Possible lattice distortions in the hubbard model for graphene

The Hubbard model on the honeycomb lattice is a well-known model for graphene. Equally well known is the Peierls type of instability of the lattice bond lengths. In the context of these two approximations we ask and answer the question of the possible lattice distortions for graphene in zero magnetic field. The answer is that in the thermodynamic limit only periodic, reflection-symmetric distortions are allowed and these have at most 6 atoms per unit cell as compared to two atoms for the undistorted lattice.     

Spontaneous parity breaking of graphene in the Quantum Hall regime

We propose that the inversion symmetry of the graphene honeycomb lattice is spontaneously broken via a magnetic-field-dependent Peierls distortion. This leads to valley splitting of the n=0 Landau level but not of the other Landau levels. Compared to Quantum Hall valley ferromagnetism recently discussed in the literature, lattice distortion provides an alternative explanation to all of the currently observed Quantum Hall plateaus in graphene.     

低维体系的不稳定性和电子关联

一维和二维体系的费米面会呈现叠套(nesting),于是,晶格结构和电子状态的重新调整(使费米面和布里渊区边界相互重合)可使体系能量降低而形成新的基态,这使低维体系具有不稳定性并产生相变,同时还会形成多种新型元激发(电荷密度波(CDW)、自施密度波(SDW)、孤子(soliton)、极化子(polaron)、分数电荷,spin bag,位相子等)。许多重要的低维体系(导电高分子、二维电子气、氧化物超导体的铜氧层等)都具有强电子耦合5研究电子关联对体系不稳定性的影响是当前凝聚态物理的重要课题。近年来,在此领域内,有两派相反的观点进行着激烈的争论:一派认为电子-电子相互作用会增强晶格不稳定性,而且增强得很多,以致不稳定性主要来自于电子-电子相互作用,电子-晶格相互作用是次要的。另一派则认为电子-电子相互作用会减弱晶格不稳定性,而不稳定性是由电子-晶格相互作用产生的。 本文首先描绘低维体系的不稳定性和各种基态及元激发的物理图象,接着介绍争论双方的论点,随后分析两派分歧的产生原因,进一步指出如何澄清这场争论。从中将说明,Hubbard模型的局限性会带来问题,解决争论的关键是正确地描述电子相互作...更多用,由屏蔽效应而形成的相互作用力程是决定性的因素。当屏蔽较弱时(长程相互作用),电子相互作用的非对角部分比对角     

Soliton wall superlattice in the quasi-one-dimensional conductor (Per)(2)Pt(mnt)(2)

We suggest a model to explain the appearance of a high resistance high magnetic field charge-density-wave (CDW) phase, discovered by Graf et al. [Phys. Rev. Lett. 93, 076406 (2004)10.1103/PhysRevLett.93.076406] in (Per)(2)Pt(mnt)(2), where Per is perylene and mnt is maleonitriledithiolate molecules. In particular, we show that the Pauli spin-splitting effects improve the nesting properties of a realistic quasi-one-dimensional electron spectrum and, therefore, a high resistance Peierls CDW phase is stabilized in high magnetic fields. In low and very high magnetic fields, a periodic soliton wall superlattice (SWS) phase is found to be a ground state. We suggest experimental studies of the predicted phase transitions between the Peierls and SWS CDW phases in (Per)(2)Pt(mnt)(2) to discover a unique SWS phase.     

The Peierls instability and superconductivity in quasi-2-d La2-x(Ba,Sr)xCuO4 compounds

The Peierls instability in the quasi-2-d La_2-x(Ba,Sr)_xCuO₄ is reexamined by taking the weak interlayer coupling into account. A two-step Peierls transition theory is developed, the general misunderstandings on the Peierls instability are therefore clarified, and a number of experimental anomalies observed in La₂CuO₄ are well explained. By suggesting an additional change of space group symmetries at the lower transition point, the calculated band structures are also intepreted. The possibility of CDW coexistence with the high-T_c superconductivity, the possible evidence for the 3-d superconductivity of the high-T_c materials are discussed with this theory.     

QUANTUM FLUCTUATION OF THE ORDER PARAMETER AND THE OPTICAL ABSORPTION IN POLYACETYLENEO

We study in this paper the effects of lattice quantum fluctuations upon the order parametert in the Peierls systems by using the Green's function technique.We start from the discrete Su-Schrief fer-Heeger model with quantized phonon field and derive a coupled system of equations for the order parameter and Green's functions.It turns out that the order parameter is reduced compared with the adiabatic value but the Peierls instability survives the quantum fluctuations in agreement with,Monte Carlo results.The band-to-band optical absorption coefficient of polyacetylene with lattice fluctuation being accounted for is also calculated and compared with the experimental data.     

Competition of charge-density waves and superconductivity in sulfur

A one-dimensional charge-density wave (CDW) instability is shown to be responsible for the formation of the incommensurate modulation of the atomic lattice in the high-pressure phase of sulfur. The coexistence of, and competition between, the CDW and the superconducting state leads to the previously observed increase of T{c} up to 17 K, which we attribute to the suppression of the CDW instability, the same phenomenology found in doped layered dichalcogenides.     

Hysteresis in the electrical properties of orthorhombic tantalum trisulphide: Evidence for an incommensurate-commensurate charge-density wave transition?

A study of electrical conduction in orthorhombic TaS₃ has revealed the existence of thermal hysteresis throughout the temperature range 55 K < T < 205 K. This is attributed to variability in the wavevector q of the charge-density wave (CDW) which develops below T_p = 215 K, and confirms the recent finding, from electron diffraction, that at temperatures not too far below T_p the CDW is incommensurate with the underlying lattice. Evidence that q becomes commensurate, at least along the chain direction at 55 K is provided by the vanishing of hysteresis at that temperature, and also by a rise in the threshold field for continuous motion of the CDW.From its dependence on temperature it is concluded that between T_p and 55 K the conduction in the linear regime is better described as that of a Peierls semi-metal, rather than that of a Peierls intrinsic semiconductor. At most temperatures within that range electrical hysteresis also is observed, and a detailed study of this leads to the tentative conclusion that translation of the CDW conveys negative charge, carried presumably by negatively-charged discommensurations. The mechanisms of conduction below 55 K remain uncertain.     

Competing periodicities in fractionally filled one-dimensional bands

We present a variable temperature scanning tunneling microscopy and spectroscopy study of the Si(553)-Au atomic chain reconstruction. This quasi-one-dimensional system undergoes at least two charge density wave (CDW) transitions, which can be attributed to electronic instabilities in the fractionally filled 1D bands of the high-symmetry phase. Upon cooling, Si(553)-Au first undergoes a single-band Peierls distortion, resulting in period doubling along the chains. This Peierls state is ultimately overcome by a competing x3 CDW, which is accompanied by a x2 periodicity in between the chains. These locked-in periodicities indicate small charge transfer between the nearly 1/2-filled and 1/4-filled bands. The presence and the mobility of atomic-scale dislocations in the x3 CDW state indicates the possibility of manipulating phase solitons carrying a (spin, charge) of (1/2, +/- e/3) or (0, +/-2e/3).     

Peierls transition with acoustic phonons and solitwistons in carbon nanotubes

We show that the Peierls instability can result in softening of acoustic phonons with small wave vectors and suggest that this unusual transition takes place in carbon nanotubes, resulting in a static twist deformation of the nanotube lattice. The topological excitations in the ordered phase are immobile and propagate only in pairs.     

Lattice dynamics and electron-phonon interaction in (3,3) carbon nanotubes

We present a detailed study of the lattice dynamics and electron-phonon coupling for a (3,3) carbon nanotube which belongs to the class of small diameter based nanotubes which have recently been claimed to be superconducting. We treat the electronic and phononic degrees of freedom completely by modern ab initio methods without involving approximations beyond the local density approximation. Using density functional perturbation theory we find a mean-field Peierls transition temperature of approximately 240 K which is an order of magnitude larger than the calculated superconducting transition temperature. Thus in (3,3) tubes the Peierls transition might compete with superconductivity. The Peierls instability is related to the special 2k(F) nesting feature of the Fermi surface. Because of the special topology of the (n,n) tubes we also find a phonon softening at the Gamma point.     

Charge-density waves in self-assembled halogen-bridged metal chains

Self-assembled growth of an ordered layer of Pt-Br-Pt chains on a Pt(110) surface is demonstrated. Upon slight doping with excess bromine, charge-density wave (CDW) domains separated by well-localized solutions are observed in the Br/Pt layer by scanning tunneling microscopy. Depending on annealing and adatom concentration, a global, long-range-ordered CDW ground state can be established. Angle-resolved UV photoemission data reveal the corresponding Fermi surface and its removal upon the Peierls transition. The CDW phase is stable to well above room temperature.     

Current induced deformation of charge density waves in orthorhombic TaS3

The low electric field ohmic resistance R of orthorhombic TaS₃ measured at 90 and 120 K well below the Peierls transition temperature depends on the product of a temperature difference ΔT applied along the sample and the sign of a previously applied current pulse if this pulse is larger than threshold for non-ohmic conductivity. This resistance change is about ΔR/RΔT ∽ 1×10^-3 K^-1 for a pure sample and ΔR/RΔT ∽ 6×10^-3 K^-1 for a slightly electron irradiated one at 90 K. The relative resistance change is insensitive to the sample length. We deduce that the CDW current changes inhomogeneously the Peierls gap E_g. ΔE_g < O at the contact where the CDW current enters and ΔE_g > O at the exit. The effect is attributed to a CDW current induced inhomogeneous deformation of the CDW itself.     

Electron-lattice interaction and the CDW state of 1T-TiSe2

The electronic band structure in the CDW state (superlattice structure) of 1T-TiSe₂ is calculated on the basis of the band-type Jahn-Teller model by extending our theory of lattice instability in the normal phase. A strong coupling between the hole-band (Se p states) around the Λ point and the electron-bands (Ti d states) around the Λ points is caused by the electron-lattice interaction. Reflecting such a strong coupling remarkable changes appear in the dispersion curves near the Fermi energy and the largest CDW gap is obtained to be 0.2 eV. We have also calculated a change of the density of states near the Fermi energy due to the superlattice formation. The result is consistent with that observed by angle-integrated photoemmision by Margaritondo et al. It is also shown that the magnitude of the lattice distortion observed at low temperatures can be explained in a way consistent with the lattice dynamics in the normal phase.     

Dephasing of Andreev pairs entering a charge density wave

A Cooper pair from a s-wave superconductor (S) entering a conventional charge density wave (CDW) below the Peierls gap dephases on the Fermi wavelength while one particle states are localized on the CDW coherence length ξ_CDW. It is thus practically impossible to observe a Josephson current through a CDW. The paths following different sequences of impurities interfere destructively, due to the different electron and hole densities in the CDW. The same conclusion holds for averaging over the conduction channels in the ballistic system. We apply two microscopic approaches to this phenomenon: (i) a Blonder, Tinkham, Klapwijk (BTK) approach for a single highly transparent S-CDW interface; and (ii) the Hamiltonian approach for the Josephson effect in a clean CDW and a CDW with non magnetic disorder. The Josephson effect through a spin density wave (SDW) is limited by the coherence length ξ_SDW, not by the Fermi wave-length. A Josephson current through a SDW might be observed in a structure with contacts on a SDW separated by a distance ξ_SDW.     

Magnetoresistance of a Two-Dimensional TbTe<Subscript>3</Subscript> Conductor in the Sliding Charge-Density Wave Regime

The magnetoresistance of a TbTe₃ two-dimensional conductor with a charge-density wave (CDW) has been measured in a wide temperature range and in magnetic fields of up to 17 T. At temperatures well below the Peierls transition temperature and in high magnetic fields, the magnetoresistance exhibits a linear dependence on the magnetic field caused by the scattering of normal charge carriers by “hot” spots of the Fermi surface. In the sliding CDW regime in low magnetic fields, a qualitative change in the magnetoresistance has been observed associated with the strong scattering of carriers by the sliding CDW.     

On the Fröhlich charge density wave and Peierls transition

The exact solution of the system of nonlinear equations for Fröhlich charge density wave (CDW) in the continuum approximation is discussed, and the temperature behaviour of the CDW condensate is investigated in a framework of the mean-field approximation. It is shown that the CDW representation in the form of a conventional harmonic wave is only correct under certain conditions, namely, not too strong an electron-phonon interaction or not too low an electron density. According to the exact solution, the CDW is essentially a nonlinear wave represented by a harmonic series. This is confirmed by the experimental data on the CDW current oscillation spectra in which the overtones are present together with the principal harmonic (the so called “narrow band noise”). At strong enough electron-phonon coupling or at low enough electron density the CDW wave vector is temperature dependent, and the BCS relation between Peierls transition temperature and energetic gap in the electron spectra at zero temperature is violated.     

Infrared study on the formation of charge density waves in (TMTSF) 2X (X = Reo4− and PF6− at atmospheric pressure

The temperature dependence of the infrared absorption spectra (4000–200 cm^?) of (TMTSF) ₂Reo₄ and (TMTSF) ₂PF₆ are investigated with the aim at using the appearance of vibronic absorptions originated by electron-molecular vibration interactions to monitor the formation of charge density waves (CDW) on the donor stacks. The presence of vibronic bands at ≈1400, 436 and 264 cm^?1 in (TMTSF) ₂Re0₄ below 180 K shows that the anion ordering transition in this salt induces the formation of commensurate CDW. The absence of vibronic bands in the PF₆^? salt at 8K indicates that no CDW formation nor an underlying 2K_f lattice distortion takes place at the metal-insulator (MI) phase transition. These results show that, at atmospheric pressure both salts do not undergo a conventional Peierls type MI transition.     

One-dimensional instability in BaVS3

The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.     

Thermal expansion measurements of the quasi-one-dimensional conductor K0.30MoO3

The charge density wave (CDW) dynamics of the quasi-one-dimensional conductor K_0.30MoO₃ shows two different regimes depending on the temperature: a strongly damped CDW motion above ∼50 K and CDW motion with almost no damping below ∼50 K. In a search for a characterization of this CDW behaviour, we performed thermal expansion measurements on K_0.30MoO₃ single crystals in the temperature range 4-250 K. In addition to the anomaly observed at the Peierls transition at 180 K along the [102] direction, an anomaly is observed at ∼50 K along the [−201] and [102] directions. The results are discussed in relation with the change in the CDW rigidity at ∼50 K.