Tunable sub-terahertz resonance absorption in high-coercivity magnetodielectric ceramics.

Recently, giant coercivities (20-42 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) at 100-300 GHz were observed for single-domain M-type hexaferrite particles with high aluminum substitution. Herein, we fabricated dense ceramics of Sr0.67Ca0.33Fe8Al4O19 and, for the first time, investigated their magnetostatic and magnetodynamic properties in the temperature range of 5-300 K. It was shown that dense ceramics maintain their high magnetic hardness (a coercivity of 10-20 kOe) and NFMR frequencies of 140-200 GHz durably in the entire temperature range. Magnetizing the initially non-magnetized ceramics leads to a considerable decrease in the resonance absorption and to almost complete vanishing of the resonance line at 5 kOe. At the same time, an efficient linear frequency tuning by the external magnetic field was observed for the remanent sample. These findings open new horizons for developing industrial terahertz electronics based on dielectric ferrimagnets.

Octahedra-Tilted Control of Displacement Disorder and Dielectric Relaxation in Mn-Doped SrTiO3 Single Crystals.

Strontium titanate SrTiO3 (STO) is a canonical example of a quantum paraelectric, and its doping with manganese ions unlocks its potential as a quantum multiferroic candidate. However, to date, the specifics of incorporation of the manganese ion into the perovskite lattice and its impact on structure-property relationships are debatable questions. Herein, using high-precision X-ray diffraction of a Mn (2 atom %)-doped STO single crystal, clear fingerprints of the displacement disorder of Mn cations in the perovskite B-sublattice are observed. Moreover, near the temperature of the antiferrodistortive transition, the off-center Mn position splits in two, providing the unequal potential barrier's distribution for possible local atomic hopping. A link with this was found via analysis of the dielectric response that reveals two Arrhenius-type relaxation processes with similar activation energies (35 and 43 meV) and attempt frequencies (1 × 1011 and ∼1.6 × 1010 Hz), suggesting similar dielectric relaxation mechanisms.

Low-temperature infrared spectroscopy of the strongly correlated semiconductor Tm0.19Yb0.81B12with dynamic charge stripes.

Tm1-xYbxB12dodecaborides represent model objects for the studies of quantum critical behavior, metal-insulator transitions (MITs) and complex charge-spin-orbital-phonon coupling phenomena. In spite of intensive investigations, the mechanism of semiconducting ground state formation both in YbB12and in the Yb-based strongly correlated electron systems remains a subject of active debates. We have performed first systematic measurements of temperature-dependent spectra of infrared conductivity of Tm0.19Yb81B12at frequencies 40-35 000 cm-1and in the temperature range 10-300 K. Analysis of the temperature evolution of the observed absorption resonances is performed allowing to associate these with the cooperative dynamic Jahn-Teller instability of the boron sub-lattice. This ferrodistortive effect of B12-complexes induces the rattling modes of the rare earth ions leading to emergence of both the intra-gap mixed-type collective excitations and the dynamic charge stripes. We estimate the temperature-dependent effective mass of charge carriers and propose the scenario of transformation of the many-body states in the multiple relaxation channels. We attribute the MIT to the localization of electrons at the vibrationally coupled Yb-Yb pairs, which is accompanied by the electronic phase separation and formation of the nanoscale filamentary structure of electron density (stripes) in Tm1-xYbxB12compounds.

Correction: High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance.

Correction for 'High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance' by Evgeny A. Gorbachev et al., Mater. Horiz., 2022, 9, 1264-1272, DOI: https://doi.org/10.1039/D1MH01797G.

Multifunctional Elastic Nanocomposites with Extremely Low Concentrations of Single-Walled Carbon Nanotubes.

Stretchable and flexible electronics has attracted broad attention over the last years. Nanocomposites based on elastomers and carbon nanotubes are a promising material for soft electronic applications. Despite the fact that single-walled carbon nanotube (SWCNT) based nanocomposites often demonstrate superior properties, the vast majority of the studies were devoted to those based on multiwalled carbon nanotubes (MWCNTs) mainly because of their higher availability and easier processing procedures. Moreover, high weight concentrations of MWCNTs are often required for high performance of the nanocomposites in electronic applications. Inspired by the recent drop in the SWCNT price, we have focused on fabrication of elastic nanocomposites with very low concentrations of SWCNTs to reduce the cost of nanocomposites further. In this work, we use a fast method of coagulation (antisolvent) precipitation to fabricate elastic composites based on thermoplastic polyurethane (TPU) and SWCNTs with a homogeneous distribution of SWCNTs in bulk TPU. Applicability of the approach is confirmed by extra low percolation threshold of 0.006 wt % and, as a consequence, by the state-of-the-art performance of fabricated elastic nanocomposites at very low SWCNT concentrations for strain sensing (gauge factor of 82 at 0.05 wt %) and EMI shielding (efficiency of 30 dB mm-1 at 0.01 wt %).

Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules.

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below T = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(T). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε')-1 down to T ≈ 0.3 K and comparing it with the data obtained for conventional quantum paraelectrics, like SrTiO3, KTaO3, we discovered clear signatures of a quantum-critical behavior of the interacting water molecular dipoles: Between T = 6 and 14 K, the reciprocal permittivity follows a quadratic temperature dependence and displays a shallow minimum below 3 K. This is the first observation of "dielectric fingerprints" of quantum-critical phenomena in a paraelectric system of coupled point electric dipoles.

High-Quality Graphene Using Boudouard Reaction.

Following the game-changing high-pressure CO (HiPco) process that established the first facile route toward large-scale production of single-walled carbon nanotubes, CO synthesis of cm-sized graphene crystals of ultra-high purity grown during tens of minutes is proposed. The Boudouard reaction serves for the first time to produce individual monolayer structures on the surface of a metal catalyst, thereby providing a chemical vapor deposition technique free from molecular and atomic hydrogen as well as vacuum conditions. This approach facilitates inhibition of the graphene nucleation from the CO/CO2 mixture and maintains a high growth rate of graphene seeds reaching large-scale monocrystals. Unique features of the Boudouard reaction coupled with CO-driven catalyst engineering ensure not only suppression of the second layer growth but also provide a simple and reliable technique for surface cleaning. Aside from being a novel carbon source, carbon monoxide ensures peculiar modification of catalyst and in general opens avenues for breakthrough graphene-catalyst composite production.

High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance.

Herein, we demonstrate for the first time compact ferrite ceramics with giant coercivity. The materials are manufactured via sintering single-domain Sr0.67Ca0.33Fe8Al4O19 particles synthesized by a citrate-nitrate auto-combustion method. The obtained ceramics show coercivities up to 22.5 kOe and natural ferromagnetic resonance frequencies (NFMR) in a sub-THz range of 160-282 GHz. At a maximum density of 95%, the sample displays coercivity of 18.5 kOe, which is the highest value among dense ferrite materials reported so far. In addition, we report an unusual blueshift of the NFMR frequency from 160 to 200 GHz, which occurs during material sintering.

Water-Activated Semiquinone Formation and Carboxylic Acid Dissociation in Melanin Revealed by Infrared Spectroscopy.

Eumelanin is a widespread biomacromolecule pigment in the biosphere and has been widely investigated for numerous bioelectronics and energetic applications. Many of these applications depend on eumelanin's ability to conduct proton current at various levels of hydration. The origin of this behavior is connected to a comproportionation reaction between oxidized and reduced monomer moieties and water. A hydration-dependent FTIR spectroscopic study on eumelanin is presented herein, which allows for the first time tracking the comproportionation reaction via the gradual increase of the overall aromaticity of melanin monomers in the course of hydration. We identified spectral features associated with the presence of specific "one and a half" C𝌁O bonds, typical for o-semiquinones. Signatures of semiquinone monomers with internal hydrogen bonds and that carboxylic groups, in contrast to semiquinones, begin to dissociate at the very beginning of melanin hydration were indicated. As such, we suggest a modification to the common hydration-dependent conductivity mechanism and propose that the conductivity at low hydration is dominated by carboxylic acid protons, whereas higher hydration levels manifest semiquinone protons.

Rotational coherence of encapsulated ortho and para water in fullerene-C60 revealed by time-domain terahertz spectroscopy.

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

Aero-Ga2O3 Nanomaterial Electromagnetically Transparent from Microwaves to Terahertz for Internet of Things Applications.

In this paper, fabrication of a new material is reported, the so-called Aero-Ga2O3 or Aerogallox, which represents an ultra-porous and ultra-lightweight three-dimensional architecture made from interconnected microtubes of gallium oxide with nanometer thin walls. The material is fabricated using epitaxial growth of an ultrathin layer of gallium nitride on zinc oxide microtetrapods followed by decomposition of sacrificial ZnO and oxidation of GaN which according to the results of X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) characterizations, is transformed gradually in ß-Ga2O3 with almost stoichiometric composition. The investigations show that the developed ultra-porous Aerogallox exhibits extremely low reflectivity and high transmissivity in an ultrabroadband electromagnetic spectrum ranging from X-band (8-12 GHz) to several terahertz which opens possibilities for quite new applications of gallium oxide, previously not anticipated.

Impedance spectroscopy of single bacterial nanofilament reveals water-mediated charge transfer.

For decades respiratory chain and photosystems were the main firing field of the studies devoted to mechanisms of electron transfer in proteins. The concept of conjugated lateral electron and transverse proton transport during cellular respiration and photosynthesis, which was formulated in the beginning of 1960-s, has been confirmed by thousands of experiments. However, charge transfer in recently discovered bacterial nanofilaments produced by various electrogenic bacteria is regarded currently outside of electron and proton conjugation concept. Here we report the new study of charge transfer within nanofilaments produced by Shewanella oneidensis MR-1 conducted in atmosphere of different relative humidity (RH). We utilize impedance spectroscopy and DC (direct current) transport measurements to find out the peculiarities of conductivity and Raman spectroscopy to analyze the nanofilaments' composition. Data analysis demonstrates that apparent conductivity of nanofilaments has crucial sensitivity to humidity and contains several components including one with unusual behavior which we assign to electron transport. We demonstrate that in the case of Shewanella oneidensis MR-1 charge transfer within these objects is strongly mediated by water. Basing on current data analysis of conductivity we conclude that the studied filaments of Shewanella oneidensis MR-1 are capable of hybrid (conjugated) electron and ion conductivity.

Sub-terahertz frequency-domain spectroscopy reveals single-grain mobility and scatter influence of large-area graphene.

The response of individual domains in wafer-sized chemical vapor deposition graphene is measured by contactless sub-terahertz interferometry, observing the intrinsic optical conductance and reaching very high mobility values. It is shown that charged scatterers limit the mobility, validating previous theoretical predictions, and sub-terahertz quality assessment is demonstrated, as necessary for large-scale applications in touchscreens, as well as wearable and optoelectronic devices.

Vibrational states of a water molecule in a nano-cavity of beryl crystal lattice.

Low-energy excitations of a single water molecule are studied when confined within a nano-size cavity formed by the ionic crystal lattice. Optical spectra are measured of manganese doped beryl single crystal Mn:Be3Al2Si6O18, that contains water molecules individually isolated in 0.51 nm diameter voids within the crystal lattice. Two types of orientation are distinguished: water-I molecules have their dipole moments aligned perpendicular to the c axis and dipole moments of water-II molecules are parallel to the c-axis. The optical conductivity σ(ν) and permittivity ɛ'(ν) spectra are recorded in terahertz and infrared ranges, at frequencies from several wavenumbers up to ν = 7000 cm(-1), at temperatures 5-300 K and for two polarizations, when the electric vector E of the radiation is parallel and perpendicular to the c-axis. Comparative experiments on as-grown and on dehydrated samples allow to identify the spectra of σ(ν) and ɛ'(ν) caused exclusively by water molecules. In the infrared range, well-known internal modes ν1, ν2, and ν3 of the H2O molecule are observed for both polarizations, indicating the presence of water-I and water-II molecules in the crystal. Spectra recorded below 1000 cm(-1) reveal a rich set of highly anisotropic features in the low-energy response of H2O molecule in a crystalline nano-cavity. While for E∥c only two absorption peaks are detected, at ~90 cm(-1) and ~160 cm(-1), several absorption bands are discovered for E⊥c, each consisting of narrower resonances. The bands are assigned to librational (400-500 cm(-1)) and translational (150-200 cm(-1)) vibrations of water-I molecule that is weakly coupled to the nano-cavity "walls." A model is presented that explains the "fine structure" of the bands by a splitting of the energy levels due to quantum tunneling between the minima in a six-well potential relief felt by a molecule within the cavity.

Quantum Behavior of Water Molecules Confined to Nanocavities in Gemstones.

When water is confined to nanocavities, its quantum mechanical behavior can be revealed by terahertz spectroscopy. We place H2O molecules in the nanopores of a beryl crystal lattice and observe a rich and highly anisotropic set of absorption lines in the terahertz spectral range. Two bands can be identified, which originate from translational and librational motions of the water molecule isolated within the cage; they correspond to the analogous broad bands in liquid water and ice. In the present case of well-defined and highly symmetric nanocavities, the observed fine structure can be explained by macroscopic tunneling of the H2O molecules within a six-fold potential caused by the interaction of the molecule with the cavity walls.

Ge/Si(001) heterostructures with dense arrays of Ge quantum dots: morphology, defects, photo-emf spectra and terahertz conductivity.

: Issues of Ge hut cluster array formation and growth at low temperatures on the Ge/Si(001) wetting layer are discussed on the basis of explorations performed by high resolution STM and in-situ RHEED. Dynamics of the RHEED patterns in the process of Ge hut array formation is investigated at low and high temperatures of Ge deposition. Different dynamics of RHEED patterns during the deposition of Ge atoms in different growth modes is observed, which reflects the difference in adatom mobility and their 'condensation' fluxes from Ge 2D gas on the surface for different modes, which in turn control the nucleation rates and densities of Ge clusters. Data of HRTEM studies of multilayer Ge/Si heterostructures are presented with the focus on low-temperature formation of perfect films.Heteroepitaxial Si p-i-n-diodes with multilayer stacks of Ge/Si(001) quantum dot dense arrays built in intrinsic domains have been investigated and found to exhibit the photo-emf in a wide spectral range from 0.8 to 5 µm. An effect of wide-band irradiation by infrared light on the photo-emf spectra has been observed. Photo-emf in different spectral ranges has been found to be differently affected by the wide-band irradiation. A significant increase in photo-emf is observed in the fundamental absorption range under the wide-band irradiation. The observed phenomena are explained in terms of positive and neutral charge states of the quantum dot layers and the Coulomb potential of the quantum dot ensemble. A new design of quantum dot infrared photodetectors is proposed.By using a coherent source spectrometer, first measurements of terahertz dynamical conductivity (absorptivity) spectra of Ge/Si(001) heterostructures were performed at frequencies ranged from 0.3 to 1.2 THz in the temperature interval from 300 to 5 K. The effective dynamical conductivity of the heterostructures with Ge quantum dots has been discovered to be significantly higher than that of the structure with the same amount of bulk germanium (not organized in an array of quantum dots). The excess conductivity is not observed in the structures with the Ge coverage less than 8 Å. When a Ge/Si(001) sample is cooled down the conductivity of the heterostructure decreases.

Direct observation of fine structure transitions in a paramagnetic nickel(II) complex using far-infrared magnetic spectroscopy: a new method for studying high-spin transition metal complexes.

Novel far-infrared (FIR) absorption spectroscopy in conjunction with multiple, fixed external magnetic fields (FIR magnetic spectroscopy, FIRMS) has been used to investigate pseudotetrahedral complexes with the formula M(PPh(3))(2)Cl(2) (M = Ni, Zn; Ph = C(6)H(5)). Crystal structures have been reported for the Ni complex; we report the structure of the Zn complex. Transmission spectra at 5 K of Ni(PPh(3))(2)Cl(2) (S = 1) at zero magnetic field exhibit absorption bands at 11.41, 15.28, and 23.0 cm(-1). The two lower frequency bands show great sensitivity to external magnetic field, and their field dependence is as expected for electron spin transitions allowing precise determination of the following parameters: |D| = 13.35(1) cm(-1), |E| = 1.93(1) cm(-1), g(x,y) = 2.20(1), g(z) = 2.00(1). Corresponding spectra of Zn(PPh(3))(2)Cl(2) (S = 0) exhibit bands only at >20 cm(-1), which show no field dependence. FIRMS is a promising technique for direct investigation of the electronic structure of high-spin transition metal complexes.