Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite

Borroni, Simone; Baldini, Edoardo; Katukuri, Vamshi M.; Mann, Andreas; Parliński, K.; Legut, Dominik; Arrell, Christopher; Mourik, Frank van; Teyssier, J.; Kozłowski, A.; Piekarz, Przemysław; Yazyev, Oleg V.; Oleś, Andrzej M.; Lorenzana, J.; Carbone, Fabrizio

doi:10.1103/physrevb.96.104308

Cited by 17 publications

(35 citation statements)

References 58 publications

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“…b, Fourier transform of both curves in a. There are no features at higher energies where a previous study observed totally-symmetric optical phonon modes excited by 3.10 eV light[30]. Our THz probe is therefore only sensitive to the low-energy electronic collective modes reported here.…”

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confidence: 76%

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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confidence: 76%

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

et al. 2020

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“…The excitation of hot carriers can activate the additional phonon mode to participate in the THz absorption that are otherwise forbidden without photo excitation of hot carriers [72][73][74][75]. The studies of the phonon spectrum of Cd3As2 show that there are many low frequency optical phonon modes, such as A1g, B1g, B2g…”

Section: E T=0mentioning

confidence: 99%

Terahertz probe of photoexcited carrier dynamics in the Dirac semimetal Cd3As2

Lu¹,

Ling²,

Xiu³

et al. 2018

Phys. Rev. B

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The relaxation dynamics of photoexcited quasiparticles of three-dimensional (3D) Dirac semimetals are vital towards their application in high performance electronic and optoelectronic devices. In this work, the relaxation dynamics of photoexcited carriers of 3D Dirac semimetal Cd3As2 are investigated by transient terahertz spectroscopy. The visible pump-THz probe spectroscopy measurement shows clear biexponential decays with two characteristic time constants. According to the pump-power and temperature dependence, these two characteristic time constants are attributed to the electron phonon coupling (1-4 ps)and anharmonic decay of hot coupled phonons to electronic uncoupled phonons (2-9 ps), respectively. An anomalous electron-optical phonon coupling reduction and a bottleneck slowing of hot optical phonons relaxation are observed with higher excitation intensities similar to that in graphene. On the other hand, the electron-optical phonon coupling can be enhanced due to the phonon frequency broadening and softening at elevated lattice temperature. Furthermore, the transient THz spectrum response is strongly modified by the phonon assisted intraband absorption of hot carriers from a pure electronic Drude model, which is evidenced by a characteristic THz absorption dip in the transient THz absorption spectrum. This absorption dip is pinned by the discrete optical phonon energy that assists the intraband transition enabled by photoexcitation of hot carriers.

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“…[18][19][20][21] In magnetite, the equilibrium optical conductivity [22][23][24] shows a broad Drude peak in the metallic phase as well as its suppression at the metal-insulator transition. The optical properties at higher energies are dominated by two features peaking at about 0.6 eV and 2 eV.…”

Section: +mentioning

confidence: 99%

Phase separation in the nonequilibrium Verwey transition in magnetite

et al. 2016

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We present equilibrium and out-of-equilibrium studies of the Verwey transition in magnetite. In the equilibrium optical conductivity, we find a step-like change at the phase transition for photon energies below about 2 eV. The possibility of triggering a non-equilibrium transient metallic state in insulating magnetite by photo excitation was recently demonstrated by an x-ray study. Here we report a full characterization of the optical properties in the visible frequency range across the nonequilibrium phase transition. Our analysis of the spectral features is based on a detailed description of the equilibrium properties. The out-of-equilibrium optical data bear the initial electronic response associated to localized photo-excitation, the occurrence of phase separation, and the transition to a transient metallic phase for excitation density larger than a critical value. This allows us to identify the electronic nature of the transient state, to unveil the phase transition dynamics, and to study the consequences of phase separation on the reflectivity, suggesting a spectroscopic feature that may be generally linked to out-of-equilibrium phase separation.

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Coherent generation of symmetry-forbidden phonons by light-induced electron-phonon interactions in magnetite

Cited by 17 publications

References 58 publications

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

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

Terahertz probe of photoexcited carrier dynamics in the Dirac semimetal Cd3As2

Phase separation in the nonequilibrium Verwey transition in magnetite

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