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Physical Review B

Entanglement, magnetism, and metal-insulator transitions in fermionic superlattices

Tiago Mendes-Santos, Thereza Paiva, Raimundo R. dos Santos

Published Jun 7, 2013• 12 citations
Publisher

Abstract

We discuss how entanglement of strongly correlated fermions is influenced by a superlattice structure by considering a one-dimensional Hubbard superlattice, made up of a repeated pattern of ${L}_{U}$ repulsive sites followed by ${L}_{0}$ free sites. Lanczos diagonalization of lattices up to 24 sites is used to calculate the von Neumann entropy and the negativity. The breakdown of particle-hole symmetry broadens the maxima of the entropy in the underdoped region, while the entanglement in the overdoped region is crucially influenced by the nature of the magnetic state, with dips at densities corresponding to repulsive-layer singlets and to $q=\ensuremath{\pi}$ (in units of inverse unit cell length, ${L}_{U}+{L}_{0}$) spin-density waves. At these special densities, the system is either a Mott insulator or a ``compressible insulator.'' We have also found that sites within the same repulsive layer (for ${L}_{U}\ensuremath{\ge}2$) are monogamically entangled with each other, which may provide a route to control entanglement simultaneously in different length scales.

DOI
10.1103/physrevb.87.214407

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We discuss how entanglement of strongly correlated fermions is influenced by a superlattice structure by considering a one-dimensional Hubbard superlattice, made up of a repeated pattern of LU{L}_{U}LU​ repulsive sites followed by L0{L}_{0}L0​ free sites. Lanczos diagonalization of lattices up to 24 sites is used to calculate the von Neumann entropy and the negativity. The breakdown of particle-hole symmetry broadens the maxima of the entropy in the underdoped region, while the entanglement in the overdoped region is crucially influenced by the nature of the magnetic state, with dips at densities corresponding to repulsive-layer singlets and to q=\ensuremathπq=\ensuremath{\pi}q=\ensuremathπ (in units of inverse unit cell length, LU+L0{L}_{U}+{L}_{0}LU​+L0​) spin-density waves. At these special densities, the system is either a Mott insulator or a ``compressible insulator.'' We have also found that sites within the same repulsive layer (for LU\ensuremath≥2{L}_{U}\ensuremath{\ge}2LU​\ensuremath≥2) are monogamically entangled with each other, which may provide a route to control entanglement simultaneously in different length scales.
We investigate the charge and spin gaps, and the spin structure in half-filled one-dimensional Hubbard superlattices with one repulsive site and L0 free sites per unit cell. For odd L0, it is correlated metal at the particle–hole symmetric point, and then turns into band insulator beyond this point. For even L0, the system has a Mott insulator phase around the particle–hole symmetric point and undergoes a metal–insulator transition with on-site repulsion U increasing. For large U, there exists a multiperiodic spin structure, which results from the ferromagnetic (antiferromagnetic) correlation between the nearest neighboring repulsive sites for odd (even) L0.
We study the metal-insulator transition in one-dimensional Hubbard superlattices, modeled by a repeated pattern of repulsive (i.e., positive on-site coupling) and free sites. The evolution of the local moment and of the charge gap (calculated from Lanczos diagonalization of chains up to 18 sites), together with a strong-coupling analysis, show that the electron density at which the system is insulating increases with the size of the free layer, relative to the repulsive one. In the insulating state, the mechanism of interaction between fermions separated by a free layer is the analog of superexchange, and the charge gap display universal features.
We consider fermions on a one-dimensional superlattice, represented by a repeated pattern of repulsive (i.e., positive Hubbard UUU) and free (i.e., U=0U\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0U=0) sites. Lanczos diagonalization of chains up to 12 sites at half filling shows that some patterns give rise to a weak form of frustration, sufficient to wipe out the spin-density-wave (SDW) correlations present in the otherwise homogeneous system. On the other hand, SDW correlations appear for some patterns above half filling, a feature absent in the homogeneous case. Below half filling, the local-moment maxima are displaced from the repulsive to the free sites. The peak in the magnetic structure factor is also shifted from $q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\pi}$, for the alternating configuration when $\ensuremath{\rho}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/3$ and 5/3.
We consider fermions in one-dimensional superlattices (SL's), modeled by site-dependent Hubbard-UUU couplings arranged in a repeated pattern of repulsive (i.e., U>0)U>0)U>0) and free (U=0)(U=0)(U=0) sites. Density matrix renormalization group diagonalization of finite systems is used to calculate the local moment and the magnetic structure factor in the ground state. We have found four regimes for magnetic behavior: uniform local moments forming a spin-density wave (SDW), ``floppy'' local moments with short-ranged correlations, local moments on repulsive sites forming long-period SDW's superimposed with short-ranged correlations, and local moments on repulsive sites solely with long-period SDW's; the boundaries between these regimes depend on the range of electronic densities $\ensuremath{\rho}$ and on the SL aspect ratio. Above a critical electronic density, ${\ensuremath{\rho}}_{\ensuremath{\uparrow}\ensuremath{\downarrow}},$ the SDW period oscillates both with $\ensuremath{\rho}$ and with the spacer thickness. The former oscillation allows one to reproduce all SDW wave vectors within a small range of electronic densities, unlike the homogeneous system. The latter oscillation is related to the exchange oscillation observed in magnetic multilayers. A crossover between regimes of ``thin'' to ``thick'' layers has also been observed.
The Hubbard model in the strong-coupling regime is mainly studied by Kondo-lattice theory or 1/d expansion theory, with d the spatial dimensionality. In two dimensions and higher, the ground state within the Hilbert subspace with no order parameter is a normal Fermi liquid except for n=1 and U/W=+infinity, with n the electron density per unit cell, U the on-site repulsion, and W the bandwidth; the cooperation between the Kondo effect, which favors a local singlet on each unit cell, and a resonating-valence-bond effect, which favors a local singlet on each pair of nearest-neighbor unit cells, stabilizes the Fermi liquid, whose ground state is a singlet as a whole, in the strong-coupling regime. In the whole Hilbert space with no restriction, the normal Fermi liquid is unstable at least against a magnetic or superconducting state. This analysis confirms an early Fermi-liquid theory of high-temperature superconductivity, F. J. Ohkawa, Jpn. J. Appl. Phys. 26, L652 (1987). The ground state for n=1 and U/W=+infinity is a Mott insulator. Actual metal-insulator transitions cannot be explained within the Hubbard model. In order to explain them, the electron-phonon interaction, multi-band or multi-orbital effects, and effects of disorder should be considered beyond the Hubbard model. The crossover between local-moment magnetism and itinerant-electron magnetism corresponds to that between a localized spin and a normal Fermi liquid in the Kondo effect and it is simply a Mott metal-insulator crossover.
We discuss the interplay between electronic correlations and an underlying superlattice structure in determining the period of charge density waves (CDW's), by considering a one-dimensional Hubbard model with a repeated (nonrandom) pattern of repulsive (U>0)(U>0)(U>0) and free (U=0)(U=0)(U=0) sites. Density matrix renormalization group diagonalization of finite systems (up to 120 sites) is used to calculate the charge-density correlation function and structure factor in the ground state. The modulation period can still be predicted through effective Fermi wave vectors kF∗{k}_{F}^{*}kF∗​ and densities, and we have found that it is much more sensitive to electron (or hole) doping, both because of the narrow range of densities needed to go from q∗=0{q}^{*}=0q∗=0 to $\ensuremath{\pi}$, but also due to sharp $2{k}_{F}^{*}\ensuremath{-}4{k}_{F}^{*}$ transitions; these features render CDW's more versatile for actual applications in heterostructures than in homogeneous systems.