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With molecular beam epitaxy and topotactic chemical reaction, we prepared NdFeAs(O,H) epitaxial thin films with various hydrogen concentrations on 5{\deg} vicinal cut MgO substrates. By measuring the resistivities along the longitudinal and transversal directions, the ab plane and the c axis resistivities (\{rho}_ab and \{rho}_c) were obtained. The resistivity anisotropy {\gamma}_\{rho}=\{rho}_c \ \{rho}_ab of NdFeAs(O,H) with various hydrogen concentrations was compared with that of NdFeAs(O,F). At the H concentrations which led to superconducting transition temperatures Tc over 40 K, {\gamma}_\r{ho} recorded ~100-150 at 50 K. On the other hand, a low {\gamma}_\{rho} value of 9 was observed with the mostly doped sample. The exponent \{beta} of the ab plane resistivity obtained by fitting a power law expression \{rho}_{ab}(T)=\{rho}_0+AT^\{beta} to the data was close to unity down to low temperature in the vicinity where the second antiferromagnetic phase locates, which may be related to the quantum critical point discussed at the over-doped side of the phase diagram., Comment: Appeared in Physical. Rev. Materials

We present stability and structure of two- and three-alpha systems embedded in dilute cold neutron matter. By solving a few-alpha Schr\"{o}dinger equation with quasiparticle properties, i.e., effective mass and induced two- and three-alpha interactions, which are evaluated in terms of Fermi polarons, it is shown that $^8 \mathrm{Be}$ and the Hoyle state become bound at densities of about $10^{-4}$ and $10^{-3}$ of the saturation density, respectively. It is also seen that, under cold neutron matter environment, both systems become smaller than the corresponding systems in vacuum. Our results would affect astrophysical models for stellar collapse and neutron star mergers, as well as relevant reaction rates for nucleosynthesis., Comment: 3 pages, 1 figure, contribution to the 16th International Symposium on Nuclei in the Cosmos (NIC-XVI), 21-25 Sep. 2021

We elucidate the fate of neighboring two and three-$\alpha$ particles in cold neutron matter by focusing on an analogy between such $\alpha$ systems and Fermi polarons realized in ultracold atoms. We describe in-medium excitation properties of an $\alpha$ particle and neutron-mediated two- and three-$\alpha$ interactions using theoretical approaches developed for studies of cold atomic systems. We numerically solve the few-body Schr\"odinger equation of $\alpha$ particles within standard $\alpha$ cluster models combined with in-medium properties of $\alpha$ particles. We point out that the resultant two-$\alpha$ ground state and three-$\alpha$ first excited state, which correspond to $^8$Be and the Hoyle state, respectively, known as main components in the triple-$\alpha$ reaction, can become bound states in such a many-neutron background although these states are unstable in vacuum. Our results suggest a significance of these in-medium cluster states not only in astrophysical environments such as core-collapsed supernova explosions and neutron star mergers but also in neutron-rich nuclei., Comment: 9 pages, 5 figures

The first excited $J^\pi=0^+$ state of $^{12}$C, the so-called Hoyle state, plays an essential role in a triple-$\alpha$ ($^4$He) reaction, which is a main contributor to the synthesis of $^{12}$C in a burning star. We investigate the Coulomb screening effects on the energy shift of the Hoyle state in a thermal plasma environment using precise three-$\alpha$ model calculations. The Coulomb screening effect between $\alpha$ clusters are taken into account within the Debye-H\"uckel approximation. To generalize our study, we utilize two standard $\alpha$-cluster models, which treat the Pauli principle between the $\alpha$ particles differently. We find that the energy shifts do not depend on these models and follow a simple estimation in the zero-size limit of the Hoyle state when the Coulomb screening length is as large as a value typical of such a plasma consisting of electrons and $\alpha$ particles., Comment: 6 pages, 2 figures, contribution to the 8th Asia-Pacific Conference on Few-Body Problems in Physics (APFB2020), 1-5 March 2021, Kanazawa, Japan

In this study, we present a rutile-related material, LiMoO2, that becomes a cluster magnet and exhibits a spin singlet formation on a preformed molybdenum dimer upon cooling. Unlike ordinary cluster magnets, the atomic dyz orbital robustly survives despite the formation of molecular orbitals, thereby affecting the magnetic properties of the selected material. Such hybrid cluster magnets with the characters of molecular and atomic orbitals realize multiple independent spins on an isolated cluster, leading to an ideal platform to study the isolated spin dimers physics., Comment: 6 pages, 4 figures

Crystal and magnetic structures of the mineral centennialite CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O crystallizes in the $P\bar{3}m1$ space group and Cu$^{2+}$ ions form a geometrically perfect kagome network with antiferromagnetic $J_1$. No intersite disorder between Cu$^{2+}$ and Ca$^{2+}$ ions is detected. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O enters a magnetic long-range ordered state below $T_\text{N}=7.2$~K, and the $\mathbf{q}=\mathbf{0}$ magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3~K is suppressed to $0.58(2)\mu_\text{B}$. Our DFT calculations indicate the presence of antiferromagnetic $J_2$ and ferromagnetic $J_d$ superexchange couplings of a strength which places the system at the crossroads of three magnetic orders (at the classical level) and a spin-$\frac{1}{2}$ PFFRG analysis shows a dominance of $\mathbf{q}=\mathbf{0}$ type magnetic correlations, consistent with and indicating proximity to the observed $\mathbf{q}=\mathbf{0}$ spin structure. The results suggest that this material is located close to a quantum critical point and is a good realization of a $J_1$-$J_2$-$J_d$ kagome antiferromagnet., Comment: 4 figures and supplemental materials

We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr$_2$RuO$_4$ by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of $\hbar\omega_\text{res}=0.56$~meV centered at a characteristic wavevector $\mathbf{Q}_\text{res}=(0.3, 0.3, 0.5)$. The resonance energy corresponds well to the superconducting gap $2\Delta=0.56$~meV estimated by the tunneling spectroscopy. The spin resonance shows the $L$ modulation with a maximum at around $L = 0.5$. The $L$ modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional $\alpha$ and $\beta$ sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of Sr$_2$RuO$_4$. We also discuss the implications on possible superconducting order parameters., Comment: arXiv admin note: substantial text overlap with arXiv:1904.01234

The triple alpha reaction is a key to $^{12}$C production and is expected to occur in weakly-coupled, thermal plasmas as encountered in normal stars. We investigate how Coulomb screening affects the structure of a system of three alpha particles in such a plasma environment by precise three-body calculations within the Debye-H\"uckel approximation. A three-alpha model that has the Coulomb interaction modified in the Yukawa form is employed. Precise three-body wave functions are obtained by a superposition of correlated Gaussian bases with the aid of the stochastic variational method. The energy shifts of the Hoyle state due to the Coulomb screening are obtained as a function of the Debye screening length. The results, which automatically incorporate the finite size effect of the Hoyle state, are consistent with the conventional result based on the Coulomb correction to the chemical potentials of ions that are regarded as point charges in a weakly-coupled, thermal plasma. We have given a theoretical basis to the conventional point-charge approach to the Coulomb screening problem relevant for nuclear reactions in normal stars by providing the first evaluation of the Coulomb corrections to the $Q$ value of the triple alpha process that produces a finite size Hoyle state., Comment: 13 pages, 3 figures

Magnetic excitations and magnetic structure of EuRbFe$_4$As$_4$ were investigated by inelastic neutron scattering (INS), neutron diffraction, and random phase approximation (RPA) calculations. Below the superconducting transition temperature $T_\text{c}=36.5$~K, the INS spectra exhibit the neutron spin resonances at $Q_\text{res}=1.27(2)$~$\text{\AA}^{-1}$ and $1.79(3)$~$\text{\AA}^{-1}$. They correspond to the $\mathbf{Q}=(0.5,0.5,1)$ and $(0.5,0.5,3)$ nesting wave vectors, showing three dimensional nature of the band structure. The characteristic energy of the neutron spin resonance is $E_\text{res}=17.7(3)$~meV corresponding to $5.7(1)k_\text{B}T_\text{c}$. Observation of the neutron spin resonance mode and our RPA calculations in conjunction with the recent optical conductivity measurements are indicative of the $s_\pm$ superconducting pairing symmetry in EuRbFe$_4$As$_4$. In addition to the neutron spin resonance mode, upon decreasing temperature below the magnetic transition temperature $T_\text{N}=15$~K, the spin wave excitation originating in the long-range magnetic order of the Eu sublattice was observed in the low-energy inelastic channel. Single-crystal neutron diffraction measurements demonstrate that the magnetic propagation vector of the Eu sublattice is $\mathbf{k}=(0, 0, 0.25)$, representing the three-dimensional antiferromagnetic order. Linear spin wave calculations assuming the obtained magnetic structure with the intra- and inter-plane nearest neighbor exchange couplings of $J_1/k_\text{B}=-1.31$~K and $J_c/k_\text{B}=0.08$~K can reproduce quantitatively the observed spin wave excitation. Our results show that superconductivity and long-range magnetic order of Eu coexist in EuRbFe$_4$As$_4$ whereas the coupling between them is rather weak., Comment: 8 pages, 4 figures

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors show quite generally that most of the iron-based superconductors are p-wave superconductors., Comment: 35 pages, 15 figures

Molecular based spin-1/2 triangular lattice systems such as LiZn$_2$Mo$_3$O$_8$ have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li$_2$$A$Mo$_3$O$_8$ ($A$ = In or Sc) is such a compound where spin-1/2 Mo$_3$O$_{13}$ clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the $A$ site. Li$_2$InMo$_3$O$_8$ undergoes conventional $120^\circ$ long-range magnetic order below $T_\text{N}=12$~K whereas isomorphic Li$_2$ScMo$_3$O$_8$ exhibits no long-range magnetic order down to 0.5~K. Here, we report exotic magnetisms in Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ investigated by muon spin rotation ($\mu$SR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ show completely different behaviors observed in both $\mu$SR and INS measurements, representing their different ground states. Li$_2$InMo$_3$O$_8$ exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the $120^\circ$ spin structure. In contrast, Li$_2$ScMo$_3$O$_8$ undergoes short-range magnetic order below 4~K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions., Comment: Sci. Rep. 9, 1826 (2019)

An epitaxial NdFeAs(O,F) thin film of 90 nm thickness grown by molecular beam epitaxy on MgO single crystal with Tc = 44.2 K has been investigated regarding a possible vortex glass-liquid tran-sition. The voltage-current characteristics show excellent scalability according to the vortex-glass model with a static critical exponent {\nu} of around 1.35 and a temperature-dependent dynamic exponent z increasing from 7.8 to 9.0 for the investigated temperature range. The large and non-constant z values are discussed in the frame of 3D vortex glass, thermally activated flux motion, and inhomogeneity broadening., Comment: 20 pages, 4 figures

We discuss herein the emergence of a large entropy change in metallic Li0.33VS2 derived from the orbitally assisted loose {\sigma} bond formation. Comprehensive structural studies based on synchrotron x-ray and neutron diffraction analyses clarify the fabrication of ribbon chains at 375 K, consisting of multiple three-centered two-electron {\sigma} bonds based on the viewpoint of local chemical bonding. Although the metallic conductivity persists down to the lowest temperature measured, exceptionally large entropy change as a metal, as much as {\Delta}S = 6.6 J/mol K, appears at the transition. Emergence of a large entropy change in a metallic state expects us the possible novel functional materials, such as a heat-storage material with rapid thermal response., Comment: 6 pages, 4 figures, to be published in Phys. Rev. B: Rapid Communications

The effect of $\alpha$-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature $T_c$ and the upper critical field $H_{c2}$, and properties more of interest for applications, like the critical current density $J_c$ and the related pinning landscape. The irradiation-induced suppression of the film $T_c$ is significantly smaller than on a similarly damaged single crystal. Moreover $H_{c2}$ behaves differently, depending on the field orientation: for H//c the $H_{c2}$ slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature $H_{c2}$, the effects on the $J_c$(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 T to 6 T after an irradiation of $2\times10^{15} cm^{-2}$. On the contrary, $J_c$(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the $J_c$(H//ab) performance is not related to a decrease of the $J_c$ anisotropy as found in other superconductors. Instead it is due to the presence of point defects that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure., Comment: 12 pages, 9 figures

The dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe$_4$As$_4$ was investigated by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations. Powder INS measurements show that the spin resonance at $Q_\text{res}=1.17(1)$~$\text{\AA}^{-1}$, corresponding to the $(\pi,\pi)$ nesting wave vector in tetragonal notation, evolves below $T_\text{c}$. The characteristic energy of the spin resonance $E_\text{res}=12.5$~meV is smaller than twice the size of the superconducting gap ($2\Delta$). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe$_4$As$_4$ is the $s_\pm$ symmetry., Comment: http://journals.jps.jp/doi/10.7566/JPSJ.86.093703

We systematically investigate the neutron-skin thickness of neutron-rich nuclei within a compressible droplet model, which includes several parameters characterizing the surface tension and the equation of state (EOS) of asymmetric nuclear matter as well as corrections due to the surface diffuseness. Such a systematic analysis helps towards constraining the EOS parameters of asymmetric nuclear matter and the poorly known density dependence of the surface tension; the latter is estimated with help of available experimental data for the neutron and proton density distributions and the nuclear masses. Validity of the present approach is confirmed by calculating realistic density distributions of Ca, Ni, Zr, Sn, Yb, and Pb isotopes within a microscopic Skyrme-Hartree-Fock+BCS method for various sets of the effective nuclear force. Our macroscopic model accompanied by the diffuseness corrections works well in the sense that it well reproduces the evolution of the microscopically deduced neutron-skin thickness with respect to the neutron number for selected sets of the effective nuclear force. We find that the surface tension of the compressible nuclear droplet is a key to bridging a gap between microscopic and macroscopic approaches., Comment: 11 pages, 7 figures

The magnetic structure and spin-wave excitations in the quasi-square-lattice layered perovskite compound La$_5$Mo$_4$O$_{16}$ were studied by a combination of neutron diffraction and inelastic neutron scattering techniques using polycrystalline sample. Neutron powder diffraction refinement revealed that the magnetic structure is ferrimagnetic in the $ab$ plane with antiferromagnetic stacking along the $c$ axis where the magnetic propagation vector is $\mathbf{k}=\left(0,0,\frac{1}{2}\right)$. The ordered magnetic moments are estimated to be $0.54(2)\mu_\text{B}$ for Mo$^{5+}$ ($4d^1$) ions and $1.07(3)\mu_\text{B}$ for Mo$^{4+}$ ($4d^2$) ions at 4 K, which are about half of the expected values. The inelastic neutron scattering results display strong easy-axis magnetic anisotropy along the $c$ axis due to the spin-orbit interaction in Mo ions evidenced by the spin gap at the magnetic zone center. The model Hamiltonian consisting of in-plane anisotropic exchange interactions, the interlayer exchange interaction, and easy-axis single-ion anisotropy can explain our inelastic neutron scattering data well. Strong Ising-like anisotropy and weak interlayer coupling compared with the intralayer exchange interaction can explain both the high-temperature magnetoresistance and long-time magnetization decay recently observed in La$_5$Mo$_4$O$_{16}$., Comment: 6 pages, 4 figures

A quantum critical point (QCP) is currently being conjectured for the BaFe$_2$(As$_{1-x}$P$_x$)$_2$ system at the critical value $x_{\rm c} \approx$ 0.3. In the proximity of a QCP, all thermodynamic and transport properties are expected to scale with a single characteristic energy, given by the quantum fluctuations. Such an universal behavior has not, however, been found in the superconducting upper critical field $H_{\rm c2}$. Here we report $H_{\rm c2}$-data for epitaxial thin films extracted from the electrical resistance measured in very high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that $H_{\rm c2}$ is sensitive to the QCP, implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is essentially band-dependent. Our results point to two qualitatively different groups of electrons in BaFe$_2$(As$_{1-x}$P$_x$)$_2$. The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also be present in many other quantum critical systems., Comment: 9 pages, 3 figures, and Supplementary materials

The Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the $\it{RE}$FeAs(O,F) ($\it{RE}$1111, where $\it{RE}$ is a rare earth element) is the family with the highest critical temperature $T_c$ but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 ($T_c$ $\sim$ 55 K). Here we focus our attention on the pinning properties of the lower-$T_c$ Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density $J_c$ at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the $\it{c}$-axis and vortex shearing prevail. When the field approaches the $\it{ab}$-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D-Abrikosov and 2D-Josephson vortices: one is determined by the formation of a vortex staircase structure and one by lock-in of the vortices parallel to the layers. This is the first study on FBS showing this behaviour in a full temperature, field, and angular range and it demonstrates that, despite the lower $T_c$ and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong $\it{ab}$-peak in $J_c$., Comment: Final version available on Scientific Reports (Open access)

We report on the systematic characterization of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ epitaxial thin films on CaF$_2$ substrate in view of their possible use for superconducting electronic applications. By using different and complementary techniques we studied the morphological characteristics of the surface, the structural properties, the magnetic response, and the superconducting properties in terms of critical temperature, critical current, and energy gaps. Particular attention was paid to the homogeneity of the films and to the comparison of their superconducting properties with those of single crystals of the same compound., Comment: 19 pages, 7 figures. Preprint version

In this work we present inelastic neutron scattering experiments which probe the single ion ground states of the rare earth pyrochlores $R_2$Ti$_2$O$_7$ ($R$ = Tb, Dy, Ho). Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ are dipolar spin ices, now often described as hosts of emergent magnetic monopole excitations; the low temperature state of Tb$_2$Ti$_2$O$_7$ has features of both spin liquids and spin glasses, and strong magnetoelastic coupling. We measured the crystal field excitations of all three compounds and obtained a unified set of crystal field parameters. Additional measurements of a single crystal of Tb$_2$Ti$_2$O$_7$ clarified the assignment of the crystal field levels in this material and also revealed a new example of a bound state between a crystal field level and an optical phonon mode., Comment: 17 pages, 7 figures

We present a model of the lattice dynamics of the rare earth titanate pyrochlores R2Ti2O7 (R=Tb, Dy, Ho), which are important materials in the study of frustrated magnetism. The phonon modes are obtained by density functional calculations, and these predictions are verified by comparison with scattering experiments. Single crystal inelastic neutron scattering is used to measure acoustic phonons along high symmetry directions for R=Tb, Ho; single crystal inelastic x-ray scattering is used to measure numerous optical modes throughout the Brillouin zone for R=Ho; and powder inelastic neutron scattering is used to estimate the phonon density of states for R=Tb, Dy, Ho. Good agreement between the calculations and all measurements is obtained, allowing confident assignment of the energies and symmetries of the phonons in these materials under ambient conditions. The knowledge of the phonon spectrum is important for understanding spin-lattice interactions, and can be expected to be transferred readily to other members of the series to guide the search for unconventional magnetic excitations., Comment: 15 pages, 11 figures

Elucidating the nature of the magnetism of a high-temperature superconductor is crucial for establishing its pairing mechanism. The parent compounds of the cuprate and iron-pnictide superconductors exhibit N\'eel and stripe magnetic order, respectively. However, FeSe, the structurally simplest iron-based superconductor, shows nematic order (Ts = 90 K), but not magnetic order in the parent phase, and its magnetic ground state is intensely debated. Here, we report inelastic neutron-scattering experiments that reveal both stripe and N\'eel spin fluctuations over a wide energy range at 110 K. On entering the nematic phase, a substantial amount of spectral weight is transferred from the N\'eel to the stripe spin fluctuations. Moreover, the total fluctuating magnetic moment of FeSe is ~ 60% larger than that in the iron pnictide BaFe2As2. Our results suggest that FeSe is a novel S = 1 nematic quantum-disordered paramagnet interpolating between the N\'eel and stripe magnetic instabilities., Comment: Supplementary information included; accepted by Nature Communications

The upper critical field Hc2 anisotropy of Ca10(PtnAs8)(Fe2-xPtxAs2)5 (n = 3, 4) single crystals with long FeAs interlayer distance (d) was studied by angular dependent resistivity measurements. A scaling of the angular dependent resistivity was realized for both single crystals using the anisotropic Ginzburg-Landau (AGL) approach with an appropriate anisotropy parameter {\gamma}. The AGL scaling parameter {\gamma} increases with decreasing temperature and reaches a value of about 10 at 0.8Tc for both single crystals. These values are much larger than those of other iron-based superconductors (FeSCs). Remarkably, the values of {\gamma}2 show an almost linear increase with the FeAs/FeSe interlayer distance d for FeSCs. Compared to cuprates, FeSCs are less anisotropic, indicating that two dimensionality of the superconductivity is intrinsically weak., Comment: Accepted in Applied Physics Letters. 5 figures

Microstructura lly clean, isov alently P-doped BaFe2As2 (Ba-122) single crystalline thin films have been prepared on MgO (001) substrates by molecular beam epitaxy. These films show a superconducting transition temperature (Tc) of over 30 K although P content is around 0.22, which is lower than the optimal one for single crystals (i.e., 0.33). The enhanced Tc at this doping level is attributed to the in-plane tensile strain. The strained film shows high transport self-field critical current densities (Jc) of over 6 MA/cm2 at 4.2 K, which are among the highest for Fe based superconductors (FeSCs). In-field Jc exceeds 0.1 MA/cm2 at m0H = 35 T for H||ab and m0H = 18 T for H||c at 4.2 K, respectively, in spite of moderate upper critical fields compared to other FeSCs with similar Tc. Structural investigations reveal no defects or misoriented grains pointing to strong pinning centers. We relate this unexpected high Jc to a strong enhancement of the vortex core energy at optimal Tc, driven by in-plane strain and doping. These unusually high Jc make P-doped Ba-122 very favorable for high-field magnet applications., Comment: 5 pages, 4 figures

Bulk magnetization, transport and neutron scattering measurements were performed to investigate the electronic and magnetic properties of a polycrystalline sample of the newly discovered ferromagnetic superconductor, CeO$_{0.3}$F$_{0.7}$BiS$_{2}$. Ferromagnetism develops below T$_{FM}$ = 6.54(8) K and superconductivity is found to coexist with the ferromagnetic state below T$_{SC}$ ~ 4.5 K. Inelastic neutron scattering measurements reveal a very weakly dispersive magnetic excitation at 1.8 meV that can be explained by an Ising-like spin Hamiltonian. Under application of an external magnetic field, the direction of the magnetic moment changes from the c-axis to the ab-plane and the 1.8 meV excitation splits into two modes. A possible mechanism for the unusual magnetism and its relation to superconductivity is discussed.

Nuclear matter under astrophysical conditions is explored with time-dependent and static Hartree-Fock calculations. The focus is in a regime of densities where matter segregates into liquid and gaseous phases unfolding a rich scenario of geometries, often called nuclear pasta shapes (e.g. spaghetti, lasagna). Particularly the appearance of the different phases depending on the proton fraction and the transition to uniform matter are investigated. In this context the neutron background density is of special interest, because it plays a crucial role for the type of pasta shape which is built. The study is performed in two dynamical ranges, one for hot matter and one at temperature zero to investigate the effect of cooling., Comment: 7 pages, 5 figures

Nuclear matter under the conditions of a supernova explosion unfolds into a rich variety of spatially structured phases, called nuclear pasta. We investigate the role of periodic network-like structures with negatively curved interfaces in nuclear pasta structures, by static and dynamic Hartree-Fock simulations in periodic lattices. As the most prominent result, we identify for the first time the {\it single gyroid} network structure of cubic chiral $I4_123$ symmetry, a well known configuration in nanostructured soft-matter systems, both as a dynamical state and as a cooled static solution. Single gyroid structures form spontaneously in the course of the dynamical simulations. Most of them are isomeric states. The very small energy differences to the ground state indicate its relevance for structures in nuclear pasta., Comment: 7 pages, 4 figures

The temperature dependence of the resistivity of epitaxial Ba(Fe_(1-x)Co_x)2As2 thin films (with nominal doping x = 0.08, 0.10 and 0.15) has been analyzed and compared with analogous measurements on single crystals taken from literature. The rho(T) of thin films looks different from that of single crystals, even when the cobalt content is the same. All rho(T) curves can be fitted by considering an effective two-band model (with holes and electrons bands) in which the electrons are more strongly coupled with the bosons (spin fluctuations) than holes, while the effect of impurities is mainly concentrated in the hole band. Within this model the mediating boson has the same characteristic energy in single crystals and thin films, but the shape of the transport spectral function at low energy has to be very different, leading to a "hardening" of the electron-boson spectral function in thin films, associated with the strain induced by the substrate., Comment: 13 pages, 4 figures

Angle-resolved transport measurements revealed that planar defects dominate flux pinning in the investigated Co-doped BaFe2As2 thin film. For any given field and temperature, the critical current depends only on the angle between the crystallographic c-axis and the applied magnetic field but not on the angle between the current and the field. The critical current is therefore limited only by the in-plane component of the Lorentz force but independent of the out-of-plane component, which is entirely balanced by the pinning force exerted by the planar defects. This one-dimensional pinning behavior shows similarities and differences to intrinsic pinning in layered superconductors., Comment: 8 pages, 6 figures

The discovery of superconductivity (SC) with a transition temperature, Tc, up to 65K in single-layer FeSe (bulk Tc =8K) films grown on SrTiO3 substrates has attracted special attention to Fe-based thin films. The high Tc is a consequence of the combined effect of electron transfer from the oxygen-vacant substrate to the FeSe thin film and lattice tensile strain. Here we demonstrate the realization of SC in the parent compound BaFe2As2 (no bulk Tc) just by tensile lattice strain without charge doping. We investigate the interplay between strain and SC in epitaxial BaFe2As2 thin films on Fe-buffered MgAl2O4 single crystalline substrates. The strong interfacial bonding between Fe and the FeAs sublattice increases the Fe-Fe distance due to the lattice misfit which leads to a suppression of the antiferromagnetic spin density wave and induces SC with bulk-Tc ?10K. These results highlight the role of structural changes in controlling the phase diagram of Fe-based superconductors., Comment: http://www.nature.com/ncomms/2013/131206/ncomms3877/full/ncomms3877.html

The anisotropy of the critical current density Jc depends in general on both the properties of the flux lines (such as line tension, coherence length and penetration depth) and the properties of the defects (such as density, shape, orientation etc.). Whereas the Jc anisotropy in microstructurally clean films can be scaled to an effective magnetic field containing the Ginzburg-Landau anisotropy term, it is in general not possible (or only in a limited field range) for samples containing extended defects. Here, the Jc anisotropy of a Co-doped BaFe2As2 sample with 45{\deg} [001] tilt grain boundaries (GBs), i.e. grain boundaries created by 45{\deg} in-plane rotated grains, as well as extended Fe particles is investigated. This microstructure leads to c-axis correlated pinning, both due to the GBs and the Fe particles and manifests in a c-axis peak in the Jc anisotropy at low magnetic fields and a deviation from the anisotropic Ginzburg-Landau scaling at higher fields. Strong pinning at ellipsoidal extended defects, i.e. the Fe particles, is discussed, and the full Jc anisotropy is fitted successfully with the vortex path model. The results are compared to a sample without GBs and Fe particles. 45{\deg} GBs seem to be good pinning centers rather than detrimental to current flow., Comment: 8 pages, 7 figures, CEC-ICMC 2013 proceeding, accepted for publication in Advances in Cryogenic Engineering (Materials)

When magnetic moments (spins) are regularly arranged in a geometry of a triangular motif, the spins may not satisfy simultaneously their interactions with their neighbors. This phenomenon, called frustration, leads to numerous energetically equivalent magnetic states (ground states), which results in exotic states such as spin liquid and spin ice. Here we report an alternative situation: a system that, classically, is to be a liquid in the clean limit freezes into a glassy state induced by quantum fluctuations. The case in point is a frustrated magnet in which spins are arranged in a triangular network of bi-pyramids. When taking into account quantum corrections, the classical degeneracy is broken into a set of local minima in a rugged energy landscape, which are separated by large energy barriers, over a finite number of degenerate, periodic, ground states. The appearance of large barriers is due to the absence of local zero-energy modes that are typical in spin-liquid candidate systems. We establish this by mapping the set of local energy minima states into a tiling with colored hexagonal tiles. We show that the system exhibits a large number of aperiodic tessellations. The configuration entropy of the local minima is extremely sensitive to boundary conditions, scaling with the boundary length rather than its volume. The low temperature thermodynamics is also discussed to compare it with other glassy materials., Comment: paper+supplementary material 28 pages. too many figures

We tested oxidized titanium layers as barriers for hybrid Josephson junctions with high $I_cR_n$-products and for the preparation of junctions for tunneling spectroscopy. For that we firstly prepared junctions with conventional superconductor electrodes lead and niobium, respectively. By tuning the barrier thickness we were able to change the junctions' behavior from a Josephson junction to tunnel-like behavior applicable for quasi-particle spectroscopy. Subsequently, we transferred the technology to junctions using Co-doped BaFe$_2$As$_2$ thin films prepared by pulsed laser deposition as base electrode and evaporated Pb as counter electrode. For barriers with a thickness of 1.5\,nm we observe clear Josephson effects with $I_cR_n$\,$\approx$\,90\,$\mu$V at 4.2\,K. These junctions behave SNS'-like and are dominated by Andreev reflection transport mechanism. For junctions with barrier thickness of 2.0\,nm and higher no Josephson but SIS'- or SINS'-like behavior with a tunnel-like conductance spectrum was observed.

To investigate the superconducting properties of iron pnictides we prepared planar hybrid SNS' junctions in thin film technology with a pnictide base electrode, a gold barrier layer and a lead counter electrode. Our design allows characterization of the electrodes and the junction independently in a 4-probe method. We show how both electrodes influence the measured spectra due to their spreading resistance. While the Pb electrode has a constant resistance above its $T_c$, the contribution of the pnictide electrode is clearly current-dependent and thus it needs a more advanced method to be corrected. We present an empirical method, which is simple to apply and allows to deal with the spreading resistance in our junctions to recalculate the actual conductance and voltage of one junction at given temperature.

The dependence of the superconducting gaps in epitaxial Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the 2D Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps \Delta_{S} and \Delta_{L} show similar monotonic trends as a function of the local critical temperature T_{c}^{A} (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e. to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T_{c} according to the empirical law \Omega_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength \lambda_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly decreases to \lambda_{tot}= 1.82 in the overdoped samples (x = 0.15)., Comment: 8 pages, 5 color figures

Surface properties of Co-doped BaFe2As2 epitaxial superconducting thin films were inspected by X-ray photoelectron spectroscopy, scanning spreading resistance microscopy (SSRM), and point contact spectroscopy (PCS). It has been shown that surface of Fe-based superconductors degrades rapidly if being exposed to air, what results in suppression of gap-like structure on PCS spectra. Moreover, SSRM measurements revealed inhomogeneous surface conductivity, what is consistent with strong dependence of PCS spectra on contact position. Presented results suggest that fresh surface and small probing area should be assured for surface sensitive measurements like PCS to obtain intrinsic properties of Fe-based superconductors., Comment: 4 pages, 4 figures

Topological materials, including topological insulators, magnets with Skyrmions and ferroelectrics with topological vortices, have recently attracted phenomenal attention in the materials science community. Complex patterns of ferroelectric domains in hexagonal REMnO3 (RE: rare earths) turn out to be associated with the macroscopic emergence of Z2xZ3 symmetry. The results of our depth profiling of crystals with a self-poling tendency near surfaces reveal that the partial dislocation (i.e., wall-wall) interaction, not the interaction between vortices and antivortices, is primarily responsible for topological condensation through the macroscopic breaking of the Z2-symmetry.

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds., Comment: 7 pages, 4 figures. Journal of Superconductivity and Novel Magnetism, in press

We present a study of the response to pulsed infrared radiation of Fe-layer shunted pnictide thin film microstructures. The thin film multilayer consisting of 20 nm thick Fe-buffer, 50 nm thick Ba(Fe,Co)2As2 film and gold protection layer were deposited on heated MgO and MgAl2O4 substrates by pulsed-laser deposition. The multilayers were patterned into 5 to 8 um wide and 5 um long microbridges by electron-beam lithography and ion-milling technique. The microbridges show Tc up to 20 K and a critical current density up to 2.56 MA/cm2 at T = 10 K. The photo-response of Fe-shunted Ba(Fe,Co)2As2 thin film microbridges to infrared radiation was studied in a wide range of incident optical power, operation temperature and bias current. We have found that the electron energy relaxation in studied multilayers is dependent on substrate material and is 1.75 times faster in case of MgAl2O4 characterized by lattice matching to pnictide film in comparison to MgO substrate., Comment: ASC 2012 submission

Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible., Comment: 7 pages, 7 figures

Magnetic excitations in a Ni$_4$ magnetic molecule were investigated by inelastic neutron scattering and bulk susceptibility ($\chi_\text{bulk}$) techniques. The magnetic excitation spectrum obtained from the inelastic neutron scattering experiments exhibits three modes at energy transfers of $\hbar\omega=0.5$, 1.35, and 1.6 meV. We show that the energy, momentum, and temperature dependences of the inelastic neutron scattering data and $\chi_\text{bulk}$ can be well reproduced by an effective spin Hamiltonian consisted of intra-molecule exchange interactions, a single-ionic anisotropy, biquadratic interactions, and Zeeman term. Under a hydrostatic pressure, the bulk magnetization decreases with increasing pressure, which along with the biquadratic term indicates spin-lattice coupling present in this system., Comment: 6 pages, 6 figures, and 2 tables

We investigate the imaginary part of the wave vector dependent dynamic spin susceptibility in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ as a function of temperature using neutron scattering. At T=5 K, two-dimensional incommensurate (IC) magnetic fluctuations are clearly observed around $\mathbf{Q}_\text{c}=(0.3,0.3,L)$ up to approximately 60 meV energy transfer. We find that the IC excitations disperse to ridges around the $(\pi,\pi)$ point. Below 50 K, the energy and temperature dependent excitations are well described by the phenomenological response function for a Fermi liquid system with a characteristic energy of 4.0(1) meV. Although the wave vector dependence of the IC magnetic fluctuations in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ is similar to that in the Fermi liquid state of the parent compound, Sr$_2$RuO$_4$, the magnetic fluctuations are clearly suppressed by the Ti-doping., Comment: 5 pages, 4 figures

Frustrated magnetic interactions in a quasi-two-dimensional [111] slab of pyrochlore lattice were studied. For uniform nearest neighbor (NN) interactions, we show that the complex magnetic problem can be mapped onto a model with two independent degrees of freedom, tri-color and binary sign. This provides a systematic way to construct the complex classical spin ground states with collinear and coplanar bi-pyramid spins. We also identify `partial but extended' zero-energy excitations amongst the ground states. For nonuniform NN interactions, the coplanar ground state can be obtained from the collinear bi-pyramid spin state by collectively rotating two spins of each tetrahedron with an angle, $\alpha$, in an opposite direction. The latter model with $\alpha \sim 30^\circ$ fits the experimental neutron data from SCGO well., Comment: 5 pages, 5 figures

To investigate the transport properties of iron based superconductors, we prepared planar hybrid superconductor-normal metal-superconductor (SNS') thin film junctions with BaFe1.8Co0.2As2 as base electrode. As counter electrode we used a lead indium alloy, while the normal metal layer was formed by thin gold films. Temperature dependent measurements of the electrical conductivity were strongly influenced by the properties of the electrodes. We developed a junction structure that allows us to characterize the electrodes, too, including the behavior of their normal state resistance in order to correct their influences on the conduction spectra. The corrected conductivity of the junction was described within an extended BTK-model and shows a behavior dominated by Andreev reflexion., Comment: 9 pages, 6 figures

We calculate the proton-nucleus total reaction cross sections at different energies of incident protons within the optical limit approximation of the Glauber theory. The isospin effect has been taken into account. The nucleon distribution is obtained in the framework of macroscopic nuclear models in a way depending on the equation of state of uniform nuclear matter near the saturation density. We find that at an energy of order 40 MeV, the reaction cross section calculated for neutron- rich isotopes significantly increases as the parameter L characterizing the density dependence of the symmetry energy increases, while at energies of order 300 and 800 MeV, it is almost independent of L. This is a feature of the optical limit Glauber theory in which an exponential dependence of the reaction cross section on the neutron skin thickness remains when the total proton-neutron cross section is small enough., Comment: 5 pages, 4 figures

By performing time-of-flight neutron scattering measurements on a large amount of single crystals of Sr$_2$RuO$_4$, we studied detailed structure of the imaginary part of the dynamic spin susceptibility over a wide range of phase space. In the normal state at T=5 K, strong incommensurate (IC) peaks were clearly observed at around $\mathbf{Q}_\text{c}=(0.3,0.3)$ up to at least $\hbar\omega=80$ meV. In addition, our data also show strong magnetic fluctuations that exist on the ridges connecting the IC peaks around the $(\pi,\pi)$ point rather than around the $\Gamma$ point. Our results are consistent with the semi-mean-field random phase approximation calculation for a two dimensional Fermi liquid with a characteristic energy of 5.0 meV. Furthermore, the IC fluctuations were observed even at room temperature.