463 research outputs found
Numerical adiabatic potentials of orthorhombic Jahn-Teller effects retrieved from ultrasound attenuation experiments. Application to the SrF2:Cr crystal
A methodology is worked out to retrieve the numerical values of all the main
parameters of the six-dimensional adiabatic potential energy surface (APES) of
a polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) from
ultrasound experiments. The method is based on a verified assumption that
ultrasound attenuation and speed encounter anomalies when the direction of
propa- gation and polarization of its wave of strain coincides with the
characteristic directions of symmetry breaking in the JTE. For the SrF2:Cr
crystal, employed as a basic example, we observed anomaly peaks in the
temperature dependence of attenuation of ultrasound at frequencies of 50-160
MHz in the temperature interval of 40-60 K for the wave propagating along the
[110] direction, for both the longitudinal and shear modes, the latter with two
polarizations along the [001] and [110] axes, respectively. We show that these
anomalies are due to the ultrasound relaxation by the system of non-interacting
Cr2+ JT centers with orthorhombic local distortions. The interpretation of the
ex- perimental findings is based on the T2g (eg +t2g) JTE problem including the
linear and quadratic terms of vibronic interactions in the Hamiltonian and the
same-symmetry modes reduced to one interaction mode. Combining the experimental
results with a theoretical analysis we show that on the complicated
six-dimensional APES of this system with three tetragonal, four trigonal, and
six orthorhombic extrema points, the latter are global minima, while the former
are saddle points, and we estimate numerically all the main parameters of this
surface, including the linear and quadratic vibronic coupling constants, the
primary force constants, the coordinates of all the extrema points and their
energies, the energy barrier between the orthorhombic minima, and the tunneling
splitting of the ground vibrational states.Comment: 8 pages, 3 figure
Pseudo Jahn-Teller Effect In The Origin Of Enhanced Flexoelectricity
The controversy between the theory and experiment in explaining the origin of enhanced flexoelectricity is removed by taking into account the pseudo Jahn-Teller effect (PJTE) which, under certain conditions, creates local dipolar distortions of dynamic nature, resonating between two or more equivalent orientations. The latter become nonequivalent under a strain gradient thus producing enhanced flexoelectricity: it is much easier to orient ready-made dipoles than to polarize an ionic solid. For BaTiO3, the obtained earlier numerical data for the adiabatic potential energy surface in the space of dipolar displacements in the Ti centers were used to estimate the flexoelectric coefficient integral in the paraelectric phase in a one-dimensional model with the strain gradient along the [111] direction: integral = -0.43 X 10(-6) Cm-1. This eliminates the huge contradiction between the experimental data of integral similar to mu Cm-1 for this case and the theoretical predictions (without the PJTE) of 3-4 orders-of-magnitude smaller values. Enhanced flexoelectricity is thus expected in solids with a sufficient density of centers that have PJTE induced dipolar instabilities. It explains also the origin of enhanced flexoelectricity observed in other solids, noticeable containing Nb perovskite centers which are known to have a PJTE instability, similar to that of Ti centers. The SrTiO3 crystal as a virtual ferroelectric in which the strain gradient eases the condition of PJTE polar instability is also discussed. (C) 2015 AIP Publishing LLC.Institute for Theoretical Chemistr
Quantum transport through single-molecule junctions with orbital degeneracies
We consider electronic transport through a single-molecule junction where the
molecule has a degenerate spectrum. Unlike previous transport models, and
theories a rate-equations description is no longer possible, and the quantum
coherences between degenerate states have to be taken into account. We present
the derivation and application of a master equation that describes the system
in the weak-coupling limit and give an in-depth discussion of the parameter
regimes and the new phenomena due to coherent on-site dynamics
Vibrational detection and control of spin in mixed-valence molecular transistors
We investigate electron transport through a mixed-valence molecular complex
in which an excess electron can tunnel between hetero-valent transition metal
ions, each having a fixed localized spin. We show that in this class of
molecules the interplay of the spins and the vibrational breathing modes of the
ionic ligand-shells allows the total molecular spin to be detected as well as
controlled by nonequilibrium transport. Due to a spin-dependent pseudo
Jahn-Teller effect electronic transitions with different spin values can be
distinguished by their vibronic conductance side peaks, without using an
external magnetic field. Conversely, we show that the spin state of the entire
molecule can also be controlled via the nonequilibrium quantized molecular
vibrations due to a vibration-induced spin-blockade.Comment: 6 pages, 3 figure
Jahn-Teller instability in C6H6+ and C6H6- revisited
The benzene cation (C6H6+) has a doublet (e_{1g}) ground state in hexagonal
ring (D_{6h}) geometry. Therefore a Jahn-Teller (JT) distortion will lower the
energy. The present theoretical study yields a model Huckel-type Hamiltonian
that includes the JT coupling of the e_{1g} electronic ground state with the
two e_{2g} vibrational modes: in-plane ring-bending and C-C bond-stretching. We
obtain the JT couplings from density functional theory (DFT), which gives a JT
energy lowering of 970 cm^{-1} in agreement with previous quantum chemistry
calculations. We find a non-adiabatic solution for vibrational spectra and
predict frequencies shifts of both the benzene cation and anion, and give a
reinterpretation of the available experimental data.Comment: 6 pages, 3 figure
Dynamics of a two-level system coupled with a quantum oscillator in the very strong coupling limit
The time-dependent behavior of a two-level system interacting with a quantum
oscillator system is analyzed in the case of a coupling larger than both the
energy separation between the two levels and the energy of quantum oscillator
(, where is the frequency of the
transition between the two levels, is the frequency of the
oscillator, and is the coupling between the two-level system and the
oscillator). Our calculations show that the amplitude of the expectation value
of the oscillator coordinate decreases as the two-level system undergoes the
transition from one level to the other, while the transfer probability between
the levels is staircase-like. This behavior is explained by the interplay
between the adiabatic and the non-adiabatic regimes encountered during the
dynamics with the system acting as a quantum counterpart of the Landau-Zener
model. The transition between the two levels occurs as long as the expectation
value of the oscillator coordinate is driven close to zero. On the contrary, if
the initial conditions are set such that the expectation values of the
oscillator coordinate are far from zero, the system will remain locked on one
level.Comment: 4 pages, 4 figures, to be published in Physical Review
Nuclear Tunnelling and Dynamical Jahn-Teller Effect in Graphene with Vacancy
We show that the substitutional vacancy in graphene forms a dynamical
Jahn-Teller center. The adiabatic potential surface resulting from the
electron-lattice coupling was computed using density-functional methods and
subsequently the Schr\"odinger equation was solved for the nuclear motion. Our
calculations show a large tunnelling splitting of about 86
cm. %, which is large as compared to the typical strain splitting. The
effect results in a large delocalization of the carbon nuclear wave functions
around the vacancy leading to a significant broadening of the Jahn-Teller
active electron states. The tunnelling splitting should be
observable in electron paramagnetic resonance and two-photon resonance
scattering experiments.Comment: 5 pages, 4 figure
High Harmonic Generation in SF: Raman-excited Vibrational Quantum Beats
In a recent experiment (N. Wagner et al., PNAS v103, p13279) on SF, a
high-harmonic generating laser pulse is preceded by a pump pulse which
stimulates Raman-active modes in the molecule. Varying the time delay between
the two pulses modulates high harmonic intensity, with frequencies equal to the
vibration frequencies of the Raman-active modes. We propose an explanation of
this modulation as a quantum interference between competing pathways that occur
via adjacent vibrational states of the molecule. The Raman and high harmonic
processes act as beamsplitters, producing vibrational quantum beats among the
Raman-active vibrational modes that are excited by the first pulse. We
introduce a rigorous treatment of the electron-ion recombination process and
the effect of the ionic Coulomb field in the electron propagation outside the
molecule, improving over the widely-used three-step model.Comment: submitted to PR
Jahn-Teller Solitons, Structural Phase Transitions and Phase Separation
It is demonstrated that under common conditions a molecular solid subject to
Jahn-Teller interactions supports stable Q-ball-like non-topological solitons.
Such solitons represent a localized lump of excess electric charge in periodic
motion accompanied by a time-dependent shape distortion of a set of adjacent
molecules. The motion of the distortion can correspond to a true rotation or to
a pseudo-rotation about the symmetric shape configuration. These solitons are
stable for Jahn-Teller coupling strengths below a critical value; however, as
the Jahn-Teller coupling approaches this critical value, the size of the
soliton diverges signaling an incipient structural phase transition. The
soliton phase mimics features commonly attributed to phase separation in
complex solids.Comment: 8 pages, 3 figure
Jahn-Teller Spectral Fingerprint in Molecular Photoemission: C60
The h_u hole spectral intensity for C60 -> C60+ molecular photoemission is
calculated at finite temperature by a parameter-free Lanczos diagonalization of
the electron-vibration Hamiltonian, including the full 8 H_g, 6 G_g, and 2 A_g
mode couplings. The computed spectrum at 800 K is in striking agreement with
gas-phase data. The energy separation of the first main shoulder from the main
photoemission peak, 230 meV in C60, is shown to measure directly and rather
generally the strength of the final-state Jahn-Teller coupling.Comment: 5 pages, 3 figure
- …