2021, Pal, Shovon, Strkalj, Nives, Yang, Chia-Jung, Weber, Mads C., Trassin, Morgan, Woerner, Michael, Fiebig, Manfred

Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric SrTiO3 thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.

2020, Slootman, Juliette, Waltz, Victoria, Yeh, C. Joshua, Baumann, Christoph, Göstl, Robert, Comtet, Jean, Creton, Costantino

Elastomers are highly valued soft materials finding many applications in the engineering and biomedical fields for their ability to stretch reversibly to large deformations. Yet their maximum extensibility is limited by the occurrence of fracture, which is currently still poorly understood. Because of a lack of experimental evidence, current physical models of elastomer fracture describe the rate and temperature dependence of the fracture energy as being solely due to viscoelastic friction, with chemical bond scission at the crack tip assumed to remain constant. Here, by coupling new fluorogenic mechanochemistry with quantitative confocal microscopy mapping, we are able to quantitatively detect, with high spatial resolution and sensitivity, the scission of covalent bonds as ordinary elastomers fracture at different strain rates and temperatures. Our measurements reveal that, in simple networks, bond scission, far from being restricted to a constant level near the crack plane, can both be delocalized over up to hundreds of micrometers and increase by a factor of 100, depending on the temperature and stretch rate. These observations, permitted by the high fluorescence and stability of the mechanophore, point to an intricate coupling between strain-rate-dependent viscous dissipation and strain-dependent irreversible network scission. These findings paint an entirely novel picture of fracture in soft materials, where energy dissipated by covalent bond scission accounts for a much larger fraction of the total fracture energy than previously believed. Our results pioneer the sensitive, quantitative, and spatially resolved detection of bond scission to assess material damage in a variety of soft materials and their applications. © 2020 authors. Published by the American Physical Society.

2020, Brausse, Felix, Bach, Florian, Krečinić, Faruk, Vrakking, Marc J.J., Rouzée, Arnaud

We report experiments on laser-assisted electron recollisions that result from strong-field ionization of photoexcited I2 molecules in the regime of low-energy electron scattering (<25  eV impact energy). By comparing differential scattering cross sections extracted from the angle-resolved photoelectron spectra to differential scattering cross sections from quantum-scattering calculations, we demonstrate that the electron-scattering dynamics is dominated by a shape resonance. When the molecular bond stretches during the evolution of a vibrational wave packet this shape resonance shifts to lower energies, both in experiment and theory. We explain this behavior by the nature of the resonance wave function, which closely resembles an antibonding molecular orbital of I2.

2021, Golias, E., Kumberg, I., Gelen, I., Thakur, S., Gördes, J., Hosseinifar, R., Guillet, Q., Dewhurst, J.K., Sharma, S., Schüßler-Langeheine, C., Pontius, N., Kuch, W.

We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L3 resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.

2022, Schweika, W., Valldor, M., Reim, J.D., Rößler, U.K.

A chiral spin liquid state is discovered in the highly frustrated, noncentrosymmetric swedenborgite compound YBaCo3FeO7, a layered kagome system of hexagonal symmetry, by advanced polarized neutron scattering from a single domain crystalline sample. The observed diffuse magnetic neutron scattering has an antisymmetric property that relates to its specific chirality, which consists of three cycloidal waves perpendicular to the c axis, forming an entity of cylindrical symmetry. Chirality and symmetry agree with relevant antisymmetric exchanges arising from broken spatial parity. Applying a Fourier analysis to the chiral interference pattern, with distinction between kagome sites and the connecting trigonal interlayer sites of threefold symmetry, the chiral spin correlation function is determined. Characteristic chiral waves originate from the trigonal sites and extend over several periods in the kagome planes. The chiral spin liquid is remarkably stable at low temperatures despite strong antiferromagnetic spin exchange. The observation raises a challenge, since the commonly accepted ground states in condensed matter either have crystalline long-range order or form a quantum liquid. We show that, within the classical theory of magnetic order, a disordered ground state may arise from chirality. The present scenario, with antisymmetric exchange acting as a frustrating gauge background that stabilizes local spin lumps, is similar to the avoided phase transition in coupled gauge and matter fields for subnuclear particles.

2021, Kuznetsov, Alexander S., Machado, Diego H.O., Biermann, Klaus, Santos, Paulo V.

Microcavity exciton polaritons enable the resonant coupling of excitons and photons to vibrations in the super-high-frequency (SHF, 3–30 GHz) domain. We introduce here a novel platform for coherent SHF optomechanics based on the coupling of polaritons and electrically driven SHF longitudinal acoustic phonons confined in a planar Bragg microcavity. The highly monochromatic phonons with tunable amplitudes are excited over a wide frequency range by piezoelectric transducers, which also act as efficient phonon detectors with a very large dynamical range. The microcavity platform exploits the long coherence time of polaritons as well as their efficient coupling to phonons. Furthermore, an intrinsic property of the platform is the backfeeding of phonons to the interaction region via reflections at the sample boundaries, which leads to quality factor × frequency products (Q×f) exceeding 1014  Hz as well as huge modulation amplitudes of the optical transition energies exceeding 8 meV. We show that the modulation is dominated by the phonon-induced energy shifts of the excitonic polariton component. Thus, the large modulation leads to a dynamical switching of light-matter nature of the particles from a mixed (i.e., polaritonic) one to photonlike and excitonlike states at frequencies up to 20 GHz. On the one hand, this work opens the way for electrically driven polariton optomechanics in the nonadiabatic, sideband-resolved regime of coherent control. Here, the bidirectionality of the transducers can be exploited for light-to-sound-to-rf conversion. On the other hand, the large phonon frequencies and Q×f products enable phonon control with optical readout down to the single-particle regime at relatively high temperatures (of 1 K).

2021, Li, Jiemin, Xu, Lei, Garcia-Fernandez, Mirian, Nag, Abhishek, Robarts, H.C., Walters, A.C., Liu, X., Zhou, Jianshi, Wohlfeld, Krzysztof, van den Brink, Jeroen, Ding, Hong, Zhou, Ke-Jin

We explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF3 using the Cu L3-edge resonant inelastic x-ray scattering. We show that the nondispersive high-energy peaks result from the Cu2+  dd orbital excitations. These high-energy modes display good agreement with the ab initio quantum chemistry calculation, indicating that the dd excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Müller ansatz calculations. The localized dd excitations and the observation of the strongly dispersive magnetic excitations suggest that the orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong local Jahn-Teller effect in KCuF3, which predominantly drives orbital ordering.

2020, Portnichenko, P.Y., Akbari, A., Nikitin, S.E., Cameron, A.S., Dukhnenko, A.V., Filipov, V.B., Shitsevalova, N.Yu., Čermák, P., Radelytskyi, I., Schneidewind, A., Ollivier, J., Podlesnyak, A., Huesges, Z., Xu, J., Ivanov, A., Sidis, Y., Petit, S., Mignot, J.-M., Thalmeier, P., Inosov, D.S.

In contrast to magnetic order formed by electrons' dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as "hidden order."Previously, the hidden order in phase II was identified as primary antiferroquadrupolar and field-induced octupolar order. Here, we present a combined experimental and theoretical investigation of collective excitations in phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in a rotating field is calculated within a localized approach using the pseudospin representation for the Γ8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at a constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of intermultipolar interactions that stabilize hidden-order phases. © 2020 authors. Published by the American Physical Society.

2021, Huang, H.Y., Singh, A., Mou, C.Y., Johnston, S., Kemper, A.F., van den Brink, J., Chen, P.J., Lee, T.K., Okamoto, J., Chu, Y.Y., Li, J.H., Komiya, S., Komarek, A.C., Fujimori, A., Chen, C.T., Huang, D.J.

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order.

2021, Sauter, E., Abrosimov, N.V., Hübner, J., Oestreich, M.

We measure the spin-lattice relaxation of donor bound electrons in ultrapure, isotopically enriched, phosphorus-doped 28Si:P. The optical pump-probe experiments reveal at low temperatures extremely long spin relaxation times which exceed 20 h. The 28Si:P spin relaxation rate increases linearly with temperature in the regime below 1 K and shows a distinct transition to a T9 dependence which dominates the spin relaxation between 2 and 4 K at low magnetic fields. The T7 dependence reported for natural silicon is absent. At high magnetic fields, the spin relaxation is dominated by the magnetic field dependent single phonon spin relaxation process. This process is well documented for natural silicon at finite temperatures but the 28Si:P measurements validate additionally that the bosonic phonon distribution leads at very low temperatures to a deviation from the linear temperature dependence of Γ as predicted by theory.