Tailoring Superconductivity in Large-Area Single-Layer NbSe2 via Self-Assembled Molecular Adlayers.

Two-dimensional transition metal dichalcogenides (TMDs) represent an ideal testbench for the search of materials by design, because their optoelectronic properties can be manipulated through surface engineering and molecular functionalization. However, the impact of molecules on intrinsic physical properties of TMDs, such as superconductivity, remains largely unexplored. In this work, the critical temperature (TC) of large-area NbSe2 monolayers is manipulated, employing ultrathin molecular adlayers. Spectroscopic evidence indicates that aligned molecular dipoles within the self-assembled layers act as a fixed gate terminal, collectively generating a macroscopic electrostatic field on NbSe2. This results in an ∼55% increase and a 70% decrease in TC depending on the electric field polarity, which is controlled via molecular selection. The reported functionalization, which improves the air stability of NbSe2, is efficient, practical, up-scalable, and suited to functionalize large-area TMDs. Our results indicate the potential of hybrid 2D materials as a novel platform for tunable superconductivity.

Coexistence of Elastic Modulations in the Charge Density Wave State of 2 H-NbSe2.

Bulk and single-layer 2 H-NbSe2 exhibit identical charge density wave order (CDW) with a quasi-commensurate 3 × 3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2 H-NbSe2 with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations for single-layers reveal that six different atomic structures are compatible with the 3 × 3 CDW distortion, although all of them lie on a very narrow energy range of at most 3 meV per formula unit, suggesting the coexistence of such structures. Our atomically resolved STM images of bulk 2 H-NbSe2 unambiguously confirm this by identifying two of these structures. Remarkably, these structures differ from the X-ray crystal structure reported for the bulk 3 × 3 CDW which in fact is also one of the six DFT structures located for the single-layer. Our calculations also show that due to the minute energy difference between the different phases, the ground state of the 3 × 3 CDW could be extremely sensitive to doping, external strain or internal pressure within the crystal. The presence of multiphase CDW order in 2 H-NbSe2 may provide further understanding of its low temperature state and the competition between different instabilities.

Comparison of normothermic and hypothermic perfusion in porcine kidneys donated after cardiac death.

BACKGROUND: Normothermic machine perfusion (NMP) is an alternative strategy for preserving kidneys donated after cardiac death (DCD). The relative efficacy of prolonged NMP compared to hypothermic machine perfusion (HMP) in DCD kidneys with moderate ischemic injury is undetermined. This study compares NMP and HMP kidney preservation in a porcine DCD model. METHODS: Ten porcine kidneys underwent NMP or HMP preservation following 45 minutes of warm ischemia and 5 hours of cold ischemia. After 8 hours of machine preservation, hemodynamic stability, renal function, perfusate biomarkers, and histologic integrity were assessed in a simulated reperfusion model. RESULTS: During simulated reperfusion, no differences were observed in oxygen consumption, urine production, creatinine clearance, fractional excretion of sodium, proteinuria, and perfusate levels of lactate dehydrogenase and aspartate aminotransferase. Resistance was no different after 30 minutes of simulated reperfusion. Histologically, NMP kidneys demonstrated increased vacuolization after preservation and greater loss of tubular integrity after simulated reperfusion. Perfusate levels of alkaline phosphatase (AP) and gamma glutamyltransferase (GGT) were higher in NMP kidneys during preservation, but upon simulated reperfusion, AP and GGT levels were higher in HMP-preserved kidneys. Peak AP and GGT during simulated reperfusion of HMP kidneys were over 14 times higher than peak AP and GGT during preservation of NMP kidneys. CONCLUSIONS: NMP provided comparable preservation of renal function as HMP and minimized AP and GGT release upon reperfusion.

Not All Patients With Locally Advanced Rectal Cancer Benefit From Neoadjuvant Therapy.

BACKGROUND: While neoadjuvant combined modality therapy (NA-CMT) is beneficial for most patients with locally advanced rectal cancer some patients may experience disease progression during treatment. The purpose of this study is to identify characteristics associated with progression during NA-CMT. METHODS: A single institution retrospective review of patients with stage II-III rectal cancer receiving NA-CMT was conducted from 2008-2019. Patients with incomplete or unknown NA-CMT treatment and those who received chemotherapy in addition to NA-CMT were excluded. Initial staging MRI was compared to post-operative pathology to determine progression. Definitions: responders (complete response or regression) and non-responders (stable disease or progression). RESULTS: 156 patients were included: 25 (16.1%) complete responders, 79 (50.6%) had evidence of regression, 34 (21.8%) were stable non-responders, and 18 (11.5%) were progressors. Those who progressed had worse overall survival. Factors associated with non-responders included black race (OR 4.5, 95% CI: 1.10-18.7) and increasing distance from the anal verge (OR 1.2, 95% CI: .2-2.9). Distance from the anal verge was determined via MRI. Recurrence was significantly more common among non-responders (15, 30.61%) when compared to responders (14, 13.46%), P = .012. CONCLUSION: Patients who progress despite NA-CMT have overall worse survival compared to patients who do respond. While this study failed to identify modifiable or predictive risk factors for progression, the multivariate logistic regression model suggests that race and tumor biology may play a role in progression. Future studies should focus on early identification of patients who may not benefit from NA-CMT in an effort to develop alternative treatment algorithms.

Evidence for ground state coherence in a two-dimensional Kondo lattice.

Kondo lattices are ideal testbeds for the exploration of heavy-fermion quantum phases of matter. While our understanding of Kondo lattices has traditionally relied on complex bulk f-electron systems, transition metal dichalcogenide heterobilayers have recently emerged as simple, accessible and tunable 2D Kondo lattice platforms where, however, their ground state remains to be established. Here we present evidence of a coherent ground state in the 1T/1H-TaSe2 heterobilayer by means of scanning tunneling microscopy/spectroscopy at 340 mK. Our measurements reveal the existence of two symmetric electronic resonances around the Fermi energy, a hallmark of coherence in the spin lattice. Spectroscopic imaging locates both resonances at the central Ta atom of the charge density wave of the 1T phase, where the localized magnetic moment is held. Furthermore, the evolution of the electronic structure with the magnetic field reveals a non-linear increase of the energy separation between the electronic resonances. Aided by ab initio and auxiliary-fermion mean-field calculations, we demonstrate that this behavior is inconsistent with a fully screened Kondo lattice, and suggests a ground state with magnetic order mediated by conduction electrons. The manifestation of magnetic coherence in TMD-based 2D Kondo lattices enables the exploration of magnetic quantum criticality, Kondo breakdown transitions and unconventional superconductivity in the strict two-dimensional limit.

Nontrivial Doping Evolution of Electronic Properties in Ising-Superconducting Alloys.

Transition metal dichalcogenides offer unprecedented versatility to engineer 2D materials with tailored properties to explore novel structural and electronic phase transitions. In this work, the atomic-scale evolution of the electronic ground state of a monolayer of Nb1- δ Moδ Se2 across the entire alloy composition range (0 < δ < 1) is investigated using low-temperature (300 mK) scanning tunneling microscopy and spectroscopy (STM/STS). In particular, the atomic and electronic structure of this 2D alloy throughout the metal to semiconductor transition (monolayer NbSe2 to MoSe2 ) is studied. The measurements enable extraction of the effective doping of Mo atoms, the bandgap evolution and the band shifts, which are monotonic with δ. Furthermore, it is demonstrated that collective electronic phases (charge density wave and superconductivity) are remarkably robust against disorder and further shown that the superconducting TC changes non-monotonically with doping. This contrasting behavior in the normal and superconducting state is explained using first-principles calculations. Mo doping is shown to decrease the density of states at the Fermi level and the magnitude of pair-breaking spin fluctuations as a function of Mo content. These results paint a detailed picture of the electronic structure evolution in 2D TMD alloys, which is of utmost relevance for future 2D materials design.

Observation of Superconducting Collective Modes from Competing Pairing Instabilities in Single-Layer NbSe2.

In certain unconventional superconductors with sizable electronic correlations, the availability of closely competing pairing channels leads to characteristic soft collective fluctuations of the order parameters, which leave fingerprints in many observables and allow the phase competition to be scrutinized. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. In this work, the existence of a soft collective mode in single-layer NbSe2 , observed as a characteristic resonance excitation in high-resolution tunneling spectra is reported. This resonance is observed along with higher harmonics, its frequency Ω/2Δ is anticorrelated with the local superconducting gap Δ, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (TC and HC2 ), which sets an unambiguous link to the superconducting state. Aided by a microscopic model that captures the main experimental observations, this resonance is interpreted as a collective Leggett mode that represents the fluctuation toward a proximate f-wave triplet state, due to subleading attraction in the triplet channel. These findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path toward unconventional superconductivity in simple, scalable, and transferable 2D superconductors.

Magnetic correlations in single-layer NbSe2.

By means of spin-resolved density functional theory calculations using both atomic orbitals and plane-wave basis codes, we study the electronic and magnetic ground state of single-layer NbSe2. We find that, for all the functionals considered, the most stable solution in this two-dimensional (2D) superconductor is the ferrimagnetic ground state with a magnetic moment of 1.09 µBat the Nb atoms and of 0.05 µBat the Se atoms pointing in the opposite direction. Our calculations show that the ferrimagnetic state precludes the development of charge density wave (CDW) order and their coexistence in the single-layer limit, unless graphene is considered as a substrate. The spin-resolved calculated density of states (DOS), a key fingerprint of the electronic and magnetic structure of a material, unambiguously reproduces the experimental DOS measured by scanning tunneling spectroscopy in single-layer NbSe2. Our work sets magnetism into play in this prototypical correlated 2D material, which is crucial to understand the formation mechanisms of 2D superconductivity and CDW order.

Proximity Effects on the Charge Density Wave Order and Superconductivity in Single-Layer NbSe2.

Collective electronic states such as the charge density wave (CDW) order and superconductivity (SC) respond sensitively to external perturbations. Such sensitivity is dramatically enhanced in two dimensions (2D), where 2D materials hosting such electronic states are largely exposed to the environment. In this regard, the ineludible presence of supporting substrates triggers various proximity effects on 2D materials that may ultimately compromise the stability and properties of the electronic ground state. In this work, we investigate the impact of proximity effects on the CDW and superconducting states in single-layer (SL) NbSe2 on four substrates of diverse nature, namely, bilayer graphene (BLG), SL-boron nitride (h-BN), Au(111), and bulk WSe2. By combining low-temperature (340 mK) scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we compare the electronic structure of this prototypical 2D superconductor on each substrate. We find that, even when the electronic band structure of SL-NbSe2 remains largely unaffected by the substrate except when placed on Au(111), where a charge transfer occurs, both the CDW and SC show disparate behaviors. On the insulating h-BN/Ir(111) substrate and the metallic BLG/SiC(0001) substrate, both the 3 × 3 CDW and superconducting phases persist in SL-NbSe2 with very similar properties, which reveals the negligible impact of graphene on these electronic phases. In contrast, these collective electronic phases are severely weakened and even absent on the bulk insulating WSe2 substrate and the metallic single-crystal Au(111) substrate. Our results provide valuable insights into the fragile stability of such electronic ground states in 2D materials.