Low-energy properties of the Kondo lattice model

Abstract

We study the zero-temperature properties of the Kondo lattice model within the dynamical mean-field theory. As an impurity solver we use the numerical renormalization group. We present results for the paramagnetic case showing the anticipated heavy-fermion physics, including direct evidence for the appearance of a large Fermi surface for antiferromagnetic exchange interaction. Allowing for the formation of a Néel state, we observe at finite doping an antiferromagnetic metal below a critical exchange interaction, which shows a crossover from a local moment antiferromagnet with a small Fermi surface for weak exchange coupling to a heavy-fermion antiferromagnet with a large Fermi surface for increasing exchange. Including lattice degrees of freedom via an additional Holstein term we observe a significant suppression of the Kondo effect, leading to a strongly reduced low-energy scale. For too large electron-phonon coupling we find a complete collapse of the heavy Fermi liquid and the formation of polarons.

Fine structure of spectra in the antiferromagnetic phase of the Kondo lattice model

“…The spectral functions are computed using the full-density-matrix NRG approach 16,17 with a discretization scheme that allows for improved spectral resolution at high energy scales 18 . Compared to prior DMFT(NRG) works 19,20 , our calculations are performed with significantly reduced spectral broadening, thus sharp features away from Fermi level are much better resolved. We use NRG discretization parameter Λ = 2 with N z = 8 interpenetrating meshes for the z-averaging 21 and the method to directly calculate the self-energy introduced by R. Bulla et al 22 The broadening parameter was α = 0.25 in most calculations 23 .…”

“…It is worth to emphasize that the spin resonances are not observed for negative (ferromagnetic) Kondo exchange cou- pling J, although the system is also antiferromagnetic. This is due to the very different topology of the quasiparticle bands ("small Fermi surface") in this case 11,32 , which is, in turn, associated with a different form of the self-energy function with no pronounced poles. Furthermore, there is no spin resonance if we enforce paramagnetic solution in the region where the AFM is the true ground state (such a comparison of AFM and PM solutions in shown in Fig.…”

“…This is the case also for high-quality continuous-time QMC calculations 11 . Some hints of the spin resonances have been observed in prior DMFT(NRG) works [32][33][34] , but have not been discussed. The spin resonances appear for any value of the NRG broadening parameter: even in calculation with no z-averaging and with large broadening parameter their presence is suggested by a broad spectral hump in one band and as a faint depression in the other.…”

Kondo effect inf-electron superlattices

“…The Kondo effect as well as the heavy-fermion state are well described, and the phase diagram of the Kondo lattice model within DMFT has been investigated by a number of authors before. [13][14][15][16][17][18][19][20][21][22][23] In the original DMFT approach, one is interested in a homogeneous lattice, so that each site can be described by the same self-energy. Because in the superlattice the system is inhomogeneous, i.e., there are different kinds of layers with different physical properties, different lattice sites cannot be approximated with the same self-energy.…”

Surface density of states of layeredf-electron materials

“…the low energy physics including heavy quasi-particles as well as magnetic phases can be described by the DMFT. [21][22][23][24][25][26][27][28][29][30][31] A detailed description of the IDMFT for superlattices can be found in reference. 15 Figure 2: (Color online) Temperature dependent LDOS close to the Fermi energy, ω = 0, for a system with a Ce-surfacelayer.…”