Impact of strong disorder on the static magnetic properties of the spin-chain compound BaCu2SiGeO7

The disordered quasi-1D magnet BaCu2SiGeO7 is considered as one of the best physical realizations of the random Heisenberg chain model, which features an irregular distribution of the exchange parameters and whose ground state is predicted to be the scarcely investigated random-singlet state (RSS). Based on extensive 29Si NMR and magnetization studies of BaCu2SiGeO7, combined with numerical Quantum Monte Carlo simulations, we obtain remarkable quantitative agreement with theoretical predictions of the random Heisenberg chain model and strong indications for the formation of a random-singlet state at low temperatures in this compound. As a local probe, NMR is a well-adapted technique for studying the magnetism of disordered systems. In this case it also reveals an additional local transverse staggered field (LTSF), which affects the low-temperature properties of the RSS. The proposed model Hamiltonian satisfactorily accounts for the temperature dependence of the NMR line shapes.Comment: 10 pages, 7 figure

Direct observation of impurity-induced magnetism in an S = 1/2 antiferromagnetic Heisenberg 2-leg spin ladder

Nuclear magnetic resonance and magnetization measurements were used to probe the magnetic features of single-crystalline Bi(Cu(1-x)Znx)2PO6 with 0<x<0.05 at temperatures between 2.6 K and 300 K. The simple lineshape of the 31P NMR signals of the pristine compound changes considerably for x>0 and we present clear evidence for a temperature dependent variation of the local magnetization close to the Zn sites. The generic nature of this observation is indicated by results of model calculations on appropriate spin systems of limited size employing QMC methods.Comment: 4 pages, 4 figure

Structure and superconductivity in the binary Re1−x_{1-x}Mox_x alloys

The binary Re$_{1-x}$Mo$_x$ alloys, known to cover the full range of solid solutions, were successfully synthesized and their crystal structures and physical properties investigated via powder x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. By varying the Re/Mo ratio we explore the full Re$_{1-x}$Mo$_x$ binary phase diagram, in all its four different solid phases: hcp-Mg ($P6_3/mmc$), $\alpha$-Mn ($I\overline{4}3m$), $\beta$-CrFe ($P4_2/mnm$), and bcc-W ($Im\overline{3}m$), of which the second is non-centrosymmetric with the rest being centrosymmetric. All Re$_{1-x}$Mo$_x$ alloys are superconductors, whose critical temperatures exhibit a peculiar phase diagram, characterized by three different superconducting regions. In most alloys the $T_c$ is almost an order of magnitude higher than in pure Re and Mo. Low-temperature electronic specific-heat data evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling across the series. Considering that several $\alpha$-Mn-type Re$T$ alloys ($T$ = transition metal) show time-reversal symmetry breaking (TRSB) in the superconducting state, while TRS is preserved in the isostructural Mg$_{10}$Ir$_{19}$B$_{16}$ or Nb$_{0.5}$Os$_{0.5}$, the Re$_{1-x}$Mo$_x$ alloys represent another suitable system for studying the interplay of space-inversion, gauge, and time-reversal symmetries in future experiments expected to probe TRSB in the Re$T$ family.Comment: 8 pages, 7 figures, accepted for publication on Physical Review Material

High-TcT_\mathrm{c} superconductivity in undoped ThFeAsN

Unlike the widely studied ReFeAsO series, the newly discovered iron-based superconductor ThFeAsN exhibits a remarkably high critical temperature of 30 K, without chemical doping or external pressure. Here we investigate in detail its magnetic and superconducting properties via muon-spin rotation/relaxation ($\mu$SR) and nuclear magnetic resonance (NMR) techniques and show that ThFeAsN exhibits strong magnetic fluctuations, suppressed below 35 K, but no magnetic order. This contrasts strongly with the ReFeAsO series, where stoichiometric parent materials order antiferromagnetically and superconductivity appears only upon doping. The ThFeAsN case indicates that Fermi-surface modifications due to structural distortions and correlation effects are as important as doping in inducing superconductivity. The direct competition between antiferromagnetism and superconductivity, which in ThFeAsN (as in LiFeAs) occurs at already zero doping, may indicate a significant deviation of the $s$-wave superconducting gap in this compound from the standard $s^{\pm}$ scenario.Comment: 6 pages, 5 figure

Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz)2_2(ClO4_4)2_2

Cu(pz)$_2$(ClO$_4$)$_2$ (with pz denoting pyrazine C$_4$H$_4$N$_2$) is a two-dimensional spin-1/2 square-lattice antiferromagnet with $T_{\mathrm{N}}$ = 4.24 K. Due to a persisting focus on the low-temperature magnetic properties, its room-temperature structural and physical properties caught no attention up to now. Here we report a study of the structural features of Cu(pz)$_2$(ClO$_4$)$_2$ in the paramagnetic phase, up to 330 K. By employing magnetization, specific heat, $^{35}$Cl nuclear magnetic resonance, and neutron diffraction measurements, we provide evidence of a second-order phase transition at $T^{\star}$ = 294 K, not reported before. The absence of a magnetic ordering across $T^{\star}$ in the magnetization data, yet the presence of a sizable anomaly in the specific heat, suggest a structural order-to-disorder type transition. NMR and neutron-diffraction data corroborate our conjecture, by revealing subtle angular distortions of the pyrazine rings and of ClO$^-_4$ counteranion tetrahedra, shown to adopt a configuration of higher symmetry above the transition temperature.Comment: 10 pages, 12 figure

Nodal-to-nodeless superconducting order parameter in LaFeAs1−x_{1-x}Px_xO synthesized under high pressure

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.Comment: 9 pages, 7 figures, Suppl. materia