The recent discovery of "polar metals" with ferroelectriclike displacements offers the promise of designing ferroelectrics with tunable energy gaps by inducing controlled metal-insulator transitions. Here we employ first-principles calculations to design a metallic polar superlattice from nonpolar metal components and show that controlled intermixing can lead to a true insulating ferroelectric with a tunable band gap. We consider a 2/2 superlattice made of two centrosymmetric metallic oxides, La_{0.75}Sr_{0.25}MnO_{3} and LaNiO_{3}, and show that ferroelectriclike displacements are induced. The ferroelectriclike distortion is found to be strongly dependent on the carrier concentration (Sr content). Further, we show that a metal-to-insulator (MI) transition is feasible in this system via disproportionation of the Ni sites. Such a disproportionation and, hence, a MI transition can be driven by intermixing of transition metal ions between Mn and Ni layers. As a result, the energy gap of the resulting ferroelectric can be tuned by varying the degree of intermixing in the experimental fabrication method.
One of the primary objectives in molecular nano-spintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this letter, we demonstrate by first principles density functional calculations, as well as second order perturbation thoery, that a strain induced change of the spin state, from S=1 $\to$ S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in a sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.
Discovering materials that display a linear magnetoelectric (ME) effect at room temperature is a challenge. Such materials could facilitate devices based on the electric field control of magnetism. Here we present simple, chemically intuitive design rules to identify a class of bulk magnetoelectric materials based on the ``bicolor'' layering of $Pbnm$ ferrite perovskites, e.g., ${\mathrm{LaFeO}}_{3}/{\mathrm{LnFeO}}_{3}$ superlattices, Ln = lanthanide cation. We use first-principles density functional theory calculations to confirm these ideas. We elucidate the origin of this effect and show it is a general consequence of the layering of any bicolor $Pbnm$ perovskite superlattice in which the number of constituent layers are odd (leading to a form of hybrid improper ferroelectricity). Our calculations suggest that the ME effect in these superlattices is larger than that observed in the prototypical magnetoelectric materials ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{BiFeO}}_{3}$. Furthermore, in these proposed materials, the strength of the linear ME coupling increases with the magnitude of the induced spontaneous polarization which is controlled by the La/Ln cation radius mismatch. We use a simple mean field model to show that the proposed materials order magnetically above room temperature.
Incorporating a dopant into a nanoparticle is a nontrivial proposition in view of the size dependent surface versus bulk energy considerations and the intrinsic proximity of the surface to the interior, which facilitates migration to the surface. If realized and controlled, however, it can open up new avenues to novel nanomaterials. Some previous studies have shown the dopability of nanosystems but only with specific surface functionalization. Here, we demonstrate the successful dopant incorporation via a new route of pulsed high energy electron induced synthesis. We choose a system Co:CdS (dilutely cobalt doped cadmium sulfide) in view of the well-known application-worthy properties of CdS and the potential possibility of its conversion to a diluted magnetic semiconductor of interest to spintronics. By using various techniques, we show that matrix incorporation and uniform distribution of cobalt are realized in CdS nanocrystals without the need for additional chemical or physical manipulation. Optical and photoluminescence properties also support dopant incorporation. Interestingly, although magnetism is realized, it is weak, and it decreases at higher cobalt concentration. First principle density functional calculations are performed to understand this counterintuitive behavior. These calculations suggest that the introduction of parent cation or anion vacancies lead to magnetic moment reduction, albeit marginally. However, with some Co impurity fraction in the octahedral interstitial site inside the wurtzite cage, the magnetic moment drops down drastically. This study reveals that defect states may have an interesting role in dopant stabilization in nanosystems, with interesting system dependent consequences for the properties.
Abstract In this article, we have systematically investigated the structural, electronic, optical and thermoelectric properties of Rb 2 Ag(Ga/In)Br 6 . The resulting negative formation energy along with the absence of imaginary phonon modes confirm the thermodynamic stability of Rb 2 Ag(Ga/In)Br 6 . In addition, the derived electronic properties by using GGA‐PBE + mBJ + SOC functional show that the direct band gap values are 1.21 eV and 1.42 eV for Rb 2 AgGaBr 6 and Rb 2 AgInBr 6 , respectively. Furthermore, the dispersed direct band nature of Rb 2 Ag(Ga/In)Br 6 leads to their outshining optical properties such as higher order (10 5 cm −1 ) absorption coefficient, appreciable optical conductivity, and low reflectivity. Moreover, the higher figure of merit values of Rb 2 Ag(Ga/In)Br 6 are resulted from their ultra‐low thermal conductivity and high electrical conductivity. Thus, Rb 2 Ag(Ga/In)Br 6 are predicted to be potential photovoltaic and thermoelectric materials.
Since the discovery of two-dimensional electron gas (2DEG) at the oxide interface of LaAlO3/SrTiO3 (LAO/STO), improving carrier mobility has become an important issue for device applications. In this paper, by using an alternate polar perovskite insulator (La0.3Sr0.7) (Al0.65Ta0.35)O3 (LSAT) for reducing lattice mismatch from 3.0% to 1.0%, the low-temperature carrier mobility has been increased 30 fold to 35,000 cm(2) V(-1) s(-1). Moreover, two critical thicknesses for the LSAT/STO (001) interface are found, one at 5 unit cells for appearance of the 2DEG and the other at 12 unit cells for a peak in the carrier mobility. By contrast, the conducting (110) and (111) LSAT/STO interfaces only show a single critical thickness of 8 unit cells. This can be explained in terms of polar fluctuation arising from LSAT chemical composition. In addition to lattice mismatch and crystal symmetry at the interface, polar fluctuation arising from composition has been identified as an important variable to be tailored at the oxide interfaces to optimize the 2DEG transport.
Layered two-dimensional (2D) transition metal phosphorous chalcogenides (TMPCs) are now in intense research focus due to their interesting ferroelectric and magnetic properties and compatibility with 2D electronic devices. Here, we have employed first-principles density functional theory calculations to investigate the electric and magnetic properties of ${\mathrm{ABP}}_{2}{\mathrm{S}}_{6}$ (A = Cu, Ni; B = Cr, Mn) TMPCs. We have systematically investigated four TMPCs compounds, namely, ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}, {\mathrm{CuMnP}}_{2}{\mathrm{S}}_{6}, {\mathrm{NiCrP}}_{2}{\mathrm{S}}_{6}$, and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$, and reported unusual antiferroelectric/ferroelectric (AFE/FE) and electronic properties. We have found a more stable ferroelectric state in van der Waals (vdW) gap with higher polarization compared to the usual ferroelectric phase in the insulating state. In case of ${\mathrm{CrMnP}}_{2}{\mathrm{S}}_{6}$ and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$, we have proposed them as polar half-metals as our analysis have revealed that ferroelectric distortions can persist in these systems in the metallic phase. Moreover, our analysis have shown that ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$ can undergo metal-to-insulator transition driven by the polar distortion. We have identified two modes namely, ${\mathrm{\ensuremath{\Gamma}}}_{2}^{\ensuremath{-}}$ ${\mathrm{\ensuremath{\Gamma}}}_{1}^{+}$, those are responsible for driving ferroelectricity and antiferroelectricity, respectively, into these systems. We could observe a rare and interesting phenomena, where the antipolar mode ${\mathrm{\ensuremath{\Gamma}}}_{1}^{+}$ that is responsible for antiferroelectric distortion leads to a ferroelectric distortion when applied excessively on the system, which is unusual. Finally, we have performed molecular dynamics simulations at various finite temperatures. In case of ${\mathrm{NiCrP}}_{2}{\mathrm{S}}_{6}$ and ${\mathrm{NiMnP}}_{2}{\mathrm{S}}_{6}$ at 300 K, we have observed that the ferroelectric state within the vdW gap is stable. Interestingly, we have discovered a hybrid inter-intra layer antiferroelectric configuration within the vdW gap for ${\mathrm{CuCrP}}_{2}{\mathrm{S}}_{6}$. The layers start to move opposite to each other due to the temperature effect, which leads to the switching of the AFE state in the first layer. Further, increase of temperature results the switching in both the layers. The reported in-gap FE/AFE states in vdW gap can be tuned by uniaxial strain along the perpendicular direction of the 2D layer and thus the materials studied here can be considered as potential piezoelectric materials.
Abstract Chiral multiferroics offer remarkable capabilities for controlling quantum devices at multiple levels. However, these materials are rare due to the competing requirements of long-range orders and strict symmetry constraints. In this study, we present experimental evidence that the coexistence of ferroelectric, magnetic orders, and crystallographic chirality is achievable in hybrid organic-inorganic perovskites [( R / S )- β -methylphenethylamine] 2 CuCl 4 . By employing Landau symmetry mode analysis, we investigate the interplay between chirality and ferroic orders and propose a novel mechanism for chirality transfer in hybrid systems. This mechanism involves the coupling of non-chiral distortions, characterized by defining a pseudo-scalar quantity, $$\xi={{{{{\bf{p}}}}}}{{\cdot }}{{{{{\bf{r}}}}}}$$ ξ=p⋅r ( $${{{{{\bf{p}}}}}}$$ p represents the ferroelectric displacement vector and $${{{{{\bf{r}}}}}}$$ r denotes the ferro-rotational vector), which distinguishes between ( R )- and ( S )-chirality based on its sign. Moreover, the reversal of this descriptor’s sign can be associated with coordinated transitions in ferroelectric distortions, Jahn-Teller antiferro-distortions, and Dzyaloshinskii-Moriya vectors, indicating the mediating role of crystallographic chirality in magnetoelectric correlations.
Metal–organic frameworks (MOFs) are hybrid crystalline compounds comprised of an extended ordered network made up of organic molecules, organic linkers and metal cations. In particular, MOFs with the same topology as inorganic perovskites have been shown to possess interesting properties, e.g., coexistence of ferroelectric and magnetic ordering. Using first-principles density functional theory, we have investigated the effect of strain on the compounds C(NH2)3Cr(HCOO)3 and (CH3CH2NH3)Mn(HCOO)3. Here, we show that compressive strain can substantially increase the ferroelectric polarization by more than 300%, and we discuss the mechanism involved in the strain enhancement of polarization. Our study highlights the complex interplay between strain and organic cations' dipoles and put forward the possibility of tuning of ferroelectric polarization through appropriate thin film growing.
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