Volume 37, Issue 3, March 2011
Index of content:
37(2011); http://dx.doi.org/10.1063/1.3570932View Description Hide Description
37(2011); http://dx.doi.org/10.1063/1.3570931View Description Hide Description
The phase diagram of Bose-Einstein condensates of dipolar excitons accumulated in lateral electrostatic traps is determined. Spatially periodic luminescence structures of the excitonBose condensate and their critical dependence on pump level and temperature are observed. Coherence in the dipolar excitonBose-Einstein condensates is studied by direct measurement of the 1-st and 2-nd order correlators with variations in pump level and temperature. These results are indicative of large-scale coherence of the excitonBose condensate.
37(2011); http://dx.doi.org/10.1063/1.3573602View Description Hide Description
A wide range of experimental methods are used in the study of two-dimensional electronic systems. Each of these methods is rather simple by itself and has, in principle, been known for along time. Nevertheless, most of them have not become part of daily practice, partly because of considerable difficulties in their use and partly because they have not become popular enough. This article is a brief review of these potentially useful methods and outlines the areas where they can be used.
37(2011); http://dx.doi.org/10.1063/1.3573604View Description Hide Description
The interaction between solitary elastic strain pulses (acoustic solitons) and localized holes in low-dimensional silicon structures is studied theoretically. It is shown that a soliton propagating through a region of hole localization converts it from one quantum state to another characterized by a different projection of the angular momentum. This effect originates in splitting of the ground hole state, which is degenerate in the absence of a perturbation, by the elastic strain. A detailed microscopic calculation of the acoustic switching of quantum mechanical states is carried out for holes localized at a quantum dot or at a shallow impurity acceptor in a quantum well. It is shown that the acoustic soliton amplitude required for complete reversal of the projected angular momentum of a hole corresponds to the amplitude of typical experimentally produced strain pulses.
37(2011); http://dx.doi.org/10.1063/1.3573648View Description Hide Description
The first results are reported from a study of a new two-dimensional electron system, a two-dimensional semimetal, that is observed in wide quantum wells based on mercury telluride, which have an inverted band spectrum. Magnetotransport experiments confirm the existence of a semimetal state in quantum wells with (013) and (112) orientations and thicknesses of . These experiments show that the band overlap . A comparison of the experimentally determined with a theoretical calculation of the energy spectrum reveals the fundamental role of strain effects in the formation of the semimetal state. Scattering processes in the two-dimensional semimetal are studied and it is found that the jump in the electron mobility during electronic metal-two-dimensional semimetal transitions is caused by shielding of electron scattering on impurities by holes. The substantial, anomalous rise in the resistivity of the two-dimensional semimetal with increasing temperature is caused by electron-hole scattering. This is the first observation of the direct effect of interparticle scattering (Landau mechanism) on the resistivity of metals. The properties of two-dimensional semimetals in the quantum Hall effect regime are examined. Primary attention is devoted to the observed suppression of strong localization under the conditions of the quantum Hall effect. It is shown that in a strong magnetic field the two-component electron-hole plasma has fundamentally different topological properties from those of an ordinary single-component (electron or hole) plasma. It is proposed that these lead to the appearance of an infinite set of conducting current states and to the suppression of localization.
37(2011); http://dx.doi.org/10.1063/1.3573664View Description Hide Description
The crystal structure, composition, galvanomagnetic, and oscillatory properties of (; ) alloys are studied for different matrices and chromium impurity concentrations. It is shown that impurity chromium ions dissolve in the lattice in amounts below , while higher chromium concentrations lead to the appearance of microscopic regions with elevated chromium contents and inclusions of chromium-tellurium compounds. Reductions in the hole concentration, conversion of the conductivity type, and stabilization of the Fermi level by the resonance level of chromium are observed with increasing chromium content. The initial rates of change of the charge carrier concentrations during doping are determined. A two-band Kane dispersion relation is used to calculate the electron concentration and Fermi level as functions of the tin content. A diagram showing the rearrangement of the electronic structure of the chromiumdoped alloys with varying matrix composition is constructed.
Anomalous low-temperature contribution to the heat capacity from hybridized electronic states on transition element impurities37(2011); http://dx.doi.org/10.1063/1.3573665View Description Hide Description
An anomalous nonmonotonic contribution to the temperature dependence of the electron heat capacity of mercury selenide is detected. This is explained in terms of hybridized electronic states on donor impurities. The observed effect is described by a theory of electron heat capacity based on a quantum Fermi-liquid approach including localization and electron-electron interactions. A quantitative interpretation of the experimental dependences yields values for the parameters of the hybridized states that are consistent with those known from other experiments. A new parameter characterizing the electron-electron interaction in the hybridized states is also found.
37(2011); http://dx.doi.org/10.1063/1.3574502View Description Hide Description
The properties of zinc oxide are examined as an analog of gallium nitride over a wide range of temperatures and possible applications. Its economic and environmental advantages are noted, as well as its radiation hardness, compared to group III nitrides. Methods for growingfilms and nanostructures with high crystal perfection are proposed. In particular, a magnetron technique for layer-by-layer growth of films is implemented which makes it possible to obtain high structural perfection and substantial thicknesses unattainable by several other methods. The feasibility of producing monochromatic UV radiation from films excited by short-wavelength radiation and electrons is demonstrated; this means that they may be useable as short-wavelength radiation sources. Efficient field emission by ZnOnanostructures and films is demonstrated and opens up the prospect of their use in vacuum microelectronics equipment. Nitrogen-doped ZnOfilms, in particular, have been used to fabricate a phototransistor with a sensitivity two orders of magnitude higher than conventional detectors. The physical basis for creating LEDs for different colors based on ZnOfilms and solid solutions with CdO is discussed. The importance of studying the physics and technology of zinc oxide-based devices is emphasized.
37(2011); http://dx.doi.org/10.1063/1.3570930View Description Hide Description
We show that the atomic layer deposition(ALD) technique has great potential for widespread use in the production of ZnOfilms for applications in electronic, photovoltaic(PV), and optoelectronic devices. The low growth temperature makes ALD-grown ZnOfilms suitable for fabrication of various semiconductor/organic hybrid structures. This opens up the possibility of novel devices based on very cheap organic materials, including organic light emitting diodes and third-generation PV cells.
37(2011); http://dx.doi.org/10.1063/1.3570929View Description Hide Description
We discuss x-ray absorption fine structure(EXAFS) data for binary dopedcompoundstructures which can be deconvolved to determine elemental bond distances and the deviations from random configurations owing to site preference occupations (SOPs). The SOP-deviation estimates can be confirmed further by independent Fourier transform infrared (FTIR) data. The limits of our model are discussed.
37(2011); http://dx.doi.org/10.1063/1.3580512View Description Hide Description
Liquid alloys of metals with mercury (amalgams) are used to synthesize the fullerides (; ; ), (; ; ; ), and . The samples are studied by x-ray diffraction,nuclear magnetic resonance, electron paramagnetic resonance, Raman scattering, and differential scanning calorimetry. The fullerides are not superconducting. Structural phase transitions are observed in them as the temperature is varied. is a superconductor with a transition temperature , while is not superconducting. are superconductors with . The cesium fullerides with an assumed composition of are either polymers or crystallize in a rhombic lattice, but none are superconducting. A maximum is observed in the fulleride with a thallium concentration . (; ) is not superconducting, but produced by means of exchange reactions with TlCl is a superconductor with .
37(2011); http://dx.doi.org/10.1063/1.3580606View Description Hide Description
A theoretical analysis and a large amount of experimental data indicate that the structure of the valence hole states in doped cuprates is more complicated than assumed in the simple Zhang-Rice singlet model. In fact, we are dealing with a competition between a hybrid -state and purely oxygen nonbonding states with - and -symmetries. Thus, as a cluster analog of a ion, the ground state of a non-Zhang-Rice hole center of this sort should be described by complicated multiplet with a set of charge, orbital, and spin order parameters, some of which are well known (e.g., spin moment or “ferromagnetic” Ising orbital momentum localized on oxygen ions) while others are unconventional or hidden (e.g., “antiferromagnetic” ordering of Ising orbital momenta localized on four oxygen atoms or a combined spin-orbital-quadrupole ordering). The non-Zhang-Rice centers are actually singlet-triplet pseudo-Jahn-Teller centers with strong vibron coupling to the lattice. The complicated structure of the ground-state multiplet of the hole centers shows up in many of the unusual properties of doped cuprates, in particular, their pseudo-gap phase.
37(2011); http://dx.doi.org/10.1063/1.3570925View Description Hide Description
Magnetoresistivity and the Hall effect measured in magnetic fields up to in electron-doped single crystal films with , 0.15, and 0.18 and different oxygen contents were studied for temperatures in the range . The behavior of the resistivity and Hall coefficient in the mixed state are discussed in terms of a flux-flow model including a vortex counterflow owing to pinning forces.