1,106 research outputs found
Thermoelectric power quantum oscillations in the ferromagnet UGe
We present thermoelectric power and resistivity measurements in the
ferromagnet UGe as a function of temperature and magnetic field. At low
temperature, huge quantum oscillations are observed in the thermoelectric power
as a function of the magnetic field applied along the axis. The frequencies
of the extreme orbits are determined and an analysis of the cyclotron masses is
performed following different theoretical approaches for quantum oscillations
detected in the thermoelectric power. They are compared to those obtained by
Shubnikov-de Haas experiments on the same crystal and previous de Haas-van
Alphen experiments. The agreement of the different probes confirms
thermoelectric power as an excellent probe to extract simultaneously both
microscopic and macroscopic information on the Fermi-surface properties.
Band-structure calculations of UGe in the ferromagnetic state are compared
to the experiment.Comment: 10 figures, 12 pages, accepted for publication in Phys. Rev.
Fermi surface instabilities in CeRh2Si2 at high magnetic field and pressure
We present thermoelectric power (TEP) studies under pressure and high
magnetic field in the antiferromagnet CeRh2Si2 at low temperature. Under
magnetic field, large quantum oscillations are observed in the TEP, S(H), in
the antiferromagnetic phase. They suddenly disappear when entering in the
polarized paramagnetic (PPM) state at Hc pointing out an important
reconstruction of the Fermi surface (FS). Under pressure, S/T increases
strongly of at low temperature near the critical pressure Pc, where the AF
order is suppressed, implying the interplay of a FS change and low energy
excitations driven by spin and valence fluctuations. The difference between the
TEP signal in the PPM state above Hc and in the paramagnetic state (PM) above
Pc can be explained by different FS. Band structure calculations at P = 0
stress that in the AF phase the 4f contribution at the Fermi level (EF) is weak
while it is the main contribution in the PM domain. By analogy to previous work
on CeRu2Si2, in the PPM phase of CeRh2Si2 the 4f contribution at EF will drop.Comment: 10 pages, 13 figure
Comment on ``Texture in the Superconducting Order Parameter of CeCoIn Revealed by Nuclear Magnetic Resonance''
The study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been of
considerable recent interest. Below the temperature which is believed to
be the transition temperature () to the FFLO phase in CeCoIn, K.
Kakuyanagi et al. (Phys. Rev. Lett. 94, 047602 (2005)) reported a composite NMR
spectrum with a tiny component observed at frequencies corresponding to the
normal state signal. The results were interpreted as evidence for the emergence
of an FFLO state. This result is inconsistent with two other NMR studies of V.
F. Mitrovi{\'c} et al. (Phys. Rev. Lett. 97, 117002 (2006)) and B.-L. Young et
al. (Phys. Rev. Lett. 98, 036402 (2007)). In this comment we show that the
findings of K. Kakuyanagi et al. do not reflect the true nature of the FFLO
state but result from excess RF excitation power used in that experiment.Comment: 1 page, to appear in PR
Quantum replica approach to the under-screened Kondo model
We extend the Schwinger boson large N treatment of the underscreened Kondo
model in a way that correctly captures the finite elastic phase shift in the
singular Fermi liquid. The new feature of the approach, is the introduction of
a flavor quantum number with K possible values, associated with the Schwinger
boson representation. The large N limit is taken maintaining the ratio k=K/N
fixed. This approach differs from previous approaches, in that we do not
explicitly enforce a constraint on the spin representation of the Schwinger
bosons. Instead, the energetics of the Kondo model cause the bosonic degrees of
freedom to ``self assemble'' into a ground-state in which the spins of K bosons
and N-K conduction electrons are antisymmetrically arranged into a Kondo
singlet. With this device, the large N limit can be taken, in such a way that a
fraction K/N of the Abrikosov Suhl resonance is immersed inside the Fermi sea.
We show how this method can be used to model the full energy dependence of the
singular Abrikosov Suhl resonance in the underscreened Kondo model and the
field-dependent magnetization.Comment: Revised draft, with plots explicitly showing logarithmic scaling of
inverse coupling constant. Small corrections prior to submission to journa
Stimulus statistics shape oscillations in nonlinear recurrent neural networks.
Rhythmic activity plays a central role in neural computations and brain functions ranging from homeostasis to attention, as well as in neurological and neuropsychiatric disorders. Despite this pervasiveness, little is known about the mechanisms whereby the frequency and power of oscillatory activity are modulated, and how they reflect the inputs received by neurons. Numerous studies have reported input-dependent fluctuations in peak frequency and power (as well as couplings across these features). However, it remains unresolved what mediates these spectral shifts among neural populations. Extending previous findings regarding stochastic nonlinear systems and experimental observations, we provide analytical insights regarding oscillatory responses of neural populations to stimulation from either endogenous or exogenous origins. Using a deceptively simple yet sparse and randomly connected network of neurons, we show how spiking inputs can reliably modulate the peak frequency and power expressed by synchronous neural populations without any changes in circuitry. Our results reveal that a generic, non-nonlinear and input-induced mechanism can robustly mediate these spectral fluctuations, and thus provide a framework in which inputs to the neurons bidirectionally regulate both the frequency and power expressed by synchronous populations. Theoretical and computational analysis of the ensuing spectral fluctuations was found to reflect the underlying dynamics of the input stimuli driving the neurons. Our results provide insights regarding a generic mechanism supporting spectral transitions observed across cortical networks and spanning multiple frequency bands
Antiferromagnetism and Superconductivity in CeRhIn
We discuss recent results on the heavy fermion superconductor CeRhIn
which presents ideal conditions to study the strong coupling between the
suppression of antiferromagnetic order and the appearance of unconventional
superconductivity. The appearance of superconductivity as function of pressure
is strongly connected to the suppression of the magnetic order. Under magnetic
field, the re-entrance of magnetic order inside the superconducting state shows
that antiferromagnetism nucleates in the vortex cores. The suppression of
antiferromagnetism in CeRhIn by Sn doping is compared to that under
hydrostatic pressure.Comment: 6 pages, 8 figures, to be published in Proc. Int. Conf. Heavy
Electrons (ICHE2010) J. Phys. Soc. Jpn. 80 (2011
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