6,305 research outputs found

### Near degeneracy and pseudo Jahn-Teller effects in mixed-valence ladders: the phase transition in NaV$_2$O$_5$

We analyze the electronic structure of a mixed-valence ladder system. We find
that structural anisotropy and complex electron correlations lead to on-rung
charge localization and insulating character. Charge fluctuations within the
rung of the ladder interact strongly to the lattice degrees of freedom, which
gives rise to large pseudo Jahn--Teller effects. The phase transition in
NaV$_2$O$_5$ should be driven by this kind of mechanism.Comment: 5 pages, 2 figures, 1 table, submitted to PR

### The Hessian biased singular value decomposition method for optimization and analysis of force fields

We present methodology (HBFF/SVD) for optimizing the form and parameters of force fields (FF) for molecular dynamics simulations through utilizing information about properties such as the geometry, Hessian, polarizability, stress (crystals), and elastic constants (crystals). This method is based on singular value decomposition (SVD) of the Jacobian describing the partial derivatives in various properties with respect to FF parameters. HBFF/SVD is effective for optimizing the parameters for accurate FFs of organic, inorganic, and transition metal compounds. In addition it provides information on the validity of the functional form of the FF for describing the properties of interest. This method is illustrated by application to organic molecules (CH2O, C2H4, C4H6, C6H8, C6H6, and naphthalene) and inorganic molecules (Cl2CrO2 and Cl2MoO2)

### The Emergence of El-Ni\~{n}o as an Autonomous Component in the Climate Network

We construct and analyze a climate network which represents the
interdependent structure of the climate in different geographical zones and
find that the network responds in a unique way to El-Ni\~{n}o events. Analyzing
the dynamics of the climate network shows that when El-Ni\~{n}o events begin,
the El-Ni\~{n}o basin partially loses its influence on its surroundings. After
typically three months, this influence is restored while the basin loses almost
all dependence on its surroundings and becomes \textit{autonomous}. The
formation of an autonomous basin is the missing link to understand the
seemingly contradicting phenomena of the afore--noticed weakening of the
interdependencies in the climate network during El-Ni\~{n}o and the known
impact of the anomalies inside the El-Ni\~{n}o basin on the global climate
system.Comment: 5 pages,10 figure

### Diurnal experiment data report, 19-20 March 1974

Temperature and wind data are presented from 70 small meteorological sounding rockets launched from eight selected launch sites in the Western Hemisphere. Table 1 gives a complete listing of the launch sites involved and the altitude of temperature and wind observations successfully completed

### Half-Metallic Ferromagnetism and the spin polarization in CrO$_2$

We present electronic structure calculations in combination with local and
non-local many-body correlation effects for the half-metallic ferromagnet
CrO$_2$. Finite-temperature Dynamical Mean Field Theory results show the
existence of non-quasiparticle states, which were recently observed as almost
currentless minority spin states near the Fermi energy in resonant scattering
experients. At zero temperatures, Variational Cluster Approach calculations
support the half-metallic nature of CrO$_2$ as seen in superconducting point
contact spectroscopy. The combination of these two techniques allowed us to
qualitatively describe the spin-polarization in CrO$_2$.Comment: 5 pages, 3 figure

### Electron correlations and bond-length fluctuations in copper oxides: from Zhang--Rice singlets to correlation bags

We perform first principles, multiconfiguration calculations on clusters
including several CuO$_6$ octahedra and study the ground-state electron
distribution and electron--lattice couplings when holes are added to the
undoped $d^9 p^6$ configuration. We find that the so-called Zhang--Rice state
on a single CuO$_4$ plaquette is nearly degenerate with a state whose leading
configuration is of the form Cu $d^9$-- O $p^5$-- Cu $d^9$. A strong coupling
between the electronic and nuclear motion gives rise to large inter-site charge
transfer effects for half-breathing displacements of the oxygen ions. Under the
assumption of charge segregation into alternating hole-free and hole-rich
stripes of Goodenough \cite{jbg_02,jbg_03}, our results seem to support the
vibronic mechanism and the traveling charge-density wave model from
Refs.\cite{jbg_02,jbg_03} for the superconductivity in copper oxides.Comment: submitted to Phys. Rev.

### Roles of Bond Alternation in Magnetic Phase Diagram of RMnO3

In order to investigate nature of the antiferromagnetic structures in
perovskite RMnO3, we study a Heisenberg J1-J2 model with bond alternation using
analytical and numerical approaches. The magnetic phase diagram which includes
incommensurate spiral states and commensurate collinear states is reproduced.
We discuss that the magnetic structure with up-up-down-down spin configuration
(E-type structure) and the ferroelectricity emerge cooperatively to stabilize
this phase. Magnetoelastic couplings are crucial to understand the magnetic and
electric phase diagram of RMnO3.Comment: 5 pages, 6 figure

### Quantum computation with devices whose contents are never read

In classical computation, a "write-only memory" (WOM) is little more than an
oxymoron, and the addition of WOM to a (deterministic or probabilistic)
classical computer brings no advantage. We prove that quantum computers that
are augmented with WOM can solve problems that neither a classical computer
with WOM nor a quantum computer without WOM can solve, when all other resource
bounds are equal. We focus on realtime quantum finite automata, and examine the
increase in their power effected by the addition of WOMs with different access
modes and capacities. Some problems that are unsolvable by two-way
probabilistic Turing machines using sublogarithmic amounts of read/write memory
are shown to be solvable by these enhanced automata.Comment: 32 pages, a preliminary version of this work was presented in the 9th
International Conference on Unconventional Computation (UC2010

### Frustrated quantum-spin system on a triangle coupled with $e_g$ lattice vibrations - Correspondence to Longuet-Higgins et al.'s Jahn-Teller model -

We investigate the quantum three spin model $({\bf S_1},{\bf S_2},{\bf S_3})$
of spin$=1/2$ on a triangle, in which spins are coupled with
lattice-vibrational modes through the exchange interaction depending on
distances between spin sites. The present model corresponds to the dynamic
Jahn-Teller system $E_g\otimes e_g$ proposed by Longuet-Higgins {\it et al.},
Proc.R.Soc.A.{\bf 244},1(1958). This correspondence is revealed by using the
transformation to Nakamura-Bishop's bases proposed in Phys.Rev.Lett.{\bf
54},861(1985). Furthermore, we elucidate the relationship between the behavior
of a chiral order parameter ${\hat \chi}={\bf S_1\cdot(S_2\times S_3)}$ and
that of the electronic orbital angular momentum ${\hat \ell_z}$ in $E_g\otimes
e_g$ vibronic model: The regular oscillatory behavior of the expectation value
$for vibronic structures with increasing energy can also be
found in that of$. The increase of the additional
anharmonicity(chaoticity) is found to yield a rapidly decaying irregular
oscillation of $$

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