Revealing the insulating gap in α'-NaV (2)O(5) with resonant inelastic x-ray scattering

We measured the low energy excitation spectrum of α'-NaV (2)O(5) across its charge ordering and crystallographic phase transition with resonant inelastic x-ray scattering (RIXS) at the V L(3) edge. Exploiting the polarization dependence of the RIXS signal and the high resolution of the data, we reveal the excitation across the insulating gap at 1?eV and identify the excitations from occupied 3d(xy) bonding orbitals to unoccupied bonding 3d(xy) and 3d(yz)/3d(xz) orbitals. Furthermore we observe a progressive change of the electronic structure of α'-NaV (2)O(5) induced by soft x-ray irradiation, with the appearance of features characteristic of sodium deficient Na(x)V (2)O(5) (x?相似文献   

Subband filling and Mott transition in Ca2-xSrxRuO4

A new concept is proposed for the paramagnetic metal insulator transition in the layer perovskite Ca2-xSrxRuO4. Whereas the pure Sr compound is metallic up to large Coulomb energies due to strong orbital fluctuations, structural changes induced by doping with Ca give rise to an interorbital charge transfer which makes the material extremely sensitive to local correlations. Using dynamical mean field theory based on finite temperature multiband exact diagonalization, it is shown that the combination of crystal field splitting and on-site Coulomb interactions leads to complete filling of the d(xy) band and to a Mott transition in the half-filled d(xz,yz) bands.     

Spontaneous 2-dimensional carrier confinement at the n-type SrTiO3/LaAlO3 interface

We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3/LaAlO3 interface as a function of the sheet carrier density n(s) via advanced first-principles calculations. Electrons localize spontaneously in Ti 3d(xy) levels within a thin (?2 nm) interface-adjacent SrTiO3 region for n(s) lower than a threshold value n(c)～10(14) cm(-2). For n(s)>n(c) a portion of charge flows into Ti 3d(xz)-d(yz) levels extending farther from the interface. This intrinsic confinement can be attributed to the interface-induced symmetry breaking and localized nature of Ti 3d t(2g) states. The sheet carrier density directly controls the binding energy and the spatial extension of the conductive region. A direct, quantitative relation of these quantities with n(s) is provided.     

Origin of the structural and magnetic anomalies of the layered compound SrFeO2: a density functional investigation

The structural and magnetic anomaly of the layered compound SrFeO2 are examined by first-principles density functional calculations and Monte Carlo simulations. The down-spin Fe 3d electron occupies the d(z(2)) level rather than the degenerate (d(xz), d(yz)) levels, which explains the absence of a Jahn-Teller instability, the easy ab-plane magnetic anisotropy, and the observed three-dimensional (0.5, 0.5, 0.5) antiferromagnetic order. Monte Carlo simulations show that the strong interlayer spin exchange is essential for the high Néel temperature.     

Ferro-type orbital state in the Mott transition system Ca2-xSrxRuO4 studied by the resonant x-ray scattering interference technique

We have succeeded in detecting ferro-type orbital states in Ca2-xSrxRuO4, which is the first outcome in a 4d Mott transition system by the resonant x-ray scattering interference technique. For x=0 (Mott insulator), the resonant signal for d(xy) orbital ordering is observed even at room temperature, in which the Jahn-Teller distortion is negligible. The signal disappears near the metal-insulator transition. On the other hand, in a metallic phase for x=0.5, orbital polarization with d(yz/zx) character dominates. With lowering temperature, the magnitude of the resonant signal slightly decreases owing to the additional influence of the gamma band with d(xy) character.     

Evolution of the 128-cm-1 Raman phonon mode with temperature in Ba(Fe1-xCox)2As 2 (x = 0.065 and 0.2)

We report electronic Raman scattering measurements on Ba(Fei_1-xCo_x)₂As₂（x = 0.065 and 0.2） single crystals with Raman shifts from 9 cm^-1 up to 600 cm^-1 in the symmetry of B_lg with respect to 1 Fe unit cell.When the crystals are cooled down,the evident quasielastic peaks of Raman spectra occur only in the crystal with x = 0.065,which is due to the contribution of orbital ordering between xz and yz Fe 3d orbitals,as we reported in another work.Here,we analyze the E_g phonon at 128 cm^-1,which has the same function form of its Raman tensors as those of xz and yz Fe 3d orbitals in these two crystals respectively.Unlike their electronic continuums,no anomalies are found in the E_g phonons of these two samples,which simply follows the expressions corresponding to the anharmonic phonon decay into acoustic phonons with the same frequencies and opposite momenta.Our results indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping and there is not any indication of coupling between charge nematicity and E_g phonon mode from our experimental results,which is consistent with the results in our previous work.     

Magnetization-direction-dependent local electronic structure probed by scanning tunneling spectroscopy

Scanning tunneling spectroscopy (STS) of thin Fe films on W(110) shows that the electronic structure of domains and domain walls is different. This experimental result is explained on the basis of first-principles calculations. A detailed analysis reveals that the spin-orbit induced mixing between minority d(xy+xz) and minority d(z(2)) spin states depends on the magnetization direction and changes the local density of states in the vacuum detectable by STS. As a consequence nanometer-scale magnetic structure information is obtained even by using nonmagnetic probe tips.     

Presence of 3d quadrupole moment in LaTiO3 studied by 47,49Ti NMR

47,49Ti NMR spectra of LaTiO3 are reexamined and the orbital state of this compound is discussed. The NMR spectra of LaTiO3 taken at 1.5 K under zero external field indicate a large nuclear quadrupole splitting. This splitting is ascribed to the presence of the rather large quadrupole moment of 3d electrons at Ti sites, suggesting that the orbital liquid model proposed for LaTiO3 is inappropriate. The NMR spectra are well explained by the orbital ordering model expressed approximately as 1/square root of 3(d(xy)+d(yz)+d(zx)) originating from a crystal field effect. It is also shown that most of the orbital moment is quenched.     

Manipulation of intrinsic quantum anomalous Hall effect in two-dimensional MoYN2CSCl MXene

Quantum anomalous Hall effect (QAHE) is an innovative topological spintronic phenomenon with dissipationless chiral edge states and attracts rapidly increasing attention. However, it has only been observed in few materials in experiments. Here, according to the first-principles calculations, we report that the MXene MoYN$_{2}$CSCl shows a topologically nontrivial band gap of 37.3~meV, possessing QAHE with a Chern number of $C = 1$, which is induced by band inversion between $ {\rm d}_{xz}$ and ${\rm d}_{yz}$ orbitals. Also, the topological phase transition for the MoYN$_{2}$CSCl can be realized via strain or by turning the magnetization direction. Remarkably, MoYN$_{2}$CSCl shows the nodal-line semimetal state dependent on the electron correlation $U$. Our findings add an experimentally accessible and tunable member to the QAHE family, which stands a chance of enriching the applications in spintronics.     

VC(001)弛豫表面构型与电子结构第一性原理研究

用第一性原理方法对VC(001)清洁表面的构型和电子结构进行了详细研究，与TiC(001)面类似，VC(001)面弛豫后形成表面皱褶，其表层V原子和C原子分别朝体相和真空方向移动. 能带计算结果表明，过渡金属碳化物(001)面的能带结构符合刚性带理论模型. 对于VC(001)面，表面态主要处在-30eV附近，其主要成分为表层C原子的2pz轨道. 此外，以表层V原子的3d轨道成分为主的表面态出现在费米能级附近，由于这些表面态以表面法线方向的轨道(3d２z和3dxz/dyz)为主要成分，因此在表面反应中将起到重要作用，从而体现出与TiC(001)面不同的反应性质. 关键词： 过渡金属碳化物 表面态 能带结构     

Fermi surface topology of Ca1.5Sr0.5RuO4 determined by angle-resolved photoelectron spectroscopy

We report angle-resolved photoelectron spectroscopy results of the Fermi surface of Ca1.5Sr0.5RuO4, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three t(2g) energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free Sr2RuO4. We find that while the Fermi surface topology of the alpha,beta (d(yz,zx)) sheets remains almost the same in these two materials, the gamma (d(xy)) sheet exhibits a holelike Fermi surface in Ca1.5Sr0.5RuO4 in contrast to being electronlike in Sr2RuO4. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in Ca1.5Sr0.5RuO4.     

Szintillationslicht von Anthrazen bei Anregung mit Deuteronen in Abhängigkeit von Einschußrichtung,Deuteronenenergie, Kristalltemperatur und Beobachtungsrichtung

The scintillation light of anthracene by bombardment with deuterons in the energy range from 7.5 MeV to 27.0 MeV at the temperature of liquid nitrogen and at room temperature has been measured. Comparing the experimental data with theoretical data based on the Voltz formula get knowledge of the quenching parameter for various directions of the incident deuterons. In difference to former measurements here only scintillation light has been taken into account which was emitted (within an angle of ±7.5°) perpendicular to one of the three crystal planes (ab), (bc′), (ac′) and the two optical planes (xy), (yz) of anthracene. By changing the direction of the incident deuterons within the above five planes the anisotropy and the fine structure peak of the scintillation light has been determined.     

High-Resolution Fourier Transform Spectrum of the D(2)O Molecule in the Region of the Second Triad of Interacting Vibrational States

The microwave torsional-rotational spectra of gauche CH(3)CD(2)OH and CH(3)CD(2)OD have been identified, assigned, and analyzed up to 70 GHz. From the observed a- and c-dipole transitions, it has been possible to determine the effective rotational coefficients and the gauche tunneling energy of the hydroxyl internal rotation. The product of inertia terms I(xy) and I(xz) were included in the analyses using the framework fixed axis method (FFAM) approach to the hydroxyl internal rotation. Further, the analyses were sensitive to selected effective centrifugal distortion coefficients. For CH(3)CD(2)OH, a-dipole lines were assigned for the first excited gauche state. As for CH(3)CH(2)OH, these lines were highly nonrigid rotor in behavior more than likely due to the resonance with the first excited state of the methyl torsion. Copyright 2000 Academic Press.     

Theoretical characterization of the ground and optically excited states of alpha'-NaV2O5

The character of the ground and optically excited states was investigated by quantum chemical calculations. We propose a rung ground state with V 3d(1)(xy)-O 2p(1)(y)-V 3d(1)(xy) character, instead of the conventional picture of one unpaired electron shared by 2 V ions. The unpaired electron on O is low-spin coupled to the V d electrons and spin density is predicted to be localized on vanadium. The absorption peak at 0.9 eV is assigned to a state with similar orbital occupations but a different spin coupling scheme, resulting in spin density localized on the bridging oxygen.     

Collective mode in a superconductor with mixed-symmetry order parameter components

We consider a superconducting state with mixed-symmetry order parameter components, e.g., d+is or d+id(') with d(') = d(xy). We argue for the existence of a new orbital magnetization mode which corresponds to oscillations of relative phase straight phi between two components around an equilibrium value of straight phi = pi / 2. It is similar to the "clapping" mode in superfluid 3He-A. We estimate the frequency of this mode omega(0)(B,T) depending on the field and temperature for the specific case of magnetic field induced d(') = d(xy) state. This mode is tunable with a magnetic field with omega(0)(B,T) approximately BDelta(0), where Delta(0) is the magnitude of the d-wave order parameter. We also estimate the velocity s(B,T) of this mode.     

Mechanism for explaining differences in the order parameters of FeAs-based and FeP-based pnictide superconductors

We put forward a scenario that explains the difference between the order-parameter character in arsenide (As) and phosphorous (P) iron-based superconductors. Using functional renormalization group to analyze it in detail, we find that nodal superconductivity on the electron pockets (hole pocket gaps are always nodeless) can naturally appear when the hole pocket at (π,π) in the unfolded Brillouin zone is absent, as is the case in LaOFeP. There, electron-electron interactions render the gap on the electron pockets softly nodal (of s(±) form). When the pocket of d(xy) orbital character is present, intraorbital interactions with the d(xy) part of the electron Fermi surface drives the superconductivity nodeless.     

光折变材料Cu∶KNSBN的晶格振动和d－d电子跃迁

比较了铜掺杂钾钠铌酸锶钡（Ｃｕ∶ＫＮＳＢＮ）和钾钠铌酸锶钡（ＫＮＳＢＮ）两样品的晶格振动和ｄ－ｄ电子跃迁谱，对于拉曼（Ｒａｍａｎ）谱，Ａ１（ｚ）对称类的差别较小，Ｅ（ｘｙ）对称类的差别最大；对于红外反射谱，两对称类的均差别较大，认为Ｃｕ２＋部分填充了晶格Ａ位和Ｃ位，可见光范围内，ｄ－ｄ电子跃迁谱表明Ｃｕ２＋在晶体中形成两个深能级２．５０ｅＶ和２．６４ｅＶ。     

c-Axis Raman scattering spectra of MgB2: observation of a dirty-limit gap in the pi bands

Raman scattering spectra from the ac face of thick MgB2 single crystals were measured in zz, xz, and xx polarizations. In zz and xz polarizations a threshold at around 29 cm(-1) forms in the below T(c) continuum but no pair-breaking peak is seen, in contrast to the sharp pair-breaking peak at around 100 cm(-1) in xx polarization. The zz and xz spectra are consistent with Raman scattering from a dirty superconductor while the sharp peak in the xx spectra argues for a clean system. Analysis of the spectra resolves this contradiction, placing the larger and smaller gap magnitudes in the sigma and pi bands and indicating that relatively strong impurity scattering is restricted to the pi bands.     

Change of electronic structure in Ca2RuO4 induced by orbital ordering

Optical conductivity spectra sigma(omega) were used to investigate the effect of orbital ordering on the electronic structure of Ca2RuO4. Our LDA+U calculation predicts Ru 4d(xy) ferro-orbital ordering at the ground state, and well explains the present sigma(omega) as well as the reported O 1s x-ray absorption spectra. Variation of temperature (T) causes a large change of spectral weight over several eV as well as collapse of a charge gap accompanied by elongation of the c-axis Ru-O bond length. These results clearly indicate that the d(xy) orbital ordering plays a crucial role in the metal-insulator transition and the T-dependent electronic structure on a large energy scale.