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We report on the measurements of the remnant magnetization, and hence critical current, in a single crystal of YBa2Cu4O8. A peak in the temperature dependence of the critical current is observed when the external magnetic field is tilted away from the a&ndash, b planes. The observed behavior is attributed to a thermally activated instability-driven vortex-lattice splitting or vortex chain formation. The nature of the peak and the possibility of a thermally-activated dimensional crossover have been discussed.

The evolution of flux distribution in MgB2 single crystals during their remagnetization was imaged with magneto-optical technique. Meissner holes, formed as the areas where the annihilation of vortices and antivortices takes place, were found at the boundary between oppositely magnetized parts of the crystal. Gradient of magnetic induction in the vicinity of Meissner holes was found to be enhanced. Finger-like structures of convex shape, formed during the penetration of magnetic flux inside the crystal, were observed and explained as an effect of inhomogeneous pinning and demagnetizing field redistribution in the sample.

Hydrostatic pressure Raman measurements at room temperature have been carried out on the SmFeAsO (Sm1111) series of oxypnictides with various substitutions (F for O and Co for Fe) and transition temperature in order to investigate lattice modifications and their connection to doping and superconductivity. Synchrotron XRD data on some of these compounds indicated that at low doping the lattice constants vary smoothly with pressure, but with further increasing of the carrier concentration there is a deviation from the normal equation of state and these effects are related with modifications in the superconducting FeAs 4 tetrahedra. The hydrostatic pressure Raman measurements indicate that the A 1g mode of the rare earth atom for the superconducting compounds deviates from the linear pressure dependence at the same pressures where the XRD results show pressure-induced lattice anomalies. A similar anomaly is found for the As phonon of the same symmetry. As in cuprates, the effect is diminished in the non-superconducting compounds and it is not related with the F substitution being present in the Sm(Fe 1− x Co x )AsO as well. The calculated Gruneisen parameters indicate a more anharmonic phonon for the Fe atom compared with the Sm and As atoms.

Mossbauer spectroscopy measurements were performed for the temperature range between 4.2 K and 300 K in a transmission geometry applying 14.41-keV resonant line in 57 Fe for PrFeAsO the latter being a parent compound of the iron-based superconductors belonging to the ‘1111’ family. It was found that an itinerant 3d magnetic order develops at about 165 K and it is accompanied by an orthorhombic distortion of the chemical unit cell. A complete longitudinal 3d incommensurate spin density wave (SDW) order develops at about 140 K. Transferred hyperfine magnetic field generated by the praseodymium magnetic order on iron nuclei is seen at 12.8 K and below, i.e., below magnetic order of praseodymium magnetic moments. It is oriented perpendicular to the field of SDW on iron nuclei. The shape of SDW is almost rectangular at low temperatures and it transforms into roughly triangular form around “nematic” transition at about 140 K. Praseodymium magnetic order leads to the substantial enhancement of SDW due to the large orbital contribution to the magnetic moment of praseodymium. A transferred field indicates presence of strong magnetic susceptibility anisotropy in the [b-c] plane while following rotation of praseodymium magnetic moments in this plane with lowering temperature. It was found that “nematic” phase region is a region of incoherent spin density wavelets typical for a critical region.

We present a detailed study of the electrical transport properties of a recently discovered iron-based superconductor: Sm$_4$Fe$_2$As$_2$Te$_{0.72}$O$_{2.8}$F$_{1.2}$. We followed the temperature dependence of the upper critical field by resistivity measurement of single crystals in magnetic fields up to 16 T, oriented along the two main crystallographic directions. This material exhibits a zero-temperature upper critical field of 90 T and 65 T parallel and perpendicular to the Fe$_2$As$_2$ planes, respectively. An unprecedented superconducting magnetic anisotropy $\gamma_H=H_{c2}^{ab}/H_{c2}^c \sim 14$ is observed near Tc, and it decreases at lower temperatures as expected in multiband superconductors. Direct measurement of the electronic anisotropy was performed on microfabricated samples, showing a value of $\rho_c/\rho_{ab}(300K) \sim 5$ that raises up to 19 near Tc. Finally, we have studied the pressure and temperature dependence of the in-plane resistivity. The critical temperature decreases linearly upon application of hydrostatic pressure (up to 2 GPa) similarly to overdoped cuprate superconductors. The resistivity shows saturation at high temperatures, suggesting that the material approaches the Mott-Ioffe-Regel limit for metallic conduction. Indeed, we have successfully modelled the resistivity in the normal state with a parallel resistor model that is widely accepted for this state. All the measured quantities suggest strong pressure dependence of the density of states.

We report the synthesis, structure, and superconducting properties of single crystals of $\mathrm{SmFeAsO}{\mathrm{H}}_{x}$. The crystals were grown at high pressure and high temperature using a cubic anvil technique. $^{1}\mathrm{H}\ensuremath{-}\mathrm{NMR}$ studies confirm the presence of H atoms in the samples. Single crystal x-ray diffraction analyses demonstrate a remarkable disorder in the $\mathrm{S}{\mathrm{m}}_{2}{\mathrm{O}}_{2}$ layers induced by hydrogen incorporation and reveal that the H positions are compatible with a ${\mathrm{H}}_{2}\mathrm{O}$-like geometry inside the crystals. We have measured the magnetotransport properties of $\mathrm{SmFeAsO}{\mathrm{H}}_{x}$ single crystals with $x=0.07$, 0.11, and 0.16 in magnetic field up to 16 T, oriented along the two main crystallographic directions. The results show an increase of the critical temperature with hydrogen content. The zero-temperature upper critical fields and the magnetic anisotropy are calculated as a function of the hydrogen content. $\mathrm{SmFeAsO}{\mathrm{H}}_{x}$ crystals present significantly higher upper critical fields and magnetic anisotropies compared to $\mathrm{SmFeAs}{\mathrm{O}}_{1\ensuremath{-}x}{\mathrm{F}}_{x}$ compounds.

Superconducting MgB$_2$ shows an E$_{2g}$ zone center phonon, as measured by Raman spectroscopy, that is very broad in energy and temperature dependent. The Raman shift and lifetime show large differences with the values elsewhere in the Brillouin Zone measured by Inelastic X-ray Scattering (IXS), where its dispersion can be accounted for by standard harmonic phonon theory, adding only a moderate electron-phonon coupling. Here we show that the effects rapidly disappear when electron-phonon coupling is switched off by Al substitution on the Mg sites. Moreover, using IXS with very high wave-vector resolution in MgB$_2$, we can follow the dispersion connecting the Raman and the IXS signal, in agreement with a theory using only electron-phonon coupling but without strong anharmonic terms. The observation is important in order to understand the effects of electron-phonon coupling on zone center phonons modes in MgB$_2$, but also in all metals characterized by a small Fermi velocity in a particular direction, typical for layered compounds.

We present a polarization-resolved and temperature-dependent Raman scattering study of $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{Ca}$, Eu). In the spin-density-wave phase, spectral weight redistribution is observed in the fully symmetric and nonsymmetric scattering channels at different energies. An anisotropic Raman response is observed in the fully symmetric channel in spontaneously detwinned ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ samples. We calculate the orbital-resolved electronic structures using a combination of density functional theory and dynamical mean field theory. We identify the electronic transitions corresponding to these two spectral features and find that the anisotropic Raman response originates from the lifted degeneracy of the ${d}_{xz/yz}$ orbitals in the broken-symmetry phase.

We report a study of the electrical transport properties of single crystals of $\mathrm{P}{\mathrm{r}}_{4}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}\mathrm{T}{\mathrm{e}}_{1\ensuremath{-}x}{\mathrm{O}}_{4}$, a recently discovered iron-based superconductor. Resistivity, Hall effect, and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohler's law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic $c$ axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of $\mathrm{P}{\mathrm{r}}_{4}\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}\mathrm{T}{\mathrm{e}}_{1\ensuremath{-}x}{\mathrm{O}}_{4}$. The model can successfully explain the strongly temperature-dependent negative Hall coefficient and the high magnetic anisotropy, assuming that the mobility of electrons is higher than that of holes.

As a follow-up to the discovery of a new family of Fe-based superconductors, namely, the RE4Fe2As2Te1-xO4 (42214) (RE = Pr, Sm, and Gd), we present a detailed ab initio study of these compounds highlighting the role of rare-earth (RE) atoms, external pressure, and Te content on their physical properties. Modifications of the structural, magnetic, and electronic properties of the pure (e.g., x = 0.0) 42214 compounds and their possible correlations with the observed superconducting properties are calculated and discussed. The careful analysis of the results obtained shows that (i) changing the RE atoms allows one to tune the internal pressure acting on the As height with respect to the Fe planes; (ii) similarly to other Fe pnictides, the 42214 pure compounds show an antiferromagnetic-stripe magnetic ground state phase joined by an orthorhombic distortion (not experimentally found yet); (iii) smaller RE atoms increase the magnetic instability of the compounds possibly favoring the onset of the superconducting state; (iv) external pressure induces the vanishing of the magnetic order with a transition to the tetragonal phase and can be a possible experimental route towards higher superconducting critical temperature (T-c); and (v) Te vacancies act on the structural parameters, changing the As height and affecting the stability of the magnetic phase.

We have studied by Raman spectroscopy the effect of doping, temperature, and hydrostatic pressure on selected Fe pnictides of the 1111 series. Two sets of RFeAsO1−x F x compounds have been examined (R=Sm and Nd) with a varying amount of doping and transition temperature. The doping dependence of the Raman active modes reveals that the rare earth phonon is correlated with the transition temperature (T c) and not with the amount of doping. As in the case of several other pnictides, the low temperature measurements indicate phonon modifications at much higher temperatures than T c even in the superconducting compounds. The application of hydrostatic pressure indicates a nonlinear behavior of the rare earth phonon, which increases with doping and in the superconducting compounds correlates with modifications in T c. The results are similar with those of the cuprates, where hydrostatic pressure has induced phonon and structure modifications at characteristic pressures where the T c dependence on pressure is also modified. All results point to some role of the lattice for superconductivity in the pnictides.

We have measured by synchrotron powder diffraction the hydrostatic pressure dependence of the structural parameters of the SmFeAsO1−xFx for four doping values (from x = 0.0 to 0.17) in order to investigate lattice modifications that might be related to doping. The response of the lattice to the applied hydrostatic pressure is anisotropic with the linear compressibility kc, along the c-axis being a factor of 2.5 greater than the corresponding ka along the a-axis, while bulk modulus increases slightly in the optimally doped compound. The analysis of the data indicates that the anisotropic compressibility of the SmFeAsO1−xFx system is doping dependent, with an increasing deviation from a linear relation between the a- and c-axis with doping, which peaks in the x = 0.17 compound with the highest Tc value studied. The width of the superconducting FeAs4 layer in this compound exhibits a non-monotonic pressure dependence, which is not evident in the non-superconducting one. This unequal compression of the subunits of the cell apparently modifies the distribution of the carriers and affects the transition temperature. Our results agree with previous measurements for the highest doping level, and furthermore provide an indication about the connection of doping with lattice anomalies.

Journal of Superconductivity and Novel Magnetism, 26 (2), ISSN:1557-1939, ISSN:0896-1107, ISSN:1557-1947, ISSN:1572-9605, ISSN:1557-1949

Iron-based superconductors Ba0.7Rb0.3Fe2As2 and CaFe1.92Co0.08As2 of the ‘122’ family have been investigated by means of the 14.41-keV Mossbauer transition in Fe versus temperature ranging from the room temperature till 4.2 K. A comparison is made with the previously investigated parent compounds BaFe2As2 and CaFe2As2. It has been found that Mossbauer spectra of these superconductors are composed of the magnetically split component due to development of spin density wave (SDW) and non-magnetic component surviving even at lowest temperatures. The latter component is responsible for superconductivity. Hence, the superconductivity occurs in the part of the sample despite the sample is single phase. This phenomenon is caused by the slight variation of the dopant concentration across the sample (crystal).

Several high-pressure techniques applied for the synthesis and crystal growth of materials important for the future and present applications are presented. For the synthesis and thermodynamic studies of novel superconductors and III–N wide-gap semiconductors three different high-pressure techniques have been used: (i) the anvil system with solid pressure medium, (ii) the nitrogen/argon gas pressure system, and (iii) the oxygen gas pressure system.

The DC magnetization and AC magnetic susceptibilities were measured for MgB 2 single crystals, unsubstituted and carbon substituted with the composition of Mg(B 0.94 C 0.06 ) 2 . AC magnetic losses were derived from the AC susceptibility data as a function of the AC amplitude and the DC bias magnetic field. From the DC magnetization loops critical current densities were derived as a function of temperature and DC field. Results show that the substitution with carbon decreases critical current densities at low external magnetic fields, in contrast to the well known effect of an increase of the critical current densities at higher magnetic fields.

We report measurements of the phonon density of states as measured with inelastic x-ray scattering in SmFeAsO$_{1-x}$F$_y$ powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO$_{1-x}$F$_y$ single crystals allow us to identify the 21 meV A$_{1g}$ in-phase (Sm,As) and the 26 meV B$_{1g}$ (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors., 4 pages, 3 figures, submitted for SNS2010 conference proceedings

Enhanced Charge-Carrier Mobility in High-Pressure-Crystallized Poly(3-hexylthiophene) Christian M€uller, Nikolai D. Zhigadlo, Avinesh Kumar, Mohammed A. Baklar, Janusz Karpinski, Paul Smith, Theo Kreouzis, and Natalie Stingelin* Department of Materials, ETH Zurich, Zurich, Switzerland Laboratory for Solid State Physics, ETH Zurich, Zurich, Switzerland Centre for Materials Research, Queen Mary University of London, London, United Kingdom Department of Materials, Imperial College London, London, United Kingdom Centre for Plastic Electronics, Imperial College London, London, United Kingdom FRIAS, School of Soft Matter Research, University of Freiburg, Freiburg, Germany

We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in undoped and near-optimally doped SmFeAsO1−x F x iron-pnictide superconductor single crystals (SC). In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition is observed. From the relaxation time in the normal state, significantly above T SDW, the second moment of the Eliashberg function is determined to be λ〈ω 2〉≈135 meV2. Below T SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2ΔSDW/k B T SDW=7±3, at 4.2 K. In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent gaps.

The recent results on the growth of the Al x Ga 1-x N bulk single crystals (0.22 ≤ x ≤ 0.91) from solution in liquid Ga under high nitrogen pressure are discussed. We focus on the influence of temperature and the choice of the Al source on the crystal growth. The experiments involving different sources of aluminum such as Al metal, pre-reacted polycrystalline Al y Ga 1-y N and AlN powder are compared. The best results were achieved using pre-reacted polycrystalline Al y Ga 1-y N or/and AIN. Single-crystal structure refinement data of these Al x Ga 1-x N crystals are presented. We also update the p-T phase diagram of (Al,Ga)N compound at high N 2 pressure for various Al content, which is the basis for (Al,Ga)N synthesis.

The thermodynamic properties of the Al-Ga-N 2 system under high N 2 pressure up to 10 kbar and 1800 °C are investigated. On the basis of the experimental p-T growth conditions for (Al,Ga)N crystals, the standard Gibbs free energy as well as the standard enthalpy and entropy of formation of the Al x Ga 1-x N crystals as a function of composition x were calculated. The a N2 -T and x-T phase diagrams for (Al,Ga)N are presented.

Superconducting technology provides most sensitive field detectors, promising implementations of qubits and high field magnets for medical imaging and for most powerful particle accelerators. Thus, with the discovery of new superconducting materials, such as the iron pnictides, exploring their potential for applications is one of the foremost tasks. Even if the critical temperature Tc is high, intrinsic electronic properties might render applications rather difficult, particularly if extreme electronic anisotropy prevents effective pinning of vortices and thus severely limits the critical current density, a problem well known for cuprates. While many questions concerning microscopic electronic properties of the iron pnictides have been successfully addressed and estimates point to a very high upper critical field, their application potential is less clarified. Thus we focus here on the critical currents, their anisotropy and the onset of electrical dissipation in high magnetic fields up to 65 T. Our detailed study of the transport properties of optimally doped SmFeAs(O,F) single crystals reveals a promising combination of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities along all crystal directions. This favorable intragrain current transport in SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a crucial requirement for possible applications. Essential in these experiments are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with sub-\mu\m^2 cross-section, with current along and perpendicular to the crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed magnetic fields. The pinning forces have been characterized by scaling the magnetically measured "peak effect".

The vortex structure in the iron pnictide single crystals Ba 1– x K x Fe 2 As 2 and Sr 1– x K x Fe 2 As 2 of the 122 type and SmFeAsO 1– xFx of the 1111 type has been investigated using the decoration method. In all of the crystals under investigation, no regular vortex lattice has been revealed in the magnetic field range up to 200 Oe. The disordered vortex structure is discussed in view of the vortex pinning in single crystals.

We have successfully grown bulk, single crystals of Al x Ga 1− x N with the Al content x ranging from 0.5 to 0.9. Samples were grown from Ga melt under high nitrogen pressure (up to 10 kbar) and at high temperature (up to 1800 °C) using a gas pressure system. The homogeneity and Al content of the crystals were investigated by X-ray diffraction and laser ablation mass spectrometry. On the basis of the high-pressure experiments, the corresponding pressure–temperature ( p – T ) phase diagram of Al–Ga–N was derived. The bandgap of the material was determined by the femtosecond two-photon absorption autocorrelation method and is equal to 5.81±0.01 eV for the Al 0.86 Ga 0.14 N crystals.

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO0.8F0.2 state-of-the-art polycrystals, with critical temperature Tc≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB2 and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to Tc, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ1=6.15±0.45 meV and Δ2=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same Tc. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.

We investigate the quasiparticle relaxation and low-energy electronic structure in a near-optimally doped pnictide superconductor with T c=49.5 K by means of femtosecond spectroscopy. Multiple relaxation processes are evident, with distinct superconducting state dynamics and a clear “pseudogap”-like feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude Δ PG=61±9 meV above and below T c. The fluence and temperature dependence of the superconducting state dynamics shows similar behaviour to the cuprate superconductors with the superconducting-condensate vaporization energy of E va/k B≈1.5 K per Fe ion.

A new modification of KOs2O6, the representative of a new structural type (Pearson symbol aP18, a=5.5668(1)A, b=6.4519(2)A, c=7.2356(2)A, space group P-1, no.2) was synthesized employing high pressure technique. Its structure was determined by single-crystal X-ray diffraction. The structure can be described as two OsO6 octahedral chains relating to each other through inversion and forming big voids with K atoms inside. Quantum chemical calculations were performed on the novel compound and structurally related cubic compound. High-pressure X-ray study showed that cubic KOs2O6 phase was stable up to 32.5(2) GPa at room temperature., Comment: 23 pages, 9 figures,6 tables. Accepted for J. Solid State Chem

A systematic micro-Raman study of the atomic substitution effects has been carried out on single crystals of MgB2, including C-doping for B and Li- or Mn-doping for Mg. The spectra of C (electron)-doped compounds exhibit remarkable differentiations with respect to pure MgB2, and new bands of E2g and A1g/B1g symmetry play the dominant role in the substituted spectra. We have observed a two-mode behavior apparently driven by the electron–phonon coupling strength and associated with an electronic topological transition of the σ bands. On the other hand, Li (hole) and Mn (magnetic impurity) doping do not seem to affect the well-defined broad band at 600 cm−1. Combining Raman investigations and Tc-interdependence shows that this band is not related with the transition temperature, questioning its relationship to superconductivity in diborides.

A systematic Raman study has been carried out on single crystals of Mg(B 1− x C x ) 2 with carbon concentrations in the range of x = 0.0–0.15. We have found remarkable differentiations of the Raman spectra with doping. Even with small amounts of carbon substitution ( x = 0.027) the E 2g -symmetry broad band at ∼600 cm −1 (for pure MgB 2 ) diminishes and new modes appear of the approximate A 1g /B 1g (∼750 cm −1 ) and E 2g (∼850 cm −1 ) symmetries, though the transition temperature is only slightly reduced ( −1 , as assumed by other investigators. The practical disappearance of the original ∼600 cm −1 mode at x ≃ 0.08 coincides with the fast reduction of T c . The doping dependence of the phonons indicates a two-mode behavior, which could be driven by the 2D–3D electronic topological transition and associated with a possible nanoscale phase separation.

We have performed local tunneling spectroscopy on high quality Mg 1 - x Al x B 2 single crystals by means of variable temperature scanning tunneling spectroscopy in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively measured, probing different amplitudes of the three-dimensional Δ π as a function of Al content (i.e. as a function of the critical temperature T C ). Temperature and magnetic field dependences of the conductance spectra were studied in S–I–N configuration: the effect of the doping resulted in a monotonous reduction of the locally measured T C down to 24 K for x = 0.2. The magnetic field dependence was studied in a local way: An estimation for upper critical field H c 2 was inferred from the evolution of the tunneling spectra with the field perpendicular to the sample surface, for different doping levels. The high spatial resolution of the STS technique allowed us to evidence possible non-homogeneities of the superconducting properties on the sample surface with variation of in the same sample depending on different local levels of doping. The locally measured upper critical field resulted to vary for different dopings, and the maximum value H c 2 ≃ 3 T was found for samples with T C = 33 K. The evolution of the density of states (DOS) was found to be characterized by two distinct regimes separated by a crossover region. Our results indicate a rapid suppression of the intrinsic term in π -band superconductivity for 0 T π -band survives uniquely due to the coupling to the σ -band. The shape of tunneling spectra suggests an important role played by the quasiparticle inter-band scattering.

Single-crystal magnesium diboride (MgB 2 ) with a transition temperature to superconductivity of T c = 38.4 K was investigated by quadrupole perturbed 11 B nuclear magnetic resonance in a field of 9 T in the normal conducting phase. Experimental values for the electric field gradient tensor, the magnetic shift tensor, and the nuclear spin–lattice relaxation time for boron were obtained. The anisotropy of the nuclear spin–lattice relaxation and the magnetic shift were determined. A positive sign of the electric field gradient was found. Our results show good agreement with published theoretical calculations and in part with experimental nuclear magnetic resonance data from polycrystalline samples.

To elucidate the effect of the mass renormalization on the superconducting state in A Os 2 O 6 , we have performed thermodynamic and transport measurements on K x Rb 1− x Os 2 O 6 with x = 0–1. We show that KOs 2 O 6 (RbOs 2 O 6 ) is an intermediate to strong-coupling superconductor with a coupling parameter λ ep ≈ 1.2 ( λ ep ≈ 1). Quantifying λ ep allows us to determine the mass enhancement over the calculated band electronic density of states in addition to the electron–phonon term. We correlate the additional enhancement with the upper critical field slope which is determined by orbital magnetism. In the normal state, the charge carriers scatter at a low-energetic phonon. This rattling motion appears as an Einstein contribution with three modes per Rb in the RbOs 2 O 6 specific heat data, suggesting the rattler being the alkali-ion.

We report on the order–disorder transition line (Bod) in the flux line lattice—illustrated by the fishtail effect—of pure and carbon doped superconducting MgB2 single crystals. Neutron irradiation offers a unique opportunity for varying the defect density (nd) in a controlled way and to determine Bod not only as a function of temperature (T), but also as a function of nd. Experimental data are compared with theoretical predictions and found to be in good agreement over most of the available parameter range, both for Bod(T) and Bod(nd). Including two-band effects in MgB2 in quite a rough way leads to even better agreement.

Single crystals of MgB 2 , with a size up to 1.5 × 1 × 0.1 mm 3 and with a weight up to 230 μg, have been grown from flux with a high-pressure cubic anvil technique. Investigations of the P – T phase diagram prove that the MgB 2 phase is stable up to 2200 °C at high hydrostatic pressure. Specific band structure of MgB 2 with two bands (π and σ) involved in superconductivity is strongly influenced by chemical substitutions. Substitutions of Al for Mg and C for B lead to increase of scattering within both π and σ bands, however, with different rates for both substituents. Therefore, different changes of the upper critical field, H c2 , and its anisotropy, γ H c 2 , for Mg 1− x Al x B 2 and MgB 2− x C x are observed. Mg 1− x Al x B 2 crystals show a moderate decrease of the superconducting transition temperature, T c , for the samples with small x and, simultaneously, significant reduction of H c2 and its anisotropy at lower temperatures, as compared to the value for unsubstituted crystals. The temperature dependence of the anisotropy is less pronounced. MgB 2− x C x crystals exhibit only slight reduction of T c with substitution and, moreover, a significant increase of H c2 for an applied field oriented both parallel, H c2∥ ab , and perpendicular, H c2∥ c , to the ab -plane. For the single crystal with x = 0.13, H c2∥ c (0) ≈ 8.5 T is more than twice as large as that for an unsubstituted compound. The anisotropy of H c2 decreases from 6 (MgB 2 ) to about 4 ( x = 0.13) at low temperatures. The corresponding H c2∥ ab (0) ≈ 34 T is close to the maximum possible enhancement of H c2 due to the chemical substitutions. Hole doping with Li decreases T c , but in much slower rate than electron doping with C and Al. For MgB 2 crystals with simultaneously substituted Li for Mg and C for B, T c decreases more rapidly than in the case when only C is substituted. The T c reduction in co-doped crystals is a sum of T c reductions for separate C and Li doping. This means that holes introduced with Li cannot counterbalance electrons added with C. The possible reason of this can be that holes coming from Li occupy π band and do not compensate the addition of electrons which, coming from C, fill the σ band. Substitution of magnetic Mn for Mg strongly suppresses T c and H c2 due to the magnetic pair breaking. However, this is not the case for the substitution of Fe for Mg, at least for low Fe concentration.

We summarize the results of point-contact Andreev-reflection (PCAR) spectroscopy in MgB2 doped by chemical substitutions, either magnetic (Mn) or non-magnetic (Al, C), obtained by us and by other groups in the last four years. Despite the variety of samples used (crystals and polycrystals of various origin) and some minor differences in the experimental techniques, these measurements have directly provided a complete and consistent picture of the effects of chemical substitutions on the gaps of MgB2 shedding light on other relevant parameters (scattering rates, DOSs) affected by doping. In Al-doped crystals and polycrystals, the gap amplitudes Δσ and Δπ – obtained through a two-band Blonder-Tinkham-Klapwijk (BTK) fit of the Andreev-reflection conductance curves – decrease on increasing the Al content x (i.e. on decreasing the critical temperature of the contacts T c A ), but remain clearly distinct with no evidence of gap merging even when T c A is of the order of 10 K. The case of C-substituted MgB2 is less clear: the merging of the two gaps at x ≃ 0.13 was observed in single crystals grown at ETH, while the persistence of two gaps up to x = 0.10 – with apparently no tendency to merging at higher x – was found in other crystals and polycrystals. Finally, in Mn-doped single crystals the effects of pair-breaking magnetic scattering strongly suppress the critical temperature and the gaps, which however remain clearly distinct from each other down to Tc ≈ 10 K. In all cases, the analysis of the Δσ and Δπ data as a function of T c A (or of the doping content x) within the two-band Eliashberg theory gives interesting information about the effects of substitutions on the different scattering channels (interband and intraband, magnetic or non-magnetic).

Electronic Raman scattering studies on MgB2 single crystals as a function of excitation and polarization have revealed three distinct superconducting features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78 cm-1 which we identify as the superconducting gaps in \pi- and \sigma-bands and as the Leggett's collective mode arising from the fluctuation in the relative phase between two superconducting condensates residing on corresponding bands. The temperature and field dependencies of the superconducting features have been established. A phononic Raman scattering study of the E2g boron stretching mode anharmonicity and of superconductivity induced self-energy effects is presented. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization., Comment: Invited review in a special issue "Superconductivity in MgB_2: Physics and Applications"

The superconducting properties of pure and carbon-substituted MgB2 single crystals were studied prior to and after fast neutron irradiation. The C substitution significantly enhances the upper critical field Hc2. In comparison with an unsubstituted crystal, Hc2 parallel to the c-axis in Mg(B0.948C0.052)2, e.g.?at 15?K, is enhanced by a factor of about 2 and the Hc2 anisotropy is reduced. After fast neutron irradiation very pronounced changes in flux pinning are observed, both for the pure and for the substituted crystal, and the impact of the induced defects on the critical current density is significantly stronger than that of substitution. On the other hand, the changes in Hc2 of the carbon-substituted crystal are much more pronounced than those caused by irradiation. The observed changes in Hc2 can be explained by taking the characteristic length scale of the defects introduced by C substitution and by neutron irradiation into account.

It has recently been established that the high temperature (high-Tc) superconducting state coexists with short-range charge-density-wave order and quenched disorder arising from dopants and strain. This complex, multiscale phase separation invites the development of theories of high temperature superconductivity that include complexity. The nature of the spatial interplay between charge and dopant order that provides a basis for nanoscale phase separation remains a key open question, because experiments have yet to probe the unknown spatial distribution at both the nanoscale and mescoscale (between atomic and macroscopic scale). Here we report micro X-ray diffraction imaging of the spatial distribution of both the charge-density-wave puddles (domains with only a few wavelengths) and quenched disorder in HgBa2CuO4+y, the single layer cuprate with the highest Tc, 95 kelvin. We found that the charge-density-wave puddles, like the steam bubbles in boiling water, have a fat-tailed size distribution that is typical of self-organization near a critical point. However, the quenched disorder, which arises from oxygen interstitials, has a distribution that is contrary to the usual assumed random, uncorrelated distribution. The interstitials-oxygen-rich domains are spatially anti-correlated with the charge-density-wave domains, leading to a complex emergent geometry of the spatial landscape for superconductivity., 11 pages, 3 figures

The lattice properties at low temperatures of two samples of NdFeAsO1-xFx (x = 0.05 and 0.25) have been examined in order to investigate possible structural phase transition that may occur in the optimally doped superconducting sample with respect to the non-superconducting low-F concentration compound. In order to detect small modifications in the ion displacements with temperature micro-Raman and high resolution synchrotron powder diffraction measurements were carried out. No increase of the width of the (2 2 0) or (3 2 2) tetragonal diffraction peaks and microstrains could be found in the superconducting sample from synchrotron XRD measurements. On the other hand, the atomic displacement parameters deviate from the expected behavior, in agreement with modifications in the phonon width, as obtained by Raman scattering. These deviations occur around 150 K for both F dopings, with distinct differences among the two compounds, i.e., they decrease at low doping and increase for the superconducting sample. The data do not support a hidden phase transition to an orthorhombic phase in the superconducting compound, but point to an isostructural lattice deformation. Based on the absence of magnetic effects in this temperature range for the superconducting sample, we attribute the observed lattice anomalies to the formation of local lattice distortions that, being screened by the carriers, can only acquire long-range coherence by means of a structural phase transition at low doping levels. (C) 2016 Elsevier B.V. All rights reserved.

We demonstrate that images of flux vortices in a superconductor taken with a transmission electron microscope can be used to measure the penetration depth and coherence length in all directions at the same temperature and magnetic field. This is particularly useful for MgB2, where these quantities vary with the applied magnetic field and values are difficult to obtain at low field or in the c direction. We obtained images of flux vortices from a MgB2 single crystal cut in the ac plane by focused ion beam milling and tilted to 45 degrees. with respect to the electron beam about the crystallographic a axis. A new method was developed to simulate these images that accounted for vortices with a nonzero core in a thin, anisotropic superconductor and a simplex algorithm was used to make a quantitative comparison between the images and simulations to measure the penetration depths and coherence lengths. This gave penetration depths Lambda(ab) = 100 +/- 35 nm and Lambda(c) = 120 +/- 15 nm at 10.8 K in a field of 4.8 mT. The large error in Lambda(ab) is a consequence of tilting the sample about a and had it been tilted about c, the errors on Lambda(ab) and Lambda(c) would be reversed. Thus obtaining the most precise values requires taking images of the flux lattice with the sample tilted in more than one direction. In a previous paper [J. C. Loudon et al., Phys. Rev. B 87, 144515 (2013)], we obtained a more precise value for Lambda(ab) using a sample cut in the ab plane. Using this value gives Lambda(ab) = 107 +/- 8 nm, Lambda(c) = 120 +/- 15 nm, xi(ab) = 39 +/- 11 nm, and xi(c) = 35 +/- 10 nm, which agree well with measurements made using other techniques. The experiment required two days to conduct and does not require large-scale facilities. It was performed on a very small sample, 30 x 15 mu m and 200-nm thick, so this method could prove useful for superconductors where only small single crystals are available, as is the case for some iron-based superconductors.

We report on the irreversible magnetic properties of MgB 2 single crystals that were modified by neutron irradiation or carbon doping. Fast neutron irradiation introduces defects with a size comparable to the superconducting coherence length. Accordingly, the effects on the irreversible properties are large. The critical current density and the irreversibility field are strongly enhanced by neutron irradiation. Additionally, a second peak that is not found in unirradiated samples, emerges and gets more pronounced with increasing neutron fluence. Carbon doping was studied for different compositions of Mg(B 1− x C x ) 2 with x = 0.038, 0.066, and 0.095. Although disorder increases with carbon doping, the critical current density decreases. On the other hand, a small fishtail is observed, which becomes more pronounced with increasing x . The superconducting (reversible) parameters are modified in a similar way both by neutron irradiation and by carbon doping. The results indicate, that the irreversible properties are mainly influenced by changes of these reversible properties in the case of carbon doping (e.g., the decrease of the coherence length reduces the pinning energy), but rather by the new defect structure in the case of neutron irradiation.

Small changes in doping or the application of a hydrostatic pressure can induce non-linear lattice distortions, lattice instabilities, and phase separation in the cuprates. In YBa2Cu3Ox, oxygen doping above optimal induces a separation into microphases originating from a modification in the buckling of the CuO2 planes. In the same compound, the excess nominal doping by Ca creates changes in the CuO2 buckling at some critical concentrations. These are correlated with the formation of domains where the additional carriers brought by Ca remain isolated surrounded by yttrium atoms. Furthermore, it was observed that hydrostatic pressure induces non-linear modifications in the spectral characteristics of the Ag-symmetry phonons in all cuprates studied (YBa2Cu3O6.5, YBa2Cu3Oover, YBa2Cu4O8, and Bi2Sr2CaCu2O8). A coexistence of phases appears at a critical pressure where changes in the superconducting transition temperature have been detected. We have also observed similar correlations between the transition temperature and spectral modifications by chemical doping in the Raman spectra of La2−xSrxCuO4. Finally, soft modes were found in the cuprates studied, with energies scaling with the maximum transition temperature of the compound.

We report on the effects of neutron irradiation on the irreversible magnetic properties of MgB2 single crystals. The size of the newly created defects is comparable to the superconducting coherence length of MgB2, which makes these defects particularly suitable for pinning flux lines. Indeed, we observe significant quantitative and qualitative modifications of the critical current density. In particular, a second peak (fishtail effects) emerges in the field dependence and is accompanied by history effects. The fishtail effect is thoroughly studied with respect to different neutron fluences and good agreement with a recent theoretical explanation by an order-disorder transition is obtained.

We report on the first scanning tunnelling spectroscopy measurements on the edge of aluminium-doped magnesium diboride (Mg1−xAlxB2,x = 0.085) single crystals. To target their edge (~30 µm thick) we have developed a low temperature STM facility featuring a home built compact STM head coupled to an inertially driven carriage. This allows linear submicronic positioning over millimetric distances. We demonstrate the capabilities of our system by showing reproducible tunnelling spectra obtained both parallel and perpendicular to the crystalline c axis. Tunnelling on the edge reveals the gap originating in the highly 2D σ band but in contrast with pure MgB2 does not show the signature of the π gap.

In the first part of the present paper we discuss the fabrication and the characterization of an MgB 2 -based SQUID magnetometer with a directly coupled large-area pick-up loop, made on an MgB 2 film deposited by an all in situ technique. The coarse structure of the SQUID was defined by optical lithography and Ar-ion milling, while the two nanobridges acting as weak links in the superconducting loop were made by focused ion beam (FIB) milling. The device was characterized at different temperatures and showed Josephson quantum interference up to 20 K as well as a noise level already compatible with the recording of an adult magnetocardiogram. In the second part, concerning the fundamental physics of MgB 2 , we present the results of very recent point-contact measurements on Mg 1− x Mn x B 2 single crystals with 34.1 ⩾ T c ⩾ 13.3 K (i.e. 0.37% ⩽ x ⩽ 1.5%). The experimental conductance curves were fitted with the generalized two-band BTK model and their behaviour in magnetic fields was studied to check if both the order parameters (OPs) of the σ and π bands were present in the whole doping range. The dependence of the OPs (evaluated through the fit) on the Andreev critical temperature of the junctions is analyzed in the framework of the two-band Eliashberg theory by including the effects of magnetic impurities. The results give an evidence of a dominant effect of the magnetic impurities on the σ-band channel.

Among the different methods of vortex study in superconductors, Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), occupies a particular place. Due to its extraordinary spatial and energy resolution STS allows the study of vortices in real space on a nanometeric scale. In practice however, the success of the observation of vortices by STS depends strongly on the surface quality. Usually highly uniform surfaces are needed to distinguish vortices. As a result of this limitation, the vortex lattice has been successfully studied only in a limited number of superconductors. Here we suggest a simple method extending the vortex analysis to 'difficult cases' of 'bad' superconductors, thin films and even granular materials. In this method the STM tip is positioned in a chosen location of the sample and the local tunneling spectroscopy is continuously performed while the magnetic field is slowly swept. The vortices entering the sample move under the tip and are detected due to the change in the shape of tunneling spectra. We show that the method allows not only to follow the vortex penetration but also to estimate essential superconducting parameters such as the penetration and the coherence lengths. Moreover, it authorizes the study of the vortex cores in the cases where scanning is difficult or impossible. (c) 2005 Elsevier Ltd. All rights reserved.