Low Electron Density Regime Research Articles

Quantized topological Hall effect in skyrmion crystal

We theoretically study the quantized anomalous Hall effect (QAHE) in skyrmion crystal (SkX) without external magnetic field. The emergent magnetic field in SkX could be gigantic as much as $\sim4000$T when its lattice constant is $\sim1$nm. The band structure is not flat but has a finite gap in the low electron-density regime. We also study the conditions to realize the QAHE for the skyrmion size, carrier density, disorder strength and temperature. Comparing the SkX and the system under the corresponding uniform magnetic field, the former is more fragile against the temperature compared with the latter since the gap is reduced by a factor of $\sim$ 1/5, while they are almost equally robust against the disorder. Therefore, it is expected that the QAHE of SkX system is realized even with strong disorder at room temperature when the electron density of the order of one per a skyrmion.

Exchange energy enhancedg-factors obtained from Landau fan diagrams at low magnetic fields

We report on measurement of the electron-hole effective $g$-factors (${g}_{\mathrm{eff}}^{*}$) depending on electron filling factors $\ensuremath{\nu}$ from magnetophotoluminescence spectroscopy at low magnetic fields $B<1$ T in low electron density regime in a GaAs/Al${}_{0.33}$Ga${}_{0.67}$As-gated quantum well. Enhancement of ${g}_{\mathrm{eff}}^{*}$ at odd $\ensuremath{\nu}$ is observed. The oscillatory behavior of ${g}_{\mathrm{eff}}^{*}$ is compared with results of a theory that takes into account the lowest-order exchange interaction of the screened Coulomb interaction. Good agreement of the observed ${g}_{\mathrm{eff}}^{*}$ with theoretical results is obtained except $\ensuremath{\nu}$ at around 3. The enhancement of ${g}_{\mathrm{eff}}^{*}$ at even $\ensuremath{\nu}$ with decrease in electron density has not been observed.

Anomalous evolution of Ar metastable density with electron density in high density Ar discharge

Recently, an anomalous evolution of argon metastable density with plasma discharge power (electron density) was reported [A. M. Daltrini, S. A. Moshkalev, T. J. Morgan, R. B. Piejak, and W. G. Graham, Appl. Phys. Lett. 92, 061504 (2008)]. Although the importance of the metastable atom and its density has been reported in a lot of literature, however, a basic physics behind the anomalous evolution of metastable density has not been clearly understood yet. In this study, we investigated a simple global model to elucidate the underlying physics of the anomalous evolution of argon metastable density with the electron density. On the basis of the proposed simple model, we reproduced the anomalous evolution of the metastable density and disclosed the detailed physics for the anomalous result. Drastic changes of dominant mechanisms for the population and depopulation processes of Ar metastable atoms with electron density, which take place even in relatively low electron density regime, is the clue to understand the result.

Overlapping-Gate Architecture for Silicon Hall Bar MOSFET Devices in the Low Electron Density and High Magnetic Field Regime

A common issue in low temperature measurements of enhancement-mode metal-oxide-semiconductor (MOS) field-effect transistors (FETs) in the low electron density regime is the high contact resistance dominating the device impedance. In that case a voltage bias applied across the source and drain contact of a Hall bar MOSFET will mostly fall across the contacts (and not across the channel) and therefore magneto-transport measurements become challenging. However, from a physical point of view, the study of MOSFET nanostructures in the low electron density regime is very interesting (impurity limited mobility [1], carrier interactions [2,3] and spin-dependent transport [4]) and it is therefore important to come up with solutions [5,6] that work around the problem of a high contact resistance in such devices (c.f. Fig. 1 (a)).

Overlapping-gate architecture for silicon Hall bar field-effect transistors in the low electron density regime

We report the fabrication and study of Hall bar field-effect transistors in which an overlapping-gate architecture allows four-terminal measurements of low-density two-dimensional electron systems while maintaining a high density at the Ohmic contacts. Comparison with devices made using a standard single gate show that measurements can be performed at much lower densities and higher channel resistances, despite a reduced peak mobility. We also observe a voltage threshold shift which we attribute to negative oxide charge, injected during electron-beam lithography processing.

Observation of two-dimensional electron gas in a Si quantum well with mobility of 1.6×106 cm2/Vs

We report the observation of a two-dimensional electron gas (2DEG) in a Si quantum well with mobility 1.6×106 cm2/Vs at carrier densities n≥1.5×1011/cm2. The 2DEG, which resides in an undoped Si/SiGe heterostructure, is capacitively induced using an insulated-gate field-effect transistor (IGFET) device structure; its mobility is determined from transport and quantum Hall effect measurements at 0.3 K. Our IGFET device makes it now possible to access by transport experiments the low electron density regime down to n∼1×1010/cm2.

Superconductivity in a model of two Hubbard chains coupled with ferromagnetic exchange interaction

We study the ground-state properties of the double-chain Hubbard model coupled with ferromagnetic exchange interaction by using the weak-coupling theory, density-matrix renormalization-group technique, and Lanczos exact-diagonalization method. We determine the ground-state phase diagram in the parameter space of the ferromagnetic exchange interaction and band filling. We find that, in high electron-density regime, the spin gap opens and the spin-singlet ${d}_{xy}$-wave-like pairing correlation is most dominant, whereas in low electron-density regime, the fully-polarized ferromagnetic state is stabilized where the spin-triplet ${p}_{y}$-wave-like pairing correlation is most dominant.

Zero-Bias Anomaly and Kondo-Assisted Quasiballistic 2D Transport

Nonequilibrium transport measurements in mesoscopic quasiballistic 2D electron systems show an enhancement in the differential conductance around the Fermi energy. At very low temperatures, such a zero-bias anomaly splits, leading to a suppression of linear transport at low energies. We also observed a scaling of the nonequilibrium characteristics at low energies which resembles electron scattering by two-state systems, addressed in the framework of two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an intrinsic phenomenon in unconfined 2D systems in the low electron-density regime.

Spin-Related Electronic Structure of Quantum Wires with an In-plane Magnetic Field

Self-consistent calculation of the electronic structure of quantum wires are implemented with an in-plane magnetic field parallel to the wire and spin-density-functional theory of Kohn and Sham is applied [Phys. Rev. A 140 (1965) 1133]. The self-consistent results show that full spontaneous spin polarization takes place in low electron density regime, even at an arbitrary small magnetic field, which demonstrates that the polarization is caused by exchange interactions. The results are consistent with recent measurements of a conductance anomaly in a quantum point contact. Moreover, it is remarkable that a large splitting of spin-related subbands occurs repeatedly whenever the Fermi energy passes the subband threshold energies, although the amplitude of the splitting becomes weaker as more subbands are occupied due to the screen effect. Self-consistent results indicate that a moderate magnetic field tends to suppress the spin polarization caused by the exchange interaction. The diamagnetic shift of the subbands is determined only for those which are close to the Fermi energy.

Spontaneous spin polarization in quantum wires

We have implemented self-consistent calculations of the electronic structure of quantum wires with an in-plane magnetic field parallel to the wire. Spin-density-functional theory of Kohn and Sham is applied. The self-consistent results show that full spontaneous spin polarization takes place in the low electron density regime, even at an arbitrary small magnetic field which demonstrates that the polarization is caused by exchange interactions. The results are consistent with recent measurements of a conductance anomaly in a quantum point contact. Moreover, it is remarkable that a large splitting of spin-related subbands occurs repeatedly whenever the Fermi energy passes the subband threshold energies, although the amplitude of the splitting becomes weaker as more subbands are occupied due to the screen effect. Self-consistent results indicate that a moderate magnetic field tends to suppress the spin polarization caused by the exchange interaction. The diamagnetic shift of the subbands is determined only for those which are close to the Fermi energy.

Simple Two‐Band s–d Model for Ferromagnetic Semiconductors with Hybridization

AbstractA simple two‐band model with hybridization between conducting s‐electrons and localized d‐(or f‐) electrons for ferromagnetic semiconductors in their low electron density regime is suggested and analyzed. Čápek's decoupling scheme is used for the time‐dependent Zubarev Green functions. A closed form is obtained for the spin‐dependent self‐energy. For a special case, the density of states is evaluated numerically.

Hydrogen Stark broadening calculations with the unified classical path theory

The unified theory has been generalized for the case of upper and lower state interaction by introducing a more compact tetradic notation. The general result is then applied to the Stark broadening of hydrogen. The thermal average of the time development operator for upper and lower state interaction is presented. Except for the time ordering it contains the effect of finite interaction time between the radiator and perturbers to all orders, thus avoiding a Lewis type cutoff. A simple technique for evaluating the Fourier transform of the thermal average has been developed. The final calculations based on the unified theory and on the one-electron theory are compared with measurements in the high and low electron density regime. The unified theory calculations cover the entire line profile from the line center to the static wing and the simpler one-electron theory calculations provide the line intensities only in the line wings.