Spectroscopy of phonons and spin torques in magnetic point contacts

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts. Our results show that impurity scattering at the N/F interface is the origin of the new single-interface spin torque effect.Comment: 4 pages, 5 figs., accepted for publication in PR

Effect of quantum interference in the nonlinear conductance of microconstrictions

The influence of the interference of electron waves, which were scattered by single impurities, on nonlinear quantum conductance of metallic microconstrictions (as was recently investigated experimentally) is studied theoretically. The dependence of the interference pattern in the conductance $G(V)$ on the contact diameter and the spatial distribution of impurities is analyzed. It is shown that the amplitude of conductance oscillation is strongly depended on the position of impurities inside the constriction.Comment: 6 pages, 4 figures, To appear in PR

Coulomb blockade in quantum dots under AC pumping

We study conductance through a quantum dot under Coulomb blockade conditions in the presence of an external periodic perturbation. The stationary state is determined by the balance between the heating of the dot electrons by the perturbation and cooling. We analyze two cooling mechanisms: electron exchange with the cold contacts and emission of phonons. Together with the usual linear Ohmic heating of the dot electrons we consider possible effects of dynamic localization. The combination of the abovementioned factors may result in a drastic change of the shape of the Coulomb blockade peak with respect to the usual equilibrium one.Comment: 12 pages, 8 figure

High-frequency oscillations in low-dimensional conductors and semiconductor superlattices induced by current in stack direction

A narrow energy band of the electronic spectrum in some direction in low-dimensional crystals may lead to a negative differential conductance and N-shaped I-V curve that results in an instability of the uniform stationary state. A well-known stable solution for such a system is a state with electric field domain. We have found a uniform stable solution in the region of negative differential conductance. This solution describes uniform high-frequency voltage oscillations. Frequency of the oscillation is determined by antenna properties of the system. The results are applicable also to semiconductor superlattices.Comment: 8 pages, 3 figure

Voltage dependent conductance and shot noise in quantum microconstriction with single defects

The influence of the interference of electron waves, which are scattered by single impurities and by a barrier on nonlinear conductance and shot noise of metallic microconstriction is studied theoretically. It is shown that the these characteristics are nonmonotonic functions on the applied bias.Comment: 18 pages,5 figure

Charge ordering and interlayer phase coherence in quantum Hall superlattices

The possibility of the existence of states with a spontaneous interlayer phase coherence in multilayer electron systems in a high perpendicular to the layers magnetic field is investigated. It is shown that phase coherence can be established in such systems only within individual pairs of adjacent layers, while such coherence does not exist between layers of different pairs. The conditions for stability of the state with interlayer phase coherence against transition to a charge-ordered state are determined. It is shown that in the system with the number of layers N\leq 10 these conditions are satisfied at any value of the interlayer distance d. For N>10 there are two intervals of stability: at sufficiently large and at sufficiently small d. For N\to \infty the stability interval in the region of small d vanishesComment: 10 page

Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF) (T_N=5.2K) HoNi2B2C single crystals in the normal and two different superconducting (SC) states (T_c=8.5K and $T_c^*=5.6K) are reported. The PC study of the electron-boson(phonon) interaction (EB(P)I) spectral function reveals pronounced phonon maxima at 16, 22 and 34meV. For the first time the high energy maxima at about 50meV and 100meV are resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitations with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the 10-meV peak in PC EPI constant \lambda_PC is evaluated as 20-30contribution of the high energy modes at 50 and 100meV amounts about 10each maxima, so the superconductivity might be affected by CEF excitations. The SC gap in HoNi2B2C exhibits a standard single-band BCS-like dependence, but vanishes at$T_c^*=5.6K<T_c, with 2\Delta/kT_c^*=3.9. The strong coupling Eliashberg analysis of the low-temperature SC phase with T_c^*=5.6K =T_N, coexisting with the commensurate AF structure, suggests a sizable value of the EPI constant \lambda_s=0.93. We also provide strong support for the recently proposed by us ''Fermi surface (FS) separation'' scenario for the coexistence of magnetism and superconductivity in magnetic borocarbides, namely, that the superconductivity in the commensurate AF phase survives at a special (nearly isotropic) FS sheet without an admixture of Ho 5d states. Above T_c^* the SC features in the PC characteristics are strongly suppressed pointing to a specific weakened SC state between T_c* and T_c.Comment: 11 pages, 8 figs, to be published in PRB, Vol.75, Iss.2

Point-contact study of the LuNi2B2C borocarbide superconducting film

We present point-contact (PC) Andreev-reflection measurements of a superconducting epitaxial c-axis oriented nickel borocarbide film LuNi2B2C (Tc=15.9 K). The averaged value of the superconducting gap is found to be 2.6 +/-0.2 meV in the one-gap approach, whereas the two-gap approach results in 2.14+/-0.36 meV and 3.0+/-0.27 meV. The better fit of the Andreev-reflection spectra for the LuNi2B2C - Cu PC obtained by the two-gap approach provides evidence for multiband superconductivity in LuNi2B2C. For the first time, PC electron-phonon interaction (EPI) spectra have been measured for this compound. They demonstrate pronounced phonon maximum at 8.5+/-0.4meV and a second shallow one at 15.8+/-0.6 meV. The electron-phonon coupling constant estimated from the PC EPI spectra turned out to be small (~ 0.1), like in other superconducting rare-earth nickel borocarbides. Possible reasons for this are discussed.Comment: 5 pages, 5 figures, V2: figs. 2 & 5 captions are corrected, and new Refs. 4, 6, 12, 13, 14 are adde

The problem of "macroscopic charge quantization" in single-electron devices

In a recent Letter by the authors [I.S. Burmistrov and A.M.M. Pruisken, Phys. Rev. Lett. 101, 056801 (2008)] it was shown that single-electron devices (single electron transistor or SET) display "macroscopic charge quantization" which is completely analogous to the quantum Hall effect observed on very different electron systems. In this investigation we present more detail on these new findings. Based on the Ambegaokar-Eckern-Schoen (AES) theory of the Coulomb blockade we introduce a general response theory that probes the sensitivity of SET to changes in the boundary conditions. This response theory defines a new set of physical observables and we establish the contact with the standard results obtained from ordinary linear response theory. The response parameters generally define the renormalization behavior of the SET in the entire regime from weak coupling with large values of the tunneling conductance all the way down to the strong coupling phase where the system displays the Coulomb blockade. We introduce a general criterion for charge quantization that is analogous to the Thouless criterion for Anderson localization. We present the results of detailed computations on the weak coupling side of the theory, i.e. both perturbative and non-perturbative (instantons). Based on an effective theory in terms of quantum spins we study the quantum critical behavior of the AES model on the strong coupling side. Consequently, a unifying scaling diagram of the SET is obtained. This diagram displays all the super universal topological features of the theta-angle concept that previously arose in the theory of the quantum Hall effect.Comment: RevTex, 22 pages, 10 figure