3,078 research outputs found
Unconventional metallic conduction in two-dimensional Hubbard-Wigner lattices
The interplay between long-range and local Coulomb repulsion in strongly
interacting electron systems is explored through a two-dimensional
Hubbard-Wigner model. An unconventional metallic state is found in which
collective low-energy excitations characteristic of the Wigner crystal induce a
flow of electrical current despite the absence of one-electron spectral weight
at the Fermi surface. Photoemission experiments on certain quarter-filled
layered molecular crystals should observe a gap in the excitation spectrum
whereas optical spectroscopy should find a finite Drude weight indicating
metallic behavior.Comment: 10 pages, accepted for publication in PR
Coexistence of Spin Density Wave and Triplet Superconductivity
We discuss the possibility of coexistence of spin density wave
(antiferromagnetism) and triplet superconductivity as a particular example of a
broad class of systems where the interplay of magnetism and superconductivity
is important. We focus on the case of quasi-one-dimensional metals, where it is
known experimentally that antiferromagnetism is in close proximity to triplet
superconductivity in the temperature versus pressure phase diagram. Over a
narrow range of pressures, we propose an intermediate non-uniform phase
consisting of alternating antiferromagnetic and triplet superconducting
stripes. Within the non-uniform phase there are also changes between two and
three dimensional behavior.Comment: Revtex4, 4 pages, 5 figure
Magic angle effects in the interlayer magnetoresistance of quasi-one-dimensional metals due to interchain incoherence
The dependence of the magnetoresistance of quasi-one-dimensional metals on
the direction of the magnetic field show dips when the field is tilted at the
so called magic angles determined by the structural dimensions of the
materials. There is currently no accepted explanation for these magic angle
effects. We present a possible explanation. Our model is based on the
assumption that, the intralayer transport in the second most conducting
direction has a small contribution from incoherent electrons. This incoherence
is modelled by a small uncertainty in momentum perpendicular to the most
conducting (chain) direction. Our model predicts the magic angles seen in
interlayer transport measurements for different orientations of the field. We
compare our results to predictions by other models and to experiment.Comment: 7 pages, 3 figures, Submitted To Phys. Rev.
Superconductivity without attraction in a quasi-one-dimensional metal
An array of one-dimensional conductors coupled by transverse hopping and
interaction is studied with the help of a variational wave function. This wave
function is devised as to account for one-dimensional correlation effects. We
show that under broad conditions our system possesses the superconducting
ground state even if no attraction is present. The superconducting mechanism is
of many-body nature and deviates substantially from BCS. The phase diagram of
the model is mapped. It consists of two ordered phases competing against each
other: density wave, spin or charge, and unconventional superconductivity.
These phases are separated by the first order transition. The symmetry of the
superconducting order parameter is a non-universal property. It depends on
particulars of the Hamiltonian. Within the framework of our model possible
choices are the triplet -wave and the singlet -wave. Organic
quasi-one-dimensional superconductors have similar phase diagram.Comment: 12 pages, 2 Encapsulated PostScript figures, revtex4; the model's
Hamiltonian is revised as compared to previous version, this revision affects
prediction of the order parameter symmetr
Effects of non-magnetic impurities on spin-fluctuations induced superconductivity
We study the effects of non-magnetic impurities on the phase diagram of a
system of interacting electrons with a flat Fermi surface. The one-loop
Wilsonian renormalization group flow of the angle dependent diffusion function
and interaction
determines the critical temperature and the nature of the low temperature
state. As the imperfect nesting increases the critical temperature decreases
and the low temperature phase changes from the spin-density wave (SDW) to the
d-wave superconductivity (dSC) and finally, for bad nesting, to the random
antiferromagnetic state (RAF). Both SDW and dSC phases are affected by
disorder. The pair breaking depends on the imperfect nesting and is the most
efficient when the critical temperature for superconductivity is maximal.Comment: 4 pages, 4 figures. Submitted to PR
EEG theta and Mu oscillations during perception of human and robot actions.
The perception of others' actions supports important skills such as communication, intention understanding, and empathy. Are mechanisms of action processing in the human brain specifically tuned to process biological agents? Humanoid robots can perform recognizable actions, but can look and move differently from humans, and as such, can be used in experiments to address such questions. Here, we recorded EEG as participants viewed actions performed by three agents. In the Human condition, the agent had biological appearance and motion. The other two conditions featured a state-of-the-art robot in two different appearances: Android, which had biological appearance but mechanical motion, and Robot, which had mechanical appearance and motion. We explored whether sensorimotor mu (8-13 Hz) and frontal theta (4-8 Hz) activity exhibited selectivity for biological entities, in particular for whether the visual appearance and/or the motion of the observed agent was biological. Sensorimotor mu suppression has been linked to the motor simulation aspect of action processing (and the human mirror neuron system, MNS), and frontal theta to semantic and memory-related aspects. For all three agents, action observation induced significant attenuation in the power of mu oscillations, with no difference between agents. Thus, mu suppression, considered an index of MNS activity, does not appear to be selective for biological agents. Observation of the Robot resulted in greater frontal theta activity compared to the Android and the Human, whereas the latter two did not differ from each other. Frontal theta thus appears to be sensitive to visual appearance, suggesting agents that are not sufficiently biological in appearance may result in greater memory processing demands for the observer. Studies combining robotics and neuroscience such as this one can allow us to explore neural basis of action processing on the one hand, and inform the design of social robots on the other
Density Induced Quantum Phase Transitions in Triplet Superconductors
We consider the possibility of quantum phase transitions in the ground state
of triplet superconductors where particle density is the tunning parameter. For
definiteness, we focus on the case of one band quasi-one-dimensional triplet
superconductors but many of our conclusions regarding the nature of the
transition are quite general. Within the functional integral formulation, we
calculate the electronic compressibility and superfluid density tensor as a
function of the particle density for various triplet order parameter symmetries
and find that these quantities are non-analytic when a critical value of the
particle density is reached.Comment: 4 pages, 3 figure
Magnetic Determination of under Accurate Alignment in (TMTSF)ClO
Cantilever magnetometry has been used to measure the upper critical magnetic
field of the quasi-one dimensional molecular organic superconductor
(TMTSF)ClO. From simultaneous resistivity and torque magnetization
experiments conducted under precise field alignment, at low
temperature is shown to reach 5T, nearly twice the Pauli paramagnetic limit
imposed on spin singlet superconductors. These results constitute the first
thermodynamic evidence for a large in this system and provide support
for spin triplet pairing in this unconventional superconductorComment: Submitted July 1, 2003, Accepted December 9, 2003, Physical Review
Letter
Point-contact tunneling involving low-dimensional spin-triplet superconductors
We modify and extend previous microscopic calculations of tunneling in
superconducting junctions based on a non-equilibrium Green function formalism
to include the case of spin-triplet pairing. We show that distinctive features
are present in the I-V characteristics of different kinds of junctions, in
particular when the effects of magnetic fields are taken into account, that
permit to identify the type of pairing. We discuss the relevance of these
results in the context of quasi one-dimensional organic superconductors like
(TMTSF)_2(PF_6) and layered compounds like Sr_2RuO_4.Comment: 4 pages, 1 figur
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