arXiv subjects

R. Kaji

Publications and source records attributed to R. Kaji.

7 recordsLinked to original sources

arXiv2026

Coherent exciton spin dynamics and three-dimensional quantum state tomography in a single InAlAs quantum dot

We investigate the coherent exciton spin dynamics in a single InAlAs/AlGaAs quantum dot using time-resolved quantum state tomography. Under two-LO-phonon quasi-resonant excitation of neutral exciton, we observe pronounced quantum beats in the circular and diagonal polarization components, reflecting the fine-structure splitting ($\Delta \approx 19.6 \ \mu\text{eV}$). By employing a global fitting procedure across three orthogonal polarization bases, we demonstrate that the spin evolution is consistently described by a unified Hamiltonian dominated by the anisotropic exchange interaction. While the initial degree of circular polarization is limited to $\approx 0.28$ due to fast relaxation processes during carrier cooling, the subsequent dynamics reveal a long-lived spin coherence ($1.1 \pm 0.2$ ns) that exceeds the exciton lifetime ($\sim 767$ ps). Our analysis reveals that the spin-formation time is significantly shorter than the instrument response function, and the absence of a discernible Overhauser shift confirms a negligible influence from the local nuclear environment under the present conditions. These results provide a quantitative benchmark for the three-dimensional reconstruction of spin trajectories using differential polarization signals, demonstrating the feasibility of using quasi-resonant excitation for stable spin initialization in semiconductor nanostructures.

cond-mat.mes-hall↗

arXiv2024

Exploration of optimal hyperfine transitions for spin-wave storage in $^{167}$Er$^{3+}$:Y$_2$SiO$_5$

The dependence of the magnetic fluctuations and the spin coherence time $T_2^{\rm hyp}$ of the lowest Stark states $^4I_{15/2}\ (Z_1)$ in $^{167}$Er$^{3+}$:Y$_2$SiO$_5$ under zero magnetic field on Er concentration is numerically investigated in the range of 10 to 100 parts per million (ppm). We investigate two primary sources of magnetic fluctuation limiting spin coherence: a constant contribution from host Y nuclei and a concentration-dependent component from dipole-dipole interactions among Er ions. Due to these two components, the Er-concentration dependence of $T_2^{\rm hyp}$ at the zero first-order Zeeman (ZEFOZ) points saturates for crystals with Er concentration below 10 ppm and no extension of the $T_2^{\rm hyp}$ is expected without an external magnetic field. Under a magnetic field, the longest $T_2^{\rm hyp}$ at a particular ZEFOZ point is expected to be over 170 s (90 s) for site 1 (site 2), which is more than $10^4$ times longer than that at zero field for 10-ppm $^{167}$Er$^{3+}$:Y$_2$SiO$_5$. Remarkably, these optimal ZEFOZ points form striking geometric patterns: a line for site 1 and a plane for site 2. This trend, which is favorable for experiments, can be explained by the anisotropy of the effective spin Hamiltonian parameters. Finally, the tolerance of the ZEFOZ point at each site with the longest $T_2^{\rm hyp}$ against the errors in the applied magnetic field vector is evaluated.

quant-ph↗

arXiv2024

Slowly generated anomalously large nuclear field in bulk n-AlGaAs

This study investigated the formation and relaxation dynamics of nuclear spin polarization in three Al$_x$Ga$_{1-x}$As bulk samples with different aluminum concentrations $x$ of 0.00, 0.05, and 0.15. The time-resolved Kerr rotation technique was primarily used. The samples with $x$ = 0.15 and 0.05 exhibited anomalously large nuclear magnetic fields BN exceeding 1 T, approximately twice the applied magnetic field. Further investigations revealed that BN formation occurred in two-stages, a rapid initial rise followed by a gradual increase toward a saturation value. Relaxation measurements revealed that the relaxation time of BN was longer for AlGaAs than for GaAs. The comparison of the results obtained under strong and weak magnetic fields indicated the suppression of quadrupole-induced relaxation. We modified the dynamics model of nuclear spin polarization and explained the two-stage formation and the accompanying large BN in AlGaAs bulks.

cond-mat.mtrl-sci↗

arXiv2018

Sign identification of electron and hole out-of-plane g factors by utilizing nuclear spin switch in single quantum nanostructures

The electron and hole g factors are the key quantities for the spin manipulations in semiconductor quantum nanostructures. However, for the individual nanostructures, the separate determination including the signs of those g factors is difficult by using some methods adopted conventionally in bulks and quantum wells. We report a convenient optical method for the sign identification of out-of-plane g factors in the individual quantum nanostructures, which utilizes the optically-induced nuclear spin switch. The method is demonstrated in typical single self-assembled In$_{0.75}$Al$_{0.25}$As/Al$_{0.3}$Ga$_{0.7}$As quantum dots and InAs/GaAs quantum rings, where the g factors with the opposite sign for electron and the same sign for hole are proved.

cond-mat.mes-hall↗

arXiv2017

Investigation of in-plane nuclear field formation in single self-assembled quantum dots

We studied the formation mechanism of the in-plane nuclear field in single self-assembled In$_{0.75}$Al$_{0.25}$As/Al$_{0.3}$Ga$_{0.7}$As quantum dots. The Hanle curves with an anomalously large width and hysteretic behavior at the critical transverse magnetic field were observed in many single quantum dots grown in the same QD sample. In order to explain the anomalies in the Hanle curve indicating the formation of a large nuclear field perpendicular to the photo-injected electron spin polarization, we propose a new model based on the current phenomenological model for dynamic nuclear spin polarization. The model includes the effects of the nuclear quadrupole interaction and the sign inversion between in-plane and out-of-plane g-factors, and the model calculations reproduce successfully the characteristics of the observed anomalies in the Hanle curves.

cond-mat.mes-hall↗

arXiv2011

Direct observation of correlation time of dynamic nuclear polarization in single quantum dots

The spin interaction between an electron and nuclei was investigated optically in a single self-assembled InAlAs quantum dot (QD). In spin dynamics, the correlation time of the coupled electron-nuclear spin system and the electron spin relaxation time play a crucial role. We examined on a positively charged exciton in a QD to evaluate these key time constants directly via the temporal evolution measurements of the Overhauser shift and the degree of circular polarization. In addition, the validity of our used spin dynamics model was discussed in the context of the experimentally obtained key parameters.

cond-mat.mtrl-sci↗

arXiv2007

Precise measurements of electron and hole g-factors of single quantum dots by using nuclear field

We demonstrated the cancellation of the external magnetic field by the nuclear field at one edge of the nuclear polarization bistability in single InAlAs quantum dots. The cancellation for the electron Zeeman splitting gives the precise value of the hole g-factor. By combining with the exciton g-factor that is obtained from the Zeeman splitting for linearly polarized excitation, the magnitude and sign of the electron and hole g-factors in the growth direction are evaluated.

cond-mat.mtrl-sci↗