Peierls Energy Gap Research Articles

Photoconduction in the Peierls conductor monoclinic TaS3

Photoconduction in the monoclinic phase of quasi-one-dimensional conductor TaS3 has been observed at T<70K. It was studied jointly with low-temperature ohmic and non-linear dark conduction. The strong sample quality dependence of both photoconduction and dark conduction at this temperature region has been observed. Together with a similarity of the main features of the photoconduction characteristic of both monoclinic (m-TaS3) and orthorhombic (o-TaS3) samples the following new peculiarities of photoconduction in m-TaS3 were found: (1) the dependence of the activation energy of photoconduction on temperature, T, (2) the change of the recombination mechanism from the linear type to the collisional one at low T with a sample quality growth, and (3) the existence of a fine structure of the electric-field dependence of photoconduction. Spectral study gives the Peierls energy gap value 2Δ⁎=0.18eV.

Effect of spin-flip scattering on current polarization in an organic spin filter

We studied the effect of spin-flip scattering to understand the spin-dependent quantum transport properties through an organic spin filter. We found that the electronic orbitals of the organic polymer are spin-mixed when the spin-flip scattering is included. The Peierls energy gap is reduced due to the spin-flip effect, which is a peculiar property of the organic ferromagnetic polymer. An analysis based on the extended Landauer–Büttiker formula shows that the spin polarization of the current through the spin filter decreases with the spin-flip scattering. However, the device keeps the spin filter function intact even when a considerable spin-flip scattering is included. We also discuss the competing effects of spin-flip scattering and ferromagnetism of the organic interlayer on the spin filter.

Micro-contact spectroscopy features of quasi-one-dimensional materials with a charge-density wave

We have applied the method of point-contact (PC) spectroscopy to conductors with a charge-density-wave (CDW) ground state. Recent experimental results of the investigation of characteristics of PC formed between a normal metal (N) and different CDW conductors are reported. For a uniform N–CDW boundary, we show that the interaction of carriers injected from the normal metal with the CDW results in their reflection without changes in the sign of the charge but with a momentum transfer 2pF to the condensate of electron–hole pairs carried away from the N–CDW interface. In the case of a non-uniform N–CDW boundary (direct point contact), a strong local CDW deformation, due to a band bending effect, was observed in materials with a semiconducting ground state. The chemical potential can be strongly modified, changing the conductivity type in the vicinity of the PC. It was shown that, in the case of a CDW conductor with a metal or semimetal ground state, the band bending effect is negligible (because of the screening of the electric field by remaining normal carriers) and the spectroscopy of the Peierls energy gap is possible.

Tunneling spectroscopy ofNbSe3in a high magnetic field

We have measured the differential current-voltage characteristics of $\mathrm{normal}\ensuremath{-}\mathrm{metal}\char21{}{\mathrm{NbSe}}_{3}$ direct point contacts (without an insulating barrier) formed along the b axis under applied magnetic field with different orientations. At $T=4.2\mathrm{K}$ two charge-density-wave (CDW) gaps, ${\ensuremath{\Delta}}_{p1}$ and ${\ensuremath{\Delta}}_{p2},$ corresponding, respectively, to the high- and low-temperature CDW's were observed simultaneously as singularities of the excess resistance, which is attributed to the reflection injected from normal-metal carriers on the Peierls energy-gap barriers. The applied magnetic field up to 8.5 T does not lead to a change in the density of states and the Peierls energy gaps, suggesting that the large magnetoresistance observed in ${\mathrm{NbSe}}_{3}$ might not result from the change in the CDW order parameter with magnetic field, but rather from the increasing scattering of carriers which are not condenced to CDW.

Charge-density-wave gaps ofNbSe3measured by point-contact spectroscopy in different crystallographic orientations

We have measured the differential current-voltage (IV) characteristics of normal-metal (Au, Cu, In)-NbSe 3 direct point contacts (without insulating barrier) formed along different crystallographic orientations. For all investigated directions, at low temperature we clearly observed two charge-density-wave gaps Δ p 1 and Δ p 2 as excess resistance singularities in the IV curves. The excess resistance is attributed to the reflection of injected carriers from the normal metal on the Peierls energy gap barriers. The analysis of these results demonstrates the two-dimensional character of the electronic spectrum in NbSe 3 with a possible strong anisotropy of the energy gap in b-c plane.

THE FERMION-BOSON MAPPING APPLIED TO LAGRANGIAN MODELS FOR CHARGE-DENSITY-WAVES IN ONE-DIMENSIONAL SYSTEMS

We use the two-dimensional Fermion-Boson mapping to perform a field theory analysis of the effective Lagrangian model for incommensurate charge-density waves (ICDW) in one-dimensional systems. We consider an approach in which both the phase of the complex phonon field and the electron field are dynamical degrees of freedom contributing to the quantum dynamics and symmetry-related features of the ICDW phenomenon. We obtain the bosonized and fermionized versions of the effective electron–phonon Lagrangian. The phase of the phonon field and the phase of the bosonized chiral density of the electron field condense as a soliton order parameter, carrying neither the charge nor the chirality of the electron–phonon system, leading to a periodic sine-Gordon potential. The phonon field is fermionized in terms of a chiral fermionic condensate and the effective model is mapped into the chiral Gross–Neveu (GN) model with two Fermi field species. The linked electron–phonon symmetry of the ICDW system is mapped into the chiral symmetry of the GN model. Within the functional integral formulation, we obtain for the vacuum expectation value of the phonon field &lt; ϕ &gt; = 0 and &lt; ϕ ϕ* &gt; ≠ 0, due to the charge selection rule associated with the chiral electron–phonon symmetry. We show that the two-point correlation function of the phonon field satisfies the cluster decomposition property, as required by the chiral symmetry of the underlying GN model. The quantum description of the ICDW corresponds to charge transport through the lattice, due to the propagation of a "Goldstone mode" carrying the effective charge of the electron–phonon system, is accomplished by an electron–lattice energy redistribution. This accounts for a dynamical Peierls's energy gap generation.

Point-contact spectroscopy probing the NbSe3 CDW gap in different crystallographic orientations

We have measured the differential current-voltage characteristics of normal metal-NbSe3 direct point contacts (without insulating barrier) formed along different crystallographic orientations under applied magnetic field with different orientations. At low temperature two energy gaps, $\Delta_{p1}$ and $\Delta_{p2}$, corresponding to the high and the low-temperature CDW were observed simultaneously as a singulanty of the excess resistance which is attributed to an analog of Andreev reflection, in which the incident electron reflects on the Peierls energy gap barriers with its charge unchanged. An applied magnetic field up to 8.5 T does not lead to a change in the density of states and in the Peierls energy gaps, suggesting that the large magnetoresistance observed in NbSe3 might not result from the change in the CDW order parameter with magnetic field but rather from the increase of scattering of non-condensed to CDW carriers.

MAGNETORESISTANCE PROPERTIES IN QUASI-TWO DIMENSIONAL CHARGE-DENSITY WAVE COMPOU ND (PO2)4(WO3)2m(m=6)

Temperature dependence of magnetoresistance in the range of 2—300K and the Δρ /ρ０-B relation at 2K were measured in quasi-two dimensional charge- density wave compound (PO2)4(WO3)2m( m=6). The enhancement of the magnetoresistance at low temperatures was analyzed by use of the magnetic breakdown model. The first Peierls energy gap in the char ge-density wave states was estimated to be about 3.0meV.

Conductivity from solitonic energy band in K 2Pt(CN) 4Br 0.3·3.2H 2O system

The coupling between electrons of the solitonic band and the phonon modes of the solitonic array opens a new Peierls energy gap in the solitonic energy band in the KCP compound, above 85 K. Considering transitions across this new energy gap, one finds the temperature dependence of the longitudinal d.c. conductivity in KCP in accordance with experiment within this temperature range.

Theory of tunneling conductance of CDW junctions

The quasi-particle transport of normal metal/insulator/CDW junctions is investigated theoretically for the configuration where the one-dimensional chains are oriented normal to the interface. The tunnel current depends strongly on the macroscopic phase of the CDW condensate and vanishes when the applied bias is smaller than the magnitude of the Peierls energy gap. The average conductance in mesoscopic systems is also calculated.

特集―分子性結晶のダイナミックス ハロゲン架橋一次元金属錯体における構造―物性相関

The series of halogen-bridged NII-X-MIV mixed valence compounds (M=Pt, Pd; X=Cl, Br, I) has attracted much interest from solid state physicists and chemists as one dimensional compounds having strong electron-lattice interactions. Their structures are well described as Peierls distorted linear chains with repeating unit…MII…X-MIV-X…. They show characteristic physical properties such as strong charge transfer absorption and luminescence with large Stokes shift that can be interpreted by using an extended Peierls-Hubbard model. The relation between the Peierls energy gap and lattice distortions for the compounds has been established by a number of structural and optical studies. The remarkable feature that the physical and structural parameters of the compounds can be controlled by changing chemical parameters such as central metals, bridging halogens, and counter ions has been also elucidated. From an expectation based on the relation, the halogen-bridged NiIII-X-NiIII compounds with an extreme limit of the MII-X-MIV compounds were synthesized. The novel structure having no Peierls distortion has been determined by detailed X-ray diffraction and physical studies. Further studies on the interesting physical properties such as an extremely strong antiferromagnetic coupling between electronic spins localized on the NiIII atoms are now in progress.

Pressure effect on the ohmic and non linear transport of K 0.3MoO 3

We have investigated the effect of hydrostatic pressure on the resistivity of the quasi-one-dimensional conductor blue bronze, K 0.3MoO 3, in both the ohmic and nonlinear regimes. The Peierls transition temperature (T p) and energy gap Δ decrease under pressure with the same rate of ∼ 1%/kbar. The threshold electric field (E T) for sliding charge density wave (CDW) conduction is pressure independent at 77 K up to 17–19 kbars. Above these pressures E T increases. The pressure dependence of the high field resistivity (E<<E T) follows that of the low field resistivity (E<<E T).

Optical investigation of CDW distortion in TaS 3

We have used bolometric infrared spectroscopy to look for metastable changes in the Peierls energy gap of TaS 3 caused by: 1) polarization of the CDW at current contacts, and b) temperature cycling. In neither case were such changes observed.

Synthetic metal superlattices

We consider theoretically a new class of superlattices consisting of alternating layers of quasi-one-dimensional (Q1D) conducting polymers. Although both compositional and doping superlattices are possible, we consider only the doping superlattices in detail here. In particular, we first obtain expressions for the two-dimensional density of electrons and holes in a solitonic as well as a polaronic doping superlattice. Next, we use these quantities to discuss the optical absorption coefficient, α(ω), and the photoconductive response of these Q1D superlattices. Due to the presence of non-linear excitations such as solitons and polarons in doped conducting polymers, the optical absorption arises not only from the usual interband transition, but also from the transitions involving localized levels in the Peierls energy gap. The absorption coefficient is a tunable quantity for the Q1D superlattices, since the effective band gap can be tailored to suit the radiaiton wavelength of interest. Finally, we describe certain means of experimentally measuring α(ω) for photon energies smaller than the Peierls gap and discuss the novel features, including the device applications of Q1D superlattices and related modulated structures.

Theory of bipolaron lattices in quasi-one-dimensional conducting polymers.

We present a theory of the bipolaron lattice in the Peierls-distorted system with nondegenerate ground states. First, we obtain an exact solution for the order parameter of a bipolaron lattice in terms of the Weierstrass \ensuremath{\zeta} function for the continuum model of Brazovskii and Kirova. Making use of the exact solution, we determine the complete band structure of a bipolaron lattice. In addition to the conduction and valence bands, we find two bipolaron bands symmetrically located about the middle of the Peierls energy gap. We also calculate the electronic density of states and the energy of formation of a bipolaron lattice as a function of the bipolaron density ${n}_{b}$. The chemical potential of a bipolaron determines the domain of stability of the lattice as a function of the confinement parameter \ensuremath{\gamma}. In the weak-coupling limit (infinite momentum cutoff \ensuremath{\Lambda}) the bipolaron-bipolaron interaction decays as a repulsive exponential while for finite \ensuremath{\Lambda} it decays inversely proportional to the distance and is attractive. This yields the possibility of phase separation in doped quasi-one-dimensional conducting polymers with nondegenerate ground states. For higher dopant concentrations we describe a possible first-order phase transition to a metallic phase consisting of an array of polaronlike distortions from a lattice of charged bipolarons. As well, the optical absorption spectrum of a bipolaron lattice is briefly discussed. Our results apply to cis-polyacetylene as well as to a class of heterocyclic compound polymers, namely, polypyrroles, polythiophenes, and their derivatives. Finally, we make contact with recent experiments which study the evolution of ordered phases during the electrochemical doping of polyparaphenylene by alkali atoms and indicate the possibility of a bipolaron lattice.

Kink lattice structure and midgap band in incommensurate Charge Density Waves

By diagonalizing the one-dimensional Fröhlich model numerically, we investigate the incommensurate charge density wave states within a mean field approximation. It is found that the midgap band inside the main Peierls energy gap always appears in nearly commensurate states, which is attributed to the formation of the kink (or soliton) lattice of the electron- and ion-density modulations. The overall band structure against band filling is shown to be self-similar and recursive. Experimental relevance of our finding is discussed.

First-order transition to a metallic state in polyacetylene: A strong-coupling polaronic metal.

We present a theory of the first-order transition to the metallic state in polyacetylene in terms of a crossover from a lattice of charged solitons to a regular array of polaronlike distortions. The polaronic metal is shown to have a strong indirect attractive interaction, ${\mathrm{U}}^{\mathrm{*}}$\ensuremath{\simeq}-2\ensuremath{\Delta}/3, between electrons in the half-filled, narrow, polaron subband within the Peierls energy gap (${\mathrm{E}}_{\mathrm{g}}$=2\ensuremath{\Delta}).