[Show abstract][Hide abstract] ABSTRACT: Optical-pump terahertz-probe spectroscopy is used to investigate ultrafast
far-infrared conductivity dynamics during the insulator-to-metal transition
(IMT) in vanadium sesquioxide (V2O3). The resultant conductivity increase
occurs on a tens of ps timescale, exhibiting a strong dependence on the initial
temperature and fluence. We have identified a scaling of the conductivity
dynamics upon renormalizing the time axis with a simple power law (alpha = 1/2)
that depends solely on the initial, final, and conductivity onset temperatures.
Qualitative and quantitative considerations indicate that the dynamics arise
from nucleation and growth of the metallic phase which can be described by the
Avrami model. We show that the temporal scaling arises from spatial scaling of
the growth of the metallic volume fraction, highlighting the self-similar
nature of the dynamics. Our results illustrate the important role played by
mesoscopic effects in phase transition dynamics.
[Show abstract][Hide abstract] ABSTRACT: We present performance characteristics of nanoscaled cobalt phthalocyanine (CoPc)-based organic field-effect transistors (OFETs) as a function of channel length. We found a channel length range which maximizes the field effect mobility in a trade-off between the decrease in the number of organic grain boundaries and the increase of the electrode–organic contact region. Further reduction of channel length is limited by fringe currents, which lead to an increased off current and to a degradation of the sub-threshold slope. From this, we define an optimal channel length of 280 nm to 1 μm for applications in submicrometric CoPc-based OFETs. Our results are particularly relevant for the miniaturization of chemical sensing OFETs, where metal phthalocyanines have proven to be excellent candidates for the fabrication of the transistor channel.
Physica Status Solidi (A) Applications and Materials 10/2014; · 1.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This article introduces magnetic field modulated microwave spectroscopy (MFMMS) as a unique and high-sensitivity technique for use in the search for new superconductors. MFMMS measures reflected microwave power as a function of temperature. The modulation induced by the external ac magnetic field enables the use of phase locked detection with the consequent sensitivity enhancement. The MFMMS signal across several prototypical structural, magnetic, and electronic transitions is investigated. A literature review on microwave absorption across superconducting transitions is included. We show that MFMMS can be used to detect superconducting transitions selectively with very high sensitivity.
Reports on Progress in Physics 09/2014; 77(9):093902. · 13.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study vortex lattice dynamics in a heterostructure that combines two type-II superconductors: a niobium film and a dense triangular array of submicrometric vanadium (V) pillars. Magnetic ac susceptibility measurements reveal a sudden increase in ac penetration, related to an increase in vortex mobility above a magnetic field, , that decreases linearly with temperature. Additionally, temperature independent matching effects that occur when the number of vortices in the sample is an integer of the number of V pillars, strongly reduce vortex mobility, and were observed for the first and second matching fields, and . The angular dependence of , and shows that matching is determined by the normal applied field component, while is independent of the applied field orientation. This important result identifies with the critical field boundary for the normal to superconducting transition of V pillars. Below , superconducting V pillars repel vortices, and the array becomes an 'antipinning' landscape that is more effective in reducing vortex mobility than the 'pinning' landscape of the normal V sites above . Matching effects are observed both below and above , implying the presence of ordered vortex configurations for 'antipinning' or 'pinning' arrays.
Superconductor Science and Technology 07/2014; 27(8):085007. · 2.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a Surface Plasmon Resonance spectroscopy study of Co-Phthalocyanine (CoPc) thin
films grown on Au layers at different substrate temperatures. We demonstrate that for quantitative
analysis, fitting of the resonance angle alone is insufficient and Whole Curve Analysis (WCA)
needs to be performed. This is because CoPc thin film dielectric constant and thickness are strongly
affected by substrate temperature, even when the total deposited mass remains fixed. Using WCA,
we are able to uniquely fit both the dielectric constants and the thicknesses of the films without
making a priori assumptions.
Journal of Applied Physics 03/2014; 115(103106). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a software package called Microscopy Image Segmentation Tool (MIST). MIST is designed for analysis of microscopy images which contain large collections of small regions of interest (ROIs). Originally developed for analysis of porous anodic alumina scanning electron images, MIST capabilities have been expanded to allow use in a large variety of problems including analysis of biological tissue, inorganic and organic film grain structure, as well as nano- and meso-scopic structures. MIST provides a robust segmentation algorithm for the ROIs, includes many useful analysis capabilities, and is highly flexible allowing incorporation of specialized user developed analysis. We describe the unique advantages MIST has over existing analysis software. In addition, we present a number of diverse applications to scanning electron microscopy, atomic force microscopy, magnetic force microscopy, scanning tunneling microscopy, and fluorescent confocal laser scanning microscopy.
Review of Scientific Instruments 03/2014; 85(3):033701-033701-6. · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nanopore-based sequencing has demonstrated a significant potential for the development of fast, accurate, and cost-efficient fingerprinting techniques for next generation molecular detection and sequencing. We propose a specific multilayered graphene-based nanopore device architecture for the recognition of single biomolecules. Molecular detection and analysis can be accomplished through the detection of transverse currents as the molecule or DNA base translocates through the nanopore. To increase the overall signal-to-noise ratio and the accuracy, we implement a new 'multi-point cross-correlation' technique for identification of DNA bases or other molecules on the single molecular level. We demonstrate that the cross-correlations between each nanopore will greatly enhance the transverse current signal for each molecule. We implement first-principles transport calculations for DNA bases surveyed across a multilayered graphene nanopore system to illustrate the advantages of the proposed geometry. A time-series analysis of the cross-correlation functions illustrates the potential of this method for enhancing the signal-to-noise ratio. This work constitutes a significant step forward in facilitating fingerprinting of single biomolecules using solid state technology.
[Show abstract][Hide abstract] ABSTRACT: The novel properties of materials produced using nanoscale manufacturing
processes often arise from interactions across interfaces between
dissimilar materials. Thus, to characterize the structure and magnetism
of nanoscale materials demands tools with interface specificity. Neutron
scattering has long been known to provide unique and quantitative
information about nuclear and magnetic structures of bulk materials.
Moreover, the specialty techniques of polarized neutron reflectometry
and small angle neutron scattering (SANS) with polarized neutron beams
and polarization analysis, are ideally and often uniquely suited to
studies of nanostructured magnetic materials. Since neutron scattering
is a weakly interacting probe, it gives quantifiable and
easily-interpreted information on properties of statistically
representative quantities of bulk, thin film and interfacial materials.
In addition, neutron scattering can provide information to complement
that obtained with bulk probes (magnetization, Kerr effect) or surface
measurements obtained with scanning probe microscopy or resonant soft
x-ray scattering. The straightforward interpretation and the
simultaneous availability of structural information, make neutron
scattering the technique of choice for the structural and physical
characterization of many novel materials, especially those with buried
interfaces, ones allowing for isotopic substitutions to decorate buried
interfaces, or cases where the magnetic response to an external stimulus
can be measured. We describe recent applications of neutron scattering
to important thin film materials systems and future opportunities.
Unquestionably, neutron scattering has played a decisive role in the
development and study of new emergent phenomena. We argue with the
advent of new techniques in neutron scattering and sample environment,
neutron scattering's role in such studies will become even more
dominant. In particular, neutron scattering will clarify and distinguish
between intrinsic vs. extrinsic origins of unusual behavior which
invariably plague novel materials. Key to realizing these opportunities
will be the development of sample environment capabilities especially
tailored to test the origins of novel phenomena, and techniques to
collect, analyze and correlate neutron event detection with time
dependent perturbations to the sample's environment.
Journal of Magnetism and Magnetic Materials 02/2014; · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sequencing by tunneling is a next-generation approach to read single-base
information using electronic tunneling transverse to the single-stranded DNA
(ssDNA) backbone while the latter is translocated through a narrow channel. The
original idea considered a single pair of electrodes to read out the current
and distinguish the bases [1, 2]. Here, we propose an improvement to the
original sequencing by tunneling method, in which $N$ pairs of electrodes are
built in series along a synthetic nanochannel. While the ssDNA is forced
through the channel using a longitudinal field it passes by each pair of
electrodes for long enough time to gather a minimum of $m$ tunneling current
measurements, where $m$ is determined by the level of sequencing error desired.
Each current time series for each nucleobase is then cross-correlated together,
from which the DNA bases can be distinguished. We show using random sampling of
data from classical molecular dynamics, that indeed the sequencing error is
significantly reduced as the number of pairs of electrodes, $N$, increases.
Compared to the sequencing ability of a single pair of electrodes,
cross-correlating $N$ pairs of electrodes is exponentially better due to the
approximate log-normal nature of the tunneling current probability
distributions. We have also used the Fenton-Wilkinson approximation to
analytically describe the mean and variance of the cross-correlations that are
used to distinguish the DNA bases. The method we suggest is particularly useful
when the measurement bandwidth is limited, allowing a smaller electrode gap
residence time while still promising to consistently identify the DNA bases
Journal of Computational Electronics 01/2014; · 1.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We find that the ohmic conductance of Co-phthalocyanine (CoPc) vertical capacitive devices is irreversibly suppressed by orders of magnitude when they are heated above 340 K. Detailed structural and transport studies imply that the changes in the conductance are due to diffusion of the top Pd electrode into the CoPc layer. This leads to a decrease in Pd electrode effective work function, which increases the potential barrier for hole injection.
[Show abstract][Hide abstract] ABSTRACT: We studied the temperature dependence of coercivity and magnetization of V2O3/Ni bilayers across the Structural Phase Transition in V2O3. We found a coercivity peak that coincides with the V2O3 phase transition on top of an overall increase of the coercivity with decreasing temperature. We propose that this sharp increase arises from a length scale competition between magnetic domains of Ni and phase coexistence during the V2O3 phase transition. This model is supported by micromagnetic simulations and shows that magnetic properties of ferromagnetic films are strongly affected by a proximal first order phase transition.
[Show abstract][Hide abstract] ABSTRACT: The pursuit of higher densities in binary storage media is facing serious operating limitations. In order to overcome these constraints, several multistate techniques have been investigated as alternatives. Here, we report on an approach to define multistate switching memory units based on magnetic nanostructures exhibiting exchange bias. Writing and reading conditions were studied in patterned antiferromagnetic/ferromagnetic thin films. We establish the necessary and sufficient requirements for this multidigit memory concept that might open up new possibilities for the exploration and design of suitable room temperature spintronic devices.
[Show abstract][Hide abstract] ABSTRACT: Using controlled ion bombardment, the contribution of interface and bulk antiferromagnetic spins to exchange bias (EB) is investigated. Several sets of ferromagnetic (FM)/antiferromagnetic (AFM) (Ni/FeF2) bilayers capped with a nonmagnetic and inert Au layer of varying thickness were grown simultaneously. He-ion bombardment was employed to selectively create defects in the EB structure at the FM/AFM interface or in the AFM bulk. Numerical simulations provide the depth profile of the ion damage. Quantitative structural and magnetic characterizations were compared before and after the bombardment revealing the relationship between interfacial and bulk located defects. These studies show that the creation of defects in the bulk of the antiferromagnet crucially affects the magnitude of EB.
[Show abstract][Hide abstract] ABSTRACT: The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties. However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution. We demonstrate an elegant experimental technique that bridges these local and global methods, giving access to mesoscale information on domain formation and evolution at time scales orders of magnitude faster than current spatially resolved approaches. Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.
[Show abstract][Hide abstract] ABSTRACT: We have developed a very sensitive, highly selective, non-destructive technique for screening inhomogeneous materials for the presence of superconductivity. This technique, based on phase sensitive detection of microwave absorption is capable of detecting 10(-12) cc of a superconductor embedded in a non-superconducting, non-magnetic matrix. For the first time, we apply this technique to the search for superconductivity in extraterrestrial samples. We tested approximately 65 micrometeorites collected from the water well at the Amundsen-Scott South pole station and compared their spectra with those of eight reference materials. None of these micrometeorites contained superconducting compounds, but we saw the Verwey transition of magnetite in our microwave system. This demonstrates that we are able to detect electro-magnetic phase transitions in extraterrestrial materials at cryogenic temperatures.
[Show abstract][Hide abstract] ABSTRACT: Hybrid magnetic arrays embedded in superconducting films are ideal systems to
study the competition between different physical (such as the coherence length)
and structural length scales such as available in artificially produced
structures. This interplay leads to oscillation in many magnetically dependent
superconducting properties such as the critical currents, resistivity and
magnetization. These effects are generally analyzed using two distinct models
based on vortex pinning or wire network. In this work, we show that for
magnetic dot arrays, as opposed to antidot (i.e holes) arrays, vortex pinning
is the main mechanism for field induced oscillations in resistance R(H),
critical current Ic(H), magnetization M(H) and ac-susceptibility Xac(H) in a
broad temperature range. Due to the coherence length divergence at Tc, a
crossover to wire network behavior is experimentally found. While pinning
occurs in a wide temperature range up to Tc, wire network behavior is only
present in a very narrow temperature window close to Tc. In this temperature
interval, contributions from both mechanisms are operational but can be
Superconductor Science and Technology 11/2013; 27(6). · 2.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have studied the shearing forces in a superconducting vortex system with artificial pinning sites using the Corbino geometry. Current was injected into the center of a Nb disc and propagated radially outward to produce a force in the azimuthal direction on the vortices, with strength proportional to 1/r. We investigated the magnetoresistance properties of the vortex lattice as a function of temperature, current, and pinning lattice configuration. The measurements show steps instead of minima at the matching field positions, indicating an unexpected influence of the pinning array on the motion of the vortex lattice. The results imply that these steps are due to a shearing transition of the vortices.
Physical Review B 11/2013; 88(17). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Phthalocyanines, a class of macrocyclic, square planar molecules, are extensively studied as semiconductor materials for chemical sensors, dye-sensitized solar cells, and other applications. In this study, we use angular dependent near-edge x-ray absorption fine structure (NEXAFS) spectroscopy as a quantitative probe of the orientation and electronic structure of H2-, Fe-, Co-, and Cu-phthalocyanine molecular thin films. NEXAFS measurements at both the carbon and nitrogen K-edges reveal that phthalocyanine films deposited on sapphire have upright molecular orientations, while films up to 50 nm thick deposited on gold substrates contain prostrate molecules. Although great similarity is observed in the carbon and nitrogen K-edge NEXAFS spectra recorded for the films composed of prostrate molecules, the H2-phthalocyanine exhibits the cleanest angular dependence due to its purely out-of-plane π∗ resonances at the absorption onset. In contrast, organometallic-phthalocyanine nitrogen K-edges have a small in-plane resonance superimposed on this π∗ region that is due to a transition into molecular orbitals interacting with the 3dx (2) -y (2) empty state. NEXAFS spectra recorded at the metal L-edges for the prostrate films reveal dramatic variations in the angular dependence of specific resonances for the Cu-phthalocyanines compared with the Fe-, and Co-phthalocyanines. The Cu L3,2 edge exhibits a strong in-plane resonance, attributed to its b1g empty state with dx (2) -y (2) character at the Cu center. Conversely, the Fe- and Co- phthalocyanine L3,2 edges have strong out-of-plane resonances; these are attributed to transitions into not only b1g (dz (2)) but also eg states with dxz and dyz character at the metal center.
The Journal of Chemical Physics 07/2013; 139(3):034701. · 3.12 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have investigated the vortex dynamics in superconducting thin film devices
with non-uniform patterns of artificial pinning centers (APCs). The
magneto-transport properties of a conformal crystal and a randomly diluted APC
pattern are compared with that of a triangular reference lattice. We have found
that in both cases the magneto-resistance below the first matching field of the
triangular reference lattice is significantly reduced. For the conformal
crystal, the magneto-resistance is below the noise floor indicating highly
effective vortex pinning over a wide magnetic field range. Further, we have
discovered that for asymmetric patterns the R vs. H curves are mostly
symmetric.This implies that the enhanced vortex pinning is due to the
commensurability with a stripe in the non-uniform APC pattern and not due to a
rearrangement and compression of the whole vortex lattice.
[Show abstract][Hide abstract] ABSTRACT: We searched for new structural, magnetic and superconductivity phases in the Pr–Si system using high-pressure high-temperature and arc melting syntheses. Both high and low Si concentration areas of the phase diagram were explored. Although a similar approach in the La–Si system produced new stable superconducting phases, in the Pr–Si system we did not find any new superconductors. At low Si concentrations, the arc-melted samples were doped with C or B. It was found that addition of C gave rise to multiple previously unknown ferromagnetic phases. Furthermore, X-ray refinement of the undoped samples confirmed the existence of the so far elusive Pr3Si2 phase.
Journal of Magnetism and Magnetic Materials 05/2013; 340(08, 2013). · 2.00 Impact Factor