Semimagnetic Quantum Research Articles

Electric field tuning of spin splitting in a quantum dot coupled to a semimagnetic quantum dot

We develop an approach for tuning the spin splitting and g-factor of a quantum dot by coupling it to semi-magnetic quantum dot and tuning the electric field. We show that spin splittings and g-factors of the states of a non-magnetic quantum dot coupled to semimagnetic quantum dot can be enhanced orders of magnitude. Evaluations are made for coupled CdTe/CdMnTe quantum dots. These effects are caused by electric field control of repulsion of spin sublevels in the non-magnetic dot due to tunnel coupling of quantum dots. Electric field control of spin splittings in quantum dots is of potential interest in connection with spin qubit rotations for quantum computation.

Hole-mediated ferromagnetism in coupled semimagnetic quantum dots

A comprehensive theory is presented on the tuning of hole states in vertically coupled quantum dots based on diluted magnetic semiconductor and on the spontaneous ferromagnetism induced by the $p\ensuremath{-}d$ exchange interaction. The characteristics of the ground states can be tuned from usual heavy-hole ($hh$)-like to light-hole ($lh$)-like by changing structural parameters. We demonstrate that $hh\ensuremath{-}lh$ coupling strongly influences the $p\ensuremath{-}d$ exchange interaction, in such a way that an increasing coupling strength enhances the system magnetization. Single-band effective mass approximation may not provide a correct prediction of ferromagnetism in coupled quantum dots, thus a multiband $\mathbf{k}.\mathbf{p}$ model was adopted that includes the $Mn$-hole exchange interaction into the same framework and a self-consistent method of the mean field simulation was used.

Magnetic field-induced dynamics of the photoluminescence bands of the II–VI semimagnetic quantum structures

The problem of quantitative analysis of photoluminescence (PL) spectra of semimagnetic quantum structures formed by superposition of a certain number of elementary components is examined. It is shown that the method of averaging can be advantageously used in such analysis. It ensures a precision in determining the single component energy sufficient for quantitative analysis. A precision of determination of both the band full width at half maximum (FWHM) and the intensity of the components is sufficient at least for qualitative analysis.

Mn2+‐assisted recombination of trions in semimagnetic quantum dots

AbstractPhotoluminescence studies of individual CdSe/ZnSe/ZnMnSe quantum dots (QDs) allow us to observe excitons and negatively charged trions in spectra recorded at liquid He temperatures. Exciton and trion lines equally split into a doublet of circularly polarized lines in a magnetic field B directed perpendicular to the sample plain. The ratio of intensities of lines in the doublet differs for excitons and trions. The difference can be explained if spin‐dependent non‐radiative recombination is taken into account. Contrary to expectations processes induced by the Coulomb interaction do not lead to such a recombination. The dominant mechanism of spin‐dependent energy transfer from photoexcited QDs to Mn ions is related to the sp‐d mixing. The efficiency of this mechanism substantially exceeds that induced by the Coulomb interaction. Suppression of the non‐radiative recombination by a magnetic field in Faraday geometry leads to the increase in QDs PL intensity.

Exciton states and tunneling in semimagnetic asymmetric double quantum wells

Exciton level structure and interwell relaxation are studied in Cd(Mn,Mg)Te-based asymmetric double quantum wells (ADQWs) by a steady-state optical spectroscopy in magnetic fields up to B = 10 T. The as grown heterostructures with CdTe QWs and nonmagnetic interwell CdMgTe barrier were subjected to a rapid temperature annealing to introduce Mn and Mg atoms from opposite barriers inside the QWs which results in a formation of the ADQW with completely different magnetic field behavior of the intrawell excitons. The giant Zeeman effect in the QW with magnetic Mn ions gives rise to a crossing of the ground exciton levels in two QWs at BC ∼ 3–6 T which is accompanied by a reverse of the interwell tunneling direction. In a single-particle picture the exciton tunneling is forbidden at B < 1 T as supported by calculations. Experimentally, nevertheless, a very efficient interwell relaxation of excitons is found at resonant excitation in the whole magnetic field range, regardless of the tunneling direction, emphasizing importance of excitonic correlations in the interwell tunneling. At nonresonant excitation an unexpectedly slow relaxation of the σ−-polarized excitons from the nonmagnetic QW to the σ+-polarized ground state in the semimagnetic QW is observed at B > BC, giving rise to a nonequilibrium distribution of excitons in ADQW. A strong dependence of the total circular polarization degree on the hh–lh splitting Δhh–lh in the nonmagnetic QW is found and attributed to the spin dependent interwell tunneling controlled by an exciton spin relaxation. Different charge-transfer mechanisms are analyzed in details and an elastic scattering due to a strong disorder is suggested as the main tunneling mechanism with the underlying influence of the valence band-mixing.

Optical control of the spin state in a semimagnetic quantum dot

AbstractIn a Mn doped quantum dot the spin state of the Mn atom can be controlled via a well chosen sequence of laser pulses, which create or annihilate excitons in the quantum dot. The change of the state consists of a flip of the Mn spin accompanied by a simultaneous spin flip of an electron or light hole induced by the exchange interaction between Mn spin and electron or hole spin. All flipping processes can be interpreted in terms of exchange‐induced Rabi oscillations. The period and amplitude of the oscillations are determined by the coupling strength of the exchange interactions, which also strongly influences the energy splitting between the involved states. External magnetic fields can be used to tune the energies of the exciton states and thereby minimize the energy splitting between the states involved in the spin flip process. Thus the switching dynamics can be optimized by applying a magnetic field. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Spin-polarized two-dimensional electron gas embedded in a semimagnetic quantum well: Ground state, spin responses, spin excitations, and Raman spectrum

We present theoretical aspects of spin-polarized two-dimensional electron gas (SP2DEG) which can be achieved in doped semimagnetic quantum wells. This original model system has been recently studied by magneto-Raman-scattering experiments, which has given access to spin-resolved excitations and spectrum of the SP2DEG. Starting from the diluted magnetic semiconductor (DMS) Hamiltonian in the presence of the Coulomb interaction between conduction electrons, we define the conditions to reach such a SP2DEG. The equilibrium state is studied at low temperature; in particular, a theory for the degree of spin polarization is derived. Dynamical spin susceptibilities are further calculated in the framework of a spin-density-functional formalism already developed in the past. We then derive spin-conserving and spin-flip excitation dispersions using a recent determination of the SP2DEG correlation energy corrected from the thickness of the well. The SP2DEG presents two key features: the spin-flip wave, whose existence is a direct consequence of the Coulomb interaction between the spin-polarized electrons with a dispersion and energy range typical of the SP2DEG obtained in DMS, and the spin-density fluctuations exhibiting a specific collective behavior when the spin polarization is increased. The dissipation spectrum through these excitations is studied in detail. Particular attention is given to the spectrum determined by resonant Raman scattering. We show, indeed, that the latter gives unique access to the spin-fluctuation spectrum of the SP2DEG.

Magnetooptical properties of a single CdMnSe/CdMgSe quantum well

The spectra of photoluminescence and reflectance in magnetic fields up to 7 T are studied for a 3.8 nm semimagnetic CdMnSe quantum well confined by two CdMgSe barriers. A noticeable magnetic shift in the σ+-polarized emission line of the heavy exciton to low energies and a decrease in the halfwidth of the line by more than one-half are detected with increasing magnetic field. It is established that a localized magnetic polaron is formed, with the polaron energy of 19.8 meV determined from the change in the degree of circular polarization in magnetic field. A σ−-polarized emission line is observed in magnetic fields ranging from 0.4 to 2 T. This line can be interpreted as being produced by the complex of two electrons, with oppositely directed spins, and a heavy hole, i.e., by the trion X− or the exciton localized at a donor, D0X. The binding energy of such complex is 10 meV.

Optical polarization of semimagnetic CdSe quantum dots with low manganese content

The authors have studied the circulary polarized photoluminescence of excitons confined in semimagnetic CdMnSe quantum dots (QDs) with nominal Mn concentrations of up to 2%. From the polarization, magnetic properties like exciton g-factors and spin relaxation times were studied. For sufficiently low doping of the QDs with Mn, it is possible to tune the exciton g-factor to zero, i.e., no Zeeman splitting occurs by applying a magnetic field. By light-controlled heating it was also observed that the QD polarization can be reduced, and even inverted. We relate this polarization inversion to the optical heating of the Mn system, which reduces the circular polarization of QD emission originating from Zeeman-split states below the initial polarization of laser-polarized electron–hole pairs.

Spin-magnetophonon level splitting in semimagnetic quantum wells

Spin-magnetophonon level splitting in a quantum well made of a semimagnetic wide gap semiconductor is considered. The semimagnetic semiconductors are characterized by a large effective $g$ factor. The resonance conditions $\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}={\ensuremath{\mu}}_{B}gB$ for the spin flip between two Zeeman levels due to the interaction with longitudinal optical phonons can be achieved at sweeping magnetic field $B$. This condition is studied in quantum wells. It is shown that it leads to a level splitting that is dependent on the electron-phonon coupling strength as well as on the spin-orbit interaction in this structure. We treat in detail the Rashba model for the spin-orbit interaction, assuming that the quantum well lacks inversion symmetry and briefly discuss other models. The resonant transmission and the reflection of light by the well are suggested as suitable experimental probes of the level splitting.

Relaxation of excitons in semimagnetic asymmetric double quantum wells

The steady-state circular-polarized photoluminescence in semimagnetic asymmetric double quantum wells based on Cd(Mn,Mg)Te is studied thoroughly in relation to the polarization of intrawell nonresonance photoexcitation in magnetic fields Bup to 9 T. In low fields B, in which the exciton in the magnetic well is higher in energy than the exciton in the nonmagnetic well, the complete interwell relaxation of excitons is observed. In fields higher than B c = 3–6 T, at which the exciton level in the magnetic well crosses the field-independent exciton level in the nonmagnetic well, the magnetic-field-induced red shift of the exciton in the magnetic well is accompanied by the establishment of a nonequilibrium distribution of excitons. This suggests that spin relaxation plays an important part in the interwell separation of excitons in the spin-dependent potential of the heterostructure. The efficiency of spin relaxation is controlled by mixing of valence band states in the nonmagnetic well and by splitting of heavy and light holes Δ hh-lh . Different modes of interwell tunneling are observed in different field regions separated by the field B c * > B c corresponding to the crossing of the localized excitons in the nonmagnetic well and free excitons in the magnetic well. Possible mechanisms of interwell tunnel relaxation are discussed.

Spin-conserving scattering of holes by magnetic ions in semimagnetic quantum wells

We have calculated the characteristic time for the direct heating of the Mn spin system by spin-polarized photoexcited holes as well as the relaxation time for a single hole in semimagnetic quantum wells in an external longitudinal magnetic field. This spin-conserving heating process is due to the $p\text{\ensuremath{-}}d$ exchange interaction-induced scattering of the holes with the Mn ions within the first heavy-hole subband. The relaxation time for a single hole determines the maximum relative change of the magnetization in the case of short (femtosecond) pulse excitation. Numerical calculations are given for the example of (Zn,Mn)Se-based quantum wells. Due to the strong spin-orbit and exchange interactions within the valence band, the photoexcited hole gas heats the Mn spin system on a nanosecond time scale. This is much more effective than for an electron gas under similar conditions, for which the corresponding characteristic time lies in microsecond range. For a single hole, the relaxation time is much smaller than the characteristic heating time and, for typical experimental conditions, lies in the picosecond range. In the frame of the developed theoretical model, we analyze the details of the dependence of the Mn heating on the hole concentration and temperature on the Mn content and on magnetic field.

Binding Energies of Ground and Excited States of Shallow Donors in Semimagnetic Parabolic Quantum Dots

Binding Energies of Ground and Excited States of Shallow Donors in Semimagnetic Parabolic Quantum Dots

Tunnel coupling in asymmetric semimagnetic double quantum wells

AbstractThe influence of the tunnel coupling on the steady‐state photoluminescence has been studied in Cd(Mg,Mn)Te‐based asymmetric double quantum wells (ADQWs) in a magnetic field up to B = 8 T. As grown structures were annealed to introduce Mn and Mg atoms from the opposite barriers inside pure CdTe quantum wells, resulting in a formation of the magnetic (MW) and nonmagnetic (NMW) wells, respectively. Radiative recombination in the ADQW with a wide barrier (9 nm) shows two bands, corresponding to excitons, localized in two decoupled QWs with a markedly different spin splitting: small splitting in the NMW due to the positive exciton Landé band g ‐factor and large splitting in the MW due to the giant Zeeman effect. Contrary, in the ADQW with a narrow barrier (3 nm) an unusual exciton splitting is observed with a negative value of the exciton g ‐factor and a strong circular polarization of the exciton bands. Simultaneously, a complete reduction of the MW emission is found, indicating strong interwell exciton relaxation. Calculations show that it is a strong electron coupling between the wells which is responsible for the observed behaviour. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Excitons and magnetic polarons in semimagnetic double quantum wells CdSe/CdMgSe/CdMnSe and CdSe/CdMnSe/CdSe

We have studied at high magnetic fields, exciton photoluminescence and reflection spectra of two groups of semimagnetic double quantum wells, CdSe/CdMgSe/CdMnSe and CdSe/CdMnSe/CdSe, with either one of the wells or a coupling barrier being semimagnetic. Owing to strong exchange interaction between band carriers and manganese ions, magnetic tuning of energy levels or the height of the coupling barrier could control the interaction of excitons of the coupled wells and their tunneling. In the first group of structures, we have observed direct and spatially indirect excitons. Photoluminescence intensity of the indirect exciton strongly increased and its half-width decreased with the magnetic field. Dependence of excitons and spin tunneling on the barrier thickness was studied. We also addressed the problem of localized magnetic polarons formation and its peculiarities for indirect and direct excitons. In the second group with a semimagnetic coupling barrier, the energy shifts of the excitons in a magnetic field exceeded significantly the calculated values obtained without account of interface effects. A proper account of both interdiffusion of Mn and Cd ions in the surface layers of the coupling barrier and decrease of antiferromagnetic coupling in these layers, allowed to fit experimental data.

From Spin Flip Excitations to the Spin Susceptibility Enhancement of a Two-Dimensional Electron Gas

The g-factor enhancement of the spin-polarized two-dimensional electron gas was measured directly over a wide range of spin polarizations, using spin flip resonant Raman scattering spectroscopy on two-dimensional electron gases embedded in Cd(1-x)Mn(x)Te semimagnetic quantum wells. At zero Raman transferred momentum, the single-particle spin flip excitation, energy Z*, coexists in the Raman spectrum with the spin flip wave of energy Z, the bare giant Zeeman splitting. We compare the measured g-factor enhancement with recent spin-susceptibility enhancement theories and deduce the spin-polarization dependence of the mass renormalization.

EFFECT OF MAGNETIC FIELDS ON BINDING ENERGY OF IMPURITY STATES IN A SEMIMAGNETIC PARABOLIC QUANTUM DOT

Using a variational approach, the binding energy of shallow hydrogenic impurities in a semimagnetic parabolic quantum dot is calculated within the effective mass approximation. The binding energy is computed for Cd 1-x in Mn x in Te / Cd 1-x out Mn x out Te structures as a function of the dot size in an external magnetic field. The results show that the impurity binding energy (i) increases with the reduction in dot sizes (ii) decreases when the magnetic field is increased for a given dot and (iii) increases to a maximum value at 100 Å and then decreases as the size of the dot increases beyond 100 Å for a realistic model. Spin polaronic shifts are estimated using a mean field theory. These results are compared with the existing literatures.

Direct and indirect excitons and magnetic polarons inCdSe∕CdMgSe∕CdMnSesemimagnetic double quantum wells

We have studied exciton photoluminescence and reflection spectra as well as formation of localized magnetic polarons and spin tunneling in $\mathrm{Cd}\mathrm{Se}∕\mathrm{Cd}\mathrm{Mg}\mathrm{Se}∕\mathrm{Cd}\mathrm{Mn}\mathrm{Se}$ asymmetric double quantum well structures with different barrier thicknesses. Measurements were performed in strong magnetic fields both in Faraday and Voigt configurations. Direct excitons from nonmagnetic CdSe and semimagnetic CdMnSe quantum wells were observed, as well as a spatially indirect exciton formed by the electron in the CdSe quantum well and the heavy hole in the CdMnSe one. The problem of the direct and indirect exciton forming localized magnetic polarons is addressed. Polarization properties of direct and indirect excitons are studied. In the case of a thin barrier, spin tunneling is observed under magnetic field. Calculations of the exciton energies are performed. The calculated values are found to be in reasonable agreement with the measured ones.

Sign reversal and light controlled tuning of circular polarization in semimagnetic CdMnSe quantum dots

Circularly polarized luminescence of CdMnSe quantum dots in magnetic fields up to 5 T is studied for nominal Mn concentrations of 0%, 1%, and 2% by using a photoelastic modulator technique. The exciton g factors as well as spin relaxation times were determined from the polarized luminescence taking into account the exciton lifetimes, which were also extracted by means of time-resolved photoluminescence spectroscopy. For quantum dots without Mn and with 2% Mn exciton g factors of −1.62 and +1.32, respectively, were found. The quantum dots with 1% Mn show a vanishing small value of g for small excitation powers. For this structure the polarization properties are dominated by the optical orientation. Interestingly, for the 1% Mn quantum dots with increasing excitation power considerable changes of the polarization and the exciton g factor were observed which are interpreted in terms of heating effects. From the power dependence indirect heating via phonons and above a critical value direct heating due to photocarriers were identified to result in drastic changes of the circular polarized quantum dot emission.

Exciton states in strongly coupled asymmetric semimagnetic double quantum dots

Exciton states in a pair of strongly coupled artificial asymmetric quantum dots (QDs) have been studied in magnetic fields up to B = 8T by means of photoluminescence spectroscopy. The QD molecules have been fabricated using a selective interdiffusion technique applied to asymmetric CdTe/(Cd,Mg,Mn)Te double quantum wells. The lateral confinement potential within the plane induced by the diffusion gives rise to effective zero-dimensional exciton localization. Incorporation of the Mn ions in only one dot results in a pair of QDs with a markedly different spin splitting. In contrast to a positive value of the exciton Lande g factor in nonmagnetic (Cd,Mg)Te-based single QDs, the ground exciton transition in the nonmagnetic QD demonstrates nearly zero g factor, thus, indicating a strong electron coupling between the dots. A new low-energy band with a strong red shift appears at high B signifying formation of the indirect exciton in accordance with our calculations.