scholarly journals Scale-free ferroelectricity induced by flat phonon bands in HfO2

Science ◽  
2020 ◽  
Vol 369 (6509) ◽  
pp. 1343-1347 ◽  
Author(s):  
Hyun-Jae Lee ◽  
Minseong Lee ◽  
Kyoungjun Lee ◽  
Jinhyeong Jo ◽  
Hyemi Yang ◽  
...  
Keyword(s):  
Domain Walls ◽  
Hafnium Dioxide ◽  
Unit Cell ◽  
Localized States ◽  
Energy Bands ◽  
Scale Free ◽  
Reduced Dimensions ◽  
Domain Wall Energy ◽  
Electric Dipoles ◽  
Ferroelectric Switching

Discovery of robust yet reversibly switchable electric dipoles at reduced dimensions is critical to the advancement of nanoelectronics devices. Energy bands flat in momentum space generate robust localized states that are activated independently of each other. We determined that flat bands exist and induce robust yet independently switchable dipoles that exhibit a distinct ferroelectricity in hafnium dioxide (HfO2). Flat polar phonon bands in HfO2 cause extreme localization of electric dipoles within its irreducible half-unit cell widths (~3 angstroms). Contrary to conventional ferroelectrics with spread dipoles, those intrinsically localized dipoles are stable against extrinsic effects such as domain walls, surface exposure, and even miniaturization down to the angstrom scale. Moreover, the subnanometer-scale dipoles are individually switchable without creating any domain-wall energy cost. This offers unexpected opportunities for ultimately dense unit cell–by–unit cell ferroelectric switching devices that are directly integrable into silicon technology.

Unexpectedly low barrier of ferroelectric switching in HfO2 via topological domain walls

2021 ◽  
Author(s):  
Duk-Hyun Choe ◽  
Sunghyun Kim ◽  
Taehwan Moon ◽  
Sanghyun Jo ◽  
Hagyoul Bae ◽  
...  
Keyword(s):  
Domain Walls ◽  
Topological Domain ◽  
Ferroelectric Switching

Electronic Structure Transformation in a Magnetized Topological Semimetal

2021 ◽  
Vol 1 ◽  
Keyword(s):  
Electronic Structure ◽  
Domain Walls ◽  
Magnetic Domain ◽  
Localized States ◽  
Structure Transformation ◽  
Nodal Lines ◽  
Topological Semimetal ◽  
Magnetic Domain Walls ◽  
Magnetic Orders ◽  
Topological Semimetals

We theoretically show that the nodal structures in topological semimetals, including Weyl points and nodal lines, can be switched by magnetic orders, accompanied by localized states at magnetic domain walls.

scholarly journals Electrostatic effects, band distortions, and superconductivity in twisted graphene bilayers

2018 ◽  
Vol 115 (52) ◽  
pp. 13174-13179 ◽  
Author(s):  
Francisco Guinea ◽  
Niels R. Walet
Keyword(s):  
Electrostatic Potential ◽  
Small Angle ◽  
Dirac Point ◽  
Unit Cell ◽  
Low Energy ◽  
Energy Bands ◽  
Dependent Change ◽  
Charge Modulation ◽  
Twisted Graphene ◽  
Narrow Bands

Bilayer graphene twisted by a small angle shows a significant charge modulation away from neutrality, as the charge in the narrow bands near the Dirac point is mostly localized in a fraction of the Moiré unit cell. The resulting electrostatic potential leads to a filling-dependent change in the low-energy bands, of a magnitude comparable to or larger than the bandwidth. These modifications can be expressed in terms of new electron–electron interactions, which, when expressed in a local basis, describe electron-assisted hopping terms. These interactions favor superconductivity at certain fillings.

A MODEL OF BOSE–EINSTEIN CONDENSATION WITH ITINERANT AND LOCALIZED STATES

2005 ◽  
Vol 19 (21) ◽  
pp. 1011-1034
Author(s):  
FUXIANG HAN ◽  
ZHIRU REN ◽  
YUN'E GAO
Keyword(s):  
Critical Temperature ◽  
Anderson Model ◽  
Localized States ◽  
Mean Field Approximation ◽  
Einstein Condensation ◽  
Model Parameters ◽  
Zeroth Order ◽  
Energy Bands ◽  
Bose Einstein Condensation ◽  
Bose Einstein

We propose a model that includes itinerant and localized states to study Bose–Einstein condensation of ultracold atoms in optical lattices (Bose–Anderson model). It is found that the original itinerant and localized states intermix to give rise to a new energy band structure with two quasiparticle energy bands. We have computed the critical temperature Tc of the Bose–Einstein condensation of the quasiparticles in the Bose–Anderson model using our newly developed numerical algorithm and found that Tc increases as na3 (the number density times the lattice constant cubed) increases according to the power law Tc≈18.93(na3)0.59 nK for na3<0.125 and according to the linear relation Tc≈8.75+10.53na3 nK for 1.25<na3<12.5 for the given model parameters. With the self-consistent equations for the condensation fractions obtained within the Bogoliubov mean-field approximation, the effects of the on-site repulsion U on the quasiparticle condensation are investigated. We have found that, for values up to several times the zeroth-order critical temperature, U enhances the zeroth-order condensation fraction at intermediate temperatures and effectively raises the critical temperature, while it slightly suppresses the zeroth-order condensation fraction at very low temperatures.

Magnetic Domains Observation from Bitter Patterns of NdFeB Alloy

2014 ◽  
Vol 802 ◽  
pp. 569-573 ◽  
Author(s):  
Raphael Lemos ◽  
Kaio Sérgio T. de Souza ◽  
Fernanda A. Sampaio da Silva ◽  
Daniel Rodrigues ◽  
José Adilson de Castro ◽  
...  
Keyword(s):  
Domain Walls ◽  
Cast Alloy ◽  
Colloid Solution ◽  
Magnetic Domains ◽  
Strong Anisotropy ◽  
Easy Magnetization ◽  
Domain Wall Energy ◽  
Magnetization Axis ◽  
Metallographic Preparation ◽  
Adequate Method

The Bitter method is an adequate method for domains observation, and depends on careful metallographic preparation and also on a proper magnetic colloid solution. In this study, Bitter patterns were obtained for a Nd-Fe-B as-cast alloy. The Nd2Fe14B grains show strong anisotropy during growing, and are larger for the direction perpendicular to the easy magnetization axis. The equilibrium distance between domain walls allows an estimate of intrinsic parameters of ferromagnetic phases as the domain wall energy and the single domain particle size.

Atomic-scale study of electric dipoles near charged and uncharged domain walls in ferroelectric films

2007 ◽  
Vol 7 (1) ◽  
pp. 57-61 ◽  
Author(s):  
Chun-Lin Jia ◽  
Shao-Bo Mi ◽  
Knut Urban ◽  
Ionela Vrejoiu ◽  
Marin Alexe ◽  
...  
Keyword(s):  
Domain Walls ◽  
Atomic Scale ◽  
Ferroelectric Films ◽  
Electric Dipoles

scholarly journals Charged Domain Wall and Polar Vortex Topologiesin a Room Temperature Magnetoelectric Multiferroic Thin Film

2021 ◽  
Author(s):  
Kalani Moore ◽  
Eoghan O'Connell ◽  
Lynette Keeney ◽  
Clive Downing ◽  
Michael Schmidt ◽  
...  
Keyword(s):  
Thin Film ◽  
Domain Wall ◽  
Room Temperature ◽  
Domain Walls ◽  
Strain Engineering ◽  
Unit Cell ◽  
Tail Domain ◽  
Aurivillius Phase ◽  
Topological Features ◽  
Dielectric Layers

Abstract Multiferroic domain walls are an emerging solution for future low-power nanoelectronics due to their combined tuneable functionality and mobility. Here we show that the magnetoelectric multiferroic Aurivillius phase Bi6TixFeyMnzO18 (B6TFMO) crystal is an ideal platform for domain wall-based nanoelectronic devices. The unit cell of B6TFMO is distinctive as it consists of a multiferroic layer between dielectric layers. We utilise atomic resolution scanning transmission electron microscopy and spectroscopy to map the sub-unit-cell polarisation in B6TFMO thin films. 180˚ charged head-to-head and tail-to-tail domain walls are found to pass through > 8 ferroelectric-dielectric layers of the film. They are structurally similar to BiFeO3 DWs but contain a large surface charge density (σ_s) = 1.09 |e|per perovskite cell, where |e| is elementary charge. Although polarisation is primarily in-plane, c-axis polarisation is identified at head-to-tail domain walls with an associated electromechanical coupling of strain and polarisation. Finally, we reveal that with controlled strain engineering during thin film growth, room-temperature vortexes are formed in the ferroelectric layer. These results confirm that sub-unit-cell topological features can play an important role in controlling the conduction properties and magnetisation state of Aurivillius phase films and other multiferroic heterostructures.

Computer simulation of ferroelastic phase transition in LaNbO4

1997 ◽  
Vol 12 (9) ◽  
pp. 2428-2437 ◽  
Author(s):  
K. Parlinski ◽  
Y. Hashi ◽  
S. Tsunekawa ◽  
Y. Kawazoe
Keyword(s):  
Phase Transition ◽  
Domain Walls ◽  
Unit Cell ◽  
Model Parameters ◽  
Domain Pattern ◽  
Degree Of Deformation ◽  
Ferroelastic Phase Transition ◽  
Tetragonal Unit Cell ◽  
Transmission Electron ◽  
Potential Parameters

A model of lanthanum orthoniobate which possesses a ferroelastic tetragonalmonoclinic phase transition is proposed. It contains only one particle per unit cell, but it is constructed consistently with symmetry changes at the phase transition. The model parameters are chosen to reproduce the bare soft mode, degree of deformation of the tetragonal unit cell to a monoclinic one, and the phase transition temperature. The ferroelastic system with free boundary conditions was simulated by the molecular dynamics technique, and the second order phase transition was reproduced. The studied annealing process shows formation of the stripe lenticular domain pattern, which has been interrupted by the appearance of a temporary band of perpendicularly oriented lenticular domains. The maps contain W′-type domain walls whose orientations are fixed only by interplay of potential parameters and not by symmetry elements. The simulated domain pattern has the same features as those observed by transmission electron microscopy.

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