Charge Density Wave Transition Temperature Research Articles

Charge density waves tuned by biaxial tensile stress.

The precise arrangement and nature of atoms drive electronic phase transitions in condensed matter. To explore this tenuous link, we developed a true biaxial mechanical deformation device working at cryogenic temperatures, compatible with x-ray diffraction and transport measurements, well adapted to layered samples. Here we show that a slight deformation of TbTe3 can have a dramatic influence on its Charge Density Wave (CDW), with an orientational transition from c to a driven by the a/c parameter, a tiny coexistence region near a = c, and without space group change. The CDW transition temperature Tc displays a linear dependence with while the gap saturates out of the coexistence region. This behaviour is well accounted for within a tight-binding model. Our results question the relationship between gap and Tc in RTe3 systems. This method opens a new route towards the study of coexisting or competing electronic orders in condensed matter.

Resonant x-ray diffraction measurements in charge ordered kagome superconductors KV3Sb5 and RbV3Sb5

We report on (resonant) x-ray diffraction experiments on the normal state properties of kagome-lattice superconductors KV3Sb5 and RbV3Sb5. We have confirmed previous reports indicating that the charge density wave (CDW) phase is characterized by a doubling of the unit cell in all three crystallographic directions. By monitoring the temperature dependence of Bragg peaks associated with the CDW phase, we ascertained that it develops gradually over several degrees, as opposed to CsV3Sb5, where the CDW peak intensity saturates promptly just below the CDW transition temperature. Analysis of symmetry modes indicates that this behavior arises due to lattice distortions linked to the formation of CDWs. These distortions occur abruptly in CsV3Sb5, while they progress more gradually in RbV3Sb5 and KV3Sb5. In contrast, the amplitude of the mode leading to the crystallographic symmetry breaking from P6/mmm to Fmmm appears to develop more gradually in CsV3Sb5 as well. Diffraction measurements close to the V K edge and the Sb L1 edge show no sensitivity to inversion- or time-symmetry breaking, which are claimed to be associated with the onset of the CDW phase. The azimuthal angle dependence of the resonant diffraction intensity observed at the Sb L1 edge is associated with the difference in the population of unoccupied states and the anisotropy of the electron density of certain Sb ions.

Experimental Investigation of Superconductivity in PdSSe Under High Pressure

AbstractAnalysis of the superconducting properties of transition metal dichalcogenides (TMDs) under high pressures offers valuable insights to guide the design and synthesis of high‐performance superconducting materials. Herein, the effect of pressure is investigated on the superconductivity of a typical van der Waals layered TMDs material, PdSSe, by measuring its transport properties. After initially increased pressure, superconductivity emerges at 10.2 GPa, with a critical superconducting temperature (Tc) of ≈5.1 K, accompanied by the diminishing charge density wave (CDW) that is originally strengthening. Then, the Tc gradually increases with increasing pressure, reaching 12.1 K at the maximum pressure. The study provides experimental evidence for the superconductivity of PdSSe, and to the best of the knowledge, this is the first report on the observation of amplified CDW phenomena under increasing pressure in nonmagnetic TMDs. The abnormal enhancement of CDW transition temperature at low pressure is consistent with the upward trend of resistance, which is related to the electron–electron interaction. Moreover, synchrotron X‐ray diffraction experiments reveal two additional structural phase transitions.

Alloying effect of rare-earth tritellurides on the charge density wave and magnetic properties

Among many van der Waals materials rare-earth tritellurides (RTe3) allow studying several phenomena like magnetic, superconducting, and charge density wave (CDW). These studies show the effect of cationic alloying of antiferromagnetic RTe3 for fully tunable near room-temperature CDW properties. DyxGd1−xTe3 and DyxTb1−xTe3 alloys were synthesized through a chemical vapor transport technique, and the rare-earth element composition was controlled by changing the ratio of rare-earth metal reagents. The results show that the lattice parameters can be continuously tuned with the composition of the rare-earth cations leading to the variation of the internal chemical pressure. Temperature-dependent Raman spectroscopy and electric transport measurement show that the CDW transition temperature (TCDW) of RTe3 alloys varies with lattice parameters/chemical pressure spanning across 300–380 K. Additional magnetism studies offer the first insights into the magnetic ordering in RTe3 alloys. The emergence of multiple magnetic transitions implies complex magnetic interactions that arise from interactions between different rare-earth elements. Overall findings introduce ways to control the CDW behavior and provide valuable insights into the magnetic ordering in RTe3 alloys, contributing to further investigation and a better understanding of their properties.

Pressure Dependence of the Structural and Superconducting Properties of the Bi-Based Superconductor Bi2Pd3Se2.

The structural and superconducting properties of the Bi-based compound Bi2Pd3Se2 were investigated over a wide pressure range. The prepared Bi2Pd3Se2 sample was a superconductor with a superconducting transition temperature, Tc, of approximately 3.0 K, which differed from a previous report (Tc of less than 1.0 K). At ambient pressure, the powder X-ray diffraction (XRD) pattern of the Bi2Pd3Se2 sample was consistent with that previously reported for Bi2Pd3Se2. The Rietveld method was used to refine the crystal structure, which had a space group of C2/m (No. 12), as reported previously. This compound showed no clear anomaly due to the charge-density-wave (CDW) transition, as seen from the temperature dependence of magnetic susceptibility. However, the temperature dependence of electrical resistivity indicated a clear anomaly, presumably because of the CDW transition in the low-pressure range; the CDW transition temperature was approximately 230 K. The XRD patterns of the Bi2Pd3Se2 sample were measured at 0.160-22.7 GPa, and the patterns were well analyzed by both the Le Bail and Rietveld refinement methods, showing no structural phase transitions in the above pressure range. The pressure dependence of Tc of Bi2Pd3Se2 was recorded based on the temperature dependence of the electrical resistance, which showed an almost constant Tc at 0-13.7 GPa, and the Tc-pressure (p) behavior was fully discussed.

Precursor region with full phonon softening above the charge-density-wave phase transition in 2H-TaSe2

Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound 2H-TaSe2 complemented by angle-resolved photoemission spectroscopy and density functional perturbation theory. Our results rule out the formation of a central-peak without full phonon softening for the CDW transition in 2H-TaSe2 and provide evidence for a novel precursor region above the CDW transition temperature TCDW, which is characterized by an overdamped phonon mode and not detectable in our photoemission experiments. Thus, 2H-TaSe2 exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions. Our ab-initio calculations explain the interplay of electron-phonon coupling and Fermi surface topology triggering the CDW phase transition and predict that the CDW soft phonon mode promotes emergent superconductivity near the pressure-driven CDW quantum critical point.

Electron-hole asymmetry in the phase diagram of carrier-tuned CsV3Sb5

In this work, we study the effect of electron doping on the kagome superconductor CsV3Sb5. Single crystals and powders of CsV3Sb5−xTex are synthesized and characterized via magnetic susceptibility, nuclear quadrupole resonance, and x-ray diffraction measurements, where we observe a slight suppression of the charge density wave transition temperature and superconducting temperature with the introduction of electron dopants. In contrast to hole doping, both transitions survive relatively unperturbed up to the solubility limit of Te within the lattice. A comparison is presented between the electronic phase diagrams of electron- and hole-tuned CsV3Sb5.

Weak dimensionality dependence of charge density wave transition in 2H-NbSe2

The effect of dimensionality on charge density wave (CDW) transition temperature (TCDW) in 2H-NbSe2 is still under debate. Raman measurements uncovered highly enhanced TCDW for few-layer samples, while scanning tunneling microscopy results suggest comparable value of bulk crystals. Here, we obtained high-quality crystals of 2H-NbSe2 with residual resistivity ratio up to 120 and processed thin flakes by mechanical exfoliation. Electrical resistance measurements were carried out on crystals with different thickness to monitor the dimensionality dependence of TCDW, superconducting Tc, and upper critical field Hc2. It is revealed that when the bulk crystal evolves into few layers, the TCDW only increases slightly, though the variations of Tc and upper critical field Hc2 are consistent with previous results. The observed weak dependence of long-range CDW order on dimensionality agrees well with the recent theoretical calculations on anharmonic spectra. These results reconcile experiment and theory, and thus shed light on the mechanism of CDW for thin flakes of 2H-NbSe2.

Signature of spin-phonon coupling driven charge density wave in a kagome magnet

The intertwining between spin, charge, and lattice degrees of freedom can give rise to unusual macroscopic quantum states, including high-temperature superconductivity and quantum anomalous Hall effects. Recently, a charge density wave (CDW) has been observed in the kagome antiferromagnet FeGe, indicative of possible intertwining physics. An outstanding question is that whether magnetic correlation is fundamental for the spontaneous spatial symmetry breaking orders. Here, utilizing elastic and high-resolution inelastic x-ray scattering, we observe a c-axis superlattice vector that coexists with the 2times2times1 CDW vectors in the kagome plane. Most interestingly, between the magnetic and CDW transition temperatures, the phonon dynamical structure factor shows a giant phonon-energy hardening and a substantial phonon linewidth broadening near the c-axis wavevectors, both signaling the spin-phonon coupling. By first principles and model calculations, we show that both the static spin polarization and dynamic spin excitations intertwine with the phonon to drive the spatial symmetry breaking in FeGe.

Emergence of a Thermal Hysteresis of Electrical Resistance by Thinning in 1T-TiSe2

AbstractWe measured the temperature dependence of the resistance in thinned samples of a layered transition metal dichalcogenide 1T-TiSe2. As a result, a thermal hysteresis of resistance was discovered in most samples with a thickness of 10 μm or less, but not in samples thicker than 100 μm. The onset temperature of this hysteresis was almost the same as the onset temperature of the charge density wave (CDW) transition, suggesting that this hysteresis is related to the CDW. All the samples with hysteresis exhibited one or a few jumps of resistance in the 110–190 K range in cooling. Moreover, the hysteresis was related not only to the history of temperature below the CDW transition temperature, but also to that above the transition temperature. To interpret these anomalous results, we discuss the effect of impurities on the domain structure of the CDW.

Q-dependent electron-phonon coupling induced phonon softening and non-conventional critical behavior in the CDW superconductor LaPt2Si2

This paper reports the first experimental observation of phonons and their softening on single crystalline LaPt2Si2 via inelastic neutron scattering. From the temperature dependence of the phonon frequency in close proximity to the charge density wave (CDW) q-vector, we obtain a CDW transition temperature of TCDW = 230 K and a critical exponent β = 0.28 ± 0.03. This value is suggestive of a non-conventional critical behavior for the CDW phase transition in LaPt2Si2, compatible with a scenario of CDW discommensuration (DC). The DC would be caused by the existence of two CDWs in this material, propagating separately in the non equivalent (Si1–Pt2–Si1) and (Pt1–Si2–Pt1) layers, respectively, with transition temperatures TCDW−1 = 230 K and TCDW−2 = 110 K. A strong q-dependence of the electron-phonon coupling has been identified as the driving mechanism for the CDW transition at TCDW−1 = 230 K while a CDW with 3-dimensional character, and Fermi surface quasi-nesting as a driving mechanism, is suggested for the transition at TCDW−2 = 110 K. Our results clarify some aspects of the CDW transition in LaPt2Si2 which have been so far misinterpreted by both theoretical predictions and experimental observations and give direct insight into its actual temperature dependence.

Tunable Physical Properties of VSe2 hexagonal disks

Transition metal dichalcogenides (TMDs) belonging to the family of two-dimensional (2D) materials have received a great deal of research interest over the past few decades due to their unique electronic, optical, mechanical, and magnetic properties as compared to their three-dimensional (3D) counterparts. Magnetism in reduced dimensions, such as in 2D materials, is an underexplored phenomenon, though it has potential for numerous applications, including the fabrication of memory devices and spintronics devices. In this work, we report the structural, morphological, and magnetic characterization of hexagonal disks of VSe2 prepared by the hydrothermal method. The structural and morphological characterization confirms the formation of hexagonal disks of 1T-VSe2. The magnetic measurements show that hexagonal VSe2 is paramagnetic in the entire range of this study, from 5–300 K, with an S–type isothermal magnetization behavior at low temperatures. The field cooling (FC) data collected at an applied magnetic field of 0.5T reveals a hump at a temperature of 118 K, which can be correlated to the charge density wave (CDW) transition point. The increase in CDW transition temperature from 107 K (bulk) to 118 K indicates a decrease in VSe2 thickness. The detailed analysis shows a tunability in the physical properties of VSe2 with its thickness.

Ultrafast Switching from the Charge Density Wave Phase to a Metastable Metallic State in 1T-TiSe_{2}.

The ultrafast electronic structures of the charge density wave material 1T-TiSe_{2} were investigated by high-resolution time- and angle-resolved photoemission spectroscopy. We found that the quasiparticle populations drove ultrafast electronic phase transitions in 1T-TiSe_{2} within 100fs after photoexcitation, and a metastable metallic state, which was significantly different from the equilibrium normal phase, was evidenced far below the charge density wave transition temperature. Detailed time- and pump-fluence-dependent experiments revealed that the photoinduced metastable metallic state was a result of the halted motion of the atoms through the coherent electron-phonon coupling process, and the lifetime of this state was prolonged to picoseconds with the highest pump fluence used in this study. Ultrafast electronic dynamics were well captured by the time-dependent Ginzburg-Landau model. Our work demonstrates a mechanism for realizing novel electronic states by photoinducing coherent motion of atoms in the lattice.

Charge fluctuations above TCDW revealed by glasslike thermal transport in kagome metals AV3Sb5 (A=K,Rb,Cs)

We present heat capacity, electrical, and thermal transport measurements of kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$. In all three compounds, the development of short-range charge fluctuations above the charge density wave (CDW) transition temperature ${T}_{\mathrm{CDW}}$ strongly scatters phonons via the electron-phonon coupling, leading to the glasslike phonon heat transport, i.e., the phonon thermal conductivity decreases weakly upon cooling. Once the long-range charge order sets in below ${T}_{\mathrm{CDW}}$, short-range charge fluctuations are quenched, and the typical Umklapp scattering dominated phonon heat transport is recovered. The charge-fluctuations-induced glasslike phonon thermal conductivity implies sizable electron-phonon coupling in $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$.

Experimental and Theoretical Studies of the Surface Oxidation Process of Rare‐Earth Tritellurides

AbstractRecent studies have established Van der Waals (vdW) layered and 2D rare‐earth tritellurides (RTe3) as superconductors and near room‐temperature charge density wave (CDW) materials. Their environmental stability raises natural concern owing to aging/stability effects observed in other tellurium‐based layered crystals. Here, the results establish the stability and environmental aging characteristics of these RTe3 systems involving a variety of metals such as La, Nd, Sm, Gd, Dy, and Ho. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results show that all the RTe3 sheets oxidize to form thin TeOx layers that are primarily confined to the surface, edges, and grain boundaries. Time‐resolved in situ Raman spectroscopy measurements are used to understand the kinetics of the oxidization process for different lanthanide metal cations and establish their relative stability/resilience to oxidization. Overall results indicate that the vdW layers show higher air stability as the 4f electron number decreases going from Ho to La, resulting in the most stable LaTe3 compared to the least stable HoTe3. Comprehensive quantum mechanical simulations reveal that environmental degradation originates from a strong oxidizing reaction with O2 molecules, while humidity (H2O) plays a negligible role unless Te vacancies are present. Moreover, the simulations explain the effects of 4f electrons on the work function and Te vacancies formation, which directly impact the aging characteristics of RTe3 layers. Interestingly, optical and electrical measurements show that the CDW response is still observed in aged RTe3 layers owing to the presence of underlying pristine/nonoxidized RTe3 layers, except CDW transition temperatures increase due to the thickness effect. Overall results offer the first in‐depth environmental aging studies on these materials, which can be applied to engineer and design their chemical stability, surface properties, and overall CDW characteristics.

Unidirectional coherent quasiparticles in the high-temperature rotational symmetry broken phase of AV3Sb5 kagome superconductors

Kagome metals AV3Sb51 (where the A can stand for K, Cs or Rb) display a rich phase diagram of correlated electron states, including superconductivity2–4 and density waves5–7. Within this landscape, recent experiments have revealed signs of a transition below approximately 35 K attributed to an electronic nematic phase that spontaneously breaks the rotational symmetry of the lattice8. Here we show that the rotational symmetry breaking initiates universally at a high temperature in these materials, towards the 2 × 2 charge density wave transition temperature. We do this via spectroscopic-imaging scanning tunnelling microscopy and study the atomic-scale signatures of the electronic symmetry breaking across several materials in the AV3Sb5 family: CsV3Sb5, KV3Sb5 and Sn-doped CsV3Sb5. Below a substantially lower temperature of about 30 K, we measure the quantum interference of quasiparticles, a key signature for the formation of a coherent electronic state. These quasiparticles display a pronounced unidirectional feature in reciprocal space that strengthens as the superconducting state is approached. Our experiments reveal that high-temperature rotation symmetry breaking and the charge ordering states are separated from the superconducting ground state by an intermediate-temperature regime with coherent unidirectional quasiparticles. This picture is phenomenologically different compared to that in high-temperature superconductors, shedding light on the complex nature of rotation symmetry breaking in AV3Sb5 kagome superconductors. The charge density wave state in kagome superconductors is not fully understood. Now, evidence suggests that the rotational symmetry of the lattice is broken before coherence of unidirectional quasiparticles is established at a lower temperature.

Unveiling the selenium content effect on the properties of TiSe2±α

We systematically examined the properties of the polycrystalline 1T-TiSe2±α series synthesized by solid-state method, combined chemical analysis and physical property characterization, including Energy-dispersive X-ray spectroscopy (EDXS), x-ray photoelectron spectroscopy (XPS) x-ray diffraction (XRD), resistivity, hall effect and magnetization measurement have been employed to study how Ti/Se ratio influences the charge density wave (CDW) formation, the crystal structure, and the electrical characteristics. The electrical resistivity of Se-rich -TiSe2 (Ti/Se ​= ​0.484) is found to be a metallic behavior with decreased CDW transition temperature. This study emphasizes how sensitive 1T-TiSe2 characteristics are to defects and stoichiometry.

Emergent charge order in pressurized kagome superconductor CsV3Sb5.

The discovery of several electronic orders in kagome superconductors AV3Sb5 (A means K, Rb, Cs) provides a promising platform for exploring unprecedented emergent physics1-9. Under moderate pressure (<2.2 GPa), the triple-Q charge density wave (CDW) order is monotonically suppressed by pressure, while the superconductivity shows a two-dome-like behaviour, suggesting an unusual interplay between superconductivity and CDW order10,11. Given that time-reversal symmetry breaking and electronic nematicity have been revealed inside the triple-Q CDW phase8,9,12,13, understanding this CDW order and its interplay with superconductivity becomes one of the core questions in AV3Sb5 (refs. 3,5,6). Here, we report the evolution of CDW and superconductivity with pressure in CsV3Sb5 by 51V nuclear magnetic resonance measurements. An emergent CDW phase, ascribed to a possible stripe-like CDW order with a unidirectional 4a0 modulation, is observed between Pc1 ≅ 0.58 GPa and Pc2 ≅ 2.0 GPa, which explains the two-dome-like superconducting behaviour under pressure. Furthermore, the nuclear spin-lattice relaxation measurement reveals evidence for pressure-independent charge fluctuations above the CDW transition temperature and unconventional superconducting pairing above Pc2. Our results not only shed new light on the interplay of superconductivity and CDW, but also reveal new electronic correlation effects in kagome superconductors AV3Sb5.

Linear nonsaturating magnetoresistance in kagome superconductor CsV3Sb5 thin flakes

Linear nonsaturating magnetoresistance (LMR) represents a class of anomalous resistivity response to external magnetic field that has been observed in a variety of materials including but not limited to topological semi-metals, high-T c superconductors and materials with charge density wave (CDW)/spin density wave orders. Here we report the observation of LMR in layered kagome superconductor and CDW material CsV3Sb5 thin flakes, as well as the dimensional crossover and temperature (T) crossover of such LMR. Specifically, in ultrathin CsV3Sb5 crystals, the magnetoresistance (MR) exhibits a crossover from LMR at low T to quadratic B dependence above the CDW transition temperature; the MR also exhibits a crossover from LMR to sublinear MR for sample thickness at around ∼20 nm at low T. We discuss several possible origins of the LMR and attribute the effect to two-dimensional CDW fluctuations. Our results may provide a new perspective for understanding the interactions between competing orders in kagome superconductors.

Simultaneous formation of two-fold rotation symmetry with charge order in the kagome superconductor CsV3Sb5 by optical polarization rotation measurement

We report the observation of two-fold rotation symmetry in charge density wave (CDW) state in the newly discovered Kagome superconductor CsV$_3$Sb$_5$. Below its CDW transition temperature ($T_{CDW}$), the polarization rotation of the reflected laser beam promptly emerges and increases close to about 1 mrad, and the rotation angle shows two-fold rotation symmetry. With femtosecond laser pulse pumping, the rotation angle can be easily suppressed and then recovers in several picoseconds accompanied with coherent oscillations. Significantly, the oscillations in the signal also experience a 180 degree periodic change. Our investigation provides clear optical evidence for the formation of nematic order with two-fold rotation symmetry just below $T_{CDW}$. The results imply a immediate development of nematicity and possible time-reversal symmetry breaking in CDW state of CsV$_3$Sb$_5$.