Taizo Masumi

Detailed examination of relaxation processes of excitons in photoluminescence spectra of Cu2O

Variations of optical absorption and especially of photoluminescence spectra of Cu 2 O with photoexcitation wavelength and temperature have been for the first time investigated in details. We have newly observed that the relation of the increase and decrease with temperature in the integrated intensity is complementary with each other between the luminescence-band V Cu and the X O -Γ 12 - exciton luminescence over T =4.2–230 K, and the relative magnitudes of the intensity of the band V Cu and V O 2+ cross over each other at the vicinity of the energy level of exciton, λ∼610 nm. We recognize the evidences of the recombination processes of the excitons coupled with vacancies levels. These phenomena indicate universal natures even among the different quality of specimens. Thus, we request a consideration based on the many-body aspect.

Cyclotron resonance of positive holes in AgBr

Abstract The first observation of cyclotron resonance of holes in AgBr is reported. Measurements have been made at 34 GHz and 1.7 K. It is shown that the energy surfaces for the hole polaron consist of a set of spheroids oriented along the [111] directions with the mass parameters m ∗ l = (1.71±0.06)m 0 and m ∗ t =(0.79±0.01)m 0 . The mobility of the hole polaron is determined to be about 1.5 × 10 5 cm 2 Vsec at 1.7 K.

Optical Properties of Cu2O Studied by Spectroscopic Ellipsometry.

The real ( e 1 ) and imaginary ( e 2 ) parts of the dielectric function of Cu 2 O have been measured by spectroscopic ellipsometry in the 1.2–5.2 eV photon-energy range at room temperature. The Cu 2 O single crystals used in this study were grown by a floating-zone melting technique. Dielectric-function spectra measured reveal distinct structures at energies of E 0 C ∼2.6 eV (“blue” exciton series), E 0 D ∼2.8 eV (“indigo” exciton series), E 1 A ∼3.5 eV and E 1 B ∼4.3 eV. The spectra are analyzed on the basis of a simplified model of the interband transitions. Results are in satisfactory agreement with the experimental data over the entire range of photon energies. Dielectric-related optical constants, such as the complex refractive index ( n * = n + i k ), absorption coefficient (α) and normal-incidence reflectivity ( R ), of Cu 2 O are also presented. Also, we obtain the static and high-frequency dielectric constants of Cu 2 O as e s =7.87 and e ∞ =6.95, respectively.The real (e 1 ) and imaginary (e 2 ) parts of the dielectric function of Cu 2 O have been measured by spectroscopic ellipsometry in the 1.2-5.2eV photon-energy range at room temperature. The Cu 2 O single crystals used in this study were grown by a floating-zone melting technique. Dielectric-function spectra measured reveal distinct structures at energies of E 0C ∼ 2.6eV (blue exciton series), E 0D ∼ 2.8eV (indigo exciton series), E 1A ∼ 3.5eV and E 1B ∼ 4.3eV. The spectra are analyzed on the basis of a simplified model of the interband transitions. Results are in satisfactory agreement with the experimental data over the entire range of photon energies. Dielectric-related optical constants, such as the complex refractive index (n * = n + ik), absorption coefficient (α) and normal-incidence reflectivity (R), of Cu 2 O are also presented. Also, we obtain the static and high-frequency dielectric constants of Cu 2 O as e s = 7.87 and e∞ = 6.95, respectively.

Optical Properties of CuO Studied by Spectroscopic Ellipsometry

The real ( e 1 ) and imaginary ( e 2 ) parts of the dielectric function of polycrystalline samples of monoclinic CuO have been measured by spectroscopic ellipsometry in the 1.2–5.0-eV photon-energy range at room temperature. The CuO crystals having a grain size of the order of 1×1 mm 2 were successfully grown by a floating-zone melting technique. Dielectric-function spectra of polycrystalline CuO reveal distinct structures at energies of E 1 ∼1.6 eV, E 2 ∼2.0 eV, E 3 ∼2.6 eV and E 4 ∼3.4 eV. These spectra are analyzed on the basis of a harmonic oscillator approximation. Dielectric-related optical constants, such as the complex refractive index ( n * = n + i k ), absorption coefficient (α) and normal-incidence reflectivity ( R ), of CuO are also presented.

Hall Mobility of Positive Holes in Cu2O

Hall mobility of photoexcited holes in the \(\varGamma_{7}^{+}\) valence band of Cu 2 O has been measured at low temperatures. On the basis of the theory of carrier-lattice interactions in polyatomic crystals the intrinsic scattering mechanisms of this substance have been clarified, for the first time, in a wide temperature range up to 400 K. From 40 K to 100 K scattering is dominated by the low-frequency LO phonon, ω 1 . Above 100 K, the effect of the high-frequency LO phonon, ω 2 , becomes remarkable. Here, ω 1 =153 cm -1 and ω 2 =660 cm -1 at 4.2 K. Mobility quenching due to the metastable self-trapping of holes possibly underlies the phenomena above 200 K. Below 40 K, the temperature dependence of mobility can be explained in terms of a combination of the effects of optical, acoustical and neutral impurity scatterings. The magnitude of the deformation potential of the \(\varGamma_{7}^{+}\) valence band is estimated to be as small as 0.7±0.3 eV.

A Novel Spectral Photoconductivity of the Y–Cu–O, Y–Ba–Cu–O Correlative with Superconductivity

An onset of “ the Novel Spectral Photoconductivity ” in accordance with the onset of “ Superconductivity ” have been observed for the first time in the Y 1 –Cu 2 –O z and Y 3- x –Ba x –Cu 3 –O 9- y systems below 150 K by applying transient pulse and d.c. techniques for conductivity measurements and by using a microwave SQUID for susceptibility measurements. Novel spectra of photoconductivity of the Y 1 –Cu 2 –O z and Y 3- x –Ba x –Cu 3 –O 9- y clearly feature the polarons and excitons in the Cu 2 O-like phase in these oxygen-deficient systems. These results suggest that “ an ensemble of united polarons and excitons ” must play substantial roles in the mechanisms of high- T c superconductivity.

Anomalous Transport Phenomena of Photoelectrons in the Ferromagnetic Semiconductor EuO

Anomalous transport phenomena of photoelectrons in pure EuO crystals have been studied over the temperature range from T =4.2 K to 80 K and in the magnetic fields up to H =50 kOe by using a fast-pulse technique with blocking electrodes. The Redfield geometry was used to determine directly the value of Hall mobility of photocarriers, µ H . Anomalous behaviours of photoconductivity such as an abrupt increase below the Curie temperature T c =69.5 K and a large negative-magnetoresistance effect around T c were observed. The value of µ H has been measured as a function of both the temperature and the magnetic field. It was directly found that the value of µ H varies with H around T c ; the µ H -value at T =63.5 K increases from 90 cm 2 /V. sec at H =0 to 160 cm 2 /V. sec at H =15 kOe. Thus, the anomalous transport phenomema in pure EuO crystals around T c have been experimentally confirmed, for the first time, to be mainly due to the mobility change. Experimental results are discussed in terms of the transport...

High Tc Magnetic Superconductor: Ho-Ba-Cu Oxide

A new material, holmium-barium-copper oxide, becomes a superconductor. Its onset temperature of the superconductivity is 93 K and the resistance vanishes below 76 K. By microwave SQUID method, a diamagnetism is found at 4.2 K.

A new time-resolved luminescence line from excitonic molecules in AgBr

SummaryA new and intrinsic luminescence line supposed to be fromthe excitonic molecules has been observed, for the first time, in the case ofthe allowed indirect transition, in pure AgBr at liquid-helium temperatures by a time-resolved spectroscopy technique.RiassuntoSi è osservata per la prima volta una nuova linea di luminescenza intrinseca che si suppone derivi da «molecole eccitoniche», nel caso ditransizioni indirette permesse in AgBr puro a temperature dell’elio liquido attraverso una tecnica di spettroscopia risolta in tempo.РезюмеВпервые в случае разрешенного непрямого перехода в чистом AgBr при гелиевых температурах наблюдалась новая люминесцентная линия, приписываемая «экситонным молекулам».

A Novel Spectral Photoconductivity of the La–Cu–O Correlative with Superconductivity

An echo-like “ Spectral Photoconductivity correlative with Superconductivity ” has been really observed also in the La 2 –Cu 1 –O z , system at 4.2∼100 K as naively expected from the results observed in the Y–Cu–O and Y–Ba–Cu–O systems by following almost similar experimental procedures. A novel spectral photoconductivity of the La 2 –Cu 1 -O z even more prominently features the polarons and excitons in the Cu 2 O-like phase in these oxygen-deficient systems. On the basis of these results together with similar data reported before on the Y–Cu–O, Y–Ba–Cu–O systems, we can reconfirm that “ an ensemble of united polarons and excitons ” must play substantial roles in the mechanisms of high- T c superconductivity.