arXiv subjects

David Vaknin

Publications and source records attributed to David Vaknin.

54 records · Page 3Linked to original sources

arXiv2014

The Effect of Cesium Carbonate on PCBM Aggregation in Films

Surface-pressure isotherms, X-ray reflectivity and X-ray near-total reflection fluorescence were used to study the properties of 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C$_{61}$ (PCBM) that was pre-mixed with cesium carbonate and spread as a film at the air-water interface. The pre-mixed PCBM with cesium carbonate demonstrated a strikingly strong effect on the organization of the film. Whereas films formed from pure PCBM solution were rough due to strong inter-molecular interactions, the films formed from the mixture were much smoother. This indicates that the cesium carbonate moderates the inter-molecular interactions among PCBM molecules, hinting that the cesium diffusion observed in inverted organic photovoltaics and the likely ensuing ionic Cs-PCBM interaction decreases aggregation tendency of PCBM. This implies that the use of cesium salts affects the morphology of the organic layer and consequently improves the efficiency of these devices.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic spectra in the tridiminished-icosahedron \{Fe$_9$\} nano-cluster by inelastic neutron scattering

Inelastic neutron scattering (INS) experiments under applied magnetic field at low temperatures show detailed low lying magnetic excitations in the so called tridiminshed iron icosahedron magnetic molecule. The magnetic molecule consists of nine iron Fe$^{3+}$ ($s = 5/2$) and three phosphorous atoms that are situated on the twelve vertices of a nearly perfect icosahedron. The three phosphorous atoms form a plane that separates the iron cluster into two weakly coupled three- and six-ion fragments, \{Fe$_3$\} and \{Fe$_6$\}, respectively. The magnetic field INS results exhibit an $S=1/2$ ground state expected from a perfect equilateral triangle of the \{Fe$_3$\} triad with a powder averaged $g$-value $=2.00$. Two sets of triplet excitations whose temperature and magnetic field dependence indicate an $S=0$ ground state with two non-degenerate $S=1$ states are attributed to the \{Fe$_6$\} fragment. The splitting may result from a finite coupling between the two fragments, single-ion anisotropy, antisymmetric exchange couplings, or from magnetic frustration of its triangular building blocks.

cond-mat.str-el↗

arXiv2014

Magnetic excitations and anomalous spin wave broadening in multiferroic FeV2O4

We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI transition, anomalous spin wave broadening is observed in the orthorhombic/CFI state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening is also observed at the zone boundary without softening, which is discussed in terms of magnon-phonon coupling.

cond-mat.str-el↗

arXiv2014

Density Profiles of Liquid/Vapor Interfaces Away from Their Critical Point

We examine the applicability of various model profiles for the liquid/vapor interface by X-ray reflectivities on water and ethanol and their mixtures at room temperature. Analysis of the X-ray reflecivities using various density profiles shows an error-function like profile is the most adequate within experimental error. Our finding, together with recent observations from simulation studies on liquid surfaces, strongly suggest that the capillary-wave dynamics shapes the interfacial density profile in terms of the error function.

cond-mat.soft↗

arXiv2013

Magnetic structures and the Ce-Fe coupling induced Fe spin reorientation in CeFeAsO single crystal

Neutron and synchrotron resonant X-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in single crystal CeFeAsO. The RXMS of magnetic reflections at the Ce $L_{\rm II}$-edge shows a magnetic transition that is specific to the Ce antiferromagnetic long-range ordering at $T_\texttt{Ce}\approx$ 4 K with short-range Ce ordering above $T_\texttt{Ce}$, whereas neutron diffraction measurements of a few magnetic reflections indicate a transition at $T^{*}\approx$ 12 K with unusual order parameter. Detailed order parameter measurements on several magnetic reflections by neutrons show a weak anomaly at 4 K which we associate with the Ce ordering. The successive transitions at $T_\texttt{Ce}$ and $T^{*}$ can also be clearly identified by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at $T^{*}\approx$ 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the \textit{ab} plane. The Fe spin reorientation transition and short-range Ce ordering above $T_\texttt{Ce}$ reflect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through $T^{*}$ and $T_\texttt{Ce}$ is proposed.

cond-mat.str-el↗

arXiv2013

Magnetoelastic coupling and charge correlation lengths in a twin domain of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0.047$): A high-resolution X-ray diffraction study

The interplay between structure, magnetism and superconductivity in single crystal Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ (x=0.047) has been studied using high-resolution X-ray diffraction by monitoring charge Bragg reflections in each twin domain separately. The emergence of the superconducting state is correlated with the suppression of the orthorhombic distortion around \emph{T}$_\texttt{C}$, exhibiting competition between orthorhombicity and superconductivity. Above \emph{T}$_\texttt{S}$, the in-plane charge correlation length increases with the decrease of temperature, possibly induced by nematic fluctuations in the paramagnetic tetragonal phase. Upon cooling, anomalies in the in-plane charge correlation lengths along $a$ ($ξ_{a}$) and $b$ axes ($ξ_{b}$) are observed at \emph{T}$_\texttt{S}$ and also at \emph{T}$_\texttt{N}$ indicative of strong magnetoelastic coupling. The in-plane charge correlation lengths are found to exhibit anisotropic behavior along and perpendicular to the in-plane component of stripe-type AFM wave vector (101)$_{\rm O}$ below around \emph{T}$_\texttt{N}$. The temperature dependence of the out-of-plane charge correlation length shows a single anomaly at \emph{T}$_\texttt{N}$, reflecting the connection between Fe-As distance and Fe local moment. The origin of the anisotropic in-plane charge correlation lengths $ξ_{a}$ and $ξ_{b}$ is discussed on the basis of the antiphase magnetic domains and their dynamic fluctuations.

cond-mat.str-el↗

arXiv2010

The magnetic form factor of iron in SrFe2As2

The iron magnetic form factor in SrFe2As2 has been determined by neutron diffraction and by density functional theory (DFT). As noted previously, the magnitude of the calculated moment using DFT is sensitive to the Fe-As distance. However, the shape of the calculated form factor is practically insensitive to the Fe-As distance, and further we show that the form factor closely resembles that of bcc iron, and agrees well with experiment. The spin density exhibits some anisotropy due to geometry and As hybridization.

cond-mat.supr-con↗

arXiv2009

Magnetic and lattice coupling in single-crystal SrFe$_2$As$_2$: A neutron scattering study

A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe$_2$As$_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_\texttt{O}$ = $T_\texttt{N}$ = (201.5 $\pm$ 0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1-2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than \emph{T}$_\texttt{O}$, we suggest that the T-O transition may be an order-disorder transition.

cond-mat.str-el↗

arXiv2009

Tweaking the spin-wave dispersion and suppressing the incommensurate phase in LiNiPO$_4$ by iron substitution

Elastic and inelastic neutron scattering studies of Li(Ni$_{1-x}$Fe$_{x}$)PO$_4$ single crystals reveal anomalous spin-wave dispersions along the crystallographic direction parallel to the characteristic wave vector of the magnetic incommensurate phase. The anomalous spin-wave dispersion ({\it magnetic soft mode}) indicates the instability of the Ising-like ground state that eventually evolves into the incommensurate phase as the temperature is raised. The pure LiNiPO$_4$ system ($x=0$), undergoes a first-order magnetic phase transition from a long-range incommensurate phase to an antiferromagnetic ground state at {\it T}$_N$ = 20.8 K. At 20% Fe concentrations, although the AFM ground state is to a large extent preserved as that of the pure system, the phase transition is second-order, and the incommensurate phase is completely suppressed. Analysis of the dispersion curves using a Heisenberg spin Hamiltonian that includes inter- and in-plane nearest and next-nearest neighbor couplings reveals frustration due to strong competing interactions between nearest- and a next-nearest neighbor site, consistent with the observed incommensurate structure. The Fe substitution only slightly lowers the extent of the frustration, sufficient to suppress the IC phase. An energy gap in the dispersion curves gradually decreases with the increase of Fe content from $\sim$2 meV for the pure system ($x=0$) to $\sim$0.9 meV for $x=0.2$.

cond-mat.str-el↗

arXiv2009

X-ray fluorescence spectroscopy from ions at charged vapor/water interfaces

X-ray fluorescence spectra from monovalent ions (Cs+) that accumulate from dilute solutions to form an ion-rich layer near a charged Langmuir monolayer are presented. For the salt solution without the monolayer, the fluorescence signals below the critical angle are significantly lower than the detection sensitivity and only above the critical angle signals from the bulk are observed. In the presence of a monolayer that provides surface charges, strong fluorescence signals below the critical angle are observed. Ion density accumulated at the interface are determined from the fluorescence. The fluorescent spectra collected as a function of incident x-ray energy near the LIII edge yield the extended absorption spectra from the ions, and are compared to recent independent results. The fluorescence data from divalent Ba2+ with and without monolayer are also presented.

cond-mat.soft↗

arXiv2009

Antiferromagnetism in the magnetoelectric effect single crystal LiMnPO$_4$

Elastic and inelastic neutron scattering studies reveal details of the antiferromagnetic tansition and intriguing spin-dynamics in the magneto-electric effect single crystal LiMnPO$_4$. The elastic scattering studies confirm the system is antiferromagnetic (AFM) below $T_N$=33.75 K with local magnetic moments (Mn$^{2+}$; $S = 5/2$) that are aligned along the crystallographic a-axis. The spin-wave dispersion curves propagating along the three principal axes, determined by inelastic scattering, are adequately modeled in the linear spin-wave framework assuming a spin-Hamiltonian that is parameterized by inter- and in-plane nearest- and next-nearest-neighbor interactions, and by easy-plane anisotropy. The temperature dependence of the spin dynamics makes this an excellent model many-body spin system to address the question of the relationship between spin-wave excitations and the order parameter.

cond-mat.str-el↗

arXiv2009

Thermally excited capillary waves at vapor/liquid interfaces of water-alcohol mixtures

The density profiles of liquid/vapor interfaces of water-alcohol (methanol, ethanol and propanol) mixtures were studied by surface sensitive synchrotron X-ray scattering techniques. X-ray reflectivity and diffuse scattering measurements, from the pure and mixed liquids, were analyzed in the framework of capillary-wave theory to address the characteristics length-scales of the intrinsic roughness and the shortest capillary-wavelength (alternatively, the upper wave-vector cutoff in capillary wave theory). Our results establish that the intrinsic roughness is dominated by average interatomic distances. The extracted effective upper wave-vector cutoff indicates capillary wave theory breaks-down at distances on order of bulk correlation lengths.

cond-mat.soft↗

arXiv2009

Preferential affinity of calcium ions to charged phosphatidic-acid surface from a mixed calcium/barium solution: X-ray reflectivity and fluorescence studies

X-ray reflectivity and fluorescence near total reflection experiments were performed to examine the affinities of divalent ions ($\mathrm{Ca^{2+}}$ and $\mathrm{Ba^{2+}}$) from aqueous solution to a charged phosphatidic-acid (PA) surface. A phospholipid (1,2-Dimyristoyl-sn-Glycero-3-Phosphate, DMPA), spread as a monolayer at the air/water interface, was used to form and control the charge density at the interface. We find that for solutions of the pure salts (i.e., $\mathrm{CaCl_{2}}$ and $\mathrm{BaCl_{2}}$), the number of bound ions per DMPA at the interface is saturated at concentrations that exceed $\mathrm{10^{-3}M}$. For a 1:1 $\mathrm{Ca^{2+}/Ba^{2+}}$ mixed solutions, we find that the bound $\mathrm{Ca^{2+}/Ba^{2+}}$ ratio at the interface is 4:1. If the only property determining charge accumulation near PA were the ionic charges, the concentration of mixed $\mathrm{Ca^{2+}/Ba^{2+}}$ at the interface would equal that of the bulk. Our results show a clear specific affinity of PA for Ca compared to Ba. We provide some discussion on this issues as well as some implications for biological systems. Although our results indicate an excess of counterion charge with respect to the surface charge, that is, charge inversion, the analysis of both reflectivity and fluorescence do not reveal excess of co-ions (namely, $\mathrm{Cl^{-}}$ or $\mathrm{I}^{-}$).

cond-mat.soft↗

arXiv2008

Spin Dynamics in the Magnetoelectric Effect LiCoPO$_4$ Compound

Inelastic neutron scattering (INS) experiments were performed to investigate the spin dynamics in magnetoelectric effect (ME) LiCoPO$_4$ single crystals. Weak dispersion was detected in the magnetic excitation spectra along the three principal crystallographic axes measured around the (0 1 0) magnetic reflection. Analysis of the data using linear spin-wave theory indicate that single-ion anisotropy in LiCoPO$_4$ is as important as the strongest nearest-neighbor exchange coupling. Our results suggest that Co$^{2+}$ single-ion anisotropy plays an important role in the spin dynamics of LiCoPO$_4$ and must be taken into account in understanding its physical properties. High resolution INS measurements reveal an anomalous low energy excitation that we hypothesize may be related to the magnetoelectric effect of LiCoPO$_4$.

cond-mat.str-el↗

arXiv2008

Extracting the Pair Distribution Function of Liquids and Liquid-Vapor Surfaces by Grazing Incidence X-ray Diffraction Mode

We show that the structure factor $S(q)$ of water can be obtained from X-ray synchrotron experiments at grazing angle of incidence (in reflection mode) by using a liquid surface diffractometer. The corrections used to obtain $S(q)$ self-consistently are described. Applying these corrections to scans at different incident beam angles (above the critical angle) collapses the measured intensities into a single master curve, without fitting parameters, which within a scale factor yields $S(q)$. Performing the measurements below the critical angle for total reflectivity yields the structure factor of the top most layers of the water/vapor interface. Our results indicate water restructuring at the vapor/water interface. We also introduce a new approach to extract $g(r)$, the pair distribution function (PDF), by expressing the PDF as a linear sum of Error functions whose parameters are refined by applying a non-linear least square fit method. This approach enables a straightforward determination of the inherent uncertainties in the PDF. Implications of our results to previously measured and theoretical predictions of the PDF are also discussed.

cond-mat.soft↗

arXiv2006

How accurate is Poisson-Boltzmann theory for monovalent ions near highly charged interfaces?

Surface sensitive synchrotron x-ray scattering studies were performed to obtain the distribution of monovalent ions next to a highly charged interface. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface to control surface charge density. Using anomalous reflectivity off and at the $L_3 $ Cs$^{+}$ resonance, we provide spatial counterion (Cs$^{+}$) distributions next to the negatively charged interfaces. Five decades in bulk concentrations are investigated, demonstrating that the interfacial distribution is strongly dependent on bulk concentration. We show that this is due to the strong binding constant of hydronium H$_3$O$^{+}$ to the phosphate group, leading to proton transfer back to the phosphate group and to a reduced surface charge. The increase of Cs$^+$ concentration modifies the contact value potential, thereby causing proton release. This process effectively modifies surface charge density and enables exploration of ion distributions as a function of effective surface charge-density. The experimentally obtained ion distributions are compared to distributions calculated by Poisson-Boltzmann theory accounting for the variation of surface charge density due to proton release and binding. We also discuss the accuracy of our experimental results in discriminating among possible deviations from Poisson-Boltzmann theory.

cond-mat.soft↗

arXiv2005

Monovalent counterion distributions at highly charged water interfaces: Proton-transfer and Poisson-Boltzmann theory

Surface sensitive synchrotron-X-ray scattering studies reveal the distributions of monovalent ions next to highly charged interfaces. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the $L_3$ Cs$^+$ resonance, we provide, for the first time, spatial counterion distributions (Cs$^+$) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H$_3$O$^+$ at the interface leads to proton-transfer back to the phosphate group by a high contact-potential, whereas high salt concentrations lower the contact-potential resulting in proton-release and increased surface charge-density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions.

cond-mat.soft↗

arXiv2005

Spin-waves in antiferromagnetic single crystal LiFePO$_4$

Spin-wave dispersions in the antiferromagnetic state of single crystal LiFePO$_4$ were determined by inelastic neutron scattering measurements. The dispersion curves measured from the (010) reflection along both {\it a}$^\ast$ and {\it b}$^\ast$ reciprocal-space directions reflect the anisotropic coupling of the layered Fe$^{2+}$ (S = 2) spin-system. The spin-wave dispersion curves were theoretically modeled using linear spin-wave theory by including in the spin-Hamiltonian in-plane nearest- and next-nearest-neighbor interactions ({\it J}$_1$ and {\it J}$_2$), inter-plane nearest-neighbor interactions ({\it J}$_\bot$) and a single-ion anisotropy ({\it D}). A weak (010) magnetic peak was observed in elastic neutron scattering studies of the same crystal indicating that the ground state of the staggered iron moments is not along (010) direction, as previously reported from polycrystalline samples studies, but slightly rotated away from this axis.

cond-mat.mtrl-sci↗