Grazing-angle reflectivity setup for the low-temperature infrared spectroscopy of two-dimensional systems

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 LaAlO3 and its SrTiO3 substrate, and to determine its Drude parameters

Lattice dynamics and spin excitations in the metal-organic framework [CH3_3NH3_3][Co(HCOO)3_3]

In metal-organic-framework (MOF) perovskites, both magnetic and ferroelectric orderings can be readily realized by compounding spin and charge degrees of freedom. The hydrogen bonds that bridge the magnetic framework and organic molecules have long been thought of as a key in generating multiferroic properties. However, the underlying physical mechanisms remain unclear. Here, we combine neutron diffraction, quasielastic and inelastic neutron scattering, and THz spectroscopy techniques to thoroughly investigate the dynamical properties of the multiferroic MOF candidate [CH$_3$NH$_3$][Co(HCOO)$_3$] through its multiple phase transitions. The wide range of energy resolutions reachable by these techniques enables us to scrutinize the coupling between the molecules and the framework throughout the phase transitions and interrogate a possible magnetoelectric coupling. Our results also reveal a structural change around 220 K which may be associated with the activation of a nodding donkey mode of the methylammonium molecule due to the ordering of the CH$_3$ groups. Upon the occurrence of the modulated phase transition around 130 K, the methylammonium molecules undergo a freezing of its reorientational motions which is concomitant with a change of the lattice parameters and anomalies of collective lattice vibrations. No significant change has been however observed in the lattice dynamics around the magnetic ordering, which therefore indicates the absence of a substantial magneto-electric coupling in zero-field

Broadband infrared study of pressure-tunable Fano resonance and metallization transition in 2H-[Formula: see text]

High pressure is a proven effective tool for modulating inter-layer interactions in semiconducting transition metal dichalcogenides, which leads to significant band structure changes. Here, we present an extended infrared study of the pressure-induced semiconductor-to-metal transition in 2H-MoTe2, which reveals that the metallization process at 13-15 GPa is not associated with the indirect bandgap closure, occurring at 24 GPa. A coherent picture is drawn where n-type doping levels just below the conduction band minimum play a crucial role in the early metallization transition. Doping levels are also responsible for the asymmetric Fano line-shape of the E-1u infrared-active mode, which has been here detected and analyzed for the first time in a transition metal dichalcogenide compound. The pressure evolution of the phonon profile under pressure shows a symmetrization in the 13-15 GPa pressure range, which occurs simultaneously with the metallization and confirms the scenario proposed for the high pressure behaviour of 2H-MoTe2

Centrosymmetry Breaking and Ferroelectricity Driven by Short-Range Magnetic Order in the Quadruple Perovskite (YMn3)Mn4O12

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 YMn3Mn4O12. This result, which explains the anomalously large thermal coefficient of the Y3+ ion in previously reported structural models, is attributed to the small size of the Y3+ 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 approximate to a(F)root 2 = 10.4352(7) angstrom, b approximate to 2b(F) = 14.6049(9) angstrom, c approximate to c(F)root 2 = 10.6961(7) angstrom, and beta = 90.110(3)degrees, where a(F) approximate to c(F) approximate to 7.45 angstrom, b(F) approximate to 7.34 angstrom, and beta approximate to 91 degrees are the unit cell parameters of the I2/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 YMn3Mn4O12

Lattice dynamics of BaFe2Se3

International audienceAbstract This paper presents a study of the lattice dynamics in BaFe 2 Se 3 . We combined first-principle calculations, infrared measurements and a thorough symmetry analysis. Our study confirms that Pnma cannot be the space group of BaFe 2 Se 3 , even at room temperature. The phonons assignment requires Pm to be the BaFe 2 Se 3 space group, not only in the magnetic phase, but also in the paramagnetic phase at room temperature. This is due to a strong coupling between a short-range spin-order along the ladders, and the lattice degrees of freedom associated with the Fe–Fe bond length. This coupling induces a change in the bond-length pattern from an alternated trapezoidal one (as in Pnma ) to an alternated small/large rectangular one. Out of the two patterns, only the latter is fully compatible with the observed block-type magnetic structure. Finally, we propose a complete symmetry analysis of the BaFe 2 Se 3 phase diagram in the 0–600 K range

Effect of chemical pressure induced by La3+/ Y3+ substitution on the magnetic ordering of (A Mn3) Mn4 O12 quadruple perovskites

We report on the successful high-pressure synthesis of single-phase powders of the quadruple perovskite (YMn3)Mn4O12, where the chemical pressure exerted by the small Y3+ ion is expected to enhance the exchange interaction between Mn3+ ions. According to this expectation, powder neutron diffraction, specific heat, and magnetization measurements give evidence of a C-type antiferromagnetic ordering of the Mn3+ ions in the octahedrally coordinated B sites at TN,B=108K, 30 K higher than in the isostructural and isovalent compound, (LaMn3)Mn4O12. Surprisingly, we found no evidence of long-range magnetic order of the square-coordinated A′ sites, although an indication of latent magnetic order is given by a broad peak of the DC magnetization between 70 and 40 K and by a sharp peak in the AC susceptibility at T∗=70K. A further unexpected feature of (YMn3)Mn4O12, not found in previously reported (AMn3)Mn4O12 compounds, is a second-order structural phase transition at Ts=200K, attributed to a partial redistribution of charge involving both the B and A′-site Mn3+ ions. The present results suggest the existence of competing magnetic orderings, which may be linked to the occurrence of magnetic ferroelectricity in the (AMn3)Mn4O12 system

Infrared phonon spectroscopy on the Cairo pentagonal antiferromagnet Bi 2 Fe 4 O 9 : A study through the pressure-induced structural transition

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Stabilizing electromagnons in CuO under pressure

International audienceAbstract Electromagnons (Electroactive spin wave excitations) could prove to be decisive in information technologies but they remain fragile quantum objects, mainly existing at low temperatures. Any future technological application requires overcoming these two limitations. By means of synchrotron radiation infrared spectroscopy performed in the THz energy range and under hydrostatic pressure, we tracked the electromagnon in the cupric oxide CuO, despite its very low absorption intensity. We demonstrate how a low pressure of 3.3 GPa strongly increases the strength of the electromagnon and expands its existence to a large temperature range enhanced by 40 K. Accordingly, these two combined effects make the electromagnon of CuO under pressure a more ductile quantum object. Numerical simulations based on an extended Heisenberg model were combined to the Monte-Carlo technique and spin dynamics to account for the magnetic phase diagram of CuO. They enable to simulate the absorbance response of the CuO electromagnons in the THz range

Ferroelectricity in the 1 µC cm −2 range induced by canted antiferromagnetism in (LaMn3)Mn4O12

International audiencePyroelectric current and field-dependent specific heat measurements on polycrystalline samples of the quadruple per-ovskite (LaMn 3)Mn 4 O 12 give evidence of ferroelectricity driven by the canted antiferromagnetic ordering of the B-site Mn 3+ 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. In the search for multiferroic materials for applications, magnetic ferroelectrics, where ferroelectricity is induced by magnetic order, are promising for their inherently strong mag-netoelectric couplings 1. The challenge is to enhance the remnant polarization, which is typically small in these materials, P ∼ 0.1µC cm −2. This has been attributed to the weakness of the spin-orbit interaction in non-collinear spin structures 2 , where P arises from the antisymmetric exchange striction (in-verse Dzyaloshinskii-Moriya interaction) between neighbouring spins. More promising are collinear spin structures, where values up to ∼10 µC cm −2 are predicted due to the comparatively strong symmetric exchange striction 3. This prediction is supported by the large value P ∼ 1µC cm −2 reached under pressure in the perovskitelike compound TbMnO 3 , characterized by a collinear E-structure 4. The symmetric exchange mechanism has been invoked to account for a remarkable P-enhancement up to 0.27 µC cm −2 in another manganese oxide , (CaMn 3)Mn 4 O 12 (CMO) 5,6 , with quadruple perovskite (QP) AA 3 B 4 O 12 structure, characterized by two distinct A and B Mn sites 7. Though, the role of this mechanism in CMO remains controversial 8,9 possibly because of a complex interplay of incommensurate helicoidal spin structure and charge and orbital ordering of the Mn 3+ and Mn 4+ ions 10. Here we show that (LaMn 3)Mn 4 O 12 11,12 (LMO) is a model system to investigate the role of symmetric exchange stric-tion on magnetic ferroelectricity. LMO shares with CMO a similar QP structure, but exhibits simpler structural and electronic properties. LMO is a single-valent Mn 3+ system with neither charge orderings nor incommensurate structural modulations 11. The Im3 cubic structure stable at high temperatures undergoes a monoclinic I2/m distortion at 653 K 12. The B-site Mn 3+ ions antiferromagnetically (AFM) order forming a canted C-type structure at T N,B =78 K; the A-sites Mn 3+ ions also order AFM at lower temperatures T N,A =21 K 11. Here we report on record high electric polarizations ∼ 0.5µC cm −2 in polycrystalline LMO samples induced by the magnetic order at T N,B , so even higher values promising for applications are expected in single-crystalline samples. The LMO samples studied in the present work are 4 mm diameter cylinders synthesized under high-pressure as described elsewhere 11. Depending on synthesis conditions, either powders or small, 0.5 mm-sized, single crystals are obtained. Powders are typically 95 % pure and contain minor LaMnO 3 and Mn 3 O 4 impurities. One powder sample was studied by x-ray diffraction as a function of temperature in a D8 Bruker diffractometer equipped with a focusing primary Ge(111) monochromator for Cu K α1 radiation and a close-cycle He cryostat. Pyrocurrent, I p , measurements were carried out on three 0.2 mm thin disks of sintered powders of the same batch. The size of the available single crystals was too small for these measurements. Electrodes were realized by depositing a 0.1µm thick Au layer on both sides of the disks by magnetron sputtering. The disks were cooled in a close-cycle cryogenic system down to T p =100 K, at which a poling field E p = 3-36 kV cm −1 was applied for 30 min. At this temperature , the dc resistance of the samples exceeds 1 GΩ, which ensures their full polarization. We subsequently cooled the samples down to 15 K, removed the poling field and applied a short-circuit for 10 min to remove space charges