Nonlinear quantum mode of terahertz electromagnetic wave amplification in resonant tunneling heterostructures

An analysis of high-frequency properties of the resonant tunneling diode (RTD) in a strong microwave electromagnetic field showed that the high-frequency current response increasing with the microwave power significantly more rapidly saturates out in the case of classical amplification mode, than in the case of “quantum” amplification mode. This makes the “quantum” mode even more attractive in comparison with the classical mode from the viewpoint of the possibility of amplification and generation in the range of subterahertz and terahertz frequencies and offers new opportunities to advance towards these frequencies.     

Geometric phase via adiabatic manipulations of the environment

It has been shown that geometric phases may be generated in a quantum system subject to noise by adiabatic manipulations of the fluctuating fields, e.g., by variation of the system-environment coupling. For a two-state quantum system, this phase is expressed in terms of the geometry of the path, traversed by the slowly varying direction and amplitude of the fluctuations. The origin of this phase and the possibilities of separating it from the known environment-induced modification of the Berry phase are discussed. The text was submitted by the authors in English.     

On the effect of the electron distribution in the near-contact region and asymmetry of the resonant-tunneling diode structure on the high-frequency response and the possibility of detecting the quantum amplification mode in an external high-frequency electric field

The conditions of the implementation of the quantum mode of microwave generation in semiconductor resonant-tunneling diode (RTD) structures were theoretically analyzed. The high-frequency response in structures with symmetric and asymmetric barriers was analyzed by numerical simulation methods. The effect of the Fermi distribution of carriers in the near-contact region was studied. It was shown that asymmetric structures with decreased impurity concentrations (10¹⁷ cm^?3) in the emitter region are optimum from the viewpoint of experimental observation of the quantum amplification mode.     

Influence of disorder on superconductivity in non-magnetic rare-earth nickel borocarbides

The effect of substitutional disorder on the superconducting properties of YNi₂B₂C was studied by partially replacing yttrium and nickel by Lu and Pt, respectively. For the two series of (Y, Lu)Ni₂B₂C and Y(Ni, Pt)₂B₂C compounds, the upper critical field H _c2(T) and the specific heat c _p(T, H) in the superconducting mixed state have been investigated. Disorder is found to reduce several relevant quantities such as T _c, the upper critical field H _c2(0) at T=0 and a characteristic positive curvature of H _c2(T) observed for these compounds near T _c. The H _c2(T) data point to the clean limit for (Y, Lu) substitutions and to a transition to the quasi-dirty limit for (Ni, Pt) substitutions. The electronic specific heat contribution γ(H) exhibits significant deviations from the usual linear γ(H) law. These deviations reduce with growing substitutional disorder but remain even in the quasidirty limit which is reached in the Y(Ni_1−x , Pt _x )₂B₂C samples for x=0.1.     

Single-well resonant-tunneling diode heterostructures based on In0.53Ga0.47As/AlAs/InP with the peak-to-valley current ratio of 22:1 at room temperature

Current-voltage (I–V) characteristics of resonant-tunneling diode In_0.53Ga_0.47As/AlAs/InP structures are studied at 300 and 77 K. The peak-to-valley current ratios were determined as 22:1 and 44:1 at temperatures of 300 and 77 K, respectively, which correspond to the maximum values for InGaAs/AlAs/InP heterostructures without an additional InAs layer of a quantum subwell in their configuration.     

Non-resonant microwave absorption studies of superconducting MgB2 and MgB2 + MgO

Non-resonant microwave absorption (NRMA) studies of superconducting MgB₂ and a sample containing ∼10% by weight of MgO in MgB₂ are reported. The NRMA results indicate near absence of intergranular weak links in the pure MgB₂ sample. A linear temperature dependence of the lower critical field H _c1 is observed indicating a non-s wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d wave symmetry is also not observed. In the MgB₂ + MgO sample, much larger low field dependent absorption is observed indicating the presence of intergranular weak links. The hysteretic behavior of NRMA is compared and contrasted in the two samples. In the pure MgB₂ sample, a large hysteresis is observed between the forward and the reverse scans of the magnetic field indicating strong pinning of flux lines. This hysteresis saturates a few degrees below T _c while in the MgB₂ + MgO sample, a much slower increase of hysteresis with decreasing temperature is observed, a signature of weaker pinning.     

Coulomb interaction and first-order superconductor-insulator transition

The superconductor-insulator transition (SIT) in regular arrays of Josephson junctions is studied at low temperatures. We derived an imaginary time Ginzburg-Landau-type action properly describing the Coulomb interaction. The renormalization group analysis at zero temperature T=0 in the space dimensionality d=3 shows that the SIT is always of the first order. At finite T, a tricritical point separates the lines of the first- and second-order phase transitions. The same conclusion holds for d=2 if the mutual capacitance is larger than the distance between junctions.     

High-frequency response and the possibility of detecting the quantum amplification mode in resonant-tunneling diode structures

The conditions of the implementation of the quantum mode of microwave generation in semiconductor resonant-tunneling diode (RTD) structures were theoretically analyzed. Based on the constructed analytical model of the steady-state current and high-frequency response in symmetric RTD structures with finite barrier widths, high-frequency properties of RTDs in an external ac electric field were analyzed by numerical simulation methods. It was shown that the quantum amplification mode can appear not only in the high-frequency region of the terahertz range, but also at relatively low frequencies due to deformation of frequency dependences in a dc electric field.     

Dephasing of Josephson qubits close to optimal points

Decoherence of Josephson qubits can be substantially reduced by tuning their parameters to optimal operation points with only quadratic coupling to fluctuations. We analyze dephasing due to 1/f noise for a two-level system detuned from an optimal point, i.e., the crossover to the linear-coupling regime, both for free induction decay and for spin-echo experiments. Influence of several noise sources is also discussed. The text was submitted by the authors in English.