Zero differential resistance in a double quantum well at high filling factors

The differential resistance r _xx in a GaAs double quantum well with two occupied size-quantization subbands have been studied at temperatures T = 1.6–4.2 K in magnetic fields B < 0.5 T. It has been found that differential resistance r _xx vanishes at the maxima of magneto-intersubband oscillations with an increase in the direct current I _dc. It has been shown that the discovered r _xx ≈ 0 state appears under the condition 2R _c E _H/ħω_c < 1/2, where R _c is the cyclotron radius of electrons at the Fermi level, E _H is the Hall electric field induced by the current I _dc, and ω_c is the cyclotron frequency.     

A new optical method for measuring the electron-phonon interaction parameter λ〈Ω2〉 using the spectral dependence of the relaxation rate

The spectral dependence of the electron-phonon relaxation rate γ_e−ph(ℏω) in metals is studied in pump-supercontinuum-probe (PSCP) experiments with femtosecond time resolution. Investigation of this spectral dependence, which exhibits a substantial slowing of the relaxation rate γ_e−ph(ℏω) near the Fermi level E _F , using the parametrization γ_e−ph(ℏω)∝λ〈Ω²〉 (ℏω−E _F )² makes it possible to determine directly the electron-phonon interaction parameter λ〈Ω²〉. The parameter λ〈Ω²〉 for YBa₂Cu₃O_7−δ is analyzed using this method. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 329–332 (10 September 1999)     

Microwave induced magnetoresistance oscillations at the subharmonics of the cyclotron resonance

The magnetoresistance oscillations that occur in a two-dimensional electron system exposed to strong microwave radiation when the microwave frequency ω coincides with the nth subharmonic of the cyclotron frequency ω _c have been investigated for n = 2, 3, and 4. It is shown that these subharmonic features can be explained within a nonequilibrium energy distribution function picture without invoking multiphoton absorption processes. The existence of a frequency threshold above which such oscillations disappear lends further support to this explanation. The text was submitted by the authors in English.     

Synchronization of high-frequency vibrations of slipping phase centers in a tin whisker under microwave radiation

Current-voltage characteristics of a system with a variable number of slipping phase centers resulting from phase separation in a tin whisker under external microwave field with a frequency Ω/2π≅35–45 GHz have been studied experimentally. Emergence and disappearance of steps with zero slope in a whisker’s current-voltage characteristic at U _m/n =(m/n)U _Ω, where m and n are integers and U _Ω is determined by Josephson’s formula _ℏΩ=2eU _Ω, have been investigated. Microwave field generated by slipping phase centers is nonharmonic, and the system of slipping phase centers permits synchronization of internal oscillations at a microwave frequency by an external field with a frequency which is the n-th harmonic of internal oscillations. The estimated microwave power generated by a whisker is 10⁻⁸ W. Stimulation of superconductivity in a current-carrying whisker has been detected. Zh. éksp. Teor. Fiz. 113, 1364–1375 (April 1998)     

Microwave photoresistance of a double quantum well at high filling factors

The effect of millimeter wave radiation on the electronic transport in a GaAs double quantum well at a temperature of 4.2 K in a magnetic field of up to 2 T has been studied. Resistance (conductance) oscillations have been shown to appear in the two-dimensional electronic system under investigation at high filling factors. The magnetic field positions of the oscillation maxima are determined by the condition Δ_SAS/? = lω_c, where Δ_SAS = (E ₂ ? E ₁) is the size quantization sublevel splitting in the quantum well, ω_c is the cyclotron frequency, and l is a positive integer. It has been found that the microwave field substantially modifies the oscillations in the double quantum well, which results in alternating two-frequency oscillations of photoresistance with the inverse magnetic field.     

Length of the Instability Intervals for Hill's Equation

The length of instability intervals is investigated for the Hill equation y′′+ω(ω− 2∈p(x)y = 0, where p(x) has an infinite Fourier series of coefficients c _n. For any small ∈ it is shown that these lengths are completely characterized by the c _n's.     

Measurement of the Magnetic Moment of the First Excited State in 93Sr Using on-line TDPAC technique

The g-factor of the first excited state of ⁹³Sr (E = 213 keV, T _1/2 = 4.6 ns) was measured by an on-line TDPAC technique with use of the strong hyperfine field in Fe metal. The Larmor frequency ω _L = (2.60 ± 0.15) × 10⁸ rad/s was obtained. The g-factor is derived as g = −0.227 ± 0.013 from g = −ℏω _L/B _hf μ _N. If the spin of the first excited state of ⁹³Sr is assumed to be 3/2, the g-factor is predicted by a simple core-excitation model as g = −0.22, which is in good agreement with the present experimental result.     

Quantum oscillations of the resistivity and hall coefficient and the quantum limit in Bi<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript> alloys in a magnetic field along the trigonal axis

The dependences of the electrical resistivity ρ and the Hall coefficient R on the magnetic field have been measured for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconductor alloys with electron concentrations in the range 1 × 10¹⁶ cm⁻³ < n < 2 × 10¹⁸ cm⁻³. It has been found that the measured dependences exhibit Shubnikov-de Haas quantum oscillations. The magnetic fields corresponding to the maxima of the quantum oscillations of the electrical resistivity are in good agreement with the calculated values of the magnetic fields in which the Landau quantum level with the number N intersects the Fermi level. The quantum oscillations of the Hall coefficient with small numbers are characterized by a significant spin splitting. In a magnetic field directed along the trigonal axis, the quantum oscillations of the resistivity ρ and the Hall coefficient R are associated with electrons of the three-valley semiconductor and are in phase with the magnetic field. In the case of a magnetic field directed parallel to the binary axis, the quantum oscillations associated both with electrons of the secondary ellipsoids in weaker magnetic fields and with electrons of the main ellipsoid in strong magnetic fields (after the overflow of electrons from the secondary ellipsoids to the main ellipsoid) are also in phase. In magnetic fields of the quantum limit ħω _c /2 ≥ E _F, the electrical conductivity increases with an increase in the magnetic field: σ₂₂(H) ∼ H ^k . A theoretical evaluation of the exponent in this expression for a nonparabolic semiconductor leads to values of k close to the experimental values in the range 4 ≤ k ≤ 4.6, which were obtained for samples of the semiconductor alloys with different electron concentrations. A further increase in the magnetic field results in a decrease of the exponent k and in the transition to the inequality σ₂₂(H) ≤ σ₂₁(H).     

Oscillator rule of the trap activation energies in NaCl crystals

The energy spectra of traps in NaCl crystals have been studied in detail by the method of thermoluminescence. Crystals of NaCl were undoped but treated thermally in different ways. The activation energies of traps form a single oscillator series, E _n =ℏω _TL (n+1/2), ℏω _TL =903 cm^-1. Contrary to other previously studied crystals with complex lattices, the corresponding line ℏω _Ram =ℏω _TL was not found in Raman spectra of NaCl. It is assumed that the oscillator rule is governed by the polaron nature of traps. The trap activation energy is determined by the vibration level from which the transition of the charge carrier to the excited luminescence centre is made possible and depends on the distance between these centres.     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.     

Rare t-quark decays t → clj+lk? and t → c?jvk induced by doublets of scalar leptoquarks within the minimal four-color-symmetry model

The rare t-quark decays t → cl _j ⁺ l _k ⁻ and t → cṽ _j k _k induced by scalar-leptoquark doublets are considered within the minimal model involving four-color quark-lepton symmetry and the Higgs mechanism of quark and lepton mass splitting. The partial widths with respect to the decays being considered and the total widths Γ(t → cl ⁺′l ⁻) = Σ_j,k Γ(t → cl _j ⁺ l _k ⁻ ) and Γ(t → cl ⁺′l ⁻) = Σ_j,kΓ(t → cṽ _j v _k ) with respect to, respectively, the charged leptonic and neutrino modes are calculated. It is shown that, at scalar-leptoquark masses higher than the t-quark mass (m _S > m _t), the branching ratios for these modes are Br(t → cl ⁺′l ⁻) ≈ (3.5−0.4) × 10⁻⁵ and Br(t → cṽ′v) ≈ (7.1−0.8) × 10⁻⁵ at m _s = 180–250 GeV and an appropriate value of the leptoquark-mixing angle (sin β ≈ 0.2) and can increase for m _S < m _t to Br(t → cl ⁺′l ⁻) ≈ 0.03−0.002 and Br(t → cl ⁺′l ⁻) ≈ 0.46−0.05 for the charged mode at m _S = 150–170 GeV for sin β ≈ 1 and sin β ≈ 0.2, respectively. In the cases being considered, t-quark decays to pairs of charged leptons can be accessible to detection at LHC. In the last case, these decays could manifest themselves (for example, in dilepton events) at the Tevatron as well. Original Russian Text ? P.Yu. Popov, A.D. Smirnov, 2006, published in Yadernaya Fizika, 2006, Vol. 69, No. 6, pp. 1006–1016.     

Magneto-intersubband zener tunneling in a wide GaAs quantum well at high filling factors

The dependence of the differential resistance r _xx on the dc current density J _dc in a wide GaAs quantum well with two occupied size quantization subbands has been investigated at the temperature T = 4.2 K in the magnetic fields B < 1 T. A peak, whose position is given by the relation 2R _c eE _H = ħω_c/2, where R _c is the cyclotron radius, E _H is the Hall electric field, and ω_c is the cyclotron frequency, has been observed in the r _xx (J _dc) curves at high filling factors. The experimental results are attributed to Zener tunneling of electrons between the Landau levels of different subbands.     

Stimulated polariton-polariton scattering and dynamic Bose-Einstein condensation of polaritons in GaAs microcavities under excitation near the exciton resonance

The dynamics of formation of the macroscopically occupied polariton mode at the bottom of the polariton band E _LP(k = 0) and its spin polarization under the quasiresonant pulse excitation of excitons (E = E _X ) with large values of quasi-momentum have been studied in planar GaAs microcavities. It has been found that the growth in the depth E _X − E _LP(k = 0) of the polariton band leads to the change in the formation mechanism for the k = 0 condensate state from the direct parametric decay of the photoexcited mode (due to the polariton-polariton interaction) to the dynamic condensation of polaritons, which results from the multiple scattering of polaritons by both phonons and polaritons. At the same time, in microcavities with E _X − E _LP(k = 0) > 3.5 meV, the direct decay of the photoexcited mode does not disappear, becoming an efficient mechanism for the filling of the states located at the k-space ring, corresponding to the energies E _LP(k) ≈ E _X − 2.6 meV.     

Effect of interlayer single-particle hoppings on the superconducting transition temperature

It is well known that the superconducting transition temperature of high-T _c cuprates depends on the number of CuO₂ planes in the unit cell. The multilayer structure implies the possibility of interlayer hopping. Under the assumption that the interlayer hopping can be specified by the parameter t _⊥(k) = t _⊥(cos(k _x ) − cos(k _y ))², the quasiparticle excitation spectrum for the bilayer cuprate in the superconducting state has been determined in the framework of the t − t′ − t″ − t _⊥ − J* model using the generalized mean-field approximation. It turns out that the interlayer hoppings does not create any additional mechanism of the Cooper paring and does not lead to an increase in T _c . The splitting of the upper Hubbard quasiparticle band attributed to the interlayer hoppings is manifested as two peaks in the doping dependence of the superconducting transition temperature at temperatures below the maximum T _c value for a single-layer cuprate. It has been found that antiferromagnetic interlayer correlations suppress the interlayer splitting. This probably leads to the common doping dependence of T _c for both single-layer and bilayer cuprates.     

Peculiarities in galvanomagnetic properties of granular YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> high-temperature superconductors in very weak magnetic fields

The character of the evolution of a system of weak links in granular high-temperature superconductors under the action of an external magnetic field H _ext has been studied by measuring the current-voltage characteristics E(j)_{Hext = const}E{(j)_{{H_{ext}} = const}} of YBa₂Cu₃O_{7 − δ} (δ ≈ 0.05) ceramic samples. The measurements have been performed at T = 77.3 K in a range of very weak magnetic fields 0 < H _ext ≲ 0.5H _c2J, where H _c2J is the upper critical field of the Josephson weak links. The results have been used to construct the field dependences of the magnetoresistance Δρ(H _ext) of the superconducting ceramics. It has been established that the parameters of the power equation E = A(j − j _cJ)^ν and the magnetoresistance Δρ are nonmonotonic functions of the external magnetic field. The presence of extrema in the curves A(H _ext), j _cJ(H _ext), ν(H _ext), and Δρ(H _ext) indicates that different systems of weak links between grain boundaries, which are capable of forming extended Josephson contacts, undergo sequential transitions to a resistive state with an increase in H _ext.     

Study of the critical current in YBaCuO films under reactor-neutron irradiation in the low-temperature helium circuit in the 25–300 K range

Changes in the temperature and dose dependences of I-V characteristics and of the critical current j _c of YBaCuO films on MgO and SrTiO₃ substrates under neutron irradiation has been studied at 25–80 K. The transport properties of YBaCuO films on MgO (M1) and SrTiO₃ (S1) substrates were found to behave differently. It is shown that the M1 films have granular structure, and their transport properties were considered within the concept of percolation over weak intergranular links. The S1 are single-crystal films, and their properties are analyzed within a resistive-state theory associated with thermal activation of Abrikosov vortices. It is shown that the degradation rate of the critical temperature T _c of S1 films is 3.5 times smaller than that of the M1 films. The dose dependence of j _c has an exponential character, j _c=j _c(0)exp(−kΦ), where k is related to the number of displaced atoms per neutron and is the same for the M1 and S1 films, irrespective of the irradiation temperature. The pinning energy has been derived from the I-V characteristics and it has been found that the U/kT ratio lies within 20–25 and is independent of neutron flux. It is shown that the radiation-induced disorder changes the pinning mechanism, from pinning at the boundaries of misoriented crystals to that at spatial inhomogeneities, apparently radiation induced. Fiz. Tverd. Tela (St. Petersburg) 40, 1961–1967 (November 1998)     

Inverse problem for the Grad-Shafranov equation with affine right-hand side

For a given domain ω ⋐ ℝ² with boundary γ = ∂ω, we study the cardinality of the set $ \mathfrak{A}_\eta \left( \Phi \right) $ \mathfrak{A}_\eta \left( \Phi \right) of pairs of numbers (a, b) for which there is a function u = u _(a,b): ω → ℝ such that ∇² u(x) = au(x) + b ⩾ 0 for x ∈ ω, u| _γ = 0, and ||∇u(s)| − Φ(s) ⩽ η for s ∈ γ. Here η ⩾ 0 stands for a very small number, Φ(s) = |∇(s)| / ∫ _γ |∇v| d γ, and v is the solution of the problem ∇² v = a ₀ v + 1 ⩾ 0 on ω with v| _γ = 0, where a ₀ is a given number. The fundamental difference between the case η = 0 and the physically meaningful case η > 0 is proved. Namely, for η = 0, the set $ \mathfrak{A}_\eta \left( \Phi \right) $ \mathfrak{A}_\eta \left( \Phi \right) contains only one element (a, b) for a broad class of domains ω, and a = a ₀. On the contrary, for an arbitrarily small η > 0, there is a sequence of pairs (a _j , b _j ) ∈ $ \mathfrak{A}_\eta \left( \Phi \right) $ \mathfrak{A}_\eta \left( \Phi \right) and the corresponding functions u _j such that ‖f _u ^j+1‖ − ‖f _u ^j ‖ > 1, where ‖f _u ^j = max _x∈ω |f _u ^j (x)| and f _u ^j (x) = a _j u _j (x) + b _j . Here the mappings f _u ^j : ω → ℝ necessarily tend as j → ∞ to the δ-function concentrated on γ.     

Shift photocurrent induced by two-quantum transitions

A theory of the shift current induced by direct two-photon and indirect one-photon absorption is developed for noncentrosymmetric crystals. A formula is derived for the microscopic shifts of Bloch electrons induced by two-quantum processes. It is shown that the ratio of the two-photon photocurrent at the photon frequency ω to the photocurrent induced by direct one-photon transitions at the photon frequency 2ω, as compared to the corresponding absorption-rate ratio, contains a large factor {ie152-1}ω/(2{ie152-2}ω − E _g), where E _g is the bandgap; i.e., these photocurrent can be comparable in order of magnitude. For crystals of T _d symmetry, the photocurrents induced by one- and two-photon absorption are compared in terms of polarization dependence.     

Properties of superconducting Nb/Al/Nb/Al?AlOx?Al?AlOx?Al/Nb/Al/Nb tunnel junctions

Modified geometry (MG) devices, Nb/Al/Nb/Al−AlO_x−Al−AlO_x−Al/Nb/Al/Nb, have been fabricated and investigated in comparison with the basic geometry (BG) double-barrier Nb/Al−AlO_x−Al−AlO_x−Al/Nb devices. The enhancement of the critical temperature in the Al film is found to be weaker for the MG devices as compared with the BG devices at temperatures nearT=4.2 K but stronger at lowT. Indication of an enhancement of dc Josephson critical current density,j _c , at bias voltageV≠0 as compared withj _c (V=0) has been observed in the MG devices for the first time.