Session N63: Organic Conductors and CDW Materials

Revealing the order parameter dynamics of 1T-TiSe2 during a photoinduced phase transition

TiSe2 is an intriguing material that hosts a charge density wave (CDW) phase below 200K and that becomes superconducting at elevated pressures or by Cu doping. The key to understand the emergence of such correlated behavioris the order parameter, which can be directly accessed by measuring the CDW gap. Here we use time and angular resolved photoelectron spectroscopy (trARPES)  to perturb the CDW phase in TiSe2 and

follow the quench and recovery of the order parameter in real time. The fluence dependence of the gap is discussed and its implication for the CDW formation mechanism are presented.   

Observation of Non-Hexagonal Domains in the Nearly-Commensurate CDW State of 1T-TaS2

The quasi-two-dimensional 1T-TaS2 is an intriguing material which hosts four distinct charge density wave (CDW) states and whose electronic properties can be manipulated or perturbed through a variety of methods including temperature, pressure, sample thickness variations, and the application of optical and voltage pulses. At low temperatures, 1T-TaS2 is in a commensurate CDW state. External perturbations, such as application of local voltage pulses or optical pulses, have led to non-hexagonal domaining and to new electronic orders emerging from within this commensurate CDW state. In this talk, we present our scanning tunneling microscopy measurements of the higher-temperature nearly-commensurate (NC) CDW state. We report observation of non-hexagonal domains in addition to the typical hexagonally-organized domains within the NC-CDW state. We compare the non-hexagonal domains observed in the NC-CDW state to those previously observed in the perturbed C-CDW state.   

Chiral Nature of the Nearly-Commensurate Charge Density Wave State in 1T-TaS2

1T-TaS₂ is a well-studied compound which plays host to a wealth of interesting physics.  Prominent in the material are four charge density wave (CDW) phases – commensurate, triclinic, nearly-commensurate, and incommensurate.  Here we present our room-temperature scanning tunneling microscopy studies of the nearly-commensurate (NC) CDW state.  The NC-CDW state involves hexagonally ordered domains.  Within a domain, the CDW spacing resembles that of the low-temperature commensurate CDW state, and domains are separated by an incommensurate CDW domain-wall network.  In our studies, we refine this characterization and show that, within a domain, the CDW state is not locked-in to the commensurate state but rather there is a continual evolution of the CDW state from domain wall to domain center.  Further, we find evidence for an intradomain CDW chirality driven by a strong coupling of the CDW to the lattice.   

Investigating the local structure of TiSe2 with symmetry-informed pair distribution function analysis

TiSe2 is an important member of the transition metal dichalcogenide family of materials known for their scientific and technological richness. TiSe2 undergoes a charge density wave (CDW) transition at 200 K, the origin of which has been hotly debated for over 40 years. Even the basic nature of the normal state and ground states as either semiconducting, semimetallic, or metallic is not entirely clear. This confusion stems in large part from the profound sample dependence of the electronic properties of TiSe2, obscuring the intrinsic material properties. To help clarify these issues, we have performed pair distribution function analysis of x-ray and neutron total scattering data collected from three different samples of TiSe2. Using symmetry mode amplitudes as the basis for structural refinements, we are able to probe the CDW transition and gain insight into the local atomic structure and its relation to the observed properties of TiSe2.    

Quantum fluidic transport of charge density waves at high temperatures

A growing body of evidence reveals that charge density waves (CDWs) often show quantum fluidic behavior in their transport and dynamics [APL 118, 184002 (2021)]. The time-correlated soliton tunneling (ST) model proposes that electron-phonon correlates within the CDW condensate (quantum solitons) act much like electrons tunneling through a Coulomb-blockade tunnel junction. Pair creation of fluidic soliton domain walls is prevented by their electrostatic energy below a Coulomb-blockade threshold field, much smaller than the classical depinning field for sliding. Above threshold, the quantum fluid flows periodically, like dripping water, via a hybrid between Zener-like and coherent Josephson-like tunneling. We summarize the time-correlated ST model and compare model simulations with experiment. The ST model shows excellent agreement with measured CDW coherent voltage oscillations, narrow band noise, and current-voltage characteristics. Remarkably, for NbS3 we find nearly precise agreement between quantum theoretical and experimental I-V curves for temperatures up to 474 K. Finally, we discuss broader implications for physics and potential applications. These include quantum reservoir computing and machine learning, as suggested by rapid natural learning phenomena in CDWs.   

Time-resolved nonlinear optical response of bulk 1T-TiSe2 single crystals

We present on the time-resolved measurement of nonlinear optical response in bulk single crystals of the transition metal dichalcogenide 1T-TiSe₂. Our experiments were conducted as a function of temperature, fluence and incident pump polarization and reveal the presence of optical phonon modes and changes eletronic relaxation across the charge density wave transition temperature at 202 K. We discuss our results in the context of potential chiral charge ordering in this material system.   

Temperature Evolution of the Nearly-Commensurate Charge Density Wave State of 1T-TaS2

The nearly-commensurate (NC) charge density wave (CDW) state of 1T-TaS2 is characterized by a hexagonal atomic lattice, hexagonal CDW lattice, and a hexagonally-ordered CDW domain structure. These features are easily captured in scanning tunneling microscopy images. Analysis of these images, as well as their Fourier transforms, allows for a sub-angstrom characterization of the NC-CDW state. In this work, we combine scanning tunneling microscopy (STM) measurements with computer-generated topographic simulations to study the temperature-dependent evolution of the NC-CDW state across its temperature range (~200 – 355 K). In particular, we detail the temperature-induced evolution of CDW domains and observed intradomain CDW chirality.   

Charge density wave order in a nonsymmorphic crystal TaTe4

Charge density wave (CDW) orders may emerge in quantum materials due to a number of possible causes: Fermi surface nesting, electron-phonon coupling, and electron-electron interactions. Interestingly, CDW orders could also modify the crystalline symmetries of a material that affect symmetry-induced topological crossings. Here using state-of-the-art angle-resolved photoemission spectroscopy (ARPES), we decipher the electronic band structure of a quasi-1D nonsymmorphic compound TaTe₄ in its (2a×2b×2c) CDW ground state. We will present experimental evidence of CDW-induced band folding that resolve previous contradictory conclusions between ARPES spectra and optical measurements. In addition, we will discuss the prospect of the crystalline symmetries introduced by the CDW order in this system towards realizing unconventional topological fermions.   

Scanning Nearfield Optical Microscopy study of 1T-TaS2

1T-TaS₂ is a transition metal dichalcogenide (TMD) with several notable features. Upon cooling, it undergoes several phase transitions to a non-commensurate charge-density-wave (CDW) state and further to a commensurate charge-density wave state. In this commensurate CDW ground state where the Ta atoms form star-of-David clusters, 1T-TaS₂ is said to be in a Mott insulating state. Furthermore, 1T-TaS₂ has been proposed to be a spin liquid candidate in this CDW ground state. In this talk, I will present data taken using scattering type scattering type scanning near-field optical microscopy (s-SNOM) on a graphene encapsulated thin flake of 1T-TaS₂, where we have imaged the near-field optical response through the metal-insulator transition down to a temperature of 50K.   

Magnetic field effects on superconductivity near frustrated Mott insulator.

Quasi-2D organic charge transfer salts are considered model systems for understanding unconventional superconductivity (SC) in cuprates and other correlated electron systems.  However, studies of SC in quasi-2D organics have been limited because the SC phase is typically reached by applying pressure.  In contrast, we perform experiments under ambient pressure on κ-[(BEDT-TTF)_0.88(BEDT-STF)_0.12]₂Cu₂(CN)₃, which is formed by partial chemical substitution of a quantum spin liquid candidate κ-(BEDT-TTF)₂Cu₂(CN)₃.  We measure linear and nonlinear dc transport over a wide range of perpendicular magnetic fields (H) and temperatures (T) down to ~0.25 K.  The results indicate the onset of SC at ~ 3 K for H=0.  The suppression of SC with H reveals, surprisingly, an insulatinglike normal state.  The underlying field-tuned superconducting quantum phase transition and the nature of the high-field normal state will be discussed and compared to those in cuprates.   

Charge Singlets and Orbital Selective Charge Density Wave Transitions

The possibility of 'orbitally selective Mott transitions' within a multi-band Hubbard model, in which one orbital with large on-site electron-electron repulsion U₁ is insulating and another orbital, to which it is hybridized, with small U_-1, is metallic, is a problem of long-standing debate and investigation. In this paper we study an analogous phenomenon, the co-existence of metallic and insulating bands in a system of orbitals with different electron-phonon coupling. To this end, we examine two variants of the bilayer Holstein model: a uniform bilayer and a 'Holstein-Metal interface' where the electron-phonon coupling (EPC) λ is zero in the 'metal' layer. In the uniform bilayer Holstein model, charge density wave (CDW) order dominates at small interlayer hybridization t₃, but decreases and eventually vanishes as t₃ grows, providing a charge analog of singlet (spin liquid) physics. In the interface case, we show that CDW order penetrates into the metal layer and forms long-range CDW order at intermediate ratio of inter- to intra-layer hopping strengths 1.4 ≤ t₃/t ≤ 3.4. This is consistent with the occurrence of an 'orbitally selective CDW' regime at weak t₃ in which the layer with λ₁≠0 exhibits long range charge order, but the 'metallic layer' with λ_-1=0, to which it is hybridized, does not.   

Fluctuating pseudospin dimers in cluster Mott insulator

The Lacunar spinel, GaTa₄Se₈, is a unique Mott insulator that hosts molecular spin-orbit J_eff-3/2 states as correlation units. The material undergoes a concurrent magnetic and structural transition at 50 K and forms a singlet ground state below the transition temperature. The magnetic ground states are entangled to the lattice distortion, and a preceding phonon mode was observed by neutron scattering measurements. However, there are no signs of the transition in the local distortion from x-ray and neutron pair distribution function measurements indicating the fluctuating dimer phase in GaTa₄Se₈. In this talk, I will discuss a series of x-ray and neutron pair distribution function measurements combining the phonon analysis to uncover the fluctuating pseudospin dimer phase in this representative cluster Mott insulator.   

Density wave mechanism with a variable wave vector from a Dirac-fermion-Landau-level perspective

A density wave is an ordered phase formed by electrons in materials under certain conditions. In the conventional Peierls' mechanism or Fermi surface nesting theory, when the temperature is below Tc, the renormalized phonon frequency touches zero at a specific wave vector, which corresponds to a periodic distortion of the lattice and a new distribution of charge or spins in real space. However, these theories have some obvious drawbacks such as the high dependence on the shape of the Fermi surface. We propose a new density wave mechanism based on the low-energy effective theory of the Landau level at the Dirac point, which does not depend on the shape of the Fermi surface, and the wave vector can change continuously according to the variation of the coupling strength. At the same time, we give a class of models applicable to this theory and perform numerical calculations and analysis for a particular model.   

Observation of hidden electronic phases in 1T-TaS2 through conductance fluctuation spectroscopy

Transition metal dichalcogenides have drawn the attention of the research community as they host multiple, competing electronic phases. One of the most studied materials is 1T-TaS₂ which has a multitude of electronically and structurally distinct phases. For example, the collapse of the low-temperature stable commensurate charge density wave (CCDW) phase under various perturbations leads to the appearance of metastable phases is a big puzzle. Using lateral strain as a tuning parameter we probe the effect of varying the interlayer coupling strength on the electronic phases in few-layer 1T-TaS₂. We observe the co-existence of insulating and metallic phases deep within the CCDW phase. Through detailed conductance fluctuation spectroscopy of the electronic ground state, we establish that the mixed-phase consists of insulating regions surrounded by one-dimensional metallic domain walls.  Our study raises the prospect of using 1T-TaS₂ as a prospect for strain-based devices.