In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams, still to be explored
in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.
The deliberate insertion of magnetic Mn dopants in the Fe sites of the optimally-doped SmFeAsO0.88-F0.12 iron-based superconductor can modify in a controlled way its electronic properties. The resulting phase diagram was investigated across a wide ra
nge of manganese contents (x) by means of muon-spin spectroscopy (muSR), both in zero- and in transverse fields, respectively, to probe the magnetic and the superconducting order. The pure superconducting phase (at x < 0.03) is replaced by a crossover region at intermediate Mn values (0.03 =< x < 0.08), where superconductivity coexists with static magnetic order. After completely suppressing superconductivity for x = 0.08, a further increase in Mn content reinforces the natural tendency towards antiferromagnetic correlations among the magnetic Mn ions. The sharp drop of Tc and the induced magnetic order in the presence of magnetic disorder/dopants, such as Mn, are both consistent with a recent theoretical model of unconventional superconductors [M. Gastiasoro et al., ArXiv 1606.09495], which includes correlation-enhanced RKKY-couplings between the impurity moments.
Muon Spin Rotation ($mu$SR) and $^{19}$F Nuclear Magnetic Resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in La$_{1-y}$Y$_{y}$Fe$_{1-x}$Mn$_x$AsO$_{0.89}$F$_{0.11}$ superconductors. While for $y = 0$ a
very low critical concentration of Mn ($x = 0.2$%) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y= 20$% it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y$=0 and for $y= 20$%. Low-energy spin fluctuations give rise to a peak in $^{19}$F NMR $1/T_1$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by $^{19}$F NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground-state are discussed in the light of the proximity of LaFeAsO$_{0.89}$F$_{0.11}$ to a quantum critical point.
Among the widely studied superconducting iron-pnictide compounds belonging to the Ln1111 family (with Ln a lanthanide), a systematic investigation of the crossover region between the superconducting and the antiferromagnetic phase for the Ln = Nd cas
e has been missing. We fill this gap by focusing on the intermediate doping regime of NdFeAsO(1-x)F(x) by means of dc-magnetometry and muon-spin spectroscopy measurements. The long-range order we detect at low fluorine doping is replaced by short-range magnetic interactions at x = 0.08, where also superconductivity appears. In this case, longitudinal-field muon-spin spectroscopy experiments show clear evidence of slow magnetic fluctuations that disappear at low temperatures. This fluctuating component is ascribed to the glassy-like character of the magnetically ordered phase of NdFeAsO at intermediate fluorine doping.
By a systematic study of the hydrogen-doped LaFeAsO system by means of dc resistivity, dc magnetometry, and muon-spin spectroscopy we addressed the question of universality of the phase diagram of rare-earth-1111 pnictides. In many respects, the beha
viour of LaFeAsO_(1-x)H_(x) resembles that of its widely studied F-doped counterpart, with H^- realizing a similar (or better) electron-doping in the LaO planes. In a x = 0.01 sample we found a long-range SDW order with T_n = 119 K, while at x = 0.05 the SDW establishes only at 38 K and, below T_c = 10 K, it coexists at a nanoscopic scale with bulk superconductivity. Unlike the abrupt M-SC transition found in the parent La-1111 compound, the presence a crossover region makes the H-doped system qualitatively similar to other Sm-, Ce-, or Nd-1111 families.
We study normal state electrical, thermoelectrical and thermal transport in polycrystalline BiS2-based compounds, which become superconducting by F doping on the O site. In particular we explore undoped LaOBiS2 and doped LaO0.5F0.5BiS2 samples, prepa
red either with or without high pressure annealing, in order to evidence the roles of doping and preparation conditions. The high pressure annealed sample exhibits room temperature values of resistivity ro around 5 mohmcm, Seebeck coefficient S around -20 microV/K and thermal conductivity k around 1.5 W/Km, while the Hall resistance RH is negative at all temperatures and its value is -10-8 m3/C at low temperature. The sample prepared at ambient pressure exhibits RH positive in sign and five times larger in magnitude, and S negative in sign and slightly smaller in magnitude. These results reveal a complex multiband evolution brought about by high pressure annealing. In particular, the sign inversion and magnitude suppression of RH, indicating increased electron-type carrier density in the high pressure sample, may be closely related to previous findings about change in lattice parameters and enhancement of superconducting Tc by high pressure annealing. As for the undoped sample, it exhibits the 10 times larger resistivity, 10 times larger |S| and 10 times larger |RH| than its doped counterpart, consistently with its insulating nature. Our results point out the dramatic effect of preparation conditions in affecting charge carrier density as well as structural, band and electronic parameters in these systems.
We report on the magnetic and superconducting properties of LaO0.5F0.5BiS2 by means of zero- (ZF) and transverse-field (TF) muon-spin spectroscopy measurements (uSR). Contrary to previous results on iron-based superconductors, measurements in zero fi
eld demonstrate the absence of magnetically ordered phases. TF-uSR data give access to the superfluid density, which shows a marked 2D character with a dominant s-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.
The evolution of the magnetic order in FeSeTe crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent DFT calculations both indicate that muons are impl
anted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase disorders progressively with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment freezes at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.
The temperature behaviour of the first critical field ($B_{C1}$) of superconducting thin film samples can be determined with high accuracy using an inductive and contactless method. Driving a sinusoidal current in a single coil placed in front of the
sample, a non zero third harmonic voltage $V_{3}$ is induced in it when Abrikosov vortices enter the sample. Conditions to be satisfied for the quantitative evaluation of $B_{C1}$ using this technique are detailed. As validation test, different type II superconductors (Nb, NbN, MgB$_{2}$ and Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-d}$ under the form of thin films) have been measured. The comparison between experimental results, data presented in literature and theoretical predictions is presented and discussed.
We present uSR investigations on SmFeAsO_1-xF_x showing coexistence of magnetic order and superconductivity only in a very narrow F-doping range. The sharp crossover between the two types of order is similar to that observed in LaFeAsO_1-xF_x, sugges
ting a common behavior for the 1111 pnictides. The analysis of the muon asymmetry demonstrates that the coexistence must be nanoscopic, i.e. the two phases must be finely interspersed over a typical length-scale of few nm. In this regime both the magnetic and the superconducting transition temperatures collapse to very low values. Our data suggest a competition between the two order parameters
