Stress concentration targeted reinforcement using multi-material based 3D printing

Topological engineering (3D printing into complex geometry) has emerged as a pragmatic approach to develop high specific strength (high strength and low density) lightweight structures. These complex lightweight structures fail at high-stress concentration regions, which can be, replaced with soft/tough material using 3D printing. It can improve mechanical properties such as strength, toughness and energy absorption etc. Here, we have developed stress concentration targeted multi-material schwarzite structures by 3D printing technique. The soft (Thermoplastic Polyurethane) material is reinforced at high-stress concentration regions of hard (Polylactic acid) schwarzite structures to enhance the specific yield strength and resilience. The mechanical properties and responses of these structures were then assessed via uniaxial compression tests. The multi-materials 3D printed composite structure shows improved mechanical properties compared to single materials architecture. The specific resilience of composites demonstrates remarkable enhancements, with percentage increases of 204.70 %, 596.50 %, and 1530.99 % observed when compared to hard primitives, and similarly impressive improvements of 182.45 %, 311.64 %, and 477.75 % observed in comparison to hard gyroids. The obtained experimental findings were comprehensively examined and validated with molecular dynamics (MD) simulations. The promising characteristics of these lightweight multi-material-based Schwarzites structures can be utilized in various fields such as energy harvesting devices, protective, safety gears, and aerospace components

Crossover and scaling in a two-dimensional field-tuned superconductor

Using an analysis similar to that of Imry and Wortis, it is shown that the apparent first order superconductor to metal transition, which has been claimed to exist at low values of the magnetic field in a two-dimensional field-tuned system at zero temperature,can be consistentlyinterpreted as a sharp crossover from a strong superconductor to an inhomogeneous state, which is a weak superconductor. The true zero-temperature superconductor to insulator transition within the inhomogenous state is conjectured to be that of randomly diluted XY model. An explaination of the observed finite temperature approximate scaling of resistivity close to the critical point is speculated within this model.Comment: 5 pages, 2 figures, corrected and modified according to referee Report

Rolling Tachyon Boundary State, Conserved Charges and Two Dimensional String Theory

The boundary state associated with the rolling tachyon solution on an unstable D-brane contains a part that decays exponentially in the asymptotic past and the asymptotic future, but it also contains other parts which either remain constant or grow exponentially in the past or future. We argue that the time dependence of the latter parts is completely determined by the requirement of BRST invariance of the boundary state, and hence they contain information about certain conserved charges in the system. We also examine this in the context of the unstable D0-brane in two dimensional string theory where these conserved charges produce closed string background associated with the discrete states, and show that these charges are in one to one correspondence with the symmetry generators in the matrix model description of this theory.Comment: LaTeX file, 37 pages; v3: references added; v4: minor change

Rolling tachyon solution of two-dimensional string theory

We consider a classical (string) field theory of $c=1$ matrix model which was developed earlier in hep-th/9207011 and subsequent papers. This is a noncommutative field theory where the noncommutativity parameter is the string coupling $g_s$. We construct a classical solution of this field theory and show that it describes the complete time history of the recently found rolling tachyon on an unstable D0 brane.Comment: 19 pages, 2 figures, minor changes in text and additional references, correction of decay time (version to appear in JHEP.

A One-Parameter Family of Hamiltonian Structures for the KP Hierarchy and a Continuous Deformation of the Nonlinear \W_{\rm KP} Algebra

The KP hierarchy is hamiltonian relative to a one-parameter family of Poisson structures obtained from a generalized Adler map in the space of formal pseudodifferential symbols with noninteger powers. The resulting \W-algebra is a one-parameter deformation of \W_{\rm KP} admitting a central extension for generic values of the parameter, reducing naturally to \W_n for special values of the parameter, and contracting to the centrally extended \W_{1+\infty}, \W_\infty and further truncations. In the classical limit, all algebras in the one-parameter family are equivalent and isomorphic to \w_{\rm KP}. The reduction induced by setting the spin-one field to zero yields a one-parameter deformation of \widehat{\W}_\infty which contracts to a new nonlinear algebra of the \W_\infty-type.Comment: 31 pages, compressed uuencoded .dvi file, BONN-HE-92/20, US-FT-7/92, KUL-TF-92/20. [version just replaced was truncated by some mailer

On The Problem of Particle Production in c=1 Matrix Model

We reconsider and analyze in detail the problem of particle production in the time dependent background of $c=1$ matrix model where the Fermi sea drains away at late time. In addition to the moving mirror method, which has already been discussed in hep-th/0403169 and hep-th/0403275, we describe yet another method of computing the Bogolubov coefficients which gives the same result. We emphasize that these Bogolubov coefficients are approximately correct for small value of the deformation parameter. We also study the time evolution of the collective field theory stress-tensor with a special point-splitting regularization. Our computations go beyond the approximation of the previous treatments and are valid at large coordinate distances from the boundary at a finite time and up-to a finite coordinate distance from the boundary at late time. In this region of validity our regularization produces a certain singular term that is precisely canceled by the collective field theory counter term in the present background. The energy and momentum densities fall off exponentially at large distance from the boundary to the values corresponding to the static background. This clearly shows that the radiated energy reaches the asymptotic region signaling the space-time decay.Comment: 37 pages, 5 figures. Section 6 is modified to clarify main accomplishments of the paper including a discussion comparing stress-tensor analysis with those preexisted in literature. Other modifications include minor changes in the text and addition of one reference. Version accepted for publication in JHE

Structure and magnetocaloric properties of La1-xKxMnO3 manganites

A technology of obtaining the single-phase ceramic samples of La1-xKxMnO3 manganites and the dependence of their structural parameters on the content of potassium has been described. Magnetocaloric effect (MCE) in the obtained samples has been measured by two independent methods: by classical direct methodic and by a method of magnetic field modulation. The values of MCE obtained by both methods have been substantially differed. The explanation of the observed divergences is given. The correlation between the level of doping and MCE value has been defined. The value of TC determined by the MCE maximum has been conformed to the literature data received by other methods.Comment: 14 pages, 6 figures, 3 table

Spacetime Energy Decreases under World-sheet RG Flow

We study renormalization group flows in unitary two dimensional sigma models with asymptotically flat target spaces. Applying an infrared cutoff to the target space, we use the Zamolodchikov c-theorem to demonstrate that the target space ADM energy of the UV fixed point is greater than that of the IR fixed point: spacetime energy decreases under world-sheet RG flow. This result mirrors the well understood decrease of spacetime Bondi energy in the time evolution process of tachyon condensation.Comment: 25 pages, 4 figures, harvma

Vortex dynamics and upper critical fields in ultrathin Bi films

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin $Bi$ films in a magnetic field are reported. These I-V's show hysteresis for all films, grown both with and without thin $Ge$ underlayers. Films on Ge underlayers, close to superconductor-insulator transition (SIT), show a peak in the critical current, indicating a structural transformation of the vortex solid (VS). These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields (B$_{c2}$) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.Comment: Phys Rev B, to be published Figure 6 replaced with correct figur

Classical A_n--W-Geometry

This is a detailed development for the $A_n$ case, of our previous article entitled "W-Geometries" to be published in Phys. Lett. It is shown that the $A_n$--W-geometry corresponds to chiral surfaces in $CP^n$. This is comes out by discussing 1) the extrinsic geometries of chiral surfaces (Frenet-Serret and Gauss-Codazzi equations) 2) the KP coordinates (W-parametrizations) of the target-manifold, and their fermionic (tau-function) description, 3) the intrinsic geometries of the associated chiral surfaces in the Grassmannians, and the associated higher instanton- numbers of W-surfaces. For regular points, the Frenet-Serret equations for $CP^n$--W-surfaces are shown to give the geometrical meaning of the $A_n$-Toda Lax pair, and of the conformally-reduced WZNW models, and Drinfeld-Sokolov equations. KP coordinates are used to show that W-transformations may be extended as particular diffeomorphisms of the target-space. This leads to higher-dimensional generalizations of the WZNW and DS equations. These are related with the Zakharov- Shabat equations. For singular points, global Pl\"ucker formulae are derived by combining the $A_n$-Toda equations with the Gauss-Bonnet theorem written for each of the associated surfaces.Comment: (60 pages