Institut des
NanoSciences de Paris

Nanostructures and quantum systems

Spin excitations in nanostructures

We focus on spin excitations in two or one dimensional electron systems confined in semiconductors (GaAs, CdTe) or magnetic semi-conductors (CdMnTe). The main technique is resonant electric Raman spectroscopy, however, through collaborations, we investigate time domain measurements of spin excitations by magneto-optic Kerr rotation or time-domain spectroscopy of THz emission.

Spin-orbit and spin-waves
(Collaboration with University of York and University of Missouri, Royal Society)

A modulation doped quantum well (the doping layer lies in one of the two barriers only) holds a structural inversion asymmetry (SIA) caused by the electric field of the ionized dopants. Hence, strong Rashba spin-orbit fields do exist. Each individual moving electron experiences an effective magnetic field proportional to its in-plane momentum, which couples to its spin. It results that a collective excitation, formed by the coherent precession of all the electron spins, looses coherence due to the spread of the individual precession frequencies and axis. We showed that a spin wave is immune against this lecherous mechanism. Indeed, because of the Coulomb interaction, the individual spin-orbit magnetic fields organize to construct a single collective magnetic field which governs the precession of the spin-wave as if the collective mode was a single object, a magnetic moment, immersed in a homogenous spin-orbit magnetic field. The phenomenon was observed for spin-modes in a GaAs quantum well [12] and for spin waves in CdMnTe [13]. The collective field was found to be enhanced by a factor 6 (electron density dependent) compared to the individual spin-orbit field. The collective Rashba constant was 20meV.Å, comparable to the one found in SrTiO3, where Rashba fields are considered as giant.

Press release here

Spin-Orbit (SO) collective field and magnetic moment µ. (a) Components of the collective SO field parallel (filled circles) and perpendicular (open circles) to q for q 8.0 µm-1, as extracted from the data and compared with theory (lines). (b) Spin-plasmon magnetic moment, experimental (squares) and theoretical (line). (c) Minimum (open diamonds) and maximum (filled diamonds) SO field versus q, compared to theoretical values (lines). (d) Spin-plasmon magnetic moment averaged over , experimental (squares) and theoretical (line), as a function of q. From Ref. [12].

[12] Giant collective spin-orbit _eld in a quantum well : Fine structure of spin plasmons. F. Baboux, F. Perez, C. A. Ullrich, I. D’Amico, J. G_omez, and M. Bernard, Phys. Rev. Lett. 109, 166401 (2012).

[13] Coulomb-Driven Organization and Enhancement of Spin-Orbit Fields in Collective Spin Excitations. F. Baboux , F. Perez, C. A. Ullrich, I. D’Amico, G. Karczewski and T. Wojtowicz, Rapid Communications, Phys. Rev. B 87, 121303(R) (2013).

THz radiation from spin precession
(Collaboration with laboratoire Pierre Aigrain, Ecole Normale, rue d’Ulm, CNANO IdF)

In the search for spin-based THz emitter, we have, with an optical femto-second pulse, excited the spin-wave modes of a 2DES embedded in a CdMnTe quantum well. The coherent spin precession starts and radiate an electromagnetic field which was collected by electro-optic sampling, thanks to the tunability in the THz range of the spin precession frequency. The latter is allowed through the adjustable Mn concentration. We demonstrated the spin origin of the observed radiation[14].

(a) Time-dependent radiated electric fields of the sample for various applied magnetic fields . The sample was excited by circularly polarized light with an energy per pulse 68nJ/cm2 and a central wavelength of 763nm. A vertical offset is added for clarity. (b) (squares) Amplitude of the transient oscillation extracted From (a), as a function of the magnetic field. (line) Calculated amplitude of the emitted field (see text). (Inset) A near-infrared (NIR) pulse (100 fs) with circular polarization is focused, with normal incidence, on the QW structure. The sample is immersed in a bath of superfluid helium inside a split-coil magnet with a static field B up to 8 T applied along the quantum well plane (parallel to the z direction). The THz transient electric field is collected along the z direction. From [14].

Press release here

See the THz team’s website

[14] Terahertz radiation from magnetic excitations in diluted magnetic semiconductors. R. Rungsawang, F. Perez, D. Oustinov, J. Gomez, V. Kolkovsky, G. Karczewski, T. Wojtowicz, J. Mad´eo, N. Jukam, S. Dhillon and J. Tignon, Phys. Rev. Lett. 110, 177203 (2013).

Spin excitations in spin-polarized two dimensional electron gas
(Collaboration with IFPAN of Varsovie, King’s College of London and University of Bath)

We introduced a model system to study the spin excitations (spin waves) of a conducting two dimensional electron system (2DES). The 2DES is embedded in a Cd1-xMnxTe quantum well. Insertion of 1% magnetic impurities of Mn creates a local exchange field which can be adjusted by the Mn spin-polarization and the Mn concentration x. Such a 2DES is strongly spin polarized at low magnetic fields, with negligible orbital Landau quantification. Thus, it mimics the spin physics of ferromagnetic diluted magnetic systems while having the advantage of high carrier mobility (µ 105 cm2/Vs). Given that, we have measured the dispersion of spin waves [1,2], showed that the carrier kinetic causes an intrinsic damping of spin waves[3] and me have modelled the responses of this spin polarized 2DES [4] together with its excitations [5]. A spin enhancement of the spin susceptibility has been evidenced [6-8]. These studies, important for the understanding of spin-resolved Coulomb interactions, have been summarized in a book chapter[9].

Comparison between Raman spectra (a) at different q values taken on sample B with the corresponding calculated spectra (b) and (c). Part (b) is obtained from the collective response and part (c) from the single-particle response. From Ref.[2]

Press release here

Related publications :

[9] Spectroscopy of spin polarized 2d carrier gas : spin resolved interactions. In Introduction to the Physics of Di- luted Magnetic Semiconductors, Perez, F. and Kossacki, P., volume 144 of Springer Series in Materials Science, 335-382. Springer (2010), Kossut, J. and Gaj, J., editors.

[8] From spin flip excitations to the spin susceptibility enhancement of a two-dimensional electron gas, Perez, F., Aku-leh, C., Richards, D., Jusserand, B., Smith, L. C., Wolverson, D., and Karczewski, G. , Phys. Rev. Lett. 99, 026403 Jul (2007).

[7] Measuring the spin polariza- tion and zeeman energy of a spin-polarized electron gas : Comparison between raman scattering and photoluminescence, Aku-Leh, C., Perez, F., Jusserand, B., Richards, D., Pacuski, W., Kos- sacki, P., Menant, M., and Karczewski, G. Phys. Rev. B 76, 155416 Oct (2007).

[6] Spin susceptibility enhancement in a two-dimensional hole gas, Boukari, H., Perez, F., Ferrand, D., Kossacki, P., Jusserand, B., and Cibert, J. Phys. Rev. B 73, 115320 Mar (2006).

[5] Spin waves in magnetic quantum wells with coulomb interaction and sd exchange coupling. Perez, F., Cibert, J., Vladimirova, M., and Scalbert, D., Phys. Rev. B 83, 075311 Feb (2011).

[4] Spin-polarized two-dimensional electron gas embedded in a semimagnetic quantum well : Ground state, spin responses, spin exci- tations, and raman spectrum. Perez, F. , Phys. Rev. B 79, 045306 Jan (2009).

[3] Intrinsic damping of spin waves by spin current in conducting two-dimensional systems. Gomez, J., Perez, F., Hankiewicz, E. M., Jusserand, B., Karczewski, G., and Wojtowicz, T., Phys. Rev. B 81, 100403(R) Mar (2010).

[2] Dynamical corrections to spin-wave excitations in quantum wells due to coulomb interactions and magnetic ions. Aku-Leh, C., Perez, F., Jusserand, B., Richards, D., and Karczewski, G., Phys. Rev. B 83, 035323 Jan (2011).

[1] Spin excitations of the spin-polarized electron gas in semimagnetic quantum wells. Jusserand, B., Perez, F., Richards, D. R., Karczewski, G., Wojtowicz, T., Testelin, C., Wolverson, D., and Davies, J. J. , Phys. Rev. Lett. 91, 086802 Aug (2003).

Spin excitations dynamics in spin-polarized two dimensional electron gas
(Collaboration with Université Montpellier II, ANR)

In the frame of an ANR project, aimed at using the previous spin waves to transmit a logical spin-based information, we studied the spin wave dynamics with pump-probe magneto-optical Kerr techniques. Two techniques were used : one pump beam to excite zone centre modes only and two crossed-polarized pump beams to excite spin waves propagating in the quantum well plane. The spin modes of a semi-magnetic systems are composed of two branches where electronic spins and Mn spins oscillate in phase (acoustic spin modes) or out of phase (optical spin modes). These two modes anticross at a particular magnetic field, where an exchange of the dynamics and the character occurs. The anticrossing gap reveals the strength of the dynamical interactions between the two coupled spin systems [10].

(a) Spin precession frequencies and (b) decay times extracted from Kerr rotation measurements under magnetic field B=1–6 T. Insets show the frequencies (a) and the intensities A of the two coupled modes in the resonance region. Symbols show the experimental data, lines are the best fit. From Ref.[10]

See the Montpellier team’s website

[11] Electron spin dephasing in Mn-based II-VI diluted magnetic semiconductors, Z. Ben Cheikh, S. Cronenberger, M. Vladimirova, D. Scalbert, F. Perez, and T. Wojtowicz. Phys. Rev. B Rapid. 88, 201306(R) (2013).

[10] Collective nature of two-dimensional electron gas spin excitations revealed by exchange interaction with magnetic ions. Barate, P., Cronenberger, S., Vladimirova, M., Scalbert, D., Perez, F., G_omez, J., Jusserand, B., Boukari, H., Ferrand, D., Mariette, H., Cib- ert, J., and Nawrocki, M., Phys. Rev. B 82, 075306 Aug (2010).