3,611 research outputs found
Theory of nonlinear phononics for coherent light-control of solids
We present a microscopic theory for ultrafast control of solids with
high-intensity terahertz frequency optical pulses. When resonant with selected
infrared-active vibrations, these pulses transiently modify the crystal
structure and lead to new collective electronic properties. The theory predicts
the dynamical path taken by the crystal lattice using first-principles
calculations of the energy surface and classical equations of motion, as well
as symmetry considerations. Two classes of dynamics are identified. In the
perturbative regime, displacements along the normal mode coordinate of
symmetry-preserving Raman active modes can be achieved by cubic
anharmonicities. This explains the light-induced insulator-to-metal transition
reported experimentally in manganites. We predict a regime in which ultrafast
instabilities that break crystal symmetry can be induced. This nonperturbative
effect involves a quartic anharmonic coupling and occurs above a critical
threshold, below which the nonlinear dynamics of the driven mode displays
softening and dynamical stabilization.Comment: updated to reflect the published versio
Cavity-mediated electron-photon superconductivity
We investigate electron paring in a two-dimensional electron system mediated
by vacuum fluctuations inside a nanoplasmonic terahertz cavity. We show that
the structured cavity vacuum can induce long-range attractive interactions
between current fluctuations which lead to pairing in generic materials with
critical temperatures in the low-Kelvin regime for realistic parameters. The
induced state is a pair density wave superconductor which can show a transition
from a fully gapped to a partially gapped phase - akin to the pseudogap phase
in high- superconductors. Our findings provide a promising tool for
engineering intrinsic electron interactions in two-dimensional materials.Comment: 11 page
Description of a computer program to calculate reacting supersonic internal flow fields with shock waves using viscous characteristics: Program manual and sample calculations
A computer program for calculating internal supersonic flow fields with chemical reactions and shock waves typical of supersonic combustion chambers with either wall or mid-stream injectors is described. The usefulness and limitations of the program are indicated. The program manual and listing are presented along with a sample calculation
On a modified-Lorentz-transformation based gravity model confirming basic GRT experiments
Implementing Poincar\'e's `geometric conventionalism' a scalar
Lorentz-covariant gravity model is obtained based on gravitationally modified
Lorentz transformations (or GMLT). The modification essentially consists of an
appropriate space-time and momentum-energy scaling ("normalization") relative
to a nondynamical flat background geometry according to an isotropic,
nonsingular gravitational `affecting' function Phi(r). Elimination of the
gravitationally `unaffected' S_0 perspective by local composition of space-time
GMLT recovers the local Minkowskian metric and thus preserves the invariance of
the locally observed velocity of light. The associated energy-momentum GMLT
provides a covariant Hamiltonian description for test particles and photons
which, in a static gravitational field configuration, endorses the four `basic'
experiments for testing General Relativity Theory: gravitational i) deflection
of light, ii) precession of perihelia, iii) delay of radar echo, iv) shift of
spectral lines. The model recovers the Lagrangian of the Lorentz-Poincar\'e
gravity model by Torgny Sj\"odin and integrates elements of the precursor
gravitational theories, with spatially Variable Speed of Light (VSL) by
Einstein and Abraham, and gravitationally variable mass by Nordstr\"om.Comment: v1: 14 pages, extended version of conf. paper PIRT VIII, London,
2002. v2: section added on effective tensorial rank, references added,
appendix added, WEP issue deleted, abstract and other parts rewritten, same
results (to appear in Found. Phys.
Metastable ferroelectricity in optically strained
Fluctuating orders in solids are generally considered high-temperature
precursors of broken symmetry phases. However, in some cases these fluctuations
persist to zero temperature and prevent the emergence of long-range order, as
for example observed in quantum spin and dipolar liquids. is a
quantum paraelectric in which dipolar fluctuations grow when the material is
cooled, although a long-range ferroelectric order never sets in. We show that
the nonlinear excitation of lattice vibrations with mid-infrared optical pulses
can induce polar order in up to temperatures in excess of 290 K. This
metastable phase, which persists for hours after the optical pump is
interrupted, is evidenced by the appearance of a large second-order optical
nonlinearity that is absent in equilibrium. Hardening of a low-frequency mode
indicates that the polar order may be associated with a photo-induced
ferroelectric phase transition. The spatial distribution of the optically
induced polar domains suggests that a new type of photo-flexoelectric coupling
triggers this effect
Transiently enhanced interlayer tunneling in optically driven high-Tc superconductors
Recent pump-probe experiments reported an enhancement of superconducting transport along the c axis of underdoped YBa2Cu3O6+δ (YBCO), induced by a midinfrared optical pump pulse tuned to a specific lattice vibration. To understand this transient nonequilibrium state, we develop a pump-probe formalism for a stack of Josephson junctions, and we consider the tunneling strengths in the presence of modulation with an ultrashort optical pulse. We demonstrate that a transient enhancement of the Josephson coupling can be obtained for pulsed excitation and that this can be even larger than in a continuously driven steady state. Especially interesting is the conclusion that the effect is largest when the material is parametrically driven at a frequency immediately above the plasma frequency, in agreement with what is found experimentally. For bilayer Josephson junctions, an enhancement similar to that experimentally is predicted below the critical temperature Tc. This model reproduces the essential features of the enhancement measured below Tc. To reproduce the experimental results above Tc, we will explore extensions of this model, such as in-plane and amplitude fluctuations, elsewhere.Deutsche Forschungsgemeinschaft; SFB 925; EXC 1074; Joachim Herz StiftungFirst author draf
Photoinduced Electron Pairing in a Driven Cavity
We demonstrate how virtual scattering of laser photons inside a cavity via two-photon processes can induce controllable long-range electron interactions in two-dimensional materials. We show that laser light that is red (blue) detuned from the cavity yields attractive (repulsive) interactions whose strength is proportional to the laser intensity. Furthermore, we find that the interactions are not screened effectively except at very low frequencies. For realistic cavity parameters, laser-induced heating of the electrons by inelastic photon scattering is suppressed and coherent electron interactions dominate. When the interactions are attractive, they cause an instability in the Cooper channel at a temperature proportional to the square root of the driving intensity. Our results provide a novel route for engineering electron interactions in a wide range of two-dimensional materials including AB-stacked bilayer graphene and the conducting interface between LaAlO3 and SrTiO3
Light dynamics in glass-vanadium dioxide nanocomposite waveguides with thermal nonlinearity
We address the propagation of laser beams in Si02-VO2 nanocomposite
waveguides with thermo-optical nonlinearity. We show that the large
modifications of the absorption coefficient as well as notable changes of
refractive index of VO2 nanoparticles embedded into the SiO2 host media that
accompany the semiconductor-to-metal phase transition may lead to optical
limiting in the near-infrared wave range.Comment: 13 pages, 3 figures, to appear in Optics Letter
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