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arXiv:1605.03041 · Physical review. B./Physical review. B

Entanglement, excitations, and correlation effects in narrow zigzag graphene nanoribbons

Imre Hagymási, Örs Legeza

Published Oct 20, 2016• 30 citations
arXivHTMLPDF

Abstract

We investigate the low-lying excitation spectrum and ground-state properties of narrow graphene nanoribbons with zigzag edge configurations. Nanoribbons of comparable widths have been synthesized very recently [P. Ruffieux et al., Nature (London) 531, 489 (2016)], and their descriptions require more sophisticated methods since in this regime conventional methods, like mean-field or density-functional theory with local-density approximation, fail to capture the enhanced quantum fluctuations. Using the unbiased density-matrix renormalization-group algorithm, we calculate the charge gaps with high accuracy for different widths and interaction strengths and compare them with mean-field results. It turns out that the gaps are much smaller in the former case due to the proper treatment of quantum fluctuations. Applying the elements of quantum information theory, we also reveal the entanglement structure inside a ribbon and examine the spectrum of subsystem density matrices to understand the origin of entanglement. We examine the possibility of magnetic ordering and the effect of magnetic field. Our findings are relevant for understanding the gap values in different recent experiments and the deviations between them.

DOI
10.1103/physrevb.94.165147

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We investigate the low-lying excitation spectrum and ground-state properties of narrow graphene nanoribbons with zigzag edge configurations. Nanoribbons of comparable widths have been synthesized very recently [P. Ruffieux et al., Nature (London) 531, 489 (2016)], and their descriptions require more sophisticated methods since in this regime conventional methods, like mean-field or density-functional theory with local-density approximation, fail to capture the enhanced quantum fluctuations. Using the unbiased density-matrix renormalization-group algorithm, we calculate the charge gaps with high accuracy for different widths and interaction strengths and compare them with mean-field results. It turns out that the gaps are much smaller in the former case due to the proper treatment of quantum fluctuations. Applying the elements of quantum information theory, we also reveal the entanglement structure inside a ribbon and examine the spectrum of subsystem density matrices to understand the origin of entanglement. We examine the possibility of magnetic ordering and the effect of magnetic field. Our findings are relevant for understanding the gap values in different recent experiments and the deviations between them.
We investigate the ground-state properties of triangular graphene nanoflakes with zigzag edge configurations. The description of zero-dimensional nanostructures requires accurate many-body techniques since the widely used density-functional theory with local density approximation or Hartree-Fock methods cannot handle the strong quantum fluctuations. Applying the unbiased density-matrix renormalization group algorithm we calculate the magnetization and entanglement patterns with high accuracy for different interaction strengths and compare them to the mean-field results. With the help of quantum information analysis and subsystem density matrices we reveal that the edges are strongly entangled with each other. We also address the effect of electron and hole doping and demonstrate that the magnetic properties of triangular nanoflakes can be controlled by an electric field, which reveals features of flat-band ferromagnetism. This may open up new avenues in graphene based spintronics.
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Density-matrix renormalization group results are reported for the excitonic transitions in zigzag graphene nanoribbons within the framework of the simple Hubbard model. This work shows that a one-dimensional quantum lattice model with open boundary conditions is able to capture the many-body features in the low-energy electronic spectra of these systems alongside the size dependence on the ribbon width. The obtained results agree with the trends of charge and spin excitation gaps available from quantum chemistry predictions.
In this article, we investigate how various parameters such as the nanoribbon length, temperature, external magnetic field, and Coulomb interactions affect fundamental quantum correlations in zigzag graphene nanoribbons. Using an effective Heisenberg model derived from the Hubbard Hamiltonian, we analyze three essential quantifiers: thermal entanglement, correlated quantum coherence, and local quantum uncertainty. Our results reveal that increasing the ribbon length leads to an exponential reduction of the exchange interaction between edge‐localized electronic states, which causes a gradual decrease of thermal entanglement. Both temperature and Coulomb repulsion significantly reduce the concurrence, while quantum coherence and local quantum uncertainty exhibit greater robustness, maintaining nonclassical correlations even at elevated temperatures. Furthermore, the application of an external magnetic field strongly influences these quantum correlations and enhances their sensitivity to thermal and geometric effects. These findings provide a comprehensive understanding of how structural and environmental parameters govern quantum correlations in graphene‐based nanostructures, offering valuable insights for the design of stable platforms for quantum information processing and spintronic applications.
Based on the effective spin-ladder model, we compute the negativity and geometric quantum discord (GQD) between two edge spins in zigzag graphene nanoribbons (ZGNRs) thermalized with a reservoir at temperature T (canonical ensemble). The results show that the entanglement only exists in antiferromagnetically coupled spin pairs, and completely vanishes between any two spins at the same ribbon edge. Intriguingly, the dominant entanglement always occurs in the nearest inter-edge coupled spin pairs, which is very robust even at room temperature. By contrast, almost all edge spin pairs can hold non-zero GQD, which strongly depends on the ribbon width. To get further insight into the properties of entanglement, the multiparticle entanglement (ME) shared between three spins is studied. The results show that ME always exists among spins located at different edges, and it is more robust than the corresponding two-spin entanglement in against the temperature. These remarkable properties make ZGNRs very promising for possible applications in quantum information processing.
Abstract We study the dynamics of entanglement between two edge spins in a zigzag graphene nanoribbon (ZGNR) which is thermalized with a reservoir at temperature T . The results show that the entanglement evolution displays oscillating behaviors in the presence of an external magnetic field. Such oscillating behaviors of entanglement strongly depend on the field frequency and relative location between two inter-edge coupled spins. At some critical field frequencies, the entanglement exhibits a periodic structure. When the temperature is low, the oscillating patterns of entanglement are quite regular and symmetrical. When the temperature is high, the patterns of entanglement evolution occur irregular distortions due to the thermal fluctuations. However, the entanglement between two inter-edge coupled spins in ZGNR still exists a nontrivial value even at room temperature.