V. V. Kapaev

Magnetoelectric phenomena in nanoelectronics

Abstract Two-well quantum structures are considered with broken space inversion and time reversal symmetries. The structures exhibit a number of magnetoelectric phenomena which on microscopic level result from energy spectrum asymmetry in quasimomentum space and from the existence of spontaneous local currents.

Collapse of resonances in semiconductor heterostructures as a transition with symmetry breaking in an open quantum system

The collapse of resonances in resonant tunneling heterostructures, which is merging of two unit-transparency resonances into a single resonance with transparency less than unity has been theoretically analyzed. The electron density distribution becomes asymmetric at the collapse point in a geometrically symmetric system. The asymmetry parameter behaves as the order parameter in the second-order phase transition. The physical mechanism of the transition is associated with the broadening of the quasistationary levels of a quantum system due to the interaction with the continuum of delocalized states. The possibility of the existence of two qualitatively different types of low-energy resonances differing in the sign of the effective range of the potential has been demonstrated.

Momentum dependence of the dimensionality of the electronic states in heterostructures

Bound states of electrons (holes) in quantum wells and wires with asymmetric barriers can exist in bounded regions of two-and one-dimensional momentum space, respectively. As the corresponding momentum increases, both the disappearance (increase of dimensionality) and appearance (decrease of dimensionality) of bound states as well as the existence of a sequence of several such transformations of dimensionality are possible. In the case of anisotropic effective masses in the quantum wells and barriers, the forms of the lines of disappearance and appearance of bound states are different from the forms of the isoenergy lines. Therefore there is a finite energy interval (i.e., electron density interval) where bound states exist on only a part of an isoenergy line. The dimensionality of the states can be controlled with an electric field; this should be observable in a number of the experiments discussed.

High-frequency response and the possibilities of frequency-tunable narrow-band terahertz amplification in resonant tunneling nanostructures

The characteristics of the high-frequency response of single- and double-well resonant tunneling structures in a dc electric field are investigated on the basis of the numerical solution of a time-dependent Schrödinger equation with open boundary conditions. The frequency dependence of the real part of high frequency conductivity (high-frequency response) in In0.53Ga0.47As/AlAs/InP structures is analyzed in detail for various values of the dc voltage Vdc in the negative differential resistance (NDR) region. It is shown that double-well three-barrier structures are promising for the design of terahertz-band oscillators. The presence of two resonant states with close energies in such structures leads to a resonant (in frequency) response whose frequency is determined by the energy difference between these levels and can be controlled by varying the parameters of the structure. It is shown that, in principle, such structures admit narrow-band amplification, tuning of the amplification frequency, and a fine control of the amplification (oscillation) frequency in a wide range of terahertz frequencies by varying a dc electric voltage applied to the structure. Starting from a certain width of the central intermediate barrier in double-well structures, one can observe a collapse of resonances, where the structure behaves like a single-well system. This phenomenon imposes a lower limit on the oscillation frequency in three-barrier resonant tunneling structures.

Mirror nesting and electron-hole asymmetry at repulsive superconducting pairing

The dependence of the superconducting order parameter Δ(k) on the momentum of the relative motion of a pair with a large total momentum K is numerically studied for the case of repulsive pairing with allowance for the kinematic and insulator constraints on the momentum transfer at scattering. The Fermi contour with nesting and mirror nesting, which is typical of cuprates and optimal for repulsion-induced superconductivity, lies in an extended vicinity of the saddle points of the dispersion law. A deviation from the mirror nesting cuts off the logarithmic singularity from below and bounds the pre-exponent in Δ(k). The effective coupling constant is determined by the degree of the electron-hole asymmetry. The suppression of the contribution of small momentum transfer processes by the impurity and electron-phonon scattering favors an increase in the order parameter amplitude. The nesting of the Fermi contour causes a Peierls singularity in the Coulomb interaction. The self-consistency equation allows the solutions that may be both antisymmetric and symmetric with respect to the momentum inversion. The maximum-amplitude antisymmetric solution in the case of a singlet pairing can be realized only for K ≠ 0.

Coulomb coupling of like charges due to negative reduced effective mass

The shape of Fermi contour high-temperature superconductors (HTS) has the important feature. Namely, the components of hole pair reduced effective mass are of opposite sign at the most part of the Fermi contour. As a result, it is possible a rise of a hole pairing due to direct Coulomb interaction. This conception is used to explain the essential distinction between the temperature of a pseudogap arising and the temperature of the superconductor transition in the case of underdoped HTS compounds.

Coulomb pairing of like-charged particles with negative effective mass in high-temperature superconductors

Quasi-steady states of pairs of like-charged quasi-particles can be formed because the electronic structure of compounds exhibiting high-temperature superconductivity has various important characteristics: a quasi-two-dimensional electron spectrum, clearly defined nesting of constant-energy lines, and the presence of a logarithmic singularity of the density of states in the immediate vicinity of the Fermi level. Thus, a situation is achieved where, in an extensive region of the Brillouin zone adjacent to the Fermi level, the principal values of the tensor of the reciprocal effective masses have opposite signs and differ appreciably in absolute value. As a result, the nature of the Coulomb correlation interaction between charge carriers of the same sign (holes in p-cuprates) varies: effective attraction may predominate, leading to the formation of long-lived states of relative motion of quasi-particles which form a pair having a quasi-momentum approximately equal to twice the Fermi quasi-momentum typical of this direction (focused pairs). Assuming that the correlation interaction is short-range (screened Coulomb interaction attenuated by filling of states inside the Fermi contour), we determine the energies and envelope functions of the relative motion of a hole pair which correspond to the density-of-states maxima of the pairs attributable to these quasi-steady states. The dependence of these quantities on the polar angle in the plane of the conducting layer reflects the symmetry of the electronic structure of the compound in the normal state and is generally consistent with a mixture of states assigned to s and d types of orbital symmetry. The quasi-steady state as a function of the doping level of the system agrees qualitatively with the concentration dependence of the temperature for the appearance of a pseudogap observed in p-cuprates at below-optimum doping levels. An estimate of the pair concentration above which a gain in correlation energy occurs gives a value which corresponds to the onset of effective pair overlap (for which the characteristic spatial scale is a few or a few tens of interatomic distances).

Double-well coherent laser with suppressed intersubband relaxation

A theory of coherent lasing on a double-well structure with asymmetric barriers is constructed. On account of the appearance of a termination point of the bottom subband, such a structural element permits strong suppression of intersubband relaxation involving the emission of an optical phonon. It is shown that such a laser possesses low threshold currents and high lasing power, and it can operate without a population inversion.

Electromagnetic-field amplification in finite one-dimensional photonic crystals

The electromagnetic-field distribution in a finite one-dimensional photonic crystal is studied using the numerical solution of Maxwell’s equations by the transfer-matrix method. The dependence of the transmission coefficient T on the period d (or the wavelength λ) has the characteristic form with M–1 (M is the number of periods in the structure) maxima with T = 1 in the allowed band of an infinite crystal and zero values in the forbidden band. The field-modulus distribution E(x) in the structure for parameters that correspond to the transmission maxima closest to the boundaries of forbidden bands has maxima at the center of the structure; the value at the maximum considerably exceeds the incident-field strength. For the number of periods M ~ 50, more than an order of magnitude increase in the field amplification is observed. The numerical results are interpreted with an analytic theory constructed by representing the solution in the form of a linear combination of counterpropagating Floquet modes in a periodic structure.

Nonlinear theory of the narrow-band generation and detection of terahertz radiation in resonant tunneling heterostructures

The nonlinear regime of high-frequency response for resonant tunneling structures in a time-periodic electric field has been investigated using a technique for solving the time-dependent Schrödinger equation based on a Floquet mode expansion of the wave functions. The dependences of current harmonic amplitudes on ac signal amplitude have been calculated and the limiting values of the generated field have been determined for singleand double-well resonant tunneling structures. The dynamic Stark effect is shown to play an important role in the formation of response. It leads to a quadratic (in ac field amplitude) shift in the positions of resonances Er in single-well structures and in double-well ones in the nonresonant case and to a splitting at resonance hν ≈ Er2–Er1 (ν is the signal frequency, Er1 and Er2 are the energies of the size-quantization levels) in double-well structures proportional to the ac signal amplitude. The phenomenon of ac signal detection by resonant tunneling structures has been investigated. The effect of resonant direct-current amplification in double-well structures has been detected at a signal frequency satisfying the condition hν ≈ Er2–Er1. In asymmetric systems, detection is shown to be possible in the absence of a dc bias, which allows zero-biased detectors based on them to be created.