Peierls Insulator Research Articles

Semimetal-antiferromagnetic insulator transition in graphene induced by biaxial strain

We report first-principles calculations on the antiferromagnetic spin ordering in graphene under biaxial strain. Using hybrid functional calculations, we found that the semimetallic graphene sheets undergo a transition to antiferromagnetic insulators at a biaxial strain of 7.7$%$ and that the band gap rapidly increases after the onset of this transition before reaching 0.9 eV at a biaxial strain of 12$%$. We examined the competition of the antiferromagnetic spin ordering with two-dimensional Peierls distortions upon biaxial strain, and found that the preceding antiferromagnetic insulator phase impedes the Peierls insulator phase. The antiferromagnetic insulator phase is destabilized upon carrier filling but robust up to moderate carrier densities. This work indicates that biaxially strained graphene represents a noble system where the electron-electron and electron-lattice interactions compete with each other in a simple but nontrivial way.

Time-domain classification of charge-density-wave insulators

Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron-lattice interactions, the Mott and the excitonic insulator caused by electron-electron interactions, and the Anderson insulator arising from electron-impurity interactions. In real materials, however, all the interactions are simultaneously present so that classification is often not straightforward. Here, we show that time- and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify-via systematic temporal discrimination of elementary electronic and structural processes-the dominant interaction. Specifically, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1T-TaS(2) is a Peierls insulator and that the ultrafast response of 1T-TiSe(2) is highly suggestive of an excitonic insulator.

DMRG analysis of the SDW-CDW crossover region in the 1D half-filled Hubbard-Holstein model

In order to clarify the physics of the crossover from a spin-density-wave (SDW) Mott insulator to a charge-density-wave (CDW) Peierls insulator in one-dimensional (1D) systems, we investigate the Hubbard-Holstein Hamiltonian at half filling within a density matrix renormalisation group (DMRG) approach. Determining the spin and charge correlation exponents, the momentum distribution function, and various excitation gaps, we confirm that an intervening metallic phase expands the SDW-CDW transition in the weak-coupling regime.

Electron correlation and electron momenta in polyacetylene

In a uniform noninteracting electron gas, (here are two nonanalytic points in the structure factor S(k), which is the Fourier transform of the electron pair correlation function g(r). One of these is the origin of k or momentum space, while the other occurs at k = 2kf, where pf= hkf is the Fermi momentum. In a Peierls insulator, attention is most often focused on the nonanalytic region around k = 2kf, which in turn is modified by the Peierls distortion. By studying here the interpretation of the linear π-plasmon dispersion observed by Ritsko et al. in polyacetylene (CH)x and also, more recently, in a specimen containing a polydiacetylene backbone, attention focuses naturally on the zero momentum non-analyticity. Using a simple model of the π-electron Fermi liquid in (CH) x paralleling that used by Feynman for the Bose liquid 4He at T = 0, the observed plasmon dispersion is related to the structure factor S(k) of the interacting π-electron liquid. Analysis of the resulting form of S(k) makes it plain that both Pauli Principle correlations between parallel spin π-electrons, and Coulomb repulsions, are essential ingredients in interpreting the linear dispersion of the π-plasmon.

Propagation of a ballistic nuclear wavepacket on an adiabatic potential surface of a one-dimensional Br-bridged Pd complex without a self-trapped exciton state

The photoexcited state of one-dimensional Br-bridged Pd complex, which is a Peierls insulator, was studied using a femtosecond luminescence spectroscopy from near to mid-infrared region. The ultrafast decay with a time constant of $240\ifmmode\pm\else\textpm\fi{}40\text{ }\text{fs}$ at all photon energies and delay of the maximum positions up to 160 fs with decreasing the photon energy were observed. These behaviors are ascribed to a slide down motion of the nuclear wavepacket on an adiabatic potential energy surface leading to the Mott-insulator domain without a local potential minimum corresponding to a self-trapped exciton.

Micrometer x-ray diffraction study ofVO2films: Separation between metal-insulator transition and structural phase transition

In order to clarify whether ${\text{VO}}_{2}$ is a Mott insulator or a Peierls insulator, the metal-insulator transition (MIT) and the structural phase transition (SPT) are simultaneously monitored for ${\text{VO}}_{2}$ films by current-voltage curve and diffraction measurements using a synchrotron micro-x-ray beam. In the regime showing a metallic conductivity below the SPT temperature (approximately $70\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$), only the diffraction planes of the monoclinic structure are observed, while planes of the tetragonal structure are absent. This observation reveals the presence of a monoclinic and metal phase between the MIT and the SPT as a characteristic of a Mott insulator.

A Route to Metallization of the Monumental Peierls Insulator TTF-TCNQ

A Route to Metallization of the Monumental Peierls Insulator TTF-TCNQ

Competing Ground States of Metal–Halide Ladders

Based on a symmetry argument, we investigate the ground-state properties of newly synthesized metal-halide ladder compounds (C_8_H_6_N_4_)[Pt(C_2_H_8_N_2_)X]_2_(ClO_4_)_4_ 2H_2_O (X=Cl, Br, I). Employing a fully dressed two-band Peierls-Hubbard model, we systematically reveal possible charge- or spin-ordered states. Numerical phase diagrams demonstrate a variety of competing Peierls and Mott insulators with particular emphasis on the transition between two types of mixed-valent state of Pt^II^ and Pt^IV^ driven by varying interchain hopping integrals and Coulomb interactions.

The excitonic ground state of the half-filled Peierls insulator

We point out that the half-filled Peierls insulator, celebrated for its soliton excitations andits application to trans(polyacetylene), is an excitonic insulator in which collectively boundelectron–hole pair excitations (excitons) are mixed into the ground state. Unlike the boundelectron pairs of the Bardeen–Cooper–Schrieffer (BCS) superconductor, however, the excitonicpairs can be photoionized leading to the direct observation of the excitonic energy gap2Δ in the optical conductivity. A deeper understanding is provided of the discovery of Kuper in1955 of a BCS-like gap equation describing the thermodynamic properties of the Fröhlich(1954) one-dimensional charge-density-wave state.

Phonon Dispersion Relations in Two-dimensional Peierls Phase

The phonon dispersion relations in the two-dimensional Peierls phase (Peierls insulator) are studied numerically, focusing on a two-dimensional SSH (Su–Shrieffer–Heeger) model having a half-filled ...

Linear and nonlinear optical properties of one-dimensional Mott insulators consisting ofNi-halogen chain andCuO-chain compounds

We studied the linear and nonlinear optical responses in the one-dimensional (1D) Mott insulators of the halogen $(X)$-bridged nickel compounds (the $\mathrm{Ni}X$-chain compounds), $[{\mathrm{Ni}(\mathrm{chxn})}_{2}X]$${Y}_{2}$ [$X,Y=\mathrm{Cl}$; $X,Y=\mathrm{Br}$; $X=\mathrm{Cl}$, $Y={\mathrm{NO}}_{3}$: (chxn)=cyclohexanediamine] and the copper oxide $(\mathrm{CuO})$ chain compounds, ${A}_{2}{\mathrm{CuO}}_{3}$ ($A=\mathrm{Sr}$ and $\mathrm{Ca}$). The excitation profiles of the photoconductivity as well as the photoluminescence efficiency measurements show that charge-transfer (CT) excited states in the $\mathrm{Ni}X$-chain compounds form excitonic bound states, while the excitonic effect is relatively small in ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ and negligible in ${\mathrm{Ca}}_{2}{\mathrm{CuO}}_{3}$. The relatively large excitonic effect in the $\mathrm{Ni}X$-chain compounds is attributable to the strong 1D confinement of the electronic states. The temperature dependence of the ${\ensuremath{\epsilon}}_{2}$ spectra reveals that the spectral widths ${\ensuremath{\Gamma}}_{\mathrm{CT}}$ of the CT bands are dominated mainly by the electron--lattice interaction, which is smaller in the $\mathrm{Ni}X$-chain compounds than in the $\mathrm{CuO}$-chain ones. The ${\ensuremath{\chi}}^{(3)}(\ensuremath{-}\ensuremath{\omega};0,0,\ensuremath{\omega})$ spectra of the 1D Mott insulators were obtained by the electroreflectance spectroscopy. The maximum values of $\ensuremath{\mid}\mathrm{Im}{\ensuremath{\chi}}^{(3)}(\ensuremath{-}\ensuremath{\omega};0,0,\ensuremath{\omega})\ensuremath{\mid}$ in the 1D Mott insulators$(\ensuremath{\sim}{10}^{\ensuremath{-}5}--{10}^{\ensuremath{-}8}\phantom{\rule{0.3em}{0ex}}\mathrm{esu})$ were considerably larger than those in other 1D semiconductors such as 1D band insulators of silicon polymers, and 1D Peierls insulators of $\ensuremath{\pi}$-conjugated polymers and halogen-bridged Pt compounds $(\ensuremath{\sim}{10}^{\ensuremath{-}8}--{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}\mathrm{esu})$. To elucidate the enhancement of $\ensuremath{\mid}\mathrm{Im}{\ensuremath{\chi}}^{(3)}(\ensuremath{-}\ensuremath{\omega};0,0,\ensuremath{\omega})\ensuremath{\mid}$ in the 1D Mott insulators, we have compared the nature of the photoexcited states of the 1D Mott insulators with those of the 1D band and Peierls insulators. In the 1D Mott insulators, the odd and even CT excited states are nearly degenerate. This degeneracy induces the large transition dipole moment between these two states and then leads to the enhancement of ${\ensuremath{\chi}}^{(3)}$. Such a feature in the 1D Mott insulators is independent of the magnitude of the excitonic effect, although the excitonic effect sharpens the ${\ensuremath{\chi}}^{(3)}$ spectrum and enhances the maximum value of $\ensuremath{\mid}{\ensuremath{\chi}}^{(3)}\ensuremath{\mid}$. In the 1D band and Peierls insulators, on the other hand, the splitting between the lowest excited state with odd parity and the second-lowest one with even parity is as large as the exciton binding energy. It leads to the diminution of the transition dipole moment between these two excited states and hence of ${\ensuremath{\chi}}^{(3)}$. These differences of the photoexcited states between the 1D Mott insulators and others have been explained in terms of the 1D extended Peierls--Hubbard model.

Tuning the electronic structure from charge-transfer insulator to Mott-Hubbard and Peierls insulators in one-dimensional halogen-bridged mixed-metal compounds

The electronic structures of one-dimensional (1D) halogen-bridged mixed-metal compounds, $[{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}{(\mathrm{chxn})}_{2}X]{X}_{2}$ ($\mathrm{chxn}$= cyclohexanediamine; $X=\mathrm{Cl}$,$\mathrm{Br}$; $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$), are investigated through optical and magnetic measurements. The results reveal that the system changes from charge-transfer (CT) insulator to Mott-Hubbard (MH) insulator at around $x\ensuremath{\sim}0.3$, and from MH insulator to Peierls insulator at $x\ensuremath{\sim}0.9$ with increasing $x$. From the analysis of optical conductivity spectra using the 1D two-band extended Peierls-Hubbard model, electronic parameters such as Coulomb repulsion energy, $pd$ hybridization, CT energy, and electron-lattice interaction are determined.

Mechanism and observation of Mott transition in VO2-based two- and three-terminal devices

When holes of about 0.018% are induced into a conduction band (breakdown of critical on-site Coulomb energy), an abrupt first-order Mott metal–insulator transition (MIT) rather than a continuous Hubbard MIT near a critical on-site Coulomb energy U/Uc=1, where U is on-site Coulomb energy between electrons, is observed on an inhomogeneous VO2 film, a strongly correlated Mott insulator. As a result, discontinuous jumps of the density of states on the Fermi surface are observed and inhomogeneity inevitably occurs. The off-current and temperature dependences of the abrupt MIT in a two-terminal device and the gate effect in a three-terminal device are clear evidence that the abrupt Mott MIT was induced by the excitation of holes. Raman spectra measured by a micro-Raman system show an MIT without the structural phase transition. Moreover, the magnitude of the observed jumps ΔJobserved at the abrupt MIT is an average over an inhomogeneous measurement region of the maximum true jump, ΔJtrue, deduced from the Brinkman–Rice picture. A brief discussion of whether VO2 is a Mott insulator or a Peierls insulator is presented.

Quantum lattice dynamical effects on single-particle excitations in one-dimensional Mott and Peierls insulators

As a generic model describing quasi-one-dimensional Mott and Peierls insulators, we investigate the Holstein-Hubbard model for half-filled bands using numerical techniques. Combining Lanczos diagonalization with Chebyshev moment expansion we calculate exactly the photoemission and inverse photoemission spectra and use these to establish the phase diagram of the model. While polaronic features emerge only at strong electron-phonon couplings, pronounced phonon signatures, such as multi-quanta band states, can be found in the Mott insulating regime as well. In order to corroborate the Mott to Peierls transition scenario, we determine the spin and charge excitation gaps by a finite-size scaling analysis based on density-matrix renormalization group calculations.

Optical properties of potassium-doped polyacetylene

The optical reflectance of polyacetylene doped with high concentrations of potassium has been measured, and the optical properties estimated by Kramers–Kronig analysis. The samples, doped by a vapor-phase technique to y≈0.08 and 0.18, showed strong doping-induced infrared absorption at ∼900, 1270, and 1390 cm −1. These experiments were motivated by calculations of the infrared absorption which showed that whereas there are only small differences in the infrared spectra for isolated solitons and isolated polarons, there is a dramatic difference between the high-concentration polaron and soliton lattices. The shapes of the absorption spectra are rather similar but the absorption for the polaron lattice is 13 orders of magnitude weaker than that of the soliton lattice. The observation of these features at high doping levels is inconsistent with both the polaronic metal and strongly disordered Peierls insulator models for highly doped polyacetylene and furthermore suggests that solitonic defects maintain their integrity to very high doping levels.

Ground-state phase diagram of the one-dimensional half-filled extended Hubbard model

We revisit the ground-state phase diagram of the one-dimensional half-filled extended Hubbard model with on-site (U) and nearest-neighbor (V) repulsive interactions. In the first half of the paper, using the weak-coupling renormalization-group approach $(g$-ology) including second-order corrections to the coupling constants, we show that bond-charge-density-wave (BCDW) phase exists for $U\ensuremath{\approx}2V$ in between charge-density-wave (CDW) and spin-density-wave (SDW) phases. We find that the umklapp scattering of parallel-spin electrons disfavors the BCDW state and leads to a bicritical point where the CDW-BCDW and SDW-BCDW continuous-transition lines merge into the CDW-SDW first-order transition line. In the second half of the paper, we investigate the phase diagram of the extended Hubbard model with either additional staggered site potential $\ensuremath{\Delta}$ or bond alternation $\ensuremath{\delta}.$ Although the alternating site potential $\ensuremath{\Delta}$ strongly favors the CDW state (that is, a band insulator), the BCDW state is not destroyed completely and occupies a finite region in the phase diagram. Our result is a natural generalization of the work by Fabrizio, Gogolin, and Nersesyan [Phys. Rev. Lett. $83,$ 2014 (1999)], who predicted the existence of a spontaneously dimerized insulating state between a band insulator and a Mott insulator in the phase diagram of the ionic Hubbard model. The bond alternation $\ensuremath{\delta}$ destroys the SDW state and changes it into the BCDW state (or Peierls insulating state). As a result the phase diagram of the model with $\ensuremath{\delta}$ contains only a single critical line separating the Peierls insulator phase and the CDW phase. The addition of $\ensuremath{\Delta}$ or $\ensuremath{\delta}$ changes the universality class of the CDW-BCDW transition from the Gaussian transition into the Ising transition.

Topological doping of a three-dimensional Peierls system: Predicted structure of dopedBaBiO3

At hole concentrations below x=0.4, Ba_(1-x)K_xBiO_3 is non-metallic. At x=0, pure BaBiO3 is a Peierls insulator. Very dilute holes create bipolaronic point defects in the Peierls order parameter. Here we find that the Rice-Sneddon version of Peierls theory predicts that more concentrated holes should form stacking faults (two-dimensional topological defects, called slices) in the Peierls order parameter. However, the long-range Coulomb interaction, left out of the Rice-Sneddon model, destabilizes slices in favor of point bipolarons at low concentrations, leaving a window near 30% doping where the sliced state is marginally stable.

Dynamical density-matrix renormalization-group method

A density-matrix renormalization-group (DMRG) method for calculating dynamical properties and excited states in low-dimensional lattice quantum many-body systems is presented. The method is based on an exact variational principle for dynamical correlation functions and the excited states contributing to them. This dynamical DMRG is an alternate formulation of the correction vector DMRG but is both simpler and more accurate. The finite-size scaling of spectral functions is discussed and a method for analyzing the scaling of dense spectra is described. The key idea of the method is a size-dependent broadening of the spectrum. The dynamical DMRG and the finite-size scaling analysis are demonstrated on the optical conductivity of the one-dimensional Peierls-Hubbard model. Comparisons with analytical results show that the spectral functions of infinite systems can be reproduced almost exactly with these techniques. The optical conductivity of the Mott-Peierls insulator is investigated and it is shown that its spectrum is qualitatively different from the simple spectra observed in Peierls (band) insulators and one-dimensional Mott-Hubbard insulators.

Self-trapped exciton defects in a charge density wave: Electronic excitations ofBaBiO3

In the previous paper, it was shown that holes doped into BaBiO3 self-trap as small polarons and bipolarons. These point defects are energetically favorable partly because they undo locally the strain in the charge-density-wave (Peierls insulator) ground state. In this paper the neutral excitations of the same model are discussed. The lowest electronic excitation is predicted to be a self-trapped exciton, consisting of an electron and a hole located on adjacent Bi atoms. This excitation has been seen experimentally (but not identified as such) via the Urbach tail in optical absorption, and the multi-phonon spectrum of the ``breathing mode'' seen in Raman scattering. These two phenomena occur because of the Franck-Condon effect associated with oxygen displacement in the excited state.

Polaron and bipolaron defects in a charge density wave: A model for lightly dopedBaBiO3

${\mathrm{BaBiO}}_{3}$ is a prototype ``charge ordering system'' forming interpenetrating sublattices with nominal valence ${\mathrm{Bi}}^{3+}$ and ${\mathrm{Bi}}^{5+}.$ It can also be regarded as a three-dimensional version of a Peierls insulator, the insulating gap being a consequence of an ordered distortion of oxygen atoms. When holes are added to ${\mathrm{BaBiO}}_{3}$ by doping, it remains insulating until a very large hole concentration is reached, at which point it becomes superconducting. The mechanism for insulating behavior of more lightly doped samples is formation of small polarons or bipolarons. These are self-organized point defects in the Peierls order parameter, which trap carriers in bound states inside the Peierls gap. We calculate properties of the polarons and bipolarons using the Rice-Sneddon model. Bipolarons are the stable defect; the missing pair of electrons come from an empty midgap state built from the lower Peierls band. Each bipolaron distortion also pulls down six localized states below the bottom of the unoccupied upper Peierls band. The activation energy for bipolaron hopping is estimated.