Magneto-intersubband Oscillations Research Articles

Contribution of Oscillations of the Fermi Level to the Shubnikov–de Haas and Magneto-Intersubband Oscillations in Single HgTe Quantum Wells

Contribution of Oscillations of the Fermi Level to the Shubnikov–de Haas and Magneto-Intersubband Oscillations in Single HgTe Quantum Wells

Interplay of Landau Quantization and Interminivalley Scatterings in a Weakly Coupled Moiré Superlattice.

Double-layer quantum systems are promising platforms for realizing novel quantum phases. Here, we report a study of quantum oscillations (QOs) in a weakly coupled double-layer system composed of a large-angle twisted-double-bilayer graphene (TDBG). We quantify the interlayer coupling strength by measuring the interlayer capacitance from the QOs pattern at low temperatures, revealing electron-hole asymmetry. At high temperatures when SdHOs are thermally smeared, we observe resistance peaks when Landau levels (LLs) from two moiré minivalleys are aligned, regardless of carrier density; eventually, it results in a 2-fold increase of oscillating frequency in D, serving as compelling evidence of the magneto-intersub-band oscillations (MISOs) in double-layer systems. The temperature dependence of MISOs suggests that electron-electron interactions play a crucial role and the scattering times obtained from MISO thermal damping are correlated with the interlayer coupling strength. Our study reveals intriguing interplays among Landau quantization, moiré band structure, and scatterings.

Optical realization of magneto-intersubband oscillations

We report on the optical realization of the magneto-intersubband oscillations that have been measured in the sub-terahertz transmittance of a GaAs quantum well with two subbands occupied. Following their dc analogue, the oscillations are periodic in the inverse magnetic field with the period governed by the subband gap. Their magnitude and polarization dependence accurately follow the presented simplified version of the dynamic magneto-intersubband oscillations equation that naturally combines dc magneto-intersubband oscillations with microwave-induced resistance oscillations. Simultaneously measured photoresistance also reveals its strong sensitivity to the sign of the circular polarization, proving the used theoretical modeling.

Magneto-Intersubband Oscillations under the Conditions of Overlapping Landau Bands

Magneto-intersubband oscillations in a highly mobile two-subband electron system with one-dimensional periodic modulation of the potential under the conditions of overlapping Landau bands have been experimentally investigated. A significant modification of magneto-intersubband oscillations relative to the amplitude and phase is found: the amplitude decreases and the reversal of magneto-intersubband oscillations occurs in some ranges of magnetic fields. The obtained experimental data can be explained by a two-humped structure of the electron energy spectrum in Landau bands.

Transformation of energy spectrum and wave functions on the way from a 2D-to-3D topological insulator in HgTe quantum wells

A magnetotransport and quantum capacitance of the two-dimensional (2D) electron gas in $\mathrm{HgTe}/{\mathrm{Cd}}_{x}{\mathrm{Hg}}_{1\ensuremath{-}x}\mathrm{Te}$ quantum wells of a width $(20.2--46.0)$ nm are experimentally investigated. It is shown that the first energy subband of spatial quantization is split due to the spin-orbit interaction and the split branches are single spin, therewith the splitting strength increases with the increase of the quantum well width. The electron effective masses in the branches are close to each other within the actual density range. Magnetointersubband oscillations (MISO) observed in the structures under study exhibit the growing amplitude with the increasing electron density that contradicts to the expected decrease of wave-function overlap for the rectangular quantum well. To interpret the data obtained, we have used a self-consistent approach to calculate the electron energy spectrum and the wave function within framework of the $kP$ model. It has been, in particular, shown that the MISO amplitude increase results from the increasing overlap of the wave functions due to their shift from the gate electrode with the gate voltage increase known as a phenomenon of the negative electron polarizability. The results obtained from the transport experiments are supported by quantum capacitance measurements.

Magneto-intersubband oscillations in two-dimensional systems with an energy spectrum split due to spin-orbit interaction

In the present paper we study magneto-intersubband oscillations (MISO) in HgTe/Hg$_{1-x}$Cd$_x$Te single quantum well with "inverted" and "normal" spectra and in conventional In$_{1-x}$Ga$_x$As/In$_{1-y}$Al$_y$As quantum wells with normal band ordering. For all the cases when two branches of the spectrum arise due to spin-orbit splitting, the mutual arrangement of the antinodes of the Shubnikov-de Haas oscillations and the maxima of MISO occurs opposite to that observed in double quantum wells and in wide quantum wells with two subbands occupied and does not agree with the theoretical predictions. A "toy" model is supposed that explain qualitatively this unusual result.

Microwave-Induced Magneto-Intersubband Scattering in a Square Lattice of Antidots

The effect of microwave radiation on low-temperature electron magnetotransport in a square antidot lattice with a period of d = 0.8 micrometer based on a GaAs quantum well with two occupied energy subbands E1 and E2 is investigated. It is shown that, owing to a significant difference between the electron densities in the subbands, commensurability oscillations of the resistance in the investigated antidot lattice are observed only for the first subband. It is found that microwave irradiation under the cyclotron resonance condition results in the formation of resistance oscillations periodic in the inverse magnetic field in the region of the main commensurability peak. It is established that the period of these oscillations corresponds to the period of magneto-intersubband oscillations. The observed effect is explained by the increase in the rate of intersubband scattering caused by the difference between the electron heating in the subbands E1 and E2.

Modulation of Magneto-Intersubband Oscillations in a One-Dimensional Lateral Superlattice

Low-temperature magnetotransport in a quasi-two-dimensional electron system based on a selectively doped GaAs quantum well with two occupied quantum-confinement subbands with one-dimensional periodic modulation of a potential is investigated. It is shown that commensurability oscillations of the resistance in this electron system coexist with magneto-intersubband oscillations. It is found that, in addition to commen-surability oscillations, the one-dimensional periodic potential in the two-subband electron system leads to the modulation of the magneto-intersubband oscillation amplitude. The experimental results are explained by the formation of Landau bands in the quasi-two-dimensional electron system with one-dimensional periodic modulation of the potential.

Features of Magneto-Intersubband Oscillations in HgTe Quantum Wells

Magneto-intersubband oscillations (MISOs) in single quantum wells of the gapless semiconductor HgTe have been studied experimentally. It has been shown that, in contrast to MISOs in double quantum wells based on broadband semiconductors GaAs and InGaAs and theoretical predictions, magnetic fields at which antinodes of high-frequency oscillations are observed coincide with minima rather than with maxima of MISOs of the magnetoresistance ρxx. It has been assumed that this feature is due to the suppression of the resonance of the probability of transitions between Landau levels of two spectral branches split by the strong spin—orbit coupling.

Magneto-transport property of an AlInN/AlN/GaN heterostructure

This paper reports a research of the magneto-transport property of an AlInN/AlN/GaN heterostructure. A large positive magnetoresistance (MR) up to 79% under 9 T is observed, which is indicative of potential sensor application. The observed positive MR is attributed to the presence of the parallel conduction. Meanwhile, the Shubnikov-de Hass (SdH) oscillations of the longitudinal MR appear up at high magnetic field. From the SdH oscillations, beating patterns are observed in the AlInN/AlN/GaN heterostructure. Neither the magneto-intersubband oscillation nor the zero-field spin splitting can explain the observed beating patterns, which can be attributed to the sample inhomogeneity.

Beats of Quantum Oscillations of the Resistance in Two-Subband Electron Systems in Tilted Magnetic Fields

Electron transport in single GaAs quantum wells of widths from 22 to 46 nm with two populated quantum-confinement subbands ES and EAS is investigated at a temperature of T = 4.2 K in tilted magnetic fields B 72° and α > 85°, respectively. The origin of this unexpected behavior of magnetointersubband oscillations in tilted magnetic fields is discussed.

Spin-orbit splitting of the conduction band in HgTe quantum wells: Role of different mechanisms

Spin-orbit splitting of conduction band in HgTe quantum wells was studied experimentally. In order to recognize the role of different mechanisms, we carried out detailed measurements of the Shubnikov-de Haas oscillations in gated structures with a quantum well widths from 8 to 18 nm over a wide range of electron density. With increasing electron density controlled by the gate voltage, splitting of the maximum of the oscillation Fourier spectrum f0 into two components f1 and f2 and the appearance of the low-frequency component f3 was observed. Our analysis shows that the components f1 and f2 are determined by the electron densities n1 and n2 in spin-orbit split subbands while the f3 component results from magneto-intersubband oscillations so that it is determined by the difference between these densities Δn. This allows us to obtain all three values n1, n2 and Δn independently. Comparison of the data obtained with results of self-consistent calculations carried out within the framework of four-band kP model shows that the main contribution to spin-orbit splitting comes from the Bychkov-Rashba effect. Contribution of the interface inversion asymmetry to the splitting of the conduction band turns out to be four-to-five times less than that for the valence band in the same structures.

Microwave-induced zero-resistance states in a high-mobility two-subband electron system

In this study, a selectively-doped GaAs/AlAs heterostructure was used to fabricate a high-mobility two-subband electronic system with a substantially different concentration of electrons in the subbands. We observe microwave photoresistance at high numbers of magneto-intersubband oscillations (MISO). The system under study demonstrates microwave-induced resistance oscillations (MIRO) and MISO interference. The MIRO in the studied two-subband system appears in lower magnetic fields compared to the MISO. This is an indication of some additional mechanism that exists in the two-subband system and is responsible for the MISO amplitude damping in low magnetic fields, while it does not affect the MIRO amplitude. Zero resistance states appear in the system under study under microwave irradiation in a narrow range of magnetic fields near the MISO maxima.

Magnetointersubband resistance oscillations in GaAs quantum wells placed in a tilted magnetic field

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 ($E_1\approx E_2<<E_3$). At zero in-plane magnetic fields magneto-intersubband oscillations (MISO) between the $i^{th}$ and $j^{th}$ subbands are observed and obey the relation $\Delta_{ij}=E_j-E_i=k\cdot\hbar\omega_c$, 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 $\hbar\omega_c \ll \Delta_{12}$ 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 $\hbar\omega_c \gg \Delta_{12}$ relatively weak oscillating variations of the electron spectrum with the in-plane magnetic field are observed. At $\hbar\omega_c \approx \Delta_{12}$ 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 $\hbar\omega_c \gg \Delta_{12}$

Magneto-intersubband oscillations of the kinetic coefficients of a two-dimensional system with a spin-orbit interaction

The temperature dependence of the Shubnikov–de Haas oscillations of longitudinal magnetoresistance of a two-dimensional electron system with the Rashba spin-orbit interaction is analyzed by using the expression for conductivity derived by S. G. Novokshonov and A. G. Groshev, Phys. Rev. B 74, 245333 (2006). It is shown that in the above system there exist magneto-intersubband oscillations of the kinetic coefficients characteristic for multisubband conductors. The period of these oscillations is determined by the spin precession frequency at the Fermi level ΩBF. In the case under consideration, the intersubband spin splitting depends on electron energy. This results in a weak temperature dependence of the magneto-intersubband oscillation amplitude, which is determined by Λ* = (2π2kBT/ℏωc)ℏ ΩBF / εF. This dependence is considerably weaker in comparison to the temperature smoothing of the Shubnikov–de Haas oscillation amplitude due to a small parameter (ℏ ΩBF /εF ≪ 1).

Zero-resistance states in bilayer electron systems induced by microwave irradiation

We experimentally demonstrate the existence of microwave-induced zero-resistance states (ZRS) in bilayer electron systems created in wide quantum wells. In contrast to single-layer two-dimensional electron systems, ZRS are developed from the strongest magneto-intersubband oscillation peaks inverted by microwave radiation. Our experimental work is discussed within the theories of microscopic mechanisms of photoresistance, and the calculations reveal that the condition for absolute negative resistivity correlates with the appearance of ZRS.

Nonlinear transport phenomena in a two-subband system

We study non-linear transport phenomena in a high-mobility bilayer system with two closely spaced populated electronic subbands in a perpendicular magnetic field. For a moderate direct current excitation, we observe zero-differential-resistance states with a characteristic 1/B periodicity. We investigate, both experimentally and theoretically, the Hall field-induced resistance oscillations which modulate the high-frequency magneto-intersubband oscillations in our system if we increase the current. We also observe and describe the influence of direct current on the magnetoresistance in the presence of microwave irradiation.

Evidence for zero-differential resistance states in electronic bilayers

We observe zero-differential resistance states at low temperatures and moderate direct currents in a bilayer electron system formed by a wide quantum well. Several regions of vanishing resistance evolve from the inverted peaks of magneto-intersubband oscillations as the current increases. The experiment, supported by a theoretical analysis, suggests that the origin of this phenomenon is based on instability of homogeneous current flow under conditions of negative differential resistivity, which leads to formation of current domains in our sample, similar to the case of single-layer systems.

Thermally activated intersubband scattering and oscillating magnetoresistance in quantum wells

Experimental studies of magnetoresistance in high-mobility wide quantum wells reveal oscillations which appear with an increase in temperature to 10 K and whose period is close to that of Shubnikov-de Haas oscillations. The observed phenomenon is identified as magnetointersubband oscillations caused by the scattering of electrons between two occupied subbands and the third subband which becomes occupied as a result of thermal activation. These small-period oscillations are less sensitive to thermal suppression than the largeperiod magnetointersubband oscillations caused by the scattering between the first and the second subbands. Theoretical study, based on consideration of electron scattering near the edge of the third subband, gives a reasonable explanation of our experimental findings.

Zero differential resistance in a double quantum well at high filling factors

The differential resistance rxx in a GaAs double quantum well with two occupied size-quantization subbands have been studied at temperatures T = 1.6–4.2 K in magnetic fields B < 0.5 T. It has been found that differential resistance rxx vanishes at the maxima of magneto-intersubband oscillations with an increase in the direct current Idc. It has been shown that the discovered rxx ≈ 0 state appears under the condition 2RcEH/ħωc < 1/2, where Rc is the cyclotron radius of electrons at the Fermi level, EH is the Hall electric field induced by the current Idc, and ωc is the cyclotron frequency.