Saturday, January 21, 2017

Abstract-Spectroscopic ellipsometry determination of optical and electrical properties of aluminum doped zinc oxide

  • Department of Physics and Astronomy & Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo, Ohio 43606, USA

An aluminum doped zinc oxide (ZnO:Al) thin film is prepared on soda lime glass by radio frequency (RF) magnetron sputtering. Optical properties of ZnO:Al in the form of complex dielectric function (ε = ε1 + iε2) spectra are studied from 0.4 to 6 meV and 0.035 to 5.89 eV using spectroscopic ellipsometry. The film is found to have an optical band gap of 3.62 ± 0.01 eV and an extrapolated DC dielectric constant of 9.072. Resistivity, scattering time, mobility, and carrier concentration are found to be (2.250 ± 0.007) × 10−3 Ω-cm, 5.1 ± 0.7 fs, 20.8 cm2V−1s−1, and 1.3 × 1020 cm−3 respectively. Resistivity and scattering time resulting from fitting to measurements are found to depend on the spectral range modeled, such that the inclusion of increasingly longer wavelengths results in a convergence to direct electrical property measurements

Friday, January 20, 2017

Graphene’s sleeping superconductivity awakens

Since its discovery in 2004, scientists have believed that graphene may have the innate ability to superconduct. Now Cambridge researchers have found a way to activate that previously dormant potential.

Researchers have found a way to trigger the innate, but previously hidden, ability of graphene to act as a superconductor – meaning that it can be made to carry an electrical current with zero resistance.
The finding, reported in Nature Communications, further enhances the potential of graphene, which is already widely seen as a material that could revolutionise industries such as healthcare and electronics. Graphene is a two-dimensional sheet of carbon atoms and combines several remarkable properties; for example, it is very strong, but also light and flexible, and highly conductive.
Since its discovery in 2004, scientists have speculated that graphene may also have the capacity to be a superconductor. Until now, superconductivity in graphene has only been achieved by doping it with, or by placing it on, a superconducting material - a process which can compromise some of its other properties.
But in the new study, researchers at the University of Cambridge managed to activate the dormant potential for graphene to superconduct in its own right. This was achieved by coupling it with a material called praseodymium cerium copper oxide (PCCO).
Superconductors are already used in numerous applications. Because they generate large magnetic fields they are an essential component in MRI scanners and levitating trains. They could also be used to make energy-efficient power lines and devices capable of storing energy for millions of years.
Superconducting graphene opens up yet more possibilities. The researchers suggest, for example, that graphene could now be used to create new types of superconducting quantum devices for high-speed computing. Intriguingly, it might also be used to prove the existence of a mysterious form of superconductivity known as “p-wave” superconductivity, which academics have been struggling to verify for more than 20 years.
The research was led by Dr Angelo Di Bernardo and Dr Jason Robinson, Fellows at St John’s College, University of Cambridge, alongside collaborators Professor Andrea Ferrari, from the Cambridge Graphene Centre; Professor Oded Millo, from the Hebrew University of Jerusalem, and Professor Jacob Linder, at the Norwegian University of Science and Technology in Trondheim.
“It has long been postulated that, under the right conditions, graphene should undergo a superconducting transition, but can’t,” Robinson said. “The idea of this experiment was, if we couple graphene to a superconductor, can we switch that intrinsic superconductivity on? The question then becomes how do you know that the superconductivity you are seeing is coming from within the graphene itself, and not the underlying superconductor?”
Similar approaches have been taken in previous studies using metallic-based superconductors, but with limited success. “Placing graphene on a metal can dramatically alter the properties so it is technically no longer behaving as we would expect,” Di Bernardo said. “What you see is not graphene’s intrinsic superconductivity, but simply that of the underlying superconductor being passed on.”
PCCO is an oxide from a wider class of superconducting materials called “cuprates”. It also has well-understood electronic properties, and using a technique called scanning and tunnelling microscopy, the researchers were able to distinguish the superconductivity in PCCO from the superconductivity observed in graphene.
Superconductivity is characterised by the way the electrons interact: within a superconductor electrons form pairs, and the spin alignment between the electrons of a pair may be different depending on the type - or “symmetry” - of superconductivity involved. In PCCO, for example, the pairs’ spin state is misaligned (antiparallel), in what is known as a “d-wave state”.
By contrast, when graphene was coupled to superconducting PCCO in the Cambridge-led  experiment, the results suggested that the electron pairs within graphene were in a p-wave state. “What we saw in the graphene was, in other words, a very different type of superconductivity than in PCCO,” Robinson said. “This was a really important step because it meant that we knew the superconductivity was not coming from outside it and that the PCCO was therefore only required to unleash the intrinsic superconductivity of graphene.”
It remains unclear what type of superconductivity the team activated, but their results strongly indicate that it is the elusive “p-wave” form. If so, the study could transform the ongoing debate about whether this mysterious type of superconductivity exists, and – if so – what exactly it is.
In 1994, researchers in Japan fabricated a triplet superconductor that may have a p-wave symmetry using a material called strontium ruthenate (SRO). The p-wave symmetry of SRO has never been fully verified, partly hindered by the fact that SRO is a bulky crystal, which makes it challenging to fabricate into the type of devices necessary to test theoretical predictions.
“If p-wave superconductivity is indeed being created in graphene, graphene could be used as a scaffold for the creation and exploration of a whole new spectrum of superconducting devices for fundamental and applied research areas,” Robinson said. “Such experiments would necessarily lead to new science through a better understanding of p-wave superconductivity, and how it behaves in different devices and settings.”
The study also has further implications. For example, it suggests that graphene could be used to make a transistor-like device in a superconducting circuit, and that its superconductivity could be incorporated into molecular electronics. “In principle, given the variety of chemical molecules that can bind to graphene’s surface, this research can result in the development of molecular electronics devices with novel functionalities based on superconducting graphene,” Di Bernardo added.
The study, p-wave triggered superconductivity in single layer graphene on an electron-doped oxide superconductor, is published in Nature Communications. (DOI: 101038/NCOMMS14024). 

A compact source of extremely strong terahertz fields

Researchers of the PTE SZKK High-Field Terahertz Research Group developed a compact, semiconductor terahertz radiation source, which enabled to increase the efficiency by more than hundred times. The source is pumped at an infrared wavelength and utilizes a unique optical grating for the generation of terahertz pulses with extremely high electric field. The work opens up new perspectives for strong-field control of matter and for constructing compact particle accelerators for materials science and medicine.
The publication has also been highlighted on the cover of the Optica journal.
Original publication: J. A. Fülöp et al., Optica 3, 1075 (2016)

Abstract-Terahertz circular fiber polarizers using suspended-core spiral fibers

Yue-yu Xiao and Shao-fan Lu

A terahertz (THz) fiber circular polarizer based on suspended core fibers is proposed and analyzed. The THz mode guides within a subwavelength-scale core, which is suspended in a polymer tube. The dry air inner cladding not only suppresses the absorption loss of the device but also supports the leaky cladding modes. The circularly polarized core mode with the same handedness of the grating couples to the leaky cladding modes suffers from loss near resonant frequencies due to the spiral structure of the fiber. The principles and characteristics of the proposed circular polarizer are theoretically derived. Parameters that have influence on the performances are analyzed by numerical simulations. Simulation results show that the proposed THz fiber polarizer exhibits a broad single circular polarization bandwidth and a relatively low absorption loss.
© 2017 Optical Society of America
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Abstract-Terahertz Transparent Electrode Using Tripod Metal Aperture Array

Abubaker M. Tareki,  Robert G. Lindquist,  Wonkyu Kim, Martin S. Heimbeck, Junpeng Guo

In this paper, a high transmission terahertz (THz), polarization insensitive electrode was designed and fabricated to modulate a liquid crystal electro-optic device in the THz regime. The designed electrode consists of a subwavelength periodic tripod aperture array structure patterned in a gold film deposited on a transparent dielectric substrate. The maximum feature size of the structure is maintained below a fraction of the thickness of the electro-optic device to allow a fringing field to uniformly excite the entire electrode area. The transmission reaches as high as 93% in measurements