扶手椅形碳纳米管的Peierls相变研究

根据描述电子-声子相互作用的Frhlich哈密顿量,推导出了扶手椅形碳纳米管(ABT(n))的Peierls相变温度的一般表达式,并利用该式计算了ABT(8)和ABT(10)的Peierls相变温度,结果表明ABT(n)的Peierls相变温度将随管径的增大而减小,并且ABT(n)在远离室温下都不会发生Peierls相变,仍然保持其金属性. 关键词：     

First-order calculation of the phonon propagator of a quasi-one-dimensional electron-phonon system

Using Green function technics the phonon propagator is calculated consistently in the first order with respect to both the square of the electron-phonon interaction matrix element and the matrix element of the Coulomb interaction of the electronic subsystem. It is shown that the so-called self-energy and exchange contributions lead to an increase of the Peierls temperatureT _p in comparison with the result of the usual random-phase approximation (RPA), whereas the Umklapp processes tend to lowerT _p .     

Superconductivity and Peierls instability in coupled linear chain systems

We study the superconducting transition temperature (T_c) and the Peierls instability temperature (T_p) using Eliashberg type equations for both T_c and T_p self consistently with finite interchain coupling. We show that T_c > T_p below a critical electron-phonon coupling constant which depends on the bare phonon frequency. This determines an upper bound on T_c so that for higher transition temperatures T_p > T_c and superconductivity is unlikely. Higher values of T_c are possible if the interchain coupling is increased above a critical value where the Peierls instability is suppressed.     

Fluctuation conductivity near a Peierls lattice distortion

There have been a number of recent theories for the fluctuation conductivity due to phonon softening near a one-dimensional Peierls lattice distortion. In these, the conductivity is evaluated for one or more Feynman response diagrams, with a resulting fluctuation conductivity which is positive, negative, or negligible. We calculate the conductivity using a conserving response theory which derives all of the response diagrams for any particular approximation to the electron propagator for the solid in thermodynamic equilibrium. Using this method in the mean-field theories to date, valid in principle for sufficiently high temperatures, the fluctuation conductivity due to phonon softening is positive and proportional to ($\text{T - T}{}_{\mathrm{p}}\text{)}{}^{\mathrm{<mtext>-</mtext><mtext>3</mtext><mtext>2</mtext>}}$, where T_p is the mean-field temperature scale in the one-dimensional Peierls system. This conductivity contribution is not due to the Fröhlich sliding mode.     

Defect induced fluctuations in the paraelectric phase of KTa0.94Nb0.06O3

First order Raman lines observed in the paraelectric phase of KTa_0.94Nb_0.06O₃ are shown to be the result of odd symmetry microdistortions or fluctuations with a correlation length that extends to several unit cells. The possibilities that the lines result from impurity induced or disorder induced Raman scattering are considered and discarded. From simple theoretical considerations it is shown that the integrated intensity of distortion induced first order Raman lines, normalized by the corresponding phonon occupation numbers, is proportional to the average distortion intensity. This proportionality is independent of the correlation length of the distortion field provided it is larger than a certain minimum.Using this relation and the experimentally measured integrated intensity of the TO₂ line we find that the average distortion intensity is proportional toT/(T _c—T). This result is compared to two models which qualitatively predict the existence of microdistortions or fluctuations in the paraelectric phase. The experimentally observed temperature dependence is found to be consistent with the defect induced fluctuations model and inconsistent with continuous phase transitions models.     

Conical emission: pT and system dependences and the first result with identified particles from STAR

We present the analysis of three-particle correlation with respect to reaction system, transverse momentum (p _T ) of associated and trigger particles measured by STAR experiment at RHIC. Our preliminary results indicate presence of conical emission of charge particles correlated with high p _T trigger particles in central A+A collisions. The measured conical emission angle is independent of p _T of associate hadrons, possibly consistent with Mach cone shock wave mechanism. The first result on PID three-particle correlation with identified associated particles is also reported.     

A New Type of Charge-Density-Wave Pinning in Orthorhombic TaS3 Crystals with Quenching Defects

Diminishing in the concentration of quenching defects during thermocycling of orthorhombic TaS₃ samples in the temperature range below the Peierls transition temperature T < T_P is observed. It makes it possible to study the character of pinning of the charge density wave by these defects. A number of fundamental differences from pinning by ordinary local pinning centers—impurities and point defects—have been found. We conclude that quenching defects are extended (non-local) objects (presumably, dislocations) that can diffuse from the crystal during low-temperature thermocycling due to their strong interaction with the charge density wave, which is intrinsic for Peierls conductors. The presence of these defects leads to a previously unknown non-local type of the charge-density-wave pinning that acts on T_P and the threshold field for the onset of charge-density-wave sliding, E_T, differently in comparison with the local pinning centers.

     

Specific heat studies of the spin-peierls transition

An experimental study is presented of the specific heat of the spin—Peierls systems TTF-TBT(M), M = Cu, Au. The linear term, γT, expected for the uniform one-dimensional antiferromagnet at low temperatures is observed above the Peierls transition. Specific heat anomalies associated with the transition are found at T_p(Cu) = 12.4K and T_p(Au) = 2.06K. The transition for TTF-BDT(Cu) is mean-field like; whereas the transition for TTF-BDT(Au) is broadened by one-dimensional fluctuation effects.     

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.     

First result of net-charge jet-correlations from STAR

We presented results on azimuthal correlation of net-charge with high p _T trigger particles. It is found that the net-charge correlation shape is similar to that of total-charge. On the near-side, the net-charge and total-charge p _T spectra have similar shape and both are harder than the inclusives. On the away-side, the correlated spectra are not much harder than the inclusives, and the net-charge/total-charge ratio increases with p _T and is similar to the inclusive ratio.     

Critical behaviour of Raman frequency shifts for translational modes near the melting point of ammonia solid I based on Pippard relations

A correlation between thermal expansivity α _p and frequency shifts (1/ν) (?ν/?P) _T was established for the ammonia solid I near its melting point. For this correlation, calculated frequencies for the translational modes of ν _T (100?cm^?1) and ν _T (130?cm^?1) were used to acquire the Raman frequency shifts at pressures of 0, 1.93 and 3.07?kbar in ammonia solid I. In this crystalline system, values of the slope dP_m /dT were deduced for the fixed pressures from linear plots of α _p vs. (1/ν) (?ν/?P) _T . Slope values at zero pressure are in very good agreement with those obtained experimentally. It is also shown that the spectroscopic modification of the Pippard relations applies satisfactorily to ammonia solid I.     

Energy spectrum of excitations in quasi-one-dimensional conductors with a charge-density wave

A well-formed energy gap Δ is observed in the energy spectrum of the quasi-one-dimensional orthorhombic conductor TaS₃ at temperatures much lower than the Peierls transition temperature T _P . As the temperature increases, in the region T>T _P /2 there is a growth of the density of states in the gap and a relative decrease of the density at energies greater than Δ. In addition, absorption lines which probably correspond to soliton states in a charge-density wave are observed in the gap. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 4, 246–250 (25 February 1996)     

Phase diagram for three-dimensional correlated site-bond percolation

The Coniglio-Stanley-Klein model is a random bond percolation process between the occupied sites of a lattice gas in thermal equilibrium. Our Monte Carlo simulation for 40³ and 60³ simple cubic lattices determines at which bond thresholdp _Bc , as a function of temperatureT and concentrationx of occupied sites, an infinite network of active bonds connects occupied sites. The curvesp _Bc (x, T) depend only slightly onT whereas they cross over if plotted as a function of the field conjugate tox. Except close toT _c we find 1/p _Bc to be approximated well by a linear function ofx, in the whole interval between the thresholdx _c (T) of interacting site percolation atp _Bc =1 and the random bond percolation limitx=1 atp _Bc =0.248±0.001. Thisx _c (T) varied between 0.22 forT=0.96 (coexistence curve) and 0.3117±0.0003 forT= (random site percolation). At the critical point (T=T andx=1/2) we confirmed quite accurately the predictionp _Bc =1-exp(–2J/k _B T _c ) of Coniglio and Klein. As a byproduct we found 0.89±0.01 for the critical exponent of the correlation length in random percolation.     

How to make sense of the jet correlations results at RHIC?

We review the di-hadron correlation results from RHIC. A consistent physical picture was constructed based on the correlation landscape in p _T , Δφ, Δη and particle species. We show that the data are consistent with competition between fragmentation of survived jets and response of the medium to quenched jets. At intermediate p _T where the medium response are important, a large fraction of trigger hadrons do not come from jet fragmentation. We argue that these hadrons can strongly influence the interpretation of the low p _T correlation data. We demonstrate this point through a simple geometrical jet absorption model simulation. The model shows that the correlation between medium response hadrons dominates the pair yield and mimics the double hump structure of the away-side Δφ distribution at low p _T . This correlation was also shown to lead to complications in interpreting the results on reaction plane dependence and three particle correlations. Finally, we briefly discuss several related experimental issues which are important for proper interpretations of the experimental data.     

Electric field dependent dielectric function in NbSe3

Dielectric function of quasi-one-dimensional conductor NbSe₃ has been measued in the vicinity of Peierls transition near T_p=145K by use of novel technique. The low frequency noise in V/I/ characteristics is attributed to the Gaussian type of position fluctuations of pinned charge density wave (CDW) and fundamental frequency of the noise is found to be identical with spring frequency of restoring forces.     

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.     

The Peierls instability in one-dimensional conductors with arbitrary bandfilling

Mean field calculations of the Peierls instability have been carried out for arbitrary bandfilling both in the case of free electrons as well as for the tightbinding approximation. New expressions for Δ(0) and T_p are derived which change the experimentally derived λ to a more reasonable value. Δ(0)/k_BT_p may differ significantly from the BCS value 1.76 in the tightbinding approximation.     

Vortex pinning in YBa2Cu3O7?x films

Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa₂Cu₃O_7−x (YBCO) films is provided by complex impedance measurements at temperature 8 K<T<T _c and magnetic field 0<B<6 T. Below the vortex lattice melting transition B_g(T) but above a threshold field B_p≈8(1-T/T _c ) T, the inductance of vortices increases as B², much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region B_g(T<B<B^*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ^*≡ξ(B=B^*)≈450?. The value of ξ^* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at B_g.     

DC Conductivity and Peierls instability in the quasi-one-dimensional organic charge density wave conductor (fluoranthene)2X

Due to the high anisotropy of the dc conductivity (σ_|/σ_⊥ ≈ 10⁴) the organic conductor (fluoranthene)₂X can be regarded as a model system for studying the Peierls instability in quasi-one-dimensional systems. The temperature dependence of the dc conductivity σ_| (T) along the highly conducting crystal axis exhibits the typical behaviour of a quasi-one-dimensional metal with a Peierls transition at about 180 K to a charge density wave (CDW) ground state. As expected for a highly one-dimensional conductor the exact transition temperature depends on three-dimensional coupling effects and therefore on the size of the counterion X^? = PF, AsF, SbF. Above the Peierls transition σ_| (T) can be described quantitatively within a model of CDW fluctuations leading to a pseudo gap in the electronic density of states. Below, the existence of a real energy gap at the Fermi level with a BCS-like temperature dependence determines the charge transport over more than eight orders of magnitude in the electrical resistance. For the intrinsic energy gaps 2 Δ (0), which characterize the ground state of the Peierls semiconductor, values of 120-180 meV have been found for different crystals.