Session A7: Undergraduate Research/SPS I

Identifying the relation between trapping force of Laser Tweezers and Size of Microsphere particles

Optical trapping technique is the method in which micron and sub-micron particles can be studied. In this technique the laser pressure radiation creates the essential force to trap particles. This force depends on several parameters including the particles' indices of refraction, the specification of the beads surrounding environment and the characteristics of the implemented laser. On this work we present the outcome of the experiment we designed to analyze the trapping force. In this experiment we used micro sized beads with different indices of refraction, changed the viscosity of the surrounding environment of the beads and adjusted the power of the laser implemented. By analyzing the motion of the beads in several trials, the trapping force was estimated and its dependency to the parameters mentioned above was identified. Finally the outcomes of our experiment were compared with the theoretical reported results.   

Scanning Tunneling Microscopy of Charge Density Wave states in TbTe$_{\mathrm{3}}$

Charge density wave (CDW) states are broken symmetry states which involve a periodic lattice distortion and an opening of a band gap. While these states are prevalent in condensed matter systems, often coexisting with other states such as superconductivity, much is still not understood about the microscopic properties of CDWs or their onset through T$_{\mathrm{CDW}}$. For these reasons we use scanning tunneling microscopy (STM) to study CDW states in TbTe$_{\mathrm{3}}$ where T$_{\mathrm{CDW}}$ \textasciitilde 335 K. We will present temperature dependent STM data through T$_{\mathrm{CDW}}$ as well as our efforts in modeling the combined effects of crystal lattice structure, CDW states, and wave-vector mixing to understand the periodicities detected in our topographic images.   

Thermal Properties of Nd-Doped PrOs$_4$Sb$_{12}$ Extracted From Measurement of Specific Heat

PrOs$_4$Sb$_{12}$ has attracted interest due to its unconventional heavy fermion superconductivity, interest that has increased once the Nd-doped compound was shown at certain concentrations of Nd to simultaneously display ferromagnetism and superconducting properties at low temperatures. In order to better understand the more exotic low temperature behavior exhibited by this system, it is necessary to characterize its normal-state properties. Therefore, the molar specific heat of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ is measured using finite heat pulse relaxation calorimetry. A curve-fit of the temperature-dependent molar specific heat allows an estimation of the Debye and Einstein temperatures, as well as the electronic specific heat coefficient. These properties are examined across x to determine the behavior of the system with respect to Nd-concentration. The results from the measurements will be discussed during the presentation.   

Pressure Dependence of the Magnetic Response of the S $=$ 1 Polymeric Chain [Ni(HF2)(3-Clpy)4]BF4.

[Ni(HF2)(3-Clpy)4]BF4 (py $=$ pyridine) is an S $=$ 1 antiferromagnetic polymeric chain with a single-ion anisotropy (zero-field splitting) of D/kB $=$ 4.3 K and an intrachain exchange interaction value of J/kB $=$ 4.86 K at ambient pressure [1]. The ratio of these parameters (D/J $=$ 0.88) places this system close to a quantum critical point at D/J $\approx $ 1, which falls between the Haldane and the Large-D phases. The temperature dependence of the low-field (1 kG) magnetic susceptibility was measured as a function of pressure, up to 1.49 GPa, using a homemade piston-clamp cell [2]. The data indicate the antiferromagnetic component is suppressed with increasing pressure. [1] J.L. Manson et al., Inorg. Chem. 51 (2012) 7520. [2] M.K. Peprah, PhD thesis, University of Florida (2015).   

High pressure differential conductance measurements of (Pb,Sn)Se

Topological transitions have been recognized as a new type of quantum phase transition.  Recently, a number of papers have reported scanning tunneling microscope (STM) measurements of the Landau level spectra of topologically non-trivial materials.  Such measurements can offer substantial insight into the nature of the transition between topologically distinct phases.  Although applied pressure represents an attractive means to drive a topological quantum phase transition, STM measurements can not be performed under high pressure conditions.  In this talk, I will discuss our recent attempts to observe Landau level spectra in compressed (Pb,Sn)Se using differential conductance measurements. \\* \\* Acknowledgements: TAP supported by REU NSF DMR-1461019. Pressure cell development and measurements at high magnetic fields supported by the National High Magnetic Field Laboratory User Collaboration Grants Program.  Synthesis, characterization, and high pressure measurements supported by NSF DMR-1453752.   

Phase Transitions in Nanostructured Mn$_{\mathrm{0.18}}$TaS$_{\mathrm{2}}$

Phase transitions in a sample of Mn-intercalated TaS$_{\mathrm{2}}$ were investigated. The concentration of manganese relative to tantalum was determined to be 18{\%}. The phase transitions of the sample were explored using a variety of techniques: Curie-Weiss, Critical Scaling, Arrott-Noakes, and Kouvel-Fisher. All but the first method include the use of critical exponents defined using the spontaneous magnetization and susceptibility in zero applied field. The sample was found to undergo a transition from paramagnetism to an ordered state below 100 K. Two of the aforementioned methods were converted to computational methods. These same methods for determining the transition temperature and critical exponents may also indicate the existence of a second transition very close to the first. These results have been extracted using the theory of scaling.   

Lattice thermal conductance of quantum wires with disorder

We model the lattice thermal conductance in long quantum wires connected to two large heat baths at different temperatures in the harmonic approximation. The thermal conductance is computed with the Landauer formula for phonons, where it is related to the sum over all transmission probabilities for phonons through the wire. The net transmission probability is computed using a recursive Green function technique, which allows one to study long wires efficiently. We consider several different kinds of disorder to reduce the lattice thermal conductivity: periodic rectangular holes of varying sizes and shapes, periodic triangular holes, and narrow bands, averaged over randomness to account for variance in manufacturing. Depending on the model, the thermal conductance was reduced by 80 percent or more from the perfectly ordered wire case. Funded by NSF grant DMR-1461019.   

3-D matrix template-assisted growth of oriented oxide nanowire arrays using glancing angle pulsed laser deposition

Nanosphere lithography is a combination of different methods to nanofabrication. In this work nanosphere lithography is used to study the growth of Zinc Oxide Nano-columns (ZnO NCs) on different diameter Silica Nanosphere (SNS) self-assembled templates. ZnO NCs are promising building blocks for many existing and emerging optical, electrical, and piezoelectric devices, specifically, the seeded growth of other oxide materials. Recently, reports have shown a ferroelectric phase of zinc stannate (ZnSnO3) and while lead zirconium titanate oxide (PZT) has been the main material of interest in ferroelectric and piezoelectric applications, the toxicity of lead has been of great concern. The possibility of developing lead free piezoelectric materials is of great interest in the ferroelectric community. Langmuir-Blodgett method was used to construct a self-assembled monolayer of SNSs on silicon substrates. Oriented ZnO NCs were grown on top of the spheres using the glancing angle pulsed laser deposition technique. Columns were formed in a spatially ordered closed-packed hexagonal configuration. Growth of ZnO NCs was studied as function of ambient Oxygen pressure with SNS size ranging from 250-1000 nm. Cross-sectional Scanning Electron Microscopy and X-ray diffraction (XRD) were used to study the template structure. Relative aspect ratios were studied and showed tunability of column dimensions with sphere size. XRD revealed ZnO NC arrays were c-axis oriented with hexagonal wurtzite structure.   

Observation of Interlayer Excitons in Monolayer MoSe2-WSe2 Heterostructure on BN Substrate

Interlayer excitons have previously been observed in monolayer MX2 heterostructures exhibiting type II band alignment. Specically, interlayer excitons in MoSe2-WSe2 heterostructures have been thoroughly characterized with photoluminescence (PL) and photoluminescence excitation spectroscopy (PLE). However, electrical control of the interlayer exciton exhibits PL intensity dependence that is inconsistent with the dipole and electric field model - possibly owing to carrier charge effects - and requires further elucidation. The addition of BN substrate, which has been shown to (1) smooth the surface and (2) reduce carrier charge inhomogeneity of graphene devices, presents itself as a potential solution. In this preliminary study, we fabricated an MoSe2-WSe2 heterostructure on BN substrate. Photoluminescence (PL) measurements on the device confirm the presence of interlayer excitons at approximately 1.40 eV, consistent with MoSe2-WSe2 heterostructures. Furthermore, the PL characterization reveals unreported spectral features for both the interlayer and intralayer excitons.   

Magneto-Optical Study of Lithographically Patterned Ferromagnetic Multilayer (Co/Pt)$_{\mathrm{8}}$ Micro-Channels

Controlled domain movement in magnetic structures has become promising for applications in magnetic memory systems and data processing. This study examines magnetic domain nucleation and propagation within a series of lithographically patterned Co/Pt micro-channels with perpendicular magnetic anisotropy (PMA). Magnetic domains are nucleated and then manipulated using out-of-plane sweep protocols and studied in situ using magneto-optical Kerr effect (MOKE) imaging. Co/Pt multilayers were fabricated with optical lithography and sputter deposition. Effects of channel width and annealing are presented. Annealing the Co/Pt after fabrication as a function of time and temperature resulted in increasing the coercivity of the unpatterned film, decreasing the coercivity of the micro-channels, and reducing the average domain size in both. Atomic force microscopy (AFM) characterization of the micro-channels showed non-uniform deposition near feature edges. MOKE imaging demonstrated that the feature edges had a much lower coercivity (70G) than the middle of the channel/pad (150G) or the unpatterned film (250G). We found that an oscillating field protocol to re-initialize soft domains near feature edges proved to be more effective than a traditional field sweep to initialize a domain wall in the channel. Once a domain wall was formed, we explored a combination of constant and pulsed field protocols to manipulate the domain wall.   

Nuclear Magnetic Resonance Study of 3D Dirac Semimetal, Na3Bi

Dirac semimetals (DS) are a hot topic of research in topological materials because their unique properties indicate a potential in electronic applications. The electron band structure of ordinary semimetals differ from insulators and conductors as the top of the valence band and bottom of the conduction band have a small overlap. In DS, this overlap occurs only at discrete points, known as Dirac points. At the Dirac points, the relationship of energy to momentum (dispersion relation) is linear, allowing electrons near the Dirac points to behave as massless particles. Of particular interest are the three-dimensional Dirac semimetals, where this interesting band structure is present along all three dimensions. We attempt to probe the local conditions of a three-dimensional DS, Na$_3$Bi, using nuclear magnetic resonance (NMR) to perturb the spin states of the nuclei. Studying each of the nuclei sites of Na$3$Bi using NMR can provide insight into the interactions among the nuclei and between the nuclei and the surrounding electrons. Na$_3$Bi has a complex NMR spectrum which requires further study to understand.   

A MATLAB GUI to study Ising model phase transition

We have created a MATLAB based graphical user interface (GUI) that simulates the single spin flip Metropolis Monte Carlo algorithm. The GUI has the capability to study temperature and external magnetic field dependence of magnetization, susceptibility, and equilibration behavior of the nearest-neighbor square lattice Ising model. Since the Ising model is a canonical system to study phase transition, the GUI can be used both for teaching and research purposes. The presence of a Monte Carlo code in a GUI format allows easy visualization of the simulation in real time and provides an attractive way to teach the concept of thermal phase transition and critical phenomena. We will also discuss the GUI implementation to study phase transition in a classical spin ice model on the pyrochlore lattice.   

Spin wave Feynman diagram vertex computation package

Spin wave theory is a well-established theoretical technique that can correctly predict the physical behavior of ordered magnetic states. However, computing the effects of an interacting spin wave theory incorporating magnons involve a laborious by hand derivation of Feynman diagram vertices. The process is tedious and time consuming. Hence, to improve productivity and have another means to check the analytical calculations, we have devised a Feynman Diagram Vertex Computation package. In this talk, we will describe our research group’s effort to implement a Mathematica based symbolic Feynman diagram vertex computation package that computes spin wave vertices. Utilizing the non-commutative algebra package NCAlgebra as an add-on to Mathematica, symbolic expressions for the Feynman diagram vertices of a Heisenberg quantum antiferromagnet are obtained. Our existing code reproduces the well-known expressions of a nearest neighbor square lattice Heisenberg model. We also discuss the case of a triangular lattice Heisenberg model where non collinear terms contribute to the vertex interactions.   

A critical comparison of electrical methods for measuring spin-orbit torques

Direct (DC) and alternating current (AC) transport measurements of spin-orbit torques (SOTs) in heavy metal-ferromagnet heterostructure with perpendicular magnetic anisotropy have been proposed and demonstrated [1,2]. A DC method measures the change of perpendicular magnetization component while an AC method probes the first and second harmonic magnetization oscillation in responses to an AC current ($\sim$1 kHz). Here we conduct both types of measurements on $\beta$-Ta/CoFeB/MgO in the form of patterned Hall bars (20 $\mu$m linewidth) and compare the results. Experiments results are qualitatively in agreement with a macro spin model including Slonzewski-like and a field-like SOTs. However, the effective field from the ac method is larger than that obtained from the DC method. We discuss the possible origins of the discrepancy and its implications for quantitatively determining SOTs. [1] L. Liu, C. Pai, Y. Li, H. Tseng, D. Ralph, and R Buhrman, Science 336, 1126 (2012). [2] J. Kim, J. Sinha, M. Hayashi, M. Yamanouchi, S. Fukami, T.Szuki, S. Mitani, and H. Ohno, Nature Mater. 12, 3522 (2013).   

Analysis of Methods to Excite Head-Tail Motion Within the Cornell Electron Storage Ring

The main accelerator complex at Cornell consists of two rings around which electrons and positrons move:~the synchrotron, where the particles are accelerated to 5 GeV, and the Storage Ring, where the particles circulate a ta Þxed energy, guided by quadrupole and dipole magnets, with a steady energy due to a sinusoidal voltage source. Keeping the beam stable in the Storage Ring is crucial for its lifetime.~A long-lasting, invariable beam means more accurate experiments, as well as brighter, more focused X-rays for use in the Cornell High Energy Synchrotron Source (CHESS). The stability of the electron and positron beams in the Cornell Electron Storage Ring (CESR) is important for the development of accelerators and for usage of the beam in X-ray science and accelerator physics.~Bunch oscillations tend to enlarge the beam's cross section, making it less stable.~We believe that one such oscillation is ``head-tail motion,'' where the bunch rocks back and forth on a pivot located at the central particle.~In this project, we write a simulation of the bunch that induces head-tail motion with a vertical driver.~We also excite this motion physically in the storage ring, and observe a deÞnite head-tail signal. In the experiment, we saw a deÞnite persistence of the drive-damp signal within a small band around the head-tail frequency, indicating that the head-tail frequency is a natural vertical mode of the bunch that was being excited. The signal seen in the experiment matched the signal seen in the simulation to within an order of magnitude.