On-site Coulomb Energy Research Articles

Evidence for a Correlated Insulator to Antiferromagnetic Metal Transition in CrN

We investigate the electronic structure of chromium nitride (CrN) across the first-order magnetostructural transition at T(N)∼286 K. Resonant photoemission spectroscopy (PES) shows a gap in the 3d partial density of states at the Fermi level and an on-site Coulomb energy U∼4.5 eV, indicating strong electron-electron correlations. Bulk-sensitive high-resolution (6 meV) laser PES reveals a clear Fermi edge indicating an antiferromagnetic metal below T(N). Hard x-ray Cr 2p core-level PES shows T-dependent changes across T(N) which originate from screening due to coherent states as substantiated by cluster model calculations using the experimentally observed U. Electrical resistivity confirms an insulator above T(N) (E(g)∼70 meV) becoming a disordered metal below T(N). Thus, CrN transforms from a correlated insulator to an antiferromagnetic metal, coupled to the magnetostructural transition.

Structural, Electronic, and Electrochemical Properties of Cathode Materials Li2MSiO4 (M = Mn, Fe, and Co): Density Functional Calculations

For Li2FeSiO4, its P21 space group makes it possibly perfect as a new cathode material for Li-ion batteries (Nishimura et al. J. Am. Chem. Soc. 2008, 130, 13212). For this type of Li2MSiO4 (M = Mn, Fe, and Co), the structural, electronic, and electrochemical properties have been investigated, using the density functional theory with the exchange-correlation energy treated as the generalized gradient approximation (GGA) plus on-site Coulomb energy correction (+U). Within the GGA+U framework, the fully lithiated Li2MSiO4 as well as the delithiated LiMSiO4 and MSiO4 are all semiconducting, and the band gap lowers with the extraction of lithium ions. The fully lithiated compounds are all stabilized at their ferromagnetic phase, while the delithiated compounds are all stabilized when antiferromagnetic. Starting from the P21 structure, the fully delithiated MSiO4 has better stability than that obtained from Pmn21 structure. In Li2FeSiO4, the possibility of reversibly extracting more than one lithium ion is enha...

Electronic structure and magnetism of the double perovskite SrKFeWO6

The crystal structure，electronic structure and magnetism of the double perovskite Sr 2 FeWO 6 and SrKFeWO 6 have been investigated under the framework of density functional theory (DFT) with the generalized gradient approximation taking into account the on-site Coulomb energy (GGA+U) using the projected augmented wave (PAW) method. Structure relaxation results show that K doping of Sr 2 FeWO 6 stabilizes FeO 6 ，WO 6 octahedra and makes the Fe—O—W angles close to 180°，indicating the enhancement of superexchange interaction. From the electronic structure calculation，it was found that the contribution to the total density of states (DOS) from K itself is small. Due to the K doping，the valence and magnetic moment of B -site cation Fe are enhanced and the hybridization between Fe and O becomes stronger，as well as the band gap is enlarged. Nevertheless，it does not cause considerable change in B ’-site cation W. The process of transfer of electrons is dominated by Fe-Fe in SrKFeWO 6 compared with Fe-W charge transfer in Sr 2 FeWO 6 before doping.

Optical Modulation of Effective On-Site Coulomb Energy for the Mott Transition in an Organic Dimer Insulator

We report an optical modulation of the effective on-site Coulomb energy U on a dimer (U_{dimer}) for achieving the Mott insulator-to-metal transition in kappa-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Br, as investigated by pump-probe spectroscopy. A reduction of U_{dimer} is optically induced by molecule displacement in the dimer under intradimer excitation. The mechanism of this metallization differs greatly from the photodoping-type mechanism reported previously. In contrast, a faster transition via the photodoping mechanism is detected for interdimer excitation. A metallic-domain-wall oscillation originating from the modulation of U_{dimer} was also observed near the critical end point of the Mott transition line.

System-parameter dependence of the metallic phase of the non-doped 2D Hubbard model

Naito et al. reported that some non-doped T′-214-type compounds drive high- T c superconductivity. The compounds are considered to be metallic since on-site Coulomb energy U is moderate and the Fermi surface is much deformed in these compounds. In order to confirm this picture and extract electronic structure information, we have examined the phase diagram of the metallic state of the 2D Hubbard model as a function of U and t′ (with t″ we fixed at − t′/2 here; t′ and t″ are the second- and third-neighbor transfer energies, respectively) by means of the variational Monte–Carlo method. We employed a Jastrow-type Gutzwiller trial wave function. In the studied range of U = 2–12, the boundary value for | t′| at which SDW disappears increases almost linearly with U. Jump-wise transition to the Mott insulator state was not observed. Using the boundary curve and experimental band parameter values, we estimate U ∼ 5 for T′-214 compounds. Preceding works are discussed in the last part.

Crossover from Metal to Insulator and Charge Order Transition of (TMTTF)2SbF6 under Constant Pressure

Crossover from Metal to Insulator and Charge Order Transition of (TMTTF)₂SbF₆ under Constant Pressure

T′- and t″-dependence of the bulk-limit superconducting condensation energy of the 2D Hubbard model

The 2D Hubbard model having the 2nd- and 3rd-neighbor transfer energies t′ and t″ is investigated by use of the variational Monte Carlo method. At the nearly optimal doping with on-site Coulomb energy U = 6 (energy unit is t) the condensation energy E cond for the d-wave superconductivity (SC) is computed for lattices of sizes from 10 × 10 to 28 × 28 with the aim to get its bulk-limit value. t″ is fixed at − t′/2. Outside and in the neighborhood of the SDW region of –0.16 ⩽ t′ ⩽ −0.08 the SC E cond dominates over the SDW E cond. At t′ = −0.05 and −0.10 we obtained a definitely finite bulk-limit SC E cond of the order of the experimental value for YBCO. At t′ = 0 E cond nearly vanishes. For t′ ⩽ –0.18, the SC E cond strongly oscillates as a function of the lattice size, when periodic boundary conditions (b.c.’s) are imposed to both axes. In the case of periodic and anti periodic b.c.’s, a finite bulk-limit value is obtained at t′ = −0.22. E cond tends to vanish with further decrease of t′. With our results the SC of LSCO is understandable with t′∼−0.10. The t′ values of Hg1201, Tl2201 and Na-CCOC seem close to −0.20 so that they locate in the boundary zone of SC indicated in the present work. Slightly larger U improves the situation by increasing E cond.

Ferromagnetism in GaN:Gd: A Density Functional Theory Study

First-principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the generalized gradient approximation (GGA) of the density functional theory with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic p-d coupling is found to be over 2 orders of magnitude larger than the s-d exchange coupling. The experimental colossal magnetic moments and room-temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4f spins via p-d coupling involving holes introduced by intrinsic defects such as Ga vacancies.

Superconducting condensation energy computed for the 2D Hubbard model with Bi2212-type band

The 2D Hubbard model is the simplest model that possibly explains the occurrence of high-Tc superconductivity (SC) in cuprates. To demonstrate this possibility with a condition of moderate on-site Coulomb energy U, we have computed the SC condensation energy Econd by means of the variational Monte Carlo method employing a slightly modified Gutzwiller trial wave function which also takes account of the Bi2212-type band, i.e., t′∼−0.34 and t″∼0.23 (t, t′ and t″ are defined in the main text; energy unit is t). The competing SDW Econd strongly depended on the employed lattice size L×L but approached the bulk-limit value when L⩾16. With proper assumption on the value of t″, the t′ region where the SDW Econd are larger than the SC Econd shrank to −0.16⩽t′⩽−0.08. In −0.40<t′<−0.16 the SC Econd was found predominant. Concerning the lattice-size dependence the SC Econd in the neighborhood of the Bi2212-type set was found to tend to increase with increasing L when L⩾16, taking about 1/3 or 1 times the experimental SC Econd of YBCO. This was computed using periodic boundary conditions (bc’s) for the two directions. The average of two results for periodic and antiperiodic bc’s imposed to both x- and y-directions for t′=−0.31 and t″=0.21 proved to make the L-dependence so mild that the extrapolation to the finite bulk limit looks plausible. Together with the previous positive result with t′∼−0.05 and t″=0, this result clearly supports the applicability of the 2D Hubbard model to the SC in the whole group of cuprates. Incidentally, more elaborate Gutzwiller–Jastrow-type trial functions were also examined but found to bring in only minor differences.

Correlation effects for semiconducting single-wall carbon nanotubes: A density matrix renormalization group study

In this paper, we report the applicability of the density matrix renormalization group(DMRG) approach to the cylindrical single wall carbon nanotube (SWCN) for purpose of its correlation effect. By applying the DMRG approach to the $t$+$U$+$V$ model, with $t$ and $V$ being the hopping and Coulomb energies between the nearest neighboring sites, respectively, and $U$ the onsite Coulomb energy, we calculate the phase diagram for the SWCN with chiral numbers ($n_{1}=3, n_{2}=2$), which reflects the competition between the correlation energy $U$ and $V$. Within reasonable parameter ranges, we investigate possible correlated groundstates, the lowest excitations and the corresponding correlation functions in which the connection with the excitonic insulator is particularly addressed.

High-resolution valence-band XPS spectra of the nonconductors quartz and olivine

High-resolution core-level and valence-band x-ray photoelectron spectra (XPS) of the nonconductor silicates, quartz, and two olivines (${[\mathrm{Mg},\mathrm{Fe}]}_{2}\mathrm{Si}{\mathrm{O}}_{4}$, Mg rich and Fe rich) have been obtained with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. Observed total linewidths are about $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for several of the peaks in these spectra, much narrower than previously obtained. The quartz and olivine valence-band spectra are very different, even though the dominant contributions to both come from molecular orbitals of $\mathrm{Si}{\mathrm{O}}_{4}$ groups. High-quality calculations of the band structure using pseudopotential density functional theory with generalized gradient approximation (GGA) for quartz yield a theoretical XPS valence-band spectrum in the Gelius approximation that is in excellent agreement with the experimental spectrum. $\mathrm{GGA}+U$ (where $U$ is the on-site Coulomb energy) calculations on Mg-rich and Fe-rich olivines yield theoretical XPS spectra in good qualitative agreement with experiment. The valence-band spectral contributions are readily assigned using the calculated partial density of states. For example, the Fe $3d$ ${t}_{2g}$ and ${e}_{g}$ orbitals in $M1$ and $M2$ sites of the olivine are located at the top of the olivine valence band and are readily resolved in both observed spectra and theoretical calculations. The valence-band spectrum of quartz is about $3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ more dispersed than the valence bands of the olivines and considerably greater than the $1.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ difference calculated previously. The difference in dispersion arises mainly from two effects. The dispersion of the quartz valence band is primarily a response to the nature of metal atoms in the second coordination sphere of the central Si atom of $\mathrm{Si}{\mathrm{O}}_{4}$ tetrahedra. For example, the $\mathrm{Si}\ensuremath{-}\mathrm{O}$ moiety is more negatively charged in olivine than in quartz because of the transfer of electrons from Mg to $\mathrm{O}\ensuremath{-}\mathrm{Si}$. The calculations indicate very small Mg $2s$ contributions to the forsterite $({\mathrm{Mg}}_{2}\mathrm{Si}{\mathrm{O}}_{4})$ valence band. Hence all Mg $2s$ electrons effectively reside on O atoms (O $2p$ orbitals) and the $\mathrm{Mg}\ensuremath{-}\mathrm{O}$ bond is ionic. Polymerization (network formation) also contributes to dispersion in that it results in large splittings in the Si $3s$ and Si $3p$ partial density of states. Fe $3d$ electrons of olivines are of both ``bonding'' and ``nonbonding'' according to the calculations. In the Mg-rich olivine, the great majority of Fe $3d$ electrons are nonbonding and give rise to separate Fe $3d$ ${t}_{2g}$ and ${e}_{g}$ peaks at the top of the valence band. The same no-bonding contributions are observed in Fe-rich olivine, but O $2p$ contributions near the top of the valence band indicate appreciable $\mathrm{Fe}\ensuremath{-}\mathrm{O}$ bonding.

Two Domains of the Parameter Space of the 2D Hubbard Model for High-T c Superconductors

High-Tc superconducting cuprates have two types of Fermi surfaces: simple-2D-tight-binding-band type (LSCO type) and the much deformed one (Bi2212 type). The difference is attributed to that of band parameter values, i.e., t′ ∼ −0.1 and t″ ∼ 0 versus t′ ∼ −0.3 and t″ ∼ 0.2 in terms of the second- and third-neighbor transfer energies t′ and t″, respectively (energy unit is the nearest-neighbor transfer energy t). Assuming a moderate value of on-site Coulomb energy U ∼ 6 and performing the variational Monte Carlo computation, we found that the two superconducting parameter domains exist in fact around these parameter sets, respectively, in which superconductivity predominates over spin density wave (SDW) due to the latter being at the brink of vanishing. Stripes were obtained in the first domain but tend to disappear in the second. In the latter domain there seems to exist parameter sets for which superconductivity appears without doping.

Spin dependent transport of “nonmagnetic metal/zigzag nanotube encapsulating magnetic atoms/nonmagnetic metal” junctions

Towards a novel magnetoresistance (MR) device with a carbon nanotube, we propose “nonmagnetic metal/zigzag nanotube encapsulating magnetic atoms/nonmagnetic metal” junctions. We theoretically investigate how spin-polarized edges of the nanotube and the encapsulated magnetic atoms influence on transport. When the on-site Coulomb energy divided by the magnitude of transfer integral, U/|t|, is larger than 0.8, large MR effect due to the direction of spins of magnetic atoms, which has the magnitude of the MR ratio of about 100%, appears reflecting such spin-polarized edges.

Control of the electronic state in a series of Pd(dmit) 2 salts, a strongly correlated electron system with a quasi-triangular lattice structure

Pressure effects on β′-type Pd(dmit) 2 anion radical salts, which belong to a strongly correlated electron system with a quasi-triangular lattice of dimerized Pd(dmit) 2 units, were studied with X-ray crystal structure analysis and resistivity measurements. The electronic state of this system is governed by various parameters including the effective on-site Coulomb energy on the dimer ( U eff.), the band width ( W), the degree of frustration, and the degree of band-filling. X-ray crystal structure analysis under hydrostatic pressure at low temperatures was performed for the Et 2Me 2P salt The tight-binding band calculations based on structural data indicate that the application of hydrostatic pressure reduces the electron correlation effect and leads the system from the Mott insulating state to the metallic state accompanied by the superconductivity. Further application of hydrostatic pressure induces another non-metallic behavior. This would originate from a partial nesting of the anisotropic Fermi surface associated with a structural transition which removes the glide plane and the two-fold axis. Based on cation dependence in the pressure effect, we propose a possible interplay of the correlation and the frustration, and discuss the uniaxial strain effect for this unique strongly correlated electron system.

Mott Gap Excitations in Twin-FreeYBa2Cu3O7−δ(Tc=93 K) Studied by Resonant Inelastic X-Ray Scattering

Mott gap excitations in the optimally doped high-T(c) superconductor YBa(2)Cu(3)O(7-delta) (T(c)=93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of the momentum transfer. Theoretical calculations based on the one-dimensional and two-dimensional Hubbard model reproduces the observed spectra when different values of the on-site Coulomb energy are assumed. The Mott gap of the CuO chain site is found to be much smaller than that of the CuO2 plane site.

Soft-X-ray Raman scattering of La 1− xSr xTiO 3

The electronic structure of doped Mott-insulator La 1− x Sr x TiO 3 has been studied by resonant soft-X-ray emission spectroscopy (SXES). The Raman scattering of the e g- and t 2g-resonant SXES spectra indicates features due to the d–d transitions corresponding to the crystal field splitting (10Dq) and half on-site Coulomb energy ( U dd/2), respectively. The U dd does not change much around two metal–insulator transitions at x = 0.05 and 0.95 in La 1− x Sr x TiO 3, while the 10Dq increases as a function of x. These results are in good agreement with the prediction of dynamical mean field theory calculation.

The role of two types of trellis layer for metal–insulator transition and antiferromagnetic order in the one-dimensional conductor V6O13

V6O13 with a metal–insulator transition at Tc ≃ 150 K and an antiferromagnetic order at TN ≃ 50 K has a ddyz-type single trellis layer and a dxy-type double trellis layer, where x and y correspond to the a- and b-axis, respectively. Magnetic properties above Tc are consistent with theoretical results for a one-dimensional Hubbard model with the uniform electron concentration and a bandwidth comparable to the effective on-site Coulomb energy. Below Tc, in the light of properties of δ-phase vanadium bronzes, the spin-singlet state is considered to appear in the double trellis layer and the one-dimensional Heisenberg chain-like state is formed in the single layer, accompanied with valence order; and at TN, the antiferromagnetic order takes place in the single layer. The temperature dependence of the electrical resistivity for the one-dimensional chain above Tc is mainly due to antiferromagnetic spin fluctuations, and those for the normal directions nearly follow 1/T which is likely due to the renormalization effect of the Fermi surface by the fluctuations. The Hall coefficient above Tc is found to be roughly linear in T.

Electronic structure for the metal–insulator transition in La 1− xSr xTiO 3 probed by resonant soft-X-ray emission spectroscopy

The electronic structure of doped Mott-insulator La 1− x Sr x TiO 3 has been studied by resonant soft-X-ray emission spectroscopy (SXES). At the t 2g-resonance SXES spectra, the d–d transition whose Raman shift is about 2.0 eV reflects the magnitude of half on-site Coulomb energy ( U dd). The U dd does not change much around the metal–insulator transition at x=0.05 in La 1− x Sr x TiO 3. This result is thought to be in accord with the prediction of dynamical-mean-field theory.

Consistent picture for high Tc and SDW obtained from the moderate- U Hubbard model

Using the two-dimensional Hubbard model with a moderate value of on-site Coulomb energy U=5 (energy unit is t, nearest-neighbor transfer energy), we have computed, by means of the variational Monte Carlo method, the condensation energy E cond of the d-wave superconducting state for square lattices of sizes from 10 × 10 to 22 × 22. The second-neighbor transfer energy t ′ was fixed at −0.05 in the middle of the t ′ region where SDW was found vanishing or very weak in a preceding work. With the electron density kept at about 0.84, E cond was found to have only a weak dependence on the lattice size, indicating the existence of a finite bulk-limit value, which is close to the observed value. Although a considerable size effect was found even in largest lattice cases, the above results suggest that the high- T c superconductivity in the cuprates at the optimal doping can be explained in terms of the present model.

First-principles studies of the electronic structure and magnetism in fayalites: M2SiO4 (M=Fe and Co)

The electronic structure and magnetism in fayalites M2SiO4 (M=Fe and Co) have been studied by density functional theory within the generalized gradient approximation with the on-site Coulomb energy taken into account (GGA+U). Stable insulating antiferromagnetic solutions are obtained. It is found that the gap opening in these fayalites results mainly from strong on-site Coulomb interactions of the magnetic atoms. In the band structures, the conduction bands originate mainly from the M(3d) orbitals of M2SiO4. The top of valence bands consists of the Fe (3d) orbitals in Fe2SiO4, and of the Co(3d)–O(2p) hybridization in Co2SiO4. All calculated orbital magnetic moments are very small.