We studied a doping series of (110)-oriented AlAs quantum wells (QWs) and observed transport evidence of single anisotropic-mass valley occupancy for the electrons in a 150 Åwide QW. Our calculations of strain and quantum confinement for these samples predict single anisotropic-mass valley occupancy for well widths $W$ greater than 53 Å. Below this, double-valley occupation is predicted such that the longitudinal mass axes are collinear. We observed mobility anisotropy in the electronic transport along the crystallographic directions in the ratio of 2.8, attributed to the mass anisotropy as well as anisotropic scattering of the electrons in the X-valley of AlAs.
Terahertz photoconductivity of $100~μ$m and $20~μ$m Hall bars fabricated from narrow AlAs quantum wells (QWs) of different widths is investigated in this paper. The photoresponse is dominated by collective magnetoplasmon excitations within the body of the Hall structure. We observed a radical change of magnetoplasma spectrum measured precisely for AlAs QWs of width ranging from $4$~nm up to $15$~nm. We have shown that the observed behavior is a vivid manifestation of valley transition taking place in the two-dimensional electron system. Remarkably, we show that photoresponse for AlAs QWs of wi…
By using different widths for two AlAs quantum wells comprising a bilayer system, we force the X-point conduction-band electrons in the two layers to occupy valleys with different Fermi contours, electron effective masses, and g-factors. Since the occupied valleys are at different X-points of the Brillouin zone, the interlayer tunneling is negligibly small despite the close electron layer spacing. We demonstrate the realization of this system via magneto-transport measurements and the observation of a phase-coherent, bilayer $ν$=1 quantum Hall state flanked by a reentrant insulating phase.
Employing state-of-the-art molecular beam epitaxy techniques to grow thin, modulation-doped AlAs quantum wells, we have achieved a low temperature mobility of 5.5 m$^2$/Vs with out-of-plane occupation, an order of magnitude improvement over previous studies. However, due to the narrow well width, mobilities are still limited by scattering due to interface roughness disorder. We demonstrate the successful implementation of a novel technique utilizing thermally-induced, biaxial, tensile strain that forces electrons to occupy the out-of-plane valley in thicker quantum wells, reducing interface ro…
The spectra of plasma and magnetoplasma excitations in a two-dimensional system of anisotropic heavy fermions were investigated for the first time. The spectrum of microwave absorption by disk-like samples of stressed AlAs quantum wells at low electron densities showed two plasma resonances separated by a frequency gap. These two plasma resonances correspond to electron mass principle values of $(1.10 \pm 0.05) m_0$ and $(0.20 \pm 0.01) m_0$. The observed results correspond to the case of a single valley strongly anisotropic Fermi surface. It was established that electron density increase resu…
We demonstrate that, in a quasi-two-dimensional electron system confined to an AlAs quantum well and occupying two conduction-band minima (valleys), a parallel magnetic field can couple to the electrons' orbital motion and tune the energies of the two valleys by different amounts. The measured density imbalance between the two valleys, which is a measure of the valley susceptibility with respect to parallel magnetic field, is enhanced compared to the predictions of non-interacting calculations, reflecting the role of electron-electron interaction.
We report the observation of commensurability oscillations in an AlAs two-dimensional electron system where two conduction-band valleys with elliptical in-plane Fermi contours are occupied. The Fourier power spectrum of the oscillations shows two frequency components consistent with those expected for the Fermi contours of the two valleys. From an analysis of the spectra we deduce $m_l/m_t=5.2\pm0.5$ for the ratio of the longitudinal and transverse electron effective masses.
Two-dimensional electrons in AlAs quantum wells occupy multiple conduction-band minima at the X- points of the Brillouin zone. These valleys have large effective mass and g-factor compared to the stan-dard GaAs electrons, and are also highly anisotropic. With proper choice of well width and by applying symmetry-breaking strain in the plane, one can control the occupation of different valleys thus rendering a system with tuneable effective mass, g-factor, Fermi contour anisotropy, and valley degeneracy. Here we review some of the rich physics that this system has allowed us to explore.
By measuring the angles at which the Landau levels overlap in tilted magnetic fields (the coincidence method), we determine the splitting of the conduction-band valleys in high-mobility two-dimensional (2D) electrons confined to AlAs quantum wells. The data reveal that, while the valleys are nearly degenerate in the absence of magnetic field, they split as a function of perpendicular magnetic field. The splitting appears to depend primarily on the magnitude of the perpendicular component of the magnetic field, suggesting electron-electron interaction as its origin.
Degenerate conduction-band minima, or `valleys', in materials such as Si, AlAs, graphene, and MoS$_2$ allow them to host two-dimensional electron systems (2DESs) that can access a valley degree of freedom. These multivalley 2DESs present exciting opportunities for both pragmatic and fundamental research alike because not only are they a platform for valleytronic devices, but they also provide a tool to tune and investigate the properties of complex many-body ground states. Here, we report ultra-high quality, modulation doped AlAs quantum wells containing 2DESs that occupy two anisotropic valle…
Spectra of magnetoplasma excitations have been investigated in a two-dimensional electron systems in AlAs quantum wells (QWs) of different widths. The magnetoplasma spectrum have been found to change profoundly when the quantum well width became thinner than $5.5$~nm, indicating a drastic change in the conduction electron energy spectrum. The transformation can be interpreted in terms of transition from the in-plane strongly anisotropic $X_x - X_y$ valley occupation to the out-of-plane isotropic $X_z$ valley in the QW plane. Strong enhancement of the cyclotron effective mass over the band valu…
This paper details a complete formalism for calculating electron subband energy and degeneracy in strained multi-valley quantum wells grown along any orientation with explicit results for the AlAs quantum well case. A standardized rotation matrix is defined to transform from the conventional- cubic-cell basis to the quantum-well-transport basis whereby effective mass tensors, valley vectors, strain matrices, anisotropic strain ratios, and scattering vectors are all defined in their respective bases. The specific cases of (001)-, (110)-, and (111)-oriented aluminum arsenide (AlAs) quantum wells…
We investigate the possibility of using phonon-drag imaging for the study of 2D electrons in (110) AlAs quantum wells. Our numerical simulations show that direct information on the strain and quantum confinement dependent valley occupancy of the electrons, on the anisotropic effective mass tensor and on electron-phonon coupling parameters can be obtained.
A doping series of AlAs (001) quantum wells with Si delta-modulation doping on both sides reveals different dark and post-illumination saturation densities, as well as temperature dependent photoconductivity. The lower dark two-dimensional electron density saturation is explained assuming deep binding energy of Delta_DK = 65.2 meV for Si-donors in the dark. Persistent photoconductivity (PPC) is observed upon illumination, with higher saturation density indicating shallow post-illumination donor binding energy. The photoconductivity is thermally activated, with 4 K illumination requiring post-i…
In a number of widely-studied materials, such as Si, AlAs, Bi, graphene, MoS2, and many transition metal dichalcogenide monolayers, electrons acquire an additional, spin-like degree of freedom at the degenerate conduction band minima, also known as "valleys". External symmetry breaking fields such as mechanical strain, or electric or magnetic fields, can tune the valley-polarization of these materials making them suitable candidates for "valleytronics". Here we study a quantum well of AlAs, where the two-dimensional electrons reside in two energetically degenerate valleys. By fabricating a str…
We have investigated the magnetophonon resonance (MPR) effect in a series of single GaAs quantum well samples which are symmetrically modulation doped in the adjacent short period AlAs/GaAs superlattices. Two distinct MPR series are observed originating from the $Γ$ and X electrons interacting with the GaAs and AlAs longitudinal optic (LO) phonons respectively. This confirms unequivocally the presence of X electrons in the AlAs quantum well of the superlattice previously invoked to explain the high electron mobility in these structures (Friedland et al. Phys. Rev. Lett. 77,4616 (1996).
We demonstrate tuning of two-dimensional (2D) plasmon spectrum in modulation-doped AlAs quantum wells via the application of in-plane uniaxial strain. We show that dramatic change in the plasma spectrum is caused by strain-induced redistribution of charge carriers between anisotropic $X_x$ and $X_y$ valleys. Discovered piezoplasmonic effect provides a tool to study the band structure of 2D systems. We use piezoplasmonic effect to measure how the inter-valley energy splitting depends on the deformation. This dependency yields the AlAs deformation potential of $E_2 = (5.6 \pm 0.3)$~eV.
Measurements on a two-dimensional electron system confined to an AlAs quantum well reveal that for a given electron density the valley susceptibility, defined as the change in valley population difference per unit strain, is enhanced as the system makes a transition from partial to full spin polarization. This observation is reminiscent of earlier studies in which the spin susceptibility of AlAs electrons was observed to be higher in a single-valley system than its two-valley counterpart.
The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy exceeding 1.63 eV is indirect in momentum space, leading to long-lived exciton states with potential applications in quantum information. Optical access to these excitons is provided by mixing of the $Γ$- and X-conduction band valleys, from which control of their spin states can be gained. This access is used here for studying the exciton spin-level structure by resonant spin-flip Raman scattering, allowing us to accurately measure the anisotropic hole and isotropic electron $g$ factors. The spin-flip mechanisms for …
We report experimental results on a quantum point contact (QPC) device formed in a wide AlAs quantum well where the two-dimensional electrons occupy two in-plane valleys with elliptical Fermi contours. To probe the closely-spaced, one-dimensional electric subbands, we fabricated a point contact device defined by shallow-etching and a top gate that covers the entire device. The conductance versus top gate bias trace shows a series of weak plateaus at integer multiples of $2e^2/h$, indicating a broken valley degeneracy in the QPC and implying the potential use of QPC as a simple "valley filter" …
The influence of illumination on a high mobility two-dimensional electron gas with high concentration of charge carriers is studied in selectively doped single GaAs quantum wells with short-period AlAs/GaAs superlattice barriers at a temperature T = 4.2 K in magnetic fields B < 2 T. It is shown that illumination at low temperatures in the studied heterostructures leads to an increase in the concentration, mobility, and quantum lifetime of electrons. An increase in the quantum lifetime due to illumination of single GaAs quantum wells with modulated superlattice doping is explained by a decrease…
We report measurements of the spin susceptibility in dilute two-dimensional electrons confined to a 45$Å$ wide AlAs quantum well. The electrons in this well occupy an out-of-plane conduction-band valley, rendering a system similar to two-dimensional electrons in Si-MOSFETs but with only one valley occupied. We observe an enhancement of the spin susceptibility over the band value that increases as the density is decreased, following closely the prediction of quantum Monte Carlo calculations and continuing at finite values through the metal-insulator transition.
We calculate using the Boltzmann transport theory the density dependent mobility of two-dimensional (2D) electrons in GaAs, SiGe and AlAs quantum wells as well as of 2D holes in GaAs quantum wells. The goal is to precisely understand the recently reported breakthrough in achieving a record 2D mobility for electrons confined in a GaAs quantum well. Comparing our theory with the experimentally reported electron mobility in GaAs quantum wells, we conclude that the mobility is limited by unintentional background random charged impurities at an unprecedented low concentration of $\sim10^{13} \mathr…
The temperature dependence of phonon-induced resistance oscillations has been investigated in two-dimensional electron system with moderate mobility at large filling factors at temperature range T = 7.4 - 25.4 K. The amplitude of phonon-induced oscillations has been found to be governed by quantum relaxation time which is determined by electron-electron interaction effects. This is in agreement with results recently obtained in ultra-high mobility two-dimensional electron system with low electron density [A. T. Hatke et al., Phys. Rev. Lett. 102, 086808 (2009)]. The shift of the main maximum o…
Direct electron spin resonance (ESR) on a high mobility two dimensional electron gas in a single AlAs quantum well reveals an electronic $g$-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 Gauss. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin orbit-interaction and a modulation of the electron wavevector caused by the microwave electric field. This contrasts markedly to conventional ESR that detects through the microwave magnetic field.
We report transport mobility measurements for clean, two-dimensional (2D) electron systems confined to GaAs quantum wells (QWs), grown via molecular beam epitaxy, in two families of structures, a standard, symmetrically-doped GaAs set of QWs with Al$_{0.32}$Ga$_{0.68}$As barriers, and one with additional AlAs cladding surrounding the QWs. Our results indicate that the mobility in narrow QWs with no cladding is consistent with existing theoretical calculations where interface roughness effects are softened by the penetration of the electron wave function into the adjacent low barriers. In contr…
We report transport measurements of composite Fermions at filling factor $ν=3/2$ in AlAs quantum wells as a function of strain and temperature. In this system the composite Fermions possess a valley degree of freedom and show piezoresistance qualitatively very similar to electrons. The temperature dependence of the resistance (R) of composite Fermions shows a metallic behavior (dR/dT > 0) for small values of valley polarization but turns insulating (dR/dT < 0) as they are driven to full valley polarization. The results highlight the importance of discrete degrees of freedom in the transport pr…
The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $Γ$-valley electron and the ex…
Thanks to their multi-valley, anisotropic, energy band structure, two-dimensional electron systems (2DESs) in modulation-doped AlAs quantum wells (QWs) provide a unique platform to investigate electron interaction physics and ballistic transport. Indeed, a plethora of phenomena unseen in other 2DESs have been observed over the past decade. However, a foundation for sample design is still lacking for AlAs 2DESs, limiting the means to achieve optimal quality samples. Here we present a systematic study on the fabrication of modulation-doped AlAs and GaAs QWs over a wide range of AlxGa1-xAs barrie…
The dependences of the transport scattering time τt, quantum lifetime τq, and their ratio τt/τq on the density ne of the electron gas in modulation-doped single GaAs quantum wells with AlAs/GaAs short-period super-lattice barriers are investigated. The experimental dependences are explained in terms of electron scattering by remote ionized donors with an effective two-dimensional concentration n*R and background impurities with a three-dimensional concentration nB. An expression for n*R(ne) is obtained including the contribution of X-valley electrons localized in AlAs layers to the suppression…
We measure the effective mass (m*) of interacting two-dimensional electrons confined to an AlAs quantum well while we change the conduction-band valley occupation and the spin polarization via the application of strain and magnetic field, respectively. Compared to its band value, m* is enhanced unless the electrons are fully valley and spin polarized. Incidentally, in the fully spin- and valley-polarized regime, the electron system exhibits an insulating behavior.
We measure the effective Lande g-factor of high-mobility two-dimensional electrons in a modulation-doped AlAs quantum well by tilting the sample in a magnetic field and monitoring the evolution of the magnetoresistance oscillations. The data reveal that |g| = 9.0, which is much enhanced with respect to the reported bulk value of 1.9. Surprisingly, in a large range of magnetic field and Landau level fillings, the value of the enhanced g-factor appears to be constant.
Cascade spin dynamics of excitons localized in indirect-band-gap (In,Al)As/AlAs quantum dots with type-I band alignment
Ioffe Institute, Russian Academy of Sciences（俄罗斯科学院约费研究所）；Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences（俄罗斯科学院西伯利亚分院拉扎诺夫半导体物理研究所）
We report magnetotransport measurements of fractional quantum Hall states in an AlAs quantum well around Landau level filling factor nu = 3/2, demonstrating that the quasiparticles are composite Fermions (CFs) with a valley degree of freedom. By monitoring the valley level crossings for these states as a function of applied symmetry-breaking strain, we determine the CF valley susceptibility and polarization. The data can be explained well by a simple Landau level fan diagram for CFs, and are in nearly quantitative agreement with the results reported for CF spin polarization.
We report measurements of the interaction-induced quantum Hall effect in a spin-polarized AlAs two-dimensional electron system where the electrons occupy two in-plane conduction band valleys. Via the application of in-plane strain, we tune the energies of these valleys and measure the energy gap of the quantum Hall state at filling factor $ν$ = 1. The gap has a finite value even at zero strain and, with strain, rises much faster than expected from a single-particle picture, suggesting that the lowest energy charged excitations at $ν=1$ are "valley Skyrmions".
In this study we used selectively-doped GaAs/AlAs heterostructure to fabricate a high-mobility two-subband electronic system with substantially different concentration of electrons in 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. MIRO in the studied two-subband system appear in lower magnetic fields comparing to MISO. This is an indication of some unknown mechanism that exists in the two-subband system and is responsible for M…
We calculate the phase diagram of the two component fractional quantum Hall effect as a function of the spin or valley Zeeman energy and the filling factor, which reveals new phase transitions and phase boundaries spanning many fractional plateaus. This phase diagram is relevant to the fractional quantum Hall effect in graphene and in GaAs and AlAs quantum wells, when either the spin or valley degree of freedom is active.
In modern GaAs/Al$_x$Ga$_{1-x}$As heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor $δ$-layers placed on both sides of the well. Each $δ$-layer is located within a narrow GaAs well, flanked by narrow AlAs layers which capture excess electrons from donors. We show that each excess electron is localized in a compact dipole atom with the nearest donor. Nevertheless, excess electrons screen both the remote donors and background impurities. When the fraction of remote donors filled by excess electrons $f$ is small, …
We observe minima of the longitudinal resistance corresponding to the quantum Hall effect of composite fermions at quantum numbers $p=1$, 2, 3, 4, and 6 in an ultraclean strongly interacting bivalley SiGe/Si/SiGe two-dimensional electron system. The minima at $p=3$ disappear below a certain electron density, although the surrounding minima at $p=2$ and $p=4$ survive at significantly lower densities. Furthermore, the onset for the resistance minimum at a filling factor $ν=3/5$ is found to be independent of the tilt angle of the magnetic field. These surprising results indicate the intersection …
The interplay between quantum Hall ordering and spontaneously broken "internal" symmetries in two-dimensional electron systems with spin or pseudospin degrees of freedom gives rise to a variety of interesting phenomena, including novel phases, phase transitions, and topological excitations. Here we develop a theory of broken-symmetry quantum Hall states, applicable to a class of multi-valley systems, where the symmetry at issue is a point group element that combines a spatial rotation with a permutation of valley indices. The anisotropy of the dispersion relation, generally present in such sys…
Prototypes for hot-carrier solar cells based on type-II InAs/AlAsSb multiple quantum wells are examined for AC photoconductivity as a function of lattice temperature and photoexcitation energy to determine the photoexcited charge carrier transport. These samples previously exhibit an excitation energy onset of a metastable regime in their short time charge carrier dynamics that potentially improves their applicability for hot-carrier photovoltaic applications. The transport results illustrate that the AC photoconductivity is larger in the dynamic regime corresponding to the metastability as a …
In modern GaAs/Al$_x$Ga$_{1-x}$As heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor $δ$-layers placed on both sides of the well. Each $δ$-layer is located within a narrow GaAs layer, flanked by narrow AlAs layers which capture excess electrons from donors but leave each of them localized in a compact dipole atom with a donor. Still excess electrons can hop between host donors to minimize their Coulomb energy. As a result they screen the random potential of donors dramatically. We numerically model the pseudogro…
Motivated by interesting recent experimental results, we consider theoretically charged-impurity scattering-limited 2D electronic transport in (100), (110), and (111)-Si inversion layers at low temperatures and carrier densities, where screening effects are important. We show conclusively that, given the same bare Coulomb disorder, the 2D mobility for a given system increases monotonically with increasing valley degeneracy. We also show that the temperature and the parallel magnetic field dependence of the 2D conductivity is strongly enhanced by increasing valley degeneracy. We analytically co…
Electrons' multiple quantum degrees of freedom can lead to rich physics, including a competition between various exotic ground states, as well as novel applications such as spintronics and valleytronics. Here we report magneto-transport experiments demonstrating how the valley degree of freedom impacts the fractional quantum states (FQHSs), and the related magnetic-flux-electron composite fermions (CFs), at very high magnetic fields in the extreme quantum limit when only the lowest Landau level is occupied. Unlike in other multivalley two-dimensional electron systems such as Si or monolayer gr…
We determine the energy splitting of the conduction-band valleys in two-dimensional (2D) electrons confined in silicon metal oxide semiconductor (Si-MOS) Hall-bar transistors. These Si-MOS Hall bars are made by advanced semiconductor manufacturing on 300 mm Si wafers and support a 2D electron gas of high quality with a maximum mobility of 17.6$\times$10$^3$cm$^2$/Vs and minimum percolation density of 3.45$\times$10$^{10}$cm$^{-2}$. Because of the low disorder, we observe beatings in the Shubnikov-de Haas oscillations that arise from the energy-split two low-lying conduction band valleys. From …
Fractional quantum Hall states (FQHSs) at even-denominator Landau level filling factors ($ν$) are of prime interest as they are predicted to host exotic, topological states of matter. We report here the observation of a FQHS at $ν=1/2$ in a two-dimensional electron system of exceptionally high quality, confined to a wide AlAs quantum well, where the electrons can occupy multiple conduction-band valleys with an anisotropic effective mass. The anisotropy and multi-valley degree of freedom offer an unprecedented tunability of the $ν=1/2$ FQHS as we can control both the valley occupancy via the ap…
We present transport measurements on high-mobility bilayer graphene fully encapsulated in hexagonal boron nitride. We show two terminal quantum Hall effect measurements which exhibit full symmetry broken Landau levels at low magnetic fields. From weak localization measurements, we extract gate-tunable phase coherence times $τ_ϕ$ as well as the inter- and intra-valley scattering times $τ_i$ and $τ_*$. While $τ_ϕ$ is in qualitative agreement with an electron-electron interaction mediated dephasing mechanism, electron spin-flip scattering processes are limiting $τ_ϕ$ at low temperatures. The anal…
Valley-Landscape Engineering in Bilayer WSe$_2$ Gate-All-Around Transistors
Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS)（物质·材料纳米构筑研究中心，日本国立材料研究所）；Kwansei Gakuin University（关西学院大学）
The fundamental properties of valleys are recently attracting growing attention due to electrons in new and topical materials possessing this degree-of-freedom and recent proposals for val-leytronics devices. In silicon MOSFETs, the interest has a longer history since the valley degree of freedom had been identified as a key parameter in the observation of the controversial "metallic behaviour" in two dimensions. However, while it has been recently demonstrated that lifting valley degeneracy can destroy the metallic behaviour, little is known about the role of intervalley scattering. Here, we …
Monolayer transition metal dichalcogenides (MX2) are promising candidates for future electronics. Although the transport properties (e.g. mobility) at low electric field have been widely studied, there are limited studies on high-field properties, which are important for many applications. Particularly, there is lack of understanding of the physical origins underlying the property differences across different MX2. Here by combining first-principles calculations with Monte Carlo simulations, we study the high-field electron transport in defects-free unstrained and tensilely strained MX2 (M=Mo, …
Motivated by theoretical predictions that spatially complex concentration modulations of Si and Ge can increase the valley splitting in quantum wells, we grow and characterize Si/SiGe heterostructures with a thin, pure Ge layer at the top of the quantum well using chemical vapor deposition. We show that these heterostructures remain hosts for high-mobility electron gases. We measure two quantum wells with approximately five monolayers of pure Ge at the upper barrier, finding mobilities as high as 70,000 cm$^2$/Vs, compared to 100,000 cm$^2$/Vs measured in samples with no Ge layer. Activation e…
We report measurements of the spin susceptibility in dilute (rs up to 10) AlAs two-dimensional (2D) electrons occupying a single conduction-band valley with an anisotropic in-plane Fermi contour, characterized by longitudinal and transverse effective masses, ml and mt. As the density is decreased, the spin susceptibility is significantly enhanced over its band value, reflecting the role of interaction. Yet the enhancement is suppressed compared to the results of quantum Monte Carlo based calculations that take the finite thickness of the electron layer into account but assume an isotropic effe…
We study quantum Hall ferromagnets with a finite density topologically charged spin textures in the presence of internal degrees of freedom such as spin, valley, or layer indices, so that the system is parametrised by a $d$-component complex spinor field. In the absence of anisotropies, we find formation of a hexagonal Skyrmion lattice which completely breaks the underlying SU(d) symmetry. The ground state charge density modulation, which inevitably exists in these lattices, vanishes exponentially in $d$. We compute analytically the complete low-lying excitation spectrum, which separates into …
We report low temperature magnetotransport measurements on a high mobility (μ=325,000 cm^2/V sec) 2D electron system on a H-terminated Si(111) surface. While low magnetic field data indicate a six-fold valley degenerate system, we observe the integral quantum Hall effect at all filling factors ν<=6 and find that ν=2 develops in an unusually narrow temperature range. An extended, exclusively even numerator, fractional quantum Hall hierarchy occurs surrounding ν=3/2, consistent with two-fold valley-degenerate composite fermions (CFs). We determine activation energies and estimate the CF mass.
