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The influence of possible presence of Co2+ ion pairs in a bulk Zn1-xCoxO mixed crystal on the low-frequency part of the Raman spectrum is discussed. Two effects can be taken into account in the theoretical considerations when analyzing the energy level scheme corresponding to Co ions. The first is a local lattice deformation in the vicinity of Co2+ ion due to a presence of the second ion, smaller than the host ZnO lattice cation. Such deformation creates a trigonal field, which can only slightly modify the energy levels of v ion. The second effect, which results from an antiferromagnetic superexchange interaction between two Co2+ ions is responsible for a new set of energy levels. The Raman data taken at low temperature on the sample corresponding to the composition x = 0.016 demonstrated the presence of two structures at about 6 cm(-1) and 13 cm(-1). These structures may be interpreted as electronic transitions between the ground state and the first excited state of a single Co2+ ion in the substitution site of ZnO lattice and as a similar transition for Co2+ ion pair, respectively.

36th International School on the Physics of Semiconducting Compounds, Jaszowiec, POLAND, JUN 09-15, 2007; International audience; Low-temperature Raman spectra of a bulk mixed crystal Zn1-xCoxO (x = 0.016) are shown and analyzed. Besides the common modes of the ZnO host lattice, electronic transitions related to the presence of Co2+ ions at the Zn sites are observed. In spite of the low Co concentration the presence of collective excitations corresponding to Co and CoO precipitates is also revealed.

The Mn/ZnO(0001) system was investigated by synchrotron radiation photoemission. The Mn/ZnO interface with 4 ML of manganese deposited onto the ZnO surface was annealed up to 500◦C. No Mn capping layer was found at the surface after annealing as was confirmed by scanning Auger spectroscopy experiment. We used a resonant photoemission to extract the Mn3d partial density of states in photoemission spectra. The Mn3d states contribute to the electronic structure of the system within 10 eV of the Fermi level. They show three features: a main peak at 3.8–4.5 eV, a valence structure at the top of the valence band (1–3 eV), and a broad satellite situated between 5.5 and 9 eV below EF. The satellite/main branching ratio was determined to be 0.43, which is a fingerprint of strong hybridization between the Mn3d electrons and the valence band of the crystal. The hybridization effect in Zn1−xMnxO surface alloy is comparable to Zn1−xMnxS and much higher than in Zn1−xMnxSe, Zn1−xMnxTe, and Ga1−xMnxAs semimagnetic compounds.

Origin of a fast component of the photoluminescence (PL) decay of Mn 2+ intra-shell 4T1 ? 6A 1 transition in bulk and quantum dot structures is discussed based on the results of PL, PL kinetics and ...

A lamellar semimagnetic semiconductor, Sn 1− x Mn x Se 2 , has been investigated by photoemission, both in the UV and X-ray photoenergy ranges. The partial Mn 3d DOS has been determined from resonant photoemission spectra; the valence of the several constituting atomic species was ascertained from XPS results.

The electronic structure of the Hg-chalcogenides HgTe and HgSe was studied by means of high resolved photoemission along the ΓΣ K -direction with the aim of investigating prototype materials for the inverted band structure model in detail. The samples were prepared by cleavage of the non polar (110)-surface plane to minimize the effects of surface charges. Assuming free electron parabola for the final states and considering reciprocal umklapp vectors the bulk band structure of the complete valence band was determined. This enabled a comparison to a state of the art–theoretical band structure calculation and to older experimental results obtained with different methods. In the case of HgTe with large spin–orbit splitting also the individual Γ 7 , Γ 6 , and Γ 8 -bands could be resolved at the valence band maximum, confirming the model of the inverted band structure directly. An attempt was made to apply this direct method also to HgSe.

Resonant photoemission spectroscopy has been applied to study the group of Pb 1− x RE x B VI (RE=Eu, Gd; B=Te, Se, S) diluted magnetic semiconductors. Energy distribution curves, as well as constant-initial-state or constant-final-state photoemission spectra have been taken for photon energies near to the intra-ion Eu 4d→4f or Gd 4d→4f transitions. The Eu 4f and Gd 4f contributions to the electronic structures of the crystals have been determined by comparison of photoemission spectra taken on and off resonance. The localized character of the Eu(Gd) 4f 7 shell has been confirmed by the analysis of the shape of the corresponding spectral feature. The influence of the host crystal ionicity on the position of the RE 4f shells with respect to the valence band is discussed.

Electronic states of 4f samarium ions were investigated by photoemission spectroscopy in samarium-rich CdSmTe sample obtained by MBE. The photon energy of synchrotron radiation allowed to investigate Fano-type resonance and antiresonance. The energy distribution curve spectra were attributed to the Sm 4d-4f transition. The shape of the constant initial states spectra was compared with this one obtained for atomic samarium. PACS numbers: 73.20.At, 79.60.-i, 79.60.Bm

Single Zn 1− x Co x S crystals with x up to about 0.285 were grown by the chemical vapour transport method using iodine as the transporting agent. The obtained single crystal grains were typically of up to 3–6 mm in size. The crystal quality was studied with the help of high-resolution diffractometry. The FWHM of rocking curves was of the order of 40″. Lattice constants measured by both, the powder diffractometry and high-resolution diffractometry, are reported and compared with the literature data. The extrapolated lattice constant for hypothetical CoS compound of sphalerite-type structure is 5.343(1) A. The interatomic distances in Zn 1− x Co x S and in related Fe and Mn containing sulphides are briefly discussed.

Resonant photoemission spectra of Snι1-xGdxTe (x = 0.02 and 0.08) measured for the photon energy range 142 to 151 eV show the valence band density of states distribution and the Gd 4f derived maximum. The energy position of the J = 0 component of the Gd 4f maximum was determined and used as a measure of the Gd 4f shell binding energy. The electrostatic model of core level shifts was used to interpret the difference in the Gd 4f binding energies observed for x = 0.02 and x = 0.08.

The magnetization of ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ samples with $x=0.019$, 0.026, and 0.060 was measured at 20 mK in fields up to 50 kOe, and at 0.6 K in fields up to 180 kOe. The 20 mK data show the magnetization steps (MST's) arising from pairs and from triplets. The pair MST's are used to obtain the dominant Eu-Eu antiferromagnetic exchange constant, ${J/k}_{B}=\ensuremath{-}0.264\ifmmode\pm\else\textpm\fi{}0.018$ K. The exchange constant for triplets is the same. Comparison of the magnetization curves with theoretical simulations indicates that the Eu ions are not randomly distributed over all the cation sites. The deviation from a random distribution is much smaller if $J$ is assumed to be the nearest-neighbor exchange constant ${J}_{1}$ rather than the next-nearest-neighbor exchange constant ${J}_{2}$. On this basis, $J$ is tentatively identified as ${J}_{1}$. However, the possibility that ${J=J}_{2}$ cannot be excluded completely. To obtain agreement with the data, it must be assumed that the Eu ions tend to bunch together. Comparision with microprobe data indicates that the length scale for these concentration variations is smaller than a few $\ensuremath{\mu}$m. The theoretical simulations in the present work improve on those performed earlier by including clusters larger than three spins.

X-ray photoelectron spectroscopy experiments on PbTe:Yb, PbS:Yb, PbTe, and PbS crystals have been performed using the monochromatized Al Kα radiation. The valence band spectrum of PbTe:Yb exhibits one set of peaks associated with divalent Yb initial states, but two sets associated with divalent and trivalent Yb are observed for PbS:Yb. The valency of Yb in PbTe:Yb is 2 (within an accuracy of the experiment) but in PbS:Yb the mixed valency of Yb is seen. These conclusions are confirmed by an analysis of Yb 4d spectra in PbTe:Yb and PbS:Yb crystals.

X-ray photoelectron spectroscopy experiments on SnTe crystals doped by Sm, Eu, and Yb were performed to determine the valency of these ions in the SnTe matrix. The analysis of the X-ray photoelectron spectroscopy valence-band spectra revealed that Eu and Yb are divalent while Sm is trivalent in SnTe. These conclusions were confirmed by studies of the binding energies of core levels (4d-levels of Yb and 3d-levels of Sm and Eu), which are not presented here.

X-ray absorption experiments were performed at the Kedge of Co and Zn in Zn1-xCoxS and Zn1—mCoxSe compounds with x = 0.25, 0.16, 0.10, 0.05, 0.00 in the sulphides samples and x = 0.07, 0.02, 0.00 in the selenide ones. Analysis of the extended X-ray absorption fine structure oscillations using phase and amplitudes either from McKale code or from standard samples, gave the distance, number of atoms and the Debye-Waller factors for nearest neighbours. We found that the cation-anion distances Zn—S(Se) or Co—S(Se) are systematically shorter in ZnS than in ZnSe matrix, the Zn-anion distance is always larger than the Co-anion one without any significant Co content dependence. This allowed us to estimate the covalent radius of Co in the studied matrices to be 0.025 A smaller than the Zn covalent radius and stated that covalent radius of Co is independent of the type of surrounding atoms. In addition the Debye—Waller factors indicated a better ordering in ternary compounds than in the binary standard ones in agreement with rocking curve measurements. PACS numbers: 61.10.Lx, 87.64.Fb, 78.70.Dm

Using complementary x-ray and electron-optical methods, a ZnSe(Co) crystal with natural face was investigated. X-ray diffraction methods such as double-crystal x-ray reflection topography, double-crystal diffractometry for rocking curve measurements, precise lattice constants measurements by the Bond technique were used for crystal structure characterization and x-ray fluorescence method for studies of chemical composition along the crystal. The scanning electron microscopic image of the crystal surface and reflection diffraction of the high energy electrons enriched the crystal structure characterization

The fundamental reflectivity spectra of monocrystalline CdTe, implanted with Ag ions at room temperature and with Er ions at liquid nitrogen temperature, are investigated in the 0.5-6.0 eV energy range. The analysis of the obtained spectra leads to the suggestion that temperature of implantation influences the obtained results much more decisively than values of other parameters. For the implantation carried out at 300 K no significant changes in reflectivity spectra are observed regardless of magnitude of the ion dose (up to 5 x 1015 /cm2 ) and this fact, in our opinion, is due to the self-annealing effect. For samples implanted at temperature 77 K with comparable doses of ions, however, the characteristic changes of shape and intensity of reflection coefficient spectra appear. The manner of this changes gives evidence that temperature 77 K is low enough to make the radiation induced lattice defects stable (frozen-in) which are responsible for the observed behaviour of CdTe fundamental reflectivity spectra. PACS numbers: 61.80.Jh, 61.70.Tm, 78.20.Ci

The magnetization and magnetic susceptibility of Bridgman-grown Sn1-xGdx Te with values of x up to 0.09 have been measured over a temperature range from 2 to 300 K and in magnetic fields up to 5.5 T. The magnetic susceptibility data followed the Curie-Weiss relation with a small Curie temperature that indicated a weak antiferromagnetic coupling among Gd ions. The magnetic field dependence of the magnetization was fitted to a modified Brillouin function with parameter values that agreed fairly well with those from Curie-Weiss plots. The value of the exchange parameter was larger than in Pb1-xG dxTe. The samples were p-type with carrier concentrations up to 1.3 x 10 21 cm'. The ferromagnetic or spin-glass phase due to the RKKY interaction was not observed.

We report on the investigation of structural and magnetic properties of the cobalt-based diluted magnetic semiconductor with wurtzite structure: Zn 1− x Co x O. Single crystals were grown by chemical vapour transport. From the Raman measurements in the temperature range 10–300 K on samples with Co concentration of 5%, we detect the presence of very small quantities of hcp-Co and also the presence of anti-ferromagnetic CoO impurities. This result is in agreement with the behaviour of magnetisation measurements under pulsed magnetic fields. High frequency–high field ESR reveals clearly the large crystal field splitting of the energy levels of isolated Co 2+ ions.

The temperature dependence of the magnetic specific heat has been studied experimentally and theoretically in the semimagnetic semiconductor ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ for $x=0.027$ and 0.073, over the temperature range from 0.5 to $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in magnetic fields up to $2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. There was a maximum in the magnetic specific heat between 1 and $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ even in zero and low magnetic fields; this maximum shifted toward higher temperatures with increasing magnetic field. The experimental data have been analyzed in the framework of a model in which we assume that the ground states of europium ions are split even without an external magnetic field. We present arguments which support this assumption and we show that it is possible to find a physical mechanism leading to the splitting which can explain the experimental results.

We report on the magnetic properties of thoroughly characterized Zn(1-x)Co(x)O epitaxial thin films, with low Co concentration, x = 0.003-0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.

Based on the results of optically detected magnetic resonance (ODMR) and time-resolved investigations we relate the observed lifetime shortening of intra-shell Mn2+ emission to spin dependent magnetic interactions between localized spins of Mn2+ ions and spins/magnetic moments of free carriers. We show that this mechanism is active both in bulk and in low dimensional structures, such as quantum wells (QWs), quantum dots (QDs) and nanostructures. © 2004 Elsevier B.V. All rights reserved.

Magnetization steps from ${\mathrm{Mn}}^{2+}$ pairs in several single crystals of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{O}$ $(0.0056l~xl~0.030),$ and in one powder $(x=0.029),$ were observed. They were used to determine the four largest exchange constants (largest $J$'s), and the single-ion axial anisotropy parameter D. The largest two exchange constants, ${J}_{1}{/k}_{\mathrm{B}}=\ensuremath{-}18.2\ifmmode\pm\else\textpm\fi{}0.5\mathrm{K}$ and ${J}_{1}^{\ensuremath{'}}{/k}_{\mathrm{B}}=\ensuremath{-}24.3\ifmmode\pm\else\textpm\fi{}0.5\mathrm{K},$ were obtained from large peaks in the differential susceptibility, $dM/dH,$ measured in pulsed magnetic fields H up to $500\mathrm{kOe}.$ These two largest $J$'s are associated with the two inequivalent classes of nearest neighbors (NN's) in the wurtzite structure. The 29% difference between ${J}_{1}$ and ${J}_{1}^{\ensuremath{'}}$ is substantially larger than 13% in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{S}$ and 15% in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}.$ The pulsed-field data also indicate that, despite the direct contact between the samples and a superfluid-helium bath, substantial departures from thermal equilibrium occurred during the 7.4-ms pulse. The third- and fourth-largest $J$'s were determined from the magnetization M at 20 mK, measured in dc magnetic fields H up to 90 kOe. Both field orientations $\mathbf{H}\ensuremath{\Vert}\mathbf{c}$ and $\mathbf{H}\ensuremath{\Vert}[101\ifmmode\bar\else\textasciimacron\fi{}0]$ were studied. (The $[101\ifmmode\bar\else\textasciimacron\fi{}0]$ direction is perpendicular to the c axis, $[0001].)$ By definition, neighbors which are not NN's are distant neighbors (DN's). The largest DN exchange constant (third-largest overall) has the value ${J/k}_{\mathrm{B}}=\ensuremath{-}0.543\ifmmode\pm\else\textpm\fi{}0.005\mathrm{K},$ and is associated with the DN at $\mathbf{r}=\mathbf{c}.$ Because this is not the closest DN, this result implies that the $J$'s do not decrease monotonically with the distance r. The second-largest DN exchange constant (fourth-largest overall) has the value ${J/k}_{\mathrm{B}}\ensuremath{\approx}\ensuremath{-}0.080\mathrm{K}.$ It is associated with one of the two classes of neighbors that have a coordination number ${z}_{n}=12,$ but the evidence is insufficient for a definite unique choice. The dependence of M on the direction of $\mathbf{H}$ gives ${D/k}_{\mathrm{B}}=\ensuremath{-}0.039\ifmmode\pm\else\textpm\fi{}0.008\mathrm{K},$ in fair agreement with $\ensuremath{-}0.031\mathrm{K}$ from earlier electron paramagnetic resonance work.

Based on the results of optically detected magnetic resonance and time-resolved investigations we relate the observed lifetime shortening of intra-shell Mn 2 + emission to spin dependent magnetic interactions between localized spins of Mn 2 + ions and spins/magnetic moments of free carriers. We show that this mechanism is active in both bulk and in low dimensional structures, such as quantum wells, quantum dots, and nanostructures.

Physical review / B 61(8), R5058 - R5061 (2000). doi:10.1103/PhysRevB.61.R5058, Angular-resolved photoemission measurements of the nonpolar (110)-cleavage face of HgTe single crystals have been performed along the Σ line to determine details of the band structure near the valence band maximum (VBM). Three bands are observed between VBM and 1 eV binding energy, instead of the two observed for a positive energy gap semiconductor CdTe. Their energy separations and positions relative to the Fermi energy are investigated at the Γ point and at slightly off-normal emission, applying room and low temperature of 40 K. In contrast to the heavily debated results of HgSe [K.-U. Gawlik et al., Phys. Rev. Lett. 78, 3165 (1997)] the clear observations for HgTe are consistent with the model of an inverted band structure, reflecting a semiconductor with a negative band gap., Published by Inst., Woodbury, NY

The magnetic susceptibility of Pb1-xCexA (A = S, Se and Te) crystals with 0.006 ≤ x ≤ 0.036 were studied in the temperature range from 20 mK up to room temperature. X-band (~9.5 GHz) Electron Paramagnetic Resonance (EPR) showed small shifts in the effective Lande factors that were attributed to crystal-field admixture. The EPR measurements were correlated with the magnetic susceptibility data and resulted in estimating the crystal-field splitting E(8) - E(7) of the lowest 2 F5/2 manifold for Ce 3+ ions in PbA (A = S, Se and Te) of about 340 K, 440 K and 540 K for Pb1-xCexTe, Pb1-xCexSe, and Pb1-xCexS, respectively. The values for the crystal-field splitting deduced from the magnetic data were found to be in agreement with the calculated ones based on the point charge model. Moreover, the de- Haas van-Alphen magnetic oscillations in the susceptibility measurements of Pb1-xCexTe (x~ 0.05 and 0.07) were observed at ultra-low temperature (20 mK); The oscillations were investigated and the values of the oscillatory period for Pb1-xCexTe (x = 0.0048 and 0.007) are reported.

Currently, ZnO-based diluted magnetic semiconductors (DMS) attract much attention, mainly because of the predicted possibility of ferromagnetism at 300 K. Among these materials, Zn1-xCoxO takes a prominent position. In spite of numerous investigations, a general understanding of its magnetic properties is still missing. Moreover, its detailed magnetic behavior depends on the sample fabrication and after growth treatment. We present a systematic analysis of structural Zn1-xCoxO properties by micro-Raman scattering for a wide variety of different sample parameters. Bulk mixed crystals (x < 0.05) and thin layers (0.03 < x < 0.12) were investigated between 1.6 K and 295 K. Beside the common eigenmodes of the ZnO host lattice, we observe disorder-induced lattice vibration modes and structures, which we assign to vibration modes of Co-related precipitates. We present their dependence on the composition, the applied preparation technique, and the annealing process. Furthermore, the possible impact of such precipitate formation on the magnetic properties is discussed.

Electron probe microanalyses were performed on SnTe crystal grown by the Bridgman method doped with 1.8 wt% Gd. Scanning electron microscopic observations and electron probe microanalysis carried out on the (110) and (100) oriented samples revealed two types of precipitate enriched in gadolinium. It was suggested that the dominant type contained a mixture of Gd4Te7 phase and Te0.8Gd0.2 eutectic. The composition of the second precipitate type was found to be a eutectic enriched with gadolinium.