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A few years ago Avetissian {\it et al.} \cite{Avetissian2014,Avetissian2015} discovered that the exponential growth rate of the stimulated annihilation photons from a singlet positronium Bose-Einstein condensate should be proportional to the square root of the positronium number density, not to the number density itself. In order to elucidate this surprising result obtained via a field-theoretical analysis, we point out that the basic physics involved is the same as that of resonant subharmonic transitions between two quantum oscillators. Using this model, we show that nonlinearities of the type discovered by Avetissian {\it et al.} are not unique to positronium and in fact will be encountered in a wide range of systems that can be modeled as nonlinearly coupled quantum oscillators., Comment: 5 pages and 1 figure

Hidden matter that interacts only gravitationally would oscillate at characteristic frequencies when trapped inside of Earth. For small oscillations near the center of the Earth, these frequencies are around 300 $\mu$Hz. Additionally, signatures at higher harmonics would appear because of the non-uniformity of Earth's density. In this work, we use data from a global network of gravimeters of the International Geodynamics and Earth Tide Service (IGETS) to look for these hypothetical trapped objects. We find no evidence for such objects with masses on the order of 10$^{14}$ kg or greater with an oscillation amplitude of 0.1 $r_e$. It may be possible to improve the sensitivity of the search by several orders of magnitude via better understanding of the terrestrial noise sources and more advanced data analysis.

We present measurements of the Joule heating of a 2D hole gas (2DHG) formed in a 30nm GaAs quantum well. The hole density is in the range (4.6-18.9)*10^9cm^-2 and exhibits an apparent metal-to-insulator transition (MIT) with a critical density 6*10^9 cm^-2. In the limit of zero heating power density P, the GaAs lattice is within 2 mK of the 6 mK base temperature of our dilution refrigerator determined by He-3 melting curve thermometry. Throughout the range of heating power densities used (1 to 10^6 fW/cm^2), the temperature rise of the lattice is estimated to be negligible compared to the temperature rise of the hole gas. We argue that the hole scattering rate is only a function of the hole temperature, with little dependence on the lattice or impurity temperatures in the relevant temperature range below 150 mK. We have therefore made measurements of the hole resistivity at negligible heating power density (P<5fW/cm^2) as a function of measured lattice temperature in the range 6 to 150 mK. We then use the hole resistivity measured in a cold lattice to estimate the temperature of the 2D hole gas as a function of P. In the low hole density insulating phase, the heating power density P(T) that heats the hole gas to a temperature T exhibits a dependence P ~ T^2 for T<30mK, gradually changing to P ~ T^4 for higher temperatures. On the metallic side of the MIT, P is proportional to T^5 or T^6 with a magnitude roughly 4 times less than the power density required to heat the insulating phase to the same temperature. Our measurements are within a factor of two of the available quantitative theoretical predictions for the hole energy loss rate as a function of temperature., Comment: 10pages, 4figures

We have studied the magnetotransport properties of a high mobility two-dimensional hole gas (2DHG) system in a 10nm GaAs quantum well (QW) with densities in range of 0.7-1.6*10^10 cm^-2 on the metallic side of the zero-field 'metal-insulator transition' (MIT). In a parallel field well above B_c that suppresses the metallic conductivity, the 2DHG exhibits a conductivity g(T)~0.3(e^2/h)lnT reminiscent of weak localization. The experiments are consistent with the coexistence of two phases in our system: a metallic phase and a weakly insulating Fermi liquid phase having a percolation threshold close to B_c.

We have investigated the effect of an in plane parallel magnetic field (B_||) on two high mobility metallic-like dilute two-dimensional hole gas (2DHG) systems in GaAs quantum wells. The experiments reveal that, while suppressing the magnitude of the low temperature resistance drop, B_|| does not affect E_a, the characteristic energy scale of the metallic resistance drop. The field B_c at which the metallic-like resistance drop vanishes is dependent on both the width of quantum well and the orientation of B_||. It is unexpected that E_a is unaffected by B_|| up to Bc depite that the Zeeman energy at B_c is roughly equal to E_a.