130 research outputs found
Andreev interferometry as a probe of superconducting phase correlations in the pseudogap regime of the cuprates
Andreev interferometry - the sensitivity of the tunneling current to spatial
variations in the local superconducting order at an interface - is proposed as
a probe of the spatial structure of the phase correlations in the pseudogap
state of the cuprate superconductors. To demonstrate this idea theoretically, a
simple tunneling model is considered, via which the tunneling current is
related to the equilibrium phase-phase correlator in the pseudogap state. These
considerations suggest that measurement of the low-voltage conductance through
mesoscopic contacts of varying areas provides a scheme for accessing
phase-phase correlation information. For illustrative purposes, quantitative
predictions are made for a model of the pseudogap state in which the phase (but
not the amplitude) of the superconducting order varies randomly, and does so
with correlations consistent with certain proposed pictures of the pseudogap
state.Comment: 9 pages, 5 figures; 3 references adde
Probing d-wave pairing correlations in the pseudogap regime of the cuprate superconductors via low-energy states near impurities
The issue of probing the pseudogap regime of the cuprate superconductors, specifically with regard to the existence and nature of superconducting pairing correlations of d-wave symmetry, is explored theoretically. It is shown that if the d-wave correlations believed to describe the superconducting state persist into the pseudogap regime, but with pair-potential phase fluctuations that destroy their long-range nature, then the low-energy quasiparticle states observed near extended impurities in the truly superconducting state should also persist as resonances in the pseudogap regime. The scattering of quasiparticles by these phase-fluctuations broadens what was (in the superconducting state) a sharp peak in the single-particle spectral function at low energy, as we demonstrate within the context of a simple model. This peak and its broadening are, in principle, accessible via scanning tunneling spectroscopy near extended impurities in the pseudogap regime. If so, such experiments would provide a probe of the extent to which d-wave superconducting correlations persist upon entering the pseudogap regime, thus providing a stringent diagnostic of the phase-fluctuation scenario
Exact microscopic analysis of a thermal Brownian motor
We study a genuine Brownian motor by hard disk molecular dynamics and
calculate analytically its properties, including its drift speed and thermal
conductivity, from microscopic theory.Comment: 4 pages, 5 figure
Resonant states and order-parameter suppression near pointlike impurities in d-wave superconductors
We examine the role of order-parameter suppression in the development of low-energy peaks ͑i.e., resonances͒ in the tunneling density of states near a nonmagnetic impurity in a d-wave superconductor. Without order-parameter suppression, the zero-energy resonance appears only in the unitary ͑i.e., strong impurity͒ limit. However, suppression makes the resonance appear even when the impurity is much weaker. To model this situation, we make the physical hypothesis that the order parameter is reduced whenever one electron of a Cooper pair encounters the impurity, a hypothesis that retains the exact solvability of the problem. In this way, we determine that suppression of the order parameter drives the effective strength of the impurity towards the unitary limit. We determine the order-parameter reduction variationally, and show that the ratios between the main energy scales-the bandwidth and superconducting gap-strongly affect this reduction and, in consequence, the position and width of the resonance
Low-energy quasiparticle states near extended scatterers in d-wave superconductors and their connection with SUSY quantum mechanics
Low-energy quasiparticle states, arising from scattering by single-particle
potentials in d-wave superconductors, are addressed. Via a natural extension of
the Andreev approximation, the idea that sign-variations in the superconducting
pair-potential lead to such states is extended beyond its original setting of
boundary scattering to the broader context of scattering by general
single-particle potentials, such as those due to impurities. The
index-theoretic origin of these states is exhibited via a simple connection
with Witten's supersymmetric quantum-mechanical model.Comment: 5 page
High mobility in a van der Waals layered antiferromagnetic metal
Magnetic van der Waals (vdW) materials have been heavily pursued for
fundamental physics as well as for device design. Despite the rapid advances,
so far magnetic vdW materials are mainly insulating or semiconducting, and none
of them possesses a high electronic mobility - a property that is rare in
layered vdW materials in general. The realization of a magnetic high-mobility
vdW material would open the possibility for novel magnetic twistronic or
spintronic devices. Here we report very high carrier mobility in the layered
vdW antiferromagnet GdTe3. The electron mobility is beyond 60,000 cm2 V-1 s-1,
which is the highest among all known layered magnetic materials, to the best of
our knowledge. Among all known vdW materials, the mobility of bulk GdTe3 is
comparable to that of black phosphorus, and is only surpassed by graphite. By
mechanical exfoliation, we further demonstrate that GdTe3 can be exfoliated to
ultrathin flakes of three monolayers, and that the magnetic order and
relatively high mobility is retained in approximately 20-nm-thin flakes
Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator
The breaking of time-reversal symmetry by ferromagnetism is predicted to
yield profound changes to the electronic surface states of a topological
insulator. Here, we report on a concerted set of structural, magnetic,
electrical and spectroscopic measurements of \MBS thin films wherein
photoemission and x-ray magnetic circular dichroism studies have recently shown
surface ferromagnetism in the temperature range 15 K K,
accompanied by a suppressed density of surface states at the Dirac point.
Secondary ion mass spectroscopy and scanning tunneling microscopy reveal an
inhomogeneous distribution of Mn atoms, with a tendency to segregate towards
the sample surface. Magnetometry and anisotropic magnetoresistance measurements
are insensitive to the high temperature ferromagnetism seen in surface studies,
revealing instead a low temperature ferromagnetic phase at K.
The absence of both a magneto-optical Kerr effect and anomalous Hall effect
suggests that this low temperature ferromagnetism is unlikely to be a
homogeneous bulk phase but likely originates in nanoscale near-surface regions
of the bulk where magnetic atoms segregate during sample growth. Although the
samples are not ideal, with both bulk and surface contributions to electron
transport, we measure a magnetoconductance whose behavior is qualitatively
consistent with predictions that the opening of a gap in the Dirac spectrum
drives quantum corrections to the conductance in topological insulators from
the symplectic to the orthogonal class.Comment: To appear in Phys. Rev.
Predictive models for Alzheimer's disease diagnosis and MCI identification: The use of cognitive scores and artificial intelligence algorithms
The paper presents a comprehensive study on predictive models for Alzheimer's disease (AD) and mild cognitive impairment (MCI) diagnosis, implementing a combination of cognitive scores and artificial intelligence algorithms. The research includes detailed analyses of clinical and demographic variables such as age, education, and various cognitive and functional scores, investigating their distributions and correlations with AD and MCI. The study utilizes several machine-learning classifiers, comparing their performance through metrics like accuracy, precision, and area under the ROC curve (AUC). Key findings include the influence of gender on AD prevalence, the potential protective effect of education, and the significance of functional decline and cognitive performance scores in the models. The results demonstrate the effectiveness of ensemble methods and the robustness of the models across different data subsets, highlighting the potential of artificial intelligence in enhancing diagnostic accuracy for Alzheimer's disease and mild cognitive impairment
Probing d-wave pairing correlations in the pseudogap regime of the cuprate superconductors via low-energy states near impurities
The issue of probing the pseudogap regime of the cuprate superconductors,
specifically with regard to the existence and nature of superconducting pairing
correlations of d-wave symmetry, is explored theoretically. It is shown that if
the d-wave correlations believed to describe the superconducting state persist
into the pseudogap regime, but with pair-potential phase-fluctuations that
destroy their long-range nature, then the low-energy quasiparticle states
observed near extended impurities in the truly superconducting state should
also persist as resonances in the pseudogap regime. The scattering of
quasiparticles by these phase-fluctuations broadens what was (in the
superconducting state) a sharp peak in the single-particle spectral function at
low energy, as we demonstrate within the context of a simple model. This peak
and its broadening is, in principle, accessible via scanning tunneling
spectroscopy near extended impurities in the pseudogap regime. If so, such
experiments would provide a probe of the extent to which d-wave superconducting
correlations persist upon entering the pseudogap regime, thus providing a
stringent diagnostic of the phase-fluctuation scenario.Comment: 8 pages, 2 figure
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