Name: angle-resolved photoemission spectroscopy at ultra-low temperatures
Uploaded: 2012-10-09T12:55:00
Description: Watch this Scientific Journal Video about Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures at JoVE.com

We gratefully acknowledge the help of Rolf Follath, Roland H&uuml;bel, K. M&ouml;hler, Dmytro Inosov, J&ouml;rg Fink, Andreas Koitzsch, Bernd B&uuml;chner, Andrei Varykhalov, Emile Rienks, Oliver Rader, Setti Thirupathaiah, Denis Vyalikh, Sergey Molodtsov, Clemens Laubschat, Ramona Weber, Hermann D&uuml;rr, Wolfgang Eberhardt, Christian Jung, Thomas Blume, Gerd Reichardt, David Batchelor, Kai Godehusen, Martin Knupfer, Stefan Le&szlig;ny, Dirk Lindackers, Stefan Leger, Ralf Voigtl&auml;nder, Ronny Sch&ouml;nfelder, who conceived the "1-cubed" project, designed, constructed and commissioned the beamline and end-station as well as provided organizational and user support.
The project "1-cubed ARPES" has been funded by BMBF grant "Highest Resolution ARPES" as well as directly by BESSYII and IFW-Dresden. This particular work was supported by the DFG priority program SPP1458, grants ZA 654/1-1, BO1912/3-1 and BO1912/2-2. E.C. and B.P.D. thank the Faculty of Science at the University of Johannesburg for travel funding. A.V., R.F. and M.C. acknowledge support from the EU -FP7/2007-2013 under grant agreement N. 264098 - MAMA.

1.	Mounting the Sample
<ol>
 <li>This experiment uses synchrotron radiation produced by the BESSY storage ring of Helmholtz-Zentrum Berlin. The photons travel a beamline to our end-station where a sample is mounted.</li>
	<li>Begin with a single crystal of the material to be investigated, here strontium ruthenate. Use silver-based epoxy to glue the sample to the sample holder. The silver-based epoxy ensures good thermal and electrical contact.</li>
	<li>Glue an aluminum top-post to the surface of the single-cry...

<ol>
	<li>Borisenko, S. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=One-Sign+Order+Parameter+in+Iron+Based+Superconductor.">One-Sign Order Parameter in Iron Based Superconductor.</a> Symmetry. 4, 251-264 (2012).</li><li>Maeno, Y., Hashimoto, H., Yoshida, K., Nishizaki, S., Fujita, T., Bednorz, J. G., Lichtenberg, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superconductivity+in+a+layered+perovskite+without+copper.">Superconductivity in a layered perovskite without copper.</a> Nature (London). 372, 532 (1994).</li><li>Singh, D. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Relationship+of+Sr2RuO4+to+the+superconducting+layered+cuprates.">Relationship of Sr2RuO4 to the superconducting layered cuprates.</a> Phys. Rev. B. 52, 1358 (1995).</li><li>Zabolotnyy, V. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+and+bulk+electronic+structure+of+the+unconventional+superconductor+Sr2RuO4:+unusual+splitting+of+the+&#946;+band.">Surface and bulk electronic structure of the unconventional superconductor Sr2RuO4: unusual splitting of the &#946; band.</a> New Journal of Physics. 14, 63039 (2012).</li><li>St&#246;we, K., Wagner, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Crystal+Structure+and+Calculated+Electronic+Band+Structure+of+ZrTe3.">Crystal Structure and Calculated Electronic Band Structure of ZrTe3.</a> Journal of Solid State Chemistry. 138, 160-168 (1998).</li><li>Zabolotnyy, V. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Momentum+and+temperature+dependence+of+renormalization+effects+in+the+high-temperature+superconductor+YBa2Cu3O7-&#948;.">Momentum and temperature dependence of renormalization effects in the high-temperature superconductor YBa2Cu3O7-&#948;.</a> Phys. Rev. B. 76, 064519 (2007).</li><li>Zabolotnyy, V. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Disentangling+surface+and+bulk+photoemission+using+circularly+polarized+light.">Disentangling surface and bulk photoemission using circularly polarized light.</a> Phys. Rev. B. 76, 024502 (2007).</li><li>Kordyuk, A. A., Borisenko, S. V., Kim, T. K., Nenkov, K. A., Knupfer, M., Fink, J., Golden, M. S., Berger, H., Follath, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Origin+of+the+Peak-Dip-Hump+Line+Shape+in+the+Superconducting-State+(&#960;,0)+Photoemission+Spectra+of+Bi2Sr2CaCu2O8.">Origin of the Peak-Dip-Hump Line Shape in the Superconducting-State (&#960;,0) Photoemission Spectra of Bi2Sr2CaCu2O8.</a> Phys. Rev. Lett. 89, 077003 (2002).</li><li>Inosov, D. S., Fink, J., Kordyuk, A. A., Borisenko, S. V., Zabolotnyy, V. B., Schuster, R., Knupfer, M., B?chner, B., Follath, R., D?rr, H. A., Eberhardt, W., Hinkov, V., Keimer, B., Berger, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Momentum+and+Energy+Dependence+of+the+Anomalous+High-Energy+Dispersion+in+the+Electronic+Structure+of+High+Temperature+Superconductors.">Momentum and Energy Dependence of the Anomalous High-Energy Dispersion in the Electronic Structure of High Temperature Superconductors.</a> Phys. Rev. Lett. 99, 237002 (2007).</li><li>Inosov, D. S., Schuster, R., Kordyuk, A. A., Fink, J., Borisenko, S. V., Zabolotnyy, V. B., Evtushinsky, D. V., Knupfer, M., B?chner, B., Follath, R., Berger, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Excitation+energy+map+of+high-energy+dispersion+anomalies+in+cuprates.">Excitation energy map of high-energy dispersion anomalies in cuprates.</a> Phys. Rev. B. 77, 212504 (2008).</li><li>H&#252;fner, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Photoelectron Spectroscopy, Principles and Applications. , (1996).</li></ol>

As is shown above, the implemented method is very efficient in studying the low-energy electronic structure of single crystals. Recent instrumental improvements have turned ARPES from a mere characterization and band-mapping tool into a sophisticated many-body spectroscopy. A modern experiment delivers information about the electronic structure of a solid or a nano-object with a new level of precision. Access to the Fermi surface in the case of a metal, energy gaps of semiconductors and insulators, their surface st...

Nowadays ARPES is widely used to determine the electronic structure of solids. Usually, different variations of this method are defined by the source of the radiation needed to excite the electrons. We use synchrotron radiation since it offers a unique opportunity to tune the polarization and the excitation photon energy in a wide energy range and is characterized by high intensity, small bandwidth (uncertainty in energy hn) and it can be focused to a narrow beam to collect photoelectrons from a spot of a few tens of microns. Synchrotron radiation is generated in electron storage rings forcing electrons circulating in the ring with an energy of the order of 2 GeV ** to pass through periodic arrangements of strong magnets (undulators). The magnetic field deflects the electrons and when such fast electrons change their direction they emit radiation. Exactly this radiation is then directed into the so called beamline where it is further monochromatized by a diffraction grating and focused on the surface of the sample by several mirrors. There are many such facilities around the world. Our end-station is located at one of the beamlines of the BESSY storage ring which belongs to the Helmholtz-Zentrum Berlin.
The heart of this ARPES facility is the electron-energy analyzer (Figure 1). Since we are interested in both the kinetic energy and angle at which electrons leave the surface, it is very convenient to detect them in one measurement. A very simple principle makes this approach a reality. As in a basic experiment with an optical lens, which focuses a plane wave into the point in the back focal plane thus performing spatial Fourier transformation, the electron optical lens projects electrons which left the surface at particular angle to a point in the focal plane (Figure 1). In such a way we gain access to the reciprocal, i.e. momentum, space. The distance from the forward direction in the focal plane corresponds to the angle and thus to the momentum of the photoelectron. Now the electrons have to be analyzed in terms of energy. For this purpose the entrance slit of the hemispherical analyzer is placed exactly in the focal plane of the electron optical lens. Voltages on two hemispheres are chosen such that only electrons with particular kinetic energy (pass energy) will be guided exactly in the middle of two hemispheres and land on the central line of the two dimensional detector. Those which are faster will hit the detector closer to the outer hemisphere; those which are slower will be deflected towards the inner hemisphere. In such a way we can get the photoemission intensity distribution as a function of angle and kinetic energy simultaneously.
The main advantage of our approach over existing methods is the use of the He3 cryomanipulator. There are at least two reasons to carry out the measurements at low temperatures. The higher the temperature of the material, the more smeared out the electronic states become in energy and momentum. To determine the electronic structure with high precision this temperature broadening has to be avoided. Also, many physical properties are temperature dependent, some ordering phenomena set in at low temperatures and the knowledge of the electronic structure in the ground state of the system, i.e. at T=0, is of fundamental importance. One of the most effective ways to cool down the sample down to tenths of a Kelvin is to liquefy He3 gas. In many experiments reaching sub-Kelvin temperatures is not a problem, since thermal radiation, the main enemy of ultra-low temperatures, can be effectively shielded. Unfortunately, this is not the case in photoemission experiments. We need to provide free access for the incoming light and outgoing electrons. This is realized by specially designed slits in three radiation shields, having different temperatures. In order to compensate for the heat load caused by the photon beam and room-temperature radiation, the cooling power of the cryostat should be very high. This is achieved by the very large pumping speed of two pumps which reduce the vapor pressure above the liquid He3, thus cooling the cold finger and sample. The design specifications of our He3 system make it the most powerful worldwide. It is perhaps the only place on the planet where one can see a 1 K cold surface through a room temperature window, the "coldest visible".
The sketch of the modern photoemission experiment is shown in Figure 1. The synchrotron beam (dashed green line) illuminates the 1 K cold surface of the sample and excites photoelectrons. Electrons are projected to the entrance slit of the hemispherical analyzer, sorted in terms of angle (yellow, magenta and cyan traces correspond to different tilt angles) and then are analyzed in terms of kinetic energy. Figure 2 shows the typical intensity distribution as a function of tilt angle and kinetic energy. Such an intensity distribution is indeed expected as the comparison with band structure calculations of this material shows (right panel). This is our window into reciprocal space.
By scanning voltages on the lens and hemispheres and rotating the sample around the vertical axis (polar angle) we can explore the broad binding energy range as well as wide regions of the reciprocal space with unprecedented detail. In particular, plotting intensity at the Fermi level as a function of both components of the in-plane momentum, calculated from tilt and polar angles, we have direct access to the Fermi surface (FS).
* Under "uncertainty" we understand the experimenter's best estimate of how far an experimental quantity might be from the "true value."
** The low-energy ring can have an energy of ~0.8 GeV, the high-energy one - up to 8 GeV.

<table border="1">
 <tbody>
 <tr>
 <td>Name of Reagent/Material</td>
 <td>Company</td>
 <td>Catalogue Number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>Single crystals of ZrTe3 and TiSe2</td>
 <td></td>
 <td></td>
 <td>grown by Dr Helmut Berger, EPFL, Lausanne</td>
 </tr>
 <tr>
 <td>Single crystals of Sr2RuO4</td>
 <td></td>
 <td></td>
 <td>grown by the group of Dr Antonio Vecchione</td>
 </tr>
 <tr>
 <td></td>
 <td></td>
 <td></td>
 <td>SAMPLES ZrTe3, TiSe2, Sr2RuO4</td>
 </tr>
 </tbody>
</table>

angle-resolved photoemission spectroscopy at ultra-low temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (&omega;) and momentum (k) and the probability to find them in a particular state with given &omega; and k is described by the spectral function A(k, &omega;). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.
In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

The ultra-low temperatures of our setup together with the high resolution of the beamline and analyzer allow us to record spectra with very high overall resolution. This is illustrated in Figure 3. The usual test of the energy resolution is to measure the width of the Fermi edge of a metal. In this case it is a freshly evaporated indium film. The full width at half maximum (FWHM) of the Gaussian, which when convoluted with the step-function precisely describes the edge, is of the order of 2 meV. Of more ...

Watch this Scientific Journal Video about Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures at JoVE.com

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The overall goal of this method is to determine the low-energy electronic structure of solids at ultra-low temperatures using Angle-Resolved Photoemission Spectroscopy with synchrotron radiation.

The physical properties of a material are defined by its  electronic structure. Electrons in solids are characterized by energy (&omega;)  and momentum ...

angle-resolved-photoemission-spectroscopy-at-ultra-low-temperatures

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High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary interiors, design materials with novel properties, understand the mechanical behavior of materials exposed to very high stresses as in explosions or impacts. Synthesis and structural analysis of materials at extreme conditions of pressure and temperature entails remarkable technical challenges. In the laser heated diamond anvil cell (LH-DAC), very high pressure is generated between the tips of two opposing diamond anvils forced against each other; focused infrared laser beams, shined through the diamonds, allow to reach very high temperatures on samples absorbing the laser radiation. When the LH-DAC is installed in a synchrotron beamline that provides extremely brilliant x-ray radiation, the structure of materials under extreme conditions can be probed in situ. LH-DAC samples, although very small, can show highly variable grain size, phase and chemical composition. In order to obtain the high resolution structural analysis and the most comprehensive characterization of a sample, we collect diffraction data in 2D grids and combine powder, single crystal and multigrain diffraction techniques. Representative results obtained in the synthesis of a new iron oxide, Fe4O5 1 will be shown.

The laser heated diamond anvil cell combined with synchrotron micro-diffraction techniques allows researchers to explore the nature and properties of new phases of matter at extreme pressure and temperature (PT) conditions. Heterogeneous samples can be characterized in situ under high pressure by 2D mapping and combined powder, single-crystal and multigrain diffraction approaches.

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Scanning-probe Single-electron Capacitance Spectroscopy

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As in other electric-field-sensitive scanned-probe techniques4, the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants1,2. The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz&frac12; at 0.3 K 5.

Scanning-probe single-electron capacitance spectroscopy facilitates the study of single-electron motion in localized subsurface regions. A sensitive charge-detection circuit is incorporated into a cryogenic scanning probe microscope to investigate small systems of dopant atoms beneath the surface of semiconductor samples.

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Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators

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Squeezed vacuum is a Gaussian state as its quadrature distributions follow a Gaussian statistics. However, it has been shown that number of protocols require non-Gaussian states5. Generating directly such states is a difficult task and would require strong &#967;3 non-linearities. Another procedure, probabilistic but heralded, consists in using a measurement-induced non-linearity via a conditional preparation technique operated on Gaussian states. Here, we detail this generation protocol for two non-Gaussian states, the single-photon state and a superposition of coherent states, using two differently phase-matched parametric oscillators as primary resources. This technique enables achievement of a high fidelity with the targeted state and generation of the state in a well-controlled spatiotemporal mode.

We describe the reliable generation of non-Gaussian states of traveling optical fields, including single-photon states and coherent state superpositions, using a conditional preparation method operated on the non-classical light emitted by optical parametric oscillators. Type-I and type-II phase-matched oscillators are considered and common procedures, such as the required frequency filtering or the high-efficiency quantum state characterization by homodyning, are detailed.

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A protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. Procedures and examples to determine the adhesion force and free energy of these molecules on solid supports and bio-interfaces are provided.

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The paper describes a method for producing extreme nanowires by melt infiltration into carbon nanotubes and how 1D systems may be characterized and investigated using Resonance Raman Spectroscopy to determine vibrational and optical excitation energies.

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Free-form Light Actuators &amp;#8212; Fabrication and Control of Actuation in Microscopic Scale

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A Simple Dewar/Cryostat for Thermally Equilibrating Samples at Known Temperatures for Accurate Cryogenic Luminescence Measurements

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and a CCD spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, fast sample freezing, fast alignment of the sample, accurate and stable sample temperatures, and small size and portability of the Dewar/cryostat cryogenic unit. When coupled with the fast data acquisition rates of the CCD spectrograph, this Dewar/cryostat is capable of supporting cryogenic luminescence spectroscopic measurements on luminescent samples at a series of known, stable temperatures in the 77-300 K range. A temperature-dependent study of the oxygen quenching of luminescence in a rhodium(III) transition metal complex is presented as an example of the type of investigation possible with this Dewar/cryostat. In the context of this apparatus, a stable temperature for cryogenic spectroscopy means a luminescent sample that is thermally equilibrated with either liquid nitrogen or gaseous nitrogen at a known measureable temperature that does not vary (&#916;T &#60; 0.1 K) during the short time scale (~1-10 sec) of the spectroscopic measurement by the CCD. The Dewar/cryostat works by taking advantage of the positive thermal gradient dT/dh that develops above liquid nitrogen level in the Dewar where h is the height of the sample above the liquid nitrogen level. The slow evaporation of the liquid nitrogen results in a slow increase in h over several hours and a consequent slow increase in the sample temperature T over this time period. A quickly acquired luminescence spectrum effectively catches the sample at a constant, thermally equilibrated temperature.

A simple liquid nitrogen Dewar/cryostat apparatus comprised of a small fused silica optical Dewar, a thermocouple, and a charge-coupled device (CCD) spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, freezing, and alignment, accurate and stable sample temperatures, and small size/portability.

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A protocol for determining the vitreous phase densities of micro- to pico-liter size drops of aqueous mixtures at cryogenic temperatures is described.

We demonstrate a method for determining the vitreous phase cryogenic temperature densities of aqueous mixtures, and other samples that require rapid ...