128 research outputs found
Superconducting Spin Qubits
We propose and theoretically investigate spin superconducting qubits. Spin
superconducting qubit consists of a single spin confined in a Josephson
junction. We show that owing to spin-orbit interaction, superconducting
difference across the junction can polarize this spin. We demonstrate that this
enables single qubit operations and more complicated quantum gates, where spins
of different qubits interact via a mutual inductance of superconducting loop
where the junctions are embedded. Recent experimental realizations of Josephson
junctions made of semiconductor quantum dots in contact with superconducting
leads have shown that the number of electrons in the quantum dot can be tuned
by a gate voltage. Spin superconducting qubit is realized when the number of
electrons is odd. We discuss the qubit properties at phenomenological level. We
present a microscopic theory that enables us to make accurate estimations of
the qubit parameters by evaluating the spin-dependent Josephson energy in the
framework of fourth-order perturbation theory.Comment: 11 pages, 8 figure
Statistics of Transmission Eigenvalues for a Disordered Quantum Point Contact
We study the distribution of transmission eigenvalues of a quantum point
contact with nearby impurities. In the semi-classical case (the chemical
potential lies at the conductance plateau) we find that the transmission
properties of this system are obtained from the ensemble of Gaussian random
reflection matrices. The distribution only depends on the number of open
transport channels and the average reflection eigenvalue and crosses over from
the Poissonian for one open channel to the form predicted by the circuit theory
in the limit of large number of open channels.Comment: 8 pages, 3 figure
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
Resonant Tunneling in a Dissipative Environment
We measure tunneling through a single quantum level in a carbon nanotube
quantum dot connected to resistive metal leads. For the electrons tunneling
to/from the nanotube, the leads serve as a dissipative environment, which
suppresses the tunneling rate. In the regime of sequential tunneling, the
height of the single-electron conductance peaks increases as the temperature is
lowered, although it scales more weekly than the conventional 1/T. In the
resonant tunneling regime (temperature smaller than the level width), the peak
width approaches saturation, while the peak height starts to decrease. Overall,
the peak height shows a non-monotonic temperature dependence. We associate this
unusual behavior with the transition from the sequential to the resonant
tunneling through a single quantum level in a dissipative environment.Comment: 5 pages, 5 figure
Topological Gauge Structure and Phase Diagram for Weakly Doped Antiferromagnets
We show that the topological gauge structure in the phase string theory of
the {\rm t-J} model gives rise to a global phase diagram of antiferromagnetic
(AF) and superconducting (SC) phases in a weakly doped regime. Dual confinement
and deconfinement of holons and spinons play essential roles here, with a
quantum critical point at a doping concentration . The complex
experimental phase diagram at low doping is well described within such a
framework.Comment: 4 pages, 2 figures, modified version, to appear in Phys. Rev. Let
0-pi transitions in Josephson junctions with antiferromagnetic interlayers
We show that the dc Josephson current through
superconductor-antiferromagnet-superconductor (S/AF/S) junctions manifests a
remarkable atomic scale dependence on the interlayer thickness. At low
temperatures the junction is either a 0- or pi-junction depending on whether
the AF interlayer consists of an even or odd number of atomic layers. This is
associated with different symmetries of the AF interlayers in the two cases. In
the junction with odd AF interlayers an additional pi-0 transition can take
place as a function of temperature. This originates from the interplay of
spin-split Andreev bound states. Experimental implications of these theoretical
findings are discussed.Comment: 4 pages, 2 figure
Modulated Floquet Topological Insulators
Floquet topological insulators are topological phases of matter generated by
the application of time-periodic perturbations on otherwise conventional
insulators. We demonstrate that spatial variations in the time-periodic
potential lead to localized quasi-stationary states in two-dimensional systems.
These states include one-dimensional interface modes at the nodes of the
external potential, and fractionalized excitations at vortices of the external
potential. We also propose a setup by which light can induce currents in these
systems. We explain these results by showing a close analogy to px+ipy
superconductors
Influence of e-e scattering on the temperature dependence of the resistance of a classical ballistic point contact in a two-dimensional electron system
We experimentally investigate the temperature (T) dependence of the
resistance of a classical ballistic point contact (PC) in a two-dimensional
electron system (2DES). The split-gate PC is realized in a high-quality
AlGaAs/GaAs heterostructure. The PC resistance is found to drop by more than
10% as T is raised from 0.5 K to 4.2 K. In the absence of a magnetic field, the
T dependence is roughly linear below 2 K and tends to saturate at higher T.
Perpendicular magnetic fields on the order of a few 10 mT suppress the
T-dependent contribution dR. This effect is more pronounced at lower
temperatures, causing a crossover to a nearly parabolic T dependence in a
magnetic field. The normalized magnetic field dependencies dR(B) permit an
empiric single parameter scaling in a wide range of PC gate voltages. These
observations give strong evidence for the influence of electron-electron (e-e)
scattering on the resistance of ballistic PCs. Our results are in qualitative
agreement with a recent theory of the e-e scattering based T dependence of the
conductance of classical ballistic PCs [ Phys. Rev. Lett. 101 216807 (2008) and
Phys. Rev. B 81 125316 (2010)].Comment: as publishe
Noise in a Quantum Point Contact due to a Fluctuating Impurity Configuration
We propose a theoretical model for the low-frequency noise observed in a
quantum point contact (QPC) electrostatically defined in the 2D electron gas at
a GaAs-AlGaAs interface. In such contacts electron scattering by soft impurity-
or boundary potentials coherently splits an incoming wave function between
different transverse modes. Interference between these modes have been
suggested to explain observed non-linearities in the QPC-conductance. In this
study we invoke the same mechanism and the time-dependent current due to soft
dynamical impurity scattering in order to analyze the low-frequency
(telegraph-like) noise which has been observed along with a nonlinear
conductance. For the simplified case of a channel with two extended (current
carrying) modes, a simple analytical formula for the noise intensity is
derived. Generally we have found qualitative similarities between the noise and
the square of the transconductance. Nevertheless, incidentally there may be
situations when noise is suppressed but transconductance enhanced.Comment: 9 revte
Interplay of electromagnetic noise and Kondo effect in quantum dots
We investigate the influence of an electromagnetic environment, characterized
by a finite impedance , on the Kondo effect in quantum dots. The
circuit voltage fluctuations couple to charge fluctuations in the dot and
influence the spin exchange processes transferring charge between the
electrodes. We discuss how the low-energy properties of a Kondo quantum dot
subject to dynamical Coulomb blockade resemble those of Kondo impurities in
Luttinger liquids. Using previous knowledge based on the bosonization of
quantum impurity models, we show that low-voltage conductance anomalies appear
at zero temperature. The conductance can vanish at low temperatures even in
presence of a screened impurity spin. Moreover, the quantitative determination
of the corresponding Kondo temperature depends on the full frequency-dependent
impedance of the circuit. This is demonstrated by a weak-coupling calculation
in the Kondo interaction, taking into account the full distribution of
excited environmental modes.Comment: 10 pages, 4 figures, revised version, new titl
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