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2.1. Quantum materials for energy

2.1.1. Superconductivity

Superconducting  transition temperatures as high as 130 K in copper oxide materials open the way to lossless transport of electrical energy and to applications in fast and energy-saving information technology. Even after 25 years of research in this field, the understanding of high-temperature superconductivity remains a great challenge. This is due to the fact that the properties of these materials arise from complexity. Insight into the mechanism underlying high temperature superconductivity promises to develop design principles to enhance the critical temperature in superconductors. Understanding the interaction leading to superconductivity is still controversial for high-Tc materials. What is widely accepted is that either spin fluctuations, phonons, or a combined excitation of both are responsible for the microscopic interaction leading to formation of a superconducting ground state. Analysis of these excitations has therefore a high priority in these materials. The strong interaction of the electrons with charge, spin, and orbital degrees of freedom lead to a number of competing ground states and the superconducting state is one of them while others are energetically very close (Figure 1). Ultrafast excitation of the superconductors provides a completely new approach to understand the various competing states around the superconducting state by driving the system out of equilibrium into neighboring states for a short period of time. In this way, even new states that cannot be observed in equilibrium may be created.

Ultrashort synchrotron radiation pulses as delivered by BESSY-VSR will provide a unique probe to study these phenomena. While in the ground state, angle-resolved photoemission spectroscopy and neutron scattering are among the momentum and excitation specific tools, time-domain methods such as time-resolved resonant soft x-ray diffraction have been established as promising complementary methods to analyze such excitations and their dynamics.

In a recent experiment, e.g., it was shown that transient superconductivity can be stimulated by selective optical excitation in a material, where a competing ground state, the so-called charge-ordered state, wins over the superconducting state [1]. The understanding of charge order in the cuprates, on the other hand, has recently been pushed forward by resonant soft x-ray diffraction, which is a method that exploits the tunability of synchrotron radiation and provides particular sensitivity to this subtle type of ordering [2].

Figure 1. Charge order competing with superconductivity: Cooling below the superconducting ordering temperature Tc results in a decay of charge order, as demonstrated by the decrease of the corresponding diffraction peak intensity. (Courtesy E. Weschke, HZB).

Angle-resolved photoelectron spectroscopy (ARPES) plays a particular role for the investigation of superconductivity as it addresses the superconducting quasiparticles in a straightforward way including control of the quasiparticle momentum. In laboratory-based experiments, the dynamics of pair breaking and the time evolution of the superconducting gap was studied [3]. Another example is the recent time- and angle-resolved photoemission work by the group of U. Bovensiepen (Universität Duisburg-Essen) on the parent compound EuFe2As2 Fe-pnictide superconductor, which vividly demonstrates the influence of a bosonic excitation on the spectral function near the Fermi level (Figure 2).

However, only in a limited momentum range due to the rather small photon energy in laboratory-based experiments could be reached. BESSYVSR will readily allow extending this type of studies to cover the full Fermi surface of the materials. Together with the time structure envisaged with BESSY-VSR the method will open a new field of time-resolved studies of subtle ordering phenomena not only in superconductors, but in complex materials in general. For this particular case, the destruction of charge order in favor of superconductivity can be studied in detail.

Femtosecond time-resolved soft x-ray scattering and spectroscopy, which is currently carried out at the femtoslicing facility FEMTOSPEX at BESSY II, provides element specificity and momentum sensitivity. This method is very valuable to analyze the coupling between a particular boson excitation W(k) and the material's ground state. Limitations of the current experimental facility are two-fold and exemplify the novel opportunities offered by BESSY-VSR:  (1) Most experiments at the femtoslicing facility FEMTOSPEX at BESSY II were carried out in transmission geometry which strongly limits the materials that can be investigated. (2) The photon flux hinders systematic studies which provide similarly detailed information as, e.g., time-resolved ARPES does already now. The first point has improved recently by implementation of a reflection geometry. In fact the group of U. Bovensiepen has very recently carried out first proof of principle experiments using time-resolved soft-x-ray spectroscopy in reflection geometry on a 122 Fe-pnictide material. The second point is more fundamental and here BESSY-VSR will enable novel insight. With a photon pulse duration of 1 ps the modifications in the photo-excited state could be probed (see Figure 2). This would increase the opportunities for time-resolved soft-x-ray pulses tremendously due to the improved time-resolution. In addition, the time resolution of 100 fs at the femtoslicing facility with a will allow studying the coherent nature and phase of bosonic excitation in an element specific manner because frequency and phase can be resolved in time (see Figure 2). Thereby, boson-mediated changes in the material become accessible in a mode-specific and time-resolved mode. It is important to note that the increased flux at the femtoslicing facility at BESSY-VSR uniquely enables systematic experiments beyond the current heroic efforts for selected samples. It is the systematics and level of detail that can be expected by ps to femtosecond experiments at BESSY-VSR that could boost studies of the ultrafast dynamics in complex materials, which are in the focus of current efforts in material science.

Figure 2. Panel (a) shows the time-dependent photoelectron intensity in femtosecond time-resolved ARPES measured on a parent compound of the 122 Fe-pnictide high-Tc superconductor EuFe2As2. The transient changes in the spectral weight in the vicinity of the Fermi level depicted in panel (b) are strongly modified by coupling to a bosonic excitation. [taken from ref. 4, Avigo et al., J. Phys.: Condens. Matter 25, 094003 (2012)].

The time structure of BESSY-VSR will, in addition, allow for angle-resolved time-of-flight electron spectrometers to be employed. The flexible fill pattern combined with a mechanical chopper or an electronic gating scheme for the spectrometer gives access to a 1.25 MHz source with variable pulse length down to the fs regime, ideally matched to the performance of spectrometers such as the ArTOF. With the advent of femtosecond table top lasers and X-ray FELs it has now become possible to determine the structural dynamics from time resolved diffraction experiments and obtain in separate experiments dynamic information on the electronic or magnetic properties. However, it has remained a conceptual challenge to access simultaneously the dynamics of the valence electrons in combination with the chemical state and the local coordination atom by atom. The extreme high transmission in combination with the angle resolving capabilites of the ArTOF will now open a new route to extract simultaneously the structural dynamics in an element selective way through time resolved photoelectron diffraction (trPED) and determine the temporal evolution of the chemical state trough time resolved ESCA (trESCA) or even the femtosecond charge transfer dynamics through excited state core-hole-clock spectroscopy. All these experiments require femtosecond and picosecond pulse length at moderate photon flux per pulse as provided by BESSY-VSR to avoid space charge in combination with high repetition rates. Ideal candidates for phase transition dynamics are mixed valence and correlated materials that exhibit geometric distortions, charge separation and orbital order on the nanometer scale and are prone to spectacular ultrafast switching of materials properties in metal-insulator transitions as well as magnetic switching. In this context, layered transition metal dichalcogenides are known to exhibit structural distortions accompanied by charge separation manifested through the appearance of characteristic charge-density wave (CDW) phases. A great number of experiments addressed the evolution of the electronic system after optical excitation using high-harmonic generation (HHG) and free-electron laser (FEL) sources in these systems [5-7] or the structural dynamics using femtosecond electron diffraction [8]. However the complete pathways of the photoexcitation could only be resolved by the simultaneous determination of the structural and electronic dynamics in the system via simultaneous trPED and trESCA.

References for section 2.1.1.
[1] D. Fausti et al., Science 331, 189 (2011).

[2] G. Ghiringhelli et al., Science 337, 821 (2012), H. H. Wu et al., Nature Commun. 3, 1023 (2012).

[3] Ch. L. Smallwood et al., Science 336, 1137 (2012), Cortes et al., Phys. Rev. Lett. 107, 097002 (2011).

[4] Avigo et al., J. Phys.: Condens. Matter 25, 094003 (2012).

[5] L. Perfetti et al., Phys. Rev. Lett. 97, 067402 (2006).

[6] S. Hellmann et. al., Phys. Rev. Lett. 105, 187401 (2010).

[7] S. Hellmann et. al., Nature Communications 3, 1069 (2012).

[8] M. Eichberger et. al., Nature 468 (2010).