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Ultra-fast X-ray Lasers Illuminate Elusive Atomic Spins

A quick flash of light can make ordinary materials extraordinary, potentially inducing qualities such as the perfect efficiency of superconductivity even at room temperature. But these subatomic transformations are infamously fleeting—they vanish in just trillionths of a second.

Now, an international team of scientists has used synchronized infrared and x-ray laser pulses to simultaneously manipulate and reveal the ultra-fast magnetic properties of this promising quantum landscape. The rapid, light-driven switching between magnetic states, explored here with unprecedented precision, could one day revolutionize the reading and writing of data in computers and other digital devices.

The study, published May 9, 2016, in the journal Nature Materials, was the result of a combined effort of a large international team, including researchers from China, the U.S., Germany, Japan, Spain, and the UK.

Nowadays, large X-ray free electron laser (FEL) facilities, such as LCLS and SACLA, can supply high quality x-ray pulses with remarkable short time length of femtoseconds. The challenge is how to use such powerful tool to detect the dynamic response of the spins. That is, a specialized x-ray detection system or 'camera' is needed. The scientists developed a highly specialized resonant inelastic X-ray scattering (RIXS) spectrometer for FEL, which used millimeter-sized silicon crystals to measure the exact energy of the rebounding x-rays, allowing reduction of the transient electronic and magnetic qualities.

In this work, the scientists used a strontium-iridium-oxygen compound (Sr2IrO4), selected for its strong magnetic interactions. An infrared laser was used to manipulate the spin, and time-resolved RIXS (tr-RIXS) was used to catch the spin in motion.The data revealed a clear difference in the propagation and timescale of the magnetic phenomena, with the inter-layer correlations taking hundreds of times longer to recover than those within each layer. Further, and more importantly, the magnetic dynamics showed significant difference between the equilibrium state and the transient state, where the low energy magnon spectral weight was largely enhanced. This indicates that the low energy spin dynamics is strongly coupled to the carriers, which come from photo-doping in this case.

These findings demonstrate the strength and precision of tr-RIXS, which opens the door to reveal femtosecond magnetic dynamics in as yet unseen detail.This made one more step closer to perfecting a recipe for manipulating materials on ultra-fast time scales.

This study entitled as "Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4" was published in Nature Materials.

The study was supported by the National Science Foundation, the Ministry of Science and Technology of China, the Chinese Academy of Sciences and also grants from the U.S., Germany, Japan, Spain, and the UK.

Institute of Physics
Xuerong Liu

Key word:
Magnetism; time resolved; spin excitations;

A new technique, namely time-resolved inelastic X-ray scattering (tr-RIXS), was developed to measure the spin dynamics of the laser induced transient states in ultra-fast time scale.

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