arXiv subjects

Nikolai D. Zhigadlo

Publications and source records attributed to Nikolai D. Zhigadlo.

21 records · Page 2Linked to original sources

arXiv2014

Quantum oscillations of the superconductor LaRu$_2$P$_2$ : comparable mass enhancement $λ\approx 1$ in Ru and Fe phosphides

We have studied the angular dependent de Haas-van Alphen oscillations of LaRu$_2$P$_2$ using magnetic torque in pulsed magnetic fields up to 60T. The observed oscillation frequencies are in excellent agreement with the geometry of the calculated Fermi surface. The temperature dependence of the oscillation amplitudes reveals effective masses m*($α$)=0.71 and m*($β$)=0.99 m$_e$, which are enhanced over the calculated band mass by $λ^{cyc}$ of 0.8. We find a similar enhancement $λ^γ \approx 1$ in comparing the measured electronic specific heat ($γ= 11.5$ mJ/mol K$^2$) with the total DOS from band structure calculations. Remarkably, very similar mass enhancements have been reported in other pnictides LaFe$_2$P$_2$, LaFePO ($T_c \approx 4K$), and LaRuPO, independent of whether they are superconducting or not. This is contrary to the common perceptions that the normal state quasi-particle renormalizations reflect the strength of the superconducting paring mechanism and leads to new questions about pairing in isostructural and isoelectronic Ru- and Fe-pnictide superconductors.

cond-mat.supr-con↗

arXiv2014

Critical current oscillations in the intrinsic hybrid vortex state of SmFeAs(O,F)

In layered superconductors the order parameter may be modulated within the unit cell, leading to non-trivial modifications of the vortex core if the interlayer coherence length $ξ_c(T)$ is comparable to the interlayer distance. In the iron-pnictide SmFeAs(O,F) ($T_c \approx 50$K) this occurs below a cross-over temperature $T^\star \approx 41$K, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around $T^\star$, hybrid vortices between these two characteristics appear. Only in this region around $T^\star$ and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the c-axis critical current $j_c(H)$ periodic in $\frac{1}{\sqrt{H}}$ due to a delicate balance of intervortex forces and interaction with the layered potential. $j_c(H)$ shows pronounced maxima when a hexagonal vortex lattice is commensurate with the crystal structure. The narrow temperature window in which oscillations are observed suggests a significant suppression of the order parameter between the superconducting layers in SmFeAs(O,F), despite its low coherence length anisotropy ($γ_ξ\approx 3-5$).

cond-mat.supr-con↗

arXiv2010

High magnetic field scales and critical currents in SmFeAs(O,F) crystals: promising for applications

Superconducting technology provides most sensitive field detectors, promising implementations of qubits and high field magnets for medical imaging and for most powerful particle accelerators. Thus, with the discovery of new superconducting materials, such as the iron pnictides, exploring their potential for applications is one of the foremost tasks. Even if the critical temperature Tc is high, intrinsic electronic properties might render applications rather difficult, particularly if extreme electronic anisotropy prevents effective pinning of vortices and thus severely limits the critical current density, a problem well known for cuprates. While many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed and estimates point to a very high upper critical field, their application potential is less clarified. Thus we focus here on the critical currents, their anisotropy and the onset of electrical dissipation in high magnetic fields up to 65 T. Our detailed study of the transport properties of optimally doped SmFeAs(O,F) single crystals reveals a promising combination of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities along all crystal directions. This favorable intragrain current transport in SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a crucial requirement for possible applications. Essential in these experiments are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with sub-μ\m^2 cross-section, with current along and perpendicular to the crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed magnetic fields. The pinning forces have been characterized by scaling the magnetically measured "peak effect".

cond-mat.supr-con↗