Showing 1–34 of 34 results for author: Rai, R

Compositional gradient engineering for enhanced ferroelectricity in ultrathin AlScN

Authors: Zekun Hu, Haiwen Zhang, Rajeev Kumar Rai, Yuhong Cao, Xiaolei Tong, Pedram Yousefian, Hyunmin Cho, Bongjun Choi, Chao-Chuan Chen, Yunfei He, Kefei Bao, Chloe Leblanc, Eric A. Stach, Roy Olsson, Deep Jariwala

Abstract: Ferroelectric AlScN is promising for CMOS-compatible non-volatile memory, but thickness scaling is limited by leakage, premature breakdown, and defect-mediated failure. Here we show that compositional grading within a continuous wurtzite AlN-AlScN lattice mitigates these limitations by distributing structural and polarization discontinuities across the film thickness, reducing defect formation and… ▽ More Ferroelectric AlScN is promising for CMOS-compatible non-volatile memory, but thickness scaling is limited by leakage, premature breakdown, and defect-mediated failure. Here we show that compositional grading within a continuous wurtzite AlN-AlScN lattice mitigates these limitations by distributing structural and polarization discontinuities across the film thickness, reducing defect formation and local field concentration. In a 20 nm graded heterostructure, monotonic Sc incorporation and AlN-rich boundaries produce reversible ferroelectric switching, an as-grown metal-polar state, a 21% higher breakdown field, 10% enhanced remanent polarization, and 40x higher resistivity relative to homogeneous AlScN. Time-domain PUND measurements reveal strongly suppressed post-switching leakage, consistent with reduced defect-assisted and polarization-coupled conduction. This improved dielectric robustness enables ferroelectric functionality in 5 nm graded stacks containing only a 2 nm $\mathrm{Al}_{0.64}\mathrm{Sc}_{0.36}\mathrm{N}$ region, with measurable switching near 1 V. These results establish compositional grading as a defect- and field-management strategy for scalable ultrathin wurtzite ferroelectrics. △ Less

Submitted 10 June, 2026; originally announced June 2026.

Comments: 6 main figures

Improving the electrical conductivity of Pt nanowires deposited by focused electron beam induced deposition using thermal annealing

Authors: Rajendra Rai, Ujjwal Dhakal, Binod DC, Yoichi Miyahara

Abstract: We investigated the electrical conductivity of platinum nanowires with heights ranging from 2 nm to 200 nm, deposited by focused electron beam induced deposition (FEBID). Post-deposition processing was employed to enhance the electrical conductivity of the platinum nanowires. Thermal annealing of as-deposited nanowires in air at 225$^{\circ}$C for 4 hours increased electrical conductance by up to… ▽ More We investigated the electrical conductivity of platinum nanowires with heights ranging from 2 nm to 200 nm, deposited by focused electron beam induced deposition (FEBID). Post-deposition processing was employed to enhance the electrical conductivity of the platinum nanowires. Thermal annealing of as-deposited nanowires in air at 225$^{\circ}$C for 4 hours increased electrical conductance by up to five orders of magnitude. After annealing, 22.5 $\mathrm{μm}$-long nanowires with a height of 36 nm exhibited resistances of approximately 10 k$Ω$. This nanowire underwent a reduction in height to one-quarter of its original value, a reduction in width to one half, and a reduction in cross-sectional area by approximately one order of magnitude. The platinum-to-carbon weight ratio increased from 35:65 to 85:15. The electrical resistance decreased monotonically as temperature was lowered from room temperature to 100 mK, confirming that annealed FEBID platinum nanowires are promising building blocks for nanoelectronic devices operating at millikelvin temperatures. △ Less

Submitted 19 January, 2026; originally announced January 2026.

Comments: 18 pages, 12 figures

Low-Field Ferroelectricity in 10 nm AlBScN Thin Films

Authors: Xiaolei Tong, Pedram Yousefian, Ziyi Wang, Meenakshi A. Saravanan, Rajeev Kumar Rai, Giovanni Esteves, Eric A. Stach, Roy H. Olsson III

Abstract: Ferroelectric aluminum scandium nitride (Al1-xScxN, AlScN) offers CMOS-compatible integration but suffers from high coercive fields and leakage currents that hinder thickness scaling. Further reduction in thickness is essential for low-voltage embedded nonvolatile memory applications. Boron incorporation into AlScN (AlBScN) suppresses leakage current in films down to 40 nm, yet its ferroelectric c… ▽ More Ferroelectric aluminum scandium nitride (Al1-xScxN, AlScN) offers CMOS-compatible integration but suffers from high coercive fields and leakage currents that hinder thickness scaling. Further reduction in thickness is essential for low-voltage embedded nonvolatile memory applications. Boron incorporation into AlScN (AlBScN) suppresses leakage current in films down to 40 nm, yet its ferroelectric characteristics in ultrathin films remains unexplored. This letter demonstrates robust ferroelectric switching in 10 nm sputtered AlBScN capacitors with a low coercive field and approximately two orders of magnitude lower leakage than AlScN. Notably, ferroelectric switching was observed at 2.2 MV/cm in capacitance-voltage measurements, and symmetric polarization reversal occurred near 4.6 MV/cm in positive-up-negative-down (PUND) measurements using 2 μs pulses. Moreover, Weibull analysis revealed a breakdown-to-coercive-field ratio (EBD/Ec) of ~2.2. These findings demonstrated AlBScN as a promising candidate for CMOS back-end-of-line (BEOL) compatible ferroelectric applications with improved energy consumption and reduced leakage current. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Evolution of electronic and magnetic properties in Mn- and Co-alloyed ferromagnetic kagome metal Fe3Sn2

Authors: Prajwal M. Laxmeesha, Rajesh Dutta, Rajeev Kumar Rai, Sharup Sheikh, Michael F. DiScala, Uditha M. Jayathilake, Alexander Velič, Tarush Tandon, Tessa D. Tucker, Christoph Klewe, Haile Ambaye, Timothy Charlton, Tien-Lin Lee, Eric A. Stach, Kemp W. Plumb, Alexander X. Gray, Steven J. May

Abstract: Kagome metals are an intriguing class of quantum materials as the presence of both flat bands and Dirac points provides access to functional properties present in strongly correlated and topological materials. To fully harness these electronic features, the ability to tune the Fermi level relative to the band positions is needed. Here we explore the structural, electronic and magnetic impacts of s… ▽ More Kagome metals are an intriguing class of quantum materials as the presence of both flat bands and Dirac points provides access to functional properties present in strongly correlated and topological materials. To fully harness these electronic features, the ability to tune the Fermi level relative to the band positions is needed. Here we explore the structural, electronic and magnetic impacts of substitutional alloying within ferromagnetic kagome metal Fe3Sn2 in thin films grown by molecular beam epitaxy. Transition metals Mn and Co are chosen as substitutes for Fe to reduce or increase the d-band electron count, thereby moving the Fermi level accordingly. We find that Co is not incorporated into the Fe3Sn2 structure but instead results in a two-phase Fe-Co and (Fe,Co)Sn composite. In contrast, Fe3-xMnxSn2 films are realized with x up to 1.0, retaining crystalline quality comparable to the parent phase. The incorporation of Mn repositions the flat bands relative to the Fermi level in a manner consistent with hole-doping, as revealed by hard x-ray photoemission and density functional theory. The Fe3-xMnxSn2 films retain room temperature ferromagnetism, with x-ray magnetic circular dichroism measurements confirming that the Fe and Mn moments are ferromagnetically aligned. The ability to hole-dope this magnetic kagome metal provides a platform for tuning properties such as anomalous Hall and Nernst responses. △ Less

Submitted 3 February, 2026; v1 submitted 28 October, 2025; originally announced October 2025.

Comments: 23 pages, 4 figures, APL Materials 14, 011114 (2026)

Coercive Field Reduction in Ultra-thin Al1-XScXN via Interfacial Engineering with a Scandium Electrode

Authors: Yinuo Zhang, Rajeev Kumar Rai, Giovanni Esteves, Yubo Wang, Deep M. Jariwala, Eric A. Stach, Roy H. Olsson III

Abstract: Aluminum scandium nitride (AlScN) ferroelectrics are promising for next-generation non-volatile memory applications due to high remnant polarization compared with Pb(ZrxTi1-x)O3 and doped-HfO2 material systems, as well as their fast switching and scalability to nanometer thicknesses. As AlScN films are thinned to 10 nm thickness, coercive field has been shown to substantially increase, which hinde… ▽ More Aluminum scandium nitride (AlScN) ferroelectrics are promising for next-generation non-volatile memory applications due to high remnant polarization compared with Pb(ZrxTi1-x)O3 and doped-HfO2 material systems, as well as their fast switching and scalability to nanometer thicknesses. As AlScN films are thinned to 10 nm thickness, coercive field has been shown to substantially increase, which hinders low voltage operation. We demonstrate that interfacial engineering through bottom electrode selection and strain management reduces this coercive field increase with scaling and improves ferroelectric performance. Here, we demonstrate robust ferroelectricity in ultra-thin AlScN capacitors deposited on a Sc bottom electrode under both alternating current and direct current conditions. The coercive field is reduced by over 20 percent compared to capacitors with an Al bottom electrode. Furthermore, the difference in dynamic switching behavior over a decade of frequency was evaluated by applying the KAI model. At frequencies lower than 16.7 kHz, the capacitors with Sc and Al bottom electrodes exhibit comparable KAI exponents of 0.030 and 0.028, indicating similar switching kinetics. However, at higher frequencies, the capacitor with an Al bottom electrode shows a significantly higher exponent of 0.063, indicating a stronger frequency dependence, whereas the capacitor with a Sc bottom electrode maintains a stable exponent of 0.030, suggesting a lower frequency dependence during faster switching scenarios. The Scanning Electron Nanobeam Diffraction technique was selected to measure the strain difference in AlScN thin films grown on templates with different lattice mismatch, providing a correlation between lattice mismatch, film strain and switching behavior in ultra-thin film systems. △ Less

Submitted 22 September, 2025; v1 submitted 11 June, 2025; originally announced June 2025.

Demonstration of highly scaled AlScN ferroelectric diode memory with storage density > 100 Mbit/mm$^2$

Authors: Zekun Hu, Hyunmin Cho, Rajeev Kumar Rai, Kefei Bao, Yinuo Zhang, Zhaosen Qu, Yunfei He, Yaoyang Ji, Chloe Leblanc, Kwan-Ho Kim, Zirun Han, Zhen Qiu, Xingyu Du, Eric A. Stach, Roy Olsson, Deep Jariwala

Abstract: Wurtzite nitride ferroelectric materials have emerged as promising candidates for next-generation memory applications due to their exceptional polarization properties and compatibility with conventional semiconductor processing techniques. Here, we demonstrate the first successful areal scaling of Aluminum Scandium Nitride (AlScN) ferroelectric diode (FeDiode) memory down to 40 nm device diameters… ▽ More Wurtzite nitride ferroelectric materials have emerged as promising candidates for next-generation memory applications due to their exceptional polarization properties and compatibility with conventional semiconductor processing techniques. Here, we demonstrate the first successful areal scaling of Aluminum Scandium Nitride (AlScN) ferroelectric diode (FeDiode) memory down to 40 nm device diameters while maintaining ON/OFF > 60. Using a 20 nm thick Al0.64Sc0.36N ferroelectric layer, we evaluate both metal-insulator-ferroelectric-metal (MIFM) and metal-ferroelectric-metal (MFM) architectures for scaled resistive memory devices. Our scaled devices exhibit an enhanced breakdown-to-coercive field ratio exceeding 2.6 due to increased breakdown field. The MIFM devices demonstrate stable 3-bit non-volatile multistate behavior with clearly distinguishable resistance states and retention exceeding 4*10^4 seconds at 85 C. By achieving more than a million-fold areal scaling with enhanced performance metrics, this work establishes AlScN-based FeDiode memory as a highly promising platform for non-volatile storage with potential for direct integration into CMOS technology. △ Less

Submitted 30 August, 2025; v1 submitted 17 April, 2025; originally announced April 2025.

Comments: 4 figures and 1 table

Epitaxial growth and magnetic properties of kagome metal FeSn/elemental ferromagnet heterostructures

Authors: Prajwal M. Laxmeesha, Tessa D. Tucker, Rajeev Kumar Rai, Shuchen Li, Myoung-Woo Yoo, Eric A. Stach, Axel Hoffmann, Steven J. May

Abstract: Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flat bands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high quality heterostructures is required. Here we report the syn… ▽ More Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flat bands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high quality heterostructures is required. Here we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution X-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices. △ Less

Submitted 21 January, 2024; originally announced January 2024.

Comments: The following article has been submitted to the Journal of Applied Physics

Effect of Spin Fluctuations on Magnetoresistance and Anomalous Hall Effect in the Chiral Magnet Co8Zn8Mn4

Authors: P. Saha, P. Das, M. Singh, R. Rai, S. Patnaik

Abstract: The beta Mn type Co-Zn-Mn alloys have seized significant attention due to their ability to host skyrmions at room temperature. Here we analyse the unconventional magneto-transport properties of Co8Zn8Mn4 single crystals with a Curie temperature of 275 K. A negative magnetoresistance is obtained over a wide temperature range of 50K to 300K. The deviation of the isothermal magnetoresistance (MR) cur… ▽ More The beta Mn type Co-Zn-Mn alloys have seized significant attention due to their ability to host skyrmions at room temperature. Here we analyse the unconventional magneto-transport properties of Co8Zn8Mn4 single crystals with a Curie temperature of 275 K. A negative magnetoresistance is obtained over a wide temperature range of 50K to 300K. The deviation of the isothermal magnetoresistance (MR) curves from linearity to non-linearity as one approaches higher temperatures points towards the transition from the dominance of magnons to spin fluctuations. In the paramagnetic phase, the change in the shape of the MR curve has been explained using the Khosla and Fischer model. The relationship between the anomalous Hall effect (AHE) and longitudinal resistivity reveals the dominance of the skew-scattering mechanism, which is inexplicable based on the theories of semi-classical magneto-transport. We experimentally determine that the spin fluctuation is the source of the skew-scattering mechanism in Co8Zn8Mn4. In general skew-scattering mechanisms predominate in compounds with high conductivity, but our findings demonstrate that this is not always the case and that other aspects also require equal consideration. Our work throws new light on the predominant scattering mechanism in chiral magnets with skyrmionics phase at low conductivity. △ Less

Submitted 1 October, 2023; originally announced October 2023.

Clausius' theorem and the Second law in the process of isoenergetic thermalization

Authors: Vansh Narang, Renuka Rai, Ramandeep S. Johal

Abstract: Isoenergetic thermalization amongst $n$ bodies is a well-known irreversible process, bringing the bodies to a common temperature $T_F$ and leading to a rise in the total entropy of the bodies. We express this change in entropy using the Clausius formula over a reversible path connecting $T_F$ with $T_f$ which corresponds to the entropy-preserving temperature of the initial nonequilibrium state. Un… ▽ More Isoenergetic thermalization amongst $n$ bodies is a well-known irreversible process, bringing the bodies to a common temperature $T_F$ and leading to a rise in the total entropy of the bodies. We express this change in entropy using the Clausius formula over a reversible path connecting $T_F$ with $T_f$ which corresponds to the entropy-preserving temperature of the initial nonequilibrium state. Under the assumption of positive heat capacities of the bodies, the Second law inequality simply follows from the fact that $T_F > T_f$. We extend this approach to the continuum case of an unequally heated rod, illustrating with the special case of the rod with constant heat capacity and a linear temperature profile. An interpolating profile between the discrete and the continuum models is studied whereby $T_f$, given by the geometric mean temperature over $n$ elements, is shown to approach the identric mean of the lowest and the highest temperatures as $n$ becomes large. We also discuss the case of negative heat capacity in a two-body set up where isoenergetic thermalization may be forbidden by the Second law. However, the alternate scheme in which first work is performed reversibly on the system and then an equivalent amount of energy is extracted in the form of heat, brings the system to an energy-preserving common temperature. △ Less

Submitted 29 August, 2024; v1 submitted 2 August, 2023; originally announced August 2023.

Comments: 17 pages, 2 figures. New section on negative heat capacity added. Main framework remains same

Journal ref: Phys. Rev. E 110, 054103 (2024)

Giant electromechanical response from defective non-ferroelectric epitaxial BaTiO3 integrated on Si 100

Authors: Sandeep Vura, Shubham Kumar Parate, Subhajit Pal, Upanya Khandelwal, Rajeev Kumar Rai, Sri Harsha Molleti, Vishnu Kumar, Rama Satya Sandilya Ventrapragada, Girish Patil, Mudit Jain, Ambresh Mallya, Majid Ahmadi, Bart Kooi, Sushobhan Avasthi, Rajeev Ranjan, Srinivasan Raghavan, Saurabh Chandorkar, Pavan Nukala

Abstract: Lead free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values o… ▽ More Lead free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients (M31) at frequencies as large as 5 kHz (1.04 x 10-14 m2 per V2 at 1 kHz, and 3.87 x 10-15 m2 per V2 at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained even after cycling the devices >5000 times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO3 and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates very well with the observed giant electrostrictive response. These films show large coefficient of thermal expansion (2.36 x 10-5/K), which along with the giant M31 implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films. △ Less

Submitted 6 March, 2023; originally announced March 2023.

Comments: 26 pages, 4 figures, 8 supplementary figures

Optical linear-nonlinear and dispersion parameters of thermally evaporated SnS thin films as absorber material for solar cells

Authors: Vinita, P. Arun, Chandra Kumar, R. Rai, B. K. Singh

Abstract: In this manuscript, we report the results of optical properties of SnS thin films, deposited on FTO coated glass substrates at room temperature by thermal evaporation technique. In addition, the effect of film thickness on the optical behavior of FTO/SnS is analyzed and obtained results are compared with data of SnS films grown on glass and ITO substrates. Our study indicates that the properties o… ▽ More In this manuscript, we report the results of optical properties of SnS thin films, deposited on FTO coated glass substrates at room temperature by thermal evaporation technique. In addition, the effect of film thickness on the optical behavior of FTO/SnS is analyzed and obtained results are compared with data of SnS films grown on glass and ITO substrates. Our study indicates that the properties of SnS film are independent of the substrate material. Further, the influence of the film thickness on the other optical parameters including, linear and third order nonlinear optical constants and dispersion parameters have also been investigated using the transmission, reflection, and absorption spectra. It is found that the optical band gap decreases from 2.07 to 1.30 eV with increase in SnS film thickness, whereas the refractive index increases with increasing thickness. Additionally, the oscillator energy, and the dispersion energy are estimated using WempleDiDomenico approach. The dispersion energies are in the range of 7.20 to 4.59 eV, while the oscillator energies of the thin films are in the range of 5.49 to 2.24 eV. Moreover, the nonlinear refractive index, and optical susceptibility are calculated by using the empirical relation of Tichy and Ticha. The volume of data suggests optical properties of SnS thin films are strongly dependent on film thickness. △ Less

Submitted 19 January, 2023; originally announced January 2023.

Comments: 24 pages, 11 figures

Modulation-Doping a Correlated Electron Insulator

Authors: Debasish Mondal, Smruti Rekha Mahapatra, Abigail M Derrico, Rajeev Kumar Rai, Jay R Paudel, Christoph Schlueter, Andrei Gloskovskii, Rajdeep Banerjee, Frank M F DeGroot, Dipankar D Sarma, Awadhesh Narayan, Pavan Nukala, Alexander X Gray, Naga Phani B Aetukuri

Abstract: Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2 - especially without inducing structural changes - has been a long-standing challenge. In this work, we design and synthesize modu… ▽ More Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2 - especially without inducing structural changes - has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard x-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5x10^21 cm^(-3) without inducing any measurable structural changes. We find that the MIT temperature (T_MIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions. △ Less

Submitted 7 January, 2023; originally announced January 2023.

Comments: Main paper 21 pages, 5 Figures. Supporting Information, 18 Pages,15 SI Figures and 1 SI Section

Coupled autonomous thermal machines and efficiency at maximum power

Authors: Ramandeep S. Johal, Renuka Rai

Abstract: We show that coupled autonomous thermal machines, in the presence of three heat reservoirs and following a global linear-irreversible description, provide a unified framework to accommodate the variety of expressions for the efficiency at maximum power (EMP). The efficiency is expressible in terms of the Carnot efficiency of the global set up if the intermediate reservoir temperature is an algebra… ▽ More We show that coupled autonomous thermal machines, in the presence of three heat reservoirs and following a global linear-irreversible description, provide a unified framework to accommodate the variety of expressions for the efficiency at maximum power (EMP). The efficiency is expressible in terms of the Carnot efficiency of the global set up if the intermediate reservoir temperature is an algebraic mean of the hot and cold temperatures. We give an explanation of the universal properties of EMP near equilibrium in terms of the properties of symmetric algebraic means. For the case of broken time reversal symmetry, a universal second order coefficient of 6/49 is predicted in the series expansion of EMP, analogous to the 1/8 coefficient in the time-reversal symmetric case. △ Less

Submitted 1 December, 2021; originally announced December 2021.

Comments: Revtex4-2,7 pages, 2 figures, 1 table

Journal ref: Phys. Rev. E 105, 044145 (2022)

Crystal and Magnetic Structure of Polar Oxide HoCrWO$_6$

Authors: C. Dhital, D. Pham, T. Lawal, C. Bucholz, A. Poyraz, Q. Zhang, R. Nepal, R. Jin, R. Rai

Abstract: Polar magnetic oxide HoCrWO$_6$ is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO$_6$ forms the polar crystal structure (space group Pna2$_1$ (#33)) due to the cation ordering of W$^{6+}$ and Cr$^{3+}$. There is an antiferromagnetic transition at TN = 24.5 K along with the magnetic entropy change (~5 J.Kg.$^{-1}$K$^{-1}$ at 70 kOe).… ▽ More Polar magnetic oxide HoCrWO$_6$ is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO$_6$ forms the polar crystal structure (space group Pna2$_1$ (#33)) due to the cation ordering of W$^{6+}$ and Cr$^{3+}$. There is an antiferromagnetic transition at TN = 24.5 K along with the magnetic entropy change (~5 J.Kg.$^{-1}$K$^{-1}$ at 70 kOe). Neutron diffraction measurement indicates that both Cr and Ho sublattices are ordered with the moment of 2.32(5)$μ_B$ and 8.7(4)$μ_B$ at 2 K, respectively. While Cr forms A-type collinear antiferromagnetic (AFM) structure with magnetic moment along the $b$ axis, Ho sublattice orders in a non-coplanar AFM arrangement. A comparison with isostructural DyFeWO$_6$ and DyCrWO$_6$ indicates that the magnetic structure of this family of compounds is controlled by the presence or absence of eg electrons in the transition metal sublattice. △ Less

Submitted 16 July, 2020; originally announced July 2020.

Comments: 7 figures, 22 pages. Journal of magnetism and magnetic materials, 2020

Growth evolution of self-affine thermally evaporated KBr thin films: A fractal assessment

Authors: R. Rai, R. P. Yadav, Triloki, Nabeel Jammal, A. K. Singh, B. K. Singh

Abstract: In this article, fractal concepts were used to explore the thermally evaporated potassium bromide thin films of different thicknesses 200, 300, and 500 nm respectively; grown on aluminium substrates at room temperature. The self-affine or self similar nature of growing surfaces was investigated by autocorrelation function and obtained results are compared with the morphological envelope method. Th… ▽ More In this article, fractal concepts were used to explore the thermally evaporated potassium bromide thin films of different thicknesses 200, 300, and 500 nm respectively; grown on aluminium substrates at room temperature. The self-affine or self similar nature of growing surfaces was investigated by autocorrelation function and obtained results are compared with the morphological envelope method. Theoretical estimations revealed that the global surface parameters such as, interface width and lateral correlation length are monotonically decreased with increasing film thickness. Also, from height profile and A-F plots, it has been perceived that irregularity/ complexity of growing layers was significantly influenced by thickness. On the other hand, the fractal dimension and local roughness exponent, estimated by height-height correlation function, do not suggest such dependency. △ Less

Submitted 16 June, 2018; originally announced June 2018.

X-ray diffraction line profile analysis of KBr thin films

Authors: R. Rai, Triloki, B. K. Singh

Abstract: In the present work, the microcrystalline characteristics of KBr thin films have been investigated by evaluating the breadth of diffraction peak. The Williamson-Hall, the Size-Strain Plot and the single line Voigt methods are employed to deconvolute the finite crystallite size and microstrain contribution from the broaden X-ray profile. The texture coefficient and dislocation density have been det… ▽ More In the present work, the microcrystalline characteristics of KBr thin films have been investigated by evaluating the breadth of diffraction peak. The Williamson-Hall, the Size-Strain Plot and the single line Voigt methods are employed to deconvolute the finite crystallite size and microstrain contribution from the broaden X-ray profile. The texture coefficient and dislocation density have been determined along each diffraction peak. Other relevant physical parameters such as stress, Young's modulus and energy density are also estimated using Uniform Stress-Deformation and Uniform Deformation Energy Density approximation of Williamson-Hall method. △ Less

Submitted 4 October, 2017; originally announced October 2017.

Near-equilibrium universality and bounds on efficiency in quasi-static regime with finite source and sink

Authors: Ramandeep S. Johal, Renuka Rai

Abstract: We show the validity of some results of finite-time thermodynamics, also within the quasi-static framework of classical thermodynamics. First, we consider the efficiency at maximum work (EMW) from finite source and sink modelled as identical thermodynamic systems. The near-equilibrium regime is characterized by expanding the internal energy upto second order (i.e. upto linear response) in the diff… ▽ More We show the validity of some results of finite-time thermodynamics, also within the quasi-static framework of classical thermodynamics. First, we consider the efficiency at maximum work (EMW) from finite source and sink modelled as identical thermodynamic systems. The near-equilibrium regime is characterized by expanding the internal energy upto second order (i.e. upto linear response) in the difference of initial entropies of the source and the sink. It is shown that the efficiency is given by a universal expression $2 η_C / (4-η_C)$, where $η_C$ is the Carnot efficiency. Then, different sizes of source and sink are treated, by combining different numbers of copies of the same thermodynamic system. The efficiency of this process is found to be ${\boldsymbolη}_0 = η_C/ (2-γη_C)$, where the parameter $γ$ depends only on the relative size of the source and the sink. This implies that within the linear response theory, EMW is bounded as ${η_C}/{2} \le {\boldsymbolη}_0 \le {η_C}/{(2 - η_C)}$, where the upper (lower) bound is obtained with a sink much larger (smaller) in size than the source. We also remark on the behavior of the efficiency beyond linear response. △ Less

Submitted 27 August, 2015; v1 submitted 1 December, 2014; originally announced December 2014.

Comments: 11 pages, no figures. New discussion added on universality beyond linear response

Journal ref: Published version: EPL, 113 (1) 10006 (2016)

Optical and structural properties of CsI thin film photocathode

Authors: Triloki, R. Rai, B. K. Singh

Abstract: In the present work performance of cesium iodide thin film photocathode is studied in detail. The optical absorbance of cesium iodide thin films have been analyzed in the spectral range of 190 nm to 900 nm. The optical band gap energy of 500 nm thick cesium iodide film is calculated using Tauc plot from absorbance data. Refractive index is estimated from envelope plot of transmittance data using S… ▽ More In the present work performance of cesium iodide thin film photocathode is studied in detail. The optical absorbance of cesium iodide thin films have been analyzed in the spectral range of 190 nm to 900 nm. The optical band gap energy of 500 nm thick cesium iodide film is calculated using Tauc plot from absorbance data. Refractive index is estimated from envelope plot of transmittance data using Swanepoel's method. Absolute quantum efficiency measurement has been carried out in the wavelength range of 150 nm to 200 nm. Crystallographic nature and surface morphology are investigated by X-ray diffraction, transmission electron microscopy and atomic force microscopy techniques. In addition, elemental composition result gained by energy dispersive X-ray analysis is also reported in the present work. △ Less

Submitted 16 January, 2015; v1 submitted 19 September, 2014; originally announced September 2014.

Bounds on Thermal Efficiency from Inference

Authors: Ramandeep S. Johal, Renuka Rai, Guenter Mahler

Abstract: The problem of inference is applied to the process of work extraction from two constant heat capacity reservoirs, when the thermodynamic coordinates of the process are not fully specified. The information that is lacking, includes both the specific value of a temperature as well as the label of the reservoir to which it is assigned. The estimates for thermal efficiency reveal that uncertainty rega… ▽ More The problem of inference is applied to the process of work extraction from two constant heat capacity reservoirs, when the thermodynamic coordinates of the process are not fully specified. The information that is lacking, includes both the specific value of a temperature as well as the label of the reservoir to which it is assigned. The estimates for thermal efficiency reveal that uncertainty regarding the exact labels, reduces the maximal efficiency below the Carnot value, its minimum value being the well known Curzon-Ahlborn value. We also make an average estimate of the efficiency {\it before} the value of the temperature is revealed. It is found that if the labels are known with certainty, then in the near-equilibrium limit the efficiency scales as 1/2 of Carnot value, while if there is maximal uncertainty in the labels, then the average estimate for efficiency drops to 1/3 of Carnot value. We also suggest how infered properties of the incomplete model can be mapped to a model with complete information but with an additional source of thermodynamic irreversibility. △ Less

Submitted 27 May, 2013; originally announced May 2013.

Comments: pdflatex, 6 pages (3 figures)

Structural characterization of as-deposited cesium iodide films studied by X-ray diffraction and transmission electron microscopy techniques

Authors: Triloki, P. Garg, R. Rai, B. K. Singh

Abstract: In the present work, cesium iodide (CsI) thin films of different thickness have been prepared by thermal evaporation technique. The crystallite size and grain size of these films are compared by using X-ray diffraction (XRD) profile analysis as well as by transmission electron microscopy (TEM) counting, respectively. These two methods provide less deviation between crystallite size and grain size… ▽ More In the present work, cesium iodide (CsI) thin films of different thickness have been prepared by thermal evaporation technique. The crystallite size and grain size of these films are compared by using X-ray diffraction (XRD) profile analysis as well as by transmission electron microscopy (TEM) counting, respectively. These two methods provide less deviation between crystallite size and grain size in the case of thin CsI films of 4 nm, but there is comparatively large difference in case of thicker CsI films (20 nm, 100 nm and 500 nm). It indicates that dislocations are arranged in a configuration which causes small orientational difference between two adjacent coherent regions. The size obtained from XRD corresponds to two separate regions, whereas in the TEM micrograph the two regions may seem to correspond one region particularly in case of thicker films. Other physical parameters such as strain, stress and deformation energy density are also estimated precisely for the prominent XRD peaks of thicker CsI films in the range $2θ= 20^{0}-80^{0}$ by using a modified Williamson-Hall (W-H) analysis assuming uniform deformation model (UDM), uniform deformation stress model (UDSM) and uniform deformation energy density model (UDEDM). △ Less

Submitted 2 November, 2013; v1 submitted 23 November, 2012; originally announced November 2012.

Comments: This paper has been accepted for publication in Nuclear Inst. and Methods in Physics Research, A

Spin-Charge-Lattice Coupling through Resonant Multi-Magnon Excitations in Multiferroic BiFeO3

Authors: M. O. Ramirez, A. Kumar, S. A. Denev, Y. H. Chu, J. Seidel, L. Martin, S-Y. Yang R. C. Rai, X. Xue, J. F. Ihlefeld, N. Podraza, E. Saiz, S. Lee, J. Klug, S. W. Cheong, M. J. Bedzyk, O. Auciello, D. G. Schlom, J. Orenstein, R. Ramesh, J. L. Musfeldt, A. P. Litvinchuk, V. Gopalan

Abstract: Spin-charge-lattice coupling mediated by multi-magnon processes is demonstrated in multiferroic BiFeO3. Experimental evidence of two and three magnons excitations as well as multimagnon coupling at electronic energy scales and high temperatures are reported. Temperature dependent Raman experiments show up to five resonant enhancements of the 2-magnon excitation below the Neel temperature. These… ▽ More Spin-charge-lattice coupling mediated by multi-magnon processes is demonstrated in multiferroic BiFeO3. Experimental evidence of two and three magnons excitations as well as multimagnon coupling at electronic energy scales and high temperatures are reported. Temperature dependent Raman experiments show up to five resonant enhancements of the 2-magnon excitation below the Neel temperature. These are shown to be collective interactions between on-site Fe d-d electronic resonance, phonons and multimagnons △ Less

Submitted 7 January, 2009; v1 submitted 24 March, 2008; originally announced March 2008.

Comments: 11 pages including figures

Linear and Nonlinear Optical constants of BiFeO_3

Authors: Amit Kumar, R. Rai, Nikolas Podraza, Sava Denev, Mariola Ramirez, Ying-Hao Chu, Jon Ihlefeld, Tassilo Heeg, Jurgen Schubert, Darrell Schlom, Joseph Orenstein, R. Ramesh, Robert Collins, Janice Musfeldt, Venkatraman Gopalan

Abstract: Using spectroscopic ellipsometry, the refractive index and absorption versus wavelength of the ferroelectric antiferromagnet Bismuth Ferrite, BiFeO_3 is reported. The material has a direct band-gap at 442 nm wavelength (2.81 eV). Using optical second harmonic generation, the nonlinear optical coefficients were determined to be d_15/d_22 = 0.20 +/- 0.01, d_31/d_22 = 0.35 +/- 0.02, d_33/d_22 = -11… ▽ More Using spectroscopic ellipsometry, the refractive index and absorption versus wavelength of the ferroelectric antiferromagnet Bismuth Ferrite, BiFeO_3 is reported. The material has a direct band-gap at 442 nm wavelength (2.81 eV). Using optical second harmonic generation, the nonlinear optical coefficients were determined to be d_15/d_22 = 0.20 +/- 0.01, d_31/d_22 = 0.35 +/- 0.02, d_33/d_22 = -11.4 +/- 0.20 and |d_22| = 298.4 +/- 6.1 pm/V at a fundamental wavelength of 800 nm. △ Less

Submitted 10 January, 2008; originally announced January 2008.

Comments: 4 pages, 3 figures

Optical Properties and Magnetic Field-Induced Phase Transitions in the Ferroelectric State of Ni$_3$V$_2$O$_8$

Authors: R. C. Rai, J. Cao, S. Brown, J. L. Musfeldt, D. Kasinathan, D. J. Singh, G. Lawes, N. Rogado, R. J. Cava, X. Wei

Abstract: We use a combination of optical spectra, first principles calculations, and energy dependent magneto-optical measurements to elucidate the electronic structure and to study the phase diagram of Ni$_3$V$_2$O$_8$. We find a remarkable interplay of magnetic field and optical properties that reveals additional high magnetic field phases and an unexpected electronic structure which we associate with… ▽ More We use a combination of optical spectra, first principles calculations, and energy dependent magneto-optical measurements to elucidate the electronic structure and to study the phase diagram of Ni$_3$V$_2$O$_8$. We find a remarkable interplay of magnetic field and optical properties that reveals additional high magnetic field phases and an unexpected electronic structure which we associate with the strong magneto-dielectric couplings in this material over a wide energy range. Specifically, we observed several prominent magneto-dielectric effects that derive from changes in crystal field environment around Ni spine and cross-tie centers. This effect is consistent with a field-induced modification of local structure. Symmetry-breaking effects are also evident with temperature. We find Ni$_3$V$_2$O$_8$ to be an intermediate gap, local moment band insulator. This electronic structure is particularly favorable for magneto-dielectric couplings, because the material is not subject to the spin charge separation characteristic of strongly correlated large gap Mott insulators, while at the same time remaining a magnetic insulator independent of the particular spin order and temperature. △ Less

Submitted 30 October, 2006; originally announced October 2006.

Comments: 12 figures, 11 pages, accepted in Physical Review B

Observation of 300 K High Energy MagnetoDielectric Response in the Bilayer Manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

Authors: J. Cao, R. C. Rai, S. Brown, J. L. Musfeldt, R. Tackett, G. Lawes, X. Wei, Y. J. Wang, M. Apostu, R. Suryanarayanan, A. Revcolevschi

Abstract: We observed a large HEMD effect in the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$, a direct consequence of field driven spin-glass insulator to ferromagnetic metal transition. The remnants of the transition can be used to achieve dielectric contrast at room temperature. This discovery suggests that electronic mechanisms such as the metal-insulator transition, charge or… ▽ More We observed a large HEMD effect in the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$, a direct consequence of field driven spin-glass insulator to ferromagnetic metal transition. The remnants of the transition can be used to achieve dielectric contrast at room temperature. This discovery suggests that electronic mechanisms such as the metal-insulator transition, charge ordering, and orbital ordering can be exploited to give substantial dielectric contrast in other materials. △ Less

Submitted 29 October, 2006; originally announced October 2006.

Comments: 4 pages, 4 figures

Statistical mechanics of thermal contact between system and bath with long-range interactions

Authors: Ramandeep S. Johal, Renuka Rai

Abstract: In this paper, we address the possibility of generalising the standard analysis of thermal contact between a sample system and a heat bath, by including long range interactions between them. As a concrete example, both system and bath are treated within the long range Ising model. For this model, we derive the equilibrium probability distribution of the energy of the sample system. Equilibrium p… ▽ More In this paper, we address the possibility of generalising the standard analysis of thermal contact between a sample system and a heat bath, by including long range interactions between them. As a concrete example, both system and bath are treated within the long range Ising model. For this model, we derive the equilibrium probability distribution of the energy of the sample system. Equilibrium properties of the system magnetisation and stability of the solutions is discussed. We find existence of a metastable phase below a critical temperature of the bath. △ Less

Submitted 14 August, 2006; originally announced August 2006.

Comments: 10 pages, Revtex, one figure

Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

Authors: J. Cao, J. T. Haraldsen, R. C. Rai, S. Brown, J. L. Musfeldt, Y. J. Wang, X. Wei, M. Apostu, R. Suryanarayanan, A. Revcolevschi

Abstract: We measured the magneto-optical response of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ in order to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associate… ▽ More We measured the magneto-optical response of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ in order to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. We discuss field- and temperature-induced trends within the framework of the Tomioka-Tokura global electronic phase diagram picture and suggest that controlled disorder near a phase boundary can be used to tune the magnetodielectric response. Remnants of the spin-glass insulator to ferromagnetic metallic transition can also drive 300 K color changes in (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$. △ Less

Submitted 13 June, 2006; originally announced June 2006.

Comments: 9 pages, 8 figures

Magneto-Dielectric Effect in the S = 1/2 Quasi-Two Dimensional Antiferromagnet K2V3O8

Authors: R. C. Rai, J. Cao, J. L. Musfeldt, D. J. Singh, R. Jin, Z. X. Zhou, B. C. Sales, D. Mandrus, X. Wei

Abstract: We report the optical and magneto-optical properties of K2V3O8, an S=1/2 quasi-two-dimensional Heisenberg antiferromagnet. Local spin density approximation electronic structure calculations are used to assign the observed excitations and analyze the field dependent features. Two large magneto-optical effects, centered at ~1.19 and 2.5 eV, are attributed to field-induced changes in the V 4+ d to… ▽ More We report the optical and magneto-optical properties of K2V3O8, an S=1/2 quasi-two-dimensional Heisenberg antiferromagnet. Local spin density approximation electronic structure calculations are used to assign the observed excitations and analyze the field dependent features. Two large magneto-optical effects, centered at ~1.19 and 2.5 eV, are attributed to field-induced changes in the V 4+ d to d on-site excitations due to modification of the local crystal field environment of the VO5 square pyramids with applied magnetic field. Taken together, the evidence for a soft lattice, the presence of vibrational fine structure on the sharp 1.19 eV magneto-optical feature,and the fact that these optical excitations are due to transitions from a nearly pure spin polarized V d state to hybridized states involving both V and O, suggest that the magneto-dielectric effect in K2V3O8 is driven by strong lattice coupling. △ Less

Submitted 10 February, 2006; v1 submitted 20 December, 2005; originally announced December 2005.

Comments: Zipped file containing 8 pages, 12 figures, in press PRB

Observation of field-dependent magnetic parameters in the magnetic molecule {Ni4Mo12}

Authors: J. Schnack, M. Brueger, M. Luban, P. Koegerler, E. Morosan, R. Fuchs, R. Modler, Hiroyuki Nojiri, Ram C. Rai, Jinbo Cao, J. L. Musfeldt, Xing Wei

Abstract: We investigate the bulk magnetic, electron paramagnetic resonance, and magneto-optical properties of {Ni4Mo12}, a magnetic molecule with antiferromagnetically coupled tetrahedral {Ni4Mo12} in a diamagnetic molybdenum matrix. The low-temperature magnetization exhibits steps at irregular field intervals, a result that cannot be explained using a Heisenberg model even if it is augmented by magnetic… ▽ More We investigate the bulk magnetic, electron paramagnetic resonance, and magneto-optical properties of {Ni4Mo12}, a magnetic molecule with antiferromagnetically coupled tetrahedral {Ni4Mo12} in a diamagnetic molybdenum matrix. The low-temperature magnetization exhibits steps at irregular field intervals, a result that cannot be explained using a Heisenberg model even if it is augmented by magnetic anisotropy and biquadratic terms. Allowing the exchange and anisotropy parameters to depend on the magnetic field provides the best fit to our data, suggesting that the molecular structure (and thus the interactions between spins) may be changing with applied magnetic field. △ Less

Submitted 14 December, 2005; v1 submitted 19 September, 2005; originally announced September 2005.

Comments: 10 pages, 12 figures, submitted to Phys. Rev. B. More information at http://www.physik.uni-osnabrueck.de/makrosysteme/

Journal ref: Phys. Rev. B 73 (2006) 094401

Dynamics of the Electro-Reflective Response of TaS3

Authors: R. C. Rai, J. W. Brill

Abstract: We have observed a large (~1%) change in infrared reflectance of the charge-density-wave (CDW) conductor, orthorhombic TaS3, when its CDW is depinned. The change is concentrated near one current contact. Assuming that the change in reflectance is proportional to the degree of CDW polarization, we have studied the dynamics of CDW repolarization through position dependent measurements of the varia… ▽ More We have observed a large (~1%) change in infrared reflectance of the charge-density-wave (CDW) conductor, orthorhombic TaS3, when its CDW is depinned. The change is concentrated near one current contact. Assuming that the change in reflectance is proportional to the degree of CDW polarization, we have studied the dynamics of CDW repolarization through position dependent measurements of the variation of the electro-reflectance with the frequency of square wave voltages applied to the sample, and have found that the response could be characterized as a damped harmonic oscillator with a distribution of relaxation (i.e. damping) times. The average relaxation time, which increases away from the contacts, varies with applied voltage as t0 ~ 1/V^p with p ~ 3/2, but the distribution of times broadens as the voltage approaches the depinning threshold. Very low resonant frequencies (~ 1 kHz) indicate a surprisingly large amount of inertia, which is observable in the time dependence of the change in reflectance as a polarity dependent delay of ~ 100 microsec. △ Less

Submitted 28 April, 2004; originally announced April 2004.

Comments: submitted to Phys. Rev. B

Electro-Reflectance Spectra of Blue Bronze

Authors: R. C. Rai, V. A. Bondarenko, J. W. Brill

Abstract: We show that the infrared reflectance of the quasi-one dimensional charge-density-wave (CDW) conductor K0.3MoO3 (blue bronze) varies with position when a voltage greater than the CDW depinning threshold is applied. The spatial dependence and spectra associated with these changes are generally as expected from the electro-transmission [B.M. Emerling, et al, Eur. Phys. J. B 16, 295 (2000)], but th… ▽ More We show that the infrared reflectance of the quasi-one dimensional charge-density-wave (CDW) conductor K0.3MoO3 (blue bronze) varies with position when a voltage greater than the CDW depinning threshold is applied. The spatial dependence and spectra associated with these changes are generally as expected from the electro-transmission [B.M. Emerling, et al, Eur. Phys. J. B 16, 295 (2000)], but there are some differences which might be associated with changes in the CDW properties on the surface. We have examined the electro-reflectance spectrum associated with CDW current investigation for light polarized parallel to the conducting chains for signs of expected current-induced intragap states, and conclude that the density of any such states is at least an order of magnitude lower than expected. △ Less

Submitted 24 July, 2003; v1 submitted 3 July, 2003; originally announced July 2003.

Comments: 1)submitted to Eur. Phys. J B 2) revised (July 24) to a) better emphasize results and b) with new figure insets to make paper more self-contained

Field-tuned Collapse of an Orbital Ordered and Spin-polarized State: Colossal Magnetoresistance in Bilayered Ruthenate

Authors: G. Cao, L. Balicas, X. N. Lin, S. Chikara, V. Duairaj, E. Elhami, J. W. Brill, R. C. Rai

Abstract: Ca3Ru2O7 with a Mott-like transition at 48 K features different in-plane anisotropies of magnetization and magnetoresistance. Applying magnetic field along the magnetic easy-axis precipitates a spin-polarized state via a first-order metamagnetic transition, but does not lead to a full suppression of the Mott state, whereas applying magnetic field along the magnetic hard axis does, causing a resi… ▽ More Ca3Ru2O7 with a Mott-like transition at 48 K features different in-plane anisotropies of magnetization and magnetoresistance. Applying magnetic field along the magnetic easy-axis precipitates a spin-polarized state via a first-order metamagnetic transition, but does not lead to a full suppression of the Mott state, whereas applying magnetic field along the magnetic hard axis does, causing a resistivity reduction of three orders of magnitude. The colossal magnetoresistivity is attributed to the collapse of the orbital ordered and spin-polarized state. Evidence for a density wave is also presented. △ Less

Submitted 3 July, 2003; originally announced July 2003.

Comments: 10 pages plus 4 figures

Generalized bit cumulants for chaotic systems: Numerical results

Authors: Renuka Rai, Ramandeep S. Johal

Abstract: We propose generalized bit cumulants for chaotic systems, within nonextensive thermodynamic approach. In this work, we apply the first and second generalized cumulants to one dimensional logistic and logistic-like family of maps. We propose generalized bit cumulants for chaotic systems, within nonextensive thermodynamic approach. In this work, we apply the first and second generalized cumulants to one dimensional logistic and logistic-like family of maps. △ Less

Submitted 20 September, 1999; originally announced September 1999.

Comments: 4 pages, 12 figures, e-mail: raman%phys@puniv.chd.nic.in

Nonextensive thermodynamic formalism for chaotic dynamical systems

Authors: Ramandeep S. Johal, Renuka Rai

Abstract: A nonextensive thermostatic approach to chaotic dynamical systems is developed by expressing generalized Tsallis distribution as escort distribution. We explicitly show the thermodynamic limit and also derive the Legendre Transform structure. As an application, bit variance is calculated for ergodic logistic map. Consistency of the formalism demands a relation between box size ($ε$) and degree o… ▽ More A nonextensive thermostatic approach to chaotic dynamical systems is developed by expressing generalized Tsallis distribution as escort distribution. We explicitly show the thermodynamic limit and also derive the Legendre Transform structure. As an application, bit variance is calculated for ergodic logistic map. Consistency of the formalism demands a relation between box size ($ε$) and degree of nonextensivity, given as $(1-q)\sim -1/{\rm ln} ε$. This relation is numerically verified for the case of bit variance as well as using basic definition of Tsallis entropy. △ Less

Submitted 15 September, 1999; originally announced September 1999.

Comments: 11 pages, 5 figures e-mail: raman%phys@puniv.chd.nic.in

Stochastic Resonance in Maps and Coupled Map Lattices

Authors: Prashant M. Gade, Renuka Rai, Harjinder Singh

Abstract: We demonstrate the phenomenon of stochastic resonance (SR) for discrete-time dynamical systems. We investigate various systems that are not necessarily bistable, but do have two well defined states, switching between which is aided by external noise which can be additive or multiplicative. Thus we find it to be a fairly generic phenomenon. In these systems, we investigate kinetic aspects like hy… ▽ More We demonstrate the phenomenon of stochastic resonance (SR) for discrete-time dynamical systems. We investigate various systems that are not necessarily bistable, but do have two well defined states, switching between which is aided by external noise which can be additive or multiplicative. Thus we find it to be a fairly generic phenomenon. In these systems, we investigate kinetic aspects like hysteresis which reflect the nonlinear and dissipative nature of the response of the system to the external field. We also explore spatially extended systems with additive or parametric noise and find that they differ qualitatively. △ Less

Submitted 9 April, 1997; originally announced April 1997.

Comments: 6 pages revtex document, 15 tex figures. (Submitted to Phys. Rev. E)