol/mJ)2, collapses remarkably for large-N systems, although this relation has been regarded to be commonly applicable to the Fermi liquids. Instead, based on the Fermi-liquid theory we propose a new relation, A /gamma2=1 x 10(-5) with A =A/1/2N(N-1) and gamma =gamma/1/2N(N-1). This new relation exhibits an excellent agreement with the data for the whole range of degenerate heavy fermions.     

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.     

Superconductivity in heavy fermion systems

The heavy fermion state in the f-electron systems is due to competition between the RKKY interaction and the Kondo effect. The typical compound is CeCu₆. To understand the electronic state, we studied the Fermi surface properties via the de Haas–van Alphen (dHvA) experiment and energy band calculation for CeSn₃,CeRu₂Si₂,UPt₃, and nowadays, transuranium compounds. Pressure is also an important technique to control the electronic state. The Néel temperature T_N decreases with increasing pressure P and becomes zero at the critical pressure for . The typical compound is an antiferromagnet CeRhIn₅, which we studied from the dHvA experiment under pressure. A change of the 4f-electronic state from localized to itinerant is realized at , revealing the first-order phase transition, together with a divergent tendency of the cyclotron mass at P_c. It is stressed that appearance of superconductivity in CeRhIn₅ is closely related to the heavy fermion state. It is also noted that the parity-mixed novel superconducting state might be realized in a pressure-induced superconductor CeIrSi₃ without inversion symmetry in the crystal structure.     

Triplet superconducting correlations in oxide heterostructures with a composite ferromagnetic interlayer

The superconducting current induced by the penetration of the long-range triplet component of superconducting correlations into a composite ferromagnetic interlayer has been detected in mesa-heterostructures based on oxide cuprate superconductors YBa₂Cu₃O_{7 ? δ} and Au/Nb bilayer films with the composite oxide interlayer that is made of ferromagnetic films of manganite La_0.7Sr_0.3MnO₃ and ruthenate SrRuO₃ and has a thickness much larger than the length of correlations determined by the exchange field. The deviation of the superconducting current in the mesa-heterostructure with the fraction of the second harmonic of 13% from a sinusoidal current-phase relation has been detected; this deviation can also be due to the generation of the triplet component of superconducting correlations in the ferromagnet.     

A variational theory of fermion matter with spin dependent correlations

A variational wave function is constructed for a system of fermions interacting with a spin dependent potential. The correlations due to the repulsive core of the potential are described by a spin independent Jastrow product ansatz and the correlations due to the longer range part of the potential, which are assumed to be spin dependent, are described by an independent pair ansatz. A cluster expansion is derived for the variational energy and a set of hypernetted chain (HNC) equations obtained to sum the cluster series in terms of the elementary diagrams. Neglecting the elementary diagrams, the HNC equations are solved numerically for the spin dependent potential, V3, in neutron matter. The introduction of spin dependent correlations is found to give a small lowering of the variational energy in the HNC approximation. The results are very sensitive to an accurate treatment of the many-body terms within the HNC approximation, however, and it is shown that additional approximations can easily lead to an exaggeration of the effect of the spin dependent correlations.     

Positive cross correlations in a three-terminal quantum dot with ferromagnetic contacts

We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads, for instance, to positive zero-frequency cross correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect with respect to polarization of the contacts and tunneling rates.     

Magnetoresistance of a ferromagnetic metal nanocomposite with nonspherical granules

It is experimentally established that the magnetoresistance of a Fex(SiO₂)_1?x nanocomposite (x≈0.6) in a strong magnetic field is described by a logarithmic function of the field strength. This field dependence is inconsistent with the well-known theory of the giant magnetoresistance in ferromagnetic nanocomposites. A model is developed according to which the unusual behavior of the magnetoresistance is explained by nonsphericity of the material grains, exhibiting a broad variety of shapes. The experimental results agree with conclusions and predictions of the proposed model.     

Cluster dual fermion approach to nonlocal correlations

A general cluster dual fermion approach to nonlocal correlations in crystals is formulated. The scheme allows the treatment of long-range correlations beyond the cluster dynamical mean field theory and nonlocal effects in realistic calculations of multiorbital systems. The simplest approximation is shown to exactly correspond to the free-cluster dynamical mean field theory. This approach is applied to the one-dimensional Hubbard model. Already the first dual-fermion correction to the free cluster leads to a drastic improvement of the calculated Green’s function. The text was submitted by the authors in English.     

Cluster dual fermion approach to nonlocal correlations

A general cluster dual fermion approach to nonlocal correlations in crystals is formulated. The scheme allows the treatment of long-range correlations beyond the cluster dynamical mean field theory and nonlocal effects in realistic calculations of multiorbital systems. The simplest approximation is shown to exactly correspond to the free-cluster dynamical mean field theory. This approach is applied to the one-dimensional Hubbard model. Already the first dual-fermion correction to the free cluster leads to a drastic improvement of the calculated Green’s function.     

Spin-spin correlations in ferromagnetic nanosystems

Using exact diagonalization, Monte-Carlo, and mean-field techniques, characteristic temperature scales for ferromagnetic order are discussed for the Ising and the classical anisotropic Heisenberg model on finite lattices in one and two dimensions. The interplay between nearest-neighbor exchange, anisotropy and the presence of surfaces leads, as a function of temperature, to a complex behavior of the distance-dependent spin-spin correlation function, which is very different from what is commonly expected. A finite experimental observation time is considered in addition, which is simulated within the Monte-Carlo approach by an incomplete statistical average. We find strong surface effects for small nanoparticles, which cannot be explained within a simple Landau or mean-field concept and which give rise to characteristic trends of the spin-correlation function in different temperature regimes. Unambiguous definitions of crossover temperatures for finite systems and an effective method to estimate the critical temperature of corresponding infinite systems are given.     

Understanding the heavy fermion phenomenology from a microscopic model

We solve the 3D periodic Anderson model using a two impurity cluster dynamical mean field theory. We obtain the temperature versus hybridization phase diagram. Approaching the quantum critical point (QCP) both the Néel and lattice Kondo temperatures decrease and they do not cross at the lowest temperature we reached. While strong ferromagnetic spin fluctuation on the Kondo side is observed, our result suggests the critical static spin susceptibility is local in space at the QCP. We observe in the crossover region a logarithmic temperature dependence in the specific heat coefficient and spin susceptibility.     

On a negative thermal expansion in heavy fermion systems

We explain negative thermal expansion observed in some heavy fermion systems at low temperatures within a two-component Fermi-liquid picture: α may become negative if Fermi-level falls on a region of steep increase in the density-of-states. The results obtained account for the correlation between the sign of thermal expansion and that of thermopower, and give certain information about the band structure of these systems.     

Pressure effects on heavy fermion alloys

In this Letter, the effects of pressure on heavy fermion alloys are studied in the framework of the Yoshimori–Kasai model under the coherent potential approximation. A unified picture is presented for both the electron-type heavy fermion systems and the hole-type heavy fermion systems. The density of states of f-electrons under the applied pressure and its variation with the alloy concentration are calculated self-consistently. The slave-boson parameter and the Kondo temperature are also obtained. These calculations show the incorporated results of alloying effects and pressure effects, in agreement with the experiments qualitatively.