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A ferroelectric is a material with a polar structure whose polarity can be reversed (switched) by applying an electric field.sup.1,2. In metals, itinerant electrons screen electrostatic forces between ions, which explains in part why polar metals are very rare.sup.3-7. Screening also excludes external electric fields, apparently ruling out the possibility of ferroelectric switching. However, in principle, a thin enough polar metal could be sufficiently penetrated by an electric field to have its polarity switched. Here we show that the topological semimetal WTe.sub.2 provides an embodiment of this principle. Although monolayer WTe.sub.2 is centro-symmetric and thus non-polar, the stacked bulk structure is polar. We find that two- or three-layer WTe.sub.2 exhibits spontaneous out-of-plane electric polarization that can be switched using gate electrodes. We directly detect and quantify the polarization using graphene as an electric-field sensor.sup.8. Moreover, the polarization states can be differentiated by conductivity and the carrier density can be varied to modify the properties. The temperature at which polarization vanishes is above 350 kelvin, and even when WTe.sub.2 is sandwiched between graphene layers it retains its switching capability at room temperature, demonstrating a robustness suitable for applications in combination with other two-dimensional materials.sup.9-12.Two- and three-layer WTe.sub.2 exhibits spontaneous out-of-plane electric polarization that can be switched electrically at room temperature and is sufficiently robust for use in applications with other two-dimensional materials., Author(s): Zaiyao Fei [sup.1] , Wenjin Zhao [sup.1] , Tauno A. Palomaki [sup.1] , Bosong Sun [sup.1] , Moira K. Miller [sup.1] , Zhiying Zhao [sup.2] [sup.3] , Jiaqiang Yan [...]

The stability of spontaneous electrical polarization in ferroelectrics is fundamental to many of their current applications, which range from the simple electric cigarette lighter to non-volatile random access memories (1). Research on nanoscale ferroelectrics reveals that their behaviour is profoundly different from that in bulk ferroelectrics, which could lead to new phenomena with potential for future devices (2-4). As ferroelectrics become thinner, maintaining a stable polarization becomes increasingly challenging. On the other hand, intentionally destabilizing this polarization can cause the effective electric permittivity of a ferroelectric to become negative (5), enabling it to behave as a negative capacitance when integrated in a heterostructure. Negative capacitance has been proposed as a way of overcoming fundamental limitations on the power consumption of field-effect transistors (6). However, experimental demonstrations of this phenomenon remain contentious (7). The prevalent interpretations based on homogeneous polarization models are difficult to reconcile with the expected strong tendency for domain formation (8,9), but the effect of domains on negative capacitance has received little attention (5,10-12). Here we report negative capacitance in a model system of multidomain ferroelectric-dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model, we show that domain-wall motion not only gives rise to negative permittivity, but can also enhance, rather than limit, its temperature range. Our first-principles-based atomistic simulations provide detailed microscopic insight into the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation., Negative capacitance in ferroelectrics arises from the imperfect screening of the spontaneous polarization (5,10,13,14). Imperfect screening is intrinsic to semiconductor-ferroelectric, and even metal-ferroelectric, interfaces because of their finite effective screening [...]

The complex interplay of spin, charge, orbital and lattice degrees of freedom provides a plethora of exotic phases and physical phenomena (1-5) . In recent years, complex spin topologies have [...]

The anisotropy parameter values β of the L shell fluorescent X-rays (Lι ([L.sub.3] - [M.sub.1]), L[β.sub.6] ([L.sub.3] - [N.sub.1]), L[α.sub.1,2] ([L.sub.3] - [M.sub.5.4]) and L[β.sub.2,15] ([L.sub.3] - [N.sub.5,4]) of Bi, Pb, Ta, Lu, and Yb excited by 22.6 keV photons were measured. These values were used to determine the kinematic coefficient ratios α(Lι/L[α.sub.1,2]), α(Lι/L[β.sub.6]), and α(L[α.sub.1,2]/L[β.sub.2,15]). The Lι and L[β.sub.6] X-rays were found to be anisotropically emitted whereas L[α.sub.1,2] and L[β.sub.2,15] X-rays showed less anisotropic spatial distributions. Also, the polarization degrees of Lι and Lα X-rays were measured for the present elements and observed that the Lι lines were found to be strongly polarized whereas the Lα lines showed less polarization. PACS Nos.: 78.70.En, 61.80.Cb, 07.85.Nc, 32.30.Rj. Resume: Nous mesurons les valeurs β du parametre d'anisotropie des rayons X fluorescents de Lα couche L, (Lι ([L.sub.3]-[M.sub.1]), L[β.sub.6]([L.sub.3] - [N.sub.1]), L[α.sub.1,2] ([L.sub.3] - [M.sub.5.4]) et L[β.sub.2,15] ([L.sub.3]-[N.sub.5.4]), des elements Bi, Pb, Ta, Lu et Yb excites par des photons de 22.6 keV. Nous utilisons les valeurs obtenues pour determiner les rapports des coefficients cinematique α(Lι/L[α.sub.1,2]), α(Lι/L[β.sub.6]) et α(L[α.sub.1,2]/L[β.sub.2,15]). Nous trouvons que les rayons X Lι et L[β.sub.6] sont emis de facon anisotrope, alors que L[α.sub.1,2] et L[β.sub.2.45] montrent moins d'anisotropie spatiale. Nous mesurons egalement le degre de polarisation des rayons X Lι et Lα et notons que les lignes de Lι sont fortement polarisees, alors que celles de Lα le sont beaucoup moins. [Traduit par Lα Redaction], 1. Introduction The K shell and [L.sub.1] and [L.sub.2] magnetic subshell vacancy states excited with charged particles (protons, electrons, and heavy ions) or photons whose total angular momenta are J [...]

Ferroelectric and ferromagnetic materials exhibit long-range order of atomic-scale electric or magnetic dipoles that can be switched by applying an appropriate electric or magnetic field, respectively. Both switching phenomena form the basis of non-volatile random access memory (1), but in the ferroelectric case, this involves destructive electrical reading and in the magnetic case, a high writing energy is required (2). In principle, low-power and high-density information storage that combines fast electrical writing and magnetic reading can be realized with magnetoelectric multiferroic materials (3). These materials not only simultaneously display ferroelectricity and ferromagnetism, but also enable magnetic moments to be induced by an external electric field, or electric polarization by a magnetic field (4,5). However, synthesizing bulk materials with both long-range orders at room temperature in a single crystalline structure is challenging because conventional ferroelectricity requires closed-shell [d.sup.0] or [s.sup.2] cations, whereas ferromagnetic order requires open-shell [d.sup.n] configurations with unpaired electrons (6). These opposing requirements pose considerable difficulties for atomic-scale design strategies such as magnetic ion substitution into ferroelectrics (7,8). One material that exhibits both ferroelectric and magnetic order is BiFe[O.sub.3], but its cycloidal magnetic structure (9) precludes bulk magnetization and linear magnetoelectric coupling (10). A solid solution of a ferroelectric and a spin-glass perovskite combines switchable polarization (11) with glassy magnetization, although it lacks long-range magnetic order (12). Crystal engineering of a layered perovskite has recently resulted in room-temperature polar ferromagnets (13), but the electrical polarization has not been switchable. Here we combine ferroelectricity and ferromagnetism at room temperature in a bulk perovskite oxide, by constructing a percolating network of magnetic ions with strong superexchange interactions within a structural scaffold exhibiting polar lattice symmetries at a morphotropic phase boundary (14) (the compositional boundary between two polar phases with different polarization directions, exemplified by the PbZr[O.sub.3]-PbTi[O.sub.3] system) that both enhances polarization switching and permits canting of the ordered magnetic moments. We expect this strategy to allow the generation of a range of tunable multiferroic materials., Several approaches to room-temperature multiferroicity have been explored. Composite multiferroics, which are multiphase mixtures of magnetic and ferroelectric materials, have displayed the largest magnetoelectric effects, originating from stress-mediated coupling (15). [...]

Diodes are devices that conduct electric current mainly in one direction. An electrically polar film that acts as a diode for superconducting current could lead to electronic devices that have ultralow power consumption. A superconducting diode effect., Author(s): Toshiya Ideue, Yoshihiro Iwasa Author Affiliations: One-way supercurrent achieved in an electrically polar film An essential process in modern electronics is rectification, whereby bidirectional electric current is converted to [...]