Negative magnetization and magnetic ordering of rare earth and transition metal sublattices in NdFe0.5Cr0.5O3.

We investigate the effect of alloying at the 3dtransition metal site of a rare-earth-transition metal oxide, by considering NdFe0.5Cr0.5O3mixed perovskite with two equal and random distribution of 3d ions, Cr and Fe, interacting with an early 4f rare earth ion, Nd. Employing temperature- and field- dependent magnetization measurements, temperature-dependent x-ray diffraction, neutron powder diffraction, and Raman spectroscopy, we characterize its structural and magnetic properties. Our study reveals bipolar magnetic switching (arising from negative magnetization) and magnetocaloric effect which underline the potential of the studied mixed perovskite in device application. The neutron diffraction study shows the absence of spin reorientation transition over the entire temperature range of 1.5-320 K, although both parent compounds exhibit spin orientation transition. We discuss the microscopic origin of this curious behavior. The neutron diffraction results also reveal the ordering of Nd spins at an unusually high temperature of about 40 K, which is corroborated by Raman measurements.

Viscoelastic response of fluid trapped between two dissimilar van der Waals surfaces.

Employing grand canonical Monte-Carlo and molecular dynamics simulations, the viscoelastic response of trapped fluid under molecularly thin confinement by walls having different wall-fluid interaction strengths, is investigated. With increase in slit asymmetry, given by the ratio of interaction strengths of the wall having strong wall-fluid interaction to that of the wall with weak wall-fluid interaction, a crossover in effective density of the fluid film, from rarer (R) to denser (D) than the bulk density is observed. Upon increasing asymmetry further, the dense fluid (F) layers undergo bond-orientational (S) ordering. The variation of viscoelastic relaxation time with scaled asymmetry shows a universal behavior, independent of slit width, with two distinct regimes. Below a critical value of asymmetry, the viscoelastic relaxation time is a slowly varying function of asymmetry, comparable with the structural relaxation time. Beyond the critical asymmetry, on the other hand, viscoelastic response time shows a sharp increase upon increasing asymmetry, deviating markedly from the structural relaxation time. Interestingly the critical asymmetry value is found to correlate with R to D crossover. The microscopic origin of the two-regime universal behavior of viscoelastic response time is found to stem from the fact that below critical asymmetry, the overall viscoelastic behaviour of the slit is dominated by that of the fast relaxing layer close to the weakly attracting surface, while above the critical asymmetry, the relaxation behaviour is guided by the dense fluid layer adjacent to the strongly attracting wall. In vicinity of fluid to ordering transition, the loss and storage moduli merge for low frequencies as in gel-like mechanical behaviour. The storage modulus takes over the loss modulus in the phase co-existence region even before the long ranged order sets in. Our findings bear important implications for fluid transport in hetero-structured geometry in nanotechnology.

Competition between heavy fermion and Kondo interaction in isoelectronic A-site-ordered perovskites.

With current research efforts shifting towards the 4d and 5d transition metal oxides, understanding the evolution of the electronic and magnetic structure as one moves away from 3d materials is of critical importance. Here we perform X-ray spectroscopy and electronic structure calculations on A-site-ordered perovskites with Cu in the A-site and the B-sites descending along the ninth group of the periodic table to elucidate the emerging properties as d-orbitals change from partially filled 3d to 4d to 5d. The results show that when descending from Co to Ir, the charge transfers from the cuprate-like Zhang-Rice state on Cu to the t(2g) orbital of the B site. As the Cu d-orbital occupation approaches the Cu(2+) limit, a mixed valence state in CaCu(3)Rh(4)O(12) and heavy fermion state in CaCu(3)Ir(4)O(12) are obtained. The investigated d-electron compounds are mapped onto the Doniach phase diagram of the competing RKKY and Kondo interactions developed for the f-electron systems.

Evidence of a field-induced Berezinskii-Kosterlitz-Thouless scenario in a two-dimensional spin-dimer system.

Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex-antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S=½ ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.

Ferroic properties in bi-component perovskites: artificial superlattices and naturally forming compounds.

The use of four different metal cations in a bi-component perovskite ABO3 structure with 50 : 50 substitution at A sublattice as well as B sublattice, opens up the door for materials designing, with the aim to improve ferroic properties. This can be achieved following two different routes; one using the concept of artificially grown superlattices with alternating layers of ABO3 and A'B'O3 perovskites in a periodic set-up and another, through synthesis of naturally grown bulk double perovskites with ordered arrangement of A and A' cations, simultaneously with that of B and B' cations. The tremendous progress in layered deposition techniques as well as advances in solid state chemistry methods, has made both routes equally plausible and an area of much activity. This review summarizes some of the recent progress in this field, with a special emphasis on two computational studies, (i) one on ultra-thin 1-1 superlattices built out of paraelectric and ferroelectric components, showing tunable piezoelectric properties, and (ii) another on CrOs-based double perovskites which show multiferroic behavior, achieved through layered ordering of A and A' cations.

Vegard's law-like behavior for Mn(m)Tc(n) alloy clusters: a first-principles prediction.

With a view to gaining an understanding of the alloying tendency of bimetallic nanoalloy clusters of isoelectronic constituents, we studied the structural and mixing behavior of MnmTcn alloy clusters with m + n = 13 for all possible compositions, using first-principles electronic structure calculations. Our study reports a favorable mixing tendency for the alloy clusters. The average bond lengths of the minimum energy structures show an overall linear variation with concentration, indicating a Vegard's law-like variation for the nanoalloy clusters, though the optimized structures undergo a structural transition from a closed and compact structure for the Mn-rich alloy clusters to an open layered-like structure for the Tc-rich alloy clusters. We work out a continuous and smooth interplay between hybridization and magnetization properties of the alloy clusters, which plays a vital role in the Vegard's law-like variation in their average bond lengths.

Layered structures and nanosheets of pyrimidinethiolate coordination polymers.

We report the synthesis, crystal structure and exfoliation of a new member of an important family of layered compounds: lamellar pyrimidinethiolate coordination polymers. Conductivity measurements and DFT calculations of iron(II) pyrimidine-2-thiolate show that this material and a related compound are insulators.

Evolution of electronic and magnetic properties in four polytypes of BaRuO3: a first-principles study.

Using density functional theory, we explore the evolution of the electronic and magnetic properties of BaRuO3 in four different phases, 9R, 4H, 6H and 3C, obtained by synthesizing under different pressure conditions. The four different phases differ in the differential proportion of hexagonal versus cubic close stacking of the BaO3 layers, leading to important changes in the structure. By computing the electronic and magnetic properties of the four different phases, and the optical properties of 4H and 9R phases, we find that density functional based calculations are to a large extent able to explain the change in properties of the four different polytypes.

Zhang-Rice physics and anomalous copper states in A-site ordered perovskites.

In low dimensional cuprates several interesting phenomena, including high Tc superconductivity, are deeply connected to electron correlations on Cu and the presence of the Zhang-Rice (ZR) singlet state. Here, we report on direct spectroscopic observation of the ZR state responsible for the low-energy physical properties in two isostructural A-site ordered cuprate perovskites, CaCu(3)Co(4)O(12) and CaCu(3)Cr(4)O(12) as revealed by resonant soft x-ray absorption spectroscopy on the Cu L(3,2)- and O K-edges. These measurements reveal the signature of Cu in the high-energy 3+ (3d(8)), the typical 2+ (3d(9)), as well as features of the ZR singlet state (i.e., 3d(9)L, L denotes an oxygen hole). First principles GGA + U calculations affirm that the B-site cation controls the degree of Cu-O hybridization and, thus, the Cu valency. These findings introduce another avenue for the study and manipulation of cuprates, bypassing the complexities inherent to conventional chemical doping (i.e. disorder) that hinder the relevant physics.

Pressure and temperature control of spin-switchable metal-organic coordination polymers from ab initio calculations.

We explore a combination of density-functional theory with supplemented Coulomb U (DFT+U) and ab initio molecular dynamics simulations to investigate the spin-crossover (SCO) phenomenon in coordination polymers. We demonstrate the applicability of the method for the case of bimetallic metal-organic framework Fe(2)[Nb(CN)(8)]·(4-pyridinealdoxime)(8)·2H(2)O [see S. Ohkoshi et al. Nat. Chem. 3, 564 (2011)]. Our study shows that this approach is capable of capturing the SCO transitions driven by pressure as well as temperature. In addition to discovering novel spin-state transitions, magnetic states involving changes in the long-range magnetic ordering pattern are achieved, thereby offering the tunability of spin states as well as the long-range order of the spins. We compare the SCO transition in the Fe-based framework with a computer designed Mn-based variant.

Size control of charge-orbital order in half-doped manganite La0.5Ca0.5MnO3.

Motivated by recent experimental results, we study the effect of size reduction on half-doped manganite, La(0.5)Ca(0.5)MnO(3), using the combination of density-functional theory (DFT) and dynamical mean-field theory (DMFT). We find that upon size reduction the charge-ordered antiferromagnetic phase, observed in bulk, is destabilized, giving rise to the stability of a ferromagnetic metallic state. Our theoretical results, carried out on a defect-free nanocluster in isolation, establish the structural changes that follow upon size reduction to be responsible for this. Our study further points out the effect of size reduction to be distinctively different from application of hydrostatic pressure. Interestingly, our DFT+DMFT study additionally reports the correlation-driven stability of the charge-orbitally ordered state in bulk La(0.5) Ca(0.5) MnO(3), even in the absence of long-range magnetic order.

A microscopic view on the Mott transition in chromium-doped V(2)O(3).

V(2)O(3) is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V(2)O(3) with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state.

Inequivalent routes across the Mott transition in V2O3 explored by X-ray absorption.

The changes in the electronic structure of V2O3 across the metal-insulator transition induced by temperature, doping, and pressure are identified using high resolution x-ray absorption spectroscopy at the V pre-K edge. Contrary to what has been taken for granted so far, the metallic phase reached under pressure is shown to differ from the one obtained by changing doping or temperature. Using a novel computational scheme, we relate this effect to the role and occupancy of the a{1g} orbitals. This finding unveils the inequivalence of different routes across the Mott transition in V2O3.

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.

Electronic structure, phonons, and dielectric anomaly in ferromagnetic insulating double pervoskite La2NiMnO6.

Using first-principles density functional calculations, we study the electronic and magnetic properties of the ferromagnetic insulating double perovskite compound La2NiMnO6, which has been reported to exhibit an interesting magnetic field sensitive dielectric anomaly as a function of temperature. Our study reveals the existence of very soft infrared active phonons that couple strongly with spins at the Ni and Mn sites through modification of the superexchange interaction. We suggest that these modes are the origin for the observed dielectric anomaly in La2NiMnO6.

Effects of two energy scales in weakly dimerized antiferromagnetic quantum spin chains.

By means of thermal expansion and specific heat measurements on the high-pressure phase of (VO)(2)P(2)O(7), the effects of two energy scales of the weakly dimerized antiferromagnetic S=1/2 Heisenberg chain are explored. The low-energy scale, given by the spin gap Delta, is found to manifest itself in a pronounced thermal expansion anomaly. A quantitative analysis, employing the density-matrix renormalization-group approach for transfer matrices calculations, shows that this feature originates from changes in the magnetic entropy with respect to Delta, partial differentialS(m)/partial differentialDelta. This term, inaccessible by specific heat, is visible only in the weak-dimerization limit, where it reflects peculiarities of the excitation spectrum and its sensitivity to variations in Delta.

Electronic structure and microscopic model of V(2)GeO(4)F(2)-a quantum spin system with S = 1.

We present first-principles density functional calculations and downfolding studies of the electronic and magnetic properties of the oxide-fluoride quantum spin system V(2)GeO(4)F(2). We discuss explicitly the nature of the exchange paths and provide quantitative estimates of magnetic exchange couplings. A microscopic modelling based on analysis of the electronic structure of this systems puts it in the interesting class of weakly coupled alternating chain S = 1 systems. Based on the microscopic model, we make inferrences about its spin excitation spectra, which needs to be tested by rigorous experimental study.

Origin of ferromagnetism and its pressure and doping dependence in Tl2Mn2O7.

Using the Nth order muffin-tin obital downfolding technique, we investigate the origin of ferromagnetism in pyrochlore Tl2Mn2O7. It is found to be driven by a hybridization induced spin polarization of delocalized charge carriers derived from Tl-s and O-p states. The mean-field estimate of the ferromagnetic transition temperature Tc estimated using computed exchange integrals are found to be in good agreement with measurements. We find an enhancement of Tc for moderate doping with nonmagnetic Sb and a suppression of Tc upon application of pressure, both in agreement with experimental findings.

Na2V3O7: a frustrated nanotubular system with spin-1/2 diamond ring geometry.

Following the recent discussion on the puzzling nature of the interactions in the nanotubular system Na(2)V(3)O(7), we present a detailed ab initio microscopic analysis of its electronic and magnetic properties. By means of a nontrivial downfolding study we propose an effective model in terms of tubes of nine-site rings with the geometry of a spin-diamond necklace with frustrated inter-ring interactions. We show that this model provides a quantitative account of the observed magnetic behavior.