Z_3 Quantum Criticality in a spin-1/2 chain model

The stability of the magnetization $m=1/3$ plateau phase of the XXZ spin-1/2 Heisenberg chain with competing interactions is investigated upon switching on a staggered transverse magnetic field. Within a bosonization approach, it is shown that the low-energy properties of the model are described by an effective two-dimensional XY model in a three-fold symmetry-breaking field. A phase transition in the three-state Potts universality class is expected separating the $m=1/3$ plateau phase to a phase where the spins are polarized along the staggered magnetic field. The Z$_3$ critical properties of the transition are determined within the bosonization approach.Comment: 5 pages, revised versio

QED3 theory of underdoped high temperature superconductors

Low-energy theory of d-wave quasiparticles coupled to fluctuating vortex loops that describes the loss of phase coherence in a two dimensional d-wave superconductor at T=0 is derived. The theory has the form of 2+1 dimensional quantum electrodynamics (QED3), and is proposed as an effective description of the T=0 superconductor-insulator transition in underdoped cuprates. The coupling constant ("charge") in this theory is proportional to the dual order parameter of the XY model, which is assumed to be describing the quantum fluctuations of the phase of the superconducting order parameter. The principal result is that the destruction of phase coherence in d-wave superconductors typically, and immediately, leads to antiferromagnetism. The transition can be understood in terms of the spontaneous breaking of an approximate "chiral" SU(2) symmetry, which may be discerned at low enough energies in the standard d-wave superconductor. The mechanism of the symmetry breaking is analogous to the dynamical mass generation in the QED3, with the "mass" here being proportional to staggered magnetization. Other insulating phases that break chiral symmetry include the translationally invariant "d+ip" and "d+is" insulators, and various one dimensional charge-density and spin-density waves. The theory offers an explanation for the rounded d-wave-like dispersion seen in ARPES experiments on Ca2CuO2Cl2 (F. Ronning et. al., Science 282, 2067 (1998)).Comment: Revtex, 20 pages, 5 figures; this is a much extended follow-up to the Phys. Rev. Lett. vol.88, 047006 (2002) (cond-mat/0110188); improved presentation, many additional explanations, comments, and references added, sec. IV rewritten. Final version, to appear in Phys. Rev.

Intelligent LED Light

Cílem bakalářské práce je navrhnout a realizovat prototyp inteligentního LED osvětlení s možností míchání barev studené bílé, teplé bílé, červené, zelené a modré barvy. Vytvořit komunikaci, ovládácí jednotky včetně W(LAN) jednotky, která bude umožňovat komunikaci mimo tuto uzavřenou sběrnici. V práci je prvně popsáno světlo a jeho historie. Návrh samotného zařízení se skládá z 3D modelu prototypu LED osvětlení zabudovaného ve stropě a následného výběru konkrétních použitých obvodů. Při návrhu bylo dbáno na nedostatky dnešních chytrých elektroinstalací. V závěru práce je popsána problematika ruční osazení desek plošných spojů a výsledky měření navržených LED budičů.The aim of the bachelor thesis is to design and implement a prototype of intelligent LED lighting with the possibility of mixing cold white, warm white, red, green and blue colours. To create communication, control units including a W(LAN) unit that will allow communication outside this closed bus. This thesis is the first to describe light and its history. The design of the device itself consists of a 3D model of a prototype LED light embedded in the ceiling, followed by the selection of the specific circuits used. In the design, the shortcomings of today’s smart wiring systems were taken into account. The thesis concludes with a description of the hand-fitting of the circuit boards and the measurement results of the designed LED exciters.