| 1. |
- Chikina, Alla, et al.
(författare)
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Band-Order Anomaly at the gamma-Al2O3/SrTiO3 Interface Drives the Electron-Mobility Boost
- 2021
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Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 15:3, s. 4347-4356
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Tidskriftsartikel (refereegranskat)abstract
- The rich functionalities of transition-metal oxides and their interfaces bear an enormous technological potential. Its realization in practical devices requires, however, a significant improvement of yet relatively low electron mobility in oxide materials. Recently, a mobility boost of about 2 orders of magnitude has been demonstrated at the spinel-perovskite gamma-Al2O3/SrTiO3 interface compared to the paradigm perovskite-perovskite LaAlO3/SrTiO3 interface. We explore the fundamental physics behind this phenomenon from direct measurements of the momentum-resolved electronic structure of this interface using resonant soft-X-ray angle-resolved photoemission. We find an anomaly in orbital ordering of the mobile electrons in gamma-Al2O3/SrTiO3 which depopulates electron states in the top SrTiO3 layer. This rearrangement of the mobile electron system pushes the electron density away from the interface, which reduces its overlap with the interfacial defects and weakens the electron-phonon interaction, both effects contributing to the mobility boost. A crystal-field analysis shows that the band order alters owing to the symmetry breaking between the spinel gamma-Al2O3 and perovskite SrTiO3. Band-order engineering, exploiting the fundamental symmetry properties, emerges as another route to boost the performance of oxide devices.
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| 2. |
- Ekström, Erik, 1989-
(författare)
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Epitaxial growth, structure, and thermoelectric properties of CaMn- and V-based oxides
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is focused on physical vapor deposition, epitaxy, and structural characterization of CaMnO3 and VO2. Environmentally friendly and abundant materials are important for energy savings in applications, e.g., thermoelectrics for waste-heat recycling and thermochromic materials for passive indoor-temperature regulation. CaMnO3 is thermoelectric and VO2 is thermochromic at attractive temperatures for waste heat recycling and passive indoor-temperature regulation, respectively. The two material systems in this thesis were grown by reactive magnetron sputtering. A two-step synthesis process was investigated where rock-salt (Ca,Mn)O was grown by sputtering followed by an annealing step to form the perovskite CaMnO3. CaMnO3 was alloyed with Nb with the purpose to enhance the thermoelectric properties by increasing the number of free carriers, resulting in CaMn1-xNbxO3 (x = 0 – 0.10) films. CaMnO3 was grown on Al2O3 and LaAlO3 while VO2 was grown on muscovite (mica), phase-pure films of the metastable B phase and the stable M1 phase could be grown by controlling the gas flow and pressure. The mica substrate is a material with weakly interacting van der Waals layers in the structure that enable van der Waals epitaxy for the stable VO2 film in M1 phase. The thickness effect on electrical properties in the thermochromic region was investigated as well as proving van der Waals epitaxy. This thesis therefore provides an overall contribution to the understanding of film structure and the control thereof and how it affects the properties of the film.
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| 3. |
- Jovari, Pal, et al.
(författare)
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Atomic structure of glassy Mg60Cu30Y10 investigated with EXAFS, x-ray and neutron diffraction, and reverse Monte Carlo simulations
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:5, s. 054208-
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Tidskriftsartikel (refereegranskat)abstract
- Short range order of amorphous Mg60Cu30Y10 was investigated by x-ray and neutron diffraction, Cu and Y K-edge x-ray absorption fine structure measurements, and the reverse Monte Carlo simulation technique. We found that Mg-Mg and Mg-Cu nearest neighbor distances are very similar to values found in crystalline Mg2Cu. The Cu-Y coordination number is 1.1 +/- 0.2, and the Cu-Y distance is similar to 4% shorter than the sum of atomic radii, suggesting that attraction between Cu and Y plays an important role in stabilizing the glassy state. Thermal stability and structure evolution upon annealing were also studied by differential scanning calorimetry and in situ x-ray powder diffraction. The alloy shows a glass transition and three crystallization events, the first and dominant one at 456 K corresponding to eutectic crystallization of at least three phases: Mg2Cu and most likely cubic MgY and CuMgY.
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| 4. |
- Kerdsongpanya, Sit, et al.
(författare)
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Anomalously high thermoelectric power factor in epitaxial ScN thin films
- 2011
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 99:23, s. 232113-
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Tidskriftsartikel (refereegranskat)abstract
- Thermoelectric properties of ScN thin films grown by reactive magnetron sputtering on Al2O3(0001) wafers are reported. X-ray diffraction and elastic recoil detection analyses show that the composition of the films is close to stoichiometry with trace amounts (similar to 1 at. % in total) of C, O, and F. We found that the ScN thin-film exhibits a rather low electrical resistivity of similar to 2.94 mu Omega m, while its Seebeck coefficient is approximately similar to-86 mu V/K at 800 K, yielding a power factor of similar to 2.5 x 10(-3) W/mK(2). This value is anomalously high for common transition-metal nitrides.
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| 5. |
- Prawiroatmodjo, Guenevere E.D.K., et al.
(författare)
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Transport and excitations in a negative-U quantum dot at the LaAlO3/SrTiO3 interface
- 2017
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- In a solid-state host, attractive electron-electron interactions can lead to the formation of local electron pairs which play an important role in the understanding of prominent phenomena such as high T c superconductivity and the pseudogap phase. Recently, evidence of a paired ground state without superconductivity was demonstrated at the level of single electrons in quantum dots at the interface of LaAlO3 and SrTiO3. Here, we present a detailed study of the excitation spectrum and transport processes of a gate-defined LaAlO3/SrTiO3 quantum dot exhibiting pairing at low temperatures. For weak tunneling, the spectrum agrees with calculations based on the Anderson model with a negative effective charging energy U, and exhibits an energy gap corresponding to the Zeeman energy of the magnetic pair-breaking field. In contrast, for strong coupling, low-bias conductance is enhanced with a characteristic dependence on temperature, magnetic field and chemical potential consistent with the charge Kondo effect.
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| 6. |
- Sillassen, Michael, et al.
(författare)
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Low-Temperature Superionic Conductivity in Strained Yttria-Stabilized Zirconia
- 2010
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Ingår i: ADVANCED FUNCTIONAL MATERIALS. - : John Wiley and Sons, Ltd. - 1616-301X .- 1616-3028. ; 20:13, s. 2071-2076
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Tidskriftsartikel (refereegranskat)abstract
- Very high lateral ionic conductivities in epitaxial cubic yttria-stabilized zirconia (YSZ) synthesized on single-crystal SrTiO3and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order similar to 1 Omega(-1)cm(-1)) are observed at 500 degrees C for 58-nm-thick films on MgO. The results indicate a superposition of two parallel contributions - one due to bulk conductivity and one attributable to conduction along the film substrate interface. Interfacial effects dominate the conductivity at low temperatures (andlt;350 degrees C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk-like conductivity is observed. The films have a negligible grain-boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150-500 degrees C are of great fundamental importance but may also be technologically relevant for low-temperature applications.
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| 7. |
- Xu, Jianxiao, et al.
(författare)
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High thermoelectric performance of reduced lanthanide molybdenum oxides densified by spark plasma sintering
- 2010
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 500:1, s. 22-25
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Tidskriftsartikel (refereegranskat)abstract
- Four highly reduced molybdenum oxides LnMo(8)O(14) (Ln = La, Ce, Nd and Sm) containing bicapped Mo-8 clusters were synthesized via solid state reaction followed by spark plasma sintering. The thermoelectric properties were investigated, and NdMo8O14 exhibits the best performance with the maximum power factor of 177 mu W/mK(2) at 1000K. The highest ZT of NdMo8O14 was determined to be around 0.1 at 1000 K.
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