Session H5: Condensed Matter Physics and Materials Research

Dephasing time in graphene due to interaction with flexural phonons

We investigate decoherence of an electron in graphene caused by electron-flexural phonon interaction. We find out that the flexural phonons can produce dephasing rate comparable to the electron-electron one. The problem appears to be quite special because there is a large interval of temperatures where dephasing rate cannot be obtained using the golden rule. We evaluate this rate for a wide range of density ($n$) and temperature ($T$) and determine several asymptotic regions with temperature dependence crossing over from $\tau_{\phi}^{-1}\sim T^{2}$ to $\tau_{\phi}^{-1}\sim T$ when temperature increases. We also find $\tau_{\phi}^{-1}$ to be a non-monotonous function of $n$. These distinctive features of the new contribution can provide an effective way to identify flexural phonons in graphene through the electronic transport by measuring the weak localization corrections in magnetoresistance.   

The Hole-istic: Superconductor Theory and Experiment

Do superconductors emit x-rays when they quench? Do holes lead double lives, undressing and pairing up as electrons when it gets cold? Can the London penetration depth be explained by holes lowering their kinetic energy and getting... well... fat? An experimental search is underway for the x-ray radiation predicted by Hirsch's hole theory of superconductivity. Originally proffered 25 years ago as a model for high temperature superconductors,, the theory as it now stands applies to all superconducting materials. The basics of the hole theory of superconductivity will be presented, followed by a review of our experiment's design. You'll come away feeling more comfortable with covalent bonding, hopping amplitudes, Hamiltonians and coherent states. You'll learn about pulsed magnetic fields and x-ray detection techniques. You'll be the envy of your friends at parties as you describe both superconductor theory and cutting edge experiments on the frontier of modern physics with confidence and aplomb.   

Electrical transport properties of graphene field effect devices due to electron irradiation on PMMA/graphene

We study the change of the transport properties of graphene field effect transistor devices as a result of electron irradiation on a Poly Methyl Methacrylate (PMMA)/graphene bilayer and subsequent removal of PMMA. We observed that PMMA spun on graphene did not substantially degrade the transport properties of graphene. Instead, the PMMA/graphene bilayer showed slightly improved transport properties than a single graphene device. After electron irradiation on a PMMA/graphene bilayer and subsequent removal of the PMMA, the transport properties deteriorated. In addition, we observed the emergence of defects in graphene by Raman spectroscopy. We conclude that changes in the transport properties due to electron irradiation on PMMA on top of graphene stem from adsorption of atoms or molecules during the depolymerization process induced by energetic electrons. Furthermore, we argue that hydrogen, fragmented from PMMA, is the main element adsorbed on graphene.   

The pumpistor: understanding the flux-pumped dc SQUID by its electrical impedance

Parametric amplifiers based on superconducting circuits have experienced recent popularity. It is possible to produce superconducting circuits which may sustain and amplify coherent states of microwaves close to the quantum limit. Such systems currently enable experiments regarding qubit readout, vacuum squeezing, and quantum feedback. To this end, we describe a circuit understanding of the flux-driven dc SQUID. This is useful for developing insight into how these devices perform as active elements, providing parametric gain. We describe three- and four-wave mixing effects, and report on experimental progress. This understanding lends itself to many more testable predictions of otherwise complicated quantum systems.   

Energetic stability of SrTiO3 on GaAs(001) interfaces

The successful growth of epitaxial SrTiO$_{3}$ (STO) film on Si substrate using Molecular Beam Epitaxy (MBE) has proved that it is feasible to monolithically integrate the functional oxides with high mobility compound semiconductors [1,2]. While STO has been also deposited on GaAs without amorphous interfacial layers, the exact interface structure has been controversial; while Scanning Transmission Electron Microscopy (STEM) analysis indicates As/Sr interface layers , X-ray diffraction (XRD) measurement shows signs of Ga/SrO interface. Using ab initio calculations, we demonstrate that forming a fully oxidized layer directly on top of GaAs substrate is thermodynamically unstable. Instead, an oxygen-depleted Sr metal layer stabilizes the SrTiO$_{3}$/GaAs interface, in accordance with STEM measurement. We also show that the interface structure observed by XRD is possible under oxygen-rich conditions. The identification of different interface structures and the corresponding growth conditions can be useful for development of growth processes of oxide/semiconductor heterostructures. \\[4pt] [1] R. Contreras-Guerrero et al, J. Cryst. Growth 378, 238 (2013)\\[0pt] [2] R. F. Klie et al, Appl. Phys. Lett. 87, 143106 (2005).   

Transport mechanisms in epitaxial SrTiO$_{3-\delta }$/Si (001) with varying oxygen deficiency

Epitaxial SrTiO$_{3}$ (STO) films were grown on p-Si (001) substrates using molecular beam epitaxy (MBE). Oxygen vacancies were introduced by controlling the oxygen pressure during growth resulting in oxygen deficient SrTiO$_{3-\delta }$ with $\delta $ up to 0.004. The single phase STO/Si films were of high crystalline quality as verified by x-ray diffraction (XRD), transmission electron microscopy (TEM), and had a surface roughness less than 0.5 nm (RMS) as measured by atomic force microscopy (AFM). Transport measurements in a Van der Pauw configuration showed semiconducting behavior. The competing effects of disorder and increased carrier concentration (n-type measured by Hall) due to oxygen vacancies influence the conduction behavior. Low oxygen pressure during growth induces more oxygen vacancies and a larger number of carriers (n-type, measured by Hall) but also leads to more disordered films. Transport in these more disordered films is strongly localized and can be fit to a Variable Range Hopping (VRH) model. Transport in films with a smaller number of oxygen vacancies is thermally activated. We consider competing effects in STO/Si: lattice mismatch with Si, strain and defects due to oxygen vacancies, structural dislocations and the bulk STO antiferrodistortive phase transition at 105K.   

First-principles studies of physical properties of IV-VI derived semiconductor heterostructures and superlattices for thermoelectric applications

Doped PbTe is of great interest in high temperature thermoelectric devices. Looking at the qualitative improvements of the figure of merit, here we study the crystal and electronic structure of bulk PbTe, Thallium and Bismuth delta-doped layers in PbTe along the [100] crystalline directions and PbTe/SnTe hererostructures. The thermoelectric properties as a function of carrier concentrations and temperature were studied by solving the semiclassical Boltzmann transport equations in conjunction with ab initio electronic structure calculations, performed within Density Functional Theory. Based on maximally-localized Wannier functions basis set and the ab initio band energies, results for the Seebeck coefficient and figure of merit are presented and compared with available experimental data. Most cases showed good agreement between the calculated properties and experimental available data. Our predictions for temperature and concentration dependences of the figure of merit revealed a promising use of PbTe derived superlattices and heterostructures for thermoelectric devices applications.   

Entropy and magnetic properties of Ni-Mn-In magnetocaloric materials

Materials showing the magnetocaloric effect (MCE) have been a source of growing interest because of their potential for an environmentally friendly and energy efficient refrigeration technology. Recently, alloys based on Ni-Mn-Z (Z$=$In, Sb, Sn) have been reported to show a large MCE across the martensitic transformation where a first order structural transition is coupled to a magnetic transition. In this study, Ni-Mn-In materials with compositions NiMnIn, NiMnInand NiMnInwere analyzed both through magnetization and field-dependent calorimetry. For measurements across the first-order transformation region, we designed a modified method for relaxation calorimetry. Based on these measurements we identified individual contributions to the entropy change, including an antiferromagnetic state at low temperatures and a large change at the first order transition. The NiMnInresults also include a large anomalous nonmagnetic contribution.We will conclude by discussing the relative cooling power, and the prospects of these materials for practical applications. We gratefully acknowledge the support by the National Science Foundation under Grant No. DMR-1108396, and by the Robert A. Welch Foundation (Grant No. A-1526).   

NMR Study of Cu$_{2}$Se Superionic Conductor

We will report NMR measurements of Cu$_{2}$Se which address the unusual movements of Cu ions in this compound. This material has attracted great attention recently because of attempts to identify high performance thermoelectric materials. Cu$_{2}$Se is a superionic conductor with fast ionic motion at high temperatures and a structural phase transition around 410K. NMR is a powerful local probe which can provide information about electronic and structure plus dynamical properties of the compounds. Here we have performed $^{63}$Cu and $^{65}$Cu NMR at low and high temperatures. The spectra at low temperatures indicate slow Cu ionic motion is initiated at 90K, coinciding with the recent reports of a new phase transition at this temperature. The high temperature spectra show motional narrowing above room temperature. Over the range of 360-400K the gradual phase transition is clearly shows itself through broadening of the spectra. The Knight shift continues to increase at high temperatures which is a measure of electronic gap and charge carriers thermally induced at these temperatures.   

Quantitative Study of Na+ and/or Cl- doped LiFePO4 as a Cathode Material for Li-Ion Batteries

In light of the growing need for high performance batteries, Li-Ion cells with phospho-olivine cathode materials have received much attention. The overall goal of this research project is to improve the cathode material of lithium ion batteries, by doping lithium iron phosphate (LiFePO4) with sodium (Na+) and/or chloride (Cl-) ions. The objective is to quantitatively study the effects of Na+-Cl- doping on the electrochemical properties of LiFePO4 in order to find the most effective doping amounts. Therefore, several Na+, and Cl- doped samples, as well as Na+-Cl- co-doped samples, and a non-doped LiFePO4 sample for comparison purposes, were prepared using a solid-state reaction. All samples were subsequently carbon coated. After preparation, the sample powders were tested using an X-ray diffractometer to give indication about the containing chemical compounds, as well as impurities. In order to measure the performance of the achieved cathode material, several cathodes were prepared from each sample, and then placed into button batteries. The subsequent testing of the batteries using an Arbin BT2000 battery testing system has been rendering data including the cell voltage, cell capacity, and conductivity.   

Determining the Stability Zone for Single Phase Synthesis of Fe doped Bismuth Titanate Water-Splitting Photocatalysts

Due to their ferroelectricity, high dielectric constant and excellent photocatalyitic properties bismuth titanate (BTO) polymorphs comprise an assortment of phenomenological wide band gap semiconductors that are used in everything from device applications to discoloration of organic pollutants. Recently, the potential for BTO as a high performance water-splitter has gathered serious attention. Without modification BTO already reduces water molecules upon UV irradiation. The band alignment is such that merely increasing the density of states will not only raise the Fermi level closer to the oxidation potential of water, but also reduce the band gap so that its photocatalytic property can be activated by visible light. Unfortunately, engineering the bands of BTO by inclusion of dopants is stifled by a phase transition-prone chemical potential landscape. We have performed a DFT study pertaining to substitutional Fe doping of the cubic pyrochlore phase (Bi2Ti2O7). We aimed to understand the correlation effects that lead to an experimentally observed Fe doping threshold concentration. We found that different precursor materials had significant effects on single phase synthesis of BTO. We present the formation energy analysis of Fe doping configurations that aided in identification of the configurations associated with formation of secondary phases. We conclude that maintaining our proposed conditions for single phase synthesis will increase the Fe doping threshold and simplify the design of an enhanced BTO alloy.   

Divine Proportion Shape Invariance of DLCA Fractal Aggregates: An Analytical Theory

A restricted hierarchical model for Diffusion Limited Cluster Aggregates (DLCA) is presented that accurately predicts analytically the fractal dimension, scaling prefactor and shape of the aggregates. This three parameter description is both necessary and sufficient for a complete description of fractal aggregate morphology. We show that aggregate shape is poorly described by the principal radii of gyration and best described by side length ratios of circumscribing rectangular solids with side directions determined by the principal radii of gyration. Remarkably, we find that aggregate shape is described by the Fibonacci series and Divine Proportion in two spatial dimensions and their generalization in three.