Tomoya Uruga

Toward the Ultimate Limit of Phase Change in Ge2Sb2Te5

The limit to which the phase change memory material Ge(2)Sb(2)Te(5) can be scaled toward the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge(2)Sb(2)Te(5) has an increasingly dominant effect on the materials ability to change phase, and a profound increase in the crystallization temperature is observed when the Ge(2)Sb(2)Te(5) layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress, we have maintained the bulk crystallization temperature in Ge(2)Sb(2)Te(5) films just 2 nm thick.

Distortion-triggered loss of long-range order in solids with bonding energy hierarchy

An amorphous-to-crystal transition in phase-change materials like Ge-Sb-Te is widely used for data storage. The basic principle is to take advantage of the property contrast between the crystalline and amorphous states to encode information; amorphization is believed to be caused by melting the materials with an intense laser or electrical pulse and subsequently quenching the melt. Here, we demonstrate that distortions in the crystalline phase may trigger a collapse of long-range order, generating the amorphous phase without going through the liquid state. We further show that the principal change in optical properties occurs during the distortion of the still crystalline structure, upsetting yet another commonly held belief that attributes the change in properties to the loss of long-range order. Furthermore, our results suggest a way to lower energy consumption by condensing phase change inducing energy into shorter pulses or through the use of coherent phonon excitation.

Local structure of crystallized GeTe films

The structure of crystallized GeTe films has been studied by x-ray absorption fine structure spectroscopy. We find that in addition to Ge–Te bonds (2.20 and 3.13 A) ∼10% of Ge–Ge bonds are present. Our results indicate that the crystallized GeTe film consists of GeTe crystallites with 10% Ge vacancies, separated by a quasiamorphous-Ge phase.

Beamline for Surface and Interface Structures at SPring-8

The main components of a new beamline for surface and interface crystal structure determination at SPring-8 are briefly described. Stages for the beamline monochromator are modified for making an incident X-ray intensity more stable for surface X-ray experiments. Absolute photon flux densities were measured with an incident photon energy. A new ultrahigh vacuum system is introduced with preliminary X-ray measurements from an ordered oxygen on Pt (111) surface.

Ionic multilayers at the free surface of an ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, probed by x-ray reflectivity measurements.

The presence of ionic multilayers at the free surface of an ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide ([TOMA(+)][C(4)C(4)N(-)]), extending into the bulk from the surface to the depth of approximately 60 A has been probed by x-ray reflectivity measurements. The reflectivity versus momentum transfer (Q) plot shows a broad peak at Q approximately 0.4 A(-1), implying the presence of ionic layers at the [TOMA(+)][C(4)C(4)N(-)] surface. The analysis using model fittings revealed that at least four layers are formed with the interlayer distance of 16 A. TOMA(+) and C(4)C(4)N(-) are suggested not to be segregated as alternating cationic and anionic layers at the [TOMA(+)][C(4)C(4)N(-)] surface. It is likely that the detection of the ionic multilayers with x-ray reflectivity has been realized by virtue of the greater size of TOMA(+) and C(4)C(4)N(-) and the high critical temperature of [TOMA(+)][C(4)C(4)N(-)].

Driving force behind adsorption-induced protein unfolding: a time-resolved X-ray reflectivity study on lysozyme adsorbed at an air/water interface.

Time-resolved X-ray reflectivity measurements for lysozyme (LSZ) adsorbed at an air/water interface were performed to study the mechanism of adsorption-induced protein unfolding. The time dependence of the density profile at the air/water interface revealed that the molecular conformation changed significantly during adsorption. Taking into account previous work using Fourier transform infrared (FTIR) spectroscopy, we propose that the LSZ molecules initially adsorbed on the air/water interface have a flat unfolded structure, forming antiparallel beta-sheets as a result of hydrophobic interactions with the gas phase. In contrast, as adsorption continues, a second layer forms in which the molecules have a very loose structure having random coils as a result of hydrophilic interactions with the hydrophilic groups that protrude from the first layer.

Electronic structure of Eu@C60 studied by XANES and UV-VIS absorption spectra

Eu endohedral C60, Eu@C60, has been extracted with aniline from soot prepared by arc-heating of a graphite/Eu2O3 composite rod and obtained at high concentration by combining sublimation and repeated high-performance liquid chromatography. The laser desorption time-of-flight mass spectrum showed a pronounced peak of Eu@C60+. The UV–VIS absorption spectrum of this sample has a red-shift in the onset (>900 nm) in comparison with those for C60 and C70, as expected from electron transfer from the Eu atom to the C60 cage. The valence state of the Eu atom in Eu@C60 has been determined to be +2 by Eu LIII-edge XANES.

Why phase-change media are fast and stable : A new approach to an old problem

Present-day multimedia strongly relies on re-writable phase-change optical memories. We find that, different from current consensus Ge2Sb2Te5 (GST), the material of choice in digital versatile discs–random access memory (DVD-RAM), possesses a structure similar to ferroelectric GeTe, namely that Ge and Sb atoms are located off-center giving rise to a net dipole moment. Amorphisation of both GeTe and GST results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in the short-range order. We demonstrate that the order-disorder transition in GeTe and GST is primarily due to a flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions cycles.

The Mixed Valence States in the Unconventional Heavy Fermion Compound SmOs4Sb12

The X-ray absorption spectra of the heavy fermion compounds SmFe 4 P 12 and SmOs 4 Sb 12 have been measured near the Sm L 3 -edge at selected temperatures below 300 K. The spectra show the valence of Sm ions in SmFe 4 P 12 to be trivalent between 14 and 300 K with no significant temperature dependence. In contrast, that in SmOs 4 Sb 12 is 2.83 at 300 K and shows a significant decrease below 150 K, indicating an important role of the nonmagnetic Sm 2+ ions in the heavy fermion state that are insensitive to an applied magnetic field.

Construction and Commissioning of BL37XU at SPring-8

Trace element analysis beamline (BL37XU) at SPring‐8 was designed for application to various X‐ray fluorescence analyses such as XRF imaging, XAFS, TXRF and XRF holography. The beamline has of two branches, one being a SPring‐8 standard undulator‐beamline optics branch (Branch A) and the other a high‐energy branch (Branch B). In the experimental hutches, several kinds of experimental device are equipped. The end‐stations have been opened to public use since November 2002, and various experiments have been carried out.