Quasi One-dimensional Conductor Research Articles

Volume dependence of the charge density wave phase transitions

ABSTRACT We analyse the volume dependence of the charge density wave phase transition temperature T CDW for 16 one and two dimensional materials such as the rare earth tritellurides TbTe 3 , ErTe 3 , TmTe 3 DyTe 3 , the transition metal dichalcogenide 2H-NbSe 2 , and the quasi-one dimensional conductors K 0.3 MoO 3 , NbSe 3 , ( TaSe 4 ) 2 I . The volume dependence is taken in the form of the logarithmic derivative of T CDW in function of the strain components. We point out some characteristics of this parameter. This parameter is dimensionless with an absolute value of about 10 obtained from the measurements of the thermal expansion, the longitudinal elastic constants and pressure dependence for 11 one- and two-dimensional CDW systems. In contrast a huge positive value of about 100 is found at the lower charge density wave transition in TbTe 3 , ErTe 3 , TmTe 3 and DyTe 3 .

Temperature variation of photoconductivity in the layered quasi one-dimensional compound TiS3: Semiconducting and unconventional behavior

The layered quasi one-dimensional compound TiS3 is remarkable for its optical properties, especially, photoconductivity. Up to now, photoconductivity in TiS3 was studied only at room temperature. Here, we report photoconductivity, δσ, of the TiS3 whiskers in the temperature range of 5–300 K under irradiation in the near-infrared region. With a temperature decrease from room temperature down to T ∼ 100 K, δσ grows moderately, dominated by the variation of the mobility of the electrons. The estimates based on the values of δσ give the recombination time ∼3–25 μs at 300 K. The results show that above ∼100 K, TiS3 behaves as a semiconductor, where electron–hole pairs are excited across the gap, while the recombination goes through transitions of the excited electrons to a donor level with a long lifetime. Below 60 K, a drop of δσ is likely to reveal a phase transition of electrons into a collective state. Another feature in δσ(T) is seen around 17 K, resembling the temperature variation of the conductivity. This feature indicates a drop in the mobility of the electrons. While the behavior of δσ(T) is consistent with the transition of electrons into a collective state, it is strikingly different from δσ(T) of the known quasi one-dimensional conductors with charge-density waves.

Effect of lattice mismatch on film morphology of the quasi-one dimensional conductor K0.3MoO3.

High quality epitaxial thin films of the quasi-one dimensional conductor K0.3MoO3 have been successfully grown on SrTiO3(100), SrTiO3(110), and SrTiO3(510) substrates via pulsed laser deposition. Scanning electron microscopy revealed quasi-one dimensional rod-shaped structures parallel to the substrate surface, and the crystal structure was verified by using X-ray diffraction. The temperature dependence of the resistivity for the K0.3MoO3 thin films demonstrates a metal-to-semiconductor transition at about 180 K. Highly anisotropic resistivity was also observed for films grown on SrTiO3(510).

Mechanically-induced Shapiro steps: the effect of vibration on the dynamics of a charge density wave

Problem formulating. The quasi one-dimensional conductor TaS3 of the orthorhombic modification shows strong electromechanical coupling of the sliding charge density wave (CDW) with the lattice of crystals. However, until now the effect of the dynamic deformation of the sample on the CDW sliding has not been studied. Goal. Search and study of a new effect: the influence of sample vibration on CDW dynamics. Result. The conductivity of TaS3 samples is studied as a function of the electric field and the frequency of mechanical deformation of the sample. It is found that when the vibration frequency of the piezoelectric actuators deforming the sample coincides with one of the mechanical resonances of the sample, suppression of the threshold field and features in the form of Shapiro steps are observed on the I–V characteristics. The effect is similar (though not equivalent) to that of RF voltage applied directly to the sample at the same frequency. Evidence is provided that these features are not coupled with the effect of electrical interference. Practical meaning. The result opens up a new area of electromechanical effects in quasi one-dimensional conductors with CDW and can be used for detecting and processing of mechanical oscillations in the microwave range.

Charge-Order Phase Transition in the Quasi One-Dimensional Organic Conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the {hbox {(TMTTF)}}_2X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor {hbox {(TMTTF)}}_2 {hbox {NO}}_3. We found that the {hbox {(TMTTF)}}_2 {hbox {NO}}_3 compound develops a peculiar anisotropy with a doubled periodicity (ab'-plane) of the ESR linewidth below about T_{text {CO}}= ({250pm 10})~hbox {K}. This behavior is similar to observations in the related compounds {hbox {(TMTTF)}}_2X (X = {hbox {PF}}_6, {hbox {SbF}}_6 and {hbox {AsF}}_6), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous {hbox {(TMTTF)}}_2 {hbox {ClO}}_4, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below T_{text {CO}} approx 250~hbox {K}.

Seebeck coefficient in low-dimensional fluctuating charge density wave systems

We study the role of charge density-wave fluctuations on the temperature dependence of Seebeck coefficient in quasi-one dimensional conductors with a Peierls instability. The description of low-dimensional incommensurate charge density-wave fluctuations as obtained by a generalized Ginzburg-Landau approach for arrays of weakly coupled chains is embodied in the numerical solution of the semi-classical Boltzmann transport equation. The energy and temperature dependence of the scattering time of electrons on fluctuations can then be extracted and its influence on the Seebeck coefficient calculated. The connexion between theory and experiments carried out on molecular conductors is presented and critically discussed.

Unveiling the bosonic nature of an ultrashort few-electron pulse

Quantum dynamics is very sensitive to dimensionality. While two-dimensional electronic systems form Fermi liquids, one-dimensional systems—Tomonaga–Luttinger liquids—are described by purely bosonic excitations, even though they are initially made of fermions. With the advent of coherent single-electron sources, the quantum dynamics of such a liquid is now accessible at the single-electron level. Here, we report on time-of-flight measurements of ultrashort few-electron charge pulses injected into a quasi one-dimensional quantum conductor. By changing the confinement potential we can tune the system from the one-dimensional Tomonaga–Luttinger liquid limit to the multi-channel Fermi liquid and show that the plasmon velocity can be varied over almost an order of magnitude. These results are in quantitative agreement with a parameter-free theory and demonstrate a powerful probe for directly investigating real-time dynamics of fractionalisation phenomena in low-dimensional conductors.

Conductance quantization suppression in the quantum Hall regime

Conductance quantization is the quintessential feature of electronic transport in non-interacting mesoscopic systems. This phenomenon is observed in quasi one-dimensional conductors at zero magnetic field B, and the formation of edge states at finite magnetic fields results in wider conductance plateaus within the quantum Hall regime. Electrostatic interactions can change this picture qualitatively. At finite B, screening mechanisms in narrow, gated ballistic conductors are predicted to give rise to an increase in conductance and a suppression of quantization due to the appearance of additional conduction channels. Despite being a universal effect, this regime has proven experimentally elusive because of difficulties in realizing one-dimensional systems with sufficiently hard-walled, disorder-free confinement. Here, we experimentally demonstrate the suppression of conductance quantization within the quantum Hall regime for graphene nanoconstrictions with low edge roughness. Our findings may have profound impact on fundamental studies of quantum transport in finite-size, two-dimensional crystals with low disorder.

Möbius Kondo insulators

Heavy fermion materials have recently attracted attention for their potential to combine topological protection with strongly correlated electron physics. To date, the ideas of topological protection have been restricted to the heavy fermion or "Kondo" insulators with the simplest point-group symmetries. Here we argue that the presence of nonsymmorphic crystal symmetries in many heavy fermion materials opens up a new family of topologically protected heavy electron systems. Re-examination of archival resistivity measurements in nonsymmorphic heavy fermion insulators Ce$_3$Bi$_4$Pt$_3$ and CeNiSn reveals the presence of low temperature conductivity plateau, making them candidate members of the new class of material. We illustrate our ideas with a specific model for CeNiSn, showing how glide symmetries generate surface states with a novel Mobius braiding that can be detected by ARPES or non-local conductivity measurements. One of the interesting effects of strong correlation, is the development of partially localization or "Kondo breakdown" on the surfaces, which transforms Mobius surface states into quasi-one dimensional conductors, with the potential for novel electronic phase transitions.

Various Quantum Ring Structures: Similarity and diversity

Similarity and diversity of various quantum ring structures are investigated by classifying energy dispersions of three different structures: an electrostatic quantum ring, a magnetic quantum ring, and a magnetic-electric quantum ring. The wave functions and the eigenenergies of a single electron in the quantum ring structures are calculated by solving the Schrdinger equation without any electron-electron interaction. Magnetoconductance is studied by calculating a two-terminal conductance while taking into account the backscattering via the resonance through the states of the quantum rings at the center of a quasi-one dimensional conductor. It is found that the energy spectra for the various quantum ring structures are sensitive to additional electrostatic potentials as well as to the effects of a nonuniform magnetic field. There are also characteristics of similarity and diversity in the energy dispersions and in the single-channel magnetoconductance.

Rapid magnetic oscillations and magnetic breakdown in quasi-1D conductors

We review the physics of magnetic quantum oscillations in quasi-one dimensional conductors with an open Fermi surface, in the presence of modulated order. We emphasize the difference between situations where a modulation couples states on the same side of the Fermi surface and a modulation couples states on opposite sides of the Fermi surface. We also consider cases where several modulations coexist, which may lead to a complex reorganization of the Fermi surface. The interplay between nesting effects and magnetic breakdown is discussed. The experimental situation is reviewed.

Spectroscopic characterization of charge carrier anisotropic motion in twisted few-layer graphene.

Graphene, a layer of carbon atoms in a honeycomb lattice, captures enormous interest as probably the most promising component of future electronics thanks to its mechanical robustness, flexibility, and unique charge carrier quasiparticles propagating like massless high energy Dirac fermions. If several graphene layers form a stack, the interaction between them is, on the one hand, weak, allowing realization of various registries between the layers and, on the other hand, strong enough for a wide range tuning of the electronic properties. Here we grow few layer graphene with various number of layers and twist configurations and address the electronic properties of individual atomic layers in single microscopic domains using angle-resolved photoelectron spectromicroscopy. The dependence of the interlayer coupling on the twist angle is analyzed and, in the domains with tri-layers and more, if different rotations are present, the electrons in weaker coupled adjacent layers are shown to have different properties manifested by coexisting van Hove singularities, moiré superlattices with corresponding superlattice Dirac points, and charge carrier group velocity renormalizations. Moreover, pronounced anisotropy in the charge carrier motion, opening a possibility to transform strongly coupled graphene bilayers into quasi one-dimensional conductors, is observed.

Variable range hopping conductivity in nanocrystalline films of K0.3MoO3

We present the morphology and electrical conductivity of thin films of quasi one-dimensional conductor K0.3MoO3 produced by pulsed laser deposition. Atomic force microscopy reveals granular nature of the films with the texture depending on the substrate and deposition conditions. While the films with better texture have higher conductivity, they all show universal temperature dependence in the temperature range 20K–300K. The results are analyzed within the models of co-tunneling variable range hopping (VRH) conductivity in granular media based on the Efros–Shklovskii (ES–VRH) model. We suggest two different approaches to interpret the data, either by two ES–VRH regimes with crossover around 45K or by a modified ES–VRH regime with T2 pre-factor up to T~100K and an activated one above. The second approach yields a unique value of ES–VRH parameter T0=1000K in all examined films.

Charge density wave soliton liquid

We investigate the charge density wave transport in a quasi–one-dimensional conductor, orthorhombic tantalum trisulfide (o-TaS3), by applying a radio-frequency ac voltage. We find a new ac-dc interference spectrum in the differential conductance, which appears on both sides of the zero-bias peak. The frequency and amplitude dependences of the new spectrum do not correspond to those of any usual ac-dc interference spectrum (Shapiro steps). The results suggest that CDW phase dynamics has a hidden degree of freedom. We propose a model in which phase solitons behave as liquid. The origin of the new spectrum is that the solitons are depinned from impurity potentials assisted by an ac field when a small dc field is applied. Our results provide a new insight as regards our understanding of an elementary process in CDW dynamics.

Strain-induced formation of ultra-coherent CDW in quasi one-dimensional conductors

We have developed techniques for stretching whiskers up to 1.5–3%, including those of nanometer cross-section. An exceptionally high coherence of the CDW transport for TaS3 and NbS3 has been observed under the uniaxial strain, ε. For TaS3 samples with transverse dimensions ∼0.1–1μm for ε approaching εс the CDW coherence falls down and a new, ultra-coherent, CDW phase begins to form. At ε~εс the two phases coexist within a very narrow range δε~2×10−4. Further stretching results in a complete vanishing of the transport of the incoherent CDW. The threshold fields, Et, of the ultra-coherent CDW, as well as the dissipation, appear by an order of magnitude lower; giant negative differential resistance is observed. The narrow-band noise shows up to 10 harmonics of the fundamental frequency. RF irradiation results in complete synchronization of the CDW sliding. The CDW transformation shows features of the 1st order transition. We also report hysteresis of resistance vs. ε, from which we conclude that the transition cannot be associated with the lock-in of the CDW: the strain draws the CDW away from 4-fold commensurability with the lattice.

Critical dynamics and domain motion from permittivity of the electronic ferroelectric (TMTTF)2AsF6

The quasi one-dimensional organic conductor (TMTTF)2AsF6 shows the charge ordering transition at TCO=101K to a state of the ferroelectric Mott insulator which is still well conducting. We present and interpret the experimental data on the gigantic dielectric response in the vicinity of TCO, concentrating on the frequency dependence of the inverse 1/ε of the complex permittivity ε=ε′+iε′′. Surprisingly for a ferroelectric, we could closely approach the 2nd order phase transition and to deeply reach the critical dynamics of the polarization. We could analyze the critical slowing-down when approaching TCO from both sides and to extract the anomalous power law for the frequency dependence of the order parameter viscosity. Moreover, below TCO we could extract a sharp absorption feature coming from a motion of domain walls which shows up at a frequency well below the relaxation rate.

Thermal fluctuations effect on the charge density wave dynamics in quasi-one dimensional conductors

The effect of thermal fluctuations on the charge density wave (CDW) dynamics properties of a one-dimensional conductor is studied numerically in the weak pinning limit. In good agreement with a recent experimental work on the CDW conductor K0.3MoO3, the addition of thermal fluctuations affects the CDW dynamics of the system. The results are discussed in the context of damping mechanisms of the collective mode arising from dissipative normal currents induced by dynamic deformations of the CDW and in the context of CDW dislocations.

Eigen-like hydrated protons traveling with a local distortion through the water nanotube in new molecular porous crystals {[MIII(H2bim)3](TMA)·20H2O}n (M = Co, Rh, Ru)

In molecular porous crystals {[M(III)(H(2)bim)(3)](TMA)·20H(2)O}(n) (M = Co, Rh, Ru), the structural property of confined water network and the dynamics of mobile hydrated protons have been examined by the measurement of infrared spectrum and microwave conductivity. The water network undergoes first order phase transition from the ice nanotube (INT) to the water nanotube (WNT) around 200 K, while the infrared spectral features for these states are almost equivalent. Consequently, the water molecules in WNT dynamically fluctuate in the vicinity of the regulated position in INT with maintaining the O-O distance. The additional band observed around 2200 cm(-1) reveals the emergence of an Eigen-like protonic hydrate, around which the O-O distance locally shrinks to ~2.56 Å. The microwave conductivity exhibiting activation-type behavior, isotope effect and anisotropy indicates that the water nanotube is a quasi one-dimensional high proton conductor. Together with the neutron experimental results, we have clarified that the proton and protonic hole are generated by the self-dissociation in some water molecules just hydrated to the carboxylate oxygen atom of trimesic acid. The Eigen-like hydrated proton and protonic hole contribute to the intrinsic proton conduction accompanying local distortions. The carrier density dominated by the intrinsic ionic equilibrium is not large, whereas the actual mobility, which is higher than 1 × 10(-2) (cm(2)/Vs), yields the present high proton conductivity.

Tunable harmonic vibrations of quasi one-dimensional conductors induced by sliding charge-density waves

We demonstrate that samples of quasi one-dimensional compounds show torsional vibrations induced by the sliding charge-density wave (CDW). Narrowband noise at the fundamental frequency is found in the vibrations spectra. The result suggests an approach to studies of the CDW dynamics and presents CDW conductors as tunable (ultra)sonic acoustoelectronic generators---unique solids vibrating under dc voltage.

Proton Dynamics in Water Nanotube of New Molecular Porous Crystal

Triple Layered Hydration Structure Called Water Nanotube Is Formed in the Nanochannel of New Molecular Porous Crystal {[CoIII(H2bim)3](TMA) 20H2O}n. The Formation of Regulated Water Network Is Identified by the Infrared Spectroscopy, X-Ray and Neutron Crystal-Structure Analyses. Furthermore, the High Conductivity Exhibiting Activation-Type Behavior Has Been Observed by the Microwave Measurement Employing Single Crystal with Millimeter-Order Length. According to the Observation of Strong Isotope Effect and Distinct Anisotropic Conduction, the Water Nanotube Is Confirmed to Be a Quasi One-Dimensional High Proton Conductor.