Phase transitions in a frustrated XY model with zig-zag couplings

We study a new generalized version of the square-lattice frustrated XY model where unequal ferromagnetic and antiferromagnetic couplings are arranged in a zig-zag pattern. The ratio between the couplings $\rho$ can be used to tune the system, continuously, from the isotropic square-lattice to the triangular-lattice frustrated XY model. The model can be physically realized as a Josephson-junction array with two different couplings, in a magnetic field corresponding to half-flux quanta per plaquette. Mean-field approximation, Ginzburg-Landau expansion and finite-size scaling of Monte Carlo simulations are used to study the phase diagram and critical behavior. Depending on the value of $\rho$, two separate transitions or a transition line in the universality class of the XY-Ising model, with combined $Z_2$ and U(1) symmetries, takes place. In particular, the phase transitions of the standard square-lattice and triangular-lattice frustrated XY models correspond to two different cuts through the same transition line. Estimates of the chiral ($Z_2$) critical exponents on this transition line deviate significantly from the pure Ising values, consistent with that along the critical line of the XY-Ising model. This suggests that a frustrated XY model or Josephson-junction array with a zig-zag coupling modulation can provide a physical realization of the XY-Ising model critical line.Comment: 11 pages, 9 figures, RevTex, to appear in Phys. Rev.

Vascular adaptations to 8-week cycling training in older men.

Contains fulltext : 36674.pdf (publisher's version ) (Closed access)AIM: Because age-related changes in the large conduit arteries (increased wall thickness, and attenuated arterial compliance and endothelial function) are associated with cardiovascular pathology, prevention is of paramount importance. The effects of endurance training (i.e. walking or cycling) in older humans are assessed in cross-sectional studies, examining the brachial and carotid arteries (supplying non-trained areas). The purpose of this study was to assess the effects of 8-week endurance training in older men on conduit artery characteristics in the trained and non-trained vascular beds. METHODS: In eight healthy sedentary older men (70 +/- 3 years), characteristics of the large conduit arteries [common femoral (CFA), superficial femoral (SFA), carotid (CA), and brachial artery (BA)] were measured before and after 8-week cycling training. Functional [arterial compliance and flow-mediated dilation (FMD)] and structural (diameter and intima-media thickness) conduit artery properties were measured using echo-Doppler. Peak blood flow, representing structural peripheral adaptations, was measured using venous occlusion plethysmography. RESULTS: After training, peak leg blood flow was increased (P < 0.01) and baseline diameter and flow were increased in the CFA (P < 0.05). Cycling training enhanced arterial compliance of the SFA (P = 0.03), but did not affect the FMD (P = 0.32) or the intima-media thickness of the SFA. Exercise training did not alter characteristics of the BA or CA. CONCLUSION: Eight weeks of endurance training in older men altered functional and structural characteristics of the lower extremity vasculature, whereas no changes are reported for the conduit arteries in the non-trained areas (BA or in the CA)

Artifact formation due to ethyl thio-incorporation into silylated steroid structures as determined in doping analysis

Trimethylsilylation of target substances in a mixture of N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA), ammonium iodide and ethanethiol is frequently applied for the application of gas chromatography–mass spectrometry (GC–MS) in steroid analysis. However, artifacts were formed when using this mixture to silylate the steroids androsterone and etiocholanolone obtained from a urine matrix. The artifacts were identified as ethyl thio-containing products of the respective trimethylsilyl derivatives. The conversion of the studied products increased slowly as a function of time, was dependent on the presence of the urine matrix and was significantly accelerated by adding diethyl disulfide to the reagent before incubation. Also ethyl thio-incorporation into testosterone and epitestosterone was established. A mechanism for ethyl thio-incorporation is proposed. The conversion achieved after 120-h sample storage at room temperature was insufficient to significantly influence the analysis of androsterone and etiocholanolone under the studied conditions. However, the results provide fundamental insight into the mechanism of silylation and the occurring side-reactions. Moreover, when investigating the formation of new metabolites, the ethyl thio-incorporation can lead to misinterpretation