Department of Physics

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In this work, we report the synthesis of Cu 2 ZnGeS 4 (CZGS) thin films using a paste of binary metal sulfides by doctor blade technique. The precursor to deposit the films was obtained by mixing appropriate amounts of CuS, ZnS and GeS with triethanolamine and dilute acetic acid. Initially, the effect of annealing temperature on the phase formation was investigated using the films developed with the precursor containing stoichiometric ratios of metal sulfides. Based on the results obtained for structural and compositional properties of the films using the stoichiometric precursor we further proceeded with the fabrication of Cu-poor CZGS films by varying the binary sulfide concentrations in the precursor. CZGS films deposited with precursor molar ratio, CuS:ZnS:GeS = 1.6:1.2:1, resulted in [Cu]/([Zn] + [Ge]) and [Zn]/[Ge] ratios, 0.77 and 1.21, respectively. Formation of polycrystalline CZGS tetragonal structure was confirmed by X-ray diffraction and the Raman spectroscopy. Scanning electron microscopy revealed flake-like morphology along with pyramidal shaped crystals. The optical band gap of the films was around 1.8 eV. The films were photosensitive and exhibited one order increase in photoconductivity under 100 mW/cm 2 illumination. The fabrication of Cu-poor CZGS films is reproducible by upscaling the amounts of binary sulfides and dispersing medium which provides an attractive option for cost-effective deposition over large area. © 2019

World University root node

Thin Solid Films

Facile synthesis of Cu 2 ZnGeS 4 thin films from binary metal sulfides and study of their physical properties

The physics of topological singularities, namely, exceptional points (EPs), has been a key to a wide range of intriguing and unique physical effects in non-Hermitian systems. In this context, exploration of the mutual interactions among the states in four-level systems around fourth-order EPs (EP4s) is lacking. Here we report a four-level parameter-dependent perturbed non-Hermitian Hamiltonian, mimicking quantum or wave-based systems, to explore the physical aspects of an EP4 analytically as well as numerically. The proposed Hamiltonian exhibits different orders of interaction schemes with the simultaneous presence of different higher-order EPs. Here an EP4 has been realized by mutual interaction between four states with proper parameter manipulation. We comprehensively investigate the dynamics of corresponding coupled eigenvalues with stroboscopic parametric variation in the vicinity of the embedded EP4 to establish a successive state-switching phenomenon among them, which proves to be robust even in the presence of different orders of EPs. Implementing the relation of the perturbation parameters with the coupling control parameters, we report a region to host multiple EP4s in a specific system. The chiral behavior of successive state exchange has also been established near the EP4. The proposed scheme, which is enriched with physical aspects of EP4s, should provide a unique light manipulation tool in any anisotropic multistate integrated system. © 2019 American Physical Society.

Fulltext

Physical Review A

Higher-order topological degeneracies and progress towards unique successive state switching in a four-level open system

We consider two non-Markovian models: random telegraph noise and non-Markovian dephasing. The memory in these models is studied from the perspective of quantum Fisher information flow. This is found to be consistent with the other well-known witnesses of non-Markovianity. The two noise channels are characterized quantum information theoretically by studying their gate and channel fidelities. Further, the quantum coherence and its balance with mixedness is studied. This helps to put in perspective the role that the two noise channels can play in various facets of quantum information processing and quantum communication. © 2019 American Physical Society.

Facets of quantum information under non-Markovian evolution

The present work is devoted to the characterization of the Leggett-Garg inequality (LGI) for three flavored neutrino oscillations in the presence of both matter and charge-conjugation and parity violating effects. This study complements and completes the recent one put forward in arXiv:1710.05562 by relaxing the stationary condition. At variance with the latter case, the LGI contains interference terms which cannot be expressed in terms of experimentally measurable quantities, thus drawing a clear-cut distinction between the two scenarios, as well as highlighting the role of the stationary assumption on such systems. We find that the additional terms are small for a high energy neutrino beam compared to the maximum value attained by the Leggett-Garg parameter. © 2019 authors. Published by the American Physical Society.

Physical Review D

Leggett-Garg inequality in the context of three flavor neutrino oscillation

At Moriond 2019, Belle collaboration announced their new measurements on RD and RD∗ which are in agreement with their Standard Model (SM) predictions within 1.2σ. After inclusion of these measurements, the discrepancy between the world averages and the SM predictions of RD-RD∗ comes down from 4.1σ to 3.1σ. Here we do a global fit by taking all measurements in b → cτν¯ transition. We find that there are seven allowed new physics solutions, each with different Lorentz structure. Further we show that it is possible to distinguish between them by means of the five observables: the τ polarization fraction in the decay B → Dτν¯, the precision measurement of RD, the forward-backward asymmetries in the decays B → (D,D∗)τν¯ and the branching ratio of Bc → τν¯. © Owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0).

Proceedings of Science

A global fit to b → cτν¯ anomalies after Moriond 2019

We studied the effect of biaxial tensile and compressive strain on the electronic and optical properties using the density function theory. Electronic properties are investigated in terms of the electronic band structure and density of states. CdO monolayer exhibits a direct bandgap ∼ 1.35 eV using a modified Becke–Johnson approximation. The value of bandgap decreases under both biaxial tensile and compressive strains. Moreover, direct to indirect electronic bandgap transition is observed above 8% tensile strain, whereas direct bandgap nature is conserved under compressive strain. The effective mass of an electron is nearly insensitive to strain, whereas hole effective mass shows linear increment with strain and a sudden jump at the electronic transition point of above 8% tensile strain. The noticed low reflectivity in unstrained and compressive strained conditions suggests that the CdO monolayer may be a promising material for transparent conducting oxide applications. © 2019, The Minerals, Metals & Materials Society.

Journal of Electronic Materials

Strain Modulated Optoelectric Properties of CdO Monolayer

Metal halide perovskites are actively pursued as photoelectrodes to drive solar fuel synthesis. However, currently, these photocathodes suffer from limited stability in water, which hampers their practical application. Here, we report a high-performance solution-processable photocathode composed of cesium formamidinium methylammonium triple-cation lead halide perovskite protected by an Al-doped ZnO (AZO) layer combined with a Field's metal encapsulation. Careful selection of charge transport layers resulted in an improvement in photocurrent, fill factor, device stability and reproducibility. The dead pixels count reduced from 25 to 6% for the devices with an AZO layer, and in photocathodes with an AZO layer the photocurrent density increased by almost 20% to 14.3 mA cm-2. In addition, we observed a 5-fold increase in the device lifetime for photocathodes with AZO, which reached up to 18 h before complete failure. Finally, the photocathodes are fabricated using low-cost and scalable methods, which have promise to become compatible with standard solution-based processes. © 2019 American Chemical Society.

ACS Applied Materials and Interfaces

Triple-Cation-Based Perovskite Photocathodes with AZO Protective Layer for Hydrogen Production Applications

Numerical Simulations are employed to create amorphous nano-films of a chosen thickness on a crystalline substrate which induces strain on the film. The films are grown by a vapor deposition technique. Using the exact relations between the Hessian matrix and the shear and bulk moduli we explore the mechanical properties of the nano-films as a function of the density of the substrate and the film thickness. The existence of the substrate dominates the mechanical properties of the combined substrate-film system. Scaling concepts are then employed to achieve data collapse in a wide range of densities and film thicknesses. © 2018

Scaling theory of the mechanical properties of amorphous nano-films

We investigated complex magnetic properties of multifunctional LaCrO3-LaFeO3 system. The magnetic measurements substantiate the presence of competing complex magnetic ordering against temperature, showing paramagnetic to ferrimagnetic transition at ∼300 K, followed by antiferromagnetic (AFM) transition near ∼250 K superimposed on ferrimagnetic phase. The onset of weak ferrimagnetic ordering is attributed to the competing complex interaction between two AFM LaCrO3-LaFeO3 sublattices. The low-Temperature AFM ordering is also substantiated by temperature-dependent Raman measurements, where the intensity ratio of 700 cm-1 Raman active mode showed a clear enhancement with lowering the temperature. The non-saturating nature of magnetic moments in LaCrO3-LaFeO6 suggests the predominating AFM ordering in conjunction with ferrimagnetic ordering between 250 K-300 K up to 5 T magnetic field. A complex magnetic structure of LaCrO3-LaFeO3 is constructed, emphasizing the metastable magnetic phase near room temperature and low-Temperature antiferromagnetic state. © 2020 IOP Publishing Ltd.

Materials Research Express

Anomalous magnetic behavior and complex magnetic structure of proximate LaCrO3-LaFeO3 system

Semiconducting Zn chalcogenide monolayers are important members of the 2D family of materials due to their unique electronic properties. In this paper, we focus on strain-modulated electronic properties of monolayers of ZnX, with X being O, S, Se, and Te. ZnO and ZnS monolayers have a hexagonal graphene-like planar structure, while ZnSe and ZnTe monolayers exhibit slightly buckled silicene and germanene-like structures, respectively. Density functional theory calculations find the hexagonal ZnO monolayer to be dynamically stable. However, ZnS, ZnSe, and ZnTe monolayers are predicted to be less stable with small imaginary frequencies. The application of tensile strain to these monolayers, interestingly, yields stability of dynamically less stable structures together with the modification in the nature of the bandgap from direct to indirect. For a tensile strain of about 8%, a closure of the bandgap in ZnTe is predicted with the semiconductor-metal transition. The results, therefore, find strain-induced stability and modification in electronic properties of monolayers of Zn chalcogenides, suggesting the use of these monolayers for novel device applications. © 2019 Author(s).