Orbital Domain Dynamics in Magnetite below the Verwey Transition

Kukreja, Roopali; Hua, Nelson; Ruby, Joshua; Barbour, Andi; Hu, Wen; Mazzoli, Claudio; Wilkins, S. B.; Fullerton, Eric E.; Shpyrko, Oleg

doi:10.1103/physrevlett.121.177601

Cited by 23 publications

(11 citation statements)

References 27 publications

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“…Using these results, we now construct an invariant polynomial under the space group operations of the high-symmetry group that couples the IRs X 3 and ∆ 5 [64,69], consistent with previous experimental observations [2,23,66,70,71]. The allowed terms in the polynomial are, to second-order, F (2) = |X| 2 + |∆| 2 , where X = (X 1 , X 2 , X 3 ) and ∆ = (∆ 1 , ∆ 2 ).…”

Section: Supplementary Note 7: Group Theory Aspects Of the Verwey Trasupporting

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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Section: Supplementary Note 7: Group Theory Aspects Of the Verwey Trasupporting

confidence: 76%

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

et al. 2020

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“…The jamming behavior is attributed to ultraslow ballistic motions as a result of relaxations driven by an internal stress field. A similar jamming behavior has been reported previously in ferroelectric PbTiO 3 /SrTiO 3 superlattice thin films, materials exhibiting charge density waves, and magnetic and orbital ordering [41][42][43]. On the other hand, the exponential (β = 1) and the stretched exponential (β < 1) indicate diffusive and subdiffusive relaxation usually found in glass-forming liquids and colloidal suspensions, respectively [44][45][46].…”

Section: Resultssupporting

confidence: 84%

Domain fluctuations in a ferroelectric low-strain BaTiO3 thin film

Zhong

Jangid

et al. 2020

Phys. Rev. Materials

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A ferroelectric BaTiO 3 thin film grown on a NdScO 3 substrate was studied using x-ray photon correlation spectroscopy (XPCS) to characterize thermal fluctuations near the a/b to a/c domain structure transformation present in this low-strain material, which is absent in the bulk. XPCS studies provide a direct comparison of the role of domain fluctuations in first-and second-order phase transformations. The a/b to a/c domain transformation is accompanied by a decrease in fluctuation timescales, and an increase in intensity and correlation length. Surprisingly, domain fluctuations are observed up to 25°C above the transformation, concomitant with the growth of a/c domains and coexistence of both domain types. After a small window of stability, as the Curie temperature is approached, a/c domain fluctuations are observed, albeit slower, potentially due to the structural transformation associated with the ferroelectric to paraelectric transformation. The observed time evolution and reconfiguration of domain patterns highlight the role played by phase coexistence and elastic boundary conditions in altering fluctuation timescales in ferroelectric thin films.

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“…In the case of iron, the transition 1s → 4p yields the K edge at about 7.119 keV and it has been used to monitor charge and orbital orderings [7,22]. In contrast, the L edges (2p → 3d) are located at much lower energies and experiments at these edges have been used to monitor orbital ordering as well as changes in the magnetic ordering [6,46].…”

Section: A Resonant X-ray Scatteringmentioning

confidence: 99%

“…As shown in Refs. [6,52], the crystallographic structure in the proximity of the VT (i.e., at 110 K) is not stable and continuous rearrangements of the domains occur. Thus, we cannot claim that the AS concerns only lattice distortion and not orbital order.…”

Section: A Resonant X-ray Scatteringmentioning

confidence: 99%

“…These subsystems are also visible in the vicinity of T V . The approach of T V from high (or low) temperatures is not homogeneous, as revealed by recent x-ray correlation spectroscopy [6]. In addition, although sudden changes of many physical properties occur at T V , the charge and orbital orders persist above the transition and disappear at 130 K [7].…”

Section: Introductionmentioning

confidence: 99%

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Magnetic field induced structural changes in magnetite observed by resonant x-ray diffraction and Mössbauer spectroscopy

Kołodziej¹,

Bialo

Tabiś

et al. 2020

Phys. Rev. B

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When a magnetic field is applied to a single crystal of magnetite at 124 K > T > 50 K, the monoclinic c M axis, which is the easy magnetization axis, switches to one of the 100 cubic directions, nearest to the direction of the magnetic field, and the phenomenon known as an axis switching (AS) occurs. A global symmetry probe, resonant x-ray scattering, and a local probe, Mössbauer spectroscopy, are used to better understand the mechanism of axis switching. The behavior of three subsystems ordered below the Verwey transition temperature T V , i.e., lattice distortion, an orbital, and charge orderings, was observed via resonant x-ray scattering as a function of an external magnetic field. This was preceded by calculation of selected peak intensities using the FDMNES code. The Mössbauer spectroscopy studies confirmed that the magnetic field triggers electronic rearrangements and atomic displacements. The structure observed after the process of axis switching is very similar to the one obtained after cooling below T V with the magnetic field applied along one of the initial 100 cubic directions and distinct from the cooling in the absence of a magnetic field. From all the experimental observations of the phenomenon done so far, it is clear that AS starts from the fluctuations between octahedral iron orbitals that ultimately lead to the Verwey transition, but also to the higher-temperature trimeron dynamics. Therefore, further observation of the axis switching may be a key point to the understanding of a majority of strongly correlated electronic behavior in magnetite as well as in other transition metal oxides.

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Orbital Domain Dynamics in Magnetite below the Verwey Transition

Cited by 23 publications

References 27 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

Domain fluctuations in a ferroelectric low-strain BaTiO3 thin film

Magnetic field induced structural changes in magnetite observed by resonant x-ray diffraction and Mössbauer spectroscopy

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