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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Two-dimensional ferromagnetic van der Waals CrCl 3 monolayer with enhanced anisotropy and Curie temperature

Among the recently widely studied van der Waals layered magnets Cr X 3 ( X = Cl, Br, I), CrCl 3 monolayer (ML) is particularly puzzling: it is shown by experiments to have only an in-plane magnetic easy axis and, furthermore, all previous first-principles calculation results contradict this. Through systematic first-principles calculations, we unveil that the in-plane shape anisotropy that dominates over the weak perpendicular magnetocrystalline anisotropy is responsible for the in-plane magnetic easy axis of CrCl 3 ML. Further, to tune the in-plane ferromagnetism of CrCl 3 ML into the desirable perpendicular one, we propose substituting Cr with isovalent tungsten (W). We find that CrWCl 6 has a strong perpendicular magnetic anisotropy and a high Curie temperature up to 76 K. Our work not only gives insight into understanding the two-dimensional ferromagnetism of van der Waals MLs but also sheds new light on engineering their performances for nanodevices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Density functionals combined with van der Waals corrections for graphene adsorbed on layered materials

Standard density functionals like Perdew-Burke-Ernzerhof (PBE) or Strongly Constrained and Appropriately Normed (SCAN) need a correction to account for long-range van der Waals (vdW) interaction. The damped Zaremba-Kohn model (dZK) starts from a formula for the vdW interaction of a distant atom with a solid surface, both with known dielectric properties, damps this formula at short range, and then treats an adsorbed molecule or atomic layer as a collection of renormalized atoms. Here, we extend this model to graphene adsorbed on semiconducting layered materials [bulk graphite and hBN, and multilayer transition metal dichalcogenides (TMD)] by including the C 4 asymptotic term and multiple electrostatic image effects due to the two surfaces of the substrate slabs in the vdW calculations. The resulting SCAN-vdW-dZK and PBE-vdW-dZK give approximately the same results for the systems considered here, in agreement with available reference values. The predicted binding energies are roughly 25% lower than those from SCAN+rVV10 (revised Vydrov and Van Voorhis 2010), and ~15% lower than those from PBE+rVV10. Since SCAN+rVV10 usually overbinds, the predicted binding energies by SCAN-vdW-dZK and PBE-vdW-dZK are expected to be closer to the true values. The predicted equilibrium binding distances from SCAN-vdW-dZK and PBE-vdW-dZK are slightly larger (~ 0.1 Å) than those from SCAN+rVV10, and close to those from SCAN. The binding energy depends upon the number of substrate layers more strongly in vdW-dZK than in rVV10. The C 4 -term contributions can be 40% of the total vdW interactions, and the C 5 term contributes about 10%. The effects of images and the back surfaces of slabs can contribute about 4–10%. The vdW interaction energy power laws from the vdW-dZK model for graphene adsorbed on multilayer MoS 2 show slowly varying decay to the pairwise exponent –4 with increasing separation $\textit{D}$, very similar to those obtained from the random-phase approximation and renormalization group approaches by Ambrosetti et al. Both the PBE-vdW-dZK and SCAN-vdW-dZK give a greater increase in interlayer binding energy when the TMD substrate changes from monolayer to four-layer for the graphene/TMD adsorption systems. This is consistent with the relevant electron-energy-loss spectroscopy experimental results showing increased dielectric response from the substrate with increasing substrate layer number.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

van der Waals corrected density functionals for cylindrical surfaces: Ammonia and nitrogen dioxide adsorbed on a single-walled carbon nanotube

In this work, we extend the damped Zaremba-Kohn model (dZK) for long-range dispersion interaction between a molecule and a planar surface to molecules adsorbed on a curved cylindrical surface, and employ this extended model as an additive correction to the semilocal density functionals PBE (Perdew-Burke-Ernzerhof) and SCAN (strongly constrained and appropriately normed). The resulting PBE+vdW (van der Waals)-dZK and SCAN+vdW-dZK are applied to two systems, NH3 and NO2 molecules adsorbed on a single-wall carbon nanotube (CNT), for calculations of binding energies and equilibrium distances. For comparison, the results from vdW nonlocal functionals, such as SCAN+rVV10 and PBE+rVV10, are also presented. The binding energies from PBE+rVV10 (Vydrov and Van Voorhis), SCAN+rVV10, PBE+vdW-dZK, and SCAN+vdW-dZK are about 70–115 meV for the system of CNT + NH 3 and 300–500 meV for the system of CNT + NO 2 . The results from PBE+vdW-dZK and SCAN+vdW-dZK are closer to each other than those from PBE+rVV10 and SCAN+rVV10 are. The relatively closer results from PBE+vdW-dZK and SCAN+vdW-dZK indicate the consistency of our developed vdW–dZK model for cylindrical surfaces. All methods, including PBE, SCAN, PBE+rVV10, SCAN+rVV10, PBE+vdW-dZK, and SCAN+vdW-dZK, give approximately the same binding energy differences between two adsorption configurations (types I and II) for the two systems. This implies that the two adsorption sites have approximately the same adsorption stability. The exponent of the vdW interaction power law from our vdW-dZK model for the two systems is about 0 at short distance, largely due to the damping factor, and tends slowly to –4 to –4.5 at distances D about 20–50 Å. At even larger distances, the vdW power-law exponent approaches –5. This feature is very similar to the one calculated with random-phase approximation and renormalization group approaches, supporting the applicability of our methods. Our developed vdW-dZK method provides a highly efficient and reliable method for large systems with cylindrical surfaces, such as vdW interactions with nanotubes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Electronic structure of rhombohedral Cr X 3 ( X = Br, Cl, I) van der Waals crystals

We perform a comprehensive analysis of both the chemical and correlated electronic structure reconstruction of rhombohedral Cr X 3 (X = Br, Cl, I) van der Waals bulk crystals. Using the generalized gradient approximation (GGA) plus dynamical mean-field theory we explicitly demonstrate the importance of local dynamical correlations for a consistent understanding of emergent Kondo quasiparticles and Mott localized electronic states, showing the interplay between material-dependent one-electron GGA line-shape and multiorbital electronic interactions. To probe the correlated paramagnetic electronic state we performed x-ray absorption spectroscopy measurements for CrCl 3 and CrBr 3 bulk crystals. Furthermore, our correlated many-body study is relevant to understanding the electronic structure reconstruction of paramagnetic Cr-trihalide crystals and should be widely applicable to other van der Waals magnetic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Observation of a van Hove singularity of a surface Fermi arc with prominent coupling to phonons in a Weyl semimetal

A van der Waals coupled Weyl semimetal NbIrTe 4 is investigated by combining scanning tunneling microscopy/spectroscopy and first-principles calculations. Here we observe a sharp peak in the tunneling conductance near the Fermi energy (E F ). Comparison with calculations indicates that the peak originates from a van Hove singularity (vHs) associated with a Lifshitz transition of the surface Fermi-arc state. Interestingly, our tunneling spectroscopy also shows signatures of strong electron-boson coupling. This is potentially due to an anomalously enhanced charge susceptibility coming from the near-E F vHs formation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

van der Waals bilayer energetics: Generalized stacking-fault energy of graphene, boron nitride, and graphene/boron nitride bilayers

The structure, thermodynamics, and band gaps in graphene/graphene, boron nitride/boron nitride, and graphene/boron nitride bilayers are determined using several different corrections to first-principles approaches to account for the dispersion interactions. While the density functional dispersion correction, van der Waals density functional, meta–generalized gradient approximation, and adiabatic fluctuation-dissipation theorem methods (ACFDT-RPA) all lead to qualitatively similar predictions, the best accuracy is obtained through the application of the computationally expensive ACFDT-RPA method. We present an accurate ACFDT-RPA-based method to determine bilayer structure, generalized stacking-fault energy (GSFE), and band gaps as a function of the relative translation states of the two layers. The GSFE data clearly identify all of the stable and metastable bilayer translations as well as the barriers between them. Furthermore, this is key for predicting the sliding, formation, and adhesion energies for homo- and hetero-bilayers, as well as for the determination of defects in such multilayer van der Waals systems. These, in turn, provide an accurate approach for determining and manipulating the spatial variation of electronic structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ab initio phonon dispersion in crystalline naphthalene using van der Waals density functionals

Acene molecular crystals are of current interest in organic optoelectronics, both as active materials and for exploring and understanding new phenomena. Phonon scattering can be an important facilitator and dissipation mechanism in charge separation and carrier transport processes. Here, we carry out density functional theory (DFT) calculations of the structure and the full phonon dispersion of crystalline naphthalene, a well-characterized acene crystal for which detailed neutron-diffraction measurements, as well as infrared and Raman spectroscopy, are available. We evaluate the performance, relative to experiments, of DFT within the local density approximation (LDA); the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE); and a recent van der Waals-corrected nonlocal correlation (vdW-DF-cx) functional. We find that the vdW-DF-cx functional accurately predicts lattice parameters of naphthalene within 1%. Intermolecular and intramolecular phonon frequencies across the Brillouin zone are reproduced within 7.8% and 1%, respectively. As expected, LDA (PBE) underestimates (overestimates) the lattice parameters and overestimates (underestimates) phonon frequencies, demonstrating their shortcomings for predictive calculations of weakly bound materials. If the unit cell is fixed to the experimental lattice parameters, PBE is shown to lead to improved phonon frequencies. Our study provides a detailed understanding of the phonon spectrum of naphthalene, and highlights the importance of including van der Waals dispersion interactions in predictive calculations of lattice parameters and phonon frequencies of molecular crystals and related organic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Synergy of van der Waals and self-interaction corrections in transition metal monoxides

Density functional theory in principle predicts correct ground-state properties for all materials. However, the rocksalt structure of MnO has been obtained only by the high-level random phase approximation and diffusion Monte Carlo (DMC). In this work, we propose and test for MnO, FeO, CoO, and NiO that a semilocal density functional can solve this problem by properly including both self-interaction and van der Waals corrections. The importance of the latter was previously unanticipated. The MnO structural energy difference from SCAN+rVV10+U agrees well with that from DMC. Here SCAN is a recent semilocal exchange-correlation functional, rVV10 is the revised Vydrov-VanVoorhis long-range van der Waals correction, and the onsite Hubbard U is taken from linear response.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Wilsonian effective field theory of two-dimensional Van Hove singularities

We study two-dimensional fermions with a short-range interaction in the presence of a Van Hove singularity. It is shown that this system can be consistently described by an effective field theory whose Fermi surface is subdivided into regions as defined by a factorization scale, and that the theory is renormalizable in the sense that all of the counterterms are well defined in the IR limit. The theory has the unusual feature that the renormalization-group equation for the coupling has an explicit dependence on the renormalization scale, much as in theories of Wilson lines. In contrast to the case of a round Fermi surface, there are multiple marginal interactions with nontrivial RG flow. The Cooper instability remains strongest in the BCS channel. Here, we also show that the marginal Fermi-liquid scenario for the quasiparticle width is a robust consequence of the Van Hove singularity. Our results are universal in the sense that they do not depend on the detailed properties of the Fermi surface away from the singularity.

2-dimensional systems↗
DOE OSTI2019

Chemical disorder and spin-liquid-like magnetism in the van der Waals layered 5 d transition metal halide Os 0.55 Cl 2

Here, single crystals of the van der Waals layered 5d transition-metal compound Os 0.55 Cl 2 were grown and characterized by x-ray diffraction, magnetization and heat-capacity measurements, and atomic resolution electron microscopy. The crystals are stable in air and easily cleaved. The structure is derived from the CdCl 2 structure type, with triangular layers of transition metal sites coordinated by edge-sharing octahedra of Cl and separated by a van der Waals gap. On average, only 55% of the metal sites are occupied by Os, and evidence for short- and long-ranged vacancy orders is observed by diffraction and real-space imaging. Magnetization data indicate magnetocrystalline anisotropy due to spin-orbit coupling, antiferromagnetic correlations, and no sign of magnetic order or spin freezing down to 0.4 K. Heat-capacity measurements in applied magnetic fields show only a broad, field-dependent anomaly. The magnetic susceptibility and heat capacity obey power laws at low temperature and low field with exponents close to 0.5. The power law behaviors of the low-temperature heat capacity and magnetic susceptibility suggest gapless magnetic fluctuations prevent spin freezing or ordering in Os 0.55 Cl 2 . Divergence of the magnetic Gruneisen parameter indicates nearness to a magnetic quantum critical point. Similarities to behaviors of spin-liquid materials are noted, and in total the findings suggest Os 0.55 Cl 2 may be an example of a quantum spin liquid in the limit of strong chemical disorder.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Compact interaction potential for van der Waals nanorods

Here, we studied the van der Waals interactions of two finite, solid, cylindrical rods at arbitrary angle and position with respect to each other. An analytic interpolative formula for the interaction potential energy is constructed based on various asymptotic cases. In conclusion, the potential can be readily used for numerical and analytic descriptions of multiwall carbon nanotubes, metallic nanorods, rod-shaped colloids, or any other similar objects with significant van der Waals interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Stacking Domain Wall Magnons in Twisted van der Waals Magnets

In this report using bilayer CrI 3 as an example, we demonstrate that stacking domain walls in van der Waals magnets can host one-dimensional (1D) magnon channels, which have lower energies than bulk magnons. Interestingly, some magnon channels are hidden in magnetically homogeneous background and can only be inferred with the knowledge of stacking domain walls. Compared to 1D magnons confined in magnetic domain walls, 1D magnons in stacking domain walls are more stable against external perturbations. We show that the relaxed moiré superlattices of small-angle twisted bilayer CrI 3 is a natural realization of stacking domain walls and host interconnected moiré magnon network. Our Letter reveals the importance of stacking domain walls in understanding magnetic properties of van der Waals magnets and extends the scope of stacking engineering to magnetic dynamics.

2-dimensional system↗
DOE OSTI2021

Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet

Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile playground for the discovery and exploration of physical phenomena and new physics. In this Letter, we report the observation of an anomalous thermal Hall effect (THE) with κ xy ~ 1 × 10 -2 WK -1 m -1 in an insulating van der Waals ferromagnet VI 3 . The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. In combination with theoretical calculations, we show that VI 3 exhibits a dual nature of the THE, i.e., dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures and by phonons induced by the magnon-phonon coupling at lower temperatures. Our results not only position VI 3 as the first ferromagnetic system to investigate both anomalous magnon and phonon THEs, but also render it as a potential platform for spintronics-magnonics applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Emergence of the Chern Supermetal and Pair-Density Wave through Higher-Order Van Hove Singularities in the Haldane-Hubbard Model

While advances in electronic band theory have brought to light new topological systems, understanding the interplay of band topology and electronic interactions remains a frontier question. In this work, we predict new interacting electronic orders emerging near higher-order Van Hove singularities present in the Chern bands of the Haldane model. We classify the nature of such singularities and employ unbiased renormalization group methods that unveil a complex landscape of electronic orders, which include ferromagnetism, density waves, and superconductivity. Importantly, we show that repulsive interactions can stabilize the long-sought pair-density-wave state and an exotic Chern supermetal, which is a new class of non-Fermi liquid with anomalous quantum Hall response. Further, this framework opens a new path to explore unconventional electronic phases in two-dimensional chiral bands through the interplay of band topology and higher-order Van Hove singularities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Molecule-surface interaction from van der Waals-corrected semilocal density functionals: The example of thiophene on transition-metal surfaces

Semilocal density functional approximations are widely used. None of them can capture the long-range van der Waals (vdW) attraction between separated subsystems, but they differ remarkably in the extent to which they capture intermediate-range vdW effects responsible for equilibrium bonds between neighboring small closed-shell subsystems. The local density approximation (LDA) often overestimates this effect, while the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) underestimates it. The strongly constrained and appropriately normed (SCAN) meta-GGA often estimates it well. All of these semilocal functionals require an additive nonlocal correction such as the revised Vydrov-Van Voorhis 2010 (rVV10) to capture the long-range part. This work reports adsorption energies and the corresponding geometry of aromatic thiophene (C 4 H 4 S) bound to transition-metal surfaces. The adsorption process requires a genuine interplay of covalent and weak binding and requires a simultaneously accurate description of surface and adsorption energies with the correct prediction of the adsorption site. All these quantities must come from well-balanced short- and long-range correlation effects for a universally applicable method for weak interactions with chemical accuracy. Here, our methods indicate that the correct interplay is not present in any combination of recent meta-GGA's and rVV10. The simple short-range damping of the vdW correction scheme that is practically successful in the combination of GGA's and vdW approximations is less transferable in SCAN+rVV10 or in the revised version, revSCAN+rVV10. In addition, we present accurate random-phase-approximation-quality adsorption energies from a model based on the one of Zaremba and Kohn.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Observation of van der Waals phonons in the single-layer cuprate ( Bi , Pb ) 2 ( Sr , La ) 2 CuO 6 + δ

Interlayer van der Waals (vdW) coupling is generic in two-dimensional materials such as graphene and transition-metal dichalcogenides, which can induce very low-energy phonon modes. Using high-resolution inelastic hard x-ray scattering, we uncover the ultralow energy phonon mode along the Cu-O bond direction in the high- T c cuprate (Bi,Pb) 2 (Sr,La) 2 ⁢CuO 6+$\delta$ (Bi2201). The energy and full-width half-maximum (FWHM) of these modes are independent of temperature, while their intensity decreases with doping in accordance with an increasing c -axis lattice parameter. Here, we compare the experimental results to first-principles density functional theory simulations and identify the observed mode as a van der Waals phonon, which arises from the shear motion of the adjacent Bi-O layers. This shows that Bi-based cuprate has vibrational properties similar to graphene and transition-metal dichalcogenides, which can be exploited to engineer novel heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Treatment and aging studies of GaAs(111)B substrates for van der Waals chalcogenide film growth

GaAs(111)B are commercially available substrates widely used for the growth of van der Waals chalcogenide films. Wafer-scale, high-quality crystalline films can be deposited on GaAs(111)B substrates using molecular beam epitaxy. However, two obstacles persist in the use of GaAs(111)B: first, the surface dangling bonds make it challenging for the growth of van der Waals materials; second, the As-terminated surface is prone to aging in air. This study investigated a thermal treatment method for deoxidizing GaAs(111)B substrates while simultaneously passivating the surface dangling bonds with Se. By optimizing the treatment parameters, we obtained a flat and completely deoxidized platform for subsequent film growth, with highly reproducible operations. Furthermore, through first-principle calculations, we find that the most energetically favorable surface of GaAs(111)B after Se passivation consists of 25% As atoms and 75% Se atoms. Finally, we discovered that the common storage method using food-grade vacuum packaging cannot completely prevent substrate aging, and even after thermal treatment, aging still affects subsequent growth. Therefore, we recommend using N2-purged containers for better preservation.

Engineering↗
DOE OSTI2013

Experimental validation of the van Herk margin formula for lung radiation therapy

Purpose: To validate the van Herk margin formula for lung radiation therapy using realistic dose calculation algorithms and respiratory motion modeling. The robustness of the margin formula against variations in lesion size, peak-to-peak motion amplitude, tissue density, treatment technique, and plan conformity was assessed, along with the margin formula assumption of a homogeneous dose distribution with perfect plan conformity.Methods: 3DCRT and IMRT lung treatment plans were generated within the ORBIT treatment planning platform (RaySearch Laboratories, Sweden) on 4DCT datasets of virtual phantoms. Random and systematic respiratory motion induced errors were simulated using deformable registration and dose accumulation tools available within ORBIT for simulated cases of varying lesion sizes, peak-to-peak motion amplitudes, tissue densities, and plan conformities. A detailed comparison between the margin formula dose profile model, the planned dose profiles, and penumbra widths was also conducted to test the assumptions of the margin formula. Finally, a correction to account for imperfect plan conformity was tested as well as a novel application of the margin formula that accounts for the patient-specific motion trajectory.Results: The van Herk margin formula ensured full clinical target volume coverage for all 3DCRT and IMRT plans of all conformities with the exception of small lesions in soft tissue. No dosimetric trends with respect to plan technique or lesion size were observed for the systematic and random error simulations. However, accumulated plans showed that plan conformity decreased with increasing tumor motion amplitude. When comparing dose profiles assumed in the margin formula model to the treatment plans, discrepancies in the low dose regions were observed for the random and systematic error simulations. However, the margin formula respected, in all experiments, the 95% dose coverage required for planning target volume (PTV) margin derivation, as defined by the ICRU; thus, suitable PTV margins were estimated. The penumbra widths calculated in lung tissue for each plan were found to be very similar to the 6.4 mm value assumed by the margin formula model. The plan conformity correction yielded inconsistent results which were largely affected by image and dose grid resolution while the trajectory modified PTV plans yielded a dosimetric benefit over the standard internal target volumes approach with up to a 5% decrease in the V20 value.Conclusions: The margin formula showed to be robust against variations in tumor size and motion, treatment technique, plan conformity, as well as low tissue density. This was validated by maintaining coverage of all of the derived PTVs by 95% dose level, as required by the formal definition of the PTV. However, the assumption of perfect plan conformity in the margin formula derivation yields conservative margin estimation. Future modifications to the margin formula will require a correction for plan conformity. Plan conformity can also be improved by using the proposed trajectory modified PTV planning approach. This proves especially beneficial for tumors with a large anterior–posterior component of respiratory motion.

61 RADIATION PROTECTION AND DOSIMETRY↗