The Center for Materials Physics and Technology performs basic and applied research on functional, structural, biological, and electronic material systems. Research includes the study of the fundamental physics and properties of materials and systems across wide ranges of length and time scales. The Center pioneers new methods for studying these systems including original experimental techniques for the development of electronic devices, as well as the development of new computational methods for modeling systems. The Center develops innovative scientific and engineering solutions for systems ranging from the atomic scale through the macroscopic, and from basic physics through the prototyping of devices for naval applications.
Five sections comprise the Center for Materials Physics and Technology. Click the boxes for additional information.
Recent Publications
2016

Calculation of Vibrational and Electronic ExcitedState Absorption Spectra of ArsenicWater Complexes Using Density Functional Theory 

Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery XXII 
2016 
Show abstract »Calculations are presented of vibrational and electronic excitedstate absorption spectra for AsH20 complexes using density function theory (DFT) and timedependent density functional theory (TDDFT). DFT and TDDFT can provide interpretation of absorption spectra with respect to molecular structure for excitation by electromagnetic waves at frequencies within the IR and UVvisible ranges. The absorption spectrum corresponding to excitation states of AsH20 complexes consisting of relatively small numbers of water molecules should be associated with response features that are intermediate between that of isolated molecules and that of a bulk system. DFT and TDDFT calculated absorption spectra represent quantitative estimates that can be correlated with additional information obtained from laboratory measurements and other types of theory based calculations. The DFT software GAUSSIAN was used for the calculations of excitation states presented here.

Determination of Anisotropic Mechanical Properties of G10 Composite Via Direct Strain Imaging 

Polymer Testing 
2016 
Show abstract »The mechanical properties of the glassepoxy composite material known as "G10," which is primarily used for electrical insulation purposes in electromechanical systems, have long been a topic of uncertainty and controversy. In the present paper, we demonstrate the application of an experimental methodology that exploits the Direct Strain Imaging (DSI) full field method to fully assess the elastic anisotropic mechanical properties of G10. Tension and compression experiments were conducted for three distinct inclinations of the specimen orthotropic axes relative to the loading directions, while strain and displacement were monitored at relevant locations on the specimens via digital image capturing and subsequent application of DSI. Based on these experiments, we present the stressstrain experimental data, the associated data post processing, the engineering properties, the anisotropic compliance matrix and the ultimate strength properties of the G10 composite material. Confidence bounds on the material properties based on the results from repeated tests on identical specimens are also reported. Published by Elsevier Ltd.

Diffraction at GaAs/Fe3Si Core/Shell Nanowires: the Formation of Nanofacets 

AIP Advances 
2016 
Show abstract »GaAs/Fe3Si core/shell nanowire structures were fabricated by molecularbeam epitaxy on oxidized Si(111) substrates and investigated by synchrotron xray diffraction. The surfaces of the Fe3Si shells exhibit nanofacets. These facets consist of well pronounced Fe3Si{111} planes. Density functional theory reveals that the Siterminated Fe3Si{111} surface has the lowest energy in agreement with the experimental findings. We can analyze the xray diffuse scattering and diffraction of the ensemble of nanowires avoiding the signal of the substrate and polycrystalline films located between the wires. Fe3Si nanofacets cause streaks in the xray reciprocal space map rotated by an azimuthal angle of 30 degrees compared with those of bare GaAs nanowires. In the corresponding TEM micrograph the facets are revealed only if the incident electron beam is oriented along [1 (1) over bar0] in accordance with the xray results. Additional maxima in the xray scans indicate the onset of chemical reactions between Fe3Si shells and GaAs cores occurring at increased growth temperatures. (C) 2016 Author(s).

Direct Observation of Photoexcited Hole Localization in CdSe Nanorods 

ACS Energy Letters 
2016 
Show abstract »Quantumconfined 1D semiconductor nanostructures are being investigated for hydrogen generation photocatalysts. In the photoreaction, after fast electron transfer, holes that remain in the nanostructure play an important role in the total quantum yield of hydrogen production. Unfortunately, knowledge of hole dynamics is limited due to lack of convenient spectroscopic signatures. Here, we directly probe hole localization dynamics within CdSe nanorods (NRs) by combining transient absorption (TA) and timeresolved terahertz (TRTS) spectroscopy. We show that when methylene blue is used as an electron acceptor, the resulting electron transfer occurs with a time constant of 3.5 Â± 0.1 ps and leaves behind a delocalized hole. However, the hole quickly localizes in the Coulomb potential well generated by the reduced electron acceptor near the NR surface with time constant of 11.7 Â± 0.2 ps. Our theoretical investigation suggests that the hole becomes confined to a ~ Â± 0.8 nm region near the reduced electron acceptor and the activation energy to detrap the hole from the potential well can be as large as 235 meV.

Electrical Detection of the Helical Spin Texture in a ptype Topological Insulator Sb2Te3 

Scientific Reports 
2016 
Show abstract »The surface states of 3D topological insulators (TIs) exhibit a helical spin texture with spin locked at right angles with momentum. The chirality of this spin texture is expected to invert crossing the Dirac point, a property that has been experimentally observed by optical probes. Here, we directly determine the chirality below the Dirac point by electrically detecting spinmomentum locking in surface states of a ptype TI, Sb2Te3. A current flowing in the Sb2Te3 surface states generates a net spin polarization due to spinmomentum locking, which is electrically detected as a voltage on an Fe/Al2O3 tunnel barrier detector. Measurements of this voltage as a function of current direction and detector magnetization indicate that hole spinmomentum locking follows the righthand rule, opposite that of electron, providing direct confirmation that the chirality is indeed inverted below Dirac point. The spin signal is linear with current, and exhibits a temperature dependence consistent with the semiconducting nature of the TI film and freezeout of bulk conduction below 100 K. Our results demonstrate that the chirality of the helical spin texture of TI surface states can be determined electrically, an enabling step in the electrical manipulation of spins in next generation TIbased quantum devices.

GridBased Partitioning for Comparing Attractors 

Physical Review E 
2016 
Show abstract »Stationary dynamical systems have invariant measures (or densities) that are characteristic of the particular dynamical system. We develop a method to characterize this density by partitioning the attractor into the smallest regions in phase space that contain information about the structure of the attractor. To accomplish this, we develop a statistic that tells us if we get more information about our data by dividing a set of data points into partitions rather than just lumping all the points together. We use this method to show that not only can we detect small changes in an attractor from a circuit experiment, but we can also distinguish between a large set of numerically generated chaotic attractors designed by Sprott. These comparisons are not limited to chaotic attractorsthey should work for signals from any finitedimensional dynamical system.

Homoepitaxial Graphene Tunnel Barriers for Spin Transport 

AIP Advances 
2016 
Show abstract »Tunnel barriers are key elements for both chargeand spinbased electronics, offering devices with reduced power consumption and new paradigms for information processing. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier. It is an insulator outofplane, possesses a defectfree, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible in environmental conditions usable for electronics applications. However, two stacked graphene layers can be decoupled using chemical functionalization. Here, we demonstrate that hydrogenation or fluorination of graphene can be used to create a tunnel barrier. We demonstrate successful tunneling by measuring nonlinear IV curves and a weakly temperature dependent zerobias resistance. We demonstrate lateral transport of spin currents in nonlocal spinvalve structures, and determine spin lifetimes with the nonlocal Hanle effect. We compare the results for hydrogenated and fluorinated tunnel and we discuss the possibility that ferromagnetic moments in the hydrogenated graphene tunnel barrier affect the spin transport of our devices. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Inverse Thermal Analysis of Steel Welds Using SolidificationBoundary Constraints 

Journal of Materials Engineering and Performance 
2016 
Show abstract »Inverse thermal analyses of structural steel deeppenetration welds are presented. These analyses employ a methodology that is in terms of numericalanalytical basis functions and constraint conditions for inverse thermal analysis of steadystate energy deposition in plate structures. These analyses provide parametric representations of weld temperature histories that can be adopted as input data to various types of computational procedures, such as those for prediction of solidstate phase transformations and mechanical response. In addition, these parameterized temperature histories can be used for inverse thermal analysis of welds corresponding to other welding processes whose process conditions are within similar regimes. The present study applies an inverse thermal analysis procedure that uses threedimensional constraint conditions whose twodimensional projections are mapped within transverse cross sections of experimentally measured solidification boundaries.

ModulusPressure Equation for Confined Fluids 

Journal of Chemical Physics 
2016 
Show abstract »Ultrasonic experiments allow one to measure the elastic modulus of bulk solid or fluid samples. Recently such experiments have been carried out on fluidsaturated nanoporous glass to probe the modulus of a confined fluid. In our previous work [G. Y. Gor et al., J. Chem. Phys., 143, 194506 (2015)], using Monte Carlo simulations we showed that the elastic modulus K of a fluid confined in a mesopore is a function of the pore size. Here we focus on the moduluspressure dependence K(P), which is linear for bulk materials, a relation known as the TaitMurnaghan equation. Using transitionmatrix Monte Carlo simulations we calculated the elastic modulus of bulk argon as a function of pressure and argon confined in silica mesopores as a function of Laplace pressure. Our calculations show that while the elastic modulus is strongly affected by confinement and temperature, the slope of the modulus versus pressure is not. Moreover, the calculated slope is in a good agreement with the reference data for bulk argon and experimental data for confined argon derived from ultrasonic experiments. We propose to use the value of the slope of K(P) to estimate the elastic moduli of an unknown porous medium. Published by AIP Publishing.

On the Feasibility of Crack Propagation Tracking and Full Field Strain Imaging Via a Strain Compatibility Functional and the Direct Strain Imaging Method 

International Journal of Impact Engineering 
2016 
Show abstract »The simultaneous tracking of the trajectory of a moving crack along with the measurement of the corresponding full field strain fields associated with this motion due to quasistatic or dynamic impact loading, is a problem of high interest from both the perspectives of the engineering practice and engineering science alike. However, full field optical methodologies have not yet been able to address the issues arising in experiments with moving boundaries such as dynamically evolving cracks. Such a problem requires solving simultaneously the problem of identifying the dynamically evolving geometry of the cracks as well as the problem of introducing these cracks into the fullfield representation of a relevant full field method. In the present work we are introducing a methodology that can solve this problem. Specifically, we describe an optical method that can be used to detect dynamically evolving discontinuities on the surface of a body even if they are invisible to the eye, by using a strain compatibility functional that makes no continuity assumptions about the underlying medium. The relevant data from this process are then introduced in a full field representation that leads to an acceptable estimation of the discontinuous strain field. The proposed method is based on the recently introduced Direct Strain Imaging method that can be used to visualize and quantify the full fields of the strain tensor components. Synthetic numerical experiments were performed to generate synthetic crack propagation data along with the associated strain fields in order to assess the feasibility of the proposed method. The approach employed for this purpose was that of applying the extended finite element method to solve the problem of a propagating crack in an elastic medium with inclusions and holes under quasistatic tension. Finally, we present the results of detecting the synthetically produced crack trajectory as well as the relevant strain fields. Published by Elsevier Ltd.

Optical Polarization and Intervalley Scattering in Single Layers of MoS2 and MoSe2 

Scientific Reports 
2016 
Show abstract »Single layers of MoS2 and MoSe2 were optically pumped with circularly polarized light and an appreciable polarization was initialized as the pump energy was varied. The circular polarization of the emitted photoluminescence was monitored as a function of the difference between the excitation energy and the Aexciton emission at the Kpoint of the Brillouin zone. Our results show a threshold of twice the LA phonon energy, specific to the material, above which phononassisted intervalley scattering causes depolarization. In both materials this leads to almost complete depolarization within ~ 100 meV above the threshold energy. We identify the extra kinetic energy of the exciton (independent of whether it is neutral or charged) as the key parameter for presenting a unifying picture of the depolarization process.

Oxygen Character in the Density of States as an Indicator of the Stability of LiIon Battery Cathode Materials 

Solid State Ionics 
2016 
Show abstract »Oxygen gas evolution from lithium battery cathode materials during charging to high voltages is a known mechanism leading to cathode instability and ultimately Liion batteries fires. We present the simple thesis that the tendency toward oxygen destabilization in metaloxide materials used as Liion battery cathodes can be correlated to the amount of oxygenderived weight in the partial density of states (PDOS) near the Fermi level (EF), which is easily calculable with density functional theory (DFT). If the oxygen PDOS dominates the total DOS at EF at any point during delithiation (charging), electrons are withdrawn from the O rather than the M states, and the O2 ions are oxidized to peroxide (Oil, and then often to O2 gas. A demonstration of the utility of this perspective is given using DET to calculate the PDOS for compounds with a broad range of structures, transition metals and degrees of lithiation: LiFePO4, Li2CuO2, LiCoO2, LiNiO2, and Li2RuO3. The resulting computational spectra correlate well to the experimental stability in the modeled compounds. We conclude that the oxygen stability of cathode materials derives from their fundamental electronic structure as function of changing voltage (and lithiation). Published by Elsevier B.V.

Parametric Models of Reflectance Spectra for Dyed Fabrics 

NextGeneration Spectroscopic Technologies IX 
2016 
Show abstract »This study examines parametric modeling of NIR reflectivity spectra for dyed fabrics, which provides for both their inverse and direct modeling. The dye considered for prototype analysis is triarylamine dye. The fabrics considered are camouflage textiles characterized by color variations. The results of this study provide validation of the constructed parametric models, within reasonable error tolerances for practical applications, including NIR spectral characteristics in camouflage textiles, for purposes of simulating NIR spectra corresponding to various dye concentrations in host fabrics, and potentially to mixtures of dyes.

Protein Composition Determines the Effect of Crowding on the Properties of Disordered Proteins 

Biophysical Journal 
2016 
Show abstract »Unlike dilute experimental conditions under which biological molecules are typically characterized, the cell interior is crowded by macromolecules, which affects both the thermodynamics and kinetics of in vivo processes. Although the excludedvolume effects of macromolecular crowding are expected to cause compaction of unfolded and disordered proteins, the extent of this effect is uncertain. We use a coarsegrained model to represent proteins with varying sequence content and directly observe changes in chain dimensions in the presence of purely repulsive spherical crowders. We find that the extent of crowdinginduced compaction is dependent not only on crowder size and concentration, but also on the properties of the protein itself. In fact, we observe a nonmonotonic trend between the dimensions of the polypeptide chain in bulk and the degree of compaction: the most extended chains experience up to 24% compaction, the most compact chains show virtually no change, and intermediate chains compress by up to 40% in size at a 40% crowder volume fraction. Freevolume theory combined with an impenetrable ellipsoidal representation of the chains predicts the crowding effects only for collapsed protein chains. An additional scaling factor, which can be easily computed from proteincrowder potential of mean force, corrects for the penetrability of extended chains and is sufficient to capture the observed nonmonotonic trend in compaction. Ac 2016 Biophysical Society

Revisiting Bangham's Law of AdsorptionInduced Deformation: Changes of Surface Energy and Surface Stress 

Physical Chemistry Chemical Physics 
2016 
Show abstract »When fluids are adsorbed on a solid surface they induce noticeable stresses, which cause the deformation of the solid. D. H. Bangham and coauthors performed a series of experimental measurements of adsorptioninduced strains, and concluded that physisorption causes expansion, which is proportional to the lowering of the surface energy Delta gamma. This statement is referred to as the Bangham effect or Bangham's law. However, it is known that the quantity that controls the deformation is actually the change in surface stress Delta f rather than surface energy Delta gamma, but this difference has not been considered in the context of adsorptioninduced deformation of mesoporous materials. We use the BrunauerEmmettTeller (BET) theory to derive both values and show the difference between them. We find the condition when the difference between the two vanishes, and Bangham's law is applicable; it is likely that this condition is satisfied in most cases, and prediction of strain based on Delta gamma is a good approximation. We show that this is the case for adsorption of argon and water on Vycor glass. Finally, we show that the difference between Delta gamma and Delta f can explain some of the experimental data that contradicts Bangham's law.

SpinOrbit Driven Peierls Transition and Possible Exotic Superconductivity in CsW2O6 

Physical Review B 
2016 
Show abstract »We study ab initio a pyrochlore compound, CsW2O6, which exhibits a yet unexplained metalinsulator transition. We find that (1) the reported lowT structure is likely inaccurate and the correct structure has a twice larger cell; (2) the insulating phase is not of a Mott or dimersinglet nature, but a rare example of a threedimensional Peierls transition, with a simultaneous condensation of three density waves; (3) the spinorbit interaction plays a crucial role, forming wellnested bands. The highT (HT) phase, if stabilized, could harbor a unique e(g) + ie(g) superconducting state that breaks time reversal symmetry, but is not chiral. This state was predicted in 1999, but not observed. We speculate about possible ways to stabilize the HT phase while keeping the conditions for superconductivity.
2015

A Multiphysics Theory for the Static Contact of Deformable Conductors with Fractal Rough Surfaces 

IEEE Transactions on Plasma Science 
2015 
Show abstract »In this paper, we present a multifield and multiscale theory leading to derivations of electric and thermal conductivities for the interface between two rough surfaces in contact, activated by mechanical load and electric current pulses. At the macroscale, the proposed approach involves multifield coupling of conduction and induction currents, with heat conduction induced by joule heating. The structural mechanics of the conducting materials are also considered. At the mesoscale and microscale, the theory contains a WeierstrassMandelbrot description of the rough contact surface profilometry and an asperitybased comprehensive model, respectively. They are both combined to derive homogenized macroscale properties for the interface boundary. The mechanical pressure and the repulsion effect from electric current through the microcontacts are accounted for as well. The results of the numerical analysis illustrate the dependence of the derived properties on the surface characteristics, external load, and electric current. Finally, the entire framework is applied to an actual conductor configuration of hollow cylinders under compression and a high current pulse to demonstrate the feasibility of the entire approach. In addition to providing typical simulation results for all selected fields present during the experiment, we also provide a comparison between the experimentally acquired resistance and the numerically derived resistance to validate the contact theory.

A Tale of Two Sites: On Defining the Carrier Concentration in GarnetBased Ionic Conductors for Advanced Li Batteries 

Advanced Energy Materials 
2015 
Show abstract »Solid electrolytes based on the garnet crystal structure have recently been identified as a promising material to enable advance Li battery cell chemistries because of the unprecedented combination of high ionic conductivity and electrochemical stability against metallic Li. To better understand the mechanisms that give rise to high conductivity, the goal of this work is to correlate Li site occupancy with Liion transport. Toward this goal, the Li site occupancy is studied in cubic garnet as a function of Li concentration over the compositions range: Li7xLa3Zr2xTaxO12 (x = 0.5, 0.75, and 1.5). The distribution of Li between the two interstitial sites (24d and 96h) is determined using neutron and synchrotron diffraction. The bulk conductivity is measured on >97% relative density polycrystalline specimens to correlate Liion transport as a function of Li site occupancy. It is determined that the conductivity changes nonlinearly with the occupancy of the octahedral (96h) Li site. It is shown that the effective carrier concentration is dependent on the Li site occupancy and suggests that this is a consequence of significant carriercarrier coulombic interactions. Furthermore, the observation of maximum conductivity near Li = 6.5 mol is explained.

Calculation of ElectronicExcitedState Absorption Spectra of Water Clusters Using TimeDependent Density Functional Theory 

Algorithms and Technologies for Multispectral, Hyperspectral, and Ultraspectral Imagery Xxi 
2015 
Show abstract »Calculations are presented of electronicexcitedstate absorption spectra for molecular clusters of H2O using timedependent density functional theory (TDDFT). Calculation of excited state resonance structure using TDDFT can provide interpretation of absorption spectra with respect to molecular structure for excitation by electromagnetic waves at frequencies within the UVvisible range. The absorption spectrum corresponding to electronic excitation states of a molecular cluster consisting of a relatively small number of water molecules should be associated with response features that are intermediate between that of isolated molecules and that of a bulk lattice. TDDFT calculated absorption spectra represent quantitative estimates that can be correlated with additional information obtained from laboratory measurements and other types of theory based calculations. The DFT software GAUSSIAN was used for the calculations of electronic excitation states presented here.

Dynamics of Intraband and Interband Auger Processes in Colloidal CoreShell Quantum Dots 

ACS Nano 
2015 
Show abstract »Conventional colloidal quantum dots (QDs) suffer from rapid energy losses by nonradiative (Auger) processes, leading to subns lifetimes in all excited states but the lowestenergy single exciton. Suppression of interband Auger decay, such as biexciton Auger recombination, has been achieved with the design of heterostructured core shell QDs. Augerlike processes are also believed to be responsible for rapid intraband hotelectron cooling in QDs. However, the simultaneous effect of shell growth on interband Auger recombination and intraband hotelectron cooling has not been addressed. Here we investigate how the growth of a CdS shell affects these two relaxation processes in CdSe/CdS core shell QDs. Using a combination of ultrafast pump push probe spectroscopy on the QD ensemble and analysis of the photon statistics from single QDs, we find that Auger losses in the biexciton state are suppressed with increasing shell thickness, while hotelectron cooling remains unaffected. Calculations conducted within an eightband k.p model confirm the experimental dependence of the biexciton Auger decay on the shell thickness, and provide insights into the factors determining the cooling rate of hot carriers.

Elastic Response of Mesoporous Silicon to Capillary Pressures in the Pores 

Applied Physics Letters 
2015 
Show abstract »We study water adsorptioninduced deformation of a monolithic, mesoporous silicon membrane traversed by independent channels of ~ 8 nm diameter. We focus on the elastic constant associated with the Laplace pressureinduced deformation of the membrane upon capillary condensation, i.e., the poreload modulus. We perform finiteelement method (FEM) simulations of the adsorptioninduced deformation of hexagonal and square lattices of cylindrical pores representing the membrane. We find that the poreload modulus weakly depends on the geometrical arrangement of pores, and can be expressed as a function of porosity. We propose an analytical model which relates the poreload modulus to the porosity and to the elastic properties of bulk silicon (Young's modulus and Poisson's ratio), and provides an excellent agreement with FEM results. We find good agreement between our experimental data and the predictions of the analytical model, with the Young's modulus of the pore walls slightly lower than the bulk value. This model is applicable to a large class of materials with morphologies ~ mesoporous silicon. Moreover, our findings suggest that liquid condensation experiments allow one to elegantly access the elastic constants of a mesoporous medium. (C) 2015 AIP Publishing LLC.

Electron Pairing Without Superconductivity 

Nature 
2015 
Show abstract »Strontium titanate (SrTiO3) is the first and best known superconducting semiconductor(1). It exhibits an extremely low carrier density threshold for superconductivity(2), and possesses a phase diagram ~ that of hightemperature superconductors(3,4)two factors that suggest an unconventional pairing mechanism. Despite sustained interest for 50 years, direct experimental insight into the nature of electron pairing in SrTiO3 has remained elusive. Here we perform transport experiments with nanowirebased singleelectron transistors at the interface between SrTiO3 and a thin layer of lanthanum aluminate, LaAlO3. Electrostatic gating reveals a series of twoelectron conductance resonancespaired electron statesthat bifurcate above a critical pairing field Bp of about 14 tesla, an order of magnitude larger than the superconducting critical magnetic field. For magnetic fields below Bp, these resonances are insensitive to the applied magnetic field; for fields in excess of Bp, the resonances exhibit a linear Zeemanlike energy splitting. Electron pairing is stable at temperatures as high as 900 millikelvin, well above the superconducting transition temperature (about 300 millikelvin). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by a model involving an attractive Hubbard interaction that describes realspace electron pairing as a precursor to superconductivity.

Energy Splitting of Image States Induced by the Surface Potential Corrugation of InAs(111) A 

Physical Review B 
2015 
Show abstract »By means of scanning tunneling spectroscopy (STS), we study the electronic structure of the IIIV semiconductor surface InAs(111) A in the field emission regime (above the vacuum level). At high sample bias voltages (approaching + 10 V), a series of well defined resonances are identified as the typical Stark shifted image states that are commonly found on metallic surfaces in the form of field emission resonances (FER). At lower bias voltages, a more complex situation arises. Up to three double peaks are identified as the first three FERs that are split due to their interaction with the periodic surface potential. The high corrugation of this potential is also quantified by means of density functional theory (DFT) calculations. Another sharp resonance not belonging to the FER series is associated with an unoccupied surface state.