Spin-Orbit-Enhanced Robustness of Supercurrent in Graphene/WS_{2} Josephson Junctions

We demonstrate the enhanced robustness of the supercurrent through graphene-based Josephson junctions in which strong spin-orbit interactions (SOIs) are induced. We compare the persistence of a supercurrent at high out-of-plane magnetic fields between Josephson junctions with graphene on hexagonal boron-nitride and graphene on WS_{2}, where strong SOIs are induced via the proximity effect. We find that in the shortest junctions both systems display signatures of induced superconductivity, characterized by a suppressed differential resistance at a low current, in magnetic fields up to 1 T. In longer junctions, however, only graphene on WS_{2} exhibits induced superconductivity features in such high magnetic fields, and they even persist up to 7 T. We argue that these robust superconducting signatures arise from quasiballistic edge states stabilized by the strong SOIs induced in graphene by WS_{2}.

Misorientation-Controlled Cross-Plane Thermoelectricity in Twisted Bilayer Graphene

The introduction of "twist" or relative rotation between two atomically thin van der Waals membranes gives rise to periodic moiré potential, leading to a substantial alteration of the band structure of the planar assembly. While most of the recent experiments primarily focus on the electronic-band hybridization by probing in-plane transport properties, here we report out-of-plane thermoelectric measurements across the van der Waals gap in twisted bilayer graphene, which exhibits an interplay of twist-dependent interlayer electronic and phononic hybridization. We show that at large twist angles, the thermopower is entirely driven by a novel phonon-drag effect at subnanometer scale, while the electronic component of the thermopower is recovered only when the misorientation between the layers is reduced to <6°. Our experiment shows that cross-plane thermoelectricity at low angles is exceptionally sensitive to the nature of band dispersion and may provide fundamental insights into the coherence of electronic states in twisted bilayer graphene.

Interplay between spin proximity effect and charge-dependent exciton dynamics in MoSe2/CrBr3 van der Waals heterostructures

Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moiré periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe2 and CrBr3 in photoluminescence, whereby the valley polarization of the MoSe2 trion state conforms closely to the local CrBr3 magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunneling, while also highlighting MoSe2 as a promising candidate to optically interface with exotic spin textures in van der Waals structures.

Long-range ballistic transport of Brown-Zak fermions in graphene superlattices

In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 10⁶ cm² V⁻¹ s⁻¹ and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1 K. We also found negative bend resistance at 1/q fractions for electrical probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field.

Here, the authors show that Brown-Zak fermions in graphene-on-boron-nitride superlattices exhibit mobilities above 10⁶ cm²/V s and micrometer scale ballistic transport.

Impact of tapered-shaped left ventricular outflow tract on permanent pacemaker implantation after the third-generation balloon-expandable valve implantation

Background

In the era of transcatheter aortic valve implantation (TAVI) for patients with lower surgical risk, conduction disturbances requiring permanent pacemaker implantation (PPI) after TAVI remain a serious concern. The association between tapered-shaped left ventricular outflow tract (LVOT) and PPI after TAVI has not been elucidated.

Purposes

This study sought to identify predictors for PPI after TAVI with the third-generation balloon-expandable valve, with focus on LVOT morphology.

Methods

Of 272 consecutive patients treated with the third-generation balloon-expandable valve, 256 patients without previous PPI or bicuspid valve were retrospectively analyzed.

Results

PPI was implanted after TAVI in 20 (7.8%) patients. Patients requiring PPI had smaller LVOT area (356.3 mm2 vs. 399.4 mm2, p=0.011). Moreover, receiver-operating characteristic (ROC) statistics showed that LVOT area /annulus area possessed significantly higher predictive ability than LVOT area (area under the curve: 0.91 [95% confidence interval [CI]: 0.84 to 0.95] vs. 0.67 [95% CI: 0.57 to 0.77], p&lt;0.001). Multivariable analysis revealed LVOT area /annulus area (odds ratio [OR]: 1.93 [95% CI: 1.38–2.71]; p&lt;0.001 per % of decreasing), the difference between membranous septum length and implantation depth (ΔMSID) (OR: 6.82 [95% CI 2.39–19.48]; p&lt;0.001 per mm of decreasing) and pre-existing complete right bundle branch block (CRBBB) (OR: 32.38 [95% CI 2.30–455.63]; p=0.002) as independent predictors of PPI. Further analysis using ROC statistics revealed LVOT area / annulus area of 88.5% and ΔMSID of 1.8 mm as the optimal cutoff points for prediction of PPI after the third-generation balloon-expandable valve implantation, with high negative predictive values of 98.1% and 99.0%, respectively. Figure shows the PPI rates stratified by the number of following predictors: LVOT area /annulus area &lt;88.5%, ΔMSID &lt;1.8 mm and pre-existing CRBBB. Patients with 2 or more predictors had significantly higher PPI rates than those with 1 or less predictor (67% [18 of 27 patients] vs. 1% [2 of 229 patients], p&lt;0.001).

Conclusions

LVOT area /annulus area, ΔMSID and pre-existing CRBBB were identified as powerful independent predictors for PPI after TAVI. Higher valve implantation is important to prevent excessive PPI especially for patients with pre-procedural tapered-shaped LVOT, short membranous septum or pre-existing CRBBB.

PPI rates stratified by predictors

Funding Acknowledgement

Type of funding source: None

Electron-Hole Crossover in Gate-Controlled Bilayer Graphene Quantum Dots

Electron and hole Bloch states in bilayer graphene exhibit topological orbital magnetic moments with opposite signs, which allows for tunable valley-polarization in an out-of-plane magnetic field. This property makes electron and hole quantum dots (QDs) in bilayer graphene interesting for valley and spin-valley qubits. Here, we show measurements of the electron-hole crossover in a bilayer graphene QD, demonstrating opposite signs of the magnetic moments associated with the Berry curvature. Using three layers of top gates, we independently control the tunneling barriers while tuning the occupation from the few-hole regime to the few-electron regime, crossing the displacement-field-controlled band gap. The band gap is around 25 meV, while the charging energies of the electron and hole dots are between 3 and 5 meV. The extracted valley g-factor is around 17 and leads to opposite valley polarization for electrons and holes at moderate B-fields. Our measurements agree well with tight-binding calculations for our device.

Neutral and charged dark excitons in monolayer WS2

Low temperature and polarization resolved magneto-photoluminescence experiments are used to investigate the properties of dark excitons and dark trions in a monolayer of WS2 encapsulated in hexagonal BN (hBN). We find that this system is an n-type doped semiconductor and that dark trions dominate the emission spectrum. In line with previous studies on WSe2, we identify the Coulomb exchange interaction coupled neutral dark and grey excitons through their polarization properties, while an analogous effect is not observed for dark trions. Applying the magnetic field in both perpendicular and parallel configurations with respect to the monolayer plane, we determine the g-factor of dark trions to be g ∼ -8.6. Their decay rate is close to 0.5 ns, more than 2 orders of magnitude longer than that of bright excitons.

Highly energy-tunable quantum light from moiré-trapped excitons

Photon antibunching, a hallmark of quantum light, has been observed in the correlations of light from isolated atomic and atomic-like solid-state systems. Two-dimensional semiconductor heterostructures offer a unique method to create a quantum light source: Moiré trapping potentials for excitons are predicted to create arrays of quantum emitters. While signatures of moiré-trapped excitons have been observed, their quantum nature has yet to be confirmed. Here, we report photon antibunching from single moiré-trapped interlayer excitons in a heterobilayer. Via magneto-optical spectroscopy, we demonstrate that the discrete anharmonic spectra arise from bound band-edge electron-hole pairs trapped in moiré potentials. Last, we exploit the large permanent dipole of interlayer excitons to achieve large direct current (DC) Stark tuning up to 40 meV. Our results confirm the quantum nature of moiré-confined excitons and open opportunities to investigate their inhomogeneity and interactions between the emitters or energetically tune single emitters into resonance with cavity modes.

Prediction of Pelvic Inclination in the Sitting Position after Corrective Surgery for Adult Spinal Deformity

Introduction

Hip dislocation rates in patients with combined total hip arthroplasty (THA) and spinal deformity fixation are significantly higher than those of THA alone. Nevertheless, there are no treatment recommendations for patients who undergo THA and require a spine deformity correction later.

Methods

Twenty-eight patients underwent spinal fixation surgery for adult spinal deformity. Sagittal spinopelvic alignment was analyzed on lateral radiographs taken preoperatively and postoperatively in the sitting and standing positions. Univariate linear regression analysis was conducted to identify the factors affecting the pelvic inclination in the sitting position after spinal fixation. Multiple regression analysis was conducted to determine the most efficient combination of radiographic parameters for predicting postoperative pelvic inclination while sitting.

Results

There were significantly weak associations between postoperative sacral slope (SS) in the sitting position and the following factors: the number of vertebral levels fused (β = 0.30, p = 0.003); the presence of sacral fixation (β = 0.22, p = 0.01); the presence of sacroiliac joint fixation (β = 0.24, p = 0.008); and preoperative SS while standing and sitting (β = 0.21, p = 0.01 and β = 0.34, p = 0.001). Postoperative lumbar lordosis (LL) while standing was strongly associated with postoperative SS in the sitting position (β = 0.67, p <.0001). The combination of postoperative LL in the standing position and preoperative SS in the sitting position was the best fit, and the adjusted R-squared was 0.82.

Conclusions

We devised a prediction formula for pelvic inclination while sitting after spinal fixation that has high predictability: postoperative SS while sitting = 11.7+ (0.4 × postoperative planned LL while standing) + (0.16 × preoperative SS while sitting). This study could be the basis for treatment recommendations for patients who have undergone THA and require a spine deformity correction later.

Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers

Excitons with binding energies of a few hundreds of meV control the optical properties of transition metal dichalcogenide monolayers. Knowledge of the fine structure of these excitons is therefore essential to understand the optoelectronic properties of these 2D materials. Here we measure the exciton fine structure of MoS2 and MoSe2 monolayers encapsulated in boron nitride by magneto-photoluminescence spectroscopy in magnetic fields up to 30 T. The experiments performed in transverse magnetic field reveal a brightening of the spin-forbidden dark excitons in MoS2 monolayer: we find that the dark excitons appear at 14 meV below the bright ones. Measurements performed in tilted magnetic field provide a conceivable description of the neutral exciton fine structure. The experimental results are in agreement with a model taking into account the effect of the exchange interaction on both the bright and dark exciton states as well as the interaction with the magnetic field.

Demonstration of a polariton step potential by local variation of light-matter coupling in a van-der-Waals heterostructure

The large oscillator strength of excitons in transition metal dichalcogenide layers facilitates the formation of exciton-polariton resonances for monolayers and van-der-Waals heterostructures embedded in optical microcavities. Here, we show, that locally changing the number of layers in a WSe₂/hBN/WSe₂ van-der-Waals heterostructure embedded in a monolithic, high-quality-factor cavity gives rise to a local variation of the coupling strength. This effect yields a polaritonic stair case potential, which we demonstrate at room temperature. Our result paves the way towards engineering local polaritonic potentials at length scales down to atomically sharp interfaces, based on purely modifying its real part contribution via the coherent light-matter coupling strength g.

Cascade of phase transitions and Dirac revivals in magic-angle graphene

Twisted bilayer graphene near the magic angle^1-4 exhibits rich electron-correlation physics, displaying insulating^3-6, magnetic^7,8 and superconducting phases^4-6. The electronic bands of this system were predicted^1,2 to narrow markedly^9,10 near the magic angle, leading to a variety of possible symmetry-breaking ground states^11-17. Here, using measurements of the local electronic compressibility, we show that these correlated phases originate from a high-energy state with an unusual sequence of band population. As carriers are added to the system, the four electronic 'flavours', which correspond to the spin and valley degrees of freedom, are not filled equally. Rather, they are populated through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moiré lattice. At each transition, a single spin/valley flavour takes all the carriers from its partially filled peers, 'resetting' them to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breaking. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state that we report here, with its strongly broken electronic flavour symmetry and revived Dirac-like electronic character, is important in the physics of magic-angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.

Observation of the Spin-Orbit Gap in Bilayer Graphene by One-Dimensional Ballistic Transport

We report on measurements of quantized conductance in gate-defined quantum point contacts in bilayer graphene that allow the observation of subband splittings due to spin-orbit coupling. The size of this splitting can be tuned from 40 to 80  μeV by the displacement field. We assign this gate-tunable subband splitting to a gap induced by spin-orbit coupling of Kane-Mele type, enhanced by proximity effects due to the substrate. We show that this spin-orbit coupling gives rise to a complex pattern in low perpendicular magnetic fields, increasing the Zeeman splitting in one valley and suppressing it in the other one. In addition, we observe a spin polarized channel of 6e^{2}/h at high in-plane magnetic field and signatures of interaction effects at the crossings of spin-split subbands of opposite spins at finite magnetic field.

Minibands in twisted bilayer graphene probed by magnetic focusing

Magnetic fields force ballistic electrons injected from a narrow contact to move along skipping orbits and form caustics. This leads to pronounced resistance peaks at nearby voltage probes as electrons are effectively focused inside them, a phenomenon known as magnetic focusing. This can be used not only for the demonstration of ballistic transport but also to study the electronic structure of metals. Here, we use magnetic focusing to probe narrowbands in graphene bilayers twisted at ~2°. Their minibands are found to support long-range ballistic transport limited at low temperatures by intrinsic electron-electron scattering. A voltage bias between the layers causes strong minivalley splitting and allows selective focusing for different minivalleys, which is of interest for using this degree of freedom in frequently discussed valleytronics.

Localized Nanoresonator Mode in Plasmonic Microcavities

Submicron-thick hexagonal boron nitride crystals embedded in noble metals form planar Fabry-Perot half-microcavities. Depositing Au nanoparticles on top of these microcavities forms previously unidentified angle- and polarization-sensitive nanoresonator modes that are tightly laterally confined by the nanoparticle. Comparing dark-field scattering with reflection spectroscopies shows plasmonic and Fabry-Perot-like enhancements magnify subtle interference contributions, which lead to unexpected redshifts in the dark-field spectra, explained by the presence of these new modes.

Ultra-long carrier lifetime in neutral graphene-hBN van der Waals heterostructures under mid-infrared illumination

Graphene/hBN heterostructures are promising active materials for devices in the THz domain, such as emitters and photodetectors based on interband transitions. Their performance requires long carrier lifetimes. However, carrier recombination processes in graphene possess sub-picosecond characteristic times for large non-equilibrium carrier densities at high energy. An additional channel has been recently demonstrated in graphene/hBN heterostructures by emission of hBN hyperbolic phonon polaritons (HPhP) with picosecond decay time. Here, we report on carrier lifetimes in graphene/hBN Zener-Klein transistors of ~30 ps for photoexcited carriers at low density and energy, using mid-infrared photoconductivity measurements. We further demonstrate the switching of carrier lifetime from ~30 ps (attributed to interband Auger) down to a few picoseconds upon ignition of HPhP relaxation at finite bias and/or with infrared excitation power. Our study opens interesting perspectives to exploit graphene/hBN heterostructures for THz lasing and highly sensitive THz photodetection as well as for phonon polariton optics.

Long carrier lifetimes are beneficial for graphene-based optoelectronics, but carrier recombination processes in graphene possess sub-picosecond characteristic times. Here, the authors report carrier lifetimes ~30 ps at low energy in graphene/hBN Zener-Klein transistors, attributed to interband Auger processes.

Intrinsic quantized anomalous Hall effect in a moiré heterostructure

The quantum anomalous Hall (QAH) effect combines topology and magnetism to produce precisely quantized Hall resistance at zero magnetic field. We report the observation of a QAH effect in twisted bilayer graphene aligned to hexagonal boron nitride. The effect is driven by intrinsic strong interactions, which polarize the electrons into a single spin- and valley-resolved moiré miniband with Chern number C = 1. In contrast to magnetically doped systems, the measured transport energy gap is larger than the Curie temperature for magnetic ordering, and quantization to within 0.1% of the von Klitzing constant persists to temperatures of several kelvin at zero magnetic field. Electrical currents as small as 1 nanoampere controllably switch the magnetic order between states of opposite polarization, forming an electrically rewritable magnetic memory.

Composite super-moiré lattices in double-aligned graphene heterostructures

When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other's properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moiré super cell.

Tunable crystal symmetry in graphene-boron nitride heterostructures with coexisting moiré superlattices

In van der Waals (vdW) heterostructures consisting of atomically thin crystals layered on top of one another, lattice mismatch and rotation between the layers can result in long-wavelength moiré superlattices. These moiré patterns can drive notable band structure reconstruction of the composite material, leading to a wide range of emergent phenomena including superconductivity^1-3, magnetism⁴, fractional Chern insulating states⁵ and moiré excitons^6-9. Here, we investigate devices consisting of monolayer graphene encapsulated between two crystals of boron nitride (BN), in which the rotational alignment of all three components is controlled. We find that bandgaps in the graphene arising from perfect rotational alignment with both BN layers can be modified considerably depending on whether the relative orientation of the two BN layers is 0° or 60°, suggesting a tunable transition between the absence or presence of inversion symmetry in the heterostructure. Small deviations (<1°) from perfect alignment of all three layers leads to coexisting long-wavelength moiré potentials, resulting in a highly reconstructed graphene band structure featuring multiple secondary Dirac points. Our results demonstrate that the interplay between multiple moiré patterns can be utilized to controllably modify the symmetry and electronic properties of the composite heterostructure.

Bacterial co-infection and early mortality among pulmonary tuberculosis patients in Manila, The Philippines

OBJECTIVE

To investigate the prevalence of bacterial co-infection and its effect on early mortality among hospitalised human immunodeficiency virus (HIV) negative pulmonary tuberculosis (PTB) patients in Manila, the Philippines.

DESIGN

A prospective observational study was conducted at a national infectious disease hospital. HIV-negative PTB patients aged 13 years hospitalised from November to December 2011 and from December 2012 to May 2013 were enrolled. Sputum samples were tested for Mycobacterium tuberculosis and six respiratory bacterial pathogens using polymerase chain reaction (PCR).

RESULTS

Of 466 patients, 228 (48.9%) were TB-PCR-positive. Overall, bacterial pathogens in purulent sputum were detected in 135 (29.0%) patients: Haemophilus influenzae was the most common bacterium (21.2%), followed by Streptococcus pneumoniae (7.9%). The prevalence of bacterial co-infection did not differ between TB-PCR-positive and -negative patients. A total of 92 (19.7%) patients died within 2 weeks. Bacterial co-infection was significantly associated with an increased risk of 2-week mortality among TB-PCR-positive patients (adjusted risk ratio [aRR] 1.67, 95%CI 1.03-2.72). This association was also observed but did not reach statistical significance among TB-PCR-negative patients (aRR1.7, 95%CI 0.95-3.02).

CONCLUSION

Bacterial co-infection is common and contributes to an increased risk of early mortality among HIV-negative PTB patients.

P5410Combination assessment of renal and hepatic dysfunction improves the predictability of prognosis in patients with acute decompensated heart failure

Background

Renal dysfunction is associated with poor mortality in patients with heart failure (HF). Hepatic dysfunction, assessed by Fibrosis-4 (FIB4) index, has also prediction ability in acute decompensated HF (ADHF) patients. We investigated whether the assessment of the combination of FIB4 index and renal dysfunction improves predictability in patients with ADHF.

Methods

We retrospectively enrolled consecutive 758 patients who admitted due to ADHF from January 2011 to February 2018 and followed up for one year. FIB4 index on admission was calculated by the formula: age (yrs) × AST[U/L] / (platelets [103/μL] × (ALT[U/L])1/2). Study subjects were divided into high FIB4 index (>3.25) and low FIB4 index (≤3.25), furthermore each group were classified by the presence/absence of CKD (estimated glomerular filtration rate <60 ml/min/1.73m). We have generated four groups; low FIB4/without CKD (n=154), low FIB4/with CKD (n=294), high FIB4/without CKD (n=56), and high FIB4/with CKD (n=254). The primary outcome was defined as all-cause mortality in one year. We performed Kaplan-Meyer analysis and multivariable Cox regression models. Furthermore, we evaluated the incremental value with C-index, net reclassification improvement (NRI) and integrated discrimination improvement (IDI) when FIB4 index and renal dysfunction added to a baseline model.

Results

In total, 106 patients died in one year. High FIB4 index and CKD showed significantly higher 1-year mortality (high FIB4 index: 19.7% vs 10.3%, p<0.001, CKD: 17.0% vs 6.7%, p<0.001, respectively). Kaplan-Meyer analysis shows that high FIB4 index with CKD showed statistically higher mortality than the others (vs low FIB4/without CKD, p<0.001, vs high FIB4/without CKD, p=0.031, vs low FIB4/with CKD, p<0.001, respectively).

Multivariate Cox regression model revealed that both high FIB4 index and CKD were an independent risk predictor of 1-year mortality (FIB4 index: p<0.001, HR 1.06, 95% CI 1.035–1.087, CKD: p=0.004, HR 1.834, 95% CI 1.213–2.773, respectively) in patients with ADHF.

A baseline model for prediction of 1-year mortality was determined by multivariable logistic regression including age, body mass index, systolic blood pressure, and serum albumin (C-index: 0.688). Adding high FIB4 index and CKD to the baseline model, all of C-index (0.738, p=0.04), NRI (0.122, p=0.067), and IDI (0.024, p=0.004) were improved.

Receiver operating characteristic curves

Conclusions

Combination assessment of renal and hepatic dysfunction could improve the predictability of prognosis in patients with ADHF.

Acknowledgement/Funding

None