Evolution of normal and superconducting properties of single crystals of Na${}_{1–\delta }$FeAs upon interaction with environment

Iron-arsenide superconductor Na${}_{1–\delta }$FeAs is highly reactive with the environment. Due to the high mobility of Na ions, this reaction affects the entire bulk of the crystals and leads to an effective stoichiometry change. Here we use this effect to study the doping evolution of normal and superconducting properties of the same single crystals. Controlled reaction with air increases the superconducting transition temperature $T_c$ from the initial value of 12 to 27 K as probed by transport and magnetic measurements. Similar effects are observed in samples reacted with Apiezon N grease, which slows down the reaction rate and results in more homogeneous samples. In both cases, the temperature-dependent resistivity ${\rho }_a\left(T\right)$ shows a dramatic change with exposure time. In freshly prepared samples, ${\rho }_a\left(T\right)$ reveals clear features at the tetragonal-to-orthorhombic ($T_s\approx 60$ K) and antiferromagnetic ($T_m=45$ K) transitions and superconductivity with onset $T_{c,\text{ons}}=16$ K and offset $T_{c,\text{off}}=12$ K. The exposed samples show $T$-linear variation of ${\rho }_a\left(T\right)$ above $T_{c,\mathrm{ons}}=30$ K ($T_{c,\mathrm{off}}=26$ K), suggesting bulk character of the observed doping evolution and implying the existence of a quantum critical point at the optimal doping. The resistivity for different doping levels is affected below $\sim$200 K suggesting the existence of a characteristic energy scale that terminates the $T$-linear regime, which could be identified with a pseudogap.

Figure 1

Generic temperature-composition phase diagram of electron-doped iron arsenide superconductors, exemplified by Ba(Fe${}_{1–x}$Co${}_x$)${}_2$As${}_2$.[19] Lines of structural, $T_s$, and magnetic, $T_m$, transitions are split with Co doping and superconductivity has maximum $T_c$ at $x_{\mathrm{opt}}\approx 0.07$, close to the composition where $T_s\left(x\right)$ and $T_m\left(x\right)$ extrapolate to zero. NaFeAs in its parent state is representative of an underdoped part of the phase diagram and the temperatures of its split structural and magnetic transitions (circles) correspond to Co-doped Ba122 with $x=0.048$. With this $x$-axis shift, the actual Co-doping phase diagram for NaFe${}_{1–x}$Co${}_x$As (down-triangles, data from Ref. 21) matches closely the phase diagram of Ba(Fe${}_{1–x}$Co${}_x$)${}_2$As${}_2$. Up triangles show $T_c$ and tentative $x$ position of the samples in which doping is achieved by environmental reaction (this study).

Figure 2

Direct current magnetization of fresh cleaved sample measured upon warming after cooling in zero magnetic field and applying a 10 Oe field (ZFC-W). Inset: magnetic hysteresis loop measured at 5 K in the same sample.

Figure 3

Temperature dependence of the normalized frequency shift in the TDR experiment ($f_0$ is the frequency shift at $T_c$.), for the sample of NaFeAs exposed to air for the indicated number of hours. Environmental reaction, caused by the oxidative deintercalation of Na,[28] increases the onset temperature of the superconducting transition from 13 K, for fresh samples, to 24 K for samples exposed for 28 hours in air (curves with symbols), only slightly broadening the transition. On further exposure, the onset $T_c$ rises to almost 28 K, however, additional features and significant broadening in the temperature dependence of the frequency shift show that the sample becomes inhomogeneous, suggesting formation of 122 phase inclusions.[28] Inset shows the evolution of the transition temperature (defined at half of the total frequency variation over the transition, $\Delta f/f_0=0.5$) with environmental reaction time. Shaded area represents samples with visible presence of multiple phases.

Figure 4

Temperature dependence of the normalized in-plane resistivity, ${\rho }_a/{\rho }_a\left(300\text{K}\right)$, for three samples of NaFeAs in a fresh state after initial sample handling and contact making. Arrows show features in the temperature dependence due to magnetic and structural transitions, and additional broad slope change features at temperatures $T_3\approx 100$ K, $T_2\approx 180$ K, and $T_1\approx 290$ K. The inset zooms onto the superconducting transition and shows the definitions of onset and offset criteria for the superconducting transition temperature.

Figure 5

(Top panel) The evolution of the temperature-dependent resistivity of the sample of Na${}_{1–\delta }$FeAs during exposure to air. Resistivity of the sample at room temperature, ${\rho }_a\left(300\text{K}\right)$, monotonically increases due to macroscopic sample degradation and formation of cracks. Bottom panel shows the same data, plotted using a normalized resistivity scale, $\rho /\rho \left(300\text{K}\right)$. The normalization procedure removes variation of the effective geometric factor and essentially reveals doping-independent resistivity close to room temperature and a strong variation below 200 K. The inset in the bottom panel zooms onto the superconducting transition range. Arrows show positions of the special features in ${\rho }_a\left(T\right)$ in the parent and 12-hour exposed samples, showing suppression of the structural/magnetic transition temperature with exposure and evolution of the crossover features.

Figure 6

Temperature dependence of the normalized frequency shift in the TDR experiment for a sample of Na${}_{1–\delta }$FeAs stored in Apiezon N grease and treated in an ultrasonic cleaner between successive runs. Oxidative deintercalation of Na increases the onset temperature of the superconducting transition from 13 K for fresh samples to 26 K with a slight transition broadening, however, without the appearance of additional features observed in air-exposed samples, Fig. 3.

Figure 7

(Top panel) The evolution of ${\rho }_a\left(T\right)$ of a Na${}_{1–\delta }$FeAs crystal during exposure to Apiezon N grease. Resistivity of the sample at room temperature, ${\rho }_a\left(300\text{K}\right)$, increases slightly, presumably due to variation of effective geometric factor due to the formation of cracks. The bottom panel shows the same data, plotted with normalized resistivity scale, $\rho /\rho \left(300\text{K}\right)$. The inset in the bottom panel zooms in on the superconducting transition range, showing increase of the superconducting transition temperature from $T_{c,\mathrm{ons}}=15$ K and $T_{c,\mathrm{off}}=12$ K in the fresh state to $T_{c,\mathrm{ons}}=30$ K and $T_{c,\mathrm{off}}=26$ K in the grease exposed state.

Figure 8

Polarized-light images of the fresh cleaved surface of a Na${}_{1–\delta }$FeAs sample, taken above structural transition at 60 K (top panel) and at 5 K, the base temperature of our experiment (bottom panel). The image shows a pattern of structural domains with domain walls running along the [100] tetragonal direction.

Figure 9

The evolution of the temperature-dependent resistivity, plotted on a normalized resistivity scale $\rho /\rho \left(300\text{K}\right)$, in a 111 system. The data for parent NaFeAs (curve 1), grease treated Na${}_{1–\delta }$FeAs (curve 2), and LiFeAs (curve 3) are representative of the underdoped, optimally doped, and overdoped regimes, respectively. The inset shows a zoom of the low-temperature range for curves 2 and 3, the line is a linear fit through $\rho \left(T\right)$ of grease-treated NaFeAs. For reference, we show the in-plane (curve 4) temperature-dependent resistivity of slightly underdoped (Ba${}_{1–x}$K${}_x$)Fe${}_2$As${}_2$, $x=0.034$, showing close to $T$-linear resistivity at low temperatures and a pseudogap crossover for both ${\rho }_a\left(T\right)$ and ${\rho }_c\left(T\right)$[18] at around 250 K.