arXiv subjects

Silvia Picozzi

Publications and source records attributed to Silvia Picozzi.

At least 73 records · Page 4Linked to original sources

arXiv2015

Mechanisms and origin of multiferroicity

Motivated by the potential applications of their intrinsic cross-coupling properties, the interest in multiferroic materials has constantly increased recently, leading to significant experimental and theoretical advancements. From the theoretical point of view, recent progresses have allowed to identify different mechanisms responsible for the appearence of ferroelectric polarization coexisting with -- and coupled to -- magnetic properties. This chapter aims at reviewing the fundamental mechanisms devised so far, mainly in transition-metal oxides, which lie at the origin of multiferroicity.

cond-mat.mtrl-sci↗

arXiv2014

Emergence of ferroelectricity and spin-valley properties in two-dimensional honeycomb binary compounds

By means of density functional theory calculations, we predict that several two dimensional AB binary monolayers, where A and B atoms belong to group IV or III-V, are ferroelectric. Dipoles arise from the buckled structure, where the A and B ions are located on the sites of a bipartite corrugated honeycomb lattice with trigonal symmetry. We discuss the emerging valley-dependent properties and the coupling of spin and valley physics, which arise from the loss of inversion symmetry, and explore the interplay between ferroelectricity and Rashba spin-spitting phenomena. We show that valley-related properties originate mainly from the binary nature of AB monolayers, while the Rashba spin-texture developing around valleys is fully controllable and switchable by reversing the ferroelectric polarization.

cond-mat.mtrl-sci↗

arXiv2014

Topological tuning in three-dimensional Dirac semimetals

We study with first-principles methods the interplay between bulk and surface Dirac fermions in three dimensional Dirac semimetals. By combining density functional theory with the coherent potential approximation, we reveal a topological phase transition in Na$_3$Bi$_{1-x}$Sb$_{x}$ and Cd$_3$[As$_{1-x}$P$_x$]$_2$ alloys, where the material goes from a Dirac semimetal to a trivial insulator upon changing Sb or P concentrations. Tuning the composition allows us to engineer the position of the bulk Dirac points in reciprocal space. Interestingly, the phase transition coincides with the reversal of the band ordering between the conduction and valence bands.

cond-mat.mes-hall↗

arXiv2014

Tuning the Ferroelectric Polarization in $AA^{\prime}$MnWO$_6$ Double Perovskites through $A$ Cation Substitution

Recent experimental and theoretical work has shown that the double perovskite NaLaMnWO$_6$ exhibits antiferromagnetic ordering owing to the Mn $d$ states, and computational studies further predict it to exhibit a spontaneous electric polarization due to an improper mechanism for ferroelectricity [King \textit{et al., Phys.\ Rev.\ B}, 2009, \textbf{79}, 224428; Fukushima \textit{et al., Phys.\ Chem.\ Chem.\ Phys.}, 2011, \textbf{13}, 12186], which make it a candidate multiferroic material. Using first-principles density functional calculations, we investigate nine isostructural and isovalent $AA^{\prime}$MnWO$_6$ double perovskites ($A$=Na, K, and Rb; $A^{\prime}$=La, Nd, and Y) with the aim of articulating crystal-chemistry guidelines describing how to enhance the magnitude of the electric polarization through chemical substitution of the $A$-site while retaining long-range magnetic order. We find that the electric polarization can be enhanced by up to 150\% in compounds which maximize the difference in the ionic size of the $A$ and $A^{\prime}$ cations. By examining the tolerance factors, bond valences, and structural distortions (described by symmetry-adapted modes) of the nine compounds, we identify the atomic scale features that are strongly correlated with the ionic and electronic contributions to the electric polarization. We also find that each compound exhibits a purely electronic remnant polarization, even in the absence of a displacive polar mode. The analysis and design strategies presented here can be further extended to additional members of this family ($B$=Fe, Co, etc.), and the improper ferroelectric nature of the mechanism allows for the decoupling of magnetic and ferroelectric properties and the targeted design of novel multiferroics.

cond-mat.mtrl-sci↗

arXiv2014

Engineering relativistic effects in ferroelectric SnTe

Spin-orbit coupling is increasingly seen as a rich source of novel phenomena, as shown by the recent excitement around topological insulators and Rashba effects. We here show that the addition of ferroelectric degrees of freedom to a semiconductor featuring topologically-non-trivial properties, such as SnTe, merges the intriguing field of spin-orbit-driven physics with non-volatile functionalities appealing for spintronics. By using a variety of modelling techniques, we show that a strikingly rich sequence of phases can be induced in SnTe, when going from a room-temperature cubic phase to a low-temperature ferroelectric structure, ranging from a topological crystalline insulator to a time-reversal-invariant $Z_2$ topological insulator to a "ferroelectric Rashba semiconductor", exhibiting a huge electrically-controllable Rashba effect in the bulk band structure.

cond-mat.mtrl-sci↗

arXiv2014

Robustness against Disorder of Relativistic Spectral Properties in Chalcogenide Alloys

In order to carefully address the interplay between substitutional disorder and spin-orbit-coupling in IV-VI alloys, we propose a novel theoretical approach that integrates the reliability of plane-wave based density-functional theory beyond the local-density approximation with the Coherent Potential Approximation. By applying the proposed method to ternary chalcogenide alloys, we predict a substantial robustness of spectral features close to the Fermi energy against substitutional disorder. Supplementing our first-principles calculations with the analysis of the $k \cdot p$ model for rock-salt chalcogenides, we show that the disorder self-energy is vanishingly small close to the band gap, thus allowing for bulk Rashba-like spin splitting to be observed in ferroelectric alloys, such as PbS$_x$Te$_{1-x}$, and protecting the band-character inversion related to the topological transition in the recently discovered Topological Crystalline Insulator Pb$_{1-x}$Sn$_x$Te.

cond-mat.mtrl-sci↗

arXiv2014

Improper origin of polar displacements at CaTiO3 and CaMnO3 Twin Walls

Recent interests towards novel functionalities arising at domain walls of ferroic materials naturally call for a microscopic understanding. To this end, first-principles calculations have been performed in order to provide solid evidence of ferroelectricity in the twin walls of nonpolar CaTiO3 and magnetic CaMnO3. We show that ferroelectricity arises from rotation/tilting octahedral distortions --- cooperatively acting at the twin wall in both considered systems --- rather than from a proper secondary ferroelectric instability, as often believed. Our results are in excellent agreement with experimental observations of domain walls in CaTiO3. In addition, we show that magnetic properties at the twin wall in CaMnO3 are also modified, thus suggesting an unexplored route to achieve and detect multiferroic ordering in a single-phase material.

cond-mat.mtrl-sci↗

arXiv2014

Hybrid improper ferroelectricity in a Multiferroic and Magnetoelectric hybrid organic-inorganic perosvkite

There is great interest in hybrid organic-inorganic materials such as metal-organic frameworks (MOFs). The compounds [C(NH$_{2}$)$_{3}$]M(HCOO)$_{3}$, where M=Cu$^{2+}$ or Cr$^{2+}$ are Jahn-Teller (JT) active ions, are MOF with perovskite topology which crystallizes in polar space group Pna2$_{1}$. In inorganic compounds, octahedral tilting and Jahn-Teller structural distortions are usually non-polar distortions. However, in this MOF cooperative interactions between the antiferro-distortive distortions of the framework and the C(NH2)$_{3}$ organic cation via hydrogen bonding breaks the inversion symmetry and induces a ferroelectric polarization.[Angew. Chem. Int. Ed. 50, 5847, 2011] Our ab-initio study supports the picture of an orbital-order-induced ferroelectricity, a rare example of dipolar ordering caused by electronic degrees of freedom. The switching of polarization direction implies the reversal of the weak ferromagnetic component. The microscopic mechanism in this JT-based MOF with ABX$_{3}$ perovskite structure displays a Hybrid Improper Ferroelectric (HIF) state, arising from a trilinear coupling between different structural deformations that comprise tilting, rotations and Jahn-Teller distortions of both the BX$_{3}$ framework and the organic cation at the A sites. Since these distortion modes in perovskite-inorganic compounds usually freeze-in at elevated temperatures, the trilinear coupling in MOF compounds may provide an interesting route towards high-temperature multiferroicity. These results offer an important starting point for tailoring multiferroic properties in this emerging class of materials for various technological applications. In particular, the high tunability of the ferroelectric polarization by means of the modification of the organic A cation has been recently shown[J. Am. Chem. Soc. 135 18126 (2013)]

cond-mat.mtrl-sci↗

arXiv2014

Ferroelectric Rashba Semiconductors as a novel class of multifunctional materials

The discovery of novel properties, effects or microscopic mechanisms in modern materials science is often driven by the quest for combining, into a single compound, several functionalities: not only the juxtaposition of the latter functionalities, but especially their coupling, can open new horizons in basic condensed matter physics as well as in technology. Semiconductor spintronics makes no exception. In this context, we have discovered by means of density-functional simulations that, when a sizeable spin-orbit coupling is combined with ferroelectricity, such as in GeTe, one obtains novel multifunctional materials - called Ferro-Electric Rashba Semi-Conductors (FERSC) - where, thanks to a giant Rashba spin-splitting, the spin texture is controllable and switchable via an electric field. This peculiar spin-electric coupling can find a natural playground in small-gap insulators, such as chalcogenides, and can bring brand new assets into the field of electrically-controlled semiconductor spintronics

cond-mat.mtrl-sci↗

arXiv2013

Ab-initio prediction of magnetoelectricity in infinite-layer CaFeO2 and MgFeO2

Density functional based simulations are employed to explore magnetoelectric effects in iron-based oxides, showing a unique layered structure. We theoretically predict CaFeO2 to be a promising magnetoelectric, showing magnetically-controlled large electric polarization, possibly even above room temperature. The cross coupling between magnetic and dipolar degrees of freedom needs, as main ingredients, Fe-site spin-orbit coupling and a spin-dependent O p - Fe d hybridization, along with structural constraints related to the non-centrosymmetric point group and the peculiar geometry characterized by "flattened" FeO4 tetrahedrons. In order to enhance magnetoelectric effects, we performed a materials-design leading to a novel and optimized system, MgFeO2, where the larger O4 tetrahedral distortion leads to a stronger polarization.

cond-mat.str-el↗

arXiv2013

Strain engineering of topological properties in lead-salt semiconductors

Rock-salt chalcogenide SnTe represents the simplest realization of a topological insulator where a crystal symmetry allows for the appearence of topologically protected metallic states with an even number of Dirac cones on high-symmetry crystal surfaces. Related rock-salt lead chalcogenides have been predicted as well to undergo a phase-transition to a topological crystalline insulating phase after band inversion induced by alloying and pressure. Here we theoretically predict that strain, as realized in thin films grown on (001) substrates, may induce such topological phase-transitions. Furthermore, relevant topological properties of the surface states, such as the location of the Dirac cones on the surface Brillouin zone or the decay length of edge states, appear to be tunable with strain, with potential implications for technological devices benefiting from those additional degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2012

Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic frameworks, MOFs)

cond-mat.mtrl-sci↗

arXiv2012

Ferroelectricity induced by cooperative orbital ordering and Peierls instability

A general mechanism by which orbital ordering, coupled to Peierls-like lattice distortions, can induce an electronic switchable polarization is discussed within a model Hamiltonian approach in the context of the modern theory of polarization. By using a Berry-phase approach, a clear picture emerges in terms of Wannier-function centers and orbital occupancies. The proposed mechanism may apply to oxide spinels whose electronic structure has effective one-dimensional character, such as CdV$_2$O$_4$, recently proposed to display multiferroic behaviour.

cond-mat.str-el↗

arXiv2012

Charge Ordering and Ferroelectricity in Half-doped Manganites

By means of density-functional simulations for half-doped manganites, such as pseudocubic Pr0.5Ca0.5MnO3 and bilayer PrCa2Mn2O7, we discuss the occurrence of ferroelectricity and we explore its crucial relation to the crystal structure and to peculiar charge/spin/orbital ordering effects. In pseudocubic Pr0.5Ca0.5MnO3, ferroelectricity is induced in the Zener polaron type structure, where Mn ions are dimerized. In marked contrast, in bilayer PrCa2Mn2O7, it is the displacements of apical oxygens bonded to either Mn3+ or Mn4+ ions that play a key role in the rising of ferroelectricity. Importantly, local dipoles due to apical oxygens are also intimately linked to charge and orbital ordering patterns in MnO2 planes, which in turn contribute to polarization. Finally, an important outcome of our work consists in proposing Born effective charges as a valid mean to quantify charge disproportionation effects, in terms of anisotropy and size of electronic clouds around Mn ions.

cond-mat.str-el↗

arXiv2012

Spin-phonon coupling effects in transition-metal perovskites:a DFT+$U$ and hybrid-functional study

Spin-phonon coupling effects, as reflected in phonon frequency shifts between ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic CaMnO$_3$, SrMnO$_3$, BaMnO$_3$, LaCrO$_3$, LaFeO$_3$ and La$_2$(CrFe)O$_6$, are investigated using density-functional methods. The calculations are carried out both with a hybrid-functional (HSE) approach and with a DFT+$U$ approach using a $U$ that has been fitted to HSE calculations. The phonon frequency shifts obtained in going from the FM to the AFM spin configuration agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational effort. We find that in the $A$MnO$_3$ materials class with $A$=Ca, Sr, and Ba, this frequency shift decreases as the A cation radius increases for the $Γ$ phonons, while it increases for R-point phonons. In La$M$O$_3$ with $M$=Cr, Fe, and Cr/Fe, the phonon frequencies at $Γ$ decrease as the spin order changes from AFM to FM for LaCrO$_3$ and LaFeO$_3$, but they increase for the double perovskite La$_2$(CrFe)O$_6$. We discuss these results and the prospects for bulk and superlattice forms of these materials to be useful as multiferroics.

cond-mat.str-el↗

arXiv2011

Mechanism of Ferroelectricity in $d^3$ Perovskites - a Model Study

By means of a model Hamiltonian approach we study the role of volume expansion, Hund's coupling and electron correlation in the standard hybridization mechanism for ferroelectricity in cubic CaMnO$_3$, a prototypical non-$d^0$ perovskite. Our results establish that the ferroelectric instability arises from a subtle balance between different energy contributions, explaining the origin of its enhancement under negative pressure. Expansion of volume is found to cause a strong reduction of the elastic energy, while leaving almost unchanged the tendency of Mn states to form covalent bonds with the surrounding oxygens. Hund's coupling with local spins of magnetic cations can reduce and even suppress the instability towards the ferroelectric state.

cond-mat.str-el↗

arXiv2011

Buckling induced Zener polaron instability in half-doped manganites

By calculating the electronic, orbital, magnetic and lattice structure of half-doped manganites we establish the central role of oxygen buckling caused by the tilting of the MnO$_6$ octahedra -- in essence a steric effect. The buckling itself does not change the Mn-Mn bonds. Instead it drives the system towards an instability where these bonds can dimerize. In presence of electron-electron interactions, this instability can fully develop and beyond a critical buckling a Zener polaron groundstate with dimerized spins, lattice and orbitals forms spontaneously, resulting in a multiferroic state.

cond-mat.str-el↗

arXiv2011

Polar distortions in hydrogen bonded organic ferroelectrics

Although ferroelectric compounds containing hydrogen bonds were among the first to be discovered, organic ferroelectrics are relatively rare. The discovery of high polarization at room temperature in croconic acid [Nature \textbf{463}, 789 (2010)] has led to a renewed interest in organic ferroelectrics. We present an ab-initio study of two ferroelectric organic molecular crystals, 1-cyclobutene-1,2-dicarboxylic acid (CBDC) and 2-phenylmalondialdehyde (PhMDA). By using a distortion-mode analysis we shed light on the microscopic mechanisms contributing to the polarization, which we find to be as large as 14.3 and 7.0\,$μ$C/cm$^{2}$ for CBDC and PhMDA respectively. These results suggest that it may be fruitful to search among known but poorly characterized organic compounds for organic ferroelectrics with enhanced polar properties suitable for device applications.

cond-mat.mtrl-sci↗