276 research outputs found
Normal state bottleneck and nematic fluctuations from femtosecond quasi-particle relaxation dynamics in Sm(Fe,Co)AsO
We investigate temperature and fluence dependent dynamics of the photoexcited
quasi-particle relaxation and low-energy electronic structure in electron-doped
1111-structure Sm(Fe_{0.93}Co_{0.07})AsO single crystal. We find that the
behavior is qualitatively identical to the 122-structure Ba(Fe,Co)_{2}As_{2}
including the presence of a normal state pseudogap and a marked 2-fold symmetry
breaking in the tetragonal phase that we relate to the electronic nematicity.
The 2-fold symmetry breaking appears to be a general feature of the electron
doped iron pnictides
Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO_{1-x}F_{x} single crystals
We investigate the quasiparticle relaxation and low-energy electronic
structure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2}
single crystals - exhibiting spin-density wave (SDW) ordering and
superconductivity respectively - using pump-probe femtosecond spectroscopy. In
the undoped single crystals a single relaxation process is observed, showing a
remarkable critical slowing down of the QP relaxation dynamics at the SDW
transition temperature T_{SDW}\simeq125{K}. In the superconducting (SC)
crystals multiple relaxation processes are present, with distinct SC state
quasiparticle recombination dynamics exhibiting a BCS-like T-dependent
superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K
indicating the existence of a pseudogap of magnitude
2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photon
energy dependence we conclude that the SC state and PG relaxation channels are
independent, implying the presence of two separate electronic subsystems. We
discuss the data in terms of spatial inhomogeneity and multi-band scenarios,
finding that the latter is more consistent with the present data.Comment: Replaced by the correct versio
Femtosecond data storage, processing and search using collective excitations of a macroscopic quantum state
An ultrafast paralell data processor is described in which amplitude mode
excitations of a charge density wave (CDW) are used to encode data on the
surface of a 1-T TaS_2 crystal. The data are written, manipulated and read
using parallel femtosecond laser pulse beams, and the operation of a database
search algorithm is demonstrated on a 2-element array.Comment: To be published in App. Phys. Let
Ultrafast switching to a stable hidden topologically protected quantum state in an electronic crystal
Hidden states of matter with novel and unusual properties may be created if a
system out of equilibrium can be induced to follow a trajectory to a state
which is inaccessible or does not even exist under normal equilibrium
conditions. Here we report on the discovery of a hidden (H) topologically
protected electronic state in a layered dichalcogenide 1T-TaS2 crystal reached
as a result of a quench caused by a single 35 fs laser pulse. The properties of
the H state are markedly different from any other state of the system: it
exhibits a large drop of electrical resistance, strongly modified single
particle and collective mode spectra and a marked change of optical
reflectivity. Particularly important and unusual, the H state is stable for an
arbitrarily long time until a laser pulse, electrical current or thermal erase
procedure is applied, causing it to revert to the thermodynamic ground state.
Major observed events can be reproduced by a kinetic model describing the
conversion of photo excited electrons and holes into an electronically ordered
crystal, thus converting a Mott insulator to a conducting H state. Its
long-time stability follows from the topological protection of the number of
periods in the electronic crystal.Comment: 21 pages and 5 figures, separate supplementary materia
Doping dependence of femtosecond quasi-particle relaxation dynamics in Ba(Fe,Co)_2As_2 single crystals: possible evidence for normal state nematic fluctuations
We systematically investigate the photoexcited (PE) quasi-particle (QP)
relaxation and low-energy electronic structure in electron doped
Ba(Fe_{1-x}Co_{x})_{2}As_{2} single crystals as a function of Co doping, 0<= x
<=0.11. The evolution of the photoinduced reflectivity transients with
proceeds with no abrupt changes. In the orthorhombic spin-density-wave (SDW)
state a bottleneck associated with a partial charge-gap opening is detected,
similar to previous results in different SDW iron-pnictides. The relative
charge gap magnitude decreases with increasing x. In the superconducting (SC)
state an additional relaxational component appears due to a partial (or
complete) destruction of the SC state proceeding on a sub-0.5-picosecond
timescale. From the SC component saturation behavior the optical SC-state
destruction energy, U_p/k_B=0.3 K/Fe, is determined near the optimal doping.
The subsequent relatively slow recovery of the SC state indicates clean SC
gaps. The T-dependence of the transient reflectivity amplitude in the normal
state is consistent with the presence of a pseudogap in the QP density of
states. The polarization anisotropy of the transients suggests that the
pseudogap-like behavior might be associated with a broken point symmetry
resulting from nematic electronic fluctuations persisting up to T~200 K at any
x. The second moment of the Eliashberg function, obtained from the relaxation
rate in the metallic state at higher temperatures, indicates a moderate
electron phonon coupling, lambda <~0.3, that decreases with increasing doping
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