Ligand field splitting of d orbitals in nine-coordinated sites of capped antiprism structure: Application to monazite

The splitting of the energy levels of d¹ ions in a field of nine ligands arranged in a capped antiprism is calculated for both the regular and distorted structure. In the regular capped antiprism, the ground state is found to be the degenerate state dxy, dx²-y²; in the distorted structure, the ground state is dx²-y². The results are consistent with experimental ESR data on the V⁴⁺: CePO₄ system.     

Excess current in point contacts on two-band superconductor MgB2 in magnetic field

A series of I(V) characteristics and bias-dependent differential resistance dV/dI(V) curves of point contacts made between a single crystal of two-band superconductor MgB₂ and Cu were measured in magnetic fields up to 9 T. The magnetic field dependences of the excess current in the I(V) curves were obtained and analyzed using Koshelev and Golubov's [Phys. Rev. Lett. 90, 177002 (2003)] theoretical results for the mixed state of a dirty two-band superconductor. Introducing a simple model for the excess current in the point contact in the mixed state, our data can be qualitatively described using the theoretical magnetic field dependence of the superconducting order parameter of the σ and π-bands and the averaged electronic density of states in MgB₂.     

Quantum Kaluza-Klein cosmologies (III)

The “ground state” proposal for the quantum state of the universe is generalized to the case of a noncompact spacelike three-hyperboloid as the configuration space. The most probable evolution of the universe must come from a gravitational instanton by quantum tunneling. We show that under some minisuperspace ansatz, there exists only S₄ × S₇ gravitational instanton in d = 11 supergravity. From the point of view of quantum cosmology this fact must be related to the fact that our observed spacetime is four-dimensional.     

Discrete symmetries in Kaluza-Klein theories

We investigate discrete symmetries in theories of higher-dimensional (d > 4) gravity and their consequences for the reduced four-dimensional theory, obtained for a ground state which is a direct product of four-dimensional Minkowski space and a compact d ? 4 dimensional internal space. If the action of pure d-dimensional gravity coupled to spinors is invariant under time reversal or reflection of an odd number of spacelike co-ordinates, the reduced four-dimensional theory has a non-trivial parity or CT symmetry not consistent with observation. A non-trivial d-dimensional charge conjugation results in an unwanted doubling of the four-dimensional fermion spectrum. As a consequence, realistic theories can only be obtained for Majorana-Weyl spinors in d = 2 mod 8 dimensions. The constraints are less stringent if supplementary fields are introduced in d dimensions. For d = 11 supergravity, for example, parity and CT invariance can be broken by a non-vanishing field strength of the totally antisymmetric three-index tensor.A ground state invariant under reflections of “internal” co-ordinates often gives rise to a non-trivial charge conjugation in four dimensions. We find that the ground state of a realistic Kaluza-Klein theory should not be invariant under any non-trivial internal co-ordinate reflection (which cannot be obtained by a gauge transformation). We finally comment on a possible solution of the strong-CP problem from Kaluza-Klein theories and discuss prospectives for finding internal spaces admitting chiral fermions.     

On the symmetry of the seminal Horodecki state

It is shown that the seminal Horodecki 2-qutrit state belongs to the class of states displaying symmetry governed by a commutative subgroup of the unitary group U(3). Taking a conjugate subgroup one obtains another classes of symmetric states and one finds equivalent representations of the Horodecki state. These results are generalized to d⊗d systems.     

Tunneling spectra for single molecules of HEX-fluorescent dye attached to DNA adsorbed on Cu(1 1 1) surfaces

We used the scanning tunneling microscope (STM) to examine single-stranded deoxyribonucleic acid (DNA) oligomers deposited on a metal surface. Because STM can be used to study the electrical properties of materials via the tunneling spectra, we used it to visualize DNA oligomers at the single molecule resolution. The 5′-hexachloro-fluorescein phosphoramidite (HEX)-labeled oligomers (sequence, AGCTTC) were observed on an atomically flat Cu(1 1 1) surface. At large tip-sample distances at large set-point biases, the lowest unoccupied molecular orbit (LUMO) peak of the empty state can be observed for the dye molecules on the tunneling spectra. When this distance becomes small, similar spectra as for the Cu substrate were observed for the dye molecule on the LUMO-related peak. Cu gave peaks at small bias voltages in the filled state. From comparison of these peaks on each subunit of the molecules, the measured values of dI/dV on HEX were smaller to those on Cu because of the large size of the HEX molecule, but the normalized values of dI/dV/(I/V) were apparently equal. We believe that the tunneling current is able to pass through the HEX molecules to the Cu substrate, thus reflecting the density of the Cu(1 1 1) surface. Molecular size therefore affects the intensity of dI/dV. LUMO-related peaks sometimes cannot be observed for HEX because of conformational differences, but Cu peaks can almost always be observed for HEX molecules. These peaks for the counter ions are almost the same as those for the Cu substrate. Thus, tunneling spectra can assist in the molecular mapping of DNA.     

Quantum field theory with an interaction on the boundary

We consider quantum theory of fields ? defined on a D dimensional manifold (bulk) with an interaction V (?) concentrated on a d < D dimensional surface (brane). Such a quantum field theory can be less singular than the one in d dimensions with an interaction V (?). It is shown that scaling properties of fields on the brane are different from the ones in the bulk. We discuss as an example fields on de Sitter space.     

Spin density wave and ferromagnetism in a quasi-one-dimensional organic polymer

The spin density wave(SDW) — charge density wave(CDW) phase transition and the magnetic properties in a half-filled quasi-one-dimensional organic polymer are investigated by the world line Monte Carlo simulations. The itinerant π electrons moving along the polymer chain are coupled radically to localized unpaired d electrons, which are situated at every other site of the polymer chain. The results show that both ferromagnetic and anti-ferromagnetic radical couplings enhance the SDW phase and the ferromagnet order of the radical spins, but suppress the CDW phase. By finite size scaling, we are able to obtain the phase transition line in the parameter space. The ferromagnetic order of the radical spins are observed to coexist with the SDW phase. As compared to the system being free of the radical coupling, the phase transition line is shifted upward in the U-V parameter space in favor of larger V, where U is the on-site repulsion and V is the nearest-neighbor interaction between the π electrons. All of these findings can be understood qualitatively by a second-order perturbation theory starting from the classical state at zero temperature in the strong coupling limit. We also address the consequences of the radical coupling for the persistent current if the polymer chain is fabricated as a mesoscopic ring.     

Point-contact spectroscopy of electron-phonon interaction in semiconductors

A kinetic theory of current-voltage characteristic of semiconductor-semiconductor and metal-semiconductor point-contact junction is presented showing (a) smooth dependence of differential resistance over the voltage R = R(V), and (b) structure in dR/dV due to phonon emission at energies ?ω < eV.     

The bound state of weakly coupled Schrödinger operators in one and two dimensions

We study the unique bound state which $\text{(}\text{?d}{}^2\text{dx}{}^2\text{) + λV}$ and ?Δ + λV (in two dimensions) have when λ is small and V is suitable. Our main results give necessary and sufficient conditions for there to be a bound state when λ is small and we prove analyticity (resp. nonanalyticity) of the energy eigenvalue at λ = 0 in one (resp. two) dimensions.     

The role of hybridization on valence transitions in the spinless Falicov-Kimball model

The spinless Falicov-Kimball model with hybridization is studied in one dimension using small-cluster exact-diagonalization calculations. The resultant exact solutions are used to examine the f-state occupation n ^f as a function of f-level energy E _f, hybridization V and d-f interaction U. In addition, the phase diagram based on the finite-size scaling analysis of the single particle excitation energy is discussed both for the symmetric (E _f = 0) and unsymmetric (E _f ≠ 0) case. A number of remarkable results are found. (i) The f-electron occupation number as a function of E _f changes continuously, confirming renormalization-group results. No discontinuous transitions are observed at finite V. (ii) In the symmetric case the insulating state persists as the ground state for all nonzero V. (iii) Outside the symmetric point the hybridization stabilizes the metallic state for lower V and the insulating state for higher V. (iv) The metal-insulator transition takes place at V = V _c > 0. The values of the critical hybridization strength V _c are of order 10^?2.     

Semiempirical determination of the atom-surface interaction

We discuss three methods of determining V(z), the lateral average (G = 0 Fourier component) of the atom-surface interaction, from the bound state spectrum found in beam scattering. One method uses the Rydberg-Klein-Rees technique, which yields the width of the potential (i.e., separation of classical turning points) as a function of energy. This method incorporates also the known asymptotic form V ～ ?C₃z^?3, whereC₃ is derived from the polarizability and dielectric function of atom and solid, respectively. The second method uses a hybrid potential, constructed from a Morse potential with shifted zero of energy connected to the asymptotic form,?C₃z^?3, requiring continuity of V and $\text{d}\text{V}\text{d}\text{z}$. The third potential is a Lennard-Jones 3–9 interaction. Results are presented for H and He scattering from LiF and NaF.     

Onsite hybridization between Ce 4f and 5d states as the indicator of the transition from Kondo insulator to metallic state in CeRhSb

We discuss and determine the conditions under which a Kondo-insulator phase is stable. We show for CeRhSb that the dependence of the onsite hybridization energy Vdf between the Ce 5d and Ce 4f states on the number of valence electrons or the unit cell volume is decisive for the hybridization Kondo gap formation. We also propose the method for determining the energy Vdf from the ab initio calculations. This approach supplies an accurate Kondo-insulator-metal transition in agreement with the recent experimental data.     

Membrabe models and generalized Z2 gauge theories

We consider models of (d?n)-dimensional membranes fluctuating in a d-dimensional space under the action of surface tension. We investigate the renormalization properties of these models perturbatively and in $\text{1}\text{n}$ expansion. The potential relationships of these models to generalized Z₂ gauge theories are indicated.     

On the order of levels in charmonium

We prove a theorem concerning the energies of the 2S and 3D states in a potential $\text{V(r) =}\text{?g}{}^2\text{r}\text{+ V}{}_{\mathrm{<mtext>c</mtext>}}\text{(r)}$, where V_c is a non-singular confining potential. If $\text{(}\text{d}\text{d}\text{r)}{}^3\text{(r}{}^2\text{V}{}_{\mathrm{<mtext>c</mtext>}}\text{)}$ is positive, then the 3D state lies above the 2S state, provided

$\text{d}\text{d}\text{r}\text{1}\text{r}\text{d}\text{d}\text{r}\text{2V}{}_{\mathrm{<mtext>c</mtext>}}\text{+r}\text{d}\text{V}{}_{\mathrm{<mtext>c</mtext>}}\text{d}\text{r}\text{< 0, ?}{}_{\mathrm{r}}\text{>0.}$

For V_c = r^α, this corresponds to 0 < α < 2.     

Zero-bias anomalies in tunneling to amorphous Co-Gd thin films

Electron tunneling spectroscopy has been applied to investigation of amorphous Co-Gd thin films. Dynamic resistivities dV/dI of Al/Al₂O₃/ Co-Gd junctions have been measured at liquid helium temperatures. All dV/dI characteristics exhibit strong peaks at V = 0 (zero-bias anomalies). These peaks have been interpreted as due to the anomalies in the electronic density of states indicating the metal-insulator transition.     

高β等离子体的离子-离子束不稳定性

在β_i《1的条件下,人们曾得到电磁效应可以稳定离子-离子束不稳定性的判据V_d/V_A<(1+β_e)^1/2。本文研究了等离子体的有限β_E、β₁效应对离子-离子束不稳定性的影响,在所研究的参数范围内,随着β_e的增加,增长率上升,并且流速比V_d/V_A所允许的不稳定区域变宽,而随着β_i的增加,依赖于流速比V_d/V_A的不同,增长率的变化也有所不同,但流速比V_d/V_A所允许的不稳定区域变窄。 关键词：     

Effect of F-inclusion in nm-thick MgO tunnel barrier

We investigated the electrical characteristics of MgO tunnel barriers while we add F during oxidation process of nm-thick Mg layer. Specifically we measured dI/dV and d²I/dV² data of MgO tunnel barriers as we add F. When comparing the data of magnesium oxyfluoride tunnel barriers to those of aluminum oxyfluoride data, we have found that effect of F-inclusion in MgO is different from that in AlO_x; F-inclusion in MgO barrier makes the barrier more symmetric while F-inclusion in AlO_x barrier makes the barrier more asymmetric. However, the d²I/dV² data of MgO did not change much even after F-inclusion, suggesting a very small amount of F-doping can make significant change in oxidation process of nm-thick Mg layer. We believe this result from the fact the oxidation process of nm-thick Mg layer is reaction-limited while the oxidation process of Al layer is diffusion-limited.     

Doping and temperature dependence of the spin susceptibility in the p-d model

The spin magnetic susceptibility of the p-d model is calculated by means of a perturbation theory in the hybridization term V through a generalized cumulant expansion (GCE). The analysis is approached from the paramagnetic metallic phase. The results qualitatively reproduce some unusual magnetic properties in the normal state of the hole-doped cuprates, supporting the scenario of a Van Hove singularity near the Fermi level. Received 15 October 1998 and Received in final form 24 March 1999     

Theory of inelastic tunneling spectroscopy of a single molecule - Competition between elastic and inelastic current

A theory of inelastic electron tunneling spectroscopy of a single molecule with scanning tunneling microscope is presented using the Keldysh Green’s function method for an adsorbate-induced resonance coupled to the molecular vibration. It is found that the correction to the tunneling current is expressed in terms of the transmission probability; the correction is negative for the elastic part of the current and positive for the inelastic one. The differential conductance (dI/dV) exhibits an increase or decrease at the threshold corresponding to the opening of inelastic channel depending on the sign of the correction, and the size of this conductance jump is scaled with the vibrational damping due to electron-hole pair excitation. The lineshape of d²I/dV²-spectra calculated using a renormalized adsorbate Green’s function evolves from an antisymmetric dip to a peak through the derivative-like one as the position of the adsorbate resonance recedes from the Fermi level of the substrate.