arXiv subjects

D. K. Campbell

Publications and source records attributed to D. K. Campbell.

52 records · Page 3Linked to original sources

arXiv2000

Re-integerization of fractional charges in the correlated quarter-filled band

Previous work has demonstrated the existence of soliton defect states with charges +/- e/2 in the limits of zero and of infinite on-site Coulomb interactions in the one-dimensional (1D) quarter-filled band. For large but finite on-site Coulomb interaction, the low temperature 2k_F bond distortion that occurs within the 4k_F bond-distorted phase is accompanied by charge-ordering on the sites. We show that a ``re-integerization'' of the defect charge occurs in this bond-charge density wave (BCDW) state due to a ``binding'' of the fractional charges. We indicate briefly possible implications of this result for mechanisms of organic superconductivity.

cond-mat.str-el↗

arXiv1999

Novel coexisting density wave ground state in strongly correlated, two-dimensional electronic materials

Two-dimensional (2D) strongly correlated electron systems underlie many of the most important phenomena in contemporary condensed matter physics, including the Quantum Hall Effect (QHE), ``high T_c'' superconductivity, and possible exotic conducting states in silicon MOSFETs. We demonstrate the existence of yet another exotic ground state in strongly correlated, 2D electronic materials: a novel, insulating bond-order/charge density wave state (BCDW) in the commensurate 1/4-filled band that persists for all anisotropies within the 2D lattice, in contradiction to the non-interacting electron prediction of the vanishing of density waves in 2D for non-1/2-filled bands. The persistence of the BCDW in the 2D lattice is a consequence of strong electron-electron (e-e) interaction and the resultant ``confinement,'' a concept recently widely debated. Our results have implications for experiments in the organic charge transfer solids (CTS), where they explain the observation of a ``mysterious'' coexistence of density waves, clarify the optical conductivity of the ``metallic'', and suggest an approach to the observed organic superconductivity.

cond-mat.str-el↗

arXiv1998

Curvature of Levels and Charge Stiffness of One-Dimensional Spinless Fermions

Combining the Bethe Ansatz with a functional deviation expansion and using an asymptotic expansion of the Bethe Ansatz equations, we compute the curvature of levels D_n at any filling for the one-dimensional lattice spinless fermion model. We use these results to study the finite temperature charge stiffness D(T). We find that the curvature of the levels obeys, in general, the relation D_n=D_0+δD_n, where D_0 is the zero-temperature charge stiffness and δD_n arises from excitations. Away from half filling and for the low-energy (gapless) eigenstates, we find that the energy levels are, in general, flux dependent and, therefore, the system behaves as an ideal conductor, with D(T) finite. We show that if gapped excitations are included the low-energy excitations feel an effective flux Φ^{eff} which is different from what is usually expected. At half filling, we prove, in the strong interacting limit and to order 1/V (V is the nearest-neighbor Coulomb interaction), that the energy levels are flux independent. This leads to a zero value for the curvature of levels D_n and, as consequence, to D(T)=0, proving an earlier conjecture of Zotos and Prelovšek.

cond-mat.str-el↗

arXiv1998

Theory of Coexisting Charge and Spin-Density Waves in (TMTTF)_2Br, (TMTSF)_2PF_6 and alpha-(BEDT-TTF)_2MHg(SCN)_4

Recent experiments indicate that the spin-density waves (SDWs) in (TMTTF)_2Br, (TMTSF)_2PF_6 and alpha-(BEDT-TTF)_2MHg(SCN)_4 are highly unconventional and coexist with charge-density waves (CDWs). We present a microscopic theory of this unusual CDW-SDW coexistence. A complete understanding requires the explicit inclusion of strong Coulomb interactions, lattice discreteness, the anisotropic two-dimensional nature of the lattice, and the correct bandfilling within the starting Hamiltonian.

cond-mat.str-el↗

arXiv1998

On Properties of Boundaries and Electron Conductivity in Mesoscopic Polycrystalline Silicon Films for Memory Devices

We present the results of molecular dynamics modeling on the structural properties of grain boundaries (GB) in thin polycrystalline films. The transition from crystalline boundaries with low mismatch angle to amorphous boundaries is investigated. It is shown that the structures of the GBs satisfy a thermodynamical criterion. The potential energy of silicon atoms is closely related with a geometrical quantity -- tetragonality of their coordination with their nearest neighbors. A crossover of the length of localization is observed. To analyze the crossover of the length of localization of the single-electron states and properties of conductance of the thin polycrystalline film at low temperature, we use a two-dimensional Anderson localization model, with the random one-site electron charging energy for a single grain (dot), random non-diagonal matrix elements, and random number of connections between the neighboring grains. The results on the crossover behavior of localization length of the single-electron states and characteristic properties of conductance are presented in the region of parameters where the transition from an insulator to a conductor regimes takes place.

cond-mat.dis-nn↗

arXiv1997

Quantum Monte Carlo in the Interaction Representation --- Application to a Spin-Peierls Model

A quantum Monte Carlo algorithm is constructed starting from the standard perturbation expansion in the interaction representation. The resulting configuration space is strongly related to that of the Stochastic Series Expansion (SSE) method, which is based on a direct power series expansion of exp(-beta*H). Sampling procedures previously developed for the SSE method can therefore be used also in the interaction representation formulation. The new method is first tested on the S=1/2 Heisenberg chain. Then, as an application to a model of great current interest, a Heisenberg chain including phonon degrees of freedom is studied. Einstein phonons are coupled to the spins via a linear modulation of the nearest-neighbor exchange. The simulation algorithm is implemented in the phonon occupation number basis, without Hilbert space truncations, and is exact. Results are presented for the magnetic properties of the system in a wide temperature regime, including the T-->0 limit where the chain undergoes a spin-Peierls transition. Some aspects of the phonon dynamics are also discussed. The results suggest that the effects of dynamic phonons in spin-Peierls compounds such as GeCuO3 and NaV2O5 must be included in order to obtain a correct quantitative description of their magnetic properties, both above and below the dimerization temperature.

cond-mat.str-el↗

arXiv1997

Complete Pseudoparticle Representation and the Conductivity Spectrum of the Hubbard Chain

We use a complete pseudoparticle operator representation to study the explicit form of the finite-frequency conductivity for the Hubbard chain. Our study reveals that the spectral weight is mostly concentrated at the ω=0 Drude peak (except at density n=1) and at an absorption starting just below twice the value of the chemical potential whose ω-dependence we evaluate exactly. We also obtain the exact ωdependence of the higher-energy, less-pronounced absorption edges, whose weights vanish for large on-site interaction U.

cond-mat↗

arXiv1996

Pseudoparticle Description of the 1D Hubbard Model Electronic Transport Properties

We extend the pseudoparticle transport description of the Hubbard chain to all energy scales. In particular we compute the mean value of the electric current transported by any Bethe-ansatz state and the transport masses of the charge carriers. We present numerical results for the optical conductivity of the model at half-filling for values of U/t=3 and 4. We show that these are in good agreement with the pseudoparticle description of the finite-energy transitions involving new pseudoparticle energy bands.

cond-mat.str-el↗

arXiv1996

A.C.-Transport Through a Two-Dimensional Quantum Point Contact

We calculate the admittance of a two-dimensional quantum point contact (QPC) using a Boltzman-like kinetic equation derived for a partial Wigner distribution function in an effective potential. We show that this approach leads to the known stepwise behavior of the admittance as a function of the gate voltage. The emittance contains both a quantum inductance determined by the harmonic mean of the velocities for the propagating electron modes and a quantum capacitance determined by the reflected modes.

cond-mat↗

arXiv1995

On a class of integrable interacting electronic systems with off-diagonal disorder

We report a class of {\it integrable} one-dimensional interacting electronic sy$ with {\it off-diagonal disorder}. For these systems, the disorder can be ``gauged away,''and the spectrum can be mapped completely onto the spectrum of the ordered problem, which can be solved by Bethe ansatz or by bosonization. We study the magnetic properties of the persistent currents in mesoscopic rings in the case in which the ordered system is a Luttinger liquid. The system can be paramagnetic or diamagnetic, depending on the amount of the disorder and the number of fermions in the system.

cond-mat↗

arXiv1994

Off-diagonal Interactions, Hund's Rules and Pair-binding in Hubbard Molecules

We have studied the effect of including nearest-neighbor, electron-electron interactions, in particular the off-diagonal (non density-density) terms, on the spectra of truncated tetrahedral and icosahedral ``Hubbard molecules,'' focusing on the relevance of these systems to the physics of doped C$_{60}$. Our perturbation theoretic and exact diagonalization results agree with previous work in that the density-density term suppresses pair-binding. However, we find that for the parameter values of interest for $C_{60}$ the off-diagonal terms {\em enhance} pair-binding, though not enough to offset the suppression due to the density-density term. We also find that the critical interaction strengths for the Hund's rules violating level crossings in C$_{60}^{-2}$, C$_{60}^{-3}$ and C$_{60}^{-4}$ are quite insensitive to the inclusion of these additional interactions.

cond-mat↗

arXiv1994

Conservation laws and bosonization in integrable Luttinger liquids

We examine and explain the Luttinger-liquid character of models solvable by the Bethe ansatz by introducing a suitable bosonic operator algebra. In the case of the Hubbard chain, this involves two bosonic algebras which apply to {\it all} values of $U$, electronic density, and magnetization. Only at zero magnetization does this lead to the usual charge - spin separation. We show that our ``pseudoparticle'' operator approach clarifies, unifies, and extends several recent results, including the existence of independent right and left equations of motion and the concept of ``pseudoparticle'' (also known as ``Bethe quasiparticle'').

cond-mat↗

arXiv1994

New perturbation theory of low-dimensional quantum liquids I: the pseudoparticle operator basis

We introduce a new operator algebra for the description of the low-energy physics of one-dimensional, integrable, multicomponent quantum liquids. Considering the particular case of the Hubbard chain in a constant external magnetic field and with varying chemical potential, we show that at low energy its Bethe-ansatz solution can be interpreted in terms of the new {\it pseudoparticle operator algebra}. Our algebraic approach provides a concise interpretation of and justification for several recent studies of low-energy excitations and transport which have been based on detailed analyses of specific Bethe-ansatz eigenfunctions and eigenenergies. A central point is that the {\it exact ground state} of the interacting many-electron problem is the non-interacting pseudoparticle ground state. Furthermore, in the pseudoparticle basis, the quantum problem becomes perturbative, {\it i.e.}, the two-pseudoparticle forward-scattering vertices and amplitudes do not diverge, and one can define a many-pseudoparticle perturbation theory. We write the general quantum-liquid Hamiltonian in the new basis and show that the pseudoparticle-perturbation theory leads, in a natural way, to the generalized Landau-liquid approach.

cond-mat↗

arXiv1994

New perturbation theory of low-dimensional quantum liquids II: operator description of Virasoro algebras in integrable systems

We show that the recently developed {\it pseudoparticle operator algebra} which generates the low-energy Hamiltonian eigenstates of multicomponent integrable systems also provides a natural operator representation for the the Virasoro algebras associated with the conformal-invariant character of the low-energy spectrum of the these models. Studying explicitly the Hubbard chain in a non-zero chemical potential and external magnetic field, we establish that the pseudoparticle perturbation theory provides a correct starting point for the construction of a suitable critical-point Hamiltonian. We derive explicit expressions in terms of pseudoparticle operators for the generators of the Virasoro algebras and the energy-momentum tensor, describe the conformal-invariant character of the critical point from the point of view of the response to curvature of the two-dimensional space-time, and discuss the relation to Kac-Moody algebras and dynamical separation.

cond-mat↗

arXiv1992

Extracting Infinite System Properties from Finite Size Clusters: `Phase Randomization/Boundary Condition Averaging'

When electron-electron correlations are important, it is often necessary to use exact numerical methods, such as Lanczos diagonalization, to study the full many-body Hamiltonian. Unfortunately, such exact diagonalization methods are restricted to small system sizes. We show that if the Hubbard $U$ term is replaced by a ``periodic Hubbard" term, the full many body Hamiltonian may be numerically exactly solved, even for very large systems (even $>$100 sites), though only for low fillings. However, for half-filled systems and large $U$ this approach is not only no longer exact, it no longer improves extrapolation to larger systems. We discuss how generalized ``randomized variable averaging'' (RVA) or ``phase randomization'' schemes can be reliably employed to improve extrapolation to large system sizes in this regime. This general approach can be combined with any many-body method and is thus of broad interest and applicability.

cond-mat↗

arXiv1992

Relaxed Excited State Geometries in a Peierls-Hubbard Model for Polyenes

Using parameter values relevant to polyacetylene and its finite polyene analogs, we determine, via exact finite size diagonalization of a 1D Peierls-Hubbard model, the relaxed geometry and optical absorption spectra of several states of interest in optical experiments (the singlet ($1^1A_g$) and triplet ($1^3B_u$) ground states, the singlet one ($n^1B_u$) and two ($2^1A_g$) photon gaps, and the triplet one ($n^3A_g$) photon gap) in order to examine the question of how well this simple model can really capture the behavior of this simple material.

cond-mat↗