Condensed matter end-stations
16 Jan 2021
Yes
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Artemis end stations for experiments in condensed matter:

Yes

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​Artemis offers the following end stations for experiments in condensed matter:


Please discuss your experimental requirements with beamline scientists Charlotte Sanders (charlotte.sanders@stfc.ac.uk) or Yu Zhang (yu.zhang@stfc.ac.uk) before you submit your proposal.

TR-A​RPES
​The analysis chamber—which can dock to either the 1-kHz or the 100-kHz beamline—is built around a SPECS Phoibos-100 hemispherical analyzer.  It offers the capabilities for pump-probe ARPES, static ARPES with a He lamp, low-energy electron diffraction (LEED) and Auger spectroscopy.  Sample cleaving, heating, and cooling can be done in the analysis chamber (see above); however, please discuss your requirements with beamline staff before you submit your proposal.
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There is also a preparation chamber, which is connected to the analysis chamber by a gate valve.  Evaporation is possible in the preparation chamber (DN40 and DN63 flanges are available for mounting evaporators), as is sample exposure to non-toxic gases via a leak valve and sample cleaving.  The sample garage can hold up to four samples at a time, and the load lock can hold up to two samples at a time.

​​The system uses flag-style sample holders.  If you would like, we can send you sample plates in advance of your beamtime for mounting samples.  Please discuss your sample mounting requirements with the beamline scientists.

The pressure in both the preparation and analysis chambers is in the low-10-10 mbar range.

Table 1: ARPES end-station parameters ​

 Parameters
ARPES end-station
​Angle of incidence of beam on sample at normal emission
​45 deg
​Analyzer slit direction
​Horizontal—radial from Γ
​Sample cooling
​~20K by open-cycle liquid-He cooling
​Sample heating
​Analysis chamber:
radiative heating; or, up to 1270 K by electron bombardment of the back of the sample.
Preparation chamber:
radiative heating; or, up to 1270 K by electron bombardment of the back of the sample.

​Manipulator degrees of freedom
​4 motorized (x, y, z, polar angle); 1 manual (azimuthal)

TR-Spin-ToF 
The goal of this development project is to offer pump-probe measurements of k vs E dependency with spin resolution, for the study of material systems with spin-polarized band structures.  The system consists of a ToF analyzer (25-cm-long drift tube) with a mini-Mott polarimeter for spin-resolved measurements.  A microchannel plate can be inserted in front of the polarimeter, by means of a linear translator, when spin-integrated measurements are needed.  The sample manipulator allows measurement temperatures down to 20 K, and up to approximately 1000 K.  There is a preparation chamber separated by a gate valve from the analysis chamber, for simple sample preparation such as sputtering and annealing.
The system has so far been used in successful experiments with a 250-kHz-repetition-rate laser source [1,2], and we are now looking to commission the system on the new 100-kHz beamline.  If you have are interested in taking part in this effort as a commissioner, please contact yu.zhang@stfc.ac.uk​.​

Photon parameters for the XUV beamlines  
The parameters listed below are intended to give a general idea of the capabilities of the XUV beamlines, when used for photoemission woth condensed matter samples.  There is some flexibility in these -although, of course, some are coupled and cannot be independently controlled.  (For example, time resolution sets limits on energy resolution, and vice versa.) Therefore, please take the numbers below as a starting point for discussion with the beamline scientists, who will try to help you find the best arrangements for your particular experiment.

Table 2: Photon parameters for the 1 kHz and new 100 kHz beamlines

​Photon parameters
​1 kHz beamline
​100 kHz beamline (under construction - commissioning planned for 2021)
​Probe energy range
​15 ~ 45 eV
​2021-22:  15 ~ 45 eV
Higher energies to be available in future calls.
​Pump energy range
​λ = 235 nm – 16 μm, tunable
​2021-22:  λ = 850 – 900 nm, 1700 – 1800 nm, and 2650 – 2950 nm.
Additional options will be available in future calls.
​Energy resolution
(limited by short-pulse characteristics and by space charge)
​250 ~ 400 meV
(Photon-energy and pulse-length dependent; discuss with beamline staff what resolution is available for your experimental parameters.)

​~ 100 meV expected
(Photon-energy and pulse-length dependent; discuss with beamline staff what resolution is available for your experimental parameters.)

​Time resolution
(limited by pulse lengths of pump and probe)

​~ Few tens of fs
​~ 50 fs expected
​Beam spotsize
at normal emission

​~ Few hundred μm diameter
​~ Few hundred μm expected; smaller spot sizes to be pursued over the course of subsequent development
​Probe flux
​~106 photons/pulse (~109 photons/second) at about 25 eV
​Expected:  ~105 photons/pulse (~1010 photons/second) at about 25 eV
​Pump fluence
​Typically several mJ/cm2/pulse (dependent on choice of pump wavelength)
​Expected: several mJ/cm2/pulse.  Under development.

​Probe polarization

​Linear polarization, s or p
​Linear polarization, s or p
​Pump polarization
​Fully controllable
​Fully controllable


​Further technical notes for preparing a user proposal

Requirements for samples (crystallinity, domain size, etc.)

Sample orientation, crystallinity, and domain size

If k-vs.-E dispersions are to be measured, the sample needs to be single-crystalline within the region of analysis (i.e., within the size of the beamspot on the sample).  If more than one crystalline domain is present inside the analysis region, the measured dispersion will be the sum of the dispersions of all the domains within the region.

Cleanliness

Within the photon energy range available at Artemis, the universal mean free path is very short [3]—meaning that measurements are extremely surface-sensitive.  It is therefore important that the sample surface be extremely clean.  Typically, samples are prepared in vacuum by cleaving, annealing, decapping, or sputtering/annealing.

Sample quality​

Photoemission linewidth becomes broad when defect density becomes high.  Good sample quality is key to acquiring data of sufficient quality for successful analysis.

Sample electronic properties

Insulating samples are, typically, inappropriate for photoemission spectroscopy, since the photoemission process then leads to sample charging.  Some exceptions exist—e.g., atomically thin insulating layers on conducting substrates are sometimes feasible.  E-mail the beamline scientists to discuss this issue.

Probe photon energy

The photoemitting (probe) laser beam is generated by high-harmonic generation in a gas jet.  You must specify your required probe photon energy in your proposal (see the beamline capabilities above) so that the probe beam can be set up and optimised before you arrive for your beamtime.  Please communicate with the beamline staff before you submit your proposal, to confirm that the probe photon energy and flux you need is available that your requested energy tolerance is feasible.  The beam photon energy can be fine-tuned during your beamtime (in steps of 2ω), but major changes to probe photon energy will require realignment and a significant loss of beamtime.

Pump-probe issues

Pump photon energy and polarization

Available photon energies are shown above.  Any polarization is possible, and polarization-dependent measurements are also possible.  Both the pump photon energy and the polarization need to be specified in the beamtime application, so that the beamline can be set up in advance of the experiment.  For polarization-dependent measurements, a motorized polarizer will be inserted into the beamline and incorporated into the experimental acquisition and control software.
Pump photon energy cannot be changed on-the-fly.  If you will require more than one pump photon energy for your experiment, contact beamline staff before submitting your proposal, so we can discuss possible options.

Acquisition time

Pump-probe techniques are intrinsically time consuming, since each delay time will constitute one full spectral acquisition—potentially on the time scale of up to a few hours, depending on laser repetition rate, on fluence limitations (especially to prevent space charge [4, 5]), and on statistics.  It is typically feasible to probe, e.g., 10 - 20 delay points selected to span the expected time scales of phenomena of interest.  If polarization dependence is to be studied, then the number of measurement points will be multiplied accordingly.  The new 100-kHz laser system will allow improved statistics, reduced space charge, and more favourable acquisition times (possibly improved by as much as 20x for some measurements).  Nevertheless, in building a feasible proposal, you should take acquisition time into account.


References

[1] Cacho, et al., “Absolute spin calibration of an electron spin polarimeter by spin-resolved photoemission from the Au(111) surface states,” Rev. Sci. Instr. 80 (2009) 043904. https://doi.org/10.1063/1.3115213
[2] Cacho, et al., “Momentum-Resolved Spin Dynamics of Bulk and Surface Excited States in the Topological Insulator Bi2Se3,” Phys. Rev. Lett., 114 (2015) 097401. https://doi.org/10.1103/PhysRevLett.114.097401​
[3] Seah and Dench, “Quantitative Electron Spectroscopy of Surfaces:  A Standard Data Base for Electron Inelastic Mean Free Paths in Solids,” Surf. Interface Analysis 1 (1979) 2. https://doi.org/10.1002/sia.740010103
[4] Hellmann, et al., “Vacuum space-charge effects in solid-state photoemission,” Phys. Rev. B 79 (2009) 035402. https://doi.org/10.1103/PhysRevB.79.035402​
[5] Zhou, et al., “Space charge effect and mirror charge effect in photoemission spectroscopy,​ ” J. Elec. Spectr. and Rel. Phenom. 142 (2005) 27. https://doi.org/10.1016/j.elspec.2004.08.004




Contact: Sanders, Charlotte (STFC,RAL,CLF)