We propose paramagnetic semiconductors as active media for refrigeration at cryogenic temperatures by adiabatic demagnetization. The paramagnetism of impurity dopants or structural defects can provide the entropy necessary for refrigeration at cryoge
nic temperatures. We present a simple model for the theoretical limitations to specific entropy and cooling power achievable by demagnetization of various semiconductor systems. Performance comparable to that of the hydrate (CMN) is predicted.
We report Coulomb drag measurements between vertically-integrated quantum wires separated by a barrier only 15 nm wide. The temperature dependence of the drag resistance is measured in the true one-dimensional (1D) regime where both wires have less t
han one 1D subband occupied. As a function of temperature, an upturn in the drag resistance is observed in three distinct devices at a temperature $T^* sim 1.6$ K. This crossover in Coulomb drag behaviour is consistent with Tomonaga-Luttinger liquid models for the 1D-1D drag between quantum wires.
We have performed quantum Monte Carlo simulations measuring the finite size and temperature superfluid response of helium-4 to the linear and rotational motion of the walls of a nanopore. Within the two-fluid model, the portion of the normal liquid d
ragged along with the boundaries is dependent on the type of motion and the resulting anisotropic superfluid density saturates far below unity at T=0.5 K. The origin of the saturation is uncovered by computing the spatial distribution of superfluidity, with only the core of the nanopore exhibiting any evidence of phase coherence. The superfluid core displays scaling behavior consistent with Luttinger liquid theory, thereby providing an experimental test for the emergence of a one dimensional quantum liquid.
Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or employ reduced-dimensional conductors like carbon nanotubes, nanowires and gated high mobility 2D electron systems. Thi
s is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomenon such as Coulomb drag: a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have observed electron drag in wires separated by a soft electrostatic barrier $gtrsim$ 80 nm. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by $sim$ 15 nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron subbands occupied in each wire, and interpret the results in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different subband occupancies, the positive drag effect can be as large as 25%.
We report on a systematic investigation of the dominant scattering mechanism in shallow two-dimensional electron gases (2DEGs) formed in modulation-doped GaAs/Al_{x}Ga_{1-x}As heterostructures. The power-law exponent of the electron mobility versus d
ensity, mu propto n^{alpha}, is extracted as a function of the 2DEGs depth. When shallower than 130 nm from the surface, the power-law exponent of the 2DEG, as well as the mobility, drops from alpha simeq 1.65 (130 nm deep) to alpha simeq 1.3 (60 nm deep). Our results for shallow 2DEGs are consistent with theoretical expectations for scattering by remote dopants, in contrast to the mobility-limiting background charged impurities of deeper heterostructures.
