36 research outputs found
In-situ spectroscopy of intrinsic Bi2Te3 topological insulator thin films and impact of extrinsic defects
Combined in-situ x-ray photoemission spectroscopy, scanning tunnelling
spectroscopy and angle resolved photoemission spectroscopy of molecular beam
epitaxy grown Bi2Te3 on lattice mismatched substrates reveal high quality
stoichiometric thin films with topological surface states without a
contribution from the bulk bands at the Fermi energy. The absence of bulk
states at the Fermi energy is achieved without counter doping. We observe that
the surface morphology and electronic band structure of Bi2Te3 are not affected
by in-vacuo storage and exposure to oxygen, whereas major changes are observed
when exposed to ambient conditions. These films help define a pathway towards
intrinsic topological devices.Comment: 8 pages, 5 figure
Optical and thermodynamic properties of the high-temperature superconductor HgBa_2CuO_4+delta
In- and out-of-plane optical spectra and specific heat measurements for the
single layer cuprate superconductor Hg-1201 at optimal doping (Tc = 97 K) are
presented. Both the in-plane and out-of-plane superfluid density agree well
with a recently proposed scaling relation rho_{s}=sigma_{dc}T_{c}. It is shown
that there is a superconductivity induced increase of the in-plane low
frequency spectral weight which follows the trend found in underdoped and
optimally doped Bi-2212 and optimally doped Bi-2223. We observe an increase of
optical spectral weight which corresponds to a change in kinetic energy of
approximately 0.5 meV/Cu which is more than enough to explain the condensation
energy. The specific heat anomaly is 10 times smaller than in YBCO and 3 times
smaller than in Bi-2212. The shape of the anomaly is similar to the one
observed in YBCO showing that the superconducting transition is governed by
thermal fluctuations.Comment: 11 pages, 13 figure
Optical properties of BaFeCoAs
We present detailed temperature dependent optical data on
BaFeCoAs (BCFA), with x = 0.14, between 4 meV and 6.5 eV.
We analyze our spectra to determine the main optical parameters and show that
in this material the interband conductivity already starts around 10 meV. We
determine the superfluid density to be 2.2 10^{7}\Delta_{1}\pm\Delta_{2}\pm$ 2 meV.Comment: 5 pages, 4 figure
Comments on the d-wave pairing mechanism for cuprate high superconductors: Higher is different?
The question of pairing glue for the cuprate superconductors (SC)is revisited
and its determination through the angle resolved photo-emission spectroscopy
(ARPES) is discussed in detail. There are two schools of thoughts about the
pairing glue question: One argues that superconductivity in the cuprates
emerges out of doping the spin singlet resonating valence bond (RVB) state.
Since singlet pairs are already formed in the RVB state there is no need for
additional boson glue to pair the electrons. The other instead suggests that
the d-wave pairs are mediated by the collective bosons like the conventional
low SC with the alteration that the phonons are replaced by another kind
of bosons ranging from the antiferromagnetic (AF) to loop current fluctuations.
An approach to resolve this dispute is to determine the frequency and momentum
dependences of the diagonal and off-diagonal self-energies directly from
experiments like the McMillan-Rowell procedure for the conventional SC. In that
a simple d-wave BCS theory describes superconducting properties of the cuprates
well, the Eliashberg analysis of well designed high resolution experimental
data will yield the crucial frequency and momentum dependences of the
self-energies. This line of approach using ARPES are discussed in more detail
in this review, and some remaining problems are commented.Comment: Invited review article published in the Journal of Korean Physical
Society; several typos corrected and a few comments and references adde
Droplet-like Fermi surfaces in the anti-ferromagnetic phase of EuFeAs, an Fe-pnictide superconductor parent compound
Using angle resolved photoemission it is shown that the low lying electronic
states of the iron pnictide parent compound EuFeAs are strongly
modified in the magnetically ordered, low temperature, orthorhombic state
compared to the tetragonal, paramagnetic case above the spin density wave
transition temperature. Back-folded bands, reflected in the orthorhombic/
anti-ferromagnetic Brillouin zone boundary hybridize strongly with the
non-folded states, leading to the opening of energy gaps. As a direct
consequence, the large Fermi surfaces of the tetragonal phase fragment, the low
temperature Fermi surface being comprised of small droplets, built up of
electron and hole-like sections. These high resolution ARPES data are therefore
in keeping with quantum oscillation and optical data from other undoped
pnictide parent compounds.Comment: 4 figures, 6 page
Spectroscopic scanning tunneling microscopy insights into Fe-based superconductors
In the first three years since the discovery of Fe-based high Tc
superconductors, scanning tunneling microscopy (STM) and spectroscopy have shed
light on three important questions. First, STM has demonstrated the complexity
of the pairing symmetry in Fe-based materials. Phase-sensitive quasiparticle
interference (QPI) imaging and low temperature spectroscopy have shown that the
pairing order parameter varies from nodal to nodeless s\pm within a single
family, FeTe1-xSex. Second, STM has imaged C4 -> C2 symmetry breaking in the
electronic states of both parent and superconducting materials. As a local
probe, STM is in a strong position to understand the interactions between these
broken symmetry states and superconductivity. Finally, STM has been used to
image the vortex state, giving insights into the technical problem of vortex
pinning, and the fundamental problem of the competing states introduced when
superconductivity is locally quenched by a magnetic field. Here we give a
pedagogical introduction to STM and QPI imaging, discuss the specific
challenges associated with extracting bulk properties from the study of
surfaces, and report on progress made in understanding Fe-based superconductors
using STM techniques.Comment: 36 pages, 23 figures, 229 reference
Pseudogap-less high T superconductivity in BaCoFeAs
The pseudogap state is one of the peculiarities of the cuprate high
temperature superconductors. Here we investigate its presence in
BaCoFeAs, a member of the pnictide family, with temperature
dependent scanning tunneling spectroscopy. We observe that for under, optimally
and overdoped systems the gap in the tunneling spectra always closes at the
bulk T, ruling out the presence of a pseudogap state. For the underdoped
case we observe superconducting gaps over large fields of view, setting a lower
limit of tens of nanometers on the length scale of possible phase separated
regions.Comment: 5 pages, 3 figure
Photo-enhanced antinodal conductivity in the pseudogap state of high-T-c cuprates
A major challenge in understanding the cuprate superconductors is to clarify the nature of the fundamental electronic correlations that lead to the pseudogap phenomenon. Here we use ultrashort light pulses to prepare a non-thermal distribution of excitations and capture novel properties that are hidden at equilibrium. Using a broadband (0.5-2 eV) probe, we are able to track the dynamics of the dielectric function and unveil an anomalous decrease in the scattering rate of the charge carriers in a pseudogap-like region of the temperature (T) and hole-doping (p) phase diagram. In this region, delimited by a well-defined T*(neq)(p) line, the photoexcitation process triggers the evolution of antinodal excitations from gapped (localized) to delocalized quasiparticles characterized by a longer lifetime. The novel concept of photo-enhanced antinodal conductivity is naturally explained within the singleband Hubbard model, in which the short-range Coulomb repulsion leads to a k-space differentiation between nodal quasiparticles and antinodal excitations. \ua9 2014 Macmillan Publishers Limited. All rights reserved