325 research outputs found
Anharmonic coherent dynamics of the soft phonon mode of a PbTe crystal
We investigate the ultrafast optical response of PbTe to an intense
single-cycle terahertz pulse, resonant with the soft transverse optical (TO)
phonon mode of the crystal. We detect multifrequency oscillations of the
reflectance anisotropy, which we associate with nonlinear motion of the TO
phonon oscillator excited directly by the terahertz pulse. Our observation of
monotonically decaying optical anisotropy together with second harmonic
oscillations of the TO mode is an evidence of a transient non-centrosymmetric
state of the crystal lattice that can be accompanied by the ferroelectric
order. We suppose that this state is induced in the PbTe crystal by the intense
terahertz pulse via alignment of the local polar nanodomains. This hypothesis
is partially supported by the observation of coherent phonons near the
satellite phonon mode frequency that are impulsively generated by a femtosecond
laser pulse and are considerably enhanced by a synchronous terahertz pulse.Comment: 7 pages, 5 figure
Quantum phase transition in ultrahigh mobility SiGe/Si/SiGe two-dimensional electron system
The metal-insulator transition (MIT) is an exceptional test bed for studying
strong electron correlations in two dimensions in the presence of disorder. In
the present study, it is found that in contrast to previous experiments on
lower-mobility samples, in ultra-high mobility SiGe/Si/SiGe quantum wells the
critical electron density, , of the MIT becomes smaller than the
density, , where the effective mass at the Fermi level tends to
diverge. Near the topological phase transition expected at , the
metallic temperature dependence of the resistance should be strengthened, which
is consistent with the experimental observation of more than an order of
magnitude resistance drop with decreasing temperature below K.Comment: Misprints corrected. As publishe
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