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Unconventional superconductivity of the heavy fermion compound UNi_1tn2Al_1tn3 [Elektronische Ressource] / von Andrey Zakharov

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Unconventional superconductivityof the heavy fermion compound UNi Al2 3Dissertationzur Erlangung des GradesDoktor der Naturwissenschaften (Dr.rer.nat.)am Fachbereich Physikder Johannes Gutenberg-Universit¨at MainzvonAndrey Zakharovgeb.in Kharkiv, UkraineMainz, 2008ContentsIntroduction 11 The superconductors UNi Al and UPd Al 32 3 2 31.1 Crystallographic structure . . . . . . . . . . . . . . . . . . . . . . 31.2 Magnetic structure . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Superconducting and magnetic properties . . . . . . . . . . . . . . 52 Experimental methods 92.1 Preparation technique . . . . . . . . . . . . . . . . . . . . . . . . 92.1.1 Molecular beam epitaxy . . . . . . . . . . . . . . . . . . . 92.1.2 The MBE preparation system . . . . . . . . . . . . . . . . 102.2 Characterization methods . . . . . . . . . . . . . . . . . . . . . . 122.2.1 RHEED . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2.2 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . 143 UNi Al thin films 192 33.1 The preparation process . . . . . . . . . . . . . . . . . . . . . . . 193.2 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.3 UNi Al thin films on Al O and MgAl O substrates . . . . . . . 222 3 2 3 2 43.4 Epitaxial UNi Al (100) thin films on YAlO substrates . . . . . . 272 3 33.4.1 Crystallographic properties. . . . . . . . . . . . . . . . . . 283.4.2 Morphology . . . . . . . . . . . . . . . . . . .

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Unconventional superconductivity
of the heavy fermion compound UNi Al2 3
Dissertation
zur Erlangung des Grades
Doktor der Naturwissenschaften (Dr.rer.nat.)
am Fachbereich Physik
der Johannes Gutenberg-Universit¨at Mainz
von
Andrey Zakharov
geb.in Kharkiv, Ukraine
Mainz, 2008Contents
Introduction 1
1 The superconductors UNi Al and UPd Al 32 3 2 3
1.1 Crystallographic structure . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Magnetic structure . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Superconducting and magnetic properties . . . . . . . . . . . . . . 5
2 Experimental methods 9
2.1 Preparation technique . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.1 Molecular beam epitaxy . . . . . . . . . . . . . . . . . . . 9
2.1.2 The MBE preparation system . . . . . . . . . . . . . . . . 10
2.2 Characterization methods . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 RHEED . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . 14
3 UNi Al thin films 192 3
3.1 The preparation process . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3 UNi Al thin films on Al O and MgAl O substrates . . . . . . . 222 3 2 3 2 4
3.4 Epitaxial UNi Al (100) thin films on YAlO substrates . . . . . . 272 3 3
3.4.1 Crystallographic properties. . . . . . . . . . . . . . . . . . 28
3.4.2 Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4 Magnetic order 35
4.1 Resonant magnetic x-ray scattering . . . . . . . . . . . . . . . . . 35
4.2 Experiments on UNi Al thin films . . . . . . . . . . . . . . . . . 362 3
4.2.1 Correlation lengths . . . . . . . . . . . . . . . . . . . . . . 38
iiiiv CONTENTS
5 Transport measurements 43
5.1 Standard R(T)-characterization . . . . . . . . . . . . . . . . . . . 43
5.2 Transport anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.3 Upper critical magnetic field H . . . . . . . . . . . . . . . . . . . 57c2
6 Tunneling spectroscopy 63
6.1 Introduction to tunneling spectroscopy . . . . . . . . . . . . . . . 63
6.2 Planar tunneling junctions . . . . . . . . . . . . . . . . . . . . . . 69
6.2.1 Concept of the preparation . . . . . . . . . . . . . . . . . . 70
6.2.2 UNi Al -based junctions . . . . . . . . . . . . . . . . . . . 712 3
6.3 Measurement technique . . . . . . . . . . . . . . . . . . . . . . . . 76
6.4 Tunneling conductivity measurements . . . . . . . . . . . . . . . . 77
6.4.1 UNi Al –AlO –(Ge)–Pb junctions . . . . . . . . . . . . . . 772 3 x
6.4.2 UNi Al –AlO –Pb cross-type junctions . . . . . . . . . . . 802 3 x
6.4.3 UNi Al –AlO –Ag mesa-junctions . . . . . . . . . . . . . . 852 3 x
6.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Summary 90
Bibliography 95List of Figures
1.1 Crystallographic structure of UNi Al . . . . . . . . . . . . . . . . 42 3
1.2 Strong coupling features of UPd Al –AlO –Pb junction and neu-2 3 x
tron inelastic scattering profile of UPd Al single crystal. . . . . . 62 3
2.1 MBE preparation chamber. . . . . . . . . . . . . . . . . . . . . . 11
2.2 Schematic picture of a RHEED experiment. . . . . . . . . . . . . 14
2.3 Schematic of a four-circle diffractometer. . . . . . . . . . . . . . . 16
2.4 Schematic of a ϕ-scan. . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5 Schematic of a Q-scan. . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1 Orientation of UNi Al thin films with respect to the substrates. . 212 3
3.2 XRD ω/2Θ-scan of a UNi Al film on Al O (110). . . . . . . . . . 232 3 2 3
3.3 SEM image of a UNi Al thin film on Al O (110). . . . . . . . . . 242 3 2 3
3.4 ϕ-scan of UNi Al thin film on Al O (110). . . . . . . . . . . . . . 242 3 2 3
3.5 R(T) dependence of UNi Al thin film on Al O (110). . . . . . . . 252 3 2 3
3.6 XRD ω/2Θ-scan of a UNi Al film on Al O (110). . . . . . . . . . 262 3 2 3
3.7 XRD ω/2Θ-scan of a UNi Al film on YAlO . . . . . . . . . . . . 292 3 3
3.8 Q-scan of the reciprocal (1kl)-plane of UNi Al . . . . . . . . . . . 302 3
3.9 RHEED-pattern of an UNi Al film on YAlO (010). . . . . . . . . 302 3 3
3.10 TEM image of UNi Al thin film. . . . . . . . . . . . . . . . . . . 312 3
3.11 ϕ-scan of UNi Al thin film on YAlO (112). . . . . . . . . . . . . 322 3 3
3.12 An AFM image of the surface of a UNi Al thin film. . . . . . . . 332 3
3.13 AFM images of UNi Al thin films of different thicknesses. . . . . 342 3
4.1 Possible orientation of the magnetic domains in UNi Al . . . . . . 372 3
4.2 TemperaturedependenceoftheintensityofthemagneticBragg-peak 37
4.3 ω-scans of the specular (h00) peaks (h=1,2,3,4) . . . . . . . . . . 39
∗4.4 Scan parallel to the a -axis around the magnetic Bragg position . 40
vvi LIST OF FIGURES
5.1 Normalized resistance R(T)/R of several UNi Al thin films. . 44max 2 3
5.2 Normalized resistance R(T)/R of several UNi Al thin films atmax 2 3
T .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44N
5.3 R(T) dependence of a non patterned UNi Al thin film. . . . . . . 452 3
5.4 Superconducting resistive transitions of a non patterned UNi Al2 3
thin film.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.5 Specificresistivityρ(T)ofanUNi Al thinfilmfordifferentcurrent2 3
directions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
5.6 Superconducting resistive transitions of different UNi Al films. . 482 3
5.7 Temperature dependencies of the energy gaps of multiband super-
conductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5.8 Fermi surface examples of a multiband superconductor. . . . . . . 51
5.9 Calculated Fermi surfaces of UNi Al and UPd Al . . . . . . . . . 522 3 2 3
5.10 Temperature dependencies of the energy gaps of multiband super-
conductors with a weak itraband coupling. . . . . . . . . . . . . . 53
5.11 Photograph of a meander patterned UNi Al thin film. . . . . . . 542 3
5.12 Specific resistivity ρ(T) of an UNi Al meander structure.. . . . . 552 3
5.13 Specific resistivity ρ(T) of an UNi Al thin film at T . . . . . . . 562 3 N
5.14 Resistive superconducting transitions of an UNi Al thin film in2 3
magnetic fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.15 Upper critical fields H(T) of an UNi Al thin film. . . . . . . . 600 2 3
5.16 Upper critical fields H(T) of an UNi Al meander structure. . . 610 2 3
6.1 Schematic of a tunneling experiment. . . . . . . . . . . . . . . . . 64
6.2 Energy diagram of NIN-junction. . . . . . . . . . . . . . . . . . . 64
6.3 Semiconductor model for a SIN tunneling process. . . . . . . . . . 66
6.4 Semiconductor model for a SIS tunneling process. . . . . . . . . . 67
6.5 Tunneling spectrum of lead with strong coupling features.. . . . . 69
6.6 Schematic of the possible barrier defects. . . . . . . . . . . . . . . 71
6.7 Schematic of a UNi Al -AlO –(Ge)–Pb planar tunnel junction. . . 722 3 x
6.8 Schematic of a UNi Al -AlO -Pb planar cross-type junction. . . . 732 3 x
6.9 Schematicofthephotomasksusedforthepreparationofthemesa-
junctions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.10 Schematic and photograph of an UNi Al -AlO -Ag mesa-junction. 752 3 x
6.11 AC-modulation set-up. . . . . . . . . . . . . . . . . . . . . . . . . 77
6.12 Differential conductivity of an UNi Al –AlO –(Ge)–Pb junction. . 782 3 xLIST OF FIGURES vii
6.13 Pb gap features in the UNi Al –AlO –(Ge)–Pb junction conduc-2 3 x
tivity, temperature and magnetic field dependence. . . . . . . . . 79
6.14 DifferentialconductivityofanUNi Al –AlO –Pbcrosstypejunction. 822 3 x
6.15 Differential conductivity of junctions with different barrier thick-
nesses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
6.16 Differential conductivity of a mesa-junction at high bias voltages. 86
6.17 Differential conductivity of a mesa-junction. . . . . . . . . . . . . 88viii LIST OF FIGURESList of Tables
3.1 Lattice misfits of the substrates . . . . . . . . . . . . . . . . . . . 22
ixx LIST OF TABLES