Quantum fidelity in the thermodynamic limit

We study quantum fidelity, the overlap between two ground states of a many-body system, focusing on the thermodynamic regime. We show how drop of fidelity near a critical point encodes universal information about a quantum phase transition. Our general scaling results are illustrated in the quantum Ising chain for which a remarkably simple expression for fidelity is found.Comment: 4 pages, 4 figures, rearranged a bit to improve presentatio

When are Zariski chambers numerically determined?

The big cone of every smooth projective surface $X$ admits the natural decomposition into Zariski chambers. The purpose of this note is to give a simple criterion for the interiors of all Zariski chambers on $X$ to be numerically determined Weyl chambers. Such a criterion generalizes the results of Bauer-Funke on K3 surfaces to arbitrary smooth projective surfaces. In the last section, we study the relation between decompositions of the big cone and elliptic fibrations on Enriques surfaces.Comment: 7 page

Assisted finite-rate adiabatic passage across a quantum critical point: Exact solution for the quantum Ising model

The dynamics of a quantum phase transition is inextricably woven with the formation of excitations, as a result of the critical slowing down in the neighborhood of the critical point. We design a transitionless quantum driving through a quantum critical point that allows one to access the ground state of the broken-symmetry phase by a finite-rate quench of the control parameter. The method is illustrated in the one-dimensional quantum Ising model in a transverse field. Driving through the critical point is assisted by an auxiliary Hamiltonian, for which the interplay between the range of the interaction and the modes where excitations are suppressed is elucidated.Comment: 2 figures, 5 page

Excitations in Spin Chains and Specific-Heat Anomalies in Yb(4)As(3)

An explanation is given for the observed magnetic-field dependence of the low-temperature specific heat coefficient of Yb(4)As(3). It is based on a recently developed model for that material which can explain the observed heavy-fermion behaviour. According to it the Yb(3+)-ions are positioned in a net of parallel chains with an effective spin coupling of the order of J = 25 K. The magnetic-field dependence can be understood by including a weak magnetic coupling J' between adjacent chains. The data require a ratio J'/J of about 10^{-4}. In that case the experimental results can be reproduced very well by the theory.Comment: 5 pages, 5 PostScript-figures, needs LaTeX2e and the graphics-packag

Dynamics of an inhomogeneous quantum phase transition

We argue that in a second order quantum phase transition driven by an inhomogeneous quench density of quasiparticle excitations is suppressed when velocity at which a critical point propagates across a system falls below a threshold velocity equal to the Kibble-Zurek correlation length times the energy gap at freeze-out divided by $\hbar$. This general prediction is supported by an analytic solution in the quantum Ising chain. Our results suggest, in particular, that adiabatic quantum computers can be made more adiabatic when operated in an "inhomogeneous" way.Comment: 7 pages; version to appear in a special issue of New J. Phy

Vitamin- und Mineralstoffsupplementierung in Deutschland

Europaweit einheitlich geltende Vorschriften für Nahrungsergänzungsmittel sehen vor, dass der Ableitung von einzusetzenden Höchstmengen auch Informationen zum Ernährungsverhalten zugrunde gelegt werden. Daher wurden das Supplementierungsverhalten in Deutschland sowie die Aufnahme von Mikronährstoffen (sowohl aus Supplementen als auch aus traditionellen Lebensmitteln) insbesondere von Personen, die mehrere Supplemente verwenden, genauer untersucht. Im repräsentativen Bundes-Gesundheitssurvey 1998 wurden im Rahmen des Ernährungssurveys insgesamt 4.030 Personen im Alter zwischen 18 und 79 Jahren in einem computergestützten persönlichen Interview zu ihrem Ernährungsverhalten befragt, was auch die Anwendung von Vitamin- und/oder Mineralstoffsupplementen beinhaltete. Etwa 43% der Bevölkerung haben angegeben, im Befragungszeitraum von 12 Monaten mindestens einmal zu supplementieren, wobei Frauen generell häufiger ihre Ernährung ergänzen als Männer. Ein relativ geringer Personenanteil verwendet mehrere Supplemente. Dies betrifft am häufigsten Vitamin- plus Multivitaminsupplemente. Von den einzelnen Inhaltsstoffen wird Vitamin C am häufigsten aus mehreren Supplementquellen aufgenommen. Eine Überschreitung der tolerablen Obergrenzen (Tolerable Upper Intake Levels) der betrachteten Mikronährstoffe kam nur selten vor. Die Anwendung von Supplementen ist in Deutschland weit verbreitet. Jedoch wendet nur ein geringer Bevölkerungsanteil täglich mehrere Supplemente an, die den gleichen Mikronährstoff enthalten. Auch bei diesen Personen bleibt die große Mehrheit unterhalb der tolerablen Obergrenzen.Recent regulations on supplement use in Europe address the necessity to include information on dietary intake to derive maximum amounts for vitamins and minerals in food supplements. Therefore, information about supplement use in Germany, as well as data on micronutrient intake (from supplements and traditional foods), in particular of persons who use several supplements, is examined in detail. As part of the representative German National Health Interview and Examination Study 1998, in the Nutrition Survey 4030 persons, aged 18–79 years, were asked about their dietary habits, including vitamin and mineral supplement use, in a personal computer-aided interview. About 43% of the population reported using supplements at least once in the observation period of 12 months. Women use supplements more frequently than men. A relatively small group uses several supplements, most often vitamin plus multivitamin supplements. Considering single nutrients, vitamin C is most commonly consumed from more than one supplement. However, a micronutrient intake above the tolerable upper intake level is rare. Use of supplements is common in Germany. Nevertheless, only a small proportion of the population uses more than one supplement containing the same micronutrient daily. Even among these persons an intake above the tolerable upper intake level is seldom

Reentrant charge order transition in the extended Hubbard model

We study the extended Hubbard model with both on-site and nearest neighbor Coulomb repulsion ($U$ and $V$, respectively) in the Dynamical Mean Field theory. At quarter filling, the model shows a transition to a charge ordered phase with different sublattice occupancies n_A \nen_B. The effective mass increases drastically at the critical $V$ and a pseudo-gap opens in the single-particle spectral function for higher values of $V$. The $V_c(T)$-curve has a negative slope for small temperatures, i.e. the charge ordering transition can be driven by increasing the temperature. This is due to the higher spin-entropy of the charge ordered phase.Comment: 4 pages, 4 EPS figures included, REVTe

Characterization of defect structures in nanocrystalline materials by X-ray line profile analysis

X-ray line profile analysis is a powerful alternative tool for determining dislocation densities, dislocation type, crystallite and subgrain size and size-distributions, and planar defects, especially the frequency of twin boundaries and stacking faults. The method is especially useful in the case of submicron grain size or nanocrystalline materials, where X-ray line broadening is a well pronounced effect, and the observation of defects with very large density is often not easy by transmission electron microscopy. The fundamentals of X-ray line broadening are summarized in terms of the different qualitative breadth methods, and the more sophisticated and more quantitative whole pattern fitting procedures. The efficiency and practical use of X-ray line profile analysis is shown by discussing its applications to metallic, ceramic, diamond-like and polymer nanomaterials

Adiabatic dynamics of an inhomogeneous quantum phase transition: the case of z > 1 dynamical exponent

We consider an inhomogeneous quantum phase transition across a multicritical point of the XY quantum spin chain. This is an example of a Lifshitz transition with a dynamical exponent z = 2. Just like in the case z = 1 considered in New J. Phys. 12, 055007 (2010) when a critical front propagates much faster than the maximal group velocity of quasiparticles vq, then the transition is effectively homogeneous: density of excitations obeys a generalized Kibble-Zurek mechanism and scales with the sixth root of the transition rate. However, unlike for z = 1, the inhomogeneous transition becomes adiabatic not below vq but a lower threshold velocity v', proportional to inhomogeneity of the transition, where the excitations are suppressed exponentially. Interestingly, the adiabatic threshold v' is nonzero despite vanishing minimal group velocity of low energy quasiparticles. In the adiabatic regime below v' the inhomogeneous transition can be used for efficient adiabatic quantum state preparation in a quantum simulator: the time required for the critical front to sweep across a chain of N spins adiabatically is merely linear in N, while the corresponding time for a homogeneous transition across the multicritical point scales with the sixth power of N. What is more, excitations after the adiabatic inhomogeneous transition, if any, are brushed away by the critical front to the end of the spin chain.Comment: 10 pages, 6 figures, improved version accepted in NJ