Charge ordering of magnetic monopoles in triangular spin ice patterns

Artificial spin ice offers the possibility to investigate a variety of dipolar orderings, spin frustrations and ground states. However, the most fascinating aspect is the realization that magnetic charge order can be established without spin order. We have investigated magnetic dipoles arranged on a honeycomb lattice as a function of applied field, using magnetic force microscopy. For the easy direction with the field parallel to one of the three dipole sublattices we observe at coercivity a maximum of spin frustration and simultaneously a maximum of charge order of magnetic monopoles with alternating charges $\pm$ 3.Comment: 7 pages, 4 figure

Conditions for free magnetic monopoles in nanoscale square arrays of dipolar spin ice

We study a modified frustrated dipolar array recently proposed by M\"{o}ller and Moessner [Phys. Rev. Lett. \textbf{96}, 237202 (2006)], which is based on an array manufactured lithographically by Wang \emph{et al.} [Nature (London) \textbf{439}, 303 (2006)] and consists of introducing a height offset $h$ between islands (dipoles) pointing along the two different lattice directions. The ground-states and excitations are studied as a function of $h$. We have found, in qualitative agreement with the results of M\"{o}ller and Moessner, that the ground-state changes for $h>h_{1}$, where $h_{1}= 0.444a$ ($a$ is the lattice parameter or distance between islands). In addition, the excitations above the ground-state behave like magnetic poles but confined by a string, whose tension decreases as $h$ increases, in such a way that for $h\approx h_1$ its value is around 20 times smaller than that for $h=0$. The system exhibits an anisotropy in the sense that the string tension and magnetic charge depends significantly on the directions in which the monopoles are separated. In turn, the intensity of the magnetic charge abruptly changes when the monopoles are separated along the direction of the longest axis of the islands. Such a gap is attributed to the transition from the anti to the ferromagnetic ground-state when $h=h_1$.Comment: 6 pages, 7 figures. Published versio

The Effectiveness of a Computer-Assisted, Cognitive-Behavior Program for Treating Anxiety Symptoms in Children with Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a complex and multifaceted, neurodevelopmental disorder that severely impacts children and families across a variety of settings. Prevalence rates of ASD are continuing to increase rapidly, with sizeable intervention and treatment costs placed on families and society. Further complicating the matter, many individuals with ASD also evidence co-occurring anxiety symptoms or disorders and tend to be at a higher risk for developing such problems when compared to other clinical populations or typically developing children. This study investigated the effectiveness of a computer-assisted CBT program, Camp Cope-A-Lot (CCAL), in regards to reducing anxiety symptoms in four participants diagnosed with ASD. Data from quantitative measures revealed inconsistent results, yet informal, qualitative feedback from parents as well as information from the researcher’s observations and progress notes appeared more promising for use of the CCAL intervention for this population of children

Determination of Terrestrial Heat Flow in Southeastern North Dakota

Terrestrial heat flow measurements have been carried out at three sites in Southeastern North Dakota. The heat flow values were calculated from temperature gradients measured in three wells and thermal conductivities measured in the lab using samples from these wells. At two of the sites values were obtained in Precambrian layers. Near Lidgerwood, North Dakota measurements in a layer of weathered Precambrian yielded a value of 1.21 HFU. At a site near Blanchard, North Dakota measurements in a Precambrian greenstone yielded a value of 0.76 HFU. At a third site near Wheatland, North Dakota, no Precambrian layer was accessible for temperature gradient measurement. Temperature gradients (42.10 and 31.56°C/km) measured in two Cretaceous sedimentary layers at this site were found to be in the same range as the gradients (45.06, 49.97 and 23.51°C/km) measured in three corresponding Cretaceous sedimentary layers at the Lidgerwood site. These contrast with the gradients (12.58, 14.22 and 13.93°C/km) measured in Cretaceous and Ordovician sedimentary layers at the Blanchard site. This contrast in these temperature gradients is reflected in the calculated heat flows. Differences in the radiogenic heat productions of the underlying Precambrian rocks is a likely explanation for the difference between the two heat flow values in the Precambrian materials at the Blanchard and Lidgerwood sites. The observed variations in the temperature gradients, and hence in the heat flows, in the sedimentary layers are probably a result of ground water movement in the different aquifers present at the three sites

Radionuclide measurements by accelerator mass spectrometry at Arizona

Over the past years, Tandem Accelerator Mass Spectrometry (TAMS) has become established as an important method for radionuclide analysis. In the Arizona system the accelerator is operated at a thermal voltage of 1.8MV for C-14 analysis, and 1.6 to 2MV for Be-10. Samples are inserted into a cesium sputter ion source in solid form. Negative ions sputtered from the target are accelerated to about 25kV, and the injection magnet selects ions of a particular mass. Ions of the 3+ charge state, having an energy of about 9MeV are selected by an electrostatic deflector, surviving ions pass through two magnets, where only ions of the desired mass-energy product are selected. The final detector is a combination ionization chamber to measure energy loss (and hence, Z), and a silicon surface-barrier detector which measures residual energy. After counting the trace iosotope for a fixed time, the injected ions are switched to the major isotope used for normalization. These ions are deflected into a Faraday cup after the first high-energy magnet. Repeated measurements of the isotope ratio of both sample and standards results in a measurement of the concentration of the radionuclide. Recent improvements in sample preparation for C-14 make preparation of high-beam current graphite targets directly from CO2 feasible. Except for some measurements of standards and backgrounds for Be-10 measurements to date have been on C-14. Although most results have been in archaeology and quaternary geology, studies have been expanded to include cosmogenic C-14 in meteorites. The data obtained so far tend to confirm the antiquity of Antarctic meteorites from the Allan Hills site. Data on three samples of Yamato meteorites gave terrestrial ages of between about 3 and 22 thousand years

Proximity effect of vanadium on spin-density-wave magnetism in Cr films

The spin-density wave (SDW) state in thin chromium films is well known to be strongly affected by proximity effects from neighboring layers. To date the main attention has been given to effects arising from exchange interactions at interfaces. In the present work we report on combined neutron and synchrotron scattering studies of proximity effects in Cr/V films where the boundary condition is due to the hybridization of Cr with paramagnetic V at the interface. We find that the V/Cr interface has a strong and long-range effect on the polarization, period, and the N\'{e}el temperature of the SDW in rather thick Cr films. This unusually strong effect is unexpected and not predicted by theory.Comment: 7 figure