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Register a new userTopological quantum transition driven by charge-phonon coupling in the Haldane Chern insulator
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Added by
Loris Maria Cangemi
Publication date
2019
fields
Physics
and research's language is
English
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In condensed matter physics many features can be understood in terms of their topological properties. Here we report evidence of a topological quantum transition driven by the charge-phonon coupling in the spinless Haldane model on a honeycomb lattice, a well-known prototypical model of Chern insulator. Starting from parameters describing the topological phase in the bare Haldane model, we show that the increasing of the strength of the charge lattice coupling drives the system towards a trivial insulator. The average number of fermions in the Dirac point, characterized by the lowest gap, exhibits a finite discontinuity at the transition point and can be used as direct indicator of the topological quantum transition. Numerical simulations show, also, that the renormalized phonon propagator exhibits a two peak structure across the quantum transition, whereas, in absence of the mass term in the bare Hadane model, there is indication of a complete softening of the effective vibrational mode signaling a charge density wave instability.
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By using the cluster perturbation theory, we investigate the effects of the local electron-phonon interaction in the quantum spin Hall topological insulator described by the half-filled Kane-Mele model on an honeycomb lattice. Starting from the topological non trivial phase, where the minimal gap is located at the two inequivalent Dirac points of the Graphene, $text{K}$ and $text{K}$, we show that the coupling with quantum phonons induces a topological-trivial quantum phase transition through a gap closing and reopening in the $text{M}$ point of the Brillouin zone. The average number of fermions in this point turns out to be a direct indicator of the quantum transition pointing out a strong hybridization between the two bare quasiparticle bands of the Kane-Mele model. By increasing the strength of charge-lattice coupling, the phonon Greens propagator displays a two peak structure: the one located at the lowest energy exhibits a softening that is maximum around the topological transition. Numerical simulations provide also evidence of several kinks in the quasiparticle dispersion caused by the coupling of the electrons with the bosonic lattice mode.
Transition metal dichalcogenides (TMDs) are a class of widely studied 2D layered materials which exist in various polymorphs. The 1T phase of MoTe2 is of prime importance as it has been reported to show quantum spin hall (QSH) behavior with a fairly large band-gap of ~ 60 meV, in contrast to most QSH materials known. It is noteworthy that though the monolayer 1T-MoTe2 was initially predicted to show the QSH behavior, recent theoretical studies claim that the few-layered counterparts also exhibit higher order topological behavior. Besides, 1T-MoTe2 also undergoes a hysteretic phase transition to the Td phase (which is a type-II Weyl semimetal) by breaking the inversion symmetry of the crystal. While the phase transition between these two topological phases is of utmost importance, its study has been mostly restricted to bulk single crystal flakes, thereby not sufficiently exploring the effect of dimensionality. We have studied the phase transition in 1T-MoTe2 as a function of flake-thickness. Though our Raman studies show a suppression of the phase transition in the thin (thickness <10 nm) flakes [similar to the report Phys. Rev. B 97, 041410 (2018)], we have experimentally demonstrated the possibility of stabilizing the desired phase (1T or Td) at room temperature by charge doping. Further, we have observed clear signatures of electron-phonon coupling in MoTe2, which evolves as a function of flake-thickness and charge doping.
Metal-insulator transitions involve a mix of charge, spin, and structural degrees of freedom, and when strongly-correlated, can underlay the emergence of exotic quantum states. Mott insulators induced by the opening of a Coulomb gap are an important and well-recognized class of transitions, but insulators purely driven by spin correlations are much less common, as the reduced energy scale often invites competition from other degrees of freedom. Here we demonstrate a clean example of a spin-correlation-driven metal-insulator transition in the all-in-all-out pyrochlore antiferromagnet Cd2Os2O7, where the lattice symmetry is fully preserved by the antiferromagnetism. After the antisymmetric linear magnetoresistance from conductive, ferromagnetic domain walls is carefully removed experimentally, the Hall coefficient of the bulk reveals four Fermi surfaces, two of electron type and two of hole type, sequentially departing the Fermi level with decreasing temperature below the Neel temperature, T_N. Contrary to the common belief of concurrent magnetic and metal-insulator transitions in Cd2Os2O7, the charge gap of a continuous metal-insulator transition opens only at T~10K, well below T_N=227K. The insulating mechanism resolved by the Hall coefficient parallels the Slater picture, but without a folded Brillouin zone, and contrasts sharply with the behavior of Mott insulators and spin density waves, where the electronic gap opens above and at T_N, respectively.
We report time- and angle-resolved photoemission spectroscopy measurements on the topological insulator Bi2Se3. We observe oscillatory modulations of the electronic structure of both the bulk and surface states at a frequency of 2.23 THz due to coherent excitation of an A1g phonon mode. A distinct, additional frequency of 2.05 THz is observed in the surface state only. The lower phonon frequency at the surface is attributed to the termination of the crystal and thus reduction of interlayer van der Waals forces, which serve as restorative forces for out-of-plane lattice distortions. DFT calculations quantitatively reproduce the magnitude of the surface phonon softening. These results represent the first band-resolved evidence of the A1g phonon mode coupling to the surface state in a topological insulator.
The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder) and Peierls (localization via distortion of a periodic 1D lattice). One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and X-ray scattering we show that the MIT in NaOsO3 is coincident with the onset of long-range commensurate three dimensional magnetic order. Whilst candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT. We discuss our results in the light of recent reports of a Mott spin-orbit insulating state in other 5d oxides.
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