Direct Probe Research Articles

Noncontact Andreev Reflection as a Direct Probe of Superconductivity on the Atomic Scale.

Direct detection of superconductivity has long been a key strength of point-contact Andreev reflection. However, its applicability to atomic-scale imaging is limited by the mechanical contact of the Andreev probe. To this end, we present a new method to probe Andreev reflection in a tunnel junction, leveraging tunneling spectroscopy and junction tunability to achieve quantitative detection of Andreev scattering. This method enables unambiguous assignment of superconducting origins of current-carrying excitations, as well as detection of higher order Andreev processes in atomic-scale junctions. We furthermore revealed distinct sensitivity of Andreev reflection to natural defects, such as step edges, even in classical superconductors. The methodology opens a new path to nano- and atomic-scale imaging of superconducting properties, including disordered superconductors and proximity to phase transitions.

Direct Cell Radiolabeling for in Vivo Cell Tracking with PET and SPECT Imaging.

The arrival of cell-based therapies is a revolution in medicine. However, its safe clinical application in a rational manner depends on reliable, clinically applicable methods for determining the fate and trafficking of therapeutic cells in vivo using medical imaging techniques—known as in vivo cell tracking. Radionuclide imaging using single photon emission computed tomography (SPECT) or positron emission tomography (PET) has several advantages over other imaging modalities for cell tracking because of its high sensitivity (requiring low amounts of probe per cell for imaging) and whole-body quantitative imaging capability using clinically available scanners. For cell tracking with radionuclides, ex vivo direct cell radiolabeling, that is, radiolabeling cells before their administration, is the simplest and most robust method, allowing labeling of any cell type without the need for genetic modification. This Review covers the development and application of direct cell radiolabeling probes utilizing a variety of chemical approaches: organic and inorganic/coordination (radio)chemistry, nanomaterials, and biochemistry. We describe the key early developments and the most recent advances in the field, identifying advantages and disadvantages of the different approaches and informing future development and choice of methods for clinical and preclinical application.

NLO electroweak and QCD corrections to the production of a photon with three charged lepton plus missing energy at the LHC

Electroweak (EW) triboson production processes with at least one heavy gauge boson are of increasing interest at the Large Hadron Collider (LHC) as direct precision probes of one of the least-tested sectors of the Standard Model (SM), the quartic couplings of the EW gauge bosons. These processes therefore offer promising opportunities for searches for indirect signals of Beyond-the-SM (BSM) physics. In this paper, we present results for fiducial cross sections at next-to-leading-order (NLO) EW and NLO QCD to $p\;p\to e^{+}\;\nu_{e}\;\mu^{+}\;\mu^{-}\;\gamma$ at the 13 TeV LHC. This signature includes the triboson production process $p\;p \to W^{+}\;Z\;\gamma$ with leptonic decays, $W^{+} \to e^{+}\;\nu_{e}$ and $Z \to \mu^{+}\;\mu^{-}$. The computation is based on the complete set of leading-order (LO) contributions of $\mathcal{O}(\alpha^5)$ and on NLO EW and NLO QCD cross sections of $\mathcal{O}(\alpha^6)$ and $\mathcal{O}(\alpha^5 \alpha_\text{s})$, respectively, and thus off-shell effects, spin correlations and non-resonance contributions are fully taken into account. We construct a Monte Carlo framework which provides total and differential cross sections for a chosen set of basic analysis cuts. We find that while NLO EW corrections enhance the fiducial LO total cross section by only $1\%$, they can significantly change some distributions in certain kinematic regions. For example, the relative NLO EW corrections to the muon transverse momentum distribution at 500 GeV amounts to $-20\%$. To illustrate how missing NLO EW corrections could masquerade as BSM physics, we show examples for the impact of dimension-8 operators in the SM Effective Field Theory framework on selected kinematic distributions.

Direct probing spiro-biloops via a partially junction-cleavable network

Direct probing spiro-biloops via a partially junction-cleavable network

Anti-symmetric Compton scattering in LiNiPO 4: Towards a direct probe of the magneto-electric multipole moment.

Magnetoelectric multipoles, which break both space-inversion and time-reversal symmetries, play an important role in the magnetoelectric response of a material. Motivated by uncovering the underlying fundamental physics of the magnetoelectric multipoles and the possible technological applications of magnetoelectric materials, understanding as well as detecting such magnetoelectric multipoles has become an active area of research in condensed matter physics. Here we employ the well-established Compton scattering effect as a possible probe for the magnetoelectric toroidal moments in LiNiPO 4. We employ combined theoretical and experimental techniques to compute as well as detect the antisymmetric Compton profile in LiNiPO 4. For the theoretical investigation we use density functional theory to compute the anti-symmetric part of the Compton profile for the magnetic and structural ground state of LiNiPO 4. For the experimental verification, we measure the Compton signals for a single magnetoelectric domain sample of LiNiPO 4, and then again for the same sample with its magnetoelectric domain reversed. We then take the difference between these two measured signals to extract the antisymmetric Compton profile in LiNiPO 4. Our theoretical calculations indicate an antisymmetric Compton profile in the direction of the t y toroidal moment in momentum space, with the computed antisymmetric profile around four orders of magnitude smaller than the total profile. The difference signal that we measure is consistent with the computed profile, but of the same order of magnitude as the statistical errors and systematic uncertainties of the experiment. While the weak difference signal in the measurements prevents an unambiguous determination of the antisymmetric Compton profile in LiNiPO 4, our results motivate further theoretical work to understand the factors that influence the size of the antisymmetric Compton profile, and to identify materials exhibiting larger effects.

Direct First Parker Solar Probe Observation of the Interaction of Two Successive Interplanetary Coronal Mass Ejections in 2020 November

We investigate the effects of the evolutionary processes in the internal magnetic structure of two interplanetary coronal mass ejections (ICMEs) detected in situ between 2020 November 29 and December 1 by the Parker Solar Probe (PSP). The sources of the ICMEs were observed remotely at the Sun in EUV and subsequently tracked to their coronal counterparts in white light. This period is of particular interest to the community as it has been identified as the first widespread solar energetic particle event of solar cycle 25. The distribution of various solar and heliospheric-dedicated spacecraft throughout the inner heliosphere during PSP observations of these large-scale magnetic structures enables a comprehensive analysis of the internal evolution and topology of such structures. By assembling different models and techniques, we identify the signatures of interaction between the two consecutive ICMEs and the implications for their internal structure. We use multispacecraft observations in combination with a remote-sensing forward modeling technique, numerical propagation models, and in situ reconstruction techniques. The outcome, from the full reconciliations, demonstrates that the two coronal mass ejections (CMEs) are interacting in the vicinity of the PSP. Thus, we identify the in situ observations based on the physical processes that are associated with the interaction and collision of both CMEs. We also expand the flux rope modeling and in situ reconstruction technique to incorporate the aging and expansion effects in a distorted internal magnetic structure and explore the implications of both effects in the magnetic configuration of the ICMEs.

Direct current magnetic Hall probe technique for measurement of field penetration in thin film superconductors for superconducting radio frequency resonators.

Superconducting Radio Frequency (SRF) cavities used in particle accelerators are typically formed from or coated with superconducting materials. Currently, high purity niobium is the material of choice for SRF cavities that have been optimized to operate near their theoretical field limits. This brings about the need for significant R & D efforts to develop next generation superconducting materials that could outperform Nb and keep up with the demands of new accelerator facilities. To achieve high quality factors and accelerating gradients, the cavity material should be able to remain in the superconducting Meissner state under a high RF magnetic field without penetration of quantized magnetic vortices through the cavity wall. Therefore, the magnetic field at which vortices penetrate a superconductor is one of the key parameters of merit of SRF cavities. Techniques to measure the onset of magnetic field penetration on thin film samples need to be developed to mitigate the issues with the conventional magnetometry measurements that are strongly influenced by the film orientation and shape and edge effects. In this work, we report the development of an experimental setup to measure the field of full flux penetration through films and multi-layered superconductors. Our system combines a small superconducting solenoid that can generate a magnetic field of up to 500 mT at the sample surface and three Hall probes to detect the full flux penetration through the superconductor. This setup can be used to study alternative materials that could potentially outperform niobium, as well as superconductor-insulator-superconductor (SIS) multilayer coatings on niobium.

On the Diversity of Mixing and Helium Core Masses of B-type Dwarfs from Gravity-mode Asteroseismology

The chemical evolution of the galaxy is largely guided by the yields from massive stars. Their evolution is heavily influenced by their internal mixing, allowing the stars to live longer and yield a more massive helium core at the end of their main-sequence evolution. Asteroseismology is a powerful tool for studying stellar interiors by providing direct probes of the interior physics of the oscillating stars. This work revisits the recently derived internal mixing profiles of 26 slowly pulsating B stars observed by the Kepler space telescope, in order to investigate how well the mixing profiles can in fact be distinguished from one another as well as provide predictions for the expected helium core masses obtained at the end of the main-sequence evolution. We find that for five of these stars the mixing profile is derived unambiguously, while the remaining stars have at least one other mixing profile which explains the oscillations equally well. Convective penetration is preferred over exponential diffusive overshoot for ≈55% of the stars, while stratified mixing is preferred in the envelope (≈39%). We estimate the expected helium core masses obtained at the end of the main-sequence evolution and find them to be highly influenced by the estimated amount of mixing occurring in the envelopes of the stars.

Prospects for multimessenger detection of binary neutron star mergers in the fourth LIGO–Virgo–KAGRA observing run

ABSTRACT The joint detection of GW170817 and GRB 170817A opened the era of multimessenger astronomy with gravitational waves (GWs) and provided the first direct probe that at least some binary neutron star (BNS) mergers are progenitors of short gamma-ray bursts (S-GRBs). In the next years, we expect to have more multimessenger detections of BNS mergers, thanks to the increasing sensitivity of GW detectors. Here, we present a comprehensive study on the prospects for joint GW and electromagnetic observations of merging BNSs in the fourth Laser Interferometer Gravitational-wave Observatory (LIGO)–Virgo–Kamioka Gravitational Wave Detector (KAGRA) observing run with Fermi Gamma-ray Space Telescope (Fermi), Neil Gehrels Swift Observatory (Swift), INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), and Space Variable Objects Monitor (SVOM). This work combines accurate population synthesis models with simulations of the expected GW signals and the associated S-GRBs, considering different assumptions about the gamma-ray burst (GRB) jet structure. We show that the expected rate of joint GW and electromagnetic detections could be up to ∼6 yr−1 when Fermi/Gamma-ray Burst Monitor (GBM) is considered. Future joint observations will help us to better constrain the association between BNS mergers and S-GRBs, as well as the geometry of the GRB jets.

Surprising Rigidity of Functionally Important Water Molecules Buried in the Lipid Headgroup Region.

Understanding water dynamics and structure is an important topic in biological systems. It is generally held in the literature that the interfacial water of hydrated phospholipids is highly mobile, in fast exchange with the bulk water ranging from the nano- to femtosecond timescale. Although nuclear magnetic resonance (NMR) is a powerful tool for structural and dynamic studies, direct probing of interfacial water in hydrated phospholipids is formidably challenging due to the extreme population difference between bulk and interfacial water. We developed a novel 17O solid-state NMR technique in combination with an ultra-high-field magnet (35.2 T) to directly probe the functionally important interfacial water. By selectively suppressing the dominant bulk water signal, we observed two distinct water species in the headgroup region of hydrated dimyristoylphosphatidylcholine (DMPC) lipid bilayers for the first time. One water species denoted as "confined water" is chemically and dynamically different from the bulk water (∼0.17 ppm downfield and a slightly shorter spin-lattice relaxation time). Another water species denoted as "bound water" has severely restricted motion and a distinct chemical shift (∼12 ppm upfield). Additionally, the bulk water is not as "free" as pure water, resulting from the fast exchange with the water molecules that weakly and transiently interact with the lipid choline groups. These new discoveries clearly indicate the existence of the interfacial water molecules that are relatively stable over the NMR timescale (on the order of milliseconds), providing an opportunity to characterize water dynamics on the millisecond or slower timescale in biomacromolecules.

How Human TET2 Enzyme Catalyzes the Oxidation of Unnatural Cytosine Modifications in Double-Stranded DNA.

Methylation of cytosine bases is strongly linked to gene expression, imprinting, aging, and carcinogenesis. The Ten-eleven translocation (TET) family of enzymes, which are Fe(II)/2-oxoglutarate (2OG)-dependent enzymes, employ Fe(IV)=O species to dealkylate the lesioned bases to an unmodified cytosine. Recently, it has been shown that the TET2 enzyme can catalyze promiscuously DNA substrates containing unnatural alkylated cytosine. Such unnatural substrates of TET can be used as direct probes for measuring the TET activity or capturing TET from cellular samples. Herein, we studied the catalytic mechanisms during the oxidation of the unnatural C5-position modifications (5-ethylcytosine (5eC), 5-vinylcytosine (5vC) and 5-ethynylcytosine (5eyC)) and the demethylation of N4-methylated lesions (4-methylcytosine (4mC) and 4,4-dimethylcytosine(4dmC)) of the cytosine base by the TET2 enzyme using molecular dynamics (MD) and combined quantum mechanics and molecular mechanics (QM/MM) computational approaches. The results reveal that the chemical nature of the alkylation of the double-stranded (ds) DNA substrates induces distinct changes in the interactions in the binding site, the second coordination sphere, and long-range correlated motions of the ES complexes. The rate-determining hydrogen atom transfer (HAT) is faster in N4-methyl substituent substrates than in the C5-alkylations. Importantly, the calculations show the preference of hydroxylation over desaturation in both 5eC and 5vC substrates. The studies elucidate the post-hydroxylation rearrangements of the hydroxylated intermediates of 5eyC and 5vC to ketene and 5-formylmethylcytosine (5fmC), respectively, and hydrolysis of hemiaminal intermediate of 4mC to formaldehyde and unmodified cytosine proceed exclusively in aqueous solution outside of the enzyme environment. Overall, the studies show that the chemical nature of the unnatural alkylated cytosine substrates exercises distinct effects on the binding interactions, reaction mechanism, and dynamics of TET2.

Study of the charged particle flow near the plasma grid in negative ion source

Flows of the charged particles in hydrogen (H) and deuterium (D) plasmas were measured with the single-tip directional Langmuir probe. The flow patterns of positive and negative ions have been constructed from the data by moving the probe near the plasma grid. The positive ion flow from driver to extraction regions showed the different velocity equivalent to the mass difference between H and D. The negative ion flow indicated the surface production of negative ions on the plasma grid, and changed the direction corresponding to the magnitude of the extraction electric field.

Methods to assess tooth gingival thickness and diagnose gingival phenotypes: A systematic review.

Measurement of the periodontal soft tissue dimension is crucial for clinical decision-making and aesthetic prognosis. However, the effectiveness of different measuring methods remains unclear. This systematic review aimed to explore the diagnostic accuracy of two non-invasive methods (namely CBCT and ultrasound) for gingival thickness measurement at different tooth positions. A systematic search was performed using PubMed (including Medline), PubMed Central, OVID, Cochrane Library, LILACS and OpenGrey. Studies focusing on comparisons between CBCT, ultrasound and direct transgingival probing were included. The means, SDs and correlation coefficients with 95% confidence intervals were extracted and analyzed using Review Manager and R software. Twelve studies were selected. No significant difference was found between CBCT measurement and transgingival probing in the anterior and posterior dentition, and a moderate correlation was observed between these two methods (r=0.41). A weak correlation was found between ultrasound measurement and transgingival probing (r=0.32), and a slight but statistically significant difference was found when comparing ultrasonic devices and transgingival probing in the posterior area. CBCT can be considered a relatively reliable method for gingival thickness measurement in both the anterior and posterior areas compared with direct probing. Ultrasonic devices provide limited accuracy in the posterior area but are relatively comparable with direct clinical assessments in the anterior area. Measurement location may affect the diagnostic accuracy and repeatability of gingival thickness measurements. Appropriate method selection in different clinical scenarios is crucial to aesthetic outcome prediction and decision-making.

Stacking‐Fault Enhanced Oxygen Redox in Li2MnO3

AbstractLattice oxygen redox yields anomalous capacity and can significantly increase the energy density of layered Li‐rich transition metal oxide cathodes, garnering tremendous interest. However, the mechanism behind O redox in these cathode materials is still under debate, in part due to the challenges in directly observing O and following associated changes upon electrochemical cycling. Here, with 17O NMR as a direct probe of O activities, it is demonstrated that stacking faults enhance O redox participation compared with Li2MnO3 domains without stacking faults. This work is concluded by combining both ex situ and in situ 17O NMR to investigate the evolution of O at 4i, 8j sites from monoclinic C2/m and 6c(1), 6c(2), 6c(3) sites from the stacking faults (P3112). These measurements are further corroborated and explained by first‐principles calculations finding a stabilization effect of stacking faults in delithiated Li2MnO3. In situ 17O NMR tracks O activities with temporal resolution and provides a quantitative determination of reversible O redox versus irreversible processes that form short covalent OO bonds. This work provides valuable insights into the O redox reactions in Li‐excess layered cathodes, which may inspire new material design for cathodes with high specific capacity.

Direct mass spectrometric observation and reaction mechanism of gas-phase initial intermediates during CL-20 decomposition

The formation and decay of initial intermediates of energetic materials (EMs) such as polynitropolyaza-caged CL-20 are responsible for the initiation of decomposition reaction network, which acts as a key mechanistic step during combustion and detonation. However, direct in situ measurements of these species have not been reported previously, and their formation mechanism remains unclear. Herein, we present direct probing of gas phase decomposition intermediates, particularly reactive initial products with parent molecule's caged skeletons from CL-20, by using in situ atmospheric pressure photoionization high-resolution mass spectrometry. The detected intermediates mainly include NO2-elimination products (C6H5O10N11, C6H6O8N10, etc.), O-elimination product (C6H6O11N12), and NO-elimination products (C6H6O11N11, etc.). Bimolecular reaction pathways involving initial intermediate C6H6O10N11 and NO2/NO are proposed and evaluated to account for the formation of initial products.

Spectroscopic evidence of multigap superconductivity in noncentrosymmetric AuBe

AuBe is a chiral, non-centrosymmetric superconductor with transition temperature $T_C$ $\simeq$ 3.25 K. The broken inversion symmetry in its crystal structure makes AuBe a possible candidate to host a mixed singlet-triplet pairing symmetry in its superconducting order parameter ($\Delta$). This possibility was investigated by transport, thermodynamic, and muon-spin rotation/relaxation experiments in AuBe. However, this issue was not addressed using direct spectroscopic probes so far. In addition, certain ambiguities exist in the description of superconductivity in AuBe based on $\mu$SR experiments reported earlier. Here we report scanning tunneling spectroscopy (STS) on AuBe down to 300 mK. We found a signature of two superconducting gaps (with 2$\Delta_1/k_{B}T_{C}$ = 4.37 and 2$\Delta_2/k_{B}T_{C}$ = 2.46 respectively) and a clean BCS-like temperature dependence of both the gaps. We have also performed band structure calculations to identify the different bands that might give rise to the observed two-gap superconductivity in AuBe.

Defect Engineering in Metal-Organic Framework Nanocrystals: Implications for Mechanical Properties and Performance.

The growth process of metal–organic framework (MOF) nanocrystals defines their properties and functions. However, defects may be prevalent during the crystallization of even seemingly perfect MOFs, such as zeolitic imidazolate framework-8 (ZIF-8), and yet direct probing of such structural defects has been challenging because of the lack of nanoscale techniques to locally examine individual nanocrystals. Here, we directly study local defects, such as missing linkers or metal vacancies, in ZIF-8 nano- and microcrystals with near-field IR nanospectroscopy combined with density functional theory calculations. We track the chemical changes during crystallization and show that structural defects like zinc cations that are bound to molecules of the reactant gradually disappear with ripening of the crystals, while dangling and missing linker defects prevail. The resulting defect-terminating groups or open-metal sites produce mechanical anisotropy and reduce the Young’s modulus, as measured via tip force microscopy with nanoscale resolution and supported by theoretical modeling. However, these structural defects also open the door for defect engineering to tune the performance of ZIF-8 by offering additional adsorption sites for targeted catalytic reactions, chemical sensing, or gas capture.

Electronic structure of 2D van der Waals crystals and heterostructures investigated by spatially- and angle-resolved photoemission

Angle-resolved photoemission is a direct probe of the momentum-resolved electronic structure and proved influential in the study of bulk crystals with novel electronic properties. Thanks to recent technical advances, this technique can now be applied for the first time for the study of van der Waals heterostructures built by stacking two-dimensional crystals. In this article we will present the current state of the art in angle-resolved photoemission measurements on two-dimensional materials and review this still young field. We will focus in particular on devices similar to those used in transport and optics experiments, including the latest developments on magic-angle twisted bilayer graphene and on the in-operando characterization of gate tunable devices.

Surface facets dependent oxygen evolution reaction of single Cu2O nanoparticles

Understanding and establishing the structure-activity relation of nanoparticles is a prerequisite for rational design of high-performance electrocatalysts. Cu 2 O nanoparticles enclosed with different crystal facets, namely, o-Cu 2 O NPs with {111} facets, c-Cu 2 O NPs with {100} facets are prepared and their electrocatalytic properties for oxygen evolution reaction (OER) in alkaline condition are evaluated at single nanoparticle level with a combination of scanning electrochemical cell microscopy and scanning electron microscopy. It is found that the o-Cu 2 O NPs have significantly superior OER electrocatalytic activity compared to c-Cu 2 O, which is almost inert. The estimated turnover frequency (TOF) at 1.97 V vs. RHE on {111} facet increases from 4 s −1 to 115 s −1 with the octahedron edge length decreasing from 1.3 µm to 100 nm. Deposition of carbon on c-Cu 2 O surface barely promotes the activity, suggesting the inherent poor electric conductivity within the nanocrystal is most likely the reason for low activity. This work provides direct probing to single transition metal oxide crystals with dramatically different activity. We endeavor to directly establish the correlation between nanoparticle surface facets with the intrinsic electrocatalytic OER activity from electrochemical measurement at single Cu 2 O nanoparticles using a combination of SECCM and SEM.

Space charge and trap energy level characteristics of SiC wide bandgap semiconductor

Charge carrier transport and accumulation in silicon carbide (SiC) wide bandgap semiconductors caused by the defect and impurity are likely to lead to serious performance degradation and failure of the semiconductor materials, and the high temperature effect makes the charge behaviors more complex. In this paper, charge carrier transport and accumulation in semi-insulating vanadium doped 4H–SiC crystal materials and the correlated temperature effect were investigated. Attempts were made to address the effect of deep trap levels on carrier transport. A combination of pulsed electro-acoustic direct space charge probing, an electrical conduction·current experiment, and x-ray diffraction measurement was employed. Space charge quantities including trap depth and trap density were extracted. The results show hetero-charge accumulation at adjacent electrode interfaces under a moderate electrical stress region (5–10 kV/mm). The charge carrier transports along the SiC bulk and is captured by the deep traps near the electrode interfaces. The deep trap energy levels originating from the vanadium dopant in SiC crystals are critical to carrier transport, providing carrier trapping sites for charges. This paper could promote the understandings of the carrier transport dynamic and trap energy level characteristic of SiC crystal materials.