K. Conder

Bio: K. Conder is an academic researcher from Paul Scherrer Institute. The author has contributed to research in topics: Superconductivity & Antiferromagnetism. The author has an hindex of 26, co-authored 88 publications receiving 1923 citations.

Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc=27K

Abstract: We report on the synthesis of large single crystals of a new FeSe-layer superconductor Cs0.8(FeSe0.98)2. X-ray powder diffraction, neutron powder-diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs0.8(FeSe0.98)2 with those of the recently discovered potassium intercalated system KxFe2Se2. The new compound Cs0.8(FeSe0.98)2 shows a slightly lower superconducting transition temperature (Tc=27.4 K) in comparison to 29.5 in K0.8(FeSe0.98)2). The volume of the crystal unit cell increases by replacing K by Cs - the c-parameter grows from 14.1353(13) A to 15.2846(11) A. For the so far known alkali metal intercalated layered compounds (K0.8Fe2Se2 and Cs0.8(FeSe0.98)2) the Tc dependence on the anion height (distance between Fe-layers and Se-layers) was found to be analogous to those reported for As-containing Fe-superconductors and Fe(Se1-xChx), where Ch=Te, S.

Synthesis, crystal structure, and chemical stability of the superconductor FeSe 1 − x

Abstract: We report on a comparative study of the crystal structure and the magnetic properties of ${\text{FeSe}}_{1\ensuremath{-}x}$ $(x=0.0--0.15)$ superconducting samples by neutron powder-diffraction and magnetization measurements. The samples were synthesized by two different methods: a ``low-temperature'' one using powders as a starting material at $T\ensuremath{\simeq}700\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and a ``high-temperature'' method using solid pieces of Fe and Se at $T\ensuremath{\simeq}1075\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The effect of a starting (nominal) stoichiometry on the phase purity of the obtained samples, the superconducting transition temperature ${T}_{\text{c}}$, as well as the chemical stability of ${\text{FeSe}}_{1\ensuremath{-}x}$ at ambient conditions were investigated. It was found that in the Fe-Se system, a stable phase exhibiting superconductivity at ${T}_{\text{c}}\ensuremath{\simeq}8\text{ }\text{K}$ exists in a narrow range of selenium concentration $({\text{FeSe}}_{0.974\ifmmode\pm\else\textpm\fi{}0.005})$.

High-temperature order-disorder transition and polaronic conductivity in PrBaCo2O5.48

Abstract: Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo{sub 2}O{sub 5.48} compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at T{sub OD}=776 K. Across the transition the perovskite structure loses its vacancy ordering, and the crystal symmetry changes from orthorhombic Pmmm to tetragonal P4/mmm. The resistivity measurements for temperatures above {approx}350 K yield high values of {rho}, indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole (polaronic) hopping is proposed.

Iron-vacancy superstructure and possible room-temperature antiferromagnetic order in superconducting Cs y Fe 2 − x Se 2

Abstract: Neutron and x-ray powder and single crystal synchrotron diffraction of ${\mathrm{Cs}}_{\text{y}}{\mathrm{Fe}}_{2\ensuremath{-}\text{x}}{\mathrm{Se}}_{2}$ show the presence of superstructure reflections with propagation vector $k=[\frac{2}{5},\frac{1}{5},1]$ with respect to the average crystal structure $I4/\mathit{mmm}$ ($a~4,c~15$ \AA{}). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation $\mathbf{A}=2\mathbf{a}+\mathbf{b}$, $\mathbf{B}=\ensuremath{-}\mathbf{a}+2\mathbf{b}$, $\mathbf{C}=\mathbf{c}$. A solution for the atomic structure is found in the space group $I4/m$ with an ordered pattern of iron vacancies corresponding to the iron deficiency $x=0.29$ and Cs stoichiometry $y=0.83$. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out all possible symmetry adapted magnetic configurations and found that the presence of antiferromagnetic ordering with the ordered magnetic moment of Fe with $\ensuremath{\simeq}$$2{\ensuremath{\mu}}_{B}$ does not contradict the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in $(\mathit{ab})$ plane or along $c$ axis. The superstructure is destroyed above ${T}_{s}\ensuremath{\simeq}500$ K by a first-order-like transition.

Synthesis of a new alkali metal-organic solvent intercalated iron selenide superconductor with Tc{\approx}45K

Abstract: We report on a new iron selenide superconductor with a TC onset of 45K and the nominal composition Lix(C5H5N)yFe2-zSe2, synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature. This superconductor exhibits a broad transition, reaching zero resistance at 10K. Magnetization measurements reveal a superconducting shielding fraction of approximately 30%. Analogous phases intercalated with Na, K and Rb were also synthesized and characterized. The superconducting transition temperature of Lix(C5H5N)yFe2-zSe2 is clearly enhanced in comparison to the known superconductors FeSe0.98 (Tc ~ 8K) and AxFe2-ySe2 (TC ~ 27-32K) and is in close agreement with critical temperatures recently reported for Lix(NH3)yFe2-zSe2. Post-annealing of intercalated material (Lix(C5H5N)yFe2-zSe2) at elevated temperatures drastically enlarges the c-parameter of the unit cell (~44%) and increases the amount of superconducting shielding fraction to nearly 100%. Our findings indicate a new synthesis road leading to possibly even higher critical temperatures in this class of materials by intercalation of organic compounds between Fe-Se layers.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Doping a Mott insulator: Physics of high-temperature superconductivity

Abstract: This article reviews the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. In contrast, the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qualitative account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temperature, which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenology over a limited temperature range, and some additional physics is needed to explain the onset of singlet formation at very high temperatures. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the $t\text{\ensuremath{-}}J$ model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint againt double occupation and it is shown that the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by first examining the U(1) formulation of the gauge theory. Some inadequacies of this formulation for underdoping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liquid phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem versus the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge separation. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liquid phase. It will be shown that inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that $d$-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high-${T}_{c}$ cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is described.

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

Abstract: The response of the worldwide scientific community to the discovery in 2008 of superconductivity at T c = 26 K in the Fe-based compound LaFeAsO1−x F x has been very enthusiastic. In short order, ot...