Recent Results on Quantum Materials with an X-ray Free Electron Laser
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Recent Results on Quantum Materials with an X-ray Free Electron Laser Joshua J. Turner Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, U.S.A. ABSTRACT We describe recent results in quantum materials through the use of a new tool: the x-ray free electron laser. We briefly describe the instrument and x-ray source capabilities and then discuss how this tool can bring new insights into materials with strong-electron correlations by ultrafast x-ray scattering from electronic structure. INTRODUCTION One of the novel tools for studying spin-electron-lattice phenomena in functional materials is the x-ray free electron laser [1]. This is especially true in the soft x-ray range, where the L-edges exist for the transition metal oxide materials and can be studied using resonant techniques. Resonant diffraction is one method of choice because of the ability to directly investigate the magnetic, charge, or orbital structure of quantum materials. Most importantly is the new available information in the time domain that using an x-ray FEL offers. By using pump-probe developments with ultra-short laser and x-ray pulses, dynamics on femtosecond time scales can be investigated that yield information about the time scales of the order parameters involved among the spin-electron-lattice phenomena. The coupling between the different mechanisms can be measured in these materials, and followed in real time. We review recent results in these systems and discuss the new understanding that is forming in the study of magnetic, charge, and orbital dynamics in these types of materials. EXPERIMENT Each individual example of a quantum material system under investigation will be described separately below. All experiments reviewed here were performed at the soft x-ray materials research (SXR) branch [2] of the Linac Coherent Light Source [1] at the SLAC National Accelerator Laboratory. This facility comprises the first x-ray laser and has been in operation since 2009, hosting the only current instruments for studies in the soft x-ray regime. In its short history, the SXR instrument been used to create solid-density plasmas [3], solve astrophysical problems [4], reconstruct single-shot images of nanomagnets [5], measure nanoscale dynamics in macroscopic, all-optical switching materials [6], and observe real-time bond breaking in chemical processes [7]. The instrument delivers short bursts of coherent [8] xrays with pulse durations from less than 10 fs to 300 fs, with a pulse energy of up to 3 mJ at a repetition rate of 120 Hz in the photon energy range of 250 eV- 2k eV. The x-ray optics are coated with B4C [9] to mitigate damage from the x-rays. The optical system includes a monochromator [10], for further narrowing the bandwidth of the lasing x-ray pulses, and a Kirkpatrick-Baez mirror pair for focusing beam sizes down to the micron level [11]. Also
important are timing diagnostics [12] for ultra-fast studies which use additional laser sources to excite the sample. All experiments were conducted using time
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