arXiv subjects

H. Suderow

Publications and source records attributed to H. Suderow.

At least 19 recordsLinked to original sources

arXiv2023

Enhancement of vortex liquid phase and reentrant behavior in NiBi3 single crystals

We investigated the vortex phase diagram of needle shaped high quality NiBi3 single crystals by transport measurements. The current is applied along the crystalline b-axis of this intermetallic quasi-1D BCS superconductor. The single crystals show a Ginzburg-Levanchuk (Gi) parameter few orders of magnitude larger than other low Tc BCS superconductors. Vortex phase diagram, critical currents and pinning forces have been extracted from the experimental data. The main findings are: 1) Enhancement of the vortex liquid phase in comparison with low Tc superconductors, 2) reentrance of the liquid phase at low fields and 3) deviation of the pinning force vs field from the usual pinning mechanisms. The interplay between weak pinning, due to quenched disorder, and the quasi-1D character of the material could be a hint to explain the lack of a single pinning mechanism.

cond-mat.supr-con↗

arXiv2022

CDW signatures in the electronic structure of LaSb2 at 13 K and metal-insulator transition

Light rare-earth antimonide LaSb2 is a material susceptible to nesting and exhibits a Charge Density Wave (CDW) at 355 K as well as superconductivity below 1.2 K. In the seek for additional CDW transitions, we have studied the temperature-dependent of LaSb2 at 200 and 13 K and infer CDW manifestations by combining angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations. ARPES measurements at 200 K show a metallic system while it appears to be semiconducting at 13 K, at odds with existing resistivity measurements. At 13 K, ARPES shows the band folding of the inner Fermi surface pockets, with considerable spectral weight on the folded band. We find a nesting vector at q = 0.25$\pm$0.02 Å -1. In addition, we observe Umklapps of other bands due to the onset of the new periodicity, together with a semiconducting behavior in the whole reciprocal space. Calculations demonstrate that the folded band is associated with the La-Sb layer and that in-plane distortion is the most probable structural modification in the system, probably affecting the whole unit cell.

cond-mat.mtrl-sci↗

arXiv2017

Influence of the multiband sign changing superconductivity on the vortex cores and on vortex pinning in the new stoichiometric high Tc CaKFe4As4

We study the superconducting density of states and vortex lattice of single crystals of CaKFe$_4$As$_4$ using a scanning tunneling microscope (STM). This material has a critical temperature of $T_c= 35\,$ K, which is one of the highest among stoichiometric iron based superconductors (FeBSC) and is comparable to $T_c$ found near optimal doping in other FeBSC. Using quasi-particle interference we identify the hole sheets around the zone center and find that two superconducting gaps open in these sheets. The scattering centers are small defects that can be localized in the surface topography and just produce quasiparticle interference, without suppressing the superconducting order parameter. This shows that sign inversion is not within hole bands, but between hole and the electron bands. Vortex core bound states show electron-hole asymmetric bound states due to proximity of the top of one of the hole bands to the Fermi level $E_F$. This places CaKFe$_4$As$_4$ in a similar situation as FeSe or related materials, with a superconducting gap $Δ$ just a few times smaller than $E_F$. On the other hand, we also identify locations showing strong suppression of the superconducting order parameter. Their size is of order of the vortex core size and vortices are pinned at these locations, leading to a disordered vortex lattice.

cond-mat.supr-con↗

arXiv2017

Tilted vortex cores and superconducting gap anisotropy in 2H-NbSe2

Superconducting vortex cores have been extensively studied for magnetic fields applied perpendicular to the surface by mapping the density of states (DOS) through Scanning Tunneling Microscopy (STM). Vortex core shapes are often linked to the superconducting gap anisotropy---quasiparticle states inside vortex cores extend along directions where the superconducting gap is smallest. The superconductor 2H-NbSe$_2$ crystallizes in a hexagonal structure and vortices give DOS maps with a sixfold star shape for magnetic fields perpendicular to the surface and the hexagonal plane. This has been associated to a hexagonal gap anisotropy located on quasi two-dimensional Fermi surface tubes oriented along the $c$ axis. The gap anisotropy in another, three-dimensional, pocket is unknown. However, the latter dominates the STM tunneling conductance. Here we measure DOS in magnetic fields parallel to the surface and perpendicular to the $c$ axis. We find patterns of stripes due to in-plane vortex cores running nearly parallel to the surface. The patterns change with the in-plane direction of the magnetic field, suggesting that the sixfold gap anisotropy is present over the whole Fermi surface. Due to a slight misalignment between the vector of the magnetic field and the surface, our images also show outgoing vortices. Their shape is successfully compared to detailed calculations of vortex cores in tilted fields. Their features merge with the patterns due to in plane vortices, suggesting that they exit at an angle with the surface. Measuring the DOS of vortex cores in highly tilted magnetic fields with STM can thus be used to study the superconducting gap structure.

cond-mat.supr-con↗

arXiv2017

Subsurface bending and reorientation of tilted vortex lattices in the bulk due to Coulomb-like repulsion at the surface

We study vortex lattices (VLs) in superconducting weak-pinning platelet-like crystals of $β$-Bi$_{2}$Pd in tilted fields with a Scanning Tunneling Microscope. We show that vortices exit the sample perpendicular to the surface and are thus bent beneath the surface. The structure and orientation of tilted VL in the bulk are, for large tilt angles, strongly affected by Coulomb-type intervortex repulsion at the surface due to stray fields.

cond-mat.supr-con↗

arXiv2016

Nodeless multiband superconductivity in stoichiometric single crystalline CaKFe$_4$As$_4$

Measurements of the London penetration depth and tunneling conductance in single crystals of the recently discovered stoicheometric, iron - based superconductor, CaKFe$_4$As$_4$ (CaK1144) show nodeless, two effective gap superconductivity with a larger gap of about 6-9 meV and a smaller gap of about 1-4 meV. Having a critical temperature, $T_{c,onset}\approx$35.8 K, this material behaves similar to slightly overdoped Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (e.g. $x=$0.54, $T_c \approx$ 34 K)---a known multigap $s_{\pm}$ superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap $s_{\pm}$ superconductivity is an essential property of high temperature superconductivity in iron - based superconductors, independent of the degree of substitutional disorder.

cond-mat.supr-con↗

arXiv2016

Field dependence of the vortex core size probed by STM

We study the spatial distribution of the density of states (DOS) at zero bias N(r) in the mixed state of single and multigap superconductors. We provide an analytic expression for N(r) based on deGennes' relation between DOS and the order parameter that reproduces well Scanning Tunneling Microscopy (STM) data in several superconducting materials. In the single gap superconductor $β$-Bi$_2$Pd, we find that N(r) is governed by a length scale $ξ_H=\sqrt{ϕ_0/2πH}$, which decreases in rising fields. The vortex core size $\cal C$, defined via the slope of the order parameter at the vortex center, $\cal C$ $\propto (d Δ/dr |_{r \to 0})^{-1}$, differs from $ξ_H$ by a material dependent numerical factor. For two gap superconductors 2H-NbSe$_{1.8}$S$_{0.2}$ and 2H-NbS$_2$, we find that $\cal C$ is field independent and has the same value for both bands. We conclude that, independently of the magnetic field induced variation of the order parameter values in both bands, the spatial variation of the order parameter close to the vortex core is the same for all bands.

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↗

arXiv2015

Magnetic field dependence of the density of states in the multiband superconductor $β$-Bi$_2$Pd

We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $β$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $β$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces.

cond-mat.supr-con↗

arXiv2014

Enhancement of long range correlations in a 2D vortex lattice by incommensurate 1D disorder potential

Long range correlations in two-dimensional (2D) systems are significantly altered by disorder potentials. Theory has predicted the existence of disorder induced phenomena such as Anderson localization and the emergence of novel glass and insulating phases as the Bose glass. More recently, it has been shown that disorder breaking the 2D continuous symmetry, such as a one dimensional (1D) modulation, can enhance long range correlations. Experimentally, developments in quantum gases have allowed the observation of a wealth of phenomena induced by the competition between interaction and disorder. However, there are no experiments exploring the effect of symmetry-breaking disorder. Here, we create a 2D vortex lattice at 0.1 K in a superconducting thin film with a well-defined 1D thickness modulation and track the field induced modification using scanning tunneling microscopy. We find that the 1D modulation becomes incommensurate to the vortex lattice and drives an order-disorder transition, behaving as a scale-invariant disorder potential. We show that the transition occurs in two steps and is mediated by the proliferation of topological defects. We find that critical exponents determining the loss of positional and orientational order are far above theoretical expectations for scale-invariant disorder and follow instead the critical behaviour which describes dislocation unbinding melting. Our data show for the first time that randomness disorders a 2D crystal, and evidence enhanced long range correlations in presence of a 1D modulation demonstrating the transformation induced by symmetry breaking disorder in interactions and the critical behaviour of the transition.

cond-mat.supr-con↗

arXiv2014

Zero-bias conductance peak in detached flakes of superconducting 2H-TaS$_2$ probed by scanning tunneling spectroscopy

We report an anomalous tunneling conductance with a zero bias peak in flakes of superconducting 2H-TaS$_2$ detached through mechanical exfoliation. To explain the observed phenomenon, we construct a minimal model for a single unit cell layer of superconducting 2H-TaS$_2$ with a simplified 2D Fermi surface and sign-changing Cooper pair wavefunction induced by Coulomb repulsion. Superconductivity is induced in the central $Γ$ pocket, where it becomes nodal. We show that weak scattering at the nodal Fermi surface, produced by non-perturbative coupling between tip and sample, gives Andreev states that lead to a zero bias peak in the tunneling conductance. We suggest that reducing dimensionality down to a few atom thick crystals could drive a crossover from conventional to sign changing pairing in the superconductor 2H-TaS$_2$.

cond-mat.supr-con↗

arXiv2014

Imaging superconducting vortex core and lattice with the scanning tunneling microscope

The observation of vortices in superconductors was a major breakthrough in developing the conceptual background for superconducting applications. Each vortex carries a flux quantum, and the magnetic field radially decreases from the center. Techniques used to make magnetic field maps, such as magnetic decoration, give vortex lattice images in a variety of systems. However, strong type II superconductors allow penetration of the magnetic field over large distances, of order of the magnetic penetration depth λ. Superconductivity survives up to magnetic fields where, for imaging purposes, there is nearly no magnetic contrast. Static and dynamic properties of vortices are largely unknown at such high magnetic fields. Reciprocal space studies using neutron scattering give insight into the collective behavior. But the microscopic details of vortex arrangements and their motion remain difficult to obtain. Direct real space visualization can be made using scanning tunneling microscopy and spectroscopy (STM/S). Instead of using magnetic contrast, the electronic density of states describes spatial variations of the quasiparticle and pair wavefunction properties. These are of order of the superconducting coherence length ξ, which is much smaller than λ. In principle, individual vortices can be imaged using STM up to the upper critical field where vortex cores, of size ξ, overlap. In this review, we describe recent advances in vortex imaging made with scanning tunneling microscopy and spectroscopy. We introduce the technique and discuss vortex images which reveal the influence of the Fermi surface distribution of the superconducting gap on the internal structure of vortices, the collective behavior of the lattice in different materials and conditions, and the observation of vortex lattice melting.

cond-mat.supr-con↗

arXiv2014

Anamalous positive magnetization signal at the onset of normal to superconductor transition in single crystal Nb sphere

We report the observation of positive magnetization signal at the onset of superconducting transition in isothemral and isomagnetic measurements in single crystal Niobium sphere. The PME displays two regimes, viz., low applied magnetic fields H < 4 Oe) and higher fields H > 120 Oe, and the PME signal is absent at the intermediate magnetic fields. Subsequently, the isothermal magnetization data shows the presence of positive signal at the superconductor to normal transition. Using this data we are able to plot the H(c3)-T(c3) line, at which the positive magnetization signal nucleates, in the phase diagram of single crystal Nb.

cond-mat.supr-con↗

arXiv2014

Conductance behavior with temperature and magnetic field in the disordered films of titanium nitride

We report in this paper the temperature and mangetic field dependence of the conductance in the polycrystalline film of titanium nitride, before and after heating at ambient conditions. The difference between the two films is the room temperature sheet resistance which remains within 15 percent and both the films show superconducting transition at lower temperatures. The zero field and the high field data, respectively, corresponds to the superconducting and the normal states. Both the films display Atshuler-Aronov zero bias anamoly in their normal states, and the superconducting gap openeing up at low fields. However the heated film has a smaller gap owing to more pronounced zero bias suppression of the density of states. The normal states in both the films are similar to the quasi-2d-disordered metal and its behavior is studied with temperature. Our data suggests that the zero bias anamoly suppresses the superconducting gap with increase in the disorder.

cond-mat.supr-con↗

arXiv2014

Local electronic properties in the superconducting and the normal phase in the disordered film of titanium nitride

We present in this paper the conductance maps at 100 mK in the disordered polycrystalline film of titanium nitride (TiN). At 5 nm, the film is close to quasi-two dimensional limit and exhibits features pertaining to the superconductor to insulator transition. We measured conductance maps at zero field and at 4 T, which represent the superconducting and the normal phase, respectively. The conductance map at 4 T is uniform, in which the conductance behavior, with logarithmic variation, resembles to the disorder enhanced electron-electron interaction in the two dimensional metallic phase. At low fields we observe the spatial variations of the conductance in the superconducting phase. At several places the superconducting energy gap fluctuates to an extent that the quasi-particle peaks are absent in the conductance curves. The conductance map over a region encompassing only few crystallites suggests that the inhomogeneities in the superconducting phase related to the spatial variations of the electronic density are across the crystalline boundaries.

cond-mat.supr-con↗

arXiv2013

Superconductivity and magnetism on flux grown single crystals of NiBi3

We present resistivity, magnetization and specific heat measurements on flux grown single crystals of NiBi3. We find typical behavior of a type-II superconductor, with, however, a sizable magnetic signal in the superconducting phase. There is a hysteretic magnetization characteristic of a ferromagnetic compound. By following the magnetization as a function of temperature, we find a drop at temperatures corresponding to the Curie temperature of ferromagnetic amorphous Ni. Thus, we assign the magnetism in NiBi$_3$ crystals to amorphous Ni impurities.

cond-mat.supr-con↗

arXiv2013

Low temperature magnetic transitions of single crystal HoBi

We present resistivity, specific heat and magnetization measurements in high quality single crystals of HoBi, with a residual resistivity ratio of 126. We find, from the temperature and field dependence of the magnetization, an antiferromagnetic transition at 5.7 K, which evolves, under magnetic fields, into a series of up to five metamagnetic phases.

cond-mat.mtrl-sci↗

arXiv2013

Supercurrent on a vortex core in 2H-NbSe$_2$: current driven scanning tunneling spectroscopy

We report current driven scanning tunneling spectroscopy (CDSTS) measurements at very low temperatures on vortices in 2H-NbSe2. We find that a current produces an increase of the density of states at the Fermi level in between vortices, and a reduction of the zero bias peak at the vortex center. This occurs well below the de-pairing current. We conclude that a supercurrent affects the low energy part of the superconducting gap structure of 2H-NbSe2.

cond-mat.supr-con↗