138 research outputs found
Point contact spectroscopy in Fe-based superconductors: recent advancements and future challenges
Iron-based superconductors (FeSC) present an unprecedented variety of
features both in the superconducting and in the normal state. Different
families differ in the value of the critical temperature, in the shape of the
Fermi surface, in the existence or absence of quasi-nesting conditions, in the
range of doping in which the antiferromagnetic (AFM) and the superconducting
phase coexist and in the structure of the order parameter in the reciprocal
space, and so on. In this paper the most important results of point-contact
spectroscopy (PCS) in Fe-based superconductors are reviewed, and the most
recent advances are described with the aim to discuss the future perspectives
and challenges of this spectroscopic technique in the characterization of the
superconducting properties of these complex compounds. One of the main
challenges, faced so far only by a few researchers in the PCS field, is to
fully explore the phase diagram of these materials, as a function of doping or
pressure, to understand the interplay between superconductivity and magnetism,
the effect of intrinsic or extrinsic inhomogeneities, the role of spin
fluctuations (SFs) in the pairing, the symmetry and the structure of the order
parameter(s).Comment: 12 pages, 4 figure
Biostratigraphic significance of the latest Cambrian-earliest Ordovician agnostoid trilobites from Northwestern Argentina
The biostratigraphic significance of the latest Cambrian-earliest Ordovician trilobite agnostoids from northwestern Argentina is summarized. A characterization of the faunas recognized below and above the Cambrian-Ordovician boundary is presented on the bases of information from sierra de Cajas (Jujuy Province, Eastern Cordillera; Lampazar and Cardonal Formations), río Volcancito (La Rioja Province, Famatina range; Volcancito Formation) and other localities assigned to the Parabolina frequens argentina Biozone. In addition, the stratigraphic meaning of each agnostoid species from the P. frequens argentina Zone is updated. The uppermost Cambrian is characterized by a distinctive agnostoid assemblage composed of Lotagnostus (Lotagnostus) sp., Lotagnostus (Semagnostus) zuninoi (HARRINGTON and LEANZA), Micragnostus vilonii HARRINGTON and LEANZA, M. calviformis HARRINGTON and LEANZA, Strictagnostus? micropeltis (HARRINGTON and LEANZA), Pseudorhaptagnostus (Machairagnostus) tmetus HARRINGTON and LEANZA, Pseudorhaptagnostus (Machairagnostus) cf. tmetus HARRINGTON and LEANZA, Pseudorhaptagnostus (Machairagnostus) sp., Gymnagnostus bolivianus (HOEK), Gymnagnostus perinflatus HARRINGTON and LEANZA, and Leiagnostus turgidulus HARRINGTON and LEANZA. This fauna is commonly recorded together with the polymeroid Beltella ulrichi (KAYSER), Onychopyge HARRINGTON, Plicatolina scalpta HARRINGTON and LEANZA, Parabolina frequens argentina (KAYSER), Parabolinella coelatifrons HARRINGTON and LEANZA, and Angelina hyeronimi (KAYSER). Based on this agnostoid-polymeroid fauna, a Pseudorhaptagnostus (Machairagnostus) - Gymnagnostus Assemblage Subzone (lower part of the P. frequens argentina Zone) is formally proposed. Agnostoid diversity drastically declines through the Cambrian-Ordovician boundary, probably as a consequence of a regional regressive-transgressive event. In the lowest Tremadoc the agnostoid records are mainly restricted to Trilobagnostus chiushuensis (KOBAYASHI) and some forms described under open nomenclature (Anglagnostus? sp., Micragnostus sp.). The upper part of the P. frequens argentina Zone is better defined by polymeroid trilobites. Jujuyaspis keideli KOBAYASHI is its most characteristic species
Design and construction of a point-contact spectroscopy rig with lateral scanning capability
The design and realization of a cryogenic rig for point-contact spectroscopy
measurements in the needle-anvil configuration is presented. Thanks to the use
of two piezoelectric nano-positioners, the tip can move along the vertical
() and horizontal () direction and thus the rig is suitable to probe
different regions of a sample \textit{in situ}. Moreover, it can also form
double point-contacts on different facets of a single crystal for achieving,
e.g., an interferometer configuration for phase-sensitive measurements. For the
latter purpose, the sample holder can also host a Helmholtz coil for applying a
small transverse magnetic field to the junction. A semi-rigid coaxial cable can
be easily added for studying the behavior of Josephson junctions under
microwave irradiation. The rig can be detached from the probe and thus used
with different cryostats. The performance of this new probe has been tested in
a Quantum Design PPMS system by conducting point-contact Andreev reflection
measurements on Nb thin films over large areas as a function of temperature and
magnetic field.Comment: 7 pages, 7 figures, published in Rev. Sci. Instru
MgB2 single crystals substituted with Li and with Li-C: Structural and superconducting properties
The effect of Li substitution for Mg and of Li-C co-substitution on the
superconducting properties and crystal structure of MgB2 single crystals has
been investigated. It has been found that hole doping with Li decreases the
superconducting transition temperature Tc, but at a slower rate than electron
doping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li for
Mg and C for B decreases more than in the case where C is substituted alone.
This means that holes introduced by Li cannot counterbalance the effect of
decrease of Tc caused by introduction of electrons coming from C. The possible
reason of it can be that holes coming from Li occupy the pi band while
electrons coming from C fill the sigma band. The temperature dependences of the
upper critical field Hc2 for Al and Li substituted crystals with the same Tc
show a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite of
much different substitution level. This indicates that the mechanism
controlling Hc2 and Tc is similar in both hole and electron doped crystals.
Electrical transport measurements show an increase of resistivity both in Li
substituted crystals and in Li and C co-substituted crystals. This indicates
enhanced scattering due to defects introduced by substitutions including
distortion of the lattice. The observed behavior can be explained as a result
of two effects, influencing both Tc and Hc2. The first one is doping related to
the changes in the carrier concentration, which may lead to the decrease or to
the increase of Tc. The second one is related to the introduction of new
scattering centers leading to the modification of the interband and/or
intraband scattering and therefore, to changes in the superconducting gaps and
to the reduction of Tc.Comment: 22 pages, 17 figures, submitted to PR
Evidence for two-gap nodeless superconductivity in SmFeAsOF from point-contact Andreev-reflection spectroscopy
Point-contact Andreev-reflection measurements were performed in
SmFeAsO_{0.8}F_{0.2} polycrystals with T_c \simeq 53 K. The experimental
conductance curves reproducibly exhibit peaks around \pm 6 mV and shoulders at
V \sim 16-20 mV, indicating the presence of two nodeless superconducting gaps.
While the single-band Blonder-Tinkham-Klapwijk fit can only reproduce a small
central portion of the conductance curve, the two-gap one accounts remarkably
well for the shape of the whole experimental dI/dV. The fits of the normalized
curves give Delta_1(0) = 6.15 \pm 0.45 meV and Delta_2(0) = 18 \pm 3 meV. Both
gaps close at the same temperature and follow a BCS-like behavior.Comment: 4 pages, 4 eps color figure
Carrier mobility and scattering lifetime in electric double-layer gated few-layer graphene
We fabricate electric double-layer field-effect transistor (EDL-FET) devices
on mechanically exfoliated few-layer graphene. We exploit the large capacitance
of a polymeric electrolyte to study the transport properties of three, four and
five-layer samples under a large induced surface charge density both above and
below the glass transition temperature of the polymer. We find that the carrier
mobility shows a strong asymmetry between the hole and electron doping regime.
We then employ ab-initio density functional theory (DFT) calculations to
determine the average scattering lifetime from the experimental data. We
explain its peculiar dependence on the carrier density in terms of the specific
properties of the electrolyte we used in our experiments.Comment: 6 pages, 3 figure
Early Cambrian archaeocyathan limestone blocks in low-grade meta-conglomerate from El Jagüelito Formation (Sierra Grande, Río Negro, Argentina
Massive grey limestone blocks containing a fairly diverse but poorly preserved archaeocyath fauna were recovered from a meta-conglomerate bed in the El Jagüelito Formation (Sierra Grande area, Eastern North Patagonian Massif, Río Negro, Argentina). This is the first documented reference of the presence of archaeocyaths in continental Argentina. Seven different taxa were identified, preliminary described and figured. Recrystallization of the skeletons due to regional low-grade metamorphism and deformation of the unit does not allow observation of key detailed features and prevents identification to genera and species. Nevertheless, the specimens studied show general affinities with archaeocyathan assemblages from the Australia-Antarctica palaeobiogeographic province and indicate a middle Early Cambrian (Atdabanian-Botomian) maximum age for the deposition of the El Jagüelito Formation protoliths. The similarities between the North Patagonian Early Paleozoic El Jagüelito Formation and those rocks from Antarctica suggest a geologic and biologic common history of these regions on the same southwest margin of Gondwana during that time
Coexistence of two order parameters and a pseudogaplike feature in the iron-based superconductor LaFeAsO_(1-x)F_x
The nature and value of the order parameters (OPs) in the superconducting
Fe-based oxypnictides REFeAsO_(1-x)F_x (RE = rare earth) are a matter of
intense debate, also connected to the pairing mechanism which is probably
unconventional. Point-contact Andreev-reflection experiments on
LaFeAsO_(1-x)F_x gave us direct evidence of three energy scales in the
superconducting state: a nodeless superconducting OP, Delta1 = 2.8-4.6 meV,
which scales with the local Tc of the contact; a larger unconventional OP that
gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases
on increasing the Tc of the contact; a pseudogaplike feature (i.e. a depression
in the conductance around zero bias), that survives in the normal state up to
T* ~ 140 K (close to the Neel temperature of the undoped compound), which we
associate to antiferromagnetic spin fluctuations (AF SF) coexisting with
superconductivity. These findings point toward a complex, unconventional nature
of superconductivity in LaFeAsO_(1-x)F_x.Comment: 19 pages, 12 figures - one figure and some insets added, minor
changes to the tex
Probing the current-phase relation in Josephson point-contact junctions between Pb 0.6 In 0.4 and Ba 0.6 K 0.4 (FeAs) 2 superconductors
The Josephson effect in point contacts between an “ordinary” superconductor Pb 0.6In0.4 (Tc≈6.6K) and single crystals of the Fe-based superconductor Ba0.6K0.4(FeAs)2 (Tc≈38.5K), was investigated. In order to shed light on the order parameter symmetry of Ba0.6K0.4(FeAs)2, the dependence of the Josephson supercurrent Is on the temperature and on sin (dφ) with d= 1 , 2 was studied. The dependencies of the critical current on temperature Ic(T) and of the amplitudes of the first current steps of the current–voltage characteristic inexp(P)(n= 0 , 1 , 2) on the power of microwave radiation with frequency f=(1.5÷8)GHz were measured. It is shown that the dependencies Ic(T) are close to the well-known Ambegaokar–Baratoff (AB) dependence for tunnel contacts between “ordinary” superconductors and to the dependence calculated by Burmistrova et al. (Phys Rev B 91, 214501 (2015)) for microshorts between an “ordinary” superconductor and a two-band superconductor with s± order parameter symmetry at certain values of the transparency of boundaries and thickness of the transition layer. It is found that the dependencies inexp(P) cannot be approximated within the resistively shunted model using the normalized microwave frequencies Ω = 2 πf/ (2 eVc/ ħ) with characteristic voltages Vc= IcRN, (RN—normal resistance of the contact) found from the low-voltage parts of the current–voltage characteristics. The reasons for this failure are discussed and a method is proposed for accurately determining the value of Ω , which takes into account all the features of the point contact affecting the period of the dependence inexp(P). An analysis of the Ic(T) and inexp(P) dependencies shows that the superconducting current of the Josephson contacts under investigation is proportional to the sin of the phase difference φ, Is= Icsin(φ). The implications of these results on the symmetry of the order parameter are also discussed
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