The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in La 1−x Ca x MnO 3 for x = 0.3 and 0.2 was investigated by dc conductivity, ellipsometry, and vacuum ultraviolet reflectance for energies between 0 and 22 eV. A stabilized Kramers-Kronig transformation yields the optical conductivity and reveals changes in the optical spectral weight up to 22 eV at the metal-to-insulator transition. In the observed energy range, the spectral weight is conserved within 0.3%. The redistribution of spectral weight in this surprisingly broad energy range has important ramifications for the effective low-energy physics. We discuss the importance of the charge-transfer, Coulomb on-site, Jahn-Teller, and long-range Coulomb screening effects to the electronic structure. Among strongly correlated materials, the manganites exhibit a wealth of novel properties. For example, some hexagonal insulating materials exhibit multiferroic behavior and the cubic doped manganites show charge ordering and the colossal magnetoresistance ͑CMR͒ effect.1,2 It is clear that the two key ingredients responsible for these diverse phenomena are, first, the high geometrical and spin frustration and, second, the large number of competing interactions, the most important of which are the electron-electron and electron-phonon interactions. 1,[3][4][5][6][7][8][9] There is a deep disagreement as to which of these interactions is the primary driving force behind either the insulating phase of the manganites or the metal-to-insulator transition in the doped manganites. Models describing these phenomena involve double exchange, Jahn-Teller ͑JT͒, superexchange, and Coulomb on-site ͑Hubbard U͒ interactions that yield effective low-energy Hamiltonians, which predict different types of quasiparticle excitations, such as spin excitations, lattice polarons, spin polarons, or orbitons. 3,4,[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] However, the effective Hamiltonians used to describe the manganites typically ignore the oxygen p bands and consider only an effective manganese d band. It is also generally assumed that the high-energy degrees of freedom can be neglected by a "down folding" of the large number of bands into a single effective band. This implies that there is no redistribution of electronic states between low-energy and high-energy degrees of freedom. On the other hand, if one considers the importance of local interactions and hybridization in correlated materials, one would expect quite pronounced effects at higher energies that are connected to charge-transfer or Mott-Hubbard physics. 15,[22][23][24] Thus, the important test for the effective low-energy picture is to study whether one finds strong exchanges of spectral weight between low and high energies.Therefore, it is crucial to test the complex nature of the band structure explicitly. The most direct experiment is to measure the dielectric response of a material as a function of temperature and doping. Unfort...
The entatic state denotes a distorted coordination geometry of a complex from its typical arrangement that generates an improvement to its function. The entatic-state principle has been observed to apply to copper electron-transfer proteins and it results in a lowering of the reorganization energy of the electron-transfer process. It is thus crucial for a multitude of biochemical processes, but its importance to photoactive complexes is unexplored. Here we study a copper complex-with a specifically designed constraining ligand geometry-that exhibits metal-to-ligand charge-transfer state lifetimes that are very short. The guanidine-quinoline ligand used here acts on the bis(chelated) copper(I) centre, allowing only small structural changes after photoexcitation that result in very fast structural dynamics. The data were collected using a multimethod approach that featured time-resolved ultraviolet-visible, infrared and X-ray absorption and optical emission spectroscopy. Through supporting density functional calculations, we deliver a detailed picture of the structural dynamics in the picosecond-to-nanosecond time range.
We investigated the fabrication and functional behaviour of conductive silver-nanowire-polymer composites for prospective use in printing applications. Silver-nanowires with an aspect ratio of up to 1000 were synthesized using the polyol route and embedded in a UV-curable and printable polymer matrix. Sheet resistances in the composites down to 13 Ω/sq at an optical transmission of about 90% were accomplished. The silver-nanowire composite morphology and network structure was investigated by electron microscopy, atomic force microscopy, profilometry, ellipsometry as well as surface sensitive X-ray scattering. By implementing different printing applications, we demonstrate that our silver nanowires can be used in different polymer composites. On the one hand, we used a tough composite for a 2D-printed film as top contact on a solar cell. On the other hand, a flexible composite was applied for a 3D-printed flexible capacitor.
We present a setup for temperature-dependent spectral generalized magneto-optical ellipsometry (SGME). This technique gives access to the electronic as well as the magnetic properties of ferromagnetic materials within one single magneto-optical measurement. It also allows the determination of the orientation of the magnetization. We show spectra of the real and the imaginary part of the refractive index N as well as the magneto-optical coupling parameter Q of permalloy and iron films for in-plane magnetization. Our findings demonstrate the relevance of SGME for the understanding of the interplay between electronic and magnetic properties of ferromagnetics.
The magnetic and electronic properties of multiferroic TbMnO 3 in the paramagnetic, antiferromagnetic, sinusoidal, and spiral-spin phases were studied by spectral generalized magneto-optical ellipsometry. The measurements show a strong anisotropy of the dielectric tensor. A redistribution of spectral weight was observed in the diagonal components of the dielectric tensor for the temperature range from 110 to T N =46 K. In the off-diagonal elements, spectral generalized magneto-optical ellipsometry shows sensitivity to the antiferromagnetic and ferroelectric phase transitions at T N = 46 K and T F = 29 K, respectively, and a persistent signal up to 6T N . DOI: 10.1103/PhysRevB.77.193105 PACS number͑s͒: 75.47.Lx, 75.25.ϩz, 75.50.Ee, 77.22.Ej Multiferroic materials exhibit phases with simultaneous magnetic and ferroelectric properties. The close coupling of the magnetization and the electric polarization in multiferroics makes these materials excellent candidates for demonstrating giant magnetoelectrical behavior, 1,2 in which large magnetic and electric responses can be induced with weak electric and magnetic fields, respectively. 3,4 Spectral generalized magneto-optical ellipsometry ͑SGME͒ is an ideal technique for studying the interplay between electric and magnetic properties in multiferroic systems, as it probes both the diagonal xx and off-diagonal xy components of the dielectric tensor. 5 Multiferroicity in TbMnO 3 is induced by spin-charge coupling. Below the Néel temperature T N = 46 K, the wavelength of the incommensurate spin density decreases with decreasing temperature down to the ferroelectric transition at T F = 29 K. In the absence of a magnetic field, this spin-density modulation gets locked in at T F . Below T F , the inversion symmetry of the lattice is broken by the bc-cycloidal arrangement of the spins, inducing a ferroelectric polarization along the c axis while the antiferromagnetic ordering along the b axis persists. 3,4,6 Here, we study the coupling between spin and charge degrees of freedom in multiferroic TbMnO 3 by examining the temperature-dependent dielectric responses xx and xy , which are the diagonal and off-diagonal components of the dielectric tensor. The magnetization of a material breaks the symmetry between left and right circularly polarized ͑LCP and RCP͒ light. While 2xx is proportional to the sum of absorptions of LCP and RCP light, 1xy is proportional to the difference. 7 Consequently, a complete magneto-optical characterization of 2xx and 1xy in a magnetic material provides detailed information regarding the coupling between the material's electronic and magnetic properties.The temperature-dependent ellipsometry was done with a custom-made setup by using an extended Sentech SE850 spectral ellipsometer inside a He flow cryostat. The cryostat has stress-free mounted windows and maintains a base pressure of about 3 ϫ 10 −8 mbar at room temperature. 8 Standard ellipsometry was performed for temperatures from 10 to 470 K in the spectral range from 0.5 to 5.5 eV. In SGME, on...
Spectral generalized magneto-optical ellipsometry is presented as an optical tool for the simultaneous measurement of the complex index of refraction ñ=n+ik, the complex magneto-optical coupling parameter Q=Qr+iQi (i.e., the Voigt-parameter), and the orientation of the saturation magnetization Ms of isotropic ferromagnetic bulk materials. For wavelengths between 220nm and 790nm and at temperatures between 4.2K and 800K measurements on iron and permalloy demonstrate the comfortable application of this technique in order to resolve the spectral response of spin-polarized carriers and bands, which can provide valuable insight about the formation of the ferromagnetic state.
Unrevealing local magnetic and electronic correlations in the vicinity of charge carriers is crucial in order to understand rich physical properties in correlated electron systems. Here, using high-energy optical conductivity (up to 35 eV) as a function of temperature and polarization, we observe a surprisingly strong spin polarization of the local spin singlet with enhanced ferromagnetic correlations between Cu spins near the doped holes in lightly hole-doped La 1.95 Sr 0.05 Cu 0.95 Zn 0.05 O 4 . The changes of the local spin polarization manifest strongly in the temperature-dependent optical conductivity at ~7.2 eV, with an anomaly at the magnetic stripe phase (~25K), accompanied by anomalous spectral-weight transfer in a broad energy range. Supported by theoretical calculations, we also assign high-energy optical transitions and their corresponding temperature dependence, particularly at ~2.5eV, ~8.7eV, ~9.7eV, ~11.3eV and ~21.8 eV. Our result shows the importance of a strong mixture of spin singlet and triplet states in hole-doped cuprates and demonstrates a new strategy to probe local magnetic correlations using highenergy optical conductivity in correlated electron systems.
We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti 1−x Ta x O 2 (x ∼ 0.05) thin films grown by pulsed laser deposition on LaAlO 3 substrates. Rutherford backscattering spectrometry (RBS), X-ray diffraction, protoninduced X-ray emission, X-ray absorption spectroscopy (XAS) and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of FM with concomitant large carrier densities was determined by a combination of superconducting quantum interference device magnetometry, * Authors for correspondence (phyandri@nus.edu.sg; eleds@nus.edu.sg; venky@nus.edu.sg). † These authors contributed equally to this work. electrical transport measurements, soft X-ray magnetic circular dichroism (SXMCD), XAS and optical magnetic circular dichroism, and was supported by first-principles calculations. SXMCD and XAS measurements revealed a 90 per cent contribution to FM from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites, though carrier activation was only 50 per cent at 5 per cent Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (V Ti ) and Ti 3+ was studied via XAS and X-ray photoemission spectroscopy, respectively. It was found that, in films with strong FM, the V Ti signal was strong while the Ti 3+ signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localized magnetic moments, V Ti sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to FM in wide-band-gap semiconducting oxides without any magnetic elements.
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