Soumyajit Sarkar

Proposed orbital ordering in MnV2O4 from first-principles calculations.

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation +U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

Proposed Orbital Ordering inMnV2O4from First-Principles Calculations

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation +U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

Cold denaturation induces inversion of dipole and spin transfer in chiral peptide monolayers

Chirality-induced spin selectivity is a recently-discovered effect, which results in spin selectivity for electrons transmitted through chiral peptide monolayers. Here, we use this spin selectivity to probe the organization of self-assembled α-helix peptide monolayers and examine the relation between structural and spin transfer phenomena. We show that the α-helix structure of oligopeptides based on alanine and aminoisobutyric acid is transformed to a more linear one upon cooling. This process is similar to the known cold denaturation in peptides, but here the self-assembled monolayer plays the role of the solvent. The structural change results in a flip in the direction of the electrical dipole moment of the adsorbed molecules. The dipole flip is accompanied by a concomitant change in the spin that is preferred in electron transfer through the molecules, observed via a new solid-state hybrid organic–inorganic device that is based on the Hall effect, but operates with no external magnetic field or magnetic material.

Signature of an antiferromagnetic metallic ground state in heavily electron-doped Sr2FeMoO6

Sr2FeMoO6 is a well-known double perovskite with exciting high-temperature magnetic properties. Through various magnetic and spectroscopic measurements, we collect compelling evidence here that thi ...

Probing the Orbital Origin of Conductance Oscillations in Atomic Chains

We investigate periodical oscillations in the conductance of suspended Au and Pt atomic chains during elongation under mechanical stress. Analysis of conductance and shot noise measurements reveals that the oscillations are mainly related to variations in a specific conduction channel as the chain undergoes transitions between zigzag and linear atomic configurations. The calculated local electronic structure shows that the oscillations originate from varying degrees of hybridization between the atomic orbitals along the chain as a function of the zigzag angle. These variations are highly dependent on the directionally and symmetry of the relevant orbitals, in agreement with the order-of-magnitude difference between the Pt and Au oscillation amplitudes observed in experiment. Our results demonstrate that the sensitivity of conductance to structural variations can be controlled by designing atomic-scale conductors in view of the directional interactions between atomic orbitals.

Comparative study of FeCr2S4 and FeSc2S4: Spinels with orbitally active A site

ion, their properties are rather dissimilar. Our study unravels the microscopicorigin of their behavior driven by the differences in hybridization of Fe d states with Cr/Sc d statesand S p states in the two cases. This leads to important differences in the nature of the magneticexchanges as well as the nearest versus next nearest neighbor exchange parameter ratios, resultinginto significant frustration effects in FeSc

Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates

Taking enantiomers for a spin There are two common ways to distinguish mirror-image molecules, or enantiomers. The first relies on their distinct interactions with circularly polarized light, the second on their interactions with a pure enantiomer of some other molecule. Now Banerjee-Ghosh et al. report a conceptually different approach to chiral resolution. Experiments showed that, depending on the direction of magnetization, chiral oligopeptides, oligonucleotides, and amino acids have enantiospecific differences in initial adsorption rates on ferromagnetic surfaces. This effect is attributed to enantiospecific induced spin polarization. Science, this issue p. 1331 Spin polarization of chiral molecules enhances the initial adsorption rate of one enantiomer onto a ferromagnetic substrate. It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.

Ionisation and (de-)protonation energies of gas-phase amino acids from an optimally tuned range-separated hybrid functional

ABSTRACT We provide a quantitative examination of the ionisation potential, protonation, and de-protonation energies of a set of 22 different amino acids. Specifically, we compare results obtained using the conventional hybrid functionals B3LYP and BHLYP (which use 20% and 50% of Fock exchange, respectively) with those obtained from the recently developed optimally tuned range-separated hybrid (OT-RSH) functional approach, as well as to literature coupled-cluster calculations with single and double excitations (CCSD) data. We find the OT-RSH results to be quantitatively close to those of the BHLYP functional and a significant improvement over those of B3LYP, with respect to the CCSD data. We conclude that the results of the OT-RSH for these quantities are as reliable and inexpensive as those of BHLYP, but possess one distinct and important advantage: they overcome the empiricism and limited predictive power associated with the arbitrary choice of the amount of Fock exchange in the BHLYP. We further discuss quantitative and qualitative trends in the optimal-tuning procedure of the amino acids studied.

Cooperative orbital ordering and Peierls instability in the checkerboard lattice with doubly degenerate orbitals

It has been suggested that the metal-insulator transitions in a number of spinel materials with partially filled t 2g d orbitals can be explained as orbitally driven Peierls instabilities. Motivated by these suggestions, we examine theoretically the possibility of formation of such orbitally driven states within a simplified theoretical model, a two-dimensional checkerboard lattice with two-directional metal orbitals per atomic site. We include orbital ordering and interatom electron-phonon interactions self-consistently within a semiclassical approximation, and onsite intraorbital and interorbital electron-electron interactions at the Hartree-Fock level. We find a stable, orbitally induced Peierls bond-dimerized state for carrier concentration of one electron per atom. The Peierls bond distortion pattern continues to be period two bond dimerization even when the charge density in the orbitals forming the one-dimensional band is significantly smaller than 1. In contrast, for carrier density of half an electron per atom the Peierls instability is absent within one-electron theory as well as mean-field theory of electron-electron interactions, even for nearly complete orbital ordering. We discuss the implications of our results in relation to complex charge, bond, and orbital ordering found in spinels.

Measurements of the stabilities of isolated retinal chromophores

The barrier energies for isomerization and fragmentation were measured for a series of retinal chromophore derivatives using a tandem ion mobility spectrometry approach. These measurements allow us to quantify the effect of charge delocalization on the rigidity of chromophores. We find that the role of the methyl group on the C13 position is pivotal regarding the ground state dynamics of the chromophore. Additionally, a correlation between quasi-equilibrium isomer distribution and fragmentation pathways is observed.