669 research outputs found
Controlled dephasing in single-dot Aharonov-Bohm interferometers
We study the Fano effect and the visibility of the Aharonov-Bohm oscillations
for a mesoscopic interferometer with an embedded quantum dot in the presence of
a nearby second dot. When the electron-electron interaction between the two
dots is considered the nearby dot acts as a charge detector. We compute the
currents through the interferometer and detector within the Keldysh formalism
and the self-energy of the non-equilibrium Green functions is found up to the
second order in the interaction strength. The current formula contains a
correction to the Landauer-B\"{uttiker} formula. Its contribution to transport
and dephasing is discussed. As the bias applied on the detector is increased,
the amplitude of both the Fano resonance and Aharonov-Bohm oscillations are
considerably reduced due to controlled dephasing. This result is explained by
analyzing the behavior of the imaginary part of the self-energy as a function
of energy and bias. We investigate as well the role of the ring-dot coupling.
Our theoretical results are consistent to the experimental observation of Buks
{\it et al.} [Nature {\bf 391}, 871 (1998)].Comment: 24 pages, 8 figure
Interplay between superconductivity and itinerant magnetism in underdoped BaKFeAs ( 0.2) probed by the response to controlled point-like disorder
The response of superconductors to controlled introduction of point-like
disorder is an important tool to probe their microscopic electronic collective
behavior. In the case of iron-based superconductors (IBS), magnetic
fluctuations presumably play an important role in inducing high temperature
superconductivity. In some cases, these two seemingly incompatible orders
coexist microscopically. Therefore, understanding how this unique coexistence
state is affected by disorder can provide important information about the
microscopic mechanisms involved. In one of the most studied pnictide family,
hole-doped BaKFeAs (BaK122), this coexistence occurs over a
wide range of doping levels, 0.16~~0.25. We used
relativistic 2.5 MeV electrons to induce vacancy-interstitial (Frenkel) pairs
that act as efficient point-like scattering centers. Upon increasing dose of
irradiation, the superconducting transition temperature decreases
dramatically. In the absence of nodes in the order parameter this provides a
strong support for a sign-changing pairing. Simultaneously, in the
normal state, there is a strong violation of the Matthiessen's rule and a
decrease (surprisingly, at the same rate as ) of the magnetic transition
temperature , which indicates the itinerant nature of the long-range
magnetic order. Comparison of the hole-doped BaK122 with electron-doped
Ba(FeCo)As (FeCo122) with similar 110~K,
0.02, reveals significant differences in the normal states, with no
apparent Matthiessen's rule violation above on the electron-doped
side. We interpret these results in terms of the distinct impact of impurity
scattering on the competing itinerant antiferromagnetic and
superconducting orders
Superconductivity and Physical Properties of CaPd2Ge2 Single Crystals
We present the superconducting and normal state properties of CaPd2Ge2 single
crystal investigated by magnetic susceptibility \chi, isothermal magnetization
M, heat capacity C_p, in-plane electrical resistivity \rho and London
penetration depth \lambda versus temperature T and magnetic field H
measurements. Bulk superconductivity is inferred from the \rho(T) and C_p(T)
data. The \rho(T) data exhibit metallic behavior and undergoes a
superconducting transition with T_c onset = 1.98 K and zero resistivity state
at T_c 0 = 1.67 K. The \chi(T) reveal the onset of superconductivity at 2.0 K.
For T>2.0 K, the \chi(T) and M(H) are weakly anisotropic paramagnetic with
\chi_ab > \chi_c. The C_p(T) confirm the bulk superconductivity below T_c =
1.69(3) K. The superconducting state electronic heat capacity is analyzed
within the framework of a single-band \alpha-model of BCS superconductivity and
various normal and superconducting state parameters are estimated. Within the
\alpha-model, the C_p(T) data and the ab plane \lambda(T) data consistently
indicate a moderately anisotropic s-wave gap with \Delta(0)/k_B T_c ~ 1.6,
somewhat smaller than the BCS value of 1.764. The relationship of the heat
capacity jump at T_c and the penetration depth measurement to the anisotropy in
the s-wave gap is discussed.Comment: 12 pages, 9 figures, 2 Tables; Submitted to PR
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