Scientific Highlights 2014

18. August 2014

A revealing mixture: The surface of an oxide insulator can host two distinct types of conducting electrons

Strontium titanate, SrTiO3, is an important material for the realization of next-generation electronic devices. A famous example is the interface of LaAlO3 grown on SrTiO3, which is metallic and magnetic at its interface, even though the individual compounds are insulating and nonmagnetic in bulk form. The physics behind how novel interface states form on SrTiO3 - and how they become endowed with such surprising properties - is not well understood. Researchers recently discovered that SrTiO3 can even host a metallic state at its surface (in other words, its interface with vacuum), which opened a new route to studying how conducting electrons behave when they are confined to a small surface or interface region of the material.

A collaboration led by scientists at PSI used detailed high-resolution angle-resolved photoemission spectroscopy (ARPES) at the Surface/Interface Spectroscopy (SIS) beamline in order to obtain the clearest view to date of the electronic structure of the metallic surface state on SrTiO3. ARPES is a uniquely powerful technique for probing and visualizing electrons' energy and momentum states, which determine a material's most fundamental electronic properties. The experiments revealed that the surface's conducting electrons come in two basic forms that behave in drastically different ways. Those that are associated with titanium 3dxy orbitals tend to be highly confined to a narrow surface region. Others associated with 3dxz and 3dyz orbitals occupy different energy levels from the 3dxy electrons and penetrate several layers or more into the subsurface region. The ARPES data show that, in addition to their spatial segregation, these two types of electrons have different conducting properties - not only relative to each other, but also relative to how they would be expected to behave in the bulk of SrTiO3. The study also found clues to how the surface state forms, including the relative influence of surface defects (against which the conducting electrons are highly robust) and other effects - perhaps structural - that more significantly alter the character of the valence electrons.
ORIGINAL PUBLICATION
Mixed Dimensionality of Confined Conducting Electrons in the Surface Region of SrTiO3
N. C. Plumb, M. Salluzzo, E. Razzoli, M. Månsson, M. Falub, J. Krempasky, C. E. Matt, J. Chang, M. Schulte, J. Braun, H. Ebert, J. Minár, B. Delley, K.-J. Zhou, T. Schmitt, M. Shi, J. Mesot, L. Patthey, and M. Radović,
Phys. Rev. Lett. 113, 086801 (2014). Published 18 August, 2014.
DOI: 10.1103/PhysRevLett.113.086801
CONTACT
Dr. Nicholas Plumb
Paul Scherrer Institut, Switzerland
E-Mail: nicholas.plumb@psi.ch


Dr. Milan Radović
Paul Scherrer Institut, Switzerland
E-Mail: milan.radovic@psi.ch

3. August 2014

Square dance of the atoms: Shedding light on ultrafast phase transitions

The exploration of the interaction of structural and electronic degrees of freedom in strongly correlated electron systems on the femtosecond time scale is an emerging area of research. One goal of these studies is to advance our understanding of the underlying correlations, another to find ways to control the exciting properties of these materials on an ultrafast time scale. So far a general model is lacking that provides a quantitative description of the correlations between the structural and electronic degrees of freedom.

Here we investigate a perovskite-type manganite, a prototypical example of a strongly correlated electron system which exhibits properties such as colossal magnetoresistance and insulator-to-metal transitions that are intrinsically related to symmetry changes of the atomic lattice and to intriguing ordering patterns of the spins, orbitals and charges. The application of an ultrashort optical pulse melts the electronic order and launches a structural phase transition. The long-range electronic and atomic order during the transition is probed directly with time-resolved resonant x-ray diffraction. Although the actual change in crystal symmetry associated with this transition occurs over different time scales characteristic of the many electronic and vibrational coordinates of the system, we find that the dynamics of the phase transformation can be well described using a single time-dependent order parameter that drives the electronic phase transition as well as the coherent motion of the atomic lattice. We hope that this concept is applicable to the general case.
Original Publication
Published 3 August 2014 in Nature Materials
DOI: 10.1038/nmat4046
Contact
Dr. Paul Beaud
Paul Scherrer Institut, Switzerland
E-Mail: paul.beaud@psi.ch

21. March 2014

Observed live with x-ray laser: electricity controls magnetism

Researchers from ETH Zurich and the Paul Scherrer Institute PSI demonstrate how the magnetic structure can be altered quickly in novel materials. The effect could be used in efficient hard drives of the future.

Data on a hard drive is stored by flipping small magnetic domains. Researchers from the Paul Scherrer Institute PSI and ETH Zurich have now changed the magnetic arrangement in a material much faster than is possible with today's hard drives. The researchers used a new technique where an electric field triggers these changes, in contrast to the magnetic fields commonly used in consumer devices. This method uses a new kind of material where the magnetic and electric properties are coupled. Applied in future devices, this kind of strong interaction between magnetic and electric properties can have numerous advantages. For instance, an electrical field can be generated more easily in a device than a magnetic one. In the experiment, the changes in magnetic arrangement took place within a picosecond (a trillionth of a second) and could be observed with x-ray flashes at the American x-ray laser LCLS. The flashes are so short that you can virtually see how the magnetisation changes from one image to the next - similar to how we are able to capture the movement of an athlete with a normal camera in a series of images with a short exposure time. In future, such experiments should also be possible at PSI's new research facility, the x-ray laser SwissFEL. The results will be published in the journal Science. They appear online in advance of print in Science Express on 6 March.

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Contact
Teresa Kubacka; Ultrafast Dynamics Group;
ETH Zurich; 8093 Zürich, Switzerland
Telephone: +41 44 633 21 56; E-mail: tkubacka@phys.ethz.ch [English, Polish]

Dr. Urs Staub; Research Group Microscopy und Magnetism;
Paul Scherrer Institute; 5232 Villigen PSI; Switzerland
Telephone: +41 56 310 44 94; E-mail: urs.staub@psi.ch [German, English]

Prof. Dr. Steven Johnson; Ultrafast Dynamics Group;
ETH Zurich; 8093 Zürich, Switzerland
Telephone: +41 44 633 76 31; E-mail: johnson@phys.ethz.ch [English, German]
Original Publication
Large-amplitude spin dynamics driven by a THz pulse in resonance with an electromagnon
T. Kubacka et al., Science Express, 6 March 2014
DOI: 10.1126/science.1242862

10. March 2014

Direct Observation of Magnetic Metastability in Individual Iron Nanoparticles

Studying the magnetization of individual iron (Fe) nanoparticles by magnetic spectromicroscopy reveals that superparamagnetic (SPM) and ferromagnetic blocked (FM) nanoparticles can coexist in the investigated size range of 8-20 nm. Spontaneous transitions from the blocked state to the superparamagnetic state are observed in single particles and suggest that the enhanced magnetic energy barriers in the ferromagnetic particles are due to metastable, structurally excited states with unexpected life times.

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Contact
Dr. Armin Kleibert, Swiss Light Source
Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Phone: +41 56 310 5527, e-mail: armin.kleibert@psi.ch

Prof. Dr. Frithjof Nolting, Swiss Light Source
Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
Phone: +41 56 310 5111, e-mail: frithjof.nolting@psi.ch
Original Publication
Direct Observation of Magnetic Metastability in Individual Iron Nanoparticles
Ana Balan, Peter M. Derlet, Arantxa Fraile Rodríguez, Joachim Bansmann, Rocio Yanes, Ulrich Nowak, Armin Kleibert, and Frithjof Nolting, Phys. Rev. Lett. 112, 107201 (2014)
DOI: 10.1103/PhysRevLett.112.107201

27. February 2014

Comprehensive study of the spin-charge interplay in antiferromagnetic La2-xSrxCuO4

The origin of the pseudogap and its relationship with superconductivity in the cuprates remains vague. In particular, the interplay between the pseudogap and magnetism is mysterious. Recent low-temperature angle-resolved photoemission spectroscopy (ARPES) experiments on the underdoped cuprate superconductors indicate the presence of a fully gapped Fermi surface (FS); even in the antiferromagnetic phase. The natural candidates for opening such a gap are competing orders such as charge or spin density waves (SDW).


A collaboration of researches from the Technion in Israel and PSI have investigated this newly discovered nodal gap in the cuprates using a combination of three experimental techniques applied to one, custom made, single crystal. The crystal is an antiferromagnetic La2-xSrxCuO4 with x=1.92%. All experiments where done in PSI. They performed angle resolved photoemission spectroscopy (ARPES) measurements as a function of temperature and found: quasi-particle peaks, Fermi surface, anti-nodal gap and below 45 K a nodal gap. Muon spin rotation measurements ensured that the sample is indeed antiferromagnetic and that the doping is close, but below, the spin-glass phase boundary. With elastic neutron scattering they determined the thermal evolution of the commensurate and incommensurate magnetic order.


The researchers found that a nodal gap opens well below the commensurate ordering at 140 K, and close to the incommensurate SDW ordering temperature of 35 K. This finding demonstrates that it is the SDW which is responsible for the nodal gap and not the standard antiferromagnetic order.

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Original Publication
Comprehensive study of the spin-charge interplay in antiferromagnetic La2-xSrxCuO4
Gil Drachuck, Elia Razzoli, Galina Bazalitski, Amit Kanigel, Christof Niedermayer, Ming Shi and Amit Keren
Nature Communications 5, Article number: 3390, 27 February 2014
DOI: 10.1038/ncomms4390
Contact
Prof. Dr. Amit Keren
Technion - Israeli Institute of Technology, Haifa, 32000, Israel
Phone: +972-54-5954696
E-Mail:keren@physics.technion.ac.il


Prof. Dr. Ming Shi
Paul Scherrer Institute, 5232 Villigen, Switzerland
Phone:+41 56 310 2393
E-mai: ming.shi@psi.ch

1. February 2014

Unique insight into carbon fibers on the nanoscale

Novel carbon materials are promising candidates for light and robust low-cost materials of the future. Understanding their mechanical properties benefits from highly resolved three-dimensional (3D) maps of their porosity and density fluctuations in uninterrupted representative volumes, but these are difficult to obtain with conventional imaging methods. Scientists at the Paul Scherrer Institut have now succeeded to produce in collaboration with Honda R&D in Germany highly resolved 3D density maps of entire sections of carbon fibers. The technique they used, called ptychographic computed tomography, offers unprecedented insights into the nanomorphology of these materials. Without the need of sectioning the fibers, their porosity can be visualized in 3D as can high-density carbon regions attributed to different degrees of graphitization, indicative of atomic structure differences in the material. Such imaging capabilities are expected to prove useful for the systematic study of the mechanical properties of carbon fibers, addressing a crucial point when designing and tailoring novel carbon materials.


Supplementary video

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Reference
Characterization of carbon fibers using X-ray phase nanotomography
A. Diaz, M. Guizar-Sicairos, A. Poeppel. A. Menzel, O. Bunk
Carbon 67 (2013) 98-103, DOI:j.carbon.2013.09.066
Contact
Dr. Ana Diaz, Swiss Light Source
Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Phone: +41 56 310 5626, e-mail: ana.diaz@psi.ch

24. January 2014

Spintronics: deciphering a material for future electronics

Topological insulators are the key to future spintronics technologies. EPFL scientists have unraveled how these strange materials work, overcoming one of the biggest obstacles on the way to next-generation applications.


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Reference
Spin texture of Bi2Se3 thin films in the quantum tunneling limit
G. Landolt, S. Schreyeck, SV. Eremeev, B. Slomski, S. Muff, J. Osterwalder, EV. Chulkov, C. Gould, G. Karczewski, K. Brunner, H. Buhmann, LW. Molenkamp and JH. Dil
Phys Rev Lett 05 Feb 2014 DOI: 10.1038/srep03857
Contact
Dr. M. Holler
Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Phone: +41 56 310 3613, e-mail: mirko.holler@psi.ch

24. January 2014

X-ray tomography reaches 16 nm isotropic 3D resolution

Tomographic microscopy has become an invaluable imaging method in both life and materials sciences. Oftentimes, high resolving power is required simultaneously with the ability to characterize large, statistically representative sample volumes. To this task, researchers at the Paul Scherrer Institut have established ptychographic computed tomography. The technique promises the ability to characterize three-dimensionally thousands of cubic microns with precision and resolution currently not achievable by other means. Using this technique, researchers at PSI now succeeded in demonstrating an isotropic 3D resolution of 16 nm, unmatched in X-ray tomography. They reported this first-of-its-kind demonstration on a 6 microns thick test object of Ta2O5-coated nanoporous glass in Scientific Reports.

The measurement was performed at the cSAXS beamline at the Swiss Light Source using a prototype instrument of the OMNY (tOMography Nano crYo) project. Whereas this prototype measures at room temperature and atmospheric pressure, the OMNY system, to be commissioned later this year, will provide a cryogenic sample environment in ultra-high vacuum without compromising imaging capabilities. The researchers believe that such a combination of ptychographic X-ray tomography with state-of-the-art instrumentation is a promising path to fill the resolution gap between electron microscopy and X-ray imaging, also in case of radiation-sensitive materials such as polymer structures and biological systems.


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Reference
X-ray ptychographic computed tomography at 16 nm isotropic 3D resolution
M. Holler, A. Diaz, M. Guizar-Sicairos, P. Karvinen, Elina Färm, Emma Härkönen, Mikko Ritala, A. Menzel, J. Raabe & O. Bunk
Scientific Reports 4, Article number: 3857, DOI: 10.1038/srep03857
Contact
Dr. Mirko Holler, Swiss Light Source
Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
Phone: +41 56 310 3613, e-mail: mirko.holler@psi.ch

5. January 2014

Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers

Super-eruptions that dwarf all historical volcanic episodes in erupted volume and environmental impact are abundant in the geological record. Such eruptions of silica-rich magmas form large calderas. The mechanisms that trigger these supereruptions are elusive because the processes occurring in conventional volcanic systems cannot simply be scaled up to the much larger magma chambers beneath super volcanoes. Over-pressurization of the magma reservoir, caused by magma recharge, is a common trigger for smaller eruptions, but is insufficient to generate eruptions from large supervolcano magma chambers. Magma buoyancy can potentially create sufficient overpressure, but the efficiency of this trigger mechanism has not been tested. Here we use synchrotron measurements of X-ray absorption to determine the density of silica rich magmas at pressures and temperatures of up to 3.6 GPa and 1,950 K, respectively. We combine our results with existing measurements of silica-rich magma density at ambient pressures to show that magma buoyancy can generate an overpressure on the roof of a large supervolcano magma chamber that exceeds the critical overpressure of 10-40 MPa required to induce dyke propagation, even when the magma is undersaturated in volatiles. We conclude that magma buoyancy alone is a viable mechanism to trigger a super-eruption, although magma recharge and mush rejuvenation, volatile saturation or tectonic stress may have been important during specific eruptions.
Reference
Supervolcano eruptions driven by melt buoyancy in large silicic magma chambers
Malfait, W.J., Seifert, R., Petitgirard, S., Perrillat, J.P., Mezouar, M., Ota, T., Nakamura, E., Lerch, P., Sanchez-Valle, C.
Nature Geoscience, online publication, January 2014 doi:10.1038/ngeo2042.
Contact
Prof. Dr. Carmen Sanchez-Valle
Inst. f. Geochemie und Petrologie
ETH Zurich
Phone: +41 44 632 43 19
E-Mail: carmen.sanchez@erdw.ethz.ch


Dr. Wim J. Malfait
EMPA
Phone:+41 58 765 49 83
E-Mail: wim.malfait@empa.ch