Atsuhiko Miyata

Direct measurement of the exciton binding energy and effective masses for charge carriers in organic–inorganic tri-halide perovskites

Direct measurement of the exciton binding energy shows that the impressive performance of perovskite solar cells arises from the spontaneous generation of free electrons and holes after light absorption.

Determination of the exciton binding energy and effective masses for methylammonium and formamidinium lead tri-halide perovskite semiconductors

The family of organic–inorganic halide perovskite materials has generated tremendous interest in the field of photovoltaics due to their high power conversion efficiencies. There has been intensive development of cells based on the archetypal methylammonium (MA) and recently introduced formamidinium (FA) materials, however, there is still considerable controversy over their fundamental electronic properties. Two of the most important parameters are the binding energy of the exciton (R*) and its reduced effective mass μ. Here we present extensive magneto optical studies of Cl assisted grown MAPbI3 as well as MAPbBr3 and the FA based materials FAPbI3 and FAPbBr3. We fit the excitonic states as a hydrogenic atom in magnetic field and the Landau levels for free carriers to give R* and μ. The values of the exciton binding energy are in the range 14–25 meV in the low temperature phase and fall considerably at higher temperatures for the tri-iodides, consistent with free carrier behaviour in all devices made from these materials. Both R* and μ increase approximately proportionally to the band gap, and the mass values, 0.09–0.117m0, are consistent with a simple k.p perturbation approach to the band structure which can be generalized to predict values for the effective mass and binding energy for other members of this perovskite family of materials.

Magnetic phases of a highly frustrated magnet, ZnCr2O4, up to an ultrahigh magnetic field of 600 T.

The Faraday rotation and magneto-optical absorption spectral measurements were conducted to reveal the full-magnetization process and map out a magnetic phase diagram of a typical geometrical frustrated magnet, ZnCr(2)O(4), by using the electro-magnetic flux compression method in ultrahigh magnetic fields up to 600 T. A fully polarized ferromagnetic phase is observed in which the absorption spectra associated with an exciton-magnon-phonon transition disappears. Furthermore, prior to the fully polarized ferromagnetic phase above 410 T, we found a novel magnetic phase above 350 T at 4.6 K followed by a canted 3∶1 phase.

Impact of microstructure on the electron–hole interaction in lead halide perovskites

Despite the remarkable progress in the performance of devices based on the lead halide perovskite semiconductor family, there is still a lack of consensus on their fundamental photophysical properties. Here, using magneto-optical transmission spectroscopy we elucidate the impact of the microstructure on the Coulomb interaction between photo-created electron–hole pairs in methylammonium lead triiodide (MAPbI3) and the triple-cation lead mixed-halide composition, Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 (Cs: cesium, MA: methylammonium, FA: formamidinium) by investigating thin films with a wide range of grain sizes from tens of nanometers to microns. At low temperatures, in which thermal fluctuations of the interactions are frozen and the rotational disorder of the organic cation is negligible, the exciton binding energy and reduced effective mass of carriers remain effectively unchanged with grain size. We conclude that the microstructure plays a negligible role in the Coulomb interaction of the photo-created electron–hole pairs, in contrast to previous reports. This renewed understanding of the relationship between these fundamental electronic properties and the microstructure is critical for future fundamental studies and improving device design.

Canted 2:1:1 Magnetic Supersolid Phase in a Frustrated Magnet MgCr2O4 as a Small Limit of the Biquadratic Spin Interaction

Ultra-high magnetic field investigation of magnetic phases of MgCr2O4, by means of magneto-optical methods, showed a distinct similarity to that in ZnCr2O4, with clear observation of a canted 2:1:1 phase prior to a half-magnetization plateau, in spite of a difference in the tetragonal lattice symmetry in antiferromagnetic phases. The appearance of the canted 2:1:1 phase regarded as the magnetic supersolid state, sandwiched between the antiferromagnetic (superfluid) and the half-magnetization plateau (solid) states, is revealed by this study as an intrinsic and a universal phenomenon in case of the infinitesimal spin–lattice coupling limit in the three-dimensional geometrically frustrated magnet system comprised of chromium spinel oxides (ACr2O4 with A = Mg, Zn, Cd, Hg).

Impact of the Halide Cage on the Electronic Properties of Fully Inorganic Cesium Lead Halide Perovskites

Perovskite solar cells with record power conversion efficiency are fabricated by alloying both hybrid and fully inorganic compounds. While the basic electronic properties of the hybrid perovskites are now well understood, key electronic parameters for solar cell performance, such as the exciton binding energy of fully inorganic perovskites, are still unknown. By performing magneto-transmission measurements, we determine with high accuracy the exciton binding energy and reduced mass of fully inorganic CsPbX3 perovskites (X = I, Br, and an alloy of these). The well-behaved (continuous) evolution of the band gap with temperature in the range of 4–270 K suggests that fully inorganic perovskites do not undergo structural phase transitions like their hybrid counterparts. The experimentally determined dielectric constants indicate that at low temperature, when the motion of the organic cation is frozen, the dielectric screening mechanism is essentially the same for both hybrid and inorganic perovskites and is do...

Ultrahigh magnetic field phases in the frustrated triangular-lattice magnet CuCrO2

The magnetic phases of a triangular-lattice antiferromagnet, CuCrO

Optical Detection of Magnetic Orders in HgCr2O4 Frustrated Spin Magnet under Pulsed High Magnetic Fields

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Ultrahigh magnetic field spectroscopy reveals the band structure of the three-dimensional topological insulator Bi2Se3

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Magnetic superfluid state in the frustrated spinel oxide CdCr2O4revealed by ultrahigh magnetic fields

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