Charge Density Wave Compound Research Articles

Effect of impurity pinning on conduction and specific heat in the Luttinger liquid

Abstract We study theoretically two effects related to impurity depinning that are common for 1D Luttinger liquid (LL) and linear-chain charge-density wave (CDW) conductors. First, we consider the electron transport through a single impurity in a 1D conducting channel and study a new regime of conduction related to LL sliding at voltage above a threshold one. The DC current in this regime is accompanied by oscillations with frequency f = I / e . This resembles the CDW depinning in linear-chain conductors, the Josephson effect, and the Coulomb blockade. Second, we found that strong pinning of the LL by impurities leads to a magnetic field dependence of the low-temperature specific heat similar to that observed experimentally in CDW compounds. We interpret this in favor of possibility of formation of the LL in linear-chain compounds.

Magnetic properties of the charge density wave compoundsRTe3(R=Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm)

The antiferromagnetic transition is investigated in the rare-earth (R) tritelluride RTe{sub 3} family of charge density wave (CDW) compounds via specific heat, magnetization and resistivity measurements. Observation of the opening of a superzone gap in the resistivity of DyTe{sub 3} indicates that additional nesting of the reconstructed Fermi surface in the CDW state plays an important role in determining the magnetic structure.

Insight on the electronic state ofSr2IrO4 revealed by cationic substitutions

We report the electrical resistivity, thermoelectric power and magnetization ofSr2−xLaxIrO4 (x = 0 and0.05) and Sr2Ir1−yRhyO4 (y = 0.05, 0.1 and 0.2) measured below room temperature. InSr2IrO4, electrons (La substitution) or holes (Rh substitution) can be doped, which lead to astrong decrease of the resistivity. In particular, a nearly-metallic state is realized in thecase of Rh doping. The thermoelectric power turns out to be metallic-like fory = 0.2. The presence of a gap in the electronic band structure is robust against these substitutions.From various experimental data, similarities with the 3D charge density wave compound,BaBiO3, are suggested. Nevertheless, rather than a charge density wave, our scenario implies thepresence of a spin density wave.

Optical properties of the Ce and La ditelluride charge density wave compounds

The La and Ce ditellurides $\mathrm{La}{\mathrm{Te}}_{2}$ and $\mathrm{Ce}{\mathrm{Te}}_{2}$ are deep in the charge-density-wave (CDW) ground state even at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We have collected their electrodynamic response over a broad spectral range from the far infrared up to the ultraviolet. We establish the energy scale of the single particle excitation across the CDW gap. Moreover, we find that the CDW collective state gaps a large portion of the Fermi surface. Based on the observation of a power-law behavior in the optical conductivity, similar to that found for the related rare-earth tritellurides but over a reduced frequency range, we suggest that interactions and umklapp processes may play a role in the onset of the CDW broken-symmetry ground state.

Electronic structure of ZrTe 3

The Fermi surface topology of the layered superconducting charge density wave compound ZrTe 3is investigated by angle resolved photoelectron spectroscopy and density functional theory. The Fermi surface is dominated by bands originating from two perpendicular systems of quasi one-dimensional chains. Nesting and opening of a pseudogap at temperatures as high as 250 K are signatures of a Peierls transition in one of these chains. The nesting properties are also studied for high pressure simulated crystal structure.

Chemical pressure and hidden one-dimensional behavior in rare-earth tri-telluride charge-density wave compounds

We report on the first optical measurements of the rare-earth tri-telluride charge-density-wave systems. Our data, collected over an extremely broad spectral range, allow us to observe both the Drude component and the single-particle peak, ascribed to the contributions due to the free charge carriers and to the charge-density-wave gap excitation, respectively. The data analysis displays a diminishing impact of the charge-density-wave condensate on the electronic properties with decreasing lattice constant across the rare-earth series. We propose a possible mechanism describing this behavior and we suggest the presence of a one-dimensional character in these two-dimensional compounds. We also envisage that interactions and umklapp processes might play a relevant role in the formation of the charge-density-wave state in these compounds.

Low-temperature specific heat of the quasi-two-dimensional charge-density wave compoundKMo6O17

Low temperature specific heat $({C}_{\mathrm{p}})$ of quasi-two-dimensional charge-density wave (CDW) compound $\mathrm{K}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$ has been studied by a relaxation method from $2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}48\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ under zero and $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ magnetic fields. The results show that no specific heat anomaly is found at $16\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ under both zero and $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ magnetic fields, although an anomaly is clearly observed in the resistivity and magnetoresistance measurements. From the data between 2 and $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the density of states at Fermi level is estimated as $0.2\phantom{\rule{0.3em}{0ex}}{\mathrm{eV}}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}\mathrm{per}\phantom{\rule{0.3em}{0ex}}\text{molecule}$ and the Debye temperature is extracted to be $418\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A bump appearing in ${C}_{\mathrm{p}}∕{T}^{3}$ is found between 4 and $48\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ centered around $12.5--15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that the phason excitations contribute to the total specific heat similarly as in quasi-one-dimensional CDW conductors. Using a modified Debye model, a pinning frequency of $0.73\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ for $\mathrm{K}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$ is estimated from the phason contribution.

Thermodynamic and transport properties ofYTe3,LaTe3, andCeTe3

Measurements of heat capacity, susceptibility, and electrical resistivity are presented for single crystals of the charge density wave compounds $\mathrm{Y}{\mathrm{Te}}_{3}$, $\mathrm{La}{\mathrm{Te}}_{3}$, and $\mathrm{Ce}{\mathrm{Te}}_{3}$. The materials are metallic to low temperatures, but have a small density of states due to the charge density wave gapping large portions of the Fermi surface. $\mathrm{Ce}{\mathrm{Te}}_{3}$ is found to be a weak Kondo lattice, with an antiferromagnetic ground state and ${T}_{N}=2.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The electrical resistivity of all three compounds is highly anisotropic, confirming the weak dispersion perpendicular to Te planes predicted by band structure calculations.

Novel Electronically Driven Surface Phase Predicted inC/Si(111)

We predict a novel electronically driven phase for the recently created C/Si(111) surface at 1/3 monolayer coverage. Whereas the isoelectronic surface Sn/Ge(111) is a 3 x 3 distorted metal and Si/SiC(0001) is an undistorted magnetic Mott insulator, the new phase combines both features. Two of three adatoms in C/Si(111) should form a distorted (3 x 3) honeycomb sublattice, the third an undistorted insulating and magnetic triangular sublattice. The generally conflicting elements, namely, band energy, favoring distortion, and strong electron correlations favoring a Mott state, actually conspire in this case. This kind of state represents the surface analog of the Fazekas-Tosatti state in the charge density wave compound 1T-TaS2.

Role of charge-density-wave fluctuations on the spectral function in a metallic charge-density-wave system

Angle-resolved photoemission spectroscopy of $\mathrm{Zr}{\mathrm{Te}}_{3}$ has been performed from room temperature $(T)$ down to $6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ (across ${T}_{\mathrm{CDW}}=63\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) to study the charge-density-wave (CDW) fluctuation effects in a metallic CDW compound having Fermi surface (FS) sheets with differing dimensionality. While spectra on the three-dimensional (3D) FS show typical Fermi--Dirac functionlike $T$ dependence, those along the quasi-one-dimensional (1D) FS show formation of a pseudogap, starting at a much higher $T$ than ${T}_{\mathrm{CDW}}$. Simultaneously, a van-Hove singularity consisting of the quasi-1D FS intersecting the 3D FS shows an increase of coherence. This demonstrates the role of CDW fluctuations on the spectral function, and relation to the dimensionality of the states, in a metallic CDW system.

Magnetic field influence on the low-temperature heat capacity of the CDW compound blue bronze [formula omitted

We have investigated the effect of magnetic field on the low-temperature heat capacity C p of the quasi-1 D compound blue bronze Rb 0.3 MoO 3 , characterized by a charge density wave (CDW) ground state. Below 1 K, C p is dominated by a quasi-linear term, γ T , characteristic of the disordered state of the CDW. Under applied field (between 0 and 2 T), there is an increase of the amplitude γ with H, and simultaneously a downward deviation from the linear regime occurs at an increasing crossover temperature. This could be due to the opening of a (pseudo)-gap in the corresponding spectral density of excitations, which develops with increasing magnetic field.

Variable-range hopping in quasi-one-dimensional electron crystals

We study the effect of impurities on the ground state and the low-temperature dc transport in a 1D chain and quasi-1D systems of many parallel chains. We assume that strong interactions impose a short-range periodicicity of the electron positions. The long-range order of such an electron crystal (or equivalently, a $4 k_F$ charge-density wave) is destroyed by impurities. The 3D array of chains behaves differently at large and at small impurity concentrations $N$. At large $N$, impurities divide the chains into metallic rods. The low-temperature conductivity is due to the variable-range hopping of electrons between the rods. It obeys the Efros-Shklovskii (ES) law and increases exponentially as $N$ decreases. When $N$ is small, the metallic-rod picture of the ground state survives only in the form of rare clusters of atypically short rods. They are the source of low-energy charge excitations. In the bulk the charge excitations are gapped and the electron crystal is pinned collectively. A strongly anisotropic screening of the Coulomb potential produces an unconventional linear in energy Coulomb gap and a new law of the variable-range hopping $-\ln\sigma \sim (T_1 / T)^{2/5}$. $T_1$ remains constant over a finite range of impurity concentrations. At smaller $N$ the 2/5-law is replaced by the Mott law, where the conductivity gets suppressed as $N$ goes down. Thus, the overall dependence of $\sigma$ on $N$ is nonmonotonic. In 1D, the granular-rod picture and the ES apply at all $N$. The conductivity decreases exponentially with $N$. Our theory provides a qualitative explanation for the transport in organic charge-density wave compounds.

Direct evidence for superconductivity in the organic charge density wave compound α-(BEDT-TTF)2KHg(SCN)4 under hydrostatic pressure

Direct evidence for superconductivity in the organic charge density wave compound <i>α</i>-(BEDT-TTF)₂KHg(SCN)₄ under hydrostatic pressure

Specific heats of the charge density wave compounds o-TaS and (TaSe ) I

Specific heats of the charge-density-wave compounds o-TaS3 and (TaSe4)2I have been measured over the wide temperature interval 10 K-300 K. Both systems exhibit strong non-Debye behavior. Very weak and broad anomalies are observed at the Peierls transition temperatures. For o-TaS3, the change in the curvature of the specific heat occurs at temperature of 40 K where glass transition has been deduced from dielectric measurements, and an extended scaling analysis suggests that the glass transition is associated with a dynamical cross over in length scales. We briefly discuss the characteristics and physical origins of the anomalies at both the Peierls and glass transitions.

Superconducting parameters of a CDW compound Lu5Ir4Si10

Superconductivity is established below 3.9K in a high quality single crystal of a charge density wave compound (TCDW=83K)Lu5Ir4Si10. The present studies show that Lu5Ir4Si10 has low anisotropy unlike that of the charge density wave (CDW) superconductor such as 2H–NbS2. The analysis of the critical current suggests that pinning is stronger in Lu5Ir4Si10 by three orders of magnitude at 1.7K as compared to that of chalcogenide superconductors. Moreover, pinning along the c-axis is stronger where the CDW transition takes place.

Fine structure in high-resolution photoemission spectra of quasi-two-dimensional 1T-TaS2

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we examined the charge density wave (CDW) compound $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ as a function of the photon energy (energy range from 11 up to 33 eV) for both quasicommensurate (QC) and commensurate (C) CDW phases. While the QC-phase normal emission ARPES spectra coincide with previous results, normal emission spectra of the C phase reveal evidence for a fine structure in the CDW-induced features between 1 eV binding energy and the Fermi level. This fine structure may be explained by the subband manifold of the Ta $5d$ band, split off due to the CDW-induced $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ superstructure.

Low temperature specific heat of blue bronze K 0.30 MoO 3

We report on specific heat measurements of the quasi-one-dimensional model system blue bronze K0.3MoO3, performed over a wide temperature interval 0.1-30 K, in its charge density wave (CDW) ground state. In this T range, the specific heat is successively determined by the presence of low-energy excitations (LEE) due to the disordered nature of the CDW ground-state, the acoustical phonon contribution which obeys an exact T3 law between 0.7 and 3.5 K, and a large excess of C / T 3 centered at 11.5-13 K. The contribution of the LEEs is of particularly low amplitude in comparison to other CDW systems. Also, feeble time-dependent effects could be detected. This implies a less disordered ground state in this compound. The C / T 3 bump, mainly of phononic origin, can be partly ascribed to a phason-excitation contribution, with a low-energy cut-off defined by the pinning energy. This interpretation is supported by the sensitivity of the low-T side of the bump to the purity of samples, and therefore to their pinning frequency. A similar effect was already measured in the specific heat of the CDW compound (TaSe4)2I, by comparison between pure and doped samples. Finally, we outline once more that some generic thermodynamic properties of CDW systems are similar to structurally disordered systems.

Magnetoresistance properties of quasi-two-dimensional charge-density wave compounds AMo6O17 (A=Na, K, and Tl)

Temperature dependence of resistivity in quasi-two-dimensional purple bronzes AMo6O17 (A=Na, K, and Tl) has been comparatively studied between 2 and 300 K at different magnetic fields. Huge positive magnetoresistance (MR) behavior below the Peierls transition temperature TP∼110 K was found in both K/Tl purple bronzes for H∥c, but only a weak one in the Na purple bronze. At lower temperature TM∼16 K, a second anomaly of the resistivity, which moved to higher temperatures under a magnetic field, was also observed in both K/Tl compounds, with no corresponding anomaly in the Na purple bronze. For H⊥c, the MR effect becomes much weaker in all three compounds. These results suggest that the second anomaly is possibly related to the second Peierls instability, and the huge MR effect results from the large modification of the third FS along the [100] direction induced by the magnetic field, in which the third FS is imperfectly nested in K/Tl purple bronzes and absent in the Na purple bronze.

High-resolution photoemission in low-dimensional conductors and superconductors

We review some recent spectroscopic results on low-dimensional systems, including high-T-c superconductors, and charge density wave compounds. We briefly discuss the reasons for the present interest in these materials, and the relevance of band mapping experiments by angle-resolved photoemission spectroscopy. The main focus of the paper is on high-energy resolution, which can be exploited to probe the quasiparticle excitations, and to study the characteristic instabilities of the Fermi surface. Gap spectroscopy in the ordered phases is a prominent part of this research, namely in the cuprates. The normal state properties, however, are often as interesting. This is particularly true in 1D, where the photoemission results indicate strong deviations from a Fermi liquid scenario. (C) 1999 Elsevier Science B.V. All rights reserved.

Glass transition from a slush to a real glass in charge-density-wave compound TaS 3

We have found by broad-band dielectric spectroscopy of charge density wave (CDW) compound TaS 3 that the main relaxation time representing CDW viscosity shows a critical slowing down in the range 55-50 K. At lower temperatures two additional processes appear that we assign to the defects in CDW superlattice. Based on very strong similarity with the glass transition in supercooled liquids, we propose a consistent scenario of the glass transition from a CDW slush to a real CDW glass. The length/time scales involved imply the existence of a new family of glasses in systems with the long-range order such as CDW.