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Exciton condensates (ECs) are macroscopic coherent states arising from condensation of electron–hole pairs1. Bilayer heterostructures, consisting of two-dimensional electron and hole layers separated by a tunnel barrier, provide a versatile platform to realize and study ECs2,3,4. The tunnel barrier suppresses recombination, yielding long-lived excitons5,6,7,8,9,10. However, this separation also reduces interlayer Coulomb interactions, limiting the exciton binding strength. Here, we report the observation of ECs in naturally occurring 2H-stacked bilayer WSe2. In this system, the intrinsic spin–valley structure suppresses interlayer tunnelling even when the separation is reduced to the atomic limit, providing access to a previously unattainable regime of strong interlayer coupling. Using capacitance spectroscopy, we investigate magneto-ECs, formed when partially filled Landau levels couple between the layers. We find that the strong-coupling ECs show dramatically different behaviour compared with previous reports, including an unanticipated variation of EC robustness with the orbital number, and find evidence for a transition between two types of low-energy charged excitations. Our results provide a demonstration of tuning EC properties by varying the constituent single-particle wavefunctions.

Exciton condensates (ECs) are macroscopic coherent states arising from condensation of electron-hole pairs

Anomalous nematic states, recently discovered in ultraclean two-dimensional electron gas, emerge from quantum Hall stripe phases upon further cooling. These states are hallmarked by a local minimum (maximum) in the hard (easy) longitudinal resistance and by an incipient plateau in the Hall resistance in nearly half-filled Landau levels. Here, we demonstrate that a modest in-plane magnetic field, applied either along $\left < 110 \right >$ or $\left < 1\bar10 \right >$ crystal axis of GaAs, destroys anomalous nematic states and restores quantum Hall stripe phases aligned along their native $\left < 110 \right >$ direction. These findings confirm that anomalous nematic states are distinct from other ground states and will assist future theories to identify their origin., 5 pages, 4 figures

Heterostructures of vertically stacked graphene double layers, separated by a thin tunnel barrier, provide a highly tunable system to explore strongly interacting electron states. This is because the interlayer Coulomb interactions can be sensitively tuned simply by varying the barrier thickness. Recent studies of double-layer graphene have shown that, in the quantum Hall effect regime, strong interlayer coupling can induce electron–hole pairing across the two layers, resulting in a superfluid phase of interlayer excitons1–3. Here, we report a series of emergent fractional quantum Hall effect (FQHE) states appearing under similar conditions. We find excellent agreement between the sequence of observable FQHE states and the theoretically proposed two-component composite-fermion (CF) model, where the CF quasiparticle construction results from both interlayer and intralayer interactions4,5. Most remarkably, we observe an additional series of incompressible states at fractional filling that do not fit within either the single- or two-component CF models. We interpret these states to result from residual pairing interactions between CFs, representing a new type of correlated ground state that is unique to graphene double-layer structures and not described by the conventional CF model. It is shown that composite fermions in the fractional quantum Hall regime form paired states in double-layer graphene. Pairing between layers gives a phase similar to an exciton condensate and pairing within a layer may lead to non-abelian states.

Charge separated interlayer excitons in transition metal dichalcogenide heterobilayers are being explored for moir\'e exciton lattices and exciton condensates. The presence of permanent dipole moments and the poorly screened Coulomb interaction make many-body interactions particularly strong for interlayer excitons. Here we reveal two distinct phase transitions for interlayer excitons in the ${\mathrm{MoSe}}_{2}/{\mathrm{WSe}}_{2}$ heterobilayer using time and spatially resolved photoluminescence imaging: from trapped excitons in the moir\'e potential to the modestly mobile exciton gas as exciton density increases to ${n}_{ex}\ensuremath{\sim}{10}^{11}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$ and from the exciton gas to the highly mobile charge separated electron-hole plasma for ${n}_{ex}g{10}^{12}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$. The latter is the Mott transition and is confirmed in photoconductivity measurements. These findings set fundamental limits for achieving quantum states of interlayer excitons.

We report on transport signatures of hidden quantum Hall stripe (hQHS) phases in high (N>2) half-filled Landau levels of Al_{x}Ga_{1-x}As/Al_{0.24}Ga_{0.76}As quantum wells with varying Al mole fraction x

Charge separated interlayer excitons in transition metal dichalcogenide heterobilayers are being explored for moiré exciton lattices and exciton condensates. The presence of permanent dipole moments and the poorly screened Coulomb interaction make many-body interactions particularly strong for interlayer excitons. Here we reveal two distinct phase transitions for interlayer excitons in the MoSe_{2}/WSe_{2} heterobilayer using time and spatially resolved photoluminescence imaging: from trapped excitons in the moiré potential to the modestly mobile exciton gas as exciton density increases to n_{ex}∼10^{11} cm^{-2} and from the exciton gas to the highly mobile charge separated electron-hole plasma for n_{ex}10^{12} cm^{-2}. The latter is the Mott transition and is confirmed in photoconductivity measurements. These findings set fundamental limits for achieving quantum states of interlayer excitons.

Monolayer semiconducting transition-metal dichalcogenides (TMDs) represent a unique class of two-dimensional (2D) electron systems. Their atomically thin structure facilitates gate tunability just like graphene does, but unlike graphene, TMDs have the advantage of a sizable band gap and strong spin-orbit coupling. Measurements under large magnetic fields have revealed an unusual Landau level (LL) structure

Monolayer (ML) semiconducting transition-metal dichalcogenides (TMDs) represent a unique class of two-dimensional (2D) electron systems. Their atomically thin structure -- just like graphene -- facilitates gate-tunability, while the sizable band gap and strong spin-orbit coupling hold promise for properties beyond graphene. Measurements under large magnetic fields have revealed an unusual LL structure, distinct from other 2D electron systems. However, owing to limited sample quality and poor electrical contact, probing the lowest Landau levels (LLs) has been challenging, and observation of electron correlations within the fractionally filled LLs regime has not been possible. Here, through bulk electronic compressibility measurements, we investigate the LL structure of ML WSe$_2$ in the extreme quantum limit, and observe fractional quantum Hall (FQH) states in the lowest three LLs. The odd-denominator FQH sequences demonstrate a systematic evolution with the LL orbital index, which has not been observed in any other system but is consistent with generic theoretical expectations. In addition, we observe an even-denominator state in the second LL that is expected to host non-Abelian statistics. Our results suggest that the 2D semiconductors can provide an experimental platform that closely resembles idealized theoretical models in the quantum Hall regime., typos in v1 corrected

A bubble phase containing several electrons per unit cell was predicted by Koulakov, Fogler, and Shklovskii in 1996 as a generalization of the Wigner crystal. While the experimental evidence of such phases with $M$=2 electrons per bubble has emerged shortly after the prediction, larger bubbles have not been identified till now. Here, the authors report on bubbles with $M$=2 and $M$=3 electrons in the third excited Landau level, revealed with the help of intentionally added alloy disorder.

We report on quantum Hall stripes (QHSs) formed in higher Landau levels of GaAs/AlGaAs quantum wells with high carrier density ($n_e > 4 \times 10^{11}$ cm$^{-2}$) which is expected to favor QHS orientation along unconventional $\left < 1\bar{1}0 \right >$ crystal axis and along the in-plane magnetic field $B_{||}$. Surprisingly, we find that at $B_{||} = 0$ QHSs in our samples are aligned along $\left < 110 \right >$ direction and can be reoriented only perpendicular to $B_{||}$. These findings suggest that high density alone is not a decisive factor for either abnormal native QHS orientation or alignment with respect to $B_{||}$, while quantum confinement of the 2DEG likely plays an important role., 4 pages

We report on magnetotransport properties of a MgZnO/ZnO heterostructure subjected to weak direct currents. We find that in the regime of overlapping Landau levels, the differential resistivity acquires a quantum correction proportional to both the square of the current and the Dingle factor. The analysis shows that the correction to the differential resistivity is dominated by a current-induced modification of the electron distribution function and allows us to access both quantum and inelastic scattering rates., 4 pages, 3 figures

We investigate quantum Hall stripes under an in-plane magnetic field ${B}_{\ensuremath{\parallel}}$ in a variable-density two-dimensional electron gas. At filling factor $\ensuremath{\nu}=9/2$, we observe one, two, and zero ${B}_{\ensuremath{\parallel}}$-induced reorientations at low, intermediate, and high densities, respectively. The appearance of these distinct regimes is due to a strong density dependence of the ${B}_{\ensuremath{\parallel}}$-induced orienting mechanism which triggers the second reorientation, rendering stripes parallel to ${B}_{\ensuremath{\parallel}}$. In contrast, the mechanism which reorients stripes perpendicular to ${B}_{\ensuremath{\parallel}}$ showed no noticeable dependence on density. Measurements at $\ensuremath{\nu}=9/2$ and $11/2$ at the same, tilted magnetic field allow us to rule out the density dependence of the native symmetry-breaking field as a dominant factor. Our findings further suggest that screening might play an important role in determining stripe orientation, providing guidance in developing theories aimed at identifying and describing native and ${B}_{\ensuremath{\parallel}}$-induced symmetry-breaking fields.

Our magnetotransport measurements of quantum Hall stripes in a high-quality GaAs quantum well in a slightly tilted magnetic field reveal that the orientation of stripes can be changed by temperature. Field-cooling and field-warming measurements, as well as observation of hysteresis at intermediate temperatures allow us to conclude that the observed temperature-induced reorientation of stripes is owing to the existence of two distinct minima in the symmetry-breaking potential. We also find that the native symmetry-breaking mechanism does not depend on temperature and that low-temperature magnetotransport data should be treated with caution as they do not necessarily reveal the true ground state, even in the absence of hysteresis.

We report on Hall field-induced resistance oscillations (HIRO) in a 60 nm-wide GaAs/AlGaAs quantum well with an \emph{in situ} grown back gate, which allows tuning the carrier density $n$. At low $n$, when all electrons are confined to the lowest subband (SB1), the HIRO frequency, proportional to the product of the cyclotron diameter and the Hall field, scales with $n^{-1/2}$, as expected. Remarkably, population of the second subband (SB2) significantly enhances HIRO, while their frequency now scales as $n^{-1}$. We demonstrate that in this two-subband regime HIRO still originate solely from backscattering of SB1 electrons. The unusual density dependence occurs because the population of SB2 steadily increases, while that of SB1 remains essentially unchanged. The enhancement of HIRO manifests an unexpected, step-like increase of the quantum lifetime of SB1 electrons, which reaches a record value of 52 ps in the two-subband regime., Comment: 5 pages, 5 figures

We report on observation of a fine structure of microwave-induced resistance oscillations in an ultraclean two-dimensional electron gas. This fine structure is manifested by multiple secondary sharp extrema, residing beside the primary ones, which emerge at high radiation power. Theoretical considerations reveal that this fine structure originates from multiphoton-assisted scattering off short-range impurities. Unique properties of the fine structure allow us to access all experimental parameters, including microwave power, and to separate different contributions to photoresistance. Furthermore, we show that the fine structure offers a convenient means to quantitatively assess the correlation properties of the disorder potential in high-quality systems, allowing separation of short- and long-range disorder contributions to the electron mobility.

We performed effective mass measurements employing microwave-induced resistance oscillation in a tunable-density GaAs/AlGaAs quantum well. Our main result is a clear observation of an effective mass increase with decreasing density, in general agreement with earlier studies which investigated the density dependence of the effective mass employing Shubnikov- de Haas oscillations. This finding provides further evidence that microwave-induced resistance oscillations are sensitive to electron-electron interactions and offer a convenient and accurate way to obtain the effective mass., 4 pages, 4 figures

We report on high-order magnetoplasmon resonances detected in photoresistance in high-mobility GaAs quantum wells. These resonances manifest themselves as a series of photoresistance extrema in the regime of Shubnikov--de Haas oscillations. Extending to orders as high as 25, the extrema exhibit alternating strength, being less (more) pronounced at even- (odd)-order magnetoplasmon modes. This experimental technique provides a sensitive and elegant means to detect and investigate multiple magnetoplasmon modes and could be applied to other systems.

Nearly half-filled Landau levels of an ultrapure two-dimensional electron gas host electronic nematic phases, commonly known as quantum Hall stripes. While original orientation of stripes is determined by the native symmetry-breaking potential of yet mysterious origin, an in-plane magnetic field can easily rotate stripes by 90 degrees. Experiments by the Minnesota/Purdue group in this paper reveal that, under a modest in-plane field, the stripe orientation can also be changed simply by making the topmost Landau level less or more than half-full. This finding reflects the strong filling factor dependence of the native symmetry-breaking potential and might provide a critical piece of information needed to finally identify its origin.

The temperature dependence of microwave-induced resistance oscillations (MIRO), according to the theory, originates from electron-electron scattering. This scattering affects both the quantum lifetime, or the density of states, and the inelastic lifetime, which governs the relaxation of the nonequilibrium distribution function. Here, we report on MIRO in an ultra-high-mobility ($\ensuremath{\mu}g3\ifmmode\times\else\texttimes\fi{}{10}^{7}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\mathrm{V}$ s) two-dimensional electron gas at $T$ between $0.3$ and $1.8$ K. In contrast to theoretical predictions, the quantum lifetime is found to be $T$ independent in the whole temperature range studied. At the same time, the $T$ dependence of the inelastic lifetime is much stronger than can be expected from electron-electron interactions.

We report on the effect of in-plane magnetic field $B_\parallel$ on stripe phases in higher ($N=2,3$) Landau levels of a high-mobility 2D electron gas. In accord with previous studies, we find that a modest $B_\parallel$ applied parallel to the native stripes aligns them perpendicular to it. However, upon further increase of $B_\parallel$, stripes are reoriented back to their native direction. Remarkably, applying $B_\parallel$ perpendicular to the native stripes also aligns stripes parallel to it. Thus, regardless of the initial orientation of stripes with respect to $B_\parallel$, stripes are ultimately aligned \emph{parallel} to $B_\parallel$. These findings provide evidence for a $B_\parallel$-induced symmetry breaking mechanism which challenge current understanding of the role of $B_\parallel$ and should be taken into account when determining the strength of the native symmetry breaking potential. Finally, our results might indicate nontrivial coupling between the native and external symmetry breaking fields, which has not yet been theoretically considered., Comment: 4 pages, 3 figures

We report on magnetotransport measurements in a two-dimensional (2D) electron gas subject to subterahertz radiation in the regime where Shubnikov--de Haas oscillations (SdHOs) and microwave-induced resistance oscillations (MIROs) coexist over a wide magnetic field range, spanning several harmonics of the cyclotron resonance. Surprisingly, we find that the SdHO amplitude is modified by the radiation in a nontrivial way, owing to the oscillatory correction which has the same period and phase as MIROs. This finding challenges our current understanding of microwave photoresistance in 2D electron gas, calling for future investigations.

We report on an observation of a fractional quantum Hall effect in an ultrahigh-quality two-dimensional hole gas hosted in a strained Ge quantum well. The Hall resistance reveals precisely quantized plateaus and vanishing longitudinal resistance at filling factors $\ensuremath{\nu}=2/3,4/3$, and $5/3$. From the temperature dependence around $\ensuremath{\nu}=3/2$ we obtain the composite fermion mass of ${m}^{\ensuremath{\star}}\ensuremath{\approx}0.4\phantom{\rule{0.16em}{0ex}}{m}_{e}$, where ${m}_{e}$ is the mass of a free electron. Owing to large Zeeman energy, all observed states are spin polarized and can be described in terms of spinless composite fermions.

We report on strong transport anisotropy in a two-dimensional hole gas in a Ge/SiGe quantum well, which emerges only when both perpendicular and in-plane magnetic fields are present. The ratio of resistances, measured along and perpendicular to the in-plane field, can exceed $3\ifmmode\times\else\texttimes\fi{}{10}^{4}$. The anisotropy occurs in a wide range of filling factors where it is determined primarily by the tilt angle. The lack of significant anisotropy without an in-plane field, easy tunability, and persistence to higher temperatures and filling factors set this anisotropy apart from nematic phases in GaAs/AlGaAs.

A recent study of a high-mobility two-dimensional hole gas in a strained Ge quantum well revealed strong transport anisotropy in the quantum Hall regime when the magnetic field was tilted away from the sample normal [Q. Shi, M. A. Zudov, C. Morrison, and M. Myronov, Phys. Rev. B 91, 201301(R) (2015)]. In the present study we demonstrate that the anisotropy persists to such high temperatures and filling factors that quantum oscillations are no longer observed. This finding rules out the formation of a stripe phase as a possible origin for the observed anisotropy. However, we also show that the observed anisotropy is not consistent with other known anisotropies, such as those arising from finite-thickness effects or surface roughness.\ud

We report on nonlinear transport measurements in a GaAs/AlGaAs quantum well exhibiting a colossal negative magnetoresistance effect. Under applied dc bias, the magnetoresistance becomes nonmonotonic, exhibiting distinct extrema that move to higher magnetic fields with increasing current. In the range of magnetic fields corresponding to the resistivity minimum at zero bias, the resistivity increases linearly with current and the rate of this increase scales with the inverse magnetic field. The latter observation is consistent with the theory, proposed more than 35 years ago, considering classical memory effects in the presence of strong, dilute scatterers.

We report on a magnetotransport study in a high-mobility 2D hole gas hosted in a pure Ge/SiGe quantum well subject to dc electric fields and high frequency microwave radiation. We find that under applied dc bias the differential resistivity exhibits a pronounced maximum at a magnetic field which increases linearly with the applied current. We associate this maximum with the fundamental peak of Hall field-induced resistance oscillations (HIRO) which are known to occur in 2D electron gases in GaAs/AlGaAs systems. After taking into account the Dingle factor correction, we find that the position of the HIRO peak is well described by the hole effective mass $m^\star \approx 0.09\,m_0$, obtained from microwave photoresistance in the same sample.

We report on a colossal negative magnetoresistance (MR) in a GaAs/AlGaAs quantum well which, at low temperatures, is manifested by a drop of the resistivity by more than an order of magnitude at a magnetic field $B\ensuremath{\approx}1$ kG. In contrast to MR effects discussed earlier, the MR reported here is not parabolic, even at small $B$, and persists to much higher in-plane magnetic fields and temperatures. Remarkably, the temperature dependence of the resistivity at $B\ensuremath{\approx}1$ kG is linear over the entire temperature range studied (from 1 to 30 K) and appears to coincide with the high-temperature limit of the zero-field resistivity, hinting on the important role of acoustic phonons.

Microwave-induced resistance oscillations (MIRO) have been extensively studied for more than a decade but, until now, have remained unique to GaAs/AlGaAs-based 2D electron systems. Here, we report on the observation of MIRO in a 2D hole gas hosted in Ge/SiGe quantum well. Our findings confirm that MIRO is a universal phenomenon and demonstrate that microwave photoresistance can be utilized to probe the energy spectrum and the correlation effects of 2D holes in Ge/SiGe quantum wells.

We have studied microwave photoresistance in a two-dimensional electron system subject to two radiation fields (frequencies $\omega_1$ and $\omega_2$) using quantum kinetic equation. We have found that when $\omega_2/\omega_1= 1 + 2/N$, where $N$ is an integer, and both waves have the same polarization, the displacement mechanism gives rise to a new, phase-sensitive photoresistance. This photoresistance oscillates with the magnetic field and can be a good fraction of the total photoresistance under typical experimental conditions. The inelastic mechanism, on the other hand, gives zero phase-sensitive photoresistance if the radiation fields are circularly polarized., Comment: 5 pages, 3 figures

In this paper, the status of application system and data integration requirement in power enterprises are discussed firstly, followed by the analysis of several issues in the use of power grid data. To solve these issues, we present a model of power grid data integration and sharing platform utilizing multi-agent based software bus technology, data adapter technology and JMS(Java Message Service) in the light of a unified power enterprise data planning. Besides, we construct the TCA-oriented agent model according to the idea of TCA(Task Control Architecture) and put forward the agent coordination and communication mode combined with KQML (Knowledge Query Manipulation Language) and SOAP (Simple Object Access Protocol), which provide a solution for the power enterprises to integrate and share their information in a unified platform.

We report on nonlinear magnetotransport in a two-dimensional electron gas hosted in a MgZnO/ZnO heterostructure. Upon application of a direct current, we observe pronounced Hall field--induced resistance oscillations (HIRO), which are well known from experiments on high-mobility GaAs/AlGaAs quantum wells. The unique sensitivity of HIRO to the short-range component of the disorder potential allows us to unambiguously establish that the mobility of our MgZnO/ZnO heterostructure is limited by impurities residing within or near the two-dimensional (2D) channel. This demonstration that HIRO can be realized in a system with a much lower mobility, much higher density, and much larger effective mass than in previously studied systems highlights the remarkable universality of the phenomenon and its great promise for use in studies of a wide variety of emerging 2D materials.