5.3 Results and Discussions .1 Transport measurement
5.3.6 Determination the Mn valence state:
The x- ray absorption spectra (XAS) are measured in sample current mode. The spectra from the structure MnO2-terminated interface (left) and LaCaO-terminated interface (right), in Figure 5.10 a). Our spectra have been normalized by subtracting the background energy (pre-edge energy) to be zero and post-edge energy around photon energy of 600eV to be one of two different structures as shown in Figure 5.10 b) and c).
Normalizing the Mn K and Mn L –edges are similar to that of O K –edge for sample current mode, However after normalization, we need to determine the valence state of Mn.
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Figure 5.11 Mn K -edge XAS spectra for LCMO/YBCOd with different interfaces at thickness of YBCO being 6nm, 10nm, 13nm, 20nm and are plotted together with spectra for the Mn2O3 (Mn3+) and MnO2 (Mn4+) standard samples and reference samples , taken in fluorescence yield mode to demonstrate how to determine the valence state of Mn.
The Figure 5.11 a) - d) show Mn K –edge XAS spectra with different interfaces at thickness of YBCO being 6nm, 10nm, 13nm, 20nm, respectively and are plotted together with spectra of the Mn2O3 (Mn3+) and MnO2 (Mn4+) standard samples and reference samples of La1-xCaxMnO3 (x= 0.3, 1) and also our LCMO sample. We have drawn the standard line (that in the present case cuts the y-axis at 0.8) where, the spectra representing the MnO2 (Mn4+) and La1-xCaxMnO3 (x= 1) met together and the same line intersects with the spectra of the Mn2O3 (Mn3+) and MnO2 (Mn4+) standard samples. This essentially indicates that the valence states of Mn3+ and Mn4+, hence the valence state of our samples are defined by pointing out the spectra on the standard line by using the linear fit as shown in Figure 5.6 b). To confirm our experiment about the charge transfer and valence state, we also double checked with Mn L –edge, our results strongly support and are consistent with our Mn K –edge spectra as discussed.
Figure 5.12 a)-c) Mn-L2,3 edge XAS spectra for LCMO/YBCOd with different interfaces with YBCO thicknesses of 2nm, 6nm and 8nm; d) Comparison of Mn L- edges XAS spectra LCMO/YBCOd with different interfaces with YBCO thicknesses of 2nm, 6nm and 8nm, and showing clearly the energy shifts after putting the YBCO thicker, taken in total electron yield mode.
The TEY signal was used to probe the electronic and magnetic structure of the MnO2
layers at the interface for the low electron escape depth (a few nanometers), and contributions from deeper layers are exponentially decreased. Figure 5.12 shows the XAS spectra of the Mn-L2,3 edge of the YBCO/LCMO heterostructures with a fixed LCMO thickness of 10nm and YBCO thicknesses of 2nm, 6nm and 8nm. The energy difference of the Mn L3-edge absorption peak between the two interfacial terminations increases with the increment of the YBCO layer thickness, with energy differences of around 90 meV, 300 meV, and 400 meV corresponding respectively to YBCO thicknesses of 2nm, 6nm and 8nm. We also noticed the spectral shifts of the Mn L3-edge moving towards higher energy for both terminated interfaces which indicates an increase in the oxidation state of the Mn ions. The spectral shifts are larger in the La0.7Ca0.3O-terminated interface than in the MnO2-terminated one. The increase of
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Mn4+ with YBCO thickness clearly reveals that the charge transfer across the interface between two terminations with different work functions, hence the magnetization decreases as the valence state of Mn (Mn4+) increases, which is also consistent with our Mn-K edge data.
Finally, to rule out the effects of the SRO layer, we also tried using the SrMnO (SMO) buffer layer to switch the termination rather than SRO, and identical results were observed.
Therefore, all the observed effects presented here are apparently not due to the SRO buffer layer.
5.4 Conclusion
To conclude this topic, this finding would have been impossible without an atomically precise interface control. We have shown that the interfaces control has played prominent role on affecting the magnetic and electronic properties of F/S heterostructures charge transfer at both interfaces in STO/LCMO/YBCOd and STO/SRO/LCMO/YBCOd structures. The charge transfer is stronger at the La0.7Ca0.3O-terminated interface than at the MnO2-terminated interface. This mechanism is responsible for the larger number of holes in the CuO2 planes in the La0.7Ca0.3O-terminated samples and hence high Tc. All the observations of superconductivity, magnetism, and XAS concur with the newly discovered charge transfer in this YBCO/LCMO system.
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Chapter 6: Summary
In this dissertation, we have tried to unveil the functional of the control the interaction in the complex oxides interfaces, whether they applied to study the two dimensional electron gas (2DEG) of LAO/STO interface or coupling between the ferromagnetism and superconducting in LCMO/YBCO system. As we have known that interfaces have emerged as focal points of current condensed matter physics. In strongly correlated oxides, heterointerfaces provide a powerful route to create and manipulate the charge, spin, orbital, and lattice degrees of freedom, suggesting new possibilities for next generation devices and creating a huge playground to discover new emergent phenomena.
The appearance of controlling the conductivity in LAO/STO interface by ferroelectric of PZT has provided potentiality to improve the quality of devices and enhance physical functionalities or create additional functionalities for future devices. One of the famous ferroelectric materials is PZT, which is widely studied recently. The other is the high mobility of 2DEG in LAO/STO interface. Therefore, PZT and LAO/STO have been chosen for investigating the ferroelectric control of the conduction at the LAO/STO heterointerface. Our structure PZT/LAO/STO shows that both PZT and LAO are grown epitaxially on STO(100) - oriented substrates and revealed clearly that we are successful in placing the ferroelectric of PZT on top of 2DEG of LAO/STO interface. In addition, our transport measurement with the ferroelectric effect sets in and the sheet resistance starts increasing with PZT thickness showing the strong impact of the intrinsic polarization (Pup) of the PZT on electron conduction at the interface. Moreover, by increasing the thickness of LAO, while thickness of PZT was kept constant (20nm), we also realized that the sheet resistance decreases when lowering the temperature and increases when the thickness of LAO is reduced. Based on our results, we can estimate that the resistance is increased by an order of magnitude when the LAO thickness is cut down to 1uc. The most important thing is that the sheet resistance can be modulated the conductivity at LAO/STO interface by not only the polarization (PZT thickness dependence) but also the different polarization states of PZT (Pup –state and Pdown –state).
Especially, another intriguing feature here is the switch of an insulating state to a conducting state. Beside that our XPS and XSTM results provide more evidence about the band bending at LAO/STO interface under the control of the polarization states of PZT. The energy shift of the band edges in LAO is smaller for the Pdown –state sample than that for the Pup –state sample which indicates that the electric field in LAO layer diminishes after the polarization of the PZT is switched from the natural Pup –state to the Pdown –state. Additionally, in the
theoretical calculation is constistent with our experimental results.
In order to search for new types of oxide interfaces, we explored the ferromagnetic La2/3Ca1/3MnO3/superconducting YBa2Cu3O7-x heterostructures of two distinct interfaces with atomically precise interface control to study the coupling between these two functional layers and identified a new mechanism of charge transfer in the YBa2Cu3O7-x/La0.7Ca0.3MnO3
(YBCO/LCMO) hetrostructures.
From our transport data, the depression of Tc indicates the enhanced interaction between superconductivity and magnetism. Based on our transport results we can conclude that controlling the interfaces is very crucial to manipulate the superconducting and other physical properties at the heterostructure of YBCO/LCMO interfaces. In addition to different Tc, different terminations also lead to different magnetic properties in these heterostructures. In our system, both the decreased Tc of YBCO with decreasing YBCO thickness and the decreased magnetization of LCMO with increasing YBCO thickness both qualitatively fit into the proximate scenario. To explain our results, we illustrate the charge transfer model at two different interfaces: La0.7Ca0.3O-terminated interface model, CuO chains are closer to MnO planes and the transfer of electrons from the MnO planes to CuO chains is relatively easier.
On the other hand, with the MnO2-terminated interface model, CuO chains are far from MnO planes. Therefore, the electron transfer from MnO planes to CuO chains is weaker. According to the present models, this is due to the change of the Mn4+ to Mn3+ ratio. For the thicker YBCO layer, the charges transfer from LCMO to YBCO layer leads to more Mn4+ ions, and thus the reduction of Mn magnetization. The same YBCO thickness, the 528 eV peak O K-edge XAS demonstrates the increase in the number of holes with increasing YBCO thickness, which certainly contribute to the enhancement of Tc.
The present work on the second topic highlights new development of multifunctional device applications of interfaces between the ferromagnetism and superconducting in YBa2Cu3O7-x/La0.67Ca0.33MnO3 and deeper understanding of underlying physics in which only shows on the terminations control such as the interaction, frustration, charge, orbital, and spin degrees of freedom, proximate effect, is critical to the superconductivity, magnetism. The above-mentioned conclusions disclose a reason for control the interaction in complex oxides interfaces. The oxide interfaces show a lot of possibilities in either promoting the usability of functionalities or creating different connectivity of structures for artificially designing new interaction mechanisms. Hence, fully understanding the physics behind these oxides interfaces systems is a critical issue for further applications.
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Supporting Materials
Appendix B. Electric field in LAO calculations
By E. A. Eliseev, Prof. A. N. Morozovska
Institute of Problems of Material Sciences and Institute of Physics, National Academy of Science of Ukraine, Kiev, Ukraine
Electrostatic quasi-stationary Maxwell equation rot E=0 should be valid in the actual frequency range, giving one the opportunity to introduce the potential of quasi-stationary electric field, Eg,f(x,z,t)g,f(x,z,t). Inside the dielectric gap potential satisfies between semiconductor STO, dielectric gap LAO and ferroelectric PZT, namely
,
0S x z Rd , (B.4a)
x,0 g
x,0 period a can be expanded in Fourier series as
The stray field (B.9) causes redistribution of the free charge density in accordance Boltzmann
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Spatial x-distributions of polarization in the stripe 180-degree domain structure, stray depolarizing field E3(x,z) existing in LAO layer and STO surface charge S(x) are shown
E existing in LAO layer. Polarization distribution has conventional rectangular shape (Figure B1a and b). Charge and field x-profiles shape and amplitude strongly depend on the period of domain structure a in PZT. Appeared that for the case aL and the stray field weakly depends on the position z in LAO and thus remains strong enough at the LAO/STO interface z 0 (Figure B1c). For the case aL and the stray field strongly depends on the position z in LAO and essentially decreases at the LAO/STO interface z0 (Figure B1d). Thus the total charge density accumulated in STO is much smaller for the case aL than the one at aL (compare Figure B1f and e).
0 20 40 60 80 100 l.c. (a, c, e) and a=10 l.c. (b,d, f). Abbreviation l.c. stands for lattice constant units.