Oxygen cost of ventilation during incremental exercise to VO2 max

Objective: Evidence of significant oxygen requirements for ventilation during exercise (exercise hyperpnoea) prompted the investigation into whether the oxygen cost of ventilation affects the presence of a whole‐body VO₂ plateau at maximal exercise. The purposes of this study were to: (i) use isocapnic hyperpnoea trials to determine the oxygen cost of ventilation (VO_2VENT) across a range of ventilation (V_E); (ii) determine the mean VO_2VENT at maximal exercise expressed as a percentage of whole‐body VO₂ max; and (iii) determine if a plateau in VO₂ is more evident when the VO_2VENT is subtracted from whole‐body VO₂ at maximal exercise. Methods: A total of 21 subjects performed a VO₂ max test on the cycle ergometer to determine the range of V_E for each subject. From the initial VO₂ max test, nine V_E values across the range of V_E were selected for each subject and the oxygen cost of each was measured. Results: The mean maximal VO_2VENT equalled 8.8 ± 3.3% of VO₂ max and ranged from 5.0% to 17.6%. VO_2VENT increased exponentially with increasing V_E, but there was considerable subject variability in the oxygen cost per litre of V_E as V_E increased. Subtracting the VO_2VENT from whole‐body VO₂ at maximal exercise increased the detection of a plateau in VO₂ at VO₂ max. Conclusions: The data of the present study indicate that the VO_2VENT is a significant portion of VO₂ max and may be a limiting factor of maximal exercise performance in some subjects.     

Inherent stochasticity during insulator–metal transition in VO2

Vanadium dioxide (VO₂), which exhibits a near-room-temperature insulator–metal transition, has great potential in applications of neuromorphic computing devices. Although its volatile switching property, which could emulate neuron spiking, has been studied widely, nanoscale studies of the structural stochasticity across the phase transition are still lacking. In this study, using in situ transmission electron microscopy and ex situ resistive switching measurement, we successfully characterized the structural phase transition between monoclinic and rutile VO₂ at local areas in planar VO₂/TiO₂ device configuration under external biasing. After each resistive switching, different VO₂ monoclinic crystal orientations are observed, forming different equilibrium states. We have evaluated a statistical cycle-to-cycle variation, demonstrated a stochastic nature of the volatile resistive switching, and presented an approach to study in-plane structural anisotropy. Our microscopic studies move a big step forward toward understanding the volatile switching mechanisms and the related applications of VO₂ as the key material of neuromorphic computing.

Resistive switching in vanadium oxides has attracted much attention because of the potential applications in bioinspired neuromorphic computing and nonvolatile memories (1–4). In a basic neural network, neurons can generate nonlinear electric spikes under external excitations, while the synapses allow for the modulation of interconnected weights between neurons. In most hardware-based neuromorphic approaches, volatile switching devices (threshold switching) were used to emulate the artificial spiking neuronal behaviors, while nonvolatile switching memories were often used to mimic the synaptic functionalities (1). VO₂, whose insulator–metal transition (IMT) is above room temperature (T_IMT ∼340 K) (5), has been widely studied as a neuristor (6). It is considered a promising candidate for energy-efficient neurons due to its threshold spiking phenomenon. In pristine VO₂ it is believed that the resistive switching can be triggered by local Joule heating across the IMT, while nonthermal switching can be induced using defect engineering (7). Recently, both volatile and nonvolatile functionalities have been achieved in VO₂ based on different switching mechanisms (8). In previous work, in situ X-ray nanomapping was used to track the out-of-plane monoclinic and rutile Bragg peaks during resistive switching (9). However, up to now the nanoscale in-plane structural anisotropy and its intrinsic stochasticity have been rarely explored (10, 11): How the system returns from metallic state to insulating state and how the structural transition influences the switching. In addition, the control of the switching usually appears to be imprecise, i.e., the voltage necessary to initiate the phase transition is not a constant (even within a single grain) (12). Therefore, it is of critical importance to understand the stochastic behavior within the neuron device and the phase transition mechanism at nanoscale under external stimuli, since the switching has to be done a large number of times during the neuron firing process. In addition, the IMT of VO₂ nanodevices can show multiple jumps, which implies that the domain structures in VO₂ may influence the transition behavior (13). Also, many physical properties, such as electrical, magnetic, and optical properties, show significant changes during the IMT process (14–16).Here, we use in situ biasing transmission electron microscopy (TEM) with ex situ electrical transport measurement to characterize the structural phase transition across the volatile switching in VO₂. In-plane resistive switching was studied in epitaxial VO₂/TiO₂ (001) nanodevices. The original in-plane structural isotropy in the VO₂ rutile structure (R phase) will lead to anisotropy in the monoclinic phase (M1 phase) after resistive switching. Multiple monoclinic domains will form when the system returns to the insulating state. The IMT phase transition and the simultaneous electrical properties change have been systematically studied. Our results demonstrate that the structural anisotropy between different M1 VO₂ domains plays an important role in the insulating state. In addition, this study provides a thorough understanding of the volatile resistive switching process and shows that the intrinsic variability is a key issue in developing stochastic neuromorphic networks.     

Reproducibility of VO2max in patients with chronic air-flow obstruction

The purpose of this study was to determine the variability in maximal oxygen consumption (VO2 max) determined from repeated exercise tests in patients with chronic air-flow obstruction (CAO). Three incremental maximal cycle ergometer tests were performed in each of 11 CAO patients who were familiar with such testing. Two tests (Test 1, Test 2) were carried out on the same day, separated by a 60-min rest period, and 1 (Test 3) was performed on a consecutive day. Group mean values for VO2 max were: 1.313 +/- 0.259, 1.311 +/- 0.281, 1.306 +/- 0.288 L/min, for Tests 1 to 3, respectively. These nearly identical values did not differ significantly. There was no systematic "fatigue" or "learning" effect from test to test. Other mean measurements obtained at maximal exercise were likewise not significantly different among the 3 tests. For tests performed on the same day, the mean of the absolute values of the individual patient VO2 max differences (delta VO2 max) was 53 +/- 30 ml. The delta VO2 max was less than 6% in 10 of the 11 patients and less than 10% in the remaining patient. For tests performed on consecutive days, delta VO2 max was 93 +/- 81 ml; delta VO2 max was less than 6% in 6 patients and less than 10% in 9 patients. From this study we concluded that repeated maximal exercise testing is highly reproducible for groups of CAO patients, although some individual patient variability is seen. Individual patient differences in delta VO2 max are less when the 2 tests are performed on the same day than when duplicate testing is performed on consecutive days.     

Operando characterization of conductive filaments during resistive switching in Mott VO2

Vanadium dioxide (VO₂) has attracted much attention owing to its metal–insulator transition near room temperature and the ability to induce volatile resistive switching, a key feature for developing novel hardware for neuromorphic computing. Despite this interest, the mechanisms for nonvolatile switching functioning as synapse in this oxide remain not understood. In this work, we use in situ transmission electron microscopy, electrical transport measurements, and numerical simulations on Au/VO₂/Ge vertical devices to study the electroforming process. We have observed the formation of V₅O₉ conductive filaments with a pronounced metal–insulator transition and that vacancy diffusion can erase the filament, allowing for the system to “forget.” Thus, both volatile and nonvolatile switching can be achieved in VO₂, useful to emulate neuronal and synaptic behaviors, respectively. Our systematic operando study of the filament provides a more comprehensive understanding of resistive switching, key in the development of resistive switching-based neuromorphic computing.

The development of smaller, faster, and more energy-efficient resistive switching devices with multifunctions is crucial for realizing neuromorphic computing systems. In these, “neurons” (neuristors) are active components which produce nonlinear electric signals under external excitations, while the “synapses” (synaptors) control the connectivity between active elements (1). The challenge in developing a brainlike computing hardware is to mimic and integrate these active components and their interconnections using electronic devices. Two different classes of materials are generally proposed to emulate neurons and synapses. Correlated insulators with metal–insulator transitions (such as vanadium dioxide [VO₂] or NbO₂) commonly feature threshold or volatile resistive switching: the transition to the metallic state can be induced by applying a voltage, and the device returns to the insulating state once the stimulus is removed. This makes them ideal candidates for reproducing neural spiking (2). On the other hand, many noncorrelated systems, such as TiO₂, HfO_x, or phase-change materials, are well known for having reversible nonvolatile resistive switching, very promising for implementing synaptic behavior (3). In the case of oxides, this switching is due to the drift of oxygen vacancies under strong electric fields. Moreover, to improve the processing speed, scalability, device density, and connectivity like those in biological systems, the development of three-dimensional (3D) cross-bar circuit is necessary (4, 5). However, fabrication of 3D stacked arrays is difficult and further studies on vertical resistive switching devices will be required.Among the different materials, VO₂ has attracted much attention because of its threshold switching behavior and its insulator–metal transition (IMT) near room temperature (6–11). The mechanism behind the volatile switching in VO₂ is believed to be a combination of thermal and/or electrical field-induced IMT in response to an external voltage/current input (12–15). However, unlike other transition-metal oxides such as TiO₂ (16, 17), the microscopic nature of the electroforming process and nonvolatile switching behavior in the Mott insulator VO₂ has not yet been explored (18). Interestingly, this provides the opportunity to realize nonvolatile resistive switching (caused by oxygen vacancy migration) coexisting with volatile resistive switching. This is of great potential, since the same material can show both neuronlike and synapselike functionalities. The nanoscale nature of the conductive filament poses a major challenge to the study of the nonvolatile switching mechanism (19). Here, combining operando transmission electron microscopy (TEM), transport measurements, and numerical simulations, we elucidate the electroforming process in Au/VO₂/Ge vertical devices. The operando TEM studies allow us to conduct simultaneous structural and transport characterizations. The atomic structure of the nanoscale conductive filament was unambiguously determined and a filament relaxation/recovery mechanism has been demonstrated. Our study unveiled the essential role of oxygen defects in the filament formation and relaxation during resistive switching.     

VO2 Max and anaerobic threshold in hypertension: a tissue Doppler study

BACKGROUND: Hypertension can impair left ventricular (LV) relaxation causing shortness of breath and reduced exercise capacity, which may affect the clinical evaluation of the symptomatic hypertensive patient. In this study we used tissue Doppler imaging (TDI) to identify correlates of anaerobic threshold (AT) and maximum oxygen uptake (VO2 Max). Our goal was to assess the feasibility of TDI as a surrogate of functional capacity in hypertensive individuals. METHODS: We studied subjects without metabolic syndrome and with normal LV function (ejection fraction (EF) >50%). Traditional echocardiographic variables were obtained before and after a cardiopulmonary exercise test. Systolic (S) and diastolic (E'and A') myocardial velocities were measured at the basal septal (bs) and posterior (bp) walls. RESULTS: After multivariate analysis, resting E'bp (r =0.56, P < 0.002) and isovolumic relaxation time (IVRT) (r =-0.49, I < 0.03) correlated with VO2 Max, while A Valsalva correlated with AT (r =-0.46, P < 0.03). Peak stress E'/A'bp correlated with age and gender corrected METs (r =-0.63, P < 0.0004) and VO2 Max (r =-0.39, P< 0.04). CONCLUSIONS: Resting E'bp and peak stress E'/A'bp correlate with VO(2) Max in hypertensive patients. TDI may be an important tool when assessing symptoms in this population.     

Oxygen-uptake (VO2) kinetics and functional mobility performance in impaired older adults

BACKGROUND: Measures of maximal oxygen uptake (VO(2max)) are limited in disabled older adults, and measures of submaximal oxygen uptake (VO(2)) may better predict functional mobility limitations. These measures may include oxygen-uptake kinetics at the onset of submaximal exercise or during recovery. We sought to determine whether the lag in oxygen uptake at the beginning of exercise (oxygen deficit) and excess oxygen uptake above rest following exercise (excess postexercise oxygen consumption) (a) predict physical performance in impaired older adults with decreased aerobic function, and (b) predict physical performance better than peak VO(2). METHODS: Two groups of community-dwelling volunteers aged 65 or older were recruited according to their performance on a maximal graded exercise test. Using the Social Security Administration criterion of disability of a peak VO(2) 18 (Unimpaired, n = 21, mean +/- SEM age 76 +/- 1 years). RESULTS: The mean +/- SEM peak VO(2) was 58% lower in the Impaired (14 +/- 1 ml/kg/min) than the Unimpaired (24 +/- 1 ml/kg/min) adults. The time constant for oxygen deficit, tc(deficit), was more than twice as high in the Impaired than the Unimpaired (p <.05), and the time constant for excess postexercise oxygen consumption, tc(EPOC), tended to be higher in the Impaired than the Unimpaired (by 43%, p =.09). Measures of submaximal oxygen-uptake kinetics were as strong or more strongly predictive of functional mobility performance than peak VO(2) in both Unimpaired and Impaired older adults. The major predictor of functional performance for the Unimpaired was a measure of oxygen deficit accruing during exercise (tc(deficit)), and for the Impaired, it was a measure of oxygen debt during recovery, tc(EPOC). CONCLUSIONS: Measurement of submaximal oxygen-uptake kinetics may provide a more practical and relevant assessment of deconditioning in frail older adults, and may eventually supplant maximal (peak) oxygen uptake as a predictor of functional disability in older adults.     

峰值氧耗量和无氧代谢阈值对慢性心力衰竭患者预后的预测价值

目的 通过心肺运动试验（CPET）检测慢性心力衰竭（CHF）患者峰值氧耗量（peak VO2）和无氧代谢阈值氧耗量（VO2AT）,并随访其预后价值。方法 选择入住同济大学附属同济医院心内科并经心脏超声确定左室射血分数（LVEF）＜0.49的CHF患者129例（经冠状动脉造影确诊的缺血性心肌病74例,扩张型心肌病55例）。对入选患者实施CPET,并对患者随访心血管死亡原因（时间中位数为33.7个月）。结果 （1）19例CHF患者因心血管原因死亡,死亡患者和非死亡患者在年龄、性别、BMI、峰值吸呼比（peak RER）差异无统计学意义的情况下（P＞0.05）,死亡患者较非死亡患者左室射血分数（LVEF）减低[（0.33±0.09） vs （0.38±0.09）,P＜0.05];左心室质量指数（LVMI）增高[（158.3±53.9） vs （133.2±40.1）,P＜0.05];peak VO2减低[（11.8±4.3） vs （14.4±3.7）ml/（kg·min）,P＜0.05];VO2AT减低[（9.3±3.2） vs （10.7±2.1）ml/（kg·min）,P＜0.05]。（2）peak VO2的ROC曲线下面积（AUC）为0.640（P＜0.05）,灵敏度为0.590,特异度为0.667,最佳阈值为peak VO2≤13.4ml/（kg·min）;VO2AT的AUC为0.600（P＞0.05）,灵敏度为0.886,特异度为0.360,最佳阈值为VO2AT≤8.2ml/（kg·min）。结论 peak VO2及VO2AT对CHF患者心血管原因死亡具有一定的预测价值,peak VO2的预测价值优于VO2AT。     

Stress-Induced In Situ Modification of Transition Temperature in VO2 Films Capped by Chalcogenide

We attempted to modify the monoclinic–rutile structural phase transition temperature (T_tr) of a VO₂ thin film in situ through stress caused by amorphous–crystalline phase change of a chalcogenide layer on it. VO₂ films on C- or R-plane Al₂O₃ substrates were capped by Ge₂Sb₂Te₅ (GST) films by means of rf magnetron sputtering. T_tr of the VO₂ layer was evaluated through temperature-controlled measurements of optical reflection intensity and electrical resistance. Crystallization of the GST capping layer was accompanied by a significant drop in T_tr of the VO₂ layer underneath, either with or without a SiN_x diffusion barrier layer between the two. The shift of T_tr was by ~30 °C for a GST/VO₂ bilayered sample with thicknesses of 200/30 nm, and was by ~6 °C for a GST/SiN_x/VO₂ trilayered sample of 200/10/6 nm. The lowering of T_tr was most probably caused by the volume reduction in GST during the amorphous–crystalline phase change. The stress-induced in in situ modification of T_tr in VO₂ films could pave the way for the application of nonvolatile changes of optical properties in optoelectronic devices.     

Peak VO2 and VE/VCO2 slope in patients with heart failure: a prognostic comparison

Background

Exercise testing with ventilatory expired gas analysis has proven to be a valuable tool for assessing patients with heart failure (HF). Peak oxygen consumption (peak VO₂) continues to be considered the gold standard for assessing prognosis in HF. The minute ventilation − carbon dioxide production relationship (VE/VCO₂ slope) has recently demonstrated prognostic significance in patients with HF, and in some studies, it has outperformed peak VO₂.

Methods

Two hundred thirteen subjects, in whom HF was diagnosed, underwent exercise testing between April 1, 1993, and October 19, 2001. The ability of peak VO₂ and VE/VCO₂ slope to predict cardiac-related mortality and hospitalization was examined.

Results

Peak VO₂ and VE/VCO₂ slope were demonstrated with univariate Cox regression analysis both to be significant predictors of cardiac-related mortality and hospitalization (P <.01). Multivariate analysis revealed that peak VO₂ added additional value to the VE/VCO₂ slope in predicting cardiac-related hospitalization, but not cardiac mortality. The VE/VCO₂ slope was demonstrated with receiver operating characteristic curve analysis to be significantly better than peak VO₂ in predicting cardiac-related mortality (P <.05). Although area under the receiver operating characteristic curve for the VE/VCO₂ slope was greater than peak VO₂ in predicting cardiac-related hospitalization (0.77 vs 0.73), the difference was not statistically significant (P = .14).

Conclusions

These results add to the present body of knowledge supporting the use of cardiopulmonary exercise testing in HF. Consideration should be given to revising clinical guidelines to reflect the prognostic importance of the VE/VCO₂ slope in addition to peak VO₂.     

Acid Solution Processed VO2-Based Composite Films with Enhanced Thermochromic Properties for Smart Windows

As a typical thermochromic material, VO₂ coatings can be applied to smart windows by modulating the transmission of near infrared (NIR) light via phase transition. However, the inherent undesirable luminous transmittance (T_lum) and solar modulation efficiency (ΔT_sol) of pure VO₂ impede its practical application. In order to solve this problem, the porous VO₂ based composite film was prepared by magnetron sputtering and subsequent acid solution process with Zn₂V₂O₇ particles used as a sacrificial template to create pores, which showed excellent T_lum (72.1%) and enhanced ΔT_sol (10.7%) compared with pure VO₂ film. It was demonstrated that the porous structure of the film caused by acid solution process could improve the T_lum obviously and the isolated VO₂ nanoparticles presented strong localized surface plasmon resonance (LSPR) effects to enhance the ΔT_sol. Therefore, this method will provide a facile way to prepare VO₂ based films with excellent thermochromic performance and thus promote the application of the VO₂ based films in smart windows.     

Influence of skinfold sum and peak VO(2) on immune function in children

OBJECTIVE: To measure the relationship of skinfold sum and peak VO(2) power with immune function in children. DESIGN: Cross-sectional, with all children tested twice during a 2 month period for peak VO(2), sum of two skinfolds, and immune function, with data from the two measures averaged and then correlated (alpha level, < or = 0.01). Immune measures included leukocyte and lymphocyte subset counts, delayed-typed hypersensitivity (DTH), global IgG antibody response over 4 weeks to pneumococcal vaccination (pIgG), salivary IgA concentration (sIgA), PHA-stimulated lymphocyte proliferation (PHA-SLP), natural killer cell activity (NKCA), and granulocyte and monocyte phagocytosis and oxidative burst activity. SUBJECTS: Seventy-three children (n=42 males, n=31 females) ranging in age from 7 to 13 y (mean+/-s.d. age, 9.9+/-1.7 y). The mean skinfold sum was 28.9+/-17.1 mm, and peak VO(2) 45.8+/-8.1 ml/kg/min. RESULTS: Peak VO(2), skinfold sum, and immune measures did not differ significantly by age or gender. Therefore, correlations were made on combined indices for all subjects. Peak VO(2) and the skinfold sum were not significantly correlated with NKCA, oxidative burst activity, plgG or DTH. Peak VO(2) was negatively correlated with monocyte phagocytosis (r=-0.30, P=0.012) and positively correlated with PHA-SLP (6.25 microg/ml; r=0.35, P=0.004). The skinfold sum was positively correlated with the total leukocyte count (r=0.39, P<0.001), granulocyte count (r=0.36, P=0.002), monocyte count (r=0.38, P=0.001), monocyte phagocytosis (r=0.41, P<0.001), granulocyte phagocytosis (r=0.35, P=0.003), and sIgA (r=0.32, P=0.006), and negatively correlated with PHA-SLP (6.25 microg/ml; r=-0.39, P=0.001). CONCLUSIONS: Data from this study indicate that a high skinfold sum is related to elevated leukocyte subset counts and monocyte/granulocyte phagocytosis, and low PHA-SLP in children.     

The different relationship of VO2max to muscle mitochondria in humans and quadrupedal animals

The relationship of VO2max to muscle oxidative capacity (estimated morphometrically as the volume of mitochondria of the entire musculature) was analyzed by compiling data on 8 species of mammals varying widely in VO2max. Over a five-fold range of weight-specific VO2max, a linear relationship between VO2max and muscle mitochondria was found. As an exception, humans seem to have 'excess' mitochondrial volume for their VO2max. However, the rate at which mitochondria operate in vivo at VO2max is found to be very similar for humans and quadrupedal animals. A larger total mitochondrial volume thus accounts for the well known finding that humans are capable of reaching VO2max with a subset of their body musculature. In quadrupedal animals the oxygen delivery system is geared to supply the entire musculature while in humans, possibly as a consequence of the upright posture, the cardiovascular system is designed to supply only a fraction of the total muscle oxidative capacity at VO2max.     

Controlled endurance exercise training and VO2max changes in older adults: a meta-analysis

This meta-analysis of controlled clinical trials quantifies the effect of aerobic exercise on VO2max (aerobic fitness) among sedentary older adults and examines the associations of such magnitude with variables of intervention. Forty-one trials including 2102 older subjects (within-group mean age of 60 years and older) were identified by searches of databases, hand searching, and cross-referencing. Outcomes were homogeneous. The pooled standardized effect size by a fixed-effect model showed a higher moderate effect (mean +/- SEM) of 0.64+/-0.05; 95% confidence interval, 0.56-0.73; p<0.001, representing a net increase in VO2max (mean +/- SEM) of 3.78+/-0.28 mL.kg-1.min-1; 95% confidence interval, 3.24-4.33; or a 16.3% improvement, compared with control groups. Greater improvement in VO2max was associated with training length more than 20 weeks and training intensity of approximately 60% but less than 70% of VO2max. Endurance training improves aerobic fitness in older adults, thus providing protective benefits for cardiovascular aging and quality of later life.     

Ileal VO(2)-O(2) alterations induced by endotoxin correlate with severity of mitochondrial injury.

Sepsis is usually associated with altered O(2) metabolism in systemic organs. Until recently, inadequate O(2) delivery was thought to be the putative mechanism underlying these metabolic alterations. However, current investigations suggest that impaired O(2) consumption due to disrupted O(2) use by mitochondria may be the culprit. Therefore, we hypothesized that endotoxin (LPS)-induced V O(2)- O(2) alterations would correlate with the severity of mitochondrial injury in a systemic organ (i.e., the ileum). Using an in situ autoperfused feline ileum preparation, we assessed V O(2)- O(2) relationships and mitochondrial ultrastructure after 2 h in LPS-treated (3 mg/kg, intravenous; n = 11) and time-matched control (n = 5) animals. Mitochondrial injury was graded in a blinded fashion on the basis of characteristics associated with established stages of cell injury. LPS-treated animals developed severe mitochondrial injury in the ileal mucosa despite unchanged regional tissue perfusion and ileal oxyhemoglobin levels compared with controls. Worsening of mitochondrial injury correlated with increases in the critical O(2) delivery (r = 0.85; p < 0.002) and decreases in the maximum O(2) extraction (r = -0.61; p < 0.02) in the ileum. These results suggest that mitochondrial injury, leading to impaired O(2) utilization, may be primarily responsible for altered V O(2)- O(2) relationships in systemic organs during sepsis.     

Evaluation of the aerobic capacity of blind people, by direct VO2 maximal measurement]

Physical activity is the basic factor to the human being in order to assume his place in society. Blind persons are often sedentary, which compromises its physical capacity. The present work evaluates the blind person's physical capacity, analysing possible differences related to sex, age and life time blindness. The sample was constituted by 27 blind (congenital and noncongenital), 18 male and 9 female, aged between 17 and 37 years old. To evaluate previous physical activity, a questionnaire, was answered which showed a dominant sedentary population. The maximum oxygen uptake was determined in a treadmill test using standard procedures (Bruce protocol). Medium results for boys were 45.85 +/- 8 ml/kg/min and for girls were 34.46 +/- 3.8 ml/kg/min. The difference between congenital and acquired blindness on the VO2max values was 2.9 ml/kg/min, not statistically significant (p = 0.468). Those findings lead us to consider that aerobic capacity mostly depends on type and intensity of physical exercise which may be temporary related to the onset of blindness and visual impairment degree.     

X-Ray Spectroscopy of Ultra-Thin Oxide/Oxide Heteroepitaxial Films: A Case Study of Single-Nanometer VO2/TiO2

Epitaxial ultra-thin oxide films can support large percent level strains well beyond their bulk counterparts, thereby enabling strain-engineering in oxides that can tailor various phenomena. At these reduced dimensions (typically < 10 nm), contributions from the substrate can dwarf the signal from the epilayer, making it difficult to distinguish the properties of the epilayer from the bulk. This is especially true for oxide on oxide systems. Here, we have employed a combination of hard X-ray photoelectron spectroscopy (HAXPES) and angular soft X-ray absorption spectroscopy (XAS) to study epitaxial VO₂/TiO₂ (100) films ranging from 7.5 to 1 nm. We observe a low-temperature (300 K) insulating phase with evidence of vanadium-vanadium (V-V) dimers and a high-temperature (400 K) metallic phase absent of V-V dimers irrespective of film thickness. Our results confirm that the metal insulator transition can exist at atomic dimensions and that biaxial strain can still be used to control the temperature of its transition when the interfaces are atomically sharp. More generally, our case study highlights the benefits of using non-destructive XAS and HAXPES to extract out information regarding the interfacial quality of the epilayers and spectroscopic signatures associated with exotic phenomena at these dimensions.