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-crystal. The top-post will be used to cleave the sample in ultra-high vacuum to expose an atomically clean surface.</li>
	<li>Mount the sample holder in the load lock.</li>
</ol>
2.	Achieving Ultra-high Vacuum and Thermal Isolation
<ol>
 <li>Begin evacuating the load lock to minimize contamination of the ultra-high vacuum chamber.</li>
 <li>Monitor the pressure. Once a pressure of about 10-8 mbar has been achieved, transfer the assembly to the preparation chamber, and subsequently to the main chamber.The cold finger and sample holder have been specially designed to guarantee the best possible thermal contact with the helium pot.</li>
 <li>These demonstration versions show how this is achieved by using conical surfaces to increase the area of contact. The conical surfaces are pressed against one another and the sample holder and cold finger are firmly fixed in place using a titanium nut and bolt.</li>
</ol>
3.	Positioning and Cooling the Sample 
<ol>
 <li>The next step is to orient the sample within the cold finger along the azimuth using the transfer arm. Fix the position of the sample by tightening the nut while applying a counterforce with the supporting arm attached to the opposite side of the chamber.</li>
 <li>Cleave the sample by moving the manipulator upwards so that the top-post is removed by interaction with the supporting arm.</li>
 <li>With the beam shutter closed, move the sample into position in the beamline using the manipulator. Once the sample is in place, make sure the cryoshields are closed properly.</li>
 <li>Start pumping on the 1-K pot and circulate the helium-3 gas in order to cool the sample to the base temperature. The temperature is measured close to the sample and will not change during the experiment.</li>
 <li>Open the beam shutter of the beamline. Use the micrometer screws on the apparatus to adjust the position of the sample so that it is at the focus of the analyzer lens. This adjustment is crucial.</li>
</ol>
4.	Collecting Data
<ol>
 <li>Once the setup is ready, switch to the angle-resolved mode of the analyzer and record the spectrum in swept mode. This will generate data for two-dimensional energy-angle plots.</li>
 <li>Construct a Fermi surface map using the data. Select polar angles that correspond to Fermi level crossings for study of the superconducting gap of strontium ruthenate.</li>
 <li>Record high-resolution spectra at the chosen polar angles above and below the superconducting transition temperature of strontium ruthenate to investigate the behavior of the superconducting gap.</li>
</ol>

<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>

No conflicts of interest declared.

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 states, band structures and momentum dependent Fermi velocities allows one to characterize the electronic structure on a general level. Comparison with ab-initio calculations yields bandwidth- and Fermi velocity renormalizations and thus determines the complexity of the material in terms of the strength of correlations. Fine structures near the Fermi level often provide the possibility to detect the fingerprints of interaction between electrons and other degrees of freedom, like phonons, plasmons, spin-fluctuations etc. Systematic momentum dependent studies can identify the dominant interaction and single out, for example, a pairing mediator in superconductors or density wave systems. More thorough investigations involve the determination of the symmetry of the order parameter, thus providing critical tests for existing theories or stimulating new approaches on a fundamental level.
As in every experimental method, there are certain drawbacks. It is known that photoelectrons are strongly scattered inside the crystal, thus having a relatively short inelastic mean free path. As a result, the escape depth can be very small, down to several lattice constants. This defines the sensitivity of the method to the surface and in some cases the electronic structure of the surface is indeed different from the bulk 6. However, ARPES offers many tools to monitor this situation. One of them is the use of various excitation photon energies. As has been mentioned previously, for a given photon energy it is possible to also estimate the component of momentum perpendicular to the surface. Observed periodic structures in ARPES spectra allow one to determine absolute kz and the corresponding momentum resolution gives uncertainty of this quantity. In such a way the experimental value of the escape depth can be estimated from the uncertainty principle &delta;kz * &delta;z ~ 1. The next tool to control the surface sensitivity of the method is variable polarization of the light. It was demonstrated earlier that using circularly polarized light it is possible to distinguish between surface and bulk contributions to the photoemission signal 7. Another advantage of using various polarizations and photon energies delivered by the synchrotron light source is the possibility to disentangle matrix element effects from genuine features of the spectral function. The matrix element is a transition probability which can suppress photoemission signal in particular regions of momentum space and result in misinterpretation of the ARPES spectra 8-10.
Obviously, the method is not very suitable for strongly 3D materials, which are difficult to cleave in situ and obtain atomically clean and flat surfaces. Finally, ARPES on insulators is much more complicated since it is necessary to compensate charging occurring because the flux of outgoing electrons cannot be compensated by electrical contact with the sample holder 11.

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 importance for detailed studies of the low-energy electronic structure of solids is the angle-resolved spectrum of the dispersing feature. Such an example is shown in the middle panel of Figure 3. A very sharp superconducting peak is observed in the iron-based superconductor LiFeAs 1 representing one of the sharpest features ever detected by ARPES. The same holds for momentum resolution. The FWHM of 0.23 &deg; is the record value for the wide angular mode of electron energy analyzers. The system has been designed to combine three achievements, 1 meV bandwidth of the beamline, 1 meV resolution of the analyzer and 1 K temperature of the sample. This aim gave the name to our system "1-cubed ARPES". If all the three components were minimized, one would expect the FWHM~ 1.4 meV. Our current measurements show that overall resolution of the order of 2 meV can be achieved.
Another representative result is our investigation of the electronic structure of the superconductor Sr2RuO4 with critical temperature 1.35 K. This material is a well-known oxide possessing a wide spectrum of interesting physical properties. It was the first discovered superconducting oxide after the cuprates 2. Its superconducting state is unusual: electrons coupled in pairs have their spins oriented in the same direction. This is the so called triplet superconductivity. It still remains not well understood and the main problem is to define the symmetry of the superconducting order parameter. In order to do that, the superconducting energy gap should be determined as a function of momentum - the exact task for ARPES. Because such low temperatures needed for superconductivity were not accessible in photoemission experiments before, it was not possible to address this problem. Here we make an attempt to do this. First of all, one needs to determine the Fermi surface. For this purpose we have recorded many cuts at different polar angles. Some of them are shown for illustration in Figure 4. Now, if we consider intensity only in the vicinity of the Fermi level, and plot it as a function of both angles, we will obtain the locus of Fermi momenta, i.e. the Fermi surface map. Such a Fermi surface map is shown in Figure 5 together with the calculated Fermi surface 3. The agreement is very good, but the experimental data show many more features. Some of them are unexpected and unusual 4. Now we can try to measure the superconducting gap. For this we have to record the spectra above and below the critical temperature, i.e. 1.35 K. In Figure 6 we show such pairs of spectra. We indeed observe some changes, compatible with the opening of the energy gap, but even the present energy resolution (or perhaps also the temperature, which can be higher right at the surface of the sample) does not allow us to draw a definite conclusion as for the energy gap in Sr2RuO4.
<img alt="Figure 1" src="/files/ftp_upload/50129/50129fig1.jpg" /> 
 Figure 1. Schematic of the experimental setup.
<img alt="Figure 2" src="/files/ftp_upload/50129/50129fig2.jpg" /> 
 Figure 2. Left panel. Example of photoemission intensity distribution as a function of kinetic energy and angle. The band structure of the solid is directly seen. Right panel. Results of the band structure calculations for the same material. Theoretical data are taken from 5.
<img alt="Figure 3" fo:content-width="4.5in" fo:src="/files/ftp_upload/50129/50129fig3highres.jpg" src="/files/ftp_upload/50129/50129fig3.jpg" /> 
 Figure 3. Performance of the experimental end-station. Left panel. Fermi edge of freshly evaporated indium film. Middle panel. Energy distribution curve (EDC) in a superconducting sample of LiFeAs. Data are taken from 1. Right panel. Momentum distribution curve (MDC) at the Fermi level of ZrTe3. Anticipated overall energy resolution of the system is expressed by the formula (1K= 0.0862 meV). The actual performance of the system is very close to the one we initially aimed for. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/50129/50129fig3large.jpg" target="_blank">Click here to view larger figure</a>.
<img alt="Figure 4" fo:content-width="5in" fo:src="/files/ftp_upload/50129/50129fig4highres.jpg" src="/files/ftp_upload/50129/50129fig4.jpg" /> 
 Figure 4. Energy-momentum cuts recorded at different polar angles (step 10) for Sr2RuO4. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/50129/50129fig4large.jpg" target="_blank">Click here to view larger figure</a>.
<img alt="Figure 5" fo:content-width="5in" fo:src="/files/ftp_upload/50129/50129fig5highres.jpg" src="/files/ftp_upload/50129/50129fig5.jpg" /> 
 Figure 5. Fermi surface map of Sr2RuO4 taken using 80 eV linearly polarized light at ~1K. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/50129/50129fig5large.jpg" target="_blank">Click here to view larger figure</a>.
<img alt="Figure 6" src="/files/ftp_upload/50129/50129fig6.jpg" /> 
 Figure 6. a, b Typical spectra taken to study the superconducting gap of Sr2RuO4. Red arrow indicates the momentum corresponding to a single energy distribution curve (EDC). T= 970 mK. c, d Shift of the leading edge of the integrated EDC. Momentum window is represented by the width of the red arrow. The gap corresponds to a FS point on the band near the BZ diagonal. e Shift of the kF EDC as a function of temperature from another point on the FS. f Typical temperature behavior of the binding energy of the leading edge in the vicinity of the crossing of two FSs.

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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17.5K Views.  IFW-Dresden. 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.

Video: Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

Synthesis and Microdiffraction at Extreme Pressures and Temperatures

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.

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary ...

Scanning-probe Single-electron Capacitance Spectroscopy

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material1,2,3. In principle, this technique can be applied to any system to resolve electron motion below an insulating surface.
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.

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the ...

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

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states are indeed the resources for implementing various protocols, ranging from enhanced metrology to quantum communication and computing. A variety of devices can be used to generate non-classical states, such as single emitters, light-matter interfaces or non-linear systems3. We focus here on the use of a continuous-wave optical parametric oscillator3,4. This system is based on a non-linear &#967;2 crystal inserted inside an optical cavity and it is now well-known as a very efficient source of non-classical light, such as single-mode or two-mode squeezed vacuum depending on the crystal phase matching. 
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.

Engineering non-classical states of the electromagnetic field is a central quest for quantum optics1,2. Beyond their fundamental significance, such states ...

Investigating Single Molecule Adhesion by Atomic Force Spectroscopy

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. At the same time the AFM tip is passivated to prevent unspecific interactions between the tip and the substrate, which is a prerequisite to study single molecules attached to the AFM tip. Analyses to determine the adhesion force, the adhesion length, and the free energy of these molecules on solid surfaces and bio-interfaces are shortly presented and external references for further reading are provided. Example molecules are the poly(amino acid) polytyrosine, the graft polymer PI-g-PS and the phospholipid POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine). These molecules are desorbed from different surfaces like CH3-SAMs, hydrogen terminated diamond and supported lipid bilayers under various solvent conditions. Finally, the advantages of force spectroscopic single molecule experiments are discussed including means to decide if truly a single molecule has been studied in the experiment.

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.

Atomic force spectroscopy is an ideal tool to study molecules at surfaces and interfaces. An experimental protocol to couple a large variety of single ...

Resonance Raman Spectroscopy of Extreme Nanowires and Other 1D Systems

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the presence of carbon nanotubes. These nanowires are extreme in the sense that they are the limit of miniaturization of nanowires and their behavior is not always a simple extrapolation of the behavior of larger nanowires as their diameter decreases. The paper then describes the methods required to obtain Raman spectra from extreme nanowires and the fact that due to the van Hove singularities that 1D systems exhibit in their optical density of states, that determining the correct choice of photon excitation energy is critical. It describes the techniques required to determine the photon energy dependence of the resonances observed in Raman spectroscopy of 1D systems and in particular how to obtain measurements of Raman cross-sections with better than 8% noise and measure the variation in the resonance as a function of sample temperature. The paper describes the importance of ensuring that the Raman scattering is linearly proportional to the intensity of the laser excitation intensity. It also describes how to use the polarization dependence of the Raman scattering to separate Raman scattering of the encapsulated 1D systems from those of other extraneous components in any sample.

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.

This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the ...

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon&#39;s energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

The manuscript describes a method of phonon-assisted quasi-resonant fluorescence spectroscopy that incorporates both laser-limited resolution and photoluminescence (PL) spectroscopy. This method utilizes optical phonons to provide linewidth-limited resolution spectra of atom-like semiconductor structures in the energy domain. The method is also easily realized with a single spectrometer optical spectroscopy setup.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we ...

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

Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted researchers&#39; attention in many fields. Most of the studies focused on macroscopic LCE structures (films, fibers) and their miniaturization is still in its infancy. Recently developed lithography techniques, e.g., mask exposure and replica molding, only allow for creating 2D structures on LCE thin films. Direct laser writing (DLW) opens access to truly 3D fabrication in the microscopic scale. However, controlling the actuation topology and dynamics at the same length scale remains a challenge.
In this paper we report on a method to control the liquid crystal (LC) molecular alignment in the LCE microstructures of arbitrary three-dimensional shape. This was made possible by a combination of direct laser writing for both the LCE structures as well as for micrograting patterns inducing local LC alignment. Several types of grating patterns were used to introduce different LC alignments, which can be subsequently patterned into the LCE structures. This protocol allows one to obtain LCE microstructures with engineered alignments able to perform multiple opto-mechanical actuation, thus being capable of multiple functionalities. Applications can be foreseen in the fields of tunable photonics, micro-robotics, lab-on-chip technology and others.

Here, we fabricate 3D polymeric micro/nano structures in which both the shape and the molecular alignment can be engineered with nanometer scale accuracy by the use of direct laser writing. Light induced deformation of several types of liquid crystalline elastomer microstructures can be controlled in the microscopic scale.

Liquid crystalline elastomers (LCEs) are smart materials capable of reversible shape-change in response to external stimuli, and have attracted ...

Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

Here, we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. Optical property evaluation of thin films and the surfaces of bulk materials is an important step in understanding new optical material systems and their applications. The method presented can estimate thickness, refractive index, and use absorptive properties of materials for detection. This metrology system uses evanescent field-based photoacoustics (EFPA), a field of research based upon the interaction of an evanescent field with the photoacoustic effect. This interaction and its resulting family of techniques allow the technique to probe optical properties within a few hundred nanometers of the sample surface. This optical near field allows for the highly accurate estimation of material properties on the same scale as the field itself such as refractive index and film thickness. With the use of EFPA and its sub techniques such as total internal reflection photoacoustic spectroscopy (TIRPAS) and optical tunneling photoacoustic spectroscopy (OTPAS), it is possible to evaluate a material at the nanoscale in a consolidated instrument without the need for many instruments and experiments that may be cost prohibitive.

Here we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. This technique evanescent field-based photoacoustics can be used to create a photoacoustic metrology system to estimate materials&#39; thicknesses, bulk and thin film refractive indices, and explore their optical properties.

Here, we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. ...

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.

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and ...

Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures

We demonstrate a method for determining the vitreous phase cryogenic temperature densities of aqueous mixtures, and other samples that require rapid cooling, to prepare the desired cryogenic temperature phase. Microliter to picoliter size drops are cooled by projection into a liquid nitrogen-argon (N2-Ar) mixture. The cryogenic temperature phase of the drop is evaluated using a visual assay that correlates with X-ray diffraction measurements. The density of the liquid N2-Ar mixture is adjusted by adding N2 or Ar until the drop becomes neutrally buoyant. The density of this mixture and thus of the drop is determined using a test mass and Archimedes principle. With appropriate care in drop preparation, management of gas above the liquid cryogen mixture to minimize icing, and regular mixing of the cryogenic mixture to prevent density stratification and phase separation, densities accurate to &#60;0.5% of drops as small as 50 pL can readily be determined. Measurements on aqueous cryoprotectant mixtures provide insight into cryoprotectant action, and provide quantitative data to facilitate thermal contraction matching in biological cryopreservation.

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 ...