Peter Kuzel

Dielectric properties of Nb-doped (~1.5 at.%) rutile single crystals were studied in the 10-300 K temperature range (at frequencies below the MHz range down to 0.3 K) in a broad frequency range, including terahertz and infrared range, to understand the origin of its giant permittivity. The results were fitted, modelled and compared with those of the undoped rutile crystal measured in the terahertz and infrared ranges. The primary effect originates from the near-electrode depletion layer of lower conductivity compared to the bulk (surface barrier-layer capacitor effect), which causes a strong thermally activated relaxation in the MHz dielectric spectra. In the higher frequency range, the main difference between doped and undoped crystals is the presence of an overdamped microwave excitation (central mode) in the doped crystal for both polarizations, persisting down to 10 K and not thermally activated. This accounts for the previously reported increased permittivity values down to 2 K – where all lower-frequency relaxations are frozen – compared to undoped crystals. It also explains why our low-frequency permittivity at 0.3 K exceeds its THz value. In similar Nb-doped rutile single crystals it was quite recently ascribed [V. A. Dinh et al., Phys. Rev. B 114, 024112, (2026)] to microwave tunneling of molecular polarons. Polar phonons are affected only by a slightly increased damping.

Time-resolved spectroscopy of diketopyrrolopyrroles carrying n -hexyl, n -butyl, or 2-(adamant-1-yl)ethyl substituents showed that the efficient singlet fission is accompanied by clearly resolved oscillations in transient spectra on the ps time scale.

Imidazolium hydrogen o-phthalate (OrtImi) is an organic ionic crystal with a helical hydrogen-bonded network. We report on the evolution of the Raman, terahertz, and infrared modes of OrtImi over a wide range of temperatures and hydrostatic pressures. We interpret the results using density functional theory and molecular dynamics within machine-learning interatomic potential framework calculations. The vibrational spectra uncovered a phase transition occurring above 3 GPa applied hydrostatic pressure. This transition is associated with a collapse of the helical network of N⁺-H···O^- hydrogen bonds transforming into a ring motif within the ab plane. Proton ordering in hydrogen bonds emerges upon cooling below 150 K, creating favorable conditions for proton-phonon coupling.

Hydrated imidazolium hemimelitate with helical hydrogen bonding network is the first amphidynamic organic crystal observed in a group of imidazolium and carboxylic acid compounds. The sublattice of acid ions forms a static network, while the dynamic part comprises imidazole ions and water molecules. A transition from positional to orientational disorder of water molecules is observed as the temperature closes to room temperature and the spatial arrangement of cations leads to an order-disorder phase transition at a temperature of 150 K, which we analyzed in a wide spectral range using THz, FIR, MIR, and Raman spectroscopies. Furthermore, DFT calculations were employed to understand the molecular dynamics and the phase transition mechanism of the studied compound. The temperature-dependent spectra also revealed proton-phonon coupling to occur below 100 K. Our findings provide valuable information, such as temperature behavior of hydrogen bonds, anharmonicity, and coupling effects for the targeted design of amphidynamic materials.

We study broadband terahertz (THz) conductivity and ultrafast photoconductivity spectra in lithographically fabricated multilayer epitaxial graphene nanoribbons grown on C-face of 6H-SiC substrate. THz near-field spectroscopy reveals local conductivity variations across nanoscale structural inhomogeneities such as wrinkles and grain boundaries within the multilayer graphene. Ultrabroadband THz far-field spectroscopy ( 0.15 – 16 THz ) distinguishes doped graphene layers near the substrate from quasineutral layers (QNLs) further from the substrate. Temperature-dependent THz conductivity spectra are dominated by intraband transitions both in the doped and QNLs. Photoexcitation then alters mainly the response of the QNLs: these exhibit very high carrier mobility and large positive THz photoconductivity with picosecond lifetime. The response of QNLs strongly depends on the carrier temperature T c : the scattering time drops by an order of magnitude down to ∼10 fs upon an increase of T c from 50 K to T c > 1000 K , which is attributed to an enhanced electron-electron and electron-phonon scattering and to an interaction of electrons with mid-gap states.

Optical pump-terahertz probe spectroscopy has been used to investigate ultrafast photo-induced charge carrier transport in 3.4 µm wide graphene ribbons upon scaling the optical pump intensity. For low pump fluences, the deposited pump energy is rapidly redistributed through carrier–carrier scattering, producing secondary hot carriers: the picosecond THz photoconductivity then acquires a negative sign and scales linearly with an increasing pump fluence. At higher fluences, there are not enough equilibrium carriers able to accept the deposited energy, directly generated (excess) carriers start to contribute significantly to the photoconductivity with a positive sign leading to its saturation behavior. This leads to a non-monotonic variation of the carrier mobility and plasmonic resonance frequency as a function of the pump fluence and, at high fluences, to a balance between a decreasing carrier scattering time and an increasing Drude weight. In addition, a weak carrier localization observed for the polarization parallel to the ribbons at low pump fluences is progressively lifted upon increasing the pump fluence as a result of the rise of initial carrier temperature.

The study of magnetic frustration in classical spin systems is motivated by the prediction and discovery of classical spin liquid states. These uncommon magnetic phases are characterized by a massive degeneracy of their ground state implying a finite magnetic entropy at zero temperature. While the classical spin liquid state is originally predicted in the Ising triangular lattice antiferromagnet in 1950, this state has never been experimentally observed in any triangular magnets. The discovery of an electric analogue of classical spin liquids on a triangular lattice of uniaxial electric dipoles in EuAl ₁₂ O ₁₉ is reported here. This new type of frustrated antipolar phase is characterized by a highly‐degenerate state at low temperature implying an absence of long‐range antiferroelectric order, despite short‐range antipolar correlations. Its dynamics are governed by a thermally activated process, slowing down upon cooling toward a complete freezing at zero temperature.