19 research outputs found
Charge-Density-Waves Tuned by Crystal Symmetry
The electronic orders appearing in condensed matter systems are originating
from the precise arrangement of atoms constituting the crystal as well as their
nature. This teneous relationship can lead to highly different phases in
condensed matter, and drive electronic phase transitions. Here, we show that a
very slight deformation of the crystal structure of TbTe can have a
dramatic influence on the electronic order that is stabilized. In particular,
we show that the Charge Density Wave (CDW) developping along the axis
in the pristine state, switches to an orientation along when the
naturally orthorhombic system is turned into a tetragonal system. This is
achieved by performing true biaxial mechanical deformation of a TbTe sample
from 250K to 375K, and by measuring both structural and electronic parameters
with x-ray diffraction and transport measurements. We show that this switching
transition is driven by the tetragonality parameter , and that the
transition occurs for , with a coexistence region for . The CDW transition temperature is found to have a linear
dependence with , with no saturation in the deformed states investigated
here, while the gap saturates out of the coexistence region. The linear
dependence of is accounted for within a tight-binding model. Our results
question the relationship between the gap and in RTe systems. More
generally, our method of applying true biaxial deformation at cryogenic
temperatures can be applied to many systems displaying electronic phase
transitions, and opens a new route towards the study of coexisting or competing
electronic orders in condensed matter
The Role of Actin Turnover in Retrograde Actin Network Flow in Neuronal Growth Cones
The balance of actin filament polymerization and depolymerization maintains a steady state network treadmill in neuronal growth cones essential for motility and guidance. Here we have investigated the connection between depolymerization and treadmilling dynamics. We show that polymerization-competent barbed ends are concentrated at the leading edge and depolymerization is distributed throughout the peripheral domain. We found a high-to-low G-actin gradient between peripheral and central domains. Inhibiting turnover with jasplakinolide collapsed this gradient and lowered leading edge barbed end density. Ultrastructural analysis showed dramatic reduction of leading edge actin filament density and filament accumulation in central regions. Live cell imaging revealed that the leading edge retracted even as retrograde actin flow rate decreased exponentially. Inhibition of myosin II activity before jasplakinolide treatment lowered baseline retrograde flow rates and prevented leading edge retraction. Myosin II activity preferentially affected filopodial bundle disassembly distinct from the global effects of jasplakinolide on network turnover. We propose that growth cone retraction following turnover inhibition resulted from the persistence of myosin II contractility even as leading edge assembly rates decreased. The buildup of actin filaments in central regions combined with monomer depletion and reduced polymerization from barbed ends suggests a mechanism for the observed exponential decay in actin retrograde flow. Our results show that growth cone motility is critically dependent on continuous disassembly of the peripheral actin network
New long-range sub-structure found in the tetragonal phase of CH3NH3PbI3 single crystals
International audienceHybrid organic-inorganic perovskites have become one of the most promising low-cost alternatives to traditional semiconductors in the field of photovoltaics and light emitting devices. It combines both attractive features of organic and inorganic materials within a single composite, for instance with excellent electronic properties. We used x-ray diffraction to reveal a sub-structure within CH 3 NH 3 PbI 3 single crystals. We could observe the presence of additional peaks with a square symmetry in several monocrystalline samples. We discuss these results in terms of two different models: a superstructure modulated in two in-plane orthogonal directions, and a model with tilted domains with a shallow angle of ~0.6°. In both cases, the modulated or tilted domains appear in regions with small lattice expansion. We show that this last model appears to be the most likely to explain our observations
Charge density waves tuned by biaxial tensile stress
International audienceAbstract The precise arrangement and nature of atoms drive electronic phase transitions in condensed matter. To explore this tenuous link, we developed a true biaxial mechanical deformation device working at cryogenic temperatures, compatible with X-ray diffraction and transport measurements, well adapted to layered samples. Here we show that a slight deformation of TbTe can have a dramatic influence on its Charge Density Wave (CDW), with an orientational transition from c to a driven by the a/c parameter, a tiny coexistence region near a = c , and without space group change. The CDW transition temperature T displays a linear dependence with while the gap saturates out of the coexistence region. This behaviour is well accounted for within a tight-binding model. Our results question the relationship between gap and T in RTe systems. This method opens a new route towards the study of coexisting or competing electronic orders in condensed matter