Spin Splitting Anisotropy in Single Diluted Magnetic Nanowire

Letter pubs.acs.org/NanoLett

Spin Splitting Anisotropy in Single Diluted Magnetic Nanowire Heterostructures Małgorzata Szymura,† Piotr Wojnar,*,† Łukasz Kłopotowski,† Jan Suffczyński,‡ Mateusz Goryca,‡ Tomasz Smoleński,‡ Piotr Kossacki,‡ Wojciech Zaleszczyk,† Tomasz Wojciechowski,† Grzegorz Karczewski,† Tomasz Wojtowicz,† and Jacek Kossut† †

Institute of Physics, Polish Academy of Sciences, Al Lotników 32/46, PL-02-668 Warsaw, Poland Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Pasteura 5, PL-02-093 Warsaw, Poland

‡

S Supporting Information *

ABSTRACT: We study the impact of the nanowire shape anisotropy on the spin splitting of excitonic photoluminescence. The experiments are performed on individual ZnMnTe/ZnMgTe core/shell nanowires as well as on ZnTe/ZnMgTe core/shell nanowires containing optically active magnetic CdMnTe insertions. When the magnetic field is oriented parallel to the nanowire axis, the spin splitting is several times larger than for the perpendicular field. We interpret this pronounced anisotropy as an effect of mixing of valence band states arising from the strain present in the core/ shell geometry. This interpretation is further supported by theoretical calculations which allow to reproduce experimental results. KEYWORDS: Nanowires, magneto-optical anisotropy, diluted magnetic semiconductors, spintronics

T

Importantly, the magnetic properties of these structures can be accessed in an optical experiment as the spin splitting is directly proportional to the magnetization.20,21 In the present work, we study systems with a paramagnetic spin response, namely ZnMnTe/ZnMgTe core/shell NWs and ZnTe/ZnMgTe core/shell NWs containing optically active CdMnTe insertions. The impact of the shape anisotropy of individual NWs on their magnetooptical properties is assessed by the micro-photoluminescence (micro-PL) technique in an external magnetic field which can be rotated in the NW plane. Most importantly, we demonstrate a strong anisotropy of the spin splitting. When the magnetic field is applied along the NW axis, the splitting is several times larger than for magnetic field applied perpendicular to the NW. This behavior is explained as a result of the splitting of the heavy and light hole bands which originates from the strain exerted by the shell onto the NW core and, in the case of the NW insertions, from the quantum confinement. The nanowires are grown in a molecular beam epitaxy chamber equipped with Cd, Zn, Mn, Mg, and Te solid source effusion cells by applying vapor−liquid−solid (VLS) mechanism.22 First, a 1 nm thick gold layer is deposited on a (111)oriented silicon substrate. Then, at 370 °C gold/silicon droplets are spontaneously formed with a typical diameters of the order of 10 nm. The droplets act as catalysts for the further growth of

he extreme shape anisotropy inherent to semiconductor nanowires (NWs) makes them exceptionally well suited for a number of applications such as single molecule sensors,1 free-standing Fabry−Perot resonators and lasers,2 and polarization-sensitive photodetectors.3 Since the NWs can be precisely positioned by AFM, electric field,4 or fluidic alignment,5 it is possible to orient the anisotropy axis on demand extending the control over its properties, especially important for applications in photonic circuits and optical interconnects. The shape anisotropy also facilitates accumulation of elastic energy in a NW without formation of dislocations. This allows fabrication of a wider range of heterostructures than in better known planar geometries. Moreover, the accumulated strain can be effectively controlled by the NW growth in core/shell morphology.6,7 In particular, varying the shell thickness and composition can be employed to tune the NW band gap8−10 in a wider range than in quantum wells or Stranski−Krastanow quantum dots (QDs). Doping NWs with transition metal atoms may result in structures with novel and exciting properties.11−15 On one hand, the shape anisotropy may give rise to a magnetization anisotropy in diluted magnetic NWs with ferromagnetic or superparamagnetic spin ordering built of Mn-doped III−V compounds.16−18 On the other hand, the sp−d exchange interaction between itinerant carriers and localized magnetic ions results in a considerable increase of the band Zeeman spin splitting with g-factors reaching values up to several hundred, depending on the density of magnetic ions and the temperature,19 even in systems with paramagnetic spin response. © 2015 American Chemical Society

Received: December 18, 2014 Revised: February 13, 2015 Published: February 24, 2015 1972

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Nano Letters the NWs. Two kinds of structures are grown: core/shell ZnMnTe/ZnMgTe nanowires (NWs) and core/shell ZnTe/ ZnMgTe nanowires with one or two CdMnTe insertions. In the case of the former structures, the ZnMnTe core is grown at about 430 °C for 30 min, which results in structures with the average length of 1.5 μm and the average diameter of 25 nm. In order to deposit the shell, the temperature is lowered to about 360 °C, allowing for a growth in the radial direction of NWs.21 The shell is deposited for 10 min, which corresponds to the average thickness of 10 nm, whereas the Mg concentration amounts to 0.3. In the case of NWs containing CdMnTe insertions, after 25 min growth of ZnTe nanowires the growth temperature is reduced from 430 to 380 °C, and Cd, Mn, and Te effusion cells are opened for 3 min. The estimated Mn concentration in the insertions basing on the Cd/Mn flux ratio is about of 0.03, and their lengths are of the order of a few nanometers. Subsequently, the upper ZnTe NW part is grown for 5 min at 380 °C followed by the deposition of ZnMgTe shell performed in the exactly the same manner as for ZnMnTe/ZnMgTe core/shell NWs. All nanowire-based heterostructures exhibit zinc-blende crystalline structure and are oriented along the (111) crystallographic direction.23,24 The morphology and the density of the NWs are assessed by scanning electron microscopy (SEM, ZEISS Auriga- CrossBeam Workstation). An initial characterization of optical properties is obtained by low-temperature cathodoluminescence (CL) measurements. For these studies, the samples are mounted on a coldfinger of a liquid helium cryostat (Kammrath and Weiss) inside a SEM chamber (Zeiss Evo HD 15). The SEM is equipped with a parabolic mirror for collection of the CL signal. The CL measurements are performed at 5 K. In order to study individual NWs, we ultrasonically remove them from the substrate and disperse on a silicon wafer. On top, a protective PMMA layer is spin-coated for measurements in the liquid helium. For the photoluminescence (PL) measurements, the sample is placed inside a specially designed pumped helium cryostat equipped with a double coil superconducting magnet. The two split coils apply the magnetic field directed along the X- and Z-axis. This enables rotating of the magnetic field vector in XZ plane electrically without any mechanical change in the experimental setup. The light passes along the Y direction through a set of quartz, strain-free windows. The excitation wavelength is either 405 nm (3.06 eV) for ZnMnTe/ZnMgTe core/shell nanowires or 532 nm (2.33 eV) for nanowires with CdMnTe insertions. The laser beam is focused with a microscope objective (3 μm spatial resolution) on the surface of the sample. The PL signal is collected with the same objective and detected with a monochromator and a CCD camera. PL measurements are performed at a temperature of 2 K, in magnetic fields up to 2 T. In order to determine the NW-axis direction of the particular nanowire, the PL linear polarization anisotropy is measured with the PL-signal passing through a rotating half-wave plate and a fixed linear polarizer. Figure 1a shows a SEM image of an ensemble of core/shell ZnMnTe/ZnMgTe nanowires. After the deposition of the shell, the total NW diameters range from 40 to 70 nm and the length is up to 1.5 μm. A micrograph of a single NW cast on a silicon wafer is shown in Figure 1b. The CL spectrum from the same spatial region is demonstrated in Figure 1c. In order to determine the origin of this light emission, we perform monochromatic spatial imaging at the photon energy corresponding to the maximum CL intensity (2350 meV) (see Figure 1d). We observe that the elongated shape of the

Figure 1. (a) SEM image of ZnMnTe/ZnMgTe core/shell NWs grown on (111)-oriented Si substrate. (b) SEM image of a single NW. (c) CL spectrum of a single NW. (d) Monochromatic CL mapping at 2350 meV (526 nm) of the same spatial area. The scale bars in both the SEM and the CL images are 200 nm.

emitting object presented in Figure 1d fits into the spatial positions of the NW seen in Figure 1b, confirming that the emission originates from the NW and not from any other structure. The PL at 2350 meV is related to excitonic recombination, not observed for pure ZnTe NWs and activated by the presence of the ZnMgTe shell.25 The broad peak centered at about 2000 meV is related to the internal Mn2+ transition.26 Because of the fact that the Mn concentration in the NWs is relatively small (only a few percent), the excitonic emission is not completely quenched by the intra-Mn2+ transition similar to the case of thin ZnMnTe epilayers.27 Analogous measurements are performed on single NWs containing a CdMnTe insertion. In Figure 2a, the SEM image

Figure 2. (a) SEM image of a single ZnTe/ZnMgTe core/shell NWs with CdMnTe insertion. (b) CL spectrum of a single insertion embedded in NW. Inset: macro-PL from ensemble of NWs containg CdMnTe insertions. (c) Monochromatic CL mapping at 1990 meV (623 nm) of the same spatial area as in (a). The scale bars in both the SEM and the CL images are 200 nm.

of such a NW is presented, whereas the CL spectrum of the same spatial region is shown in Figure 2b. Compared to the ZnMnTe NWs, the emission is observed at significantly lower energies−about 2000 meV. The monochromatic mapping at 1990 meV visualizes the position of CdMnTe insertion. As illustrated in Figure 2c, the size of the bright spot is approximately 200 nm. This is much larger than the size of the insertion because of carrier diffusion along the NW.28 1973

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Nano Letters Nevertheless, this measurement confirms that the CdMnTe insertion is successfully built into the NW. The presence of CdMnTe insertions is further confirmed by PL measurements at 10 K performed on the ensemble of NWs. A broad band in the energy range between 1800 and 2200 meV is seen (see inset to Figure 2b). In micro-PL measurement, this broad band splits into individual lines originating from single insertions. Since the photon correlation measurements performed on CdTe insertions grown in an analogous manner have shown single photon emission characteristic for quantum dots (QDs),29 we suppose that the insertions are zerodimensional objects. Moreover, the typical full width at halfmaximum (fwhm) for the observed transition is about 4 meV, as presented in Figure 3a. This value is slightly larger than

The NWs dispersed on a silicon substrate are arbitrarily oriented and their typical areal density is relatively small, usually less than 1 NW on 100 μm2. Thus, in micro-PL measurements the finding of an emitting object requires scanning with the excitation laser beam over the surface of the sample. Once an emission line is found, we determine the orientation of the studied NW with respect to the laboratory frame in order to investigate subsequently the spin splitting anisotropy of individual NW. Therefore, the first step is the measurement of PL linear polarization anisotropy (LPA) originating from the dielectric mismatch between the NW and the surrounding vacuum. This mismatch causes a strong suppression of the perpendicular component of the electric field inside the NW.3 We remark that the LPA is an inherent feature of light emission from NWs, resulting in a strong damping of the PL signal perpendicular to the NW axis. Therefore, the maximum PL (minimum PL) intensity corresponds to the linear polarization oriented parallel (perpendicular) to the NW axis (see inset in Figure 3a). In order to confirm that the NW orientation can be determined form the LPA measurement, we disperse NWs on a silicon substrate with lithographic markers. Then, we compare the polarization angle of maximum PL established from the LPA with NW orientations observed in SEM images and find a perfect correlation. In Figure 3b, we present PL intensities for a CdMnTe insertion, evaluated by fitting the PL spectrum with a Gaussian, as a function of the analyzer angle. The observed dependence I(θ) is fitted with I(θ) = A cos2(θ + φ) + B (see Figure 3b), allowing to evaluate the linear polarization degree as P = (I∥ − I⊥)/(I∥ + I⊥) = A/(A + 2B). The obtained value P for this particular insertion is 70%. From a survey of about 200 NWs and 30 NW insertions, we find that P varies from about 20% to 90%. A strong variation of these values reflects different diameters within the studied ensembles. It is important to note that when two (or more) nanowires would contribute to the same emission line, a more complex dependence of the PL intensity on the polarization angle would be observed as compared to Figure 3b because of the presence of two (or more) different anisotropy axes. In Figure 4a,b, we show the PL spectra for a CdMnTe insertion in a magnetic field applied parallel and perpendicular to the NW axis. At 2 T, in the former case, the spectrum redshifts by 9.7 meV and in the latter by only 2.2 meV. The PL peak positions, extracted by fitting a Gaussian to the spectrum, are plotted versus magnetic field in Figure 4c. An analogous behavior is observed for a core/shell ZnMnTe/ZnMgTe NW (see Figure 4d−f). The red-shift amounts in this case to about 20 and 4.6 meV for magnetic fields parallel and perpendicular to the NW axis, respectively. The large values of the red-shifts are a direct consequence of the exchange interaction between the charge carriers and localized Mn ions,19 causing spin splittings up to 2 orders of magnitude larger than for intrinsic, nonmagnetic CdTe or ZnTe. More importantly, the data shown in Figure 4 demonstrate the crucial result: the spin splitting exhibits an anisotropy with respect to the orientation of the magnetic field. In all studied insertions and NWs, the Zeeman shift is few times larger in magnetic field applied parallel than perpendicular to the NW axis. In order to confirm that the nanowire shape has, indeed, a decisive impact on the value of the giant excitonic spin splitting, a magnetic field of a constant value of 2 T is rotated by 360°. After each rotation step of 5°, the PL spectrum is measured. The results are plotted in Figure 5a for the same NW for which

Figure 3. (a) Photoluminescence emission spectrum of a single CdMnTe insertion embedded in a nanowire, measured at 1.7 K, excited at 532 nm. Inset: the analyzer alignment with respect to the nanowire axis corresponding to the minimum and maximum PL intensity. (b) Points: intensity of the PL peak, obtained from Gaussian fit plotted as a function of the analyzer angle. Solid line: fit with the square of the sine function.

observed for CdTe, CdSe, and InP QDs embedded in NWs29−31 which is due to the presence of Mn ions and magnetization fluctuations. On the other hand, the observed transitions exhibit roughly the same line width as Stranski− Krastanov CdMnTe QDs.32 Please note that the intra-Mn2+ transition is missing in the spectrum presented in Figure 3b. This is caused by the fact that the excitonic emission takes place at energies similar to or lower than the intra-Mn2+ transition. This implies that the excitation transfer from excitons to the intra-Mn2+ transition is much less efficient as compared to the situation when the intra-Mn2+ transition energy would lie below the excitonic emission.33,34 1974

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Figure 4. Magneto-photoluminescence spectra in parallel external magnetic field for a single optically active CdMnTe insertion in NW (a) and single ZnMnTe/ZnMgTe NW (d). Magneto-photoluminescence spectra in perpendicular magnetic field for a single CdMnTe insertion in NW (b) and a single ZnMnTe/ZnMgTe NW (e). Transitions energies as a function of magnetic field for these two configurations for a single CdMnTe insertion (c) and a single ZnMnTe/ZnMgTe NW (points) (f). The solid lines represent model calculations; the fitting parameters are shown in graphs: TMn = Mn spin temperature, xMn = Mn concentration, ΔLH = heavy−light hole splitting, and rs = parameter of the strain induced mixing (see text).

Figure 5. (a) Map of PL spectra collected at a magnetic field of 2 T as a function of the field rotation angle for a single ZnMnTe/ZnMgTe nanowire. (b) Calculated map of PL spectra with parameters: xMn = 0.052, TMn = 4.4 K, and ΔLH = 50 meV (c) Zeeman shift determined from (a) as a function of magnetic field angle (d) linear polarization anisotropy for the nanowire under investigation. Solid black line shows the anisotropy axis in (c, d) pointing in the same direction.

to the NW axis, respectively. In Figure 5c, the Zeeman shift determined from the spectra presented in Figure 5a is plotted as a function of the magnetic field angle. The resulting

the PL data are shown in Figure 4d−f. We observe a periodic dependence of the Zeeman shift with maxima and minima corresponding to the field oriented parallel and perpendicular 1975

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component perpendicular to the quantization axis, this term leads to a mixing of the heavy and light hole states. The second term is the Bir−Pikus Hamiltonian describing the effect of strain on the valence band states. In general, HBP includes the effects of all components of the strain tensor. As shown by theoretical calculations,9,10 the strain acting on the core of the NW is homogeneous within the entire core region, and its dominant contribution is along the nanowire axis. This tensile strain acting on ZnTe cores has been shown recently as one of the most important factors determining the emission energy of excitons.8,38 Its homogeneity allows to neglect the offdiagonal terms in HBP when calculating hole energies in a NW. Thus, the effect of strain is only the splitting of the heavy and light hole states, with the former lying lower in energy. The value of this splitting, ΔLH, depends on several geometrical factors such as the thickness and homogeneity of the shell which may differ from nanowire to nanowire. Therefore, in our considerations, ΔLH is assumed as a fitting parameter. For the insertions, we include the off-diagonal, in-plane strain term rs (see Supporting Information), which contributes also to the mixing of heavy and light hole states already at zero magnetic field. Hole energies are obtained in analytical form by diagonalization of Hhole. Electron energies do not depend on φ as a consequence of the isotropic electron spin. They are calculated by diagonalizing the Hamiltonian Helectron = Hs−d which consists only of the s−d exchange term Hs−d = (1/2)Js−dxMn⟨S⃗⟩·σ⃗, where Js−d is the s−d exchange integral and σ⃗ is the electron spin operator. Note that in the calculations we neglect the effect of confinement, relevant for the insertions, but field independent, and all energies are computed relative to the bottom of the band. Moreover, to calculate the PL transition energies, we assume that the exciton binding energy does not depend on the magnetic field. Thus, the PL transition energies are given by sums of electron and hole energies. This approach allows us to compare the results of calculations with measured Zeeman shiftstransition energies relative to the value at zero field. The calculations yield eight states, which in the case of the field applied along the Z direction (quantization axis) can be divided into four purely heavy and four purely light hole excitons with two pairs of optically active excitons with angular momentum of ±1 and two pairs of optically dark states. However, under the combined effect of strain and magnetic field with a component along the X-axis, these states are intermixed. We calculate the transition intensities as I(ω) ∝ ∫ ⟨0|px + pz)|i⟩ exp(−ℏωi/ kBT)δ(ω − ωi) dω, where |i⟩ and ⟨0| denote the initial exciton and final vacuum states, respectively, px and pz are the relevant components of the momentum operator, and T is the exciton system temperature assumed equal to bath temperature (see Supporting Information). This procedure allows us to determine the emitting ground state transition, and its energy dependence on the magnetic field is then fitted to the experimental data. In the case of the magnetic field applied along the Z-axis, the ground state does not depend on ΔLH and rs since the states are unmixed. Therefore, in this configuration, the magnetic field dependence is given by a modified Brillouin function.19 For the PL transition energies extracted from Figure 4a,d, the best fits are obtained for xMn = 0.033, TMn = 9.0 K and xMn = 0.052, TMn = 4.4 K for the CdMnTe insertion and the ZnMnTe NW, respectively. In order to fit the PL energy dependence on the perpendicular magnetic field, i.e., the transition energies extracted from Figure 4b,e, we keep xMn and TMn as obtained

anisotropy axis, shown as a solid black line, coincides with the direction of the nanowire axis determined from the linear polarization anisotropy measured on the same nanowire (Figure 5d). This fact evidence that the spin splitting anisotropy is directly associated with the nanowire shape. The observed giant splitting anisotropy may arise either from the shape induced magnetization anisotropy or from the anisotropy of excitons inside the nanowire, originating from the energetic landscape of the bands. The first effect can be, however, neglected in our case of a diluted magnetic system with the concentration of magnetic dopants being on the order of only a few percent. Therefore, the origin of the observed effects must be related to the anisotropy of excitons inside nanowires and insertions, induced by the combined effect of (i) strain effects arising from the lattice mismatch between the core and the shell semiconductors and (ii) carrier confinement. The latter however is relevant only to the insertions, which we estimate are only a few nanometers high. The confinement has most likely no significant impact on the NW excitons because of relatively large diameters of the NW cores. Furthermore, the spin splitting anisotropy cannot originate from the conduction band states, since electron states are described by pure spin wave functions, giving rise to an isotropic Zeeman Hamiltonian (see Supporting Information). On the other hand, strain and confinement lift the degeneracy of the heavy and light hole states, leading to strongly anisotropic g-factors in quantum wells35 and quantum dots.36,37 The decisive impact of strain on the band gap of ZnTe/ ZnMgTe core/shell NWs has recently been studied in a systematic manner.8 In particular, a significant decrease of ZnTe energy gap resulting from tensile strain coming from ZnMgTe shell has been observed. Combining these experimental findings with theoretical model introduced in ref 38, we conclude that the light and heavy hole splitting must take place in our structures with the heavy hole band being the ground state. It is important to note that the energy landscape of the bands in the vicinity of the Γ-point of the Brillouin zone, which is responsible for optical transitions, does not depend on the crystal orientation because of the cubic zinc-blende crystalline structure of nanowires. Therefore, the crystalline orientation cannot account for the observed anisotropic g-factors In order to reproduce the observed spin splitting anisotropy, we calculate the PL transition energies as a function of the magnetic field applied along an arbitrary direction. In the calculations, the NW axis is aligned along the Z direction, while the magnetic field is applied in the XZ plane. The angle φ gives the orientation of the magnetic field in the XZ plane with respect to the NW axis. The light is collected along the Y-axis. The parameters of the model are xMn = Mn concentration inside NW or CdMnTe insertion, TMn = the effective temperature of Mn sublattice, ΔLH = light and heavy hole splitting, and rs = parameter of the in-plane strain used only for CdMnTe insertions. The Hamiltonian for the holes reads Hhole = Hp−d + HBP. The first term is the p−d exchange interaction between the holes and manganese ions, taking the Heisenberg form: Hp−d = −(1/6)Jp−dxMn⟨S⃗⟩·j,⃗ where Jp−d is the p−d exchange integral, ⟨S⃗⟩ is average total Mn spin along the magnetic field axis, and j ⃗ is the hole angular momentum operator.39 Hp−d results in the giant spin splitting of the bands directly proportional to the magnetization M⃗ ∼ xMn⟨S⃗⟩. Therefore, Hp−d is in fact a Zeeman Hamiltonian with g-factors proportional to the magnetization. In a magnetic field with a 1976

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above and fit the data with ΔLH and rs. This procedure yields ΔLH = 40 meV and rs = 2 meV for the insertion and ΔLH = 50 meV for the NW. Moreover, using the same set of fitting parameters, we are able to reproduce the experimental results presented in Figure 5a: the dependence of the Zeeman shifts on the orientation of the field of 2 T. For the corresponding calculations presented in Figure 5b, the optical emission lines are modeled as Gaussians with the spectral width of 5 meV, as determined from the experimental data. The values and sign of ΔLH are in good agreement with theoretical calculations of strain-induced valence band mixing in ZnTe/ZnMgTe core/shell nanowires38 where it has been shown that ΔLH depends on three parameters: Mg concentration in the shell, core diameter, and shell thickness. The theory presented in ref 38 for the studied NWs yields ΔLH in the range between 20 and 60 meV, in agreement with our fitted value ΔLH = 50 meV. From a survey of six CdMnTe insertions, we find ΔLH in the range between 10 and 60 meV. Remarkably, the reported values of heavy−light hole splittings for selfassembled CdTe QDs lie in the same range.37 We note that the determination of in-plane strain parameter rs is subject to significant errors since its values affect the Zeeman shifts very weakly. Its strongest effect is a linear polarization dichroism, as observed for self-assembled QDs.37 Thus, in order to gain more insight into the effect of the in-plane strain, it is necessary to analyze the PL polarization anisotropy in a configuration where the influence of the NW shape anisotropy is negligible, i.e., with light collection along the NW growth axis. Nevertheless, our analysis shows that the values of rs are small compared to ΔLH. This may indicate a relatively large Cd/Zn intermixing region at the insertion/NW interface, which has been previously observed in nonmagnetic CdTe/ZnTe NW quantum dots.29 Our experimental procedure proves to be a powerful tool to study the effect of shape anisotropy on optical properties of single NWs and NW heterostructures. In particular, it allows to gain the insight into the energy structure of the valence band. Moreover, our results show that strain-induced tuning of the hole ground state should be possible with proper choice of shell semiconductor. Specifically, overgrowth with a quaternary Zn1−xMgxTe1−ySey shell should allow a full control over the strain in the core while maintaining a larger band gap of the shell. A strain-free core/shell heterostructure is expected for x ≈ 0.25 and y ≈ 0.18. Application of tensile strain for y > 0.18 should result in a light-hole ground state, beneficial e.g., for applications in quantum information processing.40 In conclusion, a significant anisotropy of the giant spin splitting is observed by means of micro-PL measurements performed on individual ZnMnTe/ZnMgTe nanowires and on nanowires with CdMnTe insertions. This effect is attributed to the light and heavy hole splitting resulting in the pinning of the hole spin to the NW axis. The application of a magnetic field perpendicular to this axis results in mixing of the heavy and light hole states, which can be directly monitored in the PL experiment. The light and heavy hole splitting is directly associated with the strain field resulting from the lattice mismatch between core and shell semiconductors which opens a path for an effective manipulation of this effect by means of the shell composition. Moreover, our experimental procedure can be applied to further studies of the valence band mixing in NW heterostructures and effects related to magnetization anisotropy.

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ASSOCIATED CONTENT

S Supporting Information *

Calculations of transition energies and transition intensities. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by Polish National Science Center Grants No. 2011/01/B/ST3/02287, 2011/01/ D/ST5/05039, 2012/06/A/ST3/00247, and 2013/10/E/ST3/ 00215.

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DOI: 10.1021/nl504853m Nano Lett. 2015, 15, 1972−1978

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DOI: 10.1021/nl504853m Nano Lett. 2015, 15, 1972−1978