Multiple antiferromagnetic phases and magnetic anisotropy in exfoliated CrBr3_3 multilayers

In twisted two-dimensional (2D) magnets, the stacking dependence of the magnetic exchange interaction can lead to regions of ferromagnetic and antiferromagnetic interlayer order, separated by non-collinear, skyrmion-like spin textures. Recent experimental searches for these textures have focused on CrI$_3$, known to exhibit either ferromagnetic or antiferromagnetic interlayer order, depending on layer stacking. However, the very strong uniaxial anisotropy of CrI$_3$ disfavors smooth non-collinear phases in twisted bilayers. Here, we report the experimental observation of three distinct magnetic phases -- one ferromagnetic and two antiferromagnetic -- in exfoliated CrBr$_3$ multilayers, and reveal that the uniaxial anisotropy is significantly smaller than in CrI$_3$. These results are obtained by magnetoconductance measurements on CrBr$_3$ tunnel barriers and Raman spectroscopy, in conjunction with density functional theory calculations, which enable us to identify the stackings responsible for the different interlayer magnetic couplings. The detection of all locally stable magnetic states predicted to exist in CrBr$_3$ and the excellent agreement found between theory and experiments, provide complete information on the stacking-dependent interlayer exchange energy and establish twisted bilayer CrBr$_3$ as an ideal system to deterministically create non-collinear magnetic phases

Ultrafast field-driven monochromatic photoemission from carbon nanotubes

Ultrafast electron pulses, combined with laser-pump and electron-probe technologies, allow for various forms of ultrafast microscopy and spectroscopy to elucidate otherwise challenging to observe physical and chemical transitions. However, the pursuit of simultaneous ultimate spatial and temporal resolution has been largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. State-of-the-art photon-driven sources have good monochromaticity but poor phase synchronization. In contrast, field-driven photoemission has much higher light phase synchronization, due to the intrinsic sub-cycle emission dynamics, but poor monochromaticity. Such sources suffer from larger electron energy spreads (3 - 100 eV) attributed to the relatively low field enhancement of the conventional metal tips which necessitates long pump wavelengths (> 800 nm) in order to gain sufficient ponderomotive potential to access the field-driven regime. In this work, field-driven photoemission from ~1 nm radius carbon nanotubes excited by a femtosecond laser at a short wavelength of 410 nm has been realized. The energy spread of field-driven electrons is effectively compressed to 0.25 eV outperforming all conventional ultrafast electron sources. Our new nanotube-based ultrafast electron source opens exciting prospects for attosecond imaging and emerging light-wave electronics

The heat and mass transfer characteristics of superheated steam in horizontal wells with toe-point injection technique

Abstract Little efforts were done on the heat and mass transfer characteristics of superheated steam (SHS) flow in the horizontal wellbores. In this paper, a novel numerical model is presented to analyze the heat and mass transfer characteristics of SHS in horizontal wellbores with toe-point injection technique. Firstly, with consideration of heat exchange between inner tubing (IT) and annuli, a pipe flow model of SHS flow in IT and annuli is developed with energy and momentum balance equations. Secondly, coupled with the transient heat transfer model in oil layer, a comprehensive mathematical model for predicting distributions of pressure and temperature of SHS in IT and annuli is established. Then, type curves are obtained with numerical methods and iteration technique, and sensitivity analysis is conducted. The results show that (1). The decrease in SHS temperature in annuli caused by heat and mass transfer to oil layer is offset by heat absorbtion from SHS in IT. (2). SHS temperature in both IT and annuli increases with the increase in injection pressure. (3). IT heat loss rate decreases with the increases in injection pressure. (4). Increasing pressure can improve development effect

Effect analysis of non-condensable gases on superheated steam flow in vertical single-tubing steam injection pipes based on the real gas equation of state and the transient heat transfer model in formation

Abstract Huge amount of efforts were done on saturated steam flow in wellbores with relatively little work on superheated multi-component thermal fluid (SMTF) flow in wellbores. In this paper, based on the continuity, energy and momentum balance equations, a flow model in the vertical wellbores is proposed. Then, coupled with the real gas model and transient heat flow model in formation, a comprehensive model is established for estimating thermophysical properties of SMTF in wellbores. Results show that (a) the effect of mass content of non-condensing gases on temperature profiles is negligible. The enthalpy of SMTF decreases rapidly with increasing of mass content of non-condensing gases. (b) When the injection rate is small, heat loss is the main factor on temperature drop, while when the injection rate is large enough, pressure drop becomes the dominant factor on temperature drop. (c) The two components of non-condensing gases and superheated steam in SMTF have a relatively independent mechanism of enhanced oil recovery, which should be selected based on the unique characteristics of each reservoir

New analytical equations for productivity estimation of the cyclic CO2-assisted steam stimulation process considering the non-Newtonian percolation characteristics

Abstract The research course in the estimation of productivity of cyclic steam stimulation wells can be divided into three stages: (a) the mobility of heavy oil in the cold area is neglected, (b) the mobility of heavy oil in the cold area is considered—however, it is Newtonian fluid seepage, and (c) it is conserved as non-Newtonian fluid seepage in the cold area. However, the distribution of the value of starting pressure gradient in the heated area where heavy oil is still non-Newtonian fluid is neglected. In this paper, a new model is developed for productivity estimation of cyclic steam stimulation wells with consideration of the non-Newtonian fluid flow behaviors in the heated area where the temperature is higher than the turning point. New percolation equations are developed based on the new proposed concept of “the transition region” in the heated area. The results show that: (1) when the non-Newtonian fluid characteristic is neglected, the predicted results from the new model match the results from the numerical simulator perfectly, and (2) in oil field, the non-Newtonian fluid characteristic cannot be neglected. When the non-Newtonian fluid characteristic is considered in the model, the average oil production in each cycle can match the filed data better than Yang et al.’s model. This new model laid a basic reference for oil companies and researchers involved in the area when they are designing the well pattern, spacing or estimating the productivity of oil wells

Effect of gaseous CO2 on superheated steam flow in wells

In this paper, a novel model is proposed for estimating pressure and temperature in wellbores when CO2 and superheated steam (SHS) are co-injected. Firstly, a model comprised of mass, energy and momentum conservation equations are developed. Then, Coupled with real gas model and heat transfer model in formation, a comprehensive model is established. The mass, momentum and energy balance equations are solved simultaneously with finite difference method on space and the iteration method. Finally, sensitivity analysis is conducted. Results show that (a). In order to obtain a higher superheat degree, a higher injection temperature and a lower mass fraction of CO2 are suggested. (b). Superheat degree decreases with increasing injection pressure or with increasing mass fraction of CO2. (c). Superheat degree increases with increasing mass flow rate. (d). Superheat degree decreases with increasing mass fraction of CO2

Effect of physical heating on productivity of cyclic superheated steam stimulation wells

Abstract Previous works have focused on the single factor analysis of the effects of chemical reactions of superheated steam with oil and rock minerals on the oil well productivity. However, the relationship between the factors and the contributions to productivity is still unknown. In this paper, the contribution of physical heating of superheated steam to well productivity is studied with the numerical method. Results show that: (a) the heat in the area has a very limited increase when the temperature of superheated steam continues to increase. (b) At the starting stage, the oil is heated to a higher temperature and the mobility is increased. The elastic energy becomes the dominant factor controlling the productivity of the oil well in the following stage. (c) The chemical reactions of superheated steam with oil and rock minerals are the dominant factors contributing to the productivity

Effect of critical thickness on nanoconfined water fluidity: review, communication, and inspiration

Abstract It is crucial to precisely estimate the water transport behaviors in shale formation. However, the present study on this subject is quite limited. A comprehensive literature review is conducted and some improvements are proposed. In this paper, an improved model is proposed to investigate the flow of water in nanopores of shale formation. First, a quadratic equation is proposed to build the relationship between water viscosity and contact angle. Then, the effect of critical thickness on water transport behaviors is discussed. Results show that: (a) the flow enhancement is smaller than 1 when the contact angle is smaller than 100° due to energy barrier induced by strong hydrophilicity of the nanopore wall; (b) the flow enhancement becomes infinite when the contact angle is approaching 180°; and (c) the flow enhancement increases with decreasing of critical thickness, especially for hydrophilic nanopores (the contact angle is smaller than 120°) and nanopores with a relatively small diameter (smaller than 50 nm)