Session Y16: Strongly Correlated Electrons - Experiment and Theory

Electronic Excitations in CuO from a Many-Body Point of View

The insulating transition-metal oxide CuO is considered a key to understanding the electronic structure of high-temperature superconducting cuprates, since it features similar bonding geometries. The photoemission spectra of this oxide, which is usually termed to be strongly correlated, have not been explained satisfactorily by first-principles calculations up to now. Special difficulties arise from the close entanglement of the structural and electronic degrees of freedom in this compound, which is due to the unoccupied Cu~$3d$ orbitals. In contrast to the local-density approximation of density-functional theory (DFT), which predicts CuO to be a metal, we obtain finite band gaps by means of hybrid functionals containing screened exchange and the DFT+$U$ method. Starting from these qualitatively correct band structures, we perform many-body calculations in the $GW$ approximation. The various approaches to the one-particle excitation spectra are compared to experimental results. We discuss whether or not the peaks occurring in the photoemission data are quasiparticle excitations or satellite structures, respectively.   

Pressure Effects on the Vanadium Oxides V$_{6}$O$_{11}$, V$_{7}$O$_{13}$, and V$_{8}$O$_{15}$

Members of the V$_{N}$O$_{N-1}$ Magneli Series (3~$<$~N~$<$~9) exhibit metal to insulator transitions (MIT) as well as antiferromagnetic (AFM) transitions at ambient pressure, with the exception of V$_{7}$O$_{13}$ which remains metallic to lowest measured temperatures. In this talk we present pressure and temperature dependent measurements of electrical resistivity for V$_{6}$O$_{11}$, V$_{7}$O$_{13}$ and V$_{8}$O$_{15}$ samples. For V$_{6}$O11 and V$_{8}$O$_{15}$ MIT can be suppressed by 4 and 1.3~GPa respectively. For V$_{7}$O$_{13}$ and V$_{8}$O$_{15}$ T$_{SDW}$ can be suppressed by 3.5~GPa. Composite phase diagrams will be presented and low temperature data, focusing on the possibility of quantum criticality will be discussed.   

Magnetic properties of Fe$_{1-y}$Co$_y$Si near insulator-metal transition

FeSi is a nonmagnetic narrow gap insulator with interesting temperature-dependent magnetic and optical properties. When doped with Mn, holes are introduced to the FeSi system along with S=1 magnetic moments. As a result, a non-Fermi liquid metallic state is observed due to the underscreening of the S=1 moments. Here we investigate the effects of Co doping of FeSi (Fe$_{1-y}$Co$_y$Si, 0$\leq$y$\leq$0.03) to explore the insulator-to-metal transition and to compare with Mn-doped effects. We find a systematic increase of the Curie constant and Weiss temperature of the impurity-induced low temperature susceptibility with y. The Curie constant and saturated magnetization indicates that there is an underlying competition between screening of the magnetic moments at low y and ferromagnetic ordering at higher Co-concentrations. Transport measurements are underway to explore the associated charge carrier properties in this semiconducting system on the verge of ferromagnetism.   

Signatures of Correlation Effects and Thermopower in FeSi

Correlated semiconductors have been studied intensively over the years, because they exhibit an unusual metalization process which is poorly understood. At low temperatures FeSi behaves as an ordinary semiconductor, while at high temperatures the system is a bad metal with a Curie like susceptibility. Analogies with heavy fermion Kondo insulators and mixed valence compounds, and anomalous electron phonon coupling have been invoked to account for this behavior, but lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we use realistic many-body methods to elucidate the metalization of FeSi. Our methodology accounts for all substantial anomalies observed in FeSi : lack of conservation of spectral weight in optics, Curie susceptibility and an anomalous thermoelectric power. Having quantitatively validated our approach, we propose a new scenario. In correlated insulators such as FeSi the metalization is induced by the emergence of non-quasiparticle incoherent states in the gap. This coherence-incoherence crossover is accompanied by a massive reorganization of the spin excitations. Besides the fundamental interest of our theory, our work is relevant to the design of thermoelectric materials based on correlated insulators.   

FeGa3 -- a strongly correlated insulator?

FeGa3 is one of few Fe-based nonmagnetic semiconductors, in which a small gap is produced by the hybridization of Fe 3d states with p states of a group 13 or 14 element. The role of strong electron-electron correlation effects in formation of the gap is unclear. In case of FeSi and FeSb2, a metal-insulator transition was observed at temperatures low relative to the gap energy, which is a hallmark of a Kondo nature of the gap. For FeGa3, a combination of photoemission data and DFT-based electronic structure calculations indicated a rather strong on-site effective Coulomb repulsion within the Fe 3d shell Ueff$\sim $3 eV and a sizable Ueff/W $\sim $ 0.6 (W -- band width) [1]. Interestingly, recent ARPES measurements revealed a Fe 3d derived state located around 0.4 eV away from the top of the valence band [1]. Thermodynamic and transport measurements do not give any sign of a metal-insulation transition up to 1000 K [2]. To get insight into the importance of e-e correlation effects in formation of the gap in FeGa3, we drive the system towards a metallic state by doping. The results of resistivity, specific heat and magnetization measurements on doped single crystals of FeGa3 grow by Ga-flux technique will be discussed. \\[4pt] [1] Arita M et al., Phys. Rev. B 83, 245116 (2011) \\[0pt] [2] Hadano Y et al., J. Phys. Soc. Jpn. 78, 013702 (2009)   

Hybridization wave as the cause of the metal-insulator transition in rare earth nickelates

The metal-insulator transition driven by varying rare earth ($Re$) ion in $ReNiO_3$ has been a longstanding challenge to materials theory. Experimental evidence suggesting charge order is seemingly incompatible with the strong Mott-Hubbard correlations characteristic of transition metals. We present density functional, Hartree-Fock and Dynamical Mean field calculations showing that the origin of the insulating phase is a hybridization wave, in which a two sublattice ordering of the oxygen breathing mode produces two $Ni$ sites with almost identical $Ni$ $d$-charge densities but very different magnetic moments and other properties. The high temperature crystal structure associated with smaller $Re$ ions such as $Lu$ is shown to be more susceptible to the distortion than the high temperature structure associated with larger $Re$ ions such as $La$.   

Non-thermal dynamics of the spin and charge order in striped nickelates

In the striped nickelate La$_{2-x}$Sr$_{x}$NiO$_{4}$, spin order coexists with charge order, whose periodicity is half of spin order. So far, most of the studies on the stripe phase were performed in the thermal equilibrium state by varying temperatures; the dynamics of stripe phase in the time domain when the system is driven-out-of-equilibrium has not yet been studied. Using the x-ray free electron laser (XFEL) at LCLS, we performed time-resolved optical-pump and resonant soft x-ray diffraction probe experiments to study the dynamics of the spin and charge order. Comparisons of charge and spin order dynamics will be discussed.   

The temperature evolution of NiO$_{6}$ octahedral tilts as a probe to study charge stripes in Ln$_{1:67}$Sr$_{0:33}$NiO$_{4}$ (Ln=La/Nd)

Nd$_{1:67}$Sr$_{0:33}$NiO$_{4}$ and La$_{1:67}$Sr$_{0:33}$NiO$_{4}$ exhibit long range ordered (LRO) charge stripes below Tco$\sim $240K. When the charge stripes are static and long-range ordered, they have been seen as superlattice Bragg peaks in single crystal neutron and x-ray diffraction. We used neutron atomic pair distribution function (NPDF) technique to investigate the possibility of existence of localized charges above Tco, where, the LRO of the stripes disappears. Rigid body type NiO$_{6}$ octahedral tilts were used as a probe to study the local and average structural response to charge order. The amplitude of rotation of the octahedral units was used to investigate the observed large apical oxygen thermal ellipsoids (anisotropic thermal displacement parameters) in the a-b crystal plane. We measured enhanced tilt amplitudes in the local structure, compared to the average. We will discuss the temperature dependence of the tilt amplitudes and of the correlation length of the local tilt order and explore the potential relationship to local stripe order.   

Time-Dependent Recovery of Charge and Spin Order in Striped Nickelate

Using time-dependent Ginzburg-Landau theory, we study the melting and recovery of charge and spin order in the striped nickelate La$_{1.75}$Sr$_{0.25}$NiO$_4$ in response to an ultrashort pump pulse that destroys the order. Treating the behavior of both the amplitudes and phases of the order parameters, we examine their effects on the recovery time scales of the charge and spin order. We compare the temporal dynamics of our model to experimental observations at the Linac Coherent Light Source (LCLS).   

ARPES studies on metal-insulator-transition in NiS$_{2-x}$Se$_{x}$

Understanding Metal insulator transition (MIT) is one of the most challenging issues in condensed matter physics. NiS$_{2-x}$Se$_{x}$ (NSS) is a well known system for band width controlled MIT studies while most of High-Tc superconductors (HTSCs) are described within band filling MIT picture. Cubic pyrite NiS$_{2}$ is known as a charge-transfer (CT) insulator and easily forms a solid solution with NiSe$_{2}$, which is a good metal even though it is isostrucural and isoelectronic to NiS$_{2}$. MIT is induced by Se alloying and is observed at a low temperature for x=0.5. The important merit is that there is no structure transition which often accompanies MIT. In spite of the importance of the system, even the experimental band dispersion is not known so far along with many controversies. For this reason, we performed angle resolved photoemission spectroscopy on high quality single crystals and successfully obtained Fermi surface maps of x=0.5, x=0.7 and x=0.8 systems (the metallic side). By doping dependent systematic studies on NSS and comparison with LDA calculation, we try to explain the relationship between band width and the MIT.   

Quantum criticality at a Mott-Hubbard Metal-Insulator Transition

The Mott insulator nickel disulfide undergoes an insulator to metal transition that can be driven by either hydrostatic pressure or doping-induced chemical pressure. Previously,NiS2~doped with selenium to just below the quantum critical point showed anomalous critical exponents as a function of pressure. Doping, however, introduces positional disorder and charge transfer effects from the selenium orbitals, potentially altering the critical behavior. We report here on a set of transport experiments on pure~NiS2, where the pressure and temperature scales for the critical regime require the use of a diamond anvil cell integrated with a helium dilution refrigerator, allowing us to compare the critical behavior of the pure and doped materials.   

Redefining the metal/charge-transfer insulator paradigm in transition metal oxides

The universality of the phase diagram in the variables of interaction strength and d-occupancy, shown for late transition metal oxides in Ref.[1], is examined for two series of early transition metal oxides: (SrVO$_3$, SrCrO$_3$, SrMnO$_3$) and (LaTiO$_3$, LaVO$_3$, LaCrO$_3$) using density functional theory (DFT), DFT+U and DFT+dynamical mean field theory methods. The interaction required to drive the metal-insulator transition is found to depend sensitively on the d-occupancy $N_d$, and beyond a threshold value of the d-occupancy an insulating state cannot be achieved for any practical value of the interaction. The critical $N_d$ values are determined and compared to ab initio and experimental estimates where available. Additionally, the minimal model for the transition is determined and the crucial role played by the Hunds coupling is demonstrated. \\[4pt] [1] Xin Wang, M. J. Han, Luca de' Medici, C. A. Marianetti, and Andrew J. Millis (2011). arXiv:1110.2782   

Implementation of a dynamical cluster approximation within an augmented plane-wave framework

Even though the local density approximation in combination with dynamical mean field theory (LDA+DMFT) accounts for various and interesting physics such as Mott insulator and enhancement of effective mass for real compounds, the pseudo-gap and spin density wave behaviors are missed due to the absence of spatial correlations. In order to consider the short-range spatial correlations beyond the LDA+DMFT approach for real compounds, we introduce a combination of LDA with a dynamical cluster approximation (LDA+DCA) in the framework of the full-potential linear augmented plane-wave basis and show results for SrVO3. We discuss how to implement the LDA+DCA approach in the WIEN2k density functional theory code and analyze the momentum spectral properties on SrVO3 as a result. We compare our LDA+DCA results with LDA+DMFT as well as with angle-resolved photoemission spectra (ARPES). We find that LDA+DCA results compare better with ARPES than the LDA+DMFT results due to inclusion of spatial correlations.   

Pressure induced insulator to metal transition and orbital dynamics in Sr2VO4

The unusual ground states of transition metal oxides with layered perovskite structure (K$_{2}$NiF$_{4}$ type) are driven by a complex interplay between charge, orbital, spin and lattice degrees of freedom. The S=1/2 and 3d$^{1}$ system Sr$_{2}$VO$_{4}$, a Mott insulator, is of particular interest because of its vicinity to the insulator-metal boundary. The strong two-dimensionality along with a weak Jahn-Teller distortion (slightly elongated VO$_{6}$ octahedra along c-axis) causes splitting of t$_{2g}$ orbitals (\textit{xy} orbital and doubly degenerate \textit{yz} and \textit{zx} orbitals), pressure-tuning of which is expected to reveal interesting physical properties. Our recent high pressure Raman spectroscopic investigations at low temperature describe the orbital dynamical changes in this system. The anomalous pressure dependence of the strong magnetic excitations in Raman spectra confirms the unconventional spin-orbital composite nature (magnetic octupolar type order) of the d\textit{xy}/\textit{zy} levels [PRL \textbf{103}, 067205, (2009)]. The system transforms from high temperature orbital liquid like state to the low temperature orbital ordered state via a short range orbital order precursor at 150K. With increasing pressure the orbital ordering transition temperature decreases, predicting a QCP at $\sim $8 GPa. At this pressure the system undergoes an insulator to metal transition (as observed from mid IR reflectance measurements as well as sharp rise in Raman intensity of phonon modes due to occurrence of intraband transitions).   

ab-initio calculations of electronic structure and magnetism of O2MF6 (M=Sb, Pt) : Coulomb correlation and spin-orbit interaction effects in 2p and 5d electrons

We have investigated electronic structures and magnetic properties of O$_{2}$$M$F$_{6}$ ($M$=Sb, Pt), which are composed of two building blocks of strongly correlated electrons: O$_{2}^{+}$ dioxygenyls and $M$F$_{6}^{-}$ octahedra, by employing the first-principles electronic structure band method. For O$_{2}$SbF$_{6}$, as a reference system of O$_{2}$PtF$_{6}$, we have shown that the Coulomb correlation of O(2$p$) electrons drives the Mott insulating state. For O$_{2}$PtF$_{6}$, we have demonstrated that the Mott insulating state is induced by the combined effects of the Coulomb correlation of O(2$p$) and Pt(5$d$) electrons and the spin-orbit (SO) interaction of Pt(5$d$) states. The role of the SO interaction in forming the Mott insulating state of O$_{2}$PtF$_{6}$ is similar to the case of Sr$_{2}$IrO$_{4}$ that is a prototype of a SO induced Mott system with J$_{eff}=1/2$.