Time-resolved X-ray diffraction study on superconducting YBa_1tn2Cu_1tn3O_1tn7 epitaxially grown on SrTiO_1tn3 [Elektronische Ressource] / von Andrea Lübcke

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• Physik

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• Publié par: friedrich-schiller-universitat_jena
• Publié le: 01 janvier 2007
• Nombre de lectures: 28
• Poids de l'ouvrage: 4 Mo

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Time-Resolved X-Ray Diffraction Study on
Superconducting YBa Cu O Epitaxially Grown on2 3 7
SrTiO3
Dissertation
zur Erlangung des akademischen Grades
doctor rerum naturalium (Dr. rer. nat.)
vorgelegt dem Rat der Physikalisch - Astronomischen Fakulta¨t der
Friedrich-Schiller Universit¨at Jena
von Dipl.-Phys. Andrea Lu¨bcke
geboren am 09. 12. 1978 in Grevesmu¨hlenGutachter
1. ..................
2. ..................
3. ..................
Tag der letzten Rigorosumspru¨fung .............
Tag der ¨offentlichen Verteidigung ..........
iiContents
1 Introduction 1
2 Literature Survey 3
2.1 High-Temperature Superconductors. . . . . . . . . . . . . . . . . 3
2.1.1 Universal properties . . . . . . . . . . . . . . . . . . . . . 3
2.1.2 Crystallographic response to superconductivity . . . . . . 5
2.1.3 Superconducting Stripes . . . . . . . . . . . . . . . . . . . 9
2.1.4 High T superconductors under short pulse illumination . 13c
2.2 SrTiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
2.2.1 Lattice structure . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.2 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.3 Epitaxial strain induced thin film behaviour . . . . . . . . 19
3 Experimental and Theoretical methods 21
3.1 Time-resolved X-ray diffraction technique . . . . . . . . . . . . . 21
3.2 Theory of X-ray diffraction . . . . . . . . . . . . . . . . . . . . . 23
3.2.1 YBa Cu O . . . . . . . . . . . . . . . . . . . . . . . . . . 252 3 7
3.2.2 SrTiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
4 Time-Resolved X-ray diffraction 27
4.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2 The target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.3 The X-ray optic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.4 The sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.5 The X-ray detector . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.6 The Cryostat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.7 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.8 Signal stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.9 Pump and Probe conditions . . . . . . . . . . . . . . . . . . . . . 52
5 Preparatory static measurements 54
6 Time-dependent diffraction data 58
iiiContents
6.1 Temporal response of YBa Cu O . . . . . . . . . . . . . . . . . 602 3 7
6.2 Temporal response of SrTiO . . . . . . . . . . . . . . . . . . . . 643
6.3 Summary of the Results and Conclusions . . . . . . . . . . . . . 69
7 Physical interpretation 72
7.1 YBa Cu O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 722 3 7
7.1.1 Number of broken Cooper pairs . . . . . . . . . . . . . . . 72
7.1.2 Displacive excitation of coherent phonons . . . . . . . . . 73
7.1.3 Split models. . . . . . . . . . . . . . . . . . . . . . . . . . 74
7.2 SrTiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753
7.2.1 Strain distribution . . . . . . . . . . . . . . . . . . . . . . 75
7.2.2 Microscopic origin . . . . . . . . . . . . . . . . . . . . . . 83
8 Summary and Outlook 90
A Basics of dynamical theory of X-ray diffraction 99
B Estimation of the errors of the SrTiO peak parameters 1033
B.1 Error of the peak integral . . . . . . . . . . . . . . . . . . . . . . 103
B.2 Error of peak position . . . . . . . . . . . . . . . . . . . . . . . . 104
B.3 Error of FWHM . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
iv1 Introduction
Sub-ps time resolved X-ray diffraction is extremely successful in studying the
propagationofacousticwaves, ultrafaststructuralphasetransitionsandoptical
phonons in time-domain [1–10]. These experiments were performed to demon-
strate the power of the time resolved sub-ps X-ray diffraction technique and
systems with predictable responses were used. In this PhD thesis the ultrafast
diffraction technique was applied to high-T superconductors. This is a class ofc
superconducting ceramic cuprates with extraordinary high transition tempera-
tures T . The field of High-Temperature Superconductors (HTSC) has openedc
in 1986 with the discovery of superconductivity in La Ba CuO by Bed-2−x x 4
norz and Mu¨ller [11]. Starting from a superconducting transition temperature
T ∼30 K the race for higher T s developed rapidly: Already in 1987 supercon-c c
ductivity is reported in YBa Cu O with a transition temperature T ∼ 90 K,2 3 7 c
exceeding for the first time the boiling point of nitrogen [12]. The highest
transition temperature reported up to now is 138 K at ambient pressure in
Hg Tl Ba Ca Cu O . The mechanism of HTSC is still controversely dis-1−x x 2 2 3 8.33
cussed. There is a large debate in the literature whether there is a structural
response upon entering the superconducting regime (by varying the tempera-
ture) or not. And still, no coherent answer has emerged on this point. It is
known that at the superconducting phase transition no structural phase tran-
sition occurs. However, hypothetically it could be possible that for very short
times a different crystallographic phase or different equilibrium positions for
the ions may exist. This is schematically shown in Figure 1.1 for the case that
the phase transition from superconductivity to the normal conducting state is
optically induced. The laser photons excite the groundstate A. The energy hy-
persurfacefor theexcited state might have a minimumB ata different reaction
coordinate. Thus, upon optical excitation the excited state will relax into the
metastable state B and subsequently decay again to the ground state energy
hypersurface and finally recover to the ground state A. The metastable state B
may have different crystallographic properties than the ground state. To study
this excited state a very high time resolution is necessary.
The focus of the present work was on the lattice response upon ultrafast opti-
cal breaking of Cooper pairs. As sample a thin YBa Cu O film on a SrTiO2 3 7 3
11 Introduction
B
hn
A
Reaction coordinate
Figure 1.1: Schematic energy hypersurface.
substrate was used. YBa Cu O is certainly the best studied high-T super-2 3 7 c
conductor. The reason for using a thin film rather than single crystals was the
importance of matching the extinction depths for pump and probe beam: If
only a small fraction of the probed volume is excited the (likely) very small
signal variation will be even smaller. As the experimental setup was chosen
such that the substrate necessarily remains optically unexcited, the substrate’s
reflection could be used as reference. Surprisingly, the X-ray diffraction signal
from the transparent SrTiO underneath the strongly absorbing YBa Cu O3 2 3 7
thin film has changed upon excitation. For this reason it is necessary to give
not only an introduction to important properties of the high-T film but alsoc
to those of the substrate.
A brief review of these properties will be given in Chapter 2. In Chapter 3
conclusions from the properties of the sample for the choice of experimental
andtheoretical methodswillbedrawn. Thesetupof thediffraction experiment
will be matter of Chapter 4. In Chapter 5 the results of a first static mea-
surement will be described, before in Chapter 6 the time-dependent diffraction
data will be presented. In Chapter 7 these data will be discussed and possible
explainations will be proposed.
2
Energy2 Literature Survey
In this chapter important properties of the YBa Cu O thin film and the2 3 7
SrTiO substrate are reviewed. Special attention is given to possible corre-3
lations between structural and superconducting properties as well as to the
behaviour of high-T superconductors under the illumination with ultrashortc
laser pulses. Thekindsofcorrelations between structureandsuperconductivity
reported in the literature is widely spread. Understanding both, the connec-
tion between structure and superconductivity as well as the behaviour under
shortpulseilluminationisveryimportantforthediscussionoftheexperimental
results of the present work.
2.1 High-Temperature Superconductors
2.1.1 Universal properties
Even though much of the microscopic origins of high-T superconductivity isc
unresolved, there is consensus on many common properties of the high-T su-c
perconductors. The aspects of interest for this work will be reviewed briefly.
Most of them will be discussed for the special case of YBa Cu O .2 3 7
The unit cell of YBa Cu O is shown in Figure 2.1. Its space group was de-2 3 7
termined to be Pmmm (no. 47) [13]. Common to all HTSC is their strong
anisotropy. A key role for the exceptional properties is awarded to the set of n
CuO planes that areseparated by charge reservoirs. YBa Cu O contains two2 2 3 7
CuO planes per unit cell formed by the Cu(2) atoms and the O(2) and O(3)2
atoms (see Figure 2.1 for the notation). It is believed that the carrier trans-
port and processes responsible for superconductivity are closely related to the
CuO planes. The electronic properties of the planes and those of the direction2
perpendicular to the planes, i. e. the c direction, differ strongly. Conduction
in the CuO planes is believed to be established by a charge transfer from the2
charge reservoir. In case o