626 research outputs found
Overcoming the thermal regime for the electric-field driven Mott transition in V2O3
The complex interplay among electronic, magnetic and lattice degrees of freedom in Mott-Hubbard materials leads to different types of insulator-to-metal transitions (IMT) which can be triggered by temperature, pressure, light irradiation and electric field. However, several questions remain open concerning the quantum or thermal nature of electric field-driven transition process. Here, using intense terahertz pulses, we reveal the emergence of an instantaneous purely-electronic IMT in the Mott-Hubbard vanadium sequioxide (V2O3) prototype material. While fast electronics allow thermal-driven transition involving Joule heating, which takes place after tens of picoseconds, terahertz electric field is able to induce a sub-picosecond electronic switching. We provide a comprehensive study of the THz induced Mott transition, showing a crossover from a fast quantum dynamics to a slower thermal dissipative evolution for increasing temperature. Strong-field terahertz-driven electronic transition paves the way to ultrafast electronic switches and high-harmonic generation in correlated system
Terahertz Spectroscopy of Novel Superconductors
Through the coupling of Synchrotron Radiation and Michelson interferometry, one may obtain in the terahertz (THz) range transmittance and reflectivity spectra with a signal-to-noise ratio (S/N) up to 103. In this paper we review the application of this spectroscopic technique to novel superconductors with an increasing degree of complexity: the single-gap boron-doped diamond; the isotropic multiband V3Si, where superconductivity opens two gaps at the Fermi energy; the CaAlSi superconductor, isostructural to MgB2, with a single gap in the hexagonal ab plane and two gaps along the orthogonalcaxis
Difficulties in Mid-Infrared selection of AGN in dwarf galaxies
While massive black holes (MBHs) are known to inhabit all massive galaxies,
their ubiquitous presence in dwarf galaxies has not been confirmed yet, with
only a limited number of sources detected so far. Recently, some studies
proposed infrared emission as an alternative way to identify MBHs in dwarfs,
based on a similar approach usually applied to quasars. In this study, by
accurately combining optical and infrared data taking into account resolution
effects and source overlapping, we investigate in detail the possible
limitations of this approach with current ground-based facilities, finding a
quite low (0.4 per cent) fraction of active MBH in dwarfs that are
luminous in mid-infrared, consistent with several previous results. Our results
suggest that the infrared selection is strongly affected by several limitations
that make the identification of MBHs in dwarf galaxies currently prohibitive,
especially because of the very poor resolution compared to optical surveys, and
the likely contamination by nearby sources, although we find a few good
candidates worth further follow-ups. Optical, X-ray and radio observations,
therefore, still represent the most secure way to search for MBH in dwarfs.Comment: 7 pages, 7 figures, 1 table, accepted for publication on MNRA
Chemical post-processing of magneto-hydrodynamical simulations of star-forming regions: robustness and pitfalls
A common approach to model complex chemistry in numerical simulations is via
post-processing of existing magneto-hydrodynamic simulations, relying on
computing the evolution of chemistry over the dynamic history of a subset of
particles from within the raw simulation. Here, we validate such a technique,
assessing its ability to recover the abundances of chemical species, using the
chemistry package KROME. We also assess, for the first time, the importance of
the main free input parameters, by means of a direct comparison with a
self-consistent state-of-the-art simulation in which chemistry was directly
coupled to hydrodynamics. We have found that the post-processing is highly
reliable, with an accuracy at the percent level, even when the most relaxed
input parameters are employed. In particular, our results show that the number
of particles used does not affect significantly the average properties,
although it suppresses the appearance of possibly important spatial features.
On the other hand, the choice of the integration time-step plays a crucial
role. Longer integration time-steps can produce large errors, as the
post-processing solution will be forced towards chemical equilibrium, a
condition that does not always necessarily apply. When the interpolation-based
reconstruction of chemical properties is performed, the errors further increase
up to a factor of . Concluding, our results suggest that this technique
is extremely useful when exploring the relative quantitative effect of
different chemical parameters and/or networks, without the need of re-running
simulations multiple times, but some care should be taken in the choice of
particles sub-sample and integration time-step.Comment: 11 pages, 6 figures, 3 table
Infrared evidence of a Slater metal-insulator transition in NaOsO3
The magnetically driven metal-insulator transition (MIT) was predicted by
Slater in the fifties. Here a long-range antiferromagnetic (AF) order can open
up a gap at the Brillouin electronic band boundary regardless of the Coulomb
repulsion magnitude. However, while many low-dimensional organic conductors
display evidence for an AF driven MIT, in three-dimensional (3D) systems the
Slater MIT still remains elusive. We employ terahertz and infrared spectroscopy
to investigate the MIT in the NaOsO3 3D antiferromagnet. From the optical
conductivity analysis we find evidence for a continuous opening of the energy
gap, whose temperature dependence can be well described in terms of a second
order phase transition. The comparison between the experimental Drude spectral
weight and the one calculated through Local Density Approximation (LDA) shows
that electronic correlations play a limited role in the MIT. All the
experimental evidence demonstrates that NaOsO3 is the first known 3D Slater
insulator.Comment: 4 figure
Selection rules for the orbital angular momentum of optically-produced THz radiation
In this work we theoretically study the transduction ofthe Orbital Angular Momentum (OAM)lfor infraredpump lasers into the THz domain. In the case of opti-cal rectification, the transduction of OAM occurs onlythrough a spin-orbit interaction, with the selection ruleon the OAMl=0valid for any kind of polarizationof the pump, which means that there is no transfer ofOAM along the propagation axis. In the difference fre-quency generation the selection rule for the difference∆lbetween the OAM of the pump fields with linear orcircular polarization isl=∆l, whereaslranges from∆l−2to∆l+2in both the cases of radial and azimuthalpolarization. Moreover, for THz generation in the lat-ter case, high diffraction obtained with tightly focusedpumps yieldsltending to∆l±2, whileltends to zeroin the opposite case of large pump beam
Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe_{1.84}Co_{0.16}As_2 ultrathin film: an analysis of the optical gaps in the Co-doped BaFe_2As_2 pnictide
Here we report an optical investigation in the terahertz region of a 40 nm
ultrathin BaFeCoAs superconducting film with
superconducting transition temperature T = 17.5 K. A detailed analysis of
the combined reflectance and transmittance measurements showed that the optical
properties of the superconducting system can be described in terms of a
two-band, two-gap model. The zero temperature value of the large gap
, which seems to follow a BCS-like behavior, results to be
(0) = 17 cm. For the small gap, for which (0) = 8
cm, the temperature dependence cannot be clearly established. These gap
values and those reported in the literature for the BaFeCoAs
system by using infrared spectroscopy, when put together as a function of
T, show a tendency to cluster along two main curves, providing a unified
perspective of the measured optical gaps. Below a temperature around 20 K, the
gap-sizes as a function of T seem to have a BCS-like linear behavior, but
with different slopes. Above this temperature, both gaps show different
supra-linear behaviors
Intensity and phase retrieval of IR laser pulse by THz-based measurement and THz waveform modulation
Abstract THz radiation is of great interest for a variety of applications. Simultaneously with the demonstration of high-intensity THz sources the idea to use this radiation for particle acceleration started to be investigated. THz accelerating gradients up to GV/m have been demonstrated in laboratory. THz radiation can be generated through the optical rectification process induced in non-linear crystals by a pump laser. The temporal shape of the pump laser and in general its characteristics are important aspects to be known in order to produce THz radiation via optical rectification in a controlled way. Here we present a technique that can be used to retrieve the temporal profile characteristics (envelope and phase) of the pump laser, starting from the detection of the THz waveform/spectrum and the knowledge of the physical/optical properties of the crystal used to produce it. This work also shows that the THz field can be shaped by properly acting on the pump laser phase. The possibility to opportunely shape the THz field is of great importance for many applications. Therefore this work paves the way to the possibility to coherently and dynamically control the THz field shape
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