
Affolter, J., M. Tesei, H. Pastoriza, C. Leemann, and P. Martinoli. "Observation of Isinglike critical fluctuations in frustrated Josephson junction arrays with modulated coupling energies." Physica C 369 (2002): 313–316.
Abstract: We report the results of ac sheet conductance measurements performed on fully frustrated square arrays of Josephson junctions whose coupling energy is periodically modulated in one of the principal lattice directions. Such systems are predicted to exhibit two distinct transitions: a lowtemperature Isinglike transition triggered by the proliferation of domain walls and a hightemperature transition driven by the vortex unbinding mechanism of the BeresinskiiKosterlitzThouless (BKT) theory. Both the superfluid and dissipative components of the conductance are found to exhibit features which unambiguously demonstrate the existence of a double transition whose properties are consistent with the IsingBKT scenario.



Marconi, V. I., S. Candia, P. Balenzuela, H. Pastoriza, D. Domínguez, and P. Martinoli. "Orientational pinning and transverse voltage: Simulations and experiments in square Josephson junction arrays." Phys. Rev. B 62, no. 6 (2000): 4096–4104.
Abstract: We study the dependence of the transport properties of square Josephson Junctions arrays with the direction of the applied dc current, both experimentally and numerically. We present computational simulations of currentvoltage curves at finite temperatures for a single vortex in an array of $L\times L$ junctions ($Ha^2/?_0=f=1/L^2$), and experimental measurements in $100\times1000$ arrays under a low magnetic field corresponding to $f\approx0.02$. We find that the transverse voltage vanishes only in the directions of maximum symmetry of the square lattice: the [10] and [01] direction (parallel bias) and the [11] direction (diagonal bias). For orientations different than the symmetry directions, we find a finite transverse voltage which depends strongly on the angle φ of the current. We find that vortex motion is pinned in the [10] direction ($?=0$), meaning that the voltage response is insensitive to small changes in the orientation of the current near $?=0$. We call this phenomenon orientational pinning. This leads to a finite transverse critical current for a bias at $?=0$ and to a transverse voltage for a bias at $?\not=0$. On the other hand, for diagonal bias in the [11] direction the behavior is highly unstable against small variations of φ, leading to a rapid change from zero transverse voltage to a large transverse voltage within a few degrees. This last behavior is in good agreement with our measurements in arrays with a quasidiagonal current drive.



Pastoriza, H., and P. Martinoli. "Phase transitions in triangular arrays of XY spins with variable coupling." Czechoslovak Journal of Physics 46, no. S4 (1996): 1905.
Abstract: We present Monte Carlo simulations in a generalized uniformly frustrated triangularlattice model of XY spins. The frustration is varied by changing the relative strength, $\etha$, of one of the bonds of the unit well. We found that the ground state transforms from ferromagnetic to antiferromagnetic along the variable bond direction for a critical value of $\etha=1/\sqrt(3)$. Simultaneously, the phase transition towards the paramagnetic state at high temperatures changes from purely KosterlitzThouless like to one of mixed character

