Quantum and classical mode softening near the charge-density-wave-superconductor transition of CuxTiSe2

Temperature- and x-dependent Raman scattering studies of the charge-density-wave (CDW) amplitude modes in Cu(x)TiSe(2) show that the amplitude mode frequency omega(0) exhibits identical power-law scaling with the reduced temperature T/T(CDW) and the reduced Cu content x/x(c), i.e., omega(0) approximately (1-p)(0.15) for p=T/T(CDW) or x/x(c), suggesting that mode softening is independent of the control parameter used to approach the CDW transition. We provide evidence that x-dependent mode softening in Cu(x)TiSe(2) is associated with the reduction of the electron-phonon coupling constant, and that x-dependent "quantum" (T approximately 0) mode softening suggests the presence of a quantum critical point within the superconductor phase of Cu(x)TiSe(2).     

杨-巴克斯特自旋1/2链模型的量子关联研究

量子系统各部分间的量子关联可以作为量子信息应用研究的基础资源. 而量子失协是度量量子关联大小的物理量. 由此研究杨-巴克斯特自旋1/2链模型的量子关联情况. 首先利用两个杨-巴克斯特方程的解得到相应的杨-巴克斯特自旋1/2链模型. 然后, 计算分析热平衡时杨-巴克斯特自旋1/2链模型的量子失协、几何量子失协和量子纠缠随着温度和外磁场的变化情况. 结果表明对于杨-巴克斯特自旋1/2链模型, 量子失协和几何量子失协能够比量子纠缠更好地度量量子关联.     

Anomalous fluctuations in phases with a broken continuous symmetry

It is shown that the Goldstone modes associated with a broken continuous symmetry lead to anomalously large fluctuations of the zero field order parameter at any temperature below T(c). In dimensions 2相似文献   

0.67 PbMg1/3Nb2/3O3-0.33 PbTiO3单晶中软模模式指认

应用显微拉曼技术对弛豫型铁电体0.67PbMg1/3Nb2/3O3-0.33PbTiO3(0.67PMN-0.33PT)的偏振拉曼光谱随温度变化的研究表明,在-196到600°C温度范围内0.67PMN-0.33PT存在两次相变:三方到四方(R-T)相变和四方到立方(T-C)相变。R-T相变的特征是106cm-1软模(-196°C时)随温度的升高而湮没到80cm-1"静态"模式中。依照群论分析,0.67PMN-0.33PT的80cm-1和106cm-1(-196℃时)两个模式分别为E(1TO)和A1(1TO)模。文章对三方相的对称结构(C3v点群)在背向散射配置下不同偏振方向的拉曼散射效率进行了计算。计算结果表明晶体中的A1(1TO)和E(1TO)模式能够通过使用不同的散射配置进行识别。理论计算和实验结果相比较得到了很好的吻合。     

Low-temperature broken-symmetry phases of spiral antiferromagnets

We study Heisenberg antiferromagnets with nearest- (J1) and third- (J3) neighbor exchange on the square lattice. In the limit of spin S-->infinity, there is a zero temperature (T) Lifshitz point at J(3)=1/4J(1), with long-range spiral spin order at T=0 for J3>1/4J(1). We present classical Monte Carlo simulations and a theory for T>0 crossovers near the Lifshitz point: spin rotation symmetry is restored at any T>0, but there is a broken lattice reflection symmetry for 0< or =T相似文献   

叠加激发双模SU(2)相干态的量子统计特性

构造了叠加激发双模SU(2)相干态|ψ＞=Cm(| M,ξ;m＞+eiφ| M,ξ*;m＞),用数值方法研究它的量子统计特性.结果表明:叠加激发双模SU(2)相干态两场模的光子统计分布保持反关联特征,而且呈现奇特的光放大特性;b模光场的亚泊松分布得到比较大的加强,而a模光场的亚泊松分布特性则有轻微减弱;两场模的反聚束性均得到增强;随激发量子数m的增大,两场模间的反关联性减弱.     

反对称Spin-1/2阻挫钻石链的基态和磁化行为研究

对含有次近邻节点自旋交换耦合的自旋-1/2伊辛-海森伯钻石链体系进行了研究,利用矩阵对角化和传递矩阵方法对基态磁相和宏观热力学量进行了严格求解,重点探讨了所有交换耦合均为反铁磁耦合时,体系节点伊辛自旋间次近邻相互作用的影响.研究结果表明次近邻节点伊辛自旋存在反铁磁耦合时会增强系统的阻挫效应,引入破坏平移对称性的经典亚铁磁相,使基态呈现出上上上下上上的自旋构型以及磁化曲线新颖的2/3磁化平台,丰富了体系的基态相图和宏观磁性行为.     

Critical temperature shift in weakly interacting Bose gas

With a high-performance Monte Carlo algorithm we study the interaction-induced shift of the critical point in weakly interacting three-dimensional /psi/(4) theory (which includes quantum Bose gas). In terms of critical density, n(c), mass, m, interaction, U, and temperature, T, this shift is universal: Deltan(c)(T) = -Cm(3)T(2)U, the constant C found to be equal to 0.0140+/-0.0005. For quantum Bose gas with the scattering length a this implies DeltaT(c)/T(c) = C(0)an(1/3), with C(0) = 1.29+/-0.05.     

Two resonant magnetic modes in an overdoped high T(c) superconductor

A detailed inelastic neutron scattering study of the overdoped high temperature copper oxide superconductor Y(0.9)Ca(0.1)Ba(2)Cu(3)O(7) reveals two-distinct magnetic resonant modes in the superconducting state. The modes differ in their symmetry with respect to exchange between adjacent copper oxide layers. Counterparts of the mode with odd symmetry, but not the one with even symmetry, had been observed before at lower doping levels. The observation of the even mode resolves a long-standing puzzle, and the spectral weight ratio of both modes yields an estimate of the onset of particle-hole spin-flip excitations.     

Raman scattering studies on the collapsed phase of CaCo_2As_2

In this work,Raman scattering measurements have been performed on the collapsed phase CaCo_2As_2 crystals.At least 8 Raman modes were observed at room temperature though CaCo_2As_2 is structurally similar to other 122 compounds like BaFe_2As_2.Two Raman modes are assigned to the intrinsic A_(1g)and B_(1g)of this material system respectively.The other ones are considered to originate from the local vibrations relevant to cobalt vacancies.Careful polarized measurements allow us to clearly resolve the four-fold symmetry of the B_(1g)mode,which put strong constraints on possible point group symmetries of the system with Co vacancies.The temperature-dependent measurements demonstrate that the anomalies in both frequency and width of the B_(1g)mode occur around Neel temperature T_N.The anomalies are considered to be related to the gap opening near the magnetic transition.The study may shed light on the structural and magnetic changes and their correlations with superconductivity in 122 systems.     

Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO2 and CuCrO2

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO(2) reveal that the antiferromagnetic transition observed at T(N1) = 14 K might be induced by an R3m --> pseudoproper ferroelastic transition. In that case, the group theory states that the order parameter associated with the structural transition must belong to a two-dimensional irreducible representation E(g) (x(2) - y(2), xy). Since this type of transition can be driven by a Raman E(g) mode, we performed Raman scattering measurements on CuFeO(2) between 5 and 290 K. Considering that the isostructural multiferroic compound CuCrO(2) might show similar structural deformations at the antiferromagnetic transition T(N1) = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO(2) are observed at ω(E(g)) = 352 cm(-1) and ω(A(1g)) = 692 cm(-1), while these modes are detected at ω(E(g)) = 457 cm(-1) and ω(A(1g)) = 709 cm(-1) in CuCrO(2). The analysis of the temperature dependence of the modes in both compounds shows that the frequencies of all modes increase with decreasing temperature. This typical behavior is attributed to anharmonic phonon-phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO(2) and CuCrO(2) drive the pseudoproper ferroelastic transitions observed at the Néel temperature T(N1). Finally, a broad band at about 550 cm(-1) observed in the magnetoelectric phase of CuCrO(2) below T(N2) could be associated with magnons.     

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

Magnetization plateaus of SrCu(2)(BO(3))(2) from a Chern-Simons theory

The antiferromagnetic Heisenberg model on the frustrated Shastry-Sutherland lattice is studied by a mapping onto spinless fermions carrying one quantum of statistical flux. Using a mean-field approximation these fermions populate the bands of a generalized Hofstadter problem. Their filling leads to the magnetization curve. For SrCu(2)(BO(3))(2) we reproduce plateaus at 1/3 and 1/4 of the saturation moment and predict a new one at 1/2. Gaussian fluctuations of the gauge field are shown to be massive at these plateau values.     

Investigation of the superionic behaviour of BaF 2 ( x mol% LaF 3 ) by raman and brillouin scattering and molecular dynamics simulations

High temperature Raman and Brillouin light scattering experiments have been combined with molecular dynamics simulations to provide a comprehensive study of the superionic state of BaF 2 ( x v mol% LaF 3 ) over a particularly wide range of LaF 3 dopant concentrations from x =0 to 50. Room temperature Raman spectra for x =0, 5 and 10 show the usual T 2g symmetry mode at 241 v cm m 1 , but for samples with x =20, 30 and 50 the dominant Raman mode is at higher frequencies and of E g symmetry. The temperature dependence of the Raman line-widths show initial near linear increases followed by substantial increases above temperatures ( T c ) at 1200, 850, 800, 975, 950 and 920 v K for x =0, 5, 10, 20, 30 and 50. In the Brillouin scattering experiments, the acoustic modes respectively related to elastic constants C 11 and C 44 initially showed a quasi-linear decrease in frequency with increasing temperature. Above the same characteristic values of T c , where the Raman line-widths show marked increases, there are substantial decreases in the elastic constant C 11 for all samples with x =0 to 50. Only the doped samples showed significant decreases in C 44 at corresponding values of T c . Molecular dynamics (MD) simulations have been carried out on the same systems. From the calculated mean square displacements, the diffusion coefficients ( D ) of the mobile fluorine ions were calculated as a function of temperature for each of the samples. Substantial increases in the values of D occur above the respective values of T c determined in the light scattering experiments. The MD simulations also provide details of the mechanisms of diffusion of the mobile fluorine ions. The results emphasize the role of motional effects as an explanation of the mechanisms responsible and provide a self-consistent explanation of the dominant processes in the superionic phase of doped fluorites.     

Dual Space Analyzing Based on Symmetry Properties for Phonons fo Si Quantum Dot

Phonon modes in spherical Si quantum dots (QDs) with up to 7.9 nm in diameter are calculated by using the projection operators of the group theory into valence force field model. The phonons of dot modes in each of five irreducible representations (symmetries) are classified by using a dual space analysis method. It is found that the bulk-like modes with localization radius much smaller than the dot's radius have clearly pronounced bulk specific-k point parentage, T_λ(n, k_BZ, K_cut), from specific part of the Brillouin zone (BZ) (Γ-derived, X-derived etc.) and from definite bulk band (one in six modes). In Si dots of all sizes, each specific bulk-like dot mode has specific symmetry. The bulk TO(Γ)-like and the bulk LO(Γ)-like dot modes always have T₂ and A₁ symmetries, respectively. Except the bulk-A(Γ)-like dot modes of which the frequencies blue-shift as the dot size reducing, the bulk-like Γ-derived LO and TO dot modes and bulk-like X-derived TA and LA dot modes red-shift in frequency with decreasing dot size. There is almost not LO/TO mixing for bulk-like modes. As for the surface-like modes localized at the periphery of the dot, their eigenmodes have not a dominant bulk specific-k point parentage or a dominant BZ parentage around some special point. They are a superposition of many bulk bands with k from all over the bulk BZ. They have much significant mode mixing than the bulk-like phonons. The classification of dot modes based on the symmetry of group theory will bring advantageous to the discussion of Ramam spectrum, electron-phonon interaction and other phonon-assisted effects in QDs.     

Emergent O(4) symmetry at the phase transition from plaquette-singlet to antiferromagnetic order in quasi-two-dimensional quantum magnets

Recent experiments [Guo et al., Phys. Rev. Lett. 124 206602(2020)] on thermodynamic properties of the frustrated layered quantum magnet SrCu_2(BO_3)_2-the Shastry–Sutherland material-have provided strong evidence for a lowtemperature phase transition between plaquette-singlet and antiferromagnetic order as a function of pressure. Further motivated by the recently discovered unusual first-order quantum phase transition with an apparent emergent O(4) symmetry of the antiferromagnetic and plaquette-singlet order parameters in a two-dimensional "checkerboard J-Q" quantum spin model[Zhao et al., Nat. Phys. 15 678(2019)], we here study the same model in the presence of weak inter-layer couplings. Our focus is on the evolution of the emergent symmetry as the system crosses over from two to three dimensions and the phase transition extends from strictly zero temperature in two dimensions up to finite temperature as expected in SrCu_2(BO_3)_2.Using quantum Monte Carlo simulations, we map out the phase boundaries of the plaquette-singlet and antiferromagnetic phases, with particular focus on the triple point where these two ordered phases meet the paramagnetic phase for given strength of the inter-layer coupling. All transitions are first-order in the neighborhood of the triple point. We show that the emergent O(4) symmetry of the coexistence state breaks down clearly when the interlayer coupling becomes sufficiently large, but for a weak coupling, of the magnitude expected experimentally, the enlarged symmetry can still be observed at the triple point up to significant length scales. Thus, it is likely that the plaquette-singlet to antiferromagnetic transition in SrCu_2(BO_3)_2 exhibits remnants of emergent O(4) symmetry, which should be observable due to additional weakly gapped Goldstone modes.     

Raman scattering in K2Pt(CN)4Br0.3 · 3H2O

Raman scattering experiments on K₂Pt(CN)₄Br_0.3 · 3H₂O are reported between 5 and 300 K as a function of temperature. A line of A₁ symmetry detected at 44 cm^?1 shows interesting temperature dependent properties. It is concluded from a comparison of the frequency, symmetry, and scattering intensity of this line with theoretical predictions that the excitation concerned represents the amplitude mode of the charge density wave (the line observed in infrared absorption being the phase mode). No Peierls transition is observed, but the results are consistent with a Peierls distortion present at all temperatures. The findings are correlated with inelastic neutron scattering and infrared studies. Finally, the CN stretching modes at 2189 and 2173 cm^?1 and the water mode at 3490 cm^?1 are studied as a function of temperature.     

Anomalous superfluidity in (2+1)-dimensional two-color lattice QCD

We study thermodynamics of strongly coupled lattice QCD with two colors of staggered fermions in 2+1 dimensions. The partition function of this model can be written elegantly as a statistical mechanics of dimers and baryon loops. The model is invariant under an SO(3) x U(1) symmetry. At low temperatures, we find evidence for superfluidity in the U(1) symmetry sector while the SO(3) symmetry remains unbroken. The finite temperature phase transition appears to belong to the Kosterlitz-Thouless universality class, but the superfluid density jump rho(s) (T(c)) at the critical temperature T(c) is anomalously higher than the normal value of 2T(c)/pi. We show that, by adding a small SO(3) symmetry breaking term to the model, the superfluid density jump returns to its normal value, implying that the extra symmetry causes anomalous superfluid behavior. Our results may be of interest to researchers studying superfluidity in spin-1 systems.     

Temperature-dependent Raman scattering study of cation-deficient Aurivillius phases: Bi2WO6 and Bi2W2O9

Temperature-dependent Raman scattering experiments were performed on Bi(2)WO(6) and Bi(2)W(2)O(9). Significant changes in the phonon properties of Bi(2)WO(6) were observed as the temperature was increased due to decreased distortion from the B2cb structure. It was shown that instability of the 57 cm(-1) mode that behaved as a soft mode under pressure is not responsible for the Pca2(1) to B2cb phase transition taking place in Bi(2)WO(6) at 933 K. This result confirmed that this mode is not related to the [Formula: see text] tilt mode, which disappears upon change in symmetry from Pca2(1) to B2cb. Bi(2)W(2)O(9) does not exhibit any structural phase transition in the 298-800 K range. However, the temperature dependence of Raman bands indicated that the Bi(2)W(2)O(9) structure evolves with decreasing temperature from 800 to 298 K towards a more symmetric structure that was reported above 2.8 GPa at room temperature. This structural change is driven by displacement of the W atoms and is different from that exhibited by Bi(2)WO(6) and other members of the Aurivillius family but similar to that exhibited by WO(3). Our results also show that Bi(2)W(2)O(9) belongs to the small group of compounds that show the presence of low wavenumber modes characterized by unusually small linewidths.