Session W56: Density Waves in Materials

Novel photoinduced charge density wave state and periodic lattice distortion in 1T-TaSe2

The amplitude mode in a charge density wave (CDW) material is the collective modulation of the relevant periodic lattice distortion (PLD). Here we use a femtosecond laser pulse to displacively excite the coherent amplitude mode in the CDW material 1T-TaSe₂. We then use time- and angle-resolved photoemission to track the dynamic evolution of the amplitude mode and electronic band structure – and in particular, the position and the width of the Ta 5d band. At sufficiently high fluences, and within a half-cycle of the amplitude mode oscillation, the PLD of 1T-TaSe₂ first relaxes to the position of the normal state, before overshooting to form an inverted CDW transient state. Moreover, the band width is maximum for the normal state, while it is reduced for both the ground CDW state and the inverted CDW state. Another important observation is that this inverted CDW state has larger density of states at the Fermi level than the normal state. Our observations are consistent with both density functional theory (DFT) and tight binding calculations. Our results show the possibility of using light to generate unique lattice distortions and electron correlations via ultrafast coherent phonon excitation.   

Suppression of charge density wave transitions in La(Ag1-xAux)Sb2 (0≤x≤1) by pressure.

LaAgSb₂ exhibits two CDW transitions at ambient pressure: at T₁ ~ 207 K a lattice modulation along the a-axis develops and upon further cooling, at T₂ ~ 186 K, an additional CDW ordering along the c-direction appears. [1] Here, we report resistivity measurements on La(Ag_1-xAu_x)Sb₂ (0≤x≤1) samples at ambient and high pressure that reveal suppression of both CDW transitions by pressure and Au substitution. By introducing simple pressure scale shifts for different x-values, the T(P) data fall onto universal T₁(P*) and T₂(P*) curves. Results will be compared with and contrasted to the literature data on (La_1-xR_x)AgSb₂ (R = Ce, Nd). [2]
[1] C. Song et al., Phys. Rev. B 68, 035113 (2003).
[2] M. Torikachvili et al., Phys. Rev. B 76, 235110 (2007).   

Emergence of charge density wave domain walls network in Pt doped TiSe2

Charge density wave (CDW) incommensuration in the TiSe2 family proceeds through the formation of domain walls (DWs). Previous scanning tunneling microscopy (STM) studies on Cu intercalated TiSe2 have shown that DWs host an extra population of electrons and might play an important role in the emergence of superconductivity. Here we report a low temperature STM study of Pt-doped TiSe2. A network of nanometer-thick DWs is observed and STM spectra reveal local inhomogeneity of the electronic properties which results in the emergence of pseudogaps of different size. In contrast to Cu intercalation, Pt substitution creates a network of 1D channels of low energy transport which subtract electrons from the bulk enhancing correlation effects. The interplay between electron correlation and disorder induced by DWs might be fundamental for the metal to insulator transition reported by previous transport measurements.   

Scanning Tunneling Microscopy of the Nearly-Commensurate and Incommensurate Charge Density Wave Phases of 1T-TaS2

1T-TaS₂ is an intensely-studied charge density wave (CDW) compound which hosts a wealth of interesting and diverse physics. At low temperatures, 1T-TaS₂ is in a Mott insulating, commensurate CDW state. At higher temperatures, the material can enter into triclinic, nearly-commensurate, and incommensurate CDW states. Studies across all four CDW states using multiple experimental techniques have been conducted, but considerable emphasis has been on the low-temperature commensurate CDW state. Here we present our scanning tunneling microscopy measurements on the higher-temperature nearly-commensurate and incommensurate CDW phases with a focus on domains, domain walls, and their changes with temperature.   

Charge density wave (CDW) and magnetism in RNiC2 family (R = rare earth metal)

The ternary rare-earth nickel dicarbides RNiC₂ (R - rare earth metal) with a non-centrosymmetric, orthorhombic crystal structure offer a unique opportunity to tune the ground state with varying R atom. The charge density wave (CDW) has been found for most of the members of the RNiC₂ family (R = Pr − Lu) with the Peierls temperature scaling linearly with the unit-cell volume. LaNiC₂ compound is an unconventional superconductor, SmNiC₂ undergoes a ferromagnetic transition and the rest of the compounds (apart from nonmagnetic YNiC₂ and LuNiC₂ exhibiting large positive magnetoresistance and PrNiC₂ where only a weak magnetic anomaly is observed) order antiferromagnetically below 25 K.
In this presentation, the large diversity of physical phenomena occurring in the RNiC₂ family will be discussed in terms of relations between various types of ordering on the basis of magnetic, transport and galvanomagnetic measurements. The main emphasis will be put on the CDW mutually interacting with magnetism in the polycrystalline RNiC₂ and their solid solutions.   

Core-level and Resonant Photoemission across the Charge Density Wave Transition in 1T-TiSe2

We study the electronic structure of the charge density wave (CDW; T_CDW= 200 K) material 1T-TiSe₂, using core-level photoemission and valence band resonant photoemission spectroscopy (RESPES) across the Ti L-edge threshold. While 1T-TiSe₂ was extensively studied using various techniques including angle-resolved photoemission spectroscopy (ARPES)[1], time-resolved ARPES[2] and resonant inelastic X-ray scattering[3], the Ti L-edge RESPES has not been reported. The Ti 2p and Se 3d spectra show clear shifts across the CDW transition associated with band structure changes in the CDW phase. The Ti L-edge RESPES shows a clear resonance of Ti 3d character states and a Ti LVV Auger correlation satellite. Applying the Cini-Sawatzky method provides an estimate of the effective on-site Coulomb correlation energy between two holes, U_dd= -1.5 eV. The results are consistent with core level Auger spectra of 1T-TiSe₂[4], suggesting the effective attractive Coulomb correlation energy would favour a band Jahn-Teller based model for describing the CDW transition in 1T-TiSe_2.

References :
(1) T. E. Kidd et al., PRL 88, 226402 (2002).
(2) T. Rohwer et al. Nature 471, 490 (2011).
(3) C. Monney et al. PRL 109, 047401 (2012).
(4) D. deBoer, C Haas, and G.A. Sawatzky, J. Phys. F: Met. Phys. 14, 2769 (1984).
  

High-temperature charge density wave in rare-earth tetratelluride

Charge density waves (CDWs) are broken-symmetry ground states that are driven by the electron-phonon interaction in quasi one-dimensional (1D) or two-dimensional (2D) metals. The subject has generated considerable interest as it offers important insights into electron-phonon physics, its competition or coexistence with other ground states, as well as its potential role in the phase diagram of high-Tc cuprates. Recently, a CDW state was found in a newly synthesized quasi-2D compound, rare-earth tetratelluride. Here, by high-resolution angle-resolved photoemission spectroscopy, we demonstrate that in the CDW state, the Fermi surface is fully gapped but the gap size is momentum-dependent. Surprisingly, the CDW persists to above 400 K, providing a remarkable system for the room-temperature study of 2D semiconductors with collective electronic states.   

Charge Density Wave Order in the Organic Metal α-(ET)2 KHg(SCN)4

The family of crystalline organic metals α-(ET)₂ XHg(SCN)₄, with X=K, NH₄, Tl, or Rb are of interest because three of the compounds have a coexisting charge density wave (CDW) and metallic phase at low temperatures, while one (X = NH₄) has a superconducting ground state. They are all triclinic crystals and have a quarter filled conduction band. To find the nature of the density wave state we studied α-(ET)₂ KHg(SCN)₄ using elastic x-ray scattering at the Advanced Photon Source at Argonne National Laboratory. We will present preliminary scattering data at 4 K to search for incommensurate CDW, below the thermodynamic phase transition at 8K, and contrast it with data at 104 K. We encountered serious challenges of radiation damage to these organic materials and employed a mitigation scheme by using 18 keV x-rays with reduced incident flux for extended period of time. There seems to be a weak broadening of Bragg peaks below 8 K, which would indicate some structural anomaly at this temperature.   

Origins of Charge-Density Waves in Niobium Diselenide and Tantalum Disulfide using Ultrafast Electron Diffuse Scattering

Investigations of structural dynamics on the sub-picosecond timescale can provide a detailed fundamental understanding of the non-equilibrium properties of materials. While structural transformations are related to intensity changes in Bragg peaks, thermal diffuse scattering can reveal complete information on the underlying phonon modes. Because thermal diffuse scattering intensities are many order of magnitude lower that Bragg intensities, very sensitive measurements are required.
In this talk, I will present ultrafast electron diffuse scattering to directly observe electron-phonon interactions in charge-density wave (CDW) materials with a time resolution of ~100 fs. It was found that CDW materials and high-temperature superconductors (HTS) have a similar origin and hence, gaining deep insights in CDW materials is expected to enhance our understanding of HTS. Using a combination of density functional theory and experimental studies, the rich electron-phonon interactions of two CDW materials (i.e., niobium diselenide and tantalum disulfide) were investigated. Measurements suggest photoexcitation significantly stiffens/softens acoustic modes only at certain wave-vectors, a phenomenon we have also investigated computationally.   

3D-ΔPDF analysis of three-dimensional CDW compound (CaxSr1-x)3Rh4Sn13

A three-dimensional compound series (Ca_xSr_1-x)₃Rh₄Sn₁₃ has been identified as a promising system to understand the interplay between the charge-density-waves (CDW) and the superconductivity. The second order CDW transition temperature T^* in this family can be fully suppressed with the substitution of Ca for Sr, giving rise to a Quantum Critical Point at x≈0.9 at ambient pressure. For a comprehensive understanding of the lattice instability connected to this CDW transition and superconductivity, diffuse scattering measurements were performed on a single crystal doping series in (Ca_xSr_1-x)₃Rh₄Sn₁₃ with (x=0, 0.1, 0.5, 0.6, 0.65, 0.7, 0.9) using x-ray diffraction. A 3D-ΔPDF analysis shows that there is an order-disorder transition at T^*, with local distortions still present at higher temperature.   

Optical Thermal Diffusivity Measurements in PdxErTe3

PdxErTe3 is a quasi-2D metal with two stripe ordered charge density wave (CDW) orders with of Pd intercalated between the ErTe3 layers [1]. Increasing Pd intercalation increases disorder and suppresses the CDW transitions [1,2]. We use a photo-thermal microscope for high resolution thermal diffusivity and optical reflectivity measurements by measuring phase delay in local reflectivity. This is supplemented with resistivity and thermopower measurements. Our primary observation is a decrease of thermal diffusivity and the onset of anisotropy in all transport channels below the higher temperature CDW transition. We will further discuss the relation between thermal and charge transport, and the applicability of Wiedemann-Franz law through the two CDW transitions.


[1] J. A. W. Straquadine, et al., Phys. Rev. B 99, 235138 (2019).
[2] A.Fang, et al., arXiv:1901.03471 [cond-mat.str-el].   

Density Wave Order in Superconducting Nb-Bi2Se3

Topological Insulators such as Bi₂Se₃ and Bi₂Te₃, observed to have hexagonally deformed Fermi surfaces and strong nesting, have been proposed as potential candidates for the existence of density wave order. The presence of such order becomes particularly interesting in the context of superconductivity appearing in systems such as Cu-Bi₂Se₃ and Nb-Bi₂Se₃, and of possible intercalation-induced spontaneous symmetry breaking. So far, from experiments, we have some hints of density wave order, such as from electron diffraction and angle-resolved photoemission spectroscopy in Cu-Bi₂Se₃, and the observation of multiple Fermi surfaces in torque-magnetometry based quantum oscillations in Nb-Bi₂Se₃. However, there are no direct reports of Charge Density Wave order from resistivity. Here we report, for the first time, evidence for the coexistence of superconductivity with CDW order from transport measurements on single crystals of Nb_x-Bi₂Se₃ (0 < x < 0.25). We also report our studies of specific heat, magnetization, and ²⁰⁹Bi NMR. Based on our results, we argue that the Nb d orbital might not be responsible for the observation of multiple Fermi surfaces, and that such order could simply be induced by intercalants (Bi or Nb, or both). Further work is needed in this area.   

Electronic Raman scattering spectra of various types of BEDT-TTF compounds

We measured Raman scattering spectra of various types of BEDT-TTF compounds such as a charge ordered material α-(BEDT-TTF)₂I₃, a dimer-square lattice material β-(BEDT-TTF)₂I₃ and a quantum spin-liquid material κ-(BEDT-TTF)₂Ag₂(CN)₃. These three compounds show the broad Raman structures, which can be ascribed to the electronic Raman scattering. These structures correspond to charge excitation and magnetic excitation. Comparing these electronic Raman signals, we will discuss the electronic states of these compounds. For example, α-(BEDT-TTF)₂I₃ shows a phase transition from charge ordered state to Dirac electron state. For clarification of the electronic state of α-(BEDT-TTF)₂I₃, we focus on the charge excitation in the reflectivity and Raman spectra.   

Noise spectroscopy across electric field-induced transition in an organic charge-transfer complex

Alkali-metal-TCNQ salt, K-TCNQ, is a quasi-one-dimensional organic charge-transfer complex with segregated columns of alkali cations and TCNQ anion. The compound undergoes structural transition at Tc ~ 395 K as a result of strong electron-lattice interaction inherent in quasi-one-dimensional systems. Below Tc, an electric-induced transition from insulating to metallic phase is observed and contains two threshold behaviors in current-voltage characteristics, akin to charge density wave (CDW) systems. In CDW systems, the lower threshold is attributed to Coulomb blockade for soliton nucleation and the upper one is the depinning field. The upper threshold field follows an exponential temperature-dependence like in CDW systems implying collective charge dynamics near the transition. Low-frequency conductance noise spectroscopy is employed to understand charge carrier dynamics across both the thresholds. The power spectral density (PSD) of the measured current fluctuation increases by almost three orders around the lower threshold implying possible nucleation of phase solitons and on approaching the upper threshold field possible indications of CDW sliding are seen in the behavior of PSD. The existence and dynamics of CDW in these organic charge-transfer systems will be presented.