Low-temperature Charge-density-wave Research Articles

Cooling rate dependence of charge density wave phases in 1T-TaS2 studied by scanning tunneling microscopy and x-ray diffraction

The role of dynamics in the charge density wave (CDW) phases of 1T-TaS2, especially in accessing metastable phases, is still under scrutiny. We investigated cooling rate dependence on low-temperature CDW phases in this material by scanning tunneling microscopy and x-ray diffraction. In the majority of cases, we found the typical low temperature commensurate CDW and identified no other differences between samples that were fast cooled vs slow cooled from room temperature. In rare cases (1/18 STM experiments and 1/30 XRD experiments), we found a multi-domain structure in the low temperature CDW. The domain structure and metallic behavior revealed by STM and STS measurements of this phase are in excellent agreement with the previously reported metallic mosaic CDW phase.

Features of a Low-Temperature Charge Density Wave in the Monoclinic Phase of NbS3 Manifested in the NMR and in Transport Properties

The relaxation of the transverse nuclear magnetization in the monoclinic phase of NbS3 has been studied by the 93Nb nuclear magnetic resonance method near the temperature TP2 = 150 K, at which a low-temperature charge density wave is formed. It has been shown that the critical slowing down of one of the vibrational modes of the lattice, which is quite slow even above TP2, occurs slightly below TP2. The transition at TP2 occurs not only in low-resistance samples, as thought previously, but also in high-resistance ones, and involves Nb atoms in the bulk of a sample. The transport properties of high-resistance samples, namely, the smearing of the depinning threshold for the charge density wave below TP2, confirm that the phase transition in them occurs at TP2. It has been concluded that the distortion of the lattice at TP2 is not due to the Peierls mechanism and can be attributed to the Keldysh–Kopaev transition. Another possible mechanism is the fluctuation distortion of the lattice above TP2 that prevents the sliding of the charge density wave.

Orbital-Hybridization-Driven Charge Density Wave Transition in CsV3 Sb5 Kagome Superconductor.

Owing to its inherent non-trivial geometry, the unique structural motif of the recently discovered kagome topological superconductor AV3 Sb5 (A = K, Rb, Cs) is an ideal host of diverse topologically non-trivial phenomena, including giant anomalous Hall conductivity, topological charge order, charge density wave (CDW), and unconventional superconductivity. Despite possessing a normal-state CDW order in the form of topological chiral charge order and diverse superconducting gaps structures, it remains unclear how fundamental atomic-level properties and many-body effects including Fermi surface nesting, electron-phonon coupling, and orbital hybridization contribute to these symmetry-breaking phenomena. Here, the direct participation of the V3d-Sb5p orbital hybridization in mediating the CDW phase transition in CsV3 Sb5 is reported. The combination of temperature-dependent X-ray absorption and first-principles studies clearly indicates the inverse Star-of-David structure as the preferred reconstruction in the low-temperature CDW phase. The results highlight the critical role that Sb orbitals play and establish orbital hybridization as the direct mediator of the CDW states and structural transition dynamics in kagome unconventional superconductors. This is a significant step toward the fundamental understanding and control of the emerging correlated phases from the kagome lattice through the orbital interactions and provides promising approaches to novel regimes in unconventional orders and topology.

Possible transition between charge density wave and Weyl semimetal phase in Y2Ir2O7

The subtle interplay of band topology and symmetry broken phase, induced by electron correlations, has immense contemporary relevance and potentially offers novel physical insights. Here, we demonstrate charge density wave (CDW) in bulk Y$_2$Ir$_2$O$_7$ for T < 10 K, and its transition to the Weyl semimetal (WSM) phase at higher temperatures. The CDW phase is evidenced by a) current induced nonlinear conductivity with negative differential resistance at low temperature, b) low frequency Debye like dielectric relaxation at low temperature with a large dielectric constant, and c) an anomaly in the temperature dependence of the thermal expansion coefficient. The WSM phase at higher temperature is confirmed by the DC and AC transport measurements which show an inductive response at low frequencies. More interestingly, we show that by reducing the crystallite size, the low temperature CDW phase can be eliminated leading to the restoration of the WSM phase.

Collective electronic excitations in charge density wave systems: The case of CuTe

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative.

Identification of the Mott Insulating Charge Density Wave State in 1T-TaS_{2}.

We investigate the low-temperature charge density wave (CDW) state of bulk TaS_{2} with a fully self-consistent density-functional theory with the Hubbard U potential, over which the controversy has remained unresolved regarding the out-of-plane metallic band. By examining the innate structure of the Hubbard U potential, we reveal that the conventional use of atomic-orbital basis could seriously misevaluate the electron correlation in the CDW state. By adopting a generalized basis, covering the whole David star, we successfully reproduce the Mott insulating nature with the layer-by-layer antiferromagnetic order. Similar consideration should be applied for description of the electron correlation in molecular solid.

Lattice dynamics of the topological Dirac semimetal LaAgSb2 with charge density wave ordering

LaAgSb$_{2}$ is a rare material, which offers the opportunity to investigate the complex interplay between charge density wave (CDW) ordering and topology protected electronic band structure. As both of these phenomena are governed by the structural symmetries, a comprehensive study of the lattice dynamics is highly desirable. In this report, we present the results of temperature and pressure dependent Raman spectroscopy and x-ray diffraction in single crystalline LaAgSb$_{2}$. Our results confirm that Raman spectroscopy is a highly sensitive tool to probe CDW ordering phenomenon, particularly the low-temperature second CDW transition in LaAgSb$_{2}$, which appears as a very weak anomaly in most experiments. The crystal orientation-dependent measurements provide the evolution of Raman modes with crystallographic symmetries and can be further studied through group symmetry analysis. The low-temperature x-ray diffraction data show the emergence of structural modulations corresponding to the CDW instability. The combined high-pressure Raman spectroscopy and synchrotron x-ray diffraction reveal multiple structural phase transitions through lowering of crystalline symmetries, which are also expected to lead to electronic topological transitions.

Exotic bonding interactions and coexistence of chemically distinct periodic lattice distortions in the charge density wave compound TaTe2

The superstructure of $\mathrm{TaT}{\mathrm{e}}_{2}$ is examined by total high-energy x-ray scattering and large-scale structure modeling over a broad temperature range. At room temperature, it features double zigzag chains of Ta atoms exhibiting metallic bonding. The chains are sandwiched between planes of weakly interacting Te atoms. Below 170 K, the chains appear broken to ``butterflylike'' clusters and single zigzag chains of covalently bonded Te atoms emerge while Ta-Te interactions remain largely ionic. This is a rare example of a charge density wave compound stabilized by bonding that is a mix of covalent, ionic, and metallic character giving rise to chemically distinct periodic lattice distortions. We argue that the formation of periodic lattice distortions in the Te sublattice in addition to those in the Ta sublattice favors charge delocalization, which may explain the unusually diminished resistivity and increased magnetic susceptibility exhibited by the low-temperature charge density wave phase of $\mathrm{TaT}{\mathrm{e}}_{2}$. The effect may be common for $5d$ transition metal chalcogenides containing Te because of the extended electronic orbitals of the constituent atoms.

Universal renormalization group flow toward perfect Fermi-surface nesting driven by enhanced electron-electron correlations in monolayer vanadium diselenide

Reducing the thickness of three-dimensional samples on appropriate substrates is a promising way to control electron-electron interactions, responsible for so called electronic reconstruction phenomena. Although the electronic reconstruction has been investigated both extensively and intensively in oxide heterostructure interfaces, this paradigm is not well established in the van der Waals heterointerface system. In the present study, we examine the nature of a charge ordering transition in monolayer vanadium diselenide $({\text{VSe}}_{2})$. This two-dimensional phase transition would be distinguished from that of ${\text{VSe}}_{2}$ bulk samples, driven by more enhanced electron-electron correlations. We recall that ${\text{VSe}}_{2}$ bulk samples show a charge-density-wave (CDW) transition around ${T}_{\text{CDW}}\ensuremath{\sim}105\phantom{\rule{4pt}{0ex}}\text{K}$. This bulk phase transition results from Fermi-surface nesting properties, where the low-temperature CDW state coexists with itinerant electrons of residual Fermi surfaces. Recently, angle-resolved photoemission spectroscopy measurements [Nano Lett. 18, 5432 (2018)] uncovered that the Fermi-surface nesting becomes perfect, where the dynamics of hot electrons is dispersionless along the orthogonal direction of the nesting wave vector. In addition, scanning tunneling microscopy measurements [Nano Lett. 18, 5432 (2018)] confirmed that the resulting CDW state shows essentially the same modulation pattern as the three-dimensional system of ${\text{VSe}}_{2}$. Here, we perform the renormalization group analysis based on an effective-field theory in terms of critical CDW fluctuations and hot electrons of imperfect Fermi-surface nesting. As a result, we reveal that the imperfect nesting universally flows into perfect nesting in two dimensions, where the Fermi velocity along the orthogonal direction of the nesting vector vanishes generically. We argue that this electronic reconstruction is responsible for the observation [Nano Lett. 18, 5432 (2018).] that the CDW transition temperature is much more enhanced to be around ${T}_{\text{CDW}}\ensuremath{\sim}350\phantom{\rule{4pt}{0ex}}\text{K}$ than that of the bulk sample.

Competing soft phonon modes at the charge-density-wave transitions in DyTe3

The family of rare-earth tritellurides RTe$_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [1]. What sets the heavier members of the RTe3 series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wavevector $q_{CDW,2}\parallel [100]$ of almost the same magnitude but orthogonal to the ordering wavevector $q_{CDW,1}\parallel [001]$ observed at higher temperatures [2]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe$_3$. In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at $T_{CDW,1}=308\,\rm{K}$. Surprisingly, these soft modes close to $q_{CDW,2}=(0.68,0,0)$ show strong softening near $T_{CDW,1}$ but do not exhibit any response to the lower-temperature transition at $T_{CDW,2}=68\,\rm{K}$. Our results indicate that the low-temperature CDW order is not just the 90{\deg} rotated analogue of the one appearing at high temperatures.

Anomalous Photoconduction of Low-Temperature Charge-Density Wave in NbS3-II as a Physical Basis of a Method for Detection of Weak Signals

The effect of IR radiation on photoconduction of two charge-density waves (CDWs) in the monoclinic phase of quasi-1D conductor NbS3 (NbS3-II) is studied. For the CDW-1 that emerges at a temperature of ТР1 = 360 К, threshold field Et increases with irradiation power W. For CDW-2 that emerges at a temperature of TP2 = 150 K, field Et decreases due to IR irradiation, which proves anomalous character of CDW-2 and indicates direct photoactivation of its sliding. Extremely high sensitivity of the measured signal to IR radiation is obtained using stimulation of the CDW coherence by the microwave field.

Scattering-dominated high-temperature phase of1T−TiSe2: An optical conductivity study

The controversy regarding the precise nature of the high-temperature phase of 1T-TiSe2 lasts for decades. It has intensified in recent times when new evidence for the excitonic origin of the low-temperature charge-density wave state started to unveil. Here we address the problem of the high-temperature phase through precise measurements and detailed analysis of the optical response of 1T-TiSe2 single crystals. The separate responses of electron and hole subsystems are identified and followed in temperature. We show that neither semiconductor nor semimetal pictures can be applied in their generic forms as the scattering for both types of carriers is in the vicinity of the Ioffe-Regel limit with decay rates being comparable to or larger than the offsets of band extrema. The nonmetallic temperature dependence of transport properties comes from the anomalous temperature dependence of scattering rates. Near the transition temperature the heavy electrons and the light holes contribute equally to the conductivity. This surprising coincidence is regarded as the consequence of dominant intervalley scattering that precedes the transition. The low-frequency peak in the optical spectra is identified and attributed to the critical softening of the L-point collective mode.

Charge transfer and multiple density waves in the rare earth tellurides

We use high-resolution synchrotron x-ray diffraction to uncover a second, low-temperature, charge density wave (CDW) in TbTe_3. Its T_(c2) = 41.0 +/- 0.4 K is the lowest discovered so far in the rare earth telluride series. The CDW wave vectors of the high temperature and low temperature states differ significantly and evolve in opposite directions with temperature, indicating that the two nested Fermi surfaces are separated and the CDWs coexist independently. Both the in-plane and out-of-plane correlation lengths are robust, implying that the density waves on different Te layers are well coupled through the TbTe layers. Finally, we rule out any low-temperature CDW in GdTe_3 for temperatures above 8 K, an energy scale sufficiently low to make pressure tuning of incipient CDW order a realistic possibility.

Plasmon dispersion in layered transition-metal dichalcogenides

Motivated by recent experiments, we perform a microscopic analysis of the dynamical charge response of layered transition-metal dichalcogenides that display a low-temperature charge-density wave (CDW) order. In agreement with measurements, our parameter-free results show a negative in-plane plasmon dispersion that switches to positive slope upon electron (or hole) doping. This finding is explained by the peculiar behavior of the intraband transitions, which are partially suppressed under doping, and it is not linked to the CDW order. Finally, in the direction perpendicular to the layers, we predict the reappearance around the Bragg reflections of the spectra of the first Brillouin zone, a clear effect of the crystal local-field impact. Our results give a general picture of the collective excitations in these materials suggesting a simpler reinterpretation of the experiments.

Discrete conducting states, wave-vector variation, and carrier mobilities in NbSe3microsamples

Crystals of NbSe${}_{3}$ with typical dimensions of 30 \ensuremath{\mu}m \ifmmode\times\else\texttimes\fi{} 0.03 \ensuremath{\mu}m${}^{2}$ demonstrate jumps of conductivity induced by temperature changes. The jumps are observed in both charge-density wave (CDW) states, but more detailed studies have been performed for the low-temperature CDW. The jumps reveal single phase-slip events that switch the CDW between discrete states with different wave vectors. Based on this observation, we find the temperature dependencies of the CDW wave vectors and the mobilities of the quasiparticles for type-I and type-III chains. The mobility of carriers originating from the type-I chains (i.e., ``pocket'' holes) appears to be extremely high, approaching 10${}^{5}$ cm${}^{2}/$V\ifmmode\cdot\else\textperiodcentered\fi{}s below 25 K.

Hall effect in the pinned and sliding charge density wave state ofNbSe3

Results of Hall effect measurements are reported both below and above the threshold electric field,Et, for depinning the low temperature charge density wave (CDW) inNbSe3 in a wide temperature range. At low electric fields, belowEt, we have observed a change in the sign of the Hall voltage at all temperatures lower thanTp2. Comparison between the Hall effect and the magnetoresistance behavior indicates that then-type conductivity in the low magnetic field range differs qualitatively from the p-typeconductivity in the high field range. We demonstrate that at low temperature theCDW motion significantly alters the Hall voltage. These results indicate that, inNbSe3, the CDW in the sliding state interacts essentially with holes. Possible mechanisms of thiseffect are discussed.

Coherent-noncoherent tunneling crossover inNbSe3−NbSe3point contacts

Current-voltage $(IV)$ characteristics of $\mathrm{Nb}{\mathrm{Se}}_{3}\text{\ensuremath{-}}\mathrm{Nb}{\mathrm{Se}}_{3}$ artificial contacts (point contacts) formed along the ${a}^{*}$ axis have been investigated in a wide range of contact resistances and temperatures. Depending on the contact resistance, two types of $IV$ characteristics have been observed: the first type observed for low contact resistance demonstrates a conducting peak at zero bias, while the second one for large contact resistance is characterized by a large resistance maximum near the zero bias voltage. Below the second charge-density wave (CDW) transition $(Tl59\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, CDW energy gap singularities were observed in the $IV$ characteristics of both types of junctions at $V={\ensuremath{\Delta}}_{p1}∕e$ and $V=2{\ensuremath{\Delta}}_{p2}∕e$, while the singularity at $V={\ensuremath{\Delta}}_{p2}∕e$ corresponding to the single particle tunneling of carriers noncondensed into the low-temperature CDW is absent. The experimental results are discussed in the frame of the model of interlayer coherent tunneling of the remaining charge carriers that are not condensed into the CDW.

Interplay between indirect interaction and charge-density wave in Pb-adsorbedIn(4×1)−Si(111)

The response of the electronic structure of the quasi-one-dimensional $\mathrm{In}(4\ifmmode\times\else\texttimes\fi{}1)\text{\ensuremath{-}}\mathrm{Si}(111)$ surface to the adsorption of Pb atoms is investigated by scanning tunneling microscopy and first-principles calculations. Instead of a period of even multiples of the underlying In lattice spacing $a$ along the In wire, as favored by the low temperature charge-density wave (CDW) modulation, the Pb adatoms are frequently found to occupy the odd adsorption sites separated by a distance $3a$ or $5a$. These observations indicate a strong antiphase interference between the indirect interaction of adsorbed Pb atoms and the intrinsic CDW of the $\mathrm{In}(4\ifmmode\times\else\texttimes\fi{}1)\text{\ensuremath{-}}\mathrm{Si}(111)$ surface.

EPR study of the low temperature charge density wave state of o-TaS 3

We report an EPR study of the chain conductor o-TaS 3 in the low temperature charge density wave (CDW) state. The EPR spectrum is attributed to Fe 3+ ( S=5/2) impurities. A power law for the temperature dependence of the EPR intensity, ( T − α with an exponent α∼0.8) found below ∼30 K is very close to that previously found in magnetic susceptibility measurements. The possible role of these impurities in the susceptibility data are discussed.

Inelastic x-ray scattering study of charge-density-wave dynamics in theRb0.3MoO3blue bronze

We present an inelastic x-ray scattering study of the quasi-one-dimensional blue bronze ${\mathrm{Rb}}_{0.3}{\mathrm{MoO}}_{3}.$ The charge-density-wave (CDW) dynamics is studied as a function of the temperature down to 40 K. At temperature higher than 100 K, the CDW-phase and -amplitude modes are identified, and found consistent with the inelastic neutron scattering measurements. At 40 K, the phason is no longer observed. This is discussed in the framework of current theories on the low-temperature CDW dynamics.