The intriguing electronic properties of two-dimensional materials motivates experiments to resolve their rapid, microscopic interactions and dynamics across momentum space. Essential insight into the electronic momentum-space dynamics can be obtained directly via time- and angle-resolved photoemission spectroscopy (trARPES). We discuss the development of a high-repetition rate trARPES setup that employs a bright source of narrowband, extreme-UV harmonics around 22.3 eV, and its application to sensitive studies of materials dynamics. In the bulk transition-metal dichalcogenide MoSe2 momentum-space quasiparticle scattering is observed after resonant excitation at the K-point exciton line, resulting in the time-delayed buildup of electrons at the Σ-point conduction band minimum. We will discuss this and other aspects of the non-equilibrium electronic response accessible with the extreme-UV trARPES probe.
Experiments are enlightening that the high Tc superconductivity in cuprate perovskites takes place in a particular phase showing segregation of localized and itinerant charge carriers. We have studied topological features of the Fermi surface of Bi2Sr2CaCu2O8+d (Bi2212) superconductor to investigate implication of charge segregation in the system. For the purpose we have used constant initial state angle-scanning photoemission supported by the energy distribution curves. The resulting Fermi surface measured over an extended Brillouin zone shows broken segments with partial gaps around ((pi) ,O) and shadow bands around (0.5(pi) ,0.5(pi) ) and equivalent locations. In addition data provide an evidence for new electronic states at the Fermi surface due to a one dimensional band with a small dispersion and small kF along the ((pi) ,O) direction. We argue that the depression of photointensity around ((pi) ,O) and equivalent locations with well-defined kF and one dimensional band are related with charge ordering in the CuO2 plane along the (-(pi) ,(pi) ) direction and spin fluctuations along the ((pi) ,O) direction.
We have measured the amplitude of the 1D lattice modulation forming the stripe structure in the CuO2 plane of Bi2Sr2Ca2Cu2O8+y single crystals by EXAFS. The period of the 1D modulation of the CuO2 plane has been measured by Cu anomalous diffraction. The large anharmonic content of the 1D modulation clearly shows the formation of stripes of undistorted LTO lattice. The large amplitude of the local structure modulation in plane indicates the formation of sizable potential barrier due to the linear domain walls in the plane. The size of the stripes L is such that the Fermi level is tuned to the maximum of the density of states formed by the second subband. The tuning of the Fermi level at the shape resonance of the superlattice of quantum wires is proposed to be the mechanism for the amplification of the critical temperature in high Tc cuprate superconductors.
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