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1.
Nat Commun ; 13(1): 3637, 2022 Jun 25.
Artículo en Inglés | MEDLINE | ID: mdl-35752620

RESUMEN

The relaxation time of a single-electron spin is an important parameter for solid-state spin qubits, as it directly limits the lifetime of the encoded information. Thanks to the low spin-orbit interaction and low hyperfine coupling, graphene and bilayer graphene (BLG) have long been considered promising platforms for spin qubits. Only recently, it has become possible to control single-electrons in BLG quantum dots (QDs) and to understand their spin-valley texture, while the relaxation dynamics have remained mostly unexplored. Here, we report spin relaxation times (T1) of single-electron states in BLG QDs. Using pulsed-gate spectroscopy, we extract relaxation times exceeding 200 µs at a magnetic field of 1.9 T. The T1 values show a strong dependence on the spin splitting, promising even longer T1 at lower magnetic fields, where our measurements are limited by the signal-to-noise ratio. The relaxation times are more than two orders of magnitude larger than those previously reported for carbon-based QDs, suggesting that graphene is a potentially promising host material for scalable spin qubits.

2.
Nanoscale ; 14(22): 8085-8095, 2022 Jun 09.
Artículo en Inglés | MEDLINE | ID: mdl-35611659

RESUMEN

van der Waals crystals have opened a new and exciting chapter in heterostructure research, removing the lattice matching constraint characteristics of epitaxial semiconductors. They provide unprecedented flexibility for heterostructure design. Combining two-dimensional (2D) perovskites with other 2D materials, in particular transition metal dichalcogenides (TMDs), has recently emerged as an intriguing way to design hybrid opto-electronic devices. However, the excitation transfer mechanism between the layers (charge or energy transfer) remains to be elucidated. Here, we investigate PEA2PbI4/MoSe2 and (BA)2PbI4/MoSe2 heterostructures by combining optical spectroscopy and density functional theory (DFT) calculations. We show that band alignment facilitates charge transfer. Namely, holes are transferred from TMDs to 2D perovskites, while the electron transfer is blocked, resulting in the formation of interlayer excitons. Moreover, we show that the energy transfer mechanism can be turned on by an appropriate alignment of the excitonic states, providing a rule of thumb for the deterministic control of the excitation transfer mechanism in TMD/2D-perovskite heterostructures.

3.
Science ; 376(6589): 193-199, 2022 04 08.
Artículo en Inglés | MEDLINE | ID: mdl-35389784

RESUMEN

Magic-angle twisted trilayer graphene (TTG) has recently emerged as a platform to engineer strongly correlated flat bands. We reveal the normal-state structural and electronic properties of TTG using low-temperature scanning tunneling microscopy at twist angles for which superconductivity has been observed. Real trilayer samples undergo a strong reconstruction of the moiré lattice, which locks layers into near-magic-angle, mirror symmetric domains comparable in size with the superconducting coherence length. This relaxation introduces an array of localized twist-angle faults, termed twistons and moiré solitons, whose electronic structure deviates strongly from the background regions, leading to a doping-dependent, spatially granular electronic landscape. The Fermi-level density of states is maximally uniform at dopings for which superconductivity has been observed in transport measurements.

4.
Phys Rev Lett ; 128(12): 127402, 2022 Mar 25.
Artículo en Inglés | MEDLINE | ID: mdl-35394309

RESUMEN

Nonequilibrium dynamics of strongly correlated systems constitutes a fascinating problem of condensed matter physics with many open questions. Here, we investigate the relaxation dynamics of Landau-quantized electron system into spin-valley polarized ground state in a gate-tunable MoSe_{2} monolayer subjected to a strong magnetic field. The system is driven out of equilibrium with optically injected excitons that depolarize the electron spins and the subsequent electron spin-valley relaxation is probed in time-resolved experiments. We demonstrate that both the relaxation and light-induced depolarization rates at millikelvin temperatures sensitively depend on the Landau level filling factor: the relaxation is enhanced whenever the electrons form an integer quantum Hall liquid and slows down appreciably at noninteger fillings, while the depolarization rate exhibits an opposite behavior. Our findings suggest that spin-valley dynamics may be used as a tool to investigate the interplay between the effects of disorder and strong interactions in the electronic ground state.

5.
Phys Rev Lett ; 128(8): 085001, 2022 Feb 25.
Artículo en Inglés | MEDLINE | ID: mdl-35275668

RESUMEN

A new type of self-sustained divertor oscillation is discovered in the Large Helical Device stellarator, where the peripheral plasma is detached from material diverters by means of externally applied perturbation fields. The divertor oscillation is found to be a self-regulation of an isolated magnetic field structure (the magnetic island) width induced by a drastic change in a poloidal inhomogeneity of the plasma radiation across the detachment-attachment transitions. A predator-prey model between the magnetic island width and a self-generated local plasma current (the bootstrap current) is introduced to describe the divertor oscillation, which successfully reproduces the experimental observations.

7.
Osteoporos Int ; 33(6): 1275-1284, 2022 Jun.
Artículo en Inglés | MEDLINE | ID: mdl-35091788

RESUMEN

The baseline sagittal vertical axis (SVA) and pelvic tilt (PT) are independent risk factors of osteoporosis-related fractures in women with osteoporosis. We clarified the SVA and PT to predict the incidence of osteoporosis-related fractures. PURPOSE: Sagittal alignment with osteoporosis women deteriorates with advancing age and sagittal alignment may indicate osteoporosis-related fractures in the future. However, whether the sagittal alignment predicts future osteoporosis-related fracture in patients with osteoporosis has not been clarified. We aimed to investigate the association between sagittal alignment and future osteoporosis-related fractures. METHODS: This was a retrospective cohort study. Of the 313 participants (mean follow-up period, 2.9 years), 236 were included in the analysis. At baseline, we measured bone mineral density (BMD) of the lumbar spine and the femoral neck, sagittal vertical axis (SVA), thoracic kyphosis, pelvic incidence minus lumbar lordosis, sacral slope, pelvic tilt (PT), geriatric locomotive function scale (GLFS), two-step value, and stand-up test. The information on medications and the duration of treatment were reviewed from the medical records. Additionally, participants reported their history of falls at baseline. Multiple logistic regression analysis was used to determine the association of future osteoporosis-related fracture, and adjusted Odds ratios (OR) and 95% confidence interval (CI) were calculated with all predictors as covariates. All continuous variables were calculated using standardized OR (sOR). RESULTS: Osteoporosis-related fractures occurred in 33 of 313 participants (10.5%). Multiple logistic regression analysis showed that a history of falls (OR =4.092, 95% CI: 1.029-16.265, p =0.045), SVA (sOR =4.228, 95% CI: 2.118-8.439, p <0.001), and PT (sOR =2.497, 95% CI: 1.087-5.733, p =0.031) were independent risk factors for future osteoporosis-related fractures. CONCLUSIONS: This study revealed the SVA and PT to predict osteoporosis-related fractures. TRIAL REGISTRATION NUMBER AND DATE OF REGISTRATION: UMIN000036516 (April 1, 2019).


Asunto(s)
Cifosis , Lordosis , Osteoporosis , Fracturas Osteoporóticas , Anciano , Femenino , Humanos , Cifosis/etiología , Vértebras Lumbares/diagnóstico por imagen , Osteoporosis/complicaciones , Osteoporosis/epidemiología , Fracturas Osteoporóticas/complicaciones , Fracturas Osteoporóticas/etiología , Pacientes Ambulatorios , Estudios Retrospectivos
8.
Science ; 375(6579): 430-433, 2022 01 28.
Artículo en Inglés | MEDLINE | ID: mdl-35084955

RESUMEN

In thermodynamic equilibrium, current in metallic systems is carried by electronic states near the Fermi energy, whereas the filled bands underneath contribute little to conduction. Here, we describe a very different regime in which carrier distribution in graphene and its superlattices is shifted so far from equilibrium that the filled bands start playing an essential role, leading to a critical-current behavior. The criticalities develop upon the velocity of electron flow reaching the Fermi velocity. Key signatures of the out-of-equilibrium state are current-voltage characteristics that resemble those of superconductors, sharp peaks in differential resistance, sign reversal of the Hall effect, and a marked anomaly caused by the Schwinger-like production of hot electron-hole plasma. The observed behavior is expected to be common to all graphene-based superlattices.

9.
Nat Commun ; 13(1): 213, 2022 Jan 11.
Artículo en Inglés | MEDLINE | ID: mdl-35017473

RESUMEN

The presence of "upstream" modes, moving against the direction of charge current flow in the fractional quantum Hall (FQH) phases, is critical for the emergence of renormalized modes with exotic quantum statistics. Detection of excess noise at the edge is a smoking gun for the presence of upstream modes. Here, we report noise measurements at the edges of FQH states realized in dual graphite-gated bilayer graphene devices. A noiseless dc current is injected at one of the edge contacts, and the noise generated at contacts at length, L = 4 µm and 10 µm away along the upstream direction is studied. For integer and particle-like FQH states, no detectable noise is measured. By contrast, for "hole-conjugate" FQH states, we detect a strong noise proportional to the injected current, unambiguously proving the existence of upstream modes. The noise magnitude remains independent of length, which matches our theoretical analysis demonstrating the ballistic nature of upstream energy transport, quite distinct from the diffusive propagation reported earlier in GaAs-based systems.

10.
Science ; 375(6579): 437-441, 2022 Jan 28.
Artículo en Inglés | MEDLINE | ID: mdl-34990215

RESUMEN

Strong electron correlation and spin-orbit coupling (SOC) can have a profound influence on the electronic properties of materials. We examine their combined influence on a 2-dimensional electronic system at the atomic interface between magic-angle twisted bilayer graphene and a tungsten diselenide crystal. Strong electron correlation within the moiré flatband stabilizes correlated insulating states at both quarter and half filling, and SOC transforms these Mott-like insulators into ferromagnets, evidenced by robust anomalous Hall effect with hysteretic switching behavior. The coupling between spin and valley degrees of freedom is demonstrated through the control of the magnetic order with an in-plane magnetic field, or a perpendicular electric field. Our findings establish an experimental knob to engineer topological properties of moiré bands in twisted bilayer graphene and related systems.

11.
Science ; 374(6573): 1399-1402, 2021 Dec 10.
Artículo en Inglés | MEDLINE | ID: mdl-34882473

RESUMEN

The electronic properties of graphene have been intensively investigated over the past decade. However, the singular orbital magnetism of undoped graphene, a fundamental signature of the characteristic Berry phase of graphene's electronic wave functions, has been challenging to measure in a single flake. Using a highly sensitive giant magnetoresistance (GMR) sensor, we have measured the gate voltage­dependent magnetization of a single graphene monolayer encapsulated between boron nitride crystals. The signal exhibits a diamagnetic peak at the Dirac point whose magnetic field and temperature dependences agree with long-standing theoretical predictions. Our measurements offer a means to monitor Berry phase singularities and explore correlated states generated by the combined effects of Coulomb interactions, strain, or moiré potentials.

12.
Nat Commun ; 12(1): 5741, 2021 Sep 30.
Artículo en Inglés | MEDLINE | ID: mdl-34593793

RESUMEN

Twisted two-dimensional van der Waals (vdW) heterostructures have unlocked a new means for manipulating the properties of quantum materials. The resulting mesoscopic moiré superlattices are accessible to a wide variety of scanning probes. To date, spatially-resolved techniques have prioritized electronic structure visualization, with lattice response experiments only in their infancy. Here, we therefore investigate lattice dynamics in twisted layers of hexagonal boron nitride (hBN), formed by a minute twist angle between two hBN monolayers assembled on a graphite substrate. Nano-infrared (nano-IR) spectroscopy reveals systematic variations of the in-plane optical phonon frequencies amongst the triangular domains and domain walls in the hBN moiré superlattices. Our first-principles calculations unveil a local and stacking-dependent interaction with the underlying graphite, prompting symmetry-breaking between the otherwise identical neighboring moiré domains of twisted hBN.

13.
Nat Commun ; 12(1): 5250, 2021 Sep 02.
Artículo en Inglés | MEDLINE | ID: mdl-34475394

RESUMEN

Understanding how the electron spin is coupled to orbital degrees of freedom, such as a valley degree of freedom in solid-state systems, is central to applications in spin-based electronics and quantum computation. Recent developments in the preparation of electrostatically-confined quantum dots in gapped bilayer graphene (BLG) enable to study the low-energy single-electron spectra in BLG quantum dots, which is crucial for potential spin and spin-valley qubit operations. Here, we present the observation of the spin-valley coupling in bilayer graphene quantum dots in the single-electron regime. By making use of highly-tunable double quantum dot devices we achieve an energy resolution allowing us to resolve the lifting of the fourfold spin and valley degeneracy by a Kane-Mele type spin-orbit coupling of ≈ 60 µeV. Furthermore, we find an upper limit of a potentially disorder-induced mixing of the [Formula: see text] and [Formula: see text] states below 20 µeV.

14.
Nat Commun ; 12(1): 4933, 2021 Aug 16.
Artículo en Inglés | MEDLINE | ID: mdl-34400620

RESUMEN

Engineering non-linear hybrid light-matter states in tailored lattices is a central research strategy for the simulation of complex Hamiltonians. Excitons in atomically thin crystals are an ideal active medium for such purposes, since they couple strongly with light and bear the potential to harness giant non-linearities and interactions while presenting a simple sample-processing and room temperature operability. We demonstrate lattice polaritons, based on an open, high-quality optical cavity, with an imprinted photonic lattice strongly coupled to excitons in a WS2 monolayer. We experimentally observe the emergence of the canonical band-structure of particles in a one-dimensional lattice at room temperature, and demonstrate frequency reconfigurability over a spectral window exceeding 85 meV, as well as the systematic variation of the nearest-neighbour coupling, reflected by a tunability in the bandwidth of the p-band polaritons by 7 meV. The technology presented in this work is a critical demonstration towards reconfigurable photonic emulators operated with non-linear photonic fluids, offering a simple experimental implementation and working at ambient conditions.

15.
Phys Rev Lett ; 127(5): 056802, 2021 Jul 30.
Artículo en Inglés | MEDLINE | ID: mdl-34397232

RESUMEN

Twisted bilayer graphene (TBG) provides an example of a system in which the interplay of interlayer interactions and superlattice structure impacts electron transport in a variety of nontrivial ways and gives rise to a plethora of interesting effects. Understanding the mechanisms of electron scattering in TBG has, however, proven challenging, raising many questions about the origins of resistivity in this system. Here we show that TBG exhibits high-temperature magneto-oscillations originating from the scattering of charge carriers between TBG minivalleys. The amplitude of these oscillations reveals that interminivalley scattering is strong, and its characteristic timescale is comparable to that of its intraminivalley counterpart. Furthermore, by exploring the temperature dependence of these oscillations, we estimate the electron-electron collision rate in TBG and find that it exceeds that of monolayer graphene. Our study demonstrates the consequences of the relatively small size of the superlattice Brillouin zone and Fermi velocity reduction on lateral transport in TBG.

16.
Sci Rep ; 11(1): 17037, 2021 Aug 23.
Artículo en Inglés | MEDLINE | ID: mdl-34426607

RESUMEN

Two-dimensional layered materials offer the possibility to create artificial vertically stacked structures possessing an additional degree of freedom-the interlayer twist. We present a comprehensive optical study of artificially stacked bilayers (BLs) MoS[Formula: see text] encapsulated in hexagonal BN with interlayer twist angle ranging from 0[Formula: see text] to 60[Formula: see text] using Raman scattering and photoluminescence spectroscopies. It is found that the strength of the interlayer coupling in the studied BLs can be estimated using the energy dependence of indirect emission versus the A[Formula: see text]-E[Formula: see text] energy separation. Due to the hybridization of electronic states in the valence band, the emission line related to the interlayer exciton is apparent in both the natural (2H) and artificial (62[Formula: see text]) MoS[Formula: see text] BLs, while it is absent in the structures with other twist angles. The interlayer coupling energy is estimated to be of about 50 meV. The effect of temperature on energies and intensities of the direct and indirect emission lines in MoS[Formula: see text] BLs is also quantified.

17.
Nat Commun ; 12(1): 4265, 2021 Jul 12.
Artículo en Inglés | MEDLINE | ID: mdl-34253725

RESUMEN

The quantum Hall effect is the seminal example of topological protection, as charge carriers are transmitted through one-dimensional edge channels where backscattering is prohibited. Graphene has made its marks as an exceptional platform to reveal new facets of this remarkable property. However, in conventional Hall bar geometries, topological protection of graphene edge channels is found regrettably less robust than in high mobility semi-conductors. Here, we explore graphene quantum Hall regime at the local scale, using a scanning gate microscope. We reveal the detrimental influence of antidots along the graphene edges, mediating backscattering towards upstream edge channels, hence triggering topological breakdown. Combined with simulations, our experimental results provide further insights into graphene quantum Hall channels vulnerability. In turn, this may ease future developments towards precise manipulation of topologically protected edge channels hosted in various types of two-dimensional crystals.

18.
Rev Sci Instrum ; 92(4): 043536, 2021 Apr 01.
Artículo en Inglés | MEDLINE | ID: mdl-34243406

RESUMEN

A 90 GHz W-band millimeter-wave back-scattering system is designed and installed for measuring electron scale turbulence (k⊥ρs ∼ 40). A metal lens relay antenna is used for in-vessel beam focusing, and a beam diameter of less than 40 mm is achieved in the plasma core region. This antenna can be steered at an angle of 159° ± 6°, which almost covers the plasma radius. The estimated size of the scattering volume is ∼105 mm at the edge and 135 mm at the core, respectively. A 60 m corrugated waveguide is used to achieve a low transmission loss of ∼8 dB. A heterodyne detection system for millimeter-wave circuits with probing power modulation can distinguish the scattered signal from background noise.

19.
Nat Commun ; 12(1): 3733, 2021 Jun 18.
Artículo en Inglés | MEDLINE | ID: mdl-34145226

RESUMEN

Graphene integrated photonics provides several advantages over conventional Si photonics. Single layer graphene (SLG) enables fast, broadband, and energy-efficient electro-optic modulators, optical switches and photodetectors (GPDs), and is compatible with any optical waveguide. The last major barrier to SLG-based optical receivers lies in the current GPDs' low responsivity when compared to conventional PDs. Here we overcome this by integrating a photo-thermoelectric GPD with a Si microring resonator. Under critical coupling, we achieve >90% light absorption in a ~6 µm SLG channel along a Si waveguide. Cavity-enhanced light-matter interactions cause carriers in SLG to reach ~400 K for an input power ~0.6 mW, resulting in a voltage responsivity ~90 V/W, with a receiver sensitivity enabling our GPDs to operate at a 10-9 bit-error rate, on par with mature semiconductor technology, but with a natural generation of a voltage, rather than a current, thus removing the need for transimpedance amplification, with a reduction of energy-per-bit, cost, and foot-print.

20.
Nat Mater ; 20(7): 1037, 2021 Jul.
Artículo en Inglés | MEDLINE | ID: mdl-34075204

RESUMEN

A Correction to this paper has been published: https://doi.org/10.1038/s41563-021-00997-2.

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