Instanton in disordered Peierls systems

We study disordered Peierls systems described by the Fluctuating Gap Model. We show that the typical electron states with energies lying deep inside the pseudogap are localized near large disorder fluctuations (instantons), which have the form of a soliton-antisoliton pair. Using the ``saddle-point'' method we obtain the average density of states and the average optical absorption coefficient at small energy.Comment: 10 pages, revtex, 2 Postscript figures, to appear in Phys. Lett.

Critical temperature and density of spin-flips in the anisotropic random field Ising model

We present analytical results for the strongly anisotropic random field Ising model, consisting of weakly interacting spin chains. We combine the mean-field treatment of interchain interactions with an analytical calculation of the average chain free energy (``chain mean-field'' approach). The free energy is found using a mapping on a Brownian motion model. We calculate the order parameter and give expressions for the critical random magnetic field strength below which the ground state exhibits long range order and for the critical temperature as a function of the random magnetic field strength. In the limit of vanishing interchain interactions, we obtain corrections to the zero-temperature estimate by Imry and Ma [Phys. Rev. Lett. 35, 1399 (1975)] of the ground state density of domain walls (spin-flips) in the one-dimensional random field Ising model. One of the problems to which our model has direct relevance is the lattice dimerization in disordered quasi-one-dimensional Peierls materials, such as the conjugated polymer trans-polyacetylene.Comment: 28 pages, revtex, 4 postscript figures, to appear in Phys. Rev.

Unusual magnetoelectric effect in paramagnetic rare-earth langasite

Violation of time reversal and spatial inversion symmetries has profound consequences for elementary particles and cosmology. Spontaneous breaking of these symmetries at phase transitions gives rise to unconventional physical phenomena in condensed matter systems, such as ferroelectricity induced by magnetic spirals, electromagnons, non-reciprocal propagation of light and spin waves, and the linear magnetoelectric (ME) effect - the electric polarization proportional to the applied magnetic field and the magnetization induced by the electric field. Here, we report the experimental study of the holmium-doped langasite, Ho$_{x}$La$_{3-x}$Ga$_5$SiO$_{14}$, showing a puzzling combination of linear and highly non-linear ME responses in the disordered paramagnetic state: its electric polarization grows linearly with the magnetic field but oscillates many times upon rotation of the magnetic field vector. We propose a simple phenomenological Hamiltonian describing this unusual behavior and derive it microscopically using the coupling of magnetic multipoles of the rare-earth ions to the electric field.Comment: 8 pages, 3 figure

Dielectric anomaly in NaV2O5: evidence for charge ordering

Abstract We found a high-frequency dielectric and magnetic anomaly in NaV O at the phase transition into the spin-gap state. The dielectric constant anomaly is of the antiferroelectric type, which is in agreement with the models assuming the zigzag charge ordering in the ab-plane

Subgap optical absorption in conjugated polymers

We study the quantum lattice motion in conjugated polymers using the Su-Schrieffer-Heeger (SSH) model. We find a strong reduction of the number of relevant lattice degrees of freedom for short chains (up to N = 70 for polyacetylene), which allows us to calculate fully quantum mechanically the linear optical absorption spectrum.

Statistics of Optical Spectra from Single-Ring Aggregates and Its Application to LH2

We study the statistics of the optical spectra of individual ring-shaped molecular aggregates in which the site energies and transfer interactions are perturbed by both weak random disorder and a regular modulation due to a deformation of the ring. Under these conditions, the spectrum is dominated by two lines. We present an analytical expression for the joined probability distribution of the splitting between these lines and their average position. We compare our results to recent experiments performed on the bacterial antenna system LH2. Our analysis indicates the importance of intercomplex disorder.

Quantum lattice motion and optical absorption in conjugated polymers: Nonadiabatic theory

We calculate the low-energy part of the optical-absorption spectrum of short (N=30) conjugated polymer chains described by the Su-Schrieffer-Heeger model. The calculation is based on a detailed study of the quantum lattice dynamics and accounts for the nonadiabatic mixing of different electron configurations caused by the lattice motion. We consider optical transitions to about 600 mixed electron-lattice excited states. The obtained subgap smearing of the rigid-band absorption edge agrees well with the experiment and with what we found earlier in the adiabatic approximation, while at energies close to the rigid-band transition, the nonadiabatic mixing washes out the spectral structure observed in the adiabatic theory. Finally, we show that for energies well above the rigid-band transition, the absorption spectra of both theories agree, even though the wave functions that they predict are very different.

NaV2O5: An Exotic Exciton System

We show that the phase transition which sodium vanadate undergoes at Tc = 34 K is driven by a charge ordering. The relevant effective Hamiltonian is of the Frenkel exciton type, with a very large bandwidth to molecular energy ratio. This causes strong non-Heitler-London effects and a temperature dependent gap that vanishes at Tc. In addition to the phase transition, the model qualitatively explains the observed absorption spectrum and the anomaly in the static dielectric constant. Within our model, the observed spin-gap opening at Tc results from exciton-spinon coupling.

Disorder-induced neutral solitons in degenerate ground state polymers

We study the effects of weak off-diagonal disorder on π-conjugated polymers with a doubly degenerate ground-state. We find that disorder induces a finite density of neutral solitons in the lattice dimerization of a polymer chain. Interchain interactions result in a linear potential between the solitons and, if sufficiently strong, bind them into pairs resulting in an exponential suppression of the soliton density. As neutral solitons carry spin 1/2, they contribute to the polymer’s magnetic properties. We calculate the magnetic susceptibility and suggest measurements of the magnetic susceptibility in trans-polyacetylene at low temperatures.