arXiv subjects

A. Saxena

Publications and source records attributed to A. Saxena.

At least 109 records · Page 6Linked to original sources

arXiv2012

Effects of rare earth ion size on the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites

We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appearance of the uniform shear distortion below the Jahn-Teller ordering temperature, estimate the Jahn-Teller ordering temperature and its variation between NdMnO3 and LaMnO3, and obtain the relations between distortions. We find good agreement between theoretical results and experimental data.

cond-mat.mtrl-sci↗

arXiv2011

Discrete Breathers in a Nonlinear Polarizability Model of Ferroelectrics

We present a family of discrete breathers, which exists in a nonlinear polarizability model of ferroelectric materials. The core-shell model is set up in its non-dimensionalized Hamiltonian form and its linear spectrum is examined. Subsequently, seeking localized solutions in the gap of the linear spectrum, we establish that numerically exact and potentially stable discrete breathers exist for a wide range of frequencies therein. In addition, we present nonlinear normal mode, extended spatial profile solutions from which the breathers bifurcate, as well as other associated phenomena such as the formation of phantom breathers within the model. The full bifurcation picture of the emergence and disappearance of the breathers is complemented by direct numerical simulations of their dynamical instability, when the latter arises.

physics.comp-ph↗

arXiv2011

Systematic study of projectile structure effect on fusion barrier distribution

Quasielastic excitation function measurement has been carried out for the $^{4}$He + $^{232}$Th system at $θ_{lab}$=160$^\circ$ with respect to the beam direction, to obtain a representation of the fusion barrier distribution. Using the present data along with previously measured barrier distribution results on $^{12}$C, $^{16}$O, and $^{19}$F + $^{232}$Th systems a systematic analysis has been carried out to investigate the role of target and/or projectile structures on fusion barrier distribution. It is observed that for $^{4}$He, $^{12}$C, and $^{16}$O + $^{232}$Th, reactions the couplings due to target states only are required in coupled channel fusion calculations to explain the experimental data, whereas for the $^{19}$F+ $^{232}$Th system along with the coupling of target states, inelastic states of $^{19}$F are also required to explain the experimental results on fusion-barrier distribution. The width of the barrier distribution shows interesting transition behavior when plotted with respect to the target-projectile charge product for the above systems.

nucl-ex↗

arXiv2010

Tin telluride: a weakly co-elastic metal

We report resonant ultrasound spectroscopy (RUS), dilatometry/magnetostriction, magnetotransport, magnetization, specific heat, and $^{119}$Sn Mössbauer spectroscopy measurements on SnTe and Sn$_{0.995}$Cr$_{0.005}$Te. Hall measurements at $T=77$ K indicate that our Bridgman-grown single crystals have a $p$-type carrier concentration of $3.4 \times 10^{19}$ cm$^{-3}$ and that our Cr-doped crystals have an $n$-type concentration of $5.8 \times 10^{22}$ cm$^{-3}$. Although our SnTe crystals are diamagnetic over the temperature range $2\, \text{K} \leq T \leq 1100\, \text{K}$, the Cr-doped crystals are room temperature ferromagnets with a Curie temperature of 294 K. For each sample type, three-terminal capacitive dilatometry measurements detect a subtle 0.5 micron distortion at $T_c \approx 85$ K. Whereas our RUS measurements on SnTe show elastic hardening near the structural transition, pointing to co-elastic behavior, similar measurements on Sn$_{0.995}$Cr$_{0.005}$Te show a pronounced softening, pointing to ferroelastic behavior. Effective Debye temperature, $θ_D$, values of SnTe obtained from $^{119}$Sn Mössbauer studies show a hardening of phonons in the range 60--115K ($θ_D$ = 162K) as compared with the 100--300K range ($θ_D$ = 150K). In addition, a precursor softening extending over approximately 100 K anticipates this collapse at the critical temperature, and quantitative analysis over three decades of its reduced modulus finds $ΔC_{44}/C_{44}=A|(T-T_0)/T_0|^{-κ}$ with $κ= 0.50 \pm 0.02 $, a value indicating a three-dimensional softening of phonon branches at a temperature $T_0 \sim 75$ K, considerably below $T_c$. We suggest that the differences in these two types of elastic behaviors lie in the absence of elastic domain wall motion in the one case and their nucleation in the other.

cond-mat.mtrl-sci↗

arXiv2010

Scaled free energies, power-law potentials, strain pseudospins and quasi-universality for first-order structural transitions

We consider ferroelastic first-order phase transitions with $N_{OP}$ order-parameter strains entering Landau free energies as invariant polynomials, that have $N_V$ structural-variant Landau minima. The total free energy includes (seemingly innocuous) harmonic terms, in the $n = 6 -N_{OP}$ {\it non}-order-parameter strains. Four 3D transitions are considered, tetragonal/orthorhombic, cubic/tetragonal, cubic/trigonal and cubic/orthorhombic unit-cell distortions, with respectively, $N_{OP} = 1, 2, 3 $ and 2; and $N_V = 2, 3, 4$ and 6. Five 2D transitions are also considered, as simpler examples. Following Barsch and Krumhansl, we scale the free energy to absorb most material-dependent elastic coefficients into an overall prefactor, by scaling in an overall elastic energy density; a dimensionless temperature variable; and the spontaneous-strain magnitude at transition $λ<<1$. To leading order in $λ$ the scaled Landau minima become material-independent, in a kind of 'quasi-universality'. The scaled minima in $N_{OP}$-dimensional order-parameter space, fall at the centre and at the $N_V$ corners, of a transition-specific polyhedron inscribed in a sphere, whose radius is unity at transition. The `polyhedra' for the four 3D transitions are respectively, a line, a triangle, a tetrahedron, and a hexagon. We minimize the $n$ terms harmonic in the non-order-parameter strains, by substituting solutions of the 'no dislocation' St Venant compatibility constraints, and explicitly obtain powerlaw anisotropic, order-parameter interactions, for all transitions. In a reduced discrete-variable description, the competing minima of the Landau free energies induce unit-magnitude pseudospin vectors, with $N_V +1$ values, pointing to the polyhedra corners and the (zero-value) center.

cond-mat.mtrl-sci↗

arXiv2010

On the incompatibility of strains and its application to mesoscopic studies of plasticity

Structural transitions are invariably affected by lattice distortions. If the body is to remain crack-free, the strain field cannot be arbitrary but has to satisfy the Saint-Venant compatibility constraint. Equivalently, an incompatibility constraint consistent with the actual dislocation network has to be satisfied in media with dislocations. This constraint can be incorporated into strain-based free energy functionals to study the influence of dislocations on phase stability. We provide a systematic analysis of this constraint in three dimensions and show how three incompatibility equations accommodate an arbitrary dislocation density. This approach allows the internal stress field to be calculated for an anisotropic material with spatially inhomogeneous microstructure and distribution of dislocations by minimizing the free energy. This is illustrated by calculating the stress field of an edge dislocation and comparing it with that of an edge dislocation in an infinite isotropic medium. We outline how this procedure can be utilized to study the interaction of plasticity with polarization and magnetization.

cond-mat.mtrl-sci↗

arXiv2009

Spin polarization control by electric stirring: proposal for a spintronic device

We propose a spintronic device to generate spin polarization in a mesoscopic region by purely electric means. We show that the spin Hall effect in combination with the stirring effect are sufficient to induce measurable spin polarization in a closed geometry. Our device structure does not require the application of magnetic fields, external radiation or ferromagnetic leads, and can be implemented in standard semiconducting materials.

cond-mat.mes-hall↗

arXiv2008

Atomistic studies of transformation pathways and energetics in plutonium

One of the most challenging problems in understanding the structural phase transformations in Pu is to determine the energetically favored, continuous atomic pathways from one crystal symmetry to another. This problem involves enumerating candidate pathways and studying their energetics to garner insight into instabilities and energy barriers. The purpose of this work is to investigate the energetics of two transformation pathways for the delta to alpha' transformation in Pu that were recently proposed [Lookman et al., Phys. Rev. Lett. 100:145504, 2008] on the basis of symmetry. These pathways require the presence of either an intermediate hexagonal closed-packed (hcp) structure or a simple hexagonal (sh) structure. A subgroup of the parent fcc and the intermediate hexagonal structure, which has trigonal symmetry, facilitates the transformation to the intermediate hcp or sh structure. Phonons then break the translational symmetry from the intermediate hcp or sh structure to the final monoclinic symmetry of the alpha' structure. We perform simulations using the modified embedded atom method (MEAM) for Pu to investigate these candidate pathways. Our main conclusion is that the path via hcp is energetically favored and the volume change for both pathways essentially occurs in the second step of the transformation, i.e. from the intermediate sh or hcp to the monoclinic structure. Our work also highlights the deficiency of the current state-of-the-art MEAM potential in capturing the anisotropy associated with the lower symmetry monoclinic structure.

cond-mat.mtrl-sci↗

arXiv2008

Defect-induced incompatibility of elastic strains: dislocations within the Landau theory of martensitic phase transformations

In dislocation-free martensites the components of the elastic strain tensor are constrained by the Saint-Venant compatibility condition which guarantees continuity of the body during external loading. However, in dislocated materials the plastic part of the distortion tensor introduces a displacement mismatch that is removed by elastic relaxation. The elastic strains are then no longer compatible in the sense of the Saint-Venant law and the ensuing incompatibility tensor is shown to be proportional to the gradients of the Nye dislocation density tensor. We demonstrate that the presence of this incompatibility gives rise to an additional long-range contribution in the inhomogeneous part of the Landau energy functional and to the corresponding stress fields. Competition amongst the local and long-range interactions results in frustration in the evolving order parameter (elastic) texture. We show how the Peach-Koehler forces and stress fields for any distribution of dislocations in arbitrarily anisotropic media can be calculated and employed in a Fokker-Planck dynamics for the dislocation density. This approach represents a self-consistent scheme that yields the evolutions of both the order parameter field and the continuous dislocation density. We illustrate our method by studying the effects of dislocations on microstructure, particularly twinned domain walls, in an Fe-Pd alloy undergoing a martensitic transformation.

cond-mat.stat-mech↗

arXiv2008

Geometric Control Over the Motion of Magnetic Domain Walls

We propose a method, which enables precise control of magnetic patterns, relying only on the fundamental properties of the wire and the choice of the path in the controlled parameter space but not on the rate of motion along this path. Possible experimental realizations of this mechanism are discussed. In particular, we show that the domain walls in magnetic nanowires can be translated by rotation of the magnetic easy axis, or by applying pulses of magnetic field directed transverse to the magnetic easy axis.

cond-mat.mtrl-sci↗

arXiv2006

Statics and Dynamics of an Inhomogeneously-Nonlinear Lattice

We introduce an inhomogeneously-nonlinear Schr{ö}dinger lattice, featuring a defocusing segment, a focusing segment and a transitional interface between the two. We illustrate that such inhomogeneous settings present vastly different dynamical behavior than the one expected in their homogeneous counterparts in the vicinity of the interface. We analyze the relevant stationary states, as well as their stability by means of perturbation theory and linear stability analysis. We find good agreement with the numerical findings in the vicinity of the anti-continuum limit. For larger values of the coupling, we follow the relevant branches numerically and show that they terminate at values of the coupling strength which are larger for more extended solutions. The dynamical development of relevant instabilities is also monitored in the case of unstable solutions.

nlin.PS↗

arXiv2006

Pinning frequencies of the collective modes in $α$-uranium

Uranium is the only known element that features a charge-density wave (CDW) and superconductivity. We report a comparison of the specific heat of single-crystal and polycrystalline $α$-uranium. \red{Away from the the phase transition the specific heat of the polycrystal is larger than that of the single crystal, and the aim of this paper is to explain this difference.} In the single crystal we find excess contributions to the heat capacity at 41 K, 38 K, and 23 K, with a Debye temperature, $Θ_D$ = 256 K. In the polycrystalline sample the heat capacity curve is thermally broadened ($Θ_D$ = 184 K), but no excess heat capacity was observed. The excess heat capacity, $C_ϕ$ (taken as the difference between the single crystal and polycrystal heat capacities) is well described in terms of collective-mode excitations above their respective pinning frequencies. This attribution is represented by a modified Debye spectrum with two cutoff frequencies, a pinning frequency, $ν_o$, for the pinned CDW (due to grain boundaries in the polycrystal), and a normal Debye acoustic frequency occurring in the single crystal. We explain the 50-year-old difference in Debye temperatures between heat capacity and ultrasonic measurements.

cond-mat.other↗

arXiv2005

Impact of Climate Change on Forests in India

Global assessments have shown that future climate change is likely to significantly impact forest ecosystems. The present study makes an assessment of the impact of projected climate change on forest ecosystems in India. This assessment is based on climate projections of Regional Climate Model of the Hadley Centre (HadRM3) using the A2 (740 ppm CO2) and B2 (575 ppm CO2) scenarios of Special Report on Emissions Scenarios and the BIOME4 vegetation response model. The main conclusion is that under the climate projection for the year 2085, 77% and 68% of the forested grids in India are likely to experience shift in forest types under A2 and B2 scenario, respectively. Indications are a shift towards wetter forest types in the northeastern region and drier forest types in the northwestern region in the absence of human influence. Increasing atmospheric CO2 concentration and climate warming could also result in a doubling of net primary productivity under the A2 scenario and nearly 70% increase under the B2 scenario. The trends of impacts could be considered as robust but the magnitudes should be viewed with caution, due to the uncertainty in climate projections. Given the projected trends of likely impacts of climate change on forest ecosystems, it is important to incorporate climate change consideration in forest sector long-term planning process.

q-bio.OT↗

arXiv2005

Effects of strain, electric, and magnetic fields on lateral electron spin transport in semiconductor epilayers

We construct a spin-drift-diffusion model to describe spin-polarized electron transport in zincblende semiconductors in the presence of magnetic fields, electric fields, and off-diagonal strain. We present predictions of the model for geometries that correspond to optical spin injection from the absorption of circularly polarized light, and for geometries that correspond to electrical spin injection from ferromagnetic contacts. Starting with the Keldysh Green's function description for a system driven out of equilibrium, we construct a semiclassical kinetic theory of electron spin transport in strained semiconductors in the presence of electric and magnetic fields. From this kinetic theory we derive spin-drift-diffusion equations for the components of the spin density matrix for the specific case of spatially uniform fields and uniform electron density. We solve the spin-drift-diffusion equations numerically and compare the resulting images with scanning Kerr microscopy data of spin-polarized conduction electrons flowing laterally in bulk epilayers of n-type GaAs. The spin-drift-diffusion model accurately describes the experimental observations. We contrast the properties of electron spin precession resulting from magnetic and strain fields. Spin-strain coupling depends linearly on electron wave vector and spin-magnetic field coupling is independent of electron wave vector. As a result, spatial coherence of precessing spin flows is better maintained with strain than with magnetic fields, and the spatial period of spin precession is independent of the applied electrical bias in strained structures whereas it is strongly bias dependent for the case of applied magnetic fields.

cond-mat.mtrl-sci↗

arXiv2005

Oscillating elastic defects: competition and frustration

We consider a dynamical generalization of the Eshelby problem: the strain profile due to an inclusion or "defect" in an isotropic elastic medium. We show that the higher the oscillation frequency of the defect, the more localized is the strain field around the defect. We then demonstrate that the qualitative nature of the interaction between two defects is strongly dependent on separation, frequency and direction, changing from "ferromagnetic" to "antiferromagnetic" like behavior. We generalize to a finite density of defects and show that the interactions in assemblies of defects can be mapped to XY spin-like models, and describe implications for frustration and frequency-driven pattern transitions.

cond-mat.mtrl-sci↗

arXiv2005

Asymptotic Calculation of Discrete Nonlinear Wave Interactions

We illustrate how to compute asymptotic interactions between discrete solitary waves of dispersive equations, using the approach proposed by Manton [Nucl. Phys. B 150, 397 (1979)]. We also discuss the complications arising due to discreteness and showcase the application of the method in nonlinear Schrodinger, as well as in Klein-Gordon lattices, finding excellent agreement with direct numerical computations.

nlin.PS↗

arXiv2005

Role of solvent for globular proteins in solution

The properties of the solvent affect the behavior of the solution. We propose a model that accounts for the contribution of the solvent free energy to the free energy of globular proteins in solution. For the case of an attractive square well potential, we obtain an exact mapping of the phase diagram of this model without solvent to the model that includes the solute-solvent contribution. In particular we find for appropriate choices of parameters upper critical points, lower critical points and even closed loops with both upper and lower critical points, similar to one found before [Macromolecules, 36, 5845 (2003)]. In the general case of systems whose interactions are not attractive square wells, this mapping procedure can be a first approximation to understand the phase diagram in the presence of solvent. We also present simulation results for both the square well model and a modified Lennard-Jones model.

cond-mat.soft↗

arXiv2004

Electron transmission through a short interacting wire: 0.7 conductance anomaly

We investigate tunneling through a short interacting wire. We identify two temperature regimes (a) $T_{Kondo}<T\le T^{wire}=\hbar v_F/k_Bd$ ($d$ is the length of the short wire) and (b) $T<T_{Kondo}\ll T^{wire}$. In the first regime the effective (renormalized) electron-electron interaction is smaller than the tunneling matrix element. This is the situation at finite temperature $T$ where the single particle spectrum of the wire is characterized by a multilevel "quantum dot" system with magnetic quantum number S=0 which is higher in energy than the SU(2) spin doublet $S=\pm1/2$. Due to the single particle energy we find that the tunneling electron into the wire must have an opposite spin to the one in the short wire giving rise to a conductance, $G= G_\uparrow+G_\downarrow$, $e^2/h\le G\ll 2e^2/h$. In the second regime, when $T \to0$ we have a situation that the effective (renormalized) electron-electron interaction is larger than the tunneling matrix element. This problem is equivalent to a Kondo problem. We find for $T<T_{Kondo}$ that the conductance is given by $G=2e^2/h$. These results are in agreement with recent experiments where for $T_{Kondo}<T<T^{wire}$ the conductance $G$ obeys $e^2/h\le G\ll 2e^2/h$, and for $T< T_{Kondo}$, $G=2e^2/h$. In both regimes the current is not polarized and the SU(2) symmetry is not broken.

cond-mat.mes-hall↗