61 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
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
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
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
Dynamics of Photoexcited Carriers in Ba(Fe 1âx Co x ) 2 As 2 Single Crystals with Spin-Density-Wave Ordering
Abstract We apply the femtosecond optical pump-probe spectroscopy to study the relaxation dynamics in photoexcited Co-doped Ba(Fe 1âx Co x ) 2 As 2 single crystals in the underdoped spin-density wave (SDW) state region of the x â T phase diagram. Underdoped SDW samples with Co-0 % and Co-2.5 % show a bottleneck in the relaxation as a consequence of the partial charge gap opening in the orthorhombic SDW phase, similar to previous results in other SDW iron-pnictides. Moreover, the charge gap magnitude decreases with increasing doping. The sample with Co-5.1 % displays both a SDW ordering and superconductivity at low T . We were able to observe a 2-fold anisotropy in our samples, existing up to âŒ200 K, without any applied uniaxial stress. We associate the 2-fold symmetry breaking in nominally tetragonal phase with nematic ordering of the Fe d orbitals
Ultrafast doublon dynamics in photoexcited -
Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two- dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping â/Jâ14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal- insulator transition suggests a way to modify doublon relaxation on the few- femtosecond timescale
Orbital textures and charge density waves in transition metal dichalcogenides
Low-dimensional electron systems, as realized naturally in graphene or
created artificially at the interfaces of heterostructures, exhibit a variety
of fascinating quantum phenomena with great prospects for future applications.
Once electrons are confined to low dimensions, they also tend to spontaneously
break the symmetry of the underlying nuclear lattice by forming so-called
density waves; a state of matter that currently attracts enormous attention
because of its relation to various unconventional electronic properties. In
this study we reveal a remarkable and surprising feature of charge density
waves (CDWs), namely their intimate relation to orbital order. For the
prototypical material 1T-TaS2 we not only show that the CDW within the
two-dimensional TaS2-layers involves previously unidentified orbital textures
of great complexity. We also demonstrate that two metastable stackings of the
orbitally ordered layers allow to manipulate salient features of the electronic
structure. Indeed, these orbital effects enable to switch the properties of
1T-TaS2 nanostructures from metallic to semiconducting with technologically
pertinent gaps of the order of 200 meV. This new type of orbitronics is
especially relevant for the ongoing development of novel, miniaturized and
ultra-fast devices based on layered transition metal dichalcogenides
Selective scattering between Floquet-Bloch and Volkov states in a topological insulator
The coherent optical manipulation of solids is emerging as a promising way to
engineer novel quantum states of matter. The strong time periodic potential of
intense laser light can be used to generate hybrid photon-electron states.
Interaction of light with Bloch states leads to Floquet-Bloch states which are
essential in realizing new photo-induced quantum phases. Similarly, dressing of
free electron states near the surface of a solid generates Volkov states which
are used to study non-linear optics in atoms and semiconductors. The
interaction of these two dynamic states with each other remains an open
experimental problem. Here we use Time and Angle Resolved Photoemission
Spectroscopy (Tr-ARPES) to selectively study the transition between these two
states on the surface of the topological insulator Bi2Se3. We find that the
coupling between the two strongly depends on the electron momentum, providing a
route to enhance or inhibit it. Moreover, by controlling the light polarization
we can negate Volkov states in order to generate pure Floquet-Bloch states.
This work establishes a systematic path for the coherent manipulation of solids
via light-matter interaction.Comment: 21 pages, 6 figures, final version to appear in Nature Physic
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