ISSP + Laser

Ultrahigh resolution laser-angle-resolved photoemission spectroscopy
T. Kiss1, K. Ishizaka1,2 and S. Shin1,2
Institute for Solid State Physics(ISSP), University of Tokyo, Kashiwa, Chiba 277-8581,Japan2CREST, Japan Science and Technology Corporation (JST), Kashiwa, Chiba 277-8581, Japan1

It is well known that the variety of properties that
conducting materials display stems from theelectronic structure at and near the Fermi level (EF).
One of the most fascinating examples issuperconductivity, in which pairing of two electrons
makes a tiny energy gap at EF, leading to unexpected
physical properties. Angle-resolved photoemission
spectroscopy (ARPES) is a very powerful method tomeasure electronic structure at and below EF as it canprovide the energy() and momentum(k) resolved
spectral function A(k,) of a solid. ARPES can thus
measure band dispersions and Fermi surface topology
as a function of temperature(T).[1] It has contributed
significantly to our understanding of the electronic
structure of solids. In the angle-integrated version
also, PES has given us valuable results allowing
comparisons with other techniques which probe theone-electron removal function, such as tunnelingspectroscopy. Since the discovery of high-Tc
superconductors, ARPES studies have followed aremarkable progress in energy and momentum
resolution, driven by the purpose of determining thesuperconducting gap symmetry. These studies haveusually employed synchrotron radiation or gas
discharge lamps as a photon source coupled to a
photoemission spectrometer, and for high energy
resolution achieved using a He discharge lamp (He I :21.218 eV). PES is also usually considered to be very
surface sensitive as tunneling spectroscopy, it iswell-known that the escape depth of photoelectrons
show strong kinetic-energy dependence : it is highly
surface sensitive for kinetic energies between 20- 50
eV, but can be made more bulk sensitive by changing
the kinetic energy to very low and very highenergies.[2] This can be done by simply changing theincident photon energy. Consequently, the increase of
escape depth using soft-x rays (h ~ 1000 eV ) has
been demonstrated with a resolution of ~100 meV.[3]This energy resolution is good enough to study theoverall electronic structure but is about 100 times
larger than, for example, the SC gap energy scale of alow-Tc superconductor, typically less than 1 meV. Theenergy resolution using synchrotron radiation with
20-30 eV photon energy is ~ 5 meV (recently ~ 2meV below 10eV photon energy) for solid-state
studies, which is again not enough for studying very
low energy scale low temperature electronicstructures. While PES using a He I resonance line
(21.2 eV) can produce a ~ 1-2 meV resolution, theescape depths using a photon energy of ~ 20 eV areless than 10 Å. This may not be suitable for studying
materials having a surface electronic structurecompletely different from the bulk, as is known forsome correlated-electron systems.
To achieve the ultrahigh energy-momentum
resolution and high bulk sensitivity, we havedeveloped a low-temperature ultrahigh resolution
system for polarization dependent ARPES using avacuum ultra-violet (VUV) laser ( h = 6.994 eV) asa photon source. With the aim of addressing low
energy physics, we show the system performancewith angle-integrated PES at the highest energy
resolution of 360 eV and the lowest temperature of2.9 K.[4] At present, we have been achieved 150 meVof energy resolution and 1.8 K of lowest temperaturein advanced 7 eV laser ARPES system. These newphotoemission systems demonstrate that the ultrahighresolution ARPES is quite important to understandelectronic property of materials.

Fig. 1. (a) Schematic diagram of the LPES system.
(b) Ultrahigh-resolution PES spectrum of anevaporated gold film measured at 2.9 K (red circles),together with the FD function at 2.9 K convolved by aGaussian with full width at half maximum of 360
eV (a blue line).
[1] A. Kaminski et al., Nature 416, 610 (2002).
[2] M.P. Seah and W.A. Dench, Surface. Interface.
Anal. 1, 2 (1979).
[3] A. Sekiyama et al., Nature 403, 396 (2000).
[4] T. Kiss et al., Rev. Sci. Instrum. 79, 023106(2008).