A. Shabaev

Optical pumping of the electronic and nuclear spin of single charge-tunable quantum dots.

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.

Nuclei-Induced Frequency Focusing of Electron Spin Coherence

The hyperfine interaction of an electron with the nuclei is considered as the primary obstacle to coherent control of the electron spin in semiconductor quantum dots. We show, however, that the nuclei in singly charged quantum dots act constructively by focusing the electron spin precession about a magnetic field into well-defined modes synchronized with a laser pulse protocol. In a dot with a synchronized electron, the light-stimulated fluctuations of the hyperfine nuclear field acting on the electron are suppressed. The information about electron spin precession is imprinted in the nuclei and thereby can be stored for tens of minutes in darkness. The frequency focusing drives an electron spin ensemble into dephasing-free subspaces with the potential to realize single frequency precession of the entire ensemble.

Optical control of spin coherence in singly charged (In,Ga)As/GaAs quantum dots

Electron spin coherence has been generated optically in n-type modulation doped (In,Ga)As/GaAs quantum dots (QDs) which contain on average a single electron per dot. The coherence arises from resonant excitation of the QDs by circularly polarized laser pulses, creating a coherent superposition of an electron and a trion. Time dependent Faraday rotation is used to probe the spin precession of the optically oriented electrons about a transverse magnetic field. The coherence generation can be controlled by pulse intensity, being most efficient for (2n+1)pi pulses.

Optical read-out and initialization of an electron spin in a single quantum dot

We have shown theoretically that the read-out of a single electron spin is viable through resonant excitation of a trion with circularly polarized light, and suggest a feasible protocol for the spin initialization.

Robust manipulation of electron spin coherence in an ensemble of singly charged quantum dots

Using the recently reported mode locking effect we demonstrate a highly robust control of electron spin coherence in an ensemble of (In,Ga)As quantum dots during the single spin coherence time. The spin precession in a transverse magnetic field can be fully controlled up to 25 K by the parameters of the exciting pulsed laser protocol such as the pulse train sequence, leading to adjustable quantum beat bursts in Faraday rotation. Flipping of the electron spin precession phase was demonstrated by inverting the polarization within a pulse doublet sequence.

Active bialkali photocathodes on free-standing graphene substrates

The hexagonal structure of graphene gives rise to the property of gas impermeability, motivating its investigation for a new application: protection of semiconductor photocathodes in electron accelerators. These materials are extremely susceptible to degradation in efficiency through multiple mechanisms related to contamination from the local imperfect vacuum environment of the host photoinjector. Few-layer graphene has been predicted to permit a modified photoemission response of protected photocathode surfaces, and recent experiments of single-layer graphene on copper have begun to confirm these predictions for single crystal metallic photocathodes. Unlike metallic photoemitters, the integration of an ultra-thin graphene barrier film with conventional semiconductor photocathode growth processes is not straightforward. A first step toward addressing this challenge is the growth and characterization of technologically relevant, high quantum efficiency bialkali photocathodes on ultra-thin free-standing graphene substrates. Photocathode growth on free-standing graphene provides the opportunity to integrate these two materials and study their interaction. Specifically, spectral response features and photoemission stability of cathodes grown on graphene substrates are compared to those deposited on established substrates. In addition, we observed an increase of work function for the graphene encapsulated bialkali photocathode surfaces, which is predicted by our calculations. The results provide a unique demonstration of bialkali photocathodes on free-standing substrates, and indicate promise towards our goal of fabricating high-performance graphene encapsulated photocathodes with enhanced lifetime for accelerator applications.Graphene in accelerator technology: A new material for enhanced photocathode performance and lifetimeGraphene has shown potential to unlock new capabilities of electron sources and other aspects of accelerator technology. This report focuses on integrating graphene with high performance photocathodes with the goal of extending lifetime by thousands of hours.Scientists at Los Alamos National Laboratory, USA, and colleagues succeeded in growth of chemically susceptible photocathodes on free-standing graphene substrates while maintaining state-of-the-art performance. Successful growth on graphene is a critical step toward a material-centric approach to photocathode design: enhancing lifetime without compromising efficiency or other performance metrics. Graphene, an atomically thin sheet of carbon, is an emerging material that has inspired new cathode design capabilities, including heterostructuring, resonant tunneling, and impermeable gas barriers. Conventional photocathode materials have no performance regimes. The next step is complete graphene encapsulation of photocathode films and demonstration of lifetime enhancement in the operating environment of accelerator facilities.

Optical initialization and dynamics of spin in a remotely doped quantum well

We report a high-pressure investigation of the relaxor ferroelectric lead zinc niobate PbZn{sub 1/3}Nb{sub 2/3}O{sub 3} (PZN) up to 46 GPa, which is the highest pressure yet attained in the study of relaxors. The evolution of both Raman and x-ray scattering with pressure gives evidence for important pressure instabilities, which find its expression in three successive phase transitions. The observed pressure-induced suppression of diffuse scattering above 5 GPa is similar to recent reports and supports the hypothesis that this is a general feature in relaxors at high pressures.Stable pairing states of superfluid {sup 3}He in aerogel are examined in the case with a global uniaxial anisotropy which may be created by applying a uniaxial stress to the aerogel. Due to such a global anisotropy, the stability region of an Anderson-Brinkman-Morel (ABM) pairing state becomes wider. In a uniaxially stretched aerogel, the pure polar pairing state with a horizontal line node is predicted to occur, as a three-dimensional superfluid phase, over a measurable width just below the superfluid transition at T{sub c}(P). A possible relevance of the present results to the case with no global anisotropy is also discussed.A series of Ca{sub x}CoO{sub 2} (0.15{<=}x{<=}0.40) materials have been prepared by means of an ion exchange reaction from Na{sub x}CoO{sub 2}. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion, and twinning. Systematic structural analysis, in combination with the experimental data of Na{sub x}CoO{sub 2} (0.15{<=}x{<=}0.8) and Sr{sub x}CoO{sub 2} (1.5{<=}x{<=}0.4) systems, suggests that there are two common well-defined cation ordered states corresponding, respectively, to the orthorhombic superstructure at around x=1/2 and the 3{sup 1/2}ax3{sup 1/2}a superstructure at around x=1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33<x<0.5. The TEM observations also reveal an additional periodic structural distortion with q{sub 2}=a{sup *}/2 in materials for x{<=}0.35. This structural modulation also appears in the remarkable superconducting phase Na{sub 0.33}CoO{sub 2}{center_dot}1.3H{sub 2}O.Electrical resistance, thermoelectric power, dc magnetization, ac susceptibility, and electron spin resonance (ESR) are investigated for the polycrystalline Nd{sub 1-x}Sr{sub 1+x}CoO{sub 4} (x=0.25, 0.33, and 0.60). Powder x-ray diffraction (XRD) confirms that these compounds crystallize in K{sub 2}NiF{sub 4}-type structure with space group I4/mmm. The specimens exhibit ferromagnetic and semiconducting behaviors. With Sr doping, the lattice parameter c increases, the cusp intensity related to spin-glass states weakens, and the ferromagnetic property intensifies. The transport mechanisms in high temperature range obey Arrhenius law and might be understood by small polaron models. The magnetic properties present spin-glass states at {approx}18 K and Griffiths singularity at {approx}210 K.In this work we report on a low-energy electron diffraction (LEED) study of MnO(100) thick films grown on Ag(100) in order to determine their surface geometry. The LEED results indicate a topmost layer rumple of (4.8{+-}2.0)% with the oxygen ions moving towards the vacuum side. These results are in line with other surface structure determinations carried out on the (100) surface of different oxides having rock-salt structure but are in disagreement with MEIS results reported in the literature for the MnO(100) using a MnO single crystal.The structural and magnetic properties of YVO{sub 3}, NdVO{sub 3} and TbVO{sub 3} were investigated by single-crystal and powder neutron diffraction. YVO{sub 3} shows a structural phase transition at 200 K from an orthorhombic structure with the space group Pbnm to a monoclinic one with the space group P2{sub 1}/b. But supplementary high-resolution synchrotron diffraction experiments showed that the monoclinic distortion is extremely small. A group theoretical analysis shows that this magnetic state in the monoclinic phase is incompatible with the lattice structure, unless terms of higher than bilinear order in the spin operators are incorporated in the spin Hamiltonian. This observation is discussed in the light of recent theories invoking unusual many-body correlations between the vanadium t{sub 2g} orbitals. A structural phase transition back to the orthorhombic space group Pbnm is observed upon cooling below 77 K. This transition is accompanied by a rearrangement of the magnetic structure into a mode compatible with the lattice structure. The crystal structures of NdVO{sub 3} and TbVO{sub 3} are closely similar to that of YVO{sub 3}. However, only a single magnetic phase transition was found in the vanadium sublattice down to 9.5 K. Below 60 K the magnetic moments of the Nd{supmorexa0» 3+}- and Tb{sup 3+}-ions are gradually polarized by the ordered vanadium moments. Below 11 K, we found a noncollinear order of the terbium moments.«xa0lessWe report the observation of Co{sup 3+}/Co{sup 4+} short-range charge ordering in 10% Ho-doped SrCoO{sub 3-x} by means of high resolution neutron powder diffraction. The associated one-dimensional commensurate modulation, which can be described with the propagation vector q{sub CO}=(0 0 1/2) with respect to the cubic perovskite cell Pm3m, occurs for compositions close to x=0.20, corresponding to a 1:1 Co{sup 3+}/Co{sup 4+} ratio and extends over clusters of finite size (D{approx}250 A). The bond valence sums for the Co{sup 3+} and Co{sup 4+} sites are +3.07(7) and +3.95(11) (x=0.19), very close to their nominal values +3 and +4. We attribute this astonishing observation to the one-dimensional (1D) character of the associated distortion pattern, whose elastic stabilization is eased with respect to the 3D arrays reported for other charge-ordered perovskite oxides.The compounds RNi{sub 2}Mn (R=Tb, Dy, Ho, and Er) with a MgCu{sub 2}-type structure have been synthesized. The R to transition metal atom ratio is confirmed to be 1:3 using the energy dispersive spectroscopy. The structural and magnetic properties have been investigated by various experimental methods. The x-ray diffraction patterns (XRD) can be well indexed with a cubic Laves cell and space group Fd3m. The refinement results of the XRD patterns show the presence of vacancies in the crystallographic structure. The ordering temperatures T{sub C} have been derived to be 131, 94, 75, and 50 K for R=Tb, Dy, Ho and Er, respectively, which are much higher than those of their corresponding RNi{sub 2} and RMn{sub 2} compounds. A large difference of M-T curves between zero-field-cooling and field-cooling magnetization for all samples at a certain temperature range is observed in a low field, which can be understood in the terms of narrow-domain-wall pinning and a sensitive temperature dependence of coercivity.The structure of liquid CdTe was investigated at pressures up to 23.5 GPa using synchrotron x-ray diffraction. The structure factor, S(Q), and the pair distribution function, g(r), drastically change in two pressure regions, 1.8-3.0 and 7.0-9.0 GPa, accompanied with marked increase in the average coordination number. These findings suggest that there exists at least three stable liquid forms below 23.5 GPa. The pressure interval of the structural change is much smaller compared to other liquids of tetrahedrally bonded materials. Comparing the shapes of S(Q) and g(r) and other structural parameters with the respective data for the reference materials reveals that the lowest- and intermediate-pressure forms have the same local structures as the crystalline counterpart (zinc-blende-like local structure and a NaCl-like local structure), while the highest-pressure form has a different local structure from that in the crystalline form.The charge distributions of slow atomic particles that are singly scattered, multiply scattered, recoiled, and sputtered from metal surfaces are analyzed in terms of both nonadiabatic particle-substrate electron transfer and electron transfer from electronically excited substrates. The results are compared to experimental data for 50 eV Na{sup +} ions scattered from Cu(001), and Al atoms sputtered and recoiled from Al(100). The comparison allows for a quantitative determination of the role of substrate excitations in surface charge exchange. In addition, an analysis of kinetic electron emission (KEE) is carried out using similar low-energy metal projectile-metal substrate systems. Contributions to KEE from various nonadiabatic processes are quantitatively evaluated, including the same process that is responsible for charge formation in single-scattering experiments. The results are compared to experimental KEE data induced by Na{sup +} impinging on Ru(0001). The contributions of nonadiabatic one-electron processes are shown to be small when realistic particle-substrate parameters are used. Many-electron interactions are assumed to play an important role in explaining KEE and, as an illustration, a simplified hot-spot model is outlined.Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo{sub 2}O{sub 5.48} compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at T{sub OD}=776 K. Across the transition the perovskite structure loses its vacancy ordering, and the crystal symmetry changes from orthorhombic Pmmm to tetragonal P4/mmm. The resistivity measurements for temperatures above {approx}350 K yield high values of {rho}, indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole (polaronic) hopping is proposed.Granular films composed of well defined nanometric Co particles embedded in an insulating ZrO{sub 2} matrix were prepared by pulsed laser deposition in a wide range of Co volume concentrations (0.15<x<0.43). High-resolution transmission electron microscopy (TEM) showed very sharp interfaces between the crystalline particles and the amorphous matrix. Narrow particle size distributions were determined from TEM and by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. The magnetic particle size varies little for Co volume concentrations x<0.32 and increases as the percolation limit is approached. The tunneling magnetoresistance (TMR) was successfully reproduced using the Inoue-Maekawa model. The maximum value of TMR was temperature-independent within 50-300 K, and largely increased at low T, suggesting the occurrence of higher-order tunneling processes. Consequently, the tunneling conductance and TMR in clean granular metals are dominated by the Coulomb gap and the inherent particle size distribution.Zr-rich, Nb-doped lead zirconate titanate ceramic and powder samples with composition near Pb{sub 0.99}(Zr{sub 0.95}Ti{sub 0.05}){sub 0.98}Nb{sub 0.02}O{sub 3} [PZT95/5(2Nb)] have been studied in the range of hydrostatic pressure 0-6.2 kbar and temperature 12-295 K by time-of-flight neutron powder diffraction and dielectric measurements. The combination of the two techniques has led to further insights into the properties and pressure-induced ferroelectric rhombohedral R3c (F{sub R(LT)}) to antiferroelectric orthorhombic Pbam (A{sub O}) phase transition in this material, and the diffraction results have provided a detailed view of the ionic displacements induced by changes in pressure and temperature as well as the displacements accompanying the transition. At 295 K the diffraction results revealed a sharp transition at 2.1 kbar; at 200 K this transition occurs at 1.1 kbar. The transformation is incomplete: after the initial sharp drop in the F{sub R(LT)} content at the transition, 20 wt % of the sample remains in the low-pressure F{sub R(LT)} phase. Above the transition, the fraction of F{sub R(LT)}, which exists as a minority phase in the high-pressure A{sub O} phase, continues to decrease, but even at our highest pressure of 6.2 kbar, {approx}8 wt % of the sample remains in the F{sub R(LT)} phase. Themorexa0» volume contraction at the F{sub R(LT)}-to-A{sub O} transition unexpectedly results in the retained minority F{sub R(LT)} being anisotropically clamped, with its a axis slightly expanded and c axis contracted at the transition. On pressure release to 1 bar at 295 K, only 26% of the F{sub R(LT)} phase is recovered, and this remains in the clamped state because of the surrounding majority A{sub O} phase. Heating the sample above 350 K at 1 bar followed by cooling to room temperature results in full recovery of the F{sub R(LT)} phase. The spontaneous polarization (P{sub S}) of the F{sub R(LT)} phase and its pressure and temperature dependences were determined from the ionic displacements. At 295 K, P{sub S}=38 {mu}C/cm{sup 2}--a value greater than the 31-32 {mu}C/cm{sup 2} commonly observed on ceramic PZT95/5(2Nb) samples. The difference is undoubtedly related to residual porosity in ceramic samples as well as the inability of the poling electric field to align all the polar domains. P{sub S} increases monotonically with decreasing temperature, reaching a value of {approx}44 {mu}C/cm{sup 2} at 12 K.«xa0lessThe five independent elastic moduli of single-crystalline hexagonal boron nitride (h-BN) are determined using inelastic x-ray scattering. At room temperature the elastic moduli are in units of GPa C{sub 11}=811, C{sub 12}=169, C{sub 13}=0, C{sub 33}=27.0, and C{sub 44}=7.7. Our experimental results are compared with predictions of ab initio calculations and previously reported incomplete datasets. These results provide solid background for further theoretical advances and quantitative input to model elasticity in boron nitride (BN) nanotubes.I argue that certain bosonic insulator-superfluid phase transitions as an interaction constant varies are driven by emergent geometric properties of insulating states. I examine the renormalized chemical potential and population of disordered bosons at different energy levels. These quantities define the geometric aspect of an effective low energy Hamiltonian which I employ to investigate various resonating states and quantum phase transitions. In a mean field approximation, I also demonstrate that the quantum phase transitions are in the universality class of a percolation problem.The electronic structure and physical properties of {gamma}-Sn{sub 3}N{sub 4} in the spinel structure are investigated by first-principles calculations. The calculated band structure, electronic bonding, and optical properties are compared with two well-studied spinel nitrides {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4}. {gamma}-Sn{sub 3}N{sub 4} is a semiconductor with a direct band gap of 1.40 eV and an attractive small electron effective mass of 0.17. Its optical properties are different from that of {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4} because of the difference in the conduction band minimum. The Sn K, Sn L{sub 3}, Sn M{sub 5}, and N K edges of the x-ray-absorption near-edge structure spectra in {gamma}-Sn{sub 3}N{sub 4} are calculated using a supercell approach and are found to be rich in structures. These spectra are discussed in the context of the electronic structure of the unoccupied conduction band in the presence of the electron core-hole interaction. These calculated spectra can be used for the characterization of this novel compound.The structure of the incommensurate phase of Rb{sub 2}ZnCl{sub 4} has been determined at 194 K (2 K above the lock-in transition) within the soliton regime using satellites up to fifth order. The rather anharmonic modulation functions agree with the expected steplike functions supported by theoretical arguments. In addition, the constancy of the ratio between the amplitudes of the fifth-order and first-order harmonics, a relation predicted by theory, indicate the correctness of the model and imply a value of 0.4 for the soliton density n{sub s}. A symmetry mode analysis shows that the incommensurate structure is consistent with the one of the lock-in phase in the sense that the displacement pattern of every symmetry mode remains unaltered in the transition except for a global change in the amplitudes.X-ray diffraction of SnO{sub 2} (cassiterite) at high pressures and temperatures demonstrates the existence of four phase transitions to 117 GPa. The observed sequence of phases for SnO{sub 2} is rutile-type (P4{sub 2}/mnm){yields}CaCl{sub 2}-type(Pnnm){yields}pyrite-type(Pa3){yields}ZrO{sub 2} orthorhombic phase I (Pbca){yields}cotunnite-type (Pnam). Our observations of the first three phases are generally in agreement with earlier studies. The orthorhombic phase I and cotunnite-type structure (orthorhombic phase II) were observed in SnO{sub 2} for the first time. The Pbca phase is found at 50-74 GPa during room-temperature compression. The cotunnite-type structure was synthesized when SnO{sub 2} was compressed to 74 GPa and heated at 1200 K. The cotunnite-type form was observed during compression between 54-117 GPa with additional laser heating carried out at 91 and 111 GPa. Fitting the pressure-volume data for the high-pressure phases to the second-order Birch-Murnaghan equation of state yields a bulk modulus of 259(26) GPa for the Pbca phase and 417(7) GPa for the cotunnite-type phase.We report x-ray photoelectron spectroscopy (XPS) study of Na and K adlayers on icosahedral Al{sub 70.5}Pd{sub 21}Mn{sub 8.5} (i-Al-Pd-Mn) quasicrystal. The Na 1s core-level exhibits a continuous linear shift of 0.8 eV towards lower binding energies (BE) with increasing coverage up to one monolayer (ML) saturation coverage. In the case of K/i-Al-Pd-Mn, a similar linear shift in the K 2p spectra towards lower BE is observed. In both cases, the plasmon related loss features are observed only above 1 ML. The substrate core-level peaks, such as Al 2p, do not exhibit any shift with the adlayer deposition up to the highest coverage. Based on these experimental observations and previous studies of alkali metal growth on metals, we conclude that below 1 ML, both Na and K form a dispersed phase on i-Al-Pd-Mn and there is hardly any charge transfer to the substrate. The variation of the adlayer and substrate core-level intensities with coverage indicates layer by layer growth.Using neutron elastic and inelastic scattering and high-energy x-ray diffraction, we present a comparison of 40% Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-60% PbTiO{sub 3} (PMN-60PT) with pure Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3} (PMN) and PbTiO{sub 3} (PT). We measure the structural properties of PMN-60PT to be identical to pure PT, however, the lattice dynamics are exactly that previously found in relaxors PMN and Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3} (PZN). PMN-60PT displays a well-defined macroscopic structural transition from a cubic to tetragonal unit cell at 550 K. The diffuse scattering is shown to be weak indicating that the structural distortion is long-range in PMN-60PT and short-range polar correlations (polar nanoregions) are not present. Even though polar nanoregions are absent, the soft optic mode is short-lived for wave vectors near the zone center. Therefore PMN-60PT displays the same waterfall effect as prototypical relaxors PMN and PZN. We conclude that it is random fields resulting from the intrinsic chemical disorder which is the reason for the broad transverse optic mode observed in PMN and PMN-60PT near the zone center and not due to the formation of short-ranged polar correlations. Through our comparison of PMN, PMN-60PT, and pure PT, we interpret the dynamic and static properties of themorexa0» PMN-xPT system in terms of a random field model in which the cubic anisotropy term dominates with increasing doping of PbTiO{sub 3}.«xa0lessPolarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu{sub 2}Te{sub 2}O{sub 5}X{sub 2} (X=Cl,Br). A model of the magnetic structure associated with the propagation vectors k{sup }{sub Cl}{approx_equal}(-0.150,0.422,(1/2)) and k{sup }{sub Br}{approx_equal}(-0.172,0.356,(1/2)) and stable below T{sub N}=18 K for X=Cl and T{sub N}=11 K for X=Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the four Cu{sup 2+} spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu{sup 2+} magnetic moment determined for X=Br,0.395(5){mu}{sub B}, is significantly less than for X=Cl,0.88(1){mu}{sub B} at 2 K. The magnetic structure of the chloride associated with the wave vector k{sup } differs from that determined previously for the wave vector k{approx_equal}(0.150,0.422,(1/2)) [O. Zaharko et al., Phys. Rev. Lett. 93, 217206(E) (2004)].We report the magnetic properties of the ZnL{sub 2}S{sub 4} (L=Er,Tm,Yb) olivines, in which the magnetic lanthanide ions are in a potentially frustrated geometry consisting of sawtooth chains of corner-sharing triangles. Fits to the high-temperature magnetic susceptibility yielded Curie-Weiss temperatures of {theta}{sub W}{approx_equal}-4, -13, and -75 K for the Er, Tm, and Yb compounds, respectively. None of the compounds displayed magnetic long-range order above T=1.8 K. The lack of ordering at temperatures near {theta}{sub W} may be attributed to either the low dimensionality of the structure or the frustrating effect of the triangular geometry.We have investigated the Jahn-Teller transition accompanied by orbital order-disorder transition in La{sub 1-x}Ca{sub x}MnO{sub 3} by high temperature x-ray powder diffraction with synchrotron radiation and resistivity measurements. The unit cell volume of LaMnO{sub 3} decreases with increasing temperature in a narrow temperature range below T{sub JT}{approx_equal}750 K and then undergoes a volume collapse at T{sub JT}. The transition is first order. Similar behavior is also obtained in Ca-doped La{sub 1-x}Ca{sub x}MnO{sub 3} for x=0.025 and 0.075. The amount of volume collapse, however, decreases with the doping and also the first order discontinuous transition crosses over to a quasi-continuous transition with doping. We interpret the volume contraction at the transition is due to a more efficient packing of the MnO{sub 6} octahedra in the orbitally liquid state and the crossover from the discontinuous to the quasi-continuous transition is due to the change in the anharmonic coupling parameter with the hole doping. The resistivity of LaMnO{sub 3} decreases as a function of temperature and then shows abrupt drop at T{sub JT} becoming almost temperature independent at higher temperature. The resistivity of La{sub 1-x}Ca{sub x}MnO{sub 3} also decreases at T{sub JT} but the abrupt drop becomes smeared out at higher doping. Themorexa0» similar behavior of the unit cell volume and the resistivity at the Jahn-Teller transition suggests that the volume contraction at T{sub JT} causes delocalization of e{sub g} electrons.«xa0less

A photoemission moments model using density functional and transfer matrix methods applied to coating layers on surfaces: Theory

Recent experimental measurements of a bulk material covered with a small number of graphene layers reported by Yamaguchi et al. [NPJ 2D Mater. Appl. 1, 12 (2017)] (on bialkali) and Liu et al. [Appl. Phys. Lett. 110, 041607 (2017)] (on copper) and the needs of emission models in beam optics codes have lead to substantial changes in a Moments model of photoemission. The changes account for (i) a barrier profile and density of states factor based on density functional theory (DFT) evaluations, (ii) a Drude-Lorentz model of the optical constants and laser penetration depth, and (iii) a transmission probability evaluated by an Airy Transfer Matrix Approach. Importantly, the DFT results lead to a surface barrier profile of a shape similar to both resonant barriers and reflectionless wells: the associated quantum mechanical transmission probabilities are shown to be comparable to those recently required to enable the Moments (and Three Step) model to match experimental data but for reasons very different than th...

Density of states of Cs 3 Sb calculated using density-functional theory for modeling photoemission

An analysis is presented that provides a density of states (DOS or D(E)) factor for Cs3Sb in the calculation of its quantum efficiency QE and emittance εn;rms using a Moments Approach. The analysis is based on density functional theory (DFT) adapted for the practical application of treating photoemission from bulk metal and semiconductor materials, and the interfaces between them. The Moments approach treats the processes of absorption, transmission and emission separately, for which DFT affects parameters and processes associated with each step, of which D, the optical constants n and k, and materials parameters such as effective mass mn and band gap Eg are paramount. Such factors are required to provide the components of an evaluation similar to the Tsu-Esaki formula for calculating current density over and through and over barriers, and will become more important when a proper quantum mechanical treatment of the emission barrier is considered beyond the simplistic thermal model (transmission probability is unity only for energy levels in excess of the barrier height and zero otherwise). Such features are expected to be far more consequential if the barrier supports resonant levels, e.g., heterostructures.

Analytical models of transmission probabilities for electron sources

Electron emission from coated surfaces as a result of thermal, field, and photoemission effects is often described theoretically using models dependent on the Kemble approximation for the transmission probability D(k). The validity of the approximation for the simple potential profiles (rectangular, triangular, and parabolic) is examined, and generalizations with respect to the exponential of the Gamow tunneling factor and the coefficients of D(k), which are generally ignored, are examined and extended to when the barriers become wells. As a result, unity transmission probabilities ( D(k)→1) with regard to both resonant tunneling barrier and reflectionless well behavior are contrasted. The adaptation of the findings to a general thermal-field-photoemission equation is considered. Consequences for the usage of general emission equations in beam optics code [e.g., Particle-in-Cell (PIC)] such as MICHELLE are discussed.