Session H09: Dielectric and Ferroic Oxides - Opto-Electric Responses

Femtosecond laser pulse-induced breakdown of the insulating phase in bulk V2O3: a TDDFT+DMFT study

We use the time-dependent density-functional theory + dynamical mean-field theory (TDDFT+DMFT) approach to analyze the details of the insulator-to-metal transition and ultrafast charge dynamics in bulk V2O3 excited by various femtosecond laser pulses. In this approach, to properly include the effects of strong electron-electron correlations into TDDFT calculations one uses an exchange-correlation potential obtained by solving the DMFT problem. We analyze the time-dependence of the excited charge density and of the resulting time-dependent charge conductivity of the system and demonstrate that both quantities have a power-like time-dependence. We pay a special attention to the orbital-resolved tempo-spatial charge dynamics during the metallization in V2O3 and argue that this process is strongly spatially–nonhomogeneous. We establish the time-dependence of the size of metallic domains at different values of the pulse parameters and draw consequences of the results in term of potentially-measurable experimental quantities, like femtosecond conductivity and emission.   

Large thermal conductivity switch ratio in ferroelectric materials under electric field through first-principles calculation

Barium titanate is considered as an example that has order-disorder feature due to the existence of the “central mode”, which could be a potential material that can have large thermal conductivity switch ratio if the disordered structure can be aligned to the ordered structure. Solving the Boltzmann transport equation (ShengBTE), the fully disordered structure was found to have a thermal conductivity a factor of ~4 lower than the ordered structure. Then an electric field was applied in the simulations to align the disordered structure. In doing so, the polarization hysteresis loop of barium titanate in rhombohedral phase is obtained for the first time by a full first-principles method. We found the electric field can at most increase the thermal conductivity of ordered structure by a factor of ~2. Therefore, combining both of the above effects suggests a maximum possible switch ratio of ~8. These results yield two guidelines for searching high thermal conductivity switch ratio in ferroelectric materials: the structure should be single domain, and the phase transition should be fully order-disorder rather than displacive.   

Tuning photo-induced ultrafast strain in ferroelectric devices

Among ferroic materials, ferroelectric oxides are particularly promising due to their numerous functional properties and their potential coupling. Manipulating and integrating these functionalities in devices can pave the way for innovative oxide-based electronics. Photostriction, described as a combination of both photovoltaic and inverse piezoelectric effects, is a complex physical mechanism inducing non thermal strain under illumination. Recent studies in ferroelectric thin films have reported photo-induced strain in the picosecond time range, thus opening a new route for ultrafast strain engineering and optical actuation in devices. However, the polarization is usually in as-grown state, so its contribution on the photostrictive response is not well understood.
I will present ultrafast studies on photo-induced strain in ferroelectric thin films based devices with an in-situ control of the polarization state. Our time-resolved x-ray diffraction studies performed at Advanced Photon Source (APS) revealed that both magnitude and sign of strain can be controlled by the polarization state, giving a better understanding of the ultrafast photostriction mechanism in ferroelectric devices.   

Ultrafast photostriction in ferroic compounds.

Ferroic compounds have intrinsically a large electro-mechanical coupling playing a central role in many devices in telecommunications and sensors technologies. To push the devices in the GHz-THz regime, only a control of the ferroic properties with light is possible. Among perspectives, the control of motion and strain with ultrafast light pulse should open new perspectives for light-controlled devices (actuators, mechatronics). To reach this goal, it is crucial to understand how the light energy is converted into mechanical energy (acoustic phonons) in these ferroic compounds. We then have studied the ultrafast generation/detection of coherent acoustic phonons in ferroics compounds within the pump-probe scheme. We show that ferroelectric BiFeO₃ offers unique properties to emit/detect coherent transverse (TA) and longitudinal (LA) acoustic phonons [1]. This phenomenon is attributed to an ultrafast screening of the ferroelectric polarization. Beside a rich electron-phonon coupling physics [1-3], we also evidence original ultrafast acousto-optic processes (i.e. a conversion of the probe light polarization from ordinary to extraordinary (and vice-versa) recently revealed in ferroelectrics such as LiNbO₃ and BiFeO₃ [4]. By demonstrating the manipulation of light polarization with 10-100 GHz coherent acoustic phonons, our results highlight new capabilities in using ferroelectrics in modern photoacoustics and photonics.
[1] Lejman M., Vaudel G., Infante, I. C., Gemeiner P., Gusev V., Dkhil B., Ruello P., Nature Comm. 5, 4301 (2014).
[2] Schick, D. et al., Phys. Rev. Lett. 110, 095502 (2013).
[3] H. J. Lee et al, Scientific Reports, 6, 38724 (2016)
[4] Lejman M., Vaudel G., Infante I-C., Chaban I., Pezeril T., Edely M., Nataf G., Guennou M., Kreisel J., Gusev V. E., Dkhil B., Ruello P., Nature Comm. 7, 12345 (2016).   

Electrically Tunable Polarizer based on 2D Orthorhombic Ferrovalley Materials

The concept of ferrovalley materials has been proposed recently. The existence of spontaneous valley polarization, resulting from ferromagnetism in hexagonal 2D materials makes nonvolatile valleytronic applications realizable. Here we introduce a new member of ferrovalley family with orthorhombic lattice, i.e. group-IV monochalcogenides, where intrinsic valley polarization originates from ferroelectricity. We demonstrate linearly polarized optical selectivity for valleys exists in the new ferrovalley materials. A prototype of electrically tunable polarizer is then realized. In the polarizer, a laser beam can be optionally polarized in x- or y-direction, depending on the ferrovalley state controlled by electric fields. Such a device can be further optimized to emit circularly polarized light and to realize the tunable wavelength. Therefore, 2D orthorhombic ferrovalley materials are the promising candidates to provide an advantageous platform to realize the polarizer driven by electric means, which is important in extending the practical applications of valleytronics.   

Metal-insulator transition based non-volatile, rewritable, all-optical imprinting on VO2

All-optical material manipulations are highly desirable in modern telecommunication networks for signal routing and storage, because it can eliminate the need for optical-electrical conversions that hamper data rate and generate extra energy consumptions. We demonstrate the reversible writing and erasing of metallic structures in VO₂ using a single visible laser beam by either varying the environmental pressure or tuning the laser pulse duration. Different from the conventional laser heating based phase-change materials used in optical data storage, the effects observed in VO₂ are driven by photochemical reactions related to the molecules in the air. The laser-patterned structures are stable in the air for days and can be easily read-out through their substantially altered resistance, reflectance, and structural properties and fully compatible with super-resolution optical techniques, these findings provide a new route for non-volatile photonic information processing.   

Direction Dependent Luminescence in Multiferroic CuB2O4

In materials where both space-inversion and time-reversal symmetries are simultaneously broken, luminescence intensities can be different between the opposite emission directions. However, only small directional asymmetry has been reported so far. One possible reason is that the effect has been investigated only in the paramagnetic materials where time-reversal symmetry is violated by an external magnetic field. In this presentation, I will report that the effect is robust in a noncentrosymmetric magnet CuB₂O₄ with spontaneous breaking of space-inversion and time-reversal symmetries. The sample was excited by a He-Ne laser of wavelength 633 nm, and the photoluminescence spectra in near infrared region were measured by using a grating-type optical spectrometer. We have found that the luminescence intensity changes by 70 % between the opposite directions of the emission, which is about 100 times stronger than the previously reported values. We also demonstrate that such a gigantic nonreciprocal emission of light can be applied to the imaging technique of magnetic domain structures.   

Simulation study of photo-induced phase transitions and coherent optical phonon generation in BaTiO3

The effects of above band gap ultrafast optical excitation on the structural properties and lattice dynamics of the prototypical perovskite multiferroic, BaTiO₃, are investigated using a constrained density functional method (É. D. Murray et. al. PRB 72, 060301 (2005), P. Tangney and S. Fahy, PRB 65, 054302 (2002)). We calculated the dependences of phonon frequencies and structural stability on excited carrier density. Our work shows a reduction in ionicity due to electrons being returned by O anions to Ti cations. At moderate excitation levels, this causes a decrease in polarization, a softening of several long wavelength optical phonons, and a lowering of local barriers to ferroelectric domain reversal. This indicates that a transient photo-induced lowering of both the coercive field and the phase transition temperature is possible. We suggest that pump probe spectroscopy could be used to induce a purely-displacive transition to a higher symmetry phase at low temperature and/or to generate and study the decay of coherent optical phonons. Our findings may also provide guidance to the design of optically controlled devices.   

Transport Properties of Shallow InAs Quantum Wells for Spintronic Applications

Spintronic applications of heterostructures require materials with excellent semiconducting properties, strong spin-orbit coupling, and shallow channels for optimized injection and detection of currents. To this goal, shallow n-type InAlSb/InAs/AlGaSb quantum wells have been fabricated in Hall bars followed by measurements of their sheet resistance in variable magnetic field, temperature, and under variable illumination conditions, with wavelengths of 400 nm up to 1300 nm. Here we used analysis of the Shubnikov-de Haas oscillations and the Hall effect in order to find the semiconducting and spin-orbit properties of these structures. The results were compared and contrasted with the properties of deep channel InAs/InGaAs with excellent spin-orbit coupling properties. We find that the carrier concentration of the shallow channels increases under infrared illumination, but decreases when further decreasing the wavelength. Parameters including the effective mass, the quantum scattering time, and the spin-orbit coupling are extracted and their dependence on the carrier concentration and the type of structure is discussed.   

Photoinduced gap renormalization and many-body recombination in insulating cuprates

We have studied the pump-probe response of three insulating cuprates as a function of both pump and probe photon energies, time, and fluence. The fluence dependence follows a simple and universal analytical form that includes a characteristic volume scale, which we associate with the interaction volume for a photoexcitation. This characteristic size varies strongly with pump photon energy, with a maximum just above the charge-transfer absorption peak. We also show evidence for ultrafast many-body recombination in the photocarrier kinetics, characterized by an anomalously large Auger coefficient.   

Nonlinear spin-lattice coupling in EuTiO3: a novel two-dimensional magneto-optical device for light modulation

EuTiO₃ is antiferromagnetic at low temperature, namely below T_N=5.7K. In the high temperature paramagnetic phase the strongly nonlinear coupling between the lattice and the nominally silent Eu 4f⁷ spins induces magnetic correlations which become apparent in muon spin rotation experiments and more recently in birefringence measurements in an external magnetic field. It is shown here, that high quality films of insulating EuTiO₃ deposited on a thin SrTiO₃ substrate are versatile tools for light modulation. The operating temperature is close to room temperature and admits multiple device engineering. By using small magnetic fields birefringence of the samples can be switched off and on. Similarly, rotation of the sample in the field can modify its birefringence Δn. In addition, Δn can be increased by a factor of 4 in very modest fields with simultaneously enhancing the operating temperature by almost 100K. The results can be understood in terms of paramagnon phonon interaction where spin activity is achieved via the local spin-phonon double-well potential.