Apr 07

19th ETSF Workshop on Electronic Excitations

19th ETSF Workshop on Electronic Excitations:
Complex systems in Biology and Nanoscience

I’m one of the organizer of the workshop “19th ETSF Workshop on Electronic Excitations: Complex systems in Biology and Nanoscience” that will be held in Zaragoza (Spain) from Septembre 23th to Septembre 26th.

Mar 02

Workshop: Gutzwiller wavefunction and related methods

gutzwiller_conference
I’m one of the organizer of the workshop “Gutzwiller wave functions and related methods” that will be held in Valance (France) from June 16th to June 19th. Here the French announcement, and conference web-page.

Feb 25

Review on GW and Bethe-Salpeter for molecular systems

review_organic

We just published a new review on GW and Bethe-Salpeter (BSE) for molecules. In the review we discuss electronic excitations both in TDDFT and  in the Green’s function approach.  The successes and failures of the different methods are presented. In particular we focused on charge-transfer excitations for which the photoexcited electron and the hole left behind are spatially separated. Such excitations are central to many applications in photovoltaics, photocatalysis, photosynthesis and biology.

Reference:
Phil. Trans. R. Soc. A, vol. 372 no. 2011 (2014)
The PDF is available here.

Jan 03

Excitons in second harmonic generation

excitonTogether with Myrta Gruning we just submitted a new paper on second harmonic generation(SHG)  in two-dimensional materials, h-BN and MoS2. We calculated the SHG using a real-time approach that we recently developed. We included both local field effects and electron-hole interaction at the level of screened exchange , without any additional approximation. Our approch is equivalent to the combination of GW + Bethe-Salpeter equation, widely used in linear response theory.
We found that bound excitons are clearly visible in the SHG spectra (see the simple picture on the left)  and they strongly enhance the SHG intensity.
The paper is available here:

Second Harmonic Generation in h-BN and MoS2 monolayers: the role of electron-hole interaction

Sep 26

Drawing nanostructures with povray

 

Sometime ago I discovered povray, a ray tracing program which generates stunning three-dimensional graphics. I used it to draw nanostructures and in particular nanotubes in different configurations, hereafter the plots and the corresponding scripts:
3tubesFrom left to right:
1) illuminated nanotube, STM_tips.pov
2) gated nanotube, gated_tube.pov
3) nanotube and STM tip, light_tube.pov

Sep 24

Efficient Gate-tunable light-emitting device made of defective boron nitride nanotubes

I published my first “Nature”, even if the lower level one, a Nature Scientific Reports:

tubesingleEfficient Gate-tunable light-emitting device made of defective boron nitride nanotubes: from ultraviolet to the visible

Claudio Attaccalite, Ludger Wirtz, Andrea Marini & Angel Rubio
Scientific Reports 3, Article number:2698

Boron nitride is a promising material for nanotechnology applications due to its two-dimensional graphene-like, insulating, and highly-resistant structure. Recently it has received a lot of attention as a substrate to grow and isolate graphene as well as for its intrinsic UV lasing response. Similar to carbon, one-dimensional boron nitride nanotubes (BNNTs) have been theoretically predicted and later synthesised. Here we use first principles simulations to unambiguously demonstrate that i) BN nanotubes inherit the highly efficient UV luminescence of hexagonal BN; ii) the application of an external perpendicular field closes the electronic gap keeping the UV lasing with lower yield; iii) defects in BNNTS are responsible for tunable light emission from the UV to the visible controlled by a transverse electric field (TEF). Our present findings pave the road towards optoelectronic applications of BN-nanotube-based devices that are simple to implement because they do not require any special doping or complex growth.

Sep 17

Nonlinear optics by means of the dynamical Berry-phase

Si_absX3-AB_IP_vs_MossWe present an ab-initio real-time based computational approach to nonlinear optical properties in Condensed Matter systems[1]. The equation of motions, and in particular the coupling of the electrons with the external electric field, are derived from the Berry phase formulation of the dynamical polarization.[2] The zero-field Hamiltonian includes crystal local field effects and the renormalization of the independent particle energy levels by correlation effects. We also discuss the possibility of accurately treating excitonic effects by adding a screened Hartree-Fock self-energy operator. The approach is validated by calculating the second-harmonic generation of SiC and AlAs bulk semiconductors: an excellent agreement is obtained with existing ab-initio calculations from response theory in frequency domain. We finally show applications to the second-harmonic generation of CdTe and the third-harmonic generation of Si.
In the past different methods have been proposed to study non-linear response of solids in real-time:
1). Virk and Sipe[3] presented an approach based on non-equilibrium Green’s function and a global gauge transformation to deal with the external field, I found this paper quite difficult to read.
2) The strategy of Springborg and Kirtman[4] was to optimized the phase of the Bloch functions and use the velocity gauge. Altought the idea sounds good, I think it is implementation in a plane wave code with a lot of conduction bands and in more than one dimension is very complicated.
3) Then there are the approaches based on TDDFT and velocity gauge [5]. Even if the seems very appealing the forgot the phase factor in front of the wave-function where there is the position operator that cannot be treated in a simple way. Disregarding this factor only the linear response is correct see refernces [6][7]. However they have one point stronger than our approach, the fact that they included the coupling with the Maxwell equations.
The same criticism is valid for the approached based on Kadanoff-Baym equations in velocity gauge.

References

  1. Dynamics of Berry-phase polarization in time-dependent electric fields
    Souza, I., Íniguez, J., & Vanderbilt, D. (2004). . Physical Review B, 69(8), 085106.
  2. Nonlinear optics from ab-initio by means of the dynamical Berry-phase
    C. Attaccalite, M. Grüning, http://arxiv.org/abs/1309.4012
  3. Semiconductor optics in length gauge: A general numerical approach
    Virk, K. S., & Sipe, J. E. (2007). . Physical Review B, 76(3), 035213.
  4. Analysis of vector potential approach for calculating linear and nonlinear responses of infinite periodic systems to a finite static external electric field
    Springborg, M., & Kirtman, B. (2008). . Physical Review B, 77(4), 045102.
  5. Real-space, real-time method for the dielectric function.
    Bertsch, G. F., Iwata, J. I., Rubio, A., & Yabana, K. (2000).  Physical Review B, 62(12), 7998.
  6. Equivalence of interaction Hamiltonians in the electric dipole approximation.
    Rzązewski, K., & Boyd, R. W. (2004). Journal of modern optics, 51(8), 1137-1147.

  7.  Matter-field interaction in atomic physics and quantum optics.
    Lamb Jr, W. E., Schlicher, R. R., & Scully, M. O. (1987).  Physical Review A, 36, 2763-2772.

Sep 16

Optical excitations in a paradigmatic model dipeptide

dipeptideWe just post on arxiv a new paper on the optical excitations in a paradigmatic model dipeptide, here in the figure. We studied local and charge transfer excitations by means of state of the art GW and Bethe-Salpeter equations as implemented in the FIESTA code. In particular we focused on the effect of self-consistency in GW, both eigenvalues and eigenvectors and compared our results with the ones obtained by quantum chemistry methods.

 

 

Reference:
Many-body Green’s function GW and Bethe-Salpeter study of the optical excitations in a paradigmatic model dipeptide

Carina Faber, Paul Boulanger, Ivan Duchemin, Claudio Attaccalite, Xavier Blase
http://arxiv-web3.library.cornell.edu/abs/1309.0646