We report the values and the spectral dependence of the real and imaginary parts of the dielectric permittivity of semi-insulating Fe-doped InP crystalline wafers in the 2-700 cm
(0.06-21 THz) ...spectral region at room temperature. The data shows a number of absorption bands that are assigned to one- and two-phonon and impurity-related absorption processes. Unlike the previous studies of undoped or low-doped InP material, our data unveil the dielectric properties of InP that are not screened by strong free-carrier absorption and will be useful for designing a wide variety of InP-based electronic and photonic devices operating in the terahertz spectral range.
In this review, we present analysis of absorption and circular dichroism spectra of disordered langasite crystals with the calcium gallium germanate structure, whose structures have no screw axis. We ...consider results of original research concerned with spectroscopic characteristics of undoped and transition metal-doped langasite crystals. Structural disordering on a site leads to charge disbalance on this site and defect formation. Electron transitions to “defect” states contribute to both absorption and circular dichroism spectra. In this situation, a transition metal ion can be incorporated into a “defect” site in different oxidation states, depending on the type of position and the impurity ion. It has been shown that electric dipole forbidden but magnetic dipole allowed electron transitions of impurity ions of
d
-block elements have the highest intensity in the circular dichroism spectra of doped crystals.
A novel type of sub-lattice of the Jahn-Teller (JT) centers was arranged in Ti-doped barium hexaferrite BaFe
O
. In the un-doped crystal all iron ions, sitting in five different crystallographic ...positions, are Fe
in the high-spin configuration (S = 5/2) and have a non-degenerate ground state. We show that the electron-donor Ti substitution converts the ions to Fe
predominantly in tetrahedral coordination, resulting in doubly-degenerate states subject to the Formula: see text problem of the JT effect. The arranged JT complexes, Fe
O
, their adiabatic potential energy, non-linear and quantum dynamics, have been studied by means of ultrasound and terahertz-infrared spectroscopies. The JT complexes are sensitive to external stress and applied magnetic field. For that reason, the properties of the doped crystal can be controlled by the amount and state of the JT complexes.
Abstract
Single-crystalline lead substituted M-type barium hexaferrites doped with different concentrations of Al
3+
, synthesized by flux technique are investigated at terahertz and infrared ...frequencies (8-8000 cm
−1
). The spectra of reflection coefficient (R), transmission coefficient (T), complex dielectric permittivity (
ε
` and
ε
``) were obtained using terahertz time-domain, and infrared Fourier transform spectrometers over a broadband frequency range 0.24-240 THz and at temperatures 5 K-300 K. The observed absorption lines are assigned to the electronic transitions within the fine-structured ground state of Fe
2+
ions at terahertz frequencies and to optical phonon mode at far-infrared frequencies. To analyze the origin of terahertz excitations in the compounds, we provided a model that accounts for the second-order spin-orbit interactions, the triagonal distortion of the crystal field, and the selection rules of the (C
3v
) point group symmetry of tetrahedral site-position of Fe
2+
.
The broad-band reflection spectra of YB
6
and YbB
6
hexaborides with Jahn–Teller instability of the boron cage have been measured at room temperature. An optical conductivity analysis has revealed, ...along with the Drude electronic components, heavily overdamped collective modes, which are notable in YB
6
for high dielectric contributions, Δε = 2000–5700. The fraction of nonequilibrium charge carriers in YB
6
, which is at the boundary of structural instability in the hexaboride family, reaches 85–90%, whereas this fraction in doped YbB
6
semiconductor is not higher than 25%. It has been shown that unlike the predictions of the topological Kondo insulator model, the surface “metallization” in Yb
2+
B
6
crystals can be explained by additional doping of a surface layer with Yb
3+
ions.
The BaFe12-x (Ti/Mn/Ga/In)xO19 (x = 1–7) high-entropy oxides (HEOs) were obtained by solid-phase synthesis. The correlation of the chemical composition (the level of the Fe replacement), structural ...parameters, magnetic characteristics, and terahertz (THz) properties was investigated. All studied samples were single-phase (SG: P63/mmc). Lattice parameters showed a monotonic increasing trend with an increase of (x). The key role in increasing the lattice parameters of HEOs may belong to the influence of In3+ ions, which are much larger in size than Fe3+ ions. As one of the possible reason for explanation of the lattice parameters behavior we hypothesized that partial charge transformation of the Mn3+ state to the Mn2+ state takes place in order to maintain electrical neutrality. Other cations (Ti/Ga/In) have stable oxidation state. The anisotropic nature of the increase in lattice parameters was demonstrated. No strong correlation between chemical composition and microstructural parameters of the investigated HEOs was observed. The magnetic characteristics of the BaFe12-x (Ti/Mn/Ga/In)xO19 (x = 1–7) HEOs were investigated at broad intervals of magnetic fields and temperatures. The behavior of the magnetic characteristics was shown to be the result of the magnetic structure frustration. In addition, terahertz electrodynamic properties were studied by measuring spectra of complex dielectric permittivity at frequencies 0.2–1.2 THz in the temperature interval 20–300 K. The spectra indicate the presence of a higher-frequency infrared phonon resonance, whose damping decreases with cooling, as well as an excitation below 0.2 THz that freezes out at low temperatures, the origin of which is associated with the polycrystalline nature of the materials.
Abstract
Using methods of terahertz time-domain spectroscopy dielectric response of ceramic barium hexaferrites substituted with lead (Ba1
-x
PbxFe
12
O
19
, x = 0.00-0.30) was studied in the ...frequency range of 3-110 cm
1
and at temperatures from 5 to 300 K. Obtained spectra are presented by a rich set of lines of different nature, i.e. excitations associated with electronic transitions within the fine-structure components of Fe
2+
ions, A
2u
soft optical phonon, and ferroelectric-like soft mode. The frequency of the soft mode reveals power-law temperature variation
v
SM
∼ (T
-
T
c
)
025
, which indicates a potential phase transition atr
c
. Analysis shows that
T
c
approaches zero for the concentrations x = 0.20-0.25.
Abstract
Due to their outstanding dielectric and magnetic properties, hexaferrites are attracting ever-increasing attention for developing electronic components of next-generation communication ...systems. The complex crystal structure of hexaferrites and the critical dependences of their electric and magnetic properties on external factors, such as magnetic/electric fields, pressure, and doping, open ample opportunities for targeted tuning of these properties when designing specific devices. Here we explored the electromagnetic properties of lead-substituted barium hexaferrite, Ba
1−
x
Pb
x
Fe
12
O
19
, a compound featuring an extremely rich set of physical phenomena that are inherent in the dielectric and magnetic subsystems and can have a significant effect on its electromagnetic response at terahertz frequencies. We performed the first detailed measurements of the temperature-dependent (5–300 K) dielectric response of single-crystalline Ba
1−
x
Pb
x
Fe
12
O
19
in an extremely broad spectral range of 1 Hz–240 THz. We fully analyzed numerous phenomena with a corresponding wide distribution of specific energies that can affect the terahertz properties of the material. The most important fundamental finding is the observation of a ferroelectric-like terahertz excitation with an unusual temperature behavior of its frequency and strength. We suggest microscopic models that explain the origin of the excitation and its nonstandard temperature evolution. Several narrower terahertz excitations are associated with electronic transitions between the fine-structure components of the Fe
2+
ground state. The discovered radio-frequency relaxations are attributed to the response of magnetic domains. Gigahertz resonances are presumably of magnetoelectric origin. The obtained data on diverse electromagnetic properties of Ba
1−
x
Pb
x
Fe
12
O
19
compounds provide information that makes the entire class of hexaferrites attractive for manufacturing electronic devices for the terahertz range.
Recent theoretical studies predict the suppressed ferroelectric instability in orthorhombic Pnma perovskites and experimental evidence is due. We observed significant softening at cooling of the ...lowest-frequency polar phonon at the Brillouin zone center in the Pnma antiferromagnetic fluoroperovskite NaMnF3 that is the direct proof of the theoretically predicted ferroelectric instability. In contrast to oxides where the hybridization plays the dominant role, the effective ionic charges in fluoroperovskites are close to their nominal valencies that confirms the geometric origin of the observed incipient ferroelectricity. Furthermore, below the Néel temperature, the softening phonon clearly shows a strong coupling with the magnetic subsystem as a result of dynamical modulation of the superexchange interaction. Our findings clarify microscopic mechanisms of the incipient multiferroicity in the Pnma fluoroperovskites and reveal still unexplored opportunities of this class of materials for further research and potential applications.
The BaFe
12–
x
(Ti,Mn,In,Ga)
x
O
19
(
x
= 1,
x
= 7) HEOs are prepared by a solid-phase synthesis. Correlations between the chemical composition (degree of Fe substitution), structural parameters, ...and magnetic characteristics are studied. Magnetic characteristics of the BaFe
12–
x
(Ti,Mn,In,Ga)
x
O
19
(
x
= 1,
x
= 7) HEOs are studied in a wide range of fields and temperatures. The measured IR reflectance spectra of the samples contain features caused by phonon vibrations. The dependence of characteristics of phonon lines on the concentration
x
is qualitatively analyzed; the obtained spectra are compared with those of a single-crystal BaFe
12
O
19
sample.
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