@article {Yadav2019E13,
title = {Author Correction: Spatially resolved steady-state negative capacitance (Nature, (2019), 565, 7740, (468-471), 10.1038/s41586-018-0855-y)},
journal = {Nature},
volume = {568},
number = {7753},
year = {2019},
note = {cited By 0},
pages = {E13},
publisher = {Nature Publishing Group},
abstract = {In this Letter, the first name of author Bhagwati Prasad was misspelled Bhagawati. This error has been corrected online. {\textcopyright} 2019, The Author(s), under exclusive licence to Springer Nature Limited.},
keywords = {erratum, error},
issn = {00280836},
doi = {10.1038/s41586-019-1106-6},
author = {A.K. Yadav and K.X. Nguyen and Z. Hong and P. Garc{\'\i}a-Fern{\'a}ndez and P. Aguado-Puente and C.T. Nelson and S. Das and B. Prasad and D. Kwon and S. Cheema and A.I. Khan and C. Hu and J. {\'I}{\~n}iguez and J. Junquera and L.-Q. Chen and D.A. Muller and Ramamoorthy Ramesh and S. Salahuddin}
}
@article {Hsu2019,
title = {Emergence of the Vortex State in Confined Ferroelectric Heterostructures},
journal = {Advanced Materials},
volume = {31},
number = {36},
year = {2019},
note = {cited By 2},
publisher = {Wiley-VCH Verlag},
abstract = {The manipulation of charge and lattice degrees of freedom in atomically precise, low-dimensional ferroelectric superlattices can lead to exotic polar structures, such as a vortex state. The role of interfaces in the evolution of the vortex state in these superlattices (and the associated electrostatic and elastic boundary conditions they produce) has remained unclear. Here, the toroidal state, arranged in arrays of alternating clockwise/counterclockwise polar vortices, in a confined SrTiO3/PbTiO3/SrTiO3 trilayer is investigated. By utilizing a combination of transmission electron microscopy, synchrotron-based X-ray diffraction, and phase-field modeling, the phase transition as a function of layer thickness (number of unit cells) demonstrates how the vortex state emerges from the ferroelectric state by varying the thickness of the confined PbTiO3 layer. Intriguingly, the vortex state arises at head-to-head domain boundaries in ferroelectric a1/a2 twin structures. In turn, by varying the total number of PbTiO3 layers (moving from trilayer to superlattices), it is possible to manipulate the long-range interactions among multiple confined PbTiO3 layers to stabilize the vortex state. This work provides a new understanding of how the different energies work together to produce this exciting new state of matter and can contribute to the design of novel states and potential memory applications. {\textcopyright} 2019 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
keywords = {Degrees of freedom (mechanics), Ferroelectric state, Ferroelectric superlattice, ferroelectricity, High resolution transmission electron microscopy, Interface states, Lead titanate, Long range interactions, Memory applications, Number of unit cells, Phase field models, phase transitions, Polar structures, Strontium titanates, titanium compounds, Vortex flow, Vortex state},
issn = {09359648},
doi = {10.1002/adma.201901014},
author = {S.-L. Hsu and M.R. McCarter and C. Dai and Z. Hong and L.-Q. Chen and C.T. Nelson and L.W. Martin and Ramamoorthy Ramesh}
}
@article {Das2019368,
title = {Observation of room-temperature polar skyrmions},
journal = {Nature},
volume = {568},
number = {7752},
year = {2019},
note = {cited By 31},
pages = {368-372},
publisher = {Nature Publishing Group},
abstract = {Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. For example, the recent discovery of polarization vortices and their associated complex-phase coexistence and response under applied electric fields in superlattices of (PbTiO3)n/(SrTiO3)n suggests the presence of a complex, multi-dimensional system capable of interesting physical responses, such as chirality, negative capacitance and large piezo-electric responses1{\textendash}3. Here, by varying epitaxial constraints, we discover room-temperature polar-skyrmion bubbles in a lead titanate layer confined by strontium titanate layers, which are imaged by atomic-resolution scanning transmission electron microscopy. Phase-field modelling and second-principles calculations reveal that the polar-skyrmion bubbles have a skyrmion number of +1, and resonant soft-X-ray diffraction experiments show circular dichroism, confirming chirality. Such nanometre-scale polar-skyrmion bubbles are the electric analogues of magnetic skyrmions, and could contribute to the advancement of ferroelectrics towards functionalities incorporating emergent chirality and electrically controllable negative capacitance. {\textcopyright} 2019, The Author(s), under exclusive licence to Springer Nature Limited.},
keywords = {chirality, circular dichroism, electric activity, electric capacitance, electric field, electromagnetism, Letter, Magnetism, Polarization, priority journal, room temperature, scanning transmission electron microscopy, titanium, transmission electron microscopy, X ray diffraction},
issn = {00280836},
doi = {10.1038/s41586-019-1092-8},
author = {S. Das and Y.L. Tang and Z. Hong and M.A.P. Gon{\c c}alves and M.R. McCarter and C. Klewe and K.X. Nguyen and F. G{\'o}mez-Ortiz and P. Shafer and E. Arenholz and V.A. Stoica and S.-L. Hsu and B. Wang and C. Ophus and J.F. Liu and C.T. Nelson and S. Saremi and B. Prasad and A.B. Mei and D.G. Schlom and J. {\'I}{\~n}iguez and P. Garc{\'\i}a-Fern{\'a}ndez and D.A. Muller and L.Q. Chen and J. Junquera and L.W. Martin and Ramamoorthy Ramesh}
}
@article {Stoica2019377,
title = {Optical creation of a supercrystal with three-dimensional nanoscale periodicity},
journal = {Nature Materials},
volume = {18},
number = {4},
year = {2019},
note = {cited By 9},
pages = {377-383},
publisher = {Nature Publishing Group},
abstract = {Stimulation with ultrafast light pulses can realize and manipulate states of matter with emergent structural, electronic and magnetic phenomena. However, these non-equilibrium phases are often transient and the challenge is to stabilize them as persistent states. Here, we show that atomic-scale PbTiO 3 /SrTiO 3 superlattices, counterpoising strain and polarization states in alternate layers, are converted by sub-picosecond optical pulses to a supercrystal phase. This phase persists indefinitely under ambient conditions, has not been created via equilibrium routes, and can be erased by heating. X-ray scattering and microscopy show this unusual phase consists of a coherent three-dimensional structure with polar, strain and charge-ordering periodicities of up to 30 nm. By adjusting only dielectric properties, the phase-field model describes this emergent phase as a photo-induced charge-stabilized supercrystal formed from a two-phase equilibrium state. Our results demonstrate opportunities for light-activated pathways to thermally inaccessible and emergent metastable states. {\textcopyright} 2019, This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply.},
keywords = {Coherent scattering, Dielectric properties, Lead titanate, light, Magnetic phenomena, Nano-scale periodicity, Non-equilibrium phasis, Phase field models, Photo-induced charge, Three-dimensional structure, Two phase flow, Two-phase equilibria, Ultrafast light pulse, X ray scattering},
issn = {14761122},
doi = {10.1038/s41563-019-0311-x},
author = {V.A. Stoica and N. Laanait and C. Dai and Z. Hong and Y. Yuan and Z. Zhang and S. Lei and M.R. McCarter and A. Yadav and A.R. Damodaran and S. Das and G.A. Stone and J. Karapetrova and D.A. Walko and X. Zhang and L.W. Martin and Ramamoorthy Ramesh and L.-Q. Chen and H. Wen and V. Gopalan and J.W. Freeland}
}
@article {Yadav2019468,
title = {Spatially resolved steady-state negative capacitance},
journal = {Nature},
volume = {565},
number = {7740},
year = {2019},
note = {cited By 35},
pages = {468-471},
publisher = {Nature Publishing Group},
abstract = {Negative capacitance is a newly discovered state of ferroelectric materials that holds promise for electronics applications by exploiting a region of thermodynamic space that is normally not accessible1{\textendash}14. Although existing reports of negative capacitance substantiate the importance of this phenomenon, they have focused on its macroscale manifestation. These manifestations demonstrate possible uses of steady-state negative capacitance{\textemdash}for example, enhancing the capacitance of a ferroelectric{\textendash}dielectric heterostructure4,7,14 or improving the subthreshold swing of a transistor8{\textendash}12. Yet they constitute only indirect measurements of the local state of negative capacitance in which the ferroelectric resides. Spatial mapping of this phenomenon would help its understanding at a microscopic scale and also help to achieve optimal design of devices with potential technological applications. Here we demonstrate a direct measurement of steady-state negative capacitance in a ferroelectric{\textendash}dielectric heterostructure. We use electron microscopy complemented by phase-field and first-principles-based~(second-principles) simulations in SrTiO3/PbTiO3 superlattices to directly determine, with atomic resolution, the local regions in the ferroelectric material where a state of negative capacitance is stabilized. Simultaneous vector mapping of atomic displacements (related to a complex pattern in the polarization field), in conjunction with reconstruction of the local electric field, identify the negative~capacitance regions as those with higher energy density and larger polarizability: the domain walls where the polarization is suppressed. {\textcopyright} 2019, Springer Nature Limited.},
keywords = {chemical structure, electric capacitance, electric field, electrical parameters, energy density, ferroelectric dielectric heterostructure, Letter, negative capacitance, Polarization, priority journal, scanning transmission electron microscopy, simulation, steady state, X ray diffraction},
issn = {00280836},
doi = {10.1038/s41586-018-0855-y},
author = {A.K. Yadav and K.X. Nguyen and Z. Hong and P. Garc{\'\i}a-Fern{\'a}ndez and P. Aguado-Puente and C.T. Nelson and S. Das and B. Prasad and D. Kwon and S. Cheema and A.I. Khan and C. Hu and J. {\'I}{\~n}iguez and J. Junquera and L.-Q. Chen and D.A. Muller and Ramamoorthy Ramesh and S. Salahuddin}
}
@article {Damodaran20171003,
title = {Phase coexistence and electric-field control of toroidal order in oxide superlattices},
journal = {Nature Materials},
volume = {16},
number = {10},
year = {2017},
note = {cited By 50},
pages = {1003-1009},
publisher = {Nature Publishing Group},
abstract = {Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO"3/SrTiO"3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a"1/a"2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotropic electrotoroidal compounds. Finally, application of electric fields to this mixed-phase system permits interconversion between the vortex and the ferroelectric phases concomitant with order-of-magnitude changes in piezoelectric and nonlinear optical responses. Our findings suggest new cross-coupled functionalities. {\textcopyright} 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.},
keywords = {Characterization techniques, Condensed matter physics, Electric fields, Electric-field control, Ferroelectric domains, Ferroelectric phasis, ferroelectricity, First-order phase transitions, Nonlinear optical response, Oxide superlattices, Polarization, Superlattice periods, temperature, Vortex flow},
issn = {14761122},
doi = {10.1038/NMAT4951},
author = {A.R. Damodaran and J.D. Clarkson and Z. Hong and H. B. Liu and A.K. Yadav and C.T. Nelson and S.-L. Hsu and M.R. McCarter and K.-D. Park and V. Kravtsov and A. Farhan and Y. Dong and Z. Cai and H. Zhou and P. Aguado-Puente and P. Garc{\'\i}a-Fern{\'a}ndez and J. {\'I}{\~n}iguez and J. Junquera and A. Scholl and M.B. Raschke and L.-Q. Chen and D.D. Fong and Ramamoorthy Ramesh and L.W. Martin}
}
@article {Hong20172246,
title = {Stability of Polar Vortex Lattice in Ferroelectric Superlattices},
journal = {Nano Letters},
volume = {17},
number = {4},
year = {2017},
note = {cited By 36},
pages = {2246-2252},
publisher = {American Chemical Society},
abstract = {A novel mesoscale state comprising of an ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO3/SrTiO3. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper and a lower bound to the length scale at which these states can be observed. We found that the critical length is related to the intrinsic domain wall width, which could serve as a simple intuitive design rule for the discovery of novel ultrafine topological structures in ferroic systems. {\textcopyright} 2017 American Chemical Society.},
keywords = {Article, competition, Crystal lattices, Ferroelectric superlattice, ferroelectricity, Geometric length, Neodymium compounds, Phase-field simulation, Polar vortex, Polarization, simulation, Superconducting materials, Topological structure, Topology, Vortex flow},
issn = {15306984},
doi = {10.1021/acs.nanolett.6b04875},
author = {Z. Hong and A.R. Damodaran and F. Xue and S.-L. Hsu and J. Britson and A.K. Yadav and C.T. Nelson and J.-J. Wang and J.F. Scott and L.W. Martin and Ramamoorthy Ramesh and L.-Q. Chen}
}
@article {Yadav2016138,
title = {Erratum: Observation of polar vortices in oxide superlattices (Nature (2016) 530 (198-201) DOI:10.1038/nature16463)},
journal = {Nature},
volume = {534},
number = {7605},
year = {2016},
note = {cited By 3},
pages = {138},
publisher = {Nature Publishing Group},
keywords = {erratum, error},
issn = {00280836},
doi = {10.1038/nature17420},
author = {A.K. Yadav and C.T. Nelson and S.L. Hsu and Z. Hong and J.D. Clarkson and C.M. Schlep{\"u}tz and A.R. Damodaran and P. Shafer and E. Arenholz and L.R. Dedon and D. Chen and A. Vishwanath and A.M. Minor and L.Q. Chen and J.F. Scott and L.W. Martin and Ramamoorthy Ramesh}
}
@article {Yadav2016198,
title = {Observation of polar vortices in oxide superlattices},
journal = {Nature},
volume = {530},
number = {7589},
year = {2016},
note = {cited By 248},
pages = {198-201},
publisher = {Nature Publishing Group},
abstract = {The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena. In recent years, complex spin topologies have emerged as a consequence of the electronic band structure and the interplay between spin and spin-orbit coupling in materials. Here we produce complex topologies of electrical polarization-namely, nanometre-scale vortex-antivortex (that is, clockwise-anticlockwise) arrays that are reminiscent of rotational spin topologies-by making use of the competition between charge, orbital and lattice degrees of freedom in superlattices of alternating lead titanate and strontium titanate layers. Atomic-scale mapping of the polar atomic displacements by scanning transmission electron microscopy reveals the presence of long-range ordered vortex-antivortex arrays that exhibit nearly continuous polarization rotation. Phase-field modelling confirms that the vortex array is the low-energy state for a range of superlattice periods. Within this range, the large gradient energy from the vortex structure is counterbalanced by the corresponding large reduction in overall electrostatic energy (which would otherwise arise from polar discontinuities at the lead titanate/strontium titanate interfaces) and the elastic energy associated with epitaxial constraints and domain formation. These observations have implications for the creation of new states of matter (such as dipolar skyrmions, hedgehog states) and associated phenomena in ferroic materials, such as electrically controllable chirality. {\textcopyright} 2016 Macmillan Publishers Limited. All rights reserved.},
keywords = {Erinaceidae},
issn = {00280836},
doi = {10.1038/nature16463},
author = {A.K. Yadav and C.T. Nelson and S.L. Hsu and Z. Hong and J.D. Clarkson and C.M. Schlepu{\"e}tz and A.R. Damodaran and P. Shafer and E. Arenholz and L.R. Dedon and D. Chen and A. Vishwanath and A.M. Minor and L.Q. Chen and J.F. Scott and L.W. Martin and Ramamoorthy Ramesh}
}