Interaction of both charge density waves in NbSe3 from interlayer tunneling experiments

An interlayer tunneling technique has been used for spectroscopy of charge density wave (CDW) energy gaps (Δ_1,2) in NbSe₃ subsequently opened at the Fermi surface on decreasing temperature at T _p1 = 145 K (CDW1) and at T _p2 = 60 K (CDW2). We found that the CDW2 formation is accompanied by an increase of the CDW1 gap below T _p2. The maximum enhancement of Δ₁, δΔ₁ is about 10%. The effect observed has been predicted theoretically as resulting from the joint phase locking of both CDWs with the underlying crystalline lattice below T _p2. The text was submitted by the authors in English.     

Effects of radiation damage on the charge density wave transitions in NbSe3

We have measured the electrical resistivity of NbSe₃ samples which have been radiation damaged with 2.5 MeV protons up to a defect concentration of 0.5%. We find that, unlike substitutional impurities, the defects do not destroy the charge density wave (CDW) transitions and the samples do not go superconducting. The defects become more effective scatterers below the CDW transitions so that the defect resistivity is temperature dependent. The defects pin the CDWs randomly so that carriers in the unnested regions can be scattered by the CDW. This leads to an enhancement of the defect resistivity. The resistivity of the highly damaged samples is still increasing with decreasing temperatures to below 1 K.     

Transient conductivity and conduction noise in NbSe3

In the non-Ohmic regime of NbSe₃, the transient conductivity under pulsed field and the conduction noise under dc bias were investigated. The results under pulsed-field with two steps show the number of carriers (CDWs) causing the extra conductivity is field independent. The noise spectrum of the crystal on which the pulsed experiment was carried out has only one fundamental narrow band noise and exhibits no enhancement of the noise level. It is concluded for this crystal that the CDW slides uniformly over the whole crystal. It is also confirmed from measurements under two successive pulsed field that the configuration of the CDW changes as it begins to slide.     

Incommensurate/commensurate charge-density-wave states as a source for plutonium metal behavior

Physical property behavior of plutonium (Pu) metal phases is like that of an incommensurate charge-density wave (ICDW) system where the CDW influenced distortion modulates the crystal. As incommensurates, the different Pu phases may have to be considered as superspace group structures where there is a one-dimensional modulation of the basic three-dimensional lattice. Certain Pu phases may then be classified into as many as three Bravais classes when considered in (3 + 1) dimensional space. The possible variants in Bravais class, crystals setting and bottom lines, as well as allowable differences in the number of atoms per unit cell, should permit incommensurate materials, as well as Pu phases, to appear in different variants of the basic space group structure on heating and cooling cycles. One should not expect the lower temperature phases, e.g., Pu, to return to their original distorted or modulated structures at constant rate cooling, after being distorted or modulated by CDWs in their higher temperature space group structures. This can explain the hysteresis in phase transitions noted with Pu metal and with incommensurate materials in general.

Chiral symmetry appears to be inherent to the incommensurability of a quasi-one dimensional system. All but one of the reported space group structures for Pu phases have at least a one-dimensional twofold screw axis with a center of symmetry, i.e., they show chiral symmetry. A theory suggests that chiral symmetry must permit the contraction in one or more dimensions noted with most incommensurate materials, as well as with Pu phases.

It is suggested that there is another ICDW Pu phase (αI) below ～ 60 K, and that the γ-Pu phase (Fddd) must be a composite structure. Other Pu phases appear to be composite structures also. There is evidence for a new phase, or phase mixture, which appears reproducibly between the δ and γ phases only on a cooling cycle. It is infered that this is a reappearance of the δ' phase.

Published dilatometry, internal friction and relative shear modulus results appear to confirm both incommensurate and commensurate CDW states in Pu metal phases. It is suggested that CDWs may be playing a role in f-bonding in Pu metal and that CDWs and valence fluctuations may be manifestations of the same electronic behavior.     

Magnetic susceptibility of NbSe3 and TaSe3

The magnetic susceptibility of NbSe₃ shows a decrease beginning slightly above its upper charge density wave transition (CDW) of 144 K, but no change within our resolution near the 59 K transition. The change in the density of states at the Fermi level due to the upper transition is 0.14 states-eV/Nb. TaSe₃ on the other hand has a temperature independent susceptibility. In some cases the trichalcogenides are contaminated with their corresponding dichalcogenide. Such contamination can be observed by susceptibility measurements in the case of 2HTaSe₂ but not of 2HNbSe₂. We also report an anomaly in the susceptibility of 4HaNbSe₂, which suggests a CDW transition at 45 K.     

Search for a sliding charge density wave in layered compounds

Non-linear voltage-current behavior is observed in the quasi one dimensional compound NbSe₃, due to the contribution of a sliding Charge Density Wave (CDW) to the conductivity. We have looked for a non-linear voltage-current characteristics in the incommensurate CDW state of the layered compounds 2H-TaSe₂ and 1T-TaS₂ and find no evidence for such up to $\text{1.0V}\text{cm}$ and $\text{10V}\text{cm}$, respectively. These values are several orders of magnitude higher than the minimum depinning field observed in NbSe₃.     

Stationary Josephson current in junctions involving d-wave superconductors with charge density waves: the temperature dependence and deviations from the law of corresponding states

Stationary Josephson current I _c in symmetric and non-symmetric junctions involving d-wave superconductors with charge density waves (CDWs) was calculated. It was found that, if CDWs are weak or absent, there exists an approximate proportionality between I _c and the product of superconducting order parameters in the electrodes (the law of corresponding states) for several factors affecting those quantities, such as the temperature, T, or one of the parameters characterizing the combined CDW superconducting phase (the degree of the Fermi surface dielectric gapping and the ratio between the parent superconducting and CDW order parameters). Otherwise, the dependences I _c (T) were shown to deviate from those in the absence of CDWs, and the relevant corresponding-state dependences from linearity, the deviations being especially strong at certain rotation angles of crystalline electodes with respect to the junction plane. Hence, making use of specially designed experimental setups and analyzing the I _c (T) and corresponding-state dependences, the existence of CDWs in cuprates and other non-conventional superconductors can be detected.     

The electronic structure of Fe1+xNb3-xSe10

Fe_1+xNb_3-xSe₁₀ has a crystal structure consisting of prismatic niobium chains and iron/niobium octahedral chains. A Charge Density Wave (CDW) has been observed at low temperatures, as might be expected from the similarity of the prismatic niobium chains to those present in NbSe₃. Here we report tight binding band structure calculations which indicate that the electrons responsible for conduction and the CDW reside primarily on the prismatic chains. The effects of disorder and stoichiometry in the iron/niobium octahedral chains on the electronic structure and CDW wave vector are discussed.     

2kF and 4kF charge-density waves in TTF0.4TSeF0.6-TCNQ — An X-ray study

In the quasi one-dimensional conductor TTF_0.4TSeF_0.6-TCNQ, it is found that the 2k_F CDW has three-dimensional correlations of relatively long range below the metal-insulator transition temperature ～40K, although the 4k_F CDWs are only one-dimensional in character at all temperatures. This result supports the speculation that the 2k_F CDWs and the 4k_F CDWs are present predominantly on the TCNQ and the fulvalene stacks, respectively. The 2k_F-value of the present material is similar to that of TTF-TCNQ but far from that of TSeF-TCNQ. The change of the value with the concentration of TSeF (or TTF) is nonlinear. The transverse period of the ordered 2k_F CDW along the a-axis is $\text{a}$ just below the transition temperature and becomes 4$\text{a}$ below 34.5K. A discussion is given based on the two-chain model.     

Phenomenological theory of the increase in the superconducting transition temperature in thin film layered sandwiches

A phenomenological theory is presented to account for the observed increase in the critical temperature, as a function of the number of layers, of multilayer sandwiches composed of alternating superconducting and insulating layers. The transition temperature of the layered compound NbSe₂ when cleaved down to a small number of layers and the pressure dependence of the transition temperature of bulk NbSe₂ can also be fitted into this scheme.     

Charge density wave depinning and switching in NbSe3

We have investigated the relation between charge density wave (CDW) depinning and switching in NbSe₃. In the lower CDW state we observe that the critical electric field at which switching occurs is independent of temperature. We also observe that the differential resistance of the sample is independent of applied dc bias beyond the switching threshold, and corresponds to the high-field limit of the pure resistance. We interpret our results in terms of a temperature dependent CDW domain structure in the crystal.     

Size dependence of current oscillations in the charge density wave compound (TaSe4)2I

The volume dependence of the current oscillations has been examined in the current carrying charge density wave (CDW) state of (TaSe₄)₂I. The spectral width of the oscillations broadens as the specimen cross section is increased, indicating a nonuniform distribution of CDW velocity in the spicemens. The observed behavior is contrasted with that found in the more isotropic compound NbSe₃.     

Charge density wave transport in NbSe<Subscript>3</Subscript> at low temperatures under high magnetic field

Charge density wave (CDW) depinning and sliding regimes have been studied in NbSe₃ at low temperatures down to 1.5 K under magnetic field of 19 T oriented along the c-axis. We found that the threshold field for CDW depinning becomes temperature independent below T ₀ ≈ 15 K. Also CDW current to frequency ratio characterizing CDW sliding regime increases by factor 1.7 below this temperature. The results are discussed as a crossover from thermal fluctuation to tunneling CDW depinning at T < T ₀. Besides, we found that CDW sliding strongly suppresses the amplitude of Shubnikov-de Haas oscillations of magnetoresistance.     

On the topographic and optical properties of SiC/SiO2 surfaces

The roughness of the semiconductor surface substantially influences properties of the whole structure, especially when thin films are created. In our work 3C SiC, 4H SiC and Si/a-SiC:H/SiO₂ structures treated by various oxidation a passivation procedures are studied by atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Surface roughness properties are studied by fractal geometry methods. The complexity of the analysed surface is sensitive to the oxidation and passivation steps and the proposed fractal complexity measure values enable quantification of the fine surface changes. We also determined the optical properties of oxidized and passivated samples by using visual modelling and stochastic optimization.      

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO₂(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO_2−x towards Ti₂O₃.     

Scanning tunneling microscopy with spin-polarized electrons

We present a short outline of the first STM experiments with spin-polarized electrons performed in ultrahigh vacuum by using ferromagnetic CrO₂ tips and a Cr(001) single crystal surface. A clear distinction can be made between topographic STM line scans obtained with a non-magnetic tungsten tip and those obtained with a ferromagnetic CrO₂ tip, which are modified due to an additional contribution from spin-dependent vacuum tunneling. STM therefore has the potential to measure the local electron spin polarization of the free surface as well as the spatial distribution of spins on the atomic scale.     

Growth of iron films on MgO(100) substrates at 560-600 K

The growth mechanism of heteroepitaxial thin iron films, deposited on MgO(100) substrates at 560-600 K, was investigated by means of tunnelling electron microscopy and spectroscopy. Scanning tunnelling microscopy (STM) analysis of the surface indicates the formation of long flat (about 10-20 nm) terraces for thicknesses greater than 3-4 nm. The fine structure of those samples contains sets of interpenetrating screw and helical dislocations. The onset of the formation of islands on these terraces and also their sizes were a result of stress relaxation in these films and this was supported by the Monte Carlo simulations. The resulting critical number of island layers needed to build up the gain in energy in a relaxed volume (compensating the misfit losses on its boundary) perfectly correlates with the observed STM pattern. At even larger thicknesses (greater than 9-10 nm) the surface becomes uniformly flat with a large number of mosaic voids. The differential conductivity spectra show the distinct surface peak of iron, positioned between the values for the iron crystal surface and the atomic limit. Manipulation of iron nanoparticles from the tip to the surface and back is demonstrated.     

Transient non-linear phenomena and metastable states of charge-density waves in niobium triselenide

Details are given of the transient response of NbSe₃ at 42 K to currents I(t) of various repetitive pulsed waveforms. Nonlinear conduction, due supposedly to motion of one of the two charge-density waves (CDWs) present, was measured as U(t) = IR₀ ? V, where V(t) is the voltage developed across the specimen and R₀ is its resistance in the Ohmic regime.With unidirectional pulses two threshold currents for nonlinearity were observed. On passing the lower threshold, a gradual rise (time-constant 50 μs) of U towards the d.c. value was seen; this behaviour was shown not to originate in the inertia of the CDW, and probably arose thermally. Only after the second threshold was passed did U appear to rise immediately current was applied. The existence of two thresholds accounts for a discrepancy between pulsed and d.c. measurements of conductivity noted by Brill et al. (1981), and also for a phenomenon previously attributed to “locking” between the two CDWs [6].When the pulses were alternately negative- and positive-going, |U| rose beyond its eventual (d.c.) value, towards which it subsequently decayed. A study of this “overshoot” phenomenon has shown the speciment to be left, after a current pulse, in a long-lived metastable state in which pinning stabilises some distortion of the CDW, presumably similar to that of which recent electron micrographs [11] appear to be evidence.A simple model, associating the overshoot with transitions between metastable states, adequately describes the conditions for its occurrence. However, the origin of the conduction associated with the moving CDW remains uncertain, both the Frohlich mechanism and a single-electron alternative finding some experimental support.     

Incommensurate superlattice reflections in quasi one dimensional conductors, (MSe4)2I (M=Ta and Nb)

X-ray diffraction and conductivity measurements have been made on (MSE₄)₂I, (M=Ta and Nb). A phase transition of (NbSe₄)₂I was found and the transition temperature T_c was determined to be about 210K by the conductivity measurement. For (TaSe₄)₂I, T_c was about 260K as reported previously. Superlattice reflections due to the formation of charge density wave (CDW) in (TaSe₄)₂I were observed at points $\text{q}\text{?}\text{=(±0.05, ±0.05, ±0.085)}$. The type of CDW of (NbSe₄)₂I is known to be identical to that of (TaSe₄)₂I. Temperature dependence of the threshold electric field of nonlinear conductivity is consistent with the result in previous reports.     

Effect of charge density wave on thermoreflectance spectra of 2HNbSe2

Thermoreflectance curves of 2HNbSe₂ below and above the ICDW transition temperature are presented. Modifications through the transition temperature are negligible and this is attributed to a smaller value of the CDW gap with respect to the one in 2HTaSe₂.