37 research outputs found
Spin-polarized Shapiro steps and spin-precession-assisted multiple Andreev reflection
We investigate the charge and spin transport of a voltage-biased
superconducting point contact coupled to a nanomagnet. The magnetization of the
nanomagnet is assumed to precess with the Larmor frequency, , due to
ferromagnetic resonance. The interplay between the ac Josephson current and the
magnetization dynamics leads to spin-polarized Shapiro steps at voltages
for and the subharmonic steps with
are a consequence of multiple Andreev reflection (MAR). Moreover, the
spin-precession-assisted MAR generates quasiparticle scattering amplitudes
that, due to interference, lead to current-voltage characteristics of the dc
charge and spin currents with subharmonic gap structures displaying an even-odd
effect.Comment: 5 pages, 4 figure
Input-output description of microwave radiation in the dynamical Coulomb blockade
We study microwave radiation emitted by a small voltage-biased Josephson
junction connected to a superconducting transmission line. An input-output
formalism for the radiation field is established, using a perturbation
expansion in the junction's critical current. Using output field operators
solved up to the second order, we estimate the spectral density and the
second-order coherence of the emitted field. For typical transmission line
impedances and at frequencies below the main emission peak at the Josephson
frequency, radiation occurs predominantly due to two-photon emission. This
emission is characterized by a high degree of photon bunching if detected
symmetrically around half of the Josephson frequency. Strong phase fluctuations
in the transmission line make related nonclassical phase-dependent amplitude
correlations short lived, and there is no steady-state two-mode squeezing.
However, the radiation is shown to violate the classical Cauchy-Schwarz
inequality of intensity cross-correlations, demonstrating the nonclassicality
of the photon pair production in this region.Comment: 29 pages, 4 figure
Spectral properties of superconductors with ferromagnetically ordered magnetic impurities
We present a comprehensive theoretical study of thermodynamic properties of
superconductors with a dilute concentration of magnetic impurities, with focus
on how the properties of the superconducting host change if the magnetic
moments of the impurities order ferromagnetically. Scattering off the magnetic
impurities leads to the formation of a band of Yu-Shiba-Rusinov states within
the superconducting energy gap that drastically influences superconductivity.
In the magnetically ordered system, the magnetization displays a sudden drop as
function of impurity density or magnetic moment amplitude. The drop occurs as
the spin-polarized impurity band crosses the Fermi level and is associated with
a quantum phase transition first put forward by Sakurai for the single impurity
case. Taking into account that the background magnetic field created by the
ordered impurity moments enters as a Zeeman shift, we find that the
superconducting phase transition changes from second order to first order for
high enough impurity concentration.Comment: 16 pages, 13 figure
Spontaneously broken time-reversal symmetry in high-temperature superconductors
Conventional superconductors are strong diamagnets that through the Meissner
effect expel magnetic fields. It would therefore be surprising if a
superconducting ground state would support spontaneous magnetics fields. Such
time-reversal symmetry broken states have been proposed for the
high-temperature superconductors, but their identification remains
experimentally controversial. Here we show a route to a low-temperature
superconducting state with broken time-reversal symmetry that may accommodate
currently conflicting experiments. This state is characterised by an unusual
vortex pattern in the form of a necklace of fractional vortices around the
perimeter of the material, where neighbouring vortices have opposite current
circulation. This vortex pattern is a result of a spectral rearrangement of
current carrying states near the surfaces
Transport properties of vertical heterostructures under light irradiation
Electronic and transport properties of bilayer heterostructure under light
irradiation are of fundamental interest to improve functionality of
optoelectronic devices. We theoretically study the modification of transport
properties of bilayer graphene and bilayer heterostructures under a
time-periodic external light field. The bulk electronic and transport
properties are studied in a Landauer-type configuration by using the
nonequilibrium Green's function formalism. To illustrate the behavior of the
differential conductance of a bilayer contact under light illumination, we
consider tight-binding models of bilayer graphene and graphene/hexagonal
boron-nitride heterostructures. The non-adiabatic driving induces sidebands of
the original band structure and opening of gaps in the quasienergy spectrum. In
transport properties, the gap openings are manifested in a suppression of the
differential conductance. In addition to suppression, an external light field
induces an enhancement of the differential conductance if photoexcited
electrons tunnel into or out of a Van~Hove singularity.Comment: 9 pages, 7 figure
Nonclassical photon pair production in a voltage-biased Josephson junction
We investigate electromagnetic radiation emitted by a small voltage-biased
Josephson junction connected to a superconducting transmission line. At
frequencies below the well known emission peak at the Josephson frequency
(2eV/h), extra radiation is triggered by quantum fluctuations in the
electromagnetic environment. For weak tunneling couplings and typical ohmic
transmission lines, the corresponding photon flux spectrum is symmetric around
half the Josephson frequency, indicating that the photons are predominately
created in pairs. By establishing an input-output formalism for the microwave
field in the transmission line, we give further evidence for this nonclassical
photon pair production, demonstrating that it violates the classical
Cauchy-Schwarz inequality for two-mode flux cross correlations. In connection
to recent experiments, we also consider a stepped transmission line, where
resonances increase the signal-to-noise ratio.Comment: 5 pages, 2 figures. This version accepted in Physical Review Letter
Spin-polarized currents and noise in NS junctions with Yu-Shiba-Rusinov impurities
Conventional superconductors disordered by magnetic impurities demonstrate
physical properties drastically different from their pristine counterparts. In
our previous work [Phys. Rev. B 92, 245430 (2015)] we explored spectral and
thermodynamic properties of such systems for two extreme cases: completely
random and ferromagnetically aligned impurity magnetic moments. Here we
consider transport properties of these systems, and show that they have a
potential to be used in superconducting spintronic devices. Each magnetic
impurity contributes a Yu-Shiba-Rusinov (YSR) bound state to the spectrum,
residing at sub-gap energies. Provided the YSR states form metallic bands, we
demonstrate that the tunneling current carried by these states can be highly
spin-polarized when the impurities are ferromagnetically ordered. The spin
polarization can be switched by simply tuning the bias voltage. Moreover, even
when the impurity spins are completely uncorrelated, one can still achieve
almost 100% spin polarization of the current, if the tunnel interface is
spin-active. We compute electric current and noise, varying parameters of the
interface between tunneling and fully transparent regimes, and analyze the
relative role of single-particle and Andreev reflection processes.Comment: 14 pages, 10 figure
High-sensitivity plasmonic refractive index sensing using graphene
We theoretically demonstrate a high-sensitivity, graphene-plasmon based
refractive index sensor working in the mid-infrared at room temperature. The
bulk figure of merit of our sensor reaches values above , but the key
aspect of our proposed plasmonic sensor is its surface sensitivity which we
examine in detail. We have used realistic values regarding doping level and
electron relaxation time, which is the limiting factor for the sensor
performance. Our results show quantitatively the high performance of
graphene-plasmon based refractive index sensors working in the mid-infrared.Comment: This is an author-created, un-copyedited version of an article
accepted for publication/published in 2DMaterials. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The Version of Record is available online at
https://doi.org/10.1088/2053-1583/aa70f
Optical signatures of nonlocal plasmons in graphene
We theoretically investigate under which conditions nonlocal plasmon response
in monolayer graphene can be detected. To this purpose, we study optical
scattering off graphene plasmon resonances coupled using a subwavelength
dielectric grating. We compute the graphene conductivity using the Random Phase
Approximation (RPA) obtaining a nonlocal conductivity and we calculate the
optical scattering of the graphene-grating structure. We then compare this with
the scattering amplitudes obtained if graphene is modeled by the local RPA
conductivity commonly used in the literature. We find that the graphene plasmon
wavelength calculated from the local model may deviate up to from the
more accurate nonlocal model in the small-wavelength (large-) regime. We
also find substantial differences in the scattering amplitudes obtained from
the two models. However, these differences in response are pronounced only for
small grating periods and low temperatures compared to the Fermi temperature.Comment: Accepted for publication in Physical Review B. 15 pages, 9 figure