Coupling between magnetic ordering and structural instabilities in perovskite biferroics: A first-principles study

We use first-principles density functional theory-based calculations to investigate structural instabilities in the high symmetry cubic perovskite structure of rare-earth (R $=$ La, Y, Lu) and Bi-based biferroic chromites, focusing on $\Gamma$ and $R$ point phonons of states with para-, ferro-, and antiferromagnetic ordering. We find that (a) the structure with G-type antiferromagnetic ordering is most stable, (b) the most dominant structural instabilities in these oxides are the ones associated with rotations of oxygen octahedra, and (c) structural instabilities involving changes in Cr-O-Cr bond angle depend sensitively on the changes in magnetic ordering. The dependence of structural instabilities on magnetic ordering can be understood in terms of how super-exchange interactions depend on the Cr-O-Cr bond angles and Cr-O bond lengths. We demonstrate how adequate buckling of Cr-O-Cr chains can favour ferromagnetism. Born effective charges (BEC) calculated using the Berry phase expression are found to be anomalously large for the A-cations, indicating their chemical relevance to ferroelectric distortions.Comment: 8 pages, 13 figure

Electric-field driven insulating to conducting transition in a mesoscopic quantum dot lattice

We investigate electron transport through a finite two dimensional mesoscopic periodic potential, consisting of an array of lateral quantum dots with electron density controlled by a global top gate. We observe a transition from an insulating state at low bias voltages to a conducting state at high bias voltages. The insulating state shows simply activated temperature dependence, with strongly gate voltage dependent activation energy. At low temperatures the transition between the insulating and conducting states becomes very abrupt and shows strong hysteresis. The high-bias behavior suggests underdamped transport through a periodic washboard potential resulting from collective motion.Comment: 4 pages, 4 figure

Multiferroic Properties of Nanocrystalline BaTiO3

Some of the Multiferroics [1] form a rare class of materials that exhibit magnetoelectric coupling arising from the coexistence of ferromagnetism and ferroelectricity, with potential for many technological applications.[2,3] Over the last decade, an active research on multiferroics has resulted in the identification of a few routes that lead to multiferroicity in bulk materials.[4-6] While ferroelectricity in a classic ferroelectric such as BaTiO3 is expected to diminish with the reducing particle size,[7,8] ferromagnetism cannot occur in its bulk form.[9] Here, we use a combination of experiment and first-principles simulations to demonstrate that multiferroic nature emerges in intermediate size nanocrystalline BaTiO3, ferromagnetism arising from the oxygen vacancies at the surface and ferroelectricity from the core. A strong coupling between a surface polar phonon and spin is shown to result in a magnetocapacitance effect observed at room temperature, which can open up possibilities of new electro-magneto-mechanical devices at the nano-scale.Comment: 15 pages, 5 figure

Charge transport in nanopatterned PbS colloidal quantum dot arrays

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 149-159).In this thesis, we study charge transport in nanopatterned arrays of PbS colloidal quantum dots using conventional two-probe measurements and an integrated charge sensor. PbS dots are synthesized in solution with an organic ligand or cap that serves to passivate the surface of the dot, provide a tunnel barrier as well as colloidal stability. These dots can self assemble into an array as the solvent evaporates from a drop of solution on a surface. The self-assembled arrays can be thought of as tunable artificial solids, where the coupling between the dots can be tuned by changing the ligand. Using electron beam lithography followed by a lift-off process, we develop a novel technique to nanopattern these arrays and present the first colloidal quantum dot arrays of nanoscale dimensions. Nanopatterning makes it possible to study the electrical properties intrinsic to the dots unimpeded by macroscopic defects, such as cracking and clustering that typically exist in larger-scale arrays. We find that the electrical conductivity of the nanoscale films is higher than that of drop-cast, microscopic films made of the same type of dot. We achieve unprecedented versatility in integrating the patterned films into device structures, which will be valuable both for studying the intrinsic electrical properties of the dots and for nanoscale optoelectronic applications. From two-probe measurements on the nanopatterned arrays that are approximately 15 dots wide, we observe large noise in the current as a function of time. The noise is proportional to the current when the latter is varied by applying source-drain or gate voltage in a field-effect structure or when changing temperature. Owing to the small number of current paths in the system, we often observe telegraph switching, and find that the off times follow non-poissonian statistics. We show that the results can be understood in terms of a model in which a quasi-one-dimensional percolation path is turned on and off, by charging of a dot along the path. Long organic ligands lead to highly resistive colloidal quantum dot arrays, making the low bias regime inaccessible with conventional two-probe measurements. We use an integrated charge sensor to study transport in the low bias regime as a function of the coupling between the dots. We present transport measurements on butylamine and oleic acid capped PbS dots. The resistances measured are the highest measured for colloidal quantum dots. For the native oleic acid ligand, and weak coupling between the dots, the conduction mechanism is nearest neighbor hopping, and the conductance is simply activated. At low source-drain bias voltages, the activation energy is given by the energy required to release a carrier from a trap state plus the activation over barriers resulting from site disorder. The barriers from site disorder are eliminated with a sufficiently high source-drain bias. For the shorter ligand, which gives stronger coupling, the data are consistent with Mott's variable range hopping as the conduction mechanism.by Nirat Ray.Ph. D

Assessing the impact of triaxial strain on carrier mobility and dielectric properties in cubic CsBCl3 (B = Pb, Sn, or Ge) perovskites: A first principles study

Investigating the strain and pressure dependence of perovskite materials can provide valuable insights into their structural and electronic responses, enabling the fine-tuning of their properties for various technological applications. This study investigates the influence of controlled lattice compression and expansion on the acoustic phonon-limited carrier mobility in CsBCl3 (B = Pb, Sn, Ge) perovskites, revealing tunable electronic bandgaps ranging from 0.3 to 1.2 eV by varying the B cation type and applied triaxial strain. The research demonstrates significant and monotonic carrier mobility modulation under pressure, with changes of up to 124% even at modest strain levels of −2%, along with linearly increasing exciton binding energy with lattice parameter expansion

Magnetotransport of vertex frustrated artificial spin ice structures

by Elysia Sharma,Daan M. Arroo,Nirat Ray,Lesley Cohen and Will Branfor

Monte Carlo simulation based preventive maintenance plan for a sewage pump system Case study - Nacka Municipality

Heavy rare earth chromites of the formula $LnCrO_3$ with Ln = Ho, Er, Yb, Lu and Y are shown to be multiferroic, exhibiting canted antiferromagnetism at low temperatures ($T_N$ = 113–140 K) and a ferroelectric transition in the 472–516 K range