Ultrafast X-ray diffraction probe of terahertz field-driven soft mode dynamics in SrTiO3

Kozina, Michael; Driel, Tim van; Chollet, Matthieu; Sato, Takahiro; Glownia, James M.; Wandel, Scott F.; Radović, M.; Staub, U.; Hoffmann, Matthias C.

doi:10.1063/1.4983153

Cited by 29 publications

(12 citation statements)

References 32 publications

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“…One additional frontier in this area is connected to the development of stronger low-frequency electromagnetic pulses that may be able to drive nonlinear dynamics outside a weakly perturbative regime. There is already some promising work in this area on driving dynamics in ferroelectrics, 56,71 but there is likely much more to be done.…”

Section: Discussionmentioning

confidence: 99%

Watching ultrafast responses of structure and magnetism in condensed matter with momentum-resolved probes

Johnson

Savoini

Beaud

et al. 2017

Structural Dynamics

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We present a non-comprehensive review of some representative experimental studies in crystalline condensed matter systems where the effects of intense ultrashort light pulses are probed using x-ray diffraction and photoelectron spectroscopy. On an ultrafast (sub-picosecond) time scale, conventional concepts derived from the assumption of thermodynamic equilibrium must often be modified in order to adequately describe the time-dependent changes in material properties. There are several commonly adopted approaches to this modification, appropriate in different experimental circumstances. One approach is to treat the material as a collection of quasi-thermal subsystems in thermal contact with each other in the so-called “N-temperature” models. On the other extreme, one can also treat the time-dependent changes as fully coherent dynamics of a sometimes complex network of excitations. Here, we present examples of experiments that fall into each of these categories, as well as experiments that partake of both models. We conclude with a discussion of the limitations and future potential of these concepts.

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Section: Discussionmentioning

confidence: 99%

Watching ultrafast responses of structure and magnetism in condensed matter with momentum-resolved probes

Johnson

Savoini

Beaud

et al. 2017

Structural Dynamics

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“…The soft mode can be excited directly, since there is still significant spectral overlap between the THz pump pulse and the absorption spectrum of the phonon 31 . We identify the features at 5.15 THz and 7.6 THz as two pairs of zone-center transverse and longitudinal optical (TO, LO) phonons.…”

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confidence: 99%

Terahertz-driven phonon upconversion in SrTiO3

et al. 2019

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Direct manipulation of the atomic lattice using intense long-wavelength laser pulses has become a viable approach to create new states of matter in complex materials. Conventionally, a high frequency vibrational mode is driven resonantly by a mid-infrared laser pulse and the lattice structure is modified through indirect coupling of this infrared-active phonon to other, lower frequency lattice modulations. Here, we drive the lowest frequency optical phonon in the prototypical transition metal oxide SrTiO3 well into the anharmonic regime with an intense terahertz field. We show that it is possible to transfer energy to higher frequency phonon modes through nonlinear coupling. Our observations are carried out by directly mapping the lattice response to the coherent drive field with femtosecond x-ray pulses, enabling direct visualization of the atomic displacements.In many complex condensed matter systems, small changes to the crystal lattice structure can drastically alter electronic properties including conductivity, polarization, orbital, charge, and spin-order 1-6 . Exploration of a phase diagram conventionally requires changing external parameters such as chemical composition, temperature, static pressure, strain or magnetic fields 6-9 . Additionally, optical excitation using ultrashort laser pulses has been used to dynamically change the electronic and crystal structure 2-5,10,11 . This method allows access to nonequilibrium states of matter that show unconventional and unique properties. Recent developments in mid-infrared laser sources have enabled a new route to control material properties: resonant excitation of phonon modes to dynamically alter the lattice structure and phonon-phonon coupling are exploited to coherently control inaccessible (i.e. non-infraredactive) phonon modes 12 . This approach is often dubbed "nonlinear phononics" and requires precisely tuned mid-infrared radiation and impulsive excitation 13 .Here we show a completely novel approach to alter the lattice structure by driving the lowest frequency optical phonon of the incipient ferroelectric SrTiO3 (STO) into the strongly nonlinear regime by using intense broadband terahertz (THz) radiation. Specifically, we demonstrate that it is possible to transfer energy from a low-frequency phonon mode to higherfrequency modes. We anticipate that our approach will open up a deeper understanding of quantum materials through direct manipulation of the lattice structure and the utilization of dynamic coupling of vibrational modes. STO has a low-frequency zone-center transverse optical (TO) phonon that is infrared (IR)active and highly temperature dependent [14][15][16] . This so called soft-mode phonon mediates the ferroelectric phase transition in primarily displacive ferroelectrics such as BaTiO3 or PbTiO3 17-19 .In bulk STO, while the soft mode shifts to lower frequencies with decreasing temperature, the system never reaches a ferroelectric state due to quantum fluctuations 20 . The temperature dependence and large anharmonicity make this mode ide...

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“…In the first case, a resonant excitation (RE) of the soft mode is used near the second‐order phase transition, where the arising effects are associated with an abrupt change (switching) of the order parameter due to large amplitudes and nonlinear regime of soft mode. Ultrafast lattice response was demonstrated using an intense THz pump and X‐ray probe pulses in incipient ferroelectric SrTiO 3 film, [ 8 ] ferroelectric Sn 2 P 2 S 6 crystal, [ 9 ] and BaTiO 3 film. [ 10 ] The second way is based on the assumption, that a coherent shift of the ferroelectric mode can be achieved indirectly by exciting an anharmonically coupled (AC) vibrational mode at a higher frequency.…”

Section: Figurementioning

confidence: 99%

Transient Polarization Reversal using an Intense THz Pulse in Silicon‐Doped Lead Germanate

Bilyk

Мишина

Sherstyuk

et al. 2020

Physica Rapid Research Ltrs

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The processes of dynamic polarization switching induced by the strong electric field of a nearly single‐cycle terahertz (THz) pulse are studied in a classical ferroelectric silicon‐doped lead germanate single crystal, Pb5(Ge0.74Si0.26)3O11, in THz pump–second‐harmonic‐generation (SHG) probe geometry depending on the pulse electric field strength (0.05–15.6 MV cm−1). Since the pump parameters correspond to the region of phonon excitations for a given material, THz‐induced polarization is explained by a dynamic change in the ferroelectric order parameter. Numerical solutions of the Landau–Khalatnikov equation using the thermodynamic potential constants for this material show that the effect of powerful THz pulses leads to polarization modulations and a strong nonlinear response, and is in good agreement with the experimental results. Oscillations of the nonlinear optical response after the end of the THz pulse are found, which can be associated with the excitation of the phonon mode in the crystal.

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Ultrafast X-ray diffraction probe of terahertz field-driven soft mode dynamics in SrTiO3

Cited by 29 publications

References 32 publications

Watching ultrafast responses of structure and magnetism in condensed matter with momentum-resolved probes

Watching ultrafast responses of structure and magnetism in condensed matter with momentum-resolved probes

Terahertz-driven phonon upconversion in SrTiO3

Transient Polarization Reversal using an Intense THz Pulse in Silicon‐Doped Lead Germanate

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