Finite VEVs from a Large Distance Vacuum Wave Functional

We show how to compute vacuum expectation values from derivative expansions of the vacuum wave functional. Such expansions appear to be valid only for slowly varying fields, but by exploiting analyticity in a complex scale parameter we can reconstruct the contribution from rapidly varying fields.Comment: 39 pages, 16 figures, LaTeX2e using package graphic

Charge-density wave formation in Sr_{14}Cu_{24}O_{41}

The electrodynamic response of the spin-ladder compound Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$ ($x=0, 3, 9$) has been studied from radiofrequencies up to the infrared. At temperatures below 250 K a pronounced absorption peak appears around 12 cm$^{-1}$ in Sr$_{14}$Cu$_{24}$O$_{41}$ for the radiation polarized along the chains/ladders (${\bf E}\parallel {\bf c}$). In addition a strongly temperature dependent dielectric relaxation is observed in the kHz - MHz range. We explain this behavior by a charge density wave which develops in the ladders sub-system and produces a mode pinned at 12 cm$^{-1}$. With increasing Ca doping the mode shifts up in frequency and eventually disappears for $x=9$ because the dimensionality of the system crosses over from one to two dimensions, giving way to the superconducting ground state under pressure.Comment: One name added to author list 4 pages, 2 figures, email: [email protected]

Gauge Invariant Higgs mass bounds from the Physical Effective Potential

We study a simplified version of the Standard Electroweak Model and introduce the concept of the physical gauge invariant effective potential in terms of matrix elements of the Hamiltonian in physical states. This procedure allows an unambiguous identification of the symmetry breaking order parameter and the resulting effective potential as the energy in a constrained state. We explicitly compute the physical effective potential at one loop order and improve it using the RG. This construction allows us to extract a reliable, gauge invariant bound on the Higgs mass by unambiguously obtaining the scale at which new physics should emerge to preclude vacuum instability. Comparison is made with popular gauge fixing procedures and an ``error'' estimate is provided between the Landau gauge fixed and the gauge invariant results.Comment: 23 pages, 2 figures, REVTE

The gauge invariant effective potential: equilibrium and non-equilibrium aspects

We propose a gauge invariant formulation of the effective potential in terms of a gauge invariant order parameter, for the Abelian Higgs model. The one-loop contribution at zero and finite temperature is computed explicitly, and the leading terms in the high temperature expansion are obtained. The result is contrasted to the effective potential obtained in several covariant gauge-fixing schemes, and the gauge invariant quantities that can be reliably extracted from these are identified. It is pointed out that the gauge invariant effective potential in the one-loop approximation is complex for {\em all values} of the order parameter between the maximum and the minimum of the tree level potential, both at zero and non-zero temperature. The imaginary part is related to long-wavelength instabilities towards phase separation. We study the real-time dynamics of initial states in the spinodal region, and relate the imaginary part of the effective potential to the growth rate of equal-time gauge invariant correlation functions in these states. We conjecture that the spinodal instabilities may play a role in non-equilibrium processes {\em inside} the nucleating bubbles if the transition is first order.Comment: 27 pages revtex 3.0, no figures; one reference adde

Ordering phenomena in quasi one-dimensional organic conductors

Low-dimensional organic conductors could establish themselves as model systems for the investigation of the physics in reduced dimensions. In the metallic state of a one-dimensional solid, Fermi-liquid theory breaks down and spin and charge degrees of freedom become separated. But the metallic phase is not stable in one dimension: as the temperature is reduced, the electronic charge and spin tend to arrange themselves in an ordered fashion due to strong correlations. The competition of the different interactions is responsible for which broken-symmetry ground state is eventually realized in a specific compound and which drives the system towards an insulating state. Here we review the various ordering phenomena and how they can be identified by optic and magnetic measurements. While the final results might look very similar in the case of a charge density wave and a charge-ordered metal, for instance, the physical cause is completely different. When density waves form, a gap opens in the density of states at the Fermi energy due to nesting of the one-dimension Fermi surface sheets. When a one-dimensional metal becomes a charge-ordered Mott insulator, on the other hand, the short-range Coulomb repulsion localizes the charge on the lattice sites and even causes certain charge patterns. We try to point out the similarities and conceptional differences of these phenomena and give an example for each of them. Particular emphasis will be put on collective phenomena which are inherently present as soon as ordering breaks the symmetry of the system.Comment: Review article Naturwissenschaften 200

Experimentelle Ermittlung des Elastizitätsmoduls von Cr(Re)/Al2O3-Verbundwerkstoffen

Im Maschinenbau und der Werkstoffwissenschaft kommt dem Elastizitätsmodul eine grundlegende Bedeutung zu. Die Bestimmung des E-Moduls kann mittels Zugversuch, Drei-Punkt-Biegung und Ultraschall-Impuls-Echo-Verfahren erfolgen. Entwickelt wurden weitere experimentelle und numerische Verfahren, die alle Vor- und Nachteile haben und teilweise unterschiedliche Werte für den E-Modul eines Werkstoffs liefern. Es werden die Ergebnisse experimenteller Untersuchungen sowie numerischer Modellierungen am Verbundwerkstoff Cr(Re)/Al2O3 (MMC) vorgestellt und verglichen. Die erhaltenen Ergebnisse belegen die Schwierigkeit einer genauen Ermittlung des Elastizitätsmoduls, insbesondere von Verbundwerkstoffen. Mögliche Gefügeinhomogenitäten dieser Werkstoffe wirken sich offensichtlich verschieden intensiv auf die angewandten unterschiedlichen Messprinzipien des E-Modules aus. Alle vier Verfahren nutzen verschiedene physikalische Vorgänge, um Rückschlüsse auf die elastischen Konstanten des Werkstoffs zu ziehen. Dabei spielen nicht nur lokale Phänomene wie Spannungsverteilung, Verfomungsamplitude und -geschwindigkeit eine entscheidende Rolle, auch eine akkurate Probenvorbereitung ist für die Minimierung des Ausmaßes an Fehlmessungen sehr bedeutend. Da die Drei-Punkt-Biegung sehr empfindlich auf Gefüge- und Oberflächenfehler reagiert, sollte sie nur dann eingesetzt werden, wenn keines der zerstörungsfreien Prüfverfahren zur Verfügung steht oder ein Vergleich mit schon vorhandenen Messungen dieses Verfahren erfordert. Die beiden vorgestellten FEM-Modelle können hilfreiche Hinweise bei der Werkstoffentwicklung liefern. Sie erlauben eine gute Vorhersage der elastischen Konstanten eines noch nicht entwickelten Verbundwerkstoffs