18 research outputs found
Evidence for two-gap superconductivity in (Ba,K)Fe_2As_2 by directional point contact Andreev reflection spectroscopy
Directional point-contact Andreev-reflection spectroscopy measurements on the
BaKFeAs single crystals are presented. The spectra
show significant differences when measured in the plane in comparison with
those measured in the direction of the crystal. In the latter case only a
reduced point-contact conductance around zero bias has been revealed persisting
well above and probably related to the structural and magnetic
transitions in the system. Within the plane two superconducting energy
gaps are detected below . Here a reduced conductance above could
also be found. The fits of the -plane data to the superconducting s-wave
two-gap model indicate that the smaller gap has a size below the BCS value
while the large gap reveals much higher coupling strength.Comment: published versio
Point contact Andreev reflection spectroscopy of superconducting energy gaps in 122-type family of iron pnictides
A brief overview of the superconducting energy gap studies on 122-type family
of iron pnictides is given. It seems that the situation in the hole-doped
Ba1-xKxFe2As2 is well resolved. Most of the measurements including the
presented here point-contact Andreev reflection spectra agree on existence of
multiple nodeless gaps in the excitation spectrum of this multiband system. The
gaps have basically two sizes - the small one with a strength up to the BCS
weak coupling limit and the large one with a very strong coupling with
2Delta/kTc ~ 6 - 8. In the electron doped Ba(Fe1-xCox)2As2 the most of the
experiments including our point contact measurements reveal in quite broadened
spectra only a single gap with a strong coupling strength. The high precision
ARPES measurements on this system identified two gaps but very close to each
other, both showing a strong coupling with 2Delta/kTc ~ 5 and 6, respectively.Comment: 7 pages, 4 figures, to appear in Physica C, special issue on
Fe-pnictide
Interplay between magnetism and superconductivity and appearance of a second superconducting transition in alpha-FeSe at high pressure
We synthesized tetragonal alpha-FeSe by melting a powder mixture of iron and
selenium at high pressure. Subsequent annealing at normal pressure results in
removing traces of hexagonal beta- FeSe, formation of a rather sharp transition
to superconducting state at Tc ~ 7 K, and the appearance of a magnetic
transition near Tm = 120 K. Resistivity and ac-susceptibility were measured on
the annealed sample at hydrostatic pressure up to 4.5 GPa. A magnetic
transition visible in ac-susceptibility shifts down under pressure and the
resistive anomaly typical for a spin density wave (SDW) antiferromagnetic
transition develops near the susceptibility anomaly. Tc determined by the
appearance of a diamagnetic response in susceptibility, increases linearly
under pressure at a rate dTc/dP = 3.5 K/GPa. Below 1.5 GPa, the resistive
superconducting transition is sharp; the width of transition does not change
with pressure; and, Tc determined by a peak in drho/dT increases at a rate ~
3.5 K/GPa. At higher pressure, a giant broadening of the resistive transition
develops. This effect cannot be explained by possible pressure gradients in the
sample and is inherent to alpha-FeSe. The dependences drho(T)/dT show a
signature for a second peak above 3 GPa which is indicative of the appearance
of another superconducting state in alpha-FeSe at high pressure. We argue that
this second superconducting phase coexists with SDW antiferromagnetism in a
partial volume fraction and originates from pairing of charge carriers from
other sheets of the Fermi surface
Study of Niobium Thin Films under Pressure
Niobium is widely used in many important superconducting applications. At ambient pressure, bulk Nb has the highest critical temperature, ≈ 9.25 K among the superconducting elements. Thin films of Nb show several differences in behavior in comparison with bulk Nb, e.g. substantial increase in the upper critical field (). Critical temperature of superconducting transition is usually lower for thin films than in bulk sample and depends on thickness of the film, size of grains etc. We prepared 100 nm thick niobium thin films in the high vacuum DC magnetron sputtering system, with =8.95 K at ambient pressure. In this study, we performed measurements of superconducting transition temperature by electrical resistivity measurements of Nb thin film under hydrostatic pressure of up to 30 kbar. We observed an increase of with increasing value of pressure (/dp=7.3 mK/kbar). On the other side in the case of bulk sample of Nb we observed a decrease of value (/dp=-2.5 mK/kbar) with increasing applied pressure. Difference in superconducting properties between niobium bulk and thin film under pressure is discussed
Point-Contact Spectroscopy of Crystalline Electric Field of Heterocontacts and with Pt
Point-contact spectra of single crystals and are presented. We observed maxima connected with crystalline electric field excitations in agreement with previous results for applied voltage V>8 mV. Moreover, we observed maxima at 6 mV and 3 mV , which are probably connected with phonon modes. Moreover, we observed phonon peak at 10.6 mV in . Further measurements in magnetic fields and crystalline electric field calculations are necessary
Influence of Pressure on the Electron-Phonon Interaction in Superconductors
The electron-phonon interaction is a very important and ubiquitous process in solids, affecting almost all their physical properties. In metals, where the relaxation processes depend on both electrons and phonons, all thermodynamic and transport properties are dictated by the electron-phonon interaction. A very strong manifestation of the electron-phonon interaction is related with the superconducting state. Here we report the effect of high pressure on the transport electron-phonon interaction, λ_{tr}, in superconducting systems such as YB_6 (T_c ≈ 7.5 K), Pb (T_c ≈7.2 K), Nb bulk or thin film (T_c ≈9.2 K), and in LaB_6 in which superconductivity was not yet observed. The expected pressure effect should correspond to the theoretically predicted negative pressure effect on T_c (except for Nb thin film and LaB_6). To determine the influence of pressure on λ_{tr}, we utilized the Bloch-Grüneisen fit (denoted also as "thermal spectroscopy") of the precise temperature dependence of resistivity measurements in the normal state up to a pressure of 2.8 GPa. Based on this fit the observed negative pressure effect on λ_{tr} values, dλ_{tr}/dp, are as follows: dλ_{tr}/dp ≈-0.045 GPa^{-1} for YB_6, dλ_{tr}/dp ≈-0.13 GPa^{-1} for Pb, dλ_{tr}/dp ≈-0.019 GPa^{-1} or -0.028 GPa^{-1} for Nb bulk or thin film, respectively, and dλ_{tr}/dp ≈-0.003 GPa^{-1} for LaB_6
Pressure effect on the superconducting and the normal state of β- B i2Pd
The pressure effect up to 24.0 kbar on superconducting and normal-state properties of β-Bi2Pd single crystal (Tc≈4.98K at ambient pressure) has been investigated by measurements of the electrical resistivity. In addition, we have performed the heat capacity measurements in the temperature range 0.7-300 K at ambient pressure. The recent calculations of electronic density of states, electron-phonon interaction spectral function, and phonon density of states of β-Bi2Pd [Zheng and Margine, Phys. Rev. B 95, 014512 (2017)10.1103/PhysRevB.95.014512], are used to fit the resistivity and the heat capacity data. In the superconducting state we have focused on the influence of pressure on the superconducting transition temperature Tc and upper critical field Hc2 and a negative effect with dTc/dp=-0.025K/kbar and dHc2/dp=-8mT/kbar is found. A simplified Bloch-Grüneisen model was used to analyze the pressure effect on the temperature dependence of the normal-state resistivity. The obtained results point to a decrease of the electron-phonon coupling parameter λ and to a shift of phonon frequencies to higher values with pressure. Moreover, the temperature dependence of the normal-state resistivity follows a T2 dependence above Tc up to about 25 K. Together with the enhanced value of Sommerfeld coefficient γ=13.23mJmol-1K-2 these results point to a certain role of the electron-electron interaction in the superconducting pairing mechanism in β-Bi2PdThis work was supported by projects VEGA 2/0032/16, VEGA 2/0149/16, APVV-16-0372, EU ERDF Grant No. ITMS26220120047 and by European Microkelvin Platform. Liquid nitrogen for experiments was sponsored by US Steel Košice, s.r.o. E.H. was supported by the Departamento Administrativo de Ciencia, Tecnología e Innovación, COLCIENCIAS (Colombia), programa doctorados en el exterior convocatoria No. 568-2012 and the Universidad Nacional de Colombia, División de Investigación y Extensión sede Bogotá (DIEP) Project No. 356515. H.S. and A. C. by the Spanish Ministry of Economy and Competitiveness (FIS2017-84330-R, MDM-2014-0377), by the Comunidad de Madrid through program Nanofrontmag-CM (S2013/MIT-2850), and by COST CA1621
Magnetic Phase Diagram of under High Pressure
is a Shastry-Sutherland frustrated system which exhibits very complex magnetic properties. In this contribution the phase diagram of magnetic field vs. temperature of under hydrostatic pressure up to 26.5 kbar is investigated using sensitive ac-resistance measurements. Temperature and magnetic field dependences of resistance at various pressures were carried out in a piston cylinder pressure cell between 1.7 and 14 K and in magnetic fields up to 6 T. The obtained results exhibit shifts of ordering temperatures as well as shifts of boundaries between different magnetic phases. The observed pressure dependences of can be described by the relation d /dp=+(0.16÷0.18) %/kbar. The effect of pressure on various interactions between magnetic ions in this compound is discussed