STORIES

February 2004

Development of the Angle-Resolved Photoemission (ARPES) end-station equipped with the highly efficient electron spectrometer SES-100 brought a new challenge for the beamline staff and the community of scientists interested in performing ARPES experiments. The large gap between the resolving power delivered by the existing variable line space (VLS) plane-grating monochromator (~1200, or deltaE = 50 meV at 60 eV photon energy) and the energy resolution of the SES-100 (~ 5 meV) called for a sharp boost of the resolving power of the monochromator. Although a ten-fold increase of the resolving power on a beamline designed for high throughput and low resolution seemed a huge stretch, the yearlong effort shared by the SSG and CXRO staff was a success, and a resolving power of 10,000 has been obtained.  This was achieved with a new VLS grating which, unlike the original 200 lines per millimeter grating, was blazed to operate in a negative diffraction order. The new grating, designed by Eric Gullikson of the CXRO, has both 300 lines and 600 lines per millimeter rulings on a single substrate.  The grating was ruled by T. Tanaka at Hitachi High-Technologies on a gold-coated substrate supplied by the Center. The coating process developed at the CXRO proved to be the ticket to creating a high efficiency grating (see Fig. 1). Figure 2 displays spectrum of double-excitation Rydberg series in He measured using 600 lines/mm grating. The total width of the 2p3d resonance is ~6 meV. This number translates into the resolving power of 10,000. 

June 2006

Our initial plan to install the high order light supressing filters during the two-bunch mode in August has been abruptly altered when the vacuum leak in the bellow of the rotary feed-through mounted on the sample manipulator has opened about three weeks ago. Since venting of the main chamber was unavoidable, we have decided to vent a mirror tank too and to install the filters. Four different filters (Mg, Al, Si, and B) have been installed. Al and B filters have been supplied by the LeBow Corporation and CXRO has made a Si filter. Obtaining a good Mg filter was particularly challenging due to the reactive nature of this material. This filter was manufactured by Luxel Inc. which had to go through a several attempts to sandwich thin Mg foil between the protective layers of aluminum. The final product was a Mg film (0.2 micron) sandwiched between 150-Angstrom thick aluminum layers. All filters have been attached to the rectangular frames with 1-inch wide circular apertures. 

May 2007

Some years ago, the “National Electrostatic Corporation” (Middleton, Wisconsin) has manufactured the photon beam position monitorts for the Atomic and Molecular Science brunches of BL10.0.1 The devise consists of the helically shaped wire constantly spinning inside the photon beam pipe. When photon beam hits the spinning wire it generates photoelectrons producing the electron current between the wire and the surrounding collector. If photocurrent is displayed versus the angle of rotation, it usually shows two peaks representing beam profiles along two orthogonal directions X and Y. The devise proved to be a very valuable tool for aligning photon beams. However, the model installed at BL10 utilized lubricated ball bearings. Lubrication ensured a good lifetime of a constantly spinning device. On the other hand, it prohibited use of the Beam Position Monitors (BPM`s) at other beamlines where UHV compatibility outlaws any lubricants. In attempt to get an UHV compatible BPM, the ALS has ordered custom units equipped with the dichronite ball bearings using no lubricants. These BPM`s delivered about a year ago had a very short lifetime due to the failure of non-lubricated (UHV-friendly) ball bearings. On overage, they run for less than a week.

To fix the problem at BL12 we have decided to use commercial magnetically coupled rotary feed-through (we bought mini-CF rotary manufactured by the "UHV-Design", Lewes, England) driven by the stepper motor with angular encoder. Hence, we threw away magnetically coupled feed-through (and original ball bearings) which was an integral part of the BPM as well as an asynchronous motor driving it. The stepper motor with encoder gave as a flexibility of running BPM at desired speeds, e.g. high speed for the fast feedback or low speed for the higher sensitivity. The latter option should be beneficial for the bending magnet beamlines with low photon flux. It also increases the lifetime of the BPM. Encoder has eliminated the original BPM controller with rather complicated electric circuitry providing the reference signal for calculating angular position of the spinning helical wire. Use of the commercial rotary feed-through ensures easy and quick treatment of the future failures of the bearings. Signals from the BPM (photocurrent) and encoder (angular position) are feed into the beamline software which displays beam positions (and profiles) along two orthogonal directions. 

September 2010

It looks as at BL12 a black hole has developed which traps everyone interested in topological insulators. Several research groups have plunged in and managed to send out important messages. Through recent publication in “Nature Physics” advanced online publication, September 19, 2010;  group of Prof. Zahid Hasan (Princeton University) has communicated studies of Bi2Se3 doped with Cu. This sample is the first known example of three-dimensional topological insulator which is also a superconductor in the bulk. It has a transition temperature of ~3.8 Kelvin. Superconductivity develops in the bulk of the sample while its top layer is characterized by the presence of the topological surface state. Now we dream of future measurements on these and similar samples when improved sample manipulator would be capable of reaching such low temperatures. Another flurry of activity on topological insulators took place during the two-bunch run when fully commissioned and configured to work with cleavable samples TOF spectrometer was brought to the beamline by the group of Prof. Alessandra Lanzara at UC Berkeley. For two weeks it has occupied place originally claimed by the new end-station for angle resolved photoemission. The run has produced much awaited data on spin structure of topological surface states. Experiments were conducted in collaboration with the ALS staff and members of the group lead by Prof.  Z.-X. Shen in Stanford University. BL12 is bound to see more activity on topological insulators since it is receiving ever increasing number of new proposals wishing to study these materials. Studies of more conventional samples also resulted in publications. Prof. Kevin Smith and his group (Boston University) reported their work on surfaces of ZnO (Phys. Rev. B 81, 233305). Lastly, a sole fact of “being around for a long time” has helped Alexei to score a publication when he was invited by the editors of Phys. Rev. B to write a viewpoint for “Physics” (Physics 3, 46).

December 2010

Eligibility of the “publish-or-perish” approach to measuring productivity in science is a popular discussion topic in many scientific journals, with many voting for its abolishment, for it creates unhealthy pressures. The truth, however, is that everyone is usually impressed by the gleaming publication records of celebrity scientists. Scientific facilities are not different in this respect and the Advanced Light Source can proudly report that during the period covered by the upcoming review by the Department of Energy, which is from 2008 to 2011, it has done exceptionally well supporting research relying on the technique of photoelectron spectroscopy. Its major photoemission beamlines (three ARPES beamlines: 7.0.1, 10.0.1 and 12.01, and two high-pressure XPS beamlines: 9.3.2 and 11.0.2) have contributed to 45 papers published in: “Nature”, “Science” and “Physical Review Letters” which are recognized as “premier journals”. Of these 45 papers, 24 were published in PRL, 17 were printed in Nature journals (“Nature”, “Nature Physics” and “Nature Materials”) and 4 have appeared in “Science”. To help to appreciate this achievement we should mention that “paper count” of other synchrotrons renowned for their research in photoemission is considerably lower. BESSY is next to the ALS with 29 papers, while SSRL, SRC, Swiss Light Source and Hiroshima Radiation Center can each claim about 20 papers. Interestingly, 45 high profile publications of the ALS are split almost equally between its three ARPES beamlines (7, 10 and 12). Statistics is a captive field. Someone might go farther and calculate amount of beam-time shifts, manpower, dollars, sleepless nights, etc. which have been spent for every publication.

March 2011

This year at least one invited and nine regular talks that were given by the users of BL12 at the annual March meeting of the American Physical Society (Dallas, March 21-25) contained photoemission data taken at the ALS. The talks were mainly devoted to the studies of topological insulators and graphene, which remain the hottest subjects of the condensed matter physics. Graphene has already yielded the Nobel Prize. Luckily, topological insulators will share same degree of recognition as they might constitute devices transmitting signals without losses (they are not yet “room-temperature superconductors” though, but we shall see). Hence, photoemission at beamline 12 is still at the forefront of the photoemission research. Planned addition of the spectrometer for spin-resolved photoemission will further strengthen its leadership.

May 2011

A few good and not so good things happened at BL12 during this month. We start with the good news: Crew of the photoemission branch of BL12 has doubled after Peng Zhang, a graduate student from the Institute of Physics, Chinese Academy of Science, has joined on May 2. Peng comes from the group of Prof. Hong Din. He will be stationed at BL12.0.1 for one year. More good news: Paper on the superconductivity in KC6 has appeared in Phys. Rev. Lett. (http://prl.aps.org/abstract/PRL/v106/i18/e187002). Paper on the epitaxial grows of high quality films of topological insulator Bi2Se3 has been published in Journal of Applied Physics (http://m.jap.aip.org/resource/1/japiau/v109/i10/p103702_s1). Several interesting papers have been accepted for publication. We will write about them after they come out from the print. Bad news: We noticed that electron lens in our new VG-Scienta spectrometer had a slight tilt. Close inspection of the lens concluded that it became loose. We had to remove spectrometer from the vacuum chamber and investigate the problem. We found that two out of six ceramic rings holding the lens elements together were broken. We believe they broke during the routine bake of the chamber and that the cause of the problem is the faulty design of the lens assembly which did not properly consider different thermal expansions of the materials used in the lens. Our findings will be communicated to the manufacturer. In a mean time the good solution has been found and implemented. Naturally, we were not left alone to face all the problems. Hence, we are thankful to the following staff members who gave us a helpful hand: Monroe Thomas, Derrick Crofoot, John Pepper, Kyle McCombs, and Andrew Mei.

July 2011

Two papers based on the angle-resolved photoemission (ARPES) data taken at the beamline 12.0.1 and 10.0.1 appear on the Proceedings of the National Academy of Sciences (PNAS). The paper lead by Colorado group, in collaboration with LBNL and ANL, investigates colossal magneto-resistance material (La, Sr)2Mn2O7. They find a strong variation in electronic structure at the metal-insulator transition boundary accompanies the melting of bi-stripes. The presence of localized electrons even below the critical temperature suggests that the melting bi-stripes exist in a disordered or fluctuating state. An UC-LBNL collaboration produced a paper  regarding the electronic structure of quasi-freestanding graphene at the charge neutrality point, i.e., when the chemical potential coincides with the Dirac energy. They find many body interactions in monolayer, graphene changes strongly as the doping level reaches the charge neutrality point. The electron-electron interaction results in unique renormalization, much different from ordinary metals, while the electron-phonon interaction diminishes. Both articles are also featured in LBNL News Center: http://newscenter.lbl.gov/feature-stories/2011/07/14/manganite-stripes/ and  http://newscenter.lbl.gov/feature-stories/2011/07/14/graphene-secrets/

November 2011

A disaster occurred at the photoemission branch of BL12 when a user wished to study a single crystal of antimony (Sb) by ARPES and wanted to clean the surface of the sample “in situ” using tools of the sample preparation/loading chamber attached to the main photoemission facility. Alexei--overwhelmed with a dense sequence of experiments that demanded various methods of preparation of very different samples--had no time to thoroughly investigate the accumulated knowledge on how to deal with antimony and had advised the user to check the literature. The user had found a reference(s) reporting preparation of atomically clean antimony via cycles of sputtering and annealing. Before starting the first cycle of sample preparation, Alexei decided to degas the sample and its holder by heating them to about 200C which, according to the user, was just a fraction of annealing temperature reported in the reference he had found. Disaster occurred after approximately 20 minutes of degassing when the antimony sample started to sublimate, covering inside of the preparation chamber. Since antimony is a toxic material, the entire chamber and its guts had to be disassembled and cleaned in Bldg. 77 leaving Alexei with only few days to put everything back together for the next experiment scheduled for a user coming from China. Luckily, after a great deal of hard work, he was able to meet the deadline, hence enabling the next scheduled experiment (which yielded a very impressive set of data!). As to the ARPES study of antimony, it actually has ended up with some success since the user had another sample which Alexei was able to clean after developing his own recipe of sample preparation. The moral: regardless of your workload and circumstances, always check what users want to do in the chamber and perform the literature search yourself even if the users come from a credible group/university. Since this user also conducts experiments at beamlines 4 (Merlin) and 10 (HERS), Alexei has promptly shared his experience and advised colleagues on how to deal with antimony samples (and the user). Doug Taube helped with the clean-up, and John Pepper (and his team) helped with the mechanical repairs.

April 2012

The ALS experimental floor is densely covered with shiny stainless chambers. Curiously, most of them are empty. Seriously, the majority of the ALS experiments shall be conducted under vacuum since even a small contamination of the sample by the residual atoms and molecules can surely spoil it and ruin the experiment. Recently in the photoemission chamber of BL12.0.1, we have witnessed vacuum, which is only rarely achieved on Earth: 1.7x10E-11 Torr. Getting such vacuum was not easy. We had to scrupulously inspect every inch of the chamber searching for tiny leaks. According to NASA, similar vacuum conditions exist at the surface of the Moon (Mars has only 5 Torr), while vacuum around the International Space Station is about 1E-9 Torr. Vacuum of the outer space has inspired proposals to grow clean materials in the space. In essence, a few MBE samples have been grown during several Space Shuttle missions. It would be interesting to compare the cost of growing samples in space to the cost of making them on Earth inside our chamber or any other chamber with similar vacuum. All joking aside, the users have appreciated our effort, after recognizing that their samples last longer and survive numerous cycles of cool-downs and warm-ups.

October 2012

Over the summer we have made a few significant steps towards establishing facility for spin-resolved photoemission at the ALS. Alexei has designed a couple of new chambers which will house electron spectrometer and tools for sample preparation. By the time this note was done been written the chambers were already on board of the UPS truck heading from NorCal production facility in Yreka to LBNL. Amuneal Manufacturing Company of Philadelphia has been charged with the design and production of the magnetic shields for the analysis chamber. As usual, John Pepper was around to help with the cool design ideas. It is important to remember than the only operational beamline for spin-resolved photoemission in the country is stationed at the NSLS (Brookhaven) which will be turned off soon. Hence, here at the ALS we ought to maintain a steady progress and commission our facility soon. 

March 2013

Although it might seem to the uninitiated observer that the daily routines of the beamline scientists include working on bolts and nuts or hunting for liquid helium and other scarce commodities while hoping to experience probably their most rewarding acquaintance, that is watching happy and satisfied users leaving their instruments loaded with tons of good data, the job of beamline scientist offers a lot more. From time to time we are called to share our expertise by serving on various study panels and committees. A few months ago, I was invited by the Deutsche Forschungsgemeinschaft to work with a group of experts helping to distribute millions of Euros between proposals submitted in a response to the new priority program. The job which included reviewing proposals and attending the meeting in Berlin was exhausting but very rewarding.

July 2013

The Advanced Light Source has always been recognized for its successful programs on photoemission. Our photoemission facilities are employed by a diverse scientific community representing nearly every branch of condensed matter physics. This well-established technique is posed to capitalize on the capabilities offered by the new light source facilities. Recently I have helped to organize a one day workshop “Photoemission Spectroscopy - the upcoming decades” http://pml.nist.gov/Meetings/SRI2013/program/workshop-5.html which was held during the SRI-2013 conference at NIST (June19-21, NIST, Gaithersburg, MD). The workshop, which included 13 invited presentations by speakers from the United States, France, Germany and Italy, was well attended by the conferees. The talks encompassed recent studies of magnetic and strongly correlated materials with time resolved photoemission spectroscopy; instrumentation developments enabling such studies using HHG`s and FEL`s, and theoretical aspects of pump/probe photoemission experiments. The latter topic was highlighted by James Freericks from Georgetown University. Oliver Gessner (Lawrence Berkeley National Lab.), who has lead the first time resolved photoemission work conducted at the LCLS in Stanford, talked about studies of charge transfer in dye-sensitized nano-crystals. Research opportunities provided by the new photoemission instruments, synchrotron beamlines and FEL facilities were outlined by Alexei Barinov (ELTTRA), Fausto Sirotti (SOLEIL), Serguei Molodtsov (European XFEL) and Stephen Benson (Jefferson Lab.). Christopher Smallwood (UC Berkeley), Stephan Mathias (University of Kaiserslautern), Marino Marci (University of Paris Sud) and Tomasz Durakiewic (Los Alamos National Lab.) talked about studies of strongly correlated materials using laboratory based HHG sources. They also gave an overview of the latest developments in the high harmonic generation laser sources. Much attention has been paid to the photoemission studies of electron spins in “classic” magnetic materials as well as topological insulators whose peculiar spin structure holds many promises. Martin Teichmann (Max-Born Institute Berlin and Free University Berlin) has talked about ultrafast demagnetization dynamics in gadolinium, while Dan Dessau (University of Colorado Boulder) and Nuh Gedick (Massachusetts Institute of Technology) presented photoemission studies of the helical spin structure in topological insulators. It was clear that this community practicing all aspects of time resolved photoemission spectroscopy is producing very interesting data, which is often leading to dramatic advances in various areas of fundamental and applied research. The participants emphasized the need for the development of a FEL facility targeted specifically towards ultrafast time-resolved photoemission. Decades of photoemission research at the synchrotron beamlines have proved that ~10 to~150 eV is the most useful range of photon energies. Successful photoemission experiments at an FEL will likely rely on such energies as well. 

September 2013

Two groups have consumed the bulk of the beamtime in September: Dong Qian (Shanghai Jiao Tong University) studied thin films of bismuth and Shuyun Zhou (Tsinghua University) looked at various topological insulators. Collaboration with the latter group has produced a paper in Nature Physics.  Collaboration with the Princeton University and Brookhaven Lab has resulted in the paper published in the Phys. Rev. B