New light shed on electron flips

http://thznetwork.net/index.php/archives/2033

Researchers from Berlin Joint EPR Lab at Helmholtz-Zentrum Berlin (HZB) and University of Washington (UW) derived a new set of equations that allows for calculating electron paramagnetic resonance (EPR) transition probabilities with arbitrary alignment and polarization of the exciting electromagnetic radiation. The validity of the equations could be demonstrated with a newly designed THz-EPR experiment at HZB’s storage ring BESSY II. This progress is relevant for a broad community of EPR users and is published in Physical Review Letters on January 6, 2015.

HZB-scientists Karsten Holldack, Alexander Schnegg and Joscha Nehrkorn at the BESSY II Beamline. Credit: HZB

Electron spins are quantum objects with fascinating characteristics. They can be used as sensitive probes to explore material properties at the atomic level. Electron spins behave like tiny magnets that can be aligned parallel or anti parallel to an external magnetic field. Flips between these states may be induced by electromagnetic radiation matching the energy difference of the spin states. The probability for an EPR induced spin flip critically depends on the orientation of the magnetic component of the electromagnetic radiation with respect to the external magnetic field. These probabilities can be calculated, however, up to now respective expressions have been available only for a very limited number of experimental settings.

Set of equations for unconventional geometries

Joscha Nehrkorn, Alexander Schnegg, Karsten Holldack (HZB) and Stefan Stoll (UW) now succeeded to lift this restriction and derive general expressions for the magnetic transition rates, which are valid for any excitation configuration. The expressions apply to arbitrary excitation geometry and work for linear and circular polarized as well as unpolarized radiation. “We developed a general theory for EPR transition rates of anisotropic spins systems and implemented it in a freely available computer program. Thereby, EPR users can now interpret and predict experiments and extract highly desired information which was not accessible recently” explains Joscha Nehrkorn.

Tests have been successful

To test the new theoretical expressions, the authors employed the properties of a unique THz-EPR experiment at BESSY II. They aligned the spins of iron atoms incorporated in small organic molecules to a static magnetic field and excited them by linear polarized coherent synchrotron radiation in the THz range with varying orientations of the magnetic component of the THz radiation. By comparing the polarization dependence of theoretical predicted and experimental EPR line intensities, they could verify the newly derived equations and determine the parities of ground and excited high spin iron states. “This experiment is an excellent example how broad band THz radiation from a storage ring may be used for very high frequency EPR applications, these possibilities will be further boosted by BESSY VSR, the next generation of our storage ring,” states Karsten Holldack scientist at the THz beam line.

Alexander Schnegg who coordinates the project within a priority program (SPP 1601) of the German Research Foundation (DFG) further outlines: “The achieved breakthrough in EPR methodology strongly improves the predictive power of EPR for applications in e.g. life sciences, spintronics or energy materials research and paves the way for future EPR experiments with novel excitation schemes. ”

More information: General Magnetic Transition Dipole Moments for Electron Paramagnetic Resonance (authors: J. Nehrkorn, A. Schnegg, K. Holldack and S. Stoll), Physical Review Letters.
DOI:10.1103/PhysRevLett.114.010801.

Source: Phys.org

Electronics Based on a 2-D Electron Gas

Electronics Based on a 2-D Electron Gas

Published: March 3, 2014. By Vienna University of Technologyhttp://www.tuwien.ac.at/tu_vienna/ http://www.sciencenewsline.com/articles/2014030322400025.html

Usually, microelectronic devices are made of silicon or similar semiconductors. Recently, the electronic properties of metal oxides have become quite interesting. These materials are more complex, yet offer a broader range of possibilities to tune their properties. An important breakthrough has now been achieved at the Vienna University of Technology: a two dimensional electron gas was created in strontium titanate. In a thin layer just below the surface electrons can move freely and occupy different quantum states.

Additional Image  This is Zhiming Wang. Credit: TU Vienna  This shows a strontium titanate crystal in the vacuum chamber. Credit: TU Vienna

Strontium titanate is not only a potential future alternative to standard semiconductors, it could also exhibit interesting phenomena, such as superconductivity, thermoelectricity or magnetic effects that do not occur in the materials that are used for today's electronic devices.
The Surface Layer and the Inside
This project closely links theoretical calculations and experiments. Zhiming Wang from Professor Ulrike Diebold's research team was the leading experimentalist; some of the experimental work was done at the synchrotron BESSY in Berlin. Zhicheng Zhong from Professor Karsten Held's group studied the material in computer simulations.
Not all of the atoms of strontium titanate are arranged in the same pattern: if the material is cut at a certain angle, the atoms of the surface layer form a structure, which is different from the structure in the bulk of the material. "Inside, every titanium atom has six neighbouring oxygen atoms, whereas the titanium atoms at the surface are only connected to four oxygen atoms each", says Ulrike Diebold. This is the reason for the remarkable chemical stability of the surface. Normally such materials are damaged if they come into contact with water or oxygen.
Migrating Oxygen Atoms
Something remarkable happens when the material is irradiated with high-energy electromagnetic waves: "The radiation can remove oxygen atoms from the surface", Ulrike Diebold explains. Then other oxygen atoms from within the bulk of the material move up to the surface. Inside the material, an oxygen deficiency builds up, as well a surplus of electrons.
"These electrons, located in a two dimensional layer very close to the surface, can move freely. We call this an electron gas", says Karsten Held. There has already been some evidence of two dimensional electron gases in similar materials, but until now the creation of a stable, durable electron gas at a surface has been impossible. The properties of the electrons in the gas can be finely tuned. Depending on the intensity of the radiation, the number of electrons varies. By adding different atoms, the electronic properties can also be changed.
"In solid state physics, the so-called band structure of a material is very important. It describes the relationship between the energy and the momentum of the electrons. The remarkable thing about our surface is that it shows completely different kinds of band structures, depending on the quantum state of the electron", says Karsten Held.

The electron gas in the new material exhibits a multitude of different electronic structures. Some of them could very well be suitable for producing interesting magnetic effects or superconductivity. The promising properties of strontium titanate will now be further investigated. The researchers hope that, by applying external electric fields or by placing additional metal atoms on the surface, the new material could reveal a few more of its secrets.

Computer data storage at Terahertz speeds more feasible with observation of magnetic reversal

Top, centre: While the magnetization of gadolinium (red arrow) has not yet changed, the magnetization of iron (blue arrow) has already reversed. Large picture: The laser pulse (pink) triggers magnetic reversal, while the X-ray pulse (blue) measures it. (Credit: HZB/Radu)

MY NOTE:  THIS IS ONE OF THOSE VERY INTERESTING PIECES OF INFORMATION, WHILE NOT DIRECTLY RELATED TO CURRENT TERAHERTZ SCANNING OR ASTRONOMY, IS INCLUDED HERE AS IT APPEARS IT WOULD BE OF INTEREST TO MANY READERS.

http://www.sciencedaily.com/releases/2011/04/110413101910.htm
ScienceDaily (Apr. 13, 2011) — A newly discovered magnetic phenomenon could accelerate data storage by several orders of magnitude.

With a constantly growing flood of information, we are being inundated with increasing quantities of data, which we in turn want to process faster than ever. Oddly, the physical limit to the recording speed of magnetic storage media has remained largely unresearched. In experiments performed on the particle accelerator BESSY II of Helmholtz-Zentrum Berlin, Dutch researchers have now achieved ultrafast magnetic reversal and discovered a surprising phenomenon.
In magnetic memory, data is encoded by reversing the magnetization of tiny points. Such memory works using the so-called magnetic moments of atoms, which can be in either "parallel" or "antiparallel" alignment in the storage medium to represent to "0" and "1."
The alignment is determined by a quantum mechanical effect called "exchange interaction." This is the strongest and therefore the fastest "force" in magnetism. It takes less than a hundred femtoseconds to restore magnetic order if it has been disturbed. One femtosecond is a millionth of a billionth of a second. Ilie Radu and his colleagues have now studied the hitherto unknown behaviour of magnetic alignment before the exchange interaction kicks in. Together with researchers from Berlin and York, they have published their results in Nature.
For their experiment, the researchers needed an ultra-short laser pulse to heat the material and thus induce magnetic reversal. They also needed an equally short X-ray pulse to observe how the magnetization changed. This unique combination of a femtosecond laser and circular polarized, femtosecond X-ray light is available in one place in the world: at the synchrotron radiation source BESSY II in Berlin, Germany.
In their experiment, the scientists studied an alloy of gadolinium, iron and cobalt (GdFeCo), in which the magnetic moments naturally align antiparallel. They fired a laser pulse lasting 60 femtoseconds at the GdFeCo and observed the reversal using the circular-polarized X-ray light, which also allowed them to distinguish the individual elements. What they observed came as a complete surprise: The Fe atoms already reversed their magnetization after 300 femtoseconds while the Gd atoms required five times as long to do so. That means the atoms were all briefly in parallel alignment, making the material strongly magnetized. "This is as strange as finding the north pole of a magnet reversing slower than the south pole," says Ilie Radu.
With their observation, the researchers have not only proven that magnetic reversal can take place in femtosecond timeframes, they have also derived a concrete technical application from it: "Translated to magnetic data storage, this would signify a read/write rate in the terahertz range. That would be around 1000 times faster than present-day commercial computers," says Radu.