Microscopic origins of electrical conductivity in superheated solids revealed

Scientists used terahertz radiation for measurements of strongly excited material

In-depth understanding of the electrical conductivity of matter is the key to many cutting-edge research and applications, ranging from phase-change memory in microelectronics to magnetospheres rooted in planetary interiors due to the motion of the conductive fluid. Unique states of material created by ultrafast table-top lasers or free-electron lasers (FEL) allow us to gain insight into atomic levels. However, it also requires sub-picosecond resolution to capture the details on the timescale of atomic motion. Therefore, in conductivity measurements it prevents the use of contact diagnostics such as multimeter and four-point-probe. Although ultrafast optical or X-ray measurements can provide information on high frequency electrical conductivity, they require complex models to extrapolate the intrinsic direct current (DC) conductivity of material.

The terahertz radiation (1 THz= 1012 Hz (cycles per second)) offers a unique solution to tackle this dilemma. The THz electromagnetic wave behaves like DC electric-field to the sample because the oscillation of its electric field is slow compared to the electron momentum relaxation frequencies in solid and liquid materials (typically 1013Hz or larger), and the width of each THz cycle is short enough to resolve sub-picosecond dynamics. Nevertheless, to measure the conductivity of strongly excited materials in the irreversible regime still requires high brightness THz radiation in order to penetrate the dense electron cloud as well as high sensitivity to detect the THz temporal profile in a single shot.

An international research team, led by scientists from the SLAC National Accelerator Laboratory and DESY, have recently measured the electrical conductivity of strongly heated material using the THz FEL radiation at FLASH. In this study, gold nano-foil samples were heated by the FLASH extreme ultraviolet (XUV) FEL pulses to electron temperatures up to 16,000 °C. As the thermal energy transfers from the electrons to the ions, the sample transits from cold to superheated solid and eventually melts into warm dense liquid. The researchers have determined the DC electrical conductivity by measuring the transmitted THz electric field through the heated samples. The multi-cycle THz pulses from FLASH provide continuous measurements with temporal resolutions better than 500 femtoseconds.

Read more on the DESY website

Image: Artist’s impression: origins of the electrical conductivity in superheated solids measured with THZ radiation at FLASH at DESY

Credit: Z. Chen, SLAC

IBM Investigates Microelectronics at NSLS-II

IBM researchers used the Hard X-ray Nanoprobe at NSLS-II to visualize strain in a new architecture for next-generation microelectronics

From smartphones to laptops, the demand for smaller and faster electronics is ever increasing. And as more everyday activities move to virtual formats, making consumer electronics more powerful and widely available is more important than ever.

IBM is one company at the forefront of this movement, researching ways to shrink and redesign their microelectronics—the transistors and other semiconductor devices that make up the small but mighty chips at the heart of all consumer electronics.

“As devices get smaller, it becomes more challenging to maintain electrostatic control,” said Conal Murray, a scientist at IBM’s T.J. Watson Research Center. “To ensure we can deliver the same level of performance in smaller devices, we’ve been employing new semiconductor materials and designs over the last decade.”

Read more on the NSLS-II website

Image: NSLS-II scientist Hanfei Yam is shown at the Hard X-ray Nanoprobe beamline, where IBM researchers visualised strain in a new architecture for next-generation microelectronics.

Enhancing Materials for Hi-Res Patterning to Advance Microelectronics

Scientists at Brookhaven Lab’s Center for Functional Nanomaterials created “hybrid” organic-inorganic materials for transferring ultrasmall, high-aspect-ratio features into silicon for next-generation electronic devices.

To increase the processing speed and reduce the power consumption of electronic devices, the microelectronics industry continues to push for smaller and smaller feature sizes. Transistors in today’s cell phones are typically 10 nanometers (nm) across—equivalent to about 50 silicon atoms wide—or smaller. Scaling transistors down below these dimensions with higher accuracy requires advanced materials for lithography—the primary technique for printing electrical circuit elements on silicon wafers to manufacture electronic chips. One challenge is developing robust “resists,” or materials that are used as templates for transferring circuit patterns into device-useful substrates such as silicon.

>Read more on the NSLS-II at Brookhaven Lab website

Image: (Left to right) Ashwanth Subramanian, Ming Lu, Kim Kisslinger, Chang-Yong Nam, and Nikhil Tiwale in the Electron Microscopy Facility at Brookhaven Lab’s Center for Functional Nanomaterials. The scientists used scanning electron microscopes to image high-resolution, high-aspect-ratio silicon nanostructures they etched using a “hybrid” organic-inorganic resist.

How to catch a magnetic monopole in the act

Berkeley Lab-led study could lead to smaller memory devices, microelectronics, and spintronics

A research team led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has created a nanoscale “playground” on a chip that simulates the formation of exotic magnetic particles called monopoles. The study – published recently in Science Advances – could unlock the secrets to ever-smaller, more powerful memory devices, microelectronics, and next-generation hard drives that employ the power of magnetic spin to store data.

Follow the ‘ice rules’
For years, other researchers have been trying to create a real-world model of a magnetic monopole – a theoretical magnetic, subatomic particle that has a single north or south pole. These elusive particles can be simulated and observed by manufacturing artificial spin ice materials – large arrays of nanomagnets that have structures analogous to water ice – wherein the arrangement of atoms isn’t perfectly symmetrical, leading to residual north or south poles.

>Read more on the Advanced Light Source at Berkeley Lab website

Image: Full image here. This  nanoscale “playground” on a chip uses nanomagnets to simulate the formation of exotic magnetic particles called “monopoles.” Credit: Farhan/Berkeley Lab

Spin and charge frozen by strain

In the development of next-generation microelectronics, a great deal of attention has been given to the use of epitaxy (the deposition of a crystalline overlayer on a crystalline substrate) to tailor the properties of materials to suit particular applications. Correlated electron systems provide an excellent platform for the development of new microelectronic devices due to the presence of multiple competing ground states of similar energy. In some cases, strain can drive these systems between two or more such states, resulting in phase transitions and dramatic changes in the properties of the material. Often, the specific mechanism by which strain accomplishes such a feat is unknown. This was precisely the case in lanthanum cobaltite, LaCoO3, which undergoes a strain-induced transition from paramagnet to ferromagnet, until a recent study carried out at the U.S. Department of Energy’s Advanced Photon Source (APS) revealed the intriguing microscopic phenomena at work in this system. These phenomena may play a role in spin-state and magnetic-phase transitions, regardless of stimulus, in many other correlated systems.

Lanthanum cobaltite is a perovskite, which means the structure can be thought of as made up of distorted cubes with cobalt at the cube centers, oxygen at the cube faces, and lanthanum at the cube corners. The cobalt ions have a nominal 3+ valence, meaning they lose three electrons to the neighboring oxygen ions. Bulk LaCoO3 is paramagnetic (that is, having a net magnetization only in the presence of an externally applied magnetic field) above 110 Kelvin, and non-magnetic below that temperature. In its ground state, all the electrons on a given cobalt ion are paired, meaning their magnetic spins cancel each other out. These are so-called low-spin (LS) Co3+ ions, and when all of the cobalt ions are in this form, LaCoO3 is non-magnetic.

>Read more on the Advanced Photon Source website

Image: Upper left: Resonant x-ray scattering at the cobalt K-edge. Inversion of the spectra at the reflections shown indicates the presence of charge order. Upper right: X-ray diffraction reciprocal space maps at the (002) and (003) reflection indicating the high epitaxial quality of the films. The satellite peaks result from lattice modulations associated with the reduced symmetry in the film. Lower left: Schematic crystal structure of epitaxial LaCoO3 showing the arrangement of cobalt sites with different charge and spin. The circulated charge transfer from oxygen to the different cobalt sites is also shown. Lower right: Calculated total energy as a function of the difference between the in-plane Co-O bond lengths of HS and LS cobalt ions (∆rCo-O).