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Significance Magnetoresistance has a history of revealing key electronic characteristics of materials. From early measurements on noble metals to definitive characterization of localization effects in semiconductors to recent studies of topological materials, the magnetoresistive response provides an experimental technique to explore the Fermi surface in detail, and to predict and craft physical properties through its sign, functional form, and potential quantum character. Linear magnetoresistance in density-wave systems has eluded clear explanation for over half a century. Here, we present measurements that lead to a general explanation based on unusual current paths tied to the formation of long-range charge or spin order. This mechanism potentially extends to the large magnetoresistance observed in semimetals like Bi, graphite, and WTe2., The magnetoresistance (MR) of a material is typically insensitive to reversing the applied field direction and varies quadratically with magnetic field in the low-field limit. Quantum effects, unusual topological band structures, and inhomogeneities that lead to wandering current paths can induce a cross-over from quadratic to linear MR with increasing magnetic field. Here we explore a series of metallic charge- and spin-density-wave systems that exhibit extremely large positive linear MR. By contrast to other linear MR mechanisms, this effect remains robust down to miniscule magnetic fields of tens of Oersted at low temperature. We frame an explanation of this phenomenon in a semiclassical narrative for a broad category of materials with partially gapped Fermi surfaces due to density waves.

SUMMARY We study efficient numerical implementations of crystal plasticity in the spectral representation, with emphasis on high-performance computational aspects of the simulation. For illustrative purposes, we apply this approach to a Taylor homogenization model of fcc poly-crystalline materials and show that the spectral representation of crystal plasticity is ideal for parallel implementations aimed at next-generation large-scale microstructure-sensitive simulations of material deformation. We find that multi-thread parallelizations of the algorithm provide two orders of magnitude acceleration of the calculation, whereas graphics processing unit-based computing solutions allow for three orders of magnitude speedup factors over the conventional model. Copyright © 2014 John Wiley & Sons, Ltd.

We present an implementation of the viscoplastic self-consistent (VPSC) polycrystalline model in an implicit finite element (FE) framework, which accounts for a dislocation-based hardening law for multiple slip and twinning modes at the micro-scale grain level. The model is applied to simulate the macro-scale mechanical response of a highly anisotropic low-symmetry (orthorhombic) crystal structure. In this approach, a finite element integration point represents a polycrystalline material point and the meso-scale mechanical response is obtained by the mean-field VPSC homogenization scheme. We demonstrate the accuracy of the FE-VPSC model by analyzing the mechanical response and microstructure evolution of α-uranium samples under simple compression/tension and four-point bending tests. Predictions of the FE-VPSC simulations compare favorably with experimental measurements of geometrical changes and microstructure evolution. Specifically, the model captures accurately the tension–compression asymmetry of the material associated with twinning, as well as the rigidity of the material response along the hard-to-deform crystallographic orientations.

We report on the strength of Al–Al interfaces and the effects of chemical segregation and interfacial void formation on bond strength using microcantilever bend testing. Interfaces are synthesised via hot isostatic pressing. Microcantilevers of several nominal dimensions were fabricated via focused ion beam and deformed in a nanoindenter. We find increased cantilever strength as a function of decreasing sample size, with a linear dependence of the yield strength on the inverse square root of the length scale characteristic to the cantilever cross-section. The presence of pores and chemical segregation decreases the yield strength of the material by 17% and the accommodated strain energy by 10–15% for strain values in the 6–12% range.

We present a polycrystal plasticity model based on a self-consistent homogenization capable of predicting the macroscopic mechanical response and texture evolution of α-uranium over a wide range of temperatures and strain rates. The hardening of individual crystals is based on the evolution of dislocation densities and includes effects of strain rate and temperature through thermally-activated recovery, dislocation substructure formation, and slip-twin interactions. The model is validated on a comprehensive set of compression tests performed on a clock-rolled α-uranium plate at temperatures ranging from 198 to 573 K and strain rates ranging from 10 −3 to 3600 s −1 . The model is able to reproduce the stress–strain response and texture for all tests with a unique set of single-crystal hardening parameters. We elucidate the role played by the slip and twinning mechanisms and their interactions in large plastic deformation of α-uranium as a function of strain rate and temperature.

We discuss the effect of small perturbation on nodeless solutions of the nonlinear \Schrodinger\ equation in 1+1 dimensions in an external complex potential derivable from a parity-time symmetric superpotential that was considered earlier [Phys.~Rev.~E 92, 042901 (2015)]. In particular we consider the nonlinear partial differential equation $\{ \, \rmi \, \partial_t + \partial_x^2 + g |\psi(x,t)|^2 - V^{+}(x) \, \} \, \psi(x,t) = 0$, where $V^{+}(x) = \qty( -b^2 - m^2 + 1/4 ) \, \sech^2(x) - 2 i \, m \, b \, \sech(x) \, \tanh(x)$ represents the complex potential. Here we study the perturbations as a function of $b$ and $m$ using a variational approximation based on a dissipation functional formalism. We compare the result of this variational approach with direct numerical simulation of the equations. We find that the variational approximation works quite well at small and moderate values of the parameter $b m$ which controls the strength of the imaginary part of the potential. We also show that the dissipation functional formalism is equivalent to the generalized traveling wave method for this type of dissipation., Comment: 18 pages, 6 figures

Materials with strong correlations are prone to spin and charge instabilities, driven by Coulomb, magnetic, and lattice interactions. In materials that have significant localized and itinerant spins, it is not obvious which will induce order. We combine electrical transport, X-ray magnetic diffraction, and photoemission studies with band structure calculations to characterize successive antiferromagnetic transitions in GdSi. GdSi has both sizable local moments and a partially nested Fermi surface, without confounding contributions from orbital effects. We identify a route to incommensurate order where neither type of moment dominates, but is rooted in cooperative feedback between them. The nested Fermi surface of the itinerant electrons induces strong interactions between local moments at the nesting vector, whereas the ordered local moments in turn provide the necessary coupling for a spin-density wave to form among the itinerant electrons. This mechanism echoes the cooperative interactions between electrons and ions in charge-density–wave materials, and should be germane across a spectrum of transition-metal and rare-earth intermetallic compounds.

We study the coupled thermal transport, oxygen diffusion, and thermal expansion in a generic nuclear fuel rod consisting of a ( U 1 - y Pu y ) O 2 - x fuel pellet separated by a helium gap from zircaloy cladding. Steady-state and time-dependent finite-element simulations with a variety of initial- and boundary-value conditions are used to study the effect of the Pu content, y , and deviation from stoichiometry, x , on the temperature and deformation profiles in this fuel element. We find that the equilibrium radial temperature and deformation profiles are most sensitive to x at small values of y . For larger values of y , the effects of oxygen and Pu content are equally important. Following a change in the heat-generation rate, the centerline temperature, the radial deformation of the fuel pellet, and the centerline deviation from stoichiometry track each other closely in ( U,Pu ) O 2 - x , as the characteristic time scales of the heat transport and oxygen diffusion are similar. This result is different from the situation observed in the case of UO 2 + x fuels.

The United States Department of Energy, Office of the National uclear Security Administration established the Global Threat eduction Initiative (GTRI) program in the Office of Defense Nuclear onproliferation to reduce and protect vulnerable nuclear and adiological material located at civilian sites worldwide by providng support for countries’ own national programs. An important omponent of the GTRI important is the need to convert research eactors from the use of highly -enriched uranium (HEU) to lownriched uranium (LEU), with less that 20% uranium enrichment. hese efforts result in permanent threat reduction by minimizing nd, to the extent possible, eliminating the need for HEU in civilian pplications. Dispersion fuels made from LEU are inadequate for many highower test reactors (Clark et al., 2006). To achieve the needed oading for high-power test reactors, monolithic U–Mo alloy fuels ave been proposed (Clark et al., 2006), posing the challenge of onding an aluminum cladding to the monolithic fuel foils by proesses other than the roll bonding traditionally used for dispersion uels (Clark et al., 2005, 2006). Clark et al. (2003) proposed that the fuel foils should be prouced by a hot rolling process, followed by bonding the cladding to he fuel foils. To bond the cladding, these authors investigated sevral manufacturing processes, such as: high temperature rolling, ransient liquid phase bonding, diffusion bonding via hot isostatic ressing (HIP), and friction bonding. Of these processes, the HIP onding process produced the most consistent results (Robinson t al., 2009; Clark et al., 2005) and was chosen as the preferred manfacturing method, even though HIPping requires that the material s held under vacuum or reduced atmosphere to prevent the foration of an oxide layer on the material surface.

The deformation behavior of wrought α-uranium is studied using electron backscattered diffraction and crystal plasticity modeling. We report stress–strain response and texture evolution for 12 different cases corresponding to tension and compression tests performed on three different initial textures: straight-rolled, clock-rolled and swaged α-uranium. It is seen that the response of α-uranium is highly anisotropic owing to its low-symmetry orthorhombic crystal structure and limited number of slip/twin systems. For modeling this complex system, we adapt a multiscale dislocation-based hardening law developed earlier for hexagonal metals and implement it within a viscoplastic self-consistent homogenization scheme. This hardening law performs well in capturing the anisotropic strain hardening and the texture evolution in all studied samples. Comparisons of simulations and experiments allow us to infer basic information concerning the various slip and twin mechanisms, their interactions, and their role on strain hardening and texture evolution in α-uranium.

We study the coupled thermal transport, oxygen diffusion, and thermal expansion of a typical nuclear fuel element consisting of UO2+x fuel and stainless-steel cladding. Models of thermal, mechanical and chemical properties of the materials are used in a series of finite-element simulations to study the effect of the coupled phenomena on the temperature profile, oxygen distribution and radial deformation of the fuel element. The simulations include steady-state and time-dependent regimes in a variety of initial- and boundary value conditions that include sudden changes in the power density, variable oxygen content in the atmosphere, and variable temperature of the coolant. The study reveals the difference in the characteristic times associated with these phenomena and the importance of performing coupled simulations.

In an earlier paper Cooper, Shepard, and Sodano introduced a generalized KdV equation that can exhibit the kinds of compacton solitary waves that were first seen in equations studied by Rosenau and Hyman. This paper considers the PT-symmetric extensions of the equations examined by Cooper, Shepard, and Sodano. From the scaling properties of the PT-symmetric equations a general theorem relating the energy, momentum, and velocity of any solitary-wave solution of the generalized KdV equation is derived, and it is shown that the velocity of the solitons is determined by their amplitude, width, and momentum.

We discuss the stability properties of the solutions of the general nonlinear Schroedinger equation (NLSE) in 1+1 dimensions in an external potential derivable from a parity-time (PT) symmetric superpotential $W(x)$ that we considered earlier [Kevrekedis et al Phys. Rev. E 92, 042901 (2015)]. In particular we consider the nonlinear partial differential equation $\{ i \partial_t + \partial_x^2 - V^{-}(x) +| \psi(x,t) |^{2\kappa} \} \, \psi(x,t) = 0$, for arbitrary nonlinearity parameter $\kappa$. We study the bound state solutions when $V^{-}(x) = (1/4- b^2)$ sech$^2(x)$, which can be derived from two different superpotentials $W(x)$, one of which is complex and $PT$ symmetric. Using Derrick's theorem, as well as a time dependent variational approximation, we derive exact analytic results for the domain of stability of the trapped solution as a function of the depth $b^2$ of the external potential. We compare the regime of stability found from these analytic approaches with a numerical linear stability analysis using a variant of the Vakhitov-Kolokolov (V-K) stability criterion. The numerical results of applying the V-K condition give the same answer for the domain of stability as the analytic result obtained from applying Derrick's theorem. Our main result is that for $\kappa>2$ a new regime of stability for the exact solutions appears as long as $b > b_{crit}$, where $b_{crit}$ is a function of the nonlinearity parameter $\kappa$. In the absence of the potential the related solitary wave solutions of the NLSE are unstable for $\kappa>2$., Comment: 13 pages, 7 figures

results from such a nucleon-based model will pro-vide a baseline against which to gauge effects of quarkand other degrees of freedom. This approach will alsoallow the accurate calculation of nuclear matrix ele-ments needed for some tests of the standard model, andof nuclei and processes not presently accessible in thelaboratory. This can be useful for astrophysical studiesand for comparisons to future radioactive beam experi-ments. To achieve this goal, we must both determine theHamiltonians to be used and devise reliable methods formany-body calculations using them. Presently, we haveto rely on phenomenological models for the nuclear in-teraction, because a quantitative understanding of thenuclear force based on quantum chromodynamics is stillfar in the future.In order to make statements about the correctness ofa given phenomenological Hamiltonian, one must beable to make calculations whose results reflect the prop-erties of the Hamiltonian and are not obscured by ap-proximations. Because the Hamiltonian is still unknown,the correctness of the calculations cannot be determinedfrom comparison with experiment. For this reason it isessential to have a number of different

We discuss the stability properties of the solutions of the general nonlinear \Schrodinger\ equation (NLSE) in 1+1 dimensions in an external potential derivable from a parity-time ($\PT$) symmetric superpotential $W(x)$ that we considered earlier \cite{PhysRevE.92.042901}. In particular we consider the nonlinear partial differential equation $ \{ i \, \partial_t + \partial_x^2 - V(x) + g | \psi(x,t) |^{2\kappa} \} \, \psi(x,t) = 0 \>, $ for arbitrary nonlinearity parameter $\kappa$, where $g= \pm1$ and $V$ is the well known P{\"o}schl-Teller potential which we allow to be repulsive as well as attractive. Using energy landscape methods, linear stability analysis as well as a time dependent variational approximation, we derive consistent analytic results for the domains of instability of these new exact solutions as a function of the strength of the external potential and $\kappa$. For the repulsive potential (and $g=+1$) we show that there is a translational instability which can be understood in terms of the energy landscape as a function of a stretching parameter and a translation parameter being a saddle near the exact solution. In this case, numerical simulations show that if we start with the exact solution, the initial wave function breaks into two pieces traveling in opposite directions. If we explore the slightly perturbed solution situations, a 1\% change in initial conditions can change significantly the details of how the wave function breaks into two separate pieces. For the attractive potential (and $g=+1$), changing the initial conditions by 1 \% modifies the domain of stability only slightly. For the case of the attractive potential and negative $g$ perturbed solutions merely oscillate with the oscillation frequencies predicted by the variational approximation., Comment: 16 pages, 10 figures

The aim of this work is to nd numerical solutions for dierential equations by expanding the unknown function in terms of Chebyshev polynomials and solving a system of linear equations directly for the values of the function at the extrema (or zeros) of the Chebyshev polynomial of order N. The solutions are exact at these points, apart from round-o com- puter errors and the convergence of other numerical methods used in connection to solving the linear system of equations. Applications to initial-value problems in time-dependent quantum eld theory, and second order boundary-value problems in fluid dynamics are pre- sented. of the functions rather than the Chebyshev coecients. The two approaches are formally equivalent in the sense that if we have the values of the function, the Chebyshev coecients can be calculated. In this paper we use the discrete orthogonality relation- ships of the Chebyshev polynomials to exactly discretize various continuous equations by reducing the study of the solutions to the Hilbert space of functions dened on the set of (N+1) extrema of TN(x), spanned by a dis- crete (N+1) term Chebyshev polynomial basis. In our approach we follow closely the procedures outlined by El-gendy (6) for the calculation of integrals, but extend his work to the calculation of derivatives. We also show that similar procedures can be applied for a second grid given by the zeros of TN(x). The paper is organized as follows: In Section II we re- view the basic properties of the Chebyshev polynomial and derive the general theoretical ingredients that allow us to discretize the various equations. The key element is the calculation of derivatives and integrals without ex- plicitly calculating the Chebyshev expansion coecients. In Sections III and IV we apply the formalism to obtain numerical solutions of initial-value and boundary-value problems, respectively. We accompany the general pre- sentation with examples, and compare the solution ob- tained using the proposed Chebyshev method with the numerical solution obtained using the nite-dierences method. Our conclusions are presented in Section V.

We calculate the charge form factor and the longitudinal structure function for {sup 16}O and compare with the available experimental data, up to a momentum transfer of 4 fm{sup -1} . The ground-state correlations are generated using the coupled-cluster [exp(S) ] method, together with the realistic v18 NN interaction and the Urbana IX three-nucleon interaction. Center-of-mass corrections are dealt with by adding a center-of-mass Hamiltonian to the usual internal Hamiltonian, and by means of a many-body expansion for the computation of the observables measured in the center-of-mass system. (c) 2000 The American Physical Society.

We solve numerically to order 1/N the time evolution of a quantum dynamical system of N oscillators of mass m coupled quadratically to a massless dynamic variable. We use Schwinger{close_quote}s closed time path formalism to derive the equations. We compare two methods which differ by terms of order 1/N{sup 2}. The first method is a direct perturbation theory in 1/N using the path integral. The second solves exactly the theory defined by the effective action to order 1/N. We compare the results of both methods as a function of N. At N=1, where we expect the expansion to be quite innacurate, we compare our results to an exact numerical solution of the Schr{umlt o}dinger equation. In this case we find that when the two methods disagree they also diverge from the exact answer. We also find at N=1 that the 1/N corrected evolutions track the exact answer for the expectation values much longer than the mean field (N={infinity}) result. {copyright} {ital 1997} {ital The American Physical Society}

We probe the volume collapse transition (ΔV/Vo ∼ 15%) between the isostructural γ and α phases (T ∼ 100 K) of Ce0.9Th0.1 using the Hall effect, three-terminal capacitive dilatometry, and electrical resistivity measurements. Hall effect measurements confirm the itinerant ground state as the carrier concentration increases by a factor of 7 in the α phase, γ phase (nH = 5.28 × 10(26) m(-3)), and the α phase (nH = 3.76 × 10(27) m(-3)). We were able to detect a noise spectrum consisting of avalanches while slowly varying the temperature through the hysteretic region. We surmise that the avalanches originate from intergranular stresses at the interfaces between partially transformed high-volume and low-volume phases. The statistical distribution of avalanches obey power laws with energy exponent ϵ ≃ 1.5. Hall effect measurements, combined with universal critical exponents, point to short electron mean-free percolation pathways and carrier localization at phase interfaces. Carrier localization was predicted many years ago for elemental cerium by Johansson (1974 Phil. Mag. 30 469).

We discuss the phase diagram of the Bose-Hubbard (BH) model in the leading-order auxiliary field (LOAF) theory. LOAF is a conserving non-perturbative approximation that treats on equal footing the normal and anomalous density condensates. The mean-field solutions in LOAF correspond to first-order and second-order phase transition solutions with two critical temperatures corresponding to a vanishing Bose-Einstein condensate, $T_c$, and a vanishing diatom condensate, $T^\star$. The \emph{second-order} phase transition solution predicts the correct order of the transition in continuum Bose gases. For either solution, the superfluid state is tied to the presence of the diatom condensate related to the anomalous density in the system. In ultracold Bose atomic gases confined on a three-dimensional lattice, the critical temperature $T_c$ exhibits a quantum phase transition, where $T_c$ goes to zero at a finite coupling. The BH phase diagram in LOAF features a line of first-order transitions ending in a critical point beyond which the transition is second order while approaching the quantum phase transition. We identify a region where a diatom condensate is expected for temperatures higher than $T_c$ and less than $T_0$, the critical temperature of the non-interacting system. The LOAF phase diagram for the BH model compares qualitatively well with existing experimental data and results of \emph{ab initio} Monte Carlo simulations., 10 pages, 6 figures

We show that a composite-field (diatom) Goldstone state is expected in a dilute Bose gas for temperatures between the Bose gas critical temperature where the atom Bose-Einstein condensate appears and the temperature where superfluidity sets in. The presence of superfluidity is tied to the existence of a U(1) charge-two diatom condensate in the system. By promoting the global U(1) symmetry of the theory to a gauge symmetry, we find that the mass of the gauge particle generated through the Anderson-Higgs mechanism is related to the superfluid density via the Meissner effect and the superfluid density is related to the square of the anomalous density in the Bose system., 5 pages, 2 figures

We study the temperature dependence of thermoelectric transport properties of four FeSb2 nanocomposite samples with different grain sizes. The comparison of the single crystals and nanocomposites of varying grain sizes indicates the presence of substantial phonon drag effects in this system contributing to a large Seebeck coefficient at low temperature. As the grain size decreases, the increased phonon scattering at the grain boundaries leads to a suppression of the phonon-drag effect, resulting in a much smaller peak value of the Seebeck coefficient in the nanostructured bulk materials. As a consequence, the ZT values are not improved significantly even though the thermal conductivity is drastically reduced.

We derive the Josephson relation for a dilute Bose gas in the framework of an auxiliary-field resummation of the theory in terms of the normal- and anomalous-density condensates. The mean-field phase diagram of this theory features two critical temperatures, T_c and $T^*, associated with the presence in the system of the Bose-Einstein condensate (BEC) and superfluid state, respectively. In this context, the Josephson relation shows that the superfluid density is related to a second order parameter, the square of the anomalous-density condensate. This is in contrast with the corresponding result in the Bose gas theory without an anomalous condensate, which predicts that the superfluid density is proportional to the BEC condensate density. Our findings are consistent with the prediction that in the temperature range between T_c and T^* a fraction of the system is in the superfluid state in the absence of the BEC condensate. This situation is similar to the case of dilute Fermi gases, where the superfluid density is proportional to the square of the gap parameter. The Josephson relation relies on the existence of zero energy and momentum excitations showing the intimate relationship between superfluidity and the Goldstone theorem., 13 pages, 1 figure

We study the properties of compacton-anticompacton collision processes. We compare and contrast results for the case of compacton-anticompacton solutions of the K(l,p) Rosenau-Hyman (RH) equation for l = p = 2, with compacton-anticompacton solutions of the L(l,p) Cooper-Shepard-Sodano (CSS) equation for p = 1 and l = 3. This study is performed using a Padé discretization of the RH and CSS equations. We find a significant difference in the behavior of compacton-anticompacton scattering. For the CSS equation, the scattering can be interpreted as "annihilation" as the wake left behind dissolves over time. In the RH equation, the numerical evidence is that multiple shocks form after the collision, which eventually lead to "blowup" of the resulting wave form.

We develop the auxiliary field formalism corresponding to a dilute system of spin-1/2 fermions. This theory represents the Fermi counterpart of the BEC theory developed recently by F. Cooper et al. [Phys. Rev. Lett. 105, 240402 (2010)] to describe a dilute gas of Bose particles. Assuming tunable interactions, this formalism is appropriate for the study of the crossover from the regime of Bardeen-Cooper-Schriffer (BCS) pairing to the regime of Bose-Einstein condensation (BEC) in ultracold fermionic atom gases. We show that when applied to the Fermi case at zero temperature, the leading-order auxiliary field (LOAF) approximation gives the same equations as those obtained in the standard BCS variational picture. At finite temperature, LOAF leads to the theory discussed by by Sa de Melo, Randeria, and Engelbrecht [Phys. Rev. Lett. 71, 3202(1993); Phys. Rev. B 55, 15153(1997)]. As such, LOAF provides a unified framework to study the interacting Fermi gas. The mean-field results discussed here can be systematically improved upon by calculating the one-particle irreducible (1-PI) action corrections, order by order., Comment: 12 pages, 5 figures

We discuss the equilibrium properties of dilute Bose gases using a non-perturbative formalism based on auxiliary fields related to the normal and anomalous densities. We show analytically that for a dilute Bose gas of weakly-interacting particles at zero temperature, the leading-order auxiliary field (LOAF) approximation leads to well-known analytical results. Close to the critical point the LOAF predictions are the same as those obtained using an effective field theory in the large-N approximation. We also report analytical approximations for the LOAF results in the unitarity limit, which compare favorably with our numerical results. LOAF predicts that the equation of state for the Bose gas in the unitarity limit is E / (p V) = 1, unlike the case of the Fermi gas when E / (p V) = 3/2., Comment: 11 pages, 7 figures

We rewrite the Lagrangian for a dilute Bose gas in terms of auxiliary fields related to the normal and anomalous condensate densities. We derive the loop expansion of the effective action in the composite-field propagators. The lowest-order auxiliary field (LOAF) theory is a conserving mean-field approximation consistent with the Goldstone theorem without some of the difficulties plaguing approximations such as the Hartree and Popov approximations. LOAF predicts a second-order phase transition. We give a set of Feynman rules for improving results to any order in the loop expansion in terms of composite-field propagators. We compare results of the LOAF approximation with those derived using the Popov approximation. LOAF allows us to explore the critical regime for all values of the coupling constant and we determine various parameters in the unitarity limit., Comment: 16 pages, 7 figures

Extending a Pade approximant method used for studying compactons in the Rosenau-Hyman (RH) equation, we study the numerical stability of single compactons of the Cooper-Shepard-Sodano (CSS) equation and their pairwise interactions. The CSS equation has a conserved Hamiltonian which has allowed various approaches for studying analytically the nonlinear stability of the solutions. We study three different compacton solutions and find they are numerically stable. Similar to the collisions between RH compactons, the CSS compactons reemerge with same coherent shape when scattered. The time evolution of the small-amplitude ripple resulting after scattering depends on the values of the parameters $l$ and $p$ characterizing the corresponding CSS equation. The simulation of the CSS compacton scattering requires a much smaller artificial viscosity to obtain numerical stability, than in the case of RH compactons propagation., Comment: 8 figures

We report resonant ultrasound spectroscopy (RUS), dilatometry/magnetostriction, magnetotransport, magnetization, specific-heat, and $^{119}\text{S}\text{n}$ M\"ossbauer spectroscopy measurements on SnTe and ${\text{Sn}}_{0.995}{\text{Cr}}_{0.005}\text{Te}$. Hall measurements at $T=77\text{ }\text{K}$ indicate that our Bridgman-grown single crystals have a $p$-type carrier concentration of $3.4\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$ and that our Cr-doped crystals have an $n$-type concentration of $5.8\ifmmode\times\else\texttimes\fi{}{10}^{22}\text{ }{\text{cm}}^{\ensuremath{-}3}$. Although our SnTe crystals are diamagnetic over the temperature range $2\ensuremath{\le}T\ensuremath{\le}1100\text{ }\text{K}$, 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 $0.5\text{ }\ensuremath{\mu}\text{m}$ distortion at ${T}_{c}\ensuremath{\approx}85\text{ }\text{K}$. Whereas our RUS measurements on SnTe show elastic hardening near the structural transition, pointing to co-elastic behavior, similar measurements on ${\text{Sn}}_{0.995}{\text{Cr}}_{0.005}\text{Te}$ show a pronounced softening, pointing to ferroelastic behavior. Effective Debye temperature, ${\ensuremath{\theta}}_{D}$, values of SnTe obtained from $^{119}\text{S}\text{n}$ M\"ossbauer studies show a hardening of phonons in the range 60--115 K $({\ensuremath{\theta}}_{D}=162\text{ }\text{K})$ as compared with the 100--300 K range $({\ensuremath{\theta}}_{D}=150\text{ }\text{K})$. 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 $\ensuremath{\Delta}{C}_{44}/{C}_{44}=A{|(T\ensuremath{-}{T}_{0})/{T}_{0}|}^{\ensuremath{-}\ensuremath{\kappa}}$ with $\ensuremath{\kappa}=0.50\ifmmode\pm\else\textpm\fi{}0.02$, a value indicating a three-dimensional softening of phonon branches at a temperature ${T}_{0}\ensuremath{\sim}75\text{ }\text{K}$, 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.

We present an angle-resolved photoemission spectroscopy study of the electronic structure of SnTe, and compare the experimental results to ab initio band structure calculations as well as a simplified tight-binding model of the p-bands. Our study reveals the conjectured complex Fermi surface structure near the L-points showing topological changes in the bands from disconnected pockets, to open tubes, and then to cuboids as the binding energy increases, resolving lingering issues about the electronic structure. The chemical potential at the crystal surface is found to be 0.5eV below the gap, corresponding to a carrier density of p =1.14x10^{21} cm^{-3} or 7.2x10^{-2} holes per unit cell. At a temperature below the cubic-rhombohedral structural transition a small shift in spectral energy of the valance band is found, in agreement with model predictions., 4 figures

We consider the nonlinear Dirac equations (NLDE's) in 1+1 dimension with scalar-scalar self interaction $\frac{g^2}{k+1} ({\bar \Psi} \Psi)^{k+1}$, as well as a vector-vector self interaction $\frac{g^2}{k+1} ({\bar \Psi} \gamma_\mu \Psi \bPsi \gamma^\mu \Psi)^{\frac{1}{2}(k+1)}$. We find the exact analytic form for solitary waves for arbitrary $k$ and find that they are a generalization of the exact solutions for the nonlinear Schr\"odinger equation (NLSE) and reduce to these solutions in a well defined nonrelativistic limit. We perform the nonrelativistic reduction and find the $1/2m$ correction to the NLSE, valid when $|\omega-m |\ll 2m$, where $\omega$ is the frequency of the solitary wave in the rest frame. We discuss the stability and blowup of solitary waves assuming the modified NLSE is valid and find that they should be stable for $k < 2$., Comment: 15 pages, 7 figures

We present a systematic approach for calculating higher-order derivatives of smooth functions on a uniform grid using Pad\'e approximants. We illustrate our findings by deriving higher-order approximations using traditional second-order finite-differences formulas as our starting point. We employ these schemes to study the stability and dynamical properties of K(2,2) Rosenau-Hyman (RH) compactons including the collision of two compactons and resultant shock formation. Our approach uses a differencing scheme involving only nearest and next-to-nearest neighbors on a uniform spatial grid. The partial differential equation for the compactons involves first, second and third partial derivatives in the spatial coordinate and we concentrate on four different fourth-order methods which differ in the possibility of increasing the degree of accuracy (or not) of one of the spatial derivatives to sixth order. A method designed to reduce roundoff errors was found to be the most accurate approximation in stability studies of single solitary waves, even though all derivates are accurate only to fourth order. Simulating compacton scattering requires the addition of fourth derivatives related to artificial viscosity. For those problems the different choices lead to different amounts of "spurious" radiation and we compare the virtues of the different choices., Comment: 12 figures

We develop methods for computing the dynamics of fermion pair production by strong color electric fields using the semi-classical Boltzmann-Vlasov equation. We present numerical results for a model with SU(2) symmetry in (1+1) dimension., 10 pages, 8 figures

We derive a theoretical description for dilute Bose gases as a loop expansion in terms of composite-field propagators by rewriting the Lagrangian in terms of auxiliary fields related to the normal and anomalous densities. We demonstrate that already in leading order this non-perturbative approach describes a large interval of coupling-constant values, satisfies Goldstone's theorem, yields a Bose-Einstein transition that is second-order, and is consistent with the critical temperature predicted in the weak-coupling limit by the next-to-leading order large-N expansion., Comment: 3 figures

We study the fermion pair production from a strong electric field in boost-invariant coordinates in (3+1) dimensions and exploit the cylindrical symmetry of the problem. This problem has been used previously as a toy model for populating the central-rapidity region of a heavy-ion collision (when we can replace the electric by a chromoelectric field). We derive and solve the renormalized equations for the dynamics of the mean electric field and current of the produced particles, when the field is taken to be a function only of the fluid proper time $\tau = \sqrt{t^2-z^2}$. We determine the proper-time evolution of the comoving energy density and pressure of the ensuing plasma and the time evolution of suitable interpolating number operators. We find that unlike in (1+1) dimensions, the energy density closely follows the longitudinal pressure. The transverse momentum distribution of fermion pairs at large momentum is quite different and larger than that expected from the constant field result., Comment: 8 figures

We present numerical solutions of the semi-classical Boltzmann-Vlasov equation for fermion particle-antiparticle production by strong electric fields in boost-invariant coordinates in (1+1) and (3+1) dimensional QED. We compare the Boltzmann-Vlasov results with those of recent quantum field theory calculations and find good agreement. We conclude that extending the Boltzmann-Vlasov approach to the case of QCD should allow us to do a thorough investigation of how back-reaction affects recent results on the dependence of the transverse momentum distribution of quarks and anti-quarks on a second Casimir invariant of color SU(3)., 8 figures

We consider the effect of electron-phonon coupling in semimetals in high magnetic fields, with regard to elastic modes that can lead to a redistribution of carriers between pockets. We show that in a clean three dimensional system, at each Landau level crossing, this leads to a discontinuity in the magnetostriction, and a divergent contribution to the elastic modulus. We estimate the magnitude of this effect in the group V semimetal Bismuth., Comment: 2 figures

In an earlier paper Cooper, Shepard, and Sodano introduced a generalized KdV equation that can exhibit the kinds of compacton solitary waves that were first seen in equations studied by Rosenau and Hyman. This paper considers the PT-symmetric extensions of the equations examined by Cooper, Shepard, and Sodano. From the scaling properties of the PT-symmetric equations a general theorem relating the energy, momentum, and velocity of any solitary-wave solution of the generalized KdV equation is derived, and it is shown that the velocity of the solitons is determined by their amplitude, width, and momentum., 12 pages, 5 figures

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic transition temperatures (T_M=64K and 45K, respectively). In each composition, elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's T_M. Although the new Bragg peaks appear in a discontinuous manner in the Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near the transition temperature are in favorable accord with a mean-field approximation. A Landau-theory-based fit to the pressure dependence of the transition temperature suggests the presence of a critical endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a quantum saturation temperature \theta_s=48.3 +/- 3.7K., Comment: 6 figures

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni$_2$MnGa over the temperature range $100K < T < 250K$. All measurements detect clear signatures of the premartensitic transition ($T_\mathrm{PM}\sim 247K$) and the martensitic transition ($T_\mathrm{M} \sim 196K$). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at $T_\mathrm{PM}$ located 0.3eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3eV in the UPS spectra for $T > T_\mathrm{PM}$ is due to the Ni-d minority-spin electrons. Below $T_\mathrm{M}$ this peak disappears, resulting in an enhanced density of states at energies around 0.8eV. This enhancement reflects Ni-d and Mn-d electronic contributions to the majority-spin density of states and is accompanied by significant reconstruction of the Fermi surface.