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arXiv2013

High-contrast Aharonov-Bohm oscillations in the acoustoelectric transport regime

Phase-coherent acoustoelectric transport is reported.Aharonov-Bohm oscillations in the acoustoelectric current with visibility exceeding 100% were observed in mesoscopic GaAs rings as a function of an external magnetic field at cryogenic temperatures. A theoretical analysis of the acoustoelectric transport in ballistic devices is proposed to model experimental observations. Our findings highlight a close analogy between acoustoelectric transport and thermoelectric properties in ballistic devices.

cond-mat.mes-hall↗

arXiv2013

Persistent photoconductivity in oxygen deficient YBCO/LCMO superlattices grown by PLD

We report a large persistent photoconductivity (PPC) in oxygen reduced PLD grown YBa2Cu3O7-δ/La2/3Ca1/3MnO3-y (YBCO/LCMO) superlattices scaling with the oxygen deficiency and being similar to that observed in single layer YBa2Cu3O7-δ films. These results contradict previous observations where in sputtered bilayer samples only a transient photoconductivity (TPC) was found. We argue that the PPC effect in superlattices is caused by the PPC effect due to the YBa2Cu3O7-x layers with limited charge transfer to La2/3Ca1/3MnO3-y. The discrepancy arises from the different permeability of charges across the interface and shed light on the sensitivity of oxide interface properties on details of their preparation.

cond-mat.str-el↗

arXiv2013

A nearly relaxation-free opto-electronic memory from ultra-thin graphene-MoS$_2$ binary hybrids

Ultra-thin planar heterostructures of graphene and other two-dimensional crystals have recently attracted much interest. Very high carrier mobility in a graphene-on-boron nitride assembly is now well-established, but it has been anticipated that appropriately designed hybrids could perform other tasks as well. A heterostructure of graphene and molybdenum disulphide (MoS$_2$) is expected to be sensitive to photo illumination due to the optical bandgap in MoS$_2$. Despite significant advances in device architectures with both graphene and MoS$_2$, binary graphene-MoS$_2$ hybrids have not been realized so far, and the promising opto-electronic properties of such structures remain elusive. Here we demonstrate experimentally that graphene-on-MoS$_2$ binary heterostructures display an unexpected and remarkable persistent photoconductivity under illumination of white light. The photoconductivity can not only be tuned independently with both light intensity and back gate voltage, but in response to a suitable combination of light and gate voltage pulses the device functions as a re-writable optoelectronic switch or memory. The persistent, or `ON', state shows virtually no relaxation or decay within the the experimental time scales for low and moderate photoexcitation intensity, indicating a near-perfect charge retention. A microscopic model associates the persistence with strong localization of carriers in MoS$_2$. These effects are also observable at room temperature, and with chemical vapour deposited graphene, and hence are naturally scalable for large area applications.

cond-mat.mtrl-sci↗

arXiv2013

Shake-Rattle-and-Roll: A Model of Dynamic Structural Disorder in Supported Nanoscale Catalysts

We investigate the effects of "dynamic structural disorder" (DSD) on the behavior of supported nano-scale catalysts. DSD refers to the intrinsic fluctuating, inhomogeneous structure of such nano-scale systems. In contrast to bulk materials, nano-scale systems exhibit substantial fluctuations in energy, charge, and other extensive quantities as well as large surface effects. The DSD is driven by the stochastic librational motion of the center of mass and fluxional bonding at the nanoparticle surface due to thermal coupling with the substrate. Our approach for calculating DSD is based on a combination of statistical mechanics, transient coupled-oscillator models, and real-time DFT/MD simulations. This approach treats thermal and dynamic effects over multiple time-scales, including bond-stretching and -bending vibrations, DSD, and transient tethering to the substrate at longer ps time-scales. Model calculations of molecule-cluster interactions and molecular dissociation reaction paths are presented in which the reactant molecules are adsorbed on the surface of dynamically sampled clusters. This model suggests that DSD affects both the prefactors and distribution of energy barriers in reaction rates, and thus can strongly affect catalytic activity at the nano-scale.

physics.chem-ph↗

arXiv2013

Sharpening surface of magnetic paranematic droplets

In a non-uniform magnetic field, the droplets of colloids of nickel nanorods and nanoparticles aggregate to form a cusp at the droplet surface not deforming the entire droplet shape. When the field is removed, nanorods diffuse away and cusp disappears. Spherical particles can form cusps in a similar way, but they stay aggregated after release of the field; finally, the aggregates settle down to the bottom of the drop. X-ray phase contrast imaging reveals that nanorods in the cusps stay parallel to each other without visible spatial order of their centers of mass. Formation of cusps can be explained with a model that includes magnetostatic and surface tension forces. The discovered possibility of controlled assembly and quenching of nanorod orientation under the cusped liquid surface offers vast opportunities for alignment of carbon nanotubes, nanowires and nanoscrolls, prior to spinning them into superstrong and multifunctional fibers. Magneto and electrostatic analogy suggests that similar ideal alignment can be achieved with the rod-like dipoles subject to a strong electric filed.

cond-mat.mtrl-sci↗

arXiv2013

Relativistic symmetry in deformed nuclei by similarity renormalization group

The similarity renormalization group is used to transform a general Dirac Hamiltonian into diagonal form. The diagonal Dirac operator consists of the nonrelativistic term, the spin-orbit term, the dynamical term, and the relativistic modification of kinetic energy, which are very useful to explore the symmetries hidden in the Dirac Hamiltonian for any deformed system. As an example, the relativistic symmetries in an axially deformed nucleus are investigated by comparing the contributions of every term to the single particle energies and their correlations with the deformation. The result shows that the deformation considerably influences the spin-orbit interaction and dynamical effect, which play a critical role in the relativistic symmetries and its breaking.

nucl-th↗

arXiv2013

Second comment on "Dense and nanometric electronic excitations induced by swift heavy ions in an ionic CaF2 crystal: Evidence for two thresholds of damage creation"

Controversy over nature and existence of the velocity effect [G. Szenes, Phys. Rev. B 87, 056101 (2013)], [M. Toulemonde et al., Phys. Rev. B 87, 056102 (2013)] reignited after new experimental data on swift heavy ion tracks in CaF2 was reported recently [M. Toulemonde et al., Phys. Rev. B 85, 054112 (2012)]. Here we show that results of the analysis in ref. [G. Szenes, Phys. Rev. B 87, 056101 (2013)] should be interpreted differently within analytical thermal spike model. We also propose explanation for apparently missing velocity effect.

cond-mat.mtrl-sci↗

arXiv2013

Influence of homo-buffer layer on stress control of sputtered (Ba0.45,Sr0.55)TiO3 thin films on Pt-Si

To engineer strain relaxation of sputtered BST thin films on Pt-Si wafers, homo-buffer layer method was applied to eliminate Pt hillock formation. Thin BST homo-buffer layers were deposited at room temperature and subsequently the main BST layer was deposited at 650°C, Pt hillock free BST films were obtained with homo-buffer thickness above 5 nm. Relatively good electrical properties were obtained for BST thin films with 15 and 25 nm homo-buffer layer (T= 30 % at 5V and tan δ= 0.018).

cond-mat.mtrl-sci↗

arXiv2013

Ultrafast spin-resolved spectroscopy reveals dominant exciton dynamics in conducting polymer polyaniline

The conducting polymer polyaniline (PANI) has a wide range of optoelectronic applications due to its unique electronic and optical characteristics. Although extensive works have been performed to understand the equilibrium properties, the nature of the charge type that governs its non-equilibrium optical response has been barely understood; a number of studies have debated the nature of photo-generated charge type in PANI, specifically whether it is polaron or exciton based. Here, we report experimental evidence that the charge relaxation dynamics of PANI are dominated by excitons. Utilizing ultrafast spin-resolved pump-probe spectroscopy, we observed that PANI charge dynamics are strongly spin-polarized, exhibiting a spin Pauli-blocking effect. Investigations including both spin-independent and spindependent dynamics reveal that there is no spin-flip process involved in charge relaxation. This provides compelling evidence of an exciton-dominated photo-response in PANI.

cond-mat.mes-hall↗

arXiv2013

Thin films of the spin ice compound Ho2Ti2O7

The pyrochlore compounds Ho2Ti2O7 and Dy2Ti2O7 show an exotic form of magnetism called the spin ice state, resulting from the interplay between geometrical frustration and ferromagnetic coupling. A fascinating feature of this state is the appearance of magnetic monopoles as emergent excitations above the degenerate ground state. Over the past years, strong effort has been devoted to the investigation of these monopoles and other properties of the spin ice state in bulk crystals. A tantalising prospect is to incorporate spin ice materials into devices for spintronics and devices that can manipulate the magnetic monopoles. This would require the availability of spin ice thin films. Here, we report the fabrication of Ho2Ti2O7 thin films using pulsed laser deposition. These films not only show a high crystalline quality, but also exhibit the hallmarks of a spin ice: a pronounced magnetic anisotropy and an intermediate plateau in the magnetisation along the [111] crystal direction.

cond-mat.mtrl-sci↗

arXiv2013

Experimental measurement methods and data on irradiation of functional design materials by helium ions in linear accelerator

The experimental research on the irradiation of the functional design materials by the Helium ions in the linear accelerator is conducted. The experimental measurements techniques and data on the irradiation of the functional design materials by the Helium ions with the energy up to 4 MeV, including the detailed scheme of experimental measurements setup, are presented. The new design of accelerating structure of the IH-type such as POS-4, using the method of alternate-phase focusing with the step-by-step change of the synchronous phase along the focusing periods in a linear accelerator, is developed with the aim to irradiate the functional design materials by the Helium ions. The new design of the injector of the charged Helium ions with the energy of 120 KeV at the output of an accelerating tube and the accelerating structure of the type of POS-4 for the one time charged Helium ions acceleration in the linear accelerator are researched and developed. The special chamber for the irradiation of functional design materials by the Helium ions is also created. In the process of experiment, the temperature of a sample, the magnitude of current of Helium ions beam and the irradiation dose of sample are measured precisely. The experimental measurement setup and techniques are fully tested and optimized in the course of the research on the electro-physical properties of irradiated samples and the thermal-desorption of Helium ions in a wide range of temperatures

cond-mat.mtrl-sci↗

arXiv2013

Fine structure of Surface Relief Gratings: experiment and a generic stochastic Monte Carlo model of the photoinduced mass transport in azo-polymer

We study experimentally and theoretically a double-peak fine structure of Surface Relief Gratings in azo-functionalized poly(etherimide). For the theoretical analysis we develop a stochastic Monte Carlo model for photoinduced mass transport in azobenzene functionalized polymer matrix. The long sought-after transport of polymer chains from bright to dark places of the illumination pattern is demonstrated and characterized, various scenarios for the intertwined processes of build-up of density and SRG gratings are examined. Model predicts that for some azo-functionalized materials double-peak SRG maxima can develop in the permanent, quasi-permanent or transient regimes.

cond-mat.soft↗

arXiv2013

Ultrafast charge separation in organic photovoltaics enhanced by charge delocalization and vibronically hot exciton dissociation

In organic photovoltaics, the mechanism by which free electrons and holes are generated overcoming the Coulomb attraction is a currently much debated topic. To elucidate this mechanism at a molecular level, we carried out a combined electronic structure and quantum dynamical analysis that captures the elementary events from the exciton dissociation to the free carrier generation at polymer/fullerene donor-acceptor heterojunctions. Our calculations show that experimentally observed efficient charge separations can be explained by a combination of two effects: First, the delocalization of charges which substantially reduces the Coulomb barrier, and second, the vibronically hot nature of the charge transfer state which promotes charge dissociation beyond the barrier. These effects facilitate an ultrafast charge separation even at low-band-offset heterojunctions.

physics.chem-ph↗

arXiv2013

Phase behavior of binary mixtures of hard convex polyehdra

Shape anisotropy of colloidal nanoparticles has emerged as an important design variable for engineering assemblies with targeted structure and properties. In particular, a number of polyhedral nanoparticles have been shown to exhibit a rich phase behavior [Agarwal et al., Nature Materials, 2011, 10, 230]. Since real synthesized particles have polydispersity not only in size but also in shape, we explore here the phase behavior of binary mixtures of hard convex polyhedra having similar sizes but different shapes. Choosing representative particle shapes from those readily synthesizable, we study in particular four mixtures: (i) cubes and spheres (with spheres providing a non-polyhedral reference case), (ii) cubes and truncated octahedra, (iii) cubes and cuboctahedra, and (iv) cuboctahedra and truncated octahedra. The phase behavior of such mixtures is dependent on the interplay of mixing and packing entropy, which can give rise to miscible or phase-separated states. The extent of mixing of two such particle types is expected to depend on the degree of shape similarity, relative sizes, composition, and compatibility of the crystal structures formed by the pure components. While expectedly the binary systems studied exhibit phase separation at high pressures due to the incompatible pure-component crystal structures, our study shows that the essential qualitative trends in miscibility and phase separation can be correlated to properties of the pure components, such as the relative values of the order-disorder transition pressure (ODP) of each component. Specifically, if for a mixture A+B we have that ODP_B <ODP_A and ΔODP = ODP_A - ODP_B, then at any particular pressure where phase separation occurs, the larger the ΔODP the lower the solubility of A in the B-rich ordered phase and the higher the solubility of B in the A-rich ordered phase.

cond-mat.soft↗

arXiv2013

First-principles study of band alignments in the p-type hosts BaM2X2 (M = Cu, Ag; X = S, Se)

The electronic structures of four semiconductor compounds BaCu2S2, BaCu2Se2, BaAg2S2, and BaAg2Se2 are studied by density functional theory using both semi-local and hybrid functionals. The ionization energies and electron affinities were determined by aligning the electronic states with the vacuum level by calculating the electrostatic profile within a supercell slab model. The ionization energy and electron affinity of the compounds were calculated using the Heyd-Scuseria-Ernzerhof (HSE) functionals and range from 4.5 to 5.4 eV and 3.1 to 3.4 eV, respectively. The replacement of Cu by Ag slightly increases the ionization energy and electron affinity, while the replacement of S by Se decreases the ionization energy but slightly increases the electron affinity. Overall, the low ionization energies and small electron affinities suggest that these compounds possess good p-type doping propensities. The band gaps are somewhat small to be ideal candidates for transparent semiconducting behavior. The replacement of Cu with Ag in the barium sulfide compounds can increase the band gap from 1.62 eV to 2.01 eV.

cond-mat.mtrl-sci↗

arXiv2013

Boron Nitride Monolayer: A Strain-Tunable Nanosensor

The influence of triaxial in-plane strain on the electronic properties of a hexagonal boron-nitride sheet is investigated using density functional theory. Different from graphene, the triaxial strain localizes the molecular orbitals of the boron-nitride flake in its center depending on the direction of the applied strain. The proposed technique for localizing the molecular orbitals that are close to the Fermi level in the center of boron nitride flakes can be used to actualize engineered nanosensors, for instance, to selectively detect gas molecules. We show that the central part of the strained flake adsorbs polar molecules more strongly as compared with an unstrained sheet.

cond-mat.mes-hall↗

arXiv2013

Electrophoretic mobilities of dissolved polyelectrolyte charging agent and suspended non-colloidal titanium during electrophoretic deposition

Coarse (<=20 micron) titanium particles were deposited on low-carbon steel substrates by cathodic electrophoretic deposition (EPD) with ethanol as suspension medium and poly(diallyldimethylammonium chloride) (PDADMAC) as polymeric charging agent. The preceding data demonstrated that Ti particles of ~1-12 micron size, electrosterically modified by the PDADMAC charging agent, acted effectively as colloidal particles during EPD. Owing to the non-colloidal nature of the particles and the stabilization of the Ti particles by electrosteric forces, the relevance of the zeta potential is questionable, so the more fundamental parameter of electrophoretic mobility was used. A key finding from the present work is the importance of assessing the electrophoretic mobilities of both the suspensions and solutions since the latter, which normally is overlooked, plays a critical role in the ability to interpret the results meaningfully. Further, algebraic uncoupling of these data plus determination of the deposit yield as a function of charging agent addition allow discrimination between the three main mechanistic stages of the electrokinetics of the process, which are: (1) surface saturation; (2) compression of the diffuse layer, growth of polymer-rich layer, and/or competition between the mobility of Ti and PDADMAC; and (3) little or no decrease in electrophoretic mobility of Ti, establishment of polymer-rich layer, and/or dominance of the mobility of the PDADMAC over that of Ti.

cond-mat.mtrl-sci↗

arXiv2013

On the phenomenological modelling of yield surface distortion

A simple phenomenological approach to metal plasticity, including the description of the strain-induced plastic anisotropy, is considered. The advocated approach is exemplified by a two-dimensional rheological analogy. This analogy provides insight into modelling of nonlinear kinematic hardening of Armstrong-Frederick type combined with a nonlinear distortional hardening. In the previous publications of the authors, an interpolation rule between the undistorted yield surface of a virgin material and the saturated yield surface of a pre-deformed material was considered. In the current publication, a somewhat more flexible approach is considered. Given a set of convex symmetric key surfaces which correspond to different hardening stages, the form of the yield surface is smoothly interpolated between these key surfaces. Thus, any experimentally observed sequence of symmetric convex yield surfaces can be rendered. In particular, an arbitrary sharpening of the yield locus in the loading direction combined with a flattening on the opposite side can be taken into account. Moreover, the yield locus evolves smoothly and its convexity is ensured at each hardening stage.

cond-mat.mtrl-sci↗