Ideal Two-dimensional Electron Gas Research Articles

Nearly Ideal Two-Dimensional Electron Gas Hosted by Multiple Quantized Kronig-Penney States Observed in Few-Layer InSe.

A theoretical ideal two-dimensional electron gas (2DEG) was characterized by a flat density of states independent of energy. Compared with conventional two-dimensional free-electron systems in semiconductor heterojunctions and noble metal surfaces, we report here the achievement of ideal 2DEG with multiple quantized states in few-layer InSe films. The multiple quantum well states (QWSs) in few-layer InSe films are found, and the number of QWSs is strictly equal to the number of atomic layers. The multiple stair-like DOS as well as multiple bands with parabolic dispersion both characterize ideal 2DEG features in these QWSs. Density functional theory calculations and numerical simulations based on quasi-bounded square potential wells described as the Kronig-Penney model provide a consistent explanation of 2DEG in the QWSs. Our work demonstrates that 2D van der Waals materials are ideal systems for realizing 2DEG hosted by multiple quantized Kronig-Penney states, and the semiconducting nature of the material provides a better chance for construction of high-performance electronic devices utilizing these states, for example, superlattice devices with negative differential resistance.

Temperature effects in spin-dependent Hall currents in an ideal skyrmion gas

The quest for novel reliable and fast-performing logic and memory elements in classical and quantum computing requires discoveries of new effects in novel quantum materials. Skyrmions are such magnetic textures where electron scattering can essentially change the shape and location of the skyrmion, which can be used as a memory element being more efficient than domain walls as a memory device in classical computing. Because the skyrmion motion is sensitive even to very small currents, we study electron scattering by skyrmions in an ideal skyrmion gas in a ferromagnetic environment. In such systems, the direct and Hall currents become spin-dependent. For applications, it is important to consider a Hall effect in the whole range of temperatures under the assumption of the skyrmion existence. In this study, we find the nonmonotonic temperature dependence of the direct spin-up conductivity, i.e., the conductivity for the current directed along the applied electric field due to the electrons with the spin parallel to the ferromagnetic moment. Such a behavior contradicts the traditional understanding where the temperature only increases the value of the conductivity. The spin-down Hall conductivity is found to be even more dramatic exhibiting a conductivity sign change (i.e., a change in the current direction) with temperature for small skyrmion sizes. The found effects strongly depend on Fermi energy. The most pronounced dependencies take place if the Fermi energy is slightly below and above the bottom of the upper (spin-down) energy band of an ideal two-dimensional electron gas. In addition, we also find that the direct and Hall resistivities, ${\ensuremath{\rho}}_{xx}$ and ${\ensuremath{\rho}}_{xy}$, are independent of the exchange integral $J$ for $2J>{\ensuremath{\varepsilon}}_{F}$.

Potassium-intercalated bulk HfS2 and HfSe2 : Phase stability, structure, and electronic structure

We have studied potassium-intercalated bulk HfS$_2$ and HfSe$_2$ by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy and density functional theory calculations. Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentrations $x$ are thermodynamically unstable. This leads to the coexistence of undoped and doped domains if the provided amount of the alkali metal is insufficient to saturate the whole crystal with the minimum thermodynamically stable potassium stoichiometry. Beyond this threshold concentration the domains disappear, while the alkali metal and charge carrier concentrations increase continuously upon further addition of potassium. At low intercalation levels, electron diffraction patterns indicate a significant degree of disorder in the crystal structure. The initial order in the out-of-plane direction is restored at high $x$ while the crystal layer thicknesses expand by 33-36%. Superstructures emerge parallel to the planes which we attribute to the distribution of the alkali metal rather than structural changes of the host materials. The in-plane lattice parameters change by not more than 1%. The introduction of potassium causes the formation of charge carrier plasmons. The observation of this semiconductor-to-metal transition is supported by calculations of the density of states (DOS) and band structures as well as angle-resolved photoemission spectroscopy. The calculated DOS hint at the presence of an almost ideal two-dimensional electron gas at the Fermi level for $x<0.6$. The plasmons exhibit quadratic momentum dispersions which is in agreement with the behavior expected for an ideal electron gas.

Observation of Peculiar Rashba-Type Spin-Split Band on Bi(111) Surface by High-Resolution Spin- and Angle-Resolved Photoemission Spectroscopy

Spin polarized surface states of Bi(111) film fabricated on Si(111) surface have been investigated by means of high-resolution spin-resolved photoemission spectroscopy. In contrast to the Rashba-type spin-split ideal two-dimensional electron gas in which the spin orientation is locked in-plane and tangential to the electron momentum, significant out-of-plane spin polarization has been observed in the surface states which are forming the hole pocket. The out-of-plane spin polarization is anti-symmetric with respect to Γ-M line and vanishes at the line suggesting that the out-of-plane polarization is derived from the three-fold symmetry of the Bi crystal. The magnitude of the out-of-plane spin components depends on the condition of Bi film fabrication, which is consistent with the recent x-ray diffraction results suggesting the existence of double domains rotating 180° each other on the Bi(111) film grown on Si(111) surface. [DOI: 10.1380/ejssnt.2012.153]

Electron-electron interaction in a narrow, disordered electron gas in nonequilibrium

The dephasing of nonequilibrium electrons by electron-electron interaction was studied in the diffusive regime. This was done by measuring the effect of an external bias voltage on weak localization in a quasi-one-dimensional electron gas. We find that electrons not in thermal equilibrium lose phase in quantitative agreement with the lifetime theory for an ideal two-dimensional electron gas. This is in contrast to thermalized electrons for which Nyquist phase breaking is most important.

Potential and current distribution in an ideal Hall bar.

The local potential and current distributions in a confined ideal two-dimensional electron gas with perpendicular magnetic field are calculated. The nondissipative current-carrying state of the system is described by a generalized equilibrium density operator appropriate for the boundary conditions. The Coulomb interaction of the electrons is taken into account in Hartree approximation. The results obtained by numerical computation allow the interpretation of seemingly contradictory experimental data.

Two-dimensional electron gas in a strong magnetic field. I

We obtain expressions for the thermodynamic potential, concentration, and magnetic moment of an ideal two-dimensional electron gas in a strong (quantizing the motion) magnetic field at low temperatures. The results are used to explain the integer quantum Hall effect.

Theory of the de Haas-van Alphen effect in quasi-two-dimensional conductors

According to the Peierls theory, the de Haas-van Alphen oscillations should have a saw-tooth form in an ideal two-dimensional electron gas when the chemical potential is pinned to a Landau level during the entire de Haas-van Alphen period (constant number of carriers) or when a reservoir of carriers exists (fixed chemical potential). Recent experiments indicate that neither of these conditions are fulfilled, during a single period in AsF 5 intercalated graphite. We have studied both: analytically and numerically the smoothing out of the saw tooth form of the magnetization, at finite temperature, due to an external reservoir of carriers.

Retarded transverse current-current response functions of a two-dimensional electron gas

Abstract The retarded transverse current-current response functions of an ideal two-dimensional electron gas at zero temperature are calculated. Introducing a new approximation scheme which allows one to take account of the reaction of the electromagnetic field to the induced current, i.e. the influence of the electronic induced current on the dynamics of the electromagnetic field, within the framework of the conventional linear response theory in a self-consistent way, a possible form of the dispersion relation for the transverse plasmon in the system is obtained. It is found that the energy of the transverse plasmon has a gap at k =0, if the thickness of the system is finite, in contrast to the longitudinal case.

Specific heat of 2-d electrons in strong magnetic field

It is shown that the specific heat of an ideal two-dimensional electron gas in strong magnetic field decreases exponentially at low temperatures.

Transverse plasmon in two-dimensional electrons

We calculated the retarded transverse current-current response function of an ideal two-dimensional electron gas and obtained a dispersion relation for the transverse plasmon in the system.

Inversion layer thermopower in high magnetic field

Calculates the thermopower of an ideal two-dimensional electron gas (inversion layer) in a quantising magnetic field. The authors find that the thermopower is a universal function of the reduced temperature (kBT/h(cross) omega c) which has a novel dependence on the chemical potential.

A variational approach to the ground state of the two-dimensional electron gas

Abstract In this work we calculate a variational upper bound for the ground-state energy of the two-dimensional electron gas. The calculation sheds light on the validity of approximate calculations and indicates that the “deformable jellium” model of an ideal two-dimensional electron gas may exhibit a CDW instability for rs greater than about 2. The original feature of the calculation is that some correlation effects are included in the variational bound by means of a “geminal” expansion of the many-body wave function.

Ferromagnetism of two-dimensional electron systems

The ground state energies of (1) an ideal two-dimensional electron gas, (2) a quasi-two dimensional electron system resembling electrons trapped on a liquid helium surface, (3) a quasi-two dimensional electron system resembling inversion layers of Si(100)SiO 2 system, and (4) an ideal three-dimensional electron gas are obtained by numerically evaluating the sum of “ring diagrams” contributions to the correlation energies for all magnetizations and a wide range of electron densities. The transition from nonmagnetic to ferromagnetic state is abrupt in (1) and (2) and gradual in (3) and (4).

Correlations in two-dimensional electron systems. Magnetic properties

The ground-state energies of (i) an ideal two-dimensional electron gas, (ii) a quasi-two-dimensional electron system resembling electrons trapped on a liquid-helium surface, (iii) a quasi-two-dimensional electron system resembling inversion layers of the Si(100)-Si${\mathrm{O}}_{2}$ system, and (iv) an ideal three-dimensional electron gas are obtained by numerically evaluating the sum of "ring diagram" contributions to the correlation energies for all magnetizations and a wide range of electron densities. The transition from nonmagnetic to ferromagnetic state is abrupt in (i) and (ii) and gradual in (iii) and (iv). It is argued that these transitions may occur in quasi-two-dimensional systems without violating the theorems on the nonexistence of long-range order in two dimensions because of the presence of weak long-range forces in such systems.