## Graduate Theses and Dissertations

Thesis

2013

#### Degree Name

Master of Science

#### Department

Physics and Astronomy

Ruslan Prozorov

#### Abstract

To study the electronic anisotropy in iron based superconductors, the temperature dependent London penetration depth, $\Delta \lambda$(T), have been measured in several compounds, along with the angular dependent upper critical field, H$_{c2}$(T). Study was undertaken on single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with $x$=0.108 and $x$=0.127, in the overdoped range of the doping phase diagram, characterized by notable modulation of the superconducting gap. Heavy ion irradiation with matching field doses of 6~T and 6.5~T respectively, were used to create columnar defects and to study their effect on the temperature $\Delta \lambda(T)$. The variation of the low-temperature penetration depth in both pristine and irradiated samples was fitted with a power-law function $\Delta \lambda (T)=AT^n$. Irradiation increases the magnitude of the pre-factor $A$ and decreases the exponent $n$, similar to the effect on the optimally doped samples. This finding supports the universal $s_{\pm}$ scenario for the whole doping range. Knowing that the $s _{\pm}$ gap symmetry exists across the superconducting dome for the electron doped systems, we next looked at $\lambda(T)$, in optimally - doped, SrFe$_2$(As$_{1-x}$P$_x$)$_2$, $x=$0.35. Both, as-grown ($T_c \approx$25~K) and annealed ($T_c \approx$35~K) single crystals of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ were measured. Annealing decreases the absolute value of the London penetration depth from $\lambda(0) = 300 \pm 10$~nm in as-grown samples to $\lambda(0) = 275 \pm 10$~nm. At low temperatures, $\lambda(T) \sim T$ indicates a superconducting gap with line nodes. Analysis of the full-temperature range superfluid density is consistent with the line nodes, but differs from the simple single-gap $d-$wave. The observed behavior is very similar to that of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, showing that isovalently substituted pnictides are inherently different from the charge-doped materials. In-plane resistivity measurements as a function of temperature, magnetic field, and its orientation with respect to the crystallographic $ab$-plane were used to study the upper critical field, H$_{c2}$, of two overdoped compositions of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x$=0.054 and $x$=0.072. Measurements were performed using precise alignment (with accuracy less than 0.1$^o$) of the magnetic field with respect to the Fe-As plane. The dependence of the H$_{c2}$ on angle $\theta$ between the field and the $ab$-plane was measured in isothermal conditions in a broad temperature range. We found that the shape of the H$_{c2}$ vs. $\theta$ curve clearly deviates from the Ginzburg-Landau theory.

#### DOI

https://doi.org/10.31274/etd-180810-3472

Jason Alan Murphy

en

application/pdf

72 pages

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