2,830 research outputs found
Spin excitations in layered antiferromagnetic metals and superconductors
The proximity of antiferromagnetic order in high-temperature superconducting
materials is considered a possible clue to the electronic excitations which
form superconducting pairs. Here we study the transverse and longitudinal spin
excitation spectrum in a one-band model in the pure spin density wave (SDW)
state and in the coexistence state of SDW and the superconductivity. We start
from a Stoner insulator and study the evolution of the spectrum with doping,
including distinct situations with only hole pockets, with only electron
pockets and with pockets of both types. In addition to the usual spin-wave
modes, in the partially gapped cases we find significant weight of low-energy
particle-hole excitations. We discuss the implications of our findings for
neutron scattering experiments and for theories of Cooper-pairing in the
metallic SDW state.Comment: (14 pages, 6 figures
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Sheep recognize familiar and unfamiliar human faces from two-dimensional images.
One of the most important human social skills is the ability to recognize faces. Humans recognize familiar faces easily, and can learn to identify unfamiliar faces from repeatedly presented images. Sheep are social animals that can recognize other sheep as well as familiar humans. Little is known, however, about their holistic face-processing abilities. In this study, we trained eight sheep (Ovis aries) to recognize the faces of four celebrities from photographic portraits displayed on computer screens. After training, the sheep chose the 'learned-familiar' faces rather than the unfamiliar faces significantly above chance. We then tested whether the sheep could recognize the four celebrity faces if they were presented in different perspectives. This ability has previously been shown only in humans. Sheep successfully recognized the four celebrity faces from tilted images. Interestingly, there was a drop in performance with the tilted images (from 79.22â±â7.5% to 66.5â±â4.1%) of a magnitude similar to that seen when humans perform this task. Finally, we asked whether sheep could recognize a very familiar handler from photographs. Sheep identified the handler in 71.8â±â2.3% of the trials without pretraining. Together these data show that sheep have advanced face-recognition abilities, comparable with those of humans and non-human primates
Spin-Peierls instability of the U(1) Dirac spin liquid
Quantum spin liquids are tantalizing phases of frustrated quantum magnets. A
complicating factor in their realization and observation in materials is the
ubiquitous presence of other degrees of freedom, in particular lattice
distortion modes (phonons). These provide additional routes for relieving
magnetic frustration, thereby possibly destabilizing spin-liquid ground states.
In this work, we focus on triangular-lattice Heisenberg antiferromagnets, where
recent numerical evidence suggests the presence of an extended U(1) Dirac spin
liquid phase which is described by compact quantum electrodynamics in 2+1
dimensions (QED), featuring gapless spinons and monopoles as gauge
excitations. Its low energy theory is believed to flow to a strongly-coupled
fixed point with conformal symmetries. Using complementary perturbation theory
and scaling arguments, we show that a symmetry-allowed coupling between
(classical) finite-wavevector lattice distortions and monopole operators of the
U(1) Dirac spin liquid generally induces a spin-Peierls instability towards a
(confining) 12-site valence-bond solid state. We support our theoretical
analysis with state-of-the-art density matrix renormalization group
simulations. Away from the limit of static distortions, we demonstrate that the
phonon energy gap establishes a parameter regime where the spin liquid is
expected to be stable.Comment: 23 pages, 10 figure
Effect of nodes, ellipticity and impurities on the spin resonance in Iron-based superconductors
We analyze doping dependence of the spin resonance of an s+- superconductor
and its sensitivity to the ellipticity of electron pockets, to magnetic and
non-magnetic impurities, and to the angle dependence of the superconducting gap
along electron Fermi surfaces. We show that the maximum intensity of the
resonance shifts from commensurate to incommensurate momentum above some
critical doping which decreases with increasing ellipticity. Angle dependence
of the gap and particularly the presence of accidental nodes lowers the overall
intensity of the resonance peak and shifts its position towards the onset of
the particle-hole continuum. Still, however, the resonance remains a true
\delta-function in the clean limit. When non-magnetic or magnetic impurities
are present, the resonance broadens and its position shifts. The shift depends
on the type of impurities and on the ratio of intraband and interband
scattering components. The ratio Omega_{res}/T_c increases almost linearly with
the strength of the interband impurity scattering, in agreement with the
experimental data. We also compare spin response of s+- and s++
superconductors. We show that there is no resonance for s++ gap, even when
there is a finite mismatch between electron and hole Fermi surfaces shifted by
the antiferromagnetic momentum.Comment: 12 pages, 10 figures, submitted to PR
Micromagnetometry of two-dimensional ferromagnets
The study of atomically thin ferromagnetic crystals has led to the discovery
of unusual magnetic behaviour and provided insight into the magnetic properties
of bulk materials. However, the experimental techniques that have been used to
explore ferromagnetism in such materials cannot probe the magnetic field
directly. Here, we show that ballistic Hall micromagnetometry can be used to
measure the magnetization of individual two-dimensional ferromagnets. Our
devices are made by van der Waals assembly in such a way that the investigated
ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from
encapsulated graphene. We use the micromagnetometry technique to study
atomically thin chromium tribromide (CrBr3). We find that the material remains
ferromagnetic down to monolayer thickness and exhibits strong out-of-plane
anisotropy. We also find that the magnetic response of CrBr3 varies little with
the number of layers and its temperature dependence cannot be described by the
simple Ising model of two-dimensional ferromagnetism.Comment: 19 pages, 12 figure
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Sheep recognize familiar and unfamiliar human faces from two-dimensional images.
© 2017 The Authors. One of the most important human social skills is the ability to recognize faces. Humans recognize familiar faces easily, and can learn to identify unfamiliar faces from repeatedly presented images. Sheep are social animals that can recognize other sheep as well as familiar humans. Little is known, however, about their holistic face-processing abilities. In this study, we trained eight sheep (Ovis aries) to recognize the faces of four celebrities from photographic portraits displayed on computer screens. After training, the sheep chose the âlearned-familiarâ faces rather than the unfamiliar faces significantly above chance. We then tested whether the sheep could recognize the four celebrity faces if they were presented in different perspectives. This ability has previously been shown only in humans. Sheep successfully recognized the four celebrity faces from tilted images. Interestingly, there was a drop in performance with the tilted images (from 79.22±7.5% to 66.5±4.1%) of a magnitude similar to that seen when humans perform this task. Finally, we asked whether sheep could recognize a very familiar handler from photographs. Sheep identified the handler in 71.8±2.3% of the trials without pretraining. Together these data show that sheep have advanced face-recognition abilities, comparable with those of humans and non-human primates
TCF1(+) hepatitis C virus-specific CD8(+) T cells are maintained after cessation of chronic antigen stimulation.
Differentiation and fate of virus-specific CD8(+) T cells after cessation of chronic antigen stimulation is unclear. Here we show that a TCF1(+)CD127(+)PD1(+) hepatitis C virus (HCV)-specific CD8(+) T-cell subset exists in chronically infected patients with phenotypic features of T-cell exhaustion and memory, both before and after treatment with direct acting antiviral (DAA) agents. This subset is maintained during, and for a long duration after, HCV elimination. After antigen re-challenge the less differentiated TCF1(+)CD127(+)PD1(+) population expands, which is accompanied by emergence of terminally exhausted TCF1-CD127-PD1(hi) HCV-specific CD8(+) T cells. These results suggest the TCF1(+)CD127(+)PD1(+) HCV-specific CD8(+) T-cell subset has memory-like characteristics, including antigen-independent survival and recall proliferation. We thus provide evidence for the establishment of memory-like virus-specific CD8(+) T cells in a clinically relevant setting of chronic viral infection and we uncover their fate after cessation of chronic antigen stimulation, implicating a potential strategy for antiviral immunotherapy
Electronic structure and possible pseudogap behavior in iron based superconductors
Starting from the simplified analytic model of electronic spectrum of iron -
pnictogen (chalcogen) high - temperature superconductors close to the Fermi
level, we discuss the influence of antiferromagneting (AFM)scattering both for
stoichiometric case and the region of possible short - range order AFM
fluctuations in doped compounds. Qualitative picture of the evolution of
electronic spectrum and Fermi surfaces (FS) for different dopings is presented,
with the aim of comparison with existing and future ARPES experiments. Both
electron and hole dopings are considered and possible pseudogap behavior
connected with partial FS "destruction" is demonstrated, explaining some recent
experiments.Comment: 5 pages, 4 figures, published versio
Voice-selective prediction alterations in nonclinical voice hearers
Auditory verbal hallucinations (AVH) are a cardinal symptom of psychosis but also occur in 6-13% of the general population. Voice perception is thought to engage an internal forward model that generates predictions, preparing the auditory cortex for upcoming sensory feedback. Impaired processing of sensory feedback in vocalization seems to underlie the experience of AVH in psychosis, but whether this is the case in nonclinical voice hearers remains unclear. The current study used electroencephalography (EEG) to investigate whether and how hallucination predisposition (HP) modulates the internal forward model in response to self-initiated tones and self-voices. Participants varying in HP (based on the Launay-Slade Hallucination Scale) listened to self-generated and externally generated tones or self-voices. HP did not affect responses to self vs. externally generated tones. However, HP altered the processing of the self-generated voice: increased HP was associated with increased pre-stimulus alpha power and increased N1 response to the self-generated voice. HP did not affect the P2 response to voices. These findings confirm that both prediction and comparison of predicted and perceived feedback to a self-generated voice are altered in individuals with AVH predisposition. Specific alterations in the processing of self-generated vocalizations may establish a core feature of the psychosis continuum.The Authors gratefully acknowledge all the participants who collaborated in the study, and particularly Dr. Franziska Knolle for feedback on stimulus generation, Carla Barros for help with scripts for EEG time-frequency analysis, and Dr. Celia Moreira for her advice on mixed linear models. This work was supported by the Portuguese Science National Foundation (FCT; grant numbers PTDC/PSI-PCL/116626/2010, IF/00334/2012, PTDC/MHCPCN/0101/2014) awarded to APP
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