Search

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

Complex frustrated magnetic structures in multiferroic hexaferrites are well tunable by temperature, magnetic field and doping. We investigated the influence of strong THz pulses generated by superradiant THz sources on magnetic structure and related electromagnons’ absorption in Y- and Z-type multiferroic hexaferrites. While in Z-type hexaferrite (Ba0.2Sr0.8)3Co2Fe24O41 polycrystal, the observed changes in transmission spectra were fully described by sample heating, a blue-shift of the electromagnon frequency observed in Y-type hexaferrite Ba0.2Sr1.8Co2(Fe0.96Al0.04)12O22 single-crystal could be possibly ascribed to the transition from the alternating longitudinal conical to the transverse conical magnetic structure. We elaborated a nonlinear model which explained absence of nonlinearity in Z-type hexaferrite (Ba0.2Sr0.8)3Co2Fe24O41. For Y-type hexaferrite Ba0.2Sr1.8Co2(Fe0.96Al0.04)12O22, we discuss possible transient or even permanent effects of both THz electric and magnetic fields on its magnetic structure.

Behavior of individual molecules of normal and heavy water in beryl single crystals was studied by $^1$H and $^2$H nuclear magnetic resonance spectroscopy. From temperature dependences of the spectra we deduce that type-I water molecules embedded in the beryl voids are oriented quite differently from the view established in the literature. Namely, contrary to earlier assumptions, their H-H lines deviate by about 18$^\circ$ from the hexagonal axis. We suggest that this is due to the molecules attaching to the oxygen atoms forming the beryl structural voids by a hydrogen bond. Our analysis shows that the molecules perform two types of movement: (i) rapid librations around the axis of the hydrogen bond, and (ii) less frequent orientational jumps among the twelve possible binding sites in the beryl voids. The frequencies of the librational motions are evaluated from a simple thermodynamic model, providing a good quantitative agreement with the frequencies of librations from optical experiments reported earlier., Comment: paper (27 pages, 9 figures), submitted to The Journal of Chemical Physics

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 ε̂ 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 Tc1 = 270 K and a ferroelectric one, occurring at Tc2 = 124 K. The former one is accompanied by a flat maximum in the THz-range permittivity ε̂c near Tc1, which is due to a polar excitation in the E || c spectra reflecting the dynamics of water and hydroxyl groups. The strength of this mode decreases on cooling below Tc1, and the mode vanishes below Tc2 due to hydrogen ordering. At the pseudoproper ferroelectric phase transition, two independent anomalies in permittivity were observed. First, ε̂a exhibits a peak at Tc2 = 124 K due to critical slowing down of a relaxation in the GHz range. Second, infrared and THz spectra revealed an optical phonon softening towards Tc2 which causes a smaller but pronounced maximum in ε̂b. Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.

The textbook provides an overview of the main techniques applied in pharmaceutical industry, with the focus on solid-state analysis. It discusses spectral methods, thermal analysis, and dissolution testing, explains the theoretical background for each method and shows practical examples from a real-life drug-design and quality control applications. The textbook is thus intended for both pharmacy students and early career professionals.

In recent years, due to their outstanding dielectric and magnetic properties, hexagonal ferrites (hexaferrites) have attracted considerable interest for developing electronic components of next-generation communication systems. The complex crystal structure of hexaferrites and critical dependences of their electric and magnetic properties on external factors, like magnetic or electric fields, pressure or doping, open ample opportunities for targeted tuning of these properties when designing specific devices. To that end, we explored the electromagnetic properties of the Pb-substituted barium hexaferrite, Ba1-xPbxFe12O19, a compound featuring an extremely rich set of physical phenomena that are inherent in the dielectric and magnetic subsystems of the material and are expected to have significant effect on its electromagnetic response at radio and terahertz frequencies. We performed the first detailed measurements of the AC response of single-crystalline Ba1-xPbxFe12O19 in an extremely broad spectral range from 1 Hz to 240 THz down to temperatures as low as 5 K. We fully characterized numerous microscopic phenomena that determine the broad-band dielectric response of the compound, and we analyzed their nature. This includes temperature-activated radiofrequency relaxations that were attributed to the dynamic response of magnetic/dielectric domains. The terahertz response is dominated by a ferroelectric-like soft mode with an unusual temperature behavior that we explain by means of a microscopic model. Several narrower terahertz excitations are associated with electronic transitions between the fine-structure components of the Fe2+ground state. Narrow resonances detected in the gigahertz region are presumably of magneto-electric origin. The obtained data on diverse but controllable electromagnetic properties of Ba1-xPbxFe12O19 compounds provides the researchers with information that makes the entire class of hexaferrites materials attractive for manufacturing electronic devices for the radiofrequency and terahertz ranges, such as absorbing coatings, anti-reflective coatings, absorbers, electromagnetic shields, antennas, phase shifters, filters, resonators, modulators, etc.

EuTiO 3 is an incipient ferroelectric antiferromagnet showing an anomalously strong spin-phonon coupling. We studied EuTiO 3 ceramics by terahertz (THz) time-domain spectroscopy upon varying temperature and magnetic field. We observed a large anisotropy which is due to the shift from the microwave to the sub-THz region of a ferromagnetic resonance appearing only when the external magnetic field is applied perpendicular to the magnetic component of the THz radiation.

In the antiferromagnetic phase of $\mathrm{Eu}\mathrm{Ti}{\mathrm{O}}_{3}$ ceramics, a seemingly anisotropic magnetodielectric effect (up to 2.5%) was observed via the low-frequency dielectric permittivity measured in external magnetic field below 1.9 T. We explain the effect theoretically by taking into account the demagnetizing field which effectively reduces the internal magnetic flux density by 0.6 T when the external magnetic field is applied perpendicular to the plane of the disk-shaped sample. This finding is also confirmed experimentally by magnetization measurements. The refractive index between 0.2 and 0.5 THz exhibits a large anisotropy in an external magnetic field of up to 7 T. This anisotropy is due to a shift of the ferromagnetic resonance from the microwave to the sub-THz region with external magnetic field applied perpendicular to the magnetic component of the THz radiation. This magnon observation is performed in the strong external magnetic field (above 2 T) and in both paramagnetic and antiferromagnetic phases.

We present results of a multimethod investigation of the polar antiferromagnets $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$, inspired by the colossal magnetoelectric effect present in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Both compounds crystalize in the same polar space group $R3$ as $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$, preserving the crystal symmetry at least from room temperature down to 2 K. $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$ undergo antiferromagnetic phase transitions at ${T}_{\mathrm{N}}=55$ and 52 K, and spin-flop transitions at an external magnetic field of approximately 8 and 4 T, respectively. Both compounds present an incommensurate antiferromagnetic helical structure with spins lying in the $ab$ plane, in contrast to the collinear one along the $c$ axis in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Moreover, dielectric anomalies are observed at their antiferromagnetic phase transitions, suggesting a magnetoelectric behavior. Spin and lattice dynamics studies by a combination of infrared, Raman, and terahertz spectroscopies were performed. Below ${T}_{\mathrm{N}}$, in both $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$, low-frequency spin excitations extremely sensitive to external magnetic field were observed. At least one of these magnons was simultaneously seen in Raman and THz spectra of both compounds, therefore we propose to assign them to electromagnons.

We investigated polarized infrared and Raman spectra of the multiferroic hexaferrites BaSrCoZnFe11AlO22 (Y-type) and (Ba0.5Sr0.5)3Co2Fe24O41 (Z-type) on cooling from room temperature down to 4 K, a...

Employing a combination of infrared, Raman and time-domain THz spectroscopies, we studied spin and lattice excitations in Ni 3 TeO 6 and its isostructural compounds obtained by substitution of Ni with Mn or Co. Low-temperature THz spectra revealed THz-range excitations sensitive to magnetic field; some of them can be assigned to electromagnons.

Multiferroic SrMn 7 O 12 crystals are isostructural with CaMn 7 O 12 which is known for its unusual symmetry properties and a record-high spin-induced polarization. We studied SrMn 7 O 12 ceramics by THz time-domain and Fourier-transform infrared spectroscopies upon varying temperature and magnetic field. In the magnetic phases, we observed spin waves gaining strength from polar phonons; thus, they are assigned to electromagnons. We discuss the observed persistence of several low-temperature vibrational modes above the high-temperature phase transition, and the impact of atomic substitution with regard to CaMn 7 O 12 .

${\mathrm{SrMn}}_{7}{\mathrm{O}}_{12}$ is a recently synthesized homolog of multiferroic ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$. Upon cooling, ${\mathrm{SrMn}}_{7}{\mathrm{O}}_{12}$ undergoes a series of structural and magnetic phase transitions from cubic to rhombohedral symmetry, and to an incommensurately modulated crystal structure, which is connected with charge and orbital ordering of the Mn cations. We report infrared, terahertz, and Raman spectra of ${\mathrm{SrMn}}_{7}{\mathrm{O}}_{12}$ ceramics reflecting corresponding changes in phonon selection rules, including new phonons appearing in spin-order-induced ferroelectric phases. The observed phonon activities are compared with the predictions from the factor-group analysis. In the high-temperature phase, more phonons are observed than the number predicted for the cubic symmetry. This is explained by the presence of rhombohedral clusters in the cubic phase. The strongest variations occur in THz spectra near the two magnetic phase transitions, at ${T}_{\mathrm{N}1}=87\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=63\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. These activate new modes in the spectra, with resonance frequencies and intensities changing with temperature and magnetic field. Below ${T}_{\mathrm{N}2}$, we observed a transfer of oscillator strengths from low-frequency phonons to these excitations, which we assign to electromagnons.

Water is characterized by large molecular electric dipole moments and strong interactions between molecules; however, hydrogen bonds screen the dipole–dipole coupling and suppress the ferroelectric order. The situation changes drastically when water is confined: in this case ordering of the molecular dipoles has been predicted, but never unambiguously detected experimentally. In the present study we place separate H2O molecules in the structural channels of a beryl single crystal so that they are located far enough to prevent hydrogen bonding, but close enough to keep the dipole–dipole interaction, resulting in incipient ferroelectricity in the water molecular subsystem. We observe a ferroelectric soft mode that causes Curie–Weiss behaviour of the static permittivity, which saturates below 10 K due to quantum fluctuations. The ferroelectricity of water molecules may play a key role in the functioning of biological systems and find applications in fuel and memory cells, light emitters and other nanoscale electronic devices., Ferroelectric orders hardly exist in liquid or ice state of water, despite its enormous molecular electrical polarizability. Here, Gorshunov et al. report incipient ferroelectricity in chains of interacting water molecules by placing them in the structural channels of a beryl crystal.

Dielectric and conducting properties of unintentionally doped bulk and Sn-doped thin film β - Ga 2 O 3 samples were studied using time-domain terahertz spectroscopy. Complex permittivity and optical conductivity spectra from 0.25 to 2.5 THz were obtained experimentally over a broad temperature range. The low-temperature spectra of the unintentionally doped sample were fit using a model involving two oscillators. The parameters of one of them show an unusual temperature dependence, in particular, a pronounced increase in the oscillator strength upon heating above 50 K. This is interpreted as an absorption due to thermally activated charge carriers moving in localized potential minima linked to the unintentional doping. Upon heating, the influence of this optical conductivity mechanism strongly increases, and the sample becomes opaque in the THz range near 100 K. The nanocrystalline Sn-doped Ga 2 O 3 thin film sample exhibits a much higher optical conductivity than the unintentionally doped bulk sample, and its spectra are remarkably stable over a broad temperature range (4–750 K). This first study of β- Ga 2 O 3 based on phase-sensitive THz spectroscopy reveals how the impurities influence the high-frequency conductive properties of the material.

We present a structural and spectroscopic study of the compound $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$, closely related to the polar antiferromagnet $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ known to show a colossal magnetoelectric effect and pronounced elementary magnetoelectric excitations. We prepared single crystals and polycrystalline samples of $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ showing the same polar structure as $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ from room temperature down to 4 K with the $R3$ space-group symmetry. Magnetic and dielectric measurements have indicated an antiferromagnetic phase transition at ${T}_{\mathrm{N}}\ensuremath{\approx}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, almost 20 K higher than that of $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Extensive infrared, Raman, and terahertz spectroscopy experiments were employed for investigating lattice and spin excitations, revealing all phonons predicted by the factor group analysis. Terahertz spectra below ${T}_{\mathrm{N}}$ reveal one new excitation, which is strongly influenced by external magnetic field, thus assigned to a magnon.

We investigated static and dynamic magnetoelectric properties of single crystalline ${\mathrm{BaSrCoZnFe}}_{11}{\mathrm{AlO}}_{22}$, which is a room-temperature multiferroic with Y-type hexaferrite crystal structure. Below 300 K, a purely electric-dipole-active electromagnon at $\ensuremath{\approx}1.2$ THz with the electric polarization oscillating along the hexagonal axis was observed by THz and Raman spectroscopies. We investigated the behavior of the electromagnon with applied dc magnetic field and linked its properties to static measurements of the magnetic structure. Our analytical calculations determined selection rules for electromagnons activated by the magnetostriction mechanism in various magnetic structures of Y-type hexaferrite. Comparison with our experiment supports that the electromagnon is indeed activated by the magnetostriction mechanism involving spin vibrations along the hexagonal axis.

The structural phase transition in hexagonal ${\mathrm{BaMnO}}_{3}$ occurring at ${T}_{\mathrm{C}}=130$ K was studied in ceramic samples using electron and x-ray diffraction, second harmonic generation, as well as by dielectric and lattice dynamic spectroscopies. The low-temperature phase (space group $P{6}_{3}cm)$ is ferroelectric with a triplicated unit cell. The phase transition is driven by an optical soft mode from the Brillouin-zone boundary $[q=(\frac{1}{3},\frac{1}{3},0)]$; this mode activates in infrared and Raman spectra below ${T}_{\mathrm{C}}$ and it hardens according to the Cochran law. Upon cooling below ${T}_{\mathrm{C}}$, the permittivity exhibits an unusual linear increase with temperature; below 60 K, in turn, a frequency-dependent decrease is observed, which can be explained by slowing down of ferroelectric domain wall motions. Based on our data we could not distinguish whether the high-temperature phase is paraelectric or polar (space groups $P{6}_{3}/mmc$ or $P{6}_{3}mc$, respectively). Both variants of the phase transition to the ferroelectric phase are discussed based on the Landau theory. Electron paramagnetic resonance and magnetic susceptibility measurements reveal an onset of one-dimensional antiferromagnetic ordering below $\ensuremath{\approx}220\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ which develops fully near 140 K and, below ${T}_{\mathrm{N}}\ensuremath{\approx}59$ K, it transforms into a three-dimensional antiferromagnetic order.

We have measured the terahertz–infrared (3–7000 cm−1) spectra of the optical conductivity of iron-doped single crystals of beryl, (Mn,Fe):Be3Al2Si6O18, that contain lone water molecules isolated within nanometer-sized cages formed by the ions of beryl crystal lattice. By comparing the spectra with those of dehydrated crystals, we exclude phonon resonances and reconstruct the spectra determined exclusively by vibrations of the water molecules. At liquid-helium temperatures, well-known intramolecular H2O modes are observed above 1000 cm−1 and accompanied with satellite resonances that are combinations of intramolecular and external vibrations of H2O molecules. At terahertz frequencies, a broad bump centred around 20 cm−1 (at 5 K) is observed with three rather narrow resonances at its high-frequency shoulder (38, 42 and 46 cm−1). The origin of these low-energy excitations is discussed.

We performed spectroscopic studies of water-containing beryl crystals. At terahertz frequencies, we observed a strongly temperature-dependent spectral feature, corresponding to a collective excitation caused by dipole-dipole interactions between separate H2O molecules. We associate the excitation with a soft mode signaling an incipient ferroelectric state within the water molecular subsystem. Single-particle excitations of nanocaged H2O molecules are also observed at terahertz and infrared frequencies; using DFT analysis, their origin is assigned to translational and librational vibrational states.

Mixed perovskites Sr 1−x Ba x MnO 3 were lately indicated as possible multiferroics with a strong magnetoelectric coupling. However, THz spectroscopy with varying temperature and magnetic field revealed no magnetodielectric effect. Our study brings new information on the ferroelectric phase transition.

We studied experimentally the high-temperature magnetoelectric ${({\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x})}_{3}{\mathrm{Co}}_{2}{\mathrm{Fe}}_{24}{\mathrm{O}}_{41}$ prepared as ceramics ($x=0$, 0.2) and a single crystal ($x=0.5$) using inelastic neutron scattering, THz time-domain, Raman, and far-infrared spectroscopies. The spectra, measured with varying temperature and magnetic field, reveal rich information about the collective spin and lattice excitations. In the ceramics, we observed an infrared-active magnon which is absent in ${\mathbf{E}}^{\ensuremath{\omega}}\ensuremath{\perp}z$ polarized THz spectra of the crystal, and we assume that it is an electromagnon active in ${\mathbf{E}}^{\ensuremath{\omega}}\ensuremath{\parallel}z$ polarized spectra. On heating from 7 to 250 K, the frequency of this electromagnon drops from 36 to $25\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and its damping gradually increases, so it becomes overdamped at room temperature. Applying external magnetic field has a similar effect on the damping and frequency of the electromagnon, and the mode is no longer observable in the THz spectra above 2 T, as the transverse-conical magnetic structure transforms into a collinear one. Raman spectra reveal another spin excitation with a slightly different frequency and much higher damping. Upon applying magnetic field higher than 3 T, in the low-frequency part of the THz spectra, a narrow excitation appears whose frequency linearly increases with magnetic field. We interpret this feature as the ferromagnetic resonance.

Dielectric response of perovskite Sr1-xBaxMnO3 (x = 0.43 and 0.45) ceramics was investigated using microwave, THz and infrared spectroscopic techniques in order to study the ferroelectric and antiferromagnetic phase transitions with critical temperatures TC ≈ 350 K and TN ≈ 200 K, respectively. The two lowest-frequency polar phonons are overdamped above TN and they exhibit pronounced softening on heating towards TC. Nevertheless, permittivity e' in the THz range shows only a small anomaly at TC because the phonon contribution to e' is rather small. The phonons are coupled with a central mode which provides the main contribution to the dielectric anomaly at TC. Thus, the ferroelectric phase transition has characteristics of a crossover from displacive to order-disorder type. At the same time, the intrinsic THz central peak is partially screened by conductivity and related Maxwell-Wagner relaxation, which dominates the microwave and lower-frequency spectra. Below TN, the ferroelectric distortion markedly decreases, which has an influence on the frequencies of both the central and soft modes. Therefore, e' in the THz range increases at TN on cooling. In spite of the strong spin-phonon coupling near TN, surprisingly no magnetodielectric effect was observed in the THz spectra upon applying magnetic field of up to 7 T, which is in contradiction with the theoretically expected huge magnetoelectric coupling. We explain this fact as due to the insensitivity of TN to magnetic field.

Time-domain terahertz spectroscopy was used to study the dielectric and conductive properties of thin films of four phase-change materials: GeTe, GeSb2Te4, GeSbTe2, and AgInSbTe. Both amorphous and crystalline phases were studied, and the spectra were analyzed by a model including a harmonic oscillator and the Drude term. Spectra in the crystalline phase of AgInSbTe are dominated by free-carrier motion with a scattering time of 50 fs. In the Ge-containing compounds, we observed a phonon mode and a conductive contribution of free charge carriers with a much shorter scattering time. The conductivity appears to be linked to the distortions of the crystal unit cell from cubic symmetry.

We report on results of a near-field imaging experiment with broadband terahertz pulses applied to a BaTiO3 multidomain single crystal. Data with a spatial resolution below 15 µm were collected by our scanning time-domain terahertz imaging setup. We performed the scanning by an asymmetric skewed sapphire probe metallised from two sides ended with a 60 µm × 140 µm facet. The imaging scans were performed with various angles of the polarisation of the probing field with respect to the direction of the domain walls in order to identify the orientations with optimal sensitivity. The raw experimental data were prewhitened and processed by the singular value decomposition. In this way, independent components belonging to experimental features (polarisation, noise and external perturbations) were found. The spatial resolution of the sensing probe was found significantly higher than that allowed by the outer dimensions of the probe end facet. This result was also confirmed by electromagnetic-field simulations.

We demonstrate a metamaterial exhibiting a frequency-tunable response in the terahertz domain, controlled by a bias electric field. The active part of the metamaterial consists of a periodic metallic pattern deposited on a thin epitaxially strained strontium titanate film. The role of the metallic structure is two-fold: it gives rise to the metamaterial resonance and it enables applying an electric bias to the strontium titanate layer. The strained film exhibits a pronounced dependence of its permittivity on the bias, which exerts a strong influence on the resonance. Specifically, the resonance of our structure occurs near 0.5 THz and, upon applying a bias voltage of 55 V, a relative tunability of the resonance frequency of 19% was achieved at room temperature.

Terahertz (THz) time-domain spectroscopy was used in vivo to measure the body reflectance with the aim of determining experimentally the influence of haemodialysis on the content of water in the human skin. For this purpose, an original methodology of determining the skin properties at THz frequencies from the reflectance was developed. A series of measurements were performed before and after dialysis with ten subjects. The results strongly indicate that the surface body hydration is not the main parameter determining the skin conductivity in the THz range.

We review recent results of both experimental and theoretical work aimed at developing tunable devices with optical properties in the THz range controllable by an external parameter. The presented devices include a semiconductor platelet, strontium titanate based elements and one-dimensional photonic crystals with defects. We show that it is possible to strongly modulate the complex permittivity in the THz range by light illumination, temperature and voltage, and the experimental results obtained by time-domain terahertz spectroscopy are found to be in a very good agreement with theoretical calculations. The control by illumination or applied bias enable a very fast modulation of the THz waves which is useful for prospect applications e.g. in the domain of communications. To cite this article: P. Kužel, F. Kadlec, C. R. Physique 9 (2008).

THz, infrared and inelastic neutron scattering spectra of Z-hexaferrite ceramics were measured in a broad range of temperatures. A sharp electromagnon is observed near 1THz, vanishing with increasing magnetic field until a phase transition occurs at H ≈ 2T. Then, another strong peak whose resonance frequency is linearly proportional to the magnetic field turns up from the sub-THz range. We assign this peak to a ferrimagnetic resonance.

Broadband (1 Hz to 100 THz) dielectric response of water-containing beryl crystals was measured at temperatures from 300 K down to 0.3 K. The low-energy dynamics of H 2 O molecules confined within nano-cages of the crystalline lattice was studied. We observe a THz-range soft mode indicating an incipient ferroelectric behavior due to interacting dipole moments of confined water molecules.

International audience; 2 count the nearest and next-nearest neighbor exchange interactions, two Dzyaloshinskii-Moriya interactions and an easy-axis anisotropy. 31,35 The same model successfully described the low-energy inelastic neutron scattering spectra. 30 In contrast, Komandin et al. 15 showed IR transmission spectra with an excitation at approximately 47 cm −1 which had not been previously reported by experimental or theoretical studies. The intensive discussion in the literature concerning the nature of the spin ex-citations raises the question: which excitations are pure magnons (i.e., contribute only to the magnetic perme-ability µ) and which are electromagnons (i.e., influence at least partially the permittivity ε *)? It is worth noting that according to a recent symmetry analysis, 36 BiFeO 3 allows directional dichroism and therefore spin waves can be simultaneously excited by the electric and magnetic components of electromagnetic radiation. For more details on BiFeO 3 spin dynamics, see the review of Park et al. 37 In the current work, we report spin and lattice exci-tations in BiFeO 3 ceramics, as measured by the combination of IR reflectivity and time-domain THz transmission spectroscopy, in a temperature range from 10 to 900 K. All 13 IR-active phonon modes are observed, exhibiting softening on heating. Five low-frequency spin modes are detected from 5 K up to RT, the highest two appearing at 53 and 56 cm −1. This corresponds to the frequency range where such excitations were theoretically predicted, 31,32,35 but not experimentally confirmed up to now. At 5 K, the low-energy spin dynamics in the THz range were also studied in a varying magnetic field of up to 7 T. Softening of the (electro)magnon frequencies upon increasing the magnetic field was observed. Additionally, a BiFeO 3 epitaxial thin film grown on an orthorhombic (110) TbScO 3 single crystal substrate was studied for the first time via IR reflectance spectroscopy. II. EXPERIMENTAL DETAILS BiFeO 3 ceramics were prepared by the solid-state route. A stoichiometric mixture of Fe 2 O 3 and Bi 2 O 3 powder oxides with a purity of 99.99% was ground and uniaxially cold-pressed under 20-30 MPa pressure into 8 mm diameter pellets. The pellets were then covered by sacrificial BFO powder to avoid bismuth oxide loss from the pellet, and sintered in a tube furnace at 825 • C for8 h in air. To avoid any secondary phase formation, the samples were quenched to RT. Polished disks with a diameter of 4 mm and thicknesses of ca. 600 and 338 µm were used for the IR reflectivity and THz transmission measurements, respectively. An epitaxial BiFeO 3 thin film with a thickness of 300 nm was grown by reactive molecular-beam epitaxy on a (110) single crystal substrate. The growth parameters were the same as for the samples reported in Ref. 38. Near-normal incidence IR reflectivity spectra of the BiFeO 3 ceramics and film were obtained using a Fourier-transform IR spectrometer Bruker IFS 113v in the frequency range 20-3000 cm −1 (0.6-90 THz) at RT; for the low and high temperature measurements the spectral range was reduced to 650 cm −1. Pyroelectric deuterated triglycine sulfate detectors were used for the room-and high-temperature measurements up to 900 K, whereas a He-cooled (operating temperature 1.6 K) Si bolometer was used for the low-temperature measurements down to 10 K. A commercial high-temperature cell (SPECAC P/N 5850) was used for the high-temperature experiments. The thermal radiation from the hot sample entering the interferometer was taken into account in our spectra evaluation. THz measurements from 3 cm −1 to 60 cm −1 (0.09-1.8 THz) were performed in the transmission mode with the use of a custom-made time-domain terahertz spectrometer. In this spectrometer, a femtosecond Ti:sapphire laser oscillator (Coherent, Mira) produces a train of femtosecond pulses which generates linearly polarized broadband THz pulses in a photoconduct-ing switch TeraSED (Giga-Optics). A gated detection scheme based on electrooptic sampling with a 1 mm thick [110] ZnTe crystal as a sensor allows us to measure the time profile of the electric field of the transmitted THz pulse. The same high-temperature cell as for the IR reflectivity was used for the THz range high-temperature measurements. Oxford Instruments Opti-stat optical cryostats with mylar and polyethylene windows were used for the low-temperature THz and IR measurements , respectively. THz experiments in an external magnetic field H ext ≤ 7 T were performed upon decreasing H with an Oxford Instruments Spectromag cryostat in the Voigt configuration, where the electric component of the THz radiation E THz was set perpendicular to H ext. The IR reflectivity and THz complex (relative) per-mittivity spectra ε * (ω) were carefully fit assuming the factorized form of the dielectric function based on a generalized damped-harmonic-oscillator model: 39 ε * (ω) = ε ∞ N j=1 ω 2 LOj − ω 2 + iωγ LOj ω 2 TOj − ω 2 + iωγ TOj , (1) where ω TOj and ω LOj are the frequencies of the j-th transverse optical (TO) and longitudinal optical (LO) phonons, γ TOj and γ LOj are the corresponding damping constants, and ε ∞ denotes the high-frequency (elec-tronic) contribution to the permittivity, determined from the RT frequency-independent reflectivity tail above the phonon frequencies. The reflectivity R(ω) is related to the complex dielectric function ε * (ω) by: R(ω) = ε * (ω) − 1 ε * (ω) + 1. (2) To evaluate the IR reflectance spectra of the BiFeO 3 /TbScO 3 thin film, a model corresponding to a two-layer optical system was used. 40 The IR reflectivity spectra of the bare TbScO 3 substrate were fit first at each

The knowledge of the surface resistance R s of superconducting thin film at microwave and terahertz (THz) regions is significant to design, make and assess superconducting microwave and THz electronic devices. In this paper we reported the R s of MgB2 films at microwave and THz measured with sapphire resonator technique and the time-domain THz spectroscopy, respectively. Some interesting results are revealed in the following: (1) A clear correlation is found between R s and normal-state resistivity right above T c, ρ0, i.e., R s decreases almost linearly with the decrease of ρ0. (2) A low residual R s, less than 50 μΩ at 18 GHz is achieved by different deposition techniques. In addition, between 10 and 14 K, MgB2 has the lowest R s compared with two other superconductors Nb3Sn and the high-temperature superconductor YBa2Cu3O7−δ(YBCO). (3) From THz measurement it is found that the R s of MgB2 up to around 1 THz is lower than that of copper and YBCO at the temperature below 25 K. (4) The frequency dependence of R s follows ω n , where ω is angular frequency, and n is power index. However, n changes from 1.9 at microwave to 1.5 at THz. The above results clearly give the evidences that MgB2 thin film, compared with other superconductors, is of advantage to make superconducting circuits working in the microwave and THz regions.

We present a method for simultaneous determination of the refractive index and wave impedance – or equivalently of the dielectric permittivity and magnetic permeability – of bulk samples. Two independent complex spectroscopic quantities required for an unambiguous evaluation are experimentally obtained by temporal windowing of time domain waveforms measured in the transmission or reflection geometry. We discuss several approaches that can be used for the evaluation of the complex refractive index and wave impedance; three are quantitatively analyzed and compared. Successful evaluation of the dielectric and magnetic dispersion then crucially depends on the accuracy of the wave impedance measurements.

Terahertz (THz) transmissivity and infrared (IR) reflectivity spectra of orthorhombic microwave (MW) ceramics Bi(Nb 1− x V x )O 4 (0.002 x −1 (0.09–90 THz) at room temperature. A well underdamped mode, presumably the ferroelectric soft mode, was observed at 25 cm −1 . Complex permittivity spectra obtained from the fits to our data were extrapolated down to the MW range and compared with the dielectric data near 5 GHz. The linear extrapolation of dielectric losses from THz down to the MW range is in agreement with the experimental MW losses. Addition of 3.2% of vanadium reduces the sintering temperature to 850 °C and the dielectric properties ( ɛ ′ = 42.2, Q · f = 14,000 GHz, τ f = +10 ppm/°C) remain at a level satisfactory for MW applications. Somewhat lower MW losses were observed in a sample sintered in the N 2 atmosphere.

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.

Time-domain terahertz transmission and reflection spectroscopies are described and compared. New results obtained in SrBi2Ta2O9 ceramics and thin film are presented and discussed. A central-mode type dispersion was detected near 10 cm-1 which should be responsible for a strong dielectric anomaly at the ferroelectric phase transition. * Paper originally presented at 10th European meeting on Ferroelectricity, Cambridge, U.K., August 3–8, 2003.

An approach to time-domain terahertz reflection spectroscopy is proposed and demonstrated. It allows one to obtain very accurately the relative phase of a reflected THz wave form, and consequently the complex dielectric function can be precisely extracted. The relevant setup was demonstrated to allow measurements of a variety of samples: we present results for doped silicon and for ferroelectric SrBi2Ta2O9 (bulk ceramics as well as thin film on sapphire substrates).

Room-temperature infrared reflectivities, time-domain THz transmission spectra and micro-Raman spectra of ceramics in the Ruddlesden–Popper homologous series, Srn+1TinO3n+1 (where n=1, 2, 3, 4, ∞), were studied. Increasing the number of perovskite layers (n) in the crystal system resulted in higher polarizability of the lattice and softening of the lowest frequency phonon mode, which is responsible for the observed increase in microwave permittivity, dielectric loss and temperature coefficient of resonance frequency with n. Although the infrared and Raman spectra revealed many defects in the ceramics (e.g. second phases and stacking faults), the extrapolation of experimental submillimetre dielectric data to the microwave region showed no dielectric dispersion below the phonon frequencies and prevailingly an intrinsic origin of losses in the microwave region in almost all samples (except for Sr5Ti4O13).

Time-resolved optical pump-terahertz (THz) probe experiments are currently used to obtain information about the ultrafast dynamics of photoexcited carriers in semiconductors and about the far-infrared nonlinear response during solvation in liquids. The THz dynamics in such photoexcited systems is fully characterized by a two-dimensional nonlinear susceptibility. We have developed a frequency-domain analytical method for the direct extraction of this susceptibility from the experimental data. Following effects are taken into account: dispersive propagation of radiation in a photoexcited medium, refraction on its surfaces, THz sensor responsivity, and spatio-temporal transformations of the THz pulses. Strategies for possible experiments are discussed.

We present a microscopic model, describing the THz conductivity of semiconductors with nanosized grains. The model combines free intra-grain motion and hopping on grain boundaries, and it is verified by fitting experimental spectra of phase-change materials and extracting the underlying parameters.

Spin and lattice dynamics of CaMn7O12 ceramics were investigated using infrared, THz and inelastic neutron scattering (INS) spectroscopies in the temperature range 2 to 590 K, and, at low temperatures, in applied magnetic fields of up to 12 T. On cooling, we observed phonon splitting accompanying the structural phase transition at Tc = 450K as well as the onset of the incommensurately modulated structure at 250 K. In the two antiferromagnetic phases below T_N1 = 90K and T_N2 = 48 K, several infrared-active excitations emerge in the meV range; their frequencies correspond to the maxima in the magnon density of states obtained by INS. At the magnetic phase transitions, these modes display strong anomalies and for some of them, a transfer of dielectric strength from the higher-frequency phonons is observed. We propose that these modes are electromagnons. Remarkably, at least two of these modes remain active also in the paramagnetic phase; for this reason, we call them paraelectromagnons. In accordance with this observation, quasielastic neutron scattering revealed short-range magnetic correlations persisting within temperatures up to 500K above T_N1.

International audience

We demonstrate the possibility to tune the permittivity of thin films of strontium titanate by means of an electric field. We propose to deposit on top of such films a metallic metamaterial structure designed to resonate in the terahertz range. The amplitude and frequency of this resonance will then be tunable by applying a bias.