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arXiv:cond-mat/9806280 · Physical Review Letters

Temperature-Induced Gap Formation in Dynamic Correlation Functions of the One-Dimensional Kondo Insulator: Finite-Temperature Density-Matrix Renormalization-Group Study

Tetsuya Mutou, Naokazu Shibata, Kazuo Ueda

Published Nov 30, 1998• 38 citations
arXivHTMLPDF

Abstract

Combination of the finite-temperature density-matrix renormalization group and the maximum entropy presents a new method to calculate dynamic quantities of one-dimensional many-body systems. In the present paper, density of states, local dynamic spin, and charge correlation functions of the one-dimensional Kondo insulator are discussed. Excitation gaps open with decreasing temperature and the gaps take different values depending on channels. The excitation spectra change qualitatively around the characteristic temperature corresponding to the spin gap which is the lowest energy scale of the Kondo insulator.

DOI
10.1103/physrevlett.81.4939

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Combination of the finite-temperature density-matrix renormalization group and the maximum entropy presents a new method to calculate dynamic quantities of one-dimensional many-body systems. In the present paper, density of states, local dynamic spin, and charge correlation functions of the one-dimensional Kondo insulator are discussed. Excitation gaps open with decreasing temperature and the gaps take different values depending on channels. The excitation spectra change qualitatively around the characteristic temperature corresponding to the spin gap which is the lowest energy scale of the Kondo insulator.
Recent developments in the theoretical investigation of the one-dimensional Kondo lattice model by using the density matrix renormalization group (DMRG) method are discussed in this review. Short summaries are given of the zero-temperature DMRG method, the finite-temperature DMRG method, and also the application of the finite- T DMRG to dynamic quantities. Away from half-filling, the paramagnetic metallic state is shown to be a Tomonaga-Luttinger liquid with a large Fermi surface. The size of the large Fermi surface is determined by the sum of the densities of the conduction electrons and the localized spins. The correlation exponent of this metallic phase is smaller than 1/2. At half-filling the ground state is insulating. The excitation gaps are different, depending on the channels, the spin gap, the charge gap and the quasiparticle gap. The temperature dependence of the spin and charge susceptibilities and the specific heat are discussed. Particularly interesting is the temperature dependence of various excitation spectra, which indicates unusual properties for the Kondo insulators.
Using variational matrix product states, we analyze the finite temperature behavior of a half-filled periodic Anderson model in one dimension, a prototypical model of a Kondo insulator. We present an extensive analysis of single-particle Green’s functions, two-particle Green’s functions, and transport functions creating a broad picture of the low-temperature properties. We confirm the existence of energetically low-lying spin excitations in this model and study their energy-momentum dispersion and temperature dependence. We demonstrate that charge-charge correlations at the Fermi energy exhibit a different temperature dependence than spin-spin correlations. While energetically low-lying spin excitations emerge approximately at the Kondo temperature, which exponentially depends on the interaction strength, charge correlations vanish already at high temperatures. Furthermore, we analyze the charge and thermal conductivity at finite temperatures by calculating the time-dependent current-current correlation functions. While both charge and thermal conductivity can be fitted for all interaction strengths by gapped systems with a renormalized band gap, the gap in the system describing the thermal conductivity is generally smaller than the system describing the charge conductivity. Thus, two-particle correlations affect the charge and heat conductivities in a different way resulting in a temperature region where the charge conductivity of this one-dimensional Kondo insulator is already decreasing while the heat conductivity is still increasing.
The finite-temperature density-matrix renormalization-group method is applied to the one-dimensional Kondo lattice model near half filling to study its thermodynamics. The spin and charge susceptibilities and entropy are calculated down to T=0.03t. We find two crossover temperatures near half filling. The higher crossover temperature continuously connects to the spin gap at half filling, and the susceptibilities are suppressed around this temperature. At low temperatures, the susceptibilities increase again with decreasing temperature when doping is finite. We confirm that they finally approach to the values obtained in the Tomonaga-Luttinger (TL) liquid ground state for several parameters. The crossover temperature to the TL liquid is a new energy scale determined by gapless excitations of the TL liquid. The transition from the metallic phase to the insulating phase is accompanied by the vanishing of the lower crossover temperature.
The finite-T DMRG method is applied to the one-dimensional Kondo lattice model to calculate dynamic correlation functions. Dynamic spin and charge correlations, S_f(omega), S_c(omega), and N_c(omega), and quasiparticle density of states rho(omega) are calculated in the paramagnetic metallic phase for various temperatures and hole densities. Near half filling, it is shown that a pseudogap grows in these dynamic correlation functions below the crossover temperature characterized by the spin gap at half filling. A sharp peak at omega=0 evolves at low temperatures in S_f(omega) and N_c(omega). This may be an evidence of the formation of the collective excitations, and this confirms that the metallic phase is a Tomonaga-Luttinger liquid in the low temperature limit.
Thermodynamic properties of the one-dimensional Kondo lattice model at half-filling are studied by the density matrix renormalization group method applied to the quantum transfer matrix. Spin susceptibility, charge susceptibility, and specific heat are calculated down to T=0.1t for various exchange constants. The obtained results clearly show crossover behavior from the high temperature regime of nearly independent localized spins and conduction electrons to the low temperature regime where the two degrees of freedom couple strongly. The low temperature energy scales of the charge and spin susceptibilities are determined and shown to be equal to the quasiparticle gap and the spin gap, respectively, for weak exchange couplings.
We have studied the one-dimensional Kondo chain at half filling using a density matrix formulation of the numerical renormalization group. The charge gap is larger than the spin gap for all antiferromagnetic values of the exhange coupling J. A new type of excitation, a neutral spin singlet, consists of a particle and a hole which are repulsive for J\ensuremath{\gtrsim}5t and attractive for J\ensuremath{\lesssim}5t. As J\ensuremath{\rightarrow}0, RKKY interactions become more important and the staggered susceptibility \ensuremath{\chi}(q=2kF{\mathit{k}}_{\mathit{F}}kF​) diverges. We have also studied \ensuremath{\chi}(q) and the dispersion of the low-lying spin excitations as a function of wave vector q.