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In-plane structural and electronic anisotropy of iron-based superconductors
Full text link\specialchapt{ABSTRACT}
Many iron-based superconductors undergo a tetragonal to orthorhombic change of their crystallographic lattice symmetry, as well as paramagnetic to anti-ferromagnetic ordering upon cooling through a characteristic temperature . The anisotropic structure of the orthorhombic crystal symmetry would naturally lead one to expect to find in-plane electronic anisotropy.
Upon cooling through , and going into the orthorhombic symmetry, crystals divide into many small \textit{twin domains}. Although crystallographically identical, the twin domains express four different rotations of the orthorhombic lattice within the -plane making direct measurements along an individual orthorhombic axis impossible. This complication lead to the developement of uniaxial stress and strain detwinning, which makes one of the four domain rotations far more energetically favorable than the other three, to the extent that more than 90\% of the entire crystal volume may be represented by the dominant domain. Once in this state, measurements may be made along the individual orthorhombic axes, allowing one to probe in-plane anisotropy.
Following the developement of the detwinning technique, measurements of the in-plane resistivity anisotropy between the orthorhombic and axes were made. The results, however, turned out to be the opposite of what is predicted from simple models of electrical resistivity. Many different competing theories were developed to understand this unusual behavior. The goal of my doctoral research is to understand the validitiy of these different theories and discover the primary driving force behind this unexpected result.
My experiments on the effects of doping on the in-plane resistivity anisotropy yielded an interesting result that not only is there an assymetry between electron and hole doping, but also that the sign of the anisotropy changes sign with sufficient hole doping. This result, along with the tempreature dependence of the in-plane resistivity anisotropy, provide very strong evidence that the primary source is anisotropic scattering due to magnetic spin fluctuations.</p
Low temperature London penetration depth and superfluid density in Fe-based superconductors
Full text linkThe superconducting gap symmetry of the Fe-based superconductors was studied by measurements and analysis of London penetration depth and superfluid density. Tunnel diode resonator technique for these measurements was implemented in a dilution refrigerator allowing for the temperatures down to 50 mK. For the analysis of the superfluid density, we used both experimental studies of Al-coated samples and original thermodynamic approach based on Rutgers relation. In three systems studied, we found that the superconducting gap at the optimal doping is best described in multi-gap full gap scenario. By performing experiments on samples with artificially introduced disorder with heavy ion irradiation, we show that evolution of the superconducting transition temperature and of the superfluid density are consistent with full-gap sign changing s± superconducting state. The superconducting gap develops strong modulation both in the under-doped and the over-doped regimes. In the terminal hole-doped KFe2As2, both temperature dependence of the superfluid density and its evolution with increase of the scattering rate are consistent with symmetry imposed vertical line nodes in the superconducting gap. By comparative studies of hole-doped (Ba,K)Fe2As2 and electron-doped Ca10-3-8, we show that the superconducting gap modulation in the under-doped regime is intrinsic and is not induced by the coexisting static magnetic order.</p
Sonochemical route to create superconducting and superparamagnetic nanocomposites
Full text linkThe purpose of this study is to demonstrate the capability of a single process, heterogenous sonochemistry, as a tool to produce two very different nanocomposites with tunable properties: a superconductor, MgB2 with nano-inclusions, and iron/iron oxide (Fe/Fe2O3) in an alumina (Al2O3) matrix. By modifying the sonochemical parameters, one is able to tune the magnetic properties and enhance the superconducting properties of MgB2 and the superparamagnetic (SPM) properties of the Fe/Al2O3 system. This tuning is done through the ability of sonochemical irradiation, “)))” or the exposure to high intensity ultrasound (HIU), to produce extreme environments where chemical and physical processes can occur due to acoustic cavitation. ))) has been shown to reduce particle size, increase reactivity, act as a catalyst, and improve powder homogeneity. These are the effects of ))) which are exploited in this study.
For the MgB2 study, B powder (both with and without dopants/inclusions) is exposed to HIU in a decane solution before reaction with Mg to form MgB2 nanocomposites. MgB2 is produced by encapsulation of stoichiometric amounts of B and Mg in a Ta pouch, which is then reacted and pressed using hot isostatic press (HIP). The result is a dense MgB2 pellet that is then cut, polished, and measured using a SQUID MPMS magnetometer. Critical current, Jc, values are calculated using the Bean critical state and compared at 25K and 30K. Samples, post-))), show an increase in grain connectivity, as seen by SEM, leading to an enhancement of Jc.
The SPM study utilized ))) to create SPM nanocomposites through the irradiation with HIU of the carrier matrix material, Al2O3 in the presence of a volatile organometallic precursor, Fe(CO)5. ))) results in nanosized Fe and Fe2O3 being uniformly dispersed throughout the Al2O3 matrix. Magnetic properties of these materials are measured using the SQUID MPMS magnetometer and the nanosized Fe and Fe2O3 can be seen in SEM images. Analysis of the data shows a SPM component and a general trend of the data with the adjustment of slurry loading, V% of sample to medium, and mass percentage, M% of Fe to Fe+Al2O3. The trends suggest that an increase in M% results in an increased number of SPM particles, but the V% has a optimized value where the magnetization peaks, corresponding to a peak in the moment per SPM particle.</p
London penetration depth measurements in Ba(Fe1-xTx)2As2 (T=Co,Ni,Ru,Rh,Pd,Pt,Co+Cu) superconductors
Full text linkThe London penetration depth has been measured in various doping levels of single crystals of Ba(Fe1-xTx)2As2 (T=Co,Ni,Ru,Rh,Pd,Pt,Co+Cu) superconductors by utilizing a tunnel diode resonator (TDR) apparatus. All in-plane penetration depth measurements exhibit a power law temperature dependence, indicating the existence of low-temperature, normal state quasiparticles all the way down to the lowest measured temperature, which was typically 500 mK. Several different doping concentrations from the Ba(Fe1-xTx)2As2 (T=Co,Ni) systems have been measured and the doping dependence of the power law exponent, n, is compared to results from measurements of thermal conductivity and specific heat. In addition, a novel method has been developed to allow for the measurement of the zero temperature value of the in-plane penetration depth by using TDR frequency shifts. By using this technique, the doping dependence of the zero temperature value of the penetration depth has been measured in the Ba(Fe1-xCox)2As2 series, which has allowed also for the construction of the doping-dependent superfluid phase stiffness. By studying the effects of disorder on these superconductors using heavy ion irradiation, it has been determined that the observed power law temperature dependence likely arises from pair-breaking impurity scattering contributions, which is consistent with the proposed s+- symmetry of the superconducting gap in the dirty scattering limit. This hypothesis is supported by the measurement of an exponential temperature dependence of the penetration depth in the intrinsically clean LiFeAs, indicative of a nodeless superconducting gap.</p
Study of the energy gap structure in iron-based superconductors using London penetration depth and controlled artificial disorder
Full text linkA combination of London penetration depth and artificial disorder was used to probe the energy gap structure and symmetry of a few members of the iron-based superconductor systems. Information regarding the gap structure and symmetry is an important clue which helps uncover the mechanism behind the unconventional, non-BCS-type superconductors. We used the tunnel-diode resonator method to do London penetration depth measurements with high precision down to 50~mK base temperature. The disorder is introduced by electron irradiation, which was performed at the SIRIUS facility in Ecole Polytechnique (Palaiseau France) to produce point-like disorder in the materials of study. Non-magnetic defects induced by the irradiation influence each material differently depending on its underlying susceptibility to impurity scattering. The response to irradiation provides another key clue about the gap structure and symmetry of iron-based superconductors. This dissertation describes the details of the experimental work on 16 samples from the BaKFeAs system across the superconducting dome, with the results can be explained coherently with -pairing symmetry. The same gap symmetry was also found in the CaKFeAs system. We found that this series is remarkably similar to the BaKFeAs system in many ways, consistent with other reports in literature. London penetration depth measurements and electron irradiation were also performed on FeSe, which is a unique system in the iron-based superconductor family. Surprisingly, in FeSe was \textit{enhanced} by irradiation which paints a different picture of superconductivity compared to BaKFeAs and CaK(FeNi)As. However, the FeSe experimental data could still be explained within the (extended) paradigm. In conclusion, we found a strong evidence supporting the pairing symmetry which manifested into different gap structures in several representative systems in the iron-based superconductors family.</p
Studies of anisotropy of iron based superconductors
Full text linkTo study the electronic anisotropy in iron based superconductors, the temperature dependent London penetration depth, (T), have been measured in several compounds, along with the angular dependent upper critical field, H(T). Study was undertaken on single crystals of Ba(FeCo)As with =0.108 and =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 . The variation of the low-temperature penetration depth in both pristine and irradiated samples was fitted with a power-law function . Irradiation increases the magnitude of the pre-factor and decreases the exponent , similar to the effect on the optimally doped samples. This finding supports the universal scenario for the whole doping range. Knowing that the gap symmetry exists across the superconducting dome for the electron doped systems, we next looked at , in optimally - doped, SrFe(AsP), 0.35. Both, as-grown (25~K) and annealed (35~K) single crystals of SrFe(AsP) were measured. Annealing decreases the absolute value of the London penetration depth from ~nm in as-grown samples to ~nm. At low temperatures, 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 wave. The observed behavior is very similar to that of BaFe(AsP), 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 -plane were used to study the upper critical field, H, of two overdoped compositions of Ba(FeNi)As, =0.054 and =0.072. Measurements were performed using precise alignment (with accuracy less than 0.1) of the magnetic field with respect to the Fe-As plane. The dependence of the H on angle between the field and the -plane was measured in isothermal conditions in a broad temperature range. We found that the shape of the H vs. curve clearly deviates from the Ginzburg-Landau theory.</p
Tunnel-diode resonator and nuclear magnetic resonance studies of low-dimensional magnetic and superconducting systems
Full text linkThis thesis emphasizes two frequency-domain techniques which uniquely employ radio frequency (RF) excitations to investigate the static and dynamic properties of novel magnetic and superconducting materials. The first technique is a tunnel-diode resonator (TDR) which detects bulk changes in the dynamic susceptibility, &chi = dM/dH. The capability of TDR to operate at low temperatures (less than 100 mK) and high fields (up to 65 T in pulsed fields) was critical for investigations of the antiferromagnetically correlated magnetic molecules Cr12Cu2 and Cr12Ln4 (Ln = Y, Eu, Gd, Tb, Dy, Ho, Er, Yb), and the superconductor SrFe2(As1-xPx)2 (x = 0.35). Investigations of Cr12Cu2 and Cr12Ln4 demonstrates the first implementation of TDR to experimentally investigate the lowlying energy spectra of magnetic molecules in pulsed magnetic elds. Zeeman splitting of the quantum spin states results in transitions between field-dependent ground state energy levels observed as peaks in dM/dH at 600 mK, and demonstrate good agreement with theoretical calculations using a isotropic Heisenberg spin Hamiltonian. Increasing temperature to 2.5 K, TDR reveals a rich spectrum of frequency-dependent level crossings from thermally populated excited states which cannot be observed by conventional static magnetometry techniques. The last study presented uses TDR in pulsed fields to determine the temperature-dependent upper-critical field Hc2 to investigate the effects of columnar defects arising from heavy ion irradiation of SrFe2(As1-xPx)2. Results suggest irradiation uniformly suppresses Tc and Hc2, and does not introduce additional features on Hc2(T) and the shapes of the anisotropic Hc2 curves indicates a nodal superconducting gap. The second technique is nuclear magnetic resonance (NMR) which yields site specic magnetic and electronic information arising from hyperfine interactions for select magnetic nuclei. NMR spectra and nuclear spin-lattice relaxation measurements are reported for the geometrically frustrated magnetic molecule W72V30, and for BaMn2As2 and Ba1-xKxMn2As2 (with K-concentration x = 0.04 - 0.40) which are analogs of the high Tc iron arsenides. For the magnetic molecule W72V30, 1H and 51V NMR and DC magnetization were used to investigate geometric frustration arising from antiferromagnetic interactions between 30 V4+ ions occupying the edge sites of an icosidodecahedron. This system serves as a molecular representation of the 2-dimensional kagome lattice whose finite-size allows precise quantum calculations. Analysis of W72V30 data suggests a large distribution of exchange values are necessary to characterize the field and temperature-dependent magnetic properties. For the insulating BaMn2As2 and hole-doped metallic Ba1-xKxMn2As2, both local moment antiferromagnets, 55Mn and 75As NMR spectra and spin-lattice relaxation rates 1/T1 were conducted to investigate the local magnetic and electronic properties as a function of K-concentration x. NMR independently confirms G-type antiferromagnetism from spectra measurements, while a Korringa relation in 1/T1 indicates conduction electrons in both the Mn-3d and As-4d orbitals. The observation of ferromagnetic enhancement of the 55Mn NMR signal and no appreciable shift observed in the 75As spectra, combined with the absence of a structural phase transition in neutron diraction measurements suggests, the K-doped system may exhibit a previously unseen coexistence of local-moment antiferromagnetism from the Mn2+ moments and weak ferromagnetism, possibly arising from the Mn-3d orbitals. In summary, the data presented in this work demonstrates the diversity of novel materials and physical properties which can be investigated by the RF techniques TDR and NMR.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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