1,689 research outputs found
Interaction of a Nanomagnet with a Weak Superconducting Link
We study electromagnetic interaction of a nanomagnet with a weak
superconducting link. Equations that govern coupled dynamics of the two systems
are derived and investigated numerically. We show that the presence of a small
magnet in the proximity of a weak link may be detected through Shapiro-like
steps caused by the precession of the magnetic moment. Despite very weak
magnetic field generated by the weak link, a time-dependent bias voltage
applied to the link can initiate a non-linear dynamics of the nanomagnet that
leads to the reversal of its magnetic moment. We also consider quantum problem
in which a nanomagnet interacting with a weak link is treated as a two-state
spin system due to quantum tunneling between spin-up and spin-down states.Comment: 7 pages, 4 figure
Two-instanton approximation to the Coulomb blockade problem
We develop the two-instanton approximation to the current-voltage
characteristic of a single electron transistor within the
Ambegaokar-Eckern-Sch\"on model. We determine the temperature and gate voltage
dependence of the Coulomb blockade oscillations of the conductance and the
effective charge. We find that a small (in comparison with the charging energy)
bias voltage leads to significant suppression of the Coulomb blockade
oscillations and to appearance of the bias-dependent phase shift
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<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
Exchange effects on electron transport through single-electron spin-valve transistors
We study electron transport through single-electron spin-valve transistors in
the presence of non-local exchange between the ferromagnetic leads and the
central normal-metal island. The Coulomb interaction is described with the
orthodox model for Coulomb blockade and we allow for noncollinear lead
magnetization directions. Two distinct exchange mechanisms that have been
discussed in the literature are shown to be of comparable strength and are
taken into account on equal footing. We present results for the linear
conductance as a function of gate voltage and magnetic configuration, and
discuss the response of the system to applied magnetic fields.Comment: 15 pages, 6 figure
Josephson Current and Noise at a Superconductor-Quantum Spin Hall Insulator-Superconductor Junction
We study junctions between superconductors mediated by the edge states of a
quantum spin Hall insulator. We show that such junctions exhibit a fractional
Josephson effect, in which the current phase relation has a 4\pi, rather than a
2\pi periodicity. This effect is a consequence of the conservation of fermion
parity - the number of electrons modulo 2 - in a superconducting junction, and
is closely related to the Z_2 topological structure of the quantum spin Hall
insulator. Inelastic processes, which violate the conservation of fermion
parity, lead to telegraph noise in the equilibrium supercurrent. We predict
that the low frequency noise due these processes diverges exponentially with
temperature T as T -> 0. Possible experiments on HgCdTe quantum wells will be
discussed.Comment: 4 pages, 2 figure
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
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
The Cyclotron Spin-Flip Mode as the Lowest-Energy Excitation of Unpolarized Integer Quantum Hall States
The cyclotron spin-flip modes of spin unpolarized integer quantum Hall states
() have been studied with inelastic light scattering. The energy of
these modes is significantly smaller compared to the bare cyclotron gap. Second
order exchange corrections are held responsible for a negative energy
contribution and render these modes the lowest energy excitations of
unpolarized integer quantum Hall states.Comment: Published: Phys. Rev. B 72, 073304 (2005
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
Electron-phonon interaction in transition metal diborides TB_2 (T=Zr, Nb, Ta) studied by point-contact spectroscopy
The electron-phonon interaction (EPI) in transition metal diborides TB_2
(T=Zr, Nb, Ta) is investigated by point-contact (PC) spectroscopy. The PC EPI
functions were recovered and the EPI parameters lambda<0.1 were estimated for
all three compounds. Common and distinctive features between the EPI functions
for those diborides are discussed also in connection with the superconductivity
in MgB_2.Comment: V2: minor changes, Ref.[21] added, publ. in PR
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