Anisotropic thermal expansion of AEFe2As2 (AE = Ba, Sr, Ca) single crystals

We report anisotropic thermal expansion of the parent, AEFe2As2 (AE = Ba, Sr, and Ca), compounds. Above the structural/antiferromagnetic phase transition anisotropy of the thermal expansion coefficients is observed, with the coefficient along the a-axis being significantly smaller than the coefficient for the c-axis. The high temperature (200 K < T < 300 K) coefficients themselves have similar values for the compounds studied. The sharp anomalies associated with the structural/antiferromagnetic phase transitions are clearly seen in the thermal expansion measurements. For all three pure compounds the "average" a-value increases and the c-lattice parameter decreases on warming through the transition with the smallest change in the lattice parameters observed for SrFe2As2. The data are in general agreement with the literature data from X-ray and neutron diffraction experiments

Specific heat jump at the superconducting transition temperature in Ba(Fe(1-x)Cox)2As2 and Ba(Fe(1-x)Nix)2As2 single crystals

We present detailed heat capacity measurements for Ba(Fe(1-x)Cox)2As2 and Ba(Fe(1-x)Nix)2As2 single crystals in the vicinity of the superconducting transitions. The specific heat jump at the superconducting transition temperature (Tc), Delta Cp/Tc, changes by a factor ~ 10 across these series. The Delta Cp/T$ vs. Tc data of this work (together with the literature data for Ba(Fe0.939Co0.061)2As2, (Ba0.55K0.45)Fe2As2, and (Ba0.6K0.4)Fe2As2) scale well to a single log-log plot over two orders of magnitude in Delta Cp/Tc and over about an order of magnitude in Tc, giving Delta Cp/Tc ~ Tc^2

Pressure induced superconductivity in CaFe2_2As2_2

CaFe$_2$As$_2$ has been found to be exceptionally sensitive to the application of hydrostatic pressure and superconductivity has been found to exist in a narrow pressure region that appears to be at the interface between two different phase transitions. The pressure - temperature ($P - T$) phase diagram of CaFe$_2$As$_2$ reveals that this stoichiometric, highly ordered, compound can be easily tuned to reveal all the salient features associated with FeAs-based superconductivity without introducing any disorder. Whereas at ambient pressure CaFe$_2$As$_2$ does not superconduct for $T > 1.8$ K and manifests a first order structural phase transition near $T \approx 170$ K, the application of $\sim 5$ kbar hydrostatic pressure fully suppresses the resistive signature of the structural phase transition and instead superconductivity is detected for $T < 12$ K. For $P \ge 5.5$ kbar a different transition is detected, one associated with a clear reduction in resistivity and for $P > 8.6$ kbar superconductivity is no longer detected. This higher pressure transition temperature increases rapidly with increasing pressure, exceeding 300 K by $P \sim 17$ kbar. The low temperature, superconducting dome is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This superconducting phase appears to exist when the low pressure transition is suppressed sufficiently, but before the high pressure transition has reduced the resistivity, and possibly the associated fluctuations, too dramatically

Magnetoelastic coupling and charge correlation lengths in a twin domain of Ba(Fe1−x_{1-x}Cox_{x})2_{2}As2_{2} (x=0.047x=0.047): A high-resolution X-ray diffraction study

The interplay between structure, magnetism and superconductivity in single crystal Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.047) has been studied using high-resolution X-ray diffraction by monitoring charge Bragg reflections in each twin domain separately. The emergence of the superconducting state is correlated with the suppression of the orthorhombic distortion around \emph{T}$_\texttt{C}$, exhibiting competition between orthorhombicity and superconductivity. Above \emph{T}$_\texttt{S}$, the in-plane charge correlation length increases with the decrease of temperature, possibly induced by nematic fluctuations in the paramagnetic tetragonal phase. Upon cooling, anomalies in the in-plane charge correlation lengths along $a$ ($\xi_{a}$) and $b$ axes ($\xi_{b}$) are observed at \emph{T}$_\texttt{S}$ and also at \emph{T}$_\texttt{N}$ indicative of strong magnetoelastic coupling. The in-plane charge correlation lengths are found to exhibit anisotropic behavior along and perpendicular to the in-plane component of stripe-type AFM wave vector (101)$_{\rm O}$ below around \emph{T}$_\texttt{N}$. The temperature dependence of the out-of-plane charge correlation length shows a single anomaly at \emph{T}$_\texttt{N}$, reflecting the connection between Fe-As distance and Fe local moment. The origin of the anisotropic in-plane charge correlation lengths $\xi_{a}$ and $\xi_{b}$ is discussed on the basis of the antiphase magnetic domains and their dynamic fluctuations.Comment: 7 pages, 6 figures, 34 references, submitted for publication in Physical Review

Doping dependence of heat transport in the iron-arsenide superconductor Ba(Fe1−x_{1-x}Cox_x)2_2As2_2: from isotropic to strongly kk-dependent gap structure

The temperature and magnetic field dependence of the in-plane thermal conductivity $\kappa$ of the iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ was measured down to $T \simeq 50$ mK and up to $H = 15$ T as a function of Co concentration $x$ in the range 0.048 $\leq x \leq$ 0.114. In zero magnetic field, a negligible residual linear term in $\kappa/T$ as $T \to 0$ at all $x$ shows that there are no zero-energy quasiparticles and hence the superconducting gap has no nodes in the $ab$-plane anywhere in the phase diagram. However, the field dependence of $\kappa$ reveals a systematic evolution of the superconducting gap with doping $x$, from large everywhere on the Fermi surface in the underdoped regime, as evidenced by a flat $\kappa (H)$ at $T \to 0$, to strongly $k$-dependent in the overdoped regime, where a small magnetic field can induce a large residual linear term, indicative of a deep minimum in the gap magnitude somewhere on the Fermi surface. This shows that the superconducting gap structure has a strongly $k$-dependent amplitude around the Fermi surface only outside the antiferromagnetic/orthorhombic phase.Comment: version accepted for publication in Physical Review Letters; new title, minor revision, revised fig.1, and updated reference

Low resistivity contact to iron-pnictide superconductors

Method of making a low resistivity electrical connection between an electrical conductor and an iron pnictide superconductor involves connecting the electrical conductor and superconductor using a tin or tin-based material therebetween, such as using a tin or tin-based solder. The superconductor can be based on doped AFe2As2, where A can be Ca, Sr, Ba, Eu or combinations thereof for purposes of illustration only

Diphtheria in a 13 year old adolescent girl: Management challenges

Background: Diphtheria is an acute toxic infection which is associated with a high morbidity and mortality and can pose management challenges especially in the absence of proper diagnostic and therapeutic facilities.Case report: A.S. was a 13 year old girl who presented with fever of five days duration, dysphagia and neck swelling of 4 days duration and sore throat and hoarse voice of 3days duration. Her illness started a day after returning from a 4-day holiday youth camp. She received only oral polio vaccine immunization in childhood. Significant physical examinationfindings included a swollen neck, a greyish membrane covering the soft palate and uvula with haemorrhagic spots. The pharynx, anterior nares and the nasal turbinates were inflamed and erythematous. A working diagnosis of respiratory diphtheria was made. Throat swab microscopy showed club shaped Gram positive baccilli. Appropriate culture medium for C. diphtheriawas not available. She received intravenous crystalline penicillin and metronidazole and lateroral erythromycin in an isolated ward. On the 6th day of admission she developed cardiac and neurologic complications–bradycardia (PR=40bpm), hypotension (BP=70/40mmHg), drooling of saliva and paraparesis. Electrocardiography confirmed a complete heart block. She died on the 11th day of admission while efforts were being made to raise funds for a cardiac pace maker. Conclusion: Management of this vaccine preventable disease requires a high index of suspicion and diphtheria antitoxin should be made readily available

Evaluation of the preparedness of the children’s emergency rooms (CHER) in Southern Nigeria for service delivery

Background: The Children Emergency Room (CHER) is the first point of call for many sick children. A significant proportion of childhood and under five deaths in tertiary institutions takes place in the CHER. There is thus need for a high level skilled manpower and infrastructure in the CHER in readiness for service delivery.Objective: To assesses the preparedness of the children emergency room in tertiary institutions in southern Nigeria to successful management of children presenting to the emergency rooms.Methods: This study was a cross sectional, descriptive multicentre study carried out among nine Tertiary Hospitals in Southern Nigeria. Three tertiary hospitals were randomly selected from each of the three Geo political zones (South-South, South- East and South -West) in Southern Nigeria. A structured questionnaire was used to collect data about the Children Emergency Rooms in these hospitals. The obtained data was entered and analysed using SPSS version 21 and is presented as table.Results: All the centres have an emergency room. The number of doctors in CHER ranged from 7 to 22 while the number of nurses ranged from 10 to 24 persons with a nurse: bed ratio of 1:3-15. In all the centres, the CHER had a side laboratory, well stocked emergency drug shelf, pulse oximeters, oxygen cylinders, electrical and manual suction machines, ambu bags and nebulizers. However, none of the centres has functional manual defibrillator or an Automated External Defibrillator (AED). In 5 (55.6%) of the studied centres, the doctors and nurses have training on emergency triage. Also 5 (55.6%) centres have doctors with certification in emergency care, but none of the nurses in all the centres have any certification in emergency care. Three (33.3%) centres had staff trained with skills on the use of AED while in 4(44.4%) centres they were skilled on the use of manual defibrillators. All the centres have a waiting area for patients’ relatives but only one (11.1%) has a television installed. All the CHERs have toilet facilities for patients relatives but only 5 (55.6%) have bathrooms. Running water is regularly available in the toilets of only 4 (44.4%) of the centres.Conclusion/Recommendation: We conclude that limitation abounds with regards to personnel, high technology infrastructure, personnel skill and patient friendly infrastructure. It is recommended that concerted efforts should be made by the government and all key players to make available the necessary equipment and facilities and ensure that health personnel acquire the necessary skills so that the standard of practice in our tertiary hospitals will be comparable to international best practices

Unconventional pairing in the iron arsenide superconductors

We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.Comment: 3 figures and 4 pages; final version as published