Nanoscale Imaging of Phase Transitions with Scanning Force Microscopy

Nanoscale imaging of materials through phase transitions can provide valuable insight into the local nature of the transition and the emergence of order. The scanning force microscope used in the studies presented here is an ideal instrument to investigate phase transitions with nanoscale spatial resolution. We study phase transitions in two different systems by operating in different modes: contact mode, in which we measure the local electronic properties of the sample; and non-contact mode, in which we probe the sample by monitoring the interaction between the sample and cantilever. We increased the versatility of this microscope by developing a method to control the quality factor Q of a conducting cantilever via capacitive coupling to the local environment. We show that Q may be reversibly tuned over a range of a factor of 260. We describe the underlying physics with a point-mass oscillator model. Tuning Q can enhance force-gradient sensitivity or scan speed, which we demonstrate with topographic scans of a $(VO_2)$ acquired in high vacuum. Scanning in contact mode with a conductive cantilever, we study local electronic properties of a vanadium dioxide $(VO_2)$ thin film through an insulator to metal transition. At each point in the scan, we sweep the voltage applied to the sample, obtaining current versus voltage sweeps with nanoscale resolution while inducing the insulator to metal transition. In some $(VO_2)$ grains, we see two electronic transitions, consistent with a locally stable intermediate insulating phase. We find large insulating state resistances and transition voltages at grain boundaries, underscoring the importance of Joule heating in triggering the transition in this type of measurement. Finally, we evaluate the conduction mechanism in the insulating regime, allowing the local determination of permittivity and temperature. We scan in non-contact mode with a magnetic tip to investigate the spin reorientation transition in single-crystal $Nd_2Fe_{14}B$. This ferromagnetic system undergoes the spin reorientation transition near 135 K. We achieve nanoscale magnetic resolution at both room temperature and at a variety of temperatures around the phase transition. We demonstrate the ability to resolve the magnetic domain structure and monitor its evolution through the phase transition.Physic

Microcantilever Q Control Via Capacitive Coupling

We introduce a versatile method to control the quality factor Q of a conducting cantilever in an atomic force microscope (AFM) via capacitive coupling to the local environment. Using this method, Q may be reversibly tuned to within ~ 10% of any desired value over several orders of magnitude. A point-mass oscillator model describes the measured effect. Our simple Q control module increases the AFM functionality by allowing greater control of parameters such as scan speed and force gradient sensitivity, which we demonstrate by topographic imaging of a VO$_{2}$ thin film in high vacuum.Physic

Limits to Performance Improvement Provided by Balanced Interferometers and Balanced Detection in OCT/OCM Instruments

We compare the dynamic range of OCT/OCM instruments configured with unbalanced interferometers, e.g., Michelson interferometers, with that of instruments utilizing balanced interferometers and balanced photodetection. We define the dynamic range (DR) as the ratio of the maximum fringe amplitude achieved with a highly reflecting surface to the root-mean-square (rms) noise. Balanced systems achieve a dynamic range 2.5 times higher than that of a Michelson interferometer, enabling an image acquisition speed roughly 6 times faster. This maximum improvement occurs at light source powers of a few milliwatts. At light source powers higher than 30 mW, the advantage in acquisition speed of balanced systems is reduced to a factor of 4. For video-rate imaging, the increased cost and complexity of a balanced system may be outweighed by the factor of 4 to 6 enhancement in image acquisition speed. We include in our analysis the beat-noise resulting from incoherent fight backscattered from the sample, which reduces the advantage of balanced systems. We attempt to resolve confusion surrounding the origin and magnitude of beat-noise , first described by L. Mandel in 1962. Beat-noise is present in both balanced and unbalanced OCT/OCM instruments

Role of Beat Noise in Limiting the Sensitivity of Optical Coherence Tomography

The sensitivity and dynamic range of optical coherence tomography (OCT) are calculated for instruments utilizing two common interferometer configurations and detection schemes. Previous researchers recognized that the performance of dual-balanced OCT instruments is severely limited by beat noise, which is generated by incoherent light backscattered from the sample. However, beat noise has been ignored in previous calculations of Michelson OCT performance. Our measurements of instrument noise confirm the presence of beat noise even in a simple Michelson interferometer configuration with a single photodetector. Including this noise, we calculate the dynamic range as a function of OCT light source power, and find that instruments employing balanced interferometers and balanced detectors can achieve a sensitivity up to six times greater than those based on a simple Michelson interferometer, thereby boosting image acquisition speed by the same factor for equal image quality. However, this advantage of balanced systems is degraded for source powers greater than a few milliwatts. We trace the concept of beat noise back to an earlier paper [J. Opt. Soc. Am. 52, 1335 (1962)]

Modern microwave methods in solid state inorganic materials chemistry: from fundamentals to manufacturing

No abstract available

STM imaging of symmetry-breaking structural distortion in the Bi-based cuprate superconductors

A complicating factor in unraveling the theory of high-temperature (high-Tc) superconductivity is the presence of a "pseudogap" in the density of states, whose origin has been debated since its discovery [1]. Some believe the pseudogap is a broken symmetry state distinct from superconductivity [2-4], while others believe it arises from short-range correlations without symmetry breaking [5,6]. A number of broken symmetries have been imaged and identified with the pseudogap state [7,8], but it remains crucial to disentangle any electronic symmetry breaking from pre-existing structural symmetry of the crystal. We use scanning tunneling microscopy (STM) to observe an orthorhombic structural distortion across the cuprate superconducting Bi2Sr2Can-1CunO2n+4+x (BSCCO) family tree, which breaks two-dimensional inversion symmetry in the surface BiO layer. Although this inversion symmetry breaking structure can impact electronic measurements, we show from its insensitivity to temperature, magnetic field, and doping, that it cannot be the long-sought pseudogap state. To detect this picometer-scale variation in lattice structure, we have implemented a new algorithm which will serve as a powerful tool in the search for broken symmetry electronic states in cuprates, as well as in other materials.Comment: 4 figure

Muscular coordination of single-leg hop landing in uninjured and anterior cruciate ligament-reconstructed individuals

This study compared lower-limb muscle function, defined as the contributions of muscles to center-of-mass support and braking, during a single-leg hopping task in anterior cruciate ligament-reconstructed (ACLR) individuals and uninjured controls. In total, 65 ACLR individuals and 32 controls underwent a standardized anticipated single-leg forward hop. Kinematics and ground reaction force data were input into musculoskeletal models to calculate muscle forces and to quantify muscle function by decomposing the vertical (support) and fore-aft (braking) ground reaction force components into contributions by individual lower-limb muscles. Four major muscles, the vasti, soleus, gluteus medius, and gluteus maximus, were primarily involved in support and braking in both ACLR and uninjured groups. However, although the ACLR group demonstrated lower peak forces for these muscles (all Ps <.001, except gluteus maximus, P =.767), magnitude differences in these muscles' contributions to support and braking were not significant. ACLR individuals demonstrated higher erector spinae (P =.012) and hamstrings forces (P =.085) to maintain a straighter, stiffer landing posture with more forward lumbar flexion. This altered landing posture may have enabled the ACLR group to achieve similar muscle function to controls, despite muscle force deficits. Our findings may benefit rehabilitation and the development of interventions to enable faster and safer return to sport.</p

Accuracy of a novel marker tracking approach based on the low-cost Microsoft Kinect v2 sensor

Microsoft Kinect for Windows v2 is a motion analysis system that features a markerless human pose estimation algorithm. Given its affordability and portability, Kinect v2 has potential for use in biomechanical research and within clinical settings; however, recent studies suggest high inaccuracy of the markerless algorithm compared to marker-based motion capture systems. A novel tracking method was developed using Kinect v2, employing custom-made colored markers and computer vision techniques. The aim of this study was to test the accuracy of this approach relative to a conventional Vicon motion analysis system, performing a Bland–Altman analysis of agreement. Twenty participants were recruited, and markers placed on bony prominences near hip, knee and ankle. Three-dimensional coordinates of the markers were recorded during treadmill walking and running. The limits of agreement (LOA) of marker coordinates were narrower than − 10 and 10 mm in most conditions, however a negative relationship between accuracy and treadmill speed was observed along Kinect depth direction. LOA of the surrogate knee angles were within − 1.8° 1.7° for flexion in all conditions and − 2.9° 1.7° for adduction during fast walking. The proposed methodology exhibited good agreement with a marker-based system over a range of gait speeds and, for this reason, may be useful as low-cost motion analysis tool for selected biomechanical applications. © 2018 IPE

Biomechanical Markers of Forward Hop-Landing After ACL-Reconstruction : A Pattern Recognition Approach

Biomechanical changes after anterior cruciate ligament reconstruction (ACLR) may be detrimental to long-term knee-joint health. We used pattern recognition to characterise biomechanical differences during the landing phase of a single-leg forward hop after ACLR. Experimental data from 66 individuals 12-24 months post-ACLR (28.2 ± 6.3 years) and 32 controls (25.2 ± 4.8 years old) were input into a musculoskeletal modelling pipeline to calculate joint angles, joint moments and muscle forces. These waveforms were transformed into principal components (features), and input into a pattern recognition pipeline, which found 10 main distinguishing features (and 8 associated features) between ACLR and control landing biomechanics at significance α= 0.05. Our process identified known biomechanical characteristics post-ACLR: smaller knee flexion angle; less knee extensor moment; lower vasti, rectus femoris and hamstrings forces. Importantly, we found more novel and less well-understood adaptations: smaller ankle plantar flexor moment; lower soleus forces; and altered patterns of knee rotation angle, hip rotator moment and knee abduction moment. Crucially, we identified, with high certainty, subtle aberrations indicating landing instability in the ACLR group for: knee flexion and internal rotation angles and moments; hip rotation angles and moments; and lumbar rotator and bending moments. Our findings may benefit rehabilitation and assessment for return-to-sport 12–24 months post-ACLR.</p