Spatial Structures and Giant Number Fluctuations in Models of Active Matter

The large scale fluctuations of the ordered state in active matter systems are usually characterised by studying the "giant number fluctuations" of particles in any finite volume, as compared to the expectations from the central limit theorem. However, in ordering systems, the fluctuations in density ordering are often captured through their structure functions deviating from Porod law. In this paper we study the relationship between giant number fluctuations and structure functions, for different models of active matter as well as other non-equilibrium systems. A unified picture emerges, with different models falling in four distinct classes depending on the nature of their structure functions. For one class, we show that experimentalists may find Porod law violation, by measuring subleading corrections to the number fluctuations.Comment: 5 pages, 3 figure

First-Principles Design and Discovery of New High-Entropy Materials

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Nature of the spiral state, electric polarisation and magnetic transitions in Sr-doped YBaCuFeO5_5: A first-principles study

Contradictory results on the ferroelectric response of type II multiferroic YBaCuFeO$_{5}$, in its incommensurate phase, has of late, opened up a lively debate. There are ambiguous reports on the nature of the spiral magnetic state. Using first-principles DFT calculations for the parent compound within LSDA+U+SO approximation, the multiferroic response and the nature of spiral state is revealed. The helical spiral is found to be more stable below the transition temperature as spins prefer to lie in ab plane. The Dzyaloshinskii-Moriya (DM) interaction and the spin current mechanism were earlier invoked to account for the electric polarisation in this system. However, the DM interaction is found to be absent, spin current mechanism is not valid in the helical spiral state and there is no electric polarisation thereof. These results are in good agreement with the recent single-crystal data. We also investigate the magnetic transitions in YBa$_{1-x}$Sr$_x$CuFeO$_5$ for the entire range $0\le x\le 1$ of doping. The exchange interactions are estimated as a function of doping and a quantum Monte Carlo (QMC) calculation on an effective spin Hamiltonian shows that the paramagnetic to commensurate phase transition temperature increases with doping till $x=0.5$ and decreases beyond. Our observations are consistent with experimental findings.Comment: 8 pages, 7 figure

First-Principles Correlated Approach to the Normal State of Strontium Ruthenate

The interplay between multiple bands, sizable multi-band electronic correlations and strong spin-orbit coupling may conspire in selecting a rather unusual unconventional pairing symmetry in layered Sr$_{2}$RuO$_{4}$. This mandates a detailed revisit of the normal state and, in particular, the $T$-dependent incoherence-coherence crossover. Using a modern first-principles correlated view, we study this issue in the actual structure of Sr$_{2}$RuO$_{4}$ and present a unified and quantitative description of a range of unusual physical responses in the normal state. Armed with these, we propose that a new and important element, that of dominant multi-orbital charge fluctuations in a Hund's metal, may be a primary pair glue for unconventional superconductivity. Thereby we establish a connection between the normal state responses and superconductivity in this system.Comment: 8 pages, 4 figure

Possible routes to superconductivity in the surface layers of V-doped Mg1−δ_{1-\delta}Ti2_2O4_4 through multiple charge transfers and suppression of Jahn-Teller activity

Superconductivity in the family of spinel oxides is very rare owing to their robust Mott-insulating nature. About half a century ago, LiTi$_2$O$_4$ became the first reported spinel compound to show superconductivity with a 12K transition temperature. Since then, several unsuccessful attempts were made to enhance the T$_c$ of this family of materials. However, a very recent experiment [arXiv:2209.02053] has reported superconductivity at a higher temperature (below 16K), in the V-doped Mg$_{1-\delta}$Ti$_2$O$_4$ thin surface layer while its bulk counterpart remains Mott insulating. The superconducting T$_c$ of this material is significantly higher compared to other engineered MgTi$_2$O$_4$ thin films grown on different substrates. From our first-principles analysis, we have identified that Mg-depletion significantly reduces Jahn-Teller (JT) activity and antiferromagnetic superexchange at the surface layer of V-doped MgTi$_2$O$_4$ due to considerable charge transfer between various ions. The combined effect of a degraded antiferromagnetic order and reduced JT activity weakens the Mottness of the system, leading to the emergence of superconductivity at higher temperatures.Comment: Maine article - 11 pages (single column), 4 figures. Supplemental attache