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We summarize structural instabilities exhibited by the one dimensional (1D) (arene)2X family of organic conductors in relation with their electronic and magnetic properties. With a charge transfer of one electron to each anion X these salts exhibit a quarter-filled (hole) conduction band located on the donor stacks. Compounds built with donors such as fluorenthene and perylene derivatives and anions X such PF6 or AsF6 exhibit a high temperature (TP~170K) conventional Peierls transition which is announced by a sizeable regime of 1D 2kF charge density wave fluctuations (kF is the Fermi wave vector of the 1D electron gas located on Per stacks). Surprisingly, and probably because of the presence of a multi-sheet warped Fermi surface, the Peierls transition is considerably reduced in the perylene series α-(Per)2[M(mnt)2] where X is the dithiolate molecule with M=Au, Cu, Co and Fe. A special attention is devoted in this paper to physical properties of α-(Per)2[M(mnt)2] salts which with M=Pt, Pd and Ni incorporate segregated S=1/2 1D antiferromagnetic (AF) dithiolate stacks with 1D metallic Per stacks. We analyse conjointly the structural and magnetic properties of these salts in relation with the 1D spin-Peierls (SP) instability located on the dithiolate stacks. We show that the SP instability of the Pd and Ni derivatives occurs in the classical (adiabatic limit) while the SP instability of the Pt derivative occurs in the quantum (anti-adiabatic limit). Furthermore we show that in the Pd and Ni derivatives frustrated 1st neighbour direct and 2nd neighbour indirect (through a fine tuning with the mediated 2kF RKKY coupling interaction on Per stacks) AF interactions add their contribution to the SP instability to open a singlet-triplet gap. Our analysis of the data show unambiguously that magnetic α-(Per)2[M(mnt)2] salts are a typical realization of the physics predicted for two chain 1D Kondo lattices.

Tek katmanlı bükülmüş boron tabakaları (borofen) ve bunun diğer polimorf yapıları, iki boyutlu sistemlerin bilimsel çalışmalarında büyük ilgi görmüştür. Bu tez çalışmasında, değişken yapısal davranışından dolayı incelediğimiz borofen malzemesinin farklı polimorflarını analiz ettik ve iki yeni kristal yapı fazını ön gördük. Simetrik ve asimetrik kırışık (washboard) olarak adlandırdığımzbu yapıları yoğunluk fonksiyonel teorisine dayanan ab initio yöntemlerini kullanarak analiz ettik. Simetrik kırışık borofen, Fermi seviyesi yakınında yüksek elektronik yoğunluklu bir metal gibi davranırken ve asimetrik kırışık borofen dar bant aralıklı bir yarı iletken ozellikleri göstermektedir. Asimetrik kırışık yapı aslında simetrik kırışık yapının, bor atomlarının zincirleri boyunca Peierls bozulması ile 2x1 birim hücre olarak yeniden şekillendiği bir fazdır. Bu husus,zot elektronik davranoşlar gösteren fazlara sebep olmuştur. Yoğunluk fonksiyonel perürbasyon teorisine dayanan fonon dağılım hesaplamaları her iki yapının 0 K'de stabil olduğunu ortaya koymaktadır. Ancak ab initio moleküler dinamik simülasyonları, simetrik kırışık geometrinin sadece mutlak sıfıra yakın sıcaklıklarda stabil olduğgunu göstermiştir. Normal sıcaklıklarda bu yapı deforme olarak asimetrik kırışık yapısına dönüşür. Ayrıca asimetrik kırışık yapı pozitifbir Poisson sabitine sahipken, simetrik yapının negatif Poisson özelliği gösterdiğibulunmuştur. Ayrıca bu çalışmada, bükülmüş borofenin elektron verme eğilimi göz önünde bulundurularak tek halojen atomlarının (F, Cl, Br, I) borofen ile etkileşimi analiz edildi. Olası soğurma pozisyonları test edildiğinde bor atomunun üst noktası temel durum konfigürasyonu olarak tespit edilmiştir. Bağlanma ve güclü kimyasal etkileşimin doğası, tahmini durum yoğunluğu ve yük farkı analizleri yapılarak belirlenmiştir. Tek halojen atomunun borofenin üst yüzeyi boyunca hareketi analiz edilmiş ve değişken ama yüksek difüzyon bariyerleri elde edilmiştir. Borofenin metalik özelliği soğurma sebebiyle korunurken elektriksel iletkenlik anizotropisi değişmektedir. Soğurma ve oluşum enerjisinin, atomlar arası mesafenin, yük transferinin, difüzyon bariyerleri ve bağlanma karakterinin halojen atomuna göre değişimi araştırılarak eğilimler ortaya konmuştur. Sonolarak, halojen moleküllerinin (F2, Cl2, Br2, I2 ) muhtemel ayrışmayı içeren soğurması incelenmiştir. Elde edilen sonuçlar, borofen fazlarının ve halojenli türevleri hakkındaki temel bilginin edinilmesi ve bu malzemelerin muhtemel yüksek teknoloji cihaz uygulamaları için önemlidir. The realization of buckled monolayer sheets of boron (i.e., borophene) and its other polymorphs has attracted significant interest in the field of two-dimensional systems. Motivated by their chameleonic behavior we analyzed different polymorphs of borophene and discovered two new phases with unprecedented crystal structures namely symmetric washboard and asymmetric washboard using abinitio methods based on density functional theory. While symmetric washboard borophene is a metal with high electronic density of states in the vicinity of Fermi level asymmetric washboard borophene is a narrow band gap semiconductor. Asymmetric washboard structure is actually a 2x1 reconstructed form of symmetric structure with in plane and out of plane Peierl's distortion along the chains of boron atoms which is the key reason for the contrasting electronic behavior of these phases. Phonons dispersion calculations based on density functionalperturbation theory reveal that both structures are stable at 0 K however ab initio molecular dynamics simulations showed that symmetric washboard structure is stable only at temperatures close to absolute zero and at finite temperatures this structure gets deformed transforming into asymmetric washboard structure. Moreover we discovered that asymmetric washboard structure has a positive Poissons's ratio however symmetric one has a negative Poisson's ratio. In the next work, motivated by buckled borophene's tendency to donate electrons, we analyzed the interaction of single halogen atoms (F, Cl, Br, I) with borophene. Thepossible adsorption sites are tested and the top of the boron atom is found as the ground state configuration. The nature of bonding and strong chemical interaction is revealed by using projected density of states and charge difference analysis. The migration of single halogen atoms on the surface of borophene is analyzed and high diffusion barriers that decrease with atomic size are obtained. The metallicity of borophene is preserved upon adsorption but anisotropy in electricalconductivity is altered. The variation of adsorption and formation energy, interatomic distance, charge transfer, diffusion barriers, and bonding character with the type of halogen atom are explored and trends are revealed. Lastly, the adsorption of halogen molecules (F2, Cl2, Br2, I2), including the possibility of dissociation, is studied. The obtained results are substantial for fundamental understanding and possible device implementations of borophenes and their halogenated derivatives. 65

We extend the continuous-time interaction-expansion quantum Monte Carlo method with respect to measuring observables for fermion-boson lattice models. Using generating functionals, we express expectation values involving boson operators, which are not directly accessible because simulations are done in terms of a purely fermionic action, as integrals over fermionic correlation functions. We also demonstrate that certain observables can be inferred directly from the vertex distribution, and present efficient estimators for the total energy and the phonon propagator of the Holstein model. Furthermore, we generalize the covariance estimator of the fidelity susceptibility, an unbiased diagnostic for phase transitions, to the case of retarded interactions. The new estimators are applied to half-filled spinless and spinful Holstein models in one dimension. The observed renormalization of the phonon mode across the Peierls transition in the spinless model suggests a soft-mode transition in the adiabatic regime. The critical point is associated with a minimum in the phonon kinetic energy and a maximum in the fidelity susceptibility., 12 pages, 4 figures, 1 table; final version

We use the density matrix renormalization group method to investigate the role of longitudinal quantized phonons on the Peierls transition in the spin-Peierls model. For both the $XY$ and Heisenberg spin-Peierls model we show that the staggered phonon order parameter scales as $\sqrt{\ensuremath{\lambda}}$ (and the dimerized bond order scales as $\ensuremath{\lambda}$) as $\ensuremath{\lambda}\ensuremath{\rightarrow}0$ (where $\ensuremath{\lambda}$ is the electron-phonon interaction). This result is true for both linear and cyclic chains. Thus, we conclude that the Peierls transition occurs at $\ensuremath{\lambda}=0$ in these models. Moreover, for the $XY$ spin-Peierls model we show that the quantum predictions for the bond order follow the classical prediction as a function of inverse chain size for small $\ensuremath{\lambda}$. We therefore conclude that the zero $\ensuremath{\lambda}$ phase transition is of the mean-field type.

The broad band spectrum of the optical conductivity of the quasi-one dimensional organic conductor (FA) 2 PF 6 (FA = fluoranthene) has been investigated in a wide temperature range including the Peierls transition temperature T p = 180 K. A semiconducting gap has been observed both below and above T p varying between 150 meV at 10 K and 80 meV at 300 K. The (T p − T) 1 4 temperature dependence for the order parameter has been deduced from an analysis of the integrated band intensity of phase phonons thus giving evidence that the Peierls transition is a 2nd order phase transition near the tricritical point.

The density matrix renormalization group method is used to investigate the spin-Peierls transition for Heisenberg spins coupled to quantized phonons. We use a phonon spectrum that interpolates between a gapped, dispersionless (Einstein) limit to a gapless, dispersive (Debye) limit. A variety of theoretical probes are used to determine the quantum phase transition, including energy gap crossing, a finite size scaling analysis, bond order auto-correlation functions, and bipartite quantum entanglement. All these probes indicate that in the antiadiabatic phonon limit a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type is observed at a non-zero spin-phonon coupling, $g_{\text c}$. An extrapolation from the Einstein limit to the Debye limit is accompanied by an increase in $g_{\text c}$ for a fixed optical ($q=\pi $) phonon gap. We therefore conclude that the dimerized ground state is more unstable with respect to Debye phonons, with the introduction of phonon dispersion renormalizing the effective spin-lattice coupling for the Peierls-active mode. We also show that the staggered spin-spin and phonon displacement order parameters are unreliable means of determining the transition., Comment: To be published in Phys. Rev. B

Starting from a Su-Schrieffer-Heeger-like model inferred from first-principles simulations, we show that the metal-insulator transition in In/Si(111) is a first-order grand canonical Peierls transition in which the substrate acts as an electron reservoir for the wires. This model explains naturally the existence of a metastable metallic phase over a wide temperature range below the critical temperature and the sensitivity of the transition to doping. Raman scattering experiments corroborate the softening of the two Peierls deformation modes close to the transition.

Detailed structural and magnetotransport properties of monophosphate tungsten bronze ${\text{K}}_{x}({\text{PO}}_{2}){}_{4}({\text{WO}}_{3}){}_{8}$ single crystals are reported. Both galvanomagnetic and thermal properties are shown to be consistent with a charge density wave electronic transition due to hidden nesting of the quasi-1D portion of the Fermi surface. We also observe the enhancement of electronic anisotropy due to reconstruction of the Fermi surface at the Peierls transition. The resistivity presents a thermal hysteresis suggesting a first-order nature characteristic of a strong-coupling scenario. However, other measurements such as the change of carrier density demonstrate a second-order Peierls scenario with weak-coupling features. We suggest that the structural transition driven by the residual strain in the K-P-O environment is responsible for the resistivity hysteresis and modifies the Fermi surface which then helps the rise to the second-order Peierls instability.

Antimony is an interesting elemental crystal because, in its ground state, it is stabilized by a Peierls distortion. Here we perform density-functional-theory molecular dynamics simulations of this intriguing material before and after femtosecond-laser excitation using a simulation box with N = 864 atoms and periodic boundary conditions, where the atoms are treated in the Γ-point approximation and the electrons are integrated over 8 k points. After an appropriate initialization of the atoms in the harmonic approximation we thermalize our system during 20 picoseconds. Then an intense femtosecond-laser excitation is simulated by instantaneously raising the electronic temperature to 8000 Kelvin. Our results show a laser-induced anti-Peierls transition.

We report synthesis of quasi-one-dimensional conductor NbS 3 , TEM studies and transport properties ofthe prepared samples. The grown NbS 3 whiskers are Peierls conductors known as phase II with thetransitions at T P1 ¼365 K and T P2 ¼150 K. CDW1 is stable and not so sensitive to growth conditions. Itcan slide and be synchronized by the external microwave irradiation up to 16 GHz. CDW2 stronglydepends on growth conditions. Nevertheless, it also shows sliding and synchronization.& 2012 Elsevier B.V. All rights reserved. 1. IntroductionNbS 3 , phase II, is standing out for its extremely high Peierlstransition temperature, T P ¼ 330–355 K [1]. Studies of this phasewere complicated because of the difficulties with its synthesis.The number of NbS 3 polytypes is not quite clear yet. The mostreproducible in synthesis is phase I (triclinic), high-ohmic at all T;it does not show clear evidence for CDW transport. Phase II ismonoclinic with 8Nb chains per unit cell and shows approximatetrimerisation below T

In this paper, we give an example how domain walls (DW) within an adsorbed monolayer form one-dimensional localized electronic states. We investigated in detail the Ag 3 × 3 phase on Si (111), a prototype system for a low dimensional electron gas, by means of scanning tunneling microscopy. The doubling of the periodicity along the DW-direction at low temperatures (80 K) suggests a metal–insulator transition of Peierls type. The superimposed intensity modulation in the direction across the DW can be interpreted in terms of lateral quantum well states. However, the simultaneous vanishing of both features at room temperature indicates more complicated changes in band structure and/or wave functions during the transition than described by the simple Peierls model.

Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USAReceived 20 July 2011, revised 16 September 2011, accepted 26 September 2011Published online 28 December 2011Dedicated to Thomas Frauenheim on the occasion of his 60th birthdayKeywords charge-density wave, electron transport, entropy, in adsorption, nanowire, Peierls transition, Si(111) surface

The melting curve of cI16 sodium at high pressure is investigated extensively by means of Lindemann melting criterion and ab initio molecular dynamics simulations. Different from previous theoretical results, our results show that the melting point of cI16 sodium decreases with increasing pressure. It is found that the elastic constant softening plays an important role and results in a negative-slope Lindemann melting curve of cI16 sodium. The melting mechanism is different from that of f.c.c. sodium reported in previous work, that the Peierls-like liquid transition is the origin of negative-slope melting curve. Electronic structure analysis shows that more electrons in liquid sodium localize in the voids of lattice just like in its solid state, the exclusionary effect of atomic cores on valence electrons in the liquid still takes effect.

We studied the insulator-metal transition (IMT) in single-domain, single crystalline vanadium dioxide (VO(2)) microbeams with infrared microspectroscopy. The unique nature of such samples allowed us to probe the intrinsic behavior of both insulating and metallic phases in the close vicinity of IMT, and investigate the IMT driven by either strain or temperature independently. We found that the VO(2) insulating band gap narrows rapidly upon heating, and the infrared response undergoes an abrupt transition at both strain- and temperature-induced IMT. The results are consistent with recent studies attributing the opening of VO(2) insulating band gap to a correlation-assisted Peierls transition.

Structural, energetic, and electronic properties of one-dimensional armchair and zigzag Bi nanoribbons (A- and ZBNRs) are systematically investigated within the framework of density functional theory. The presence of edge reconstruction greatly enhances the structural stability of both A- and ZBNRs. ABNRs are indirect band gap semiconductors and their indirect band gaps exhibit width-dependent odd−even oscillation, while the direct band gaps at the Gamma point monotonically decrease with increasing the ribbon width due to quantum confinement effect. The Peierls transition in the ZBNRs turns the ZBNRs from metals into semiconductors accompanying dimerized Bi atoms at both edges and stabilizing the ribbons.

The lattice dynamics in the CuIr2S4 system have been investigated through Raman spectroscopy and the induced-pressure effect. Four Raman active modes are observed experimentally. Upon cooling, these Raman spectra undergo changes due to the Peierls-like phase transition. In addition, the substitution of Ag for Cu affects the amplitude of the Raman spectra while keeping their wave number unchanged. Furthermore, the positive and negative pressure effects are induced by shrinking and expanding the lattice. It is suggested that the pressure effect is an effective proof of the orbital-induced Peierls state mechanism.

A study of electron spin resonance in uncompensated Ge:As semiconductor samples in the vicinity of the insulator-metal second-order phase transiti on reveals that the interaction of spins localized at As atoms brings about a distortion of the crystal lattice and enhances the localization. This effect occurs in the range of electron concentrations n = 3 × 1017—3.7 × 1017 cm-3, just below the critical point of the phase transition. The effect is explained in the context of a model considering the spin-Peierls transition in the random impurity sublattice of the semiconductor, and its features, as compared to other known materials where the spin-Peierls transition is observed, are understood.

We investigate the ground-state properties of polyacene in terms of an extended Peierls-Hubbard Hamiltonian with particular emphasis on its structural instability of two types: double bonds in a cis pattern and those in a trans pattern. The two Peierls-distorted states are degenerate in their energetics but quite distinct in their optics. The trans configuration is easily photoconverted into the cis one, whereas an opposite transition, if any, is hardly completed. Domain-wall (soliton) excitations play an important role in any photoinduced phase transitions, but charged and neutral ones behave quite differently.

We have numerically studied the phonon softening in a triangular lattice system with Su–Schrieffer–Heeger's (SSH) type interaction and with a three-quarter-filled electronic band. When the temperature is lowered, the simultaneous softening of multiple phonon modes is observed at a critical temperature. These softened modes have momenta parallel to the vectors connecting neighboring vertices of the hexagonal structure of the Fermi surface, being irrelevant to nesting vectors. Therefore, this phenomenon that occurs when the temperature is lowered is not a conventional Peierls transition. It is found that the Fermi surface composed of straight lines is a necessary condition for the simultaneous phonon softening. Multi-mode states are realized below the critical temperature. However, all of the softened modes are not necessarily condensed at the lowest free-energy state.

Giant nonlinear conduction in the charge ordered organic conductor θ ‐ ( BEDT ‐ TTF ) 2 MM ′ ( SCN ) 4 ( M = Cs , Rb; M ′ = Co , Zn) is analyzed in relation to the current-induced suppression of the charge ordered state characterized by a wave vector of q 2 = ( 0 k 1 2 ) . On the basis of a theory of the nonequilibrium Peierls transition, the energy gap is calculated as a function of current density J ext , which is found to depend weakly on temperature in an intermediate temperature range. This justifies the evaluation of the charge-order energy gap Δ from the nonlinear resistivity ρ assuming ρ ∝ exp [ Δ ( J ext ) / k B T ] , and Δ ( J ext ) is found to be consistent with the calculation. We compare the present results with nonequilibrium superconducting states induced by excess quasiparticles, and discuss a mechanism of current-induced suppression of the charge ordering in the title compound.

The 2D Peierls transition in an anisotropic triangular lattice is studied from the view point of phonon softening. The model is obtained by introducing a coupling in one of the diagonal directions to the isotropic square lattice, for which the multimode Peierls phase, involving not only the nesting vector component but also other Fourier components of distortion with wave vectors parallel to the nesting vector, is predicted. The results indicate that the multimode type Peierls transition can be expected as far as the diagonal coupling is weak compared to the nearest neighbor coupling in the square lattice.

The effect of an elastic spontaneous distortion of the crystal lattice of a doped semiconductor Ge:As near the insulator–metal (IM) phase transition has been discovered. The effect is manifested in the electron spin resonance (ESR) of neutral As atoms as a splitting of the single resonance absorption line. It observed at electron concentrations in the range 0.8 < n/nC < 1 at low temperatures T < 100 K (nC = 3.7 × 1017 cm-3 is the critical electron concentration for the IM phase transition). The splitting is the strongest along each of the six [110] directions, which indicates that the local lattice distortion occurs just in these directions. As a result, a sample is possibly divided into separate domains differing in the directions of compressive or tensile deformations. A study of concentration, temperature, and angular dependences of the effect has shown that the phenomenon discovered can be understood in terms of the Peierls spin transition model.

The ultrafast terahertz response to the photoexcitation in vanadium dioxide was investigated using the optical-pump terahertz-probe technique at room temperature. The optical excitation at 790 nm induced an ultrafast decrease of the transmittance of the terahertz pulses corresponding to the increase of the electronic conductivity within 0.7 ps, and then the transmittance decreases gradually up to 100 ps. This two-step behavior is very similar to the previous reports of the time resolved X-ray and electron diffractions. This fact indicates that the increase of the electronic conductivity and the change of the lattice structure proceed in parallel, and the photo-induced insulator–metal phase transition is of the Peierls type.

The orbit-dependent hopping of the 5 d shell electrons of Ir 4 + ions is used to explain the Peierls-like phase transition in the sulphide CuIr 2 S 4 system. This orbit-induced Peierls state can be suppressed by the substitution of Co ions with smaller splitting energy Δ E d γ - d ɛ at the center of BS 6 octahedron for Ir ions, and the metallic state is enhanced due to the increase of itinerant electrons with the increase of Co content in CuIr 2 - x Co x S 4 system. The specific heat for CuIr 2 - x Co x S 4 system is also investigated. The contribution of the electron specific heat becomes important at low temperatures with the increase of Co content, and the C p / T – T 2 relation at low temperatures deviates from the straight line with the increase of Co content, which is due to the scattering of the electrons by the magnetic moments at low temperatures.

An anion-radical salt (ARS) (N-Me-2,5-di-Me-Pz)(TCNQ) 2 , where Pz is pyrazine, was synthesized and its crystal structure was resolved. X-ray diffraction experiments on single crystals were performed. Heat capacity was measured in the temperature range from 2 to 300 K. Magnetisation and magnetic susceptibility were measured in the temperature range from 2 to 300 K and the low-temperature part was measured in magnetic fields from 5 mT to 5 T. The experimental results were explained in terms of dimerized Heisenberg spin 1 2 chain model. Numerical calculations were performed and compared with experimental data.

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH 4 -TCNQ have been measured at temperatures in the 12–350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH 4 -TCNQ. The enthalpy and entropy of transition are Δ trs H =(667±7) J mol −1 and Δ trs S =(2.19±0.02) J K −1 mol −1 , respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin ( S =1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as Δ trs S mag =0.61 J K −1 mol −1 and the lattice contribution as Δ trs S lat =1.58 J K −1 mol −1 . Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K −1 mol −1 ), a majority of the magnetic entropy (∼4.6 J K −1 mol −1 ) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.

The concepts spin density wave, charge density wave, and Peierls transition are reviewed with reference to various theoretical models and to their relevance for the interpretation of experimental data.

The partial oxidation of [Ni(II/II)(2)(RCS(2))(4)] (R = Et (1), n-Pr (2), and n-Bu (3)) with iodine affords the MMX chain compounds [Ni(II/III)(2)(RCS(2))(4)I](infinity) (R = Et (4), n-Pr (5), and n-Bu (6)), respectively. The crystal structures of 4-6 consist of neutral one-dimensional (1-D) chains with a repeating -Ni-Ni-I- unit. The room-temperature (RT) structure of 4 indicates a charge-polarization (CP) state {-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)- (delta << 0.5)} close to an averaged valence state judged by the Ni-I distances. In contrast, 5 and 6 exhibit a 3-fold periodicity of a -Ni-Ni-I- unit due to the disorder of the dithiocarboxylato ligands. Compounds 4-6 show typical semiconducting behavior and exhibit an intense sharp absorption band centered at 5400 cm(-1), which is attributed to a Mott-Hubbard gap due to a relatively large on-site Coulomb repulsion energy U of the nickel atoms. The high-temperature magnetic susceptibilities of 4-6 can be described by a 1-D Heisenberg antiferromagnetic chain model with |J|/k(B) ranging from 898(2) to 939(3) K. Compounds 4 and 5 undergo a spin-Peierls (SP) transition at relatively high T(sp) = 47 and 36 K, respectively, which are accompanied by superlattice reflections corresponding to a 2-fold -Ni-Ni-I- period below T(sp). By determining the superstructure of 4 at 26 K, we conclude that the valence-ordered state changes from the CP in the RT phase to the alternate charge-polarization (ACP) state of -Ni((2.5-delta)+)-Ni((2.5+delta)+)-I(-)-Ni((2.5+delta)+)-Ni((2.5-delta)+)-I(-)- in the SP phase. Such a spin-Peierls transition could not be observed for 6.

We have fabricated nanocomposites consisting of 4-Å carbon nanotubes embedded in the 0.7-nm pores of aluminophosphate-five (AFI) zeolite that display a superconducting specific heat transition at 15 K. MicroRaman spectra of the samples show strong and spatially uniform radial breathing mode (RBM) signals at 510 cm −1 and 550 cm −1 , characteristic of the (4, 2) and (5, 0) nanotubes, respectively. The specific heat transition is suppressed at >2 T, with a temperature dependence characteristic of finite-size effects. Comparison with theory shows the behavior to be consistent with that of a type II BCS superconductor, characterized by a coherence length of 14 ± 2 nm and a magnetic penetration length of 1.5 ± 0.7 μm. Four probe and differential resistance measurements have also indicated a superconducting transition initiating at 15 K, but the magnetoresistance data indicate the superconducting network to be inhomogeneous, with a component being susceptible to magnetic fields below 3 T and other parts capable of withstanding a magnetic field of 5 T or beyond.