M. Kraken

Bio: M. Kraken is an academic researcher from Braunschweig University of Technology. The author has contributed to research in topics: Quantum critical point & Antiferromagnetism. The author has an hindex of 5, co-authored 7 publications receiving 813 citations.

The electronic phase diagram of the LaO 1− x F x FeAs superconductor

Abstract: The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mossbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

The electronic phase diagram of the LaO1-xFxFeAs superconductor

Abstract: The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, e.g. in high-transition-temperature cuprates 1, heavy fermions 2 and organic superconductors3. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, e.g. charge carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently high-temperature superconductivity has been discovered in iron-pnictides providing a new class of unconventional superconductors4,5,6. Similar to other unconventional superconductors the parent compounds of the pnictides exhibit a magnetic ground state7,8 and superconductivity is induced upon charge carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of x-ray scattering, MuSR and Moessbauer spectroscopy on the series LaO1-xFxFeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and of the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in ironpnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

Commensurate spin density wave in LaFeAsO: a local probe study.

Abstract: We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of $0.25(5){\ensuremath{\mu}}_{B}$ at the iron site below ${T}_{N}=138\text{ }\text{ }\mathrm{K}$, well separated from a structural phase transition at ${T}_{S}=156\text{ }\text{ }\mathrm{K}$. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below ${T}_{N}$ are reproduced.

Spectroscopic study of metallic magnetism in single-crystalline Nb 1 − y Fe 2 + y

Abstract: We have investigated single crystals and polycrystals from the series ${\mathrm{Nb}}_{1\ensuremath{-}y}{\mathrm{Fe}}_{2+y}, \ensuremath{-}0004\ensuremath{\le}y\ensuremath{\le}0018$, by electron spin resonance, muon spin relaxation, and M\"ossbauer spectroscopy Our data establish that at lowest temperatures all samples exhibit bulk magnetic order Slight Fe excess induces low-moment ferromagnetism, consistent with bulk magnetometry, while Nb-rich and stoichiometric ${\mathrm{NbFe}}_{2}$ display spin density wave order with small magnetic moment amplitudes of order $\ensuremath{\sim}0001--001\phantom{\rule{028em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{Fe}$ This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in ${\mathrm{NbFe}}_{2}$

Magnetoelastic and structural properties of azuriteCu3(CO3)2(OH)2from neutron scattering and muon spin rotation

Abstract: Azurite, ${\text{Cu}}_{3}{({\text{CO}}_{3})}_{2}{(\text{OH})}_{2}$, has been considered an ideal example of a one-dimensional diamond chain antiferromagnet. Early studies of this material imply the presence of an ordered antiferromagnetic phase below ${T}_{N}\ensuremath{\sim}1.9\text{ }\text{K}$ while magnetization measurements have revealed a 1/3 magnetization plateau. To the best of our knowledge, no corroborating neutron-scattering results have been published to confirm the ordered magnetic moment structure. We present recent neutron-diffraction results which reveal the presence of commensurate magnetic order in azurite which coexists with significant magnetoelastic strain. The latter of these effects may indicate the presence of spin frustration in zero applied magnetic field. Muon spin rotation reveals an onset of short-range order below 3 K and confirms long-range order below ${T}_{N}$.

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2.

Abstract: The ternary iron arsenide ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at ${T}_{c}=38\text{ }\text{ }\mathrm{K}$ in $({\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ with $x\ensuremath{\approx}0.4$. The parent compound ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ crystallizes in the tetragonal ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure, which consists of $(\mathrm{FeAs}{)}^{\ensuremath{\delta}\ensuremath{-}}$ iron arsenide layers separated by ${\mathrm{Ba}}^{2+}$ ions. ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting ${\mathrm{Ba}}^{2+}$ for ${\mathrm{K}}^{+}$ ions we have introduced holes in the $(\mathrm{FeAs}{)}^{\ensuremath{-}}$ layers, which suppress the anomaly and induce superconductivity. The ${T}_{c}$ of 38 K in $({\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure.

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...

Superconductivity in iron compounds

Abstract: Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

High-temperature superconductivity in iron-based materials

Abstract: The surprising discovery of high-temperature superconductivity in a material containing a strong magnet—iron—has led to thousands of publications. By placing all the data in context, it becomes clear what we know and where we are headed.

Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure

Abstract: Superconductivity was recently observed in the binary iron-based compound, FeSe. It is now shown that under pressure, the transition temperature can rise above 36 K. In addition, no static magnetic ordering is observed for this system, contrary to FeAs superconductors. The discovery of new high-temperature superconductors1 based on FeAs has led to a new ‘gold rush’ in high-TC superconductivity. All of the new superconductors share the same common structural motif of FeAs layers and reach TC values up to 55 K (ref. 2). Recently, superconductivity has been reported in FeSe (ref. 3), which has the same iron pnictide layer structure, but without separating layers. Here, we report the magnetic and electronic phase diagram of β-Fe1.01Se as a function of temperature and pressure. The superconducting transition temperature increases from 8.5 to 36.7 K under an applied pressure of 8.9 GPa. It then decreases at higher pressures. A marked change in volume is observed at the same time as TC rises, owing to a collapse of the separation between the Fe2Se2 layers. No static magnetic ordering is observed for the whole p–T phase diagram. We also report that at higher pressures (starting around 7 GPa and completed at 38 GPa), Fe1.01Se transforms to a hexagonal NiAs-type structure and exhibits non-magnetic behaviour.