Session Z35: Focus Session: Spins in Semiconductors -- DMS: II-VI and Group IV

Photophysical Properties of Colloidal Mn(II)-Doped CdSe Nanoparticles: Exchange Fields, Exciton Storage, and Light-Induced Spontaneous Magnetization

An attractive approach to controlling spin effects in semiconductor nanostructures for applications in electronics is to use light to generate, manipulate, or read out spins. The main focus of this presentation will be on the recent demonstration of spontaneous photoinduced polarization of Mn(II) spins in doped colloidal CdSe quantum dots, an effect due to the formation of excitonic magnetic polarons. Photoexcitation generates large dopant-carrier exchange fields, enhanced by strong spatial confinement, that lead to giant Zeeman splittings of the semiconductor band structure in the absence of applied magnetic fields. These internal exchange fields allow spontaneous magnetic saturation of the Mn(II) spins to be achieved at zero external magnetic field up to ca. 50 K, and photomagnetic effects are observed all the way up to room temperature. The factors that allow this fascinating effect to be observed in colloidal Mn(II)-doped CdSe nanoparticles will be discussed. Relevant Publications: 1) Beaulac, Schneider, Archer, Bacher, and Gamelin. Science, 325, 973 (2009) 2) Beaulac, Archer, Ochsenbein, and Gamelin, Adv. Funct. Mat., 18, 3873 (2008)   

Polarized Magneto-Photoluminescence from Mn-doped ZnSe/CdSe Core/Shell Nanocrystals

We study the low temperature magneto-optical properties of Mn-doped ZnSe/CdSe core/shell nanocrystals using magnetic circular dichroism (MCD) and circularly polarized luminescence (PL) as a function of magnetic field. MCD studies reveal giant field- and temperature-dependent Zeeman splittings of the band-edge exciton, demonstrating a strong \textit{sp-d} exchange coupling of electrons and holes to the embedded paramagnetic Mn atoms [1]. Magneto-PL studies surprisingly reveal a strongly circularly polarized PL from internal Mn transitions at $\sim $2.15 eV with applied magnetic fields, which follows the same field- and temperature-dependent (Brillouin-like) magnetization of the Mn spins. Notably, the intensity of the right- and left-circularly polarized Mn PL increases and decreases with applied field, respectively, in strong contrast to similar studies in bulk ZnMnSe and in ZnCdMnSe quantum wells. We discuss the effects of strong quantum confinement on coupling between spin-polarized excitons and the local Mn spins. [1] D. A. Bussian \textit{et al.}, Nature Materials \textbf{8}, 35 (2009).   

Magnetism and Carrier Spin Polarization in Mn-doped CdSe Quantum Dots

We report the magnetic and magneto-optical properties of Mn2+ doped CdSe nanoparticles synthesized by hot colloidal solution method. Magnetic hysteresis measurements on a particle ensemble show that they are paramagnetic at room temperature, and become ferromagnetic below about 50 K. The coercivity reaches to about 0.4 Tesla at 6K. The carrier spin polarization has been investigated by circularly polarized photoluminescence. Positive circular polarization of the PL of 30{\%} at 7K has been observed. This is due to the excitonic Zeeman splitting resulting from the strong sp-d exchange interactions between the carriers and Mn dopants. The circular polarization has been investigated as a function of applied magnetic field in the 7-100 K temperature range.   

Magneto-PL Measurements of (Zn,Mn)Se Nanowires and Residual Nanostructures

Magnetic semiconductor nanowires (NWs) potentially provide model systems for studying spin polarized 1D Fermi liquids [Nano Letters {\bf 9}, 3142 (2009)]. Here, we report low temperature magneto-photoluminescence measurements of (Zn,Mn)Se NWs grown using a two-stage vapor-liquid-solid process [e.g. Appl. Phys. Lett {\bf 93}, 143106 (2008)] that yields defect-free NWs in the 1D regime. Far-field photoluminescence (PL) measurements of as-grown samples show near band edge emission as well as a broad defect band convolved with Mn emission. In a magnetic field, we observe significant Zeeman shifts in the band edge luminescence at low temperatures, indicating the presence of strong sp-d exchange. Micro-PL measurements of as-grown samples and dispersed NWs map out the magnetic field variation of spatially resolved spectra at submicron length scales, revealing spatially localized emitters with both spectrally sharp features near the band edge as well as broad defect PL. We observe blinking and spectral diffusion in the sharper spectral features. We attribute the observed PL spectra to both (Zn,Mn)Se NWs and residual (Zn,Mn)Se nano-crystallites nucleated during the NW growth process. Supported by NSF.   

Optical Studies of Single Zn$_{1-x}$Mn$_x$Se Nanowires Synthesized by Chemical Vapor Deposition

Magnetic semiconductor nano-filaments provide interesting model systems for fundamental studies of quasi-one-dimensional spintronics [1]. Here, we report the growth and characterization of single crystal Zn$_{1-x}$Mn$_x$Se nanowires (NWs) and nanobelts fabricated on Si and quartz substrates via the vapor-solid-liquid mechanism during chemical vapor deposition. We obtain NWs that are tens of $\mu$m in length, with diameters in the range 40 nm$-$100 nm. The Mn concentration can be varied over the range $0 \leq x \leq 0.5$ by controlling the substrate temperature. We carry out Raman and photoluminescence (PL) measurements on single NWs supported over the holes of a transmission electron microscope (TEM) grid. This allows us to directly correlate optical properties with structural characteristics of the NWs obtained using TEM. Room temperature micro-Raman measurements on single NWs probe the phonon modes, while low temperature PL spectra show clear evidence for the substitutional incorporation of Mn into the ZnSe lattice. This work is supported by NSF-MRSEC.   

Magnetic Polarons in Anisotropic Quantum Dots

Tunability of confinement in magnetically-doped quantum dots (QDs) allows to tailor magnetism to an extent not available in bulk semiconductors. Versatile control of magnetic ordering, along with piezomagnetism, has been predicted even at a fixed number of carriers [1]. Recent experiments on colloidal QDs revealed strongly bound magnetic polarons (MPs) [2]. Previous studies of MPs in bulk semiconductors showed that the mean-field theory predicts a spurious magnetic phase transition, which is removed by taking into account spin fluctuations [3]. Here we present our theoretical results for MPs forming in QDs with pronounced magnetic anisotropy, which influences the spin fluctuations. We apply our findings to explain some peculiarities of the magnetic behavior of type-II ZnSe/(Zn,Mn)Te QDs, where magnetic polarons are found to persist to at least 200K [4]. Supported by ONR, AFOSR, and NSF-ECCS CAREER. \\[4pt] [1] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. 101, 207202 (2008); I. Zutic and A. G. Petukhov, Nature Mater.4, 623 (2009). \\[0pt] [2] R. Beaulac et al., Science 325, 973 (2009). \\[0pt] [3] T. Dietl and J. Spalek, Phys. Rev. Lett. 48, 355 (1982). \\[0pt] [4] I. R. Sellers, R. Oszwaldowski, et al., preprint; I. R. Sellers et al., Phys. Rev. Lett. 100, 136405 (2008).   

Magnetic Polarons in type-II (Zn,Mn)Te columnar quantum dots

We present the results of a time-resolved photoluminescence (TRPL) study of type-II (Zn,Mn)Te/ZnSe quantum dots. The sample consists of 5 layers of (Zn,Mn)Te QDs separated by ZnSe spacers. We observe magnetic ordering in these QDs through measurements of the peak energy (E) of the PL vs. time. The large red shift with time ($\sim $ 40 meV at low temperatures) is attributed to the formation of magnetic polarons (MPs) in the QDs, induced by the exchange interaction between the spins of photoexcited holes and those of Mn within the QDs. The MPs are detected at temperatures up to $\sim $ 200 K, with a binding energy that is very weakly dependent on temperature. We find two separate time scales for all temperatures. The shorter time (0.7 ns) is assigned to the MP formation while the origin of the longer time (11 ns) is not well understood [1]. The TRPL measurements on a control ZnTe QD sample showed a much different behavior (initial blue shift of E over about 20 ns followed by a very gradual red shift). We also present magnetization measurements of these two samples. [1] I. R. Sellers et al. pre-print.   

Single Fluorine Impurities in ZnSe: Magnetospectroscopy and Spin Qubit Applications

We report on the optical detection and investigation of single donors in ZnSe. By isolating the donors in quantum well mesas, we are able to probe them individually using magnetospectroscopy in both Voigt and Faraday geometry. The structure of interest is the electron bound to a F$^{19}$ neutral donor, which has been proposed as a strong candidate for a semiconductor-based qubit. The donor electron is optically accessible through the bound exciton transition, allowing the possibility of ultrafast optical spin control and detection. We present our recent spectroscopic and $g^2(0)$ experimental results and discuss their spin qubit applications.   

Diluted magnetic semiconductor quantum wells: disorder and electron-electron interaction

Using an equation of motion approach for the current-current response function, we develop a theory of electron transport in diluted magnetic semiconductor (DMS) quantum wells that treats disorder and electron-electron interaction on the same footing. A first principle treatment of disorder is implemented which goes beyond the simple relaxation time approximation. Interactions within the electron liquid including correlation effects and collective excitations are accounted for through the methods of time-dependent density functional theory. We present results for transport and optical properties as well as charge and spin collective modes in DMS quantum wells and discuss the influence of charge and spin disorder and electronic many-body effects.   

Chemical trend of exchange coupling in II-VI diluted magnetic semiconductors

We present an ab-initio study of the magnetic couplings in Mn- and Co-doped II-VI DMS ZnA (A=O,S,Se,Te). We show the necessity of taking into account the strong electron correlation on the transition metal (TM) 3d level to reproduce correctly the experimental chemical trend. Within the LSDA+U (local spin density approximation with a Hubbard-type correction to TM 3d electrons), we find (i) the d-d exchange couplings between nearest-neighbor magnetic ions to be antiferromagnetic (AFM) of the order of -1 meV and (ii) the sp-d exchange constants between magnetic ions and conduction (valence) band electrons (holes) N$\alpha $ (N$\beta )$ to be FM (AFM) of the order of 0.1 eV (-1 eV). In ZnMnO and ZnCoO, the strong p-d hybridisation leads to the presence of a bound state above the valence band, the failure of the commonly-used Larson perturbation theory formulae for p-d and d-d exchange interactions [1] and prevents high-Tc ferromagnetism [2]. \newline [1] B. Larson \textit{et al. }, PRB 37, 4137 (1988) \newline [2] T. Chanier \textit{et al.} , PRB 79, 205204 (2009)   

Studies of the (013) HgTe/Cd$_x$Hg$_{1-x}$Te heterostructure in tilted high magnetic fields

Properties of 2D carriers in the symmetrically doped Cd$_x$Hg$_ {1-x}$Te/HgTe/Cd$_x$Hg$_{1-x}$Te heterostructure with the quantum well thickness of 20 nm, carrier density n = 1.6 $\times$10$^{11}$\,cm$^{-2}$, and mobility $\mu$ = 28 $m^2/Vs$ were studied in tilted magnetic fields of up to 18 T at temperature 0.6 K. The heterostructure was grown on a (013) surface of a GaAs wafer as it was expected that the quantum well quality might be better than if created on a customary (001) surface. Coincidence of quantum levels in the range of SdH oscillations at $\nu$ = 4 and 6 was observed at the tilt angle values of about 67, 78, and 83$^0$. Thus, m*g*/m$_0 $=0.786 and if the effective mass m*/m$_0$ = 0.024, the effective g-factor g*=33 in agreement with the value obtained on the (001) oriented HgTe/Cd$_x$Hg$_{1-x}$Te wells. However, in the quantum Hall regime at $\nu$ = 3 maximum of the magnetoresistance does not occur at the corresponding critical angle of 78$^0$.   

Amorphous Magnetic Ge(1-x)Mn(x) Thin Films Grown by MBE

We explored the properties of Mn doped magnetic Group IV semiconductors with the ultimate goal of providing a new structure for logic switches that have extremely low bit switching energy. Precipitate-free amorphous Ge(1-x)Mn(x) thin films have been prepared by co-depositing Ge and Mn on SiO(2)/Si using a Molecular Beam Epitaxy (MBE) system. We varied the growth temperature and Mn doping concentration (2.8{\%}, 10.9{\%} and 21.3{\%}) in order to achieve the optimal magnetic properties. The ferromagnetic saturation moments were found to increase with Mn concentration with a maximum of 0.7 Bohr magnetron per Mn in the as-grown samples. Similar to MBE-grown single crystalline and implanted amorphous GeMn, two magnetic transition temperatures around 15 K and 200 K were observed in these amorphous MBE-grown samples. The Anomalous Hall Effect (AHE) persisted up to 200 K and disappeared together with the magnetism, which confirmed the strong correlation between the magnetization and transport properties and indicated the presence of substitutional Mn ions dispersed in the Ge host. In addition, negative magnetoresistance (MR) was detected from 5K to room temperature.   

Modulated spinodal decomposition in (Ge,Mn) films grown on GaAs(001)

The field of ferromagnetic semiconductors evolves very fast nowadays for their potential use in spintronic devices. Up to now, efforts have mainly focused on Diluted Magnetic Semiconductors but Curie temperatures in these materials still remain modest. One possible route to increase at least locally transition temperatures is to use spinodal decomposition leading to transition metal-rich high T$_{C}$ nanostructures. We focus here on (Ge,Mn) considered as a model system for spinodal decomposition and compatible with Si-based microelectronics. While the growth of (Ge,Mn) films on Ge substrates leads systematically to Mn-rich self-assembled nanocolumns exhibiting high T$_{C}$, we demonstrate the fine control of spinodal decomposition in (Ge,Mn) films grown on GaAs. Using different surface preparations, we clearly identify the role of surface morphology and impurity diffusion from the substrate (Ga or As) on the nanocolumns growth and the electrical properties (MR and AHE).   

Many-body effects in the cyclotron resonance of few-electron quantum dots doped with a single magnetic impurity

The magneto-optical absorption spectrum of a II-VI cadmium telluride based quantum dot containing few electrons ($N_e=1\div5$) doped with a single magnetic impurity ($Mn^{2+}$) is studied in the presence of a magnetic field. The strongly correlated electrons interact with the magnetic ion (Mn-ion) through the spin-spin exchange interaction which 1) competes with the Zeeman splitting energies leading to the existence of different magnetic phases, 2) results in the coupling of the electron center-of-mass motion with the relative motions leading to significant changes in the cyclotron resonance spectrum as compared to the case without a Mn-ion. At the ferromagnetic-antiferromagnetic transition: 1) the ground-state energy exhibits a cusp, 2) the cyclotron resonance energies exhibit a shift, 3) the oscillator strengths are discontinuous, and 4) the number of allowed transitions increases. The cyclotron resonance spectra are obtained which are quantitative and qualitative different for different $N_e$ due to the breakdown of Kohn's theorem. The results are dependent on the position of the Mn-ion inside the quantum dot. \begin{flushleft} \small{Nga T. T. Nguyen and F. M. Peeters, Phys. Rev. B \textbf{78}, 045321 (2008); \textbf{78}, 245311 (2008); \textbf{80}, 115335 (2009).} \end{flushleft}