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...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).展开更多
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 R...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.展开更多
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 i...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).展开更多
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 requir...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.展开更多
Charge-density-wave(CDW) materials with strongly correlated electrons have broadband light absorption and ultrafast response to light irradiation, and hence hold great potential in photodetection. 1 T-TaS2 is a typica...Charge-density-wave(CDW) materials with strongly correlated electrons have broadband light absorption and ultrafast response to light irradiation, and hence hold great potential in photodetection. 1 T-TaS2 is a typical CDW material with various thermodynamically CDW ground states at different temperatures and fertile out-of-equilibrium intermediate/hidden states. In particular, the light pulses can trigger melting of CDW ordering and also forms hidden states, which exhibits strikingly different electrical conductivity compared to the ground phase. Here, we review the recent research on phase transitions in 1 T-TaS2 and their potential applications in photodetection. We also discuss the ultrafast melting of CDW ordering by ultrafast laser irradiation and the out-of-equilibrium intermediate/hidden states by optical/electrical pulse. For photodetection, demonstrations of photoconductors and bolometers are introduced. Finally, we discuss some of the challenges that remain.展开更多
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 measur...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(TiS_3). 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 TiS_3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.展开更多
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...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.展开更多
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 tensi...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.展开更多
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 ...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.展开更多
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 phas...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.展开更多
Transmission electron microscopy(TEM) study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave(CDW) state below TCDW≈ 470 K. These two structural modulations can be well explained...Transmission electron microscopy(TEM) study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave(CDW) state below TCDW≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1= 0.62a*on the a*–b*plane and q2=0.23a*on the a*–c*plane. The atomic displacements occur along the b-axis direction for q1and along the c-axis direction for q2, respectively. Moreover, the correlation between q1and q2can be generally written as q1=(q2+ a*)/2 in the CDW state, suggesting the presence of essential coupling between q1and q2. A small fraction of Ir doping on the Pt site in Sr(Pt1-xIrx)2As2(x ≤ 0.06) could moderately change these CDW modulations and also affect their superconductivities.展开更多
It was found that selenium doping can suppress the charge-density-wave(CDW) order and induce bulk superconductivity in ZrTe_3. The observed superconducting dome suggests the existence of a CDW quantum critical point(Q...It was found that selenium doping can suppress the charge-density-wave(CDW) order and induce bulk superconductivity in ZrTe_3. The observed superconducting dome suggests the existence of a CDW quantum critical point(QCP) in ZrTe_3-xSex near x ≈ 0.04. To elucidate the superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe_(3-x)Se_x single crystals(x = 0.044 and 0.051) down to 80 m K. For both samples, the residual linear term κ_0/T at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of κ_0/T manifests a multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe_(3-x)Se_x,which indicates conventional superconductivity despite of the existence of a CDW QCP.展开更多
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 materi...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.展开更多
The wave/particle duality of particles in Physics is well known. Particles have properties that uniquely characterize them from one another, such as mass, charge and spin. Charged particles have associated Electric an...The wave/particle duality of particles in Physics is well known. Particles have properties that uniquely characterize them from one another, such as mass, charge and spin. Charged particles have associated Electric and Magnetic fields. Also, every moving particle has a De Broglie wavelength determined by its mass and velocity. This paper shows that all of these properties of a particle can be derived from a single wave function equation for that particle. Wave functions for the Electron and the Positron are presented and principles are provided that can be used to calculate the wave functions of all the fundamental particles in Physics. Fundamental particles such as electrons and positrons are considered to be point particles in the Standard Model of Physics and are not considered to have a structure. This paper demonstrates that they do indeed have structure and that this structure extends into the space around the particle’s center (in fact, they have infinite extent), but with rapidly diminishing energy density with the distance from that center. The particles are formed from Electromagnetic standing waves, which are stable solutions to the Schrödinger and Classical wave equations. This stable structure therefore accounts for both the wave and particle nature of these particles. In fact, all of their properties such as mass, spin and electric charge, can be accounted for from this structure. These particle properties appear to originate from a single point at the center of the wave function structure, in the same sort of way that the Shell theorem of gravity causes the gravity of a body to appear to all originate from a central point. This paper represents the first two fully characterized fundamental particles, with a complete description of their structure and properties, built up from the underlying Electromagnetic waves that comprise these and all fundamental particles.展开更多
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 mul...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.展开更多
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 o...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.展开更多
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 calcula...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.展开更多
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 cha...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.展开更多
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 superconduct...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).展开更多
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11574338 and 12074038)NSAF(Grant No.U1530402)。
文摘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).
基金the National Key Research and Development Program of China(Grant No.2021YFA1400403)the National Natural Science Foundation of China(Grant Nos.92165205,11790311,and 11774149)+2 种基金Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302800)the support by the open project of Beijing National Laboratory for Condensed Matter Physics(Grant No.ZBJ2106110017)the Double First-Class Initiative Fund of Shanghai Tech University。
文摘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.
基金Project supported by the National Key Research and Development Project of China(Grant No.2019YFA0308500)the National Natural Science Foundation of China(Grant No.61888102)the Chinese Academy of Sciences(Grant Nos.XDB30000000 and YSBR-003).
文摘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).
基金the National Key Research and Development Program of China(Grant Nos.2021YFA1400100,2020YFA0308800,and 2019YFA0308000)the National Natural Science Foundation of China(Grant Nos.92163206,62171035,62171035,61901038,61971035,61725107,and 61674171)+1 种基金the Beijing Nova Program from Beijing Municipal Science&Technology Commission(Grant No.Z211100002121072)the Beijing Natural Science Foundation(Grant Nos.Z190006 and 4192054)。
文摘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.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.21673058 and 21822502)the Key Research Program of Frontier Sciences of Chinese Academy of Sciences(Grant No.QYZDB-SSW-SYS031)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB30000000)
文摘Charge-density-wave(CDW) materials with strongly correlated electrons have broadband light absorption and ultrafast response to light irradiation, and hence hold great potential in photodetection. 1 T-TaS2 is a typical CDW material with various thermodynamically CDW ground states at different temperatures and fertile out-of-equilibrium intermediate/hidden states. In particular, the light pulses can trigger melting of CDW ordering and also forms hidden states, which exhibits strikingly different electrical conductivity compared to the ground phase. Here, we review the recent research on phase transitions in 1 T-TaS2 and their potential applications in photodetection. We also discuss the ultrafast melting of CDW ordering by ultrafast laser irradiation and the out-of-equilibrium intermediate/hidden states by optical/electrical pulse. For photodetection, demonstrations of photoconductors and bolometers are introduced. Finally, we discuss some of the challenges that remain.
基金Project supported by the National Young 1000-Talent Planthe National Natural Science Foundation of China(Grant Nos.61322407,11474058,and61674040)
文摘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(TiS_3). 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 TiS_3 as a promising quasi-1D CDW material and open up a new window for the study of collective phases in TMTCs.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 92165205, 11790311, 12004172, 11774152, 11604366, and 11634007)the National Key Research and Development Program of China (Grant Nos. 2018YFA0306800 and 2016YFA0300401)+1 种基金the Program of High-Level Entrepreneurial and Innovative Talents Introduction of Jiangsu Province, the Jiangsu Planned Projects for Postdoctoral Research Funds (Grant No. 2020Z172)the Natural Science Foundation of Jiangsu Province, China (Grant No. BK 20160397)。
文摘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.
基金supported by the National Natural Science Foundation of China(Grant Nos.22176181,11874306,and 12174320)the Foundation of Science and Technology on Surface Physics and Chemistry Laboratory(Grant No.WDZC202101)the Natural Science Foundation of Chongqing,China(Grant No.cstc2021jcyj-msxmX0209)。
文摘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.
基金the National Key Research and Development Program of China(Grant Nos.2018YFA0305700,2017YFA0403600,and2016YFA0401804)the National Natural Science Foundation of China(Grant Nos.U1632275,U19A2093,U1932152,U1632162,12004004,11874362,11804344,11704387,and 11674325)+4 种基金the Natural Science Foundation of Anhui Province,China(Grant Nos.1908085QA18,2008085QA40,and1808085MA06)the Users with Excellence Project of Hefei Science Center CAS(Grant Nos.2018HSC-UE012,2020HSC-CIP014,2020HSC-UE015,and2021HSC-UE008)the Major Program of Development Foundation of Hefei Center for Physical Science and Technology(Grant No.2018ZYFX002)supported by the High Magnetic Field Laboratory of Anhui Province(Grant No.AHHM-FX-2020-02)supported by the Youth Innovation Promotion Association of Chinese Academy of Sciences(Grant No.2020443)。
文摘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.
基金supported by the National Natural Science Foundation of China (Grant No. 11874263)the National Key R&D Program of China (Grant No. 2017YFE0131300)Shanghai Technology Innovation Action Plan (2020-Integrated Circuit Technology Support Program 20DZ1100605,2021-Fundamental Research Area 21JC1404700)。
文摘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.
基金Project supported by the National Basic Research Program of China(Grant Nos.2011CBA00101,2010CB923002,2012CB821404,and 2011CB921703)the National Natural Science Foundation of China(Grant Nos.11190022,11274368,and 51272277)the Funds from the Chinese Academy of Sciences
文摘Transmission electron microscopy(TEM) study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave(CDW) state below TCDW≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1= 0.62a*on the a*–b*plane and q2=0.23a*on the a*–c*plane. The atomic displacements occur along the b-axis direction for q1and along the c-axis direction for q2, respectively. Moreover, the correlation between q1and q2can be generally written as q1=(q2+ a*)/2 in the CDW state, suggesting the presence of essential coupling between q1and q2. A small fraction of Ir doping on the Pt site in Sr(Pt1-xIrx)2As2(x ≤ 0.06) could moderately change these CDW modulations and also affect their superconductivities.
基金Project supported by the Natural Science Foundation of Zhejiang Province, China （Grant No Y605106） and National Basic Research Program of China （Grant No 2006CB601003）.
基金Project supported by the National Basic Research Program of China(Grant Nos.2012CB821402 and 2015CB921401)the National Natural Science Foundation of China(Grant Nos.91421101,11422429,and 11204312)+1 种基金the Program for Professor of Special Appointment(Eastern Scholar)at Shanghai Institutions of Higher Learning,China,STCSM of China(Grant No.15XD1500200)Work at Brookhaven National Laboratory was supported by the US DOE under Contract No.DESC00112704
文摘It was found that selenium doping can suppress the charge-density-wave(CDW) order and induce bulk superconductivity in ZrTe_3. The observed superconducting dome suggests the existence of a CDW quantum critical point(QCP) in ZrTe_3-xSex near x ≈ 0.04. To elucidate the superconducting state near the CDW QCP, we measure the thermal conductivity of two ZrTe_(3-x)Se_x single crystals(x = 0.044 and 0.051) down to 80 m K. For both samples, the residual linear term κ_0/T at zero field is negligible, which is a clear evidence for nodeless superconducting gap. Furthermore, the field dependence of κ_0/T manifests a multigap behavior. These results demonstrate multiple nodeless superconducting gaps in ZrTe_(3-x)Se_x,which indicates conventional superconductivity despite of the existence of a CDW QCP.
基金support from the National Key R&D Program of China(No.2017YFA0402901)the National Natural Science Foundation of China(Nos.U2032153,21727801,and 11621063)+2 种基金the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDB25000000)the International Partnership Program of Chinese Academy of Sciences(CAS)(No.211134KYSB20190063)the Collaborative Innovation Program of Hefei Science Center of CAS(No.2019HSC-CIP007).
文摘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.
文摘The wave/particle duality of particles in Physics is well known. Particles have properties that uniquely characterize them from one another, such as mass, charge and spin. Charged particles have associated Electric and Magnetic fields. Also, every moving particle has a De Broglie wavelength determined by its mass and velocity. This paper shows that all of these properties of a particle can be derived from a single wave function equation for that particle. Wave functions for the Electron and the Positron are presented and principles are provided that can be used to calculate the wave functions of all the fundamental particles in Physics. Fundamental particles such as electrons and positrons are considered to be point particles in the Standard Model of Physics and are not considered to have a structure. This paper demonstrates that they do indeed have structure and that this structure extends into the space around the particle’s center (in fact, they have infinite extent), but with rapidly diminishing energy density with the distance from that center. The particles are formed from Electromagnetic standing waves, which are stable solutions to the Schrödinger and Classical wave equations. This stable structure therefore accounts for both the wave and particle nature of these particles. In fact, all of their properties such as mass, spin and electric charge, can be accounted for from this structure. These particle properties appear to originate from a single point at the center of the wave function structure, in the same sort of way that the Shell theorem of gravity causes the gravity of a body to appear to all originate from a central point. This paper represents the first two fully characterized fundamental particles, with a complete description of their structure and properties, built up from the underlying Electromagnetic waves that comprise these and all fundamental particles.
基金support from the Ministry of Science and Technology of Chinathe National Natural Science Foundation of China and Chinese Academy of Sciences
文摘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.
基金the National Key R&D Program of China(Grant Nos.2018YFA0305700 and 2018YFA0305800)the National Natural Science Foundation of China(Grant Nos.12025408,11874400,11834016,11921004,11888101,and 11904391)+3 种基金the Beijing Natural Science Foundation,China(Grant No.Z190008)the Strategic Priority Research Program and Key Research Program of Frontier Sciences of Chinese Academy of Sciences(CAS)(Grant Nos.XDB25000000,XDB33000000 and QYZDBSSW-SLH013)the CAS Interdisciplinary Innovation Team(Grant No.JCTD-201-01)supported by the U.S.Department of Energy,Office of Science,Basic Energy Sciences,Materials Sciences and Engineering Division。
文摘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.
基金supported by the National Natural Science Foundation of China(Nos.12274013 and 11874079)the open research fund program of the State key laboratory of low dimensional quantum physics(No.KF202103)the Independent Research Project of Medical Engineering Laboratory of Chinese PLA General Hospital(No.2022SYSZZKY10).
文摘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.
基金the National Key Research and Development Program of China(No.2018YFA0703700)the National Natural Science Foundation of China(Nos.62074045 and 61774040,)+1 种基金the Shanghai Municipal Natural Science Foundation(No.20ZR1403200)the National Young 1000 Talent Plan of China.
文摘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.
基金supported by the National Key Research and Development Program of China(Grant No.2022YFA1604204)National Natural Science Foundation of China(Grant No.92265101)+1 种基金Science and Technology Innovation Program of Hunan Province(Grant No.2022RC3068)Innovation-driven Plan in Central South University(Grant No.2016CXS032)。
文摘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).
