6,530 research outputs found
How to Distinguish between Specular and Retroconfigurations for Andreev Reflection in Graphene Rings
We numerically investigate Andreev reflection in a graphene ring with one
normal conducting and one superconducting lead by solving the Bogoliubov--de
Gennes equation within the Landauer-B\"uttiker formalism. By tuning chemical
potential and bias voltage, it is possible to switch between regimes where
electron and hole originate from the same band (retroconfiguration) or from
different bands (specular configuration) of the graphene dispersion,
respectively. We find that the dominant contributions to the Aharonov-Bohm
conductance oscillations in the subgap transport are of period in
retroconfiguration and of period in specular configuration, confirming
the predictions obtained from a qualitative analysis of interfering scattering
paths. Because of the robustness against disorder and moderate changes to the
system, this provides a clear signature to distinguish both types of Andreev
reflection processes in graphene.Comment: 5 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:1201.620
The Aharonov-Bohm effect in graphene rings
This is a review of electronic quantum interference in mesoscopic ring
structures based on graphene, with a focus on the interplay between the
Aharonov-Bohm effect and the peculiar electronic and transport properties of
this material. We first present an overview on recent developments of this
topic, both from the experimental as well as the theoretical side. We then
review our recent work on signatures of two prominent graphene-specific
features in the Aharonov-Bohm conductance oscillations, namely Klein tunneling
and specular Andreev reflection. We close with an assessment of experimental
and theoretical development in the field and highlight open questions as well
as potential directions of the developments in future work.Comment: review article for "Special Issue on Graphene", to appear in "Solid
State Communications
Apparatus for high resolution microwave spectroscopy in strong magnetic fields
We have developed a low temperature, high-resolution microwave surface
impedance probe that is able to operate in high static magnetic fields. Surface
impedance is measured by cavity perturbation of dielectric resonators, with
sufficient sensitivity to resolve the microwave absorption of sub-mm-sized
superconducting samples. The resonators are constructed from high permittivity
single-crystal rutile (TiO2) and have quality factors in excess of 10^6.
Resonators with such high performance have traditionally required the use of
superconducting materials, making them incompatible with large magnetic fields
and subject to problems associated with aging and power-dependent response.
Rutile resonators avoid these problems while retaining comparable sensitivity
to surface impedance. Our cylindrical rutile resonators have a hollow bore and
are excited in TE_01(n-d) modes, providing homogeneous microwave fields at the
center of the resonator where the sample is positioned. Using a sapphire
hot-finger technique, measurements can be made at sample temperatures in the
range 1.1 K to 200 K, while the probe itself remains immersed in a liquid
helium bath at 4.2 K. The novel apparatus described in this article is an
extremely robust and versatile system for microwave spectroscopy, integrating
several important features into a single system. These include: operation at
high magnetic fields; multiple measurement frequencies between 2.64 GHz and
14.0 GHz in a single resonator; excellent frequency stability, with typical
drifts < 1 Hz per hour; the ability to withdraw the sample from the resonator
for background calibration; and a small pot of liquid helium separate from the
external bath that provides a sample base temperature of 1.1 K.Comment: 10 pages, 5 figure
Finite-element simulations of hysteretic ac losses in a magnetically coated superconducting tubular wire subject to an oscillating transverse magnetic field
Numerical simulations of hysteretic ac losses in a tubular
superconductor/paramagnet heterostructure subject to an oscillating transverse
magnetic field are performed within the quasistatic approach, calling upon the
COMSOL finite-element software package and exploiting
magnetostatic-electrostatic analogues. It is shown that one-sided magnetic
shielding of a thin, type-II superconducting tube by a coaxial paramagnetic
support results in a slight increase of hysteretic ac losses as compared to
those for a vacuum environment, when the support is placed inside; a
spectacular shielding effect with a possible reduction of hysteretic ac losses
by orders of magnitude, however, ensues, depending on the magnetic permeability
and the amplitude of the applied magnetic field, when the support is placed
outside.Comment: 7 pages, 4 figure
Analysis of a single-mode waveguide at sub-terahertz frequencies as a communication channel
We study experimentally the transmission of an electromagnetic waveguide in the frequency range from 160 to 300 GHz. Photo-mixing is used to excite and detect the fundamental TE10 mode in a rectangular waveguide with two orders-of-magnitude lower impedance. The large impedance mismatch leads to a strong frequency dependence of the transmission, which we measure with a high-dynamic range of up to 80 dB and with high frequency-resolution. The modified transmission function is directly related to the information rate of the waveguide, which we estimate to be about 1 bit per photon. We suggest that the results are applicable to a Josephson junction employed as a single-photon source and coupled to a superconducting waveguide to achieve a simple on-demand narrow-bandwidth free-space number-state quantum channel
Superconducting Vortices in Half-Metals
When the impurity mean free path is short, only spin-polarized Cooper pairs
which are non-locally and antisymmetrically correlated in time may exist in a
half-metallic ferromagnet. As a consequence, the half-metal acts as an
odd-frequency superconducting condensate. We demonstrate both analytically and
numerically that quantum vortices can emerge in half-metals despite the
complete absence of conventional superconducting correlations. Because these
metals are conducting in only one spin band, we show that a circulating spin
supercurrent accompanies these vortices. Moreover, we demonstrate that magnetic
disorder at the interfaces with the superconductor influences the position at
which the vortices nucleate. This insight can be used to help determine the
effective interfacial misalignment angles for the magnetization in hybrid
structures, since the vortex position is experimentally observable via
STM-measurements. We also give a brief discussion regarding which
superconducting order parameter to use for odd-frequency triplet Cooper pairs
in the quasiclassical theory.Comment: 9 pages, 8 figure
Observation of the Dynamical Casimir Effect in a Superconducting Circuit
One of the most surprising predictions of modern quantum theory is that the
vacuum of space is not empty. In fact, quantum theory predicts that it teems
with virtual particles flitting in and out of existence. While initially a
curiosity, it was quickly realized that these vacuum fluctuations had
measurable consequences, for instance producing the Lamb shift of atomic
spectra and modifying the magnetic moment for the electron. This type of
renormalization due to vacuum fluctuations is now central to our understanding
of nature. However, these effects provide indirect evidence for the existence
of vacuum fluctuations. From early on, it was discussed if it might instead be
possible to more directly observe the virtual particles that compose the
quantum vacuum. 40 years ago, Moore suggested that a mirror undergoing
relativistic motion could convert virtual photons into directly observable real
photons. This effect was later named the dynamical Casimir effect (DCE). Using
a superconducting circuit, we have observed the DCE for the first time. The
circuit consists of a coplanar transmission line with an electrical length that
can be changed at a few percent of the speed of light. The length is changed by
modulating the inductance of a superconducting quantum interference device
(SQUID) at high frequencies (~11 GHz). In addition to observing the creation of
real photons, we observe two-mode squeezing of the emitted radiation, which is
a signature of the quantum character of the generation process.Comment: 12 pages, 3 figure
Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions
The rise of topological phases of matter is strongly connected to their
potential to host Majorana bound states, a powerful ingredient in the search
for a robust, topologically protected, quantum information processing. In order
to produce such states, a method of choice is to induce superconductivity in
topological insulators. The engineering of the interplay between
superconductivity and the electronic properties of a topological insulator is a
challenging task and it is consequently very important to understand the
physics of simple superconducting devices such as Josephson junctions, in which
new topological properties are expected to emerge. In this article, we review
recent experiments investigating topological superconductivity in topological
insulators, using microwave excitation and detection techniques. More
precisely, we have fabricated and studied topological Josephson junctions made
of HgTe weak links in contact with two Al or Nb contacts. In such devices, we
have observed two signatures of the fractional Josephson effect, which is
expected to emerge from topologically-protected gapless Andreev bound states.
We first recall the theoretical background on topological Josephson junctions,
then move to the experimental observations. Then, we assess the topological
origin of the observed features and conclude with an outlook towards more
advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017,
published in "Topological Matter. Springer Series in Solid-State Sciences,
vol 190. Springer
Stimulating uncertainty: Amplifying the quantum vacuum with superconducting circuits
The ability to generate particles from the quantum vacuum is one of the most
profound consequences of Heisenberg's uncertainty principle. Although the
significance of vacuum fluctuations can be seen throughout physics, the
experimental realization of vacuum amplification effects has until now been
limited to a few cases. Superconducting circuit devices, driven by the goal to
achieve a viable quantum computer, have been used in the experimental
demonstration of the dynamical Casimir effect, and may soon be able to realize
the elusive verification of analogue Hawking radiation. This article describes
several mechanisms for generating photons from the quantum vacuum and
emphasizes their connection to the well-known parametric amplifier from quantum
optics. Discussed in detail is the possible realization of each mechanism, or
its analogue, in superconducting circuit systems. The ability to selectively
engineer these circuit devices highlights the relationship between the various
amplification mechanisms.Comment: 27 pages, 10 figures, version published in Rev. Mod. Phys. as a
Colloquiu
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