arXiv subjects

Ross H. McKenzie

Publications and source records attributed to Ross H. McKenzie.

At least 55 records · Page 3Linked to original sources

arXiv2010

Thermal and electrical conductivity of Fermi pocket models of underdoped cuprates

Several models of the electronic spectrum in the pseudogap state of underdoped cuprates have been proposed to explain ARPES and STM measurements, which reveal only truncated Fermi pockets instead of a full metallic Fermi surface. We consider the transport properties expected of four physically distinct models, and calculate the thermal and electrical conductivity of the electronic quasiparticles. By proposing transport currents that reflect the close correspondence between quasiparticles on the Fermi pockets in the pseudogap and those near nodes in the superconducting state, we show that measurable transport coefficients provide stringent tests of pseudogap models.

cond-mat.supr-con↗

arXiv2010

A Dark Excited State of Fluorescent Protein Chromophores, Considered as Brooker Dyes

The green fluorescent protein (GFP) chromophore is an asymmetric monomethine dye system. In the resonance color theory of dyes, a strong optical excitation arises from interactions of two valence-bond structures with a third, higher structure. We use correlated quantum chemistry to show that the anionic chromophore is a resonant Brooker dye, and that the third structure corresponds to a higher stationary electronic state of this species. The excitation energy of this state should be just below the first excitation energy of the neutral form. This has implications for excited state mechanism in GFPs, which we discuss.

physics.chem-ph↗

arXiv2009

Charge Distribution Near Oxygen Vacancies in Reduced Ceria

Understanding the electronic charge distribution around oxygen vacancies in transition metal and rare earth oxides is a scientific challenge of considerable technological importance. We show how significant information about the charge distribution around vacancies in cerium oxide can be gained from a study of high resolution crystal structures of higher order oxides which exhibit ordering of oxygen vacancies. Specifically, we consider the implications of a bond valence sum analysis of Ce$_{7}$O$_{12}$ and Ce$_{11}$O$_{20}$. To illuminate our analysis we show alternative representations of the crystal structures in terms of orderly arrays of co-ordination defects and in terms of flourite-type modules. We found that in Ce$_{7}$O$_{12}$, the excess charge resulting from removal of an oxygen atom delocalizes among all three triclinic Ce sites closest to the O vacancy. In Ce$_{11}$O$_{20}$, the charge localizes on the next nearest neighbour Ce atoms. Our main result is that the charge prefers to distribute itself so that it is farthest away from the O vacancies. This contradicts \emph{the standard picture of charge localisation} which assumes that each of the two excess electrons localises on one of the cerium ions nearest to the vacancy. This standard picture is assumed in most calculations based on density functional theory (DFT). Based on the known crystal structure of Pr$_{6}$O$_{11}$, we also predict that the charge in Ce$_{6}$O$_{11}$ will be found in the second coordination shell of the O vacancy. Although this review focuses on bulk cerium oxides our approach to characterising electronic properties of oxygen vacancies and the physical insights gained should also be relevant to surface defects and to other rare earth and transition metal oxides.

cond-mat.mtrl-sci↗

arXiv2009

A Consistent Picture of Charge Distribution in Reduced Ceria Phases

We consider the implications of the bond valence model (BVM) description of charge distribution in reduced ceria phases (CeO$_{2-x}$)\cite{Shoko2009,Shoko2009b} to the models used to describe electronic and ionic conductivity in these phases. We conclude that the BVM is consistent with both the small polaron model (SPM) and the atomistic models which describe the electronic and ionic conductivities respectively. For intermediate phases, i.e., $x \sim 0.3$, we suggest the possibility of low temperature metallic conductivity. This has not yet been experimentally observed. We contrast the BVM results and the conventional description of charge distribution in reduced ceria phases.

cond-mat.str-el↗

arXiv2009

Conical Intersections, Charge Transfer and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye

We propose a minimal model Hamiltonian for the electronic structure of a monomethine dye, in order to describe the photoisomerization of such dyes. The model describes interactions between three diabatic electronic states, each of which can be associated with a valence bond structure. Monomethine dyes are characterized by a charge-transfer resonance; the indeterminacy of the single-double bonding structure dictated by the resonance is reflected in a duality of photoisomerization pathways corresponding to the different methine bonds. The possible multiplicity of decay channels complicates mechanistic models of the effect of the environment on fluorescent quantum yields, as well as coherent control strategies. We examine the extent and topology of intersection seams between the electronic states of the dye, and how they relate to charge localization and selection between different decay pathways. We find that intersections between the S1 and S0 surfaces only occur for large twist angles. In contrast, S2/S1 intersections can occur near the Franck-Condon region. When the molecule has left-right symmetry, all intersections are associated with con- or dis-rotations and never with single bond twists. For asymmetric molecules (i.e. where the bridge couples more strongly to one end) then the S2 and S1 surfaces bias torsion about different bonds. Charge localization and torsion pathway biasing are correlated. We relate our observations to several recent experimental and theoretical results, which have been obtained for dyes with similar structure.

physics.chem-ph↗

arXiv2009

Quantum Transition State Theory for proton transfer reactions in enzymes

We consider the role of quantum effects in the transfer of hyrogen-like species in enzyme-catalysed reactions. This study is stimulated by claims that the observed magnitude and temperature dependence of kinetic isotope effects imply that quantum tunneling below the energy barrier associated with the transition state significantly enhances the reaction rate in many enzymes. We use a path integral approach which provides a general framework to understand tunneling in a quantum system which interacts with an environment at non-zero temperature. Here the quantum system is the active site of the enzyme and the environment is the surrounding protein and water. Tunneling well below the barrier only occurs for temperatures less than a temperature $T_0$ which is determined by the curvature of potential energy surface near the top of the barrier. We argue that for most enzymes this temperature is less than room temperature. For physically reasonable parameters quantum transition state theory gives a quantitative description of the temperature dependence and magnitude of kinetic isotope effects for two classes of enzymes which have been claimed to exhibit signatures of quantum tunneling. The only quantum effects are those associated with the transition state, both reflection at the barrier top and tunneling just below the barrier. We establish that the friction due to the environment is weak and only slightly modifies the reaction rate. Furthermore, at room temperature and for typical energy barriers environmental degrees of freedom with frequencies much less than 1000 cm$^{-1}$ do not have a significant effect on quantum corrections to the reaction rate.

quant-ph↗

arXiv2009

Ionic Hubbard model on a triangular lattice for Na_0.5CoO_2, Rb_0.5CoO_2 and K_0.5CoO_2: Mean-field slave boson theory

We introduce a strongly correlated mean-field theory of the ionic Hubbard model on the triangular lattice with alternating stripes of site energy using Barnes-Coleman slave bosons. We study the paramagnetic phases of this theory at three quarters filling, where it is a model of Na_0.5CoO_2, Rb_0.5CoO_2, and K_0.5CoO_2. This theory has two bands of fermionic quasi-particles: one of which is filled or nearly filled and hence weakly correlated; the other is half-filled or nearly half-filled and hence strongly correlated. Further results depend strongly on the sign of the hopping integral, t. The light band is always filled for t>0, but only becomes filled for |Delta/t|>=1.5 for t<0, where Delta is the difference in the site energies of the two sublattices. A metal--charge transfer insulator transition occurs at |Delta/t|=5.0 for t>0 and |Delta/t|=8.0 for t<0. In the charge transfer insulator complete charge disproportionation occurs: one sublattice is filled and the other in half filled. We compare our results with exact diagonalisation calculations and experiments on Na_0.5CoO_2, and discuss the relevance of our results to Rb_0.5CoO_2 and K_0.5CoO_2. In particular we propose a resolution of seemingly contradictory experimental results on Na_0.5CoO_2. Many experiments suggest that there is a charge gap, yet quantum oscillations are observed suggesting the existence of quasiparticle states at arbitrarily low excitation energies. We argue that the heavy band is gapped while the light band, which contains less than 1 charge carrier per 100 unit cells, remains ungapped.

cond-mat.str-el↗

arXiv2009

Spin Fluctuations and the Pseudogap in Organic Superconductors

We show that there are strong similarities in the spin lattice relaxation of non-magnetic organic charge transfer salts, and that these similarities can be understood in terms of spin fluctuations. Further, we show that, in all of the kappa-phase organic superconductors for which there is nuclear magnetic resonance data, the energy scale for the spin fluctuations coincides with the energy scale for the pseudogap. This suggests that the pseudogap is caused by short-range spin correlations. In the weakly frustrated metals k-(BEDT-TTF)_2Cu[N(CN)_2]Br, k-(BEDT-TTF)_2Cu(NCS)_2, and k-(BEDT-TTF)_2Cu[N(CN)_2]Cl (under pressure) the pseudogap opens at the same temperature as coherence emerges in the (intralayer) transport. We argue that this is because the spin correlations are cut off by the loss of intralayer coherence at high temperatures. We discuss what might happen to these two energy scales at high pressures, where the electronic correlations are weaker. In these weakly frustrated materials the data is well described by the chemical pressure hypothesis (that anion substitution is equivalent to hydrostatic pressure). However, we find important differences in the metallic state of k-(BEDT-TTF)_2Cu_2(CN)_3, which is highly frustrated and displays a spin liquid insulating phase. We also show that the characteristic temperature scale of the spin fluctuations in (TMTSF)_2ClO_4 is the same as superconducting critical temperature, which may be evidence that spin fluctuations mediate the superconductivity in the Bechgaard salts.

cond-mat.supr-con↗

arXiv2009

Conical Intersections and Charge Localization in a Minimal Model of a Monomethine Dye System: Degenerate Case

We investigate a minimal valence-bond-based model Hamiltonian for photoisomerization of monomethine dyes. These include a broad class of large conjugated organic molecules including diarylmethane and cyanine dyes, and the chromophores of fluorescent proteins. The model fulfills the most basic expectations for a double bond photoisomerization model: 1) it describes favorable twisting in the excited state and 2) it describes conical intersections which can mediate radidationless decay. We examine the topology, topography and extent of the conical intersection seams. Important properties of the model include 1) two regimes of the coupling ratio where the first excited state intersects the ground and a higher excited state along orthogonal twisting coordinates or along the same coordinate 2) first and higher excited state intersections occurring in regions of little or no twisting and 3) charge localization connected with excited-state twisting which changes discontinuously across a conical intersection. The model is promising as a simple extension of empirical force fields in order to describe photoisomerization processes in methine-bridged systems and as a starting point for more elaborate models which may include antisymmetric internal couplings or which may be interfaced with environmental or quantum dynamical models.

physics.chem-ph↗

arXiv2009

Interlayer magnetoresistance in an anisotropic pseudogap state

The interlayer magnetoresistance of a quasi-two-dimensional layered metal with a d-wave pseudogap is calculated semiclassically. An expression for the interlayer resistivity as a function of the strength and direction of the magnetic field, the magnitude of the pseudogap, temperature, and scattering rate is obtained. We find that the pseudogap, by introducing low-energy nodal quasiparticle contours, smooths the dependence on field direction in a manner characteristic of its anisotropy. We thus propose that interlayer resistance measurements under a strong field of variable orientation can be used to fully characterize an anisotropic pseudogap. The general result is applied to the case of a magnetic field parallel to the conducting layers using a model band structure appropriate for overdoped T$\ell$2201.

cond-mat.str-el↗

arXiv2009

A Diabatic Three-State Representation of Photoisomerization in the Green Fluorescent Protein Chromophore

We give a quantum chemical description of bridge photoisomerization reaction of green fluorescent protein (GFP) chromophores using a representation over three diabatic states. Bridge photoisomerization leads to non-radiative decay, and competes with fluorescence in these systems. In the protein, this pathway is suppressed, leading to fluorescence. Understanding the electronic structure of the photoisomerization is a prerequisite to understanding how the protein suppresses this pathway and preserves the emitting state of the chromophore. We present a solution to the state-averaged complete active space problem, which is spanned at convergence by three fragment-localized orbitals. We generate the diabatic-state representation by applying a block diagonalization transformation to the Hamiltonian calculated for the anionic chromophore model HBDI with multi-reference, multi-state perturbation theory. The diabatic states that emerge are charge-localized structures with a natural valence-bond interpretation. At planar geometries, the diabatic picture recaptures the charge transfer resonance of the anion. The strong S0-S1 excitation at these geometries is reasonably described within a two-state model, but extension to a three-state model is necessary to describe decay via two possible pathways associated with photoisomerization of the (methine) bridge. Parametric Hamiltonians based on the three-state ansatz can be fit directly to data generated using the underlying active space. We provide an illustrative example of such a parametric Hamiltonian.

physics.chem-ph↗

arXiv2009

Vertex Corrections and the Korringa Ratio in Strongly Correlated Electron Materials

We show that the Korringa ratio, associated with nuclear magnetic resonance in metals, is unity if vertex corrections for the dynamic spin susceptibility are negligible and the hyperfine coupling is momentum independent. In the absence of vertex corrections we also find a Korringa behaviour for $T_1$, the nuclear spin relaxation rate, i.e., $1/T_1\propto T$, and a temperature independent Knight shift. These results are independent of the form and magnitude of the self-energy (so far as is consistent with neglecting vertex corrections) and of the dimensionality of the system.

cond-mat.str-el↗

arXiv2009

The Jahn-Teller instability in dissipative quantum electromechanical systems

We consider the steady states of a harmonic oscillator coupled so strongly to a two-level system (a qubit) that the rotating wave approximation cannot be made. The Hamiltonian version of this model is known as the $E\otimesβ$ Jahn-Teller model. The semiclassical version of this system exhibits a fixed point bifurcation, which in the quantum model leads to a ground state with substantial entanglement between the oscillator and the qubit. We show that the dynamical bifurcation survives in a dissipative quantum description of the system, amidst an even richer bifurcation structure. We propose two experimental implementations of this model based on superconducting cavities: a parametrically driven nonlinear nanomechanical resonator coupled capacitively to a coplanar microwave cavity and a superconducting junction in the central conductor of a coplanar waveguide.

cond-mat.mes-hall↗

arXiv2009

Apparent Violation of the Wiedemann-Franz law near a magnetic field tuned metal-antiferromagnetic quantum critical point

The temperature dependence of the interlayer electrical and thermal resistivity in a layered metal are calculated for Fermi liquid quasiparticles which are scattered inelastically by two-dimensional antiferromagnetic spin fluctuations. Both resistivities have a linear temperature dependence over a broad temperature range. Extrapolations to zero temperature made from this linear-$T$ range give values that appear to violate the Wiedemann-Franz law. However, below a low-temperature scale, which becomes small close to the critical point, a recovery of this law occurs. Our results describe recent measurements on CeCoIn$_5$ near a magnetic field-induced quantum phase transition. Hence, the experiments do not necessarily imply a non-Fermi liquid ground state.

cond-mat.str-el↗

arXiv2009

Interplay of frustration, magnetism, charge ordering, and covalency in a model of Na0.5CoO2

We investigate an effective Hamiltonian for Na0.5CoO2 that includes the electrostatic potential due to the ordered Na ions and strong electronic correlations. This model displays a subtle interplay between metallic and insulating phases and between charge and magnetic order. For realistic parameters, the model predicts an insulating phase with similarities to a covalent insulator. We show that this interpretation gives a consistent explanation of experiments on Na0.5CoO2, including the small degree of charge ordering, the small charge gap, the large moment, and the optical conductivity.

cond-mat.str-el↗

arXiv2008

Mixed valency in cerium oxide crystallographic phases: Determination of valence of the different cerium sites by the bond valence method

We have applied the bond valence method to cerium oxides to determine the oxidation states of the Ce ion at the various site symmetries of the crystals. The crystals studied include cerium dioxide and the two sesquioxides along with some selected intermediate phases which are crystallographically well characterized. Our results indicate that cerium dioxide has a mixed-valence ground state with an f-electron population on the Ce site of 0.27 while both the A- and C-sesquioxides have a nearly pure f^1 configuration. The Ce sites in most of the intermediate oxides have non-integral valences. Furthermore, many of these valences are different from the values predicted from a naive consideration of the stoichiometric valence of the compound.

cond-mat.mtrl-sci↗

arXiv2007

Quasiparticles at the verge of localization near the Mott metal-insulator transition in a two-dimensional material

The dynamics of charge carriers close to the Mott transition is explored theoretically and experimentally in the quasi two-dimensional organic charge-transfer salt $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br$_x$Cl$_{1-x}$, with varying Br content. The frequency dependence of the conductivity deviates significantly from simple Drude model behavior: there is a strong redistribution of spectral weight as the Mott transition is approached and with temperature. The effective mass of the quasiparticles increases considerably when coming close to the insulating phase. A dynamical mean-field-theory treatment of the relevant Hubbard model gives a good quantitative description of the experimental data.

cond-mat.str-el↗

arXiv2007

Sensitivity of the interlayer magnetoresistance of layered metals to intralayer anisotropies

Many of the most interesting and technologically important electronic materials discovered in the past two decades have two common features: a layered crystal structure and strong interactions between electrons. Two of the most fundamental questions about such layered metals concern the origin of intralayer anisotropies and the coherence of interlayer charge transport. We show that angle dependent magnetoresistance oscillations (AMRO) are sensitive to anisotropies around an intralayer Fermi surface. Hence, AMRO can be a probe of intralayer anisotropies that is complementary to angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). However, AMRO are not very sensitive to the coherence of the interlayer transport. We illustrate this with comparisons to recent AMRO experiments on an overdoped cuprate.

cond-mat.str-el↗