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P. Szabo

Publications and source records attributed to P. Szabo.

At least 19 recordsLinked to original sources
arXiv2025

Single-gap two-band superconductivity well above the Pauli limit in non-centrosymmetric TaIr$_2$B$_2$

Non-centrosymmetric superconducting materials represent an exciting class of novel superconductors featuring a variety of unconventional properties, including mixed-parity pairing and very high upper critical fields. Here, we present a comprehensive study of TaIr$_2$B$_2$ (with $T_c$ = 5.1 K), using a set of complementary experimental methods, including bulk- and surface-sensitive techniques. We provide evidence that this system is a two-band, yet it behaves as a single-gap superconductor with a strong coupling. The upper critical field of TaIr$_2$B$_2$ significantly exceeds the Pauli limit and exhibits a nearly linear temperature dependence down to the lowest temperatures. This behavior, rarely seen in superconductors, is discussed in terms of anti-symmetric spin-orbit interaction, two-band-, and strong-coupling effects, as well as disorder.

cond-mat.supr-con↗
arXiv2023

Disorder- and magnetic field-tuned fermionic superconductor-insulator transition in MoN thin films. Transport and STM studies

Superconductor-insulator transition (SIT) driven by disorder and transverse magnetic field has been investigated in ultrathin MoN films by means of transport measurements and scanning tunneling microscopy and spectroscopy. Upon decreasing thickness, the homogeneously disordered films show increasing sheet resistance Rs, shift of the superconducting transition Tc to lower temperatures with the 3 nm MoN being the last superconducting film and thinner films already insulating. Fermionic scenario of SIT is evidenced by applicability of the Finkelsteins model, by the fact that Tc and the superconducting gap are coupled with a constant ratio, and by the spatial homogeneity of the superconducting and electronic characteristics. The logarithmic anomaly found in the tunneling spectra of the non-superconducting films is further enhanced in increased magnetic field due to the Zeeman spin effects driving the system deeper into the insulating state and pointing also to fermionic SIT.

cond-mat.supr-con↗
arXiv2023

Suppressed superconductivity in ultrathin Mo2N films due to pair-breaking at the interface

A strong disorder characterized by a small product of the Fermi vector kF and the electron mean free l drives superconductors towards insulating state. Such disorder can be introduced by making the films very thin. Here, we present 3-nm Mo2N film with k_F*l ~ 2 with a resistive superconducting transition temperature Tc = 2 K heavily suppressed in comparison with the bulk Tc. Superconducting density of states (DOS) with smeared gap-like peaks and in-gap states, so called Dynes DOS, is observed by the low temperature tunneling spectroscopy despite a sharp resistive transition. By scanning tunneling microscope the spectral maps are obtained and related to the surface topography. The maps show a spatial variation of the superconducting energy gap on the order of 20 % which is not accidental but well correlates with the surface corrugation: protrusions reveal larger gap, smaller spectral smearing and smaller in-gap states. In agreement with our previous measurements on ultrathin MoC films we suggest that the film-substrate interface introducing the local pair-breaking is responsible for the observed effects and generally for the suppression of the superconductivity in these ultrathin films.

cond-mat.supr-con↗
arXiv2021

Superconductivity in hole-doped germanium point contacts

We have observed superconductivity in heavy p-doped Ge by measuring of differential resistance dV/dI(V) of Ge - PtIr point contacts. The superconducting (SC) features disappear above 6 K or above 1 T, what can be taken as the critical temperature and the critical magnetic field, respectively. The observed dV/dI(V) spectrum with Andreev reflection like features was fitted within one-gap Blonder-Tinkham-Klapwijk model. The extracted SC gap demonstrates Bardeen-Cooper-Schrieffer-like behavior with 2 Delta/kBTc = 10+/-1 ratio, which is much higher that expected for conventional superconductors. Magnetic field suppresses Andreev reflection features, but the SC gap moderately decreases in magnetic field similarly as it was observed previously for the type-II superconductors, including nickel borocarbide and iron-based superconductors. Curiously, we have not yet observed superconductivity in n-doped Ge with a similar dopant concentration.

cond-mat.supr-con↗
arXiv2017

Suppression of the superconductivity in ultrathin amorphous Mo$_{78}$Ge$_{22}$ thin films observed by STM

In contact with a superconductor, a normal metal modifies its properties due to Andreev reflection. In the current work, the local density of states (LDOS) of superconductor - normal metal Mo$_{78}$Ge$_{22}$ - Au bilayers are studied by means of STM applied from the Au side. Three bilayers have been prepared on silicate glass substrate consisting of 100, 10 and 5 nm MoGe thin films covered always by 5 nm Au layer. The tunneling spectra were measured at temperatures from 0.5 K to 7 K. The two-dimensional cross-correlation between topography and normalized zero-bias conductance (ZBC) indicates a proximity effect between 100 and 10 nm MoGe thin films and Au layer where a superconducting gap slightly smaller than that of bulk MoGe is observed. The effect of the thinnest 5 nm MoGe layer on Au leads to much smaller gap moreover the LDOS reveals almost completely suppressed coherence peaks. This is attributed to a strong pair-breaking effect of spin-flip processes at the interface between MoGe films and the substrate.

cond-mat.supr-con↗
arXiv2017

Confined Dense Circumstellar Material Surrounding a Regular Type II Supernova: The Unique Flash-Spectroscopy Event of SN 2013fs

With the advent of new wide-field, high-cadence optical transient surveys, our understanding of the diversity of core-collapse supernovae has grown tremendously in the last decade. However, the pre-supernova evolution of massive stars, that sets the physical backdrop to these violent events, is theoretically not well understood and difficult to probe observationally. Here we report the discovery of the supernova iPTF13dqy = SN 2013fs, a mere ~3 hr after explosion. Our rapid follow-up observations, which include multiwavelength photometry and extremely early (beginning at ~6 hr post-explosion) spectra, map the distribution of material in the immediate environment (<~ 10^15 cm) of the exploding star and establish that it was surrounded by circumstellar material (CSM) that was ejected during the final ~1 yr prior to explosion at a high rate, around 10^-3 solar masses per year. The complete disappearance of flash-ionised emission lines within the first several days requires that the dense CSM be confined to within <~ 10^15 cm, consistent with radio non-detections at 70--100 days. The observations indicate that iPTF13dqy was a regular Type II SN; thus, the finding that the probable red supergiant (RSG) progenitor of this common explosion ejected material at a highly elevated rate just prior to its demise suggests that pre-supernova instabilities may be common among exploding massive stars.

astro-ph.HE↗
arXiv2016

Implementation of the Log-Conformation Formulation for Two-Dimensional Viscoelastic Flow

We have implemented the log-conformation method for two-dimensional viscoelastic flow in COMSOL, a commercial high-level finite element package. The code is verified for an Oldroyd-B fluid flowing past a confined cylinder. We are also able to describe the well-known bistability of the viscoelastic flow in a cross-slot geometry for a FENE-CR fluid, and we describe the changes required for performing simulations with the Phan-Thien-Tanner (PTT), Giesekus and FENE-P models. Finally, we calculate the flow of a FENE-CR fluid in a geometry with three in- and outlets. The implementation is included in the supplementary material, and we hope that it can inspire new as well as experienced researchers in the field of differential constitutive equations for viscoelastic flow.

cs.CE↗
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.

cond-mat.supr-con↗
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.

cond-mat.supr-con↗
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.

cond-mat.supr-con↗
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.

cond-mat.supr-con↗
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.

cond-mat.supr-con↗
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.

cond-mat.supr-con↗
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.

cond-mat.supr-con↗
arXiv2008

Short time die attach characterisation of semiconductor devices

Thermal qualification of the die attach of semiconductor devices is a very important element in the device characterization as the temperature of the chip is strongly affected by the quality of the die attach. Voids or delaminations in this layer may cause higher temperature elevation and thus damage or shorter lifetime. Thermal test of each device in the manufacturing process would be the best solution for eliminating the devices with wrong die attach layer. In this paper we will present the short time thermal transient measurement method and the structure function evaluation through simulations and measurements for die attach characterization. We will also present a method for eliminating the very time consuming calibration process. Using the proposed methods even the in-line testing of LEDs can be accomplished.

physics.gen-ph↗
arXiv2007

Characterisation of the Etching Quality in Micro-Electro-Mechanical Systems by Thermal Transient Methodology

Our paper presents a non-destructive thermal transient measurement method that is able to reveal differences even in the micron size range of MEMS structures. Devices of the same design can have differences in their sacrificial layers as consequence of the differences in their manufacturing processes e.g. different etching times. We have made simulations examining how the etching quality reflects in the thermal behaviour of devices. These simulations predicted change in the thermal behaviour of MEMS structures having differences in their sacrificial layers. The theory was tested with measurements of similar MEMS devices prepared with different etching times. In the measurements we used the T3Ster thermal transient tester equipment. The results show that deviations in the devices, as consequence of the different etching times, result in different temperature elevations and manifest also as shift in time in the relevant temperature transient curves.

cs.OH↗
arXiv2007

Thermal measurement and modeling of multi-die packages

Thermal measurement and modeling of multi-die packages became a hot topic recently in different fields like RAM chip packaging or LEDs / LED assemblies, resulting in vertical (stacked) and lateral arrangement. In our present study we show results for a mixed arrangement: an opto-coupler device has been investigated with 4 chips in lateral as well as vertical arrangement. In this paper we give an overview of measurement and modeling techniques and results for stacked and MCM structures, describe our present measurement results together with our structure function based methodology of validating the detailed model of the package being studied. Also, we show how to derive junction-to-pin thermal resistances with a technique using structure functions.

cond-mat.mtrl-sci↗