Full counting statistics of the interference contrast from independent Bose-Einstein condensates

We show that the visibility in interference experiments with Bose-Einstein condensates is directly related to the condensate fraction. The probability distribution of the contrast over many runs of an interference experiment thus gives the full counting statistics of the condensed atom number. For two-dimensional Bose gases, we discuss the universal behavior of the probability distribution in the superfluid regime and provide analytical expressions for the distributions for both homogeneous and harmonically trapped samples. They are non-Gaussian and unimodal with a variance that is directly related to the superfluid density. In general, the visibility is a self-averaging observable only in the presence of long range phase coherence. Close to the transition temperature, the visibility distribution reflects the universal order parameter distribution in the vicinity of the critical point

Critical Fields of mesoscopic superconductors

Recent measurements have shown oscillations in the upper critical field of simply connected mesoscopic superconductors. A quantitative theory of these effects is given here on the basis of a Ginzburg-Landau description. For small fields, the $H-T$ phase boundary exhibits a cusp where the screening currents change sign for the first time thus defining a lower critical field $H_{c1}$. In the limit where many flux quanta are threading the sample, nucleation occurs at the boundary and the upper critical field becomes identical with the surface critical field $H_{c3}$.Comment: 5 pages (Revtex and 2 PostScript figures), to apppear in Z. Phys.

FFLO strange metal and quantum criticality in two dimensions: theory and application to organic superconductors

Increasing the spin imbalance in superconductors can spatially modulate the gap by forming Cooper pairs with finite momentum. For large imbalances compared to the Fermi energy, the inhomogeneous FFLO superconductor ultimately becomes a normal metal. There is mounting experimental evidence for this scenario in 2D organic superconductors in large in-plane magnetic fields; this is complemented by ongoing efforts to realize this scenario in coupled tubes of atomic Fermi gases with spin imbalance. Yet, a theory for the phase transition from a metal to an FFLO superconductor has not been developed so far and the universality class has remained unknown. Here we propose and analyze a spin imbalance driven quantum critical point between a 2D metal and an FFLO phase in anisotropic electron systems. We derive the effective action for electrons and bosonic FFLO pairs at this quantum phase transition. Using this action, we predict non-Fermi liquid behavior and the absence of quasi-particles at a discrete set of hot spots on the Fermi surfaces. This results in strange power-laws in thermodynamics and response functions, which are testable with existing experimental set-ups on 2D organic superconductors and may also serve as signatures of the elusive FFLO phase itself. The proposed universality class is distinct from previously known quantum critical metals and, because its critical fluctuations appear already in the pairing channel, a promising candidate for naked metallic quantum criticality over extended temperature ranges.Comment: 3+1 figure

Efimov states near a Feshbach resonance and the limits of van der Waals universality at finite background scattering length

We calculate the spectrum of three-body Efimov bound states near a Feshbach resonance within a model which accounts both for the finite range of interactions and the presence of background scattering. The latter may be due to direct interactions in an open channel or a second overlapping Feshbach resonance. It is found that background scattering gives rise to substantial changes in the trimer spectrum as a function of the detuning away from a Feshbach resonance, in particular in the regime where the background channel supports Efimov states on its own. Compared to the situation with negligible background scattering, the regime where van der Waals universality applies is shifted to larger values of the resonance strength if the background scattering length is positive. For negative background scattering lengths, in turn, van der Waals universality extends to even small values of the resonance strength parameter, consistent with experimental results on Efimov states in $^{39}$K. Within a simple model, we show that short-range three-body forces do not affect van der Waals universality significantly. Repulsive three-body forces may, however, explain the observed variation between around $-8$ and $-10$ of the ratio between the scattering length where the first Efimov trimer appears and the van der Waals length.Comment: 17 pages, 13 figures; final version as publishe

Bose-Einstein Condensation versus Dicke-Hepp-Lieb Transition in an Optical Cavity

We provide an exact solution for the interplay between Bose-Einstein condensation and the Dicke-Hepp-Lieb self-organization transition of an ideal Bose gas trapped inside a single-mode optical cavity and subject to a transverse laser drive. Based on an effective action approach, we determine the full phase diagram at arbitrary temperature, which features a bi-critical point where the transitions cross. We calculate the dynamically generated band structure of the atoms and the associated supression of the critical temperature for Bose-Einstein condensation in the phase with a spontaneous periodic density modulation. Moreover, we determine the evolution of the polariton spectrum due to the coupling of the cavity photons and the atomic field near the self-organization transition, which is quite different above or below the Bose-Einstein condensation temperature. At low temperatures, the critical value of the Dicke-Hepp-Lieb transition decreases with temperature and thus thermal fluctuations can enhance the tendency to a periodic arrangement of the atoms.Comment: 18 pages, 12 figure

Non-local order in Mott insulators, Duality and Wilson Loops

It is shown that the Mott insulating and superfluid phases of bosons in an optical lattice may be distinguished by a non-local 'parity order parameter' which is directly accessible via single site resolution imaging. In one dimension, the lattice Bose model is dual to a classical interface roughening problem. We use known exact results from the latter to prove that the parity order parameter exhibits long range order in the Mott insulating phase, consistent with recent experiments by Endres et al. [Science 334, 200 (2011)]. In two spatial dimensions, the parity order parameter can be expressed in terms of an equal time Wilson loop of a non-trivial U(1) gauge theory in 2+1 dimensions which exhibits a transition between a Coulomb and a confining phase. The negative logarithm of the parity order parameter obeys a perimeter law in the Mott insulator and is enhanced by a logarithmic factor in the superfluid.Comment: published versio

Entwicklung, Überprüfung und Praxiseinführung des Prognosemodells ÖKO-SIMPHYT zur gezielten Bekämpfung der Kraut- und Knollenfäule (P. infestans) im ökologischen Kartoffelanbau mit dem Ziel, den Einsatz kupferhaltige Fungizide auf ein Minimum zu reduzieren

Die anhand des entwickelten Prognosemodells ÖKOSIMPHYT angepassten Spritzintervalle und Kupferaufwandmengen ermöglichten in Jahren mit niedrigem Infektionsdruck im Vergleich mit routinemäßigen Applikationen gleichwertige Erträge bei reduzierten Kupfermengen. Vereinzelt waren so Einsparungen von bis zu 1000g Kupfer möglich. Durchschnittlich wurden 0,6 Behandlungen bzw. 535g Kupfer pro Hektar im Vergleich zur wöchentlichen Behandlung eingespart. Auch der Einsatz von neuen, kupferreduzierten Fungiziden nach den Modellempfehlungen erwies sich als erfolgreich, so dass hier ein weiteres Einsparpotential zur Verfügung steht. Zugang zum Modell ÖKOSIMPHYT ist für Landwirte und Berater unter www.isip.de im Internet möglich. Die Prognose hat sich als zuverlässiges Hilfsmittel bei der Krautfäulebekämpfung erwiesen. Einzig der bislang nicht mit ausreichender Sicherheit prognostizierbare primäre Stängelbefall kann in Einzelfällen zu einer verspäteten Spritzstart-Empfehlung führen. Hier herrscht weiterhin Forschungsbedarf, um die Prognose des Primärbefalls zu optimieren. Um diese Lücke zu schließen und den primären Stängelbefall, welcher bislang durch Pflanzenschutzmaßnahmen nicht verhindert werden kann, zu regulieren, wurde die Kupferbeizung des Saatgutes eingesetzt. Die hierzu in den Versuchen verwendete ULV-Methode ermöglicht eine Beizung der Pflanzknollen mit geringen Kupfermengen (120g/ha). Durch dieses Verfahren konnte der Stängelbefall vielfach signifikant reduziert werden, was sich in Mehrerträgen von bis zu 55% wiederspiegelte. Anhand molekularbiologischer Methoden konnte gezeigt werden, dass die latente, also nicht sichtbare Verseuchung des Pflanzgutes mit dem Krautfäuleerreger ein ernst zu nehmendes und bislang unterschätztes Problem darstellt. So waren durchschnittlich 11% der untersuchten Saatkartoffeln latent befallen und nur ein geringer Anteil der Pflanzgutpartien erwies sich als befallsfrei