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Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked against GW band structure calculations and experiments

Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked against GW band structure calculations and experiments | Physics | Scoop.it

- Mikko's summary -

 

* New hybrid functional benchmarked, aims to reproduce both ground- and excited state properties within a fully ab initio framework.

 

* A well-written introductory part, with review of the standard LDA/GGA gap problem, how the fraction 'alpha' in the hybrid calculation scheme relates to macroscopic dielectric constant, and what approaches have been used for alpha.

 

* Detailed, useful computational methodology part with discussions on codes and chosen parameters.

 

* The new approach is tested on wide-gap oxide semiconductors that have current or future technological impact (e.g. in photovoltaics and catalysis): ZnO, TiO2, ZrO2 and WO2.

 

* The paper essentially addresses the question of how to tune hybrid functional scheme to be a more applicable tool that can handle in a reliable way both energetics and electronic (ground and excited state) properties.

 

Read the paper:

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.155201

Mikko Hakala's insight:

Any similar recent papers on hybrid functionals? Add your comment.

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Team reveals molecular structure of water at gold electrodes

Team reveals molecular structure of water at gold electrodes | Physics | Scoop.it
When a solid material is immersed in a liquid, the liquid immediately next to its surface differs from that of the bulk liquid at the molecular level. This interfacial layer is critical to our understanding of a diverse set of phenomena from biology to materials science. When the solid surface is charged, ...
Mikko Hakala's insight:

State of the art in the experimental research on structure of water. X-ray absorption spectroscopy reveals the structure of interfacial water (on the gold surface) with sub-nanometer sensitivity.

 

http://phys.org/news/2014-10-team-reveals-molecular-gold-electrodes.html

 

http://www.sciencemag.org/content/early/2014/10/22/science.1259437

 

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The Nobel Prize in Physics 2014

The Nobel Prize in Physics 2014 | Physics | Scoop.it

The Nobel Prize in Physics 2014 was awarded jointly to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources".

Mikko Hakala's insight:

The Nobel prize website offers both the popular and the more detailed scientific background of blue LEDs.

 

Quoting from the latter: "GaN-based LEDs result from a long series of breakthroughs in basic materials physics and crystal growth, in device physics with advanced heterostructure design, and in optical physics for the optimization of the light out-coupling."

 

The effort was worth it. The luminosity [measured in lumens] / power [measured in watts] increases roughly as follows (from the popular article):

   Light bulb, 1x 

   Fluorescent lamp, 4x 

   LED lamp, 19x

So roughly 20 times less energy consumed compared to light bulbs.

 

More detailed scientific background for blue LEDs:

http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/advanced-physicsprize2014.pdf

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Free Educational Physics Lecture Videos with Lecture Notes

Free Educational Physics Lecture Videos with Lecture Notes | Physics | Scoop.it
After teaching physics in a high school classroom for 13 years, I stepped out of the classroom to start Flipping Physics, a business dedicated to creating free, clear, concise and comedic physics educational videos.
Mikko Hakala's insight:

A nice and fun set of physics lecture videos. Many topics are covered, for example mechanics, electromagnetism, electrical circuits, refraction, interference. Some lectures also on modern physics.

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Scientists discover new magnetic phase in iron-based superconductors

Scientists discover new magnetic phase in iron-based superconductors | Physics | Scoop.it
(Phys.org) —Scientists at the U.S. Department of Energy's Argonne National Laboratory have discovered a previously unknown phase in a class of superconductors called iron arsenides. This sheds light on a debate over the interactions between atoms and electrons that are responsible for their unusual ...
Mikko Hakala's insight:

More research on iron-based superconductors (http://sco.lt/7CfptR), here on doped barium iron arsenide. The electron pairing mechanism is not understood in these unconventional superconductors. To elucidate the mechanism, one issue is to map out the precise phase diagrams.


Here neutron diffraction showed that on lowering the temperature, close to the onset of superconductivity, the structure restores 4-fold symmetry. At higher temperatures the structure has nematic ("thread-like") order with 2-fold symmetry, whereas at room temperature the symmetry is again 4-fold.


The finding appears to support the model that the nematic phase is driven by magnetic interactions and not by iron 3d orbital ordering. And this hints, according to the phys.org article, that it could be magnetism that could be the key to electron pairing. 


Pin it for later:

http://www.pinterest.com/pin/318981586080372229/


The Nature Communications article:

http://www.nature.com/ncomms/2014/140522/ncomms4845/full/ncomms4845.html


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Breakthrough in Study of High-Temperature Superconductivity | Simons Foundation

Breakthrough in Study of High-Temperature Superconductivity | Simons Foundation | Physics | Scoop.it
Experimentalists have pinpointed the microscopic structure of waves inside high-temperature superconductors, which could be the key to understanding the complex materials.
Mikko Hakala's insight:

An interesting, broad and detailed article about the current advancements in understanding the microscopic origin of superconductivity in cuprates. 

 

The article discusses recent findings by various groups. In particlar, it addresses the details of the d-wave charge density order, and how antiferromagnetism is the parent state both to superconductivity and to charge density waves.

 

Challenges remain, for example "[a recently proposed theoretical framework] is not yet refined enough to predict how the balance of charge density waves and superconductivity vary with temperature, magnetic field or type of cuprate."

 

In short, this is a clarifying expert article on a specific condensed matter topic, the origin of d-wave superconductivity and the nature of charge density waves in cuprates.

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Phase transiting to a new quantum universe

Phase transiting to a new quantum universe | Physics | Scoop.it
(Phys.org) —Recent insight and discovery of a new class of quantum transition opens the way for a whole new subfield of materials physics and quantum technologies.
Mikko Hakala's insight:

This phys.org article gives a flavor of the intriguing condensed matter physics related to ferroelectric materials and their quantum phase transitions (QPT) to paraelectric ones. In QPTs, the order-disorder transition takes place at absolute zero temperature, and is achieved by chemical or isotope substitution, or by doping.

 

In this work it was found that for ferroelectrics the physics describing the phase transition is very different from the better know ferromagnetic counterparts. In fact, "the fluctuation spectrum found in quantum critical ferroelectrics is the same as that in elementary particle physics—propagating modes in three spatial dimensions plus one time dimension."

 

Wikipedia article on quantum phase transitions:

http://en.wikipedia.org/wiki/Quantum_phase_transition

 

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Scientists capture ultrafast snapshots of light-driven superconductivity

Scientists capture ultrafast snapshots of light-driven superconductivity | Physics | Scoop.it
A new study pins down a major factor behind the appearance of superconductivity—the ability to conduct electricity with 100 percent efficiency—in a promising copper-oxide material.
Mikko Hakala's insight:

X-ray studies bring insight into developing room-temperature superconductors. This laser pump - x-ray probe experiment showed that the appearance of superconductivity is not related to lattice distortions (the distortions persist longer than the melting of charge stripes, i.e. the point where superconductivity emerges).

 

The findings are relevant for understanding generally the mechanism of superconductivity in layered copper oxide materials, and in the new field of light-induced superconductivity.

 

The experimental x-ray method used was resonant soft x-ray diffraction.

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Jacobs Physics

Jacobs Physics | Physics | Scoop.it

"Resources for teachers and students of introductory physics"

Mikko Hakala's insight:

Lots of good, practical physics content and interesting teaching discussions in this blog.

 

See for example this post about how much the teacher should be talking in front of the class:

http://jacobsphysics.blogspot.fi/2014/01/is-there-any-point-at-all-in-talking-in.html

 

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Crystals ripple in response to light: First propagating surface phonon polaritons in a van der Waals crystal

Crystals ripple in response to light: First propagating surface phonon polaritons in a van der Waals crystal | Physics | Scoop.it
Light can trigger coordinated, wavelike motions of atoms in atom-thin layers of crystal, scientists have shown. The waves, called phonon polaritons, are far shorter than light waves and can be 'tuned' to particular frequencies and amplitudes by varying the number of layers of crystal, they report in ...
Mikko Hakala's insight:

This finding may bring more degrees of freedom to design nanodevices (information flow, heat control, imaging). The phonon polariton wavelength can be adjusted, and there is no electronic dissipation because the material is insulator.  

 

The Science article: Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride

http://www.sciencemag.org/content/343/6175/1125.abstract

 

See also: http://en.wikipedia.org/wiki/Polariton

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Attosecond photonics : Nature Photonics

Attosecond photonics : Nature Photonics | Physics | Scoop.it

From the Journal: "Attosecond photonics, currently one of the most promising branches of modern photonics, is progressing at an extremely rapid pace. Although still in its infancy, it has already captured the imagination of the scientific community with its promise of enhancing our understanding of ultrafast phenomena of direct relevance to life, technology and potentially medicine." 

 

Image: SLAC National Accelerator Laboratory, Flickr.

Mikko Hakala's insight:

Focus issue on attosecond photonics.

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Faster X-ray technology observes catalyst surface at work with atomic resolution

Faster X-ray technology observes catalyst surface at work with atomic resolution | Physics | Scoop.it
By using a novel X-ray technique, researchers have observed a catalyst surface at work in real time and were able to resolve its atomic structure in detail. The new technique, pioneered at DESY's X-ray light source PETRA III, may pave the way for the design of better catalysts and other materials on ...
Mikko Hakala's insight:

High-energy X-ray diffraction is reported as an advantageous imaging technique for surface atomic structures. It can be used at realistic, in situ dynamical conditions (in real time).

 

http://www.sciencemag.org/content/343/6172/758.abstract

 

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X-rays reveal high-temperature superconductivity is caused by a mechanism distinct from the classical variety

X-rays reveal high-temperature superconductivity is caused by a mechanism distinct from the classical variety | Physics | Scoop.it
Classical and high-temperature superconductors differ hugely in their critical temperature when they lose electrical resistance.

 

Scientists have now used powerful X-rays to establish another big difference: high-temperature superconductivity cannot be accounted for by the mechanism that leads to conventional superconductivity. As this mechanism called "electron-phonon coupling" contributes only marginally to the loss of electrical resistance, other scenarios must now be developed to explain high-temperature superconductivity. The results are published on 24 November 2013 in Nature Physics.

 

The team of scientists was led by Mathieu Le Tacon and Bernhard Keimer from the Max-Planck-Institute for Solid State Research in Stuttgart (Germany) and comprised scientists from Politecnico di Milano (Italy), Karlsruhe Institute of Technology (KIT) and the European Synchrotron (ESRF) in Grenoble, France.

 

High-temperature superconductivity was discovered nearly thirty years ago and is beginning to find more and more practical applications. These materials have fascinated scientists since their discovery. For even more practical applications, the origin of their amazing properties must be understood, and ways found to calculate the critical temperature. A key element of this understanding is the process that makes electrons combine into so-called "Cooper pairs" when the material is cooled below the critical temperature. In classical superconductors, these Cooper pairs are formed thanks to electron-phonon coupling, an interaction between electrons carrying the electrical current and collective vibrations of atoms in the material.

 

To understand the role this interaction plays in high-temperature superconductors, Matthieu Le Tacon and his colleagues took up the challenge to study these atomic vibrations as the material was cooled down below its critical temperature. "Studying electron-phonon coupling in these superconductors is always a delicate task, due to the complex structure of the materials," says Alexeï Bosak, an ESRF scientist and member of the team. He adds: "This is why we developed a two-level approach to literally find a needle in the hay stack".

 

The big surprise came once the electron-phonon coupling had been probed. "In terms of its amplitude, the coupling is actually by far the biggest ever observed in a superconductor, but it occurs in a very narrow region of phonon wavelengths and at a very low energy of vibration of the atoms", adds Mathieu Le Tacon. "This explains why nobody could see it before the two-level approach of X-ray scattering was developed".

 

Because the electron-phonon coupling is in such a narrow wavelength region, it cannot "help" two electrons to bind themselves together into a Cooper pair. The next step will be to make systematic observations in many other high-temperature superconductors. "Although we now know that electron-phonon coupling does not contribute to their superconductivity, the unexpected size of the effect—we call it giant electron-phonon-coupling—happens to be a valuable tool to study the interplay between superconductivity and other competing processes. This will hopefully provide further insight into the origin of high-temperature superconductivity, still one of the big mysteries of science", concludes Mathieu Le Tacon.


Via Dr. Stefan Gruenwald
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Holey Rubber Slab Has Controllable Stiffness

Holey Rubber Slab Has Controllable Stiffness | Physics | Scoop.it
Squeezing a holey rubber slab changes its stiffness over a wide range in the direction perpendicular to the squeeze.
Mikko Hakala's insight:

 

Good materials physics article. Here's my quick summary:

 

* For metamaterials the properties (strength, reflectivity etc.) follow from macroscopic structure, not elemental composition.

* The developed silicone rubber slab has perforated alternative holes, leading to interesting non-linear stress-strain behavior.

* The material could absorb and dissipate energy (transferred to it for ex. by impacts or shocks), rather than only storing it as energy in the spring.

* In other words, the material is not 'bumpy' but 'dissipative'. Applications can be found in robotics etc. and everyday objects which require control of damping.

* The advantage of this material is that it's pure mechanical and doesn't require external energy to operate. 

 

Pin it for later: http://www.pinterest.com/pin/318981586081801924/

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Fundamentals of physics confirmed: Experiments testing Einstein's time dilation and quantum electrodynamics

Fundamentals of physics confirmed: Experiments testing Einstein's time dilation and quantum electrodynamics | Physics | Scoop.it
The special theory of relativity of Albert Einstein and quantum electrodynamics, which was formulated by, among others, Richard Feynman, are two important fundaments of modern physics. In cooperation with colleagues from several international universities and institutes, the research group of Professor ...
Mikko Hakala's insight:

 

Detecting fluorescence radiation from relativistically moving ions is here used to test two fundamental physics theories: special relativity and QED.  - Better precision obtained in the experiments, no violations found.

 

http://phys.org/news/2014-10-fundamentals-physics-einstein-dilation-quantum.html

 

Direct links to the studies:

 

Special relativity

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.120405

http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.113.120405 ;

 

Hyperfine splitting (predicted by quantum electrodynamics)

http://journals.aps.org/pra/abstract/10.1103/PhysRevA.90.030501

 

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Perovskite Solar Cell Technology of Oxford PV, The Potential Game-Changer | Sun Is The Future

Perovskite Solar Cell Technology of Oxford PV, The Potential Game-Changer | Sun Is The Future | Physics | Scoop.it
Dear Friends, Visitors/Viewers/Readers, (Please click on red links and note magenta) At InterSolar North America 2014 in San Francisco, CA, I came across
Mikko Hakala's insight:

 

Solar cell technology based on perovskites

 

Useful info in the post and interview on Oxford PV activities in this field. In the video Dr. Christopher Case describes the material and its applications in various ways in solar cell technologies. Here's my quick summary of the video's content:

 

0:45 Fastest increasing photovoltaic efficiency 

1:15 Thin film material

1:35 Applications, initial view of the company was to develop semi-transparent coatings

2:10 Can be made in almost any color

3:00 New application of perovskites: tandem solar cells (perovskite coating on conventional Si)

4:10 What are perovskites

4:45 How to tailor the properties

5:10 Material itself studied already for decades

5:50 Finding the planar form by Prof. Snaith was a breakthrough

6:40 More and more papers coming out on this material

6:55 Perovskite on Si solar cells boosts efficiency, easier to make this product than a fully integrated PV

7:40 Perovskites fundamentally and perspectives for efficiency

8:15 Potential to replace Si?

8:40 About processing: it's solution processed from inexpensive materials

9:30 Future steps. Towards full perovskite-on-perovskite tandem cells

9:50 How to get in contact, what their company's website contains

 

 

 

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Team first to detect exciton in metal

Team first to detect exciton in metal | Physics | Scoop.it
University of Pittsburgh researchers have become the first to detect a fundamental particle of light-matter interaction in metals, the exciton. The team will publish its work online June 1 in Nature Physics.
Mikko Hakala's insight:

Observation of excitons in metals is difficult due to their short lifetimes. This research reports detection on metal surfaces via multiphoton photoemission.

 

Quick info on excitons: http://en.wikipedia.org/wiki/Exciton

 

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Solution to two long-standing mysteries of cuprate superconductivity found

Solution to two long-standing mysteries of cuprate superconductivity found | Physics | Scoop.it
Scientists seeking to understand the intricacies of high-temperature superconductivity—the ability of certain materials to carry electrical current with no energy loss—have been particularly puzzled by a mysterious phase that emerges as charge carriers are added that appears to compete with superconductivity. ...
Mikko Hakala's insight:

More on the superconductivity of cuprates (see http://sco.lt/5pcQjZ).

 

At low hole doping, i.e. at the pseudogap phase, the static electron arrangement (charge density wave or "frozen" stripe patterns) and the associated nanoscale fluctuations prevent the free flow of electrons. At higher hole doping the density wave disappears and unrestricted superconductivity appears.

 

Measurements by spectroscopic imaging scanning tunneling microscope.

 

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Physicists discover how to change the crystal structure of graphene

Physicists discover how to change the crystal structure of graphene | Physics | Scoop.it
A University of Arizona-led team of physicists has discovered how to change the crystal structure of graphene, more commonly known as pencil lead, with an electric field, an important step toward the possible use of graphene in microprocessors that would be smaller and faster than current, silicon-based ...
Mikko Hakala's insight:

The paper shows that the stacking of graphene layers (in a 3-layer case) can be controlled with an external voltage. This way one obtains either metallic or semiconducting behavior. According to the article, for the first time such on-off switch is demonstrated in graphene.

 

This is in early stages but could become very important if the tuning of graphene's electrical properties can be realized at larger scale. The ultimate aim is a graphene-based transistor, with huge advantages compared to silicon-based ones.

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Solar-cell materials and efficiencies

Solar-cell materials and efficiencies | Physics | Scoop.it

What are the types and efficiencies of current phovoltaic materials? This Wikimedia Commons graph is a great, constantly updated reference to various photovoltaic technologies under research. The classification is to following types of cells:

 

1) multijunction

2) single-junction GaAs

3) crystalline Si

4) thin-film (CIGS, CdTe, amorphous and other forms of Si )

5) emerging (perovskite, quantum dot, organic, inorganic, dye-sensitized)

Follow it to stay updated on recent achievements.

 

More information:

 

* Nature Photonics has a focus issue on a set of cutting-edge solar cell technologies. It covers quantum dot, polymer, dye-sensitized and intermediate-band solar cells: http://www.nature.com/nphoton/focus/photovoltaics/index.html

The editorial 'A sunny outlook': http://www.nature.com/nphoton/journal/v6/n3/full/nphoton.2012.38.html

 * Brief info on CdTe: http://spectrum.ieee.org/nanoclast/green-tech/solar/what-makes-for-better-cdte-solar-cells  Figure: By NREL (US Department of Energy) [Public domain], via Wikimedia Commons

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Concepts for teaching superconductors

Concepts for teaching superconductors | Physics | Scoop.it

"Concepts of superconductors for teaching purposes in a solid-state physics course."

 

Free to use under Creative Commons licensing.

Mikko Hakala's insight:

Here's my visual plan to teach superconductors in an ongoing course. Free to use under Creative Commons licensing.

 

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Free Online Physics Courses

Free Online Physics Courses | Physics | Scoop.it
Get free online Physics courses from the world's leading universities. You can download these audio & video courses straight to your computer or mp3 player. For more online courses, visit our complete collection of Free Courses.
Mikko Hakala's insight:

47 courses, from introductory to modern physics.

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Jamie's Fusion Project

Jamie's Fusion Project | Physics | Scoop.it

"I am Jamie Edwards, a 13 year old amateur nuclear scientist. At the moment I am building a nuclear fusion reactor and hope to become the youngest "fusioneer". I wouldn't be doing this without the financial support from my school, Penwortham Priory Academy."

Mikko Hakala's insight:

Wow!

http://www.dailymail.co.uk/sciencetech/article-2573998/I-star-jar-13-year-old-youngest-person-world-build-NUCLEAR-FUSION-REACTOR.html

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X rays Measure Lone Molecules

X rays Measure Lone Molecules | Physics | Scoop.it
Researchers measured an atomic distance in a molecule without using a crystal, which is usually required. Instead they used a beam of isolated molecules.
Mikko Hakala's insight:

This is a great demonstrations how x-ray free electron lasers are used for diffraction imaging of single molecules. 

 

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.083002

 

X-ray scattering data collected at LCLS (https://portal.slac.stanford.edu/sites/lcls_public/)

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Vanadium dioxide 'smart glass' can be activated to block infrared light while remaining transparent to visible light

Vanadium dioxide 'smart glass' can be activated to block infrared light while remaining transparent to visible light | Physics | Scoop.it
'Smart glass' can switch from transparent to opaque at the flick of a switch and is increasingly used in cars, aircraft and homes to reduce the Sun's glare and filter out infrared light and heat.

Via José Gonçalves
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Joao Leao's curator insight, December 6, 2013 3:56 PM

That is pretty smart!