Amid Ranjkesh | Kyungpook National University (original) (raw)

Papers by Amid Ranjkesh

Applied Surface Science, 2016

Sensors, 2015

We propose a highly oriented photochromic dye film for an ultraviolet (UV)-sensing layer, where s... more We propose a highly oriented photochromic dye film for an ultraviolet (UV)-sensing layer, where spirooxazine (SO) derivatives are aligned with the liquid crystalline UV-curable reactive mesogens (RM) using a guest-host effect. For effective electrical UV sensing with a simple metal-insulator-metal structure, our results show that the UV-induced switchable dipole moment amount of the SO derivatives is high; however, their tilting orientation should be controlled. Compared to the dielectric layer with the nearly planar SO dye orientation, the photochromic dielectric layer with the moderately tilted dye orientation shows more than seven times higher the UV-induced capacitance variation.

Optics Express, 2015

We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode ... more We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode that occurred at the moment of frame change even in the optimized DC offset condition for minimum image flicker. Based on the positional dynamic behaviors of LCs by using a high-speed camera, we found that the transient blink is highly related to the asymmetric responses of the splay-bend transitions caused by the flexoelectric (FE) effect. To remove the transient blink, the elastic property adjustment of LCs was an effective solution because the FE switching dynamics between the splay-enhanced and bend-enhanced deformations are highly dependent on the elastic constants of LCs, which is the cause of momentary brightness drop.

Molecular Crystals and Liquid Crystals, 2015

SID Symposium Digest of Technical Papers, 2015

We studied the typical domain size and configuration character of randomly perturbed nematic liqu... more We studied the typical domain size and configuration character of randomly perturbed nematic liquid crystal system, or in general the system of rod-like objects which interact via a Lebwohl-Lasher-type interaction. We described their local direction with a headless unit director field. We introduced into the system the impurities of concentration p, which impose the random anisotropy field-type disorder to directors. We studied the domain-type pattern of molecules as a function of p, anchoring strength W between a neighboring director and impurity, temperature, and history of samples. In simulations, we quenched the directors either from the random or from homogeneous initial configuration. Our results show that a history of system strongly influences: i) the average domain coherence length; and ii) the range of orientational ordering in the system. In the random case, the obtained order is always short-range (SRO). On the contrary, in the homogeneous case, SRO is obtained only for strong enough anchoring W and large enough concentration p. In other cases, the ordering is either quasi long range (QLRO) or long range (LRO). We further studied field-induced memory effects for the random initial configuration. With increasing external ordering field, either QLRO or LRO is realized. This ordering is preserved even if the field is switched off, and its degree saturates for a large enough value of the field. Therefore, one can control the degree of global ordering and average domain coherence size by temporarily exposing the system to an external ordering field. Such systems could be exploited as soft matter based phase-memory devices.

2008 IEEE International Conference on Dielectric Liquids, 2008

The valuable information about the nature and strength of interactions in liquid solutions, over ... more The valuable information about the nature and strength of interactions in liquid solutions, over a range of mole fractions, can be obtained using dielectric studies. A large amount of investigation has been carried out in recent years on the dielectric measurements of polar solutions with various molecular structure and different functional groups, in order to understand and provide further information about the dielectric behavior and solution chemistry of chosen compounds. The dielectric properties of Flour benzene, Chlorobenzne, Bromobenzne , 4-Chlorobenzaldehyd and 4-flourobenzaldehyd in 1,4-dioxane at various concentrations were measured at 298 K, 308 K and 318 K.The Kirkwood -Frohlich linear correlation factor ( g) , Dipole moment and Bruggeman dielectric factor were determined.

Advances in Condensed Matter Physics, 2014

We studied numerically external field induced memory effects in randomly perturbed nematic liquid... more We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. The impurities imposing orientational disorder were randomly spatially distributed with the concentration below the percolation threshold. Simulations were carried for finite temperatures, where we varied , interaction strength between LC molecules, and impurities and external field . In the { , } plane we determined lines separating short rangequasi long range and quasi long range-long range order. Furthermore, crossover regime separating external field and random field dominated regime was estimated. We calculated remanent nematic ordering in samples at = 0 as a function of the previously experienced external field strength .

We studied numerically external field induced memory effects in randomly perturbed nematic liquid... more We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. The impurities imposing orientational disorder were randomly spatially distributed with the concentration 𝑝 below the percolation threshold. Simulations were carried for finite temperatures, where we varied 𝑝, interaction strength between LCmolecules, and impurities and external field𝐵. In the {𝐵, 𝑇} plane we determined lines separating short range— quasi long range and quasi long range—long range order. Furthermore, crossover regime separating external field and random field
dominated regime was estimated. We calculated remanent nematic ordering in samples at 𝐵 = 0 as a function of the previously experienced external field strength 𝐵.

Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied ... more Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied using Brownian molecular dynamics. A three-dimensional lattice system of rod-like
objects interacting via the Lebwohl–Lasher interaction is considered. We test the applicability of predictions originally derived in cosmology and magnetism. In the first part we focus on coarsening dynamics following the temperature driven isotropic–nematic phase transition for different quench rates. The behavior in the early coarsening regime supports predictions made originally by Kibble in cosmology. For fast enough quenches, symmetry breaking and causality give rise to a dense tangle of defects. When the degree of orientational ordering is large enough, well defined protodomains characterized by a single average domain length are
formed. With time subcritical domains gradually vanish and supercritical domains grow with time, exhibiting a universal scaling law. In the second part of the paper we study the impact of random-field-type disorder on a range of ordering in the (symmetry broken) nematic phase. We demonstrate that short-range order is observed even for a minute concentration of impurities, giving rise to disorder in line with the Imry–Ma theorem prediction only for the appropriate history of systems.

We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal... more We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the
impact of the samples history on the universal behavior.The obtained results are of interest for every randomly perturbed system
exhibiting a continuous symmetry-breaking phase transition. A semimicroscopic lattice simulation is used where the LCmolecules
are treated as cylindrically symmetric, rod-like objects interacting via a Lebwohl-Lasher (LL) interaction. Pure LC systems exhibit a
first order phase transition into the orientationally ordered nematic phase at𝑇 = 𝑇c on lowering the temperature𝑇.Theorientational
ordering of LC molecules is perturbed by the quenched, randomly distributed rod-like impurities of concentration 𝑝. Their
orientation is randomly distributed, and they are coupled with the LC molecules via an LL-type interaction. Only concentrations
below the percolation threshold are considered. The key macroscopic characteristics of perturbed LC structures in the symmetrybroken
nematic phase are analyzed for two qualitatively different histories at 𝑇 ≪ 𝑇
𝑐. We demonstrate that, for a weak enough
interaction among the LCmolecules and impurities, qualitatively different history-dependent states could be obtained. These states
could exhibit either short-range, quasi-long-range, or even long-range order.

Glassy liquid crystalline systems are expected to show significant history-dependent effects. Two... more Glassy liquid crystalline systems are expected to show significant history-dependent effects. Two model glassy
systems are the RAN and SSS (sprinkled silica spin) lattice models. The RAN model is a Lebwohl-Lasher lattice
model with locally coupled nematic spins, together with uncorrelated random anisotropy fields at each site, while
the SSS model has a finite concentration of impurity spins frozen in random directions. Here Brownian simulation
is used to study the effect of different sample histories in the low temperature regime in a three-dimensional
(d = 3) model intermediate between SSS and RAN, in which a finite concentration p < pc (pc the percolation
threshold) of frozen spins interacts with neighboring nematic spins with couplingW. Simulations were performed
at temperature T ∼ TNI /2 (TNI the bulk nematic-isotropic transition temperature) for temperature-quenched and
field-quenched histories (TQH and FQH, respectively), as well as for temperature-annealed histories (AH). The
first two of these limits represent extreme histories encountered in typical experimental studies. Using long-time
averages for equilibrated systems,we calculate orientational order parameters and two-point correlation functions.
Finite-size scaling was used to determine the range of the orientational ordering, as a function of coupling strength
W,p and sample history. Sample history plays a significant role; for given concentration p, as disorder strength
W is increased, TQH systems sustain quasi-long-range order (QLRO) and short-range order (SRO). The data
are also consistent with a long-range order (LRO) phase at very low disorder strength. By contrast, for FQH and
p  0.1, only LRO and QLRO occur within the range of parameters investigated. The crossover between regimes
depends on history, but in general, the FQH phase is more ordered than the AH phase, which is more ordered
than the TQH phase. However, at temperatures close to the isotropic-nematic phase transition of pure samples
we observe SRO for p = 0.1 even for FQH.We detect also in the QLRO phase a domain-type structural pattern,
consistent with ideas introduced by Giamarchi and Doussal [Phys. Rev. B 52, 1242 (1995)] on superconducting
flux lattices. In the weak-disorder limit the orientational correlation length obeys the Larkin-Imry-Ma scaling
ξ ∼ D
−2/(4−d).

Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanato... more Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and
4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated throughout the nematic and isotropic
phase in order to determine their refractive indices and order parameters using the Vuks method. These
factors characterize anisotropic behaviors of selected liquid crystals that are essential for their applications
and improvement. The temperature dependence of refractive indices, birefringence and order parameters in
anisotropic nematic phase, have been investigated for these four nematic liquid crystals. All the compounds
have positive and large optical anisotropies that increase with decreasing temperature.

Experimental data from dielectric investigations of solutions of flourobenzene (FB) and its mesog... more Experimental data from dielectric investigations of solutions of flourobenzene
(FB) and its mesogenic derivative (1-fluoro-4-(4-pentylcyclohexyl) benzene (FPCHB) in
1,4-dioxane are reported for various mole fractions and temperatures. The molecular dipole
moments were determined using the Guggenheim-Debye method in the temperature
range of 298.2 to 318.2 K. Both fluorinated compounds show a positive and small temperature
coefficient for the effective dipole moment. Variations of the effective dipole moment
and correlation factor, g, with mole fraction in these mixtures were investigated using
the Kirkwood-Frohlich equation. Dielectric measurements were also carried out on binary
mixtures of FPCHB with 1-butanol for various concentrations at 318.2 K. The Kirkwood
correlation factor, the Bruggeman factor, and the excess permittivity were determined.

Experimental results of dielectric investigations for solutions of the three butanediols {2,3-but... more Experimental results of dielectric investigations for solutions of the three butanediols {2,3-butanediol
(2,3BD), 1,3-butanediol (1,3BD), and 1,4-butanediol (1,4BD)}, in 1,4-dioxane (1,4DX) are reported for various
mole fractions at T = 298.2 K. Values of relative permittivity were measured at 100 kHz. The molecular
dipole moments were determined using Guggenheim method. The variations of effective dipole
moment and correlation factor, g, with mole fraction in these materials were investigated using Kirkwood–
Frohlich equation. Dielectric measurements were also carried out on binary polar mixtures of
the butanediols with 2-ethyl-1-hexanol (2EH) for various concentrations at T = 298.2 K. The Kirkwood
correlation factor, the Bruggeman factor, and the excess permittivity were determined.

Applied Surface Science, 2016

Sensors, 2015

We propose a highly oriented photochromic dye film for an ultraviolet (UV)-sensing layer, where s... more We propose a highly oriented photochromic dye film for an ultraviolet (UV)-sensing layer, where spirooxazine (SO) derivatives are aligned with the liquid crystalline UV-curable reactive mesogens (RM) using a guest-host effect. For effective electrical UV sensing with a simple metal-insulator-metal structure, our results show that the UV-induced switchable dipole moment amount of the SO derivatives is high; however, their tilting orientation should be controlled. Compared to the dielectric layer with the nearly planar SO dye orientation, the photochromic dielectric layer with the moderately tilted dye orientation shows more than seven times higher the UV-induced capacitance variation.

Optics Express, 2015

We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode ... more We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode that occurred at the moment of frame change even in the optimized DC offset condition for minimum image flicker. Based on the positional dynamic behaviors of LCs by using a high-speed camera, we found that the transient blink is highly related to the asymmetric responses of the splay-bend transitions caused by the flexoelectric (FE) effect. To remove the transient blink, the elastic property adjustment of LCs was an effective solution because the FE switching dynamics between the splay-enhanced and bend-enhanced deformations are highly dependent on the elastic constants of LCs, which is the cause of momentary brightness drop.

Molecular Crystals and Liquid Crystals, 2015

SID Symposium Digest of Technical Papers, 2015

We studied the typical domain size and configuration character of randomly perturbed nematic liqu... more We studied the typical domain size and configuration character of randomly perturbed nematic liquid crystal system, or in general the system of rod-like objects which interact via a Lebwohl-Lasher-type interaction. We described their local direction with a headless unit director field. We introduced into the system the impurities of concentration p, which impose the random anisotropy field-type disorder to directors. We studied the domain-type pattern of molecules as a function of p, anchoring strength W between a neighboring director and impurity, temperature, and history of samples. In simulations, we quenched the directors either from the random or from homogeneous initial configuration. Our results show that a history of system strongly influences: i) the average domain coherence length; and ii) the range of orientational ordering in the system. In the random case, the obtained order is always short-range (SRO). On the contrary, in the homogeneous case, SRO is obtained only for strong enough anchoring W and large enough concentration p. In other cases, the ordering is either quasi long range (QLRO) or long range (LRO). We further studied field-induced memory effects for the random initial configuration. With increasing external ordering field, either QLRO or LRO is realized. This ordering is preserved even if the field is switched off, and its degree saturates for a large enough value of the field. Therefore, one can control the degree of global ordering and average domain coherence size by temporarily exposing the system to an external ordering field. Such systems could be exploited as soft matter based phase-memory devices.

2008 IEEE International Conference on Dielectric Liquids, 2008

The valuable information about the nature and strength of interactions in liquid solutions, over ... more The valuable information about the nature and strength of interactions in liquid solutions, over a range of mole fractions, can be obtained using dielectric studies. A large amount of investigation has been carried out in recent years on the dielectric measurements of polar solutions with various molecular structure and different functional groups, in order to understand and provide further information about the dielectric behavior and solution chemistry of chosen compounds. The dielectric properties of Flour benzene, Chlorobenzne, Bromobenzne , 4-Chlorobenzaldehyd and 4-flourobenzaldehyd in 1,4-dioxane at various concentrations were measured at 298 K, 308 K and 318 K.The Kirkwood -Frohlich linear correlation factor ( g) , Dipole moment and Bruggeman dielectric factor were determined.

Advances in Condensed Matter Physics, 2014

We studied numerically external field induced memory effects in randomly perturbed nematic liquid... more We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. The impurities imposing orientational disorder were randomly spatially distributed with the concentration below the percolation threshold. Simulations were carried for finite temperatures, where we varied , interaction strength between LC molecules, and impurities and external field . In the { , } plane we determined lines separating short rangequasi long range and quasi long range-long range order. Furthermore, crossover regime separating external field and random field dominated regime was estimated. We calculated remanent nematic ordering in samples at = 0 as a function of the previously experienced external field strength .

We studied numerically external field induced memory effects in randomly perturbed nematic liquid... more We studied numerically external field induced memory effects in randomly perturbed nematic liquid crystals. Random anisotropy nematic-type lattice model was used. The impurities imposing orientational disorder were randomly spatially distributed with the concentration 𝑝 below the percolation threshold. Simulations were carried for finite temperatures, where we varied 𝑝, interaction strength between LCmolecules, and impurities and external field𝐵. In the {𝐵, 𝑇} plane we determined lines separating short range— quasi long range and quasi long range—long range order. Furthermore, crossover regime separating external field and random field
dominated regime was estimated. We calculated remanent nematic ordering in samples at 𝐵 = 0 as a function of the previously experienced external field strength 𝐵.

Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied ... more Universal behavior related to continuous symmetry breaking in nematic liquid crystals is studied using Brownian molecular dynamics. A three-dimensional lattice system of rod-like
objects interacting via the Lebwohl–Lasher interaction is considered. We test the applicability of predictions originally derived in cosmology and magnetism. In the first part we focus on coarsening dynamics following the temperature driven isotropic–nematic phase transition for different quench rates. The behavior in the early coarsening regime supports predictions made originally by Kibble in cosmology. For fast enough quenches, symmetry breaking and causality give rise to a dense tangle of defects. When the degree of orientational ordering is large enough, well defined protodomains characterized by a single average domain length are
formed. With time subcritical domains gradually vanish and supercritical domains grow with time, exhibiting a universal scaling law. In the second part of the paper we study the impact of random-field-type disorder on a range of ordering in the (symmetry broken) nematic phase. We demonstrate that short-range order is observed even for a minute concentration of impurities, giving rise to disorder in line with the Imry–Ma theorem prediction only for the appropriate history of systems.

We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal... more We study the characteristics of nematic structures in a randomly perturbed nematic liquid crystal (LC) phase. We focus on the
impact of the samples history on the universal behavior.The obtained results are of interest for every randomly perturbed system
exhibiting a continuous symmetry-breaking phase transition. A semimicroscopic lattice simulation is used where the LCmolecules
are treated as cylindrically symmetric, rod-like objects interacting via a Lebwohl-Lasher (LL) interaction. Pure LC systems exhibit a
first order phase transition into the orientationally ordered nematic phase at𝑇 = 𝑇c on lowering the temperature𝑇.Theorientational
ordering of LC molecules is perturbed by the quenched, randomly distributed rod-like impurities of concentration 𝑝. Their
orientation is randomly distributed, and they are coupled with the LC molecules via an LL-type interaction. Only concentrations
below the percolation threshold are considered. The key macroscopic characteristics of perturbed LC structures in the symmetrybroken
nematic phase are analyzed for two qualitatively different histories at 𝑇 ≪ 𝑇
𝑐. We demonstrate that, for a weak enough
interaction among the LCmolecules and impurities, qualitatively different history-dependent states could be obtained. These states
could exhibit either short-range, quasi-long-range, or even long-range order.

Glassy liquid crystalline systems are expected to show significant history-dependent effects. Two... more Glassy liquid crystalline systems are expected to show significant history-dependent effects. Two model glassy
systems are the RAN and SSS (sprinkled silica spin) lattice models. The RAN model is a Lebwohl-Lasher lattice
model with locally coupled nematic spins, together with uncorrelated random anisotropy fields at each site, while
the SSS model has a finite concentration of impurity spins frozen in random directions. Here Brownian simulation
is used to study the effect of different sample histories in the low temperature regime in a three-dimensional
(d = 3) model intermediate between SSS and RAN, in which a finite concentration p < pc (pc the percolation
threshold) of frozen spins interacts with neighboring nematic spins with couplingW. Simulations were performed
at temperature T ∼ TNI /2 (TNI the bulk nematic-isotropic transition temperature) for temperature-quenched and
field-quenched histories (TQH and FQH, respectively), as well as for temperature-annealed histories (AH). The
first two of these limits represent extreme histories encountered in typical experimental studies. Using long-time
averages for equilibrated systems,we calculate orientational order parameters and two-point correlation functions.
Finite-size scaling was used to determine the range of the orientational ordering, as a function of coupling strength
W,p and sample history. Sample history plays a significant role; for given concentration p, as disorder strength
W is increased, TQH systems sustain quasi-long-range order (QLRO) and short-range order (SRO). The data
are also consistent with a long-range order (LRO) phase at very low disorder strength. By contrast, for FQH and
p  0.1, only LRO and QLRO occur within the range of parameters investigated. The crossover between regimes
depends on history, but in general, the FQH phase is more ordered than the AH phase, which is more ordered
than the TQH phase. However, at temperatures close to the isotropic-nematic phase transition of pure samples
we observe SRO for p = 0.1 even for FQH.We detect also in the QLRO phase a domain-type structural pattern,
consistent with ideas introduced by Giamarchi and Doussal [Phys. Rev. B 52, 1242 (1995)] on superconducting
flux lattices. In the weak-disorder limit the orientational correlation length obeys the Larkin-Imry-Ma scaling
ξ ∼ D
−2/(4−d).

Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanato... more Four tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and
4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated throughout the nematic and isotropic
phase in order to determine their refractive indices and order parameters using the Vuks method. These
factors characterize anisotropic behaviors of selected liquid crystals that are essential for their applications
and improvement. The temperature dependence of refractive indices, birefringence and order parameters in
anisotropic nematic phase, have been investigated for these four nematic liquid crystals. All the compounds
have positive and large optical anisotropies that increase with decreasing temperature.

Experimental data from dielectric investigations of solutions of flourobenzene (FB) and its mesog... more Experimental data from dielectric investigations of solutions of flourobenzene
(FB) and its mesogenic derivative (1-fluoro-4-(4-pentylcyclohexyl) benzene (FPCHB) in
1,4-dioxane are reported for various mole fractions and temperatures. The molecular dipole
moments were determined using the Guggenheim-Debye method in the temperature
range of 298.2 to 318.2 K. Both fluorinated compounds show a positive and small temperature
coefficient for the effective dipole moment. Variations of the effective dipole moment
and correlation factor, g, with mole fraction in these mixtures were investigated using
the Kirkwood-Frohlich equation. Dielectric measurements were also carried out on binary
mixtures of FPCHB with 1-butanol for various concentrations at 318.2 K. The Kirkwood
correlation factor, the Bruggeman factor, and the excess permittivity were determined.

Experimental results of dielectric investigations for solutions of the three butanediols {2,3-but... more Experimental results of dielectric investigations for solutions of the three butanediols {2,3-butanediol
(2,3BD), 1,3-butanediol (1,3BD), and 1,4-butanediol (1,4BD)}, in 1,4-dioxane (1,4DX) are reported for various
mole fractions at T = 298.2 K. Values of relative permittivity were measured at 100 kHz. The molecular
dipole moments were determined using Guggenheim method. The variations of effective dipole
moment and correlation factor, g, with mole fraction in these materials were investigated using Kirkwood–
Frohlich equation. Dielectric measurements were also carried out on binary polar mixtures of
the butanediols with 2-ethyl-1-hexanol (2EH) for various concentrations at T = 298.2 K. The Kirkwood
correlation factor, the Bruggeman factor, and the excess permittivity were determined.