ABSTRACT. Understanding and controlling structure and function of liquid interfaces is a constant challenge in biology, nanoscience and nanotechnology, with applications ranging from molecular electronics to controlled drug release. Elucidating the underlying non-equilibrium physics at these interfaces at fast- and ultrafast- time scales and in atomic spatial resolution associates significant experimental challenges. Short ultrafast optical pulses synchronized with focused synchrotron X-ray pulses are the key to this endeavor, as they can access the structure and dynamics of the liquids at their surfaces and interfaces with atomistic details. X-ray reflectivity and grazing incidence diffraction provide an invaluable probe for studying the atomic scale structure at liquid–air interfaces [1]. The new ultra-fast laser system at the LISA liquid diffractometer [2] situated at beamline P08 at the PETRA III synchrotron radiation source in Hamburg provides the laser pump with X-ray probe. The femtosecond laser combined with the LISA diffractometer allows unique opportunities to investigate photo-induced structural changes at liquid interfaces from picosecond time scales with pump-probe techniques. Our studies concentrate on two aspects, the non-equilibrium dynamics due to ion creation under influence of the pump laser and temperature-dependent capillary wave behaviour [3]. For water and NaI solution UV (258 nm) laser excitation at the vapor interface of salt solutions (NaI, NaCl, RbBr and K4[Fe(Cn)6]) indicates interesting dynamics at the surface. Additionally, thermally induced capillary wave dynamics at water and mercury surfaces were investigated with time response pump-probe X-ray reflectivity studies in the time range of ns to s with IR (1030 nm) laser excitation.

[1] A. Elsen; S. Festersen, B. Runge, C.T. Koops, B. Ocko, M.; Deutsch, O. Seeck, B. M. Murphy, O. M. Magnussen, Proc. Natl. Acad. Sci, 110 (17), 6663-6668.(2013) [2] B. M. Murphy, M. Greve, B. Runge, C.T. Koops, A. Elsen, J. Stettner, O. Seeck, O. M. Magnussen, J. Synchr. Rad., 21, 45 (2014) [3] B. Runge, S. Festersen, C. T. Koops, A. Elsen, M. Deutsch, B. M. Ocko, O. H. Seeck, B. M. Murphy, and O. M. Magnussen, Phys. Rev. B 93, 165408 (2016)

ABSTRACT. In this work, following our previous work on molybdate glasses, we have employed a combination of neutron diffraction and neutron Compton scattering, augmented by ab initio harmonic lattice dynamics and Reverse Monte Carlo modelling in order to characterize the force-constant disorder in the tungsten-oxide based glasses. Specifically, we have discussed the correlations between that average interatomic force constant magnitudes inferred from neutron Compton scattering and average bond-lengths and interatomic distances obtained from diffraction data analysis. We have compared and contrasted the local structural and dynamic observables characterising the glasses and their precursor metal-oxide systems and established that, in the case of similar atomic coordination, clear correlations are visible between the bond-length and average force constant distributions in both types of systems. As far as the force-constant disorder is concerned, we have characterised glasses using two different computational protocols. Firstly, we have provided a comparative analysis of the widths of force-constant distributions of individual atomic species in glasses and their precursor metal oxides based on the distributions of the widths of nuclear momentum distributions. Secondly, we have established a scale of the softening of atom-projected vibrational densities of states induced by the force-constant disorder by placing the widths of nuclear momentum distributions on a scale spanned by the Maxwell-Boltzmann distribution of momenta in the absence of any binding potential and ordering and compositional averages of momentum distributions of the precursor metal oxide systems.

ABSTRACT. I will present our latest results on the understanding of electrodynamic properties of water and ice on a molecular level, recently summarized in [1]. In particular, I will discuss the mechanisms behind the dielectric relaxation, dynamic conductivity, and infrared spectra of both ice and water [2,3]. I will present a non-rotational mechanism of polarization, which suits well for water and alcohols [4]. Finally, anomalous dielectric properties of water at strong confinement will be discussed in the context of some practical applications for electrochemical energy storage [5].

[1] V. G. Artemov, The Electrodynamics of Water and Ice, Springer: Cham (2021) [2] V. G. Artemov, PCCP, 21, 8067 (2019) [3] V. G. Artemov, E. Uykur, S. Roh, A. Pronin, H. Ouerdane, and M. Dressel,Scientific Reports, 10, 11320 (2020) [4] V. G. Artemov, A. Ryzhov, E. Carlsen, P. O. Kapralov, H. Ouerdane, J. Phys. Chem. B, 124, 11022 (2020) [5] V. G. Artemov, E. Uykur, P. O. Kapralov, A. Kiselev, K. Stevenson, H. Ouerdane, M. Dressel, J. Phys. Chem. Lett., 11, 3623 (2020)

ABSTRACT. Raman spectroscopy is a quick, easy and non-destructive method for the characterization of materials. Also in the world of 2D materials, it readily yields information, e.g., on the number of layers, on doping, strain, interaction with the substrate and many more. In order to extract a maximal amount of information, it is important to have a theory and computational framework that allows to quantitatively describe the Raman intensities. We present an ab-initio approach, based on many-body perturbation theory including exciton-phonon coupling and non-adiabatic effects, and applications to prototype 2D materials such as hBN and MoS2.

ABSTRACT. Since the isolation of monolayer graphene, which merited the Nobel Prize for Physics in 2010, intensive research has been devoted to the investigation of the fundamental properties of layered materials.[1] The two-dimensional (2D) materials exhibit a wide range of electrical properties including semiconducting, metallic, superconducting, and allow to envision a new era of flexible electronics. While most work has been devoted to the investigation of the optical, mechanical and electrical properties of layered 2D materials, the exploration of magnetism is still in an early stage.

The recent discovery of 2D magnetic materials has opened a door to experimentally research different types of magnetic ordering previously predicted.[2] One of the most important families of 2D magnetic materials are the so-called transition-metal thiophosphates (TMTs, with chemical formula MPS3, M = Mn, Fe, Ni, Zn, Cd), which mostly exhibit antiferromagnetic Ising ordering. Alloying antiferromagnetic MnPS3 (TN = 78 K) with non-magnetic ZnPS3 which is suspected to exhibit ferroelectric character is interesting since the alloy is a potential multiferroic candidate. Moreover, studying diluted magnetically-active transition metals such as Mn on wide gap semiconductors such as ZnPS3, is of particular interest to explore potential spintronic applications and understand magnetism in disordered systems.

Raman spectroscopy is an excellent tool to investigate structural and magnetic transitions. Indeed, recent Raman measurements on 2D magnetic materials detected magnetic ordering from three distinctive mechanisms, i) folding of the Brillouin zone below TN, ii) spin-spin and spin-phonon interactions and iii) interference of the single-phonon state with electronic transitions due to spin-splitting of the electronic band structure. In the present work we report temperature- and wavelength- dependent Raman measurements on MnxZn1-xPS3 along the whole compositional range. Our results allow to determine, for the first time, the Neel temperature in Mn-rich MnxZn1-xPS3 and discuss the presence of a structural distortion in Zn-rich MnxZn1-xPS3.

[1] K. S. Novoselov, et al., Science 306, 666 (2004). [2] M. Gibertini, et al. Nat. Nanotechnol. 14, 408–419 (2019).

ABSTRACT. The moiré induced interactions between twisted atomic lattices in low-dimensional structures attracted tremendous attention recently. We study the one-dimensional moiré coupling of vibrational and electronic states in double walled carbon nanotubes. The electronic states get perturbed by the moiré potential changing the frequencies of radially oriented phonons. We find that the frequency shift is proportional to the changes in transition energies and construct a Kataura plot for double walled carbon nanotubes. This is the first manifestation of strong moiré effects onto the vibrational states. In addition to moiré coupling dielectric screening affects the optical transition energies of the inner tube in double walled nanotubes. The dielectric effect depends on the electronic type of the outer wall. Metallic outer walls provide higher screening, than semiconducting tubes. Higher screening results in a stronger red-shift of the inner tubes transition energies.

ABSTRACT. Bismuth vanadate BiVO4 is in the focus of research for photocatalytic applications because of its optical absorption edge in the visible light range. While the value of the absorption edge, around 2.5eV, is agreed on, the electronic transitions involved in the absorption process are only marginally investigated. In this work, we make use of a technique proposed by Weber et al. [2], where resonant Raman spectroscopy is exploited to study band-to-band transitions with respect to direct and indirect electronic transitions. We show how selective enhancement of 1st and 2nd order scattering processes can be used to make band-to-band transitions visible in Raman Spectra. Two different strategies are demonstrated to achieve selective enhancement: first, by tuning the input laser energy and secondly by ramping the temperature. This analysis suggests that the optical absorption in Bismuth vanadate is originated from a direct band-to-band transition in the visible light range and an indirect transition at slightly higher energy.

ABSTRACT. Tetragonal perovskites exhibit high electric polarization and piezoelectric coefficients which make then promising materials for functional devices. The polarization being coupled to the tetragonal strain, it is expected that highly elongated super tetragonal unit cells will display stronger polarity [1]. If a giant axial ratio could be associated with some magnetic ordering, then it would be possible to obtain room-temperature multiferroic materials. In this regard, PbVO3 was predicted to fulfil this requirement [2]. PbVO3 was synthesized under high-temperature and high-pressure in the tetragonal structure (P4mm space group), with c/a ≈ 1.23 that is 16 % higher than the isostructural PbTiO3 [3,4]. Lattice dynamics of PbVO3 has been scarcely studied and little is known about its Raman signature.

From a combined angle-resolved polarized spectroscopy study of the Raman intensity and first-principles density functional theory simulations in super tetragonal PbVO3 single crystal, we have assigned the eight observed transversal optical phonons at 136, 269, 374 and 508 cm-1 to E symmetry, at 188, 429 and 874 cm-1 to A1 symmetry and at 320 cm-1 to B1 symmetry. The VO stretching A1 Raman mode at 874 cm-1 appears as a polar mode and displays a large anisotropy in it's tensor values. This mode is thus crucial for finding the chemical modifications that would make the material more switchable to observe the ferroelectricity. PbVO3 exhibits a large splitting of the “silent” E-B1 modes, which are usually found at the same frequency in other polar tetragonal P4mm perovskite compounds. Contrary to the general understanding, we show that this splitting is not only due to the tetragonal anisotropy but is also associated to the VO polar distortion involved in the A1 Raman mode. The experimental results give strong evidence for a coupling between the B1 mode and specific two-phonon states.

1- M.J. Haun, E. Furman, S.J. Jang, H.A. McKinstry, L.E. Cross, J. Appl. Phys. 62 (1987) 3331-3338. 2- Y. Uratani, T. Shishidou, F. Ishii, T. Oguchi, Jpn. J. Appl. Phys., 44 (2005), 7130-7133. 3- R.V. Shpanchenko, V.V. Chernaya, A.A. Tsirlin, P.S. Chizhov, D.E. Sklovsky, E.V. Antipov, E.P. Hhlybov, V. Pomjakushin, A.M. Balagurov, J.E. Medvedeva, E.E. Kaul, Ch. Geibel, Chem. Mater. 16 (2004) 3267-3273. 4- A.A. Belik, M. Azuma, T. Saito, Y. Shimakawa, M. Takano, Chem. Mater. 17 (2005) 269-273.

ABSTRACT. New classes of nanoscale ferroelectric structures with nontrivial topological properties have been recently discovered, including polar vortices and skyrmions. These electrical counterparts of magnetic systems can host new collective dynamics and functionality that result from the unique connectivity of polarizations, holding promises for electric-field control of topological structures for data processing with ultrahigh speed and density. However, their dynamics are unknown. In this talk, I will present our discovery of collective dynamics in polar vortices by terahertz-field-pump, femtosecond x-ray diffraction techniques [1]. We reveal a soft mode in which nanoscopic circular with the picosecond oscillating vorticity, arising from complex many-body interactions between the polar order and lattice. The condensation of its dynamics at a critical strain indicates a new phase transition in polar vortices. The discovery of this tunable soft mode opens a new avenue for high-frequency dielectrics and optoelectronics applications. On-going investigation of polar skyrmions using the similar technique will be briefly discussed.

[1] Q. Li, V. A. Stoica, et al. "Subterahertz collective dynamics of polar vortices", Nature, 592, 376-380 (2021)

ABSTRACT. Topological structures in ferroelectrics have become a fascinating subject of studies promising novel functionalities of ferroelectric materials. For example, systems with nanoscopic domains (dense domain walls) have been argued to possess enhanced dielectric, piezoelectric or photovoltaic properties. Superlattices hosting ferroelectric vortex structures have been shown to be a playground for peculiar phenomena such as negative capacitance and light-induced ordering of vortices. Here we present a computational scheme for studying a fundamental feature of such topological structures – their dynamics. We use interatomic potentials derived from first-principle calculations which allow us to access collective dynamics within supercells of thousands of atoms. ‘Normal’ phonons of the bulk material as well as vibrations of topological objects, such as domain walls or vortices optimized within a given supercell, can be captured within the same formalism. The exemplary calculations include our recent studies of nanotwinned domains in BiFeO3 [1] and vortex structure in PbTiO3/SrTiO3 superlattices [2]. In both cases we were able to single out electrically active vibrations in the sub-THz range and discuss their relation to the underlying atomic structure.

[1] J. Hlinka, M. Paściak, S. Körbel, and P. Marton, Physical Review Letters 119, 057604 (2017). [2] Q. Li, V. A. Stoica, M. Paściak et al., Nature 592, 376 (2021).

ABSTRACT. Topological materials are of central interest in research, and hence witness enormous attention for developing novel and modern-type applications. Here we focus on investigating magnetic skyrmion (SKY) topologies of both the Bloch [1], the Néel [2] and the Antiskyrmion [3] type at the nanometer length scale, employing a thorough magnetic force microscopy (MFM) analysis combined with macroscopic MOKE, neutron scattering, and Anomalous Hall transport experiments [4]. Bloch-type SKYs were inspected with respect to their nucleation, stability [5] and magnetic-field-induced reorientation [6] in both the standard B20-material Fe0.5Co0.5Si and the magnetoelectric Cu2OSeO3 [7]. The latter constitutes the one and unique material known to date that hosts two different SKY phases, as proven by our MFM study. Multiferroic GaV4S8 hosts Néel-type SKYs [2,8] pinned along the [111] axes due to easy axes anisotropy; moreover severe magnetism and a pronounced magnetoelectric effect within individual domain walls in the parental GaV4Se8 Xtal could be discovered [9], similar to former investigations on GaMo4S8 [10], Finally, Antiskyrmions, were analyzed in the half-Heusler material Mn1.4PtSn up to room temperature [11] where, in addition to standard external magnetic fields and temperature variation, the skyrmion lattice formation can be elegantly tuned here through sample thickness adjustment as well [3].

References: [1] P. Milde et al., Science 340, 1076 (2013); https://doi.org/10.1126/science.1234657. [2] I. Kézsmárki et al., Nat. Mater. 14, 1116 (2015); https://doi.org/10.1038/nmat4402. [3] B.E. Zuniga Cespedes et al., Phys. Rev. B 103, 184411 (2021); https://doi.org/10.1103/PhysRevB.103.184411. [4] G. Malsch et al., ACS Appl. Nanomater. 3, 1182 (2020); https://doi.org/10.1021/acsanm.9b01918. [5] P. Milde et al., Phys. Rev. B 100, 024408 (2019); https://doi.org/10.1103/PhysRevB.100.024408. [6] P. Milde et al., Phys. Rev. B 102, 024426 (2020); https://doi.org/10.1103/PhysRevB.102.024426. [7] P. Milde et al., NanoLett. 16, 5612 (2016); https://doi.org/10.1021/acs.nanolett.6b02167. [8] A. Butykai et al., Sci. Rep. 7, 44663 (2017); https://doi.org/10.1038/srep44663. [9] K. Geirhos et al., npj Quantum Materials 5, 44 (2020); https://doi.org/10.1038/s41535-020-0247-z. [10] E. Neuber et al., J. Phys.: Condens. Matter 30, 445402 (2018); https://doi.org/10.1088/1361-648X/aae448. [11] A.S. Sukhanov et al., Phys. Rev. B 102, 174447 (2020); https://doi.org/10.1103/PhysRevB.102.174447.

ABSTRACT. The domain walls in ferroelectric materials are an interface with nanometer scale that separates two regions with different orientations of the spontaneous electrical polarization, the domains. Over the last decades the domain walls have attracted the attention of the scientific community, as these have distinct properties of those of the parent material [1]. One example of this was the observation at room temperature of conductive domain walls in thin films of multiferroic insulating BiFeO3 [2]. Following this work, it was also demonstrated that the domain walls in thin films of Pb(Zr0.2Ti0.8)O3 are also conducting [3]. This work also demonstrated that charged domain walls can be obtained in purely ferroelectric materials such as Pb(Zr0.2Ti0.8)O3. In this work, we performed molecular dynamics simulations using a shell model potential to study charged domain walls in PbTiO3 [4]. In order to stabilize the charged domain walls, we added compensating charges by constructing a superlattice formed by layers of pure PbTiO3, PbTiO3 doped with Nb5+ (adding electrons), and PbTiO3 doped with Mg2+ (adding holes). Taking advantage of our methods, in our study we investigate the effect of the charge introduced into the superlattices, more precisely how the amount of the charge added in the system affects the charged domain walls and contributes for different polarization arrangements in the domains and inside the charged domain walls.

Acknowledgements: Work was supported by the Czech Science Foundation (Project No. 20-05167Y00)

[1] G. Catalan, J. Seidel, R. Ramesh, and J. F. Scott, Rev. Mod. Phys. 84, 119 (2012). [2] J. Seidel, L. W. Martin, Q. He, Q. Zhan, Y.-H. Chu, A. Rother, M. E. Hawkridge, P. Maksymovych, P. Yu, M. Gajek, N. Balke, S. V. Kalinin, S. Gemming, F. Wang, G. Catalan, J. F. Scott, N. A. Spaldin, J. Orenstein, and R. Ramesh, Nat. Mater. 8 229 (2009). [3] J. Guyonnet, I. Gaponenko, S. Gariglio, and P. Paruch, Adv. Mater. 23, 5377 (2011). [4] M. Sepliarsky and R. E. Cohen J. Phys.: Condens. Matter 23, 435902 (2011).

ABSTRACT. In this work we combine phenomenological Landau-Devonshire (LD) theory and density functional theory (DFT) calculations to investigate the effect of La doping on the structural and ferroelectric properties of BiFeO$_3$ (BFO). First, we compute the parameters of the LD potential for BFO and La$_0.25$Bi$_ 0.75$FeO$_3$ (LBFO) by considering an expansion, around the reference cubic phase, as a function of the relevant degrees of freedom: polarization, antiphase rotations of FeO$_6$ octahedra and strain. Namely, for both systems we determine the LD coefficients by imposing that the model accurately reproduces the DFT energies and the distortion amplitudes for a set of relevant structural polymorphs. Then, we utilize this LD potential in Landau-Khalatnikov simulations of ferroelectric switching in BFO and LBFO, in order to elucidate how La doping affects the polarization switching times and voltages.

Work is funded by the Semiconductor Research Corporation and Intel, contract no. 2018-IN-2865

ABSTRACT. It is a generally accepted fact that the unique dielectric properties of relaxor ferroelectrics are related to the formation of polar nanoregions (PNRs) due to the off-centering of one of the ions. Less well recognized is the corollary that, because they are polar and therefore lack inversion symmetry, PNRs are also piezoelectric at the nanoscale and can drive macroscopic electro-mechanical resonances. These resonances provide a tool to study the underlying microscopic dynamics in those systems. In this talk, we report on the resonances observed in two related ferroelectric relaxors, K1−xLixTaO3 (KLT) and KTa1-xNbxO3 (KTN). Unexpectedly, pairs of coupled resonances rather than a single one are observed in both systems, which evolve differently with temperature due to the specific character of the off-center ions in the two systems, Li and Nb respectively. The complex resonance spectra can be described equally well by either one of two alternative but complementary models: a purely classical one based on two coupled damped harmonic oscillators and a quantum mechanical (semi-classical) one based on two discrete excitations coupled to each other through a continuum. Both reproduce the rapid evolution of the resonance spectrum across three temperature ranges, including a structural phase transition range. Together they provide complementary perspectives on the underlying physics of these and other relaxor systems. Additionally, we show that the observed spectra bear a remarkable resemblance to optical spectra observed in atomic vapors under certain excitation conditions.

Ref. J. Toulouse et al. Physical Review B 98, 134113 (2018)

ABSTRACT. A first principle DFT calculations study was carried out on the complex perovskites Ba(Mg$_{1/3}$Nb$_{2/3}$)O$_3$, Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$, Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$. Large supercells of up to 1080-atoms have been designed. Possible ordered structures deduced from Monte-Carlo simulations were tested and the displacements of the A and B site cations confronted to experimental results. A Bader charge analysis is presented and compared to electronic structure features of the electronic density of states. Crystallographic and spectroscopic features known in these systems are discussed in keep with the present analysis.

ABSTRACT. Inferred from experiments, but directly experimentally invisible: enigmatic and frequently disputed polar nanoregions provide simultaneously the definition and the intuitive understanding of relaxor ferroelectric substances since several decades. In fact, the concept exists in multiple variants and can take different forms. The purpose of this talk is twofold: (i) to stimulate discussion about the possible quantitative ramifications of the concept allowing classification of polar nanoregions and (ii) to draw attention to the dynamic aspects of selected types of polar nanoregions that can be inferred from recent experiments and atomistic simulations.

ABSTRACT. We present relaxation dynamics of relaxor ferroelectric materials investigated by means of inelastic neutron scattering and dielectric spectroscopy techniques. We focus on two representatives of relaxor ferroelectrics with distinctly different crystallographic structures: the canonical relaxor PbMg1/3Nb2/3O3 (PMN) with perovskite structure [1] and unixaial relaxor ferroelectric Sr0.61Ba0.39Nb2O6 (SBN61) having tetragonal tungsten bronze structure [2]. We will show that correlated atomic displacements manifested in inelastic diffuse scattering can be directly related to the polarization fluctuations responsible for the Vogel-Fulcher-type dielectric behavior of ferroelectric relaxors [3]. We will discuss the role of these polarization fluctuations in electrocaloric effect.

References [1] M. Pasciak, T. R. Welberry, J. Kulda et al., Polar nanoregions and diffuse scattering in the relaxor ferroelectric PbMg1/3Nb2/3O3, Phys. Rev. B 85, 224109 (2012). [2] M. Pasciak, P. Ondrejkovic, J. Kulda, et al., Local structure of relaxor ferroelectric SrxBa1−xNb2O6 from a pair distribution function analysis, Phys. Rev. B 99, 104102 (2019). [3] P. Ondrejkovic, M. Kempa, J. Kulda et al., Dynamics of nanoscale polarization fluctuations in a uniaxial relaxor, Phys. Rev. Lett. 113, 167601 (2014).

ABSTRACT. Although the sensation of pitch in sound perception is usually associated with a periodicity of the signal, comparison of extremely short acoustic pulses produces an impression of shorter pulses being higher than longer [1]. Examples will be provided within the presentation. The effect seem to contradict the uncertainty relation which requires that the pulse be long enough to encompass sufficient number of oscillation periods. To elucidate the physical mechanisms of the phenomenon we shall analyse the response of an artificial head [2] and compare them with behavioural studies with subjects. The discrimination of the duration time of the pulses seems to follow the Weber-Fechner law.

ABSTRACT. EuTiO3 is an incipient ferroelectric antiferromagnet attracting attention of multiferroic community for several reasons. First, due to its unusually strong non-linear, third order (E2H2-type) magnetoelectric coupling, which is demonstrated via a magnetodielectric (spin-phonon coupling) effect below Néel temperature of about 5 K. Second, because of the possible induction of ferroelectricity at low temperatures by biaxial strain or barium doping. Thus induced multiferroic system could exhibit interesting magnetoelectric coupling. Moreover, its affinity with SrTiO3 and BaTiO3 predetermine these compounds to form (Eu, Ba, Sr)TiO3 solid-state solution with remarkable properties. One of them is possible existence of multiferroic quantum criticalities causing an unconventional temperature dependence of low-temperature electric and magnetic susceptibility.

Apart from the brief overview of basic properties of EuTiO3, this contribution is focused on the seemingly anisotropic behaviour of the magnetodielectric effect due to the demagnetizing field [1], observation of the ferromagnetic resonance shift from microwave to THz region with external magnetic field [1], and discussion of preliminary results concerning the multiferroic quantum critical behaviour in (Eu,Ba,Sr)TiO3 solid solutions [2].

[1] D. Repček, et. al., Physical Review B 102, 144402 (2020). [2] A. Narayan, A. Cano, A. V. Balatsky, and N. A. Spaldin, Nature Materials 18, 223 (2019).

ABSTRACT. The polar properties of antiphase boundaries (APBs) and ferroelastic twin walls (FTWs) in PbZrO3 and SrTiO3 are analyzed in detail using a recently developed [1] layer group approach in order parameter space and compared with the results from the Landau-Ginzburg free energy description. It is shown that the former approach reveals the microscopic properties of the domain walls. The obtained [2] polar APB structures at particular positions inside the unit cell agree very well with recent experimental observations [3]. In contrast with it the free energy description obscures the microscopic features but still can reflect the macroscopic properties of the domain walls. The relation between the layer group approach and the Landau-Ginzburg free energy description is discussed and illustrated on several examples.

[1] W. Schranz, C. Schuster, A. Tröster and I. Rychetsky, Phys. Rev. B 102, 184101 (2020). [2] I. Rychetsky, W. Schranz and A. Troester, Symmetry and polarity of antiphase boundaries in PbZrO3, submitted to Mater. Res. Bulletin. [3] X. K. Wei, C. L. Jia, K. Roleder, and N. Setter, Mater. Res. Bull. 62, 101 (2015).

ABSTRACT. Materials that can switch between a low-spin and high-spin state under the influence of temperature or pressure show a growing interest due to their promising applications as sensors or optical switches [1]. Among them, many Fe(II) complexes as [Fe (NCS)2(tvp)2]·4CH3CN·4H2O [tvp = trans-(4,4-vinylenedipyridine)], studied in this work, have been investigated during the last years. It is tetragonal (P4/ncc) at high temperatures and about 200K it becomes orthorhombic (Pccn). This multiferroic phase transition is accompanied by a switch from the high temperature high-spin configuration to the low-spin ground state. The family of metal−organic frameworks (MOFs) based on tvp ligands develops highly porous structures, where voids (with typical cross sections of about 80 Å2) occupy a great fraction of the crystal volume (about 40% of the total volume) and show very large surface areas making them suitable candidates for flexible and tunable host−guest systems. The structures of porous MOFs are conveniently described in a bottom-up way from the primary building unit (PBU, the metal cations) through the secondary building unit (SBU, cluster of atoms involving and/or surrounding PBUs) to their porous three-dimensional (3D) arrangement of SBUs where absorbed guest molecules are disposed. The size, type, and arrangement of guest molecules determine, in part, the properties of the composite systems, modifying characteristics inherent to the host and guest subsystems when considered separately. We present a DFT study of spin cross-over transition (SCO) focused on the interplay between the crystallographic transformation and the stabilization of the spin configuration. The coupling of strain and magnetism is considered a key ingredient in many SCO phase transitions [3]; it can provide the microscopic origin of the cooperativity needed to understand the behaviour of these materials and allow the engineering of new SCO compounds [4]. [1] H. Banerjee, S. Chakraborty and T. Saha-Dasgupta, Inorganics 5, 47 (2017). [2] T. Romero-Morcillo, N. de la Pinta, L. M. Callejo, L. Piñeiro-López, M. C. Muñoz, G. Madariaga, S. Ferrer, T. Breczewski, R. Cortés and J. A. Real, Chem. Eur. J. 21, (2015). [3] Banerjee, S. Chakraborty, and T. Saha-Dasgupta, Phys. Rev. B, 90 (2014) [4] H. Banerjee, S. Chakraborty, and T. Saha-Dasgupta, Chem. Mater. 28 (2016). [5] K. Tarafder,S. Kanungo, P. M. Oppeneer and T. Saha-Dasgupta, Phys. Rev Lett., 109 (2012)

ABSTRACT. The present study is devoted to the characterization of dynamic magnetic properties of Mn12-based single-molecule magnets (SMMs) deposited on the surface of spherical silica nanoparticles. The SMMs are well-known low-dimensional magnetic systems possessing magnetic hysteresis and slow magnetization relaxations of purely molecular origin at low temperatures [1]. Surface deposition of SMMs opens up numerous possibilities for their wide applications in modern nanotechnological devices [2]. The deposition of single-molecule magnets was done using the pre-functionalization of the silica's surface with anchoring units (propyl-carboxyl acid). It gave a possibility for attaching SMMs at the surface and, by using spacer groups [3], control their surface distribution. The AC and DC magnetic measurements were applied to investigate the preservation of relaxation dynamics of Mn12-based SMMs after surface deposition. As observed, the study of time dependent DC magnetization and AC magnetic susceptibility revealed the presence of a slow relaxation process in all investigated samples. In addition, it was shown that the magnetic relaxation parameters (effective energy barrier and relaxation time) could be changed depending on the level of magnetic molecules' separation on the surface. It was found that the higher distributed SMMs on the silica surface demonstrate values of the above parameters, which are close to the literature-reported data for the analogous bulk SMMs.

[1] G. Christou, D. Gatteschi, D. N. Hendrickson, R. Sessoli, Single-Molecular Magnets, MRS Bulletin, 2000, 25, 66. [2] A. Cornia and M. Mannini, Single-Molecule Magnets on Surfaces, Springer-Verlag, Berlin Heidelberg, 2014. [3] M. Laskowska, M. Oyama, I. Kityk, M. Marszałek, M. Dulski, Ł. Laskowski, Surface functionalization by silver-containing molecules with controlled distribution of functionalities, Appl. Surf. Sci., 2019, 481, 433.

ABSTRACT. Room temperature multiferroics (BaSr)4(CoZn)2Fe36O60, Sr1Co1Zn1Fe16O27 and (BaSr)3Co2Fe24O41 are materials crystallizing in U-, W- and Z-type hexaferrite structures, respectively. They exhibit ferrimagnetic phase transitions below approx. 700 K. The spiral magnetic structure changes to the transverse conical magnetic one below approx. 400 K, and it induces a ferroelectric order. Magnetoelectric coupling in the hexaferrite systems is exceptionally high and we have demonstrated that it can be determined not only from change of the magnetization in electric field, but also by electric-field-modulated magnetic resonance studies at ~ 9 GHz. [1]. U-, W- and Z-type hexaferrite ceramics were studied by the microwave (1 MHz – 50 GHz) spectroscopy using coaxial, microstrip and coplanar waveguides. Temperature-frequency dependences of the impedance, transmission and reflection coefficients of the waveguides with a sample, as well as magnetic permeability of the materials, were measured and analyzed. Temperature evolution of the multiple microwave excitations is revealed and discussed. Anomalies at the temperature dependences of the main excitation frequencies are related to the phase transitions. Some excitations are attributed to the ferromagnetic resonances. Critical slowing-down related to the ferrimagnetic/ferroelectric phase transition is well seen in temperature dependences of the magnetic permeability and ferromagnetic resonance frequencies. At room temperature, influence of the bias magnetic field on the magnetic permeability and main excitation frequencies were studied. Essential magnetic nonlinearity and magneto-impedance effect were revealed. [1] V. Laguta, M. Kempa, V. Bovtun, J. Buršík, K. Zhai, Y. Sun, and S. Kamba. Magnetoelectric coupling in multiferroic Z-type hexaferrite revealed by electric-field-modulated magnetic resonance studies. J. Mat. Sci. 55, 7624-7633 (2020).

ABSTRACT. New device applications in materials sciences call for the accessibility of known physical properties in the nanometer scale. These materials are often functional metal oxides, that are present in different technologies such as memory storage, energy harvesting and sensors. However, the spectroscopic study of such functional thin films comes with new challenges. It is important to understand the influence of the film thickness, the substrate, and the interfaces present. All these parameters contribute to the physical phenomena probed. To control the desired properties, it is necessary to understand the electric and structural signatures responsible for such properties. Exploiting the high brilliance and the stability of the synchrotron radiation in a wide infrared spectral range available at the AILES beamline of SOLEIL, we were able to perform infrared and Terahertz spectroscopic measurements on several thin film materials promising for applications. One of these materials is the recently discovered HfZrO2 ferroelectric thin (below 20 nm) film. This unexpected ferroelectricity in such nanometric films has stimulated new research into understanding its origin and opened a path to ferroelectrics memristors. Beyond these promising applications, many fundamental aspects related to the ferroelectricity order in HfO2-based systems remain to be unveiled e.g. ferroelectric transition temperature, role of non-polar coexisting phases, imposed electrode boundary conditions, electrical cycling effects. We report the results obtained from temperature and electric field dependence of transmission infrared and THz spectroscopy measurements. Other systems will also be described.

ABSTRACT. Within the framework of this presentation, the audience will be guided through the methods of molecular engineering. The key to the bottom-up approach for molecular engineering is to design synthesis in such a way that atoms create assumed molecular structure by themselves through the self-organization process. This concept can be applied to operate a position of individual atoms and molecules, preserving the assumed distribution of the building blocks (atoms) and control the distance between them. Then, we can observe them as separate objects, investigate the interaction between them as a function of intermolecular distance and utilize the properties of individual molecules, which often differ tremendously from their bulk characteristics.

All this above sounds promising but how to achieve this in practice? Let us imagine a solid material with regularly distributed anchoring units separated by a specific distance, which can catch particular atoms or molecules and keep them separated. One association that comes to mind is some kind of a solvent since the last one is able to coordinate the molecules in such a way that they are separated and to create a solvation complex. In the case of a liquid, nevertheless, dissolved molecules are difficult to investigate since they are not immobilized. The compound we are searching for should have such a form that allows for rigid immobilization of nano-objects. This way the objects could be used for specific purposes at specific times and places. It should be some kind of a „solid solvent”.

In this lecture, the concept of 2D solid solvent will be presented. Such materials can be fabricated based on functionalized nanostructured materials, such as silica or alumina. A few examples of functional materials being in fact 2D solid solvents will be shown, along with an explanation of their structure, functionality, description of the fabrication route, and justification of the purposefulness of their production. Also, the role of molecular engineering in the fabrication process will be clarified here.