Experimental tests for macroscopic phase coherence in magnetic-quasiparticle condensates of insulating spin systems

For most kinds of already known Bose-Einstein condensates experimental evidence for the existence of a macroscopic coherent quantum state has been provided, e.g., from the observation of interference phenomena, the formation vortices, the detection of Josephson effects, or even by the manifestation of superfluid behaviour. However, none of these hallmarks for a true, macroscopic phase-coherent state has ever been reported for any insulating spin system in a solid in which magnetic bosonic quasiparticles are thought to condense close to a quantum critical point. We describe an attempt to detect superfluid behaviour in TlCuCl3, and discuss in more detail a proposal for an experiment in which the a.c. Josephson effect can be probed by using a device composed of two weakly coupled magnetic insulators with different critical fields. We conclude that the detection of the a.c. Josephson effect is feasible for a proper choice of compounds with realistic material parameters.Comment: 6 pages, 2 figure

Structure and magnetic interactions in the solid solution Ba3-xSrxCr2O8

Solid solutions of the magnetic insulators Ba3Cr2O8 and Sr3Cr2O8 (Ba3-xSrxCr2O8) have been prepared in polycrystalline form for the first time. Single crys- talline material was obtained using a mirror image floating zone technique. X-ray diffraction data taken at room temperature indicate that the space group of Ba3-xSrxCr2O8 remains unchanged for all values of x, while the cell parameters depend on the chemical composition, as expected. Magnetization data, measured from 300 K down to 2 K, suggests that the interaction constant Jd within the Cr5+ dimers varies in a peculiar way as a function of x, starting at Jd = 25K for x = 0, then first slightly dropping to Jd = 18K for x = 0.75, before reaching Jd = 62K for x = 3

Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2

We report on the magnetic properties of CsCo$_2$Se$_2$ with ThCr$_2$Si$_2$ structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that CsCo$_2$Se2$_2$ undergoes a phase transition to an antiferromagnetically ordered state with a N\'eel temperature of $T_{\rm N} \approx$ 66 K. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie-Weiss temperature of $\Theta \approx$ 51.0 K. We find that the magnetic structure of CsCo$_2$Se$_2$ consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal \textit{c}-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at $T$ = 1.5 K is found to be only 0.20(1)$\mu_{\rm Bohr}$/Co. Already in comparably small magnetic fields of $\mu_0 H_{MM}$(5K) $\approx$ 0.3 T, we observe a metamagnetic transition that can be attributed to spin-rearrangements of CsCo$_2$Se$_2$, with the moments fully ferromagnetically saturated in a magnetic field of $\mu_0 H_{\rm FM}$(5K) $\approx$ 6.4 T. We discuss the entire experimentally deduced magnetic phase diagram for CsCo$_2$Se$_2$ with respect to its unconventionally weak magnetic coupling. Our study characterizes CsCo$_2$Se$_2$, which is chemically and electronically posed closely to the $A_xFe_{2-y}Se_2$ superconductors, as a host of versatile magnetic interactions

A preliminary list of the ants of Utah

journal articleThe following list of the ants of Utah was prepared from specimens and data now in the Biology Department of the University of Utah. This list is by no means complete, yet it is representative of practically all parts of the state. A few specimens in the collection date back to 1902, and all of these specimens and numerous other specimens in the collection were identified by Professor W. M. Wheeler. Since 1937 the authors have made collections in all parts of the state in an effort to obtain material for this paper. In addition, Mr. Grundmann has made intensive collections and a study of the ants of Salt Lake County as thesis material for a Master's Degree

Effect of deconfinement on resonant transport in quantum wires

The effect of deconfinement due to finite band offsets on transport through quantum wires with two constrictions is investigated. It is shown that the increase in resonance linewidth becomes increasingly important as the size is reduced and ultimately places an upper limit on the energy (temperature) scale for which resonances may be observed.Comment: 6 pages, 6 postscript files with figures; uses REVTe

Optical properties of arrays of quantum dots with internal disorder

Optical properties of large arrays of isolated quantum dots are discussed in order to interpret the existent photoluminescence data. The presented theory explains the large observed shift between the lowest emission and absorption energies as the average distance between the ground and first excited states of the dots. The lineshape of the spectra is calculated for the case when the fluctuations of the energy levels in quantum dots are due to the alloy composition fluctuations. The calculated lineshape is in good agreement with the experimental data. The influence of fluctuations of the shape of quantum dots on the photoluminescence spectra is also discussed.Comment: 7 pages (twocolumn) LATEX, 6 Postscript figure

Self-consistent Coulomb effects and charge distribution of quantum dot arrays

This paper considers the self-consistent Coulomb interaction within arrays of self-assembled InAs quantum dots (QDs) which are embedded in a pn structure. Strong emphasis is being put on the statistical occupation of the electronic QD states which has to be solved self-consistently with the actual three-dimensional potential distribution. A model which is based on a Green's function formalism including screening effects is used to calculate the interaction of QD carriers within an array of QDs, where screening due to the inhomogeneous bulk charge distribution is taken into acount. We apply our model to simulate capacitance-voltage (CV) characteristics of a pn structure with embedded QDs. Different size distributions of QDs and ensembles of spatially perodic and randomly distributed arrays of QDs are investigated.Comment: submitted to pr

High-field magnetoexcitons in unstrained GaAs/AlxGa1-xAs quantum dots

The magnetic field dependence of the excitonic states in unstrained GaAs/AlxGa1-xAs quantum dots is investigated theoretically and experimentally. The diamagnetic shift for the ground and the excited states are studied in magnetic fields of varying orientation. In the theoretical study, calculations are performed within the single band effective mass approximation, including band nonparabolicity, the full experimental three-dimensional dot shape and the electron-hole Coulomb interaction. These calculations are compared with the experimental results for both the ground and the excited states in fields up to 50 Tesla. Good agreement is found between theory and experiment