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.     

Dephasing of Andreev pairs entering a charge density wave

A Cooper pair from a s-wave superconductor (S) entering a conventional charge density wave (CDW) below the Peierls gap dephases on the Fermi wavelength while one particle states are localized on the CDW coherence length ξ_CDW. It is thus practically impossible to observe a Josephson current through a CDW. The paths following different sequences of impurities interfere destructively, due to the different electron and hole densities in the CDW. The same conclusion holds for averaging over the conduction channels in the ballistic system. We apply two microscopic approaches to this phenomenon: (i) a Blonder, Tinkham, Klapwijk (BTK) approach for a single highly transparent S-CDW interface; and (ii) the Hamiltonian approach for the Josephson effect in a clean CDW and a CDW with non magnetic disorder. The Josephson effect through a spin density wave (SDW) is limited by the coherence length ξ_SDW, not by the Fermi wave-length. A Josephson current through a SDW might be observed in a structure with contacts on a SDW separated by a distance ξ_SDW.     

Angular studies of the magnetoresistance in the density wave state of the quasi-two-dimensional purple bronze KMo<Subscript>6</Subscript>O<Subscript>17</Subscript>

The purple molybdenum bronze KMo₆O₁₇ is a quasi-two-dimensional compound which shows a Peierls transition towards a commensurate metallic charge density wave (CDW) state. High magnetic field measurements have revealed several transitions at low temperature and have provided an unusual phase diagram “temperature-magnetic field”. Angular studies of the interlayer magnetoresistance are now reported. The results suggest that the orbital coupling of the magnetic field to the CDW is the most likely mechanism for the field induced transitions. The angular dependence of the magnetoresistance is discussed on the basis of a warped quasi-cylindrical Fermi surface and provides information on the geometry of the Fermi surface in the low temperature density wave state.     

Electronic structure and anomalous photoemission line-shape of quasi-2D oxide η-Mo4O11

The η-Mo₄O₁₁ compound is a layered two-dimensional (2D) metallic system whose reduced dimensionality originates non-linear properties as charge density wave (CDW) instabilities. We report on synchrotron radiation angle resolved photoemission spectroscopy (ARPES) measurements in order to obtain a detailed picture of the electronic structure of this material. The symmetry of the states near the Fermi level (E_F) has been discussed in relation to the photoemission symmetry selections rules. Our results are in excellent agreement with previous tight-binding calculations and support the hidden nesting concept proposed to explain the CDW instabilities exhibited by this family of compounds. In addition, a very peculiar photoemission line-shape has been found with the presence of localized non-dispersive states. Some possible explanations are discussed.     

Charge-density-wave partial gap opening in quasi-2D KMo6O17 purple bronze studied by angle resolved photoemission spectroscopy

Low dimensional (LD) metallic oxides have been a subject of continuous interest in the last two decades, mainly due to the electronic instabilities that they present at low temperatures. In particular, charge density waves (CDW) instabilities associated with a strong electron-phonon interaction have been found in Molybdenum metallic oxides such as KMo₆O₁₇ purple bronze. We report an angle resolved photoemission (ARPES) study from room temperature (RT) to T ∼40 K well below the Peierls transition temperature for this material, with CDW transition temperature T_CDW ∼120 K. We have focused on photoemission spectra along ΓM high symmetry direction as well as photoemission measurements were taken as a function of temperature at one representative k_F point in the Brillouin zone in order to look for the characteristic gap opening after the phase transition. We found out a pseudogap opening and a decrease in the density of states near the Fermi energy, E_F, consistent with the partial removal of the nested portions of the Fermi surface (FS) at temperature below the CDW transition. In order to elucidate possible Fermi liquid (FL) or non-Fermi liquid (NFL) behaviour we have compared the ARPES data with that one reported on quasi-1D K_0.3MoO₃ blue bronze.     

Resonant Raman scattering in the superconducting cuprates: Frozen-phonon versus perturbational approach

We consider in detail Raman scattering by vibration of the apical oxygen ions in the RBa₂Cu₃O₇ superconducting cuprates. The scattering intensity is very sensitive to the ratio of diagonal and off-diagonal matrix elements of electron-phonon coupling, bandstructure, and carrier concentration. Our results show a large quantitative difference between the results of frozen-phonon and perturbational approach to the Raman process. The discrepancy becomes especially large when interband transitions to the states near the Fermi level are close to resonance with the incident light. The calculation of phonon-induced ion charge fluctuations shows an analogous discrepancy. The reason for these effects is the possibility of carrier redistribution between different parts of the Fermi surface arising in the frozen-phonon approximation. Our results show that Raman scattering in superconducting superlattices is very sensitive to the properties of the states near the Fermi level. For this reason experiments performed on the superlattices can help to resolve the discrepancy. Received 8 December 1999     

Magnetoresistance of a Two-Dimensional TbTe<Subscript>3</Subscript> Conductor in the Sliding Charge-Density Wave Regime

The magnetoresistance of a TbTe₃ two-dimensional conductor with a charge-density wave (CDW) has been measured in a wide temperature range and in magnetic fields of up to 17 T. At temperatures well below the Peierls transition temperature and in high magnetic fields, the magnetoresistance exhibits a linear dependence on the magnetic field caused by the scattering of normal charge carriers by “hot” spots of the Fermi surface. In the sliding CDW regime in low magnetic fields, a qualitative change in the magnetoresistance has been observed associated with the strong scattering of carriers by the sliding CDW.     

Charge-density-wave instabilities and quantum transport in the monophosphate tungsten bronzes with m = 5 alternate structure

Resistivity, thermoelectric power and magnetotransport measurements have been performed on single crystals of the quasi two-dimensional monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m for m =5 with alternate structure, between 0.4 K and 500 K, in magnetic fields of up to 36 T. These compounds show one charge density instability (CDW) at 160 K and a possible second one at 30 K. Large positive magnetoresistance in the CDW state is observed. The anisotropic Shubnikov-de Haas and de Haas-van Alphen oscillations detected at low temperatures are attributed to the existence of small electron and hole pockets left by the CDW gap openings. Angular dependent magnetoresistance oscillations (AMRO) have been found at temperatures below 30 K. The results are discussed in terms of a weakly corrugated cylindrical Fermi surface. They are shown to be consistent with a change of the Fermi surface below 30 K. Received 23 November 1999 and Received in final form 23 March 2000     

Electromodulated infrared transmission spectra of blue bronze

The infrared transmission of the quasi-one dimensional charge-density-wave (CDW) conductor blue bronze (K_0.3MoO₃) is affected by polarization of the CDW, and therefore by application of a voltage near or above the threshold for CDW depinning. In this paper, we compare the spectra associated with the relative change in transmission taken for different temperatures and oscillating voltages. We find that the phonon spectrum is affected by CDW polarization; the linewidths or frequencies of most phonons change by cm^-1. However, no new intragap states that can be associated with current injection are observed; i.e. the spectra associated with polarization of the CDW in the crystal bulk is identical to that associated with CDW current injection near the contacts. Our results indicate that, for light polarized perpendicular to the conducting chains, the density (n), cross-section , and bandwidth of intragap states are related by: n (?cm^-1)^-1. For expected values of the cross-section and bandwidth, this implies that the intragap states can be optically excited for a time less than s. Received 21 February 2000     

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.     

High-temperature superconductors as heterostructures

The wave-function envelope method is used to describe the electronic states of the cuprate high-T _c superconductors (HTSCs). In this method the 2D electronic states of the CuO₂ layers of a unit cell play the role of quantum wells, while the 2D states of the reservoir play the role of quantum barriers. Because of the different anisotropy of the 2D effective masses of the wells and barriers, some states on the Fermi surface (line) belong to CuO₂ layers and some states belong to the reservoir layers. This behavior of the electronic states explains characteristic features of HTSCs, such as the existence of regions on the Fermi surface with strongly different relaxation times, the weak suppression of d-type superconducting pairing by nonmagnetic scattering, and the coincidence of the angular dependence of the superconducting order parameter and the angular dependence of the electronic density of states (forward scattering predominating). The change in the signs of the components of the effective masses along the Fermi surface can result in the formation of hole pairs (biholes) or electron pairs (bielectrons) on account of the Coulomb interaction in the case of a negative reduced mass of the pairs. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 211–216 (10 August 1998)     

Optical properties of the charge-density-wave polychalcogenide compounds R 2Te 5 (R=Nd, Sm and Gd)

We investigate the rare-earth polychalcogenide R₂Te₅ (R=Nd, Sm and Gd) charge-density-wave (CDW) compounds by optical reflectivity measurements. We obtain the optical conductivity through Kramers-Kronig transformation of the reflectivity spectra. From the real part of the optical conductivity we then extract the excitation energy of the CDW gap and estimate the fraction of the Fermi surface which is gapped by the formation of the CDW condensate. In analogy to previous findings on the related RTe_n (n=2 and 3) families, we establish the progressive closing of the CDW gap and the moderate enhancement of the metallic component upon chemically compressing the lattice.     

Electron-lattice interaction and the CDW state of 1T-TiSe2

The electronic band structure in the CDW state (superlattice structure) of 1T-TiSe₂ is calculated on the basis of the band-type Jahn-Teller model by extending our theory of lattice instability in the normal phase. A strong coupling between the hole-band (Se p states) around the Λ point and the electron-bands (Ti d states) around the Λ points is caused by the electron-lattice interaction. Reflecting such a strong coupling remarkable changes appear in the dispersion curves near the Fermi energy and the largest CDW gap is obtained to be 0.2 eV. We have also calculated a change of the density of states near the Fermi energy due to the superlattice formation. The result is consistent with that observed by angle-integrated photoemmision by Margaritondo et al. It is also shown that the magnitude of the lattice distortion observed at low temperatures can be explained in a way consistent with the lattice dynamics in the normal phase.     

Microscopic theory on the Raman spectra of transition metal dichalcogenides in CDW state

Microscopic mechanisms are clarified for Raman scattering of collective modes, i.e., amplitude and phase modes in the charge density wave (CDW) state of transition metal dichalcogenides. We study three phonon process in triple CDW state and effects of $\text{partial}$ destruction of the Fermi surface due to the phase transition. It has then been understood how both phase and amplitude modes are given Raman activity and how A-E splittings of both modes and the commensurability pinning of the phase mode are related to the three phonon process. We can also explain change of Raman intensity of the originally Raman active A_1g phonon mode at phase transition as interference between paramagnetic and diamagnetic contributions for 2HTaSe₂ as well as other materials.     

Novel mechanism of a charge density wave in a transition metal dichalcogenide

The charge density wave (CDW) is usually associated with Fermi surfaces nesting. We here report a new CDW mechanism discovered in a 2H-structured transition metal dichalcogenide, where the two essential ingredients of the CDW are realized in very anomalous ways due to the strong-coupling nature of the electronic structure. Namely, the CDW gap is only partially open, and charge density wave vector match is fulfilled through participation of states of the large Fermi patch, while the straight Fermi surface sections have secondary or negligible contributions.     

Electronic structure calculation of CuMn alloy

We have used a fully self-consistent multiple scatteringX _α method within the local density formalism to study the charge distribution, bonding characteristics and the density of states in CuMn alloy. The charge distribution shows almost no ionic character but significant hybridization ofs andd states is observed near the Fermi level. The crystal field splittings, ionization energies and the excitation energies are calculated and compared with experiments wherever available.     

Quasiparticle density of states of 2H-NbSe2 single crystals revealed by low-temperature specific heat measurements according to a two-component model

Low-temperature specific heat in a dichalcogenide superconductor 2H-NbSe2 is measured in various magnetic fields. It is found that the specific heat can be described very well by a simple model concerning two components corresponding to vortex normal core and ambient superconducting region, separately. For calculating the specific heat outside the vortex core region, we use the Bardeen-Cooper Schrieffer （BCS） formalism under the assumption of a narrow distribution of the superconducting gaps. The field-dependent vortex core size in the mixed state of 2H-NbSe2, determined by using this model, can explain the nonlinear field dependence of specific heat coefficient γ（H）, which is in good agreement with the previous experimental results and more formal calculations. With the high-temperature specific heat data, we can find that, in the multi-band superconductor 2H-NbSe2, the recovered density of states （or Fermi surface） below Tc under a magnetic field seems not to be gapped again by the charge density wave （CDW） gap, which suggests that the superconducting gap and the CDW gap may open on different Fermi surface sheets.     

Microscopic theory of a strongly correlated two-dimensional electron gas

The rearrangement of the Fermi surface in a diluted two-dimensional electron gas beyond the topological quantum critical point has been examined within an approach based on the Landau theory of Fermi liquid and a nonperturbative functional method. The possibility of a transition of the first order in the coupling constant at zero temperature between the states with a three-sheet Fermi surface and a transition of the first order in temperature between these states at a fixed coupling constant has been shown. It has also been shown that a topological crossover, which is associated with the joining of two sheets of the Fermi surface and is characterized by the maxima of the density of states N(T) and ratio C(T)/T of the specific heat to the temperature, occurs at a very low temperature T _⋄ determined by the structure of a state with the three-sheet Fermi surface. A momentum region where the distribution n(p, T) depends slightly on the temperature, which is manifested in the maximum of the specific heat C(T) near T _*, appears through a crossover at temperatures T ∼ T _* > T _⋄. It has been shown that the flattening of the single-particle spectrum of the strongly correlated two-dimensional electron gas results in the crossover from the Fermi liquid behavior to a non-Fermi liquid one with the density of states N(T) ∝ T ^−α with the exponent α }~ 2/3.     

Evolution of the electronic properties of transition metal nanoclusters on graphite surface

The electronic properties of nanoclusters of transition (Ni, Co, Cr) and noble (Au, Cu) metals deposited on the surface of highly oriented pyrolytic graphite (HOPG) are studied using the method of X-ray photoelectron spectroscopy. The laws of variation of a change ΔE _b in the binding energies of core-level electrons in the initial (ΔE _i) and final (ΔE _f) states of atoms in nanoclusters, the intrinsic widths γ of photoelectron lines, and their singularity indices α as functions of the metal cluster size d are determined. A qualitative difference in behavior of the ΔE _i(d) and α(d) values in metals of the two groups (Ni, Cr versus Co, Cu) is found. The values of the final-state energy (ΔE _f < 0) and the line width (Δγ > 0) in the clusters of all metals studied vary in a similar manner. It is shown that a significant contribution to E _i is due to a transfer of the valence-shell electrons at the cluster-substrate interface, which is caused by the contact potential difference. The value of an uncompensated charge per nanocluster is determined as a function of the cluster size and the number of atoms in the cluster. The behavior of ΔE _f(d) is controlled by the Coulomb energy of a charged cluster and by a decrease in the efficiency of electron screening, which is different in the metals studied. The broadening of photoelectron lines is determined by a spread of the cluster sizes and by lower electron screening in the final Fermi system. An asymmetry of the core-level electron spectra of nanoclusters can be explained using notions about the electron-hole pair excitation near the Fermi level. The effect of the structure of the density of electron states in the d band of transition metals on the asymmetry of photoelectron lines is considered and it is concluded that this structure near the Fermi level qualitatively changes with a decrease in the nanocluster size. The obtained results indicate that the behavior of the electron subsystem of clusters of the d-metals in a size range of 2–10 nm under consideration is close to the behavior of a normal Fermi system.