Search

We report on the high-pressure growth, structural characterization, and investigation of the electronic properties of single-crystalline LaFeAsO co-substituted by Sm and P, in both its normal- and superconducting states. Here, the appearance of superconductivity is attributed to the inner chemical pressure induced by the smaller-size isovalent substituents. X-ray structural refinements show that the partial substitution of La by Sm and As by P in the parent LaFeAsO compound leads to a contraction in both the conducting Fe2(As,P)2 layers and the interlayer spacing. The main parameters of the superconducting state, including the critical temperature, the lower- and upper critical fields, as well as the coherence length, the penetration depth, and their anisotropy, were determined from magnetometry measurements on a single-crystalline La0.87Sm0.13FeAs0.91P0.09O sample. The critical current density (jc), as resulting from loops of magnetization hysteresis in the self-generated magnetic field, is 2 × 106 A/cm2 at 2 K. Overall, our findings illustrate a rare and interesting case of how superconductivity can be induced by co-substitution in the 1111 family. Such approach delineates new possibilities in the creation of superconductors by design, thus stimulating the exploration of related systems under multi-chemical pressure conditions., Journal of Alloys and Compounds, 958, ISSN:0925-8388, ISSN:1873-4669

In two dimensional materials, substitutional doping during growth can be used to alter the electronic properties.

The iron-based oxypnictide superconductor SmFeAsO0.8F0.2 was synthesized under high-pressure and investigated by measuring dc magnetic susceptibility. The zero-field cooled (ZFC) magnetic susceptibility confirmed the bulk superconductivity of the sample with a critical temperature of Tc = 50 K and a significant jump in the magnetization at 4.3 K, usually attributed to the antiferromagnetic ordering of Sm3+ ions in this system. Since the occurrence of the jump depends on the cooling history, our data strongly suggest a spin-glass-like behaviour.

Using multiple Andreev reflections (MAR) effect spectroscopy of SnS junctions, we directly determine temperature dependences of the superconducting order parameters, excess Andreev current, and zero-bias conductance (ZBC) in polycrystalline samples of superconducting oxypnictides Sm0.7Th0.3OFeAs and NdO0.6H0.36FeA with almost optimal critical temperatures. It is shown that the results obtained are self-consistent and could be described in the framework of a two-band model. We estimate a dominating contribution 70–85% of the bands with the large gap to the total conductance.

The light emission properties of color centers emitting in 3.3-4 eV region are investigated for hydrostatic pressures ranging up to 5GPa at liquid helium temperature. The light emission energy decreases with pressure less sensitively than the bandgap. This behavior at variance from the shift of the bandgap is typical of deep traps. Interestingly, hydrostatic pressure reveals the existence of levels that vary differently under pressure (smaller increase of the emission wavelength compared to the rest of the levels in this energy region or even decrease of it) with pressure. This discovery enriches the physics of the color centers operating in the UV in hBN., Comment: 16 pages, 3 figures

A single-sample synthesis concept based on multi-element ceramic samples can produce a variety of local products. When applied to cuprate superconductors (SC), statistical modelling predicts the occurrence of possible compounds in a concentration range of ∼50 ppm. In samples with such low concentrations, determining which compositions are superconducting is a challenging task and requires local probes or separation techniques. Here, we report results from samples with seven components: BaO2, CaCO3, SrCO3, La2O3, PbCO3, ZrO2 and CuO oxides and carbonates, starting from different grain sizes. The reacted ceramics show different phases, particular grain growth, as well as variations in homogeneity and superconducting properties. High-Tc superconductivity up to 118 K was found. Powder x-ray diffraction (XRD) in combination with energy-dispersive spectroscopy (EDS), scanning transmission electron microscopy (STEM) can assign “Pb1223” and “(Sr,Ca,Ba)0.7-1.0CuO2” phases in inhomogeneous samples milled with 10 mm ball sizes. Rather uniform samples featuring strong grain growth were obtained with 3 mm ball sizes, resulting in Tc ∼70 K superconductivity of the “La(Ba,Ca)2Cu3Ox” based phase. Scanning SQUID microscopy (SSM) establishes locally formed superconducting areas at a level of a few microns in inhomogeneous superconducting particles captured by a magnetic separation technique. The present results demonstrate a new synthetic approach for attaining high-Tc superconductivity in compounds without Bi, Tl, Hg, or the need for high-pressure synthesis.

Superconductivity in topological materials, either at ambient or extreme conditions, has continued to intrigue scientists as a promising candidate for realizing topological superconductivity, a platform to host the long-sought Majorana fermions in condensed matter. The recent discovery of extremely large magnetoresistance (XMR) in the rare-earth monopnictides opens a new avenue to search for topologically nontrivial states therein, although contrasting opinions argue that it is the carrier compensation effect that is responsible for the observed large, nonsaturating magnetoresistance. Here we study the quantum oscillations and pressure-induced superconductivity in the topologically nontrivial candidate YBi. While the magnetotransport and quantum oscillations do reveal nearly compensated charge carriers, first-principles calculations clearly show that the electronic surface states manifest topologically nontrivial features. Upon applying external hydrostatic pressures, the magnetoresistance is found to decrease and at $P\ensuremath{\sim}2.5$ GPa, superconductivity emerges. There exists, however, a regime where XMR and superconductivity coexist in the phase diagram. YBi may therefore represent a rare system for studying the interplay between XMR, topological states, and superconductivity.

Black phosphorus (b-P) is an allotrope of phosphorus whose properties have attracted great attention. In contrast to other 2D compounds, or pristine b-P, the properties of b-P alloys have yet to be explored. In this report, we present a detailed study on the Raman spectra and on the temperature dependence of the electrical transport properties of As-doped black phosphorus (b-AsP) for an As fraction x = 0.25. The observed complex Raman spectra were interpreted with the support of Density Functional Theory (DFT) calculations since each original mode splits in three due to P-P, P-As, and As-As bonds. Field-effect transistors (FET) fabricated from few-layered b-AsP exfoliated onto Si/SiO2 substrates exhibit hole-doped like conduction with a room temperature ON/OFF current ratio of ∼103 and an intrinsic field-effect mobility approaching ∼300 cm2 V-1 s-1 at 300 K which increases up to 600 cm2 V-1 s-1 at 100 K when measured via a 4-terminal method. Remarkably, these values are comparable to, or higher, than those initially reported for pristine b-P, indicating that this level of As doping is not detrimental to its transport properties. The ON to OFF current ratio is observed to increase up to 105 at 4 K. At high gate voltages b-AsP displays metallic behavior with the resistivity decreasing with decreasing temperature and saturating below T ∼100 K, indicating a gate-induced insulator to metal transition. Similarly to pristine b-P, its transport properties reveal a high anisotropy between armchair (AC) and zig-zag (ZZ) directions. Electronic band structure computed through periodic dispersion-corrected hybrid Density Functional Theory (DFT) indicate close proximity between the Fermi level and the top of the valence band(s) thus explaining its hole doped character. Our study shows that b-AsP has potential for optoelectronics applications that benefit from its anisotropic character and the ability to tune its band gap as a function of the number of layers and As content.

Using small-angle neutron scattering we have studied the superconducting vortex lattice (VL) phase diagram in MgB2 as the applied magnetic field is rotated away from the c axis and towards the basal plane. The field rotation gradually suppresses the intermediate VL phase which exists between end states aligned with two high symmetry directions in the hexagonal basal plane for H || c. Above a critical angle, the intermediate state disappears, and the previously continuous transition becomes discontinuous. The evolution towards the discontinuous transition can be parameterized by a vanishing twelvefold anisotropy term in the VL free energy., 6 pages, 4 figures; [v2]: minor revisions to text and figures; [v3] typos fixed, journal reference added

Chiral superconductors are novel topological materials with finite angular momentum Cooper pairs circulating around a unique chiral axis, thereby spontaneously breaking time-reversal symmetry. They are rather scarce and usually feature triplet pairing: a canonical example is the chiral p-wave state realized in the A-phase of superfluid He3. Chiral triplet superconductors are, however, topologically fragile with the corresponding gapless boundary modes only weakly protected against symmetry-preserving perturbations in contrast to their singlet counterparts. Using muon spin relaxation measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral superconductor: spontaneous magnetic fields inside the superconducting state indicating broken time-reversal symmetry and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is a chiral d-wave singlet., Nature Communications, 12 (1), ISSN:2041-1723

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while $^{75}$As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_{c}$ and expected to mediate the superconducting pairing., 12 pages, 9 figures. Title changed, minor amendments in the text

Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe2-class of transition metal dichalcogenides. However, the Dirac nodes observed in PtSe2, PdTe2, and PtTe2 candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here, we report the synthesis of a new type-II Dirac semimetal NiTe2 in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry’s phase associated with these Dirac fermions. Our first-principles calculations f...

Synthesized under high pressure, hydrogen-substituted NdFeAsO${}_{0.6}$H${}_{0.36}$ (with a critical temperature of 48 K) possibly demonstrates a double-dome superconducting state in its phase diagram, thus foreshadowing extraordinary physics as compared with other iron-based oxypnictides. Ballistic Andreev transport in superconductor--normal metal--superconductor junctions provides direct information about the superconducting order parameter at any temperatures up to ${T}_{c}$, and specific boson emission. The authors discuss here the characteristics found of the multiple-band superconducting state, the boson energy, and their nature.

The occurrence of superconductivity in topological materials is considered as a promising route for realizing topological superconductors, a platform able to host the long-sought Majorana fermions in condensed matter. In this work, by using electrical transport and heat-capacity measurements, as well as first-principles band-structure calculations, we investigate the physical properties of TaOsSi, a superconductor with ${T}_{c}\ensuremath{\approx}5.8$ K. The behavior of both its upper critical field and low-temperature heat-capacity suggest the existence of two superconducting gaps. More strikingly, first-principles calculations reveal gapless topological surface states in the present material. The evolution of the electrical resistivity with pressure (up to 50 GPa) was also investigated, and a ``V-shaped'' diagram of ${T}_{c}$ vs $P$ was found. Overall, our data suggest that TaOsSi is a new system where multiband superconductivity and topological surface states coexist and, hence, it may serve as a possible candidate in the search for topological superconductivity.

Two-dimensional lateral heterojunctions based on monolayer transition-metal dichalcogenides (TMDs) have received increasing attention given that their direct band gap makes them very attractive for optoelectronic applications. Although bilayer TMDs present an indirect band gap, their electrical properties are expected to be less susceptible to ambient conditions, with higher mobilities and density of states when compared to monolayers. Bilayers and few-layers single domain devices have already demonstrated higher performance in radio frequency and photosensing applications. Despite these advantages, lateral heterostructures based on bilayer domains have been less explored. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS2-WS2 and MoSe2-WSe2 monodomains. The heterojunctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layers. A phenomenological mechanism is proposed to explain the growth mode with self-limited thickness that happens within a certain window of growth conditions. With respect to their as-grown monolayer counterparts, bilayer lateral heterostructures yield nearly 1 order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ∼103 times larger than those extracted from the as-grown monolayers, in addition to room-temperature electroluminescence. The improved performance of bilayer heterostructures significantly expands the potential of two-dimensional materials for optoelectronics.

By varying the parameters controlling the growth of crystals, including the thermodynamic variables, such as temperature, pressure, and reagent composition and the kinetic factors, namely reaction time and cooling rate, we found the most appropriate conditions for the reproducible growth of the nonstoichiometric antiperovskite superconductors MgC1-xNi3-y. Bulk single crystals of MgC1-xNi3-y were grown by a self-flux method at 3 GPa and 1700 C using a mixture of Mg, C, and Ni powders in a molar ratio 1:1.25:3. The as-grown black colored crystals, mechanically extracted from solidified lump, exhibit various irregular three dimensional shapes, with flat surfaces and maximum dimensions up to ~ (1-1.2) x (0.8-1.0) x (0.4-0.6) mm3. Single-crystal x-ray diffraction refinement confirmed the high structural perfection of the grown crystals (Space group Pm-3m, No 221, Z = 1, a = 3.7913(1) {\AA}, and V = 54.5(1) {\AA}3), but also the presence of deficiencies on the C and Ni sites. Temperature-dependent magnetization measurements showed a single-phase behaviour with a critical temperature (Tc) ranging between 6.3 and 6.8 K due to the slightly different C and Ni stoichiometries of MgC1-xNi3-y crystals. The growth of relatively large crystals reported here could provide a helpful guidance for further syntheses of various 3d-based antiperovskite intermetallics under high pressure., Comment: 4 tables, 5 figures

We have studied non-equilibrium phase transitions in the vortex lattice in superconducting MgB2, where metastable states are observed in connection with an intrinsically continuous rotation transition. Using small-angle neutron scattering and a stop-motion technique, we investigated the manner in which the metastable vortex lattice returns to the equilibrium state under the influence of an ac magnetic field. This shows a qualitative difference between the supercooled case which undergoes a discontinuous transition, and the superheated case where the transition to the equilibrium state is continuous. In both cases the transition may be described by an an activated process, with an activation barrier that increases as the metastable state is suppressed, as previously reported for the supercooled vortex lattice [E. R. Louden et al., Phys. Rev. B 99, 060502(R) (2019)]. Separate preparations of superheated metastable vortex lattices with different domain populations showed an identical transition towards the equilibrium state. This provides further evidence that the vortex lattice metastability, and the kinetics associated with the transition to the equilibrium state, is governed by nucleation and growth of domains and the associated domain boundaries., Comment: 27 pages, 10 figures. arXiv admin note: text overlap with arXiv:1812.05970

The vortex lattice (VL) in MgB2 is characterized by the presence of long-lived metastable states (MSs), which arise from cooling or heating across the equilibrium phase boundaries. A return to the equilibrium configuration can be achieved by inducing vortex motion. Here we report on small-angle neutron scattering studies of MgB2, focusing on the structural properties of the VL as it is gradually driven from metastable to equilibrium states (ESs) by an AC magnetic field. Measurements were performed using initial MSs obtained either by cooling or heating across the equilibrium phase transition. In all cases, the longitudinal correlation length remains constant and comparable to the sample thickness. Correspondingly, the VL may be considered as a system of straight rods, where the formation and growth of ES domains only occurs in the two-dimensional plane perpendicular to the applied field direction. Spatially resolved raster scans of the sample were performed with apertures as small as 80 μm, corresponding to only 1.2 × 106 vortices for an applied field of 0.5 T. These revealed spatial variations in the metastable and equilibrium VL populations, but individual domains were not directly resolved. A statistical analysis of the data indicates an upper limit on the average domain size of approximately 50 μm., New Journal of Physics, 21, ISSN:1367-2630

Bulk single crystals of SrPt3P have been grown for the first time by a self-flux method at 2 GPa and 1500 {\deg}C using the cubic-anvil, high-pressure and high-temperature technique. The grown black crystals were found to have either a plate-like or pillar-like morphology with maximum dimensions of ~ 1 - 0.6 - 0.3 mm3. According to a goniometric study the crystals are elongated in the ab-plane with the c-axis perpendicular to the plane. The crystal structure was confirmed by X-ray diffraction (P4/nmm, # 129, Z = 2, a = b = 5.7927(2) {\AA}, c = 5.3729(2) {\AA} and V = 180.290(11) {\AA}3). Temperature dependent susceptibility measurements showed a single-phase behaviour and a superconducting transition temperature of 8.6 K. This value for single crystals is slightly higher than that previously reported for polycrystalline SrPt3P. The sharpness of the susceptibility drop ({\Delta}Tc = 0.07 K) supports a high homogeneity of obtained crystals., Comment: 18 pages, 4 figures, 4 tables, accepted for J. Cryst. Growth

In this short review, we provide some new insights into the material synthesis and characterization of modern multi-component superconducting oxides. Two different approaches such as the high-pressure, high-temperature method and ceramic combinatorial chemistry will be reported with application to several typical examples. First, we highlight the key role of the extreme conditions in the growth of Fe-based superconductors, where a careful control of the composition-structure relation is vital for understanding the microscopic physics. The availability of high-quality LnFeAsO (Ln = lanthanide) single crystals with substitution of O by F, Sm by Th, Fe by Co, and As by P allowed us to measure intrinsic and anisotropic superconducting properties such as Hc2, Jc. Furthermore, we demonstrate that combinatorial ceramic chemistry is an efficient way to search for new superconducting compounds. A single-sample synthesis concept based on multi-element ceramic mixtures can produce a variety of local products. Such a system needs local probe analyses and separation techniques to identify compounds of interest. We present the results obtained from random mixtures of Ca, Sr, Ba, La, Zr, Pb, Tl, Y, Bi, and Cu oxides reacted at different conditions. By adding Zr but removing Tl, Y, and Bi, the bulk state superconductivity got enhanced up to about 122 K.

We report on the first successful growth of single crystals of the noncentrosymmetric superconductor ${\mathrm{Mo}}_{3}{\mathrm{Al}}_{2}\mathrm{C}$ obtained by means of a cubic-anvil, high-pressure, and high-temperature technique. Composition, structure, and normal-state transport properties of the crystals were studied by means of x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetic susceptibility, and resistivity measurements as a function of temperature. Variations in critical temperature (${T}_{c}$) between 8.6 and 9.3 K were observed, probably due to the slightly different carbon stoichiometry of the samples. Single-crystal x-ray refinement confirmed the high structural perfection of the grown crystals. Remarkably, the refined Flack parameter values for all the measured crystals using a $P{4}_{1}32$ space-group model were consistently close to either 0 or 1, hence indicating that the considered crystals belong to two enantiomorphic space groups, $P{4}_{1}32$ and $P{4}_{3}32$. An anomaly in the resistivity is observed at $\ensuremath{\simeq}130$ K, most likely associated with the onset of a charge-density-wave phase. The superconducting properties (and in particular the symmetry, the amplitude, and the temperature dependence of the superconducting gap) were studied by using point contact Andreev-reflection spectroscopy. The results confirm that ${\mathrm{Mo}}_{3}{\mathrm{Al}}_{2}\mathrm{C}$ is a moderately strongly-coupled superconductor with $2\mathrm{\ensuremath{\Delta}}/{k}_{\mathrm{B}}{T}_{c}\ensuremath{\simeq}4$ and unambiguously prove that the order parameter has an $s$-wave symmetry despite the asymmetric spin-orbit coupling arising from the lack of inversion symmetry.

The hunt for the benchmark topological superconductor (TSc) has been an extremely active research subject in condensed matter research, with quite a few candidates identified or proposed. However, low transition temperatures (Tc) and/or strong sensitivity to disorder and dopant levels in known TSc candidates have greatly hampered progress in this field. Here, we use Angle-resolved Photoemission Spectroscopy (ARPES) to show the presence of Dirac Nodal Lines (DNLs) and the corresponding topological surface states (TSS's) on the [010] faces of the Tc=39K s-wave BCS superconductor MgB2. Not only is this nearly triple the current record of superconducting Tc among all candidate TSc's, but the nature of these DNL states should make them highly tolerant against disorder and inadvertent doping variations. This makes MgB2 a promising high temperature platform for the study of topological superconductivity.

Similar to chemical doping, pressure produces and stabilizes new phases of known materials, whose properties may differ greatly from those of their standard counterparts. Here, by considering a series of LaFeAs$_{1-x}$P$_x$O iron-pnictides synthesized under high-pressure high-temperature conditions, we investigate the simultaneous effects of pressure and isoelectronic doping in the 1111 family. Results of numerous macro- and microscopic technique measurements, unambiguously show a radically different phase diagram for the pressure-grown materials, characterized by the lack of magnetic order and the persistence of superconductivity across the whole $0.3 \leq x \leq 0.7$ doping range. This unexpected scenario is accompanied by a branching in the electronic properties across $x = 0.5$, involving both the normal and superconducting phases. Most notably, the superconducting order parameter evolves from nodal (for $x < 0.5$) to nodeless (for $x \geq 0.5$), in clear contrast to other 1111 and 122 iron-based materials grown under ambient-pressure conditions., 9 pages, 7 figures, Suppl. material

We report the first determination of the in-plane complex optical conductivity of 1111 high-${T}_{c}$ superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and microfocused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission, we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at ${T}_{c}$ and experiences an anomalous $T$-linear softening and narrowing below ${T}^{*}\ensuremath{\approx}110\text{ }\text{ }\mathrm{K}\ensuremath{\gg}{T}_{c}$. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}\ensuremath{\approx}7$ in both materials, indicating strong pairing.

We studied a reproducible fine structure observed in dynamic conductance spectra of Andreev arrays in Sm$_{1-x}$Th$_x$OFeAs superconductors with various thorium concentrations ($x = 0.08 - 0.3$) and critical temperatures $T_c = 26-50$\,K. This structure is unambiguously caused by a multiple boson emission (of the same energy) during the process of multiple Andreev reflections. The directly determined energy of the bosonic mode reaches $\varepsilon_0 = 14.8 \pm 2.2$\,meV for optimal compound. Within the studied range of $T_c$, this energy as well as the large $\Delta_L$ and the small $\Delta_S$ superconducting gaps, nearly scales with critical temperature with the characteristic ratio $\varepsilon_0/k_BT_c \approx 3.2$ (and $2\Delta_L/k_BT_c \approx 5.3$, correspondingly) resembling the expected energy $\Delta_L + \Delta_S$ of spin resonance and spectral density enhancement in $s^{\pm}$ and $s^{++}$ states, respectively., Comment: 10 pages, 4 figures

Using small-angle neutron scattering, we investigated the behavior of a metastable vortex lattice state in MgB2 as it is driven towards equilibrium by an AC magnetic field. This shows an activated behavior, where the AC field amplitude and cycle count are equivalent to, respectively, an effective "temperature" and "time". The activation barrier increases as the metastable state is suppressed, corresponding to an aging of the vortex lattice. Furthermore, we find a cross-over from a partial to a complete suppression of metastable domains depending on the AC field amplitude, which may empirically be described by a single free parameter. This represents a novel kind of collective vortex behavior, most likely governed by the nucleation and growth of equilibrium vortex lattice domains., Comment: 5 pages plus 3 pages of supplemental material

Bulk single crystals of manganese arsenide (MnAs) were grown from melt at 1 GPa and 1100 {\deg}C by using a cubic-anvil, high-pressure, and high-temperature technique. The as-grown black colored crystals extracted from solidified lump exhibit a plate-like morphology, with flat surfaces and maximum dimensions up to ~ 3 x 2 x 0.5 mm3. The hexagonal crystal structure at room temperature was confirmed by X-ray diffraction [B81, space group P63/mmc, No 194, Z = 2, a = 3.7173(4) {\AA}, b = 3.7173(4) {\AA}, c = 5.7054(8) {\AA}, and V = 68.277(16) {\AA}3]. Temperature-dependent magnetization measurements reveal the occurrence of a first-order ferro- to paramagnetic transition at Tc = 318.5 K accompanied by a hysteresis of ~ 9 K. The successful growth of relatively large crystals reported here might be extended to various substituted analogues of MnAs, thus opening new possibilities for further exploration of this interesting system., Comment: arXiv admin note: text overlap with arXiv:1705.10491

The anti-perovskite superconductor MgC$_{0.93}$Ni$_{2.85}$ was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20\% reduction in the Fermi level density-of-states would lead to a significant ($\sim 70\%$) suppression of the superconducting $T_c$ for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller $T_c$ reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations., 11 pages, 3 figures

One of the most unique and robust experimental facts about iron-based superconductors is the renormalization of the electronic band dispersion by factor of 3 and more near the Fermi level. Obviously related to the electron pairing, this prominent deviation from the band theory lacks understanding. Experimentally studying the entire spectrum of the valence electrons in iron arsenides, we have found an unexpected depletion of the spectral weight in the middle of the iron-derived band, which is accompanied by a drastic increase of the scattering rate. At the same time, the measured arsenic-derived band exhibits very good agreement with theoretical calculations. We show that the low-energy Fermi velocity renormalization should be viewed as a part of the modification of the spectral function by a strong electronic interaction. Such an interaction with an energy scale of the whole $d$ band appears to be a hallmark of many families of unconventional superconductors.

The anti-perovskite superconductor MgC0.93Ni2.85 was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20% reduction in the Fermi level density-of-states would lead to a significant (~70%) suppression of the superconducting T c for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller T c reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations.

Muon-spin rotation data collected at ambient pressure ($p$) and at $p=2.42$ GPa in MnP were analyzed to check their consistency with various low- and high-pressure magnetic structures reported in the literature. Our analysis confirms that in MnP the low-temperature and low-pressure helimagnetic phase is characterised by an increased value of the average magnetic moment compared to the high-temperature ferromagnetic phase. An elliptical double-helical structure with a propagation vector ${\bf Q}=(0,0,0.117)$, an $a-$axis moment elongated by approximately 18% and an additional tilt of the rotation plane towards $c-$direction by $\simeq 4-8^{\rm o}$ leads to a good agreement between the theory and the experiment. The analysis of the high-pressure $\mu$SR data reveals that the new magnetic order appearing for pressures exceeding $1.5$ GPa can not be described by keeping the propagation vector ${\bf Q} \parallel c$. Even the extreme case -- decoupling the double-helical structure into four individual helices -- remains inconsistent with the experiment. It is shown that the high-pressure magnetic phase which is a precursor of superconductivity is an incommensurate helical state with ${\bf Q} \parallel b$., Comment: 9 pages, 9 figures. arXiv admin note: substantial text overlap with arXiv:1603.03367

Using intrinsic multiple Andreev reflections effect spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors ${\mathrm{Sm}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}\mathrm{OFeAs}$ with various thorium doping and critical temperatures ${T}_{C}=21--54$ K. We observe a scaling between both superconducting gaps and ${T}_{C}$. The determined BCS ratio for the large gap $2{\mathrm{\ensuremath{\Delta}}}_{L}/{k}_{B}{T}_{C}=5.0--5.7$ and its eigen-BCS ratio (in a hypothetical case of zero interband coupling) $2{\mathrm{\ensuremath{\Delta}}}_{L}/{k}_{B}{T}_{C}^{L}=4.1--4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2{\mathrm{\ensuremath{\Delta}}}_{S}/{k}_{B}{T}_{C}=1.2--1.6$, remain nearly constant within all the ${T}_{C}$ range studied. The temperature dependences ${\mathrm{\ensuremath{\Delta}}}_{L,S}(T)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas Coulomb repulsion constants ${\ensuremath{\mu}}^{*}$ are finite in Sm-based oxypnictides.

Through a direct comparison of specific heat and magnetoresistance, we critically asses the nature of superconducting fluctuations in the same nanogram crystal of SmFeAs(O,F). We show that although the superconducting fluctuation contribution to conductivity scales well within the two-dimensional lowest Landau level (2D-LLL) scheme, its predictions contrast with the inherently 3D nature of SmFeAs(O,F) in the vicinity of ${T}_{c}$. Furthermore, the transition seen in specific heat cannot be satisfactory described either by the LLL or the $XY$ scaling. Additionally we have validated, through comparing ${H}_{c2}$ values obtained from the entropy conservation construction (${H}_{c2//ab}=\ensuremath{-}19.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}/\mathrm{K}$ and ${H}_{c2//c}=\ensuremath{-}2.9\phantom{\rule{0.16em}{0ex}}\mathrm{T}/\mathrm{K})$, the analysis of fluctuation contribution to conductivity as a reasonable method for estimating the $H{}_{c2}$ slope.

Bulk single crystals of manganese phosphide (MnP) were grown from melt at 1 GPa and 1200 C by using a cubic-anvil, high-pressure, and high-temperature technique. The obtained black colored crystals exhibit a plate-like morphology, with flat surfaces and maximum dimensions up to 4 x 2 x 0.5 mm3. The orthorhombic crystal structure was confirmed by X-ray diffraction [Pnma, 62, Z = 4, a = 5.2510(4) {\AA}, b = 3.1670(3) {\AA}, c = 5.90098 (4) {\AA} and V = 98.279(14) {\AA}3]. Temperature-dependent magnetization measurements reveal the occurrence of two successive transitions: a paramagnetic to ferromagnetic transition at Tc = 290.5 K and the development of a double helimagnetic order at Ts = 44.5 K. Zero-field 31P NMR measurements in the FM and in the screw-spin AFM state show prominent features, which are compared with previous experimental data and theoretical calculations. The relatively large crystals obtained here open up new possibilities for further explorations of this interesting material., Comment: 4 tables, 7 figures

We studied iron-based superconductors of various families with critical temperatures covering almost all range $T_C = 9 - 53$ K. In natural arrays of contacts formed in these materials we observed intrinsic multiple Andreev reflections effect (IMARE). By using IMARE spectroscopy, we detected the two-gap superconductivity, determined the value of the large and the small superconducting gaps, and the corresponding BCS-ratios. The temperature dependencies of the large and the small gaps $\Delta_{L,S}(T)$ are similar for various families of the Fe-based superconductors and could be well-fitted in the framework of the two-band model by Moskalenko and Suhl. We concluded on the extended s-wave symmetry of the $\Delta_L$ order parameter (20-30 % anisotropy in k-space) and on the absence of nodes for $\Delta_S$. The BCS-ratio $2\Delta_L/k_BT_C \approx 5.2$ is nearly constant within the whole range of $T_C$ (this means that coupling rate is unchanged), reflecting the 20 % reduction of the $T_C^{local}$ in relation to the eigen $T_C^L$, and the large gap roughly corresponds to the energy of magnetic resonance $2\Delta_L \approx E_{res}$. This result requires a special theoretical consideration. Our estimation of the relative coupling constants and eigen parameters of each condensate (in a hypothetical case of a zero interband interaction) $2\Delta_L/k_BT_C^L = 4.2 - 4.8$ and $2\Delta_S/k_BT_C^S = 3.5 - 4.5$ leads to indirect conclusion that namely a strong electron-phonon interaction in each condensate described in the framework of the Eliashberg theory plays the key role in the superconductivity of iron-based oxypnictides. With it, the two condensates interact weakly with each other. The observed scaling of $\Delta_{L,S}$ with $T_C$, as was discussed above, is caused mainly by changing of the density of states $N_{L,S}$ in the bands, whereas Ln-O spacers act as charge reservoirs.

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.

We studied SNS and S-N-S-N-...-S contacts (where S is a superconductor and N is a normal metal) formed by “break-junction” technique in polycrystalline Sm1 − x Th x OFeAs superconductor samples with critical temperatures T C = 34–45 K. In such contacts (intrinsic) multiple Andreev reflections effects were observed. Using spectroscopies based on these effects, we detected two independent bulk order parameters and determined their magnitudes. Theoretical analysis of the large and the small gap temperature dependences revealed superconducting properties of Sm1 − x Th x OFeAs to be driven by intraband coupling, and $$\sqrt {V_{11} V_{22} } /V_{12} \approx 14$$ (where V ij are the electron-boson interaction matrix elements), whereas the ratio between density of states for the bands with the small and the large gap, N 2/N 1, correspondingly, was roughly of an order. We estimated “solo” BCS-ratio values in a hypothetic case of zero interband coupling (V i ≠ j = 0) for each condensate as 2ΔL, S/k B T C L,S ≤ 4.5. The values are constant within the range of critical temperatures studied, and correspond to a case of strong intraband electron-phonon coupling.

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.

The preparation and definition of stoichiometric large (3–5 mm edge length) single crystals of CeN are described. The band structure is discussed and compared with X-ray-photoemission-spectra (XPS), Bremsstrahlen-isochromat-spectroscopy (BIS) and optical reflectivity. CeN is intermediate valent with partially occupied and empty 4f states near EF. The specific heat is measured until 0.3 K and the γ term exhibits a sharp upturn toward the lowest temperatures. The Debye temperature is evaluated. The magnetic susceptibility is a large Pauli term pointing to a high density of states at EF as expected for a fractionally filled 4f1 state., Results in Physics, 3, ISSN:2211-3797

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 report a detailed $\mu$SR study of the pressure evolution of the magnetic order in the manganese based pnictide MnP, which has been recently found to undergo a superconducting transition under pressure once the magnetic ground state is suppressed. Using the muon as a volume sensitive local magnetic probe, we identify a ferromagnetic state as well as two incommensurate helical states (with propagation vectors ${\bf Q}$ aligned along the crystallographic $c-$ and $b-$directions, respectively) which transform into each other through first order phase transitions as a function of pressure and temperature. Our data appear to support that the magnetic state from which superconductivity develops at higher pressures is an incommensurate helical phase., Comment: 11 pages, 9 figures

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.

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.