Development of Laser-ARPES System for the Study of the Electronic Structure of Unconventional Superconductors

概要

Abstract
Development of Laser-ARPES System for the Study of the Electronic Structure of Unconventional
Superconductors
(非従来型超伝導体の電子構造の研究のためのレーザーARPES システムの開発)
Wumiti Mansuer

Superconductors are among the most expected materials that would make
huge changes in our lives. They present remarkable physical properties
such as zero-resistance and diamagnetism below a certain critical
temperature Tc. After many years of investigations, finally, the first
microscopic theory of superconductivity in metals was formulated which
is widely known as the BCS theory. The central idea of this theory is a
weak electron-phonon interaction which leads to the appearance of an
attractive potential between two electrons. As a consequence, they form
the Cooper Pairs which have some bosonic properties, and bosons, at a
sufficiently low temperature, can form a large Bose-Einstein condensate.
However, in 1986, the high-Tc superconductors were discovered in copperoxide system (cuprates). The maximum record of Tc has exceeded the
prediction a lot much higher and the mechanism of such high-Tc
superconductors could not be explained by the conventional BCS theory
anymore. This unexpected founding trigged the investigation boom of the
high-Tc superconductors all over the world.
For cuprate superconductors, the hybridization of copper dx2-y2 orbital
and oxygen px/py orbital forms the low-energy electronic band responsible
for the superconductivity. Replacing small amount of Cu with another
transition-metal element, one can introduce impurities right on the CuO2
planes, namely the stage of high-Tc superconductivity, and consequently
reduce the maximum critical temperature Tcmax. This provides us with a
good opportunity to investigate the relation between the energy gap and
the high-Tc superconductivity.

Physical properties of cuprate superconductors differ from each other,
depending on the number of cupper-oxide layers stacked in a unit cell. As
an example, it causes obvious increase in critical temperature Tc when the
number of cupper-oxide layers increases from 1 to 3. Such an interesting
feature of cuprates indicates a kind of interaction between cupper-oxide
layers. Interaction between two cupper-oxide layers in Bi2Sr2CaCu2O8+δ
(Bi2212) results in the splitting of electronic band dispersion near Fermi
Surface.
Angle-Resolved photoemission spectroscopy (ARPES) takes the
technique of photoemission spectroscopy (PES) one step further, and it
is the most efficient experimental technique to determine the entire
band structure of a material. ARPES can probe not only the energies but
also the momenta of the photoelectrons in solids. Low-energy laser-based
ARPES will give us more opportunities on the bulk sensitive studies with
the ultra-high energy, momentum and spacial resolutions.
In present thesis, we show our efforts on the development of a low energy
laser-based µ-ARPES system. Furthermore, with the advantages
(ultra-high spacial resolution etc.) of the µ-ARPES system, we
perform its applications on the FeSe superconductor. Secondly, we also
show our work on the upgrading of the µ-ARPES system by realizing
the polarization-tunable laser light with the help of lambda optics. Using
the advantage of polarization-tunable laser, we perform a systematic
study on the bilayer band splitting for Bi2212 cuprates.
We also report our bulk sensitive photoemission studies of unconventional
superconductors. Such as the systematic low-energy ARPES study of
Bi2.1Sr1.9Ca(Cu1-xNix)2O8+δ (x = 0, 0.01, 0.03) and the HAXPES study of
layered phosphide ZrP2-xSex superconductors.