The oil and gas industry faces increasing pressure to remove people from dangerous offshore envir... more The oil and gas industry faces increasing pressure to remove people from dangerous offshore environments. Robots present a cost-effective and safe method for inspection, repair, and maintenance of topside and marine offshore infrastructure. In this work, we introduce a new multi-sensing platform, the Limpet, which is designed to be low-cost and highly manufacturable, and thus can be deployed in huge collectives for monitoring offshore platforms. The Limpet can be considered an instrument, where in abstract terms, an instrument is a device that transforms a physical variable of interest (measurand) into a form that is suitable for recording (measurement). The Limpet is designed to be part of the ORCA (Offshore Robotics for Certification of Assets) Hub System, which consists of the offshore assets and all the robots (Underwater Autonomous Vehicles, drones, mobile legged robots etc.) interacting with them. The Limpet comprises the sensing aspect of the ORCA Hub System. We integrated th...
This article presents an overview of the research and educational programs for connected and auto... more This article presents an overview of the research and educational programs for connected and autonomous vehicles at the University of Waterloo (UWaterloo). UWaterloo is Canada’s largest engineering school, with 9,500 engineering students and 309 engineering faculty. The University of Waterloo Centre for Automotive Research (WatCAR) for faculty, staff and students is contributing to the development of in-vehicle systems education programs for connected and autonomous vehicles (CAVs) at Waterloo. Over 130 Waterloo faculty, 110 from engineering, are engaged in WatCAR’s automotive and transportation systems research programs. The school’s CAV efforts leverage WatCAR research expertise from five areas: (1) Connected and Autonomous; (2) Software and Data; (3) Lightweighting and Fabrication; (4) Structure and Safety; and (5) Advanced Powertrain and Emissions. Foundational and operational artificial intelligence expertise from the University of Waterloo Artificial Intelligence Institute com...
This paper is a collection of selected contributions of the 1st International Workshop on Mechano... more This paper is a collection of selected contributions of the 1st International Workshop on Mechanochemistry of Metal Hydrides that was held in Oslo in May 2018. In this paper, the recent developments in the use of mechanochemistry to synthesize and modify metal hydrides are reviewed. A special emphasis is made on new techniques beside the traditional way of ball milling. High energy milling, ball milling under hydrogen reactive gas, cryomilling and severe plastic deformation techniques such as High-Pressure Torsion (HPT), Surface Mechanical Attrition Treatment (SMAT) and cold rolling are discussed. The new characterization method of in-situ X-ray diffraction during milling is described.
In many strongly correlated electron metals the thermoelectric power has a non-monotonic temperat... more In many strongly correlated electron metals the thermoelectric power has a non-monotonic temperature dependence and values that are orders of magnitude larger than for elemental metals. Kelvin proposed a particularly simple expression for the thermopower in terms of the temperature dependence of the chemical potential. We consider a Hubbard model on an anisotropic triangular lattice at half filling, a minimal effective Hamiltonian for several classes of organic charge transfer salts. The finite temperature Lanczos method is used to calculate the temperature dependence of the thermopower using the Kelvin formula. We find that electronic correlations significantly enhance the magnitude of the thermopower and lead to a non-monotonic temperature dependence. The latter reflects a crossover with increasing temperature from a Fermi liquid to a bad metal. Although, the Kelvin formula gives a semi-quantitative description of some experimental results it cannot describe the directional dependence of the sign of the thermopower in some materials.
Finding microscopic models for metallic states that exhibit quantum critical properties is a majo... more Finding microscopic models for metallic states that exhibit quantum critical properties is a major theoretical challenge. We calculate the dynamical local spin susceptibility χ(T, ω) for a Hubbard model at half filling using Dynamical Mean-Field Theory, which is exact in infinite dimensions. Qualitatively distinct behavior is found in the different regions of the phase diagram: Mott insulator, Fermi liquid metal, bad metal, and a quantum critical region above the finite temperature critical point. The signature of the latter is ω/T scaling where ω is the frequency and T is the temperature. Our results are consistent with previous results showing scaling of the dc electrical conductivity and are relevant to experiments on organic charge transfer salts.
We investigate the critical behaviour of the spectral weight of a single quasiparticle, one of th... more We investigate the critical behaviour of the spectral weight of a single quasiparticle, one of the key observables in experiment, for the particular case of the transverse Ising model. Series expansions are calculated for the linear chain and the square and simple cubic lattices. For the chain model, a conjectured exact result is discovered. For the square and simple cubic lattices, series analyses are used to estimate the critical exponents. The results agree with the general predictions of Sachdev.
We present a resonating-valence-bond theory of superconductivity for the Hubbard-Heisenberg model... more We present a resonating-valence-bond theory of superconductivity for the Hubbard-Heisenberg model on an anisotropic triangular lattice. Our calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the , 0 , , and phases of BEDT-TTF 2 X [bis(ethylenedithio)tetrathiafulvalene] and BETS 2 X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order transition from a Mott insulator to a d x 2 ÿy 2 superconductor with a small superfluid stiffness and a pseudogap with d x 2 ÿy 2 symmetry.
Physical Review B Condensed Matter and Materials Physics, Dec 23, 2009
The interlayer magnetoresistance of a quasi-two-dimensional layered metal with a d-wave pseudogap... more 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 Tᐉ2201.
Good metals are characterised by diffusive transport of coherent quasi-particle states and the re... more Good metals are characterised by diffusive transport of coherent quasi-particle states and the resistivity is much less than the Mott-Ioffe-Regel (MIR) limit, $\frac{ha}{e^{2}}$, where $a$ is the lattice constant. In bad metals, such as many strongly correlated electron materials, the resistivity exceeds the Mott-Ioffe-Regel limit and the transport is incoherent in nature. Hartnoll, loosely motivated by holographic duality (AdS/CFT correspondence) in string theory, recently proposed a lower bound to the charge diffusion constant, $D \gtrsim \hbar v_{F}^{2}/(k_{B}T)$, in the incoherent regime of transport, where $v_F$ is the Fermi velocity and $T$ the temperature. Using dynamical mean field theory (DMFT) we calculate the charge diffusion constant in a single band Hubbard model at half filling. We show that in the strongly correlated regime the Hartnoll's bound is violated in the crossover region between the coherent Fermi liquid region and the incoherent (bad metal) local moment region. The violation occurs even when the bare Fermi velocity $v_F$ is replaced by its low temperature renormalised value, $v_F^*$.The bound is satisfied at all temperatures in the weakly and moderately correlated systems as well as in strongly correlated systems in the high temperature region where the resistivity is close to linear in temperature. Our calculated charge diffusion constant, in the incoherent regime of transport, also strongly violates a proposed quantum limit of spin diffusion, $D_{s} \sim 1.3 \hbar/m$, where $m$ is the fermion mass, experimentally observed and theoretically calculated in a cold degenerate Fermi gas in the unitary limit of scattering.
We present an resonating valence bond (RVB) theory of superconductivity for the Hubbard--Heisenbe... more We present an resonating valence bond (RVB) theory of superconductivity for the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show that these calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the beta, beta', kappa and lambda phases of (BEDT-TTF)_2X [bis(ethylenedithio)tetrathiafulvalene] and (BETS)_2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order transition from a Mott insulator to a d_{x^2-y^2} superconductor with a small superfluid stiffness and a pseudogap with d_{x^2-y^2} symmetry. The Mott--Hubbard transition can be driven either by increasing the on-site Coulomb repulsion, U, or by changing the anisotropy of the two hopping integrals, t'/t. Our results suggest that the ratio t'/t plays an important role in determining the phase diagram of the organic superconductors.
The effect of disorder on the quantum phase transitions induced by a transverse field, anisotropy... more The effect of disorder on the quantum phase transitions induced by a transverse field, anisotropy, and dimerization in XY spin chains is investigated. The low-energy behavior near the critical point is described by a Dirac-type equation with a random mass for which an exact analytic treatment is possible. Results obtained for the dynamical critical exponent, the specific heat, and transverse susceptibility agree with results recently obtained using a real space renormalization group decimation technique, supporting Fisher's claim that it is exact. A non-zero transverse field changes the universality class of the anisotropy transition.
We introduce a strongly correlated mean-field theory of the ionic Hubbard model on the triangular... more 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.
The metallic states of a broad range of strongly correlated electron materials exhibit the subtle... more The metallic states of a broad range of strongly correlated electron materials exhibit the subtle interplay between antiferromagnetic spin fluctuations, a pseudogap in the excitation spectra, and non-Fermi liquid properties. In order to understand these issues better in the κ-(ET)2X family of organic charge transfer salts we give a quantitative analysis of the published results of nuclear magnetic resonance (NMR) experiments. The temperature dependence of the nuclear spin relaxation rate 1/T1, the Knight shift Ks, and the Korringa ratio K, are compared to the predictions of the phenomenological spin fluctuation model of Moriya, and Millis, Monien and Pines (M-MMP), that has been used extensively to quantify antiferromagnetic spin fluctuations in the cuprates. For temperatures above TNMR ≃ 50 K, the model gives a good quantitative description of the data for the paramagnetic metallic phase of several κ-(ET)2X materials, with an antiferromagnetic correlation length which increases with decreasing temperature; growing to several lattice constants by TNMR. It is shown that the fact that the dimensionless Korringa ratio is much larger than unity is inconsistent with a broad class of theoretical models (such as dynamical mean-field theory) which neglect spatial correlations and/or vertex corrections. For materials close to the Mott insulating phase the nuclear spin relaxation rate, the Knight shift and the Korringa ratio all decrease significantly with decreasing temperature below TNMR. This cannot be described by the M-MMP model and the most natural explanation is that a pseudogap opens up in the density of states below TNMR, as in, for example, the underdoped cuprate superconductors. An analysis of the Mott insulating phase of κ-(ET)2Cu2(CN)3 is somewhat more ambiguous; nevertheless it suggests that the antiferromagnetic correlation length is less than a lattice constant, consistent with a large frustration of antiferromagnetic interactions as is believed to occur in this material. We show that the NMR measurements reported for κ-(ET)2Cu2(CN)3 are qualitatively inconsistent with this material having a ground state with long range magnetic order. A pseudogap in the metallic state of organic superconductors is an important prediction of the resonating valence bond theory of superconductivity. Understanding the nature, origin, and momentum dependence of the pseudogap and its relationship to superconductivity are important outstanding problems. We propose specific new experiments on organic superconductors to elucidate these issues. Specifically, measurements should be performed to see if high magnetic fields or high pressures can be used to close the pseudogap.
The interlayer magnetoresistance of the quasi-two-dimensional metal α-(BEDT-TTF)2KHg(SCN)4 is con... more The interlayer magnetoresistance of the quasi-two-dimensional metal α-(BEDT-TTF)2KHg(SCN)4 is considered. In the temperature range from 0.5 to 10 K and for fields up to 10 tesla the magnetoresistance has a stronger temperature dependence than the zero-field resistance. Consequently Kohler's rule is not obeyed for any range of temperatures or fields. This means that the magnetoresistance cannot be described in terms of semiclassical transport on a single Fermi surface with a single scattering time. Possible explanations for the violations of Kohler's rule are considered, both within the framework of semi-classical transport theory and involving incoherent interlayer transport. The issues considered are similar to those raised by the magnetotransport of the cuprate superconductors.
The oil and gas industry faces increasing pressure to remove people from dangerous offshore envir... more The oil and gas industry faces increasing pressure to remove people from dangerous offshore environments. Robots present a cost-effective and safe method for inspection, repair, and maintenance of topside and marine offshore infrastructure. In this work, we introduce a new multi-sensing platform, the Limpet, which is designed to be low-cost and highly manufacturable, and thus can be deployed in huge collectives for monitoring offshore platforms. The Limpet can be considered an instrument, where in abstract terms, an instrument is a device that transforms a physical variable of interest (measurand) into a form that is suitable for recording (measurement). The Limpet is designed to be part of the ORCA (Offshore Robotics for Certification of Assets) Hub System, which consists of the offshore assets and all the robots (Underwater Autonomous Vehicles, drones, mobile legged robots etc.) interacting with them. The Limpet comprises the sensing aspect of the ORCA Hub System. We integrated th...
This article presents an overview of the research and educational programs for connected and auto... more This article presents an overview of the research and educational programs for connected and autonomous vehicles at the University of Waterloo (UWaterloo). UWaterloo is Canada’s largest engineering school, with 9,500 engineering students and 309 engineering faculty. The University of Waterloo Centre for Automotive Research (WatCAR) for faculty, staff and students is contributing to the development of in-vehicle systems education programs for connected and autonomous vehicles (CAVs) at Waterloo. Over 130 Waterloo faculty, 110 from engineering, are engaged in WatCAR’s automotive and transportation systems research programs. The school’s CAV efforts leverage WatCAR research expertise from five areas: (1) Connected and Autonomous; (2) Software and Data; (3) Lightweighting and Fabrication; (4) Structure and Safety; and (5) Advanced Powertrain and Emissions. Foundational and operational artificial intelligence expertise from the University of Waterloo Artificial Intelligence Institute com...
This paper is a collection of selected contributions of the 1st International Workshop on Mechano... more This paper is a collection of selected contributions of the 1st International Workshop on Mechanochemistry of Metal Hydrides that was held in Oslo in May 2018. In this paper, the recent developments in the use of mechanochemistry to synthesize and modify metal hydrides are reviewed. A special emphasis is made on new techniques beside the traditional way of ball milling. High energy milling, ball milling under hydrogen reactive gas, cryomilling and severe plastic deformation techniques such as High-Pressure Torsion (HPT), Surface Mechanical Attrition Treatment (SMAT) and cold rolling are discussed. The new characterization method of in-situ X-ray diffraction during milling is described.
In many strongly correlated electron metals the thermoelectric power has a non-monotonic temperat... more In many strongly correlated electron metals the thermoelectric power has a non-monotonic temperature dependence and values that are orders of magnitude larger than for elemental metals. Kelvin proposed a particularly simple expression for the thermopower in terms of the temperature dependence of the chemical potential. We consider a Hubbard model on an anisotropic triangular lattice at half filling, a minimal effective Hamiltonian for several classes of organic charge transfer salts. The finite temperature Lanczos method is used to calculate the temperature dependence of the thermopower using the Kelvin formula. We find that electronic correlations significantly enhance the magnitude of the thermopower and lead to a non-monotonic temperature dependence. The latter reflects a crossover with increasing temperature from a Fermi liquid to a bad metal. Although, the Kelvin formula gives a semi-quantitative description of some experimental results it cannot describe the directional dependence of the sign of the thermopower in some materials.
Finding microscopic models for metallic states that exhibit quantum critical properties is a majo... more Finding microscopic models for metallic states that exhibit quantum critical properties is a major theoretical challenge. We calculate the dynamical local spin susceptibility χ(T, ω) for a Hubbard model at half filling using Dynamical Mean-Field Theory, which is exact in infinite dimensions. Qualitatively distinct behavior is found in the different regions of the phase diagram: Mott insulator, Fermi liquid metal, bad metal, and a quantum critical region above the finite temperature critical point. The signature of the latter is ω/T scaling where ω is the frequency and T is the temperature. Our results are consistent with previous results showing scaling of the dc electrical conductivity and are relevant to experiments on organic charge transfer salts.
We investigate the critical behaviour of the spectral weight of a single quasiparticle, one of th... more We investigate the critical behaviour of the spectral weight of a single quasiparticle, one of the key observables in experiment, for the particular case of the transverse Ising model. Series expansions are calculated for the linear chain and the square and simple cubic lattices. For the chain model, a conjectured exact result is discovered. For the square and simple cubic lattices, series analyses are used to estimate the critical exponents. The results agree with the general predictions of Sachdev.
We present a resonating-valence-bond theory of superconductivity for the Hubbard-Heisenberg model... more We present a resonating-valence-bond theory of superconductivity for the Hubbard-Heisenberg model on an anisotropic triangular lattice. Our calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the , 0 , , and phases of BEDT-TTF 2 X [bis(ethylenedithio)tetrathiafulvalene] and BETS 2 X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order transition from a Mott insulator to a d x 2 ÿy 2 superconductor with a small superfluid stiffness and a pseudogap with d x 2 ÿy 2 symmetry.
Physical Review B Condensed Matter and Materials Physics, Dec 23, 2009
The interlayer magnetoresistance of a quasi-two-dimensional layered metal with a d-wave pseudogap... more 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 Tᐉ2201.
Good metals are characterised by diffusive transport of coherent quasi-particle states and the re... more Good metals are characterised by diffusive transport of coherent quasi-particle states and the resistivity is much less than the Mott-Ioffe-Regel (MIR) limit, $\frac{ha}{e^{2}}$, where $a$ is the lattice constant. In bad metals, such as many strongly correlated electron materials, the resistivity exceeds the Mott-Ioffe-Regel limit and the transport is incoherent in nature. Hartnoll, loosely motivated by holographic duality (AdS/CFT correspondence) in string theory, recently proposed a lower bound to the charge diffusion constant, $D \gtrsim \hbar v_{F}^{2}/(k_{B}T)$, in the incoherent regime of transport, where $v_F$ is the Fermi velocity and $T$ the temperature. Using dynamical mean field theory (DMFT) we calculate the charge diffusion constant in a single band Hubbard model at half filling. We show that in the strongly correlated regime the Hartnoll's bound is violated in the crossover region between the coherent Fermi liquid region and the incoherent (bad metal) local moment region. The violation occurs even when the bare Fermi velocity $v_F$ is replaced by its low temperature renormalised value, $v_F^*$.The bound is satisfied at all temperatures in the weakly and moderately correlated systems as well as in strongly correlated systems in the high temperature region where the resistivity is close to linear in temperature. Our calculated charge diffusion constant, in the incoherent regime of transport, also strongly violates a proposed quantum limit of spin diffusion, $D_{s} \sim 1.3 \hbar/m$, where $m$ is the fermion mass, experimentally observed and theoretically calculated in a cold degenerate Fermi gas in the unitary limit of scattering.
We present an resonating valence bond (RVB) theory of superconductivity for the Hubbard--Heisenbe... more We present an resonating valence bond (RVB) theory of superconductivity for the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show that these calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the beta, beta', kappa and lambda phases of (BEDT-TTF)_2X [bis(ethylenedithio)tetrathiafulvalene] and (BETS)_2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order transition from a Mott insulator to a d_{x^2-y^2} superconductor with a small superfluid stiffness and a pseudogap with d_{x^2-y^2} symmetry. The Mott--Hubbard transition can be driven either by increasing the on-site Coulomb repulsion, U, or by changing the anisotropy of the two hopping integrals, t'/t. Our results suggest that the ratio t'/t plays an important role in determining the phase diagram of the organic superconductors.
The effect of disorder on the quantum phase transitions induced by a transverse field, anisotropy... more The effect of disorder on the quantum phase transitions induced by a transverse field, anisotropy, and dimerization in XY spin chains is investigated. The low-energy behavior near the critical point is described by a Dirac-type equation with a random mass for which an exact analytic treatment is possible. Results obtained for the dynamical critical exponent, the specific heat, and transverse susceptibility agree with results recently obtained using a real space renormalization group decimation technique, supporting Fisher's claim that it is exact. A non-zero transverse field changes the universality class of the anisotropy transition.
We introduce a strongly correlated mean-field theory of the ionic Hubbard model on the triangular... more 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.
The metallic states of a broad range of strongly correlated electron materials exhibit the subtle... more The metallic states of a broad range of strongly correlated electron materials exhibit the subtle interplay between antiferromagnetic spin fluctuations, a pseudogap in the excitation spectra, and non-Fermi liquid properties. In order to understand these issues better in the κ-(ET)2X family of organic charge transfer salts we give a quantitative analysis of the published results of nuclear magnetic resonance (NMR) experiments. The temperature dependence of the nuclear spin relaxation rate 1/T1, the Knight shift Ks, and the Korringa ratio K, are compared to the predictions of the phenomenological spin fluctuation model of Moriya, and Millis, Monien and Pines (M-MMP), that has been used extensively to quantify antiferromagnetic spin fluctuations in the cuprates. For temperatures above TNMR ≃ 50 K, the model gives a good quantitative description of the data for the paramagnetic metallic phase of several κ-(ET)2X materials, with an antiferromagnetic correlation length which increases with decreasing temperature; growing to several lattice constants by TNMR. It is shown that the fact that the dimensionless Korringa ratio is much larger than unity is inconsistent with a broad class of theoretical models (such as dynamical mean-field theory) which neglect spatial correlations and/or vertex corrections. For materials close to the Mott insulating phase the nuclear spin relaxation rate, the Knight shift and the Korringa ratio all decrease significantly with decreasing temperature below TNMR. This cannot be described by the M-MMP model and the most natural explanation is that a pseudogap opens up in the density of states below TNMR, as in, for example, the underdoped cuprate superconductors. An analysis of the Mott insulating phase of κ-(ET)2Cu2(CN)3 is somewhat more ambiguous; nevertheless it suggests that the antiferromagnetic correlation length is less than a lattice constant, consistent with a large frustration of antiferromagnetic interactions as is believed to occur in this material. We show that the NMR measurements reported for κ-(ET)2Cu2(CN)3 are qualitatively inconsistent with this material having a ground state with long range magnetic order. A pseudogap in the metallic state of organic superconductors is an important prediction of the resonating valence bond theory of superconductivity. Understanding the nature, origin, and momentum dependence of the pseudogap and its relationship to superconductivity are important outstanding problems. We propose specific new experiments on organic superconductors to elucidate these issues. Specifically, measurements should be performed to see if high magnetic fields or high pressures can be used to close the pseudogap.
The interlayer magnetoresistance of the quasi-two-dimensional metal α-(BEDT-TTF)2KHg(SCN)4 is con... more The interlayer magnetoresistance of the quasi-two-dimensional metal α-(BEDT-TTF)2KHg(SCN)4 is considered. In the temperature range from 0.5 to 10 K and for fields up to 10 tesla the magnetoresistance has a stronger temperature dependence than the zero-field resistance. Consequently Kohler's rule is not obeyed for any range of temperatures or fields. This means that the magnetoresistance cannot be described in terms of semiclassical transport on a single Fermi surface with a single scattering time. Possible explanations for the violations of Kohler's rule are considered, both within the framework of semi-classical transport theory and involving incoherent interlayer transport. The issues considered are similar to those raised by the magnetotransport of the cuprate superconductors.
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