Unusual Electronic States Research Articles

Spectroscopic signature of trimer Mott insulator and charge disproportionation in BaIrO3

We have measured the reflectivity spectra of the barium iridate $9R$ BaIrO$_3$, the crystal structure of which consists of characteristic Ir$_3$O$_{12}$ trimers. In the high-temperature phase above the transition temperature $T_c\simeq180$ K, we find that the optical conductivity involves two temperature-dependent optical transitions with an ill-defined Drude response. These features are reminiscent of the optical spectra in the organic dimer Mott insulators, implying a possible emergence of an unusual electronic state named trimer Mott insulator in BaIrO$_3$, where the carrier is localized on the trimer owing to the strong Coulomb repulsion. Along with a pronounced splitting of the phonon peak observed below $T_c$, which is a hallmark of charge disproportionation, we discuss a possible phase transition from the trimer Mott insulator to a charge-ordered insulating phase in BaIrO$_3$.

High surface conductivity of Fermi-arc electrons in Weyl semimetals

Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semi--quantized anomalous Hall effect and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi arcs surface states. Here we analyze electron--phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight binding model with the presence of quenched surface vacancies using the Coherent Potential Approximation (CPA) and Kubo--Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is a factor of 50 larger of a comparable set up with surface states of TI. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material.

Unusual coexistence of negative and positive charge transfer in mixed-valenceNaxCa1−xCr2O4

We have investigated the electronic structure of Na$_x$Ca$_{1-x}$Cr$_2$O$_4$ using x-ray absorption spectroscopy together with Anderson impurity model calculations with full multiplets. We show Na$_x$Ca$_{1-x}$Cr$_2$O$_4$ taking a novel mixed-valence electronic state in which the positive charge-transfer (CT) and the negative self-doped states coexist. While CaCr$_2$O$_4$ (one end member) exhibits a typical CT nature with strong covalent character, Na substitution causes a self-doped state with an oxygen hole. In NaCr$_2$O$_4$ (the other end member), positive CT and negative self-doped states coexist with equal weight. This unusual electronic state is in sharp contrast to the conventional mixed-valence description, in which the ground state can be described by the mixture of Cr$^{3+}$ ($3d^3$) and Cr$^{4+}$ ($3d^2$).

Depth-resolved chemical speciation of a W-B4C multilayer structure

We report results of simultaneous x-ray reflectivity and grazing incidence x-ray fluorescence measurements in combination with x-ray standing-wave-assisted depth-resolved near-edge x-ray absorption measurements to reveal new insights on chemical speciation of W in a ${\text{W-B}}_{4}\mathrm{C}$ multilayer structure. Interestingly, our results show the existence of various unusual electronic states for the W atoms especially those near to the surface and interface boundary of a thin film medium as compared to those of the bulk. These observations are found to be consistent with the results obtained using first-principles calculations. Unlike the conventional x-ray absorption measurements the present approach has an advantage that it permits the determination of the depth-resolved chemical nature of an element in the thin-layered materials at atomic length scale resolution.

Spin-orbit driven magnetic insulating state withJeff=12character in a4doxide

The unusual magnetic and electronic ground states of 5d iridates has been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in 4d oxides is less clear, with one hurdle being the existence of such compounds. Here we present experimental and theoretical results on Sr$_4$RhO$_6$ that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous ${J}_{\mathrm{eff}}\mathbf{=}1/2$ Mott iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy and find a magnetic insulating ground state with ${J}_{\mathrm{eff}}\mathbf{=}1/2$ character.

Polarization control at spin-driven ferroelectric domain walls.

Unusual electronic states arise at ferroelectric domain walls due to the local symmetry reduction, strain gradients and electrostatics. This particularly applies to improper ferroelectrics, where the polarization is induced by a structural or magnetic order parameter. Because of the subordinate nature of the polarization, the rigid mechanical and electrostatic boundary conditions that constrain domain walls in proper ferroics are lifted. Here we show that spin-driven ferroelectricity promotes the emergence of charged domain walls. This provides new degrees of flexibility for controlling domain-wall charges in a deterministic and reversible process. We create and position a domain wall by an electric field in Mn0.95Co0.05WO4. With a magnetic field we then rotate the polarization and convert neutral into charged domain walls, while its magnetic properties peg the wall to its location. Using atomistic Landau-Lifshitz-Gilbert simulations we quantify the polarization changes across the two wall types and highlight their general occurrence.

Reconstruction of band structure induced by electronic nematicity in an FeSe superconductor.

We have performed high-resolution angle-resolved photoemission spectroscopy on an FeSe superconductor (T_{c}∼8 K), which exhibits a tetragonal-to-orthorhombic structural transition at T_{s}∼90 K. At low temperature, we found splitting of the energy bands as large as 50meV at the M point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to T∼110 K, slightly above T_{s}, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives rise to a van Hove singularity within 5meV of the Fermi energy. The present result strongly suggests that this unusual electronic state is responsible for the unconventional superconductivity in FeSe.

Spatially resolved one-dimensional boundary states in graphene-hexagonal boron nitride planar heterostructures.

Two-dimensional interfaces between crystalline materials have been shown to generate unusual interfacial electronic states in complex oxides. Recently, a one-dimensional interface has been realized in hexagonal boron nitride and graphene planar heterostructures, where a polar-on-nonpolar one-dimensional boundary is expected to possess peculiar electronic states associated with edge states of graphene and the polarity of boron nitride. Here we present a combined scanning tunnelling microscopy and first-principles theory study of the graphene-boron nitride boundary to provide a first glimpse into the spatial and energetic distributions of the one-dimensional boundary states down to atomic resolution. The revealed boundary states are about 0.6 eV below or above the Fermi level depending on the termination of the boron nitride at the boundary, and are extended along but localized at the boundary. These results suggest that unconventional physical effects similar to those observed at two-dimensional interfaces can also exist in lower dimensions.

Direct observations of local electronic states in an Al-based quasicrystal by STEM-EELS.

Most quasicrystals (QCs) reveal pseudogaps in their density of states around Fermi level, and hence the stability of QCs have been discussed in terms of energetic gains in electron systems. In fact, many QCs have been discovered by tuning valence electron density based on Hume-Rothery rule. Therefore, understanding electronic structures in QCs may provide an important clue for their stabilization mechanism. Generally, it has been frequently discussed based on an interaction between Fermi surface and Brillouin zone boundary within the framework of nearly free electron model, which is believed to be an underlying physics of a Hume-Rothery's empirical criteria. However the hybridization effect also stabilize electron system, particularly in Al-transition metal system, in which a lot of quasicrystalline phases were discovered. Therefore, the electronic structures of QCs have not yet been fully understood, whereas their atomic structures have been studied well in terms of configuration entropy by scanning transmission electron microscopy (STEM) [1]. In the present work, we investigate local electronic states in Al-based QCs using electron energy loss spectroscopy (EELS) combined with STEM, by which EELS spectra with sub-Å probe and atomic structure can be obtained simultaneously. We report STEM-EELS results on AlCuIr decagonal phases [2].jmicro;63/suppl_1/i17-a/DFU069F1F1DFU069F1Fig. 1.Core-loss edges obtained from cluster-centers and cluster-edges. Al L1 (left) Ir O23, Ir N67 (center) and Cu L23 (right). Principal components analysis clearly shows up the atomic-site dependence of plasmon loss spectra in a two-dimensional map. Qualitatively, there seems to be certain correlations between the plasmon peaks and the core-loss edges, Al L1, Ir O23, Ir N67 and Cu L23, all of which reveal different behaviors at the cluster centers and the edges (Fig. 1). All results indicate the cluster centers have metallic states and the cluster edges have covalent states in comparison. First-principles calculations confirm the unusual electronic state. On the basis of the calculated DOS and charge-density map, we conclude Al-Ir pairs, which are mainly located at the cluster edges, hybridize their orbitals and Cu atoms are localized at cluster center without hybridization. We analyze a distribution of the hybridized orbitals by Fourier transformation of electron localization function. The distribution seems like a 10-fold standing wave with Fermi wave length. It suggests that the Hume-Rothery mechanism works even when hybridization effect mainly contributes to pseudogap formation. In other words, global distribution of hybridized orbitals is important to stabilize structures whereas usually only local atomic configurations are discussed. Moreover, in the 10-fold standing wave, no distinct peak appears at the cluster centers. It means hybridized orbitals located at cluster centers are unable to contribute to the Hume-Rothery mechanism. It may be the reason why the metallic regions appear at the cluster centers.

Observations of Local Electronic States in Al-based quasicrystals by STEM-EELS

Most quasicrystals (QCs) reveal pseudogaps in their density of states around Fermi level, and hence the stability of QCs have been discussed in terms of energetic gains in electron systems. In fact, many QCs have been discovered by tuning valence electron density based on Hume-Rothery rule. Therefore, understanding electronic structures in QCs may provide an important clue for their stabilization mechanism. Generally, it has been frequently discussed based on an interaction between Fermi surface and Brillouin zone boundary within the framework of nearly free electron model, which is believed to be an underlying physics of a Hume-Rothery's empirical criteria. However, the electronic structures of QCs have not yet been fully understood, particularly being in microscopic-macroscopic relations. In the present work, we investigate local electronic states in Al-based QCs using electron energy loss spectroscopy (EELS) combined with scanning transmission electron microscopy (STEM), by which EELS spectra with sub-Å probe and atomic structure can be obtained simultaneously. We report STEM-EELS results on AlCuIr decagonal phases [1]. Principal components analysis clearly shows up the atomic-site dependence of plasmon loss spectra in a two-dimensional map. Qualitatively, there seems to be certain correlations between the plasmon peaks and the core-loss edges, Al L1, Ir O23, Ir N67and Cu L23, all of which reveal different behaviours at the cluster centers and the edges. All results indicate the cluster centers have metallic states and the cluster edges have covalent states in comparison. First-principles calculations confirm the unusual electronic state. We analyse a distribution of covalent electrons by Fourier transformation of electron localization function. The distribution seems like a 10-fold charge density wave with Fermi wave length. It suggests that the Hume-Rothery mechanism is important even when hybridization effect mainly contributes to pseudogap formation. The cluster centers can be regarded as the regions which are unable to contribute to the Hume-Rothery mechanism. It may be origin of the unusual electronic states. This work was conducted in Research Hub for Advanced Nano Characterization, The University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. It is acknowledged that T. Seki is a research fellow of Japan Society for the Promotion of Science.

New electronic perovskite oxides from beyond high pressure synthesis

Materials synthesised from high pressure (HP) conditions offer a variety of possibilities for the discovery of new electronic and magnetic phenomena. Recovery of a solid to ambient pressure (AP) from its HP thermodynamic stability field effectively introduces a large negative pressure that can drive the material into an unusual electronic ground state; as illustrated by HP structural and property studies of PbRuO3 and BiNiO3. PbRuO3 is synthesised at 10 GPa and shows an orbitally ordered phase at AP and low temperatures that is suppressed at an apparent quantum critical point near 6 GPa [1]. A new structural phase emerges at much higher pressures ~30 GPa. BiNiO3 synthesised at 6 GPa shows an unusual charge order that is suppressed at 3.4 GPa; the transition is associated with a volume collapse leading to colossal negative thermal expansion [2]. Materials recovered from HP are also precursors for 'hard-soft' chemistry, where the instability of a dense precursor from `hard' HP conditions is relieved through `soft' post-synthesis modification. We recently demonstrated this concept for the HP phase SrCrO3 which on reduction gives two new phases SrCrO2.80 and SrCrO2.75 with long period oxygen vacancy, charge and spin ordered superstructures [3]. These studies have been carried out in collaboration with co-authors of the papers below.

Ytterbium(III) Porpholactones: β‐Lactonization of Porphyrin Ligands Enhances Sensitization Efficiency of Lanthanide Near‐Infrared Luminescence

The near-infrared (NIR) luminescence efficiency of lanthanide complexes is largely dependent on the electronic and photophysical properties of antenna ligands. Although porphyrin ligands are efficient sensitizers of lanthanide NIR luminescence, non-pyrrolic porphyrin analogues, which have unusual symmetry and electronic states, have been much less studied. In this work, we used porpholactones, a class of β-pyrrolic-modified porphyrins, as ligands and investigated the photophysical properties of lanthanide porpholactones Yb-1 a-5 a. Compared with Yb porphyrin complexes, the porpholactone complexes displayed remarkable enhancement of NIR emission (50-120 %). Estimating the triplet-state levels of porphyrin and porpholactone in Gd complexes revealed that β-lactonization of porphyrinic ligands lowers the ligand T1 state and results in a narrow energy gap between this state and the lowest excited state of Yb(3+) . Transient absorption spectra showed that Yb(III) porpholactone has a longer transient decay lifetime at the Soret band than the porphyrin analogue (30.8 versus 17.0 μs). Thus, the narrower energy gap and longer lifetime arising from β-lactonization are assumed to enhance NIR emission of Yb porpholactones. To demonstrate the potential applications of Yb porpholactone, a water-soluble Yb bioprobe was constructed by conjugating glucose to Yb-1 a. Interestingly, the NIR emission of this Yb porpholactone could be specifically switched on in the presence of glucose oxidase and then switched off by addition of glucose. This is the first demonstration that non-pyrrolic porphyrin ligands enhance the sensitization efficiency of lanthanide luminescence and also display switchable NIR emission in the region of biological analytes (800-1400 nm).

β-Lactonization of fluorinated porphyrin enhances LDL binding affinity, cellular uptake with selective intracellular localization

Porpholactones, with one porphyrin β,β′-double bond replaced by a lactone moiety, possess unusual electronic states and photophysical properties between porphyrin and chlorin. However, such optical properties and unique structural characteristics have rarely been investigated within biological studies. In this work, we report that β-lactonization of porphyrin decreases the lipophilicity (log Po/w), due to the lower pKa value of pyrrollic nitrogen arising from the electron-withdrawing lactone moiety, and increases singlet oxygen quantum yields. Attaching glucose conjugates to the fluorinated porpholactones results in a high binding affinity with low density lipoprotein (LDL), which facilitates cellular-uptake efficacy through an LDL-dependent pathway and selective intracellular localization (lysosome). More importantly, β-lactonization of porphyrin endows higher photocytotoxicity against Hela cells through apoptosis, this is highly associated with increasing intracellular ROS levels. These results increase the understanding of the relationship between the basic structural characteristics and biological activity of porphyrins containing a non-pyrrolic moiety and will be important to expand the scope of β-modified porphyrinoids as potential photosensitizers for photodynamic therapy.

NMR at low and ultralow temperatures.

Solid state nuclear magnetic resonance (NMR) measurements at low temperatures have been common in physical sciences for many years and are becoming increasingly important in studies of biomolecular systems. This Account reviews a diverse set of projects from my laboratory, dating back to the early 1990s, that illustrate the motivations for low-temperature solid state NMR, the types of information that are available from the measurements, and likely directions for future research. These projects include NMR studies of both physical and biological systems, performed at low (cooled with nitrogen, down to 77 K) and ultralow (cooled with helium, below 77 K) temperatures, and performed with and without magic-angle spinning (MAS). NMR studies of physical systems often focus on phenomena that occur only at low temperatures. Two examples from my laboratory are studies of molecular rotation and orientational ordering in solid C60 at low temperatures and studies of unusual electronic states, called skyrmions, in two-dimensionally confined electron systems within semiconductor quantum wells. To study quantum wells, we used optical pumping of nuclear spin polarizations to enhance their NMR signals. The optical pumping phenomenon exists only at ultralow temperatures. In studies of biomolecular systems, low-temperature NMR has several motivations. In some cases, low temperatures suppress molecular tumbling, thereby permitting solid state NMR measurements on soluble proteins. Studies of AIDS-related peptide/antibody complexes illustrate this effect. In other cases, low temperatures suppress conformational exchange, thereby permitting quantitation of conformational distributions. Studies of chemically denatured states of the model protein HP35 illustrate this effect. Low temperatures and rapid freeze-quenching can also be used to trap transient intermediate states in a non-equilibrium kinetic process, as shown in studies of a transient intermediate in the rapid folding pathway of HP35. NMR sensitivity generally increases with decreasing sample temperature. Therefore, it can be useful to carry out experiments at the lowest possible temperatures, particularly in studies of biomolecular systems in frozen solutions. However, solid state NMR studies of biomolecular systems generally require rapid MAS. A novel MAS NMR probe design that uses nitrogen gas for sample spinning and cold helium only for sample cooling allows a wide variety of solid state NMR measurements to be performed on biomolecular systems at 20-25 K, where signals are enhanced by factors of 12-15 relative to measurements at room temperature. MAS NMR at ultralow temperatures also facilitates dynamic nuclear polarization (DNP), allowing sizeable additional signal enhancements and large absolute NMR signal amplitudes with relatively low microwave powers. Current research in my laboratory seeks to develop and exploit DNP-enhanced MAS NMR at ultralow temperatures, for example, in studies of transient intermediates in protein folding and aggregation processes and studies of peptide/protein complexes that can be prepared only at low concentrations.

Heteroaryl Oxenium Ions Have Diverse and Unusual Low-Energy Electronic States

The electronic state orderings and energies of heteroaryl oxenium ions were computed using high-level CASPT2//CASSCF computations. We find that these ions have a number of diverse, low-energy configurations. Depending on the nature of the heteroaryl substituent, the lowest-energy configuration may be open-shell singlet, closed-shell singlet, or triplet, with further diversity found among the subtypes of these configurations. The 2- and 3-pyridinyl oxenium ions show small perturbations from the phenyl oxenium ion in electronic state orderings and energies, having closed-shell singlet ground states with significant gaps to an n,π* triplet state. In contrast, the 4-pyridinyl oxenium ion is computed to have a low-energy nitrenium ion-like triplet state. The pyrimidinyl oxenium ion is computed to have a near degeneracy between an open-shell singlet and triplet state, and the pyrizidinyl oxenium ion is computed to have a near-triple degeneracy between a closed-shell singlet state, an open-shell singlet state, and a triplet state. Therefore, the ground state of these latter heteroaryl oxenium ions cannot be predicted with certainty; in principle, reactivity from any of these states may be possible. These systems are of fundamental interest for probing the spin- and configuration-dependent reactivity of unusual electronic states for this important class of reactive intermediate.

Unconventional Colossal Magnetoresistance in Sodium Chromium Oxide with a Mixed‐Valence State

Spin frustration and the unusual electronic state of Cr4+ ions are the origins of unconventional colossal magnetoresistance (CMR) in NaCr2O4, a new member of the rare mixed-valence chromium oxides. CMR materials have previously been limited to almost only manganese oxides; the discovery of the CMR in a non-manganese-based material opens a new dimension in the quest for novel CMR materials.

Pressure-induced metal-insulator transition in the spin-orbit Mott insulator Ba2IrO4

Electronic transport properties in the spin-orbit Mott insulator Ba${}_{2}$IrO${}_{4}$ were studied under high pressure ($P$), up to 15 GPa. Resistivity measurements revealed a metal-insulator (M-I) transition at 13.8 GPa in Ba${}_{2}$IrO${}_{4}$. In the insulating state ($Pl13.8$ GPa), the resistivity well follows the variable-range-hopping regime, suggesting that the carrier conduction is dominated by Anderson localization. In the metallic state ($P\ensuremath{\ge}13.8$ GPa), non-Fermi-liquid behavior was observed at low temperature. Ba${}_{2}$IrO${}_{4}$ exhibits an unconventional critical exponent (\ensuremath{\delta} = 1 \ensuremath{-} 2) at the M-I transition. These results suggest that Ba${}_{2}$IrO${}_{4}$ has an unusual electronic state affected by the ``marginal quantum critical point (MQCP).''

Excitons and excitonic fine structures in Si nanowires: Prediction of an electronic state crossover with diameter changes

Si nanowires have attracted considerable attention as promising candidates for electronic, thermoelectric, photonic, and photovoltaic devices, yet there appears to be only limited understanding of the underlying electronic and excitonic structures on all pertinent energy scales. Using atomistic pseudopotential calculations of single-particle as well as many-body states, we have identified remarkable properties of Si nanowires in three energy scales: (i) In the ``high-energy'' $\ensuremath{\sim}$1-eV scale, we find an unusual electronic state crossover whereby the nature of the lowest unoccupied molecular orbital (LUMO) state changes its symmetry with wire diameters for [001]-oriented wires but not for [011]-oriented wires. This change leads to orbitally allowed transitions becoming orbitally forbidden below a certain critical diameter for [001] wires. (ii) In the ``intermediate-energy'' $\ensuremath{\sim}$10${}^{\ensuremath{-}1}$-eV scale, we describe the excitonic binding, finding that in [001] wires the diameter ($D$) dependence of excitonic gap scales as $1/{D}^{1.9}$, not as $1/{D}^{1}$ as expected. The exciton binding energy increases from 52 meV at $D=7.6$ nm to 85 meV at $D=3.3$ nm and 128 meV at $D=2.2$ nm. (iii) In the ``low-energy'' $\ensuremath{\sim}$10${}^{\ensuremath{-}3}$-eV scale, we describe dark/bright excitonic states and predict how orbitally allowed transitions [in scale (i)] become spin-forbidden due to the electron-hole exchange interaction, whereas the spin-allowed states in the orbitally forbidden diameter region remain dark. The diameter dependence of the fine-structure splitting of excitonic states scales as $1/{D}^{2.3}$ in [001] wires and as $1/{D}^{2.6}$ in [011] wires. Surface-polarization effects are found to significantly enhance electron-hole Coulomb interaction, but have a small effect on the exchange fine-structure splitting. The present work provides a road map for a variety of electronic and optical effects in Si nanowires that can guide spectroscopic studies.

Unusual electronic state of tin dopant atoms in surface layers of NiTiO3

The 119Sn Mossbauer spectra of polycrystalline NiTiO3 samples impregnated with a solution containing 0.3 at % Sn4+ are evidence that annealing in H2 converts tin into the state with the electron density |Ψ(0)|2 on 119Sn nuclei corresponding to “Sn3+” ions. The stabilization of tin atoms in such an untypical formal oxidation state occurs at a depth of no more than 2–3 nm from the surface of titanate crystallites. It was revealed that the Sn3+ ions are not subjected to spin polarization even at temperatures considerably lower than the Neel temperature of NiTiO3, which can be explained by their location in the Ni2+ positions. The formation of Sn3+ prevents the further reduction of tin to the divalent state and, hence, precludes localization of 119Sn probe cations in positions at the interface.

Carrier Dynamics in κ-Type Organic Superconductors: Time-Resolved Observation

The κ-type organic superconductors have unique physical properties, many of which are yet to be understood. Most of their physical properties are closely related to carrier dynamics, which can be directly observed by ultrafast optical measurements in their nonequilibrium states. Time-resolved temperature-dependent reflectivity changes were measured on single crystals of an isostructural pair of organic charge transfer salts κ-(ET) 2 Cu[N(CN) 2 ]X, which are ambient- (X=Br) or high-pressure (X=Cl) superconductors. The latter compound in its ground state is a Mott insulator under ambient pressure. The obtained temperature dependences of relaxation times for both salts indicated the existence of an intermediate electronic state between those of metals and insulators at all measurement temperatures. Such an unusual electronic state coexisted with the superconducting and Mott insulating states in X=Br and X=Cl, respectively. In addition, the results for X=Br revealed the formation of a temperature-dependent we...