Atomic-like spin noise in solid-state demonstrated with manganese in cadmium telluride

Spin noise spectroscopy is an optical technique which can probe spin resonances non-perturbatively. First applied to atomic vapours, it revealed detailed information about nuclear magnetism and the hyperfine interaction. In solids, this approach has been limited to carriers in semiconductor heterostructures. Here we show that atomic-like spin fluctuations of Mn ions diluted in CdT e (bulk and quantum wells) can be detected through the Kerr rotation associated to excitonic transitions. Zeeman transitions within and between hyperfine multiplets are clearly observed in zero and small magnetic fields and reveal the local symmetry because of crystal field and strain. The linewidths of these resonances are close to the dipolar limit. The sensitivity is high enough to open the way towards the detection of a few spins in systems where the decoherence due to nuclear spins can be suppressed by isotopic enrichment, and towards spin resonance microscopy with important applications in biology and materials science

Spatiotemporal electronic spin fluctuations in random nuclear fields in n-CdTe

We report on the dynamics of electron spins in n-doped CdTe layers that differs significantly from the expected response derived from the studies dedicated to electron spin relaxation in n-GaAs. At zero magnetic field, the electron spin noise spectra exhibit a two-peak structure - a zero-frequency line and a satellite - that we attribute to the electron spin precession in a frozen random nuclear spin distribution. This implies a surprisingly long electron spin correlation time whatever the doping level, even above the Mott transition. Using spatiotemporal spin noise spectroscopy, we demonstrate that the observation of a satellite in the spin noise spectra and a fast spin diffusion are mutually exclusive. This is consistent with a shortening of the electron spin correlation time due to hopping between donors. We interpret our data via a model assuming that the low temperature spin relaxation is due to hopping between donors in presence of hyperfine and anisotropic exchange interactions. Most of our results can be interpreted in this framework. First, a transition from inhomogeneous to homogeneous broadening of the spin noise peaks and the disappearance of the satellite are observed when the hopping rate becomes larger than the Larmor period induced by the local nuclear fields. In the regime of homogeneous broadening the ratio between the spin diffusion constant and the spin relaxation rate has a value in good agreement with the Dresselhaus constant. In the regime of inhomogeneous broadening, most of the samples exhibit a broadening consistent with the distribution of local nuclear fields. We obtain a new estimate of the hyperfine constants in CdTe and a value of 0.10 Tesla for the maximum nuclear field. Finally, our study also reveals a puzzle as our samples behave as if the active donor concentration was reduced by several orders of magnitudes with respect to the nominal values.Comment: 9 pages, 7 figure

Quantum limited heterodyne detection of spin noise

Spin noise spectroscopy is a powerful technique for studying spin relaxation in semiconductors. Inthis article, we propose an extension of this technique based on optical heterodyne detection of spinnoise, which provides several key advantages compared to conventional spin noise spectroscopy:detection of high frequency spin noise not limited by detector bandwidth or sampling rates of digitizers,quantum limited sensitivity even in case of very weak probe power, and possible amplificationof the spin noise signal. Heterodyne detection of spin noise is demonstrated on insulating n-dopedGaAs. From measurements of spin noise spectra up to 0.4 Tesla, we determined the distribution ofg-factors, Δg/g = 0.49%

Spectroscopie femtoseconde de semiconducteurs (Contribution à l'étude de la relaxation d'énergie et de la cohérence de spin excitonique)

La cohérence de spin excitonique est étudiée, dans une première partie, dans un échantillon de semi-conducteur massif par la technique du mélange à quatre ondes (FWM). Le couplage fort exciton-photon dans ce système mène à la formation de polaritons, quasi-particules mixtes exciton-photon, dont les propriétés mêlent les propriétés de la lumière et de la matière. Dans le CuCl, semi-conducteur I-VII, le biexciton, état lié de deux excitons, présente une forte énergie de liaison. Nous montrons qu'il est possible de créer et de sonder la cohérence de spin des excitons par une technique de FWM à trois faisceaux. En nous appuyant sur la modélisation des signaux de mélange d'onde nous montrons que la mesure ne souffre pas des forts effets de dispersion du milieu lorsque la cohérence est sondée par l'intermédiaire du biexciton. Nous trouvons que le temps de cohérence de spin égal le temps de relaxation de spin. L'énergie de photon des impulsions excitatrices est également choisie pour que les termes de corrélation exciton-exciton (amplitude de transition du biexciton) dominent les non-linéarités responsables des signaux de mélange d'onde. Nous mettons alors en évidence la génération de battements quantiques de polaritons induits par la formation du biexciton. Ces battements se révèlent être caractéristiques d'un système fortement corrélé.Dans une deuxième partie, nous nous attachons à l'étude de la relaxation d'énergie des paires électron-trou par émission de phonons optiques longitudinaux dans une structure contenant des boîtes quantiques de CdZnTe. La dynamique des répliques phonons d'une distribution de paire électron-trou photocréée est étudiée par des expériences pompe-sonde à deux couleurs et par photoluminescence résolue en temps (PL-RT). La dynamique aux temps longs (2ps<t<30ps) révélée par les deux types de mesures se trouve être fortement liée au décalage de Stokes observé entre l'absorption et l'émission des boîtes quantiques et aux mesures de photoluminescence sous excitation continue. Ces mesures nous permettent d'estimer le temps de relaxation du réseau (~ 5ps) dans la boîte quantique et de lier cette relaxation à la décohérence des excitations électroniques dans les boîtes de semiconducteurs II-VI. L'étude aux temps courts (t<2ps) des spectres de variations de transmission nous permet de déterminer que le temps d'émission d'un phonon est de 130 fs. Celui-ci est inférieur à la période d'oscillation du phonon de 165 fs. Cela signifie que la paire électron-trou perd l'énergie d'un phonon avant même la première oscillation de celui-ci. Nous montrons alors que la dynamique des répliques phonon est dominée par des effets de cinétique quantique visibles sous la forme d'un élargissement des répliques qui dépend du temps.The exciton spin coherence is studied in bulk copper chloride by means of four-wave mixing (FWM) experiments. In this large band gap semiconductor, the strong exciton-photon coupling leads to the formation of polaritons, which are mixed exciton-photon quasi-particles, i.e. elementary excitations showing mixed light and matter properties. In this I-VII semiconductor the two-exciton bound state, also called biexciton, has an important binding energy. We take advantage of this to show that it is possible to create and to probe an exciton spin coherence in a three beam FWM experiment. Calculations show that the measured spin coherence, probed through the biexciton formation, is not modified by the strongly dispersive properties of the medium. The photon energy of the pulses has to be chosen such that exciton-exciton correlations (biexciton amplitude transition) dominate the nonlinearities. When comparing FWM signal calculations to measured emission, we evidence polariton quantum beats induced by the biexciton formation. These beats are shown to be observable only in a strongly correlated system.In a second part, we present a study of the electron-hole pair energy relaxation by longitudinal-optical phonon emission in a CdZnTe nanostructure, which contains quantum dots. We study the phonon replicas of a photocreated electron-hole distribution and we measure its dynamics by two-colours pump-probe (PP) and time resolved photoluminescence experiments (TR-PL). The long-time dynamics (2ps<t<30ps) found in the PP and TR-PL experiments is shown to be linked to the Stokes shift between absorption and emission of the quantum dots. These measurements allow us to estimate the quantum dot lattice relaxation time (~ 5ps), which dominates the decoherence processes in II-VI semiconductor quantum dots. The short-time dynamics (t<2ps) in PP spectra allows us in addition to measure the optical phonon emission time (130 fs) which is found to be smaller than the optical phonon oscillation period (165 fs). This means that the electron-hole pair energy relaxes before the first phonon oscillation is finished. In this case, the dynamics of the phonon replica is strongly dominated by quantum kinetic phenomena. We evidence this through a time dependant broadening of the phonon replicas.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Ultralong spin relaxation time of donor bound electrons in n-doped CdTe measured by spin noise spectroscopy

In the recent years the spectroscopy of spin noise has won its spurs for spin dynamics related studies in semiconductors, largely because it exhibits several quite attractive features. Noticeably it enables almost perturbation-free detection of spontaneous electron spin fluctuations. In addition as the amplitude of spin noise grows when the size of the probed region is reduced, it is well adapted for spatially resolved studies. Also the spin noise spectrum is quite sensitive to internal effective fields, which allows to probe locally the existence of nuclear fields. Finally, the combination of spin noise spectroscopy and optical heterodyning has been demonstrated, which permits enhanced sensitivity and broadband detection, while keeping a high spectral resolution [1].In this poster we will present results obtained by heterodyne detection of spin noise in an n-doped CdTe epilayer with donor density n∼3×〖10〗^17cm-3. Thanks to the enhanced sensitivity gained by heterodyne detection we could detect the spin noise of electrons bound to neutral donors for probe powers as low as 5 µW. In these conditions we observe an extremely slow hopping rate W0 of electron spin between neighbouring donors, and extremely slow electron spin relaxation rate s (see Figure 1). The observed noise spectrum at zero field is characteristic of electron spin precession in the frozen nuclear field acting on the electrons bound to the donors and exhibits two components. A central lorentzian line corresponding to the electron spin component along the nuclear field, and two satellites gaussian lines corresponding to the spin precession in the nuclear field. We analyze our results in the framework of a theoretical model, which takes into account both the electron spin precession, and the hopping between donors, but slightly modified to take into account an eventual non-zero nuclear spin polarization [2]. In agreement with the theory we can see that the satellites merge with the central line as the hopping rate increases (at the highest probe power). Surprisingly W0 and s become quite small at the lowest probe power. We find that the electron spin relaxation time becomes longer than 1 µs in this regime

Spatiotemporal Spin Noise Spectroscopy

We report on the potential of a new spin noise spectroscopy approach by demonstrating all-optical probing of spatiotemporal spin fluctuations. This is achieved by homodyne mixing of a spatially phase-modulated local oscillator with spin-flip scattered light, from which the frequency and wave vector dependence of the spin noise power is unveiled. As a first application of the method we measure the spatiotemporal spin noise in weakly n-doped CdTe layers, from which the electron spin diffusion constant and spin relaxation rates are determined. The absence of spatial spin correlations is also shown for this particular system

Long coherence time of isolated Mn spin in CdTe revealed by pump-probe experiment

International audienceWe report time-resolved Kerr rotation experiments on bulk CdMnTe with very low Mn concentration. We observe spin beatings which are the result of a complex spin dynamics of the d-electrons in the hyperfine field of their nucleus and the tetragonal crystal field. We find the transverse Mn spin relaxation time to increase with the increase of the effective temperature of Mn subsystem. At higher temperatures we measure T2 up to 12 ns, which is much longer than any previously measured values

Omnidirectional spin noise spectroscopy

International audiencespin fluctuations in atomic vapors and in semiconductors. As such it is quite attractive for probing non-perturbatively electrons or nuclear spins in atomic vapours or in semiconductors, particularly when the laser is detuned from optical resonances, an aspect which has triggered considerable interest to spin noise spectroscopy in recent years. The signal in spin noise spectroscopy can be considered as arising from intensity fluctuations due to the interference between the light scattered by Raman spin-flip excitations and the probe laser. This suggests that homodyne or heterodyne detection of spin noise by mixing of the Raman signal with a local oscillator is feasible, which we have demonstrated recently [1]. This opportunity is essential to spin noise spectroscopy because it allows us to increase its sensitivity and accessible frequency range.Homodyne and/or heterodyne detection of spin noise for scattered light directions different from that of the probe should give access to spatio-temporal spin correlations [2], and therefore could considerably extend the application range of spin noise spectroscopy. Here we demonstrate non-collinear homodyne detection of spin noise in n-doped CdTe epilayers. This allows us to exploit different scattering geometries based on spin-flip Raman selection rules. We are able to measure simultaneously different fluctuating spin components by homodyne mixing in different space directions, and also for different local oscillator polarizations. [1] S. Cronenberger and D. Scalbert, Review of Scientific Instruments 87, 093111 (2016).[2] G. G. Kozlov, I. I. Ryzhov, and V. S. Zapasskii, Phys. Rev. A 95, 043810 (2017)

Exciton-spin dephasing and relaxation due to symmetry breaking in two-band bulk semiconductors

The full point group symmetry of a crystal can be broken due to internal or external effective fields. In the study of excitons, such symmetry breaking can lead to a coupling of different exciton states and if a system is prepared in an exciton state with a defined total angular momentum (pseudospin), spin beating is obtained. Looking at the fluctuations of these fields, we use the invariant expansion of an effective Hamiltonian to investigate exciton-spin-relaxation dynamics in a model two-band bulk semiconductor and discuss the respective importance of the different spin-flip processes. We find that interaction terms leading to an electron or hole spin flip give rise to a pure transverse dephasing. Terms where the electron and hole spins are simultaneously reversed lead to transitions between the spin states, which are characterized by the longitudinal relaxation time. Similar to motional narrowing in the case of free carriers, the latter process can lead to an increase in the exciton-spin-relaxation times if extrinsic electric, magnetic, or strain fields rapidly fluctuate in the sample. This effect is shown to be due to the electron-hole exchange interaction

Mn Hyperfine Beats in CdMnTe

Mn spin dynamics is measured in very diluted (Cd,Mn)Te crystals by time-resolved Kerr rotation. Spin beats due to the hyperfine interaction between the 3d electrons of the Mn ions and their own nuclei are detected. It is shown that the effect of the crystal field can be strongly suppressed for "magic" orientations of the magnetic field. This particular orientation of the field permits the optical read-out of the Mn nuclear spin state. Manganese ions trapped on a semiconductor lattice have uniform properties and relatively long spin lifetimes, which make them promising for optical manipulation. In particular, Mn2+ ions embedded in a II-VI semiconductor are S-state ions, weakly coupled to the lattice. For this reason quite long Mn electronic spin relaxation times are expected and have been observed [1]. Here we show that the spin coherence time is mainly limited by dipole-dipole interactions at a Mn concentration x=0.001, and reaches up to 15 ns. At this low concentration, fine and hyperfine structures of Mn2+ are resolved in electron spin resonance experiments. In the time-domain it corresponds to low-frequency beats as shown in Fig. 1