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At least 1,135 records · Page 63Linked to original sources
PubMed2006

Entropy-induced separation of star polymers in porous media.

We present a quantitative picture of the separation of star polymers in a solution where part of the volume is influenced by a porous medium. To this end, we study the impact of long-range-correlated quenched disorder on the entropy and scaling properties of f-arm star polymers in a good solvent. We assume that the disorder is correlated on the polymer length scale with a power-law decay of the pair correlation function g(r) approximately r-a. Applying the field-theoretical renormalization group approach we show in a double expansion in epsilon=4-d and delta=4-a that there is a range of correlation strengths delta for which the disorder changes the scaling behavior of star polymers. In a second approach we calculate for fixed space dimension d=3 and different values of the correlation parameter a the corresponding scaling exponents gammaf that govern entropic effects. We find that gammaf-1, the deviation of gammaf from its mean field value is amplified by the disorder once we increase delta beyond a threshold. The consequences for a solution of diluted chain and star polymers of equal molecular weight inside a porous medium are that star polymers exert a higher osmotic pressure than chain polymers and in general higher branched star polymers are expelled more strongly from the correlated porous medium. Surprisingly, polymer chains will prefer a stronger correlated medium to a less or uncorrelated medium of the same density while the opposite is the case for star polymers.

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PubMed2006

Maximum entropy approach to power-law distributions in coupled dynamic-stochastic systems.

Statistical properties of coupled dynamic-stochastic systems are studied within a combination of the maximum information principle and the superstatistical approach. The conditions at which the Shannon entropy functional leads to power-law statistics are investigated. It is demonstrated that, from a quite general point of view, the power-law dependencies may appear as a consequence of "global" constraints restricting both the dynamic phase space and the stochastic fluctuations. As a result, at sufficiently long observation times the dynamic counterpart is driven into a nonequilibrium steady state whose deviation from the usual exponential statistics is given by the distance from the conventional equilibrium.

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PubMed2000

Entropy balance and evidence for local spin singlets in a Kagome-like magnet

We have measured the specific heat C(T) of the S = 3 / 2 Kagome-lattice-containing compound SrCr (9p)Ga (12-9p)O (19). We find little field dependence of the low-temperature C(T), consistent with a low-energy spectrum dominated by many-body singlet excitations. At high temperatures, we recover only approximately 50% of the total R ln4 entropy.

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PubMed2000

Logarithmic correction to the bekenstein-hawking entropy

The exact formula derived by us earlier for the entropy of a four dimensional nonrotating black hole within the quantum geometry formulation of the event horizon in terms of boundary states of a three dimensional Chern-Simons theory is reexamined for large horizon areas. In addition to the semiclassical Bekenstein-Hawking contribution proportional to the area obtained earlier, we find a contribution proportional to the logarithm of the area together with subleading corrections that constitute a series in inverse powers of the area.

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PubMed2000

Continuous frequency entanglement: effective finite hilbert space and entropy control

We examine the quantum structure of continuum entanglement and in the context of short-pulse down-conversion we answer the open question of how many of the uncountably many frequency modes contribute effectively to the entanglement. We derive a set of two-photon mode functions that provide an exact, discrete, and effectively finite basis for characterizing pairwise entanglement. Our analysis provides a basis for entropy control in two-photon pulses generated from down-conversion.

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PubMed2000

Exit-time approach to epsilon-entropy.

An efficient approach to the calculation of the epsilon-entropy is proposed. The method is based on the idea of looking at the information content of a string of data, by analyzing the signal only at the instants when the fluctuations are larger than a certain threshold epsilon, i.e., by looking at the exit-time statistics. The practical and theoretical advantages of our method with respect to the usual one are shown by the examples of a deterministic map and a self-affine stochastic process.

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PubMed2000

Negative excess interfacial entropy between free and end-grafted chemically identical polymers.

We measured the temperature dependence of the interfacial tension between free melt and end-grafted chemically identical polymers. The excess interfacial entropy (Delta S(MB)) was found to be extremely small and negative. This is in sharp contrast to the case of liquid surfaces and interfaces between different liquids, where Delta S(MB) is usually much larger and positive, i.e., molecules at such surfaces and interfaces have a higher degree of freedom than in the bulk. A quantitative comparison with theoretical predictions revealed large differences which we attribute to the finite compressibility of the polymer melt, not yet taken into account by theory.

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PubMed2001

Entropy-vanishing transition and glassy dynamics in frustrated spins.

In an effort to understand the glass transition, the dynamics of a nonrandomly frustrated spin model has been analyzed. The phenomenology of the spin model is similar to that of a supercooled liquid undergoing the glass transition. The slow dynamics can be associated with the presence of extended stringlike structures which demarcate regions of fast spin flips. An entropy-vanishing transition, with the string density as the order parameter, is related to the observed glass transition in the spin model.

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PubMed2001

Entropy-driven stabilization of a novel configuration for acceptor-hydrogen complexes in GaN.

We present a model for the microscopic structure of Mg-H complexes in GaN, explaining the unusual bond angle observed in recent vibrational spectroscopy studies. The structure is not the lowest-energy configuration at T = 0, but it is stabilized at elevated temperatures due to the large entropy associated with a set of low-energy rotational excitations. The rotational excitation spectrum is calculated using a quantum-mechanical model in which the hydrogen atom moves in a weak corrugation potential. Consequences for experiment are discussed.

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PubMed2001

Asymptotic relative entropy of entanglement.

We present an analytical formula for the asymptotic relative entropy of entanglement with respect to positive partial transpose states for Werner states of arbitrary dimension. We then demonstrate its validity using methods from convex optimization. This is the first case in which the asymptotic value of a subadditive entanglement measure has been calculated.

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PubMed2001

Teleportation as a depolarizing quantum channel, relative entropy, and classical capacity.

We show that standard teleportation with an arbitrary mixed state resource is equivalent to a generalized depolarizing channel with probabilities given by the maximally entangled components of the resource. This enables the usage of any quantum channel as a generalized depolarizing channel without additional twirling operations. It also provides a nontrivial upper bound on the entanglement of a class of mixed states. Our result allows a consistent and statistically motivated quantification of teleportation success in terms of the relative entropy and this quantification can be related to a classical capacity.

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PubMed2002

Entropy of vortex cores near the superconductor-insulator transition in an underdoped cuprate.

We present a study of Nernst effect in underdoped La(2-x)Sr(x)CuO4 in magnetic fields as high as 28 T. At high fields, a sizable Nernst signal was found to persist in the presence of a field-induced nonmetallic resistivity. By simultaneously measuring resistivity and the Nernst coefficient, we extract the entropy of vortex cores in the vicinity of this field-induced superconductor-insulator transition. Moreover, the temperature dependence of the thermoelectric Hall angle provides strong constraints on the possible origins of the finite Nernst signal above T(c), as recently discovered by Xu et al. [Nature (London) 406, 486 (2000)].

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PubMed2002

Generalized nonadditive entropies and quantum entanglement.

We examine the inference of quantum density operators from incomplete information by means of the maximization of general nonadditive entropic forms. Extended thermodynamic relations are given. When applied to a bipartite spin 1 / 2 system, the formalism allows one to avoid fake entanglement for data based on the Bell-Clauser-Horne-Shimony-Holt observable, and, in general, on any set of Bell constraints. Particular results obtained with the Tsallis entropy and with an introduced exponential entropic form are also discussed.

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PubMed2002

Quantum entropy and special relativity.

We consider a single free spin- 1 / 2 particle. The reduced density matrix for its spin is not covariant under Lorentz transformations. The spin entropy is not a relativistic scalar and has no invariant meaning.

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PubMed2002

Hamilton-Jacobi formulation of Kolmogorov-Sinai entropy for classical and quantum dynamics.

A Hamilton-Jacobi formulation of the Lyapunov spectrum and Kolmogorov-Sinai (KS) entropy is developed. It is numerically efficient and reveals a close relation between the KS invariant and the classical action. This formulation is extended to the quantum domain using the Madelung-Bohm orbits associated with the Schroedinger equation. The resulting quantum KS invariant for a given orbit equals the mean decay rate of the probability density along the orbit, while its ensemble average measures the mean growth rate of configuration-space information for the quantum system.

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PubMed2002

Heat conduction and entropy production in a one-dimensional hard-particle gas.

We present large scale simulations for a one-dimensional chain of hard-point particles with alternating masses and correct several claims in recent literature based on much smaller simulations. We find heat conductivities kappa to diverge with the number N of particles. These depended strongly on the mass ratio, and extrapolations to N--> infinity, and t--> infinity, are difficult due to very large finite-size and finite-time corrections. Nevertheless, our data seem compatible with a universal power law kappa approximately N(alpha) with alpha approximately 0.33 suggesting a relation to the Kardar-Parisi-Zhang model. We finally discuss why the system leads nevertheless to energy dissipation and entropy production, in spite of not being chaotic in the usual sense.

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PubMed2003

Entanglement and entropy engineering of atomic two-qubit States.

We propose a scheme employing quantum-reservoir engineering to controllably entangle the internal states of two atoms trapped in a high-finesse optical cavity. Using laser and cavity fields to drive two separate Raman transitions between stable atomic ground states, a system is realized corresponding to a pair of two-state atoms coupled collectively to a squeezed reservoir. Phase-sensitive reservoir correlations lead to entanglement between the atoms, and, via local unitary transformations and adjustment of the degree and purity of squeezing, one can prepare entangled mixed states with any allowed combination of linear entropy and entanglement of formation.

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PubMed2003

Influence of energy and entropy on the melting of sodium clusters.

Energetic and entropic influences on the melting temperatures of size selected sodium clusters are experimentally separated. It is shown that the energetic difference between solid and liquid is the leading influence for the still puzzling features in the size dependence of sodium melting points. Additionally, this energy difference decreases towards smaller cluster sizes and causes steplike melting phase transitions to vanish. The entropy difference between solid and liquid has been found to be strongly correlated with the energy and causes a pronounced damping of the energetic influences.

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