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S Wessel

Publications and source records attributed to S Wessel.

10 recordsLinked to original sources
PubMed2006

Hard-core bosons on the kagome lattice: valence-bond solids and their quantum melting.

Using large scale quantum Monte Carlo simulations and dual vortex theory, we analyze the ground state phase diagram of hard-core bosons on the kagome lattice with nearest-neighbor repulsion. In contrast with the case of a triangular lattice, no supersolid emerges for strong interactions. While a uniform superfluid prevails at half filling, two novel solid phases emerge at densities rho=1/3 and rho=2/3. These solids exhibit an only partial ordering of the bosonic density, allowing for local resonances on a subset of hexagons of the kagome lattice. We provide evidence for a weakly first-order phase transition at the quantum melting point between these solid phases and the superfluid.

Journal Article↗
PubMed2005

Ultracold atoms in optical lattices with random on-site interactions.

We consider the physics of lattice bosons affected by disordered on-site interparticle interactions. Characteristic qualitative changes in the zero-temperature phase diagram are observed when compared to the case of randomness in the chemical potential. The Mott-insulating regions shrink and eventually vanish for any finite disorder strength beyond a sufficiently large filling factor. Furthermore, at low values of the chemical potential both the superfluid and Mott insulator are stable towards formation of a Bose glass leading to a possibly nontrivial tricritical point. We discuss feasible experimental realizations of our scenario in the context of ultracold atoms on optical lattices.

Journal Article↗
PubMed2004

Performance limitations of flat-histogram methods.

We determine the optimal scaling of local-update flat-histogram methods with system size by using a perfect flat-histogram scheme based upon the exact density of states of 2D Ising models. The typical tunneling time needed to sample the entire bandwidth does not scale with the number of spins N as the minimal N2 of an unbiased random walk in energy space. While the scaling is power law for the ferromagnetic and fully frustrated Ising model, for the +/-J nearest-neighbor spin glass the distribution of tunneling times is governed by a fat-tailed Fréchet extremal value distribution that obeys exponential scaling. Furthermore, the shape parameters of these distributions indicate that statistical sample means become ill defined already for moderate system sizes within these complex energy landscapes.

Journal Article↗
PubMed2002

Hydration dynamics of human fingernails: an ellipsometric study.

We use spectroscopic ellipsometry to obtain the complex refractive index, ñ=n+ik, of human fingernails. By studying the change of ñ upon hydration and dehydration, we reveal three different time domains with typical time constants of 4, 150, and 3200 min. A simple model that takes into account the presence of one fast and one slow process is fully consistent with the observed hydration and dehydration dynamics. We attribute these processes to "free" water incorporated between the keratin filaments and water more tightly "bound" in keratin complexes, respectively. From our model we determine the hydration profiles of "free" and "bound" water during, both, hydration and dehydration.

Chemical Phenomena↗
PubMed2001

Field-induced magnetic order in quantum spin liquids.

We study magnetic-field-induced three-dimensional ordering transitions in low-dimensional quantum spin liquids, such as weakly coupled, antiferromagnetic spin- 1/2 Heisenberg dimers and ladders. Using stochastic series expansion quantum Monte Carlo simulations, we obtain the critical scaling exponents which dictate the power-law dependence of the transition temperature on the magnetic field. These are compared with recent experiments on candidate materials and with predictions for the Bose-Einstein condensation of magnons. The critical exponents deviate from isotropic mean-field theory and exhibit different scaling behavior at the lower and upper critical magnetic fields.

Journal Article↗
PubMed2001

Order by disorder from nonmagnetic impurities in a two-dimensional quantum spin liquid.

We consider doping of nonmagnetic impurities in the spin-1/2, 1/5-depleted square lattice. This structure, whose undoped phase diagram offers both magnetically ordered and spin-liquid ground states, is realized physically in CaV4O9. Doping into the ordered phase results in a progressive loss of order, which becomes complete at the percolation threshold. By contrast, doping into the spin liquids creates a phase of weak but long-ranged antiferromagnetic order, a true order-by-disorder phenomenon. We study the phase diagram of the doped system by computing the static susceptibility and staggered magnetization using a stochastic series-expansion quantum Monte Carlo technique.

Journal Article↗
PubMed2000

Penetration pathways of fluorescent dyes in human hair fibres investigated by scanning near-field optical microscopy.

Thin cross-sections of human hairs were investigated by scanning near-field optical microscopy (SNOM) and confocal laser scanning microscopy (CLSM) after penetration of a fluorescent dye. The same samples were measured with both techniques to compare the observed structures. The images obtained from the two methods show nearly identical structures representing pathways of the dye molecules in hairs. The SNOM images provide a higher resolution than the CLSM images. Therefore, SNOM is believed to be a suitable method for investigations at a resolution of 100 nm on penetration pathways of fluorescent dyes such as the cell membrane complex pathway in cross-sections of hairs.

Fluorescent Dyes↗
PubMed1999

Hydration of human nails investigated by NIR-FT-Raman spectroscopy.

The human nail, although it is usually stable against outer influences, becomes soft and flexible after soaking in water. Frequent washing increases brittleness of nails. Hydration of nails is thought to be the most important factor influencing the physical properties of nails and possibly acts through changes in keratin structure. Here NIR-FT-Raman has been used to examine molecular structural changes of intact moisten nails. Raman spectra were obtained both in vitro from nail samples and in vivo before and after soaking in water. The water uptake of normal nail samples during the first 15 min was reflected in the increasing intensity ratio of the nu(OH)/nu(CH(2)) bands. A saturating effect appeared soon after 10 min which is explained by a defined water holding capacity. R(nu) representation of the low frequency range of the Raman spectra showed that mainly bound water is found both in dry and in wet nails. This implies water-protein interaction. Protein backbone vibration at 932 cm(-1) indicating alpha-helical proteins increased in intensity in the wet nails. The nu(S-S) which is sensitive to changes in conformation of proteins showed a 4% higher intensity. Additional protein-water interactions could lead to a slight change of the dihedral angle of the C-S-S-C bonds and to geometric changes in coiling behavior of the alpha-helical protein. Suggesting a separation between matrix proteins and fiber proteins giving them a greater freedom of flexibility. The in vivo spectra detected from the distal part of the nail resembled spectra in vitro. Raman spectra of the proximal part of the nail showed that it was fully saturated with water. The proximal part of the nail did not show changes in water content and protein structure during nail moisturizing in the Raman spectra. Our results suggest that the softening of the nail following hydration may be due to changed matrix protein molecular structure induced by water.

Humans↗
PubMed1998

Water and protein structure in photoaged and chronically aged skin.

Changes in the structural proteins and hydration during aging is responsible for altered skin morphologic and mechanical properties manifested as wrinkling, sagging, loss of elasticity, or apparent dryness. To gain insight into the age-related alterations in protein conformation and water structure, we obtained Raman spectra from the sun-protected buttock skin representing chronologic aging and the sun-exposed forearm skin representing combined effects of photoaging and chronologic aging. Ten aged individuals (five men, five women; age range 74-87) and 10 control young individuals (five men, five women; age range 22-29) entered the study. In the photoaged forearm skin the positions of protein-specific amide I, amide III, and CH stretching bands were shifted, suggesting increased protein folding. In contrast, major changes were seen only in the amide I peak in chronologically aged skin. The intensity of the 3250 cm(-1) OH stretching band was increased in photoaged skin (but not in chronologically aged skin) indicating an increased water content. R(v) representation of the low-frequency region of Raman spectra was applied to determine water structure. In the young skin and chronologically aged skin water was mostly present in the bound form. In the photoaged skin, however, an increase in intensity at 180 cm(-1) was noted, which reflects an increase in the not-protein bound water (tetrahedron water clusters). In conclusion, it seems that proteins in the photoaged skin are more compact and interact with water to limited degree. Impairment in protein hydration may add to the understanding of ultrastructural, mechanical, and biochemical changes in structural proteins in the aged skin.

Adult↗