Browsing by Author "Guo, G. Y."

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Hidden lattice instabilities as origin of the conductive interface between insulating LaAlO3 and SrTiO3
(2016-09-01)
Lee, P. W.
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Singh, V. N.
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Guo, G. Y.
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Liu, H. -J.
;
Lin, J. -C.
The metallic interface between insulating LaAlO3 and SrTiO3 opens up the field of oxide electronics. With more than a decade of researches on this heterostructure, the origin of the interfacial conductivity, however, remains unsettled. Here we resolve this long-standing puzzle by atomic-scale observation of electron-gas formation for screening hidden lattice instabilities, rejuvenated near the interface by epitaxial strain. Using atomic-resolution imaging and electron spectroscopy, the generally accepted notions of polar catastrophe and cation intermixing for the metallic interface are discounted. Instead, the conductivity onset at the critical thickness of 4-unit cell LaAlO3 on SrTiO3 substrate is accompanied with head-to-head ferroelectric-like polarizations across the interface due to strain-rejuvenated ferroelectric-like instabilities in the materials. The divergent depolarization fields of the head-to-head polarizations cast the interface into an electron reservoir, forming screening electron gas in SrTiO3 with LaAlO3 hosting complementary localized holes. The ferroelectriclike polarizations and electron-hole juxtaposition reveal the cooperative nature of metallic LaAlO3/SrTiO3.
Orbital polarization of the unoccupied states in multiferroic LiCu(2)O(2)
(2008-12-01)
Chen, C. L.
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Yeh, K. W.
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Huang, D. J.
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Hsu, F. C.
;
Lee, Y. C.
Using measurements of polarization-dependent soft-x-ray absorption and band-structure calculations including the on-site Coulomb interaction, we investigated the symmetry of the intrinsic 3d holes in the low-dimensional quantum magnet LiCu(2)O(2) with edge-sharing spin chains. The Cu L-edge and O K-edge measurements with in-plane and out-of-plane E vectors of x ray reveal the orbital character of the valence states of Cu on different sites. The unoccupied 3d states of nominal Cu(2+) exhibit predominantly xy symmetry, whereas 3d holes of 3z(2)-r(2) symmetry reside on the nominal Cu(+) sites. Both the experimental observations and theoretical calculations agree on the hybridization of the O 2p and Cu 3d states above the Fermi level. The existence of 3d holes on Cu(+) enhances the interlayer magnetic coupling.
Orbital polarization of the unoccupied states in multiferroic LiCu2O2
(2008-12-01)
Chen, C. L.
;
Yeh, K. W.
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Huang, D. J.
;
Hsu, F. C.
;
Lee, Y. C.
Using measurements of polarization-dependent soft-x-ray absorption and band-structure calculations including the on-site Coulomb interaction, we investigated the symmetry of the intrinsic 3d holes in the low-dimensional quantum magnet LiCu2O2 with edge-sharing spin chains. The Cu L-edge and O K-edge measurements with in-plane and out-of-plane E vectors of x ray reveal the orbital character of the valence states of Cu on different sites. The unoccupied 3d states of nominal Cu2+ exhibit predominantly xy symmetry, whereas 3d holes of 3z(2)-r(2) symmetry reside on the nominal Cu+ sites. Both the experimental observations and theoretical calculations agree on the hybridization of the O 2p and Cu 3d states above the Fermi level. The existence of 3d holes on Cu+ enhances the interlayer magnetic coupling.
Picosecond Creation of Switchable Optomagnets from a Polar Antiferromagnet with Giant Photoinduced Kerr Rotations
(2019-08-30)
Sheu, Y. M.
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Chang, Y. M.
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Chang, C. P.
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Li, Y. H.
;
Babu, K. R.
On-demand spin orientation with a long polarized lifetime and an easily detectable signal is the ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, we demonstrate switchable optomagnet effects in (Fe1-xZnx)(2)Mo3O8, from which we can obtain tunable magnetization (spanning from -40% to 40% of a saturated magnetization) that is created from zero magnetization in the antiferromagnetic state without magnetic fields. It is accomplishable by utilizing circularly polarized laser pulses to excite spin-flip transitions in polar antiferromagnets that have no spin canting, traditionally hard to control without very strong magnetic fields. The spin controllability in (Fe1-xZnx)(2)Mo3O8 originates from its polar structure that breaks the crystal inversion symmetry, allowing distinct on-site d-d transitions for selective spin flip. By chemical doping, we exploit the phase competition between antiferromagnetic and ferrimagnetic states to enhance and stabilize the optomagnet effects, which result in long-lived photoinduced Kerr rotations. The present study creating switchable giant optomagnet effects in polar antiferromagnets sketches a new blueprint for the function of antiferromagnetic spintronics.