A

Giacomo Prando

Phase Diagrams ofREFeAsO-xFx Materials

Macroscopic and NanoscopicExperimental Investigation

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I edizione: maggio

A Lara e alla mia famiglia

«E cos’è questa?» domandò vivacemente Guglielmotoccando una pietra che giaceva sullo scaffale.«Questa? Mi è stata donata tempo fa. Credo sialopris amatiti o lapis ematitis. Pare abbia varie virtùterapeutiche, ma non ho ancora scoperto quali. Laconosci?»«Sì,» disse Guglielmo, «ma non come medicina.»Trasse dal saio un coltellino e lo appressò lentamentealla pietra. Come il coltellino, mosso dalla sua manocon estrema delicatezza, giunse a poca distanza dallapietra, vidi che la lama compiva un movimentobrusco, come se Guglielmo avesse mosso il polso,che invece aveva fermissimo. E la lama aderì allapietra con un lieve rumore di metallo.«Vedi,» mi disse Guglielmo, «è un magnete.»«E a che serve?» chiesi.«A varie cose, di cui ti dirò (. . . )»

U. Eco, Il nome della rosa

La scienza, come la poesia, si sa che sta ad un passodalla follia (. . . )

L. Sciascia, La scomparsa di Majorana

Sapientia, cum probitate morum conjuncta, humanae mentis perfectioMotto of Collegio Ghislieri, Pavia

Contents

List of Figures 13

List of Tables 23

Preface by Prof. Pietro Carretta 25

Preface by Prof. Philippe Mendels 27

Preface by the Author 29

Introduction to Fe-based pnictide and chalcogenide materials 310.1 Pnictide and chalcogenide materials. Overview of the main features . . . . . . 33

0.1.1 Structural properties of RETmPnO compounds . . . . . . . . . . . . . 330.1.2 Magnetic RETmPnO compounds and f − d interaction . . . . . . . . 360.1.3 High-Tc superconductivity in REFeAsO1−xFx compounds and in other

pnictide and chalcogenide materials . . . . . . . . . . . . . . . . . . . 380.1.4 Problems associated with the synthesis of REFeAsO1−xFx . . . . . . 46

0.2 Outline of the dissertation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

I Properties of superconductivity in REFeAsO1−xFx deduced bymeasurements of macroscopic susceptibilities 53

I Macroscopic dc susceptibility of REFeAsO1−xFx compounds 55I.1 Contributions to dc magnetization in 1111 materials . . . . . . . . . . . . . . 56

I.1.1 Static magnetization in non-superconducting CeFeAsO1−xFx . . . . . 56I.1.2 Static magnetization in superconducting CeFeAsO1−xFx . . . . . . . 59I.1.3 Coexistence region for spin density wave and superconductivity . . . . 65

I.2 Superconducting phase fluctuations for T � Tc . . . . . . . . . . . . . . . . . 68I.2.1 The model of phase superconducting fluctuations . . . . . . . . . . . . 69I.2.2 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

I.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

9

10

II Phase diagram of vortices in REFeAsO1−xFx superconductors 77II.1 Phase diagram of flux lines and depinning energy barriers . . . . . . . . . . . 79

II.1.1 Preliminary magnetic characterization of the samples . . . . . . . . . 79II.1.2 Temperature dependence of the upper critical field Hc2 . . . . . . . . . 80II.1.3 Irreversibility line Hirr(T ) . . . . . . . . . . . . . . . . . . . . . . . . 83II.1.4 Depinning energy barriers 〈U0(H)〉pwd . . . . . . . . . . . . . . . . . 90

II.2 Analysis of the experimental results . . . . . . . . . . . . . . . . . . . . . . . 92II.2.1 Anisotropic Ginzburg-Landau model of collective pinning . . . . . . . 92II.2.2 Linking data relative to different H-regimes . . . . . . . . . . . . . . 95II.2.3 Results of the analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 98

II.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

II Local features of magnetism in REFeAsO compounds scanned bychemical substitutions and external hydrostatic pressure 101

III Phase diagram of CeFeAsO1−xFx by means of µ+SR measurements 103III.1 µ+SR investigation of CeFeAsO1−xFx . . . . . . . . . . . . . . . . . . . . . 104

III.1.1 Undoped sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109III.1.2 Lightly doped x = 0.03 and x = 0.04 samples . . . . . . . . . . . . . 112III.1.3 Coexistence of spin density wave and superconductivity for x = 0.06 . 115III.1.4 Superconductivity for x = 0.055. Comparison with SmFeAsO0.8F0.2 . 121III.1.5 Phase diagram of CeFeAsO1−xFx . . . . . . . . . . . . . . . . . . . 124

III.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

IV Magnetism of rare earth ions in CeFeAsO1−xFx studied by means of 19F-NMR 127IV.1 19F-NMR across the electronic phase diagram . . . . . . . . . . . . . . . . . 128

IV.1.1 Line-shape and paramagnetic shift. Experimental results . . . . . . . . 129IV.1.2 Theory and simulation of the 19F-NMR absorption line . . . . . . . . 133IV.1.3 Spin-lattice relaxation rate . . . . . . . . . . . . . . . . . . . . . . . 137IV.1.4 Phase diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

IV.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

V Effects of hydrostatic pressure on REFeAsO1−xFx compounds 143V.1 µ+SR measurements upon applied hydrostatic pressure . . . . . . . . . . . . 143V.2 Crossover between magnetism and superconductivity in

REFeAsO1−xFx (RE = La, Ce and Sm) . . . . . . . . . . . . . . . . . . . . . 147V.2.1 Experimental results on the La-based sample . . . . . . . . . . . . . . 149V.2.2 Experimental results on the RE-based samples (RE = Sm, Ce) . . . . . 150V.2.3 Effects of pressure in superconducting CeFeAsO0.94F0.06 . . . . . . . 152

V.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

Contents10

10

II Phase diagram of vortices in REFeAsO1−xFx superconductors 77II.1 Phase diagram of flux lines and depinning energy barriers . . . . . . . . . . . 79

II.1.1 Preliminary magnetic characterization of the samples . . . . . . . . . 79II.1.2 Temperature dependence of the upper critical field Hc2 . . . . . . . . . 80II.1.3 Irreversibility line Hirr(T ) . . . . . . . . . . . . . . . . . . . . . . . . 83II.1.4 Depinning energy barriers 〈U0(H)〉pwd . . . . . . . . . . . . . . . . . 90

II.2 Analysis of the experimental results . . . . . . . . . . . . . . . . . . . . . . . 92II.2.1 Anisotropic Ginzburg-Landau model of collective pinning . . . . . . . 92II.2.2 Linking data relative to different H-regimes . . . . . . . . . . . . . . 95II.2.3 Results of the analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 98

II.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

II Local features of magnetism in REFeAsO compounds scanned bychemical substitutions and external hydrostatic pressure 101

III Phase diagram of CeFeAsO1−xFx by means of µ+SR measurements 103III.1 µ+SR investigation of CeFeAsO1−xFx . . . . . . . . . . . . . . . . . . . . . 104

III.1.1 Undoped sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109III.1.2 Lightly doped x = 0.03 and x = 0.04 samples . . . . . . . . . . . . . 112III.1.3 Coexistence of spin density wave and superconductivity for x = 0.06 . 115III.1.4 Superconductivity for x = 0.055. Comparison with SmFeAsO0.8F0.2 . 121III.1.5 Phase diagram of CeFeAsO1−xFx . . . . . . . . . . . . . . . . . . . 124

III.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

IV Magnetism of rare earth ions in CeFeAsO1−xFx studied by means of 19F-NMR 127IV.1 19F-NMR across the electronic phase diagram . . . . . . . . . . . . . . . . . 128

IV.1.1 Line-shape and paramagnetic shift. Experimental results . . . . . . . . 129IV.1.2 Theory and simulation of the 19F-NMR absorption line . . . . . . . . 133IV.1.3 Spin-lattice relaxation rate . . . . . . . . . . . . . . . . . . . . . . . 137IV.1.4 Phase diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

IV.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

V Effects of hydrostatic pressure on REFeAsO1−xFx compounds 143V.1 µ+SR measurements upon applied hydrostatic pressure . . . . . . . . . . . . 143V.2 Crossover between magnetism and superconductivity in

REFeAsO1−xFx (RE = La, Ce and Sm) . . . . . . . . . . . . . . . . . . . . . 147V.2.1 Experimental results on the La-based sample . . . . . . . . . . . . . . 149V.2.2 Experimental results on the RE-based samples (RE = Sm, Ce) . . . . . 150V.2.3 Effects of pressure in superconducting CeFeAsO0.94F0.06 . . . . . . . 152

V.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

11

III Conclusions and open perspectives 157

IV Appendices 165

A Basics of nuclear magnetic resonance 167A.1 Larmor precession and magnetic resonance . . . . . . . . . . . . . . . . . . . 167A.2 Static local magnetic fields and paramagnetic shift . . . . . . . . . . . . . . . 169A.3 Dynamical properties and relaxation times . . . . . . . . . . . . . . . . . . . 172

A.3.1 Bloch’s equations. T1 and T2 relaxation times . . . . . . . . . . . . . 172A.3.2 Statistical description of the spin-lattice relaxation rate . . . . . . . . . 172A.3.3 Microscopic interpretation. Relaxation mechanisms . . . . . . . . . . 174

B NMR quantification of real F− content 177

C Basics of µ+ spin spectroscopy 179C.1 Positive muons µ+. Production, implantation and decay . . . . . . . . . . . . 179C.2 Zero magnetic field measurements . . . . . . . . . . . . . . . . . . . . . . . 185

C.2.1 Static distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185C.2.2 Dynamical processes . . . . . . . . . . . . . . . . . . . . . . . . . . 186

C.3 Longitudinal magnetic field measurements . . . . . . . . . . . . . . . . . . . 187

D Physical Constants 189

Bibliography 191

Acknoledgements 207

Contents 11

List of Figures

1 Sketchy crystalline structures of typical Fe-based pnictide and chalcogenidecompounds (reprinted from [Lum10] with permissions both of Authors and ofIOP Publishing. Copyright IOP Publishing. All rights reserved). From left toright: 1111, 122, 111 and 11 families. As–Fe–As and Se–Fe–Se tri-layers aredrawn with green and blue balls. . . . . . . . . . . . . . . . . . . . . . . . . 32

2 Upper image: crystallographic structure of RETmPnO compounds at room Taround the tetragonal primitive cell represented by the black box (space groupP4/nmm). Tm and Pn ions are indicated with ochre and green spheres, re-spectively. The red dots and the big violet spheres represent the O2− ions andthe RE3+ ions, respectively. Lower image: view along the c-axis of the sameimage plotted in the upper image. Both the images have been drawn by usingthe software VESTA, see [Mom08]. . . . . . . . . . . . . . . . . . . . . . . . 34

3 Phase diagrams for the SmFeAsO1−xFx family reporting quite contradictorybehaviours for the x-dependence of the structural transition temperature. Theequivalent labels Ts and TT-O are used to refer to such critical temperature inthe left graph (reprinted from [Mar09] with permissions both of Authors andof the American Physical Society, Copyright 2009 from the American PhysicalSociety) and right graph (reprinted from [Mar11] with permissions both of Au-thors and of the American Physical Society, Copyright 2011 from the AmericanPhysical Society), respectively. . . . . . . . . . . . . . . . . . . . . . . . . . 35

4 Left graph (reprinted from [Luo10] with permissions both of Authors and ofthe American Physical Society, Copyright 2010 from the American PhysicalSociety): phase diagram of the magnetic ground states for both Fe and Cesublattices in CeFeAs1−xPxO. Left graph, inset: comparison of the differentscales for the overall exchange constant among RE ions and the Kondo tem-perature represented as blue and red lines, respectively. Right graph (reprintedfrom [Jia09] with permissions both of Authors and of IOP Publishing. Copy-right IOP Publishing. All rights reserved): phase diagram for the ground stateof BaFe2(As1−xPx)2 where the progressive As1−xPx substitution suppressesthe magnetic phase and drives the system towards a superconducting state forx � 0.2. Right graph, inset: power-law exponent for the T -dependence ofresistivity ρ in the normal state. . . . . . . . . . . . . . . . . . . . . . . . . . 37

13

14

5 Left graph (reprinted from [Arm10] with permissions both of Authors and ofthe American Physical Society, Copyright 2010 from the American PhysicalSociety): sketchy phase diagram of the electron-doped cuprate superconductorsRE2−xCexCuO4. Right graph (courtesy of Dr. S. Sanna, see also [San09a]):phase diagram of SmFeAsO1−xFx obtained from µ+SR and magnetizationmeasurements displaying the same phenomenology presented in the case ofRE2−xCexCuO4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6 Left graph (reprinted from [Les09] with permissions both of Authors and ofthe American Physical Society, Copyright 2009 from the American PhysicalSociety): phase diagram of Ba(Fe1−xCox)2As2 obtained from heat-capacity,resistivity and neutron diffraction measurements. Left graph, inset: suppressionof the Fe ordered moment with increasing the Co concentration. Right graph(reprinted from [Kat10] with permissions both of Authors and of the Journalof the Physical Society of Japan): phase diagram of Fe1+ySexTe1−x obtainedfrom magnetization and neutron scattering measurements. The yellow SG areaindicated the emergence of a spin-glass-like phase. . . . . . . . . . . . . . . . 40

7 Left graph: phase diagram of LaFeAsO1−xFx obtained from µ+SR measure-ments and displaying a sharp first-order-like crossover between the magneticand the superconducting phases (courtesy of Dr. H. Luetkens. See also [Lue09]).Right graph: phase diagram obtained from neutron scattering measurements forCeFeAsO1−xFx, claiming that a quantum critical point is present at x valuesclose to 0.06 (courtesy of Dr. J. Zhao and Prof. P. Dai. See also [Zha08]). . . . 41

8 Left graph (reprinted from [Lee10] with permissions both of Authors and ofthe American Physical Society, Copyright 2010 from the American PhysicalSociety): phase diagram for the flux lines in a SmFeAsO0.85 single crystal ob-tained by means of magnetoresistivity measurements. Right graph (reprintedfrom [Pan10] with permissions both of Authors and of the American Physi-cal Society, Copyright 2010 from the American Physical Society): compari-son among the irreversibility lines obtained in different cuprate and pnictidesuperconductors, showing a wide region for the glassy phase of vortices inSmFeAsO1−xFx. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

9 Left graph (reprinted from [Dag09] with permissions both of Authors and ofthe American Physical Society, Copyright 2009 from the American PhysicalSociety): T -dependence of the superconducting gaps from Andreev-reflectionspectroscopy in SmFeAsO1−xFx. Right graph (reprinted from [Umm09] withpermissions both of Authors and of the American Physical Society, Copyright2009 from the American Physical Society): reduced Brillouin zone reportinghole (1, 2) and electron bands (3) employed in the modeling of s±-like super-conductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

I.1 Inverse magnetic susceptibility of CeFeAsO0.96F0.04 at H = 100 Oe as a func-tion of T (volume units). The continuous red line is a best-fit according to equa-tion (I.1). Upper inset: M vs. H at 2.8 K for CeFeAsO0.96F0.04. The red curveis a best-fit according to a linear function. Lower inset: magnetic susceptibilityof CeFeAsO at different applied H in the region of low T . . . . . . . . . . . . 57

14 List of Figures14

14

5 Left graph (reprinted from [Arm10] with permissions both of Authors and ofthe American Physical Society, Copyright 2010 from the American PhysicalSociety): sketchy phase diagram of the electron-doped cuprate superconductorsRE2−xCexCuO4. Right graph (courtesy of Dr. S. Sanna, see also [San09a]):phase diagram of SmFeAsO1−xFx obtained from µ+SR and magnetizationmeasurements displaying the same phenomenology presented in the case ofRE2−xCexCuO4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6 Left graph (reprinted from [Les09] with permissions both of Authors and ofthe American Physical Society, Copyright 2009 from the American PhysicalSociety): phase diagram of Ba(Fe1−xCox)2As2 obtained from heat-capacity,resistivity and neutron diffraction measurements. Left graph, inset: suppressionof the Fe ordered moment with increasing the Co concentration. Right graph(reprinted from [Kat10] with permissions both of Authors and of the Journalof the Physical Society of Japan): phase diagram of Fe1+ySexTe1−x obtainedfrom magnetization and neutron scattering measurements. The yellow SG areaindicated the emergence of a spin-glass-like phase. . . . . . . . . . . . . . . . 40

7 Left graph: phase diagram of LaFeAsO1−xFx obtained from µ+SR measure-ments and displaying a sharp first-order-like crossover between the magneticand the superconducting phases (courtesy of Dr. H. Luetkens. See also [Lue09]).Right graph: phase diagram obtained from neutron scattering measurements forCeFeAsO1−xFx, claiming that a quantum critical point is present at x valuesclose to 0.06 (courtesy of Dr. J. Zhao and Prof. P. Dai. See also [Zha08]). . . . 41

8 Left graph (reprinted from [Lee10] with permissions both of Authors and ofthe American Physical Society, Copyright 2010 from the American PhysicalSociety): phase diagram for the flux lines in a SmFeAsO0.85 single crystal ob-tained by means of magnetoresistivity measurements. Right graph (reprintedfrom [Pan10] with permissions both of Authors and of the American Physi-cal Society, Copyright 2010 from the American Physical Society): compari-son among the irreversibility lines obtained in different cuprate and pnictidesuperconductors, showing a wide region for the glassy phase of vortices inSmFeAsO1−xFx. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

9 Left graph (reprinted from [Dag09] with permissions both of Authors and ofthe American Physical Society, Copyright 2009 from the American PhysicalSociety): T -dependence of the superconducting gaps from Andreev-reflectionspectroscopy in SmFeAsO1−xFx. Right graph (reprinted from [Umm09] withpermissions both of Authors and of the American Physical Society, Copyright2009 from the American Physical Society): reduced Brillouin zone reportinghole (1, 2) and electron bands (3) employed in the modeling of s±-like super-conductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

I.1 Inverse magnetic susceptibility of CeFeAsO0.96F0.04 at H = 100 Oe as a func-tion of T (volume units). The continuous red line is a best-fit according to equa-tion (I.1). Upper inset: M vs. H at 2.8 K for CeFeAsO0.96F0.04. The red curveis a best-fit according to a linear function. Lower inset: magnetic susceptibilityof CeFeAsO at different applied H in the region of low T . . . . . . . . . . . . 57

15

I.2 FC magnetization of CeFeAsO0.945F0.055 as a function of T (volume units) atdifferent H in the high-H regime (the low-H regime is plotted in the inset).Continuous lines are best-fits according to equation (I.5). The green curvesindicate that χsc is fixed to 0. . . . . . . . . . . . . . . . . . . . . . . . . . . 59

I.3 Upper graph: M vs. H curve at 40 K for CeFeAsO0.945F0.055 (volume units).The red curve is a best-fit according to a linear function. Upper graph, inset:enlargement at low H values evidencing a component associated with magneticimpurities and saturating at µ0H � 0.5 T. Lower graph: values of M0 vs. Hfor CeFeAsO0.945F0.055 after the fitting procedure to M vs. T data accordingto equation (I.5). Data are plotted after the subtraction of the T -independentsaturated contribution associated with magnetic impurities. The red curve isa best-fit according to a linear function. Lower graph, inset: H-dependenceof Neff per Ce3+ ion resulting from the fitting procedure according to equa-tion (I.5). The weak H-dependence should be considered as an artifact fromthe fitting procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

I.4 Upper graph: M vs. T at different H for optimally-doped SmFeAsO0.8F0.2

(FC). The red continuous curves are best-fits according to equation (I.5). Thelow-T anomalies are related to the AFM ordering of Sm3+ magnetic moments.Lower graph: values of M0 = χ0H vs. H for SmFeAsO0.8F0.2 after the fittingprocedure to M vs. T data according to equation (I.5). Data are plotted after thesubtraction of a T -independent saturated contribution associated with magneticimpurities. The red curve is a best-fit according to a linear function. Lowergraph, upper inset: M vs. H curve at 56 K (volume units). The red curve is abest-fit according to a linear function. Lower graph, lower inset: H-dependenceof Neff resulting from the fitting procedure according to equation (I.5). . . . . 62

I.5 M/H vs. T at H = 5 Oe for SmFeAsO0.8F0.2 (volume units). The presence ofa small volume percentage of magnetic impurities is inferred from the separa-tion of ZFC and FC curves already far above Tc(0). Inset: linear extrapolationprocedure to deduce the value of Tc(0) � 52.3 K from the FC curve (Copyright2011 from the American Physical Society). . . . . . . . . . . . . . . . . . . . 64

I.6 M vs. T curves for five samples of CeFeAsO1−xFx across the spin densitywave - superconductivity coexistence region of the phase diagram. The mea-surements were performed at H = 5 Oe (FC) for all the measurements. Thedata are shifted along the vertical axis for the sake of clarity, with the onlyexception of CeFeAsO0.945F0.055. . . . . . . . . . . . . . . . . . . . . . . . 66

I.7 Neff vs. H estimated by a fitting procedure according to equation (I.5). See alsothe relative raw experimental curves at H = 5 Oe displayed in figure I.6. . . . 67

I.8 M vs. H curves for SmFeAsO0.8F0.2 at 52.75 K and 56 K before the subtrac-tion procedures (enlargement in the low-H regime). Inset: isothermal diamag-netic contributions Mdia vs. H at 52.75 K (see text). . . . . . . . . . . . . . . 72

I.9 Mdia vs. H for SmFeAsO0.8F0.2 at representative T values above Tc. Thecurves are obtained after the subtraction procedures described in the text. Con-tinuous lines are best-fits obtained by means of numerical integration of equa-tion (I.9). Inset: T -dependence of Hup. The dashed line is a guide for the eye(Copyright 2011 from the American Physical Society). . . . . . . . . . . . . . 73

15List of Figures 15

16

I.10 Reduced magnetization mc vs. T for SmFeAsO0.8F0.2 in the low-H regime.No crossing of curves at Tc is observed (Copyright 2011 from the AmericanPhysical Society). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

I.11 mc vs. T for SmFeAsO0.8F0.2 in the high-H regime. The crossing of curves atT � 53.2 K suggests that in this H-range the GL framework is valid (Copyright2011 from the American Physical Society). . . . . . . . . . . . . . . . . . . . 75

II.1 Left graph: simplified sketchy phase diagram for the superconducting phase intype-II superconductors (reprinted from [Bla94] with permissions both of Au-thor and of the American Physical Society, Copyright 1994 from the AmericanPhysical Society). Right graph: refinement of the phase diagram presented inthe left graph with the addition of the irreversibility line, here denoted as Bm(T )(reprinted from [Bla94] with permissions both of Author and of the AmericanPhysical Society, Copyright 1994 from the American Physical Society). . . . . 78

II.2 Magnetic susceptibility curves (volume units) at H = 5 Oe (FC) as a functionof T for the three samples. Data are plotted after subtracting a linear termroughly accounting for the contribution from impurities (Copyright 2012 fromthe American Physical Society). . . . . . . . . . . . . . . . . . . . . . . . . . 80

II.3 Hc2 vs. T/Tc(0) curves for the three samples as obtained from M vs. T data.The continuous lines are best-fits to data in the high-H regime according tolinear functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

II.4 Upper graph: raw data of the imaginary and the real components of χac in vol-ume units relative to CeFeAsO0.945F0.055 (upper and lower subgraphs, respec-tively). Measurements were performed at Hac = 1.5 Oe and νm = 478 Hz andat different applied H (Copyright 2012 from the American Physical Society).Lower graph: raw data for the real component of χac vs. T in SmFeAsO0.8F0.2

(volume units). Measurements have been performed at H = 250 Oe andνm = 1488 Hz and at different amplitudes of the alternating magnetic fieldsHac = 0.0675 − 1.5 Oe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

II.5 FLs phase diagram in SmFeAsO0.8F0.2. Both the criteria for the determinationof Hirr(T ) are presented (onset in χ′

ac: × signs, maximum in χ′′ac at νm = 37

Hz: ��� signs. Hac = 1.5 Oe for all the measurements). The dashed lines arebest fits to data according to equation (II.16). The different × signs at fixedH are relative to estimates at different νm. Hc2(T ) is denoted by � signs (seefigure II.3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

II.6 Upper graph, upper subgraph: enlargement of raw data presented in the up-per graph of figure II.4 for CeFeAsO0.945F0.055 displaying χ′′

ac vs. T curvesaround Tp. Upper graph, lower subgraph: dχ′

ac/dT vs. T curves, whose max-ima clearly display a precise correlation with Tp (Copyright 2012 from theAmerican Physical Society). Lower graph: FLs phase diagram relative to thethree studied samples (Copyright 2012 from the American Physical Society).Open symbols track Hirr(T ) as deduced from the χ′′

ac-criterion at νm = 37 Hz(dashed-dotted lines are guides for the eye). Full symbols track Hc2(T ) asdeduced from M vs. T curves (continuous lines are the best-fits reported infigure II.3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

II.7 Dependence of 1/Tp on νm in CeFeAsO0.945F0.055 (Hac = 1.5 Oe and µ0H =1.5 T). The dashed line is a best fit to data according to equation (II.18). . . . . 90

16 List of Figures16

16

I.10 Reduced magnetization mc vs. T for SmFeAsO0.8F0.2 in the low-H regime.No crossing of curves at Tc is observed (Copyright 2011 from the AmericanPhysical Society). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

I.11 mc vs. T for SmFeAsO0.8F0.2 in the high-H regime. The crossing of curves atT � 53.2 K suggests that in this H-range the GL framework is valid (Copyright2011 from the American Physical Society). . . . . . . . . . . . . . . . . . . . 75

II.1 Left graph: simplified sketchy phase diagram for the superconducting phase intype-II superconductors (reprinted from [Bla94] with permissions both of Au-thor and of the American Physical Society, Copyright 1994 from the AmericanPhysical Society). Right graph: refinement of the phase diagram presented inthe left graph with the addition of the irreversibility line, here denoted as Bm(T )(reprinted from [Bla94] with permissions both of Author and of the AmericanPhysical Society, Copyright 1994 from the American Physical Society). . . . . 78

II.2 Magnetic susceptibility curves (volume units) at H = 5 Oe (FC) as a functionof T for the three samples. Data are plotted after subtracting a linear termroughly accounting for the contribution from impurities (Copyright 2012 fromthe American Physical Society). . . . . . . . . . . . . . . . . . . . . . . . . . 80

II.3 Hc2 vs. T/Tc(0) curves for the three samples as obtained from M vs. T data.The continuous lines are best-fits to data in the high-H regime according tolinear functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

II.4 Upper graph: raw data of the imaginary and the real components of χac in vol-ume units relative to CeFeAsO0.945F0.055 (upper and lower subgraphs, respec-tively). Measurements were performed at Hac = 1.5 Oe and νm = 478 Hz andat different applied H (Copyright 2012 from the American Physical Society).Lower graph: raw data for the real component of χac vs. T in SmFeAsO0.8F0.2

(volume units). Measurements have been performed at H = 250 Oe andνm = 1488 Hz and at different amplitudes of the alternating magnetic fieldsHac = 0.0675 − 1.5 Oe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

II.5 FLs phase diagram in SmFeAsO0.8F0.2. Both the criteria for the determinationof Hirr(T ) are presented (onset in χ′

ac: × signs, maximum in χ′′ac at νm = 37

Hz: ��� signs. Hac = 1.5 Oe for all the measurements). The dashed lines arebest fits to data according to equation (II.16). The different × signs at fixedH are relative to estimates at different νm. Hc2(T ) is denoted by � signs (seefigure II.3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

II.6 Upper graph, upper subgraph: enlargement of raw data presented in the up-per graph of figure II.4 for CeFeAsO0.945F0.055 displaying χ′′

ac vs. T curvesaround Tp. Upper graph, lower subgraph: dχ′

ac/dT vs. T curves, whose max-ima clearly display a precise correlation with Tp (Copyright 2012 from theAmerican Physical Society). Lower graph: FLs phase diagram relative to thethree studied samples (Copyright 2012 from the American Physical Society).Open symbols track Hirr(T ) as deduced from the χ′′

ac-criterion at νm = 37 Hz(dashed-dotted lines are guides for the eye). Full symbols track Hc2(T ) asdeduced from M vs. T curves (continuous lines are the best-fits reported infigure II.3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

II.7 Dependence of 1/Tp on νm in CeFeAsO0.945F0.055 (Hac = 1.5 Oe and µ0H =1.5 T). The dashed line is a best fit to data according to equation (II.18). . . . . 90

17

II.8 H-dependence of 〈U0(H)〉pwd in the three investigated samples. Continuouslines are best-fits to data according to a 1/H dependence. . . . . . . . . . . . 91

II.9 3D plot displaying the dependence of 〈U0(H, T )〉pwd (blue full circles) on bothT and H relatively to LaFeAsO0.9F0.1. The projections on the H − T planetrack Hirr(T ) while the projections on the U0 − H plane correspond to dataplotted in figure II.8. With the additional points (+ signs) relative to the T -dependence of Hc2, the H − T plane represents the phase diagram of FLsshown in the lower graph of figure II.6. . . . . . . . . . . . . . . . . . . . . . 92

II.10 Hirr(T ) relative to the three samples normalized with respect to the relativevalue of 〈Hc2(0)〉pwd. Continuous lines are best-fits according to equation (II.34)(Copyright 2012 from the American Physical Society). . . . . . . . . . . . . . 95

II.11 Representation of the quantity between curly brackets in equation (II.43) inthe low-H limit. The dashed lines are guides for the eye and they are used toextrapolate the intercept values back to H = 0 Oe (Copyright 2012 from theAmerican Physical Society). . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

II.12 Collapse of 1/λ2ab(0) vs. H data after a proper normalization according to the

value of 1/λ2ab(0) at H = 250 Oe. Inset: H-dependence of the λab(0) values.

Open (full) symbols are relative to data from the low-H (high-H) regime. . . . 99

III.1 Electrostatic potential of REFeAsO materials (reprinted from [Mae09] with per-missions both of Authors and of the American Physical Society, Copyright 2009from the American Physical Society). The µ+ crystallographic site “A”, equiv-alent to “1”, is positioned right on the FeAs layers. . . . . . . . . . . . . . . . 107

III.2 Upper graph: ZF-µ+SR spectra for CeFeAsO. Continuous lines are best-fits ac-cording to equation (III.5) and to table III.1. Lower graph: short-t enlargementof the ZF-µ+SR spectra shown in the upper graph. . . . . . . . . . . . . . . . 110

III.3 Upper graph: T -dependence of Vm for CeFeAsO. The continuous black lineis a best-fit function according to equation (III.3). The red filled circles repre-sent λL(T ). Lower graph: T -dependence of Bµ1(T ) in CeFeAsO measured bya cosine-like fitting to the transversal component of the spectra shown in fig-ure III.2. The continuous line is a best-fit according to equation (III.10). Thered full triangles represent λTr

1 (T ). . . . . . . . . . . . . . . . . . . . . . . . 111III.4 Upper graph: ZF-µ+SR spectra for CeFeAsO0.97F0.03. Continuous lines are

best-fits according to equation (III.5) and to table III.1. Lower graph: Bµ1

vs. T in CeFeAsO0.97F0.03 measured by a Bessel-like fitting to the transversalcomponent of the spectra shown in the upper graph (red full triangles). Thecontinuous red line is a best-fit according to equation (III.10). Lower graph,inset: Vm(T ) for CeFeAsO0.97F0.03. The continuous line is a best-fit functionaccording to equation (III.3). . . . . . . . . . . . . . . . . . . . . . . . . . . 113

III.5 Upper graph: ZF-µ+SR spectra for CeFeAsO0.96F0.04. Continuous lines arebest-fits according to equation (III.5) and to table III.1. Lower graph: λTr

1 (T )for CeFeAsO0.96F0.04. Lower graph, inset: Vm(T ) for CeFeAsO0.96F0.04. Thecontinuous line is a best-fit function according to equation (III.3). . . . . . . . 114

III.6 Upper graph: ZF-µ+SR spectra for CeFeAsO0.94F0.06. Continuous lines arebest-fits according to equation (III.5) and to table III.1. Lower graph: λTr

1 (T )in the x = 0.04 and x = 0.06 samples. Lower graph, inset: Vm(T ) forCeFeAsO0.94F0.06. The line is a best-fit function according to equation (III.3). 117

17List of Figures 17

18

III.7 Sketchy representation of the model of phase segregation of superconductivityand magnetism (reprinted from [Par09] with permissions both of Authors andof the American Physical Society, Copyright 2009 from the American Phys-ical Society). The nanoscopic coexistence is realized whether the order-of-magnitude of the mean distance among magnetic domains (green droplets) isclose to d ∼ 1 nm, namely the spatial range of the magnetic dipolar field gener-ated by uncompensated Fe moments on the domain walls (red arrows). Underthese circumstances, both µ1- and µ2-like implanted muons feel a static mag-netic local field giving rise to the phenomenology observed in figure III.6. . . . 118

III.8 Upper graph: TF-µ+SR spectra for CeFeAsO0.94F0.06 (H = 200 Oe). Up-per graph, upper subgraph: data in the paramagnetic regime T > Tc. Uppergraph, lower subgraph: superconducting mixed regime T < Tc. Continuousred lines are best-fits according to equation (III.13). Lower graph: fitting re-sults according to equation (III.13). Lower graph, upper subgraph: λ(T ) for thenon-magnetic oscillating term. Lower graph, lower subgraph: Bµ(T ) at the µ+

site. The red dashed line is a guide to the eye according to a double-exponentmean-field-like function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

III.9 ZF-µ+SR spectra for CeFeAsO0.945F0.055. Continuous lines are best-fits ac-cording to equation (III.5) and to table III.1. . . . . . . . . . . . . . . . . . . 121

III.10TF-µ+SR spectra for CeFeAsO0.945F0.055 (H = 150 Oe). Upper graph: para-magnetic regime at 40 K. Lower graph: mixed regime at 5 K. Continuous redlines are best-fits according to equation (III.12). . . . . . . . . . . . . . . . . . 122

III.11Upper graph: fitting results for TF-µ+SR spectra of CeFeAsO0.945F0.055 re-ported in figure III.10 according to equation (III.15). Upper graph, upper sub-graph: σsc vs. T . Upper graph, lower subgraph: Bµ vs. T . Lower graph:fitting results for TF-µ+SR spectra of SmFeAsO0.8F0.2 (not shown) accordingto equation (III.15). Lower graph, upper subgraph: σsc vs. T . Lower graph,lower subgraph: Bµ vs. T . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

III.12Phase diagram of CeFeAsO1−xFx family resulting from the µ+SR measure-ments reported in this chapter (Copyright 2011 from the American PhysicalSociety). The labels “M” and “SC” stands for magnetic spin density wave andsuperconductivity, respectively, while the meaning of “LR” and “SR” is long-and short-range order for the magnetic phase, respectively. The x = 0.07 sam-ple is not discussed in the thesis since its behaviour is extremely similar to thex = 0.06 one. The point deep into the superconducting region is extracted fromthe phase diagram reported in [Zha08]. . . . . . . . . . . . . . . . . . . . . . 124

IV.1 Main graph: inverse magnetic susceptibility of CeFeAsO0.965F0.035 as a func-tion of T at H = 20 Oe (volume units). The continuous line is a best-fit ac-cording to equation (IV.1). Inset: magnetic susceptibility of CeFeAsO0.95F0.05

as a function of T at H = 5 Oe (volume units). The continuous line is a best-fitaccording to equation (IV.2). . . . . . . . . . . . . . . . . . . . . . . . . . . 128

18 List of Figures18

18

III.7 Sketchy representation of the model of phase segregation of superconductivityand magnetism (reprinted from [Par09] with permissions both of Authors andof the American Physical Society, Copyright 2009 from the American Phys-ical Society). The nanoscopic coexistence is realized whether the order-of-magnitude of the mean distance among magnetic domains (green droplets) isclose to d ∼ 1 nm, namely the spatial range of the magnetic dipolar field gener-ated by uncompensated Fe moments on the domain walls (red arrows). Underthese circumstances, both µ1- and µ2-like implanted muons feel a static mag-netic local field giving rise to the phenomenology observed in figure III.6. . . . 118

III.8 Upper graph: TF-µ+SR spectra for CeFeAsO0.94F0.06 (H = 200 Oe). Up-per graph, upper subgraph: data in the paramagnetic regime T > Tc. Uppergraph, lower subgraph: superconducting mixed regime T < Tc. Continuousred lines are best-fits according to equation (III.13). Lower graph: fitting re-sults according to equation (III.13). Lower graph, upper subgraph: λ(T ) for thenon-magnetic oscillating term. Lower graph, lower subgraph: Bµ(T ) at the µ+

site. The red dashed line is a guide to the eye according to a double-exponentmean-field-like function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

III.9 ZF-µ+SR spectra for CeFeAsO0.945F0.055. Continuous lines are best-fits ac-cording to equation (III.5) and to table III.1. . . . . . . . . . . . . . . . . . . 121

III.10TF-µ+SR spectra for CeFeAsO0.945F0.055 (H = 150 Oe). Upper graph: para-magnetic regime at 40 K. Lower graph: mixed regime at 5 K. Continuous redlines are best-fits according to equation (III.12). . . . . . . . . . . . . . . . . . 122

III.11Upper graph: fitting results for TF-µ+SR spectra of CeFeAsO0.945F0.055 re-ported in figure III.10 according to equation (III.15). Upper graph, upper sub-graph: σsc vs. T . Upper graph, lower subgraph: Bµ vs. T . Lower graph:fitting results for TF-µ+SR spectra of SmFeAsO0.8F0.2 (not shown) accordingto equation (III.15). Lower graph, upper subgraph: σsc vs. T . Lower graph,lower subgraph: Bµ vs. T . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

III.12Phase diagram of CeFeAsO1−xFx family resulting from the µ+SR measure-ments reported in this chapter (Copyright 2011 from the American PhysicalSociety). The labels “M” and “SC” stands for magnetic spin density wave andsuperconductivity, respectively, while the meaning of “LR” and “SR” is long-and short-range order for the magnetic phase, respectively. The x = 0.07 sam-ple is not discussed in the thesis since its behaviour is extremely similar to thex = 0.06 one. The point deep into the superconducting region is extracted fromthe phase diagram reported in [Zha08]. . . . . . . . . . . . . . . . . . . . . . 124

IV.1 Main graph: inverse magnetic susceptibility of CeFeAsO0.965F0.035 as a func-tion of T at H = 20 Oe (volume units). The continuous line is a best-fit ac-cording to equation (IV.1). Inset: magnetic susceptibility of CeFeAsO0.95F0.05

as a function of T at H = 5 Oe (volume units). The continuous line is a best-fitaccording to equation (IV.2). . . . . . . . . . . . . . . . . . . . . . . . . . . 128

19

IV.2 Upper graph: fitting results for the 19F-NMR absorption line at different rep-resentative T for CeFeAsO0.965F0.035 (experimental points are not shown forthe sake of clarity). Upper graph, inset: raw data relative to representative Tvalues. Lower graph: shape of the 19F-NMR line for CeFeAsO0.95F0.05 atT = 30 K. The continuous black curve is a best-fit to data according to thefunction reported in equation (IV.3). The two single contributions are also re-ported as continuous coloured lines and labelled correspondingly to the relativecomponents of the anisotropic fK tensor. . . . . . . . . . . . . . . . . . . . . 131

IV.3 Main graph: T -dependence for the ab and c components of the fK tensor for allthe investigated samples at µ0H = 3.9 T (main graph and inset, respectively). . 132

IV.4 Main graph: Kab and Kc as a function of the M/H curve reported in fig-ure IV.1 with T as an implicit parameter (data relative to CeFeAsO0.965F0.035).The continuous curves are best-fits according to linear functions. The blackdashed line roughly accounts for the contribution to K from a transferred con-tact term. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

IV.5 Main graph: 19F-NMR resonance line of CeFeAsO0.965F0.035 at T = 45 Kand µ0H = 5 T. The continuous line is a simulation of experimental dataaccording to the procedure explained in the text (partial powder average withπ/6 ≤ ϕ ≤ 5π/6). The dashed line is obtained by extending the powderaverage procedure to the whole solid angle. . . . . . . . . . . . . . . . . . . . 136

IV.6 Main graph: recovery laws for CeFeAsO0.965F0.035 at selected T . The contin-uous curves are best-fits according to a single-exponential function, see equa-tion (IV.22). Inset: enlargement at short-t values displaying the good quality ofthe fitting procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

IV.7 Main graph: T -dependence of 1/T1. The black dashed line is guide to the eyeaccording to the trend T−4/3. The two arrows indicate the anomalies associatedwith the transition to the spin density wave phase. No anomaly is observed inCeFeAsO0.95F0.05 and CeFeAsO0.945F0.055 in correspondence to the onset ofsuperconductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

IV.8 Main graph: T -dependence of 1/T1 in the three investigated samples. The con-tinuous lines are best-fits according to the phenomenological function reportedin equation (IV.23). Inset: fitting results for the parameter ∆CF expressed in Kdegrees. The black dashed line is a linear extrapolation back to x = 0. . . . . . 140

IV.9 Phase diagram of CeFeAsO1−xFx after the 19F-NMR measurements. The la-bels “M” and “SC” stands for spin density wave and superconductivity, respec-tively. The dashed red line relative to Tc in the coexistence region is indicativeof some uncertainty in the positioning of the emergence of superconductivity.The point relative to the undoped sample is borrowed from the phase diagramreported in figure III.12 obtained by means of µ+SR on the sample series fromthe University of Genova. On the other hand, the point deep into the supercon-ducting region is extracted from the phase diagram reported in [Zha08]. . . . . 141

V.1 The MP35N double-wall piston-cylinder pressure cell developed at the Labo-ratory for Muon Spin Spectroscopy (Paul Scherrer Institut) and used for µ+SRmeasurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

19List of Figures 19

20

V.2 Upper graph: TF-µ+SR spectra for SmFeAsO at P = 2.05 GPa (H = 50 Oe).The two curves are relative to T values well below and well above TN � 132.5K. The continuous lines are best-fits according to the function reported in equa-tion (V.3). Upper graph, inset: short-t enlargement displaying an oscillatingtransverse signal from the magnetic phase of the sample. Lower graph: ZF-µ+SR spectra for SmFeAsO at P = 2.05 GPa. The continuous lines are bestfits to data according to the function reported in equation (V.5) while the dashedline is a rough estimate of the contribution from the PC itself. Lower graph,inset: short-t enlargement displaying an oscillating transverse signal from themagnetic phase of the sample. . . . . . . . . . . . . . . . . . . . . . . . . . . 146

V.3 Main graph: magnetic susceptibility of LaFeAsO0.945F0.055 at two differentvalues of applied P (H = 5 Oe, ZFC, volume units). The label 10−4 GPacorresponds to ambient P conditions with the sample mounted in the unloadedPC. Inset: enhancement of Tc upon increasing P estimated from FC curves (notshown). The dashed line is a guide for the eye. . . . . . . . . . . . . . . . . . 148

V.4 Vm for LaFeAsO0.945F0.055 at several values of P up to 2.35 GPa. Blue squaresigns represent measurements at ambient P , where the open symbols are rel-ative to the calibration measurement while the filled ones are relative to themeasurement with the sample mounted in the unloaded PC. The continuouslines are best-fits according to equation (V.6). Inset: gradual suppression of TN

on increasing P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

V.5 Main graph: Vm for SmFeAsO0.925F0.075 estimated from TF-µ+SR data forseveral values of P up to 1.6 GPa. The dashed line is a guide to the eye accord-ing to equation (V.6). Upper inset: short-t ZF-µ+SR depolarization at ambientP . The continuous red line is a best-fit according to the function reported inequation (V.5). Lower inset: Bµ(T ) at the µ+ site estimated from ZF-µ+SRdata for two values of P . Continuous lines are best-fits according to the functionreported in equation (V.9). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

V.6 Upper graph: Vm for CeFeAsO0.96F0.04 estimated from TF-µ+SR data. Filledsymbols are relative to GPD measurements while open ones are relative tothe GPS characterization (already shown in the lower graph of figure III.5, in-set). The dashed line is a guide to the eye according to equation (V.6). Uppergraph, inset: TF-µ+SR depolarization at 15 K and 100 K at ambient P (sam-ple mounted in the unloaded PC). Continuous lines are best-fits according toequation (V.3). Lower graph: λTr vs. T at two different values of P . Open andfilled squares are relative to the ambient P measurements at GPS and GPD, re-spectively. No dependence on the applied P is deduced within the experimentalerror. Lower graph, inset: ZF-µ+SR spectra at some representative T values forthe measurement at 2.35 GPa. . . . . . . . . . . . . . . . . . . . . . . . . . . 153

20 List of Figures20

20

V.2 Upper graph: TF-µ+SR spectra for SmFeAsO at P = 2.05 GPa (H = 50 Oe).The two curves are relative to T values well below and well above TN � 132.5K. The continuous lines are best-fits according to the function reported in equa-tion (V.3). Upper graph, inset: short-t enlargement displaying an oscillatingtransverse signal from the magnetic phase of the sample. Lower graph: ZF-µ+SR spectra for SmFeAsO at P = 2.05 GPa. The continuous lines are bestfits to data according to the function reported in equation (V.5) while the dashedline is a rough estimate of the contribution from the PC itself. Lower graph,inset: short-t enlargement displaying an oscillating transverse signal from themagnetic phase of the sample. . . . . . . . . . . . . . . . . . . . . . . . . . . 146

V.3 Main graph: magnetic susceptibility of LaFeAsO0.945F0.055 at two differentvalues of applied P (H = 5 Oe, ZFC, volume units). The label 10−4 GPacorresponds to ambient P conditions with the sample mounted in the unloadedPC. Inset: enhancement of Tc upon increasing P estimated from FC curves (notshown). The dashed line is a guide for the eye. . . . . . . . . . . . . . . . . . 148

V.4 Vm for LaFeAsO0.945F0.055 at several values of P up to 2.35 GPa. Blue squaresigns represent measurements at ambient P , where the open symbols are rel-ative to the calibration measurement while the filled ones are relative to themeasurement with the sample mounted in the unloaded PC. The continuouslines are best-fits according to equation (V.6). Inset: gradual suppression of TN

on increasing P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

V.5 Main graph: Vm for SmFeAsO0.925F0.075 estimated from TF-µ+SR data forseveral values of P up to 1.6 GPa. The dashed line is a guide to the eye accord-ing to equation (V.6). Upper inset: short-t ZF-µ+SR depolarization at ambientP . The continuous red line is a best-fit according to the function reported inequation (V.5). Lower inset: Bµ(T ) at the µ+ site estimated from ZF-µ+SRdata for two values of P . Continuous lines are best-fits according to the functionreported in equation (V.9). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

V.6 Upper graph: Vm for CeFeAsO0.96F0.04 estimated from TF-µ+SR data. Filledsymbols are relative to GPD measurements while open ones are relative tothe GPS characterization (already shown in the lower graph of figure III.5, in-set). The dashed line is a guide to the eye according to equation (V.6). Uppergraph, inset: TF-µ+SR depolarization at 15 K and 100 K at ambient P (sam-ple mounted in the unloaded PC). Continuous lines are best-fits according toequation (V.3). Lower graph: λTr vs. T at two different values of P . Open andfilled squares are relative to the ambient P measurements at GPS and GPD, re-spectively. No dependence on the applied P is deduced within the experimentalerror. Lower graph, inset: ZF-µ+SR spectra at some representative T values forthe measurement at 2.35 GPa. . . . . . . . . . . . . . . . . . . . . . . . . . . 153

21

V.7 Upper graph: M /H for CeFeAsO0.94F0.06 at two values of P (H = 5 Oe, ZFC,volume units). Continuous lines are best-fits according to equation (V.10). Thelabel 10−4 GPa corresponds to ambient P with the sample mounted in the un-loaded PC. Upper graph, upper inset: Tc vs. P estimated from FC curves (notshown). The dashed line is a guide for the eye. Upper graph, lower inset: sup-pression of the value of the magnetic moment of Ce3+ ions upon increasing P .Lower graph: Vm in CeFeAsO0.94F0.06 estimated from TF-µ+SR (see also thelower graph of fig. III.8, lower subgraph). The continuous lines are best-fits ac-cording to equation (V.6). Lower graph, inset: Bµ vs. T in the superconductingphase from TF-µ+SR (see also the lower graph of figure III.8, lower subgraph). 154

B.1 Integrated intensity of the spin echo as a function of t in SmOF (left graph)and in CeFeAsO0.93F0.07 “Ge” (right graph). Both the measurements are per-formed at room temperature. The two continuous lines are best-fits of dataaccording to a Gaussian-like decay in SmOF and to a exponential-like decay inCeFeAsO0.93F0.07 “Ge”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

C.1 ISIS particles accelerator scheme. . . . . . . . . . . . . . . . . . . . . . . . . 180

C.2 π+ meson decay according to equation (C.1) (reprinted from [Cox87] with per-missions both of Author and of IOP Publishing. Copyright IOP Publishing. Allrights reserved). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

C.3 Larmor precession of sµ+ around the local magnetic field B (reprinted from[Cox87] with permissions both of Author and of IOP Publishing. CopyrightIOP Publishing. All rights reserved). . . . . . . . . . . . . . . . . . . . . . . 182

C.4 µ+ decay corresponding to the emission of the positron with maximum linearmomentum and energy according to equation (C.4) (reprinted from [Cox87]with permissions both of Author and of IOP Publishing. Copyright IOP Pub-lishing. All rights reserved). . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

C.5 µ+ asymmetric decay (reprinted from [Cox87] with permissions both of Authorand of IOP Publishing. Copyright IOP Publishing. All rights reserved). Curvelabeled as a = 1: only the positron’s maximum emission energy is considered.Curve labeled as a = 1/3: the positron emission is averaged on all the possibleenergies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

C.6 Position of the detectors of positrons around the sample in the longitudinal con-figuration (reprinted from [Dal97] with permissions both of Authors and of IOPPublishing. Copyright IOP Publishing. All rights reserved). The position of theµ+ spin is rotated by 180◦ with respect to its initial direction. . . . . . . . . . 184

C.7 Zero-field relaxation functions caused by different distributions of static lo-cal magnetic fields: (a) Gaussian Kubo-Toyabe, (b) Lorebtzian Kubo-Toyabe(reprinted from [Cox87] with permissions both of Author and of IOP Publish-ing. Copyright IOP Publishing. All rights reserved). . . . . . . . . . . . . . . 185

C.8 Depolarization functions caused by a dynamical Gaussian process in the strongcollision limit (reprinted from [Dal97] with permissions both of Authors and ofIOP Publishing. Copyright IOP Publishing. All rights reserved). . . . . . . . . 186

21List of Figures 21

22

C.9 Effect of the application of an external longitudinal magnetic field in the caseof a static distribution of local fields (reprinted from [Dal97] with permissionsboth of Authors and of IOP Publishing. Copyright IOP Publishing. All rightsreserved). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

C.10 Comparison between the effects of the application of an external longitudinalmagnetic field HLF on (a) a static distribution of local magnetic fields of widthσ ≡ ∆ and (b) on a dynamical process with frequency ν ≡ νc (reprinted from[Cox87] with permissions both of Author and of IOP Publishing. CopyrightIOP Publishing. All rights reserved). . . . . . . . . . . . . . . . . . . . . . . 188

22 List of Figures22