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We present a theory whose goal is to account for the recent exciting hysteresis experiments on the spin cluster crystal Mn₁₂-acetate that indicate the occurrence of spin tunneling. We have found the tunneling probability P for the tunneling of spin from one spin state into another at a spin level crossing in response to a swept applied magnetic field H. It is shown that P is a function of only one parameter, the dimensionless sweep rate ρ given by ρ ≡ 2ℏ/Δ²₀, where 2 is the rate at which the two energy levels approach each other as the field is swept (being proportional to the sweep rate dH/dt). Δ₀ is the minimum energy gap at the anti-crossing, Δ₀/ℏ being the tunneling rate thereat. Numerical integration leads to P(ρ) = 1 − exp [ − [(π)/(2ρ)]]. We find that, contrary to widespread belief, the experiments cannot be understood in terms of tunneling of individual spin clusters in the presence of a static applied transverse field. Rather, the required transverse field must have dynamic nuclear spins and other dynamic spin clusters as its source.

We study the ground-state phase diagram and the critical properties of interacting bosons in one dimension by means of a quantum Monte Carlo technique. The direct experimental realization is a chain of Josephson junctions. For finite-range interactions we find a novel intermediate phase which shows neither solid order nor superfluidity. We determine the location of this phase and study the critical behaviour of the various transitions. For on-site interaction only, we map out the phase diagram as a function of the hopping strength and the chemical potential.

We propose a laser cooling scheme that allows to cool a single atom confined in a harmonic potential to the trap ground state |0⟩. The scheme assumes strong confinement, where the oscillation frequency in the trap is larger than the effective spontaneous decay width, but is not restricted to the Lamb-Dicke limit, i.e. the size of the trap ground state can be larger than the optical wavelength. This cooling scheme may be useful in the context of quantum computations with ions and Bose-Einstein condensation.

This letter describes the combined use of high-energy X-ray diffraction and neutron diffraction to investigate the problem of the structure of liquid FeCl₃. The results confirm the picture of liquid iron chloride consisting of closely packed Fe₂Cl₆ bitetrahedral molecules which have an atomic structure consistent with the electron diffraction results in the vapor. Comparison of the X-ray and neutron results at low Q suggests a paramagnetic moment about half that expected for a free Fe³⁺ ion, and confirms the picture suggested previously for topological and chemical short-range and intermediate-range order in the melt. Comparison of the X-ray diffraction data with molecular-dynamics results suggests a charge transfer intermediate between an atomic and fully ionic charge distribution, consistent with recent ab initio molecular orbital calculations.

Conformation space renormalisation group theory is applied to study the size distribution of living or self-assembling polymers in good solvents. In dilute solution the aggregate size distribution is found to be of the Schulz-Zimm type. It crosses over to an (almost) exponential distribution in the semi-dilute regime. The growth exponent, describing the power law dependence of the mean aggregate size on the concentration, changes from α = ½ − (1/16) in dilute to α = ½ + (1/16) in semi-dilute solution, to first order in = 4 − d with d ⩽ 4 the dimensionality of space.

We explore the rich scaling behavior of copolymer networks in solution. We establish a field-theoretic description in terms of composite operators. Our 3rd-order resummation of the spectrum of scaling dimensions brings about remarkable features: Convexity of the spectra allows for a multifractal interpretation. This has not been conceived for power of field operators of ϕ⁴ field theory before. The 2D limit of the mutually avoiding walk star apparently corresponds to results of a conformal Kac series. Such a classification seems not possible for the 2D limit of other copolymer stars. The 3rd-order calculation of a large collection of exponents furthermore allows for a consistency check of two complementary schemes: epsilon expansion and renormalization at fixed dimension.

A numerical vibrational-mode analysis has been performed for structural models of vitreous silica constructed using molecular dynamics. Dispersion curves, associated with remnants of phonon-like behaviour, exhibit pseudo-zone boundaries revealing evidence for two pseudo-periods in the structure, the dominant one being equal to the height of SiO₄ tetrahedra, and the other to the corresponding height of SiSi₄ tetrahedra. A new model for the boson peak is proposed: the excess modes are the lowest optic band states hybridized with acoustic states.

I analyze the topological structures generated by diffusion-limited aggregation (DLA), using the recently developed "branched growth model". The computed bifurcation number B for DLA in two dimensions is B ≈ 4.9, in good agreement with the numerically obtained result of B ≈ 5.2. In high dimensions, B → 3.12; the bifurcation ratio is thus a decreasing function of dimensionality. This analysis also determines the scaling properties of the ramification matrix, which describes the hierarchy of branches.

Strong dynamical scaling violations exist in quenched two-dimensional systems with vector O(3) order parameters. These systems support non-singular topologically stable configurations (skyrmions). By tuning the stability of isolated skyrmions to expand or shrink, we find dramatic differences in the dynamical multiscaling spectrum of decaying moments ⟨|ρ|ⁿ⟩ of the topological charge density distribution and in particular in the decay of the energy density ∼ ⟨|ρ|⟩. We present a simple two–length-scale model for the observed exponents in the case when isolated skyrmions expand. No such simple model is found when isolated skyrmions shrink.

The infinite-size version of the density-matrix renormalization-group approach in real space is applied to the Kondo lattice model in one dimension, providing a more accurate determination of the zero-temperature magnetic properties for electron densities n ⩽ 0.5. The paramagnetic state of the free model evolves into a state with ferromagnetic correlations, by increasing the strength of the Kondo coupling, in agreement with previous numerical results. The change in the magnetic properties is produced by the competition between the RKKY mechanism, which favours the paramagnetic state in the weak-coupling region, and the Kondo screening, which leads to the formation of tightly bound singlets in the strong-coupling region, enforcing ferromagnetic correlations in the system.

Applying k-resolved inverse photoemission (KRIPES) to the √3×√3 R30°-reconstructed 6H-SiC(0001) face, we have observed a sharp surface state U located at 1.10 ± 0.05 eV above the Fermi level at the centre of the surface Brillouin zone. Its bandwidth of 0.34 ± 0.05 eV is in good agreement with the 0.35 eV predicted by first-principle calculations based on a Si-adatom model. However, LDA calculations predict a half-filled Σ₁ state and a metallic character for this reconstruction. Together with recent ARUPS data, our results reveal that the one-electron band Σ₁ is split into two bands, giving a semiconducting surface with a reduced indirect bandgap around 2.0 eV at the ' point. Many-body correlation effects may give rise, in the limit of strong localization, to this bandgap opening.

We have observed anisotropic standing waves on the Be(100 surface using a low-temperature scanning tunnelling microscope (STM). Such charge density oscillations result from interference of electronic states near scattering centers. Since the charge density on Be(100 is mainly provided by a surface state located near the edge of the first Brillouin zone, the wave functions are significantly different from plane waves. It is shown how this difference is reflected in the STM images. By numerical post-processing we find that the observed wave patterns exhibit a multiperiodic structure with individual periods differing by inverse lattice vectors. This result constitutes a direct experimental illustration of the Bloch theorem for electrons in a periodic potential.

We report temporal conductance instabilities in the far-from-equilibrium transport through quantum point contacts. Random telegraph switching between conduction states is shown to have a maximum probability when the chemical potential of the injection electrode is close to a subband minimum. We show the RTS in these far-from-equilibrium experiments must have a different origin to that proposed for RTS in near-equilibrium measurements.

We study a flux lattice which is parallel to superconducting layers, allowing for dislocations and for disorder of both short wavelength and long wavelength. We find that the long-wavelength disorder of strength has a significant effect on the phase diagram—it produces a first-order transition within the Bragg glass phase and leads to melting at strong . This then allows a Friedel scenario of 2D superconductivity.

With the help of exact ground states obtained by a polynomial algorithm we compute the domain wall energy Δ at zero temperature for the bond-random and the site-random Ising spin glass model in two dimensions. We find that in both models the stability of the ferromagnetic and the spin glass order ceases to exist at a unique concentration p_c for the ferromagnetic bonds. In the vicinity of this critical point, the size and concentration dependence of the first and second moment of the domain wall energy are, for both models, described by a common finite-size scaling form. Moreover, below this concentration the stiffness exponent turns out to be slightly negative, θ_S = − 0.056(6), indicating the absence of any intermediate spin glass phase at non-zero temperature.

It is shown that the magnon spectrum of magnetically ordered crystals can be calculated using a frozen magnon scheme and spin density functional theory in the local approximation. Ab initio calculated magnon spectra for Fe and Ni are presented. Kohn anomalies are predicted for the magnon spectrum of Fe. Considering the magnons as true low-lying thermal excitations and using mean-field semi-classical statistics at elevated temperatures, the finite-temperature magnetization and Curie temperature T_C are calculated, which compare well with experiment for Fe, Co, and Ni.

Steps and terraces on GaAs (001) vicinal surfaces misoriented towards (111) or (11) are studied after MBE growth for a large range of misorientation angle (0.2° to 2°) using ex situ contact mode AFM. The terrace width distribution and correlation length are well explained by thermodynamical equilibrium of interacting steps. The step-step interaction is much larger than in elemental materials and is dominated by dipole rather than elastic interaction. Step bunching is never observed on clean surfaces for any misorientation. However, for Si-doped layers, the surface disorder increases and formation of macrosteps occurs at very high doping.

We report the correlation analysis of various redshift surveys which shows that the available data are consistent with each other and manifest fractal correlations (with dimension D ≃ 2) up to the present observational limits ( ≈ 150 h⁻¹Mpc) without any tendency towards homogenization. This result points to a new interpretation of the number counts that represents the main subject of this letter. We show that an analysis of the small-scale fluctuations allows us to reconcile the correlation analysis and the number counts in a new perspective which has a number of important implications.