437 research outputs found
Thermoelectric properties of junctions between metal and strongly correlated semiconductor
We propose a junction of metal and rare-earth compound semiconductor as the
basis for a possible efficient low-temperature thermoelectric device. If an
overlayer of rare earth atoms differing from the bulk is placed at the
interface, very high values of the figure of merit ZT can be reached at low
temperature. This is due to sharp variation of the transmission coefficient of
carriers across the junction at a narrow energy range, which is intrinsically
linked to the localized character of the overlayer f-orbital.Comment: RevTeX 3.0, 4 pages, 3 postscript figures. To be published in Applied
Physics Letter
Interacting Dirac Materials
We investigate the extent to which the class of Dirac materials in
two-dimensions provides general statements about the behavior of both fermionic
and bosonic Dirac quasiparticles in the interacting regime. For both
quasiparticle types, we find common features for the interaction induced
renormalization of the conical Dirac spectrum. We perform the perturbative
renormalization analysis and compute the self-energy for both quasiparticle
types with different interactions and collate previous results from the
literature whenever necessary. Guided by the systematic presentation of our
results in Table~\ref{Summary}, we conclude that long-range interactions
generically lead to an increase of the slope of the single-particle Dirac cone,
whereas short-range interactions lead to a decrease. The quasiparticle
statistics does not qualitatively impact the self-energy correction for
long-range repulsion but does affect the behavior of short-range coupled
systems, giving rise to different thermal power-law contributions. The
possibility of a universal description of the Dirac materials based on these
features is also mentioned.Comment: 19 pages and 12 Figures; Contains 6 Appendice
Suppressed reflectivity due to spin-controlled localization in a magnetic semiconductor
The narrow gap semiconductor FeSi owes its strong paramagnetism to
electron-correlation effects. Partial Co substitution for Fe produces a
spin-polarized doped semiconductor. The spin-polarization causes suppression of
the metallic reflectivity and increased scattering of charge carriers, in
contrast to what happens in other magnetic semiconductors, where magnetic order
reduces the scattering. The loss of metallicity continues progressively even
into the fully polarized state, and entails as much as a 25% reduction in
average mean-free path. We attribute the observed effect to a deepening of the
potential wells presented by the randomly distributed Co atoms to the majority
spin carriers. This mechanism inverts the sequence of steps for dealing with
disorder and interactions from that in the classic Al'tshuler Aronov approach -
where disorder amplifies the Coulomb interaction between carriers - in that
here, the Coulomb interaction leads to spin polarization which in turn
amplifies the disorder-induced scattering.Comment: 6 figures Submitted to PR
Stripe phases in high-temperature superconductors
Stripe phases are predicted and observed to occur in a class of
strongly-correlated materials describable as doped antiferromagnets, of which
the copper-oxide superconductors are the most prominent representative. The
existence of stripe correlations necessitates the development of new principles
for describing charge transport, and especially superconductivity, in these
materials.Comment: 5 pp, 1 color eps fig., to appear as a Perspective in Proc. Natl.
Acad. Sci. US
Controlling ferromagnetic ground states and solitons in thin films and nanowires built from iron phthalocyanine chains
Iron phthalocyanine (FePc) is a molecular semiconductor whose building blocks are one-dimensional ferromagnetic chains. We show that its optical and magnetic properties are controlled by the growth strategy, obtaining extremely high coercivities of over 1 T and modulating the exchange constant between 15 and 29 K through tuning the crystal phase by switching from thin films with controlled orientations, to ultralong nanowires. Magnetisation measurements are analysed using concepts and formulas with broad applicability to all one-dimensional ferromagnetic chains. They show that FePc is best described by a Heisenberg model with moments preferentially lying in the molecular planes. The chain Hamiltonian is very similar to that for the classic inorganic magnet CsNiF3, but with ferromagnetic rather than antiferromagnetic interchain interactions. The data at large magnetic fields are well-described by the soliton picture, where the dominant degrees of freedom are moving one-dimensional magnetic domain walls and at low temperatures and fields by the “super-Curie-Weiss” law characteristic of nearly one-dimensional xy and Heisenberg ferromagnets. The ability to control the molecular orientation and ferromagnetism of FePc systems, and produce them on flexible substrates, together with excellent transistor characteristics reported previously for phthalocyanine analogues, makes them potentially useful for magneto-optical and spintronic devices
Ultra-high-resolution software-defined photonic terahertz spectroscopy
A novel technique for high-resolution 1.5 µm photonics-enabled terahertz (THz) spectroscopy using software control of
the illumination spectral line shape (SLS) is presented. The technique enhances the performance of a continuous-wave
THz spectrometer to reveal previously inaccessible details of closely spaced spectral peaks. We demonstrate the technique by performing spectroscopy on LiYF4:Ho3+, a material of interest for quantum science and technology, where
we discriminate between inhomogeneous Gaussian and homogeneous Lorentzian contributions to absorption lines
near 0.2 THz. Ultra-high-resolution (<100 Hz full-width at half maximum) frequency-domain spectroscopy with
quality factor Q > 2 × 109
is achieved using an exact frequency spacing comb source in the optical communications
band, with a custom uni-traveling-carrier photodiode mixer and coherent down-conversion detection. Software-defined
time-domain modulation of one of the comb lines is demonstrated and used to resolve the sample SLS and to obtain a
magnetic field-free readout of the electronuclear spectrum for the Ho3+ ions in LiYF4:Ho3+. In particular, homogeneous
and inhomogeneous contributions to the spectrum are readily separated. The experiment reveals previously unmeasured
information regarding the hyperfine structure of the first excited state in the 5
I8 manifold complementing the results
reported in Phys. Rev. B 94, 205132 (2016)
Optical investigation of the metal-insulator transition in
We present a comprehensive optical study of the narrow gap
semiconductor. From the optical reflectivity, measured from the far infrared up
to the ultraviolet spectral range, we extract the complete absorption spectrum,
represented by the real part of the complex optical
conductivity. With decreasing temperature below 80 K, we find a progressive
depletion of below cm, the
semiconducting optical gap. The suppressed (Drude) spectral weight within the
gap is transferred at energies and also partially piles up over a
continuum of excitations extending in the spectral range between zero and
. Moreover, the interaction of one phonon mode with this continuum leads
to an asymmetric phonon shape. Even though several analogies between
and were claimed and a Kondo-insulator scenario was also invoked for
both systems, our data on differ in several aspects from those of
. The relevance of our findings with respect to the Kondo insulator
description will be addressed.Comment: 17 pages, 5 figure
Anisotropic three-dimentional magnetic fluctuations in heavy fermion CeRhIn5
CeRhIn5 is a heavy fermion antiferromagnet that orders at 3.8 K. The
observation of pressure-induced superconductivity in CeRhIn5 at a very high Tc
of 2.1 K for heavy fermion materials has led to speculations regarding to its
magnetic fluctuation spectrum. Using magnetic neutron scattering, we report
anisotropic three-dimensional antiferromagnetic fluctuations with an energy
scale of less than 1.7 meV for temperatures as high as 3Tc. In addition, the
effect of the magnetic fluctuations on electrical resistivity is well described
by the Born approximation.Comment: 4 pages, 4 figure
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