Session B29: Organic Conductors and CDW Materials

Charge density wave melting in TiSe2

TiSe₂ is an intriguing material that hosts a (2x2x2) charge density wave (CDW) phase below 200K and which can become superconducting when breaking the CDW long range order by elevated pressures or Cu doping. The ideal tool to study such a material is time and angular resolved photoelectron spectroscopy (trARPES), as it allows to directly acess the dynamics of the two spectral representation of the CDW state, the folded Se4p* band and the CDW gap. In this work we study these quantities in dependence of pump fluence, going from a perturbation of the CDW order in the low fluence regime to a complete phase transition in the high fluence regime.

    

Scanning Tunneling Microscopy of Hf-doped ZrTe3

ZrTe3 is a quasi-one-dimensional material which hosts a CDW state below ~63 K and a superconducting state below ~2 K. While the crystal structure is quasi-two-dimensional, the in-plane electronic properties are notably anisotropic, with higher electrical conductivity along the a crystal axis. The CDW state is believed driven primarily by Fermi surface nesting of near parallel Fermi surface sheets along the a* direction. In this work, we present our scanning tunneling microscopy measurements on Hf-doped ZrTe3 both above and below the CDW transition, characterizing properties of the CDW state as well as atomically-resolved defects.

*This work is supported by NSF Grant No. DMR-1904918.   

Scanning tunneling microscopy and spectroscopy imaging of a kagome metal ScV6Sn6

Kagome metals are an exciting platform to realize novel exotic phenomena, such as dispersionless “flat” bands, Dirac and Weyl fermions, and various density waves. Recently synthesized family of kagome metals ReV₆Sn₆ comprised of a non-magnetic V kagome net offers the possibility to tune magnetism and emergent electronic states by the choice of the rare-earth Re element. In this talk, I will discuss our experiments on cleaved bulk single crystals of kagome metal ScV₆Sn₆, the first material in this family to exhibit a bulk charge density wave. We use low-temperature spectroscopic-imaging scanning tunneling microscopy (SI-STM) to investigate its surface structure and its low-energy electronic properties. From quasi-particle interference imaging, we reveal a Dirac cone dispersion near Fermi level. We further explore the existence of the charge density wave at the surface and the effect of magnetic field on the electronic band structure.

    

Author not Attending

Solid state systems can exhibit a superlattice charge modulation due to the broken translation symmetry, which is known as charge density wave. The interplay of charge density wave with other order parameter can produce exotic states, and enrich the phase diagram in condensed matter systems. While the relation between charge ordering and superconductivity is widely studied, It is not yet understood the relation betwen CDW and magnetism, possibly due to the lack of model system. In this talk, I will talk about our recent scanning tunneling microscopy studies of FeGe, a kagome antiferromagnet featuring CDW below the Neel temperature. Using spin-polarized STM, we unrevil the non-trivial correlation between charge ordering and magnetism in FeGe.   

Observation of the massive Lee-Fukuyama phason in a charge density wave insulator (TaSe4)2I

The lowest-lying fundamental excitation of an incommensurate charge density wave (CDW) material is widely believed to be a massless phason, i.e. a collective modulation of the phase of the CDW order parameter. However, as first pointed out by Lee and Fukuyama in 1978, long-range Coulomb interactions should push the phason energy up to the plasma energy of the CDW condensate, making the phason massive and a fully gapped spectrum. Whether such behavior occurs in a CDW system has been unresolved for more than four decades. Using time-domain THz emission spectroscopy, we investigate this issue in the material (TaSe₄)₂I, a classical example of a quasi-one-dimensional CDW insulator. Upon transient photoexcitation at low temperatures, we find the material strikingly emits coherent, narrow-band THz radiation. The frequency, polarization and temperature-dependence of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interaction (U). Our observations constitute the first direct evidence of the massive "Lee-Fukuyama" phason and highlight the potential applicability of fundamental collective modes of correlated materials as compact and robust sources of THz radiation.   

Ultrafast formation of topological defects and transient charge density waves in rare earth tri-tellurides

Ultrafast excitations in solids can lead to novel dynamics and to the emergence of material phases that are out of reach in thermal equilibrium. We use ultrafast x-ray diffraction to study the dynamics of charge density waves (CDW) in LaTe₃, where a “competing” CDW has been observed following laser excitation. LaTe₃ is a layered compound exhibiting a CDW along its ‘c’ axis. Upon laser excitation we measure suppression of the super-lattice Bragg peak and the formation of domain walls. These walls break down, forming 1D topological defects (vortex lines), the signatures of which we resolve using the extremely high temporal and momentum resolution provided by the free electron laser. The suppression of the stable CDW induces a transient enhancement of the intensity at a perpendicular wave-vector (‘a’ axis) with a timescale of a few picoseconds; However, our results strongly suggest that this modulation never develops long range order. By studying the dynamics of the ‘a’ and ‘c’ modulations we find indications that the vortex lines of the ‘c’ order prolong the lifetime of the ‘a’ axis fluctuations. Our results provide an important insight into the delicate interplay between competing phases and suggest routes to control emergent phases through topological defects.   

Imaging the charge density wave in rare-earth tellurides

Rare-earth tritellurides (RTe₃; R = rare-earth element) are quasi-2D materials showing unidirectional, incommensurate charge density waves (CDWs). The heavier rare-earth elements in RTe₃ can have a second CDW perpendicular to the first one, making it a bidirectional CDW. The CDW state and the transition temperature can be tuned by different rare-earth elements: from La-Tb, a unidirectional CDW is observed, but for Dy-Tm, the CDW is bidirectional. While previous studies suggest an interplay between the two CDWs [1], a proper understanding of the dynamics of the second CDW evolution is still lacking. Moreover, the incommensurate nature of the CDW is unclear [2,3]. In this study, we investigate LaTe₃ with a unidirectional CDW at room temperature and ErTe₃ that transitions to a unidirectional CDW at 265 K and a bidirectional CDW below 155 K.  We use atomic-resolution STEM-HAADF imaging, 4D STEM, and electron diffraction to study the effects of nanoscale confinement, nature of the incommensurate lattice modulations, and evolution of the two CDW orders as we vary the temperature from 300 K to 120 K.

[1]        A. A. Sinchenko, et al., Phys. Rev. B 93, 235141 (2016).

[2]        A. Tomic, et al., Phys. Rev. B 79, 085422 (2009).

[3]        A. Fang, et al., Phys. Rev. Lett. 99, 046401 (2007).   

Emergent collective excitations in ErTe3

Rare-earth tritellurides, RTe₃, studied for long time a canonical charge density wave (CDW), while their square-net nature was suggested to host unique quantum geometry. This has led to the recent discovery by our group of the Axial Higgs mode in LaTe₃ and GdTe₃, suggesting the CDW is unconventional. Here we explore ErTe₃, which shows the orthogonal bidirectional CDW, using Raman scattering. Surprisingly, we find the amplitude mode corresponding to primary and secondary CDW has different underlying symmetry of the order parameter strongly suggesting that the onset secondary CDW is not just the 90 degree rotated analogue of the primary CDW order. Furthermore, the circular polarized Raman study shows the appearance of the secondary CDW mode only in the crossed circular polarization configuration confirms that it has angular momentum. Our results shed light on the unconventional nature of the CDW in ErTe₃ and highlight the rich physics associated with these square-net quantum materials.   

Rotational-Anisotropy Nonlinear Harmonic Generation Study of the Charge Density Wave Phase of 1T-TiSe2

The charge density wave (CDW) phase of the transition metal dichalcogenide 1T-TiSe₂ features a non-trivial symmetry configuration at T_CDW ≈ 200 K, with varying interpretations on the existence of a chiral CDW state that putatively onsets ~10 K below T_CDW. The nonlinear optical technique of rotational anisotropy-nonlinear harmonic generation (RA-NHG), in which the n^th harmonic of an incident laser field is measured as a function of incoming and outgoing polarization, can reveal symmetry changes deriving from the lattice, charges, and spins that may be less obvious from conventional scattering probes. Here, we present a RA-NHG study of 1T-TiSe₂ as a function of temperature and wavelength, and discuss how our findings relate to the debate on the existence of a chiral CDW in this material.   

Temperature Evolution of Intradomain Chirality in the Nearly-Commensurate CDW State in 1T-TaS2

The discovery and manipulation of chiral charge density wave (CDW) states has opened the possibility for new electronic applications based on chiral domain switching. Recent work has provided atomic-scale details of the evolution of the CDW state from domain center to domain wall and uncovered an intradomain chiral nature to the nearly-commensurate charge density wave state (NC-CDW) in 1T-TaS2.[1] As opposed to a chiral CDW state originating from orbital ordering, as seen in related transition metal dichalcogenides, the intradomain chirality in 1T-TaS2 is proposed to originate from a strong coupling of the CDW state to the atomic lattice. Here we present our temperature-dependent scanning tunneling microscopy measurements across the temperature range of the NC-CDW state: from just below the ~355 K transition to the incommensurate CDW state to just above the ~180 K transition to the low-temperature commensurate CDW state. We examine the atomic-scale evolution of CDW domains and intradomain chirality with temperature.

[1] Singh et al., Physical Review B, 106, L081407, 2022.

    

An exactly solvable model of pinned charge density waves

Recent advances in the resolution of experimental measurements on charge density waves (CDW) [1,2] give cause to re-examine the theoretical understanding of such systems, particularly the effects of impurities [3]. Here, we present an exactly solvable model of a CDW coupled to quenched disorder, which we use to demonstrate a novel crossover between weakly and strongly disordered regimes. In contrast to previous studies utilizing the large-N limit [4], our approach preserves the Abelian symmetry which allows for a Kosterlitz-Thouless transition in the clean system. By treating the quenched disorder and thermal fluctuations independently and on equal footing, we draw conclusions about their interplay across the phase diagram of the theory. 

[1]    M. Mitrano, et al., Phys. Rev. B 100, 205125 (2019).

[2]    S. Lee, et al., Phys. Rev. Lett. 127, 027602 (2021).

[3]    D. F. Mross and T. Senthil, Phys. Rev. X 5, 031008 (2015).

[4]    L. Nie, G. Tarjus, and S. A. Kivelson, Proc. Natl. Acad. Sci. 111, 7980 (2014).   

Strain Tuning of Charge Density Wave Order in ErTe3

Utilizing both x-ray diffraction (XRD) and transport techniques we study the ability of anisotropic strain to manipulate the charge density wave (CDW) order in ErTe₃. ErTe₃ belongs to the rare-earth tritelluride family, RTe₃ (R=La-Pr, Sm, Gd-Tm), which are quasi-2D materials comprising nearly-square Te nets that exhibit unidirectional incommensurate CDW states. These materials are a model system to explore open questions regarding CDW formation and its interrelation with superconductivity and possible nematic phase in related materials such as the cuprates.

Although RTe₃ is weakly orthorhombic and the primary CDW forms along the longer in-plane direction, Kohn anomalies are nevertheless observed at equivalent wavevectors in both in-plane directions, suggesting the material is ‘almost tetragonal’. Here, we demonstrate that the material can indeed be biased to an effectively tetragonal state via strain tuning opening a pathway to realizing nematic behavior in a fundamentally orthorhombic material. Present verification of the ability to strain tune the CDW orientation in this material makes it an ideal candidate to host a possible vestigial nematic phase.   

Giant elastocaloric effect at low temperatures above the ferroquadrupolar phase transition in TmVO4

Ferroquadrupole order of local atomic states provides a realization of electronic nematic order. A key physical property associated with such order is the quadrupole (nematic) susceptibility, which diverges approaching the phase transition. Here, we present a new method to measure this quantity using an elastocaloric effect (ECE) technique. We choose the representative cooperative Jahn-Teller system TmVO₄, which undergoes ferroquadrupole order at 2.2 K, as a proof of principle. A simple Maxwell relation relates the ECE that we measure to the temperature derivative of the elastic modulus c₆₆, the softening of which also heralds the phase transition. A comparison of ECE data with results obtained from ultrasound measurements demonstrates that the temperature dependence of the quadrupole strain susceptibility approaching the critical temperature is indeed faithfully captured by the ECE measurement. We also compare the experimentally reconstructed entropy landscape in temperature-strain space to that from theory to further establish the extent to which the ECE measurement captures the physics of the material. Furthermore, because the specific material that we have studied orders at such a low temperature, and because the coupling to strain is necessarily large for Jahn-Teller materials, the ECE signature at low temperatures is very large. Observation of this giant ECE at low temperatures establishes the potential for using similar low-temperature ferroquadrupolar/nematic materials for elastocaloric cooling at cryogenic temperatures.