arXiv subjects

P. Samuely

Publications and source records attributed to P. Samuely.

At least 37 records · Page 2Linked to original sources

arXiv2016

Fermionic scenario for the destruction of superconductivity in ultrathin MoC films evidenced by STM measurements

We use sub-Kelvin scanning tunneling spectroscopy to investigate the suppression of superconductivity in homogeneously disordered ultrathin MoC films. We observe that the superconducting state remains spatially homogeneous even on the films of 3 nm thickness. The vortex imaging suggests the global phase coherence in our films. Upon decreasing thickness, when the superconducting transition drops from 8.5 to 1.2 K, the superconducting energy gap follows perfectly Tc. All this is pointing to a two-stage fermionic scenario of the superconductor-insulator transition (SIT) via a metallic state as an alternative to the direct bosonic SIT scenario with a Cooper-pair insulating state evidenced by the last decade STM experiments.

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arXiv2015

Finite quasiparticle lifetime in disordered superconductors

We investigate the complex conductivity of a highly disordered MoC superconducting film with $k_Fl\approx 1$, where $k_F$ is the Fermi wavenumber and $l$ is the mean free path, derived from experimental transmission characteristics of coplanar waveguide resonators in a wide temperature range below the superconducting transition temperature $T_c$. We find that the original Mattis-Bardeen model with a finite quasiparticle lifetime, $τ$, offers a perfect description of the experimentally observed complex conductivity. We show that $τ$ is appreciably reduced by scattering effects. Characteristics of the scattering centers are independently found by the scanning tunneling spectroscopy and agree with those determined from the complex conductivity.

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arXiv2014

High pressure effect on superconductivity of YB6

Pressure effect on superconducting properties of two YB6 samples (Tc = 5.9 and 7.5 K) were investigated by measurements of electrical resistivity, magnetic susceptibility, and X-ray diffraction in the pressure range up to 320 kbar. Magnetoresistivity measurements down to 60 mK and up to 47 kbar have shown a negative pressure effect on Tc as well as on the third critical field Hc3 with the slopes dlnTc/dp = -0.59%/kbar and dlnHc3/dp = -1.1%/kbar, respectively. The magnetic susceptibility measurements evidenced that the slope of dlnTc/dp gradually decreases with pressure reaching 3 times smaller value at 112 kbar. The lattice parameter measurements revealed the volume reduction of 14% at 320 kbar. The pressure-volume dependence is described by the Rose-Vinet equation of state. The obtained relative volume dependence dlnTc/dlnV analyzed by the McMillan formula for Tc indicates that the reduction of the superconducting transition temperature is mainly due to hardening of the Einstein-like phonon mode responsible for the superconducting coupling. This is confirmed by the analysis of the resistivity measurements in the normal state up to T = 300 K performed at pressures up to 28 kbar.

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arXiv2013

Far-infrared electrodynamics of thin superconducting NbN film in magnetic field

We studied a thin superconducting NbN film in magnetic fields up to 8 T above the zero- temperature limit by means of time-domain terahertz and scanning tunneling spectroscopies in order to understand the vortex response. Scanning tunneling spectroscopy was used to determine the optical gap and the upper critical field of the sample. The obtained values were subsequently used to fit the terahertz complex conductivity spectra in the magnetic field in the Faraday geometry above the zero temperature limit. These spectra are best described in terms of the Coffey-Clem self-consistent solution of a modified London equation in the flux creep regime.

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arXiv2013

Heat Capacity of Single Crystal CuxTiSe2 Superconductors

We present heat capacity measurements on a series of superconducting Cu$_x$TiSe$_2$ single crystals with different Cu content down to 600 mK and up to 1 T performed by ac microcalorimetry. The samples cover a large portion of the phase diagram from an underdoped to a slightly overdoped region with an increasing superconducting critical temperature and the charge density wave (CDW) order gradually suppressed. The electronic heat capacity as a function of normalized temperature $T/T_c$ shows no difference regardless of the concentration of copper, i.e., regardless of how much the CDW order is developed in the samples. The data analysis reveals consistently a single s-wave gap with an intermediate coupling strength $2Δ/k_BT_c$ = 3.7 for all samples.

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arXiv2013

Point-contact spectroscopy of the phononic mechanism of superconductivity in YB6

Lortz et al. [Phys. Rev. B 73, 024512 (2006)] have utilized specific heat and resistivity measurements as "thermal spectroscopies" to deconvolve the spectrum of the electron-phonon interaction in YB$_6$ assuming a major role of the low frequency phonon mode in mediating superconductivity. Here, we present direct point-contact spectroscopy studies of the superconducting interaction in this system. As a result the normalized superconducting gap reveals a strong coupling with $2Δ/k_BT_c = 4$ and moreover the spectra contain nonlinearities typical of the electron-phonon interaction at energies around 8 meV. The in-magnetic-field measurements evidence that the phonon features found in the second derivative of the current-voltage characteristics are due to the energy dependence of the superconducting energy gap as their energy position shrinks equally as the gap is closing. This is a direct proof that the superconducting coupling in the system is due to the low energy Einstein-like phonon mode associated with the yttrium ion vibrations in a perfect agreement with determinations from bulk measurements.

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arXiv2013

Type II superconductivity in SrPd2Ge2

Previous investigations have shown that SrPd2Ge2, a compound isostructural with "122" iron pnictides but iron- and pnictogen-free, is a conventional superconductor with a single s-wave energy gap and a strongly three-dimensional electronic structure. In this work we reveal the Abrikosov vortex lattice formed in SrPd2Ge2 when exposed to magnetic field by means of scanning tunneling microscopy and spectroscopy. Moreover, by examining the differential conductance spectra across a vortex and estimating the upper and lower critical magnetic fields by tunneling spectroscopy and local magnetization measurements, we show that SrPd2Ge2 is a strong type II superconductor with κ>> sqrt(2). Also, we compare the differential conductance spectra in various magnetic fields to the pair breaking model of Maki - de Gennes for dirty limit type II superconductor in the gapless region. This way we demonstrate that the type II superconductivity is induced by the sample being in the dirty limit, while in the clean limit it would be a type I superconductor with κ << sqrt(2), in concordance with our previous study (T. Kim et al., Phys. Rev. B 85, (2012)).

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arXiv2011

Evidence for conventional superconductivity in SrPd2Ge2

Electronic structure of SrPd2Ge2 single crystals is studied by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and band-structure calculations within the local-density approximation (LDA). The STS measurements show single s-wave superconducting energy gap Δ(0) = 0.5 meV. Photon-energy dependence of the observed Fermi surface reveals a strongly three-dimensional character of the corresponding electronic bands. By comparing the experimentally measured and calculated Fermi velocities a renormalization factor of 0.95 is obtained, which is much smaller than typical values reported in Fe-based superconductors. We ascribe such an unusually low band renormalization to the different orbital character of the conduction electrons and using ARPES and STS data argue that SrPd2Ge2 is likely to be a conventional superconductor, which makes it clearly distinct from isostructural iron pnictide superconductors of the "122" family.

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arXiv2011

Superconducting energy gap in MgCNi3 single crystals: Point-contact spectroscopy and specific-heat measurements

Specific heat has been measured down to 600 mK and up to 8 Tesla by the highly sensitive AC microcalorimetry on the MgCNi3 single crystals with Tc ~ 7 K. Exponential decay of the electronic specific heat at low temperatures proved that a superconducting energy gap is fully open on the whole Fermi surface, in agreement with our previous magnetic penetration depth measurements on the same crystals. The specific-heat data analysis shows consistently the strong coupling strength 2D/kTc ~ 4. This scenario is supported by the direct gap measurements via the point-contact spectroscopy. Moreover, the spectroscopy measurements show a decrease in the critical temperature at the sample surface accounting for the observed differences of the superfluid density deduced from the measurements by different techniques.

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arXiv2010

Specific-heat measurements of superconducting NbS2 single crystal in an external magnetic field: Study on the energy gap structure

The heat capacity of a 2H-NbS2 single crystal has been measured by a highly sensitive ac technique down to 0.6 K and in magnetic fields up to 14 T. At very low temperatures data show excitations over an energy gap (2DS/kBTc \approx 2.1) much smaller than the BCS value. The overall temperature dependence of the electronic specific heat Ce can be explained either by the existence of a strongly anisotropic single-energy gap or within a two-gap scenario with the large gap about twice bigger than the small one. The field dependence of the Sommerfeld coefficient shows a strong curvature for both principal-field orientations, parallel and perpendicular to the c axis of the crystal, resulting in a magnetic field dependence of the superconducting anisotropy. These features are discussed in comparison to the case of MgB2 and to the data obtained by scanning-tunneling spectroscopy. We conclude that the two-gap scenario better describes the gap structure of NbS2 than the anisotropic s-wave model.

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arXiv2009

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.

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arXiv2009

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 Ba$_{0.55}$K$_{0.45}$Fe$_2$As$_2$ single crystals are presented. The spectra show significant differences when measured in the $ab$ plane in comparison with those measured in the $c$ direction of the crystal. In the latter case only a reduced point-contact conductance around zero bias has been revealed persisting well above $T_c$ and probably related to the structural and magnetic transitions in the system. Within the $ab$ plane two superconducting energy gaps are detected below $T_c$. Here a reduced conductance above $T_c$ could also be found. The fits of the $ab$-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.

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arXiv2008

Possible two-gap superconductivity in NdFeAs(O,F) probed by point-contact Andreev-reflection spectroscopy

Systematic studies of the NdFeAsOF superconducting energy gap via the point-contact Andreev-reflection (PCAR) spectroscopy are presented. The PCAR conductance spectra show at low temperatures a pair of gap-like peaks at about 4 - 7 mV indicating the superconducting energy gap and in most cases also a pair of humps at around 10 mV. Fits to the s-wave two-gap model of the PCAR conductance allowed to determine two superconducting energy gaps in the system. The energy-gap features however disappear already at T* = 15 to 20 K, much below the particular Tc of the junction under study. At T* a zero-bias conductance (ZBC) peak emerges, which at higher temperatures usually overwhelms the spectrum with intensity significantly higher than the conductance signal at lower temperatures. Possible causes of this unexpected temperature effect are discussed. In some cases the conductance spectra show just a reduced conductance around the zero-bias voltage, the effect persisting well above the bulk transition temperature. This indicates a presence of the pseudogap in the system.

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arXiv2006

Point-contact spectroscopy of Al- and C-doped MgB2. Superconducting energy gaps and scattering studies

The two-band/two-gap superconductivity in aluminium and carbon doped MgB$_2$ has been addressed by the point-contact spectroscopy. Two gaps are preserved in all samples with $T_c's$ down to 22 K. The evolution of two gaps as a function of the critical temperature in the doped systems suggest the dominance of the band-filling effects but for the increased Al-doping the enhanced interband scattering approaching two gaps must be considered. The magnetic field dependences of the Andreev reflection excess currents as well as zero-energy density of states determined from the experimental data are used to analyze the intraband scattering. It is shown, that while the C-doping increases the intraband scattering in the $π$-band more rapidly then in the $σ$ band, the Al-doping does not change their relative weight.

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arXiv2006

Influence of Al doping on the critical fields and gap values in magnesium diboride single crystals

The lower ($H_{c1}$) and upper ($H_{c2}$) critical fields of Mg$_{1-x}$Al$_{x}$B$_2$ single crystals (for $x = 0$, 0.1 and $\gtrsim 0.2$) have been deduced from specific heat and local magnetization measurements, respectively. We show that $H_{c1}$ and $H_{c2}$ are both decreasing with increasing doping content. The corresponding anisotropy parameter $Γ_{H_{c2}}(0) = H^{ab}_{c2}(0)/H^c_{c2}(0)$ value also decreases from $\sim 5$ in pure MgB$_2$ samples down to $\sim 1.5$ for $x \gtrsim 0.2$ whereas $Γ_{H_{c1}}(0)=H^c_{c1}(0)/H^{ab}_{c1}(0)$ remains on the order of 1 in all samples. The small and large gap values have been obtained by fitting the temperature dependence of the zero field electronic contribution to the specific heat to the two gap model for the three Al concentrations. Very similar values have also been obtained by point contact spectroscopy measurements. The evolution of those gaps with Al concentration suggests that both band filling and interband scattering effects are present.

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arXiv2005

Comment on: "Band Filling and Interband Scattering Effects in MgB2: Carbon versus Aluminium Doping"

In a recent Physical Review Letter Kortus et al. claim that the observed decrease of $T_c$ in Al and C doped MgB$_2$ samples is mainly due to the band filling by the electron doping but for a fuller understanding of the different behavior of the superconducting energy gaps in Al and C doped samples it is necessary to also include an increased interband scattering for the carbon doped samples. We argue in the following that this latter statement is misleading and is based on the assumption that for carbon doping the two superconducting gaps merge near 10 % carbon doping: an assumption that contradicts most of the existing experimental data.

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arXiv2004

Systematic study of the two band/two gap superconductivity in carbon-substituted MgB2 by point-contact spectroscopy

Point-contact measurements on the carbon-substituted Mg(B$_{1-x}$C$_x$)$_2$ filament/powder samples directly reveal a retention of the two superconducting energy gaps in the whole doping range from $x = 0$ to $x \approx 0.1$. The large gap on the $σ$-band is decreased in an essentially linear fashion with increasing the carbon concentrations. The changes in the the small gap $Δ_π$ up to 3.8 % C are proportionally smaller and are more difficult to detect but for the heavily doped sample with $x \approx 0.1$ and $T_c = 22$ K both gaps are still present, and significantly reduced, consistent with a strong essentially linear, reduction of each gap with the transition temperature.

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arXiv2003

Two band/two gap superconductivity in carbon-substituted MgB2 evidenced by point-contact spectroscopy

The Andreev reflection measurements of the superconducting energy gap in the carbon-substituted MgB$_2$ are presented. Despite the strong suppression of the transition temperature by 17 K in comparison with the pure MgB$_2$, the same reduced value of the small superconducting energy gap with $2Δ/kT_c \approx$ 1.7 has been systematically observed. This indicates that the two band/two gap superconductivity is still preserved here.

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