arXiv subjects

Z. Zajicek

Publications and source records attributed to Z. Zajicek.

7 recordsLinked to original sources

arXiv2025

Drastic field-induced resistivity upturns as signatures of unconventional magnetism in superconducting iron chalcogenides

Electronic scattering is a powerful tool to identify underlying changes in electronic behavior and incipient electronic and magnetic orders. The nematic and magnetic phases are strongly intertwined under applied pressure in FeSe, however, the additional isoelectronic substitution of sulphur offers an elegant way to separate them. Here we report the detailed evolution of the electronic and superconducting behaviour of FeSe$_{0.96}$S$_{0.04}$ under applied pressure via longitudinal magnetoresistance studies up to 15T. At intermediate pressures, inside the nematic phase, the resistivity displays an upturn in zero magnetic field, which is significantly enhanced in the magnetic field, suggesting the stabilization of a spin-density wave phase, which competes with superconductivity. At higher pressures, beyond the nematic phase boundaries, the resistivity no longer displays any clear anomalies in the zero magnetic field, but an external magnetic field induces significant upturns in resistivity reflecting a field-induced order, where superconductivity and magnetic anomalies are enhanced in tandem. This study highlights the essential role of high magnetic fields in stabilizing different electronic phases and revealing a complex interplay between magnetism and superconductivity tuned by applied pressure in FeSe$_{1-x}$S$_{x}$.

cond-mat.supr-con↗

arXiv2024

Unveiling the quasiparticle behaviour in the pressure-induced high-$T_c$ phase of an iron-chalcogenide superconductor

Superconductivity of iron chalocogenides is strongly enhanced under applied pressure yet its underlying pairing mechanism remains elusive. Here, we present a quantum oscillations study up to 45 T in the high-$T_c$ phase of tetragonal FeSe$_{0.82}$S$_{0.18}$ up to 22 kbar. Under applied pressure, the quasi-two dimensional multiband Fermi surface expands and the effective masses remain large, whereas the superconductivity displays a three-fold enhancement. Comparing with chemical pressure tuning of FeSe$_{1-x}$S$_x$, the Fermi surface enlarges in a similar manner but the effective masses and $T_c$ are suppressed. These differences may be attributed to the changes in the density of states influenced by the chalcogen height, which could promote stronger spin fluctuations pairing under pressure. Furthermore, our study also reveals unusual scattering and broadening of superconducting transitions in the high-pressure phase, indicating the presence of a complex pairing mechanism.

cond-mat.supr-con↗

arXiv2023

Multi-band description of the upper critical field of bulk FeSe

The upper critical field of multi-band superconductors can be an essential quantity to unravel the nature of superconducting pairing and its interplay with the electronic structure. Here we experimentally map out the complete upper critical field phase diagram of FeSe for different magnetic field orientations at temperatures down to 0.3 K using both resistivity and torque measurements. The temperature dependence of the upper critical field reflects that of a multi-band superconductor and requires a two-band description in the clean limit with band coupling parameters favouring interband over intraband interactions. Despite the relatively small Maki parameter in FeSe of $α\sim 1.6$, the multi-band description of the upper critical field is consistent with the stabilization of a FFLO state below $T/T_{\rm c}\sim 0.3$. We find that the anomalous behaviour of the upper critical field is linked to a departure from the single-band picture, and FeSe provides a clear example where multi-band effects and the strong anisotropy of the superconducting gap need to be taken into account.

cond-mat.supr-con↗

arXiv2022

Unconventional localization of electrons inside of a nematic electronic phase

The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally-dependent correlation effects, enhanced interband spin-fluctuations or a Lifshitz-like transition which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface.

cond-mat.supr-con↗

arXiv2022

Robust superconductivity and fragile magnetism induced by the strong Cu impurity scattering in the high-pressure phase of FeSe

Superconductivity in FeSe is strongly enhanced under applied pressure and it is proposed to emerge from anomalously coupled structural and magnetic phases. Small impurities inside the Fe plane can strongly disrupt the pair formation in FeSe at ambient pressure and can also reveal the interplay between normal and superconducting phases. Here, we investigate how an impurity inside the Fe plane induced by the Cu substitution can alter the balance between competing electronic phases of FeSe at high pressures. In the absence of an applied magnetic field, at low pressures the nematic and superconducting phases are suppressed by a similar factor. On the other hand, at high pressures, above 10 kbar, the superconductivity remains unaltered despite the lack of any signature in transport associated to a magnetic phase in zero-magnetic field. However, by applying a magnetic field, the resistivity displays an anomaly preceding the activated behaviour in temperature, assigned to a magnetic anomaly. We find that the high-pressure superconducting phase of FeSe is robust and remains enhanced in the presence of Cu impurity, whereas the magnetic phase is not. This could suggest that high-$T_{\rm c}$ superconductivity has a sign-preserving order parameter in a presence of a rather glassy magnetic phase.

cond-mat.supr-con↗

arXiv2022

The drastic effect of the impurity scattering on the electronic and superconducting properties of Cu-doped FeSe

Non-magnetic impurities in iron-based superconductors can provide an important tool to understand the pair symmetry and they can influence significantly the transport and the superconducting behaviour. Here, we present a study of the role of strong impurity potential in the Fe plane, induced by Cu substitution, on the electronic and superconducting properties of single crystals of FeSe. The addition of Cu quickly suppresses both the nematic and superconducting states, and increases the residual resistivity due to enhanced impurity scattering. Using magnetotransport data up to 35 T for a small amount of Cu impurity, we detect a significant reduction in the mobility of the charge carriers by a factor of ~3. While the electronic conduction is strongly disrupted by Cu substitution, we identify additional signatures of anisotropic scattering which manifest in linear resistivity at low temperatures and $H^{1.6}$ dependence of magnetoresistance. The suppression of superconductivity by Cu substitution is consistent with a sign-changing $s_{\pm}$ order parameter. Additionally, in the presence of compressive strain, the superconductivity is enhanced, similar to FeSe.

cond-mat.supr-con↗

arXiv2019

Anomalous high-magnetic field electronic state of the nematic superconductors FeSe$_{1-x}$S$_x$

Understanding superconductivity requires detailed knowledge of the normal electronic state from which it emerges. A nematic electronic state that breaks the rotational symmetry of the lattice can potentially promote unique scattering relevant for superconductivity. Here, we investigate the normal transport of superconducting FeSe$_{1-x}$S$_x$ across a nematic phase transition using high magnetic fields up to 69 T to establish the temperature and field-dependencies. We find that the nematic state is an anomalous non-Fermi liquid, dominated by a linear resistivity at low temperatures that can transform into a Fermi liquid, depending on the composition $x$ and the impurity level. Near the nematic end point, we find an extended temperature regime with $T^{1.5}$ resistivity. The transverse magnetoresistance inside the nematic phase has as a $H^{1.55}$ dependence over a large magnetic field range and it displays an unusual peak at low temperatures inside the nematic phase. Our study reveals anomalous transport inside the nematic phase, driven by the subtle interplay between the changes in the electronic structure of a multi-band system and the unusual scattering processes affected by large magnetic fields and disorder

cond-mat.supr-con↗