D. Hägele

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=80u2002K. 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.

Proof of the Cases p≤ 7 of the Lieb-Seiringer Formulation of the Bessis-Moussa-Villani Conjecture

It is shown that the polynomial λ (t)=Tr[(A+ tB)p] has nonnegative coefficients when p≤ 7 and A and B are any two complex positive semidefinite n× n matrices with arbitrary n. This proves a general nontrivial case of the Lieb-Seiringer formulation of the Bessis-Moussa-Villani conjecture which is a long standing problem in theoretical physics.

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.

Ultrafast spin noise spectroscopy

We propose an extension of optical spin noise spectroscopy that expands the so far accessible frequency range from a few gigahertz to several terahertz employing pairs of ultrafast femtosecond-laser pulses. The method is suitable for probing noise signals with very high bandwidths and signals centered at zero frequency. A time-resolved version of noise spectroscopy for detecting noise after a pump event follows naturally from the scheme. The analytical description of ultrafast spin noise spectroscopy along with numerical simulations proves the method a powerful spectroscopic tool.

Discrete-time windows with minimal RMS bandwidth for given RMS temporal width

abstract We derive a family of discrete window functions for the N-point Fourier transform forapplication in spectral analysis that optimize the root mean square (RMS) frequency width s ω for a given temporal RMS width s t . The window family yields as a byproduct the minimumtime-bandwidth product s ω s t for given s t and N. The new windows interpolate for decreasings t between the popular Cosine-window and a nearly Gaussian window. The new “confinedGaussian” window function g ðcGÞk (with k¼0;…;N 1) is extremely well approximated byg ðacGÞk pGðkÞ Gð 1=2Þ½GðkþNÞþGðk NÞ =½Gð 1=2þNÞþGð 1=2 NÞ with the GaussianGðxÞ¼exp½ δt 2 ðxð N 1Þ=2Þ 2 =ð4s 2 Þ ,thetemporalwidths s t ,andtimestepδt.& 2014 Elsevier B.V. All rights reserved. 1. IntroductionIn this paper we solve the problem of window func-tions with minimum time-bandwidth product for theN-point Fourier transform. The problem appears in digitalspectral analysis whenever time and frequency localiza-tion is important. The encoding of audio signals into theMP3 or related formats is probably the most notableexample where such requirements need to be met [1].Ingeneral, the application of windows is not limited to audiosignals. In fact, many different windows are in use fordigital image processing [2] and signal processing theirchoice being determined byapplication purposes like side-band suppression, frequency estimation, or peak-areaestimation (for a recent review on the use of windowssee [3]). Suggestions for windows with special propertiesappear on a regular basis (e.g. [4,5]).While Harris mentions already in his famous 1978review the truncated-Gaussian window family for its closeto optimal root mean square (RMS) time-bandwidthproduct [6], the question of windows with ultimately thesmallest time-bandwidth product had not been settled.The subject of an optimal time-bandwidth product is wellunderstood in the case of a Fourier pair f(t) and FðωÞ ofcontinuous functions with s

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 45u2009mT. 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 293u2009K up to 500u2009K by time-resolved Kerr-rotation spectroscopy. We find extraordinarily long spin lifetimes exceeding 1u2009ns at 500u2009K. 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 15u2009K up to 500u2009K 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.