arXiv subjects

Itay Mangel

Publications and source records attributed to Itay Mangel.

5 recordsLinked to original sources

arXiv2024

The two critical temperatures conundrum in La$_{1.83}$Sr$_{0.17}$CuO$_4$

The in-plane and out-of-plane superconducting stiffness of LSCO rings appear to vanish at different transition temperatures, which contradicts thermodynamical expectation. In addition, we observe a surprisingly strong dependence of the out-of-plane stiffness transition on sample width. With evidence from Monte Carlo simulations, this effect is explained by very small ratio $α$ of interplane over intraplane superconducting stiffnesses. For three dimensional rings of millimeter dimensions, a crossover from layered three dimensional to quasi one dimensional behavior occurs at temperatures near the thermodynamic transition temperature $T_{\rm c}$, and the out of-plane stiffness appears to vanish below $T_{\rm c}$ by a temperature shift of order $αL_a/ξ^\parallel$, where $L_a/ξ^\parallel$ is the sample's width over coherence length. Including the effects of layer-correlated disorder, the measured temperature shifts can be fit by $α=4.1\times 10^{-5}$ near $T_{\rm c}$, which is significantly lower than its previously measured value near zero temperature.

cond-mat.supr-con↗

arXiv2023

The Ground-state Inter-plane Superconducting Coherence Length of La$_{1.875}$Sr$_{0.125}$CuO$_4$ Measured by a "Xiometer"

A long excitation coil piercing a superconducting (SC) ring is used to generate ever increasing persistent current in the ring, until the current destroys the order parameter. Given that the penetration depth $λ$ is known, this experiment measures, hypothetically, the coherence length $ξ$, hence the name "Xiometer". We examine various aspects of this theoretically driven hypothesis by testing niobium rings with different dimensions, and by comparing the results to the known values of $ξ$. We then apply the method to two La$_{1.875}$Sr$_{0.125}$CuO$_4$ rings at $T \rightarrow 0$. In one, the current flows in the CuO$_2$ planes hence it is set by $ξ_{ab}$. In the other, the current must cross planes and is determined by $ξ_{c}$. We find that $ξ_{c}=1.3 \pm 0.1$~nm, and $ξ_{ab}<2.3~$nm indicating that at low temperatures the Cooper pairs are three dimensional.

cond-mat.supr-con↗

arXiv2023

Superconducting Stiffness and Coherence Length of FeSe$_{0.5}$Te$_{0.5}$ Measured in Zero-Applied Field

Superconducting stiffness $ρ_s$ and coherence length $ξ$ are usually determined by measuring the penetration depth $λ$ of a magnetic field and the upper critical field $H_{c2}$ of a superconductor (SC), respectively. However, in magnetic SC, e.g. some of the iron-based, this could lead to erroneous results since the internal field could be very different from the applied one. To overcome this problem in Fe$_{1+y}$Se$_x$Te$_{1-x}$ with $x \sim 0.5$ and $y \sim 0$ (FST), we measure both quantities with the Stiffnessometer technique. In this technique, one applies a rotor-free vector potential $\textbf{A}$ to a superconducting ring and measures the current density $\textbf{j}$ via the ring's magnetic moment $\textbf{m}$. $ρ_s$ and $ξ$ are determined from London's equation $\textbf{j}=-ρ_s\textbf{A}$ and its range of validity. This method is particularly accurate at temperatures close to the critical temperature $T_c$. We find weaker $ρ_s$ and longer $ξ$ than existing literature reports, and critical exponents which agree better with expectations based on the Ginzburg-Landau theory.

cond-mat.supr-con↗

arXiv2022

Mixed superconducting state without applied magnetic field

A superconducting (SC) mixed state occurs in type-II superconductors where the upper critical field Hc2 is higher than the thermodynamic critical field Hc. When an applied field is in between these fields, the free energy depends weakly on the order parameter which therefore can be small (SC state) or zero (normal state) at different parts of the sample. In this paper we demonstrate how a normal state along a line traversing a superconductor can be turned on and off externally in zero field. The concept is based on a long, current-carrying excitation coil, piercing a ringshaped superconductor. The ring experiences zero field, but the vector potential produced by the coil generates a circular current that destroys superconductivity along a radial line starting at preexisting nucleation points in the sample. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil, and different cooldowns yield the same normal lines. We suggest potential applications of this magnetic-field-free mixed state.

cond-mat.supr-con↗

arXiv2020

Stiffnessometer, a magnetic-field-free superconducting stiffness meter and its application

We provide a detailed account for a new method to measure superconducting stiffness $ρ_{s}$, critical current density $j_c$, and coherence length $ξ$, in one apparatus, without subjecting the sample to magnetic field or attaching leads. The method is based on the London equation $\mathbf{j}=-ρ_{s}\mathbf{A}$, where ${\bf j}$ is the current density and ${\bf A}$ is the vector potential. Using a rotor free $\bf{A}$ and a measurement of $\bf{j}$ via the magnetic moment of a superconducting ring, we determine $ρ_{s}$. By increasing $\mathbf{A}$ until the London equation fails we determine $j_c$ and $ξ$. The method is sensitive to very small stiffness, which translates to penetration depth $λ\lesssim 1$~mm. It is also sensitive to low critical current density $j_c \sim 10^3$ Amm$^{-2}$ or long coherence length $ξ\sim 1$~$μ$m. Naturally, the method does not suffer from demagnetization factor complications, the presence of vortices, or out-of-equilibrium conditions. Therefore, the absolute values of the different parameters can be determined. We demonstrate the application of this method to La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.17$.

cond-mat.supr-con↗