Charge density wave | EPFL Graph Search

A charge density wave (CDW) is an ordered quantum fluid of electrons in a linear chain compound or layered crystal. The electrons within a CDW form a standing wave pattern and sometimes collectively carry an electric current. The electrons in such a CDW, like those in a superconductor, can flow through a linear chain compound en masse, in a highly correlated fashion. Unlike a superconductor, however, the electric CDW current often flows in a jerky fashion, much like water dripping from a faucet due to its electrostatic properties. In a CDW, the combined effects of pinning (due to impurities) and electrostatic interactions (due to the net electric charges of any CDW kinks) likely play critical roles in the CDW current's jerky behavior, as discussed in sections 4 & 5 below. Most CDW's in metallic crystals form due to the wave-like nature of electrons – a manifestation of quantum mechanical wave-particle duality – causing the electronic charge density to become spatially modulated, i.e

Crystalline and correlated phases in two-dimensional transition metal dichalcogenides

Diego José Pasquier

This thesis is dedicated to the study of various aspects of the electronic structure of two-dimensional transition metal dichalcogenides (TMDs) of chemical composition MX

_2

(where M is a transition metal atom and X= S, Se, Te), using a combination of \textit{ab inito} density-functional methods. We first address the relative stability of the

1T

and

1H

phases of two-dimensional TMDs as a function of the column of the transition metal atom in the periodic table. Using a Wannier-function approach, we calculate crystal field and ligand field parameters for a broad range of members of this family of materials. Taking TaS

_2

as an example, we show how the splitting of the

d

electron states arises from an interplay of electrostatic effects and hybridization with the ligands'

s

p

and

d

states. We show that the ligand field alone cannot explain the stabilization of the

1H

polymorph for

d^1

and

d^2

TMDs, and that band structure effects are dominant. We present trends of the calculated parameters across the periodic table, and argue that these allow developing simple chemical intuition. Secondly, we study the occurrence of charge density wave phases and periodic lattice distortion in metallic

1T

transition metal dichalcogenides. The phonon dispersion and fermiology of representative examples with different

d

electron counts are studied as a function of doping. Two qualitatively different behaviours are found as a function of the filling of the

t_{2g}

subshell. We argue that away from half-filling, weak-coupling nesting arguments are a useful starting point for understanding, whereas closer to half-filling a strong-coupling real-space picture is more correct. Using Wannier functions, it is shown that strong metal-metal bonds are formed and that simple bond-counting arguments apply. Thirdly, the recently synthesized

1T

phase of NbSe

_2

, in monolayer form, is investigated from first principles. We find that

1T

-NbSe

_2

is unstable towards the formation of an incommensurate charge density wave phase, whose periodicity can be understood from the Fermi surface topology. We investigate different scenarios for the experimentally observed superlattice and insulating behaviour, and conclude that the star-of-David phase is the most stable commensurate charge density wave phase. We study the electronic properties of the star-of-David phase at various levels of theory and confirm its Mott insulating character, as speculated and in analogy with TaS

_2

. The Heisenberg exchange couplings are found to be ferromagnetic, which suggests a parallel with the so-called flat-band ferromagnetism in certain multiband Hubbard models. Finally, we address the possibility of the occurrence of the excitonic insulator phase in single-layer TiSe

_2

. The relative role of electron-electron and electron-phonon interactions in driving the charge density wave in layered and two-dimensional TiSe

_2

has been disputed and is still unresolved. We calculate the electronic structure and finite-momentum exciton spectrum from hybrid density functional theory. We find that in a certain range of parameters, excitonic effects are strong and the material is close to a pure excitonic insulator instability. A possible necessary condition for the physical realization of a pure excitonic insulator is proposed.

EPFL2019

Scanning tunneling microscopy of the charge density wave in 1T-TiSe2 in the presence of single atom defects

Philipp Aebi, Helmuth Berger

We present a detailed low-temperature scanning tunneling microscopy (STM) study of the commensurate charge density wave (CDW) in 1T-TiSe2 in the presence of single atom defects. We find no significant modification of the CDW lattice in single crystals with native defect concentrations where some bulk probes already measure substantial reductions in the CDW phase transition signature. A systematic analysis of STM micrographs combined with density functional theory modeling of atomic defect patterns indicate that the observed CDW modulation lies in the Se surface layer. The defect patterns clearly show there are no 2H-polytype inclusions in the CDW phase, as previously found at room temperature [A. N. Titov et al., Phys. Solid State 53, 1073 (2011)]. They further provide an alternative explanation for the chiral Friedel oscillations recently reported in this compound [J. Ishioka et al., Phys. Rev. B 84, 245125 (2011)].

Amer Physical Soc2015

Doping Nature of Native Defects in 1T-TiSe2

Philipp Aebi, Helmuth Berger

The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of T-CDW approximate to 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature-dependent resistivity, and strongly perturbate the CDW phase. Here, we study the structural and doping nature of such native defects combining scanning tunneling microscopy or spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies, and Se substitutions by residual iodine and oxygen.

Amer Physical Soc2014

Crystalline and correlated phases in two-dimensional transition metal dichalcogenides

Diego José Pasquier

This thesis is dedicated to the study of various aspects of the electronic structure of two-dimensional transition metal dichalcogenides (TMDs) of chemical composition MX

_2

(where M is a transition metal atom and X= S, Se, Te), using a combination of \textit{ab inito} density-functional methods. We first address the relative stability of the

1T

and

1H

_2

as an example, we show how the splitting of the

d

electron states arises from an interplay of electrostatic effects and hybridization with the ligands'

s

p

and

d

states. We show that the ligand field alone cannot explain the stabilization of the

1H

polymorph for

d^1

and

d^2

1T

transition metal dichalcogenides. The phonon dispersion and fermiology of representative examples with different

d

electron counts are studied as a function of doping. Two qualitatively different behaviours are found as a function of the filling of the

t_{2g}

1T

phase of NbSe

_2

, in monolayer form, is investigated from first principles. We find that

1T

-NbSe

_2

_2

_2

. The relative role of electron-electron and electron-phonon interactions in driving the charge density wave in layered and two-dimensional TiSe

_2

EPFL2019