We study the interaction-enhanced spin gaps in the two-dimensional electron gas confined in GaAs/AlGaAs single heterojunctions subjected to weak magnetic fields. The values are obtained from the chemical potential jumps measured by magnetocapacitance. The gap increase with parallel magnetic field indicates that the lowest-lying charged excitations are accompanied with a single spin flip at the odd-integer filling factor ν = 1 and ν = 3, in disagreement with the concept of skyrmions.

In the framework of the density functional theory of freezing proposed in our previous works, we calculate the phase diagram of two-dimensional system of particles interacting through the repulsive shoulder potential. This potential consists of the hard core and repulsive shoulder of the larger radius. It is shown that at low densities the system melts through the continuous transition in accordance with the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario, while at high densities the conventional first order transition takes place.A large number of papers studying the melting transition in two dimensions have been published during last decades. They include results of real experiments, computer simulations and various theoretical approaches. This is dictated by the growing interest to the behavior of the nanoconfined systems. Confining drastically changes the spatial distribution and the ways of dynamic rearrangement of the molecules in the system. The confined fluids microscopically relax and flow with characteristic times that differ from the bulk fluids. These effects play important role in the thermodynamic behavior of the confined systems and can considerably change the topology of the phase diagram. In general, the motivation for the study of the confined systems follows from the fact that there are a lot of real physical, chemical and biological processes which drastically depend on the properties of such systems [1][2][3][4][5][6].It is not surprising that the spatial ordering of molecules depends on the dimensionality of the space to which it is confined. Mermin [7] has shown that in in two dimensions (2D) the long-range crystalline order can not exist because of the thermal fluctuations and transforms to the quasi-long-range order. On the other hand, the real long range bond orientational order does exist in this case. At high temperatures one can find the conventional isotropic fluid.The melting scenario in two dimensions is a subject of long lasting controversy. Now it is widely believed that the Kosterlitz, Thouless, Halperin, Nelson, and Young theory (KTHNY theory) [8][9][10][11] correctly describes the melting transition in 2D. In the framework of the KTHNY theory the two-dimensional melting occurs in the way which is fundamentally different from the melting transition of three-dimensional systems. In 2D, the bound dislocation pairs dissociate at some temperature T m transforming the quasi-long range translational order in the short-range, and long-range orientational order into the quasi-long range order. The new phase with the quasi-long range orientational order is called the hexatic phase. After consequent dissociation of the disclination pairs at some temperature T i the system transforms into the isotropic liquid. Both transitions are continuous, in contrast with the conventional first order three dimensional melting.The unambiguous confirmations of the KTHNY theory have been obtained, for example, from the recent experiments on the colloidal model system with repulsive magnetic di...

Experimental data on quantum phase transitions in two-dimensional systems (superconductorinsulator, metal-insulator, and transitions under conditions of integer quantum Hall effect) are critically analyzed.

We present measurements of the energy relaxation time, τ ε , of electrons in a single AlGaAs/GaAs heterojunction in a quasi-equilibrium state using microwave time-resolved spectroscopy at 4.2 K. We find the relaxation time has a power-law dependence on the carrier density of the two-dimensional electron gas, τ ε ∝ n γ s with γ = 0.40 ± 0.02 for values of the carrier density, n s , from 1.6 × 10 11 to 6.6 × 10 11 cm −2 . The results are in good agreement with predictions taking into account the scattering of the carriers by both piezoelectric and deformation potential acoustic phonons. We compare these results with indirect measurements of the energy relaxation time from energy loss measurements involving Joule heating of the electron gas.

Quantum corrections to conductivity and quantum Hall effect in GaAs-GaAlAs multiple quantum well structures Kulbachinskii, V.A.; de Visser, A.; Kadushkin, V.I.; Kytin, V.G.; Shangina, E.L.