arXiv subjects

Jasper Knoester

Publications and source records attributed to Jasper Knoester.

27 records · Page 2Linked to original sources

arXiv2001

Optical excitation of interacting electron-hole pairs in disordered one-dimensional semiconductors

We apply the optimal fluctuation method to the calculation of the optical absorption in disordered one-dimensional semiconductors below the fundamental optical gap. We find that a photon energy exists at which the shape of the optimal fluctuation undergoes a dramatic change, resulting in a different energy dependence of the absorption rate above and below this energy. In the limit when the interaction of an electron and a hole with disorder is stronger than their interaction with each other, we obtain an analytical expression for the optical conductivity. We show that to calculate the absorption rate, it is, in general, necessary to consider a manifold of optimal fluctuations, rather than just a single fluctuation. For an arbitrary ratio of the Coulomb interaction and disorder, the optimal fluctuation is found numerically.

cond-mat.dis-nn↗

arXiv1999

Peierls transition with acoustic phonons and twist deformation in carbon nanotubes

We consider the Peierls instability due to the interaction of electrons with both acoustic and optical phonons. We suggest that such a transition takes place in carbon nanotubes with small radius. The topological excitations and the temperature dependence of the conductivity resulting from the electron-lattice interactions are considered.

cond-mat.soft↗

arXiv1999

Magnetic Susceptibility Due to Disorder-Induced Neutral Solitons in Interacting Polymer Chains

We study the magnetic response due to neutral solitons induced by disorder in polymer materials. We account for interchain interactions, which, if sufficiently strong, result in a bond-ordered phase, in which the neutral solitons are bound into pairs. We analytically calculate the corresponding pair size distribution. As the spins of the solitons have a distance dependent antiferromagnetic coupling, this allows us to calculate the magnetic susceptibility in the ordered phase. At low temperatures, the result deviates from the usual Curie behavior in a way that depends on the relative strength of the disorder and the interchain interactions. We compare our results to the observed magnetic susceptibility of trans-polyacetylene and we suggest new experiments extending towards lower temperatures.

cond-mat.soft↗

arXiv1998

Disorder-induced neutral solitons in degenerate ground state polymers

In this letter, we study the effects of weak off-diagonal disorder on 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 {\it trans}-polyacetylene at low temperatures.

cond-mat.soft↗

arXiv1998

Instanton calculation of the density of states of disordered Peierls chains

We use the optimal fluctuation method to find the density of electron states inside the pseudogap in disordered Peierls chains. The electrons are described by the one-dimensional Dirac Hamiltonian with randomly varying mass (the Fluctuating Gap Model). We establish a relation between the disorder average in this model and the quantum-mechanical average for a certain double-well problem. We show that the optimal disorder fluctuation, which has the form of a soliton-antisoliton pair, corresponds to the instanton trajectory in the double-well problem. We use the instanton method developed for the double-well problem to find the contribution to the density of states from disorder realizations close to the optimal fluctuation.

cond-mat↗

arXiv1998

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.

cond-mat.stat-mech↗

arXiv1997

Phase diagram of disordered spin-Peierls systems

We study the competition between the spin-Peierls and the antiferromagnetic ordering in disordered quasi-one-dimensional spin systems. We obtain the temperature vs disorder-strength phase diagram, which qualitatively agrees with recent experiments on doped CuGeO_3.

cond-mat↗

arXiv1997

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.

cond-mat↗

arXiv1997

Disorder-induced solitons in conjugated polymers

We show that weak off-diagonal disorder in degenerate ground state conjugated polymers results in a finite density of randomly positioned kinks (solitons and antisolitons) in the lattice dimerization. For realistic values of the disorder, these kinks should clearly show up in the optical and magnetic properties.

cond-mat↗