Zemma, E., and J. Luzuriaga. "Turbulent Flow Around an Oscillating Body in Superfluid Helium: Dissipation Characteristics of the Nonlinear Regime." Journal of Low Temperature Physics 172, no. 34 (2013): 256–265.
Abstract: By examining the resonance curves of an oscillator submerged in superfluid liquid helium, it is found that their shape is affected by two distinct dissipation regimes when the amplitude is large enough to generate turbulence in the liquid. In a resonance curve, the central part close to resonance, may be in a turbulent regime, but the response is of much lower amplitude away from the resonance frequency, so that the oscillation can still be in the linear regime for frequencies not exactly at resonance. This introduces an ambiguity in estimating the inverse quality factor Q âˆ’1 of the oscillator. By analyzing experimental data we consider a way of matching the two ways of estimating Q âˆ’1 and use the information to evaluate the frictional force as a function of velocity in a silicon paddle oscillator generating turbulence in the superfluid.

D'Anna, G., M.  O. Andre, W. Benoit, E. Rodriguez, D. S. Rodriguez, J. Luzuriaga, and J. V. Wasczak. "Fluxline response in 2HNbSe2 investigated by means of the vibrating superconductor method." Physica C 218, no. 12 (1993): 238–244.
Abstract: We measure transverse AC losses in the low and highamplitude regime of 2HNbSe2 single crystals using vibrating superconductor methods. The measurements are sensitive to small deviations of the critical state. The data constitute evidence for a peak effect of the critical current as a function of the temperature in this compound. We construct in the HT phase diagram the ''peakeffect'' line which is supposed to mark an abrupt crossover in the vortexpinning regime.

Zemma, E., and J. Luzuriaga. "Measurements of Turbulence Onset and Dissipation in Superfluid Helium with a Silicon Double Paddle Oscillator." Journal of Low Temperature Physics 166, no. 34 (2012): 171–181.
Abstract: We have studied experimentally the response of a silicon single crystal double paddle oscillator submerged in superfluid helium from the lambda point to 1.55 K. Measuring the resonance frequency and dissipation on three modes of this high Q system allows us to study the dissipation at the onset of turbulence in the flow around the paddle. The critical velocity V c for turbulence onset decreases with temperature. If we use the density of the normal component of the superfluid to obtain a Reynolds number Re associated with V c we find a value which is largely temperature independent. This result is different from the behavior previously found by other authors below 1 K, where the quantized vorticity (extrinsic nucleation) is observed at velocities more than an order of magnitude greater. In our temperature range, we conclude that the transition is governed by the normal fraction acting as a classical fluid. The laminar regime shows a dissipation that is proportional to the viscous drag calculated by well known formulas for an object oscillating in a liquid. We also find a decrease in resonance frequency in the turbulent regime which is clearly observed but hard to reproduce from run to run.

Herbsommer, J. A., J. Luzuriaga, and S.  W. Cheong. "Vortex glass melting in single crystal La1.825 Sr0.075CuO4." Physica C 258, no. 12 (1996): 169–174.
Abstract: The vortex phase diagram in single crystalline La1.85Sr0.075CuO4 has been studied using an ACsusceptibility technique. A peak in the outofphase (x?) component of the susceptibility indicates a transition from a pinned flux lattice (FLL) to an unpinned one. This peak is frequency dependent for all the values of the magnetic field measured (0.01 to 4 T), and this, as well as the general behavior found in the cuprates, has prompted us to interpret our data as evidence for a vortexglass to liquid transition in the FLL. The activation energies obtained can be fitted to a theory developed by Vinokur et al. Measurements with the magnetic field at an angle with the CuO planes may also be understood qualitatively within this framework.

Herbsommer, J. A., J. Luzuriaga, L. Civale, G. Nieva, G. Pasquini, H. Lanza, and P. Levy. "Angular variation of pinning near the irreversibility temperature in single crystal Y Ba2Cu3O7? with splayed columnar defects." Physica C 304, no. 12 (1998): 112–118.
Abstract: We have studied vortex pinning through ac susceptibility measurements in single crystals of YBCO with columnar defects (CDs). The CDs have 0, 10 and 20 splay angle and average direction 108 off the caxis. By studying the angular variation we can compensate for the anisotropy and effects of twins, etc. Using a simple expression we can obtain the angular spread when the field direction is outside the splay angle. An increase of pinning when the field direction is inside
the angle defined by the CDs can be attributed to vortex entanglement due to the splay and the suppression of the sliding of double kinks.

Schmoranzer, D., M. J. Jackson, and J. Luzuriaga. "On the Nonlinear Damping of Mechanical Oscillators in Flows of 4He." Journal of Low Temperature Physics 175, no. 1 (2014): 97–103.
Abstract: In the studies of both classical and quantum turbulence, significant attention is devoted to the investigation of the behavior of various submerged resonators. Upon entering the turbulent regime, the oscillators start to experience a significant drag force, which varies nonlinearly with velocity. We present an empirical way of modeling such systems, and calculate the expected resonant response of such oscillators near the fundamental frequency as a function of the applied driving force. We apply the model to the crossover from linear to nonlinear drag forces and compare with previous models as well as selected experimental data on the transition to turbulence in oscillatory flow of 4He.

Zemma, E., and J. Luzuriaga. "Anomalous Trajectories of H2 Solid Particles Observed Near a Sphere Oscillating in Superfluid Turbulent 4He." Journal of Low Temperature Physics 173, no. 12 (2013): 71–79.
Abstract: Using a relatively low cost apparatus, consisting of a glass dewar and a digital camera capable of taking images at 240 frames per second we have observed trajectories of frozen H2 particles which follow the flow of liquid helium below 2 K, around a sphere oscillating at 38 Hz. In some of the images the motion is compatible with laminar flow, while at high amplitudes, where we can reach Reynolds numbers of a few thousand in the normal component, the flow is clearly turbulent. In some of the videos taken we find particles being suddenly accelerated to several times the velocity of the oscillating sphere.

Herbsommer, J. A., V. F. Correa, G. Nieva, H. Pastoriza, and J. Luzuriaga. "Vortex dynamics in Bi2Sr2CaCu2O8+δ single crystals with planar defects." Solid State Communications 120, no. 23 (2001): 59–63.
Abstract: We have studied the effects of caxis correlated planar defects on the vortex lattice of Bi$2$Sr$2$CaCu$2Ο8$ single crystals using Bitter decoration, transverse ac permeability and Xray measurements. Our results show that the low field low temperature phase coherent vortex lattice exhibits anisotropic pinning in the presence of these planar defects. Frequency dependent ac permeability measurements show that the ordered solidliquid transition is first order as in samples free of correlated defects. However, the melting point is detected only when the excitation field is along the defects.

Rodriguez, D. S., E. Rodriguez, J. Luzuriaga, J. V. Waszczak, and S. W. Cheong. "Role of anisotropy in the vortex lattice of superconductors highQ oscillator measurements in 2HNbSe2 and La1.825Sr.075CuO4." Physica C 229, no. 12 (1994): 59–64.
Abstract: Measurements of the response of the fluxline lattice in NbSe2 and La1.825Sr0.075CuO4 show important differences between these two materials. In particular, we have studied the magneticfield and angular dependence of the response of a highQ mechanical oscillator in fields of up to 1 T. The features seen in NbSe2 seem to be well explained in terms of a change in the pinning regime, usually termed the ''peak effect'' in the critical current, using the collectivepinning model of Larkin and Ovchivnikov within GinzburgLandau anisotropic theory. On the other hand the behavior found in the highT(c) material LSCO seems to fall naturally into a description which takes into account the possibility of phase transitions in the vortex lattice and the quasitwodimensional character of the superconductivity.

Luzuriaga, J. "Even in translation, Richter's 'science' unimpressive." Physics Today 58, no. 1 (2005): 14–15.
