S. Takeyama

Cathodoluminescence study of diluted magnetic semiconductor quantum well/micromagnet hybrid structures

Cathodoluminescence (CL) was studied in hybrid structures consisting of a diluted magnetic semiconductor (DMS) Cd1−xMnxTe (x=0.06 and 0.09) quantum well buried 300 A below the surface on which Fe islands with micrometric dimensions were deposited. The CL at T=10u200aK collected from areas far away from the Fe island was consistent with the photoluminescence spectra obtained prior to Fe deposition as were the raster scans and spot excited CL spectra taken in nonmagnetized structures close to the Fe islands. After a magnetization at a magnetic field of 3 T, the CL peak related to DMS quantum well (QW) shifts by up to 4 meV to lower energy only when the exciting beam is focused close to edges of an island. The observed shifts are interpreted as due to a fringe field, affecting the DMS QW, of magnetic domains formed in the Fe islands. The experiments prove a feasibility of the concept of usage of the fringe fields to achieve further confinement of excitons in submicron DMS/ferromagnet hybrid structures.

Biexciton spin states of diluted magnetic semiconductor quantum wells in high magnetic fields

An exciton and a biexciton photoluminescence in (Cd,Mn)Te/CdTe/(Cd,Mg)Te asymmetric quantum wells were investigated under magnetic fields. A quantum beat between an exciton and a biexciton has been observed by a transient four-wave-mixing spectroscopy. The spin-energy structures have been clarified by spectra of the four-wave-mixing signals. We have observed decrease of the intensity ratio of a biexciton to an exciton on increasing magnetic field. The instability of the spin singlet biexciton state is determined by the inner effective magnetic field produced by the local Mn magnetic moments.

Dynamical spin properties of exciton and biexciton in CdMnTe/CdTe/CdMgTe single quantum well

Abstract Exciton and biexciton optical properties in II–VI semiconductor single quantum well have been studied by magneto-photoluminescence, and time-resolved four-wave-mixing spectroscopies. The polarization dependence of both measurements can clearly distinguish biexciton from other bound states and show the dynamical spin properties of exciton and biexciton.

Biexcitons in CdMnTe/CdTe/CdMgTe single quantum well

Abstract We have performed two-pulse and three-pulse time-integrated as well as spectrally resolved four-wave-mixing measurements on a single CdMnTe/CdTe/CdMgTe quantum well. The polarization dependence of both four-wave-mixing measurements can be explained well by the five-level model of the exciton–biexciton system including two-biexciton states with parallel spin and anti-parallel spin orientations.

Spectral anomalies of exciton photoluminescence at v = 1 and 2/3 in a modulation-doped n-type CdTe/(Cd, Mg, Mn)Te single quantum well

Spin-polarized magnetophotoluminescence measurements have been conducted in a modulation-doped n-type CdTe/(Cd, Mg, Mn)Te single quantum well at 1.5 K under 2 s long pulsed magnetic fields up to 60 T. For the σ - polarization, a new peak was observed to emerge at ν ∼ 1 on the higher energy side of a main peak. The former peak became larger than the latter with increasing magnetic field, while the latter peak had a local maximum at ν ∼ 0.9 and attenuated. Furthermore, magnetic field dependence of the latter peak energy showed step-like behavior around ν ∼ 0.9. The intensity of the former peak was observed to increase monotonically with field, showed a kink at v ∼ 2/3 and saturated, i.e. showed a shoulder-like structure there. With changing the angle between the direction of the sample surface normal and the magnetic field, both features at ν ∼ 1 and 2/3 exhibited tendencies to fade out, which suggests that the former is associated with spin-singlet charged exciton and the latter with fractional quantum Hall effect at v = 2/3.

Submillimeter wave ESR study of Cd1-xMnxTe

Abstract Submillimeter wave ESR experiments have been performed on Cd 1− x Mn x Te at x =0.35. A crossover from the damped ESR signal to the collective spin-wave mode was found around 15 T. In addition to the host resonance, a new peak was observed for the sample doped with 0.3% of Fe together with 38% of Mn. The correlation between the host resonance and the new peak observed in the frequency–field relation indicates the existence of magnetic clusters interacting with host spins.

Biexciton Dynamical Behavior In (Cd,Mn)Te/CdTe/(Cd,Mg)Te Asymmetric Quantum Wells

Exciton and biexciton photoluminescence in (Cd,Mn)Te/CdTe/(Cd,Mg)Te asymmetric quantum wells have been studied by time-resolved photoluminescence spectroscopy. The biexciton luminescence shows slower rise-time and longer decay as compared to the exciton luminescence. This result is explained by the biexciton localization at an unavoidable interface disorder. The biexciton luminescence intensity decreases with increasing magnetic field, while there are observed no changes of the luminescence decay rate. These results suggest that the spin-singlet biexciton formation rate is suppressed by applying a magnetic field, whereas the biexciton decay remains unaffected.

High-pressure and high-magnetic-field study of energy transfer from excitons into local d electrons in a CdTe/(Cd, Mn)Te quantum well structure

Photoluminescence measurements have been conducted for a CdTe/Cd1-xMnx Te (x = 0.4) single-quantum-well structure at low temperatures under pressures to 0.49 GPa and magnetic fields to 60 T. At the ambient pressure, a new emission was induced by the application of a magnetic field. The emission has been assigned to exciton emission from the barrier layer, which is suppressed below 9 T due to the energy transfer from the exciton to local d electrons. At 0.49 GPa, the emission recovered at 44 T. In the field region where the energy transfer occurs, an anomalous red-shift of the exciton energy was observed clearly for the case of the ambient pressure. The alloy potential fluctuation effect and the magnetopolaron effect are examined as candidates for the mechanism to cause this phenomenon.

Interface local spin states in a CdTe/(Cd, Mn)Te quantum well

Magneto-photoluminescence spectra obtained experimentally for a 1.9 nm thick CdTe quantum well with semi-magnetic barriers of Cd1−xMnx Te( x =0 :24) in the range of a magnetic 6eld B = 0–50 Tand applied pressure up to 1 :5 GPa have been analyzed through the interface � -potential model. In the model, the interface potential is expressed as A�� (z ±Lw=2), where � is the spin projection ±1=2 and Lw is the quantum well width. Our results show that the amplitude of the interface potential A is saturated at 15 Tand is not changed up to 50 T . T his behavior is described well by the modi6ed Brillouin function, which suggests that the magnetic ions in the interface region are isolated rather than clustered, in contrast to those in the barrier layers. The potential amplitude A is found to be enhanced by the pressure. c

Magnetic polaron bifurcation in asymmetric diluted magnetic semiconductor quantum wells

Abstract Calculations of exciton energies are presented in a 3.6 nm Cd 0.78 Mg 0.22 Te − CdTe − Cd 0.86 Mn 0.14 Te asymmetric quantum well. The results of the calculations are compared to recent photoluminescence (PL) experiments. The analysis of the PL spectra in a magnetic field reveals the occurrence of a transition with an energy lower than the “free” exciton transition in the quantum well. This additional transition is attributed to the formation of a “structural-asymmetry induced” bifurcated magnetic polaron state. The magnetic-field dependence of the bifurcated polaron state is calculated and found to be consistent with the experimental results.