Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J44: Organic Conductors and CDW MaterialsLive
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Sponsoring Units: DCMP Chair: Wencan Jin, Auburn University |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J44.00001: Domains, Domain Ordering, and Phase Slips in the Nearly-Commensurate Charge Density Wave Phase of 1T-TaS2 Boning Yu, Manoj k Singh, Bishnu Sharma, James Huber, ghilles Ainouche, Michael Boyer The nearly-commensurate (NC) charge density wave (CDW) state of 1T-TaS2 consists of hexagonally-ordered domains. Within these domains, the CDW state is reported to be commensurate with the lattice. However, at domain walls, the CDW state undergoes phase slips. We use scanning tunneling microscopy (STM) to study the nearly-commensurate CDW state so as to detail, at the atomic-scale, domains, domain ordering, and CDW phase slips in the NC-CDW phase of 1T-TaS2. In particular, we examine how these imaged real-space features manifest themselves in Fourier transforms of STM topographic images. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J44.00002: Origin of Stripe CDW Structures and Multivalley Free Energy Landscape in MX2 Systems Keiji Nakatsugawa, Tatsuhiko N. Ikeda, Satoshi Tanda Transition metal dichalcogenides (MX2) exhibit various CDW phases depending on temperature and crystal symmetry. The appearance of stripe CDW phases with anisotropic domain walls [1] has been explained by inter-layer interaction. However, stripe domain walls were observed experimentally in monolayer 1T-TaS2 [2]. Therefore, a new model for the appearance of anisotropic phases is necessary. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J44.00003: The Landau-Peierls distortion in the linear acetylenic carbon chain: an ab-initio study of the interplay between electronic correlations and anharmonic lattice dynamics Davide Romanin, Lorenzo Monacelli, Raffaello Bianco, Ion Errea, Francesco Mauri, Matteo Calandra The linear acetylenic carbon chain (or carbyne) is the simplest example of a 1D carbon system and the prototype of a charge-density wave (CDW). As a matter of fact, at the harmonic level, it undergoes a Landau-Peierls phase transition from an orderered metallic phase (cumulene) to a distorted insulating phase (polyyne) due to an unstable phonon mode with momentum 2kF (where kF is the Fermi momentum). Its existence is still controversial: while both the Coleman and the Mermin-Wagner-Hohenberg theorems prevent polyyne from existing at any temperature, Landau and Peierls showed that the long-range nature of interatomic forces stabilizes the distorted system. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J44.00004: Charge order from structured coupling in VSe2 Jans Henke, Felix Flicker, Jude Laverock, Jasper Van Wezel Charge order - ubiquitous among correlated materials - is customarily described purely as an instability of the electronic structure. However, the resulting theoretical predictions often do not match high-resolution experimental data. A pertinent case is 1T-VSe2, whose single-band Fermi surface and weak-coupling nature make it qualitatively similar to the Peierls model underlying the traditional approach. Despite this, its Fermi surface is poorly nested, the thermal evolution of its charge density wave (CDW) ordering vectors displays an unexpected jump, and the CDW gap itself evades detection in direct probes of the electronic structure. We demonstrate that the thermal variation of the CDW vectors is naturally reproduced by the electronic susceptibility when incorporating a structured, momentum-dependent electron-phonon coupling, while the evasive CDW gap presents itself as a localized suppression of spectral weight centered above the Fermi level. Our results showcase the general utility of incorporating a structured coupling in the description of charge ordered materials, including those that appear unconventional. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J44.00005: Charge-Density-Wave Transition in VSe2 from First Principles Josu Diego, Ion Errea, Santiago Blanco-Canosa Among transition metal dichalcogenides (TMDs), VSe2 is particularly interesting, as it develops a purely 3-dimensional (3D) charge-density wave (CDW) at TCDW=110 K. High resolution inelastic x-ray (IXS) experiments have proved for the first time ever that the CDW transition in this compound is characterized by the collapse of an acoustic mode at the critical wave vector qCDW=(2.25 0 0.7) r.l.u. and critical temperature TCDW=110 K. This softening behaviour is ratified by our fully anharmonic ab initio calculations, which show that van der Waals interactions are crucial to melt the CDW. Our work also highlights the important role of out-of-plane interactions to describe 3D CDWs in TMDs. Finally, our first principles calculations determine a large momentum-dependence of the electron-phonon interaction in VSe2, peaking at the CDW wavevector, in the presence of nesting. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J44.00006: Charge density wave in BaFe2Al9 William Meier, Bryan C Chakoumakos, Andrew Christianson, Qiang Zhang, Shang Gao, Matthew Brandon Stone, Satoshi Okamoto, German D Samolyuk, Raphael Hermann, Michael A McGuire, Brian Craig Sales Charge density waves (CDW) are modulations of the crystal lattice that develop in crystalline materials on cooling. Generally, CDW compounds have strongly 1-d or 2-d character. In contrast, BaFe2Al9 is a hexagonal intermetallic compound with a distinctly 3-d electronic structure. Despite this, a hysteretic phase transition is observed in single crystals centered around 100K in magnetic susceptibility and resistance measurements. Diffraction and Mössbauer experiments reveal this transition is due to the formation of a CDW via a first order phase transition with a large lattice strain (-1.5% along the c-axis). Details of this modulated phase will be discussed as well as why charge order is favored over magnetic order in this metallic, iron-containing compound. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J44.00007: Electronic Structure and doping dependence of the electron nematic series SrxBa1-xNi2As2 Dushyant Narayan, PEIPEI HAO, Kyle Gordon, Andrew Linn, Haoxiang Li, Bryan Berggren, Hope Whitelock, Chris Eckberg, Prathum Saraf, John Collini, Johnpierre Paglione, Daniel Dessau The interplay between competing ordered states and superconductivity has long been identified as key to understanding high-temperature superconductivity. In the case of the iron pnictide superconductors, superconductivity coexists with SDW order and electronic nematicity. BaNi2As2, a structural analogue of the high temperature pnictide superconductor BaFe2As2 has recently been reported to host CDW order and electronic nematicity in conjunction with superconductivity. Interestingly, as a function of Sr doping, CDW and nematicity in the system is suppressed. Upon suppression of this order to absolute zero, a dramatic, six-fold enhancement of the superconducting transition temperature is observed. This system could thus serve as a model system to study the enhancement of superconductivity in the vicinity of quantum critical fluctuations. Here we present detailed high-resolution ARPES measurements of the SrxBa1-xNi2As2 series as a function of temperature, comparing the measured band dispersion and Fermi surfaces to density functional theory calculations of the electronic structure carried out in different phases. Utilizing both the experimental and theoretical data, we discuss various mechanisms for the CDW and electronic nematicity. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J44.00008: Study of temperature-dependent lattice parameter in the organic metal α-(ET)2 KHg(SCN)4 Raju Ghimire, Owen Ganter, Brett Laramee, Zahir Islam, Ulrich Welp, Jennifer N Neu, Charles C Agosta Each of the organic metals in the family α-(ET)2XHg(SCN)4, with X=K, NH4, Tl, or Rb displays a correlated electron ground state at low-temperature. Although they are isostructural and exhibit almost identical band structures and Fermi surfaces, three of the salts have a low-temperature coexisting charge density wave (CDW) and metallic phase, while one (X=NH4) has a superconducting ground state. To understand the nature of the density wave state, we studied α-(ET)2KHg(SCN)4 using elastic x-ray scattering at the Advanced Photon Source at Argonne National Laboratory. Here we will present preliminary temperature-dependent data for the lattice parameter (b-axis) at 296K, 104K, and 4K. In addition, we will discuss scattering data at 4 K to search for incommensurate CDW, below the thermodynamic phase transition at 8K. We will also use density-functional theory and the x-ray results at different temperatures to investigate changes in the band structure and calculate the Lindhard response function as a function of temperature to predict the amplitude and the orientation of the CDW. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J44.00009: Robust quantum spin liquid state in hydrogen-bonded organic Mott insulators Kenichiro Hashimoto, Masaaki Shimozawa, Minoru Yamashita, Akira Ueda, Hatsumi Mori, Takahiko Sasaki κ-H3(Cat-EDT-TTF)2 is a hydrogen-bonded organic Mott insulator that provides a new class of quantum spin liquids (QSLs), where the strong coupling between the localized spins and the hydrogen atoms leads to a quantum paramagnetic and quantum paraelectric (QPE) state. Although this material has a 2D spin-1/2 triangular lattice, its anisotropy parameter t'/t = 1.25 is far from unity. This raises a question as to whether the geometric frustration of the tranigular lattice is an important factor for realizing the QSL state in this system. Here, we investigate a series of κ-H3(Cat-X)2 (X = EDT-TTF, EDT-ST, EDT-d4-TTF, and EDSe-TTF), where the substitution of X affects the anisotropy of the triangular lattice as well as the hydrogen-bond dynamics. Our dielectric and thermal-transport measurements reveal that all the materials exhibit a QSL and QPE state in spite of the large t'/t (for instance, t'/t = 1.84 for X = EDSe-TTF), indicating that the coupling between the π electrons and the hydrogen atoms plays an important role for stabilizing the QSL state. We also find that the QPE behavior is strongly enhaced in X = EDT-ST that is located near a regime where the hydrogen atoms are localized at low temperatures, suggesting the presence of a QCP related to the hydrogen-bond dynamics. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J44.00010: Electrical transport and resistance noise spectroscopy in an organic semiconductor Nicholas Jerla, Dasharath Adhikari, Yulong Huang, Shenqiang Ren, Sambandamurthy Ganapathy The quasi-two-dimensional organic charge-transfer salt, κ-(BEDT-TTF)2Cu[N(CN)2]Cl, exhibits a rich phase diagram under application of external stimuli such as pressure and temperature leading to the formation of metallic, semiconducting, or superconducting phases. At ambient pressure, the system can be thermally driven from a semiconducting phase to a paramagnetic Mott insulator at TMI ~ 50 K and into an anti-ferromagnetic Mott insulator at TN ~ 25 K. The Mott insulator gap forms due to the electronic correlation in the half filled molecular π orbitals as the temperature is varied. The microscopic charge transport behavior across the semiconductor-insulator transition is studied using electrical transport and noise spectroscopy in single crystals of the material. The power spectral density of the resistance fluctuations across the phase transition reveals information about the dynamical behavior of the competing phases that evolve over different time scales. The power spectral density, the second spectrum, and probability density function of the resistance fluctuations are analyzed to gain insights into the roles of correlation, phase separation, and domain dynamics near phase transitions. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J44.00011: Born-Oppenheimer Treatment of the Quadratic SSH Model at Half Filling Stepan Fomichev, Mona Berciu In order to tackle stronger electron-phonon coupling and address derivation inconsistencies, we extend the Su-Schrieffer-Heeger (SSH) model to second order in atomic displacements, and study it semi-analytically using the Born-Oppenheimer approximation. Including only linear terms in the SSH expansion can lead to un-physical "phase separation'' behavior at strong coupling, as we show using mean-field and DMRG approaches. Moreover, while the canonical SSH model successfully predicts Peierls distortion, the lattice is often treated semiclassically, as there is no known analytical solution for quantum phonons -- whereas to physically stabilize a dimerized chain, the phonon spectrum must be modified in response to electron influence. Using the quadratic SSH model, we find the undimerized phonon spectrum to be highly unstable due to a large Kohn anomaly at $2k_F$. This is lifted by Peierls dimerization, and a gap appears between the optical and acoustic modes. We proceed to study the multi-parameter phase space of the SSH chain, varying the coupling and phonon frequency, to analyze the impact on the phonon spectrum gap, optical phonon softening, and the zero-point energy. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J44.00012: Atomic-Scale Studies of the Nearly-Commensurate and Incommensurate Charge Density Wave Phases of 1T-TaS2 Michael Boyer, Manoj k Singh, Boning Yu, Bishnu Sharma, James Huber The charge density wave (CDW) compound 1T-TaS2 hosts interesting and diverse physics including possible Mott insulating and quantum spin liquid states. At low temperatures 1T-TaS2 is in a commensurate CDW state and transitions into triclinic, nearly-commensurate, and incommensurate CDW states with increasing temperature. While studies, using multiple experimental techniques, have investigated all four CDW phases, considerable emphasis has been placed on studying the low-temperature commensurate CDW state. Here we present our scanning tunneling microscopy studies on the higher-temperature nearly-commensurate and incommensurate CDW states with a focus on domains, domain walls, and their changes with temperature. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J44.00013: Visualizing Fermi surface anisotropy underpinning a unidirectional charge density wave Nastaran Alamgir Tehrani, Bryan Rachmilowitz, Ilija Zeljkovic, Raman Sankar, Fangchang Chou, Ziqiang Wang Transition metal dichalcogenide 2H-NbSe2 has emerged as a fruitful playground for investigating charge density wave (CDW) transitions, with several different charge ordering wave vectors and geometries reported in this system. Here, we use spectroscopic-imaging scanning tunneling microscopy (SI-STM) to visualize and compare the underlying band structure of a unidirectional (1Q) and a triangular (3Q) 2a0 CDW phase in strained NbSe2. In the 1Q CDW state, we discover a small anisotropy in the scattering of electrons along the CDW direction, indicative of a distorted Fermi surface. Our results provide an additional input for a microscopic theory of CDW transitions in NbSe2. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J44.00014: Predicted monopole-antimonopole pair annihilation in an organic salt α-(BEDT-TTF)2I3 induced by linearly polarized light Keisuke Kitayama, Yasuhiro Tanaka, Masao Ogata, Masahito Mochizuki Magnetic monopoles have not been observed yet since Dirac proposed a concept of them because the experimental observation needs to deal with extremely large energy of ~1016GeV. On the other hand, researchers have discovered that Dirac cones in band structures of materials can behave as sources (monopoles) or sinks (antimonopoles) of magnetic fields. Here we theoretically predicted that the monopole-antimonopole pair annihilation can occur in a photodriven α-(BEDT-TTF)2I3 which possesses slightly gapped Dirac cones in its band structure. We calculated quasienergies and Berry curvatures of nonequilibrium steady states in the driven α-(BEDT-TTF)2I3 under application of linearly polarized light by using the Floquet theory and found the pair annihilation of Dirac points accompanied by a phase transition from the Dirac semimetal to the normal insulator. This phenomenon can be realized at low energy of ~1eV. Therefore our results provide an opportunity to study the monopole-antimonopole pair annihilation in feasible experiments instead of the high-energy experiments. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J44.00015: Anisotropic Charge Density Waves in Thermal Quench of 2H-TaSe2 Tatsuhiko N. Ikeda, Koji Kawahara, Keiji Nakatsugawa, Satoshi Tanda Transition-metal dichalcogenides such as 2H-TaSe2, known for various charge density waves (CDWs), have attracted renewed attention for their variety of nonequilibrium CDW states, including the hidden states [1]. Nakatsugawa et.al. [2] reinvestigated the phenomenological Ginzburg-Landau (GL) model for CDWs in 2d within a constrained order-parameter (OP) manifold, finding many metastable states including stripe-CDW states. There remain two questions: whether other metastable states exist outside the constrained manifold and what metastable states among others are realizable by physical perturbations such as thermal quench. To answer these questions, we performed a real-space time-dependent GL simulation for 2H-TaSe2, without constraint on OP configurations. Temperature sweeping with white thermal noise was considered, and an anisotropic CDW with lowered symmetry was found under strong noise, although the model was initially fully symmetric. Thus, our time-dependent GL approach is useful for describing CDW dynamics and finding new states and contributes to the unified understanding of various CDWs including the hidden ones. |
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