arXiv subjects

P. Szabó

Publications and source records attributed to P. Szabó.

14 recordsLinked to original sources

arXiv2024

Two-gap superconductivity in the noncentrosymmetric La$_3$Se$_4$ compound

Point-contact Andreev reflection spectroscopy at low temperatures and high magnetic fields has been performed on a noncentrosymmetric La$_3$Se$_4$ superconductor with a critical temperature $T_c$ = 8 K. Two superconducting energy gaps $Δ_1$ and $Δ_2$ with $2Δ_{1}/k_{B} T_{c}$ ~ 5.8 and $2Δ_{2}/k_{B} T_{c}$ ~ 2.3, are directly observed in some of the spectra. The temperature and magnetic field effects help to resolve a two-gap structure even on the most frequent spectra where at low temperatures only a single gap is apparent, reflected in a pair of maxima around the zero bias. Two-gap superconductivity consistently with the point contact Andreev reflection spectroscopy is also supported by the heat capacity and the Hall probe magnetization measurements.

cond-mat.supr-con↗

arXiv2021

Extreme in-plane upper critical magnetic fields of heavily doped quasi-two-dimensional transition metal dichalcogenides

Extreme in-plane upper critical magnetic fields $B_{c2//ab}$ strongly violating the Pauli paramagnetic limit have been observed in the misfit layer $(LaSe)_{1.14}(NbSe_2)$ and $(LaSe)_{1.14}(NbSe_2)_2$ single crystals with $T_c$ = 1.23 K and 5.7 K, respectively. The crystals show a two-dimensional to three-dimensional transition at temperatures slightly below $T_c$ with an upturn in the temperature dependence of $B_{c2//ab}$, a temperature dependent huge superconducting anisotropy and a cusplike behavior of the angular dependence of $B_{c2}$. Both misfits are characterized by a strong charge transfer from LaSe to $NbSe_2$. As shown in our previous work, $(LaSe)_{1.14}(NbSe_2)$ is electronically equivalent to the highly doped $NbSe_2$ monolayers. Then, the strong upper critical field can be attributed to the Ising coupling recently discovered in atomically thin transition metal dichalcogenides with strong spin-orbit coupling and a lack of inversion symmetry. A very similar behavior is found in $(LaSe)_{1.14}(NbSe_2)_2$, where the charge transfer is nominally twice as big, which could eventually lead to complete filling of the $NbSe_2$ conduction band and opening superconductivity in LaSe. Whatever the particular superconducting mechanism would be, a common denominator in both misfits is that they behave as a stack of almost decoupled superconducting atomic layers, proving that Ising superconductivity can also exist in bulk materials.

cond-mat.supr-con↗

arXiv2021

Zeeman driven superconductor insulator transition in strongly disordered MoC film. STM and transport studies in transverse magnetic field

Superconductor insulator transition in transverse magnetic field is studied in the highly disordered MoC film with the product of the Fermi momentum and the mean free path $k_F*l$ close to unity. Surprisingly, the Zeeman paramagnetic effects dominate over orbital coupling on both sides of the transition. In superconducting state it is evidenced by a high upper critical magnetic field $B_{c2}$, by its square root dependence on temperature, as well as by the Zeeman splitting of the quasiparticle density of states (DOS) measured by scanning tunneling microscopy. At $B_{c2}$ a logarithmic anomaly in DOS is observed. This anomaly is further enhanced in increasing magnetic field, which is explained by the Zeeman splitting of the Altshuler-Aronov DOS driving the system into a more insulating or resistive state. Spin dependent Altshuler-Aronov correction is also needed to explain the transport behavior above $B_{c2}$.

cond-mat.supr-con↗

arXiv2020

TESS observations of Cepheid stars: first light results

We present the first analysis of Cepheid stars observed by the TESS space mission in Sectors 1 to 5. Our sample consists of 25 pulsators: ten fundamental mode, three overtone and two double-mode classical Cepheids, plus three Type II and seven anomalous Cepheids. The targets were chosen from fields with different stellar densities, both from the Galactic field and from the Magellanic System. Three targets have 2-minute cadence light curves available by the TESS Science Processing Operations Center: for the rest, we prepared custom light curves from the full-frame images with our own differential photometric FITSH pipeline. Our main goal was to explore the potential and the limitations of TESS concerning the various subtypes of Cepheids. We detected many low amplitude features: weak modulation, period jitter, and timing variations due to light-time effect. We also report signs of non-radial modes and the first discovery of such a mode in an anomalous Cepheid, the overtone star XZ Cet, which we then confirmed with ground-based multicolor photometric measurements. We prepared a custom photometric solution to minimize saturation effects in the bright fundamental-mode classical Cepheid, $β$ Dor with the lightkurve software, and we revealed strong evidence of cycle-to-cycle variations in the star. In several cases, however, fluctuations in the pulsation could not be distinguished from instrumental effects, such as contamination from nearby sources which also varies between sectors. Finally, we discuss how precise light curve shapes will be crucial not only for classification purposes but also to determine physical properties of these stars.

astro-ph.SR↗

arXiv2019

Sub-kelvin Andreev reflection spectroscopy of superconducting gaps in FeSe

Point contact Andreev reflection studies have been conducted on FeSe single crystals by lowering the temperatures down to 0.5 K. The point contact Andreev reflection spectra were analyzed in the framework of the two-band model. As a result, the presence of two anisotropic superconducting gaps in FeSe were certainly established and their BCS-like temperature dependencies were obtained. The weights of each gap have been determined and the anisotropy parameter has been calculated. It is shown, that sub-kelvin temperatures are necessary to ascertain details of the superconducting gap structure, especially for multiband materials when Andreev reflection spectroscopy is applied.

cond-mat.supr-con↗

arXiv2019

Observation of quantum corrections to conductivity up to optical frequencies

It is well known that conductivity of disordered metals is suppressed in the limit of low frequencies and temperatures by quantum corrections. Although predicted by theory to exist up to much higher energies, such corrections have so far been experimentally proven only for $\lesssim$80 meV. Here, by a combination of transport and optical studies, we demonstrate that the quantum corrections are present in strongly disordered conductor MoC up to at least $\sim$4 eV, thereby extending the experimental window where such corrections were found by a factor of 50. The knowledge of both, the real and imaginary parts of conductivity, enables us to identify the microscopic parameters of the conduction electron fluid. We find that the conduction electron density of strongly disordered MoC is surprisingly high and we argue that this should be considered a generic property of metals on the verge of disorder-induced localization transition.

cond-mat.mes-hall↗

arXiv2019

Single-gap superconductivity in Mo$_8$Ga$_{41}$

In this paper, potential two-gap superconductivity in Mo$_8$Ga$_{41}$ is addressed in detail by means of thermodynamic and spectroscopic measurements. Combination of highly sensitive ac-calorimetry and scanning tunneling spectroscopy (STS), as bulk and surface sensitive probes, utilized on the same piece of crystal reveals that there is only one intrinsic gap in the system featuring strong electron-phonon coupling. Traces of multiple superconducting phases seen by STS and also in the heat capacity measured in high magnetic fields on a high-quality and seemingly single-phase crystal might mimic the multigap superconductivity of Mo$_8$Ga$_{41}$ suggested recently in several studies.

cond-mat.supr-con↗

arXiv2018

On the origin of in-gap states in homogeneously disordered ultrathin films. MoC case

Many disordered superconducting films exhibit smeared tunneling spectra with evident in-gap states. We demonstrated that the tunneling density of states in ultrathin MoC films is gapless and can be described by the Dynes version of the BCS density of states with a strong broadening parameter Gamma accounting for the suppression of coherence peaks and increased in-gap states. The thinner the film, the lower the Tc and the superconducting energy gap Delta and the larger the Gamma. MoC films of 3 nm thickness deposited simultaneously on silicon and sapphire substrates reveal very similar scalar disorder, evidenced by the equal sheet resistance, but exhibit different superconducting characteristics of Tc, Delta and Gamma, suggesting that pair breaking responsible for the dissipation channel and the suppression of superconductivity originates on the film-substrate interface. It indicates that sapphire is a stronger pair breaker. Interface pair breaking can be operative in other cases as well.

cond-mat.supr-con↗

arXiv2017

Unconventional superconductivity in the strong-coupling limit for the heavy fermion system CeCoIn$_5$

We present scanning tunneling spectroscopy measurements of the local quasiparticles' excitation spectra of CeCoIn$_5$ between 440mK and 3K in samples with a bulk $T_{\rm c}=2.25$K. The spectral shape of our low-temperature tunneling data, quite textbook nodal-gap conductance, allow us to confidently fit the spectra with a d-wave density of states considering also a shortening of quasiparticles' lifetime term $Γ$. The $Δ(0)$ value obtained from the fits yields a BCS ratio $2Δ/kT_{\rm c} =7.73$ suggesting that CeCoIn$_5$ is an unconventional superconductor in the strong coupling limit. The fits also suggest that the height of coherence peaks in CeCoIn$_5$ is reduced with respect to a pure BCS spectra and therefore the coupling of quasiparticles with spin excitations should play a relevant role. In addition, the tunneling conductance shows a depletion at energies smaller than $Δ$ for temperatures larger than the bulk $T_{\rm c}$, giving further support to the existence of a pseudogap phase that in our samples span up to $T^{*}\sim 1.2 T_{\rm c}$. The phenomenological scaling of the pseudogap temperature observed in various families of cuprates, $2Δ/kT^{*} \sim 4.3 $, is not fulfilled in our measurements. This suggests that in CeCoIn$_5$ the strong magnetic fluctuations might conspire to close the local superconducting gap at a smaller pesudogap temperature-scale than in cuprates.

cond-mat.supr-con↗

arXiv2016

Single gap superconductivity in beta-Bi2Pd

beta-Bi2Pd compound has been proposed as another example of a multi-gap superconductor [Y. Imai et al., J. Phys. Soc. Jap. 81, 113708 (2012)]. Here, we report on measurements of several important physical quantities capable to show a presence of multiple energy gaps on our superconducting single crystals of beta-Bi2Pd with the critical temperature Tc close to 5 K. The calorimetric study via a sensitive ac technique shows a sharp anomaly at the superconducting transition, however only a single energy gap is detected. Also other characteristics inferred from calorimetric measurements as the field dependence of the Sommerfeld coefficient and the temperature and angular dependence of the upper critical magnetic field point unequivocally to standard single s-wave gap superconductivity. The Hall-probe magnetometry provides the same result from the analysis of the temperature dependence of the lower critical field. A single-gapped BCS density of states is detected by the scanning tunneling spectroscopy measurements. Then, the bulk as well as the surface sensitive probes evidence a standard conventional superconductivity in this system where the topologically protected surface states have been recently detected by ARPES [M. Sakano et al., Nature Comm. 6, 8595 (2015)] .

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

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.

cond-mat.supr-con↗

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)).

cond-mat.supr-con↗

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.

cond-mat.supr-con↗