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  • 1
    Electronic Resource
    Electronic Resource
    Overcompensated impurities in antiferromagnetic Heisenberg chains : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 6071-6073 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An impurity of spin S'=1/2 is introduced in the Babujian-Takhtajian Heisenberg model of spin S in one dimension. The impurity is located on the mth link of the chain and interacts with both neighboring sites. The interaction between impurity and Heisenberg chain is of a special type so that the integrability is preserved. The diagonalization of the transfer matrices leads to the Bethe ansatz equations of the model. The thermodynamics of the system is studied. For ferromagnetic coupling in the Heisenberg chain, the impurity is locked into the critical behavior of the lattice, i.e., at low temperatures the specific heat is proportional to T1/2 (ferromagnetic magnons) and the susceptibility diverges as T−2 (with logarithmic corrections). For antiferromagnetic coupling and S =1/2 the impurity is just one more site in the chain. If S (approximately-greater-than)1/2 the collective properties of impurity and lattice give rise to critical behavior in complete analogy to the overcompensated n-channel Kondo problem. The zero-temperature entropy is finite in zero-field, but zero if the field is nonzero, giving rise to an essential singularity at T=H=0. As a consequence a two-peak structure arises in the specific heat in a small but finite field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350049
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  • 2
    Electronic Resource
    Electronic Resource
    Isolated ferromagnetic bonds in the two-dimensional spin-1/2 Heisenberg antiferromagnet (abstract) : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 4902-4902 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Several mechanisms for high-temperature superconductivity invoke the strong antiferromagnetic correlations within the CuO planes. In particular, properties of the high-Tc compounds are believed to be related to defects in the planes, e.g., static vacancies, ferromagnetic bonds, and mobile holes. We consider the effects of isolated ferromagnetic links on an otherwise antiferromagnetic square lattice. The addition of holes in La2CuO4 (by doping with Sr) introduces a local ferromagnetic exchange coupling between Cu spins. We describe the Heisenberg antiferromagnet within the linearized spin-wave theory; the problem of a single ferromagnetic bond embedded in an otherwise antiferromagnetic square lattice can then be solved exactly. The procedure is similar to the one employed in Ref. 1 for a vacancy. The longitudinal terms, involving Sz, tend to enhance the ordered staggered magnetic moment, competing in this way with the transverse terms, involving Sx and Sy, which represent the quantum fluctuations and suppress the sublattice magnetization. The interplay between these two interactions is discussed as a function of the ferromagnetic coupling strength.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348220
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  • 3
    Electronic Resource
    Electronic Resource
    Magnetic and thermodynamic properties of nonsuperconducting (Y,Pr)Ba2Cu3O6 : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 5376-5378 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The concentration dependence of the room-temperature lattice constants and the concentration and temperature dependence of the specific heat C(T) and the magnetic susceptibility have been measured on the tetragonal, nonsuperconducting phase of Y1−xPrxBa2Cu3O6. The effective paramagnetic moment μeff is independent of x and has an average value of 2.7±0.2 μB. For PrBa2Cu3O6, the Néel temperature TN for Pr moment ordering is 10.5 K as compared to TN=17 K for the orthorhombic PrBa2Cu3O7 phase. The TN is depressed rapidly with Y doping in the oxygen-depleted compound and goes to zero at x≈0.4–0.5. At low temperatures and x≤0.5, C(T)/T vs T2 is nonlinear and shows a rapid decrease of C(T)/T indicative of strong magnetic correlations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348037
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  • 4
    Electronic Resource
    Electronic Resource
    Superconducting fluctuations in the Hubbard model with an attractive on-site interaction in one dimension (1990)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 5740-5742 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The low-temperature thermodynamics of the one-dimensional Hubbard model with attractive on-site interaction of the electrons is discussed within the framework of the Bethe ansatz for the wave function. At low T the dominant states consist of spin-paired electrons (Cooper-pair-like) and excitations of such pairs (without breaking up the singlet bound states). A critical field Hc (energy required to depair a Cooper pair) is obtained at T=0 (note that the superconducting Tc =0), which disappears for T≠0. There is no magnetic response at T=0 for H〈Hc . The one-electron correlation function falls off exponentially with distance, while the singlet pair–singlet pair correlation decreases with a power law. The specific heat is linear in temperature, which is possibly a consequence of the dimension. The elementary excitations of the system are discussed. The excitation spectrum shows some analogies to that of resonant valence bonds in two dimensions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.345948
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  • 5
    Electronic Resource
    Electronic Resource
    Groundstate properties of the degenerate supersymmetric t−J model with crystalline field splitting in one dimension : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6645-6647 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The one-dimensional t−J model is considered, consisting of electrons with four spin-components on a lattice with nearest neighbor hopping t constrained by the excluded multiple occupancy of the sites, exchange J between neighboring sites and a crystal field splitting Δ. This model is integrable at the supersymmetric point. The exact solution is presented via Bethe's ansatz and groundstate properties as a function of Δ are discussed. The model could correspond to a two-band situation with one band being the z2 and the other one the x2−y2 orbitals of Cu2+.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352541
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  • 6
    Electronic Resource
    Electronic Resource
    Applications of the overcompensated n-channel Kondo problem : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 5806-5808 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We discuss three applications of the overcompensated n-channel Kondo problem for an impurity of spin S =1/2 in the context of the exact numerical solution (as a function of field, temperature, and number of channels) of the Bethe-ansatz equations. The entropy has an essential singularity as H and T tend to zero, giving rise to critical behavior in the susceptibility and the specific heat. The specific heat in a small constant (nonzero) field shows a double peak structure, arising from the singularity in the entropy. This gives rise to giant γ values of the specific heat. The applications discussed here are (i) the quadrupolar Kondo effect, (ii) the low temperature properties of a two-level system interacting with conduction electrons, and (iii) a S =1/2 magnetic impurity embedded in a SU(2)-invariant Heisenberg chain of arbitrary spin (Takhtajan–Babujian model). The divergent quadrupolar susceptibility in (i) gives rise to a local tetragonal distortion of the lattice below a critical temperature Tc, which depends on the strength of the spin-lattice coupling. The same divergence destabilizes the low-temperature strong-coupling fixed point of (ii) for a symmetric double well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350143
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  • 7
    Electronic Resource
    Electronic Resource
    The heavy fermion compound YbAgCu4 : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5412-5414 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We re-examine the experimental data for the specific heat, the magnetic susceptibility, magnetization, and inelastic neutron scattering for the moderately heavy-electron compound YbAgCu4 within the framework of the single-ion Anderson model. Such an analysis has previously given excellent agreement between theory and experiment for numerous light heavy-fermion compounds. We extend here previous interpretations for the specific heat and magnetization within the Coqblin–Schrieffer model to a common set of model parameters by incorporating a weak valence admixture. The crystalline field splitting in this compound appears to be small compared to the Kondo temperature. Existing exact solutions of the Anderson model within the framework of Bethe's ansatz are used for the thermodynamic properties. A simple resonant-level model is introduced to parametrize the Kondo resonance and to qualitatively interpret the inelastic neutron scattering cross section.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353699
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  • 8
    Electronic Resource
    Electronic Resource
    A simple theory of the Kondo hole : 35th annual conference on magnetism and magnetic materials (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 5478-5480 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Kondo hole is an isolated nonmagnetic impurity in a heavy-fermion lattice. Experimentally, the Kondo hole can be realized by La or Th atoms replacing a Ce ion in a stoichiometric Ce compound or by Th impurities in a U–heavy-fermion system. Adding impurities to a Kondo lattice gradually destroys the coherence of the heavy-fermion ground state. Perturbations of the coherent state are conveniently studied in the f-electron density of states. We consider a dispersionless orbitally nondegenerate f band locally hybridized with one nearest-neighbor hopping tight-binding conduction band on a simple cubic lattice. A hybridization gap opens in the symmetric case, and the Kondo hole introduces a bound state in the gap. We study the effects of the scattering on the local density of f states in the neighborhood of the nonmagnetic impurity. The effects of the correlations within the f band are introduced via a self-energy, evaluated to second-order perturbation in U, using the d=∞ treatment of Schweitzer and Czycholl [Solid State Commun. 74, 735 (1990)].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347971
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  • 9
    Electronic Resource
    Electronic Resource
    Ferromagnetic instability in a mean-field slave-boson approach for the Anderson lattice : 37th Annual conference on magnetism and magnetic materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 5400-5402 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We consider a stoichiometric metallic Anderson lattice with orbital degeneracy in the U→∞ limit. A Gutzwiller type of mean-field approximation is formulated in terms of three slave bosons per site in analogy to Kotliar and Ruckenstein's approach for the Hubbard model. In the orbitally nondegenerate case the paramagnetic solution becomes unstable towards ferromagnetism if the valence is smaller than a critical one. This instability is suppressed with increasing orbital degeneracy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354045
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  • 10
    Electronic Resource
    Electronic Resource
    Local density of states in the vicinity of a Kondo hole : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 5803-5805 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The substitution of magnetic ions by nonmagnetic impurities in a Kondo lattice gradually destroys the coherence of the heavy fermion groundstate. These nonmagnetic impurities are frequently referred to as Kondo holes. We consider a simple cubic Anderson lattice without orbital degeneracy and study the effects of the scattering off the Kondo hole in the local density of f states in the neighborhood of the nonmagnetic impurity. The correlations within the f band are introduced via a self-energy, evaluated to second order perturbation in U. We use the 1/d expansion method of Schweitzer and Czycholl to leading order (d=∞) in which the k integrations are properly carried out, but the k dependence of the self-energy is neglected. For a Kondo insulator we find a δ-function-like boundstate in the gap. The spectral weight of the boundstate decreases rapidly with increasing distance from the impurity. In the metallic case we obtain a resonance of finite width in the pseudogap of the lattice, which again is localized in the neighborhood of the Kondo hole. These states only appear in the coherent phase and disappear in the continuum at higher temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350142
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