Atomic motions untangled

The pursuit of capturing motion in a movie bears an obvious fascination irrespective of the time scales involved. In the atomic and molecular world where the masses are so light and the distances small the relevant time scale shifts to the subpicosecond range and the motions become frantic especially for larger molecular systems. In the material class of strongly correlated electron materials the intricate balance of competing structural, magnetic and charge interactions complicates the picture when it comes down to disentangle the coupled processes. In order to advance the understanding of the underlying correlations in these materials current efforts focus on the interaction of the atomic, electronic, and magnetic subsystems on their relevant time scales. In particular, femtosecond x-ray or electron diffraction received considerable attention in the recent past because they enable direct access to the evolving atomic and electronic structure. Here, we study specific lattice modulations coupled to the melting of charge and orbital order in a manganite by means of femtosecond x-ray diffraction. By using a carefully chosen set of reflections combined with structure factor calculations we are able to identify the involved atomic motions.

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Soft x-ray photoelectron spectroscopy on buried complex oxide interfaces: a new method to diagnose authentic protected electronic structures

Exotic phenomena at interfaces of complex oxides are highly promising for future solid-state electronics applications. A prominent example is the interface of two wide band gap insulators formed by growing a LaAlO3 layer on TiO2-terminated SrTiO3 substrate. When the LaAlO3 thickness exceeds 3 unit cells this system undergoes a sharp insulator-to-metal transition with a two-dimensional electron gas (2DEG) appearing at the interface. A team of scientists from the Paul Scherrer Institute and University of Geneva, Switzerland, has investigated the LaAlO3/SrTiO3 interface with photoelectron spectroscopy. The researchers have for the first time unambiguously directly detected the 2DEG signal at the Fermi level with its sharp onset between the insulating and conducting interfaces. Furthermore, they determined the spatial localization depth of the conducting sheet. The problem of absorption and inelastic scattering of the photoelectrons as they pass through the LaAlO3 overlayer was solved by using advanced synchrotron-based instrumentation delivering a high photon flux, and operating in the soft-X-ray energy range where the photoelectron escape depth increases. Furthermore, in consequence of the very small number of electrons in the 2DEG, their photoelectron signal was boosted by tuning the photon energy to resonant excitation of the core electrons, through a Fanolike process. This work represents a significant methodological advance in resonant photoemission spectroscopy of buried interfaces.

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Imaging fluctuations with X-ray microscopy

X-rays allow an inside look at structures that cannot be imaged using visible light. They are used to investigate nanoscale structures of objects as varied as single cells or magnetic storage media. Yet, high-resolution images impose extreme constraints on both the X ray microscope and the samples under investigation. Researchers at the Technische Universität München, Germany, and the Paul Scherrer Institut in Villigen, Switzerland, now showed how to relax these conditions without loss of image quality. They further showed how to image objects featuring fast fluctuations, such as the rapid switching events that determine the life time of data storage in magnetic materials. They demonstrated their method with an experiment at the Swiss synchrotron SLS and with computer simulations. The results have been published in the science journal Nature.

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Three-Dimensional Electron Realm in Crystalline Solids Revealed with Soft-X-Rays

The electronic band structure E(k) as energy E of the electrons depending on its wavevector k is the cornerstone concept of the quantum solid state theory. The main experimental method to investigate E(k) is the angle-resolved photoelectron spectroscopy (ARPES). However, a small photoelectron escape depth of a few Å largely restricts the applications of ARPES to two-dimensonal crystals. A team of SLS scientists working at the ADRESS beamline have recently extended this technique to the much wider world of three-dimensional (3D) crystalline systems. They used soft-X-ray synchrotron radiation in the soft-X-ray photon energy range around 1 keV, where the photoelectron escape depth and concomitantly the 3D k-vector definition increase by a factor of 3-5. The problem of a dramatic loss of the photoexcitation cross-section by a few orders of magnitude in this energy range has been overpowered with ADRESS beamline, which delivers high photon flux exceeding that of the closest competitors by up to 2 orders of magnitude. The pilot soft-X-ray ARPES research has been carried out on the paradigm transition-metal-dichalcogenide VSe2. Unparalleled definition of the 3D k-vector has allowed the scientists to explore in 3D the Fermi surface of VSe2 determining the whole host of its unusual macroscopic properties including the electric conductivity anomalies. The textbook calrity of the experimental FS (figure) manifests excellent definition of the 3D k vector and regular photoexcitation matrix elements achieved at soft-X-ray energies. Autocorrelation analysis of 3D warping of the experimental Fermi surface has revealed its nesting as the precursor for the exotic 3D charge density waves in VSe2. This pilot research has been followed in the last year by a plethora of other breakthrough experiments exploiting the advantages of soft-X-ray ARPES on the probing depth and 3D k-vector definition, such as the electronic structure of Mn ferromagnetic impurities in the paradigm spintronic material GaMnAs, buried 2D electron gas in LaAlO3/SrTiO3 hererostructures, standing X ray waves excited ARPES of multilayer heterostructures allowing depth resolved electronic structure profiling, etc.

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Magnetic nano-chessboard puts itself together

Researchers switch the quantum properties of magnetic molecules

Researchers from the Paul Scherrer Institute and the Indian Institute of Science Education and Research (Pune, India) have managed to ‘turn off’ the magnetization of every second molecule in an array of magnetized molecules and thereby create a ‘magnetic chessboard’. The magnetic molecules were so constructed that they were able to find their places in the nano-chessboard by themselves. Thus the nano-chessboard effectively built itself together. The researchers were able to manipulate the quantum state of just a part of the molecules in a specific way. Being able to specifically alter the state of individual quantum objects is an important prerequisite for the development of quantum computers. Such computers would rely on the laws of quantum physics and could perform some calculational tasks very much faster than present-day computers. However, today we are very far from being able to build quantum computers that are in reality comparably powerful for particular calculations. The scientists have published their results in the journal Advanced Materials.

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X-rays provide insights into volcanic processes

Experiments performed at the Paul Scherrer Institute (PSI) investigate processes inside volcanic materials that determine whether a volcano will erupt violently or mildly.

In the experiments, an international team of scientists used a laser-based heating system to heat small pieces of volcanic material similarly to conditions present at the beginning of a volcanic eruption. They used X-rays from the Swiss Light Source (SLS) at the Paul Scherrer Institute (PSI) to observe, in real time, what happens to the rock as it goes from the solid to the molten state. A determining factor to the type of eruption that occurs is how fast gas bubbles form inside the material. These studies indicate that the type of eruption taking place may be established as early as the first few seconds of bubble growth. The researchers have published their results in the online journal Nature Communications.

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New Insights into Superconducting Materials

An American-Swiss research team has used a new X-ray technique at Swiss Light Source (SLS) of the Paul Scherrer Institute (PSI) to investigate the magnetic properties of atomically thin layers of a parent compound of a high-temperature superconductor. It turns out that the magnetic properties of such thin films differ by only a surprisingly small degree from those of macroscopically thick samples. In their experiment, the researchers investigated the material using X-ray light from the SLS and found out how the energy of the light changed as it passed through the sample. The “RIXS” technique that is available at PSI is the first of its kind that is sensitive enough to enable investigations on such thin films. At the present time, superconducting materials are already being studied by this technique. In the future, it could be used for studying the processes occurring in very thin superconductors and contribute to an understanding of the fascinating phenomenon of high-temperature superconductivity. The results of the present work have recently appeared in the journal Nature Materials.

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Ultra-short X-ray laser pulses precisely surveyed for the first time

X-ray lasers belong to a modern generation of light sources from which scientists in widely different disciplines expect to obtain new knowledge about the structure and function of materials at the atomic level. On the basis of this new knowledge, it could then be possible one day to develop better medicines, more powerful computers or more efficient catalysts for energy transformation. The scientific value of an X-ray laser stands or falls on the quality of the ultra-short X-ray pulses it produces and which researchers use to illuminate their samples. An international team led by scientists from the Paul Scherer Institute, PSI, has now precisely measured these pulses. In so doing, they have laid the foundation for a scientifically optimal utilisation of X-ray lasers – not least, of the planned SwissFEL at PSI. The results of this work have recently been published in the scientific journal Nature Communications.

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Controversy clarified: Why two insulators together can transport electricity

How can two materials which do not conduct electricity create an electrically conducting layer when they are joined together? Since this effect was discovered in 2004, researchers have developed various hypotheses to answer this question – each with its own advocates, who defend it and try to prove its validity. Now, an international team under the leadership of researchers at the Paul Scherrer Institute has probably settled the controversy. They have shown that it is the combination of the properties of both materials that produces the effect, and therefore disproved an alternative hypothesis, which proposes that the materials mix at the interface to create a new, conducting material. The materials under study are so-called perovskites, members of a large class of materials with interesting electrical or magnetic properties that could play a significant role in electronics and computing in the future. The results have been published in the scientific journal Nature Communications.

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Physicists observe the splitting of an electron inside a solid

An electron has been observed to decay into two separate parts, each carrying a particular property of the electron: a spinon carrying its spin – the property making the electron behave as a tiny compass needle – and an orbiton carrying its orbital moment – which arises from the electron’s motion around the nucleus. These newly created particles, however, cannot leave the material in which they have been produced. This result is reported in a paper published in Nature by an international team of researchers led by experimental physicists from the Paul Scherrer Institute (Switzerland) and theoretical physicists from the IFW Dresden (Germany). 

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Three-Dimensional Spin Rotations in a Monolayer Electron System

In the emerging field of spintronics, the generation, injection, and in particular the control of highly spin polarized currents are main issues to be solved. Lifting of spin degeneracy by the spin-orbit interaction at surfaces, known as Rashba effect, represents a promising approach, since it generates two spin-polarized bands without the necessity of an external field. In our recent study, we realize such a system for a metallic surface layer on a semiconductor: Au/Ge(111). Based on a combination of spin-resolved photoemission with 3D detection and advanced density functional calculations, we have unveiled the complex spin texture at the Fermi surface. Due to the heavy atoms involved, this surface system shows a significant band splitting related to the spin-orbit interaction. Importantly, strong deviations from conventional Rashba physics occur. This pertains most prominently to a strong out-of-plane spin orientation. The spin pattern moreover shows in-plane rotations of the spin near mirror symmetry planes, which is observed for the first time. These findings pinpoint the complex spin texture found at surfaces, resulting from the potential landscape at the atomic scale. Interestingly, the spin texture bears close relationship to that predicted for the Fermi surface of topological insulators. Thus, these findings relate to a variety of topical solid-state issues, and could be stimulus for future studies of spin manipulation with regard to spintronics applications.

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Creating magnetism takes much longer than destroying it

Researchers at the Paul Scherrer Institute are finding out how long it takes to establish magnetism and how this happens. Establishing a magnetically ordered phase in the metallic alloy iron-rhodium takes much longer than the reverse process of demagnetization. This fact was established by researchers of the Paul Scherrer Institute (PSI), Switzerland, together with colleagues of an international collaboration. Magnetism is established in a two-step process. Initially, small magnetic regions form, but have random orientation. Subsequently, these regions rotate until they all have a common orientation. This is reported in an article which has recently been published in the renowned journal “Physical Review Letters”. The result comes from basic research, but has relevance for the computer industry, as it shows which processes limit the speed of magnetic data storage and where improvements might be made.

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Liquids in narrow spaces

How does spatial confinement affect the microscopic structure of liquids? This is a question which is receiving increasing attention from condensed matter physicists. Liquids are characterized by a short-ranged, so-called local structure, and it has been predicted theoretically about 25 years ago that confinement induces anisotropy in the local structure, and hence many properties, of liquids. However, this prediction had not been experimentally verified previously. Here, we have combined x-ray scattering experiments from colloid-filled nanofluidic channel arrays, a technique developed at PSI, with a state-of-the-art inhomogeneous liquid-state theory, to provide the first experimental verification of the theoretically predicted anisotropic local structure of confined liquids. The simultaneous experimental and theoretical description of confined liquids at this level allows accurate studies of the interaction mechanisms in liquids under spatial confinement.

Origin of the Large Polarization in Multiferroic YMnO₃ Thin Films

Multiferroic materials have attracted much interest because of their ability to control magnetism by the application of an electric field. This ability is expected to reduce the power required by electronic devices and to increase their speed. However, the number of multiferroic materials discovered so far has been small, and ferromagnetism and ferroelectricity in the known materials are often much weaker than required for practical applications. Hence, it is important to find novel multiferroic materials. In 2011, we succeeded in fabricating a YMnO₃ multiferroic film having a dielectric polarization exceeding that of previous multiferroic thin films, and performed x-ray diffraction measurements to investigate the magnetic structure and lattice strain of this film. We discovered that the spin of Mn ions has two coexisting magnetic structures, namely the cycloidal and E-type antiferromagntic (AF) orderings, and that the lattice strain due to E-type AF orderings is the origin of the large electric polarization. The ability to produce multiferroic YMnO₃ thin films and the understanding of the mechanism inducing the large electric polarization is expected to support the design of multiferroic materials for practical applications in the future.

A close look at correlated electrons in heavy-fermion metal through ARPES

Showing astonishing properties like magnetism, superconductivity, Kondo and heavy-fermion (HF) behavior, rare-earth intermetallic compounds have been at the forefront of modern solid state physics for many years. Most of these properties are related to a delicate interplay between the partially filled 4f-shell and conduction electrons. Studying HF system YbRh₂Si₂ we made the observation of crystal-electric field (CEF) splittings of a 4f state by means of k-resolved photoemission. Their interaction with extended valence bands can force the localized CEF-split 4f states to become dispersive and induce Fermi level crossings in specific parts of the k-space. This can change the ground-state symmetry as well as the occupancy, number, energy separation, energy order and degeneration of the CEF-split magnetic 4f states k-dependently, i.e. very different from the widely believed scenario based on non-interacting atomic-like 4f orbitals. We got direct access to the Fermi surface of this system and: (i) detected its strong f-character, (ii) disentangled its topology and features reflecting f-d coupling at the surface and bulk of the material, (iii) explored evolution of the iso-energy surfaces closely below the Fermi energy that indeed change dramatically at the meV range.

New insights into the cell’s protein factory

Eukaryotic ribosomes are among the most complex cellular machineries of the cell. These large macromolecular assemblies are responsible for the production of all proteins and are thus of pivotal importance to all forms of life. Two independent research groups at the ETH Zürich and the Institute of Genetics and Molecular and Cellular Biology in Strasbourg have obtained new insights into the atomic structure of the eukaryotic ribosome. The results have been published in the journal Science. All diffraction data were measured with synchrotron light at the Swiss Light Source macromolecular crystallography beamline X06SA at the Paul Scherrer Institute.

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Bilayer manganites reveal polarons in the midst of a metallic breakdown

The origin of colossal magnetoresistance (CMR) in manganese oxides is among the most challenging problems in condensed- matter physics today. The true nature of the low-temperature electronic phase of these materials is heavily debated. By combining photoemission and tunnelling data, we show that in the archetypal bilayer system La_2-2xSr_1+2xMn₂O₇, polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state of bilayer manganites supports a vanishing coherent quasi-particle spectral weight at the Fermi level throughout k-space. The incoherence of the charge carriers, resulting from strong electron–lattice interactions and the accompanying orbital physics, offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopies. The stacking number N is the key factor for true metallic behaviour, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in this system.

Investigation of a new method for the diagnosis of cancer in breast tissue

Collaboration between research, hospital and industry aimed at transferring innovative procedure into daily practice.


The Paul Scherrer Institute (PSI) has developed a new breast cancer diagnostic method, and is now carrying out first tests on non-preserved human tissue in conjunction with the Kantonsspital Baden AG. This new method should be able to reveal structures that cannot be seen using conventional mammography. Standard procedures only determine the extent to which X-rays are attenuated by various tissue structures. In contrast to this, the new method also makes use of the fact that X-rays actually consist of waves, and that their properties change slightly as they travel through tissue. These changes are now measurable and can contribute to the creation of a more meaningful image of the object under investigation. Scientists from the research department at Philips are currently investigating the use of this process as the basis for application in medical practice, and in mammography in particular. The researchers have reported on their results in the online edition of the “Investigative Radiology” journal.
The aim of any mammography investigation is to detect tumours in the female breast as early as possible, so that treatment can start in good time. A good mammography procedure is therefore expected to recognise as many tissue changes as possible and to distinguish tumour tissue clearly from any other tissue. At the same time, the radiation dose administered during the investigation must be kept as low as possible.

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X-ray methods help to understand brain disorders better

An international team of researchers from Denmark, Germany, Switzerland and France has developed a new method for making detailed X-ray images of brain tissue, which has been used to make the myelin sheaths of nerve fibres visible. Damage to these protective sheaths can lead to various disorders, such as multiple sclerosis. The facility for creating these images of the protective sheaths of nerve cells is being operated at the Swiss Light Source (SLS), at the Paul Scherrer Institute. The research team has reported on its work in the online version of the scientific journal NeuroImage.

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Observation of Orbital Currents in CuO

Although high-temperature (Tc) superconductivity was discovered in the cuprates 25 years ago, there is still no consensus on its microscopic origin.
The peculiar properties of the normal state are widely thought to hold the key to understanding the electronic behavior of the cuprates, including superconductivity. For this reason considerable attention has been paid to the pseudo-gap region of the phase diagram. One basic and interesting theoretical approach to describe the pseudo-gap phase predicts the existence of time-reversal symmetry breaking because of orbital currents. However, experimental verification of symmetry-breaking by orbital currents in cuprates is very difficult. So far there is indirect evidence, which is strongly debated. We have performed soft X-ray resonant diffraction with polarization analysis, a technique pioneered at the Swiss Light Source, on cupric oxide (CuO) to study orbital currents. We have been able to unambiguously observe orbital currents in a copper-oxygen plaquette, the building block of high-temperature cuprates.
The recorded diffracted intensities from our sample have a complex dependence on the X-ray polarization that can be explained only by the presence of an ordered (antiferro-type) pattern of orbital currents between the copper and the oxygen atoms.
Our observations provides strong encouragement for models based on orbital current ordering and related phenomena in high-temperature superconductors.

Observation of a ubiquitous three-dimensional superconducting gap function in optimally doped Ba_0.6K_0.4Fe₂As₂

The iron-pnictide superconductors have a layered structureformed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional¹ (3D) electronic structure of these high-temperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the k_z capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on Ba_0.6K_0.4Fe₂As₂ samples. We found a marked k_z dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer Δ₁ and interlayer Δ₂ pairing. The anisotropy ratio Δ₁/Δ₂, determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling J_a/J_ab in the parent compound². The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrains the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.

Reference

LaAlO₃ - Buckling under pressure to hand over the charges

In this paper, we report on the change in the atomic structure of the conducting interface between the insulators LaAlO₃ and SrTiO₃ as a function of the LaAlO₃ layer thickness. We discovered that the atoms at the interface buckle in an attempt to counteract the internal electric field produced when these two insulators touch one another. Despite the partial neutralizing effect of the buckling, the electrical potential becomes sufficiently high above a critical thickness to pull electrons out of the LaAlO₃ and provide the conducting layer. The electric field then collapses and the buckling is suppressed. These findings, made possible using x-ray scattering techniques capable of identifying atomic positions down to one millionth of one millionth of a meter, explain at a structural level the discoveries of the conductivity at the interface in 2004 and the need for a minimum LaAlO₃ thickness to induce it, in 2006. This phenomenon may have important technological applications in future nanoscale electronics based on metal-oxides."

Reference

Direct Determination of Large Spin-Torque Nonadiabaticity in Vortex Core Dynamics

We use a pump-probe photoemission electron microscopy technique to image the displacement of vortex cores in Permalloy discs due to the spin-torque effect during current pulse injection. Exploiting the distinctly different symmetries of the spin torques and the Oersted-field torque with respect to the vortex spin structure we determine the torques unambiguously, and we quantify the amplitude of the strongly debated nonadiabatic spin torque. The nonadiabaticity parameter is found to be β = 0:15 +/- 0:07, which is more than an order of magnitude larger than the damping constant α, pointing to strong nonadiabatic transport across the high magnetization gradient vortex spin structures.

Read the viewpoint "the alphabet of spin in nanostructures" by Rolf Allenspach and Philipp Eib

Röntgenpreis for X-Ray research goes to Christian David

On 26th November 2010, Christian David, scientist at the Laboratory for Micro and Nanotechnology, received the Röntgenpreis for research in radiation science. David pioneered a method to enhance the quality of X-ray images. He received the award jointly with Franz Pfeiffer from Technische Universität München who worked closely together with him.

The award
The Röntgen Prize is awarded annually by the University of Giessen (Germany) for new and outstanding scientific work in fundamental research in the fields of radiation physics or radiation biology. The award is named after Wilhelm Conrad Röntgen, the physicist who discovered X-rays and was a professor at Giessen during the period 1879–1888. The award was established jointly by the Pfeiffer Vacuum GmbH company and the Dr. Erich Pfeiffer-Stiftung and Ludwig-Schunk-Stiftung e.V. foundations.

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Moving Monopoles Caught on Camera - researchers make visible the movement of monopoles in an assembly of nanomagnets

For decades, researchers have been searching for magnetic monopoles – isolated magnetic charges, which can move around freely in the same way as electrical charges – since magnetic poles normally only occur in pairs. Now a team of researchers at the Paul Scherrer Institute PSI in Switzerland and University College Dublin have managed to create monpoles in the form of quasiparticles in an assembly of nanoscale magnets and to observe how they move using a microscope at the Swiss Light Source (SLS) to make the magnetic structures visible. As with the elementary monopoles, which were first predicted by the british physicist Paul Dirac in 1931, each monopole is connected by a ‘string’ to a monopole of opposite charge. The two monopoles can nevertheless move independently of each other. These results are not only of scientific interest, but could also provide a basis for the development of future electronic devices. These results will be published online in Nature Physics on 17 October.

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High-resolution method for computed nano-tomography developed

A novel nano-tomography method developed by a team of researchers from the Technische Universität München (TUM), the Paul Scherrer Institute (PSI) and the ETH Zurich opens the door to computed tomography examinations of minute structures at nanometer resolutions. The new method makes possible, for example, three-dimensional internal imaging of fragile bone structures. The first nano-CT images generated with this procedure was published in the renowned journal Nature on September 23, 2010. This new technique will facilitate advances in both life sciences and materials sciences.

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New X-ray technique distinguishes between that which previously looked the same

A new method forms the basis for the widespread use of an X-ray technique which distinguishing types of tissue that normally appear the same in conventional X-ray images

Traditional X-ray images can clearly distinguish between bones and soft tissue, with muscles, cartilage, tendons and soft-tissue tumours all look virtually identical. The phase-contrast technique developed a few years ago at the Paul Scherrer Institute enables X-ray images to be produced that clearly distinguish between these tissue types. Researchers at the Paul Scherrer Institute and the Chinese Academy of Science have now further developed the technique to such an extent that, in the future, it will be as simple to use as conventional X-rays. They anticipate that the process will help tumours to be detected in medical practices and could also help identify hazardous objects in luggage at airports. The researchers are reporting their findings this week in the online edition of the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

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Understanding plastic semiconductors better

New method allows important insights into polymer semiconductors

Semiconductors made from polymer materials are becoming increasingly important for the electronics industry – as a basis for transistors, solar cells or LEDs – showing important advantages when compared to conventional materials: they are lightweight, flexible and very cheap to produce. Usually, they consist of more than one substance as they get their particular electric properties only when several materials are blended. But in order to find the optimal material, one has to know how different polymers mix together (or don’t) and how the various components contribute to the properties of the material. Researchers from the Paul Scherrer Institute (Switzerland) and the University of Cambridge (United Kingdom) have developed a method that allows them to determine the detailed structure of the material – both in the bulk and on the surface. The investigations were performed at the Swiss Light Source SLS of the Paul Scherrer Institute.

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Observation of a d-wave nodal liquid in highly underdoped Bi₂Sr₂CaCu₂O_8+δ

A key question in condensed-matter physics is to understand how high-temperature superconductivity emerges on adding mobile charged carriers to an antiferromagnetic Mott insulator. We address this question using angle-resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the Mott insulator and the d-wave superconductor in Bi₂Sr₂CaCu₂O_8+δ. Despite a temperature-dependent resistivity characteristic of an insulator, the excitations in this intermediate state have a highly anisotropic energy gap that vanishes at four points in momentum space. This nodal-liquid state has the same gap structure as that of the d-wave superconductor but no sharp quasiparticle peaks. We observe a smooth evolution of the excitation spectrum, along with the appearance of coherent quasiparticles, as one goes through the insulator-tosuperconductor transition as a function of doping. Our results suggest that high-temperature superconductivity emerges when quantum phase coherence is established in a nonsuperconducting nodal liquid.

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Confinement-Induced Orientational Alignment of Quasi-2D Fluids

Extreme confinement is known to induce ordering of the fluid, thereby affecting its properties. However, experimental studies are hampered by the confining surfaces. In this work we show that x-ray scattering experiments on artificial fluids under extreme confinement, making use of colloidal fluids confined in diffraction gratings, can be used to determine both the average density profile and the fluid's local structure. In particular, the experiment shows how extreme confinement induces orientational alignment of the fluid, while still preserving a fluid-like structure.

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Watching atoms move: an ultrafast phase transition

One approach to advance our understanding of the complex interactions between different degrees of freedom in strongly correlated systems is to use time-resolved methods to study the response of a material after it has been driven out of equilibrium. Ultrafast optical techniques have demonstrated considerable potential to unravel the correlations that drive the interesting physics in such materials. Phonon dynamics in these studies are only indirectly observed via the electronic response, and are not generally able to unambiguously disentangle the dynamics of the lattice from those of the electronic subsystem. By using femtosecond x-ray diffraction to probe directly the structural response of photoexcited manganite, we have found evidence of an ultrafast laser-induced structural phase transition driven directly by electronic excitation and occuring on a sub-picosecond time scale.

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Advanced phase contrast imaging using a grating interferometer

Conventional absorption based X-ray microtomography can become limited for objects showing only very weak attenuation contrast at high energies. However, a wide range of samples studied in biology and materials science can produce significant phase shifts of the X-ray beam and thus phase contrast X-ray imaging can provide substantially increased contrast sensitivity. A Differential Phase Contrast (DPC) imaging facility, based on grating interferometry, has been installed at the TOMCAT beamline, with the aim of having a high-throughput of samples in terms of fast data acquisition and post-processing. We have made hardware and software advancements to enable a range of PEC tomographic imaging methods to be applied, such as local and 'widefield' PEC tomography. Darkfield imaging, based on the mechanism of small-angle scattering, provides simultaneous and complementary information about a sample at the micron and the sub-micron length scales. The technique allows the visualisation of the soft tissue features of a rat brain, for example, with a contrast impossible to obtain with conventional absorption-based imaging.

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A fast selenium derivatization strategy for crystallization

and phasing of nucleic acid structures The growing number of biologically important nucleic acid sequences (DNA and RNA) demands a fast and reliable method for their de novo three-dimensional structure determination. In this work, we described a fast and inexpensive strategy for the crystallization and phasing of structures of nucleic acid and nucleic acid/protein complexes.

In the early 1990's, covalent modification of nucleic acid using Selenium was introduced as a new approach to facilitate crystal structure determination of nucleic acid (in case of failure of classical molecular replacement or heavy atoms derivatives techniques, such modification allows to use the powerful multiwavelength anomalous dispersion technique to tackle the phase problem). Hovewer, due to a cumbersome and expensive synthesis of such Seleno-labeled nucleic acid, only few structures have been determined.

We have developed an efficient strategy for crystallization and structure determination of nucleic acids by exploiting the similar crystallization properties of 2'-SeCH3- and 2'-OCH3-modification.

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Putting the squeeze on phonons

Photon squeezing has been the subject of intense interest in the field of quantum optics, since it serves as a unique demonstration of the quantum nature of light. On a practical level, squeezing offers opportunities to make interferometric measurements much more precise than would normally be allowed by quantum uncertainty limits. In principle, the physics of squeezing may be applied to many different types of bosons. Our work demonstrates phonon squeezing by using femtosecond laser excitation of bismuth to create squeezed phonon states and then femtosecond x-ray diffraction to watch how the atomic position variance in the crystal evolves in time.

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Electrons with opposite spins move in opposite directions

In one dimension, there are only two ways to move: left or right. This leads to some peculiar properties for one-dimensional systems on the atomic scale. In our paper we present a one-dimensional conductor forming on a bismuth surface, which effectively separates the electrons going through it according to their spin, a kind of rotation around the electron's axis. It turns out that electrons going to the left have exactly the opposite spin as electrons going to the right. Such a situation could have useful applications in the field of spintronics, a novel type of electronics which is based on the electron's spin rather than its charge and which could lead to more effective computers or even quantum computing. The state reported here is in several ways similar to so-called edge states appearing in the recently discovered quantum spin Hall effect but instead of being found for a sandwich structure of semiconductors at very low temperatures, it is found on a simple, clean surface, is truly one-dimensional and, most remarkably, even exists at room temperature.

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Exciting Heavy Metal

Retrieving Structures in Photocatalysis Photocatalysts play an important role in a broad range of applications, from photochemical conversion of light energy into chemical energy through to initiating novel chemical reactions. One family of compounds that has attracted much attention is the dinuclear d8-d8 platinum, rhodium and iridium complexes that have a highly reactive electronic excited state. When photo-excited with light these systems have been shown to abstract H-atoms from a variety of substrates and initiate electron transfer processes. In this work we exam-ine the structure of the triplet excited state of a diplatinum member of this photocatalyst family.

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Building blocks of an artificial kagome spin ice: Photoemission electron microscopy of arrays of ferromagnetic islands

Arrays of dipolar coupled ferromagnetic islands arranged in specific geometries provide ideal systems to directly study frustration. We have examined with photoemission electron microscopy the magnetic configurations in three basic building blocks of an artificial kagome spin ice consisting of one, two, and three rings. The kagome spin ice arrangement is particularly interesting because it is highly frustrated and the three interactions at a vertex are equivalent. Employing dipolar energy calculations, we are able to make a full characterization of the magnetic states and therefore identify the lowest energy states. Experimentally we find that the ice rule is always obeyed even at low dipolar coupling strengths. However, as the number of rings increases there is a drastic decrease in the ability to achieve the low-energy states via demagnetization, a behavior also identified in the magnetization reversal. This carries the implication that the ground state will never be achieved in the infinite system. Finally, we show that at low coupling, the applied field direction governs the resulting states. This work opens the door to a novel class of systems for future spintronic applications.

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Super-Resolution X-ray Microscopy unveils the buried secrets of the nanoworld

A novel super-resolution X-ray microscope developed by a team of researchers from the Paul Scherrer Institut (PSI) and EPFL in Switzerland combines the high penetration power of x-rays with high spatial resolution, making it possible for the first time to shed light on the detailed interior composition of semiconductor devices and cellular structures. The first super-resolution images from this novel microscope will be published online July 18, 2008 in the journal Science. “Researchers have been working on such super-resolution microscopy concepts for electrons and x-rays for many years,” says EPFL Professor and team leader Franz Pfeiffer. “Only the construction of a dedicated multi-million Swiss-franc instrument at PSI's Swiss Light Source allowed us to achieve the stability that is necessary to implement our novel method in practice.”

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Coherent Diffraction Imaging Using Phase Front Modifications

We introduce a coherent diffractive imaging technique that utilizes multiple exposures with modifications to the phase profile of the transmitted wave front to compensate for the missing phase information. This is a single spot technique sensitive to both the transmission and phase shift through the sample. Along with the details of the method, we present results from the first proof of principle experiment. The experiment was performed with 6.0 keV X-rays, in which an estimated spatial resolution of 200 nm was achieved.

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Is Smaller Stronger?

In 2004 researcher discovered that a single crystalline metal is stronger when the sample volume is reduced to the micron or even submicron range. In an ongoing debate on the origin of this phenomenon classical deformation theories are questioned. The suspicion that structural defects, i.e. deviations from perfect crystalline structure would play an important role in the smaller is stronger effect, could not be verified because of the lack of an appropriate measuring technique. In “Time resolved Laue diffraction of deforming micro pillars” the microstructure of micron sized Au pillars is followed in real time using a micro focused white X-ray beam at the microXAS beamline of the Swiss Light Source. This newly developed technique demonstrates the occurrence of crystal rotation and shows that the increased strength of the smallest Au pillars can be explained by plasticity starting on a slip system that is geometrically not predicted, but selected because of the character of the preexisting defect structure. Time resolved Laue diffraction presents itself as a powerful technique to investigate the fundamentals of the “smaller is stronger” paradigm.

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The exciting story of TiSe2

In this story of TiSe2, experiment and theory meet to provide an explanation for a long-standing enigma. In this system, the electrons rearrange themselves spontaneously at low temperature, resulting in a new periodicity from that of the original lattice. This phase change is driven by a decrease in the total energy of the system. However, the nature of this transition has been a matter of controversy for a long time. Recently, physicists from Neuchâtel rejuvenated an old theory to explain their photoemission data taken at the SLS on TiSe2 and to provide a promising solution to this problem.

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X-ray dark-field imaging using a grating interferometer

A type of X-ray imaging that shows detail otherwise lost, and which is compatible with conventional radiography instrumentation is now feasible, reports a study published online in Nature Materials. This technique offers unprecedented resolution for several applications, including medical imaging, security screening and industrial non-destructive testing.

Dark-field imaging is commonly used in visible light microscopy, and it enables details to be resolved that are otherwise smeared out in the direct reflection mode or bright field. The quality of the dark-field image depends on the intensity of the light scattered by the object. With X-rays, however it has always been difficult to have a high enough signal-to-noise ratio, and therefore the use of X-ray dark-field imaging has usually been restricted to very-high-intensity light sources, such as synchrotrons.

In a collaborating research team led by Franz Pfeiffer (Laboratory for Synchrotron Radiation) and Christian David (Laboratory for Micro-and Nanotechnology) has shown that by using an appropriate arrangement of certain optical components it is possible to obtain a high signal-to-noise ratio even with conventional X-ray tubes, as they demonstrate by revealing the very fine structure of bones in a chicken wing.

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Pushing atoms on a swing

The typical time scale of atomic motion during fundamental physical processes such as phase transitions in solids or molecular dynamics in chemical reactions ranges from ten to hundreds of femtoseconds. The direct observation of these processes on an atomic length scale therefore requires utrashort light pulses at wavelengths capable of resolving the underlying atomic structures. For these reason significant efforts have been undertaken in the past decades to develop femtosecond sources operating in the hard x-ray spectral domain. At the Swiss Light Source we have recently commissioned an undulator source offering spatially and temporally stable x-ray pulses of ~100 fs duration that are tunable in the angstrom range. The temporal characteristics of the x-ray pulses are determined studying high-amplitude phonon dynamics of photo-excited bismuth. Optical control of real space atomic motion is successfully demonstrated.

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The conducting meat in the insulating sandwich

In 2004, it was discovered that when a layer of LaAlO3 (LAO) is in contact with a layer of SrTiO3 (STO), an ultrathin layer of highly conducting material is formed where they contact one another, despite the fact that both LAO and STO are insulators. The underlying physics responsible for this phenomenon is still much disputed, despite a worldwide concerted research drive since then to explain it. Using x-rays, the atomic structure of the interface between LAO and STO has been revealed. For the first time, the exact positions and chemical compositions of each atomic layer were defined. Using simple arguments regarding electrostatic energy minimization and the known sizes of the contributing ions, it was shown that the conducting layer consists of about three monolayers of a graded mixture of STO and LAO, which is predicted to be conducting. Thus this fascinating and potentially technologically important phenomenon could be explained based on structural arguments alone.

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Making the invisible visible

Using x-rays scientists have learned to make the invisible visible. Since almost 100 years doctors use the difference in x-ray absorption between bones and tissue to diagonose their patients. Using modern synchrotrons it has become possible to do such imaging with much reduced side effects. The new phase contrast method uses the fact that, like visible light, x-rays are deflected when traveling through objects with different densities. This allows making small density variations visible even in weakly absorbing tissue. This novel technology may now find broad applications outside of synchrotrons. Using a specially designed grating interferometer such phase contrast imaging can now be done with ordinary x-ray tubes. The technique was first tested on an insect, but after further tests and development it may well be used for human patients. Here it may soon make invisible tumors visible and allow early therapy.

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A virus in a nutshell

Nature has found remarkable ways to protect sensitive objects. One example is the seed of a nut which is protected by its shell. Another are viruses from the cypovirus family. They are hidden inside tiny natural crystals where they can survive harsh conditions until they meet their target, the gut of the silkworm. Here the virus is released from the crystal causing a virus infection of the worm. Researchers have now unraveled the structure of these natural protein crystals. This allows them to understand how and why the virus particles are so well protected by its crystal. This knowledge might not only help farmers by protecting the silkworm from the cypovirus family, much more it may lead to novel concepts for dedicated drug delivery. Imagine a beneficial drug introduced into such a crystal designed to specifically delivery its cargo to a target, the center of a disease in the human body.

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Shining light on superconductors

More than 20 years ago researchers in Switzerland discovered that certain materials transport electrical current without any loss. For this they need to be cooled, but because they do so at relatively high temperatures (up to -150°C) they are called high temperature superconductors. How exactly the electrons transport current in such materials is still a mystery. But the electrons can be studied using the photoelectric effect where light of high energy knocks an electron out of the material. Measuring the electrons direction and energy provides the necessary information on the motion of the electron in the material. Combining experiments at the SLS and laboratory experiments, researchers now solved a longstanding puzzle. They show that the direction of electron motion is closely related to the arrangement of the atoms in the crystal. Even small distortions of the crystal symmetry are relevant and influence the electrons motion. The solved a long standing dispute about the origin of the so called shadow band. But the search is not over yet, identifying the mechanism leading to loss less currents in these materials needs more experiments. And as in the past synchrotron light will surely be part of this search.

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A microscope without a lens

It is known since a long time that x-rays, which are nothing but light of very short wave length, can be used for microscopy. This is particularly attractive because due to the small wave length x-rays allow studying objects which are invisibly small in an optical microscope. Here we present a truly new kind of x-ray microscope, one which does not need lenses and can nevertheless investigate objects of arbitrary size. This so called ptychographic microscope needs coherent (laser-like) x-rays from a defined source and produces an interference pattern on a CCD camera. Translating the sample and then stitching together many such interference patterns allows to easily reconstruct the object which produced the diffraction pattern. Although similar techniques were known, this technique is new since it needs no lens, can image objects which attenuate the x-rays and shift their phase, and can image arbitrarily large objects. Furthermore the reconstruction in the computer is significantly faster, simple and unambiguous. A real breakthrough in microscopy accomplished without using a single lens.

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Looking inside fossilised embryos

Although only recently discovered, the fossil record of embryonic development has already begun to challenge cherished hypotheses on the origin of major animal groups. Synchrotron-based X-ray Tomographic Microscopy has provided unparalleled insight into the anatomy and preservation of these fossil remains and this has allowed us to test competing hypotheses on their nature. With knowledge of both adults and embryos from the time of diversification of the major animals groups, it is now possible to test models of developmental evolution based on modern model organisms using information from their long-extinct ancestors.

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How to avoid atomic sandpaper

When growing thin films of novel materials, smooth surfaces are a must. How else could one stack them layer by layer, as needed in optical coatings, sensors or conductors? One method known to produce atomically smooth films is Pulsed Laser Deposition (PLD). In PLD, a pulsed laser beam hits a bulk target. With every pulse, it creates a jet (or “plume”) of high energy atoms from the target. When these condense on a smooth substrate, they recreate the target material, but now as a thin film. Per laser pulse only about 1/100th of an atomic layer is deposited on the substrate. Although the fact that PLD produces atomically smooth films has long been known, the reasons for this were somewhat speculative. A recent experiment measured the roughness of PLD films, carefully adding one laser pulse after another. This provided the clue to the puzzle: when the first atoms land on the smooth substrate they form small islands of the material. Subsequent pulses lead either to formation of more small islands if the particles land on an uncovered part of the monolayer, or, importantly, cause already formed islands to break up if the energetic particles land on top of them. This avoids growth of "islands on top of islands", which would gradually result in rough films - that is, atomic sandpaper. Now that the process is understood, one can optimally tune PLD for growing thin films of materials which are then used in devices.

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Seeing ever finer Details with X-Rays

X-ray radiographic absorption imaging is an invaluable tool in medical diagnostics and materials science. For biological tissue samples, polymers, or fiber composites, however, the use of conventional X-ray radiography is limited due to their weak absorption. This is resolved at highly brilliant X-ray synchrotron or micro-focus sources by using phase-sensitive imaging methods to improve contrast. The requirements of the illuminating radiation mean, however, that hard x-ray phase-sensitive imaging has until now been impractical with more readily available x-ray sources, such as x-ray tubes. With the present work we show how a setup consisting of three transmission gratings can efficiently yield quantitative differential phase contrast images with conventional x-ray tubes. As opposed to existing techniques, the method requires no spatial or temporal coherence, is mechanically robust, and can be scaled up to large fields of view. The new method provides all the benefits of contrast enhanced phase-sensitive imaging, but yet is fully compatible with conventional absorption radiography. It is applicable to x-ray medical imaging, industrial non-destructive testing, and to other low-brilliance radiation, such as neutrons or atoms.

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Phase Imaging with Neutrons

Neutrons are usually considered as small massive particles with a size of about 10^-15 meters. Due to the wave-particle duality of quantum mechanics, however, they can equivalently be considered as matter wave packets whose spatial extent may be large enough to show interference effects similar to what can be observed with visible laser light. Measurements of the neutron wave packet's phase shift induced by different interactions with matter thus have a long and distinguished history in the exploration of the fundamental properties of quantum mechanics. Here we report how a setup consisting of three gratings can be used to produce images depicting the quantum-mechanical phase shifts of neutron wave packets induced by the influence of macroscopic objects. Applications aiming at the imaging of the magnetic domain structures inside macroscopic objects based on the neutrons interaction with the local magnetic field can be envisioned. Furthermore, this work could provide the basis for bridging the gap between imaging and quantum-optical investigations with other matter waves, such as protons, atoms, or molecules.

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Controlling the ribosome

Ribosomes can be thought of as factories that build proteins from a set of genetic instructions. Translation relies on two selection processes: a) charging of tRNA by selection of the correct aminoacid to be covalently bound to it, b) the selection of the tRNA as specified by the codon of the mRNA. Aminoacyl-tRNA synthetases catalyse the first of these steps using hydrolysis of ATP. In the present study the ribosome of Thermus thermophilus was cocrystallised with initiator tRNAfMet and a structured mRNA fragment which codes for threonyl-tRNA synthetase. The thrS mRNA fragment consists of the translation operator domain flanked by two single stranded regions which constitute the ribosome binding site. Crystals containing functional ribosome in complex with initiator tRNAfMet and either thrS mRNA, mk27 mRNA from the bacteriophage T4, or in the absence of mRNA were obtained under similar experimental conditions. Highly complete and redundant data were collected from 300 to 5.5 Å at beamline X06SA. The electron densities derived from the diffraction images of the crystallised ribosome complexes suggest a general way in which mRNA control elements must be placed on the ribosome to perform their regulatory task.

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Magnets shine in a different color

Measuring -or feeling- magnetic interactions looks simple at first glance: holding two magnets close to each other gives an immediate idea. How about the case when the 'magnets of interest' are tiny and amount to nothing more than atoms? X-rays generated at the Swiss Light Source allow 'zooming in' on magnetic interactions relevant at inter-atomic scale: we bring forward the first evidence of local spin flips of atomic moments in a 'photon-in photon-out' scattering experiment.

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Dance of the domains

In a ferromagnet regions exist where all the atoms orient their atomic compass neddles into one direction. These regions are called domains. A recent experiment succeeded in exciting such domains and watching their vibration and subsequent relaxation to the original state. This is the Dance of the domains. Such processes happen extremly fast, taking on the order of a few hundered picoseconds (ps), or half of a billionth of a second. The experiment showed that even in a structure as simple as a square there are three different excitations, which are associated with the domain, the walls seperating two domains and the vortex, created at the intersection of two domains. It was possible to analyze all of these processes quantitativly and determine the frequencies. Fortunately the frequencies are high, several GHz. The domains dance at a high beat, which is important in every day life, because these domains are used to store information on the hard-disk of your PC, and the faster the beat, the faster a PC can store data.

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