Session X56: Electronic Charge and Orbital Ordering

Role of Coulomb correlations in the charge density wave of CuTe

A quasi-one-dimensional layered material CuTe undergoes a charge density wave (CDW) transition in Te chains with a modulation vector of q_CDW = (0.4, 0.0, 0.5). Here, using first-principles calculations, we demonstrate that the correlation effect of Cu is critical to stabilizing the 5 × 1 × 2 modulation of Te chains. The phonon calculation with the strong Coulomb correlation exhibits the imaginary phonon frequency at q_ph0 = (0.4, 0.0, 0.5), indicating the structural instability. The corresponding lattice distortion of the soft mode agrees well with the experimental modulation. These results demonstrate that the CDW transition in CuTe originates from the interplay of the Coulomb correlation and electron-phonon interaction. Furthermore, we investigated the stability of the CDW state in a CuTe monolayer, Similarly to its bulk structure, we find the phonon soft mode at q = (0.4, 0.0), indicating structural instability, which only appears with the correlation effect on Cu. Interestingly, by reducing the interlayer interactions, tuning of the CDW modulation may be possible, as demonstrated by the modulation pattern in quasi-one-dimensional Te chains being different from that in the bulk counterpart.
Reference
S. Kim, B. Kim, K. Kim, Physical Review B, 100, 054112 (2019)   

Optical Enhancement of Superconductivity via Targeted Destruction of Charge Density Waves

It has been experimentally established that the occurrence of charge density waves is a common feature of various under-doped cuprate superconducting compounds. The observed states, which are often found in the form of bond density waves (BDW), often occur in a temperature regime immediately above the superconducting transition temperature. Motivated by recent optical experiments on superconducting materials, here, we propose a new approach for the enhancement of superconductivity by the targeted destruction of the BDW order. Since BDW states are usually found in competition with superconductivity, suppression of the BDW order enhances the tendency of electrons to form Cooper pairs after reaching a steady-state. By investigating the optical coupling of gapless, collective fluctuations of the BDW modes, we argue that the resonant excitation of these modes can melt the underlying BDW order parameter.   

Charge density waves in a quantum plasma

We analyze the instability of an unpolarized uniform quantum plasma consisting of two oppositely charged fermionic components with varying mass ratios against charge and spin density waves. Using density functional theory, we treat each component with the local spin density approximation and a rescaled exchange-correlation functional. Interactions between different components are treated with a mean-field approximation. In both two and three dimensions, we find leading unstable charge density wave modes in the second-order expansion of the energy functional, which would induce the transition to quantum liquid crystals. The transition point and the length of the wave vector are computed numerically. Discontinuous ranges of the wave vector are found for different mass ratios between the two components, indicating exotic quantum phase transitions. Phase diagrams are obtained, and a scaling relation is proposed to generalize the results to two-component fermionic plasmas with any mass scale. We discuss the implications of our results and directions for further improvement in treating quantum plasmas.   

A Quantum Monte Carlo Study of the Effect of Strain on Charge Density Wave Order in the Holstein Model

We investigate charge ordering in the Holstein model in the presence of anisotropic hopping, t_x,t_y=(1-δ),(1+δ), as a model of the effect of strain on charge density wave (CDW) materials. Using Quantum Monte Carlo simulations, we show that the CDW transition temperature is relatively insensitive to moderate anisotropy δ≤0.3, but begins to decrease more rapidly at δ≥0.4. However, the density correlations change significantly for moderate δ. Accompanying mean-field theory calculations show a similar qualitative structure, with the transition temperature relatively constant at small δ and a more rapid decrease for larger strains. We also obtain the density of states N(ω), which provides clear signal of the charge ordering transition at large strain, where finite size scaling of the charge structure factor is extremely difficult because of the small value of the order parameter.   

Charge Density Waves on a Decorated Honeycomb Lattice

Tight binding models like the Hubbard Hamiltonian are most often explored in the context of uniform intersite hopping t. The electron-electron interactions, if sufficiently large compared to this translationally invariant t, can give rise to ordered magnetic phases and Mott insulator transitions, especially at commensurate filling. The more complex situation of non-uniform t has been studied within a number of situations, perhaps most prominently in multi-band geometries where there is a natural distinction of hopping between orbitals of different degree of overlap. In this paper we explore related questions arising from the interplay of multiple kinetic energy scales and electron-phonon interactions. Specifically, we use Determinant Quantum Monte Carlo to solve the Holstein Hamiltonian on a `decorated honeycomb lattice', consisting of hexagons with internal hopping t coupled together by t'. This modulation of the hopping introduces a gap in the Dirac spectrum and affects the nature of the topological phases. We determine the range of t/t' values which support a charge density wave phase about the Dirac point of uniform hopping t=t' as well as the critical transition temperature T_c.   

Charge-density-wave melting in the one-dimensional Holstein model

We study the real-time dynamics in the half-filled Holstein model starting from different initial states that are charge-density-wave (CDW) ordered. The regime where the relaxation dynamics is dominated by electron-phonon coupling is considered (complementary to the case studied in [1] where strong electron interactions were present) and we focus on the far-from-equilibrium regime. Here, a clear separation of time scales between electron relaxation and phonon equilibration is identified. In the transient dynamics we observe effects like a temporal self trapping of the electrons. The study of such regimes is enabled by extending the time-evolving block decimation algorithm with local basis optimization, previously applied to single-polaron dynamics [2], to a half-filled system.

[1] Hashimoto and Ishihara, PRB 96, 035154 (2017)
[2] Brockt et al., PRB 92, 241106(R) (2015)   

Tuning of physical properties through chemical intercalation in two dimensional FexNbTe2

The layered materials with weak van der Waals interlayer interaction have created a rich platform to exhibit exotic properties such as charge density wave (CDW), topological properties, 2D magnetism, and superconductivity. NbTe₂ is a layered materials with the same structure as 1T’ WTe₂ phase. It has a CDW transition above 550K, and meanwhile exhibits superconducting transition below 0.74K. By intercalation of magnetic iron atoms, we are expecting to observe the interplay among superconductivity, CDW and magnetism in this NbTe₂ system. Indeed, through manipulation of the synthetic conditions, we observed drastic changes of crystal symmetry and physical properties in the same material Fe_xNbTe₂ with ground state from spin glass insulator to antiferromagnetic metal down to 2K, with more exotic magnetoresistivity and Hall behavior observed. The details of these results will be presented and the interplay of multiple orders in this system will be further elaborated.
  

Unity-order refractive index tunability at room temperature in 1T-TaS2, a strongly correlated material

Strongly correlated materials could greatly respond to a stimulus due to their collective behavior. Thus, strongly correlated materials are well studied in the past for their tunable properties, especially electrical and thermal. However, their optical properties remain to be explored. In this work, we study the optical properties of 1T-TaS₂, a quasi-2D material supporting charge density waves (CDW). 1T-TaS₂ is the only material to exhibit a nearly commensurate CDW phase at room temperature and its electrical and thermal properties are shown to exhibit a large change with temperature, DC and AC biases, pressure, and light. Here, we study the optical properties of 1T-TaS₂ under DC and AC biases, different intensities of optical illumination, and at different temperatures. Under all these conditions, we observe a unity-order change in the refractive index of thin films of 1T-TaS₂. We hypothesize that the stacking of CDWs in this material changes with stimulus resulting in its optical constants to change. ARPES and STM measurements are underway to verify the hypothesis. Our discovery of the large tunability of optical constants of 1T-TaS₂ presents opportunities in studying light-matter interaction in strongly correlated materials and its application to nanophotonic devices.   

Probing Finite-Momentum Charge Collective Modes in Electron Doped SrTiO3

Electron doped strontium titanate, SrTiO₃, becomes a bulk superconductor at carrier densities as low as 5.5×10¹⁷ cm^-3. The pairing mechanism in this dilute material, which is unconventional and widely debated, has recently been explained in terms of hybridization between the longitudinal optic phonons and the electronic plasmon collective modes. Using momentum-resolved inelastic electron scattering (M-EELS), we observed sub-200 meV acoustic and optic phonons, which have previously been implicated in the superconducting pairing, and a valence plasmon from the free carriers. For a carrier density of 2.137×10²⁰ cm^-3, the plasmon blueshifts with decreasing temperature, while its width and dispersion deviate from RPA predictions. In this talk, I will discuss evidence for hybridization of these modes for different carrier densities.   

Understanding the relationship between the structural and dielectric changes at the order-disorder transition in TEA(TCNQ)2

Triethylammonium bis-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)₂) shows an interesting dielectric behaviour, at approximately 220 K, where both the capacitance and the loss exhibit clear anomalies. This corresponds to the order-disorder transition where it is believed that the TEA cations freeze within the structure promoting both a structural change which manifests itself also as a change in dielectric properties. To understand this further, we have coupled quasi-elastic neutron scattering and other techniques to provide information on how these TEA cations are actually moving and what motions freeze out. We find that two things happen; firstly at 100 K we believe the methyl groups start rotating but at 220 K, the TEA cations rotate with a spehrical radius of ~1.7 Angstroms.   

The Raman study of the structural transition in metallic LiOsO3 single crystal under high pressure up to 40 GPa

LiOsO₃ is a novel metal that undergoes a second order phase transition from a centrosymetric R-3c structure to a polar R3c structure at T_s=140 K [1]. In this work, we report the measurements of Raman scattering including polarized and unpolarized models on LiOsO₃ single crystal at different pressure up to 40 GPa with a diamond anvil cell. There are four phonon peaks in the phase at ambient condition; new peaks emerge for P > 8.5 GPa. The feature of the high-pressure phase resembles that of LiOsO₃ below T_s at ambient condition [2]. The polar phase appears to be stabilized under high pressure, which is consistent with the observation of dT_s/dP >0 from the resistivity measurement [3]. Results from the symmetry analysis and first-principles calculations will also be presented.
  

Evolution of a structural dimerization across a pressure-induced insulator-metal transition in the spin-orbit Mott insulator GaTa4Se8

The interplay between electronic correlations and spin-orbit coupling often gives rise to novel quantum phases of matter. When spin-orbit coupling is strong and orbital degeneracies are present, the lattice also plays an important role in determining magnetic ground states. In the Lacunar spinel GaTa₄Se₈, electronic correlations localize a single unpaired electron on tetrahedral clusters of Ta ions, while spin-orbit coupling generates J_eff=3/2 magnetic degrees of freedom. At ambient pressures and T=50 K, GaTa₄Se₈ undergoes a magnetic transition to a valence bond solid state accompanied by a structural dimerization. Hydrostatic pressures can induce an insulator to metal transition, but the evolution of magnetic correlations and the crystal structure across this transition is not known. I will present a series of low temperature, high pressure x-ray diffraction measurements following the evolution of the structural dimerization in GaTa4Se8 across the pressure-induced insulator to metal transition. These measurements shed light on the coupling of spin-orbital pseudo spins with the crystal lattice.   

NMR Investigation of Ferroquadrupolar Order in TmVO4

TmVO₄ undergoes a tetragonal to orthorhombic structural phase transition below 2.2K due to a cooperative Jahn-Teller distortion. The Tm ions experience a crystal-field effect with a non-Kramer’s ground state doublet, and undergo ferroquadrupolar order that breaks the C₄ symmetry of the lattice. This material is therefore is a model system to investigate Ising nematic ordering without competing phases. While previous ⁵¹V NMR has been hampered by significant line broadening due to demagnetization fields at low temperature, here we present improved results in a single crystal of ellipsoidal shape for which this broadening is absent. These are the first NMR measurements in which an ellipsoidal sample is used that has been carefully cut by a focused ion beam (FIB).   

Electronic Structure of the Electron Nematic BaNi2As2

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 Sr_xBa_1-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.   

Effects of Electron Irradiation on Nematic-Like Phase Transition in BaNi2P4

We present the results of an experimental study of a tetragonal-to-orthorhombic phase transition with rather high transition temperature (T_TO ~380 K) in the clathrate material BaNi₂P₄. The transition is accompanied by formation of structural domain patterns which are revealed in polarized optics studies. The electrical resistivity shows metallic behavior both above and below T_TO. The transition temperature is notably affected by the low-temperature (20 K) irradiation with relativistic 2.5 MeV electrons. Additional insight into the nature of the transition is obtained from Hall effect measurements and band structure calculations. Similarity to the behavior found in the parent compounds of iron based superconductors will be discussed.