Orbital fluctuations and orbital flipping in RVO3 perovskites

The effect of the average R-site ionic radius IR and variance on the orbital and magnetic order in R3+-doped YVO3 was studied in Y1-xLaxVO3 and Y1-x(La0.2337Lu0.7663)xVO3 with fixed IR. The orbital flipping temperature T{CG} increases nonlinearly with increasing R-site variance, indicating that the V-O-V bond angle is not the primary driving force stabilizing the C-type orbitally ordered phase. The suppressed thermal conductivity in the G-type orbitally ordered phase signals some remaining orbital randomness that is enhanced by t{2} and et hybridization in {3}T{1g} site symmetry.     

Transition from orbital liquid to Jahn-Teller insulator in orthorhombic perovskites RTiO3

Following the same strategy used for RVO3, thermal conductivity measurements have been made on a series of single-crystal perovskites RTiO3 (R=La,Nd,...,Yb). Results reveal explicitly a transition from an orbital liquid to an orbitally ordered phase at a magnetic transition temperature, which is common for both the antiferromagnetic and ferromagnetic phases in the phase diagram of RTiO3. This spin/orbital transition is consistent with the mode softening at T_{N} in antiferromagnetic LaTiO3 and is supported by an anomalous critical behavior at T_{c} in ferromagnetic YTiO3.     

Fermi surface of 3d1 perovskite CaVO3 near the Mott transition

We present a detailed de Haas-van Alphen effect study of the perovskite CaVO3, offering an unprecedented test of electronic structure calculations in a 3d transition metal oxide. Our experimental and calculated Fermi surfaces are in good agreement, but only if we ignore large orthorhombic distortions of the cubic perovskite structure. Subtle discrepancies may shed light on an apparent conflict between the low energy properties of CaVO3, which are those of a simple metal, and high energy probes which reveal strong correlations that place CaVO3 on the verge of a metal-insulator transition.     

Intrinsic coupling of orbital excitations to spin fluctuations in Mott insulators

We show how the general and basic asymmetry between two fundamental degrees of freedom present in strongly correlated oxides, spin and orbital, has very profound repercussions on the elementary spin and orbital excitations. Whereas the magnons remain largely unaffected, orbitons become inherently coupled with spin fluctuations in spin-orbital models with antiferromagnetic and ferro-orbital ordered ground states. The composite orbiton-magnon modes that emerge fractionalize again in one dimension, giving rise to spin-orbital separation in the peculiar regime where spinons are faster than orbitons.     

The Mott Anderson metal insulator transition in n orbital models

The interplay of Coulomb interactions and disorder is formulated on the basis of local gauge invariant models withn orbitals per site. Universality classes of correlation-influenced metal insulator transitions are examined from the conducting region of an approximately semi-elliptical band. The microscopic 1/n expansion is brought into direct correspondence with the 1/(k _F ?) expansion of Altshuler et al. Based on1. the assumption that theO(1/n)-expansion can be exponentiated and2. on the renormalization group β-function for finite length scaling, critical exponents are derived in leading order ofd-2 (d is dimensionality). The density of states-at-E _F as the order parameter vanishes at the critical pointE _c like \(\left| {E_F - E_c } \right|^{\beta _{MA} } \) with \(\beta _{MA} = \frac{1}{{4(d - 2)}}\) for the interacting real matrix (orthogonal) ensemble and \(\beta _{MA} = \frac{1}{{2(d - 2)}}\) for the unitary ensemble. If the bare Coulomb interactionU _b (q)∝q ^1-d is replaced by a general long range interactionV _b (q)∝q ^?x withx>0 ford=2, β_MA depends on the “interaction range exponent”x like β_MA(x) = xβ_MA(1). Also in leading order ofd?2, the conductivity exponent ist=1 for both models with or without time reversal invariance respectively. This implies a correlation-induced crossover fromt=1/2 for unitary Anderson localization (broken time reversal invariance) tot=1 at the Mott Anderson transition.     

Compressibility of orthorhombic perovskites. The effect of transition metal ions (TMI)

Interest in perovskites evenly spans Materials Science and Geophysics. Due to their inimitably lattice flexibility enabling small as well as large ions to be accommodated, perovskites have become a base structure for new technological applications. Understanding the mechanisms governing their evolution at non-ambient conditions (such as high-pressure and high-temperature) is fundamentally important both for devising functional materials and in order to provide the most reliable possible deep-Earth model. With particular attention being paid to the chemical nature of the constituent ions, a suite of orthorhombic perovskites has been selected and contrasted using several parameterizations and models. A new perspective on the pressure-induced distortion of orthorhombic perovskite structures has enabled their compressional behaviour to be redefined.     

Ferroelectricity in the magnetic E-phase of orthorhombic perovskites

We show that the symmetry of the spin zigzag chain E phase of the orthorhombic perovskite manganites and nickelates allows for the existence of a finite ferroelectric polarization. The proposed microscopic mechanism is independent of spin-orbit coupling. We predict that the polarization induced by the E-type magnetic order can potentially be enhanced by up to 2 orders of magnitude with respect to that in the spiral magnetic phases of TbMnO3 and similar multiferroic compounds.     

Mott physics near the insulator-to-metal transition in NdNiO3

An optical study of NdNiO(3) ultrathin films with insulating and metallic ground states reveals new aspects of the insulator-to-metal transition that point to Mott physics as the driving force. In contrast with the behavior of charge-ordered systems, we find that the emergence of the Drude resonance across the transition is linked to a spectral weight transfer over an energy range of the order of the Coulomb repulsion U, as the energy gap is filled with states instead of closing continuously.     

Mott transition in anharmonic confinement

Two effects are identified that affect the visibility of the Mott transition in an atomic gas in an optical lattice confined in a power-law potential. The transition can be made more pronounced by increasing the power law, but at the same time, experimental uncertainty in the number of particles will induce corresponding fluctuations in the measured condensate fraction. Calculations in two dimensions indicate that a potential slightly more flat-bottomed than a quadratic one is to be preferred for a wide range of particle number fluctuation size.     

Raman scattering in the Mott insulators LaTiO3 and YTiO3: evidence for orbital excitations

Raman scattering is used to observe pronounced electronic excitations around 230 meV--well above the two-phonon range--in the Mott insulators LaTiO3 and YTiO3. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.     

Mott insulation in transition metal compounds

The conditions for the occurance of Mott insulation and its disappearance in transition metal compounds are investigated. It is found, that transition metal compounds with less than fived-electrons may be good examples of Mott Insulators, while those with more than fived-electrons should more properly be termed charge transfer compounds.     

Subpicosecond time-resolved Mott transition in CuCl

The Mott transition in CuCl from a metallic phase of free electrons and holes towards an insulating phase of bound particles (excitons or biexcitons) has been studied by time-resolved luminescence with a resolution slightly better than 1 ps. The phase change takes place in a very short but finite time (about 3 ps) at a carrier density N 10¹⁹ cm^-3.     

The Mott transition in cold semiconductors

We discuss the Mott transition when a semiconductor is externally excited at low temperatures. We demonstrate that hysteresis is inevitable unless a phase separation occurs at higher temperatures or lower average excitation densities than necessary for the predicted ionization catastrophe. The critical temperature and pair density for hysteresis in Ge is of the order 6K and 3 · 10¹⁶ cm^-3, respectively, for a model involving dynamical screening.     

利用X射线磁性圆二色吸收谱计算3d过渡族磁性原子的轨道和自旋磁矩

在4个方面研究了实验数据的预处理和应用加和定则中的问题.1)外磁场对样品电流法测量的吸收谱强度的影响.发现外磁场H<200×10^-4T时，信号强度正比于H^-β；当H>200×10^-4T时，尽管外磁场继续增加，但信号强度基本保持不变.2)不同方向的电磁铁剩磁会导致吸收谱的分离.这种分离与入射光的偏振态和样品的磁性无关，可以通过乘以一个常数很好地消除这种分离.3)通过XPSPEAK 4.1对实验数据拟合，写出了吸收谱的解析函数.利用解析函数的积分值，建立一种相对“客观"的标准，判断在一定的实验条件下，不同的数值积分方法的准确性.4)以误差函数作为吸收谱的背景函数，建立了一套完整的X射线磁性圆二色的数据处理方法.最后用Bode积分法计算出20nm厚Co膜的轨道和自旋磁矩分别为0.141μ_B和1.314μ_B. 关键词： X射线磁性圆二色 加和定则 台阶函数 吸收谱拟合     

Hole dynamics in spin and orbital ordered vanadium perovskites

A theory of doped perovskite vanadates with spin and orbital orders is presented. Mobile holes are strongly renormalized by spin excitations (magnons) in the spin G-type and orbital C-type (SG-OC) order, and orbital excitations (orbitons) in the spin C-type and orbital G-type (SC-OG) one. Hole dynamics in a staggered t(2g) orbital array is distinguished from that in the antiferromagnetic order and the e(g) orbital one. The fragile character of the (SG-OC) order in Y1-xCaxVO3 is attributed to the orbiton softening induced by a reduction of the spin order parameter.     

Universal fluctuations in orbital diamagnetism

Bohr–van Leuween theorem has attracted the notice of physicists for more than 100 years. The theorem states about the absence of magnetisation in classical systems in thermal equilibrium. In this paper, we discuss about fluctuations of magnetic moment in classical systems. In recent years, this topic has been investigated intensively and it is not free from controversy. We have considered a system consisting of a single particle moving in a plane. A magnetic field is applied perpendicular to the plane. The system is in contact with a thermal bath. We have considered three cases: (a) particle moving in a homogeneous medium, (b) particle moving in a medium with space-dependent friction and (c) particle moving in a medium with space-dependent temperature. For all the three cases, the average magnetic moment and fluctuations in magnetic moment have been calculated. Average magnetic moment saturates to a finite value in the case of free particle but goes to zero when the particle is confined by a 2D harmonic potential. Fluctuations in magnetic moment shows universal features in the presence of arbitrary friction inhomogeneity. For this case, the system reaches equilibrium asymptotically. In the case of space-dependent temperature profile, the stationary distribution is non-Gibbsian and fluctuations deviate from universal value for the bounded system only.     

Mott transition and Kondo screening in f-electron metals

We study how a finite hybridization between a narrow correlated band and a wide conduction band affects the Mott transition. At zero temperature, the hybridization is found to be a relevant perturbation, so that the Mott transition is suppressed by Kondo screening. In contrast, a first-order transition remains at finite temperature, separating a local-moment phase and a Kondo-screened phase. The first-order transition line terminates in two critical end points. Implications for experiments on f-electron materials such as the cerium alloy Ce0.8La0.1Th0.1 are discussed.