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

P4551Prognostic utility of Palliative Prognostic Index for prediction of 30-day and 1-year outcome in patients with acute decompensated heart failure

Background

The prognosis of heart failure remains poor similar to the terminal cancer patients, although recent progress in medical treatment. Palliative Prognostic Index (PPI) is a widely used prognostic index for terminal cancer patients (PPI includes: Palliative Performance Scale, oral intake, oedema, dyspnea at rest and delirium), suggesting the short-term prognostic marker of terminal cancer patients.

Purpose

The purpose of this study was to evaluate the impact of PPI on 30-day mortality, 1-year mortality and 1-year events (including all-cause mortality, readmission due to heart failure and new onset of cerebral infarction after hospital discharge) among acute decompensated heart failure (ADHF) patients.

Method

Study subjects comprised of consecutive 764 patients who admitted due to ADHF and followed up for 1-year. PPI were calculated at the time of hospital admission. Study subjects were divided into two groups based on the PPI: L-PPI (PPI<6) and H-PPI (6≤PPI). We calculated the C-index, net reclassification improvement (NRI) and the integrated discrimination improvement (IDI) to evaluate the improvement of prediction ability on 30-day mortality.

Result

H-PPI showed significantly higher 30-day mortality than L-PPI [7.9% vs 2.0%, log rank p<0.001, Hazard retio (HR): 1.26, 95% confidential interval(CI): 1.14–1.37, p<0.001], 1-year mortality [20.0% vs 12.7%, log rank p=0.022, HR 1.15, 95% CI 1.09–1.21, p<0.001]and 1-year events [45.5% vs 31.1%, log rank p<0.001, HR 1.13, 95% CI 1.09–1.17, p<0.001]. Multivariate cox proportional hazard models adjusted with several covariates revealed that PPI was an independent predictor of 30-day mortality (HR: 1.23, 95% CI: 1.10–1.36, p<0.001), 1-year mortality (HR: 1.10, 95% CI: 1.04–1.16, p<0.001) and 1-year events (HR: 1.11, 95% CI: 1.07–1.15, p<0.001), respectively.

A reference model for prediction of 30-day mortality was determined including left ventricular ejection fraction and serum albumin concentration by multivariable logistic regression analysis. (P<0.05) (C-index: 0.720) Adding PPI to the reference model (C-index: 0.773) significantly improved both NRI (0.458, p=0.038) and IDI (0.046, p=0.007), respectively.

Conclusion

We suggest that assessment of PPI showed good prognostic ability for 30-day and 1-year outcome, while PPI provided additional prognostic information in patients with ADHF.

P2434Digital zoom decreases radiation exposure dose up to 30% in percutaneous coronary intervention

Background

Interventional cardiology is gaining greater popularity worldwide with each passing year. Reduction of exposure dose is a very imminent and an important issue in cardiology procedure. Although a newer radiation reduction technique, device and procedure are very valuable and expected, we should consider about therapy technique, radiation technique, devices, and the way to protection. Digital zoom digitally enlarges images in real time by up to 2.5-fold at lower doses than those used with traditional field of view changes. In our phantom examination the average dose reduction of digital zoom was 27%.

Methods and results

This study is designated as single-center, retrospective, not-randomized, observation study. 2101 eligible cases were collected. We assigned the cases of PCI without the use of Digital zoom to the Conventional group and those involving the use of Digital zoom to the Digital zoom group. There were 806 patients in the Conventional group and 1195 in the Digital zoom group. Because we had begun using Digital zoom from January 2015 onwards, all patients in the Conventional group had undergone PCI from January 2013 to December 2014 and all patients in the Digital zoom group had undergone PCI from January 2015 to December 2016. In addition, we calculated the RAK/minute and DAP/minute for an accurate assessment. To minimize the difference of characteristics between two groups, propensity score including all baseline variables was performed. Furthermore, Predictors of radiation exposure were investigated using multivariable least square methods. Inter group differences were observed in DAP, RAK, DAP/min, and RAK/min (Digital zoom group vs conventional group: DAP, 16000 cGy cm2 [from 1st quartile to 3rd quartile; 10300–24400] vs 20700 [13400–29500], p<0.001; DAP/min, 557 cGy cm2/min [392–737] vs 782 [571–1010], p<0.01; RAK, 1590 Gy [990–2410] vs 1850 [1220–2720], p<0.01; RAK/min, 54.7 Gy/min [38.5–73.2] vs 71.2 [51.5–93.0], p<0.01). Even after propensity score matching, intergroup differences in DAP (810 cases), DAP/min (811 cases), RAK (746 cases), and RAK/min (744 cases) persisted. Furthermore, the least squares method showed that Digital zoom is an important predictor of DAP (β=0.17, p<0.01) and RAK (β=0.12, p<0.01).

Conclusion

Digital zoom is an old and cost-free technique, but one of most powerful reduction of exposure method. Propensity score adjustment and least square methods show that digital zoom is one of independent effective method.

Energy Spectrum of Two-Dimensional Excitons in a Nonuniform Dielectric Medium

We demonstrate that, in monolayers (MLs) of semiconducting transition metal dichalcogenides, the s-type Rydberg series of excitonic states follows a simple energy ladder: ε_{n}=-Ry^{*}/(n+δ)^{2}, n=1,2,…, in which Ry^{*} is very close to the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is accounted for only by δ. This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe_{2} ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schrödinger equation when approximating the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other MoSe_{2}, WS_{2}, MoS_{2} MLs encapsulated in hBN, we estimate an apparent magnitude of δ for each of the studied structures. Intriguingly, δ is found to be close to zero for WSe_{2} as well as for MoS_{2} monolayers, what implies that the energy ladder of excitonic states in these two-dimensional structures resembles that of Rydberg states of a three-dimensional hydrogen atom.

Probing and Manipulating Valley Coherence of Dark Excitons in Monolayer WSe_{2}

Monolayers of semiconducting transition metal dichalcogenides are two-dimensional direct-gap systems which host tightly bound excitons with an internal degree of freedom corresponding to the valley of the constituting carriers. Strong spin-orbit interaction and the resulting ordering of the spin-split subbands in the valence and conduction bands makes the lowest-lying excitons in WX_{2} (X being S or Se) spin forbidden and optically dark. With polarization-resolved photoluminescence experiments performed on a WSe_{2} monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic exchange interaction in combination with the applied in-plane and/or out-of-plane magnetic fields enables one to probe and manipulate the valley degree of freedom of the dark excitons.

Interaction-Induced Shubnikov-de Haas Oscillations in Optical Conductivity of Monolayer MoSe_{2}

We report polarization-resolved resonant reflection spectroscopy of a charge-tunable atomically thin valley semiconductor hosting tightly bound excitons coupled to a dilute system of fully spin- and valley-polarized holes in the presence of a strong magnetic field. We find that exciton-hole interactions manifest themselves in hole-density dependent, Shubnikov-de Haas-like oscillations in the energy and line broadening of the excitonic resonances. These oscillations are evidenced to be precisely correlated with the occupation of Landau levels, thus demonstrating that strong interactions between the excitons and Landau-quantized itinerant carriers enable optical investigation of quantum-Hall physics in transition metal dichalcogenides.

Control of the Exciton Radiative Lifetime in van der Waals Heterostructures

Optical properties of atomically thin transition metal dichalcogenides are controlled by robust excitons characterized by a very large oscillator strengths. Encapsulation of monolayers such as MoSe_{2} in hexagonal boron nitride (hBN) yields narrow optical transitions approaching the homogenous exciton linewidth. We demonstrate that the exciton radiative rate in these van der Waals heterostructures can be tailored by a simple change of the hBN encapsulation layer thickness as a consequence of the Purcell effect. The time-resolved photoluminescence measurements show that the neutral exciton spontaneous emission time can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. The inhibition of the radiative recombination can yield spontaneous emission time up to 10 ps. These results are in very good agreement with the calculated recombination rate in the weak exciton-photon coupling regime. The analysis shows that we are also able to observe a sizable enhancement of the exciton radiative decay rate. Understanding the role of these electrodynamical effects allows us to elucidate the complex dynamics of relaxation and recombination for both neutral and charged excitons.

Integrated impedance bridge for absolute capacitance measurements at cryogenic temperatures and finite magnetic fields

We developed an impedance bridge that operates at cryogenic temperatures (down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed circuit board containing the device under test and thereby parallel to the magnetic field. The measured amplitude and phase of the output signal allows for the separation of the total impedance into an absolute capacitance and a resistance. Through a detailed noise characterization, we find that the best resolution is obtained when operating the HEMT amplifier at the highest gain. We obtained a resolution in the absolute capacitance of 6.4 aF/Hz at 77 K on a comb-drive actuator while maintaining a small excitation amplitude of 15 k_BT/e. We show the magnetic field functionality of our impedance bridge by measuring the quantum Hall plateaus of a top-gated hBN/graphene/hBN heterostructure at 60 mK with a probe signal of 12.8 k_BT/e.

Electronic Compressibility of Magic-Angle Graphene Superlattices

We report the first electronic compressibility measurements of magic-angle twisted bilayer graphene. The evolution of the compressibility with carrier density offers insights into the interaction-driven ground state that have not been accessible in prior transport and tunneling studies. From capacitance measurements, we determine the chemical potential as a function of carrier density and find the widths of the energy gaps at fractional filling of the moiré lattice. In the electron-doped regime, we observe unexpectedly large gaps at quarter- and half-filling and strong electron-hole asymmetry. Moreover, we measure a ∼35  meV minibandwidth that is much wider than most theoretical estimates. Finally, we explore the field dependence up to the quantum Hall regime and observe significant differences from transport measurements.

Excited States in Bilayer Graphene Quantum Dots

We report ground- and excited-state transport through an electrostatically defined few-hole quantum dot in bilayer graphene in both parallel and perpendicular applied magnetic fields. A remarkably clear level scheme for the two-particle spectra is found by analyzing finite bias spectroscopy data within a two-particle model including spin and valley degrees of freedom. We identify the two-hole ground state to be a spin-triplet and valley-singlet state. This spin alignment can be seen as Hund's rule for a valley-degenerate system, which is fundamentally different from quantum dots in carbon nanotubes, where the two-particle ground state is a spin-singlet state. The spin-singlet excited states are found to be valley-triplet states by tilting the magnetic field with respect to the sample plane. We quantify the exchange energy to be 0.35 meV and measure a valley and spin g factor of 36 and 2, respectively.

Universal quantized thermal conductance in graphene

The universal quantization of thermal conductance provides information on a state's topological order. Recent measurements revealed that the observed value of thermal conductance of the 5 2 state is inconsistent with either Pfaffian or anti-Pfaffian model, motivating several theoretical articles. Analysis has been made complicated by the presence of counter-propagating edge channels arising from edge reconstruction, an inevitable consequence of separating the dopant layer from the GaAs quantum well and the resulting soft confining potential. Here, we measured thermal conductance in graphene with atomically sharp confining potential by using sensitive noise thermometry on hexagonal boron-nitride encapsulated graphene devices, gated by either SiO₂/Si or graphite back gate. We find the quantization of thermal conductance within 5% accuracy for ν = 1 ; 4 3 ; 2 and 6 plateaus, emphasizing the universality of flow of information. These graphene quantum Hall thermal transport measurements will allow new insight into exotic systems like even-denominator quantum Hall fractions in graphene.

Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum photonic technologies. The ability to tailor quantum emitters via site-selective defect engineering is essential for realizing scalable architectures. However, a major difficulty is that defects need to be controllably positioned within the material. Here, we overcome this challenge by controllably irradiating monolayer MoS2 using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion exposed MoS2 flake with high-quality hBN reveals spectrally narrow emission lines that produce photons in the visible spectral range. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron-hole complexes at defect states generated by the local helium ion exposure. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and interacting exciton lattices that may allow the exploration of Hubbard physics.

Semiconductor Nanostructure Design for Thermoelectric Property Control

We present the methodologies for developing high-performance thermoelectric materials using nanostructured interfaces by reviewing our three studies and giving the new aspect of nanostructuring results. (1) Connected Si nanocrystals exhibited ultrasmall thermal conductivity. The drastic thermal conductivity reduction was brought by phonon confinement and phonon scattering. Here, we present discussion about the new aspect for phonon transport: not only nanocrystal size but also shape can contribute to thermal conductivity reduction. (2) Si films including Ge nanocrystals demonstrated that phonon and carrier conductions were independently controlled in the films, where carriers were easily transported through the interfaces between Si and Ge, while phonons could be effectively scattered at the interfaces. (3) Embedded-ZnO nanowire structure demonstrated the simultaneous realization of power factor increase and thermal conductivity reduction. The [Formula: see text] increase was caused by the interface-dominated carrier transport. The nanowire interfaces also worked as phonon scatterers, resulting in the thermal conductivity reduction.

High-Quality Magnetotransport in Graphene Using the Edge-Free Corbino Geometry

We report fabrication of graphene devices in a Corbino geometry consisting of concentric circular electrodes with no physical edge connecting the inner and outer electrodes. High device mobility is realized using boron nitride encapsulation together with a dual-graphite gate structure. Bulk conductance measurement in the quantum Hall effect (QHE) regime outperforms previously reported Hall bar measurements, with improved resolution observed for both the integer and fractional QHE states. We identify apparent phase transitions in the fractional sequence in both the lowest and first excited Landau levels (LLs) and observe features consistent with electron solid phases in higher LLs.

Sensitivity of the superconducting state in thin films

For more than two decades, there have been reports on an unexpected metallic state separating the established superconducting and insulating phases of thin-film superconductors. To date, no theoretical explanation has been able to fully capture the existence of such a state for the large variety of superconductors exhibiting it. Here, we show that for two very different thin-film superconductors, amorphous indium oxide and a single crystal of 2H-NbSe₂, this metallic state can be eliminated by adequately filtering external radiation. Our results show that the appearance of temperature-independent, metallic-like transport at low temperatures is sufficiently described by the extreme sensitivity of these superconducting films to external perturbations. We relate this sensitivity to the theoretical observation that, in two dimensions, superconductivity is only marginally stable.

Expression analysis of cellulose synthases that comprise the Type II complex in hybrid aspen

Gene duplication in plants occurs via several different mechanisms, including whole genome duplication, and the copied genes acquire various forms and types. The cellulose synthase (CesA) family functions in cellulose synthesis complex (CSC) formation, which is involved in the synthesis of primary and secondary cell walls in plants. In the genome of Populus, 17 CesA have been annotated, and some of them appeared through whole genome duplication. The nucleotide sequence of the duplicated genes changed during subsequent evolution, and functional differentiation of genes might have occurred. To gain insight into the evolutionary fate of the duplicated CesA, expression analysis with quantitative reverse transcription polymerase chain reactions and promoter-reporter assays was performed on three duplicated gene pairs whose products have been reported to form a single CSC. Changes in expression of each gene at different developmental stages were detected and divergent expression patterns in different organs and tissues observed between the gene pairs. Among the tested genes, expression of PttCesA3-C was apparently lower than that of its counterpart, PttCesA3-D. The results suggest that the six CesA are approaching sub-functionalisation or non-functionalisation. Furthermore, the level of functionalisation may vary among the three pairs of genes, and functional specialisation of each CesA should have been achieved, at least partially, through differences in expression of genes.

Measuring Hall viscosity of graphene's electron fluid

An electrical conductor subjected to a magnetic field exhibits the Hall effect in the presence of current flow. Here, we report a qualitative deviation from the standard behavior in electron systems with high viscosity. We found that the viscous electron fluid in graphene responds to nonquantizing magnetic fields by producing an electric field opposite to that generated by the ordinary Hall effect. The viscous contribution is substantial and identified by studying local voltages that arise in the vicinity of current-injecting contacts. We analyzed the anomaly over a wide range of temperatures and carrier densities and extracted the Hall viscosity, a dissipationless transport coefficient that was long identified theoretically but remained elusive in experiments.

Photonic crystals for nano-light in moiré graphene superlattices

Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moiré superlattice. The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus-obtained twisted bilayer graphene (TBG). We studied the propagation of plasmon polaritons in TBG by infrared nano-imaging. We discovered that the atomic reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway for controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials, eliminating the need for arduous top-down nanofabrication.

Tuning superconductivity in twisted bilayer graphene

Materials with flat electronic bands often exhibit exotic quantum phenomena owing to strong correlations. An isolated low-energy flat band can be induced in bilayer graphene by simply rotating the layers by 1.1°, resulting in the appearance of gate-tunable superconducting and correlated insulating phases. In this study, we demonstrate that in addition to the twist angle, the interlayer coupling can be varied to precisely tune these phases. We induce superconductivity at a twist angle larger than 1.1°—in which correlated phases are otherwise absent—by varying the interlayer spacing with hydrostatic pressure. Our low-disorder devices reveal details about the superconducting phase diagram and its relationship to the nearby insulator. Our results demonstrate twisted bilayer graphene to be a distinctively tunable platform for exploring correlated states.