ABSTRACT. I will discuss how coherent electromagnetic radiation at Tera-Herz frequencies can be used to drive complex solids periodically, controlling the coupling between their collective excitations. These drives are shown to give rise to non-thermal states with unconventional types of microscopic order and new functional properties. Important examples involve the nonlinear control of the crystal lattice, used to induce magnetic order, ferroelectricity and non-equilibrium superconductivity at high temperatures.

ABSTRACT. We present an experimental study on how thermally induced ultrafast charge-carrier injection affects the exciton formation dynamics in bulk (thickness around 20 nm) transition metal dichalcogenides (TMDs). For this purpose, we studied bulk WS2 deposited on a gold substrate employing a pump-push-probe (PPP) scheme. One pump pulse induces thermal injection of electrons from the gold into the conduction band of the semiconductor, one delayed push pulse excites hot electrons, holes and corresponding excitons in the WS2, and a probe detects WS2 A-exciton dynamics. The transient reflection shows qualitatively different dynamics in the first hundreds of femtosecond by varying the fixed delay between pump and push pulse by sub-ps steps. To verify that this effect stems from the ultrafast electron injection at the Schottky interface, we repeated the same PPP measurements in a bulk WS2 sample deposited on a SiO2 substrate. In that case the transient signal showed no change upon variation of the pump-push delay on a sub-ps time scale.

ABSTRACT. Due to their quantum character, the vortices in superconductors are topologically stabilized objects which are hot candidates for carriers of elementary information in the next generation of logical devices. However, the vortex mass (a non-trivial function of material, temperature, and magnetic field) is a particularly controversial issue. Its theoretical estimates are scattered over more than eight orders of magnitude whereas only two experimental determinations have been performed. Here, we present relevant experiments leading to an evaluation of the vortex mass in the most common high-temperature superconductor YBaCuO in the form of a thin film with a thickness of L = 107 nm, and with CuO2 planes parallel to the surface. Our method was inspired by the standard determination of cyclotron mass of charge carriers in semiconductors in magnetic field through measuring the cyclotron resonance. In the case of vortices, we probed their interaction with circularly polarized far-infrared light under external magnetic field of up to 11 T. It differed for the clockwise and anti-clockwise polarized light cases, resulting in the so-called circular dichroism, which manifests itself as a difference in the transmittance of the light through the superconducting sample. The ratio of transmittances of opposite helicity can be related to the mass of the vortices. The sample parameters have been determined by usual techniques or taken from the literature. Additional film properties in the far-infrared range have been established in a separate experiment using standard time-domain THz spectroscopy. With such inputs, we show that the theory of the vortex mass developed by Kopnin and coworkers matches our experimental data without any additional fitting parameter. For YBaCuO at 45 K, the vortex mass in the zero-frequency limit amounts to 2.4 x 10E8 electron masses per centimeter. The presentation will focus on the experimental approaches which led to these results.

ABSTRACT. Interacting degrees of freedom in solids underlie new emergent ground states and competing phases with potential for new functionalities. Vibrations of the atomic lattice, i.e. phonons, can couple to electrons, magnetic, or orbital degrees of freedom. In particular, electron-phonon coupling (EPC) received a lot of attention as a microscopically understood origin of superconductivity. Furthermore, EPC has recently been in the focus of investigations of materials with competing phases. Reduced phonon lifetimes, are typically related to nesting where large sections of the Fermi surface are connected by a single phonon wave vector. The hallmark of nesting is a singularity or at least strong peak in the electronic susceptibility for a particular phonon wave vector q connecting parallel sections of the Fermi surface. Such phonon anomalies received a lot of attention recently in the context of charge density wave formation, which is often driven by a soft phonon mode triggering a structural distortion at the charge-density wave transition. In some charge-density wave compounds, such as 1D conductors, this behavior is indeed related to the nesting of the Fermi surface [1]. Yet in others, such as 2H-NbSe2, nesting is absent [2,3] and cannot explain the soft-mode properties. While the charge-density wave in 2H-NbSe2 originates from EPC [4], it has been proposed that the phonon softening and broadening on cooling towards the charge-density wave transition temperature T_CDW can be explained only by taking into account lattice fluctuations [2], and anharmonic effects may play an important role [5]. In principle, the phonon momentum, q, dependence of the EPC itself, expressed in the EPC matrix elements g_(k+q,k), can determine the wavevector of the phonon broadening in the absence of Fermi surface nesting. Yet, g_(k+q,k) is expected to be temperature-independent for temperatures of the order of up to 103 K.

Here, we propose a scenario in which the electron momentum, k, dependence of the EPC matrix elements comes into play. In such a scenario, g_(k+q,k), is particularly large for electrons on certain parts of the Fermi surface. We show that such k-selective EPC can be strong enough to significantly broaden phonons even when k-integrated quantities like the electronic susceptibility χ_q lack particular features at the phonon momentum q. In this case, the broadening can sensitively depend on the temperature-induced changes of electronic states at the Fermi energy. This scenario can explain large temperature-dependent phonon linewidths in the absence of both nesting and anharmonicity. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and density functional perturbation theory to demonstrate this scenario on the electron-phonon superconductor YNi2B2C (T_c=15.2 K) focusing on strong-coupling acoustic phonons in the normal state at low temperatures, i.e. T = 20 K. We show that our experimental results agree well with ab-initio lattice dynamical and electronic band structure calculations, which allows us to use these calculations to gain insights into the microscopic origin of EPC. The calculations highlight the importance of 2D electronic joint density of states. It is defined as the usual electronic joint density of states but evaluated for 2D slices of the reciprocal space at a specific component of k. The results highlight the decisive role of the interplay between the k-dependence of the EPC combined with a strongly k-dependent 2D-eJDOS, and explain strongly temperature-dependent phonon broadening in the absence of Fermi surface nesting and lattice anharmonicity.

[1] R. Comes et al., Physica Status Solidi B-Basic Research 71, 171 (1975). [2] F. Flicker and J. van Wezel, Nat Commun 6, 7034 (2015). [3] F. Weber et al., Physical Review B 97, 235122 (2018). [4] F. Weber et al., Physical Review Letters 107, 107403 (2011). [5] M. Leroux et al., Physical Review B 92, 140303 (2015).

ABSTRACT. The interaction between an ensemble of non-interacting two-level quantum systems and a bosonic field is theoretically described by the Dicke-model which predicts a quantum phase transition in the thermodynamic limit when the strength of the interaction reaches a sufficiently large critical value [1,2]. However, the experimental realization of this so-called superradiant phase transition is challenging. Recently superradiance of magnetic oscillators was observed in yttrium-iron-garnet films by measuring the coherent radiation of magnetic moments [3]. Here, based on the idea of Ref. [4] we present a method to study the superradiant phase transition in SmFe3(BO3)4, where isolated rare-earth quasi-spins (Sm) play the role of the two-level system and the bosonic field is provided by the spin-waves (i.e. magnons) of the antiferromagnetically ordered Fe ions. At low temperatures (T=3K) we observe an avoided crossing of the optically active low-frequency iron magnon and the Sm quasispin excitations with a coupling of about 70% of the critical value needed for the superradiant transition. The strength of the coupling was tuned by varying density and population of the Sm two-level systems. [1]Phys. Rev. A 8, 2517 (1973). [2]Phys. Rev. A 7, 831 (1973). [3]Commun. Phys. 4, 97 (2021). [4]Science 361, 794 (2018).

ABSTRACT. Understanding the nature and furthermore controlling the mechanisms of coupling between lattice, magnetic and orbital orders in magnetoelectric materials remains a fundamental challenge to achieve in order to open the path toward technological applications. The dynamic of collective order of lattice, spin and orbital results in low-energy excitations as phonon, magnon and crystal fields, respectively and the coupling between theses orders can give rise to hybrid excitation as electromagnon. These excitations lie in the THz and FIR energy range and carry either an electric or a magnetic dipole that can be directly excited by the electromagnetic wave of light making infrared spectroscopy a technique of choice to investigate them. Moreover, hydrostatic pressure can be used as a tuning parameter in order to understand the coupling mechanisms at play and improve the resulting properties suitable for applications. However, the low-energy range, the small intensity of the excitations and the sample limited size (typically a few hundred microns) required for high-pressure measurement make this kind of experiment very challenging. We will show that, at the AILES beamline of synchrotron SOLEIL, the combination of the high brilliance and stability of the synchrotron beam in the low energy range with a highly focusing high-pressure/low-temperature setup, allows one to measure and follow, as a function of pressure, phonons, magnons, crystal fields but also electromagnons in a wide frequency range down to the diffraction limit. These measurements reveal unique information about the dynamical coupling of orders in magnetoelectric materials. The cases of several oxides will be presented.

ABSTRACT. Multiferroics, where both magnetic and ferroelectric orders coexist and are coupled to one another, have attracted great interest. In type-II multiferroics, the ferroelectric polarization is a consequence of a particular magnetic order in which magnetic interactions break inversion symmetry of the crystal. In this class of materials, the magnetic origin of ferroelectricity is responsible for a strong coupling between both orders which, for example, makes it possible to rotate the direction of the spontaneous electric polarization via an external magnetic field. TbMnO3 is a typical representative of type-II multiferroics, where a cycloid spiral spin ordering is stabilized below Tc = 28 K [1]. Due to the Dzyaloshinskii-Moriya interaction, TbMnO3 is ferroelectric below Tc [1]. There is also a strong dynamic magnetoelectric coupling in TbMnO3, leading to the existence of electric dipole active magnons termed electromagnons [2, 3]. One way to alter the magnetic interactions in TbMnO3 is by chemical substitution of Mn3+ ions for the identically sized Fe3+ ions, giving rise to the fully soluble TbMn1-xFexO3 system. For x ≤ 0.05, the phase sequence was found to be the same as TbMnO3 [4]. In this work, the influence of substituting Mn3+ ions by Fe3+ in TbMnO3 on the magnetic properties and spin and lattice dynamics of oriented single crystals of TbMn1-xFexO3 with x = 0.02 and 0.04 is systematically investigated. Muon spin relaxation spectroscopy reveals a short-range magnetic order up to 200 K, the magnetic susceptibility indicates the creation of a short-range magnetic order at least below 100 K, far above the Néel temperature. The temperature dependence of phonon frequencies exhibits a deviation from the classical anharmonic behavior due to spin-phonon coupling below 70 K, with electromagnon absorption becoming pronounced also below this temperature. The high-frequency electromagnon exhibits significant softening with decreasing temperature, not observed in pure TbMnO3 [3]. Several experimental results will be presented to clarify the impact that the presence of Fe3+ has in the magnetoelectric coupling.

[1] Kimura, T., et al. "Magnetic control of ferroelectric polarization." nature 426.6962 (2003): 55-58.

[2] Pimenov, A., et al. "Possible evidence for electromagnons in multiferroic manganites." Nature physics 2.2 (2006): 97-100.

[3] Takahashi, Y., et al. "Evidence for an electric-dipole active continuum band of spin excitations in multiferroic TbMnO 3." Physical review letters 101.18 (2008): 187201.

[4] Vilarinho, R., et al. "On the ferroelectric and magnetoelectric mechanisms in low Fe3+ doped TbMnO3." Journal of Magnetism and Magnetic Materials 439 (2017): 167-172.

ABSTRACT. Rare-earth orthoferrites (RFeO3, R = rare-earth) have renewed the scientific interest due to their remarkable magnetic properties, spin-reorientation transitions and, more recently, by the discovery of ferroelectricity and multiferroicity in some of them [1]. The magnetic structure of RFeO3 can be regarded as the superposition of interacting Fe3+ and R3+ spin sublattices. The Fe3+ spins strongly interact and order at much higher temperatures than the R3+ spins, that order antiferromagnetically only at low temperatures, usually below 10 K. At TN between 623 and 740 K, the Fe3+ spin sublattice undergoes a magnetic phase transition into an AxFyGz canted antiferromagnetic Fe3+ spin structure. For R = Pr, Nd, Sm, Tb, Ho, Er, Tm, and Yb, there is also a spin-reorientation transition into the CxGyFz spin structure [1]. Between TN and the temperature of R3+ spin ordering, the R3+ sublattice behaves paramagnetically, and can be polarized by the molecular magnetic field of the net ferromagnetic moment of the ordered Fe3+ sublattice. In the special cases with R = Nd, Sm, and Er, this ordering occurs in opposite direction of the Fe3+ sublattice, yielding a decrease of the total net magnetization that eventu-ally vanishes at the compensation temperature, and reverses its sign below [2]. This behavior has been associated with the reinforcement of the R-R interactions. Despite the intensive research of the magnetic properties of these compounds, the origin of a set of anomalies ob-served in the M(T) curves, not ascribed to any magnetic phase transitions, have not been the subject of discussion or experimental analysis [3]. This work combines a systematic molecular field analysis of the temperature depend-ence of the magnetization, and Raman-active spin excitations and phonons coupled to the magnetic structure by the spin-orbit mediated spin-phonon coupling. The use of a compre-hensive molecular field model enabled to disclose remarkable magnetic interactions, namely the increase of Fe-sublattice magnetization and its interaction with the R-sublattice [3]. These were seen to be distinctly mirrored by the anomalies in the temperature dependence of Ra-man-active magnons and the FeO6 octahedral rotation mode frequency, the later via spin-phonon coupling. This coupling also allows to unravel the distinct magnetic interaction be-tween both Nd3+/Tb3+ and the Fe3+ spin sublattices through the anomalies observed in the temperature dependence of the corresponding rare-earth oscillations, ascertained to be cou-pled with the FeO6 octahedra rotational modes. Contrarily, such anomalous features are not shown in the temperature dependence of both Eu3+ and Gd3+ oscillation modes, the later due to the lack of spin-orbit coupling. The work here reported provides significant experimental evidence, in a specific set of RFeO3, regarding the Fe3+ spins structure below room tempera-ture, its interaction with the rare-earth spins sublattices, and its magnetostructural coupling and gives a renewed overview concerning the mechanisms underlying the complex magnetic properties of RFeO3.

References [1] C. Weingart et al., Physical Review B, 86 094413 (2012) [2] S. Yuan et al., Physical Review B, 87 184405 (2013) [3] M. Eibschütz et al., Physical Review, 156 562-577 (1967)

ABSTRACT. Planar transmission lines, namely microstrip lines and coplanar waveguides, can be employed together with a vector network analyzer for characterization of microwave excitations and relaxations in solids. The temperature- and magnetic-field dependence of these anomalies can give information on the dynamics in the materials and their phase transitions. Using high-frequency electromagnetic simulation software is essential for analyzing the measured transmission and reflection coefficients.

In this contribution, we will present experimental data (100 MHz - 50 GHz) of selected magnetic, multiferroic and ferroelectric materials (focussing on hexaferrites). Simulation of the planar transmission lines and their electromagnetic response including the sample will be performed with the HFSS simulation software. Where possible, the results will be compared with data from other high-frequency techniques.

ABSTRACT. Despite its importance for the understanding of fundamental aspects of condensed matter dynamics, the mesoscopic scale of matter (100-1 nm)1 has been un-explored so far, also due to a lack of an appropriate instrument. This technological and scientific gap has been progressively closed in recent years with the development of two complementary instruments, the so called miniTIMER and TIMER set-ups. Both, by wave-front splitting and recombining an extreme ultraviolet (EUV) pulse from a free electron laser, are able to generate a transient excitation grating with spatial periodicity down to the 10’s nm regime. A so short spatial scale requires a third EUV probing pulse; such a multi-EUV-pulse capability has been implemented at the TIMER instrument, with a dedicated delay line equipped with purposely designed multilayer mirrors2. The use of EUV pulse also enables the fascinating option of element sensitivity, thus adding the “chemical resolution” on top of nanometer-femtosecond spatial-temporal resolution. In this talk, I will review the most recent experimental results obtained with the combined use of the two instruments, which range from the study of heat3 and spin diffusions4 at the tents of nm scale to the attenuation of acoustic phonons in scientifically and technologically relevant systems5-7.

1) F. Bencivenga et al., Nature volume 520, pages205–208 (2015) 2) R. Mincigrucci et al., NIMA, Vol. 907, pp. 132-148 (2018) 3) A. Maznev et al., in preparation 4) D. Ksenzov et al., Nano Letters, Vol. 21 - 7, pp. 2905-2911 (2021) 5) F. Bencivenga et al., Science Advances, Vol. 5 - 7, eaaw5805 (2019) 6) G. Baldi et al., in preparation 7) A. Maznev et al., APL submitted

ABSTRACT. The importance of anharmonicity for describing fundamental materials properties, starting from finite heat conductivity due to phonon-phonon scattering, can hardly be overemphasized. For crystalline matter, the principal microscopic gauge is constituted by the broadening in energy of the phonon dispersions, corresponding to q-dependent phonon lifetimes.

Here the case of elemental Al at temperatures up to the melting point will be considered. Experimental data obtained by inelastic neutron scattering will be compared to calculations of q-dependent line broadenings on the basis of density-functional theory. While the standard approach of perturbation theory gives significant discrepancies, the agreement with spectra computed by ab initio molecular dynamics is satisfactory [1]. Further, an analysis of the atomic interaction constants will show how to construct numerically efficient phenomenological potentials that accurately reproduce anharmonic properties as computed by DFT at very small computational effort, and finally the limitations of perturbation-derived linewidths will be elucidated.

[1] A. Glensk et al., Phys. Rev. Lett. 123, 235501 (2019)

ABSTRACT. It is well known that dielectric properties of various ferroelectric structures strongly depend on external dc and ac electric fields. In order to obtain initial dielectric permittivity (permittivity in the limit of zero probing electric field) and to detect the contribution of probing electric field during dielectric measurements, we have developed thermal noise impedance spectroscopy method. This approach is the most suitable for the study of systems with strong nonlinearity, such as thin films and polydomain structures, where the probing voltage induces huge electric field and/or changes the system itself. Here we introduce the novel approach for measurements of real and imaginary parts of dielectric permittivity at different frequencies by thermal noise impedance spectroscopy method [1], which is based on simple thermal noise method known from 1960s [2]. Dielectric permittivity and dielectric loss tangent were obtained by two methods, the traditional impedance spectroscopy measurements with various ac signals and the thermal noise impedance spectroscopy method. We demonstrate the second one on etalon RC circuits and on BaTiO3 and SrTiO3 bulk and thin film structures, which are the proper targets for this new technique.

1. P.S. Bednyakov, I.V. Shnaidshtein, and B.A. Strukov, Ferroelectrics 500, 203-217(2016). 2. J.J. Brophy and S.L. Webb. Phys. Rev. 128, 584 (1962).

ABSTRACT. Novel materials for application in optoelectronic devices are currently the subject of intensive research. One of the new solutions can be silica-based composite materials. In this solution, mesoporous silica in the form of thin films with vertically aligned channels is a matrix that hosts various functional groups. The composition and concentration of functional groups can be changed to tune the material's nonlinear optical properties. This type of material containing propyl-copper-phosphonate functional groups in five different concentrations was prepared and deeply characterized. To characterize the nonlinear optical properties of obtained materials, we used methods such as second harmonic generation (SHG) and third harmonic generation (THG). The transmission electron microscopy and X-ray diffraction were used to confirm obtaining the hexagonally ordered porous structure of the silica matrix. Vibrational spectroscopy supported by DFT calculations revealed the molecular structure, which was crucial for the proper interpretation of research results. We have found that the system geometry strongly influences its NLO properties, and the SHG and THG signals strongly depend on the degree of functionality of the material. Thus NLO properties of such materials can be tuned at the stage of sample synthesis in a very precise way by selecting functional group types and their concentration in the silica matrix. Another option for obtaining materials with enhanced nonlinear optical response is a synthesis of optically active nanocrystals under strong spatial confinements inside silica pores. In this case, we used SBA-15 porous silica powder as the matrix and its pores as nanoreactors. The idea of silica nanoreactors is based on the thermal decomposition of the initial compound, such as SBA-15 silica possessing nickel phosphonate units anchored inside the silica channels by propyl chains. This procedure induces an acentricity of electronic charge density, which is crucial for the second harmonic generation (SHG) and, to some extent, for the third harmonic generation (THG). Using thermogravimetry, transmission electron microscopy (TEM), Raman vibrational analysis, and N2 sorption analysis, we analyzed the thermal behavior of the initial functional material. All research that is connected with second and third harmonic generation enables the structural changes in these materials and their influence on their optical properties to be explored. Nonlinear optical measurements confirmed our assumption that material prepared using silica nanoreactors could have precisely tailored properties, including nonlinear optical properties.

ABSTRACT. Broadband dielectric spectra (1 Hz - 100 THz) of the (1−x)BaTiO3-xBaZrO3 (BZT-x) ceramic solid solution studied in the whole composition range and broad temperature range [1-5] reveal three main excitations: soft mode (SM), central mode (CM) and dielectric relaxation. Except for x = 1, all the spectra are characterized, in addition to the phonon modes, by an overdamped CM in the microwave range which weakens on decreasing temperature. The far-infrared SM, as well as the THz CM, soften partially only in BaTiO3, for all other compositions they stay almost temperature independent and do not contribute substantially to the low-frequency permittivity maximum, which exists for all x ≤ 0.9. The most important dielectric contribution is brought by one (for the diffuse ferroelectric compositions, x = 0.1 - 0.2, by two) Cole-Cole relaxation, which emerges below the CM in the GHz range, broadens on cooling, obeys the Arrhenius law with a common activation energy of Ea ≈ 2100 +/- 200 K (181 +/- 17 meV) for all relaxor compositions (x = 0.4 - 0.8) and passes without any anomaly in frequency through the temperature of dielectric maximum. In agreement with molecular-dynamic simulations, it is assigned to hopping of Ti ions in the frozen barium-titanate clusters [6]. The relaxation broadening can be described by a temperature-independent distribution of similar activation energies for all the relaxor compositions. This leads to conclusion that the polar nano-regions (PNRs) within the barium-titanate clusters remain very small, temperature independent and presumably frozen below room temperature. This differs strongly from the usual lead-containing relaxors, in which the PNRs grow on cooling and follow the gradual Vogel-Fulcher freezing [7,8]. [1] D. Nuzhnyy, J. Petzelt, M. Savinov, T. Ostapchuk, V. Bovtun, M. Kempa, J. Hlinka, V. Buscaglia, M. T. Buscaglia, P. Nanni, Phys. Rev. B 2012, 86, 014106. [2] J. Petzelt, D. Nuzhnyy, M. Savinov, V. Bovtun, M. Kempa, T. Ostapchuk, J. Hlinka, G. Canu, and V. Buscaglia, Ferroelectrics 2014, 469, 14. [3] J. Petzelt, D. Nuzhnyy, V. Bovtun, M. Kempa, M. Savinov, S. Kamba, J. Hlinka, Phase Transitions 2015, 88, 320. [4] V. Bovtun, D. Nuzhnyy, M. Kempa, T. Ostapchuk, V. Skoromets, J. Suchanicz, P. Czaja, J. Petzelt, S. Kamba, Phys. Rev. Mater. 2021, 5, 014404. [5] J. Petzelt, V. Bovtun, D. Nuzhnyy, M. Kempa, M. Savinov, M. Pasciak, S. Kamba, G. Canu, V. Buscaglia, to be published in Phys. Status Solidi B 2021 [6] D. Wang, A. A. Bokov, Z.-G. Ye, J. Hlinka, L. Bellaiche, Nature Commun. 2016, 7, 11014. [7] S. Kamba, V. Bovtun, J. Petzelt, I. Rychetsky, R. Mizaras, A. Brilingas, J. Banys, J. Grigas, M. Kosec, J. Phys.: Cond. Matter 2000, 12, 497. [8] D. Nuzhnyy, J. Petzelt, V. Bovtun, M. Kempa, S. Kamba, J. Hlinka, B. Hehlen, Phys. Rev. B 2017, 96, 174113.

ABSTRACT. In order to substitute lead zirconate-titanate (PZT) based materials, due to its poisonous nature, some promising piezoelectric and friendly environment compounds are attracting growing attention, namely KxNa(1-x)NbO3 [1]. For the particular case of x=0.5, K0.5Na0.5NbO3 (KNN), the high-temperature cubic symmetry changes to a non-symmetric ferroelectric tetragonal structure at T3=700 K, becoming orthorhombic at T2=465 K, and finally stabilizing in a rhombohedral symmetry below T1=135 K [2]. Recently, theoretical calculations have predicted piezoelectric response enhancement when T3 becomes closer to T2 [3], in which sintering conditions could play an important role [4,5].

In this work, we revisit the phase transition sequence and the effect of the sintering process on the structure, lattice dynamics, and dielectric/polar properties of KNN ceramics prepared by conventional sintering, spark plasma sintering, and spark plasma texturing. From a comparative analysis of the overall experimental obtained results, we have observed that the phase transition sequence includes an unreported structural and polar phase at low temperatures, independently of the processing method. Moreover, apparent changes in the stability temperature interval of the different phases have been ascertained. The dielectric strength and the emergence of a dielectric relaxation phenomenon are also noticeably dependent on the processing method, which can be understood as an effect of the different grain sizes. The results here reported clearly point out the possibility to change physical properties following different sintering routes in order to improve device performance.

References: [1] I. Coondoo et al., J. Advanced Dielectrics, 03, 1330002 (2013) [2] B. Orayech et al., J. Appl. Cryst., 48, 318-333 (2015) [3] D. Damjanovic, Appl. Physics Letters, 97, 062906 (2010) [4] R. Pinho et al., Appl. Mater. Today 19, 100566 (2020) [5] M.M. Gomes et al., Ceram. Int., 47, 8308-8314 (2021)

ABSTRACT. Lawsonite [CaAl2Si2O7(OH)2·H2O] is a rare mineral found in metamorphic rocks, occurring at subduction zones, supposedly at depths of up to 250 km. It contains 11.5 wt% water in its crystal structure. At room temperature, it is orthorhombic, crystallizing in the Cmcm space group. The structure contains a silicate tetrahedra framework with four chemical formulas in the unit cell, each of them with one water molecule embedded in a structural cavity; these cavities form channels aligned along the c axis. The static and dynamic orientations of the water molecules play a key role in two low-temperature phase transitions—a structural one at T_c1=270 K (Pmcn space group), and a ferroelectric one at T_c2=124 K (P2₁cn). We studied the lattice dynamics in a single crystal of lawsonite using THz time-domain transmittance and infrared reflectance spectroscopies. In addition, dielectric properties between 1 Hz and 1 GHz were measured, and static polarization was obtained from pyrocurrent measurements.

The ferroelectric order appears below T_c2 in the crystallographic a-direction. Low-frequency permittivity reaches a value of up to ε'~200. The dielectric anomaly weakens in the microwave region, and it is not observable in the THz range. Also, in the b direction, a rise in permittivity ε' up to ~250 was observed, peaking ca. 15 K below T_c2, and extending in frequency up to the THz and infrared ranges, where a soft phonon mode was observed. Pyrocurrent measurements revealed a polarization of up to 50 nC/cm2. A number of phonons harden with heating, which is obviously linked to the anomalous temperature dependence of the unit cell parameter a. Along the c direction, an anomaly near T_c1 in THz-range permittivity was observed, which is a part of a broader far-infrared absorption interval; the spectra can be described by a pair of coupled phonons. The observed pyrocurrent signal (~12 pA) at T_c1 for electric field applied along b direction might be due to polarized structural domain walls. Our results indicate complex proton and crystal dynamics, suggesting that the ferroelectric phase transition is of mixed displacive / order-disorder type.

ABSTRACT. Quantum paraelectrics, such as strontium titanate, potassium tantalate, etc., attract the attention of researchers due to both their applied and fundamental significance. The possibility of indicating polar [1] and magnetic phases [2] by creating defects, by cationic substitution and by mechanical stress, allows one to speak about the prospects of their use in various reconfigurable devices, sensors and memory elements. Also, much attention is paid to the resistive switching effect [3]. From a fundamental point of view, interest in quantum paraelectrics is caused by the specific features of phase dynamics, mechanisms of phase transitions, and exotic ground states, such as the Mueller "phase" [4] or dipole and spin glasses [5]. The aim of this study is to establish the effect of dipolar orderings on the oscillator parameters of lattice excitations and to reveal the role of various mechanisms and interactions in the formation of the phases. A thin (~ 100 nm) film of the Sr0.98Ba0.02TiO3 solid solution was prepared by pulsed laser deposition on a MgO substrate. The ceramic sample used as a target was obtained by the solid-phase reaction method. Both samples were studied using X-ray diffraction, Fourier-transform infrared (FTIR), terahertz and Raman spectroscopy techniques. We discover differences in the temperature behavior of the spectral response of the thin film in comparison with the bulk ceramic sample, and a clear presence of a lower-symmetry phase. The spectral signature of the low-symmetry phase increases dramatically during cooling. In the Raman spectra of the thin film, IR-active lines forbidden by the selection rules appear, indicating a local symmetry breaking and the appearance of polar regions at T = 150 K. These changes in the Raman response are accompanied by a shift of the ferroelectric soft mode towards low frequencies and by the appearance of additional absorption lines below T = 120 K in the IR spectra. A significant transformation of the spectral response with decreasing temperature is connected with the percolation of polar regions during the formation of low-symmetry phases. However, the emergence at high frequencies, above 1000 cm-1, of additional absorption lines in the Raman spectra cannot be explained by two-phonon processes, as was proposed in [6], and is probably a consequence of a more complex interaction of phonons at the boundaries of the Brillouin zone. Arguments in favor of the formation of the R3c phase predicted by the phenomenological theory [7] are discussed.

The research was supported by RSF 2021(data analysis and spectroscopic measurements of the bulk sample) (project № 21-12-00358), RFBR (bulk sample synthesis, spectroscopic measurements of thin film and X-ray characterization of samples) (project № 17-02-01247) and MECOUP (thin film synthesis).

References 1. O. E. Kvyatkovskii. Quantum Effects in Incipient and Low-Temperature Ferroelectrics (A Review) // Physics of the Solid State. 2001. 43, 8. 1401–1419. 2. I.R. Shein, A.L. Ivanovskii. First principle prediction of vacancy-induced magnetism in non-magnetic perovskite SrTiO3 // Physics Letters A. 2007. 371. 155–159 3. Jian-kun Li,Chao Ma, Kui-juan и др. Temperature-dependent resistance switching in SrTiO3 // Applied Physics Letters. 2016 108, 242901 4. K.A. Muller. Macroscopic quantum phenomena // Ferroelectrics. 1996. 183,1 11-24. 5. Kleemann Disordered Multiferroics //Solid State Phenomena.2012. Vol. 189 pp 41-56 6. A. B. Shi and W. Z. Shen. Phase transition temperature of SrTiO3 ultrathin films: An annealing study by ultraviolet Raman spectroscopy 7. V. B. Shirokov and Yu. I. Yuzyuk, B. Dkhil. Phenomenological theory of phase transitions in epitaxial BaxSr1−xTiO3 thin films // PRB. 2009. 79, 144118

ABSTRACT. Disentangling the Phase sequence and correlated critical properties in Bi0.7La0.3FeO3 by structural studies

M. M. Gomes1,T. T. Carvalho1, B. Manjunath1, R. Vilarinho1, A. S. Gibbs2, K. S. Knight2, J. A. Paixao3, P. B. Tavares4, A . Almeida1, J. Agostinho Moreira1

1 IFIMUP, Departamento de Física e Astronomia da Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, 4169-007 Porto, Portugal 2 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 OQX, England, United Kingdom 3 CFisUC, Department of Physics, University of Coimbra, P-3004-516 Coimbra, Portugal 4 Centro de Química-Vila Real, ECVA, Chemistry Department, Universidade de Trás-os-Montes e Alto Douro, 5000-801 Vila Real, Portugal

Bismuth ferrite is one of the few room temperature magnetoelectric multiferroics exhibiting simultaneous magnetic and polar ordering at room conditions. But due to its antiferromagnetic character and spiral spin cycloidal structure with an incommensurate modulation wavelength of 62-64 nm, bulk BFO shows very low magnetoelectric coupling. Substitution at the Bi site, especially by La has been reported to show improved magnetic and electric properties due to the effective suppression of the spin cycloid structure and leakage currents. Previous works have shown that Bi0.7La0.3FeO3 is both ferromagnetic and ferroelectric at room temperature [1]. But, there are contradictory reports on the crystal structure [2] and origin of modulation of this composition and further detailed studies were performed to understand the complex physics behind the same.

In this work we have studied the high temperature phase sequence of Bi0.7La0.3FeO3 (BLF30) using high resolution powder neutron diffraction (NPD) and Raman spectroscopy as a function of temperature. Calculation of lattice parameters and modulation wave vector were performed by Pawley refinement of the NPD data. The analysis revealed that BLF30 exhibits an incommensurately modulated orthorhombic Pn21a(00g)000 phase at room temperature. Above T1 = 543 K, the low temperature modulated Pn21a(00g)000 evolves monotonically into a fractional growing Pnma structure up to TN = 663 K. At 663 K, the low temperature canted antiferromagnetic phase is suppressed concurrently with the switching of the former onto a non-modulated Pn21a structure that continues to coexist with the Pnma one, until the latter reaches a 100% fraction of the sample volume at high temperatures above 733 K. Raman mode frequencies also revealed anomaly at these temperatures.

Neutron diffraction and Raman results provide adequate evidence for the existence of a spin-phonon coupling in BLF30. Magnetoelastic coupling is observed below to TN, involving the canted antiferromagnetic ordering of the spin irons and the symmetric stretching mode of FeO6 octahedra. The correlation between magnetic data from VSM measurements and the structural results from NPD provides a definite evidence for the magnetic origin of the modulation. The fits to the temperature dependent magnetization from VSM, and the intensity of the magnetic peaks obtained from NPD experiments revealed a critical exponent (β) value of 0.38. The lower limit T1 = 543 K of the phase coexistence temperature marking the emergence of the Pnma phase, is also associated with the temperature wherein the modulation magnitude starts to decrease.

References [1] T. T. Carvalho, J. R. Fernandes, J. P. De La Cruz, J. V. Vidal, N. A. Sobolev, F. Figueiras, S. Das, V. S. Amaral,A. Almeida, J. Agostinho Moreira, P. B. Tavares, J Alloy Compd 554, 97 (2013). [2] D. A. Rusakov, A. M. Abakumov, K. Yamaura, A. A. Belik, G. Van Tendeloo, and E. Takayama-Muromachi, Chemistry of Materials 23, 285 (2011).

ABSTRACT. Zirconia and hafnia based thin films have attracted considerable attention in the last decade due to their ferroelectric behavior at few nanometer scale, which can enable the downscaling and design of a next-generation functional memory devices. The present work combines experimental structural studies with DFT calculations to disclose a novel rhombohedral R3m epitaxially-strained phase of (111)-oriented ZrO2 thin films grown by ion-beam sputtering deposition technique on (111)-Nb:SrTiO3 substrates. Comprehensive local and macroscopic ferroelectric characterization reveals that these ZrO2 films display a switchable ferroelectric polarization reaching 20.2 μC/cm2 with a coercive field of 1.5 MV/cm. Moreover, the time dependent polarization reversal characteristics of Nb:STO/ZrO2/Au film capacitors exhibit the typical bell-shape curves associated with domains reversal feature of ferroelectric films. The estimated activation field is comparable to the coercive field obtained from polarization-electric field hysteresis loops. Interestingly, the studied films show a ferroelectric behavior per se, i.e. a technological advantage over the previously studied conventional orthorhombic ZrO2 films where it is indispensable to apply wake-up cycles to induce ferroelectricity.