Light scattering from the critical modes of the Verwey transition in magnetite

Borroni, Simone; Teyssier, J.; Piekarz, Przemysław; Kuzmenko, A. B.; Oleś, Andrzej M.; Lorenzana, J.; Carbone, Fabrizio

doi:10.1103/physrevb.98.184301

Cited by 8 publications

(11 citation statements)

References 45 publications

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“…The presence of phonon anharmonicity and strong electronphonon coupling was also demonstrated by means of Raman measurements [65]. Anomalies of the lattice vibrations that originate from strong coupling to electronic excitations and the spectroscopic signatures of diffusive modes in the electronic contribution to the Raman response function were observed [66]. Ultrafast broadband optical spectroscopy revealed that impulsive photoexcitation of particle-hole pairs couples to the fluctuations of the ordering field and coherently generates phonon modes of the ordered phase above T V [67].…”

Section: Introductionmentioning

confidence: 91%

“…Experimentally, the phonon dispersion curves in magnetite were studied by inelastic neutron scattering (INS) [52][53][54] and by inelastic x-ray scattering (IXS) [39]. The Fe-projected phonon density of states (DOS) was measured using nuclear inelastic scattering (NIS) [55][56][57] and detailed studies of the optical modes at the Brillouin zone center were performed by spontaneous Raman scatter-ing and infrared reflectivity measurements [58][59][60][61][62][63][64][65][66][67]. The presence of phonon anharmonicity and strong electronphonon coupling was also demonstrated by means of Raman measurements [65].…”

Section: Introductionmentioning

confidence: 99%

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Trimeron-phonon coupling in magnetite

et al. 2021

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Using density functional theory, we study the lattice dynamical properties of magnetite (Fe3O4) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and phonon density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic Cc structure revealing the strong coupling between charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise which, remarkably, can be understood assuming that the strong trimeron-phonon coupling can be extended above the Verwey transition, with lattice dynamics influenced by the short-range trimeron order instead of the average cubic structure. Our results establish the validity of trimerons (and trimeron-phonon coupling) in explaining the physics of magnetite much beyond their original formulation.

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Section: Introductionmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Trimeron-phonon coupling in magnetite

et al. 2021

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“…Precursor effects were found to anticipate the discontinuous transition over temperature intervals extending far above T V [12][13][14][15][16][17][18][19][20][21]. Diffuse scattering of neutrons and x-rays with local maxima at incommensurate points in reciprocal space was shown to survive to room temperature and attributed to the persistence of local and dynamical electronic and structural correlations in the cubic phase [13][14][15][16].…”

Section: Introductionmentioning

confidence: 93%

“…Examination of the scattering intensity indicates a prevalent contribution of the X 3 mode to the inherent symmetry of the critical fluctuations [15]. Recently, by means of femtosecond broadband spectroscopy, some of us have demonstrated that modes of the low-temperature phase can be photoinduced also above the critical temperature of the material, owing to their strong coupling to charge correlations on the Fe sites [17]. Deeper considerations on the intrinsic features of short-range order above T V are prevented by sample dependent effects.…”

Section: Introductionmentioning

confidence: 99%

Mapping the lattice dynamical anomaly of the order parameters across the Verwey transition in magnetite

et al. 2017

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We present inelastic neutron scattering data across the Verwey transition in magnetite, obtained for a single crystal via a detwinning method. We provide direct evidence of the influence of the charge order on the transverse-acoustic phonons, associated with discontinuous hardening and narrowing at the transition temperature, and energy splitting for different polarizations. This contrasts with the behavior of the transverse-optical X 3 mode, which does not present any critical anomaly, contrary to theoretical expectations. Our data indicate that the incommensurate fluctuations occurring above the critical temperature become locked to the lattice at the transition point, through a mechanism similar to the crystallization of a two-dimensional liquid on a solid surface. Our results also contribute to clarify the different dynamics and mutual interactions of the electronic and structural modes in the Verwey transition.

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“…These excitations slightly harden with decreasing temperature and are centered around 1.5 and 4.2 meV at 7 K. Since they appear below the charge gap for single-particle excitations [17], it is natural to ascribe them to distinct lowenergy collective modes at the Brillouin zone center. Intriguingly, these excitations have never been observed in any previous study on magnetite [16,[18][19][20][21][22]. Thus, it is pivotal to identify their origin and clarify their potential involvement in the Verwey transition.We make use of advanced density-functional theory (DFT) calculations of the phonon dispersions in the lowtemperature Cc symmetry of magnetite to compare the energy of the two observed excitations with that of longwavelength lattice modes (see Methods and Supplementary Note 2).…”

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confidence: 96%

Discovery of the soft electronic modes of the trimeron order in magnetite

et al. 2020

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The Verwey transition in magnetite (Fe 3 O 4 ) is the first metal-insulator transition ever observed [1] and involves a concomitant structural rearrangement and charge-orbital ordering. Due to the complex interplay of these intertwined degrees of freedom, a complete characterization of the low-temperature phase of magnetite and the mechanism driving the transition have long remained elusive. It was demonstrated in recent years that the fundamental building blocks of the charge-ordered structure are three-site small polarons called trimerons [2]. However, electronic collective modes of this trimeron order have not been detected to date, and thus an understanding of the dynamics of the Verwey transition from an electronic point of view is still lacking. Here, we discover spectroscopic signatures of the low-energy electronic excitations of the trimeron network using terahertz light. By driving these modes coherently with an ultrashort laser pulse, we reveal their critical softening and hence demonstrate their direct involvement in the Verwey transition. These findings represent the first observation of soft modes in magnetite and shed new light on the cooperative mechanism at the origin of its exotic ground state.Along with his groundbreaking discovery in 1939, Verwey postulated the emergence of a charge ordering of Fe 2+ and Fe 3+ ions as the mechanism driving the dramatic conductivity drop at T V ∼ 125 K [1]. A vast number of subsequent experimental and theoretical investigations, including those by Anderson [3], Mott [4], and many others, have stimulated a still unresolved debate over a complete description of the Verwey transition [5,6]. In particular, several seemingly incompatible findings related to the intricate low-temperature phase of magnetite have been reported: the crucial role of Coulomb repulsion [7], the necessity of including electron-phonon coupling [4,8,9], small charge disproportionation [7,10,11], anomalous phonon broadening with the absence of a softening towards T V [12], and the observation of structural fluctuations that are connected to the Fermi surface nesting [13] and that persist up to the Curie transition temperature (T C ∼ 850 K) [14].The last decade witnessed significant progress in understanding the Verwey transition from a structural point of view. Most notably, a refinement of the lowtemperature charge-ordered structure as a network of three-site small polarons, termed trimerons, was given by x-ray diffraction [2] ( Fig. 1a). A trimeron consists of a linear unit of three Fe sites accompanied by distortions of the two outer Fe 3+ ions towards the central Fe 2+ ion. An orbital ordering of coplanar t 2g orbitals is also established on each ion within the trimeron (Fig. 1b). This picture of the trimeron order has been crucial for determining the correct noncentrosymmetric Cc space group of magnetite and explaining its spontaneous charge-driven ferroelectric polarization [2,6,15]. Nevertheless, despite extensive research, no soft modes of the trimeron order have been detected to date. ...

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Light scattering from the critical modes of the Verwey transition in magnetite

Cited by 8 publications

References 45 publications

Trimeron-phonon coupling in magnetite

Trimeron-phonon coupling in magnetite

Mapping the lattice dynamical anomaly of the order parameters across the Verwey transition in magnetite

Discovery of the soft electronic modes of the trimeron order in magnetite

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