One-dimensional Kondo Insulator Research Articles

Matrix product study of spin fractionalization in the one-dimensional Kondo insulator

The Kondo lattice is one of the classic examples of strongly correlated electronic systems. We conduct a controlled study of the Kondo lattice in one dimension, highlighting the role of excitations created by the composite fermion operator. Using time-dependent matrix product state methods, we compute various correlation functions and contrast them with both large-N mean-field theory and the strong-coupling expansion. We show that the composite fermion operator creates long-lived, charge-e and spin-1/2 excitations, which cover the low-lying single-particle excitation spectrum of the system. Furthermore, spin excitations can be thought to be composed of such fractionalized quasiparticles with a residual interaction which tend to disappear at weak Kondo coupling. Published by the American Physical Society 2024

Low-energy excitations and transport functions of the one-dimensional Kondo insulator

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.

Local quantum criticality of a one-dimensional Kondo insulator model

The continuous quantum phase transition and the nature of quantum critical point (QCP) in a modified Kondo lattice model with Ising anisotropic exchange interactions is studied within the density-matrix renormalization group algorithm. We investigate the effect of quantum fluctuations on critical Kondo destruction QCP, by probing static and dynamic properties of the magnetic order and the Kondo effect. In particular, we identify that local Kondo physics itself becomes critical at the magnetic phase transition point, providing unbiased evidences for local quantum criticality between two insulators without resorting to the change of Fermi surface.

Synthesis and Properties of Single-Crystal FeSi Nanowires

We report for the first time the chemical synthesis of free-standing single-crystal nanowires (NWs) of FeSi, the only transition-metal Kondo insulator and the host structure for ferromagnetic semiconductor Fe(x)Co(1-x)Si. Straight and smooth FeSi nanowires are produced on silicon substrates covered with a thin layer of silicon oxide through the decomposition of the single-source organometallic precursor trans-Fe(SiCl3)2(CO)4 in a simple chemical vapor deposition process. Unlike typical vapor-liquid-solid (VLS) NW growth, FeSi NWs form without the addition of metal catalysts, have no catalyst tips, and depend strongly on the surface employed. X-ray spectroscopy verifies the identity and the room-temperature metallic nature of FeSi NWs. Room-temperature electrical transport measurements using NW devices show an average resistivity of 210 micro Omega cm, similar to the value for bulk FeSi. Investigations into the low-temperature physical properties of the first one-dimensional Kondo insulator and the possible new NW growth mechanism are underway. This unique synthetic approach to FeSi NWs will be generally applicable to many other transition-metal silicides.

Kondo hole in one-dimensional Kondo insulators

Properties of a nonmagnetic impurity in Kondo insulators are investigated by considering a one-dimensional Kondo lattice model with depletion of a localized spin. The ground-state phase diagram determined by the Lanczos method shows that the magnetic moment is more stable than in ordinary metals. Temperature dependence of impurity susceptibilities is also studied by using the finite-temperature density-matrix renormalization group.

Critical Ising modes in low-dimensional Kondo insulators

We present an Ising-like intermediate phase for one-dimensional Kondo insulator systems. Resulting from a spinon splitting, its low-energy excitations are critical Ising modes, whereas the triplet sector has a spectral gap. It should occur as long as the RKKY oscillation amplitude dominates over any direct exchange between localized spins. The chiral fixed point, however, becomes unstable in the far Infra-Red limit due to prevalent fluctuations among localized spins which induce gapless triplet excitations in the spectrum. Based on previous numerical results, we obtain a paramagnetic disordered state ruled by the correlation length of the single impurity Kondo model.

Erratum: Temperature-Induced Gap Formation in Dynamic Correlation Functions of the One-Dimensional Kondo Insulator: Finite-Temperature Density-Matrix Renormalization-Group Study [Phys. Rev. Lett. 81, 4939 (1998)

Erratum: Temperature-Induced Gap Formation in Dynamic Correlation Functions of the One-Dimensional Kondo Insulator: Finite-Temperature Density-Matrix Renormalization-Group Study [Phys. Rev. Lett. 81, 4939 (1998)

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

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.

Thermodynamic Properties of the One-Dimensional Kondo Insulators Studied by the Density Matrix Renormalization Group Method

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.

Hole doping and disorder effects on the one-dimensional Kondo lattice, for ferromagnetic Kondo couplings

We investigate the one-dimensional Kondo lattice model for ferromagnetic Kondo couplings. The so-called ferromagnetic two-leg spin ladder and the $S=1$ antiferromagnet occur as one-dimensional Kondo insulators. Both exhibit a spin gap. But, in contrast to the strong-coupling limit, the Haldane state which characterizes the two-leg spin-ladder Kondo insulator cannot fight against very weak exterior perturbations. First, by using standard bosonization techniques, we prove that an antiferromagnetic ground state occurs by doping with few holes; it is characterized by a form factor of the spin-spin correlation functions which exhibits two structures, respectively, at $q=\ensuremath{\pi}$ and ${q=2k}_{F}.$ Second, we prove precisely by using renormalization-group methods that the Anderson localization inevitably takes place in that weak-coupling Haldane system, by the introduction of quenched randomness; the spin-fixed point rather corresponds to a ``glass'' state. Finally, a weak-coupling analog of the $S=1$ antiferromagnet Kondo insulator is proposed; we show that the transition into the Anderson-localization state may be avoided in that unusual weak-coupling Haldane system.

Numerical renormalization-group study of a Kondo hole in a one-dimensional Kondo insulator.

We have studied a Kondo hole in a one-dimensional Kondo insulator at half-filling using a density matrix formulation of the numerical renormalization group. The Kondo hole introduces midgap states. The spin density introduced by the hole is localized in the vicinity of the hole. It resides primarily in the f spins for small exchange coupling J and in the conduction spins for large J. We present results on the spin gap, charge gap, and neutral gap. The presence of the Kondo hole reduces the charge gap but not the spin gap relative to a Kondo insulator without defects. For small J, the spin gap is smaller than the charge gap, while for large J, the spin gap is larger than the charge gap. RKKY interactions are reduced by the Kondo hole as can be seen in the staggered susceptibility. \textcopyright{} 1996 The American Physical Society.

Dynamical correlation functions of one-dimensional Kondo insulators

Dynamical correlation functions of the one-dimensional Kondo lattice model (1D KLM) are calculated as a reference for dynamic properties of Kondo insulators. In the strong coupling limit of the 1D KLM, the antiferromagnetic dynamic spin susceptibility consists of a peak at the lowest spin excitation energy Δ s and multiple peaks in the higher energy region. The optical conductivity has a dominant peak just above the charge excitation energy Δ c. The ratio Δ c/Δ s is larger than 3/2, and is a useful quantity to measure the electron correlation in a Kondo insulator.

The interplay of the Kondo effect and RKKY interactions in the one-dimensional Kondo insulator

Abstract We have studied the one-dimensional Kondo chain at half-filling using a density matrix formulation of the numerical renormalization group. As the exchange coupling J → 0, RKKY interactions become more important, and the correlation length and the staggered susceptibility χ(q = 2kF) diverge.

Magnetic structure factors and susceptibilities of the one-dimensional Kondo insulator

Abstract The Kondo insulator CeNiSn shows substantial antiferromagnetic correlations along one axis, but does not show analogous structure in the corresponding spin susceptibility. In an attempt to understand this behavior, we perform quantum Monte Carlo simulations of the one-dimensional Kondo insulator. We first describe the model and the quantum Monte Carlo technique used, and then compare our results with the experimental data. The model appears inadequate to describe the observed CeNiSn susceptibility behavior.

Numerical renormalization group study of the one-dimensional Kondo insulator.

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 exchange coupling.r. A new type of excitation, a neutral spin singlet, consists of a particle and a hole which are repulsive for J≥5t and attractive for J≤5t. As J→0, RKKY interactions become more important and the staggered susceptibility χ(q-2k F ) diverges. We have also studied χ(q) and the dispersion of the low-lying spin excitations as a function of wave vector q