43 research outputs found
Comeback of epitaxial graphene for electronics: large-area growth of bilayer-free graphene on SiC
We present a new fabrication method for epitaxial graphene on SiC which
enables the growth of ultra-smooth defect- and bilayer-free graphene sheets
with an unprecedented reproducibility, a necessary prerequisite for wafer-scale
fabrication of high quality graphene-based electronic devices. The inherent but
unfavorable formation of high SiC surface terrace steps during high temperature
sublimation growth is suppressed by rapid formation of the graphene buffer
layer which stabilizes the SiC surface. The enhanced nucleation is enforced by
decomposition of polymer adsorbates which act as a carbon source. With most of
the steps well below 0.75 nm pure monolayer graphene without bilayer inclusions
is formed with lateral dimensions only limited by the size of the substrate.
This makes the polymer assisted sublimation growth technique the most promising
method for commercial wafer scale epitaxial graphene fabrication. The
extraordinary electronic quality is evidenced by quantum resistance metrology
at 4.2 K with until now unreached precision and high electron mobilities on mm
scale devices.Comment: 20 pages, 6 Figure
Electron Counting Capacitance Standard with an improved five-junction R-pump
The Electron Counting Capacitance Standard currently pursued at PTB aims to
close the Quantum Metrological Triangle with a final precision of a few parts
in 10^7. This paper reports the considerable progress recently achieved with a
new generation of single-electron tunnelling devices. A five-junction R-pump
was operated with a relative charge transfer error of five electrons in 10^7,
and was used to successfully perform single-electron charging of a cryogenic
capacitor. The preliminary result for the single-electron charge quantum has an
uncertainty of less than two parts in 10^6 and is consistent with the value of
the elementary charge.Comment: 16 pages, 9 figures, 1 tabl
Space-borne Bose-Einstein condensation for precision interferometry
Space offers virtually unlimited free-fall in gravity. Bose-Einstein
condensation (BEC) enables ineffable low kinetic energies corresponding to
pico- or even femtokelvins. The combination of both features makes atom
interferometers with unprecedented sensitivity for inertial forces possible and
opens a new era for quantum gas experiments. On January 23, 2017, we created
Bose-Einstein condensates in space on the sounding rocket mission MAIUS-1 and
conducted 110 experiments central to matter-wave interferometry. In particular,
we have explored laser cooling and trapping in the presence of large
accelerations as experienced during launch, and have studied the evolution,
manipulation and interferometry employing Bragg scattering of BECs during the
six-minute space flight. In this letter, we focus on the phase transition and
the collective dynamics of BECs, whose impact is magnified by the extended
free-fall time. Our experiments demonstrate a high reproducibility of the
manipulation of BECs on the atom chip reflecting the exquisite control features
and the robustness of our experiment. These properties are crucial to novel
protocols for creating quantum matter with designed collective excitations at
the lowest kinetic energy scales close to femtokelvins.Comment: 6 pages, 4 figure
Simulation of SVPWM Based Multivariable Control Method for a DFIG Wind Energy System
This paper deals with a variable speed device toproduce electrical energy on a power network based on adoubly-fed induction machine used in generating mode(DFIG) in wind energy system by using SVPWM powertransfer matrix. This paper presents a modeling and controlapproach which uses instantaneous real and reactive powerinstead of dq components of currents in a vector controlscheme. The main features of the proposed model comparedto conventional models in the dq frame of reference arerobustness and simplicity of realization. The sequential loopclosing technique is adopted to design a multivariable controlsystem including six compensators for a DFIG wind energysystem to capture the maximum wind power and to inject therequired reactive power to the generator. In this paperSVPWM method is used for better controlling of converters.It also provides fault ride through method to protect theconverter during a fault. The time-domain simulation of thestudy system is presented by using MATLAB Simulink to testthe system robustness, to validate the proposed model and toshow the enhanced tracking capability