Magneto-intersubband resistance oscillations (MISO) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields tilted from the normal to the 2D electron layer at different temperatures $T$
. Decrease of MISO amplitude with temperature increase is observed. At moderate tilts the temperature decrease of MISO amplitude is consistent with decrease of Dingle factor due to reduction of quantum electron lifetime at high temperatures. At large tilts new regime of strong MISO suppression with the temperature is observed. Proposed model relates this suppression to magnetic entanglement between subbands, leading to beating in oscillating density of states. The model yields corresponding temperature damping factor: $A_{MISO}(T)=X/sinh(X)$, where $X=2pi^2kTdelta f$ and $delta f$ is difference frequency of oscillations of density of states in two subbands. This factor is in agreement with experiment. Fermi liquid enhancement of MISO amplitude is observed.
Shubnikov de Haas resistance oscillations of highly mobile two dimensional helical electrons propagating on a conducting surface of strained HgTe 3D topological insulator are studied in magnetic fields B tilted by angle $theta$ from the normal to the
conducting layer. Strong decrease of oscillation amplitude A is observed with the tilt: $A sim exp(-xi/cos(theta))$, where $xi$ is a constant. Evolution of the oscillations with temperature T shows that the parameter $xi$ contains two terms: $xi=xi_1+xi_2 T$. The temperature independent term, $xi_1$, describes reduction of electron mean free path in magnetic field B pointing toward suppression of the topological protection of the electron states against impurity scattering. The temperature dependent term, $xi_2 T$, indicates increase of the reciprocal velocity of 2D helical electrons suggesting modification of the electron spectrum in magnetic fields.
Transport properties of highly mobile 2D electrons are studied in symmetric GaAs quantum wells placed in titled magnetic fields. Quantum positive magnetoresistance (QPMR) is observed in magnetic fields perpendicular to the 2D layer. Application of in
-plane magnetic field produces a dramatic decrease of the QPMR. This decrease correlates strongly with the reduction of the amplitude of Shubnikov de Haas resistance oscillations due to modification of the electron spectrum via enhanced Zeeman splitting. Surprisingly no quantization of the spectrum is detected when the Zeeman energy exceeds the half of the cyclotron energy suggesting an abrupt transformation of the electron dynamics. Observed angular evolution of QPMR implies strong mixing between spin subbands. Theoretical estimations indicate that in the presence of spin-orbital interaction the elastic impurity scattering provides significant contribution to the spin mixing in GaAs quantum wells at high filling factors.
Effect of dc electric field on transport of highly mobile 2D electrons is studied in wide GaAs single quantum wells placed in titled magnetic fields. The study shows that in perpendicular magnetic field resistance oscillates due to electric field ind
uced Landau-Zener transitions between quantum levels that corresponds to geometric resonances between cyclotron orbits and periodic modulation of electron density of states. Magnetic field tilt inverts these oscillations. Surprisingly the strongest inverted oscillations are observed at a tilt corresponding to nearly absent modulation of the electron density of states in regime of magnetic breakdown of semiclassical electron orbits. This phenomenon establishes an example of quantum resistance oscillations due to Landau quantization, which occur in electron systems with a constant density of states.
The magnetotransport of highly mobile 2D electrons in wide GaAs single quantum wells with three populated subbands placed in titled magnetic fields is studied. The bottoms of the lower two subbands have nearly the same energy while the bottom of the
third subband has a much higher energy ($E_1approx E_2<<E_3$). At zero in-plane magnetic fields magneto-intersubband oscillations (MISO) between the $i^{th}$ and $j^{th}$ subbands are observed and obey the relation $Delta_{ij}=E_j-E_i=kcdothbaromega_c$, where $omega_c$ is the cyclotron frequency and $k$ is an integer. An application of in-plane magnetic field produces dramatic changes in MISO and the corresponding electron spectrum. Three regimes are identified. At $hbaromega_c ll Delta_{12}$ the in-plane magnetic field increases considerably the gap $Delta_{12}$, which is consistent with the semi-classical regime of electron propagation. In contrast at strong magnetic fields $hbaromega_c gg Delta_{12}$ relatively weak oscillating variations of the electron spectrum with the in-plane magnetic field are observed. At $hbaromega_c approx Delta_{12}$ the electron spectrum undergoes a transition between these two regimes through magnetic breakdown. In this transition regime MISO with odd quantum number $k$ terminate, while MISO corresponding to even $k$ evolve $continuously$ into the high field regime corresponding to $hbaromega_c gg Delta_{12}$
The effects of microwave radiation on the transport properties of atomically thin $La_{2-x}Sr_xCuO_4$ films were studied in the 0.1-13 GHz frequency range. Resistance changes induced by microwaves were investigated at different temperatures near the
superconducting transition. The nonlinear response decreases by several orders of magnitude within a few GHz of a cutoff frequency $ u_{cut} approx$ 2 GHz. Numerical simulations that assume an ac response to follow the dc V-I characteristics of the films reproduce well the low frequency behavior, but fail above $ u_{cut}$. The results indicate that two-dimensional superconductivity is resilient against high-frequency microwave radiation, because vortex-antivortex dissociation is dramatically suppressed in two-dimensional superconducting condensates oscillating at high frequencies.
The lifetime of two dimensional electrons in GaAs quantum wells, placed in weak quantizing magnetic fields, is measured using a simple transport method in broad range of temperatures from 0.3 K to 20 K. The temperature variations of the electron life
time are found to be in good agreement with conventional theory of electron-electron scattering in 2D systems.
The longitudinal resistivity of two dimensional (2D) electrons placed in strong magnetic field is significantly reduced by applied electric field, an effect which is studied in a broad range of magnetic fields and temperatures in GaAs quantum wells w
ith high electron density. The data are found to be in good agreement with theory, considering the strong nonlinearity of the resistivity as result of non-uniform spectral diffusion of the 2D electrons. Inelastic processes limit the diffusion. Comparison with the theory yields the inelastic scattering time of the two dimensional electrons. In the temperature range T=2-10(K) for overlapping Landau levels, the inelastic scattering rate is found to be proportional to T^2, indicating a dominant contribution of the electron-electron scattering to the inelastic relaxation. In a strong magnetic field, the nonlinear resistivity demonstrates scaling behavior, indicating a specific regime of electron heating of well-separated Landau levels. In this regime the inelastic scattering rate is found to be proportional to T^3, suggesting the electron-phonon scattering as the dominant mechanism of the inelastic relaxation.
Warming in complex physical systems, in particular global warming, attracts significant contemporary interest. It is essential, therefore, to understand basic physical mechanisms leading to overheating. It is well known that application of an electri
c field to conductors heats electric charge carriers. Often an elevated electron temperature describes the result of the heating. This paper demonstrates that an electric field applied to a conductor with discrete electron spectrum produces a non-equilibrium electron distribution, which cannot be described by temperature. Such electron distribution changes dramatically the conductivity of highly mobile two dimensional electrons in a magnetic field, forcing them into a state with a zero differential resistance. Most importantly the results demonstrate that, in general, the effective overheating in the systems with discrete spectrum is significantly stronger than the one in systems with continuous and homogeneous distribution of the energy levels at the same input power.
Zero differential resistance state is found in response to direct current applied to 2D electron systems at strong magnetic field and low temperatures. Transition to the state is accompanied by sharp dip of negative differential resistance, which occ
urs above threshold value $I_{th}$ of the direct current. The state depends significantly on the temperature and is not observable above several Kelvins. Additional analysis shows lack of the linear stability of the 2D electron systems at $I>I_{th}$ and inhomogeneous, non-stationary pattern of the electric current in the zero differential resistance state. We suggest that the dc bias induced redistribution of the 2D electrons in energy space is the dominant mechanism leading to the new electron state.
