Intraband population inversion and amplification of IR radiation through charge-carrier injection into quantum wells and quantum dots

A mechanism is proposed for obtaining intraband population inversion of electrons in size-quantization levels through the injection of electron-hole pairs into the i region of a heterostructure with quantum wells or quantum dots. Key elements of the mechanism are the simultaneous generation of interband (hv≈E _g ) near-IR radiation and the presence of a “metastable” level. In quantum wells such a level can be produced by making use of the weak overlap of the wave functions of electrons in the levels of a quantum well of complicated configuration and exploiting the characteristic features of the interaction of electrons with optical phonons in polar semiconductors. In quantum dots such a level forms as a result of the phonon bottleneck effect. Estimates are made of the gain for mid-IR radiation in intraband optical transitions of electrons. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 392–399 (10 September 1998)     

Analysis of adiabatic transfer in cavity quantum electrodynamics

A three-level atom in a Λ configuration trapped in an optical cavity forms a basic unit in a number of proposed protocols for quantum information processing. This system allows for efficient storage of cavity photons into long-lived atomic excitations, and their retrieval with high fidelity, in an adiabatic transfer process through the ‘dark state’ by a slow variation of the control laser intensity. We study the full quantum mechanics of this transfer process with a view to examine the non-adiabatic effects arising from inevitable excitations of the system to states involving the upper level of Λ, which is radiative. We find that the fidelity of storage is better, the stronger the control field and the slower the rate of its switching off. On the contrary, unlike the adiabatic notion, retrieval is better with faster rates of switching on of an optimal control field. Also, for retrieval, the behaviour with dissipation is non-monotonic. These results lend themselves to experimental tests. Our exact computations, when applied to slow variations of the control intensity for strong atom–photon couplings, are in very good agreement with Berry’s superadiabatic transfer results without dissipation.     

Cooper pairing and superfluidity in the Emery model

The energy E of the system as a function of the gauge phase Φ is calculated by exact diagonalization in a two-dimensional Cu₄O₈ cluster and by the slave-boson method for large systems. It is shown that motion of carriers with charge 2e, i.e., Cooper pairs, is observed for certain values of the parameters in the Hamiltonian. This motion is identified from the onset of a characteristic maximum of E(Φ) at Φ≈Φ₀/2, where Φ₀ is the flux quantum. The phase diagram is constructed and the range of values of the model parameters where the effect is observed is determined. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 78–82 (25 January 1996)     

Temporal structure of the muon disk at large distances from the axis of extensive air showers with E 0⩾6 ×1016eV

Preliminary results are reported from an investigation of the temporal structure of the muon disk in extensive air showers (EASs) with primary energy E ₀≥6×10¹⁶eV at distances 100–1500 m from the axis. The investigation is performed at the Yakutsk array using the large muon detector, which commenced operation in November 1995, with a planned area 184 m² and a detection threshold E _μ≈0.5· sec θ GeV. For E ₀>10¹⁸eV it is found that the thickness of the muon disk tends to decrease. This requires substantial changes in our notions of the development of EASs. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 361–366 (25 March 1998)     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Interference and Interaction in multi-wall carbon nanotubes

We report equilibrium electric resistance R and tunneling spectroscopy (dI/dV)measurements obtained on single multi-wall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence (l_φ)and elastic-scattering lengths (l_e)are deduced. Because l_e is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density of states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at ≈1–10 Kfor all measured nanotubes. R(T) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of the order ≈10 meV, a pseudogap is observed in dI/dV which agrees with Luttinger-liquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-liquid as well as Luttinger-liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 4 August 1999     

Quantum orientational melting and the phase diagram of a mesoscopic system

The “phase diagram” of a two-dimensional mesoscopic system of bosons is investigated. An example of such a system is a system of indirect magnetoexcitons in semiconductor double quantum dots. Quantum Monte Carlo calculations show the existence of quantum orientational melting. At zero (quite low) temperature, as quantum fluctuations of the particles intensify, two quantum disordering phenomena occur with increasing de Boer parameter q. First, at q≈10⁻³ the system passes to a radially ordered but orientationally disordered state, where different shells of a cluster rotate relative to one another. Then at q≈0.16 a transition to a superfluid state occurs. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 817–822 (10 December 1998)     

Quasi-one-dimensional antiferromagnet LiCuVO4

An antiferromagnetic transition is observed in the quasi-one-dimensional metal-oxide compound LiCuVO₄. A wide peak is observed in the temperature dependence of the magnetic susceptibility at T _M =28 K, and the magnetic susceptibility exhibits a sharp drop at T _N ≈4 K. As the magnetic field increases, the antiferromagnetic-ordering temperature T _N =2.3 K at first increases somewhat and then decreases rapidly. The exchange interaction in the chains of copper-oxygen octahedra is estimated to be J ₁=22.5 K. The interchain interaction is estimated to be J ₂∼1 K. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 828–831 (10 June 1999)     

Ultrahigh-energy cosmic rays, superheavy long-lived particles, and matter creation after inflation

Cosmic rays of the highest energy, above the Greisen-Zatsepin-Kuzmin (GZK) cutoff of the spectrum, may originate in decays of superheavy long-lived particles. We conjecture that these particles may be produced naturally in the early Universe from vacuum fluctuations during inflation and may constitute a considerable fraction of cold dark matter. We predict a new cutoff in the ultrahigh-energy cosmic ray spectrum E _cutoff<m _inflaton≈10¹³ GeV, the exact position of the cutoff and the shape of the cosmic ray spectrum beyond the GZK cutoff being determined by the QCD quark/gluon fragmentation. The Pierre Auger Project installation may in principle observe this phenomenon. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 255–259 (25 August 1998)     

Exact self-consistent plane-symmetric solutions of the equations of two interacting fields (spinor field and scalar field)

A spinor field interacting with a zero-mass neutral scalar field is considered for the case of the simplest type of direct interaction, where the interaction Lagrangian has the formL _int =1/2 ϕ_αϕ_,α F(S) whereF(S) is an arbitrary function of the spinor field invariantS=ψψ. Exact solutions of the corresponding systems of equations that take into account the natural gravitational field in a plane-symmetric metric are obtained. It is proved that the initial system of equations has regular localized soliton-type solutions only if the energy density of the zero-mass scalar field is negative as it “disengages” from interaction with the spinor field. In two-dimensional space-time the system of field equations we are studying describes the configuration of fields with constant energy densityT ₀₀ , i.e., no soliton-like solutions exist in this case. Russian People’s Friendship University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 69–75, July, 1998.     

Electron spin resonance studies at the phase transition in YbInCu4

In the intermetallic compound YbInCu₄ the Gd³⁺-ESR and the static susceptibility were measured in the temperature range 1.5K–300 K, i.e. both below and above the valence phase transition which occurs in this material at T_ph≈50 K. The Gd³⁺ resonance is mainly determined by exchange interaction of Gd³⁺ ions with fluctuating Yb³⁺ ions via conduction electrons (RKKY coupling) both below and above this transition. Arguments are presented that at low temperature YbInCu₄ is a dense Kondo system (mixed valent state).     

Band structure and dominating electron-scattering mechanisms in Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis>−<Emphasis Type="Italic">y</Emphasis></Subscript>Mn<Subscript>x</Subscript>Fe<Subscript>y</Subscript>Se

Magnetic and kinetic properties as well as transmission and absorption spectra of Hg _1−x−y Mn _x Fe _ySe (0.09 ≤ x ≤ 0.099 and 0.001 ≤ y ≤ 0.01) crystals are investigated at H ≈ 0.5–6 kOe in the temperature range T = 77–300 K. The band parameters are determined on the basis of experimental data. It is found that in the crystals under study at T ≈ 300 K, electron scattering by polar optical phonons dominates, direct optical band-to-band transitions occur, and replacement of a part of Mn atoms by Fe for x + y = 0.1 results in an increase in E_g ^op with Fe content. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 35–39, March, 2007.     

Nature of the low-temperature anomalies in the physical properties of the intermediate-valent compound SmB6

The transport properties (Hall coefficient, thermopower, and resistivity) of high-quality single-crystal samples of the classical mixed-valent compound SmB₆ are investigated over a broad temperature range (1.6–300 K) in magnetic fields up to 45 T for the first time following the quasioptical measurements in the 0.6–4.5 meV frequency range [B. Gorshunov, N. Sluchanko, A. Volkov et al., submitted to Phys. Rev. B (1998)]. Measurements in the intrinsic conduction region permit determination of the gap width E _g ≈20 meV and evaluation of the behavior of the mobility and concentration of light and heavy charge carriers, as well as the temperature dependence of the carrier relaxation time, in samarium hexaboride. The results of experimental investigations in the “impurity” conduction region (E _ex≈3.5 meV) are discussed within the Kikoin-Mishchenko exciton-polaron model of charge fluctuations. Arguments supporting the formation of a metallic state with an electron-hole liquid in SmB₆ at liquid-helium temperatures are presented. Zh. éksp. Teor. Fiz. 115, 970–978 (March 1999)     

Bethe-Salpeter equation with the sine-Gordon interaction

An attempt is made to study the interaction Hamiltonian,H _int =Gψ ²(x)U(φ(x)) in the Bethe-Salpeter framework for the confined states of theψ particles interactingvia the exchange of theU field, whereU(φ) = cos (gφ). An approximate solution of the eigenvalue problem is obtained in the instantaneous approximation by projecting the Wick-rotated Bethe-Salpeter equation onto the surface of a four-dimensional sphere and employing Hecke’s theorem in the weak-binding limit. We find that the spectrum of energies for the confined states,E =2m+B (B is the binding energy), is characterized byE ∼n ⁶, wheren is the principal quantum number.     

Scattering of atoms in the field of counterpropagating light waves. Effect of initial conditions

The scattering of an atom in the field of counterpropagating light waves is studied under conditions such that the state of the atom is a superposition of the ground and excited states. For the case in which this superposition is created by the field of a traveling wave, the momentum distribution function of the atom after scattering by a standing wave is found analytically in the approximation of a short interaction time, when the atom’s motion can be neglected. Longer interactions of the atom with the field are studied numerically. We also consider the case of counterpropagating light waves consisting of Gaussian or supergaussian pulses. Zh. éksp. Teor. Fiz. 113, 563–572 (February 1998)     

Dynamics of four-wave mixing on excitons in a quantum optical microcavity

A theory of degenerate four-wave mixing in a semiconductor optical cavity with quantum wells is constructed. The nonlinear response of a microcavity can be four to five orders of magnitude stronger than that of an isolated quantum well. For P ² E-and P ³-type nonlinearities the damped diffracted signal oscillates with a period determined by the Rabi splitting. For a biexcitonic mechanism of nonlinearity, the signal contains damped overtones of the Rabi splitting and the biexciton binding energy. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 749–754 (10 December 1996)     

Condition of strong magnetic field in the mode of quantum reluctance oscillations

The volume (3D), quasi-two-dimentional (Q2D), and two-dimentional (2D) oscillations of degenerated electrons of the Shubnikov-de Haase reluctance reveal common properties. The region of quantum oscillations is bounded (in the magnetic field) from below by the weak field condition (ω_cτ_t≤1) and from above by the quantum limit condition (ξ_F≥ħω_c/2). The monotonic oscillation component is saturated for the occupied main conduction E_m-subband and excitation E_p-subband of dimensional quantization for the Q2D and 2D electron systems in strong magnetic fields. The reluctance of the Q2D system in the quantum limit changes according to a law ∼B ^α with α ≈ 2.6–2.8. The oscillation amplitude is described by an exponential dependence on the magnetic field strength and temperature, similarly to the 3D case. This is caused by the identity of physical conditions and the topology of resonant escape of the Landau levels out of the Fermi surface. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 33–39, March, 2006.     

Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/Al_xGa_1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 10¹¹ cm⁻² the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 139–144 (10 August 1997)     

Kinetics of the b{su1}Σ{sk{su\s+}/{in{itg}}→\ga{su1}\gD{in{itg}} fluorescence of molecular oxygen in solutionsfluorescence of molecular oxygen in solutions

The kinetics of sensitized fluorescence of O₂ has been recorded for the first time in solutions at transition from the second singlet excited state b{su1}Σ{sk{su\s+}/{in{itg}} to the first excited state \ga{su1}\gD{in{itg}} (b→a. We measured the quantum yield of the b→a-fluorescence at γ≈1.93 μm and assessed its probability with respect to the a→ X-phosphorescence at 1.27 μm. We show that the lifetime of the b→a-fluorescence (105 nsec for CCl₄ and 140 nsec for CS₂) becomes much shorter on addition of solvents with large frequencies of intramolecular vibrations. The measured radiative velocity constant of the b→a-transition K_b→a is by three orders of magnitude larger than for the phosphorescence (≈1.27 μm) of the a→X-transition from the first singlet to the ground triplet state. The method of recording IR fluorescence is promising for determining O₂ in solutions and biosystems. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp 5–8, January–February, 1998.     

CO2 and C2H4 grafting to the native defects of ion-bombarded porous silica

• )₄, though it is expected to relax via formation of peroxidic bridges), and the oxygen-bridge vacancy (which in the absence of relaxation can be described as a diradical center (≡Si^•)₂– the E^′′ center). These radicals react with the residual atmosphere according to completely new pathways: the bombardment in a CO₂ atmosphere results in the formation of ester-like and carboxylate groups, stable up to 500 °C at least, inserted in the SiO₂ network at the oxygen-bridge vacancies. The bombardment in a C₂H₄ atmosphere results in more complex configurations: the oxygen-bridge vacancy reacts at room temperature with ethylene forming a Lewis adduct which, after heating at 500 °C, presumably reverts to a -CH₂-CH₂- bridge in between silicon atoms; the silicon-link vacancy likely reacts with C₂H₄ forming CH₃CHO. These conclusions, based on experimental data (mainly infrared spectroscopy), are also supported by extended quantum mechanical calculations (density-functional methods and ab initio molecular dynamics). Received: 6 March 1998/Accepted: 21 September 1998