2,715 research outputs found
Numerical and approximate analytical results for the frustrated spin-1/2 quantum spin chain
We study the frustrated phase of the quantum spin-
system with nearest-neighbour and next-nearest-neighbour isotropic exchange
known as the Majumdar-Ghosh Hamiltonian. We first apply the coupled-cluster
method of quantum many-body theory based on a spiral model state to obtain the
ground state energy and the pitch angle. These results are compared with
accurate numerical results using the density matrix renormalisation group
method, which also gives the correlation functions. We also investigate the
periodicity of the phase using the Marshall sign criterion. We discuss
particularly the behaviour close to the phase transitions at each end of the
frustrated phase.Comment: 17 pages, Standard Latex File + 7 PostScript figures in separate
file. Figures also can also be requested from [email protected]
Onset of incommensurability in quantum spin chains
In quantum spin chains, it has been observed that the incommensurability
occurs near valence-bond-solid (VBS)-type solvable points, and the correlation
length becomes shortest at VBS-type points. Besides, the correlation function
decays purely exponentially at VBS-type points, in contrast with the
two-dimensional (2D) Ornstein-Zernicke type behavior in the other region with
an excitation gap. We propose a mechanism to explain the onset of the
incommensurability and the shortest correlation length at VBS-like points. This
theory can be applicable for more general cases.Comment: 9 pages, 2 figure
Diode laser assisted surface nitriding of Ti-6Al-4v: properties of the nitrided surface
In the present study, a detailed investigation of mechanical and electrochemical properties of laser-surface-nitrided Ti-6Al-4V has been carried out. Laser treatment is carried out by melting the surface of Ti-6Al-4V substrate using a high power CW diode laser with nitrogen as shrouding environment. The effect of laser parameters (applied power and gas flow rate) on the properties of the nitrided surface was evaluated. The microhardness of the nitrided surface was improved to a maximum of 1175 VHN in the present set of laser processing conditions as compared to 280 VHN of as-received substrate. Surface nitriding increased the potential for pit formation (E pit) significantly as compared to as-received Ti-6Al-4V. Immersion in Hank's solution showed calcium phosphate deposition from the solution. The optimum process parameters for laser surface nitriding were derived
Scaling of NonOhmic Conduction in Strongly Correlated Systems
A new scaling formalism is used to analyze nonlinear I-V data in the vicinity
of metal-insulator transitions (MIT) in five manganite systems. An exponent,
called the nonlinearity exponent, and an onset field for nonlinearity, both
characteristic of the system under study, are obtained from the analysis. The
onset field is found to have an anomalously low value corroborating the
theoretically predicted electronically soft phases. The scaling functions above
and below the MIT of a polycrystalline sample are found to be the same but with
different exponents which are attributed to the distribution of the MIT
temperatures. The applicability of the scaling in manganites underlines the
universal response of the disordered systems to electric field
Microscopic Non-Universality versus Macroscopic Universality in Algorithms for Critical Dynamics
We study relaxation processes in spin systems near criticality after a quench
from a high-temperature initial state. Special attention is paid to the stage
where universal behavior, with increasing order parameter emerges from an early
non-universal period. We compare various algorithms, lattice types, and
updating schemes and find in each case the same universal behavior at
macroscopic times, despite of surprising differences during the early
non-universal stages.Comment: 9 pages, 3 figures, RevTeX, submitted to Phys. Rev. Let
Laser surface nitriding of Ti-6Al-4V for bio-implant application
The present study aims at enhancing the biocompatibility of Ti-6Al-4V by laser surface nitriding. Laser surface nitriding has been carried out by melting of sand blasted Ti-6Al-4V substrate using a high power continuous wave DIODE laser with nitrogen as shrouding environment (at a pressure of 5l/min). Following laser treatment, a detailed characterization of the surface has been conducted. Microhardness and biocompatibility have been evaluated. Laser surface nitriding led to formation of dendrites of TiN on the surface. The microhardness is improved to 900- 950 VHN (in laser surface nitriding) as compared to 260 VHN of as-received substrate. Biocompatibility behavior showed a better cell viability in laser surface nitrided Ti-6Al-4V sample as compared to as-received one
Frustrated antiferromagnetic quantum spin chains for spin length S > 1
We investigate frustrated antiferromagnetic Heisenberg quantum spin chains at
T=0 for S=3/2 and S=2 using the DMRG method. We localize disorder and Lifshitz
points, confirming that quantum disorder points can be seen as quantum remnants
of classical phase transitions. Both in the S=3/2 and the S=2 chain, we observe
the disappearance of effectively free S=1/2 and S=1 end spins respectively. The
frustrated spin chain is therefore a suitable system for clearly showing the
existence of free end spins S'=[S/2] also in half-integer antiferromagnetic
spin chains with S>1/2. We suggest that the first order transition found for
S=1 in our previous work is present in all frustrated spin chains with S>1/2,
characterized by the disappearance of effectively free end spins with S'=[S/2].Comment: 6 pages, 8 ps figures, uses RevTeX, submitted to PR
Phase diagram and hidden order for generalized spin ladders
We investigate the phase diagram of antiferromagnetic spin ladders with
additional exchange interactions on diagonal bonds by variational and numerical
methods. These generalized spin ladders interpolate smoothly between the
chain with competing nn and nnn interactions, the chain with
alternating exchange and the antiferromagnetic chain. The Majumdar-Ghosh
ground states are formulated as matrix product states and are shown to exhibit
the same type of hidden order as the af chain. Generalized matrix product
states are used for a variational calculation of the ground state energy and
the spin and string correlation functions. Numerical (Lanczos) calculations of
the energies of the ground state and of the low-lying excited states are
performed, and compare reasonably with the variational approach. Our results
support the hypothesis that the dimer and Majumdar-Ghosh points are in the same
phase as the af chain.Comment: 23 pages, REVTEX, 7 figure
Shock statistics in higher-dimensional Burgers turbulence
We conjecture the exact shock statistics in the inviscid decaying Burgers
equation in D>1 dimensions, with a special class of correlated initial
velocities, which reduce to Brownian for D=1. The prediction is based on a
field-theory argument, and receives support from our numerical calculations. We
find that, along any given direction, shocks sizes and locations are
uncorrelated.Comment: 4 pages, 8 figure
Laser surface treatment of Ti-6Al-4V for bio-implant application
The present study aims at enhancing the wear resistance of Ti-6Al-4V by laser surface melting and nitriding and subsequently, studying the influence of laser surface treatment on the corrosion resistance in a simulated body fluid and also the bio-compatibility. The laser surface treatment is carried out using a high power continuous wave diode laser with argon and nitrogen as shrouding gas. Laser surface melting leads to an increased volume fraction of acicular martensite and a decreased volume fraction of the β phase in the microstructure. Laser surface nitriding leads to the formation of titanium nitride dendrites. The micro-hardness could be improved up to a maximum of 450 Hv in laser surface melting and 900-950 Hv in the case of laser surface nitriding as compared to 260 Hv of the as-received substrate. Surface melting increases the corrosion potential (Ecorr) and primary potential for pit formation (Epp1) significantly as compared to the as-received Ti-6Al-4V. However, when processed under similar conditions, surface nitriding shifts Ecorr marginally in the more noble direction, and increased Epp1 as compared to Ti-6Al-4V. The biocompatibility behaviour shows a superior cell viability on surface nitriding and an inferior cell viability on surface melting as compared to the as-received Ti-6Al-4V
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