Session Z23: Strongly Correlated Electrons II

Yb- and Ce- based, dilute rare earth intermetallic compounds: Fully ordered compounds that approach the single Kondo-impurity limit

Dilute, rare earth intermetallic compounds are ordered structures in which the rare earth comprises less than $5\%$ atomic of the compound, but still fully occupies a unique crystallographic site. Whereas for local moment bearing R-members these series offer the possibility of studying the RKKY interaction for relatively large R – R spacing, the R = Yb and Ce members offer a unique opportunity to study the Kondo lattice for concentrations that start to approach the single ion limit. In this talk we will present data on six Yb-based, and two Ce-based, dilute rare earth intermetallic compounds. Despite the rather different behavior of the analogous Gd-based members, the thermodynamic and transport properties of the Yb-based materials are rather similar. This new family of heavy Fermion materials promises to be a fruitful testing ground for current theories of correlated electron physics.   

The low temperature thermodynamic properties and Hall effect in YbAgGe close to the field-induced quantum

We present temperature and field dependent heat capacity and magnetization data ($T \geq 50$ mK, $H \leq 11.5$ T) as well as field dependent Hall effect data ($T \geq 50$ mK, $H \leq 18$ T) for YbAgGe, a heavy-fermion compound with a field induced quantum critical point. These data clearly indicate that the same electronic degrees of freedom are responsible for the features seen in both specific heat and magnetization data. In addition, they further refine the different boundaries suggested for the $H - T$ phase diagram of YbAgGe through previous, magneto-transport measurements, and allow for further understanding of different phases on the $H - T$ phase diagram, in particular, clearly disconnecting the field-induced quantum critical point in YbAgGe from any sort of saturation of the Yb moment in higher applied magnetic field.   

Study of low carrier density heavy fermion Ce$_{3}$Au$_{3}$Sb$_{4}$

Ce$_{3}$Au$_{3}$Sb$_{4}$ is a narrow gap semiconductor with a well localized Ce$^{3+}$ state. The specific heat measurements, however, show a Kondo impurity like behavior with a large increase of the specific heat coefficient at low temperature, suggesting a heavy fermionic character in this low carrier concentration system. Magnetic susceptibility, resistivity, and specific heat data will be shown for single crystal Ce$_{3}$Au$_{3}$Sb$_{4}$. The dilution study with La substituted on the Ce site will be also presented to further discuss the Kondo effect in this system.   

Rare-earth nitrides: an LSDA+U study

The class of rare-earth (Ce-Lu) nitrides all form in the rocksalt crystal structure. One member in particular, GdN, has received considerable theoretical and experimental interest because it appears to be a bulk ferromagnetic semiconductor. The other members of this class of compounds have received much less attention. Electronic structure calculations for the entire series are presented here using a full-potential linear muffin-tin orbital (FP-LMTO) method within the LSDA+U approach. In this approach the localized orbitals have their Coulomb interactions treated in a screened Hartree-Fock mean field approximation while the remaining orbitals are treated in the standard local spin density approximation. $U$ terms were also added to the rare-earth 5$d$ states to correct for the underestimation of the band gap in LSDA. In a cubic field, the 4$f$ states split into triply degenerate $t_{1u}$ and $t_{2u}$ states and a singly degenerate $a_{2u}$ state. The large Hubbard $U$ is found to dominate crystal field splitting to determine the order of minority and majority spins. Avoiding partially filled states at the Fermi level is the dominant principle, yielding narrow gap semiconducting or semimetallic band structures. In a few cases, namely those which correspond to configurations deviating by 2 electrons from a completely empty, completely full, or half-full configuration, however, an $f$-band is forced to cross the Fermi level, possibly resulting in heavy-fermion metallic behavior.   

Electronic structure of CrN: a Mott insulator

It was recently reported by D. Gall et al. [J. Appl. Phys. 91, 5882, 2002] that CrN in the rocksalt structure has an optical band gap of about 0.7 eV, though in local spin density approximation, this materials is predicted to be metallic. We examine this possibility using the LSDA+U approach in the fully localized limit implemented in our full-potential muffin-tin orbital method. Slater integral \(F^{0}=U\) is screened such that the position of the occupied 3$d$ levels agrees well with the photoelectron spectra of CrN. We find that a band gap opens in the band structure. The actual value of the gap obviously depends on the choice of $U$. To understand the origin of the gap it is essential to study how the $d$ states split in cubic symmetry and what their filling is. Cr in forming CrN is trivalent and hence has three $3d$ electrons. The Cr $e_g$, form antibonding states in the conduction band. Adding $U$ tends to push these empty states further up for both spins. The $t_{2g}$ on the other hand form weaker $\pi$ bonds with N $2p$, which in LSDA occur near the Fermi energy. Adding a Hubbard $U$ now shift the majority spin electrons by $-U/2$ and the minority spin ones by $U/2$ and remove them from the Fermi level. This works because the three fold degenerate majority spin state $t_{2g\uparrow}$ becomes completely filled while the minority $t_{2g\downarrow}$ becomes empty. The valence band maximum then has predominantly N$2p$ character, which makes CrN a charge transfer type Mott-insulator.   

Free Magnetic Moments in Disordered Metals

The screening of magnetic moments in metals, the Kondo effect, is found to be quenched with a finite probability in the presence of nonmagnetic disorder. Numerical results for a disordered electron system show that the distribution of Kondo temperatures deviates strongly from the result expected from random matrix theory even in the diffusive regime. A pronounced second peak emerges for small Kondo temperatures, showing that the probability that magnetic moments remain unscreened at low temperatures increases with disorder. Analytical calculations, taking into account correlations between eigenfunction intensities yield a finite width for the distribution that survives the thermodynamic limit. Experimental consequences for the electron dephasing in disordered mesoscopic metals and the thermodynamic properties of heavy-fermion compounds are discussed.   

d-wave Kondo liquids

Conventional heavy Fermi liquid phases of Kondo lattices involve the formation of a ``Kondo singlet'' between the local moments and the conduction electrons. This Kondo singlet is usually taken to be in an internal s-wave angular momentum state. Here we explore the possibility of Fermi liquid phases where the Kondo singlet has internal angular momentum that is d- wave. Such states are readily acessed in a slave boson mean field formulation, and are energetically favorable when the Kondo interaction is between a local moment and an electron at a nearest neighbor site. The properties of the d- wave Kondo lattice are studied. Effective mass and quasiparticle residue show large angle dependence on the Fermi surface. We suggest that such d-wave Kondo pairing may provide a useful route to thinking about correlated Fermi liquids with strong anisotropy along the Fermi surface.   

Randomly Depleted Kondo Lattices

How is the ``heavy Fermi liquid'' of the Kondo lattice connected to the Kondo impurity ``local Fermi liquid'' as the concentration of f-moments is varied? We study the ensuing interplay of strong correlations and disorder within an $SU(N)$ model in a $N\rightarrow \infty$ limit. Although normal Fermi liquid behavior is well defined close to the two limits of Kondo-lattice and Kondo-impurity, we find that in between them there are strongly in-homogeneous phases with large fractions of quasi-free moments that result in a violation of the usual Fermi-liquid behavior. The full spatial dependence of the mean field solutions allows us to study the depletion driven evolution of inhomogeneities in local quantities like the local susceptibility and the local spectral function in detail. We describe the relevance of our results to recent experiments on $Ce_{1-x}La_{x}CoIn_{5}$.   

Multipole properties of one-dimensional $f$-electron systems

By exploiting a density matrix renormalization group method, we investigate the ground-state properties of a one-dimensional three-orbital Hubbard model on the basis of a $j$-$j$ coupling scheme. Here we focus on the case where the $f$-electron number per site is one ($f^1$). When three orbitals are degenerate, we observe a peak at $q$=0 in $\Gamma_{3g}$ quadrupole correlation, indicating a ferro-orbital state. Namely, $f$ electron occupies an itinerant $\Gamma_8^b$ orbital to gain kinetic energy, while localized $\Gamma_8^a$ and $\Gamma_7$ orbitals are found to be almost empty. Furthermore, we find a peak at $q$=$\pi$ in $\Gamma_{4u}$ dipole correlation, suggesting an antiferromagnetic state. On the other hand, when we take account of the level splitting between $\Gamma_8$ and $\Gamma_7$ orbitals, due to the competition between itinerant and localized orbitals, we observe a characteristic change of $\Gamma_{3g}$ quadrupole correlation into an incommensurate structure in accordance with the change of the orbital structure. We will also discuss a key role of multipole degrees of freedom in $f^2$- and $f^3$-electron systems.   

Correlation Behavior in Nanoassembled Spin Lattices

The single-impurity Kondo problem, in which an isolated magnetic impurity in a non-magnetic metallic host has its spin screened by that of the conduction electrons, has been extensively studied both theoretically and through bulk experiments. Only recently, however, have new methods allowed detailed experimental probing of single-impurity Kondo effect of individual magnetic atoms. When many magnetic impurities are present in a bulk conductor or on its surface, the interactions between them may engender novel collective effects. Using a scanning tunneling microscope (STM) we assembled and studied atomically precise periodic arrangements of (magnetic) Co atoms and (non-magnetic) CO molecules on the Cu(111) surface. We observe signs of spin correlation effects when the lattice row spacing for the Co lattices approached half of the Fermi wavelength for the Cu(111) surface electrons. Removing the central atom in each lattice and directly probing the energetics of the resulting hole provides a novel method to search for global spin correlations and investigate Kondo hole behavior in these systems.   

Studies on Single Crystal CeCo3B2

Magnetization, resisitivity, and specific heat measurements, under pressure, on weak ferromagnet CeCo3B2 are reported. This hexagonal structure with space group P6/mmm (D$^{1}_{6h})$, and one formula unit per unit cell, forms in the CaCu$_{5}$-type structure. At ambient pressure, dc and ac susceptibility displayed an ordering below a T$_{curie}$ of $\sim $210K, with the highly anisotropic ordering along the c axis, which is weakly suppressed with the application of pressure(- 4 K/GPa). Along the ordering axis, the saturation moment, taking into account the linear behavior at higher fields, reaches only $\sim $0.01$\mu _{B}$ at 2K. This work was supported by NSF-DMR 0433560.   

Evolving Magnetism from self damage in $\alpha $ and $\delta $-Pu

As a consequence of the unusual nature of plutonium's electronic structure, point- and extended-defects are expected to, and do exhibit extraordinary properties. Low temperature magnetic susceptibility measurements on Pu and fcc-Pu(Ga) show that the magnetic susceptibility increases as a function of time, yet upon annealing the specimen returns to its initial magnetic susceptibility. This excess magnetic susceptibility (EMS) arises from the $\alpha $-decay and U recoil damage cascades which produce vacancy and interstitials as point and extended defects. The time dependence of the EMS leads to an estimate of the number of atoms influenced by each $\alpha $-decay, as well as a temperature dependence well described by a Curie-Weiss law. These results will be described in context of recent time dependent studies of the elastic modulus in delta Pu. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.   

Volume collapse of Cerium

The gamma-to-alpha (low-density fcc to high-density fcc) transition in cerium alloys is measured as a function of magnetic field using specific heat, magnetization, and thermal expansion. The dependence of the transition on magnetic field is discussed within the context of the entropy of the 4f moments in the gamma phase.   

Evidence for Kondo effect in Zn:Fe

Zn films doped with $^{57}$Fe (atomic concentration c between 0.2 and 2.0 at \%) have been prepared by the co-evaporation of the two metals onto a cooled substrate ($\rm T_s$ = 80 K). \emph{In- situ} electrical resistance measurements on these films show resistance minima at $\rm T_K$ ($\rm T_K$ = 10 K for c = 1.0 at \%) which disappear in an external magnetic field $\rm B_{ex}$ = 5 T and, therefore, are interpreted as Kondo minima. $^{57}$Fe M\"{o} ssbauer effect studies on the films with c = 0.6 at \% taken at T = 4.3 K and for various $\rm B_{ex}$ values, reveal that the effective paramagnetic Fe moments, $\mu$, are $\mu$ $<$ 0.1 $\mu_{B}$ for $\rm B_{ex}$ $=$ 3 T. For larger magnetic fields the magnitude of $\mu$ is increasing with increasing $\rm B_{ex}$ [$\mu$ $\sim$ 0.30(3) $\mu_{B}$ for $\rm B_{ex}$ = 7 T], as expected for a spin-compensated Kondo state.