A. Annadi | University of Chicago (original) (raw)
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Papers by A. Annadi
Nature Communications, 2013
Scientific Reports, 2015
The 2D electron gas at the polar/non-polar oxide interface has become an important platform for s... more The 2D electron gas at the polar/non-polar oxide interface has become an important platform for several novel oxide electronic devices. In this paper, the transport properties of a wide range of polar perovskite oxide ABO3/SrTiO3 (STO) interfaces, where ABO3 includes LaAlO3, PrAlO3, NdAlO3, NdGaO3 and LaGaO3 in both crystalline and amorphous forms, were investigated. A robust 4 unit cell (uc) critical thickness for metal insulator transition was observed for crystalline polar layer/STO interface while the critical thickness for amorphous ones was strongly dependent on the B site atom and its oxygen affinity. For the crystalline interfaces, a sharp transition to the metallic state (i.e. polarization catastrophe induced 2D electron gas only) occurs at a growth temperature of 515 °C which corresponds to a critical relative crystallinity of ~70 ± 10% of the LaAlO3 overlayer. This temperature is generally lower than the metal silicide formation temperature and thus offers a route to integrate oxide heterojunction based devices on silicon.
Scientific Reports, 2014
High-k dielectric oxides are supposedly ideal gate-materials for ultra-high doping in graphene an... more High-k dielectric oxides are supposedly ideal gate-materials for ultra-high doping in graphene and other 2D-crystals. Here, we report a temperature-dependent electronic transport study on chemical vapor deposited-graphene gated with SrTiO₃ (STO) thin film substrate. At carrier densities away from charge neutrality point the temperature-dependent resistivity of our graphene samples on both STO and SiO₂/Si substrates show metallic behavior with contributions from Coulomb scattering and flexural phonons attributable to the presence of characteristic quasi-periodic nano-ripple arrays. Significantly, for graphene samples on STO substrates we observe an anomalous 'slope-break' in the temperature-dependent resistivity for T = 50 to 100 K accompanied by a decrease in mobility above 30 K. Furthermore, we observe an unusual decrease in the gate-induced doping-rate at low temperatures, despite an increase in dielectric constant of the substrate. We believe that a complex mechanism is at play as a consequence of the structural phase transition of the underlying substrate showing an anomalous transport behavior in graphene on STO. The anomalies are discussed in the context of Coulomb as well as phonon scattering.
A Collection of Extended Abstracts of the Nature Conference Frontiers in Electronic Materials, June 17 th to 20 th 2012, Aachen, Germany, 2012
Physical Review B, 2014
ABSTRACT
Applied Physics Letters, 2014
Applied Physics Letters, 2014
ABSTRACT We report magnetotransport properties of double heterointerfaces in LaAlO3/SrTiO3/LaAlO3... more ABSTRACT We report magnetotransport properties of double heterointerfaces in LaAlO3/SrTiO3/LaAlO3/SrTiO3(001) (LAO/STO/LAO/STO). A strong nonlinearity in the Hall resistivity is found when the temperature is below 80 K. This effect is attributed to multichannel conduction of interfacial charges generated in double heterostructures of LAO/STO where two-dimensional electron gas (2DEG) is produced. The multichannel conduction is confirmed by back gating modulation of Hall effect. Our result suggests the possibility to achieve coupled bilayer 2DEG layers in LAO/STO superlattices.
Physical Review Letters, 2011
Physical Review B, 2013
ABSTRACT Localization of electrons in the two-dimensional electron gas at the LaAlO_{3}/SrTiO_{3}... more ABSTRACT Localization of electrons in the two-dimensional electron gas at the LaAlO_{3}/SrTiO_{3} interface is investigated by varying the channel thickness in order to establish the nature of the conducting channel. Layers of SrTiO_{3} were grown on NdGaO_{3} (110) substrates and capped with LaAlO_{3}. When the SrTiO_{3} thickness is ≤6 unit cells, most electrons at the interface are localized, but when the number of SrTiO_{3} layers is 8–16, the free carrier density approaches 3.3×10^{14} cm^{−2}, the value corresponding to charge transfer of 0.5 electrons per unit cell at the interface. The number of delocalized electrons decreases again when the SrTiO_{3} thickness is ≥20 unit cells. The ∼4 nm conducting channel is therefore located significantly below the interface. The results are explained in terms of Anderson localization and the position of the mobility edge with respect to the Fermi level.
Physical Review B, 2013
ABSTRACT The confinement of the two-dimensional electron gas (2DEG), preferential occupancy of th... more ABSTRACT The confinement of the two-dimensional electron gas (2DEG), preferential occupancy of the Ti 3d orbital, and strong spin-orbit coupling at the LaAlO3/SrTiO3 interface play a significant role in its emerging properties. Here we report a fourfold oscillation in the anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) at the LaAlO3/SrTiO3 heterointerface. We evaluate the carrier confinement effects on the AMR and find that the fourfold oscillation appears only for the case of 2DEG systems while it is twofold for the three-dimensional system. The AMR behavior is further found to be sensitive to applied electric field, emphasizing the significance of spin-orbit coupling at the interface. These confinement effects suggest that the magnetic interactions are predominant at the interface, and the gate electric field modulation of AMR shows the tunability of magnetic interactions. As the fourfold oscillation fits well to the phenomenological model for a cubic symmetry system, this suggests that the origin of the oscillation may be linked to the anisotropy in magnetic scattering arising from the interaction of electrons with the localized magnetic moments coupled to the crystal symmetry. The observed large PHE further indicates the in-plane nature of magnetic ordering that arises from the in-plane Ti 3dxy orbitals.
Nature Communications, 2011
Nature Communications, 2013
Scientific Reports, 2015
The 2D electron gas at the polar/non-polar oxide interface has become an important platform for s... more The 2D electron gas at the polar/non-polar oxide interface has become an important platform for several novel oxide electronic devices. In this paper, the transport properties of a wide range of polar perovskite oxide ABO3/SrTiO3 (STO) interfaces, where ABO3 includes LaAlO3, PrAlO3, NdAlO3, NdGaO3 and LaGaO3 in both crystalline and amorphous forms, were investigated. A robust 4 unit cell (uc) critical thickness for metal insulator transition was observed for crystalline polar layer/STO interface while the critical thickness for amorphous ones was strongly dependent on the B site atom and its oxygen affinity. For the crystalline interfaces, a sharp transition to the metallic state (i.e. polarization catastrophe induced 2D electron gas only) occurs at a growth temperature of 515 °C which corresponds to a critical relative crystallinity of ~70 ± 10% of the LaAlO3 overlayer. This temperature is generally lower than the metal silicide formation temperature and thus offers a route to integrate oxide heterojunction based devices on silicon.
Scientific Reports, 2014
High-k dielectric oxides are supposedly ideal gate-materials for ultra-high doping in graphene an... more High-k dielectric oxides are supposedly ideal gate-materials for ultra-high doping in graphene and other 2D-crystals. Here, we report a temperature-dependent electronic transport study on chemical vapor deposited-graphene gated with SrTiO₃ (STO) thin film substrate. At carrier densities away from charge neutrality point the temperature-dependent resistivity of our graphene samples on both STO and SiO₂/Si substrates show metallic behavior with contributions from Coulomb scattering and flexural phonons attributable to the presence of characteristic quasi-periodic nano-ripple arrays. Significantly, for graphene samples on STO substrates we observe an anomalous 'slope-break' in the temperature-dependent resistivity for T = 50 to 100 K accompanied by a decrease in mobility above 30 K. Furthermore, we observe an unusual decrease in the gate-induced doping-rate at low temperatures, despite an increase in dielectric constant of the substrate. We believe that a complex mechanism is at play as a consequence of the structural phase transition of the underlying substrate showing an anomalous transport behavior in graphene on STO. The anomalies are discussed in the context of Coulomb as well as phonon scattering.
A Collection of Extended Abstracts of the Nature Conference Frontiers in Electronic Materials, June 17 th to 20 th 2012, Aachen, Germany, 2012
Physical Review B, 2014
ABSTRACT
Applied Physics Letters, 2014
Applied Physics Letters, 2014
ABSTRACT We report magnetotransport properties of double heterointerfaces in LaAlO3/SrTiO3/LaAlO3... more ABSTRACT We report magnetotransport properties of double heterointerfaces in LaAlO3/SrTiO3/LaAlO3/SrTiO3(001) (LAO/STO/LAO/STO). A strong nonlinearity in the Hall resistivity is found when the temperature is below 80 K. This effect is attributed to multichannel conduction of interfacial charges generated in double heterostructures of LAO/STO where two-dimensional electron gas (2DEG) is produced. The multichannel conduction is confirmed by back gating modulation of Hall effect. Our result suggests the possibility to achieve coupled bilayer 2DEG layers in LAO/STO superlattices.
Physical Review Letters, 2011
Physical Review B, 2013
ABSTRACT Localization of electrons in the two-dimensional electron gas at the LaAlO_{3}/SrTiO_{3}... more ABSTRACT Localization of electrons in the two-dimensional electron gas at the LaAlO_{3}/SrTiO_{3} interface is investigated by varying the channel thickness in order to establish the nature of the conducting channel. Layers of SrTiO_{3} were grown on NdGaO_{3} (110) substrates and capped with LaAlO_{3}. When the SrTiO_{3} thickness is ≤6 unit cells, most electrons at the interface are localized, but when the number of SrTiO_{3} layers is 8–16, the free carrier density approaches 3.3×10^{14} cm^{−2}, the value corresponding to charge transfer of 0.5 electrons per unit cell at the interface. The number of delocalized electrons decreases again when the SrTiO_{3} thickness is ≥20 unit cells. The ∼4 nm conducting channel is therefore located significantly below the interface. The results are explained in terms of Anderson localization and the position of the mobility edge with respect to the Fermi level.
Physical Review B, 2013
ABSTRACT The confinement of the two-dimensional electron gas (2DEG), preferential occupancy of th... more ABSTRACT The confinement of the two-dimensional electron gas (2DEG), preferential occupancy of the Ti 3d orbital, and strong spin-orbit coupling at the LaAlO3/SrTiO3 interface play a significant role in its emerging properties. Here we report a fourfold oscillation in the anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) at the LaAlO3/SrTiO3 heterointerface. We evaluate the carrier confinement effects on the AMR and find that the fourfold oscillation appears only for the case of 2DEG systems while it is twofold for the three-dimensional system. The AMR behavior is further found to be sensitive to applied electric field, emphasizing the significance of spin-orbit coupling at the interface. These confinement effects suggest that the magnetic interactions are predominant at the interface, and the gate electric field modulation of AMR shows the tunability of magnetic interactions. As the fourfold oscillation fits well to the phenomenological model for a cubic symmetry system, this suggests that the origin of the oscillation may be linked to the anisotropy in magnetic scattering arising from the interaction of electrons with the localized magnetic moments coupled to the crystal symmetry. The observed large PHE further indicates the in-plane nature of magnetic ordering that arises from the in-plane Ti 3dxy orbitals.
Nature Communications, 2011