BROWSE VOLUME

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January 2009

Volume 35, Issue 1,  pp. 1-93

XVII Ural International Winter School on Physics of Semiconductors

Low Temp. Phys. 35, 1 (2009) (1 page)

Online Publication Date: 10 February 2009

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01.30.Rr, 01.30.Bb, 75.50.-y, 74.70.-b

Electrons near a graphene edge

V. A. Volkov and I. V. Zagorodnev

Low Temp. Phys. 35, 2 (2009) (4 pages)

Online Publication Date: 10 February 2009

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To describe the behavior of a 2D electron near a graphene edge, one needs to know the boundary conditions for the effective (envelope) wave function, which obeys an equation of the Weyl–Dirac type. These conditions, which are deduced from general arguments, amount to a linear relation on the components of the spinor comprised of the envelope functions. In neglect of the intervalley interaction, the boundary conditions depend only on one real parameter, which contains a boundary of the general type. The spectrum of electronic edge states for the linear boundary is obtained.
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73.21.-b, 71.70.Ej, 71.18.+y

Two-dimensional electron systems in HgTe quantum wells

Z. D. Kvon, E. B. Olshanetsky, N. N. Mikhailov, and D. A. Kozlov

Low Temp. Phys. 35, 6 (2009) (9 pages)

Online Publication Date: 10 February 2009

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A brief review is given on the results of research on the properties of two-dimensional (2D) electron systems in quantum wells based on CdHgTe/HgTe/CdHgTe double heterojunctions with surface orientations of (100) and (013). The main features of the energy spectrum are described. The results of experiments yielding information about the parameters of the energy spectrum are presented. From cyclotron resonance measurements the effective mass of the 2D electrons in HgTe quantum wells with inverted band structure is obtained as a function of the density of such electrons, Ns, in the range 2.2×1011  cm−2<=Ns<=9.6×1011  cm−2. This density dependence indicates an appreciable nonparabolicity of the spectrum: the mass increases with increasing Ns in that range from a value (0.026±0.005)m0 to (0.0335±0.005)m0. The giant spin splitting observed in asymmetric HgTe quantum wells is discussed, and the results of experimental research on the transition from a quantum Hall liquid to an insulator and the plateau-plateau transition in 2D electron systems in HgTe quantum wells are recounted. The 2D electron-hole system observed recently in the quantum wells under study, which is the first realization of a 2D semimetal, is described. It is found that it arises in lightly doped quantum wells with an inverted band structure and surface orientation (013). The magnetotransport is found to exhibit a number of features (positive magnetoresistance, variable-sign Hall effect, anomalous behavior in the quantum Hall effect regime) due to the simultaneous existence of 2D electrons and holes.
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73.21.Fg, 73.20.At, 73.40.Lq, 73.43.Nq, 73.43.Qt

Spin-dependent tunneling conductance in two-dimensional structures at zero magnetic field

I. V. Rozhansky and N. S. Averkiev

Low Temp. Phys. 35, 15 (2009) (6 pages)

Online Publication Date: 10 February 2009

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The influence of the spin-orbit interaction on the tunneling between two-dimensional electron layers is considered. A general expression for the tunneling current is obtained with the Rashba and Dresselhaus effects and also elastic scattering of charge carriers on impurities taken into account. It is shown that the particular form of the tunneling conductance as a function of the voltage between layers is extremely sensitive to the relationship between the Rashba and Dresselhaus parameters. This makes it possible to determine the parameters of the spin-orbit interaction and the quantum scattering time directly from measurements of the tunneling conductance in the absence of magnetic field.
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71.70.Ej, 71.55.-i, 73.21.-b, 72.25.-b

Resonance variation of the kinetic coefficients in a two-dimensional electron system with spin-orbit interaction

I. I. Lyapilin

Low Temp. Phys. 35, 21 (2009) (3 pages)

Online Publication Date: 10 February 2009

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A study is made of the resonance variation of the kinetic coefficients in two-dimensional systems with spin-orbit interaction upon saturation of the combined resonance transitions.
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73.63.-b, 71.70.Ej

Spin effects and quantum corrections to the conductivity of two-dimensional systems

A. V. Germanenko

Low Temp. Phys. 35, 24 (2009) (8 pages)

Online Publication Date: 10 February 2009

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A summary of a lecture delivered at the XVII Ural International Winter School on Physics of Semiconductors (Ekaterinburg, 2008) is presented. The basic concepts of quantum corrections to the conductivity of two-dimensional systems are briefly laid out. The interference quantum correction and the Altshuler-Aronov correction due to the electron-electron interaction are examined. Particular attention is devoted to systems with spin-orbit interaction. The main ideas are illustrated with experimental results.
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71.70.Ej, 73.23.-b

Magnetotransport in two-dimensional n-InGaAs/GaAs double-quantum-well structures near the transition from the insulator to the quantum Hall effect regime

Yu. G. Arapov, I. V. Karskanov, G. I. Harus, V. N. Neverov, N. G. Shelushinina, and M. V. Yakunin

Low Temp. Phys. 35, 32 (2009) (12 pages)

Online Publication Date: 10 February 2009

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The temperature and magnetic-field dependence of the components of the conductivity and resistivity tensors are calculated with the quantum corrections due to the weak localization effect and electron-electron interaction taken into account in the diffusion and ballistic regimes. The corrections to the conductivity from the weak localization and electron-electron interaction and also the influence of spin and oscillation effects are taken into account by a renormalization of the transport relaxation time of the electron momentum, with the result that the Drude conductivity becomes temperature dependent. A calculation of the components of the conductivity and resistivity tensors is carried out with the use of the theoretical values of the parameters of the theory of quantum corrections, which are determined solely by the values of the carrier density and mobility of a particular sample. The results of the calculation are compared with experimental results for two-dimensional n-InGaAs/GaAs structures with double quantum wells. It is shown that taking only the quantum corrections into account with the theoretical values of the parameters does not permit even a qualitative description of experiment, and it is therefore necessary to take additional temperature-dependent contributions to the Drude conductivity into account.
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73.63.Hs, 73.40.Kp, 73.43.Qt, 72.20.Fr, 72.20.My, 71.30.+h, 72.80.Jc

Quantum Hall effect in an InAs/AlSb double quantum well

M. V. Yakunin, S. M. Podgornykh, and Yu. G. Sadofyev

Low Temp. Phys. 35, 44 (2009) (4 pages)

Online Publication Date: 10 February 2009

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Double quantum wells (DQWs) were first implemented in the InAs/AlSb heterosystem, which is characterized by a large Landé g factor |g|=15 of the InAs layers forming the well, much larger than the bulk g factor |g|=0.4 of the GaAs in conventional GaAs/AlGaAs DQWs. The quality of the samples is good enough to permit observation of a clear picture of the quantum Hall effect (QHE). Despite the small tunneling gap, which is due to the large barrier height (1.4  eV), features with odd filling factors nu=3,5,7,… are present in the QHE, due to collectivized interlayer states of the DQW. When the field is rotated relative to the normal to the layers, the nu=3 state is suppressed, confirming the collectivized nature of that state and denying that it could owe its existence to a strong asymmetry of the DQW. Previously the destruction of the collectivized QHE states by a parallel field had been observed only for the nu=1 state. The observation of a similar effect for nu=3 in an InAs/AlSb DQW may be due to the large bulk g factor of InAs.
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73.43.-f, 73.40.Kp, 73.40.Lq, 71.18.+y

Doped nanoparticles for optoelectronics applications

M. Godlewski, E. Wolska, S. Yatsunenko, A. Opalińska, J. Fidelus, W. Lojkowski, M. Zalewska, A. Klonkowski, and D. Kuritsyn

Low Temp. Phys. 35, 48 (2009) (5 pages)

Online Publication Date: 10 February 2009

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Nanoparticles of wide-band-gap materials doped with transition metal ions or rare earth ions are intensively studied for their possible applications in a new generation of light sources for an overhead illumination. In this work we discuss mechanisms of emission enhancement in nanoparticles doped with rare earth or/and transition metal ions. Arguments are presented that phosphors of nanosize may emit light more efficiently and thus be applied in practical optoelectronic devices.
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78.67.Bf, 78.55.-m

On the origin of room-temperature ferromagnetism in wide-gap semiconductors

A. Korbecka and J. A. Majewski

Low Temp. Phys. 35, 53 (2009) (5 pages)

Online Publication Date: 10 February 2009

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The emerging field of semiconductor spintronics would be dramatically boosted if a semiconductor exhibiting room-temperature ferromagnetism could be found. Here, we discuss the recent stage of research, paying particular attention to the understanding of the observed room-temperature ferromagnetism in the wide-gap semiconductors GaMnN and ZnMnO. Since spinodal decomposition has been observed in these structures, we consider the possibilities of influencing density fluctuations of the alloys to obtain ferromagnetic semiconductors with required functionalities. We contrast these compounds with (In,Mn)As and (Ga,Mn)As, where the ferromagnetism is well understood, albeit well below room temperature.
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75.30.Et, 75.50.Pp, 75.50.Dd

Peculiarities of the ferromagnetic ordering in diluted magnetic insulators

K. Kikoin

Low Temp. Phys. 35, 58 (2009) (4 pages)

Online Publication Date: 10 February 2009

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The present state of experimental and theoretical research on diluted magnetic insulators (thin oxide films doped with transition metals) is discussed. The fundamentally irremovable thermodynamic nonequilibrium character of these films makes it possible for long-range ferromagnetic order to be established in them or for a highly nonuniform multiphase state (superparamagnet) to form. In all cases a fundamental role is played by concomitant defects (oxygen vacancies, shallow donors and acceptors).
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75.70.Ak, 75.50.Pp, 61.72.jd, 75.50.Dd, 75.30.Hx, 75.20.Ck

Diluted magnetic semiconductors based on II–VI, III–VI, and IV–VI compounds

G. V. Lashkarev, V. I. Sichkovskiyi, M. V. Radchenko, V. A. Karpina, P. E. Butorin, O. I. Dmitriev, V. I. Lazorenko, E. I. Slyn'ko, P. M. Lytvyn, R. Jakiela, W. Knoff, T. Story, and P. Aleshkevych

Low Temp. Phys. 35, 62 (2009) (9 pages)

Online Publication Date: 10 February 2009

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Chemical and phase composition, magnetic susceptibility, SIMS, magnetic force microscopy, and neutron diffraction data for Ge1−x−ySnxMnyTe, InSe<Mn>, and ZnO<Co, Mn> single crystals are investigated over a wide range of temperatures and magnetic fields. For Ge1−x−ySnxMnyTe the existence of ferromagnetic (FM) ordering with a Curie temperature TC~50  K, due to an indirect exchange interaction between Mn ions via the degenerate hole gas, is established. It is shown that at T<50  K the ferromagnetic regions of the crystal form a spin-glass phase. In InSe<Mn> it is found that hysteresis loops of the magnetic moment M(H) are observed up to 350  K. They attest to the existence of ferromagnetic ordering, which is apparently due to ferromagnetic clusters in which a superexchange of the Mn ions via the Se anions is proposed, and to an indirect interaction via the 2D electron gas. At T<70  K a period doubling of the magnetic sublattice of alpha-MnSe second-phase inclusions is observed, and their distribution in the layered structure of the InSe<Mn> host matrix has a regular character, forming a self-organized FM/AFM superlattice. In ZnO<Co, Mn> the temperature dependence of M obeys a Curie law. When the solubility limit of Co in ZnO is exceeded, hysteresis loops are observed as a consequence of the appearance of a ferromagnetic second phase. In ZnO<Mn> samples and also in some ZnO<Co> samples with Co content below the solubility limit an antiferromagnetic (AFM) interaction takes place.
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75.30.Cr, 75.30.Kz, 75.30.Et, 75.60.Ej, 75.50.Dd, 75.50.Pp

Observation and interpretation of the low-temperature features of the phonon thermal conductivity of mercury selenide crystals doped with impurities of 3d transition elements

A. T. Lonchakov, V. I. Okulov, and S. Yu. Paranchich

Low Temp. Phys. 35, 71 (2009) (5 pages)

Online Publication Date: 10 February 2009

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The temperature dependence of the thermal conductivity of mercury selenide containing iron and cobalt impurities is investigated experimentally. An analysis of the phonon contributions to the thermal conductivity at low temperatures in HgSe:Fe crystals reveals a significant impurity-related anomaly of the phonon thermal conductivity—a dip in its temperature dependence near 15  K. On the basis of a proposed theoretical description it is shown that this anomaly is explained by the contribution to the thermal conductivity from the scattering of phonons on electrons in hybridized impurity states.
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61.72.uj, 61.72.sd, 66.70.Df, 71.38.-k

Ultrasonic investigations of the Jahn–Teller effect in a ZnSe:Fe2+ crystal

V. V. Gudkov, A. T. Lonchakov, I. V. Zhevstovskikh, V. I. Sokolov, and V. T. Surikov

Low Temp. Phys. 35, 76 (2009) (3 pages)

Online Publication Date: 10 February 2009

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The temperature dependence of the absorption and velocity of transverse ultrasonic waves propagating in the [110] direction in a ZnSe:Fe2+ crystal with the iron impurity of n=2.2×1019  cm−3 is investigated in the temperature interval 1.4–100  K at a frequency of 53  MHz. For the slow transverse mode an absorption peak at 7.3  K and a decrease of the phase velocity below 40  K are found, while no such features are observed for the fast mode. The anomalies are interpreted as manifestations of the Jahn–Teller effect. The softening of the elastic modulus (C11–C12)/2 indicates that the local distortions in ZnSe:Fe2+ are tetragonal. Under the assumption that the absorption maximum is of a relaxational nature, the temperature dependence of the relaxation time is recovered from the data, and the values of the potential barrier height and vibronic frequency are obtained.
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71.70.Ej, 62.65.+k, 81.40.Jj, 62.20.de, 43.58.Ry, 61.72.S-

Soft x-ray emission spectra and ferromagnetism in wide-gap doped semiconductors

T. P. Surkova, V. R. Galakhov, and É. Z. Kurmaev

Low Temp. Phys. 35, 79 (2009) (4 pages)

Online Publication Date: 10 February 2009

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A study is made of the resonant and nonresonant L x-ray emission spectra of impurities in the semiconducting compounds ZnS:Mn, ZnO:Mn, ZnO:Co, and Co2O:Mn. An analysis of the Mn L2,3 x-ray emission spectra of Zn1−xMnxS (x=0.1–0.3) reveals that the Mn impurities do not form clusters in the ZnS lattice. Studies of the Mn L2,3 spectra and electronic structure of epitaxial films of Zn0.8Mn0.2O annealed at different temperatures show that the cause of the observed suppression of ferromagnetism at T>600  °C is segregation of Mn atoms. In this case the Mn atoms occupy both Zn sites and interstitial positions. For Zn1−xCoxO (x=0.02, 0.06, and 0.10) the absence of free carriers that could mediate an exchange interaction between Co ions is established. Mn L2,3 x-ray emission measurements show that in Mn-doped oxides Cu2O synthesized at 650 and 800  °C the Mn atoms are found both in interstitial positions and occupy Cu sites, but the configurations of these defects depend on the synthesis temperature. A decrease of the Curie temperature with increasing synthesis temperature may be explained as a manifestation of antiferromagnetic superexchange between substituent Mn atoms via oxygen.
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78.70.En, 75.70.Ak, 68.55.ag, 75.30.Kz, 61.72.Cc, 75.30.Et

Donor exciton of cobalt and its interaction with lattice vibrations in the semiconductor crystal ZnO:Co

N. B. Gruzdev, V. I. Sokolov, and G. A. Yemelchenko

Low Temp. Phys. 35, 83 (2009) (6 pages)

Online Publication Date: 10 February 2009

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Vibrational states interacting with a donor exciton in the compound ZnO:Co are revealed by the sensitive method of field exciton-vibrational spectroscopy. The vibrational modes of the electroabsorption spectrum of the compound ZnO:Co in the region of the donor exciton are given an interpretation based on the existing data on the symmetrized local density of states of the compounds ZnO and ZnO:Ni3+. The results are compared with the known data for II–VI:Ni compounds in the case of an acceptor exciton. The position of the donor level of the Co2+ ion relative to the bottom of the conduction band in the given compound is determined and found to conform well to the universal trend for donor levels of 3d ions in II–VI compounds.
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71.35.-y, 71.20.Nr, 61.72.uj

Statistical model structure of A1−xZxB2 Laves phase C15 system—the superconducting alloy Ce1−xLaxRu2

B. V. Robouch, A. Marcelli, N. L. Saini, and A. Kisiel

Low Temp. Phys. 35, 89 (2009) (5 pages)

Online Publication Date: 10 February 2009

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The local structure of the Ce1−xLaxRu2 system measured by EXAFS is re-examined and correlated to the statistical ad hoc model recently applied to the sphalerite, wurtzite, and other intermetallic ternary alloys. Deconvolution of the EXAFS data shows that the Ce1−xLaxRu2 ternary system is essentially a mixture of CeRu2 and LaRu2 binary alloys with a small proportion of the Ce0.5La0.5Ru2 ternary configuration, which is maximum for the intermediate concentration. Moreover, the analysis reveals that while the LaRu2 configuration exhibits a Bernoulli random distribution, the presence of a Ce atom affects both the CeRu2 and Ce0.5La0.5Ru2 distributions, strongly favoring the configuration with the CeCe pair, while keeping rare that with a single Ce ion.
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61.66.Dk, 74.20.-z, 78.70.Dm, 74.70.Ad