New probes for condensed matter research at the Paul Scherrer Institute

New neutron spectrometer EIGER

Neutron spectroscopy with its cross section that is known exactly is an exceptional technique for studies of the elementary excitations of condensed matter systems. Up to now there were four spectrometers using cold neutrons from SINQ, but none for neutrons from its thermal moderator. The cold spectrometers are two triple-axis spectrometers (RITA-2 and TASP), one direct geometry time-of-flight spectrometer (FOCUS) and one inverse geometry time-of-flight backscattering instrument (MARS). Cold spectrometers are best suited to study excitations up to energies of about 5 meV. However, there is also a strong demand for experiments, which require larger energy transfers. In 2012 construction of the new thermal triple-axis spectrometer EIGER was completed. EIGER will extend the available energy range up to 50 meV.

The most important quality factors for a triple-axis-instrument are intensity and (low) background. Since the new instrument is the first thermal triple-axis instrument at a continuous spallation source it was a special challenge to design proper shielding for the monochromator in order to keep the fast neutron and gamma-ray background sufficiently low. Additionally, it is very desirable to be able to perform experiments with strong cryo-magnets, therefore, the shielding had to be built from non-magnetic materials. The final design is an optimized composite structure of different materials: boron for the absorption of thermal neutrons, tungsten for the absorption of fast neutrons and the attenuation of gamma-rays and additional special heavy (non-magnetic) concrete to absorb gamma radiation.

The principle layout for the primary spectrometer controlling the incident neutron beam consists of a sapphire filter, which reduces the contribution of fast neutrons at energies above 80 meV, an adjustable virtual source with a maximum width of 40 mm and a double focusing monochromator. The double focusing monochromator is made of 15×9 pieces of pyrolytic graphite (PG) with a mosaicity of about 30’ and a size of 20×20 mm² each. The graphite is fixed on Al-lamellas, which can be rotated for horizontal focusing of the beam and bent for vertical focusing. As the secondary spectrometer, analyzing the energies of the scattered neutrons also by Bragg scattering from PG crystals, components of the former cold triple-axis spectrometer Drüchal at SINQ are used. A picture of the instrument is presented in Figure 1, with a helium cryostat (orange) at the position of the sample.

The resolution of the instrument at a final energy of 14.7 meV with double focusing monochromator and horizontal focusing analyzer was determined to 0.64 meV, in agreement with Monte-Carlo simulations of the instrument, and the total flux at the sample position was estimated with a calibrated monitor (5.8 ·10⁶ n·cm^-2·sec^-1 at the same final energy).

During the commissioning phase in 2012 half a dozen experiments have already been performed on EIGER. When normalized by the source flux, its performance is among those of the best spectrometers of this type in the world. Efficiency gains for experiments at SINQ in the energy range 2-10 meV will be significant and new science will be done on materials with excitations at energies above that range and up to 50 meV, which were previously inaccessible. Among the first experiments were studies of magnetic excitations and phonons in quantum and frustrated magnets, magnetic semiconductors and novel superconductors. The unique combination of neutron and photon spectroscopy (like RIXS described below) at PSI and their complementarity now enable studies of the dynamics in solids from 0 to several eV on samples that are as diverse as single atomic layers of model two-dimensional insulators and large single crystals of superconductors.

Resonant inelastic-X-ray scattering RIXS

Resonant inelastic X-ray scattering (RIXS) is a powerful spectroscopy technique, which is atomic site selective. This is due to the involvement of a core-hole in the intermediate state created by resonant excitation to the conduction band. With RIXS the electronic structure of condensed matter is probed by determining the energy and symmetry of charge neutral electronic excitations (e.g. crystal field, charge transfer, phonon or spin excitations) in strongly correlated materials such as transition metal oxides and rare earth systems. Variation of the scattering angle can be used in RIXS to study the dispersion of these excitations as a function of momentum transfer, thereby enabling characterization of their localized vs. delocalized character.

The SAXES instrument at the ADRESS beamline of the SLS (see Figure 4) is a high-resolution X-ray emission spectro­meter following an optical scheme based on two variable line spacing (VLS) spherical gratings with radius of 58.55 m and 60 m for an average groove density of 3200 lines/mm and 1500 lines/mm, respectively. With an input arm around 1 m and exit arm of up to 4 m, SAXES utilizes a total length of 5 m. The detector used is a liquid nitrogen cooled two-dimensional CCD camera with a pixel size of 13.5 μm and effective spatial resolution of about 24 μm. The long dispersion arm and advanced detector enable extremely high resolving power of better than 10’000 below 1100 eV, in particular 17’000 at the Ti L₃ edge (470 eV) and 12’000 at the Cu L₃ edge (930 eV). The total operative energy range of SAXES is 400 – 1600 eV with a typical spot size on the sample below 5 μm.

In a recent highlight the SAXES instrument was used to map momentum dispersive spectra from collective spin- as well as orbital- excitations in the spin-chain compound Sr₂CuO₃ with Cu L₃ edge RIXS (see Figure 5). From this study it was shown for the first time that in a one-dimensional spin system the electronic degree of freedom can split up in its spin and orbital parts. Another powerful demonstration of the high sensitivity and capabilities of the RIXS set-up at SLS is the detection of spin waves in isolated single atomic La₂CuO₄ layers.

Future experiments at the SwissFEL

The PSI has proposed the construction of a hard x-ray free-electron laser facility, the SwissFEL, which will produce intense, ultra-short pulses of coherent radiation in the energy range from 2 keV to 13 keV, with a future extension to the soft x-ray range (see Figure 6). The science case for the SwissFEL project emphasizes the dynamics of condensed matter systems, chemical systems and the damage-free imaging of nanostructures. The Swiss parliament has approved the proposal by the end of 2012. The preparation of the site has already started. The goal is to produce the first beam by fall 2016. Pilot experiments should start in spring 2017. The specifications of the experimental stations have been discussed with user groups in a series of workshops held at PSI. A Conceptual Design Report will be soon available (more information).

In order to prepare for SwissFEL and to start work in the exciting area of ultra-fast spectroscopy several teams from Swiss universities and PSI are using some of the few existing x-ray free electron lasers worldwide to do first experiments. Here, the ability to manipulate matter on ultra-short time scales offers potential breakthroughs in future device technologies as well as a better understanding of fundamental material properties. For this purpose, it is of immense importance to be able to selectively drive excitations of interest. A team of researchers from PSI, the ETH, Stanford (SLAC) and Berkley has recently demonstrated this with an experiment performed in July 2012 at the x-ray free electron laser LCLS (Linac Coherent Light Source at Stanford, US). They showed that a short THz-frequency pulse may be used to excite a coherent electromagnon in multiferroic TbMnO₃. Electromagnons are hybrid excitations of a magnon and a phonon that couple polarization with magnetism in multiferroic materials. The material response was studied by time dependent resonant magnetic soft x-ray diffraction. This experiment demonstrates that newly available methods of creating short, intense THz-frequency pulses make it possible to coherently control magnetic moments. X-ray free electron lasers allow us to study these excitations in real time. With the upcoming SwissFEL, studies of the detailed mechanism of switching electric polarization through selective excitations will become possible.