Electronic Charge Density Wave Research Articles

Quantum nonlinear phononics route towards nonequilibrium materials engineering: Melting dynamics of a ferrielectric charge density wave

Negative nonlinear electron-phonon coupling involving an infrared-active phonon mode can lead to an instability towards the formation of a polar lattice distortion with ferrielectric (FE) moments accompanied by an electronic charge-density wave (CDW). Analyzing a small model system in and out of thermal equilibrium, we investigate the FE-CDW and its melting dynamics following an ultrashort laser pulse that drives the ionic dipoles. We observe nonequilibrium coherent phonon amplitude mode oscillations that soften towards the transition to the normal phase. Our case study serves as a first step towards a microscopic understanding of quantum nonlinear phononics as a basis for nonequilibrium control in quantum materials.

Theory of hydrodynamic transport in fluctuating electronic charge density wave states

We describe the collective hydrodynamic motion of an incommensurate charge density wave state in a clean electronic system. Our description simultaneously incorporates the effects of both pinning due to weak disorder and also phase relaxation due to proliferating dislocations. We show that the interplay between these two phenomena has important consequences for charge and momentum transport. For instance, it can lead to metal-insulator transitions. We furthermore identify signatures of fluctuating density waves in frequency and spatially resolved conductivities. Phase disordering is well known to lead to a large viscosity. We derive a precise formula for the phase relaxation rate in terms of the viscosity in the dislocation cores. We thereby determine the viscosity of the superconducting state of BSCCO from the observed melting dynamics of Abrikosov lattices and show that the result is consistent with dissipation into Bogoliubov quasiparticles.

Short range smectic order driving long range nematic order: example of cuprates.

We present a model for describing the combined presence of nematic and ‘smectic’ or stripe-like orders seen in recent scanning tunneling microscopy (STM) experiments on cuprates. The smectic order is treated as an electronic charge density wave with an associated Peierls distortion or a ‘Pomeranchuk wave’. This primary order is restricted to nanoscale domains by disorder effects, while the secondary coupling to strain generates the nematic order with a considerably longer range. A variety of experimental results are shown to be consistent with our theoretical predictions.

Quantum plasmonics

Quantum plasmonics is a rapidly growing field of research that involves the study of the quantum properties of light and its interaction with matter at the nanoscale. Here, surface plasmons—electromagnetic excitations coupled to electron charge density waves on metal–dielectric interfaces or localized on metallic nanostructures—enable the confinement of light to scales far below that of conventional optics. We review recent progress in the experimental and theoretical investigation of the quantum properties of surface plasmons, their role in controlling light–matter interactions at the quantum level and potential applications. Quantum plasmonics opens up a new frontier in the study of the fundamental physics of surface plasmons and the realization of quantum-controlled devices, including single-photon sources, transistors and ultra-compact circuitry at the nanoscale. Surface-plasmon polaritons are hybrid particles that result from strong coupling between light and collective electron motion on the surface of a metal. This Review presents an overview of the quantum properties of surface plasmons, their role in controlling light–matter interactions at the quantum level and potential applications.

Competing phases in the cuprates: Charge vs spin order

We have investigated magnetic and charge instabilities of the cuprates within the Gutzwiller approximation RPA (GA+RPA). Interestingly, in GA the dressed Hubbard U is not a single parameter, but has different forms in the spin and charge responses, with distinct doping dependencies. While there are a number of competing magnetic instabilities for hole-doped cuprates, we fail to find any purely electronic charge density waves. The dominant magnetic instabilities are associated with ‘double nesting’, and the phase diagrams are material dependent, with LSCO differing from other cuprates.

Quantum Melting of the Hole Crystal in the Spin Ladder ofSr14−xCaxCu24O41

We have used resonant soft x-ray scattering to study the effects of discommensuration on the hole Wigner crystal (HC) in the spin ladder Sr(14-x)CaxCu24O41 (SCCO). As the hole density is varied the HC forms only with the commensurate wave vectors L(L) = 1/5 and L(L) = 1/3; for incommensurate values it "melts." A simple scaling between L(L) and temperature is observed, tau1/3/tau1/5 = 5/3, indicating an inverse relationship between the interaction strength and wavelength. Our results suggest that SCCO contains hole pairs that are crystallized through an interplay between lattice commensuration and Coulomb repulsion, reminiscent of the "pair density wave" scenario.

Effects of acoustic-mode localization under dimensional crossover of an electron gas

We have considered the semiconductor heterostructure that localizes acoustic modes within the quantum-well layer. It has been shown that additional confinement of the electrons in the plane of the layer gives rise to a supplementary localization (in a second direction) of acoustic modes. That means, in the many-electron system interacting with the lattice, the crossover from two- to one-dimensional electron-gas behavior leads to the appearance of a collective excitation which consists of lattice vibrations and electron charge-density waves, accompanied by an electrostatic field. The spectrum of this excitation is quite similar to those of acoustic phonons. The excitation is localized in two directions and is one dimensional in character. Splitting of this branch of the excitation from phonon branches of the structure and its additional localization increases with the wave vector of the excitation q. The two-dimensional electron gas forming a conductive channel with submicrometer dimensions completely localizes the excitation within this channel. If the electron gas is confined further to a one-dimensional channel, the excitation reveals maxima in the splitting and localization in the region q\ensuremath{\approx}${2\mathrm{k}}_{\mathrm{f}}$, ${\mathrm{k}}_{\mathrm{f}}$ being the Fermi wave vector. In this region the dispersion curve of the excitation shows rotonlike behavior and the density of states has a peak (T=0).

X-ray evidence of charge density wave modulations in the magnetic phases of (TMTSF) 2PF 6 and (TMTTF) 2Br

We have performed X-ray scattering investigations of (TMTSF) 2PF 6 and (TMTTF) 2 Br in their magnetic ground states. Very weak 2k F and 4k F satellite reflections are observed in the spin density wave (SDW) ground state of (TMTSF) 2PF 6. They are probably the signature of a purely electronic charge density wave (CDW) modulation of about one tenth of electron per molecule. Our X-ray results together with magnetic measurements show that the ground state of (TMTSF) 2PF 6 will exhibit an unprecedented 2k F mixed CDW-SDW modulation. 4k F satellite reflections are observed in the antiferromagnetic (AF) ground state of (TMTTF) 2Br. In this salt the 4k F CDW modulation is probably set by magnetoelastic coupling of the lattice degrees of freedom with the AF modulation.

Structure and Bonding in Cubic IV‐VI Crystals VII. On the Influence of Defects on the Stability of the Cubic Structure

AbstractThe influence of defects (vacancies) on the phase transition is qualitatively treated within the framework of simple one‐dimensional models. Generalizing Gömez‐Santos' and Ynduráin's treatment to a diatomic chain, their stabilizing the cubic phase rôle can be explained as destroying the coherence of the electronic charge density wave driving the Peierls instability. The degree of coherence destroying necessary to suppress the phase transition can be quite small due to the extrinsic Peierls gap (ionic energy).

Charge-density wave and magnetoresistance anomaly in graphite

The magnetoresistance of graphite exhibits a sharp increase at a certain critical field ${H}_{c}$ (${H}_{c}\ensuremath{\ge}22$ T), where ${H}_{c}$ is sensitive to temperature. This anomaly was ascribed to the formation of an electronic charge density wave by Yoshioka and Fukuyama (YF). To obtain an expression of ${H}_{c}$ versus $T$, it is very important to estimate the screening constant of the Coulomb interaction beyond the Thomas-Fermi approximation which was employed in YF theory. By introducing the non---Thomas-Fermi term in the screening constant we can obtain an expression for ${H}_{c}$ versus $T$ which is in good agreement with the observed results. The Hamiltonian is diagonalized by using a decoupling procedure in the Hartree-Fock scheme. With the use of these results, the transverse magnetoresistance is calculated, and the calculation describes the qualitative features of the observed results.