58 research outputs found
Uniaxial magnetic anisotropy tuned by nanoscale ripple formation: ion-sculpting of Co/Cu(001) thin films
We have investigated the growth of surface nanostructures on a Co/Cu(001) film and the growth of Co films on a nanostructured Cu(001) substrate as well as the effect of nanoscale pattern formation on the film magnetic properties. Here we demonstrate by scanning tunneling microscopy measurements and magneto-optic Kerr effect hysteresis curves that low-temperature grazing-incidence ion sputtering can be used to induce the formation of nanoscale ripples which reduce the four-fold symmetry of the Co film to two-fold, thus generating a strong in-plane uniaxial magnetic anisotropy. The nanostructures and the associated uniaxial magnetic anisotropy were found to be stable up to room temperature
Effect of 3d-doping on the electronic structure of BaFe2As2
The electronic structure of BaFe2As2 doped with Co, Ni, and Cu has been
studied by a variety of experimental and theoretical methods, but a clear
picture of the dopant 3d states has not yet emerged. Herein we provide
experimental evidence of the distribution of Co, Ni, and Cu 3d states in the
valence band. We conclude that the Co and Ni 3d states provide additional free
carriers to the Fermi level, while the Cu 3d states are found at the bottom of
the valence band in a localized 3d10 shell. These findings help shed light on
why superconductivity can occur in BaFe2As2 doped with Co and Ni but not Cu.Comment: 18 pages, 8 figure
Doping-Dependent and Orbital-Dependent Band Renormalization in Ba(Fe_1-xCo_x)_2As_2 Superconductors
Angle resolved photoemission spectroscopy of Ba(Fe1-xCox)2As2 (x = 0.06,
0.14, and 0.24) shows that the width of the Fe 3d yz/zx hole band depends on
the doping level. In contrast, the Fe 3d x^2-y^2 and 3z^2-r^2 bands are rigid
and shifted by the Co doping. The Fe 3d yz/zx hole band is flattened at the
optimal doping level x = 0.06, indicating that the band renormalization of the
Fe 3d yz/zx band correlates with the enhancement of the superconducting
transition temperature. The orbital-dependent and doping-dependent band
renormalization indicates that the fluctuations responsible for the
superconductivity is deeply related to the Fe 3d orbital degeneracy.Comment: 5 pages, 4 figure
Doping evolution of superconducting gaps and electronic densities of states in Ba(Fe1-xCox)2As2 iron pnictides
An extensive calorimetric study of the normal- and superconducting-state
properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state
Sommerfeld coefficient increases (decreases) with Co doping for x
0.06), which illustrates the strong competition between magnetism and
superconductivity to monopolize the Fermi surface in the underdoped region and
the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All
superconducting samples exhibit a residual electronic density of states of
unknown origin in the zero-temperature limit, which is minimal at optimal
doping but increases to the normal-state value in the strongly under- and
over-doped regions. The remaining specific heat in the superconducting state is
well described using a two-band model with isotropic s-wave superconducting
gaps.Comment: Submitted to Europhysics Letter
Upper critical magnetic field in Ba_0.68K_0.32Fe_2As_2 and Ba(Fe_0.93Co_0.07)_2As_2
We report measurements of the temperature dependence of the radio-frequency
magnetic penetration depth in Ba_0.68K_0.32Fe_2As_2 and
Ba(Fe_0.93Co_0.07)_2As_2 single crystals in pulsed magnetic fields up to 60 T.
From our data, we construct an H-T phase diagram for the inter-plane (H || c)
and in-plane (H || ab) directions for both compounds. For both field
orientations in Ba_0.68K_0.32Fe_2As_2, we find a concave curvature of the
Hc2(T) lines with decreasing anisotropy and saturation towards lower
temperature. Taking into account Pauli spin paramagnetism we can describe
Hc2(T) and its anisotropy. In contrast, we find that Pauli paramagnetic pair
breaking is not essential for Ba(Fe_0.93Co_0.07)_2As_2. For this electron-doped
compound, the data support a Hc2(T) dependence that can be described by the
Werthamer Helfand Hohenberg model for H || ab and a two-gap behavior for H ||
c.Comment: 7 pages, 8 figure
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
Effect of transition-metal substitution in iron-based superconductors
We study theoretically the current debatable issue about the effect of
transition-metal (TM) substitution in iron-based superconductors through
treating all of the TM ions as randomly distributed impurities. The extra
electrons from TM elements are localized at the impurity sites. In the mean
time the chemical potential shifts upon substitution. The phase diagram is
mapped out and it seems that the TM elements can act as effective dopants. The
local density of states (LDOS) is calculated and the bottom becomes V-shaped as
the impurity concentration increases. The LDOS at the Fermi energy
is finite and reaches the minimum at the optimal doping level.
Our results are in good agreement with the scanning tunneling microscopy
experiments.Comment: 5 pages, 4 figure
Strong nodeless pairing on separate electron Fermi surface sheets in (Tl, K)FeSe probed by ARPES
We performed a high-resolution angle-resolved photoemission spectroscopy
study of the TlKFeSe superconductor (
K). We show the existence of two electronlike bands at the M point
which cross the Fermi level at similar Fermi wave vectors to form nearly
circular electronlike Fermi surface pockets. We observe a nearly isotropic
8.5 meV superconducting gap () on these Fermi
surfaces. Our analysis of the band structure around the Brillouin zone centre
reveals two additional electronlike Fermi surfaces: a very small one and a
larger one with comparable to the FS pockets at M. Interestingly, a SC
gap with a magnitude of 8 meV also develops along the latter FS. Our
observations are consistent with the s-wave strong coupling scenario.Comment: 5 pages, 4 figure
AFe2As2 (A = Ca, Sr, Ba, Eu) and SrFe_(2-x)TM_(x)As2 (TM = Mn, Co, Ni): crystal structure, charge doping, magnetism and superconductivity
The electronic structure and physical properties of the pnictide compound
families OFeAs ( = La, Ce, Pr, Nd, Sm), FeAs ( = Ca,
Sr, Ba, Eu), LiFeAs and FeSe are quite similar. Here, we focus on the members
of the FeAs family whose sample composition, quality and single
crystal growth are better controllable compared to the other systems. Using
first principles band structure calculations we focus on understanding the
relationship between the crystal structure, charge doping and magnetism in
FeAs systems. We will elaborate on the tetragonal to
orthorhombic structural distortion along with the associated magnetic order and
anisotropy, influence of doping on the site as well as on the Fe site, and
the changes in the electronic structure as a function of pressure.
Experimentally, we investigate the substitution of Fe in
SrFeAs by other 3 transition metals, = Mn, Co, Ni.
In contrast to a partial substitution of Fe by Co or Ni (electron doping) a
corresponding Mn partial substitution does not lead to the supression of the
antiferromagnetic order or the appearance of superconductivity. Most calculated
properties agree well with the measured properties, but several of them are
sensitive to the As position. For a microscopic understanding of the
electronic structure of this new family of superconductors this structural
feature related to the Fe-As interplay is crucial, but its correct ab initio
treatment still remains an open question.Comment: 27 pages, single colum
Neutron lifetime measurement with pulsed cold neutrons
The neutron lifetime has been measured by comparing the decay rate with the
reaction rate of He nuclei of a pulsed neutron beam from the spallation
neutron source at the Japan Proton Accelerator Research Complex (J-PARC). The
decay rate and the reaction rate were determined by simultaneously detecting
electrons from the neutron decay and protons from the He(n,p)H reaction
using a gas chamber of which working gas contains diluted He. The measured
neutron lifetime was 898\,\pm\,10\,_{\rm stat}\,^{+15}_{-18}\,_{\rm sys}\,s.Comment: 28 pages, 20 figures, will be submitted to PTE
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