J. Rudolph

Laser threshold reduction in a spintronic device

We experimentally demonstrate a threshold reduction of a semiconductor laser by optically pumping with spin-polarized electrons. Calculations show that a threshold reduction by 50% is possible even for electrically pumped lasers.

Room-temperature threshold reduction in vertical-cavity surface-emitting lasers by injection of spin-polarized electrons

We experimentally demonstrate the reduction of the laser threshold of a commercial GaAs∕(AlGa)As vertical-cavity surface-emitting laser (VCSEL) by optical injection of spin-polarized electrons at room temperature. Calculations with a rate-equation model reproduce the measured reduction of 2.5% for injected electrons with 50% spin polarization. The model predicts an improved threshold reduction of 50% in otherwise identical VCSELs grown on a (110) substrate due to the enhanced spin lifetime in such structures.

Anisotropic electron spin relaxation in bulk GaN

Electron spin dynamics in n-type c-oriented wurtzite GaN epilayers is studied by time-resolved Kerr-rotation measurements at T=80 K. The electron spin lifetime shows a sudden increase if an external magnetic field is applied in the sample plane. This enhancement is explained by anisotropic Dyakonov–Perel spin relaxation in bulk GaN as a direct consequence of the anisotropy of spin-orbit coupling in semiconductors with wurtzite structure.

Long room-temperature electron spin lifetimes in highly doped cubic GaN

We report on very long electron spin relaxation times in highly n-doped bulk zincblende GaN exceeding 500 ps up to room-temperature. Time-resolved Kerr-rotation measurements show an almost temperature independent spin relaxation time between 80 and 295 K confirming an early prediction of Dyakonov and Perel for a degenerate electron gas.

Temperature dependent low-field measurements of the magnetocaloric ΔT with sub-mK resolution in small volume and thin film samples

We present temperature dependent ΔT measurements of the magnetocaloric effect in a thin film sample of Gd, employing magnetomodulation and detection of thermal radiation. A bulk sample of the metamagnetic material LaFe11.05Co0.91Si1.04 shows a strong broadening of the ΔT peak for increasing field amplitudes between 4 and 45 mT. Bulk Gd in comparison shows only a weak broadening. All investigated samples exhibit a clear quadratic dependence of ΔT on the external field Hext at the ΔT peak maximum, contrary to earlier predictions. An analytic expression is derived that interpolates between the Hext2-behavior at low and the well-known Hext2/3-behavior at high fields.

Long electron spin coherence in ion‐implanted GaN: The role of localization

The impact of Ga and Au ion implantation on the electron spin dynamics in bulk wurtzite GaN is studied by time‐resolved Kerr‐rotation spectroscopy. The spin relaxation time increases strongly by up to a factor of 20 for increasing implantation doses. This drastic increase is caused by a transition from delocalized to localized electrons. We find a characteristic change in the magnetic field dependence of spin relaxation that can be used as a sensitive probe for the degree of localization.

High temperature electron spin dynamics in bulk cubic GaN: Nanosecond spin lifetimes far above room-temperature

The electron spin dynamics in n-doped bulk cubic GaN is investigated for very high temperatures from 293 K up to 500 K by time-resolved Kerr-rotation spectroscopy. We find extraordinarily long spin lifetimes exceeding 1 ns at 500 K. The temperature dependence of the spin relaxation time is in qualitative agreement with predictions of Dyakonov-Perel theory, while the absolute experimental times are an order of magnitude shorter than predicted. Possible reasons for this discrepancy are discussed, including the role of phase mixtures of hexagonal and cubic GaN as well as the impact of localized carriers.

Temperature dependence of the electron Landé g-factor in cubic GaN

The temperature dependence of the electron Lande g-factor in bulk cubic GaN is investigated over an extremely broad temperature range from 15 K up to 500 K by time-resolved Kerr-rotation spectroscopy. The g-factor is found to be approximately constant over the full investigated temperature range. Calculations by k·p-theory predict a negligible temperature dependence g(T) in complete agreement with the experiment as a consequence of the large band-gap and small spin orbit splitting in cubic GaN.

Magneto-optical studies of Gd-implanted GaN: No spin alignment of conduction band electrons

Gd-implanted wurtzite GaN as a candidate for a ferromagnetic dilute magnetic semiconductor is investigated by time-resolved magneto-optical spectroscopy. We observe a strong increase of the electron spin lifetimes for increasing Gd doses, while the electron spin Larmor precession frequency is independent of the Gd concentration. These findings are well explained by carrier localization at defects and a negligible interaction with Gd ions. The data show that Gd-implanted GaN cannot be used for an electron spin aligner.

Two-dimensional higher order noise spectroscopy up to radio frequencies

Going beyond the usual determination of the frequency-resolved power spectrum of an electrical noise signal, we implement a setup for the determination of a frequency-resolved two-dimensional correlation spectrum. We demonstrate measurements of two-dimensional correlation spectra with sampling rates up to 180 MSamples/s and real-time numerical evaluation with up to 100% data coverage. As an example, the purely Gaussian behavior of 1/f resistor noise is demonstrated with unprecedented sensitivity by verifying the absence of correlations between different frequencies. Unlike the usual power spectrum, the correlation spectrum is shown to contain information on both the homogeneous and inhomogeneous linewidths of a signal, suggesting applications in spin noise spectroscopy and signal analysis in general.