Metamagnetism and Lifshitz Transitions in Models for Heavy Fermions

We investigate metamagnetic transitions in models for heavy fermions by considering the doped Kondo lattice model in two dimensions. Results are obtained within the framework of dynamical mean field and dynamical cluster approximations. Universal magnetization curves for different temperatures and Kondo couplings develop upon scaling with the lattice coherence temperature. Furthermore, the coupling of the local moments to the magnetic field is varied to take into account the different Land\'e factors of localized and itinerant electrons. The competition between the lattice coherence scale and the Zeeman energy scale allows for two interpretations of the metamagnetism in heavy fermions: Kondo breakdown or Lifshitz transitions. By tracking the single-particle residue through the transition, we can uniquely conclude in favor of the Lifshitz transition scenario. In this scenario, a quasiparticle band drops below the Fermi energy which leads to a change in topology of the Fermi surface.Comment: 8 pages, 7 figure

The modulated spin liquid: a new paradigm for URu2_2Si2_2

We argue that near a Kondo breakdown critical point, a spin liquid with spatial modulations can form. Unlike its uniform counterpart, we find that this occurs via a second order phase transition. The amount of entropy quenched when ordering is of the same magnitude as for an antiferromagnet. Moreover, the two states are competitive, and at low temperatures are separated by a first order phase transition. The modulated spin liquid we find breaks $Z_4$ symmetry, as recently seen in the hidden order phase of URu$_2$Si$_2$. Based on this, we suggest that the modulated spin liquid is a viable candidate for this unique phase of matter.Comment: 4 pages, 2 figure

Coherence scale of the Kondo lattice

It is shown that the large-N approach yields two energy scales for the Kondo lattice model. The single-impurity Kondo temperature, $T_K$, signals the onset of local singlet formation, while Fermi liquid coherence sets in only below a lower scale, $T^{\star}$. At low conduction electron density $n_c$ ("exhaustion" limit), the ratio $T^{\star}/T_K$ is much smaller than unity, and is shown to depend only on $n_c$ and not on the Kondo coupling. The physical meaning of these two scales is demonstrated by computing several quantities as a function of $n_c$ and temperature.Comment: 4 pages, 4 eps figures. Minor changes. To appear in Phys. Rev. Let

Effect of Disorder on Fermi surface in Heavy Electron Systems

The Kondo lattice model with substitutional disorder is studied with attention to the size of the Fermi surface and the associated Dingle temperature. The model serves for understanding heavy-fermion Ce compounds alloyed with La according to substitution Ce{x}La{1-x}. The Fermi surface is identified from the steepest change of the momentum distribution of conduction electrons, and is derived at low enough temperature by the dynamical mean-field theory (DMFT) combined with the coherent potential approximation (CPA). The Fermi surface without magnetic field increases in size with decreasing x from x=1 (Ce end), and disappears at such x that gives the same number of localized spins as that of conduction electrons. From the opposite limit of x=0 (La end), the Fermi surface broadens quickly as x increases, but stays at the same position as that of the La end. With increasing magnetic field, a metamagnetic transition occurs, and the Fermi surface above the critical field changes continuously across the whole range of x. The Dingle temperature takes a maximum around x=0.5. Implication of the results to experimental observation is discussed.Comment: 5 pages, 5 figure

Nonlocal interactions in doped cuprates: correlated motion of Zhang-Rice polarons

In-plane, inter-carrier correlations in hole doped cuprates are investigated by ab initio multiconfiguration calculations. The dressed carriers display features that are reminiscent of both Zhang-Rice (ZR) CuO4 singlet states and Jahn-Teller polarons. The interaction between these quasiparticles is repulsive. At doping levels that are high enough, the interplay between long-range unscreened Coulomb interactions and long-range phase coherence among the O-ion half-breathing vibrations on the ZR plaquettes may lead to a strong reduction of the effective adiabatic energy barrier associated to each polaronic state. Tunneling effects cannot be neglected for a relatively flat, multi-well energy landscape. We suggest that the coherent, superconducting quantum state is the result of such coherent quantum lattice fluctuations involving the in-plane O ions. Our findings appear to support models where the superconductivity is related to a lowering of the in-plane kinetic energy

Upper limit on the Ks ---> 3 pi0 decay

The search for CP-violating K_S\to3\pi^0 decay was performed with SND detector at VEPP-2M e^+e^- collider. The total amount of data corresponding to 7 million produced K_S mesons was analyzed. No candidate events were found, giving an upper limit of the decay branching ratio: Br(K_S\to3\pi^0) < 1.4\cdot10^{-5} at 90% confidence level.Comment: 6 pages, 4 figures, to be published in Physics Letters

Two energy scales and slow crossover in YbAl3

Experimental results for the susceptibility, specific heat, 4f occupation number, Hall effect and magnetoresistance for single crystals of YbAl$_{3}$ show that, in addition to the Kondo energy scale $k_{B}T_{K}$ $$ 670K, there is a low temperature scale $T_{coh}<50$K for the onset of coherence. Furthermore the crossover from the low temperature Fermi liquid regime to the high temperature local moment regime is slower than predicted by the Anderson impurity model. These effects may reflect the behavior of the Anderson Lattice in the limit of low conduction electron density.Comment: Ten pages, including three figure

Electronic states and magnetic excitations in LiV2O4: Exact diagonalization study

Motivated by recent inelastic neutron scattering experiment we examine magnetic properties of LiV2O4. We consider a model which describes the half-filled localized A1g spins interacting via frustrated antiferromagnetic Heisenberg exchange and coupled by local Hund's interaction with the 1/8-filled itinerant Eg band, and study it within an exact diagonalization scheme. In the present study we limited the analysis to the case of the cluster of two isolated tetrahedrons. We obtained that both the ground state structure and low-lying excitations depend strongly on the value of the Hund's coupling which favors the triplet states. With increasing temperature the triplet states become more and more populated which results in the formation of non-zero residual magnetic moment. We present the temperature dependence of calculated magnetic moment and of the spin-spin correlation functions at different values of Hund's coupling and compare them with the experimental results.Comment: 7 pages. 6 eps figure