Condensed matter physics

Artemis has two experimental stations for condensed matter physics: one for time- and angle-resolved photoemission spectroscopy (ARPES) and one for ultrafast demagnetisation.

ARPES end-station

The ARPES end station

The Artemis ARPES end-station is designed for time- and angle-resolved photoemission spectroscopy. The end-station consists of a main mu-metal chamber for photoemission with a base pressure of 2x10^-10mbar. This chamber is equipped with a hemispherical electron analyser (SPECS Phoibos 100). The energy- and angle-resolved measurements are performed with a two-dimensional CCD detector, achieving an ultimate energy resolution of ~5 meV and angular resolution of <1°. Our current energy resolution is 130 meV, limited by the bandwidth of the XUV harmonic generated with a 30fs pulse.

A liquid-helium-cooled, five-axis manipulator (azimuthal and polar angles) enables us to orientate the sample crystallographic axis with the measurement plane and to cool the sample to 14 K (in <30 minutes) or e-beam heat it to 1000 K. The surface crystallographic order is checked using a low-energy electron diffraction (LEED) analyser. A helium discharge lamp emitting at 21.2 eV enables off-line characterisation of the sample surface. This chamber is also equipped with a wobble stick for in situ sample cleaving.

The main chamber is connected to the beamline with a window valve and to a sample preparation chamber. This second UHV chamber is dedicated to Ar ion sputtering, e-beam heating and thin film growth. Further flanges are available for users’ evaporators and sample storage capability is available. The sample is introduced to the preparation chamber through a fast load-lock chamber (sample turn-around time of 30 min) and then transferred to the experimental chamber with a transfer arm.

Time- and angle-resolved photoemission with XUV pulses

Time-resolved photoemission measurement on TaS2 with 17eV

Until recently, tr-ARPES measurements with lasers have typically used only near UV radiation (<7 eV) and pulses of 100 fs or longer. The low photon energy meant that only a small subset of electrons, with certain energies and travelling in certain directions, could be ejected from the material and detected. The long pulselength meant that it was impossible to see the fastest changes to the material.

The Artemis beamline is one of the first in the world to overcome these limitations by using XUV pulses from high-order laser harmonics, with 20 eV photon energy and 30 fs time resolution. XUV pulses are created through the technique of high harmonic generation. A short pulse laser is focused into a gas-jet and interacts with the gas, producing even shorter pulses of coherent radiation in the 10-100 nm wavelength range. The higher photon energy enables electrons with a much wider range of energy and momentum space to be detected, meaning that each snapshot of electronic structure has a much wider field of view.

Our first time-resolved photoemission measurement with XUV pulses was performed on TaS₂ at low temperature. The sample was pumped with 1.55 eV (800 nm) photon energy and probed with the 11^th harmonic from the beamline. At zero time delay a clear collapse of the Mott gap was observed, characteristic of a photo-induced insulator-to-metal transition. The rigid shift oscillation observed at positive time delay is an amplitude mode of the charge density wave corresponding to the breathing of the Ta cluster. This work was published in Physical Review Letters (link opens in a new window).

We have now demonstrated both time- and angle- resolved photoemission measurements with XUV pulses on a number of materials, including graphene and topological insulators - see our list of publications for links to recently submitted work on arXiv. 

Ultrafast demagnetisation end-station

This photoemission chamber is dedicated to time-resolved studies on ferromagnetic systems and will be available for users from early 2014. In-situ Magneto Optic Kerr Effect (MOKE), photoemission and spin-resolved photoemission are the probe techniques available for measuring the magnetic properties of the sample. The main UHV chamber is equipped with a spin-resolved time-of-flight (ToF-Spin) analyser combining time-of-flight with a Mott polarimeter for the spin detection. The analyser has been successfully used at 6.2 eV photon energy with ultrafast laser sources [1-3]. Operation at higher photon energy will be available in the future after further commissioning time. Due to the 1 kHz repetition rate of the laser system, spin-resolved measurements can only be performed with large angular acceptance.

Standard photoemission with high statistics are also accessible by inserting an electron detector (MCP) at the end of the ToF drift tube. Connected to the monochromatised XUV beamline, this gives access to the valence band electronic structure and to the Linear Magnetic Dichroism measured at the M-edge of the ferromagnetic 3d-transition metal (Ni, Co, Fe).

The chamber is equipped with pulsed magnetic coils to reverse the sample magnetisation between measurements. Sample preparation equipment (sputtering, annealing) to grow thin ferromagnetic films is available. The MOKE measurement can be used as a reference measurement to characterise the macroscopic demagnetisation.

1 - C Cacho et al., Spin-resolved two-photon photoemission on Fe77B16Si5 alloy (link opens in a new window), J Elec Spec Rel Phenom 169 62-66 (2009)

2 - C Cacho et al., Absolute spin calibration of an electron spin polarimeter by spin-resolved photoemission from the Au(111) surface states (link opens in a new window), Rev. Sci. Instrum. 80, 043904 (2009)

3 - W. Wang et al., Fe t2g band dispersion and spin polarization in thin films of Fe3O4(0 0 1)/MgO(0 0 1): Half-metallicity of magnetite revisited (link opens in a new window), Phys. Rev. B 87, 085118 (2013)