Dimensional Charge Density Wave Research Articles

Charge filling induced quasi-two dimensional charge/spin density wave channel in a ballistic transport device

Charge filling controlled mean field metal–insulator phase transition is examined in the context of two dimensional Fermi surface nesting and van Hove singularity induced charge density wave (CDW), spin density wave (SDW) condensates. In the framework of a coherent ballistic transport model utilizing the Non-Equilibrium Green Function approach (NEGF), a three terminal device with metallic gate, source, drain and CDW/SDW channel is simulated and studied. Within the validity of mean field approximation, we exposit the commensurability and boundary effects. The efficacy of the Hubbard model for (quasi) two dimensional Charge and Spin Density Wave materials is discussed. A two orbital generalization of the effective Hamiltonian is proposed for transport calculations in rare earth Tellurides RTe 3.

Non-Maxwell–Boltzmann dependence of channel carrier concentration in a quasi one dimensional charge density wave channel in the ballistic transport regime

With the aid of a coherent transport model utilizing the non-equilibrium Green function approach, a three terminal device with metallic gate, source and drain and a quasi one dimensional charge density wave (CDW) channel is simulated focussing on the transistor behaviour brought about by a sweep of the channel potential or equivalently the chemical potential in the channel. The channel is strongly insulating only at half-filling and moving to lower and higher carrier concentrations both incur a mean field phase transition to a conducting state. With the aid of conductance calculations for a pinned CDW condensate, we present calculations for the sub-threshold slope in terms of the hopping parameter or equivalently the width of the tight-binding chain. The effects of source to drain bias and length are examined. The conductance profiles are analyed in relation to transmission profiles. The observed CDW profiles are explained in terms of filling and Fermi surface nesting. Boundary conditions, gap equations and response functions are shown to reveal the commensurability conditions and size of the transport gap. The channel carrier concentration is modulated in an athermal (non-Maxwellian–Boltzmann) fashion, thereby making it an interesting prospect for steep transistors.

Charge density waves in low-dimensional material

Charge density waves (CDWs) have triggered off extensive research in low-dimensional systems. The discovery of CDW offers a new crucial clue to understanding the intrinsic mechanisms of low-dimensional electron-phonon coupling and electron correlation. In addition, the physical properties of low-dimensional material such as magnetism and superconductivity can be fine-tuned with accurately and effectively controlled CDW phase. At the beginning,we briefly introduce the basic properties of CDW in one-dimensional and quasi one-dimensional materials, revealing the physical proprieties of the CDW, for instance, the excited state and the manipulation technologies. Then, focusing on the CDW in a two-dimensional system, we mainly introduce the recent research progress and the generation mechanism of CDW of two-dimensional materials. The interaction between CDW and Mott insulator and between superconductivity and other orders such as spin density wave and pair density wave provide a new perspective to research the multi-electron collective excitation and electron interaction. The manipulation of multi-electron collective excitation and electron-phonon interaction in CDW through doping, high pressure and laser pulse is also introduced and shares similarity with the one-dimensional system. Finally, in this article we propose a potential research application of two dimensional CDW.

Transport and thermal behavior of the charge density wave phase transition in CuTe

Copper monotelluride CuTe is of current interest due to the discovery of the quasi-one dimensional charge density wave (CDW) behavior below the transition temperature ${T}_{\mathrm{CDW}}\ensuremath{\simeq}335\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. To explore the transport and thermodynamic properties and provide experimental insights into the underlying origins of the CDW, we have carried out a combined study by means of the electrical resistivity, Seebeck coefficient, thermal conductivity, as well as specific heat measurements on single crystalline CuTe. The CDW phase transition has been characterized by marked features near ${T}_{\mathrm{CDW}}$ from all measured physical quantities. In particular, the observed Seebeck coefficient and electronic thermal conductivity exhibit a pronounced reduction as cooling the temperature across ${T}_{\mathrm{CDW}}$, indicative of the partially gapped Fermi surfaces associated with the CDW formation. From the examination of the excess specific heat in the vicinity of ${T}_{\mathrm{CDW}}$, we obtained evidence for the strong-coupling character of the CDW, suggesting that the electron-phonon coupling plays an important role for the CDW instability in CuTe.

Infrared spectroscopy study of the in-plane response of YBa2Cu3O6.6 in magnetic fields up to 30 Tesla

With Terahertz and Infrared spectroscopy we studied the in-plane response of an underdoped YBa2Cu3O6.6 single crystal with Tc=58(1) K in high magnetic fields up to B=30 Tesla applied along the c-axis. Our goal was to investigate the field-induced suppression of superconductivity and to observe the signatures of the three dimensional (3d) incommensurate copper charge density wave (Cu-CDW) which was previously shown to develop at such high magnetic fields. Our study confirms that a B-field in excess of 20 Tesla gives rise to a full suppression of the macroscopic response of the superconducting condensate. However, it reveals surprisingly weak signatures of the 3d Cu-CDW at high magnetic fields. At 30 Tesla there is only a weak reduction of the spectral weight of the Drude-response (by about 3%) that is accompanied by an enhancement of two narrow electronic modes around 90 and 240 cm-1, that are interpreted in terms of pinned phase modes of the CDW along the a- and b-direction, respectively, and of the so-called mid-infrared (MIR) band. The pinned phased modes and the MIR band are strong features already without magnetic field which suggests that prominent but short-ranged and slowly fluctuating (compared to the picosecond IR-time scale) CDW correlations exist all along, i.e., even at zero magnetic field.

Elastic anomalies at the first order transition in Lu5Ir4Si10

Ultrasonic measurements on a single crystal of the intermetallic compound Lu5Ir4Si10 are presented. Large anomalies in the velocity and ultrasonic attenuation of the elastic modes are observed at the structural phase transition TS∼80 K. The velocity of the longitudinal C11 and C33 mode exhibits a second order transition behavior. In contrast the velocities of the shear C44 and C66 modes exhibit a first order transition behavior. The longitudinal C11 mode is strongly coupled to the ordering system below TS. Similarities in the longitudinal elastic anomalies with known reduced dimensional charge density wave systems suggest that the nature of the structural phase transition TS in Lu5Ir4Si10 could be associated with a charge density wave phase transition.

Voltage-induced metal-insulator transition in a one-dimensional charge density wave

We present a theoretical investigation of the voltage-driven metal insulator transition based on solving coupled Boltzmann and Hartree-Fock equations to determine the insulating gap and the electron distribution in a model system -- a one dimensional charge density wave. Electric fields that are parametrically small relative to energy gaps can shift the electron distribution away from the momentum-space region where interband relaxation is efficient, leading to a highly non-equilibrium quasiparticle distribution even in the absence of Zener tunneling. The gap equation is found to have regions of multistability; a non-equilibrium analog of the free energy is constructed and used to determine which phase is preferred.

Inhomogeneous phases in coupled electron-hole bilayer graphene sheets: Charge Density Waves and Coupled Wigner Crystals

Recently proposed accurate correlation energies are used to determine the phase diagram of strongly coupled electron-hole graphene bilayers. The control parameters of the phase diagram are the charge carrier density and the insulating barrier thickness separating the bilayers. In addition to the electron-hole superfluid phase we find two new inhomogeneous ground states, a one dimensional charge density wave phase and a coupled electron-hole Wigner crystal. The elementary crystal structure of bilayer graphene plays no role in generating these new quantum phases, which are completely determined by the electrons and holes interacting through the Coulomb interaction. The experimental parameters for the new phases lie within attainable ranges and therefore coupled electron-hole bilayer graphene presents itself as an experimental system where novel emergent many-body phases can be realized.

Sliding state of the quasi-two dimensional CDW system TbTe3 probed by coherent x-ray diffraction

Coherent x-ray diffraction experiments have been performed to probe the sliding state of a quasi-two dimensional charge density wave (CDW) in TbTe3. The 2kF satellite reflection associated with the CDW has been observed with respect to external dc currents, on both sides of the threshold current. These measurements illustrate how the periodic lattice distortion associated to the CDW supports the collective motion of a sliding CDW in a quasi-two dimensional system. A comparison with the quasi-one dimensional NbSe3 system is done.

Direct observation of single-electron solitons and Friedel oscillations in a quasi-one dimensional material with incommensurate charge-density waves

We have performed scanning tunneling microscopy experiments in the quasi-one dimensional charge density wave (CDW) system NbSe3, where we could image and study in detail individual solitons corresponding to the self-trapping of a single electron. Our analysis shows that the type of soliton we observed is an “Amplitude Soliton”, characterized by a vanishing CDW amplitude at the soliton center and by a π-shift of its phase along the chain direction. Pairs of solitons or multiple solitons were also observed. Such observations could be made only in the high-temperature CDW phase. Additionally, one-dimensional Friedel oscillations around charged defects could also be imaged and are found to be superimposed to the CDW. The distinction between amplitude solitons and Friedel oscillations can be made without any ambiguity.

Photoinduced Dynamics in the Charge Density Wave Compound 4HB-TaSe2

We report on ultrafast photoindued charge density wave (CDW) dynamics in the transition-metal dichalcogenide 4Hb -TaSe2 , studied with ultrafast electron diffraction. Fluence dependence of the lattice superstructure suppression show a phase transition from the commensurate to the incommensurate phase of 4Hb -TaSe2 . Unusually long recovery times of perturbed charge density waves indicate th importance of a coupling between the two dimensional CDWs.

Unconventional Charge Density Wave Arising from Electron–Phonon Interaction

We report of a theoretical study on quasi-one dimensional unconventional charge density wave (UCDW) driven by electron–phonon interaction. Within mean field theory, we find that the wavevector dependence of the coupling leads to a momentum dependent single particle gap on the Fermi surface. The presence of small energy single particle excitations around the gap nodes significantly changes the optical conductivity compared to the conventional CDW result. In addition to that, the collective phase excitation arising from fluctuation of the order parameter leads to further qualitative changes of the conductivity and results in an effective mass that is nonmonotonic in temperature.

High magnetic field magnetoresistance anomalies in the charge density wave state of the quasi-two dimensional bronze KMo6O17

We report high magnetic field magnetoresistance measurements performed in pulsed fields up to 55T on the quasi-two dimensional charge density wave conductor KMo 6 O 17 . Magnetoresistance curves show several anomalies below 28 T. First order transitions to smaller gap states take place at low temperature above 30T. A phase diagram T(B) has been obtained. The angular dependence of the anomalies is reported.

Theory for Optical Absorption, Direct and Indirect Excitons in BaBiO3

The infrared and visible spectral shapes of optical absorption in a three dimensional charge density wave (CDW) state of BaBiO 3 (BBO) are theoretically studied by using an extended Peierls-Hubbard model, so as to clarify the direct and indirect exciton effects in this material. For this purpose we introduce the adiabatic approximation for phonons and the Hartree-Fock approximation for inter-electron coulombic interactions. The electron-hole correlation on the Bi atoms and the classical fluctuation of the oxygen sub-lattice coordinates are also taken into account, so as to obtain exciton effects as well as thermal fluctuations of the lattice. The energy splitting due to CDW formation in the 6s band of Bi is found to be very large (2.0 eV), whereas, the minimum gap is much smaller (0.55 eV) than it. This large optical gap appears in the visible region as a dominant peak of the optical conductivity, and it is mainly due to the direct exciton. While the smaller gap appears in the infrared region, and it is d...

Quantum fluctuations, Berry's phase and the one dimensional charge density wave

We show that the static long-range order of a charge density wave (CDW) state in a one-dimensional system is destroyed by slow quantum fluctuations of the phonon mode responsible for this state by a mechanism involving Berry's geometric phase.

Elastic chain in a random potential: Simulation of the displacement correlation function and relaxation.

We simulate the low temperature behaviour of an elastic chain in a random potential where the displacements $u(x)$ are confined to the {\it longitudinal} direction ($u(x)$ parallel to $x$) as in a one dimensional charge density wave--type problem. We calculate the displacement correlation function $g(x)=< (u(x)-u(0))^2>$ and the size dependent average square displacement $W(L)=<(u(x)-\bar{u})^2>$. We find that $g(x)\sim x^{2\eta}$ with $\eta\simeq3/4$ at short distances and $\eta\simeq3/5$ at intermediate distances. We cannot resolve the asymptotic long distance dependence of $g$ upon $x$. For the system sizes considered we find $g(L/2)\propto W\sim L^{2\chi}$ with $\chi\simeq2/3$. The exponent $\eta\simeq3/5$ is in agreement with the Random Manifold exponent obtained from replica calculations and the exponent $\chi\simeq2/3$ is consistent with an exact solution for the chain with {\it transverse} displacements ($u(x)$ perpendicular to $x$).The distribution of nearest distances between pinning wells and chain-particles is found to develop forbidden regions.

Quasi one dimensional charge density wave instabilities in the family of monophosphate tungsten bronzes: (PO 2) 4(WO 3) 2m

We present a X-ray diffuse scattering investigation of the members m=4,6,7 of a new series of tungsten bronzes (PO 2) 4(WO 3) 2m, which exhibit two-dimensional electronic properties. Suprisingly, at ambient temperature these materials show quasi-one-dimensional (1D) charge density wave (CDW) fluctuations, as proved by computer simulations of the X-ray patterns. These fluctuations can be associated with particular chains of WO 6 octahedra, contributing to flat portions of the Fermi surface of these materials. In P 4W 12O 44 these fluctuations condense into successive phase transitions at 120 K and 62K where the electrical resistivity shows giant anomalies. In P 4W 14O 50 they condense at 189 K into a triply incommensurate and non-sinusoidal modulation exhibiting harmonics up to the 6 th order. Below 47 K another triply incommensurate and non-sinusoidal modulation is stabilized. Different quasi-1D fluctuations are involved in the 80 K and 52 K CDW phase transitions of P 4W 8O 32. The origin of these 1D CDW instabilities and their 3D ordering at the various phase transitions are discussed in the framework of nesting mechanisms of the Fermi surface of these bronzes.

The two dimensional electron solid charge density wave

A brief presentation of the physics and physical realisations of classical and quantum 2 dimensional electron solids is followed by a tourist guide to some of the experiments

Imperfect nesting and density of states in charge and spin density waves

In actual three dimensional charge density wave (CDW) and spin density wave (SDW), imperfect nesting plays a crucial role unlike in the one demensional system. We review here a variety of physical properties associated with imperfect nesting. Perhaps imperfect nesting is most directly accessible through the study of electron tunneling.

Electromechanical properties of charge density waves and spin density waves

We review a recent theory of sound propagation in the quasi one dimensional charge density wave (CDW) or spin density wave (SDW). The elastic moduli of a CDW or an SDW depend strongly on whether the CDW (or the SDW) is pinned or unpinned. When the CDW (or the SDW) is unpinned, the elastic moduli are the same as in the normal state, while the elastic moduli increase in the CDW (or the SDW) when it is pinned. The electric field dependence of Young's modulus in a CDW is then interpreted in terms of the depining process in a CDW. Our predictions are compared with the experiments in (TMTSF) 2PF 6.