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Light-induced control of the Metal Insulator Transition in Magnetite

English title Light-induced control of the Metal Insulator Transition in Magnetite
Applicant Carbone Fabrizio
Number 165560
Funding scheme Project funding
Research institution Institut de physique de la matière condensée EPFL - SB - ICMP
Institution of higher education EPF Lausanne - EPFL
Main discipline Condensed Matter Physics
Start/End 01.04.2016 - 31.03.2019
Approved amount 200'000.00
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Keywords (5)

Verwey transition; femtosecond ; Electron diffraction; Metal insulator transition; magnetite

Lay Summary (Italian)

Lead
Controllare le proprietà di conduzione/isolamento della magnetite tramite impulsi di luce
Lay summary

L'obiettivo di questo progetto di ricerca è di utilizzare impulsi luminosi per modificare le proprietà elettriche di uno dei materiali piu' usati dall'uomo: la magnetite, o ossido di Ferro. Quando viene raffreddata sotto i 120 gradi kelvin, la magnetite cambia la sua struttura cristallina e allo stesso tempo cessa di condurre l'elettricità e diventa un isolante. Le sue proprietà magnetiche invece sono sostanzialmente preservate attraverso questa transizione di fase. La natura stessa di questo fenomeno è dibattuta dai fisici dello stato solido dal 1934. L'ipotesi piu' accreditata è quella di Verwey, che diede cosi' il nome alla transizione di fase, secondo il quale lo stato di isolante è provocato da un ordinamento della carica elettrica nel materiale. Nel nostro laboratorio, abbiamo osservato i cambiamenti di colore che avvengono nella magnetite quando con un impulso laser ultracorto (50 milionesimi di milardesimi di secondo) si induce la transizione di fase. Grazie a questi esperimenti abbiamo osservato che l'ordinamento di carica responsabile per lo stato isolante di bassa temperatura avviene in cooperazione con certe distorsioni della struttura cristallina del materiale. Sorprendentemente, abbiamo anche osservato che impulsi di luce di certi colori sono in grado di indurre la fase di bassa temperatura mentre il cristallo si trova a temperatura ambiente, causando quello che si puo' chiamare un congelamento foto-indotto del materiale. Nell'ambito di questo progetto, faremo degli esperimenti mirati ad osservare simultaneamente i cambiamenti di colore della magnetite e le distorsioni strutturali che accompagnano la transizione di fase indotta da impulsi luminosi di diverse energie. L'obiettivo finale è di acquisire un controllo delle proprietà di questo materiale tramite la luce che permetterebbe di sviluppare nuovi dispositivi elettro-ottici e allargare il campo di applicazione della magnetite

Direct link to Lay Summary Last update: 25.03.2016

Responsible applicant and co-applicants

Employees

Collaboration

Group / person Country
Types of collaboration
Dr Damien Mc Grouther, Kelvin Nanocharacterisation Centre/ University of Glasgow Great Britain and Northern Ireland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
- Exchange of personnel
Prof Andrea Ferrari, Cambridge Great Britain and Northern Ireland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Prof. Ronnow, LQM EPFL Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Profl Oleg Yaziev Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Steve Johnson ETHZ Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Prof. Lorenzana Italy (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure

Associated projects

Number Title Start Funding scheme
128269 The dynamics of chemical bonds by ultrafast electron diffraction 01.09.2010 SNSF Professorships
157956 NCCR MUST: Molecular Ultrafast Science and Technology (phase II) 01.07.2014 National Centres of Competence in Research (NCCRs)

Abstract

In this project, we aim at demonstrating the ability of tailored light pulses to switch the current transport properties of magnetite (Fe3O4) between metallic and the insulating phase, and in both directions (i.e. from the low-temperature phase to the high-temperature phase and vice-versa).Phase transitions termed “Inverse freezing” or “inverse melting” happen when a phase having a higher symmetry occurs below the critical temperature and not above. This has been shown to happen in magnetic thin films and some organic molecules. More recently, the possibility to induce such a phenomenology in strongly correlated materials with laser pulses has been demonstrated by revealing signatures of superconductivity in the photoexcited high-temperature phase of a cuprate. Driving phase transitions with light pulses is a promising field of research as it allows to reveal transient states otherwise “hidden” in equilibrium experiments, and it also holds promises for the discovery of new optoelectronic functionalities. Light in fact can modify the emergent properties of a solid via the interplay of structural and electronic degrees of freedom. Upon laser excitation, the lattice as well as the electronic excitation relaxation processes are not independent from each other, but rather provoke new transient states of matter having peculiar charge or structural ordering properties. Furthermore, the dynamical study of a system manifesting a symmetry-breaking phase transition (PT) has fundamental implications in very diverse fields of research, ranging from condensed matter physics to cosmology.In a recent work, we have shown that fs blue-light pulses can transiently freeze the conduction band carriers of magnetite in their low-temperature ordered state, while the crystal is above the critical temperature of the phase transition. Magnetite is the earliest discovered magnetic material and is currently used in numerous applications. The ability to switch its transport properties as our experiments suggest open new fascinating scenarios for optoelectronic devices.To reveal the intimate interplay between the atomic motions and the electronic structure changes responsible for such a light-induced inverse transition, combined structural and spectroscopic information is needed. For this reason, we plan to investigate single crystals and thin films of magnetite by means of fs-electron diffraction experiments combined with fs-optical spectroscopy.The metal-insulator transition (MIT) in magnetite has been recently investigated by means of ultrafast X-ray diffraction. However, in those experiments 800 nm excitation was used; in our recent work, we demonstrate that coherent structural and electronic effects can only be induced by resonant photoexcitation of the material, in particular in tune with its charge transfer at 3.1 eV. The coherent oscillation assigned to the combination of the ?5 structural mode and the charge density wave (CDW) mode (associated with the charge/orbital ordering) has been found to undergo a sizeable softening across the Verwey transition. However, the critical mode of the CDW alone, whose oscillation period should lengthen when crossing TV, could not be observed. Electron diffraction experiments could successfully disentangle the structural and electronic contributions to the Verwey transition and directly reveal the critical mode of the charge/orbital ordering. The oscillation periods of the structural ?5 mode and of the critical mode should be around 400 fs and few ps, respectively, both accessible to the time-resolution of our electron diffraction setup. Furthermore, electron diffraction offers the possibility to record a wider reciprocal space range revealing the cooperation of the different structural degrees of freedom across the phase transition.In our laboratory at the EPFL, we implemented a state of the art table-top electron diffraction set-up, capable of fs-resolved experiments both in the transmission and in the reflection geometry. An overall temporal resolution around 300 fs for bunches containing up to 10^5 electrons at a repetition rate of 20 kHz was demonstrated. It is known that the group velocity mismatch (GVM) due to the different speeds of electrons and photons imposes a limitation on the time resolution. The effect of GVM can be overcome by tilting the optical pulses to match the velocity of the laser and electrons on the surface of a solid sample. In our laboratory, we recently achieved the tilting of the 800 nm optical pulses, improving the time resolution by a factor of 10. In order to resonantly photoexcite the material at 3.1 eV, corresponding to the charge transfer from the O 2p to the Fe 3d level in magnetite, we will implement tilting of the 400 nm optical pulses. We also propose to excite specific structural modes involved in the Verwey transition selectively by THz radiation and probing with electron pulses which should provide direct access to the critical mode.
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