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Pyroelectric current and field-dependent specific heat measurements on polycrystalline samples of the quadruple perovskite (LaMn3)Mn4O12 give evidence of ferroelectricity driven by the canted antiferromagnetic ordering of the B-site Mn3+ ions at T N , B = 78 K with record large remnant electric polarization up to 0.56 μC cm−2. X-ray diffraction measurements indicate an anomalous behavior of the monoclinic β angle at T N , B , which suggests that the polarization lies in the ac-plane, where the moments are collinear, and that symmetric exchange striction is the mechanism of spin-driven ferroelectricity. Polarization values up to ∼3–6 μC cm−2 are expected in single crystals or epitaxial films, which would enable the development of practical multiferroic applications. [ABSTRACT FROM AUTHOR]

Pyroelectric current and field-dependent specific heat measurements on polycrystalline samples of the quadruple perovskite (LaMn3)Mn4O12 give evidence of ferroelectricity driven by the canted antiferromagnetic ordering of the B-site Mn3+ ions at T N , B = 78 K with record large remnant electric polarization up to 0.56 μC cm−2. X-ray diffraction measurements indicate an anomalous behavior of the monoclinic β angle at T N , B , which suggests that the polarization lies in the ac-plane, where the moments are collinear, and that symmetric exchange striction is the mechanism of spin-driven ferroelectricity. Polarization values up to ∼3–6 μC cm−2 are expected in single crystals or epitaxial films, which would enable the development of practical multiferroic applications. [ABSTRACT FROM AUTHOR]

In the quest for thinner and more efficient ferroelectric devices, Hf 0.5 Zr 0.5 O 2 (HZO) has emerged as a potential ultrathin and lead-free ferroelectric material. Indeed, when deposited on a TiN electrode, 1-25 nm thick HZO exhibits excellent ferroelectricity capability, allowing the prospective miniaturization of capacitors and transistor devices. To investigate the origin of ferroelectricity in HZO thin films, we conducted a far-infrared (FIR) spectroscopic study on 5 HZO films with thicknesses ranging from 10 to 52 nm, both within and out of the ferroelectric thickness range where ferroelectric properties are observed. Based on X-ray diffraction, these HZO films are estimated to contain various proportions of monoclinic (m-), tetragonal (t-), and polar orthorhombic (polar o-) phases, while only the 11, 17, and 21 nm thick are expected to include a higher amount of polar o-phase. We coupled the HZO infrared measurements with DFT simulations for these m-, t-, and polar o-crystallographic structures. The approach used was based on the supercell method, which combines all possible Hf/Zr mixed atomic sites in the solid solution. The excellent agreement between measured and simulated spectra allows assigning most bands and provides infrared signatures for the various HZO structures, including the polar orthorhombic form. Beyond pure assignment of bands, the DFT IR spectra averaging using a mix of different compositions (e.g., 70% polar o-phase +30% m-phase) of HZO DFT crystal phases allows quantification of the percentage of different structures inside the different HZO film thicknesses. Regarding the experimental data analysis, we used the spectroscopic data to perform a Kramers-Kronig constrained variational fit to extract the optical functions of the films using a Drude-Lorentz-based model. We found that the ferroelectric films could be described using a set of about 7 oscillators, which results in static dielectric constants in good agreement with theoretical values and previously reported ones for HfO 2 -doped ferroelectric films.

We present a high-pressure investigation of the semiconductor-to-metal transition in MoS 2 and WS 2 carried out by synchrotron-based far-infrared spectroscopy, to reconcile the controversial estimates of the metallization pressure found in the literature and gain new insight into the mechanisms ruling this electronic transition. Two spectral descriptors are found indicative of the onset of metallicity and of the origin of the free carriers in the metallic state: the absorbance spectral weight, whose abrupt increase defines the metallization pressure threshold, and the asymmetric line shape of the E 1 u peak, whose pressure evolution, interpreted within the Fano model, suggests the electrons in the metallic state originate from n-type doping levels. Combining our results with those reported in the literature, we hypothesize a two-step mechanism is at work in the metallization process, in which the pressure-induced hybridization between doping and conduction band states drives an early metallic behavior, while the band gap closes at higher pressures.

A new optical setup is described that allows the reflectivity at grazing incidence to be measured, including ultrathin films and two-dimensional electron systems (2DES) down to liquid-helium temperatures, by exploiting the Berreman effect and the high brilliance of infrared synchrotron radiation. This apparatus is well adapted to detect the absorption of a 2DES of nanometric thickness, namely that which forms spontaneously at the interface between a thin film of LaAlO 3 and its SrTiO 3 substrate, and to determine its Drude parameters.

By means of single-crystal X-ray diffraction, we give direct crystallographic evidence of a centrosymmetry breaking below T S = 200 K, concomitant with the onset of a commensurate structural modulation in the quadruple perovskite YMn 3 Mn 4 O 12 . This result, which explains the anomalously large thermal coefficient of the Y 3+ ion in previously reported structural models, is attributed to the small size of the Y 3+ ion, which causes its underbonding within the dodecahedral coordination polyhedron. The present data are consistent with a commensurate superstructure described by an I-centered pseudo-orthorhombic cell with polar Ia symmetry and a ≈ a F √2 = 10.4352(7) Å, b ≈ 2 b F = 14.6049(9) Å, c ≈ c F √2 = 10.6961(7) Å, and β = 90.110(3)°, where a F ≈ c F ≈ 7.45 Å, b F ≈ 7.34 Å, and β ≈ 91° are the unit cell parameters of the I 2/ m structure observed at room temperature. Consistent with the above polar structure, at lower temperature, T * = 70 K, we observe in polycrystalline samples an anomaly of the direct current (DC) and alternating current (AC) magnetization, concomitant with the appearance of a net electric polarization, as indicated by pyrocurrent and dielectric constant measurements. These results, complemented by electrical transport measurements, suggest a magnetic ferroelectricity driven by short-range magnetic order in YMn 3 Mn 4 O 12 .