Surface energy bands observed in photoemission from a (1 × 1)-N structure on Cr (100)

The surface structure obtained on Cr (100) by diffusion of nitrogen from the bulk is studied by angle resolved photoemission. It is found that nitrogen is bonded in an ordered (1 × 1) configuration with a crystallographic point group C_4v. The data display a strongly dispersive two dimensional Bloch character for the N2p derived resonances. For these bands the symmetry at the $\text{Γ}$ point and the dispersion along the $\text{Δ}$ and $\text{Σ}$ lines of the surface Brillouin zone are determined. The results indicate a strong interaction of the nitrogen atoms with the substrate and favor the intersticial fourfold hollow position.     

N2 Monolayers on graphite: Specific heat and vapor pressure measurements — thermodynamics of size effects and steric factors

Calorimetric and vapor pressure measurements of N₂ monolayers on Grafoil have been made in the regions recently explored by neutron scattering. The phase transition at T = 48 K involves approximately symmetric specific heat anomalies of 2̃ K FWHM, and the peak position shifts about 1 K from $\text{1}\text{3}$ to $\text{2}\text{3}$ of one monolayer. In spite of the peak width and temperature shift the transition is intrinsically first order, due to melting of the √3 registered structure into a disordered phase in the presence of 2D vapor. The shift and broadening are shown to be a consequence of size effects primarily due to large edge-to-area ratios of the ordered phase domains. A formula relating the domain size to the shift in transition temperature gives r = 50' A for the mean radius of the domains. At film densities above $\text{2}\text{3}$ monolayer the peak broadens and shifts to a higher temperature until a critical region is reached near T= 85 K. The combined heat capacity and vapor pressure data indicate that there is no two phase region at coverages near monolayer completion and that the transition near T = 85 K is higher than first order. Effects of molecular orientation and surface-normal vibration appear in certain regions of temperature and density. A phase diagram is constructed which is consistent with both calorimetric and neutron scattering results.     

Light scattering study of dispersion and attenuation of hypersound in adamantane

The Brillouin scattering techniques have been used to measure the velocity dispersion of hypersonic acoustic waves in the “high temperature” disordered cubic phase of adamantane. Shear waves, characteristic of the C₄₄ elastic constant, show no significant dispersion. Longitudinal waves propagating in the (001) plane show strong velocity dispersion. The measures have been performed at the same temperature T = 295.7 K. Using a classical single relaxation time model for the dispersion as a function of frequency at temperature T, the L-mode data have been correctly fitted.The importance of the dispersion $\text{(C}{}_{\mathrm{\infty }}\text{? C}{}_{\mathrm{o}}\text{)}\text{C}{}_0$ for the elastic constants is 20% for C₁₁, 51% for C₁₂ #1% for C₄₄ and ?2.8% for $\text{(C}{}_{11}\text{? C}{}_{12}\text{)}\text{2}$. The fitted relaxation time is τ ? 9 × 10^?11 sec.     

Chlorine adsorption on copper: II. Photoemission from Cu(001)c(2 × 2)-C1 and Cu(111)(√3 × √3)R30°-Cl

We have studied high-resolution angle-resolved and photon-polarization dependent photoemission from chlorine adsorbed on Cu(OOl) and Cu(111). Chlorine forms a c(2 × 2) saturation overlayer on Cu(OO1) and adsorbs dissociatively as revealed by LEED and XPS. Several two-dimensional energy bands on Cu(001)c(2 × 2)-Cl could be iden along the $\text{}\text{?}$gG M? and $\text{}\text{?}$gG M? lines of the surface Brillouin zone, their respective mirror symmetry and their orbital character could be determined. An interpretation of these bands is given in terms of the interaction of the ordered overlayers with particular substrate bulk bands. Besides the appearance of adsorbate-induced two-dimensional bands drastic changes are resolved in the substrate d-band emission region. These can be explained almost exclusively by surface umklapp processes involving reciprocal lattice vectors of the ordered adsorbate mesh. Supplementary studies of the Cu(111) (√3 × √3)R30°-Cl system support our ideas. We discuss some important implications of our results for the interpretation of angle-resolved photoenussion spectra from ordered adsorbate layers.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.     

Raman spectra of two new modifications of germanium: Allo-germanium and 4H-Ge

The Raman spectra of allogermanium and 4H germanium have been measured. Allogermanium shows a band which mimics the density of states of diamond-like Ge(Ge?I). 4H-Ge shows a peak at 287 cm^-1 which can be assigned to an E₂ phonon. This phonon corresponds to a TO-mode of Ge-I at the $\text{(}\text{π}\text{2}\text{a}\text{) (1|1)}$ point of the Brillouin zone.     

Elastic constants of single crystals of the three phases of In---Pb alloys

In a study of the elastic behaviour of the InPb alloys, the elastic stiffness tensor components of crystals of each of the three phases (fct, $\text{c}\text{a}\text{> 1}$; fct, $\text{c}\text{a}\text{> 1}$; fcc) have been obtained as a function of temperature from pulse superposition measurements of ultrasonic wave velocities. Comparison of the elastic stiffness constants obtained for a fct ($\text{c}\text{a}\text{> 1}$) 5 atm.% Pb alloy with those of In itself and those of InTl and InCd alloys, establishes for this phase that alloying with Pb, as with TI and Cd, enhances the softening of the acoustic [110] phonon mode, polarization [1$\text{1}$0] near the Brillouin zone centre. The elastic properties of a 17 atm.% Pb crystal, which is in the fct ($\text{c}\text{a}\text{> 1}$) phase, are quite different from those shown by In alloys in the fct ($\text{c}\text{a}\text{> 1}$ phase; in particular the response to a shear stress is remarkably isotropie: there is no phonon mode softening in this alloy. Neither is there softening of this mode (which corresponds at the zone centre to the shear stiffness $\text{1}\text{2}$(C₁₁;C₁₂)) in crystals of the fee phase — in complete contrast to the dominating influence of the softening of $\text{1}\text{2}$(C₁₁;C₁₂) in the InTl and InCd fee alloys. In fact for a fcc In-75 atm.% Pb alloy the anistropy ratio for shear $\text{2C}{}_{44}\text{(C}{}_{11}\text{C}{}_{12}\text{)}$ is close to unity. The transitions between the three phases of the InPb alloys are markedly first order and acoustic mode softening has a much smaller influence on the elastic behaviour of the fct ($\text{c}\text{a}\text{< 1}$) and fcc InPb alloys than it has on the fct ($\text{c}\text{a}\text{< 1}$) InPb, InCd and InTl alloys.     

The Rayleigh phonon dispersion curve on Ni(100) in the ΓM (〈100〉) direction

The dispersion curve of the Rayleigh phonon on Ni(100) has been measured by high resolution electron energy loss spectroscopy throughout the two-dimensional Brillouin zone in the $\text{ΓM}$ direction. Lattice dynamical calculations, fitted to the experimental data, confirm a stiffening of the force constant between first and second layer of the crystal of the order of 20%.     

The adsorption of benzene and naphthalene on the Rh(111) surface: A LEED,AES and TDS study

The adsorption of benzene and naphthalene on the Rh(111) single-crystal surface has been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). Both benzene and naphthalene form two different ordered surface structures separated by temperature-induced phase transitions: benzene transforms from a ($\text{3}\text{1}\text{1}\text{3}$) structure, which can also be labelled c($\text{2}\text{3}\text{× 4}$)rect, to a (3 × 3) structure in the range of 363–395 K, while naphthalene transforms from a ($\text{3}\text{3}\text{× 3}\text{3}$)R30° structure to a (3 × 3) structure in the range 398–423 K. Increasing the temperature further, these structures are found to disorder at about 393 K for benzene and about 448 K for naphthalene. Then, a first H₂ desorption peak appears at about 413 K for benzene and 578 K for naphthalene and is interpreted as due to the occurrence of molecular dissociation. All these phase transitions are irreversible. The ordered structures are interpreted as due to flat-lying or nearly flat-lying intact molecules on the rhodium surface, and they are compared with similar structures found on other metal surfaces. Structural models and phase transition mechanisms are proposed.     

Determination of the lattice vibrations of imidazole by neutron scattering

The external molecular vibrations of deuterated imidazole have been investigated at 12 K for wave vectors along the three principal directions of the Brillouin zone by neutron scattering and by Raman scattering at the zone center. A complete symmetry assignment is obtained. The dominant role of the N-H … N hydrogen bond within the intermolecular interactions is reflected in a relatively simple manner in the shape of the dispersion curves because of the chain-like arrangement of hydrogen bonds. The experiment yields directly a stretching force constant of 0.33 mdyn Å^?1 for this bond. This value is compatible with monopole-dipole and dipole-dipole interactions as major contributions to the attractive interactions of the hydrogen bond of this length (2.86 Å). Relatively strong static monopoles and dipoles are confirmed by a $\text{CNDO}\text{2}$ calculation and are consequently incorporated into the interaction model.     

Structural Lifshitz point in the quasi d = 1 magnetostrictive XY model

We obtain, within a framework in which the crystalline degrees of freedom (assumed essentially three-dimensional) are adiabatically treated and the magnetic degrees of freedom are exactly (approximately) treated in the disordered (ordered) phase (s), the peculiar phase diagram of the d = 1 first-neighbour spin-$\text{1}\text{2}$ magnetostrictive XY model in the presence of a magnetic field along the Z-axis. The structural instability wavevector continuously varies along the (2nd order) critical line. This variation presents two non-trivial points: one of them corresponds, in the phase diagram, to a Lifshitz point, where the uniform and dimerized phases converge with a (complex) modulated one; the other one presents characteristics which, to the best of our knowledge, have never been observed.     

Conductivity of disordered structures

The theory of the disordered alloy conductivity on the basis of Kadanoff-Baym equations was constructed. In the case of the alloy with the short-range impurity potential the “tails” of density of states should appear at low temperatures and conductivity becomes proportional to $\text{[φ(T)/T]}\text{exp}\text{{?λ/T}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{}}$. The theory is applicable at arbitrary impurity concentrations and at any impurity potential strength.     

Diffuse LEED intensities of disordered crystal surfaces: III. LEED investigation of the disordered (110) surface of gold

The LEED pattern of clean (101) surfaces of Au show a characteristic (1 × 2) superstructure. The diffuseness of reflections in the reciprocal [010] direction is caused by one-dimensional disorder of chains, strictly ordered into spatial [101&#x0304;] direction. There is a transition from this disordered superstructure to the normal (1 × 1) structure at 420 + 15°C. The angular profiles of the $\text{(0}\text{1}\text{2}\text{)}$ and (01) beam are measured at various temperatures and with constant energy and angles of incidence of the primary beam. The beam profiles are deconvoluted approximately with the instrument response function.     

Simple group-theoretical method to determine the symmetry of soft modes in general second order phase transitions

It is shown how one can obtain, extending Birman's method, the symmetry of the soft modes in general second order phase transitions which occurs at $\text{k}\text{≠0}$ points in Brillouin zone accompanying the primitive cell volume enlargement. This method is much simpler than the usual Landau method which utilizes the free energy expansion and its minimization. Gd₂(MoO₄)₃ is taken as an example. The case in which several arms of star-k contribute to the transition is also discussed using the example of C⁶_4h→C⁶_2h.     

On the spectrum of singlet excitations of the S = 12 linear Heisenberg antiferromagnet

We note that available numerical data for the spectrum of small antiferromagnetic Heisenberg rings with nearest neighbour interaction, and $\text{S}\text{=}\text{1}\text{2}$ facilitate an extrapolation giving some idea of the form of the lower edge of the spectrum of singlet excitations for the infinite system. The result we obtain is radically different from the one given by Ovchinnikov in two respects: the singlet excitations are roughly equally distributed over the whole Brillouin zone, and the shape is better approximated by the formula valid for the triplet spectrum. A proof that there is a continuum of singlet excitations for every wave number is given.     

ESR of copper tutton salts around 0.1 K

A remarkable H/T effect in the intensity of ESR absorption is reported for the case of two Cu Tutton salts studied at 9.4 GHz and at temperatures down to 37 mK. Results can be reasonably expressed by the Brillouin function for $\text{S =}\text{1}\text{2}$.     

Ordered steps on Bi(∼ 0001) vicinal surfaces by laser pulse anneal

LEED patterns characteristic of an ordered array of equally spaced steps of minimum height resulted from pulsing disordered Bi(～ 0001) vicinal surfaces. Ten XeCl eximer laser pulses at λ = 308 nm, of 10 ns duration and $\text{4 mJ}\text{cm}{}^2$ each and at 0.5 s intervals optimized ordering on the Bi(101&#x0304;16) surface, but additional exposure degraded the surface crystallinity as did a twentyfold increase in power density.     

Zero-point energy of some cubic metals

A screened shell phenomenological model has been used for the estimation of the zero-point energies of four alkali, three noble and six b.c.c. transition metals. The calculated values are compared with the reported values. It has been found that a good phenomenological model along with a consideration of proper number of wave vectors in the irreducible ($\text{1}\text{48}$) part of the Brillouin zone, can yield satisfactory results.     

Dispersion of empty surface states on Ni(1 1 0)

We have used k-resolved bremsstrahlung isochromat spectroscopy at $\text{h}\text{?}\text{ω=9.5eV}$ to investigate Ni(1 1 0) along the ΓKL bulk mirror plane. Empty surface states of both the crystal-induced and image-potential-induced type have been detected besides two bulk direct transitions. We studied their different behaviour against oxygen contamination and mapped their energy dispersion E(k_∥) along the ΓY direction of the surface Brillouin zone.