Reliable postprocessing improvement of van der Waals heterostructures

The successful assembly of heterostructures consisting of several layers of different 2D materials in arbitrary order by exploiting van der Waals forces has truly been a game changer in the field of low dimensional physics. For instance, the encapsulation of graphene or MoS2 between atomically flat hexagonal boron nitride (hBN) layers with strong affinity and graphitic gates that screen charge impurity disorder provided access to a plethora of interesting physical phenomena by drastically boosting the device quality. The encapsulation is accompanied by a self-cleansing effect at the interfaces. The otherwise predominant charged impurity disorder is minimized and random strain fluctuations ultimately constitute the main source of residual disorder. Despite these advances, the fabricated heterostructures still vary notably in their performance. While some achieve record mobilities, others only possess mediocre quality. Here, we report a reliable method to improve fully completed van der Waals heterostructure devices with a straightforward post-processing surface treatment based on thermal annealing and contact mode AFM. The impact is demonstrated by comparing magnetotransport measurements before and after the AFM treatment on one and the same device as well as on a larger set of treated and untreated devices to collect device statistics. Both the low temperature properties as well as the room temperature electrical characteristics, as relevant for applications, improve on average substantially. We surmise that the main beneficial effect arises from reducing nanometer scale corrugations at the interfaces, i.e. the detrimental impact of random strain fluctuations

Breakdown of the interlayer coherence in twisted bilayer graphene

Coherent motion of the electrons in the Bloch states is one of the fundamental concepts of the charge conduction in solid state physics. In layered materials, however, such a condition often breaks down for the interlayer conduction, when the interlayer coupling is significantly reduced by e.g. large interlayer separation. We report that complete suppression of coherent conduction is realized even in an atomic length scale of layer separation in twisted bilayer graphene. The interlayer resistivity of twisted bilayer graphene is much higher than the c-axis resistivity of Bernal-stacked graphite, and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation and incoherent conduction is a main transport channel between the layers of twisted bilayer graphene.Comment: 5 pages, 3 figure

Nanoscale imaging of equilibrium quantum Hall edge currents and of the magnetic monopole response in graphene

The recently predicted topological magnetoelectric effect and the response to an electric charge that mimics an induced mirror magnetic monopole are fundamental attributes of topological states of matter with broken time reversal symmetry. Using a SQUID-on-tip, acting simultaneously as a tunable scanning electric charge and as ultrasensitive nanoscale magnetometer, we induce and directly image the microscopic currents generating the magnetic monopole response in a graphene quantum Hall electron system. We find a rich and complex nonlinear behavior governed by coexistence of topological and nontopological equilibrium currents that is not captured by the monopole models. Furthermore, by utilizing a tuning fork that induces nanoscale vibrations of the SQUID-on-tip, we directly image the equilibrium currents of individual quantum Hall edge states for the first time. We reveal that the edge states that are commonly assumed to carry only a chiral downstream current, in fact carry a pair of counterpropagating currents, in which the topological downstream current in the incompressible region is always counterbalanced by heretofore unobserved nontopological upstream current flowing in the adjacent compressible region. The intricate patterns of the counterpropagating equilibrium-state orbital currents provide new insights into the microscopic origins of the topological and nontopological charge and energy flow in quantum Hall systems

Balance Control and Exercise-Based Interventions in Older Adults

Loss of balance and consequential falling, caused by natural degenerations in the sensory and motor systems with aging, are critical issues that require constant research exploration to ultimately improve the quality of life in older populations. Balance can be simply classified into static and dynamic balance, and the latter is more associated with common causes of falling in older adults. There are numerous ways to improve dynamic balance, and exercise training has been considered the most beneficial intervention for that purpose. Specifically, aquatic exercises have been suggested as a promising modality because several properties of water, including buoyance and hydrostatic pressure, impart direct benefits to older adults during the exercise. However, it is still inconclusive whether aquatic exercises are more effective than land exercises at improving dynamic balance. Further, slips and trips are the most predominant causes of falls in older adults, and they often require a rapid, accurate action to avoid a potential fall. This process is called reactive balance (i.e., compensatory balance reaction). It also can be enhanced by exercise interventions; however, it is unclear what type of exercise is most effective at improving reactive balance. In this dissertation, we compared the impacts of exercise environments on dynamic balance, and then explored what type of exercise intervention improves reactive balance the most in older adults. These studies revealed that both aquatic and land exercises have equivalent effects on improving dynamic balance, and reactive balance improved most successfully after one or more reactive balance exercises were provided. In addition, power training was the second most effective intervention for improving reactive balance. The findings from this dissertation suggest that when exercise-based interventions are used to improve dynamic balance, the exercise environments can be selected based on the purpose of the intervention or each participant’s subjective decision. Moreover, practitioners may wish to implement task-specific reactive balance training on the preferential basis for the intervention aiming at reactive balance. Also, power training, which reflects the mechanism of the targeted reactive balance task, can be jointly or adjunctly utilized to improve reactive balance, which is critical for decreasing falls in older adults

Effects of Height Increasing Heel Insole on Lower Extremity Joint Mechanics

High-heel shoes alter foot pressure distribution and lower extremity kinetic and kinematics. Unlike general women’s high-heeled shoes, height increasing heel insoles are cushioned, which may contribute to impact absorption. The purpose of this study is to compare the kinetic and kinematics of gaits with and without height increasing heel insoles. Two male subjects performed level walking under two conditions: wearing sports shoes with and without height increasing heel insoles which added additional 5 cm of the heel height (total 6 cm). The vertical ground reaction force (GRF), angle, moment, and power of ankle, knee, and hip during the stance phase of the right foot were calculated. As results, higher heel height decreased the GRF under the heel and increased the GRF under the forefoot. The overall plantarflexed foot posture during the stance phase led to the overall reduction in the peak plantar flexor moment, which was persisted into the terminal stance. Also, it limited the power output and concentric contraction of the plantar flexor muscles, including gastrocnemius and soleus. The angle, moment, and power of the knee and hip were similar for both heel height conditions. Walking with height increasing heel insoles caused changes in ankle kinetic and kinematics that begin in the early stance phase. Increased heel height altered entire ankle angle during the stance phase, caused more plantarflexion, and shortened gastrocnemius and soleus muscles. Therefore, the propulsive ability was restricted, leading to the decreases of ankle moment and power

Even denominator fractional quantum Hall states in higher Landau levels of graphene

An important development in the field of the fractional quantum Hall effect has been the proposal that the 5/2 state observed in the Landau level with orbital index $n = 1$ of two dimensional electrons in a GaAs quantum well originates from a chiral $p$-wave paired state of composite fermions which are topological bound states of electrons and quantized vortices. This state is theoretically described by a "Pfaffian" wave function or its hole partner called the anti-Pfaffian, whose excitations are neither fermions nor bosons but Majorana quasiparticles obeying non-Abelian braid statistics. This has inspired ideas on fault-tolerant topological quantum computation and has also instigated a search for other states with exotic quasiparticles. Here we report experiments on monolayer graphene that show clear evidence for unexpected even-denominator fractional quantum Hall physics in the $n=3$ Landau level. We numerically investigate the known candidate states for the even-denominator fractional quantum Hall effect, including the Pfaffian, the particle-hole symmetric Pfaffian, and the 221-parton states, and conclude that, among these, the 221-parton appears a potentially suitable candidate to describe the experimentally observed state. Like the Pfaffian, this state is believed to harbour quasi-particles with non-Abelian braid statistic