Anisotropy of 2H-NbSe_(2)in the superconducting and charge density wave states

Anisotropy is an important feature of layered materials,and a large anisotropy is usually related to the two-dimensional charac teristics.We investigated the anisotropy of the layered transition metal dicalcogenide 2H-NbSe_(2)in the superconducting and charge density wave(CDW)states using magnetotransport measurements.In the superconducting state,the normalized H_(c2)^(‖c)/H_(p)is independent of the thickness of 2H-NbSe_(2),while H_(c2)^(‖ab)/H_p increases significantly with decreasing thickness,where H_p is the Pauli limiting magnetic field and H_(c2)^(‖c)anu H_(c2)^(‖ab)are the upper critical fields in the c and ab directions,respectively.It is found that the superconducting anisotropy parameterγH_(c2)=H_(c2)^(‖ab)/H_(c2)^(‖c)increases with reduction in the thickness of 2H-NbSe_(2).In the CDW state,the angular(θ)dependence of magnetoresistance,R(H,θ)scales with H(cos^(2)θ+γ_(CDW)^(-2)sin^(2)θ)^(1/2),which decreases with increasing temperature and disappears at about 40 K.It is found that the CDW anisotropy parameterγ_(CDW)is much larger than the effective mass anisotropy but does not change a lot for ultrathin and bulk samples.Our results suggest the existence of three-dimensional superconductivity and quasi-two dimensional CDWs in bulk 2H-NbSe_(2).

Electronic states of domain walls in commensurate charge density wave ground state and mosaic phase in 1T-TaS_(2)

Domain walls(DWs)in the charge-density-wave(CDW)Mott insulator 1T-TaS_(2)have unique localized states,which play an important role in exploring the electronic properties of the material.However,the electronic states in DWs in 1TTaS_(2)have not been clearly understood,mostly due to the complex structures,phases,and interlayer stacking orders in the DW areas.Here,we explored the electronic states of DWs in the large-area CDW phase and mosaic phase of 1T-TaS_(2)by scanning tunneling spectroscopy.Due to the different densities of DWs,the electronic states of DWs show distinct features in these phases.In the large area CDW phase,both the domain and the DWs(DW1,DW2,DW4)have zero conductance at the Fermi level;while in the mosaic phase,they can be metallic or insulating depending on their environments.In areas with a high density of DWs,some electronic states were observed both on the DWs and within the domains,indicating delocalized states over the whole region.Our work contributes to further understanding of the interplay between CDW and electron correlations in 1T-TaS_(2).

Manipulating charge density wave state in kagome compound RbV_(3)Sb_(5)

Owing to the unique electronic structure,kagome materials AV_(3)Sb_(5)(A=K,Rb,Cs)provide a fertile platform of quantum phenomena such as the strongly correlated state and topological Dirac band.It is well known that RbV_(3)Sb_(5)exhibits a 2×2 unconventional charge density wave(CDW)state at low temperature,and the mechanism is controversial.Here,by using scanning tunneling microscopy/spectroscopy(STM/STS),we successfully manipulated the CDW state in the Sb plane of RbV_(3)Sb_(5),and realized a new3(1/2)×3(1/2)modulation together with the ubiquitous 2×2 period in the CDW state of RbV_(3)Sb_(5).This work provides a new understanding of the collective quantum ground states in the kagome materials.

Charge Density Wave States and Structural Transition in Layered Chalcogenide TaSe_(2-x)Te_x

The structural features and three-dimensional nature of the charge density wave （CDW） state of the layered chalcogenide 1T-TaSe2-xTex （0≤x≤2.0） are characterized by Cs-corrected transmission electron microscopy measurements. Notable changes of both average structure and the CDW state arising from Te substitution for Se are clearly demonstrated in samples with x〉0.3. The commensurate CDW state characterized by the known star-of-David clustering in the 1T-TaSe2 crystal becomes visibly unstable with Te substitution and vanishes when x=0.3. The 1T-TaSe2-xTex （0.3≤x≤1.3） samples generally adopt a remarkable incommensurate CDW state with monoclinic distortion, which could be fundamentally in correlation with the strong qq-dependent electron-phonon coupling-induced period-lattice-distortion as identified in TaTe22. Systematic analysis demonstrates that the occurrence of superconductivity is related to the suppression of the commensurate CDW phase and the presence of discommensuration is an evident structural feature observed in the superconducting samples.

Charge density wave states in phase-engineered monolayer VTe_(2)

Charge density wave(CDW)strongly affects the electronic properties of two-dimensional(2D)materials and can be tuned by phase engineering.Among 2D transitional metal dichalcogenides(TMDs),VTe_(2)was predicted to require small energy for its phase transition and shows unexpected CDW states in its T-phase.However,the CDW state of H-VTe_(2)has been barely reported.Here,we investigate the CDW states in monolayer(ML)H-VTe_(2),induced by phase-engineering from T-phase VTe_(2).The phase transition between T-and H-VTe_(2)is revealed with x-ray photoelectron spectroscopy(XPS)and scanning transmission electron microscopy(STEM)measurements.For H-VTe_(2),scanning tunneling microscope(STM)and low-energy electron diffraction(LEED)results show a robust 2√3×2√3CDW superlattice with a transition temperature above 450 K.Our findings provide a promising way for manipulating the CDWs in 2D materials and show great potential in its application of nanoelectronics.

Observation of multiple charge density wave phases in epitaxial monolayer 1T-VSe_(2) film

As a special order of electronic correlation induced by spatial modulation, the charge density wave(CDW) phenomena in condensed matters attract enormous research interests. Here, using scanning-tunneling microscopy in various temperatures, we discover a hidden incommensurate stripe-like CDW order besides the(■) CDW phase at low-temperature of 4 K in the epitaxial monolayer 1T-VSe_(2) film. Combining the variable-temperature angle-resolved photoemission spectroscopic(ARPES) measurements, we discover a two-step transition of an anisotropic CDW gap structure that consists of two parts △_(1) and△_(2). The gap part ?1 that closes around ~ 150 K is accompanied with the vanish of the(√7×√3) CDW phase. While another momentum-dependent gap part △_(2) can survive up to ~ 340 K, and is suggested to the result of the incommensurate CDW phase. This two-step transition with anisotropic gap opening and the resulted evolution in ARPES spectra are corroborated by our theoretical calculation based on a phenomenological form for the self-energy containing a two-gap structure △_(1) +△_(2), which suggests different forming mechanisms between the(√7×√3) and the incommensurate CDW phases. Our findings provide significant information and deep understandings on the CDW phases in monolayer 1T-VSe_(2) film as a two-dimensional(2D) material.

Tunable charge density wave in TiS3 nanoribbons

Recently, modifications of charge density wave（CDW） in two-dimensional（2D） show intriguing properties in quasi-2D materials such as layered transition metal dichalcogenides（TMDCs）. Optical, electrical transport measurements and scanning tunneling microscopy uncover the enormous difference on the many-body states when the thickness is reduced down to monolayer. However, the CDW in quasi-one-dimensional（1D） materials like transition metal trichalcogenides（TMTCs） is yet to be explored in low dimension whose mechanism is likely distinct from their quasi-2D counterparts.Here, we report a systematic study on the CDW properties of titanium trisulfide（TiS3）. Two phase transition temperatures were observed to decrease from 53 K（103 K） to 46 K（85 K） for the bulk and 〈 15-nm thick nanoribbon, respectively,which arises from the increased fluctuation effect across the chain in the nanoribbon structure, thereby destroying the CDW coherence. It also suggests a strong anisotropy of CDW states in quasi-1D TMTCs which is different from that in TMDCs.Remarkably, by using back gate of-30 V ～ 70 V in 15-nm device, we can tune the second transition temperature from110 K（at-30 V） to 93 K（at 70 V） owing to the altered electron concentration. Finally, the optical approach through the impinging of laser beams on the sample surface is exploited to manipulate the CDW transition, where the melting of the CDW states shows a strong dependence on the excitation energy. Our results demonstrate TiS3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.

Structural and electrical transport properties of charge density wave material LaAgSb_(2)under high pressure

Layered lanthanum silver antimonide LaAgSb_(2)exhibits both charge density wave(CDW)order and Dirac-cone-like band structure at ambient pressure.Here,we systematically investigate the pressure evolution of structural and electronic properties of LaAgSb_(2)single crystal.We show that the CDW order is destabilized under compression,as evidenced by the gradual suppression of magnetoresistance.At P_(C)~22 GPa,synchrotron x-ray diffraction and Raman scattering measurements reveal a structural modification at room-temperature.Meanwhile,the sign change of the Hall coefficient is observed at 5 K.Our results demonstrate the tunability of CDW order in the pressurized LaAgSb_(2)single crystal,which can be helpful for its potential applications in the next-generation devices.

Effect of strain on charge density wave order inα-U

The effect of strain on charge density wave(CDW)order inα-U is investigated within the framework of relativistic density-functional theory.The energetical stability ofα-U with CDW distortion is enhanced by the tensile strain along a and b axes,which is similar to the case of negative pressure and normal.However,the tensile strain along c axis suppresses the energetical stability of CDW phase.This abnormal effect could be understood from the emergence of a new onedimensional atomic chain along c axis inα-U.Furthermore,this effect is supported by the calculations of Fermi surface and phonon mode,in which the topological objects and the dynamical instability show opposite behaviors between strains along a/b and c axes.

Competition of Quantum Anomalous Hall States and Charge Density Wave in a Correlated Topological Model

We investigate the topological phase transition driven by non-local electronic correlations in a realistic quantum anomalous Hall model consisting of d_(xy)–d_(x^(2)-y^(2)) orbitals. Three topologically distinct phases defined in the noninteracting limit evolve to different charge density wave phases under correlations. Two conspicuous conclusions were obtained: The topological phase transition does not involve gap-closing and the dynamical fluctuations significantly suppress the charge order favored by the next nearest neighbor interaction. Our study sheds light on the stability of topological phase under electronic correlations, and we demonstrate a positive role played by dynamical fluctuations that is distinct to all previous studies on correlated topological states.

Persistence of charge density wave against variation of band structures in V_(x)Ti_(1−x)Se_(2)(x=0–0.1)

Charge density wave(CDW)is a phenomenon that occurs in materials,accompanied by changes in their intrinsic electronic properties.The study of CDW and its modulation in materials holds tremendous significance in materials research,as it provides a unique approach to controlling the electronic properties of materials.TiSe_(2) is a typical layered material with a CDW phase at low temperatures.Through V substitution for Ti in TiSe_(2),we tuned the carrier concentration in V_(x)Ti_(1-x)Se_(2) to study how its electronic structures evolve.Angle-resolved photoemission spectroscopy(ARPES)shows that the band-folding effect is sustained with the doping level up to 10%,indicating the persistence of the CDW phase,even though the band structure is strikingly different from that of the parent compound TiSe_(2).Though CDW can induce the band fold effect with a driving force from the perspective of electronic systems,our studies suggest that this behavior could be maintained by lattice distortion of the CDW phase,even if band structures deviate from the electron-driven CDW scenario.Our work provides a constraint for understanding the CDW mechanism in TiSe_(2),and highlights the role of lattice distortion in the band-folding effect.

Charge density wave order and electron-boson coupling in ternary superconductor Bi_(2)Rh_(3)Se_(2)

The newly discovered ternary chalcogenide superconductor Bi_(2)Rh_(3)Se_(2) has attracted growing attention,which provides an opportunity to explore the interplay between charge density wave(CDW)order and superconductivity.However,whether the phase transition around 240 K can be attributed to the formation of CDW remains controversial.To help resolve the debate,we study the electronic structure of Bi_(2)Rh_(3)Se_(2) by angle-resolved photoemission spectroscopy,focusing on the nature of its high-temperature phase transition around 240 K.Our measurements demonstrate that the phase transition at 240 K is a second-order CDW phase transition.Our results reveal(i)a 2×2 CDW order in Bi_(2)Rh_(3)Se_(2),accompanied by the reconstruction of electronic structure,such as band folding,band splitting,and opening of CDW gaps at and away from the Fermi level;(ii)the existence of electron-boson coupling,which is manifested as an apparent kink and peak-dip-hump structure in dispersion.Our observations thus enable us to shed light on the nature of CDW order and superconductivity in Bi_(2)Rh_(3)Se_(2).

Molecular interactions induced collapse of charge density wave quantum states in 2H tantalum disulfide nanosheets

2H-tantalum disulfide(2H-TaS_(2))is a layered metallic transition metal dichalcogenide(TMD)that has recently been studied from the perspective of new physics phenomena,including simultaneous lattice distortion and charge density modulation known as the charge density wave(CDW)phase.Here we explored the collapse of CDW states in few-layer 2H-TaS_(2)induced by molecular interactions using Raman spectroscopy.Our results indicate that the CDW states disappear in few-layer 2H-TaS_(2)with rhodamine 6G(R6G)adsorbed due to the charge transfer,which is reflected by the change of behaviors of lattice vibrational modes in 2HTaS_(2).We observed the 2-phonon mode that signifies the CDW formation in 2H-TaS_(2),becomes a phonon-hardened mode when R6G molecules are absorbed on its surface.R6G adsorption further induces the breakdown of the Raman polarization selection rule in 2H-TaS_(2),which results in the alteration of the A_(1g)phonon mode polarization state of 2H-TaS_(2).This study can shed light not only on the underlying mechanisms of CDW states but also on controlling the CDW states under a variety of environmental conditions.

Emergence of charge density wave and Ising superconductivity in centrosymmetric monolayer 1T-HfTe_(2)

Understanding and control of many-body collective phenomena such as charge density wave(CDW)and superconductivity in atomically thin crystals remains a hot topic in material science.Here,using first-principles calculations,we find that 1T-HfTe_(2)possessing no CDWs in the bulk form,unexpectedly shows a stable 2×2 CDW order in the monolayer form,which can be attributed to the enhancement of electron–phonon coupling(EPC)in the monolayer.Meanwhile,the CDW induces a metal-to-insulator transition in monolayer 1T-HfTe_(2)through the accompanying lattice distortion.Remarkably,Ising superconductivity with a significantly enhanced in-plane critical field can emerge in centrosymmetric monolayer 1T-HfTe_(2)after the CDW is suppressed by electron doping.The Ising paring is revealed to be protected by the spin–orbital locking without the participation of the inversion symmetry breaking which is a must for conventional 2H-NbSe2-like Ising superconductors.Our results open a new window for designing and controlling novel quantum states in two-dimensional(2D)matter.

Charge density wave phase suppression in 1T-TiSe_(2) through Sn intercalation

Taking advantage of the unique layered structure of TiSe2,the intrinsic electronic properties of two-dimensional materials can easily be tuned via heteroatomic engineering.Herein,we show that the charge density wave(CDW)phase in 1T-TiSe_(2) single-crystals can be gradually suppressed through Sn atoms intercalation.Using angle-resolved photoemission spectroscopy(ARPES)and temperature-dependent resistivity measurements,this work reveals that Sn atoms can induce charge doping and modulate the intrinsic electronic properties in the host 1T-TiSe_(2).Notably,our temperature-dependent ARPES results highlight the role exciton-phonon interaction and the Jahn-Teller mechanism through the formation of backfolded bands and exhibition of a downward Se shift of 4p valence band in the formation of CDW in this material.

Charge density wave and weak Kondo effect in a Dirac semimetal CeSbTe

Using angle-resolved photoemission spectroscopy(ARPES) and low-energy electron diffraction(LEED), together with densityfunctional theory(DFT) calculation, we report the formation of charge density wave(CDW) and its interplay with the Kondo effect and topological states in CeSbTe. The observed Fermi surface(FS) exhibits parallel segments that can be well connected by the observed CDWordering vector, indicating that the CDWorder is driven by the electron-phonon coupling(EPC) as a result of the nested FS. The CDW gap is large(~0.3 eV) and momentum-dependent, which naturally explains the robust CDWorder up to high temperatures. The gap opening leads to a reduced density of states(DOS) near the Fermi level(EF), which correspondingly suppresses the many-body Kondo effect, leading to very localized 4 f electrons at 20 K and above. The topological Dirac cone at the X point is found to remain gapless inside the CDW phase. Our results provide evidence for the competition between CDWand the Kondo effect in a Kondo lattice system. The robust CDWorder in CeSbTe and related compounds provide an opportunity to search for the long-sought-after axionic insulator.

Layer-dependent charge density wave phase transition stiffness in 1T-TaS_(2)nanoflakes evidenced by ultrafast carrier dynamics

Novel physical properties emerge when the thickness of charge density wave(CDW)materials is reduced to the atomic level,owing to the significant modification of the electronic band structure and correlation effects.Here,we investigate the layer-dependent CDW phase transition and evolution of the nonequilibrium state of 1T-TaS_(2)nanoflakes using pump-probe spectroscopy.Both the low-energy single-particle and collective excitation relaxations exhibit sharp changes at〜210 K,indicating a phase transition from commensurate CDW to nearly commensurate CDW state.The single particle process reveals that the phase transition stiffness(PTS)is thickness-dependent.Moreover,a small PTS is observed in thin nanoflakes,which is attributed to the reduced thickness that increases the fluctuation and inhibits the nucleation and growth of discommensurations.In addition,the phase mode vanishes when the discommensuration network appears.Our results suggest that the carrier dynamics could be an efficient operational approach to measuring the quantum phase transition in correlated materials.

Superconductivity and unconventional density waves in vanadium-based kagome materials AV_(3)Sb_(5)

Recently,the discovery of vanadium-based kagome metal AV_(3)Sb_(5)(A=K,Rb,Cs)has attracted great interest in the field of superconductivity due to the coexistence of superconductivity,non-trivial surface state and multiple density waves.In this topical review,we present recent works of superconductivity and unconventional density waves in vanadium-based kagome materials AV_(3)Sb_(5).We start with the unconventional charge density waves,which are thought to correlate to the time-reversal symmetry-breaking orders and the unconventional anomalous Hall effects in AV_(3)Sb_(5).Then we discuss the superconductivity and the topological band structure.Next,we review the competition between the superconductivity and charge density waves under different conditions of pressure,chemical doping,thickness,and strains.Finally,the experimental evidence of pseudogap pair density wave is discussed.

A density-wave-like transition in the polycrystalline V_(3)Sb_(2) sample with bilayer kagome lattice

Recently,transition-metal-based kagome metals have aroused much research interest as a novel platform to explore exotic topological quantum phenomena.Here we report on the synthesis,structure,and physical properties of a bilayer kagome lattice compound V_(3)Sb_(2).The polycrystalline V_(3)Sb_(2) samples were synthesized by conventional solid-state-reaction method in a sealed quartz tube at temperatures below 850℃.Measurements of magnetic susceptibility and resistivity revealed consistently a density-wave-like transition at Tdw≈160 K with a large thermal hysteresis,even though some sample-dependent behaviors were observed presumably due to the different preparation conditions.Upon cooling through Tdw,no strong anomaly in lattice parameters and no indication of symmetry lowering were detected in powder x-ray diffraction measurements.This transition can be suppressed completely by applying hydrostatic pressures of about 1.8 GPa,around which no sign of superconductivity was observed down to 1.5 K.Specific-heat measurements revealed a relatively large Sommerfeld coefficientγ=18.5 mJ·mol^(-1)·K^(-2),confirming the metallic ground state with moderate electronic correlations.Density functional theory calculations indicate that V_(3)Sb_(2) shows a non-trivial topological crystalline property.Thus,our study makes V_(3)Sb_(2) a new candidate of metallic kagome compound to study the interplay between density-wave-order,nontrivial band topology,and possible superconductivity.

Quasiparticle density of states of 2H-NbSe_2 single crystals revealed by low-temperature specific heat measurements according to a two-component model

Low-temperature specific heat in a dichalcogenide superconductor 2H-NbSe2 is measured in various magnetic fields. It is found that the specific heat can be described very well by a simple model concerning two components corresponding to vortex normal core and ambient superconducting region, separately. For calculating the specific heat outside the vortex core region, we use the Bardeen-Cooper Schrieffer （BCS） formalism under the assumption of a narrow distribution of the superconducting gaps. The field-dependent vortex core size in the mixed state of 2H-NbSe2, determined by using this model, can explain the nonlinear field dependence of specific heat coefficient γ（H）, which is in good agreement with the previous experimental results and more formal calculations. With the high-temperature specific heat data, we can find that, in the multi-band superconductor 2H-NbSe2, the recovered density of states （or Fermi surface） below Tc under a magnetic field seems not to be gapped again by the charge density wave （CDW） gap, which suggests that the superconducting gap and the CDW gap may open on different Fermi surface sheets.