Electron-hole Liquid Research Articles

Observation of the electron-hole liquid inSi1−xGex/Siquantum wells by steady-state and time-resolved photoluminescence measurements

Observation of the electron-hole liquid in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Si</mml:mtext></mml:mrow><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mtext>Ge</mml:mtext></mml:mrow><mml:mi>x</mml:mi></mml:msub><mml:mo>/</mml:mo><mml:mtext>Si</mml:mtext></mml:mrow></mml:math>quantum wells by steady-state and time-resolved photoluminescence measurements

Electron-hole liquid and excitonic molecules in quasi-two-dimensional SiGe layers of Si/SiGe/Si heterostructures

The electron-hole liquid (EHL) in SiGe layers of Si/Si1 − xGex/Si quantum-confinement heterostructures is discovered. It is composed of quasi-two-dimensional holes in the quantum well formed by the SiGe layer and quasi-three-dimensional electrons, which occupy a wider region of space centered on this layer. The densities of electrons and holes in the EHL are determined to be p0 ≈ 8.5 × 1011 cm−2 and n0 ≈ 4.8 × 1018 cm−3, respectively. It is demonstrated that the gas phase consists of excitons and excitonic molecules. The conditions on the band parameters of the structure under which the formation of the EHL of this kind and biexcitons is possible are formulated.

Determination of carrier diffusion coefficient and lifetime in single crystalline CVD diamonds by light‐induced transient grating technique

AbstractWe report on a contactless, all‐optical study of carrier diffusion and recombination kinetics in single‐crystalline diamond layers using the light‐induced transient grating (LITG) technique. Decay times of transient diffraction grating yielded carrier lifetime of τR ≈ 3 ns and bipolar diffusion coefficient Da = 12 cm2/s at 300 K. The latter value of Da was found to be 4–5 times lower than the ambipolar diffusivity based on electron and hole mobilities, measured by photo‐electrical time‐of‐flight (ToF) technique. This discrepancy was attributed to the bandgap renormalization at high excess carrier densities and its impact on carrier diffusion. The significant decrease of low temperature diffusivity pointed out to a contribution of many‐body effects which are tentatively attributed to the formation of electron–hole liquid (EHL) at T &lt; 150 K.

Surface Recombination Velocity Measurements of Efficiently Passivated Gold-Catalyzed Silicon Nanowires by a New Optical Method

The past decade has seen the explosion of experimental results on nanowires grown by catalyzed mechanisms. However, few are known on their electronic properties especially the influence of surfaces and catalysts. We demonstrate by an optical method how a curious electron-hole thermodynamic phase can help to characterize volume and surface recombination rates of silicon nanowires (SiNWs). By studying the electron-hole liquid dynamics as a function of the spatial confinement, we directly measured these two key parameters. We measured a surface recombination velocity of passivated SiNWs of 20 cm s(-1), 100 times lower than previous values reported. Furthermore, the volume recombination rate of gold-catalyzed SiNWs is found to be similar to that of a high-quality three-dimensional silicon crystal; the influence of the catalyst is negligible. These results advance the knowledge of SiNW surface passivation and provide essential guidance to the development of efficient nanowire-based devices.

Guided Plasmons in Graphenep−nJunctions

Spatial separation of electrons and holes in graphene gives rise to the existence of plasmon waves confined to the boundary region. A theory of such guided plasmon modes within hydrodynamics of electron-hole liquid is developed. For plasmon wavelengths smaller than the size of charged domains, plasmon dispersion is found to be omega proportional to q(1/4). The frequency, velocity, and direction of propagation of guided plasmon modes can be easily controlled by the external electric field. In the presence of a magnetic field, a spectrum of additional gapless magnetoplasmon excitations is obtained. Our findings indicate that graphene is a promising material for nanoplasmonics.

Temperature-dependent photoluminescence of arsenic-doped Si nanocrystals

The characteristics of temperature-dependent photoluminescence (PL) from Si nanocrystals and effects of arsenic-doping (As-doping) were investigated. The Si nanocrystals on a p-type Si substrate were prepared by low pressure chemical vapor deposition and post-deposition thermal oxidation. The As-doping process was carried out using the gas-phase-doping technique. Temperature-dependent PL from Si nanocrystals exhibited considerable differences between samples with/without As-doping. Phase transition between electron–hole liquid and free exciton was observed in the undoped Si nanocrystals, leading to the increase in PL intensity with temperature less than 50 K. Electron emission from As-doped Si nanocrystals to the p-Si substrate was responsible for the significant increase in PL intensity with temperature greater than 50 K. Characteristics of light emission from Si nanocrystals will facilitate the development of silicon-based nanoscaled light-emitting devices.

Recombination Dynamics of Spatially Confined Electron−Hole System in Luminescent Gold Catalyzed Silicon Nanowires

We study by time-resolved low temperature photoluminescence (PL) experiments of the electronic states of silicon nanowires (SiNWs) grown by gold catalyzed chemical vapor deposition and passivated by thermal SiO(2). The typical recombination line of free carriers in gold-catalyzed SiNWs (Au-SiNWs) is identified and studied by time-resolved experiments. We demonstrate that intrinsic Auger recombination governs the recombination dynamic of the dense e-h plasma generated inside the NW. In a few tens of nanoseconds after the pulsed excitation, the density of the initial electronic system rapidly decreases down to reach that of a stable electron-hole liquid phase. The comparison of the PL intensity decay time of Au-SiNWs with high crystalline quality and purity silicon layer allows us to conclude that the Au-SiNW electronic properties are highly comparable to those of bulk silicon crystal.

Intra-Landau-level excitations of the two-dimensional electron–hole liquid

The intra-Landau-level excitations of the two-dimensional electron–hole liquid arecharacterized by two branches of the energy spectrum. The acoustical plasmon branch within-phase oscillations of electrons and holes has a linear dispersion law in the range of smallwavevectors, with a velocity which does not depend on the magnetic field strength, andmonotonically increases with saturation at higher values of the wavevectors. The opticalplasmon branch with oscillations of electrons and holes in opposite phases has a quadraticdependence in the range of long wavelength, a weak roton-type behaviour at theintermediary values of the wavevectors and monotonically increases with saturationsimilar to the case of the acoustical branch. The influence of the supplementaryin-plane electric field leads to the drift of the charged particles in the crossedelectric and magnetic fields and to the energy spectrum as in the reference frame,where the e–h system is moving with the drift velocity. A perturbation theoryusing the Green function method is developed on the basis of a small parameterv2(1−v2), wherev2 is the fillingfactor and (1−v2) displays the phase space filling effect.

Disproportionation and electronic phase separation in parent manganiteLaMnO3

Nominally pure undoped parent manganite LaMnO_3 exhibits a puzzling behavior inconsistent with a simple picture of an A-type antiferromagnetic insulator (A-AFI) with a cooperative Jahn-Teller ordering. We do assign its anomalous properties to charge transfer instabilities and competition between insulating A-AFI phase and metallic-like dynamically disproportionated phase formally separated by a first-order phase transition at T_{disp}=T_{JT}\approx 750 K. The unconventional high-temperature phase is addressed to be a specific electron-hole Bose liquid (EHBL) rather than a simple "chemically" disproportionated R(Mn^{2+}Mn^{4+})O_3 phase. New phase does nucleate as a result of the charge transfer (CT) instability and evolves from the self-trapped CT excitons, or specific EH-dimers, which seem to be a precursor of both insulating and metallic-like ferromagnetic phases observed in manganites. We arrive at highly frustrated system of triplet (e_g^2)^3A_{2g} bosons moving in a lattice formed by hole Mn^{4+} centers. Starting with different experimental data we have reproduced a typical temperature dependence of the volume fraction of high-temperature mixed-valent EHBL phase. We argue that a slight nonisovalent substitution, photo-irradiation, external pressure or magnetic field gives rise to an electronic phase separation with a nucleation or an overgrowth of EH-droplets. Such a scenario provides a comprehensive explanation of numerous puzzling properties observed in parent and nonisovalently doped manganite LaMnO_3 including an intriguing manifestation of superconducting fluctuations.

Phase transitions in nonequilibrium electron-hole systems of Si/SiGe/Si nanoheterostructures

The exciton condensation in a Si1−xGex solid solution layer of Si/Si1−xGex/Si heterostructures with the formation of electron-hole liquid has been investigated by low-temperature photoluminescence spectroscopy. In the temperature range above the critical temperature of the transition from an exciton gas to electron-hole liquid, a Mott transition from an exciton gas to electron-hole plasma has been found and investigated.

Exciton condensation in the compressively strained SiGe layers of Si/SiGe/Si heterostructures

The photoluminescence spectra of type-I alignment Si/Si 1 − x Ge x /Si heterostructures contained thin Si 1 − x Ge x layers ( d = 25–70 nm) are studied under various temperatures and excitation intensities. It was shown, excitation intensity increase at low temperatures leads to the exciton condensation resulting electron–hole liquid (EHL) formation in Si 1 − x Ge x layer. Electron–hole pair density n 0 and binding energy of the EHL relative to exciton gas φ decrease noticeably while х increases. The decrease in the binding energy and density of the electron–hole liquid is attributed to splitting of conduction and valance bands due to internal strains in the Si 1 − x Ge x layer. The Mott transition (from the exciton gas to electron–hole plasma) occurs above the critical temperatures for high excitation intensities.

Exciton Gas Compression and Metallic Condensation in a Single Semiconductor Quantum Wire

We study the metal-insulator transition in individual self-assembled quantum wires and report optical evidence of metallic liquid condensation at low temperatures. First, we observe that the temperature and power dependence of the single nanowire photoluminescence follow the evolution expected for an electron-hole liquid in one dimension. Second, we find novel spectral features that suggest that in this situation the expanding liquid condensate compresses the exciton gas in real space. Finally, we estimate the critical density and critical temperature of the phase transition diagram at n{c} approximately 1 x 10;{5} cm;{-1} and T{c} approximately 35 K, respectively.

Exciton condensation in strong magnetic fields

The density correction to the chemical potential of excitons in a strong magnetic field was calculated. The possibilities of Bose-Einstein condensation of excitons and their condensation into electron-hole liquid (EHL) were studied. Magnetic field ranges in which these processes can be observed were determined.

The plasma oscillations in a two-dimensional electron-hole liquid

The plasma oscillations in a two-dimensional electron-hole liquid

Cooperative effects in SiGe/Si quantum wells

Studies of the luminescence spectra of Si 1− x Ge x /Si quantum wells with a low Ge content have been carried out in a wide pumping power range. It was shown that a Si 1− x Ge x /Si quantum well spectrum variation observed at low temperatures and high pumping powers is caused by cooperative effects—formation of the electron-hole liquid and multiexciton complexes in quantum wells.

Electron-hole liquid in strained SiGe layers of silicon heterostructures

The electron-hole liquid has been found in strained SiGe thin films of Si/Si1−x Gex/Si heterostructures. The density and binding energy of the electron-hole liquid have been determined. Owing to the presence of internal strains in the SiGe layer, the density and binding energy are significantly smaller than the respective quantities for the electron-hole liquid in a bulk single crystal of the solid solution of the same composition. The critical temperature of the transition from the exciton gas to the electron-hole liquid is estimated using the experimental data. The Mott transition (from the exciton gas to electron-hole plasma) occurs above the critical temperatures for high excitation intensities.

Influence of Coulomb scattering of electrons and holes between Landau levels on energy spectrum and collective properties of two-dimensional magnetoexcitons

This study is concerned with a two-dimensional electron–hole system in a strong perpendicular magnetic field with special attention devoted to the influence of the virtual quantum transitions of interacting particles between the Landau levels. It is shown that virtual quantum transitions of two Coulomb interacting particles from the lowest Landau levels to excited Landau levels with arbitrary quantum numbers n and m and their transition back to the lowest Landau levels in the second order of the perturbation theory result in indirect attraction between the particles supplementary to their Coulomb interaction. The influence of this indirect interaction on the chemical potential of the Bose–Einstein condensed magnetoexcitons and on the ground state energy of the metallic-type electron–hole liquid (EHL) is investigated in the Hartree–Fock approximation. The supplementary electron–electron and hole–hole interactions being averaged with direct pairing of operators increases the binding energy of magnetoexciton and the energy per pair in the EHL phase. The terms obtained in the exchange pairing of operators give rise to repulsion. Together with the Bogoliubov self-energy terms arising from the electron–hole supplementary interaction they both influence in the favor of BEC of magnetoexcitons with small momentum. The influence of the excited exciton bands on the energy spectrum and on the wave function of the lowest magnetoexciton band is studied in the second order of the perturbation theory. The BEC of magnetoexcitons in the superposition state is considered. The generalized Bogoliubov transformations, the BCS-type ground state wave function and the phase-space filling factors of the lowest and first excited Landau levels are determined.

Correlation effects in partially ionized mass asymmetric electron-hole plasmas

The effects of strong Coulomb correlations in dense three-dimensional electron-hole plasmas are studied by means of unbiased direct path integral Monte Carlo simulations. The formation and dissociation of bound states, such as excitons and biexcitons, is analyzed and the density-temperature region of their appearance is identified. At high density, the Mott transition to the fully ionized metallic state (electron-hole liquid) is detected. Particular attention is paid to the influence of the hole to electron mass ratio M on the properties of the plasma. Above a critical value of about M=80 formation of a hole Coulomb crystal was recently verified [Bonitz, Phys. Rev. Lett. 95, 235006 (2005)] which is supported by additional results. Results are related to the excitonic phase diagram of intermediate valent Tm[Se,Te], where large values of M have been observed experimentally.

Excitonic versus Electron–Hole Liquid Phases in Tm[Se,Te] Compounds

We discuss, from a theoretical point of view, excitonic phases at the pressure induced semiconductor–semimetal transition in TmSe 0.45 Te 0.55 , focusing, in particular, on the stability against an electron–hole liquid. The electron–hole pair density parameter r s ( E g , T ) is calculated within the quasi-static plasmon pole approximation as a function of temperature T and energy gap E g and converted into - E g ( r s , T ). A comparison of this quantity, which is the electron–hole pair chemical potential, with the exciton binding energy reveals that excitons should be suppressed in contrast to experimental evidence for excitonic phases. We suspect therefore inter-valley exciton scattering and exciton–phonon scattering to substantially stabilise excitons in TmSe 0.45 Te 0.55 .

Influence of excited Landau levels on a two-dimensional electron–hole system in a strong perpendicular magnetic field

The study of the quantum states of a two-dimensional electron–hole system in a strong perpendicular magnetic field is carried out with special attention to the influence of virtual quantum transitions of interacting particles between the Landau levels. These virtual quantum transitions from the lowest Landau levels to excited Landau levels with arbitrary quantum numbers n and m and their reversion to the lowest Landau levels in second order perturbation theory result in an indirect attraction between the particles. The influence of the indirect interaction on the magnetoexciton ground state, on the chemical potential of the Bose–Einstein condensed magnetoexcitons, and on the ground state energy of the metallic-type electron–hole liquid is investigated in the Hartree–Fock approximation. The coexistence of different phases is suggested.