Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3.

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature TS2 of the second structural transition associated with the order-order magnetic phase transition. Below TS2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI3 indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered the exclusive domain of antiferromagnets.

Unusual features of lattice dynamics in lawsonite near its phase transitions.

Lattice dynamics of a single crystal of lawsonite were studied over a broad range of frequencies (1 Hz to 20 THz) using impedance, THz time-domain and infrared spectroscopies. Based on polarized spectra of complex permittivity [Formula: see text] measured as a function of temperature between 10 K and 500 K, we analyzed the properties of the two known phase transitions-an antiferrodistortive one near [Formula: see text] and a ferroelectric one, occurring at [Formula: see text]. The former one is accompanied by a flat maximum in the THz-range permittivity [Formula: see text] near [Formula: see text], which is due to an overdamped polar excitation in the [Formula: see text] spectra reflecting the dynamics of water and hydroxyl groups. The strength of this mode decreases on cooling below [Formula: see text], and the mode vanishes below [Formula: see text] due to hydrogen ordering. At the pseudoproper ferroelectric phase transition, two independent anomalies in permittivity were observed. First, [Formula: see text] exhibits a peak at [Formula: see text] due to critical slowing down of a relaxation in the GHz range. Second, infrared and THz spectra revealed an optical phonon softening towards [Formula: see text] which causes a smaller but pronounced maximum in [Formula: see text]. Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.

Mass of Abrikosov vortex in high-temperature superconductor YBa[Formula: see text]Cu[Formula: see text]O[Formula: see text].

For more than four decades, mass of Abrikosov vortices defied experimental observations. We demonstrate a method of its detection in high-temperature superconductors. Similarly to electrons, fluxons circulate in the direction given by the magnetic field, causing circular dichroism. We report the magneto-transmittance of a nearly optimally doped thin YBa[Formula: see text]Cu[Formula: see text]O[Formula: see text] film, measured using circularly polarized submillimeter waves. The circular dichroism emerges in the superconducting state and increases with dropping temperature. Our results confirm the dominant role of quasiparticle states in the vortex core and yield the diagonal fluxon mass of [Formula: see text] electron masses per centimeter at 45 K and zero-frequency limit, and even larger off-diagonal mass of [Formula: see text]/cm.

Subterahertz collective dynamics of polar vortices.

The collective dynamics of topological structures1-6 are of interest from both fundamental and applied perspectives. For example, studies of dynamical properties of magnetic vortices and skyrmions3,4 have not only deepened our understanding of many-body physics but also offered potential applications in data processing and storage7. Topological structures constructed from electrical polarization, rather than electron spin, have recently been realized in ferroelectric superlattices5,6, and these are promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics underlying the functionality of such complex extended nanostructures. Here, using terahertz-field excitation and femtosecond X-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders-of-magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices3. A previously unseen tunable mode, hereafter referred to as a vortexon, emerges in the form of transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond timescales. Its frequency is considerably reduced (softened) at a critical strain, indicating a condensation (freezing) of structural dynamics. We use first-principles-based atomistic calculations and phase-field modelling to reveal the microscopic atomic arrangements and corroborate the frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens opportunities for electric-field-driven data processing in topological structures with ultrahigh speed and density.

Targeted chemical pressure yields tuneable millimetre-wave dielectric.

Epitaxial strain can unlock enhanced properties in oxide materials, but restricts substrate choice and maximum film thickness, above which lattice relaxation and property degradation occur. Here we employ a chemical alternative to epitaxial strain by providing targeted chemical pressure, distinct from random doping, to induce a ferroelectric instability with the strategic introduction of barium into today's best millimetre-wave tuneable dielectric, the epitaxially strained 50-nm-thick n = 6 (SrTiO3)nSrO Ruddlesden-Popper dielectric grown on (110) DyScO3. The defect mitigating nature of (SrTiO3)nSrO results in unprecedented low loss at frequencies up to 125 GHz. No barium-containing Ruddlesden-Popper titanates are known, but the resulting atomically engineered superlattice material, (SrTiO3)n-m(BaTiO3)mSrO, enables low-loss, tuneable dielectric properties to be achieved with lower epitaxial strain and a 200% improvement in the figure of merit at commercially relevant millimetre-wave frequencies. As tuneable dielectrics are key constituents of emerging millimetre-wave high-frequency devices in telecommunications, our findings could lead to higher performance adaptive and reconfigurable electronics at these frequencies.

Materials with on-demand refractive indices in the terahertz range.

We demonstrate the possibility to create materials with chosen refractive indices and a strong birefringence in the terahertz range by etching of patterns with appropriate filling factors in a dielectric substrate. We show that by using deep inductive plasma etching of silicon wafers, it is possible to achieve a birefringence as high as 1.2 in an 80 microm thick layer. The resulting stacks were used as building blocks for a photonic crystal displaying sharp defect mode peaks in transmittance.

Optical pump-terahertz probe spectroscopy of dyes in solutions: probing the dynamics of liquid solvent or solid precipitate?

The optical pump-terahertz probe spectroscopy was used together with ab initio calculations and molecular dynamics simulations to investigate ultrafast dynamics following electronic excitation of Coumarin 153 and TBNC (2,11,20,29-tetra-tert-butyl-2,3-naphtalocyanine) dyes in polar solvents. By scanning the terahertz waveform for different pump-probe delays this experimental technique allows us to obtain two dimensional spectra directly reflecting the temporal response of the system. A distinct signal was obtained for TBNC in chloroform, 2-propanol, and n-butanol, while no signal was recorded for Coumarin 153 in either of these solvents. We explain the nonequilibrium signal detected in TBNC solutions by the presence of a solid, polycrystalline phase of the dye resulting from irradiating the solution by intense optical pulses.