Search

By means of first-principles calculations within the density-functional theory, we find that stable face-centered-cubic (fcc) ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids can be formed. For both solids, two possibly stable geometrical phases (identified as phase $A$ and phase $B$) with different lattice parameters can exist, where phase $A$ has a lattice parameter smaller than phase $B$. In phase $A$, ${\text{B}}_{80}$ clusters are significantly distorted and two or four intercluster covalent bonds are formed for ${\text{K}}_{3}{\text{B}}_{80}$ or ${\text{Mg}}_{3}{\text{B}}_{80}$, respectively. In phase $B$, ${\text{B}}_{80}$ units are slightly distorted and no intercluster covalent bonds exist. The phase $A$ of ${\text{Mg}}_{3}{\text{B}}_{80}$ bears the largest cohesive energy among them and is more stable than the fcc ${\text{B}}_{80}$ solid. The charge population analysis shows that K and Mg are ionized and donate electrons to the other boron atoms of ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids. The different ionic radii of K and Mg lead to major geometrical differences between ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids and the competition of the covalent and ionic bondings could explain the emergence of two different geometrical phases for both. The electronic structural calculations reveal that both fcc ${\text{K}}_{3}{\text{B}}_{80}$ and ${\text{Mg}}_{3}{\text{B}}_{80}$ solids are metals.

By means of first-principles density functional theory calculations, we find that hydrogen-passivated ultrathin silicon nanowires (SiNWs) along [100] direction with symmetrical multiple surface dangling bonds (SDBs) and boron doping can have a half-metallic ground state with 100% spin polarization, where the half-metallicity is shown quite robust against external electric fields. Under the circumstances with various SDBs, the H-passivated SiNWs can also be ferromagnetic or antiferromagnetic semiconductors. The present study not only offers a possible route to engineer half-metallic SiNWs without containing magnetic atoms but also sheds light on manipulating spin-dependent properties of nanowires through surface passivation., Comment: 4 pages, 5 figures

By means of ab initio calculations within the density functional theory, we have found that B80 fullerenes can condense to form stable face-centered-cubic fcc solids. It is shown that when forming a crystal, B80 cages are geometrically distorted, the Ih symmetry is lowered to Th, and four boron-boron chemical bonds are formed between every two nearest neighbor B80 cages. The cohesive energy of B80 fcc solid is 0.23 eV/atom with respect to the isolated B80 fullerene. The calculated electronic structure reveals that the fcc B80 solid is a metal. The predicted solid phase would constitute a form of pure boron and might have diverse implications. In addition, a simple electron counting rule is proposed, which could explain the stability of B80 fullerene and the recently predicted stable boron sheet., 5 pages, 4 figures

A set of general constructing schemes is unveiled to predict a large family of stable boron monoelemental, hollow fullerenes with magic numbers 32+8k (k>=0). The remarkable stabilities of these new boron fullerenes are then studied by intense ab initio calculations. An electron counting rule as well as an isolated hollow rule are proposed to readily show the high stability and the electronic bonding property, which are also revealed applicable to a number of newly predicted boron sheets and nanotubes., 4 pages, 5 figures

The magnetic behaviors of a spin-1/2 alternating Heisenberg antiferromagnetic chain in a staggered transverse magnetic field is studied by means of the density-matrix renormalization group method and Jordan-Wigner transformation. Quantum phase transitions of different types are observed in the S=1 Neel and XY-like gapless phases, which result from the competitions between the staggered transverse field and magnetic orders induced by anisotropy and alternating interactions. The results are compared with the mean-field and some exactly resolved results., Comment: 5 pages, 4 figures, published in Phys. Lett. A, 373,1687

A molecular solid ${\mathrm{C}}_{50}{\mathrm{Cl}}_{10}$ with possible crystalline structures, including the hexagonal-close-packed (hcp) phase, the face-centered-cubic (fcc) phase, and a hexagonal monolayer, is predicted in terms of first-principles calculation within the density functional theory. The stable structures are determined from the total-energy calculations, where the hcp phase is shown to be more stable than the fcc phase and the hexagonal monolayer in energy per molecule. The energy bands and density of states for hcp and fcc ${\mathrm{C}}_{50}{\mathrm{Cl}}_{10}$ are presented. The results show that ${\mathrm{C}}_{50}{\mathrm{Cl}}_{10}$ molecules can form either a hcp or fcc indirect-gap band insulator or an insulating hexagonal monolayer.

Based on Keldysh's nonequilibrium Green function method, the spin-dependent transport properties in a ferromagnet-quantum dot (QD)-ferromagnet coupled system are investigated. It is shown the spin current shows quite different characteristics from its electrical counterpart, and by changing the relative orientation of both magnetizations, it can change its magnitude even sign. The current-induced spin transfer torque (CISTT) is uncovered to be greatly enhanced when the bias voltage meets with the discrete levels of the QD at resonant positions. The relationship between the CISTT, the electrical current and the spin current is also addressed., Comment: 21 pages, 8 figures

First- and second-order phase transitions, Fulde-Ferrel (FF) inhomogeneous superconducting (SC) state and quantum criticality in ferromagnet/superconductor/ferromagnet double tunnel junctions are investigated. For the antiparallel alignment of magnetizations, it is shown that a first-order phase transition from the homogeneous BCS state to the inhomogeneous FF state occurs at a certain bias voltage $V^{\ast}$; while the transitions from the BCS state and the FF state to the normal state at $% V_{c}$ are of the second-order. A phase diagram for the central superconductor is presented. In addition, a quantum critical point (QCP), $% V_{QCP}$, is identified. It is uncovered that near the QCP, the SC gap, the chemical potential shift induced by the spin accumulation, and the difference of free energies between the SC and normal states vanish as $% |V-V_{QCP}|^{z\nu}$ with the quantum critical exponents $z\nu =1/2$, 1 and 2, respectively. The tunnel conductance and magnetoresistance are also discussed., Comment: 5 pages, 4 figures, Phys. Rev. B 71, 144514 (2005)

Spin-polarized transport through a marginal Fermi liquid (MFL) which is connected to two noncollinear ferromagnets via tunnel junctions is discussed in terms of the nonequilibrium Green function approach. It is found that the current-voltage characteristics deviate obviously from the ohmic behavior, and the tunnel current increases slightly with temperature, in contrast to those of the system with a Fermi liquid. The tunnel magnetoresistance (TMR) is observed to decay exponentially with increasing the bias voltage, and to decrease slowly with increasing temperature. With increasing the coupling constant of the MFL, the current is shown to increase linearly, while the TMR is found to decay slowly. The spin-valve effect is observed., Comment: 5 pages, 6 figures, Phys. Rev. B 71, 064412 (2005)

The spin-dependent transport of the electrons tunneling through a resonant tunneling structure with ferromagnetic multi-terminal under dc and ac fields is explored by means of the nonequilibrium Green function technique. A general formulation for the time-dependent current and the time-averaged current is established. As its application the systems with two and three terminals in noncollinear configurations of the magnetizations under dc and ac biases are investigated, respectively. The asymmetric factor of the relaxation times for the electrons with different spin in the central region is uncovered to bring about various behaviours of the TMR. The present three-terminal device is different from that discussed in literature, which is coined as a spin transistor with source. The current-amplification effect is found. In addition, the time-dependent spin transport for the two-terminal device is studied. It is found that the photonic sidebands provide new channels for the electrons tunneling through the barriers, and give rise to new resonances of the TMR, which is called as the photon-asisted spin-dependent tunneling. The asymmetric factor of the relaxation times is observed to lead to additional resonant peaks besides the photon-asisted resonances., Comment: 32 pages,14 figures

The responses of the electrical current and the current-induced spin transfer torque (CISTT) to an ac bias in addition to a dc bias in a magnetic tunnel junction are investigated by means of the time-dependent nonquilibrium Green function technique. The time-averaged current (time-averaged CISTT) is formulated in the form of a summation of dc current (dc CISTT) multiplied by products of Bessel functions with the energy levels shifted by $m\hbar \omega _{0}$. The tunneling current can be viewed as to happen between the photonic sidebands of the two ferromagnets. The electrons can pass through the barrier easily under high frequencies but difficultly under low frequencies. The tunnel magnetoresistance almost does not vary with an ac field. It is found that the spin transfer torque, still being proportional to the electrical current under an ac bias, can be changed by varying frequency. Low frequencies could yield a rapid decrease of the spin transfer torque, while a large ac signal leads to both decrease of the electrical current and the spin torque. If only an ac bias is present, the spin transfer torque is sharply enhanced at the particular amplitude and frequency of the ac bias. A nearly linear relation between such an amplitude and frequency is observed., Comment: 13 pages,8 figures

Spin-dependent transport in ferromagnet-unconventional superconductor-ferromagnet double tunnel junctions is investigated. It is shown that the spin accumulation can induce the phase inhomogeneity in the central superconductor at lower temperatures when the magnetizations of both ferromagnet electrodes are antiparallel. A scaling behavior for the critical voltage where superconductivity is suppressed is uncovered for four different gap symmetries (i.e., s, ${d}_{{x}^{2}\ensuremath{-}{y}^{2}},$ ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i\ensuremath{\varepsilon}s,$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i\ensuremath{\varepsilon}{d}_{\mathrm{xy}}$ waves, with $\ensuremath{\varepsilon}$ the mixing parameter among different orbitals). It is found that the gap symmetry of the central superconductor does not alter qualitatively the spintronic transport property of the tunnel junction although there are quantitative changes.

The structural properties and phase stability of the four common polytypes of boron nitride, cubic zinc blende (c-BN), hexagonal (h-BN), wurtzite (w-BN) and rhombohedral (r-BN), are studied by ab initio calculations. Electronic energies are calculated using an ultra soft pseudopotential method under the density-functional theory, and phonon dispersions are calculated using the first-principles force-constant method. The $p\ensuremath{-}T$ phase diagrams of these four boron nitride phases are constructed with the quasiharmonic approximation. Direct compression simulations are then performed to find probable phase transformation paths among these polytypes, with additional energy calculations of plausible transition structures. The c-BN phase is the most thermodynamically stable in ambient conditions among these four polytypes, and the transformation between r-BN and c-BN has the smallest energy barrier. Direct transformation between h-BN and c-BN is far less favorable than indirect transformation, with w-BN or r-BN as an intermediate. The presence of structural defects is a key attribute in reducing the energy barrier of phase transformation. The results in this work offer theoretical clues to experimental data on c-BN film growth, particularly the absence of w-BN.

The spin-dependent low-frequency and weakly nonlinear electrical transport of spin-polarized tunneling ferromagnet/insulator (semiconductor) single and double junctions are investigated on the basis of B\"uttiker's gauge invariant, charge conservation nonlinear transport theory. We have applied the tight-binding approximation method to calculate the Green function. The energy and the angle (between the molecular field and the vertical axis) dependences of the weakly nonlinear dc transport coefficient and the linear low frequency ac transport coefficient indicate interesting characteristics of quantum tunneling. In the calculation the charge conservation and the gauge invariance are always kept.

We have systematically studied both classical and quantum chaotic behaviors of two colliding harmonic oscillators. The classical case falls in Kolmogorov-Arnold-Moser class. It is shown that there exists an energy threshold, above which the system becomes nonintegrable. For some values of the initial energy near the threshold, we have found that the ratio of frequencies of the two oscillators affects the Poincar\'e sections significantly. The largest Lyapunov character exponent depends linearly on the ratio of frequencies of the two oscillators away from the energy threshold in some chaotic regions, which shows that the chaotic behaviors of the system are mainly determined by the ratio. In the quantum case, for certain parameters, the distribution of the energy level spacings also varies with the ratio of frequencies of the two oscillators. The relation between the energy spectra and the ratio of frequencies of the two oscillators, the interaction constant, and the semi-classical quantization constant, is also investigated respectively., Comment: Revtex, 15 pages, uuencoded figures attached

We study a versatile structurally favorable periodic $sp^2$-bonded carbon atomic planar sheet with $C_{4v}$ symmetry by means of the first-principles calculations. This carbon allotrope is composed of carbon octagons and squares with two bond lengths and is thus dubbed as octagraphene. It is a semimetal with the Fermi surface consisting of one hole and one electron pocket, whose low-energy physics can be well described by a tight-binding model of $\pi$-electrons. Its Young's modulus, breaking strength and Poisson's ratio are obtained to be 306 $N/m$, 34.4 $N/m$ and 0.13, respectively, which are close to those of graphene. The novel sawtooth and armchair carbon nanotubes as well as unconventional fullerenes can also be constructed from octagraphene. It is found that the Ti-absorbed octagraphene can be allowed for hydrogen storage with capacity around 7.76 wt%.

A program package is described which can compute ab-initio without the input of any complicated results the characters, subgroup-symmetry adapted projective (both single- and double-valued) irreducible representations of the little co-group and the full representation matrices of the space group, the wave-vector selection rules, the Clebsch-Gordan (CG) series and CG coefficients in 230 space groups, as well as the characters, irreducible representations and CG coefficients of the 32 point groups. The tables for characters, representation matrices and CG coefficients are printed out in an easily recognizable form with exact values in the form of (p/q) 1 2 exp(iπn/m).