Light-enhanced Charge Density Wave Coherence in a High-Temperature Superconductor

In high-T$_{C}$ cuprates, superconductivity and charge density waves (CDW) are competitive, yet coexisting orders. To understand their microscopic interdependence a probe capable of discerning their interaction on its natural length and time scales is necessary. Here we use ultrafast resonant soft x-ray scattering to track the transient evolution of CDW correlations in YBa$_{2}$Cu$_{3}$O$_{6+x}$ following the quench of superconductivity by an infrared laser pulse. We observe a picosecond non-thermal response of the CDW order, characterized by a large enhancement of spatial coherence, nearly doubling the CDW correlation length, while only marginally affecting its amplitude. This ultrafast snapshot of the interaction between order parameters demonstrates that their competition manifests inhomogeneously through disruption of spatial coherence, and indicates the role of superconductivity in stabilizing topological defects within CDW domains.Comment: 29 pages, 9 figures, Main text and Supplementary Material

Enhanced charge density wave coherence in a light-quenched, high-temperature superconductor

Superconductivity and charge density waves (CDWs) are competitive, yet coexisting, orders in cuprate superconductors. To understand their microscopic interdependence, a probe capable of discerning their interaction on its natural length and time scale is necessary. We use ultrafast resonant soft x-ray scattering to track the transient evolution of CDW correlations in YBa2Cu3O6+x after the quench of superconductivity by an infrared laser pulse. We observe a nonthermal response of the CDW order characterized by a near doubling of the correlation length within ≈1 picosecond of the superconducting quench. Our results are consistent with a model in which the interaction between superconductivity and CDWs manifests inhomogeneously through disruption of spatial coherence, with superconductivity playing the dominant role in stabilizing CDW topological defects, such as discommensurations

Design and performance of the traveling-wave beam chopper for the SSRL injector

A pulsed, split-parallel plate chopper has been designed built, and installed as part of the preinjector of the SSRL Injector. Its function is to allow the linear accelerator three consecutive S-band bunches from the long bunch train provided by a RF gun. A permanent magnet deflector (PMD) at the chopper entrance deflects the beam into an absorber when the chopper pulse is off. The beam is swept across a pair of slits at the beam output end when a 7 kV, 10-ns rise-time pulse passes in the opposite direction through the 75 {Omega} stripline formed by the deflecting plates. Bunches exiting the slits have their trajectories corrected by another PMD, and enter the linac. Beam tests demonstrate that the chopper functions as expected. 9 refs., 5 figs

COHERENT X-RAY SEEDING SOURCE FOR DRIVING FELS* A CONCEPT-BY-3.0 and by the respective authors Seeding FELs

Abstract The success of the hard X-ray self-seeding experiment (HXRSS) at the LCLS is very important in that it provided narrow, nearly transform-limited bandwidth from the FEL, fulfilling a beam quality requirement for experimental applications requiring highly monochromatic X-rays. Yet, because the HXRSS signal is generated from random spikes of noise, it is not a truly continuous monochromatic seed signal and even higher FEL performance would be achieved using a continuous seed source. We propose developing such a source using a low-Q X-ray cavity to achieve a continuous, narrowband X-ray seed signal. The low-Q cavity works like a return path for the fields, produced in the undulator situated within an X-ray cavity. We do not assume that Xray fields can be coherently stored in the cavity because of the high tolerances on the cavity length. But we assume that the undulator works as a very high gain amplifier, which compensates amplitude loss due to X-ray reflections in the cavity. The cavity may consist of several elements, which can reflect X-rays by several degrees to make a total of 360 degrees. For example, the elements could be four crystals with a corresponding Bragg angle of about 45 degrees each with additional small angle correcting elements. In this case, the amplitude loss is due to the small bandwidth of the reflected fields. The frequency spectrum of the final X-ray signal will be determined by the bandwidth of the reflected elements. This is not a very new idea. A regenerative-amplifier FEL (RAFEL) has been even demonstrated in the infrared wavelength region A CONCEPT The basic schematic is shown in As with classical FEL, the beam energy (a few GeV) corresponds to the radiation wavelength. The beam energy spread and beam emittance must not be above the usual FEL requirement. The electron bunch pattern may consist of an initial train of relatively low current bunches followed by a high current bunch. The bunch spacing depends upon the total length of the undulators inside the cavity. However there is no strong requirement on the arrival time because the reflected X-ray pulse length is increased (~ps) due to the frequency filtering (because of the multiple reflections inside the crystal)

Coherent X-Ray Seeding Source for Driving FELS

The success of the hard X-ray self-seeding experiment (HXRSS) at the LCLS is very important in that it provided narrow, nearly transform-limited bandwidth from the FEL, fulfilling a beam quality requirement for experimental applications requiring highly monochromatic X-rays. Yet, because the HXRSS signal is generated from random spikes of noise, it is not a truly continuous monochromatic seed signal and even higher FEL performance would be achieved using a continuous seed source. We propose developing such a source using a low-Q X-ray cavity to achieve a continuous, narrowband X-ray seed signal. The low-Q cavity works like a return path for the fields, produced in the undulator situated within an X-ray cavity. We do not assume that X

Tracking Study for Top-off Safety Validation at SSRL

A tracking study was performed at SSRL to identify necessary controls and to prove the safety of top-off operation from radiation hazard under such conditions. The safety rationale, tracking setup and the results are presented. Top-off operational mode has become a trend for existing and planned third-generation storage ring light sources for the many benefits such as increased average brightness, improved thermal stability and elimination of the interruption to user experiments due to traditional injection [1, 2]. Unlike the traditional decay mode injection which happens a few times a day and during which the photon beamline shutters are closed, top-off mode injection requires photon beamline shutters to remain open during injection and occurs much more frequently, from once every 5 seconds to once every 30 minutes. Therefore injection may be transparent to user experiments and the stored current variation can be significantly reduced. For a facility equipped with a full-energy injector, the biggest challenge to the implementation of the top-off mode may be the control of radiation hazard. Studies at ALS and SSRL [2, 3] have shown that a single injected electron pulse that enters the photon beamline and exits the radiation shield wall would cause unacceptable radiation doses on the experimental floor. For the protection of users and experimental equipment, it is hence a prerequisite for top-off operation to establish controls that absolutely prevent such occurrences. Similar to other facilities such as ALS and APS [2, 4], tracking simulations were conducted at SSRL to identify the control measures, define the specifications and prove the radiation safety. However, a different approach toward the proof of safety is taken at SSRL. In this paper we first describe the SSRL accelerator complex with emphasis on the aspects related to top-off in section 2. The general considerations and requirements for top-off are presented in section 3. Section 4 and 5 give a detailed description of the tracking setup and results. Concluding remarks are given in section 6