Tailoring Magnetism in Quantum Dots

Abstract

We study magnetism in magnetically doped quantum dots as a function of confining potential, particle numbers, temperature, and strength of Coulomb interactions. We explore possibility of tailoring magnetism by controlling the electron-electron Coulomb interaction, without changing the number of particles. The interplay of strong Coulomb interactions and quantum confinement leads to enhanced inhomogeneous magnetization which persist at higher temperatures than in the noninteracting case. The temperature of the onset of magnetization can be controlled by changing the number of particles as well as by modifying the quantum confinement and the strength of Coulomb interactions. We predict a series of electronic spin transitions which arise from the competition between the many-body gap and magnetic thermal fluctuations.PACS numbers: 75.75.+a,75.50.Pp, Magnetic doping of semiconductor quantum dots (QDs) provides an interesting interplay of interaction effects in confined geometries [1,2,3,4,5,6,7,8] and potential spintronic applications [9]. In the bulk-like dilute magnetic semiconductors the carrier-mediated ferromagnetism can be photoinduced [10,11] and electrically controlled by gate electrodes [12], suggesting possible nonvolatile devices with tunable optical, electrical, and magnetic properties [9]. QDs allow for a versatile control of the number of carriers, spin, and the effects of quantum confinement which could lead to improved optical, transport, and magnetic properties as compared to their bulk counterparts [1,13,14]. Unlike in the bulk structures, adding a single carrier in a magnetic QD can have important ramifications. An extra carrier can both strongly change the total carrier spin and the temperature of the onset of magnetization which we show can be further controlled by modifying the quantum confinement and the strength of Coulomb interactions.We study the magnetic ordering of carrier spin and magnetic impurities in (II,Mn)VI QDs identified as a versatile system to demonstrate interplay of quantum confinement and magnetism [4,5,6,15,16,17,18]. Because Mn is isoelectronic with group-II elements it does not change the number of carriers which in QDs are controlled by either chemical doping or by external electrostatic potential applied to the metallic gates. The latter allows confinement of the carriers in a dot with tunable size and shape [2]. By using real space finite-temperature local spin density approximation (LSDA) [19] we study temperature (T ) evolution of magnetic properties of QDs over a large parameter space. This approach allow us to consider QDs with varying number of interacting electrons (N ) and Mn impurities (N m ) which already for small N and N m becomes computationally inaccessible to the exact diagonalization techniques [18,20]. We extend the previous studies of Coulomb interactions in magnetic QDs with N m = 1, 2 at T = 0 [18] and T > 0 results using either Thomas-Fermi approximation or by applying Hund's rule with up to 6 carriers [17]. We reveal that the interplay of strong Cou...

Single-exciton spectroscopy of single Mn doped InAs quantum dots

“…From the theory side there is a lot of interest on the effect of number of carriers on the magnetic properties of a dot doped with Mn atoms. 9,11,16,25,[37][38][39] A priori, the ground state of a negatively charged InAs dot doped with a single Mn should be the ionized A − acceptor, with spin properties identical to those of Mn in neutral CdTe. 14 Band-to-band transitions should yield zero-field PL spectra with 6 peaks.…”

Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots

“…[1][2][3][4] Compared to their bulk counterparts, [5][6][7][8][9] magnetically doped semiconductor QDs could provide control of the magnetic ordering, [10][11][12][13][14][15][16] with the onset of magnetization at substantially higher temperatures. [17][18][19][20][21] Experiments typically focus on Mn-doped II-VI and III-V QDs, in which it is possible to include both single [22][23][24][25] and several magnetic impurities, [17][18][19][20][21][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] having similarities with nuclear spins.…”

Direct Measurements of Magnetic Polarons in Cd1–xMnxSe Nanocrystals from Resonant Photoluminescence

“…As such, local spin-spin interactions can be greatly enhanced by strong quantum confinement, which compresses carrier wavefunctions to nanometer-scale volumes and therefore increases |ψ e,h (r)| 2 . The extent to which these exchange interactions can be enhanced and controlled via quantum confinement is an area of significant current interest and has recently been studied in a variety of magnetically-doped semiconductor nanostructures, including nanoribbons [12, 13], nanoplatelets [14], epitaxial quantum dots [14][15][16][18][19][20][21][22][23][24][25][26], and colloidal nanocrystals [1-8, 27, 28, 30].A particularly striking consequence of sp-d interactions in II-VI semiconductors is the formation of exciton magnetic polarons (EMPs), wherein the effective magnetic exchange field from a single photogenerated exciton -B ex -induces the collective and spontaneous ferromagnetic alignment of the magnetic dopants within its wavefunction envelope, generating a net local magnetization even in the absence of any applied field [31][32][33]. In turn, these aligned local moments act back on the exciton's spin, which lowers the exciton's energy, further localizes the exciton, and further stabilizes the polaron.…”

“…In turn, these aligned local moments act back on the exciton's spin, which lowers the exciton's energy, further localizes the exciton, and further stabilizes the polaron. The stability and binding energy of EMPs therefore depends on the detailed interplay between many factors including the exciton lifetime, the polaron formation time, the exchange field B ex , sample dimensionality, and temperature.EMPs and collective magnetic phenomena have been experimentally studied in a variety of Mn 2+ -doped semiconductor nanostructures, including CdMnSe and CdMnTe-based epilayers and quantum wells [34][35][36][37][38][39][40][41][42], selfassembled CdMnSe and CdMnTe quantum dots grown by molecular-beam epitaxy [14][15][16][18][19][20][21][22][23][24][25], and most recently in CdMnSe nanocrystals synthesized via colloidal techniques [8,28]. Common measurement techniques include the analysis of conventional (i.e., non-resonant) PL [8, 14, 15,18,21,28,39] and time-resolved PL [8, 16,28,34,35,40].…”