Epitaxially strained [001]-(PbTiO3_3)1_1(PbZrO3_3)1_1 superlattice and PbTiO3_3 from first principles

The effect of layer-by-layer heterostructuring and epitaxial strain on lattice instabilities and related ferroelectric properties is investigated from first principles for the [001]-(PbTiO$_3$)$_1$(PbZrO$_3$)$_1$ superlattice and pure PbTiO$_3$ on a cubic substrate. The results for the superlattice show an enhancement of the stability of the monoclinic r-phase with respect to pure PbTiO$_3$. Analysis of the lattice instabilities of the relaxed centrosymmetric reference structure computed within density functional perturbation theory suggests that this results from the presence of two unstable zone-center modes, one confined in the PbTiO$_3$ layer and one in the PbZrO$_3$ layer, which produce in-plane and normal components of the polarization, respectively. The zero-temperature dielectric response is computed and shown to be enhanced not only near the phase boundaries, but throughout the r-phase. Analysis of the analogous calculation for pure PbTiO$_3$ is consistent with this interpretation, and suggests useful approaches to engineering the dielectric properties of artificially structured perovskite oxides.Comment: 8 pages, 5 figure

What Makes Some People Think Astrology Is Scientific?

Citizens in both North America and Europe are apt to read horoscope columns in newspapers and magazines. While some people read these casually and purely for entertainment, some believe that astrology has scientific status and can provide real insight into events and personality. Using data from a European survey, this article explores some of the reasons why some people think that astrology is scientific and how astrology is viewed in relation to other knowledge-producing practices. Three hypotheses in particular are tested. The first is that some Europeans lack the necessary scientific literacy to distinguish science from pseudoscience. The second is that people are confused about what astrology actually is. The third is derived from Adorno’s work on authoritarianism and the occult and postulates that those who adhere to authoritarian values are more likely to believe in astrological claims. Support is found for all three hypotheses. </jats:p

Resonance Energy Transfer from Monolayer WS2 to Organic Dye Molecules Conversion of Faint Visible Red into Bright Near Infrared Luminescence

The synergetic combination of transition metal dichalcogenides TMDCs with organic dye molecules in functional heterostructures is promising for various optoelectronic applications. Here resonance energy transfer RET from a red emitting WS2 monolayer 1L WS2 to a layer of near infrared NIR emitting organic dye molecules is demonstrated. It is found that the total photoluminescence PL yield of the heterostructures is up to a factor of eight higher as compared to the PL yield of pristine 1L WS2. This is attributed to the efficient conversion of the mostly non radiative excitons in 1L WS2 into radiative excitons in the dye layer. A type I energy level alignment of the 1L WS2 dye interface assures the emission of bright PL. From excitation density dependent PL experiments, it is concluded that RET prevails against defect assisted non radiative recombination as well as Auger type exciton exciton annihilation in 1L WS2. The work paves the way for employing organic dye molecules in heterostructures with TMDCs in nanoscale light emitting devices with improved efficiency and tunable colo

Anomalous enhancement of tetragonality in PbTiO3 induced by negative pressure

Using a first-principles approach based on density-functional theory, we find that a large tetragonal strain can be induced in PbTiO3 by application of a negative hydrostatic pressure. The structural parameters and the dielectric and dynamical properties are found to change abruptly near a crossover pressure, displaying a ``kinky'' behavior suggestive of proximity to a phase transition. Analogous calculations for BaTiO3 show that the same effect is also present there, but at much higher negative pressure. We investigate this unexpected behavior of PbTiO3 and discuss an interpretation involving a phenomenological description in terms of a reduced set of relevant degrees of freedom.Comment: 9 pages, with 9 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/st_pbti/index.htm

Lattice instabilities of PbZrO3/PbTiO3 [1:1] superlattices from first principles

Ab initio phonon calculations for the nonpolar reference structures of the (001), (110), and (111) PbZrO_3/PbTiO_3 [1:1] superlattices are presented. The unstable polar modes in the tetragonal (001) and (110) structures are confined in either the Ti- or the Zr-centered layers and display two-mode behavior, while in the cubic (111) case one-mode behavior is observed. Instabilities with pure oxygen character are observed in all three structures. The implications for the ferroelectric behavior and related properties are discussed.Comment: 12 pages, 2 figures, 7 tables, submitted to PR

First-principles study of lattice instabilities in the ferromagnetic martensite Ni2_2MnGa

The phonon dispersion relations and elastic constants for ferromagnetic Ni$_2$MnGa in the cubic and tetragonally distorted Heusler structures are computed using density-functional and density-functional perturbation theory within the spin-polarized generalized-gradient approximation. For $0.9<c/a<1.06$, the TA$_2$ tranverse acoustic branch along $[110]$ and symmetry-related directions displays a dynamical instability at a wavevector that depends on $c/a$. Through examination of the Fermi-surface nesting and electron-phonon coupling, this is identified as a Kohn anomaly. In the parent cubic phase the computed tetragonal shear elastic constant, C$^\prime$=(C$_{11}-$C$_{12}$)/2, is close to zero, indicating a marginal elastic instability towards a uniform tetragonal distortion. We conclude that the cubic Heusler structure is unstable against a family of energy-lowering distortions produced by the coupling between a uniform tetragonal distortion and the corresponding $[110]$ modulation. The computed relation between the $c/a$ ratio and the modulation wavevector is in excellent agreement with structural data on the premartensitic ($c/a$ = 1) and martensitic ($c/a$ = 0.94) phases of Ni$_2$MnGa.Comment: submitted to Phys. Rev.

First-principles study of the structural energetics of PdTi and PtTi

The structural energetics of PdTi and PtTi have been studied using first-principles density-functional theory with pseudopotentials and a plane-wave basis. We predict that in both materials, the experimentally reported orthorhombic $B19$ phase will undergo a low-temperature phase transition to a monoclinic $B19'$ ground state. Within a soft-mode framework, we relate the $B19$ structure to the cubic $B2$ structure, observed at high temperature, and the $B19'$ structure to $B19$ via phonon modes strongly coupled to strain. In contrast to NiTi, the $B19$ structure is extremely close to hcp. We draw on the analogy to the bcc-hcp transition to suggest likely transition mechanisms in the present case.Comment: 8 pages 5 figure

Strong-correlation effects in Born effective charges

Large values of Born effective charges are generally considered as reliable indicators of the genuine tendency of an insulator towards ferroelectric instability. However, these quantities can be very much influenced by strong electron correlation and metallic behavior, which are not exclusive properties of ferroelectric materials. In this paper we compare the Born effective charges of some prototypical ferroelectrics with those of magnetic, non-ferroelectric compounds using a novel, self-interaction free methodology that improves on the local-density approximation description of the electronic properties. We show that the inclusion of strong-correlation effects systermatically reduces the size of the Born effective charges and the electron localization lengths. Furthermore we give an interpretation of the Born effective charges in terms of band energy structure and orbital occupations which can be used as a guideline to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure

Theory of structural response to macroscopic electric fields in ferroelectric systems

We have developed and implemented a formalism for computing the structural response of a periodic insulating system to a homogeneous static electric field within density-functional perturbation theory (DFPT). We consider the thermodynamic potentials E(R,eta,e) and F(R,eta,e) whose minimization with respect to the internal structural parameters R and unit cell strain eta yields the equilibrium structure at fixed electric field e and polarization P, respectively. First-order expansion of E(R,eta,e) in e leads to a useful approximation in which R(P) and eta(P) can be obtained by simply minimizing the zero-field internal energy with respect to structural coordinates subject to the constraint of a fixed spontaneous polarization P. To facilitate this minimization, we formulate a modified DFPT scheme such that the computed derivatives of the polarization are consistent with the discretized form of the Berry-phase expression. We then describe the application of this approach to several problems associated with bulk and short-period superlattice structures of ferroelectric materials such as BaTiO3 and PbTiO3. These include the effects of compositionally broken inversion symmetry, the equilibrium structure for high values of polarization, field-induced structural phase transitions, and the lattice contributions to the linear and the non-linear dielectric constants.Comment: 19 pages, with 15 postscript figures embedded. Uses REVTEX4 and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/sai_pol/index.htm

The capacity of social policies to combat poverty among new social risk groups

This paper considers groups who are most likely to be vulnerable to new social risks and tests the effects of social policies on their poverty levels. Specifically, the paper conducts multi-level regression analyses across 18 OECD countries near the year 2004, analyzing the effects of social policies on the likelihood of being poor of low-skilled young women and men aged 18-30, and of those at risk of possessing obsolete skills, namely low-educated men aged 55-64. The analyses are conducted by combining both macro-level policy data and household-and person-level micro-data from the Luxembourg Income Study (LIS) cross-national database. The central question asks which policies - active labor market policies (ALMP), passive labor market policies (PLMP), employment protection legislation (EPL), family policies, government daycare spending - are effective at combating new social risks. In addition to analyzing social policies, the paper also considers union density and representation of women in national parliaments as two measures that depict agents who are most intent on combating old and new social risks, respectively. The findings show that active labor market policies (ALMP) are the most important predictor of a decrease in poverty levels among the low skilled. The negative effect of passive labor market policies (PLMP) on poverty is only significant for the older male group. Family policies are related to a reduction in poverty for both low-skilled young women and men. Union density is significant in reducing the odds of poverty of the older male group, while it is insignificant for both younger males and females - thereby reflecting a measure of prevention against old, but not new, social risks. Additionally, the paper's findings support the hypothesis that the greater the representation of women in national parliaments, the greater the chances of having policies that are associated with a reduction of poverty across age levels. Lastly, gross public social spending as a measure of overall welfare generosity is found to be associated with a reduction in poverty only of the older male group, but not that of the younger groups. The paper's analyses suggest that some social policies remain geared toward older segments of society, leaving the younger population at greater financial and therefore social risk